内蒙古边家大院铅锌银矿床深部正长花岗岩年代学与形成环境研究

Zircon U-Pb dating and geochemistry of the syenogranite from the Bianjiadayuan Pb-Zn-Ag deposit of Inner Mongolia and its tectonic implications

文章对内蒙古边家大院铅锌银多金属矿床深部正长花岗岩进行了LA-ICP-MS锆石U-Pb同位素测年,并对花岗岩的主量元素、微量元素和Sr-Nd同位素组成做了分析研究。结果显示,边家大院正长花岗岩加权平均年龄为(140.31±0.34)Ma,为大兴安岭南段早白垩世岩浆活动集中期产物;研究区至少有两期岩浆活动,早期酸性岩浆侵位,约10 Ma后中基性岩浆侵位,成岩与成矿同期进行。花岗岩地球化学特征具有高Si O2、K2O,低Mg O、Ca O、Ti O2的主量元素特征,A/CNK在0.98~1.19,属于准铝-弱过铝系列;富集Rb、Th、U、K等大离子亲石元素(LILE),亏损Sr、P、Ti等高场强元素(HFSE);稀土总量ΣREE较大,δEu为0.12~0.14,强烈Eu负异常;(87Sr/86Sr)i和(143Nd/144Nd)i初始比值,分别介于0.7066~0.7077和0.5121~0.5122(t=140 Ma),εNd(t)为-5.0‰^-6.6‰,成岩物质来自于中元古界下地壳铁镁质源岩的部分熔融。研究表明边家大院正长花岗岩是在早白垩世受到了蒙古—鄂霍次克海"剪刀式"闭合造山后的伸展条件和岩石圈减薄作用的影响,形成于高温低压环境的A型花岗岩。高温低压环境还可能与该区域岩石圈发生拆沉作用有关。

In this study,a series of analyses such as LA-ICP-MS zircon U-Pb isotopic dating and major elements,trace elements and Sr-Nd isotope composition investigation were performed for the syenogranite located in the deep layer of the Bianjiadayuan Pb-Zn-Ag polymetallic deposit,Inner Mongolia.Formed during the magmatic concentration period of early Cretaceous in southern Da Hinggan Mountains,the syenogranite in this deposit has age of(140.31±0.34)Ma.There were at least two periods of magmatic activity in the study area:Acid magma invaded in the early period,whereas intermediate magma and basic magma invaded about10Ma later.Ore-forming and rock-forming activities occurred over the same period.Geochemistry of major elements in the syenogranite is characterized by high SiO2and K2O and low MgO,CaO and TiO2with A/CNK ratio between0.98and1.19,suggesting metaluminous-weakly peraluminous series.The syenogranite is enriched in LILE such as Rb,Th,U and K and depleted in HFSE such as Sr,P and Ti.TheΣREE values are slightly high.TheδEu lies between0.12and0.14,exhibiting significant negative Eu anomalies.The initial ratio of(87Sr/86Sr)i is between0.7066and0.7077,while the initial ratio of(143Nd/144Nd)i is between0.5121and0.5122(t=140Ma);εNd(t)values vary in the range of-5.0to-6.6.Therefore,the petrogenetic materials were the products of partial melting of mafic-ultramafic source rock in middle Proterozoic lower crust.The analyses reveal that the syenogranite in the Bianjiadayuan deposit is A-type granite formed in the environment of high temperature and low pressure with the impact of the post-orogenic extension of Mongolia-Okhotsk scissor-type closed orogeny and lithospheric thinning in early Cretaceous.The high temperature and low pressure environment was probably related to the regional lithosphere demolition effect.