Zircon U-Pb Geochronology and Geochemistry of Mesozoic Volcanic Rocks from Dasijiazi Area at Zhangwu, West Liaoning Province
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摘要: 通过对辽西彰武县以东大四家子乡高城窝堡村义县组标准剖面中生代火山岩锆石U-Pb年代学研究表明, 其火山岩年龄为122.4±0.4Ma, 属早白垩世.对该区域23件典型火山岩样品的地球化学研究表明, 除3件流纹岩样品外, 其余样品具有高镁埃达克岩地球化学特征(SiO2=56.46%~65.14%、Al2O3=14.60%~17.19%、Mg#=50~59、Sr=501~700μg/g、Yb=1.04~1.54μg/g、Y=12.0~17.5μg/g、Eu/Eu*=0.85~0.97、Sr/Y=29~46、LaN/YbN=13~28), 同位素上具有高的初始87Sr/86Sr(0.705464~0.705812)比值, 低的εNd(122Ma)(-6.12~-12.80)值特征, 同时样品中存在具有反环带的辉石斑晶, 辉石中稀土元素含量分布存在着从核部到边部逐渐降低的趋势, 且存在负铕异常(Eu/Eu*=0.64~0.76).结合前人对彰武义县组下部火山岩的研究, 笔者倾向认为该套火山岩的成因是拆沉作用与岩浆混合作用共同作用的结果, 即拆沉作用导致软流圈地幔物质上涌加热下地壳形成的长英质岩浆, 与来自地幔由拆沉作用形成的埃达克质高镁安山岩浆混合形成.Abstract: Zircon U-Pb dating by LA-ICPMS reveals that the age of Mesozoic volcanic rocks from the Zhangwu area in west Liaoning Province is 122.4± 0.4 Ma, which belongs to Early Cretaceous. Geochemical compositions of 23 typical volcanic samples are studied. The results show that most samples except three rhyolites have similar geochemical characteristics of high-Mg adakite (SiO2 =56.46%-65.14%, Al2O3 =14. 60%--17.19%, Mg# = 50--59, Sr=501- 700μg/g, Yb= 1.04- 1.54μg/g, Y= 12.0- 17.5μg/g, Eu/Eu* = 0.85 - 0.97, Sr/Y = 29 - 46, LaN/YbN = 13 - 28). They also have high initial 87Sr/86Sr (0. 706 39-0. 706 47) and low Nd (122 Ma) values (- 6.43 to - 12.26) that are inconsistent with the original ones from slab melting. Pyroxene phenocrysts are characterized by reversed compositional zonning. Rare earth element concentrations decrease from pyroxene core to rim and also show negative Eu anomalies (Eu/Eu*= 0.64-0. 76). Combined with the previous studies on volcanic rocks from the low part of Yixian Formation in Zhangwu area, we propose that our samples from Zhangwu area resulted from delamination and magma mixing. A crustally derived felsic magma resulted from partial melting of the low crust, heated by asthenosphere upwelling and subsequently mixed with a mantle-derived high-Mg adakitie melt during magma storage or ascent to surface to form the high-Mg adakites in the Zhangwu area.
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Key words:
- high-Mg adakites /
- magma mixing /
- delamination /
- Mesozoic volcanic rocks /
- West Liaoning Province
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图 1 辽西彰武地区区域地质简图及采样点分布(地质简图据1973年辽宁省第一区域地质测量队所绘地质图绘制)
Fig. 1. Geological sketch map in Zhangwu area, West Liaoning Province and sampling points
图 2 彰武流纹岩样品ZW21 LA-ICP-MS锆石U-Pb同位素年龄谐和图及阴极发光图像
Fig. 2. U-Pb concordant diagram of zircons from ZW21 and zircon CLimage.Solid circleindicates location of analytical spot along with 206Pb/238U age
图 3 彰武地区火山岩SiO2-(K2O +Na2O) 图解(据Le Maitre et al., 1989绘制)
虚线代表碱性和亚碱性岩石系列的分界线, 引自Irvine and Baragar, 1971.●代表黄华等(2007)研究的样品; ■代表本研究的样品
Fig. 3. TAS diagram of Zhangwu volcanic rocks
图 4 彰武地区火山岩稀土元素模式分配图(a)和微量元素蛛网图(b),灰色区域为黄华等(2007)研究的样品
Fig. 4. Chondrite-normalized REE patterns (a) and primitive mantle-normalized spider diagrams (b) of Zhangwu volcanicrocks. Gray area indicates range of samples studied by Huang et al. (2007)
图 5 彰武火山岩Y-Sr/Y(a)与MgO-SiO2(b)判别图(Defant et al., 2001,●代表黄华等(2007)研究的样品，■代表本研究的样品
Fig. 5. Y vs. Sr/Y (a) and MgO vs. SiO2(b) diagrams for Zhangwu volcanic rocks
图 6 彰武火山岩中单斜辉石反环带背散射照片
(圆圈表示分析点位置; core, mantle, rim分别指核部、幔部、边部)
Fig. 6. Representative back-scattered electron image of one reversely zoned clinopyroxene phenocryst from Zhang wu volcanic rocks
图 7 彰武安山岩样品ZW14中单斜辉石不同环带的稀土元素分布(a) 与计算出与不同环带平衡的岩浆稀土元素分布(b).计算中核部和边部采用稀土元素在单斜辉石与长英质岩浆之间的分配系数(Barth et al., 2002), 幔部采用稀土元素在单斜辉石与玄武质岩浆之间的分配系数(Mcdade et al., 2003)
Fig. 7. Chondrite-normalized REE pattern of one rever selyzoned clinopyroxene phenocryst from ZW 14 (a) and calculated REE pattern of melts in equilibrium with different zones (b).The calculation uses REE partition coefficients between clinopyroxene and felsic melt (Barth et al., 2002) for the core and rim and those between clinopyroxene and basaltic melt (McDade et al., 2003) for the mantle
表 1 彰武流纹岩样品ZW21错石U-Pb同位素分析结果
Table 1. Zircon U-Pb LA-ICPMS data for ZW21
表 2 彰武中生代火山岩样品主量元素(%)和微量元素(μg/g) 分析结果
Table 2. The analyzed data of major elements (%) by XRF and trace elements (μg/g) by ICP-MS
表 3 彰武中生代火山岩样品同位素分析结果
Table 3. Istopic date of Sr-Nd for Zhangwu volcanic rocks
表 4 辉石反环带主量元素分析结果
Table 4. The analyzed date of major elements(%) of reversely zoned pyroxene phenocrysts by electron microprobe
表 5 ZW14中单斜辉石反环带微量元素分析结果
Table 5. The analyzed date of trace elements (μg/g) of reversely zoned clinopyroxene phenocry st from ZW14 by LA-ICPMS
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