LA-ICP-MS analysis on chromite: a guide for the geodynamic setting of formation of ultramafic massifs in the Bulgarian Rhodopes

Chromite is a well-established indicator for the petrogenesis and geodynamic setting of ultramafic rocks. However, despite of its higher resistance to alteration relative to other minerals, its chemistry and structure can be substantially affected by secondary processes (i.e., sub-solidus re-equilibrium, metamorphism or hydrothermal alteration). The effects of these processes are minimized in chromite forming monomineralic bodies (i.e., chromitites). In the Dobromir tsi Ultramafic Massif (Central Rhodope, southern Bulgaria) chromitite bodies of uncer tain origin occur within highly metamorphosed harzburgite-dunite. Metamorphic alteration modified original chemistry and structure of almost chromite grains, although few core relics of chromite from the more massive part of the bodies preserved those pristine compositions. LA-ICP-MS analysis reveals cores characterized by high Cr0.67-0.74) with Ti =1280-1641 ppm, Mn =1,267-1,400 ppm, Ni = 835-1,011 ppm, V =672-723 ppm, Zn =428-456 ppm, Co =224-244 ppm, Ga =30-35 ppm, and Sc = 3-6 ppm, and cores with lower Cr0.55-057) characterized by higher Ti (2041-2555 ppm), Mn (1575-1747 ppm), V (792-840 ppm), Zn (522-569 ppm) and Ga (47-49 ppm), slightly lower Ni (858-957 ppm) but similar Co (220-238 ppm) and Sc (3 ppm). These minor and trace element compositions of chromite of the Dobromir tsi chromitites are in accordance with that for podiform chromitites hosted in the mantle section of ophiolites originated in supra-subduction zone settings. Their compositions definitively differ from layered chromitites of mafic continental intrusions or komatiites.