Triassic Magmatic Rocks Research Articles

Tectonic evolution of the Qiangtang Basin (northern Tibet): Constraints from detrital zircon geochronology

The Mesozoic evolution of the Qiangtang Basin in the Tibetan Plateau, which potentially hosts oil and gas resources, is not fully understood, particularly in the context of the opening and closure of the Tethyan oceans. To better understand the tectonic evolution of the Qiangtang Basin and the history of sedimentation, we obtained 788 new U-Pb detrital zircon ages from 12 sandstone samples. Our results show that the provenance of the Mesozoic strata varies both in space and time. In the central part of the basin (Central Qiangtang), older sedimentary rocks are characterized by unimodal age spectra (300–233 Ma) that indicate a proximal arc source. Trace-element compositions of Permian to Triassic zircons from Lower Triassic strata are characteristic of crystallization in arc and backarc environments, thus supporting previous suggestions that the Qiangtang Basin formed as a backarc basin during subduction of the Paleo-Tethys oceanic lithosphere. Younger (Upper Triassic and Jurassic) strata in the Qiangtang Basin received detrital zircons from diverse sources that included Cambrian to Triassic magmatic rocks and Paleozoic strata in the Central Qiangtang, and Triassic flysch deposits in the Songpan-Garze Terrane. We suggest that this change in provenance — from a unimodal arc source that produced zircons in Lower–Middle Triassic rocks to the diverse provenance recognized in the Upper Triassic strata — marks the closure of the Paleo-Tethys Ocean during the Late Triassic. Jurassic sedimentation in the Qiangtang Basin occurred during the expansion of the Meso-Tethys Ocean (Early–Middle Jurassic) and the subsequent subduction of this ocean (since the Callovian). During the Early Cretaceous, the closure of the Meso-Tethys Ocean led to an uplift and a sedimentary hiatus that marked the end of marine sedimentation in the Qiangtang Basin.

Geochemical characteristics, tectonic environment and significance of Triassic magmatic rocks in the Erdaokan silver deposit, Heilongjiang Province

Geochemical characteristics, tectonic environment and significance of Triassic magmatic rocks in the Erdaokan silver deposit, Heilongjiang Province

Provenance of Late Carboniferous–Triassic clastic rocks in the Western Kunlun orogenic belt, western China: Implications for the tectonic evolution of the Paleo-Tethys Ocean

The Western Kunlun orogenic belt in the northwest Tibetan Plateau is composed of the North Kunlun, South Kunlun, Tianshuihai, and Karakorum terranes, and has records about the tectonic evolution of the Paleo-Tethys Ocean. To provide new insights into the Paleo-Tethys orogeny in Western Kunlun, we present detrital zircon U-Pb and Lu-Hf isotopic data from the Late Carboniferous–Triassic sedimentary successions in the Tianshuihai terrane. Zircons with ages of 350–250 Ma are abundant in the Late Carboniferous–Late Permian samples and yielded εHf(t) values ranging from + 14.9 to −18.9. This indicates continuous arc-related magmatism in the Tianshuihai terrane during the Early Carboniferous–Late Permian. The Upper Permian Huangyangling Group primarily comprised Paleozoic detrital zircons predominantly sourced from the Tianshuihai terrane. Other strata contain higher proportion of Precambrian detrital zircons compared to the Huangyangling Group. This feature indicates that detritus from these strata were primarily originated from the North Kunlun, South Kunlun, and Tianshuihai terranes. We present a synthesis of provenance analysis, late Paleozoic–Triassic magmatic rocks, and Triassic metamorphic records associated with the Mazha–Kangxiwa suture zone. These data indicate that the Paleo-Tethys Ocean between the South Kunlun and Tianshuihai terranes opened at ca. 350 Ma and closed during the Middle Triassic, following continuous Carboniferous–Early Triassic subduction. Our study provides an important example of the provenance of clastic rocks in the orogen, which can be used to reconstruct information about eroded magmatic rocks and provide insights into the tectonic evolution of the orogen.

Final suture of the Palaeo-Asian Ocean domain: Insights from the geochemistry and geochronology of the early to Middle Permian Elitu Formation volcanics

The consumption of the Palaeo-Asian Ocean domain triggered substantial resources and environmental effects, but the final suture timing remains debated. This study presents the zircon U-Pb age and Hf-O isotope, whole–rock element and Sr–Nd–Pb isotope data from the Elitu Formation trachyandesite and rhyolitic tuff in the Bainaimiao arc belt. The zircon U-Pb ages (260–280 Ma) indicate that the Elitu Formation volcanism erupted at the Kungurian to Capitanian. Trachyandesites exhibit an arc-like whole–rock trace element composition and have enriched Sr–Nd–Pb isotopic composition (ISr(t) = 0.7062–0.7065; εNd(t) = −8.8 to −9.4; (206Pb/204Pb)t = 17.14–17.31) and high zircon δ18O (6.08‰–7.59‰) values, which are derived from the partial melting of the lithosphere mantle metasomatised by ancient sediment-derived melts. Rhyolitic tuffs have variable εHf(t) (−7.15 to +10.47) and low δ18O (4.99‰–6.21‰) values and exhibit affinities with I-type felsic rocks. They mainly originate from the partial melting of the basement of the Bainaimiao arc belt. Our compiled geochemical and geochronological data indicate that there were significant differences between the Early to Middle Permian Elitu Formation volcanic rocks and intrusive counterparts and the Early to Middle Triassic magmatic rocks from the Bainaimiao arc belt, and the Late Permian was the key tectonic transition period. The Late Permian transition was compatible with the regional crustal-shortening deformation, greenschist facies metamorphism and shift of deposition environment. This study argues that the Elitu Formation volcanism should be subduction-related and predate the final suture of the Palaeo-Asian Ocean domain at the Changhsingian. The suture of the Palaeo-Asian Ocean domain was not the original trigger of the Kungurian climate change in the North China Craton.

