North Star Basalt
The North Star Basalt is the first formation of the Warrawoona Group belonging Talga Talga Subgroup in Western Australia ( Pilbara craton ). With an age of around 3500 million years BP , the predominantly basaltic formation comes from the Paleoarchean (outgoing Isuum ).
Occurrence
The North Star Basalt occurs in greenstone belts (such as the Marble Bar Greenstone Belt and the Coppin Gap Greenstone Belt ) of the West Pilbara Terrane .
stratigraphy
The volcanic cycle of the approximately 2000 meter thick North Star Basalt begins with ultramafic, komatiitic basalts and serpentinitized peridotites . This is followed by the actual main mass of tholeiitic basalts, which occasionally form pillow lava and are interspersed with numerous storage corridors made of gabbro and dolerite . Separated by a thin layer of chert with silicified ribbon ores , ultramafites appear again. This is followed by storage corridors of green diorite , over which clastic sediments (quartz sandstones) lie. The formation closes with extrusive volcanic rocks, which are covered by a mighty chert band of the concordantly following McPhee formation .
The sedimentary sequence can be intruded by both the Muccan Granite Complex and the Mount Edgar Granite Complex .
Due to the penetration of the granite complexes , the rocks of the North Star basalt were metamorphosed , the physical conditions ranged from the green slate to the lower amphibolite facies. As a result, the original basalts are mostly amphibolites or amphibolite slates.
Dating
Van Koolwijk and colleagues (2001) were able to assign an age of 3490 ± 15 million years BP to an ultramafic, retromorphosed pyroxenite lens on hornblende using the Ar-Ar dating method .
Individual evidence
- ^ Van Koolwijk et al .: AGSO-GA Record 2001/37 . 2001, p. 102 .