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Field relations of felsic gneiss and leucosome. (a) Migmatitic character of the felsic gneiss. (b), (c) Leucosomes generally occur as innumerable thin layers in the felsic gneiss, which are generally aligned subparallel to the gneissosity or foliation. (d) Leucosome in felsic gneiss cross-cut these early structures at a moderate to steep angle. (e) Coarse-grained euhedralsubhedral garnet grains within the granitic leucosome. (f) Block of dark residuum surrounded by leucosome.
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Granitic leucosomes are widely distributed within felsic gneiss in the North Qaidam ultrahigh-pressure (UHP) metamorphic terrane in western China, which is crucial to understanding the relationships between partial melting, metamorphic evolution and orogenic processes. We have applied of petrology, whole-rock geochemistry and Sr-Nd isotope, zircon...
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... areas, the felsic gneiss (granitic gneiss) is mainly composed of garnet, biotite, K-feldspar, plagioclase, quartz, rare phengite and amphibole. The accessory phases consist of rutile, zircon and monazite. Outcrops of felsic gneiss throughout this re- gion preserve features diagnostic of anatexis and, in local areas, display strong migmatization (Fig. 2a). The migmatitic gneiss comprises a mix- ture of pale plagioclase-rich quartzo-feldspathic leucosomes, which represent the former sites of melt segregation and/or accumulation, and dark melanosome, which represents the crystal-rich residuum from which melt was extracted. The residuum may be darker in color due to a higher concentration ...
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... minerals (predominantly biotite and amphibole), although it is commonly intermediate to leucocratic in color. Quartzofeldspathic leucosomes generally occur as innumerable thin layers in the felsic gneiss. The proportion of thin leucosomes varies greatly from outcrop to outcrop and the leucosomes range in thickness from millimeters to decimeters (Fig. 2b and c). The leucosomes are gen- erally aligned subparallel to the gneissosity or foliation (Fig. 2b and c), but also locally cross-cut these early structures at a moderate to steep angle (Fig. 2d). The light-colored leucosomes are commonly coarser-grained than their host residuum. The leucosomes containing quartz + plagioclase + K-feldspar ...
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... in color. Quartzofeldspathic leucosomes generally occur as innumerable thin layers in the felsic gneiss. The proportion of thin leucosomes varies greatly from outcrop to outcrop and the leucosomes range in thickness from millimeters to decimeters (Fig. 2b and c). The leucosomes are gen- erally aligned subparallel to the gneissosity or foliation (Fig. 2b and c), but also locally cross-cut these early structures at a moderate to steep angle (Fig. 2d). The light-colored leucosomes are commonly coarser-grained than their host residuum. The leucosomes containing quartz + plagioclase + K-feldspar surround crystals of pink garnet, which represent peritectic phases (Fig. 2e). The felsic component ...
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... gneiss. The proportion of thin leucosomes varies greatly from outcrop to outcrop and the leucosomes range in thickness from millimeters to decimeters (Fig. 2b and c). The leucosomes are gen- erally aligned subparallel to the gneissosity or foliation (Fig. 2b and c), but also locally cross-cut these early structures at a moderate to steep angle (Fig. 2d). The light-colored leucosomes are commonly coarser-grained than their host residuum. The leucosomes containing quartz + plagioclase + K-feldspar surround crystals of pink garnet, which represent peritectic phases (Fig. 2e). The felsic component locally contains lenses, blocks and/or melanocratic layers (Fig. 2f), which are considered ...
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... the gneissosity or foliation (Fig. 2b and c), but also locally cross-cut these early structures at a moderate to steep angle (Fig. 2d). The light-colored leucosomes are commonly coarser-grained than their host residuum. The leucosomes containing quartz + plagioclase + K-feldspar surround crystals of pink garnet, which represent peritectic phases (Fig. 2e). The felsic component locally contains lenses, blocks and/or melanocratic layers (Fig. 2f), which are considered to be the segregated neosome consisting of the leucosome and the surrounded residuum (or melanosome). These features are principal evidence for in situ partial ...
