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Volume 32 Issue 6
Dec.  2021
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Bárbara Bueno Toledo, Valdecir de Assis Janasi. Petrogenesis of Granites from the Ediacaran Socorro Batholith, SE Brazil: Constraints from Zircon Dating, Geochemistry and Sr-Nd-Hf Isotopes. Journal of Earth Science, 2021, 32(6): 1397-1414. doi: 10.1007/s12583-021-1494-z
Citation: Bárbara Bueno Toledo, Valdecir de Assis Janasi. Petrogenesis of Granites from the Ediacaran Socorro Batholith, SE Brazil: Constraints from Zircon Dating, Geochemistry and Sr-Nd-Hf Isotopes. Journal of Earth Science, 2021, 32(6): 1397-1414. doi: 10.1007/s12583-021-1494-z

Petrogenesis of Granites from the Ediacaran Socorro Batholith, SE Brazil: Constraints from Zircon Dating, Geochemistry and Sr-Nd-Hf Isotopes

doi: 10.1007/s12583-021-1494-z
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  • Whole rock elemental and Sr-Nd isotope geochemistry and in situ zircon Hf isotope geochemistry were used to identify the sources of the Neoproterozoic granites from the Socorro batholith, Socorro-Guaxupé Nappe (SGN), South Brasilia Orogen, Brazil. Zircon trace elements and Hf isotope geochemistry provided information about sources and crystallization (T, fO2) conditions. Three main types of granites built the bulk of the batholiths, beginning with probably pre-collisional ~640-630 Ma charnockites, and ending with ~610 Ma voluminous post-collisional high-K calc-alkaline (HKCA) I-type granites (Bragança Paulista-type). Several types of leucogranites were generated from 625 to 610 Ma, spanning the interval from collisional to post-collisional tectonics. Two charnockite bodies occur in the study area: the ~640 Ma Socorro charnockite has remarkable chemical similarities with Bragança Paulista-type granites, but higher εNd(t)=-6.1 and average zircon εHf(t)=-9.1 and lower 86Sr/87Srt (0.709 3) values, indicative of more juvenile and water-poor source. The ~633 Ma Atibaia charnockite has distinct geochemical signature (lower Mg# and Sr content; higher Zr), more negative εNd(t)=-14.1, similar average zircon εHf(t)=-8.9, and much higher 86Sr/87Srt=0.719 7, probably reflecting a larger component from old crust. The predominant ~610 Ma Bragança Paulista-type granites were emplaced in a post-collisional setting, and correspond to porphyritic biotite-hornblende monzogranites of high-K calc-alkaline character with 61 wt.%-67 wt.% SiO2, high Mg# (39-42), Sr/Y (19-40), La/Yb (12-69), highly negative εNd(t) (-12.3 to-12.9) and zircon εHf(t) (-12 to -17) and 87Sr/86Srt=0.711 9-0.713 1. These features are interpreted as indicative of magma generation in a thickened crust, where melts from enriched mantle sources emplaced in the lowermost crust, heated host old continental crust rocks (gneisses and granulites) and partially mixed with their melting products. Leucogranite plutons (SiO2 > 72 wt.%) occurring in the southern portion of the batholith have a range of geochemical and isotope signatures, reflecting melting of crustal sources in space and time between ~625 Ma (Bocaina Pluton) and ~610 Ma (Bairro da Pedreira Pluton). Highly negative εNd(t) (-16.2) and average zircon εHf(t)=-16, and high 87Sr/86Srt(0.715 6-0.717 1) are consistent with relatively old ortho-and paragneiss sources similar to those which generated regionally abundant migmatites and anatectic granites in the collisional to post-collisional setting.
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    Trouw, R. A. J., Peternel, R., Ribeiro, A., et al., 2013. A New Interpretation for the Interference Zone between the Southern Brasília Belt and the Central Ribeira Belt, SE Brazil. Journal of South American Earth Sciences, 48: 43-57. https://doi.org/10.1016/j.jsames.2013.07.012 doi:  10.1016/j.jsames.2013.07.012
    Valeriano, C. D. M., Mendes, J. C., Tupinambá, M., et al., 2016. Cambro- Ordovician Post-Collisional Granites of the Ribeira Belt, SE-Brazil: A Case of Terminal Magmatism of a Hot Orogen. Journal of South American Earth Sciences, 68(1): 269-281. https://doi.org/10.1016/j.jsames.2015.12.014 doi:  10.1016/j.jsames.2015.12.014
    Virmond, L. A., 2019. Petrochronology of Anatectic Rocks from Nazaré Paulista (SP), Southern Socorro Guaxupé Nappe: [Dissertation]. Universidade de São Paulo, São Paulo
    Watson, E. B., Harrison, T. M., 1983. Zircon Saturation Revisited: Temperature and Composition Effects in a Variety of Crustal Magma Types. Earth and Planetary Science Letters, 64(2): 295-304. https://doi.org/10.1016/0012-821x(83)90211-x doi:  10.1016/0012-821x(83)90211-x
    Wernick, E., Didier, J., Artur, A. C., et al., 1984a. Caracterização da Zona Marginal Charnockítica do Complexo Socorro nos Arredores da Cidade Homônima, SP/MG. 33° Congresso Brasileiro de Geologia, 6: 2919-2934
    Wernick, E., Hormann, P. K., Artur, A. C., et al., 1984b. Aspectos Petrológicos do Complexo Granítico Socorro (SP/MG): Dados Analíticos e Discussão Preliminar. Revista Brasileira de Geociências, 14(1): 23-29. https://doi.org/10.25249/0375-7536.19842329 doi:  10.25249/0375-7536.19842329
    Whalen, J. B., Hildebrand, R. S., 2019. Trace Element Discrimination of Arc, Slab Failure, and A-Type Granitic Rocks. Lithos, 348/349(11): 105179. https://doi.org/10.1016/j.lithos.2019.105179 doi:  10.1016/j.lithos.2019.105179
    Zhao, K., Xu, X. S., Erdmann, S., 2017. Crystallization Conditions of Peraluminous Charnockites: Constraints from Mineral Thermometry and Thermodynamic Modelling. Contributions to Mineralogy and Petrology, 172(5): 26. https://doi.org/10.1007/s00410-017-1344-2 doi:  10.1007/s00410-017-1344-2
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Petrogenesis of Granites from the Ediacaran Socorro Batholith, SE Brazil: Constraints from Zircon Dating, Geochemistry and Sr-Nd-Hf Isotopes

doi: 10.1007/s12583-021-1494-z

Abstract: Whole rock elemental and Sr-Nd isotope geochemistry and in situ zircon Hf isotope geochemistry were used to identify the sources of the Neoproterozoic granites from the Socorro batholith, Socorro-Guaxupé Nappe (SGN), South Brasilia Orogen, Brazil. Zircon trace elements and Hf isotope geochemistry provided information about sources and crystallization (T, fO2) conditions. Three main types of granites built the bulk of the batholiths, beginning with probably pre-collisional ~640-630 Ma charnockites, and ending with ~610 Ma voluminous post-collisional high-K calc-alkaline (HKCA) I-type granites (Bragança Paulista-type). Several types of leucogranites were generated from 625 to 610 Ma, spanning the interval from collisional to post-collisional tectonics. Two charnockite bodies occur in the study area: the ~640 Ma Socorro charnockite has remarkable chemical similarities with Bragança Paulista-type granites, but higher εNd(t)=-6.1 and average zircon εHf(t)=-9.1 and lower 86Sr/87Srt (0.709 3) values, indicative of more juvenile and water-poor source. The ~633 Ma Atibaia charnockite has distinct geochemical signature (lower Mg# and Sr content; higher Zr), more negative εNd(t)=-14.1, similar average zircon εHf(t)=-8.9, and much higher 86Sr/87Srt=0.719 7, probably reflecting a larger component from old crust. The predominant ~610 Ma Bragança Paulista-type granites were emplaced in a post-collisional setting, and correspond to porphyritic biotite-hornblende monzogranites of high-K calc-alkaline character with 61 wt.%-67 wt.% SiO2, high Mg# (39-42), Sr/Y (19-40), La/Yb (12-69), highly negative εNd(t) (-12.3 to-12.9) and zircon εHf(t) (-12 to -17) and 87Sr/86Srt=0.711 9-0.713 1. These features are interpreted as indicative of magma generation in a thickened crust, where melts from enriched mantle sources emplaced in the lowermost crust, heated host old continental crust rocks (gneisses and granulites) and partially mixed with their melting products. Leucogranite plutons (SiO2 > 72 wt.%) occurring in the southern portion of the batholith have a range of geochemical and isotope signatures, reflecting melting of crustal sources in space and time between ~625 Ma (Bocaina Pluton) and ~610 Ma (Bairro da Pedreira Pluton). Highly negative εNd(t) (-16.2) and average zircon εHf(t)=-16, and high 87Sr/86Srt(0.715 6-0.717 1) are consistent with relatively old ortho-and paragneiss sources similar to those which generated regionally abundant migmatites and anatectic granites in the collisional to post-collisional setting.

