造山带榴辉岩的变质作用P-T-t轨迹研究进展

Recent progresses in metamorphic P-T-t path of orogenic eclogites

板块俯冲和折返是板块构造理论的重要组成部分,也是固体地球科学研究的一个前沿方向。俯冲/碰撞造山带内的高压-超高压变质岩经历了俯冲至地球深部而后折返至地壳浅部的复杂地质过程,反演它们的P-T-t轨迹是高压-超高压变质作用研究的重要课题,也是揭示造山带构造演化的一个重要途径。造山带榴辉岩作为高压-超高压变质岩中的一类典型代表,常保留多个变质阶段的矿物组合,较完整地记录了造山带演化的关键信息,是反演P-T-t轨迹的理想样品。根据详细的岩相学观察和矿物成分分析,能识别出榴辉岩不同变质阶段的矿物组合。在此基础上,选取合适的地质温压计或采用相平衡模拟,可获得岩石相应变质阶段的温压条件(P-T)。如何获得多阶段变质年龄(t)和如何匹配变质作用的温压条件和年龄信息,是反演榴辉岩P-T-t轨迹的两个关键问题。本文针对榴辉岩中的锆石、金红石、榍石和褐帘石这四个适用于U-Pb定年的副矿物,从结晶条件和封闭温度等角度评估它们可能记录的年龄信息。结果表明,受限于结晶的温压条件或结晶后的改造作用,单种矿物定年难以获得多阶段年龄,一些实际研究中需要采用多种矿物或多个同位素体系实施定年分析。对于将副矿物从岩石中分选出来再实施原位微区定年的方法,测定微量元素和包裹体矿物组合并不总能准确限定这些定年副矿物结晶于哪个变质阶段,这将影响副矿物年龄意义的解析。观察分析定年副矿物在榴辉岩中的结构,能够厘清它们与岩石主要组成矿物之间的成因联系,进而可通过主要组成矿物限定副矿物的结晶温压条件和年龄意义。因此,具有岩相学结构制约的副矿物原位定年分析(“岩石年代学”方法)是耦合变质岩石学(P-T)和地质年代学(t)结果的重要手段,应在今后的变质作用研究中得到更多的重视和运用。与此同时,进一步研究变质分解/重结晶、熔/流体交代、脆/韧性变形等地质作用对副矿物定年同位素体系的影响。变质作用的时间信息不仅包括通过同位素定年获得的绝对年龄,也包括变质作用所持续的时间。受制于定年测试的精度,一些短时间尺度(≤1Myr)的变质作用难以通过同位素年代学方法进行限制。基于矿物的成分扩散、结构演变、力学性质转变等模拟方法,可用于揭示和限定这类变质事件的时间尺度,以此精细刻画岩石的变质P-T-t轨迹,有助于深入理解造山带榴辉岩的形成过程和机制。

Slab subduction and exhumation are essential components of plate tectonic theory and they are important branches of solid earth science study.High pressure(HP)and ultrahigh pressure(UHP)metamorphic rocks in subduction/collision orogens have experienced complex geological processes involving subduction to deep earth followed by exhumation to shallow crust.Deciphering P-T-t path of HP-UHP metamorphic rocks is not only an important research topic in HP-UHP metamorphism,but also a key approach for revealing the tectonic evolution of subduction/collision orogens.Eclogite,one of the representative of HP-UHP metamorphic rocks,typically preserves several stages of mineral assemblage,which can integrally record crucial information of orogenic evolution.Therefore,orogenic eclogite can be served as a kind of ideal rock type for retrieving P-T-t path of HP-UHP metamorphism.Metamorphic mineral assemblage of different stages can be recognized through detailed petrographic observation and mineral composition analysis,thus the metamorphic P-T evolution of a rock can be decoded by using applicable thermobarometers and/or phase equilibria modelling.For an eclogite,two key problems are involved in depicting its P-T-t path,i.e.,how to obtain multistage metamorphic ages and how to link these metamorphic ages(t)to corresponding metamorphic conditions(P-T).In this paper,four potential accessory minerals(zircon,rutile,titanite and allanite)suitable for U-Pb dating in eclogites are reviewed in terms of their crystallization conditions and closure temperature,to evaluate the metamorphic ages they possibly record.The result shows that dating a single kind of accessory mineral can hardly obtain multistage metamorphic ages under some circumstances.Therefore,combined dating of several accessory minerals or applying multichronometric methods are potential solutions for the first problem above.For in situ dating of separated accessory minerals,trace elements and Raman mineral inclusion analyses are not adequate to constrain the crystallization conditions of the dating minerals in some cases.Detailed analysis of petrographic texture of the accessory minerals in the eclogites can clarify the genetic relationship between them and other major minerals,which further constrains their crystallization conditions and thus geological meanings of their ages.Therefore,texturally controlled in situ microanalysis(i.e.,petrochronology method)of the dating mineral is a key approach for coupling the results of the metamorphic petrology(P-T)and geochronology(t).The petrochronology method should be applied to future study more frequently.Meanwhile,further study should be carried out,in order to investigate the effects of some geological processes(e.g.,metamorphic decomposition/recrystallization,fluid/melt metasomatism,brittle/ductile deformation)on the isotopic systems of the accessory minerals.The timing of metamorphism includes not only the absolute age determined by radiometric geochronology,but also the timescale of the metamorphic process.Due to the limitations of analytical precision,some metamorphic processes with the short duration(≤1Myr)can hardly be resolved by radiometric dating methods.Modelling of compositional diffusion,textural evolution and visco-elastic rheology of some related minerals are potential methods to reveal these metamorphic events,which contributes to a refined metamorphic P-T-t path and comprehensive understanding on the petrogenesis of orogenic eclogites.