Discovery of rhyolitic tuffaceous slate in the southwestern margin of Yangtze Craton:Zircon U-Pb ages(2491 Ma)and tectonic-thermal events

The Mesoproterozoic Dongchuan Group that is widely exposed in Yimen area,central Yunnan Province is a series of sedimentary sort of low-grade metamorphic rocks interbedded with volcanic rocks,which are closely related to the early tectonic evolution of the Earth.However,its formation era,sedimentary filling sequence,and geotectonic characteristics have always been in dispute.In this study,several rhyolitic tuffaceous slate interlayers with a centimeter-level thickness were found in the previously determined Heishan Formation of the Dongchuan Group located to the western part of Yimen-Luoci fault zone.This paper focuses on the study of the rhyolitic tuffaceous slate in Qifulangqing Village,Tongchang Township,Yimen County.LA-ICP-MS zircon dating was conducted,achieving the crystallization age of magma of 2491±15 Ma and the metamorphic ages of about 2.3 Ga,2.0 Ga,and 1.8 Ga for the first time.Meanwhile,according to in-situ Hf isotope analysis,the zirconεHf(t)values were determined to range from−3.0 to 7.6,with an average of 2.7.Furthermore,the first-stage Hf model age(TDM1)was determined to be 2513−2916 Ma,indicating that the provenance of the rhyolitic tuffaceous slate is the depleted mantle or juvenile crust between the Middle Mesoarchean and the Late Neoarchean.Therefore,it is believed that the strata of the slate were deposited in the Late Neoarchean,instead of the Mesoproterozoic as determined by previous researchers.Accordingly,it is not appropriate to group the strata into the Mesoproterozoic Dongchuan Group.Instead,they should be classified as the Maolu Formation of the Neoarchean Puduhe Group given the lithologic association and regional information.Furthermore,the magma ages of 2491±15 Ma are highly consistent with the eras of the large-scale Late Neoarchean orogenic magmatic activities on the northern margin of the Yangtze Craton,and thus reflect the orogenic process consisting of subduction and collision from Late Neoarchean to Early Paleoproterozoic.The magmatic activities during this period were possibly caused by the convergence of the supercontinent Kenorland.Meanwhile,the metamorphic ages of 2.3 Ga,2.0 Ga,and 1.8 Ga are highly consistent with three metamorphic ages of 2.36 Ga,1.95 Ga,and 1.85 Ga of the northern margin of the Yangtze Craton,indicating that the strata experienced Paleoproterozoic tectonic-thermal events.The study area is located on the eastern margin of Qinghai-Tibet Plateau,and thus was possibly re-transformed by magmatism subjected to the subduction of the Meso-Tethys Ocean during the Early Cretaceous.The discoveries made in this study will provide strong petrological and chronological evidence for analyzing the early crustal evolution of the Yangtze block.

Protoconodont fossils for refining the Cambrian bottom and the contribution to shale gas formation along the southwest margin of Yangtze Block

It has been an intense debate on the exact boundary between Ediacaran and Cambrian in the southwest Yangtze Block.The calibration of this critical boundary has a remarkable influence on the further investigation of the break-up of the Rodinia Supercontinent,the early life evolution,and the mechanism of the phosphorite deposit.Ediacaran and Cambrian strata and fossils are widely distributed in Anning,Yunnan Province in China.In recent years,the Xiaowaitoushan Member from the Lower Yuhucun Formation has been studied.Through this interval with continuous collections,the first appearance datums(FADs)of the protoconodont(Fomitchella cf.inchoate Yang et He,Protohertzina cf.anabarica Missarzhevsky)and globular embryos fossil(Olivooides sp.)earlier than these in the Lower Cambrian strata of the Meishucun Formation were discovered.This discovery indicates that the Xiaowaitoushan Member has included more FADs than the previously discovered single FAD of Anabarites primitivus Qian et Jiang,and the Ediacaran–Cambrian boundary in southwest China should be replaced below the Point“A”of the Meishucun Formation in Yunnan Province.The Point“B”of the Meishucun Formation is younger than the suggested age 541 Ma of the Ediacaran–Cambrian boundary and can no longer reference the Global Boundary Stratotype Section and Point(GSSP)correlation in southwest China.It can be suggested based on the previous stratigraphy and palaeontology studies from northern Sichuan and southern Shaanxi and the FAD of the globular embryos fossils that the Ediacaran‒Cambrian boundary in the southwest Yangtze Block should be placed at the base of the Xiaowaitoushan Member;other phosphorite strata refer to Xiaowaitoushan Member.The discovery of the FADs of the shelly fossils in the Xiaowaitoushan Member provides new evidence for the global correlation of the Ediacaran–Cambrian boundary in the southwest Yangtze Block.The conodont discoloration index(CAI)of the specimens in Anning is between 2 and 3,which indicates that the organic matter in Xiaowaitoushan Member is matured and has high potential to form a shale gas reservoir.