A palaeomagnetic study of Jurassic intrusives from southern New South Wales: further evidence for a pre-Cenozoic dipole low

Abstract

The results of rock magnetic, thermal demagnetization and Thellier palaeointensity studies are presented for two high-level intrusions from southern New South Wales, Australia. The Gingenbullen Dolerite (GB) (34.5°S, 150.3°E) and the Gibraltar Microsyenite (GS) (34.5°S, 150.4°E) were emplaced during the second of three major phases of igneous activity that affected the southern Sydney Basin and are K/Ar dated at 172 and 178 Ma, respectively. The magnetic mineralogy of the two intrusions is different: the GB is dominated by single-domain (SD)/pseudo-single-domain grains of magnetite, whereas the GS has both SD magnetite and haematite components, although the haematite component does not record a stable remanence. The GB records two opposed components of magnetization, above 100 °C, occupying distinct parts of the blocking temperature spectrum. The lower blocking temperature (LBT) component resides between 150 and 400 °C and is of normal polarity, having a mean direction of D/I = 021°/−79° (α95 = 8°, k = 63), with a corresponding VGP at Lat/Long = 54°S/137°E (dp = 12, dm = 13). This component is believed to be a TCRM or TVRM related to the initial opening of the Tasman Sea at ~90 Ma. The higher blocking temperature (HBT) ChRM component is recognized between 450 and 580 °C and is of reverse polarity, with a mean direction of D/I = 174°/+81° (α95 = 11°, k = 33) and a corresponding VGP at Lat/Long = 52°S, 153°E (dp = 17, dm = 18). The ChRM for the GS resides in the 100–450 °C region and, although the scatter is high, has a similar mean direction (D/I = 167°/+79°; α95 = 24°, k = 26) to the HBT component of the GB, with a pole position at Lat/Long = 54°S/158°E (dp = 30, dm = 31). There is no evidence of the GB LBT component in samples from the GS, suggesting that this intrusion was magnetically unaffected by the later event, which is also seen for other intrusions in the Sydney Basin. The VGPs calculated from the ChRM directions both plot on the Jurassic segment of the Australian APWP (Dunlop et al. 1997), providing further palaeomagnetic confirmation for the K/Ar ages. Thellier palaeointensity results were of variable quality, as expected for rocks of this age. The mean VDM values calculated for the ChRMs of the GB and GS were 1.3 × 1022 and 4.1 × 1022 Am2, respectively (approximately 16 and 51 per cent of the present value). On application of a cooling-rate correction estimate, the GB result falls to a minimum of 0.87 × 1022 Am2 (approximately 11 per cent of the present-day value), representing an overestimate of up to 56 per cent. These values, arising from a time window previously lacking in Thellier estimates, provide further confirmation of the existence of a pre-Cenozoic dipole low. The VDM values derived from the overprint in the GB samples yield an average of 4.8 × 1022 Am2 (from a range between 44 and 81 per cent of the present-day mean VDM) and are tentatively attributed to the Cretaceous Normal Superchron (CNS). These results may document evidence for a previously unrecognized low dipole moment during the bulk of the CNS, a possibility that requires further investigation by performing palaeointensity work on igneous rocks with radiometric ages between 110 and 90 Myr that carry unequivocal primary TRMs.