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Abstract:The Tula A2‐subtype granite pluton is located between the Altun fault and its branching fault. According to the geological, geochemical, REE and trace elements characteristics, it belongs to the A2(PA) subtype granite. The SHRIMP U‐Pb zircon dating gives a result of 385.2±8.1 Ma, which is located between the Middle and Late Devonian in the international stratigraphic chart, and can be regarded as the crystallization age of the Tula granite. The study indicates that the Tula area was in a local extensional environment in the end of the Middle Devonian, and that environment was probably related to the synchronized strike‐slip activity of the Altun fault.

Three suites of granitic rocks are present in the early Paleozoic North Qaidam ultrahigh-pressure (UHP) metamorphic belt, which has eclogite ages of 466-495 Ma. The first suite consists of quartz monzodiorite + granodiorite + monzogranite, the second of monzogranite + two-mica granite + muscovite granite + syenogranite, and the third of granodiorite + monzogranite + biotite granite. The country rocks into which these bodies were intruded consist chiefly of medium- and high-grade Proterozoic gneisses and schists and Paleozoic sedimentary and volcanic rocks. Monzogranite of the first suite has a zircon SHRIMP age of 473 Ma, whereas syenogranite in the second suite has an age of 446 Ma, and granodiorite in the third suite has an age of 397 Ma. Rocks of the first and third suites have similar mineralogies consisting of plagioclase, hornblende, quartz, alkali feldspar, and biotite, whereas those of the second suite consist of potassium feldspar, quartz, muscovite, biotite, and plagioclase. Rocks of the first and third suites have I-type compositions with SiO2 = 61-69 wt%, Na2O/K2O 1, ANK <1, and Eu/Eu* = 0.7-1.0, whereas the second suite consists of S-type granites, with SiO2 = 70-76 wt%, Na2O/K2O <1, ANK >1, and Eu/Eu* = 0.1-0.3. The first suite is interpreted as island arc or active continental margin magmatism, the second as igneous activity related to collision between the Qaidam and Qilian blocks, and the third as post-collisional plutonic activity.

Abstract Two types of granitic intrusives occur in the lower Paleozoic strata in north Qilian. One is the hornblende‐bearing granitic body in Jingzichuan, characterized by the REE total of 90×10−6106×10−6, LREE/HREE ratio of less than 8 (ranging from 6.9 to 7.8) and zircon SHRIMP age of 464 Ma. These intrusives have island‐arc affinity. The other is biotite‐bearing granitic body in Huangyanghe with the REE total of 214×10−6250×10−6, LREE/HREE ratio of more than 8 (ranging from 8.2 to 9.7) and zircon LA‐ICP‐MS age of 383 Ma. They were tectonically intruded in a post‐coilisional or within‐plate environment.

Abstract There is a large ductile shear zone, 2 km wide and more than 350 km long, in the South Qilian Mountains, western China. It is composed of volcanic, granitic and calcareous mylonites. The microstructures of the ductile shear zone show nearly E‐W extending subvertical foliation, horizontal and oblique stretching lineations, shearing sense from sinistral to oblique sinistral strike‐slip from east to west, “A” type folds and abundant granitic veins. Measured lattice preferred orientations (LPOs) of the mylonitic and recrystallized quartz of the granitic mylonite in the west segment suggest a strong LPO characterized by the dominant slip systems {10times10} formed at high temperature (>650°C). K‐feldspar of the mylonite shows an 39Ar/40Ar high‐temperature plateau age of 243.3±1.3 Ma, and biotite, 250.5±0.5 Ma, which represent the formation age of the ductile shear zone. The 39Ar/40Ar plateau ages of 169.7±0.3 Ma and 160.6±0.1 Ma and the 39Ar/40Ar isochron ages of 166.99±2.37 Ma and 160.6±0.1 Ma of biotites in the mylonite represent the subsequent deformation age. These ages indicate that this ductile shear zone is similar to the Altun and South Kunlun sinistral ductile shear zones in its ages of movement, formation, reactivation and duration.

Abstract Coesite was discovered as inclusions in zircon separates from pelitic gneiss associated with a large eclogite body in the North Qaidam ultrahigh‐pressure (UHP) terrane. Some graphite inclusions were also found. This finding suggested the occurrence of in‐situ UHP metamorphism and that the terrane was most likely recrystallized at pressures below the diamond stability field. It supported other previous indirect UHP evidence, such as polycrystalline quartz inclusions in eclogitic garnet, quartz lamellae in omphacite and P‐Testimates for both eclogite and garnet peridotite. The U‐Pb and Sm‐Nd ages of the North Qaidam eclogite indicated that subduction of continental crust occurred in the Early Palaeozoic, which probably recorded a collision between the Sino‐Korean and Yangtze plates.

