Zircon U-Pb age and Hf-O isotope insights into genesis of Permian Tarim felsic rocks,NW China: Implications for crustal melting in response to a mantle plume

Deciphering the contribution of crustal materials to A-type granites is critical to understanding their petrogenesis. Abundant alkaline syenitic and granitic intrusions distributed in Tarim Large Igneous Province, NW China, offer a good opportunity to address relevant issues. This paper presents new zircon Hf-O isotopic data and U-Pb dates on these intrusions, together with whole-rock geochemical compositions, to constrain crustal melting processes associated with a mantle plume. The ∼280 Ma Xiaohaizi quartz syenite porphyry and syenite exhibit identical zircon δ¹⁸O values of 4.40 ± 0.34‰ (2σ) and 4.48 ± 0.28‰ (2σ), respectively, corresponding to whole-rock δ¹⁸O values of 5.6‰ and 6.0‰, respectively. These values are similar to mantle value and suggest an origin of closed-system fractional crystallization from Tarim plume-derived melts. In contrast, the ∼275 Ma Halajun A-type granites have higher δ¹⁸O values (8.82–9.26‰) than the mantle. Together with their whole-rock ε_Nd(t) (−2.0–+0.6) and zircon ε_Hf(t) (−0.6–+1.5) values, they were derived from mixing between crust- and mantle-derived melts. These felsic rocks thus record crustal melting above the Tarim mantle plume. At ∼280–275 Ma, melts derived from decompression melting of Tarim mantle plume were emplaced into the crust, where fractional crystallization of a common parental magma generated mafic-ultramafic complex, syenite, and quartz syenite porphyry as exemplified in the Xiaohaizi region. Meanwhile, partial melting of upper crustal materials would occur in response to basaltic magma underplating. The resultant partial melts mixed with Tarim plume-derived basaltic magmas coupled with fractional crystallization led to formation of the Halajun A-type granites.     

Early Neoproterozoic mafic Rocks in the North Lhasa Terrane,Tibet: A relic of the Mozambique Ocean

The Mozambique Ocean is postulated to mark the huge oceanic basin of the East African Orogen and separated the continental components of eastern and western Gondwana, but only limited studies have considered the production of its oceanic crust. Here we present a combined analysis of petrological, geochronological, geochemical, and Sr–Nd–Hf isotopic data for the Early Neoproterozoic gabbros in the North Lhasa terrane, Tibet. Zircons from the gabbros yielded concordant ages of ca. 913–901 Ma. The gabbros display N‐MORB compositions with limited subduction input and are characterized by high positive zircon ε_Hf(t) (+5.3 to +12.1) and whole‐rock ε_Nd(t) (+5.6 to +6.5) values. They have not experienced significant crustal contamination but their compositions have been modified by extensive fractional crystallization. They are most likely derived from relatively high degree partial melting (>20%) of a spinel lherzolite source in a depleted mantle. Combined with observations from previous studies, we suggest that the generation of these gabbros was probably related to the incipient formation of oceanic crust in a back‐arc basin and they may mark a relic of the eastern Mozambique Ocean. To our knowledge, they are the oldest oceanic relic in the Tibetan Plateau.     

Subduction of the Indian lower crust beneath southern Tibet revealed by the post-collisional potassic and ultrapotassic rocks in SW Tibet

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 ε_Hf(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‰, ε_Hf(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 ε_Hf(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 ε_Hf(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.     

Phanerozoic amalgamation of the Alxa Block and North China Craton: Evidence from Paleozoic granitoids,U–Pb geochronology and Sr–Nd–Pb–Hf–O isotope geochemistry

