Reconnaissance isotopic geochemistry of anorthosite mangerite-charnockite-granite (AMCG) complexes, Grenville Province, Canada

Abstract

Anorthosite-mangerite-charnockite-granite (AMCG) complexes with ages of 1.13–1.16 Ga constitute the most widespread and voluminous igneous suites in the Grenville Province. The complexes are widely believed to comprise mantle-derived anorthosite and related basic rocks together with mangerites, charnockites and granites of largely crustal origin. This paper presents results of geochemical and Nd, Sr and Pb isotopic studies of basic and silicic samples from five AMCG complexes in the Grenville Province. Anorthositic members all have positive ϵ Nd (+0.8 to +4.5) indicating time-integrated incompatible element depleted sources, in agreement with earlier studies. Most granitoid samples, with one notable exception, also have positive ϵ Nd (+1.6 to +3.4) indicating sources with short crustal residence times. Granitoids from the Atikonak River complex in southern Labrador have ϵ Nd −3.7 to −2.2 revealing clear evidence that partial melting of relatively old crust was involved in their formation. For both anorthositic and granitoid samples there is broad correlation of decreasing ϵ Nd with increasing Sr i imparting confidence in a model requiring at least two fundamentally different end-members. Isotopic studies of these Grenville Province AMCG complexes, excluding Atikonak, are in accord with a two-source origin in which the crustal sources were not more than a few hundred Ma old at the time they were partially melted; however, a single source origin is not excluded for complexes like Mealy Mountains that evidently developed in newly-formed crust. The petrology and geochemistry of AMCG complexes is broadly consistent with processes whereby crustal partial melting at depth is driven by proximity to basic magma. Extraction of granitoid partial melts is expected to leave crustal residues of appropriate composition to produce anorthositic magmas when assimilated by subjacent basic magmas.