Isotopic, major and trace element constraints on the sources of granites in an 1800-Ma-old igneous complex near St. Cloud, Minnesota

Thesis/Dissertation · Thu Jan 01 04:00:00 EST 1987

OSTI ID:6019527

Spencer, K J

A suite of basic to granitic rocks was emplaced near St. Cloud, MN about 1800 Ma ago. These are strongly LREE enriched and were derived from LREE enriched sources. Nd-Sm systematics suggest that LREE enrichment occurred during the Early Proterozoic. Initial Pb ratios for basic rocks through granites are similar to inferred 1800 Ma old mantle Pb, and suggest sources that became U/Pb enriched during the Proterozoic or latest Archean. These sources had long-term Th/U ratios similar to inferred mantle or average crustal values. Oxygen isotopes indicate that granitic rocks incorporated a small previous crustal component. Petrogenetic modeling suggest these processes: Granodiorite represents an evolved high Mg-andesite or a partial melt of a basic precursor distinct from the nearby basic rocks. Granites and quartz monzonites were derived at temperatures of ca. 800-950 (e.g. water-undersaturated) and at 15 to 20 km depth. The sources are inferred to have been quartz saturated with respect to tholeiite. Because granites have similar isotopic histories as basic, sources of granites were ultimately derived from the mantle. REE and Nd isotope systematics allow these scenarios: (1) 40-75% of the Nd in the rocks was derived from continental crust recycled into the mantle. (2) The mantle source had a chondritic REE geochemistry but was enriched in LREE shortly before melting. (3) The rocks were in fact derived by melting a basic lower crust that included a small fraction of intercalated sedimentary rocks.

• Research Organization: State Univ. of New York, Stony Brook (USA)
• OSTI ID: 6019527
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580400* -- Geochemistry-- (-1989)
ACTINIDES
CHEMISTRY
EARTH CRUST
EARTH MANTLE
ELEMENTS
ENRICHMENT
FEDERAL REGION V
GEOCHEMISTRY
GEOLOGIC DEPOSITS
GEOLOGIC MODELS
GRANITES
IGNEOUS ROCKS
INTRUSION
ISOTOPE RATIO
ISOTOPES
LEAD
METALS
MINNESOTA
NEODYMIUM
NORTH AMERICA
OXYGEN ISOTOPES
PLUTONIC ROCKS
QUANTITY RATIO
RARE EARTHS
ROCKS
SAMARIUM
TECTONICS
THORIUM
URANIUM
USA