U-Pb Age and Geochemistry of the Tholeiitic Trachy-Andesite of Gouveia (MG) Compared with Regional Pedro Lessa and Formiga Tholeiitic Basic Rocks: Fragments of a Tonian Large Igneous Province (LIP)

Abstract

In Gouveia (MG), a tholeiitic high-K trachy-andesitic subvolcanic rock, with phenocrysts of plagioclase and sanidine, and a matrix rich in feldspars, biotite, magmatic epidote and quartz, outcrops in the form of irregular bodies and subverticalized dykes. The U-Pb age of 938.5 +/- 4.5 Ma found for the Gouveia trachy-andesite is practically the same as for the regional metabasic rocks of the Pedro Lessa suite (933 +/- 20 Ma) and the Formiga dykes (940 + / - 50 Ma), pointing to the grouping of all these rocks in the same magmatic event. The combined treatment of the geochemistry of the trachy-andesitic rock with that of the regional Pedro Lessa and Formiga rocks, reveals cogeneticity among these lithotypes. The basaltic magma that generates these rocks, with an OIB signature (typical of mantle plume), would came from partial melting of a garnet-bearing peridotite, passed through lithospheric contamination and differentiated through the fractional crystallization process until the investigated trachy-andesite. This basic/intermediate magmatism of Tonian age is inserted in the magmatic context of a large igneous province (LIP) rebuilt in São Francisco and Congo cratons.

Keywords

• Trachy-andesite
• Gouveia
• Pedro Lessa
• Formiga
• U-Pb
• geochemistry
• LIP