Efficient segregation of high-silica granitic melt from complementary cumulate caused by high-temperature gas sparging from mafic recharge

Abstract

Segregation of granitic melts from their reservoir is a key process in the genesis of granites. However, the melt segregation mechanism is still enigmatic because the ability for the segregation of the melts from their magma reservoir decreases with decreasing temperature. Two complementary lithologic units have been identified in the Beidashan pluton of NE China, based on reverse mineral zoning, rimward temperature-increasing pattern in minerals, and their contrasting whole rock Rb/Ba and Zr/Hf ratios, Ba and Zr concentrations, and alkali feldspar BaO and biotite F contents. One of the units represents a crystallized crystal-poor granitic melt, and the other represents a melt-poor crystal mush with many igneous mafic enclaves. The genesis of the two units can be explained by the removal of the crystal-poor high-silica granitic melt from a complementary mushy reservoir. We also show that granitic melts can be efficiently segregated from their mushy reservoir. This is caused by fluxed melting and melt expulsion in a temperature-increasing condition driven by high-temperature gas sparging from mafic recharge when it quenched at the base of the overlying felsic mushy reservoir. The segregated granitic melts formed by this mechanism are significantly higher in temperature than their water-saturated solidus and, thus can ascend to a very shallow crustal level away from their mushy reservoir. Our model can explain many observations that rare complementary cumulates are reported accompanying high-silica granites. Furthermore, the study proposes a robust petrological, mineralogical, and geochemical identification signature of high-temperature granitic cumulates after removing complementary high-silica granitic melt.