Triassic Granites Research Articles

Extensional settings favour initial REE enrichment in the parent granites of ion-adsorption REE deposits: implications from Late Permian to Triassic granites in South China

To understand the generation of ion-adsorption REE deposits it is important to clarify the mechanism of REE enrichment in granites. We investigated Late Permian to Triassic granites from South China to investigate these issues. Ore-related granites with high REE contents (192–467 ppm) intruded in the Late Permian have high K 2 O and Zr + Ce + Nb + Y contents, high Ga/Al and K/Na ratios and low CaO contents, and are classified as aluminous A-type granite derived from melting of felsic igneous rocks in a high-temperature extensional setting. Ore-unrelated Triassic Pingtian granite with relatively low REE contents (81.7–127 ppm) is peraluminous with occurrence of muscovite, indicating an S-type affinity. Its high Rb/Sr and low CaO/Na 2 O ratios indicate magma origin from a metapelitic source related to a compressional setting owing to continental collision. In extensional settings in South China, the high-temperature conditions promoting melting of REE-rich accessory minerals and F-rich conditions improving the solubility of REE in the melts could be the reasons for the initial REE enrichment in these granites. Given that parent rocks for ion-adsorption REE deposits are all generated in extensional settings, it is considered that multiple tectonic stages and long-term extensional setting in South China contributed to generation of the specific ion-adsorption REE deposits in South China.

Mineral chemistry and geothermobarometry of metasedimentary rocks of Central Menderes Massif, Western Türkiye: Metamorphic evolution and source of metapelitic rocks

The Menderes Massif (MM) is the largest metamorphic massif of Türkiye. This study was carried out to determine the origins, metamorphic conditions and age of garnet-mica schists covering large areas in the Central Menderes Massif (CMM). Detrital zircon dating of a garnet-mica schist derived from a quartzofeldspathic source measured the youngest zircon age to be 450 ± 61 Ma (102 % concordant, Th/U = 0.02) Ma and the oldest zircon age to be 1529 ± 18 Ma (97 % concordant, Th/U = 0.02) Ma. The ƐHf(t) values of the selected zircon grains vary between −27.42 and −3.43, and the 176Lu/177Hf isotope values vary between 0.2820 and 0.2827, and these values are consistent with the values obtained from the orthogneisses and paragneisses of the massif. The presence of five Ordovician zircon grains in garnet-mica schists and the intrusive contact with Triassic granites indicate that the source rock of the schists may have been deposited in the Late Ordovician - Triassic interval. Mineral chemical analyses show that garnets are almandine, biotites are Fe-biotite and white micas are muscovite-phengite in composition, and they are affected by metamorphism (greenschist - lower amphibolite facies) under ∼5 kbar pressure and ∼600 °C temperature conditions. For the first time, rutile UPb dating shows that garnet-mica schists underwent metamorphism 43.1 ± 6.1 Ma ago. PT conditions and metamorphic ages obtained from garnet-mica schists show that the overlying units underwent metamorphism in the greenschist-lower amphibolite facies in the Lutetian (Eocene) period.

Late Mesozoic ductile deformation and exhumation along the Shangdan suture zone in the Qinling Orogenic Belt, China

The Qinling Orogenic Belt (QOB) is a multi‐stage orogenic belt recording subduction and collision processes. The ductile shearing deformation with different properties and ages is developed on the boundary of the different litho‐tectonic units, which can be used to decipher the tectonic evolution of the QOB. Previous studies mainly concentrated on the pre‐Late Mesozoic shearing deformation, while the Late Mesozoic shearing deformation received less attention. It restricts the understanding of the Late Mesozoic tectonic evolution of the QOB. Hence, we conducted detailed field structural analysis and laboratory analysis of the Shagou shear zone (SGSZ) in the middle part of the Shangdan suture zone (SDSZ). The SGSZ, developed in the southern margin of the Baliping pluton, is an S–SSW‐dipping sinistral strike‐slip ductile shear zone with a top‐to‐the‐ESE shear sense. The mylonite protoliths are voluminous Late Triassic granites (200–215 Ma) and a small amount of Neoproterozoic granites (759 Ma) and Early Cretaceous granites (145 Ma). Microstructures and geothermobarometer results suggest that the ductile shearing deformation occurred under high greenschist–low amphibolite‐facies conditions (400–650°C) at middle crustal depths (13–21 km). Zircon U–Pb and amphibole‐biotite 40Ar/39Ar dating results constrain the timing of shearing deformation to 135–145 Ma and the rapid exhumation of the SGSZ to 115–135 Ma. Combining with the regional data, we consider that the sinistral strike‐slip shearing deformation in the SGSZ controls and accommodates the south‐eastward extrusion of the East SQB under the intracontinental compression. Subsequently, the intracontinental extension led to the rapid exhumation of the SGSZ.

