| Zankan iron deposit, as one of the medium to large iron deposits in the western Kunlun iron belt, part of its roof and floor ore formations in main ore belts are volcanic-subvolcanic rocks. This paper selects the volcanic rocks beneath No.3 main ore belts(ZKA2, n=5) and the subvolcanic rocks which invade No.1 main ore belts(ZKA9, n=15). Through research the petrology, major elements, trace elements, zircon U-Pb dating, zircon Hf isotope of the rocks to determine their petrology classification, formation age, material source and the tectonic background,etc, in order to explore their geodynamic context further and to explore the causes of iron ore to use as reference.Petrographic studies show that these are all dacite porphyry rock and its chemical composition falling rhyolite-dacite area. SiO2 content of 20 samples was 65.52%-74.32%(average 70.23%), A12O3 content is 13.32%-15.65%(average 14.39%), aluminum saturation index A/CNK=0.71-1.18, alkali transfer coefficient A/NK= 1.13 to 1.25, on the whole belongs to a metaluminous to weakly peraluminous potassium calc-alkaline rocks; Trace element shows that enriched in Nd, Th, U, La, Y and so on. Lost of HFSEs, such as Sr, P, Ba, Ti and Nb, etc. Chondrite normalized REE distribution patterns of Zankan dacite porphyry shows LREE enrichment, Eu weak loss (8Eu=0.35-0.77) and the rightward feature. Zircon U-Pb dating results shows that the two samples were obtained consistent ages, were 533 ± 10Ma and 527.4 ± 9.0Ma,belongs to Cambrian, which is classified as predecessors of this Precambrian Bulunkuole group is quite different; Magma source discrimination diagrams show both its original magma adding deposits and fluid, indicating that occur in subduction mantle wedge position. On the tectonic discrimination diagram, all samples have shown a continental margin or island arc background, which can be thought of this set of dacite porphyry formed in the continental margin arc to the background, combined with its U-Pb age and heterogeneous Hf isotope values, suggesting the "original Tethys" ocean may already begun all samples have shown a continental margin or island arc background, which can be thought of this set of dacite porphyry formed in the continental margin arc to the background, combined with its U-Pb age and heterogeneous Hf isotope values, suggesting the "original Tethys" ocean may already begun to subduction from north to south in the early Paleozoic.From its geological features, the formation of iron deposits dominated by volcanic-sedimentary mineralization.As follows:(1) Ore controlled by the formations obviously, roof and floor of the main ore body are biotite quartz schist, some ore output space is considered closely related to marine volcanic or intrusive rocks produced in. (2) Orebodies generally layered, layered-like distribution, and gradual transition relations with the top floor.(3) The ore has original sedimentary structures obviously, often have banded, striped structure, and the bands, stripes parallel to the wall rock sedimentary bedding.(4) Ore formations are metamorphic uneven and higher iron background values named Bulunkuole group, mainly greenschist facies, partial to amphibolite facies,near wall rock mostly are quartz biotite schist, amphibolite schist, metamorphic rocks and gypsum powder layer, the original rocks are basic volcanic in a set of iron-clastics,carbonates mezzanine partial development. Thus this study suggests that the formation of ore age should be consistent with the original iron deposits rocks era.Based on morphological and geological characteristics of Zankan iron deposit, combined with ore formations of volcanic rock package (536.7 ± 3.4Ma) ages and volcanic rocks ore sandwich (536.4 ± 4Ma) ages which obtained by our research group in 2013. We believe that this set of dacite porphyry and ore body to be formed in the same era, the early Paleozoic, amended the previous understanding of the local iron deposit was formed in Precambrian (Proterozoic). |
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