REMELTING AND REMOBILIZATION OF MAGMATIC ROCKS IN ARCS

Thermo-mechanical models of magmatic arcs suggest that intermittent intrusion of magma batches should lead to remelting and remobilization of earlier intrusive rocks as a result of fluctuations in temperature and water content. However, examples of remelting and remobilization of earlier intrusive rocks, formed during arc-building, are surprisingly rare. We first investigate the evolution of magmatic rocks of the Palaeoproterozoic St Peter Suite, in the Gawler Craton, South Australia. This suite records multiple intrusions, magma hybridization, and the remelting and remobilization of these intrusions to form migmatites and newly-formed leucocratic magmas. We then present results from Sierra de Quilmes, Argentina, where the country rock to the 470Ma Famatinian arc underwent at least two melting events caused by Bt-dehydration reaction, forming complex migmatites, presumably as a result of temperature fluctuations. Finally, we present results from Kangaroo Island, South Australia, where water fluxing of hot rocks of the 500Ma Delamerian orogen led to several overprinting anatectic events in short succession, without requiring significant temperature fluctuations. Here, anatectic rocks have typically 20 to 50% or more neosome volume, rare sillimanite, late-formed randomly oriented muscovite, and lack any other anhydrous peritectic minerals. Early anatectic melts solidified and were subsequently overprinted by later anatectic events associated with changes in the nature and orientation of structures. Both the oldest and youngest anatectic rocks reveal a continuous age spread between ca. 495 and 465 Ma. The age range is interpreted to indicate the duration of anatexis. These examples record many of the key processes expected in arcs, including the prediction that early intrusive arc rocks remelt to form younger more fractionated magmas.