40Ar/39Ar chronology of the Nain anorthosites, Canada

Abstract

A hornblende 40Ar/39Ar age of 1328 ± 8 Ma has been obtained from the contact aureole of the Bird Lake Massif, confirming it as one of the oldest members of the Nain plutonic suite. This age constrains the timing of the early stage magma activity of the Nain plutonic suite.Plagioclase samples from the intrusions yield either U-shaped, or staircase-shaped 40Ar/39Ar age spectra, and most of them have two isochrons of similar age but greatly different initial argon composition. Most plagioclase closure ages are >95% of either the hornblende age from a contact aureole or coexisting biotite ages, and appear to reflect single cooling events for each intrusion. Two major stages of emplacement for the Nain anorthosites are identified: an older anorthosite event and a main anorthosite event, in agreement with results from earlier field studies. The Port Manvers Run, Paul Island, and Nukasorsuktokh Island intrusions yield much younger plagioclase ages than any previously established intrusion ages, probably reflecting previously unknown later, local heating events.The total duration of the major anorthositic magma activity in the Nain region is about 23 Ma, estimated from the Bird Lake massif margin at 1328 ± 8 Ma and the Kiglapait intrusion at 1305 ± 2 Ma (Sm/Nd, U/Pb), implying an emplacement rate of about 0.0022 km3/year. This rate is far less than the effusion rate of magma on some of the Phanerozoic rifted margins and supports the "aborted rifting" model for the generation of massif anorthosite.