Massimo Raveggi

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

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.

Evidence for hybridisation in the Tynong Province granitoids, Lachlan Fold Belt, eastern Australia

ABSTRACT The role of mafic–felsic magma mixing in the formation of granites is controversial. Field evidence in many granite plutons undoubtedly implies interaction of mafic (basaltic–intermediate) magma with (usually) much more abundant granitic magma, but the extent of such mixing and its effect on overall chemical features of the host intrusion are unclear. Late Devonian I-type granitoids of the Tynong Province in the western Lachlan Fold Belt, southeast Australia, show typical evidence for magma mingling and mixing, such as small dioritic stocks, hybrid zones with local host granite and ubiquitous microgranitoid enclaves. The latter commonly have irregular boundaries and show textural features characteristic of hybridisation, e.g. xenocrysts of granitic quartz and K-feldspars, rapakivi and antirapakivi textures, quartz and feldspar ocelli, and acicular apatite. Linear (well defined to diffuse) compositional trends for granites, hybrid zones and enclaves have been attributed to magma mixing but could also be explained by other mechanisms. Magmatic zircons of the Tynong and Toorongo granodiorites yield U–Pb zircon ages consistent with the known ca 370 Ma age of the province and preserve relatively unevolved ϵHf (averages for three samples are +6.9, +4.3 and +3.9). The range in zircon ϵHf in two of the three analysed samples (8.8 and 10.1 ϵHf units) exceeds that expected from a single homogeneous population (∼4 units) and suggests considerable Hf isotopic heterogeneity in the melt from which the zircon formed, consistent with syn-intrusion magma mixing. Correlated whole-rock Sr–Nd isotope data for the Tynong Province granitoids show a considerable range (0.7049–0.7074, ϵNd +1.2 to –4.7), which may map the hybridisation between a mafic magma and possibly multiple crustal magmas. Major-element variations for host granite, hybrid zones and enclaves in the large Tynong granodiorite show correlations with major-element compositions of the type expected from mixing of contrasting mafic and felsic magmas. However, chemical–isotopic correlations are poorly developed for the province as a whole, especially for 87Sr/86Sr. In a magma mixing model, such complexities could be explained in terms of a dynamic mixing/mingling environment, with multiple mixing events and subsequent interactions between hybrids and superimposed fractional crystallisation. The results indicate that features plausibly attributed to mafic–felsic magma mixing exist at all scales within this granite province and suggest a major role for magma mixing/mingling in the formation of I-type granites.

Source and significance of the felsic magmatism in the Paleoproterozoic to Mesoproterozoic Broken Hill Block, New South Wales

Major, trace, rare-earth elements and isotopic (Sm – Nd and Rb – Sr) data from the ca 1704 – 1685 Ma Alma, Farmcote, Rasp Ridge and Hores-Potosi felsic magmatic gneisses and the ca 1600 Ma syn-orogenic felsic intrusions of the Willyama Supergroup in the Broken Hill Block of western New South Wales are documented. The ca 1704 – 1685 Ma felsic melts were generated by anatexis of the Willyama Supergroup metasediments with variable degrees of mixing of a juvenile, mantle-derived component represented by coeval high Fe – Ti metatholeiitic rocks. This is consistent with previous interpretations for bimodal magmatism occurring in an extensional environment with an elevated geothermal gradient driven by lithospheric thinning and mafic magmatism. Interpretations involving the presence of a mafic underplate as a source for the ca 1704 – 1685 Ma felsic melts are not supported by these data. The ca 1600 Ma syn-orogenic felsic intrusions are a direct product of partial melting of the sedimentary sequences of the Willyama Supergroup as a response to the high-temperature, low-pressure amphibolite- to granulite-facies metamorphic event accompanying the Olarian Orogeny. The presence of an Archean basement for the Willyama Supergroup remains unclear, although if this basement exists it was not sampled during the period of felsic magmatic activity that took place in the Broken Hill Block between ca 1710 and 1600 Ma.