Petrogenesis of the cogenetic Stewart pegmatite-aplite, Pala, California: Regional implications

Abstract

The Stewart pegmatite-aplite dike in Pala, California (USA) is well known as a source of lithium, gem minerals, and unusual phosphate minerals. We reinterpret the petrogenesis of the dike based on a combination of new regional and detailed geochemical isotopic and textural data. The Stewart dike, like other pegmatites in the Pala district and other major pegmatite districts in the northern Peninsular Ranges batholith, is enclosed within gabbro/mafic tonalite. The 40 Ar/ 39 Ar method of dating on muscovite from the dike, and U/Pb dating of zircon from the gabbro yield essentially the same age. Initial 87 Sr/ 86 Sr is similar for the dike, 0.7042, and the gabbro, 0.7036-0.7037, indicating a juvenile and likely common source for both.The extreme mineralogic, lithologic, and textural variations within the dike are interpreted to have resulted from in situ mineral segregation, autometasomatism, and migration of volatiles within an essentially closed system. Contacts between the pegmatite dike and the host gabbro are diffuse.All previous interpretations of the Stewart pegmatite dike invoked an allogenic origin, formed by fluids derived externally from a nearby or distant granitic body, with the fluids subsequently migrating to, and intruding, their gabbro/mafic tonalite host. These previously proposed sources of pegmatite-forming fluid(s) are ruled out here on the basis of similar ages of the host and pegmatite, similar initial 87 Sr/ 86 Sr isotopes, unrealistic 25-40 km distances between any proposed potential source and the host gabbro-tonalite pluton, and mineralogic and textural relationships indicating internal differentiation. For these reasons, we suggest an autogenic origin, whereby the gem-bearing pegmatites are petrogenetically limited to their gabbroic hosts and likely represent late-stage, internal segregation of magmatic fluids. We note that nearly every gem-bearing pegmatite in the northern Peninsular Ranges batholith is hosted within gabbro/mafic tonalite bodies, not within the more silicic plutons.

Episodes of fast crystal growth in pegmatites

“…To explore crystal growth rates in natural systems, we conducted a case study of quartz crystals from the Stewart pegmatite in the Pala district of southern California, USA. The pegmatites in this district are Cretaceous-aged and occur as dikes hosted in gabbroic plutons of the same age 9,10 . The pegmatites are thought to represent late-stage fluids fractionated from a hydrous gabbroic pluton that then form a tabular, compositionally zoned pegmatite body 9 .…”

“…The pegmatites in this district are Cretaceous-aged and occur as dikes hosted in gabbroic plutons of the same age 9,10 . The pegmatites are thought to represent late-stage fluids fractionated from a hydrous gabbroic pluton that then form a tabular, compositionally zoned pegmatite body 9 . At its widest, the Stewart body is represented by a 50-m-wide dike with the outer layers composed of feldspar-rich granite (the upper and lower intermediate zones), a transitional perthite zone, and a coarse-grained internal core containing spodumene, lepidolite, heulandite, petalite, and albite ( Fig.…”

“…The contacts between the chimneys and the pegmatite body are sharp, suggesting the chimneys represent late-stage fluids that diked into a mostly solidified upper zone of the pegmatite body. These chimneys make up~50% by volume of the perthite zone and thus represent an important phase in the petrogenetic evolution of the pegmatite 9 . Based on conductive cooling models, Webber et al 6 suggest that the pegmatitic body cooled from 650°C to below 550°C within~9 years after emplacement.…”

“… a Schematic diagram of the Stewart pegmatite modified from Morton et al 9 . A 2 m tall person for scale on the lower left.…”

“…Therefore, k Ge can be estimated by constraining how Si partitions into quartz relative to a fluid, which can be constrained from Si solubility in the fluid. Because the quartz precipitated in miarolitic cavities, a water-dominated fluid is assumed rather than a hydrous silicate melt as the growth medium 8 , 9 , 38 , 39 . At 540 °C and 150–200 MPa 6 , 10 (5–7 km emplacement depth) and using the Si solubility data of Fournier and Potter 40 and Burnham et al 41 , we arrive at a k Si of ~200.…”