哀牢山-大象山变质杂岩带中斜长角闪岩的地球化学、同位素年代学及其地质意义

Geochemistry,geochronology and geological implication of amphibolites in Ailao Shan-Day Nui Con Voi metamorphic complex belt,southeastern Tibetan Plateau

哀牢山-大象山变质杂岩带位于青藏高原东南缘,是西南三江地区重要的北西向造山带。杂岩带主要由各种类型的副片麻岩、片岩、石英岩、大理岩和斜长角闪岩构成,岩石发生强烈的糜棱岩化。本文在哀牢山变质杂岩带东南段元江和金平地区以及大象变质杂岩带北段老街和Pho Rang地区发现了中二叠-早三叠世的斜长角闪岩。岩石地球化学和锆石U-Pb年龄研究表明,这些斜长角闪岩可分为4组:第1组为元江斜长角闪岩,具有类似于E-MORB的稀土和微量元素配分曲线特征,锆石U-Pb定年显示其原岩形成年龄为272.5±1.7Ma;第2组为勐桥-马鞍底斜长角闪岩,相比于元江斜长角闪岩(第1组)具有更高的轻/重稀土分馏程度,其稀土和微量元素配分曲线类似于E-MORB,微量元素比值(Nb/Yb、Th/Yb等)显示具有沿MORB-OIB序列演化的趋势,勐桥和马鞍底斜长角闪岩中的Ti含量和Ti/Y比值等特征分别类似于峨眉山低Ti玄武岩和高Ti玄武岩。锆石U-Pb定年结果表明,勐桥-马鞍底斜长角闪岩形成于265.2±1.0Ma^266.2±1.0Ma和250.4±1.5Ma^248.7±1.6Ma,其中晚期岩石中含有261.2±1.5Ma^257.9±1.6Ma的继承锆石,该年龄与峨眉山玄武岩(约260Ma)近于同期。上述证据表明勐桥-马鞍底斜长角闪岩岩浆演化过程中可能混染了部分具有OIB属性的峨眉山地幔柱物质成分;第3组为大象山老街-Pho Rang斜长角闪岩,具有类似于OIB的稀土和微量元素配分曲线特征,微量元素含量和比值(Ti O2=3.28%~4.31%,Nb/La=0.84~1.01,Ti/Y>500等)特征显示与峨眉山高Ti玄武岩相似的地球化学属性,表明峨眉山玄武岩在哀牢山-大象山变质杂岩带内广泛分布,该岩石成分可能为勐桥-马鞍底斜长角闪岩(第2组)的端元组分之一;第4组为大象山Pho Rang斜长角闪岩,该组岩石具有与元江斜长角闪岩(第1组)相似的E-MORB属性特征。进一步的研究表明,哀牢山-大象山变质杂岩带中具有类似于E-MORB属性的斜长角闪岩均表现出不同程度的Nb、Ta元素亏损和Rb、Ba等大离子元素富集,微量元素比值(Nb/Yb(0.72~5.29),Th/Yb(0.11~0.87),La/Nb(0.91~8.83))等特征类似于岛弧玄武岩,这些特征指示其原岩岩浆可能是俯冲环境下地幔楔岩石部分熔融的产物。结合哀牢山-大象山变质杂岩带、哀牢山-马江缝合带以及扬子地块之间的时-空关系,本文推测哀牢山-大象山变质杂岩带内的E-MORB类型岩浆岩形成于东古特提斯支洋(即哀牢山-马江洋)向东的俯冲过程,其俯冲持续时间为中二叠-早三叠世(272~248Ma)。

The Ailao Shan-Day Nui Con Voi metamorphic complex belt(ADB),located on the southeastern margin of Tibetan Plateau,is a vital NW trend orogenic belt in southeastern Tibetan Plateau.The ADB mainly consists of various paragneisses,schists,quartzites,marbles,amphibolites,which have experienced extensive mylonitization.In this paper,we discovered the Middle PermianEarly Triassic amphibolites in the Yuanjiang,Jinping,Lao Cai and Pho Rang areas within the ADB.The amphibolites can be subdivided into four groups by the integrated studies of the geochemistry and zircon U-Pb geochronology.Group 1 is distributed in Yuanjiang area.Its REE and trace element patterns are similar to those of E-MORB.LA-ICP-MS zircon U-Pb dating of these rocks yields an age of 272.5±1.7 Ma.Group 2 is collected from Mengqiao and Maandi.Compared with Yuanjiang amphibolites(Group 1),it shows higher ratios of LREE/HREE,and the REE and trace element patterns are similar to the enriched mid-ocean ridge basalts(EMORB).The signatures of trace element ratios(e.g.Nb/Yb,Th/Yb,etc.)show the trend from MORB to OIB.The Mengqiao and Maandi amphibolites are similar to Emeishan Low-Ti basalts and High-Ti basalts,respectively,based on their Ti contents and Ti/Y ratios.The zircon U-Pb dating results yield weighted mean ages of 265.2±1.0 Ma^266.2±1.0 Ma and 250.4±1.5 Ma^248.7±1.6 Ma for Mengqiao and Maandi amphibolites,in which the late rocks contain 261.2±1.5 Ma^257.9±1.6 Ma of inherited zircon.These ages of protoliths and inherited zircon are similar to Emeishan basalt(~260 Ma).All of these evidences indicate that the Mengqiao-Maandi mafic rocks are mainly derived from the mixing of mantle source components(E-MORB-like)and Emeishan plume material(OIB-like).Group 3 is distributed in Lao Cai and Pho Rang in Day Nui Con Voi,and has OIB-like characteristics of rare earth and trace elements.The contents and radio of trace elements(Ti O2=3.28%~4.31%,Nb/La=0.84~1.01,Ti/Y>500,and so on)are similar to high-Ti(HT)Emeishan basalt,indicating that the Emeishan basalt is widely distributed in the ADB,and these rocks may be one of the end-member components of the Mengqiao-Maandi amphibolites(Group 2).Group 4 is distributed in Pho Rang with geochemical features similar to the Yuanjiang amphibolites(Group 1).Further research suggests that the amphibolites(E-MORBlike)in ADB are characterized by varying degrees of negative Nb,Ta anomalies and positive Rb,Ba anomalies.The ratios of incompatible trace elements(Nb/Yb(0.72~5.29),Th/Yb(0.11~0.87),La/Nb(0.91~8.83),and so on)are similar to island arc basalts.Such geochemical signatures indicate that these rocks were derived from partial melting of mantle wedge in the subduction zone.In combination with previous studies on the time-space distribution between the ADB,Ailao Shan-Song Ma suture and Yangtze block,we proposed that the East Paleo-Tethys Branch Ocean(Ailao Shan-Song Ma Ocean)subducted eastward beneath the Yangtze Block during the Middle Permian-Early Triassic(272~248 Ma),and accompanied formation of E-MORB-like magmas in the ADB.