| The Qilian orogenic belt is located in the northeastern margin of the Tibetan Plateau and it is a typical Caledonian orogenic belt in west China. The Qilian orogen is surrounded by the Ordos block to the east, the Tarim block to the west, the North Qaidam and the West Qinling belt to the south and the Alxa block to the north. It is an important part of the Central Orogenic Belt of China. The Early Paleozoic intrusive rocks are widely distributed in the Qilian orogen, which recorded the important information about the convergence of the Qaidam, Central Qilian and Alxa blocks. However, previous studies manily focused on the North Qilian area, while the works on the Central Qilian is relatively limited. The eastern part of the Central Qilian connects the North Qilian suture zone to the north and the North Qaidam-West Qinling belt to the south, thus it is a key area to understand the tectono-magmatic evolution of the Qilian orogenic belt and the relationship between the Qilian orogen and its adjacent belts. This paper reports zircon U-Pb ages and Hf isotopic data, and whole-rock major and trace elemental and Sr-Nd isotopic data for Early Palaeozoic intrusive rocks (the Jishishan, Bamishan, Heishishan, Binglingsi, Ledu and Shichuan plutons, a total of 19 lithological units) in the eastern part of the Central Qilian. Combined previous works, we have established the geochronological frame of the intrusive rocks in the eastern Central Qilian and have discussed petrogenesis of different types of intrusive rocks, the geodynamic process responsible for the generation of intrusive rocks in the eastern Central Qilian and Early Paleozoic tectonic evolution of the earstern part of the Qilian orogen. The main results obtained are as follows:1. The results of U-Pb zircon dating for 19 samples from 17 lithological units of the the Jishishan, Bamishan, Heishishan, Binglingsi, Ledu and Shichuan plutons show that their magma crystallization ages are 472-427 Ma (mainly 460-430 Ma). Combined with previously published data, the intrusive magmatic activities of the eastern Central Qilian can be divided into three stages:(1) Late Cambrian (-490 Ma), (2) Early to Middle Ordovician (472-467 Ma) and (3) Late Ordovician to Middle Silurian (459-427 Ma). Late Cambrian and Early to Middle Ordovician rocks are mainly intermediate to mafic intrusive rocks, while Late Ordovician to Middle Silurian rocks include adakitic rocks, moderately to strongly peraluminous granites, typical I-type granites and granodiorite, diorite, mafic-ultramafic rocks and basic dykes. The intrusive magmatic activities in the southern part of the eastern Central Qilian occurred relatively earlier than those in the northern part of the eastern Central Qilian. Moreover, the mafic rocks mainly crop out in the southern part of the eastern Central Qilian.2. Early-Middle Ordovician medium-and fine-grained diorites from the Jishishan pluton possess magma crystallization ages of 472-471 Ma and 467 Ma, respectively. They have low SiO2 contents (52.07-56.86%), high Mg# (50-57) and depleted Sr-Nd-Hf isotopic compositions ((87Sr/86Sr)i=0.7038-0.7041, εNd(0=+3.6 to+4.6, εHf(t)=+10-6 to+12.9), and are characterized by enrichment of LILE (e.g., Ba, K, Sr) and LREE, indicating that they were probably derived from metasomatic mantle. Furthermore, the Jishishan dioritic rocks have high Ba/Th (142-816), Sr/Nd (42.7-63.8) and Sr/Th (212-1425) ratios, suggesting that their mantle source had been metasomatized by subduction-related fluids.3. Late Ordovician-Early Silurian Bamishan monzogranite (459 Ma), Heishishan granodiorite and biotite granite (440 Ma) and Binglingsi biotite granite (432 Ma) have geochemical characteristics similar to those of adakites. The Bamishan monzogranite has high Na2O (4.79-5.78%) and Ba (859-2116 ppm) contents, low Th/La (0.09-0.14) and Th/Zr (0.02-0.04) ratios, and depleted Sr-Nd-Hf isotopic compositions ((87Sr/86Sr)i=0.7040-0.7050, εNd(0=+2.8 to+3.2, εHf(0=+6.0 to+8.6). We suggest that the Bamishan monzogranite was produced by fractional crystallization of oceanic slab (altered MORB+minor subducted sediments)-derived melts. The Bamishan quartz diorite (451 Ma) has Ba and Sr contents and Sr-Nd-Hf isotopic compositions similar to those of the Bamishan