Detrital Zircon Provenance Of The Silurian And Devonian In South Qingling, And The Northwestern Margin Of Yangtze Terrane And Its Tectonic Implications

The Qinling orogen, dividing the North China block and the South China block, is thought to have evolved by the multistage rifting and collision over a prolonged history, with two mountain chains and two sutures. Between the Middle Paleozoic Shangdan and Triassic Mianlue sutures is the South Qinling orogen, which is predominated by Paleozoic metasedimentary and metavolcanic rocks and Triassic granitoids. South to the Mianlue suture is the South China block, which is actually a composite terrane formed by the assembly of the Yangtze and Cathaysian blocks through the Jiangnan orogen around 820 Ma. Middle Paleozoic rifting and then seafloor spreading led to breakaway of the South Qinling belt from the South China block. Synchronously, separation of East Asian crustal fragments from Gondwanaland and openning the Palaeo-Tethys. Accordingly, it can be concluded that the Middle Paleozoic is a very important period in the evolution of South Qinling belt, the South China block, as well as Gondwana. This dissertation presents a study of detrital zircon U-Pb geochronology and Hf isotope geochemistry of quartz arenites of the Lower Devonian in northwestern margin of Yangtze terrane and Middle Paleozoic sandstones in South Qinling, putting some constraints on their provenance and exploring the tectonic and paleogeograhic relationship between the South Qinling, South China block and Gondwanaland.U-Pb detrital zircon geochronology from Lower Devonian quartz arenites of the northwestern margin of the Yangtze block yields dominant early Neoproterozoic (0.85-1.0 Ga), Pan-African (0.5-0.65 Ga) and middle Neoproterozoic (0.68-0.8 Ga) age populations and minor Mesoproterozoic to middle Mesoarchean (1.0-3.0 Ga) ages. Middle Mesoarchean to Mesoproterozoic rocks, however, are widespread in the South China block. Although Hf isotopic compositions show both juvenile crustal growth and crustal reworking for all the age groupings, the crust growth, essentially mantle-derived, occurred mainly around 3.1 Ga,1.9 Ga and 1.0 Ga, respectively. Zircon typology and youngest grain ages indicate that this suite of quartz arenites was the product of multiphase reworking. Abundant magmatic zircon detritus with concordant U-Pb Grenvillian and Pan-African ages, together with accompanying variousεHf(t) values, indicate an exotic provenance for the quartz arenite external to the South China block. Qualitative comparisons of age spectra for the late Neoproterozoic sediments of the Cathaysian Block, early Paleozoic sediments of pre-rift Tethyan Himalaya sequence in North India and lower Paleozoic sandstone from the Perth Basin in West Australia, show that they all have two the largest age clusters representing Grenvillian and Pan-African orogenic episodes. The resemblance of these age spectra and zircon typology suggest that the most likely source for the Lower Devonian quartz arenites of the South China block was the East African Orogen and Kuunga Orogen for their early Grenvillian and Pan-African populations, whereas the Hannan-Panxi arc, Jiangnan orogen, and the Yangtze block basements might have contributed to the detrital zircon grains of the Neoproterozoic and Pre-Grenvillian ages. Hf isotopic data indicate that the crustal evolution of the drainage area matches well with the episodic crust generation of Gondwana [Kemp et al.,2006]. These results imply that the previously suggested position of the SCB in Gondwana should be re-valuated, and the South China block should be linked with North India and West Australia as a part of East Gondwana during the assembly of Gondwana, rather than a discrete continent block in the paleo-Pacific.Collectively, the dominant detrital zircon age populations within Siluro-Devonian sandstones in South Qinling are Caledonian age (ca.400-500 Ma), ca.750-900 Ma and ca. 900-1000 Ma, with peak value of 450 Ma,820 Ma and 970 Ma, respectively. MostεHf(t)of detrital zircons from Silurian lie below the line of chondritic uniform reservoir (CHUR), indicating they are the products of reworking preexistent crust. The Hf isotopic character of Devonian resemble Silurian's besides the presence of positiveεHf(t) value in the majority of Devonian detrial zircons with Caledonian age. Most zircons with Caledonian and 750-900 Ma age in Siluro-Devonian are angular or sub-angular, showing the provenance was near. After precluding the possibility of North Qinling behaved as the provenance,the Paleozoic orogenic belt and Jiangnan orogen are inferred as the provenance of providing the detrital zircons with Caledonian and 750-900 Ma age. The presence of Silurian fossils in Devonian strata and positiveεHf(t) value of Caledonian zircons reveal the Devonian's source is the combination of underlying Silurian and igneous rocks formed in the process of rifting between South Qinling and SCB. Of importance, these facts solve the question Where and When South Qinling rifted from SCB, indicating the South Qinling are close to Cathaysian block before splitting.