Timescale of magma recharge for a crystal-rich mush: Perspectives from the compositional zoned quartz in Xiangshan caldera, SE China

Abstract

Crystal-rich (>30 vol%) volcanic rocks, which are generally thought to be formed by the eruption of crystal-rich mush after its reactivation, have provided an important lens through which to view the connection of volcanic and plutonic rocks in the calderas worldwide. The crystal-rich rhyolitic porphyritic lava is widely exposed in Cretaceous calderas in the southeast China. However, the petrogenesis of crystal-rich porphyritic lava still remains controversial. Xiangshan caldera is a uranium mineralization related volcanic-plutonic complex, which is composed of first subcycle rhyodacite and second subcycle tuff, crystal-rich rhyolitic porphyritic lava, granite porphyry and quartz monzonitite porphyry. All the volcanic and plutonic rocks have consistent zircon U-Pb ages (135 to 133 Ma). Sr-Nd-Hf-O isotopic characteristics show that they all mainly originated from crust derived intermediate-silicic magma. Zircon εHf(t) values for inherited zircons from rhyolitic porphyritic lava and rhyodacite range from −6.9 to +9.5. Correspondingly, zircon Hf two-stage model ages of inherited zircons are 2.12 to 1.10 Ga, suggesting their derivation from the Neoproterozoic juvenile crust and the Archean to Paleoproterozoic ancient crust. The rhyolitic porphyritic lavas show higher SiO2 contents and Rb/Sr ratios, and more obvious negative anomalies of Eu/Eu*, Ba, Sr, P and Ti; The granite porphyry show lower SiO2 contents and Rb/Sr ratios, with weak negative anomalies of Eu/Eu*, Ba, Sr, P and Ti. Combined with similar Sr-Nd-Hf-O isotopic composition, they may be derived from the same magma reservoir. Lower εNd(t), εHf(t), δ18O values and older model ages of tuff indicate more contribution of older crustal material. The cathodoluminescence (CL) brightness is strongly correlated with Ti concentration in quartz. The CL of selected quartz grains from crystal-rich rhyolitic porphyritic lava and granite porphyry show brightness zoned mantles (Ti = 19 to 77 ppm) and bright rims (high-Ti; >70 ppm). Some grains have dark cores (low-Ti; <9 ppm). However, the Ti contents of the quartz in the tuff of the second subcycle are homogeneous. Resemble the crystal-rich rhyolitic porphyritic lavas, the rims of quartz phenocrysts in the granite porphyry have consistent CL-brightness with the quartz in groundmass and have small embayments caused by growth impediments in them, which is due to the pressure release. It indicates that the granite porphyry also migrated upward rather than in situ cumulate. The repeated rise of CL brightness and Ti contents in the mantle of quartz phenocrysts recorded the rise of temperature and/or the injection of more primitive magma, indicating several times of magma recharge in the magma reservoir. The timescale of Ti diffusion in the compositional zoned mantle of quartz documented an ephemeral reactivation and remobilization of the crystal-rich mush within several to 16 thousand years with at least 7 times of magma recharge which lasted for 17 years to >11 thousand years each time.