Search

Magnesium isotopic compositions, along with new Sr–Nd–Pb isotopic data and elemental analyses, are reported for 12 Miocene tourmaline-bearing leucogranites, 15 Eocene two-mica granites and 40 metamorphic rocks to investigate magnesium isotopic behaviors during metamorphic processes and associated magmatism and constrain the tectonic-magmatic-metamorphic evolution of the Himalayan orogeny. The gneisses, granulites and amphibolites represent samples of the Indian lower crust and display large range in δ26Mg from −0.44‰ to −0.09‰ in mafic granulites, −0.44‰ to −0.10‰ in amphibolites, and −0.70‰ to −0.03‰ in granitic gneisses. The average Mg isotopic compositions of the granitic gneisses (−0.19 ± 0.34‰), mafic granulites (−0.22 ± 0.17‰) and amphibolites (−0.25 ± 0.24‰) are similar, indicating the limited Mg isotope fractionation during prograde metamorphism from granitic gneisses to mafic granulites and retrograde metamorphism from mafic granulites to amphibolites. The Eocene two-mica granites and Miocene leucogranites are characterized by large variations in elemental and Sr–Nd–Pb isotopic compositions. The leucogranites and two-mica granites have their corresponding (87Sr/86Sr)i varying from 0.7282 to 0.7860 and 0.7163 to 0.7191, (143Nd/144Nd)i from 0.511888 to 0.512040 and 0.511953 to 0.512076, 207Pb/204Pb from 15.7215 to 15.7891 and 15.7031 to 15.7317, 208Pb/204Pb from 38.8521 to 39.5286 and 39.2710 to 39.4035, and 206Pb/204Pb from 18.4748 to 19.0139 and 18.7834 to 18.9339. However, they have similar Mg isotopic compositions (−0.21‰ to +0.06‰ versus −0.24‰ to +0.09‰), which did not originate from fractional crystallization nor source heterogeneity. Based on hornblende/biotite/muscovite dehydration melting reaction and Mg isotopic variations in two-mica granites and leucogranites with the proceeding metamorphism, along with elemental discrimination diagrams, Eocene two-mica granites and Miocene leucogranites could be related to hornblende dehydration melting and muscovite dehydration melting, respectively. Mg isotopic compositions of Eocene two-mica granites become heavier compared to the source because of residues of isotopically light garnet in the source; while those of Miocene leucogranites become lighter because of entrainment of isotopically light garnet from the source region. Thus, a new model for crustal anatexis and Himalayan orogenesis was proposed based on the Mg isotope fractionation in the leucogranites and metamorphic rocks. This model emphasizes a successive process from Indian continental subduction to rapid exhumation of the Higher Himalayan Crystalline Series (HHCS). The former underwent high-temperature (HT) and high-pressure (HP) granulite-facies prograde metamorphism, which resulted in the hornblende dehydration melting and the formation of Eocene two-mica granites; while the latter experienced amphibolite-facies retrogression and decompression, which resulted in the muscovite dehydration melting and the formation of Miocene leucogranites.

The tectonic transition from subduction to collision is a fundamental process during orogenesis, yet the magmatic expression of this transition and related deep geodynamic processes remain unclear. This study focuses on a newly identified volcanic belt within the Moyun–Zaduo–Sulu area of the North Qiangtang Block and presents new zircon U-Pb data that indicate that this belt formed during the Middle Triassic (247–241 Ma), a time characterized by a regional transition from subduction to collisional tectonism. The volcanic belt is located to the south of a Permian to Early Triassic arc and is dominated by high-K calc-alkaline and peraluminous rhyolites. These rhyolites have low Mg#, Nb/Ta, and δEu values, contain low contents of Sr, have high Rb/Sr and whole-rock eNd(t) values, and show positive zircon eHf(t) values, all of which suggest that they formed from magmas generated by the dehydration melting of juvenile crustal material. The migration of Middle Triassic volcanism in this region was most likely caused by rollback of the subducting Longmucuo–Shuanghu Tethyan oceanic slab. Combining our new data with previously published results of numerical modeling of subduction–collisional processes and regional data from north-central Tibet yields insights into the magmatic expressions and related deep geodynamics of the transition from oceanic subduction to continental collision. This combination of data also suggests that variations in oxygen fugacity can be used as a proxy for the discrimination of magmatism related to subduction, the transition from subduction to collision, and collisional tectonism.

