云南民乐铜矿床中玄武岩和流纹斑岩的成因:年代学和地球化学制约

Petrogenesis of the basalts and rhyolite porphyries of the Minle copper deposit,Yunnan:Geochronological and geochemical constraints

本文对云南民乐铜矿床中宋家坡组流纹斑岩进行了锆石SIMSU-Pb年代学,流纹斑岩和玄武岩的地球化学和Nd同位素研究工作,目的是揭示民乐铜矿床中赋矿建造次火山岩和玄武岩的成因和构造环境。宋家坡组玄武岩通常显示出较高的Fe2O3、Al2O3和Na2O含量,随SiO2含量增加,MgO、CaO和P2O5含量减小。玄武岩具有较高的εNd(t)值(+2.9～+3.5),岩石稀土总量较低(∑REE=50.1×10-6～60.6×10-6)并在球粒陨石标准化的稀土元素图解上显示出比较平坦的REE模式,在蛛网图上表现出Nb-Ta轻微亏损Sr富集的特征。这些岩石总体上表现出亚碱性玄武质岩石特征,原始岩浆很可能来源于长期亏损、富斜长石而贫石榴石的尖晶石地幔源区部分熔融。本文认为这些玄武岩形成过程中岩浆经历了结晶分异作用和不同程度的地壳混染作用。宋家坡组流纹斑岩的锆石SIMSU-Pb年龄为234.8±2.4Ma。这些岩石与一般流纹岩相比具有较高的SiO2、(Na2O+K2O)和Al2O3含量,在球粒陨石标准化的稀土元素图解上显示出轻稀土元素(LREE)富集而重稀土元素相对平坦的模式,并具有轻微Eu负异常的特征。在原始地幔标准化的微量元素蛛网图上,这些岩石的Nb-Ta、Sr、P和Ti表现出明显亏损的特点。上述流纹斑岩显示出I型花岗岩的特征,其εNd(t)为-1.9至-0.51之间。这些岩石的母岩浆很可能是弱碱性的流纹安山质岩浆,可能是由区域分布的中-新元古代镁铁质-中性的下地壳部分熔融而来,然后加入了少量的同时代玄武质岩浆。这些岩浆而后经历了较充分的结晶分异作用形成了流纹斑岩。民乐矿区玄武岩和流纹斑岩表现出岛弧火山岩的特征,很可能是在碰撞晚期-碰撞后环境下形成。

SIMS U-Pb zircon ages for rhyolite porphyries, and geochemical and Nd isotopic data are reported for rhyolite porphyries and the related basalts from the Songjiapo Formation of the Minle copper deposit of Yunnan Province, SW China. The aims are to constrain the origin and petrogenesis of these rocks. The basalts are commonly high in Fe203, A1203 and Na20 contents. In the Harker diagrams, MgO, CaO and P205 contents of the basalts decrease with increasing SiO2 contents. These rocks with high end （t） values （ ＋ 2. 9 - ＋ 3. 5 ）, are characterized by relatively low total REE contents （ 50. 1 × 10^-6- 60. 6 × 10^-6 ） with relatively flat REE patterns in the chondrite-normalized diagram and with slightly negative Nb-Ta and positive Sr anomalies in the primitive-normalized spidergram. The parental magma for the basalts exhibits affinity with a sub-alkaline basaltic magma generated by melting of a depleted, plagioclase-rich and garnet-free mantle source in the spinel field. It is suggested that the basalts originated by fractional crystallization of the parental magma plus varying degrees of crustal contamination. Cameca SIMS zircon U-Pb dating indicates that the rhyolite porphyries were emplaced at 234.8 ＋- 2.4Ma. These rocks have high SiO2, （ Na20 ＋ K20） and AI203 contents compared with the normal rhyolites, and are enriched in LREE and show relatively flat HREE patterns with slightly negative Eu anomalies in the ehondrite-normalized diagram and significantly negative Nb-Ta, St, P and Ti anomalies in the primitive mantle-normalized spidergrams. The rhyolite porphyries display geochemical characteristics of I-type granites and exhibit variably initial eNa （t） values ranging from - 1.9 to -0. 51. The parental magma for these rocks exhibits affinity with a mildly alkaline intermediate to felsic magma, which was probably generated by partial melting of the regional Mesoproterozoic to Neoproterozoic marie to intermediate lower crust and thereafter mixed with small amounts of the Middle Triassic basaltic magmas. The rhyolite porphyries were then formed by extensive crystal fractionation of the mildly alkaline magma. The basalts and the rhyolite porphyries from the Minle copper deposit display the characteristics of arc volcanic rocks, which possibly formed in a late-collisonal to post-collisional environment.