郯庐断裂山东段(胶东)中生代中酸性火山岩的锆石U-Pb年代学及地球化学特征

Zircon U-Pb age and geochemistry of Mesozoic intermediate and acidic volcanic rocks from the Shandong segment( Jiaodong area) of the Tan-Lu fault

对胶东青山群中酸性火山岩进行了LA-ICP-MS锆石U-Pb同位素年代学研究.结果表明,3个粗面岩-租面英安岩和1个流纹岩的年龄分别为119.4±0.9 Ma、118.2±1.0 Ma、120.2±0.9Ma和120.0±0.8Ma,都形成于早白垩世120～118 Ma之间.研究表明郯庐断裂带山东段及其两侧中生代青山群火山岩的年龄与苏皖段断裂带内火山岩的年龄相似,明显小于苏皖段断裂带附近火山岩盆地的火山岩年龄.其结果表明在中国东部岩石圈减薄的大背景下,受郯庐断裂控制的岩浆喷发事件持续的时间可能更长.胶东青山群中酸性火山岩表现出富钾、富碱、贫镁、贫钛和低Ni、Cr的地球化学特征,轻稀土元素富集,重稀土元素亏损.按地球化学组成可进一步分为粗面岩-粗面英安岩和流纹岩两类,前者富集大离子亲石元素Rb、Ba、K,亏损高场强元素Nb、Ta、Ti和P;而后者则强烈亏损Ba、Sr等大离子亲石元素和Ti、P等高场强元素.粗面岩-粗面英安岩相对流纹岩具有高的Sr和Ba含量以及La/Nb、Ba/Nb比值,而具有低的Rb/Ba比值和Eu负异常,说明它们来源于不同的岩浆源区.推测粗面岩-粗面英安岩可能是下地壳（俯冲扬子下地壳或者古老华北下地壳）部分熔融和富集地幔部分熔融混合的产物;流纹岩是由于区域拉张环境和高热异常背景下,下地壳发生深熔产生的岩浆喷出地表形成的,岩浆在上升过程中发生有分离结晶（大量斜长石的分离结晶）过程.

In this paper, LA-ICP-MS zircon U-Pb dating was carried out for four samples of volcanic rocks from Qingshan Group in ]iaodong area and, as a result, their concordant ages were obtained, which are 119.4 ± 0.9 Ma, 118.2 ± 1.0 Ma, 120.2 ± 0.9 Ma and 120.0 ± 0.8 Ma, respectively. The results show that all the volcanic rocks in Jiaodong area were formed from about 120 Ma to 118 Ma in the Early Cretaceous period. The ages of Qingshan Group volcanic rocks from Shandong Province controlled by Tan-Lu fault are similar to those from the Su-Wan segment of the Tan-Lu fault zone, but younger than those around the Tan-Lu fault zone, which indicates that the duration of volcanic eruption along Tan-Lu fault is probably longer than the duration in other areasunder the background of lithospheric thinning in eastern China. Geochemically, all the intermediate and acidic volcanic rocks have high K20, ALK, LREE values and low MgO, TiO2, Ni, Cr, HREE values. According to the geochemical features, the volcanic rocks can be subdivided into trachyte-trachydaeite and rhyolite. The tra- chyte-trachydacite is characterized by LILE enrichment（Rb, Ba, K）and HFSE depletion（Nb, Ta, Ti, P）, but the rhyolite exhibits stronger depletion of Ba, Sr and HFSE （Ti, P）. Compared with the rhyolite, the trachyte-tra- chydacite possesses higher Sr and Ba values as well as La/Nb and Ba/Nb ratios, but lower Rb/Ba ratios and 3Eu values. All these data indicate that they might have been derived from different magma sources. It is inferred that the trachyte-trachydacite rocks were derived from the mixture of the partial melting of the lower crust （Yangtze Craton or North China Craton）and enriched lithospherie mantle. In addition, the rhyolites were relat- ed to the partial melting of the lower crust in the regional extension environment and high geothermal anomaly background with crystallization differentiation during the magma evolution.