Origin and tectonic evolution of the Qilian Precambrian basement on NW China were investigated using zircon U-Pb ages with collaborating stratigraphic and paleontological evidence. Zircon grains were separated from two schists, two granitic gneisses and one mylonized gneiss and dated with SHRIMP. Seventy percent of sixty-one detrital zircon ages from two schists ranges from 0.88 Ga to 3.09 Ga, mostly within 1.0 Ga to 1.8 Ga with a peak at 1.6 Ga to 1.8 Ga, and twenty percent varies from 2.0 Ga to 2.5 Ga. A few falls in the Archean and Neoproterozoic periods. The two granitic gneisses were dated 930±8 Ma and 918±14 Ma, whereas the mylonized granitic gneiss was dated 790±12 Ma. These ages represent two periods of magmatisms, which can be correlated with the early and late stages of magmatisms associated with the Jinningian movement on the Yangtze Blocks.The results from this and previous studies indicate that the ages of the Precambrian detrital zircons from the Qilian Block are widely distributed in the Proterozoic era, distinct from the North China Block which was stable in the Neo-Mesoproterozoic era. By contrast, the age histograms of the detrital zircons from the Qilian Block is similar to those from Precambrian basement of the Yangtze Craton. Therefore, it is suggested that the Qilian Block had a strong affinity toward the Yangtze Craton and might belong to the supercontinent Gondwana in the Neoproterozoic time. This inference is supported by Nd model age (T DM ), stratigraphic, and paleontological evidence. It is further considered that the Qilian Block was rifted from the supercontinent Gondwana during late Sinian to form an isolated continent in the Proto-Tethyan Ocean, moving towards the Alaxa Block in the North China Craton. The part of Proto-Tethyan Ocean between the Qilian and Alaxa Blocks should correspond to the so-called Paleo-Qilian Ocean. Following the closure of the Paleo-Qilian Ocean in the early Paleozoic, the Qilian Block collided with the Alaxa Block to form the North Qilian Orogenic Belt. Based on this tectonic explanation, the North Qilian ophiolites should represent parts of lithosphere from the Proto-Tethyan Ocean. Lithological and geochronological evidence also indicates that the Qilian Block underwent continental reactivation possibly induced by the deep northward subduction of the North Qaidam Block in early Paleozoic time.

High-grade gneisses of the north Qaidam high-pressure/ultrahigh-pressure metamorphic terrane enclose minor eclogites and ultramafi c rocks. In combination with petrological data, sensitive high-resolution ion microprobe U-Pb geochronology on the granulitized gneisses in the Luliangshan and Xitieshan, western north Qaidam Mountains, reveals a polyphase tectonothermal history including Early Neoproterozoic and Early Paleozoic events. The rocks investigated are two granulite-facies paragneisses and one orthogneiss that both surround garnet peridotite in the Luliangshan, and two paragneisses that enclose eclogite in the Xitieshan. The inherited zircon cores from paragneiss and orthogneiss yield ages between ca. 1000 Ma and ca. 2500 Ma, representing Mesoproterozoic to Archaean source material for these gneisses. A ca. 900 Ma age obtained from one orthogneiss and one paragneiss is interpreted as the age of simultaneous Early Neoproterozoic magmatism and metamorphism. This Early Neoproterozoic tectonothermal event is similar in age to the Jinning orogeny, which is commonly recognized in the metamorphic basement of the south China block, and suggests that the north Qaidam Mountains have an affi nity to the south China block. Early Paleozoic metamorphism is recorded in all gneiss samples. In conjunction with cathodoluminescence imagery and mineral inclusions, U-Pb dating of zircons reveals two Early Paleozoic age groups: ca. 450 Ma represents the age of high-pressure granulite metamorphism (Grt + Rt inclusions in zircon; kyanite porphyroblasts), whereas ca. 425 Ma refl ects the time of medium-pressure, granulite-facies metamorphism (Pl + Sil inclusions in zircon) and associated anatexis during decompression. The ages obtained suggest that the granulitefacies metamorphism lasted for ~25 m.y. and was related to the Early Paleozoic continental collision between the Qilian and Qaidam blocks, and to subsequent thermal relaxation and exhumation.

