Tethyan Tectonic Domain Research Articles

Tracing the sedimentary response to the rifting and opening of the Meso‐Tethys Ocean

ABSTRACTKnowledge of the rifting and opening process of the Meso‐Tethys Ocean is important for understanding the evolution of the Tethys tectonic domain and for an in‐depth understanding of Tethys dynamics. Sedimentary rocks are faithful recorders of surface expressions of tectonic events and are thus expected to have recorded the rifting and opening of the Meso‐Tethys Ocean. This study presents petrography, sedimentology, zircon U–Pb geochronological data and Hf isotope data for the Carboniferous–Permian Shiquanhe stratigraphic succession in the Lhasa Terrane of the Tibetan Plateau. The data indicate that the 300 to 273 Ma Lagar and Angjie formations in the lower section of the Carboniferous–Permian Shiquanhe succession were deposited in the stable passive margin of northern Gondwana with sediment sources in Gondwanaland. However, the 273 to 252 Ma Xiala Formation in the upper section of the Carboniferous–Permian Shiquanhe succession was deposited in a tectonically active setting characterized by intense magmatic activity. Considering the transition from the Lagar and Angjie formations to the Xiala Formation, a significant change in depositional setting from tectonically stable to tectonically active occurred at ca 273 Ma. Similar Early–Middle Permian changes in depositional setting have also been identified in the South Qiangtang, Baoshan and Oman regions in the Tethyan tectonic domain. These widespread changes in depositional setting, together with Early–Middle Permian tectonic uplift and episodes of subsidence, as well as rift‐related magmatism in the South Qiangtang, Baoshan, Himalayan, Zagros, Oman and Turkey regions once located in northern Gondwana, constitute a complete record of continental rifting–ocean opening associated with the diachronous opening of the Meso‐Tethys Ocean during the Early–Middle Permian.

Zircon U–Pb ages and Hf–O isotopes of Xiaomasa Miocene granite in the Ximeng area, western Yunnan, southwest China: Constraints on petrogenesis and tectonic setting

Characteristic granites of the East Tethyan Tectonic Domain (ETTD) are primarily exposed along the Himalayan orogenic belt and Gangdese batholith in the north, extend from western Yunnan in the centre, and through the Burma batholith to Peninsular Malaysia in the south. Western Yunnan is an important part of the ETTD magmatic arc, featuring characteristic granites that mainly developed between the Jurassic and Paleogene. Nonetheless, compared to other regions of the ETTD, Miocene magmatism has been poorly documented in western Yunnan. The Xiaomasa granite occurs in the Ximeng area of western Yunnan. In this study, we present U–Pb ages, and Hf–O isotope data for zircon crystals from the Xiaomasa granite pluton. Laser Ablation–Multicollector–Inductively Coupled Plasma–Mass Spectrometry (LA–MC–ICP–MS) zircon U–Pb dating yields precise emplacement ages of 15.8–16.1 Ma for the Xiaomasa granite, indicating that Miocene granitic magmatism occurred in western Yunnan. Zircons from the Xiaomasa granite have negative εHf(t) values ranging from −6.6 to −11.6, relatively high δ18O values (6.8%–10.2‰), and two‐stage Hf isotopic model ages (1.51–1.83 Ga). Zircon Hf–O isotopic characteristics indicate that the Xiaomasa granite was mainly derived from partial melting of ancient continental crust. The Ximeng Precambrian metamorphic basement is a probable material source for the granite. The Xiaomasa granite was probably produced by partial melting of the thickened Ximeng ancient metamorphic basement under a reduced pressure environment, reflecting a cumulative effect of the escape of Sundaland caused by the dynamics of the continuous India–Asia collision.

Origin of the Songpan-Garze Terrane on the northeastern Tibetan Plateau, eastern segment of the Tethyan tectonic domain: A Middle Permian−Early Triassic intracontinental rifting system

The initiation of a continental rift or the birth of an oceanic basin, and their long-term evolution, are challenges in the understanding of the evolution of regional and global tectonics. Rifts develop within the interior or margins of plates, essentially as extensional structures, and they have been studied widely since the discovery of plate tectonics. The eastern segment of the Paleo-Tethys traversed central Asia, its structural features, timing, geological setting, and geodynamic mechanisms are poorly understood. The Songpan-Garze Terrane (SGT) on the northeastern Tibetan Plateau has been regarded as either a remnant of the Paleo-Tethys Ocean or a continental rift basin. Different interpretations of the SGT lead to various tectonic reconstructions between Gondwana and Laurasia. Here, sedimentology, geochemistry, and geochronology have been applied to the southern and northern parts of the SGT to reconstruct the initiation of a Permian rift system. In this area, for the first time, field investigation reveals that the SGT is characterized by a continuous succession of Permian−Lower Triassic terrestrial-neritic deposits and ocean island basalt (OIB)-type basalts, and that it developed as an intracontinental rift basin in the Yangtze Block. Zircons from gabbros with OIB-type geochemical signatures intruded in the southern SGT yield U-Pb ages of 263−254 Ma, constraining the timing of the initial rifting. Proterozoic felsic rocks collected from the southern SGT yield U-Pb ages of 761−865 Ma, and they are petrologically and geochemically comparable to Neoproterozoic basement rocks along the western margin of the Yangtze Block. The Middle−Late Permian volcano-sedimentary sequences of the SGT and their geochemistry and geochronology are comparable to rift sequences in the Panxi region of the western Yangtze Block. Thus, it has been proposed that the SGT originated as an intracontinental rift within the Yangtze Block during the Middle Permian−Early Triassic, and that it continued to deepen as a rift basin through the Middle−Late Triassic.