The North China Craton (NCC) has been considered to be part of the supercontinent Columbia. The nature of the NCC western boundary, however, remains strongly disputed. A key question in this regard is whether or not the Alxa Block is a part of the NCC. It is located in the vicinity of the inferred boundary, and therefore could potentially resolve the issue of the NCC's relationship to the Columbia supercontinent. Some previous studies based on the Alxa Block's geological evolution and detrital zircon ages suggested that it is likely not a part of the NCC. The lack of evidence from key igneous rock units, however, requires further constraints on the tectonic affinity of the western NCC and Alxa Block and on the timing of their amalgamation.In this study, new zircon U–Pb age and Hf–O isotopes and whole-rock geochemical and Sr–Nd–Pb isotopic data for the Paleozoic granitoids in or near the eastern Alxa Block were used to constrain the petrogenesis of these rocks and the relationship between the Alxa Block and NCC. Secondary ion mass spectrometry (SIMS) U–Pb zircon dating indicates that the Bayanbulage, Hetun, Diebusige and South Diebusige granitoids were formed at ca. 423 Ma, 345 Ma, 345 Ma and 337 Ma, respectively. The Late Silurian (Bayanbulage) quartz diorites have variable SiO₂ (58.0–67.9 wt.%), and low Sr/Y (20–24) values, while the Early Carboniferous (Hetun, Diebusige and South Diebusige) monzogranites have high SiO₂ (71.5–76.7 wt.%) and Sr/Y (40–94) values. The Late Silurian quartz diorites display relatively homogeneous and high zircon δ¹⁸O (8.5–9.1‰) and ε_Hf(t) (− 8.6 to − 5.3) values, high whole-rock ε_Nd(t) values (− 9.2 to − 7.6) and highly radiogenic Pb isotopes (²⁰⁶Pb/²⁰⁴Pb = 18.13–18.25), whereas the Early Carboniferous monzogranites exhibit relatively low and variable zircon δ¹⁸O (5.7–7.2‰) and ε_Hf(t) (− 23.1 to − 7.4) values, low whole-rock initial ⁸⁷Sr/⁸⁶Sr (0.7043–0.7070) and ε_Nd(t) (− 19.1 to − 13.5) values and variable Pb isotopes (²⁰⁶Pb/²⁰⁴Pb = 16.06–18.22). The differences in whole rock Nd model ages and Pb isotope compositions of the Paleoproterozoic–Permian rocks in either side of the west fault of the Bayanwulashan–Diebusige complexes suggest that the Alxa Block is not a part of the NCC, and that the western boundary of the NCC is probably located on this fault. Furthermore, the linear distribution of the Early Paleozoic–Early Carboniferous granitoids, the high zircon δ¹⁸O values of the Late Silurian quartz diorites, the Early Devonian metamorphism and the foreland basin system formed during the collision between the Alxa Block and the NCC indicate that a Paleozoic cryptic suture zone likely existed in this area and records the amalgamation of the Alxa Block and North China Craton. Together with detrital zircon data, the initial collision was considered to have possibly occurred in Late Ordovician.     

Subduction of Indian continent beneath southern Tibet in the latest Eocene (~ 35 Ma): Insights from the Quguosha gabbros in southern Lhasa block

Geophysical data illustrate that the Indian continental lithosphere has northward subducted beneath the Tibet Plateau, reaching the Bangong–Nujiang suture in central Tibet. However, when the Indian continental lithosphere started to subduct, and whether the Indian continental crust has injected into the mantle beneath southern Lhasa block, are not clear. Here we report new results from the Quguosha gabbros of southern Lhasa block, southern Tibet. LA-ICP-MS zircon U–Pb dating of two samples gives a ca. 35 Ma formation age (i.e., the latest Eocene) for the Quguosha gabbros. The Quguosha gabbro samples are geochemically characterized by variable SiO₂ and MgO contents, strongly negative Nb–Ta–Ti and slightly negative Eu anomalies, and uniform initial ⁸⁷Sr/⁸⁶Sr (0.7056–0.7058) and ε_Nd(t) (− 2.2 to − 3.6). They exhibit Sr–Nd isotopic compositions different from those of the Jurassic–Eocene magmatic rocks with depleted Sr–Nd isotopic characteristics, but somewhat similar to those of Oligocene–Miocene K-rich magmatic rocks with enriched Sr–Nd isotopic characteristics. We therefore propose that an enriched Indian crustal component was added into the lithospheric mantle beneath southern Lhasa by continental subduction at least prior to the latest Eocene (ca. 35 Ma). We interpret the Quguosha mafic magmas to have been generated by partial melting of lithospheric mantle metasomatized by subducted continental sediments, which entered continental subduction channel(s) and then probably accreted or underplated into the overlying mantle during the northward subduction of the Indian continent. Continental subduction likely played a key role in the formation of the Tibetan plateau at an earlier date than previously thought.     

Petrogenesis of Cretaceous igneous rocks from the Duolong porphyry Cu–Au deposit,central Tibet: evidence from zircon U–Pb geochronology,petrochemistry and Sr–Nd–Pb–Hf isotope characteristics

