GEOLOGY OF WESTERN PACIFIC OPHIOLITES AND SUPRASUBDUCTION ZONE SYSTEMS: INSIGHT TO ORIGIN OF CORDILLERAN TERRANES

Many (most?)ophiolites were derived from supra-subduction zone (SSZ)regimes, i.e.,trench-arc-backarc basin systems. Western Pacific(WPAC),Eocene to Recent SSZ rocks give insight to origin of far-traveled Cordilleran ophiolites and oceanic arc terranes, swept in from the Pacific Basin and accreted to North America. Some ophiolites have rocks similar to mid-ocean ridge basalt (MORB); others have rocks common to arc-backarc series of SSZ systems. Cenozoic WPAC trench/arc/backarc geology offers insight to origin of some of these terranes. Here I compare a true ophiolite with SSZ rocks that serve as a model for ophiolite origin.

Palau Islands,south end of Palau-Kyushu Ridge (PKR), expose Eoc. - Oligo. basalt, basaltic andesite,andesite flows, pillows, dikes, and breccias and minor andesite/dacite intrusive plugs. All have island arc tholeiite (IAT) chemistry. Similar rocks occur in nearby Palau Trench. PKR represents earliest WPAC SSZ crust. Blocks rifted from PKR, now 1100 km east, form part of the Mariana forearc including Guam and Saipan.

Mariana Trench-wall rocks near Guam comprise: serpentinised depleted harzburgite (HZB), dunite (DUN), pyroxenites (PXITE),gabbro/norite (GN), diabase, basalt - i.e. an ophiolite series. Unlike typical MORB, primary amphibole is common. Rocks have IAT chemistry or are depleted mantle. Also present are boninite and granitoids. Coarsely crystalline mafic/ultramafics experienced near-solidus penetrative deformation that formed gneissose, blastomylonitic and mylonitic textures. Some have hydrous retrograde imprints on higher-T assemblages. Tonalite-trondhjemite-granitoid (TTG) rocks are a minor but sigificant component.

Zambales Range, Luzon ophiolite has 15-20 km thick depleted, tectonized, serpentinised DUN and HZB. Above Petrologic Moho is 600-800 m of layered Ol, Opx, Cpx cumulates with rare anorthosite. CPXite veins are common. Above Geophysical Moho is 2-3 km of layered GN; above this is 1-2 km of massive GN with rare TTG. Non-sheeted diabase, mixed with small TTG plutons, and basalt comprise upper 1 - 1.5 km. Rocks have IAT chemistry. The Zambales has remnants of a SSZ system. Its Eocene age is correlative with early WPAC SSZ crustal generation. All rock types in Mariana Trench are found in situ in Zambales Ophiolite