苏鲁造山带东北端石榴辉石麻粒岩的Sm-Nd同位素年代学及其大地构造含义

 大量含石榴石的基性麻粒岩透镜体出露于苏鲁变质带的北部及邻近地区,它们可能是再变质的高压变质岩石。在详细的岩相学研究的基础上,确定采自莱西和文登的样品WD01、WD04、ML06 是由高压麻粒岩经中-高压麻粒岩相再变质形成的,而采自威海的样品WH1 是由柯石英榴辉岩经中-高压麻粒岩相再变质形成的。Sm-Nd 同位素年代学研究也证实了二者的重大差别。3 个高压麻粒岩样品的矿物-全岩内部等时线年龄分别是1 846+ /-76Ma,1 743+ /-79Ma 和1 752+ /-30Ma,TDM 模式年龄是3.3Ga,3.0Ga 和2.8Ga.上述数据说明原岩形成在太古宙,而1 800Ma 是麻粒岩相降压变质事件的记录,这与华北克拉通前寒武纪高压麻粒岩的年代学一致。威海样品的Sm-Nd 同位素特征则完全不同。矿物和全岩形不成等时线,表现出它们之间的同位素不平衡。ε_Nd(0)值高达+ 127,TDM 模式年龄是1.3Ga.这与Jahn(1994,1996)对威海同类样品的测定结果相同。可以推测威海样品的原岩是元古宙岩石,在后来复杂的变质过程中,在水岩相互作用和岩浆及重熔作用的影响下,同位素系统发生重大变化。同位素年代学为苏鲁变质带和华北克拉通的界限是昆嵛山岩浆-变质杂岩带提供了依据。

Sm-Nd GEOCHRONOLOGY AND PETROGRAPHY OF GARNET PYROXENE GRANULITES IN THE NORTHERN SULU REGION AND THEIR GEOTECTONIC IMPLICATION

Abundant garnet-bearing granulite lenses are widely distributed in the northern part of the Sulu region and adjacent area. They are possibly re-metamorphosed high-pressure metamorphic rocks. On the basis of detailed petrographical study, samples WD01, WD04 and ML06 from Laixi and Wendeng were high-pressure granulites, and WH1 from Weihai was coesite-bearing eclogite. Three high-pressure granulite samples give Sm-Nd mineral-WR isochron ages of 1846+/-76Ma, 1743+/-79Ma and 1752+/-30Ma. TDM ages are 3.3Ga, 3.0Ga and 2.8Ga. The Sm-Nd mineral-WR isochron ages are interpreted as metamorphic age of middle-high pressure granulite facies, representing an isotopic re-homogeneity during uplifting of high-pressure granulites from deep continent crust. It is important that Sm-Nd chronological characteristics are same as Archaean high-pressure granulites in the North China craton. However, sample WH1 from Weihai demonstrates abnormal Sm-Nd characteristics. Its whole-rock ε_Nd(0)value is +129. TDM age is 1.3Ga, and constrains the minimum age of re-metamorphosed eclogite protolith formation to mid-Proterozoic. This result is identical to that reported by Jahn(1994), showing complicated process of metamorphism and metasomatism. The data of this paper further provide evidences to define the boundary between the North China craton and UHPM belt in eastern Shandong and to understand geotectonic nature of the boundary.