The sanukitoid series: magmatism at the Archaean–Proterozoic transition

Abstract

A specific type of granitoid, referred to as sanukitoid (Shirey & Hanson 1984), was emplaced mainly across the Archaean–Proterozoic transition. The major and trace element composition of sanukitoids is intermediate between typical Archaean TTG and modern arc granitoids. However, among sanukitoids, two groups can be distinguished on the basis of the Ti content of the less differentiated rocks of the suite: high- and low-Ti sanukitoids. Melting experiments and petrogenetic modelling show that they may have formed by either (1) melting of mantle peridotite previously metasomatised by felsic melts of TTG composition, or (2) by reaction between TTG melts and mantle peridotite (assimilation). Rocks of the sanukitoid suite were emplaced at the Archaean–Proterozoic boundary, possibly marking the time when TTG-dominated granitoid magmatism changed to a more modern-style, arc-dominated magmatism. Consequently, the intermediate character of sanukitoids is not only compositional but chronological. The succession of granitoid magmatism with time is integrated in a plate tectonic model where it is linked to the thermal evolution of subduction zones, reflecting the progressive cooling of Earth: (1) the Archaean Earth’s heat production was high enough to allow the production of large amounts of TTG granitoids formed by partial melting of recycled basaltic crust (‘slab melting’); (2) at the end of the Archaean, due to the progressive cooling of the Earth, the extent of slab melting was reduced, resulting in lower melt:rock ratios. In such conditions the slab melts can be strongly contaminated by assimilation of mantle peridotite, thus giving rise to low-Ti sanukitoids. It is also possible that the slab melts were totally consumed in reactions with mantle peridotite, subsequent melting of this ‘melt-metasomatised mantle’ producing the high-Ti sanukitoid magmas; (3) after 2·5 Ga, Earth heat production was too low to allow slab melting, except in relatively rare geodynamic circumstances, and most modern arc magmas are produced by melting of the mantle wedge peridotite metasomatised by fluids from dehydration of the subducted slab. Of course, such changes did not take place exactly at the same time all over the world. The Archaean mechanisms coexisted with new processes over a relatively long time period, even if they were subordinate to the more modern processes.

Li isotopes and trace elements as a petrogenetic tracer in zircon: insights from Archean TTGs and sanukitoids

“…The amounts of crustal contamination and composition of metasomatizing fluids (aqueous or silicate melt) are uncertain (Martin et al 2009;Moyen et al 1997;Shirey and Hanson 1984;Smithies and Champion 1999;Stern 1989;Stern and Hanson 1991;Stevenson et al 1999Stevenson et al , 2009.…”

Petrography, geochemistry and geochronology of the potassic granitoids of the Rio Itacambiruçu Supersuite: implications for the Meso- to Neoarchean evolution of the Itacambira-Monte Azul block

“…However, some characteristics resemble those of TTGs, such as the negative anomalies of Nb, Ta, P and Ti observed on the primitive mantle-normalized diagrams. The absence of mafic microgranular enclaves, the low concentrations of ferromagnesian oxides, Ba, Sr and transition elements, and the low Ba/Rb ratios (< 4) obtained in these rocks are incompatible with the signatures of the sanukitoid granitoid series (Shirey & Hanson 1984, Martin et al 2009) and the Closepet granites (Moyen et al 2003).…”

Rio Maria granodiorite and associated rocks of Ourilândia do Norte – Carajás province: Petrography, geochemistry and implications for sanukitoid petrogenesis

“…The granitoids completely overlapping with those of sanukitoid affinity, and especially low-Ti sanukitoids from Martin et al (2009). However, the trace elements of granodioritic variety turn out to absence of negative Th anomaly and exhibit higher HREE depleted in relation to Rio Maria sanukitoids, what may only be related to slightly different magma origin and evolution.…”

“…In general, such rocks emerged worldwide mostly from the Archean-Proterozoic transition (between 2.9 and 2.5 Ga), especially during the late-stages of TTG magmatism and prior to the more "classical" arc-related BADR suite (Basalt-Andesite-Dacite-Rhyolite; Martin et al 2009). Thereby, the sanukitoids share geochemical features with Archaean TTGs, like light rare earth elements (LREE) enrichments and strong heavy rare earth elements (HREE) depletion, leading to steep rare earth elements (REE) patterns; and of BADR suite, such as K 2 O-and large-ion lithophile elements (LILE) enrichment following the calc-alkaline trends from mafic (diorite/gabbro) to felsic (granite) end-members.…”