Detrital Zircon Geochronology Of The Upper Proterozoic-Mesozoic Clastic Rocks In The Mid-Upper Yangtze Region

Our knowledge on the evolution of the continental crust relies heavily on the understandingof the formation age of various types of rocks that are exposed to the surface of the Earth. That,however, is frequently hampered by a variety of geological factors. Detrital zircons preserved inclastic rocks are a mixture of zircons derived from various source rocks, and thus provide us withan ideal way to study the average age composition of the continental crust, to understand thegrowth and modification of continents and to reconstruct paleogeography of the past Earth. Inthis paper we present results of petrographic, geochemical and LA-ICPMS detrital zircongeochronological study on representative profiles of the Nanhua, Silurian, Devonian, Triassic andJurassic systems in the Mid-Yangtze and sedimentary provenance analysis in an attempt to probeinto the age composition of the Yangtze basement and paleogeography during periods of tectonicregime transition.Detrital zircons from the Yangtze sediments are largely of magmatic origin, as indicated bytypical oscillatory zoning, depletion of light rare earth elements (REEs) and enrichment of heavyREEs and significant negative Eu anomaly and positive Ce anomaly, with Th/U ratios mostly＞0.3. The uniform characters argue against trace element as a viable sedimentary provenanceindicator. Nevertheless, light REEs and total REE contents of these zircons average at 70 ppmand 723 ppm, respectively, and are surprisingly lower than those of zircons from granitoids (207ppm and 1800 ppm, respectively) reported in literature. If the REE composition of zircons fromgranitoids is universal, this difference requires that LREE/HREE fractionation have happenedduring deposition and diagenesis as granitoids are the major source of detrital zircons. However,this interpretation is not viable because zircons are highly stable mineral that are unlikely toundergo REE fractionation during sedimentary processes. Therefore, the previously reportedchemical characteristics of zircons from granitoids may not be representative, whereas detritalzircon REE composition presented in this work is a more reasonable estimation for crustal zirconcomposition.Proterozoic successions from different localities display distinct zircon age patterns. Thosefrom W Hubei-SE Hubei regions consistently show prominent peaks at～2500 Ma,～2000Ma and～800 Ma, this age pattern is interpreted to represent the shallow basement of theMid-Yangtze region in the context of paleogeographical background and geochronological dataof inherited zircon from the granitoids that formed in the Yangtze craton. Therefore, the YangtzeBlock is composed of 16％Neoproterozoic (～800 Ma) rocks, 23％Paleoproterozoic (～2000Ma)rocks and 61％Neoarchean (2500～2400 Ma) rocks. In contrast, Proterozoic successionsfrom the northern and western margin of the Yangtze Block are characterized by much youngerage populations, such as the 1600～900 Ma one, in particular the universal presence of 1100～900 Ma zircons. They also lack～2000 Ma and～2500 Ma age populations. These age patternsare consistent with the wide occurrence of coeval magmatic rocks and support the existence ofextensive oceanic subduction and arc-magmatism all around the Yangtze Block during the Late Mesoproterozoic-Early Neoproterozoic.The Upper Triassic sandstones from the Pingle and Jianghan basins show unimodal agepatterns with prominent peak at 1900～1800 Ma, which match that of the Cathaysia Block, thenorthern portion of the Cathaysia, and are distinct from that of the Yangtze Block. The NorthChina Craton is also not a viable source as it contains abundant Archean basement rocks.Derivation from the Cathaysia Block is most favorable, and is supported by paleocurrent studies.Besides the presence of～2500 Ma and Paleozoic age population, the Late Triassic-EarlyJurassic sandstones from the Sichuan Basin also contain abundant 1900～1800 Ma zircons,which are not available from either the interior of Yangtze Block or the Qinling-Dabie Orogen,and are close to those of the Triassic Pingle and Jianghan sediments, point to partial derivationfrom Cathaysia. In the Triassic flysch from the Songpan-Ganzi basin, West China, a prominent1900～1800 Ma age peak is also present, suggest that the sediments might also have beensourced by the Cathaysia Block. This requires a westward draining trans-continental fluvialsystem, which is more than 1000 km long. The abrupt occurrence of these Cathaysian zircons inthe interior, marginal and even the outside of the Yangtze Block imply that the Cathaysia blockhave been rapidly uplifted, probably as a result of the oblique collision starting from the east.Detritus shed from the Cathaysia Block was transported along the continental margin of theSouth China Block across the Jianghan and Sichuan basins and finally reached theSongpan-Ganzi remnant basin.In combination with published geochronological data in South China, detrital zircongeochronology reveals the universal presence of Paleoproterozoic events in both the Yangtze andthe Cathaysia blocks that are correlated with different tectonic setting. Paleoproterozoic events inthe Yangtze block are characterized by a 2100～2000Ma magmatic event and a 2000～1900Mametamorphic event, with the former probably associated with ocean-continent subduction and thelatter with subsequent arc-continent collision and orogeny in the northern margin of the Yangtzeblock. The Cathaysia block has underwent intensive magmatism during the period of 1900～1800Ma, which might have led to the formation of the oldest continental crust. Although both theYangtze and the Cathaysia blocks are likely part of the Columbia supercontinent, they havedifferent evolutionary history.