Middle Triassic Granitoids Research Articles

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.

Late Paleozoic–Early Mesozoic granitic rocks in Eastern Peninsular Malaysia: New insights for the subduction and evolution of the Paleo-Tethys

• The granitoids in East Malaya were emplaced at 289–244 Ma. • The granitoids were formed in response to the Paleotethyan subduction . • The slab roll-back of the Eastern Paleotethyan initially occur at ∼ 270 Ma. • Subduction-related granitoids in East Malaya can extend to SW China and W Borneo. Well-preserved Late Paleozoic–Early Mesozoic igneous rocks in Peninsular Malaysia are important for investigating the Paleotethyan tectonic evolution. This study presents new zircon U-Pb dating, whole-rock geochemical, and Sr-Nd-Pb-Hf isotopic data for granitoids identified from the East Malaya. By utilizing all available data, the Permian–Middle Triassic granitoids in Eastern Peninsular Malaysia can be divided into three groups. Group A (289–270 Ma) I-type granites have higher ε Nd (t) values of − 6.6 to − 2.6 and zircon ε Hf (t) values of − 7.1 to + 4.9 than Group B S-type granites (285–272 Ma) with ε Nd (t) = − 10.1 and zircon ε Hf (t) = − 23.1 to + 1.5. Group C (254–244 Ma) with A-type characteristics, has ε Nd (t) and zircon ε Hf (t) values ranging from − 7.3 to − 3.8 and − 8.9 to + 2.2, respectively. The Pb isotopic values for three groups are similar with ( 206 Pb/ 204 Pb) i = 18.71–19.43, ( 207 Pb/ 204 Pb) i = 15.66–15.71 and ( 208 Pb/ 204 Pb) i = 38.27–39.43. Group A was derived from metaigneous rocks with a juvenile mafic component, while Group B was from metasedimentary rocks. Group C had a mixed source of “ancient” meta-mafic rocks with a juvenile mafic component. This study proposes that the Late Paleozoic–Early Mesozoic granitoids were formed in response to the subduction of the Paleotethyan Ocean, and that a tectonic transition from subduction to slab roll-back might occur at ∼ 270 Ma. Subduction-related granitoids in East Malaya can northernly extend to the Tachileik Batholith in eastern Myanmar and Jinghong in SW China, and southernly link with W Borneo.

Two magma fractionation paths for continental crust growth: Insights from the adakite-like and normal-arc granites in the Ailaoshan fold belt (SW Yunnan, China)

Abstract Geochemical similarities between the continental crust and arc magmas have led to the inference that subduction zones may be the primary sites of crustal growth. Thus, it is necessary to unravel the petrogenetic mechanism(s) of granitoid generation in subduction-related settings to understand crustal growth through magmatic differentiation processes. In this study, we focused on granitoid generation in oceanic-continental subduction zones. We analyzed the whole-rock geochemistry and Sr-Nd isotopes, together with zircon U-Pb-Hf-O isotopes, of the newly identified Middle Triassic granitoids in the Ailaoshan high-grade metamorphic complex (Yunnan, SW China). All the studied granite samples were characterized by large ion lithophile element (e.g., Rb, Sr, and Ba) enrichments and high field strength element (e.g., Nb, Ta, and Ti) depletions, similar to arc-type rocks. They also showed a range of whole-rock Sr-Nd, (87Sr/86Sr)i = 0.7020–0.7048, εNd(t) = +0.6 to +4.2, and zircon Hf-O, εHf(t) = +10.3 to +18.1, δ18Ozircon = 5.09‰–6.65‰, isotope compositions, which overlap with those of previously reported coeval (ca. 237–235 Ma) hornblende diorite and granodiorite, the formation of which was interpreted to have originated from a mantle wedge metasomatized by a sediment-derived melt. Furthermore, the fractionation trends of some of the granitic samples and diorite-granodiorite suite overlap. They can be divided into two geochemical groups: Group 1 has intermediate to high SiO2 (66.9–73.8 wt%) and K2O (3.40–5.42 wt%) and low MgO (0.19–1.09 wt%) contents and shows depletion in heavy rare earth elements (HREEs; e.g., Yb and Y), resulting in adakite-like high Sr/Y (61–183) and La/Yb (47–90) ratios. Group 1 shows positive SiO2 versus Sr/Y and La/Yb correlations and negative SiO2 versus HREE and Y correlations, implying fractionation of a garnet-bearing assemblage. The negative correlations between SiO2 and εNd(t) and Nb/La reveal a crustal assimilation trend. Group 2 has relatively high SiO2 (72.6–76.5 wt%) and low K2O (1.93–3.82 wt%) and MgO (0.05–0.83 wt%) contents and shows depletion in middle REEs (MREEs; e.g., Gd and Dy) with low Sr/Y (1–10) and La/Yb (4–11) ratios. Group 2 granites show negative Gd/Yb versus SiO2 correlation, which indicates significant fractionation of an amphibole-bearing assemblage. Our results suggest that both group 1 and 2 granites were formed in a subduction setting from a common mantle-derived parental dioritic magma, but they experienced two distinct fractionation processes. While group 1 granites were likely formed by crustal assimilation and high-pressure (lower-crustal) garnet-dominated fractionation, group 2 granites were generated through low-pressure (middle-/upper-crustal) amphibole-/plagioclase-dominated fractionation. We suggest that these two fractionation trends are critical to crustal growth and the development of a more fractionated (felsic) upper crust.