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... at a moderate to steep angle (Fig. 2d). The light-colored leucosomes are commonly coarser-grained than their host residuum. The leucosomes containing quartz + plagioclase + K-feldspar surround crystals of pink garnet, which represent peritectic phases (Fig. 2e). The felsic component locally contains lenses, blocks and/or melanocratic layers (Fig. 2f), which are considered to be the segregated neosome consisting of the leucosome and the surrounded residuum (or melanosome). These features are principal evidence for in situ partial ...
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... rocks in the NQD UHP terrane ( Chen et al., 2012;Song et al., 2014aSong et al., , 2014bYu et al., 2009Yu et al., , 2012Yu et al., , 2014. In this study, field evidence for partial melting at the NQD UHP terrane is provided by sub- stantial numbers of thin veins in the felsic gneisses, which locally dis- play features diagnostic of migmatization (Fig. 2a, b, c and d). Remarkably, local coarse-grained patches consist of plagioclase + quartz ± K-feldspar leucosomes that commonly surround garnet (Fig. 2e). These patches are interpreted to be in situ neosomes consisting of leucosomes surrounding peritectic phases and are evi- dence of partial melting. Felsic veinlets composed of K-feldspar + quartz ± ...
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... field evidence for partial melting at the NQD UHP terrane is provided by sub- stantial numbers of thin veins in the felsic gneisses, which locally dis- play features diagnostic of migmatization (Fig. 2a, b, c and d). Remarkably, local coarse-grained patches consist of plagioclase + quartz ± K-feldspar leucosomes that commonly surround garnet (Fig. 2e). These patches are interpreted to be in situ neosomes consisting of leucosomes surrounding peritectic phases and are evi- dence of partial melting. Felsic veinlets composed of K-feldspar + quartz ± plagioclase ± muscovite (Fig. 5f), which strongly argue for the existence of felsic melts (Vernon, 2010), further indicate the exis- tence ...
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... of HP-UHP felsic rocks produces a residual mineral assemblage of biotite + plagioclase + garnet by the following reaction: phengite + clinopyroxene + quartz = .biotite + plagioclase + garnet + melt. The mineral assemblage of biotite + plagioclase + gar- net + amphibole is commonly observed in the Xitieshan melanosome layers of migmatic gneiss (Fig. 2e), which was suggested to be stable below 2 GPa ( Auzanneau et al., 2006). These observations provide strong evidence that partial melting of the felsic gneissic rocks mainly occurred during retrogression stages and melts crystallized under granulite-facies conditions rather than during the UHP metamorphism in Xitieshan ...
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... It can be further divided into the early Paleozoic high-pressure to ultra-high pressure metamorphic complex (Song et al., 2014;Zhao et al., 2017), the Oulongbuluke microcontinental block (Wu et al., 2019;Sun et al., 2020), and the Zongwulong orogenic belt (Fig. 1b). The early Paleozoic high-pressure to ultra-high pressure metamorphic complex consists of garnet peridotite, eclogite, schist, and high-grade gneiss, overlaid with Devonian strata and the Tianjianshan volcanic group, formed on the active continental margin (Song et al., 2006(Song et al., , 2014Yu et al., 2013;Yu et al., 2015). The Oulongbuluke microcontinental block has a double- Tang et al., 2023;Yan et al., 2022). ...
