Indo-Asian Collision Zone Research Articles

Clockwise rotation of the Tarim basin driven by the Indian plate impact. Part II*

In the previous article**, data were given on the clockwise rotation of the Tarim Basin at a speed of 0.461° per million years around a virtual axis within the structure. Additional fieldwork and new evidence confirm earlier findings about the asymmetry of the Indo-Asian collision zone. These data are additional arguments in favor of the rotation of the Tarim Basin and lithospheric interactions along the Tarim boundaries. Conclusions are based on detailed geological and geophysical data.

Eocene palms from central Myanmar in a South-East Asian and global perspective: evidence from the palynological record

AbstractIn the Palaeogene, pollen assemblages at low and mid latitudes are characterized by abundant palm and palm-like (PPL) taxa. Although these taxa have been widely reported, their occurrence in the Palaeogene of Myanmar remains poorly documented. Here we report on the morphology of PPL pollen along a middle to upper Eocene sedimentary sequence in central Myanmar and discuss their nearest living relatives (NLRs). Principal components analysis (PCA) indicates that Palmaepollenites kutchensis, Dicolpopollis and Longapertites were dispersed from freshwater plants, whereas the parent taxon of Proxapertites operculatus was probably a member of the coastal vegetation in the manner of Spinizonocolpites. This, with sedimentological data, suggests a palaeoenvironmental change from a brackish, tidally influenced environment to a fully freshwater setting through the late Eocene. Additionally, we mapped and compared the geographical distribution of selected Eocene palm taxa and their NLRs, and found that their distributions shrank after the Eocene. Moreover, in the Palaeogene, species diversity of selected PPL taxa seems lower in Myanmar than in the Indian subcontinent and other regions in South-East Asia. We hypothesize that in the Eocene the Indo–Asian collision zone formed a ‘hotspot’ for palm diversity, which is reflected in species-rich palynofloras. However, the local palm diversity declined after the Eocene, whereas, at the global level, palm distribution was distinctly reduced between the Eocene and the present. We propose that the retreat of the palms may have occurred as early as the Eocene – Oligocene Transition (EOT), but this remains to be confirmed by the study of EOT pollen records in tropical regions.

Fault zones and stress fields of the Earth’s crust in the vicinity of Ulaanbaatar (Mongolia) at the modern stage of tectogenesis

The seismic and emanation activity of fault zones at a geodynamic test site arranged in the vicinity of Ulaanbaatar to investigate dangerous geological processes in a heavily populated area of Mongolia is analyzed. It has been found that, at the modern stage of tectogenesis, destruction of the Earth’s crust occurs in the area of the strike-slip fault transformed from the Indo-Asian collision zone. Thus, a network of fault zones with seismic and emanation activity is developed in this area. In the vicinity of Ulaanbaatar, there are four systems of fault zones. The spatial pattern of the systems shows a nodal junction of comparatively large rightlateral and left-lateral strike-slip faults of submeridional and sublatitudinal orientation. These strike-slips are barely connected with a network of ancient faults. The northwestern and northeastern zones developed by compression and tension, on the contrary, inherit disjunctive structures of the most recent age located in a nodal junction at the tops of dihedral angles. Determination of the pattern of recent faulting is essential to estimate the seismic hazard for the capital of Mongolia, where more than one-third of the nation lives.

Zircon U–Pb ages of the Mianning–Dechang syenites, Sichuan Province, southwestern China: Constraints on the giant REE mineralization belt and its regional geological setting

