CR-PYROPE GARNETS IN THE LITHOSPHERIC MANTLE. I. COMPOSITIONAL SYSTEMATICS AND RELATIONS TO TECTONIC SETTING

Abstract

Chrome-pyrope garnet is a minor but widespread phase in ultramafic rocks of the continental lithosphere; it complex chemistry preserves a record of events related to fluid movements in the mantle, including melt extraction and metasomatism. We have examined the major-element and trace-element composition of >12600 Cr-pyrope (Cr2O3 > 1 wt%) xenocrysts in volcanic rocks to evaluate their compositional ranges and interelement relationships. Samples have been divided into three major groups (Archon, greater than or equal to 2.5 Ga; Proton, 2.5-1 Ga; Tecton, <1 Ga) depending on the age of the last major tectonothermal event in the crust penetrated by the host volcanic rock. Relative depths of garnets within each sample have been determined by measurement of Nickel Temperature (TN). Mn, Ni and Zn contents of Cr-pyrope garnets are controlled by T-dependent partitioning between garnet and mantle olivine. the expected correlation of mg-number with T is largely masked by effects of bulk composition and crystal chemistry. The Cr content of garnet is a primary indicator of the degree of depletion of the host rock; Fe, Y, Ti and Ga show negative correlations with Cr, suggesting that all have been removed as part of the primary depletion process. In garnets with T-Ni < 1200 degrees C, the average degree of depletion as measured by these elements decreases from Archon to Proton to Tecton. High-temperature metasomatism, reflecting the introduction of asthenospheric melts, produces strong positive correlations between Fe, Zr, Ti, Y and Ga, and leads to 'refertilization' of previously depleted rocks. The prominent Ca-Cr correlation ('lherzolite trend') seen in garnets from clinopyroxene-bearing rocks is controlled primarily by the Cr/Al of the host rock, and Ca shows a strong negative association with Mg. The position and slope of the lherzolite trend vary with temperature and tectonic setting, suggesting that the P/T ratio exerts a control on Ca/Cr in lherzolite garnets. Garnets with less Ca than the lherzolite trend ('subcalcic garnets') are largely confined to Archon suites, where they typically are concentrated in the 130-180 km depth range. The few subcalcic garnets from Proton suites typically are lower in Cr and occur at shallower depths (100-120 km). Subcalcic garnets are absent in Tecton suites analysed in this work. The complexity of the geochemical relationships illustrated here, and their variation with temperature and tectonic setting, suggests that it is possible to define meaningful compositional populations of garnets, which can be used to map the stratigraphy and structure of the lithospheric mantle.