The Caledonian North Qilian orogenic belt lies between the North China plate and the Qaidam mi-croplates, and resulted from the collision among the Qaidam microplate, mid-Qilian block and the North China plate. The orogen initiated from the rifting of the Late Proterozoic Rodinia, and then it experi-enced stages of Cambrian rift basin and Ordovician archipelagic oceanic basin, and foreland basin during Silurian to Early-Middle Devonian. The average ratios of Al/(Al+Fe+Mn), Al/(Al+Fe), δ Ce, Lan/Ybn and Lan/Cen from cherts of Cambrian Heicigou Formation are 0.797, 0.627, 1.114, 0.994 and 1.034 re-spectively. In the NAS standardized REE distribution pattern, the cherts from Xiangqianshan is slightly HREE enriched, and the cherts from Ganluci and Shiqingdong are plane. All of these features indicated that Cambrian cherts of the Heicigou Formation originated from a continental margin rift background. On the contrary, the average ratios of Al/(Al+Fe+Mn), Al/(Al+Fe), δ Ce, Lan/Ybn, Lan/Cen of the Ordovician chert from Dakecha, Cuijiadun, Shihuigou, Laohushan, Heicigou, Maomaoshan, Bianmagou, Da-chadaban, Baiquanmen, Jiugequan and Angzanggou, are respectively 0.72, 0.58, 0.99, 1.09 and 0.96 respectively. Their NAS standardized REE distribution patterns of most Ordovician cherts are plane mode or slightly HREE enriched. The REE distribution pattern of few samples of cherts are slightly LREE enriched. Characteristics of sedimentary geochemistry and tectonic evolution demonstrated that the Cambrian-Ordovician cherts, associated with rift, oceanic, island arc and back-arc volcanic rocks, was not formed in a typical abyssal oceanic basin or mid-oceanic ridge. On the contrary, they formed in a deepwater basin of continental margin or a archipelagic ocean tectonic setting. Several Early Paleo-zoic ophiolite belts in North Qilian and adjacent periphery Qaidam microplate imply that an archipelagic ocean during Ordovician existed in the east of Pro-Tethys.
DU YuanSheng1,2, ZHU Jie1, GU SongZhu1, 2, XU YaJun1 & YANG JiangHai1 1 Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China
Typical contourite deposits associated with submarine turbidite fan deposits are recognized for the first time from the Lower Devonian Liptrap Formation at Cape Liatrap, Victoria in southeast Australia. The contourites are well integrated within the turbidite fan deposits and are characterized by thin (5-8 cm), lenticular, well-sorted coarse-grained siltstones to fine-grained sandstones with current-ripples and cross beddings. The palaeocurrent directions of the turbidite fan and contourites are perpendicular to each other, with the former directed generally westward while the latter varying from 165° to 190° southward. In view of the facies types and architecture, we suggest that the turbidite fan was developed at the base of a westward inclined palaeo-slope, at the front of which the contourites were deposited as a result of southward flowing deep-sea contour (geostrophic) currents. The depositional setting inter- preted for the Liptrap Formation thus may provide a provisional model for the Lower Devonian conti- nental slope and abyssal basin environment in the southeastern part of the Melbourne Trough.
The detrital modes of the sandstones from the south basin of Dabie（大别） orogenic belt during the Early-Middle Jurassic are complex due to multiple supplying.Detrital compositions of medium-coarse sandstones from the Lower-Middle Jurassic,Huangshi（黄石）,show three main provenances：recycled ororgen,volcanic arc（arc orogen）,and continental block on the bases of the Dickinson discriminating diagrams of modal composition and analysis of sandstone petrofacies.Combined with sparse published paleocurrent data,geotectonic setting of the study area,and the unpublished detrital zircon age data,the positions of the main provenances are inferred as follows：coastal orogenic belt（COB） in the East China provided detritus as the recycled orogen provenance;Dabie orogenic belt（DOB） to the north was the source of volcanic arc（arc orogen） materials;and the continental block clasts were likely from the uplift of the Yangtze basement（UYB）（Jiangnan（江南） uplift）.Six sedimentary cycles are recognized in the studied section,one in Wuchang（武昌） Formation,and five in Huajiahu（花家湖） Formation.During the deposition,the three main provenances play different roles.The quantitative petrofacies analysis revealed distinct changes of the effect from each source,that is,61.9%（Cycle Ⅰ）→54.5%（Cycle Ⅲ）→50%（Cycle Ⅳ）→57.9%（Cycle Ⅴ）→14.3%（Cycle Ⅵ）,14.3%（Cycle Ⅰ）→21.2%（ Cycle Ⅲ）→40%（Cycle Ⅳ）→36.8%（Cycle Ⅴ）→14.3%（Cycle Ⅵ）,and 23.8%（Cycle Ⅰ）→24.2%（Cycle Ⅲ）→10%（Cycle Ⅳ）→5.3%（Cycle Ⅴ）→71.4%（Cycle Ⅵ） from COB,DOB,and UYB,respectively.The results show that（1） COB behaved in considerable activation during the EarlyJurassic but weakened in the Middle Jurassic;（2） the vigorous tectonic uplift of DOB initiated in Middle Jurassic;and（3） UYB was in stabilization basically from the Early to Middle Jurassic.
