西藏东巧蛇绿岩中玄武质岩石成因和构造背景探讨
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本文为国家自然科学基金项目(41720104009)、中国地质调查局项目(DD20160023- 01)、中国地质调查局项目(DD201190060)和地质部行业基金项目(201511022)联合资助的成果。


Genesis and tectonic environment of basaltic rocks from the Dongqiao ophiolite in Tibet
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    摘要:

    蛇绿岩记录了大洋裂解、俯冲消减和大陆增生的一系列过程,西藏北部班公湖-怒江缝合带中的蛇绿岩记录了青藏高原的地体拼合和隆升历史。班公湖- 怒江缝合带中段的东巧蛇绿岩十分发育,地表出露较好,主要由地幔橄榄岩、堆晶杂岩、辉长辉绿岩和玄武岩组成。玄武岩呈多个露头产出,每个露头从十几平方米至数十平方米不等,辉绿岩呈脉状(或岩墙)产在玄武岩中。本文对东巧西出露的玄武岩和辉绿岩开展了详细的野外地质考察和室内岩相学、年代学和地球化学研究分析,探讨了该蛇绿岩的成因以及形成背景。研究表明,辉绿岩和玄武岩属低钾拉斑系列和钙碱性系列。辉绿岩轻稀土含量略高于N- MORB,兼具IAT和N- MORB的特征;玄武岩轻稀土相比重稀土稍富集,具有E- MORB的特征。样品的Nb/U值(30~48)均大于地壳平均值(10)以及(Th/Nb)N值(0. 58~0. 86)均小于1,表明岩石未遭受地壳混染。辉绿岩和玄武岩3个样品的锆石U- Pb年龄值均显示较大的变化区间,结合前人班怒带蛇绿岩年代学的研究成果,将其分为三组,第一组(156~239Ma)代表蛇绿岩自身的年龄,第二组(37~141Ma)为晚于蛇绿岩形成的年龄段,推测其为后期的岩浆事件成因,第三组(277~2454Ma)为早于蛇绿岩形成的年龄,推测其成因为原始地幔中残留的早期俯冲板片所携带的地壳中的锆石。通过岩石地球化学成分对比研究,认为辉绿岩和玄武岩为弧后盆地玄武岩(BABB)。结合前人地幔橄榄岩的研究,本文认为东巧玄武质岩石形成于俯冲带(SSZ)的弧后盆地扩张时期,并受到俯冲带流体不同程度的影响。

    Abstract:

    Ophiolites record a series processes of oceanic decomposition, subduction and continental accretion. Ophiolites of the Bangongco- Nujiang Suture in northern Tibet record the history of terrain assemblage and uplift of the Qinghai- Tibet Plateau. Ophiolites of the middle section of the Bangongco- Nujiang Suture is well developed and exposed, mainly composed of mantle peridotite, cumulate complex, gabbro diabase and basalt. Basalt occurs in several outcrops, each of which ranges from more than ten square meters to tens of square meters. Diabase occurs in veins (or dikes) in basalt. This study carried out detailed field geological investigation and laboratory lithofacies, chronology and geochemistry analysis of basalts and diabase outcropped in the west of Dongqiao to understand the origin and formation background of the ophiolite. Research shows that diabase and basalt belong to low- K tholeiite series and calc- alkaline series. The content of LREE in diabase is slightly higher than that in N- MORB, which has the characteristics of IAT and N- MORB. Basalt is enriched in HREE relative to LREE, and has the characteristics of E- MORB. Nb/U values of the samples (30~48) are higher than the average values of the crust (10) and (Th/Nb)N values are less than 1, indicating that the rocks were not contaminated by the crust. Zircon U- Pb ages of three diabase and basalt samples show large variation ranges. Combined with previous chronological study of ophiolites in the Bangongco- Nujiang Suture, it can be divided into three groups. The first group (156~239 Ma) represents the age of ophiolite itself. The second group (37~141 Ma) is later than the age of ophiolites formation, indicating that the formation of the second group (37~141 Ma)resulted from late magmatic events. The third group (277~2454 Ma)is earlier than the age of ophiolites formation, suggesting that the zircons are from the primitive mantle which carried by the early subduction plates from the crust. Comparative lithogeochemical study shows that diabase and basalt are considered to be back- arc basin basalt (BABB). Based on the previous studies of mantle peridotite, this study can conclude that basaltic rocks of Dongqiao formed during the spreading period of back- arc basin and was influenced by subduction zone fluids to varying degrees (SSZ).

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陈晓坚,杨经绥,董玉飞,熊发挥,卢雨潇,李观龙,薄容众.2019.西藏东巧蛇绿岩中玄武质岩石成因和构造背景探讨[J].地质学报,93(10):2509-2530.
CHEN Xiaojian, YANG Jingsui, DONG Yufei, XIONG Fahui, LU Yuxiao, LI Guanlong, BO Rongzhong.2019. Genesis and tectonic environment of basaltic rocks from the Dongqiao ophiolite in Tibet[J]. Acta Geologica Sinica,93(10):2509-2530.

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  • 收稿日期:2019-07-12
  • 最后修改日期:2019-08-16
  • 录用日期:2019-08-19
  • 在线发布日期: 2019-09-04
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