Mantle Heterogeneity in Eastern Papua New Guinea: Implications for Plate-tectonic Evolution of the Southeast Asia and Southwest Pacific Regions

Khin Maung Wai School of Earth Sciences, Flinders University of South Australia,

GPO Box 2100, Adelaide 5001, Australia

J. Foden Department of Geology and Geophysics, Adelaide University, Adelaide 5005, Australia

M. J. Abbott School of Earth Sciences, Flinders University of South Australia,

GPO Box 2100, Adelaide 5001, Australia

A. E. Grady School of Earth Sciences, Flinders University of South Australia,

GPO Box 2100, Adelaide 5001, Australia

Cretaceous to Eocene ophiolitic rocks in Papua New Guinea (Papuan Ultramafic Belt, Marum, Sadowa, Kutu and Uyaknji complexes) are supposed to be remnants of narrow ocean basins that developed during multi-branched rifting of a microcontinent following southwest subduction during the Jurassic or early Cretaceous or after a Permo-Triassic compressional event. Possibly depend on the branch closing time their ages of emplacement may be different. These ophiolites obducted onto the continental fragment to form composite terrane. Their emplacement will be related with a change in plate motion and emplaced shortly after their creation.


Geochemical data confirm the presence of two distinct compositional groups in the mafic lavas of ophiolites
in Papua New Guinea, New Zealand, New Caledonia, eastern India and Burma:(1) tholeiitic basalts (2) transitional tholeiitic basalts. Multi-stage melting is considered in the ophiolitic rocks of Papua New Guinea. The SIC (Sadowa Igneous Complex) samples contain low Th/La (0.01-0.07), low La/Ta (9.7-18.75) and are in the range of MORB-like basalts. La/Ta ratios of the SIC are similar to the range of Bela ophiolitic basalts, Pakistan (La/Ta ratios=9.62-18.04, Sarwar, 1992) and these values are akin to N-MORB and
E-MORB (N-MORB, La/Ta=15; E-MORB, La/Ta=10; Wood et al., 1979). These SIC basalts contain wide range of Zr/Nb ratios (3.89-30) and most of them (Zr/Nb ratios=9.09-13.75) are akin to E-type MORB (Zr/Nb=9; Sun and McDonough, 1989) whereas one sample (Zr/Nb=30) in the middle section of the SIC is more comparable to N-type MORB (Zr/Nb=32; Sun and McDonough,1989) and some samples (Zr/Nb=3.89-8.75) are akin to OIB (Zr/Nb-5.83; Sun and McDonough, 1989). The SIC data plot within the MORB-OIB array and a wide range of 143Nd/144Nd ratios (0.51270-0.51294) are quite comparable with those of oceanic island basalts, it suggested that the SIC is a hot spot type component. N-type MORBs are found only in
the middle section of the complex and is similar to Red Sea (e.g. Barrat et al., 1990). Comparison of the Nd isotopic compositions of the SIC(eNd=+1.02 to +6.3) and Papuan Ultramafic Belt (eNd=+7.4) reveals a much larger degree of heterogeneity in the Sadowa Igneous Complex. eNd values of the SIC (+1.02 to +6.3) are almost identical with Proterozoic Purtuniq ophiolite (eNd=+1.0 to +6.1), Canada which is a plume pudding type mantle or a heterogeneous highly depleted mantle plume (Hegner and Bevier, 1991).

Discussion and conclusions

During the Cretaceous to Eocene the Ninetyeast ridge was an active transform fault between India and Austral-Antarctic plates, and the Australia-Pacific boundary transform fault developed in the western Pacific. Tasman Sea, Coral Sea and other small ocean basins open in New Caledonia, Newzealand and Papua New Guinea. Cretaceous to Eocene ophiolitic rocks from Southeast Asia and Southwest Pacific may be related with these transform faults. Geochemical data confirm that they are formed in small ocean basins, possibly at or near a ridge transform intersection. A ridge-transform setting is compatible with intraplate character of some of the transitional basalts indicating that they were part of the Australian plate. Their occurrence suggests that continenet-continent collision (e.g., Wai et al., 1994) rather than continent-island-arc collision.


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