Multiple intrusions and remelting-remobilization events in a magmatic arc: The St. Peter Suite, South Australia

Abstract

Granitoids with juvenile signatures are common in arc environments and contribute to growth of the continental crust. Thermo-mechanical models of arcs suggest that intermittent intrusion of magma batches leads to magma hybridization, remelting, and remobilization of earlier intrusive rocks driven by fluctuations in temperature and water fluxing. While there are numerous examples in the literature of multiple intrusions and magma hybridization, field examples of remelting and remobilization of earlier intrusive rocks within an arc are rare. Here, we investigate the evolution of magmatic rocks of the Paleoproterozoic St. Peter Suite, emplaced along the SW margin of the Gawler craton, South Australia, a typical calc-alkaline arc suite. Magmatic rocks recording multiple intrusions and multiple magma interactions have undergone in situ remelting and remobilization forming migmatites. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb zircon dating yielded crystallization ages of 1647 ± 12 Ma for a tonalitic gneiss representing the oldest intrusive suite, and 1604 ± 12 Ma for a leucogranite representing the youngest intrusive suite. Both these suites developed synmagmatic foliation and magmatic banding defined by broadly parallel dikes, elongated enclaves, and schlieren. The rock suites record two deformation events associated with anatexis. The first event, D 1 , was responsible for a dominant approximately E-W–striking foliation (S 1 ) parallel to magmatic foliation, and associated with a dominantly sinistral shearing that affected the older suite. This deformation was associated with the first anatectic event, as demonstrated by the association between leucosomes and structures. The second deformation event, D 2 , affected both suites and was characterized by isolated F 2 folds and shear planes filled with leucosomes subparallel to axial planar foliation. Leucosomes interconnected and gave rise to magma extraction channels tens of meters long. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb titanite dating of a late leucosome sample, collected from within the older magmatic suite, yielded an age of 1605 ± 7 Ma, coinciding with the crystallization age of the younger suite rather than postdating it, as expected. We interpret these results to indicate that crystallization of the younger suite and the second anatectic event occurred in the time encompassed by the error of these young ages. Leucosomes from both anatectic phases lack anhydrous peritectic phases and are interpreted to represent low-temperature anatexis resulting from water fluxing. Combined, these results suggest that a protracted and complex intrusive history can be made significantly more complex by anatexis, giving rise to evolved magmas after older ones, erasing earlier intrusive relationships, and establishing new ones. Rocks of the St. Peter Suite record many of the key processes expected in arcs, including the prediction that early intrusive arc rocks remelt to form younger and more fractionated magmas.