The effects of mafic-felsic magma interaction on magma diversity: insights from an early Paleozoic hornblendite-quartz monzonite suite in the South China block

Abstract

Understanding the mechanisms responsible for the interplay between mafic and felsic magmas is the key to retrieving information on their sources, and characterizing the exchange of mass between them. In order to characterize compositional and mineralogical changes in the mafic end-member during mafic-felsic magma interaction and to better understand the nature of early Paleozoic intracontinental magmatism in the South China Block (SCB), a detailed study was conducted on an early Paleozoic hornblendite-quartz monzonite suite in the SCB. The amphibole phenocrysts in the hornblendite are zoned with respect to major and trace elements. From the brown core to the light-green rim, these amphibole phenocrysts display significant increases in Si, Mg, and Mn, coupled with abrupt decreases in Al, Ti, Na, K, and most of the trace elements, but only minor variations in Ca, Fe, Co, and Ni. The light-green matrix amphiboles in the hornblendite have similar compositions to the outer rim of amphibole phenocrysts (except Na). It is important to note that the amphibole grains in the quartz monzonite have significantly higher rare-earth element (REE) contents than the amphibole grains in the hornblendite. There is convincing evidence to support a significant transfer of incompatible elements (e.g., K, Na, LILE, LREE, U, and Th) from the felsic magma to the mafic magma, such as (1) the absence of high-Ca plagioclase in hornblendite, with the majority of feldspar grains being albite (Ab96–97) and orthoclase (Or94–96), and (2) uniform Sr-Nd-Hf isotope compositions (initial 87Sr/86Sr = 0.7081–0.7098; eNd(t) = −6.8 to −6.3; weighted mean zircon eHf(t) = −8.0 to −7.4) for the hornblendite and quartz monzonite samples. It is, therefore, suggested that during mafic-felsic magma interaction, water was transferred from the quartz monzonite magma to the coeval hornblendite magma and promoted the formation of the amphibole crystals in the latter. The incompatible elements transferred from the quartz monzonite magma to the hornblendite magma were mainly incorporated into the late-crystallized anhedral phases in the hornblendite (e.g., orthoclase, sodic plagioclase, quartz, zircon, and apatite). This study suggests that water, which behaves as a supercritical fluid in most mafic-felsic magmas, may play a key role in the exchange of mass between mafic and felsic magmas, and this may be extended to the petrogenesis of biotite-amphibole aggregations in intermediate-felsic magmas.