Bárbara Bueno Toledo, Valdecir de Assis Janasi. Petrogenesis of Granites from the Ediacaran Socorro Batholith, SE Brazil: Constraints from Zircon Dating, Geochemistry and Sr-Nd-Hf Isotopes. Journal of Earth Science, 2021, 32(6): 1397-1414. doi: 10.1007/s12583-021-1494-z
Citation: Bárbara Bueno Toledo, Valdecir de Assis Janasi. Petrogenesis of Granites from the Ediacaran Socorro Batholith, SE Brazil: Constraints from Zircon Dating, Geochemistry and Sr-Nd-Hf Isotopes. Journal of Earth Science, 2021, 32(6): 1397-1414. doi: 10.1007/s12583-021-1494-z
  • The largest volumes of granitic magmas are formed in convergent plate boundaries, from oceanic plate consumption to collision and orogen collapse (Moyen et al., 2017; Barbarin, 1999). Collision is considered as the initial impact of two or more continental plates, and is characterized by high-pressure metamorphism (Liégeois et al., 1998). The post-collisional period has great complexity, and encompasses events occurring after the collision, but still related to it (Liégeois, 1998; Bonin, 1990). The "post-collisional" period is thus referred in this paper as that part of the orogen-building period that immediately follows ocean plate consumption and crustal thickening.

    High-K calc-alkaline (HKCA) magmatism is typical of the post-collisional setting (Barbarin, 1999; Liégeois, 1998), but occurs in different tectonic contexts of convergent plate margins, including continental margin magmatic arcs (e.g., Pitcher, 1983). Their use as proxies of past tectonic environments is therefore equivocal, as exemplified by recent propositions suggesting that even some calc-alkaline batholiths classically admitted as subduction-related could have been generated in collisional events associated with slab failure (or "slab breakoff") (e.g., Hildebrand and Whalen, 2014).

    In the Ribeira belt, SE Brazil, the Neoproterozoic granitic magmatism is dominated by volumetrically large batholiths of HKCA metaluminous, I-type granitic rocks bearing hornblende as a main mafic mineral, with ages ranging from 640 to 600 Ma. The Socorro batholith, with about 2 200 km of exposure area (Artur, 2003), the focus of this study, corresponds to one of the most expressive of these granitic batholiths. The tectonic setting of these extensive elongated HKCA batholiths is still uncertain. They have been considered as products of active continental margin magmatic arcs (Heilbron et al., 2017, 2013, 2004; Trouw et al., 2013; Janasi and Ulbrich, 1991; Campos Neto et al., 1984a), but recent works associated some of them with the post-collisional period (Toledo et al., 2018; Janasi et al., 2016; Meira et al., 2015) and therefore not directly linked to active ocean plate consumption.

    Modern geochronological and isotopic data (in situ Hf isotope determination in zircon, U-Pb in situ dating by SHRIMP) to support inferences on the sources of magmas and their tectonic significance are relatively scarce in Socorro batholith. A recent work by our group (Toledo et al., 2018) reported five U-Pb zircon ages by SHRIMP, ranging from 608±7 (HKCA Bragança Paulista-type granite) to 642±4 Ma (Socorro charnockite).

    This article aims to contribute to determination of sources, crystallization conditions, magma evolution and tectonic environment of the granitic rocks that make up the Socorro batholith based on new elemental and Sr-Nd isotope whole rock geochemistry, Hf isotope and trace-element geochemistry of zircon, Ti in zircon thermometry and Zr saturation temperatures.

  • The Socorro-Guaxupé Nappe (SGN) is a high-metamorphic grade allochthonous terrane interpreted to represent the root of a Late Neoproterozoic magmatic arc, developed at the active margin of a continental plate (e.g., Campos Neto and Caby, 2000, 1999). The SGN is constituted by para-/orthogneisses and migmatites and (in its basal portions) granulites, intruded by several types of younger Neoproterozoic (630-600 Ma) "syn-orogenic" granites. It was emplaced on top of a nappe system dominated by metasedimentary rocks of both passive margin (the lower nappes) and active margin (the middle nappes) at the southern margin of the São Francisco Craton (Campos Neto et al., 2011). High- pressure metamorphism affecting basement slices and marking the collisional event is recorded in retro-eclogites dated at ~625 Ma (e.g., Coelho et al., 2017). Timing of metamorphism related the collisional tectonics is estimated at 625-600 Ma (Rocha et al., 2018; see also Martins et al., 2009). Shallow-level post-orogenic granites dominantly of A-type character (the Itu granitic province; Janasi et al., 2009) intrude the southern portion of the SGN.

    Geologic terranes located immediately south of the SGN and related to the Ribeira belt (Apiaí and São Roque domains) are also intruded by large volumes of "syn-tectonic" HKCA granites, and by the post-orogenic granites of the post-orogenic Itu granitic province, which straddles the limits between the SGN and the Ribeira belt. Domains located further E-SE in the Ribeira belt (the Oriental terrane, Janasi et al., 2016) also have significant volumes of Neoproterozoic granites, but the most voluminous "syn- orogenic" batholiths are typically younger (~590-565 Ma; e.g., Heilbron et al., 2017), as is the post-orogenic granite magmatism (520-500 Ma; Valeriano et al., 2016).

  • The Socorro batholith corresponds to an extensive area dominated by granitic rocks elongated in the N30E direction in the southern portion of SGN (ca. 60 km×25 km; total ca. 2 200 km2). Our new map of the southern part of the batholith (Fig. 1) was based on the field survey at the 1 : 50 000 scale supported by reprocessing of available aerogamaspectrometric survey (CPRM) and focused on the delimitation of circumscribed granite bodies. Different types of high-grade, mostly migmatitic ortho and paragneisses that constitute the country rocks of the granite bodies were not distinguished.

    Figure 1.  Geological map of the southern portion of the Socorro batholith.

    About all granitic rocks from the Socorro batholith are coarse-grained and/or porphyritic, and many show some degrees of deformation producing grain reduction at the contacts of the primary minerals. Modal counting of a representative number of samples would therefore be too laborious and imprecise, and we used geochemical classification diagrams (Fig. 2, Debon and Le Fort, 1988; De la Roche et al., 1980) to name the rocks.

    Figure 2.  Geochemical classification diagrams of (a) P-Q (after Debon and Le Fort, 1988) and (b) R1-R2 (after De la Roche et al., 1980) for Socorro batholith samples.

    The most voluminous rocks in the Socorro batholith constitute the Bragança Paulista type, corresponding to porphyritic hornblende-biotite granites of HKCA character, and occur mainly in the NE part of the map (Fig. 1). The color index varies from 15 to 20, with abundant large K-feldspar megacrysts averaging (3-4) cm×(1.5-2) cm set in a massive, coarse matrix where plagioclase occurs as the more abundant (~30%) or the only feldspar, and is accompanied by quartz (~19%), biotite and hornblende. Titanite (~1%), zircon, apatite and magnetite are the main accessory minerals. Naranjo and Vlach (2018) determined the modal composition of a sample collected in the same outcrop of our ATB-13 sample, and reported plagioclase (29%), quartz (17%), K-feldspar (32%, with 23% as megacrysts and 9% as matrix), biotite (10%), hornblende (10%), ilmenite (1%), and minor amounts (total ~1%) of allanite, zircon, apatite, magnetite, titanite and sulfides.

    The Bragança Paulista-type granites were mostly emplaced at ~610 Ma, based on two SHRIMP U-Pb zircon ages reported in our previous work (610±7 and 608±7 Ma; Toledo et al., 2018), which are equivalent to the result obtained in one sample by Ebert et al. (1996) (610±10 Ma). A new age determination presented in this contribution yields a similar age (cf. item 5.1).

    Three bodies of leucogranite appear in the map (Fig. 1), and are named Bocaina, Bairro da Pedreira and Fazenda São Sebastião plutons. All the three types are classified as syenogranites (Debon and Le Fort, 1988) and granites (De la Roche et al., 1980) (Fig. 2). The Bocaina Pluton is constituted by a pink coarse- grained to porphyritic biotite-syenogranite with up to 4 cm long K-feldspar megacrysts and low (~6) color index. It corresponds to the typical "Salmão-type" granite referred by Campos Neto et al. (1984a), who reported typical mineral proportions as 33% quartz, 34% K-feldspar, 28% plagioclase, 4% biotite and accessory apatite, zircon, opaque minerals and monazite totaling 1%. Our previous U-Pb dating of sample BRP-03 by SHRIMP indicated a crystallization age of 624±4 Ma, with inherited zircon cores of Mesoproterozoic to Paleoproterozoic ages (1 500 and 1 780 Ma; Toledo et al., 2018).

    The Bairro da Pedreira Pluton occurs at the southern portion of the map (Fig. 1), and the best exposures are found in the Atibaia quarry, located at its southern border. The pluton has an elongated shape along the N-S direction (Fig. 1). The main rock type is a foliated inequigranular syenogranite, with color index around 8. The rock foliation is defined by alignment of ~2 cm-long alkali feldspar megacrysts and also by the elongation of quartz, marking a magmatic to solid-state foliation. Main accessory minerals are zircon, apatite, opaque minerals and monazite. A SHRIMP U-Pb zircon age of 613±2 Ma was obtained in this work for a typical syenogranite from the Atibaia quarry (see below item 5.1).

    The Fazenda São Sebastião Pluton is located NE of the city of Itatiba at the southwestern end of the Socorro batholith. It is dominated by a foliated inequigranular hololeucocratic granite with large quartz crystals (up to 1-2 cm) and color index around 4. Biotite, muscovite and opaque minerals are present in low concentration.