A combined methodology for reconstructing source-to-sink basin evolution, exemplified by the Triassic Songpan–Ganzi basin, central China

Source-to-sink evolution of a basin is a key to understand sedimentary processes, especially in a complex regional orogenic setting. Detrital zircon populations can be traced from their primary sources to their depositional settings. The resulting interpretations are enhanced by calculation of the adjacent orogen's paleoaltimetry, which provides additional insights into paleogeography. In this study, we present a combined methodology which aims to reconstruct source-to-sink evolution by the analysis of detrital zircon age distribution in sandstones, together with the calculation of paleo-elevation of surrounding orogens based on the chemical compositions of coeval magmatic rocks. We test the method using detrital zircon U–Pb geochronological data sets from the Triassic Songpan–Ganzi basin in central China, combined with whole-rock geochemical data from intermediate-composition magmatic rocks in adjacent crustal blocks. Application of the combined methodology supports a syn-collisional basin model for the formation of the Triassic Songpan-Ganzi basin in preference to a continental back-arc basin. The clastic sediments, mainly deep-marine turbidites, accumulated in a remnant Paleotethyan Ocean that was surrounded by the converging North China Block, South China Block, East Kunlun Orogenic Belt and the Qiangtang Block. The North China Block and the North Qaidam Block were major proto-sources of detrital zircons to the basin, contributing on average 12 % and 15 %, respectively. Triassic magmatic rocks in the East Kunlun and Qiangtang regions were major sources of igneous zircons, up to 68 % for the former and up to 56 % for the latter. Despite being located at a calculated elevation of ca. 4000 m, the Qinling Orogenic Belt contributed only ca. <10 % of the zircons, mostly restricted to the eastern depocenter of the basin. In contrast, supply from the North Qiangtang Block, despite its calculated lower elevation (1000–3000 m), accounts for 2–10 % of the detrital zircons in the basin, suggesting high erosion rates of this block. The minimal supply of zircons from the South China Block, restricted to 3–6 % in the central and western depocenters, is inconsistent with the zircon abundances predicted in the alternative back-arc basin model of the Songpan–Ganzi basin.

Origin of Quartz Diorite and Mafic Enclaves in the Delong Gold-Copper Deposit and Evaluation of the Gold-Copper Mineralization Potential

The Triassic Paleo-Tethyan magmatic belt in the East Kunlun Orogen (EKO) hosts a small number of porphyry-skarn deposits. The controls of these deposits, especially those in the eastern EKO, are poorly understood. In this contribution, we report new petrological, zircon U-Th-Pb-Hf isotopic, whole-rock elemental with Sr-Nd isotopic, and mineral chemistry data of the Delong quartz diorite and mafic enclaves to constrain their petrogenesis and metal fertility. The quartz diorite and mafic enclaves are emplaced in the Late Triassic (ca. 234 Ma). They are medium-K, metaluminous, enriched in large-ion lithophile elements (e.g., Rb, Ba, Th) and light rare earth elements (e.g., La, Ce, Nd), and relatively depleted in high field strength elements (e.g., Nb, Ta, Ti, P) and heavy rare earth elements (e.g., Gd, Er, Tm, Yb). The quartz diorite show similar (87Sr/86Sr)i (0.712584~0.713172) and more depleted εNd(t) (−6.4~−5.7) and εHf(t) (−2.3~+2.6) to those of mafic enclaves ((87Sr/86Sr)i = 0.712463~0.713093; εNd(t) = −6.4~−6.0; εHf(t) = −9.4~−4.8). Geochemical compositions of zircon, amphibole, and biotite yield high water content (5.3 wt.%~6.9 wt.% and 6.1 wt.%~7.3 wt.% based on amphibole, respectively) and high redox state for both the quartz diorite and mafic enclaves. These data, together with petrography, indicate the Delong intrusion was formed by mingling of magmas from enriched mantle and lower continental crust with juvenile materials. The oxidized and water-rich features of these magmas denote they have potential for porphyry Cu (±Au ± Mo) deposits, as do some Triassic magmatic rocks in the eastern EKO that show similar geochemical and petrographic characteristics with the Delong intrusion.

