Geotermobarometria y Quimica Mineral del Pluton Sienito "Pajeú" Serra Talhada, Pernambuco, NE de Brasil

CRISTINA NANCY REYES

Resumo


The Pajeu Syenitic Complex, Serra Talhada, Pernambuco province, northeastern Brazil is tectonically emplaced within the framework of Neoproterozoic (Brasiliano). In the present work were studied whole rock and specific mineral chemical data (85’24” – 816’31” S and 3819’8” – 3836’2” W). Two distinct petrographic facies have been identified. The predominant porphyritic syenite facies is characterized by perthite microcline megacrysts (up to 8 cm long) and fine grained syenite; both facies have distributed melanocratic microgranular enclaves of syenitic composition. The mineral assemblage comprising alkali feldspar, plagioclase, quartz, biotite, hornblende and accessory amounts of zircon, opaque minerals, epidote, allanite, sphene etc. the geochemical signatures define overlapping calc-alkaline and shoshonitic affinities and metaluminous to peraluminous nature. The complex is characterized by silica enrichment (62.2% - 72.4%), moderate alumina (14% - 16.6%) and high alkali abundance (8.6% - 9.1%). The enclaves have low silica (54.2% - 56.4%) and corresponding enrichment in mafic phases. High alkali abundance, preponderance of K2O over Na2O and TiO2 variation between 0.22% to 0.57% are in agreement with shoshonitic affinities. Mineral chemistry of amphiboles enables their classification as edenite – magnesian hornblende. The micas are mainly magnesian biotite. Plagioclase is usually oligoclase (An20) and K-feldspar is orthoclase. The geochemical signatures of biotite and amphibole are in perfect agreement with the other shoshonitic rocks of northeastern of Brazil. Temperatures estimateds for calcic plagioclase and amphibole pairs indicate a temperature range of 646-671C for porphyritic syenite and 629-650C for enclaves. Geobarometric estimates, using Al in amphiboles indicate equation pressure of 1,48 Kbar to 2,26 Kbar for porphyritic syenite and 1,01 Kbar for enclaves. Other pressure estimates yielding 2,25 Kbar to 2,91 Kbar and 1,76 Kbar respectively, though at variance with the earlier estimates correspond with epizonal emplacement of the pluton.



Palavras-chave


shoshonitic; peraluminous; clac-alkaline

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DOI: https://doi.org/10.22456/1807-9806.21174

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