51 Pegmatites: Most Evolved Components of the Continental Crust

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

DIRECT DATING OF PEGMATITES BY LA-MC-ICP-MS U/PB COLUMBITE GEOCHRONOLOGY

Paper number: 51-6
Program: Theme Sessions
Time: 9:40 AM – 10:00 AM

Authors

Abstract

Rare element-bearing pegmatites enriched in U, Th, Zr, Hf, Nb, Be, Y, and Bi occur in southern New York emplaced in the Bedford gneiss (Kinkel-K, Bailys-B) and outcrops of the Manhattan Schist (Fort George-FG, Harlem River drive-H) of Lower Paleozoic age. Zircon from these rocks is generally metamict (“cyrtolite”) and thus not suitable for U/Pb dating, but some pegmatites contain columbite-(Fe) (K, B) or columbite-(Mn) (FG, H) that were investigated for geochronology by LA-MC-ICP-MS. The results we obtained are similar to the TIMS U/Pb zircon geochronology on LCT pegmatites from Connecticut and Maine (Bradley et al. 2013).

Columbite-(Fe) from K is characterized by low Ta (0.10) and Mn (0.29) apfu, contains 578-657 ppm U and displays U/Th ratio of 110-123. Three crystals yielded a Concordia age of 371±6.8, 372±5.5, and 382.6±5.8 Ma. Columbite-(Fe) from B displays higher Ta (0.14), Mn (0.43) apfu, U (765-834 ppm) and U/Th ratio (177-185). It is younger than K with a Concordia age of 271.8±5.4 Ma. The difference in age between these mineralogically similar pegmatites despite their proximity does not imply affiliation, but the reoccurrence of P-T-X conditions that favor the formation of the rare element pegmatites.

Columbite-(Mn) from FG displays Ta (0.22), Mn (0.66) apfu, U (494-510 ppm), and (77-87) U/Th ratio and its Concordia age is 371.7±7.6 Ma. Columbite-(Mn) from H displays Ta and Mn at the same level with FG, contains 105-272 ppm U and U/Th ratio is 9-51. The Concordia age is 380±13 Ma.

The oldest pegmatites, 382.6±5.8 Ma and 371.7±7.6 Ma, were intruded during the Neoacadian igneous event in northern Appalachians; the younger pegmatite, 271.8±5.4 Ma, was probably emplaced after the cessation of the collision of Laurentia-Africa during the Alleghanian orogeny. At this time the crust relaxed and fluids from the mantle made the generation of partial melts possible. Surprisingly pegmatites temporally associated with the Taconic orogeny were not found. This work further validates the use of columbite as a useful geochronometer and development of a matrix-matched standard is currently in progress.

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

doi: 10.1130/abs/2016NE-272105

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