U/Pb zircon ages of basement gneisses and discordant felsic dykes from Vestranden, westernmost Baltic Shield and central Norwegian Caledonides
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Baltic Shield
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Summary Most of the granitic sheets and veins in the Loch Coire migmatite complex do not have chemical compositions in accordance with a formation by local anatexis. Regional metasomatism by fluids from outside the exposed confines of the complex was an important factor in the development of the sheets and veins, although there was probably some overlap with magmatic conditions.
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Abstract The Mt. Mica pegmatite is famous for producing gem tourmaline for nearly 200 years. The dike, ranging in thickness from 1 to 8 m 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. Potassium 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 pressure-temperature conditions inferred for a Sebago Migmatite Domain (SMD) assemblage of sillimanite, quartz, muscovite, biotite, and alkali feldspar formed at 650 °C and 3 kb. Gradational contact between the leucosomes and pegmatite suggests that the pegmatitic melt was at the same temperature. Coromoto Minerals LLC began mining in 2003 and the mine now extends down-dip for over 100 m to a depth of 33 m. 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 H 2 O content of 1.16 wt.% of the pegmatite melt. REE plots of bulk pegmatite versus leucosomes from the migmatite are strikingly similar. Chondrite-normalized REE patterns of leucosomes and pegmatite are very flat with no Eu anomaly, whereas the Sebago granite is more strongly LREE-enriched and displays a pronounced negative Eu anomaly. Spider diagrams of leucosomes and pegmatite versus 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.
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ABSTRACT The current classification of granitic pegmatites, originally introduced by Černý (1991a), has been the accepted system for grouping pegmatites of diverse mineralogy and chemistry for nearly three decades. Despite its general acceptance, several issues have been highlighted (Müller et al. 2022) which have imposed some limitations on its use and therefore necessitated the need to reevaluate its methodology. A new classification for granitic pegmatites is proposed in an attempt to be more inclusive of pegmatite types omitted in previous classification schemes. The new approach utilizes a more comprehensive suite of accessory minerals and defines three pegmatite groups which are genetically related to granite plutons and the anatexis of metaigneous and metasedimentary protoliths. Pegmatites belonging to Groups 1 and 2 are generated from the residual melts of S-, A-, and I-type granite magmatism (RGM) as well as being direct products of anatexis (DPA), whereas Group 3 pegmatites are only derived by anatexis.
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