底侵玄武质下地壳的熔融:来自安徽沙溪adakite质富钠石英闪长玢岩的证据

Melting of the underplated basaltic lower crust:Evidence from the Shaxi adakitic sodic quartz diorite-porphyrites,Anhui Province,China

沙溪石英闪长玢岩具有高Al2O3、Sr、Sr/Y、La/Yb,低Y、YbSr正异常,Eu弱负异常—正异常,表现出与adakite岩类似的地球化学特征,与庐枞火山盆地双庙组粗面玄武岩的微量元素特征和NdSr同位素组成类似。研究表明:(1)沙溪侵入岩不是由俯冲的洋壳熔融形成,而是由底侵的玄武质下地壳熔融形成,该玄武质下地壳的物质来源与双庙组玄武岩的来源相似,都为富集地幔;(2)燕山晚期,长江中下游地区可能存在玄武质岩浆的底侵作用,地壳发生垂向增生,沙溪地区地壳厚度曾大于40km,但白垩纪至现在,沙溪地区地壳明显减薄;(3)沙溪铜(金)矿床不是与大洋板片俯冲有关的斑岩铜(金)矿床,而可能是与底侵的玄武质下地壳熔融有关的斑岩铜(金)矿床。

The Shaxi sodic quartz diorite-porphyrites have the geochemical chara cteristics as adakite,such as high Al 2 O 3 ,Sr,Sr /Y and La /Yb,low Y and Yb,Sr positive anomaly,and Eu weakly negative abnomaly or positive abnormaly;their characteristics o f trace elements and the composition of Nd-Sr isotope are similar to those of the Shuangmiao Formation basalts in Luzong volcanic basin.On the basis of petrology and geochemistry,it is suggested that:(1)Shaxi intrusive rocks were not forme d by the melting of subducted oceanic slab,but by the melting of underplating basaltic lower crust whose source was the enriched mantle,the same source as the Shuangmiao For-mation basalts;(2)in late Yanshanian,basaltic magma u nderplating might take place in the middle and lower reaches of the Changjiang River,the crust then accreted vertically,and the thickness of crust in Shaxi regio n was larger than 40km for a time,but from C retaceous to now,it has become thin;(3)the copper(gold)porphyry deposit of Shaxi was not related to su bducted oceanic slab,but related to the melting of the underplated basaltic lower crust.