پترولوژی (Jun 2025)
Chromian spinel geochemistry as a petrogenetic indicator in the chromitites and peridotites of the Bajgan Complex, Western Makran
Abstract
IntroductionEconomic chromite deposits are very common in ophiolite belts. These deposits usually occur in the uppermost horizons of the ophiolite mantle sequence or within the ultramafic cumulitic rocks at the base of the crustal sequence (González-Jiménez et al., 2014a; González-Jiménez et al., 2014b; Uysal et al., 2018; Arai, 2021).The largest chromite deposit in Iran lies within the Sorkhband Ultramafic Complex (Faryab chromite mine). In addition, numerous peridotite masses are exposed within the Bajgan Complex and the Colored-mélange Complex, as well as around Rudan, which are host for minor chromite deposits. In this study, chromite masses from different areas of the Bajgan Complex and the Colored-mélange Complex in western Makran were sampled and their field characteristics, petrography and chemical composition were studied. These data were applied to identify the nature of the rocks, the petrological changes and the tectono-magmatic setting as well.Field Evidence and PetrographyThe study area located in the south of Kerman Province and the western part of the Makran Zone. The peridotite outcrops occurred within the Bajgan metamorphic complex and around Kahnuj, Kuhshah, Kuh-e-Sefid, and Rudan. These rocks mainly contain minor chromite reserves. These peridotite masses occur as relatively small to large blocks ranging in size from a few meters to several kilometers, often in fault contact with metamorphic units of the Bajgan Complex. The southwestern Kahnuj peridotite outcrop with a dunite composition covers an area of ov 10 square kilometers. The Kuhshah peridotite masses cover an area of over 50 square kilometers consisting mainly of dunite and to a lesser extent harzburgite, pyroxenite and wehrlite.Dunites exhibit a variety of textures, including porphyroclastic, granoblastic, and mylonitic, reflecting varying degrees of deformation and recrystallization. Harzburgites consist mainly of olivine (80-85 %Vol), orthopyroxene (10-15 %Vol), clinopyroxene (less than 2-3 %Vol) and spinel (1-3 %Vol). Harzburgites often have a porphyroclastic texture, and the effects of plastic deformation in olivine and orthopyroxene crystals are visible as wave extinction and kink bands. Chromitites often occur as irregular lenses or masses within dunite units. These rocks are mostly massive to semi-massive and disseminated in texture. Analytical Methods Following the mineralogical studies using Polarized light microscopy, polished thin sections were prepared from a number of mantle peridotite and chromitite samples. In order to determine the chemical composition of chromite minerals, point analysis of minerals was carried out in the Experimental Laboratory of Geophysics and Volcanology, Department of Seismology and Tectonophysics, INGV, Rome (Italy). In this method, the main elemental composition of the minerals was obtained by Jeol JXA 8200 electron microprobe (EMP). The analysis conditions included an accelerating voltage of 15 keV, a sample current of about 7.5 nA, a counting time of 10s and 5s on the peaks and background, respectively, and a beam size of 2-3 mm.GeochemistryIn the dunites, the Al2O3 content of the chromian spinels varies from 21.89 to 25.26 wt.% and Mg# [Mg/ (Mg + Fe2+)] varies from 0.19 to 0.63 (Table 1), while in the harzburgite chromian spinels, the Al2O3 and Mg# contents are higher (Al2O3: 43.01 to 48.28 wt.%; Mg#:0.71 to 0.76). The value of Cr# [Cr/ (Cr + Al)] in the dunites and harzburgites, varies from 0.50 to 0.75 and 0.24 to 0.30, respectively. In chromitites, the Cr# content has a relatively wide range, based on which they are classified into two groups: high-Cr and high-Al chromitites. The Kutak chromitite has relatively high amounts of Al2O3 (20.52 to 23.63 wt%), while the Al2O3 content in the Kuhshah, Faryab and colored-mélange chromitites varies between 2.45 and 10.47 wt.%. Similarly, the Cr2O3 content in the Kutak chromitites varies between 45.96 and 52.26, while it is higher in other chromitites in the region, it has a higher value and varies between 60 and 70.93 wt.%. Therefore, Kutak chromitite with Cr# between 0.57 and 0.63 are in the high-Al chromitites range, and Kuhshah, Faryab and colored-mélange chromitites (Cr# between 0.80 and 0.95) are in the high-Cr chromitites group. DiscussionThe chemical composition of chromian spinel is widely used to determine the origin of magma and also to determine the tectonic setting of peridotites (Dick and Bullen, 1984; Kamenetsky et al., 2001). The chromian spinel in the ophiolitic peridotites and chromitites studied in the western Makran region shows a relatively wide range in terms of Cr# content. The high Mg# and low Cr# content in the chromian spinels of harzburgite rocks indicate the remnants of a relatively depleted mantle melt. The Cr# content of the chromian spinels in dunites and high-Al chromitites can often be explained by melt-rock reaction and chemical equilibrium with similar MORB melts, while the Cr# content in high-Cr chromitites points to the effective role of Mg-rich magmas or boninitic melts in the melt-rock reaction. The chemical compositions of chromite as well as the chemistry of the parental magma calculated using the Al2O3 content and the FeO/MgO ratio indicate that the genesis of the mantle peridotites of the Bajgan complex evolved in a supra-subduction environment (SSZ). As the obtained data of this study display the chemical diversity of chromites indicates a complex history of chemical and tectonic evolution, which is related to the heterogeneity of the mantle origin, different degrees of partial melting of the host rock, the compositional diversity of the reacting melts (MORB-like melts to boninitic melts), the evolution of the tectonic setting during the initial stages of subduction, and the development of the fore-arc tectonic environment.ConclusionThe study of chromite deposits in the Bajgan complex and colored-mélange complex in western Makran reveals significant variations in chromian spinel chemistry, reflecting diverse magmatic and tectonic processes. High-Al chromitites (e.g., Kutak) suggest melt-rock reactions with MORB-like melts, while high-Cr chromitites (e.g., Kuhshah, Faryab) indicate interaction with boninitic melts in a supra-subduction zone (SSZ) setting. The chemical diversity of chromites points to a complex mantle evolution involving heterogeneous sources, varying degrees of partial melting, and changing tectonic conditions during subduction initiation and fore-arc development. These findings enhance understanding of ophiolite formation and chromite genesis in the Makran zone.
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