Double bar formation
The double-bar formation is the third and final formation of the from the paleoarchean Western Australia derived Coonterunah subgroup . Thus it is at the base of the Pilbara Supergroup belonging Warrawoona Group of Eastern Pilbara Craton ( Pilbara craton ).
Occurrence
The presence of the double-bar formation is on the south of the Carlindi-granite complex located Pilgangoora greenstone belt limited in the other greenstone belts of Eastern Pilbara Craton missing.
stratigraphy
The double bar formation, which is up to 2,500 meters thick on average, is of magmatic origin and predominantly extrusive in nature. It consists mainly of fine-grained, tholeiitic basalts that occasionally form pillow lava . Similar to the lower table-top formation , sub-volcanic intrusives such as gabbros also occur here . In places, thin, volcanoclastic intermediate layers of fine-grain, mafic tuffs have also been preserved. The rocks were later silicified to different degrees, recrystallized and metamorphosed (green slate to amphibolite facies). The double bar formation concordantly overlays the acidic volcanic rocks of the Coucal formation with a basal double chert layer . The formation ends with an erosive unconformity and then in Pilgangoora greenstone belt of the already at Kelly Group belonging Strelley Pool Chert covered.
Individual evidence
- ↑ MG Green: Early Archaean crustal evolution: evidence from ~ 3.5 trillion year old greenstone successions in the Pilgangoora Belt, Pilbara Craton, Australia . Sydney 2001 (PhD thesis, University of Sydney).