THE ZONED RAMONA PLUTONIC COMPLEX, SAN DIEGO COUNTY, CALIFORNIA: A MID-CRUSTAL MAGMA CHAMBER BENEATH A CRETACEOUS STRATOVOLCANO?

The zoned Ramona plutonic complex (RPC) was first described in 1941 by Richard Merriam (Ramona ring-dike). The complex is about 130 km² and roughly elliptical with NE elongation. Internal contacts with surrounding plutons and lower amphibolite grade country rocks, as well as contacts between plutons, dip steeply. Magmatic foliation dipping 70° to 90° generally parallels the external shape of the complex and internal contacts between its plutons. Screens of metamorphic rocks within the RPC are concordant with the elliptical structure. The eastern part of the RPC intrudes a Late Jurassic-Early Cretaceous suture between eastern Jurassic S-type plutons and a western zone of Early Cretaceous I-type granitic and gabbroic plutons.

Compositional zoning of the RPC is irregular but reverse in sense: a mafic tonalite core (Las Bancas plutonic suite) is surrounded concentrically by plutons that are progressively less mafic outward (tonalites of Japatul Valley and Alpine suites; tonalite-granodiorite of La Posta suite). The outermost zone consists of a string of small gabbro bodies on the south and west sides and five small leucogranite plutons on the SE and east sides. The compositions and structural arrangement of plutons suggest that the RPC represents a large mid-crustal (~5-14 km) magma chamber into which batches of magma were emplaced over a period of at least ~9 m.y.

The RPC includes one of the westernmost “precursor” plutons of the La Posta plutonic suite, best known for the family of large plutons in the eastern zone of the Peninsular Ranges batholith. Western La Posta plutons are smaller, more mafic, have lower Sr/Y and ⁸⁷Sr/⁸⁶Sr (Sr_i) ratios, and are older than their eastern-zone counterparts. A distinctive feature of the RPC is the presence of La Posta pegmatite dike swarms on the south and west sides. Gem-bearing pegmatite dikes are hosted by an ~800-m zone of magma mingling between 101-Ma Las Bancas and Japatul Valley tonalite plutons, a conduit for late-stage La Posta magma.

Ridgetops in the southern RPC are capped by remnant fluvial deposits of exotic, auriferous Tertiary gravels of the Ballena River system. Enigmatic sedimentary deposits (QTf) consisting of massive sandstone with Poway-type metavolcanic and quartzite clasts occur on erosional surfaces at 1,200 to 2,400 ft elevation.