Post-collisional K-rich volcanic rocks (KVRs) can provide an opportunity to constrain the architecture of the lithosphere and the mechanisms of plateau uplift. However, their petrogenesis and geodynamic setting remain in dispute. Lithium concentrations and isotopic compositions of 87 potassic, ultrapotassic and Mg-rich potassic volcanic rocks (PVRs, UPVs, and MPRs, respectively) in SW Tibet, along with new Pb–Sr–Nd isotope data and whole-rock analyses, are used to constrain their mantle source and genesis. These rocks are characterized by very similar δ7Li values: PVRs vary from −4.9‰ to +3.2‰, UPVs from −3.9‰ to +1.7‰, and MPRs from −1.2‰ to +3.5‰. They can be classified into two groups: Group I (19 out of 87 samples) with heavier δ7Li values (+1.0‰ to +3.5‰) similar to those reported for mid-ocean-ridge and ocean-island basalts (MORBs and OIBs, respectively), and Group II (68 out of 87 samples) with lighter values (−4.9‰ to +1.0‰) similar to those of Indian lower crust. These variable isotopic compositions may record the isotopic signature of the early-middle Miocene subcontinental lithospheric mantle (SCLM). This paper demonstrates the existence of isotopically light mantle domains beneath the Lhasa terrane, which were ascribed to the interaction with fluids/melts derived from the subducted Indian lower crust. The modeling curves of Indian lower crust with a metasomatized mantle composition fully account for compositional variations in the PVRs, UPVs, and MPRs. They were generated by the partial melting of SCLM, which was metasomatized by fluids/melts derived from the subducted Indian lower crust (ca. 4–14%, ca. 4–10%, and ca. 6–10% for the PVRs, UPVs, and MPRs, respectively). The Li isotopic data indicate that the Indian lower crust was subducted beneath the central Lhasa subterrane, and this sheds new light on the formation of the Tibet Plateau.

The Tethyan domain from China to Iran hosts many important sediment-hosted Pb–Zn deposits but most have been poorly documented. This study summarizes the salient features of these deposits and discusses the type of ore, tectonic setting, and important ore controls, on the basis of new geological observations and previous publications. The Tethyan domain is characterized by the young and extensive Himalayan–Tibetan and Zagros orogens that formed through collisions between the India/Arabia and Eurasia continents since the Late Cretaceous or early Cenozoic. Abundant Mississippi Valley-type (MVT) and subordinate clastic-dominated (CD, also known as SEDEX) Pb–Zn deposits occur in this domain, including in central and eastern Himalayan–Tibetan orogen in China, the Indian passive margin in southern Pakistan, and various tectonic units of Iran. Economically important deposits contain 0.1–21 Mt Pb + Zn and have total metal resources of ~75 Mt with ~48% being oxidized ores. All major deposits known in this domain are MVTs (i.e., the Jinding, Huoshaoyun, Mehdiabad, and Angouran deposits). Mississippi Valley-type Pb–Zn deposits occur in continental-collision-related fold-and-thrust belts and forelands, where deposits are mostly located on the margin of the Eurasian continent, with some in the Indian and Arabian continental margins. Clastic-dominated Pb–Zn deposits occur in central Iran and southern Pakistan, hosted by deep-water siliciclastic sequences of the early Cambrian rifted continental margin of Gondwana and the Jurassic passive continental margin of India, respectively. The youngest mineralized rocks and ages constrain that some important MVT deposits (e.g., the Jinding, Chaqupacha, and Angouran deposits) were formed after a main phase of regional compression, during a regional, large-scale strike-slip or crustal-extension stage in a continental collision setting. In sense of lithologic structure, important ore controls for MVT deposits include evaporite diapir structure, carbonate/evaporite dissolution–collapse structure, pre-existing barite, and porous dolostone. Much of the primary sulfide ore in this domain has been oxidized by supergene processes. This is particularly pronounced in the newly discovered Huoshaoyun deposit, where almost all sulfides have been oxidized to smithsonite and cerussite. An understanding of tectonic setting, ore controls, and supergene processes is essential in exploring for MVT deposits in this domain.

