51 Pegmatites: Most Evolved Components of the Continental Crust

Wednesday, 23 March 2016
8:00 AM - 12:00 PM
Meeting Room 1

A CASE FOR THE PRODUCTION OF HIGHLY EVOLVED GRANITIC MELT BY ANATEXIS: PEGMATITES IN THE OXFORD PEGMATITE FIELD, MAINE.

Paper number: 51-2
Program: Theme Sessions
Time: 8:20 AM – 8:40 AM

Authors

Abstract

Most of the pegmatites in the Oxford Field are enriched in Lithium, Cesium, and to a much lesser extent Tantalum. They are genetically related to collisional tectonics that formed the late-Paleozoic Alleghenian Orogeny. Mt. Mica pegmatite occurs near Paris, Maine in migmatite of the Sebago Migmatite Domain (SMD), which now largely replaces the Sebago Pluton.

 Recent comparative mineralogical and geochemical studies of Mt. Mica pegmatite and leucosomes from the host migmatite (Simmons et al. 2013) lead us to propose that some of the pegmatites in the Oxford Pegmatite Field formed from melts derived by anatexis.

 Pegmatites are parallel to subparallel to the foliation of the migmatite and the leucosomes of the migmatite show gradational contact with pegmatite where they are 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. At Mt. Mica, garnet-biotite thermometry from sample pairs in the country rock at the contact yield a temperature range of 650-690°C (Clark et al., 2013), which is consistent with the P-T conditions inferred for an SMD assemblage of sillimanite, quartz, muscovite, biotite and alkali feldspar (650°C and 3 kb) by Guidry et al., (2013). Gradational contact between leucosomes and pegmatite suggests that the pegmatitic melt was at the same temperature.

 Plots of the chemistry of the bulk pegmatite (from a 45-core composite) vs. leucosomes from the migmatite are strikingly similar in REE content. Chondrite normalized REE patterns of leucosomes and pegmatite have very flat patterns with no Eu anomaly, whereas Sebago granite is more strongly LREE-enriched and displays a pronounced negative Eu-anomaly. Also, spider diagrams of leucosomes and pegmatite vs. average crust show very similar patterns.

 In conclusion, in contrast to suggestions of previous authors that Mt. Mica must be derived by fractional crystallization from the Sebago Granite pluton (or any other pluton), we believe the evidence suggests that the Mt. Mica pegmatitic melt could be derived directly from partial melting of the metapelitic rocks of the SMD.  Thus the pegmatites are likely results of anataxis and not formed from the Sebago pluton via fractional crystallization.

Geological Society of America Abstracts with Programs. Vol. 48, No. 1

doi: 10.1130/abs/2016NE-272580

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