‘Boninitic’ clasts from the Mesozoic olistostromes and turbidites of Angelokastron (Argolis, Greece)

The Lower Unit of the ophiolitic sequence of Northern Argolis comprises turbiditic sediments and olistostromes. both containing ophiolitic clasts. mainly crystal fragments (clinopyroxene. plagioclase. Cr-spinel, amphibole) in the turbidites and cumulitic intrusives (quartz noritic amphibole-bearing gabbros). subvolcanic rocks (dolerites) and various effusive lithologies (mainly Si-rich basalts to basaltic andesites) in the olistostromes. The volcanic rocks belong to three groups. In rare cases the lavas are niineralogically and chemically comparable with MORB; most of them. and the subvolcanic rocks, contain primary quartz and amphibole, orthopyroxene, Ca-rich plagioclase and clinopyroxene_+ Cr-spinels. All rocks are Si- and Mg-rich and have high concentrations of ‘compatible’ and very low concentrations of ’incompatible’ elements. The REE profiles are characteristically U-shaped. Many of the observed features are comparable with those of suhduction-related lavas and, in particular, with present day boninites and ophiolitic boninitic rocks. The gabbroic rocks have mineralogical and chemical analogies with the dolerites and lavas, thus it may be argued that the gabbros represent the intrusive counterparts of the ‘boninitic’ volcanic clasts. The mineral clasts occurring in the turbidites are chemically comparable with those analysed in the ophiolitic clasts of the overlying olistostrome. It may be concluded that the ophiolitic clasts of both olistostromes and turbidites were derived from a subduction-related sequence. An island arc back-arc system might explain the occurrence of both boninitic and MORB-type lithologies in the olistostrome of Angelokastron. This may support the hypothesis of the onset of compressive tectonics along the Pindos Ocean during the Jurassic. X; 1996 by John Wiley & Sons, Ltd. Recrired 3 Murch 199.5; Accrpred 10 Ocroher 199.5

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