The 1600 km-long Gangdese magmatic belt features extensive Paleocene–Eocene I-type intrusive rocks and coeval volcanic successions, which can be divided into Group I (~69–53 Ma), Group II (~53–49 Ma), and Group III (~49–43 Ma), corresponding to Neo-Tethyan slab rollback, Neo-Tethyan slab breakoff, and ongoing Indian-Asian collision, respectively. The magmas from these three groups show significant variations in geochemical and isotopic compositions, which provide the information of the growth of continental crust in southern Tibet. The most voluminous magmatism in the Gangdese belt occurred during ~53–49 Ma. High zircon saturation temperature (up to 800 °C) and Ti in zircon temperature (up to 980 °C) estimations suggest there is a period of thermal anomaly during ~53–49 Ma. Starting from ~53 Ma, magmas have increased K2O contents, and their zircons have decreased Th/U ratios, and Y and Yb contents. Zircons from Group II have the most heterogeneous Hf isotopic compositions (eHf(t) = −5.3 to 15.1). These are evident of ingress of asthenosphere mantle in the arc, extensive crustal melting, and magma mixing. Magma underplating during this time is the main mechanism for the growth of continental crust. With the Indian-Asian collision going on, the magmas in Group III show high Th/Y and La/Yb ratios and K2O contents, but significantly low Tzr and T(ti-zr) values (mostly below 750 °C). These features suggest the water-fluxed melting of early arc residues occurred in the late stage of growth of continental crust. The crust has been thickened and nearly mature at this stage. This study has great implication on understanding of growth of continental crust in orogenic belts.

The Neotethyan tectonics of the Zagros orogenic belt, SW Iran remains still hotly debated in comparing with its western counterparts. One major issue concerns the timing and nature of the Sanandaj-Sirjan magmatic-metamorphic belt (SSMB), which is made predominantly of metamorphic rocks and Jurassic to Early Cretaceous large plutonic complexes. The Alvand and Qory are two largest plutonic complexes locating in north-central and southern segments, respectively, of the SSMB. Careful LA-ICP-MS U/Pb analyses of the magmatic zircons from the Alvand plutonic complex reveal a smooth spectra, along which the concordant age increase gradually from 120 to 190 Ma; while that of Qory is step-like consisting of two stages, a Jurassic and a late Early Cretaceous ones, respectively. New geochemical data, combined with zircon Lu/Hf results suggest that (1) the Alvand granitoids mostly resulted from a long-lived, successive injection of juvenile-crust-sourced magma batches without obvious fractionation crystallization (FC); but (2) the two stages granitoids of the Qory complex both generated by FC of juvenile-crust-sourced magmas; and (3) the gabbros of the Alvand complex are geochemically of E-MORB-affinity while those of the Qory complex are typical continental arc mafic rocks. Previously published petrological and 40Ar/39Ar data have identified a broken, Jurassic to Early Cretaceous high-pressure metamorphic belt to the southwest of the SSMB, which likely represents the closed, southeastern equivalent of the northern Neotethyan Ocean, north of the Taurides-Anatolia-Armenia block. Thus, the SSMB in Iran, the Kapan belt in Caucasus, and the Serbo-Macedonian belt in northern Turkey form a huge Jurassic to Early Cretaceous continental margin arc system recording northeastwards subduction of the older Northern Neotethyan Ocean beneath Eurasia. The Albian-Cenomanian ophiolites such as Koy, Kermanshah, and Neyriz in Iran represent the eastern counterpart of the younger Southern Neotethyan Ocean, south of the Taurides-Anatolia-Armenia block. During the subduction of the Old Neotethys, an intraplate transform fault likely opened and generated a slab-window beneath the Alvand region, which provided a constant energy source to steadily heat the low crust. This model satisfactorily interprets the unusual geochronological framework and geochemistry of the Alvand complex.

