A 40 MA GEOCHEMICAL RECORD OF SPATIAL AND TEMPORAL CHANGES IN CASCADE ARC MAGMATISM

Magmatism in the Cascade Arc began approximately 40 Ma when the rate of subduction of the oceanic plate slowed, and that activity continues today. Geochemical data for 1643 samples were compiled from 34 volcanic and plutonic units within the Cascade Arc, primarily in Washington and northern Oregon. These data were examined for spatial and temporal trends that could reveal changes in subduction parameters including crustal composition, crustal thickness, the dip of the subducting slab, and the rate of subduction.

Crustal thickness beneath the Cascade Arc was calculated using the covariance of CaO/Na₂O of lavas with crustal thicknesses observed from modern arcs worldwide. Geophysical estimates of crustal thickness in modern rock units exhibit a correlation of 0.83 with CaO/Na₂O values adjusted to a MgO content of 6 wt. percent (Plank & Langmuir, 1988), and this regression was used to determine crustal thickness for 27 rock units that formed over the last 40 Ma. Western rock units exhibit crust up to 11 km thicker than units 2° to the east, attributed to a decrease in the relative thickness of the mantle wedge closer to the trench assuming a slab dip of 12°, as well as to the longevity of magmatic activity in the western Cascades. Crustal thickness varied regionally, but remained relatively constant over time, with the exception of an apparent thinning event around 25 Ma, in which many rock units decreased from 40 km to 28 km. Many of the rock units dated around 25 Ma were located farther east where a greater slab depth and thicker mantle wedge implies thinner crust. It is also possible that the thinning reflects an increase in dip of the subducting slab related to a temporarily slower rate of subduction.

There is marked variation in trace element geochemistry along the Cascade Arc. Ba/Nb ratios increase and become more varied above 48°N. Higher Ba and Sr concentrations combined with low Nb suggest a greater degree of melting, consistent with slowing in subduction and/or a thicker mantle wedge. Ba/Nb ratios have also increased from about 40 to more than 150 and become significantly more variable towards the present. No other incompatible trace elements mirrored this pattern, which indicates greater enrichment of High Cascade lavas in the present, compared to earlier units of the Cascade Arc.