José Renato Nogueira

PYROXENES AND THE P-T PATH IN GRANULITE FACIES OF SW MINAS GERAIS, BRAZIL

RESUMO PIROXENOS E O PERCURSO P-T EM FACIES GRANULITO DO SW DE MINAS GERAIS, BRASIL Piroxenios de facies granulito do Macico de Guaxupe apresentam exsolucoes caracteristicas na forma de lamelas paralelas a (001) em clinopiroxenios calcicos e, por vezes, na forma de tres grupos de lamelas paralelas a. (001) - lamela relativamente espessa, b. (100) - lamela fina, c. -(001) lamela muito fina. A lamela espessa (001) representa pigeonita, posteriormente convertida para ortopiroxenio. Composicionalmente, os piroxenios sao muito ricos em ferro, e o KD OPX-CPX = 0,65 se encaixa dentro da gama de variacao dos piroxenios metamorficos. Estes piroxenios se equilibraram em condicoes de facies granulito sob temperaturas estimadas entre 786°C e 833°C, o que esta de acordo com estimativas anteriores feitas para o terreno granulitico de Guaxupe. O resfriamento a partir destas temperaturas produziu as texturas de exsolucao observadas. Considerando que as condicoes para o equilibrio dos piroxenios coincide com as temperaturas de pico metamorfico, e possivel que estes tenham se resfriado ao longo de um caminho P-T-t de descompressao isotermica estabelecido para os granulitos de Guaxupe. Palavras- chave: piroxenios. granulitos, exsolucao ABSTRACT Granulite facies pyroxenes from the Guaxupe Massif show characteristic exsolution in the form of lamellae parallel with (001) in calcic clinopyroxenes.and at times three sets of lamellae parallel with a. (001) -relatively broad lamellae , b. (100) - fine lamellae, and c. (001) - very fine lamellae. The broad (001) lamellae represent pigeonite, later inverted to orthopyroxene.Compositionally, the pyroxenes are very iron-rich, and KD OPX-CPX = 0.65 is within the range for metamorphic pyroxenes. These pyroxenes equilibrated in granulite facies yield temperature estimates of 786°C to 833° C, which is in keeping with previous estimates for the Guaxupe granulite terrain. Cooling from these temperatures resulted in the observed exsolutions. Since the conditions for the equilibra- tion of the pyroxenes coincide with peak metamorphic temperatures, it is possible that they cooled along the isothermal decompression (ITD) path established for the Guaxupe granulites. Keywords: pyroxenes, granulites, exsolution INTRODUCTION Exsolution of pyroxenes, both ig- neous and metamorphic, is a common feature in rocks that have cooled slowly from higher temperatures. The exsolution takes the form of simple to complex lamellae in igneous rocks in the form of blebs as well, that generally follow the (100) or (001) orientations, depending on the pyroxene (Deer et al 1992). In the case of metamorphic rocks, granulite facies pyroxenes often contain exsolution lamellae that have re- sulted owing to cooling subsequent to peak metamorphic temperatures. Integrated total analysis of such pyroxenes have yielded compositions either related to their pre-meta- morphic igneous history or to the high temperature granulite event itself (Bohlen and Essene, 1978; Sandiford and Powell, 1986). In the latter case, temperatures should be close to maximum values prior to cooling and exsolution, and this exsolution provides insight into the nature of crystallographic transformations in the pyroxenes. In the granulite facies terrain that covers the area extending from Sao Paulo to Minas Gerais State coexisting orthopy- roxene and clinopyroxene have been studied and analysed for geothermometry (Oliveira, 1978; Santos, 1987), and exsolu- tion in mangerite pyroxenes have been reported from the Caldas Massif further to the south( Janasi, 1995). Within the framework of a broader study of the Guaxupe granuiite ter- rain, a closer examination of pyroxenes, mainly from a char- nockite-mangerite suite, reveals interesting details with re- spect to their cooling from peak metamorphic temperatures. Bearing in mind the importance of pyroxenes in the granulite facies, the petrography of coexisting orthopyroxene and cal- cic clinopyroxene from this region in SW Minas Gerais are reported here, and the temperature for their equilibration is estimated based on the method of Wood and Banno (1973).