September 2015 LIP of the Month

Mafic–ultramafic suites of the Irindina Province, Northern Territory, Australia: Implications for the Neoproterozoic to Devonian evolution of central Australia

Madeline L. Wallace ^a , Simon M. Jowitt ^a, Ahmad Saleem ^b

a School of Earth, Atmosphere and Environment, Monash University, Melbourne, VIC 3800, Australia;  Emails: maddiewallace@hotmail.com  &  simon.jowitt@monash.edu

b MMG, Level 23, 28 Freshwater Place, Southbank, Melbourne, VIC 3006, Australia;  Email: Ahmad.Saleem@mmg.com

This LIP of the Month is based on a recent publication in Lithos; Wallace, M. L, Jowitt, S. M., & Saleem, A. (2015) Petrogenesis of mafic-ultramafic suites within the Irindina and Aileron provinces of the Northern Territory: implications for the Mesoproterozoic to Paleozoic evolution of Australia. Lithos, 234–235, 61–78. Readers are directed there for more information.

Introduction

Mafic–ultramafic magmas can form in a variety of tectonic settings as reflected by their magma geochemistry, which varies depending on the nature and composition of the mantle region that their parental magmas were derived from (e.g., Pearce, 2008). Mafic magmatism can provide a record of magmatic and tectonic events, key evidence for tectonic reconstructions and insights into the metallogenic prospectivity of an area (e.g., Ernst and Jowitt, 2013; Jowitt and Ernst, 2013; Jowitt et al., 2014).

Geochemical discrimination diagrams using immobile trace element ratios have been utilised to assign probable tectonic settings to mafic–ultramafic magmatic events. Here, we use these approaches on samples from the Arunta region of the Northern Territory, a poorly understood and tectonically complex part of Australia. The area has undergone multiple and prolonged deformation and tectonic events, leading to a large degree of geological complexity and a significant amount of uncertainty with regards to the tectonic evolution of the region (Scrimgeour, 2003). Recent research in this area has focussed on improving our understanding of the Arunta region, driven by the important record that this area has for both the Proterozoic and Phanerozoic evolution of Australia. The recent discovery of magmatic Ni–Cu–PGE sulphides in mafic intrusions in the Eastern Arunta also highlights the region's prospectivity for mafic intrusion-hosted sulphide mineralisation, research that complemented work by Hoatson et al. (2005) and Claoué-Long and Hoatson, 2005, who completed a seminal study on the mafic–ultramafic magmatic events in the Arunta.

This research presents new geochemical and petrological data for comprehensive rock and chip samples from outcropping mafic–ultramafic intrusive rocks within the Harts Range Group (Irindina Province) and the surrounding Aileron metamorphic rocks of the Northern Territory, Australia (Fig. 1). The data from these samples was used to identify: (1) different magmatic suites using lithogeochemistry; (2) the petrogenetic processes behind the formation and (3) the tectonic settings of these magmatic events. The latter point, in conjunction with existing geochronological data, advances our understanding of the evolution of the Irindina and Aileron provinces and the geological history of this part of Australia.

Regional Geology

Irindina Province

The Arunta region is divided into the Palaeoproterozoic to Mesoproterozoic Warumpi and Aileron Provinces and the Neoproterozoic to Cambrian Irindina, Amadeus and Georgina Provinces (Whelan et al., 2012; Fig. 1). The region has undergone a protracted tectonic and metamorphic history, with at least 8 different documented deformational events stretching from the Stafford Event (1810–1790 Ma) to the Alice Springs Orogeny (450–300 Ma; Schofield and Huston, 2012).

The study area lies along the boundary of the Irindina and Aileron province, in the south-eastern part of the Arunta region. Basement rocks in the area are interpreted to be part of the Aileron Province and consist of layered felsic and mafic gneisses that have undergone amphibolite to granulite facies metamorphism (Ding and James, 1985). Structurally overlying the Aileron is the Irindina basin that entirely consists of the Harts Range Group (HRG; Maidment et al., 2013), a series of metasediments and amphibolites with anorthosite and ultramafic intrusives (Ding and James, 1985; Sivell, 1988). The HRG, previously referred to as the Irindina Supracrustal Assemblage (Ding and James, 1985), is restricted to the Irindina Province (Fig. 1; Scrimgeour, 2003) and structurally overlies Palaeoproterozoic Aileron basement material, as identified by the correlation of the basement stratigraphy to rocks west of the Irindina Province (Maidment et al., 2013).