Linking beryllium mineralization to fluid-rock reactions: A case study of the Madumeng quartz vein-type beryllium mineralization in the Southern Great Xing'an Range, Northeast China

The Southern Great Xing'an Range (SGXR) in Northeast China is a significant polymetallic mineralization belt. Although several beryllium (Be) deposits have been reported in the SGXR, the timing and mechanisms of Be mineralization are poorly understood. To determine the mechanisms of newly-discovered Be mineralization in the Madumeng area, we carried out monazite–zircon–apatite U–Pb dating, whole-rock and mineral geochemical, and monazite Nd isotopic analyses of the beryl-bearing quartz veins and host Late Permian–Early Triassic granite. Monazite from the beryl-bearing quartz veins yielded U–Pb ages of 138–137 Ma, and the host granite yielded zircon and apatite U–Pb ages of 251 ± 1 and 256 ± 27 Ma, which indicate that the Madumeng Be mineralization formed later than the host granite. Monazite in the beryl-bearing veins has similar εNd(t) values (−1.31 to +4.27) to those of Early Cretaceous highly fractionated granite related to Be mineralization in the SGXR, which suggests the ore-forming material was derived from exsolved fluids associated with Early Cretaceous, deep-seated, highly fractionated granites. Fluid-rock reactions had a key role in generating the Madumeng Be mineralization, and the intensity of these reactions was recorded by the whole-rock and mineral geochemistry. The alteration during fluid-rock reactions increased the pH and Ca contents of the fluids. Mica and beryl record the release of Ca and Fe from the granite into the fluid. The released Ca2+ ions combined with F− ions in the fluid, leading to instability of Be–F complexes and subsequent precipitation of beryl. Our findings indicate that the Early Cretaceous was an important period of Be mineralization in the SGXR and highlight the contribution of fluid-rock reactions to Be precipitation in quartz vein-type Be mineralization.

Genesis of W mineralization in the Yangla Cu-polymetallic deposit (NW Yunnan, China): Constraints from scheelite microstructure, trace element, U-Pb dating and Sr isotope geochemistry

The Yangla Cu-polymetallic deposit is the largest skarn Cu deposit in the Jinshajiang suture zone. Recently, an new W orebody was identified in the depths of the Cu orebodies. However, the genetic relationship between W and Cu orebodies, along with the tungsten ore-forming processes, remains unclear. This study delves into the careful investigation of three types of scheelites focusing on microstructure, trace element compositions and Sr isotope geochemistry. The aim is to elucidate the source and evolution of ore-forming fluids and ore genesis in the multi-episodic metallogenic systems. Primary scheelite (Sch-I) generally displays oscillatory zoning in CL images with the lowest REE (3.8–142 ppm) and highest Sr (604–1480 ppm) contents. Secondary scheelite (Sch-III), commonly precipitated as overprinting rims on primary crystal, has the highest REE (145–1071 ppm) and lowest Sr (376–793 ppm) contents. In contrast, Sch-II represents a transitional stage from primary to secondary, with REE and Sr contents at intermediate levels. Sch-I is characterized by LREE enrichment and a positive Eu anomaly in the chondrite-normalized REE patterns, whereas Sch-III shows MREE-rich patterns with a weak positive Eu anomaly. Scheelite U-Pb dating (30.3 ± 1.5 Ma) indicates that the W mineralization is not genetically related to Triassic granitoids in the Yangla district. Instead, it is likely associated with magmatic activities and tectono-thermal events in the Jinshajiang fault zone during the Cenozoic. The high initial 87Sr/86Sr ratios (0.71771–0.72229) in the scheelite, significantly higher than those of Triassic granites and ore-hosting marble, suggest that the geochemical features of Sch-I are inherited from the parental ore-forming fluids. These fluids are likely derived from the mixed hydrothermal fluids involved by the Neoproterozoic strata and/or concealed magmatism.

The Petrographic Characteristics and U-Pb Zircon Isotope Age of Felsic Extrusive Rocks in the Western Thanh Hoa Area, Song Ma Suture Zone

Felsic extrusive rocks are widely distributed in the western region of Thanh Hoa, primarily comprising rhyolite and rhyolitic tuff formations. They show porphyritic textures, and are composed of 15–25% phenocrysts and 75–85% groundmass. Phenocrysts are mainly euhedral–subhedral plagioclase (5-10 vol.%), K-feldspar (5-10 vol.%) and quartz (5-10 vol.%). The groundmass consists of plagioclase (30-45 vol.%), K-feldspar (25-40 vol.%), quartz (30-40 vol.%), and minor biotite with random distribution. In terms of geological context, these extrusive rocks overlie Silurian-Devonian sequences of sandstone and siltstone interbedded with shale and limestone lenses, and were intruded by the Early Triassic granite of the Song Ma complex. Zircons selected from rhyolite were LA-ICP-MS U-Pb dated, yielding an age of 253 Ma. This age is the timing of the magma crystallization, corresponding to the Late Permian - Early Triassic Indosinian tectonomagmatic event. These findings together with historical study results, prove that the Indosinian magmatic event widely occurred in the Truong Son belt, the Song Ma domain and the Kon Tum area. The eruption of the rhyolite is linked to the amalgamation between the South China and Indochina blocks.