Late Triassic magmatic rocks in the southern East Kunlun Orogenic Belt, northern Tibetan Plateau: Petrogenesis and tectonic implications

ABSTRACT The East Kunlun Orogenic Belt (E-KOB) documents successive subduction and accretion processes of the Paleo-Tethyan Ocean. However, the tectonic regime of the E-KOB at the late orogenic stage remains ambiguous. This study presents new results of whole-rock geochemistry and zircon U-Pb-Hf isotopes of the Xidatan pluton in the southern E-KOB. Detailed investigation reveals that the pluton is composed of quartz monzodiorite, host monzogranite, and igneous microgranular enclaves (IMEs). Zircon U-Pb dating results suggest that they are coeval magmatic rocks emplaced at ca. 206–203 Ma. Geochemical studies reveal that the quartz monzodiorite and IMEs share a common magma source, evidenced by consistent compositions of major elements, congruently enriched LREEs and LILEs, and consistently moderately depleted HFSEs and HREEs. Enriched to slightly juvenile zircon Hf isotopes (εHf(t) = −4.24 to +0.49) indicate that the primitive magmas of the quartz monzodiorite and IMEs involve the addition of mantle-derived magmas. The quartz monzodiorite and IMEs have identical ages, and consistent isotope features with the host monzogranite, together with the observation of ingestion of xenocrysts from host felsic rocks into the IMEs indicating that they were produced through the mixing of the mantle-derived mafic magmas and host felsic magmas. In contrast, The host monzogranite has moderately high SiO2 and Na2O+K2O, but low Fe2O3 T and MgO contents. It shows crust-like trace element patterns such as strong enrichments in LREEs and LILEs, but remarkable depletions in HREEs. Along with enriched zircon Hf isotopes (εHf(t) = −3.51 to −0.25), the host monzogranite was most likely to have originated from partial melting of lower crustal mafic rocks induced by underplated mantle-derived magmas. Considering abundant Late Triassic igneous rocks across the E-KOB, these new data provide further evidence that the Late Triassic magmatism in the E-KOB may be related to the delamination of thickened orogenic root under an extensional setting.

Permian–Triassic magmatic rocks in the Middle Gobi volcanic-plutonic belt, Mongolia: revisiting the scissor-like closure model of the Mongol-Okhotsk Ocean

Permian–Triassic magmatic rocks in the Middle Gobi volcanic-plutonic belt, Mongolia: revisiting the scissor-like closure model of the Mongol-Okhotsk Ocean

A TRIASSIC OROGENIC GOLD MINERALIZATION EVENT IN THE PALEOPROTEROZOIC METAMORPHIC ROCKS: EVIDENCE FROM TWO TYPES OF RUTILE IN THE BAIYUN GOLD DEPOSIT, LIAODONG PENINSULA, NORTH CHINA CRATON

Abstract Rutile grains often occur in different types of gold deposits, and their U-Pb ages have been widely used to determine the formation time of gold mineralization. However, the origin of rutile grains in the gold deposits remains controversial. In this paper, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of U-Pb ages and trace elements on rutile grains were applied to investigate the metamorphic and hydrothermal processes of the Baiyun gold deposit (70 t Au, avg grade: 3 g/t) in the Liaodong Peninsula in the northeastern part of the North China craton. Rutile grains in the hydrothermal altered gold schist from the Baiyun deposit yielded two group ages of 1924 ± 18 and 237.0 ± 1.8 Ma, respectively. Combined with our systematic U-Pb zircon geochronological results of the ore-hosting schists and post-ore dikes, we suggest these rutile ages record a Paleoproterozoic metamorphic event and a Triassic hydrothermal gold mineralization event, respectively. The metamorphic and hydrothermal rutile grains have no obvious textural differences, but they show distinct trace element contents of Zr, W, Nb, and Ta. Combined with previous published data, we propose that high W (&amp;gt;1,000 ppm) and low Zr (&amp;lt;200 ppm) contents in rutile can be used to distinguish hydrothermal rutile from metamorphic and magmatic rutile. The newly identified ca. 237 Ma hydrothermal event is much older than the ca. 227 to 210 Ma Triassic magmatic rocks in the region, which precludes a temporal and genetic link between the Baiyun gold mineralization and the regional Mesozoic magmatism. Rather, the ca. 237 Ma gold mineralization may be associated with the Triassic orogenic metamorphism, and Baiyun is an orogenic gold deposit. The Triassic gold deposits in the northern margin of the North China craton formed by orogenesis between the Siberian craton and the North China craton. After a hiatus, the large-scale gold deposits formed during the Early Cretaceous in the North China craton due to a westward subduction of the paleo-Pacific plate beneath the craton since the Early Jurassic. Our study highlights that rutile in gold deposits may be inherited from the host rocks and/or formed by hydrothermal fluids. Distinguishing between these two different rutile generations requires a combination of in situ age dating and trace element geochemistry in petrogenetic context.

