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Título

STRUCTURAL CONTROL OF GRANITE PEGMATITES HOSTED IN THE RIBEIRÃO DA FOLHA FORMATION, ARAÇUAÍ OROGEN, BRAZIL.

Texto do resumo

In its type-area, the ophiolite-related sequence of the Ribeirão da Folha Formation is located in the northern Araçuaí orogen, hosts late-collisional S-type granites of the G4 Supersuite, known locally as Mangabeiras Suite, and related bodies of granitic pegmatites (Pedrosa-Soares, 1997). Although previously mapped at 1:25.000 scale (Queiroga 2006), the region lacked of works focusing on the structural control of granitic pegmatites. An area of approximately 106 Km² was covered by geological mapping at 1:20.000 scale during an elective course for undergraduate students. As result, we individualize the Lower and Upper members of the Ribeirão da Folha Formation. The Lower Member consists of amphibolite and serpentinite or other metaultramafic varieties, overlapping by quartz-mica schist with Fe-rich garnet, staurolite and kyanite. The Upper Member comprises coarse-grained mica-quartz schist and quartzite found in higher altitude areas. The Mangabeiras Suite consists of granitoids with equigranular quartz, plagioclase, muscovite, and biotite. Southern pegmatites are composed mainly of quartz, biotite, muscovite, plagioclase and microcline, where the intergrowth of quartz and feldspar marks the graphic texture. Tourmaline, cassiterite, columbite and tantalite comprises the accessory mineralogy. The structural framework is defined by three distinct planar structures, whose cross-cutting relationship, distribution and mode of occurrence denotes the tectonic history of the mapped area. The Sn is a relict foliation completely transposed by the predominant penetrative ductile Sn+1 foliation (amphibolite facies, kyanite zone, in correspondence with Sn foliation described by Queiroga, 2006). It striking-EW and gently dipping east, possibly verging to the north. The Sn+2 foliation occurs at ductile-ruptile to ruptile-ductile transitional crustal levels (reaching up the greenschist facies), appears as crenulation cleavage in schistose rocks and spaced cleavage in sandy rocks, predominantly oriented NS and dipping north. The set of geological map, field relationships and structural data suggest no parental relationship between the granitic pegmatites and the Mangabeiras Suite. The Mangabeiras pluton appears elongated in map view, with its main axis parallel to the ductile Sn+1 penetrative foliation, both being cutted by Sn+2 cleavage. It seems plausible associate the generation of Sn+1 foliation to the collisional stage of the Araçuaí orogen, striking EW as tectonic inheritance of the Guanhães Block, located at south. The Mangabeiras pluton must represent the final magmatism of the collisional stage in this region. On the other hand, Sn+2 cleavage and the pegmatites bodies show close association, leading to believe that their spatial and temporal generation/emplacement are related to collapse stage of the Araçuaí orogen. The abrupt contact between the granitic pegmatites and the host rocks of the Ribeirão da Folha Formation, without the occurrence of metasomatic minerals assemblages, suggests two mechanisms that operated during the emplacement of these pegmatites (according to Chadwick, 1958): i) permissive emplacement, ii) or local derivation. Acknowledgements to FAPEMIG (project APQ-02811-21), to Mineração Ribeirão das Folhas Ltda, DEGEO/EM/UFOP and all undergraduate students of the GEO391_2023_1.

Palavras Chave

Structural Controls; Granite Pegmatites; Araçuaí orogen; Ribeirão da Folha region.