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Volume 30 Issue 6
Dec.  2019
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Yuanku Meng, Fahui Xiong, Jingsui Yang, Zhao Liu, Kieran A. Iles, Paul T. Robinson, Xiangzhen Xu. Tectonic Implications and Petrogenesis of the Various Types of Magmatic Rocks from the Zedang Area in Southern Tibet. Journal of Earth Science, 2019, 30(6): 1125-1143. doi: 10.1007/s12583-019-1248-3
Citation: Yuanku Meng, Fahui Xiong, Jingsui Yang, Zhao Liu, Kieran A. Iles, Paul T. Robinson, Xiangzhen Xu. Tectonic Implications and Petrogenesis of the Various Types of Magmatic Rocks from the Zedang Area in Southern Tibet. Journal of Earth Science, 2019, 30(6): 1125-1143. doi: 10.1007/s12583-019-1248-3

Tectonic Implications and Petrogenesis of the Various Types of Magmatic Rocks from the Zedang Area in Southern Tibet

doi: 10.1007/s12583-019-1248-3
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  • In this study, we report systematically field observations, geochronology, whole-rock geochemistry and Sr-Nd-Hf isotopic dataset on the various types of magmatic rocks collected from the Zedang area. Chemically, the diabase and gabbro have a low-K calc-alkaline affinity, whereas the basalt and plagiogranite have medium to high-K calc-alkaline characteristics. In addition, the basalts are highly enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), but strongly depleted in high strength field elements (HFSE), indicating that their magma source probably was derived from a subduction- or arc-related setting. In contrast, both the gabbro and diabase mainly demonstrate an N-MORB-like affinity consistent with normal mid-oceanic ridge basalt (N-MORB) origin. The zircon U-Pb dating results suggest that the basalts were crystallized earlier at ca. 158-161 Ma (Oxfordian stage), but the gabbro was crystallized at ca. 131 Ma (Hauterivian stage of Early Cre-taceous). The zircon U-Pb dating results correspond with the field observations that the veins of gabbro intruded basalt. Furthermore, the plagiogranite has a weighted mean age of ca. 160 Ma (MSWD=2.1) consistent with the basalt within the uncertainty. The basalt and the plagiogranite have significantly positive εHf(t) values (+5.8 to +15.6 and +8.6 to +16.1, respectively), suggesting that they were originated from partial melting of a depleted source. However, basalt and plagiogranite are characterized by the wide variations of εHf(t) values indicating minor amounts of exotic crustal material input during the later magma evolution. Additionally, the basalt shows duplex geochemical features of island-arc and mid-oceanic ridge basalt, corresponding to the supra-subduction zone-(SSZ) type affinity. To sum up, two distinct magmatic events identified in this study probably suggest an intra-oceanic arc system ex-isting in the Zedang area during the Late Jurassic, but the intra-oceanic arc subduction extinguished in the Early Cretaceous as suggested by the N-MORB-like gabbro and diabase. Integrated with regional background and different rock types, as well as geochemical features, we conclude that intra-oceanic arc subduction setting developed during the Late Jurassic in the Zedang area, southern Tibet.
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Tectonic Implications and Petrogenesis of the Various Types of Magmatic Rocks from the Zedang Area in Southern Tibet

doi: 10.1007/s12583-019-1248-3
    Corresponding author: Fahui Xiong

Abstract: In this study, we report systematically field observations, geochronology, whole-rock geochemistry and Sr-Nd-Hf isotopic dataset on the various types of magmatic rocks collected from the Zedang area. Chemically, the diabase and gabbro have a low-K calc-alkaline affinity, whereas the basalt and plagiogranite have medium to high-K calc-alkaline characteristics. In addition, the basalts are highly enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), but strongly depleted in high strength field elements (HFSE), indicating that their magma source probably was derived from a subduction- or arc-related setting. In contrast, both the gabbro and diabase mainly demonstrate an N-MORB-like affinity consistent with normal mid-oceanic ridge basalt (N-MORB) origin. The zircon U-Pb dating results suggest that the basalts were crystallized earlier at ca. 158-161 Ma (Oxfordian stage), but the gabbro was crystallized at ca. 131 Ma (Hauterivian stage of Early Cre-taceous). The zircon U-Pb dating results correspond with the field observations that the veins of gabbro intruded basalt. Furthermore, the plagiogranite has a weighted mean age of ca. 160 Ma (MSWD=2.1) consistent with the basalt within the uncertainty. The basalt and the plagiogranite have significantly positive εHf(t) values (+5.8 to +15.6 and +8.6 to +16.1, respectively), suggesting that they were originated from partial melting of a depleted source. However, basalt and plagiogranite are characterized by the wide variations of εHf(t) values indicating minor amounts of exotic crustal material input during the later magma evolution. Additionally, the basalt shows duplex geochemical features of island-arc and mid-oceanic ridge basalt, corresponding to the supra-subduction zone-(SSZ) type affinity. To sum up, two distinct magmatic events identified in this study probably suggest an intra-oceanic arc system ex-isting in the Zedang area during the Late Jurassic, but the intra-oceanic arc subduction extinguished in the Early Cretaceous as suggested by the N-MORB-like gabbro and diabase. Integrated with regional background and different rock types, as well as geochemical features, we conclude that intra-oceanic arc subduction setting developed during the Late Jurassic in the Zedang area, southern Tibet.