The Chakabeishan (CKBS) deposit is a newly discovered pegmatite-type lithium-beryllium deposit in the northern Qaidam tectonic belt of the northern Tibet Plateau. In recent years, some studies have discussed the genesis of the deposit, but the topic remains unclear. This study constrains the genesis of CKBS deposit based on detailed field observations and mineralogical studies, and presents fluid inclusion data and Li isotopic compositions of Li-rich pegmatites and Li-poor pegmatites. The mineralization characteristics of CKBS deposit shows a trend from weak Be mineralization to Li mineralization in the south to north direction, which shows the magmatic differentiation feature. Four types of fluid inclusions are identified: (1) triphase crystal-bearing inclusions (type-1), (2) biphase CO2-rich inclusions (type-2), (3) biphase aqueous inclusions (type-3), and (4) monophase liquid inclusions (type-4). The Li-rich pegmatites mainly contain type-1 inclusions and type-2 inclusions, while Li-poor pegmatites contain type-3 inclusions and some type-4 inclusions. Microthermometric and Laser Raman spectroscopy analyses show that Li-rich pegmatites formed in a low to medium temperature (210.5 ∼ 381.3 ℃), low to medium salinity (5.23 ∼ 17.82 % NaCl equiv.) NaCl-H2O-CO2 system, whereas Li-poor pegmatites formed in a low to medium temperature (206.4 ∼ 413.4 ℃), low to medium salinity (1.05 ∼ 10.49 % NaCl equiv.) NaCl-H2O system, which indicates a fluid immiscibility between them. In addition, lithium is greatly enriched in Li-rich pegmatites under the positive effects of CO2. The Li-rich pegmatites display high Li content (8362.4 ∼ 15509.6 ppm) but low δ7Li values (1.61 ∼ 1.80 ‰). In contrast, the Li-poor pegmatites display low Li content (23.9 ∼ 231.9) but high δ7Li values (0.79 ∼ 12.32 ‰). This means that the lithium isotopic fractionation is produced by the melt-fluid immiscibility in CKBS deposit, in which a Li-rich system with rich water and poor silicate and a Li-poor system with poor water and rich silicate were formed during the process. Therefore, the CKBS deposit may have formed under the combined effects of magmatic differentiation, melt-fluid immiscibility, and fluid immiscibility.
... Previous data show that the Proto-Tethys Ocean in the NQ began to subduct northward during the Early to Middle Cambrian, producing long-lived voluminous arc-related magmatic rocks [5,10,17,[21][22][23]. In the Middle and Late Ordovician, the Qaidam Block was dragged to initiate deep subduction, evolved to a continental subduction/collision orogeny stage, and then evolved to a post-orogenic collapse stage around the Late Silurian [4,5,[14][15][16][17][24][25][26][27][28]. The Maoniushan Formation molasses deposits, which are widely exposed in the NQ, represent the end of the Proto-Tethys tectonic cycle and the beginning of the Paleo-Tethys tectonic cycle [29][30][31]. ...
... Following the subduction processes, the ancient Qaidam Ocean finally closed in the Middle to Late Ordovician (ca. 460-450 Ma), dragging the Qaidam Block into continental deep subduction, which implies that the NQ evolved into a continental subduction/collision orogeny stage [15,[24][25][26][27][28]. The eclogites and granites in the North Qaidam UHP/HP Metamorphic Belt recorded the continental deep subduction, which constrained the timing of the continental subduction/collision at 420-460 Ma [43]. ...
The transition from the Proto- to the Paleo-Tethys is still a controversial issue. This study reports a new petrology, zircon U–Pb geochronology, and whole-rock geochemistry of volcanic rocks from the Maoniushan Formation in the Nankeke area, northern Qaidam (NQ) of the Tibetan Plateau, to provide new evidence for the transition from the Proto- to the Paleo-Tethys oceans. The volcanic suite consists mainly of rhyolitic crystal lithic tuff lavas and minor basalts. Zircon U–Pb data indicate that the bimodal volcanic rocks were formed during the Early Devonian (ca. 410–409 Ma). Geochemically, the basalts have low contents of SiO2 (48.92 wt.%–51.19 wt.%) and relatively high contents of MgO (8.94 wt.%–9.99 wt.%), TiO2 (1.05 wt.%–1.29 wt.%), K2O (2.35 wt.%–4.17 wt.