The Himalayan Mianning–Dechang (MD) rare earth element (REE) belt in western Sichuan Province, southwestern China, is approximately 270km long and 15km wide, and contains total reserves of more than 3Mt of light REEs (LREEs), comprising one giant (Maoniuping), one large (Dalucao), two small–medium-sized (Muluozhai and Lizhuang), and numerous smaller REE deposits. The belt occurs within the eastern Indo-Asian collision zone (EIACZ), where its location is controlled by large-scale strike-slip faults and tensional fissure zones. Himalayan carbonatite–syenite complexes consist predominantly of alkaline syenite stocks and carbonatite sills or dikes that host REE mineralization. Previous studies have reported inconsistent ages for alkaline magmatism syenite formation and REE mineralization. Here, we present new results of sensitive high-resolution ion micro-probe U–Pb dating of zircons from syenites from the Dalucao, Maoniuping, Lizhuang and Diaoloushan areas, the first systematic and precise age determinations for these rocks in the MD belt. The new data give concordant ages of 12.13±0.19 and 11.32±0.23Ma for the Dalucao deposit, 22.81±0.31 and 21.3±0.4Ma for Maoniuping, 26.77±0.32Ma for Muluozhai, and 27.41±0.35Ma for Lizhuang. These ages, which should be regarded as maximum ages for the REE mineralization in the study area, can be split into two groups, i.e. 11–12Ma in the southern part of the MD belt and 12–27Ma in the northern part, suggesting a progression of magmatism from north to south. These data suggest that the majority of carbonatite–syenite magmatism within the EIACZ occurred during the main stage of Himalayan metallogenesis. The ages presented in this study suggest that strike-slip shear along the MD belt was initiated at ca. 27Ma and ended ca. 12Ma. This timing is consistent with movements along the adjacent Ailaoshan–Red River strike-slip fault in southeastern Tibet (to the south of the MD belt) and one of the three Cenozoic strike-slip faults in eastern Tibet. Ascent of an asthenospheric mantle diapir beneath the EIACZ in the Cenozoic may have provided a thermal mechanism for the generation of magmas that formed the carbonatite–syenite complexes in the study area. Alternatitvely, the magmas may have been generated by decompression melting associated with the transition from a transpressional to a transtensional regime at 38–40Ma. The precise age results for syenite magmatism in the study area indicate that this transition occurred prior to carbonatite–syenite magmatism and the formation of the MD REE belt, which is consistent with the regional tectonic model.

Distributed deformation around the eastern tip of the Kunlun fault

Whether active strain within the Indo-Asian collision zone is primarily localized along major strike-slip fault systems or is distributed throughout the intervening crust between faults remains uncertain. Despite refined estimates of slip rates along many of the major fault zones, relatively little is known about how displacement along these structures is accommodated at fault terminations. Here, we show that a systematic decrease in left-lateral slip rates along the eastern ~200 km of the Kunlun fault, from >10 mm/year to <1 mm/year, is coincident with high topography in the Anyemaqen Shan and with a broad zone of distributed shear and clockwise vorticity within the Tibetan Plateau. Geomorphic analysis of river longitudinal profiles, coupled with inventories of cosmogenic radionuclides in fluvial sediment, reveal correlated variations in fluvial relief and erosion rate across the Anyemaqen Shan that reflect ongoing differential rock uplift across the range. Our results imply that the termination of the Kunlun fault system is accommodated by a combination of distributed crustal thickening and by clockwise rotation of the eastern fault segments.

Late Miocene–Pliocene range growth in the interior of the northeastern Tibetan Plateau

The time-space patterns of deformation throughout the Indo-Asian collision zone can place constraints on the processes responsible for the development of high topography. Although most agree that high topography associated with the Tibetan Plateau expanded throughout the Cenozoic, it is increasingly being recognized that portions of the present-day plateau experienced a protracted history of deformation starting before or shortly after collision. Deciphering the history of deformation in these regions is central to understanding the dynamics of plateau formation. Here, we report new constraints on the timing of shortening along the southern margin of the Gonghe Basin complex, a broad Tertiary–Quaternary depocenter within the interior region of the northeastern Tibetan Plateau. Deformation of basin strata, lithostratigraphic patterns, and changes in paleocurrents record the growth of structures along the southern margin of the basin. A novel combination of magnetostratigraphy and cosmogenic burial ages from fluvial deposits provides a chronology that suggests that sediment accumulation initiated at ca. 20 Ma and that indicates the basin-bounding structures became active during the late Miocene, between ca. 10 and 7 Ma. The probable onset of basin development in the early Miocene is similar to other regions of the northeastern Tibetan Plateau, and it appears to herald the onset of widespread contractional deformation in the region. Moreover, late Miocene activity on thrusts bounding the southern margin of Gonghe Basin was broadly synchronous with the rise of mountain ranges elsewhere along the periphery of the plateau, suggesting a coordinated pulse of growth of high topography during this time.

Thermochronology of the PoSen complex, northern Vietnam: Implications for tectonic evolution in SE Asia

