ZONAL STRUCTURE OF LAYERED PEGMATITES: THIN VERSUS THICK DIKES

Subhorizontal pegmatites at the Little Three mine, Ramona, California, are typical of the miarolitic pegmatites in San Diego County and elsewhere. The dikes are thin (~ 2 m thick), with strike lengths on the order of km, and internally zoned in a layered alternation of textures and mineral assemblages. Based on a model of pegmatite emplacement at liquidus temperature (700°C) at depths of 7 km and a geothermal gradient of 20°C/km, these dikes would have cooled to 450°C, the lowest plausible solidus temperature, in ~ 18 days. In contrast, the subhorizontal and internally layered Tanco pegmatite at Bernic Lake, Manitoba, is a giant, with lateral dimensions of ~ 1.5 x 1.0 km, and an average thickness of ~ 40 m. The magma body that formed Tanco is well constrained to an emplacement depth of ~ 8 km. With a late Archean geotherm of 25°C/km, the Tanco body would have cooled to 450°C at its center in ~ 30 years. Despite this large difference in cooling rate, the thin and thick pegmatites have nearly identical internal features: the textures of each internal zone, the sequence of the internal zonation, and the volumetric proportions of zones are the same. From the bottom up, the pegmatites contain a thick footwall zone of mostly albite + quartz but with abundant graphic K-feldspar crystals that are elongate perpendicular to the contacts. Aplite, both massive and layered, and granitic (California) or sodic (Tanco) forms a drape over the wall zone assemblage. An intermediate zone comprised of feldspars, micas, and Li-aluminosilicates radiates upward from the aplite into massive quartz, which exhibits a prominent enrichment in beryl at the contacts between the quartz unit and the intermediate zone or aplite below it. Down from the hanging wall, a thin wall zone gives way to exceedingly coarse clusters of K-feldspar. These radiate downward as “pillars” that meet the lower intermediate zone beneath the quartz body. Downward crystallization of plagioclase- and Li-aluminosilicate assemblages forms “walls” between pillars, and hence isolates the most fractionated, remnant pools of melt in “cells”. As a result of this cellular structure, the coarse, pegmatitic upper intermediate zones appear to be discontinuous vertically and laterally, whereas the units that formed upward from the footwall are vertically and laterally continuous.