扬子陆块西南缘3.8 Ga碎屑锆石及其地质意义

Detrital Zircons of 3.8 Ga in Southwestern Yangtze Block and Its Geological Implications

古老锆石和岩石的发现,是探索地球早期地质演化的关键.为进一步揭示扬子陆块基底物质组成和早期地壳形成演化,采用LA-ICP-MS锆石微区U-Pb测年,对扬子陆块西南缘禄丰地区东川群变质砂岩进行了年代学研究,发现3822±21 Ma的古老碎屑锆石.这是目前在扬子陆块获得的第2颗>3.8 Ga的古老锆石,也是目前在该地区发现的最老锆石.变质砂岩碎屑物质主要包括4个峰值年龄(~2320 Ma、~2162 Ma、~2036 Ma和~1915 Ma),2颗最年轻的锆石年龄基本限定了东川群早期最大沉积时限,与区域上火山岩时代相吻合.另外还包含少量中-晚太古代(2.6~2.9 Ga)和始太古代(3.7~3.8 Ga)的碎屑物质.Hf同位素组成显示这些碎屑锆石具有不同成因,其中2674~3822 Ma的碎屑锆石总体具有正的εHf(t)值和2.9~3.9 Ga的两阶段模式年龄,暗示扬子陆块在冥古宙-古太古代时期就有一定规模的新生陆壳分布.古元古代(1.9~2.4 Ga)的岩浆活动除有少量古元古代(2.3~2.4 Ga)新生地壳组分熔融外,大多为太古宙(2.5~3.7 Ga)古老地壳部分熔融.中元古代更多表现为古老地壳的熔融和物质再循环.研究结果为深化扬子陆块早前寒武纪地质演化认识提供了新资料.

Discovery of ancient zircons and rocks is the key to exploring the early geological evolution of the earth.In order to further decipher the basement material composition and early crustal formation and evolution of the Yangtze Block,it conducted the chronological study on the metamorphic sandstone of the Dongchuan Group in the Lufeng area of southwestern Yangtze Block with LA-ICP-MS zircon U-Pb dating,and the detrital zircon of 3822±21 Ma was found.This is the second zircon with the age older than 3.8 Ga in the Yangtze Block,and it’s also the oldest age record that has been found in the southwestern Yangtze Block.The clastic material of the metasandstone mainly includes four age peaks(~2320 Ma,~2162 Ma,~2036 Ma and^1915 Ma).Two youngest zircons basically limit the maximum deposition age of the Dongchuan Group,which coincides with the volcanic rock age of Yinmin Formation in the region.In addition,it also contains a small amount of clastic materials with the ages of 2.6-2.9 Ga and 3.7-3.8 Ga.The Hf isotopic composition shows that these detrital zircons have different origins.The clastic zircons with the age of 2674-3822 Ma have positive εHf(t)values and two-stage Hf model age of 2.9-3.9 Ga,suggesting a certain scale of juvenile crust distribution within the Yangtze Block during the Hadean-Archean period.The Paleoproterozoic(1.9-2.4 Ga)magmatic activities were characterized by partial melting of the Archean(2.5-3.7 Ga)ancient crust,except for a small amount of melting of the Paleoproterozoic(2.3-2.4 Ga)juvenile crust.The Mesoproterozoic is more manifested in the melting and recycling of ancient crust.This study provides new information for deepening the understanding of the early geological evolution of the Yangtze Block.