Ediacaran post-collisional high-silica volcanism associated to the Florianopolis Batholith, Dom Feliciano Belt, southernmost Brazil: lithofacies analysis and petrology

摘要

The Cambirela Silicic Volcanic Sequence is the only Neoproterozoic volcanic manifestation in the Florianopolis Batholith. The sequence is associated with the late stages of the post-collisional magmatism emplaced in the Dom Feliciano Belt during the Brazilian/Pan-African Orogenic Cycle that was responsible for the amalgamation of the West Gondwana supercontinent. Distinct from other ignimbrite occurrence, that are generally associated with caldera and sedimentary sequences, the Cambirela Silicic Volcanic Sequence is associated with shallow granitic bodies in a plutono-volcanic system. The sequence is arranged along a high geomorphological feature with NNE-SSW orientation, distributed in a total area of 88 km(2), and is constituted by effusive and pyroclastic units. The pyroclastic unit is dominant in the sequence and characterized by ignimbrites with 15-50% of quartz, K-feldspar and plagioclase crystals, crystal and lithic fragments immersed in a tuffaceous matrix, presenting incipient eutaxitic to parataxitic structure. Due to its geotectonic structure and the absence of associated caldera and sedimentary sequences, these ignimbrites were classified as fissure ignimbrites. The effusive unit is composed by rhyolites with 3% of quartz, K-feldspar and plagioclase phenocrysts immersed in an equigranular fine matrix. These units were divided into five lithofaceis: porphyritic rhyolite (pR), lapilli-tuff with low-crystal content and eutaxitic structure (lceLT), lapilli-tuff with high-crystal content and eutaxitic structure (hceLT), lapilli-tuff with parataxitic structure (pLT) and crystal-rich lapilli-tuff (crLT). Based on geochemical data, these were classified as high-silica rhyolites, related to the high-K calc-alkaline series, close to the shoshonitic series. The sequence has a slightly metaluminous to peraluminous character and high contents of alkalis, FeOt/(FeOt + MgO) and agpaitic indexes. These characteristics are associated to the A-type magmatism in post-collisional settings in southern Brazil. Regarding to the rheology, the sequence present glass transition temperatures ranging from 731 to 781 degrees C, liquidus temperatures from 940 to 968 degrees C and viscosities from 7.72 to 8.90 log Pa.s. These values are similar to the chronocorrelated riha Granite, but slightly different from the Ana Dias Rhyolite. The rheological and geochronological similarities between the Cambirela Silicic Volcanic Sequence and the Ilha Granite, together with the contact relationships and interaction observed in the field between these units, indicate that these melts probably belonged to the same magma chamber constituting a magma mush located in the shallow portion of the crust.