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OS PICRITOS ALCALINOS DA REGIÃO DE IPORÁ: IMPLICAÇÕES NA GÊNESE DOS COMPLEXOS DO TIPO CENTRAL DO SUL DE GOIÁS

1994; Sociedade Brasileira de Geologia; Volume: 24; Issue: 2 Linguagem: Inglês

10.5327/rbg.v24i1.497

2317-4889

José C.M. Danni,

Geochemistry and Elemental Analysis

In the Ipora region, Goias State, the occurrence of several subvolcanic intrusions (sills, dykes and plugs) of alkaline picrites are recognized as part of the Upper Cretaceous Alkaline Province. The picrites have cumulate porphiritic texture, and were formed by 10 to 25% (vol) of olivine (Fo 88 ) macrocrystals and some clinopyroxenes (O to 5%), set in a microcristalline to quenched matrix formed by olivine, diopside, plagioclase, ulvospinel, K-feldspar, nepheline, Ti-biotite and leucite (altered to analcite). Chemically these rocks are ultrabasic alkaline, subsaturated, with ol and ne in the norm. They present MgO/MgO+FeO ratios (molecular %) between 0.72 and 0.80, and high values of Cr, Ni, V, Ba, Sr, and light REE. Variations of the major and trace elements are controlled by olivine fractionation. Calculation by mass subtraction of 15-20% of olivine from the picrites results in Mg-basalt compositions with MgO = 13% and MgO/MgO+FeO ratio of 0.71, that may represent the parental magma from wich the alkaline central complexes of the Ipora region fractionated.