摘要
New geochronologic data from midcontinental Laurentia demonstrate that emplacement of the 1476-1470 Ma Wolf River granitic batholith was not an isolated igneous event,but was accompanied by regional metamorphism,deformation,and sedimentation.Evidence for such metamorphism and deformation is best seen in siliciclastic sedimentary rocks of the Baraboo Interval,which were deposited closely following the 1.65-1.63 Ga Mazatzal orogeny.In Baraboo Interval strata,muscovite parallel to slatey cleavage,in hydrothermal veins,in quartzite breccia,and in metamorphosed paleosol yielded ^(40)Ar/^(39)Ar plateau ages of 1493-1465 Ma.In addition,U-Th-total Pb dating of neoblastic overgrowths on detrital monazite gave an age of 1488±20 Ma,and recrystallized hematite in folded metapelite gave a mean U/Th-He age of 1411±39 Ma.Post-Baraboo,arkosic polymictic conglomerate,which contains detrital zircon with a minimum peak age of 1493 Ma,was intruded by a 1470 Ma granite porphyry at the northeastern margin of the Wolf River batholith.This episode of magmatism,regional deformation and metamorphism,and sedimentation,which is designated herein as the Baraboo orogeny,provides a midcontinental link between the Picuris orogeny to the southwest and the Pinware orogeny to the northeast,completing the extent of early Mesoproterozoic(Calymmian)orogenesis for 5000 km along the southern margin of Laurentia.This transcontinental orogen is unique among Precambrian orogenies for its great width(-1600 km),the predominance of ferroan granites derived from partial melting of lower continental crust,and the prevalence of regional high T-P metamorphism related to advective heating by granitic magmas emplaced in the middle to upper crust.
基金
Detrital zircon analyses were funded in part by the USGS National Cooperative Geologic Mapping Program under award G16AC00143(2016)to EKS.Support for the Arizona LaserChron Center,where detrital zircon analyses of samples 07ES15 and 05ES15 were performed,was provided by NSFEAR 1649254.Support for monazite geochronology was partially provided by an Institute for Lake Superior Geology student research grant and NSF-EAR 0620101 to AVL.
