July 2005 LIP of the Month

Corresponds to events #101-103 in LIP record database.

The Marnda Moorn LIP, a late Mesoproterozoic large igneous province in the Yilgarn craton, Western Australia

Michael TD Wingate¹ and Robert T Pidgeon²

¹Tectonics Special Research Centre, University of Western Australia, Crawley, WA 6009, Australia
²Dept of Applied Geology, Curtin University of Technology, Perth, WA 6845, Australia

New and recent SHRIMP U-Pb geochronology has revealed the existence of a previously unrecognised Large Igneous Province (LIP) of late Mesoproterozoic age that extends over a large area of western Australia. In keeping with recent precedent (e.g. the 1075 Ma Warakurna and 510 Ma Kalkarindji LIPs), and after consultation with a local Aboriginal elder, we have named it the Marnda Moorn LIP, after the Balladong words used to describe the black rocks that intrude Yilgarn craton granites. This brief contribution is an excerpt from a paper currently in preparation - stand by for future updates!

Recent geochronological studies show that dyke swarms concentrated around the margins of the Yilgarn craton (Hallberg, 1987), and also some that extend well into the craton interior, are similar in age at about 1210 Ma (Table 1).

Most intrusions consist of dolerite (diabase) or gabbro, although more felsic compositions, such as quartz diorite (e.g. Qui et al., 1999), are also present. Most dykes in the Yilgarn craton are very poorly exposed and strongly weathered, making investigations difficult. Prior to our recognition of the precisely coeval nature of these intrusions, the dykes were grouped into geographically distinct dyke swarms.

A high-resolution aeromagnetic study (Isles and Cooke, 1990) delineated an extensive swarm of undeformed NE-trending dolerite dykes intruded into Archean rocks of the southeastern Yilgarn craton (Figure 1).

Because the swarm extends parallel to the Fraser Mobile Belt, the eastern segment of the Albany-Fraser Orogen, it was referred to by Wingate et al. (2000) as the Fraser Dyke Swarm. Owing to deep weathering, only a single exposure of one dyke is available, in the Defiance open-cut at the Victory Gold Mine, Kambalda (Figure 2).

The dyke is 30 to 35 m wide, subvertical, undeformed, and has 3 to 5 m wide chilled margins against Archean metavolcanic host rocks. Baddeleyite from a granophyric segregation in the dyke centre provided a mean ²⁰⁷Pb/²⁰⁶Pb age of 1212 ± 10 Ma (Wingate et al., 2000).

The Fraser swarm is probably continuous, beneath regolith, with undeformed dolerite dykes of the Gnowangerup swarm (Myers, 1990a) also known as the Ravensthorpe swarm), which subparallels the southern Yilgarn margin (Figure 1). Myers (1990a) noted that some dykes become progressively recrystallized towards the craton margin and others are strongly deformed within the orogen, implying that at least some dykes were emplaced prior to the youngest deformation in the Albany-Fraser orogen. Two dolerite samples yielded zircon ages of 1203 ± 15 Ma and ~1238 Ma (Evans, 1999), although the latter result is based on only two analyses.

Although many dolerite dykes in the Boyagin swarm (Myers, 1990b) trend NNW along the western Yilgarn margin, dykes are present in a wide variety of orientations (Prider, 1948; Lewis, 1994; Pidgeon and Cook, 2003). Close to the Darling Fault, mafic intrusions commonly occur as anastomosing or interconnected irregular bodies that in places appear to follow previously existing fractures and 'net-vein' the host Archean granitoids (e.g. Davis, 1942; Prider, 1948a,b; Pidgeon and Cook, 2003). Based on dyke orientations, cross-cutting relationships, alteration (including clouded feldspars), and paleomagnetic data, the Boyagin swarm probably includes dykes of significantly different ages (Giddings, 1976; Halls and Wingate, 2000). Most mafic intrusions within about 100 km of the Darling Fault zone (Figure 1) are extremely uniform in composition, strongly uralitised (Davis, 1942; Halls and Wingate, 2000) and generally devoid of dateable U-bearing minerals. However, a quartz dolerite dyke in the Darling Scarp near Perth yielded zircon with an age of 1214 ± 5 Ma (Pidgeon and Cook, 2003), and another 100 km inland, near York, furnished a zircon age of 1204 ± 10 Ma (Pidgeon and Nemchin, 2001).

About 200 to 300 km E and NE of Perth, the Wheatbelt swarm contains very poorly-exposed, roughly E-trending diorite and dolerite dykes with zircon ages of 1202 ± 13, 1209 ± 8, and 1216 ± 11 Ma (Evans, 1999). Qiu et al. (1999) obtained a zircon age of 1215 ± 11 Ma for a ~1 x 5 km, E-trending quartz diorite intrusion.

In the far northwest corner of the Yilgarn craton, mainly ENE-trending dykes were grouped as the Muggamurra swarm by Myers (1990b). Two ENE-trending gabbro dykes yielded precise zircon ages of 1207 and 1211 Ma (details of data and precision to be published elsewhere). However, the Muggamurra 'swarm' also encompasses dolerite dykes and plugs dated at 1075 Ma by SHRIMP and/or paleomagnetism and related to the Warakurna LIP (Wingate et al., 2004).

Collectively, the dated intrusions have ²⁰⁷Pb/²⁰⁶Pb ages between 1202 and 1216 Ma, and an average age of 1210 Ma. They extend over at least 400,000 km² in the western and southern Yilgarn craton; whether any occur in the northeast Yilgarn is unknown. Interestingly, dykes of this age have not been reported from within the Capricorn orogen, nor from the Pilbara craton. Although the wide distribution and short duration of the Marnda Moorn event is consistent with a plume origin, so far we have not identified a consistent geometric pattern of emplacement, other than the apparent concentration of dykes around the present cratonic margins, which implies that plate boundary stresses at this time were a major influence on dyke emplacement. Emplacement of the Marnda Moorn LIP coincided with the second stage of compressional tectonism and magmatism in the Albany-Fraser-Musgrave orogen. The location of the Gnowangerup-Fraser dykes, adjacent and parallel to the Albany-Fraser orogen, suggested to Wingate et al. (2000) that they were emplaced into lines of weakness that originated during tectonic loading and downwards flexure of the craton margin. Plate reorganization at about 1.2 Ga is also indicated by a major bend in the apparent polar wander (APW) path for Australia.

Only preliminary paleomagnetic measurements are available for rocks of the Marnda Moorn LIP, owing to poor exposure and strong weathering. The 1212 Ma Fraser dyke, exposed in mine workings, yielded a steep downwards-directed magnetization that is shown to be primary by a positive baked-contact test (Figure 2). The well-dated paleopole for this dyke, together with paleomagnetic data from ~1205 Ma metamorphic rocks of the Albany-Fraser orogen, reliably places Australia at high latitudes at 1210 Ma (Pisarevsky et al., 2003). This result argues strongly against previously-proposed Meso- and Paleoproterozoic reconstructions of Australia and Laurentia, because Laurentia occupied equatorial latitudes at this time.

The Marnda Moorn LIP is not similar in age to any other significant mafic event recognized so far, but is another example of LIP emplacement during assembly of the Rodinia supercontinent (Hanson et al., 2004).

Acknowledgements

The authors are indebted to Walter and Barry McGuire, for sharing aspects of their Aboriginal culture and heritage, and for suggesting a suitable and meaningful name for the LIP.

References