姚安和马厂箐富碱侵入岩体的地球化学特征

Geochemical characteristics of the Yao'an and Machangqing alkaline-rich intrusions

姚安岩体和马厂箐岩体属哀牢山-金沙江富碱侵入岩带的组成部分,位于NW-WNW向哀牢山-金沙江深大断裂东侧。姚安岩体主要为正长斑岩,马厂箐岩体主要为花岗斑岩两岩体化学成分上具有富碱、高钾的特点。岩石富轻稀土和Rb、Sr、Ba、Th、La等大离子亲石元素而亏损Nb、Ta和Ti等高场强元素。岩石的Rb/Sr、La/Nb和Ba/Nb以及LREE/HREE比值高,铕异常不明显,ISr值偏高,εNd值偏低。研究表明:(1)姚安岩体和马厂箐岩体形成于喜山期大陆弧伸展环境。两岩体的岩浆均起源于富集地幔(EMII),其壳幔混合特征,主要是俯冲带地幔楔内壳-幔物质源区混合的结果;(2)尽管姚安和马厂箐岩体在成岩构造背景、成岩时代、成岩方式和源区特征以及成分上高碱富钾等方面具相似性,但它们在岩性和组成上仍存在较大差异。这种差异可能是由源区物质部分熔融程度不同,以及岩浆结晶时温压和氧逸度等条件的不同所造成;(3)金矿化主要与相对高温、低压和高氧逸度的碱性系列岩浆活动有关,而铜矿化则主要与相对低温、高压和低氧逸度的碱性-亚碱性过度系列岩浆活动有关。

The Yao'an and Machangqing alkaline-rich intrusions, lying in east margin of the NW-WNW Ailaoshan-Jinshajiang fault zone, form parts of the Ailaoshan-Jinshajiang alkaline-rich intrusive belt. The Yao'an intrusion consists mainly of syenite porphyry, and the Machangqing intrusion consists mainly of granite porphyry. Chemically these two intrusions are characterized by enrichment in alkaline and potassium. They enrich in LREE and LILE (such as Rb, Sr, Ba, Th, La etc.), depict HFSE (such as Nb, Ta and Ti etc.), have high Rb/Sr, La/Nb, Ba/Nb and LREE/HREE ratios, and don't show obvious Eu anomalies. The I-Sr values of these two intrusions are relatively high, and epsilon(Nd) values are relatively low. It is suggested that the intrusions were derived from partial melting of an enriched upper mantle (EMII), a mixture between normal mantle and subducted oceanic crust, in a continental arc of extension tectonic setting. Although these two intrusions share similar characteristics on tectonic setting in the Himalayan period, age and source region, they have some difference on lithology and chemical composition, which was probably related to the difference on physical-chemical condition (such as T, P and fO(2) etc.) during magmatic crystallization, and on different melted degress of source rocks. An mineralization is considered to be associated with the intrusions of alkaline series while Cu mineralization with the intrusions of alkaline-subalkaline series in Ailaoshan- Jinshajiang alkaline intrusive belt.