Evolução geoquímica e petrogênese de charnockitos ígneos no cinturão móvel costeiro do Brasil

Abstract

Orthopyroxene-bearing felsic and mafic rocks are important components of the transition zone between the São Francisco Craton and the Late Proterozoic Costeiro Mobile Belt in southeastern Bahia, Brazil. These granitoids are also important economically, hosting gemstone-bearing pegmatites. In the studied area, the granitoids are mainly explored as dimension stones, but pegmatites also occur, locally, containing gemstones of good quality (aquamarine, beryl and alexandrite). When a crustal thermal anomaly is triggered by under- or intra-plating of a basaltic magma trapped in the deep crust, it crystallizes and recrystallizes under granulite-facies conditions. This will produce a non-restitic mafic granulite, such as the mixtures commonly exhumed as xenoliths in certain continental alkaline volcanic rocks (Clemens et al. 1986). Any rock can, of course, be metamorphosed to granulite grade, irrespective of its origin. However, there is confusion as to what constitutes a charnockite. There are several varieties with very different origins, but which have commonly, and mistakenly, been lumped together as a single phenomenon. They are all orthopyroxene-bearing quartz-feldspathic rocks with granitoid composition. Some are metagreywackes and metagranitoids produced during fluid-absent granulite-facies metamorphism, with or without melt loss. Another group (e.g., the famous Indian "incipient" charnockites) are the products of carbonic dehydration reactions operating in tonalitic to granitic rocks at conditions typical of the upper amphibolite facies (Clemens 1992). The CO 2-rich fluids mostly have local origins and only produce patch and vein charnockite because the fluids gained access to the rocks only along fractures, etc. The last kind is the so-called "massif" charnockites. These are not metamorphic rocks at all, but granitic intrusions that were extremely hot and had relatively low H2O contents. They preserve their early-crystallized pyroxene. A great number of the world's granitic rocks in fact once contained near-liquidus pyroxene, but this is not preserved due to the protracted evolution of the magmas, with increasing H2O and KAISi3O8 activities, as the magmas cool and chemically envolve. If you know what to look for, you can very commonly spot the pseudomorphs in thin sections. Several petrogenetic schemes have been proposed for the origin of A-type granites, including: (1) fractionation products of alkaline basalts (Turner et al. 1992); (2) melting of lower-crustal source rocks under fluxing of mantle-derived volatiles (Bailey 1978); (3) low degrees of partial melting of F - and/or Cl-enriched dry, granulitic residue from which a granitoid melt was previously extracted (Clemens et al. 1986); and (4) melting of a tonalitic l-type granite (Creaser et al. 1991); The terranes hosting the granitoids investigated comprise the following sequences: Caraiba-Paramirim Complex, Paraíba do Sul Complex and Macaúbas Group. The Caraíba-Paramirim Complex represents the Archean/Paleoproterozoic basement, exposed in the northeastern portion of the area. The Paraíba do Sul Complex, a gneissic-migmatitic unit, comprises metasedimentary rocks of upper amphibolite to granulite facies. Paleoproterozoic ages have been traditionally suggested for the deposition of these sediments, with an overprint of Transamazonian and Brasiliano tectono-metamorphic events. In the study area, this complex is represented by kinzigitic gneiss, intruded by the neoproterozoic granitoids. Sm-Nd isotopic data for the Paraíba do Sul paragneiss indicate T DM model ages between 1.61 and 1.74 Ga (Celino et al. 2000), which are interpreted as an upper limit for the sources of the original sediments. Therefore, the deposition of these rocks most probably occurred during the Meso- or Neoproterozoic. The Macaúbas Group is a Neoproterozoic sedimentary sequence deposited on a continental passive margin. Orthopyroxene-bearing granitoids - denominated Salomão Suite (SLS) - comprises a group of chamockitic rocks with relict magmatic textures. SiO 2 values range from 52% to 71%. The rocks are enriched in Ti, P, Zr, Ba and REE. The SLS granitoids are metaluminous, with conspicuous enrichment in LIL and HFS elements, plotting mainly in the field of A-type granitoids in a FeO*/MgO versus (Zr+Nb+Ce+Y) diagram. These features indicate some degree of mantle contribution to the parental melt. Fractional crystallization was probably the main differentiation process during the evolution of the charnockitic suite (Celino 1999).Thermobarometric calculations suggest temperatures between 600 and 700° C and pressure of 4,5 kbar (-20 km depth) for the crystallization of these rocks (Celino & Botelho 2000). Field evidence, supplemented by mineralogical, textural and chemical data, suggests an origin by partial melting of deep continental crust under dry conditions of granulite facies. We agree with a petrogenetic model (Landenberger & Collins 1996) involving partial melting of a mafic-intermediate lower crust of similar composition to the I-type source, which was dehydrated, but not melt depleted, at time of production of the slightly earlier I-type granites, leaving alkali feldspar, rather than biotite, as the dominant K-bearing phase in the source, at elevated temperatures, produced a hot, dry magma of slightly different composition from the I-type magma, and these differences were magnified by fractional crystallization. The emplacement of post-tectonic intrusions (included SLS) followed and was related to NS reactivated zones. The I-type granitoids, that presumably fractionated from crustal magmas, are indicative of early and/or rapid uplift and crustal thinning in the orogenic hinterland (Celino et al. 2000), with concomitant upper mantle anatexis by pressure release melting. A final episode of magmatism was activated in consequence of the proceeding post-collisional uplift. This episode is characterized by numerous bodies of A-type granitoids and the charnockites of the Salomão Suite (SLS).