%), and K2O/Na2O ratios (1.04–2.56), showing the characteristics of calc-alkaline basalts. Their rare earth element (REE) patterns and trace element spider diagrams are characterized by enrichments in LREEs (LREE/HREE = 18.31–21.34) and large ion lithophile elements (LILEs; Rb, Th, and K) and depletion in high-field-strength elements (HFSEs; Nb, Ta, P, and Ti), with slight negative Eu anomalies (Eu/Eu* = 0.82–0.86), which are similar to Etendeka continental flood basalts (CFB). These features suggest that the basalts were most likely derived from low degree (1%–5%) partial melting of the asthenospheric mantle, contaminated by small volumes of continental crust. In contrast, the felsic volcanics have high SiO2 (68.41 wt.%–77.12 wt.%), variable Al2O3 (9.56 wt.%–12.62 wt.%), low MgO, and A/CNK ratios mostly between 1.08 and 1.15, defining their peraluminous and medium-K calc-alkaline signatures. Their trace element signatures show enrichments of LREEs and LILEs (e.g., Rb, Th, U, K, and Pb), depletion of HFSEs (e.g., Nb, Ti, Ta, and P), and negative Eu anomalies (Eu/Eu* = 0.22–0.66). These features suggest that the felsic volcanics were derived from partial melting of the middle crust, without interaction with mantle melts. Considering all the previous data and geochemical features, the Maoniushan Formation volcanic rocks in NQ formed in a post-collisional extensional setting associated with asthenospheric mantle upwelling and delamination in the Early Devonian. Together with the regional data, this study proposed that the Proto-Tethys Ocean had closed and evolved to the continental subduction/collision orogeny stage during the Middle to Late Ordovician, evolved to the post-collisional extensional stage in the Early Devonian, and finally formed the Zongwulong Ocean (branches of the Paleo-Tethys Ocean) in the Late Carboniferous, forming the tectonic framework of the Paleo-Tethys Archipelagic Ocean in the northern margin of the Tibetan Plateau.
... In contrast, there is little evidence of the older Palaeozoic magmatism at ca. 500-480 Ma, which occurred to the north ( Figure 7f). Much of this Silurian-Devonian magmatism was related to melting of different types of subducted crust, during exhumation (Yu et al., 2015b;Yang et al., 2020b;Xu et al., 2022). Individual zircons in granitoids have magmatic rims ≤ 20 Myr younger than metamorphic cores, interpreted as the interval between peak UHP metamorphism and melting during exhumation (Yang et al., 2020b). ...
... To further constrain the melting P-T conditions when the potassic and ultrapotassic rocks formed, five mafic samples with relatively high MgO content (8.84-10.87 wt%) and Mg# (69-74) were chosen to Zindler and Hart (1986) and Hofmann (1997); global oceanic sediments come from Plank and Langmuir (1998); post-collision magmatic rocks of the Tibetan Plateau comes from Hou et al. (2006) and reference therein; potassic volcanic rocks in SW Tibet come from Miller et al. (1999); paragneiss of North Qaidam UHP terrane comes from Yu et al. (2015). ...
... Some of the Xitieshan eclogites with MORB-like geochemical features were metamorphosed from Neoproterozoic oceanic crust (Zhang et al., 2013a). This area contains voluminous felsic veins, which are considered to originate from partial melting of eclogites and felsic gneisses during exhumation (Chen et al., 2012;Yu et al., 2015). ...
... Partial melting of metamorphic rocks may cause modifications of contents of REE and HFSE, particularly LREE (Stepanov et al., 2014). Recently studies showed that partial melting of metamorphic rocks are only present in the Xitieshan and Luliangshan terranes, which are characterized by the presence of voluminous felsic veins occured in both eclogites and gneisses (Chen et al., 2012;Yu et al., 2015). The evidence for partial melting of the eclogite has not been found in the Yuka and Dulan terranes, and the studied samples have no significant Ce anomalies (Fig. 5), suggesting these rocks have not suffered significant influence of partial melting. ...
... Zircon REE chondrite-normalized patterns (McDonough and Sun, 1995) from the meta-granites display steeply rising slope from LREE to HREE (Fig. 7A), negative Eu anomalies (Eu/Eu* = 0.03 to 0.46) and positive Ce anomalies (Ce/Ce* = 0.08 to 12.14), all of which are consistent with unaltered igneous zircons (Hoskin and Schaltegger, 2003;Wu and Zheng, 2004). Moreover, low Hf/Y ratios (1.12 to 15.03), high Th (56.6 to 7085 ppm), and variable U (120 to 4151 ppm) contents with an average Th/U ratio of 0.58 ( Fig. 7D; Yu et al., 2015;Rubatto, 2002) confirms their magmatic origin. ...
