The Studies Of Paleozoic Chert From North Qilian Mts.

Many chert sequences are located at key stratigraphic intervals or in structurallyallochthonous packages, and because such sequences provide critical informationregarding the formation of sedimentary basins and tectonic orogens it is important todetermine precisely their original depositional position. Chemical methods of delineationenvironments are becoming increasingly sophisticated and useful, in part because evenslight post-depositional recrystallization may obscure primary rock textures.Hereâ… synthesize major, trace, and rare earth element (REE) data in bulk chert from31xequences from continental rift and island arc environments, ranging in age from LateProterozoic, and develop a set of depositional chemical criteria applicable to cherts. Thecriteria allow for analytical limitations and diagenetic modifications (precluding use of Si,Ca, Mn, P, Sr, and Ba, and other elements) intrinsic to chert chemistry. Major elementratios, capitalizing on the affiliations of Al₂O₃ and TiO₂ to terrigenous matter and ofFe₂O₃ to metalliferous input, can distinguish continental margin from ridge-proximalsedimentation. Trace element data are too few to interpret rigorously, with substantialoverlap occurring between depositional regimes, but Zr, Cr and Rb also can be selected todistinguish terrigenous matter and metalliferous input. Of course, REE ratios (i.e.,La_N/Ce_N) allow the best resolution of all the depositional regimes and are independent ofdiagenetic modification.Rare earth element (REE), combining with major and trace element abundances andrelative fractionatons in thirteen cherts sampled by North Qilian Mountains indicate thattwo kinds of cherts in continental rift can be resolved: the cherts in central edifice withCe/Ce^*=1, La_N/Yb_N=4.3, and La_ex～45%; and the cherts in crevasse volcanism withCe/Ce^*=0.92, La_N/Yb_N=1.1, and La_ex～60%. Also the other eighteen samples can beresolved into three kinds: the cherts in trench wedge with Ce/Ce^*=0.98 and La_N/Yb_N=1.2:the cherts in continental arc with Ce/Ce^*=0.95 and La_N/Yb_N=0.73, and the cherts inback-arc basin with Ce/Ce^*=0.93 and La_N/Yb_N=0.4. Meanwhile, a type of chert in centraledifice can be identified with HREE enriched, related to Baiyinchang-type massivesulfide deposits; another type of chert in back-arc basin related to Shijuligou-type rich-Cudeposits. The resolutions provide a potentially powerful new tool for the similar depositsfinding.