The PoSen complex, located closely adjacent to the southwestern margin of the Red River shear zone represents the uplifted basement of north Vietnam and may record the motion of the shear zone. However, its thermochronological history has not been fully examined yet. Here we applied U–Pb and 40Ar/ 39Ar dating methods to reveal its thermochronological history. U–Pb analysis of composite zircon grains by TIMS yielded an average age of 760 ± 25 Ma, clustering on the concordia line. Twelve SHRIMP U–Pb analyses also yielded a consistent result of 751 ± 7 Ma. Along with the geochemical features, the U–Pb dating results suggest the PoSen complex was a late Proterozoic magmatic complex, which could correspond to the Chengjiang orogeny, a widespread thermal event in southwest China. Results of 40Ar/ 39Ar dating of micas and K-feldspars were in the range of 36–30 Ma, revealing a rapid cooling and exhumation history of the PoSen complex during the late Paleogene. The time span of cooling and exhumation of the PoSen complex is slightly older than the main cooling phases of the Ailao Shan–Red River (ASRR) metamorphic massifs (28–17 Ma), but is synchronous with the early igneous activity stage in the eastern Indo-Asian collision zone of southeast China and north Vietnam. Owing to the ongoing debate about the initiation and offset of the ASRR shear zone, the tectonic force for the late Paleogene cooling of the PoSen complex is still inconclusive. The rapid exhumation of the PoSen complex could be in response to either the detachment of the Neo-Tethyan slab or a transpressional phase of continental subduction along the ASRR shear system in the eastern Indo-Asian collision zone.

Millennial slip rates along the eastern Kunlun fault: Implications for the dynamics of intracontinental deformation in Asia

The role of major strike-slip faults in the Indo-Asian collision zone is central to our understanding of the ways in which continental crust and lithosphere deform in response to continental collision. We investigated how slip varies along the eastern segments of the Kunlun fault in northeastern Tibet. Millennial slip rates were determined based on landforms that are offset by the fault and that were dated using a combination of 14 C and cosmogenic radionuclide exposure dating techniques. We developed estimates for slip rates at four new locations along the fault in addition to four previously published sites. All of these sites are located along the eastern 300 km of the fault system, and our results reveal a systematic eastward decrease in slip rate along this portion of the fault since the late Pleistocene. This displacement gradient is consistent with the termination of the Kunlun fault near ∼102°E. Coincident variations in fault slip rates and geometry reflect complex kinematics along the fault zone. Although other faults exist in the region, our observations suggest that none of these accomplishes transfer of slip from the primary Kunlun fault system. Instead, we interpret that either the eastern Kunlun fault is relatively young and propagating eastward, or that left-lateral slip is absorbed by interaction of the fault zone with regional rotation of the eastern fault tip. Both of these scenarios contrast with previous interpretations and indicate that the Kunlun fault does not accommodate the eastward extrusion of the central Tibetan Plateau lithosphere.

The Himalayan Mianning–Dechang REE belt associated with carbonatite–alkaline complexes, eastern Indo-Asian collision zone, SW China

The Himalayan Mianning–Dechang REE belt, western Sichuan, SW China, is approximately 270 km long and 15 km wide, and contains total reserves of > 3 Mt of LREE, including one giant (Maoniuping), one large (Dalucao) and a number of small-medium REE deposits (Muluozhai, Lizhuang) and occurrences. The belt is tectonically located within a continental collision zone, i.e., the eastern Indo-Asian collision zone. REE mineralization is associated with Himalayan carbonatite–alkaline complexes, which consist of carbonatitic sills or dykes and associated alkaline syenite stocks. Available dating data define a Himalayan metallogenic epoch (40–10 Ma). The magmatic–hydrothermal REE systems were controlled by Cenozoic large-scale strike-slip faulting and the resultant tensional fissure zones, developed in a transform phase from a transpressional to a transtensional regime. Alteration is characterized by fenitization, which formed fenite halos enveloping the REE orebodies. Associated REE mineralization occurs as complex vein systems consisting of various veinlets, stringer and stockwork zones, and breccia-pipe systems. The REE orebodies show various shapes from plate-like, lenticular to pipe-like in different districts. Ore types are dominated by pegmatitic, carbonatitic, breccia, and stringer (stockwork), and disseminated ores, mainly composed of barite + fluorite + aegirine-augite + calcite + bastnaesite assemblages. Bastnaesite precipitation (275–325 °C) appears in three assemblages: early clastics-bearing barite + arfvedsonite + calcite + bastnaesite; intermediate quartz + fluorite + barite + bastnaesite; and late fluorite + barite + bastnaesite assemblages. These were followed by low-temperature pyrite + galena + sphalerite assemblages. Fluid inclusion studies indicate that gangue quartz and fluorite hosts numerous melt/fluid inclusions with a large proportion of sulfate (i.e., BaSO 4, K 2SO 4, and CaSO 4), in addition to CaCO 3 and CaF 2, yielding abnormally high (up to 750 °C) but wide-ranging homogenization temperatures (750–125 °C). The similarity of Sr–Nd isotopic compositions for gangue minerals with those of the host carbonatite–syenite suggests that ore-forming fluids, especially the F −, HCO 3 2−, SO 4 2− components, were derived from carbonatite–syenite magmas. The O–D–C isotopic data from gangue calcite, fluorite and quartz indicate that the fluids were of orthomagmatic origin, but also involved mixing with an external fluid. On the basis of our synthesis, we propose a possible genetic model for REE mineralization, in which the hydrothermal system temporally underwent a complicated evolution from separation of high-T sulfate-bearing NaCl–KCl brine resulting in effective precipitation of REE-fluorocarbonate and sulfate, to subsequent mixing with low-temperature meteoric water precipitating minor sulfide assemblages. Spatially, this sequence of events generated a three-tier REE mineralized architecture, in which pipe-like breccia orebodies grade downwards from shallow to deep structural levels into large veinlet ore systems, and veinlet-disseminated orebodies.

