武夷地块黎川铝质A型花岗岩成因及其地质意义

Genesis and Geological Significance of Aluminous A-type Granite in Lichuan Area,Wuyi Block

华南早古生代花岗岩的成因和后碰撞伸展环境的形成机制仍存在争议。选择华南早古生代黎川花岗岩开展锆石U-Pb同位素、全岩地球化学研究,探讨其成因并分析后碰撞伸展环境的起始时间及形成机制。黎川岩体形成于443 Ma,属于志留纪兰多维列世岩浆活动的产物。样品属于高硅、富钾的高钾钙碱性花岗岩系列;其A/CNK值为1.04~1.39,显示过铝质花岗岩特征。样品微量元素显示亏损Ba、Nb、Sr、P、Ti,富集Rb、Th、U、K、Pb等元素;轻稀土相对富集,强Eu负异常(δ_(Eu)=0.27~0.74)。其具有低ε_(Nd)(t)值(-8.4~-9.8)、高(^(87)Sr/^(86)Sr)i值(0.47883~0.72364)、高10^(4)×Ga/Al(3.24~4.19)比值的特征。上述特征表明,黎川花岗岩属于A型岩浆岩范畴。元素和同位素地球化学特征显示,黎川过铝质A型花岗岩可能是前寒武纪变质沉积岩部分熔融的产物。结合华南早古生代武夷-云开造山运动的高级变质岩和花岗岩的年代学特征,可知华南早古生代武夷地块自志留纪兰多维列世开始就从同碰撞环境转变为后碰撞伸展环境;该伸展环境可能是受到地壳迅速减薄,热地幔岩浆底侵导致造山带深部和壳幔相互熔融的影响。

The genesis of the Early Paleozoic granites in South China and the formation mechanism of the post-collision extensional environment are still controversial.The early Paleozoic Lichuan granite of South China was selected to carry out zircon U-Pb isotopic and whole-rock geochemistry studies,to further reveal its genesis,and analyzed the starting time and formation mechanism of the post-collision extensional environment.The Lichuan intrusion was formed at 443 Ma,which belonged to the product of the Silurian Landovian magmatic activity.The samples belong to the high potassium calc-alkaline granite series with high silicon and rich potassium.Its A/CNK value is 1.04~1.39,showing the characteristics of peraluminous granite.The trace elements in the sample showed that Ba,Nb,Sr,P,Ti were depleted and Rb,Th,U,K,Pb were enriched.Relative enrichment of light rare earths,strong negative Eu ano-maly(δ_(Eu)=0.27~0.74).It has a lowε_(Nd)(t)value(-8.4~-9.8),high(^(87)Sr/^(86)Sr)ivalue(0.47883~0.72364),high 10^(4)×Ga/Al(3.24~4.19)ratios.The above characteristics indicate that the Lichuan granite belongs to the category of A-type magmatic rocks.The elemental and isotopic geochemical characteristics indicate that the Lichuan peraluminous A-type granite may be the product of partial melting of the precambrian metasedimentary rocks.Combined with the geochronological characteristics of the high-grade metamorphic rocks and granites of the early Paleozoic Wuyi-Yunkai orogeny in South China,it is found that the early Paleozoic Wuyi block in South China has changed from a co-collisional environment to a post-collisional extensional environment since the Silurian Landovian.The extensional environment may be affected by the rapid crustal thinning and underplating of hotmantle magma induced themelting of the orogenic root and crust-mantle interaction.