Mesozoic tectonic transition from the Tethyan to Paleo-Pacific domains in South China: New insights from Mesozoic igneous rocks in the western Nanling Range

• The studied Mesozoic igneous in the Nanling Rang include post-orogenic ultramafic–mafic rock, subduction-related mafic rocks and other calc-alkaline intermediate-felsic rocks. • Those igneous rock assemblages in the Nanling Rang reflect the tectonic transition of the SCB. • The Nanling Rang was a Tethyan domain before ca. 170 Ma but a Paleo-Pacific domain after that. • The Mesozoic evolution of the SCB involved four stages: post-orogenic extension, slab foundering, and tectonic transition from subduction to rollback, followed by slab rollback. Mesozoic tectono-magmatic evolution of the Nanling Range in the South China Block (SCB) remains controversial, especially about the role of the Tethyan and Paleo-Pacific dynamics in the generation of Mesozoic igneous rocks. The Mesozoic magmatic events have produced a diverse variety of rocks, dominantly granites and spatially associated mafic–ultramafic rocks, lamprophyres and diorites. We present new zircon U-Pb ages and in-situ Hf isotope data, along with whole-rock elemental and Sr-Nd isotopic data for the Mesozoic igneous rocks from the western Nanling Range. The two ca. 200 Ma ultramafic–mafic rocks have low SiO 2 contents (40.00–44.94 wt%) and typical ocean island basalt (OIB)-like trace elemental affinities with positive Nb and Ta anomalies, positive ε Nd (t) (4.47 to 4.88) and variable ε Hf (t) values (–3.65 to 11.91), suggesting that they were generated by partial melting of an asthenospheric mantle source. The ca. 165 Ma Jiangyong lamprophyre dyke has relatively high SiO 2 content (51.70 wt%) and arc-like trace elemental affinities with significantly negative Nb and Ta anomalies and variable ε Hf (t) values (–9.10 to 2.65). They were derived from a lithospheric mantle source previously modified by subducted component, along with a contribution from the asthenospheric mantle. Granites from the ca. 146–137 Ma Taibao granitic pluton have high SiO 2 contents (63.41–66.66 wt%), variable (La/Yb) N and Sr/Y values, and negative to positive ε Hf (t) values (−4.16 to 0.86). These granites have compositions transitional from adakite to arc-type rocks and were likely generated by partial melting of Mesoproterozoic meta -igneous rocks, with an addition of a mantle-derived component. Compared with those granites, diorites of the ca. 99 Ma Eryue pluton exhibit moderate SiO 2 contents (59.90–60.30 wt%), a wide range of zircon ε Hf (t) values (−12.04 to 0.50) and negative ε Nd (t) values (–8.61 to –8.57), and they were probably formed by magma mixing of crust-mantle materials associated with back-arc extension. Our new data, together with other geological evidence, provide further insights for the evolution of the Tethyan and Paleo-Pacific tectonic domains during the Late Triassic to Late Cretaceous in the SCB. In the early stage, the Tethyan tectonic domain, ultramafic–mafic rocks from the Ludong and Fuchuan plutons in the Nanling Range was related to the post-orogenic extension. This domain was gradually changed to the Paleo-Pacific domain during the period from Late Triassic to Early Jurassic and the tectonic transition was accomplished at ∼ 170 ± 5 Ma. In the later stage, the Paleo-Pacific tectonic domain, the Jiangyong lamprophyre, Taibao granites and Eryue diorites were produced via the crustal thickening, localized extension, and large-scale lithospheric extension due to the advancing, slab-foundering, and rollback of the oceanic slab during the period from Middle Jurassic to Late Cretaceous.