    The Atibaia charnockite is a small NE-elongated body (3 km×0.5 km) in the southern portion of the Socorro batholith, first reported by Oliveira et al. (1986). The rock is medium- to coarse-grained, with magmatic foliation, with greenish-brown color and monzogranitic composition. Mafic minerals are clinopyroxene, orthopyroxene, amphibole and traces of biotite; accessory minerals are zircon, apatite and opaque minerals. Associated with these charnockites apparently as a marginal facies are heterogeneous medium-grained equigranular granites, locally with traces of garnet, possibly related to contamination with country-rock migmatites. Our previous U-Pb zircon SHRIMP dating yielded a weighted average 206Pb/238U age of 633±6 Ma for this charnockite (Toledo et al., 2018). This age marginally overlaps, but its central value is slightly younger than that of the charnockites described in the northern part of the batholith (Socorro charnockite; Artur, 2003; Wernick et al., 1984a), that were dated at 642±4 Ma (Toledo et al., 2018).

    The paragneisses correspond to a package dominated by migmatitic garnet-biotite gneisses, locally sillimanite-bearing, with bands of less aluminous character (biotite gneiss) and cm- to dm-sized amphibolite. These rocks are locally densely intruded by veins, dikes and small bodies of garnet-biotite granite (Nazaré Paulista-type); detailed descriptions can be found in Janasi et al. (2005) and Martins et al. (2009).

    Orthogneisses are predominant in the western portion of the mapped area (Fig. 1), and correspond to domains of strongly foliated metaplutonic rocks of varied compositions (granitic to dioritic) and textures (equigranular to porphyritic), dominantly with metaluminous character (bearing biotite± hornblende). Hololeucocratic varieties are frequent near the city of Itatiba, where they may be difficult to separate from similar less foliated rocks that are considered in this work as a younger granite body (the Fazenda São Sebastião Pluton).

  • Whole-rock geochemical data (major, minor and trace- elements by XRF; REE, Th, U and additional trace-elements by ICP-MS) and Sr-Nd isotope data analyzed by TIMS were mainly used in this work. In situ determinations and Hf isotope ratios in zircon by LA-ICP-MS and U-Pb ages by SHRIMP were also obtained. Zircon crystals were extracted from 6 to 8 kg granite samples using standard techniques of milling, magnetic and dense liquids separation. Prior to the microanalyses, cathodoluminescence (CL) and optical microscopy images were obtained from the epoxy mounts with 50-100 crystals, and carefully studied to identify textural features of the zircon crystals.

    All analyses were performed in the laboratories of Instituto de Geociências, Universidade de São Paulo, Brazil. Details of the analytical protocols of all methods employed are presented in ESM1.

  • The Bragança Paulista-type granites show a SiO2 range of 60.7 wt.%-66.3 wt.%, reflecting their more primitive character compared to other granites of the batholith, which is also reflected in higher Mg# (molar 100×MgO/(MgO+FeOT))=(39-42), CaO (4.5 wt.%-3.2 wt.%), Fe2O3 (6.2 wt.%-4.3 wt.%), TiO2 (0.7 wt.%-1.2 wt.%) and P2O5 (Fig. 3). The Ba and Sr contents are relatively high at 1 000 ppm-1 600 ppm and 540 ppm-800 ppm, respectively. The content of K2O, although high (3.6 wt.%-5.4 wt.%), is on average lower than those of associated more differentiated granites. Zirconium is relatively constant at 200 ppm- 300 ppm over the entire silica range.

    Figure 3.  Variation diagrams of multiple major and trace elements using SiO2 as differentiation index for granites from the Socorro batholith.

    Our chemical data (ESM3) reveal important differences between the three leucogranite types. All samples from the leucogranite plutons have high SiO2 content: ~73 wt.% in Bocaina and Bairro da Pedreira, with the highest content (~75 wt.%) in the Fazenda São Sebastião Pluton. The content of K2O is high and very similar in the three plutons at ~5 wt.%-6 wt.% (Fig. 3). More pronounced differences are in Mg# and Th content, which attain the highest values in samples from the Bairro da Pedreira Pluton (Th=64 ppm; Mg#=28-36), and the lowest values in the high-silica sample from Fazenda São Sebastião (Th=7 ppm; Mg#=22-25), with the Bocaina sample showing intermediate values. The Sr contents are always lower than those of the Bragança Paulista-type granites, with an anomalously high content for one sample of the Fazenda São Sebastião Pluton (BRP-74; 496 ppm), which also has low Y (0.5 ppm).

    The geochemical characteristics of the two charnockites are very distinct. The Atibaia charnockite is more evolved, with higher contents of SiO2 (70 wt.%), K2O (5.8 wt.%), and Zr (402 ppm) and much lower Sr (170 ppm) and Mg# (25-27). The Socorro charnockite is chemically very similar to the Bragança Paulista-type granites, with SiO2 61.6 wt.%, CaO 4.6 wt.%, MgO 2.2 wt.% and Mg# 40-44, and relatively high contents of K2O (3.7 wt.%), Ba (1 300 ppm) and Sr (800 ppm).

  • Chondrite-normalized REE patterns of Bragança Paulista- type granites (Fig. 4, ESM4) are moderately fractionated with (La/Yb)N=25-47 and discreet negative Eu anomalies (average Eu/Eu*=0.80). The MREE to HREE fractionation is moderate with (Gd/Yb)N=2.8-3.8.

    Figure 4.  REE patterns (normalized to the chondrite of Boynton, 1984) for granites from the Socorro batholith.

    The REE patterns of leucogranites from the Bocaina and Bairro da Pedreira plutons are strongly fractionated ((La/Yb)N= 166 and 176, respectively); the MREE/HREE fractionation is also strong ((Gd/Yb)N=8.9 and 7.9. Leucogranites from Fazenda São Sebastião Pluton are also fractionated, with (La/Yb)N=163 and 83; the MREE/HREE fractionation is less pronounced ((Gd/Yb)N=2.7-2.9). Significant contrasts exist in the behavior of Eu. Samples from Bairro da Pedreira show pronounced negative anomalies (Eu/Eu*=0.50-0.58), while in the single Bocaina Pluton sample (BRP-03) it is discreet (Eu/Eu*=0.8). Samples from the Fazenda São Sebastião Pluton have positive anomalies, discreet in BRP-32 (Eu/Eu*=1.6) and very pronounced in BRP-74 (Eu/Eu*=12), indicative of feldspar accumulation.

    The Atibaia charnockite has a fractionated REE pattern ((La/Yb)N=82) with (Gd/Yb)N=4.8 and a moderate to weak negative Eu anomaly (Eu/Eu*=0.7). In contrast, the Socorro charnockite shows a less fractionated REE pattern ((La/Yb)N= 22; (Gd/Yb)N=2.3) with poorly developed negative Eu anomaly (Eu/Eu*=0.9), similar to the patterns of Bragança Paulista-type granites.

  • Results of whole-rock Sr and Nd isotope determinations in selected samples from the Socorro batholith are shown in Tables 1 and 2. Initial ratios were calculated at the age of crystallization of each sample (for non-dated samples, calculation was made for an age of 610 Ma). Whole-rock Sr isotope determinations are available in the literature for Bragança Paulista-type and Salmão-type granites from the Socorro batholith (Tassinari, 1988) and were used for comparison.

    Samples Type Unit Rb (ppm) Sr (ppm) 87Rb/86Sr 87Sr/86Sr 87Sr/86Srt T1 (Ma) ε(t1)
    BRP-12 Charnockite Socorro charnockite 111 799 0.402 0.712 999 0.709 326 640 79.2
    ATB-08 Atibaia charnockite 162 175 2.692 0.744 067 0.719 688 635 226.3
    ATB-13 Bragança Paulista granites Bragança Paulista-type granite 145 626 0.673 0.717 751 0.711 894 610 115.2
    BRP-08 Bragança Paulista-type granite 153 658 0.674 0.718 842 0.713 077 600 131.8
    BRP-27 Bragança Paulista-type granite 158 712 0.643 0.717 425 0.711 832 610 114.3
    BRP-31A Bragança Paulista-type granite 140 713 0.570 0.717 035 0.712 074 610 117.7
    BRP-31D Enclave 170 634 0.779 0.718 359 0.711 585 610 110.8
    BRP-14A Leucogranites Bairro da Pedreira Pluton 207 303 1.986 0.732 928 0.715 592 612 167.7
    BRP-14D Hybrid rock 204 248 2.389 0.737 242 0.716 461 610 180.0
    BRP-14E Xenolith 220 608 1.050 0.721 050 0.711 917 610 115.5
    BRP-32 Faz. São Sebastião Pluton 148 125 3.428 0.740 503 0.710 684 610 98.0
    BRP-03 Bocaina Pluton 183 183 2.904 0.742 946 0.717 098 624 189.3

