Sub-arc Mantle Reservoirs Through Time in Cascadia

Inheritance from pre-existing mantle domains and fluid/melt contributions from active subduction together produce the geochemical signatures of mantle-derived arc basalts. In this context, we evaluate the evolution of Cascadia mantle sources by documenting the isotopic and compositional characteristics of primitive basalts along a transect across the Eo-Oligocene Proto-Cascadia (EOPC) arc at ~44.5-45.5° N. Primitive EOPC flows, dikes, and sills are exposed across a ~300km transect that includes the Oregon Coast Range (OCR) in the Cascadia forearc, the Western Cascades (WC) flanking of the modern arc, and the John Day and Eastern Clarno (JD-EC) formations east of the Cascades. Like the modern arc, EOPC was built upon accreted terranes of western N. America and within the Columbia embayment; lithosphere of oceanic affinity that outcrops as the Siletzia terrane in the forearc and extends beneath the arc to the backarc. Potential mantle source reservoirs for EOPC magmas include contributions from: (1) mantle domains related to pre-existing underlying terranes; (2) distinct N. America asthenosphere; and (3) depleted Pacific-like upper mantle. In addition, the geochemical characteristics of EOPC magmas have likely been overprinted by subduction processes. Incompatible trace elements show that primitive magmas from each of the three EOPC settings have distinguishable mantle sources. WC magmas have a restricted compositional range that closely mirrors that of the primitive low-K tholeiites of the modern Cascades, indicating an early existence of this mantle source. Enrichments in fluid mobile elements (e.g. elevated Ba/Zr) are consistent with a significant subduction component for WC and JD-EC magmas. By contrast, OCR samples do not share this subduction character and have low Ba/Zr relative to typical MORB values. Isotopically, the EOPC are distinguished from most modern Pacific MORB by slightly lower ɛNd and higher 87Sr/86Sr, overlapping values found in the primitive basalts of the modern Cascades and High Lava Plains. The JD-EC magmas extend to lower ɛNd and higher 87Sr/86Sr perhaps reflecting a greater N. American lithosphere influence to the east. WC and JD-EC samples show a limited range in 206Pb/204Pb and a steep trend in 206Pb/204Pb vs. 207Pb/204Pb, displaying generally higher 207Pb/204Pb for a given 206Pb/204Pb than modern Pacific MORB. This trend also is shown in modern Cascade and HLP basalts and suggests a contribution from subducted Pacific Northwest sediments that overlap the WC and JD-EC lavas in Pb isotopic composition. The OCR samples also have high 207Pb/204Pb, but trend to much higher 206Pb/204Pb than the other EOPC lavas. In this respect, they mirror the data for the Paleocene-Eocene Siletzia basement.