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Volume 32 Issue 6
Dec.  2021
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Pedro Quelhas, João Mata, Ágata Alveirinho Dias. Magmatic Evolution of Garnet-Bearing highly Fractionated Granitic Rocks from Macao, Southeast China: Implications for Granite-Related Mineralization Processes. Journal of Earth Science, 2021, 32(6): 1454-1471. doi: 10.1007/s12583-020-1389-4
Citation: Pedro Quelhas, João Mata, Ágata Alveirinho Dias. Magmatic Evolution of Garnet-Bearing highly Fractionated Granitic Rocks from Macao, Southeast China: Implications for Granite-Related Mineralization Processes. Journal of Earth Science, 2021, 32(6): 1454-1471. doi: 10.1007/s12583-020-1389-4

Magmatic Evolution of Garnet-Bearing highly Fractionated Granitic Rocks from Macao, Southeast China: Implications for Granite-Related Mineralization Processes

doi: 10.1007/s12583-020-1389-4
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  • The widespread W-(Mo)-Sn-Nb-Ta polymetallic mineralization in Southeast (SE) China is genetically associated with Mesozoic highly fractionated granitic rocks. Such rocks have enigmatic mineralogical and geochemical features, making its petrogenesis an intensely debated topic. To better understand the underlying magma evolution processes, petrography, garnet chemistry and whole-rock major and trace element data are reported for Jurassic highly fractionated granitic rocks and associated microgranite and aplite-pegmatite dikes from Macao and compared with coeval similar granitic rocks from nearby areas in SE China. Despite the fact that the most evolved rocks in Macao are garnet-bearing aplite-pegmatite dikes, the existence of coeval two-mica and garnet-bearing biotite and muscovite granites displaying more evolved compositions (e.g., lower Zr/Hf ratios) indicates that the differentiation sequence reached higher degrees of fractionation at a regional scale. Although crystal fractionation played an important role, late-stage fluid/melt interactions, involving F-rich fluids, imparted specific geochemical characteristics to Macao and SE China highly fractionated granitic rocks such as the non-CHARAC (CHArge-and-RAdius-Controlled) behavior of trace elements, leading, for example, to non-chondritic Zr/Hf ratios, Rare Earth Elements (REE) tetrad effects and Nb-Ta enrichment and fractionation. Such process contributed to the late-stage crystallization of accessory phases only found in these highly evolved facies. Among the latter, two populations of garnet were identified in MGI (Macao Group I) highly fractionated granitic rocks: small grossular-poor euhedral grains and large grossular-rich skeletal garnet grains with quartz inclusions. The first group was mainly formed through precipitation from highly evolved Mn-rich slightly peraluminous melts under low-pressure and relatively low temperature (~700 ℃) conditions. Assimilation of upper crust metasedimentary materials may have contributed as a source of Mn and Al to the formation of garnet. The second group has a metasomatic origin related to the interaction of magmatic fluids with previously crystallized mineral phases and, possibly, with assimilated metasedimentary enclaves or surrounding metasedimentary strata. The highly fractionated granitic rocks in Macao represent the first stage in the development of granite-related W-(Mo)-Sn-Nb-Ta mineralization associated with coeval more evolved lithotypes in SE China.
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Magmatic Evolution of Garnet-Bearing highly Fractionated Granitic Rocks from Macao, Southeast China: Implications for Granite-Related Mineralization Processes

doi: 10.1007/s12583-020-1389-4

Abstract: The widespread W-(Mo)-Sn-Nb-Ta polymetallic mineralization in Southeast (SE) China is genetically associated with Mesozoic highly fractionated granitic rocks. Such rocks have enigmatic mineralogical and geochemical features, making its petrogenesis an intensely debated topic. To better understand the underlying magma evolution processes, petrography, garnet chemistry and whole-rock major and trace element data are reported for Jurassic highly fractionated granitic rocks and associated microgranite and aplite-pegmatite dikes from Macao and compared with coeval similar granitic rocks from nearby areas in SE China. Despite the fact that the most evolved rocks in Macao are garnet-bearing aplite-pegmatite dikes, the existence of coeval two-mica and garnet-bearing biotite and muscovite granites displaying more evolved compositions (e.g., lower Zr/Hf ratios) indicates that the differentiation sequence reached higher degrees of fractionation at a regional scale. Although crystal fractionation played an important role, late-stage fluid/melt interactions, involving F-rich fluids, imparted specific geochemical characteristics to Macao and SE China highly fractionated granitic rocks such as the non-CHARAC (CHArge-and-RAdius-Controlled) behavior of trace elements, leading, for example, to non-chondritic Zr/Hf ratios, Rare Earth Elements (REE) tetrad effects and Nb-Ta enrichment and fractionation. Such process contributed to the late-stage crystallization of accessory phases only found in these highly evolved facies. Among the latter, two populations of garnet were identified in MGI (Macao Group I) highly fractionated granitic rocks: small grossular-poor euhedral grains and large grossular-rich skeletal garnet grains with quartz inclusions. The first group was mainly formed through precipitation from highly evolved Mn-rich slightly peraluminous melts under low-pressure and relatively low temperature (~700 ℃) conditions. Assimilation of upper crust metasedimentary materials may have contributed as a source of Mn and Al to the formation of garnet. The second group has a metasomatic origin related to the interaction of magmatic fluids with previously crystallized mineral phases and, possibly, with assimilated metasedimentary enclaves or surrounding metasedimentary strata. The highly fractionated granitic rocks in Macao represent the first stage in the development of granite-related W-(Mo)-Sn-Nb-Ta mineralization associated with coeval more evolved lithotypes in SE China.

Pedro Quelhas, João Mata, Ágata Alveirinho Dias. Magmatic Evolution of Garnet-Bearing highly Fractionated Granitic Rocks from Macao, Southeast China: Implications for Granite-Related Mineralization Processes. Journal of Earth Science, 2021, 32(6): 1454-1471. doi: 10.1007/s12583-020-1389-4
Citation: Pedro Quelhas, João Mata, Ágata Alveirinho Dias. Magmatic Evolution of Garnet-Bearing highly Fractionated Granitic Rocks from Macao, Southeast China: Implications for Granite-Related Mineralization Processes. Journal of Earth Science, 2021, 32(6): 1454-1471. doi: 10.1007/s12583-020-1389-4
  • From genesis to emplacement, granitic magmas can undergo significant degrees of differentiation leading to the formation of highly fractionated rocks such as fine-grained granites, aplites and pegmatites (Wu et al., 2017; London and Kontak, 2012; Neiva and Ramos, 2010; Neiva et al., 2008; Simmons and Webber, 2008). Aplites and pegmatites usually occur as segregations along the margins of cupolas of granitic plutons and/or sharply discordant dikes intruding igneous and metamorphic rocks (London, 2014; London and Kontak, 2012). Such highly evolved rocks are characterized by compositions close to the granite system minimum or eutectic (London et al., 2012; Jahns and Tuttle, 1963; Tuttle and Bowen, 1958), which confer them an utmost importance in the assessment of processes occurring during the latest stages of granitic magmas differentiation (Dill, 2015; London, 2008). Since pegmatites have an extremely coarse but variable grain-size, it is often difficult to obtain representative whole-rock chemical analyzes. Considering this, and the fact that aplites are compositionally similar to pegmatites for most, but volatile, elements (Dill, 2015), studies on the late stages of differentiation of granitic magmas often rely on the geochemistry of aplitic facies (e.g., Antunes et al., 2013; Neiva et al., 2012, 2008).

