"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. FLORENCIA; BJERG, ERNESTO A.

Heidelberg

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

Universidad de Heidelberg

Chemical analyses of chromite crystals from metaperidotites of the Río de Las Tunas Belt, Central Andes of Argentina were performed with a Jeol SEM 6310 scanning electron microscope at the Karl-Franzens University of Graz, Austria. These crystals show chemical zonation with Cr-rich cores and Fe-rich rims (Gargiulo et al. 2013). The chromite component predominates in the core together with the highest concentration of MnO, TiO2 and MgO. The core is a ferritchromite and its MnO content can reach up to 2.56 wt.%. This allows considering these crystals as Mn-rich-chromite or mangano-ferritchromite (Grieco and Merlini 2012). The analyzed crystals show enrichment in the magnetite component towards the rim where gradually have chromian-magnetite to magnetite (sensu stricto) compositions in detriment of the chromite component together with MnO and TiO2 lower concentrations. Therefore, the highest content of MnO is always related to the ferritchromite compositions. Barnes (2000) has demonstrated that concentration of Mn, Ti and Zn in the core or in the rim of chromite crystals are mostly related with diffusion process of these elements during the replacement of the original magmatic crystal due to reactions with the silicates of the host rock during metamorphism in greenschist to amphibolite facies conditions, enriching the magnetite component. Because above 550ºC chromite compositions equilibrate with the relict olivine in metaperidotite (Barnes 2000), it is possible to estimate temperatures (T) through the classic olivine-spinel geothermometer (Sack and Ghiorso 1991). The studied crystals from Río de Las Tunas metaperidotites are in equilibrium with olivine in a paragenesis corresponding to the amphibolite facies (Gargiulo et al. 2013). The estimated T is in the range of 717º to 647ºC. Correlating the distribution of the compositions with the isotherms of Sack and Ghiorso (1991) we interpret that the cores of the analyzed crystals were equilibrated with the mineral association of the host rock between 700º and 600ºC in amphibolite facies conditions; meanwhile the enrichment in the magnetite component towards the rim was developed below 550ºC. Temperatures above 600ºC were high enough to favor the diffusion process allowing a better distribution of the ions in the crystalline framework. Thus, the magmatic composition of these crystals it is not preserved in the cores since they were completely replaced by the ferritchromite during metamorphism. It is important to note that the Mn content (2.56 wt.%) in the analyzed crystals allow to consider them as Mn-rich-chromite or mangano-ferritcromite. According to the chemical variation shown by the profiles performed in these crystals together with the textural relationship and mineral paragenesis in the studied metaperidotites, these chemical variations are interpreted as the consequence of the metamorphic cycle that affected this rocks during middle to upper Paleozoic times.