Coherent Chemical Variation Trends of the 55—25 Ma Magmatic Rocks in SE Tibet: N—S Direction Lithospheric Stretching of Eurasia during Early Stage of India—Eurasia Collision

The progressive indentation of India into Eurasia generated an E-W-trending orthogonal collision belt and a N-S-trending oblique collision belt. Compiling available data reveals that ∼70% of the Cenozoic igneous rocks in eastern and southeastern Tibet are concentrated within an ENE-trending, ∼550–km long and ∼250–km wide magmatic zone (CMZ) that once separated the orthogonal and oblique collision belts. The Latitude 26°N Line is now its southern boundary. The onset timing of magmatism of the CMZ varies gradually from ∼55 Ma in the westernmost part to ∼27 Ma in the easternmost. Then the magmatism successively occurred and suddenly stopped at ∼25 Ma. The segmented and coherent chemical variation trends found suggest that the CMZ magmatic rocks were formed due to partial melting of the heterogeneous upper mantle and crusts of Eurasia. Subduction of Paleo- and Neotethyan oceanic plates generated this compositional and mineralogical heterogeneity. Combined with available geophysical data, the CMZ was diachronously formed in response to asthenosphere upwelling induced by NNW—SSE-direction lithosphere stretching. The difference in responses of the orthogonal and oblique collision belts to the indentation of the Indian continent has led to this lithosphere stretching.