The Laojunshan Formation is a suite of molasse formed during the rapid uplift of the North Qilian Orogenic Belt (NQOB). Forty-one samples of sandstone have been collected from the Sunan and Minle sections in the western sector and the Gulang and Jingyuan sections in the eastern sector of the NQOB belt. Geochemical analyses of those samples indicated: 1) The MgO+Fe2O3T and Al2O3/SiO2 values are higher, and K2O/Na2O ratios are lower in the western sector than those in the eastern sector. 2) All of them are depleted in Nb and Ta elements. The samples from the western sector are depleted in Rb element and enriched with Sc, Co, Ni, V, and Cr elements in the Upper Crust-normalized patterns. However, those from the eastern sector are depleted in Sr without enrichments of Sc, Co, Ni, V, and Cr. 3) All of the samples display a right-inclined REE pattern af- ter Chondrite-normalized REE pattern. But LaN/YbN and Eu/Eu* ratios of the samples from the western sector are lower than those of the samples from the eastern sector. These geochemical characteristics suggest the prominent input of mafic clast with minor granitic rocks into the Sunan area, felsic clast into the Gulang and Jingyuan area, and both mafic and felsic clast into the Minle area. The angular shapes of gravels imply that these ill-sorted sediments were deposited near their sources without recy- cling. Geochemical features above also demonstrated that no major chemical weathering occurred for the western provenance, but deposits in the eastern sector resulted from low or middle degree chemical weathering. Evidences combining tectonic discriminations and comparisons with potential provenances revealed that sediments in the Sunan area were derived mainly from the North Qilian Continental arc, whereas sediments in the Minle, Gulang, and Jingyuan areas were derived not only from the North Qilian Continental arc but also from the basement of the Middle Qilian block. Integrated with the characteristics of development of Silurian and Devonian, these imply that the oro
The Biyang Depression, lying in the eastern Nanxiang Basin, has a dustpan-shape with faults developed to the southwest and southeast. The Paleogene Hetaoyuan Formation in this depression hosts one of the oldest soda-deposits, the Anpeng deposit. These sodium carbonate-bearing strata consist of lutites, muddy dolomites, and bedded soda-deposits, which are dominated by nahcolite with little trona, mirabilite, and halite. In the top Unit 3 and lower Unit 2 of Hetaoyuan Formation where the sodium carbonates are concentrated, dolomitic shale, muddy carbonate, and sodium carbonate can be separated based on their distinct A1203, TiO〉 Na20, MgO＋CaO, and LOI （loss on ignition） contents. By using Na20/（MgO＋CaO） ratio as an indicator for the relative concentration of Na carbonates to Mg-Ca carbonates, the muddy dolomites can be classified into two groups. One group with Na20/（MgO＋CaO）〈0.6 exhibits a positive correlation between this ratio and AI203＋TiO2 content, an proxy for ter- rigenous clastic input and the other group with Na20/（MgO＋CaO）〉0.6 presents a negative correlation. As the clastic propor- tion decreases, the former indicates that it tends to form pure Mg-Ca carbonates reflecting evaporation less than or equal to re- plenishment for the lake water in a relative humid climate, and the latter reveals a trend to form sodium carbonates with lake water＇s evaporation more than replenishment in a relatively dry climate. From muddy dolomite to sodium carbonate samples, with Na20/（MgO＋CaO） ratio increasing, the A1 normalized Zr, Ti, La, K, Rb, and Ba tend to be enriched and the post-Archean Average Shale （PAAS） normalized positive Eu anomaly becomes more obvious. Integrated with the high B contents and the report of searlesite in the bedded sodium carbonates and the regional extensional tectonic regime, the above geochemical characteristics were interpreted to signify hydrothermal contribution to forming these soda-deposits through the deep fault systems of basin margins.
YANG JiangHaiYI ChengLongDU YuanShengZHANG ZongHengYAN JiaXin
