Rare-earth element modelling of Archean meta-igneous and igneous rocks, Lake Despair area, northwestern Ontario

Archean felsic to intermediate meta-igneous rocks from the Lake Despair area, northwestern Ontario, have highly-fractionated REE patterns (LaN/LuN 10–100). They are rich in LREEs (ca. 40–100 × chondrites) and poor in HREEs (ca. 1–7 × chondrites). Simple models for the REEs suggest eclogite—quartz eclogite parents for these rocks. The Footprint Gneiss, the major rock type of the Rainy Lake Batholith, was formed by limited melting (ca. 10%) of a quartz eclogite under hydrous conditions. The putative parent may have been transformed basalt derived from primitive, LREE-rich Archean mantle. The mafic metavolcanic rocks have a REE chemistry similar to modern island-arc or mid-ocean ridge tholeiites. Felsic metavolcanic rocks, and granodiorite from the Northwest Bay complex, have REE abundances compatible with an origin by partial melting (ca. 10%), under hydrous conditions, of quartz eclogite of tholeiitic REE chemistry.The Jackfish Lake plutonic complex, which is composed mostly of diorite, has REE abundances that are best described by 10% melting of eclogite (with tholeiitic-REE chemistry) under hydrous conditions. A small portion of the diorite magma was subsequently fractionate, largely by the early crystallization of amphibole, and formed more leucocratic rock types (e.g. leucodiorite and granodiorite).