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Description
The Horsethief Canyon pluton is located approximately 50 km east of San Diego, California. The pluton intruded metamorphosed clastic and calc-silicate rocks and intruded against pre-existing gabbro plutons. The pluton exhibits an uncommon form of compositional zoning, referred to as reverse zoning. The pluton can be subdivided into three zones on the basis of petrographic analysis. A border zone of tonalite-quartz diorite, which is relatively enriched in modal quartz and hydrous ferromagnesian silicates, grades into a transitional of biotite-pyroxene quartz diorite. The transitional zone grades into an interior zone of quartz diorite and hornblende norite. The rocks from the interior of the pluton are relatively rich in anhydrous ferromagnesian silicates and relatively poor in quartz. Some samples from the interior of the pluton exhibit cumulus textures and contain two generations of clinopyroxenes. The major element variation patterns for rocks of the three zones, when compared with the chemical compositions of possible contaminant rocks, do not allow for an unambiguous choice between a crystal fractionation or a combined fractionation-assimilation origin for the rocks of the pluton. Trace element patterns are also ambiguous, although Rb variations suggest at least a partial assimilated component. Comparison of petrographic and microprobe data with experimental phase equilibria data suggests that a gradient in H2O content existed within the magma chamber during crystallization. A possible reaction, when quantified with microprobe data, will explain the textural and mineralogic variations observed petrographically. Two possible models will explain the observed mineralogic zoning and the inferred gradient of H2O content within the pluton. Both models involve initial clinopyroxene fractionation from a quartz tholeiitic or andesitic magma at 5 to 10 kb pressure. One model is based upon the development of concentration gradients within a stagnant melt layer above a convecting magma volume in a crystallizing magma chamber. Subsequent crystallization at 2 to 3 kb total pressure preserved the concentration gradients within the chamber. Later "auto-intrusion" of a lower cumulus crystal mush produced the inner mafic zone of the pluton. The alternative model involves the establishment of a chemical potential gradient in H2O between a relatively dry, homogeneous, convecting magma and the surrounding country rock. There is also the influx of silicic material by partial melting and mechanical mixing of country rock (metasediments). Fluxes of silicic material and H2O into the magma were aided by mass transport across a magmatic boundary layer next to the convecting magma cell. With time, convection ceased, and the residual magma chamber crystallized as a relatively dry inner core. "Auto-intrusion" of a cumulus crystal mush produced isolated mafic hornblende norite in the inner zone of the pluton.
