Chronology and Geochemistry of Gonjo Triassic Volcanic Rocks in Northern Part of Nujiang-Lancangjiang-Jinshajiang and Its Geological Significance
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摘要: 三江北段贡觉三叠纪火山岩是金沙江古特提斯洋向西俯冲消减及随后的弧陆碰撞造山作用的产物，主要由玄武岩、安山质熔结凝灰岩和安山岩等组成，对探讨金沙江古特提斯构造-岩浆演化过程具有重要意义.对贡觉火山岩进行了LA-ICP-MS锆石U-Pb定年和全岩主、微量元素研究.其中代表性样品安山质熔结凝灰岩的LA-ICP-MS锆石U-Pb定年结果显示，锆石23个分析点的206Pb/238U年龄为243~233 Ma，206Pb/238U加权平均年龄为238.3±1.3 Ma（MSWD=1.7，n=23），代表了岩浆的结晶年龄为中-晚三叠世.地球化学分析表明，火山岩以低钛（0.52%~0.91%）、中镁（3.38%~7.22%）、富钠（3.63%~4.66%）、贫钾（0.09%~4.05%）、富集轻稀土元素（LREE/HREE=4.09~13.27）和大离子亲石元素（Th、U、La、Sm）、亏损高场强元素Nb和Ta（Nb=2.51×10-6~24.5×10-6、Ta=0.22×10-6~1.89×10-6）为特征，La/Nb值（2.13~2.89）较高、Nb/Th值（1.12~1.62）相对较低，具有与岛弧型或晚碰撞造山型火山岩相似的主-微量元素组成特征.贡觉中-晚三叠世火山岩代表了金沙江古特提斯演化过程中晚碰撞造山阶段的岩浆响应，推测是晚碰撞过程中残留的金沙江洋壳物质与岩石圈地幔物质相互作用的结果.Abstract: The Gonjo Triassic volcanic rocks in the northern area of the Nujiang-Lancangjiang-Jinshajiang are the products of subduction of the Jinshajiang Paleo-Tethyan Ocean to the west and subsequent arc-continent collision-orogeny effect. It is mainly composed of basalt, andesite fused tuff and andesite, etc.The study of Gonjo volcanic rocks is of great significance for the exploration of the Jinshajiang Paleo-Tethyan tectonic-magmatic evolution process. In this paper, it presents experiments on Gonjo volcanic rocks of LA-ICP-MS zircon U-Pb dating and master-trace element research. The LA-ICP-MS zircon U-Pb dating of representative samples of andesite fused tuff shows that the age of 206Pb/238U of 23 analysis points of zircon is 243-233 Ma. The LA-ICP-MS zircon U-Pb age of the representative sample of andesite fused tuff is 238.3±1.3 Ma, which indicates that the magmatic crystallization age is Middle-Late Triassic. Geochemical analysis shows that volcanic rocks contains low titanium (0.52%-0.91%), medium magnesium (3.38%-7.22%), rich sodium (3.63%-4.66%), depleted potassium (0.09%-4.05%), enriched light rare earth elements (LREE/HREE=4.09-13.27) and large ionic lithophilic elements (Th, U, La, Sm), deficient high field strength elements Nb and Ta (Nb=2.51×10-6-24.5×10-6, Ta=0.22×10-6-1.89×10-6), relatively high La/Nb (2.13-2.89) ratio and relatively low Nb/Th (1.12-1.62), which possesses the composition characteristics of main-trace elements similar to island arc or late collision orogenic volcanic rocks. It is concluded that the Middle-Late Triassic Gonjo volcanic rocks represent the magmatic response of the Jinshajiang during the evolution of Paleo-Tethys, which is probably the result of the interaction between the Jinshajiang oceanic crust material and the lithospheric mantle material remaining during the late collision.
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Key words:
- zircon U-Pb dating /
- geochemistry /
- island arc volcanic rock /
- Jinshajiang suture zone /
- geochronology
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图 1 三江北段大地构造单元划分
Ⅰ.扬子地块；Ⅱ.三江造山系；Ⅱ1.甘孜-理塘结合带；Ⅱ2.德庆-乡城岩浆弧；Ⅱ3.中咱-中甸地块；Ⅱ4.金沙江-哀牢山结合带；Ⅱ5.江达-德钦-维西陆缘弧；Ⅱ6.昌都-思茅地块；Ⅲ.班公-怒江-昌宁-孟连对接带；Ⅲ1.左贡临沧-勐海岩浆弧；Ⅲ2.昌宁-孟连结合带；Ⅳ.冈底斯-腾冲陆缘造山系；Ⅳ1.冈底斯-腾冲岩浆弧；Ⅳ2.嘉黎-波密结合带；Ⅳ3.察隅-盈江岩浆弧；Ⅴ.印度陆块区.据王保弟等（2018）
Fig. 1. Division of tectonic units in the northern part of Nujiang-Lancang-Jinshajiang
图 3 贡觉火山岩显微镜下照片
Cal.方解石；Px.辉石；Fs.长石；Qz.石英；Ol.橄榄石；Hbl.角闪石
Fig. 3. Microscope photos of Gonjo volcanic rocks
图 6 贡觉火山岩SiO2含量与其他主量元素的相关性图解
Fig. 6. Correlations between SiO2 content and other major elements of Gonjo volcanic rocks
图 7 贡觉火山岩REE配分图（a）和微量元素蛛网图（b）
球粒陨石和原始地幔标准化值、N-MORB、E-MORB及OIB数据据Sun and McDonough (1989)
Fig. 7. REE pattern (a) and trace element spider diagram (b) of Gonjo volcanic rocks
图 8 贡觉火山岩的岩石分类图解（a）和10TiO2-10K2O-Al2O3图解（b）
Ⅰ.大洋玄武岩；Ⅱ.大陆玄武岩、安山岩；Ⅲ.岛弧、造山带玄武岩、安山岩.a.据王保弟等（2013）；b.据赵崇贺（1989）
Fig. 8. Rock classification diagram(a)and 10TiO2-10K2O-Al2O3 diagram(b)of Gonjo volcanic rocks
图 9 贡觉火山岩2Nb-Zr/4-Y（a）和Hf/3-Th-Nb/16（b）构造环境图解
a.据Meschede（1986）；AⅠ.板内碱性玄武岩，AⅡ.板内碱性玄武岩+板内拉斑玄武岩，B.E-MORB，C.板内拉斑玄武岩+火山弧玄武岩，D.火山弧玄武岩+N-MORB. b.据Wood（1980）；A.N-MORB，B.E-MORB，C.板内碱性玄武岩，D.岛弧钙碱性玄武岩
Fig. 9. 2Nb-Zr/4-Y(a)and Hf/3-Th-Nb/16(b)diagrams of Gonjo volcanic rock
图 10 贡觉火山岩源区的特征微量元素判别图解
据李曙光等（1993）；IAB.岛弧玄武岩，MORB.洋中脊玄武岩，OIB.洋岛玄武岩
Fig. 10. Discriminant diagrams of typical trace elements in the source area of Gonjo volcanic rocks
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