    Table 1.  Results of Sr isotope determinations for the studied samples

    Samples Type Unit Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd Error (2σ) ε(0) fSm/Nd TDM (Ma) T1 (Ma) εNd(t1)
    BRP-12 Charnockite Socorro charnockite 9.0 52.3 0.105 0.511 938 0.000 004 -13.7 -0.47 1 541 640 -6.1
    ATB-08 Atibaia charnockite 8.4 56.7 0.090 0.511 469 0.000 005 -22.8 -0.54 1 933 635 -14.1
    ATB-13 Bragança Paulista granite Bragança Paulista granite 7.9 45.4 0.104 0.511 607 0.000 004 -20.1 -0.47 2 006 610 -12.9
    BRP-27 Bragança Paulista granite 9.6 64.2 0.090 0.511 640 0.000 005 -19.5 -0.54 1 734 610 -11.2
    BRP-31A Bragança Paulista granite 9.6 61.2 0.095 0.511 601 0.000 004 -20.2 -0.52 1 850 610 -12.3
    BRP-31D Bragança Paulista enclave 11.9 50.2 0.143 0.511 783 0.000 005 -16.7 -0.27 2 753 610 -12.5
    BRP-08 Bragança Paulista granite 12.6 76.6 0.099 0.511 599 0.000 004 -20.3 -0.49 1 927 610 -12.7
    BRP-14A Leucogranite Bairro da Pedreira Pluton 10.9 81.8 0.081 0.511 344 0.000 004 -25.2 -0.59 1 949 612 -16.2
    BRP-14D Hybrid rock 8.9 62.7 0.086 0.511 374 0.000 004 -24.7 -0.56 1 992 610 -16.0
    BRP-14E Xenolith 13.1 76.3 0.104 0.511 553 0.000 004 -21.2 -0.47 2 064 610 -13.9
    BRP-32 Faz. São Sebastião Pluton 3.1 26.8 0.069 0.511 504 0.000 004 -22.1 -0.65 1 621 610 -12.2
    BRP-03 Bocaina Pluton 6.3 44.2 0.086 0.511 355 0.000 004 -25.0 -0.57 2 009 625 -16.2

    Table 2.  Results of Nd isotope determinations for the studied samples

    There is wide variation in 87Rb/86Sr ratios, from 0.4 in Socorro charnockite to 3.4 in Fazenda São Sebastião leucogranite sample BRP-32. The 87Sr/86Srt ratios of Bragança Paulista-type granites show small variation, from 0.711 9 (sample ATB-13) to 0.713 1 (sample BRP-08); these values are within the same range of literature data (0.711 2-0.713 6; Tassinari, 1988). The two charnockites have different ratios, higher (0.719 7) in the Atibaia as compared to the Socorro charnockite (0.709 3). Samples from the three leucogranite plutons show variable signatures, the higher values for the Bocaina Pluton (0.717 1; within the range obtained by Tassinari (1988) for Salmão-type granite (0.713 8-0.718 6)); 87Sr/86Srt is 0.715 6 for the Bairro da Pedreira and 0.710 7 for the Fazenda São Sebastião sample.

    Results of Nd isotope determinations are presented in Table 2. The values of εNd(t) are all negative, with the lowest values observed in leucogranites from the Bocaina and Bairro da Pedreira plutons (both -16.2) and the highest in the Socorro charnockite (-6.1). All Bragança Paulista-type granites (including the mafic enclave BRP-31D) have intermediate values (-12.3 to -12.9), which are similar to the leucogranite from Fazenda São Sebastião (BRP-32; εNd(t)= -12.0) and Atibaia charnockite (-14.1). An enclave of porphyritic granites from Atibaia quarry (Bairro da Pedreira Pluton; sample BRP-14D) has εNd(t)= -16.0, identical to the host syenogranite, while a sample from a zone where this granite is hybridized with a mafic rock has a slightly less negative εNd(t)= -13.9.

    The Nd TDM ages calculated using DePaolo's model (DePaolo, 1981) are concentrated in the 1.8-2.0 Ga interval; an exception is the Socorro charnockite, with 1.5 Ga. Most samples have 147Sm/144Nd ratios between 0.09 and 0.13, within the range of typical crustal rocks (Dickin, 2005); the Nd TDM calculated for rocks with values lower and higher than these respond for anomalous values, respectively younger (1.6 Ga for Fazenda São Sebastião Pluton leucogranite, with 147Sm/144Nd=0.07) and higher (2.7 Ga for Bragança Paulista mafic enclave BRP-31D with 147Sm/144Nd=0.14), with no geological meaning.

  • In addition to the five samples dated in our previous study (Toledo et al., 2018), we present here two new U-Pb zircon ages obtained by SHRIMP: one in a sample of Bragança Paulista-type granite from a small occurrence at the southwesternmost part of the batholith (BRP-31A) and the other in the Bairro da Pedreira Pluton (BRP-14A). Analytical results are presented as ESM2.

    In the Bragança Paulista-type sample (BRP-31A) twelve points were analyzed, of which three are > 5% discordant. Excluding the youngest (597±12 Ma) and the three oldest (633±12, 634±12 and 642±12 Ma) results, a concordia age (Fig. 5) can be obtained with the remaining eight spots and yields 612±4 Ma (errors at 2σ; MSWD=0.30), which is assumed as the best estimate of the age of this sample.

    Figure 5.  Concordia ages of dated samples.

    The seventeen analyses obtained in zircons from the Bairro da Pedreira Pluton spread over the concordia for ca. 70 Ma (610-680 Ma), excluding five extreme results: the oldest one, indicative of inheritance (749±14 Ma), and four younger ages varying from 531 to 598 Ma, probably related to Pb loss. Other four spots with higher ages (634 to 680 Ma) are not part of a coherent age group and were also excluded for age calculation. The group of eight coherent results defines a concordia age (Fig. 5) of 613±2 Ma (MSWD=0.23) that is considered the best estimate of the magmatic crystallization age of this sample.

  • The Hf isotope ratios in zircon were recalculated at their age of crystallization. The Hf isotope signature of zircons from Bragança Paulista-type granite (samples BRP-31A and ATB-13) is relatively homogeneous with negative εHf(t) values varying from -12 to -17 and an average of -14.5 (Table 3; Fig. 6). An inherited crystal with crystallization age of 1 440 Ma from ATB-13 (spot 13.1) has εHf(t)= -3.1. Zircons from BRP-08 sample have more negative εHf(t) values (-15 to -20) and an average of -17.9 (Fig. 6).

