东北准噶尔哈旦逊石炭纪弧岩浆及堆晶柱形成机制

Generation Mechanism of Carboniferous Arc Magma and Cumulate Column in Middle Arc Crust,Hadanxun of Northeast Junggar

为了阐明东北准噶尔弧岩浆形成机制和破译中地壳P波高速块体,对哈旦逊石炭纪侵入杂岩进行了锆石U-Pb年龄(324.8~323.1 Ma)和δ^(18)O及ε_(Hf)(t)、矿物化学、主微量元素和Nd-Sr同位素分析.其稀土元素分布模式上凹,从闪长岩(夹带角闪石岩堆晶)到二长花岗岩的主微量元素变异可以用两阶段分离结晶模拟.结果显示侵入杂岩高Sr/Y、低Y,锆石δ^(18)O加权平均值为6.42‰,来源于俯冲洋壳板片.运用角闪石全铝压力计计算得到角闪石岩堆晶种群1、种群2的结晶深度分别为26~22 km、20~18 km,表明堆晶构成了中地壳P波高速块体.侵入杂岩具有高的正的锆石ε_(Hf)(t)(加权平均值为12.99)、ε_(Nd)(t)(6.22~6.55)和(^(87)Sr/^(86)Sr)i(0.7037~0.7040),源岩不含洋壳残片和阿尔泰陆壳物质.东北准噶尔是一个洋内弧,大洋板块的俯冲持续到石炭纪.

Secondary-ion mass spectrometric U-Pb age(324.8-323.1 Ma)andδ^(18)O and laser-ablation multi-collector ICP-MSε_(Hf)(t)of zircons,mineral chemistry and whole-rock geochemistry and Nd-Sr isotopes of the Carboniferous intrusive complex of Hadanxun were analyzed in this study in order to expound the mechanism of arc magma generation and decipher high P-wave velocity body in the middle crust of Northeast Junggar.Their rare earth element distribution patterns are concave-up.The major and trace element variations from diorite(with entrained hornblendite cumulate)to monzogranite,with decreasing Dy/Yb and primitive-mantle normalized NbN/TaN,can be reproduced with quantitative modeling of fractional crystallization of hornblendite(Stage 1)and feldspar-dominated minerals(Stage 2).They exhibit high Sr/Y,low Y and mean zirconδ^(18)O of 6.42‰,suggesting derivation from subducting oceanic-slab.Al-in-hornblende barometer reveals that the hornblendite cumulate(population 1:26-22 km depth;population 2:20-18 km)constitutes high P-wave velocity body in the middle crust of Northeast Junggar.The Carboniferous intrusive complex of Hadanxun exhibits high positive zirconε_(Hf)(t)(weighted mean=12.99)and depletedε_(Nd)(t)(6.22-6.55)and(^(87)Sr/^(86)Sr)i(0.7037-0.7040),precluding oceanic crust and continental-crustal materials from Chinese Altay as source component.Thus,the Northeast Junggar was an intra-oceanic arc and the oceanic-plate subduction continued until the Carboniferous.