U-Pb zircon and monazite ages of Middle Proterozoic rocks, northern Blue Ridge, Virginia

Conference · Mon Mar 01 04:00:00 EST 1993 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5722986

Aleinikoff, J N; Walter, M ^[1]; Lyttle, P T; Burton, W C; Leo, G W; Nelson, A E; Schindler, J S; Southworth, C S ^[2]

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1. Geological Survey, Denver, CO (United States)
2. Geological Survey, Reston, VA (United States)

New geologic mapping of bedrock in the northern Blue Ride of Virginia and Maryland has resulted in the identification of at least ten different Middle Proterozoic granitic gneisses. The authors new U-Pb zircon data on eleven samples indicate crystallization ages ranging from about 1,055 to 1,145 Ma. The results are grouped as follows: Group 1--biotite granite gneiss, porphyroblastic granite gneiss, pink leucogranite gneiss, white leucogranite gneiss, and garnetiferous leucogranite gneiss; Group 2--biotite granite gneiss, hornblende monzogranite gneiss, and coarse-grained granitic gneiss; and, Group 3--porphyroblastic granite gneiss. Isotopic data from multigrain fractions and single zircons from a leucocratic layered gneiss are scattered, with Pb/Pb ages ranging from 1,092 to 1,139 Ma; these results support field evidence suggesting that the protolith of the layered gneiss was a composite of porphyroblastic gneiss and younger granite(s) that was highly sheared and homogenized. The geochronology agrees with structural interpretations and limited cross-cutting relations; rocks in Groups 2 and 3 are much more strongly deformed than rocks of Group 1. The name Marshall Metagranite is retained, but restricted to biotite and coarse-grained gneisses of Group 2, which resemble gneisses in the type area. U-Pb ages of monazites from several samples have a wide spread (1,033 to 1,106 Ma). In all samples but one, monazites are younger than coexisting zircons and are coeval with the next younger intrusive event. These data suggest that several of the monazite dates are not cooling ages but represent the times of metamorphic growth during subsequent intrusive events. Because the monazite ages have not been reset to a single date, the authors conclude that the regional granulite metamorphism never exceeded about 700C after 1,106 Ma in this part of the Grenville terrane.

• OSTI ID: 5722986
• Report Number(s): CONF-9303211--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 25:2
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
ACTINIDE ISOTOPES
AGE ESTIMATION
DEVELOPED COUNTRIES
GEOLOGIC AGES
GEOLOGIC FORMATIONS
GEOLOGIC HISTORY
GEOLOGY
GEOTHERMOMETRY
GNEISSES
ISOTOPE DATING
ISOTOPES
LEAD ISOTOPES
MATERIALS
METAMORPHIC ROCKS
METAMORPHISM
MINERALS
MONAZITES
NORTH AMERICA
PETROGENESIS
PETROLOGY
PHOSPHATE MINERALS
PRECAMBRIAN ERA
RADIOACTIVE MATERIALS
RADIOACTIVE MINERALS
RARE EARTH COMPOUNDS
ROCKS
SILICATE MINERALS
THORIUM MINERALS
URANIUM ISOTOPES
USA
VIRGINIA
ZIRCON