    Spot # Age (t, Ma) 176Hf/177Hf 2σ 176Lu/177Hf 2σ 176Hf/177Hft εHf(0) εHf(t) Hf TDM
    Bragança Paulista-type granite (BRP-31 A), UTM: 7455943×315560, 23 K
    1.1 612 0.282 00 0.000 08 0.000 621 0.000 005 0.281 99 -27.0 -14.1 2 389
    2.1 612 0.282 09 0.000 08 0.000 580 0.000 060 0.282 08 -24.0 -10.9 2 189
    4.1 612 0.282 05 0.000 08 0.000 670 0.000 050 0.282 05 -25.0 -12.3 2 280
    5.1 612 0.282 02 0.000 08 0.001 960 0.000 080 0.282 00 -26.0 -13.9 2 379
    6.1 612 0.281 93 0.000 09 0.000 250 0.000 020 0.281 93 -29.0 -16.4 2 535
    8.1 612 0.282 00 0.000 08 0.000 589 0.000 005 0.281 99 -27.0 -14.1 2 389
    9.1 612 0.282 00 0.000 08 0.000 235 0.000 004 0.281 99 -27.0 -13.9 2 380
    10.1 612 0.282 05 0.000 08 0.000 340 0.000 020 0.282 05 -25.0 -12.2 2 271
    11.1 612 0.282 06 0.000 08 0.001 157 0.000 006 0.282 04 -25.0 -12.2 2 270
    12.1 612 0.281 91 0.000 08 0.000 350 0.000 006 0.281 90 -30.0 -17.2 2 582
    Leucogranite Pluton 2 (BRP-14 A), UTM: 7448974×332634, 23 K
    1.1 613 0.281 91 0.000 08 0.000 399 0.000 008 0.281 91 -30.0 -17.2 2 582
    3.1 613 0.281 81 0.000 09 0.000 650 0.000 050 0.281 81 -33.0 -20.8 2 809
    6.1 613 0.282 00 0.000 10 0.002 300 0.000 500 0.281 98 -27.0 -14.7 2 432
    6.2 749 0.281 99 0.000 09 0.001 200 0.000 200 0.281 97 -27.0 -11.7 2 349
    7.1 613 0.281 91 0.000 09 0.000 390 0.000 010 0.281 90 -30.0 -17.2 2 582
    8.1 613 0.281 86 0.000 08 0.001 100 0.000 100 0.281 85 -32.0 -19.2 2 710
    9.1 613 0.281 92 0.000 08 0.000 470 0.000 030 0.281 92 -30.0 -16.8 2 562
    13.1 613 0.281 85 0.000 08 0.000 600 0.000 200 0.281 84 -32.0 -19.4 2 719
    14.1 613 0.282 12 0.000 08 0.000 618 0.000 006 0.282 11 -23.0 -9.8 2 122
    16.1 613 0.281 78 0.000 08 0.000 705 0.000 005 0.281 77 -35.0 -21.9 2 876
    Bragança Paulista-type granite (ATB-13), UTM: 7464639×338956, 23 K
    1.1 608 0.282 05 0.000 08 0.000 500 0.000 010 0.282 04 -25.0 -12.3 2 278
    4.1 608 0.281 94 0.000 08 0.000 490 0.000 040 0.281 94 -29.0 -16.2 2 521
    5.1 608 0.281 99 0.000 08 0.000 552 0.000 004 0.281 99 -27.0 -14.5 2 412
    5.2 608 0.282 04 0.000 08 0.001 130 0.000 070 0.282 03 -25.0 -13.0 2 316
    6.1 608 0.281 98 0.000 08 0.000 772 0.000 005 0.281 97 -28.0 -14.9 2 440
    8.1 608 0.282 00 0.000 10 0.001 610 0.000 030 0.282 01 -26.0 -14.6 2 417
    9.1 608 0.281 97 0.000 08 0.000 900 0.000 100 0.281 96 -28.0 -15.3 2 465
    10.1 608 0.281 96 0.000 09 0.001 200 0.000 300 0.281 95 -28.0 -15.8 2 495
    13.1 1 440 0.281 80 0.000 08 0.000 699 0.000 007 0.281 78 -34.0 -3.1 2 336
    Atibaia charnockite (ATB-08), UTM: 74479343×36105, 23 K
    1.1 633 0.282 30 0.000 10 0.000 810 0.000 070 0.282 31 -16.0 -3.1 1 713
    2.1 633 0.282 23 0.000 09 0.000 630 0.000 020 0.282 22 -19.0 -5.5 1 865
    3.1 633 0.282 07 0.000 08 0.000 690 0.000 070 0.282 06 -25.0 -11.2 2 224
    4.1 633 0.282 08 0.000 09 0.000 604 0.000 006 0.282 07 -24.0 -10.8 2 199
    5.1 633 0.282 00 0.000 08 0.000 520 0.000 020 0.281 99 -27.0 -13.6 2 375
    6.1 633 0.282 02 0.000 08 0.000 650 0.000 040 0.282 01 -26.0 -12.9 2 334
    7.1 633 0.282 20 0.000 10 0.000 540 0.000 090 0.282 18 -20.0 -6.5 1 930
    8.1 633 0.282 10 0.000 10 0.001 000 0.000 300 0.282 05 -25.0 -10.2 2 165
    10.1 633 0.282 13 0.000 09 0.001 096 0.000 009 0.282 11 -22.0 -9.2 2 101
    11.1 633 0.282 10 0.000 10 0.000 796 0.000 006 0.282 11 -23.0 -10.2 2 160
    11.2 633 0.282 10 0.000 10 0.000 553 0.000 005 0.282 06 -25.0 -10.1 2 153
    Bragança Paulista-type granite (BRP-08), UTM: 7479933×350622, 23 K
    2.1 610 0.281 917 0.000 023 0.000 305 5 0.000 001 9 0.281 913 -30.2 -16.9 2 566
    4.1 610 0.281 900 0.000 023 0.000 357 9 0.000 001 0 0.281 896 -30.8 -17.6 2 604
    6.1 610 0.281 829 0.000 026 0.000 539 2 0.000 007 0 0.281 823 -33.3 -20.1 2 766
    7.1 610 0.281 977 0.000 031 0.000 378 5 0.000 008 1 0.281 973 -28.1 -14.8 2 436
    10.1 610 0.281 832 0.000 038 0.000 451 2 0.000 001 0 0.281 827 -33.2 -20.0 2 757
    Bragança Paulista-type granite (BRP-31 A), UTM: 7455943×315560, 23 K
    9.1 610 0.281 867 0.000 033 0.000 836 8 0.000 020 0 0.281 857 -32.0 -18.9 2 690
    12.1 610 0.281 927 0.000 032 0.000 581 6 0.000 017 2 0.281 920 -29.9 -16.7 2 552
    12.2 610 0.281 834 0.000 037 0.000 551 2 0.000 005 0 0.281 827 -33.2 -20.0 2 756
    16.1 610 0.281 926 0.000 023 0.000 301 1 0.000 000 0 0.281 923 -29.9 -16.6 2 545
    16.2 610 0.281 917 0.000 032 0.000 641 0 0.000 001 7 0.281 910 -30.2 -17.1 2 574
    Socorro charnockite (BRP-12), UTM: 7504203×345870, 23 K
    5.1 642 0.282 242 0.000 026 0.000 201 5 0.000 006 0 0.282 239 5 -18.7 -4.7 1 822
    6.1 642 0.282 084 0.000 039 0.000 525 8 0.000 001 6 0.282 077 7 -24.3 -10.4 2 183
    7.1 642 0.282 169 0.000 027 0.000 659 1 0.000 008 0 0.282 161 3 -21.3 -7.4 1 996
    8.1 642 0.282 050 0.000 032 0.000 982 7 0.000 043 0 0.282 038 4 -25.5 -11.8 2 270
    9.1 642 0.281 955 0.000 041 0.001 332 4 0.000 075 0 0.281 938 9 -28.9 -15.3 2 490
    11.1 642 0.282 216 0.000 030 0.000 590 1 0.000 001 9 0.282 208 7 -19.7 -5.8 1 890
    11.2 642 0.282 186 0.000 027 0.000 274 7 0.000 010 1 0.282 182 7 -20.7 -6.7 1 949
    12.2 642 0.282 204 0.000 029 0.000 484 6 0.000 026 0 0.282 198 2 -20.1 -6.1 1 914
    12.1 642 0.282 054 0.000 032 0.000 582 1 0.000 006 0 0.282 046 3 -25.4 -11.5 2 252
    13.1 642 0.282 032 0.000 046 0.001 433 4 0.000 002 0 0.282 015 2 -26.2 -12.6 2 322
    Leucogranite Pluton 1 (BRP-03), UTM: 7456685×338634, 23 K
    1.1 624 0.281 810 0.000 026 0.000 145 5 0.000 006 0 0.281 809 -34.0 -20.3 2 789
    2.1 624 0.281 860 0.000 033 0.000 312 8 0.000 001 5 0.281 856 -32.3 -18.7 2 684
    4.1 624 0.281 827 0.000 028 0.000 431 6 0.000 007 0 0.281 822 -33.4 -19.9 2 760
    5.1 624 0.281 534 0.000 052 0.002 384 1 0.000 021 5 0.281 506 -43.8 -31.0 3 449
    6.1 624 0.281 924 0.000 029 0.000 264 7 0.000 001 0 0.281 921 -30.0 -16.4 2 541
    7.1 1 675 0.281 905 0.000 026 0.000 209 0 0.000 002 0 0.281 898 -30.7 6.4 1 911
    8.1 624 0.282 003 0.000 041 0.000 167 9 0.000 001 0 0.282 001 -27.2 -13.5 2 363
    9.1 624 0.281 782 0.000 027 0.000 243 4 0.000 007 0 0.281 779 -35.0 -21.4 2 854
    12.1 1 777 0.281 438 0.000 047 0.000 653 9 0.000 009 0 0.281 416 -47.2 -8.4 2 932
    14.1 1 502 0.281 368 0.000 068 0.001 158 3 0.000 012 3 0.281 336 -49.6 -17.5 3 290
    UTM, K and the numbers refer to the geographical position of the sample.

    Table 3.  Results of LA-ICPMS in situ Hf isotope determination in zircon crystals from Socorro batholith samples

    Figure 6.  The εHf(t) isotope data vs. U-Pb age from dated samples of Socorro batholith (error bars=3ε units).

    Zircons from the Bairro da Pedreira leucogranite (BRP-14A) show a wide spread of εHf(t) values from -9 to -20 (Fig. 6) and a strongly negative average of -16.2. Zircons from the leucogranite of the Bocaina Pluton (sample BRP-03) also have a wide spread of εHf(t) (-14 to -31; Fig. 5); the average εHf(t) is identical to that of the Bairro da Pedreira (-16.1). The inherited grains, with 1.5-1.8 Ga (cf. Toledo et al., 2018), show varied εHf(t) values, from positive (+6.4; spot 7.1) to strongly negative (-17.5; spot 14.1). The lowest value obtained for a magmatic (~620 Ma) zircon (εHf(t)= -31, spot 5.1) is from a CL-dark, slightly zoned grain.

    Zircons from Atibaia charnockite have less negative εHf(t) values compared to other samples from the Socorro batholith, with a moderate spread varying from -13 to -2, and most results in the -12 to -9 range. The less negative value (-2) is from spot 1.1, a crystal with irregular zoning and corroded rims. The average εHf(t) is -8.9. The εHf(t) of zircons from Socorro charnockite is similar, with an average of εHf(t)= -9.1 and ranges from -5 to -15.

  • Chemical analyses of zircon crystals were performed by LA-ICPMS in raster mode, following homogeneous zones within the crystals. All results and location of the rasters are shown in ESM4.

    Chondrite-normalized REE patterns of zircons from Bragança Paulista-type granites (Fig. 7) show typical patterns with strong HREE enrichment, negative Eu anomalies (median of (Eu/Eu*)N=0.43) and positive Ce anomalies (Ce/Ce*). The MREE/HREE fractionation is reflected by (Gd/Yb)N=0.07. In variation diagrams using Hf as a fractionation index there is a slight trend of increasing Th/U while Y is nearly constant (Fig. 8).

    Figure 7.  Zircons REE patterns (median) from Socorro batholith samples. The whole set of patterns for each sample is shown in ESM5.

    Figure 8.  Variation diagrams for zircon trace-elements from samples of Socorro batholith.

    Zircons from Bairro da Pedreira Pluton (BRP-14A) show the highest contents of HREE with discreet positive Ce anomalies (Ce/Ce*) and pronounced negative Eu anomalies (median (Eu/Eu*)=0.34). The MREE/LREE fractionation is reflected by (Gd/Yb)N=0.06. Zircons from Bocaina Pluton (BRP-03) have lower HREE contents (compared with BRP-14A) with negative Eu anomalies (median (Eu/Eu*)=0.35) and positive Ce anomalies (Ce/Ce*=6.64). The (Gd/Yb)N ratio is 0.08, the highest of the set analyzed. Both leucogranite samples have low Th/U ratios that show a negative correlation with Hf. BRP-14A zircons show elevated Y and Hf contents with a positive correlation (Fig. 8).