    In general, granitic magmas contain water between 3 wt.% to 6 wt.% and the partial pressures of magmatic gases can achieve several hundred bars (Lowenstern, 1994). As crystallization proceeds, the vapor pressure increases and the hydrostatic head of the magma may be exceeded, producing a separate vapor/fluid phase at the late stages of magmatic evolution (Chen and Grapes, 2007). At this stage, large amounts of the vapor/ fluid phase can mobilize and remove significant quantities of chemical elements from high-silica magmas to other locations (inside and/or outside of the pluton), leaving marked effects on the chemical composition of the residual magma (e.g., Monecke et al., 2007, 2002; Wu et al., 2004; Jahn et al., 2001; Irber, 1999; Bau, 1996; Clarke, 1992; Masuda et al., 1987). On the other hand, increasing contents of fluxing agents (H2O, Li, B, F, P, and/or Cl; Bau, 1997, 1996; London, 1992) significantly reduce the viscosity and solidus temperature of the magma, promoting concentration of rare or large-ion lithophile elements (LILE) in the residual melt, which results in crystallization of several mineral phases not found in less evolved rocks (e.g., topaz, phosphates, lithium micas, tourmaline, cassiterite, Mn-rich garnet, columbite-tantalite, microlite; Dill, 2015; Neiva et al., 2012). In addition, they also directly or indirectly contribute to a lower nucleation rate and higher crystal growth rates, causing the formation of pegmatitic textures (Nabelek et al., 2010).

    The Mesozoic geology of the Cathaysia Block is characterized by widespread granitic rocks (Fig. 1), which are closely associated with large-scale polymetallic mineralization (Mao et al., 2013; Zhou et al., 2006). Highly evolved Jurassic granites are common and tend to occur as small stocks hosting different kinds of mineralization (Mo, W, Sn, U, REE, Nb-Ta, Cu, Pb and Zn; e.g., Cao et al., 2018; Jiang H et al., 2018; Jiang W C et al., 2018; Wu et al., 2017; Xiang et al., 2017; Qiu et al., 2016; Zhou et al., 2016; Xu et al., 2015; Zhao et al., 2002). Despite the inherent economic potential that has prompted numerous and significant studies of granites, their origin and evolution, as well as the controlling factors of their distinct geochemistry and mineralogy, are still enigmatic. This is particularly true for the Pearl River Delta region in southern Guangdong Province (Southeast China), where Macao is located.

    Figure 1.  (a) Map of China showing the location of the studied region (GS(2019)1686); (b) geological map of SE China showing the distribution of Mesozoic granitic rocks (modified after Yang et al., 2018; Zhou et al., 2006). SJPF. Shaoxing-Jiangshan-Pingxiang fault; ZDF. Zhenghe-Dapu fault zone.

    This study deals with the petrography, mineralogy and geochemistry of highly evolved fine- to medium-grained non- porphyritic granitic facies and aplites from Macao, in order to understand the dynamics of highly evolved granitic systems in this area of SE China. The magmatic evolution stage at which pegmatites were formed will be interpreted in the light of the geochemistry of aplite and microgranite dikes. The objectives are: (1) to understand the relationship between highly fractionated granites and the aplite-pegmatite dikes; (2) to identify and describe the evolution processes responsible for the mineralogical and geochemical characteristics of these rocks; (3) to assess the role of fluids on the late-stage evolution of granitic magmas; (4) to unravel the origin of garnet occurring in the highly evolved granitic rocks in Macao and; (5) to establish the relationship between these rocks and the wide spread highly fractionated, and often mineralized, Jurassic granites in SE China. To achieve these, petrography, mineral chemistry and whole-rock elemental geochemical data are presented and discussed in this paper.

  • The territory of Macao is part of the Cathaysia Block, which, during the Early Neoproterozoic, amalgamated to the Yangtze Block in the northwest, along the Jiangshan-Shaoxing suture zone, forming the South China Block (SCB; e.g., Charvet, 2013; Wang et al., 2013; Ye et al., 2007; Li et al., 2002). Macao is located to southern Guangdong Province, along the coast of SE China, ca. 50 km to the west of Hong Kong, on the western margin of the Pearl River Delta. In this region, there is a predominance of granitic intrusions belonging to the ~3 500 km long and ~800 km wide NE-trending Mesozoic Southeast China Magmatic Belt (Fig. 1; Zhou et al., 2006). Jurassic to Cretaceous granitoids, in particular, were formed due to intense magmatic activity during the Yanshanian Orogeny, which is thought to be related to the subduction of the Paleo-Pacific Plate under the Eurasian Plate (e.g., Quelhas et al., 2021; Wang et al., 2013; Jiang et al., 2009; Li and Li, 2007; Li et al., 2007; Zhou et al., 2006; Zhou and Li, 2000).

    The Mesozoic volcanic-plutonic rocks intruded Late Paleozoic and younger Early to Middle Jurassic sedimentary successions deformed and metamorphosed by several tectonic events (Quelhas et al., 2021; Huang et al., 2013; Sewell et al., 2000; Ribeiro et al., 1992; Sewell et al., 1992). To the west of Macao, along the southern coastal region of the Guangdong Province, a few Late Jurassic amphibole-bearing granites occur as stocks/small plutons (Huang et al., 2013). However, most of the granites in the region form larger plutons or batholiths composed of biotite granites (rarely containing amphibole) spatially associated with volumetrically minor tourmaline-, muscovite- and garnet-bearing granites, mainly emplaced between 166–155 Ma (Quelhas et al., 2020; Huang et al., 2013).

    Several studies have contributed to the knowledge on the geology of Macao for the past few decades (Quelhas et al., 2021, 2020; Ribeiro et al., 2010, 1992; Carrington da Costa and Lemos, 1964; Costa, 1944; Neiva, 1944). The territory (~30 km2) is composed by several granitic intrusions belonging to a Jurassic batholith extending about 50 km to the north, which intrudes Paleozoic sedimentary strata (Quelhas et al., 2021; Huang et al., 2013; Ribeiro et al., 1992). Remnants of the wall-rock are present as metasedimentary enclaves of Devonian age enclosed within the granites (Quelhas et al., 2021). The dominant lithology is biotite granite, with minor Microgranular Mafic Enclaves (MME), that is intruded by a relatively diversified swarm of granitic and quartz dikes (and veins) and by younger Middle Jurassic to Cretaceous dacite dikes (Fig. 2). The granitic rocks consist of two chemically and age distinct groups of I-type granites: Macao Group I (MGI; 164.5±0.6 to 162.9±0.7 Ma) and Macao Group II (MGII; 156.6±0.2 to 155.5±0.8 Ma) granites (Quelhas et al., 2020). It has been proposed that these granitic magmas were generated by partial melting of infracrustal medium- to high-K basaltic Paleoproterozoic to Mesoproterozoic protoliths, though each group had a different magmatic evolution history (Quelhas et al., 2021). Each group, in its turn, can be subdivided in "weakly to moderately fractionated granites" (Zr/Hf > 25) and "highly fractionated granites" (Zr/Hf≤25; Quelhas et al., 2021, 2020). This study will focus on the MGI highly fractionated granitic rocks, including the associated microgranite, aplite and pegmatite dikes. The MGI highly fractionated granites (Figs. 3b3f) either occur as large bodies of medium- to coarse-grained non-porphyritic biotite granites gradually changing into less fractionated coarse-grained porphyritic varieties or as small stocks of fine-grained granites in the margins of the main granitic intrusions (Fig. 2). In the latter case, aplites and pegmatites tend to be particularly abundant, occurring either as sharply discordant dikes (Fig. 3j) or as segregations within the highly fractionated granites (Fig. 3i). The contacts between aplite-pegmatite dikes and the host granites vary between straight and irregular sharp to gradual. The microgranite dikes tend to be larger (decimeter to meter-wide) than the aplite dikes, have sharp contacts with the host granite and their texture is uniform (Fig. 3g). The aplites, on the other hand, occur as smaller dikes (centimeter to decimeter-wide), with typical sugary texture and often show gradual variation in grain size to pegmatitic textures (Fig. 3j).