Geochemistry and zircon U–Pb dating of Early Jurassic syenogranite in the Kaoshan area, southern part of the Zhangguangcai Range, NE China, and tectonic implications

Early Jurassic Kaoshan medium‐ and fine‐grained syenogranites emplaced along the Yitong–Shulan Fault Zone in the southern part the Zhangguangcai Range, central Jilin Province, intruded Middle Triassic granitoids and Carboniferous‐Devonian intrusive rocks. They have medium‐ and fine‐grained textures and contain K‐feldspar, quartz, plagioclase, and biotite. They are peraluminous (A/CNK = 1.13–1.20) and have high values of SiO2 (74.60–75.82 wt%), total alkalis (Na2O + K2O = 8.28–9.18 wt%), light rare earth elements (20.89–56.14 ppm), large‐ion lithophile elements, and FeO/MgO = 2.78–5.93. In contrast, they feature low values of CaO (0.40–0.49 wt%), TiO2 (0.05–0.14 wt%), P2O5 (0.02–0.04 wt%), Sr (24.20–62.59 ppm), heavy rare earth elements (5.14–11.80 ppm), high‐field‐strength elements, and δEu (0.41–0.76), and the calculated zircon saturation temperatures (Tzr) range from 804 to 870°C, all of which are similar to the geochemical signatures of anorogenic A1‐type granites. Zircon U–Pb geochronology analyses from the medium‐ and fine‐grained syenogranites yield concordia ages of 193.18 ± 0.95 Ma (MSWD = 6.7, n = 18) and 190.29 ± 0.92 Ma (MSWD = 8.7, n = 17), respectively, and belong to the Early Jurassic Sinemurian stage. In situ Hf isotope analyses of dated zircons yield εHf(t) values ranging from +17.0 to +9.7 and Hf model ages (TDMC) of 225–614 Ma. Their whole‐rock (87Sr/86Sr)i (0.704740–0.704133) and εNd(t) values (+2.8 to +0.2) are similar to those of many coeval A2‐type granites in this region, and they form an A‐type granite assemblage. Consequently, the Kaoshan medium‐ and fine‐grained syenogranites were probably derived from partial melting of juvenile lower crust and might be related to the subduction of the Palaeo‐Pacific Plate.

Middle to Late Triassic granitic magmatism in the East Kunlun Orogenic Belt, NW China: Petrogenesis and implications for a transition from subduction to post‐collision setting of the Palaeo‐Tethys Ocean

In this study, we present new geochronological and petrogenetic data for the Triassic granitoids of the East Kunlun Orogenic Belt (EKOB), in order to constrain their precise ages, petrogenesis, and tectonic settings. LA‐ICP‐MS zircon U–Pb data indicate that the Triassic granitoids were emplaced in two stages: (a) Middle Triassic (247–240 Ma), represented by a suite of porphyritic granites and granodiorites; and (b) Late Triassic (234–227 Ma), forming an intrusive rock association of K‐feldspar granites, granodiorites, and porphyritic granites. Geochemical analyses and mineral associations suggest that all the Triassic granitoids belong to I‐type granites but have different origins. The Middle Triassic granitoids have high SiO2, low to moderate Mg# values (25–37 for XSG porphyritic granite; 45–47 for DB granodiorite), low Sr/Y ratios (2.2–4.6 for XSG porphyritic granite; 17.8–20.8 for DB granodiorite), and relatively restricted zircon εHf(t) values (+2.4 − +4.6 for the ca. 247 Ma porphyritic granite, and − 8.0 to −1.5 for the ca. 240 Ma granodiorite), indicating that they were dominantly generated from partial melting of different crust sources (either juvenile or ancient) in a normal lower crust level. In contrast, the Late Triassic granitoids have high SiO2, K2O, and Y contents, low MgO and HREE contents, and variable zircon εHf(t) values (from negative to positive, −4.9 to +3.3), implying a strong crustal–mantle interaction that occurred during the Late Triassic, and this stage of granitoids were derived from a complex magma source possibly a mixture of mantle‐derived and ancient crustal‐derived materials. By combining these new data with the previous data, we conclude that the two stages of Triassic granitoids were emplaced in an active continental margin setting and a post‐collisional extension setting, respectively. Moreover, this study suggests a tectonic shift of the Palaeo‐Tethys Ocean in the EKOB from subduction during the Middle Triassic to a post‐collision during the Late Triassic.

