Geochemical and isotopic constraints on the age and origin of the Nidar Ophiolitic Complex, Ladakh, India: Implications for the Neo-Tethyan subduction along the Indus suture zone
Geochemical and isotopic constraints on the age and origin of the Nidar Ophiolitic Complex, Ladakh, India: Implications for the Neo-Tethyan subduction along the Indus suture zone
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DOI：
10.1016/j.tecto.2007.11.049
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发表时间：
2008-04
期刊：
影响因子：
2.9
通讯作者：
T. Ahmad;Tsuyoshi Tanaka;H. Sachan;Y. Asahara;R. Islam;P. Khanna
中科院分区：
文献类型：
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作者：
T. Ahmad;Tsuyoshi Tanaka;H. Sachan;Y. Asahara;R. Islam;P. Khanna
该缝合带被认为是印度板块相对于亚洲板块向北移动时通过俯冲闭合的新特提斯残余洋。这两个板块最终在始新世中期相撞。Nidar蛇绿岩杂岩底部为超基性岩序列，中部为辉长岩序列，顶部为火山-沉积组合。早期的研究认为Nidar蛇绿岩杂岩代表了一个基于岩性组合的海底序列。然而，目前的研究基于新的矿物和全岩地球化学和同位素数据（关于大块岩石和矿物分离），表明它们的形成和就位是在洋内俯冲环境中。深成岩和火山岩的稀土元素（REE）模式接近平坦或略亏缺。辉长岩的稀土元素和蜘蛛图显示出强烈的Sr和Eu正异常，可能是斜长石聚集的标志。蜘蛛图中高场强元素（HFSE）的损耗可能与在俯冲板块中保留高场强元素的相的稳定和/或钛磁铁矿相的分离结晶有关。这些岩石的高放射性成因Nd同位素比和低放射性成因sr同位素比排除了大陆物质对其成因的任何影响，暗示了海洋内环境。9点矿物-全岩Sm-Nd等时线对应的年龄为140±32 Ma，初始143nd /144Nd为0.513835±0.000053 （ENdt=+7.4）。根据保存完好的放射虫化石，该年龄与上覆火山-沉积（含放射虫岩）序列的早白垩世Hauterivian（132±2 ~ 127±1.6 Ma）至Aptian（121±1.4 ~ 112±1.1 Ma）年龄一致（Kojima， S., Ahmad， T., Tanaka， T., Bagati， T.N., Mishra， M., Kumar， R. Islam， R., Khanna， P.P., 2001）。见：大阪微古生物学家新闻（NOM），特别卷，12,257-270 .)和基于39ar /40Ar的Nidar-Spontang蛇长岩的110-130 Ma冷却年龄（mah<s:1>， G., Berttrand， H., Guillot， S., Villa, i.m., Keller， F., Capiez， P., 2004）。化学。青烟。[医学杂志]，2003,273-303 .]由于这些辉长岩和火山岩被解释为与弧相关，新的Sm-Nd年龄数据可能表明新特提斯海洋的洋内俯冲可能在~ 140±32 Ma之前就开始了，因为这个日期被解释为弧岩浆的结晶年龄。目前和已发表的来自印度河缝合带的弧岩浆岩年龄数据可以共同表明，随着弧成熟度的增加，弧的岩浆活动是幕式的，从弧未成熟时的基性岩浆活动（~140±32 Ma），到弧未成熟时的中度（安山岩/花岗闪长岩）岩浆活动（~100 Ma），再到印度板块和亚洲板块碰撞时的~ 50-45 Ma的长英质（流纹岩/闪长岩）岩浆活动。
The Nidar ophiolite complex is exposed within the Indus suture zone in eastern Ladakh, India. The suture zone is considered to represent remnant Neo-Tethyan Ocean that closed via subduction as the Indian plate moved northward with respect to the Asian plate. The two plates ultimately collided during the Middle Eocene. The Nidar ophiolite complex comprises a sequence of ultra-mafic rocks at the base, gabbroic rocks in the middle and volcano-sedimentary assemblage on the top. Earlier studies considered the Nidar ophiolite complex to represent an oceanic floor sequence based on lithological assemblage. However, present study, based on new mineral and whole rock geochemical and isotopic data (on bulk rocks and mineral separates) indicate their generation and emplacement in an intra-oceanic subduction environment. The plutonic and volcanic rocks have nearly flat to slightly depleted rare earth element (REE) patterns. The gabbroic rocks, in particular, show strong positive Sr and Eu anomalies in their REE and spidergram patterns, probably indicating plagioclase accumulation. Depletion in high field strength elements (HFSE) in the spidergram patterns may be related to stabilization of phases retaining the HFSE in the subducting slab and / or fractional crystallization of titano-magnetite phases. The high radiogenic Nd- and low radiogenic Sr-isotopic ratios for these rocks exclude any influence of continental material in their genesis, implying an intra-oceanic environment. Nine point mineral–whole rock Sm–Nd isochron corresponds to an age of 140±32 Ma with an initial143Nd/144Nd of 0.513835±0.000053 (ENdt=+7.4). This age is consistent with the precise Early Cretaceous age of Hauterivian (132±2 to 127±1.6 Ma) to Aptian (121±1.4 to 112±1.1 Ma) for the overlying volcano-sedimentary (radiolarian bearing chert) sequences based on well-preserved radiolarian fossils (Kojima, S., Ahmad, T., Tanaka, T., Bagati, T.N., Mishra, M., Kumar, R. Islam, R., Khanna, P.P., 2001. Early Cretaceous radiolarians from the Indus suture zone, Ladakh, northern India. In: News of Osaka Micropaleontologists (NOM), Spec. Vol., 12, 257–270.) and cooling ages of 110–130 Ma based on39Ar/40Ar for Nidar–Spontang ophiolitic rocks (Mahéo, G., Berttrand, H., Guillot, S., Villa, I. M., Keller, F., Capiez, P., 2004. The South Ladakh Ophiolites (NW Himalaya, India): an intra-oceanic tholeiitic arc origin with implications for the closure of the Neo-Tethys. Chem. Geol., 203, 273–303.). As these gabbroic and volcanic rocks are interpreted to be arc related, the new Sm–Nd age data may indicate that intra-ocean subduction in the Neo-Tethyan ocean may have started much before ∼140±32 Ma as this date is interpreted as the age of crystallization of the arc magma. Present and published age data on the arc magmatic rocks from the Indus suture zone may collectively indicate episodic magmatism with increasing maturity of the arc from more basic (during ~140±32 Ma) when the arc was immature through intermediate (andesitic/granodioritic) at ~100 Ma to more felsic (rhyolitic/dioritic) magmatism at ~50–45 Ma, when the Indian and the Asian plates collided.