三江造山带兰坪盆地西缘火山岩U-Pb年代学与地质意义

Zircon U-Pb Geochronology and Geological Significance of Volcanic Rocks from the Western Margin of Lanping Basin, Sanjiang Orogenic Belt, China

兰坪盆地西缘火山岩由安山岩、安山质火山角砾岩、杏仁状玄武岩、安山质凝灰岩组成,安山岩LA-ICP-MS锆石U-Pb加权平均年龄为(241.0±3.7)Ma,表明了安山岩的形成时代为中三叠世。安山岩、玄武岩具有一致的稀土元素配分模式,均表现为右倾型,具有弱的负铕异(0.77~1.09),铈异常不明显(0.91~0.99),表明火山岩源于相同的岩浆源区。火山岩为钙碱性玄武岩系列,富集大离子亲石元素(Rb、Ba、Th、U)、亏损高场强元素(Nb、Ta、Ti、P),高Al_2O_3、低TiO_2,显示出类似于岛弧火山岩的特征。研究区火山岩来源于交代岩石圈地幔源区低等程度的部分熔融,原始岩浆经历了以斜方辉石为主的分离结晶作用,并在上升过程中经历了地壳物质的混染作用。火山岩总体上显示板内玄武岩的特征并处于伸展环境,具有俯冲带的地球化学信息,可能与三江造山带古特提斯主洋盆向东俯冲过程中发生的板片断离有关。

Volcanic rocks from the western margin of the Lanping Basin are composed of andesite, andesitic volcanic breccia, amygdaloidal basalt and andesitic tuff. The weighted average age of LA-ICP-MS zircon U-Pb of andesite is （241.0 ± 3.7） Ma, indicating that the formation age of andesite is the middle Triassic epoch. The andesite and basalt in this suite of volcanic rocks share the same REE pattern, and both display right leaning, possessing a weak negative Eu anomaly （0.77-1.09） and an unobvious Ce anomaly （0.91 -0.99）, which indicates that the volcanic rocks originate from the same magmatic source. The volcanic rocks from this area fall into the category of talc-alkaline basalt series, enriched with LILE （e.g., Rb, Ba, Th, and U） and deficient in HFSE （e.g., Nb, Ta, Ti, and P）, with high A1203 and low TiO2, presenting the features similar to those of island arc volcanic rocks. The volcanic rocks from this area are originated from the partial melting of the low grade of metasomatized lithospheric mantle source region; the protomagma has experienced the action of fractional crystallization dominated by orthopyroxene, and also experienced the hybridism of crust materials during the ascending process. Generally, the volcanic rocks display the characteristics of intraplate basalts and are in an extensional environment, with the geochemical information of subduction zones, which may be related to the slab break-off occurred during the eastward subduction of the Paleo-Tethyan major oceanic basin in the Sanjiang orogenic belt.