ARCADAS/ÁGUAS DE LINDÓIA METAMAFIC/ULTRAMAFIC BELT, SOUTHEASTERN BRAZIL - SOME NOTES

Abstract

The studied metaultramafic rocks occur in the supracrustal sequences on the south of Sao Francisco Craton, at Sao Paulo State portion of the Itapira/Amparo Metamorphic Belt, between the parallels 22o 25’ e 22o 45’ S and the meridians 46o 55’ e 46o 35’ W, preciselly at Serra das Aguas Claras (SAC), situated to the WNW of Aguas de Lindoia city, and at the District of Arcadas (ARC), Amparo city. In the Arcadas/Aguas de Lindoia metamafic/ultramafic belt, the bodies occur like tabular to lenticular intercalations, parallel to the main structure, inside or between metasedimentary rocks of Itapira Complex and orthogneisses or neoproterozoic granitoid rocks. These lithotypes are not enclosed by basement rocks (Amparo Complex), only between these and metasediments and neoproterozoic granitoids. The rocks are represented by metaperidotites, olivine pyroxenites, anthophyllite schists/fels, tremolite/actinolite schists/fels, clinopyroxene-amphibole banded schists, hornblende schists, cummingtonite/grunerite schists, glimmerites, chlorite schists, talc schists, amphibolites and banded diopside amphibolites. Between these two concentration areas of metaultramafic rocks, there are several lenticular bodies, parallel to the general structure, which it suggests that both concentrations were linked and they were disrupted because of tangential and directional tectonics. Syn to tard granitic mobilizations, can be observed the formation of phlogopite and/or biotite schists in the edges of metaultramafic bodies or tectonic discontinuites. Post granitic magma mobilization, i.e., when temperatures were lower than 650 oC, we can analyse processes of amphibole chloritization and talcification (anthophyllite, tremolite and cummingtonite), formation of talc-phlogopite schists, chlorite schists, talc-chlorite schists, talc schists or fels and anthophyllite or tremolite/actinolite amianthus, making metasomatism evident (Fig. 1, 2). At SAC the glimmerites are generally at the edges and along shear zones internal to the lenses; the amphibole-talc schists are in the inner portions and can form preserved cores among biotite schists and talc schists (Fig.3). The hornblende/actinolite schists constitute smaller lenses, even at the edge of metaultramafic bodies or isolated in the granitoid rocks around. Mineralogical transformation noticed in the metamafic and metaultramafic rocks, in association with infiltration features of granitic material and quartz, makes it evident that metamorphic evolution happened at opened chemical system. The variation diagrams suggest SiO2 entry in the system, which is compatible with the talcification process of these rocks. The distribution patterns of rare earth elements and incompatibles, normalized to chondrite C1, show Ce negative anomaly, suggesting oceanic environment for these rocks or metasomatic modification caused by fluid passage. The mobility of major elements (Si, Al, Mg, Fe, Ca, K, Na), can be seen at field and at laboratory observing the processes of phlogopitization, talcification, chloritization, etc., however the quantification is too dificult due to the absence of a rock with preserved former composition. The zirconium, minor elements and, probably, the rare earth elements, mobility can be exemplified with the presence of diminute zircon crystals in quartzvenulation, suggesting in situ crystallization and the entrance of these elementes across fluid solution. Petrography showed that in SAC metaultramafic bodies, retrometamorphic transformations do not allowed preservation of olivine and pyroxene. Bodies with olivine and pyroxenes vestiges occur just at neighbourhood of SAC. Minerals forming the rocks of SAC exhibit minor compositional variance, insinuating better re-equilibrium during retrometamorphism, compared to ARC rocks. This can be due to a major fluid passage, what facilitated generalized phlogopitization, chloritization and talcification and the amianthus formation. The metamafic and metaultramafic lithotypes of ARC region are similar to those of SAC, although the first ones have better preserved mineral assemblage of metamorphic peak. Isotopic Sm/Nd analyses did not permit accurate calculus in relation to eNd. They make evident a strong contamination, which could have happened during magma emplacement or during the evolution of tectonic-metamorphic processes. Rb/Sr method demonstrated that the region was intensively affected by a neoproterozoic event, which was responsible for isotopic changes of palaeoproterozoic and archaean rocks, anatexy and amphibolite facies metamorphism. The ARC occurrences were less affected by greenschist transformation and by metasomatism, which are associated to the final phase of solidification of evolved granites and late anatetic material (Late Proterozoic, possibly with age around 600 Ma). Although, mineralogical evolution is similar to that of SAC, insinuating the same origin and evolution. The metamafic and metaultramafic rocks that appear between the both concentration areas (SAC and ARC) make evident a physical connection of these bodies, before tangential and directional tectonics. Zanardo (1987) and Lazarini (2000) suggested that the rocks treated here were ophiolites disrupted by the tectonics mentioned, but, in face of chemical changes that affected these rocks, it isn´t possible to certify their origin. The authors are grateful to Fapesp, by research assistance (Process 2001/10034-2), and to CNPq (fellowship).