Early Mesozoic Anatexis‐Induced Strain Partitioning and Gneiss Doming in the Yunkai Massif, South China: A Response to Contrasted Dynamics of Paleo‐Pacific and Paleo‐Tethys Subductions?

Abstract

This study aims to decipher the structural impact of late anatexis in the core of polycyclic basements, like the Cathaysia Block (South China). Based on field mapping and detailed structural and kinematic analyses in the southern part of the Yunkai massif, we document anatexis-induced strain partitioning and gneiss dome formation. The exhumation of migmatites within gneiss domes occurred in a context of NE-SW regional extension coupled with syn-anatexis (a) horizontal lateral flow and vertical one in their cores, (b) strike-slip dominated flow at the limbs of domes, and (c) constrictive lateral flow at the external parts. Petrographic and microstructural analyses suggest that the cores of the gneiss domes and plutons recorded magmatic to sub-solidus deformation while strain localization at the top of gneiss domes is characterized by retrogressive shearing. Zircons U-Pb dating of diatexites and late-kinematic plutons give coherent crystallization ages of ca. 240 Ma, suggesting that the regional partial melting and gneiss doming occurred in the Triassic and probably as early as Late Permian, rather than during the Early Paleozoic as previously thought. A regional NE-SW lateral flow in the deep crust is fully compatible with NW-SE horizontal shortening, as also exemplified for Late Permian to Middle Triassic times in the whole Cathaysia and the Xuefengshan belt. Therefore, the Yunkai massif should no longer be considered as belonging to the Indosinian belt, instead, orogen-parallel crustal flow in the Yunkai massif looks rather controlled by the subduction of the Paleo-Pacific plate at the eastern margin of the South China Block.