ماهیت ماگمای مادر کرومیت در افیولیت سبزوار (شمال شرق ایران)

Significance of chromite, as the unique source for chromium, a metal with key role in industry, has inspired many geologists to investigate its genesis and natural distribution as an important contribution to its exploration in many parts of the world. To do this, geochemistry of chromite, which is considered to be the reflection of the nature of its parent magma and the geotectonic setting in which it is generated, is extensively used by many researchers. To reveal the nature of chromite’s parent magma, its chemical composition is used as a petrogenetic indicator. In this regard many authors have classified chromite deposits as either high-Al (Al2O3 ≥ 25 wt %) or high-Cr (Cr2O3 ≥ 45 to 60 wt %) and have demonstrated that Al-rich chromites are generated from tholiitic and Cr-rich ones from Mg-rich boninitic magmas, two melts generated in distinct and different geotectonic setting. In this study on the basis of geochemistry of chromite samples from three nearby mining districts in Sabzevar Ophiolite Range, in the North-East of Iran, the nature of chromite parent magma and its geotectonic setting is investigated. Chromite samples are analyzed by EPMA method. The layered chromitite bodies, embedded in serpenitnized dunite, are displayed to belong to high-Cr deposits. Considering high Cr# (86-88), moderate Mg# (44-51), low TiO2 content (0.1-0.2 wt %) and other geochemical features of chromite, the parent magma dominantly accords with a high Mg# boninitic melt. According to the results of the previous works and petrography, geochemistry (especially Cr#, Mg#, Fe+3 and TiO2 contents) and field observations represented in this work, the chromitites must be generated in a suprasubduction zone setting. Sea floor hydrothermal processes have probably caused especially, the Mg# not to be as high as expected.