Contrasting Sources and Related Metallogeny of the Triassic and Jurassic Granitoids in the Chifeng–Chaoyang District, Northern Margin of the North China Craton: A Review with New Data

Understanding of the mechanism between magma sources and metallogeny is still vague. As an important gold and molybdenum producing area, the Chifeng–Chaoyang district, located at the northern margin of the North China Craton (NCC), is a key place for this issue. New geochemical data relating to Taijiying gold-deposit-related granites are presented. These data, coupled with previous studies, are used to explore the relationship between magma sources and mineralization processes. Two major magmatic periods, the Middle Triassic (220–230 Ma) and Late Jurassic (150–160 Ma), are identified based on the compiled data. The Triassic magmatic rocks are mostly fractionated I-type and A-type granites, including monzogranite, biotite granite, and syenogranite. They have low initial 87Sr/86Sr values (0.7050), moderately enriched εNd(t)–εHf(t) values (−8.5 and −5.6), and relatively young Nd–Hf model ages (TDM2-TDMC) (1.47–1.57 Ga). These features indicate that more Archean–Paleoproterozoic mantle-derived materials were involved in their sources. On the other hand, Jurassic granites are high-K calc-alkaline of the calc-alkaline series and mainly consist of granite, monzogranite, leucogranite, and granodiorite. They have high Na2O/K2O, Sr/Y, and La/Yb ratios and low Y and Yb contents. The adakitic features suggest the existence of a thickened lower crust. Their significant negative εNd(t)–εHf(t) values (−15.0 and −12.8) and older Nd–Hf model ages (TDM2–TDMC) (2.17–2.11 Ga) are consistent with their derivation from thickened ancient lower crust, indicating the initial activation of NCC. It is proposed that the change in the main source resulted from the tectonic transition during the early Mesozoic initial decratonization, that is, from the post-collisional extension to the subduction of the Paleo-Pacific plate beneath the East Asia plate from the Triassic to the Jurassic. Comparative analysis suggests that the medium–large-scale gold deposits with a high grade are closely related to the Triassic granites; however, most molybdenum deposits formed in the Jurassic. The decratonization of the NCC in the early Mesozoic experienced tectonic transition and controlled the gold and molybdenum mineralizations in the different stages by the changing magma sources. This pattern is beneficial to understanding the metallogenesis in the Chifeng–Chaoyang district.

Two-Stage Orogenic Cycle of the Eastern Paleo-Asian Ocean from Early Palaeozoic to Early Triassic: Constraints from Magmatic Rocks of the Southeastern Central Asian Orogenic Belt

The evolution of the eastern Paleo-Asian Ocean (PAO) has controlled the formation of the southeastern Central Asian Orogenic Belt (CAOB). However, the evolution history and final closure time of the eastern PAO still remain controversial, which greatly restricts understanding of the formation process of the CAOB. To address these issues, we provide detailed zircon chronology and Hf isotope and geochemical data of Paleozoic to Triassic magmatic rocks in the southeastern CAOB. We have identified four periods of magmatism as evidenced by: Early Silurian quartz diorites (434.7 Ma), Early Devonian monzogranites (394.2 Ma), Middle Permian granites (260.2–264.5 Ma) and Late Permian-Early Triassic syenogranite (250.8–253.6 Ma). These rocks have features of low MgO and mantle-compatible elements, are enriched in Th, U, K, Pb, Sr, Zr and Hf and depleted in Nb, Ta, La, Ce, P, and Ti. The quartz diorites belong to the medium-K calc-alkaline series with εHf(t) values of −0.76 to 2.21, indicating that they may be derived from partial melting of mafic lower crust with minor contribution of mantle magma. The monzogranites and syenogranite have high Zr + Nb + Ce + Y (260–390; 261–461 ppm, respectively), total alkali contents (9.98–10.80; 8.46–9.29 wt.%, respectively), and high zircon saturation temperature (807–840; 810–885 °C). They can be classified as A-type granites. Monzogranites have εHf(t) values (between −1.20 and +3.34); hence, we believe that they were derived from the crust modified by mantle-derived fluids or melts. Syenogranite have high εHf(t) values (5.49–11.36), and we suggest that they were derived from the juvenile lower crust that originated from the depleted mantle. The granites have high Sr/Y ratios (118–257), low Y (1.42–2.82 ppm), and Yb (0.31–0.41 ppm), consistent with the features of adakite. Considering the εHf(t) isotopic values (2.99–8.50), we suggest that they originated from thickened juvenile lower crust. Combining the results from our own and previous studies, we propose a new evolution model of the eastern PAO from Paleozoic to Triassic. It can be divided into two stages: (1) Late Cambrian to Early Devonian; (2) Early Permian to Triassic. The first stage is the evolution of the Bainaimiao ocean (secondary ocean basin of the PAO), which closed in the Late Silurian and led to the Bainaimiao arc accretion to the North China Cratons. The second stage is the final closure of the eastern PAO during the Late Permian (~254 Ma).

