THE IGNEOUS GEOCHEMISTRY AND TECTONIC SETTING OF METABASITES FROM THEEMO METAMORPHICS, PAPUA-NEW-GUINEA - A RECORD OF THE EVOLUTION AND DESTRUCTION OF A BACKARC BASIN

Metabasites from the Emo Metamorphics, occurring as thrust sheets bene ath the Papuan ophiolite of Papua New Guinea, fall into four petrograp hic groups; Group 1 (garnetiferous blueschists), Group 2 (amphibolites ), Group 3 (lawsonite blueschists) and Group 4 (greenschists). Most sp ecimens shows a polymetamorphic history with textures grading from cru dely foliated to mylonitic. Many contain primary igneous clinopyroxene , suggesting that metamorphism occurred under relatively low P-H2O. Th is may explain the coherent magmatic geochemical signatures shown by m any major elements normally mobilised during metamorphism. Geochemical ly the rocks are all tholeiitic in character and fall into three suite s; Suite 1 are LREE depleted, resembling N-MORB with Zr/Nb values of > 40, Suite 2 are LREE enriched resembling E-MORB and having significan tly lower Zr/Nb values of 8-16. Suite 3 includes ferrobasalts possibly comagmatic with Suite 2 but arbitrarily separated by P2O5 contents of > 0.5 and Zr contents of > 250 ppm. The three suites show significant ly lower FeO and higher Al2O3 than a Galapagos MORB comparative suite ; these, and other trace element characteristics are normally associat ed with back-are basin basalts. Two new Ar-39-Ar-40 dates are presente d, and a tectonic model developed integrating the geochemical and geoc hronological data with current models of the evolution of the New Guin ea Orogen.