Possible relationship of the Iron Mountain metavolcanic assemblage to the El Cajon Mtn ring complex, San Diego Country, California: Justin Kerl, B.S. Candidate, Department of Geological Sciences, San Diego State University - The Cretaceous Santiago Peak Volcanics are exposed along the western edge of the Peninsular Ranges batholith(PRB) and are interpreted as the volcanic cover of the batholitic intrusions. Ring dikes are also a common feature in the western PRB and were first described almost 70 years ago (Merriam, 1941). Ring dikes and ring complexes are interpreted as subvolcanic feeder systems that represent the transition from the plutonic realm to the volcanic realm in magmatic arcs (Johnston et al., 2002). However, there has been very little work on the petrology of the ring complexes in the PRB, and in particular how they might link the intrusive and extrusive history of the magmatic arc.
This study is focused on the El Cajon Mountain ring complex which was mapped by Todd (2004) and represents the possible roots of a central-vent volcano. The hypothesis tested in this study is that the El Cajon ring complex is the root of a volcano that fed surface volcanic deposits of the nearby Santiago Peak Volcanics at Iron Mountain. The Iron Mountain metavolcanics occur as a strongly foliated and steeply dipping screen completely surrounded by batholith wallrock. Zircon U-Pb ages from the ring dikes and metavolcanic rocks yield similar ages of ~120 Ma. Here, I present whole rock major and trace element concentration data from the dikes and volcanics to test if they could be closely related to one another as indicated by the similarity of the zircon ages.
Ten samples of Iron Mountain metavolcanic rocks and two samples of El Cajon Mountain ring dikes were analyzed in this study. Whole rock chemical analyses were obtained in the SDSU Geological Sciences laboratory by X-ray fluorescence analysis. Nine of the ten Iron Mountain samples are high silica rocks with SiO2 contents averaging 72.9±2.7 wt% (1 sigma sd) corresponding to a rhyolite composition for the volcanics. The two El Cajon Mountain ring dikes similarly yield high SiO2 contents of 74.5 and 77.7 wt%. A plot of SiO2 verse K2O plot indicates the rock are arc tholeiites or calc-alkaline in character consistent with a volcanic arc origin. A plot of immobile trace elements (Nb/Y vs. Zr/TiO2) also supports a possible close relationship of the metavolcanic rocks and ring dikes.
Johnson, S.E., Schmidt, K.L., and Tate, M.C., 2002, Ring complexes in the Peninsular Ranges Batholith, Mexico and the USA: magma plumbing systems in the middle and upper crust: Lithos, v. 61, p. 187- 208. Merriam, R., 1941, A Southern California ring-dike: American Journal of Science, v. 239, p. 365-371.
Todd, 2004, Preliminary Geologic Map of the El Cajon 30´ x 60´ Quadrangle, Southern California, USGS Open-File Report 2004-1361, Detailed Description of Map Units, version 1.0.