ISOTOPIC TRANSECT OF THE OLIGOCENE PROTO-CASCADIA ARC: SPATIAL AND TEMPORAL VARIABILITY OF SUB-ARC MANTLE RESERVOIRS

Geochemical mantle signatures in arc basalts may be inherited from pre-existing mantle domains as well as being acquired by active subduction processes. This work evaluates the persistence of mantle domains by comparing the isotopic and compositional characteristics of primitive basalts from a transect across the Proto-Cascadia (PC) arc to pre-existing mantle domains and to the present arc.

The early Oligocene Cascadia margin experienced widespread forearc and backarc magmatism coeval with subduction-induced volcanism of the Western Cascades. Magmatism in the PC forearc is now preserved as numerous mafic dikes and sills in the Oregon Coast Range. Volcanic rocks and intrusions of the deeply eroded Western Cascades flank the modern High Cascades and represent the remains of the more westerly axis of the arc that was active between ~42-10 Ma. Volcanic rocks behind the arc axis are preserved mainly as the silicic ignimbrites and tuffaceous sedimentary rocks of the John Day Formation, with related sparse mafic flows and dikes.

Like the modern arc, PC is built across the transition between accreted terranes of western North America (NA) and the Columbia embayment, mantle lithosphere of oceanic affinity that outcrops as the Siletzia terrane in the forearc and extends beneath the arc to the backarc. Potential sources of heterogeneity in primitive Cascade magmas include: (1) the persistence of mantle related to these different terranes; (2) the advection of distinct asthenophere from under N. America due to subduction and attendant convection; and (3) Pacific mantle flow through slab windows at the edges of the downgoing plate, facilitated by slab rollback. Subduction processes overprint all of this potential patchwork.

The PC arc shows systematic across arc variation, with ²⁰⁶Pb/²⁰⁴Pb decreasing forearc to backarc. PC forearc samples describe a trend generally parallel to Siletzia (elevated ²⁰⁶Pb/²⁰⁴Pb), but displaced to slightly lower e_Nd; high ²⁰⁶Pb/²⁰⁴Pb ratios are also found in HFSE enriched basalts of the Columbia Segment of the modern Cascades, consistent with an inherited mantle source related to Siletzia. The PC data do not suggest advection of Pacific mantle when compared to modern ridge data, but may indicate a North American influence, as suggested by lower e_Nd.