Cenozoic basin histories determined by fission-track dating of basement granites, South Island, New Zealand

Abstract

New apatite (56), sphene (5) and zircon (22) fission-track ages from granitoid rocks are combined with K-Ar ages (both new and published) to reveal a thermo-tectonic history of the West Coast, South Island, New Zealand. Although many of the intrusives are thought to have been emplaced in Palaeozoic time, later thermal events are revealed as a result of resetting of the various isotopic clocks. Modal ages of 105, 95 and 75 Ma on biotite (K-Ar), zircon and apatite (fission track), respectively, represent a Mesozoic cooling/uplift of the granites. A few zircon and many apatite ages contain further overprinting, representing later uplift during the Miocene. On the basis of these events, the West Coast can be subdivided into three tectonic regions. Several of the present granitoid mountains were the sites of NE-SW elongate sedimentary basins in the Early Tertiary. A direct correlation can be made between coal rank (percent vitrinite reflectance) of Eocene coals immediately above basement granites and the underlying granite apatite “age”. Granites, with associated overlying coals having a percent vitrinite reflectance of > 0.7, have been buried sufficiently deeply for the apatites to be reset prior to basin uplift in the Miocene. Those coals with a reflectance of generally < 0.55 overlie granites that have retained a Late Mesozoic age. Intermediate or mixed ages (55 Ma) are found where the vitrinite reflectance of coal lies between these two values, although there is some overlap. These are the product of partial annealing, a conclusion supported by confined track-length distributions. Ages on samples on either side of major faults suggest that these faults may have been active both during basin development and Miocene uplift; hence, their sense of movement has reversed with time.