桂中都安-马山煌斑岩成因及其构造意义

Genesis and Tectonic Significance of Lamprophyre in Du’an-Mashan,Central Guangxi

为精确厘定桂中地区都安-马山带煌斑岩的形成时代并探讨其源区属性、构造环境及其动力学背景,对其进行了LA-ICP-MS锆石U-Pb定年、^(40)Ar-^(39)Ar金云母定年、Hf同位素分析测试和全岩地球化学研究.结果表明,锆石U-Pb定年未能成功限定煌斑岩的成岩年龄,但煌斑岩中发育大量2578~1650 Ma的捕获锆石,指示其存在太古宙-古元古代变质基底.金云母^(40)Ar-^(39)Ar定年结果限定了煌斑岩的侵位年龄为100.4±0.99 Ma.Hf同位素研究表明,ε_(Hf)(t)有正有负,且正值多于负值,表明岩浆源区主要以新生地壳为主,存在部分古老地壳的再循环.全岩地球化学分析显示,煌斑岩具有较低的SiO_(2)含量(47.66%~50.93%)及高的K_(2)O含量(4.98%~6.77%)、富集LILE和LREE而亏损HFSE(如Nb、Ta和Ti),表现出富集地幔和俯冲流体交代作用的特征.桂中煌斑岩形成过程中,地壳混染作用十分有限,主要形成过程为:流体交代的富集地幔,在燕山晚期岩石圈的拉张-伸展背景下,部分熔融形成煌斑岩岩浆,同时NNW向的南丹-昆仑关大断裂为岩浆的侵位提供了通道.研究区煌斑岩形成于太平洋和印度板块向欧亚板块俯冲碰撞的动力学背景下.

In order to determine the formation age of lamprophyre accurately in Du’an-Mashan belt,Central Guangxi,and to discuss its source property,tectonic environment and dynamic setting,LA-ICP-MS zircon U-Pb dating,^(40)Ar^(39)Ar phlogopite dating,ε_(Hf) isotope and whole-rock geochemistry were obtained.It shows that zircon U-Pb dating fails to determine the fomation age of lamprophyres,however,a mass of captured zircons ranging from 2578 Ma to 1650 Ma indicates the existence of the Archean-Paleoproterozoic metamorphic basement.The phlogopite ^(40)Ar/^(39)Ar age of 100.4±0.99 Ma displayes the formation age of the lamprophyres.Zircons yield both positive and negativeε_(Hf)(t)values(with mostly positive),suggesting that the protolith was derived from newborn crust that incorporated a few evolved ancient continental crustal components.Geochemical data exhibit that lamprophyres have low SiO_(2)(47.66%-50.93%)and high K_(2)O(4.98%-6.77%)contents,with relatively high LILE and LREE and low HFSE(such as Nb,Ta and Ti)contents,indicating the characteristics of enriched mantle and subducted fluid metasomatism.During the formation of lamprophyres in Central Guangxi,crustal contamination is very limited,and the main formation process is as follows:under the background of lithospheric extension in the Late Yanshanian,the enriched mantle which was metasomatic was partially melted to form lamprophyre magma,and the Nandan-Kunlunguan fault extending in the NNW direction provided a channel for the magma fluid.The lamprophyres formed under the dynamic setting of subduction and collision of Pacific and Indian plate to Eurasian plate.