Anatexis and resultant magmatism of the Ama Drime Massif: Implications for Himalayan mid-Miocene tectonic regime transition

Abstract

In the middle Miocene, the Himalayan Orogen underwent a remarkable tectonic regime transition from a north-south extension to an east-west extension, but the underlying mechanisms of the Himalayan mid-Miocene tectonic regime transition are still debated. In this study, we conducted an integrated petrological, geochronological, and geochemical study on the high-grade metamorphic rocks and leucogranites from the Ama Drime Massif (ADM) to detect the anatexis and magmatism associated with the east-west extension. According to geochronological results, the structurally higher ADM paragneisses and the structurally lower ADM orthogneisses underwent partial melting in 22–12.5 Ma and 14–13 Ma, respectively, accompanied by the formation of 12 Ma tourmaline-muscovite leucogranites and 12–11 Ma two-mica leucogranites. By comparison, the tourmaline-muscovite leucogranites and the ADM paragneisses have compatative protolith ages and Sr-Nd isotopic compositions. Likewise, the two-mica leucogranites and the ADM orthogneisses are comparable in protolith ages and Sr-Nd isotopic compositions. For whole-rock geochemistry, the two-mica leucogranites are characterized by higher TiO 2 , TFeO, MgO, CaO, Sr, Ba, LREE, Th, Zr/Hf, but lower Rb, Rb/Sr, and weak Eu anomalies compared to the tourmaline-muscovite leucogranites. Based on the spatial, temporal, and protolith relationships between the gneisses and leucogranites in the ADM, the tourmaline-muscovite leucogranites originated from muscovite dehydration melting of the paragneisses and then experienced strong fractional crystallization, while the two-mica leucogranites resulted from biotite dehydration melting of the orthogneisses. Anatexis and resultant magmatism in the ADM reveal that the mid-Miocene tectonic regime transition changed not only the style of the Himalayan extensional structures but also the crustal thermal structure of the Himalayan Orogen. In this contribution, a potential lateral delamination model is proposed to accommodate the heating and coeval rapid east-west extension in the ADM region and to explain the Himalayan mid-Miocene tectonic regime transition- • The ADM underwent various anatexis and magmatism during 14–11 Ma. • The anatexis between the paragneisses and orthogneisses was decoupled in the ADM and produced various leucogranites. • The Himalayan east-west extension was accompanied by the change of crustal thermal state. • A new model of lateral delamination is proposed to explain the Himalayan mid-Miocene tectonic regime transition.