Voluminous Jurassic granitoids within the Sanandaj–Sirjan Zone (SSZ) provide insight into the magmatic arc formed in the active margin of Eurasia. Here, we present new in situ zircon U–Pb, whole-rock major and trace element, and Sr–Nd isotopic data for the Gorveh Plutonic Complex (GPC) of the northwestern SSZ in Iran. Six samples from the plutons within the GPC yielded zircon U–Pb ages that range from 151 to 146 Ma. These plutons can be subdivided into two groups based on their geochemistry. Group 1 rocks (the Mobarak Abad diorites and the Gorveh gabbros and diorites) contain relatively high concentrations of the high field strength elements (HFSE; Nb, Ta, Zr, and Ti) and have low Th/Nb (0.20–0.56) and moderate Sm/Yb ratios (1.51–2.32), low (87Sr/86Sr)i values (0.70354–0.70622), and high eNd(t) values (2.3–5.4). These features indicate that the Group 1 rocks formed from magmas derived from a subduction-modified region of the subcontinental lithospheric mantle. The Group 2 plutons (the Bolban Abad granites and the Gorveh quartz monzonites) have A-type granites affinities, including high K2O + Na2O and Zr + Nb + Ce + Y concentrations, and high FeOtot/MgO and 10,000 × Ga/Al ratios. These A-type rocks are enriched in Rb, Th, and K, and depleted in Ba, U, Nb, Ta, Sr, P, and Ti. The Group 2 plutons have different Sr–Nd isotopic compositions to each other, indicating they were derived from different sources and record different igneous processes. The Gorveh quartz monzonites have high (87Sr/86Sr)i ratios (0.70552–0.70617), negative eNd(t) values (−1.1 to −5.4), and extremely low concentrations of MgO (0.32–0.35 wt%), suggesting they were derived from an igneous quartzo-feldspathic crustal source. In comparison, the Bolban Abad granites have positive eNd(t) values and contain high concentrations of SiO2 and low concentrations of MgO, suggesting that they formed from Group 1 magmas that subsequently underwent assimilation and fractional crystallization processes. Combining these new data with the results of previous research, we conclude that this Jurassic magmatism was the result of the formation of a slab window within the subducting Neo-Tethys slab, a process that caused the partial melting of overlying continental lithospheric material.

The concentrations and isotopic geochemistry of Li are potentially useful geochemical tracers of geological processes. To fully utilize Li isotopes as geochemical tracers, it is necessary to characterize the Li isotopic compositions of the various geological reservoirs. However, the Li isotopic composition of the juvenile lower crust is currently poorly constrained. Given that lithospheric architecture of the Tibetan Plateau includes Indian upper/lower crust, Tibetan upper/lower crust and juvenile lower crust, it is necessary to determine the Li isotopic composition for each geological endmember underneath southern Tibet. Among them, the juvenile lower crust was formed directly by underplating of mantle-derived basaltic magma, which is likely to be the critical factor to control the Cu-Au mineralization in southern Tibet and is responsible for crustal thickening beneath southern Tibet. Here, we report the Li concentration and isotopic composition of the juvenile lower crust in southern Tibet. Based on whole-rock major element, trace element, and Sr–Nd–Pb isotopic data, we infer that the Yeba basalts and Gangdese gabbros were derived from partial melting of metasomatized lithospheric mantle, and have compositions similar to the juvenile lower crust. In contrast, the Dianzhong andesites and Gangdese diorites originated from partial melting of the juvenile lower crust. Therefore, these units may be considered representative of the juvenile lower crust. The juvenile lower crust has Li concentrations of 7.1–37.2 ppm (mean = 15.4 ppm), consistent with the Li concentration for the lower crust (13 ppm). Li isotopic compositions (δ7Li) vary from +0.8‰ to +6.6‰ (mean = 3.0‰), similar to values for the EMI/EMII mantle. The Li isotopic compositions of the analyzed samples were not significantly affected by alteration, metamorphism, crustal assimilation, or magmatic differentiation, and therefore represent the isotopic compositions of the juvenile lower crust. The Li systematics of the juvenile lower crust may be attributed to partial melting of subcontinental lithospheric mantle that has undergone metasomatism by Li-rich fluids derived from subducted oceanic crust and marine sediments. Our study also demonstrates near-identical Li isotopic compositions for juvenile lower crust and metasomatized lithospheric mantle, resulting from the lack of Li isotope fractionation during basalt generation and differentiation.

