Summary
Metabasites from the Emo Metamorphics, occurring as thrust sheets beneath the Papuan ophiolite of Papua New Guinea, fall into four petrographic groups; Group 1 (garnetiferous blueschists), Group 2 (amphibolites), Group 3 (lawsonite blueschists) and Group 4 (greenschists). Most specimens shows a polymetamorphic history with textures grading from crudely foliated to mylonitic. Many contain primary igneous clinopyroxene, suggesting that metamorphism occurred under relatively low\(P_{H_2 O} \). This may explain the coherent magmatic geochemical signatures shown by many major elements normally mobilised during metamorphism. Geochemically the rocks are all tholeiitic in character and fall into three suites; Suite 1 are LREE depleted, resembling N-MORB with Zr/Nb values of > 40, Suite 2 are LREE enriched resembling E-MORB and having significantly lower Zr/Nb values of 8-16. Suite 3 includes ferrobasalts possibly comagmatic with Suite 2 but arbitrarily separated byP2O5 contents of > 0.5 and Zr contents of > 250 ppm. The three suites show significantly lower FeO* and higher AI2O3 than a Galapagos MORB comparative suite; these, and other trace element characteristics are normally associated with back-arc basin basalts. Two new39Ar-40Ar dates are presented, and a tectonic model developed integrating the geochemical and geochronological data with current models of the evolution of the New Guinea Orogen.
Zusammenfassung
Metabasite aus den Emo-Metamorphiten, die als Crustsheets unter dem Papua-Ophiolit von Papua-Neuguinea vorkommen, lassen sich auf petrographischer Basis in vier Gruppen unterteilen: Gruppe 1 (granatführende Blauschiefer) Gruppe 2 (Amphibolite), Gruppe 3 (Lawsonite-führende Blauschiefer) und Gruppe 4 (Grünschiefer). Die meisten Proben lassen eine polymetamorphe Entwicklung erkennen, mit Texturen, die vom grob gefältelten zum mylonitischen reichen. Viele enthalten primären magmatischen Klinopyroxen, was darauf hinweist, daß die Metamorphose bei relativniedrigen\(P_{H_2 O} \) stattgefunden hat. Das dürfte auch die eindeutig magmatischen geochemischen Signaturen erklären, die viele Hauptelemente, die sonst während der Metamorphose mobilisiert werden erkennen lassen. Geochemisch sind alle Gesteine als tholeütisch zu bezeichnen und lassen sich in drei Suiten unterteilen: Suite I sind LREE verarmt, und ähneln E-MORB mit signifikant niedrigen Zr-Nb Werten von 8-16. Suite 111 umfaßt Ferrobasalte, die möglicherweise mit Suite 11 komagmatisch sind aber durch P2O5 Gehalte von > 0.5 und Zr-Gehalte von > 250 ppm sich von den ersteren unterscheidet. Die drei Suiten zeigen signifikant niedrigere FeO und höhere Al2O3 als eine MORB vergleichbare Suite von Galapagos. Diese und andere Spurenelement-Verteilungsmuster sind üblicherweise mit Backarc-Becken Basalten assoziiert. Zwei neue39Ar/40Ar Altersbestimmungen werden vorgelegt und ein tektonisches Modell wird entwickelt, das die geochemischen und die geochronologischen Daten mit gängigen Modellen zur Entwicklung des Orogens von Neuguinea integriert.
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Worthing, M.A., Crawford, A.J. The igneous geochemistry and tectonic setting of metabasites from the emo metamorphics, Papua New Guinea; A record of the evolution and destruction of a backarc basin. Mineralogy and Petrology 58, 79–100 (1996). https://doi.org/10.1007/BF01165765
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DOI: https://doi.org/10.1007/BF01165765