Geochemical Features and Tectonic Setting of the Mesozoic Intrusions in Eastern Tianshan
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摘要: 东天山百灵山西辉长岩、似斑状花岗岩近年来才被发现，为了解其成因及相关大地构造背景，对其进行地质学、地球化学、锆石U-Pb定年及Sr-Nd同位素分析.LA-ICP-MS锆石U-Pb测年结果显示，百灵山西辉长岩、似斑状花岗岩分别形成于236 Ma和228 Ma.辉长岩具有较低的SiO2（43.50%~46.03%）含量，较高的CaO（11.40%~13.24%）、Fe2O3T（9.62%~11.84%）和MgO（6.02%~10.58%；Mg#=53~69）含量以及Ce/Pb、Ti/Zr、Ti/Y及Ba/Th比值，且富集LREE、LILE，表明其形成于受板片脱水流体交代地幔楔的部分熔融.似斑状花岗岩表现出I型花岗岩的特征，且具有较高的SiO2（71.14%~72.71%），强富集LREE[（La/Yb）N=12.61~28.45]和LILE（例如Rb，K和Pb），表明其形成于下地壳的部分熔融.而较高的Mg#（39~41）值、Ti/Y（154.40~306.18）比值，显示其源区有地幔物质的加入.结合前人研究成果，可知东天山地区在240 Ma以后已经进入陆内演化阶段，百灵山西辉长岩及似斑状花岗岩均形成于陆内环境.Abstract: The Early Mesozoic magmatism of the eastern Tianshan was just reported recently, the source and tectonic setting of which are poorly understood. Formed during the Early Mesozoic, the Bailingshanxi gabbro and porphyritic granite were recently discovered. In this study, new geochemistry, zircon U-Pb dating and whole-rock Sr-Nd isotope data are provided for these intrusions. Zircon LA-ICP-MS U-Pb dating results reveal that the gabbro and porphyritic granite emplaced at ca. 236 Ma and 228 Ma, respectively. The Bailingshanxi gabbro has low SiO2 (43.50%-46.03%) content, and high CaO (11.40%-13.24%), Fe2O3T(9.62%-11.84%) and MgO (6.02%-10.58%; Mg#=53-69) contents. Meanwhile, it is characterized by high Ce/Pb, Ti/Zr, Ti/Y and Ba/Th ratios, and rich in LREE and LILE (e.g., Ba and Sr), indicating the Bailingshanxi gabbro was originated from the partial melting of the metasomated mantle wedge. Showing high SiO2 (71.14%-72.71%), LREE ((La/Yb)N=12.61-28.45) and LILE (e.g., Rb, K and Pb), the Bailingshanxi porphyritic granite was likely derived from the partial melting of the lower crust. Additionally, it has high Mg# (39—-41) value and Ti/Y (154.40-306.18) ratios, implying mantle component played an important role in the formation of the porphyritic granite. Integrating with previous studies, the Eastern Tianshan is probably under inter-plat environment after 240 Ma, and the Bailignshanxi gabbro and porphyritic granite were formed during this stage.
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Key words:
- eastern Tianshan /
- early mesozoic /
- Bailingshanxi gabbro /
- porphyritic granite /
- geochemistry
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图 1 （a）东天山地质特征及重要矿床分布；（b）百灵山西地区地质简图
a. 据王京彬等（2006），Deng et al.（2017）修改；b. 据新疆地质调查院1：5万地质图
Fig. 1. (a) Simplified tectonic map of the eastern Tianshan belt; (b) geologic map of the Bailingshanxi area
图 2 （a）百灵山西辉长岩手标本及（b）镜下照片; （c）百灵山西似斑状花岗岩手标本及（d）镜下照片
Ol. 橄榄石；Pl. 斜长石；Px. 辉石；Qtz. 石英；Kfs. 钾长石；Bt. 黑云母
Fig. 2. Photographs (a, c) and photomicrographs (b, d) of the Bailingshanxi gabbro (a, b) and porphyritic granite (c, d)
图 3 （a）百灵山西辉长岩锆石CL图像；（b）百灵山西似斑状花岗岩锆石CL图像；（c，d）百灵山西辉长岩及（e，f）似斑状花岗岩U-Pb年龄谐和图及加权平均年龄
Fig. 3. The CL images of zircon grains from the Bailingshanxi gabbro (a) and porphyritic granite (b); U-Pb concordia and weighted average diagrams of the zircons from (c, d) Bailingshanxi gabbro and (e, f) porphyritic granite
图 5 （a）K2O vs. SiO2图解；（b）A/NK vs. A/CNK图解
Fig. 5. (a) K2O vs. SiO2 diagram and (b) A/NK vs. A/CNK diagram for the Bailingshanxi intrusions
图 6 百灵山西辉长岩、似斑状花岗岩球粒陨石标准化REE模式图和原始地幔标准化微量元素蛛网图
Fig. 6. Chondrite-normalized REE and primitive-mantle-normalized trace element diagrams for the Bailingshanxi gabbro and porphyritic granite
图 7 百灵山西辉长岩、似斑状花岗岩ISr-εNd(t)图解
DM. 亏损地幔；MORB. 洋中脊玄武岩；OIB. 大洋岛弧玄武岩；UC. 上地壳；OIB、MORB区域来自于Wang et al., （2018a）
Fig. 7. ISr-εNd(t) diagrams for the Bailingshanxi
图 8 （a）百灵山西辉长岩及似斑状花岗岩的Zr/Nb-Zr及（b）百灵山西辉长岩的Hf-3×Ta-Rb/30图解
IAB. 岛弧玄武岩；WPA. 板内拉斑玄武岩；WPT. 碱性板内玄武岩；N-MORB. 正常型洋中脊玄武岩；E-MORB. 富集型洋中脊玄武岩；OIB. 大洋岛弧玄武岩
Fig. 8. (a) Zr/Nb vs. Zr diagram for Bailingshanxi gabbro and porphyritic granite and (b) Hf-3×Ta-Rb/30 diagram for Bailingshanxi gabbro
图 9 白山似斑状花岗岩Zr-10 000×Ga/Al及10 000×Ga/Al-（Zr+Nb+Ce+Y）相关图解
FG. 分异的I、S型花岗岩；OGT. 未分异I、S型花岗岩；Zr与Zr+Nb+Ce+Y数量级为10-6
Fig. 9. Zr vs. 10 000×Ga/Al and 10 000×Ga/(Al-Zr+Nb+Ce+Y) diagrams for Baishan porphyritic granite
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