滇西银厂坪花岗斑岩岩体的地球化学及年代学特征

银厂坪花岗斑岩岩体位于扬子陆块的西南缘,处于鲜水河—小江碱性岩带和金沙江—哀牢山富碱斑岩带之间。其主量元素具有高碱,高Na2O/K2O值,低TiO2特征,属于高钾钙碱性系列岩石。微量元素富集LILE,LREE和过渡金属元素(Cu、Ni、Co),具有明显的"Ta-Nb-Ti"负异常,Eu负异常不明显。锆石LA-ICP-MS U-Pb年龄表明该区岩石的结晶年龄为(37.39±0.33)Ma。εHf(t)值为-1.8～+1.8,两阶段模式年龄为996～1 226 Ma。该区岩石具有较低的εNd(t)值(-7.4～-3.6),较高的(87 Sr/86 Sr)i比值(0.706 077～0.706 231),以及具有造山带铅特征的207 Pb/204 Pb值(15.567～15.574)和208 Pb/204 Pb值(38.321～38.335)。对银厂坪岩体的元素地球化学以及Sr-Nd-Pb-Hf同位素组成特征分析研究认为,研究区的花岗斑岩形成于近似大陆弧环境,同位素地球化学属于EMI-EMII过渡型源区,岩体形成于壳-幔混合层。银厂坪岩体的地球化学属性更靠近安宁河碱性岩带,是该带分异末端的酸性端员组分。

Geochronology,geochemistry and geological significance of Yinchangping granite porphyry,Yunnan Province

Yinchangping granite porphyry intrusive outcrops around the southwestern segment of the Yangtze block,which is between the Xianshuihe-Xiaojiang alkaline rock zone and Jinshajiang-Ailaoshan alkali-rich porphyry rock zone.The major elements of this area show high alkaline,high ratio of Na2O/K2O and lower TiO2,presenting typical geochemical characteristic of high-K calc-alkaline rocks.Trace elements characteristics include LILE,LREE(with enrichments) and transitional elements(Cu,Ni and Co),as well as significant "Ta-Nb-Ti" negative anomaly and indistinctive Eu anomaly.LA-ICP-MS zircon U-Pb dating shows that the emplacement age of Yinchangping granite porphyry is(37.39±0.33) Ma: εHf(t) value is in the range of-1.8-+1.8 with two-stage Hf model age of 996-1226 Ma;εNd(t) value is a little low within the range of-7.4——3.6;(87Sr/86Sr)i(0.706077-0.706231) is high;207Pb/204Pb(15.567-15.574) and 208Pb/204Pb(38.321-38.335) present a typical characteristic of orogenic zone.All these analyses suggest that Yinchangping granite porphyry intrusion belongs to the Anninghe alkaline rocks zone which is formed in the environment of near continental arc,where there is not only the geochemical characteristic of EM I-EM II type source enrichment in mantle but also the porphyritic magma formed in "crust-mantle mixed layer".Yinchangping granite porphyry is the acid component at the end of fractional crystallization in Anninghe alkaline rock zone.