Formation of gabbronorites in the Purang ophiolite (SW Tibet) through melting of hydrothermally altered mantle along a detachment fault
Formation of gabbronorites in the Purang ophiolite (SW Tibet) through melting of hydrothermally altered mantle along a detachment fault
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DOI：
10.1016/j.lithos.2014.06.019
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发表时间：
2014-09
期刊：
影响因子：
3.5
通讯作者：
Chuan-Zhou Liu;Chang Zhang;Liu Yang;L. Zhang;Weiqiang Ji;Fu-Yuan Wu
中科院分区：
文献类型：
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作者：
Chuan-Zhou Liu;Chang Zhang;Liu Yang;L. Zhang;Weiqiang Ji;Fu-Yuan Wu
它包括一个约800平方公里的大型超镁铁质地块，但明显没有深部和火山剖面。普朗橄榄岩地块以黑斑岩为主，偶有辉长岩和玄武岩侵入，并有少量贫斜辉长岩。它们的特点是al2o3和CaO含量较低，可以用~ 15-25%的等压间歇熔炼在20 kbar的压力下模拟。尖晶石Cr#值估计为普朗橄榄岩的部分熔融程度约为12-19%。坡令橄榄岩中斜辉石具有可变的LREE模式，其HREE含量可以通过~ 10 ~ 16%的衰竭地幔源分馏熔融来重现。普朗辉长岩不含橄榄石，相对于斜长石具有较早的正辉石和斜辉石结晶。辉长岩中的辉石比橄榄岩中的辉长岩具有更多的可变Mg#。斜长石的特点是钙长石含量很高，为94-99。矿物组成表明，蒲朗辉长岩是由高硅含水熔体结晶而成，其不相容元素明显缺失。这种含水熔体可能来源于蛇纹岩化橄榄岩，是在软流圈上涌引发的缓慢扩张山脊上沿滑脱断层融化而成的。拆离断裂在海底掘出浦令橄榄岩地块为洋核杂岩，在缝合带内侵位保存。因此，我们认为普朗蛇绿岩形成于洋中脊，而不是与俯冲有关的环境。
The Purang ophiolite tectonically was thrust over an Upper Cretaceous mélange south of the Yarlung-Zangbo Suture Zone (YZSZ) in SW Tibet. It comprises a large ultramafic massif of ~ 800 km2, but plutonic and volcanic sections are notably absent. The Purang peridotite massif is dominated by harzburgites with minor clinopyroxene-poor lherzolites, which are occasionally intruded by gabbronoritic and basaltic dykes. They are characterized by low Al2O3and CaO, which can be modeled by ~ 15–25% degrees of isobaric batch melting at a pressure of 20 kbar. Spinel Cr# values give an estimated ~ 12–19% degrees of fractional melting for the Purang peridotites. Clinopyroxenes in Purang peridotites show variable LREE patterns and their HREE contents could be reproduced by ~ 10–16% degrees of fractional melting from a depleted mantle source. The Purang gabbronorites are olivine-free, with early crystallization of orthopyroxene and clinopyroxene relative to plagioclase. Pyroxenes in gabbronorites have more variable Mg# than those in peridotites. Plagioclases are characterized by very high anorthite contents of 94–99. Mineral compositions suggest that the Purang gabbronorites crystallized from high-silica hydrous melts, which are strongly depleted in incompatible elements. Such hydrous melts could be derived from serpentinized peridotite via melting along a detachment fault at a slow-spreading ridge that was triggered by the upwelling asthenosphere. The detachment fault exhumed the Purang peridotite massif as an oceanic core complex at the seafloor, which was emplaced and preserved in the suture zone. Therefore, we suggest that the Purang ophiolite was formed in the mid-ocean ridge rather than subduction-related settings.