PETROCHEMISTRY AND GEOCHRONOLOGY OF THE MESQUITE PASS SILL, SAN BERNARDINO COUNTY, CALIFORNIA

The middle Miocene Mesquite Pass sill lies along the southeastern margin of the Mesquite Range where it is up to 100 meters thick with an exposed strike length of 3 kilometers. The northerly extent of the sill is obscured by alluvial cover but a northwest striking high-angle fault can be projected to cut it. Faulting has obliquely cut out the sill to the south. In its thickest and best exposed portions it is internally unstrained. To the south, an attenuated extension of the sill less than a meter thick is interpreted to have participated in a dilatational event (Kingston Range detachment) after its intrusion. The abrupt thinning of the sill is marked by a series of en echelon, nearly vertical west-striking faults. These faults appear to be responsible for left-lateral separation of the thinned and disrupted rocks. The sill is generally concordant with west-dipping early Cambrian pelitic sedimentary host rocks, which lie in the footwall of the easternmost fault of the Mesozoic compressional system affecting the Mesquite Range. Where best exposed the sill displays large-scale layering which manifests itself in textural, color and chemical variations. The layers are variable in thickness but are broadly tens of meters thick. Columnar jointing is common normal to compositional layering. XRF whole-rock analysis of samples from the sill reveals a bimodal data population. Utilizing the LeBas (1986) classification, one group of samples plots as a trachydacite/trachyandesite (SiO₂=60-66%) while the second approximates a rhyolite (SiO₂>72%). Detailed analysis shows lithologic variation controlled by both stratigraphic position within the sill as well as location along strike. In the thickest portion of the sill, silica enrichment occurs near both the upper and lower contacts suggesting either fractionation or assimilation of silica-rich wallrock. In the attenuated portions of the sill, silica content increases to the south. Ar/Ar age determinations for the two different rock types gave statistically indistinguishable ages (12.80+/- 0.12 Ma for the rhyolite and 12.99+/-0.14 Ma for the trachydacite). Age dates and structural relationships constrain detachment within the Mesquite valley to post 12.8 Ma.