MAGMA MINGLING AND MAFIC ENCLAVES: STRUCTURAL RELATIONS WITHIN UPPER CRUSTAL PLUTONS IN THE PENINSULAR RANGES BATHOLITH, MESA GRANDE 7.5′ QUADRANGLE, NORTHERN SAN DIEGO COUNTY, CALIFORNIA

The California Geological Survey currently is mapping the Mesa Grande 7.5’ quadrangle in northern San Diego County as part of an ongoing effort with the USGS to produce seamless geologic maps of 7.5’ and 30’ x 60’ quadrangles in California. New mapping revealed spectacular exposures of agmatite, a field term for commingled fine-grained gabbroic dikes and felsic host rocks (Paterson et al., 2010; Dave Tucker, 2010).

In Carney Canyon, agmatite consists of felsic granitic rocks that contain sub-equal volumes of fine-grained gabbroic rocks, mostly as ribbon-like inclusions (mafic enclaves). The agmatite body measures about 5 km long and 1.5 km wide, and strikes NNE, approximately parallel to the strike of steeply dipping foliation and contacts in this area. It is bounded on the east by a pluton of Alpine tonalite and a lensoid body of gabbro. The western contact is with a pluton of Japatul Valley tonalite, mapped along the west side of Pamo Valley, a NNE-oriented fault-controlled valley that exhibits evidence of brittle and ductile deformation extending northward to the Elsinore Fault Zone.

The Alpine tonalite pluton and one or more gabbro plutons, including Black Mountain to the east, are cut by leucogranite dikes emplaced sub-parallel to the plutonic contacts. These dikes may emanate from the same source as numerous, small- to- medium-size leucogranite plutons that are spatially associated with the gabbro plutons east of Carney Canyon.

Possible parental magmas for the agmatite in Carney Canyon are the Alpine tonalite and the Cuyamaca Gabbro. If Alpine tonalite was the granitic parent, it may have undergone some degree of crystal fractionation to more silicic compositions. Alternatively, if the granitic parent was derived from the same magma source that produced the leucogranite dikes and plutons, the agmatite may have intruded a fault zone between tonalitic and gabbroic magmas. Smaller, but similar, bodies of agmatite in San Diego County have been attributed to mingling of approximately coeval granitic and gabbroic magmas (e.g., Todd and Hernandez, 2014).

The area is within the western zone of plutons in the Peninsular Ranges batholith that crystallized at pressures between 3 and 4 kb. Detailed mapping will improve our understanding of structures and emplacement histories of upper crustal plutons in the western zone of the PRB.