A structural, geochronological and geochemical investigation of the St Peter Suite, Gawler Craton, South Australia

2017-02-28T00:10:49Z (GMT) by Symington, Neil John
The Gawler Craton is an equidimensional crustal province located on the southern margin of Australia. It is comprised of a sheared, irregularly shaped, dominantly metasedimentary Archaean nucleus surrounded by Proterozoic-aged plutonic and orogenic rocks (Daly et al., 1998; Hand et al., 2007). The Gawler Craton has undergone three major orogenies; the Sleafordian Orogeny (2480-2420Ma), the Kimban Orogeny (1730-1690Ma) and the Kararan Orogeny (1570-1540Ma) (Betts and Giles, 2006; Daly et al., 1998; Hand et al., 2007). Voluminous magmatism dominated the period between the Kimban and Kararan Orogenies. This included the I-type Tunkillia Suite (1690-1670 Ma) and St Peter Suite (1631-1608 Ma), and the A-type Hiltaba Suite and its volcanic equivalent the Gawler Range Volcanics (1595-1575 Ma) (Hand et al., 2007). This period in the evolution of the Gawler Craton marks the transition from St Peter Suite magmatism at an inferred former plate margin to intracontinental magmatism associated with the Hiltaba tectonothermal event (Betts and Giles, 2006; Betts et al., 2009; Betts et al., 2007; Hand et al., 2007; Swain et al., 2008). Tectonic models reconstructing the history of the Gawler Craton and Proterozoic Australia have been created (Betts and Giles, 2006; Betts et al., 2009; Betts et al., 2002; Giles et al., 2004; Myers et al., 1996; Wade et al., 2006) although lack of basement outcrop and paucity of data especially at former plate margins means that these models are often poorly resolved (Stewart and Betts, 2010). Additional structural and geochronological data from former plate margins during this significant period is thus required to better constrain tectonic reconstructions. The evolution of arc magmas above subduction zones is a complex multistage process that is still poorly understood despite considerable research and numerous publications (Winter, 2010). It is thought that dehydration of the downgoing slab releases LILE-rich fluid into the overriding mantle causing partial melting forming tholeiitic basaltic magmas (Hawkesworth et al., 1993; Stern, 2002). The production of intermediate to silicic calc-alkaline magmas, which are so voluminously preserved in the upper crust in volcanic arcs, from primary tholeiitic magmas is complex with numerous processes at work (Winter, 2010). Fractional crystallisation (Bachmann and Bergantz, 2004; Gill, 1981; Grove et al., 2003) and assimilation of crustal material and crust-derived magmas during ascent (Hildreth and Moorbath, 1988) are thought to be the dominant processes. The role of fractional melting of earlier arc rocks is less well understood and has been mostly inferred from geochemistry (Tamura and Tatsumi, 2002; Vogel et al., 2004), petrological features within lavas (Bachmann et al., 2002; Murphy et al., 2000) and zircon recycling within arc magmas (Weinberg and Dunlap, 2000; White et al., 2012). This thesis investigates the Palaeoproterozoic arc represented by the calc-alkaline St Peter Suite at two localities: Rocky Point and Westall. These two outcrops have undergone more strain than other St Peter Suite rocks investigated elsewhere (Chalmers, 2009; Wolfram, 2011). Previous investigations on the St Peter Suite have focused on its geochemistry and geochronology (Chalmers, 2009; Flint et al., 1990; Knight, 1997; Rahilly, 2011; Swain et al., 2008; Wolfram, 2011). This research aims to use a multidisciplinary approach, including structural, geochemical and geochronological analysis, to decipher the igneous and structural evolution of the St Peter Suite at these localities thus contributing to constraining the timing and evolution of this plate margin at a time preceding the intracontinental A-type Hiltaba magmatism, and contributing to better understanding magmatic arc processes such as remelting, magma mingling and mixing and continental crust formation.