Refining metamorphic-time paths with deformation conditions: The exhumation history of the southernmost part of Brasília Orogen, Brazil

Abstract

In the southeastern Brazil, a thick section of lower crust crops out in the Socorro-Guaxupé Nappe, part of a Neoproterozoic orogenic collage in the southernmost part of the Brasília Orogen. Migmatites, predominantly diatexites, and granulites underwent ultra-high temperature conditions, reaching around 1000 °C and 10 kbar during their metamorphic peak. However, these rocks exhibit distinct structural characteristics within the studied area. Late-stage foliation in migmatites displays a more dispersed distribution with a predominant NW-SE orientation, whereas in granulites, the foliation predominantly trends in a WSW-ESE direction. Lineation in the migmatites is defined by feldspars and hydrated minerals, such as biotite and hornblende with a WNW-ESE direction, whereas in the granulites it is defined by anhydrous minerals like pyroxenes and feldspars, exhibiting a WSW-ENE orientation. The quartz c-axis fabric opening-angle for the residuum of granulite shows the highest values (135–154°), followed by the leucosome of granulites (113–130°) and then leucosome of migmatites (86–97°). This implies that the quartz c-axis fabric in granulites records two distinct temperature regimes: the residual portion aligns closely with the metamorphic peak temperatures, ranging between 869 and 923 °C, while another stage corresponds to temperatures near the crystallization point of the leucosome, ranging between 796 and 853 °C. The migmatites exhibit quartz c-axis fabric temperature between 633 and 685 °C. Therefore, the solid-state deformation in granulites occurred at higher temperatures compared to migmatites, which remained in suprasolidus conditions over a broader temperature range (>300 °C). Thus, both units reached the metamorphic peak at same crustal level, but their exhumation histories are contrasting: while granulites were crystallized, migmatites remained partially molten, preserving distinct deformation geometries.