Origins of massif-type anorthosites and related rocks-isotopic evidence from the southern Nain Anorthosite Complex, Labrador

Conference · Tue Jan 01 04:00:00 EST 1985 · Geol. Soc. Am., Abstr. Programs; (United States)

OSTI ID:6273105

Simmons, E C; Synder, G A; Kalamarides, R I; Wiebe, R A

Leuconoritic rocks approaching magma compositions occur extensively throughout the southern Nain Anorthosite Complex, as fine-grained dikes and chill margins and even as coarser-grained samples from the intrusions themselves. These rocks are silica-saturated, with high A1203 (>19.5 WT%), low atomic Mg/Mg+Fe (<0.5), and have high total REE abundances with only small positive Eu anomalies. Sr-isotope data for these samples indicate a surprisingly large range of initial ratios at T=1.3 Ga: 0.7052-0.710; the Rb/Sr ratios for these samples are almost constant (approx. 0.03), resulting in a vertical array on a Rb/Sr isochron plot. Nd-isotope data for three of the samples, which cover the range of Sr-initial ratios yield element of values between -9.0 and -11.0; delta 180 values for two samples are 6.4 and 7.4. The low Mg/Mg+Fe ratios of these magmas indicate that they are not primary, mantle-derived melts; either these magmas resulted from extensive amounts of fractional crystallization of mantle-derived magmas, or they formed by partial melting of mafic/ultramafic material in the lower crust. The O-isotope data and large amounts of unsupported radiogenic Sr preclude the involvement of upper crustal materials in the generation of these magmas. The data are consistent with the involvement, either as a component in the source region, or as a contaminant, of material which had a low Sm/Nd and high Rb/Sr for a significant period of geologic time prior to partial melting, but a low Rb/Sr ratio at the time of partial melting.

• Research Organization: Colorado School of Mines, Golden (USA)
• OSTI ID: 6273105
• Report Number(s): CONF-8510489-
• Conference Information: Journal Name: Geol. Soc. Am., Abstr. Programs; (United States) Journal Volume: 17
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580400* -- Geochemistry-- (-1989)
ALKALI METAL ISOTOPES
ALKALINE EARTH ISOTOPES
ANORTHOSITES
CANADA
CHEMICAL COMPOSITION
CHEMISTRY
GABBROS
GEOCHEMISTRY
IGNEOUS ROCKS
ISLANDS
ISOTOPE RATIO
ISOTOPES
MAGMA
NEODYMIUM ISOTOPES
NEWFOUNDLAND
NORTH AMERICA
PETROGENESIS
PLUTONIC ROCKS
ROCKS
RUBIDIUM ISOTOPES
STRONTIUM ISOTOPES