CORDILLERAN SUPRA-SUBDUCTION ZONE (SSZ) OPHIOLITES: THE FORE-ARC CONNECTION

Ophiolites are distinct assemblages of submarine volcanic and plutonic rocks, which are thought to represent oceanic crust. It is now recognized that most ophiolites form during transient episodes of subduction initiation, and are called supra-subduction zone (SSZ) ophiolites. It is also recognized that in situ analogues of onland ophiolites may be found in the forearcs of primitive modern volcanic arcs (e.g., IBM arc). Forearc volcanic rocks exhibit a range in compositions (like ophiolites) and are typically enriched in fluid-mobile elements (FME). Highly depleted boninites, which must represent extensive fluid-fluxed melting, are found only in this setting. In contrast, volcanic rocks formed at mid-ocean ridges and back-arc basins documents a limited range in compositions with low FME contents. Recent studies have strengthened the connection between ophiolites and forearcs by documenting the occurrence of forearc basalts with MORB-like compositions that underlie the boninite suite lavas. Forearc basalts form by decompression melting before the onset of extensive hydrous flux through the mantle wedge. In ophiolites, this magma series forms arc tholeiites that underlie boninite suite lavas, as well as the extensive layered gabbro suites that are cross-cut by boninite-derived wehrlites.

Cordilleran SSZ ophiolites include the Ordovician Trinity ophiolite and the mid-Jurassic Coast Range, Josephine, Cedros and Smartville ophiolites. The dominance of SSZ ophiolites in the Cordillera, and their common association with primitive arc terranes, requires a complex history of subduction initiation and arc accretion that may have occurred far from the current continental margin. Serpentine matrix mélanges containing fragments of oceanic crust suggest that subduction may have initiated on long-offset fracture zones, similar to events in the IBM forearc.