The microstructure of layered ultramafic cumulates: Case study of the Bear Creek intrusion, Trinity ophiolite, California, USA

In the Trinity ophiolite, California, USA, several mafic-ultramafic plutons intruded a peridotitic host 435 to 405 m.y. ago in tectonic setting interpreted as an arc-related spreading centre. One of these intrusions, Bear Creek area, exposes basal ultramafic cumulates with igneous layering comprising alternation uncommonly thin (down few mm) layers dunite, peridotite and pyroxenite that might be specific this setting. These offer excellent opportunity characterise microstructure uncommon from lower crust using EBSD (Electron Backscatter Diffraction). This “high-frequency” rests on underlying lherzolites grades upward more massive pyroxenites (i.e. clinopyroxenites minor olivine-rich layers). Our field observations data together preservation magmatic features suggest environment was tectonically stable after emplacement cumulates. A detailed microstructural investigation all minerals allows us decipher their plastic deformation history. structural reference frame defined by compositional elongation direction surrounding peridotites, olivine presents [010]-fibre fabric rarely [001](010) fabric. Clinopyroxene shows concentration [010] axes normal plane [100] [001] defining girdles. Orthopyroxene mostly has and/or subnormal scattered along girdle layering. All show strong We interpret formation developed planar microstructures result processes, high contribution crystal settling. To lesser extent, compaction could have been operating may linked rare evidence deformation. Clusters within girdles pyroxene CPOs preferentially appear close peridotites N115°). analysis shape-preferred orientation Creek's revealed preferential alignment cpx long axis N°115 direction. lineation preferred are both consistent stretching acquired during solid-state mantle ophiolite. weak magma flux early ongoing but limited regional stress responsible for clusters. Although direct coupling between asthenospheric flow cannot invoked context melt intrusion lithosphere, highlights applied still active similar intrusion. new data, previously presented petrological fit scenario evolution ophiolite which segment single large batch primitive boninitic-andesitic melt. results emphasise importance considering initial original shape anisotropy when investigating later rocks.