Rock magnetism and palaeomagnetism of the Oddanchatram anorthosite, Tamil Nadu, South India

Abstract

Rock magnetic and palaeomagnetic data are reported on the Proterozoic massif-type anorthosite body from Oddanchatram, on the northern slopes of the Kodaikanal (Palani) Ranges, Tamil Nadu. Alternating field demagnetization treatment indicated a very stable direction of magnetization. Progressive heating revealed a remanence that became unblocked at 580°C. The demagnetization analysis yields a characteristic component with a mean direction: Dm= 97.3° Im =− 28.8° (k= 51, α95= 7.2° and N= 9 sites). In addition to this characteristic component, specimens from a few sites show the presence of a low coercivity component with a mean palaeomagnetic direction of D= 36.7°; I=−29.4° (k= 63, α95= 4.4° and N= 18). The isothermal remanent magnetization acquisition curves indicate magnetite as the main carrier of the stable remanence. The direction of the characteristic component corresponds to a pole position at 9.7°N, 1.7°E (dp= 4.4°, dm= 7.9°). The location of this pole is comparable to palaeomagnetic poles reported for the period of 1100–1000 Ma for a range of formations from the Indian shield. The pole for the secondary component (45°S, 23°E) fits well with poles for 550 Ma, associated with the Pan-African thermal event that extensively caused granulite grade metamorphism in the Southern Granulite Terrain. Despite the high temperature (>700 °C) that prevailed during the Pan-African event, the remanence magnetization of 1100–1000 Ma is preserved in the Oddanchatram anorthosite. It is inferred that the northern boundary of the terrain affected by the dominant 550 Ma granulitic metamorphism lies close to the southern margin of the Oddanchatram anorthosite body. Thus Oddanchatram anorthosite escaped the high temperatures metamorphism. The weak secondary magnetization found in a few specimens is attributed to hydrothermal activity associated with the intrusion of pink granites, immediately south of the Oddanchatram anorthosite body. A mid-Proterozoic magnetization age of the Oddanchatram anorthosite supports the view that this body was emplaced/remobilized during the Eastern Ghat Orogeny (∼1000 Ma) and this body can be seen as a western extension of the string of massif-type anorthosites in the Eastern Ghat Mobile Belt.