Structure and spatial-temporal relationships of Eocene-Oligocene potassic magmatism linked to the Ailao Shan-Red River shear zone and post-collisional extension

Abstract

Eocene potassic magmatic rocks are abundantly developed in the Jinshajiang–Ailaoshan tectono-magmatic belt that stretches from eastern Tibet over western Yunnan to Vietnam. This study reports results from adakite-like potassic rocks from the Ailaoshan-Red River shear zone (ASRR-SZ) investigated by geochemistry, Zircon U–Pb geochronology, electron microprobe, Lu–Hf isotopic and EBSD analysis. High SiO2 (68.95–72.38%), K2O (4.38–5.80%) and low MgO (0.29–0.51%), Y (5.58–14.19 ppm) and Yb (0.4–1.34 ppm) contents as well as high Sr/Y (70–285) and La/Yb (11.5–47.5) ratios show that these rocks belong to high-K calc-alkaline to shoshonitic adakite-like rocks. Zircon U–Pb ages reveal that the adakite-like potassic rocks with different deformation characteristics emplaced between 39 and 30 Ma. εHf values of magmatic zircons of these adakite-like rocks range from −2 to +6, and two-stage model ages TDM = 750–1500 Ma supports the interpretation of its formation by mixing and partial melting of enriched mantle-derived melt and magmas derived from thickened ancient crust. The potassic magmatic activity lowered the rheological strength of the lithosphere, which was one of the triggers of initiation of the ASRR-SZ. Temperature and pressure calculated by amphibole-plagioclase thermobarometry reveal average emplacement depths of ca. 18 km of granodiorites, indicating exhumation from nearly 18 km depth by apparent vertical movement of ASRR-SZ. In combination with the results of the Eocene potassic magmatism in the Jinshajiang–Ailaoshan tectono-magmatic belt, we suggest that the adakite-like potassic rocks were formed during the Eocene post-collisional extensional tectonic setting, which were closely related to the rollback and/or detachment of the Neo-Tethys oceanic slab.