Geochemistry and Texture of Clinopyroxene Phenocrysts from Paleoproterozoic Picrobasalts, Karelian Craton, Fennoscandian Shield: Records of Magma Mixing Processes

Sergei Svetov,Svetlana Chazhengina,Alexandra Stepanova

doi:10.3390/min10050434

Abstract

This paper presents an integrated major and trace element data and crystal size distribution analysis for zoned clinopyroxene phenocrysts hosted in variolitic and massive picrobasalts of the Suisaari Formation, Karelian Craton, Eastern Fennoscandian Shield. Clinopyroxenes in variolitic and massive lavas occur as unzoned, reverse, and normally zoned crystal. Oscillatory-zoned clinopyroxenes are only observed in variolitic lavas. The obtained data were examined in order to evaluate the contribution of magmatic processes such as magma mixing, contamination and fractional crystallization to the formation of various zoning patterns of clinopyroxene phenocrysts. Clinopyroxene phenocrysts in both variolitic and massive lavas originate from similar primary melts from a single magmatic source. The obtained data on composition and texture of clinopyroxene phenocrysts together with the crystal size distribution (CSD) analysis suggest that crystallization of the massive lavas mainly involves fractionation in a closed magmatic system, whereas the crystallization of the variolitic lavas is determined by processes in an open magmatic system. The results provide novel information on the evolution of Paleoproterozoic magmatic systems in the Karelian Craton.

Highlights

Pyroxenes are abundant rock-forming minerals in volcanic rocks
Major element composition data obtained for 350 clinopyroxenes, including phenocrysts and microcrysts from the variolitic and massive lavas of the Yalguba Ridge, indicated that they all corresponded to augite [36], with En content ranging from 31% to 52%
According to the zoning patterns, clinopyroxene phenocrysts clinopyroxene phenocrysts were categorized into four groups: normal-zoned with Mg-rich cores; were categorized into four groups: normal-zoned with Mg-rich cores; reverse-zoned with Mg-poor reverse-zoned with Mg-poor cores; oscillatory-zoned with an Mg-poor core, Mg-rich mantle, and cores; oscillatory-zoned with an Mg-poor core, Mg-rich mantle, and Mg-poor rim; and “homogenous”

Summary

Introduction

Pyroxenes are abundant rock-forming minerals in volcanic rocks. They exhibit diverse varieties of textural, zoning and compositional features that record the crystallization history of magma from early crystallization (where pyroxenes form large phenocrysts) to the final stage (in which pyroxene microcrysts crystallize in the groundmass) [1,2,3]. Numerous studies of pyroxenes from geochemically diverse volcanic rocks of various geodynamic settings [2,7,8,9,10,11,12] have demonstrated that their composition and texture are reliable indicators by which to estimate the evolution and ascent histories of magmas. This approach has been successfully applied to decipher the Minerals 2020, 10, 434; doi:10.3390/min10050434 www.mdpi.com/journal/minerals

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Conclusion