New major and trace elemental, Sr–Nd–Pb isotope, and zircon U–Pb geochronological and Hf–O isotope data of post-collisional potassic and ultrapotassic volcanic rocks (PVRs and UPVs, respectively) along with geochemical data of PVRs, UPVs, and Mg-rich potassic rocks (MPRs) in the literature are used to constrain their mantle source and genesis. The PVRs, UPVs, and MPRs share similar geochemical features but with some discrepancies, suggesting that they were derived from subcontinental lithospheric mantle (SCLM) with isotopic heterogeneity resulting from the varying contributions of subducted Indian lower crust into the mantle source (ca. 6–20%, ca. 8–30%, and ca. 9–30%, respectively). The zircon Hf–O isotopic compositions of these rocks can be classified into two groups, including Group I rocks with high δ18O (6.7–11.3‰), low eHf(t) (− 17.0 to − 12.0), and old Hf crustal model ages (1.87–2.19 Ga) that indicate an ancient SCLM source, and Group II rocks with δ18O values of 6.8–10.7‰, eHf(t) values of − 11.8 to − 6.3, and younger Hf crustal model ages (1.50–1.86 Ga). The negative correlation defined by δ18O and eHf(t) of Group II samples suggests a two-component mixing between mantle- and crust-derived melts, in which the latter would be the subducted Indian lower crust as indicated by the similar negative eHf(t) values between Group II samples (− 11.8 to − 6.3) and the High Himalayan gneiss (− 14.2 to + 0.3). Thus we propose two enrichment events to account for the Hf–O isotopic compositions of the PVRs and UPVs/MPRs: the first involves the enrichment of the overlying SCLM that was metasomatized by fluids derived from dehydration of the subducted Indian lower crust, and the second invokes the enrichment of the overlying SCLM metasomatized by melts of the already dehydrated different proportions of the Indian lower crust. We argue that break-off of the northwards subducted Indian Plate in the early Miocene caused the asthenospheric upwelling under the Indian plate through slab window, resulting in varying degrees of partial melting of the overlying metasomatized heterogeneous SCLM to produce the primitive magmas of the PVRs, UPVs, and MPRs in an extensional setting. These observations and interpretations imply that the Indian lower crust was subducted beneath the Lhasa terrane in the Early–Middle Miocene.

Mississippi Valley type (MVT) Pb–Zn deposits can occur in orogenic thrust belts. However, the relationship between MVT ore-forming processes and thrusting is unclear. The 1500-km-long Sanjiang Metallogenic Belt in Tibetan Plateau is an important thrust-controlled MVT ore province with 860 Mt at 0.76–2.3% Pb, 0.3–6.1% Zn. The Zhaofayong MVT ore cluster in the Changdu area is a typical sample. The orebodies in this ore cluster are hosted in limestone, controlled by secondary faults to regional thrusts and forming along these faults. Two Pb–Zn mineralization stages in this cluster are recognized. Stage I is characterized by coarse and euhedral galena + sphalerite + calcite + fluorite + barite and Stage II by fine grained sphalerite + galena + pyrite + calcite. Sm–Nd isotopic dating of calcite forming in Stage I yields isochron ages of 41.1–38.1 Ma, suggesting the mineralization formed during extension following the first regional compression in the Changdu area. The connection between Stage I mineralization and the regional thrusting in the Changdu area can extend to the whole Sanjiang belt. Two stages of regional Pb–Zn mineralization are recognized between 65 Ma and 30 Ma and between 30 Ma and 16 Ma in the belt. The two Pb–Zn mineralization stages are consistent with those regional episodic thrusting activities and both of them immediately occurred after the episodic thrusting. An interpretation of the regional Pb–Zn mineralization is that regional compression forced the movement of hydrothermal fluids along regional thrust-nappe detachment faults and subsequent post-thrust extension caused the migration of hydrothermal fluids to the ore forming locations. The two mineralization stages in the Sanjiang Belt indicate complex processes related to India–Eurasia collision and the gradually younger mineralization ages from southeast to northwest indicate the collision follows the same direction.

