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

Abstract

Major tectono-metamorphic differences between two pre-2900Ma 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 3320Ma by repeated cycles of plume-related mafic–felsic volcanism onto 3725–3600Ma continental crust. Intrusion of syn-volcanic tonalite–trondhjemite–granodiorite (TTG) sheets and subvolcanic laccoliths at ca. 3490–3430Ma caused locally significant syn-volcanic doming, but had only limited penetration into the uppermost greenstone succession. A major deformation event at 3315Ma 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 2950Ma, 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. 3280–3250Ma 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. 3480Ma crust, or was derived from a source region of this age, but the basal contact of the group is obscured by intrusive 3270Ma 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. 3160Ma 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. 3020Ma Cleaverville Formation. The Sholl domain, containing the 3130–3115Ma 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 3280Ma, and is interpreted to have been deposited on oceanic-type crust in the post-3240Ma rift. Four granitoid complexes in the WP were mainly formed between 3015 and 2970Ma, 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.