XENOLITH-RICH DIKES IN THE COMONDÚ ARC, BAHÍA CONCEPCIÓN PENINSULA, BAJA CALIFORNIA SUR, GULF OF CALIFORNIA: RESULTS FROM BAJA BASINS IRES

An Oligocene to Miocene section through the Comondú arc on the Bahía Concepción Peninsula in the Gulf of California exposes an unusual igneous dike system that contains abundant large xenoliths and xenocrysts. We present evidence that these were derived from the early Cretaceous granitoid basement as well as Oligocene to Miocene sedimentary and volcanic rocks that unconformably overlie the basement and lie structurally below the dikes. Field characteristics, petrography and SEM elemental map data from one of these dikes are compared with units sampled by the dike.

The dike crosscuts an anorthosite intrusion, has chilled margins, and its width and the size of the xenoliths decrease over the 35 m long exposure of the dike. Abundant, large xenoliths of granitoid basement (≤70 cm) and smaller volcanic rock fragments (<6 cm) are obvious in outcrop.

The granitoid basement consists of large quartz with zircon inclusions, potassium feldspar, plagioclase, large (≤ 2 mm) oxidized sphene, apatite, and a chloritized mafic mineral. This is disconformably overlain by the Oligocene Salto Formation¹ with sandstones and rhyolite ignimbrites that both contain small (<1 mm) broken volcanic quartz, pumice, silicic volcanic lithics, and biotite. The overlying Miocene Pelones Formation¹ consists of intermediate-composition tuff breccias and lapilli tuffs with plagioclase and clinopyroxene.

Petrography and SEM mapping shows that the dike contains rock fragments of the granitoid basement, and contains all of its components in disaggregated form. This disaggregation process is not yet understood. The dike contains small grains of broken volcanic quartz and biotite as well as large (≤ 8 mm) glassy pumice clasts derived from Salto Formation. The presence of glassy pumice clasts indicates that any alteration evident in xenoliths or xenocrysts must have occurred before the dike was emplaced. Plagioclase-clinopyroxene volcanic lithic fragments and free crystals were derived from the Pelones Formation. No xenoliths of the host anorthosite are present in the dike, although electron microprobe work is needed to determine whether any plagioclase was derived from the anorthosite, and to determine the composition of the fine-grained igneous groundmass of the dike.

¹McFall, 1968, Stanford Univ. Pubs. Geol. Sci., 10-5.