Digital paleogeographic reconstruction of the eastern Tethyan tectonic domain from the Middle Permian to the Middle Triassic

Paleogeographic reconstruction studies the evolution of the earth's surface tectonic process, the pattern of land and sea, climate, and biology in time and space in geological history. With the advent of the big data era, the accuracy of the digital paleogeographic reconstruction is essential. Although there are various global paleogeographic reconstruction models, there is still a lack of paleogeographic reconstruction integrating tectonic, lithofacies, paleontology, and paleoclimate data. In this paper, a case study of the comprehensive paleogeographic reconstruction is carried out in the Middle Permian-Middle Triassic eastern Tethyan tectonic domain, which is highly concerned and rich in geological data accumulation. The digital paleogeographic map is completed by the combination of automatic mapping with machine learning and manual correction. We use the newly upgraded paleomagnetic, geological and paleontological data to reconstruct the paleoposition of the East Asian blocks at 260, 250 and 240 Ma, which shows the Paleo-Tethys Ocean (PTO) had a wider east-west range than the previous version due to adopting the method of Torsvik et al. (2008) to reconstruct the paleolongitude of South China at 260 Ma. The new model shows the PTO was semi-closed with several narrow seaways and was divided into north and south branches at 260 Ma. Based on the updated lithofacies paleogeographic map data of China and marine fossils data, we updated the paleogeographic map of the Eastern Tethyan tectonic domain on the basis of the global paleogeographic map, combining automatic mapping with machine learning and manual revision of paleotopography and paleocoastline. Based on the new tectonic paleogeographic reconstruction, we revised the paleoclimate zonation pattern of the East Tethys ocean using climate-sensitive lithologies.

Subduction and carbonate platform interactions

AbstractPlate tectonics, as the unifying theory in Earth sciences, controls the functioning of important planetary processes on geological timescales. Here, we present an open‐source workflow that interrogates community digital plate tectonic reconstructions, primarily in the context of the planetary deep carbon cycle. We present an updated plate tectonic reconstruction covering the last 400 million years of Earth evolution and explore components of the plate–mantle system that is involved in the exchange and storage of carbon. First, the workflow enables us to estimate subduction zone lengths through time, which represent the “tap” of carbon that is released at convergent tectonic margins. Second, we explore the role of Andean‐style versus intra‐oceanic subduction regimes during Pangea assembly and breakup. Third, we provide an improved model for carbonate platform evolution since the Devonian and evaluate the interaction of subduction zones and buried carbonate platforms. Last, we present a new model for estimating oceanic age, carbon content in the upper oceanic crust, and estimated (carbon‐containing) sediment thicknesses through time and present methods to track the subduction of this material through time. These components of the deep carbon cycle are key mechanisms controlling, or at least modulating, atmospheric CO2 on geological timescales and hence strongly influencing long‐term climate. We find that the mid to Late Cretaceous greenhouse climates were likely driven by increased subduction fluxes of volatiles and increased subduction zone interactions with carbonate platforms in the Tethyan tectonic domain. Our work highlights the importance of community digital plate tectonic reconstructions as a framework for studying key systems, such as the deep carbon cycle, that influence the life‐support mechanisms on our planet.

Petrogenesis and tectonic implications of late Permian and Triassic granitoids on Hainan Island, South China

Hainan Island is a key part of the eastern Tethyan region and contains detailed geological information that can enhance our understanding of the tectonic evolution of this region since the late Palaeozoic. In this paper, we report geochronological, geochemical, and Sr–Nd isotopic data for granitoids from Baomei and Chahe in the northern part of Hainan Island. Three new zircon U–Pb ages of 243.3 ± 1.7 Ma, 237.1 ± 5.1 Ma, and 257.0 ± 4.0 Ma were obtained for these granitoids. The late Permian Chahe granite (257 Ma) is weakly peraluminous to peraluminous, and has rare earth element patterns and Ga/Al and FeOT/MgO ratios typical of A‐type granites formed in an extensional tectonic setting. The Middle Triassic Baomei granitoids (243–237 Ma) are weakly peraluminous, and exhibit a negative correlation between P2O5 and SiO2, which is typical of I‐type granites. All the granitoids have EM II‐type‐enriched mantle Sr–Nd isotopic compositions, which indicate that they were derived from a mantle source that had experienced enrichment and metasomatism. Hainan Island was in an extensional tectonic setting from the late Permian to Middle Triassic when widespread A‐ and I‐type granites were generated. We speculate that Hainan Island was mainly affected by the Tethyan tectonic domain during the Permian–Triassic, which involved closure of the Palaeo‐Tethys Ocean, collision of the Indochina and South China blocks, and subsequent post‐collisional extension.

New insights on the age of the Mengyejing Formation in the Simao Basin, SE Tethyan domain and its geological implications

Knowledge on the origin and mechanism of the large potash formation in the southeast Tethyan tectonic domain are still under debate, one of which mainly due to the controversial ages of the potash-bearing strata. Here we present a detailed detrital zircon U-Pb geochronologic-magnetostratigraphic study of the potash-bearing Mengyejing (MYJ) Formation (Fm) from the Simao Basin. 948 paleomagnetic cores and 4 detrital zircon U-Pb samples were collected from a 932-m thick Jiangcheng section. Thermal demagnetization isolated 857 primary characteristic remnant magnetization (ChRM) directions, indicated by positive reversals and fold tests and petrologic examinations. Seven normal and seven reversed polarity zones were identified and correlated well with chrons C27r to C34n of the geomagnetic polarity timescale (GPTS2012), yielding the first known age sequence of >112 to ∼63 Ma for the MYJ Fm. The ages for the potash bearing strata in the Simao commenced ∼20 Ma earlier than those in the adjacent Khorat basins, opposite to the traditional viewpoint that the former would rather correspond to the middle and lower parts of the latter. Given the paleogeographic reconstruction of likely a pan-Simao-Khorat basin during the Cretaceous, ∼85 Ma would be another important potential period for future potash exploration in the Simao Basin.