Petrogenesis of the Late Triassic Habake granitic pluton, the West Kunlun Orogen, Western China: Tectonic implications for the evolution of the Paleo-Tethys

Recent discoveries of rare metal pegmatite deposits in the West Kunlun region are believed to be related to Triassic granites. However, the tectonic background of the mineralization in this region is currently unclear. Additionally, the mineralization potential of the granite must be determined to assess whether further exploration is warranted. This paper reports whole-rock elemental and Sr-Nd isotopic compositions, and zircon U-Pb ages and Lu-Hf isotopic compositions for the Habake granitic pluton. The zircon U–Pb dating revealed that these rocks formed in the Late Triassic (210–205 Ma). The two-mica granites are strongly peraluminous high-K calc-alkaline series rocks, with initial 87Sr/86Sr ratios of 0.71294 ∼ 0.71428, εNd(t) values of −7.79 ∼ -8.15, and εHf(t) values of −1.77 ∼ −32.22; these values are similar to those of Triassic BaryanHar Group graywacke rocks. The other granites are weakly peraluminous high-K calc-alkaline series rocks, with initial 87Sr/86Sr ratios of 0.70820 ∼ 0.70823 and 0.70942 ∼ 0.71041, εNd(t) of −3.81 ∼ -4.11 and −5.33 ∼ -5.40, and εHf(t) of −0.90 to +7.02 and −1.39 to −7.68, indicating that the granodiorite and biotite granite were respectively derived from partial melting of the juvenile crust and older lower crust in continental collision stage. Our new data show that the end of the continental collision between the South Kunlun Terrane (SKT) and Tianshuihai terrane (TST) likely occurred after 209 Ma. After analysis of the ore-forming parent rocks of the rare metal pegmatites, the graywacke-derived Habake two-mica granites, has lower Li contents that too low(<150 ppm), indicating that they did not form the rare metal pegmatites.

Crustal evolution of Paleozoic-Mesozoic granitoid in Dakrong-A Luoi area, Truong Son belt, central Vietnam: evidence from zircon U-Pb geochronology, geochemistry, and Hf isotope composition

ABSTRACT The Dakrong-A Luoi area is located in the South Truong Son belt in the Indochina block and has been affected by multiple stages of magmatic-metamorphic events. In this paper, Paleozoic-Mesozoic granitoids from the Dakrong-A Luoi area were studied using LA ICP-MS U-Pb zircon geochronologies, Hf isotopes on zircon and whole-rock geochemistry of the Paleozoic to examine the tectonic-magmatic history of the Southern Truong Son belt. The magmatism history of the area can be divided into three phases, at ~ 452 Ma (Ordovician), ~259 Ma (Permian), and ~ 243 Ma (Triassic). The Ordovician granites were exposed in the narrow belt of NW-SE trending, while the later two granite phases were exposed in E-W trending. The Ordovician granites show consistent LA-ICP-MS zircon ꜪHf(t) values from + 6.2 to + 8.3. Their corresponding Hf model ages (TDM2) from 0.9 Ga to 1.0 Ga suggest that the Ordovician granite was derived from the partial melting of ancient crustal rocks, which is related to the subduction of the Tam Ky-Phuoc Son (TKPS) ocean beneath the Truong Son belt. The Permian granites have Hf isotopic natures (ꜪHf(t)=-1.6 to 1.3, TDM2 = 1.2–1.3 Ga) of partial melting from ancient mantle components with minor Paleoproterozoic ancient continental crust The Triassic granites have negative ɛHf(t) values ranging from −11.9 to −7.0 and Hf model ages of 1.7–2.0 Ga suggesting that they were a product of partial melting from the Paleoproterozoic crust, corresponding to subduction/collision of the Paleo-Tethys ocean beneath the Indochina block along the Song Ma suture.

Geochemical characteristics and detrital zircon U-Pb ages of the Yimin Formation, Kelulun Depression, Hailar Basin and constraints on uranium mineralization.

