Regional Variation Of Geochemical Characteristics And Tectonic Implication Of The Early Eocene Basic Rocks In Southern Tibet

The geodynamical implications of the Linzizong volcanic rocks and the sameperiod intrusive rocks, which widely exposed in the southern Lhasa Terrane, Tibet,remain poorly understood so far. A better understanding on such issue will help us toexplore the subduction history of the Yarlung-Zangbo Tethyan oceanic lithosphere andsubsequent India-Asia collisional process. Key to explore this issue is to constrain themagmatic origin and nature of the basic rocks from the Linzizong volcanic rocks andthe same period intrusive rocks. This paper reports new data and summarizes theexisting in the literature (including whole-rock major and trace element, zircon U-Pbage and zircon Hf isotope) of the basic rocks from southern Tibet (5446Ma). Incombination with the data, the origin and petrogenesis of these Early Eocene basicrocks and their constraints on the process of India-Asia collision have been explored.The Early Eocene basic rocks from southern Tibet are calc-alkalin, high-Kcalc-alkaline and shoshonite, enriched in large-ion lithophile elements (LILEs),depleted in high field strength elements (HFSEs). From the eastern to the western,Geochemical characteristics of the Early Eocene basic rocks in southern Tibet didn’thave significant changes. We found that Nd, Nb, Zr, Hf and the degrees of REEfractionation increase from south to north, and series form medium-to high-Kcalc-alkaline, and Mg#decrease from south to north. All samples enriched in large-ionlithophile elements (LILEs), depleted in high field strength elements (HFSEs), similarto things of arc-type volcanic rocks. Early Eocene basic rocks in southern Tibet aremost likely derived from the mantle that was metasomatized by subduction-relatedcomponents, the samples from south are closer to the trench, is more affect by thefluid which is release from the subducted oceanic crust. And the northern samples isalso accompanied with significant contributions from sub-slab asthenospheric mantle.Because the previous impact of subsduction, the south samples mainly show arcbasalt features. In combination with the coeval zonal magmatic flare-up, This papersuggests that the basic rocks from the Northern part of our study area are indicative ofthe slab break-off of the northward subduction of the Yarlung-Zangbo Tethyanoceanic lithosphere at ca.52Ma, indicating that the initial India-Asia collision shouldhave occurred before60Ma.