John Foden

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 short-comings 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 87Sr/86Sr ratios of many A-type rocks are entirely consistent with an origin by extended fractionation of bsaltic 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°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 ϵNd (+ 2 to −3) and low initial 87Sr/86Sr (0.703-0.706), these A-type magmas are well modelled as the products of ∼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.

The Timing and Duration of the Delamerian Orogeny: Correlation with the Ross Orogen and Implications for Gondwana Assembly

The Antarctic Ross and the Australian Delamerian orogenies are the consequence of stress transfer to the outboard trailing edge of the newly assembled Gondwana supercontinent. This tectonic reorganization occurred in the Early to Middle Cambrian on completion of Pan‐African deformation and subduction along the sutures between eastern and western Gondwanan continental fragments. Before this, Neoproterozoic to Early Cambrian rocks in eastern Australia were formed in a passive margin and record dispersion of Rodinia with consequent opening of the proto‐Pacific. Our new U‐Pb and Rb‐Sr geochronology shows that in the South Australian (Adelaide Fold Belt) domain of the Delamerian Orogen, contractional orogenesis commenced at \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Possible role of amphibole in the origin of andesite: some experimental and natural evidence

514\pm 3

Field setting, mineralogy, chemistry, and genesis of arc picrites, New Georgia, Solomon Islands

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Sm-Nd isotopic evidence for the provenance of sediments from the Adelaide Fold Belt and southeastern Australia with implications for episodic crustal addition

490\pm 3

The geochemistry and petrogenesis of K-rich alkaline volcanics from the Batu Tara volcano, eastern Sunda arc

\end{document} Ma, terminated by rapid uplift, cooling, and extension in association with posttectonic magmatism. Integration of new and published U‐Pb and 40Ar‐39Ar geochronology from the entire Ross‐Delamerian belt shows that although both the Delamerian and Ross have a synchronous late magmatic and terminal cooling history, the Ross commenced its convergent orogenic history at ∼540 Ma. This was 25 m.yr. before Delamerian deformation began. During the Early Cambrian, eastern Australia was still in a state of extension (or transtension), with opening of the Kanmantoo Basin and associated anorogenic, largely mafic magmatism. This basin received sediment from the already exposed Ross Orogen to the south. The simultaneous first occurrence of strain fabrics and subduction‐related magmatism (including boninite, granite, and andesite lavas) at ∼514 Ma in New Zealand, Victoria, South Australia, New South Wales, and Tasmania implies that the Delamerian Orogeny was driven by ridge‐push forces transmitted on the initiation of westward‐dipping subduction. Subsequent eastward slab rollback at 490 Ma may have occurred when the new slab had reached the transition zone at 650‐km depth, resulting in upper plate extension and anorogenic Basin and Range–style magmatism in South Australia and Tasmania (Mount Read belt). The delayed onset of subduction in the Australian sector of the margin implies that westward motion of the Australian portion of eastern Gondwana continued to be accommodated during the late Early Cambrian by subduction or deformation along either the Mozambique Suture or at the northern end of the South Prince Charles Mountains–Prydz Bay suture.