The sandstone uranium deposits in the Kelulun Depression are the first commercially viable uranium deposits discovered in the Hailar Basin and the ore-bearing strata corresponding to the Lower Cretaceous Yimin Formation. However, the source of sedimentary matter, uranium source conditions, and uranium mineralization processes in the region have not been characterized. Accordingly, we analyzed the lithology, whole-rock geochemistry, zircon U-Pb ages, and trace elements of the Yimin Formation sandstones. The Yimin Formation sandstones were primarily composed of detrital grains with low compositional maturity. A geochemical analysis indicated that the parent rocks are felsic igneous rocks formed at an active continental margin with a moderately high degree of weathering. The detrital zircon U-Pb ages of the Yimin Formation 215-287 Ma with a peak at 230-260 Ma. Based on chronological, geochemical, and lithological data, we conclude that the Yimin Formation matter is derived from the Adunchulu Uplift on the western side of the Kelulun Depression and its parent rocks are Triassic granites. The Adunchulu uplift since the late Early Cretaceous and weathering and denudation of its uranium-rich granites provided ample matter and uranium. Therefore, the Kelulun Depression is a promising area for the exploration of sandstone uranium deposits.

Origin and tectonic setting of middle–late Triassic Lalingzaohuo granitoids in the western East Kunlun Orogen, northern Tibetan Plateau

ABSTRACT Triassic granitoids are widespread in the East Kunlun Orogen (EKO), with Middle Triassic granitoids being dominant and Late Triassic granitoids being subordinate. This paper presents new whole-rock geochemical and Sr – Nd isotope, and zircon U – Pb age and Lu – Hf isotope data for eight granitoids in the Lalingzaohuo area, western EKO, northern Tibetan Plateau. These granitoids yield Middle and Late Triassic ages of 245–239 and 223–220 Ma, respectively. The granitoids from the Lalingzaohuo area have features typical of I-type granites and belong to the high-K calc-alkaline series. They exhibit variable rare earth and trace element patterns, and Sr – Nd – Hf isotopic compositions, indicative of different petrogenetic processes. The Middle Triassic samples have higher ISr values of 0.71440–0.71572 and lower εNd(t) values of −6.18 to −4.26 than the Late Triassic samples (ISr = 0.71139–0.71348; εNd(t) = –3.23 to −1.51), which are consistent with I-type granitoids derived from the lower crust. Geochemical and isotopic data indicate that the two stages of I-type granitoids had a significant mantle contribution. The Middle Triassic granitoids in the EKO, particularly the western EKO, have adakitic characteristics and formed by partial melting of lower continental crust. The T2–T3 granites formed by oceanic slab breakoff related to subduction and mantle convection, and the Late Triassic granites formed by lithospheric thinning and asthenospheric upwelling after slab breakoff.

Subduction initiation of Paleo-Pacific Ocean and final closure of Paleo-Asian Ocean: constraints from Triassic granitoids in Jiamusi Block, NE China

ABSTRACT The process of transformation and superposition from the Paleo-Asian Ocean to the Paleo-Pacific Ocean tectonic systems have been a critical scientific issue. However, the final closure of the Paleo-Asian Ocean and the initial subduction time of the Paleo-Pacific Ocean, together with the conversion between them in Northeast China, still remain controversial. The Triassic granites in the Jiamusi Block are located in the key position of Northeast China, recording the conversion process of two tectonic domains. Through systematic geochronology and geochemical studies, it can provide important constraints for revealing the superposition and conversion mechanism of the Paleo-Asian Ocean and the Paleo-Pacific Ocean and the early subduction history of the Paleo-Pacific Ocean plate. In this study, the granites in Jiamusi Block were formed in the Early–Middle Triassic (250–245 Ma), characterized by high Si, rich alkali, poor Mg, and are obviously enriched in light rare earth elements and large ion lithophile elements, depleting in high field strength elements such as Nb, Ta, and Ti. They have typical characteristics of continental magmatic arc, deriving from partial melting of amphibolite facies and garnet amphibolite facies mafic lower crust. Combined with zircon Hf isotope results, zircon εHf(t) values are −6.52 to + 9.79, and the two-stage Hf model ages are 0.6–1.7 Ga, indicating that the magma should be derived from a relative younger crustal source, and some ancient materials may be added. Moreover, based on the spatial-temporal framework and Hf isotope characteristics of magmatism in Northeast China, we believe that the Jiamusi Block belonged to an exotic block. Meanwhile, combined with geochronological and geochemical data of igneous rocks on the Jiamusi Block, we suggest that the evolution of Jiamusi Block was dominated by the Paleo-Pacific Ocean rather than Paleo-Asian Ocean during the Triassic, which means the subduction of Paleo-Pacific Ocean plate initiated at least in Early Triassic.

