鲁西早前寒武纪花岗岩—绿岩地体稀土元素地球化学

REE GEOCHEMISTRY OF THE EARLY PRECAMBRIAN GRANITEGREENSTONE TERRAIN IN WESTERN SHANDONG, CHINA

本文总结了鲁西早前寒武纪花岗岩—绿岩地体的稀土元素地球化学特征。26亿年前形成的泰山群岩石中,稀土元素在云母片岩中最富集,是球粒陨石中稀土元素总量的40倍,在透闪片岩(科马提岩)中最低,是球粒陨石的4倍;稀土元素分馏作用在云母片岩中最明显,La_N/Yb_N为98.56,而在斜长角闪岩和透闪片岩中最不明显,La_N/Yb_N为1.42左右;斜长角闪岩Sm/Nd比值一般大于0.33,属轻稀土亏损型,泰山群中斜长角闪岩之原岩与大洋拉斑玄武岩相似。鲁西早前寒武纪侵入岩,随着时代由老至新的演化,稀土元素总量愈来愈高,且愈来愈富集轻稀土;侵入岩的δEu则愈来愈小,但作者所统计的50件侵入岩样品,其中有46件大于0.52;鲁西晚太古代的侵入岩属无铕异常型,早元古代的侵入岩为负铕异常型;岩石的化学成分表明,鲁西早前寒武纪侵入岩与Ⅰ型花岗岩相当,且均属轻稀土富集型;鲁西晚太古代的英云闪长岩和花岗闪长岩的La_N/Yb_N比值是球粒陨石的20倍,并根据投影,推设这两种岩石来自于玄武质岩石——角闪岩25—50％的部分熔融,而玄武质岩石也是从地幔中部分熔融所产生。

This paper sums up REE geochemical characteristics of the early Precambrian granitegreenstone terrain in western Shandong, China. In the Taishan Group with an age of 2.6 Ga, REEs are enriched most in the mica schists in which the REE contents are 40 folds as those in the chondrite and depleted in tremolite-schists(komatiites) in which the REE contents are only 4 folds as those in the chondrite. REE fractionation is distinct in the mica schists(La_N/Yb_N=98.56) but indistinct in the amphibolites and tremolitc—schists (La_N/Yb_N=1.42). The Sm/Nd ratios in the amphibolites are usually greater than 0.33, which show LREE-depieted patterns. The protoliths of the amphibolites in the Taishan Group are similar to ocean tholeiitic basalts. The bulk contents of REEs in the Precambrian intrusive rocks from old to young are increased with the LREE contents much more enriched, while the values of δEu become smaller but most are greater than 0.52 according to the results obtained from 50 statistical samples. The chondrite normalized REE distribution patterns show no Eu anomalies in the late Archaean intrusive rocks but show negative Eu anomalies in the early Proterozoic ones. Petrochemical compositions indicate that these early Precambrian intrusive rocks correspond to the I—type granites and consistently show LREE-enriched patterns. The La_N/Yb_N ratios for the late Archaean tonalites and granodiorites are 20 folds as high as those for the chondrite, and based on their perspective results, it is suggested that these two rock types probably derived from basaltic rocks-now the amphibolites with partial melting of 25—50％, while the basaltic rocks were formed by partial melting of the mantle.