    Zircons from Atibaia charnockite (sample ATB-08) have a very strong negative Eu anomaly (median Eu/Eu*=0.05) (Fig. 7) and a positive Ce anomaly. HREE are higher than the Socorro charnockite and (Gd/Yb)N=0.06. Zircons from the Socorro charnockite sample have a pronounced positive Ce anomaly and strong enrichment in HREE. The negative Eu anomaly is well-defined (Eu/Eu*=0.45). (Gd/Yb)N=0.03 is the lowest of all analyzed samples. The Hf contents are low and show a negative correlation with Gd/Yb (Fig. 8).

  • Estimates of the physicochemical parameters (T, P, fluid activities) are fundamental for understanding magmatic petrogenesis. Naranjo and Vlach (2018) presented detailed estimates based on mineral equilibria and accessory mineral saturation for a representative sample of Bragança Paulista granite collected from our ATB-13 outcrop.

    Emplacement pressure was estimated at 510±50 MPa based on the Al-in-hornblende geobarometer (calibration of Mutch et al., 2016), and fO2 at NNO+1 based on the Mg# of biotite. A ~220 ℃ temperature range for magma crystallization was suggested, from ~975 ℃ (liquidus T, from apatite saturation) to ~755 ℃ (from plagioclase-hornblende equilibria); zircon saturation temperatures at ~800 ℃ or less are close to solidus, and taken as indicative that the magmas were Zr-undersaturated.

  • Liquidus temperatures of the voluminous I-type, metaluminous calc-alkaline granitic magmas making up large volumes of continental crust are still a major issue in granite petrogenesis. High color index granodiorites and quartz monzonites with M ~15-25 and SiO2 in the 62 wt.%-68 wt.% range are predominant in many large batholiths, and this is the case in the Socorro batholith. Melts with equivalent composition are not produced as low temperature products of partial melting of any crustal protoliths, as indicated by experimental petrology and the study of migmatites (e.g., Barbarin, 1999). Liquidus temperatures calculated even at high (> 8 wt.%) water contents from thermodynamic modeling (Rhyolite-MELTS; Gualda et al., 2012) are typically higher than 1 000 ℃.

    Apatite saturation temperatures (TAp) were calculated for all samples analyzed for whole-rock geochemistry, which are inferred to be P-saturated, as suggested by negative correlations between P2O5 and differentiation indices (e.g., SiO2). Early crystallization of apatite is also indicated by petrography, as it is commonly included in early-crystallized main minerals (e.g., hornblende, plagioclase). The TAp calibration by Harrison and Watson (1984) yields T=950-1 000 ℃ (average=977±15 ℃, n=12, identical to the value obtained in the sample studied by Naranjo and Vlach (2018). The TAp for the Socorro charnockite is within the same range, while it is slightly lower for the more differentiated Atibaia charnockite (930-940 ℃). The TAp of the leucogranites varies from 890 to 780 ℃, with two exceptions being rocks with unusually high P2O5 which yield TAp=940 ℃ (Table 4).

    Unit Sample TAp_sat TZr_satWH TZr_satB Ap-ZrWH Ap-ZrB WH-B T (aTi=1) aTi(sat)WH aTi(sat)B a(SiO2)B
    (℃)
    BP ATB-13 988 807 753 181 235 54 832 1.00 1.00 0.80
    BRP-08 964 811 752 153 212 59 847 1.00 1.00 0.80
    BRP-31A 983 796 732 187 251 64 831 1.00 1.00 0.80
    SCh BRP-12 958 789 730 169 228 59 812 1.00 1.00 0.80
    Ach ATB-08 944 866 830 78 114 36 788 0.62 0.77 0.80
    LGP BRP-14A 888 824 783 64 105 41 728 0.51 0.67 1.00
    LGB BRP-03 844 790 745 54 99 45 727 0.63 0.87 1.00
    The value of aSiO2 assumed as 0.8 for charnockites and Bragança Paulista granites, and 1.0 for leucogranites, based on Rhyolite-MELTS modeling. aTi(sat)WH and aTi(sat)B are aTiO2 calculated using the model by Borisov and Aranovich (2020) at zircon saturation temperatures using Watson and Harrison (1983) and Boetcher et al. (2013), respectively; aTi=1 is the temperature at which TiO2 saturation is achieved.

    Table 4.  Zircon saturation temperatures by Watson and Harrison (TZr_satWH) and by Boenhke (TZrB) and apatite saturation temperatures (TAp_sat) and values of aTiO2 and aSiO2 used for calculation of (Ti-Zr)

    Zircon saturation temperatures (TZr) have been largely used as reliable proxies for near-liquidus temperatures of granitic magmas, but are subject to several shortcomings. Demonstration that zircon is an early-precipitating phase is often tricky from either petrographic or geochemical evidence. When present, inherited zircon adds Zr which was not dissolved in the melt to whole-rock compositions, and they can be particularly abundant in low-temperature granites (Miller et al., 2003) Additionally, choice of a calibration is not straightforward; for instance, the recalibration of Boehnke et al. (2013) (B13) yields results that are systematically lower by over 50 ℃ when compared to the original calibration of Watson and Harrison (1983) (WH83), and the latter continues to be adopted by some authors.

    We present in Fig. 9 comparison of TZr calculated by the two calibrations for selected granitic rocks from the Socorro batholith. Bragança Paulista-type granites have Zr contents varying from 250 ppm to 350 ppm; no obvious trend of depletion with fractionation over their small SiO2 range (62 wt.%-66 wt.%) is observed. Accordingly, TZr remains essentially constant at ~800 ℃ with the WH83 calibration (total range is 790-830 ℃); B13 results average 750 ℃. The highest TZr are shown by the Atibaia charnockite (WH83: 860-870 ℃); the Socorro charnockite has TZr within the range of the Bragança Paulista granites. Considered together, leucogranites show a clear trend of decreasing Zr contents with differentiation (Fig. 9). Although these rocks are not comagmatic, and are even in part of different age, this behavior is suggestive, as expected, that these highly fractionated granites are zircon saturated. The WH83 temperatures are 820-790 ℃ for the Bairro da Pedreira and Bocaina plutons, the same range of the Bragança Paulista granites, in spite of their significantly more differentiated character (72 wt.%-74 wt.% SiO2). The yet more silica-rich leucogranites from the São Sebastião Pluton have lower TZr (< 770 ℃). The B13 temperatures for the leucogranites are lower by ~40 ℃.

    Figure 9.  Variation diagram showing TZr, TTi-Zr vs. SiO2 from samples of Socorro batholith. The temperatures obtained by Watson and Harrison (1983) thermometer are not plotted but are ~50 ℃ higher than those obtained by Boehnke et al. (2013).

    The Ti contents in zircon are a function of temperature, and a Ti-in-zircon thermometer (TTi-Zr) was calibrated by Ferry and Watson (2007). In theory, temperatures recorded by zircon should spread over a range from TZr to solidus, reflecting melt cooling and crystallization. They have a dependency on activities of SiO2 and TiO2, both of which, in turn, increase with crystallization.

    Recent experimental work by Borisov and Aranovich (2020) allows calculations of aTiO2 as a function of magma composition and temperature. We therefore tried to get an approach to aTiO2 in the different granites for which we have Ti analyses in zircon instead of using a fixed value (e.g., 0.7 for more mafic I-type granites; Collins et al., 2016). We assumed for calculations of TTi-Zr the aTiO2 of the melt at TZr (B13 calibration; taking into consideration that WH83 is 40-50 ℃ higher, and temperatures will not be constant, but dropping as zircon crystallizes). These values vary from 1.0 for Bragança Paulista-type granites and Socorro charnockite to 0.7-0.8 in the samples from Bairro da Pedreira granite and Atibaia charnockite (Table 4).

    Quartz is a late phase in Bragança Paulista granites and charnockites; Rhyolite-MELTS modeling indicates that at 500 MPa and 4 wt.%-6 wt.% H2O it would crystallize below 750 ℃, i.e., at or below TZr; accordingly, aSiO2 was set to 0.8 in these rocks. On the other hand, aSiO2 was set to 1 in the silica-rich leucogranites, where quartz is a near liquidus phase.

    With a few exceptions, calculated TTi-Zr extends from TZr (B13) to solidus at ~650 ℃ (Fig. 9). Scarcity of temperatures between B13 and WH83 TZr is suggestive that the lower temperatures estimated by the first calibration are more adequate for the studied granites. More importantly, the values confirm the low TZr obtained for the more primitive Bragança Paulista granites and Socorro charnockite which should result, in our view, from a zircon undersaturated character of these magmas. Moreover, our new temperature estimates imply that the solidus of the Bragança Paulista granites is ~100 ℃ lower than the ~755 ℃ value estimated by Naranjo and Vlach (2018) on the basis of plagioclase-hornblende equilibria. Rhyolite-MELTS modelings are in agreement with this, and indicate that the crystal contents of a representative sample (BRP-31A) at 750 ℃ would be < 35 wt.% and < 50 wt.% at 6 wt.% and 4 wt.% H2O, respectively, and quartz would precipitate at T < 720 ℃.

  • The Socorro and Atibaia charnockites are early components of the Socorro batholith, respectively dated at 640 and 630 Ma. Both show transitional contacts with pyroxene-free equivalents of similar composition, but in spite of similar mafic mineralogy with the presence of orthopyroxene, indicative of relatively low H2O contents (e.g., Zhao et al., 2017 demonstrated that peraluminous charnockite melts may have up to 5.6 wt.% H2O), they have different chemical compositions.

