Generation Mechanism of Carboniferous Arc Magma and Cumulate Column in Middle Arc Crust, Hadanxun of Northeast Junggar
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摘要:
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δ18O及εHf(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δ18O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εHf(t) (加权平均值为12.99)、εNd(t) (6.22~6.55)和(87Sr/86Sr)i (0.703 7~0.704 0)，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.
Abstract:Secondary-ion mass spectrometric U-Pb age (324.8-323.1 Ma) and δ18O and laser-ablation multi-collector ICP-MS εHf(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 zircon δ18O 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 zircon εHf(t) (weighted mean=12.99) and depleted εNd(t) (6.22-6.55) and (87Sr/86Sr)i (0.703 7-0.704 0), 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.
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图 1 东北准噶尔(a)和哈旦逊侵入杂岩(b)地质简图
Fig. 1. Geological sketch maps of the Northeast Junggar (a) and the Hadanxun intrusive complexes (b)
图 2 哈旦逊侵入杂岩南岩体的野外关系
a. 岩性从山丘顶部的灰色二长闪长岩渐变到山脚的肉红色石英二长岩；b. 灰色闪长岩变化到较浅色的二长闪长岩，闪长岩夹带着直径约3 m的深色角闪石岩堆晶的碎块
Fig. 2. Photos showing field relation of the southern pluton
图 3 哈旦逊石炭纪侵入杂岩的显微照片所展示的岩相特征
南岩体：闪长岩(图a)、闪长岩夹带的角闪石岩堆晶HDX79(图b和图c)、二长闪长岩(图d)、石英二长岩(图e)；哈旦逊北岩体北西侧的二长花岗岩(图f). 在图c左边，种群2(population 2)角闪石与种群1(population 1)角闪石呈压痕状边界，并且轻微交代后者. 笔圈标记电子探针分析靶点. 矿物代号：Plg.斜长石；Kf.钾长石；Hb.角闪石；Bt.黑云母；Qz.石英；Ch.微斜‒条纹长石；Mt.磁铁矿；Ilm.钛铁矿；Oxd.金属氧化物
Fig. 3. Photomicrographs showing petrography of the Carboniferous intrusive complex of Hadanxun
图 4 哈旦逊石炭纪侵入杂岩的锆石U-Pb谐和图
Fig. 4. U-Pb concordia diagrams of zircons from the Carboniferous intrusive complex of Hadanxun
图 5 哈旦逊石炭纪侵入杂岩的锆石的阴极发光图像所揭示的岩相学特征
每个阴极发光图像包含以下信息：初次流喷溅区域(长方形)和分析锆石的编号及其测定206Pb/238U年龄或δ18O测定值(括弧中数值)；激光剥蚀区域(红色圆圈)和分析锆石的编号及其εHf(t)测定值(括弧中红色数值)
Fig. 5. Cathodoluminescence images showing petrographic characteristics of zircon from the Carboniferous intrusive complex of Hadanxun
图 6 哈旦逊石炭纪侵入杂岩的锆石的εHf(t)±2σ (a)和δ18O±2σ (b)相对概率密度分布图
Fig. 6. Relative probability density plots of zircon εHf(t)±2σ (a) and δ18O±2σ (b) from the Carboniferous intrusive complex of Hadanxun
图 7 哈旦逊石炭纪侵入杂岩的长石的An (钙长石): Ab (钠长石): Or (正长石)三角图(a)和每个黑云母分子的Mg vs. (Fe2++ Fe3++Mn)离子协变图(b)
