Abstract
In Archean greenstone belts, magmatism is dominated by intrusive and volcanic rocks with tholeiitic affinities, as well as tonalite- and granodiorite-dominated large-volume batholiths, i.e., tonalite–trondhjemite–granodiorite (TTG) suites. These intrusions are associated with poorly documented mineralization (Cu-Au porphyries) that, in the Neoarchean Abitibi Subprovince (>2.79 to ~2.65 Ga), Superior Province, Canada, are associated with diorite bearing plutons, i.e., tonalite–trondhjemite–diorite (TTD) suites. The importance of TTG versus TTD suites in the evolution of greenstone belts and of their magmatic-hydrothermal systems and related mineralization is unconstrained. The aim of this study was to portray the chemistry and distribution of these suites in the Abitibi Subprovince. The study used data compiled by the geological surveys of Québec and Ontario to evaluate the chemistry of TTG and TTD suites and uncovered two coeval magmas that significantly differentiated (fractional crystallization mostly): 1) a heavy rare earth elements (HREE)-depleted tonalitic magma from high pressure melting of an hydrated basalt source; and 2) a hybrid HREE-undepleted magma that may be a mixture of mantle-derived (tholeiite) and tonalitic melts. The HREE-depleted rocks (mostly tonalite and granodiorite) display chemical characteristics of TTG suites (HREE, Ti, Nb, Ta, Y, and Sr depletion, lack of mafic unit, Na-rich), while the other rocks (tonalite and diorite) formed TTD suites. Tonalite-dominated magmatism, in the Abitibi Subprovince, comprises crustal melts as well as a significant proportion of mantle-derived magmas and this may be essential for Cu-Au magmatic-hydrothermal mineralizing systems.
Highlights
The Abitibi Subprovince is the largest continuous greenstone belt of the Canadian Shield (Figure 1) [1]
These data are compiled in the Sigeom dataset, which can be downloaded online free of charge [11]
Granodiorite-granite (30.5%)—units with granite and/or granodiorite as the main lithology; Tonalite-granodiorite (21.7%)—units dominated by tonalite and granodiorite intrusive rocks with similar or distinct ages; Tonalite (20.6%)—units dominated by tonalite rocks and locally referred to as leucotonalite, i.e., quartz-rich tonalite and trondhjemite; Tonalite-diorite (6.5%)—diorite and tonalite dominated intrusions; Diorite (6.2%)—diorite-dominated plutons; Tonalite-granodiorite-(diorite) (1.0%)—tonalite- and granodiorite-rich intrusions with minor amounts of diorite; Syenite, monzodiorite, carbonatite, among other (8.2%)—intrusions of the syntectonic period that contain more K than the intrusions considered here; Syenite, monzodiorite, granodiorite, tonalite (4.9%)—syntectonic intrusions that crosscut tonalite-dominated intrusions of the synvolcanic and syntectonic periods; Unclassified (0.4%)—units identified as intermediate-felsic intrusions
Summary
The Abitibi Subprovince is the largest continuous greenstone belt of the Canadian Shield (Figure 1) [1]. In the Abitibi Subprovince, an additional rock type (diorite) is observed as part of tonalite plutons associated with Cu-Au magmatic-hydrothermal systems [5,6]. The petrogenetic evolution, as well as the geodynamic and economic significance of the tonalite, granodiorite, and diorite suite, is not fully resolved [4]. The aim of this study is twofold: 1) to examine the chemistry and distribution of these intrusive rocks in greenstone belts, using the Abitibi Subprovince. Côté-Gold Au-(Cu) deposits associated, respectively, with the Chibougamau pluton and the Chester intrusive complex [5,6] (Figure 1). Granitoid intrusions of the synvolcanic period are mostly tonalite–trondhjemite–granodiorite (TTG) suites [9,10], which are felsic (>64 wt% SiO2), aluminous (>14–16 wt% Al2O3 for 70 wt% SiO2), and Fe-Mg-poor
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