    The Socorro charnockite is more primitive (62 wt.%-64 wt.% SiO2), and chemically similar to the least evolved components of the younger Bragança Paulista-type HKCA granites, whereas the Atibaia charnockite has SiO2 ~68 wt.%-70 wt.%, intermediate between the latter and the leucogranites. Some aspects of the chemical signature of the Atibaia charnockite differ from those of calc-alkaline granitoids, and are akin to A-type granites (e.g., lower Mg# and Sr; higher Zr and correspondently higher TZr).

    Granulitic sources are usually implied in the genesis of charnockitic magmas, and we examine the isotope signatures of the granulites that are exposed in the lowermost portions of the SGN, inferred to have formed at T > 900 ℃ and P up to 12 kbar (e.g., Motta et al., 2021; Rocha et al., 2018; Tedeschi et al., 2018). Two suites of charnockitic rocks form the northern part of the SGN, both dated at ~625 Ma (Janasi, 2002) were proposed to derive from two distinct granulite sources, one with younger crustal age, characterized by 87Sr/86Sr ~0.707 and initial εNd= -4 to -7 (São Pedro de Caldas) and another with 87Sr/86Sr ~0.710 and εNd= -10 to -12 (Divinolândia). Published Nd isotope data for granulites from the basal levels of the SGN are indicative that these two sources are distinct, with younger granulites (with Nd TDM ~1.5 Ga) forming the eastern part and older granulites (Nd TDM > 2.1 Ga) the western part of the exposed lower crust (Janasi, 2002).

    The Socorro charnockite has the least negative Nd and Hf isotope signatures of the batholith (respectively, εNd(t)= -6 and average zircon εHf(t)= -9), and initial 87Sr/86Sr ~0.709. Its source can be inferred to have a crustal age similar to that of the São José do Rio Pardo Suite (Janasi, 2002), although with slightly higher time-integrated Rb/Sr (reflected in the higher initial 87Sr/86Sr). Importantly, its chemical composition differs from both 625 Ma charnockitic suites described by Janasi (2002), which show "within-plate" geochemical signatures and are therefore more similar to the Atibaia charnockite.

    The Atibaia charnockite, in turn, differs markedly from the charnockites described by Janasi in its isotope signatures, especially the very high initial 87Sr/86Sr that requires a source with high Rb/Sr similar to that of the contemporaneous peraluminous Nazaré Paulista-type anatectic garnet-biotite granites, mostly derived from metasedimentary sources. An interesting feature of the Atibaia charnockite sample is an apparent decoupling between the Hf and Nd isotope signatures, since its εHf(t) is similar to that of the Socorro charnockite, while its εNd(t) is much more negative (-14). Its Sr-Nd isotope signature (Fig. 10) overlaps the field of the crust-derived Nazaré Paulista granites, suggesting similar (albeit less hydrated) sources.

    Figure 10.  The Sr-Nd(t) isotope variation of selected samples and from granites occurrences in the Ribeira belt. Values are calculated at the crystallization ages of the granites.

  • The three leucogranite types described in this work correspond to the most fractionated rocks of the Socorro batholith (> 72 wt.% SiO2) and were dated at 612 Ma (Bairro da Pedreira Pluton, contemporaneous of Bragança Paulista-type HKCA granites) and 625 Ma (Bocaina Pluton). They show some significant geochemical contrasts, but seem to correspond to products of pure crustal melting (e.g., high K2O, SiO2; strongly enriched Nd and Hf isotopes; high 87Sr/86Srt, high Th/La ratio). The trace-element ratios of granites from the Bocaina and Bairro da Pedreira plutons are typical of fractionated rocks with HREE depletion (high La/Yb, Gd/Yb), that could indicate the presence of residual garnet in the source of these granites.

    Granites from the ~625 Ma Bocaina Pluton have a moderately peraluminous character and show a wide range of εHf(t) in magmatic zircon (-31 to -14). Such high variability suggests mixing of magma batches derived from multiple crustal sources (e.g., Farina et al., 2018; Kemp et al., 2007). Models envisaging granitic magma evolution in transcrustal magma systems (e.g., Cashman et al., 2017) are consistent with the conception that early-formed minerals residing in a given magma body may have crystallized in distinct magma chambers at deeper level. Alternatively, Farina et al. (2018) admitted that isotopically distinct magma batches might coexist in a single granitic magma chamber, recording magmatic domains that were physically separated and whose Hf isotope systems evolved independently.

    Three inherited crystals from the Bocaina Pluton with U-Pb ages of ~1 502, 1 675 and 1 777 Ma, were analyzed, and show widely different εHf(t) values, -17.5, +6.4 and -8.4, respectively. This large dispersion of εHf(t) values is suggestive of contribution from sources formed within a relatively short time interval (1.5-1.8 Ga), but varying from juvenile to reworked.

    Figure 10 allows comparison of the Sr-Nd isotope signature of the Bocaina and Bairro da Pedreira leucogranites with that of the Nazaré Paulista garnet-biotite granites, typical products of crustal melting in the southern SGN (Martins et al., 2009; Martins, 2006), which define a field at high initial 87Sr/86Sr (0.716-0.728) and εNd(t)= -13 to -18. The Bocaina and Bairro da Pedreira leucogranites overlap with the more negative εNd(t) and lower initial 87Sr/86Srt in this field, indicating that their parent melts may have derived from melting of para- and orthogneisses similar to the sources of contemporaneous anatectic granites. The similar age and overlapping Hf isotope signature of inherited zircon grains from Nazaré Paulista leucogranites (Virmond, 2019) reinforce the interpretation of a common crustal source.

    The Fazenda São Sebastião Pluton has the lowest calculated initial 87Sr/86Sr (0.710 7) of the leucogranites and the least negative εNd(t) (-12), in the same range of the Bragança Paulista-type granites (Fig. 11). Its elemental geochemistry (lower Ba, Sr, Th, Y and Yb contents compared to the other leucogranites; positive Eu anomalies) is indicative of alkali-feldspar accumulation from a strongly differentiated magma.

    Figure 11.  The εNd(t) isotope variation vs. time (Ga) of selected samples and from metamorphic rocks from Socorro-Guaxupé Nappe.

  • The most voluminous rocks from Socorro batholith are the Bragança Paulista-type granites: I-type granites with high-K calc- alkaline character and compositions ranging from (Hbl)-Bt- monzogranites to granodiorites and quartz monzonites, dated at ~610 Ma.

    The petrogenesis of I-type granites is still the subject of much debate. Some authors admitted that they are generated at high temperatures (≥900 ℃), either by fractionation of juvenile basaltic magmas (e.g., Jagoutz and Klein, 2018; Müntener and Ulmer, 2018; Lee and Bachmann, 2014; Jagoutz and Schmidt, 2012), or by fluid-absent partial melting of basaltic rocks (Clemens et al., 2020; Sisson et al., 2005), or else as hybrid magmas formed by mixing of mantle- and crust-derived melts (Annen et al., 2006; Hildreth and Moorbath, 1988). Reliable estimates of magma temperatures are a major issue on current controversies (e.g., Collins et al., 2020, 2016; Castro, 2014; Annen et al., 2006). The Zr saturation and Ti-in-zircon temperatures of the HKCA Bragança Paulista granites are low (B13 TZr=730-780 ℃), within the same range of Cordilleran I-type granites from western North America, which was interpreted by Collins et al. (2016) as evidence that they correspond to low-temperature melts derived from water-present melting of basalt underplates.

    Rhyolite-MELTS (Gualda et al., 2012) modeling of a composition equivalent to Bragança Paulista granite BRP-31A (ESM6) indicates liquidus temperatures ~1 050 ℃ at 4 wt.%-6 wt.% H2O; however, the amount of crystals remains low down to 800 ℃ (20 wt.% for 6 wt.% H2O; 35 wt.% for 4 wt.% H2O), and is restricted to mafic minerals down to ~820 ℃ at 6 wt.% water; moreover, plagioclase, K-feldspar and quartz saturate within a small temperature range. Several models admitted that the high mafic mineral content of I-type granites could be related to incorporation of residua from the source, either as restites (Chappell et al., 1987), peritectic minerals (Clemens et al., 2011) or assimilated refractory mafic lithologies (Carvalho et al., 2017). However, no textural evidence exists that the mafic minerals (e.g., hornblende, biotite) are relict in these rocks, and apatite saturation temperatures (Table 4), which are successfully used as a geothermometer in mafic to intermediate charnockitic rocks from the SGN (e.g., Janasi, 2002), are > 900 ℃.

    The trace-element signature of the Bragança Paulista granites is typical of high-K calc-alkaline granites, with high contents of LILE (K, Ba, Sr) and low HFSE (Zr, Hf, Nb). Strongly fractionated REE patterns with weak negative Eu anomalies and high Sr/Y ratios are consistent with melt equilibration in a thickened crust with residual garnet (cf. further discussion in next section). Their isotope signature is consistent with a major contribution from sources with relatively long crustal residence (e.g., Nd TDM=1.7-2.0 Ga; very negative average zircon εHf(t)= -14.5 to -17.9). A comparison with regional granites with HKCA affinities shows that the Bragança Paulista granites lie in an intermediate position in a continuous parabolic trend with decreasing εNd(t) and increasing initial 87Sr/66Sr defined by granites of the São Roque Domain (Janasi et al., 2016) and more evolved "late-orogenic" plutons of the Agudos Grandes batholith (Leite et al., 2007) (Fig. 10). In detail, the Bragança Paulista granites lie just slightly off the trend, showing initial 87Sr/66Sr a little higher for a given εNd(t).

