Protracted extraction of high-silica melts from an upper-crustal magma reservoir recorded by the Wuchagou volcanic rocks in central Great Xing'an Range, NE China

Abstract

High-silica volcanic rocks and their plutonic counterparts carry critical information concerning the evolution of shallow magmatic systems and construction of silicic upper continental crust, yet their origins remain debatable. Here, we examine high-silica volcanic rocks from the Lower Cretaceous Manitu and Baiyin'gaolao formations of the Wuchagou volcanic field, central Great Xing'an Range (GXR), to constrain the genesis of these high-silica volcanic rocks and the evolution of the Wuchagou magmatic system. Geochronological data suggest that the Manitu and Baiyin'gaolao volcanic rocks represent a prolonged eruption from the Wuchagou magmatic system, spanning Period I (135–129 Ma ) and Period II (127–122 Ma ) magmatism in the central GXR. Their slightly depleted ε Nd (t) values and relatively young model ages coincide with a juvenile crustal origin. We suggest that the Manitu and Baiyin'gaolao high-silica volcanic rocks are the evolved compositions of their coexisting intermediate rocks given the similarities in both formation age and Sr-Nd isotopic composition and systematic geochemical variations. Clinopyroxene- and amphibole-based thermobarometers respectively yield low average pressures of ~3.3 kbar and ~ 1.8 kbar with low temperatures of ~1013 °C and 875 °C. Also, Clinopyroxene- and amphibole-based hygrometry yield relatively high H 2 O contents of ~2.7–4.1 wt% and ~ 3.1–4.6 wt%. Together, they indicate the presence of a hydrous upper-crustal magma reservoir feeding the eruptions of the Manitu and Baiyin'gaolao high-silica volcanic rocks. Combining the behavior of highly incompatible elements (Rb and Th) with trace element modeling results, we show that the Wuchagou high-silica volcanic rocks formed by protracted extraction of high-silica melts in the upper crust, with complementary residual cumulates partially disguised in the Early Cretaceous felsic plutons in the central GXR. Our results highlight the significance of prolonged upper-crustal differentiation of magmas derived from juvenile crust in the development of the high-silica upper continental crust. • High-silica volcanic rocks extracted from a long-lived upper-crustal reservoir. • Complementary residual cumulates disguised in contemporaneous felsic plutons. • Crystal-melt segregation contributed to development of high-silica upper crust.