Lithostratigraphy and structure of the early Archaean Doolena Gap greenstone belt, East Pilbara Terrane, Western Australia

Abstract

The early Archaean East Pilbara Terrane of the Pilbara Craton represents the archetypical granite dome – greenstone keel terrain and preserves much of the primary geological features of dome-and-keel formation. Here we present the first detailed lithostratigraphic and structural transect from marginal gneisses of the Muccan Granitic Complex through 6km of steeply dipping strata of the western Doolena Gap greenstone belt.Based on overprinting relationships, we identified five deformational events (D1–D5) that can be correlated between four structural domains: (i) marginal orthogneisses of the polyphase Muccan Granitic Complex, (ii) a dominantly mylonitic shear zone (South Muccan Shear Zone) representing a granite dome – greenstone keel detachment zone, (iii) a Central Fold Belt of dominantly mafic greenschists, and (iv) a lower greenschist- to sub-greenschist facies southern Low-Strain Belt.The D1 event in the Low-Strain Belt is recorded in syn-depositional normal faulting (f1) during eruption of deep water low vesicular pillow basalts of the ca. 3.47Ga Mount Ada Basalt, followed by a conformable transition towards shallow water stromatolites in the Duffer Formation. We interpret the D1 event as the result of upper-crustal extension and subsequent uplift above the Muccan Granitic Complex.The D2 event shows the most intense deformation with syn-anatectic tight folds within the Muccan Granitic Complex and tight to isoclinal transposed folds (F2) and fluid-induced normal faults (f2) within the Central Fold Belt. The D2 event resulted in a regional EW composite S1/2 foliation, and in the development of the mylonitic South Muccan Shear Zone along the dome-keel interface and within the Central Fold Belt, mineral lineations (L2) in conjunction with increasing southward D2l-tectonite. This D2 event is not observed in a ∼1km-thick sandstone package (ascribed to the <3.43Ga Strelley Pool Formation) in the Low-Strain Belt that was deposited over an angular unconformity indicating 45° SSW tilting of the underlying Mount Ada Basalt and Duffer Formation. This constrains the D2 event to between 3.47 and 3.43Ga.We propose that the D1 and D2 events record locally significant initiation of Muccan Granitic Complex doming and associated sagduction of greenstone keel rocks, suggesting that partial convective overturn initiated early in the evolution of the East Pilbara Terrane.