Celi Zanon, Rômulo Machado, Ruy Paulo Philipp, Umberto G Cordani
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Abstract
Basic acid dike swarms of Piratini and Pinheiro Machado areas in the Dom Feliciano belt are intrusive in the Pelotas Batholith. They were emplaced in a shallower crustal level than the batholith. U-Pb age of 560 Ma was obtained for them. We investigated in this paper the origin of these rocks (basic, intermediate, and acid) based on mineral chemistry (K-feldspar, plagioclase, clinopyroxene, amphibole and biotite), and on petrographic and field data. The decrease in TiO2 content from core to edge of clinopyroxene (diopside-augite) suggests that its crystallization occurred from a liquid sub-saturated in silica. The high values of mg# (0.71-0.94) of the core of amphibole crystals of intermediate dikes are comparable to the experimental data of amphiboles equilibrated with primitive basalt magmas. Biotite in the discriminant diagram (FeOtotal versus Al2O3) occupy the fields of the alkaline and peraluminous suites and at the boundary of the calc-alkaline and slightly peraluminous fields. The alkaline character in TAS diagram together with the plot of part of biotite samples in the field of rocks of the alkaline suite of Abdel-Rahman (1994) suggests connection with magmatism of mantle origin that was modified by processes of crustal contamination.
Dom Feliciano带的Piratini和Pinheiro Machado地区的碱性酸性岩脉群侵入了Pelotas岩基。它们被安置在比岩基更浅的地壳层。测得它们的U-Pb年龄为560 Ma。本文根据矿物化学(钾长石、斜长石、斜辉石、角闪石和黑云母)、岩石学和野外资料,对这些岩石(基性、中质和酸性)的成因进行了研究。斜辉石(透辉石-辉石)的TiO2含量从核心到边缘呈下降趋势,表明其结晶发生于二氧化硅中亚饱和的液体。中间岩脉角闪孔晶体岩心的高mg值(0.71 ~ 0.94)与原始玄武岩岩浆平衡的角闪孔实验数据相当。判别图(FeOtotal vs . Al2O3)中的黑云母占据碱性和过铝质层以及钙碱性和轻度过铝质层的边界。TAS图的碱性特征以及Abdel-Rahman(1994)碱性套岩野外部分黑云母样品图表明其与经地壳污染过程修饰的幔源岩浆活动有关。
期刊介绍:
The Brazilian Academy of Sciences (BAS) publishes its journal, Annals of the Brazilian Academy of Sciences (AABC, in its Brazilianportuguese acronym ), every 3 months, being the oldest journal in Brazil with conkinuous distribukion, daking back to 1929. This scienkihic journal aims to publish the advances in scienkihic research from both Brazilian and foreigner scienkists, who work in the main research centers in the whole world, always looking for excellence.
Essenkially a mulkidisciplinary journal, the AABC cover, with both reviews and original researches, the diverse areas represented in the Academy, such as Biology, Physics, Biomedical Sciences, Chemistry, Agrarian Sciences, Engineering, Mathemakics, Social, Health and Earth Sciences.