Ah. Fetter et Sa. Goldberg, AGE AND GEOCHEMICAL CHARACTERISTICS OF BIMODAL MAGMATISM IN THE NEOPROTEROZOIC GRANDFATHER MOUNTAIN RIFT BASIN, The Journal of geology, 103(3), 1995, pp. 313-326
The Grandfather Mountain Formation (GMF) is one of several remnants of
Neoproterozoic intracratonic rifting and associated magmatism in the
southern Appalachian Blue Ridge province. New U-Pb zircon ages and iso
topic data are presented to constrain the timing of rift basin formati
on and the source of concurrent magmatism. U-Pb data for three concord
ant zircon fractions from the stratigraphically highest metarhyolite w
ithin the GMF yield an age of 742 +/- 2 Ma (2 sigma). This age is inte
rpreted as the time of crystallization, dating an episode of volcanism
and sedimentation during the latter stages of rift basin development.
The GMF rests nonconformably on Mesoproterozoic basement and Brown Mo
untain metagranite. A U-Pb age of 765 +/- 7 Ma (2 sigma) for Brown Mou
ntain metagranite, based on five zircon fractions, places a maximum li
mit for the onset of rift basin formation. Volcanic rocks of the GMF h
ave initial epsilon(Nd) values ranging from -2.0 to 2.8 and Mesoproter
ozoic T-DM model ages of 1124 to 1392 Ma. Nd evolution trends for fels
ic volcanic and plutonic rocks of the Crossnore Complex in North Carol
ina and southern Virginia are similar to those for samples of Mesoprot
erozoic basement rock. Pb isotopic data for felsic volcanic and pluton
ic rocks indicate that Mesoproterozoic crust is a possible source of t
he magmatism, as Pb-206/Pb-204-Pb-207/Pb-204 values plot along a simil
ar crustal growth curve. The GMF belongs to an early stage of abortive
, Neoproterozoic rifting (c. 750 Ma) along the eastern margin of Laure
ntia. Early Paleozoic passive margin development, associated with the
development of Iapetus Ocean, cannot be directly related to Neoprotero
zoic volcanism and rifting in the Grandfather Mountain basin. We sugge
st that the c. 750 Ma rift sequences preserved along both margins of L
aurentia are similarly linked to the breakup of Rodinia and the breako
ut of Laurentia as a relatively discrete land mass.