Diversification of Archean tonalite-trondhjemite-granodiorite suites in a mushy middle crust

Tonalite-trondhjemite-granodiorite (TTG) suites are the dominant component of Earth’s first continents, but their origins are debated. The trace element concentrations of these rocks are conventionally linked to their source depths and inferred geodynamic settings with the implicit assumption that TTG compositions are source-controlled. Alternatively, their variable compositions may be caused by fractional crystallization in TTG plutons after emplacement and less clearly linked to source depth. Most TTGs in Archean mid-crustal exposures are the dominant component of igneous gray gneiss complexes; the processes that influence the evolution of TTG magmas in this setting are poorly understood. We present a petrological–geochemical model that explains the coexistence of TTGs in the middle crust with low-pressure and high-pressure geochemical trends, irrespective of tectonic setting or depth of the TTG source. We propose that mid-crustal TTGs were long-lived crystal mushes with compositions controlled by the separation of early-crystallizing plagioclase and melt. Using phase equilibrium modeling, we demonstrate that a suite of TTGs from the southern Superior Province in Canada represents variably plagioclase-rich and melt-rich fractions from a common parent magma. The behavior of plagioclase may have a strong influence on the geochemical trends of TTGs, including the degree of rare earth element fractionation. Our results suggest that trace element compositions of TTGs may not primarily reflect the depth of the source and cannot be used alone to infer Archean geodynamic settings.

Age and isotopic character of Early Proterozoic basement gneisses in the southern Monashee Complex, southeastern British Columbia

The southern Monashee Complex is a fault-bounded exposure of upper-amphibolite-grade basement gneisses (core gneisses) and unconformably overlying mantling metasedimentary gneisses. The eastern margin is marked by the Early Eocene ductile to brittle Columbia River fault zone; the western and southern margins are marked by the Monashee Decollement and structurally higher Selkirk allochthon.The basement gneisses are exposed in the cores of large, northeast-verging nappes that subsequently have been overprinted towards the east by the Columbia River fault zone. The basement gneisses are a supracrustal sequence intruded by at least two distinct orthogneisses: (i) a biotite granite gneiss ("gray gneiss") dated by U/Pb zircon at 1874 ± 21 Ma; and (ii) a ±hornblende–biotite K-feldspar augen gneiss dated by U/Pb zircon at 1934 ± 6 Ma.The supracrustal gneisses are predominantly heterogeneous biotite–quartz–feldspar gneiss interlayered with less common pelitic schist and calc-silicate gneiss. U/Pb zircon data on detrital zircon populations from this heterogeneous supracrustal sequence give 207Pb*/206Pb* ages of less than 2.2 Ga. Whole-rock Pb isotopic data indicate an age of approximately 2.0 Ga. Whole-rock Sm/Nd model ages on the two intrusive suites indicate separate sources, the 1874 Ma gneiss having been produced from similar-age juvenile Early Proterozoic material (TDM ≈ 2.2 Ga). In contrast, Nd data from the 1934 Ma augen gneiss clearly indicate interaction with a component of older (late Archean) material (TDM ≈ 2.8 Ga). Whole-rock Sm/Nd data from the supracrustal gneisses follow this same pattern, with one group (seven samples) similar to the 1874 Ma gneiss (with TDM ≈ 2.3–2.6 Ga) and a second group (five samples) showing provenance or derivation from an Archean source (TDM = 2.8–3.3 Ga). The age of the intrusive suites, combined with the Nd data, strongly argues for a correlation with the Early Proterozoic Wopmay orogenic belt in northern Canada.