Constraints on the tectonic evolution of the southern central Asian orogenic belt from early Permian–middle Triassic granitoids from the central Dunhuang orogenic belt, NW China

The Dunhuang orogenic belt (DOB), located in the southern Central Asian Orogenic Belt (CAOB), exposes Precambrian and Paleozoic intrusive-metamorphic rocks. To better understand its evolution, we carried out detailed petrological and geochronological investigations on the major lithologies of the central DOB. The central DOB can be divided into two terranes, the north and the south, with contrasting geological history. The north is characterized by Precambrian crystalline rocks (ca. 3.1–1.6 Ga) overprinted by Devonian tectono-thermal events. The Precambrian rocks show evidence for major juvenile crust growth with additions of older crustal materials at 2.7–2.5 Ga, a major crustal reworking event at 2.0–1.9 Ga, and juvenile additions with minor crustal reworking at 1.8–1.7 Ga. In contrast, the south is characterized by early Devonian (ca. 406 Ma) TTG-gneiss and amphibolite gneiss, and some Late Paleozoic–early Mesozoic granitoid rocks (ca. 284–238 Ma). The Late Paleozoic–early Mesozoic granitoid rocks from the south were emplaced in two phases: early Permian (ca. 284–275 Ma) and late Permian–middle Triassic (ca. 255–238 Ma). They belong to the high-K calc-alkaline I-type series which were generated from mixing juvenile materials with Mesoproterozoic continental crust. The early Permian and the late Permian–middle Triassic granitoid rocks are considered as arc/subduction-related and post-collisional rocks, respectively. These suggest that the final assembly of the DOB with the Beishan or final assembly within the DOB happened in the middle Permian (ca. 275–255 Ma). The DOB formed by accretion/collision of multiple terranes/microcontinents, similar to other areas in the CAOB.

Subduction-modified mantle-derived Triassic high-Mg andesites in the Sanjiang Tethys, eastern Tibet

Andesites in the Yangla district of the Sanjiang Tethys, eastern Tibet, are characterized by relatively high MgO and with rich large-ion lithophile elements (LILEs), indicating similarities to typical high-Mg andesites formed in continental collisional zones. Zircon U-Pb age of 232 ± 1 Ma for these andesites shows that they occurred synchronously with the regionally widespread late Middle Triassic granitoids and bimodal volcanic rocks. Primitive mantle-normalized spidergrams for these andesites show depletion in high field-strength elements (HFSEs), with Sr-Nd-Pb isotopic compositions that are comparable to both arc- and MORB-like high-Mg rocks generated from subduction-modified sources. Modelling results suggest that an input of ~20% sediment-derived melts into mantle-derived melts could be possibly responsible for the Sr-Nd isotopic characteristic of these andesites. MORB- and arc-like compositions likely indicate a metasomatized mantle wedge modified by subduction processes during the closure of the Paleo-Tethys Ocean and subsequent continental collision. It is speculated that the late Middle to Late Triassic intra-continental re-activation associated with regional tectonic collapse and thermal relaxation triggered the melting of this previously subduction-modified mantle, leading to the formation of the high-Mg andesites in the Yangla district.

Generation of coeval metaluminous and muscovite-bearing peraluminous granitoids in the same composite pluton in West Qinling, NE Tibetan Plateau

There is considerable controversy over the nature of (strongly) peraluminous granitoids and their relationship with coeval metaluminous granitoids. The origin of peraluminous granitoids is of great importance for an understanding of granite petrogenesis, as well as the formation and evolution of the continental crust. This study reveals the presence of contemporaneous metaluminous granitoids and muscovite-bearing peraluminous granitoids in the Guomaying composite pluton (GCP) at approximately 243–242 Ma in West Qinling in the NE Tibetan Plateau. The Middle Triassic granitoids in the GCP can be petrographically and geochemically divided into two groups. The group I samples are characterized by slightly higher K2O/Na2O ratios and are metaluminous with lower Zr-saturation temperatures (740–756 °C). In contrast, the group II samples have lower K2O/Na2O ratios and mostly strongly peraluminous compositions, with marginally higher Zr-saturation temperatures (762–778 °C). Both groups share similar trace elements and Sr–Nd–Pb–Hf isotopic compositions, pointing to middle to lower crustal origins with an amphibolitic residue. The metaluminous granitoids are interpreted as partial melting of amphibolites in the middle to lower crust beneath West Qinling, and the muscovite-bearing peraluminous granodiorites were also generated by partial melting of an amphibolitic source, with addition of a feldspar-rich component in the source. The hydrous basaltic melts derived from partial melting of a metasomatized mantle wedge source were also incorporated into the magma mush. It is inferred that disparate magma sources, entrained peritectic assemblages, and various degrees of antecryst recycling collectively exerted influence on the granitic magma chemistry of the GCP. Consequently, our data suggest that muscovite-bearing strongly peraluminous granitoids, which are widely regarded as the products of recycled metasedimentary rocks, can also originate from an amphibolite-dominated source with involvement of a feldspar-rich component.

