Date of Award

1986

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Abstract

The Coongan Formation, defined and described here, is an approximately 3.5 Ga-old unit of felsic volcanic and volcaniclastic rocks, representing a major episode of early Archean felsic volcanism, in the predominantly basaltic Warrawoona Group, eastern Pilbara Block, Western Australia. The Coongan Formation is divided into three members: the Duffer and Panorama Members and the Strelley Pool Chert. The oldest and volumetrically greatest part of the Coongan Formation is the Duffer Member, which consists largely of volcaniclastic breccia, conglomerate, tuff, and felsic lava. Vertical sequences, up to 5 km thick, indicate that the Duffer Member represents coarse-grained, shoaling-upward debris-aprons that flanked felsic volcanic centers. The Panorama Member, consisting predominantly of silicified volcaniclastic sandstone, tuff, siltstone, chert-clast conglomerate and banded chert, caps or interfingers with the upper part of the Duffer Member in many sections, or, in other localities, is developed to the exclusion of the Duffer Member. Sequences of the Panorama Member, generally less than 300 m thick, represent a spectrum of environments, including fan (?)-delta and laterally flanking shoreface environments as well as subaqueous settings under the direct influence of pyroclastic volcanism. After a period of erosion, ortho- and biochemical sediments represented by the Strelley Pool Chert accumulated on a broad, post-volcanic platform. Petrographic techniques, developed here for analysis of highly silicified sandstones, indicate that felsic volcanic sources provided most of the detritus to the Panorama Member. No evidence for granitic detritus was found in either the Duffer or Panorama Members, indicating that granitoid plutons in the eastern Pilbara Block were not exposed during the felsic volcanic episode. The maximum temperature attained during alteration of Panorama sandstones was 200 to 300 C, based on the present mineralogy of these rocks. Paleocurrent evidence sugests that sediment was dispersed from felsic centers, now mostly eroded, that coincided with the present positions of parts of Archean granitoid batholiths in the eastern Pilbara Block. These data suggest that the oldest granitoid rocks in the batholiths are genetically related to felsic volcanic rocks of the Coongan Formation, and support similar hypotheses made by others based on geochemical data.

Pages

338

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