Mafic-ultramafic magmatic events within the Irindina Province

The tectonic evolution of the Arunta province is marked by a number of mafic–ultramafic magmatic events. Within the Aileron, Proterozoic magmatic events identified include the Edmirringee (~1830 Ma), Mount Hay (~1810 Ma), Hart (~1780 Ma), Woman-in-White (~1680 Ma), Andrew Hills Young (~1635 Ma), the lamprophyre-related Mordor (~1135 Ma; Barnes et al., 2008), the Warakurna (~1070 Ma; Wingate et al., 2004) and the Elizabeth Hills (~975 Ma) events, none of which were previously identified in the eastern Arunta or within the Irindina basin. Little research has been undertaken on the mafic and ultramafic units in the eastern Aileron region, the most recent being by Whelan et al. (2010), who focused on the geochemical and isotopic characteristics of three mafic suites emplaced into the HRG. The oldest identified is the ~600 Ma Riddoch Amphibolite, split into a metagabbroic light rare earth element (LREE)-depleted and quartz-rich LREE-enriched subsuites. The second suite identified is the ~506 Ma gabbroic Stanovos Igneous Suite, which shares particular geochemical affinities with the Kalkarindji LIP. Basalts and dolerites formed as part of the Kalkarindji LIP event are geochemically similar to continental tholeiites except they are high field strength element (HFSE)-depleted (especially Ti, P and Nb) relative to other incompatible elements and are enriched in Th and U (Glass, 2002). The Stanovos suite shares some, but not all, characteristics with the Kalkarindji LIP. In addition, an Early Cambrian detrital zircon age for the protoliths of the HRG metabasites and metasediments tentatively suggests that some of the rocks may be genetically related to the Kalkarindji event (Buick et al., 2005; Evins et al., 2009). The youngest event, the 409 ± 9 Ma Lloyd Gabbro magmatic event, was contemporaneous with the Alice Springs Orogeny (Whelan et al., 2010) and is associated with the emplacement of olivine gabbro and gabbronorite units from magmas not usually associated with orogenesis — indicating that the relationship between this magmatism and the Alice Springs orogenic cycle recorded in this area is uncertain.

Trace element variation and subdivision of suites

The vast majority of the samples analysed during this study are subalkaline and basaltic (Fig. 2). The main subdivision of samples into suites was undertaken using a (Gd/Yb)PM vs. (Nb/La)PM diagram (Fig. 3), as these ratios represent the depth of melting within the mantle and the influence of arc lithosphere respectively (e.g., Jowitt and Ernst, 2013; Wang et al., 2002), where PM denotes normalisation to the primitive mantle values of McDonough and Sun (1995). Discriminations were further corroborated by observing the distribution of suites in other classification diagrams that are omitted here for brevity; the reader is referred to Wallace et al. (2015) for more details. In all, a total of six mafic-ultramafic suites were identified in the study area (Table 1); the petrogenetic history of these suites is discussed below.

Regional associations and tectonic implications for the geodynamic evolution of the NAC

Without tightly constrained geochronological evidence for the timing and evolution of the discussed suites, the geochemical data discussed here can only be placed in their proper context using associations with similar but dated suites in the surrounding study area. Correlating the geochemistry of these suites with previously identified and dated suites in the study area certainly allows an increased understanding of the temporal relationships between different magmatic events in this area and enables the identification of the tectonic evolution of the region during the emplacement and eruption of these mafic–ultramafic magmas. The characteristics of each of the suites identified during this study are also provided in Table 1.

~1070 Ma

Suite D has negligible HFSE depletions compared to the other suites; this lack of HFSE depletions provides strong evidence that this suite intruded before the development of the arc-type material (most probably related to the ~600 Ma event described below) that was assimilated by all other magmatic suites in this area. In addition, the Suite D samples are nearly geochemically identical to the Alcurra dolerite dykes within the Musgrave Province of central Australia and the related Stuart dolerite of the Arunta region, both of which are part of the wider ~1070 Ma Warakurna LIP (Pirajno and Hoatson, 2012; Wingate et al., 2004; Schmidt et al., 2006) which includes the mineralised Giles magmatic event. No other geochemically similar mafic–ultramafic units to the Warakurna have been identified to date in the Eastern Arunta or the Irindina, suggesting that this LIP event may extend further into the Northern Territory than has previously been identified.

Similar trace element systematics and the close spatial distribution of the Suite D and F samples (Fig. 1) suggest that Suite F represents more alkaline magmas, derived from a deeper source, but formed during the Warakurna LIP event.

~600 Ma

Suite E formed at ~600 Ma from magmas generated by high degree partial melting of the mantle at shallow depths, most likely within an extensional environment. This suite most likely represents the metagabbroic member of the ~600 Ma Riddoch Amphibolite (Whelan et al., 2010; Whelan et al., 2012), which is interlayered with the LREE-enriched quartz-rich Riddoch Amphibolite unit that hosts the Basil and Polly Cu–Co–Ag dominated volcanogenic massive sulphide (VMS) prospects (Whelan et al., 2010). The Co- and Cu-rich (and Pb-poor) nature of this mineralisation suggests it represents the more mafic Cyprus type (e.g., Jowitt et al., 2012) end of the VMS spectrum (Sharrad et al., 2013). Such deposits are commonly associated with boninitic to low-Ti island arc tholeiitic, mid ocean ridge basalt (MORB) or back-arc basin (BAB) basalt related magmatism (Piercey, 2010). Our preferred model involves a BAB forming as a response to west-dipping subduction in northern Queensland during the ~600 Ma Delamerian cycle, an event that may have caused BAB development as far away as the Irindina Province (Fig. 4C; Kositcin et al., 2009).  

