MELTING REGIMES AND SUB-ARC MANTLE DOMAINS IN CASCADIA AS SEEN THROUGH PRIMITIVE BASALT COMPOSITIONS

Primitive basalts in the modern Cascades arc are of 3 main types: calcalkaline (CAB), low-K tholeiite (LKT), and high-field strength element enriched (HFSE) compositions. They provide clues to the mantle sources and melting regimes that drive arc magmatism. Multiple primitive magma types occur the length of the modern arc, at individual long-lived volcanoes, and in the ancestral Cascades arc, putting to rest the notion that a single type of arc basalt fuels subduction magmatism.

The distribution of primitive basalt types corresponds with segmentation in the arc as expressed in vent distribution and basement type. Paleozoic to Mesozoic metamorphic basement underlies the Northern segment (Mt. Meagher to Glacier Peak), where subduction is ~orthogonal, volcanism is concentrated at main edifices, and primitive basalts are mainly calcalkaline. The oceanic Siletzia terrane underlies the Columbia segment (Rainier to Jefferson); the arc is broad with widely scattered vents, and HFSE basalts occur along with ubiquitous CAB and LKT. The Central and South segments are underlain by accreted Paleozoic to Mesozoic terranes and differ in the amount of extension related to clockwise rotation in the forearc and westward and northward encroachment of the Basin and Range. The Central segment (Three Sisters to Mt. McLoughlin) has extensional structures and is distinguished by anomalously abundant and mafic volcanism primarily of the LKT type. Medicine Lake and Mt Lassen share characteristics with the central segment. Mt Shasta is distinct, with CAB, LKT and also a high-Mg primitive compositions.

The spectrum of basalts can be explained by changes in degree of melting plus a subduction-engendered fluid flux component. For example, small degrees of melting produce Nb (and Ba) enrichment in the HFSE basalt. Larger degrees of melting can dilute those enrichments to yield the LKT type and fluid-fluxed melting yields the CAB type. Distinct radiogenic isotope character of different basalt types along the arc, however, indicates that changes in melting regime are superimposed on different mantle domains, some of which are inherited from the ancestral Cascades architecture with increasing subduction contamination in time, plus influx of mantle from the northeastern Basin and Range related to slab rollback.