Formation of HP–LT rocks and their tectonic implications in the western Tianshan Orogen, NW China: geochemical and age constraints
Formation of HP–LT rocks and their tectonic implications in the western Tianshan Orogen, NW China: geochemical and age constraints
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DOI：
10.1016/s0024-4937(02)00153-6
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发表时间：
2003
期刊：
影响因子：
3.5
通讯作者：
Jun Gao;R. Klemd
中科院分区：
文献类型：
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作者：
Jun Gao;R. Klemd
中国西北部西天山高压低温变质带形成于伊犁-中天山板块南侧古生代增生楔内。它包括各种岩性，包括镁铁质变火山岩、变火山碎屑岩、变杂砂岩、大理石和蛇纹岩。变质火山岩包括榴辉岩、绿辉石、绿帘石和蓝片岩。这些岩石的微量元素和稀土元素特征表明，MORB到OIB的亲和力，建议这些岩石的原岩类似于沉积岩的设置。榴辉岩的ε Nd值（+8.9）表明其原岩来源于长期亏损的地幔，即N-MORB源区，支持了这些样品的微量元素和稀土元素特征。蓝片岩榴辉岩石香肠Sm-Nd等时线年龄为343±44 Ma（Omp-Gln-Grt-WR）和346±3 Ma（Grt-Gln）。从含绿辉石-多硅白云母的蓝片岩中获得了十字石的明确40 Ar/39 Ar平台年龄为344±1 Ma，与榴辉岩的Sm-Nd等时线年龄一致，表明氩损失或过量氩都没有影响十字石。同一样品中多硅白云母的~（40）Ar/~（39）Ar坪年龄为331±2 Ma。高压变质岩是由古生代南天山洋B型俯冲作用形成的。然而，他们被折返到更高的地壳水平，在塔里木和伊犁-中天山板块碰撞约331 Ma前，类似于“阿尔卑斯型”的构造体制。
The Western Tianshan high-pressure/low-temperature metamorphic belt in northwestern China formed within a Paleozoic accretionary wedge on the south side of the Yili–Central Tianshan plate. It comprises various lithologies, including mafic metavolcanic rocks, metavolcaniclastic rocks, metagreywacke, marble and serpentinite. The metavolcanics comprise eclogite, omphacitite, epidosite and blueschist. The trace and rare-earth element characteristics of these rocks indicate MORB to OIB affinities suggesting a seamount-like setting for the protoliths of these rocks. The εNdvalue of the eclogite (+8.9) indicates that the protolith was derived from a long-term depleted mantle, i.e. an N-MORB source, supporting the trace and rare-earth element characteristics of these samples. Sm–Nd isochron dating of 343±44 Ma (Omp–Gln–Grt–WR) and an age of 346±3 Ma (Grt–Gln) were determined using an eclogite boudin within blueschist. A well-defined40Ar/39Ar plateau age of 344±1 Ma was obtained for crossite from an omphacite–phengite-bearing blueschist and is concordant with the Sm–Nd isochron age for the eclogite, suggesting that neither argon loss nor excess argon affected the crossite. A well-defined40Ar/39Ar plateau age of 331±2 Ma was determined for phengite from the same sample. The high-pressure metamorphic rocks were formed by the B-type subduction of a Paleozoic ‘South Tianshan Ocean’ about 344 Ma ago. They were, however, exhumed to higher crustal levels during the collision of the Tarim and Yili–Central Tianshan plates about 331 Ma ago, similar to an ‘Alpine-type’ tectonic regime.