Petrogenesis of late Permian to Middle Triassic magmatic rocks on northern Hainan Island, South China: Implications for crust–mantle interaction and the tectonic evolution of the Paleo-Tethys

Geochronological, geochemical, and Sr–Nd–Hf isotope data are reported for the late Permian to Middle Triassic Juding gabbros, the Xinxing gabbroic–dioritic rocks, and granites and mafic enclaves from the northern part of Hainan Island, South China. Our data were used to constrain the petrogenesis of these rocks, the crust–mantle interactions involved, and the implications for the tectonic evolution of the area. Zircon U–Pb dating gave crystallization ages of ca. 235 Ma for the Juding gabbros, ca. 252 Ma for the Xinxing gabbroic–dioritic rocks, and 253–238 Ma for the granites and mafic enclaves. The Juding gabbros are characterized by weak negative Nb, Ta, and Ti anomalies, low initial 87Sr/86Sr ratios (0.704603–0.704630), and high εNd(t) values (+3.7 to +3.8). The Juding gabbros are inferred to have been derived by partial melting of depleted asthenospheric mantle, with these melts mixing with melts produced from enriched lithospheric mantle. The Xinxing gabbroic–dioritic rocks have relatively intermediate compositions, with initial 87Sr/86Sr ratios of 0.706600–0.708664, εNd(t) values of −6.3 to −5.1, and εHf(t) values of −3.3 to −0.6. The Xinxing gabbroic-dioritic rocks probably derived by partial melting of enriched lithospheric mantle that was modified by subduction-related melts and/or fluids. The granites from the northern part of Hainan Island have extremely high SiO2, low MgO and Ni contents, with initial 87Sr/86Sr ratios of 0.704794–0.711158, εNd(t) values of −8.5 to −6.6, and εHf(t) values of −15.0 to −1.2. They were probably derived by partial melting of a variety of ancient crustal sources with some addition of mantle material. The mafic enclaves in these granites have relatively low SiO2, high MgO and Ni contents, with initial 87Sr/86Sr ratio of 0.707882, εNd(t) value of −6.4, and εHf(t) values of −9.8 to −3.9. They seem to represent the remnants of a mafic component derived from enriched mantle. The gabbroic–dioritic rocks and gabbros formed in response to the subduction and break off of the Paleo-Tethyan oceanic slab. The late Permian to Middle Triassic magmatic rocks from the northern part of Hainan Island record complex multistage crust–mantle interactions and lithosphere–asthenosphere interaction, probably related to subduction during closure of the Paleo-Tethys Ocean and the consequential collision of the South China and Indochina blocks.

Petrogenesis and Tectonic Implications of the Triassic Granitoids in the Ela Mountain Area of the East Kunlun Orogenic Belt

The East Kunlun Orogenic Belt is located in the western part of the Central Orogenic Belt of China, with a large number of Triassic igneous rocks parallel to the Paleo-Tethys ophiolite belt, which provides a large amount of geological information for the tectonic evolution of the Paleo-Tethys Ocean. The granitoids studied in this paper are located in the Ela Mountain area in the eastern part of the East Kunlun Orogenic Belt. Zircon U-Pb dating results show that these different types of granitoids were crystallized in the Triassic. The 247.5 Ma porphyritic granites from Zairiri (ZRR) displayed calc-alkaline I-type granite affinities, with the zircon εHf(t) values being mainly positive (−0.5 to + 3.8, TDM2 of 1309–1031 Ma), indicating that they are derived from the partial melting of the juvenile crust and mixed with ancient crustal components. The 236.8 Ma Henqionggou (HQG) granodiorites and 237.5 Ma Daheba (DHB) granodiorites are high-K calc-alkaline I-type granite, and both have mafic microgranular enclaves (MMEs), showing higher and more varied Mg# (39.73–62.73), combined with their negative Hf isotopes (εHf(t) = −2.6 to −1.6, TDM2 = 1430–1369 Ma), suggesting that their primary magmas were the products of partial melting of the Mesoproterozoic lower crust that mixed with mantle-derived rocks. The 236.4 Ma DHB porphyritic diorites showed characteristics of high-K calc-alkaline I-type granitoids, with moderate SiO2 contents, medium Mg# values (40.41–40.65), with the Hf isotopes (εHf(t) = −2.9 to −0.5; TDM2 = 1451–1298 Ma) indistinguishably relative to contemporaneous host granodiorites and MMEs. The petrographic and geochemical characteristics indicate that the porphyritic diorites are the product of well-mixed magma derived from the Mesoproterozoic lower crust and lithospheric mantle. Based on the results of this paper and previous data, the chronology framework of Late Permian–Triassic magmatic rocks in the eastern part of the East Kunlun Orogenic Belt was constructed, and the magmatic activities in this area were divided into three peak periods, with each peak representing an extensional event in a particular tectonic setting, for example, P1 (slab roll-back in subduction period; 254–246 Ma), P2 (slab break-off in transition period of subduction and collision; 244–232 Ma), P3 (delamination after collision; 230–218 Ma).

