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The Neo-Tethyan subduction that operated before the India- Asia collision resulted in an Andean-type convergent margin in South Asia and was associated with extensive arc magmatism that formed the Transhimalayan batholiths. Magmatism in the Gangdese Batholith, the largest batholith exposed in the Lhasa terrane of southern Tibet, is considered to have lasted from the early Jurassic to Eocene. However, eastward correlation of the Gangdese Batholith is uncertain because it is truncated by the eastern Himalayan syntaxis. Here, we report new data from the Lohit Batholith, NE India, including: (i) zircon U-Pb ages of five granitoids from ca. 148 to 96 Ma; and (ii) zircon Hf isotopes of these rocks that yield high and positive εHf(T) values. We argue that the Lohit Batholith is the eastward extension of the Gangdese Batholith, and can be correlated southward to the Wuntho-Popa arc in West Burma, thus linking a prolonged Neo-Tethyan magmatic arc system from southern Tibet to Southeast Asia. [ABSTRACT FROM AUTHOR]

Abstract: Before the continental collision of India with Asia, northward subduction of the Tethyan oceanic lithosphere beneath South Asia resulted in widespread arc magmatism in the Lhasa terrane of southern Tibet from Early Jurassic to Eocene time. The detailed magmatic history of this Transhimalayan arc system, however, remains unclear. Here we report in-situ detrital zircon U–Pb and Hf isotopic analyses of sedimentary rocks from the Xigaze fore-arc basin, which developed along the southern margin of the Lhasa terrane. The zircon U–Pb results allow us to better constrain the depositional age of the Xigaze Group, which consists from bottom to top of the Chongdui, Ngamring, Padana and Qubeiya formations deposited between ca. 116 and 65Ma, with the main stage (i.e., the Ngamring Formation) being deposited between ca. 107 and 84Ma. The majority of these zircons are characterized by high 176Hf/177Hf isotopic ratios and positive ε Hf(t) values that are similar to those of magmatic zircons from the Gangdese batholith, suggesting the latter was a predominant source provenance of the fore-arc sediments. In the younger sequences, i.e., the upper Ngamring and Padana/Qubeiya formations, zircons that record either pre-Mesozoic U–Pb ages or negative ε Hf(t) values become more abundant, implying additional sources from the northern Lhasa and/or Qiangtang terranes due to northward development of the fluvial systems. The overall zircon U–Pb and Hf isotopic data furthermore suggest that during fore-arc deposition, the exposed Gangdese arc was dominated by igneous rocks formed between 130 and 80Ma, associated with lesser amounts zircon with ages between 190 and 150Ma, thus recording two important stages of arc magmatism that are only sporadically preserved in modern outcrops as a result of extensive erosion related to uplift during Mesozoic–Cenozoic time. [Copyright &y& Elsevier]

Abstract: Before the Indian collision with Asia, northward subduction of the Neo-Tethyan oceanic lithosphere produced an Andean-type convergent margin in South Asia characterized by arc magmatism starting from the early Jurassic and lasting until the Eocene. The magmatic arc is now represented by widespread intrusive bodies or the so-called Transhimalayan batholiths in the Lhasa terrane of southern Tibet that have been divided into two main magmatic suites, i.e., the northern plutonic belt and the southern Gangdese Batholith. Their temporal distribution, concerning how exactly the magmatic suites correlate eastwards and then southeastwards around the eastern Himalayan syntaxis, however, remains poorly constrained. Here we report the first combined zircon U–Pb and Hf isotopic study of the Transhimalayan batholiths from the eastern part of the Lhasa terrane (∼95–97.5°E and ∼28.5–30°N), SE Tibet. Zircon U–Pb dating results of 24 granitoids indicate that the rocks were emplaced principally in the Early Cretaceous (∼133–110 Ma) and subordinately in the Paleocene (∼66–57 Ma), long after an older granite intrusion in the earliest Jurassic (∼198 Ma). The zircon ε Hf(T) values range from +5 to −20, yielding Hf crustal model ages (T DM C) between 0.8 and 2.4 Ga that peak at ∼1.7 Ga, suggesting a major episode of crustal growth in the Proterozoic and a predominantly, or even exclusively, crustal source for the granitoid petrogenesis. These zircon U–Pb and Hf isotopic constraints, together with whole-rock geochemical characteristics, allow us to correlate them to S-type granitoids in the northern plutonic belt, rather than to the I-type Gangdese Batholith from the central part of the southern Lhasa terrane. In contrast to the Gangdese magmatism that formed with a significant juvenile mantle input related to the Neo-Tethyan subduction, these S-type granitoids show geochemical similarities to the North American Cordilleran Interior batholiths that did not form directly from a subduction setting. Thus, we attribute the petrogenesis of the widespread Early Cretaceous granitoids in the northern belt and eastern Himalayan batholiths to a postcollisional regime due to the Late Jurassic–Early Cretaceous continental collision between the Lhasa and Qiangtang terranes. Under this framework, the S-type magmatism was generated in response to collision-induced crustal thickening, which may have capabilities of causing crustal anatexis by itself in the region. However, continued interplay with the Neo-Tethyan subduction zone processes such as back-arc extension could also have played a crucial, and long-lasting, role in the magma generation. [Copyright &y& Elsevier]