Metamorphism of bauxites on Naxos, Greece

Abstract

This thesis presents the results of a petrological-mineralogical and geochemical study of the metamorphosed karstbauxites on the island of Naxos, Greece. The bauxites have been subject to an Eocene highpressure metamorphism (M1), followed by a Late Oligocene-Miocene medium-pressure metamorphism (M2). P-T conditions during M1 were confined to the stability field of diaspore. Metamorphic conditions during M2 ranged from lower greenschist grade in SE Naxos to upper amphibolite grade in the central part of the island. The metabauxites cover this metamorphic interval and show the following characteristic mineral assemblages with increasing grade; diaspore-chloritoid, corundum-chloritoid, corundum-staurolite and corundum-green spinel. Relicts of the high-pressure M1-mineralogies are locally found in the metabauxites up to medium amphibolite grade M2-conditions. Microprobe studies indicate that the metabauxitic minerals are highly aluminous. Many minerals (e.g. chloritoid, staurolite, biotite, margarite) display a large compositional variation which is thought to reflect the initial chemical inhomogeneity of the bauxites. The petrological and chemical data provide strong evidence that chemical equilibrium was commonly attained on a local scale and that the metabauxites essentially behaved as closed systems during metamorphism. The restricted mobility of elements during metamorphism is also evident from the fact that vertical trace element patterns characteristic of non-metamorphosed karstbauxites have been preserved in the Naxos deposits after metamorphism. In chemical composition the Naxos metabauxites closely resemble Jurassic karstbauxites in Greece and Yugoslavia, supposing that this is their depositional age.