Thesis (Ph. D.)--University of Wisconsin - Madison, 2005. / (UnM)AAI3186163. Source: Dissertation Abstracts International, Volume: 66-08, Section: B, page: 4128. Supervisor: John W. Valley.

Thesis (Ph.D.)--Yale University, 2006. / (UMI)AAI3243705. Adviser: Mark Thomas Brandon. Source: Dissertation Abstracts International, Volume: 67-12, Section: B, page: 6968.

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 asthenosphere. Such characters also indicate that the magma source of the mantle in the Nain area must have been depleted in Rb, Nb and Zr before the melting process.

The Nain Anorthosite Complex is a Proterozoic plutonic suite in Labrador. This complex is composed over 24 anorthositic and troctolitic rocks, but its regional geochronology and tectonic environment were uncertain. The $\sp{40}$Ar/$\sp{39}$Ar method was applied to this complex, and the results shed light on these problems. The early anorthositic magmatic activity of the Nain complex is represented by three major anorthosite massifs along Tikkoatokhakh Bay, and their ages are constrained by a reset $\sp{40}$Ar/$\sp{39}$Ar hornblende age of 1328 $\pm$ 10 Ma in the contact aureole, as well as closure ages from other minerals. This age is younger than a previously estimated age of about 1.4 Ga. It signifies the beginning of large scale anorthositic magmatism. Plagioclase $\sp{40}$Ar/$\sp{39}$Ar closure ages from various anorthosite intrusions reveal two stages of emplacement for the Nain anorthosites. These stages correspond to the Older Anorthosite Event and the Main Anorthosite Event as identified from field evidence. This agreement indicates the overall feasibility of the field criteria used to classify the relative ages of anorthosites. Some contradictions between the two results are also observed, raising questions for further study. The cooling history study is largely based on the closure ages and temperatures of minerals from the Kiglapait Intrusion. This troctolitic layered intrusion yields a hornblende $\sp{40}$Ar/$\sp{39}$Ar age of 1298 $\pm$ 15 Ma, very close to a previous Sm/Nd result of 1305 Ma reported by DePaolo. Different minerals from the Kiglapait Intrusion behave surprisingly well and are concordant. Together, they not only allow the reconstruction of the late cooling history for the Kiglapait Intrusion, but, combined with the pre-intrusion country rock temperature and $\sp{40}$Ar/$\sp{39}$Ar age data from other anorthosite intrusions, reveal a continuous, slow cooling of the Nain region. This cooling trend reflects a late uplift during and after the large scale anorthositic magmatism, with an uplift rate dropping continuously from about 0.15 mm/yr to 0.07 mm/yr. This slowing uplift was probably related to the subsidence of a mantle plume upwelling that originally initiated the magmatism.