~506 Ma

The geochemistry of Suite B is identical to that of the previously identified ~506 Ma Stanovos Igneous Suite (Whelan et al., 2010). Suite B samples are derived from shallow depths within the mantle by high degree partial melts before assimilating crustal material. Previous research on the Stanovos suite identified variable LREE and LILE enrichments and HFSE depletions, with εNd values of +0.7 to −0.7 (Whelan et al., 2010) and strong affinities to the low-Ti flood basalts of the ~507 Ma Kalkarindji LIP. The major geochemical and isotopic differences between the Stanovos and Kalkarindji suites can be explained by a model where Kalkarindji magmas assimilated older arc-related crustal material, such as the Riddoch Amphibolite BAB material within the study area. The ~600 Ma age of this arc event can also explain the differences between the two LIP events that are recorded in this area. The 1070 Ma Warakurna LIP-affiliated Suites D and F do not provide any evidence of the assimilation of arc material, whereas the ~506 Ma Suite B/Stanovos/Kalkarindji event has a distinct arc-type signature that is related to assimilation of the intervening ~600 Ma Riddoch Amphibolite-related arc event. Suite B samples are typical of the homogenised (i.e., high energy mixing between magma and contaminants) systems associated with voluminous mafic LIP systems (e.g., Jowitt and Ernst, 2013; Jowitt et al., 2014) and support the Stanovos representing a hitherto unidentified CFB province within the broader Kalkarindji LIP. Coeval felsic magmatism in the region (Lawley, 2005) may also be related to the Kalkarindji LIP and may have been generated by melting of crustal material during the intrusion and emplacement of Kalkarindji magmas, as is evident in other LIP events (e.g., the Warakurna LIP). Suite A was formed from alkaline magmas generated by low degree partial melting of a deep mantle source, indicating that the Kalkarindji may be similar to several other documented LIP events that contain both shallow and deeper derived magmas that may have been erupted or emplaced contemporaneously (e.g., the Siberian, Franklin, and High Arctic LIPs; Arndt et al., 1998; Jowitt and Ernst, 2013; Jowitt et al., 2014). Given the evidence presented in this paper, it is possible that Suite A represents an alkaline phase of the Kalkarindji LIP derived from a deeper garnet-bearing source, although this requires geochronological verification.

~409 Ma

The Suite C samples used during this study are from outcrops previously identified as the Lloyd Gabbro (Whelan et al., 2010) with one of the Ni-enriched samples from the Baldrick Ni–Cu–PGE magmatic sulphide prospect. This suite has a MORB-like affinity although the tectonic setting during this event remains uncertain. One possibility is that the emplacement of extension-related mafic magmas during a compressional event may indicate the presence of extensional stresses related to continued E–W back-arc spreading.

Tectonic history of the Irindina area

The magmatic events responsible for the mafic–ultramafic units in the study area record the changing tectonism of this part of Australia. Pre-magmatic modification of the sub-continental lithospheric mantle occurred beneath the Arunta region possibly related to a ~1850 Ma northdipping subduction zone. This was followed by a number of tectonothermal events focussed on the southern margin of the Arunta province. One of these was the ~1070 Ma mantle plume or thermal anomaly-related melting of the sub-continental lithospheric mantle and intrusion of the crustally contaminated Suite D-associated Alcurra and Stuart dyke swarms in the Musgrave and Arunta regions respectively. The single anomalous Suite F sample identified in this study may also have intruded during this event. This was followed by Delamerian (~600 Ma) subduction along the eastern margin of Australia, forming a back-arc basin in Queensland that possibly extended south through the Arunta region (e.g., Kositcin et al., 2009). This Delamerian BAB caused shallow mantle melting and generated the contamination-free Suite E Riddoch Amphibolite back-arc basin basalts and the Cyprus-type VMS mineralisation at Basil. Subsequent destabilisation of the sub-continental lithospheric mantle in the Halls Creek region was associated with the Kalkarindji LIP magmatism, with large volumes of melts from a shallow mantle source and lesser volumes of melts from a deeper source forming beneath the Irindina sub-basin before intruding into the crust becoming contaminated and forming Suites B and A in the study area. The last magmatism in the study area was related to the inversion of the Irindina basin. This inversion resulted in melting of a deep garnet-bearing mantle source region and in the intrusion of the crustally contaminated Suite C Lloyd Gabbro. This tectonic history of the study area is summarised in Fig. 4 and is discussed briefly below:

a)       Initial subduction modification of the sub-continental lithospheric mantle beneath the Arunta Region, possibly occuring during ~1850 Ma north dipping subduction.

