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Description
A mafic complex forms the north peak and the southern ridge of McGinty Mountain which is located approximately 0.8 mile north of Jamul, San Diego County, California. The mafic rocks are in contact to the west with an unnamed xenolithic granodiorite and to the east with a unit of Woodson Mountain granodiorite which forms the southeastern ridge and a second peak of McGinty Mountain. A major fault trends along Beaver Hollow valley which is located northeast of McGinty Mountain and forms the eastern contact between Woodson Mountain granodiorite and a unit of Green Valley tonalite which underlies the floor of the valley. Two faults cut the gabbroic rocks, the northern fault separating a small northern protrusion from the main part of the gabbroic body and the other fault cutting across the mafic complex between the wide northern portion of the body and the narrow southern extension. Small gabbroic bodies are isolated to the east of the main mafic complex. Both the eastern and the western contacts of the main body dip under this body at angles varying from 22º to 81° and may converge inward at relatively shallow depths. The ultramafic rocks which grade from hypersthene-olivine peridotites to calcic plagioclase-olivine troctolites form a distinct zone of initial crystal settling within the gabbroic body. The mafic rocks consist predominantly of calcic plagioclase, diopsidic augite and olivine. Depending on the proportion of each constituent, the rock types range from anorthosites and gabbroic anorthosites to anorthositic gabbros, olivine gabbros and gabbros. Typically the contacts between rock types are intricate and gradational. Hornblende gabbro was found in a single locality where contacts between mafic rock types are sharp. In addition to the less calcic plagioclase and green hornblende, diopsidic augite is present as relic cores. Pegmatite and aplite dikes cut all batholithic units in the area. The gabbroic rocks are diked by Woodson Mountain granodiorite; whereas, the lamprophyre dikes, mafic pegmatite dikes and common banding structures in the gabbroic bodies are terminated at contacts. Small, isolated bodies of gabbro in the Woodson Mountain unit, gabbro adjacent to the contact with xenolithic granodiorite, and inclusions in the xenolithic unit may have been partially assimilated during emplacement of granitic plutons. Post-emplacement addition of fluids from the Woodson Mountain magmas extensively altered the pyroxene in the gabbros along portions of their contacts. Observed petrographic and structural features are compatible with emplacement of batholithic units from a combination of processes: (1) the introduction of a H2O- and CaAl-rich leucogabbroic magma into the lower crust, individual gabbroic plutons probably originating from a single mantle derivative but not intruding simultaneously; (2) partial melting, assimilation and anatexis of pelitic country rock to form quartz monzonitic magmas; (3) mixing of heated granitic magmas with both partially crystallized and solid gabbros to form hybrid batholithic rocks with intermediate compositions; and (4) diapiric movement of quartz-monzonitic and hybrid magmas, rafting up solidified gabbroic bodies to an ultimate emplacement higher in the crust.
