GEOLOGICAL AND PETROGRAPHICAL CHARACTERISTICS OF SORAP MASSIF GABBROS, RAS-KOH OPHIOLITE, BALOCHISTAN, WESTERN PAKISTAN

This study discussed the Sorap Massif which is the ophiolitic fragment composed of the upper mantle and lower crustal section of an ophiolitic sequence. An ophiolitic sequence in Sorap Massif consists of harzburgite, dunite, large distant units of serpentinized wehrlite intrusion and outcrop of confined layered gabbro covered by the Quaternary sand dunes. The basal part of gabbroic intrusion is in contact with mantle rocks and the upper part is juxtaposed with the Kuchakki Volcanic Group. On outcrop-level and in hand specimen, the gabbros exhibit needle-like ferromagnesian minerals including hornblende associated with plagioclase and pyroxene. Petrographically the gabbros are classified into norite, gabbro norite and gabbro. The mineral constituents of norite are plagioclase, orthopyroxene and amphibole, gabbro norite is consists of equal constituents of orthopyroxene and clinopyroxene, plagioclase and amphibole and the gabbro is composed of clinopyroxene, plagioclase and amphibole. The high constituents of minerals such as pyroxene, amphibole and low constituents of olivine in the Sorap gabbros indicates that these gabbros are formed by the immature part of the oceanic plate with dehydration of the oceanic plate subduction.

Chromite chemistry as an indicator of petrogenesis and tectonic setting of the Ranomena ultramafic complex in north-eastern Madagascar

The Ranomena ultramafic complex in NE Madagascar consists of layered gabbro, harzburgite, orthopyroxenite, clinopyroxenite, garnet websterite and chromitite-layered peridotite. This study of the Ranomena chromite chemistry aims to better understand the petrogenesis and palaeotectonic environment of the complex. The chromite from the Ranomena chromitite is unzoned/weakly zoned and has a Cr# (Cr/(Cr + Al)) of 0.59–0.69, a Mg# (Mg/(Fe + Mg)) of 0.37–0.44, and low Al2O3(15–23 wt %) suggesting derivation from a supra-subduction zone arc setting. Calculation of parental melt composition suggests that the parental magma composition of the Ranomena chromitite was similar to that of a primitive tholeiitic basalt formed at a high degree of mantle melting, suggesting the parental melt composition was equivalent to that of an island-arc tholeiite (IAT). The parental magma of the Ranomena chromite had a FeO/MgO ratio of 0.9 to 1.8, suggesting arc derivation. The parental magma was Al- and Fe-rich, similar to a tholeiitic basaltic magma. The composition of orthopyroxene from the chromitite indicates a crystallization temperature range of 1250–1300°C at 1.0 GPa. The chemistry of the chromite in the Ranomena chromitite further suggests that the complex formed in a supra-subduction zone arc tectonic setting.