    Figure 2.  Simplified geological map of Macao showing the distribution of the two groups of granitic rocks and the location of samples analyzed in this study (modified after Quelhas et al., 2021, 2020).

    Figure 3.  Representative hand specimens and thin-sections of the Macao Group I (MGI) weakly to moderately fractionated (a) and highly fractionated (b)–(l) granitic rocks. (a) Large microcline megacrysts in a coarse-grained porphyritic biotite granite; (b) medium- to coarse-grained non-porphyritic garnet-bearing biotite granite; (c) garnet associated with biotite in a medium- to coarse-grained non-porphyritic garnet-bearing biotite granite; (d) fine- to medium-grained non-porphyritic garnet-bearing biotite granite, with two crystals of red-brownish garnet; (e) garnet-quartz intergrowth in a fine- to medium-grained non-porphyritic garnet-bearing biotite granite; (f) lath-shaped biotite crystals in a fine- to medium-grained non-porphyritic garnet-bearing biotite granite; (g) fine-grained equigranular microgranite dike; (h) microcline and quartz anhedral grains and graphic texture in a microgranite dike; (i) pod-shaped pegmatite associated with a fine- to medium-grained non-porphyritic garnet-bearing biotite granite; (j) aplite-pegmatite dike with the pegmatite in the center surrounded by aplite; (k) two euhedral garnet grains in an aplite dike; (l) microtexture of the aplite dike, showing subhedral tabular plagioclase and anhedral quartz and K-feldspar grains. Qtz. Quartz; Pl. plagioclase; Mc. microcline; Bt. biotite; Grt. garnet.

  • Mineral analyses were performed on carbon-coated polished thin sections using a JEOL SUPERPROBE™, model JXA-8200, in wavelength dispersive mode at the Departamento de Geologia da Faculdade de Ciências da Universidade de Lisboa (Portugal). Minerals were analyzed with an acceleration voltage of 15 kV and a current of 25 nA, using a 5 µm wide beam. The analyses were calibrated using the composition of in-house standard materials, with reproducibility errors being lower than 2% and ordinarily around 1%. Matrix effects were corrected using the ZAF software provided by JEOL. Information about standards and detection limits are given in Supplementary File 1a.

  • Whole-rock major and trace element concentrations were obtained at Activation Laboratories, Ltd. (Ancaster, Ontario, Canada) using the geochemical analytical package 4E-Research. Major oxide content was analyzed using Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) using a Thermo Jarrell-Ash ENVIRO II ICP and/or Spectro Cirros ICP. Trace elements, including Rare Earth Elements (REE), were obtained using Inductively Coupled Plasma Mass-Spectrometry (ICP-MS) on a Perkin Elmer SCIEX ELAN 6000, 6100 or 9000 ICP/MS and by Instrumental Neutron Activation Analysis (INAA). In-house standards and several certified reference materials of granitic to dioritic compositions (s.l.) from USGS (United States Geological Survey), GSJ (Geological Survey of Japan) and CCRMP (Canadian Certificate Reference Material Project) were used. Errors associated with the accuracy are ≤4% for major elements and better than 9% for the REE and the most widely used incompatible elements. Duplicate measurements of samples indicate that errors associated with reproducibility were generally lower than 5% for both major and trace elements. Information about detection limits is given in Supplementary File 1B. For detailed information regarding analytical and control procedures consult the Actlabs website (www.actlabs.com).

  • The petrographic and mineralogical features of Macao granites have been described in previous studies (Quelhas et al., 2021, 2020). Here we focus more specifically, and with more detail, on the mineralogy and textures of MGI highly fractionated granitic rocks, including microgranites and aplite-pegmatites.

    The MGI weakly to moderately fractionated facies have a strong porphyritic character (Fig. 3a) in contrast with the MGI highly fractionated granites, which mainly occur as coarse-to medium-grained non-porphyritic garnet-bearing biotite- microcline facies (Figs. 3b, 3c) gradually changing into slightly porphyritic facies in Taipa (Fig. 2). The MGI highly fractionated granites also occur as small intrusions of fine-to medium-grained non-porphyritic garnet-bearing biotite-microcline granite (Figs. 3d3f) in Macao Peninsula and Coloane (Fig. 2). Under the microscope, the texture is xenomorphic. Alkali feldspar is present as anhedral to subhedral tabular perthite grains (< 2 cm), showing Carlsbad twinning and weak undulatory extinction. Sodic feldspar blebs are usually weakly developed, with irregular contours, rarely showing polysynthetic twins. It is often poikilitic, including fine crystals of anhedral quartz and subhedral tabular zoned plagioclase. Plagioclase is less abundant than alkali feldspar, occurring as anhedral to subhedral tabular weakly sericitized grains (< 1 cm), with polysynthetic twinning (rarely combined with Carlsbad twins) and occasionally showing continuous normal zoning. Quartz grains are inequigranular and anhedral (< 1 cm; Fig. 3e), with irregular to sutured boundaries and strong undulatory extinction. The grains are frequently fractured and sub-granulated. Biotite occurs as aggregates of subhedral plates in coarse-grained facies (Fig. 3c) and as subhedral elongated lath-shaped crystals in fine-grained facies (Fig. 3f), pleochroic between pale yellow and green and moderately chloritized. Some crystals have abundant pleochroic haloes surrounding inclusions of zircon, monazite, xenotime and thorite. In coarse-grained facies, garnet occurs as small light brown subhedral crystals (< 0.5 mm; Fig. 3c) associated with biotite, whereas in fine-grained facies it occurs as light brown anhedral to subhedral large skeletal crystals (< 5 mm; Figs. 3d, 3e), forming symplectitic intergrowths with quartz. Other accessory minerals include: ilmenite, magnetite, fluorite, apatite, allanite, thorite, fergusonite, gadolinite, columbite-tantalite and sulfides.

