Rhyodacites of Kulshan Caldera, north Cascades of Washington;post-caldera lavas that span the Jaramillo

Abstract

Kulshan caldera (4.5X8 km), at the northeast foot of Mount Baker, isfilled with rhyodacite ignimbrite (1.15 Ma) and postcaldera lavas andis only the third Quaternary caldera identified in the Cascade arc. Agravity traverse across the caldera yields a steep-sided, symmetrical,complete Bouguer anomaly of -16 mGal centered over the caldera.Density considerations suggest that the caldera fill, which is incisedto an observed thickness of 1 km, may be about 1.5 km thick and isflat-floored, overlying a cylindrical piston of subsided metamorphicrocks. Outflow sheets have been stripped by advances of theCordilleran Ice Sheet, but the climactic fallout (Lake Tapps tephra)is as thick as 30 cm some 200 km south of the caldera. Ten precalderaunits, which range in (super 40) Ar/ (super 39) Ar age from 1.29 to1.15 Ma, are dikes and erosional scraps that probably never amountedto a large edifice. A dozen postcaldera rhyodacite lavas and dikesrange in age from 1.15 to 0.99 Ma; rhyodacites have subsequently beenabsent, the silicic reservoir having finally crystallized. At least 60early Pleistocene intermediate dikes next intruded the caldera fill,helping energize an acid-sulfate hydrothermal system and constitutingthe main surviving record of an early postcaldera andesite-dacite pilepresumed to have been large. Most of the pre- and postcalderarhyodacites were dated by (super 40) Ar/ (super 39) Ar or K-Armethods, and 13 were drilled for remanent magnetic directions. Inagreement with the radiometric ages, the paleomagnetic data indicatethat eruptions took place before, during, and after the JaramilloNormal Polarity Subchron, and that one rhyodacite with transitionalpolarity may represent the termination of the Jaramillo. Most of thebiotite-hornblende-orthopyroxene-plagioclase rhyodacite lavas, dikes,and tuffs are in the range 68-73% SiO (sub 2) , but there were largecompositional fluctuations during the 300-kyr duration of therhyodacite episode. The rhyodacitic magma reservoir was wider (11 km)than the caldera that collapsed into it (8 km).

61 samples from this reference