Late Cretaceous-Tertiary tectonics of the southwest Pacific: Insights from U-Pb sensitive, high-resolution ion microprobe (SHRIMP) dating of eclogite facies rocks from New Caledonia

摘要

We present new U-Pb sensitive, high-resolution ion microprobe (SHRIMP) dates for zircon from eclogite facies samples from northern New Caledonia, which have important consequences for the Late Cretaceous to Eocene tectonic evolution of the southwest Pacific region. Two high-pressure metapelites contain zoned zircon crystals. Cores of these crystals represent inherited magmatic zircon grains, which yield an age of 85 Ma in both samples. Igneous zircon domains from a metamorphosed mafic cumulate and a felsic metasedimentary rock were also dated at 55 Ma. The igneous zircon of uniform age in rocks of sedimentary origin indicates that sedimentation took place from a restricted source area soon after magmatism. Therefore the protoliths of the eclogite facies rocks are considered to have formed between 85 and 55 Ma in a back-arc basin. This time period matches the age of the unmetamorphosed Poya Terrane of western New Caledonia and provides further evidence for a direct link between the two terranes. In the metapelites, irregular zircon rims that cross cut the oscillatory zoning of the cores have low trace element and Th/U contents and contain inclusions of high-pressure minerals providing evidence that these zircon rims formed during eclogite facies metamorphism. Therefore the 44.1 ± 0.9 Ma and 44.5 ± 1.2 Ma ages obtained from these rims represent burial related to subduction of the back-arc basin. It is suggested that subduction initiated soon after deposition of the youngest sediments at 55 Ma and stopped at 44 Ma during attempted subduction of continental crust, now represented as the Norfolk Ridge. Therefore the subduction system that produced the high-pressure rocks could only have operated for a maximum of 11 m.y. and would have been relatively hot. The 44 Ma age constraint for peak metarnorphism also significantly predates (by 10 m.y.) the timing of obduction of the New Caledonia Ultramafic Nappe, discounting any link between obduction and high-pressure metarnorphism. Combining our results with previously documented geological data, we present a revised model for the geological development of New Caledonia, which involves multiple episodes of compression and extension during the Eocene.