Geochronology and cooling history of the Nain Anorthosite Complex from an argon-40/argon-39 study

• Main Author: Yu, Yang
• Format: Text
• Language: English
• Published: ScholarWorks@UMass Amherst 1992
• Subjects: Geochemistry|Geology|Mineralogy; Nain
• Online Access: https://scholarworks.umass.edu/dissertations/AAI9219517

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 ...