Origin of Reverse Zoned Cr-Spinels from the Paleoproterozoic Yanmenguan Mafic–Ultramafic Complex in the North China Craton

Abstract

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.

Contrasting Textural and Chemical Signatures of Chromitites in the Mesoarchaean Ulamertoq Peridotite Body, Southern West Greenland

“…The surrounding olivine may have buffered the trivalent cation diffusion with chromite, because it contains lesser Al, Cr, and Fe 3+ . This observation is also similar to the accessory spinels in the ultramafic rocks from the Yanmenguan mafic-ultramafic complex [42]. In comparison to the chromites enveloped by Al-rich phases, the core compositions of some of the chromites surrounded by olivine may be the least modified.…”

“…Continuous elemental exchange from postcumulus to subsolidus stages may have caused the Al enrichment along the chromite rims. In the Yanmenguan mafic-ultramafic complex in China, similar zonations characterize the chromite inclusions in enstatite, tschermakite, and phlogopite [42]. Olivine-hosted chromites also exhibit zonation, but to a lesser extent due to its low Cr and Al contents relative to pyroxenes and amphiboles.…”

Distinctive melt activity and chromite mineralization in Luobusa and Purang ophiolites, southern Tibet: constraints from trace element compositions of chromite and olivine

“…In olivine, two elements, such as Al and Sc, or Li and P, are normally coupled to substitute for Mg and/or Si or to occupy vacancies to maintain charge balance [79][80][81]. Meanwhile, trivalent cations in olivine generally diffuse more slowly than divalent ones [33,37] Partition coefficients of V, Cr, Mn and Zn in olivine are highly variable from compatible to incompatible and appear to be susceptible to subsolidus reequilibration that is controlled not only by the temperature, but also by the distribution of the dominant divalent cations in coexisting chromite [17,33,34,73,82,83].…”

“…Greater depths of formation are typically associated with higher temperatures. Efficient subsolidus exchange between chromite and olivine requires high initial temperatures of crystallization and long cooling times in relatively large melt chambers or channels [31,34,84]. Obviously, the insignificant subsolidus reequilibrium between chromite and olivine in the Purang dunites and chromitites suggests that they crystallized at a lower temperature and possibly experienced a faster uplift/emplacement than those in the Luobusa samples.…”

“…They are thus traditionally thought to be important petrogenetic indicators [25][26][27], particularly chromite, which is more resistant to alteration and weathering than olivine. Recently, some authors have used trace elements and Li, Mg and Fe isotopes to gain new insights into the genesis of chromite deposits [4,10,16,[28][29][30][31][32] and have shown that such elements can be used as indices of partial melting, fractional crystallization, melt-rock interaction and chemical diffusion [10,21,[32][33][34]. Incompatible trace elements in olivine and chromite are sensitive to melting and differentiation processes, whereas compatible trace elements are mostly used to identify source compositions [35,36].…”