Whole-rock and Nd isotopic geochemistry of Neoarchaean granitoids and their bearing on the evolution of the Central Hearne supracrustal belt, Western Churchill Province, Canada

Abstract

The Central Hearne supracrustal belt (CHSB), Hearne domain, Western Churchill Province forms a large (ca. 30,000 km 2), greenschist grade Neoarchaean (2711–2660 Ma) terrane of predominant metavolcanic and less common metasedimentary rocks intruded by three groups of plutonic rocks. Pre-tectonic group 1 plutons (>2690 Ma) include rare gabbro, dominant diorite to tonalite and rare granodiorite and granite. These intrude and incorporate angular inclusions of cogenetic volcanic rocks and have variably developed foliations. Syn-tectonic group 2 plutons (2690–2679 Ma) comprise rare gabbro and diorite, predominant tonalite and granodiorite and rare granite, and exhibit N–S-trending, ductiley deformed supracrustal schlieren-rich intrusive margins. Post-tectonic group 3 plutons are rare, and comprise either potassic, biotite monzogranite or alkalic nepheline syenite and rare carbonatite. Abundant tonalite to granodiorite, biotite ± hornblende – bearing mineralogies and metaluminous and generally low-medium K 2O compositions, indicate that most rocks are I- or M-type granitoids. Molecular Na–Ca–K variations and AFM indices indicate transitional calc-alkaline – trondjhemitic trends with both tholeitic and calc-alkaline affinities. Rare earth and incompatible element variations suggest that most granitoids exhibit volcanic arc- or tonalite–trondhjemite–granodiorite (TTG)-like abundances with multi-element patterns varying gradationally from La-poor (La N/Yb N < 12) to La-rich compositions (La N/Yb N > 12). Nd isotopic values overlap with contemporaneous depleted mantle indicating that the granitoids represent melts derived from predominantly juvenile mantle or crust. La-poor rocks likely formed through low- P anatexis of amphibolitic crust with plagioclase + amphibole ± clinopyroxene present, whereas high-La rocks were generated via high- P partial melting of a garnet + clinopyroxene-bearing protolith (plausibly a subducted slab). Granitoid evolution from early dominant low- P, tholeiitic and calc-alkaline melts, to later, predominant high- P, high-Al 2O 3 TTG melts, reflects a change in the tectonomagmatic setting at ca. 2690 Ma. Construction of proto-arc crust from ca. 2711–2690 Ma, and extension of the leading edge of normal (ca. 40 km) Archaean oceanic crust, in response to lithospheric processes analogous to those of the Eocene SW Pacific Ocean, resulted in asthenospheric upwelling, intrusion of mantle-derived melts into the lower crust, and their subsequent ascent and fractionation. Partial melting at the base of the extended, thick oceanic crust likely yielded sparse high-La melts at this time. At ca. 2690 Ma, a change from an extensional to a shortening regime resulted in initiation of subduction of adjacent oceanic lithosphere, partial melting of the eclogitic downgoing slab and generation of voluminous, high- P, La-rich granitoid magmas. This yielded less abundant low- P, La-poor melts emplaced into the “infant-arc” crust during and following tectonism. The complete sequence was intruded by late potassic granites derived from tonalitic lower crust and alkalic magmas that originated in the lithospheric mantle.