Synchronous formation of the‘forearc’Bay of Islands ophiolite and its basal high-temperature metamorphic sole constrained by U-Pb zircon ages  

作　　者：Weiyao Yan John F.Casey 

机构地区：[1]Department of Earth and Atmospheric Sciences,University of Houston,Houston,TX,USA

出　　处：《Geoscience Frontiers》2023年第6期234-272,共39页地学前缘(英文版)

基　　金：Funding to J.F.Casey for mapping,sample collections,and geochemical work in the Bay of Islands region were derived from U.S.National Science Foundation grants EAR80-26445,EAR-83-09535,EAR-88-04756;a University of Houston Departmental of Earth and Atmospheric Sciences grant in 2017.

摘　　要：The welded metamorphic sole at the base of the Bay of Islands Ophiolite Complex(BOIC)in the Northern Appalachians of Newfoundland shows a typical inverted pressure-temperature(P-T)metamorphic gradient from HT-MP granulite to LT-LP greenschist facies.It incorporates mafic volcanic/plutonic protoliths mixed with pelagic,hemi-pelagic and coarser epiclastic sedimentary protoliths.New LA-ICP-MS U–Pb concordia ages,trace elements,and Ti-in-zircon geothermometry for -250 zircon analyses from three metabasites of the upper HT sole amphibolites with N-MORB-like protoliths are reported.Two samples collected within meters of the ophiolite peridotite-sole contact of the Blow Me Down Mountain and North Arm Mountain massifs yielded the oldest comparable concordia ages of 487.7±2.6 Ma and 489.1±3.1 Ma,respectively,that are both within error of the igneous age of 488.3±1.5 Ma of the directly overlying BOIC ophiolite,which formed at a supra-subduction zone(SSZ)forearc spreading center.A third slightly younger age of 484.2±2.4 Ma was obtained for an upper HT amphibolite sample with similar phase assemblages but collected30 m below the peridotite contact of the Table Mountain massif.Zircon crystals analyzed have similar size and morphologies,subparallel rare earth element(REE)variation patterns,and steep heavy REE-enrichments((Lu/Gd)_(cn)>20),significant positive Ce anomalies(dominantly>5)and slight positive to dominantly negative Eu anomalies(1.2–0.4).Zircon shows Th/U mean values of 0.37–0.48 with little to no rim to core variation.Minimum Ti-in-zircon mean crystallization temperatures range from764–787℃.These neocrystallized zircon crystals appear to be derived from thin leucosomes within the three amphibolites.Two other samples also from the upper HT sole show evidence of inherited detrital zircon with core dates spanning the Cambrian Notre Dame Arc through older Laurentian-like basement and rift age ranges.Subcretion of the sole took place below a hot forearc asthenospheric wedge,that is,a consequence of the newl

关 键 词：Ophiolite metamorphic sole U-Pb zircon ages Hot subduction interface Supra-subduction forearc spreading center Arc-continent collision Taconic Orogeny 

分 类 号：P587[天文地球—岩石学] P597.3[天文地球—地质学]