LATE CENOZOIC MAGMATISM OF THE NORTHWESTERN U.S. – THE ROLE OF SUB-CONTINENTAL LITHOSPHERIC MANTLE (SCLM)

The nature/origin of bimodal magmatism across SE Oregon-Idaho-NW Wyoming is often attributed in some way to passage of North America over the Yellowstone hotspot (i.e., the tail of the Siletzia hotspot). Many observations are inconsistent with this view. Notably, [1] lateral variations in basement geology/age correlate with distinctions in basalt geochemistry and eruptive history; [2] east of the ancient craton boundary, silicic volcanism appears to be eastwardly age-progressive, whereas onset of basaltic eruptions is delayed but, once commenced, continue sporadically to the present showing no consistent age progression (the same is true in a westerly sense in Oregon); [3] basalt compositions are distinct from those in virtually all oceanic hotspot tracks, precluding their direct origin from comparable deep mantle upwellings.

These patterns suggest that the lithospheric plate strongly influences magmatic processes, with different influences between the craton to the east of the ID-OR border and accreted oceanic terranes to the west. Specifically, the thicker cratonic lithosphere precludes significant melt production from the Yellowstone plume, and favors infiltration of basaltic magmas within the crust/upper mantle and generation of voluminous silicic magmas in part due to remelting of early mafic inputs. Geochemistry of the mafic magmas is consistent with melting of ancient subcontinental lithospheric mantle (SCLM), rather than the plume, and this process can be attributed to decompressional melting of such domains in response to lithospheric extension that has propagated eastward across the region (and moreso in the Basin & Range province to the south).

Thinner lithosphere beneath the western accreted terranes may allow more efficient melting of upwelling sub-lithospheric mantle. However, geochemistry of basalts there implies that their sources (SCLM?) have been modified to varied extent by ancient(?) additions of subduction-like components, and this precludes their formation dominantly from a typical plume upwelling.