大兴安岭南段北大山岩体的年代学和地球化学:对岩石成因及成矿潜力的指示

Geochronology and geochemistry of the Beidashan pluton in the southern Great Xing an Range,Northeast China:Implication for its petrogenesis and metallogenic potential

大兴安岭南段发育包括维拉斯托、黄岗、安乐、大井、毛登和边家大院等锡多金属矿床,是我国北方最重要的锡多金属成矿带。北大山岩体是该锡多金属成矿带规模最大,出露最完全的花岗质侵入体,其北部主要为石英二长斑岩,南部为黑云母花岗岩,且南部岩相中常见电气石和绿柱石,被认为是区内锡多金属矿床的成矿母岩。然而,目前对该岩体岩石成因及其稀有金属成矿潜力的认识却存在较大争议。本文在对北大山岩体开展岩相学观察和锆石U-Pb定年的基础上,通过全岩地球化学和锆石Hf同位素分析,结合MELTS热力学模拟计算,试图阐明该岩体的成因类型、源区特征和演化过程,并讨论其成矿潜力。锆石U-Pb定年结果显示,北大山岩体北部的石英二长斑岩形成于143.4±1.3Ma,南部黑云母花岗岩形成于142.6±1.3Ma,与大兴安岭南段早白垩世锡多金属成矿年龄峰值相一致。北大山岩体中含自形富水矿物角闪石及黑云母、富碱(K_(2)O+Na_(2)O=8.58%~9.34%)、ACNK/CNK值介于0.97~1.02,P_(2)O_(5)含量低(<0.14%)且与SiO_(2)含量呈负相关,指示该岩体为高钾钙碱性I型花岗岩。岩体的锆石Hf同位素组成较为亏损(ε_(Hf)(t)值平均6.81,n=20),且全岩锆饱和温度较高(平均值为813℃),指示其为新生地壳物质高温熔融的产物。主量元素变化关系和MELTS模拟结果表明,北大山岩体为高钾钙碱性岩浆体系不同程度分离结晶的产物,其中北部石英二长斑岩样品之间结晶分异程度较低,而南部黑云母花岗岩的结晶分异程度较高。北大山岩体的形成时代、源区特征和氧化还原条件(△FMQ-2.5)与大兴安岭南段稀有金属花岗岩类似,具有一定的锡多金属成矿潜力,但其初融温度、挥发分组成(相对富B贫F)、分异演化程度(结晶分异和熔体-流体相互作用程度相对较低)明显不同于维拉斯托矿床成矿碱长花岗斑岩,不会是该矿床的成矿母岩。

The southern section of the Great Xing an Range where many tin-polymetallic deposits(including the Weilasituo,Huanggang,Anle,Dajing,Maodeng and Bianjiadayuan)developed is one of the most important tin-polymetallic metallogenic belts in northern China.The Beidashan pluton,the largest and most well-exposed granitic pluton in the tin-polymetallic metallogenic belt,is composed of quartz monzonite porphyry in the north and biotite granites with tourmaline and beryl in the south.This pluton was generally considered as the parent magma of tin-polymetallic deposits in the area due to their similar spatial and temporal relations.However,the petrogenesis and metallogenic potential of the pluton are still controversial.Based on lithological observation and zircon U-Pb dating,we conduct systematic bulk-rock geochemistry,zircon Hf isotope analysis and MELTS thermodynamic simulation calculation upon the pluton to clarify its genetic type,source area characteristics and evolution process,and then discuss its metallogenic potential.Zircon U-Pb dating results show that the quartz monzonite porphyry in the north of the pluton was formed at 143.4±1.3Ma,and the biotite granite in the south was formed at 142.6±1.3Ma,which are consistent with the Early Cretaceous tin-polymetallic mineralization peak age in the southern section of the Great Xing an Range.The Beidashan pluton contains idiomorphic hydrous mineral(e.g.,amphibole and biotite),enriched in alkali(K_(2)O+Na_(2)O=8.58%~9.34%)with ACNK/CNK=0.97~1.02,and depleted in P_(2)O_(5)content(<0.14%)with a negative correlation between P_(2)O_(5)and SiO_(2),indicating it belongs to the high-K calc-alkaline I-type granite.The zircon Hf isotopic composition of the pluton is relatively deficient(with an averageε_(Hf)(t)=6.81,n=20),and the whole rock zirconium saturation temperature is high(averaged at 813℃),indicating it is a product of high-temperature melting from new crustal materials.Major elements covariation and MELTS simulation results indicate that the pluton had undergone different degrees of fractional crystallization in magmatic systems,such as the quartz monzonite porphyry underwent a higher degree of fractional crystallization than the biotite granite.The age,characteristics of the magma source and redox conditions(△FMQ-2.5)of the Beidashan pluton are similar to those of granites with rare metal mineralization,and thus exhibit a certain degree of tin-polymetallic mineralization potential for this pluton.However,the initial melting temperature,volatile composition(relatively rich B but poor F),differentiation and evolution degree(relatively low degree of crystal differentiation and melt-fluid interaction)of the pluton are obviously different from those of the Weilasito alkali feldspar granite porphyry,demonstrating that it should not be the metallogenic parent rock for the Weilasituo deposit.