... The North Qaidam high-pressure to ultrahigh-pressure (HP-UHP) metamorphic belt consists of garnet peridotite, eclogite, schist, and medium-to high-grade gneiss, in addition, which is overlain by post-Devonian strata and Tianjianshan volcanic group, which is suggested to be formed in an active continental margin (Song et al., 2006(Song et al., , 2014Yu et al., 2013;Yu et al., 2015). The Precambrian basement of the QJM comprises the Paleoproterozoic-Mesoproterozoic Dakendaban Group, Mesoproterozoic Wandonggou Group, and Paleoproterozoic Delingha Complex, which is unconformably overlain by Paleozoic-Mesozoic strata and the Neoproterozoic Quanji group (Chen et al., 2009(Chen et al., , 2012Li et al., 2019b;Sun et al., 2020;Wang et al., 2009a;Wu et al., 2019; Gong et al., 2019;Liao et al., 2018;Wang et al., 2018) et al., 2014). ...
... From 510 to 440 Ma, the northward subduction of the South Qilian Ocean formed the active continental margin of the QJM and QLM, resulted in the formation of the Tanjianshan volcanic Group (Song et al., 2006(Song et al., , 2014Zhang et al., 2017). From 440 to 400 Ma, as the closure of the South Qilian Ocean, continental collision between the Qilian and Qaidam Blocks resulted in the HP-UHP metamorphic belt (Song et al., 2006(Song et al., , 2014(Song et al., , 2019Sun et al., 2021;Yu et al., 2013Yu et al., , 2015. Moreover, starting from the Carboniferous, the northern subduction of the Kunlun Ocean, a part of the Paleo-Tethys Ocean, formed the Kunlun-Qaidam active arc-continental margin. ...
In recent years, a newly rare metal metallogenic belt has been discovered in the North Qaidam Terrane (NQT), NW China, of which the Chakabeishan (CKBS) Li-Be pegmatitic deposit is an important part. In this study, CMS (chemical composition-mineral assemblage-structural geology) classification is applied on CKBS pegmatitic rocks, based on their CMS features, the rocks are identified as pegmatite type. The CKBS pegmatites exhibit unique internal vertical zonation characteristics, and six zones are identified: biotite-muscovite-albite zone, graphic zone, layered tourmaline-albite zone, beryl-albite zone, spodumene-beryl-albite zone, and spodumene-albite zone. In this article, we present U-Pb zircon ages, Hf isotope dating, whole-rock geochemistry, and mineral geochemistry to elucidate pegmatite evolution. The zircon U-Pb ages of the spodumene-bearing pegmatite are 258.1±1.4 Ma and 219.6±6.3 Ma, indicate that the magma experienced two stage crystallization. The negative εHf(t) values range from -14.55 to -9.4 with T2DM model ages of 1845-2173 Ma, indicate that the magma originated from ancient rocks anatexis. The first anatectic event occurred during the Paleo-Tethys Ocean subduction process, as the magma originated from the anataxis of ancient metasedimentary rocks, provided enough rare elements (Li, Be, Nb, Ta). However, the second anatectic event occurred during post-orogenic stage and intruded in the host rocks forming pegmatite veins in the Late Triassic. Subsequently, intruded magma experienced mild fractionated crystallization, melt-fluid immiscibility and subsequent metasomatism, forming the vertical zonings.
