辽宁青城子铅锌多金属矿田晚三叠世岩浆岩年代学、地球化学及地质意义

Geochronology,geochemistry and geological significance of Late Triassic magmatism in Qingchengzi orefield,Liaoning

辽宁青城子铅锌矿田产出在辽东裂谷凹陷带,是中国北方重要的铅锌金银多金属矿田。研究表明,矿区内发育的以双顶沟黑云母二长花岗岩和新岭花岗岩为代表的晚三叠世花岗质岩浆事件,应与本区的矿化作用密切相关。LA-ICPMS锆石U-Pb定年结果显示双顶沟岩体侵位时间为224.2±1.2Ma。双顶沟岩体岩石化学组成呈现SiO269.07%～71.31%,K2O3.53%～5.22%,Na2O3.87%～4.14%,属于钙碱性岩石系列。Al2O3为12.46%～14.48%,A/CNK<1,具有准铝质特征。微量元素数据显示稀土总量较高,轻重稀土元素之间强烈分馏,[(La/Yb)N=35.43～79.01,LREE/HREE=23.09～35.10],Eu负异常不明显(δEu=0.68～0.97)。富集大离子亲石元素Rb、Th、K、Pb等,而亏损高场强元素Nb、Ta、P、Ti等。具有高Sr,Ba含量,低的Y,Yb含量特征,属于具有类似埃达克质亲和性的I型花岗岩,具有高Nb/Ta(18.4～21.2),上述岩石地球化学特征,表明其岩浆源区残留固相矿物组合中包含可观的石榴子石和金红石,可能起源于加厚下地壳的部分熔融,并可能存在与幔源基性岩浆混合作用。推测扬子和华北板块深俯冲过程中板片断离可能是诱发与成矿有关岩浆的地球动力学原因。

The Qingchengzi orefield which lies in geotectogene of Liaodong rift is an important polymetallic mineral district in northern China for its clustering several large Pb-Zn,Ag and Au deposits.A phase of Late Triassic magmatism,represented by Shuangdinggou biotite monzogranite and Xinling granite,is intimately related to polymetallic mineralization.LA-ICPMS zircon U-Pb age of 224.2±1.2Ma was obtained for the Shuangdinggou intrusion.Geochemical data for the Shuangdinggou intrusion reveal SiO2 content of 69.07%～71.31%,while K2O accounts for 3.53%～5.22% and Na2O for 3.87%～4.14%,indicating it belongs to high K calc-alkaline series.Al2O3 content ranges between 12.46%～14.48% and A/CNK1 suggest metaluminous feature.Trace element geochemistry of the biotite monzogranite displays high total REE content and demonstrate strong fractionation between light and heavy REE elements [（La/Yb） N=35.43～79.01,LREE/HREE=23.09～35.10] and minor negative Eu anomalies（δEu=0.68～0.97） .The biotite monzogranite also shows LILE（such as Rb,Th,K,Pb） enrichment and HFSE（including Nb,Ta,P,Ti） depletion,with unusual high Ba and Sr,low Y and Yb abundances manefesting adakitic geochemical signature.Based on above-stated petrological and geochemical data it is inferred that the biotite monzogranitic magma might be derived from the partial melting of a thickened lower crust with prominent garnet and rutile retained in the residual assemblages,and magma mixing is probably involved based on the unusually high Nb/Ta ratio（18.4～21.2） .It is suggested that slab break-off during the process of Yangtze Craton and North China Craton continental deep subduction in Late Triassic may be responsible for the ore-related magmatism.