Petrogenesis of contemporary felsic-mafic rock associations: an integration of geochemistry and structural geology in post-collision sintectonic granites of Santa Catarina

Abstract

The early post-collisional magmatism of the Brasiliano/Pan-African Cycle in the eastern part of Santa Catarina (ca. 630-620 Ma), southern Brazil, is marked by large granitic massifs with associated mafic rocks. It is composed of a plutonic association of coeval monzogranites and syenogranites with abundant mafic microgranular enclaves and syn-plutonic dykes, interpreted as co-mingling products and, smaller bodies of contemporaneous gabbros and diorites. Field relations define a chronological order of magmatic events, where the granitic plutons represent diverses magmatic pulses, which were intruded and mingled with mafic magmas before their complete crystallization. The plutonism was controlled in successive stages by a litospheric scale discontinuity, the Southern Brazilian Shear Belt (SBSB), which includes and encloses diverse anastomosed shear zones of kilometer-size thickness and with mostly transcurrent kinematics. Temperature conditions compatible with green-schist to lower amphibolite fácies were identified in zones of major concentration of deformation, as in the Major Gercino Shear Zone, where heat was periodically produced as consequence of the continued magmatic activity. The early Neoproterozoic magmatism is then defined as syntranscurrent, and microstructures indicative of high-temperature solid-state deformation are described in the basic and granitic rocks. Basic rocks have a medium to high-K tholeiitic affinity and show mineralogical and geochemical features, including isotopic ones that resemble the high-Ti-P basalts of the Serra Geral Formation. Their high contents of K, Rb, Sr and Ba, as well as negative anomalies of Nb and Ta in multielementar diagrams indicate mantle sources enriched in LREE and modified by previous subduction, wich is coherent with its post-collisional character. The granitoids result from the interaction of tholeiitic magmas with crustal materials in different proportions, hence may be considered as granitoids of tholeiitic affinity. The role of transcurrent tectonics seems to be essential for promoting the generation of mantle-derived magmas and their interaction with crustal material, with consequent generation of abundant granitoids. The compositional resemblance of this granitoids with A-type ones makes them different from the medium to high-K granitoids of mature magmatic arcs, and this difference probably reflects the tholeiitic character of their primary magmas, typical of the early magmatic stages of post-collisional settings. The present project aims the investigation of the origin, diversity and tectonic control of the early post-collision magmatism of southern Brazil, looking for to elucidate the relation between crustal and mantle sources and the role of the tectonic in the origin of the granitic magmas and in the evolution of this petrotectonic association. The focus of this project is the identification of the diverses sources involved in the generation of basic and acid coeval magmatism of the early post-collisional stage, with emphasis in the investigation of the mixing processes that occur under stress fields with diverse magnitudes. Detailed studies will be conducted in associations of felsic and mafic coeval rocks situated in regions of low and high deformation (PortoBelo, Garopaba-PauloLopes and Camboriú), all related to different segments of the Southern Brazilian Shear Belt Major Gercino Shear Zone, in the Santa Catarina State. The comparative study of these occurrences, combining structural geology (detailed field study in selected areas, petrography and microstructural analysis) and geochemistry (major and trace elements analysis in rock and minerals and isotopic Sr-Nd determinations), will allow the identification and quantification of processes and sources involved in the generation and evolution of the basic-acid plutonic association of the early post-collisonal magmatism. (AU)