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

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.