Abstract
The Sanshui Basin is located at the northern continental margin of the South China Sea and characterized by a continental rift basin. The bimodal volcanic rocks in Sanshui Basin record the early Cenozoic magmatic activity in the South China Block, but the magmatic evolution that produced the bimodal volcanic rocks is poorly understood. Clinopyroxenes in bimodal volcanic rocks in the Sanshui Basin provide an opportunity to investigate magma during magma ascent. In this work, we classified nine types of clinopyroxene phenocrysts according to composition and texture in cogenetic basalt-trachyandesite-comenditic trachyte, while the composition of unzoned clinopyroxene have an evolution sequence of diopside-hedenbergite-aegirine along with an increase in trace element contents with a decrease of Mg#, indicating that the genesis of clinopyroxene was dominated by fractional crystallization in a closed magma system. However, the clinopyroxenes with reversed zoning and multiple zoning record the process of magma mixing and recharge indicating an open magma system. While fractional crystallization is the dominant process, magma mixing, recharge, and crystal settling were also found to influence magma evolution. Thermobarometric calculations showed that clinopyroxene crystallized a several structural levels in the crust during magma ascent. In this study, we established a magma plumbing system that provides new constraints for the magma evolution in the Sanshui Basin.
Highlights
The study of volcanic rocks could help us investigate deep magma chamber processes and their evolutionary history
The texture and composition of various clinopyroxenes in bimodal volcanic rocks from the Sanshui Basin reveal the complexity of the deep magmatic process and the evolutionary history of magma ascent
(1) Clinopyroxenes in bimodal volcanic rocks from the Sanshui Basin can be classified into nine types according to their optical characteristics, major and trace element contents, with four types in basalts, one type in trachyandesite, and four types in comenditic trachyte
Summary
The study of volcanic rocks could help us investigate deep magma chamber processes and their evolutionary history. Magma chamber processes such as magma mixing, recharge, crystal mush remobilization, fractional crystallization, and assimilation are all part of the magma evolution process and can trigger volcanic eruptions [1,2,3]. These processes may not always be documented in the whole-rock composition of volcanic rocks if the mixing process is efficient [2,4,5,6]. Granular scale studies of clinopyroxene with different textures and compositions can be used to distinguish open- and closed-magma systems and provide the evidence necessary to unravel the evolution of magma systems and establish magma plumbing systems [1,2,11]
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