Different melting conditions and petrogenesis of peraluminous granites in western Qinling, China, and tectonic implications

The Qinling orogen records the prolonged amalgamation history between the North China Block (NCB) and the South China Block (SCB) accompanied by major granitoid intrusions during the Triassic. However, the petrogenesis of these granitoids, especially that of the peraluminous granites, remains a matter of debate. Detailed study of these peraluminous granites, which are the products of crustal melting, will give insights into the melting process and petrogenesis of peraluminous magmas and will further constrain the regional geological evolution. In this study, Xiahe (XH) tourmaline-bearing two-mica granite and Baijiazhuang (BJZ) two-mica granite were selected for systematic analysis of their mineralogy, petrology, zircon U-Pb geochronology, and geochemistry. The zircon U-Pb dating showed that the XH and BJZ granites were emplaced at approximately 242 Ma and approximately 214 Ma, respectively. The presence of muscovite and tourmaline, high peraluminosity at >1.1, low oxygen fugacity (ƒO2) conditions below the fayalite–magnetite–quartz (FMQ) buffer, and low zircon saturation temperatures indicate sedimentary protolith (S-type) granitic affinity. The low CaO/Na2O ratios <0.4 suggest that the granites formed by partial melting of a pelitic source. The low heavy rare earth element (HREE) concentration and weak negative Eu anomaly in the XH granite implies garnet residue in the magma source, whereas an obvious Eu anomaly in the BJZ granite indicates no residual garnet. The Rb/Sr ratios remained constant in the XH granite with decreasing Sr and Ba concentrations; however, these ratios increased in the BJZ granite. This observation reflects that the XH (BJZ) granite was the product of fluid-present (fluid-absent) muscovite melting. This finding is in accord with the different geochemical features and the estimates of zircon and monazite saturation temperature between the XH and BJZ granites. In western Qinling, low-Sr/high-REE (group A) and high-Sr/low-REE (group B) peraluminous granites have been identified. We propose that the former were generated during fluid-absent muscovite melting, whereas the latter were derived from fluid-present muscovite melting. This hypothesis was further confirmed by higher zircon and monazite saturation temperature estimates in addition to higher REE, Th, Nb, and Ta concentrations in the group A granitoids. From regional geological considerations, it can be concluded that the Middle Triassic granitoids in western Qinling formed during subduction of the A'nimaqing–Mianlue oceanic crust, whereas the Late Triassic granitoids are related to syn- or post-collisional processes.

Southward subduction of the Mongolia–Okhostk Ocean: Insights from Early–Middle Triassic intrusive rocks from the Jiawula–Tsagenbulagen area in NE China

New zircon U–Pb age data, Hf isotopes, whole‐rock major and trace elements, and Sr–Nd isotopic data for Early–Middle Triassic intrusive rocks from the Jiawula and Tsagenbulagen area in the southern part of the Erguna Massif provide valuable constraints on the southward subduction of the Mongolia–Okhostk Ocean lithosphere. The Jiawula–Tsagenbulagen granitoids mainly consist of granodiorite, quartz monzonite, and biotite granite. These granitoids, long believed to be mainly Late Jurassic–Early Cretaceous (ca. 150–139 Ma) and minor Permian (ca. 278 Ma and 254–253 Ma), are newly found to contain a latest Early Triassic to Middle Triassic generation (zircon U–Pb age: ca. 248–244 Ma). Geochemically, the Early to Middle Triassic granitoids are characterized by high SiO2 (74–77 wt.%) and Al2O3 (12.2–15.95 wt.%), low Mg# values (12.98–37.32), and variable Na2O/K2O ratios (0.62–1.75). They are metaluminous to peraluminous and belong to the high‐K calc‐alkaline series. Moreover, they show enrichment in light rare earth elements and large‐ion lithophile elements and depletion in heavy rare earth elements and high‐field‐strength elements (e.g., Nb, Ta, and Ti). They have an increased amount of SiO2/Al2O3 but lower V/Th ratios and negative correlations between V/Th and SiO2/Al2O3 ratios, indicating that the magmas experienced various degrees of biotite and/or plagioclase fractional crystallization. Their whole‐rock εNd(t) and zircon εHf(t) values ranged from +1.07 to +4.15 and from +0.5 to +2.9, respectively, indicating that the primary magmas were probably generated by partial melting of juvenile crustal material with the help of mantle‐derived mafic magmas. These magmas subsequently underwent fractional crystallization, magma mixing, and a small amount of crustal contamination during their emplacement. The geochemical characteristics of these Early–Middle Triassic intrusive rocks have affinities with intrusive rocks from active continental margin settings. Therefore, we conclude that the Early–Middle Triassic magmatism in the Erguna Massif was generated within an Andean‐type arc setting related to the southwards subduction of the Mongol–Okhotsk oceanic plate beneath the Erguna Massif. We summarized previous age data of the Early to Late Triassic magmatic rocks from Erguna and Central Mongolia and identified arc setting granites and post‐orogenic granites. Thus, a tectonic evolutionary model was proposed for the geological observations in the Erguna and Central Mongolia massifs, involving the Triassic continuous southward subduction in the Erguna Massif and the Late Triassic slab‐rollback and back‐arc basin extension in the south‐eastern Central Mongolia Massif.

