Migmatite and leucogranite in a continental-scale exhumed strike-slip shear zone: Implications for tectonic evolution and initiation of shearing

Abstract

Plutons within continental strike-slip shear zones bear important geological processes on late-stage plate transpression and continent-continent collision and associated lateral block extrusion. Where, when, and how intrusions and shearing along transpressional strike-slip shear zones respond to plate interactions, however, are often debated. In this study, we investigated migmatite associated leucogranite and pegmatite from the exhumed >1000-km-long Ailao Shan-Red River left-lateral strike-slip shear zone in Southeast Asia that was active during India-Eurasia plate convergence. Most zircons from the migmatites and leucogranitic intrusions present inherited core-rim structure. The depletion of rare earth element patterns and positive Eu anomalies suggest that leucosomes and leucogranites are the result of crustal anatexis. Zircon rims from the foliated migmatites and leucogranites record U-Pb ages of 41−28 Ma, revealing the timing of the Cenozoic crustal anatexis event along this strike-slip shear zone. Ages of the magmatic zircons from the unfoliated pegmatites provide the timing of the termination of a high-temperature tectono-thermal event and ductile left-lateral shearing at 26−23 Ma. The Cenozoic crustal anatexis along the Ailao Shan-Red River strike-slip shear zone indicates that thickened crust underneath the shear zone involved previously subducted crust. We propose that the Cenozoic thermal state has an important effect on the crustal anatexis and thus on the rheological behavior of the lithosphere by thermal weakening, which plays an essential role in localizing the initiation of the deep-seated lower-crustal shear zone.