Thrust-controlled, sediment-hosted, Himalayan Zn–Pb–Cu–Ag deposits in the Lanping foreland fold belt, eastern margin of Tibetan Plateau

Cenozoic economic Zn–Pb–Cu–Ag ore deposits occur in the Lanping foreland fold belt (Lanping basin) of the Eastern Indo-Asian collision zone (EIACZ), eastern margin of the Tibetan plateau. They are sediment-hosted and controlled by two Cenozoic thrust–nappe systems developed in the western and eastern segments of the Tertiary basin, respectively, where thrusting was towards the centre of the basin in an approximately E–W direction and juxtaposes Triassic–Cretaceous over Tertiary strata. In the western system, Cu deposits occur as veins in the root zone, hosted by ~ NS-striking faults in Jurassic intense folds. Zn deposits occur in its front zone, controlled by second-order faults of major thrust faults and thrust-related strike-slip faults. In the eastern system, Zn–Pb–Cu–Ag deposits are located in the front zone, controlled by an E-dipping thrust fault and its second-order faults. The giant Jinding Zn–Pb deposit is controlled by a trap structure related to regional thrusting. Homogenization temperatures of fluid inclusions from these deposits are predominately < 200 °C, with salinities of 1.6 to 27.7 wt.% NaCl equiv. Ore sulfides have Pb isotopes similar to the rocks in the basin and δ 34S CDT values ranging from − 30.4‰ to 11.2‰. H–O isotopes of fluid inclusions are similar to those of basinal brines, and C–O isotopes of hydrothermal carbonate minerals are partly close to those of carbonates and sedimentary organisms. These data suggest that the ore-forming fluids and materials mainly derive from the fluids and various rocks of the basin, respectively. The sediment-hosted Zn–Pb–Cu–Ag deposits in the Lanping foreland fold belt share some similarities with MVT-, SST-, SSC-, and SEDEX-type deposits in that there is a lack of any igneous affinity and instead must relate to activities of basinal fluids. The deposits show, however, a set of unique features: (1) they were formed in a strongly deformed foreland basin within a continent collision orogen and (2) closely related to regional thrust–nappe structure; (3) they are fault-controlled, commonly without strong selection for type of host rocks; and (4) contain a variety of metals including Zn, Pb, Cu, Ag, and minor Sr, Co, etc. It is suggested that the Zn–Pb–Cu–Ag deposits controlled by the thrust systems in the Lanping foreland belt are a new sub-type within the family of sediment-hosted base metal deposits.

Long recurrence interval of faulting beyond the 2005 Kashmir earthquake around the northwestern margin of the Indo-Asian collision zone

The 2005 Kashmir earthquake in Pakistan occurred on a previously mapped active fault around the northwest margin of the Indo-Asian collision zone. To address the quantitative contribution of the earthquake to plate convergence, we performed paleoseismological trench excavations at Nisar Camp site near Muzaffarabad across the middle section of the 2005 surface rupture. The fault strands exposed in the trench cut late Holocene fluvial deposits and record evidence of both the 2005 and a penultimate event, supported by the presence of colluvial deposits and a downdip increase in displacement along the fault strands. The 2005 event produced a net slip of 5.4 m, and the penultimate earthquake exhibits a similar amount of slip. Radiocarbon ages and historical accounts loosely constrain the timing of the penultimate event between 500 and 2200 yr B.P.; however, the exposed section encompasses ~4 k.y. of stratigraphy, suggesting an average interevent interval of ~2 k.y. for the 2005 type events. We thus conclude that the 2005 event did not occur on the plate boundary megathrusts, but on intraplate active faults within the Sub-Himalaya. Consequently, the accumulated elastic strain around the complex northwestern margin of the Indo-Asian collision zone has not been significantly released by the 2005 earthquake.

