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Author(s):
Luana Moreira Florisbal
Total Authors: 1
Document type: Doctoral Thesis
Press: São Paulo.
Institution: Universidade de São Paulo (USP). Instituto de Geociências
Defense date:
Examining board members:
Valdecir de Assis Janasi; Miguel Angelo Stipp Basei; Maria do Carmo Pinto Gastal; Pedro Oyhantçabal; Silvio Roberto Farias Vlach
Advisor: Valdecir de Assis Janasi; Maria de Fatima Aparecida Saraiva Bitencourt
Abstract

The Catarinense Shield comprises several granitic plutons linked in space and time with translithospheric shear zones. The Major Gercino Shear Zone (MGSZ) is a megastructure consisting of several anastomosing segments that controlled the ascent and emplacement of successive granitic pulses. The connection between shearing and source of the magmas is still unclear and gives rise to distinct interpretations. The Porto Belo region corresponds to the highest deformation site in the MGSZ, where the early-phase post-collisional magmatism is represented by high-K subalkaline granitoids (Quatro Ilhas Granitoids, QIG) that were intruded by peraluminous muscovite-biotite granite (Mariscal Granite, MG); these two associations were dated by LA-MC-ICPMS at 630-615 Ma and 610 Ma, respectively. The camboriú region, situated within a low strain zone immediately to the north of MGSZ, also features high-K, subalkaline metaluminous biotite \'+ ou -\'hornblende granite (the ~620 Ma Rio pequeno Granite, RPG) intruded by peraluminous muscovite-biotite granites (Serra dos Macacos Granite, SMG, dated at ~610 Ma). In spite of the similar crystallization ages, the pattern of zircon inheritance is remarkably different in the two regions. In the Porto Belo region granites the inheritance ages are predominantly Neoproterozoic (900,700-650 Ma) with rare paleoproterozoic (2.0-2.2 Ga) ages, just identified in the peraluminous MG. On the other hand, in the camboriú region granites, Neoproterozoic (730-650 Ma) inheritance ages were also identified in both RPG and SMG, besides Mesoproterozoic ages (1.6 Ga) restricted to the RPG and Paleoproterozoic (2.1-1.8 Ga) and Archean (3.4-2.9 Ga) to the SMG. The Sm-Nd isotopic signature also distinguishes the granites outcropping north of the MGSZ, with strongly negative \'épsilon Nd IND. (t)\' (RPG: -12 to -16; SMG: -22 to -24), from the granites within the MGSZ (\'\'épsilon\'Nd IND. t\' = -6 to -10 in QIG and MG). Such differences are reflected in the Sm-Nd model ages T(DM), higher for SMG (2.5-2.6 Ga) and RPG (1.7-2.1 Ga) as compared to QIG (~1.5 Ga) and MG (2.2 Ga) \'\'ANTPOT. 87 Sr\'/\'ANTPOT. 86Sr\' IND. (t)\' ratio is relatively low in RPG and SMG (0.708-0.711); in Particular for the SMG, with high Sm-Nd T(DM), these values imply sources with low timeintegrated Rb/Sr ratios. Higher \'\'ANTPOT. 87sr\'/\'ANTPOT. 86Sr\' IND. (t)\' ratios characterize granites from the Porto Belo region, especially the QIG (0.712-0.725); lower values are shown by associated mafic rocks (0.708-0.710). The Pb-Pb isotope signature of K-feldspars from RPG and SMC are broadly similar, with low \'ANTPOT. 206 Pb\'/\'ANTPOT. 204Pb\'=16.0-16.7, \'ANTPOT 207Pb\'/\'ANTPOT. 204Pb\'=15.3-15.6 and \'ANTPOT. 208Pb\'/\'ANTPOT. 204Pb\'=36.6-37.5 Ratios, whereas the QlG and MG are much more radiogenic \'ANTPOT. 206Pb\'/\'ANTPOT. 204 Pb= 18.0-18.6; \'ANTPOT. 207Pb\'/\'ANTPOT. 204Pb\'= 15.6-16.0 ; \'ANTPOT. 208Pb\'/\'ANTPOT. 204Pb= 37.7-38.8). The geochemical and isotope characteristics of the early post-collisional magmatism in both domains point to dominantly crustal sources, but the occurrence of coeval mafic rocks attests to the participation of tholeiitic magmas as an important component at least in some of the less differentiated rocks. Considering the existence of a major tectonic structure, as well as the contrasted U-Pb, Sm-Nd, Rb-Sr and Pb-Pb data obtained from granites on either side of it, which point to distinct sources, the interpretation that this structure is a suture seems a viable hypothesis. On the other hand, the clearly transcurrent character of the MGSZ, as well as the post-collisional signature and similar age of granitic magmatism on both sides of it argue against its interpretation as a suture younger than 630 Ma. Likewise, the isotopic differences could be attributed to pretranscurrence tectonic interleaving related to a ca. 650 Ma main collision, as well as to the juxtaposition of different crustal segments resulting from dextral transcurrence. (AU)

FAPESP's process: 07/02387-9 - Petrogenesis of contemporary felsic-mafic rock associations: an integration of geochemistry and structural geology in post-collision sintectonic granites of Santa Catarina
Grantee:Luana Moreira Florisbal
Support type: Scholarships in Brazil - Doctorate