摘要
从中生代到新生代,华北东部岩石圈地幔发生了减薄以及地球化学性质置换,而扬子地块东部中生代岩石圈地幔也表现出类似的过程,对中生代火山岩的地球化学研究有助于了解这一变化过程以及发生置换时的时空关系。庐枞火山岩出露于扬子地块东部,为一套包括粗玄岩–玄武粗安岩–粗面岩的富碱橄榄安粗岩系。研究了双庙组基性火山岩,这些岩石富集Rb,K,Sr,Th和轻稀土元素,亏损高场强元素。(87Sr/86Sr)i=0.7060～0.7063,εNd(t)=-3.9～-6.2,(206Pb/204Pb)i=17.788～18.125,(207Pb/204Pb)i=15.511～15.546,(208Pb/204Pb)i=37.735～38.184。在喷出地表过程中,火山岩没有受到明显的地壳物质混染,因此元素和同位素组成反映了地幔源区的地球化学特征。其地幔源区具有同位素富集特征,表明火山岩源区曾受到地壳物质的影响,是富集地幔部分熔融的产物,并经历明显的结晶分异作用。庐枞火山岩的岩浆成分和源区特征反映该地区在晚中生代岩石圈地幔的伸展和软流圈地幔上涌的演化过程。
The sub-continental lithospheric mantle （SCLM） under the eastern part of North China was thinned and the geochemical characteristics were replaced during Mesozoic and Cenozoic. The Mesozoic SCLM under eastern Yangtze block shows the similar procedure as that in North China craton. This paper reports the geochemical characteristics of Mesozoic volcanics, which will be helpful for understanding the characteristics of Yangtze SCLM and the time-space relationship of the replacement characteristics from Mesozoic to Cenozoic. The Luzhong volcanics, occurring in eastern part of China and belonging to Yangtze block, are a set of Late-Mesozoic alkali-rich shoshonitic rocks, which include traehybasalts-basaltic traehyandesite-traehyte. The Suangmiao group, part of the Luzhong set, was studied in detail in this paper. The volcanics were slightly contaminated by crust materials during the formation, and the elemental and isotopic compositions can well reflect their mantle source. The basaltic rocks are characterized by enriched Rb, K, Sr, Th and LREE, and depleted HFSEs. They also have enriched Sr-Nd isotopic compositions with （^87Sr/^86Sr）i = 0.7060～0.7063, εNd（t） = -3.9～-6.2, （^206pb/^204Pb）i = 17.788～18.125, （^207pb/^204Pb）i = 15.511～5.546, （^208Pb/^204Pb）i = 37.735～38.184. These data suggest that the magma was partially melted from enriched mantle source, which was effected by lower crust materials within the source, and followed by crystal fractionation. The magma composition and mantle source characteristics of Luzhong volcanics imply the extension of lithospheric mantle and asthenosphere upwelling in the Late Mesozoic.
出处
《高校地质学报》
CAS
CSCD
北大核心
2007年第2期235-249,共15页
Geological Journal of China Universities
基金
自然科学基金项目(40673008)
国家自然科学基金青年基金项目(40203004)
