Geochronology of the granite-greenstone terranes in the Murchison and Southern Cross Provinces of the Yilgarn Craton, Western Australia

Abstract

The Archaean Yilgarn Craton of Western Australia consists of a high-grade gneiss terrain (Western Gneiss Terrain) and three granite-greenstone provinces: the Murchison, Southern Cross and Eastern Goldfields Provinces. Most of the previous geochronological studies were conducted in the Eastern Goldfields Province, particularly in the Norseman-Kalgoorlie region, and in the northern part of the Western Gneiss Terrain (i.e. Narryer Gneiss Complex). The remaining parts of the Yilgarn Craton are less well dated. Due, in part, to the lack of precise and accurate geochronological data, the origin and evolution of the craton is still controversial. This study focuses primarily on the granite-greenstone terrains of the Yilgarn Craton, in particular those of the Murchison and Southern Cross Provinces. U-Pb zircon geochronology was carried out using the Sensitive High Resolution Ion Microprobe (SHRIMP). Sm-Nd isotopic systematics of selected granitoids were also determined to trace their source and the timing of separation from the mantle. These investigations provide geochronological constraints on the timing of formation of the supracrustal rocks and emplacem nt of granitoids, as well as information about the underlying basement in the studied areas. The data are used to test the stratigraphic subdivision schemes for the Murchison Province, to determine stratigraphic positions of the Gum Creek and Lake Johnston greenstone belts in the Southern Cross Province, and to discuss the spatial and temporal distributions of the volcanic and plutonic episodes and timing of mineralisation. The late Archaean granite-greenstone terrains of the Yilgarn Craton developed during three distinct thermal events at ca. 3.0-2.90 Ga, ca. 2.8 Ga and ca. 2.76-2.60 Ga. There are two major volcanic and plutonic episodes in the Yilgarn Craton. Volcanic rocks formed between ca. 3.0-2.90 Ga and ca. 2.76-2.70 Ga, and plutonic rocks between ca. 3.0-2.90 and ca. 2.76-2.60 Ga. The ca. 2.8 Ga volcanic event is spatially and temporally restricted and is not accompanied by synvolcanic plutonism of the same age. Geochronological comparison across the Yilgarn Craton indicates that the granite­ greenstone terrains have shared a common evolutionary history since at least ca. 3.0 Ga and that the gneiss terrain in the western Yilgarn Craton was not brought in tectonic contact with the granite-greenstone terrains (e.g. the Murchison Province) until ca. 2.70-2.60 Ga when cratonisation occurred. Xenocrystic zircons record, not only the three thermal events in the lower crust noted above, but also provide useful information about the age of the basement to the granite-greenstone terrains. The presence of ca. 3.0-3.9 Ga xenocrysts, combined with evidence from geophysical and Pb and Sm-Nd isotopic data, imply that all granite-greenstone terrains in the Yilgarn Craton formed above, or at least in association with, a pre­ existing continental crust. In the Murchison Province, two greenstone sequences, the ca. 3.0 Ga Luke Creek and the ca. 2.8 Ga Mount Farmer Groups had been previously proposed. However, the results of this study, in combination with previous geochronology, reveal that three greenstone sequences were developed in this province during separate volcanic episodes at ca. 3.0-2.93 Ga, ca. 2.8, and ca. 2.76-2.70 Ga. The Mount Farmer Group is shown to be ca. 2.7 Ga, not ca. 2.8 Ga as had previously been thought. The present study provides evidence for the existence of a third province-wide greenstone succession at ca. 2.8 Ga, which lies unconformably between the other two sequences. This observation indicates that the ca. 2.8 Ga stratigraphic unit cannot be the uppermost component of the Luke Creek Group as formerly suggested. The new results support the newly proposed, informal, process-based stratigraphic classification for the northern part of the province (Hallberg et al., personal communication, 1997; see also Pidgeon and Hallberg, in press). Most granitoids in the Murchison Province were emplaced between ca. 2.76 and 2.60 Ga. Intrusion of the oldest external recrystallised monzogranites, which occupy large areas surrounding the greenstone belts, and the oldest small to medium-sized internal plutons that truncate the greenstones and tectonic features associated with folding, commenced at ca. 2.76 Ga. These intrusions are synchronous with the onset of deposition of the youngest greenstone sequence in the province, indicating that they developed in a single thermal-magmatic event. No consistent geochronologic relationship was found between regional and internal plutons and between the ages and sizes of granitoids, although the oldest plutons tend to be small. Similarly, no correlation was found between the degree of deformation and the timing of intrusion. It is apparent that field-based observations at isolated outcrops cannot be used to predict the relative order of granitoid emplacement. The ca. 2.7 Ga ages obtained for the Gum Creek greenstone belt do not support the previous suggestion that the greenstones from this belt can be correlated with the ca. 2.93-3.0 Ga sequence found in the Murchison Province, and with the lower sequence of similar age recognised in the Southern Cross Province. The studied rock units apparently belong to the ca. 2.7 Ga upper greenstone sequence in the Southern Cross Province, which is broadly synchronous with the youngest greenstone sequence developed in the Murchison Province. The Lake Johnston greenstone belt has an age of ca. 2.9 Ga, and thus forms part of the lower sequence of the Southern Cross Province. Ages determined for the granitoids in the Gum Creek greenstone belt are between ca. 2638-2722 Ma, similar to those dated elsewhere within the Southern Cross and Murchison Provinces. Most Nd model ages for the studied granitoids lie between ca. 2.93 and 3.10 Ga, which are typically a few hundreds of million years older than their emplacement ages, but are similar to, or slightly older than, the oldest (ca. 2.91-2.94 Ga) xenocrysts in these samples and the oldest (ca. 2.93-2.98 Ga) greenstone sequence in the Murchison Province. The results suggest that the crustal rocks, with an average age of 2.93-2.98 Ga, are the main sources of the granitoids. The time interval between ca. 2.98 and 3.0 Ga appears to be a major period of crustal formation in the Murchison Province. Although the data imply different sources for the various granitoid suites in the Murchison Province, the voluminous regional and the main suite of the internal plutons, appear to have been derived from the same, or isotopically similar, source regions. The stratabound volcanic massive sulphide mineralisation at Golden Grove in the Murchison Province is constrained to have formed between 2953±7 and 2945±4 Ma, predating similar mineralisation in the Eastern Goldfields Province by about 250 million years. The formation of komatiites and associated Ni mineralisation at the Maggie Hays nickel prospect in the Lake Johnston greenstone belt, is constrained to lie between 2921±4 and 2903±5 Ma, about 200 million years before their counterparts in the adjacent Eastern Goldfields Province. Samples from the Kanowna Belle mine (- 20 km northeast of Kalgoorlie in the Eastern Goldfields Province), which is the first discovery of a major gold deposit hosted by felsic porphyry in the Yilgarn Craton, were dated in order to constrain the two stages of mineralisation identified by petrographic, fluid inclusion, and stable isotope studies. The early stage of mineralisation has an age of 2695±4 Ma, which is ca. 60 million years older than the accepted age of 2630±10 Ma for the main gold forming event in the Yilgarn Craton, making it the oldest recorded gold mineralisation event in the region. Younger events occurred between ca. 2630 and 2650 Ma, broadly synchronous with the principal period of gold mineralisation in the region. These results, combined with existing data, indicate that significant gold mineralisation occurred over at least 90, and possibly as much as 130 million years, in the Yilgarn Craton. Show more