Eruptive history and geochronology of Mount Mazama and the Crater Lakeregion, Oregon

Abstract

Geologic mapping, K-Ar, and (super 40) Ar/ (super 39) Ar agedeterminations, supplemented by paleomagnetic measurements andgeochemical data, are used to quantify the Quaternary volcanic historyof the Crater Lake region in order to define processes and conditionsthat led to voluminous explosive eruptions. The Cascade arc volcanoknown as Mount Mazama collapsed during its climactic eruption ofapproximately 50 km (super 3) of mainly rhyodacitic magmaapproximately 7700 yr ago to form Crater Lake caldera. The Mazamaedifice was constructed on a Pleistocene silicic lava field, amidemonogenetic and shield volcanoes ranging from basalt to andesitesimilar to parental magmas for Mount Mazama. Between 420 ka and 35 ka,Mazama produced medium-K andesite and dacite in 2:1 proportion. Theedifice was built in many episodes; some of the more voluminousoccurred approximately coeval with volcanic pulses in the surroundingregion, and some were possibly related to deglaciation followingmarine oxygen isotope stages (MIS) 12, 10, 8, 6, 5.2, and 2. Magmas asevolved as dacite erupted many times, commonly associated with orfollowing voluminous andesite effusion. Establishment of the climacticmagma chamber was under way when the first preclimactic rhyodacitesvented ca. 27 ka. The silicic melt volume then grew incrementally atan average rate of 2.5 km (super 3) k.y. (super -1) for nearly 20 k.y.The climactic eruption exhausted the rhyodacitic magma and brought upcrystal-rich andesitic magma, mafic cumulate mush, and wall-rockgranodiorite. Postcaldera volcanism produced 4 km (super 3) ofandesite during the first 200-500 yr after collapse, followed at ca.4800 yr B.P. by 0.07 km (super 3) of rhyodacite. The average eruptionrate for all Mazama products was approximately 0.4 km (super 3) k.y.(super -1) , but major edifice construction episodes had rates ofapproximately 0.8 km (super 3) k.y. (super -1) . The long-termeruption rate for regional monogenetic and shield volcanoes wasapproximately 0.07 km (super 3) k.y. (super -1) , but onlyapproximately 0.02 km (super 3) k.y. (super -1) when the two majorshields are excluded. Plutonic xenoliths and evidence forcrystallization differentiation imply that the amount of magmaintruded beneath Mount Mazama is several times that which has beenerupted. The eruptive and intrusive history reflects competitionbetween (1) crystallization driven by degassing and hydrothermalcooling and (2) thermal input from a regional magma flux focused atMazama. Before ca. 30 ka, relatively small volumes of noneruptedderivative magma crystallized to form a composite pluton because theupper crust had not been heated sufficiently to sustain voluminousconvecting crystal-poor melt. Subsequently, and perhaps notcoincidentally, during MIS 2, a large volume of eruptible silicicmagma accumulated in the climactic chamber, probably because ofheating associated with mantle input to the roots of the system assuggested by eruption of unusually primitive magnesian basalticandesite and tholeiite west of Mazama.

117 samples from this reference