PLAGIOGRANITES IN THE HELLENIC OPHIOLITES

Abstract

Plagiogranites have been studied from the areas of Guevgueli, Samothraki Island, Koziakas Mountains, Rhodes Island and Crete. They are typical trondhjemites (plus Qz-diorites in Guevgueli) while an unusual Qz-monzonite (geochemically defined) occurs at Rhodes. Furthermore, a leucogranite assemblage is associated with the plagiogranite in Samothraki; it has been derived by partial melting of the plagiogranite during a collision event. Geochemical results show that all the plagiogranitic rocks are differentiates of a mafic magma. In addition, at Guevgueli and Samothraki, a subsequent filter pressing process resulted in flowage of the leucocratic melt into cracks of the country rocks with the formation of a net-veined structure. The plagiogranites from Samothraki, Koziakas and Crete have chemistries similar to Volcanic Arc Granites, whereas the Guevgueli plagiogranite has an affinity closer to Ocean Ridge Granites. However, this variation is attributed to heterogeneities of the subducted lithosphere since all the above plagiogranites are associated with ophiolites of marginal basin origin.