Whole-rock RbSr isochrons and pseudo-isochrons from turbidite suites from the Torlesse accretionary prism, New Zealand

Abstract

Whole-rock RbSr dating of three suites of greywacke and argillite from the Torlesse accretionary prism terrane of New Zealand provides age constraints on the timing of deposition of the sediments in the absence of fossil ages. The three suites produced four linear arrays. Two arrays are interpreted as isochrons with age significance (135 ± 3 and 114 ± 4 Ma) reflecting the diachronous deformation and metamorphism of separate thrust packets in the accretionary prism. Two arrays appear to be too old (154 ± 9 and 144 ± 8 Ma) with respect to other age data which suggests these rocks are Early Cretaceous in age; these linear arrays are consequently interpreted as pseudo-isochrons. A fourth suite from an overlying trench slope basin deposit also yields a pseudo-isochron, the age of which (153 ± 18 Ma) is older than the known age of deposition (c. 98–119 Ma).

Recent RbSr dating of Late Triassic Torlesse rock suites and suites from other accretionary prism terranes, have shown that meaningful ages can be obtained from rocks which have undergone complex deformation and low-grade metamorphism as a consequence of subduction and accretion. By comparison, the mixed provenance (and possibly different burial conditions), of Late Jurassic-Early Cretaceous Torlesse rocks has hampered complete Sr isotopic homogenisation. This study shows that RbSr dating can be applied to suites from these terranes, but a clear knowledge of the provenance (from petrography and geochemistry), tectonic setting, and any additional age constraints, are required to interpret the data.