Zhidan Zhao

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.

40Ar-39Ar geochronology of Cenozoic Linzizong volcanic rocks from Linzhou Basin, Tibet, China, and their geological implications

Whole-rock and mineral separate Ar-Ar dating was carried out for the Linzizong volcanic rocks at Linzhou Basin in Tibet to constrain the time span of volcanism and the corresponding stratigraphic sequence. Sampling was based on detailed geologic mapping and stratigraphic sequence of Dianzhong, Nianbo, Pana Formations, systematically from the bottom to near the top. The results indicate that the Linzizong volcanic rocks erupted from Paleocene to middle of Eocene (64.43· 43.93 Ma). Among them, the Pana Formation formed from ca. 48.73 to 43.9 Ma, the Nianbo Formation around 54 Ma and the Dianzhong Formation from 64.4 to 60.6 Ma. In combination with evidence from the geochemical characteristics of the volcanic rocks, and from stratigraphy in southern Tibet, it is postulated that the age of the lowest member in the Dianzhong Formation of the Linzizong volcanic rock, which overlies unconformably the Late Cretaceous Shexing Formation, likely corresponds to the inception of the collision between Indian and Asian continents in southern Tibet.

Miocene volcanism in the Lhasa block, Tibet: spatial trends and geodynamic implications §

Miocene ultrapotassic (UP), shoshonitic (P) and calc-alkaline (CA) volcanism is recognized widely over the Tibetan plateau.This volcanism occurred after the end of India’s oceanic lithosphere subduction beneath the plateau. Ambiguities about the distribution and significance of this volcanism arise in part from a paucity of geochronologic and geochemical data.We have sampled two volcanic fields in the Lhasa block: (1) the eastern edge of the Zabuye Salt Lake (central part of the block), (2) the southwestern edge of the Yangbajin Graben.Major and trace elements display high K2O (6.06^6.54%) over a wide range of SiO2 (75.03^55.30%), Na2O (1.32^3.30%) and LREE/HREE enrichment with a significant rare earth element fractionation (266 (La/Yb)n 6 42) as well as negative Nb, Ta, and Ti anomalies.These features indicate that our samples are chemically identical to the post-collision Miocene UP to P volcanic rocks already identified all over the Tibetan plateau and the southern plateau specifically but distinct from the CA rocks found in the Lhasa block.The Miocene volcanics found in the Lhasa block present greater chemical variation than the rest of the Tibetan plateau. 40 Ar/ 39 Ar ages on both sanidine and biotite gave ages ranging from 16.16 G 0.12 Ma to 16.01 G 0.12 Ma and 10.60 G 0.14 Ma to 10.88 G 0.17 Ma (2 c, excluding systematic errors, relative to Fish Canyon sanidine at 28.02 Ma) for Zabuye Salt Lake and Yangying geothermal field volcanics respectively. These results are consistent with the range of previously published ages from this part of the Tibetan plateau.The presence of N^S-trending dikes and the location of the volcanic fields suggest that local E^W extension in the Lhasa block was coeval.We propose that the synchronous UP, P and CA volcanism in the Lhasa block between 23 and 8 Ma is the result of progressively deepening northward subduction of Indian continental lithosphere producing asthenospheric upwelling.Convective thinning (delamination) of lithospheric mantle fueled by the hot asthenosphere induced partial melting of an enriched sub-continental mantle lithosphere as well as eclogitic lower crust.This mechanism could also explain the local E^W extension observed in the same period (18^10 Ma).We also propose that lateral migration of the deepening Indian continental lithosphere mantle slab could explain the observed younger volcanism in the eastern part of the Lhasa terrane.

Fragments of hot and metasomatized mantle lithosphere in Middle Miocene ultrapotassic lavas, southern Tibet

Uplift of the Tibetan Plateau and its infl uence on our global climate have been the focus of numerous studies. Miocene potassic to ultrapotassic volcanism is widespread in southern Tibet and has been generally attributed to convective removal of collision-thickened Asian lithosphere, which is also responsible for the uplift of the plateau. An implicit assumption of this model is the existence of a hydrous, metasomatized (i.e., phlogopite bearing) lithospheric mantle that remained after the convective thinning and was subsequently heated to form small-volume melts. If such a lithospheric mantle was present in the Miocene, it implies further change since that time, as seismic velocities indicate that cold and strong upper mantle occurs beneath the thick crust in southern Tibet. Here we describe peridotite xenoliths entrained in Middle Miocene ultrapotassic lavas from Sailipu, southern Tibet. The results suggest the existence of hot, highly metasomatized lithospheric mantle beneath southern Tibet during the Middle Miocene, and thus support the idea that convective thinning of the lithosphere was responsible for the uplift of the plateau. The relict mantle was later removed or squeezed northward by the underthrusting Indian continental lithosphere, which terminated magmatism in southern Tibet and played a role in creating the entire plateau.

