辽西北票蓝旗组火山岩锆石U-Pb年龄和Hf同位素组成

辽西北票常河营子地区有中生代蓝旗组火山岩分布,其中上部安山质角砾熔岩的锆石LA-ICPMS U-Pb年龄分析结果表明,其结晶年龄为159.4±3.4Ma,属晚侏罗世.锆石~(176)Hf/~(177)Hf比值介于0.282098～0.282789之间,ε_(Hf)(t)值为-20.4～+4.1,主体分布在华北克拉通地壳演化线之上,位于古元古代地壳演化范围内,所给出的亏损地幔年龄(t_(DM))和平均地壳模式年龄(t_(crust))分别为0.7～1.6Ga和0.9～2.5Ga.结合已发表蓝旗组中酸性火山岩的岩石地球化学及Sr-Nd同位素组成特征,我们认为安山质火山岩源于古老(如晚太古代)下地壳玄武质岩石的部分熔融,其形成过程可能与中生代幔源岩浆底侵作用有关.

In-situ U-Pb dating and Hf isotopic analyses of zircons in the volcanic rock of the Lanqi Formation in the Beipiao area, western Liaoning Province

Large-scale volcanic rocks of the Lanqi Formation are developed in the Changheyingzi area, western Liaoning Province. LA-ICPMS zircon U-Pb dating gives the Late Jurassic eruption age for andesitic breccia lava of the Lanqi Formation as 159.4 ± 3.4 Ma. In-situ zircon Hf analyses show that Lanqi Formation have ~(176)Hf/~(177)Hf values of 0.282098 to 0.282789, ε_(Hf)(t) values of -20.4 to + 4.1, t_(DM) (depleted mantle model age) of 0.7 ～ 1.6Ga and t_(crust)(average crustal model age) of 0.9 ～2.5Ga, respectively. Combining the published geochemical data of the volcanic rocks, especially their Sr-Nd isotopic compositions, we interpret the andesitic volcanic rocks as products mainly derived from the partial melting of Neoarchean basaltic rocks in the lower crust due to the basaltic magmas underplating in Mesozoic time.