Prefolding and overprint magnetic signatures in Precambrian (∼2.9–2.7 Ga) Igneous rocks from the Pilbara Craton and Hamersley Basin, NW Australia

Abstract

A total of 446 oriented samples have been collected from an Archaean intrusion and the Fortescue Group volcanics and intrusions in the Pilbara and Hamersley regions of northwestern Australia. The paleomagnetism and rock magnetism of these samples have been investigated using susceptibility and hysteresis properties and both alternating field (af) and thermal demagnetization. The most consistent paleomagnetic results were achieved by first applying af demagnetization and then stepwise thermal demagnetization. Evidence of the effects of lightning are widespread which accounts for the efficacy of this particular demagnetization strategy. Application of the fold test has resulted in placing significant constraints on the acquisition ages of some of the magnetizations and their corresponding pole positions. A prefolding pole position from the “Millindinna Complex,” is latitude = 11.9°S, longitude = 161.3°E (dp = 6.8°, dm = 8.4°) and is correlated with a radiometric age of 2.86±0.02 Ga. Folding in the “Millindinna Complex” occurred prior to the deposition of the Fortescue Group. New geochronological data indicate that the extrusion of the flood basalts commenced about 2.8 Ga. This provides a minimum age for the magnetization of the “Millindinna Complex.” Deformation responsible for the formation of the mobile belt separating the Pilbara and Yilgarn blocks during the Ophthalmian Orogeny has affected the Fortescue Group and produced a moderate degree of folding in the southern part of the Hamersley Basin. In the region of “Millindinna Complex” outcrop, the Fortescue Group is generally flat lying. Prefolding pole positions from the Fortescue Group are for the Mount Roe Basalt, latitude = 52.4°S, longitude = 178.0°E (dp = 6.4°, dm = 9.1°) and for the Mount Jope Volcanics, latitude = 40.5°S, longitude = 128.7°E (dp = 19.9°, dm = 20.8°). The overprint magnetization in the Mount Jope Volcanics yields a pole position, latitude = 35.0°S, longitude = 211.5°E (dp = 2.8°, dm = 5.5°) and is interpreted as having been acquired during folding relating to the formation of the Ophthalmian Fold Belt.

Magnetic petrophysical results from the Hamersley Basin and their implications for interpretation of magnetic surveys

“…Our work confirms the presence of a regional magnetic overprint directed upward upwards the northwest, which is similar to findings of previous studies in the basin (Schmidt & Embleton, 1985;Clark & Schmidt 1986;Li et al 1993). We note that there are at least 16 formations in the Mount Bruce Supergroup but previous studies were based on samples collected from only two formations in the Fortescue Group, namely the Mt Jope Volcanics (Schmidt & Embleton 1985) and the Jeerinah Formation (Li et al 1993), and three formations in the Hamersley Group, namely the Brockman Iron and Weeli Wolli Formations (Clark & Schmidt 1986) and the Wittenoom Dolomite (Li et al 1993).…”

“…A similar direction was also revealed by previous studies on volcanic rocks of the Fortescue Group(Schmidt & Embleton 1985;Li et al 1993), the Wittenoom Dolomite(Li et al 1993), and fresh dolerite dykes and BIFs(Clark & Schmidt 1986) …”

“…We note that there are at least 16 formations in the Mount Bruce Supergroup but previous studies were based on samples collected from only two formations in the Fortescue Group, namely the Mt Jope Volcanics (Schmidt & Embleton 1985) and the Jeerinah Formation (Li et al 1993), and three formations in the Hamersley Group, namely the Brockman Iron and Weeli Wolli Formations (Clark & Schmidt 1986) and the Wittenoom Dolomite (Li et al 1993). With the more extensive stratigraphic coverage in this study, our results not only reaffirm the presence of this northwesterly upward regional magnetic overprint in the Mount Bruce Supergroup, but also extend this overprint to as far as the Mount McGrath Formation in the Wyloo Group.…”

U–Pb geochronology and paleomagnetism of the Westerberg Sill Suite, Kaapvaal Craton – Support for a coherent Kaapvaal–Pilbara Block (Vaalbara) into the Paleoproterozoic?

“…In addition, new U-Pb baddeleyite ages of ca. 2990-2978 Ma for the Usushwana Complex (Gumsley et al 2015) also contradicts the link tothe 2860 ± 20 Ma Millindina Complex, although it may be related to the Hlagothi Complex (Schmidt and Embleton, 1985;Gumsley et al, 2013).…”

Records of geomagnetism, climate, and tectonics across a Paleoarchean erosion surface

“…Paleomagnetic studies of drill core offer unprecedented access to records of ancient remanent magnetization that are largely unavailable from weathered and lightning-remagnetized surface sam- Hale and Dunlop, 1984;Schmidt and Embleton, 1985;Schmidt and Clark, 1994;Layer et al, 1998;Wingate, 1998;Li, 2000;Wingate and Giddings, 2000;Yoshihara, 2001;Strik et al, 2003;Suganuma et al, 2006;Tarduno et al, 2007;de Kock et al, 2009;Biggin et al, 2011). Asterisks indicate poles that have been reversed in polarity.…”