... Furthermore, the Datonggou biotite granite has similar characteristics to experimental melts of tonalite produced at a pressure of 4 kbar (Fig. 11D). Although multiple stages of granites were emplaced before the Mid-Devonian in different terranes of the NQOB, the ε Nd(382 Ma) and/ or ε Hf(382 Ma) values of the Datonggou biotite granite are similar to those of Neoproterozoic gneissic granites and Devonian-Ordovician granitoids in this region ( Fig. 6C and D), including gneisses in the Lüliangshan (e.g., Yu et al., 2013b), melanosome and leucosomes in the Xitieshan and Dulan gneisses (e.g., Yu et al., 2015), metaluminous mica granites in the Lüliangshan batholith (Yang et al., 2020), and granites from Xitieshan and Dulan (Sun et al., 2020;Wu et al., 2014). Considering that the inherited zircons in Datonggou biotite granite have similar ages and Hf isotopic compositions with zircons of Ordovician-Silurian granitoids, the Datonggou biotite might have been generated by partial melting of Ordovician-Silurian granitoids. ...
Post-collisional magmatic intrusions and their mafic microgranular enclaves (MMEs) can provide significant insights into granite petrogenesis, crust-mantle interaction, and the reworking and growth of the continental crust. A combined petrological and geochemical study was carried out on MMEs and their host Niubiziliang I-type felsic composite pluton (granite and granodiorite) and aluminous A2-type Datonggou biotite granite from the Niubiziliang batholith in the northwestern segment of the North Qaidam Orogenic Belt (NQOB), China. This study presents whole-rock major- and trace elements and SrNd isotopic compositions, electron probe microanalyses of plagioclase, hornblende, and biotite, and zircon UPb dating and Hf isotopes.
Zircon UPb dating demonstrates that the studied rocks formed during the Late Devonian (ca. 382 Ma). The Niubiziliang composite felsic pluton samples yield a metaluminous calc-alkaline to high-K calc-alkaline I-type granitic composition. The MMEs from Niubiziliang composite felsic pluton have low SiO2 (52–64 wt%) and high MgO (2.8–4.9 wt%) concentrations. Both MMEs and host granites are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and are depleted in P and high field strength elements (HFSEs). All MMEs and host granitoids samples from Niubiziliang have indistinguishable Sr-Nd-Hf isotopic compositions characterized by initial Sr isotope ratios of 0.70545–0.70633 (MMEs), 0.70555–0.70742 (host rock), whole-rock εNd (382 Ma) values of −2.95 to +1.6, −0.25 to +3.5, and zircon εHf(382Ma) values of +2.2 to +10.2, +1.3 to +10.4, respectively. The Niubiziliang composite felsic pluton is a product of mixing juvenile crust-derived and depleted mantle-derived melts in a 40:60 proportion. MMEs crystallized from the same source as the host granite, and they were formed by self-mixing with the earlier crystallized cumulates when they were affected by the emplacement of subsequent hot magmas.
The Datonggou biotite granite has high TFe2O3/(TFe2O3 + MgO), TiO2/MgO, Ga/Al, and Y/Nb ratios, high zircon saturation temperatures (802–846 °C), high HFSE concentrations (e.g., Zr, Nb, and Y), moderate (Na2O + K2O) concentrations, and low Eu and Sr concentrations. Interstitial biotite has high Fe2O3 but low MgO concentrations, similar to those of aluminous A-type granite. The above-mentioned characteristics imply that the Datonggou biotite granite is an aluminous A2-type granite. Considering their high initial Sr isotopic composition (0.72218–0.72652), low εNd (382 Ma) (−6.73 to −6.31), and comparatively old two-stage Nd model ages (1640–1674 Ma), we suggest the biotite granite was generated by partial melting of Ordovician-Silurian granitoids with tonalitic compositions at HT-LP conditions. The formation of coeval I- and aluminous A2-type granitoids in this region was probably related to the lithospheric mantle delamination and associated asthenospheric upwelling at ca. 390 Ma. The I-type granitoids and MMEs from the Niubiziliang area represent a net addition of juvenile mantle to the crust and, given the wide distribution of contemporaneous I-type granitoids, the post-collisional magmatism has significantly contributed to the growth of the continental crust in the NQOB.
