PRELIMINARY GEOCHRONOLOGY AND GEOCHEMISTRY OF THE HICKS BUTTE COMPLEX, CENTRAL CASCADES, WASHINGTON: POSSIBLE LINKS BETWEEN POLYGENETIC MESOZOIC ARC ROCKS OF THE CENTRAL CASCADES AND THE BLUE MOUNTAINS PROVINCE James MacDonald, Florida Gulf Coast University, and Mark Pecha, University of Arizona

Geologic province sketch map displaying the location of the Washington’s Mesozoic units south of the North Cascades with respect to the Blue Mountains, Klamath Mountains, and Northern Sierra Nevada.

Simplified geologic map displaying pre-Cenozoic tectonic elements of the central and northwest Cascades. Terranes east of the Straight Creek-Fraser River fault have been dextrally displaced to the south. Modified from Miller et al. (1993), Tabor (1994), and Brown and Dragovich (2003)

Simplified geologic map displaying pre-Cenozoic tectonic elements of the central and northwest Cascades palinspasticly restored for dextral movement along the Straight Creek-Fraser River fault zone. Modified from Miller et al. (1993), Tabor (1994), and Brown and Dragovich (2003)

Restoration of northern Cordillera strike- slip faulting at 100 Ma. BT = Baker terrane IO = Ingalls ophiolite Hicks Butte complex would be outboard of IO Wyld et al., 2006

Simplified geologic map displaying pre-Cenozoic tectonic elements of the central and northwest Cascades. Terranes east of the Straight Creek-Fraser River fault have been dextrally displaced to the south. Modified from Miller et al. (1993), Tabor (1994), and Brown and Dragovich (2003)

Simplified geologic map displaying the tectonic elements east of the Straight Creek fault and south of the Cascades Crystalline Core. From the north to the south, the Late Jurassic arc rocks within these inliers include:  Hicks Butte complex 153 ± 2 Ma (Miller et al., 1993)  Quartz Mountain stock ± 1.2 Ma (Miller et al., 1993; MacDonald, 2006)  Indian Creek complex 154 ± 2 Ma (Miller et al., 1993) Modified from Tabor et al. (1982, 1987, 1993, 2000), Miller (1985, 1989) and Miller et al. (1993)

Simplified geologic map of the Hicks Butte inlier.

Tonalite, diorite, and quartz diorite are the most common rock types within the Hicks Butte complex. Lesser hornblende gabbro, dacite, and hornblendite also occur. All of these rock are deformed to gneiss or amphibolite. Tonalite cut by a dike of hornblende gabbro Dacite intruding tonalite and hornblende gabbro gneiss

Relatively undeformed tonalite displaying a hypidiomorphic- granular texture. Sub- to anhedral hornblende is surrounded by subhedral feldspar and anhedral quartz and lesser oxide. Low temperature hydrothermal alteration is evident. Undeformed dacite predominantly display allotriomorphic-granular texture. Sub- to anhedral feldspar is slightly microporphyritic and surrounded by anhedral quartz and interstitial hornblende. Resorption of feldspar and quartz occurs.

Tonalite gneiss cut by a dike of hornblende gabbro gneiss -- displaying boudinage. Annotated folded and faulted dike of hornblende gabbro that is located within tonalite gneiss

Tonalite gneiss cut by two dikes of dacite– one displaying boudinage and the other slightly folded..

100 μm Igneous zircons from a Hicks Butte complex tonalite (HB-1). Zircon cores typically display sector to complex zoning. Rims display oscillatory zoning. Spots 4R and 4C yield Carboniferous ages.

 11 igneous zircons from the tonalite yield a U-Pb age of ± 6.8 Ma (2σ; total error = 4.5 %; MSWD = 0.2).  This corroborates the published date of 153 ± 2 Ma from a tonalite gneiss from this complex (Miller et al., 1993).  This sample also contains 3 concordant Paleozoic age zircon spot analyses (317 ± 12; 356 ± 12; & 464 ± 8 Ma).

Igneous zircons from a Hicks Butte complex dacite (HB-12). Zircon cores typically display oscillatory, sector or complex zoning. Rims display oscillatory zoning. Few zircons are not zoned.

28 igneous zircons from the dacite yield a U-Pb age of ± 2.4 Ma (2σ; total error = 1.7%; MSWD = 1.1).

Simplified geologic map of the Hicks Butte inlier displaying new and published ages.

Mesozoic ages in some Washington Cascades plutons Hicks Butte inlier Hicks Butte complex Ma U-Pb zircon 144 ± 2.4 Ma dacite (Miller et al., 1993; This study) Manastash inlier Quartz Mt. stock 157 Ma U-Pb zircon Hereford Meadow amphibolite 143 ± 0.7 Ma Ar-Ar hbl (Miller et al., 1993; MacDonald, 2006) Rimrock Lake inlier Indian Creek complex 154 Ma U-Pb zircon 142 ± 9 Ma K-Ar hbl (Conley, 1980; Miller et al., 1993) Modified from Tabor et al. (1982, 1987, 1993, 2000), Miller (1985, 1989) and Miller et al. (1993)

Complex ranges from mafic to felsic in composition. Includes dacites. All samples from the complex are low-K, tholeiitic Le Maitre (2002)Compiled by Rollinson (1993)

All samples from the complex are calcic 150 Ma = metaluminous 144 Ma = peraluminous Frost et al. (2001)

150 Ma = “normal” Sr/Y ratios 144 Ma = high Sr/Y ratios of 197 and and 419 Samples from the complex are predominantly magnesian. Two are ferroan. Frost et al. (2001)Drummond and Defant (1990)

144 MA dacite are low in Ni, Cr, and Mg#

Chondrite-normalized diagram for Hicks Butte complex samples. Normalization values are from McDonough and Sun (1995)

Conclusions: 1)Main phase of the Hicks Butte complex is Ma and Paleozoic xenocrystic zircons suggest that the complex is not exotic to the North American craton 2)144 Ma high Sr/Y dacite intrudes the Ma portion of the complex 3) Ma preliminary geochemistry suggests partial melting of a hydrous depleted mantle source in an arc setting 4)Two high Sr/Y 144 Ma dacites have geochemistry that is consistent with partial melting of an enriched mantle source in an arc setting with garnet as a residual phase. Possibly low oxygen activities?

Regional links: 1)Ca. 144 Ma high Sr/Y magmatism within the central Cascades is coeval to the Blue Mountains province 2)Ca. 144 Ma high Sr/Y magmatism within the central Cascades could be related to widespread orogenic thickening and partial melting that has been identified within the Klamath and Blue Mountains (Barnes et al., 1996, 2006; Schwartz et al., 2011) Discussions with Bob Miller and Joe Dragovich were of a great help. Josh Schwartz kindly provided a draft of his Lithos paper Funding was provided by Florida Gulf Coast University (MacDonald) and NSF (Pecha).