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北阿尔金西段出露大量中酸性侵入岩，为探讨西段地区花岗质岩石的成因、形成环境以及北阿尔金地区岩浆活动特点，本文选取北阿尔金西段的正长花岗岩体和闪长岩体作为研究对象，进行了岩石学、地球化学、锆石U-Pb年代学及Hf同位素等方面的研究。研究结果表明，正长花岗岩体形成时代为495.7~502.0Ma，铝饱和指数A/CNK为0.86~1.09小于1.1，P2O5与SiO2含量呈负相关，具Ⅰ型花岗岩特征。轻稀土富集而重稀土亏损，具有明显的负Eu异常，微量元素特征显示富集Rb、Ba、Th、U、K等元素，相对亏损Nb、Ta、P、Ti等元素。岩石地球化学及锆石Hf同位素特征暗示其源区物质成分不均一，源岩主要为0.95~1.4Ga的新生地壳（变中基性岩），并有1.42~1.83Ga的古老地壳物质的参与。闪长岩体的侵位时代为497.1±3Ma，轻稀土富集而重稀土亏损，较弱的负Eu异常，相对富集Rb、Ba、Th、U、K等元素，而亏损Nb、Ta、P、Ti。锆石eHf（t）值为-1.61~+2.16，二阶段模式年龄tDM2为1.34~1.56Ga，暗示其源岩可能主要为新生地壳，并有古老地壳物质参与成岩。北阿尔金西段正长花岗岩、闪长岩均形成于早古生代，具弧型花岗岩特点，结合区域构造背景，认为其形成可能与北阿尔金洋壳俯冲有关，产于陆弧环境。

Quantitative analyses of the spatial distribution of fault structures can provide a theoretical basis for forecasting prospective ore deposits. Characteristics and complexity of fault structure distribution in the Qitianling area, Southern Hunan Province, China, were quantitatively calculated and appraised by fractal and multifractal methods to evaluate the relation between fault structures and ore-prospecting potential. The results show that the lengths of faults can be modeled as multifractals. Multifractal spectra evidently reflect the characteristics of the scaling of fault structures. The box-counting dimension value (D) of fault structures is equal to 1.656, as indicates complexity of the spatial distribution of faults and favorable structural conditions for the formation of ore deposits. Moreover, the D values of sub-regions were calculated and isopleths of their fractal dimension values were plotted accordingly. Overlay analyses of isopleths of fractal dimension values and distributions of known ore deposits show that areas with the larger fractal dimension values of fault structures have more ore deposits. This spatial coupling relationship between D values and ore deposits can be used to forecast and explore other ore deposits. On the basis of complexity theory for ore-forming systems, three exploration targets with high D values were delineated as prospective ore deposits.

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-T estimates 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.

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 74.9×10−6 for spinel pyroxene cumulate, 179.7×10−6 for hornblende cumulate, 226.9 × 10−6 for hornblende cumulate crystal, 289.9×10−6 for micrograined dioritic mixtite and 131.2×10−6 for 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.

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.

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.

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.

The Qilian Caledonian orogenic belt on the north margin of the Qinghai-Tibet Plateau were formed by convergence and collision of the Alxa terrain, Qilian terrain and Qaidam terrain. The 350-km-long, WNW–ESE-trending North Qaidam ultrahigh-pressure (UHP) metamorphic belt, lying between the Qilian and Qaidam terrains, was formed between 495 and 440 Ma by deep subduction of the South Qilian Sea and the Qaidam continental crust beneath the Qilian terrain. The UHP belt was exhumed by a process of ‘oblique extrusion’ during transformation from ‘normal’ to ‘oblique’ intracontinental subduction between the Qilian and Qaidam terrains. Exhumation began at 470–460 Ma and was completed by 406–400 Ma. Exhumation structures are well-preserved in the UHP rocks and record extensive retrograde metamorphism.

