The Tectonic Affinity Of The Zongnaishan-shalazhashan Zone In Northern Alxa And Its Implications

The Central Asian Orogenic Belt (CAOB) is one of the largest Phanerozoic accretionary orogen in the world, and it is situated between the Siberian Craton to the north and the Sino-Korean-Tarim cratons to the south. The formation of CAOB is related to the evolution of the Paleo-Asian Ocean and its continental margins. Very large volumes of granitic rocks and volcanic rocks with positived(t) values and sm(t) values were emplaced during the accretionary process of the Central Asian Orogenic Belt. However, the architecture, timing and boundary of the accretionary orogenesis are debated due to difficulties in identifying the nature of this huge and complex accretionary belt. The Alxa region is located in the boundary area between the central segment of the southern CAOB and the Alxa Block and connects the Beishan-Tianshan orogen to the west and the northern margin orogen of the North China Craton to the east. This thesis presents new geochronological, petrological, geochemical and zircon Hf isotopic data of the Precambrian rocks and Late Paleozoic intrusions from the Zongnaishan-Shalazhashan in northern Alxa and discusses the tectonic evolution and boundary between the CAOB and Alxa-North China Craton.Using zircon U-Pb dating, the Late Paleozoic intrusions in Zongnaishan-Shalazhashan zone can be broadly grouped as three episodes:Late Carboniferous (-301Ma, the Wenduermaodao granodiorites), Middle Permian (272-264Ma, Tamusu K-feldspar granites, Hurentaolegai K-feldspar granites and Baogeqi gabbros), and Late Permian-Early Triassic (257-247Ma, Shalataoerhan-Wuliji granite batholith, Sharijimiao quartz monzodiorites and Tamusu diorites). Moreover, the Late Permian-Early Triassic granitoids are identified as the most intense magmatism event in the zone, and make up the main intrusive body of Zongnaishan-Shalazhashan Mountains. The remarkably high zircon Hf isotopes (sHf(t)=+6.6-+10.1) and characteristics of high silica adakites of the Late Carboniferous granodiorites suggest that they were mainly derived from "hot" basaltic slab-melts of the subducted oceanic crust. The Middle Permian gabbros exhibited typical cumulate textures and were derived from the partial melting of depleted mantle. The Middle Permian K-feldspar granites are slightly peraluminous with high-K calc-alkaline and low emit) values from-0.9to+5.3, exhibiting typical features of highly differentiated I-type granites. These granites were most likely derived from juvenile materials mixed with old crustal materials. The Late Permian granodiorites, monzogranites and quartz monzodiorites are characterized as metaluminous to slightly peraluminous and calc-alkaline-high-K calc-alkaline. TheseⅠ-type granitoids were mainly derived from juvenile crustal materials, as evidenced by their high εHf(t) values (+3.3to+8.9). The Hawula complex which located in the southwest verge of the Zongnaishan-Shalazhashan zone are composed of metamorphosed supracrustal rocks and meta-intrusive rocks. Most zircon grains are euhedral and show oscillatory zoning, suggesting magmatic origins. The crystallization ages of1459±11Ma and1458±3Ma were obtained for the biotite plagioclase gneiss and the augen biotite-monzonitic gneisses in Aertengtuoya area respectively, which is interpreted as the best estimate of the crystallization age of their igneous protolith. U-Pb dating of the magma zircons from the orthogneiss (amphibole monzonitic gneisses in the Gashunhuduge area) yield an age of1433±17Ma. Thus, the Zongnaishan-Shalazhashan zone was a terrene with Mesoproterozoic metamorphic basement of1.4-1.5Ga.The Zongnaishan-Shalazhashan zone has a Mesoproterozoic metamorphic basement of1.4～1.5Ga, wihich is similar to the Mesoproterozoic micro-continents in the CAOB, but different from the Alxa Block which existed Late Neoarchean-Paleoproterozoic rocks. The Late Paleozoic granitoids in the Zongnaishan-Shalazhashan zone have positive εHf(t) values and young Hf model ages, indicating a juvenile nature of the basement of the terrane. Especially for the largest batholiths here, the Shalataoerhan-Wuliji granite batholith has positive εHf(t) values of+3.3to+11.6and young Hf model ages of1063to533Ma, indicating that the magma is mainly derived from juvenile crustal materials. Importantly, these characteristics are similar to those of the granitoids in the CAOB, but contrasted with those of the coeval magmatic rocks (εHf(t)=-20～-4, TDMC=2200～1500Ma;εNd(t)=-16.4～-8.7, TDMC=2399～1751Ma) in the Nuoergong-Honggueryulin Zone of the Alxa Block. The juvenile nature of the Zongnaishan-Shalazhashan Zone or terrane is consistent with the most prominent features of the CAOB but different from the NCC and the Alxa Block. These findings suggest that the Zongnaishan-Shalazhashan terrane belongs to the CAOB rather than the Alxa Block and that its boundary with the Alxa block can be regarded as the southernmost boundary of the CAOB. These studies, by interaction of regional geology, provide new constraints on the tectonic evolution of the southern CAOB during the Late Paleozoic and the location of the southernmost margin of the CAOB.