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

Webber, Karen L.; Simmons, William B.; Falster, Alexander U.; Foord, Eugene E.

doi:10.2138/am-1999-5-602

Cited by 135 publications

(53 citation statements)

References 13 publications

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“…In-situ crystallization of very small gahnite crystals records the instantaneous composition of the melt, which fast cooling conditions preserved. This is consistent with rapid growth (on the order of 10 −5 -10 −6 cm/s; Webber et al 1999) and rapid cooling rates inferred for aplite minerals in composite pegmatite-aplite dikes from San Diego County, California (Jahns and Tuttle 1963;Webber et al 1997Webber et al , 1999.…”

Section: Compositional Zoning In Gahnite and Melt Evolutionsupporting

confidence: 80%

Melt evolution in felsic dikes inferred from the composition of gahnite in two new occurrences, Pala Chief and Elizabeth R composite dikes, California

2016

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We report the occurrence and composition of gahnite ([Zn,Fe,Mg,Mn]Al 2 O 4 ) in aplites of the Pala Chief and Elizabeth R layered pegmatite-aplite dikes, Pala District, California, and the significance for felsic melt evolution. The aplite is characterized by alternating bands of white, finegrained plagioclase + quartz + muscovite +/− black tourmaline and thin laminations defined by concentrations of reddishbrown garnet (Bline rock^). Accessory gahnite occurs as extremely fine-grained (50-250 μm) green to bluish-green crystals. Gahnite compositions in both occurrences are similar and defined by end-member ranges Ghn 83.5-90.9 Hc 8.7-15 Spl 0-1.4 . Zinc contents in gahnite are high (36.7-39.5 wt.% ZnO for Pala Chief, 37.2-40.2 wt.% ZnO for Elizabeth R), the MgO and MnO contents are negligible (< 0.7 wt.%), and Fe is the main substitution present (< 6.9 wt.% FeO). Gahnite is chemically zoned and characterized by higher Zn (< 2 wt.% ZnO) and lower Fe, Mg, and Mn contents in rims compared to cores, which shows the substitution mechanism, and reflects fast growth during melt evolution via fractional crystallization followed by fast cooling. Compositional variations are greater within individual crystals than among crystals and between laminations, which indicate a homogeneous melt at the aplite scale. Compared with the composition of gahnite from pegmatites worldwide, the studied gahnite reflects a moderate degree of melt evolution. This study confirms the incompatible character of Zn in evolving pegmatite-aplite melts and shows the usefulness of gahnite as a petrogenetic indicator. Because gahnite is a resistant mineral, surficial findings of gahnite with very low Mg contents and Zn contents similar or higher than those measured here may indicate a source of granitic pegmatite-aplite.

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Section: Compositional Zoning In Gahnite and Melt Evolutionsupporting

confidence: 80%

Melt evolution in felsic dikes inferred from the composition of gahnite in two new occurrences, Pala Chief and Elizabeth R composite dikes, California

2016

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“…This texture is similar to that described from San Diego Co. California pegmatites that have been attributed to rapid crystal growth (Webber et al 1997(Webber et al , 1999. Schorl crystals also radiate around some pockets and the convergence direction of their long axes serves as a pocket indicator.…”

Section: Muscovite and Lepidolitesupporting

confidence: 78%

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

Simmons¹,

Falster²,

Webber³

et al. 2017

New England Intercollegiate Geological Conference 2017

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INTRODUCTIONThis fieldtrip will visit two world renowned gem-producing pegmatites, in the Oxford County pegmatite field of western Maine: Havey and Mount Mica pegmatites. This is intended to be primarily an instructional fieldtrip lead by pegmatite experts from the Maine Mineral and Gem Museum (MMGM) MP 2 (Mineralogy, Pegmatology, Petrology) research group. This is an opportunity to learn about the latest research and advances in the field of pegmatology in Maine. Mt. Mica and the Havey pegmatites are enriched in Be, B, and Li and over the last ten years have produced significant finds of gem tourmaline. The Havey produces a very consistent mint green elbaite with lesser amounts of pink and some rare blue gem elbaite. Mt. Mica has produced a huge quantity of gem elbaite in a wide range of colors. Some of the elbaite crystals recently mined from Mt. Mica are exceptionally large and many are fine museum-quality mineral specimens.This field trip guide is divided into the following four sections:1. Field trip summary with the leaders identified (A1- Maine (357595.99 m E, 4918691.40 m N). The field trip will begin with a brief discussion in the parking lot. We encourage carpooling to limit the number of vehicles entering the mines. A convoy of vehicles will travel approximately one hour to the Havey Quarry located off of Levine Road in Poland (395997.39 m E, 4880613.82 m N). Plan to spend approximately 2hrs at this location, starting with a discussion and later an opportunity to collect. Departing the Havey Quarry, the convoy will travel north approximately 40 minutes to the Mt. Mica Quarry located off of Mt. Mica road in Paris (382422.66 m E, 4902813.15 m N). Plan to spend approximately 2 hrs. at this site, which will begin with a discussion and end with an opportunity to collect on the dumps afterwards. There will not be an opportunity to go underground. At 4:00 pm, participants will leave the mine and travel north approximately 40 minutes back to Bethel. We encourage all participants to attend the Friday night reception at the Maine Mineral and Gem Museum. For this field trip, be prepared with a lunch as we will not be stopping along the way. There will be bathroom facilities available at each of the mine sites. Enrollment will be limited to 25 participants. Expect cold and unpredictable weather and dress accordingly. Please note: both quarries we are visiting are not open to the public except via previous arrangement. 1 A1-2 SIMMONS, FALSTER, WEBBER, FELCH, BRADLEY GENERAL GEOLOGY OF THE OXFORD COUNTY PEGMATITE FIELD IN MAINE REGIONAL GEOLOGYThe New England Appalachians formed during a prolonged Wilson Cycle between the breakup of the supercontinent Rodinia in the Neoproterozoic and the staged assembly of the supercontinent Pangea in the Phanerozoic. The pre-Silurian rocks mainly fall into two groups: those of Laurentian or peri-Laurentian affinity, toward the west, and those of peri-Gondwanan affinity, toward the east . These rocks were juxtaposed via mainly convergent plate motions, deformed, metamorphosed, a...

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“…The temperature range is based on estimates of the crystallization interval of Li-bearing pegmatites (e.g., Wyllie and Tuttle 1964;Vaughan 1964;Glyuk and Trufanova 1977;Stewart 1978;Martin 1983;Sirbescu and Nabelek 2003;Maneta and Baker 2014). The duration of the experiments was based on the fast diffusion of Li, which was estimated at 10 −11 m/s at 600 °C (Zhang et al 2010), and cooling models applied to the crystallization of pegmatitic dikes (Webber et al 1999).…”

Section: Experimental Conditions In the Piston-cylinder Apparatusmentioning

confidence: 99%

Evidence for lithium-aluminosilicate supersaturation of pegmatite-forming melts

Maneta

Baker

Minarik

2015

Contrib Mineral Petrol

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

Cited by 135 publications

References 13 publications

Melt evolution in felsic dikes inferred from the composition of gahnite in two new occurrences, Pala Chief and Elizabeth R composite dikes, California

Melt evolution in felsic dikes inferred from the composition of gahnite in two new occurrences, Pala Chief and Elizabeth R composite dikes, California

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

Evidence for lithium-aluminosilicate supersaturation of pegmatite-forming melts

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