Petrogenesis of Eocene Lamprophyre Dykes in Northern Qiangtang Terrane, Tibetan Plateau: Implications for the Tethyan Mantle Metasomatism and Tectonic Evolution

Post-collisional (ultra)potassic lamprophyre dykes are the key probes for understanding mantle metasomatism and reconstructing tectonic evolution. In this study, we present new petrological, geochronological, geochemical and zircon Lu-Hf isotopic data for lamprophyre dykes in the northern Qiangtang terrane (central Tibet), aiming to constrain their petrogenesis and geodynamic setting. The studied lamprophyres are minettes with phenocrysts of siderophyllite and phlogopite, which intrude into Triassic granite of 236.9 Ma. These lamprophyres yield zircon U-Pb ages of 39.7–40.9 Ma. They exhibit high contents of K2O (7.61–8.59 wt.%) and ultrapotassic features with high K2O/Na2O (11.43–14.38) ratios. They are characterized by increased values of Mg# (69.1 to 72.1) and high concentrations of compatible elements (e.g., Cr = 277–529 ppm, Ni = 232–322 ppm), which are diagnostic of mantle-derived primitive magma. The studied lamprophyres have a high abundance of rare earth elements (∑REE = 902–1061 ppm) with significantly fractionated REE patterns ((La/Yb)N = 66.3–100.6), and they are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), but depleted in high field strength elements (HFSE) (e.g., Nb, Ta and Ti) and heavy rare earth elements (HREE) with enriched zircon Hf isotopes (εHf(t) from −6.40 to 3.80). This indicates their derivation from an enriched mantle source which was metasomatized by subduction-related fluids and sediment-derived melts. A petrogenetic study suggests that the lamprophyres were generated by the partial melting of a phlogopite-bearing lherzolite within the garnet stability field. We propose that the Cenozoic ultrapotassic mafic rocks in the central Tibetan Plateau originated in the lithospheric mantle metasomatized by the subduction-related components, and are the magmatic response to the detachment of the subducted Tethyan slab.

Petrogenesis and geological implications of the qiyishan triassic granitoids in east Beishan orogen, inner Mongolia, NW China: evidence from geochronology, geochemistry and Nd-Hf isotopes

The Qiyishan deposit is a large-scale Rb polymetallic deposit in the Beishan orogen. However, there remain debates regarding its metallogenic age and tectonic setting. In addition, studies of Triassic tectono-magmatic events in the Beishan orogen are still insufficient, and conducting genesis studies on the Qiyishan Triassic granitoids will help to enhance the understanding of Triassic magmatism and tectonic evolution in the Beishan orogen. In this contribution, we report new data for the ore-forming granitoids of Qiyishan deposit, including zircon U-Pb ages, major and trace element concentrations and Nd-Hf isotope compositions to define the ages and genesis of the Qiyishan granitoids and discuss their origin and geodynamic implications. Zircon U-Pb dating of the Qiyishan ore-forming granitoids yielded three ages of 217.5 ± 1.3 Ma, 217.2 ±0.8 Ma, and 207.5 ± 2.0 Ma, respectively. The age of Rb mineralization can be constrained to 207.5± 2.0 Ma, while the age of W-Sn-Mo mineralization is considered to be slightly younger than approximately 217 Ma. The Characteristics of major and trace elements of the rock samples indicate that the Qiyishan granites can be classified to highly fractionated I-type granite, and characterised by a transition to A-type like granite to some extent. The granites were not only affected by fractional crystallisation, but also underwent magmatic-hydrothermal interaction. The zircon εHf(t) values of the Qiyishan granitoids ranged from 3.28 to 16.07 and the Hf model age (TDMc) ranged from 0.216 to 1.042 Ga, revealing that the Qiyishan granitoids originated from the partial melting of both mantle and crustal sources. εNd(t) values ranged from −0.52 to −0.25, with Nd model ages of 0.998 Ga to 1.007 Ga. These results indicate that the granitoids originated from the mantle-derived magmas intruding into the lower crust within an intracontinental extensional environment. Combining the previous studies of Triassic granites in the Beishan orogen and this work, the Triassic granites exhibit a transition from I-type to A-type along the northeast direction, indicating a decrease in the contribution of ancient crustal to the magma source. We propose that the Qiyishan granitoids formed in a transitional tectonic environment, signifying the shift from post-orogenic to intracontinental extensional settings in beishan orogen during late Triassic.

