The Serra da Graciosa A-type Granites and Syenites,southern Brazil Part 2:Petrographic and mineralogical evolution of the alkaline and aluminous associations

摘要

The Serra da Graciosa Granites and Syenites comprise five distinct plutons in the Brasiliano/Pan-African Graciosa A-type Province,southern Brazil.Six petrographic series can be identified in these plutons:(1)Alkaline series 1,composed of amphibole-bearing alkali feldspar syenites with varied mafic mineralogy and quartz contents,from alkali feldspar syenites with calcic amphibole,clinopyroxene,olivine and allanite to alkali feldspar quartz syenites with sodic-calcic amphibole and chevkinite-perrierite and to alkali feldspar granites with sodic amphibole;(2)Alkaline series 2,characterized by amphibole-bearing alkali feldspar granites,with limited modal variations but amphibole compositions also varying from calcic to sodic;(3)Alkaline series 3,of limited occurrence,which includes alkali feldspar syenites with olivine and clinopyroxene and no amphibole;(4)Aluminous series 1,of widespread occurrence,with various petrographic facies of biotite granites with amphibole;(5)Aluminous series 2,characterized by alkali feldspar granites with biotite and only minor amphibole;(6)Monzodiorites,typically with biotite,calcic amphibole and augitic clinopyroxene,partially mingled with granitic magmas.The mafic minerals present are,in general,Fe-rich with correspondingly low Mg and Al contents.In Alkaline series 1,amphiboles crystallized in progressively more oxidizing and alkaline conditions,while in Alkaline series 2,the initial conditions were somewhat more oxidizing and shifted to reducing in the final stages.In Aluminous series 1 and Aluminous series 2,amphiboles are calcic and comparatively homogeneous.The amphiboles in the monzodioritic rocks,while also homogeneous,are more Mg-rich and show compositions quite distinct from the calcic varieties in the other associations,and this is also the case for clinopyroxene.Mg# in biotite decreases from the monzodioritic rocks to Aluminous series 1 and further to Aluminous series 2.Contrasting evolution of the various associations suggests that several coeval magmatic series are present in the Serra da Graciosa granites.