Reworked pre-Grenville crust and timing of Grenville orogenesis in the southeastern Llano Uplift, Texas: Results from U-Pb geochronometry

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

OSTI ID:5935889

Reese, J F; Roback, R C; Walker, N W ^[1]

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1. Univ. of Texas, Austin, TX (United States). Geological Sciences

Zircon geochronometry of metaigneous units in the southeastern Llano Uplift (1) show that 1355-1215 Ma rocks were tectonically reworked during the Grenville orogeny, (2) further constrain the timing of Grenville orogenesis along the southern margin of North America, and (3) necessitate revision of previously proposed stratigraphic and structural relations within the Llano Supergroup. Multigrain, abraded zircon fractions from a sample of quartzofeldspathic Valley Spring Gneiss (VSG) define a discordia trajectory with an upper intercept of 1355 [plus minus] 3 Ma, interpreted as the igneous crystallization age of the protolith. This age is the oldest thus far documented for Llano Supergroup protoliths and is substantially older than a recently reported age (1232 [plus minus]4 Ma) from a different part of the VSG. The lithologic and chronometric similarities of the old VSG to rocks to the Mid-continent Granite-Rhyolite terrane raise the possibility of genetic relations between these units. Thus components of the VSG are tentatively interpreted to represent a deformed and metamorphosed part of the southernmost Mid-Continent Granite-Rhyolite terrane. Alternatively, this rock could be part of an ensialic arc built upon the margin of southern North America and later deformed during the Grenville orogeny. Analyses from four abraded zircon fractions from a quartzofeldspathic gneiss in the structurally overlying Packsaddle Schist yield an upper intercept of 1215 [plus minus] 3 Ma, interpreted as the igneous protolith crystallization age. This age is the youngest yet determined for a polydeformed metamorphic rock in the Llano Uplift and more tightly brackets the timing of Grenville deformation in the southeastern Llano Uplift to post-1215 Ma and pre-1098 Ma.

• OSTI ID: 5935889
• 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
AGE ESTIMATION
CONTINENTAL CRUST
CRYSTALLIZATION
DEVELOPED COUNTRIES
EARTH CRUST
GEOLOGIC FORMATIONS
GEOLOGY
GROUND UPLIFT
LITHOLOGY
METAMORPHIC ROCKS
MINERALS
NORTH AMERICA
OROGENESIS
PETROLOGY
PHASE TRANSFORMATIONS
ROCKS
SILICATE MINERALS
STRATIGRAPHY
TECTONICS
TEXAS
USA
ZIRCON