The Bilihe gold deposit, along the northern margin of the North China Craton (NCC), is hosted in Permian intrusions that were emplaced into Bainaimiao Group greenschist- to amphibolite-facies Mesoproterozoic basement rocks, the early Paleozoic Bainaimiao Arc, and late Paleozoic sedimentary and volcanic cover rock sequences. The Middle Ordovician to Early Silurian calc-alkaline arc comprises volcanic sequences of basalt, andesite, and felsic lavas, and an intrusive complex ranging from gabbro through granite. The deposit is genetically related to ilmenite-series and highly fractionated quartz diorite porphyry host rocks. The most fractionated variety of the quartz diorite porphyry is granitic, which contains dendritic quartz phenocrysts and occurs at the top part of the intrusions. Gold mostly occurs as Au grains (> 990 fineness) in trails in dendritic quartz, and the gold trails typically occur along certain crystallographic elements of dendritic quartz. The gold is believed to have crystallized directly from magma at temperatures of at least 750–800 °C. The Fe2O3/(Fe2O3 + FeO) ratio of the quartz diorite ranges from 0.21 to 0.29, and there is significantly more ilmenite than magnetite, suggesting that the magma was moderately reduced. There are also post-ore syenogranite porphyries in the deposit area. Zircon U-Pb LA-ICP-MS dating in this study shows that two samples of the volcanic wallrock have ages of 274 ± 3 Ma and 270 ± 4 Ma, two samples of gold-forming quartz diorite porphyries have ages of 261 ± 2 Ma and 259 ± 3 Ma, with 440–400 Ma and 1344–1316 Ma cores, and one syenogranite porphyry sample is dated at 253 ± 3 Ma. The mineralized quartz diorite porphyries exhibit a high-K calc-alkaline composition, high Mg# (46–51), and a high content of compatible elements (V: 114–164 ppm; Cr: 80–93 ppm; Ni: 37–42 ppm) and Rb, Th, and U, as well as depletions in Ba, Sr, and HFSE (e.g., Ti, Nb, and Ta), suggesting melt from a slab fluid-metasomatized enriched mantle. The LREE enrichment, the intermediate oxidation state, and the presence of the Silurian and Mesoproterozoic zircon cores indicate that crustal materials also played a significant role in porphyry formation. In addition, the rocks have slightly negative eNd(t) (− 2.5–0.1), but variable (87Sr/86Sr)i (0.70328–0.70573). Based on the above features, we propose that quartz diorite porphyries were produced by partial melting of the metasomatized mantle coupled with the assimilation of crustal materials of the Bainaimiao Group, Late Ordovician-Silurian subduction-related plutons, and Late Carboniferous-Early Permian marine sedimentary rocks. The assimilation of rocks of the Bainaimiao Group, which have an elevated gold content of 26.3 ppb, is speculated to have played important roles in the formation of a rarely documented magmatic gold deposit. In contrast, syenogranite porphyries are characterized by high contents of K2O + Na2O (10.3–10.6 wt%), Al2O3 (16.3–16.5 wt%), K, Hf, and Zr; low CaO (0.6–1.2 wt%), Mg# values (17–21), compatible elements (V: 16–21 ppm; Cr: 4.9–7.9 ppm; Ni: 2.8–4.5 ppm), and Eu, Sr, and HFSE; and positive eNd(t) (1.4–1.5) with low (87Sr/86Sr)i (< 0.70431), indicating that the porphyries were formed by partial melting of the subduction-modified lower crust with a minor input of asthenospheric materials. It is concluded that the quartz diorite porphyries formed during the southward subduction of the Paleo-Asian ocean underneath the northern margin of the NCC. This confirms that the northern margin was not a passive continental margin, but an active margin with subduction and development of a continental arc. In addition to Bilihe, there are at least four more gold deposits (Hadamiao, Changshanhao, Saiyinwusu, and Chaihulanzi) that formed at the northern margin of the NCC from Middle Permian to Early Triassic. Such a timing indicates that they may also be ultimately related to the tectonic regime imposed by southward subduction of the Paleo-Asian ocean. Based on the new understanding of the tectonic setting, with significant known mineral occurrences along the length of the margin, it is inferred that the > 1000-km-long northern margin of NCC may have a large potential for subduction-related mineralization.

