MINERALOGY, INTERNAL ZONATION, AND THERMAL HISTORY OF THE LITTLE THREE PEGMATITE, RAMONA, CALIFORNIA

The Little Three pegmatite dike, Ramona, California is an asymmetrically zoned, composite pegmatite-aplite body typical of the subhorizontal miarolitic pegmatite class. The dike consists of sodic plagioclase (Pl), alkali feldspar (Kfs), quartz (Qtz), minor tourmaline (Tur), plus accessory muscovite (Ms) and garnet (Gt). A point count on a complete cross section of the dike (1,392 points on a 1 cm² grid) yielded a modal composition (vol%) of Pl₃₀Kfs₃₂Qtz₃₁Tur₅Ms_<2Gt_<1, and an aluminum saturation index (ASI) of 1.28. Modal abundances of Kfs and Pl vary inversely across the dike, but Pl and Tur co-vary as do Kfs and Ms. The modal variation of Qtz is smaller than that of the feldspars.

Two-feldspar (Fsp) geothermometry was performed for logical Kfs – Pl pairs, including integrated cryptoperthite and mesoperthite Kfs compositions, using published Margules models for the ternary feldspar solvus. Though extrapolated far from the conditions at which the algorithms were calibrated (700°-900°C), a large number of Fsp pairs converge for all components via one or several Margules models at a compositional uncertainty of 0.02 mole fractions of components. The resultant temperature range for the aplite is 416°-353°C, and that of the overlying pegmatite/graphic granite is 455°-355°C.

Textures and zonation of Kfs and Pl were examined by electron microprobe, including EM-cathodoluminescence (CL) imaging. Blue-dominant CL intensity for the feldspars apparently arises from structural strain and/or defects. CL variability in Kfs from the aplite corresponds to the distribution of tartan twinning, but some Kfs in the aplite also exhibits hourglass sector zoning. CL in Pl reveals systematic growth zonation that mostly correlates with compositional zonation from An₇ (bright cores) to ~ An₁ (dark rims). The darkest CL zone of Pl occurs along the latest edges of crystals in the direction of growth from margin to center of the dike. In total, the CL and textural features from feldspars lead us to conclude that the highest temperatures obtained (425°-455°C) reflect primary crystallization temperatures for this dike, whereas the lower temperatures (< 400°C) likely record recrystallization events.