Two contrasting granite−greenstone terranes in the Pilbara Craton, Australia: evidence for vertical and horizontal tectonic regimes prior to 2900 Ma

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Abstract

Major tectono-metamorphic differences between two pre-2900 Ma granite–greenstone terranes of the Pilbara Craton indicate evolution under fundamentally different tectonic regimes.

The East Pilbara Granite–Greenstone Terrane (EP) contains a typical dome and basin regional structural pattern, lacking any preferred orientation of domical granitoid complexes, intervening synclinal greenstone belts, or faults. The EP greenstone succession contains no low-angle thrusting, is demonstrably autochthonous throughout the EP, and most of its stratigraphic components are common to all the greenstone belts. The oldest greenstone successions of the EP were deposited between 3515 and 3320 Ma by repeated cycles of plume-related mafic–felsic volcanism onto 3725–3600 Ma continental crust. Intrusion of syn-volcanic tonalite–trondhjemite–granodiorite (TTG) sheets and subvolcanic laccoliths at ca. 3490–3430 Ma caused locally significant syn-volcanic doming, but had only limited penetration into the uppermost greenstone succession. A major deformation event at 3315 Ma is consistent with gravity-driven diapiric doming of the TTG complex and sinking of denser, overlying greenstones into adjacent synclines. This diapiric deformation established the main features of the present dome and basin pattern. Subsequent monzogranite intrusion was almost entirely restricted to the cores of the granitoid domes and, together with renewed diapirism at 3240 and 2950 Ma, amplified some of these domes into almost vertical cylindrical structures. The crustal evolution of the EP cannot be satisfactorily explained using plate-tectonic models.

The West Pilbara Granite–Greenstone Terrane (WP) exhibits a strong northeast-oriented structural grain defined by the elongation of granitoid complexes, the trend of its two main greenstone belts, and by numerous closely spaced east- and northeast-striking faults. Unlike the EP, the WP is a collage of three separate, fault-bounded tectono-stratigraphic domains, each of which contains a unique stratigraphic succession and a separate set of tectonic structures. The oldest greenstones of the WP belong to the ca. 3280–3250 Ma Roebourne Group that is restricted to the Karratha domain. The Roebourne Group is an ultramafic–felsic volcanic succession with overlying, subordinate clastic metasedimentary rocks. Isotopic data indicate that the group was deposited on ca. 3480 Ma crust, or was derived from a source region of this age, but the basal contact of the group is obscured by intrusive 3270 Ma tonalite and granodiorite. The Roebourne Group is interpreted to have formed on EP basement during rifting of the EP, after which the EP and WP evolved in different tectonic environments. At ca. 3160 Ma the Karratha domain was tectonically over-ridden by oceanic-type crust of the Regal Formation, indicating collision with an unknown plate to the north. The Regal Formation forms the lower part of the Cleaverville domain, the upper component being the unconformable, post-collision, ca. 3020 Ma Cleaverville Formation. The Sholl domain, containing the 3130–3115 Ma Whundo Group, is separated from the Karratha and Cleaverville domains by a major, long-lived fault zone, the Sholl Shear Zone (SSZ). The Whundo Group is a juvenile, predominantly basaltic pile showing no evidence of contamination by crust older than 3280 Ma, and is interpreted to have been deposited on oceanic-type crust in the post-3240 Ma rift. Four granitoid complexes in the WP were mainly formed between 3015 and 2970 Ma, and have no age equivalents in the EP. No diapiric domes are present in the WP, where all deformation is interpreted to be the result of successive episodes of horizontal compression, consistent with evolution in “Phanerozoic-style” plate-tectonic environments.

Introduction

Archaean terranes older than 2900 Ma commonly exhibit regional dome and basin patterns, whereas post-2900 Ma terranes are typically characterized by linear structural patterns. This may be evidence of a change in tectonic processes over time, and other workers have used different lines of evidence to argue for evolving tectonic processes during the Archaean (e.g. Choukroune et al., 1995, Davies, 1998). The Pilbara Craton of Western Australia preserves an essentially complete geological record from 3520 to 2400 Ma, making it one of the best areas in the world to study Paleo- to Neoarchaean crustal evolution.

