PYROXENITE VEINS WITHIN SSZ PERIDOTITES – EVIDENCE OF MELT-ROCK INTERACTION (EGIINGOL MASSIF), MAJOR AND TRACE ELEMENT COMPOSITION OF MINERALS

A A Karimov,V A Belyaev,M A Gornova

doi:10.5800/gt-2017-8-3-0269

Abstract

Evidence of melt-rock reaction between suprasubduction zone (SSZ) peridotites and island arc boninititc and tholeiitic melts are identified. This process is the cause of replacive dunites and pyroxenite veins forming, which are represent the ways of island-arc melts migration. The peridotite-melt interaction is confirmed by compositional features of rocks and minerals. Influence of boninitic melt in peridotites of South Sandwich island arc leads to increasing of TiO2 and Cr-number (Cr#) in spinels [Pearce et al., 2000] e.g. REE patterns of clinopyroxene from Voykar are equilibrium to boninitic melts [Belousov et al., 2009]. We show that pyroxenites are formed sequential, orthopyroxenites are originated firstly, websterites – after, and the main forming process is interaction of SSZ peridotites with percolating boninite-like melts.

Highlights

Evidence of melt-rock reaction between suprasubduction zone (SSZ) peridotites and island arc boninititc and tholeiitic melts are identified
Influence of boninitic melt in peridotites of South Sandwich island arc leads to increasing of TiO2 and Cr-number (Cr#) in spinels [Pearce et al, 2000] e.g. REE patterns of clinopyroxene from Voykar are equilibrium to boninitic melts [Belousov et al, 2009]
Cr# increasing without FeO and Fe2O3 growth is typical of upper-amphibolite facies metamorphism [Muntener et al, 2000]

Summary

COMPOSITION OF MINERALS

Pyroxenite veins within SSZ peridotites – evidence of melt-rock interaction (Egiingol massif), major and trace element composition of minerals. The Egiingol rocks include the serpentinized harzburgite and dunite, which are geochemicaly similar to SSZ peridotites and evidence of interaction with boninite-like melts [Gornova et al, 2008, 2010]. Spinels of harzburgite and pyroxenite correspond (Cr#-Mg# ratio, TiO2 and Fe2O3 content) to the spinel composition of peridotites from SSZ settings and differ from boninitic spinels (Fig. 1). Orthopyroxenes from thin orthopyroxenite veins and host harzburgites have similar composition (Mg#, Al2O3 and Cr2O3 content), they correspond to the field of SSZ peridotites (Fig. 2, A, B). Comparing with Opx from orthopyroxenites, orthopyroxenes of websterite have lower Mg# and wider Mg#, Al2O3 and Cr2O3, they partially fall into the high-Ca boninites field (Fig. 2, A, B).

Calculated melts in equilibrium with CPX from websterites of Egiingol

Discussion