Rock magnetism and palaeomagnetism of the Oddanchatram anorthosite, Tamil Nadu, South India
Rock magnetism and palaeomagnetism of the Oddanchatram anorthosite, Tamil Nadu, South India
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DOI：
10.1111/j.1365-246x.2003.02116.x
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发表时间：
2003-12
影响因子：
2.8
通讯作者：
K. Satyanarayana;B. Arora;A. Janardhan
中科院分区：
文献类型：
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作者：
K. Satyanarayana;B. Arora;A. Janardhan
报道了泰米尔纳德邦Kodaikanal （Palani）山脉北坡Oddanchatram元古代块状斜长岩的岩石地磁和古地磁资料。交变场退磁处理显示出非常稳定的磁化方向。逐渐加热发现，在580摄氏度时，残余物变得畅通。消磁分析得到一个具有平均方向的特征分量：D-m=97.3度Im=-28.8度（k=51, alpha(95)=7.2度，N=9个位点）。除了这个特征分量外，少数地点的样品还存在一个低矫顽力分量，平均古地磁方向为D=36.7°；I=29.4度（k=63, alpha(95)=4.4度，N=18）。等温剩磁采集曲线表明，磁铁矿是稳定剩磁的主要载体。特征分量方向对应的极点位置为9.7°n, 1.7°e (d（p)=4.4, d(m)=7.9）。这个磁极的位置与报道的1100-1000 Ma时期印度地盾一系列地层的古磁极相当。次级分量的极（45°，23°）与550 Ma的极相吻合，与泛非洲热事件有关，该热事件广泛引起了南部麻粒岩的麻粒岩级变质作用。尽管在泛非事件期间普遍存在高温（700摄氏度），但Oddanchatram斜长岩中保留了1100-1000 Ma的剩余磁化强度。推断受550 Ma麻质变质作用影响的北边界靠近Oddanchatram斜长岩体南缘。因此奥丹恰拉姆斜长岩逃脱了高温变质作用。在少数样品中发现的弱次级磁化归因于与粉红色花岗岩侵入有关的热液活动，这些花岗岩位于Oddanchatram斜长岩体的正南。Oddanchatram斜长岩的中元古代磁化年龄支持了该岩体在东高止造山运动期间（类似于1000 Ma）的侵位/再活动的观点，该岩体可视为东高止活动带地块型斜长岩系列的向西延伸。
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 580degreesC. The demagnetization analysis yields a characteristic component with a mean direction: D-m=97.3degrees Im=-28.8degrees (k=51, alpha(95)=7.2degrees 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=36.7degrees; I=29.4degrees (k=63, alpha(95)=4.4degrees 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 9.7degreesN, 1.7degreesE (d(p)=4.4, d(m)=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 (45degreesS, 23degreesE) 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 (700degreesC) 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 (similar to1000 Ma) and this body can be seen as a western extension of the string of massif-type anorthosites in the Eastern Ghat Mobile Belt.