大别山超高压变质岩形成深度的同位素限制

大别山超高压变质岩形成深度是各国地质学家十分关心的问题。它不仅影响对碰撞造山带形成机制和演化过程的认识,而且影响对地球深部状况及地球动力学的研究。该文对大别山超高压变质岩已有同位素资料进行了分析与讨论。大别山榴辉岩的εNd为-6.2~-17,εSr为18~42,且显示明显的Nd同位素的不平衡现象。大别山榴辉岩的氧同位素组成研究表明,这些榴辉岩的原岩在超高压变质前,不同程度地与贫18O的大气降水(或海水)发生过氧同位素交换,且在超高压变质过程中依然保留了这些痕迹。除一个样品外,大别-苏鲁地区的榴辉岩的3He/4He比值都落在0.79×10-7~9.35×10-7范围内,显示陆壳岩石来源He的重要贡献。所有Sr-Nd、O和He同位素研究均表明:超高压变质岩保存着表壳岩石原岩的同位素特征,而未显示变质时受到地幔物质的明显影响。对于超高压变质岩的上述同位素特征,有人认为是由于大别山造山带俯冲和折返的速度太快造成的。由于造山带俯冲和折返的速度太快,表壳岩石原岩变质时来不及与地幔物质发生交换,故没有留下地幔物质参与的痕迹。该研究认为这种解释有些勉强,因为大别造山带俯冲和折返时间至少需要15Ma.在如此长的时间内,在100多公里地幔深处高于700℃的高温下发生超高压变质作用,表壳岩石原岩不可能不与地幔物质发生同位素交换。相反,如果认为大别山超高压变质岩就在地壳内形成,则大别山超高压变质岩同位素的所有特征就很好解释了。

ISOTOPIC CONSTRAINTS ON THE FORMATION DEPTH OF ULTRAHIGH PRESSURE METAMORPHIC ROCKS IN THE DABIE REGION

The formation depth of metamorphic rocks in the Dabie ultra-high pressure metamorphic (UHPM) zone is a question attrating attention of geologists in the world. It has some bearing not only on our understanding of formation mechanism and evolution precesses of collision orogenic belt,but also on studies on earth's interior and geodynamic processes.
In this study, the isotopic data of metamorphic rocks in the Dabie UHPM zone are analyzed and discussed to give constraints on the formation depth of in the Dabie UHPM zone.
The ε_sr of eclogite in the Dabie UHPM zone varies from 18 to 42,and ε_Nd from -6.1 to -17,showing remarkable isotopic disequilibrium. The oxygen isotope studies indicate that the protoliths of these UHPM rocks have experienced oxygen isotope exchange with meteoric water(or sea water) depleted in 18O before metamorphism and remained unchanged in the processes of metamorphism. Except for one from the Bixiling,all samples of eclogite from Dabie UHPM zone have the 3He/4He ratios falling within the range between 0.79×10-7 and 9.35×10-7,indicating an important contribution of He from continental crust. All studies of Sr,Nd,O and He isotopes indicate that the UHPM rocks have retained the isotopic characteristics of the original surface rocks. No significant infulence of mantle derived materials has been found in them.
Some researchers attributed the above isotopic characterisitics, to rapid subduction and exhumation that no isotopic exchange between the two would take place,with no remains of mantle materials in these UHPM rocks. However,this assumption is not justified with pressent knowledge. It was estimated that the whole process of UHPM has spanned a time of at least 15Ma. During such a long period,it seems impossible that no isotopic exchange between mantle materials and original surface rock would occur at a depth of ≥ 100km where the temperature would rise to ≥ 700℃.Conversely,the isotopic characteristics will be well explained by assuming that the UHPM occurred in the crust instead in the upper mantle.