Inhomogeneous crust-mantle interaction and Triassic tectonic escape of a Proterozoic microplate: A tale of the Bikou Terrane

Abstract

Compressional stress in collisional orogenic belts open a free boundary for the lateral extrusion of rigid blocks. Here we trace the history of tectonic escape of a Proterozoic microplate and the associated inhomogeneous crust-mantle processes from magmatic records in the Bikou Terrane, which is the largest microblock located among the North China Block, South China Block and Songpan-Ganzi Orogenic Belt. Zircon U-Pb geochronology indicates that the Triassic granitoid intrusions in the Bikou Terrane emplaced at 220–210 Ma. Petrographical, whole-rock geochemical and isotopical features suggest that the granitoids derived from the partial melting of lower crust, with inhomogeneous contribution of mafic magma derived from partial melting of subcontinental lithospheric mantle. Compared with the Yangba pluton in northeastern Bikou Terrane, the five intrusions in the southwestern Bikou Terrane display relatively lower REE concentrations, more negative Eu anomalies (δEu = 0.24–0.93), and lower Nb/Ta and Zr/Hf ratios with the values of 14–20 and 37–45. Furthermore, they display higher (87Sr/86Sr) i from 0.7050 to 0.7075 and lower εNd(t) from −9.4 to −4.7. These features imply that the granitic intrusions in southwestern Bikou Terrane were primary derived from the partial melting of the lower crust with lesser input of mantle materials than the Yangba pluton, indicating inhomogeneous crust-mantle interaction in the Bikou Terrane. We calculated the Moho depth of northeast and southwest Bikou Terrane as ~60 km and ~ 40–50 km, respectively. We propose that the Bikou Terrane escaped southwestward in Late Triassic following a transition in the stress regime of compression from NE-SW to NW-SE, in addition to the slab break-off from east to west. The lateral extrusion resulted in the inhomogenous crust-mantle interaction.