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 b...