Paper No. 33-3
Presentation Time: 8:55 AM
MAGMATIC DIVERSITY IN THE BORING VOLCANIC FIELD, PORTLAND BASIN, OREGON AND WASHINGTON: IMPLICATIONS FOR SUBDUCTION ZONE MAGMATISM
In the Cascades volcanic arc, magmatic diversity is greatest in the sector bordering the Columbia River. This is manifest to full extent in the frontal arc (Portland Basin) Boring Volcanic Field (BVF) that comprises some 80 dispersed monogenetic centers as young as 58 ka. Voluminous early (<3 Ma) low-K tholeiites (LKT), many sourced in the High Cascades, were followed by distinctive OIB-like (OIB) and calcalkalic (CAB) magma types (ranging from basalt to andesite; 47-60% SiO2), and including rare potassic absarokites (ABS) erupted from local vents. Geochemical parameters (e.g., Ba/Nb, Sr/Y, K/Ti, etc.; >1400 analyses) define distinctions between primitive end member (MgO > ca. 8%) magma types. Notably, LKTs and OIBs seem to have little or no subduction contribution. These data preclude cogenetic relations between the groups, and implicate formation of the respective magmas by melting of multiple distinct mantle sources. Higher Mg#s and olivine Fo content typify primitive CAB and ABS lavas, suggesting they have more refractory mantle sources compared to LKT or EIB magmas. Many of the samples defy easy classification, and at least some likely are hybrid mixtures of multiple end member magmas and/or their derivatives. Textures and mineral compositions indicate that magma mixing was common, especially in more evolved lavas. Overall, the eruption of primitive lava types implies that deep-seated magmas of each type often could ascend with little impediment. Mapping and detailed geochronology reveal a general northwesterly migration of vent localities over time - presumably in response to ongoing deformation due to oblique subduction. The distribution of magma chemical types is less systematic, implying that the underlying plumbing system is dominated by isolated conduits linked to heterogeneous mantle sources beneath the frontal arc region (as well as the arc itself). Estimated depths of magma segregation from the mantle are greatest for primitive LKTs and progressively shallower for OIBs, CABs, and ABSs. Thus, magmatism may be driven by decompression melting of LKT source mantle, and other magma types may form by melting shallower mantle domains (some with an inherited SZ signature) in response to infiltration of voluminous hot LKT magmas. Implications for the other respective magma source domains will be discussed.