Abstract Zircon SHRIMP ages of the Aolaoshan granite on the south margin of the Qilian Mts. range from 445±15.3 to 496±7.6 Ma (averaging 473 Ma), belonging to the Early Ordovician. Geochemically, the granite is similar to I‐type granite and, tectonically, was formed in an island‐arc environment based on relevant diagrams for structural discriminations. Considering also the regional geology, the authors suggest that the granite is part of an ultrahigh‐pressure belt on the south margin of the Qilian Mts. and that its formation bears a close relationship to this belt.

Abstract A contact zone sandwiched between an arc and an oceanic crust was discovered in the Laohushan area in the present study. It consists of a series of north‐dipping imbricated thrust sheets and is exposed on the surface as a narrow arcuate belt, which extends for about 30 km in an E‐W direction and measures about 1–3 km wide. Lithologically, it can be divided into four subzones. Subzone 1 consists of meta‐andesite and metasandstone; subzone 2, psammitic schists; subzone 3, psammitic and pelitic schists, quartz diorite and hornfelses; and subzone 4, metagabbro, epidote amphibolite and pelitic schists. The metamorphism has the following grading sequence: low greenschist facies in subzone 1 ŕ high greenschist facies in subzone 2 ŕ low amphibolite facies in subzone 3 ŕ epidote amphibolite facies in subzone 4. Petrographic and geochemical evidence shows that rocks in subzones 1, 2 and 3 are arc rocks, whereas those of subzone 4 are oceanic crustal rocks. The metamorphic mineral assemblages and especially mineral chemistry of the bluish green amphibole from the pelitic schists and epidote amphibolite of subzone 4 suggest that the rocks of the contact zone were metamorphosed at a pressure of up to 0.69 GPa. It is thought that the Late‐Mid Ordovician oceanic lithosphere of a back‐arc basin was underthrust northerly beneath an arc to a depth of 20–23 km, where the basaltic rocks and gabbro were converted to epidote amphibolite and metagabbro respectively. Then, the root rocks of the arc and these metamorphosed oceanic rocks were brought up to shallower depths by thrust faults to form a contact zone between the arc and the oceanic crust in the Laohushan area.

Abstract:Enclaves in intermediate‐acid plutons from Tongling can be divided into three types: xenoliths, relics and magmatogenic enclaves. The magmatogenic enclaves consist of cumulates, micrograined dioritite mixtite and dioritic chilled border enclaves. Petrologically, relics with eyed and meta‐poikilitic texture are characterized by high content of biotite (>80%) and low content of cordierite and grossular. The cumulates with accumulate texture consist of a great amount of pyroxene, hornblende and minor spinel and phlogopite. The micrograined dioritic mixtite is composed of more hornblendes and feldspar and less needle apatites and an ellipsoid basic core included in plagioclase. The chilled border enclaves have the same mineral association, but more dark minerals than the host rocks consisting of plagioclase, quartz, alkaline feldspar, hornblende and biotite. Geochemically, the relics exhibit high REE content (455.8×10−6) and high ratio of LREE/HREE, more obvious Eu negative anomaly and are rich in Cr and Ni, poor in Ba, Nb and Sr. All kinds of magmatogenic enclaves have the same REE pattern without Eu anomaly, and the total REE is 74.9×10−6for spinel pyroxene cumulate, 179.7×10−6for hornblende cumulate, 226.9 × 10−6for hornblende cumulate crystal, 289.9×10−6for micrograined dioritic mixtite and 131.2×10−6for chilled border enclaves. Calculation with the thermometer and barometer for minerals shows that the temperature and pressure of formation are 1126 °C and 8.44 kbar (7.15–9.04 kbar) for the spinel pyroxene cumulate, 1065 °C‐1029 °C and 8.23 kbar (7.13–9.50 kbar) for the hornblende pyroxene cumulate, 959 °C and 3.40 kbar (2.86–4.09 kbar) for the micrograined dioritic mixtite enclaves, and 951 °C and 0.59 kbar for the dioritic chilled border enclaves, respectively. Thus, the following conclusions can be drawn. Relic enclaves were derived from the crustal partial melting and all cumulate enclaves originated from deep magma chambers near the Moho in the lower crust, which are cumulate fragments formed by fractional crystallization of basic magma; mixtite came from shallow magma chambers in the middle‐upper crust, formed by the mixture of mantle‐derived magma and acid magma derived from the crustal partial melting; and the chilled border enclaves came from the border fragments of early intrusive bodies.