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Zircon geochronology reveals polyphase magmatism and crustal anatexis in the Buchan Block,NE Scotland:Implications for the Grampian Orogeny

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Geoscience Frontiers 2017年第6期8卷 1469-1478页

作者： T.E.Johnson C.L.Kirkland d.r.viete S.Fischer S.M.reddy N.J.Evans B.J.Mcdonald department of Applied Geology The Institute for Geoscience Research(TIGeR)CET-Curtin NodeJohn de Laeter CentreCurtin UniversityPerthWA 6102Australia Morton K.Blaustein department of Earth&Planetary Sciences Johns Hopkins UniversityOlin Hall3400 N Charles StreetBaltimoreMD 21218USA School of Earth&Environmental Science University of St AndrewsIrvine BuildingNorth StreetSt AndrewsKY16 9ALScotlandUK

The type locality for high-temperature,low-pressure regional metamorphism,the Buchan Block in NE Scotland,exhibits profound differences to the rest of the Grampian Terrane.These differences have led some to regard the Buchan Block as an exotic crustal fragment comprising Precambrian basement gneisses and cover rocks thrust into their current position during Grampian orogenesis.Although rocks of the Buchan Block are now generally correlated with dalradian strata elsewhere,the origin of the gneisses and the cause of the high heat flow and associated magmatism is debated.We report SIMS U-Pb and LA-ICPMS Hf isotopic data in zircon from high-grade rocks from the northeast(Inzie Head Gneiss)and northwest(Portsoy)corners of the Buchan Block.Around Inzie Head,upper amphibolite to granulite facies metasedimentary gneisses coexist with diorite sheets that were emplaced contemporaneously with partial melting of their host rocks,at least locally.U-Pb geochronology indicates a crystallisation age for the diorite of 486±9 Ma.Highly-deformed diorites within the Portsoy Gabbro have a crystallisation age of 493±8 Ma.Ages of ca.490 Ma for magmatism and high-grade metamorphism,which are broadly contemporaneous with ophiolite obduction and the onset of orogenesis,are significantly older than the established peak of Grampian metamorphism(ca.470 Ma).We propose a new model for the Grampian Orogeny involving punctuated tectonothermal activity due to tectonic switching during accretionary orogenesis.rollback of a NW-dipping subduction zone at ca.490 Ma produced a back-arc environment(the Buchan Block)with associated arc magmatism and high dT/dP metamorphism.Arrival of an outboard arc resulted in shortening(the initial phase of the Grampian Orogeny)at ca.488 Ma.rollback of a NW-dipping subduction zone to the SE of the ca.488 Ma suture began at 473 Ma and led to lithospheric-scale extension,decompression melting and advective heating of the middle crust,producing the widespread ca.470 Ma Grampian(classic Barrovian and Buchan)regional metam

关键词： dalradian Grampian Orogeny Buchan Block Zircon geochronology Magmatism Metamorphism

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Mesozoic-Cenozoic extension of the Bohai Sea： contribution to the destruction of North China Craton

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Frontiers of Earth Science 2014年第2期8卷 202-215页

作者： Guiting HOU K. r. HArI The Key Laboratory of Orogenic Belts and Crustal Evolution School of Earth and Space Sciences Peking University Beijing 100871 China School of Studies in Geology and Water resource Management Pt. Ravishankar Shukla University Raipur 492010 India

The Bohai Sea is a Late Mesozoic-Cenozoic feature of the basin-mountain system located in eastern North China Craton （NCC）. The Late Mesozoic thinning of the lithosphere signals the early destruction of the NCC. The onset of the destruction was due to the delamination of thick lithosphere of the craton, represented by the NW- trending grabens in an en-echelon arrangement west to Tanlu Fault, and by the NNE-trending grabens within the Tanlu Fault Zone. The Late Mesozoic NW-trending grabens are overprinted by structures related to the Cenozoic NE-trending pull-apart basin with very thick Mesozoic-Cenozoic sediments in the eastern NCC. C- frequency diagrams of growth faults and the extension factor （fl） of four sections across the basin suggest that the extension migrated from the margin to the center of Bohai Sea, and that the Mesozoic and Cenozoic extension factors for Bohai Sea are higher than that of the margin. These evidences suggest that the greatest extension occurred in the center of Bohai Sea, which is consistent with the thinnest crust being found in the center of the sea. The extension ratios and tectonic evolution of the Bohai Sea suggest that it is the key region for the destruction of the NCC, as evidenced by the topography. However, the NCC experienced two stages of destruction with the late stage related to the tectonic regime of Northeast Asia.

关键词： Bohai Sea extension pull-apart lithosphericthinning destruction of craton

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