GEOCHEMISTRY OF GARNET METAGABBROS FROM THE MINDYAK OPHIOLITE MASSIF, SOUTHERN URALS

Abstract

Garnet metagabbro blocks crop out in a tectonic breccia at the contact of the mantle and transition zone sequences of the Mindyak plagioclase-spinel lherzolite ophiolite massif which is situated along the Main Uralian Fault orogenic suture in the Southern Urals. The garnetiferous blocks contain varying proportions of grossular-rich garnet, diopside, amphibole, and accessory minerals (Fe-Ti oxides, titanite, apatite, and zircon). Major and trace element data indicate that protoliths of the garnet metagabbro blocks could have formed by different stages of crystallization of MORB-type melts: Type 1 being plagioclase ± olivine ± clinopyroxene cumulates (high mg#, depleted HREE with flat chondrite-normalized patterns, and positive Eu anomalies Eu/Eu* = 1.33–1.51); Type 2 being solidified differentiated inter-cumulus liquid giving Fe-rich gabbros (lower mg#, elevated Fe2O3T, HFSE and HREE). In addition, low 87Sr/86Sr480 (0.7037–0.7043) and high eNd480 (5.5–7.8) support a depleted, sub-oceanic, mantle source for the protoliths. Post-crystallization rodingitization resulted in compositional changes, notably depletion in Si and alkalis, and enrichment in Ca and Al which are reflected in the metamorphic mineral assemblages above. An Early Devonian metamorphic event affected the already altered oceanic protoliths, this was followed by retrogression to amphibolite, and subsequently greenschist, facies.