Zircon U–Pb Ages and Geochemistry of Granite Porphyries in the Yangla Cu Deposit, SW China: Constraints on Petrogenesis and Tectonic Evolution of the Jinshajiang Suture Belt

Located in the eastern part of the Tethyan tectonic domain, the Jinshajiang Suture Belt (JSB), northwestern Yunnan, China, is notable for its large-scale distribution of Jurassic to Triassic granitoids that are genetically related to the evolution of the Paleo-Tethys Ocean and polymetallic mineralization. In this study, geochronological and geochemical analyses were conducted on three samples of these granite porphyries (GPs) using laser ablation inductively coupled plasma mass spectrometry and zircon U–Pb aging to reveal ages of 213±15, 198.4±8.6, and 195.3±6.4 Ma, respectively. These ages are younger than the emplacement ages of the granodiorites, at 208–239 Ma, suggesting that magmatic activities in the Yangla mining district likely continued for ~44 Ma. These GPs are rich in large-ion lithophile elements such as Rb, Ba, Th, U, K, and La but are deficient in high field strength elements such as Ta, Nb, Ce, Zr, Hf, and Ti. Significant Pb enrichment and P depletion were noted, as were varying degrees of metallogenic element enrichment in the order of Cu>Pb>Zn. The total content of rare earth elements (ΣREEs) of the GPs is in the range of 50.41–127.27 ppm and the LREE/HREE ratio is in the rage of 4.46–10.54. The GPs are rich in LREEs, with a high degree of differentiation noted between the LREEs and HREEs. The δEu (EuN/Eu∗) and δCe (CeN/Ce∗) values, at 0.53–0.86 and 0.79–0.98, indicate weak and slightly weak negative anomalies, respectively. The geochemical characteristics of the GPs indicate that these bodies are slightly metaluminous to peraluminous S-type granites in a calc–alkaline series that formed in a late-collisional or postcollisional tectonic setting. Three-component mixing of magmas including those of upper crust, lower crust, and mantle materials in addition to subsequent partial melting could have been responsible for the generation of these GPs in an epithermal low-pressure setting at <5 kbar.

U–Pb dating of detrital zircons in the eastern Guangdong Basin, South China, and constraints on the tectonic transformation from the Early to Middle Jurassic

Controversies exist regarding the mechanism of formation of basins located on the continental margin of South China as well as when they formed. It was ascertained based on clastic petrology, geochemical analysis, and zircon U–Pb dating that the sedimentary provenances in the eastern Guangdong Basin are mainly felsic igneous rocks from the late Early Jurassic to the Middle Jurassic. The late Early Jurassic Qiaoyuan Formation mainly shows major age peaks at approximately 238 Ma, 259 Ma, and 1858 Ma, and the Middle Jurassic Tangxia Formation shows major age peaks at approximately 169 Ma and 172 Ma. From the late Early Jurassic to the Middle Jurassic in the eastern Guangdong Basin, the source region changes from southwestern South China and southern South China to the eastern Nanling Range. It was determined by comparing the detrital zircon ages of the Qiaoyuan Formation and the Tangxia Formation with those of the late Paleozoic to early Mesozoic basins, and analyzing both the geochemical data and sedimentation, that the eastern Guangdong Basin changed from the basin-arc foreland basin of the late Early Jurassic to the back-arc extension basin of the Middle Jurassic. The changes in early Mesozoic detrital zircon age peaks indicate that the tectonic regime of the eastern Guangdong Basin ended the transformation from the Tethyan tectonic domain to the paleo-Pacific tectonic domain in the early Middle Jurassic (approximately 172 Ma).

Plate tectonic control on the formation and tectonic migration of Cenozoic basins in northern margin of the South China Sea

