Petrogenesis and structure of the Buck Creek mafic-ultramafic suite,southern Appalachians:Constraints on ophiolite evolution and emplacement in collisional orogens

摘要

Integration of detailed field,penological,structural,and geochemical observations of the Buck Creek mafic-ultramafic suite,which is among the larger southern Appalachian ultramafic exposures,points to an igneous origin as a mid-ocean-ridge cumulate massif,with subsequent emplacement in a deep subduction-zone setting.These results help to clarify the unresolved role of"alpine-type"ultramafic bodies in the southern Appalachians,a problem complicated by their generally small and fragmented nature and by significant tectonic overprinting.As an ophiolite fragment,the Buck Creek rocks are unusual in their preservation of relatively high-pressure,anhydrous conditions,and thus they provide constraints on mechanisms of ophiolite emplacement.Field support for a cumulate origin for the Buck Creek suite includes centimeter-to meter-scale interlayering of dunite,troctolite,anorthosite,and spinel-rich layers,and local gradation between Buck Creek troctolitic rocks and surrounding amphibolites.Relict anorthitic plagioclase,forsteritic olivine,and augitic clinopyroxene define a"cumulate triangle"on an MgO versus Al2O3 diagram:meta-troctolite and meta-dunite compositions define a linear array between olivine and plagioclase compositions,and many amphibolites fall within this triangle,reflecting gabbroic protoliths.The relatively abundant troctolite cumulates,scarce pyroxene in the ultramafic rocks,and the high Al2O3/TiO2 and low Cr numbers in spinel best match LOT-or L-type(lherzolite)ophiolites,consistent with crystallization in a slow-spreading mid-ocean-ridge setting.Sapphirine-bearing,spinel symplec-tites in the meta-troctolites suggest that Buck Creek rocks remained anhydrous to-800 °C and-0.9-1.1 GPa,representing conditions atypical of ophiolite emplacement Comparisons to the Zermatt-Saas ophiolite and Bergen eclogite suggest rapid subduction of Buck Creek rocks to depths of-30 km,where partial hydration,perhaps facilitated by pro-grade dehydration reactions in surrounding rocks,caused strain softening that permitted ductile disaggregation and aided in their emplacement into the overlying accretionary complex.Alternatively,emplacement might have resulted from a switch in subduction polarity,with hydration following emplacement.Uplift,shortening,and hydration within the accretionary sequences are responsible for the dominant structural features.