Narigongma is a poorly studied Mo-rich (similar to 0.06 wt.%) post-collisional porphyry Cu deposit located in southern Qinghai Province, Tibet, 400 km northwest of the Yulong porphyry Cu-Mo-Au belt. The Narigongma deposit has a similar age (43-40 Ma) to porphyry deposits in the Yulong belt, but different ore assemblages. The Narigongma deposit is associated with Eocene granodiorite and granite intrusions that were emplaced into a Permian volcanic-sedimentary rock sequence. An similar to 43.3 Ma biotite granite stock (P1 porphyry) is the earliest Eocene intrusion, and this was itself intruded by a number of smaller, similar to 43.6 Ma fine-grained granite porphyry stocks (P2 porphyry) and several post-ore quartz diorite porphyry dikes (similar to 41.7 Ma). The main Cu-Mo mineralization at Narigongma is associated with the P1 porphyry. Hydrothermal alteration surrounding the deposits is generally characterized by concentric zones that range from an inner potassic zone outward to phyllic and argillic alteration zones, and an outer propylitic zone. Hypogene mineralization at Narigongrna was characterized by early-stage precipitation of molybdenite during potassic alteration and late-stage deposition of chalcopyrite during phyllic alteration. Deposition of both the Mo and Cu mineralization stages was caused by decreasing temperature. A high degree of crystallization of the P1 porphyry occurred prior to fluid saturation that produced Mo enrichment in the residual melt due to the incompatible behavior of Mo. However, compatible Cu was sequestered by the crystallizing phases and resulted in the generation of a high-Mo/Cu magmatic-hydrothermal fluid and the final Mo +/- Cu mineralization assemblage. Zircon epsilon(Hf)(t) values of +4.1 to +7.9 are indicative of magma derivation from a depleted source. These isotopic data, coupled with other geochemical characteristics of the Narigongma porphyry, such as high SiO2 and K2O contents, low MgO contents and compatible element abundances, and highly fractionated rare earth element patterns, indicate a mixing model for the origin of the porphyry bodies. Generation of the post-collisional ore-forming porphyries occurred in two stages: (1) partial melting of metasomatized phlogopite-bearing lithospheric mantle that generated potassic to ultra-potassic mafic melts, and (2) underplating of such melts beneath thickened juvenile lower crust, which triggered partial melting of the lower crust to produce the ore-forming, high-K adakitic magma. (C) 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Different hypotheses have been proposed for the origin and pre-Cenozoic evolution of the Tibetan Plateau as a result of several collision events between a series of Gondwana-derived terranes (e.g., Qiangtang, Lhasa and India) and Asian continent since the early Paleozoic. This paper reviews and reevaluates these hypotheses in light of new data from Tibet including (1) the distribution of major tectonic boundaries and suture zones, (2) basement rocks and their sedimentary covers, (3) magmatic suites, and (4) detrital zircon constraints from Paleozoic metasedimentary rocks. The Western Qiangtang, Amdo, and Tethyan Himalaya terranes have the Indian Gondwana origin, whereas the Lhasa Terrane shows an Australian Gondwana affinity. The Cambrian magmatic record in the Lhasa Terrane resulted from the subduction of the proto-Tethyan Ocean lithosphere beneath the Australian Gondwana. The newly identified late Devonian granitoids in the southern margin of the Lhasa Terrane may represent an extensional magmatic event associated with its rifting, which ultimately resulted in the opening of the Songdo Tethyan Ocean. The Lhasa-northern Australia collision at similar to 263 Ma was likely responsible for the initiation of a southward-dipping subduction of the Bangong-Nujiang Tethyan Oceanic lithosphere. The Yarlung-Zangbo Tethyan Ocean opened as a back-arc basin in the late Triassic, leading to the separation of the Lhasa Terrane from northern Australia. The subsequent northward subduction of the Yarlung-Zangbo Tethyan Ocean lithosphere beneath the Lhasa Terrane may have been triggered by the Qiangtang-Lhasa collision in the earliest Cretaceous. The mafic dike swarms (ca. 284 Ma) in the Western Qiangtang originated from the Panjal plume activity that resulted in continental rifting and its separation from the northern Indian continent. The subsequent collision of the Western Qiangtang with the Eastern Qiangtang in the middle Triassic was followed by slab breakoff that led to the exhumation of the Qiangtang metamorphic rocks. This collision may have caused the northward subduction initiation of the Bangong-Nujiang Ocean lithosphere beneath the Western Qiangtang. Collision-related coeval igneous rocks occurring on both sides of the suture zone and the within-plate basalt affinity of associated mafic lithologies suggest slab breakoff-induced magmatism in a continent-continent collision zone. This zone may be the site of net continental crust growth, as exemplified by the Tibetan Plateau. (C) 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

