The onset of plate tectonics is a heavily debated topic[1,2], but geological evidence from paleomagnetism, development of passive margins, metamorphic rocks and granitoid suggests that it was progressively established from 3.2 to 2.5 Ga. Archean granitoid archives from many cratons show a change in rock chemistry between 3.2 and 2.5 Ga from intrusions of the sodic tonalite-trondhjemite-granodiorite (TTG) series to granitic intrusions with increasing potassic content[2,3,4].
The elevated Na/K, Sr/Y and high La/Yb in many TTGs is ascribed to partial melting of mafic rocks at depth, leaving a stable garnet residue[5,6]. Through time there was a decrease in TTG Na+Ca and Sr contents, which led to the distinction between high-Sr TTG (deeper, high-P melts with residual garnet) and low-Sr transitional TTG
(lower-P melts with residual plagioclase) sub-types[3,4,5,6]. Associated depletion in heavy rare earth elements
(HREE) and residual garnet requires melting of source mafic rocks in a range of 1 to 3 GPa[7,8], but depending on the source composition stability of garnet can be achieved at pressures bellow 1 GPa. Contemporaneously with the transitional TTG, a specific and restricted type of granitoid, known as sanukitoid began to appear in nearly all cratons, where they are late additions accompanied in some cases by K-rich granitoids. These sanukitoids are mostly diorites to granodiorites that have a clear mantle signature, especially in their high MgO contents, but are also enriched in incompatible elements, most notably Ba and Sr, and sometimes high-field strength elements[3,4,10]. Melting experiments and petrogenetic modelling show that they may have formed either by peridotitic melting of mantle previously metasomatised by felsic melts of TTG composition, or by reaction between TTG melts and mantle.
Although there is no reported genetic relationship between sanukitoids and Iron Ore-Copper-Gold (IOCG) deposits, they share the same geochemical signature with enrichment in trace and minor elements typical of both basic (Co, Ni) and felsic (LREE, LILE, U, F) sources[11,12]. The source of Cu, Au, incompatible elements, and volatiles in IOCG deposits is commonly attributed to metasomatism of the underlying subcontinental lithospheric mantle (SCLM) during previous subduction events on continental margins[13,14]. Precambrian IOCG deposits occur inboard of lithospheric boundaries in necked transitions between thick Archean and thinner Proterozoic underlying mantle that focused and guided melting of this previously subduction-metasomatized mantle. In contrast, Phanerozoic IOCG deposits are preferentially located in extensional or transtensional zones of arcs[13,14]. The existence of “modern-style” subduction in the Archean is contentious[1,2,4] and the stagnant lid tectonic hypothesis is at least equally plausible to explain Archean geodynamics[15,16] and the evolving nature of Archean magmas. Thus, understanding the origin and timing of Archean IOCGs and their temporal and chemical connection with the evolving Archean magmatism could help constrain the tectonic setting at the time.
The Carajás Mineral Province (CMP) of the Amazon craton in northern Brazil includes the only Archean IOCG deposit discovered so far. In this region, a long history of Archean granitoid magmatism (3.01 to 2.5 Ga) records the same compelling change in rock chemistry reported in other cratons. Here we report on ages, Hf isotopic composition and trace elements of zircons from modern drainages within the CMP, which reflect the nature of the exposed Archean rocks. We combine these results with available zircon crystallization ages and bulk-rock chemistry of granitoids to define a temporal evolution linked to the tectonic evolution of the region. We argue that emplacement of the 2.87-2.83 Ga transitional TTG and sanukitoid magmas records early fertilization and melting of the lithospheric mantle, which was re-melted at c. 2.75 Ga to generate the IOCG mineral systems in the transition from drip to mobile tectonics in the Amazon Craton. This differs from the models of mantle enrichment via subduction proposed for the younger IOCGs.
GEOLOGICAL SETTING OF CARAJÁS MINERAL PROVINCE (CMP) AND ITS IOCG DEPOSITS
The CMP region encompasses three main rock associations: i) Archean basement rocks (3.00-2.83 Ga) represented by TTG-greenstone belt associations, with minor sanukitoids and K-rich granitoids[17,18,19,20]; ii) the Carajás basin composed of a meta-volcanosedimentary sequence, including thick Superior-type BIFs coeval with the intrusion of A-type granitoids and layered mafic-ultramafic complexes (2.78-2.72 Ga), alkaline high-K intrusions (c. 2.56 Ga) and younger low-grade siliciclastic cover rocks[21,22]; and iii) widespread Paleoproterozoic (c. 1.88 Ga) A-type intrusions (Fig. 1). To the north, these lithologies are bounded by high-grade rocks (gneisses and migmatites) of Paleoproterozoic age (2.50-1.96 Ga) within older Archean terranes. Interpretations based on geological, bulk-rock geochemistry and isotopic data have led to two contrasting interpretations for the tectonic setting of the Carajás Basin: an intracontinental rift forming the Carajás basin[21,22] or a compressional arc-related setting.
In the CMP, the region to the south of the Carajás basin, the environment related to the emplacement of the
3.0 to 2.83 Ga TTGs is preserved, with the dome-and-keel structures typical of many granite-greenstone terranes such as the Mogno dome[26,27] in Fig. 1. The dome-and-keel terrane is bound to the north by a high- to low grade 200-km wide anastomosing set of E-W to ESE-WNW strike-slip shear zones and faults (Itacaiúnas shear belt) developed around 2.85 Ga. This shear belt was later reactivated by sinistral motion at the time of development of Carajás basin at 2.78-2.72 Ga, accompanied by the intrusion of syn-kinematic A-type granites[20,28,29].
The IOCG deposits are located along or near these reactivated shear zones, in proximity to intrusions of different compositions, and are characterized by Fe-oxides and enrichment in Cu-Au-REE-(U-Y-Ni-Co-Pd-Sn-Bi-Pb-Ag- Te). They have been affected by intense proximal K-Fe and distal Na-Ca hydrothermal alteration with late- stage chlorite and carbonates, associated with hypersaline fluids (250°-570°C) and late sulphide deposition with different sulfidation states. Geochronology points to two main IOCG mineralization events at 2.75-2.68 Ga and 2.62-2.50 Ga, the first associated with reactivated regional shear zones and syn-kinematic intrusions of A-type granitoids and the later to a second reactivation stage accompanied by minor alkaline high-K granitoids.