Rb-Sr resetting ages and chemical characterization of turbidites in an accretionary wedge: Torlesse Complex, Otaki Gorge, New Zealand

Whole-rock geochemical and Rb-Sr isotopic analyses have been undertaken on turbidites from three discrete localities, each about 2 km apart across strike, within the Torlesse Complex, a Permian-Cretaceous accretionary wedge in New Zealand. Overall, the three suites show similar petrographic and geochemical compositions to turbidites from elsewhere in the Torlesse Complex, but between the suites, there are detectable differences for most major and trace elements, particularly Sr, MgO, and Na₂O. The individual suites define three parallel Rb-Sr whole-rock isochrons with identical dates of about 205 Ma. The presence of Monotis bivalves provides a depositional age of Norian (219-225 Ma). Hence, the isochrons suggest that the Rb-Sr clock was reset some 15-20 m.y. after deposition, during low-grade metamorphism associated with accretion and deformation of the sediment pile. There is a wide variation in the initial ⁸⁷Sr/⁸⁶Sr ratios from 0.70683 to 0.70955, which is best explained by changes in the composition of the detritus being provided from the source area, as detected by the chemical differences. Sea water is not considered to have played a significant role in contributing to the initial ratios. The failure of some samples collected away from the main sample localities to plot on their respective isochrons suggests that domains of Sr isotopic equilibration are small, in the order of a few cubic meters. As a result of this study, we have extended the Rb-Sr whole-rock dating method of sediments, now widely applied to shales and cherts, to the more complex turbidite sequences from accretionary wedges and have shown that meaningful dates can be obtained. This should provide an invaluable method of dating these sparsely fossiliferous terranes.