Crustal evolution of Grenville terranes in the central and southern Appalachians: The Pb isotope perspective for Grenville tectonics

Conference · Wed Jan 01 04:00:00 EST 1992 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5689524

Parks, J E; Jenks, P J; Sinha, A K ^[1]

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1. Virginia Polytechnic Inst. and State Univ., Blacksburg, VA (United States). Dept. of Geological Sciences

Grenville age rocks are recognized in eastern North America from Newfoundland to Georgia and are part of a globe encircling belt. Whole rock Pb-U-Th isotope systematics permit recognition of distinct lead isotopic reservoirs that may reflect juxtaposition of different crustal blocks. Pb isotopic analyses of over 80 samples of basement rocks from PA to GA define at least two distinct groups. Group 1 includes rocks from VA, Md, and Pa, while Group 2 includes rocks from GA and NC. Initial Pb isotopic compositions of the basement rocks of the Virginia Blue Ridge (of Group 1) are used as a reference for comparing all other isotopic ratios. These reference isotopic values are Pb-206/Pb-204: 16.63--18.64, Pb-207/Pb-204: 15.38--15.58, Pb-208/Pb-204: 33.52--39.51. The isotopic deviation in percent (Delta 207/204, Delta 208/204) show a remarkable separation of the basement massifs into separate U-Th-Pb provinces. For example, Delta 207/204 and Delta 208/204 values for other Group 1 rocks range from [minus]4.4 to 3.6 and [minus]30.9 to 11.1 respectively. Rocks of Group 2 have a higher percent deviation from the reference. For example, the Carvers Gap Gneiss of NC has a Delta 207/204 range of 4.63--13.25 and a Delta 208/204 range of 91.78--414.78. Limited published lead isotopic data for other Grenville age basement massifs from Newfoundland to Texas have similarities and differences in terms of their time-integrated Th/U and U/Pb relative to the reference values presented here. The use of Pb isotopes to determine source region characteristics for crustal provinces has far reaching implications. Differences between juvenile crust and reworked older crust, leading to similarities or differences in reservoirs can be used as a method for determining the accretionary history of distinct terranes during the Grenville Orogeny.

• OSTI ID: 5689524
• Report Number(s): CONF-921058--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 24:7
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
ACTINIDES
AGE ESTIMATION
APPALACHIAN MOUNTAINS
BASEMENT ROCK
CHEMISTRY
CONTINENTAL CRUST
EARTH CRUST
ELEMENTS
GEOCHEMISTRY
GEOLOGIC FORMATIONS
ISOTOPE DATING
ISOTOPE RATIO
LEAD
METALS
MOUNTAINS
ORIGIN
TECTONICS
THORIUM
URANIUM