Mn-rich-chromite in Río de Las Tunas mafic-ultramafic belt, Central Andes of Argentina: its significance in the context of regional metamorphism

GARGIULO, M. F.; BJERG, E. A.

Heidelberg

Workshop; 23rd International Colloquium on Latin American Earth Sciences (LAK); 2014

Universidad de Heidelberg

Chemicalanalyses of chromite crystals from metaperidotites of the Río de Las TunasBelt, Central Andes of Argentinawere performed with a Jeol SEM 6310 scanning electron microscope at theKarl-Franzens University of Graz, Austria. These crystals show chemicalzonation with Cr-rich cores and Fe-rich rims (Gargiulo et al. 2013). Thechromite component predominates in the core together with the highestconcentration of MnO, TiO2 and MgO. The core is a ferritchromite andits MnO content can reach up to 2.56 wt.%. This allows considering thesecrystals as Mn-rich-chromite or mangano-ferritchromite (Grieco and Merlini2012). The analyzed crystals show enrichment in the magnetite component towardsthe rim where gradually have chromian-magnetite to magnetite (sensu stricto)compositions in detriment of the chromite component together with MnO and TiO2lower concentrations. Therefore, the highest content of MnO is always relatedto the ferritchromite compositions. Barnes (2000) has demonstrated thatconcentration of Mn, Ti and Zn in the core or in the rim of chromite crystalsare mostly related with diffusion process of these elements during thereplacement of the original magmatic crystal due to reactions with thesilicates of the host rock during metamorphism in greenschist to amphibolitefacies conditions, enriching the magnetite component. Because above 550ºCchromite compositions equilibrate with the relict olivine in metaperidotite(Barnes 2000), it is possible to estimate temperatures (T) through the classicolivine-spinel geothermometer (Sack and Ghiorso 1991). The studied crystalsfrom Río de Las Tunas metaperidotites are in equilibrium with olivine in aparagenesis corresponding to the amphibolite facies (Gargiulo et al. 2013). Theestimated T is in the range of 717º to 647ºC.Correlating the distribution of the compositions with the isotherms of Sack andGhiorso (1991) we interpret that the cores of the analyzed crystals wereequilibrated with the mineral association of the host rock between 700º and 600ºCin amphibolite facies conditions; meanwhile the enrichment in the magnetitecomponent towards the rim was developed below 550ºC.Temperatures above 600ºCwere high enough to favor the diffusion process allowing a better distributionof the ions in the crystalline framework. Thus, the magmatic composition ofthese crystals it is not preserved in the cores since they were completelyreplaced by the ferritchromite during metamorphism. It is important to notethat the Mn content (2.56 wt.%) in the analyzed crystals allow to consider themas Mn-rich-chromite or mangano-ferritcromite. According to the chemicalvariation shown by the profiles performed in these crystals together with thetextural relationship and mineral paragenesis in the studied metaperidotites,these chemical variations are interpreted as the consequence of the metamorphiccycle that affected this rocks during middle to upper Paleozoic times.