    Microgranite dikes tend to be more abundant in some specific areas (e.g., northeastern areas of Taipa main intrusions; Fig. 2) and display a wide range of widths (from centimeter to meter-wide). These rocks are leucocratic (in some cases slightly pinkish due to K-feldspar alteration; Fig. 3g) and have a fine-grained (< 0.1 cm), moderately equigranular xenomorphic texture (Figs. 3g, 3h). Alkali feldspar is represented by fine-grained anhedral microperthite with weak undulatory extinction (Fig. 3h) and, in some cases, with poikilitic texture, hosting inclusions of quartz and plagioclase. Sodic blebs are rarely well-developed to the point of showing polysynthetic twinning. Plagioclase is less abundant than alkali feldspar, occurring as fine-grained subhedral tabular crystals with polysynthetic twinning. Quartz is present as fine-grained intergranular anhedral grains with undulatory extinction and as worm-like intergrowths with alkali feldspar, producing abundant graphic textures (Fig. 3h). Biotite is present as rare green to light yellow pleochroic fine-grained anhedral to subhedral flakes. The flakes are weakly to strongly chloritized and often show deformation features such as broken crystals and bent cleavages. Garnet occurs as light brown euhedral to subhedral isolated hexagonal and/or dodecahedron-shaped crystals (< 500 µm), in textural equilibrium with surrounding minerals. Other accessory minerals include: Fe-Ti oxides, sulfides and fluorite.

    Pegmatites occur essentially as dikes cutting fine-grained granitic facies, occurring, in some cases, as pod-shaped bodies (up to 1 meter in diameter; Fig. 3i). Alternation between aplite and pegmatite textures is common in these dikes (Fig. 3j). The mineralogy of pegmatites is mostly composed by euhedral to subhedral large crystals (centimeter to decimeter-large) of quartz, feldspar and biotite (Fig. 3i) and by millimeter-sized accessory garnet. Aplite dikes have centimeter- to decimeter-wide dimensions and are characterized by a very fine-grained (< 0.1 cm) moderately equigranular xenomorphic texture with abundant garnet crystals as accessory mineral (Fig. 3k). The major mineral components in the mineralogy of aplites are: alkali feldspar, represented by anhedral microperthitic microcline crystals, rarely poikilitic (hosting smaller plagioclase and quartz grains), with weak undulatory extinction and weakly to well-developed sodic blebs; subhedral tabular plagioclase crystals with frequent polysynthetic twinning (Fig. 3l), and; anhedral crystals of quartz with moderate to strong undulatory extinction and irregular to slightly sutured boundaries (Fig. 3l). Biotite is also present as a minor component. It occurs as weakly to strongly chloritized anhedral to subhedral flakes showing strong pleochroism between pale yellow and brownish green, with a few pleochroic haloes. Garnet occurs as light brown euhedral to subhedral isolated octagon and/or dodecahedron-shaped crystals (< 500 µm; Fig. 3k), in textural equilibrium with surrounding minerals. Other accessory minerals include Fe-Ti oxides, fluorite and sulfides.

  • The results of major element analysis of garnet grains from the MGI highly fractionated granitic rocks are listed in Supplementary File 2 and shown in Fig. 4. Overall, the abundance of garnet in the MGI highly fractionated granitic rocks increases from non-porphyritic garnet-bearing biotite granites (1 vol.%–2 vol.%) to aplite dikes (around 5 vol.%).

    Figure 4.  Ternary diagram showing the chemical composition of garnets from MGI highly fractionated granitic rocks in terms of the almandine, spessartine and grossular molecules.

    All garnet grains have high MnO (12.8 wt.%–28.1 wt.%) and FeO (8.1 wt.%–20.9 wt.%), but low MgO (< 0.2 wt.%), TiO2 (< 0.7 wt.%) and low to moderate CaO (0.8 wt.%–12.4 wt.%) contents. Al2O3 is nearly constant in all the analyzed grains (19.7 wt.%–20.7 wt.%). They are mostly almandine-spessartine (63.7%–97.5% of the total molecular composition; Fig. 4). However, garnet grains from a fine-grained garnet-bearing biotite granite depict a considerably higher grossular component (24.4%–34.5%; sample C71, see Fig. 4 and Supplementary File 2).

    Electron microprobe data show that, while some crystals have relatively constant compositions, others have weak to moderate compositional zoning (Supplementary File 2). Among the latter, most grains are characterized by Mn-rich and Fe-poor cores and Mn-poor and Fe-rich rims. More rarely grains display oscillatory zoning patterns with variable MnO, FeO and CaO contents from rim to rim.

  • The whole-rock elemental data are listed in Supplementary File 3 (for details on the location of the analyzed samples see Supplementary File 4). According to their normative composition, the MGI highly fractionated granitic rocks vary between syenogranite to alkali feldspar granite (Fig. 5a). They have high SiO2=73.47 wt.%–76.92 wt.%, moderate Al2O3 (12.17 wt.%–13.12 wt.%) and are relatively high in total alkalis, with K2O=3.92 wt.%–5.01 wt.%, Na2O=3.22 wt.%–4.26 wt.% and K2O+Na2O=7.65 wt.%–8.96 wt.%, plotting into the calc-alkaline series (Fig. 5b). Their A/CNK values range from 0.97 to 1.13, most of the samples being weakly peraluminous (Fig. 5c). They also have low contents of FeOT (1.08–2.55; total Fe expressed as divalent), MgO (0.02 wt.%–0.08 wt.%), CaO (0.32 wt.%–0.99 wt.%), P2O5 (< 0.07 wt.%), and TiO2 (0.01 wt.%–0.08 wt.%), which tend to decrease with increasing SiO2 contents. The FeOT/(FeOT+MgO) ratios are relatively high, plotting into the ferroan granites field (Fig. 5d).

    Figure 5.  Classification diagrams for MGI and SE China Jurassic highly fractionated granitic rocks. (a) Q'-ANOR normative composition diagram (Streckeisen and Le Maitre, 1979), where Q'=Q/(Q+Or+Ab+An)×100; ANOR=An/(Or+An)×100; (b) Na2O+K2O–CaO vs. SiO2 (Frost and Frost, 2008); (c) aluminous saturation index [ASI or A/CNK defined as the molar ratio Al2O3/(CaO+Na2O+K2O)] vs. SiO2; (d) FeOT/(FeOT+MgO) vs. SiO2 (Frost and Frost, 2008); SE China Jurassic highly fractionated granitic rocks with Zr/Hf≤25 (Jiang and Zhu, 2017; Zhang et al., 2017, 2015; Huang et al., 2015, 2013; Guo et al., 2012; Li et al., 2007).

    In the primitive mantle-normalized multi-element variation diagram (Fig. 6a), the MGI highly fractionated granitic rocks are characterized by enrichment in large ion lithophile elements (LILE; e.g., Rb, K, Pb, Th and U), with conspicuous negative Ba, Sr, P, Eu and Zr anomalies. While the MGI weakly to moderately fractionated granites tend to show depletion in high field strength elements (HFSE; e.g., Nb, Ta, and Ti), the MGI highly fractionated granites show enrichment in Nb, Ta, Pb, U, Y and heavy REE (HREE; Figs. 6a, 6b). The latter are also characterized by low Nb/Ta (2.71–5.65) and Zr/Hf (12.24–25.00) ratios similar to Jurassic highly fractionated granites outcropping in other regions of SE China (Jiang and Zhu, 2017; Zhang et al., 2017, 2015; Huang et al., 2015, 2013; Tao et al., 2013; Guo et al., 2012; Li et al., 2007).

