Petrogenesis and Geological Significance of Late Jurassic Volcanic Rocks in Mami Area, Central Tibetan Plateau
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摘要: 在青藏高原中部中拉萨地块之上新识别了一套晚侏罗世-早白垩世早期岩浆岩，本次对其中麻米地区出露的火山岩进行了锆石U-Pb定年与全岩主微量元素分析研究工作.测年结果显示麻米火山岩形成于晚侏罗世（152~150 Ma），岩石地球化学具有高SiO2、高全碱含量（Na2O+K2O）、低MgO、低P2O5的特征，并显示明显的Eu、Sr、Ba等元素的亏损，具有高分异I型花岗质岩石的特征.结合区域上报道的同期中酸性侵入岩资料，麻米酸性火山岩起源于古老地壳物质重熔并经历了广泛的结晶分异作用.本文研究表明中拉萨地块上晚侏罗世-早白垩世早期岩浆作用形成于洋壳俯冲背景，应该是新特提斯洋北向俯冲过程中引发的弧型岩浆事件.Abstract: A new Late Jurassic-Early Cretaceous magmatic belt has been identified in the central Tibetan Plateau. This paper reports two zircon U-Pb ages and also new whole-rock major and trace element data of the newly discovered volcanic rocks in Mami area. These volcanic rocks were dated as Late Jurassic (152-150 Ma). The studied samples are characterized by high SiO2 content, high alkali, low MgO, and low P2O5 abundances, with significant depletion in Eu, Sr, and Ba, suggesting a geochemical affinity with highly fractionated I-type granites. It is concluded that the volcanic rocks were derived by partial melting of ancient mafic lower crust, followed by fractional crystallization. Our research propose that Late Jurassic-Early Cretaceous magmatic magmatism occurred in an oceanic subduction setting, likely related to the northward subduction of the Neo-Tethys oceanic lithosphere.
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Key words:
- central Tibetan Plateau /
- Late Jurassic /
- volcanics /
- zircon U-Pb dating /
- geochemistry
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图 1 青藏高原构造单元划分图（a）；研究区地质图及本文采样位置（b）；凝灰岩野外露头照片（c）；凝灰岩镜下照片（d）
引用年龄数据引自：姜昕等（2010）、杜德道等（2011）、张璋等（2011）、Li et al.(2014a, 2014b)、Liu et al.（2014）、Cao et al.（2016）、Wu et al.（2016）、闫晶晶等（2017）、尼玛次仁等（2017）、叶春林等（2018）
Fig. 1. Tectonic subdivision of the Tibetan Plateau(a); simplified geological map of the Mami area, Tibet(b); field photograph(c); petrographic photograph(d)
图 2 锆石CL图像、锆石U-Pb谐和图和年龄分布
Fig. 2. Cathodoluminescence (CL) images, U-Pb concordia plots and age distribution for zircons
图 3 岩石地球化学分类图解
a.TAS图解；b.K2O-SiO2图解；c.Th-Co图解；d.A/NK-A/CNK图解；引用数据引自：杜德道等（2011）、Cao et al.（2016）、尼玛次仁等（2017）、闫晶晶等（2017）
Fig. 3. Geochemical classifcation diagrams
图 5 岩石球粒陨石标准化稀土元素模式（a）；原始地幔标准化微量元素蛛网图（b）
标准化值引自Sun and McDonough（1989）
Fig. 5. Chondrite-normalized rare earth element(a); primitive-mantle-normalized trace element patterns(b)
图 6 岩石微量元素与元素比值变化图解
Fig. 6. Geochemical composition variation diagrams of trace elements and element ratios
图 7 南羌塘板块与中拉萨地块晚侏罗世-早白垩世早期岩浆岩年龄分布频谱图
Fig. 7. Frequency distribution of magmatic age data of southern Qiangtang and central Lhasa terranes
图 8 岩石地球化学分类图解
a.(FeO*/MgO)-Zr+Ce+Nb+Y图解，据Whalen et al.（1987）；b.P2O5-SiO2图解；c.Th-Rb图解，据Li et al.（2007）
Fig. 8. Geochemical classification diagrams
图 9 岩石地球化学判别图解
a.Rb/Sr-Sr图解；b.Ba-Sr图解，据Rollinson（1993）；Pl.斜长石；Kfs.钾长石；Bi.黑云母；Amp.角闪石
Fig. 9. Geochemical discrimination diagrams
图 10 构造环境判别图解
a.Nb-Y图解；b.Rb-Y+Nb图解，据Pearce et al.（1984）；syn-COLG.同碰撞花岗岩；VAG.火山弧花岗岩；WPG.板内花岗岩；ORG.洋脊花岗岩
Fig. 10. Tectonic setting distinguish diagram
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