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QUÍMICA MINERAL E EVOLUÇÃO PETROLÓGICA DO SIENITO PIQUIRI: MAGMATISMO SHOSHONÍTICO, NEOPROTEROZÓICO, PÓS-COLISIONAL NO SUL DO BRASIL

2001; Sociedade Brasileira de Geologia; Volume: 31; Issue: 2 Linguagem: Inglês

10.5327/rbg.v31i2.935

2317-4889

Larissa Zitto Stabel, Lauro Valentim Stoll Nardi, J. Plá Cid,

Paleontology and Stratigraphy of Fossils

The Piquiri Syenite is a Neoproterozoic intrusion, related to the post-collisional magmatism of the Brasiliano Cycle in the Rio Grande do Sul State, Southern Brazil. It is related to the shoshonitic magmatism in the molassic Camaqua Basin, described as the Lavras do Sul Shoshonitic Association. This study is based mainly on field data, petrography and microprobe determinations. The lithological variation, including monzonitic to monzogranitic fast crystallized rocks in the borders and perthite syenitic rocks in the core, is attributed mainly to non-equilibrated plagioclase and quartz in the fine-grained rocks. Diopside and augite, as well as magnesiumhornblende and edenite are the dominant ferromagnesian phases, typical of shoshonitic syenites. Biotites are xenocrysts derived from lamprophiric magmas co-mingled with the syenitic host or late crystallized magmatic products. Andesine and diopside are present only in the fine-grained borders where quartz is more abundant, albitic plagioclase occurs also as resorbed portions inside mesoperthitic alkali feldspar. Mesoperthites are the dominant feldspars in the core of the intrusion. Very fine inclusions of K-rich augite are found in diopside from lamprophyric enclaves, whereas the syenite have clinopyroxenes with up to 0.35wt% of K 2 O. These K-rich clinopyroxenes were crystallized under pressures of about 5 GPa, suggesting that these magmas were produced from K-enriched mantle sources at depths around 150 km. The evolution of Fe/Mg ratios in the ferromagnesian phases and of plagioclases to mesoperthites and eventually to albite + K-feldspar, as well as the presence of ilmenite + magnetite mostly associated to intense amphibole generation, are interpreted as produced by increasing aH 2 O and fO 2 during magma crystallization. Early crystallized apatite and zircon are consistent with a metaluminous alkaline silica-saturated syenite. Flow-segregation and co-mingling processes produced a large amount of complex structures such as mafic microgranular enclaves, schlieren, cumulative mafic bands and layers, fine-grained autoliths and an oriented fabric.