    Figure 6.  Primitive mantle (PM) normalized trace element spidergram (PM values are from Sun and McDonough, 1989) (a); and chondrite normalized rare earth elements (REE) patterns (chondrite values are from McDonough and Sun, 1995) (b) of MGI and SE China Jurassic highly fractionated granitic rocks.

    The MGI highly fractionated granitic rocks also have moderate total REE contents (ΣREE=65.29 ppm–180.32 ppm) and show relatively flat (garnet-bearing biotite granites) to slightly positive slopes (garnet-bearing microgranite and aplite dikes) on chondrite-normalized REE patterns (Fig. 6b), with (La/Yb)N=0.12–2.41, well-pronounced Eu anomalies (Eu/Eu*=0.02–0.19) and showing evidence for the tetrad effect (TE1, 3=1–1.12) as defined by Irber (1999). With the increase of SiO2 content and decrease in Zr/Hf ratios, the HREE contents tend to increase and the Eu anomalies become more negative.

    Overall, the MGI highly fractionated granitic rocks have similar compositions to those of the SE China Jurassic highly fractionated. However, SE China Jurassic garnet-bearing biotite, muscovite and two-mica granites show more pronounced Eu and Sr anomalies (Figs. 6a, 6b), suggesting higher degrees of fractionation than those characterizing MGI highly fractionated granitic magmas. Some SE China Jurassic garnet-bearing muscovite granites can reach a strong peraluminous character (A/CNK up to ~2.2; Fig. 5c).

  • The crystal fractionation processes underlying the evolution of Macao granites, in particular the influence of different major and accessory phases, have been addressed in detail by Quelhas et al. (2021). In this discussion, the aim is to unravel the differentiation sequence within the MGI highly fractionated rocks and frame it within the evolution of Jurassic highly fractionated granites in SE China. For this, we use Zr/Hf ratios, Rb, Sr, Ba and Eu concentration variation diagrams to assess the position of the different granitic facies in the fractionation series.

    The Zr/Hf ratio has been regarded as a valuable differentiation index for granitic systems (Zaraisky et al., 2009; Irber, 1999) and was successfully applied to Macao granitic rocks in order to separate weakly to moderately fractionated facies (Zr/Hf ratios > 25), and highly fractionated ones (Zr/Hf ratios≤25; Quelhas et al., 2021). As expected from their low Zr/Hf ratios, all the MGI highly fractionated granitic rocks plot into the strong differentiated granites field of the ternary diagram Ba-Sr-Rb (Fig. 7a). The trace element variation diagrams shown in Figs. 7b, 7c clearly show the influence of feldspar (s.l.) fractionation in the differentiation history of MGI and SE China granites. However, the variation in Ba contents points to a stronger influence of K-feldspar fractionation rather than plagioclase. Within the MGI granitic rocks, Sr and Ba is progressively depleted throughout the liquid line of descent producing the following lithological sequence: biotite granites, garnet-bearing biotite granites, microgranite dikes and lastly aplite dikes. Thus, the most fractionated magma in Macao is represented by the aplite-pegmatite dikes. However, coeval SE China Jurassic granites can reach even higher degrees of fractionation (Fig. 7). Indeed, while SE China biotite granites represent similar degrees of evolution as those of the MGI granites, SE China two-mica granites (Zr/Hf down to 12.24) can reach extremely low Ba (down to 3.53 ppm) and Sr (down to 0.94 ppm) contents and even more pronounced Eu anomalies (Eu/Eu* down to 0.03; Fig. 6b) than those characterizing the MGI highly fractionated granitic rocks (Eu/Eu* down to 0.13; Zr/Hf down to 8.88; Fig. 6b).

    Figure 7.  Trace element fractionation diagrams for MGI and SE China Jurassic highly fractionated granitic rocks. (a) Ba-Rb-Sr ternary diagram (El Bouseily and El Sokkary, 1975); (b) Sr vs. Eu; (c) Ba vs. Eu. Vectors represent the fractionation effect of different minerals calculated using partition coefficients from the literature (Bea et al., 1994; Bacon and Druitt, 1988; Nash and Crecraft, 1985). Kfs. Alkali feldspar; Pl. plagioclase; Bt. biotite; Hbl. hornblende. Symbols are as in Fig. 5.

    It has been shown by Quelhas et al. (2021) that fractionation of accessory phases such as allanite, monazite, xenotime, titanite, apatite and zircon, played an important role in the evolution of Macao granitic magmas. It is for example the case of the decrease in Th contents with increasing degree of evolution, i.e., with the decrease of Zr/Hf ratios (Fig. 8a), which can be explained by fractionation of Th-bearing phases like xenotime and thorite. However, fractionation of these phases cannot explain the increase in Sm/Nd ratios (from 0.20 to 0.77) observed for progressively more evolved MGI granitic rocks. Such increase in Sm/Nd, together with other peculiar chemical features of the MGI highly fractionated granitic rocks, is also observed for SE China Jurassic highly fractionated rocks (Sm/Nd=0.18–0.69), suggesting that Jurassic highly evolved magmas underwent evolution processes other than crystal fractionation.

    Figure 8.  Trace element variation diagrams for MGI and SE China Jurassic highly fractionated granitic rocks. (a) Th vs. Zr/Hf; (b) Y vs. Zr/Hf; (c) Nb vs. Zr/Hf; (d) Nb/Ta vs. K/Rb; (e) Nb/Ta vs. Ta; (f) Nb/Ta vs. TE3, 1. The dashed lines represent the values of Nb/Ta (< 5) and K/Rb (< 150) which fingerprint the magmatic-hydrothermal transition in peraluminous granites (Ballouard et al., 2016); the dashed line for TE3, 1 represents the value above which the degree of the tetrad effect is considered significant (Irber, 1999). Symbols are as in Fig. 5.

  • As explained above, there are compositional characteristics of MGI granites, such as a significant enrichment in Nb, Ta, Pb, U, Y and HREE (Figs. 6a, 6b), that are hardly explained by crystal fractionation processes. A considerable enrichment in Rb is also observable for rocks with Zr/Hf < 25 (Rb=214 ppm–1 578 ppm), leading to a significant decrease of K/Rb ratios in the melts (Fig. 8d). Such Rb enrichment in highly evolved granites is known to reflect autometasomatic processes caused by residual magmatic solutions (Gerstenberger, 1989).

    It has long been known that highly evolved granitic magmas are characterized by physicochemical properties transitional between pure silicate melts and aqueous fluids (e.g., Monecke et al., 2007; Wu et al., 2004; Monecke et al., 2002; Jahn et al., 2001; Irber, 1999; Bau, 1996; Masuda et al., 1987). Chemical complexation with a wide variety of ligands, like non-bridging oxygen (e.g., F, B, P, etc.), changes significantly the melt structure and, consequently, the mineral/melt partition coefficients, promoting non-CHARAC (CHArge-and-RAdius-Controlled) behavior of HFSE (Bau, 1996).