Yulong Deposit, Eastern Tibet: A High-Sulfidation Cu-Au Porphyry Copper Deposit in the Eastern Indo-Asian Collision Zone

The Yulong ore body is the largest Cu deposit (6.22 million metric tons [Mt] at 0.99% Cu) in the 300 km long Himalayan porphyry copper belt, and is controlled by major Cenozoic strike-slip faults in the eastern Indo-Asian collision zone. It is associated with a steeply dipping, pipe-like multiphase (42-35 Ma) monzogranitic stock. The host rocks are potassic calc-alkaline or shoshonitic, and show geochemical affinities with adakites. They appear to have been derived from a thickened lower crustal source in East Tibet. The Yulong deposit consists of a ring-shaped, high-grade Cu-Au zone overlying and/or surrounding a porphyry-type Cu-Mo ore body. Cu-Mo mineralization produced a steeply dipping, pipe-like, veinlet-disseminated ore body within the stock. Associated hydrothermal alteration produced K-silicate and quartz-sericite assemblages within the stock, and contemporaneous propylitic alteration in the Upper Triassic sandy-slate wall rock. Fluid inclusion and δ18O-δD data indicate that the ore-forming fluid was supercritical, and exsolved from a high-level magma chamber at >620°C; it then separated into a hypersaline aqueous liquid and a coexisting low-salinity vapor at 340°-600°C. The high-grade Cu-Au zone (3 Mt at 4.74% Cu, and 4.5 g/t Au) is dominated by a supergene chalcocite-malachite blanket resting on an underlying supergene/hypogene sulfide transition unit and a hypogene pyrite-chalcopyrite sulfide unit. The Cu-Au zone was controlled by a subhorizontal or gently outward dipping breccia horizon developed along the marginal fracture zone near the roof of the stock, produced by hydrothermal brecciation due to regional uplift and/or fluid boiling. Alteration associated with the hypogene Cu-Au mineralization was texture-destructive advanced argillic alteration, characterized by associations of quartz, kaolinite, dickite, endellite, montmorillonite, hydromica, and minor alunite. It mainly developed within the breccia horizons, and partially over-printed the early-formed K-silicate zone and the quartz-sericite zone. Associated mineralization was of the high-sulfidation epithermal-type, characterized by chalcocite, tennantite, covellite, bornite, and minor pyrite, which formed the main ore body in the high-grade Cu-Au zone. Epithermal fluids also caused the dissolution of early-formed sulfides and remobilization of Cu-Mo, the latter transported into the intense advanced argillic alteration halo within the mineralized stock. This late-stage alteration and mineralization is attributed to a CO2-rich, low-temperature (<350°C), low-salinity (<12 wt% NaCl equiv.) meteoric fluid, involving input of magmatic fluid. Based on alteration, mineralization, fluid inclusion and stable isotopic data, a two-stage genetic history has been reconstructed for the Yulong deposit. It spans (1) a magmatic hydrothermal environment reflecting the emplacement of the monzogranite stock and Cu-Mo introduction through (2) hydrothermal fluid infiltration of breccia zones to epithermal overprinting.

Impact of riser reconstructions on estimation of secular variation in rates of strike–slip faulting: Revisiting the Cherchen River site along the Altyn Tagh Fault, NW China

Slip rates are heavily contested for many of the first-order strike–slip faults within the Indo–Asian collision zone. Rates determined geodetically are generally lower than those reported using reconstructions of offset landforms, and it is unclear if this discrepancy reflects true secular variation in slip history, systematic errors in interpretation, or both. Here I examine the methods used to derive slip rates from reconstructions of displaced fluvial risers, and show that such rates are subject to potentially important uncertainties that have largely been overlooked. Review of published data demonstrates that the slip rate can vary by a factor of 1.2 to 5 for the same site, depending on whether the reconstruction uses the age of the upper or lower terrace for the initiation of riser offset. To reduce this epistemic uncertainty, I have developed six geomorphic indices that can be used to identify the most accurate end-member reconstruction: (1) comparison of riser offset with inset channel width; (2) similarity of riser and tread displacements; (3) morphological analysis of the scarp profile; (4) riser deflection in plan view; (5) diachroneity of terrace abandonment; and (6) whether riser crests or bases yield the slip vector. Without the use of these geomorphic observations the epistemic uncertainty associated with current methods for determining slip rate from offset fluvial risers is likely to be so large that there is no resolvable discrepancy between the GPS and geologic rates within the Indo–Asian collision zone. Application of three of these indices to new field mapping of the Cherchen He (River) Site (∼ 86.4°E) along the central Altyn Tagh Fault yields a revised slip rate of 9.4 ± 2.3 mm/yr, equivalent within error to the rates previously derived from GPS and paleoseismic studies. Although reinterpretation of published data from the Sulamu Tagh site (∼ 87.4°) is consistent with this revised rate, additional work is needed to reconcile this result with some of the rates reported near Aksay (∼ 94°E).