monzogranite, but their MgO, FeOT, Cr, Ni and REE (e.g., Y) contents and Dy/Yb ratios are higher and Sr/Y and La/Yb ratios are lower. Combined with petrographic characteristics, we suggest that the Bamishan quartz diorite was also derived from oceanic slab, but it contains cumulus phases (hornblende and accessory minerals (e.g., titanite)). The Heishishan and Binglingsi adakitic rocks were characterized by relatively evolved Sr-Nd-Hf isotopic compositions (εNd(0=-3.9 to-0.2), and have slightly concaveupward REE patterns. They probably resulted from partial melting of mafic lower crust at pressure of>10 kbar (depth>35 km), and there were residual hornblende and garnet in the source.4. Late Ordovician-Middle Silurian Jishishan biotite granite (452 Ma) and two mica granite (455 Ma), Ledu biotite granite (441 Ma) and two mica granite (431 Ma) and Shichuan monzogranite (427 Ma) has higher SiO2 contents (> 70%). They are mainly moderately to strongly peraluminous rocks and display transitional I-S type characteristics. Through comparing the chemical compositions of these peraluminous granites with those of Lachlan S-type granitoids, Himalayan leucogranites and experimental melts, we suggest that they were probably derived from metagreywackes or quartzofeldspathic meta-igneous rocks. Isotopic data indicate that the Jishishan biotite granite and two mica granite and the Ledu biotite granite and two mica granite mainly resulted from melting of Precambrian basement (orthogneisses) in the Central Qilian block, while the Shichuan monzogranite could be generated by melting of metagreywackes at pressure of>5 kbar. The peraluminous granites have obvious negative Eu anomalies, relatively high Zr saturation temperatures (>750℃ for each granite type, on average), and relatively low Pb (20.4-46.0 ppm) and high Ba (182-1049 ppm) contents, suggesting that they were mainly produced by biotite dehydration melting under fluid-absent conditions. Geochemical and isotopic data show that the chemical composition of these peraluminous granites were not significantly influenced by addition of mantle-and/or slab-derived magmas, peritectic assemblage entrainment or restite unmixing. Combined with previous studies, this paper argues that the chemical composition (especially the major element compositions) of high SiO2, moderately to strongly peraluminous granite is mainly controlled:by source compositions and melting conditions (temperature, pressure, water fugacity, etc.).5. This paper has also discussed the petrogenesis of Late Ordovician-Early Silurian intermediate-mafic intrusive rocks. The Bamishan hornblende gabbro (449 Ma) and gabbroic dike and the Heishishan hornblende gabbro (438 Ma) all have low SiO2 (49.53-51.39%) and high MgO (5.44-14.07%, Mg#=57-74) contents, and enriched Sr-Nd isotopic compositions ((87Sr/86Sr)i=0.7058-0.7069, εNd(t)=1-9 to 0.0), suggesting that they were dominantly derived from enriched lithospheric mantle. Among them, the Bamishan gabbro has higher MgO (12.38-14.07%), Cr (1159-1163 ppm) and Ni (220-353 ppm) than those of mantle-derived primary arc basaltic magma. Combined with petrographic characteristics, they were suggested to be cumulates from the water-rich parental basaltic magma. The Bamishan gabbroic dike is shoshonitic rock and has relatively high K2O content (2.73%), K2O/Na2O (1.22) and Rb/Sr (0.15) ratios and low Ba/Rb (6.77) ratios, suggesting that it was derived from a phlogopite-bearing mantle source. Moreover, the Bamishan gabbroic dike possesses relatively high (87Sr/86Sr)i ratio (0.7058), Th content (6.90 ppm) and low Ba/Th (92.9) and Sr/Th (93.5) ratios, suggesting that its mantle source had been metasomatized by sediment-derived melts. The Heishishan gabbro possesses relatively low MgO (5.44%) and high Al2O3 (18.36%) contents, which are similar to those of low magnesium-high alumina basaltic rocks. It could be generated by strong fractional crystallization of mafic minerals (olivine, pyroxene) from mantle-derived basaltic magma. The Heishishan quartz diorite has high Mg# (50-61) and enriched Sr-Nd isotopic compositions ((87Sr/86Sr);=0.7051-0.7062, εNd(0=-2-2 to+0.5), indicating that they were derived from enriched lithospheric mantle. The Bamishan quartz dioritic dike (432 