Asia Collision Zone Research Articles

Late Miocene to present synchronous extension and contraction in the eastern Pamir: Insights from inversion of thermochronologic data across the southern Muztaghata dome

Late Cenozoic gneiss domes cover ∼30% of the surface of the Pamir salient in the northwestern end of the India−Asia collision zone. The highest peaks of the Pamir are in the east, where the ∼250-km-long, ∼N−S-trending Kongur Shan extensional system controls the topography. We combined 115 new apatite (U-Th-Sm)/He and zircon (U-Th)/He single-grain dates from 18 samples and previous thermochronologic data with three-dimensional thermokinematic models to constrain the thermo-tectonic history of the southern portion of the Muztaghata dome, one of the largest gneiss domes in the eastern Pamir. The new cooling dates from the western boundary of the southern Muztaghata dome generally increase with distance from the southern Kongur Shan fault and are related to normal faulting along the fault at near-surface levels over the last 6.5 m.y. The new dates across the central−eastern portion of the dome outline the previously recorded U-shaped date pattern at a higher spatial resolution. The modeling indicates that this pattern is most likely the result of uplift and erosion above a flat-ramp-flat thrust fault at depth over the last 7 m.y. Modeling does not resolve how topographic changes may have affected the observed distribution of cooling dates, but it indicates a faster thrust-slip rate associated with an increase in relief and a slower one associated with steady-state topography. Our results suggest that the modern topography along the southern Muztaghata dome, similar to the rest of the eastern Pamir salient, is shaped by normal faulting at shallow depth, but its growth may still be governed by contraction and crustal thickening at depth.

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.

Origin of a Miocene alkaline–carbonatite complex in the Dunkeldik area of Pamir, Tajikistan: Petrology, geochemistry, LA–ICP–MS zircon U–Pb dating, and Hf isotope analysis

Intrusive carbonatite complexes host the world’s largest rare-earth element (REE) deposits. However, the genesis of carbonatite complexes remains unclear. A recently discovered carbonatite–syenite complex in the Dunkeldik area of Pamir, Tajikistan, is interpreted as a carbonatite-related REE deposit. The deposit is hosted in an alkaline–carbonatite belt comprising alkali basalt, alkali trachyte, syenogranite, syenitic porphyry, and carbonatite. The present study focuses on a 1–3-m-wide, 25-m-long, NE-trending carbonatite dike and associated syenitic porphyries. The carbonatite comprises mainly calcite with minor apatite, bastnäsite, allanite, and zircon. Relatively high proportions of anhydrite, gypsum, fluorite, quartz, and barite post-date the formation of the carbonatite–syenite complex. The syenitic porphyries contain phenocrysts of microcline, microperthite, and fluorite ± nepheline or quartz in a feldspathic groundmass.Carbonatite samples are characterized by low SiO2 (<5 wt%), FeO (<1.84 wt%), and MgO (<0.25 wt%), and relatively high CaO (29.39–35.04 wt%) concentrations, distinguishing them from primary magnesiocarbonatites. The associated syenitic porphyries yield high SiO2 (54.26–69.59 wt%), Al2O3 (12.73–16.08 wt%), and alkali (K2O + Na2O = 10.84–13.48 wt%; K2O > Na2O) concentrations, and low MgO (0.14–0.97 wt%) and TiO2 (0.21–0.50 wt%) concentrations. The carbonatite and syenitic porphyries show similar mantle-normalized incompatible-trace-element trends, characterized by negative gradients, extreme large-ion lithophile element (e.g., Sr and Ba) and light REE enrichment, and depletion in high-field-strength elements (e.g., Nb, Ta, P, Zr, Hf, and Ti). Laser-ablation–inductively coupled plasma–mass spectrometry U–Pb dating of zircon from carbonatite and syenite porphyry samples yields weighted mean ages of 10.66 ± 0.23 Ma (2σ) and 10.76 ± 0.09 Ma (2σ), respectively. Zircon grains in the carbonatite have moderately radiogenic Hf isotope compositions (εHf = −7.9 to +0.9). Zircons from carbonatite and syenitic porphyry samples show similar REE compositional trends, characterized by enrichment in heavy REEs relative to light REEs. Zircon grains from carbonatite and syenitic porphyry yield Th/U ratios of 0.77–1.18 and 0.85–1.62, respectively. We therefore interpret zircon grains in the carbonatite as xenocrysts inherited from the associated syenites.The similar emplacement ages and trace-element compositions of the spatially associated syenites and carbonatite suggest an origin involving liquid immiscibility. The geochemistry and Hf isotope data indicate that the magmas were derived from thickened lower crust that received an input of enriched mantle-sourced material hybridized by fluids derived from an ancient subducted oceanic slab. We infer that the carbonatite and syenitic porphyries formed in a post-collisional setting during the Miocene. The upwelling of an asthenospheric mantle diapir during the Cenozoic induced the partial melting of metasomatized lithospheric mantle and lower crust beneath the western margin of the Indian–Asian collision zone. The large-scale Karakorum strike-slip fault may have acted as a conduit for the emplacement of the syenite–carbonatite magma.

