Abstract

A sequence of varied igneous rocks, of Triassic age, occurs in a complex stack of thrust sheets in the Othris Mountains of eastern Greece. The igneous rocks, which are picritic basalts, diabases, keratophyric tuffs, and pillow lavas, are conformably overlain by marine sediments typical of continental margins, and tectonically overlain by several thrust sheets of ophiolitic rock. The igneous rocks are undersaturated with silica. They are similar in chemistry to igneous rocks erupted on the margins of the Red Sea, which are located on a young, accreting plate margin. The igneous rocks of the Othris Mountains are interpreted as evidence of the development of an accreting plate margin in eastern Greece in Triassic time. The ophiolites overlying them probably came from an ocean basin created at that margin.In contrast to the rocks on the margins of the Red Sea, igneous rocks on many other modern oceanic margins are saturated with silica. This contrast is tentatively related to the speed with which an accreting plate margin develops. Silica-saturated igneous rocks are probably erupted at rapidly separating continental margins, whereas silica-undersaturated igneous rocks are probably erupted in the early stages of activity at plate boundaries which begin to spread only slowly.

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