太行山南段洪山火山岩的锆石U-Pb年龄、地球化学特征及Hf同位素组成

洪山岩体是太行山南段中生代侵入岩的重要组成部分，具有碱性岩特征。洪山岩体南北两侧尚出露有火山岩类，在以往的研究中涉及较少。首次报道了洪山火山岩的锆石 U-Pb 年代学、全岩元素地球化学和锆石 Hf 同位素数据。锆石 U-Pb 定年结果表明，洪山火山岩形成年龄为(132.8±1.1) Ma，与洪山正长岩基本同期。其岩石类型为粗面质岩屑晶屑凝灰岩，具有富Al₂O₃(15.13%~16.78%)、高TFeO(5.61%~8.69%)、富碱(K₂O+Na₂O =9.72%~11.13%)的含量特征，无Eu异常(δEu=0.97~1.02)，轻重稀土分馏明显(LREE/HREE=8.61~12.30)，富集Rb、Sr、Th、K等大离子亲石元素(LILEs)，而亏损Nb、Ta、Ti等高场强元素(HFSEs)。洪山凝灰岩锆石测得ε_Hf (t) 值变化范围为-20.51~-12.34 ，对应的地壳模式年龄(T^C_DM)为1.9~2.5 Ga。洪山凝灰岩与洪山正长岩是同期-同源的岩浆作用的产物，形成于早白垩世由挤压环境向伸展环境转变的构造格局，是EMI型富集地幔部分熔融形成富碱的基性幔源岩浆上升，提供的热量诱发古老下地壳物质重熔，又与该壳源熔体混合形成母岩浆，再经历镁铁质矿物的分离结晶作用形成的。

Zircon U-Pb Ages,Geochemistry Characteristics and Hf Isotopic Compositions of the Hongshan Volcanic Rocks from Southern Taihang Mountains

The Hongshan complex is an important component of the Mesozoic intrusive rocks in the North Taihang Mountains, and is especially significant in alkaline rocks. Previous research restricted our overall understanding of Hongshan complex for lack in the volcanic rocks which are found on both sides of the Hongshan complex. In this paper, we firstly present the zircon U-Pb age data, whole-rock geochemistry and zircon Hf-isotope data of the Hongshan volcanic rocks. According to the U-Pb dating results, the eruption of tuffs on north of the Hongshan complex is dated at (132.8±1.1) Ma, indicating that the age is also close to the emplacement timing of the Hongshan complex and the intrusive rocks in North Taihang Mountains, even in other North China region. The tuffs are mainly composed of detritus, vitric, little crystal fragment and plastic lava. They are generally rich in Al₂O₃ (15.13%-16.78%), TFeO (5.61%-8.69%) and alkalis (K₂O+Na₂O,9.72%-11.13%), with normal Eu (δEu,0.97-1.02) and obvious fractionation between LREE and HREE (LREE/HREE,8.61-12.30). These rocks are all rich in LREE and LILEs (Rb, Sr, Th, K, etc), depleted in HFSEs (Nb, Ta, Ti, etc). The ε_Hf (t) values of the selected zircons vary between -20.51 and -12.34,with wide range of Hf-isotope crustal model ages from 1.9 to 2.5 Ga. Based on the above data, we suggest that the Hongshan volcanic rocks were most likely generated in a setting associated with westward subduction of the paleo-Pacific Plate to Eurasian Plate in Late Mesozoic, and originated from partial melting of an EMI-type mantle source, where mantle wedge-derived magmas may have provided the heat and material for the anatexis of ancient crust of the southern Taihang Mountains, resulted in hybrid melts. Such hybrid melts followed by a coupled fractionation of ferromagnesian phases are ultimately evolved into the Hongshan volcanic rocks.