广西丹池成矿带芒场矿田岩浆岩源区特征及锆石U-Pb年龄分析

Source characteristics of magmatic rocks and zircon U-Pb age in the Mangchang ore field,Danchi metallogenic belt,Guangxi

丹池成矿带是我国重要锡多金属成矿带,过去对丹池成矿带成岩成矿研究主要集中在大厂矿田及五圩矿田,而对丹池成矿带北部的芒场矿田岩浆作用时代、源区特征及其成矿性缺少分析。芒场矿田岩浆活动强烈,发育隐伏斑状花岗岩及有关的花岗斑岩脉和隐伏细粒花岗斑岩及有关的白云母花岗斑岩脉。本文分析花岗斑岩脉U-Pb年龄及花岗斑岩脉和白云母花岗斑岩脉主、微量元素组成,以探讨岩浆活动时代、岩浆源区特征及其成矿潜力。芒场矿田花岗斑岩脉锆石U-Pb年龄为89.1±0.9Ma(MSWD=0.9),和丹池成矿带内大厂矿田岩浆活动时代相近,表明丹池成矿带岩浆活动时代都发生于90Ma左右。白云母花岗斑岩脉具高A/CNK比值(2.69~2.88),含高铝硅酸盐矿物白云母及在Th-Rb图上沿S型花岗岩趋势线分布,和大厂矿田S型黑云母花岗岩的特征基本一致,表明其主要为S型花岗岩。花岗斑岩脉形成时代晚于白云母花岗斑岩脉,但其具有更低的SiO2含量和更高的MgO、Fe2O3T、CaO和TiO2含量,且在SiO2与TiO2、Fe2O3T、Al2O3和P2O5关系图中分布于不同区域,没有线性变化关系,显示两者不是同一岩浆结晶分异演化形成的,而为不同沉积变质岩部分熔融形成的。白云母花岗斑岩脉富Al2O3、K2O、Rb、Cs、Sn、W、Nb和Ta,在Rb/Ba-Rb/Sr图上位于富粘土源区,为强风化作用形成的富粘土质富稀有金属源区部分熔融形成的产物。华南西部基底发育经强风化作用形成的富粘土质富稀有金属元素沉积变质源区,为华南西南缘大规模锡矿床的形成提供了物质基础。

The Danchi metallogenic belt is an important tin-polymetallic metallogenic belt in Guangxi,China.Previous work focused mainly on the Sn-polymetallic deposits in the Dachang and the Wuxu ore fields,while less work has been carried out on the ages,source features and ore potential fertility of the felsic igneous rocks in the Machang ore field.Magmatic activities are strong in the Mangchang ore field,where concealed small grain granites and porphyritic granites and their related muscovite granite porphyry dykes and porphyry granite dykes were found,respectively.Zircon LA-ICP-MS U-Pb ages of a porphyry granite dyke,and major and trace element compositions of granite porphyry dikes and muscovite porphyry granite dykes are analyzed in order to reveal the ages of their magmatic activities,source features and fertility of the different igneous felsic rocks in the Mangchang ore field.The granite porphyry dyke has a zircon U-Pb age of 89.1±0.9 Ma CMSWD=0.9 D,which is similar to the zircon ages of felsic igneous rocks and cassiterite U-Pb ages of the ore bodies in the Dachang ore field of the same metallogenic belt.This similar zircon and cassiterite U-Pb ages in the Mangchang and Dachang ore fields suggest that the magmatic activities and related Sn-polymetallic mineralization occurred at about90 Ma in the Danchi metallogenic belt.The muscovite granite porphyry dyke is characterized by high A/CNK C2.69~2.88 D,with a mineral assemblage containing muscovite and being distributed along the S-type granite evolution line on the Th vs.Rb diagram,which is the same as those of the Longxiangai biotite granite in the Dachang ore field,indicating that both of them could be classified as S-type granite.The granite porphyry dykes which emplaced later than the muscovite granite dyke have lower SiO2 and higher MgO,Fe2 O3,CaO and TiO2 concentrations than the muscovite granite dyke and the both types of dykes distribute on different domains on the diagrams of SiO2 vs.TiO2,Fe2 O3,Al2 O3 and P2 O5 without evolution trend,suggesting that they are not derived from the same magma chamber,but from partial melting of different metasedimentary rocks.The muscovite granite porphyry dyke is characterized by high concentrations of Al2 O3,K2 O,Rb,Cs,Sn,W,Nb,and Ta and is located in the domain of clay rich sedimentary source on the Rb/Ba vs.Rb/Sr diagram,suggesting that it was sourced from partial melting of clay and rare metal rich sedimentary source formed by strong weathering.Our results indicate that the basement of southwestern China contains clay and rare metal rich metasedimentary rocks,which could provide enough ore forming material for the formation of large and giant SnCWD and other rare metal deposits in southwestern China and the adjacent area.