湘南道县辉长岩包体的锆石LA-ICP-MS定年、Hf同位组成及其地质意义

LA-ICP-MS Zircon U-Pb Dating and Hf Isotope Feature of Gabbro Xenolith and Its Geological Significance in Huziyan Basalt of Daoxian County,Southern Hunan Province

道县虎子岩碱性玄武岩筒产出较多的辉长岩包体。用于锆石定年的辉长岩样品的岩石化学特征显示:其K_2O、Fe_2O_3、TiO_2高于其他同类样品,稀土元素、微量元素总量也相对较高,样品的K*值(2K_N/(Ta_N+La_N))为7.60>1,显示岩浆具有亲岛弧性质,反映源区地幔受到较强的交代作用;Nb*值(2Nb_N/(K_N+La_N))为0.11<1,反映基性岩浆侵入时受到了地壳混染;辉长岩中的锆石具有清晰的韵律环带,Th/U值=0.91~4.35,具有岩浆锆石的特征;锆石^(206)Pb/^(238)U加权平均年龄为(201.50±0.53)Ma,代表了辉长岩的结晶年龄;ε_(Hf)(t)值为-0.88~4.45,投点落入球粒陨石附近,暗示强烈的壳幔交换作用;Hf模式年龄平均为751 Ma,反映早期地壳对岩石源区的贡献。Hf同位素组成特征反映辉长质岩浆是在印支晚期局部拉张背景下,岩石圈上地幔上隆减压熔融的产物。

There are many gabbro xenoliths in Huziyan alkali basalt pipe in Daoxian County. The geochemical analysis of one gabbro dating sample show that it has high content of K_2O,Fe_2O_3 and TiO_2,compared with other gabbro samples,which suggests that the gabbro xenolith is product of crystallizationdifferentiation of late-stage magma. The K＊ value（ 2K_N/（ Ta_N＋ La_N）） is 7. 60（ 1）,displaying the magma source experienced metasomatism. The Nb＊ value（ 2 Nb_N/（ K_N＋ La_N）） is 0. 11（ 1）,implying crust contamination during basic magma intrusion. Zircon grains are chosen from gabbro xenolith for LA-ICP-Ms U-Pb dating and Hf isotope analysis. Zircon grains in the gabbro have clear crystallization zoning,the ratio of Th / U is0. 91 ～ 4. 35,these features imply that these zircon grains have magmatic origin. The average age of 206 Pb /238 U is（ 201. 50 ± 0. 53） Ma（ 1σ,MSWD = 4. 6）,which represents the emplacement age of the gabbro xenolith.According to Hf isotope compositions,the calculated model age is 751 Ma,indicating the earlier crustal material gave contributions to source area,the values of ε_（Hf）（ t） are- 0. 88 ～ 4. 45,in the figure of ε_（Hf）（ t）- t,the points of zircon grains lie in the area neighbored to evolution curve of chondrite mantle,which are different apparently from their Nd isotope geochemistry. Geochemical characteristics show that the deeply strong material exchange progress between deep crust and mantle in southern Hunan area. In Late Indosinian epoch,there occurred mantle uplift induced by partial extensions,which resulted in progressively the mantle melting to from gabbro magma. The gabbro magma underplated at the bottom of lithosphere and became one part of its consistent.