Petrography and chemistry of the bed sediments of the Red River in China and Vietnam: Provenance and chemical weathering

The Red (Hong) River straddles southwestern China and northern Vietnam and drains the eastern Indo-Asian collision zone. We collected bed sediments from its tributaries and main channel and report the petrographic point counts of framework grains and major oxide compositions as well as organic and inorganic carbon contents. The Q:F:Rf ratios and Q:F:(L–L c) ratios of the bed-load indicate quartz-poor, mineralogically immature sediments of recycled orogen provenance. The weathering indices based on major oxides — the chemical index of alteration (CIA) and the weathering index of Parker — are also consistent with the recycled sedimentary nature of the bed sediments. Using geographic information system (GIS) we calculated for each sample basin such parameters as temperature, precipitation, potential evapotranspiration, runoff, basin length, area, relief, and areal exposure of igneous, metamorphic and sedimentary rocks. Statistically meaningful correlations are obtained between the two weathering indices, between CIA and sedimentary to metamorphic rock fragments ratio, S / (S + M), and between CIA and sedimentary rock cover, but otherwise correlations are poor. The bed sediments preserve signatures of their provenance, but the effect of weathering is not clearly seen. Subtle differences in the bed sediments are observed between the Red and the Himalayan rivers (Indus, Ganges, and Brahmaputra) as well as between sub-basins within the Red River system and are attributed mainly to differences in lithology.

The Himalayan collision zone carbonatites in western Sichuan, SW China: Petrogenesis, mantle source and tectonic implication

Major and trace element compositions, and Sr–Nd–Pb and O–C isotope data for Cenozoic carbonatites (WSC) in western Sichuan, east Tibet, China are presented in this paper. The WSC occur in the eastern Indo-Asian collision zone (EIACZ), and occur as sills or dykes in the contemporaneous syenitic intrusions, which form a 270-km long, NS-trending belt of carbonatite–alkalic complexes, controlled by strike-slip faults that accommodated and adjusted to the collision strain. These carbonatite–alkalic complexes with Cenozoic potassic felsic rocks and calc-alkaline (shoshonitic) lamprophyres form a semi-continuous potassic igneous province with magmatism peaking at ∼35 Ma in the EIACZ. The WSC are characterized by low SiO 2 (< 10.22 %), FeO (< 1.20 %) and MgO (< 0.73 %), and a wide range of CaO content (40.7∼55.4 %) that distinguish them from primary magnesiocarbonatites. Overlapping emplacement ages and Sr–Nd isotopic compositions and similar mantle-normalized trace element patterns to spatially associated syenites suggest an origin for the carbonatites by liquid immiscibility. The WSC are extremely enriched in LILE (Sr, Ba) and light REE, but relatively depleted in high-field strength elements (Nb, Ta, P, Zr, Hf, Ti), suggesting a metasomatized mantle source. Their δ 18O V-SMOW (6.4∼10.5‰) and δ 13C V-PDB (− 3.9 to − 8.5‰) values are similar to those of primary, mantle-derived carbonatites, also suggesting a mantle genesis. However, they have extremely low å Nd( t) of − 3.2 to − 18.7 and relative high ( 87Sr / 86Sr) i of 0.706020∼0.707923, as well as a wide range of 207Pb / 204Pb (15.362 ∼ 15.679) and 208Pb / 204Pb ratios (38.083∼39.202), which distinguish them from most carbonatites around the world. Their Sr–Nd, Sr–Pb and Nd–Pb isotopic signatures indicate that some carbonatites underwent Sr–Nd–Pb contamination by crustal materials, but the least-contaminated carbonatites were derived from a transitional source between EMI and EMII components. The T DM and modeling on Sr–Nd isotopic compositions indicate that the formation of this source probably was related to recycling of oceanic crust and pelagic/terrigenous sediments with various mass ratios. Partial melting of the mantle source was most likely triggered by a Cenozoic asthenospheric mantle diapir related to Indian–Asian continent collision at 65–45 Ma. Rising and emplacement of carbonatitic magmas with coeval potassium-rich magmas took place in the tectonic regime of the transition from transpression to transtension at Eocene/Oligocene boundary in the EIACZ.

