Modeling of Intraplate Basaltic Magma Crystallization and the Evaluation of the Influence of Metamorphism on Rock Composition: The Anyui Gabbro-Dolerite Complex, Western Chukotka

Abstract

The petrography, mineralogy and geochemistry of metamorphosed Permian–Triassic to Early Triassic rocks of the Anyui gabbro-dolerite complex, composing sills in metaterrigenous rocks of the Keperveem and Malyi Anyui uplifts of western Chukotka, were studied to determine the composition of the parental melt of these rocks and to assess the mobility of elements during their metamorphism. To solve these problems, the methods of petrological and geochemical modeling of melt crystallization were applied using the COMAGMAT version 3.72 program. It was established that the rocks (hypabyssal gabbros, gabbrodiorites, and diorites) are derivatives of a single parental melt formed in a large lower crustal magma chamber. These rocks are shown to have crystallized from intraplate continental tholeiitic basaltic parental melt that had a moderately differentiated composition with Mg# 52.1 corresponding to the Cpx–Pl cotectic and exhibited signals of crustal contamination. During regional metamorphism to the greenschist facies, the contents of a number of major, minor, and trace elements in the most of the studied rocks have been changed, with the estimated relative mobility of elements increasing as follows: Eu, V < Mn < Zn, U, Co < Cu, Pb < Sr < Fe, Ba, K, Rb < Ni < Cs < Mg < Ca, Na < Li. The elements immobile during metamorphism were Si, Al, Ti, P, REE (except Eu), Y, Sc, Nb, Ta, and probably also Zr, Hf, and Th (although the contents of the latter in some rocks may reflect the presence of xenogenic accessory minerals). The COMAGMAT program was applied to model the phase crystallization sequence established based on petrographic and mineralogical data on rocks, and the parameters of the compositions of the coexisting minerals during the fractionation stages of the parental melt before magnetite started to crystallize. The application of the petrological–geochemical modeling method in combination with data on the geochemistry and mineralogy of the gabbroids thus allows one to evaluate not only the compositions of the magmas and melts and their changes during fractionation but also an input/output of elements during metamorphism and the degree of their mobility.