Title:
 Rhyodacites of Kulshan Caldera, north Cascades of Washington;post-caldera lavas that span the Jaramillo 
Reference Number:
 7390 
ISSN:
 0377-0273
Publication Year:
 2004
Publisher:
 Elsevier, Amsterdam, Netherlands (NLD)
GeoREF Number:
 2007-002412 (View Original Data File)
Authors:
 Hildreth, Wes; Lanphere, Marvin A; Champion, Duane E; Fierstein, Judy
Descriptors:
 absolute age; Ar/Ar; basement; breccia; calderas; Cascade Range;Cenozoic; chemical composition; collapse structures; Cordilleran icesheet; dates; dikes; domes; emplacement; eruptions; fluctuations;geochemistry; geomorphology; geophysical methods; geophysicalprofiles; geophysical surveys; gravity methods; gravity profiles;igneous rocks; intrusions; Jaramillo Subchron; K/Ar; Kulshan Caldera;landform evolution; lava flows; lower Pleistocene; magmatism;magnetization; Mount Baker; paleoenvironment; paleomagnetism;Pleistocene; Quaternary; remanent magnetization; rhyodacites; surveys;United States; volcanic features; volcanic rocks; volcanism;volcanoes; Washington
Source:
 Journal of Volcanology and Geothermal Research, vol.130, no.3-4,pp.227-264, 29 Feb 2004
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).
Copyright:
 GeoRef, Copyright 2007, American Geological Institute. Referenceincludes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam,Netherlands

Sample Summary:
Sample ID: Major Data: Trace Data: Isotope Data:
618 Yes Yes No
628 Yes Yes No
212 No No No
145 Yes Yes No
82-2 No No No
630 Yes Yes No
287 Yes Yes No
227 No No No
188 Yes Yes No
583 Yes Yes No
275C Yes Yes No
622 Yes Yes No
626 No No No
380 No No No
636A Yes Yes No
201 No No No
333 No No No
493 Yes Yes No
500 Yes Yes No
316 Yes No No
145 Yes No No
212 Yes Yes No
356 Yes No No
276B Yes No No
93 Yes No No
91 Yes No No
81 Yes No No
99 Yes No No
89 Yes No No
10 Yes No No
391 Yes No No
572 Yes No No
492 Yes Yes No
s129 Yes Yes No
s626 Yes Yes No
n627 Yes Yes No
297 Yes Yes No
621A Yes Yes No
96 Yes Yes No
189 Yes Yes No
321 Yes Yes No
137 Yes Yes No
290 Yes Yes No
451 Yes Yes No
452 Yes Yes No
80 Yes Yes No
82 Yes Yes No
448B Yes Yes No
631 Yes Yes No
228 Yes Yes No
378 Yes Yes No
455 Yes Yes No
73 Yes Yes No
190 Yes Yes No
291 Yes Yes No
n315 Yes Yes No
s316 Yes Yes No
299 Yes Yes No
356A Yes Yes No
636B Yes Yes No
636C Yes Yes No