Petrogenesis, emplacement setting, and magmatic-hydrothermal mineralization of the peralkaline Flowers River Igneous Suite, Hopedale Block, Labrador

Abstract

The peralkaline Flowers River Igneous Suite intrudes the southernmost region of the Mesoproterozoic Nain Plutonic Suite composite batholith in north-central Labrador. The Flowers River complex comprises a voluminous series of peralkaline granite ring intrusions and their coeval volcanic assemblage, the latter of which has been identified as a target for rare metal exploration. Peralkaline igneous complexes throughout Labrador show a ubiquitous spatial association with earlier anorthosite-mangerite-charnockite-granite (AMCG)-affinity plutonism, suggesting the geodynamic conditions responsible for generating the latter may have systematically conditioned those sites to subsequently produce highly evolved, incompatible element-enriched magmas. Laser ablation inductively coupled plasma-mass spectrometry (LA ICP-MS) U-Pb zircon geochronology is used to define a high-resolution magmatic and hydrothermal timeline for the Flowers River complex and its host suites. The Flowers River Granite intruded the local Nain Plutonic Suite rocks after ca. 8 Myr of quiescence, though trace element compositions indicate these rocks share a common, incompatible element-enriched source. Protracted fractionation of the liquids derived from this source gave rise to two contrasting styles of magmatic-hydrothermal (Zr-Nb-Y-)REE mineralization, both hosted by the cogenetic volcanic rocks overlying the plutonic complex.