A new 3.59 Ga magmatic suite and a chondritic source to the east Pilbara Craton

Abstract

The Pilbara Craton, Western Australia hosts one of the best-preserved Paleoarchean granite-greenstone terrains on Earth, and is inferred to have developed on an older (>3.8 Ga), possibly Hadean, continental substrate. Such ancient crust has, however, never been identified in outcrop. Here, we show that metamorphosed gabbroic, leucogabbroic and anorthositic rocks of the South Daltons area, in the western part of the Shaw Granitic Complex, formed at 3.59–3.58 Ga and were intruded by granitic magma at 3.44 Ga. The 3.59–3.58 Ga gabbroic rocks, here named the Mount Webber Gabbro, represent the oldest, unambiguous igneous rock emplacement in the Pilbara Craton and significantly predate the oldest volcanic activity of the 3.53–3.23 Ga Pilbara Supergroup within the East Pilbara Terrane. We interpret the Mount Webber Gabbro samples to represent fragments of a dismembered layered mafic intrusion. Mantle-like zircon δ18O and Hf isotope signatures indicate derivation from a chondritic to near chondritic mantle at ~3.59 Ga, and do not support the existence of a >3.8 Ga basement to the East Pilbara Terrane. These results strengthen the notion of an approximately chondritic >3.5 Ga mantle beneath the Pilbara Craton, and provide further evidence that recent estimates of Archean stabilised continental volumes, based on the assumption of crust extraction from a global, convecting depleted mantle reservoir, may be overestimated.