Di-Cheng Zhu

Lhasa terrane in southern Tibet came from Australia

The U-Pb age and Hf isotope data on detrital zircons from Paleozoic metasedimentary rocks in the Lhasa terrane (Tibet) defi ne a distinctive age population of ca. 1170 Ma with e Hf (t) values identical to the coeval detrital zircons from Western Australia, but those from the western Qiangtang and Tethyan Himalaya terranes defi ne an age population of ca. 950 Ma with a similar e Hf (t) range. The ca. 1170 Ma detrital zircons in the Lhasa terrane were most likely derived from the Albany-Fraser belt in southwest Australia, whereas the ca. 950 Ma detrital zircons from both the western Qiangtang and Tethyan Himalaya terranes might have been sourced from the High Himalaya to the south. Such detrital zircon connections enable us to propose that the Lhasa terrane is exotic to the Tibetan Plateau system, and should no longer be considered as part of the Qiangtang‐Greater India‐Tethyan Himalaya continental margin system in the Paleozoic reconstruction of the Indian plate, as current models show; rather, it should be placed at the northwestern margin of Australia. These results provide new constraints on the paleogeographic reconstruction and tectonic evolution of southern Tibet, and indicate that the Lhasa terrane evolved as part of the late Precambrian‐early Paleozoic evolution as part of Australia in a different paleogeographical setting than that of the Qiangtang−Greater India−Tethyan Himalaya system.

Magmatic record of India-Asia collision

New geochronological and geochemical data on magmatic activity from the India-Asia collision zone enables recognition of a distinct magmatic flare-up event that we ascribe to slab breakoff. This tie-point in the collisional record can be used to back-date to the time of initial impingement of the Indian continent with the Asian margin. Continental arc magmatism in southern Tibet during 80–40 Ma migrated from south to north and then back to south with significant mantle input at 70–43 Ma. A pronounced flare up in magmatic intensity (including ignimbrite and mafic rock) at ca. 52–51 Ma corresponds to a sudden decrease in the India-Asia convergence rate. Geological and geochemical data are consistent with mantle input controlled by slab rollback from ca. 70 Ma and slab breakoff at ca. 53 Ma. We propose that the slowdown of the Indian plate at ca. 51 Ma is largely the consequence of slab breakoff of the subducting Neo-Tethyan oceanic lithosphere, rather than the onset of the India-Asia collision as traditionally interpreted, implying that the initial India-Asia collision commenced earlier, likely at ca. 55 Ma.

The 132 Ma Comei-Bunbury large igneous province: Remnants identified in present-day southeastern Tibet and southwestern Australia

We report 11 new U-Pb zircon ages obtained by sensitive high-resolution ion microprobe (SHRIMP) and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP–MS) for a large province of Early Cretaceous Comei igneous rocks consisting of basaltic lavas, mafi c sills and dikes, and gabbroic intrusions together with subordinate layered ultramafi c intrusions and silicic volcanic rocks exposed in the Tethyan Himalaya, southeastern Tibet. Available zircon U-Pb ages obtained from various rocks in this province, which has an areal extent of ~40,000 km 2 (~270 km × 150 km), indicate that the magmatism occurred ca. 132 Ma ago, coeval with the Bunbury Basalt in southwestern Australia. Such a striking similarity in emplacement age, in combination with the tectonic reconstruction of eastern Gondwana ca. 132 Ma ago, allows us to propose that the extensive Comei igneous rocks in southeastern Tibet and the Bunbury Basalts in southwestern Australia may represent the erosional and/or deformational remnants of a large igneous province, which we call the Comei-Bunbury LIP. We argue that this newly identifi ed LIP was likely caused by the Kerguelen mantle plume, which started in the Early Cretaceous and may have played a role in the breakup of eastern Gondwana and the development of the 132 Ma old Weissert oceanic anoxic event.

Presence of Permian extension- and arc-type magmatism in southern Tibet: Paleogeographic implications

The geographical location of the Lhasa terrane in the Permian remains a subject of debate. The recognition of the Permian basalts in the Tethyan Himalaya and the Permian volcanic rocks in the Lhasa terrane in southern Tibet together with the geochemistry of these rocks offer some new insights. The Permian basalts in the Tethyan Himalaya show a geochemical affi nity with tholeiitic continental fl ood basalts, and are interpreted to have formed in an extensional setting. The new geochemical data and the geographical distribution of these basalts indicate that they probably represent the easternmost extent of the Panjal continental fl ood basalt province. All of the Permian basalts in the Lhasa terrane show a calcalkaline , high-alumina basalt affi nity, with signifi cant negative Nb-Ta-Ti anomalies. These geochemical features, combined with the recent documentation of the Permian Songdo eclogite and sedimentological observations, indicate the existence of a subduction system beneath the central Lhasa subterrane in the Permian. The presence of both extension- and arc-type magmatism of Permian age in present-day southern Tibet is inconsistent with the general view that the Lhasa terrane did not rift away from the northern margin of the Greater India until the Late Permian or Triassic. Instead, we suggest that the central Lhasa subterrane may have been a microcontinent isolated in the Paleo-Tethyan Ocean basin, at least during the Carboniferous‐Middle Permian time.

