High-precision Zircon U-Pb Geochronological Studies Of The Permian Ash Beds From China And North America

The Permian Period is a critical interval during the geological history,which was characterized by a series of far-reaching global changes in paleogeography,paleoclimate and biodiversity.A high-resolution geochronologic framework is essential to assess timings and rates of these changes to evaluate their temporal relationships.Existing high-precision radioisotopic dates have been insufficient to establish a high-resolution temporal framework for the marine Guadalupian,the terrestrial Permian sequence in North China and the transitional sequence around the Permian-Triassic boundary in southwestern China.The Permian sections in South China,North China and West Texas,USA were sampled for volcanic ashes to conduct geochronological study by the zircon U-Pb chemical abrasion-isotope dilution-thermal ionization mass spectrometry(CA-ID-TIMS)technique.More than 20 high-precision zircon U-Pb dates were obtained in this thesis,and the following important progress has been achieved.(1)The zircon U-Pb dates has provided the most reliable and high-precision age constraints for the Guadalupian stages,constraining the base-Guadalupian(base-Roadian)at 272.95 ± 0.11 Ma,base-Wordian at 266.9 ± 0.4 Ma and base-Capitanian at 264.28±0.16 Ma.The Illawarra Reversal in West Texas was estimated at 267.4 ± 0.4 Ma to 266.5 ± 0.3 Ma.The pre-Lopingian biocrisis may have taken place during the last ca.1 myr in the Capitanian,coincident with the Emeishan Large Igneous Province(LIP).The emplacement of the Emeishan LIP in South China may have caused environmental pressure that led to a rapid evolutionary rate of the conodonts in the late Capitanian and hampered ecosystem restoration in the early Wuchiapingian.Besides,these dates have provided important constraints on the Guadalupian correlation between South China and North America.(2)The zircon U-Pb dates has provided age constraints on the Permian floral turnovers,paleoclimate changes and tectonic activities of the North China Block.The lower part of the Taiyuan Formation was assigned to the latest Carboniferous,the upper part of the Taiyuan Formation,Shansi Formation and the Lower Shihhotse Formation to the Asselian,the Upper Shihhotse Formation to the Sakmarian to early Kungurian,the Sunjiagou Formation to the late Capitanian to Lopingian based on these dates.The results for the first time reveal a ca.20 myr hiatus ranging from early Kungurian to latest Guadalupian in the topmost part of the Upper Shihhotse Formation possibly derived from a strong tectonic uplift during the collision between the North China Block and the Mongolian Terrane which resulted in the closure of the Paleo-Asian Ocean.The new temporal framework indicates a distinct paleoclimate shift from early Cisuralian to Guadalupian which led to a great loss of the highly diverse and abundant Cathaysian floras of the humid condition and then the widespread invasion of the monotonous Angaran floras under an arid condition into North China.The similar floral and climate shift history between the Euramerican continent and the North China Block suggests that the frequent extensive volcanisms during the Cisuralian may have been the major trigger of global warming and aridification.(3)The Permian-Triassic mass extinction is not a globally synchronous extinction event.The high-precision zircon U-Pb dates suggest that the mass extinction at the terrestrial and transitional sections in southwestern China took place at the earliest Triassict(251.776±0.055 Ma),coincident with the oneset of intrusive activity of the Siberian LIP.The intrusive magmatism into the host sedimentary rocks with abundant organic materials liberated large amounts of greenhouse gases,which intensified the global warming and aridity and may have finally triggered the extinction of the Cathaysia flora in South China.The terrestrial mass ectinction in southwestern China postdated the main marine extinction in South China for ca.165 kyr,while that in northern high latitudes predated the main marine extinction for ca.370 kyr.The delayed ecosystem collapse in the low paleo-latitudes during the environmental deterioration may have been attributed to their higher biodiversity and more complex ecosystem structures.