云南祥云马厂箐富碱斑岩体的地球化学特征及其形成的构造环境

云南祥云马厂箐岩体处于扬子板块西缘与NW向的金沙江—哀牢山深大断裂带东侧交汇部位,属于哀牢山—金沙江富碱侵入岩带的组成部分。马厂箐岩体主要为花岗斑岩、二长斑岩和正长斑岩,岩体化学成分具有富碱、富铝和高钾的特点。岩石富集大离子亲石元素(Rb、Sr、Ba、Th和U)和轻稀土元素(LREE)、相对亏损高场强元素(Ta、Nb和Ti),且Ta、Nb和Ti具"TNT"负异常;LREE/HREE值为8.04～23.99,δEu值为0.72～0.88,负Eu异常不明显;n(87Sr)/n(86Sr)值范围为0.7072～0.7086,均值为0.7080,高于原始地幔现代值0.7045;n(143Nd)/n(144Nd)值范围为0.5121～0.5124,均值0.5123,低于原始地幔现代值0.512638;ISr值范围为0.7061～0.7075,均值为0.7070;εNd值范围为(3.1～(10.2,均值为(6.8。马厂箐岩体既不属于传统的含碱性暗色矿物的碱性岩类,也不同于典型的A型花岗岩,应属于富碱侵入岩类;岩体的岩浆起源于壳—幔物质混合的一种所谓EMⅡ型富集地幔源;其壳幔混合特征,主要是俯冲进入上地幔的地壳物质与地幔物质发生了源区混合作用的结果;形成于碰撞后的板内构造环境。

Geochemistry Characteristics of Machangqing Alkaline-rich Intrusives, Yunnan, and Its Forming Tectonic Settings

Located at the joint of western margin of the Yangtze plate and eastern side of the NW-trending Ailao Mts.-Jinsha River major fault, the Machangqing intrusives is a part of alkaline-rich intrusion belt of the Ailao Mts.-Jinsha River. The Machangqing intrusives mainly consist of granite porphyry, monzonite porphyry and syenite-porphyry.The compositions of the rocks are riched in alkaline, aluminum, potassic, large ion lithophile elements(Rb, Sr, Ba,Th and U) and light rare earth elements, depleted in Ta, Nb and Ti, with "TNT" negative anomaly. The value of LREE/HREE ranges from 8.04 to 23.99, δEu from 0.72 to 0.88,is no conspicuous Eu anormaly;n(~(87)Sr) / n(~(86)Sr) ranges from 0.7072 to 0.7086,averaging 0.7080, higher than present mantle value (0.7045);n(~(143)Nd)/n(~(144)Nd) varies in the range of 0.5121～0.5124,averaging 0.5123, lower than present mantle value (0.512638);I_(Sr) varies in the range of 0.7061～0.7075,averaging 0.7070;(_(Nd)varies in the range of -3.1～-10.2,averaginjg-6.8. The Machangqing intrusives are neither traditional alkaline rocks with some dark alkaline minerals nor typical A-type granite, but alkaline-rich intrusions. The magma of the intrusives is derived from the mixture of crust-mantle, as being called EMⅡ-type. The intrusives show the characteristics of crust-mantle mixture, resulting from mixing in the mantle source with the crust subduction formed in a post-collisional intraplate.