Karen L. Webber

Cooling rates and crystallization dynamics of shallow level pegmatite-aplite dikes, San Diego County, California

The George Ashley Block (GAB) is a rockslide block located in the Pala pegmatite district of Southern California. It is layered, asymmetric, pocket containing, and peraluminous. The GAB consists of quartz (42 vol%), Na-rich plagioclase (27%), potassium feldspar (24%), muscovite (7%), Mn-rich garnet (2%), biotite (1%), and a trace of tourmaline and gahnite. It contains only small amounts of the incompatible elements that characterize differentiated pegmatite bodies. P2O5, MnO, and F are present in amounts of <1 wt% each; B, Be, Ce, Li, Nb, Nd, and Th are <100 ppm each. More than 90% of the garnet grains in the GAB are zoned toward Mn-rich rims, and a symmetrical change in garnet-core composition occurs across the body. The mean X site contents for garnet (in at%) are 57% Fe, 40% Mn, 3.1% Mg, and 0.4% Ca. The Mn contents of garnet range from 30 to 55 at%; Fe contents vary inversely with Mn and range from about 66 to 43 at%. It is concluded that the bulk chemistry yields little information about fractionation, but the garnet, muscovite, and biotite mineral chemistry is more useful. There may have been two separate injections of magma to form the GAB. *E-mail: wkleck@lightspeed.net †Deceased: January 8, 1998. 0003-004X/99/0506–0695

Pegmatite genesis: state of the art

05.00 695 American Mineralogist, Volume 84, pages 695–707, 1999 FIGURE 1. Location map for the George Ashley Block pegmatite. KLECK AND FOORD: GAB PEGMATITE 696 tain” was used by Jahns and Wright (1951), but the name “Heriot Mountain” appears on the most recent Pechanga Quadrangle for the same geographic feature. This area is within the Pala pegmatite district, San Diego Co., California. The Pala Indian Tribe now controls most of Heriot Mountain and is acquiring (or has acquired) the George Ashley property. At present, access has been closed to these tribal properties. Preliminary, bulk-chemical compositions and a description of some line-rock features of the GAB were published by Webber et al. (1997). Kleck (1994, 1996) presented a brief description of the GAB and some preliminary chemical data. Simpson (1965) reported one analysis and a composition computed from modal mineralogy for pegmatites in the Ramona pegmatite district. Jahns and Tuttle (1963) presented normative mineralogy (but not the actual oxide analyses) for the Katerina dike, Pala district, California; Himalaya dike, Mesa Grande district, California; and the Upper Mack dike, Rincon district, California. Jahns and Tuttle (1963) also described some of the structures in the pegmatite bodies of Southern California pegmatite districts. Jahns and Wright (1951) described the Pala pegmatite district (which includes Heriot Mountain) and provided a basic description of many pegmatite bodies in this district. That work emphasized structures, petrology, and mineralogy; no analyses of the rocks and no systematic chemistry of the minerals were reported. Analyses of minerals from Southern California pegmatite bodies (mainly pocket minerals) were reported in several publications; representative references can be found in Jahns (1955) and in Foord et al. (1989).

A1: Lithium-Boron-Beryllium Gem Pegmatites, Oxford Co., Maine: Havey and Mount Mica Pegmatites

No one universally accepted model of pegmatite genesis has yet emerged that satisfactorily explains all the diverse features of granitic pegmatites. Genesis from residual melts derived from the crystallization of granitic plutons is favoured by most researchers. Incompatible components, fluxes, volatiles and rare elements, are enriched in the residual melts. The presence of fluxes and volatiles, which lower the crystallization temperature, decrease nucleation rates, melt polymerization and viscosity, and increase diffusion rates and solubility, are considered to be critical to the development of large crystals. A number of new concepts have shed light on problems related to pegmatite genesis. Cooling rates calculated from thermal cooling models demonstrate that shallow-level pegmatites cool radically more rapidly than previously believed. Rapid cooling rates for pegmatites represent a quantum shift from the widely held view that the large crystals found in pegmatites are the result of very slow rates of cooling and crystal growth. Experimental and field evidence both suggest that undercooling and disequilibrium crystallization dominate pegmatite crystallization. London’s constitutional zone refining model of pegmatite evolution involves disequilibrium crystallization from an undercooled, flux-bearing granitic melt. The melt is not necessarily flux–rich and the model does not require the presence of an aqueous vapor phase. Experimental studies of volatile- and flux-rich melts and fluid inclusion studies suggest that volatile-rich silicate melts may persist to temperatures well below 500 °C and even down to 350 °C. Studies of melt inclusions and fluid inclusions have led some researchers to suggest that the role of immiscible fluids must be considered in any model regarding pegmatite genesis. Fluid saturation is thought to occur early in the crystallization history of pegmatites. Two types of melt inclusions along with primary fluid inclusions have been found coexisting in pegmatite minerals. Advances by Petr Cerný in pegmatite classification are in wide use and the fractionation trends of Nb, Ta and other HFSE and K, Rb, Cs, Li, Ga and Tl are now well understood. How pegmatitic melts are produced, the types of source rocks involved and how melt generation relates to plate tectonic models are challenging areas for future investigations. Also, the roles of regional zoning, anatexis, and chemical quenching in pegmatite genesis are areas for future pegmatite research.

