大兴安岭中段索伦地区早白垩世花岗岩锆石U-Pb年代学、地球化学及构造意义

Zircon U-Pb Geochronology,Geochemistry and Tectonic Significance of Early Cretaceous Granites in Suolun Area of Central Da Hinggan Mountains

对大兴安岭中部索伦地区花岗岩进行了年代学和地球化学研究,探讨了其成因类型及构造环境。LAICP-MS锆石U-Pb定年结果显示,明水河林场、索伦马场、索伦镇岩体年龄分别为(136±1),(134±1),(128±1)Ma,形成于早白垩世。明水河林场岩体高硅、富碱,贫钙、镁、钛和磷,A/CNK介于0.83～0.90,具有强烈的Eu负异常(δEu=0.33～0.38),明显富集Rb、Th、U、K等元素而亏损Ba、Sr、P、Ti等元素,具有A型花岗岩的特征。索伦马场和索伦镇岩体属高钾钙碱性系列,A/CNK介于0.88～1.0,富集大离子亲石元素LILE和LREE,亏损高场强元素HFSE,显示高分异I型花岗岩的特征。岩石地球化学组成显示索伦地区花岗岩岩浆源区为新增生的年轻地壳物质部分熔融。结合晚中生代大兴安岭地区的构造演化特征,认为索伦地区花岗岩形成于张性构造背景,可能与古太平洋俯冲作用引起的板内伸展作用有关。

We conducted geochronological and geochemical studies of the granites in Suolun area of central Da Hinggan Mountains,in order to discuss its rock type and tectonic setting.LA-ICP-MS zircon U-Pb dating results show that the rock ages of Mingshuihe forest farm,Suolun racecourse and Suolun town are(136±1)Ma,(134±1)Ma and(128±1)Ma,respectively.That means the granites in Suolun area are formed in the Early Cretaceous.The granites of Mingshuihe forest farm are characterized by high silicon,enrichment of alkaline,but depletion in calcium,magnesium,titanium and phosphorus.The A/CNK values range from0.83 to 0.90,show a peralkaline affinity.This series of rocks are enriched in Rb,Th,U,K,but depleted in Ba,Sr,P,Ti.Rare earth elements are characterized by strong negative Eu anomalies(δEu=0.33-0.38).The rock samples show an affinity to A-type granite.The geochemical features of the Suolun racecourse and Suolun town plutons show that they belong to high-K calc-alkaline rock series and are enriched in LILE and LREE,but depleted in HFSE.These chemical compositions of the rocks display that they are highly fractionated I-type granite.Petrological and geochemical compositions of the granites in Suolun area suggest that the magma source of the rocks was derived from partially melted by newly-accreted young crustal material.Combined with the tectonic evolution characteristics of the Da Hinggan Mountains in Late Mesozoic,we conclude that the granites in Suolun area are formed in the extensional tectonic setting,which may be related to the intraplate extension caused by the subduction of the Paleo-Pacific Ocean.