Magmatic Products by Ocean Floor Spreading in MAR : Preliminary Analyses of Peridotites from IODP Exp.304/305 at Atlantis Massif, MAR 30°N

金沢大学大学院自然科学研究科4the International Symposium of the Kanazawa University 21st-Century COE Program,　Promotion Envirnmental Research in Pan-Japan Sea Area -Young Researchers\u27 Network- , DATE:March 8-10,2006, PLACE: Kanazawa Excel Hotel Tokyu, Japan, Sponsors: Japan Sea Research Institute / UNU-IAS(United Nation University Institute of Advanced Studies), Ishikawa Prefectural Government, City of Kanazaw

Structural and magmatic history of upper mantle peridotites in the Bay of Islands Complex, Newfoundland

The Bay of Islands Complex (BOIC) represents a lower Ordovician ophiolite located in western Newfoundland. In the northern part of the BOIC, a 6 km thick mantle section is exposed in the Table Mountain (TM) massif. The structural, micro-structural (including olivine lattice data), and geochemical variation within the mantle section were investigated in order to delineate the tectonic and magmatic history of the peridotites. -- The field structures indicate the presence of six structural units which are defined by distinct trends of the high temperature mineral stretching lineation of harzburgites, lherzolites, and dunites. On the basis of microstructural evidence, the formation of four of these units is assigned to spreading-related events, the remaining two to detachment tectonics. A high level and a low level zone preserving microstructural evidence for melt infiltration are present. The high level peridotites also show mineral chemical evidence for melt infiltration. -- Peridotite microstructures are classified into several 'type microstructures' interpreted to reflect differences in stress, accumulated strain, temperatures of deformation, and presence or absence of melt during deformation. The microstructural behaviour of orthopyroxene and spinel is investigated in more detail. The range of deformation mechanisms inferred from the morphology of orthopyroxene correlates largely with the temperature of deformation. In areas of lower accumulated strain, spinel grains, together with spatially associated phases, appear to preserve morphologies discriminating between different magmatic processes, i.e. spinel formed during partial melting, trapping of melt, and transient melt movement. -- The process of melt infiltration into peridotite is further investigated and two end-member processes can conceptionally be distinguished: (i) simple trapping of primitive melt; (ii) extensive flushing of melt through peridotite associated with fractionation of melt and reaction with the host. Mineralogical and geochemical trends associated with both processes are discussed and compared to the TM peridotites. -- Relative age relationships between the spreading-related structural units support the plate thickening model for formation of oceanic lithosphere. By applying thermal models for formation of oceanic lithosphere to the TM mantle section, the distance from the ridge axis, at which each unit has been structurally frozen in, can be estimated. The additional use of data concerning the relative sense of shear in the peridotites and the relative position of the ridge axis with respect to the TM massif indicates that the high level, early mantle structures in TM are related to plate driven flow. Later, deeper mantle structures formed during forced flow. The latest spreading-related structures are assigned to an off-axis, diapiric motion of the upper mantle which was stopped at the base of the lithosphere at approximately 50 km from the ridge axis. The lower melt infiltration zone is correlated with this event. -- The history of the BOIC can be explained as an evolution during a decreasing spreading rate. The decreasing spreading rate is considered responsible for the emergence of a late, transient, far-reaching forced flow field driven by the buoancy of partially melted peridotite below the ridge during a period of restricted extensional plate movement. As a further consequence of restricted extension, magmatic products ponded at the base of the oceanic crust, resulting in the formation of thick dunite sequences in the late history of the BOIC. This late history is best preserved in the Blow Me Down massif of the BOIC which, based on the preservation of ridge-parallel flow structures up to a depth of several kilometres below the Moho, must have been located close to the ridge axis when spreading ceased. The presence of lherzolites instead of harzburgites at the base of all four massifs of the BOIC may be explained as the result of slower mantle upwelling prevailing during very slow spreading in the final extensional history of the BOIC. Later, the plates converged, causing detachment of the ophiolite. -- The oceanic basin where the BOIC was generated may represent a pull-apart basin (Andaman Sea type) which opened during strike-slip dominated tectonics within an older, arc-type oceanic lithosphere. The boundary between the young lithosphere of the BOIC and the older arc-type lithosphere would correlate with the proposed fracture zone in the Lewis Hills massif