Fig. 7. The triangular diagram of An (anorthite): Ab (albite): Or (orthoclase) in feldspar (a) and Mg vs. (Fe2++ Fe3++Mn) ions per formular of biotite (b) from the Carboniferous intrusive complex of Hadanxun
图 8 哈旦逊石炭纪侵入杂岩的角闪石的每矿物分子的Si vs. Mg/(Mg + Fe2+)(a)、Si/Altotal vs. (Na + K)A(b)及Mg/(Mg + Fe2+) vs. Al2O3 (c)图解
Fig. 8. Classification diagrams of Si vs. Mg/(Mg + Fe2+) (a), Si/Altotal vs. (Na + K)A (b) and Mg/(Mg + Fe2+) vs. Al2O3 (c) of hornblendes from the Carboniferous intrusive complex of Hadanxun
图 9 哈旦逊石炭纪侵入杂岩的Al2O3 (a)、MgO (b)、TFe2O3 (c)、TiO2 (d)和CaO (e)与SiO2的协变图
Fig. 9. Diagrams of Al2O3 (a), MgO (b), TFe2O3 (c), TiO2 (d) and CaO (e) vs. SiO2 from the Carboniferous intrusive complex of Hadanxun
图 10 哈旦逊石炭纪侵入杂岩的Sr (a)、Cr (b)、Ni (c)和V (d)与SiO2的协变图
Fig. 10. Diagrams of Sr (a), Cr (b), Ni (c) and V (d) vs. SiO2 from the Carboniferous intrusive complex of Hadanxun
图 11 哈旦逊石炭纪侵入杂岩的球粒陨石标准化稀土元素分布模式(a)和Dy/Yb vs. SiO2协变图(b)
图a据Boynton，1984；第一阶段和第二阶段分离结晶的残余熔体的微量元素含量模拟值($ {C}_{\mathrm{l}}^{1} $、$ {C}_{\mathrm{l}}^{2} $)也展示于图a
Fig. 11. Chondrite-normalized rare earth element distribution pattern (a) and Dy/Yb vs. SiO2 covariation diagram (b) of the Carboniferous intrusive complex of Hadanxun
图 12 哈旦逊石炭纪侵入杂岩的多元素原始地幔标准化微量元素蛛网图
图a据Sun and McDonough，1989；第一阶段和第二阶段分离结晶的残余熔体的微量元素含量模拟值($ {C}_{\mathrm{l}}^{1} $、$ {C}_{\mathrm{l}}^{2} $)分别展示于图b和图c
Fig. 12. Multi-element primitive mantle-normalized trace element spider diagrams of the Carboniferous intrusive complex of Hadanxun
图 13 哈旦逊石炭纪侵入杂岩的Sr/Y-Y协变图
埃达克岩和岛弧火山岩的投影区域据Defant and Drummond (1990)
Fig. 13. Sr/Y vs. Y diagram of the Carboniferous intrusive complex of Hadanxun
图 14 哈旦逊石炭纪侵入杂岩的εNd(t) vs. (87Sr/86Sr)i图及其与其他岩石的投影范围对比
a. 东北准噶尔的浅部地壳岩石；b. 阿尔曼太蛇绿岩(数据来自黄萱等，1997)、阿尔泰陆壳物质(Wang et al.，2009)及阿尔泰南缘的玛因鄂博强过铝质花岗岩基(t = 283 Ma)(周刚，2007). 图a的对比岩石包括沿着F3断裂处于哈旦逊北北西~25 km的老山口中泥盆统北塔山组火成岩(Liang et al.，2016)、东北准噶尔索尔库都克的托让格库都克组埃达克岩和富Nb玄武岩(Niu et al.，2006)、东准噶尔西北部的石炭纪花岗岩类(Liu et al.，2019). MORB和OIB的区域(Zindler and Hart，1986)也显示于图a、图b. 所有火成岩的εNd(t)和(87Sr/86Sr)i值都校正到其结晶年龄，除了托让格库都克组的埃达克岩和富Nb玄武岩及阿尔曼太蛇绿岩，后两者的εNd(t)和(87Sr/86Sr)i值校正到290 Ma
Fig. 14. εNd(t) vs. (87Sr/86Sr)i diagram of the Carboniferous intrusive complex of Hadanxun in comparison to representative rocks
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