Investigating the Formation of Hot-Dry Rock in Gonghe Basin, Qinghai, China

The Gonghe Basin, Qinghai Province, China, has rich geothermal and hot-dry rock resources. Through a magnetotelluric survey line with 400 points, combined with regional geology data, the deep geoelectrical structural background and thermal source mechanisms of the Gonghe Basin were explored. The results showed that (1) a deep structure with high conductivity may exist at a depth of 15 km in the basin, and could be compared to the layer-shaped, low-velocity, high-conductivity structure in the eastern part of the Qinghai–Tibet Plateau; (2) the rushing reverse fault played a crucial role in heat control and conduction from the hot field; and (3) high-temperature heat storage existed, including four layers of geothermal resources. This study proposed a triple-polymorphism model of hot-dry rock in the area; that is, the high-conductivity layer in the Middle–Late Cenozoic crust was the principal heat source; the Middle–Late Triassic granite was the essential heat-storing body, as well as a parent rock to the hot-dry rock; and the Cenozoic sedimentary rock was the effective caprock. This model is critical to understanding geothermal causes, predicting geothermal resources, and planning, on the Qinghai–Tibet Plateau.

Three-Dimensional Electromagnetic Imaging of Geothermal System in Gonghe Basin

To better understand the geothermal system of the Gonghe Basin, we deployed 471 magnetotelluric survey points with an average distance of 2~3 km, covering the eastern and southern areas of the Basin. We used ModEM inversion software to carry out 3D inversion of 431 survey points and established a 3D-electrical model at a depth of 50 km in the area. The resistivity model shows that the low resistivity in the shallow part of the basin is related to the Cenozoic loose sedimentary cover, while the resistivity values of the mountains around the basin and the magmatic rock uplift zone are higher. The electrical model also shows that the high-conductivity layer is widely distributed in the middle and lower crust (15~35 km) of the basin, and direction of the high-conductivity layer is consistent with that of NW–SE fault in the basin. These high-conductivity layers may be the principal reason for the high heat flow values in the Gonghe Basin. Our resistivity model also shows that there is an obvious discontinuity between high- and low-resistivity blocks at different depths in the middle and upper crust. These discontinuities are consistent with the faults observed on the surface, which are related to the strong topographic relief. Our electrical model shows that these faults in the middle and upper crust are connected with the high-conductivity layer as the channel of heat transfer to the shallow part. Finally, the heat energy is enriched in the Triassic granite to form dry hot rock (HDR). The 3D-magnetotelluric imaging results depict the 3D-distribution characteristics of the geothermal system in the eastern and southern parts of the Gonghe Basin.

Genesis and exhumation of the Kongur-Muztaghata and Maeryang gneiss domes in NE Pamir since the Mesozoic

The Kongur-Muztaghata-Maeryang terrane in NE Pamir is considered to be the western extension of the Songpan-Ganze terrane located in the northern Tibetan Plateau. The Kongur-Muztaghata gneiss dome (KMGD) is situated in the north while the Maeryang gneiss dome (MYGD) is in the south. The KMGD comprises Triassic granites and granitic gneiss in the core and Early Paleozoic-Triassic sediments in the mantle that underwent Barrovian-type and Buchan-type metamorphisms. Based on geochemical and geochronological data, the Kongur-Muztaghata magmatic arc was formed around ∼252–204 Ma due to northward subduction of the Paleo-Tethys Jinsha oceanic slab. The collision of the Kongur-Muztaghata magmatic arc and the Qiangtang terrane occurred subsequently. Previous research suggested that the KMGD was formed in the Miocene (21–8 Ma). However, our new in-situ monazite U–Pb data for the mantled metasediment shows that the KMGD was initially formed at ∼198 Ma.The MYGD is comprised of an Early Paleozoic-Triassic metasediment mantle and a Cambrian anatexis complex core that underwent Barrovian-Buchan metamorphisms. Our new structural, geochemical, and geochronological data suggest that the protolith of the Maeryang orthogneiss was formed around ∼519-513 Ma, with the surrounding Early Paleozoic metavolcanic rocks erupted at ∼519-508 Ma. Together, they formed the Early Cambrian magmatic complex. In-situ U–Pb dating of monazites and zircon metamorphic rims for the Triassic metamorphic rocks in the mantle indicate that the Barrovian-Buchan metamorphism in the MYGD occurred around ∼206-187 Ma, likely caused by anatexis in the deep crust of the gneiss dome core. Thus, we propose that the KMGD and MYGD underwent a two-stage exhumation: the initial uplift during the Late Triassic-Early Jurassic thermo-tectonic event associated with the Cimmerian orogeny and the late rapid exhumation since the Miocene driven by the collision between the Eurasian and Indian plates.

