Petrografia e química mineral dos diques máficos da região Crixás-Goiás, estado de Goiás

Abstract

Paleoproterozoic mafic dikes intrude Archean rocks of the Goiás Block along two main trends, NE and NW. Based on petrographic features they are classified in three groups: diabase, metabasite and amphibolite. Chemical analyses display tholeiitic affinity and permit the great majority of rocks to be classified as basalt. Some samples, mainly diabase, are basaltic andesites. Diabase, metabasite, and at least part of the amphibolites seem to be cogenetic. Diabase and metabasite contain Carich (augite) and Capoor pyroxenes (enstatite and pigeonite). In both types the wollastonite content (Wo) decreases and the ferrosilite component (Fs) increases from core to rim of the pyroxenes. This compositional variation is less pronounced in the Capoor pyroxenes. Cores and rims of plagioclases of diabase and metabasite vary from labradorite to bytownite. In the amphibolites the plagioclase ranges from oligoclase to andesine. Several geothermometers indicate similar crystallization temperatures for diabase and metabasite. Data from pyroxene and plagioclase point out average values of 1100ºC - 1200ºC and 1100°C respectively. Igneous and secondary amphiboles (derived from pyroxenes) exhibit a large compositional range, mainly due to variable contents of FeO and MgO. Chemical analyses of opaque minerals identified magnetite and ilmenite.