Petrogenesis of Early Cretaceous bimodal volcanic rocks in the Fanchang Basin, SE China: an energy-constrained assimilation-fractional crystallization model

摘要

Volcanic rocks in the Middle-Lower Yangtze River Valley (MLYRV) constitute a bimodal magmatic suite, with a significant compositional gap (between 50% and 63% SiO_2) between the mafic and felsic members. The suite is characterized by a rela tively wide spectrum of rock types, including basalts, trachytes, and rhyolites. The basaltic rocks have low-to-moderate SiO_2 contents of 46.00-50.01%, whereas the trachytes and rhyolites possess SiO_2 contents in the range of 63.08-77.61%. Rocks of the bimodal suite show moderate enrichment of LILEs, negative Nb, Ta, and Ti anomalies, and are significantly enriched in LREEs. The basalts were most likely generated by parental mafic magmas derived from enriched lithospheric mantle with minor assimilation of crustal materials involving coeval crystal fractionation during magma evolution. The results of ener gy-constrained assimilation and fractional crystallization simulations demonstrate that the felsic magma was produced by the mixing of 5-20% lower crustal anatectic melts with an evolved mafic magma (～48% SiO_2) and accompanied by exten sive clinopyroxene, plagioclase, biotite, and Fe-Ti oxide fractionation. Our model for the genesis of felsic rocks in bimodal suites is different from the traditional models of crustal melting and fractional crystallization or assimilation-fractional crystallization of basaltic liquids.