SHRIMP U-Pb dating was carried out on zircons from the Niuxinshan gneissic granite and the Leigongshan gneissic tonalite in the North Qilian orogen, NW China. The results yield weighted averaged 206 Pb/ 238 U ages of 776±10 Ma and 774±23 Ma respectively. Igneous morphology, oscillatory zonings, and relatively high Th/U ratios for these zircons suggest magmatic origin. These ages are interpreted as timing of magma emplacement and thus representing a Neoproterozoic (~775 Ma) magmatic event in the North Qilian area. It is suggested that this magmatism is probably related to breakup of the supercontinent Rodinia. This finding, together with the similar ages of 750 to 800 Ma reported for neighboring terranes of South-central Qilian and North Qaidam, is of significance to understanding of the Rodinia evolution in West China.

Sixty-two geologically meaningful U-Pb dates were obtained by using SHRIMP technique for the detrital zircons in three metasedimentary rocks from stratigraphically uppermost parts of the Longshoushan Group in the present study. Eighty percents of these dates range from 1.7 Ga to 2.2 Ga with a peak at 1.8-2.0 Ga and twenty percents from 2.3 Ga to 2.7 Ga. The youngest detrital zircon is dated at 1724±19 Ma which is interpreted as the maximum depositional age of the metasedimentary rocks. Therefore, the age for the diagenesis and lithification of the original sedimentary rocks of the Longshoushan Group before the metamorphism must be younger than 1724±19 Ma. Comparison of the age histograms of these detrital zircons with the ages of the igneous rocks on the surrounding older massifs suggests that the sediments of the Longshoushan Group were most likely derived from the Alaxa Block and Tarim Craton. This implies that the affinity between Alaxa Block and Tarim Craton was strong and that they might have been a unified craton during middle-early Proterozoic time.

<p>Field relationships, mineralogy, petrology, geochemistry, geochronology, and Nd-Hf-O isotopes of the mafic-ultramafic rocks from the east part of the Qilian block are studied in the present work. The Aganzhen intrusive body only exposed in the Zhigoumen, Shiguanzi, Xianggoumen outcrops and includes Hornblende peridotite, wehrlite, olivine-bearing pyroxenite, hornblende-bearing pyroxenite, websterite, clinopyroxenite, hornblendite, olivine-bearing gabbro. The gabbroic rocks are also layered or massive cumulates with rock types varying continuously from noritic gabbro through hornblende gabbro to dioritic norite. Contact metamorphic zones are well developed between the Aganzhen intrusive body and the country rock. Major element contents of Aganzhen ultramafic-mafic rocks show subalkalic series and are characterized by low SiO<sub>2</sub> contents (38.09-54.96 %), low TiO<sub>2</sub> contents (0.09-0.72 %), low P<sub>2</sub>O<sub>5</sub> contents (0.00-0.36 %) and alkali contents (Na<sub>2</sub>O+K<sub>2</sub>O 0.01-5.35 %), but high MgO contents (9.68-33.06 %), Ni contents (116-1505 ppm), Cr contents (713-2808 ppm). Similar LREE-rich pattern ((Ce/Yb)<sub>N</sub> =0.95-3.80 except two Samples) and tiny Eu anomaly (Eu/Eu* =0.6-1.2) indicate the Aganzhen ultramafic-mafic rocks have the same magma source. Trace elements are enriched in LILE (Rb, Th, U, K), relatively depleted in HFSE (Nb and Ta), and the La/Yb, Ce/Yb, Th/Yb, Nb/La, La/Sm values suggest the limited crustal contamination during the rise of the magma. The ε<sub>Nd</sub> (430 Ma) values are −6.9–+2.5 and T<sub>DM</sub> values are 3.6–1.4 Ga. The SHRIMP ages are 433±2 Ma for the Zhigoumen websterite(101-2101A), 434±3 Ma for Shiguanzi hornblendite(101-2104A) and 412±3 Ma for the Xianggoumen serpentinite(101-2107A). In situ zircon O-Hf isotope, the δ<sup>18</sup>O compositions of vary from +9.03 to +9.50 (except three points +11.33, +12.38, +12.44) and ε<sub>Hf</sub>(t) value is +0.29 to +4.13 for the Zhigoumen pyroxenite(101-2101A), the δ<sup>18</sup>O compositions of vary from +6.39 to +7.12 and ε<sub>Hf</sub>(t) value is +7.76 to +13.26 for Shiguanzi gabbro(101-2104A), and the δ<sup>18</sup>O compositions of vary from +4.68 to +5.31 and ε<sub>Hf</sub>(t) value of +0.28 to +2.79 for the Xianggoumen serpentinite(101-2107A). According to the above datum, we suggest that middle Paleozoic magmatisms last ~20 m.y. (434-412 Ma) on the northern margin of the Qilian Block was related to the Early Paleozoic continental collision between the Qilian and Alax blocks, and to subsequent subduction and thermal underplating.</p>