太行山南段洪山岩体中粗面斑岩地球化学特征及LA-ICP-MS锆石U-Pb定年

Geochemistry and LA-ICP-MS zircon U-Pb dating for trachyte porphyry of Hongshan intrusion in the southern Taihang Mountains

在详细野外调研的基础上,对位于太行山南段洪山岩体中粗面斑岩的岩石学、地球化学和年代学特征进行了研究。该区斑岩中Si O2含量为63.57%~67.85%,平均为65.19%;里特曼指数σ值为5.6~8.2;K2O/Na2O为1.00~1.74,平均为1.24;A12O3为16.75%~18.77%,平均为18.21%。铝饱和指数A/CNK为1.00~1.06,属于碱性正长岩类岩石。岩石稀土元素总量(∑REE)为33.75×10-6~144.30×10-6,(La/Yb)N=9.18~33.28,呈现轻稀土元素(LREE)富集、重稀土元素(HREE)相对亏损的特点。δEu=0.99~1.16,有微弱正Eu异常。在原始地幔标准化微量元素蜘蛛图解中,样品均显示强烈富集大离子亲石元素(LILEs)、相对亏损高场强元素(HFSEs)的特征。同时,在该地区获得的粗面斑岩LA-ICP-MS锆石U-Pb年龄为128.6±1.0Ma。锆石εHf(t)值为-16.0^-11.4,对应的地壳模式年龄(TDM)为1258~1432Ma。这些数据表明,洪山粗面斑岩与洪山其他中生代侵入岩是同期岩浆作用的产物,形成于早白垩世由造山挤压环境向伸展环境转变期,是EM1型富集地幔部分熔融形成富碱的岩浆上升侵位,并受古老下地壳物质混染形成的。华北规模庞大的中生代岩浆作用可能与部分熔融并消耗富集岩石圈地幔层有关,这可能也是岩石圈地幔减薄的重要方式之一。

Based on detailed field investigation, the authors studied the petrology, geochemistry and geochronology of trachyte por- phyry in Hongshan intrusion. The SiO2 and A1203 of the rock range from 63.57% to 67.85% and 16.75% to 18.77%, 65.19% and 18.21% on average respectively. Rittmann index （σ） ranges 5.6-8.2, the K2O/Na2O ratio ranges 1.00-1.74, with an average of 1.24. A/CNK varies from 1.00 to 1.06. These data suggest that it belongs to alkaline syenite rock. The REE content is between 33.75-10 6 and 144.30×10^-6, with （La/Yb）N ratios of 9.18-33.28 and δ Eu of 0.99-1.16. The intrusion is remarkably characterized by enriched LREE and depleted HREE. The chondrite-normalized REE patterns show weak positive Eu anomaly. In the primitive mantle nor-malized trace element spider diagrams, the samples show strong enrichment of LILEs and relative depletion of HFSEs. In addition, tra- chyte porphyry＇s LA-ICP-MS zircon U-Pb dating indicates that the trachyte porphyry were emplaced at ca.128.6±1.0Ma. The zircon ε.Hf（t） values （-16.0- -11.4） yielded zircon Hf crustal modal ages of 1258-1432Ma. Based on the data obtained, the authors hold that Hongshan volcanic rocks originated from partial melting of an EMI-type mantle source, contaminated by LCC. Trachyte por- phyry formed in the same time as the Hongshan intrusion in the Mesozoic Cretaceous. Magmatic activities took place in the transfor- mation period of the collision environment, causing the partial melting of the mantle and producing the trachyte porphyry. This might have been one of the important ways oflithospheric mantle thinning.