云开地区早古生代竹雅-石板辉长岩锆石U-Pb定年与Lu-Hf同位素特征

摘要

华夏地块西段的云开地区出露少量早古生代基性侵入岩,多以小岩株状侵入前寒武纪变质地层或与同期片麻状花岗岩共生,岩石类型主要为辉长岩和苏长辉长岩.LA-ICP-MS锆石U-Pb测年显示,信宜竹雅和高州石板辉长岩结晶年龄约为445Ma.辉长岩岩石地球化学以富MgO、CaO,相对贫TiO2为特征,具有极低的稀土元素总量(∑REE=20.8×10-6~26.6×10-6),轻稀土元素略微富集,亏损Nb、Zr、Ti、P等高场强元素.锆石εHf(t)均为弱的负值,Hf同位素一阶段模式年龄介于1.1~1.4Ga之间,显示富集地幔源区特征.构造环境分析表明,辉长岩与火山弧玄武岩相似.综合前人研究认为,华夏地块早古生代存在一系列具典型弧火山岩特征的基性岩类,这对早古生代构造运动的认识具有重要的限定作用.%Early Paleozoic mafic intrusive rocks in Yunkai terrane commonly occur as small stocks that intruded into pre-Cambrian strata and/or contemporary gneissic granite, with main rock types being gabbro and norite gabbro. The gabbros in Zhuya and Yunlu yielded 206Pb/238U weighted age of ca. 445Ma. The gabbros show relatively high MgO, CaO, low TiO2 content, very low ∑REE (20.8×10-6~26.6×10-6), weak enrichment of LREE and strong depletion of HFSE (Nb-Zr-Ti-P). Zircons give negative εHf(t) and TDM model ages from 1.1Ga to 1.4Ga, showing the features of an enriched mantle source. Tectonic setting analysis shows that the gab?bors acted like volcanic arc rocks. Previous studies show many Early Paleozoic mafic rocks in Cathaysia Block with characteristic fin?gerprints of arc volcanic rock features, and hence the Early Paleozoic geotectonic evolution history in Yunkai terrane (even Cathaysia Block) should be reconsidered.