    MGI highly fractionated granitic rocks are characterized by non-chondritic ratios of Zr/Hf (12.24–25), K/Ba (498–9 560), K/Rb (101–158), La/Nb (0.15–2.18), La/Ta (0.69–11.32) and Nb/Ta (2.71–7.00; Fig. 8) strongly suggesting non-CHARAC behavior of trace elements, which is well evidenced in the Y/Ho vs. Zr/Hf diagram (Fig. 9a), where SE China Jurassic highly fractionated granitic rocks show a similar behavior.

    Figure 9.  Trace element discriminant diagrams for MGI and SE China Jurassic highly fractionated granitic rocks. (a) Y/Ho vs. Zr/Hf (after Bau, 1996); (b) Nb/Ta vs. Zr/Hf (after Ballouard et al., 2016); (c) Zr/Hf vs. SiO2 (after Zaraisky et al., 2009). Symbols are as in Fig. 5.

    Additionally, MGI and SE China Jurassic highly fractionated granitic rocks have REE patterns characterized by the presence of tetrad effects (TE1, 3 up to 1.13 and 1.22, respectively). Different processes have been proposed for the formation of the REE tetrad effect: (1) fractionation of accessory minerals (e.g., Pan, 1997; Pan and Breaks, 1997; Bau, 1996; McLennan, 1994; Zhao and Cooper, 1993; Yurimoto et al., 1990; Jolliff et al., 1989); (2) post-magmatic water-rock interactions (e.g., Takahashi et al., 2002; Masuda and Akagi, 1989), and (3) late-stage fluid/melt interactions (e.g., Tao et al., 2013; Wu C Z et al., 2011; Monecke et al., 2007; Wu F Y et al., 2004; Monecke et al., 2002; Zhao et al., 2002; Jahn et al., 2001; Irber, 1999; Bau, 1996), in particular with F-rich fluids (Badanina et al., 2006; Veksler et al., 2005).

    Bau (1996) has shown that the tetrad effect cannot be adequately modelled with mineral/melt partition coefficients, which are smooth functions of ionic radii. Bau (1997) and Irber (1999) examined in the Nd-Sm decoupling and the tetrad effect in detail using Rayleigh model calculations and reported that mineral fractionation involving accessory phases failed to reproduce the tetrad effect-like REE pattern. This is also true for Macao, where Quelhas et al. (2021) showed that the observed increase of Sm/Nd ratios in MGI granitic rocks follows a distinct trend direction than that of monazite fractionation, an accessory phase that has been considered by some authors as capable of generating the REE tetrad effect (Duc-Tin and Keppler, 2015; Yurimoto et al., 1990).

    Another argument against mineral fractionation as a common cause for the tetrad effect is based on accessory minerals that display tetrad effects that are similar to those of the host rocks (Wu et al., 2011; Zhao et al., 2002), indicating that the accessory mineral tetrad effect was inherited from a pre-existing melt tetrad effect rather than through a process of mineral fractionation.

    Post-magmatic water-rock interaction is also unlikely since no REE tetrad effects have been observed in hydrothermally altered rocks that are not highly evolved (Jahn et al., 2001). Plus, analyses on some separated minerals show the same tetrad effects as their host rocks (Nardi et al., 2012; Wu et al., 2011; Zhao et al., 2002; Irber, 1999). This feature has thus a magmatic origin, inherited from the crystallization of magmas with the tetrad effect.

    Fluid/melt interactions promote enrichment of volatiles and alkaline elements in the highly evolved granitic magmas, consequently lowering the solidus temperatures of the residual granitic system and leading to the formation of a co-existing magmatic-hydrothermal system (e.g., Peretyazhko and Savina, 2010; Monecke et al., 2007; Irber, 1999; Pan, 1997; London et al., 1989). The volatile-enriched late magmatic fluids, having "W" type tetrad REE patterns, interact with the highly evolved melt and lead to the "M" type tetrad effect in the granites (Jahn et al., 2001; Irber, 1999), as observed for some of the more evolved granites, microgranites and aplites from Macao (Fig. 6b).

    Fluorine (F) has been considered an important constituent of fluids inducing the REE tetrad effect on magmas with which they interact (Jiang H et al., 2018; Xiang et al., 2017; Yang et al., 2017; Zhang et al., 2017; Bau and Dulski, 1995). Interestingly, some of the biotite grains from the MGI highly fractionated granites with the tetrad effect have relatively high F (up to 2.13 wt.%), similar to the F contents of muscovite from muscovite-bearing granites from SE China (F up to 2.25 wt.%; Zhang et al., 2017), and large crystals of purple fluorite have also been found in quartz veins cutting MGI granitic rocks (Quelhas et al., 2021). In addition, the more evolved coeval granites from surrounding areas of Macao show a progressive increase in Zr/Hf and Y/Ho ratios towards the Fluorite veins field (Fig. 9a). All these suggest that the late-stage fluid exsolved by the MGI and Jurassic SE China highly evolved granitic melts was rich in F.

    Furthermore, the fluid/melt partition coefficients of Ta and Nb are low (DNb and DTa are 0–0.1 and 0.002–0.08, respectively; Zajacz et al., 2008; London et al., 1988), meaning that these elements are more compatible with the granitic magma than with the fluid and, thus, exsolution of the fluid from magma will increase the concentration of these elements in the magma (Linnen, 1998; Linnen and Keppler, 1997). The Nb and Ta contents of MGI granitic rocks remain nearly constant with decreasing Zr/Hf ratios, but suddenly increase for the MGI highly fractionated samples along with SE China granitic rocks with Zr/Hf≤25 and Nb/Ta < 5 (Figs. 8c, 8e). Moreover, the fluid/melt partition coefficients mentioned above show more than one order of magnitude difference between Nb and Ta. This might explain the observed decrease in Nb/Ta ratios for MGI and SE China Jurassic highly fractionated rocks (Figs. 8d8f), which is accompanied by an increase in the degree of the REE tetrad effect (Fig. 8f). Therefore, we suggest that, along with the REE tetrad effect and the non-CHARAC behavior of trace elements, the main drive for Nb-Ta enrichment and fractionation in the MGI and SE China highly fractionated granitic magmas resulted from the exsolution and interaction of a F-rich fluid phase with the melt. Such process may also explain the increase in Y and HREE (Figs. 6, 8b) in the MGI highly fractionated granitic rocks, which favored the precipitation from the magma of mineral phases like fergusonite, gadolinite and columbite-tantalite. It is noteworthy that the highest enrichment in Y and HREE is observed in the aplites (Figs. 6b, 8b), where these mineral phases are more frequently found among MGI granitic rocks.

  • Overall, garnet from MGI highly fractionated granitic rocks occurs as euhedral to anhedral red-brownish grains, characterized by high FeO and MnO contents and often showing chemical zoning with spessartine-rich cores and almandine-rich rims or oscillatory zoning (Fig. 4; Supplementary File 2). Nevertheless, different garnet textures and chemistry point to the existence of two distinct types: small grossular-poor euhedral grains (Figs. 3k, 4) and much larger grossular-rich anhedral garnet grains with quartz inclusions (Figs. 3e, 4). The first type mostly occurs in aplitic facies, whereas the second type is mainly restricted to the fine- to medium-grained non-porphyritic garnet-bearing biotite granite (sample C71; see Fig. 4 and Supplementary File 2).

