西藏罗布莎康金拉铬铁矿区地幔橄榄岩锆石SHRIMP U-Pb年龄及地质意义

本文对罗布莎蛇绿岩康金拉11号铬铁矿矿体的近矿围岩纯橄岩和方辉橄榄岩分别进行了锆石SHRIMP U-Pb年代学研究。结果显示两个样品的年龄都非常发散:纯橄岩的时代跨越早白垩世到太古宙,年龄125.6±2.6~1791±23Ma,4粒锆石的加权平均年龄为130.0±2.8Ma(2.1%)。而方辉橄榄岩的年龄208.9±3.7~2770±44Ma,从晚三叠世到太古宙,2粒锆石的加权平均值为209.7±5.2Ma(2.5%),锆石的复杂年代学结果反映该岩石经历的多阶段地质演化过程。激光拉曼分析显示纯橄岩和方辉橄榄岩中的锆石含磷灰石、长石和石英为主的地壳物质包裹体,暗示其地壳成因,记录了地幔经历了地壳混染过程。这些物质存在于由洋底扩张形成的蛇绿岩中,表明俯冲下去的大陆地壳物质在地幔循环中被运移到海底扩张脊并出露地表。结合我们在康金拉地幔橄榄岩和铬铁矿中发现的大量异常地幔矿物,可能显示了一个地壳物质俯冲至地幔,而后从洋脊回到地表的物质循环过程,佐证了豆荚状铬铁矿多阶段多期次的演化模式。

Zircon SHRIMP U-Pb Age of the Mantle Peridotite in the Kangjinla Chromite Deposit, Tibet and Its Geological Significance

SHRIMP U-Pb dating was performed on the dunite and harzburgite samples collected from the Kangjinla No.11 chromite in the Luobusha ophiolite, Tibet. Age results of the two samples are not coupled. Dunite sample has an age range from from 125.6±2.6 Ma to 1791±23 Ma (from Archean to Early Cretaceous), with a minimum weighted average value of 130.0±2.8 Ma (2.1%) for four zircon samples. Harzburgite has an age range from 208.9±3.7 Ma to 2770±44 Ma (from Archean to Late Triassic) with a weighted average value of 209.7±5.2 Ma (2.5%) for two zircon samples. The complex characteristics of zircon grains imply the multi-stage geological evolution. Laser Raman Spectroscopy analysis shows that zircons in dunite and harzburgite contain inclusions of crustal material consisting of apatite, feldspar and quartz, indicating its crustal genesis and recording a process of crustal contamination of the mantle. In combination with a large amount of unusual mantle minerals discovered in the Kangjinla chromite and mantle peridotite, there might be a cycle process that crustal material returned surface from oceanic ridge after subducting to mantle, which also proves a multi-stage formation model of the podiform chromitite.