Yuanku Meng, Fahui Xiong, Jingsui Yang, Zhao Liu, Kieran A. Iles, Paul T. Robinson, Xiangzhen Xu. Tectonic Implications and Petrogenesis of the Various Types of Magmatic Rocks from the Zedang Area in Southern Tibet. Journal of Earth Science, 2019, 30(6): 1125-1143. doi: 10.1007/s12583-019-1248-3
Citation: Yuanku Meng, Fahui Xiong, Jingsui Yang, Zhao Liu, Kieran A. Iles, Paul T. Robinson, Xiangzhen Xu. Tectonic Implications and Petrogenesis of the Various Types of Magmatic Rocks from the Zedang Area in Southern Tibet. Journal of Earth Science, 2019, 30(6): 1125-1143. doi: 10.1007/s12583-019-1248-3
  • The Zedang ophiolite rocks, which are mainly composed of dunite and harzburgite, mafic cumulates and basalts, extend ~20 km in length and ~4 km in width, covering an area of 45 km2 (Chen et al., 2015; Li et al., 2014). Locally, rock types of magmatic veins range from pyroxenite to gabbronorite rocks that intruded into the mantle peridotites. At the lithological base of the ophiolite, a thin mélange zone was identified. The mélange complex comprises dismembered volcanic rocks and epi-metamorphic Triassic shallow-bathyal carbonate-clastic rocks with minor amounts of chert (Xiong et al., 2015). In addition, extensive serpentinite rocks formed by water interaction. Field observations show that the harzburgite rocks in the south of mafic volcanic rocks typically are massive textured (Figs. 1c, 2a, 2b). The Zedang harzburgite rocks mostly show coarse-grained and granular textures, and mainly consist of orthopyroxene and olivine with minor clinopyroxene, chromian spinel and magnetite.

    Figure 2.  Field photographs showing the various types of magmatic rocks in the Zedang ophiolite;(a) harzburgite outcrop in the study area;(b) field relationship of the basalt cover over the harzburgite;(c) outcrop relationship of the plagiogranite and basalt;(d) fresh clinopyroxene minerals in the basalt;(e) gabbro outcrop in the study area;(f) diabase as a vein intruded into the peridotite.

    The Zedang sub-terrane contains a thick section of magmatic rocks (Figs. 2b2f), with sporadically distributed granitic rocks intruding into andesite rocks (Aitchison et al., 2007). Basalt consists of phenocryst minerals, such as hornblende, pyroxene and plagioclase marked by crystals with grain-sized of 0.5 to 4 mm, and vesicular structures also can be observed in the outcrops (Fig. 2d). Thin section microstructures show that most of the plagioclase crystals or phenocrysts suffered from sericitization, but hornblende and pyroxene grains that show euhedral to subhedral morphologies were subjected to a weak alteration only. The basalt matrix has an intergranular texture and is commonly altered to chlorites and sericites (Fig. 3a).

    Figure 3.  Microstructural photographs showing the various types of magmatic rocks in the Zedang ophiolite (under cross-polarized light);(a) basalt with pyroxene and sericitization plagioclase;(b) gabbro with plagioclase and pyroxene;(c) plagiogranite consisting of plagioclase and quartz;(d) diabase consisting of pyroxene and hornblende and plagioclase;abbreviations: Py. pyroxene;Pl. plagioclase;Qtz. quartz;Hbl.hornblende.

    Gabbro occurs as a vein (~1–2 m in width and~10–30 m in length) that intruded into the basalt lavas (Fig. 2e). Less than~10% of the gabbro displays classic gabbroic textures with equigranular pyroxene (~50%) and plagioclase (~40%) grains (Fig. 3b). Vein-like plagiogranite has a clear boundary with its surrounding basalt and thus demonstrates its intrusion character (Fig. 2c). Microstructures demonstrate that the plagiogranite contains strongly altered plagioclase (~80%), quartz (~15%) and minor amounts of accessory minerals (~5%) (Fig. 3c). In addition, the plagioclase crystals show typical mechanical twinning that indicates a structural shearing during the later emplacement process (Fig. 3c). The diabase, which intrudes into the peridotite as the vein (Fig. 2f), has porphyritic textures and vesicular structures, and its phenocrysts are composed of slight-alteration plagioclase (~80%) and rounded-corroded pyroxene (clinopyroxene and orthopyroxene) (~20%) in size of 0.5–2 mm (Fig. 3d).

  • Representative rock pieces were handpicked for whole-rock geochemistry and Sr-Nd isotope measurements. Before the studied samples separation and processing, the rocks were carefully cleaned, and then crushed to 200 mesh powder in an agate mortar for geochemistry analyses. Major oxide elements were conducted using X-ray fluorescence (XRF-2100) spectrometry. The analytical precision of the major elements is ±10%, but some elements are ±5%. Trace elements were assayed utilizing the ICP-MS method (Agilent 7500a instrument). The two standard samples GSR-3 and GSR-5, as well as three internal standard samples, were measured simultaneously to verify the consistency and accuracy of the analytical results. The uncertainties of analytical results are estimated to be ±10% for trace elements with concentrations ≤10 ppm, and ±5% for that ≥10 ppm.

    Zircon crystals were separated from the crushed rocks using gravitational and magnetic separation methods at the Yuneng Company in Langfang, Hebei Province. They were handpicked and mounted in an epoxy resin disk, and then polished to half-section for cathodoluminescence (CL) images. High-resolution CL pictures were performed at the Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources (KLDEDMNR), Chinese Academy of Geological Sciences, with a Mono CL-4 instrument for inspecting internal textures and structures of zircon crystals in order to select appropriate domains (no cracks and inclusions) for U-Pb and in situ Lu-Hf isotope measurements. Zircon U-Pb dating was performed at the Tianjin Institute of Geology and Mineral Resources of China Geological Survey using a Neptune MC-ICP-MS instrument attached to a New Wave 213 nm excimer laser-ablation system (laser spot size at ca. 30 μm). Zircon GJ-1 (608.5±1.05 Ma of 207Pb/206Pb) and 91500 (1 062.4±0.8 Ma of 206Pb/238U age and 1 065.4±0.6 Ma of 207Pb/206Pb age) were adopted as internal and external standards for U-Pb dating corrections, respectively (Jackson et al., 2004; Andersen, 2002). In situ Lu-Hf isotope analyses were performed using the Nu Plasma HR MC-ICP-MS technique with a laser spot of 40 μm and laser pulse frequency of 8–10 Hz. In this experiment, the 176Hf/177Hf value of standard zircon sample 91500 (value=0.282 308±0.000 012) corresponds with the value that is obtained by the solution method (0.282 302±0.000 008) within the uncertainty (Goolaerts et al., 2004). The model ages (tDM) ages were calculated using the average continental crust value (176Lu/177Hf value=0.015) (Vervoort and Blichert-Toft, 1999). Analytical detailed procedures and processes are shown in Zhang Y et al. (2016) and Griffin et al.(2002, 2000).

