Rubidium-strontium age determinations of muscovites and biotites from pegmatites of the Blue Ridge and Piedmont

Abstract

Eighteen mica specimens from pegmatites of the Blue Ridge and Piedmont provinces yield Rb/Sr apparent ages of 250 to 510 million years (m.y.). Muscovites from the Piedmont generally appear to be younger (avg. about 285 m.y.) than those from the Blue Ridge (avg. about 315 m.y.). Highly discordant 'ages' were found for coexisting biotite and muscovite from two Blue Ridge pegmatites; the biotite 'ages' are about 275 m.y. while the muscovites both yield 'ages' of 510 m.y. Loss of radiogenic Sr 87 from the biotite seems the most likely explanation of the differences; some of this may have been gained by the muscovite, thereby raising its 'age.' Some possible causes of discordance seem excluded by this study; these include (a) different crystallization times for biotite and muscovite, (b) preferential loss of rubidium from the muscovite, (c) original Sr 87 abundances in the pegmatitic fluids markedly higher than 7.02 per cent, and (d) assimilation by the crystallizing pegmatites of country rock with an anomalously high Sr •* abundance. In general, the study supports the previous suggestion by others that widespread metamorphism and pegmatite emplacement took place in the Blue Ridge in two periods, about 500 and 350 m.y. ago; however, the 'ages' found for the eight Piedmont samples range from 255 to 330 m.y. and do not support the idea of a single 250-m.y. event in this region.

Alpha-decay damage in minerals of the pyrochlore group

“…A Rb-Sr age of 289 m.y. was determined for the Rutherford 4+2 pegmatite by Deuser and Hertzog 1962. This is consistent with radiometric ages (250-330 m.y.)…”

“…This is consistent with radiometric ages (250-330 m.y.) of post-metamorphic pegmatites and plutons of the Piedmont province of the Southern Appalachians (Deuser and Hertzog 1962;Fullagar 1971;Wright et al 1975).…”

Implications for Thrust‐Related Shortening Punctuated by Extension From P‐T Paths and Geochronology of Garnet‐Bearing Schists, Southern (Çine) Menderes Massif, SW Turkey

“…The ratios of 208 Pb + /Th+ versus ThO 2 + /Th + for Th‐Pb geochronology and 206 Pb + /U + versus UO + /U + for U‐Pb dating form the calibration, with the slope and intercept of the curve controlled by the age of the standard grain. Although the Amelia has a reported range of ages from 279±14 Ma (Rb‐Sr age by Deuser & Herzog, ) to 250‐255 Ma (Kohn & Vervoort, ), we used the TIMS reference age of 274.6±0.6 Ma (Peterman et al, ) and avoided analyzing recrystallized areas. We separated the analyses based on the calibration in two sessions.…”

Ion microprobe 232Th-208Pb ages from high common Pb monazite, Morefield Mine, Amelia County, Virginia: Implications for Alleghanian tectonics

“…Radiometric dating methods applied to Morefield Mine pegmatite minerals yield a wide range of ages that differ by ϳ170 myrs (table 1) 270Ϯ13 Ma, and 261Ϯ13 Ma) has been interpreted to reflect a redistribution of Sr during metamorphism (Deuser and Herzog, 1962;Smerekanicz and Dudas, 1999). Mica and albite K-Ar ages obtained from the same locality are also complicated (Laughlin, 1966(Laughlin, , 1968Smerekanicz and Dudas, 1999); three muscovite ages average 248Ϯ4 Ma, whereas two albite ages are 416Ϯ21 Ma (blocky blue albite) and 297Ϯ9 Ma (moonstone variety) (Laughlin, 1966;Laughlin, 1968).…”