Structure and emplacement history of a multiple-center, cone-sheet-bearingring complex: The Zarza Intrusive Complex, Baja California, Mexico

The Cretaceous Zarza Intrusive Complex, located in the Peninsular Ranges of Baja California Norte, Mexico, is perhaps the best-preserved multiple-cent er, cone-sheet-bearing ring complex documented in North America. The 7 km(2 ) elliptical complex hosts three nested, non-concentric intrusive centers t hat are successively younger to the south, The northern and central centers show the same evolutionary sequence of (1) intrusion of concentric gabbroi c cone sheets, (2) intrusion of massive core gabbros, and (3) development o f subvertical, ductile ring faults. Ring-fault kinematics indicate that bot h centers moved down relative to the surrounding country rocks, suggesting collapse into an underlying magma chamber, The southern center is composed of approximately equal proportions of gabbro and tonalite and lacks cone sh eets. Aluminum-in-hornblende barometry on the tonalite indicates a maximum emplacement depth of 2.3 +/- 0.6 kbar. The Zarza Intrusive Complex is surro unded by a ductile deformation aureole, and bedding is inward dipping and i nward younging around the entire complex. Excellent preservation of the int rusive history allowed us to evaluate the origin of the aureole, and the th ree most applicable models are (1) collapse of the complex into its underly ing magma chamber, (2) sinking of the complex and its chamber after solidif ication, and (3) formation of the aureole prior to emplacement of the compl ex. The preserved structural and intrusive relationships provide informatio n on the dynamic evolution of subvolcanic magma chambers and suggest that t he complex may have been overlain by a caldera.