Abstract

Lead-alpha measurements for zircon and monazite from Woodson Mountain Granodiorite, Ramona and Cuyamaca Peak quadrangles, give ages of about 110 m.y. Bonsall Tonalite, Bonsall and Cuyamaca Peak quadrangles, gives ages of about 120 m.y. Green Valley Tonalite, Escondido and Cuyamaca Peak quadrangles, gives ages of 130 m.y. Prebatholithic volcanic rocks from the Santiago Peak and Escondido quadrangles give ages of about 150 m.y. Granite and granodierite from the Palm Desert quadrangle give ages of 110 and 105 m.y. A quartz diorite from the Superstition Hills, Plaster City quadrangle, indicates 155 m.y. Detrital zircon in the prebatholithic Bedford Canyon Formation, Corona South quadrangle, and Julian Schist, Santa Ysabel quadrangle, give composite ages of 750–1000 m.y. A metasedimentary migmatite gneiss from the Orocopia Mountains, Canyon Spring quadrangle, yields a zircon age of 2400 m.y.

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