Zircon xenocrysts in Tibetan ultrapotassic magmas: Imaging the deep crust through time

Zircons entrained in mantle-derived magmas offer a unique opportunity to identify cryptic magmatic episodes in the deep crust and thus to image lithospheric thickening and crustal evolution. We investigated zircon xenocrysts from mantle-derived ultrapotassic rocks in southern Tibet to evaluate their potential as a probe of crustal evolution. Similar age (Proterozoic–Paleozoic) distributions of these zircons and those in the Lhasa terrane detrital spectra demonstrate the continental origin of xenocrysts with high U/Yb. Timeprogressive variations in zircon e Hf (t) reveal three major magmatic pulses ca. 90, 50, and 20 Ma, suggesting signifi cant crustal growth in the Lhasa terrane at those times. This is consistent with major mantle inputs previously documented from surface rocks in the Lhasa terrane. Increasing Dy N /Yb N and U/Yb since ca. 55 Ma are interpreted to refl ect progressive crustal thickening in response to the India-Asia convergence. Zircon xenocrysts with varying U-Pb ages and heterogeneous Hf isotopes indicate assimilation of Lhasa terrane crust in the genesis of ultrapotassic magmas.

SHRIMP U-Pb zircon dating for the dacite of the Sangxiu Formation in the central segment of Tethyan Himalaya and its implications

Petrology and SHRIMP U-Pb zircon chronology are reported for the dacite of the Sangxiu Formation in the central segment of Tethyan Himalaya to the southeast of Yangzuoyong Co. Twenty-one measured zircon grains from a dacite sample of the Sangxiu Formation can be divided into two groups, which include long columnar magmatic zircons of 133±3.0 Ma, representing the age of volcanism in the Sangxiu Formation, and inherited zircons consisting of core and overgrowth rim, in which ages of the three cores are 2244±16, 1153±33 and 492±25 Ma respectively; and the age of one overgrowth rim is 132.7±5.2 Ma. These ages are considered to represent the remelting of accretionary crust at different periods resulting from the volcanism of Sangxiu Formation. These volcanic rocks in the Sangxiu Formation, for which age is close to that of Bunbury basalt in southwestern margin of Australia, and older than that of Rajmahal basalt in northeastern India, maybe considered as the products of early activity of the hotspot represented by the Rajmahal basalt.

Erratum: Corrigendum: Magmatic record of India-Asia collision

Scientific Reports 5 Article number: 14289; 10.1038/srep14289 published online: September232015; updated: December182015. In Supplementary Figure 1a, the Linzizong volcanic rock samples ‘12LZ14-1’ and ‘12LZ13-1’ should read ‘13LZ14-1’ and ‘13LZ13-1’ respectively. The correct Supplementary Figure 1a appears below as Fig. 1. Figure 1 In Table S1, samples ‘12LZ13-1@02’ and ‘12LZ14-1@02’ should read ‘13LZ13-1@02’ and ‘13LZ14-1@02’ respectively.

Constraining quantitatively the timing and process of continent-continent collision using magmatic record: Method and examples

