Late Eocene-Miocene tectono-magmatic response to the Indian- Eurasian plate collision: constraints from structural analysis, and Sr-Nd and Hf geochemistry of leucocratic intrusions along the Ailao Shan Red-River shear zone, SE Tibet

The over 1000 km Ailao Shan-Red River (ASRR) shear zone is one of the most important geological discontinuities in Southeast Asia. Great controversies remain on the nature of the shear zone and its role in shaping the tectonic framework of Southeast Asia. Our observation reveals the existence of the Paleogene high potassic alkaline rocks and calc-alkaline intrusions (>30Ma) and the late Oligocene to early Miocene calc-alkaline granitic rocks (28-21Ma). The former are concordant dykes and are generally strongly sheared into mylonitic rocks. The latter are either concordant and show weak strain fabric, or discordant and show no strain fabric. Meanwhile, they have distinct REE, Sr-Nd, Hf isotope signatures and are different in mineralizing features. The Paleogene intrusions are characterized by enriched LREE and depleted HREE without any Eu anomalies (whole rock). Whole rock Sr-Nd (87Sr/86Sr(i): 0.7069 to 0.7098; ɛNd(t): -7.98 to -3.31) and in situ Zircon Hf isotope (-0.79 to +6.2) analyses yield a binary mixing trend between the mantle- and supracrustal-derived melts for the Paleogene magma. Here our new data suggest that most of the Paleogene magmatic rocks are either sheared high potassium alkaline rocks or deformed calc-alkaline intrusions. They are identical to and are the deformed counterparts of rocks from the two Paleogene mineralizing magmatic provinces on both sides of the ASRR shear zone, i.e. the Jinping-Fan Si Pan province and the Dali-Beiya province. These two types of leucocratic rocks are formed as the result of post-collisional delamination of a thickened crust, and deformed and offset by the left lateral shearing along the ASSR shear zone. The late Oligo-Miocene calc-alkaline granitic rocks are localized within the ASRR shear zone. They are in overall concordant to the mylonitic foliation in the shear zone and preserve microstructures typical of syn- to late kinematic emplacement. They have negative Eu anomalies, variable but mostly higher Sr ratios (87Sr/86Sr(i): 0.7070 to 0.713), lowerɛNd(t) (-9.27 to -6.81) and negative Hf (-10.2 to -1.1). These characteristics suggest an origin from crustal melts for the late Oligocene-early Miocene barren magmatism. Their differences in Sr-Nd isotopes from the country gneisses suggest that they are derived from a source different from the country rocks. We conclude that shearing along the ASRR shear zone is coeval with the magmatism between 28 Ma and 21 Ma. Furthermore, the tectono-magmatic relationship also suggests that the shearing was limited to the crust level, instead of the entire lithosphere. Extrusion of the crust of the Indochina block did not occur until shearing started, subsequent to the post collisional collapse. The transformation from extension-related to extrusion related tectonic regimes contributed to the transition from localized high rates sedimentation to widespread low rates deposition in the entire South China Sea.