Magmatic processes inferred from chemical composition, texture and crystal size distribution of the Heikongshan lavas in the Tengchong volcanic field, SW China

Abstract

The products of volcanic eruption contain abundant information about the magmatic processes in a magma chamber. In this paper, we study the magmatic processes associated with the Heikongshan volcano in the Tengchong volcanic field (TVF), Yunnan Province of southwestern China, through chemical composition, texture and crystal size distribution (CSD) analysis of volcanic lavas. The Heikongshan lavas comprise five flow units associated with three different episodes of volcanic eruption since the middle Pleistocene. Bulk rock chemical analyses indicate that unit I is a basaltic trachyandesite, units II–IV are of trachyandesite, and unit V is composed of trachyte. The textures of these lavas can be further classified into three main groups accordingly, each represented by different phenocryst and groundmass assemblages, reflecting the changes of physical environment in the magma chamber. CSD analysis of plagioclase phenocrysts yields a mixture of CSD curves, some of which are nearly straight lines, while others are weakly concave-up. These concave-up CSD curves are consistent with magma mixing between host magma and early residual dacitic magma. In specifics, the CSD curve of large plagioclase phenocrysts (>1.6mm) in unit I reflects the characteristic of early dacitic magma. Whereas, the CSD curves of small plagioclase phenocrysts (<1.6mm) in units II–IV yield straight lines, indicating a relatively steady open system in which they formed. In contrast, influenced by the plagioclase crystals in early dacitic magma, the CSD curves of large plagioclase phenocrysts (>1.6mm) in units II–IV deviated from their original trend and formed in concave-up shape. Finally, in the last episode when unit V formed, the magma temperature decreased, resulting in the formation of relatively large amount of small-sized plagioclase crystals. Such process is reflected in the CSD diagram by a relatively higher value of intercept and the steeper slope.