Geochemical evolution of the Minto block: a 2.7 Ga continental magmatic arc built on the Superior proto-craton

Abstract

Abstract A 400 km-long by 100 km-wide geological transect across the Minto block of northeastern Superior Province reveals five plutonic suites and minor amphibolite- to granulite-grade supracrustal remnants. The oldest regionally significant plutonic rocks include 3.0±0.1 Ga foliated to gneissic tonalite characterized by high Na K ratios, high Al contents, light REE enrichment, and initial geNd= +1.8. The Leaf River plutonic suite is the most widespread, with crystallization ages of about 2725 ± 5 Ma across the transect. The suite comprises massive to foliated pyroxene- and hornblende-bearing granodiorite, tonalite, granite, diorite, and minor gabbro-pyroxenite and syn-plutonic mafic dykes. The rocks are I-type, calc-alkaline, have variable Na K ratios, moderate to high Ba and Sr contents, and REE patterns with variable light REE enrichment depletion of heavy REEs with increasing silica, and neutral, negative, or positive Eu anomalies. Initial ϵNd values for the suite range from −0.5 to +1.3 (mean = +0.4), and initial 87 Sr 86 Sr ratios (ISr) are ∼0.7020. The diatexite plutonic suite (2713 Ma) occurs within a 150 km-wide zone, and consists of orthopyroxene±garnet granodiorite containing 25–50% inclusions of granulite-grade mafic, supracrustal, and tonalitic inclusions. The variable major- and trace-element chemistry of the suite, with generally high Ni and Cr contents, reflects heterogeneous source materials and the presence of non-liquid components. Initial ϵNd values are from +0.1 to −1.7. Monzogranite and orthopyroxene granite dykes, plugs and plutonic masses, both ∼2690 Ma, comprise the last important plutonic suites. Both biotite monzogranite and orthopyroxene granite are light REE-enriched, but the former have negative Eu anomalies, and the latter positive. Initial ϵNd values range from −0.7 to −3.2 for monzogranite (ISr=0.7016 to 0.7067), and +0.7 to −0.2 for orthopyroxene granite (ISr=0.701 to 0.702). The initial ϵNd and ISr values of the ∼ 2.7 Ga plutonic suites are lower and higher, respectively, than estimatesf of the contemporaneous depleted mantle, suggesting significant and wide-scale reworking of older, tight REE-enriched, high Rb SR lithosphere. Early tonalite orthogneiss from Goudalie domain may be derived by partial melting of mafic crust at high pressures. The lithological and geochemical diversity of the 2725 Ma Leaf River plutonic suite indicate a range of parental magma compositions and petrogenetic processes, but fractional crystallization of mantle-derived basaltic melts which had assimilated small amounts of significantly older lower crust may have been the dominant mechanism. Crustal-inclusion-charged granodioritic diatexite (2713 Ma) was probably derived by partial melting of heterogeneous lower crustal materials under relatively dry conditions. Late biotite monzogranite was generated by partial melting of older, sialic crust, and orthopyroxene granite by deeper melting or fractional crystallization of more juvenile source materials with garnet as a residual mineral. We infer from the presence of coeval mafic dykes in all plutonic suites that heat for melting was ultimately derived from intrusion of basaltic magmas at depth. The ∼ 2.7 Ga magmatism in the Minto block occurred during a period of active production and accretion of juvenile oceanic terranes in the southern Superior Province. The northwest-directed subduction regime inferred from the southern Superior Province arc terranes may also have been responsible for the magmatism that acted on 3 Ga lithosphere in the northern Superior Province. Melting within the mantle wedge beneath the proto-cratonic lithosphere and in the lower crust occurred within a tectonic setting similar to that of modem continental magmatic arcs.