b)       Mantle plume upwelling ~1070 Ma causes melting of the sub-continental lithospheric mantle, with mantle-derived magmas becoming crustally contaminated upon intruding the crust and forming the Alcurra and Stuart dyke swarms in the Musgrave and Arunta regions respectively. Note that this ~1070 Ma event may also be related to a long-lived thermal anomaly as suggested by Smithies et al. (2014) and others, although for simplicity a single plume source is depicted here.

c)       Delamerian cycle subduction (~600 Ma) along the eastern margin of Australia forms a back-arc basin in Queensland that extends to the Arunta region. Back-arc basin basalts form by shallow mantle melting and intrude into the crust without having undergone crustal contamination.

d)       Destabilisation of the sub-continental lithospheric mantle in the Halls Creek region results in the upwelling of the Kalkarindji diapir. Large volumes of mantle melts from a shallow source and lesser volumes of mantle melts from a deeper source form beneath the Irindina sub-basin and intrude into the crust, where they assimilate significant crustal and back-arc basin material.

e)       Following compressive exhumation of the Irindina basin, magmas derived from a deep source intrude the crust where they become crustally contaminated prior to their emplacement into the upper crust, and east-west extension in response to back-arc spreading could still be occurring.

Although these mafic–ultramafic units provide new insights into the complex geological history of this region, further research is required to understand the temporal relationship of Suite A with other suites and to determine the extent and, more comprehensively, the nature and metallogenic prospectivity of the multiple magmatic events within this region.

Conclusions

1. Six magmatic suites, named A–F, with differing petrogenetic histories have been identified in the Irindina Province of the Northern Territory of Australia and record the changing tectonism of the study area between ~1070 and ~400 Ma.

2. Each of these suites had different mantle sources, with Suites A and F derived from a deeper source region than Suites A, B, C and D. Suites C and D are derived from an enriched mantle source region. Although all samples appear to have formed in an extensional setting, Suite E has a geochemical signature consistent with a subduction influence. This subduction related material was subsequently assimilated by suites (A, B, C and F) that formed after the development of Suite E. Suites A, B, C and F are related to a series of known magmatic events within and around the Irindina Province.

3. Suite D, the earliest mafic–ultramafic magmatic event in the study area, is geochemically similar to the ~1070 Ma Alcurra and Stuart dolerites of the Musgrave and Arunta regions, part of the Warakurna LIP and probably formed through similar processes (melting of subduction modified SCLM). Suite F most likely intruded during this event as well.

4. Suite E represents the ~600 Ma LREE-depleted Riddoch Amphibolite, most likely formed within a back arc basin associated with Delamerian west-dipping subduction in Queensland, eastern Australia. This extension of back arc basin magmatism generated the contamination-free mafic portion of the Riddoch Amphibolite (i.e., Suite E) and may be related to the formation of hydrothermal VMS mineralisation such as that within the Basil Cu–Co deposit.

5. Destabilisation of the sub-continental lithospheric mantle in the Halls Creeks region generated magmas that formed Suite B. These voluminous magmas represent the ~506 Ma Stanovos Igneous Suite, forming part of the Kalkarindji LIP, and assimilated Riddoch-like arc-related material prior to emplacement.

6. Suite A is related to Suite B, and formed by melting of a deep garnet bearing source and represents a potentially contemporaneous alkaline phase of the Kalkarindji LIP.

7. The last magmatic event in the study area was related to compressive exhumation of the Irindina basin and the formation of the enriched Suite C magmas as part of the ~409 Ma Lloyd Gabbro event. These magmas underwent significant crustal contamination and sulphur saturation, hosting magmatic Ni–Cu–PGE sulphide mineralisation at the Blackadder and Baldrick prospects.

This study represents the first step in furthering our understanding of the tectonism, magmatism, and metallogenic prospectivity of this poorly-studied but geologically important region of Australia. Future research, including developing better constrained links between tectonic setting and mineralisation and more detailed isotopic and geochronological studies to link the timing of magmatism, the deposition of sediments and deformation is needed to place these intrusive events better into context. This research will not only improve our understanding of the geological history of this region and Australia, but will also act as a guide for mineral exploration in this poorly explored but certainly prospective region.

Acknowledgments

MMG Limited is thanked for providing samples and in-kind and analytical support during this project. We also thank two anonymous reviewers for constructive comments that improved the Lithos manuscript and Andrew Kerr for editorial handling of our Lithos paper.

References