Petrogenesis of K-rich and high-mg adakitic rocks in the North Dabie terrane, China: products of early cretaceous orogenic collapse

Abstract

ABSTRACT Understanding orogenic collapse is key to comprehending the evolution of continental collision orogens. The Dabie orogen is a collapsed continental collision orogen that provides an ideal window for revealing orogenic collapse. Here, we conduct fieldwork, whole-rock geochemistry, and zircon U-Pb dating of the Daoshichong pluton in the North Dabie terrane to elucidate the collapse mechanism of the Dabie orogen. The Daoshichong pluton is composed of quartz monzodiorite and mafic microgranular enclaves (MMEs) with an Early Cretaceous (~130 Ma) crystallization age. The Daoshichong pluton is strongly deformed and ductile, displaying a gneissic structure. The quartz monzodiorites are K-rich and high-Mg adakitic rocks. They contain 60.2–63.3 wt.% SiO2 and have high K2O (2.88–3.42 wt.%) and MgO (2.08–3.04 wt.%) contents with relatively high Mg# values (44.4–49.3). They exhibit high Sr (673–819 ppm) and low Y (11.4–20.9 ppm) and heavy rare earth elements (e.g. Yb: 0.94–1.87 ppm) contents, resulting in high Sr/Y (35.1–72.0) and (La/Yb)N (15.7–36.1) ratios. The MMEs exhibit geochemical characteristics similar to those of the Early Cretaceous mafic – ultramafic rocks of the Dabie orogen. They have low SiO2 contents (48.1–51.9 wt.%) and high MgO contents (3.27–5.62 wt.%) with high Mg# values (47.3–51.2). They also exhibit relatively high Ni (16.6–63.6 ppm), Cr (30.5–164.6 ppm), Y (18.2–25.8 ppm) and Yb (1.50–1.94 ppm) contents and high Sr/Y ratios (53.7–109.5). Evidence from fieldwork and geochemistry distinctly indicates magma mixing during magma evolution between the MMEs and the quartz monzodioritic host. Thus, the Daoshichong quartz monzodioritic host and the MMEs can be interpreted as a mixture of mantle-derived magma and over-thickened lower continental crust-derived adakitic melt. Previous studies suggest that the Early Cretaceous (~130 Ma) high-Mg adakite-like rocks are distributed only along the eastern margin of the Dabie orogen (i.e. along the Tan – Lu Fault). This study is the first to report a homochronous (~130 Ma) K-rich and high-Mg adakite-like pluton in the core of the Dabie orogen. Our results suggest that a whole delamination of the over-thickened orogenic root caused the orogenic collapse, subsequently triggering crust – mantle interactions, dome structure, and magmatism in the Dabie orogen.