TEXTURAL AND CHEMICAL CONSTRAINTS ON THE ORIGIN OF EPIDOTE IN GRANITIC PEGMATITES

Although magmatic epidote may crystallize in tonalite and granodiorite at sufficiently high pressures (≥ 5 kb), it is less common in granite, presumably due to lower CaO. Epidote is apparently even rarer in granitic pegmatites, and where present is commonly interpreted as secondary. In order to evaluate the origin of epidote in granitic pegmatites, we investigated epidote samples from 7 localities: Golden Dam, AZ; Wright’s Quarry, MD; Lick Ridge, NC; Searchlight pegmatite district, NV; Rincon pegmatite district, CA; Strzegom, Poland; and Rizhao, China. Most of these pegmatites are known or inferred to have crystallized at relatively low pressure (<2 kb), based on the presence of miarolitic cavities. Based on hand sample characteristics, epidotes were characterized as either “pocket” or “fracture” related, with most pocket varieties interpreted to be magmatic. Evidence for the magmatic interpretation includes: 1) euhedral crystal shapes; 2) lack of alteration in adjacent feldspars; 3) textural relationships (e.g., intergrowths with quartz and feldspars); and 4) presence of oscillatory zoning. Microprobe analyses show a range in pistacite (Ps) content from Ps_12.7 to Ps_31.6, with considerable overlap between the types. Zoned crystals may have cores enriched in rare-earth elements (REE) with ΣREE ranging from 13.2 to 25.7 wt%. Such REE-rich grains are absent in the fracture-related types. Collectively, our observations and data are consistent with a magmatic origin for epidote in the pocket-related types, and its presence must be unrelated to pressure. We suggest, alternatively, that bulk composition is the major factor controlling epidote crystallization in these pegmatites. One possibility in this regard is that these pegmatites were relatively poor in P₂O₅ and/or F, such that minimal apatite (a major Ca-bearing phase in many pegmatites) or fluorite formed. Any available Ca could then be available to form epidote.