The tectonic evolution history of the South China Sea (SCS) is important for understanding the interaction between the Pacific Tectonic Domain and the Tethyan Tectonic Domain, as well as the regional tectonics and geodynamics during the multi-plate convergence in the Cenozoic. Several Cenozoic basins formed in the northern margin of the SCS, which preserve the sedimentary tectonic records of the opening of the SCS. Due to the spatial non-uniformity among different basins, a systematic study on the various basins in the northern margin of the SCS constituting the Northern Cenozoic Basin Group (NCBG) is essential. Here we present results from a detailed evaluation of the spatial-temporal migration of the boundary faults and primary unconformities to unravel the mechanism of formation of the NCBG. The NCBG is composed of the Beibu Gulf Basin (BBGB), Qiongdongnan Basin (QDNB), Pearl River Mouth Basin (PRMB) and Taixinan Basin (TXNB). Based on seismic profiles and gravity-magnetic anomalies, we confirm that the NE-striking onshore boundary faults propagated into the northern margin of the SCS. Combining the fault slip rate, fault combination and a comparison of the unconformities in different basins, we identify NE-striking rift composed of two-stage rifting events in the NCBG: an early-stage rifting (from the Paleocene to the Early Oligocene) and a late-stage rifting (from the Late Eocene to the beginning of the Miocene). Spatially only the late-stage faults occurs in the western part of the NCBG (the BBGB, the QDNB and the western PRMB), but the early-stage rifting is distributed in the whole NCBG. Temporally, the early-stage rifting can be subdivided into three phases which show an eastward migration, resulting in the same trend of the primary unconformities and peak faulting within the NCBG. The late-stage rifting is subdivided into two phases, which took place simultaneously in different basins. The first and second phase of the early-stage rifting is related to back-arc extension of the Pacific subduction retreat system. The third phase of the early-stage rifting resulted from the joint effect of slab-pull force due to southward subduction of the proto-SCS and the back-arc extension of the Pacific subduction retreat system. In addition, the first phase of the late-stage faulting corresponds with the combined effect of the post-collision extension along the Red River Fault and slab-pull force of the proto-SCS subduction. The second phase of the late-stage faulting fits well with the sinistral faulting of the Red River Fault in response to the Indochina Block escape tectonics and the slab-pull force of the proto-SCS.

Provenance of the East Guangdong Basin and Yong’an Basin in southeast China: Response to the Mesozoic tectonic regime transformation

The East Guangdong Basin and Yong’an Basin, located in the south-eastern South China Block, were influenced by both the Tethyan tectonic domain and the Paleo-Pacific tectonic domain, and the records of magmatic activity, tectonic deformation, and basin sedimentation in these areas preserve important geological information about the Mesozoic tectonic setting of the South China Block. In this study, detrital zircon U-Pb dating was conducted on the Late Triassic to Middle Jurassic sequences in the East Guangdong Basin and Yong’an Basin to study the sediment provenances, constrain the timing of tectonic regime transformation and discuss the mechanism driving this transformation in South China. The results show that the detrital zircon U-Pb ages from Late Triassic-Early Jurassic samples from the East Guangdong Basin form six major age groups of 2300–2600 Ma, 1800–2000 Ma, 650–1050 Ma, 410–480 Ma, 350–250 Ma, and 250–210 Ma, while the samples from the Yong’an Basin form two groups of 650–1050 Ma and 410–480 Ma. The detrital zircon U-Pb ages from the Middle Jurassic samples from the two basins mainly form three age groups of 1650–2100 Ma, 250–200 Ma, and 180–160 Ma. Provenance analysis suggests that the Late Triassic and Early Jurassic detritus in the East Guangdong Basin was derived from the south-western or southern margin of the Yangtze Block, Hainan Island, the Yunkai Massif, and the Nanling tectonic belt in the South China Block and that the detritus in the Yong’an Basin was sourced from the southern Wuyi Massif to the north of the basin. In contrast, the Middle Jurassic detrital zircons were mainly sourced from uplifted regions in north-eastern Fujian and south-western Zhejiang and were derived from early Mesozoic magmatic and metamorphic rocks, which are widely distributed in the South China Block. The variations in the main age groups of the detrital zircons in the study areas and the corresponding provenance areas combined with tectonic information, paleo-current data, and magmatic activities indicate that the tectonic regime of the South China Block experienced a transition from the Tethyan tectonic domain to the Paleo-Pacific tectonic domain in the early Mesozoic and the transition occurred between the Early Jurassic and completed at 165 ± 5 Ma. The driving force for the transformation was the westward subduction of the Paleo-Pacific Plate beneath the South China Block, resulting in Middle-Late Jurassic magmatism and extrusion and uplift of the eastern part of the South China Block.

Chemical composition, genesis and exploration implication of garnet from the Hongshan Cu-Mo skarn deposit, SW China

