Mineralogy, geochemistry and geotectonic significance of mantle peridotites with high-Cr chromitites in the Neyriz ophiolite from the outer Zagros ophiolite belts, Iran

摘要

The Neyriz ophiolite containing chromitite pods from the outer Zagros ophiolite belt was studied in the Abadeh Tashk area. It appears as four detached massifs in an area with 125 km~2 in south of Iran and is comprised predominantly of peridotites. Harzburgites and dunites are the most ultramafics in the massifs with rare mafic and gabbroic rocks. Mineralogical composition of chromian spinel in chromitite and host peridotite of the Neyriz ophiolite in comparison with those of the Nain ophiolite, to tracing their geographical variations between outer and inner sectors of the Zagros ophiolite belt, shows that there are remarkable and striking compositional variations between these two ophiolites. Chromian spinels of the Neyriz mantle peridotite and chromitite pods are characterized by higher contents of Cr# (56-79; average, 71) in harzburgites and dunites, and higher Cr# (73-82), Mg# (62-71) and lower Al (9.1-13.9 wt% A1_2O_3) and Ti (up to 0.08 wt% TiO_2) in chromitite pods with respect to available data on samples from the Nain ophiolite (Cr# 40-61 of spinels in mantle harzburgites; Cr#: 59-73, Mg#: 60-70, Al_2O_3: 13.6-22.37 wt%, TiO_2: 0.13-0.40 wt% of spinels in chromitites). Based on geochemical affinities, we contend that the Neyriz mantle peridotites were largely affected by percolating hydrous boninitic melts, to produce high-Cr chromitites, whereas chromitites of the Nain mantle peridotites carry geochemical imprints of boninitic melts with MORB-like affinity, suggesting their distinct geotectonic setting. The presence of the high concentrations of wolfram (W: 275-1276 ppm) in the Neyriz mantle peridotites in comparison with those of the Nain mantle peridotites (W < 3 ppm), which is the one of the most striking geochemical features of these rocks, provides an additional evidence for the significant role of recycling of continental material such as subducted pelagic sediments into the sources of subduction zone magmas. Combining with the lithological variations of the Central Iran ophiolites from NE (harzburgite dominate Nain ophiolite with limited and small-size chromitite pods) to SW (harzburgite-dunite dominate Neyriz ophiolite with large potential of high-Cr chromitites), discernible geochemical tendencies of chromitites reveal that the Neyriz chromitites from the Zagros southern sectors (outer Zagros) are considered to be the product of typical of fore-arc-related boninitic melts, whereas the Nain chromitites of the inner Zagros ophiolite belt are associated with the back-arc basin-related boninitic parentage with affinities to MORB-like melts.