Metasomatized lithosphere–asthenosphere interaction during slab roll-back: Evidence from Late Carboniferous gabbros in the Luotuogou area, Central Tianshan
Metasomatized lithosphere–asthenosphere interaction during slab roll-back: Evidence from Late Carboniferous gabbros in the Luotuogou area, Central Tianshan
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DOI：
10.1016/j.lithos.2012.08.015
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发表时间：
2012-12
期刊：
影响因子：
3.5
通讯作者：
G. Tang;Qiang Wang;D. Wyman;Zheng‐Xiang Li;Yigang Xu;Zhenhua Zhao
中科院分区：
文献类型：
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作者：
G. Tang;Qiang Wang;D. Wyman;Zheng‐Xiang Li;Yigang Xu;Zhenhua Zhao
晚石炭世火成岩在天山西部分布广泛，但其形成的构造背景仍有争议。本文报道了骆头沟地区辉长岩的斜长石40ar /39Ar年龄、地球化学、Sr-Nd同位素和LA-ICPMS斜辉石微量元素数据。洛头沟辉长岩的年龄为晚石炭世（312±1Ma）40Ar/39Ar, εNd(t)值在+3.7 ~ +7.8之间变化较大。具有板块内岩浆岩和岛弧岩浆岩的地球化学特征，即TiO2含量较高（0.6 ~ 2.2wt）。Nb （4.2 ~ 24ppm）和Zr （51.4 ~ 283ppm）含量变化，Nb/La比值略高（0.24 ~ 1.8，大部分为> ~ 0.7），Nb呈负向正异常。辉长岩含分带斜辉长岩，岩心富镁、富铬。根据富cr （>3000ppm）斜辉石岩心微量元素数据和斜辉石/玄武岩液体分配系数计算，其母岩浆显示出不相容元素的富集，以及显著的负至微正Nb异常，表明地幔源中俯冲相关成分的影响。这些特征表明，骆头沟辉长岩极有可能是软流圈与交代岩石圈地幔相互作用形成的。它们最可能是由软流圈幔源熔体和交代岩石圈幔源熔体混合形成的。混合是由俯冲的古准噶尔大洋板块回滚引发的软流圈上升流的结果，而不是晚石炭世与地幔柱相关的裂陷或碰撞后断裂的结果。
Late Carboniferous igneous rocks are widespread in the western Tianshan, but the tectonic settings for these rocks remain controversial. We report a plagioclase40Ar/39Ar age, and geochemical, Sr–Nd isotope and LA–ICPMS clinopyroxene trace element data for gabbros in the Luotuogou region. The tholeiitic Luotuogou gabbros give a Late Carboniferous (312±1Ma)40Ar/39Ar age and are characterized by high and variable εNd(t) values ranging from +3.7 to +7.8. They have geochemical features of both intra-plate and island arc magmatic rocks, i.e., relatively high TiO2(0.6–2.2wt.%), Nb (4.2–24ppm) and Zr (51.4–283ppm) contents combined with variable and slightly high Nb/La ratios (0.24–1.8, mostly>0.7), and negative to positive Nb anomalies. The gabbros contain zoned clinopyroxenes, with Mg- and Cr-rich cores. Their parental magmas, as calculated using trace element data from Cr-rich (>3000ppm) clinopyroxene cores and clinopyroxene/basaltic liquid partition coefficients, show enrichments in incompatible elements, and prominent negative to slightly positive Nb anomalies, indicative of the influence of subduction-related compositions in their mantle source. These features indicate that the Luotuogou gabbros were most likely formed by interactions between asthenospheric and metasomatized lithospheric mantle. They were most plausibly formed by mixing between the asthenospheric mantle-derived and metasomatized lithosphere mantle-derived melts. Mixing was the result of asthenosphere upwelling triggered by roll-back of the subducted Paleo-Junggar Oceanic Plate rather than mantle plume-related rifting or post-collisional break-off during the Late Carboniferous.