北太行紫荆关基性岩体的成因: 岩石学和地球化学证据

刘玲; 陈斌; 刘安坤

北太行紫荆关基性岩体的成因: 岩石学和地球化学证据

北京大学造山带与地壳演化教育部重点实验室, 地球与空间科学学院, 北京 100871

基金项目:

国家自然科学基金项目 40625005

详细信息

作者简介:
刘玲(1985-), 女, 硕士, 主要从事岩浆作用岩石学与地球化学研究

通讯作者:
陈斌, E-mail: binchen@pku.edu.cn

中图分类号: P581
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出版历程
- 收稿日期: 2008-11-09
- 刊出日期: 2009-01-25

Petrogenesis of the Zijingguan Mafic Pluton, Northern Taihang Orogen: Constraints from Petrology and Geochemistry

Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, School of Earth and Space Sciences, Peking University, Beijing 100871, China

摘要

摘要: 通过对北太行紫荆关晚中生代基性岩体的岩石学和地球化学特征研究, 讨论其岩浆源区特征及成因.结果表明, 紫荆关岩体主要由橄榄二辉石岩、角闪辉长岩和闪长岩等岩石组成.橄榄二辉石岩具有明显的堆晶结构, 可能是堆晶成因, 而角闪辉长岩和闪长岩可能是堆晶形成之后的残余岩浆的结晶.闪长岩中的斜长石显示复杂的结构和成分环带, 相对富钠的核部见富钙斜长石叠加, 后者又向富钠斜长石演化.闪长岩中的单斜辉石也显示复杂的成分环带, 主要是最边部的MgO含量比紧邻的内侧辉石更高.这表明在闪长岩形成过程中有新的幔源高温富镁的岩浆注入.以上各岩石类型具有类似的地球化学性质, 包括较强的稀土分异((La/Yb)_N比值为5.76~23.13), 高Sr/Y比值(50.95~146.13), 富集K、Sr、Ba等大离子亲石元素和LREE, 亏损Nb、Zr、Ti等高场强元素和HREE, Eu异常不明显.都显示富集的Sr-Nd同位素特征, I_Sr=0.7058~0.7066, ε_Nd(t) =-13.8~-18.2.这些特征表明紫荆关基性岩体的母岩浆来源于EM1型富集地幔的部分熔融.橄榄辉石岩和角闪辉长岩具有类似的Nd同位素成分(主要为ε_Nd(t) =-15.0~-16.6), 但形成较晚的闪长岩的ε_Nd(t) 值却比较高(-13.8~-14.6), 也支持新的幔源岩浆在晚期的注入.
- 基性岩体 /
- 斜长石环带 /
- Sr-Nd同位素 /
- 岩浆补充 /
- 太行山
Abstract: Through the petrological and geochemical study carried out for the Zijingguan mafic pluton (North Taihang), this paper is to understand their magma sources and petrogenesis.The Zijingguan mafic rocks are mainly made up of olivine-bearing pyroxenites, hornblende gabbros and diorites.Cumulate textures can be seen in the olivine-bearing pyroxenites, suggesting a cumulus origin of them.Hornblende gabbros and diorites are probably crystallized products from the residual melts after the accumulation process.Plagioclases from diorites show complicated textures and compositional zoning, with relatively sodium-rich core mantled by calcium-rich plagioclase, the latter evolved to sodium-rich plagioclase towards the rim with descent of magma temperature.Similarly, clinopyroxenes from diorite show complicated compositional zoning, with the MgO abundances in the rim being slightly higher than that of immediate inner part of pyroxenes.These textures suggest that the magma chamber had experienced a process of magma recharge from the mantle source.The different rock types of the Zijingguan mafic pluton share many common geochemical features like highly fractionated REE patterns ((La/Yb)_N=5.76-23.13) and high Sr/Y ratios (50.95-146.13), highly enriched LILE (K, Sr, Ba) and LREE, and depleted HFSE (Nb, Zr, Ti) and HREE, with minor Eu anomalies in the REE patterns.They show enriched Sr-Nd isotopic compositions, with I_Sr=0.7058-0.7066, ε_Nd(^t) =-13.8 to-18.2.These geochemical features suggest that the mafic pluton originated from partial melting of an EM1 type mantle source.Olivine-bearing pyroxenites have isotopic composition identical to that of hornblende gabbros (mostly ε_Nd(^t) =-15 to-16.6), while the later-crystallizing diorites have a slightly higher ε_Nd(^t) =-13.8 to-14.6, which is in agreement with the model of magma recharge during magma evolution.
- mafic pluton /
- compositional zoning of plagioclase /
- Sr-Nd isotopes /
- magma recharge /
- Taihang Mountains