The Duolong porphyry Cu–Au deposit (5.4 Mt at 0.72% Cu, 41 t at 0.23 g/t Au) was recently discovered in the southern Qiangtang terrane, central Tibet. Here, new whole‐rock elemental and Sr–Nd–Pb isotope and zircon Hf isotopic data of syn‐ and post‐ore volcanic rocks and barren and ore‐bearing granodiorite porphyries are presented for a reconstruction of magmas associated with Cu–Au mineralization. LA–ICP–MS zircon U–Pb dating yields mean ages of 117.0 ± 2.0 and 120.9 ± 1.7 Ma for ore‐bearing granodiorite porphyry and 105.2 ± 1.3 Ma for post‐ore basaltic andesite. All the samples show high‐K calc‐alkaline compositions, with enrichment of light rare earth elements (LREE) and large ion lithophile elements (LILE: Cs and Rb) and depletion of high field strength elements (HFSE: Nb and Ti), consistent with the geochemical characteristics of arc‐type magmas. Syn‐ and post‐ore volcanic rocks show initial Sr ratios of 0.7045–0.7055, ε_Nd(t) values of −0.8 to 3.6, (²⁰⁶Pb/²⁰⁴Pb)_t ratios of 18.408–18.642, (²⁰⁷Pb/²⁰⁴Pb)_t of 15.584–15.672 and positive zircon ε_Hf(t) values of 1.3–10.5, likely suggesting they dominantly were derived from metasomatized mantle wedge and contaminated by southern Qiangtang crust. Compared to mafic volcanic rocks, barren and ore‐bearing granodiorite porphyries have relatively high initial Sr isotopic ratios (0.7054–0.7072), low ε_Nd(t) values (−1.7 to −4.0), similar Pb and enriched zircon Hf isotopic compositions [ε_Hf(t) of 1.5–9.7], possibly suggesting more contribution from southern Qiangtang crust. Duolong volcanic rocks and granodiorite porphyries likely formed in a continental arc setting during northward subduction of the Bangong–Nujiang ocean and evolved at the base of the lower crust by MASH (melting, assimilation, storage and homogenization) processes. Copyright © 2014 John Wiley & Sons, Ltd.     

Late Triassic crustal growth in southern Tibet: Evidence from the Gangdese magmatic belt

The Gangdese magmatic belt, located in the southern margin of the Lhasa terrane and carrying significant copper and polymetallic mineralization, preserves important information relating to the tectonics associated with Indian–Eurasian collision and the crustal growth of southern Tibet. Here we investigate the Quxu batholith in the central domain of the Gangdese magmatic belt and report the occurrence of hornblende gabbros for the first time. We present petrologic, zircon U–Pb–Hf isotopic and bulk-rock chemistry data on these rocks. The hornblende gabbros display sub-alkaline features, and correspond to tholeiite composition. They also show medium K calc-alkaline to low K affinity. The rocks show enrichment in LILEs and LREEs, but are depleted in HFSEs, indicating a subduction-related active continental margin setting for the magma genesis. Our computations show that the gabbroic pluton was emplaced in the middle-lower crustal depth of ca. 18 km. Zircons from the hornblende gabbros yield crystallization age of ca. 210 Ma, revealing a late Triassic magmatic event. Combined with available data from the Gangdese magmatic belt, our study suggests that the northward subduction of the Neo-Tethys oceanic crust beneath the southern margin of the Lhasa terrane might have been initiated not later than the Norian period of Triassic. Zircons from the hornblende gabbro show positive ε_Hf(t) values of 9.56 to 14.75 (mean value 12.44), corresponding to single stage model ages (T_DM1) in the range of 256 Ma to 459 Ma, attesting to crustal growth in the southern Lhasa terrane associated with the subduction of the Neo-Tethys oceanic crust.     

Early Neoproterozoic tectonic evolution of the Erguna Terrane (NE China) and its paleogeographic location in Rodinia supercontinent: Insights from magmatic and sedimentary record

Owing to the lack of early Neoproterozoic geological and geochronological data, most Rodinia supercontinent reconstruction models do not include the Amuria Block in the Central Asian Orogenic Belt (CAOB), and the Amuria Block was varying attributed to the North China, Siberian or Tarim tectonic affinities. In this study, we identified one early Neoproterozoic granitic pluton (964–947 Ma) and one early Neoproterozoic sedimentary unit (<906 Ma) in the Erguna Terrane. The samples (964–947 Ma) are I-type granitoids, and show high zircon in-situ ε_Hf(t) (−2.1–10.0) and whole-rock ε_Hf(t) (1.4–4.8) and high ε_Nd(t) (−2.3 to −0.8). These granitoids are characterized by high Zr saturation temperature (T_Zr) (701–835 °C) and no inherited zircons, suggesting high-degree of partial melting of their source rocks. The granites were likely formed by biotite-/muscovite dehydration melting of subalkaline mafic lower crust in a continental arc setting. Detrital zircons of the sandstone sample define an age peak at 923–906 Ma. Early Neoproterozoic age data compilation from the four Amuria microcontinents (i.e., Erguna, Xing'an, Songnen and Jiamusi terranes) in NE China indicate the presence of two major magmatic flare-ups at 964–880 Ma and 850–740 Ma. Considering that early Neoproterozoic magmatic rocks are absent in the Siberian and North China cratons but widespread in the Tarim Craton, we suggested that the Erguna Terrane was part of the Tarim Craton in the Early Neoproterozoic. The Erguna Terrane may have undergone a two-staged Neoproterozoic tectonic evolutionary history: (1) early Neoproterozoic arc accretion in response to the Rodinia assembly, and (2) middle Neoproterozoic break-away from the SW Tarim Craton associated with the Rodinia breakup.     