Himalayan Cu–Mo–Au mineralization in the eastern Indo–Asian collision zone: constraints from Re–Os dating of molybdenite

We present new Re–Os molybdenite age data on three porphyry Cu–Mo–Au deposits (Yulong, Machangqing, and Xifanping). These deposits are associated with the Himalayan adakitic magmatism that occurred in a continental collision environment, controlled by large-scale Cenozoic strike-slip faults in the eastern Indo–Asian collision zone. Three distinct episodes of Cu–Mo–Au mineralization are recognized. At Yulong, Re–Os isotopic data of four molybdenite samples from sulfide-quartz veins in the quartz–sericite alteration zone yield an isochron with an age of 40.1±1.8 Ma (2σ), coincident to a zircon sensitive high-mass resolution ion microprobe (SHRIMP) age of 40.9±0.1 Ma for the host monzogranite. The molybdenite Re–Os dates, together with K–Ar, Rb–Sr, U–Pb, and 40Ar/39Ar dates on the pre- and intra-ore porphyries, suggest that Cu–Mo–Au mineralization formed during the late stage (∼40 Ma) of regional porphyry magmatism, but hydrothermal activity probably lasted to at least ∼36 Ma. At Machangqing, molybdenite Re–Os data from the K–silicate and quartz–sericite alteration zones yield an isochron with an age of 35.8±1.6 Ma (2σ), which is identical to the zircon SHRIMP and bulk-rock Rb–Sr ages (35∼36 Ma) of the host granite, but older than bulk-rock K–Ar dates (31∼32 Ma) for associated Au-bearing quartz syenite with advanced argillic alteration. At Xifanping, five molybdenite samples from the K–silicate alteration zone yield the youngest Re–Os isochron age in the area, at 32.1±1.6 Ma (2σ). The Re–Os molybdenite dates here are younger than K–Ar ages (33.5∼34.6) for hydrothermal biotite and actinolite. There is a positive correlation between the absolute age of the deposits and their Cu and Au reserves in the eastern Indo–Asian collisional zone. Episodic stress relaxation probably caused multiple magmatic intrusions, which most likely resulted in three episodes of Cu–Mo–Au mineralization in the eastern Indo–Asian collision zone.

Potassic Magmatism in Western Sichuan and Yunnan Provinces, SE Tibet, China: Petrological and Geochemical Constraints on Petrogenesis

Potassic volcanism in the western Sichuan and Yunnan Provinces, SE Tibet, forms part of an extensive magmatic province in the eastern Indo-Asian collision zone during the Paleogene (40–24Ma). The dominant rock types are phlogopite-, clinopyroxeneand olivinephyric calc-alkaline (shoshonitic) lamprophyres. They are relatively depleted in Na2O, Fe2O3, and Al2O3 compared with the late Permian–early Triassic Emeishan continental flood basalts in the western part of the Yangtze craton, and have very high and variable abundances of incompatible trace elements. Primitive mantle-normalized incompatible element patterns have marked negative Nb, Ta and Ti anomalies similar to those of K-rich subduction-related magmas, although the geodynamic setting is clearly post-collisional. Spatially, some incompatible trace element abundances, together with inferred depths of melt segregation based on the Mg-15 normalized compositions of the samples, display progressive zonation trends from SW to NE with increasing distance from the western boundary of the Yangtze craton. Systematic variations in major and trace element abundances and Sr–Nd–Pb isotope compositions appear to have petrogenetic significance. The systematic increases in incompatible trace element abundances from the western margin to the interior of the Yangtze craton can be explained by progressively decreasing extents of partial melting, whereas steady changes in some incompatible trace element ratios can be attributed to changes in the amount of subduction-derived fluid added to the lithospheric mantle of the Yangtze craton. The mantle source region of the lamprophyres is considered to be a relatively refractory phlogopite-bearing spinel peridotite, heterogeneously enriched by fluids derived from earlier phases of late Proterozoic and Palaeozoic subduction beneath the western part of the Yangtze craton. Calculations based on a non-modal batch melting model show that the degree of partial melting ranges from 0 6% to 15% and the proportion of subduction-derived fluid added from 0 1% to 0 7% (higher-Ba fluid) or from 5% to 25% (lowerBa fluid) from the interior to the western margin of the Yangtze craton. Some pre-existing lithospheric faults might have been reactivated in the area neighbouring the Ailao Shan–Red River (ASRR) strike-slip belt, accompanying collision-induced extrusion of the Indo-China block and left-lateral strike-slip along the ASRR shear zone. This, in turn, could have triggered decompression melting of the previously enriched mantle lithosphere, resulting in calc-alkaline lamprophyric magmatism in the western part of the Yangtze craton.

Cenozoic rifting and volcanism in eastern China: a mantle dynamic link to the Indo–Asian collision?

