Identification for the Mesozoic OIB-type Basalts in Central Qinghai-Tibetan Plateau:Geochronology,Geochemistry and Their Tectonic Setting
Identification for the Mesozoic OIB-type Basalts in Central Qinghai-Tibetan Plateau:Geochronology,Geochemistry and Their Tectonic Setting
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发表时间：
2006
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通讯作者：
Zhu Dicheng
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作者：
Zhu Dicheng
青藏高原中部蛇绿岩的岩石类型、构造背景和地幔源区性质目前尚不清楚。本文对青藏高原中部龙木错-双湖蛇绿混杂岩带加错村、班公湖-怒江缝合带和那曲盆地西部的多玛村和塔仁本村的玄武岩进行了单斜辉石Ar-Ar测年、SHRIMP锆石测年、地球化学和Sr-Nd-Pb同位素结果，以确定它们的时代、构造背景和地幔源区性质。现有资料表明，双湖尕措村玄武岩岩浆活动发生在中晚三叠世单斜辉石Ar-Ar定年结果表明，班公湖-怒江缝合带的多玛、塔仁本等地的玄武岩岩浆活动可能发生在早白垩世晚期（~110 Ma）。这与化石限制是一致的。出露于不同蛇绿混杂岩带中的中生代玄武岩在成分上具有OIB型而非MORB型，其中加科玄武岩的地球化学特征介于峨眉山高钛玄武岩和夏威夷碱性玄武岩之间，那曲盆地西部中晚三叠世加家组玄武岩与夏威夷碱性玄武岩和多玛早白垩世玄武岩的地球化学特征相似，班公湖-怒江缝合带的塔仁本村与夏威夷碱性玄武岩具有很大的可比性。结合地球化学约束和沉积序列，我们认为加柯OIB型玄武岩可能形成于增生楔上的裂谷环境，而不是洋壳板内裂谷环境，而加加组玄武岩可能喷发于被弧内和弧前盆地沉积所包围的陆棚斜坡裂谷环境。班公湖-怒江缝合带中的多玛玄武岩和塔仁本玄武岩形成于洋岛环境，表明班公湖-怒江洋壳仍存在于110 Ma左右，这意味着该洋壳的闭合时间比前人提出的晚侏罗世-早白垩世要晚得多。地球化学特征表明，地壳混染和陆下岩石圈地幔物质对青藏高原中部中生代玄武岩的形成作用不明显，因此，某些相对新鲜样品的Nd、Pb同位素组成可能代表青藏高原中部中生代玄武岩的地幔源区特征。根据206 Pb/204 P的高比值（18.5），认为当时青藏高原中部中生代特提斯地幔中不存在“Dupal”异常。然而，在获得更多数据后，应进一步证实这一观察结果。
Rock-type, tectonic setting and nature of mantle source region for the “so-called" ophiolite in central Qinghai-Tibetan Plateau is poorly understand at present. We present Ar-Ar dating of clinopyroxene, SHRIMP zircon dating, geochemical and Sr-Nd-Pb isotopic results for the basalts from Gaco village in Longmu Co-Shuanghu ophiolitic melange zone, Duoma and Tarenben villages from Bangong Co-Nujiang suture zone and western Nagqu basin in central Qinghai-Tibetan Plateau in order to constrain their age, tectonic setting and nature of mantle source region. Present data show that the basaltic magmatism in Gaco village, Shuanghu occurred in the Mid-Late Triassic (232.5 Ma) as indicated by the Ar-Ar dating of clinopyroxene in the basalt, and that the basaltic magmatism in Duoma, Tarenben villages in Bangong Co-Nujiang suture zone may occurred in the late of Early Cretaceous (～110 Ma) indicated by the SHRIMP zircon and Ar-Ar age, which is consistent with the fossil constraint. The Mesozoic basalts exposed in different ophiolitic melange zones have OIB affinities rather than MORB-type in composition, in which the geochemical features of Gaco basalts are transitional between Emeishan high-Ti basalt and Hawaiian alkaline basalt, the Mid-Late Triassic basalts in Gajia Formation from western Nagqu basin are closely geochemically similar to Hawaiian alkaline basalt and the Early Cretaceous basalts in Duoma, Tarenben villages from Bangong Co-Nujiang suture zone are largely comparable to those of Hawaiian alkaline basalt. With consideration of geochemical constraint and sedimentary sequence, we propose that Gaco OIB-type basalts may form at a rift setting constructed on accretionary wedge rather than an intra-plate on oceanic crust, and that Gajia Formation basalts are probably erupted at a rift setting on continental shelf-slope floored by intra-and fore-arc basin sediments. Duoma and Tarenben basalts in Bangong Co-Nujiang suture zone are interpreted to have formed in an oceanic island setting, which may indicate that the Bangong Co-Nujiang oceanic crust is still presented at about 110 Ma, implying that the closure time of this ocean is considerably lag behind the Late Jurassic-Early Cretaceous proposed by previous works. Geochemical signatures indicate that crust contamination and sub-continental lithospheric mantle material played insignificant role in the generation of the Mesozoic basalts in central Qinghai-Tibetan Plateau, it is therefore possible that the Nd, Pb isotopic compositions of some relative fresh samples can be used to represent the characteristics of mantle source region for the Mesozoic basalts in central Qinghai-Tibetan Plateau. The present available data show that “"Dupal” anomaly is likely not presented in the Mesozoic Tethyan mantle beneath central Qinghai-Tibetan Plateau at that time according to the high ~ 206 Pb/ ~ 204 P ratios (18.5). However, this observation should be further confirmed when more data are available.