The Pilbara Craton comprises two major tectonic units: an assemblage of pre-2800 Ma granite–greenstone terranes, and an unconformably overlying succession of volcanic and sedimentary rocks that were deposited in the 2770–2400 Ma Hamersley Basin (Fig. 1; Trendall, 1990). The “greenstones” are metamorphosed sedimentary and volcanic rocks, and their hypabyssal equivalents that Hickman (1983) collectively assigned to the Pilbara Supergroup. The “granites” range in composition from diorite and tonalite–trondhjemite–granodiorite (TTG) to syenite, but are dominantly monzogranite and granodiorite, and form structural complexes of multiple intrusions. About 65% of the granite–greenstone basement is concealed by the relatively flat-lying Hamersley Basin succession such that only in the northern part of the craton is this basement exposed over a very large area (Fig. 1). Regional gravity and magnetic data (Blewett et al., 2000) have been used to interpret the underlying granite–greenstone geology of the entire craton (Fig. 2).

Since 1995, the granite–greenstone basement in the northern part of the Pilbara Craton (previously referred to as the “Pilbara Block,” Hickman, 1983) has been the subject of a detailed mapping program by the Geological Survey of Western Australia (GSWA) in collaboration with the Australian Geological Survey Organization (AGSO, recently renamed Geoscience Australia). The mapping has been supported by precise geochronology and geophysical surveys, and has provided a vast amount of new data. The present paper highlights the contrasting geological styles between the western and eastern granite−greenstone terranes.

Section snippets

Regional geology

The granite–greenstone basement in the northern part of the Pilbara Craton contains three granite−greenstone terranes separated by two late tectonic clastic sedimentary basins (Fig. 1, Fig. 2; Hickman, 2001a, Van Kranendonk et al., 2002. The ca. 3280–2920 Ma West Pilbara Granite–Greenstone Terrane (WP) is separated from the ca. 3515–2830 Ma East Pilbara Granite–Greenstone Terrane (EP) by the ca. 3010–2940 Ma Mallina Basin within the Central Pilbara Tectonic Zone (CPTZ; Fig. 1). Southeast of the

“Pilbara Block”

Hickman (1983) provided the first regional geological description of the north Pilbara granite−greenstones, then referred to as the “Pilbara Block.” He interpreted the geology of the Pilbara Block to have resulted from early, dominantly volcanic deposition (Warrawoona Group) on a basement of earlier granite–greenstones, followed by successive periods of deformation, and related sedimentation, volcanism, and granitoid intrusion. Despite recognition of important local stratigraphic variations at

Geology of the EP and WP

Recognition that there is no geological basis for dividing the EP into separate tectono-stratigraphic domains, and that the model of westward-migrating depositional basins across the north Pilbara Craton is not supported by geological or geochronological data, requires reassessment of the basic assumptions and interpretations that have underpinned recent schemes of sequence stratigraphy (Krapez, 1993, Krapez and Eisenlohr, 1998). Lithostratigraphy is not model-dependant, and is thus used in

Discussion

There exists general consensus that the ca. 3280–2920 Ma WP was formed through Phanerozoic-style plate-tectonic processes (Krapez, 1993, Smith et al., 1998, Krapez and Eisenlohr, 1998, Van Kranendonk et al., 2002), but the extent to which such processes also operated in the older EP is currently far more controversial. In the case of the EP, Van Kranendonk et al., 2002, Van Kranendonk et al., 2004 have argued that proponents of horizontal tectonic processes (e.g. Alpine-style thrusting;

Conclusions

The WP and the EP evolved in very different tectonic environments, leading to major differences in rheology, buoyancy and crustal architecture. The terrane-scale structural geology of the EP cannot be satisfactorily interpreted in terms of Phanerozoic-style plate-tectonics. Long-lived, episodic magmatism in the EP is attributed to successive mantle plumes beneath evolving and thickening continental crust, and deformation is attributed to diapiric rise of relatively buoyant granitoids and

Acknowledgements

Although the interpretations and conclusions reached in this paper are the author’s own, they are based on the results of a major collaborative project between the GSWA and Geoscience Australia since 1995. During the 6 year term of the project the author has had countless stimulating discussions on Pilbara geology with geologists involved in this project, in particular Martin Van Kranendonk, R. Hugh Smithies, Ian R. Williams, Leon Bagas, Terry Farrell, David N. Nelson, Caroline A. Strong,

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