102nd Annual Meeting of the Cordilleran Section, GSA, 81st Annual Meeting of the Pacific Section, AAPG, and the Western Regional Meeting of the Alaska Section, SPE (8–10 May 2006)

Paper No. 12
Presentation Time: 1:00 PM-5:00 PM

MAGMA ASCENT MODEL FOR A LARGE DACITE DIKE, LITTLE CHUCKWALLA MOUNTAINS, CALIFORNIA


HAMANE, Angelique, Geological Sciences, California State University Los Angeles, 5151 State University Drive, Los Angeles, CA 90032 and MAYO, Dave, Department of Geological Sciences, California State University, Los Angeles, 5151 State University Drive, Los Angeles, CA 90032, angel_hamane@sbcglobal.net

The Little Chuckwalla Mountains (LCM) expose a 25 Ma volcanic and sedimentary sequence cut by a swarm of near-vertical, NW-striking hypabyssal dikes. The largest dike (porphyritic dacite > 50 m thick) is exposed over 2 km along strike. Regional deformation rotated the dike ~ 30º around a horizontal axis normal to the dike. Geochemical, textural, and mineralogical variations at three inferred paleodepths in the dike help establish a qualitative emplacement model.

Geochemical data reveal that the LCM dike is a high-K dacite that is enriched in light rare earth elements and depleted in heavy rare earth elements relative to MORB. The dike originated in a subduction-zone setting from a mantle source enriched in subcontinental lithosphere.

Groundmass plagioclase laths average 0.15 mm, 0.10 mm, and 0.05 mm in length at paleodepths of 1100, 300, and 0 m, respectively, reflecting slower cooling at depth. Hornblende phenocrysts at the 1100-m paleodepth indicate > 4.0 wt. % H2O, P=110-160 MPa, and T=750-910ºC. Reaction rims < 0.01 mm thick around hornblende phenocrysts in contact with the groundmass imply hornblende was outside of the hornblende stability field. Thin oxidation rims suggest a relatively fast ascent rate of 35-50 m/hr. Fayalitic olivine is present at 0 m paleodepth. Fayalite-rich olivine commonly occurs in volcanic and shallow intrusive felsic rocks characterized by anhydrous, high-temperature magmas with elevated Fe/(Fe+Mg) ratios. The LCM dike Fe/(Fe+Mg) values range from 0.70 to 0.80. Fayalite stability in the dike is attributed to a high Fe/(Fe+Mg) ratio and relatively low pressure conditions.

Geochemical analyses suggest a mantle source from a subduction zone setting. Petrographical analyses support a magma ascent model for the LCM dacite dike. Subduction of oceanic crust led to a H2O-rich environment in which hornblende was stable. Thin reaction rims indicate a relatively fast magma ascent rate. Fayalitic olivine at 0 m indicates the loss of magma H2O vapor through an open conduit system, which resulted in an effusive eruption.