| The North Qaidam tectonic belt is located at the northeastern margin of the Tibetan Plateau, NW China, and is one of the most important polymetallic belts in the northwest of China. It is characterized by the presence of island arc volcanic rocks, HP-UHP metamorphic belt and ophiolite complex. The Xitieshan area is located at the central part of the North Qaidam tectonic belt. The Tanjianshan Group, hosting most of the Xitieshan massive sulfide deposits, has been divided into four informal units from northeast to southwest, they are: 1) unit a, or the lower volcanic-sedimentary rocks, comprises bimodal volcanic rocks(unit a-1) and sedimentary rocks(unit a-2); 2) unit b, or intermediate-basic volcaniclastic rocks; 3) unit c, purplish red sandy conglomerate; 4) unit d, or basic volcanic rocks, from base to up, comprises the lower basic volcaniclastic rocks(unit d-1), middle clastic sedimentary rocks(unit d-2), upper basic volcaniclastic rocks(unit d-3), and uppermost basic lava(unit d-4). In recent years, several general reviews of the Xitieshan Tanjianshan Group are published. However, detailed descriptions are limited, and many geologic parameters, critical for the Tanjianshan Group, are unknown. In this study, stratigraphy, petrology, mineralogy, major/trace element and isotope geochemistry, zircon U-Pb dating, integrated Ar-Ar isotope analysis from the different units of the Tanjianshan Group have been carried out 1) to provide a descriptive field-based geologic database of the Xitieshan Tanjianshan Group; 2) to understand the formation age of Tanjianshan Group; 3) to elucidate the petrogenesis and tectonic setting of the Tanjianshan Group; 4) to establish the Early Paleozoic tectonic evolution model for the Tanjianshan Group; 5) to understand the regional tectonic setting that led to the formation of the Xitieshan deposit, as well as the relationship between mineralization and the different evolutionary process of volcanism; 6) to provide temporal constraints on the ductile shear deformation and associated mineralization in the Xitieshan district; 7) to elucidate the relationship among the sedimentation, mineralization and volcanism in the Xitieshan deposit, and to provide insight into the genesis of the Xitieshan deposit. The main conclusions are as listed as following:1. The formation age of the Tanjianshan Group is identified.Nine samples in this study from the different units of the Tanjianshan Group are used to analysised zircon U-Pb age using LA-ICPMS. Sample 2011XTS02-2 from unit a-1 yeilded an age of 461.5±5.2 Ma, which provides the best estimate of the crystallization age and the maximum sedimentation age of volcano-sedimentation. Three felsic volcanic rock samples from unit a-1 yielded 454.1±4.2 Ma, 454.2±3.9 Ma and 452.1±5.5 Ma, respectively, which are interpreted as the two stage crystallization age of the felsic volcanic rocks. Sample DC09-1 from unit a-2 was analyzed and given concordant data with a wide range of 207Pb/206 Pb or 206Pb/238 Pb ages(from 452 ± 7 to 1759 ± 74 Ma). The younger age group includes four grains ranging from 452±7 to 453±8 Ma with a mean value of 452±8 Ma, which is interpreted as the maximum age of deposition for unit a-2 volcaniclastic rocks. Thirty grains from sample DC13-1(from unit b) yielded the ages ranging from 451±8 to 3350±14 Ma. The minimum peak value of 451±8 Ma was given from four grain data, representing the maximum age for deposition. Mafic volcanic rock sample 2011XTS03-2(from unit d-4) yielded a narrow age range of 451-468 Ma, giving a weighted mean 206 Pb/238 U age of 462.7±3.7 Ma that provides the best estimate of the erupting age of volcanic rock. Mafic intrusive rock sample 14XTS03-1(from unit d-4) yielded a weighted average age of 452.8±5.6 Ma, which is interpreted as the crystallization age of the mafic intrusive rocks in the Tanjianshan Group. Aubergine pebbly sandstone sample 2010XTS-05 from formation c yielded ages ranging from 429 to 2636 Ma,forming a significant peak at 431 Ma. The minimum peak age of 431 Ma imply that the deposition age of the formation c should be later 431 Ma and not belongs to Late Ordovician. Therefore, the formation age of the Tanjianshan Group should be the Middle to Late Ordovician.2. The petrogenesis of volcanic rocks from the Tanjianshan Group is identified.The Tanjianshan Group includes two volcanic sedimentary circles from bottom to up, and they are calc-alkali bimodal island volcanic rocks in the first circle and tholeiitic series N-MORB like rocks in the second circle. Mafic volcanic rocks in unit a-1 display Si O2 contents of 46–54 wt.