    Whole-rock Sr-Nd analytical procedures are shown in Li et al.(2011a, b). Whole-rock Sr-Nd isotopic compositions were assayed using the Triton TI-TIMS method (Pu et al., 2004). The Sr-Nd compositional measurements of the studied basalts were assayed at the state key laboratory for mineral deposits research (SKLMDR) of Nanjing University (Jiangsu Province, China).

  • The major oxide elements from the Zedang magmatic rocks were recalculated to 100 wt.% on a volatile-free basis.

    The analytical results of major and trace elements are given in Table S1. The Zedang basalts show high concentrations of Al2O3 (16.41 wt.%–16.95 wt.%), MgO (5.67 wt.%–7.03 wt.%), Cr (49.3 ppm–113 ppm) and Ni (31.4 ppm–51.5 ppm). High MgO contents (5.67 wt.%–7.03 wt.%) correspond with the typical mid-oceanic basalt (6.56 wt.%) (Xia et al., 1998). The SiO2 contents of the basalt vary from 44.36 wt.% to 46.83 wt.%, which is typical for basaltic rocks;likewise other major element oxides measures, correspond to basaltic compositions (Table S1). The basalts have intermediate loss on ignition (LOI) contents of 3.60 wt.% to 4.48 wt.%, which indicates that they were subjected to variable alterations. Additionally, the basalts are characterized by low TiO2 (0.86 wt.%–0.90 wt.%) compared to the mid-oceanic ridge basalt (1.0 wt.%–1.5 wt.%) (Sun et al., 1979). Gabbros exposed in the Zedang sub-terrane as veins (Fig. 2e) yield SiO2 contents between 49.97 wt.% to 51.36 wt.% and potentially belong to a typical gabbroic suite chemically. They have relatively moderate to low LOI values of 2.33 wt.% to 2.98 wt.% (Table S1). Moreover, the gabbro has high Ni contents (103 ppm–122 ppm) and variable Cr contents of 456 ppm–171 ppm. The plagiogranite that intruded the basalt as vein (Fig. 2c) shows variable but narrow SiO2 contents of 70.88 wt.% to 74.79 wt.% (Table S1). The plagiogranite is commonly fresh (Figs. 2c, 3c), with low LOI values of 1.49 wt.% to 3.63 wt.%. Additionally, the plagiogranite is characterized by moderate Al2O3 contents (13.91 wt.%–14.53 wt.%) and high Na2O/K2O ratios of 1.1–2.08, corresponding to a Na-rich affinity. The low MgO contents (0.32 wt.%–0.48 wt.%) and higher Mg# [Mg/(Mg+Fe)] values (37–51) than the pure crust (Rapp and Watson, 1995) imply a mantle origin of the plagiogranite. Furthermore, the plagiogranite demonstrates greatly variable Cr contents of 20.6 ppm–203 ppm and low Ni contents (3.54 ppm–4.84 ppm). Molar ratios Al2O3/(CaO+Na2O+K2O) (A/CNK) range from 0.94 to 1.27, revealing a metaluminous to peraluminous feature. Few plagiogranite is characterized by the peraluminous feature that might be related to later chemical composition changes. The SiO2 contents range from 48.30 wt.% to 49.87 wt.% for the diabase. Compared to the basalt, the diabase has lower Ni (18.9 ppm–30.0 ppm), Cr (22.7 ppm–25.4 ppm) and V (263 ppm–268 ppm) contents (Table S1). The diabase shows high Na2O contents (6.09 wt.%–8.05 wt.%), but low K2O contents (0.4 wt.%–0.54 wt.%), with high ratios of Na2O/K2O, suggesting a Na-rich source, which is probably related to sea-water alteration and exchanges.

    In the chemical discriminative diagram (Fig. 4a), the studied samples can be divided into two parts, the tholeiite and calc-alkaline fields. The samples collected from the diabase vein and solidified basalt lavas document a tholeiitic affinity, but gabbro and plagiogranite show a calc-alkaline one. Based on the SiO2 versus K2O plot, the various types of magmatic rocks are further divided into low-K tholeiite, medium-K calc-alkaline and high-K calc-alkaline fields, respectively. Compared to the plagioclase and basalt, the gabbro and diabase show low-K affinity (Fig. 4b). In the SiO2 versus K2O+Na2O (TAS) diagram (Fig. 4c), the analyzed samples collected from the basalt and gabbro plot in the basaltic (gabbro) field (B) consistent with the thin section analyses and field observations;however, the diabase samples mainly locate in the basaltic trachyandesite field (S2), but one sample plots in the phonotephrite field (U2);the plagiogranite samples fall into the granite (rhyolite) field (R). Figure 4c also shows that the basalt, plagiogranite and gabbro samples belong to the sub-alkaline series, but the diabase samples show alkaline characteristics. The TAS diagram coincides with the Zr/TiO2 versus Nb/Y diagram (Fig. 4d). In addition to the above geochemical diagrams, the Harker diagrams show that MgO and SiO2 have no obvious covariant relations with other major elements (Figs. 5a5f), indicating that fractional crystallization played a minor or negligible role in the formation of the various types of magmatic rocks in the Zedang region. In addition, the gabbro and basalt samples plot into the high-Al and high-Mg fields, respectively. Generally, the high-Al basaltic rocks are related to oceanic subduction (Meng et al., 2016a; Ferlito, 2011; Lu and Sang, 2002). Therefore, the basalt might have formed in a subduction-related setting.