In this study, we present new mineral chemical data on the garnet from the Hongshan Cu-Mo deposit, one of the largest skarn deposits in the Sanjiang Tethyan tectonic domain, in SW China. The aims are to unravel the formation mechanism of the oscillatory-zoned garnet at Hongshan, and to explore possible applications using garnet trace element to distinguish different types of skarn deposits.Two generations of garnets, i.e., poorly-zoned garnet I (Grt I) and the well-zoned garnet II (Grt II), are identified from the endoskarn and exoskarn, at Hongshan based on field and petrographic observations. EPMA and LA-ICP-MS analyses show that Grt I contains a narrow compositional range (Adr40.07-61.25 Grs36.65-57.08), and chondrite-normalized HREE-enriched and LREE-depleted patterns with minor positive Eu anomalies. In contrast, Grt II shows a wide compositional range (Adr27.78-67.53 Grs30.67-70.92 to almost pure andradite Adr94.41-99.99 Grs0.00-4.21) with interlayered Al-rich and Fe-rich zones. In Grt II, its Al-rich zone is chemically similar to Grt I, whereas its Fe-rich zone exhibits chondrite-normalized LREE-enriched and HREE-depleted patterns with distinct positive Eu anomalies. The positive REE3+ vs. Mg correlations, and the poor REE3+ vs. Y and Ca correlations, altogether indicate that the REE3+ incorporated into garnet was co-influenced by the menzerite-type substitution mechanism (Ca2+-1VIIIREE3++1VIIIAl3+-1VIMg2++1VI), fluid chemistry, and the physicochemical conditions of precipitation. The higher U contents and HREE-enriched patterns of Grt I suggest that it was formed under a relatively reducing and near-neutral pH condition. Varying U contents and REE patterns across different oscillatory zones indicate that Grt II was probably formed under alternating physicochemical fluctuations led by repeating crack-seal cycles in the hydrothermal system. Besides, chemical data compilation suggests that the garnet from different types of skarn deposits has different compositions, and can be used as mineral prospecting tools. We suggested that discrimination plots, such as δEu vs. V and δCe vs. U, can distinguish Cu skarn deposit from other skarn deposit types, whilst the U vs. W plot can discriminate between Cu, W, W-Sn and W-Mo skarn deposits.

A long-lived Indian Ocean slab: Deep dip reversal induced by the African LLSVP

A slab-like high seismic velocity anomaly (referred as SEIS) has been inferred beneath the central-southern Indian Ocean in a recent tomographic inversion. Although subduction has previously been suggested regionally by surface observations, the new inversion is consistent with a north-dipping slab extending from the upper mantle to the core mantle boundary (CMB). We propose that SEIS anomaly originated from an oceanic plate in the Paleo-Tethys that was consumed by a south-dipping intra-oceanic subduction zone during the Triassic and Jurassic periods. SEIS challenges traditional concepts of the dynamics of slab descent by its relatively shallow depths and a present-day polarity opposite to the geometry of subduction. Geodynamic models show the upwelling mantle flow exerted by a thermochemical pile can hold and stagnate the descending SEIS slab at shallow depths for more than 100 Myr. The spatial distribution of resistance from the upwelling mantle flow can reverse the slab dip, producing a structure consistent with seismic inversions as well as with our proposed tectonic scenario and geology constraining the Tethyan tectonic domain. The results suggest that slabs can descend through the lower mantle at rates substantially lower than 1 cm/yr, and even reverse their polarity through interactions with background mantle flow.

Early Paleozoic arc–back-arc system in the southeastern margin of the North Qilian Orogen, China: Constraints from geochronology, and whole-rock elemental and Sr-Nd-Pb-Hf isotopic geochemistry of volcanic suites

The North Qilian Orogen is considered to represent an oceanic suture zone formed by closure of the North Qilian Ocean, which formed the northern part of the Proto-Tethys Ocean along the northern margin of the eastern Gondwana during the Early Paleozoic. However, the Early Paleozoic subduction system remains controversial, especially along the southeastern margin where it is connected to the Qinling Orogen. Here we present zircon U-Pb geochronology and whole-rock elemental and Sr-Nd-Pb-Hf isotopic data on a suite of andesitic–dacitic volcanic rocks from the Chenjiahe Group and basalts–basaltic andesites from the Hongtubao Formation in the southeast margin of the North Qilian Orogen. SHRIMP and LA-ICP-MS zircon U-Pb dating of dacite and basaltic andesite sample yield crystallization ages of ca. 443Ma and 438Ma. The andesitic–dacitic volcanic rocks exhibit calc-alkaline feature, with LREE- and LILE-enrichment, Nb–Ta depletion, moderate whole-rock (87Sr/86Sr)i (0.7055–0.7071), εNd(t) (−4.5 to 0.3) and εHf(t) (−0.3 to 5.6) values, and enriched Pb-isotopic compositions. Our data suggest magma derivation from ancient crustal sources involving juvenile materials by magmatic underplating, followed by MASH processes under a continental arc setting. The basalts–basaltic andesites are characterized by medium- to low-K tholeiitic, LREE- and LILE-enrichment and Nb-Ta depletion. Their whole-rock Sr-Nd-Pb-Hf isotopes show depleted mantle features with lower (87Sr/86Sr)i (0.7044–0.7047), positive εNd(t) (3.0–5.9) and εHf(t) (11.1–11.3) values, and Nd-Pb-Hf isotopic features that exhibit affinity with the Tethyan tectonic domain. These features could reflect partial melting of an asthenospheric mantle that witnessed recent subduction-related metasomatism and experienced fractional crystallization in an initial back-arc setting. We suggest that the Early Paleozoic arc–back-arc system was formed during the southward oceanic subduction beneath the Qilian Block in the southeastern margin of the North Qilian Orogen and that the Early Paleozoic subduction system might be inconsistent in different parts of the North Qilian Orogen.