Petrogenesis and Tectonic Setting of the Madeng Dacite, SW Sanjiang Indosinian Orogen: Evidence from Zircon U-Pb-Hf Isotopes, and Whole-Rock Geochemistry and Sr-Nd Isotopes

The Sanjiang Indosinian orogen, located in the eastern part of the Paleo-Tethys tectonic domain, is a critical region to study the Paleo-Tethyan Ocean evolution. Middle Permian–Late Triassic magmatic rocks are widespread in the Deqin–Weixi–Madeng area of southwestern (SW) Sanjiang Indosinian orogen, yet their petrogenesis and tectonic setting remain disputed. In this study, LA-ICP-MS zircon U-Pb age and Hf isotopes, and whole-rock elemental and Sr-Nd isotope geochemistry of Madeng dacite were studied. The Madeng dacite was dated at ca. 241.7 and 243.4 Ma. The samples had high Al2O3 (12.91 to 14.39 wt.%) but low MgO (0.62 to 1.76 wt.%) contents, and were alkali-rich (Na2O + K2O = 6.97 to 8.66 wt.%) with A/CNK &gt; 1.1, strongly resembling peraluminous S-type granites. The rocks were enriched in Rb, K, Th, U and LREE, but depleted in Ba, Sr, Nb, Ta, P and Ti, and showed obvious negative Eu anomalies, suggesting fractionation of Ti-bearing minerals (e.g., rutile and ilmenite) and plagioclase. The dacite had an initial 87Sr/86Sr value of 0.705698 to 0.710277, and negative εNd(t) (−11.28 to −10.64) and εHf(t) (−13.99 to −8.60), indicating a continental meta-sedimentary source. Their average Nb/Ta (12.24) and Th/U (4.65) were also consistent with continental crust. According to the lithological assemblage and geochemical features, we propose that the Deqin–Weixi–Madeng area intermediate-felsic magmatism was generated in a subduction-related tectonic setting.

Age and origin of tuffites from the Hekou Formation in the Napo basin, Southwest Guangxi and its tectonic implications

桂西南那坡盆地位于八布-Song Hien构造带，发育早-中三叠世岩浆岩和巨厚海相碎屑岩系，是研究华南地块南缘古特提斯造山过程的关键地区。本文对那坡盆地中三叠统河口组中-上部的两套沉凝灰岩开展了详细的岩石学、地球化学和锆石U-Pb-Hf同位素分析研究。两件沉凝灰岩样品LA-ICP-MS锆石U-Pb定年结果分别为240.6±1.0Ma和236.3±0.8Ma，指示河口组碎屑岩沉积时代为中三叠世拉丁期，部分为晚三叠世卡尼期早期。全岩地球化学结果显示，该沉凝灰岩属于亚碱性英安质，富集轻稀土（LREEs）和大离子亲石元素（LILEs）、Rb、Th和U，亏损高场强元素（HFSEs），具有显著的Nb、Ta和Ti负异常。沉凝灰岩锆石具有较低的ε_Hf（t）值（-18.6~-6.3）和较老的地壳模式年龄（t_DM^C=1664~2434Ma），结合锆石微量元素特征认为其形成于俯冲环境。整体上，那坡盆地河口组沉凝灰岩表现出俯冲作用形成的岛弧火山岩地球化学特征，推断其来源于桂西南凭祥地区的中-晚三叠世酸性火山岩。结合已有区域地质调查成果，认为华南地块南缘八布-Cao Bang古特提斯分支洋盆于晚二叠世至晚三叠世早期向北俯冲，随后自西向东呈剪刀式逐渐闭合。华南地块与北越地块完全碰撞拼合的时间发生在晚三叠世中晚期。;The Napo basin, Southwest Guangxi, locates in the Babu-Song Hien tectonic belt and shows widespread Early-Middle Triassic magmatic and massive marine clastic rocks, which plays a key role in studying the evolution of the Paleo-Tethys orogeny along the southern margin of South China block. In this paper, we have carefully analyzed the petrology, geochemistry and zircon U-Pb-Hf isotope for two sets of tuffites in the middle-upper part of the Middle Triassic Hekou Formation, Napo basin. LA-ICP-MS zircon U-Pb dating results of the two tuffite samples yield the ages at 240.6±1.0Ma and 236.3±0.8Ma, respectively, indicating that the clastic rocks of Hekou Formation should be deposited during the Middle Triassic Ladinian stage, some of which may extend to the early Carnian of Late Triassic. Whole-rock geochemical results indicate that the tuffites are subalkaline dacitic, enriched in light rare earth elements (LREEs), large ion lithophile elements (LILEs), Rb, Th and U, depleted in high field strength elements (HFSEs), and have significant negative anomalies of Nb, Ta and Ti. The tuffite zircons have lower ε_Hf(t) values (-18.6~-6.3) and older crustal model ages (t_DM^C=1664~2434Ma). Combined with trace elements, we suggest these zircons may have been formed in subduction-related setting. In summary, the tuffite samples from the Hekou Formation in Napo basin display the geochemical characteristics of island-arc volcanic rock, which are inferred to be derived from the Middle-Late Triassic acidic volcanic rocks in Pingxiang area, Southwest Guangxi. Given the existing geological survey results, it is concluded that the Babu-Cao Bang Paleo-Tethys branch ocean along the southern margin of the South China block should subduct northward during the Late Permian-early Late Triassic, and subsequently closed with scissor-style from west to east. The collision between the South China block and North Vietnam terrane should occur in the middle to late stage of Late Triassic.

