Sm-Nd isotopic evidence on the age of eclogitization in the Zermatt-Saas ophiolite
Sm-Nd isotopic evidence on the age of eclogitization in the Zermatt-Saas ophiolite
复制标题
采尔马特-萨斯蛇绿岩中榴辉岩化年龄的 Sm-Nd 同位素证据
DOI：
10.1111/j.1525-1314.1994.tb00013.x
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发表时间：
1994
影响因子：
3.4
通讯作者：
A. Barnicoat
中科院分区：
文献类型：
--
作者：
S. Bowtell;R. Cliff;A. Barnicoat
西阿尔卑斯泽马特-萨斯蛇绿岩内的岩石在阿尔卑斯碰撞前的俯冲过程中经历了榴辉岩相变质作用。这些岩石的变质历史是明确的，榴辉岩组合之后是蓝片岩组合的有限生长，蓝片岩组合是蓝帘石和副长石。随后的绿片岩蚀变发生在断层、脉和变质沉积层附近。远离这样的水源，退化是非常有限的。对一个基本不退退的榴辉岩变质岩的Sm-Nd同位素分析表明，榴辉岩相变质作用发生在52±18 Ma。很大的不确定性是由于在石榴石中以包裹体的形式存在极少量的富钕绿帘石。由于认为石榴石对Sm和Nd的闭合温度为bb0 ~ 600dC，因此认为高压后扩散引起的复位不重要。考虑到分析样品的细粒原岩及其在榴辉岩相条件下的广泛变形，前变质同位素变化的不完全均匀化也被认为不太可能是年轻的原因。第三纪榴辉石化时代意味着基于白垩纪高压变质作用停止的早期阿尔卑斯演化模式需要修正。
Rocks within the Zermatt-Saas ophiolite of the western Alps have undergone eclogite facies metamorphism during subduction prior to the Alpine collision. The metamorphic history of these rocks is well defined, with eclogitic assemblages being followed by the limited growth of blueschist assemblages of glaucophane and paragonite. Subsequent greenschist alteration occurs adjacent to faults, veins and metasediments. Away from such sources of water, retrogression is very limited. Sm-Nd isotopic analyses of an essentially unretrogressed eclogitic metabasalt suggest that eclogite facies metamorphism occurred at 52 ± 18 Ma. The large uncertainty is due to the presence of very small amounts of Nd-rich epidote present as inclusions within garnet. As the closure temperature of garnet to Sm & Nd is thought to be >600dC, resetting due to post-high-pressure diffusion is thought to be insignificant. Given the fine-grained protolith to the sample analysed, and its extensive deformation under eclogite facies conditions, incomplete homogenization of pre-metamorphic isotopic variations is also considered unlikely to be responsible for the young age. A Tertiary age of eclogitization means that models of early Alpine evolution based on the cessation of high-pressure metamorphism in the Cretaceous need to be revised.