新疆中天山巴仑台地区晚石炭世+ε_(Nd)辉长岩体的岩石学和同位素地球化学研究  

Petrology and geochemistry of a +ε_(Nd) gabbro body in Baluntai region, central Tianshan mountains,Xinjiang.

作　　者：朱永峰[1] 郭璇[1] 周晶[1] 

机构地区：[1]北京大学地球与空间科学学院造山带与地壳演化教育部重点实验室,北京100871

出　　处：《岩石学报》2006年第5期1178-1192,共15页Acta Petrologica Sinica

基　　金：国家973"中国西部中亚型造山与成矿"2001CB409802课题;国家自然科学基金项目(批准号40572033)资助.

摘　　要：新疆巴仑台附近出露的辉长岩(骆驼沟岩体)是中天山地区出露最大的晚石炭世基性侵入体,该岩体分带明显,由边部的细粒辉长玢岩(冷凝边)逐渐过渡到内部的中粗粒辉长玢岩。中粗粒辉长玢岩的ε_(Nd)值(+5．10～+5．74)和初始^(87)Sr/^(86)Sr比值(0．7055～0．7058)变化很局限,但边缘相细粒辉长玢岩的ε_(Nd)值(+4．25～+7．28)变化较大而初始^(87)Sr/^(86)Sr值(0．7058～0．7059)变化较小。三个细粒辉长玢岩全岩样品构成一条Rb-Sr等时线,等时线年龄321±10Ma(MSWD=1．5,初始^(87)Sr/^(86)Sr值0．705725±0．000084),这与位于该地区以西大面积出露的粗面安山岩的锆石SHRIMP年龄(313Ma)接近。与晚石炭世粗面安山岩类似,骆驼沟辉长岩富集La、Sr和Ba。高的+ε_(Nd)值表明形成骆驼沟辉长岩的岩浆具有亏损地幔的性质,但明显偏高的初始^(87)Sr/^(86)Sr比值说明骆驼沟辉长岩不是亏损地幔直接部分熔融的产物,而可能是亏损地幔来源的岩浆与富La、Ba、Sr的长英质岩浆混合(或者与相应火山岩同化混染)的结果。The Luotuogou gabbro in the Baluntai region of the Central Tianshan Mountains is a large mafic intrusive body formed during the Late Carboniferous period. This pluton is clearly zoned with a thin rim （ width 50 - 200m）. The rim is composed by the fine-grained porphyritic gabbro, which gradually changed to medium-and coarse-grained gabbro （ the major body of the Luotuogou pluton）. The medium to coarse-grained gabbro samples have high and less variable ＋ εNd values （ ＋ 5. 10 - ＋5.74） and high initial ^87Sr/^86Sr values （0. 7055 -0. 7058）. The fine-grained gabbro samples have largely variable εNd values （ ＋4. 25 - ＋7. 28） and relatively high initial ^87Sr/^86Sr ratios （0.7058 - 0.7059 ）. Three samples of the fine-grained gabbro composed a Rb-Sr isochron, which yielded an age of 321 ± 10Ma （MSWD = 1.5, initial ^87Sr/^86Sr =0.705725 ±0.000084）. This age is close to the zircon SHRIMP age of the trachandesite （313Ma） distributed in the northwest of Baluntai region. Similar to the Late Carboniferous trachyandesite, the Luotuogou gabbro is enriched in La, Sr and Ba. High ＋ εNd values indicate that the studied gabbro has a geochemical nature of depleted mantle. However, the obvious high initial ^87Sr/^86Sr ratios suggest that the Luotuogou gabbro was not derived directly from the partial melting of the lithosphere mantle but could be formed by the mixing （ or AFC process） between magma from depleted mantle and the La-, Ba-, and Sr-rich felsic magma （or volcanic rocks）.

关 键 词：辉长岩 SR-ND同位素 石炭纪 巴仑台 中天山 新疆 

分 类 号：P597.3[天文地球—地球化学]