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.

Geochronology and Geochemistry of the Ore-Forming Porphyries in the Lailisigao'er-Lamasu Region of the Western Tianshan Mountains, Xinjiang, NW China: Implications for Petrogenesis, Metallogenesis, and Tectonic Setting

Porphyry-type Cu (Mo, Zn) deposits have been discovered along the late Paleozoic Kokirqin arc in the western Tianshan Mountains of China, part of the Central Asian Orogenic Belt. The deposits include the Lailisigao’er Mo-Cu deposit, the 3571 Cu deposit, and the Lamasu Cu-Zn deposit. The ore-forming porphyries from these three deposits are predominantly intermediate-felsic and belong to calc-alkaline and transitional series. Laser ablation–ICPMS zircon U-Pb dating on ore-forming porphyries from the Lailisigao’er and 3571 deposits yields ages of Ma and Ma, respectively. The trace element compositions of these porphyries from these three deposits are similar to those formed in a continental arc setting and are characterized by enrichment of large ion lithophile elements and depletion of high field strength elements and heavy rare earth elements ) coupled with slightly negative Eu anomalies. These rocks also show high (87Sr/86Sr)t (0.70722–0.71028) and low ϵNd(t) values (−3.71 to +0.17), coupled with depletion of Ba relative to Th and elevated Th/Ce, Nb/Y, and Th/Yb ratios, suggesting that the porphyry magma originated from a partial melting of subducted sediments, mixed with minor melts produced by partial melting of mantle wedge components and involvement also of lower continental crust during emplacement. These three deposits belong to the first metallogenic group in the Chinese Tianshan, which formed from the Middle Devonian to the early Carboniferous in a continental arc environment related to a subducted oceanic slab; this group is distinguishable from a second group that formed in the Permian during a late collisional stage, in which regional collisional compression changed to extension.

Early Paleozoic Tectonic Evolution of the South Tianshan Collisional Belt: Evidence from Geochemistry and Zircon U-Pb Geochronology of the Tie’reke Monzonite Pluton, Northwest China

We report an Early Paleozoic hornblende quartz monzonitic pluton from the Tie’reke region in the central South Tianshan Collisional Belt (STCB). Laser ablation ICP-MS U-Pb zircon dating reveals that the pluton was emplaced during the Late Silurian at ∼ Ma. Our data, together with those from coeval intrusive rocks in the eastern STCB and the eastern Northern Margin of the Tarim Block (NMTB), indicate a major late Early Paleozoic magmatic event in the region. This magmatic event is supported by a detrital zircon U-Pb age population of 462–395 Ma obtained from a Cenozoic sandstone sample from the Kangsu region and an Early Paleozoic metasandstone sample from the Jigen region. Geochemically, the Tie’reke pluton is intermediate in composition, with SiO2 contents ranging from 60.73 to 64.73 wt%, and belongs to the alkali-calcic and shoshonitic series. The pluton displays relative depletion of Nb, Ta, P, and Ti and enrichment of large-ion lithophile elements (Ba, K, and Rb), typical of continental arc–related igneous rocks. Whole-rock Sr-Nd and zircon Hf isotopic data reveal that the magma was derived dominantly from partial melting of the Paleoproterozoic continental crust, with input from juvenile materials from a depleted-mantle wedge. In general, geochronological, geochemical, and isotopic features of the Late Silurian igneous rocks in the present STCB and NMTB, coupled with detrital zircon U-Pb geochronological data from the two sedimentary rocks, suggest that the northern margin of the Paleozoic Tarim Block was an Andean-type active continental margin during Middle Ordovician to Middle Devonian time. Given the coeval magmatism in the Central Tianshan Block, which necessitates a northward subduction of the Paleozoic South Tianshan Ocean, we propose a double-subduction model for the evolution of the Paleozoic South Tianshan Ocean during the Late Ordovician to Middle Devonian period. During the Late Devonian to Middle Carboniferous, the northern margin of the Paleozoic Tarim Block was likely characterized by tectonomagmatic quiescence, whereas the Central Tianshan Block was still extensively affected by arc-type magmatism, furthering the northward subduction of the Paleozoic South Tianshan Ocean.