Cr-pyrope xenocrysts with oxide mineral inclusions from the Chompolo lamprophyres (Aldan shield): Insights into mantle processes beneath the southeastern Siberian craton

Abstract

AbstractPyrope xenocrysts (N = 52) with associated inclusions of Ti- and/or Cr-rich oxide minerals from the Aldanskaya dyke and Ogonek diatreme (Chompolo field, southeastern Siberian craton) have been investigated. The majority of xenocrysts are of lherzolitic paragenesis and have concave-upwards (normal) rare earth element (REEN) patterns that increase in concentration from light REE to medium–heavy REE (Group 1). Four Ca-rich (5.7–7.4 wt.% CaO) pyropes are extremely low in Ti, Na and Y and have sinusoidal REEN spectra, thus exhibiting distinct geochemical signatures (Group 2). A peculiar xenocryst, s165, is the only sample to show harzburgitic derivation, whilst demonstrating a normal-to-weakly sinusoidal REEN pattern and the highest Zr (93 ppm) and Sc (471 ppm). Chromite–magnesiochromite, rutile, Mg-ilmenite and crichtonite-group minerals comprise a suite of oxide mineral inclusions in the pyrope xenocrysts. These minerals are characteristically enriched in Cr with 0.6–7.2 wt.% Cr2O3 in rutile, 0.7–3.6 wt.% in Mg-ilmenite and 7.1–18.0 wt.% in the crichtonite-group minerals. Complex titanates of the crichtonite group enriched in large ion lithophile elements (LILE) are high in Al2O3 (0.9–2.2 wt.%), ZrO2 (1.5–5.4 wt.%) and display a trend of compositions from the Ca–Sr-specific varieties to the Ba-dominant species (e.g. lindsleyite). In the pyrope xenocrysts the oxides coexist with silicates (clino- and orthopyroxene and olivine), hydrous silicates (talc, phlogopite and amphibole), carbonate (magnesite), sulfides (pentlandite, chalcopyrite, breakdown products of monosulfide and bornite solid solutions), apatite and graphite. P–T estimates imply the inclusion-bearing pyrope xenocrysts have been derived from low-temperature peridotite assemblages that resided at temperatures of ~600–800°C and a pressure range of ~25–35 kbar in the graphite stability field. Pyrope genesis is linked to the metasomatic enrichment of peridotite protoliths by Ca–Zr–LILE-bearing percolating fluid–melt phases containing significant volatile components. These metasomatic agents are probably volatile-rich melts or supercritical C–O–H–S fluids that were released from a Palaeo-subduction slab.