The Indo–Asian continental collision is known to have had a great impact on crustal deformation in south-central Asia, but its effects on the sublithospheric mantle remain uncertain. Studies of seismic anisotropy and volcanism have suggested that the collision may have driven significant lateral mantle flow under the Asian continent, similar to the observed lateral extrusion of Asian crustal blocks. Here we present supporting evidence from P-wave travel time seismic tomography and numerical modeling. The tomography shows continuous low-velocity asthenospheric mantle structures extending from the Tibetan plateau to eastern China, consistent with the notion of a collision-driven lateral mantle extrusion. Numerical simulations suggest that, at the presence of a low-viscosity asthenosphere, continued mass injection under the Indo–Asian collision zone over the past ∼50 My could have driven significant lateral extrusion of the asthenospheric mantle, leading to diffuse asthenospheric upwelling, rifting, and widespread Cenozoic volcanism in eastern China.

Geochronology and geochemistry of deep-drill-core samples from the basement of the central Tarim basin

The Tarim basin between the Tibetan plateau to the south and Tian Shan to the north in the Indo-Asian collision zone is little deformed as indicated by flat-lying Cenozoic strata across much of the basin. Due to the lack of direct observations from its crystalline basement, the geologic setting for the existence of such a rigid Cenozoic block remains elusive. Hypotheses for the nature of the Tarim basement include (1) Precambrian basement, (2) late Paleozoic trapped oceanic basin, (3) a late Precambrian failed rift, and (4) a Precambrian oceanic plateau. These models make specific predictions about the age and composition of the Tarim basement. To test these hypotheses, we conduct geochemical and geochronologic analyses of samples recovered from a deep well that reached a depth of >7000 m and drilled into the crystalline basement for ∼35 m beneath the central Tarim basin. Mineralogical composition and major element analysis suggest that the crystalline from the drill core is a diorite. Under think sections the rocks samples consist of fine-grained (0.1–0.4 mm in the longest dimension) and medium-grain domains (2–3 mm in the longest dimension). The contact between the two domains is sharp and the change in grain size across the boundary is abrupt. The rock under thin section shows undeformed igneous textures. Rare earth element patterns and isotopic compositions of Sr and Nd suggest that the central Tarim diorite was derived from an arc setting. The minimum age of the diorite is determined by 40Ar/ 39Ar dating of hornblende, which yields three ages from three different samples: 790.0±22.1, 754.4±22.6, and 744.0±9.3 Ma, respectively (uncertainty is reported at 1 σ). The older age is associated with the fine-grained sample while the younger ages are associated with the medium-grained samples. We are unable to determine whether the different ages are due to argon loss as the rock was located in partial retention zone or caused by different phases of igneous intrusion. In any case, the initiation of the pluton intrusion in the central Tarim region must predate 790.0±22.1 Ma. The possible existence of a Proterozoic magmatic arc (>790 Ma) in the central Tarim region may be spatially correlated to a late Precambrian blueschist belt in the southern Tian Shan and a 970–920 Ma plutonic belt in the central and eastern Altyn Tagh range east and west of the drilling site where our samples were obtained. This regional correlation implies the existence of an east-trending Precambrian subduction system underneath the Tarim basin, which might be related to a north-dipping subduction zone. Our new data do not support the Tarim to be floored by a remnant oceanic basin or an oceanic plateau. Our results in conjunction with the fact that the Tarim interior has experienced major deformation as recent as the Jurassic do not support the hypotheses that the Tarim basement is compositionally different from and mechanically stronger than its surrounding areas during Cenozoic deformation. Differences in thermal structures or stress state could be alternative explanations for the highly inhomogeneous distribution of strain across central Asia during the Cenozoic Indo-Asian collision.

A tectonic model for Cenozoic igneous activities in the eastern Indo–Asian collision zone

Geochronologic dating and compilation of existing age data suggest that Cenozoic activities in the eastern Indo–Asian collision zone of southeast China and Indochina occurred in two episodes, each with distinctive geochemical signatures, at 42–24 Myr and 16–0 Myr. The older rocks are localized along major strike–slip faults such as the Red River fault system and erupted synchronously with transpression. The younger rocks are widely distributed in rift basins and are coeval with east–west extension of Tibet and eastern Asia. Geochemical data suggest that the early igneous phase was generated by continental subduction while the late episode was caused by decompression melting of a metasomatically altered, depleted mantle. The magmatic gap between the two magmatic sequences represents an important geodynamic transition in the evolution of the eastern Indo–Asian collision zone, from processes controlled mainly by crustal deformation to that largely dominated by mantle tectonics.