Geochronological and geochemical constraints on the genesis of the Tieshan complex in the Edong district,Eastern China with implications for Fe and Cu skarn mineralization

Fe and Cu skarn deposits constitute the most important skarn type worldwide, whereas the controlling factors that lead to the difference in metal associations remain not well known. The Fe- and Cu-hosting Tieshan complex in the Edong district provides a good opportunity for comparative study on the genetic differences between Fe and Cu skarn deposits. In this study, integrated studies of geochronology, geochemistry and Sr-Nd-Hf isotope compositions were conducted on the complex. LA-ICP-MS zircon U-Pb dating results show that the Tieshan complex was emplaced in the time interval of 135 ± 3 to 144 ± 1 Ma. Multiphase rocks from the complex can be broadly subdivided into two suites. The Fe-Cu-related suite, which consists of diopside diorite, quartz diorite, quartz diorite porphyrite and porphyritic granodiorite, possesses low SiO₂ (53.5–67.1 wt.%), K₂O (2.44–3.53 wt.%) and Rb (45−83 ppm) contents, but high Sr (1132−2684 ppm), Ba (1073−1656 ppm) contents and negligible Eu anomalies, with very high Sr/Y (>90) ratios, similar to typical high Ba-Sr granitoids. The rock suite has initial ⁸⁷Sr/⁸⁶Sr values of 0.70648 to 0.70737, ε_Nd(t) values of −12.3 to −8.2 and ε_Hf(t) values of −16 to −7, comparable to values of the Early Cretaceous mafic rocks in the Edong district and adjacent areas, indicating that it might be largely derived from an enriched lithospheric mantle source, along with minor involvement of lower-crustal components. By contrast, the Fe-related suite, which is composed of quartz diorite, quartz diorite porphyrite and granodiorite porphyry, is characterized by relatively high SiO₂ (63.0–71.0 wt.%) and K₂O contents (3.36–5.53 wt.%), and a wide range of Sr (158−1135 ppm), Ba (762−1366 ppm) contents and Sr/Y (11–99) ratios. In combination with the presence of abundant inherited zircon grains, the lower ε_Nd(t) (−12.4 to −9.3) and ε_Hf(t) (−25 to −15) values indicate a greater degree of lower-crustal contribution for the Fe-related suite. In addition, the calculated zircon Ce (Ce/Ce* and Ce⁴⁺/Ce³⁺) and Eu (Eu/Eu*) anomalies suggest that the Fe-Cu-related suite has much higher oxygen fugacity (fO₂) than the Fe-related suite. This study highlights fO₂ and fractionation degree of magma as useful indicators for differentiating Fe and Cu skarn mineralization.     

In situ zircon U–Pb and Hf–O isotopic results for ca. 73 Ma granite in Hainan Island: Implications for the termination of an Andean-type active continental margin in southeast China

We report in the paper integrated analyses of in situ zircon U–Pb ages, Hf–O isotopes, whole-rock geochemistry and Sr–Nd isotopes for the Longlou granite in northern Hainan Island, southeast China. SIMS zircon U–Pb dating results yield a crystallization age of ∼73 Ma for the Longlou granite, which is the youngest granite recognized in southeast China. The granite rocks are characterized by high SiO₂ and K₂O, weakly peraluminous (A/CNK = 1.04–1.10), depletion in Sr, Ba and high field strength elements (HFSE) and enrichment in LREE and large ion lithophile elements (LILE). Chemical variations of the granite are dominated by fractional crystallization of feldspar, biotite, Ti–Fe oxides and apatite. Their whole-rock initial ⁸⁷Sr/⁸⁶Sr ratios (0.7073–0.7107) and ε_Nd(t) (−4.6 to −6.6) and zircon ε_Hf(t) (−5.0 to 0.8) values are broadly consistent with those of the Late Mesozoic granites in southeast China coast. Zircon δ¹⁸O values of 6.9–8.3‰ suggest insignificant involvement of supracrustal materials in the granites. These granites are likely generated by partial melting of medium- to high-K basaltic rocks in an active continental margin related to subduction of the Pacific plate. The ca. 73 Ma Longlou granite is broadly coeval with the Campanian (ca. 80–70 Ma) granitoid rocks in southwest Japan and South Korea, indicating that they might be formed along a common Andean-type active continental margin of east–southeast Asia. Tectonic transition from the Andean-type to the West Pacific-type continental margin of southeast China likely took place at ca.70 Ma, rather than ca. 90–85 Ma as previously thought.     