... Instead, these geochemical features are generally similar to those of the continental crust components such as the continental eclogite, orthogneiss and syn-exhumation granite in the North Qaidam orogen (Figs. 8a and 9), which exhibit enriched to depleted Sr-Nd-Hf isotopes (Sun et al., 2020;Wang et al., 2014;Yang et al., 2020;Yu et al., 2015aYu et al., , 2015bZhang et al., 2017b) and high zircon δ 18 O values . According to the tectonic location and formation timing of these mafic magmatic rocks, the most possible candidate for their sources is the orogenic lithospheric mantle beneath the North Qaidam orogen, which might contain recycled crustal components due to previous deep subduction of continental crust into the mantle. ...
... The results of orthogneiss, continental eclogite, orogenic peridotite, syn-exhumation and post-collisional granites from this orogen are also shown for comparison. Data for orthogneiss are from Yu et al. (2015a), Liu et al. (2014), andFu et al. (2015); data for continental eclogite are from Yu et al. (2015b), Liu et al. (2014), and Zhang et al. (2015aZhang et al. ( , 2016Zhang et al. ( , 2017b; data for orogenic peridotite are from Chen et al. (2017b); data for syn-exhumation granites are from Sun et al. (2020); data for post-collisional granites are from our unpublished data. Note: the studied syn-exhumation mafic magmatic rock in Fig. 9a is 09QL56, the post-collisional mafic magmatic rocks in the lower right corner and the top left corner in Fig. 9a are 15NQ269 and 15NQ113, respectively. ...
... In contrast, the continental eclogite-derived melts were dominated by felsic veins, so we chose these felsic veins as the other metasomatic agent. The average compositions of syn-exhumation granites and felsic veins of the continental eclogite (Sun et al., 2020;Yu et al., 2015aYu et al., , 2015bZhang et al., 2017b) were then used for model calculations of melt-peridotite reaction. We assume that the metasomatized lithospheric mantle has experienced aggregated fractional melting at different stages to produce the mafic magmas. ...
Syn-exhumation and post-collisional mafic magmatic rocks are generally exposed in continental collision orogens. Their geochemical compositions can not only reflect the nature of orogenic lithospheric mantle but also provide a window to trace crust-mantle interaction in continental collision zones. Herein, we present a combined study of whole-rock major-trace elements and Sr-Nd-Hf isotopes as well as zircon UPb ages and HfO isotopes for the Paleozoic mafic magmatic rocks from the North Qaidam orogen, northeastern Tibet. Zircon UPb dating for the rocks yields two groups of ages of 420 ± 8 to 395 ± 2 Ma and 383 ± 5 to 368 ± 3 Ma, corresponding to the syn-exhumation and post-collisional stages, respectively. These mafic magmatic rocks are characterized by arc-like trace element distribution patterns, enriched to depleted radiogenic Sr-Nd-Hf isotope compositions and high zircon δ¹⁸O values, which are comparable to those of the orthogneiss and continental eclogite in this orogen. This indicates their derivation from the orogenic lithospheric mantle modified by the subducted continental crust during the continental collision. Significant differences in radiogenic isotope compositions between the syn-exhumation and post-collisional mafic magmatic rocks suggest their mantle sources were mainly metasomatized by the orthogneiss- and continental eclogite-derived melts, respectively. Quantitative modelling of trace element compositions shows that about 93.0:6.0:1.0 and 91.7:2.8:5.5 mixture of the mantle peridotite, orthogneiss- and continental eclogite-derived melts with 23% and 28% aggregated fractional melting can closely match the syn-exhumation and post-collisional mafic magmatic rocks, respectively. The reaction of the continental crust-derived melts with mantle peridotite would result in the heterogeneous orogenic lithospheric mantle. Therefore, the syn-exhumation and post-collisional mafic magmatic rocks provide a snapshot of the compositional variations in the orogenic lithospheric mantle and the crust-mantle interaction in the continental collision zone.