    Figure 4.  (a) FeOT/MgO versus SiO2 diagram (after Miyashiro, 1974);(b) SiO2 versus K2O plot (after Peccerillo and Taylor, 1976);(c) TAS diagram (after Middlemost, 1994), dotted line for Irvine line (after Irvine and Baragar, 1971), below dotted line for subalkaline, above dotted line for alkaline;(d) Nb/Y versus Zr/TiO2×0.000 1 diagram (after Winchester and Floyd, 1977). Pc. picrobasalt;B. basalt (gabbro);O1. basaltic andesite (gabbroic diorite);O2. andesite (diorite);O3. dacite (granodiorite);R. rhyolite (granite);S1. trachybasalt;S2. basaltic trachyandesite (monzo diorite);S3. trachyandesite (monzonite);T. trachyte (syenite) or trachydacite (quartz monzonite);F. nephelinite or melilitite;U1. tephrite or basanite;U2. phonotephrite;U3. tephriphonolite;Ph. phonolite.

    Figure 5.  (a)–(f) Major oxides geochemical Harker diagrams (high-aluminum and high-magnesium fields are after Sisson and Grove, 1993).

    The chondrite-normalized REE diagrams reveal that the basalt and plagiogranite show relatively enriched light REE, with (La/Yb)N ratios varying from 3.0–4.0 to 8.1–9.6, slightly depleted heavy REE and negligibly Eu anomalies (Figs. 6a, 6c). These samples exhibit enrichment in Sr, K, Rb and La and depletion in HFSE, with significantly negative Nb-Ta and Ti anomalies, corresponding to typical characteristics of subduction-related magmas (Figs. 6b, 6d) (Kelemen et al., 1990). In addition, the basalt and plagiogranite are enriched in lead (Figs. 6b, 6d).

    Figure 6.  Chondrite-normalized rare earth elements (REE) patterns and primitive mantle-normalized multiple trace element diagrams;the chondrite values and the primitive mantle values are from McDonough and Sun (1995).

    Additionally, compared to the basalt and plagiogranite, both the diabase and gabbro show relatively flat LREE patterns and weak enrichment in HREE relative to middle REE (MREE) (Figs. 6e, 6g), similar to the N-MORB-like patterns. Overall, the REE patterns from La to MREE are flat, whereas the patterns are from MREE to HREE are not flat but show an enrichment in HREE. Furthermore, diabase and gabbro exhibit enrichment in Sr and K, and negative Nb, Ta and Ti anomalies (Figs. 7f, 7h). Trace elements results show that the concentrations of Cr and Ni (Table S1) in the Zedang magmatic rocks are low relative to the primary mantle (Ni 300 ppm–400 ppm, Cr 300 ppm–500 ppm;Frey et al., 1978).

    Figure 7.  (a)–(d) Representative zircon CL images of the magmatic rocks (white font for sample number, yellow font for zircon U-Pb age, red font for εHf(t) values, black circle for zircon U-Pb dating field and white dotted circle for Lu-Hf isotope field);(e)–(h) Tera-Wasserberg concordia plots for LA-ICP-MS zircon U-Pb data.

  • In situ zircon LA-ICP-MS U-Pb isotope data are given in Table S2. All analytical zircon grains are colorless and transparent with prismatic morphologies, suggesting an igneous origin. The CL images of zircons show concentric zoning or rhythmic stripes (Figs. 7a7d) (zircons from mafic rocks display tabular zoning and from felsic rocks oscillatory zoning). Dating spots without visible inclusions and cracks were selected for zircon U-Pb dating and in situ Lu-Hf isotope analyses (Figs. 7a7d).

    Zircon grains separated from the basalts are mostly prismatic and euhedral shapes (Figs. 7a7b), and are 60–180 μm in length, with length/width ratios of 1 : 1 to 3 : 1. Twenty analytical spots were performed on 20 zircon grains from sample 13YZ-27-5, yielding concordant 206Pb/238U ages ranging from 137 to 166 Ma, with a weighted mean age of 161.4±0.9 Ma (MSWD=2.0) (Fig. 7e). Thirteen analyses were performed on 13 zircon grains from 13YZ-31-9, yielding concordant 206Pb/238U ages ranging from 151 to 184 Ma, with a weighted mean age of 158.3±3.9 Ma (MSWD=6.9) (few dating points are excluded due to high discordance) (Fig. 7f). Two dating results show that the basalt was crystallized during Late Jurassic (158.3–161.4 Ma).

    Zircon grains from plagiogranite and gabbro mostly are euhedral in shape (Figs. 7c, 7d) and are 100 to 200 μm in length, showing length/width ratios of 1 : 1–2 : 1. Twenty-four spots were performed on 24 zircon grains from sample 13YZ-41-2, yielding concordant 206Pb/238U ages ranging from 158 to 170 Ma, with a weighted mean age of 160.2±0.94 Ma (MSWD=2.1) (Fig. 7g). Twenty-three analyses were carried out on 23 zircon crystals from 13YZ-27-3, yielding 206Pb/238U ages ranging of 128–138 Ma, with a weighted mean age of 131±1 Ma (MSWD=4.8) (Fig. 7h).

  • The representative samples were selected for zircon Lu-Hf isotope measurements, and analytical results are shown in Table S3. Zircons from the basalt samples have 176Hf/177Hf ratios ranging from 0. 283 124 to 0.282 757, yielding εHf(t) values of +2.0 to +15.6. Model ages calculations relative to the depleted mantle (DM) range of 231–626 Ma, whereas the crustal model ages (tDMC) vary from 231 to 1 121 Ma (Table S3). Furthermore, the rocks from the plagiogranite also show similar zircon εHf(t) values (+8.6 to +16.2) compared to those of the coeval Gangdese batholith (Fig. 8a) (Hou et al., 2015; Ji et al., 2009). Furthermore, the wide ranges of zircon εHf(t) values suggest that the basalt and plagiogranite originate from heterogeneous magma sources or are subjected to alteration or contamination during later evolution. It is well-known that relatively narrow ranges of εHf(t) values (homogeneous Hf isotope compositions) indicate a single magma source (Meng et al., 2019; Wang et al., 2019b; Griffin et al., 2002).

