榴辉岩石榴子石反应边的震击成因初探

doi:10.13745/j.esf.sf.2025.8.64

摘要/Abstract

摘要：

造山带高压变质岩分布于古地震断裂内及其附近。一般认为高压变质作用是在俯冲地壳中沿断裂浸入的流体作用下进行的。但是关于榴辉岩石榴子石反应边的研究表明它们是缺乏流体参与的变质反应的特征结构。山东青岛仰口榴辉岩中高压矿物的星射状生长结构,富含原岩矿物包裹体的筛状结构,以及斜长石假象和镁铁矿物之间的反应边结构,均反映出榴辉岩的不平衡结晶性质。这类结构指示岩石是在快速过程中形成的,具有动力变质作用的时长特点。斜长石假象的相图也表明,仰口榴辉岩是由缺乏自由流体的辉长岩转变而来的,矿物生长过程涉及的元素扩散必然是在固态介质中进行的。鉴于仰口的各种榴辉岩(反应边榴辉岩,榴辉岩角砾岩及碎裂岩脉)记录了冲击波挤压和释放波解压的过程,可以认为高压相变是受地震波冲击作用的驱动而不是俯冲过程中压力和温度缓慢升高所致。在冲击应力作用下,水不饱和岩石中元素扩散速率相对静态热驱动的固态扩散高出多个甚至十多个数量级,使高压矿物瞬时生长。

关键词: 反应边, 不平衡, 榴辉岩化, 反应速率, 震击, 固态扩散

Abstract:

High-pressure metamorphic rocks are distributed along fossil seismic faults in orogenic belts. It is generally believed that high-pressure metamorphism occurs due to fluid infiltration into deep faults within subducted slabs. However, studies of garnet coronas in eclogites show that these textures are characteristic of mineral growth through fluid-absent metamorphic reactions, reflecting disequilibrium during the formation of the eclogite assemblages. Phase diagram modeling of plagioclase pseudomorphs from various localities indicates that the high-pressure assemblages are H₂O-undersaturated. On the other hand, the corona textures, like the stellate and micropoikilitic textures, suggest crystallization in rapid processes. The rapid growth of high-pressure minerals under H₂O-undersaturated conditions requires unusually high diffusion and reaction rates, which can be explained by the effect of seismic shock waves. This is supported by accumulating evidence, here particularly referenced to data at Yangkou in the Chinese Su-Lu high-pressure metamorphic belt.

Key words: corona, disequilibrium, eclogitization, reaction rate, seismic shock, solid-state diffusion

中图分类号:

P578.947

杨建军. 榴辉岩石榴子石反应边的震击成因初探[J]. 地学前缘, 2025, 32(6): 276-285.

YANG Jianjun. Seismic shock induced formation of garnet coronas in eclogites: A perspective[J]. Earth Science Frontiers, 2025, 32(6): 276-285.

图/表 4

图1 辉长岩转变为榴辉岩的反应边结构(a据文献[25]修改;b引自文献[30]) a—山东青岛仰口含柯石英榴辉岩中斜长石假象(Jd+Ph+Ky细粒集合体)与辉石假象(Omp+Qz+Rt细粒集合体)之间的石榴子石反应边(矿物代号参照文献[29])。反应边由3个亚层构成,下方靠近斜长石的淡色亚层,中间含大量金红石细粒的褐红色亚层和上方靠近辉石假象只含少量金红石的褐红色亚层,后者含柯石英包裹体。退变质部分硬玉被Ab+Amp取代。b—极地乌拉尔变橄长岩中的反应边。从橄榄石到斜长石假象形成4层反应边,顺序为斜方辉石→石榴子石→单斜辉石→石榴子石。斜长石假象中针状黝帘石呈放射状集合体。

Fig.1 Corona textures in eclogitized gabbros. a modified after [25]; b adapted from [30].

图2 等温(500 ℃)下斜长石假象的p-M(H2O)(压力-水含量(摩尔分数))假切面(引自文献[26]) 用于计算的总组成为:SiO2含量65.00%, Al2O3含量16.00%, CaO含量5.76%, MgO含量1.27%, FeO含量2.79%, K2O含量0.87%, Na2O含量8.01%, Fe2O3含量0.30% (构成NCKFMASHO体系)。水饱和线以下(其左边不含H2O相的区域),含水量最高的矿物组合以钠云母为特征,含水量最低的矿物组合以钾长石为特征。观察到的变辉长岩和含石英反应边榴辉岩斜长石假象中的矿物组合分别位于水含量在2%~3%(摩尔分数)及压力为0.82~1.12 GPa和大于1.26 GPa的区域。

Fig.2 Isothermal p-M(H2O) pseudosection at 500 ℃ for the plagioclase pseudomorphs. Adapted from [26].

图3 仰口含柯石英榴辉岩角砾岩石榴子石中金红石构成的“爆炸结构”(引自文献[52]) a—仰口榴辉岩角砾岩石榴子石碎裂颗粒中呈放射状排布的针状、棒状和粒状金红石构成的“爆炸结构”(单偏光);b—同a(正交偏光)

Fig.3 Explosion pattern formed by rutile granules in a garnet grain in the coesite-bearing eclogite breccia at Yangkou. Adapted from [52].

图4 仰口榴辉岩石榴子石反应边形成过程示意图 a—辉长岩原岩由普通辉石+斜长石+石英构成;b—震击非晶化阶段(详见正文),辉石假象和斜长石假象之间存在Mg、Ca、Fe、Na、Al和Si等元素的化学位梯度驱使它们向低化学位方向扩散;c—非晶质结晶为榴辉岩。辉石假象结晶为绿辉石+柯石英+金红石,斜长石假象结晶为硬玉+多硅白云母+蓝晶石+柯石英(退变质部分硬玉为富钠斜长石+角闪石取代),二者之间形成石榴子石反应边。此处的矿物组合分布参照图1a。全部石英颗粒均假设由柯石英退变而来。

Fig.4 Schematic illustration for the formation of garnet coronas between augite and plagioclase during eclogitization of a gabbro at Yangkou

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