Geochemistry, Geochronology and Lu‐Hf Isotopes of Peraluminous Granitic Porphyry from the Walegen Au Deposit, West Qinling Terrane

AbstractWalegen Au deposit is closely correlated with granitic intrusions of Triassic age, which are composed of granite and quartz porphyries. Both granite porphyry and quartz porphyry consist of quartz, feldspar and muscovite as primary minerals. Weakly peraluminous granite porphyry (A/ CNK=1.10–1.15) is enriched in LREE, depleted in HREE with Nb‐Ta‐Ti anomalies, and displays subduction‐related geochemistry. Quartz porphyry is strongly peraluminous (A/CNK=1.64–2.81) with highly evolved components, characterized by lower TiO2, REE contents, Mg#, K/Rb, Nb/Ta, Zr/Hf ratios and higher Rb/Sr ratios than the granite porphyry. REE patterns of quartz porphyry exhibit lanthanide tetrad effect, resulting from mineral fractionation or participation of fluids with enriched F and Cl. LA‐ICP‐MS zircon U‐Pb dating indicates quartz porphyry formed at 233±3 Ma. The ages of relict zircons from Triassic magmatic rocks match well with the detrital zircons from regional area. In addition, ∊Hf(t) values of Triassic magmatic zircons from the granite and quartz porphyries are –14.2 to –9.1 (with an exception of +4.1) and –10.8 to –8.6 respectively, indicating a crustal‐dominant source. Regionally, numerous Middle Triassic granitoids were previously reported to be formed under the consumption of Paleotethyan Ocean. These facts indicate that the granitic porphyries from Walegen Au deposit may have been formed in the processes of the closing of Paleotethyan Ocean, which could correlate with the arc‐related magmatism in the Kunlun orogen to the west and the Qinling orogen to the east.

Zircon U–Pb ages, Hf isotope data, and tectonic implications of Early–Middle Triassic granitoids in the Ailaoshan high-grade metamorphic belt of Southeast Tibet

The Ailaoshan tectonic belt, where the effects of the Paleo-Tethyan ocean evolution and Indian–Eurasian plate collision are superimposed, is one of the most significant geological discontinuities in western Yunnan province of southeast Tibet. An Ailaoshan micro-block within the belt is bounded by the Ailaoshan suture zone to the west and the Red River Fault to the east, and consists of low- and high-grade metamorphic belts. Late Permian–Middle Triassic granitoids that are widely distributed to the west of the Ailaoshan suture zone and within the Ailaoshan micro-block may yield significant information on the Tethyan tectonic evolution of the Ailaoshan tectonic belt. This study reports new LA–ICP–MS zircon U–Pb geochronology and Hf isotope data of four granitoids from the Ailaoshan high-grade metamorphic belt. Zircon grains from the Yinjie granitoid do not have inherited cores and yield a weighted mean U–Pb age of 247.1 ± 2.0 Ma. The zircon e Hf(t) values range from 7.8 to 12.1, and Hf model ages from 775 to 546 Ma, indicating that the granitoid was derived from juvenile crust. The rims of zircons from the Majie and Yuanjiang granitoids yield weighted mean U–Pb ages of 239.5 ± 1.8 and 237.9 ± 2.6 Ma, respectively, whereas the cores yield ages of 1608–352 Ma. The e Hf(t) values of zircon rims range from −20.4 to −5.3, yielding Hf model ages from 2557 to 1606 Ma and suggesting that the source magma of the Majie and Yuanjiang granitoids was derived from ancient crust. An additional granitoid located near the Majie Village yields a zircon U–Pb age of 241.2 ± 1.0 Ma. Based on our geochronological and geochemical data, combined with geological observations, we propose that the Ailaoshan micro-block was derived from the western margin of the Yangtze block, and is comparable to the Zhongzan and Nam Co micro-blocks. The presence of late Permian mafic rocks with rift-related geochemical characteristics within the Ailaoshan micro-block, together with granitoids derived from partial melting of ancient/juvenile crust, indicates the presence of an Ailaoshan rift. This possible rift may correspond to the Ganzi–Litang Ocean to the northwest and the Jinping–Song Da rift to the southeast. It is suggested that westward subduction of the Jinshajiang–Ailaoshan–Song Ma oceanic lithosphere triggered the separation of the Zhongzan, Ailaoshan, and Nam Co micro-blocks from the western passive continental margin of the Yangtze block through the opening of the Ganzi–Litang–Ailaoshan–Jinping–Song Da ocean/rift. This ocean/rift may represent a subsidiary branch of the Paleo-Tethyan Ocean along the western margin of the Yangtze block.

