Summer Intern Conference

Material in this volume may be copied without restraint for library, abstract service, education, or personal research purposes; however, republication of any paper or portion thereof requires the written permission of the authors as well as appropriate acknowledgement of this publication. Any opinions, findings, and conclusions or recommendations expressed in this volume are those of the author(s) and do not necessarily reflect the views of the National Aeronautics and Space Administration. INTRODUCTION: The Pilbara Craton in Northwestern Australia is home to some of the oldest and least metamorphosed rocks on Earth. One example of such a group of rocks in this area is the 3.51 Ga Coucal Formation in the Coonterunah Group. The Coucal Formation, which is predominantly basalt with thin interbedded chert layers, is slightly over 700 m thick (Fig 1): Fig 1: Stratigraphic section of the Coucal Formation. The formation is underlain by the Table Top Formation and overlain by the Double Bar Formation, both of which are composed of metabasalt, metadolerite, and amphibolites [1]. Much of the geology above the Coucal Formation is thought to be hydrothermal in origin based on the rocks' textures and mineralogy [2]. This study investigates the origins of the cherts in the Coucal Formation. METHODS: Hand samples and thin sections of the chert layers were first examined in macroscopically and then under a petrographic microscope. After locating promising textural associations in thin section, more work was carried out on the JEOL 5910LV Scanning Electron Microscope (SEM) at a potential of 15 kV. Thin sections were imaged either as backscatter electron (BSE) images (BEI) or scanning electron images (SEI) at various magnifications ranging from 90x to 1500x. Elemental data was collected using the SEM's X-ray Energy Dispersion Spectrum (EDS). The basalt analyses consisted of whole rock analysis done by X-ray fluorescence. We then used this data for making normative calculations for the basalt composition. RESULTS: The basalts of the Coucal Formation are extrusive volcanics. They are fine grained and pillowed. Their compositions are relatively homogeneous, and the majority of the basalts are trachytes, trachyandesites, or trachybasalts based on normative calculations. Of the 17 basalt samples analyzed, four of them are very unexpectedly quartz rich. One sample falls in between these two extremes and is an alkali rhyolite based on the normative calculation (Fig 2). Sample, C28, was very quartz depleted and we were unable to identify the rock type based on chemistry. Fig …