Detailed field work in two outcrops of the Nazaré Paulista anatectic granite, SE Brazil

Abstract

Two exposures of the anatectic Nazare Paulista garnet-biotite granite and its country rocks were studied in detail in order to identify the different granite bodies and their relations to each other. In outcrop NP74, three main granite bodies and smaller aplitepegmatite layers were distinguished, all intrusive but subconcordant with the country-rock migmatitic (garnet)-biotite gneisses. The first granite body (from NW to SE; 74 g1), approximately 43 m thick, is made up of veined gray garnet-biotite granite; garnet leucogranite veins form a network cutting the gray granite, and surround the abundant gneiss xenolith and schlieren; the roof zone is a approximately 8 m thick garnet leucogranite with abundant xenoliths that can have formed by concentration of the vein-forming material. The approximately 12 m thick 74 g2 body occurs after a thin gneiss septum; it is made up of a garnet leucogranite with local pegmatite patches. The approximately 20 m thick 74 g3 granite forms the southern end of the outcrop, after a large exposure of country rock intruded by pegmatite layers; it is made up of (+ or -biotite)-garnet leucogranite with local fibrolite. In outcrop NP40, country-rock migmatites only appear as weathered rocks surrounding the granite exposure. Here, a approximately 15 m wide body of veined gray granite (40 g2) is intruded in its central portion by a white leucogranite (40 g3) possibly formed by collection of differentiated material from the veins, and on both sides by heterogeneous, schlieren-rich aplite-pegmatite garnet leucogranite (40 g1 and 40 g4). These two outcrops reveal that a wide variety of Nazare Paulista granite exists at the detailed scale, forming small bodies that can have their mutual relationships identified. The veined gray granite, usually considered as the "typical" Nazare Paulista granite, has wide distribution in the region, but is locally subordinated to more leucocratic varieties. The latter can be directly related to the collection of the vein material next to xenoliths or at the roof of the gray granite bodies, but they also form independent bodies which may have a totally different genesis. Structural evidence of syn-magmatic extension in outcrop NP74 suggests that some magma migration occurred in an extending collisional belt; however, the timespan of anatexis in the region may have been large (>10 Myr?), and still demands further precise age determinations.