    The origin of garnet in granitic rocks has been debated for many years, with different models being proposed: (1) refractory restite phase (René and Stelling, 2007); (2) peritectic entrainment phase from the zone of partial melting (Villaros et al., 2009; Stevens et al., 2007); (3) phenocrysts formed at high pressure (P≥7 kb) from mantle-derived magmas (Harangi et al., 2001; Green, 1992); (4) xenocrysts derived from partially assimilated metapelitic rocks (Lackey et al., 2012; Scallion et al., 2011; Erdmann et al., 2009; Fourcade et al., 2001); (5) precipitated phase from highly fractionated Mn-enriched peraluminous melts or from post-magmatic fluids through autometasomatic processes at low to moderate pressures (Speer and Becker, 1992; Whitworth, 1992; du Bray, 1988; Harrison, 1988; Kontak and Corey, 1988).

    The occurrence of garnet in MGI granitic rocks is restricted to the highly fractionated facies, precluding the three first above-mentioned origins. Despite peritectic garnet, texturally similar to the large skeletal garnet crystals of this study (Fig. 3e), has been reported in some S-type granites (e.g., Taylor and Stevens, 2010), this origin is unlikely in the case of MGI highly fractionated granites, as garnet is absent in the less evolved members of this group (i.e., weakly to moderately fractionated facies). In addition, garnet formed at high-pressure usually has low MnO < 3 wt.% (Harangi et al., 2001), in contrast with the high MnO contents of garnet from MGI (MnO=12.8 wt.%–28.1 wt.%). All these, together with the fact that garnet grains, in particular those from the first group (i.e., grossular-poor garnet; Fig. 4) have no or few mineral inclusions and occur in textural equilibrium with other mineral phases, indicates crystallization from the host granitic magmas.

    It has been recently proposed that the presence of garnet in Macao granites is suggestive of an origin through partial melting of sedimentary sources (Shellnut et al., 2020). Such interpretation is not consistent with the data and petrogenetic model published by Quelhas et al.(2021, 2020), in which Macao granites are interpreted as being derived from an infracrustal igneous source, for several reasons: (1) Sampling carried out by Shellnut et al. (2020) is concentrated in two specific areas of Macao, not being representative of the whole territory; (2) Shellnut et al. (2020) proposed an interpretation mainly based on the mineralogy and geochemistry of highly evolved granites from Macao (SiO2=71.3 wt.%–79.1 wt.%; Rb/Sr=2–23), but did not take into account the characteristics of the coeval less evolved members of the magmatic sequence outcropping in Macao and surrounding areas (Quelhas et al., 2021, 2020; Zhang et al., 2015; Huang et al., 2013; Sewell et al., 1997). For instance, garnet only occurs in the highly fractionated granites of MGI, but not in more primitive co-genetic granites, clearly indicating a late-stage origin (Quelhas et al., 2021; this study); (3) aluminous phases characteristic of S-type granites (e.g., muscovite and cordierite) are absent in Macao granites (Quelhas et al., 2021; this study), but hornblende, typical of I-type granites (Chappell and White, 1992), is present in coeval biotite granites and granodiorites in adjacent areas of Macao (e.g., Huang et al., 2013; Sewell et al., 1997); (4) Shellnut et al. (2020) has also suggested that the wide age range of inherited zircons present in weakly peraluminous granites (A/CNK < 1.1) from Macao favors a sedimentary source. Yet, the percentage of inherited grains is much smaller compared with the magmatic zircon population (~95%; Quelhas et al., 2020), unlike what is commonly reported for highly peraluminous S-type granites (e.g., Miller et al., 2007, 2003); (4) the fact that two samples reported by Shellnut et al. (2020) have A/CNK slightly above 1.1, can be explained by higher fractionation degrees of more primitive I-type metaluminous to weakly- peraluminous magmas leading to an increase of the A/CNK of residual melts; (5) Shellnut et al. (2020) also used major [e.g., Molar Al2O3/(MgO+FeOT) vs. molar CaO/(MgO+FeOT)] and trace (Rb/Ba vs. Rb/Sr) element ratios to argue for a sedimentary source. Due to high degrees of crystal fractionation, such elemental ratios in highly fractionated granites are not representative of the chemical composition of primary magmas and, consequently, not reliable to infer the source composition. Taking all these into account, the origin of garnet in Macao granites is not related with melting of sedimentary sources and the MGI highly fractionated granites (Zr/Hf < 25) crystallized from highly evolved I-type granitic magmas.

    The MGI highly fractionated rocks are slightly peraluminous (A/CNK up to 1.13) and have relatively high MnO/(MnO+FeOT) ratios (up to 0.86), conditions that favored the crystallization of Mn-rich garnet with percentages of spessartine molecule varying between 29.3% and 65.7%. Such features indicate that its crystallization occurred under low-pressure conditions (1–3 Kb; Dahlquist et al., 2007; Speer and Becker, 1992; Clemens and Wall, 1988; Miller and Stoddard, 1982; Green, 1977), in agreement with the intrusion of MGI granitic rocks in the upper crust. It has been proposed that assimilation of Mn-rich peraluminous materials in granitic magmas can promote crystallization of magmatic garnet (Scallion et al., 2011). Indeed, Quelhas et al. (2021) has reported the occurrence of Paleozoic metasedimentary enclaves enclosed within the MGI highly fractionated granites and also a relatively high percentage of inherited zircon with similar age in these granites (see also Quelhas et al., 2020). These evidences, together with the observed increase in initial 87Sr/86Sr ratios with degree of evolution observed for MGI, have led Quelhas et al. (2021) to propose that MGI granitic magmas assimilated metasedimentary materials during ascension and emplacement through assimilation-fractional crystallization (AFC) processes. Thus, assimilation of metasedimentary materials could have been a contributing factor to the formation of garnet in the MGI highly fractionated granitic magmas.

    Zoning in garnet is strongly constrained by temperature variation during crystal growth (e.g., Li et al., 2017; Yardley, 1977). It has been suggested that magmatic garnets in granitic rocks crystallized above ~700 ℃ have "spessartine inverse bell-shaped profiles" (Mn-poor cores and Mn-rich rims) or are unzoned, whereas garnets exhibiting "spessartine bell-shaped profile" must have been formed in highly felsic magmas (SiO2=73 wt.%–76 wt.%) crystallizing below ~700 ℃ (Dahlquist et al., 2007). The garnet crystals in MGI highly fractionated granitic rocks are either unzoned or characterized by a "spessartine bell-shaped profile" (Mn-rich cores and Mn-poor rims) suggesting their crystallization either above or below 700 ℃, respectively. This is endorsed by the range of zircon saturation temperatures (TZr=691.1–735.4 ℃; Supplementary File 3) calculated for the MGI highly fractionated granitic rocks. Although the presence of significant amounts of inherited zircon component can lead to an overestimation of magma temperatures (e.g., Miller et al., 2003), this is not the case for Macao granitic rocks where the dominant zircon population is of magmatic origin (Quelhas et al., 2020).