    Figure 8.  (a) Zircon U-Pb ages versus εHf(t) values for the basalt and plagiogranite (Gangdese batholith εHf(t) ranges are after Ji et al., 2009);(b) εNd(t) versus (87Sr/86Sr)i diagram for the Zedang basalts (data of Neo-Tethys ophiolite are from Zhang et al., 2005; Xu and Castillo, 2004; Mahoney et al., 1998;data of the Yeba basalts are after Zhu et al., 2008;and data for the upper and lower crust are from Ma et al., 2013 and Wen et al., 2008);(c) 143Nd/144Nd versus 87Sr/86Sr diagram for the Zedang basalts (after Winter, 2010);data of New Britain, Marians arc, Japan, the northern volcanic zone of Andes arc are all from http://georoc.mpch-mainz.gwdg.de/georoc/.

  • Six representative basalt samples collected from the Zedang ophiolite and Zedang sub-terrane were subjected to Sr-Nd isotope analysis. The measured results are presented in Table S4. The basalts have significantly positive εNd(t) values (+6.74 to +7.12) and initial 87Sr/86Sr ratios (0.702 96–0.703 03) (Figs. 8b, 8c). The whole-rock Sr-Nd isotopes suggest that the basalts are derived from partial melting of the depleted mantle (mantle array), corresponding to the Neo-Tethys ophiolite (Fig. 8b). The mid-oceanic basalts show εNd(t) values varying from +8 to +12 (DePaolo and Wasserburg, 1976a, b). Compared to the modern mid-oceanic basalts, the Zedang basalts show relatively lower εNd(t) values. Due to our interpretation, the Zedang basalt might have experienced minor contamination with crustal sediments. This argument is also supported by Moyen and Laurent (2018) who proposed that the injection or participation of minor amounts of crustal sediments will lead to the εNd(t) values lower than the ideal linear depleted mantle at ca. +2. In Fig. 8c, the basalts from the Zedang plot into the fields where equivalent samples from Japan and the northern volcanic zone of the Andes arc are located. Furthermore, the coeval intermediate-basic Yeba volcanic rocks distributed in the Gangdese arc show different Sr-Nd isotope features, indicating that the contemporary volcanic rocks from the Gangdese arc have different magma sources and are formed due to heterogeneous petrogenetic mechanisms.

  • As discussed above, the wide ranges of Hf isotope compositions might indicate a complicated magma source (Griffin et al., 2002). In addition, some researchers suggest that contamination by crustal material plays a role in the evolution of the southern Tibet ophiolites (Liu et al., 2015; Yamamoto et al., 2013).

    Crustal contamination of mantle material essentially is a consequence of the following main mechanisms: (ⅰ) crustal recycling into the mantle via subduction zone input;(ⅱ) lithospheric delamination;(ⅲ) crust-mantle interaction (Du et al., 2018; Pearce, 2008). The (Th/Ta)PM and (La/Nb)PM ratios are useful in the discriminating level of crustal contamination (normalized values after Sun and McDonough, 1989) (Peng et al., 1994). If parental magmas were mixed with the upper crust or sediments, the ratios of (Th/Ta)PM and (La/Nb)PM would both be higher than 2;if the ratio of (Th/Ta)PM would approach to or is equal to 1 and (La/Nb)PM would be higher than 1, indicating mafic lower crustal material participation during magma evolution (Peng et al., 1994) is indicated. The calculated results reveal that (Th/Ta)PM and (La/Nb)PM ratios of the basalt samples are between 2.60 and 5.27 and between 2.37 and 6.72, respectively. Both ratios are higher than 2 indicating upper crustal material or sediment participation during basalt magma evolution process. In addition, the basalt samples show arc-type geochemical features characterized by enrichment in light REE and Pb element, but depletion in HFSE (Nb, Ta, and Ti). These geochemical fingerprints also suggest contamination by crustal material. However, the diabase and gabbro samples show N-MORB-like patterns (Fig. 6), which are different from typical arc-related or crustal geochemical features. The Sr-Nd isotopes support the assimilation fractional crystallization (AFC) or magma mixing during the basalt magma evolution (Figs. 9a, 9b). Furthermore, inherited zircons of Early Jurassic age which are marked by oscillatory zoning textures indicate that a felsic magma source is involved (Figs. 7a, 7b) and also point to crustal contamination. Compared to felsic rocks, zircons derived from mafic rocks show tabular zoning textures and small grain sizes, suggesting a high-temperature crystallization process. These Early Jurassic inherited zircons marked by highly positive εHf(t) values are probably derived from adjacent Gangdese arc (Fig. 8a).

    Figure 9.  (a), (b) SiO2 versus εNd(t) and initial 87Sr/86Sr diagrams of the basal in the Zedang terrane;FC. fractional crystallization, and AFC. assimilation and fractional crystallization;(c), (d) SiO2 versus La (ppm) and Rb/La versus Rb (ppm) diagrams (after Schiano et al., 2010; Brophy, 2009).

    We argue that the basalt experienced crustal material commination during subsequent ascent, whereas the diabase and gabbro did not experience crustal contamination during the magma evolution. Consequently, the basalt is formed due to different magmatic-tectonic mechanisms and geodynamic processes compared to the diabase and gabbro.

