科研之友
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摘要
Felsic and mafic microgranular enclaves occurs hosted in Paleoproterozoic felsic volcanic and plutonic rocks of the Inn Group and Rio Dourado Intrusive Suite that crop out in the northeast of the Brazilian state of Mato Grosso as part of the Uatuma Supergroup in the SE Amazonian Craton. The volcanic rocks consist of dacites, rhyolites and ignimbrites of the Inn Group among which dacites yielded a crystallization age of 1895 +/- 7 Ma. The granitoids are composed of syenogranites, and monzogranites along with subordinate granodiorites. The granite pluton of syenogranite composition belongs to the Rio Dourado Intrusive Suite and was previously dated at 1876 +/- 39 Ma. Mafic enclaves show petrographic features that suggest magma mingling processes and may be classified as olivine-clinopyroxene gabbro, clinopyroxene-hornblende gabbro, biotite-clinopyroxene diorite, biotite-hornblende diorite, and biotitequartz diorite. Geochemical patterns of dioritic enclaves fit with those of the mafic rocks of the Santa Ines Intrusive Suite and suggest that they are comagmatic. The felsic enclaves consist of quartz monzonites, monzogranites and quartz-rich granitoids, with geochemical patterns and ages compatible with those of the Irid Group. U-Pb dating of a granodioritic enclave hosted by a syenogranite of the Rio Dourado Intrusive Suite yielded three distinct ages: a Paleoproterozoic magmatic age around 1.87 Ga; a Paleoproterozoic inherited age of 2001 +/- 7 Ma; and Neoarchean inherited ages ranging from 2512 to 2767 Ma. The felsic enclaves may have been derived from crustal melts, and contain restitic zircon grains, or chilled margins with zircon removed from the wall rocks. The recognition of diverse geochemical patterns of felsic and mafic enclaves suggests that the study area can represent the top of a shallow complex magma chamber that experienced a number of different replenishment events from diverse sources. The mafic enclaves and their coeval enclosing volcanic rocks suggest that the Uatuma Supergroup felsic magmatism and the Santa Ines mafic magmatism could be contemporaneous. In confirming that, this assumption would allow us to recognize a bimodal character of the Uatuma magmatism in the study area as well as the important role that mantle melts played on its genesis.
- 出版日期2017-10
