The contribution of geochronology to understanding the paleozoic geological history of Australia

Abstract

From the viewpoint of isotope geochronology, studies of the Paleozoic geological history of Australia can be divided into three main eras: (i) the era of stratigraphy, when the ages of igneous and metamorphic events were inferred primarily from stratigraphic relationships; (ii) the era of K-Ar and Rb-Sr, when the broad time frame of many of the main thermal events was mapped out; and (iii) the era of SHRIMP, when U-Pb microanalysis, in combination with techniques such as Ar-Ar, Sm-Nd and Lu-Hf, made it possible to study not only the most recent events in the thermal history but also cooling histories and the ages of precursors and protoliths. The work during the last era was the most ambitious and controversial. As age measurements became more precise, their accuracy and the accuracy of the calibration of the geologic time-scale were called into question. Isotopic analyses of eastern Australian granitic rocks, undertaken to help understand magma genesis and the age of the source rocks from which the magmas were derived (the basement of the Lachlan Orogen), were interpreted by different researchers in fundamentally different ways. Ages measured on detrital minerals from Paleozoic sedimentary rocks were interpreted as indicating erosion of quite different source regions. These debates did not detract from the main achievements, however. There is a large and expanding database of high-quality age measurements on thermal events throughout the Australian Paleozoic. In many cases, this provides directly determined ages for igneous, metamorphic and deformation events previously dated only approximately using stratigraphy. The distribution of coeval magmatism and the progressive migration of that activity have been mapped out. Isotopic compositions have been used to estimate the relative contributions of crust and mantle components to magmas and the ages of those components. In some cases, geochronology has changed the geological paradigm. For example, in central Australia, the Alice Springs Orogeny is now known to be a major intraplate tectonothermal event, and granulite grade metasediments in the east of the Paleoproterozoic Arunta Inlier have been shown to be deformed and metamorphosed equivalents of some of the lower units of the adjacent Centralian Superbasin, part of a previously unrecognised incipient Paleozoic transcontinental rift. In combination with field studies, dating of mineralisation has shown that ore genesis occurred not randomly, but in a series of distinct metallogenic episodes, many of the major Paleozoic mineral deposits in Australia having formed in response to far-field stresses associated with orogenic events, commonly late in a tectonic cycle. The principal episode of mineralisation (480-440 Ma) followed the end of the major tectonothermal event that coincided with the protracted collision between east and west Gondwana, the Pan-African/ Ross/Delamerian Orogeny.