低δ^(18)O岩浆岩的成因

On the origin of low δ^(18)O magmatic rocks

具有显著低于正常幔源岩浆δ18O值的岩浆岩是地球上比较罕见的一种岩石,其形成需要特殊的岩浆过程。虽然岩浆分异过程基本上不改变氧同位素组成,但水岩反应可以明显改变岩石的氧同位素组成,高温水岩反应会导致岩石的氧同位素组成显著降低,这得到了理论计算的支持。对黄石高原流纹岩和冰岛玄武岩这两个典型的低δ18O岩浆实例的长期研究发现,在地表遭受了高温热液蚀变的低δ18O岩石部分熔融或者被岩浆同化是形成低δ18O岩浆的重要机制,这通常通过裂谷构造带破火山口的垮塌来实现。塞舌尔和碾子山代表了结晶于低δ18O岩浆的花岗岩实例,其形成需要源区有亏损18O的地壳物质参与。大别-苏鲁造山带超高压变质岩亏损18O的幅度和这些亏损18O岩石的分布面积都非常令人瞩目。对这些超高压变质岩的原岩进行氧同位素研究,能够为其这些低δ18O岩石的成因提供有力的制约。目前已经在北淮阳花岗岩中报道了具有新元古代年龄和低δ18O值的锆石。研究显示,这些花岗岩可能经历了两次高温水岩反应,第一次导致了低18O岩浆的形成和低δ18O值锆石的结晶,第二次则进一步降低了其它矿物的δ18O值,导致了氧同位素不平衡。对这些低δ18O花岗岩的深入研究,特别是进行原位微区分析,不仅有助于我们深入认识低δ18O岩浆的形成机理,而且有助于进一步理解水岩反应中元素和同位素的地球化学行为。

Magmatic rocks with δ18O values significantly lower than normal mantle-derived magmas can be generated by a series of special geological processes during the formation and evolution of crustal magmas.While there is negligible effect of magma differentiation on the oxygen isotope composition of magmas,theoretical calculation suggests that high-T water-rock interaction is a significant process to decrease the oxygen isotope ratios of silicate rocks.Decades of geochemical studies on two classical cases of low δ18O magmas,Yellowstone rhyolites and Iceland basalts,have demonstrated that the mechanism to form the low δ18O magmas is through the partial melting of low δ18O rocks or the assimilation of normal δ18O magmas by low δ18O rocks during magma emplacement.In either case,there are preexisting low δ18O rocks prior to the low δ18O magmatism.The low δ18O rocks can be generated by high-T hydrothermal alteration of surface water（seawater or meteoric water）.The low δ18O magmatism usually takes place in rifting tectonic zones,with development of caldera collapse and subsequent remelting of low δ18O rocks.The granites at Seychelles in Indian Ocean and Nianzhishan in Northeast China represent two examples of low δ18O granites crystallized from low δ18O magmas.Their formation calls for the involvement of 18O-depleted materials in the magma source.The extent of 18O-depletion in the Dabie-Sulu metamorphic rocks and their extensive occurrences is very remarkable.The most negative δ18O rock-forming minerals were found in granite in the Dabie orogen,but with positive δ18O values for zircons of Neoproterozoic U-Pb ages.It is inferred that the low δ18O granite would have experienced two-stage water-rock interaction at high temperatures.The first leads to the formation of low δ18O magma and thus the crystallization of low δ18O zircons,and the second is characterized by the high-T meteoric-hydrothermal alteration and thus the further decrease of δ18O values for rock-forming minerals.More studies on the low δ18O granites,particularly microscale in-situ oxygen isotope analyses,will be helpful to the understanding of the mechanism of low δ18O magmatism.This can place basic geochemical constraints on the elements and isotopes behavior in the proceses of water-rock interaction.