Characteristics and petrogenesis of the “Pale Microgranular Masses” in ophiolite-type chromitites in northern Xinjiang

Abstract Massive “Pale Microgranular Masses” (PMMs) with mafic to ultramafic compositions have been discovered in representative chromite deposits (i.e., the Sartohay, Jingyu, Tangbale, Saleinuohai, Hongguleleng and Qingshui deposits) and related ophiolite belts. Based on a detailed study of the occurrence, petrology (alteration minerals) and whole-rock geochemistry, these PMMs can be subdivided into six categories: (olivine-rich) vitrobasaltic rocks, gabbro-diabasic rocks, ultramafic rocks, tuffaceous-pyroclastic rocks, skarns/rodingites without protolith structure, and metasomatites. The wall rocks of the PMMs are mainly harzburgites, subordinate chromitites and lherzolites. According to comprehensive analyses of the occurrence, wall rock types, metasomatic relationships and fabrics, the PMMs are considered to be xenoliths in ultramafic magma. The geochemical characteristics of the associated alteration minerals indicate that altered augite-endiopsides and regenerated diopsides, grossularite-andradites, and chrysotiles are dominant in the PMMs, and the chlorites in the PMMs can be divided into four categories: brunsvigite-pycnochlorites, diabantites, sheridanite-chlorophaeite-pycnochlorites and clinochlore-penninites. The PMMs have been mostly or completely subjected to rodingitization and exhibit Ca-rich and Si-undersaturated features in terms of major element compositions. These characteristics indicate that the PMMs were formed by metasomatism during the capture process by peridotites, with an import of mafic components (dominantly magnesium) and an export of felsic components, accompanied by serpentinization and chloritization. The trace element characteristics of the PMMs are obviously influenced by the wall rock type. PMMs in contact with peridotites generally have higher rare earth element (REE) and trace element contents than the primitive mantle, are characterized by slightly left-sloping chondrite-normalized REE patterns with weakly positive to negative Eu anomalies, and exhibit various degrees of Sr and K depletions. PMMs in contact with chromitites have lower REE and trace element contents than the primitive mantle, distinctly left-sloping chondrite-normalized REE patterns with obviously positive Eu anomalies, and Sr and K enrichments. The Eu, Sr and K anomalies show an obvious complementary relationship between the peridotites and the captured PMMs and metasomatites, which is most likely indicative of Ca-metasomatism by the ultramafic magma. The breakdown of clinopyroxene in peridotites liberated Ca, which accumulated and infused into the metasomatites. Consequently, the results of this study suggest that no less than two different genetic mechanisms might account for the generation of PMMs. The dominant characteristics of the fabric resulted from the processeses of xenolith formation and metasomatism by ultramafic magma in the early stage, whereas the characteristics of the mineral assemblages and geochemical components resulted from rodingitization in the later stage.