Abstract
We conducted petrological and mineral chemistry investigations of Cr-spinel in ultramafic rocks of the Yanmenguan mafic–ultramafic complex in the North China Craton. The Cr-spinel grains occur as inclusions in enstatite, tschermakite, phlogopite, and olivine, or as interstitial grains among the aforementioned silicate minerals, and show concentric or asymmetrical textures. Back-scattered electron and elemental images and compositional profiles of the spinel grains indicate the presence of Cr- and Fe-rich cores and Al- and Mg-rich rims. The host silicate minerals display a decrease in Al and Mg contents accompanied by an increase in Cr and Fe away from the spinel. These textures and compositional variations suggest that subsolidus elemental exchange more likely gave rise to the compositional zonation, resulting in the transfer of Al and Mg from the silicate minerals to the spinel. The Mn, Ni, and Ti contents in spinel and the major elements of olivine-hosted spinel are relatively stable during subsolidus elemental diffusion and thus are more reliable tracers of primary high-temperature processes. The temperature estimates reveal that the subsolidus diffusion might have occurred at 600–720 °C, which could be linked to the regional metamorphic event.
Highlights
IntroductionChromium-bearing spinels are stable and highly refractory minerals that are very resistant to alteration, weathering, and melt percolation, in comparison with paragenetic silicate minerals [1,2]
Chromium-bearing spinels are stable and highly refractory minerals that are very resistant to alteration, weathering, and melt percolation, in comparison with paragenetic silicate minerals [1,2].They are good petrogenetic indicators of the source characteristics, evolution, and tectonic setting of magmas [3,4,5,6]
We report reverse zoning texture in Cr-spinels from ultramafic rocks of the Yanmenguan mafic–ultramafic complex
Summary
Chromium-bearing spinels are stable and highly refractory minerals that are very resistant to alteration, weathering, and melt percolation, in comparison with paragenetic silicate minerals [1,2]. They are good petrogenetic indicators of the source characteristics, evolution, and tectonic setting of magmas [3,4,5,6]. The normally zoned Cr-spinels have been reported in ophiolites [10], layered mafic–ultramafic complexes [11], komatiites [12], mantle xenoliths [13,14], and metamorphosed ultramafic massifs [15]. Their formation is generally ascribed to high temperature hydrothermal alteration [16] or metamorphism [15,17], evidenced by the presence of secondary phases such as chlorite and antigorite surrounding the Cr-spinel or relict magnetite [15,18,19]
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