DERIVATION OF SOME A-TYPE MAGMAS BY FRACTIONATION OF BASALTIC MAGMA - AN EXAMPLE FROM THE PADTHAWAY RIDGE, SOUTH AUSTRALIA

Various petrogenetic schemes have been proposed for A-type granitic and volcanic rocks. many involving melting of I-type granites or their residual sources. These models can be shown to have several shortcomings and whilst there may be various kinds of A-type magmas their high temperatures suggest that mantle melts be invoked in any petrogenetic model. In addition, we suggest that the bimodal association with mafic rocks, the chemical characteristics and typically low initial Sr-87/Sr-86 ratios of many A-type rocks are entirely consistent with an origin by extended fractionation of basaltic magmas. Interstitial felsic material from layered mafic intrusions also provides evidence for this origin, as do new data presented here for suite of A-type granites and volcanics from the Padthaway Ridge in South Australia. Mineralogical, chemical and isotopic arguments show that the Padthaway suite evolved from the basaltic magma that also formed contemporaneous gabbroic plutons which themselves contain A-type felsic fractionates. Olivine and pyroxene assemblages of these hypersolvus granites and volcanics attest to temperatures of 900-1000-degrees-C and water undersaturated conditions with final H2O < 3 wt.%. Curvilinear geochemical trends and negative Eu anomalies indicate a history of protracted fractionation involving pyroxene and feldspar. Consistent with their high epsilon-Nd (+2 to -3) and low initial Sr-87/Sr-86 (0.703-0.706), these A-type magmas are well modelled as the products of approximately 90% crystallization of contemporary, mantle-derived, parental magma. Our calculations, supported by gravity data, suggest that A-type suites. which typically intrude in non-compressional settings, may mark episodes of crustal growth during which considerable mantle material is added to the crust.