Formation of low-δ18O magmas of the Kangerlussuaq Intrusion by addition of water derived from dehydration of foundered basaltic roof rocks
Formation of low-δ18O magmas of the Kangerlussuaq Intrusion by addition of water derived from dehydration of foundered basaltic roof rocks
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DOI：
10.1007/s00410-015-1134-7
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发表时间：
2015-04
影响因子：
3.5
通讯作者：
M. Riishuus;C. Harris;D. Peate;C. Tegner;J. Wilson;C. Brooks
中科院分区：
文献类型：
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作者：
M. Riishuus;C. Harris;D. Peate;C. Tegner;J. Wilson;C. Brooks
东格陵兰Kangerlussuaq侵入体从边缘的石英正长岩(石英正长岩)到中心的辉长岩(霞石正长岩)呈同心分带，没有明显的侵入接触。在岩浆温度下，共生矿物的δ~(18)O值符合氧同位素平衡。大多数侵入岩形成于低δ~(18)O岩浆，岩浆δ~(18)O值一般从石英诺尔马克岩中的3.3K‰上升到Foyaite中的5.6T‰。岩浆δ~(18)O值最低，约为−_(1.0)‰，来自诺尔马岩的上部，那里有高密度的熔融玄武岩包体(从侵入体的顶板停止)。正长岩中的角闪石的δD值从典型的含水地幔矿物的值到低得多的值(−86到−157‰)，包体和围岩的全岩样品也是如此(−125到−148‰)。较低的岩浆δ18O和δD值与采场玄武岩顶板释放的低δ18O和δD流体的持续并入、交换和向上逃逸相一致。物质平衡表明，所涉及的总水量为岩浆体积的7%，但局部达到30%。只有当沉没的玄武岩中含有大部分水而不是含水矿物时，才能满足对低δ18O值的大量水的需求。即使有了这一要求，停止的玄武岩的体积也将等于岩浆的体积。残留岩浆与污染程度逐渐减少的二氧化硅不饱和熔体的反复补充，导致了低压热分水岭的逐渐转移。残余岩浆(pH_2O~1)含水饱和，使液相线下降，抑制了结晶作用。
The Kangerlussuaq Intrusion in East Greenland is concentrically zoned from quartz nordmarkite (quartz syenite) at the margin, through pulaskite, to foyaite (nepheline syenite) in the centre, with no apparent intrusive contacts. The δ18O values of coexisting minerals are consistent with oxygen isotope equilibrium at magmatic temperatures. Most of the intrusion formed from low-δ18O magma; magma δ18O values generally increased upwards from about 3.3 ‰ in the quartz nordmarkites to 5.6 ‰ in the foyaites. The lowest magma δ18O value of about −1.0 ‰ is from the upper part of the nordmarkites, where there is a high concentration of foundered basaltic xenoliths (stoped from the roof of the intrusion). The amphiboles in the syenites have δD values that range from those typical of hydrous mantle-derived minerals to much lower values (−86 to −157 ‰), as do whole-rock samples of xenolith and country rock (−125 to −148 ‰). The low magma δ18O and δD values are consistent with continuous incorporation, exchange and upward escape of low-δ18O and δD fluids released from stoped basaltic roof material. Mass balance suggests that the integrated amount of water involved was 7 wt% of the volume of the magma, but locally reached 30 wt% water. The requirement for large amounts of water with low δ18O value is satisfied only if the foundered basalt contained most of its water in cavities as opposed to hydrous minerals. Even with this requirement, the volume of stoped basalt would have been equal to the volume of the magma. Repeated recharge of the residual magma with progressively less contaminated silica undersaturated melt resulted in a gradual shift across the low-pressure thermal divide. Crystallisation was suppressed by the depression of the liquidus due to water saturation of the residual magma (pH2O ~1 kbar).