A shift of mantle sources for the post-collisional lavas and tectonic links with synchronous deformation in the SE Tibetan Plateau

Abstract

Strong post-collisional deformation and contemporaneous magmatic flare-ups during the Cenozoic indicate large-scale continental lithospheric modification in the southeastern (SE) Tibetan Plateau. However, the relationship between regional deformation and magmatism is poorly understood. In this paper, we report for the first time the presence of 32–17 Ma mafic rocks within the Ailaoshan-Red River shear zone (ARSZ) and provide new insights into the correlation between deformation and magmatism. Using 369 Cenozoic mafic and felsic rocks (this study and literature data) in and around the ARSZ, we document a continuous magmatic record that delineates a consecutive depletion of their mantle sources through time, which is correlated with the development of regional deformation. The spatial distributions, rock associations, and geochemical features of these rocks indicate a three-stage magmatic evolution in the SE Tibetan Plateau. During the earliest episode (40–32 Ma), the strong emplacement of both mafic and felsic rocks shows a planar-shaped geographic distribution. The second episode (32–17 Ma) is characteristic of voluminous mylonitized granites and minor basaltic rocks generally along the ARSZ. The latest episode (<17 Ma) magmatic rocks are sporadically exposed with dominant OIB-type basaltic rocks and minor A-type granites. From the earliest to the latest episodes, the mafic rocks exhibit progressive depletion in SrNd isotopic compositions, with initial Sr isotopic ratios and εNd(t) values from 0.70509 to 0.71151 and − 12.5 ~ −0.3 to 0.70313–0.70734 and − 3.9 ~ +8.1, respectively. These data, together with the covariations in the (Nb/La)PM and Zr/Nb ratios, signify a dramatic mantle-source shift from enriched to depleted through time for these Cenozoic mafic rocks. Mafic rocks from the earliest episode were generated from an enriched lithospheric mantle through convective thinning. In contrast, the OIB-type basalts from the latest episode were pure asthenospheric magma from decompression melting. Therefore, we propose an interdependent evolution between the shift of mantle sources and the development of synchronous deformation in and around the ARSZ. Our discovery of 32–17 Ma mafic rocks stands as the first direct evidence for the key role of the synchronous development of the lithospheric-scale ARSZ in the shift of mantle sources, asthenospheric melt transportation, and heating-induced crustal magma generation.