Investigation of silicate micro-inclusions from Orhaneli and Harmancik chromitites (NW Turkey): new ultrahigh-pressure evidence from Western Tethyan ophiolitic chromitites

Abstract

In this study, mainly silicate and some sulfide micro-inclusions hosted in magnesiochromites of chromitites from Orhaneli Ophiolite Complex and Harmancık Peridotites in northwestern Turkey are studied. Existence of randomly distributed anhedral (globular), selectively oriented lamellar/needle-shaped silicate micro-inclusions and several other cubic/octahedral inclusions hosted in magnesiochromites are noted along with Ir-group platinum-group element (IPGE) bearing phases and base metal sulfides (BMS). The microscopic observations, Raman, electron dispersive and wavelength dispersive spectrometry data have shown that the randomly distributed anhedral (globular) silicate inclusions mainly consist of forsterite and diopside. Selectively oriented lamellar/needle-shaped silicate micro-inclusions are determined as diopsides. Some of the cubic/octahedral inclusions are identified as (cubic) Mg-silicates, while some other inclusions are identified as euhedral to subhedral clinopyroxene and Na-K-Ca and Mg bearing silicates. An important amount of the studied cubic/octahedral inclusions are identified as negative crystals of cubic/octahedral form (some in form of trigonal or macle crystals). The oriented lamellae/needles of diopside are found both in the Orhaneli and Harmancık chromitites. Cubic/octahedral Mg-silicates and negative crystals are only detected in a multi-textured chromitite sample (with dunitic-orbicular, semi-massive and massive textures), which lacks the oriented diopside lamellae/needles. Although no other ultrahigh-pressure (UHP) phases are found, the existence of oriented diopside lamellae/needles in the Orhaneli and Harmancık chromitites are interpreted as the first in-situ evidence of UHP conditions and possibly their deep mantle recycling history. It may be speculated that the occurrence of micro-lamellae/needles of diopside in the Orhaneli chromitites within the cumulate dunites result from entrapment of recycled relics of UHP chromitites within the upwelling primordial melts that replenished the evolved magmas ponding in the MTZ. A step-by-step textural interpretation of the multi-textured Harmancık chromitite sample also suggested at least two stages of chromite formation, the former represented with the tightly packed nodular chromite nuclei within the dunitic orbicules, and the latter with surrounding semi-massive to massive (tightly packed) chromitites. The former stage can clearly be traced to be finally incorporated within the latter. The cubic/octahedral Mg-silicates and negative crystals are both seen within the first stage chromitite nuclei of the dunitic orbicules and the second stage surrounding chromitite. Occurrences of these inclusions are also questioned for a possible link between the textural evolution of chromitite and the UHP processes, and possibilities for finding a field (macroscopic) evidence for discriminating UHP chromitites. Due to some conflicting findings, it is concluded that more data are needed for reaching a final decision.