%, Mg O of 4–15 wt.%, Fe2O3 of 10–12 wt.%, and Al2O3 of 9–16 wt.%, with smoothly pronounced LREE enrichment patterns(La/Sm N = 2.76–3.36; Gd/Yb N = 1.71–2.71), and the significant Nb, Ta, Ti, Zr, and Hf anomalies. They are calc-alkalic island volcanic rocks. The isotopic data give the highest ?Sr(t) values with ranging from 22.8 to 36.8, and the low ?Nd(t) values from –4.3 to 0.2. Based on the petrographical and geochemical relationships, felsic volcanic rocks are divided into two types: RP1 and RP2. The RP1 suite rocks are calc-alkali rhyolite, with pronounced LREE-enrichments, negative Eu anomalies, elevated HSFE, negative Nb, Ta, and Ti anomalies, and positive Zr and Hf anomalies. The isotopic data give εNd value of-8.2 and-7.7, and εSr value of 323.6 and 314.5 for sample D19-4 and D19-5, respectively, and suggest that they mainly originated from the partial melting of continental crust. The RP2 suite rocks are rhyodacite-dacite and nearly rhyolite, with the low REE abundances with LREE-enriched chondrite-normalized patternand a [La/Sm]N ratio of 2.85 and the low HFSE, negative Nb, Ta, and middle Ti anomalies, and weakly positive Zr and Hf anomalies. The isotopic data give εNd value of 0.69 and 0.09, and εSr value of 13.9 and 40.3, and suggest that they mainly dirived from the fractional crystallization of the mafic mantle material, and the partially crustal contamination. Mafic volcanic rocks from unit d-1 have Si O2 contents of 45 to 50 wt.%, Al2O3 of 15.7–22.7 wt.%, Fe2O3 of 6.5–12 wt.%, and Mg O of 5.7–6.2 wt.%. They have elevated REE patterns, with high LREE(La, Ce, Nd, and Sm) values, and in conjunction with moderately HREE fractionated patterns(La/Sm N = 1.72–2.04; Gd/Yb N = 1.82–2.00). Nb and Ta are elevated with respect to Th and La, forming a hump on the primitive mantle-normalized plot, which is typical feature of OIB. Their ?Sr(t) values range from 1.5 to 7.4, while the ?Nd(t) value is very consistent with a value of 3.3. Mafic volcanic rocks in unit d-2 show Si O2 contents of 44–49 wt.%, Mg O of 6–8 wt.%, Fe2O3 of 12–14 wt.% and Al2O3 of 12–14 wt.%, and are characterized by unfractionated or weakly depleted LREE and mildly fractionated HREE(La/Sm N = 0.74–1.17; Gd/Yb N = 1.12–1.27). Most samples are plotted along the boundary between the N-MORB and BABB or the MORB field. They have the highest ?Nd(t) values of 5.6 to 6.2, and moderately varied ?Sr(t) values of 10.1 to 22.5. Mafic volcanic rocks in unit d-3 display Si O2 contents of 46–49 wt.%, Mg O of 7–11 wt.%, Fe2O3 of 8–11 wt.%, and Al2O3 of 13–18 wt.%. Their features of the trace element patterns are similar to those of unit a-1 and d-1, and with smoothly and moderately LREE enrichment and the intermediate Nb, Ta, Ti, Zr, and Hf negative anomalies. Their isotopic data give the lower positive ?Nd(t) values of 2.0 and 2.4, and varied ?Sr(t) values from 8.9 to 17.5. Mafic volcanic rocks from unit a-1 to unit d-3, to unit d-1, and to unit d-2 mainly resulted from the continuous upwelling of the basaltic mantle with the back-arc rifting.3. The tectonic setting of the Tanjianshan Group was a back-arc basin.The complex nature of the volcanism in the Tanjianshan Group, covering IAB of unit a, OIB-like of unit d-1 and MORB-like of unit d-2 suggests a complex geodynamic setting. Individual components of this assemblage can be recognized in a diverse range of geodynamic settings including island