Early Paleozoic magmatism and metallogeny related to Proto-Tethys subduction: Insights from volcanic rocks in the northeastern Altyn Mountains,NW China

Remnants of the Proto-Tethys are mainly preserved in the region between south of the North China-Tarim Block and north of Qiangtang-Sibumasu/Baoshan Blocks. Magmatic-metallogenic events related to the Proto-Tethyan subductions were rarely reported, and the subduction history and polarity of the Proto-Tethyan are still under debate. Here, we presented new data of zircon UPb ages, whole-rock Sr–Nd–Pb isotopes, major and trace elements and zircon Hf isotopes for the volcanic rocks in the northeastern Altyn Mountains. Information of over 14 volcanic-hosted deposits/prospects in the region has been compiled. These volcanic ore hosts consist mainly of basaltic andesite, andesite, dacite and rhyolite rocks. The andesite and rhyolite rocks are newly zircon UPb dated to be Late Cambrian-Early Ordovician (andesite: 490.5 ± 5.2 Ma; rhyolite: 492.6 ± 2.9 Ma and 491.6 ± 5.6 Ma), representing the timing of volcanism and VMS (Volcanogenic Massive Sulfide) mineralization. All the volcanic rocks belong to the high-K calc-alkaline and shoshonite series: the andesite rocks from the Kaladawan area in north of the region display arc geochemical affinities and contain (⁸⁷Sr/⁸⁶Sr)_i (0.7082–0.7083) and ε_Nd(t) (−9.7 to −7.6), indicating that they were likely formed by partial melting of the mantle wedge with subducted sediment inputs. The rhyolite rocks from the Kaladaban area in south of the region are characterized by high SiO₂ (64.46–78.55 wt%), low alkali (Na₂O + K₂O, 3.46–7.17 wt%), and contain (⁸⁷Sr/⁸⁶Sr)_i (0.7063–0.7095), ε_Nd(t) (−6.6 to −1.5), and zircon ε_Hf(t) (−5.5 to 5.4), indicating that they were likely derived from partial melting of the lower crust with depleted mantle inputs. Rock assemblage and geochemistry suggest that volcanic rocks in the northeastern Altyn Mountains may have formed in a continental arc setting. Their spatial distributions with respect to the ophiolites in the region suggest that the subduction was likely south-dipping. This subduction-related arc magmatism may have formed the many important VMS and porphyry–skarn deposits in the region.     

Detrital zircon U–Pb geochronology,trace-element and Hf isotope geochemistry of the metasedimentary rocks in the Eastern Himalayan syntaxis: Tectonic and paleogeographic implications

The origin of the Greater Himalayan Sequence in the Himalaya and the paleogeographic position of the Lhasa terrane within Gondwanaland remain controversial. In the Eastern Himalayan syntaxis, the basement complexes of the northeastern Indian plate (Namche Barwa Complex) and the South Lhasa terrane (Nyingchi Complex) can be studied to explore these issues. Detrital zircons from the metasedimentary rocks in the Namche Barwa Complex and Nyingchi Complex yield similar U–Pb age spectra, with major age populations of 1.00–1.20 Ga, 1.30–1.45 Ga, 1.50–1.65 Ga and 1.70–1.80 Ga. The maximum depositional ages for their sedimentary protoliths are ~ 1.0 Ga based on the mean ages of the youngest three detrital zircons. Their minimum depositional ages are ~ 477 Ma for the Namche Barwa Complex and ~ 499 Ma for the Nyingchi Complex. Detrital zircons from the Namche Barwa Complex and Nyingchi Complex also display similar trace-element signatures and Hf isotopic composition, indicating that they were derived from common provenance. The trace-element signatures of 1.30–1.45 Ga detrital zircons indicate that the 1.3–1.5 Ga alkalic and mafic rocks belt in the southeastern India is a potential provenance. Most 1.50–1.65 Ga zircons have positive ε_Hf(t) values (+ 1.2 to + 9.0), and most 1.70–1.80 Ga zircons have negative ε_Hf(t) values (− 7.1 to − 1.9), which are compatible with those of the Paleo- to Mesoproterozoic orthogneisses in the Namche Barwa Complex. Provenance analysis indicates that the southern Indian Shield, South Lhasa terrane and probably Eastern Antarctica were the potential detrital sources. Combined with previous studies, our results suggest that: (1) the Namche Barwa Complex is the northeastern extension of the Greater Himalaya Sequence; (2) the metasedimentary rocks in the Namche Barwa Complex represent distal deposits of the northern Indian margin relative to the Lesser Himalaya; (3) the South Lhasa terrane was tectonically linked to northern India before the Cambrian.     