Tectono-magmatic response to the geometric evolution of slab breakoff in the Paleo-Tethys Ocean: Constraints from Late Triassic granites in the Qiangtang block, northern Tibet

Slab breakoff can explain the postcollisional igneous and metamorphic events in many collisional orogens and triggers the transition from initial continental collision to deep subduction. However, the relationship between the deep-seated geometry of slab breakoff and shallow tectono-magmatic events remains unclear. In this study, we report new geochronological, geochemical, and Hf isotope data for granitic rocks from the Qiangtang block. Three granitic samples yielded zircon U-Pb ages of 226−214 Ma and mostly gave negative zircon εHf(t) values of −20.1 to +0.57. These granitoids are K-rich, calc-alkaline, magnesian, and metaluminous to weakly peraluminous. We propose that the granites were generated by partial melting of compositionally heterogeneous lower crust in the Southern Qiangtang block. Our new data, along with previously published data for igneous and metamorphic rocks in the Qiangtang block, suggest that the Longmu Co−Shuanghu suture zone represents a remnant of the closure of the Paleo-Tethys Ocean, and that the Late Triassic tectono-magmatic evolution was controlled by slab breakoff after continental collision. The contrasting magmatic and metamorphic ages show that the two stages of tectono-magmatic activity in the Late Triassic were due to a change in the subduction angle of the continental lithosphere from steep (225−215 Ma) to shallow (215−205 Ma) during slab breakoff. Furthermore, we infer that slab breakoff of the oceanic lithosphere caused extensive crustal melting and reworking and initiated far-field subduction. Our results provide new insights into the slab breakoff processes that control the geometry of subducting continental lithosphere and related postcollisional magmatic and metamorphic events in collision zones.

Late Triassic granites with mafic microenclaves in the East Kunlun Orogenic Belt, northwestern China: petrogenesis and implications for continental crust evolution and geodynamic evolution

The magmatic source of the Late Triassic granites in the East Kunlun Orogenic Belt (EKOB) is controversial, which restricts our understanding of crustal evolution and geodynamic evolution in the EKOB. Therefore, this study conducts zircon U‒Pb isotope dating, major and trace geochemical analysis, electron microprobe analysis of feldspar and amphibole, and in situ zircon Hf isotope analysis of Xiangride host granites and their mafic microgranular enclaves (MMEs) in the EKOB to reveal their magmatic source and geodynamic implications. Zircon U‒Pb dating shows that the Xiangride host granites and the MMEs crystallized simultaneously at ca. 228–223 Ma. The MMEs show an igneous texture, finer grain size, higher crystallization temperature and water content, and lower oxygen fugacity than those of the host granite, suggesting that they were probably derived from two distinct primitive magmas. Therefore, the MMEs are considered the products of magma mixing between granitic and mafic magmas. The Xiangride host granites show high Sr/Y (42.0–73.1) and (La/Yb)N (12.7–30.7) ratios, showing affinity with adakites derived from a thickened lower crust. Combined with the indistinguishable εHf(t) values (−3.47 to +0.08) from the coeval mafic rocks, it is concluded that they are derived from partial melting of juvenile thickened lower crust. The adakitic features of the Xiangride host granites and widespread coeval granites indicate the existence of a thickened lower crust before 228 Ma and delamination of the lower lithosphere is likely the geodynamic process resulting in the postcollisional extension regime.

Petrogenesis of the Triassic Sihaiping granite in the South Qinling orogen, central China: Implications for Mo-W mineralization

Late Triassic granites and related magmatic-hydrothermal Mo-W mineralization are common in the South Qinling orogen of central China, for which the Sihaiping granite pluton is a typical example. However, the petrogenesis of the Sihaiping granite remains enigmatic. Unraveling its petrogenesis can aid in our understanding of the Mo-W metallogeny in the South Qinling orogen. In this study, we present in situ zircon U-Pb ages, whole-rock geochemistry, and Sr-Nd-Hf isotopes of the Sihaiping granite. Zircon U-Pb dating by laser ablation inductively coupled plasma mass spectroscopy yielded Late Triassic ages of 206.6 ± 0.94, 206.5 ± 0.96, and 205.9 ± 0.75 Ma. The granite displayed high SiO2, K2O, and Rb contents, but low MgO, P2O5, TiO2, Sr, Ba, and Eu contents, indicating highly fractionated I-type granite affinity. The magma was interpreted to have been derived from Proterozoic crystalline basement in the South Qinling given its relatively low (87Sr/86Sr)i (0.704006 to 0.705039), negative εNd(t) (–4.9 to –3.5), and zircon εHf(t) (–6.2 to +1.6) values, with Nd model ages ranging from 1.39 to 1.27 Ga and two-stage Hf model ages ranging from 1.36 to 1.04 Ga. The strong fractional crystallization and suitable oxygen fugacity (ΔFMQ values mainly concentrated between +2 to +8) in the magmatic system played a major role in the Mo-W mineralization, indicating that the pluton has significant Mo(-W) metallogenic potential. Overall, the findings suggest that the Triassic Sihaiping granite and its related Mo-W mineralization likely formed in the transitional stage from syn- to post-collision between the South China Block and South Qinling along the Mianlue suture.