Ma) is metaluminous, high-K calcalkaline rock and was probably generated by melting of mafic lower crust. The Ledu quartz diorite (446 Ma) and granodiorite (452 Ma) have relatively low SiO2 contents (59.43-62.84%) and variable K20/Na2O (0.88-1.54 and 0.79-1.25, respectively) ratios and high Mg# (40-50 and 48-50, respectively), suggesting that they were probably derived by melting of mafic lower crust with minor addition of mantle-derived magma and minor contamination by Precambrian basement.6. There are some differences between Early Paleozoic intrusive rocks in the Central Qilian and the North Qilian in their petrogenesis, ages and distributions. For the North Qilian, Early Paleozoic mafic intrusive rocks are mainly exposed in its western segment. However, for the Central Qilian, the mafic intrusive rocks mainly crop out in its eastern segment. Moreover, the mafic intrusive rocks in the Central Qilian were generally derived from more enriched mantle sources. The magma crystallization ages of the intermediate intrusive rocks in the Central Qilian and the North Qilian are similar. But unlike in North Qilian, there are intermediate dikes and slab-drived intermediate intrusive rocks in the Central Qilian. Petrogenesis of I-type, S-type and transitional I-S type granites in the Central Qilian and the North Qilian are similar. However, the slab-derived adakitic rocks formed earlier in the Central Qilian, and adakitic rocks derived from delaminated lower crust have not been reported in the Central Qilian. Based on these differences, and combined with the temporal and spatial distribution of Early Paleozoic ophiolites and subduction-and collision-related magmatic rocks in North Qaidam and the northern margin of the West Qiling and mafic-intemediate-felsic rocks in the eastern Central Qilian, this paper proposes that the generation of Early Paleozoic magmatism in the eastern Central Qilian was related to the northward subduction of Early Paleozoic oceanic slab exsisted between the Central-South Qilian and Qaidam-West Qinling and subsequent collision or post-collisional processes.7. According to the results of this paper, and combined with the regional data, we argue that Early-Middle Ordovician intermediate-mafic intrusive rocks in the eastern Central Qilian formed in subduction setting, and they were produced by partial melting of mantle wedge induced by dehydration of subducted slab. Late Ordovician-Middle Silurian intrusive rocks in the eastern Central Qilian could form in the post-collisional setting, and their petrogenesis and distributions suggest that their magma generation was probably related to slab break-off. After slab break-off, asthenosphere upwelling induced the partial melting of the broken slab (MORB+minor overlying sediments), lithospheric mantle and middle-lower crust, and then various types of rocks including slab-derived adakitic rocks, lower crust-derived adakitic rocks, gabbroic rocks, dioritic rocks and I-type, S-type and transitional I-S type granites formed.8. Early Paleozoic tectonic evolution of the eastern Qilian is divided into the following four stages:(1) During the Late Cambrian to the early stage of the Middle Ordovician, ancient ocean plate on both sides of the eastern Central Qilian subducted northward, leading to the generation of subduction related magmatic rocks in the eastern Central Qilian and eastern North Qilian; (2) During the middle stage to late stage of the Early Ordovician, ancient ocean plate on both sides of the eastern Central Qilian closed due to the Gulang Movement and then continent-(or arc-) continent collision occurred; (3) During the Late Ordovician to the late stage of the Early Silurian, slab break-off occurred; (4) During the late stage of the Early Silurian to the early stage of the Middle Silurian, the eastern Central Qilian was still influenced by slab-break off, melting of continental crust due to gradually propagation of heat toward north resulted in the generation of Ledu two mica granite and Shichuan monzogranite; after obvious crustal thickening, delamination of the lower crust took place in the North Qilian, which resulted in the formation of high-Mg adakitic rocks in the North Qilian. |
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