Pitfalls of classifying ancient magmatic suites with tectonic discrimination diagrams: An example from the Paleoproterozoic Tunkillia Suite, southern Australia

The 1690–1670 Ma felsic Tunkillia Suite was generated through the central Gawler Craton, southern Australia, during or after the waning stages of the 1730–1690 Ma Kimban Orogeny. Previous models for the generation of the Tunkillia Suite used trace element tectonic discrimination diagrams to suggest a subduction-related petrogenesis. Subsequently, Tunkillia Suite magmatism was used to infer an active plate margin in paleotectonic reconstruction models. In part, this led to the suggestion of a long-lived active margin on the southern edge of paleo-Australia that was correlated with the long-lived active margin of south-western Laurentia. The more detailed geochemical and isotopic analyses used in this study highlight the shortcomings of analysis based upon simple application of trace-element tectonic discrimination diagrams. The combination of detailed geochemical assessment and new geochronology demonstrates that some intrusions previously used to characterise the tectonic setting of the Tunkillia Suite are in fact unrelated. Tunkillia Suite intrusions are typically high-K, alkali-calcic and magnesian and are dominantly felsic (>70 wt% SiO2), moderately peraluminous granitoids. Trace and rare earth element abundances display large variations. Sr and Y concentrations range from 40 to 550 ppm and from 4.7 to 41 ppm, respectively. Eu/Eu* values range from 0.2 to 1.6 and combined with REE patterns demonstrate the varying composition of restite associated with melt generation of the Tunkillia Suite. The majority of the Tunkillia Suite display chemical characteristics typical of a plagioclase- or hornblende-dominated restite with only three samples demonstrating garnet-rich restite composition. Nd-isotope characteristics of Tunkillia Suite intrusions suggest a crustal column of variable age exists in the Gawler Craton, as variation in ɛNd(T) values (−6.3 to +2.6) cannot be accounted for simply through assimilation and fractional crystallisation processes. The limited SiO2 compositional range of the Tunkillia Suite inhibits unequivocal tectonic classification due to the inability to determine mantle melt composition and infer mantle melting conditions leading to the generation of the Tunkillia Suite. Geochemical, isotopic and mineralogical characteristics of the Tunkillia Suite are most consistent with a ‘late- to post-tectonic’ setting for petrogenesis. The term ‘late- to post-tectonic’ is used in this instance as a genetic descriptor based upon the comparison with Phanerozoic granitoid suites formed in such a setting after collisional orogenesis. Contrary to the suggestion of previous studies, a subduction-related arc setting is not readily reconcilable with the chemical and mineralogical characteristics of the Tunkillia Suite. Refereed/Peer-reviewed