Northward subduction of the Indian Plate beneath the Kohistan island arc, Pakistan Himalaya: new evidence from isotopic data
Northward subduction of the Indian Plate beneath the Kohistan island arc, Pakistan Himalaya: new evidence from isotopic data
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DOI：
10.1144/0016-764902-068
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发表时间：
2003-05
影响因子：
2.7
通讯作者：
S. M. Bignold;P. Treloar
中科院分区：
文献类型：
--
作者：
S. M. Bignold;P. Treloar
大洋内科希斯坦岛弧形成于白垩纪，是印度板块北移的结果。传统的断言认为该弧是建立在北倾俯冲带上方的，这一说法受到了另一种假说的挑战，该假说认为该弧是建立在南倾俯冲带之上的。这一假设部分是基于弧形镁铁质岩石中存在明显的“Dupal”特征，部分是基于弧形北部出露的玻安岩的存在。只有在赤道地区从地幔提取的岩石中才能发现“Dupal”特征，其放射性成因铅的水平增加。然而，这里提供的新的放射性成因同位素数据表明，年轻弧的镁铁质火山岩的同位素特征不是Dupal类型的，而是海底沉积物俯冲和脱水所产生的流体的函数。科希斯坦弧内的玻尼质火山岩是弧北部成分多样的层序中体积较小的一部分。博尼岩和高镁安山岩主要发现于弧前地区，但并不仅限于此，越来越多的证据表明，这些岩石也可能喷发到弧后地区。沉积学和地球化学数据表明，科希斯坦玻安岩喷发成了一个短暂的弧后盆地。玻安岩喷发到弧后，以及没有‘Dupal’特征，都与科希斯坦向北俯冲一致，尽管弧初起的纬度仍未确定。
The intra-oceanic Kohistan island arc was initiated during the Cretaceous as a result of northward movement of the Indian Plate. The conventional assertion that the arc was erected above a north-dipping subduction zone has been challenged by an alternative hypothesis, which suggests that the arc was built above a south-dipping subduction zone. This hypothesis is based partly on the apparent presence of a ‘Dupal’ signature in mafic rocks of the arc, and partly on the occurrence of boninites exposed in the northern part of the arc. The ‘Dupal’ signature, with enhanced levels of radiogenic lead, is found only in rocks extracted from the mantle in equatorial regions. However, new radiogenic isotope data, presented here, suggest that the isotopic signature of the mafic volcanic rocks of the juvenile arc, rather than being ‘Dupal’-type, is a function of fluids derived from subduction and dehydration of sea-floor sediments. Boninitic volcanic rocks in the Kohistan arc form a volumetrically minor part of a compositionally diverse sequence in the northern part of the arc. Boninites and high-Mg andesites are found mostly, but not exclusively, in the fore-arc region, and evidence is accumulating that these may also be erupted into back-arc regions. Sedimentological and geochemical data suggest the Kohistan boninites to have been erupted into a short-lived back-arc basin. Both the eruption of boninites into the back-arc and the lack of a ‘Dupal’ signature would be consistent with northward subduction beneath Kohistan, although the latitude at which the arc was initiated remains unresolved.