Contrasting origins of A-type granites in the Late Triassic-Early Jurassic Pitou complex, southern Jiangxi province: Implications for Mesozoic tectonic evolution in South China

The origin of A-type granites is purely circumstantial. Granitic complexes are quite common and generally composed of different type granites. I- and A-type granite association in a granitic complex is often described in the literature. Here we present a granitic complex, i.e. the Pitou complex in southern Jiangxi province, South China that is composed of different varieties of A-type granites. Detailed zircon LA-ICP-MS U Pb chronology and in situ Hf isotope, mineral chemistry and whole-rock element and Sr Nd isotope data of this granitic complex are used to explore their origin and to further constrain the geodynamics of Mesozoic magmatism in South China. Our new data indicate that the Pitou complex was emplaced during the Late Triassic (237 Ma) (central to northern part) and Early Jurassic (188–185 Ma) (southern part), respectively and that both the Late Triassic and Early Jurassic granites belong to A-type but show contrasting origins. The Late Triassic granites show an association of alkali-feldspar granite-syenogranite-monzogranite and the rock type of the Early Jurassic granites is alkali-feldspar granite. All the granites are composed mainly of K-feldspar, quartz, plagioclase and biotite (ferri-biotite or annite) with biotite occurring as anhedral crystal, interstitial to quartz and feldspar. Both the Late Triassic and Early Jurassic granites have high SiO 2 (>70 wt%) and total alkalis (K 2 O + Na 2 O > 8 wt%) with high FeO T /(FeO T + MgO) and low Mg#. They are enriched in rare earth elements (except Eu) and depleted in Sr and Ba. They display Zr + Y + Ce + Nb >350 ppm, 10,000 × Ga/Al >2.6 and have high zircon saturation temperatures (>800 °C). Biotites in the granites are Fe-rich with very low Fe 3+ /Fe 2+ , indicating fairly reducing conditions. However, the Late Triassic and Early Jurassic granites also show some geochemical differences. The Late Triassic granites have lower FeO T /(FeO T + MgO) and higher Mg# than the Early Jurassic granites, with the Mg# of the former consistent with pure crustal melts and the Mg# of the latter much lower than pure crustal melts. In addition, the Late Triassic granites have lower zircon saturation temperatures (up to 843 °C, average 823 °C) than the Early Jurassic granites (up to 946 °C, average 875 °C). Furthermore, the Late Triassic granites have high 87 Sr/ 86 Sr (t) (0.7132–0.7226) and negative ε Nd (t) (−10.3 to −10.9) and ε Hf (t) (in situ zircon) (−11.5 to −11.6) that are similar to those of the early Paleozoic granitoids in the region whereas the Early Jurassic granites have low 87 Sr/ 86 Sr (t) (down to 0.7034) and positive ε Nd (t) (up to +3.0) and ε Hf (t) (in situ zircon) (up to +8.3) that are similar to those of the Early Jurassic gabbroic rocks in the region. Geochemical data and major element modeling suggest that the Late Triassic A-type granites were formed by shallow (ca. 15–20 km depth) dehydration melting of the early Paleozoic granitoids triggered by intraplating of basaltic magmas and the Early Jurassic A-type granites were produced by extensive fractionation (ca. 90% to 95%) from gabbroic magmas by removal of plagioclase, amphibole, K-feldspar, clinopyroxene and magnetite. We further suggest the origin of the Late Triassic A-type granites was related to the transtension of the regional NE–trending strike-slip faults caused by the early Indosinian continental collisions and the Early Jurassic A-type granites were emplaced in a back-arc extensional setting coupled with the slab rollback and subsequent slab break-off of the subducted Palaeo-Pacific plate. Our new data confirm that the early Mesozoic magmatism in South China was controlled by continental collisions during the Late Permian to Triassic amalgamation in Southeast Asia whereas the late Mesozoic magmatism was related to the northwestward subduction of the Palaeo-Pacific plate. A transition in the dominant tectonic regime from the Tethyan to the Pacific tectonic domain most likely took place during the Rhaetian. • The Pitou granitic complex emplaced during the Late Triassic and Early Jurassic. • Both Late Triassic and Early Jurassic granites are A-type but show contrasting origin. • Late Triassic granites formed by shallow dehydration melting of Paleozoic granitoids. • Early Jurassic granites formed by extensive fractionation from gabbroic magmas. • Origin of both A-type granites suggests different geodynamic mechanisms.