Compositions, Characteristics, Chronological Framework And Origin Of Early-Precambrian Metamorphic Basement In Western Alxa Block

The westernmost North China Craton (NCC), termed the Alxa block, is bounded by the North-Qilian Orogen to the south, the Tarim Block to the northwest, and the Central Asian Orogenic Belt to the north. The Alxa block is largely covered by deserts, with Precambrian basement rocks being sporadically exposed, and it remains the least studied area in China. The Alxa Block was traditionally considered as part of the NCC. However, the geological evolutionary history of the NCC remains controversy. Some workers proposed that the NCC was amalgamated by a number of micro-continental blocks and the cratonization was completed at ca.2.5Ga, while the other workers proposed that the NCC was formed by accretion and amalgamation of the western block and eastern block at ca.1.85Ga. To understand the origin and affinity of the Alxa block, consequently, it is of great importance to compare contemporary stratigraphy, tectonics, magmatism and metamorphism of the Alxa block with those of the NCC. Based on these, we can further understand the role of the Alxa block in the early Precambrian evolution of NCC and provide the evidences for the accretion and amalgamation history of the entire NCC. In the present thesis, we present field geology, petrology, geochemistry, U-Pb zircon dating and Hf isotope data on the metamorphic basement rocks of the Longshoushan complex and Beidashan complex in the western Alxa block, aiming to explore the origination and early Precambrian evolution history of the Alxa block.The NW-SE trending Longshoushan complex, which locate in the southwest verge of the Alxa block, are composed of para/orthogneisses, migmatites, schists, leptynites, amphibolites and marbles. All these metamorphic basement rocks experienced amphibolite-facies metamorphism. Based on the zircon textures shown by CL images, U-Pb dating reveals that three Paleoproterozoic magmatic episodes (～2.04,～2.17and～2.33Ga) and two subsequent regional metamorphism events (～1.85and1.90～1.95Ga) for metamorphic plutonic rocks and ampibolite in Longshoushan complex.～1.85Ga amphibolite-facies metamorphism represents the formation time of metamorphic basement of the Alxa block. U-Pb dating of the detrital zircons from metasedimentary rocks in Longshoushan complex yields207Pb/206Pb ages between2.0Ga and2.2Ga, and a few metamorphic zircon rims give～1.95Ga metamorphic age, suggesting a sedimentary time between1.95～2.0Ga, and the sedimentary provenance most probably derived from the plutonic rocks in Longshoushan complex. In situ zircon Lu-Hf isotopic analyses show that most magmatic zircons with ages of～2.0Ga and-2.17Ga yield positive εHf(t) values (Avg.2.52) with two-stage Hf model ages (TDMC) between2.45～2.65Ga (with a peak at～2.5Ga), indicating that these Paleoproterozoic plutonic rocks were derived from the reworking of～2.5Ga juvenile crust. Zircons from the plutonic rocks and metasedimentary rocks yielded similar εHf(t) and TDMC values, further indicating the sedimentary provenance most probably derived from the plutonic rocks in Longshoushan complex.The Beidashan complex, which lie to the north of Longshoushan, are mainly composed of biotite-plagioclase gneiss, amphibole-plagioclase gneiss, granodioritic gneiss, amphibolites, marbles and minor micaschists, leptynites, quartzite. Mineralogical features show these metamorphic rocks underwent amphibolite-facies metamorphism. Peak metamorphic assemblage (garnet+amphibolite+plagioclase+quartz) in a Garnet-bearing amphibolite in the Beidashan complex gives a metamorphic condition of0.8-0.9Gpa and690～721℃. Six orthogneisses collected from the Beidashan complex contain high SiO2(64.35～70.15wt.%), high Na2O (4.35～5.22wt.%), high Mg#(34.8～51.5), low K2O (1.15～3.53wt.%) and K2O/Na2O ratios (0.22～0.81), with high Sr (591～969ppm) content and Sr/Y rations (52～134), low Y (6.11～13.9ppm), low Yb (0.5～1.61ppm), low Nb (0.41～7.57ppm) content, showing a similar geochemical character with typical Neoarchean TTG. LA-MC-ICPMS and SHRIMP zircon U-Pb dating yielded the emplacement ages of the Beidashan TTG between2.50～2.55Ga and subsequent high-grade metamorphic event between2.47～2.53Ga. The short time interval～(10-50Ma) between initial magmatism and high-grade regional metamorphism signifies that they were related to the same tectonothermal event, similar to the～2.5Ga TTG magmatism and subsequent high-grade regional metamorphism reported in western and eastern block of NCC. The Beidashan TTG also recorded another～1.85Ga metamorphism, coincided with the amphibolite-facies metamorphism in Longshoushan complex, indicating that the～1.85Ga metamorphism was widely distributed in western Alxa block and it might represent the time of the cratonization of entire western Alxa block. Hf isotopic analyses revealed that the～2.5Ga zircons from TTG gneisses contain εHf(t) values mainly between0.8and5.0, with TDMC mainly between2.7～3.0Ga. A few～2.5Ga zircons have values of εHf(t) close to the depleted mantle evolutionary line. These data indicate a main Mesoarchean-Neoarchean (2.7-3.0Ga) crust growth and a minor～2.5Ga juvenile crust formation in western Alxa block.In combination with early Precambrian magmatic-metamorphic events in the Longshoushan and Beidashan complex, the western Alxa block show very similar sequence of events to those of the other part of the NCC, and they both experienced main2.7～3.0Ga crust growth,～2.5Ga TTG magmatic-metamorphic event,2.0～2.3Ga magmatic events,1.9～1.95Ga and～1.85Ga high-grade metamorphic events. The western Alxa block is not a separated Paleoproterozoic terrane from the NCC but an integrated part of the NCC from Neoarchean to Paleoproterozoic, and it is most possibly constitute the western extension of the Paleoproterozoic Khondalite Belt, rather than the Yinshan Block. We tentatively propose the Khondalite Belt would extend westward from the Qianlishan-Helanshan, through the Bayanwulashan in the eastern Alxa block, to the Beidashan and Longshoushan in the western Alxa block. This model also implies that the Yinshan block is much smaller than that previously recognized. Moreover, The Dunhuang block, where the～2.5Ga TTG magmatic-metaorphism and～1.85Ga HP granulite-facies metamorphism had been recently identified, show similar early-Precambrian magmatic-metamorphic events with the Alxa block, indicating the Dunhuang block and the western Alxa block might exist as a single block during the Neoarchean to Paleoproterozoic and they were dissected later by the Altyn Tagh fault. The Paleoproterozoic Khondalite Belt possibly extends through the Altyn Tagh fault to the Dunhuang block.