Cumulate hornblendite enclaves in diorite–porphyrite intrusions from the Shuangyashan, Northeast China, and implications for the transition from lower crust to upper mantle in subduction setting

Abstract

A suite of hornblendite (amphibole proportion ≥90%) enclaves were found in Late Cretaceous diorite–porphyrite stocks intruding the lower Cretaceous coal strata around the Shuangyashan City, eastern Heilongjiang Province, Northeast China. The enclaves have similar mineralogy and may be divided into clinopyroxene-bearing and clinopyroxene-free hornblendites and both of them show cumulate textures: pargasitic amphibole and clinopyroxene are cumulus and anorthitic plagioclase is intercumulus. The accumulation might have occurred in the magma chamber, minor clinopyroxene enclosed in amphibole was earlier crystallized, followed by a large amount of amphibole, and the residual melt trapped between cumulate crystals finally formed the intercumulus plagioclase. Probably, such a crystallization process could produce layered cumulates: lower part is dominated by clinopyroxene-bearing hornblendite and upper part is composed of clinopyroxene-free one. The enclaves have similar geochemistry and imply a hydrous basaltic parental magma enriched Rb, Ba, Th and other large-ion lithophile elements. The generation of the parental magma might be related to the subduction of Paleo-Pacific plate beneath eastern Eurasian continent at that time. Estimated crystallization pressure, P-wave velocity, and density for the cumulate hornblendite enclaves are generally in agreement with the values of the local crust–mantle transition zone, suggesting that these cumulates may have the origin in the transition from the lower crust to upper mantle in arc and back-arc settings.