Study on Geochemistry of Hornblendic Rocks and Their Anatectic Derivatives of the Fuping Group in the Xiaojue Area, Hebei Province
河北小觉地区阜平岩群斜长角闪岩及深熔产物的稀土和Nd同位素地球化学研究

The Neoarchaean high amphibolite facies Fuping Group consists mainly of meta arkosic leuco leptite, semipelitic biotite leptite gneiss, hornblendic rocks, marbles and calcsilicate rocks, which have been partly subjected to differential anatexis. In the Xiaohuilonggou section, northwest of the Xiaojue town , the hornblendic rocks occur in a relatively high proportion and can be divided into two types. One is thick bedded amphibolite, consisting of plagioclase, hornblende, and locally small amounts of quartz, garnet and biotite, with some accessory minerals such as zircon and apatite. In which, there are some felsic patches of various shapes and scales as the products of the initial stage of anatexis. The anarectic derivatives are low in REE and HFS element contents and somewhat high in LREE/HREE ratio compared with their parent rock. The main reason is that in the anatectic process the accessory minerals , especially zircon, behaved as restitic crystals. The other is bedded or boudinage like amphibolite interbedded with biotite leptite gneiss. It is composed mainly of plagioclase and hornblende, without or only with very little accessory minerals. So being different from what mentioned above, the newly formed felsic derivatives are higher in REE contents and lower in LREE/HREE ratio than the amphibolite. In the both cases, the felsic derivatives are obviously lower in LREE/HREE ratio than the typical Archaean TTG rocks because of that the anatexis of the Fuping Group happened under middle low pressure conditions. It is also observed that the thick bedded amphibolite and its anatectic product are quite different in Nd isotopic compositions, showing the preservation of isotopic disequilibrium. Some felsic materials nearby the bedded amphibolite, being very high in LREE/HREE ratio and quite different in Nd isotopic composition from the amphibolite, are considered to be formed by the anatexis of the biotite leptite gneiss.