滇东南马关和屏边地区新生代玄武岩地球化学特征及深部动力学意义  

Geochemical characteristics and geodynamic significance of Cenozoic basalts from Maguan and Pingbian, southeastern Yunnan Province

作　　者：黄行凯[1,2,3] 莫宣学[3] 喻学惠[3] 李勇[3] 和文言[3] 

机构地区：[1]有色金属矿产地质调查中心,北京100012 [2]北京矿产地质研究院,北京100012 [3]地质过程与矿床资源国家重点实验室,中国地质大学地球科学与资源学院,北京100083

出　　处：《岩石学报》2013年第4期1325-1337,共13页Acta Petrologica Sinica

基　　金：国家重点基础研究973项目(2009CB421000);国家自然科学基金项目(40572046;40972049);"十二五"国家科技支撑计划项目(2011BAB06B04-03)联合资助

摘　　要：滇东南马关和屏边地区新生代玄武岩是青藏高原后碰撞岩浆作用的重要组成部分。玄武岩全岩的39Ar/40Ar定年结果显示,马关地区存在两期玄武岩浆作用,其时代分别为12.9±0.2Ma和21.2±1.2Ma;屏边玄武岩的时代非常年轻,<1.7Ma,与越南、南海地区以及滇西腾冲新生代玄武岩浆作用的时代一致。根据全岩化学与标准矿物分类命名方法,确定马关玄武岩包括橄榄拉斑玄武岩、碱性橄榄玄武岩和碧玄岩三种岩石类型;而屏边玄武岩均属碧玄岩。采用有关熔体分离压力估算的方法,分别估算了两地玄武岩岩浆的起源深度,结果为:马关从橄榄拉斑玄武岩-碱性橄榄玄武岩-碧玄岩,其起源深度分别为57～73km、82km和67.5～87km。屏边碧玄岩的起源深度为79～88km,略高于马关地区的碧玄岩。全岩稀土微量与Sr-Nd同位素结果表明,两地新生代玄武岩具较低的初始87Sr/86Sr同位素比值和较高的143Nd/144Nd比值,所有玄武岩均具有正的εNd值,以及富集LREE、LILE和Nb、Ta等高场强元素,无Eu异常,且具较高的Nb/U比值等地球化学特征,证明这两地的玄武岩具有相似于OIB的亏损地幔源区,显示出MORB(或OIB)型与EMⅠ(和EMⅡ)富集端元的混合特征。结合稀土元素模拟与地幔包体研究成果,提出该区新生代玄武岩源区应该处于石榴子石相-尖晶石相的过渡区域,可以通过石榴石二辉橄榄岩经1%部分熔融所产生的熔体与尖晶石相二辉橄榄岩经2%～5%部分熔融形成的熔体的混合形成。作为对始于65Ma左右的印度-欧亚大陆巨型碰撞的一种响应,沿金沙江-哀牢山-红河断裂带发生了显著的块体旋转、挤出或逃逸以及构造应力场的巨大反转与调整,诱发青藏高原下部深部物质向周边发生迁移。在西太平洋俯冲带"吸力"的影响下,加速了软流圈物质向东南方向的流动和上涌,最终形成马关、屏边地区新生代岩浆岩。因此,该区新生代Cenozoic basaltic magmatism in Maguan and Pingbian area, southeastern Yunnan Province, is an important component of post-collisional magamatism of eastern Tibetan Plateau. Whole-rock 39Ar/40Ar dating results reveal two pulses of magma eruption occured in Maguan area, in 12.9±0.2Ma and 21.2±1.2Ma, respectively. Basalts in Pingbian area are very young products of lately erupted magma, with age less than 1.7Ma, which is well synchronous to the magmatism distributed in Tengchong, Vietnam, Hainan and South China Sea area. According to CIPW normalization and whole-rock chemistry, basalts from Maguan can be classified into olivine tholeiite, alkaline olivine basalts and basanite, which originate from depths of 57～73km, 82km and 67.5～87km, respectively. Basalts from Pingbian are exclusively basanite, with segregation depth ranging from 79km to 88km. Basalts show OIB-like geochemical characteristic, with enrichment in light rare earth elements, large ion lithophlie elements and high field strength elements （Nb, Ta）, high Nb/U ratios and without Eu anomaly. Relatively high initial 143Nd/144Nd and low 87Sr/86Sr ratios and positive εNd collectively ascribe the source region of basalts to a hybrid of depleted asthenospheric mantle and EMⅠ （and/or EMⅡ） type enriched mantle, which is akin to those of basalts from northern Vietnam and South China Sea area. Simulation results and Re-Os isotopic characteristics illustrate that the source region of basalts situates at the transition zone of spinel and garnet, and the basaltic melt is a mixture of partial melt of garnet-facies lherzolite and spinel-facies lherzolite, with melt degree of 1% and 2%～5%,respectively. As a response to the huge collision between India and Eurasian plate onset around 65Ma, significant block rotation, extrusion or escape took place along Jinshajiang-Ailao Shan-Red River strike-slip fault belt, which resulted in the migration of deep material beneath Tibetan Plateau toward surrounding area. The suction of Western Pacific subduction zone

关 键 词：新生代玄武岩 深部物质流 马关 屏边 滇东南 

分 类 号：P588.145[天文地球—岩石学]