Zircon U–Pb ages, geochemistry, and Sr–Nd–Pb isotopic compositions of Middle Triassic granodiorites from the Kaimuqi area, East Kunlun, Northwest China: implications for slab breakoff

The Qimantage area of Northwest China lies in the western part of the East Kunlun Orogenic Belt, and is dominated by late Permian to Late Triassic granitoids. Among these, the Middle Triassic granitoids are mainly distributed south of the North Kunlun Fault, and consist of two main granitic assemblages: the Kaimuqi assemblage in the east and the Mositu assemblage in the west. To better constrain the Indosinian tectonic evolution of this area, we present data on the geochronology, geochemistry, and petrology of ore-bearing granodiorites from the Kaimuqi area in eastern Qimantage. The granodiorite samples have porphyritic or fine-grained textures. Laser ablation inductively coupled plasma mass spectrometry U–Pb zircon dating yields emplacement ages of 238–242 Ma, interpreted here as the result of the Middle Triassic magmatism. The granodiorites are mostly of the high-K calc-alkaline series, and are enriched in light rare earth elements, depleted in heavy rare earth elements such as Nb, Ta, P, and Ti, and have weak negative Eu (Eu/Eu*) anomalies. The Kaimuqi granodiorites have lower SiO2 and Sr contents, and higher Na2O/K2O ratios than the Mositu granodiorites. They also show initial 87Sr/86Sr ratios of 0.712151–0.715436, εNd(t) values of −7.4 to −6.3, and two-stage Nd model ages of 1.53–1.61 Ga. Together with their radiogenic Pb isotopic ratios for 206Pb/204Pb(t) (18.271–18.622), 207Pb/204Pb(t) (15.637–15.651), and 208Pb/204Pb(t) (38.452–37.870), these data indicate both mantle and crustal contributions to the source of the granodiorites. Field investigations show that Middle Triassic granitoids in both the Mositu and Kaimuqi assemblages contain large numbers of mafic microgranular enclaves, which supports an interpretation of mantle and crustal magmatic mixing. Based on a comparison of these results with data from coeval granites in the Mositu assemblage, we propose that the Middle Triassic granitoids in the Qimantage area were produced at ca. 240 Ma, as a result of the end of subduction and the initiation of collision during the Variscan–Indosinian orogeny. Magma mixing may be interpreted as the result of slab breakoff in a subduction zone environment, which led to fluid metasomatism and induced partial melting of an enriched lithospheric mantle, resulting in the formation of voluminous granitic magma.

Geochronological and geochemical constraints on the Erguna massif basement, NE China – subduction history of the Mongol–Okhotsk oceanic crust

We present new geochronological and geochemical data for granites and volcanic rocks of the Erguna massif, NE China. These data are integrated with previous findings to better constrain the nature of the massif basement and to provide new insights into the subduction history of Mongol–Okhotsk oceanic crust and its closure. U–Pb dating of zircons from 12 granites previously mapped as Palaeoproterozoic and from three granites reported as Neoproterozoic yield exclusively Phanerozoic ages. These new ages, together with recently reported isotopic dates for the metamorphic and igneous basement rocks, as well as Nd–Hf crustal-residence ages, suggest that it is unlikely that pre-Mesoproterozoic basement exists in the Erguna massif. The geochronological and geochemical results are consistent with a three-stage subduction history of Mongol–Okhotsk oceanic crust beneath the Erguna massif, as follows. (1) The Erguna massif records a transition from Late Devonian A-type magmatism to Carboniferous adakitic magmatism. This indicates that southward subduction of the Mongol–Okhotsk oceanic crust along the northern margin of the Erguna massif began in the Carboniferous. (2) Late Permian–Middle Triassic granitoids in the Erguna massif are distributed along the Mongol–Okhotsk suture zone and coeval magmatic rocks in the Xing’an terrane are scarce, suggesting that they are unlikely to have formed in association with the collision between the North China Craton and the Jiamusi–Mongolia block along the Solonker–Xra Moron–Changchun–Yanji suture zone. Instead, the apparent subduction-related signature of the granites and their proximity to the Mongol–Okhotsk suture zone suggest that they are related to southward subduction of Mongol–Okhotsk oceanic crust. (3) A conspicuous lack of magmatic activity during the Middle Jurassic marks an abrupt shift in magmatic style from Late Triassic–Early Jurassic normal and adakite-like calc-alkaline magmatism (pre-quiescent episode) to Late Jurassic–Early Cretaceous A-type felsic magmatism (post-quiescent episode). Evidently a significant change in geodynamic processes took place during the Middle Jurassic. Late Triassic–Early Jurassic subduction-related signatures and adakitic affinities confirm the existence of subduction during this time. Late Jurassic–Early Cretaceous post-collision magmatism constrains the timing of the final closure of the Mongol–Okhotsk Ocean involving collision between the Jiamusi–Mongolia block and the Siberian Craton to the Middle Jurassic.

U–Pb zircon geochronology of Early Permian to Late Triassic rocks from Singapore and Johor: A plate tectonic reinterpretation

The results of U–Pb zircon geochronology of Permian to Middle Triassic granitoids from Singapore and neighbouring Johor are presented. Rhyolite from Pulau Sibu is 276±5Ma, gabbros from Singapore give ages of 260±2 to 249±2Ma, granites from Singapore give ages from 249±2 to 230±6Ma and rhyolite from Telkuma gives an age of 238±2Ma.U–Pb detrital zircon age spectra from a Late Triassic conglomerate from Singapore show Carboniferous, Middle Triassic and Late Triassic spikes at 350, 245 and 217Ma respectively. These new data, when combined with published U–Pb zircon ages, allow a revision of the plate tectonic evolution of the Malaysian Peninsula region. A tin-bearing magmatic arc evolved in the Eastern Malaysian Peninsula part of Indochina from the Early Permian to the Middle Triassic when the Palaeo Tethys Ocean was subducted and Sibumasu continental lithosphere was rifted from Pangea. This opened up the Neo Tethys Ocean. That the granitoid ages show a progressive decrease in age from Pulau Sibu (276±5Ma) to Singapore (230±6Ma) suggests that during this Andean-type orogenic phase, the subduction zone rolled back and steepened causing the locus of mainly I-type magmatism to migrate from east to west (in present day coordinates) across the Eastern and Central Belts and stretch the lithosphere as it did so. In the Late Triassic, Sibumasu collided with Indochina causing the Western Belt (Sibumasu) to over-thrust the Central Belt (Indochina) along the Bentong–Raub Suture. The lower half of the thickened crust (formally the tin-bearing Indochina Plate) was partially melted and predominantly S-type tin-bearing granites were intruded into the upper crust (formally Sibumasu). This Late Triassic collisional phase is the Indosinian Orogeny in Peninsular Malaysia.

Petrogenesis of middle Triassic post-collisional granite from Jiefangyingzi area, southeast Inner Mongolia: Constraint on the Triassic tectonic evolution of the north margin of the Sino-Korean paleoplate

Zircon U–Pb dating and geochemical data are reported for the Jiefangyingzi and Yongshenghao granitic plutons from the Jiefangyingzi area to the northeast (NE) of Chifeng city, Inner Mongolia, with the objective of revealing the tectonic nature of the northern margin of the Sino-Korean paleoplate in early Mesozoic. Zircons from these granitic intrusions are euhedral and subeuhedral in shape with typical oscillatory zoning, indicating magmatic origin. The dating results indicate that these plutons formed in late middle Triassic (229±2Ma and 229±1Ma respectively), rather than in late Paleozoic, as previously thought. The Jiefangyingzi and Yongshegnhao granitic plutons are composed primarily of granodiorite and granite, with compositions of SiO2=63.23–68.91wt.%, K2O=3.06–4.51wt.%, and A/CNK=0.94–0.99. Chemically, they belong to the high-K calc-alkaline series and meta-aluminous I-type granites. Zircon εHf(t) values of both plutons range from +2.1 to +5.0 and +1.4 to +3.9, respectively, whereas the whole-rock εNd(t) values of Yongshenghao pluton range from −1.2 to −0.7, indicating the participation of depleted mantle material in their magmatic source. The petrological and geochemical characteristics suggest that these plutons are post-collisional granitoids. Combined with regional geological data, we conclude that these late middle Triassic granitoids formed during the transitional period from the post-collision setting to the post-orogenic extension setting after the collision between the Siberian and Sino-Korean paleoplates.