Subduction initiation of the Neo-Tethys oceanic lithosphere by collision‐induced subduction transference

Collision-induced subduction transference may have triggered subduction of the Neo-Tethys oceanic lithosphere. To verify this proposition, we present an integrated geochemical and geochronological studies of new and published data of the Permian-Triassic magmatism in the Lhasa Terrane. The newly identified Chongni mafic intrusions (gabbros and diorites) in the North Lhasa Terrane (NLT) were emplaced during the Early Permian (ca. 275 Ma). The mafic rocks have (87Sr/86Sr)t ratios between 0.7037 and 0.7042, and εNd(t) values between + 5.2 and + 5.8. The zircons of the Chongni mafic rock samples have εHf(t) values from + 9.1 to + 12.9. The isotopic data in conjunction with trace element features suggest that the Chongni mafic intrusions have been derived mainly by partial melting of a metasomatized mantle wedge source. The newly identified Quxu granodiorites and hornblende gabbros in the South Lhasa Terrane (SLT) were emplaced in the Late Triassic (ca. 228–203 Ma). The granodiorites are characterized by (87Sr/86Sr)t ratios of 0.7036–0.7037 and εNd(t) values of +6.6 to +6.7, while the hornblende gabbro samples have (87Sr/86Sr)t ratios of 0.7033–0.7035 and εNd(t) values of +7.3 to +7.4. The zircons of the granodiorite and hornblende gabbro samples revealed εHf(t) values of +10.3 to +14.0 and +10.8 to +16.4, respectively. The isotopic data in combination with trace element signatures suggest that the granodiorites were derived from a juvenile basaltic lower crust with a minor input of mantle components, while the hornblende gabbros are interpreted to have formed during partial melting of a formerly metasomatized depleted mantle wedge source. The Early Permian and Late Triassic magmatic rocks consistently show arc-related signatures, which probably record the northward subduction of the Sumdo Paleo-Tethys and the Neo-Tethys oceanic slabs, respectively. This evidence, in combination with that of other studies on the Permian-Triassic magmatism across the Lhasa Terrane, indicates that arc magmatism within the NLT switched to collisional magmatism at ca. 260 Ma due to the collision of oceanic islands/plateaus and, further on, the collision of the SLT with the NLT. Furthermore, the change from ca. 265–255 Ma alkaline magmatism to ca. 240–200 Ma calc‐alkaline magmatism within the SLT may reflect the geodynamic transition from lithospheric extension to Andean-style orogenesis. Therefore, we propose that subduction initiation of the Neo-Tethys oceanic lithosphere in the Tibetan Plateau resulted from the southward transference of the Sumdo Paleo-Tethys subduction zone, involving the collision of oceanic islands/plateaus and, subsequently, the collision of the SLT with the NLT. Additionally, the geological records of the Tethyan orogenic system suggest that the subduction initiation of the Neo-Tethys oceanic lithosphere is a uniform response to the collision between Cimmeria and Eurasia.

Ore-hosting igneous rocks in the Xiahe–Hezuo district, West Qinling orogen, China, and their relationships with gold mineralization

• The Gulangdi Formation in West Qinling orogen (WQO) was formed prior to Triassic. • Gold-bearing igneous rocks in WQO were crystallized prior to gold mineralization . • Gold-bearing igneous rocks were generated by subduction of the Mian–Lue Ocean. • The Mian–Lue Ocean started the subduction process since ca. 251 Ma. • The gold metallogeny in WQO postdates regional magmatism by 40 to 20 m.y. The Xiahe–Hezuo district in the West Qinling orogen hosts a large number of gold deposits, most of which are spatially associated with igneous rocks. However, the nature and origin of these igneous rocks and their possible relation with gold mineralization remain poorly understood. In this study, the ore-hosting igneous rocks from the Zaozigou, Yidi’nan, and Gangcha gold deposits are selected to perform geochronological and geochemical analyses, aiming to clarify their petrogenesis and their role in gold mineralization. The quartz diorite porphyry from the Zaozigou gold deposit, the quartz diorite from the Yidi’nan gold deposit, and the andesite from the Gangcha gold deposit yield LA-ICP-MS zircon U-Pb ages of 250 ± 2, 251 ± 1, and 247 ± 1 Ma, respectively, representing the earliest Triassic magmatic rocks in the West Qinling orogen. The quartz diorite porphyry and quartz diorite display relatively high ( 87 Sr/ 86 Sr) i ratios (0.707188–0.710693), negative ε Nd (t) (−11.7 to −6.2) and zircon ε Hf (t) values (−13.0 to −1.6), and ancient two-stage Nd and Hf model ages (2073–1357 Ma), indicating they were generated by partial melting of ancient crustal materials. The andesite exhibits slightly positive ε Nd (t) values (0.8–1.8) and negative ε Hf (t) values (−20.6 to −2.2), indicating it was derived by partial melting of a metasomatized lithospheric mantle with significant crustal contamination. All of these igneous rocks show enrichments in large ion lithophile and light rare earth elements, depletions in high field strength elements, and high Mg # values (>50), indicating their generation in a subduction-related magmatic arc. These imply that the northward subduction of the Mian–Lue Ocean beneath the West Qinling orogen may had already started since the earliest Triassic. The geological and geochronological evidences suggest these igneous rocks were not genetically associated with the ore-forming process, and they merely provided the space for gold deposition.