Geochemistry, zircon U–Pb geochronology and Hf isotopes of Jurassic-Cretaceous granites in the Tengchong terrane, SW China: implications for the Mesozoic tectono-magmatic evolution of the Eastern Tethyan Tectonic Domain

ABSTRACTRecently identified Early Jurassic, Early Cretaceous, and Late Cretaceous granites of the Tengchong terrane, SW China, help to refine our understanding of the Mesozoic tectonic-magmatic evolutionary history of the region. We present new zircon U–Pb geochronological, Lu–Hf isotopic and geochemical data on these rocks. The zircon LA-ICP-MS U–Pb ages of the Mangzhangxiang, Laochangpo, and Guyong granites, and Guyong granodioritic microgranular enclaves are 185.6, 120.7, 72.9, and 72.7 Ma, respectively. Geochemical and Hf isotopic characteristics suggest the Mangzhangxiang and Laochangpo S-type granites were derived from partial melting of felsic crust and that the Guyong I-type granite and associated MMEs were generated through magma mixing/mingling. Mesozoic magmatism in the Tengchong terrane can be divided into three episodes: (1) the Triassic syn- and post-collisional magmatic event was related to the closure of the Palaeo-Tethyan Ocean, as represented by the Changning-Menglian suture zone; (2) the Jurassic to Early Cretaceous magmatism was related to the subduction of the Meso-Tethyan oceanic crust, as represented by the Myitkyina ophiolite belt; and (3) the Late Cretaceous magmatism was related to the subduction of the Neo-Tethyan oceanic crust, as represented by the Kalaymyo ophiolite belt.

Petrogenesis of granitoids in the Dewulu skarn copper deposit: implications for the evolution of the Paleotethys ocean and mineralization in Western Qinling, China

The Tethyan tectonic domain hosts numerous world-class mineral deposits. Among these, the Dewulu skarn copper deposit in Western Qinling, China belongs to the Paleotethys ore belt. The skarn and orebodies here occur as stratoids or lenses at the contact between the Triassic Dewulu intrusive complex and Permian marine clastic and carbonates. Alteration minerals include prograde skarns (garnet, diopside, wollastonite), plagioclase, hornblende, actinolite, tremolite, epidote, chlorite, calcite, quartz and sericite. The main ore types include early disseminated skarn-type replacement orebodies and late-stage quartz-sulfide veins. Chalcopyrite is the major ore mineral, along with pyrite, bornite and sphalerite. The Dewulu intrusive complex comprises quartz diorite, quartz diorite porphyry and dioritic mafic microgranular enclaves (MME). The MMEs are spheroidal in shape, and have igneous mineral assemblages, acicular apatites, complex oscillatory zoned plagioclase and quartz megacrysts surrounded by mafic minerals. The MMEs are metaluminous and calc-alkaline to high-K calc-alkaline, and possess relatively high Ni, Cr and MgO contents and Mg# values. They display sub-parallel patterns in trace element spider diagrams and rare earth element (REE) plots. They are also characterized by the enrichment of Rb, U and Th, depletion of Ba, Sr, Nb and Ta and negative Eu anomaly. Zircon LA-ICP-MS U–Pb dating of the dioritic MME yields an age of 247.0±2.2Ma, coeval with the host quartz diorite, quartz diorite porphyry and ore-related sericite 40Ar/39Ar plateau ages within analytical uncertainties. Oxygen fugacity estimated from trace element compositions of zircons from the dioritic MME shows FMQ±3.3. The zircons have negative εHf(t) values in a range of −8.0 to −3.3, corresponding to two-stage model ages ranging from 1.48 to 1.78Ga. The integrated data from petrology, geochronology and bulk geochemistry suggest that the Early Triassic granitoids associated with Cu skarn mineralization at Dewulu were products of arc magmatism and involved magma mixing in an active continental margin setting. The magma was sourced through partial melting of enriched sub-continental lithospheric mantle that had been previously modified by slab-derived melt during the continuous northward subduction of the Paleotethys oceanic slab.

Mineralogy and mineral chemistry of Bi-minerals: Constraints on ore genesis of the Beiya giant porphyry-skarn gold deposit, southwestern China

The Beiya deposit, located in the Sanjiang Tethyan tectonic domain (SW China), is the third largest Au deposit in China (323t Au @ 2.47g/t). As a porphyry-skarn deposit, Beiya is related to Cenozoic (Himalayan) alkaline porphyries. Abundant Bi-minerals have been recognized from both the porphyry- and skarn- ores, comprising bismuthinite, Bi–Cu sulfosalts (emplectite, wittichenite), Bi–Pb sulfosalts (galenobismutite, cosalite), Bi–Ag sulfosalt (matildite), Bi–Cu–Pb sulfosalts (bismuthinite derivatives), Bi–Pb–Ag sulfosalts (lillianite homologs, galena-matildite series), and Bi chalcogenides (tsumoite, the unnamed Bi2Te, the unnamed Ag4Bi3Te3, tetradymite, and the unnamed (Bi, Pb)3(Te, S)4). Native bismuth and maldonite are also found in the skarn ores. The arsenopyrite geothermometer reveals that the porphyry Au mineralization took place at temperatures in the range of 350–450°C and at log fS2 in the range of −8.0 to −5.5, respectively. In contrast, the Beiya Bi-mineral assemblages indicate that the skarn ore-forming fluids had minimum temperatures of 230–175°C (prevailing temperatures exceeding 271°C) and fluctuating fS2–fTe2 conditions. We also model a prolonged skarn Au mineralization history at Beiya, including at least two episodes of Bi melts scavenging Au. We thus suggest that this process was among the most effective Au-enrichment mechanisms at Beiya.

