Fluorite-bearing early and middle proterozoic granites, Okak Bay area, labrador: Geochronology, geochemistry and petrogenesis

Abstract

Three discrete age groups of relatively fractionated, fluorite-bearing, post-tectonic Proterozoic granitoid rocks have been identified in the Okak Bay area of Labrador on the basis of U-Pb zircon geochronology. Two of the groups occur entirely within the Archean Nain Province; the third occurs largely within the Early Proterozoic Churchill Province (Torngat Orogen). None of the three episodes of granitoid magmatism can be directly linked to orogenic activity in its immediate surroundings. Although all three granite groups are broadly similar, petrologically and chemically, to the felsic rocks of the anorogenic Elsonian (≈1450−1290 Ma) Nain Plutonic Suite (NPS), only one of the dated units can be correlated confidently with Elsonian magmatism. The other two groups represent significantly older anorogenic or postorogenic magmatism. The oldest unit, the Wheeler Mountain granites of the Nain Province has yielded a U-Pb age of about 2135 (2134± 3 1, 2137±2) Ma. A major tectonothermal event of this age has not been previously identified in northern Labrador, and the origin of the granites is uncertain. Three small intrusions on offshore islands (White Bear, Saddle, Opingiviksuak) in the Nain Province were intruded about 1775 (1774± 2 1,1776± 4 2, 1774± 2 1) Ma. These granites may have been intruded post-kinematically into a stable Archean foreland representing a delayed effect of a preceding collisional event (Torngat Orogeny) between Nain and Churchill Province to the west. The youngest unit dated, at about 1318 (1319±2, 1316± 2 3) Ma, is Umiakovik Lake batholith which crops out largely within the eastern Churchill Province, and is one of the largest granitoid intrusions of the NPS. The age confirms that it formed an integral part of the Elsonian magmatic event, but both Umiakovik Lake batholith and nearby 1320 Ma Makhavinekh Lake pluton are significantly older than some basic members of the NPS in the coastal area to the southeast. Nd and Sr isotopic data demonsrate that Archean protoliths formed significant components of the source materials for all three of these Proterozoic granitoid magmas. This source was supplemented by one or more Proterozoic episodes of lower crustal intrusion and underplating by juvenile mantle-derived magmas that occurred in the region.