... As a typical example of continental deep subduction and ultra-high pressure (UHP) metamorphism in the world, the North Qaidam orogen has attracted considerable attention (Chen et al., 2008(Chen et al., , 2012(Chen et al., , 2013Li et al., 2018aLi et al., , 2018bMattinson et al., 2009;Shi et al., 2006;Song et al., 2006Song et al., , 2014Song et al., , 2015Song et al., , 2019Yu et al., 2012Yu et al., , 2015Yu et al., , 2019aYu et al., , 2019bYu et al., , 2019cZhang et al., 2008Zhang et al., , 2009Zhang et al., , 2012Zhang et al., , 2015Zhang et al., , 2016Zhang et al., , 2017. Compared with the several investigations on UHP metamorphism and related high-grade metamorphic rocks, studies on the granitoid magmatism in the North Qaidam remains largely inadequate and controversial. ...
... The North Qaidam orogen is located at the northern margin of the Tibet Plateau, and is bound to the north by the Qilian block and to the south by the Qaidam block ( Fig. 1a-b). Tectonically, the North Qaidam orogen can be further subdivided from north to south into the Oulongbuluke microblock and UHP metamorphic belt ( Fig. 1c) (Chen et al., 2008(Chen et al., , 2012(Chen et al., , 2013Kang et al., 2015;Li et al., 2018aLi et al., , 2018bLu et al., 2018;Song et al., 2006Song et al., , 2014Wang et al., 2019Wang et al., , 2021Wu et al., 2006Wu et al., , 2019Yu et al., 2015Yu et al., , 2017Yu et al., , 2019cZhang et al., 2009Zhang et al., , 2016Zhang et al., , 2017. ...
... The UHP metamorphic belt in the North Qaidam orogen is mainly composed of ortho-and para-gneisses, metasandstones, basites, and minor lentoid eclogites within the country rocks. The protoliths of the ortho-and para-gneisses and eclogites were formed during the late Mesoproterozoic to the early Neoproterozoic and are hence considered as the products of assembly and dispersal of the supercontinent Rodinia related to Grenvillian orogenesis in western China (Chen et al., 2008(Chen et al., , 2012Mattinson et al., 2009;Song et al., 2006Song et al., , 2014Yu et al., 2015Yu et al., , 2019cZhang et al., 2008Zhang et al., , 2009Zhang et al., , 2012Zhang et al., , 2017. This belt is considered as a typical early Paleozoic UHP metamorphic belt generated through continental deep subduction based on the discovery of UHP index minerals such as coesite inclusions within zircons from pelitic gneisses and continental-type eclogites, or within continental-type eclogite slices in the Dulan and Yuqia, diamond-bearing zircons within garnet peridotite from Lvliangshan (Song et al., 2014;Yu et al., 2019a;Zhang et al., 2017). ...
Granitoids constitute the major theme in evaluating the growth and reworking of continental crust on Earth. Here we present U-Pb geochronology, geochemistry and Sr-Nd-Hf isotopes of the early Paleozoic granitoids emplaced during pre- and syn-collisional stages in the North Qaidam orogen to gain insights on the granitoid typology, genetic mechanism, as well as the implications for the evolution of continental crust. The pre- and syn-collisional granitoids in this region all belong to I-type granite and are derived from different continental crustal sources including late Mesoproterozoic to early Neoproterozoic metamorphic crystalline basement, and juvenile continental crust formed during early Paleozoic oceanic subduction. Granitic magmas derived from the two sources underwent a series of magmatic processes such as mixing or assimilation, which lead to the transitional geochemical and isotopic features, suggesting that besides source components, magmatic processes from melt extraction to granitoid emplacement also exerted an important influence on the formation of these granitoids and crustal maturation. Although S-type granitoids are commonly taken as the fingerprint for continental collision, our study emphasizes that the role of I-type granites formed in syn-collisional setting should not be underestimated. Besides subducted slab processes such as roll-back, retreat, or break-off, we propose a novel geodynamic model which envisages that the lateral inhomogeneity in lithospheric thickness triggered by continental collision within the overriding plate above subduction zone controlled the magmatism. Both growth and reworking of continental crust occur during oceanic subduction, whereas syn-collision setting is dominated by continental crust reworking as in the North Qaidam orogen.
