A review of the alkaline rocks of Australia.

Abstract

Alkaline rocks are known from all major tectonic provinces in Australia except for the Precambrian Pilbara Block. Most occurrences are intrusions of limited extent and, with the exception of the Cainozoic alkali basalts of the Tasman fold belt of eastern Australia, are volumetrically insignificant compared with the volume of sub-alkaline rocks observed in each province. The alkaline rocks range from late Archaean to Holocene. Where reliable isotopic dates are available the intrusive ages mostly appear to be broadly synchronous with, or closely postdate ages of, cratonization for each of the provinces, a noteable exception being the Tertiary lamproites of the West Kimberley region. The emplacement of many of the pre-Cainozoic rocks, particularly the basic-ultrabasic suites, appears to be influenced by, and in some cases clearly controlled by, initiation of reactivation of deep crustal fractures. The distribution of many of the Cainozoic, and more recently some of the Mesozoic, igneous rocks of eastern Australia has been ascribed to northward movement of the Australian continent over hot-spots associated with regional uplift. The alkaline rocks of Australia include carbonatites and a spectrum of silicate rocks belonging to one of three series: an ultra-potassic (K2O/Na2O >3) series of dominantly ultrabasic to basic rocks which include kimberlite, lamprophyre and lamproite (some of which are diamondiferous); a series of potassic (K2O/Na2O = 1-3), dominantly basic to intermediate rocks (shoshonite association), including monzonites, latites and lamprophyres; a sodic series (Na2O > K2O) of dominantly alkali basalt composition but ranging from mafic, e.g. olivine nephelinites, to felsic compositions, e.g. syenites, phonolites, trachytes. The dramatic increase in the volume of alkaline compared to subalkaline rocks in the Cainozoic may reflect decreasing degrees of partial melting of the modern mantle and lower crust with time, perhaps as a response to falling mantle T. High geothermal gradients experienced in pre-Cainozoic times generated large volumes of sub-alkaline igneous rocks (typically tholeiitic basalts and non-alkaline granitoids). Alkaline rocks are associated with newly initiated or reactivated deep crustal fractures. Some appear to have been emplaced during the early stages of a thermal event, correlating with the early stages of continental rifting. Others have been generated under a cooling geotherm following extensive high-P-T metamorphism and crustal thickening (cratonization). (Authors' abstract)-C.N.