    A simple explanation for the observed parabolic trend in the Sr-Nd isotope space would be mixing of older crustal sources with high time-integrated Rb/Sr (such as those of the Socorro leucogranites that lie at the more evolved end of the trend) and younger sources similar to those that generated the Socorro charnockite (Fig. 11). The zircon Hf isotope signatures are consistent with such interpretation. Indeed, they show a continuous and overlapping trend from less negative εHf(t) shown by the charnockites to more negative values of the leucogranites, with the three Bragança Paulista samples lying in the middle.

    A single inherited (~1.4 Ga) zircon grain was found in Bragança Paulista granite sample ATB-13, and it plots within the extrapolated evolution line of the sources of these granites in the εHf vs. t space, as inferred from the magmatic zircons (Fig. 12). This may have important meaning, since as yet scarce Hf isotope data from inherited zircon of our leucogranite BRP-03 and the anatectic Nazaré Paulista granites (Virmond, 2019) point to the potential existence of Statherian-Calymnian (1.8-1.5 Ga) juvenile crust (with positive εHf) at depth and/or as a source of detritus to the metasediments of the SGN.

    Figure 12.  The εHf(t) isotope variation vs. time (Ga) of selected samples and from metamorphic rocks from Guaxupé Nappe (enclave, mafic granulites, gneiss and opdalite from Tedeschi et al., 2018) and Nazaré Paulista leucogranites (Virmond, 2019; Martins et al., 2009).

    The presence of an important mantle-derived mafic component in the Bragança Paulista granites is suggested by their primitive character (up to 20% of mafic minerals; 62 wt.%-66 wt.% SiO2; high Mg#, CaO, Fe2O3, P2O5) and the occasional presence of mafic microgranular enclaves with igneous texture. In fact, the probable site for generation of the melts from which these granites were formed is the lowermost crust, where basalt influxes from the mantle are typically trapped and result in "hot zones" in which basalts and their differentiates may mix with crust-derived partial melts (e.g., Annen et al., 2006; Hildreth and Moorbath, 1988). Existing Nd and Hf isotope data for mafic granulites of Neoproterozoic age from the lower portions of the SGN, potentially representing basalt underplates, show slightly negative values (εNd(t)= -5 to -9; εHf(t)= - 8 to -12; Tedeschi et al., 2018; Janasi, 2002), within the range of the most primitive charnockites (Janasi, 2002; this work). As recognized in several other provinces, such "crustal" contribution could reflect introduction of crust-derived material in the mantle during subduction, and therefore the extent of upper plate crustal contributions would be lower (Jagoutz and Klein, 2018; Moyen et al., 2017).

  • The oldest magmatic rocks recorded in the Socorro batholith are foliated charnockites rocks that show transitional contact to texturally similar orthogneisses (Wernick et al., 1984b). The age of the Socorro charnockite (~642 Ma) overlaps those of regional orthogneisses (in part also of charnockitic character) considered as associated with pre-collisional tectonics (subduction-related? Hackspacher et al., 2003). The Atibaia charnockite is slightly younger (~633 Ma) and the distinct geochemistry (lower Mg# and Sr content; higher Zr) can indicate also some change in the tectonic setting (end of the period of plate consumption? Toledo et al., 2018).

    The voluminous Bragança Paulista-type granites and coeval leucogranites (e.g., Bairro da Pedreira leucogranite) have ages (~610 Ma) and geochemical signatures indicative of a post-collisional tectonic environment. The Bocaina Pluton with an older age (~624 Ma), possibly reflects a syn-collisional period. This is consistent with dating of the syn-collisional UHP metamorphism in the SGN (ca. 630-625 Ma) (Rocha et al., 2018) and in retro-eclogites of the Andrelândia Nappe System (collision starting at ~625 Ma; Coelho et al., 2017).

    The compositions of granites from the Socorro batholith were plotted in the diagrams proposed by Whalen and Hildebrand (2019) to discriminate A-type, subduction-related and post-collisional (slab failure) I-type granites. Most samples fall in the slab failure field (Fig. 13), and contrast with younger (~580 Ma) "oxidized" granites from the Itupeva Pluton (Monsalve- Hernández and Janasi, unpublished data), which is part of the Itu granitic province (IGP; Janasi et al., 2009) and plots in the A-type field (Fig. 13). The post-orogenic IGP comprises shallow-level plutons that intrude the SGN migmatites and the Socorro batholith after significant denudation (e.g., Naranjo and Vlach, 2018), and reflect the period of orogen collapse.

    Figure 13.  Tectonic discrimination diagramas (after Whalen and Hildebrand, 2019) comparing HKCA granites from the Socorro batholith and Itu batholith (Itupeva Pluton).

    The Bragança Paulista-type granites have fractionated REE patterns with weak negative Eu anomalies, and high Sr/Y=19-40. These geochemical features are typical of magmas equilibrated at high crustal pressures, where heavy rare earth elements (HREE) and Y are strongly partitioned into residual garnet while of Sr and Eu (elements that partition into plagioclase) are incorporated into the melt (e.g., Gromet and Silver, 1987). This is consistent with the interpretation that melt generation occurred in a thickened crust in a post-collisional environment, as a result of slab failure (Hildebrand and Whalen, 2017). These qualitative relationships were recently translated in empirical formulas that could be applied to magmatic arcs (Profeta et al., 2015) and continental collisional magmatism (Hu et al., 2017), relating Sr/Y and La/Yb to Moho depth. Applying the formulation of Hu et al. (2017) to the average Sr/Y (34) and La/Yb (32) of the Bragança Paulista-type granites would indicate a crustal thickness of 51 and 54 km, respectively, assuming near- Moho magma equilibration. An alternative explanation for high Sr/Y in granitic rocks is plagioclase accumulation (raising the whole-rock Sr content), since many plutons may correspond to crystal-rich magma reservoirs (e.g., Laurent et al., 2020). However, the consistently high Sr/Y typical of all samples of Bragança Paulista-type granites and the correlation with high La/Yb (which is not affected by feldspar accumulation) are suggestive that it should be a primary feature of the melts.

    The rocks from the leucogranite plutons also have distinctive (La/Yb)N (120-360), Sm/Yb (6-25), Gd/Yb (4-11) and Sr/Y (30-970) that are even higher than those of the Bragança Paulista-type granites, and could reflect generation in a thickened crust. However, the positive Eu anomaly and extremely high Sr/Y shown by samples of the Fazenda São Sebastião leucogranite clearly results from feldspar accumulation.

  • Whole-rock elemental and Sr-Nd isotope geochemistry and zircon Hf isotope geochemistry by LA-ICP-MS reveal that the Ediacaran (~640-610 Ma) granite magmatism that formed the Socorro batholith derived from old (in part Paleoproterozoic) crustal sources, with some contribution from a Mesoproterozoic crust (as indicated by zircon inheritance). A juvenile (Neoproterozoic) mantle component may be present, as suggested by the relatively primitive (< 65 wt.% SiO2) composition of important volumes of these granites.

    The most abundant components of the batholith are ~610 Ma Bragança Paulista-type granites, that are relatively primitive, but have high contents of LILE and very negative initial εNd and εHf indicative of a strong contribution from old crust sources of Mesoproterozoic and Paleoproterozoic age. A contribution from (enriched) mantle is suggested by high mafic mineral content, presence of mafic microgranular enclaves and inferred high liquidus (apatite saturation) temperatures. Taking into account the age inferred for the main collisional event and evidence of generation in thickened crust, the Bragança Paulista-type granites are considered of post-collisional character.

    Charnockites occur as the oldest (~640-630 Ma) manifestations in the Socorro batholith, and may be pre-collisional. The Socorro charnockite has more radiogenic Nd and Hf isotope signatures than Bragança Paulista-type granites, suggestive of water-poor (granulitic?) sources of younger crustal age; a mantle contribution is likely, given its primitive character. The slightly younger Atibaia charnockite, on the other hand, is more evolved, and its geochemistry indicates a source that may be similar to that of the contemporaneous peraluminous Nazaré Paulista-type anatectic garnet-biotite granites, mostly derived from metasedimentary sources.

    Leucogranites occur as three separated bodies in the southern part of the batholith and, in spite of important chemical and isotope differences, are interpreted as products of crustal anatexis. Zircon εHf(t) data are consistent with derivation from multiple and isotopically distinct magma batches, including sources with 1.8-1.5 Ga, previously unidentified in the SGN.

  • This research was supported by the FAPESP (Nos. 2015/01817-6 and 2019/17550-0) to Valdecir de Assis Janasi. Valdecir de Assis Janasi acknowledges a CNPq Productivity Research (No. 306102/2019-6). The authors acknowledge the assistance of Vasco Loios (CPGeo-USP) with mineral separation, Kei Sato (CPGeo-USP) with acquisition and processing of geochronological data, Sandra Andrade and José Vinícius Martins (Geoanalítica Core Facility Laboratories, USP) with zircon trace element analysis. The final publication is available at Springer via https://doi.org/10.1007/s12583-021-1494-z.

    Electronic Supplementary Materials: Supplementary materials containing ESM1 (Methods) and ESM2 (New zircon U-Pb age determinations by SHRIMP), ESM3 (Major and trace elements of granites from the Socorro batholith by X-ray), ESM4 (Rare earth and additional trace elements of the Socorro Batholith by ICPMS vertical), ESM5 (Zircon trace-elements data), ESM6 (Rhyolite-MELTS modeling BRP-31a), are available in the online version of this article at https://doi.org/10.1007/s12583-021-1494-z.

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