  • Multiple studies have revealed that in general mafic magmas are originated from spinel-lherzolite usually exhibits a relatively flat REE pattern (Liu et al., 2015; Workman and Hart, 2005). Basaltic melts that source from a low degree of partial melting, however, show a relatively high fractionation of HREE (Liu et al., 2015; Saccani et al., 2013). In this research, the basalts are weakly LREE enriched compared to typical continental arc mafic rocks, with LaN/YbN ratios ranging from 2.81 to 3.73 (Table S1) (continental arc rocks have higher (La/Yb)N ratios >5.0) (Gao and Zhang, 2012; Sun and McDonough, 1989). In the magma source discriminative diagrams, the basalts have formed by means of ~20% partial melting of garnet-spinel peridotite, whereas the diabase and gabbro are close to the spinel lherzolite partial melting curve, suggesting that the diabase and gabbro are originated from ~10% and >20% partial melting of spinel lherzolite, respectively (Figs. 10a, 10b). Therefore, the simulating partial melting curves also indicate that the gabbro- and diabase-forming melts were generated in greater depths (and thus experienced higher pressures) than the melts that formed basalt. In the study area, the plagiogranite shows low K2O/Na2O ratios ranging from 0.48 to 0.91, but high Rb (83.7 ppm–103 ppm) and K2O (2.35 wt.%–3.51 wt.%) contents, of which geochemical features are inconsistent with typical oceanic plagiogranite (Ishizaka and Yanagi, 1975). Plagiogranite, consisting of quartz and plagioclase with small amounts of mafic minerals, is, in general, an accessory rock component of ophiolites. Coleman and Peterman (1975) regarded the plagiogranite as the crystallization differentiation products of basaltic magma. However, some studies refuted this hypothesis and argued that plagiogranite can also form by the following possibilities: (i) partial melting of down-going oceanic crust (Kang et al., 2015);(ii) partial melting of juvenile crustal material or tonalite (Li et al., 2009; Popov et al., 2002);(iii) immiscible models of magma (Natland et al., 2002; Shastry et al., 2002). Multiple studies have revealed that the partial melting of down-going oceanic crust is a widespread mechanism to explain the petrogenesis of plagiogranite (Kuibida et al., 2013; Brophy and Pu, 2012; Rollinson, 2009). In this study, the representative samples collected from the plagiogranite have an affinity associated with partial melting rather than the crystallization differentiation of mafic magma (Figs. 9c, 9d). Thus, combined with the discriminative diagrams and field observations, as well as Hf isotope features, we argue that the plagiogranite might have been formed by partial melting of down-going oceanic crust.

    Figure 10.  (a) La/Yb versus Sm/Yb diagram (after Johnson et al., 1990) and (b) La/Sm versus Sm/Yb diagram (after Aldanmaz et al., 2000);(c), (d) Th versus Ba/Th diagrams of the magmatic rocks from the Zedang terrane (after Dilek et al., 2008). MORB. Mid-ocean-ridge basalt;E-MORB. enriched mid-oceanic ridge basalt;N-MORB. normal mid-oceanic ridge basalt;PM. primitive mantle;DMM. depleted MORB mantle.

    As mentioned above, plagiogranite shows relatively high K2O and Rb contents and low Ni, Co and Cr contents (Table S1), and thus a crustal component is indicated to play a major role in its evolution. If the plagiogranite is sourced from partial melting of pure oceanic crust, it will show low K and high Cr, Ni, Co and V contents. Therefore, the possibility of the partial melting of the pure oceanic crust will be excluded. This model is supported by the discriminative diagrams. In Fig. 10c, the sediment input and metasomatism take effect in the formation of the plagiogranite. Combined with Figs. 9c9d and 10c, we argue that the plagiogranite might be originated from the partial melting of a subducted oceanic crust, that experienced crustal contamination and that was partially metasomatized. This inference is supported by geochemical data (Table S1). The gabbro and diabase are derived from partial melting of a fluid-metasomatized lithospheric mantle (Fig. 10d). However, the basalts show a complicated curve, potentially indicating that the sediments and fluids both participate in the evolution of lithosphere mantle (Fig. 10b) from which the basaltic melts derive. In addition, the geochemical diagrams (Figs. 10a, 10b) also suggest the crustal material (sediments) participating in the evolution of the basalt. Additionally, the basalt and plagiogranite are characterized by a wide range of Hf isotope compositions, suggesting a hybrid magma chamber (Fig. 8a). Although the crustal sediments participated in the evolution of the plagiogranite and basalt, the highly positive εHf(t) values and young model ages (Table S3) are contradictory with the sediments input. It is well recognized that the sediments are marked by enriched radioactive isotope compositions (Sr-Nd-Hf). If the abundant amounts of sediments would have been assimilated by melts of the depleted magma chamber, the isotope polarity of the whole magma chamber will be changed greatly. Similar examples have been proved in the Gangdese batholith (Fig. 8a). The magmatic rocks from the Gangdese arc show significantly depleted Hf isotopes prior to India-Asia collision, but the isotope features were changed considerably after India-Asia collision due to Indian old crustal material injection (Meng et al., 2018b; Hou et al., 2015; Zhu et al., 2011; Ji et al., 2009) (Fig. 8a). Therefore, the sediments from the Neo-Tethys oceanic basin play a minor role;whereas the depleted mantle compositions dominate the system.

    According to the biostratigraphy evidence, Badengzhu (1979) firstly argued that the Zedang sub-terrane was generated in Late Cretaceous;however, Aitchison et al. (2007) suggested that the volcanic rocks of the Zedang sub-terrane were crystallized at ca. Late Jurassic, as Early Cretaceous to Early Late Jurassic (ca. 166 to 125 Ma) radiolarians have been identified in the red-bedded cherts that underlie the volcanic sedimentary sequences (Zhang et al., 2014). In the Zedang sub-terrane, direct dating data of the widely distributed magmatic rocks also indicate emplacement ages of Late Jurassic (Zhang et al., 2014; Wang et al., 2012; McDermid et al., 2002). McDermid et al. (2002) systematically dated dacite breccia and veins and quartz diorites, and presented zircon U-Pb ages of 157 to 163 Ma that are slightly older than hornblende 40Ar-39Ar ages of ca. 152–158 Ma. Subsequently, Wang et al. (2012) presented a zircon U-Pb age of ca. 157.5 Ma collected from a granodiorite sample in the Zedang sub-terrane. Zhang et al. (2014) presented new ages on hornblendite, gabbro, andesite and tonalite from the north part of the Zedang ophiolite, with the dating results varying from 155 to 160 Ma. In this study, zircons from the plagiogranite sample yield a U-Pb age (160.2 Ma) that is identical to those of the basalts (158.3 to 161.4 Ma) (Figs. 7e7g), indicating that the plagiogranite was formed coevally with the mafic volcanic rocks in the Zedang sub-terrane. As a consequence of our current results, combined with the already existing published data, an occurrence of widespread magmatic pulses during Late Jurassic in the Zedang sub-terrane is indicated (Figs. 7e7g). It is notable, however, that the gabbro, dated at ca. 131±1 Ma, is much younger than the basalt and plagiogranite (Fig. 7h), suggesting two distinct magmatic events. The dating results are also supported by the field observations that the gabbroic melts intrudes the basalt, forming vein-like systems (Fig. 2e). The geochemical compositions and mineral features of the basalts in the Zedang sub-terrane suggest that they originate from partial melting of a hydrous depleted mantle in an active continental margin setting (Chen et al., 2015). In this study, the diagram (Fig. 10d) also supports the model that the fluids and sediments are key factors to the formation of the basalt.

