摘要
灵山岛位于胶东半岛以南约16km处,构造上处于扬子和华北克拉通结合部位——苏鲁造山带东端。早白垩世晚期,灵山岛岩浆活动极为强烈,大量发育流纹岩和火山角砾岩,不整合覆盖于底部碎屑沉积岩层之上;同时,区内广泛发育基性岩墙(辉绿玢岩),沿北东-南西向侵入于碎屑沉积岩地层之中。本文选取流纹岩和辉绿玢岩样品进行了岩石地球化学和年代学研究。结果表明,流纹岩具有富钾(K_2O=4.10%～4.42%)、富碱(Na_2O+K_2O=8.83%～9.06%)、贫钙(CaO=0.10%～0.46%)、低钛(TiO 2=0.08%～0.09%)、低镁(MgO=0.12%～0.15%)和铁(Fe_2O_3~T=0.79%～0.83%)的特征,属于弱过铝质高钾钙碱性岩石系列;岩石稀土总量较低(∑REE=109.0×10^(-6)～128.8×10^(-6)),轻重稀土元素分异较弱((La/Yb)N=6.42～8.09),Eu显著负异常(δEu=0.27～0.28)。辉绿玢岩SiO_2含量为(51.17%～51.97%),具有富碱(Na_2O+K_2O=5.01%～6.07%)和高Mg#值(67.6～69.4)的特征,属于钾玄岩系列;岩石稀土总量较高(∑REE=160.6×10^(-6)～173.5×10^(-6)),轻重稀土元素分馏较为明显((La/Yb)N=11.1～11.6),显示弱Eu正异常(δEu=1.12～1.18),它们在球粒陨石标准化稀土元素图解和原始地幔标准化微量元素图解上与OIB类似。上述地球化学特征指示流纹岩和辉绿玢岩可能均形成于伸展减压背景下。LA-ICP-MS锆石U-Pb定年结果表明,流纹岩和辉绿玢岩的形成时代分别为118±2Ma和109±3Ma,属于早白垩世岩浆活动的产物。锆石Hf同位素分析结果显示,流纹岩具有负的ε_(Hf)(t)值(-31.0～-24.5),表明其来源于古老地壳物质部分熔融。辉绿玢岩的岩浆锆石具有负的ε_(Hf)(t)值(-31.2～-28.8)和正的ε_(Hf)(t)值(+7.1～+8.1),指示其来源于有深部亏损软流圈地幔物质加入的富集地幔源区。综合本文研究结果和同时期区域构造演化推测,受伊泽奈崎板块和古太平洋板块俯冲方向改变的影响,中生代期间中国东部构造体制发生转变。早白垩世晚期,华北东部处于伸展构造背景,形成了一系列与之对应的超浅成相-喷出相辉绿玢岩岩墙和流纹岩,是燕山运动在胶东地区的地质表现。
The Lingshan Island is located approximately 16km in the south of Jiaodong Peninsula. It is known as the eastern part of the Dabie-Sulu orongenic belt conjuncting the North China Craton （NCC） and the Yangtze Block （YB）. In Late Early Cretaceous, magmatic activities strongly occurred in this place, developing numerous rhyolites and volcanic breccias in the upper part of the island that unconformably overlies the clastic sedimentary rocks in the lower part. What＇s more, mafic dyke swarms （diabase porphyrite） widely intrude into the clastic sedimentary rocks along the NE-SW trend. In this study, petrogeochemical and chronological analyses were conducted on the rhyolite and diabase porphyrite. The results show that the rhyolite samples have high K2O contents （4.10%-4.42%） and are alkali-rich （Na2O＋K2O=8.83%-9.06%）, with low contents of CaO （0.10%-0.46%）, TiO2 （0.08%-0.09%）, MgO （0.12%-0.15%） and Fe2O3T （0.79%-0.83%）. All the samples are slightly peraluminous and belong to calcic-alkaline rock series. Their chondrite-normalized REE patterns are characterized by slightly enrichment of LREEs （（La/Yb）N=6.42-8.09）, relatively low REE contents （∑REE=109.0×10-6-128.8×10-6） and negative Eu anomalies （δEu=0.27-0.28）. The diabase porphyrite samples have low SiO2 contents （51.17%-51.97%）, high alkali contents （Na2O＋K2O=5.01%-6.07%） and Mg# values （67.6-69.4）, belonging to shoshonitic rock series. Their chondrite-normalized REE patterns show relatively enrichment of LREEs （（La/Yb）N=11.1-11.6） and relatively high REE contents （∑REE=160.6×10-6-173.5×10-6）, with slightly positive Eu anomalies （δEu=1.12-1.18）. In the primitive mantle-normalized trace element patterns and chondrite-normalized REE patterns, these diabase porphyrite samples display OIB affinity. All these geochemical features reveal that the rhyolite and diabase porphyrite were generated in an extentional setting with low pressure. Futhermore, LA-ICP-MS zircon U-Pb dating results show that the rhyolite and diabase porphyrite were produced by Early Cretaceous magmatism, with formation age of 118±2Ma and 109±3Ma, respectively. Zircon Lu-Hf isotopic analyses indicate that the rhyolite was derived from an ancient crustal origin with negative εHf（t） values of -31.0--24.5, while the diabase porphyrite was derived from the enriched mantle source with negative εHf（t） values of -31.2--28.8 which mingled by deep depleted mantle components with positive εHf（t） values of ＋7.1-＋8.1. All these signatures, together with regional contemporary tectonic events suggest that, affected by the direction-changed of subduction of Izanagi plate and paleo-Pacific plate, tectonic regime in eastern China was transformed during Mesozoic. In Late Early Cretaceous, the eastern NCC was under extensional regime, producing a series of rhyolites and diabase porphyrite dykes, which represent geological response to the Yanshan Movement in Jiaodong Peninsula.
作者
敖文昊
冯涛
赵燕
翟明国
孙勇
AO WenHao;FENG Tao;ZHAO Yan;ZHAI MingGuo;SUN Yong(State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi＇ an 710069, China;Pangea Gold Corporation Limited, Kowloon, Hong Kong 999077, China;Key Laboratory of Computational Geodynamics, College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beifing 100049, China)
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2018年第6期1612-1640,共29页
Acta Petrologica Sinica
基金
国家"973"项目(2012CB416606)
西北大学大陆动力学国家重点实验室科技部专项联合资助
关键词
灵山岛
早白垩世
燕山运动
克拉通破坏
Lingshan Island
Early Cretaceous
Yanshan Movement
Cratonic destruction
