Diques básicos en las Sierras de Tandil

TERUGGI, M.E.; KILMURRAY, J.; RAPELA, C.W.; DALLA SALDA, L.H.

Revista Asociacion Geológica Argentina

Asociación Geológica Argentina

Lugar: Buenos Aires; Año: 1974 vol. 29 p. 41 - 60

0004-4822

The occurrence of thick (50 m average) diabase dykes in the Precambrian migmatites and granitoids of the Tandil basement is here reported for the first time. The dykes have been intruded along vertical fractures connected to the structures of the tectonic domain. The diabases are made up of acid labradorite and augite or salitic augitic clinopyroxenes. Plagioclases are systematically transformed into a grain scaly aggregate, whilst pyroxenes may be fresh, amphibolitized and/or chloritized. Ilmenite partly replaced by leucoxene, and tare apatite, are found as accesories. Geochemical analyses show that the diabases are somewhat low in their contents of SiO2 while they are relatively rich in alcalis; consecuently, in Kuno’s diagrams they fall in the field of alcaline basalts, with occasional trend towards high alumina basalts. Similar results are obtained by calculating the Barth-Niggli katanorm and applying Irvine and Baragar’s normative classification. Chemical analyses of five trace elements (Ni, Co, Cu, Zn, Cr) are also given. The petrological part of this paper deals with the metamorphic transformations that have affected the diabases, whose problematic nature is discussed. Finally, datings of the intrusives are presented and their position in the evolutive sequence of the Tandil basement is considered and compared with that of Guiana.