First discovery of a Palaeoproterozoic A-type granite in southern Wuyishan terrane,Cathaysia Block: evidence from geochronology,geochemistry, and Nd–Hf–O isotopes

In this study, a combined study of zircon U–Pb and Hf–O isotopes, as well as whole-rock major and trace elements and Nd isotopes has been conducted for Yangjia gneissic granite from the southern Wuyishan terrane, Southeast China, to constrain its petrogenesis and provide a new window for investigating the tectonic evolution of the Cathaysia basement. U–Pb dating for magmatic zircons yields a ²⁰⁷Pb/²⁰⁶Pb age of ca. 1.80 Ga, interpreted as the emplacement age of the Yangjia granite. The granites have relatively high K₂O, Rb, Ga, Zr, Nb, Y, and Ce contents and show low Al₂O₃, CaO, and Ba concentrations. Their 10,000*Ga/Al ratios range between 2.8 and 3.2. Zircons from the granite have ε_Hf(t) values ranging from ?13.2 to ?7.2, corresponding to T^Hf_DM2 model ages of 2.99 Ga to 2.72 Ga. The zircon δ¹⁸O values range between 6.7‰ and 9.1‰ with an average of 7.7‰. In addition, the whole-rock ε_Nd(t) values of the granites range from ?6.5 to ?5.4 and the T^Nd_DM2 model ages from 2.73 Ga to 2.82 Ga. All these geochemical and Nd–Hf–O isotopic signatures suggest an A-type affinity for the Yangjia granites, and they were likely generated by partial melting of Palaeoproterozoic parametamorphic rocks of the Wuyishan terrane in a post-collisional extensional setting. When our data is combined with existing geochronological data, it provides further evidence for the Palaeoproterozoic basement in the southern Wuyishan terrane, which records a rapid tectonic transition from post-collision to intraplate extension (1.80–1.77 Ga) related to the break-up of the supercontinent Columbia.     

Late Paleozoic magmatism and metallogenesis in the Aqishan-Yamansu belt,Eastern Tianshan: Constraints from the Bailingshan intrusive complex

The Aqishan-Yamansu belt in the Eastern Tianshan (NW China) contains many intermediate to felsic intrusive rocks and spatially and temporally associated Fe (-Cu) deposits. Zircon U-Pb dating of the Bailingshan granitoids, including diorite enclaves (in granodiorite), diorite, monzogranite and granodiorite, and andesitic tuff from the Shuanglong Fe-Cu deposit area yielded ages of 329.3 ± 2.1 Ma, 323.4 ± 2.6 Ma, 313.0 ± 2.0 Ma, 307.5 ± 1.7 Ma and 318.0 ± 2.0 Ma, respectively. These new ages, in combination with published data can be used to subdivide magmatism of the Bailingshan intrusive complex into three phases at ca. 329–323 Ma, ca. 318–313 Ma and ca. 308–297 Ma. Of the analyzed rocks of this study, the Shuanglong diorite enclave, diorite and andesitic tuff show calc-alkaline affinities, exhibiting LILE enrichment and HFSE depletion, with negative Nb and Ta anomalies. They have high MgO contents and Mg^# values, with depleted ε_Hf(t) and positive ε_Nd(t) values, similar crustal-derived Nb/Ta and Y/Nb ratios, low Th/Yb and Th/Nb, and high Ba/La ratios, which are consistent with them being sourced from a depleted mantle wedge metasomatized by slab-derived fluids and crustal contamination. However, the monzogranite and granodiorite are metaluminous with characteristics of low- to high-K calc-alkaline I-type granites. The granitic rocks are enriched in LILE, depleted in HFSE and have significant Eu anomalies, with high Y contents and low Sr/Y ratios, resembling typical of normal arc magmas. Depleted ε_Hf(t) and positive ε_Nd(t) values with corresponding young T_DM^C ages of zircons, as well as Nb/Ta, Y/Nb, Th/U and La/Yb ratios suggest that the granitic rocks were probably formed by re-melting of juvenile lower crust or pre-existing mantle-derived mafic–intermediate igneous rocks. Integrating published data, we conclude that the Bailingshan granitoids (excluding the Shuanglong diorite and diorite enclave) were derived from re-melting of juvenile lower crust and mantle-derived mafic–intermediate igneous rocks, with mantle components playing a more prominent role in the formation of the younger and more felsic rocks. A comprehensive review, including our new data, suggests that the Aqishan-Yamansu belt formed as a fore-arc basin during the Carboniferous (ca. 350–300 Ma) when the Kangguer oceanic slab subducted beneath the Yili-Central Tianshan block. The ongoing southward subduction of the slab resulted in the closure of the Aqishan-Yamansu fore-arc basin (ca. 320–300 Ma), due to slab steepening and rollback followed by slab breakoff and rebound. During the Aqishan-Yamansu fore-arc basin inversion, the main phase of the Bailingshan granitoids emplaced in the Aqishan-Yamansu belt, accompanied by contemporary Fe and Fe-Cu mineralization.     

