No abstract

In the Cleaverville area of Western Australia, the Regal, Dixon Island, and Cleaverville Formations preserve a Mesoarchean lower-greenschist-facies volcano-sedimentary succession in the coastal Pilbara Terrane. These formations are distributed in a rhomboidal-shaped area and are unconformably overlain by two narrowly distributed shallow-marine sedimentary sequences: the Sixty-Six Hill and Forty-Four Hill Members of the Lizard Hills Formation. The former member is preserved within the core of the Cleaverville Syncline and the latter formed along the northeast-trending Eighty-Seven Fault. Based on the metamorphic grade and structures, two deformation events are recognized: D 1 resulted in folding caused by a collisional event, and D 2 resulted in regional sinistral strike-slip deformation. A previous study reported that the Cleaverville Formation was deposited at 3020 Ma, after the Prinsep Orogeny (3070-3050 Ma). Our SHRIMP U-Pb zircon ages show that: (i) graded volcaniclastic-felsic tuff within the black shale sequence below the banded iron formation in the Cleaverville Formation yields an age of (3 114 ±14) Ma; (ii) the youngest zircons in sandstones of the Sixty-Six Hill Member, which unconformably overlies pillow basalt of the Regal Formation, yield ages of 3090-3060 Ma; and (iii) zircons in sandstones of the Forty-Four Hill Member show two age peaks at 3270 Ma and 3020 Ma. In this way, the Cleaverville Formation was deposited at 3114-3060 Ma and was deformed at 3070-3050 Ma (D 1 ). Depositional age of the Cleaverville Formation is at least 40-90 Myr older than that proposed in previous studies and pre-dates the Prinsep Orogeny (3070-3050 Ma). After 3020 Ma, D 2 resulted in the formation of a regional strike-slip pull-apart basin in the Cleaverville area. The lower-greenschist-facies volcano-sedimentary rocks are distributed only within this basin structure. This strike-slip deformation was synchronous with crustal-scale sinistral shear deformation (3000-2930 Ma) in the Pilbara region.