The Mealy dykes, Labrador: petrology, age, and tectonic significance

Abstract

The Mealy dykes constitute a major olivine diabase swarm in the Mealy Mountains, Grenville Province, Labrador. The dykes intrude anorthosites and monzonites of the Mealy Mountains complex and outcrop over an area of at least 4000 km2. The swarm strikes east-northeast and may constitute the largest group of unmetamorphosed diabase dykes in the Grenville Province.The Mealy dykes are moderately iron-rich olivine tholeiites, typically nonporphyritic. Thick dykes have coarse-grained interiors that show some fractionation but variations in bulk chemical composition are relatively modest. Olivine (Fo47–Fo27), plagioclase (cores An63–An47, rims An47–An30), and low-Ca augite are the principal silicate minerals. Coexisting magnetite and ilmenite suggest that solidus temperatures were about 980 °C and that subsolidus oxide re-equilibration continued to near 530 °C or perhaps lower.A whole-rock Rb–Sr (errorchron) age for the Mealy dykes yielded 1380 ± 54 Ma, Sr1 = 0.7028 ± 0.0002, which, within precision estimates, is the same as published ages for other basic rocks in the region including Shabogamo gabbros, Seal Lake basalts and diabases, Harp dykes, and Michael gabbros. Together, these rocks signify a major episode of crustal dilation in central and southern Labrador that succeeded widespread anorthosite–monzonite–quartz monzonite intrusion. K–Ar ages on hornblendes, biotites, and whole rocks imply that if prograde heating of the Mealy Mountains terrane occurred at all during the Grenvillian event it was restricted to greenschist grade or lower.