汉诺坝-阳原火成碳酸岩成因探讨

Genesis of carbonatite from Hannuoba and Yangyuan

大多数幔源硅酸盐岩浆都含少量碳酸盐岩浆,这些少量的碳酸盐岩浆在地幔演化中起了非同寻常的作用。本文报道了发现于汉诺坝、阳原地区新生代玄武岩中鲜见的火成碳酸岩。碳酸岩脉贯穿于玄武岩及其捕虏体橄榄岩,并导致橄榄岩强烈的碳酸盐化现象。碳酸岩脉主要由方解石组成(90%以上),岩石类型为方解石碳酸岩,含少量被裹挟的地幔橄榄石、斜方辉石、单斜辉石和尖晶石等矿物。碳酸岩化橄榄岩由原先的黄、绿色变为紫褐色,灰白色网状碳酸岩细脉穿插其中。碳酸岩脉和碳酸盐化橄榄岩的全岩稀土含量很低(∑REE=8.7×10-6～13.7×10-6),球粒陨石标准化REE模式呈LREE略微富集(～10×球粒陨石)分布模式,微量元素也只显示轻微富集(数倍于原始地幔),它们的δ13C均为负值(-11.2‰～-12.3‰),δ18O均为正值(22.0‰～22.6‰)。碳酸岩的Sr、Nd、Pb同位素组成均显示富集(87Sr/86Sr=0.7078～0.7079,143Nd/144Nd=0.5129,206Pb/204Pb=18.0,207Pb/204Pb=15.5,208Pb/204Pb=38.0)。由于碳酸盐岩浆喷出地表后易于风化,导致REE、微量元素和同位素组成明显偏离原生火成碳酸岩。但从张北少数新鲜碳酸岩所具有的原生火成碳酸岩的C、O同位素组成(δ13C=-5.7‰～-7.3‰,δ18O=8.5‰～10.1‰)特征,以及接沙坝碳酸岩的正εNd(5.3～5.5)为亏损地幔的特征,表明汉诺坝碳酸岩与玄武岩的同源性——它们都来自地幔。

Most of the mantle-derived magmas are silicate melt,with very few others,such as carbonate magmas. However these rare carbonate magmas tell quite different but important stories on mantle evolution. Here we report a new case of mantle-derived carbonate magma found in the Cenozoic basalts in Hannuoba and Yangyuan,North China. The carbonate melts occur in small veins,which cut through the basalts and mantle peridotite xenoliths. The carbonatite veins are mainly dominated by calcite (>90%),with a little amount of mantle minerals,e.g. olivine,clinopyroxene,orthopyroxene and spinel. The xenoliths are altered by the carbonate melt,and their colors changed from originally yellow or green to purplish brown,with grayish white network stringers of carbonatite cutting through. Whole rock chemical compositions reveal that the carbonatite veins have low rare earth elements (∑REE =8.7×10-6~13.7×10-6) and trace elements,with slightly enriched LREE patterns. Stable isotopes of C and O in the carbonatite veins are δ13C=-11.2‰^-12.3‰ and δ18O=22.6‰~22.8‰,respectively. Only a few fresh samples from Hannuoba have mantle carbonatite C and O isotopic ratios (δ13C=-5.7‰^-7.3‰,δ18O =8.5‰~10.1‰),while most samples show significant characteristics of weathering. Radioactive isotopes of Sr,Nd,Pb are 87Sr/86Sr=0.7078~0.7079,143Nd/144Nd=0.5129,206Pb/204Pb=18.0,207Pb/204Pb=15.5,208Pb/204Pb=38.0,respectively. εNd of two samples in Jieshaba vary in 5.3~5.5,and reveal their DM mantle origin. The carbonatite indicate that they have the same origin of depleted mantle as the basalt,but weathering after magma eruption modifies the carbonatite.