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图 1 北太行紫荆关地区地质简图

Fig. 1. Geological sketch map of the relief framework in Zijingguan, northern Taihang orogen

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图 2 二辉石岩(ZJ017) 显微照片.橄榄石和辉石具有角闪石反应边, 橄榄石和辉石呈堆晶, 斜长石呈他形充填(a); 粗粒角闪石颗粒包裹橄榄石(b)

Fig. 2. Micrographs of pyroxenite (ZJ017), with olivine and clinopyroxene rimmed by amphibole overgrowth, and interstitial plagioclase (a), and olivine included in amphibole megacryst (b)

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图 3 闪长岩中斜长石和单斜辉石的结构和成分环带

a、b、c为斜长石; d、e、f为单斜辉石; 图中圆圈内标注的数字为斜长石的An值或单斜辉石的Mg^#

Fig. 3. Micrographs showing the compositional and textual disequilibrium in plagioclase and clinopyroxene crystals from diorite

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图 4 紫荆关基性岩体的AFM图(Irvine and Baragar, 1971)

A=Na₂O+K₂O; F=FeO+0.899 8Fe₂O₃; M=MgO

Fig. 4. AFM plots of the Zijingguan gabbroic rocks

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图 5 紫荆关基性岩体的主量元素变化图

Fig. 5. Variation diagrams for major elements of the Zijingguan gabbroic rocks

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图 6 紫荆关辉长岩体的微量元素变化图

Fig. 6. Variation diagrams for trace elements of the Zijingguan gabbroic rocks

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图 7 紫荆关辉长岩体的稀土元素配分型式图(图a, b, c)和微量元素蛛网图(图d, e, f)(球粒陨石和原始地幔值来自Sun and Mcdonough, 1989)

Fig. 7. Chondrite-normalized REE patterns and primitive mantle-normalized spidergrams of the Zijingguan gabbroic rocks

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图 8 紫荆关基性岩体的ε_Nd(t) -I_Sr图(EM1地幔的位置引自Zindler and Hart, 1986)

Fig. 8. Plot of I_Sr vs. ε_Nd(t) for the Zijingguan mafic pluton

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图 9 紫荆关基性岩体(a) SiO₂-ε_Nd(t) 和(b) SiO₂-I_Sr (t) 变化图

Fig. 9. Plots of (a) SiO₂ vs. ε_Nd(t) and (b) SiO₂ vs. I_Sr (t) for the Zijingguan mafic rocks

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表 1 紫荆关基性岩体中闪长岩的斜长石电子探针分析(%)

Table 1. Microprobe analysis data of plagioclase from diorite of the Zijingguan mafic pluton

表 2 紫荆关基性岩体中闪长岩的单斜辉石电子探针分析(%)

Table 2. Microprobe analysis data of clinopyroxene from diorite of the Zijingguan mafic pluton

表 3 闪长岩结晶温度和相关电子探针分析(%)

Table 3. The crystallizing temperature and related microprobe analysis data of diorite

表 4 二辉石岩的结晶温度和相关电子探针分析(%)

Table 4. The crystallizing temperature and related microprobe analysis data of pyroxenite

表 5 紫荆关基性岩体的主量(%)和微量(μg/g)元素分析数据

Table 5. Major and trace elements of the Zijingguan mafic rocks

表 6 紫荆关基性岩体的Sr-Nd同位素数据

Table 6. Sr-Nd isotopic data of the Zijingguan mafic rocks

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