Generation of leucogranites via fractional crystallization: A case from the Late Triassic Luoza batholith in the Lhasa Terrane,southern Tibet

Leucogranites are important in understanding intracrustal differentiation and regional tectonic evolution, but how these rocks form remains a matter of much debate. The Luoza batholith in the central Lhasa subterrane provides an important opportunity to address these issues as this batholith consists of normal calc-alkaline S-type granodiorites (Group 1) and highly fractionated S-type leucogranites (Group 2). Laser Ablation Inductively Coupled Plasma Mass Spectrometry zircon U-Pb dating indicates that these rocks were emplaced at ca. 221–205 Ma. The Group 1 samples (ca. 205 Ma) have low SiO₂ content (64–67 wt%) and low differentiation index values (DI = 75–76), and are slightly depleted in Ba, P, Sr, and Ti compared to the Group 2 samples (ca. 221 Ma and 213 Ma), which have high SiO₂ content (74–75 wt%), high differentiation index values (91–94), and are significantly depleted in Ba, P, Sr, and Ti. Samples from Group 1 and Group 2 exhibit similar zircon ε_Hf(t) values (−15.6 to −1.4), both of which are significantly higher than melts from the pure ancient basement in the central Lhasa subterrane (−20.5 to −10.6). The Group 1 samples can be interpreted as having been derived from the partial melting of metagreywacke within the ancient basement in the central Lhasa subterrane with contributions from mantle- or juvenile- crust-derived components, whereas the Group 2 samples may have formed via varying degrees of fractional crystallization of different minerals (e.g., plagioclase ± K-feldspar, biotite, zircon, allanite/monazite, titanite, and apatite) from the parental magmas, represented by the granodioritic samples in Group 1, in shallow crustal magma chambers. In combination with sedimentary records, the high-temperature and low-pressure conditions indicated by the Group 1 samples can be geodynamically linked to a back-arc extensional setting in response to the southward subduction of the Bangong-Nujiang Tethys Ocean seafloor. Our work implies that fractional crystallization is likely a feasible mechanism for the development of leucogranites and for explaining the compositional diversity of the granite during intracrustal differentiation.     

Lithium isotopic evidence for subduction of the Indian lower crust beneath southern Tibet

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.     

Late Triassic post-collisional granites related to Paleotethyan evolution in northwestern Lao PDR: Geochronological and geochemical evidence

The late Paleozoic to early Mesozoic granites exposed in northwestern Lao PDR provide important constraints on the tectonic evolution of the Eastern Paleotethyan Ocean and regional correlation with the giant granitic belt in Southeast Asia. New geochronological data show that the granites have Late Triassic zircon U–Pb ages of 231–220 Ma. They are dominated by monzogranite and biotite granite with an I-type geochemical affinity. These granites are enriched in LREEs and LILEs and depleted in HFSEs. The geochemical variations for these granites indicate the fractional crystallization of plagioclase, K-feldspar, biotite, apatite, and Fe–Ti oxides. Their initial ⁸⁷Sr/⁸⁶Sr ratios range from 0.7021 to 0.7105 and ε_Nd (t) values from −1.6 to −7.3. Zircon in-situ ε_Hf (t) values are in range of −6.0–+5.3 (peaks at −2.8 and +2.5, respectively), Hf model ages of 0.67–1.64 Ga (peaks at 0.83 Ga and 1.45 Ga, respectively), and δ¹⁸O values of 6.6‰–8.0‰, similar to the Late Triassic granitoids from the Eastern Province in Southeast Asia. These granites originated from a mixed source of ancient metamorphic rocks with juvenile mafic crust. The Late Triassic granites in northwestern Lao PDR formed in a post-collisional setting in response to the thickened crustal collapse during the assembly of the Sibumasu with Indochina blocks. These granites can southerly link with the Eastern granite province of the Eastern Paleotethyan Domain in Southeast Asia.     

Geochemical variations of the Late Mesozoic granitoids in the southern margin of North China Craton: A possible link to the tectonic transformation from compression to extension

The composition of the continental crust of the North China Craton (NCC) is more felsic than that of the average bulk crust, which is regarded to be the result of the delamination of the thickened lower crust during Mesozoic. However, whether the thickened continental crust existed and when the delamination event happened along the southern margin of the NCC are still debated. Here, we report geochronological, geochemical and Sr-Nd-Hf-Pb isotopic evidence that granitoids from the Late Jurassic Wuzhangshan pluton and the Early Cretaceous Huashani complex were derived by partial melting of the lower crust with different thickness. Our new data shows that the two lithofacies of the Wuzhangshan pluton were mainly formed between ca. 157 and 156 Ma, whereas the five lithofacies of the Huashani complex were mainly emplaced between ca. 132 and 125 Ma. The Wuzhangshan pluton and the earlier four lithofacies granitoids of the Huashani complex (ca. 160–125 Ma) both display adakitic geochemical features, which are characterized by as high SiO₂ (63.26–72.71 wt%), Al₂O₃ (13.97–16.89 wt%) and Sr (413–1218 ppm) contents, and low Y (6.30–14.98 ppm) and Yb_N (1.55–4.43), and high Sr/Y (33−112) and (La/Yb)_N (11.53–29.72) ratios. They also have high (⁸⁷Sr/⁸⁶Sr)i (0.7066–0.7086), and low εNd(t) (−9.9 to −18.8) and εHf(t) (−11 to −26) values, and two-stage Nd and Hf model ages ranging from 2.4 to 1.7 Ga and 2.7 to 1.7 Ga, respectively. In contrast, the late Early Cretaceous (ca. 125–110 Ma) granitoids have higher SiO₂ (71.30–76.78 wt%) and lower Sr (64–333 ppm) contents, and lower Sr/Y (17–29) and (La/Yb)_N (13.25–18.36) ratios, and similar εNd(t) (−10 to −16) and relatively higher εHf(t) (−10 to −14) values. These geochemical variations suggest that the ca.160–125 Ma granitoids were most probably produced by partial melting of thickened crust (>45 km) with eclogite, garnet amphibolite or amphibolite residues, whereas that the ca. 125–110 Ma granitoids were formed by partial melting of the thinner crust (<33 km). We thus suggest that the NCC likely underwent a synchronous tectonic transformation at ca. 125 Ma from a compressional setting with thickened crust to an intensive extensional setting with thinner crust at ca. 125 Ma, which demonstrates that the lower crust was most likely delaminated.     

