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Amazonitização em granito resultante da intrusão de pegmatitos

2000; Sociedade Brasileira de Geologia; Volume: 30; Issue: 4 Linguagem: Inglês

10.5327/rbg.v30i4.896

2317-4889

Hanna Jordt Evangelista, Júlio Cézar Mendes, Ana Luisa Cosso Lima,

Mineralogy and Gemology Studies

Borrachudos-type granite from Santa Maria do Itabira in Minas Gerais is cut by amazonite-bearing pegmatites, being marginally transformed into a green granite due to amazonitization. This paper deals with the processes and the mineralogical and chemical variations involved in the transformation of the normal granite into a green granite. The meter-wide pegmatite is composed of decimeter-large amazonite crystals of intense green color, cleavelandite, biotite, quartz and small amounts of black tourmaline, galena, fluorite and anatase. The alkaline anorogenic granite is composed of quartz, nearly pure albite, microcline with triclinicities of 0.68 to 0.93, Fe-biotite and accessory minerals. The mean SiO 2 content is 74 weight %, but in quartz-poor samples it decreases to 62%. MgO is very low (< 0.05 weight %), Fe 2 O 3total , reaches 4.4% and alkalis 8.5%. A strong negative Eu anomaly could be due to the retention of Eu in the calcic plagioclases left over in restmc rocks which generated the alkaline magma by partial melting. The intensity of the green color in the granite increases towards the pegmatite contact. Chemically this color variation is accompanied by an increase in the Pb (and to a lesser extent also in Rb) contents, while no difference could be registered for the major and other trace elements. Pb varies from 37 ppm to 302 ppm, reaching 406 ppm in the amazonite. It is possible that the subsolidus introduction of not only Pb but also of water from the pegmatite into the country rock was responsible for the generation of H,O - Pb pairs in the microcline, which are postulated in the current literature as being the cause of the green color of amazonites. Considering the relatively low temperatures of pegmatitic magmas and the massive structure of the granite the metasomatic process of amazonitization was very efficient since the aureoles locally reached a width equal to the width of the pegmatite.