HR: 0830h
AN: V31C-0946    [PDF]
TI: Episodes of Middle to Late Jurassic Arc Development in the Klamath Mountains, California and Oregon
AU: * Barnes, C G
EM: Cal.Barnes@ttu.edu
AF: Texas Tech Univ., Dept. of Geosciences, Lubbock, TX 79409
AU: Allen, C M
EM: charlotte.allen@anu.edu.au
AF: Australia National Univ., Research School of Earth Sciences, Canberra, ACT 0200 Austria
AU: Snoke, A W
EM: snoke@uwyo.edu
AF: Univ. of Wyoming, Dept. of Geology & Geophysics, Laramie, WY 82071
AB: The sequence of tectonostratigraphic terranes in the Klamath Mountains provides a rich history of arc magmatism that spans at least 400 m.y. The Middle to Late Jurassic record can be organized into episodes that consist of arc formation and development, contractional deformation (thrusting), plutonism that lacks significant crustal influence, and younger plutonism (with accompanying extension?) with widespread crustal assimilation and/or anatexis. The first episode began with formation of the Middle Jurassic western Hayfork arc (WH; 179 to 171 Ma). It is characterized by water-rich calc-alkaline andesite, but ranges from basalt to dacite. WH magmatism ended with a regional thrusting event. Following thrusting, within uncertainty of dating, plutons of the Ironside Mountain suite (IM) were emplaced. They show a distinctive decrease in Mg/(Mg+Fe), a paucity of early hydrous phases, and increases in LILE. Neither the WH nor IM rocks show evidence for significant crustal contribution. The period 169 to 162 Ma was characterized by regional metamorphism and emplacement of small calc-alkaline plutons. Then from 162 to 159 Ma, large calc-alkaline to calcic plutons were emplaced (Wooley Creek suite). All plutons of this age show isotopic evidence for crustal anatexis and/or assimilation. Scarce 159 Ma garnet-bearing andesitic dikes suggest equilibration at circa 800 MPa, with a crustal thickness of at least 30 km. Wooley Creek magmatism was coeval with outboard growth of the supra-subduction zone Josephine ophiolite (166 to 159 Ma). Extension associated with ophiolite formation may have increased the flux of mafic magma necessary for crustal melting in the Wooley Creek suite. The next episode began with development of the Rogue/Chetco arc along the outboard margin of the Josephine ophiolite (155 to 160 Ma). This arc was generally similar to the WH arc, with water-rich, calc-alkaline magmatism. Collapse and underthrusting of the Josephine ophiolite-Rogue arc assemblage beneath inboard terranes occurred from 155 to 150 Ma; this contractional deformation was the Nevadan orogeny. Thrusting was followed by renewed calcic and calc-alkaline plutonism from 150 to 146 Ma, some of which was similar to the dry, LILE-rich IM suite. These plutons are also similar to the IM in that they show little evidence for interaction with crustal rocks. In contrast, tonalitic to granodioritic plutonism from 145 to 136 Ma requires primarily crustal sources, specifically the underthrust rocks of the Josephine-Rogue assemblage. Each episode began with arc magmatism that ended in contractional deformation. Contraction was followed by localized emplacement of mantle-derived plutons. Later magmatism was partly crustal in origin and, at least in the case of the Wooley Creek suite, associated with extension. This suggests that the time lag between post-arc contraction and emplacement of crustally-derived magmas reflects the time necessary for fresh, mantle-derived magma to heat the lower crust to its solidus.
DE: 8105 Continental margins and sedimentary basins
DE: 8150 Plate boundary--general (3040)
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting