Carboniferous Island Arc And Associated Rocks From The Mision Calamaju Area Baja California Mexico

Approximately 6,100 m of metamorphosed volcanic and sedimentary rocks are exposed near Mision Calamajue (latitude 29° 25' longitude 114° 15'). Lower greenschist metamorphism and shear cleavage parallel to axial plane folation is pervasive in the area. The lowermost 1,000 m represent a coarsening-upward sequence of thin-bedded to massive silty claystone interlayered with limestone, pebble conglomerate and bedded chert and capped by a coarse-grained chert litharenite. Conodonts recovered from near the top of this underlying unit include Gnathodus bilineatus (Roundy) of Early Chesterian age. The overlying 2,300 m consist of intercalated basalt, hornblende andesite and dacite submarine flows, overlain by 400 m of dacite to rhyolite tuff, volcaniclastic sediments, pebbly mudstone, pebble conglomerate and metaquartzite boulder conglomerate. These rocks represent an island-arc system. Overlying the volcanic sequence is 2,000 m of very thinly-bedded clayey siltstone containing widely spaced intervals of limestone. Conodont fragments recovered from this overlying unit limit the upper age of the volcanic strata to Carboniferous. A Pb/Pb minimum age of 262 Ma was provided by the analysis of U/Pb from one discordant zircon subfraction. This is consistent with the Carboniferous age provided by the conodonts. A highly sheared, 400 m thick basic volcanic or hypabyssal unit interfingers with this 2,000 m thick thin-bedded clayey siltstone and represents a mylonitized zone. The study area of Mision Calamajue may be a southern extension of the Carboniferous inner-arc basin between the Antler Orogenic Highland and the Klamath-Northern Sierra Nevada island-arc. Palinspastic reconstruction along the Mojave-Sonora Megashear(s) may juxtapose the Mision Calamajue area with other island-arc sequences in the western United States.