The petrology and timing of the Anabama Granite and associated igneous activity, Olary Region, SA

Abstract

Two ideologies of thought exist when models of granite genesis are considered. Do they represent the products of direct fractionation of a basaltic mantle melt, or, do they form in accordance with the restite model of White and Chappell (1977)? Assimilation and fractional crystallization (AFC) modelling of Nd - and Sr - isotopic data from the Anabama Granite, of this study, and data from the granites of the southern Adelaide Fold Belt, Antarctica and the Lachlan Fold Belt of New South Wales, all of approximately the same age, appears to reflect mixed sources with components derived both from an average Delamerian basalt composition and an average Archean crust composition. Results indicate that the Anabama Granite mostly represents primitive Delamerian basalt, contaminated by 12- 14 % Archean crustal material. Field relationships of the Anabama Granite indicate that it was the site of multiple magmatic intrusions, between approximately 490- 425 Ma. These intrusions are represented by several episodes of hydrothermal alteration and crosscutting dykes. A long-lived thermal source, not represented in the southern Adelaide Fold Belt, may be responsible for this ongoing magmatic activity. Examples of these dykes are the lamprophyre dyke, dated at 457 ± 18 Ma, which is similar in composition and appearance to the lamprophyres near Truro (South Australia) and the dacite porphyry dyke which crosscuts all other lithologies and was dated at 425 ± 13Ma. This age corresponds to the onset of thermal activity in the Lachlan Fold Belt, and therefore, leads to the suggestion that the region where the Anabama Granite outcrops may represent the western margin of the thermal perturbation responsible for the production of granitic melts in the Lachlan Fold Belt at around 400 Ma. Differences in source regions for the Anabama Granite, the granites of Antarctica and those of the Lachlan Fold Belt are recognized by the different Nd- and Sr - isotopic ratios, although all granites may represent the same process of formation, that being AFC. The dacite porphyry's isotopic signature indicates a more primitive source than that suggested for the Anabama Granite, and therefore its genesis does not represent a remelting of the Anabama Granite or of its source region. Geochemically, the Anabama Granite is similar to the Reedy Creek Granodiorite of the southern Adelaide Fold Belt and the Wanda Granodiorite of western Victoria. It can also be classified as an I-type granite using the criteria established by Chappell and White (1974). Geophysical gravity modelling of the Anabama Granite was carried out and it was found that the granite extends to a depth of approximately 15 km and dips uniformly to the north west. Thus giving an indication that fracture propagation, rather than plutonism, is the mechanism of granitic melt transport through the upper crust for the Anabama Granite and granites of the southern Adelaide Fold Belt.