Zircon U–Pb and Molybdenite Re–Os Ages of the Lakange Porphyry Cu–Mo Deposit, Gangdese Porphyry Copper Belt, Southern Tibet, China

AbstractThe Lakange porphyry Cu–Mo deposit within the Gangdese metallogenic belt of Tibet is located in the southern–central part of the eastern Lhasa block, in the Tibetan Tethyan tectonic domain. This deposit is one of the largest identified by a joint Qinghai–Tibetan Plateau geological survey project undertaken in recent years. Here, we present the results of the systematic logging of drillholes and provide new petrological, zircon U–Pb age, and molybdenite Re–Os age data for the deposit. The ore‐bearing porphyritic granodiorite contains elevated concentrations of silica and alkali elements but low concentrations of MgO and CaO. It is metaluminous to weakly peraluminous and has A/CNK values of 0.90–1.01. The samples contain low total REE concentrations and show light REE/heavy REE (LREE/HREE) ratios of 17.51–19.77 and (La/Yb)N values of 29.65–41.05. The intrusion is enriched in the large‐ion lithophile elements (LILE) and depleted in the HREE and high field‐strength elements (HFSE). The ore‐bearing porphyritic granodiorite yielded a Miocene zircon U–Pb crystallization age of 13.58 ± 0.42 Ma, whereas the mineralization within the Lakange deposit yielded Miocene molybdenite Re–Os ages of 13.20 ± 0.20 and 13.64 ± 0.21, with a weighted mean of 13.38 ± 0.15 Ma and an isochron age of 13.12 ± 0.44 Ma. This indicates that the crystallization and mineralization of the Lakange porphyry were contemporaneous. The ore‐bearing porphyritic granodiorite yielded zircon εHf(t) values between −3.99 and 4.49 (mean, −0.14) and two‐stage model ages between 1349 and 808 Myr (mean, 1103 Myr). The molybdenite within the deposit contains 343.6–835.7 ppm Re (mean, 557.8 ppm). These data indicate that the mineralized porphyritic granodiorite within the Lakange deposit is adakitic and formed from parental magmas derived mainly from juvenile crustal material that partly mixed with older continental crust during the evolution of the magmas. The Lakange porphyry Cu–Mo deposit and numerous associated porphyry–skarn deposits in the eastern Gangdese porphyry copper belt (17–13 Ma) formed in an extensional tectonic setting during the India–Asia continental collision.

Episodic Mesozoic constructional events of central South China: constraints from lines of evidence of superimposed folds, fault kinematic analysis, and magma geochronology

ABSTRACTThe South China block (SCB), located in the convergence zones of the western Pacific subduction tectonic domain and Tethyan tectonic domain, has experienced complex tectonic processes, including the continent–continent collision caused by the closure of the Palaeo-Tethys Ocean, and the subduction of the Palaeo-Pacific plate. However, due to complex intracontinental deformation and abundant magmatism, there are serious divergences in the corresponding records of the tectonic transformation from Palaeo-Tethys to Palaeo-Pacific. We have analysed the map-scale superimposed fold system developed in central SCB and also inverted the palaeo-stress field based on the fault–slip vectors. On this basis, the deformation styles and superposition mechanism of two-stage folds were recovered to establish the tectonic stress field in early Mesozoic and tectono-magmatic events by combining the chronology of the accompanying syntectonic magma. The early E–W/WNW-trending folds and Triassic magmatic system were identified; these were controlled by the NE–SW-trending compressive stress field, to coordinate with the collisions between the SCB and the Indochina block in the southwest, and the North China block in the north. The late NE/NNE-trending folds superimposed on the early folds in an orthogonal way to form a large-scale dome-basin superimposed fold system, which were controlled by the WNW–ESE-trending compressive stress field. According to the strata relations involved in deformation and the chronology data of magmatic rocks, it could be determined that NE/ENE-trending folds would be formed during the Mid- to Late Jurassic, corresponding to the westward subduction events of the Palaeo-Pacific plate. The establishment of the large-scale superimposed fold styles and the identification of fold deformation in the Triassic and Jurassic are important for understanding the early Mesozoic tectonics of South China, and even for all East Asia continent. In particular, it can provide important temporal and spatial constraints to explain the complex deformation process and geodynamic settings of South China in the early Mesozoic.