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Contamination of massif anorthosite and Precambrian crustal evolution in central Labrador: A combined trace element and strontium, neodymium and lead isotopic study
Abstract
Nain craton tonalites show overall similarities to Uivak gneisses of northern Labrador, but are K$\sb2$O-poor and have variable LIL element contents--ascribed to variable degrees of Late Archean migmatization. Orthogneisses from the southern part of the Nain Province and from the Rae Province in the area studied show the greatest signs of LIL element depletion. Few mafic igneous rocks of Archean or Early Proterozoic age in this area appear to represent primary liquids. Many of these rocks are compositionally alkali basalts, high-Mg tholeiites, basaltic komatiites, or contain appreciable cumulus olivine. Most have nearly chondritic elemental ratios, although a few are more akin to those in modern-day MORB. Fine-grained metasedimentary rocks have geochemical signatures appropriate for their being derived from the quartzofeldspathic orthogneisses. Nd, Sr and Pb isotopic data were obtained on samples of troctolites, norites, anorthosites and plagioclase megacrysts from across the entire central portion of the exposed NPS. Primitive initial Nd and Sr isotopic ratios are preserved in olivine-bearing plutons and in a plagioclase megacryst from the western side of the massif. Significant variation in initial $\varepsilon\sb{\rm Nd}$ occurs across the NPS, from about $-$4 to $-$7 for magmas emplaced into the Rae Province (west), decreasing to $-$8 to $-$15 for magmas intrusive into Nain craton (east) and underscores the role of pre-existing lithosphere in controlling massif anorthosite isotopic compositional variation. This is strongly supported by preliminary Pb isotopic data. Mixing models between an estimated parent magma derived from a depleted source and specific samples of crust from country rock contacts reproduce the observed data, but require 30-60% contamination. A uniform level of most primitive (least contaminated) isotopic anorthosites from the western section and troctolites implies a 'ceiling' of $\varepsilon\sb{\rm Nd} \sim-$3, $\varepsilon\sb{\rm Sr} \sim$ +5 at 1.3 Ga. A model is proposed involving enrichment of a previously depleted lithospheric source at a minimum of 2.0 Ga, which is then inherited by the parental magmas to the anorthosite suite at 1.3 Ga. Magmas then assimilate either Archean or Proterozoic lower crust (depending on location). Calculated assimilation proportions are thereby reduced by approximately 15%. (Abstract shortened by UMI.)
Subject Area
Geochemistry|Geology
Recommended Citation
Hamilton, Michael Andrew, "Contamination of massif anorthosite and Precambrian crustal evolution in central Labrador: A combined trace element and strontium, neodymium and lead isotopic study" (1993). Doctoral Dissertations Available from Proquest. AAI9316658.
https://scholarworks.umass.edu/dissertations/AAI9316658