Genesis of lamprophyres from Machangqing, western Yunnan: Constraints from geochemistry, geochronology and Sr-Nd-Pb-Hf isotopes.
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摘要:
滇西马厂箐煌斑岩脉分为早晚两期，早期为云斜煌岩，晚期为云煌岩。对煌斑岩样品进行锆石LA-MC-ICP-MS U-Pb定年，获得33.77±0.11Ma的岩浆侵位年龄，表明马厂箐煌斑岩为马厂箐复式杂岩体中期岩浆结晶作用的产物，也是金沙江-哀牢山断裂带新生代早期岩浆活动的产物。岩石地球化学分析表明马厂箐煌斑岩高钾(K2O/Na2O=2.88~4.60)、富碱(K2O+Na2O=6.40%~7.55%)、高Mg#(74.3~76.9)，富集大离子亲石元素(LILE)(K、Rb、Ba)和轻稀土元素(LREE)，亏损高场强元素(HFSE)(Nb、Zr、P)和重稀土元素。同位素以高(87Sr/86Sr)i(0.707406~0.706506)，低εNd(t)(-1.78~-7.64)，富集放射性Pb (208Pb/204Pb=38.87~39.35，207Pb/204Pb=15.6390~15.6431，206Pb/204Pb=18.6579~18.8093)为特征。176Hf/177Hf为0.282712~0.282864，εHf(t)为-1.4~4.0，εNd(t)与εHf(t)存在Nd-Hf同位素解耦。根据岩石地球化学和同位素特征以及区域构造演化历史分析，马厂箐煌斑岩起源于金沙江洋壳俯冲交代作用形成的富集地幔，区域上软流圈物质上涌可能促使EMII的部分熔融形成煌斑岩岩浆，而金沙江-哀牢山断裂及其与其他断裂的交汇处则为岩浆上侵提供了良好的通道。
Abstract:Lamprophyres occur as early and late dykes in Machangqing, western Yunnan. The early lamprophyres are mostly kersantons and the late are minettes. Zircon LA-MC-ICP-MS U-Pb dating yields an age of 33.77±0.11Ma, which indicate lamprophyres are the product of middle stage magmatic process of Machangqing complex and early pulse of Cenozoic magmatism in the Jinshajiang-Ailaoshan fault belt. Geochemically, the lamprophyres have high values of potassium (K2O/Na2O=2.88~4.60), alkaline (K2O+Na2O=6.40%~7.55%) and Mg#(74.3~76.9) and are enriched in large ion lithophile elements (K, Rb, Ba) and LREEs and depleted in the high field strength elements (Nb, Zr, P) and HREEs. The isotopes of lamprophyres are characterized by high (87Sr/86Sr)i(0.707406~0.706506), low εNd(t) (-1.78~-7.64) but enriched radioactive Pb (208Pb/204Pb=38.87~39.35, 207Pb/204Pb=15.6390~15.6431, 206Pb/204Pb=18.6579~18.8093). The ratios of 176Hf/177Hf range from 0.282712 to 0.282864, with εHf(t) values varying from -1.4 to 4.0. The εNd(t) and εHf(t) values show a Nd-Hf decoupling feature. Based on the geochemistry, isotopes and the regional tectonic evolution history, the lamprophyres were derived from EMII produced by an interaction between the Jinshajiang oceanic crust and the mantle peridotite while the heat was provided by the upwelling asthenosphere. The Jinshajiang-Ailaoshan fault belt is likely to act as migration channels for magmatic emplacement.
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Key words:
- Lamprophyres /
- Geochemistry /
- Geochronology /
- Sr-Nd-Pb-Hf isotopes /
- Genesis /
- Machangqing
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图 1
马厂箐矿区地质简图(底图据西南冶金地质勘探公司310地质队，1981①修改)
Figure 1.
Geological sketch map of the Machangqing ore-field
图 2
马厂箐煌斑岩部分锆石CL图像和U-Pb年龄谐和图
Figure 2.
Concordia U-Pb age diagram and CL images of zircons from Machangqing lamprophyres
图 4
马厂箐煌斑岩球粒陨石标准化REE模式图(a，球粒陨石值据Boynton, 1984)和原始地幔标准化微量元素蛛网图(b，原始地幔值据McDonough et al., 1992)
Figure 4.
Chondrite-normalized REE distribution patterns (a, normalized data after Boynton, 1984) and primitive mantel normalized spidergrams (b, normalized data after McDonough et al., 1992) of lamprophyres from Machangqing
图 5
马厂箐煌斑岩87Sr/86Sr-206Pb/204Pb和143Nd/144Nd-206Pb/204Pb相关图解(底图据Rollison, 2000)
Figure 5.
The 87Sr/86Sr-206Pb/204Pb and 143Nd/144Nd-206Pb/204Pb diagrams of the lamprophyres from Machangqing (after Rollison, 2000)
图 6
马厂箐煌斑岩207Pb/204Pb-206Pb/204Pb相关图解(底图据Rollison, 2000)
Figure 6.
The 207Pb/204Pb-206Pb/204Pb diagrams of the lamprophyres from Machangqing (after Rollison, 2000)
图 7
马厂箐煌斑岩中锆石Hf同位素组成
Figure 7.
Zircon Hf isotopic compositions of lamprophyres from Machangqing
图 8
马厂箐复式杂岩体同位素年龄统计图
Figure 8.
Statistical diagram of isotopes ages for the complex from Machangqing
图 9
马厂箐煌斑岩Nb/U-Nb图解(底图据姜耀辉等, 2006)
Figure 9.
Nb/U-Nb diagrams of lamprophyres from Machangqing (after Jiang et al., 2006)
图 10
马厂箐煌斑岩La/Yb-Na/Ta图解
Figure 10.
La/Yb-Na/Ta diagrams of lamprophyres from Machangqing
图 11
马厂箐煌斑岩构造环境判别图(底图据Müller et al., 1992)
Figure 11.
Discrimination diagrams of tectonic setting of lamprophyres from Machangqing (after Müller et al., 1992)
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