LA-ICP-MS mineral chemistry of titanite and the geological implications for exploration of porphyry Cu deposits in the Jinshajiang – Red River alkaline igneous belt, SW China

The Jinshajiang–Red River alkaline igneous belt in the eastern Indian–Asian collision zone, of southwestern China, hosts abundant, economically important Cu–Mo–Au mineralization of Cenozoic age. Major- and trace-element compositions of titanites from representative Cu-mineralized intrusions determined by LA-ICP-MS show higher values for Fe2O3/Al2O3, ΣREE + Y, LREE/HREE, Ce/Ce*, (Ce/Ce*)/(Eu/Eu*), U, Th, Ta, Nb and Ga, and lower values for Al2O3, CaO, Eu/Eu*, Zr/Hf, Nb/Ta and Sr than those for titanites from barren intrusions. Different ΣREE + Y, LREE/HREE, U, Th, Ta and Nb values of titanites between Cu-mineralized and barren intrusions were controlled mainly by the coexisting melt compositions. However, different Sr concentrations and negative Eu anomalies of titanites between Cu-mineralized and barren intrusions were most probably caused by different degrees of crystallization of feldspar from melts. In addition, different Ga concentrations and positive Ce anomalies of titanites between Cu-mineralized and barren intrusions were most likely caused by different magmatic fO2 conditions. Pronounced compositional differences of titanites between Cu-mineralized and barren intrusions can provide a useful tool to help discriminate between ore-bearing and barren intrusions at an early stage of exploration, and, thus, have a potential application in exploration for porphyry Cu deposits in the Jinshajiang – Red River alkaline igneous belt, and to other areas.

Origin of the ore-forming fluids of the Tongchang porphyry Cu–Mo deposit in the Jinshajiang–Red River alkaline igneous belt, SW China: Constraints from He, Ar and S isotopes

The Jinshajiang–Red River alkaline igneous belt with abundant Cu–Mo–Au mineralization, in the eastern Indian–Asian collision zone, is an important Cenozoic magmatic belt formed under an intra-continental strike-slip system in southwestern (SW) China. The Tongchang deposit is a representative porphyry Cu–Mo deposit in southern segment of the Jinshajiang–Red River alkaline igneous belt, with 8621t Cu @ 1.24wt.% and 17,060t Mo @ 0.218wt.%. In this study, He, Ar and S isotopic compositions of the Tongchang deposit were determined. He and Ar isotopic compositions suggest that the ore-forming fluids, with 3He/4He ratios varying from 0.17 to 1.50Ra and 40Ar/36Ar ratios from 299.1 to 347.3 for the deposit, are a mixture between a crust-derived fluid (MASW) with near atmospheric Ar and crustal He, and a mantle-derived fluid. However, the δ34S values of the hydrothermal pyrite samples ranging from 1.0‰ to 1.5‰ with an average of 1.2‰, indicate that the sulfur in the ore-forming fluids of the Tongchang deposit was primarily derived from the magma or indirectly mantle-derived without assimilation of crustal sulfur. In combination with previously published He and Ar isotopic data of the Yulong and Machangqing deposits in northern and central segments of the Jinshajiang–Red River alkaline igneous belt, respectively, the ore-forming fluids of the Yulong and Machangqing deposits are obviously richer in 3He and 40Ar, and poorer in 36Ar in comparison with the Tongchang deposit, implying that more mantle-derived fluids were involved in the ore-forming fluids of the Yulong and Machangqing deposits than those for the Tongchang deposit. This might be one of the most important factors producing larger scales of mineralization in the Yulong and Machangqing deposits than the Tongchang deposit.

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).

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.

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.

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.

Miocene 40Ar/ 39Ar ages from the Karakorum Batholith and Shyok Mélange, northern Pakistan, indicate late Tertiary uplift and southward displacement

The magmatic arc of the Karakorum Mountains is separated from the Cretaceous—Tertiary Kohistan arc sequence by an ophiolitic mélange—the Shyok Mélange. Two views of the age of the Shyok Mélange and Karakorum arc are: (a) that they represent a mid-Mesozoic arc-trench complex which collided with the Kohistan arc in the Cretaceous before the closure of the main Tethys ocean to the south; and (b) that they represent a late Tertiary arc-trench complex which closed in the late Miocene, representing final subduction of Tethyan sea-floor. Whether final destruction of ocean crust took place in the Mesozoic or Tertiary has important implications for current models of the development of the Indian—Asian collision zone. Here we wish to report ages of between 20 and 8 Ma, suggesting that final assembly and uplift did not occur until the late Tertiary; and that the Shyok Mélange is the structural equivalent of the Main Central Thrust of the Himalaya to the east. Like the Main Central Thrust, the Shyok Mélange appears to mark intraplate continental subduction which is mainly responsible for Tertiary granitoid intrusion, deformation and uplift in the High Himalaya and Karakorum to the north.