南秦岭勉略带铧厂沟火山岩锆石U-Pb年代学及地球化学研究

摘要

陕西省西南部铧厂沟火山岩以英安岩为主(～ 90vol％),夹玄武岩构造透镜体(～ 10vol％).玄武岩SiO2含量为43.6％ ～ 54.7％,具有低K、Ti,高Na、Mg的特征；稀土总量为24×10-6 ～29×10-6,中稀土轻微富集,Eu、Sr轻微正异常；具有正Rb、Ba异常及负Nb、Zr异常,LaN/YbN值为1.81 ～2.87,Th/Yb值为0.19 ～0.23,Th/Nb值为0.11 ～0.20,Nb/La值为0.26～0.70,Hf/Th值为0.50 ～ 0.67,显示亚碱性弧玄武岩的特征.英安岩SiO2含量为59.5％～ 72.3％,稀土总量较低(116×10-6～187 × 10-6),为右倾式配分模式,Eu负异常,富集大离子亲石元素(如Rb、Ba、Th、K等),亏损高场强元素(如Nb、P、Ti、Ta等),显示弧火山岩地球化学特征.获得玄武岩的锆石SHRIMP U-Pb年龄为801.7±4.7Ma (MSWD=1.18;n=12),英安岩的锆石LA-ICP-MS U-Pb年龄为802.1±5.3Ma (MSWD=1.02;n=19),二者在误差范.内一致.因此,铧厂沟火山岩是一套火山弧环境的亚碱性玄武质-英安质火山岩组合,表明新元古代曾有大洋板块向南俯冲到扬子古板块北缘之下；这套火山岩裹挟于泥盆系沉积地层中,与泥盆系地层一起,共同组成了一套由晚古生代-三叠纪勉略洋闭合所致的构造混杂岩带.%The Huachanggou volcanic rocks in Southwest Shanxi are mainly dacite ( ~ 90vol% ) structurally interlayered with basalt tectonic lenses ( ~ 10vol% ). The basalts have SiO2 contents of 43. 6% ~54. 7% , characterized by low contents of K and Ti, and high contents of Na and Mg. Their REE concentrations range 24 × 10 -6 ~ 29 × 10 -6 with slightly enriched LREE and MREE, and slightly positive Eu and Sr anomalies. They show positive Rb and Ba anomalies and negative Nb and Zr anomalies, and yield (La/ Yb)N, Th/Yb, Th/Nb, Nb/La and Hf/Th ratios of 1.81 ~2.87, 0.19 ~0.23, 0.11 ~0.2, 0.26 ~0.7 and 0.50 ~0.67, respectively, resembling the characteristics of sub-alkaline arc basalts. The dacites have SiO2 contents of 59. 5% ~ 72. 3% , REE concentrations of ll6×l0-6 ~187 ×l0-6 and negative Eu anomalies, and are rich in LILE (e. g. Rb, Ba, Th and K) , but depleted in HFSE (e. g. Nb, P, Ti and Ta) , similar to the volcanic rocks developed in volcanic arcs. The basalt sample yields zircon SHRIMP U-Pb age of 801. 7 ±4. 7Ma (MSWD = 1. 18; n = 12); which is same to the zircon LA-ICP-MS U-Pb age of 802. 1 ±5. 3Ma (MSWD = 1. 02; n = 19) obtained from the dacite. Hence, the volcanic rocks at Huachanggou area constitute a sub-alkaline basalt-to-dacite suite which suggests that there existed a southward subduction of an oceanic plate beneath the Yangtze craton in Neoproterozoic; and the Devonian sedimentary strata, together with their structurally sandwiched slices of the Neoproterozic volcanic rocks, form a tectonic melange resulted from Late Paleozoic-Triassic suturing of the Mian-Lue Ocean.