Warrawoona Large Igneous Province, 3530–3427 Ma

Abstract

The 3530–3427 Ma Warrawoona Group is the oldest of the three groups that make up the Pilbara Supergroup. This 3530–3235 Ma supergroup comprises the Paleoarchean greenstone succession of the East Pilbara Terrane and is preserved in 20 greenstone belts. The group was erupted across the entire area of the 3800–3530 Ma Pilbara crust (Chap. 2 ), and its thickness varies between 10 and 15 km. Variations are partly due to local erosional unconformities that were formed from 3460 Ma onwards when granite–greenstone domes of the terrane began to rise at different rates. Apart from thin sedimentary units deposited between 3490 and 3474 Ma and between 3459 and 3450 Ma, the Warrawoona Group is volcanic. The succession is composed of successive ultramafic–mafic–felsic volcanic cycles in which felsic volcanism was contemporaneous with intrusion of tonalite–trondhjemite–granodiorite (TTG). Field exposures reveal that many of the TTG intrusions were subvolcanic to the felsic volcanic formations. The East Pilbara Terrane is now exposed across 40,000 km2 of the northeast section of the Pilbara Craton, with concealed parts estimated to occupy an additional 60,000 km2. With an interpreted total volume of volcanic rocks exceeding 1,000,000 km3, the Warrawoona Group easily meets the volume requirement for a large igneous province (LIP). The 3530–3300 Ma stratigraphic successions of the Pilbara and Kaapvaal Cratons are remarkably similar, even including a common volcanic hiatus between about 3426 and 3350 Ma. In the Pilbara, two lines of evidence indicate a magmatic event commencing abruptly at 3530 Ma: firstly, a major peak in the frequency of zircon aged between 3530 and 3490 Ma, preceded by an almost total lack of zircons dated between 3550 and 3530 Ma; and secondly, Lu–Hf isotope evidence for a surge of mantle-derived juvenile magmas between 3530 and 3490 Ma. This sudden magmatic activity is interpreted to coincide with the arrival of the first of a series of mantle plumes that had major impacts on the Paleoarchean crustal evolution of the craton.