扬子陆块西缘安益南华纪火山岩的厘定及其地质意义

Determination of the Nanhua System volcanic rocks in Anyi on the western margin of the Yangtze Block

扬子陆块西缘安益大湾山地区出露一套由变质玄武岩等组成的变质基性火山岩,前人将其归为中元古界,并作为寻找磁铁矿的主要对象。调查发现,安益大湾山变质基性火山岩与下伏浅变质岩系间发育一套稳定沉积的砾岩。应用LA-ICP-MS技术对其底砾岩之上最底部的变质玄武岩进行了锆石U-Pb年龄测定,获得了781.3±1.9Ma的岩浆锆石^(206)Pb/^(238)U年龄加权平均值和1008±14Ma、1142±15Ma、2714±10Ma的继承性岩浆锆石^(207)Pb/^(206)Pb年龄,指示该套变质基性火山岩形成于南华纪,并将其从浅变质岩系中解离出来,对比为澄江组。继承性锆石年龄数据指示,扬子地块西缘安益地区存在新太古界和中元古界物质记录。结合前人研究成果和近来获得的年龄数据,将滇中澄江组的时代界定于820～740Ma,并将南华系的底界界定于820Ma。大湾山中-大型磁铁矿产于扬子地块西缘澄江组的变质基性火山岩中,其主成矿期为南华纪,可能属于热液氧化物-铜-金矿床。

Metamorphic basic volcanic rocks composed of metamorphic basalts and some other rocks are exposed in the Dawanshan area of Auyi on the western margin of the Yangtze Block. Due to the lack of geochronologic data, previous researchers classified these rocks as belonging to the Middle Proterozoic and considered them as the main target for magnetite prospecting. The authors fouud that a stable sedimentary conglomerate is developed between the metamorphic basic volcanic rocks and the underlying metamorphic rocks in the Dawanshan area of Auyi. LA-ICP-MS tech was applied to measuring the U-Pb age of zircons from metamorphic basalt above most conglomerates. The 206Pb/238U weighted average values of 781.3±1.9Ma, and 1008±14Ma, 1142±15Ma, 2714±10Ma for 207Pb/206Pb ages of inherited magma zircon were obtained, suggesting that the metamorphic basic volcanic rocks were formed in Nanhua epoch. The authors separated them from the shallow metamorphic rock series and assigned them to Chengjiang Formation through correlation. The ages of the inherited zircons and the geological characteristics of this region suggest that the Neoarchean and Mesoproterozoic material records exist in the Auyi area on the western margin of the Yangtze Block. Based on previous research results and the age of tuff obtained from Chengjiang Formation by the authors, the temporal boundary of Chengjiang Formation in cennal Yumlan was defined at 820-740Ma,and the age of the bottom of Nanhua System was defined at 820Ma. Dawaushan middlelarge magnetite was produced in the metamorphic basic volcanic rocks of Chengjiang Formation on the western margin of the Yangtze Block. The main ore-forming activity occurred in the Nanhua period and may belong to the IOCG deposit.