The relationship between potassic felsic intrusions, cospatial and cotemporal with potassic mafic magmatism, and strike–slip displacement along the Ailao Shan-Red River shear zone in western Yunnan, SW China has been highly controversial. We report 22 new SHRIMP zircon U–Pb ages from 19 potassic felsic intrusions located as far as 150 km east and 50 km west of the shear zone to constrain temporal relationships. The results show that the felsic intrusions in western Yunnan were emplaced between 36.9 ± 0.3 Ma and 32.5 ± 0.3 Ma. This age range is significantly shorter than previous dating of 38 Ma to 23 Ma using K–Ar and Ar–Ar methods from 15 intrusions, indicating that the latter are reset or cooling ages. Potassic felsic intrusions are coeval with potassic mafic volcanic rocks (36.6 Ma to 33.4 Ma) in the same area, supporting a possible genetic link between the two end members. The dated intrusions occur over a 200 km wide zone across the Ailao Shan-Red River shear zone; therefore they are unlikely genetically related to shearing. Rather, potassic mafic magmatism in western Yunnan may be related to delamination and melting of previously subduction-modified mantle lithosphere along the Mesozoic Jinsha suture reactivated in the Eocene–Early Oligocene, following the India-Asia continental collision; potassic felsic magmas were crustal melts hybridized with mafic liquids. Delamination in turn facilitated and localized initiation of the Ailao Shan-Red River shear zone at ca. 32 Ma during continental extrusion.

Among the Phanerozoic magmatic pulses that substantially modified the structure and composition of the subcontinental lithosphere of the North China Craton (NCC), the Mesozoic suite includes a wide variety of high Sr/Y and La/Yb rocks. The spatio-temporal distribution and characterization of these rocks are fundamental to evaluating the various models of decratonization of the NCC. Here we report petrologic, geochemical, Sr–Nd–Pb and U–Pb zircon isotopic data on the Mesozoic volcanic rocks from northern Taihang orogen in the eastern NCC. The magmatic zircons in these rocks display high Th/U values (0.4–1.7) and yield a 206Pb/238U age range of 152 Ma to 138 Ma, with a weighted mean of 145.6 Ma. The cores of inherited zircon xenocrysts yield discordant ages of 1840 Ma and 2013 Ma suggesting derivation from ancient crustal sources. Most of the volcanic rocks of our study posses moderately high initial 87Sr/86Sr ratios ranging from 0.7060 to 0.7062, and relatively low initial 143Nd/144Nd values from 0.5116 to 0.5117, defining a tight cluster in 87Sr/86Sr versus 143Nd/144Nd plots. They show low initial 206Pb/204Pb (16.590–16.807), 207Pb/204Pb (15.247–15.269) and 208Pb/204Pb (36.656–36.820) values. The associated hornblendites have isotopic compositions identical to those of the volcanic rocks and define continuous variation trends reflecting common parent magma. Geochemical modeling shows that the high Sr/Y and La/Yb signatures of the Mesozoic volcanic rocks from the northern Taihang orogen are primarily the result of fractional crystallization of mantle-derived melts. We evaluate the magma tectonics through the MASH (melting, assimilation, storage, homogenization) model with subsequent evolution of the hybrid magmas and their emplacement controlled by structural conduits. Our interpretation is in deviation from previous models involving partial melting in a thickened lower crust, and crustal delamination, but provides a more robust mechanism to account for the field relations including the occurrence of a wide compositional variety of lithologies in the same region, distinct petrological features such as reverse zoning of some phenocryst phases, geochemical characteristics including smooth fractionation trends, as well as the presence of multiple age populations of inherited zircons.