Late Paleozoic – Mesozoic subduction-related magmatism at the southern margin of the Siberian continent and the 150 million-year history of the Mongol-Okhotsk Ocean

The paper reviews geological, geochronological and geochemical data from the Late Paleozoic – Mesozoic magmatic complexes of the Siberian continent north of the Mongol-Okhotsk suture. These data imply that these complexes are related to the subduction of the Mongol-Okhotsk Ocean under the Siberian continent. We suggest that this subduction started in the Devonian, prior to the peak of magmatic activity. Studied magmatic complexes are of variable compositions possibly controlled by changes of the subduction regime and by possible input from enriched mantle sources (hot spots).The oceanic lithosphere of the Mongol-Okhotsk Ocean had shallowly subducted under the Siberian continent in the Devonian. Steeper subduction in the Early – Late Carboniferous led to switching from an extensional to compressional tectonic regime resulting in fold-thrust deformation, to the development of duplex structures and finally to the thickening of the continental crust. This stage was marked by emplacement of voluminous autochthonous biotite granites of the Angara-Vitim batholith into the thickened crust. The igneous activity in the Late Carboniferous – Early Permian was controlled by the destruction of the subducted slab. The allochthonous granitoids of the Angara-Vitim batholith, and the alkaline granitoids and volcanics of the Western Transbaikalian belt were formed at this stage. All these complexes are indicative of extension of the thickened continental crust. A normal-angle subduction in the Late Permian – Late Triassic caused emplacement of various types of intrusions and volcanism. The calc-alkaline granitoids of the Late Permian – Middle Triassic Khangay batholith and Late Triassic Khentey batholith were intruded near the Mongol-Okhotsk suture, whereas alkaline granitoids and bimodal lavas were formed in the hinterland above the broken slab. The Jurassic is characterized by a significant decrease of magmatic activity, probably related to the end of Mongol-Okhotsk subduction beneath the studied area.The spatial relationship of the Late Permian – Middle Triassic granitoids, and the Late Triassic granitoids is typical for an active continental margin developing above a subduction zone. All the Late Carboniferous to Late Jurassic mafic rocks are geochemically similar to subduction-related basalts. They are depleted in Nb, Ta, Ti and enriched in Sr, Ba, Pb. However, the basaltoids located farther from the Mongol-Okhotsk suture are geochemically similar to a transition type between island-arc basalts and within-plate basalts. Such chemical characteristics might be caused by input of hot spot related enriched mantle to the lithospheric mantle modified by subduction. The Early Permian and Late Triassic alkaline granitoids of southern Siberia are of the A2-type geochemical affinities, which is also typical of active continental margins. Only the basaltoids generated at the end of Early Cretaceous are geochemically similar to typical within-plate basalts, reflecting the final closure of the Mongol-Okhotsk Ocean.

Paleozoic and Mesozoic Basement Magmatisms of Eastern Qaidam Basin, Northern Qinghai‐Tibet Plateau: LA‐ICP‐MS Zircon U‐Pb Geochronology and its Geological Significance

Abstract:The eastern margin of the Qaidam Basin lies in the key tectonic location connecting the Qinling, Qilian and East Kunlun orogens. The paper presents an investigation and analysis of the geologic structures of the area and LA‐ICP MS zircon U‐Pb dating of Paleozoic and Mesozoic magmatisms of granitoids in the basement of the eastern Qaidam Basin on the basis of 16 granitoid samples collected from the South Qilian Mountains, the Qaidam Basin basement and the East Kunlun Mountains. According to the results in this paper, the basement of the basin, from the northern margin of the Qaidam Basin to the East Kunlun Mountains, has experienced at least three periods of intrusive activities of granitoids since the Early Paleozoic, i.e. the magmatisms occurring in the Late Cambrian (493.1±4.9 Ma), the Silurian (422.9±8.0 Ma‐420.4±4.6 Ma) and the Late Permian‐Middle Triassic (257.8±4.0 Ma‐228.8±1.5 Ma), respectively. Among them, the Late Permian – Middle Triassic granitoids form the main components of the basement of the basin. The statistics of dated zircons in this paper shows the intrusive magmatic activities in the basement of the basin have three peak ages of 244 Ma (main), 418 Ma, and 493 Ma respectively. The dating results reveal that the Early Paleozoic magmatism of granitoids mainly occurred on the northern margin of the Qaidam Basin and the southern margin of the Qilian Mountains, with only weak indications in the East Kunlun Mountains. However, the distribution of Permo‐Triassic (P‐T) granitoids occupied across the whole basement of the eastern Qaidam Basin from the southern margin of the Qilian Mountains to the East Kunlun Mountains. An integrated analysis of the age distribution of P‐T granitoids in the Qaidam Basin and its surrounding mountains shows that the earliest P‐T magmatism (293.6–270 Ma) occurred in the northwestern part of the basin and expanded eastwards and southwards, resulting in the P‐T intrusive magmatism that ran through the whole basin basement As the Cenozoic basement thrust system developed in the eastern Qaidam Basin, the nearly N‐S‐trending shortening and deformation in the basement of the basin tended to intensify from west to east, which went contrary to the distribution trend of N‐S‐trending shortening and deformation in the Cenozoic cover of the basin, reflecting that there was a transformation of shortening and thickening of Cenozoic crust between the eastern and western parts of the Qaidam Basin, i.e., the crustal shortening of eastern Qaidam was dominated by the basement deformation (triggered at the middle and lower crust), whereas that of western Qaidam was mainly by folding and thrusting of the sedimentary cover (the upper crust).