| The Tianshui area is located among the east Qinling, west Qinling and Qilian Mountains, which is a key to understand the tectonic regime and evolutionary history of the subduction and collision among three plates. The regional geology can be tectonicly divided into north and south tectonic belt with different rock associations, metamorphism and deformation by the Guanzizhen-Wushan ophiolitic melange belt. These differences show that the north and south tectonic belt has possessed different evolutionary history. The northern belt mainly consists of Precambrian metamorphic basement and Lower Paleozoic Liziyuan Group, which composed of metamorphic volcanic-sedimentary assemblage of greenschist facies. The southern tectonic belt mainly consists of Upper Devonian Dacaotan Group in north, and Middle-Upper Devonian Shuajiaba Group in south. Although many studies have documented the comparability of Guanzizhen-Wushan ophiolite zone and Shangdan zone, there are still much dispute on the plate tectonics and evolution history of the Guanzizhen-Wushan ophiolitic melange belt. The detailed study of the sedimentary rocks distributed along the suture zone, which was deposited during subduction-collision, is significant to reveal the westward extension of Shangdan zone, the process of the tectonic emplacement of Guanzizhen-Wushan ophiolite, especially the tectonic evolution of the south and north blocks. Based on the combined investigation of regional geology, sedimentary geology and structural geology, the geology and geochemistry of the Dacaotan Group were studied in this thesis. After all, the tectonic setting and implications of the clastic rocks as well as the restriction of evolution of Shangdan suture zone in the Tianshui area has been discussed.Based on the application of the study of how to discriminate the tectonic setting of fine grained clastic rocks by geochemistry, the tectonic setting of clastic rocks are classified into four types: oceanic island arc (OIA), continental island arc (CIA), active continental margin (ACM) and passive margin (PM). As for the major elements: OIA sandstones are characterized by the high abundances of TFe2O3+MgO(8-14×10-2), TiO2(0.8-1.4× 10-2), and low Al2O3/SiO2(0.24-0.33) and K2O/Na2O(0.2-0.4) ratios. The CIA sandstones can be distinguished from the OIA type by their lower TFe2O3+MgO(5-8×10-2) and TiO2(0.5-0.7× 10-2), and higher Al2O3/SiO2(0.15-0.22) and K2O/Na2O(0.4-0.8). The ACM sandstones are characterized by low TFe2O3+MgO(2-5 × 10-2), TiO2(0.25-0.45 × 10-2) and K20/Na20 ration 1. PM sandstones are generally enriched in SiO2, and depleted in Na2O, CaO, and TiO2. As for the trace elements: OIA sandstones are characterized by extremely low abundances of La(8.72±2.5× 10-6), Th(2.27±0.7 × 10-6), U(1.09±0.21 X 106), Zr(96± 20× 10-6), Nb(2.0±0.4× 10-6), low Th/U(2.1 ± 0.78) and high La/Sc(0.55 ± 0.22), La/Th(4.26±1.2), Ti/Zr(56.8±21.4), Zr/Th(48.0±13.4) ratios. The CIA sandstones are characterized by high abundances of La(24.4±2.3× 10-6), Th( 11.1 ±1.1 × 10-6), U(2.53± 0.24× 10-6), Zr(229±27× 10-6) and Nb(8.5±0.8X 10'6), and can be easily identified by the La-Th-Sc and La/Sc-Ti/Zr diagrams. ACM and PM sandstones are discriminated by the Th-Sc-Zr/10 and Th-Co-Zr/10 diagrams and associated parameters of Th/Zr, Th/Sc et al. In general, with high abundance of Zr(298±80× 10-6), Zr/Th(19.1 ±5.8) and lower Ba(253± 64× 10-6), Rb(61 ±19 × 10-6), Sr(66±22× 10-6) and Ti/Zr(6.74±0.9) ratios, the PM type sandstones are distinguished from the ACM sandstones. As for the rare earth elements: OIA sandstones are characterrized by lower E REE abundance(58 ± 10 × 10-6), slight enrichment of LREE, and the absence of a negative Eu anomaly(1.04±0.11). CIA sandstones are discriminated by higher ￡ REE abundance(146 ± 20 × 10-6) , La/Yb ratio(ll ±3.6) and a slight negative Eu negative(0.79±0.13). Both of the ACM and PM sandstones are all characterized by the high EREE abundance (186× 10-6 and 21 O× 10-6) and enrichment of LREE, and distinguished negative Eu anomaly(0.60 and 0.56).The Upper Devonian Dacaotan Group in Tianshui area is terrestrial coarse clastic deposit, including unmetamorphic heavy bedded-lumpy conglomerate, glutenite, middle-fine grained quartzose sandstone, pelitic siltstone, graywacke and heterogeneous metamorphic greenschist, chlorite-quartz schist and phyllite, which are reconstructed as graywackes. Geochemistry of the meta-sandstones can be described as follows: the useful discriminating parameters such as the abundances of total Fe2O3(5.71-8.39 * 10'2), TiO2(0.6-0.82 * 10'2) and Al2O3/SiO2(0.15-0.24) ratio are similar to the sandstones depositing in a CIA setting. Most of samples from Tianshui area are plotted into the field of the island arc and/or ACM setting by using discrimination diagrams for major elements of sandstones. The abundances of Th(ll.68-17.36 × 10-6), Zr( 174-295 × 10-6), Hf(4.8-8.5 * 10"6), Sc(ll. 1-19.3 * 10"6),V(70 -146 × 10-6), Zn(59.6-86.0 x 10'6) and the La/Th(l.85-2.80), La/Sc(l.95-3.20), Th/Sc(0.90-1.55), Ti/Zr( 12.88-20.71), and Zr/Hf(33.59-41.11) ratios suggest that the Tianshui sandstones were formed in a CIA setting and/or ACM setting. The useful discrimination diagrams show that most of samples of the sandstones are dropping into the field of the island arc area. In addition to, they are characterized by the higher E REE(116-233 * 10"6)abundance and enrichment of LREE, and higher negative Eu (Eu/Eu*=0.61-0.68) anomaly. Especially the chondrite-normalized REE patterns indicate that they are accordant with that of the CIA or the ACM sandstones. Although there are a lot of dispute on the interpretations and implications of the geochemistory of the sandstones, the geochemistry of the sandstones from Tianshui area were studied in details, especially combined with the studies of sedimentary geology and regional geology. It is reasonablly inferred that the Upper Devonian sandstones from the Tianshui area were formed in a foreland basin setting, which implies that the contact between North China block and Qinling block were not later than late Devonian.
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