A suite of Early Paleozoic island arc volcanic rocks, consisting mainly of basaltic lavas and some intermediate-silicic rocks, crops out in the North Qaidam basin, northeastern Tibet Plateau. These rocks have been extensively metamorphosed under greenschist facies conditions, and hence differ from the older Proterozoic intermediate-mafic volcanic rocks, which have generally experienced amphibolite facies metamorphism. The Early Paleozoic volcanic rocks are divided into three groups based on their geochemical characteristics: (1) VTG-I, consists of island arc tholeiitic basalts, basaltic andesites and andesites that crop out in Jilusu and Shuangkoushan; (2) VTG-II, which are high-Al, alkaline and sub-alkaline basalts in Jilusu; and (3) VTG-III, an assemblage of highly depleted, MORB-type basalts, andesites and dacites that occurs mostly in Shuangkoushan. Groups VTG-I and -II are considered to be the products of mature island arcs produced in two stages. In the first stage, following initiation of ocean–continent subduction, the island arc tholeiites were generated by partial melting of the subsiding oceanic crust and mantle wedge beneath the island arc. As subduction progressed and accelerated, calc-alkaline and high-Al basalts were formed and the crust beneath the island arc was thickened. The highly depleted MORB-type lavas (VTG-III) have boninitic affinities and are believed to have been generated from depleted mantle material modified by subduction zone fluids, possibly in the fore-arc region. The island arc tholeiites are dated at 514.2±8.5 Ma whereas UHP eclogite in the North Qaidam Belt has been dated at 494 Ma. These ages suggest that ocean–continent subduction took place in the North Qaidam Basin in the Early Paleozoic and was followed shortly by continent–continent collision and subduction, which produced the UHP rocks.

Using both TIMS and SHRIMP methods, U–Pb isotopic dating was carried out on zircons from eclogites and their host gneisses in the Xitieshan area of the north Qaidam mountains, western China. Zircons from both eclogites and retrograded eclogites with metamorphic characteristics gave ages of 485–500 Ma, representing the age of eclogite facies metamorphism. Euhedral zircons from the eclogites, which show a magmatic character (Th/U>1), yielded ages of 750–800 Ma, which are believed to represent the magmatic crystallization age of the protlith. TIMS dating of zircons from granitic gneiss enclosing retrograded eclogite gave an upper intercept age of 952±13 Ma and a lower intercept age of 478±44 Ma. The former probably represents the magmatic crystallization age of the granitic protolith, and the latter is in agreement with the age of metamorphic zircons from the eclogite within the margin of error. This implies that the eclogites and their enclosing gneisses in Xitieshan area as a whole underwent HP–UHP metamorphism during an early Paleozoic orogeny to form one part of the north Qaidam HP–UHP metamorphic belt. These results also suggest an ‘in situ’ rather than ‘tectonic'relationship between the eclogites and their host gneisses.

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 a...

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.

An Early Palaeozoic UHP metamorphic belt was recently discovered in the North Qaidam Mountains in the north-eastern part of the Tibet Plateau. The belt contains abundant eclogite with lesser amounts of garnet peridotite and gneiss. The paragneiss contains abundant zircon with coesite inclusions whereas the eclogite has garnet with coesite pseudomorphs and omphacite with exsolved quartz. The calculated peak metamorphic conditions for the eclogite are T = 730 °C and P = 2.8 GPa. The garnet peridotites are magnesium-rich with Mg#s [100 × Mg/(Mg + Fe)] up to 92. Olivine in the peridotites is highly magnesian (Fo > 91.5) and TiO2-poor and is typical of low-T garnet peridotites. Calculated peak metamorphic conditions for the peridotites are T = 837 °C and P = 2.5 GPa. U–Pb and Sm–Nd isotope dating indicate that continental subduction started during the early Ordovician (∼ 495 Ma). The North Qaidam UHP, together with the North Qilian HP belt about 300 km to the north, constitute a pair of genetically related early Palaeozoic subduction zones along the northern margin of the Tibet Plateau.

The pyroxenite xenoliths in the volcanic rocks of Hoh Xil consist of clinopyroxenes and orthopyroxenes. The mineral composition of these pyroxenes is similar to that of mantle xenoliths including peridotite and pyroxenite from China and abroad, and different from that of granulites. The pyroxenes formed at 1101–1400°C (averaging 1250°C) and under 30–60 kb (averaging 46 kb). We deduced that the magma was derived from the mantle at a depth of more than 150 km, which fits in with the geophysical conclusion that the low-velocity layer existed in the mantle under 150 km.