arcs and plume related oceanic islands, whereas modern back-arc basins have also been documented to include all these distinct suites. For example, the Lau basin and the Mariana trench include N-MORB, subduction influenced MORB, island arc, and OIB-like basalts. The wide range of geochemical compositions preserved in the Tanjianshan Group assemblage provides strong evidence for its origin as a back-arc basin. In essence, different basic volcanic rocks assemblages are the products of different stages in the process of the Ordovician back-arc basin development.4. Tectonic model for the Tanjianshan GroupThe presence of volcanic rocks from the Tanjianshan Group, HP-UHP metamorphic rocks and an ophiolite complex records a series of Early Paleozoic tectonic thermal events in North Qaidam, including the oceanic crust subduction, continental crust subduction, and arc-continent or continent-continent collision. Several stages are identified in the Early Paleozoic tectonic evolution of the Tanjianshan Group: 1) the passive continental margin formed along the southwest Qilian block; 2) the subduction of the South Qilian oceanic crust; 3) the volcanic island arc occurred; 4) the onset rifting of the back-arc basin; 5) the further spreading of the back-arc basin, and formed the N-MORB like rocks; 6) the back-arc basin closed, and subsequently begun arc-continent or continent-continent collision. In essence, the tectonic environment forming the Tanjianshan Group at the Xitieshan area has experienced changes from compression to extension with a final return to compressional setting.5. Back-arc basin metallogenic system for the Tanjianshan GroupThe petrogenetic evolution of the volcanic units of the Tanjianshan Group at the Xitieshan mine area suggested a successively gradual shift from the subduction-related IAB(unit a), to the typical BABB(unit d-3), to the OIB(unit d-1), and lastly to the N-MORB(unit d-2), is characterized by the typical back-arc basin basalt systematics. The difference depends primarily on the maturity of the back-arc basin, and the evolutionary process of the back-arc tectonic environment of the Ordovician Tanjianshan Group is a key to understanding the metallogenic processes in North Qaidam, i.e. the Valu Fa Ridge of the Lau basin. In nascent back-arc basin, Kuroko-type mineralization at Okinawa trough influenced by continental crust was produced. During the early stage of interarc spreading, Lau basin-type deposits were formed. During the third stage(back-arc basin development mature), the Noranda-type deposits occurred and controlled by basaltic crust, and the sulfide composition was close to mid-ocean ridge. Unit d-2 of the Tanjianshan Group in the Xitieshan area, to some extent, is likely considered to be the analogue tectonic setting of a mid-ocean ridge deposit.6. Nature and formation age of the ductile shear zoneDynamic evidences for the ductile shear deformation, including the asymmetric fold, S-C fabric, the rotated phenocrysts, mica fish structure, flattening and/or stretching of geologic markers, suggest that it is kinematically characterized by dextral compressed shear. The muscovite ages from the mylonitized granitic gneisses(sample 10XTS-02) range from 391±2 Ma to 405±2 Ma, and yielded plateau age of 398±4 Ma and inverse isochronal age 399±4 Ma, which are considered to be the formation age of the dextral ductile shear zone in the Xitieshan area. This ductile shear deformation event controlled the general shape of the ore zones at the Xitieshan deposit, which exhibit a series of lenses.7. New relative evidence for the genesis of the Xitieshan massive sulfide depositThe Xitieshan district has many features similar to the Bathurst district of Canada, the Iberian Pyrite Belt of Spain, and the Wolverine deposit in Canada, these deposit are considered as the typical VSHMS deposits. It is therefore reasonable that the Xitieshan deposit should be ascribed to the VSHMS deposits. |
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