Tel:  61 2 6249 0593
Fax: 61 2 6279 8253
Canberra, ACT 0200
Australia
andrew.glikson@anu.edu.au
copy to: aygsearch@cos.com.au
---------------------------------
(1) IMPACT RESEARCH
    CURRENT ACTIVITIES/PROGRESS, June-August, 1998

From Andrew Gilson <andrew.glikson@anu.edu.au>


4. Mingar Dome, Pilbara Craton, Western Australia

Field relations, Landsat-5-Thematic Mapper multispectral images
and geophysical data were examined in connection with the
near-perfectly circular 80 km-diameter large Mingar Dome, western
Pilbara Craton, north western Western Australia, with the aim of
elucidating its origin. The dome consists of Fortescue Group (2.76
Ga) mafic volcanics and minor felsic volcanics and sediments
unconformably overlying core inliers of granite (3.24 Ga),
migmatite and amphibolite exposed by the Fortescue River at the
center of the structure. Local pseudotachylite microbreccia vein
systems occur immediately below the basal unconformity of the
Fortescue Group on the Fortescue River, consisting of angular
quartz grains and deformed feldspar and containing granitic
fragments from the surrounding basement. No penetrative
deformation lamella diagnostic of high velocity shock were
observed, and the veins were possibly triggered by earthquakes. No
direct evidence has been observed for a meteoritic impact
connection of the Mingar Dome. A spin-off of the present study has
been the Landsat-5-TM correlation and identification of a range of
Fortescue Group and Hamersley Group volcanic and sedimentary rock
types, allowing spectral fingerprinting of a range of lithological
types, including separation of carbonates, shales, iron formations
and a range of volcanic rock types using band ratios and principal
components.

E-Mail: chall@tsrc.uwa.edu.au
Degree:Ph.D.
B.Sc. (Hons) & M.Sc., Otago University, New Zealand

Keywords:Sedimentology, Geochemistry, Fortescue Group, Archaean, Pilbara, Palaeogeography, Bellary Formation, Mount Roe Basalt, Hardey Formation
Supervisors: Prof. Chris Powell & Dr. Annette George Tectonics Special Research Center

Project Description
Palaeogeography, sedimentology and tectonics of the Fortescue Group, southern Hamersley Province, Western Australia.
The project aims to provide a palaeo-reconstruction of the southern margin of the Pilbara Craton approximately 2765 Million years ago when the Fortescue Group was deposited, unconformably, on the granite-greenstone terrane. This will be achieved by:
Applying basin analysis techniques to the 3 lowermost formations of the Fortescue Group (Bellary Formation, Mount Roe Basalt and Hardey Formation) in order to determine distribution and facies.

Palaeocurrent analysis and petrography of the Hardey Formation (quartz-rich sandstone) to determine provenance..
Geochemistry of the Hardey Formation to determine provenance and combined with points (1) & (2) will set up a tectonic discrimination diagram for Archaean sandstones..
Geochemistry (major and trace element) of basalt flows from the Bellary Formation and Mount Roe Basalt to determine tectonic setting..
Provide an age for the Bellary Formation using zircons from a tuffaceous unit. Method of analysis will be by SHRIMP..

Brief Summary of results to date
Regional mapping of the Bellary Formation at 1:15,000 scale has identified a series of basalt flows intercalated with fluvial and lacustrine sediments .The base of the Bellary Formation is not seen, but is assumed to be unconformable with the Archaean Granite-Greenstone terrane.Basalt flows 1-4metres thick, with autobrecciated flow tops, at the base of the formation are overlain by upward fining turbidites (Tb-Td). Flute Casts within the turbidites indicate a source region to the east.A poorly sorted conglomerate, with well rounded boulders of granite, chert, banded chert and basalt overlies the turbidites but is not laterally continuous.Above the turbidites and conglomerate are more basalt flows with coarse, but well sorted, quartzose sandstones and very fine siliceous mudstones. The thickness of the Bellary Formation is approximately 400 metres. Preliminary geochemistry results indicate that the basalts have calc-alkaline to tholeiitic affinities.
Conformably above the Bellary Formation is the Mount Roe Basalt, found only on the Bellary and Rocklea Domes. Two new locations of the Mount Roe Basalt have been identified on the Rocklea Dome. The distribution of the basalt on this dome may have been controlled by NNE trending growth faults, active during deposition of the lower Fortescue Group. Geochemistry results are similar to the Bellary Formation with calc-alkaline to tholeiitic affinities and LREE enrichment. More results are pending.
The Hardey Formation is an extensive quartz-rich sandstone with localised conglomerates at the base. The base of the Hardey Formation is either unconformable with the basement or disconformable with the Mount Roe Basalt Restored palaeocurrent results from trough cross beds, parting lineations, flute casts and ripples indicate a source region from the east, north east. The average quartz composition from the Milli Milli and Rocklea Domes is 70-72 % with the remaining 30 ­28 % made up by plagioclase and K-feldspar. The Hardey Formation on the Bellary Dome has a higher component of quartz (83%) which is similar to the percentage of quartz in the Bellary Formation (82%). The percentage of heavy minerals is very low, and restricted to zircon, titanite, apatite and Fe-oxide minerals. These results suggest that the source the Hardey Formation was most likely of granitic origin.
Initial palaegeographic reconstructions suggest that the lower Fortescue Group was deposited in a non-marine environment such as an alluvial braid plane, at the onset of rifting approximately 2.76 Billion years ago. Normal faults active during the rifting formed graben and half-grabens which acted as local depocentres for the Bellary and Mount Roe Basalt. The disconformity between the Mount Roe Basalt and Hardey Formation indicates a pause in the rifting, followed by erosion of a granitic source to the east and deposition of a fluvial sandstone and silts. Conformably above the Hardey Formation are pillow basalts, basalts, volcanoclastic sediments and komatiites of the upper Fortescue Group.
References
Hall, C.E., Cooper, A.F., & Parkinson, D.L. (1995) Early Cambrian carbonatite in Antarctica. J.Geol.Soc.Lon. 152, 721-728 Baxter, L.J., Chisholm, J.M. & Hall, C.E. (1993) Ore block modelling and structural geology: Their integration. In: Robertson,I., Shaw, W., Arnold, C., & Lines, K. (eds) Proceedings of the Internation Mining Geology Conference. 31-34