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首页> 外文期刊>Journal of Asian earth sciences >Relative contributions of crust and mantle to the generation of the Tianshan Carboniferous rift-related basic lavas, northwestern China
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Relative contributions of crust and mantle to the generation of the Tianshan Carboniferous rift-related basic lavas, northwestern China

机译：地壳和地幔对中国西北天山石炭系裂谷相关基本熔岩生成的相对贡献

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摘要

The Tianshan Carboniferous-Permian rift-related volcanism in northwestern China represents a newly recognized large igneous province extending over at least 1.5 × 10~6 km~2. The volcanic successions comprise thick piles of basaltic lavas and subordinate intermediate and silicic lavas and pyroclastics, and are interpreted to result from a mantle plume head with component of ε_(Nd)(t) ≈ +5, ~(87)Sr/~(86)Sr(t) ≈ 0.704 and La/Nb ≈ 0.9. On the basis of petrogeochemical data, the Carboniferous basic lavas can be generally incorporated into low-Ti/Y (LT, Ti/Y < 500) magma type that can be further divided into three subtypes: LT1, LT2 and LT3. The chemical evolution of the LT1, LT2 (in central Tianshan) and LT3 (in western Tianshan and Jungar) lavas is controlled by an olivine (ol) + clin-opyroxene (cpx) fractionation, but gabbroic fractionation accounts for the chemical variation of the LT3 lavas from eastern Tianshan. Elemental and isotopic data suggest that the chemical variation of Tianshan Carboniferous basic lavas cannot be explained by crystallization from a common parental magma. The Sr-Nd isotopic variation of the crustally contaminated LT3 lavas is related to the nature of lithosphere through which the plume-derived melts have erupted. The involvement of an older (Precambrian) lithosphere led the LT3 lavas in western Tianshan to have lower to negative ε_(Nd)(t) (-1-2 to +6.1) and variable ~(87)Sr/~(86)Sr(t) (0.7036-0.7061), whereas the LT3 lavas from eastern Tianshan and Jungar are characterized by high ε_(Nd)(t) (+4.2 to + 9.7) and low ~(87)Sr/~(86)Sr(t) (0.7035-0.7044), that are related to the contamination of upper crust containing early Paleozoic and Devonian arc-basin volcanic rocks and/or to a pre-Carboniferous subduction enrichment of the litho-spheric mantle source region. The observed geochemical variations in the Tianshan data are consistent with an AFC process. The Tianshan Carboniferous rift-related volcanic rocks display a spatial petrogeochemical variation in which predominantly uncon-taminated LT1 and less-contaminated LT2 tholeiitic lavas erupted in central Tianshan rift and predominantly the strongly contaminated LT3 tholeiites erupted in the circumjacent regions of the central Tianshan rift. The LT1 and LT2 lavas were generated by a higher degree (10-30%) of partial melting in the garnet stability field of the mantle plume compared to the LT3 lavas. The lower degree ( < 10%) of partial melting in the spinel-garnet transition zone of the mantle plume, as is characteristic of the LT3 lavas, may be the result of a relatively lower geotherm.
机译：中国西北部与天山石炭纪-二叠纪裂谷有关的火山活动是一个新近认可的大火成岩省，至少延伸了1.5×10〜6 km〜2。火山演替过程包括厚厚的玄武岩熔岩，次要的中，硅质熔岩和热碎屑岩，并解释为是由地幔柱顶形成的，其地幔成分为ε_（Nd）（t）≈+5，〜（87）Sr /〜（ 86）Sr（t）≈0.704和La / Nb≈0.9。根据岩石化学数据，通常可以将石炭纪的基本熔岩合并为低Ti / Y（LT，Ti / Y <500）岩浆类型，可以进一步分为三种亚型：LT1，LT2和LT3。 LT1，LT2（在天山中部）和LT3（在天山中部和准Jun尔）熔岩的化学演化受橄榄石（ol）+克林-芴（cpx）分级控制，但是辉长岩分级解释了熔岩的化学变化。来自天山东部的LT3熔岩。元素和同位素数据表明，天山石炭纪碱性熔岩的化学变化不能用普通母岩浆的结晶来解释。地壳污染的LT3熔岩的Sr-Nd同位素变化与岩石圈的性质有关，通过该岩石圈喷出了羽状熔体。较旧的（前寒武纪）岩石圈的介入导致天山西部的LT3熔岩具有较低至负的ε_（Nd）（t）（-1-2至+6.1）和可变的〜（87）Sr /〜（86）Sr （t）（0.7036-0.7061），而来自天山东部和准gar尔的LT3熔岩的特征是ε_（Nd）（t）高（+4.2至+ 9.7）和低〜（87）Sr /〜（86）Sr（ t）（0.7035-0.7044），与含有早古生代和泥盆纪弧盆火山岩的上地壳的污染和/或岩石圈地幔源区的前石炭纪俯冲富集有关。天山数据中观测到的地球化学变化与AFC过程一致。与天山石炭纪裂谷有关的火山岩表现出空间岩石化学变化，其中主要是未污染的LT1和污染较少的LT2熔岩喷发于天山中部裂谷，主要是受严重污染的LT3冲蚀岩在中天山裂谷周围喷出。与LT3熔岩相比，LT1和LT2熔岩是通过地幔柱石榴石稳定性场中较高程度的部分熔融（10-30％）生成的。地幔柱的尖晶石-石榴石过渡带中较低的部分融化程度（<10％），这是LT3熔岩的特征，可能是相对较低的地热导致的。

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