Chemical characteristics of the Nain Anorthosites and their parent magmas

• Main Author: Xue, Suizhou
• Format: Text
• Language: English
• Published: ScholarWorks@UMass Amherst 1992
• Subjects: Geochemistry|Geology|Mineralogy; Nain
• Online Access: https://scholarworks.umass.edu/dissertations/AAI9219515

The origin of massif anorthosite is related to Precambrian crustal evolution. The chemical composition of anorthosite is fundamental to an understanding of the geochemical evolution of the earth in the Proterozoic Era. Chemical studies of Proterozoic anorthosite massifs in the Nain Plutonic Suite, Labrador, delineate their composition ranges and variation, and illustrate the variety of magmas and their source characteristics. Three massifs along Tikkoatokhakh Bay (TIK) are mainly noritic anorthosites and show no mineralogical or chemical differences. The Kikkertavak (KIK) and Port Manvers Run (PMR) intrusions are mainly troctolitic anorthosites and contain more mafics than the TIK bodies. Chemically, the TIK bodies can definitely be distinguished from those at KIK and PMR, the former containing a low An content in plagioclase, high alkalies and LIL in rock. The KIK rocks are similar to the PMR ones in most cases, except for lower Rb/Sr and higher K/Rb ratios in the PMR. Based on petrographic and major element data, three different magma types exist in the study area. The sodic noritic magma accumulated plagioclase in TIK. The troctolitic magmas in KIK and PMR are similar to known troctolitic bodies such as the Kiglapait Intrusion, the Hettasch Intrusion and the intrusion at Paul Island, and their feldspar rich cumulates. A moderate clinopyroxene component in the PMR magma suggests that the PMR magma could be gabbroic-troctolitic. The inversion from plagioclase to melt composition via D$\sp{\rm PL/L}$ was used to calculate REE and trace element concentrations in the parental magmas of the Nain anorthosites. The estimated magmas, noritic in TIK, troctolitic in KIK and gabbroic-troctolitic in PMR, are consistent with those indicated from the major element data. The magmas show that their chemical characters seem to have mostly Plume-MORB nature. High Ba, Sr, Ti and low Rb, Nb, Zr appear to indicate incorporation of a lindsleyite phase from the source, which is ascribed to metasomatism of lithosphere from the ...