Ophiolite metamorphism in northeast Borneo

Abstract

The Darvel Bay ophiolite consists of a conformable sequence of mantle harzburgite, a gabbro layer of about 2 km thickness, a basalt layer and associated chert-spilite association of Late Cretaceous to Eocene age, and overlying Miocene melange and olistostrome deposits. The ophiolite is an extension into Borneo of the recently inactive Sulu Archipelago non-volcanic arc. Generally the gabbro layer has been converted to banded plagioclase-amphibole gneisses and the basalt layer to crudely foliated epidote amphibolite. Igneous relicts occur sporadically, especially in the lower gabbro horizons. They have either cumulus granular or gabbroic textures and contain diopsidic augite, bronzite, and a dark brown pargasitic hornblende. The labradorite is occasionally clouded. The partitioning of Fe/Mg between the two pyroxenes and the hornblende suggests an igneous paragenesis and the former allocation of these pyroxene-bearing rocks to the granulite facies is now rejected. The basites have undergone metamorphic re-equlibration, frequently sporadic and imperfec, through amphibolite facies, characterized by hornblende, to greenschist facies, characterized by actinolite. Igneous labradorite frequently persists to the lower grades. The partitioning of Ca/Na between amphibole and plagioclase may be correlated with metamorphic grade. Sub sea-floor retrogressive metamorphism, combined with large scale metasomatism resulting from the circulation of hot brine, appears to be the most appropriate mechanism to explain the sodium enrichment in the basalt layer and the imperfect metamorphic re-equilibration of the ophiolite generally.