Tectonothermal evolution of contrasting metamorphic complexes in northwest Spitsbergen (Biskayerhalvoya): Evidence from 40Ar/39Ar and Rb-Sr mineral ages

摘要

Contrasting metamorphic sequences are exposed within a fault-bounded horst on Biskayerhalvøya, northwestern Spitsbergen. The Biskayerfonna Group is composed of variably pelitic metasedimentary rocks together with interlayered amphibolite. Amphibole within this sequence records 40Ar/39Ar isotope correlation ages of 429 and 437 m.y., which are interpreted to date cooling through appropriate argon closure temperatures following a late Caledonian amphibolite-facies metamorphism. Muscovite and biotite record slightly younger 4OAr/39Ar and Rb-Sr ages (400-430 m.y.), which are interpreted to date cooling through lower closure temperatures. The Biskayerfonna Group is tectonically imbricated with an older metamorphic complex (Richarddalen Group) composed of variably retrogressed eclogite, metagranite, foliated amphibolite, and various metasedimentary rocks. Amphibole within massive retrogressive assemblages derived from eclogite and wihin foliated amphibolite records 4OAr/39Ar isotope correlation ages between 500 and 542 m.y. These are interpreted to date cooling through appropriate closure temperatures following the initial high-pressure metamorphism. Rb-Sr ages recorded by muscovite and most biotite within the Richarddalen Group are markedly younger, ranging between 410 and 430 m.y. These ages likely record cooling following a late Caledonian metamorphic overprint that occurred at approximately the same time as initial prograde metamorphism of the Biskayerfonna Group; however, maximum overprint temperatures in the Richarddalen Group must have been less than prograde conditions in the Biskayerfonna Group because amphibole in the former shows no evidence of any isotopic rejuvenation. These relationships suggest that structural imbrication of the contrasting metamorphic sequences must have occurred during or immediately following late Caledonian metamorphism at ca. 429-437 Ma but prior to regional cooling between ca. 402 and 430 Ma. Phengitic muscovite within the Richard-dalen Group records 4OAr/39Ar plateau ages between 440 and 480 m.y. and displays no spectral evidence of subsequent rejuvenation. This suggests that the late Caledonian metamorphism must have been of variable intensity within the Richarddalen Group.