Cycles in the Warrawoona group (3.47-3.4 Ga)

Cycles in the Warrawoona group (3.47-3.4 Ga): Report of the results of the field work in the Coongan and Warrrawoona greenstone belts, Pilbara Block, Westem Australia.

De Leeuw, Diana

Molengraaff Fonds

2000-09

This report contains the results of the data that were obtained during field work in the oldest recognized greenstone unit, the Warrawoona Group (3.47-3.4 Ga), in the Coongan greenstone belt and the Warrawoona greenstone belt, in the East Pilbara in Western Australia. The rocks that constitute the greenstone belts are assumed to be deposited into a sedimentary basin. In both of the studied greenstone belts rock units can be grouped into cycles. A cycle mainly consists of a volcanic base with a sedimentary top or contains sediments only. In the studied part of the Coongan greenstone belt eight different cycles were recognized. In the studied part of the Warrawoona greenstone belt three different cycles were found. Facing direction in the Coongan greenstone belt is to the east, facing direction in the Warrawoona greenstone belt points to the southeast. On base of this younging direction and the occurrence of rather comparable cycles in both greenstone belts, the cycles of the Coongan and Warrawoona greenstone belts are assumed to be correlated with each other, with cycles in the Warrawoona greenstone belt lying on top of the cycles in the Coongan greenstone belt. From cycle 1 in the Coongan greenstone belt up to cycle 3 in the Warrawoona greenstone belt the basin infill tells the story of the basin. First a basin was formed into an extending basement. During the first cycles the basin was mainly filled with submarine basaltic/andesitic flows and sediments, after which the upper part was eroded. During beginning of repeated infill with coarse erosive material, the basin collapsed and growth faults occurred. In the north of the area a rhyodacitic volcano (dated at 3.467-3.468 Ga) is assumed to have been active at this time. During the upper cycles the basin was mainly filled with sedimentary rocks. In the sedimentary package only one cycle is present with submarine mafic flows, m which volcanic exhalatives occur. In the upper sedimentary cycles slumps occur. In the Warrawoona area large erosion products were found in the cycle that is assumed to be lying on top of the sedimentary package of cycles in the Coongan area. During cycle 2 the sedimentary period changes to a period with mafic flows again in cycle 3. This whole basin evolution is assumed to be happened between about 3.47 (age Duffer) and 3.4 (age of felsic intrusives) Ga. Younger units are present in the Warrawoona greenstone belt, but have not been studied for this report. After deposition in the basin the total basin was tilted to the east by an unknown process. After that the batholiths are assumed to be intruded and several shear events took place. Stratigraphy and facing direction in the studied parts of the basin do not support the diapiric model of Collins et al. (1998). Facing is directed throughout the studied Warrawoona group to the east or southeast and no symmetric repetition was found in the studied parts of the greenstone belts. Therefore the mid-crustal detachment model is preferred above the diapiric model. This implies that sub-horizontal forces were already active in the early Archaean.

http://resolver.tudelft.nl/uuid:f015ddeb-9007-4b91-9371-44c8eb0f2d30

Vrije Universiteit Amsterdam

Geoscience Reports

report

(c) Diana de Leeuw