Based on the main driving force of plate motion (the slab pull force generated by the descent of the oceanic plate in subduction zones) and the three primary mechanisms for magma generation (adding fluid, increasing temperature, and decreasing pressure), the continent-continent collisional process has been divided into three stages, including initial collision, ongoing collision, and tectonic transition. These stages are characterized by normal calc-alkaline andesitic magma (dehydration of the oceanic crust to release fluids), the migration of calc-alkaline magma toward the trench (dehydration of the oceanic crust or an increase in temperature) or small-scale crust-derived peraluminous magma (heat from intra-crustal shearing), and extensive magmatism with compositional diversity induced by slab break-off (increasing temperature and decreasing pressure), respectively. On the basis of the obtained age of slab break-off, the timing of the initial continent-continent collision can be quantitatively back-dated using the convergence rate, depth of slab break-off, and subduction angle. The spatio-temporal migration of the magmatic activity of the Gangdese Batholith, the onset of magmatic flare-up, and the increase of magma temperature at 52–51 Ma documented by the volcanic rocks of the Linzizong Pana Formation were most likely the result of the break-off of the Yarlung-Zangbo Neo-Tethyan oceanic lithosphere at approximately 53 Ma. This proposed age of slab break-off suggests that the initial India-Asia collision likely occurred at approximately 55–54 Ma, which is consistent with the collision ages constrained by other abundant geological data (60–55 Ma). This magmatic method has been applied to the Bitlis orogenic belt in southern Turkey in the Arabia-Eurasia continental collision zone, yielding an age range of approximately 29–22 Ma for the initial Arabia-Asia continental collision that is close to the collision ages recently obtained by apatite fission-track dating (approximately 20 Ma) and regional tectonic shortening (approximately 27 Ma). The intense folding of the Upper Cretaceous and the angular unconformity between the overlying Linzizong volcanic rocks in the southern Lhasa Terrane (90−69 Ma) are not related to the initial continental collision between India and Asia, but can be interpreted as the consequences of the strong coupling between the hot and young subducting oceanic crust immediately south of the spreading ridge and the overriding lithosphere or the subduction of the Neo-Tethys oceanic plateaux or seamounts. The tectonic event documented by the angular unconformity between the Linzizong Dianzhong Formation and the Nianbo Formation lasted approximately 3 Ma and likely marks the initial India-Asia collision. The significant deceleration of the Indian continent at approximately 51 Ma can be attributed to the disappearance of the slab pull force in the subduction zone due to the break-off of the Yarlung-Zangbo Neo-Tethyan oceanic lithosphere. The descent of the eclogitized lower crust of the northern Indian continent provides the main driving force for the current northward motion of Indian plate. The weak deformation of the lithospheric plate in the overriding plate of the India-Asia collisional zone between 60 and 40 Ma can be attributed to the high-angle subduction related to the rollback of the Yarlung-Zangbo Neo-Tethyan oceanic lithosphere after the initial India-Asia continental collision, the presence of the thick crust and high elevation on the southern margin of the Lhasa Terrane, and the decoupling between the mid-upper and lower crust and between the lower crust and lithospheric mantle of the Indian continent.

Geochronology, geochemistry and implications of Au-mineralized porphyries in the Linzhou basin, Gangdese belt, Tibet

The Linzhou porphyry bodies are distributed within or along the edge of Linzhou basin, which is located to the south of Namling-Luobadui-Milashan fault in the central Gangdese magmatic arc. The porphyry bodies can be divided into two groups including the early intruded the Sexing Formation, the Dianzhong Formation and the late intruded the upper Linzizong volcanic successions. The single zircon from the early porphyry body (Hutouya) yielded a restrictedly concordant weighted mean 206Pb/238U age of 58.7±1.1 Ma (2σ) and is ascribed to the collisional stage porphyries when India collided with Asia. The early porphyries are geochemically similar to Linzizong volcanic rocks, and are interpreted to the predominately remelting of crustal components, and to a lesser extent, the contributions from subducted oceanic crust were also involved in their generation. The early porphyries with Au mineralization at collisional stage indicate that the collisional stage intrusives (60-50 Ma) in the Gangdese have potential for gold.

One or Two Early Cretaceous Arc Systems in the Lhasa Terrane, Southern Tibet

Spatial and temporal variations of arc-related magmatism are key to determining the subduction polarity of incompletely preserved arc systems. Petrographic, geochronological, geochemical, and isotope data of Early Cretaceous volcanic rocks from the northern Lhasa Terrane around Yanhu indicate south dipping subduction of the Bangong Tethys. Two distinct calc-alkaline magmatic successions are recognized: older medium-K basalts and Mg-rich andesites (131–116 Ma), and younger high-K basalts and trachyandesites (116–110 Ma). The medium-K basalts exhibit a typical arc signature, whereas the medium-K andesites show higher MgO contents relative to arc andesites. The medium-K series are interpreted as partial melting of a metasomatized mantle wedge source at lower pressure and greater water activity in generating the Mg-rich andesites. The high-K series are characterized by enrichments in highly incompatible elements and are considered as low-degree partial melting of asthenosphere mantle source that was previously metasomatized. All samples show arc-related signatures, which indicate the development of what we term the Baingoin-Yanhu arc in the northern Lhasa Terrane. This observation, in combination with the distribution of Early Cretaceous arc magmatism across the Lhasa Terrane, which prior to subsequent deformation had a width of at least 600 km, requires the existence of two arc systems flanking the Lhasa Terrane and related to opposed oceanic plate subduction: north dipping subduction of the Neo-Tethys and south dipping subduction of the Bangong Tethys. Compositional change from medium-K to high-K calc-alkaline volcanism around Yanhu records changing mantle geodynamics, which we infer to reflect rollback and breakoff of the south dipping Bangong Tethyan slab.