Memorial of Eugene Edward Foord, 1946-1998

Mt. Mica pegmatite is famous for gem tourmaline production for nearly 200 years. The dike, ranging in thickness from 1 to 8 meters and dipping 20° SE, has a simple zonal structure consisting of a wall zone and core zone. The wall zone is essentially devoid of K-feldspar. The outer portion of the pegmatite consists of quartz, muscovite, albite (An 1.8) and schorl. Muscovite is the dominant K-bearing species in the outer portion of the pegmatite. K-feldspar only appears in the core zone adjacent to pockets. The pegmatite is subparallel to the foliation of the enclosing migmatite, and leucosomes show a gradational contact with the pegmatite where juxtaposed. Texturally, the pegmatite and leucosomes appear to be in equilibrium with no change in grain size or composition where the two are in contact. Garnet-biotite thermometry of the migmatite at the contact yields an average temperature of 630°C, which is consistent with the P-T conditions inferred for a Sebago Migmatite Domain (SMD) assemblage of sillimanite, quartz, muscovite, biotite and alkali feldspar of 650°C and 3 kb. Gradational contact between leucosomes and pegmatite suggests that the pegmatitic melt was at the same temperature. Coromoto Minerals began mining in 2003 and the mine now extends down dip for over 100 meters to a depth of 33 meters. A very detailed and accurately surveyed geologic map produced by owner/operator Gary Freeman during mining shows the total area of pegmatite removed, the spatial distribution and aerial extent of pockets, massive lepidolite (compositions near trilithionite) pods, microcline, and xenoliths. The map was analyzed using image analysis and thickness values of the units to calculate the total volumes of pegmatite mined, lepidolite pods and all pockets found. Forty-five drill cores were taken across the pegmatite from the hanging wall to foot wall contacts along a transect intentionally avoiding lepidolite pods and miaroles. Cores were pulverized, thoroughly mixed and homogenized and the percent Li content calculated from the mapped volume was added to produce a sample that was representative of the bulk composition of Mt. Mica. The sample was then analyzed by fusion ICP spectroscopy for major and trace elements and DCP spectroscopy for B and Li. Structural water was determined by LOI. Water content was calculated using the calculated volume of open space (pocket volumes), assuming that the pockets were filled with water-rich fluid. This fluid content was added to LOI water (above 500°C) to estimate a maximum H2O content of 1.16 wt. % of the pegmatite melt. REE plots of bulk pegmatite vs. leucosomes from the migmatite are strikingly similar. Chondrite normalized REE patterns of leucosomes and pegmatite are very flat with no Eu anomaly, whereas Sebago granite is more strongly LREE-enriched and displays a pronounced negative Eu-anomaly. Spider diagrams of leucosomes and pegmatite vs. average crust show very similar patterns. These results suggest that the Mt. Mica pegmatitic melt did not form by fractional crystallization of the older Sebago pluton, but instead was derived directly from partial melting of the metapelitic rocks of the SMD. Batches of anatectic melt accumulated and coalesced into a larger volume that subsequently formed the pegmatite. This is the first chemical evidence presented for the formation of an LCT type pegmatite by direct anatexis.

The role of Diffusion-controlled Oscillatory Nucleation in the Formation of Line Rock in Pegmatite–plite Dikes

A chemical and paragenetic study was performed on gadolinite-group minerals occurring in miarolitic pink granite and granophyric leucogranite of the subvolcanic Hercynian plutons at Baveno and Cuasso al Monte, Southern Alps, Italy. In the localities investigated, gadolinite-group minerals are hosted in massive pegmatite, in aplite, and in miarolitic cavities having different degrees of evolution. The petrological relations indicate that progressive crystallization has occurred from magmatic through to hydrothermal conditions. At Baveno, Ce-rich gadolinite-(Y) (with ∑REE >Y) formed during the primitive stages of pegmatite crystallization. Gadolinite-(Y) (with SREE <Y) formed in pegmatites and granophyric aplites during primitive to moderately evolved stages of these dikes. Gadolinite-(Y) (with ∑REE <Y) and hingganite-(Y), which contains a significant amount of the datolite component, occur in miarolitic cavities together with several rare-element accessory phases. During the latest stages, datolite formed with zeolites. At Cuasso al Monte, gadolinite is found only in primitive to highly evolved miarolitic cavities. The cores of these crystals consist of Nd-rich gado- linite-(Y) (with ∑REE >Y). Gadolinite-(Y) (with ∑REE <Y) formed during intermediate stages of evolution, and hingganite-(Y) is dominant in highly evolved miarolitic cavities together with several rare-element phases. The chemical differences observed in the gadolinites from the two localities may indicate a different parental magma composition and reflect a difference in the crystallization processes. In contrast to Baveno, the crystallization at Cuasso al Monte occurred under open-system conditions, which prevented the formation of a zeolite (datolite-bearing) stage and generated a typical medium- to low-temperature hydrothermal mineral assemblage consisting of quartz, fluorite, barite, sulfides, and carbonates. The large variations in the Y/Dy ratio observed in the studied samples may be due to a change in the fluorine abundance in hydrothermal fluids related to paragenetic effects and mixing processes.