    Geochemical discriminative correlations between the gabbro and volcanic rocks support our proposed idea that the gabbro has a distinct formational setting compared to the plagiogranite and basalts. In the Ta/Yb versus Th/Yb plot (Fig. 11a), the basalts are plotted in two distinguishing compositional fields, "continental arc" and "intra-oceanic arc", while the locations of diabase and gabbro are close to an N-MORB setting. Other discriminative diagrams also suggest the basalt was formed in an arc-related setting or transitional setting, whereas the diabase and gabbro belong to a typical MORB setting (Figs. 11b11d). In the normalized REE patterns and spider diagrams (Figs. 6a6d), the basalt and plagiogranite show enrichment of LREE and Pb, but depletion in HSFE (Ti, and Nb-Ta), suggesting a subduction-related setting (Kelemen et al., 1990). The diabase and gabbro have N-MORB-like characteristics (Figs. 6e, 6g). Furthermore, the plagiogranite has low Sr/Y (5.32–10.12) and (La/Yb)N (8.09–9.65) values inconsistent with typical adakite-like rocks (Table S1) (Richards and Kerrich, 2007; Defant and Drummond, 1990), suggesting a normal arc setting in the Zedang sub-terrane.

    Figure 11.  Tectonic setting discrimination diagrams for the Zedang magmatic rocks;(a) Th/Yb versus Ta/Yb diagram (after Pearce, 1983);(b) V versus Ti diagram (after Shervais, 1982);(c) Zr versus Ba diagram (after Floyd, 1991);(d) TiO2 (wt.%) versus TFeO/MgO diagram (after Wei, 2007; Pearce, 1987);the fields of arc tholeiite, calc-alkaline basalt, mid-ocean-ridge basalt, continental flood basalt, and ocean-island and alkali basalts were drawn by Rollinson (1993) and Shervais (1982);OIB. oceanic island basalt;IAB. island arc basalt;BABB. back-arc basin basalt;IAT. island arc tholeiite.

    In addition to the above descriptions, the melts also experienced magmatic differentiation due to a fractionation crystallization process. Figures. 12a12b suggests that the mafic volcanic rocks experience fractionation of clinopyroxene and orthopyroxene, but the plagiogranite results from fractionation of plagioclase during magma evolution.

    Figure 12.  (a) Cr versus V diagram;Sr versus Rb/Sr diagram;mineral partition coefficients are after Rollinson (1993).

  • The Zedang sub-terrane, which is located in the eastern part of the YZSZ, is mainly composed of ophiolite and related arc-type rocks. As discussed before, the petrogenesis and tectonic setting of the Zedang ophiolite is discussed controversially since their formation is for instance explained either by the duplex subduction or a single subduction process (Zhang et al., 2014; Wei, 2007).

    Some of volcanic rocks from the Zedang sub-terrane are marked by high K2O contents that are ascribed as the shoshonite series as suggested by Aitchison et al. (2007). It is well-known that shoshonite rocks commonly form in the intra-oceanic arcs (the Izu-Bonin-Mariana (IBM) and Fiji arc systems) (Zhang et al., 2014; Wei, 2007; Sun and Stern, 2001; Gill and Whelan, 1989);the authors concluded that the Zedang sub-terrane represents a remnant of an intra-oceanic arc that occurred within the Neo-Tethys ocean basin. Plagiogranite contains relatively high amounts of K2O as shown in the K2O versus SiO2 diagram (Fig. 4b);basalt, however, is located within the shoshonite to high-K calc-alkaline fields, similar to the volcanic rocks classified as shoshonites and reported by Aitchison et al. (2007). Presence of abundant hornblende crystals in the basalt indicates that the basalt was sourced from a hydrous mantle (intra-oceanic subduction setting);the best alternative is a mantle wedge metasomatized by fluids or hydrous melts derived from a down-going slab. As mentioned above, the basalts belong to typical high-Al basalt (Fig. 6f), corresponding to the subduction- related hydrous setting (Meng et al., 2016a; Sisson et al., 1996; Sisson and Grove, 1993). Trace element components and geochemical features of the gabbro and diabase from the Zedang sub-terrane imply that they were produced in a MORB-like setting (Figs. 6 and 11). Similarly, the tectonic discriminative diagrams show that the Zedang basalts plot into the intra-oceanic arc field (Figs. 11a11d) rather than in fields typical for continental arc or back-arc basin setting (BABB). Thus, the geochemical data suggest that the Zedang various types of magmatic rocks were formed in a different tectonic environment that was distinct from the basalts exposed in the Jurassic Yeba Formation of the Lhasa terrane, which was interpreted to have generated in an active continental margin (Ma X X et al., 2017; Zhu et al., 2008). The depletion of the Zr and Hf and εNd(t) versus (87Sr/86Sr) also suggest that the source of the Zedang basalts is different from that of the Yeba volcanic rocks that were produced in a continental arc (Figs. 8b, 8c). The basalts in this study might be products of an intra-oceanic arc setting.

