The Thessaloniki Gabbros

Abstract

The Thessaloniki gabbroic rocks show cumulus features and, following the I.U.G.S. classification scheme, the most widespread petrographical types have been referred to as gabbronorite, olivine gabbronorite and normal gabbro. X.R.F. analyses for thirtysix rocks, representative of all rock types, and forty electron probe and X.R.F analyses of the main constituent minerals, plagioclase, olivine and Ca-rich and Ca-poor pyroxenes are presented. Optical and X-ray data for some of these minerals are also given. The plagioclase is unzoned with a composition ranging between 91–95 per cent An. The clinopyroxene belongs to the diopside-salite series and occupies the compositional range between Ca47Mg46Fe7 and Ca46Mg42Fe12. The orthopyroxene belongs to the bronzite-hypersthene series and ranges in composition from Ca3Mg79Fe18 to Ca2Mg65Fe33. Olivine ranges in composition from Fo73·5 to Fo81·5 (mole per cent). The Al2 values, Tschermak's molecule contents and KD values of the pyroxenes suggest that rlatively low temperature and low pressure prevailed during the formation of these rocks. The pyroxene geothermometer indicates equilibration temperatures in the range 998–1071 °C. The tholeiitic nature of the rocks is indicated by the normative composition of pyroxene, the contents in Si, Al and Ti and also by the coexisting Ca-rich and Ca-poor pyroxenes which frequently contain exsolution lamellae. Following Yoder & Tilley's (1962) magma classification, these rocks belong to the olivine-tholeiitic magma type. However, features such as very low alkali and high calcium contents as well as Fo-rich olivines and An-rich plagioclase do not allow clear identification with one of the known eruptive basaltic magma types and indicate that these rocks have been derived from a magma rich in Ca and Mg. These rocks do, however, exhibit similarities with some Archaean basic rocks believed to be derived from peridotitic or basaltic-komatiite type magmas.