Petrology of the Paloma Valley ring complex, southern California batholith

Abstract

The Paloma Valley ring complex is one of the numerous plutons that make up the Cretaceous southern California batholith. The complex is composite, consisting of (1) an older, single ring dike and two subsidiary short-arced inner dikes, and (2) a younger set of thin short-arced dikes largely inside the older ring dike. The older ring dike, composed of granodiorite-quartz monzonite, has nearly vertical walls and is elliptical in plan; its long axis (14 km) is oriented west-northwest. It was emplaced in and contains numerous fragments of gabbro. The more than 200 short-arced granitic pegmatite dikes, mainly ranging from 0.2 to 1 m in thickness, define a domal ring dike set with moderately to steeply dipping outer dikes and nearly horizontal inner dikes. The younger dikes cut both the older ring dike and the gabbro. Spatially associated with the younger dikes are bodies of fine-grained granophyre that contain stringers of granitic pegmatite. The granophyre has an Mg content similar to that of the younger ring-dike rock, but contains less K and more Fe. The older ring dike is interpreted to have been magmatically emplaced in an elliptical zone of ring fracturing in gabbro; the magma made room by stoping the denser gabbro with little assimilation. Upon a release of pressure, a set of domal fractures formed, along which volatile-rich magma was emplaced, forming the younger ring dikes. Granophyre resulted from pressure-quenching through a loss of volatiles. Residual volatiles, or volatiles that were introduced later, recrystallized parts of the granophyre and caused the formation of pegmatite stringers.