Capricorn Orogen

Basement rocks of the Gascoyne Province and the overlying sedimentary rocks of the Edmund and Collier Basins were first systematically mapped by the Geological Survey of Western Australia (GSWA) in the 1960s and 1970s. Products from this program included 1:250 000 scale maps and Explanatory Notes, and a number of early Reports. Systematic 1:100 000 scale field mapping from 1996 to 2017 has produced over 44 first and second edition printed maps, Explanatory Notes for all lithostratigraphic units and events, as well as numerous Reports and Records outlining the protracted tectonic history of this part of the Orogen. The province and basins are covered by recent aeromagnetic and radiometric data at 400 m line spacing, 2.5 km spaced gravity, and Landsat TM and DEM-derived imagery, as well as crustal-scale geophysical data including seismic reflection and magnetotelluric (MT) data, and regional-scale passive seismic data. Together with abundant SHRIMP U–Pb zircon, monazite and xenotime geochronology, whole-rock and mineral isotope geochemistry, and whole-rock lithogeochemistry, these data help define the tectonic, depositional and mineralization history of the Orogen.

Links to GSWA’s and other publications are provided below. The Capricorn Orogen 2016 Geological Information Series package is the most up-to-date compilation for the project.

The Gascoyne Province forms the medium- to high-grade metamorphic core of the orogen. The oldest component is the Glenburgh Terrane, interpreted to be an exotic microcontinent. The terrane comprises heterogeneous granitic gneisses of the 2555–2430 Ma Halfway Gneiss and voluminous intermediate to granitic rocks of the 2005–1975 Ma Dalgaringa Supersuite, which forms an extensive granite batholith along the southern margin of the terrane. The Dalgaringa Supersuite magmatic rocks are interpreted to have been generated in a continental margin arc setting along the (present-day) southern margin of the terrane during the final amalgamation of the West Australian Craton. Following the assembly of the West Australian Craton, the province was structurally and thermally reworked during at least five punctuated intraplate orogenic events over a period of more than one billion years. Many of the events were accompanied by the intrusion of voluminous syntectonic felsic magmatic rocks. Granitic rocks associated with the Capricorn, Mangaroon and Edmundian Orogenies show similar calc-alkaline geochemical signatures and were generated and emplaced entirely within an intraplate tectonic setting. The protracted crustal reworking history of the Province has led to a highly differentiated orogenic crust, whereby the shallow portion has been enriched in high-heat producing elements such as U, Th and K, resulting in warmer crustal geotherms that may have weakened the lithosphere, making the orogen more susceptible to intraplate reworking.

The 1679–1067 Ma Edmund and Collier Basins are the youngest depositional elements of the Capricorn Orogen and contain 4–12 km of siliciclastic, carbonate and minor volcaniclastic sedimentary rocks deposited in a variety of fluvial to deep-marine environments. The rocks have been divided into six informal depositional packages, each defined by basal unconformities or major marine flooding surfaces and are the result of differential fault movements or fluctuations in sea level. Voluminous mafic sills were intruded into the basins at various times, locally inflating the stratigraphic thickness by up to 60%. The Edmund Group was intruded by the Waldburg and Narimbunna Dolerites, and the Edmund and Collier Groups were both intruded by the c. 1070 Ma Kulkatharra Dolerite.

The Edmund Basin has a half-graben architecture formed by the normal reactivation of older basement faults and sutures during the latter part of the Mangaroon Orogeny. Regional-scale extension during this time period has been calculated at ~ 7–8%. Paleocurrent directions and detrital zircon studies show that sediment supply was largely from the north. The Edmund Group sedimentary rocks were deformed during the Mutherbukin Tectonic Event prior to the deposition of the overlying Collier Group. The architecture of the Collier Basin is still poorly known, but the age and isotopic composition of detrital zircons indicate that the sediments were sourced directly from the underlying rocks of the Edmund Basin. Both groups were deformed during the Edmundian Orogeny, the Kuparr Event, and the Mulka Tectonic Event. Total regional-scale shortening during all postdepositional orogenic events is calculated at ~ 21%.

The Edmund and Collier Basins contain a wide range of mineral occurrences including supergene manganese and lead, minor gold and phosphate, as well as Western Australia's largest stratabound Pb–Ag–Cu–Au deposit at Abra. Many of these deposits are associated with major crustal-scale faults that have been reactivated multiple times.

Detailed descriptions of all lithological units and tectonic events in the Capricorn orogeny can be accessed through the Explanatory Notes System (ENS).

Reports

Records

Maps

Further articles and posters are available on the Department’s eBookshop.

Much of this work is supported by the Western Australian Government Exploration Incentive Scheme.

For more information contact:
geological.survey@dmirs.wa.gov.au