    The second type of garnet has significantly higher CaO contents, which translates into a higher grossular component (24.4%–34.5%; Fig. 4). These garnet crystals occur with a distinct habit, characterized by large sizes (up to 5 mm; Fig. 3d) and skeletal shapes forming garnet-quartz intergrowths (Fig. 3e), suggesting late-stage replacement of the primary granite mineralogy by garnet through metasomatic processes (Kontak and Corey, 1988). Such process would require the local gain of Fe, Ca, Al and Mn provided by the interaction of magmatic fluids with previously crystallized mineral phases (i.e., quartz, biotite, K-feldspar and plagioclase), though an extraneous component derived from the interaction of these fluids with assimilated metasedimentary enclaves or surrounding metasedimentary strata cannot be ruled out. In the latter case, it has been proposed that the circulating fluid breaks down and dissolves plagioclase from the surrounding lithologies, becomes more aluminous, acidic and Ca-bearing, eventually re- entering and introducing foreign elements (e.g., Ca, Fe and Mg) into the highly evolved granitic body (Martin and De Vito, 2014). This could also account for the oscillatory zoning of some garnet crystals (e.g., sample c71_b5_grt; Supplementary File 2), as this kind of compositional zoning often reflects oscillatory changes in the fluid composition that may be internally and/or externally controlled (e.g., Ranjbar et al., 2016; Ríos Reyes et al., 2009). Oscillatory zoning requires that the fluid flow is episodic and, possibly, accompanied by periods of reduced growth rates between individual pulses of channelized hydrothermal fluid flow (Dziggel et al., 2009).

    The transition from magmatic to metasomatic garnets in MGI highly fractionated granites thus reinforces the role of fluids as a controlling factor in the chemistry and mineralogy of late-stage highly evolved granitic rocks. We propose that increasing CaO contents of late-stage crystallized garnets in granitic rocks may reflect a progressively stronger influence of fluids in their composition.

  • The Jurassic magmatic activity, particularly at 165–150 Ma, produced widespread highly differentiated granitic magmas in the Cathaysia Block, some of them clearly more evolved than the Macao counterparts.

    The Zr/Hf and Nb/Ta ratios have been considered good indicators of potential mineralization in granitic rocks (Ballouard et al., 2016; Zaraisky et al., 2009). The MGI weakly to moderately fractionated granites straddle the limit between "barren granites" and "mineralized granites" fields (Figs. 9b and 9c). The MGI highly fractionated granitic rocks, on the other hand, have trace element features similar to Sn-W-(U)-Mo-Be related granites (Figs. 9b, 9c), suggesting that this facies has the potential for mineralization and most likely corresponds to the first stage of tungsten (W) mineralization in SE China during the Jurassic (e.g., Mao et al., 2013; Wang et al., 2011). Moreover, spessartine-rich garnet tends to be less abundant in biotite granites (such as those represented by MGI highly fractionated granites) than it is in the two-mica granites and muscovite granites, which are the main host rocks of tungsten deposits in SE China (Zhang et al., 2017; Guo et al., 2012). In MGI highly fractionated granites, however, no W-bearing phases have been observed. Thus, higher degrees of fractional crystallization coupled with fluid/melt interactions progressively enriched the granitic magmas in W (and also Sn, U, Nb-Ta and REE; Jiang et al., 2017; this study), which has been confirmed by increasing whole-rock W concentrations from the garnet-free biotite granites to garnet-bearing muscovite granites in the Nanling Range, SE China (Zhang et al., 2017). This might explain why the large-scale polymetallic mineralization is closely associated with the muscovite-bearing granites rather than biotite granites in Macao and SE China.

    We thus propose that the stage of evolution represented by the MGI highly fractionated granitic rocks corresponds to the onset of fluid/melt interaction in a highly evolved granitic system, which led to enhanced hydrothermal activity and consequent widespread mineralization in the more evolved lithotypes (e.g., two-mica granites and garnet muscovite granites) of the Jurassic differentiation sequence in SE China (Figs. 9b, 9c; e.g., Jiang and Zhu, 2017; Zhang et al., 2017, 2015; Huang et al., 2015, 2013; Guo et al., 2012; Li et al., 2007). Although other authors have proposed different Zr/Hf threshold values for the beginning of mineralization in a granitic system (Zr/Hf=46 and Zr/Hf=30; Zaraisky et al., 2009; and Ballouard et al., 2016, respectively), our data suggest that the values of Zr/Hf≤25, characterizing the non-CHARAC behavior of these elements (Bau, 1996), along with those of Nb/Ta < 5 (Ballouard et al., 2016; this study), mark a significant increase in mineralization potential of granitic systems due to fluid/melt interactions at late-stage magma evolution.

  • (1) The differentiation sequence in Macao is as follows: biotite granites, garnet-bearing biotite granites, microgranite dikes and lastly aplite (and pegmatite) dikes. Our study suggests that the ~160 Ma granitic magmas in SE China reached even higher degrees of evolution, forming two-mica granites and garnet-bearing biotite and muscovite granites in surrounding areas of Macao.

    (2) High degrees of fractionation coupled with fluid/ melt interactions, involving magmatic F-rich fluids, was the cause for the observed non-CHARAC behavior of trace elements, leading, for example, to non-chondritic Zr/Hf ratios, REE tetrad effect and Nb-Ta enrichment and fractionation in the MGI and SE China highly fractionated granitic magmas.

    (3) Two populations of garnet, with distinct textural and chemical features, were identified in MGI highly fractionated granitic rocks: small grossular-poor euhedral grains and large grossular-rich skeletal garnet grains with quartz inclusions. The first group was formed through precipitation from highly evolved Mn-rich slightly peraluminous melts under low- pressure and relatively low temperature (~700 ℃) conditions. Assimilation of upper crust metasedimentary materials may have contributed as a source of Mn and Al to the formation of garnet. The second group has a metasomatic origin related to the interaction of magmatic fluids with previously crystallized mineral phases (i.e., quartz, biotite, K-feldspar and plagioclase) and, possibly, with assimilated metasedimentary enclaves or surrounding metasedimentary strata. We propose that increasing CaO contents of late-stage crystallized garnets in granitic rocks may reflect a progressively stronger influence of fluids in their composition.

    (4) The stage of evolution represented by the MGI highly fractionated granitic rocks corresponds to the onset of fluid/melt interaction in a highly evolved granitic system, which led to enhanced hydrothermal activity and consequent W-(Mo)-Sn-Nb-Ta polymetallic mineralization in the more evolved lithotypes (e.g., two-mica granites and garnet muscovite granites) of the Jurassic magmatic differentiation sequence in SE China. Zr/Hf≤25 and Nb/Ta < 5 may be important thresholds to identify granite-related mineral deposits, particularly in the SE China region.

  • This research was supported by the Macao Science and Technology Development Fund (No. FDCT 043/2014/A1), and the financial FCT support (No. UIDB/50019/2020–IDL). We are very grateful to Varon Lou (Lou U. Tat) and Ricardo Borges for their important help during the field and lab work and to Pedro Rodrigues for skilled assistance during electron microprobe analyses. We thank the two anonymous reviewers for their useful comments that greatly improved the manuscript. The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1389-4.

    Electronic Supplementary Materials: Supplementary materials (Supplementary Files 1, 2, 3, 4) are available in the online version of this article at https://doi.org/10.1007/s12583-020-1389-4.

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