Holocene volcanism: south slope of Mount Adams volcano, Cascade Arc, USA

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University of New Brunswick


The south- slope Holocene volcanic cinder cones and lava flows of Mount Adams volcano in the southern part of t h e Washington Cascade Arc, USA, present a petrologically limited calcalkaline rock series consisting of alkali olivine basalts , basaltic andesites and andesites. The rocks display mostly porphyritic vesicular textures, with anhydrous mineral assemblages - ol+pl (alkali olivine basalt), ol+pl+cpx (basalt) pl+cpx+opx+ol (basaltic andesite), and cpx+opx+pl (andesite). The anhydrous nature of the Mount Adams rocks contrasts with the hydrous assemblages of Mt. St. Helens, 50 km to the west. The chemical data identifies the rocks as medium to high-K calcalkaline volcanics, with normative hy and qz (basaltic andesites and andesites), and normative ne and ol (basalts). Consideration and evaluation of atomic absorbtion spectrophotometry (AAS), x-ray fluorescence spectrome try (XRF), electron probe microanalysis (EPHA) and petrographic data analyses and observations allows the conclusion of high pressure, low Foz melt generation of mafic high alumina basalt (HAB) precursor type magmas in the mantle wedge above the subducted, dehydrating , Juan de Fuca plate, followed by melt fractionation with ascension in conduits localized by deep crustal fractures , to give rise to the observed linearity , of Holocene flows and domes of homogenous composition, over large north-south distances on the stratovolcano . The presence of silicic andesitic flows issuing from higher levels of the volcano, while more mafic cones and flows of basal tic andesite and basalt of the same age originate at lower elevations, supports the thesis of the presence of a thermally and compositionally stratified magma chamber , Hildreth (1978), McBirney (1984). An empirical relationship is suggested for the observed hydrous to anhydrous style of arc volcanism with increasing distance from the subduction zone. As such, volcanism and its particular occurrence in time and space is seen as a function of distance from the subduction zone, angle of dip and convergence, and the nature of dehydration reactions progressing down slab.