Imaging the Late Triassic lithospheric architecture of the Yidun Terrane, eastern Tibetan Plateau: Observations and interpretations

The present-day lithospheric architecture of modern and ancient orogens can be imaged by geophysical techniques. For ancient orogens, unravelling their architecture at the time of formation is hindered by later tectono-magmatic events. In this paper, we use spatial variations in radiogenic isotopic compositions of Late Triassic magmatism from the Yidun Terrane, eastern Tibetan Plateau, to establish its lithospheric architecture during the Triassic. Comprehensive geochemical and isotopic data of Late Triassic magmatic rocks from four transects across the Yidun Terrane document eastward enrichment in whole-rock Nd, Sr, and zircon Hf isotopic compositions. Mafic and felsic rocks of major plutons show coherent and nonlinear trends in the Zr and P2O5 systematics and have limited variation of isotopic compositions. This indicates that Late Triassic magmatic differentiation was dominated by fractionation of mantle-derived mafic magmas. The spatial isotopic trends result from changing mantle sources, including variable contributions of isotopically depleted asthenospheric mantle and isotopically enriched subcontinental lithospheric mantle (SCLM) to magma sources. The spatial variation of mantle sources suggests a westward thinning of the SCLM during the Triassic. We propose that this architecture is most likely associated with eastward subduction of oceanic lithosphere of the Jinshajiang Ocean located at the west of the Yidun Terrane, immediately prior to the Late Triassic magmatism.

Middle Triassic arc magmatism in the southern Lhasa terrane: Geochronology, petrogenesis and tectonic setting

The voluminous Triassic magmatism within the Lhasa Terrane, Tibetan Plateau, has been studied for decades yet the petrogenesis of the magmas involved and the tectonic setting of this magmatism and associated events remain uncertain. This study presents new zircon U–Pb geochronological, Lu–Hf isotopic, and whole-rock geochemical data for the Middle Triassic volcanic rocks within the southern region of the Lhasa terrane. Zircon U–Pb dating of three samples from this area yield consistent ages of 233.9 ± 1.3, 232.5 ± 1.6, and 231.5 ± 1.6 Ma, representing the Middle Triassic magmatism in the southern Lhasa terrane. The whole-rock geochemistry of these volcanic rocks indicate they are enriched in the large ion lithophile elements (LILE) and the light rare earth elements (LREE). They have high εNd(t) (from +4.3 to +5.4) and zircon εHf(t) (from +16.6 to +21.8) values as well as low initial 87Sr/86Sr (0.7035–0.7037) values. These characteristics indicate that the Middle Triassic igneous rocks in this region formed from magmas generated by the partial melting of arc-type mantle wedge material. These magmas underwent fractional crystallization but did not assimilate significant amounts of ancient crustal material prior to emplacement and eruption. Combining our new data with the results of previous petrological, stratigraphic, paleomagnetic, and paleontological data from this region indicates that the Triassic magmatic rocks of the southern Lhasa terranes formed as a result of tectonism associated with the opening of the Neo-Tethyan Ocean. The driving force of the extension was most likely driven by correlated with the southward-subduction of the Banggonghu–Nujiang Oceanic plate.

Petrogenesis and tectonic implications of Middle Triassic basalts and rhyolites in the northern Qiangtang Block, central Tibet

• Triassic rocks in northern Qiangtang Block include basalts-rhyolites (237–235 Ma). • Basalts were originated from metasomatized mantle sources . • Rhyolites were generated from newly formed basaltic lower crust . • The formation of studied suite was related to Paleo-Tethys oceanic slab subduction . Triassic magmatic rocks are widely exposed in the northern Qiangtang Block in Tibet. However, their petrogenesis and relationship to the evolution of Tethys Ocean are matters of debate. Here we report zircon U-Pb ages, mineral compositions, whole-rock major and trace elements, and Nd isotope for newly reported Triassic volcanics in order to provide constraints on the formation of these rocks in Tethys ocean’s evolution. The volcanic suite consists of grey-porphyritic basalts (B1), black-aphyric basalts (B2), and rhyolites . SIMS, LA-ICPMS zircon U-Pb age dating shows that they were erupted at 237–235 Ma. All rocks show no continuous trends in major-element compositions, and are enriched in Light Rare Earth Elements (LREE) relative to heavy REE (HREE) with various Eu-anomalies. Basalts have narrow εNd(t) range (−0.90 to −0.01), slightly higher than those of rhyolites (−4.36 to −0.66). Given similar Nd-isotope and REE patterns, B1 and B2 basalts were probably produced from a common parental magma by mineral accumulation and fractional crystallization during late- and early-stages of magma evolution, respectively. Based on indicative element-ratios of slab-derived materials, we suggest that parental magmas of these basalts were derived by partial melting of metasomatized mantle source with subducted oceanic slab component. Rhyolites were generated by partial melting of basaltic lower-crust as a result of underplating by magma that corresponds to the studied basalts. Given spatial–temporal distribution of Triassic magmatic-metamorphic rocks, and geological background, we suggest that magmatism of studied suite was triggered by northward subduction of the Paleo-Tethys ocean, and recorded late-stage of Paleo-Tethys Wilson cycle.