沂水杂岩中超镁铁质岩的岩石地球化学特征

Petrology and geochemistry of ultramafic rocks in the Yishui complex,Shandong Province

主要以捕虏体形式存在于沂水岩浆杂岩和变质杂岩中的超镁铁质岩石不发育鬣刺结构,岩石化学组成以高MgO和低SiO2、TiO2、K2O含量为主要特征。按岩石中是否含有橄榄石大致可以分为橄榄辉石岩和尖晶角闪二辉石岩两种,前者以强烈发育蛇纹石化为特征,矿物组合以单斜（透）辉石＋橄榄石为主（偶见斜方辉石）,蚀变矿物组合为蛇纹石±铬铁矿＋磁铁矿±角闪石±尖晶石等;后者以局部发育滑石化为特征,矿物组合以斜方（古铜）辉石＋单斜（透）辉石＋尖晶石为主,其次是角闪石＋磁铁矿±滑石等。岩石总体以稀土元素总量（∑REE）相对较低、LREE/HREE=1.64~4.40为特征,稀土元素的球粒陨石标准化配分图解显示所有样品均具Eu和Ce的负异常,除3个橄榄辉石岩样品外,多数样品无明显的轻稀土元素、轻重稀土元素和重稀土元素分异。岩石的微量元素组成以不相容元素Rb、Ba、U、Nb、Sr、Zr等具有明显不同的异常为特征：Ba、Nb呈现负异常,而Rb、U呈现正异常,Sr部分呈正异常,Zr和Ti负异常出现在橄榄辉石岩中,其他样品无Zr异常。样品YS0631的SHRIMP锆石U-Pb定年结果显示其变质锆石年龄值为2 560~2 605 Ma;另有一颗结晶锆石的年龄值为2 719 Ma,εHf（t）值为8.2,亏损地幔模式年龄为2 680 Ma。综上所述,该超镁铁质岩石源于地幔,形成于新太古代早期,随后遭受深熔及岩浆作用影响,经历了变质作用的改造。

Ultramafic rocks occur mainly as inclusions or xenoliths in Yishui magmatic and metamorphic complexes,without typical spinifex texture.Their chemical compositions are characterized by high MgO（22.23%~27.49%,20%） and low SiO2（38.60%~49.40%）,TiO2（0.11%~0.39%,0.5%） and K2O（0.02%~0.39%,0.5%） content.As for other oxide compositions,total iron（FeOT） values are in the range of 7.68%~11.14%,CaO content ranges 5.15%~12.86%,and Al2O3 content varies remarkably in the range of 1.56%~8.94%.The CaO/Al2O3 ratios of most samples are 0.61~1.36（≈1）,with the exception of three samples,whose CaO/Al2O3 ratios are 7.70~8.20（1） and MgO/FeOT ratios are 2.00~3.33.MnO（0.13%~0.20%） and Na2O（0.12%~1.03%） content is low.The rocks can be roughly divided into two types,i.e.,olivine-pyroxenite and spinel-hornblende-pyroxenite.The former containing olivine shows strong serpentinization,with a mineral assemblage of calcic clinopyroxene＋olivine＋minor orthopyroxene,followed by the altered mineral assemblage of serpentine±chromite＋magnetite±hornblende±spinel.The latter containing no olivine has features of local talcization,and its mineral assemblage consists of（bronze） pyroxene ＋ clinopyroxene（diopside）＋spinel,followed by hornblende＋magnetite±talc±serpentine.In general,the ultramafic rocks have fairly low REE content（∑REE） =11.45×10-6~22.63×10-6） with LREE/HREE ratios in the range of 1.64~4.40.The chondrite-normalized REE patterns of all samples show negative anomalies of Ce and Eu.Most samples show indistinct fractionation of LREE,LREE-HREE and HREE,with the exception of three olivine-containing samples showing relatively high LREE/HREE ratios（3.35~4.40）.The trace elements in the ultramafic rocks show significantly different anomalies of incompatible elements Rb,Ba,U,Nb,Sr and Zr.Ba and Nb assume negative anomalies,and Rb and U as well as part of Sr display positive anomalies;in addition,negative Zr and Ti anomalies appear in olivine-pyroxenite,and there exist no Zr and Ti anomalies.SHRIMP zircon U-Pb dating of the ultramafic rock（YS0631） shows that the metamorphic zircon U-Pb ages are 2 560~2 605 Ma,with a crystal zircon age of 2 719 Ma.Hf isotopic composition of the sample has Hf（t） value of 8.2,indicating the depleted mantle model age of 2 680 Ma.In summary,the ultramafic rocks originated from the mantle,formed in the early Neoarchean,and were then subjected to anatexis and magmatic effects,thus experiencing metamorphic transformation.