大别——苏鲁高压-超高压变质带的“构造增压壳内成因”研究

Studies of the formation of Dabie-Sulu high-and ultrahigh-pressure metamorphic(H/UHPM) belt from the “tectonic pressure increasing model caused in the crust”

研究发现,高压-超高压变质岩基本都发育于全球或区域规模的大型构造带,产于强烈构造变形带中相对较弱应变的部位。苏鲁—大别超高压变质岩具有明显的构造变形,这成为追索构造应力-应变作用和探讨形成构造环境与演化过程的重要线索。本研究用成岩成矿深度的"构造校正测算方法",进行野外观察、岩石变形-应力测算及构造校正测算。其结果显示,超高压变质岩形成深度在23~55km之间。大别—苏鲁超高压变质岩的锆石SHRIMP年龄显示,具有环带的内核高压矿物年龄大于680 Ma,而其含柯石英幔圈里超高压变质矿物在(231±4)Ma形成,角闪岩相等退变质矿物形成在(211±4)Ma,可见超高压变质发生在陆缘和陆内的地质环境。综合研究其岩石矿物的Sr-Nd、O和He同位素含量,有力证明了岩石的壳内成因特征。深钻孔岩心的岩石矿物学系统显示,超高压变质岩及其几公里宽范围里的各类围岩普遍含有柯石英等高压-超高压矿物包体。上述实际资料,用超高压变质的"深俯冲-折返"模式已经不能得到科学的解释。本文提出大别—苏鲁超高压变质岩"构造增压壳内成因模式",认为这些超高压变质作用可能发育在陆内地块之间的强烈构造挤压环境。在230Ma左右,由于构造压力与重力压力叠加致使局部达到超高压及相应温度等条件,特别是当p≥2.8GPa时,变质的物理化学条件得到满足,可以在23~55km深处发生超高压变质作用,之后经应力松弛和拆离构造,岩石又逐步抬升并发生退变质作用。也可以说,该超高压变质岩具有构造物理化学成因。

By studying the space-time distribution pattern,the high-/ultrahigh-pressure metamorphic（H/UHPM）rocks were mostly developed in the active tectonic zone in global scale.The rocks found in the Dabie-Sulu UHPM belt clearly show tectonic deformation.In the intense tectonic deformation belt,H/UHPM rocks developed relatively weaker deformation points.Field observation combined with rock deformation and stress calculation,together with structural correction calculation,yielded rock formation depth wear 23-55km.These geological data demonstrate that the tectonic structure played a major role in diagenesis.Furthermore,the SHRIMP age of Dabie-Sulu ultrahigh-pressure metamorphic rocks indicates the mineral band kernel age is greater than 680 Ma;and the ultrahigh-pressure metamorphic coesite curtain circle and amphibolite metamorphisms occurred wear at 231±4 Ma and 211±4 Ma,respectively.Visible,ultrahigh-pressure metamorphism occurs in the marginal and intra-continental geological environment,producing rocks whose Sr,Nd,O and He isotope content has the characteristics of the crust.UHPM rocks and surrounding rocks display such geological features that they generally contain high-pressure mineral inclusions such as coesite.These facts reveal that at about 230 Ma,the Dabie-Sulu body,after the generalized ultrahigh-pressure metamorphism had occurred,gradually raised up again and underwent retrogressive metamorphism.In this paper,on the Dabie-Sulu UHPM rock formation,the＂tectonic pressure increasing model causes in crust model＂ is proposed.It hypothesizes that the ultrahigh-pressure metamorphism and strong tectonic compression developing environment might only happen at the depth of 23-55km,where local ultrahigh pressure and corresponding temperatures are reached to the condition of tectonic stress superposed with gravity stress,and especially when at p≥2.8 GPa certain physicochemical conditions are satisfied.It can be stated that the tectonic physicochemical factors played a major role in UHPM rock diagenesis.