北黄海盆地花岗斑岩锆石U-Pb年代学、地球化学及其地质意义

北黄海盆地X1和X2钻井岩心揭示上侏罗统中有侵入的花岗质岩石,镜下鉴定其为花岗斑岩。对花岗斑岩进行了主量、微量和稀土元素测试分析,样品的Si O2含量介于70.22%~71.88%之间,K2O/Na2O比值介于0.95~1.68之间,A/CNK比值介于0.75~1.12之间,有高钾钙碱性、弱碱质-偏铝质性质。微量元素组成富集Rb、Th、U等元素,相对亏损Nb、Eu、Sr等元素。稀土总量较高(∑REE=50.38×10-6~162.27×10-6),轻稀土(LREE)相对富集(LREE/HREE=6.86~25.43,(La/Yb)N=10.06~57.76),具有明显的Eu负异常(?Eu=0.65~0.83)。样品的Th含量高,与Rb呈正相关性,综合主量元素特征(A/CNK1.1),判别其为I型花岗岩。LA-ICP-MS锆石U-Pb测年结果表明X1井花岗斑岩年龄为110±1 Ma,X2井花岗斑岩年龄108±1 Ma、107±1 Ma,形成于早白垩世晚期。结合区域构造背景及岩石地球化学特征认为,北黄海盆地花岗斑岩形成于板块碰撞后的伸展构造背景。

U-Pb Zircon Chronology and Geochemistry of Granite Porphyry in North Yellow Sea Basin and their Geological Significance

Granite porphyries intrude Upper Jurassic strata were observed in the drilling cores of X1 and X2 in North Yellow Sea Basin.Major and trace element analyses show that the granite porphyries have SiO2 of 70.22％-71.88％,K2O/Na2O values of 0.95-1.68 and A/CNK values of 0.75-1.12.The rocks are high-K calc-alkaline and weak alkaline-metaluminous.The rocks are enriched in Rb,Th and U and relatively depleted in Nb,Eu and Sr.The rocks are rich in REE (∑REE=50.38×10-6-162.27×10-6) and characterized by relatively enriched LREE (LREE/HREE=6.86-25.43 and (La/Yb)N of 10.06-57.76),with obvious negative Eu anomalies (δEu values are 0.65-0.83).LA-ICP-MS zircon U-Pb dating results show that the granite porphyry of X1 well was formed at 110±1 Ma,and granite porphyry of X2 well at 108±1 Ma to107±1 Ma.The granites were emplaced in the late stage of Early Cretaceous.Based on regional tectonic setting and geochemical characteristics,the granite porphyries were formed in the extensional fault tectonic setting after plate collision.