    The concentrations of Y, Ti, V and Zr trace elements in the basalts are a useful tool in distinguishing tectonic settings (as demonstrated by Meng et al., 2019; Zhang et al. 2014; Pearce and Norry, 1979; Pearce and Cann, 1973). The Ti versus V and TiO2 versus TFeO/MgO diagrams show that the basalts plot into the MORB to arc tholeiitic basalt fields (Figs. 11b, 11d;Shervais, 1982). The classic arc-type geochemical characteristics (such as depletion in Nb, Ta, Ti, Zr and Hf) shown by the basalts indicate that they were generated in a subduction-related setting (Figs. 6a, 6b). In combination with the results of Early Jurassic granites reported from the southern Lhasa terrane (Guo et al., 2013; Tang et al., 2010; Ji et al., 2009; Yang et al., 2008; Qu et al., 2007; Zhang et al., 2007; Chu et al., 2006), we argue that the Neo-Tethyan oceanic northward subduction began prior to Early Jurassic.

    The competing question is whether the various types of magmatic rocks from the Zedang ophiolite were produced in a continental margin or an intra-oceanic arc setting (Chen et al., 2015). The duplex geochemical features of the basalt rocks suggest a complex tectonic setting. Compared to the continental arc and within-plate basalt (WPB), in this study, the basalts are tholeiitic with low K2O (<2.5 wt.%), Th contents, LaN/YbN and initial 87Sr/86Sr ratios, and are depleted in Nd isotope, similar to basalts that derived from an intra-oceanic arc setting and unlike typical basalts that form in a continental margin arc (the central volcanic zone of the Andes arc) (Wilson, 1989). In the REE patterns, the Zedang basalts demonstrate light LREE enrichment (Fig. 6a), similar to the basalts from the Zao volcano of the northeastern Japan arc and Bima Group volcanic rocks (Kang et al., 2014; Wilson, 1989), whereas different from both continental arc rocks in the central volcanic zone of the Andes and typical intra-oceanic arcs (the Tonga-Kermadec and Marianas arcs) (Smith et al., 2010; Peate and Pearce, 1998). Furthermore, a subduction system, which was formed in an intra-oceanic subduction, was identified in the YZSZ (Kang et al., 2014; McDermid et al., 2002; Aitchison et al., 2000). The duplex subduction geodynamic mechanism extended to India and Karakoram in the west and to India and Burma in the east (Kang et al., 2014; Zhang et al., 2014; Dai et al., 2013). Our results indicate that the various types of magmatic rocks from the Zedang terrane were formed in two distinct settings (intra-oceanic and N-MORB-like settings). The arc-type rocks (granitoids with minor amounts of mafic rocks) of the adjacent Gangdese region were formed in a passive continental margin setting that was related to the Neo-Tethyan oceanic subduction (Meng et al., 2018a, 2016a; Ji et al., 2009). To decipher the tectonic scenarios of the magmatic rocks from the Zedang sub-terrane, further studies on volcanic as well as related sedimentary rocks and their depositional environment are required.

    In addition to geochemical data, our hypothesis is also supported by geophysical evidence (Van der Voo et al., 1999). The geophysical evidence (tomography) identified several deep high-speed abnormal zones within the lithosphere of the southern Tibet. Wei (2007) and Van der Voo et al. (1999) argued that the high-speed abnormal zones represented the relics of the subducted oceanic crust, and deduced an intra-oceanic arc system. On the other hand, the plagiogranite yields low Sr/Y and LaN/YbN ratios which are inconsistent with typical adakites as proposed by Defant and Drummond (1990). However, Wei (2007) reported adakites in the Zedang sub-terrane, and considered them as the partial melting products of down-going oceanic crust (amphibolite to eclogite facies), suggesting the intra-oceanic subduction existing in the Zedang sub-terrane.

    Based on the above discussion, we suggest that there is an intra-oceanic arc system existing in the Zedang area during Late Jurassic (ca. 158–161 Ma). Multi-stage subduction model can better explain the geochemical data of the various types of magmatic rocks in the Zedang area, southern Tibet.

  • Conclusions from this study are drawn as follows. (1) In the Zedang sub-terrane, the intra-oceanic arc subduction system existed during the Late Jurassic (ca. 158–161 Ma). (2) Two magmatic events (158–161 and 131 Ma) are identified in the Zedang area. The Early magmatic event characterized by the occurrence of basalts (158.3–161.4 Ma) and plagiogranite (160.2 Ma) indicates an intra-oceanic subduction setting;the later event documented by gabbro reveals an N-MORB-like setting, and suggests that the final magmatic pulses of the intra-oceanic arc system occurred in Early Cretaceous. (3) Petrological, geochemical data and Sr-Nd-Hf isotopes together suggest that the diabase and gabbro were sourced from a depleted mantle that was metasomatized by the slab-released fluids. The basalts were sourced from the partial melting of the depleted mantle that was metasomatized by slab-released fluids and down-going sediments. The plagiogranite was originated from the partial melting of the Neo-Tethyan oceanic crust and subjected to crustal magma mixing.

  • Firstly, we are grateful to the guest editors Profs. Xu-Ping Li and Hans-Peter Schertl for their warm-hearted invitation on this special issue. Many thanks go to two reviewers for their constructive and suggestive comments which improved the manuscript greatly. In addition, we thank Prof. Zhiqin Xu for her academic guidance during the southern Tibet field survey. The research was financially co-supported by the Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources (Nos. J1901-7, J1901-16), the State Scholarship Fund (No. 201904180031), the National Key Research and Development Project of China (No. 2016YFC0600310), the 2nd Tibetan Plateau Scientific Expedition (No. 2019QZKK0802), the National Natural Science Foundation of China (Nos. 41672046, 41641015, 41762005, 41720104009, 41703036), the China Geological Survey (No. DD201190060), and the International Geological Correlation Project (No. IGCP-649). The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1248-3.

    Electronic Supplementary Materials: Supplementary materials (Tables S1–S4) are available in the online version of this article at https://doi.org/10.1007/s12583-019-1248-3.

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