GEORGE ASHLEY BLOCK PEGMATITE

The George Ashley Block pegmatite body (GAB for short) is a rockslide block from a dike located in the Pala Pegmatite District, northern San Diego Co., CA. It is pegmatite-aplite layered, peraluminous, and contains miarolitic cavities. The mineral composition (all minerals in volume %, (xx-xx = variation); note-values are from thin-sections which are a small comparative volume) is quartz-42(36-69)%, Na-rich plagioclase-27(16-48)%, K-feldspar-24(0-44)%, muscovite-7(0-15)%, Mn-Fe-rich garnet-2(<1-12)%, biotite-1(0-2)%, and a trace of tourmaline and gahnite (gahnite scattered; tourmaline only near the core).

Across the dike, SiO₂, Al₂O₃, Na₂O, and K₂O typically sum approximately 97 wt% (oxides in wt%). The less abundant major elements are CaO, 0.1 to 0.5; Fe₂O₃, 0.4 to 4.8; MgO, <0.1 to 0.2; P₂O_5, <0.3; MnO <0.1 to 3.8; TiO₂ <0.1; and F <0.1. The minor elements B, Be, Ce, Cu, Ga, La, Nd, Nb, Pb, Sc, Sr, Th, Y, Yb, and Zn are present and are <100 ppm each; Ba varies between 5 and 240 ppm and Li between 50 and 210 ppm. The variation in mineral content, rock-chemistry, and mineral composition across the dike is complex and remarkable.

The events leading to the study of the George Ashley Block pegmatite body began with a 1991 GSA field trip to the Pala District (led by Gene Foord) and later field trips to other pegmatites by Gene and I during the summer of 1992. Our thoughts were compatible, and we agreed to pursue a detailed study of a Pala pegmatite body. I would do the field work, sampling, and petrology; Gene would do the mineral and rock chemistry. After that, we would co-author a paper describing the results.

I found the GAB in 1993. It was an isolated, rockslide block about 3x8x10 m at the west base of Hariot Mt.; the top (8x10 m) was a cross-section of the parent pegmatite body. It was relatively fresh, typical of the Pala pegmatites, and could be correctly sampled. I sampled the block (20 samples) with a gas-powered, concrete saw. Splits were shipped to Gene (at the USGS analysis laboratory in Denver, CO) for mineral and rock analysis. I produced and worked on nineteen thin-sections. Through 1993-96 we worked on the samples, gathered data, and shared information. We did an initial draft of a paper in mid-1996. I did the final draft after Gene’s long illness and death in January, 1998 (Kleck and Foord, 1999).