Geochronology and geochemistry of igneous rocks in the Bailingshan area: Implications for the tectonic setting of late Paleozoic magmatism and iron skarn mineralization in the eastern Tianshan,NW China

The late Paleozoic Bailingshan intrusions and volcanic rocks are located in the Aqishan–Yamansu arc belt in the southern part of the eastern Tianshan and are associated with an important group of iron skarn deposits. The exposed intrusive rocks are mainly granodiorite, monzonitic granite, and granite. Zircon U–Pb dating of the Tugutublak Formation tuffaceous dacitic lava yields an age of 324 Ma, whereas dates of the Bailingshan granodiorite, monzonitic granite, and granite yields ages of 317 Ma, 313 Ma, and 307 Ma, respectively. The results indicate that the Bailingshan granitoids were emplaced soon after the eruption of the Tugutublak dacite. All these rocks studied show calc-alkaline to high-K calc-alkaline and metaluminous affinities, with A/CNK values ranging 0.83–1.10. They are enriched in Rb, K, and Pb, depleted in Nb, Ta, Ti, and P, and contain low Sr/Y (4.16–23.7) and Sr (109.0–347.0 ppm) values, displaying typical arc geochemical affinities. The tuffaceous dacitic lava has low Nb/Ta (10.3–14.1) values, a wide range of Mg^# (6–64), positive zircon ε_Hf(t) (3.2–7.5) values, and elevated whole-rock ε_Nd(t) (2.03–4.41), but low I_Sr values (0.70427–0.70530), indicating that the source magma may have been derived from the juvenile lower crust with minor mantle input. The Bailingshan I-type intrusions also exhibit a mixed source signal, as constrained by Nb/Ta ratios, Mg^#, and isotopes characteristics. Because the granodiorite, monzonitic granite, and granite intrusions have higher zircon ε_Hf(t) (3.3–7.5, 11.8–13.5, and 10.2–14.4, respectively) and ε_Nd(t) (3.90, 5.78, and 5.94, respectively) values than those of the tuffaceous dacitic lava, it is suggested that mantle-derived materials may have played a more prominent role with their petrogenetic evolution. Integrating our new geological, age, geochemical and isotopic data we propose that the Aqishan–Yamansu iron skarn belt may have formed in a back-arc position or within an intra-arc basin generated by the southward subduction of the Kanggur oceanic plate beneath the Yili–Central Tianshan block during the late Paleozoic, with felsic-intermediate magmatism occurring during the basin inversion.     

The Sr,Nd, and Hf isotopic geochemistry of rocks of the gabbro–diorite–tonalite association from the Eastern Segment of the Middle Urals as an indicator of the age of the continental crust in this area

According to isotopic analysis of rocks of the Reft gabbro–diorite–tonalite complex (Middle Urals), gabbro and related diorite and dikes and vein-shaped bodies of plagiogranitoids, crosscutting gabbro, are similar to the depleted mantle substance in ε_Nd(T) = 8.6–9.7 and ε_Hf(T) = 15.9–17.9. Their model Hf ages are correlated with the time of crystallization. Here, the tonalites and quartz diorites constituting most of the Reft massif are characterized by lower values: ε_Nd(T) = 3.7–6.0, ε_Hf(T) = 11.1–12.7, and T _DM values significantly exceeding the age datings. This is evidence that Neoproterozoic crustal rocks were a source of parental magma for these rocks. The primary ⁸⁷Sr/⁸⁶Sr ratio in rocks of both groups is highly variable (0.70348–0.70495). The data obtained allow us to reach the conclusion that the Reft gabbro–diorite–tonalite complex was formed as a result of nearly synchronous processes occurring in the crust and the mantle within a limited area.