Palaeomagnetic and rock magnetic study of a deeply exposed continental section in the Charnockite Belt of southern India: implications to crustal magnetisation and palaeoproterozoic continental nucleii

This study has investigated palaeomagnetism and magnetic properties of a tract of continental basement in south India metamorphosed to granulite facies close to the Archaean–Proterozoic boundary (∼2600 Ma) at former depths of 20–35 km. The metamorphism is characterised by extensive development of charnockite produced by flushing of CO2-rich fluids through older crust. Rock magnetic studies (thermomagnetic, high and low temperature susceptibility, hysteresis and IRM) show that very low-Ti metamorphic magnetite is the ubiquitous ferromagnet in all rock types. It has predominantly multidomain characteristics but is of high magnetic stability probably due to exsolution precipitation and microfracture filling producing anisotropic grain shapes. Magnetic susceptibility shows little increase with temperature falling to zero at ∼550 °C implying that induced magnetisation drops to a weak paramagnetic effect near this isotherm. Susceptibilities and intensities of magnetisation increase from amphibolite to granulite facies and by an order of magnitude within charnockites in the lower dehydrated crust. Induced magnetisations are able to explain ∼50% of observed magnetic anomalies of deep origin. Intensities of magnetisation and Qn values are high, especially in charnockite where metasomatism has resulted in new magnetite formation, and it is concluded this facies is able to explain observed magnetic anomalies provided that viscous remnant magnetisation is enhanced at depth to contribute the remaining magnetisation. The magnitude of anisotropy of magnetic susceptibility (AMS) is also increased by charnockite formation, as are degrees of lineation and foliation. However, directions of AMS in these anhydrous rocks are essentially random and reflect the static nature of the charnockite metasomatism. reciable Phanerozoic overprinting is evident in this terrane and the dominant feature of the palaeomagnetic record is a west to north shallow swathe of component directions with a minor antiparallel sequence between means summarised by A1 (D/I=280/−8°), A2 (D/I=319/−11°) and A3 (D/I=1/−10°). This swathe is interpreted as an uplift-related cooling acquisition following charnockite formation at ∼2600 Ma and a remanence age of ∼2600–2400 Ma is implied by the absence of most of the swathe in early Palaeoproterozoic and younger dyke swarms cutting the region. A second group of component directions (‘B’, mean D/I=238/−87°) is similar to magnetisations found in dykes linked to an age of ∼2370 Ma and interpreted as somewhat younger than the ‘A’ swathe. We argue that the entire magnetic record was acquired before final basement uplift prior to 2000–1800 Ma. Collective late Archaean–early Proterozoic palaeopoles from India define a low to high latitude movement of the shield at ∼2600–2300 Ma which is replicated in contemporaneous results from the other ancient Gondwana nucleii of southern Africa and Australia. The cratonic nucleii of these three shields have the oldest laterally-extensive supracrustal covers and comprise the core of the protocontinent of ‘Ur’. The 2900–2200 Ma palaeopoles are consistent with close proximities and subsequent movements between them were apparently limited because continental proximities, but not configurations, were similar within the (Palaeozoic) supercontinent of Gondwana.