When crust comes of age: on the chemical evolution of Archaean, felsic continental crust by crustal drip tectonics

Abstract

The secular evolution of the Earth's crust is marked by a profound change in average crustal chemistry between 3.2 and 2.5 Ga. A key marker for this change is the transition from Archaean sodic granitoid intrusions of the tonalite–trondhjemite–granodiorite (TTG) series to potassic (K) granitic suites, akin (but not identical) to I-type granites that today are associated with subduction zones. It remains poorly constrained as to how and why this change was initiated and if it holds clues about the geodynamic transition from a pre-plate tectonic mode, often referred to as stagnant lid, to mobile plate tectonics. Here, we combine a series of proposed mechanisms for Archaean crustal geodynamics in a single model to explain the observed change in granitoid chemistry. Numeric modelling indicates that upper mantle convection drives crustal flow and subsidence, leading to profound diversity in lithospheric thickness with thin versus thick proto-plates. When convecting asthenospheric mantle interacts with lower lithosphere, scattered crustal drips are created. Under increasing P-T conditions, partial melting of hydrated meta-basalt within these drips produces felsic melts that intrude the overlying crust to form TTG. Dome structures, in which these melts can be preserved, are a positive diapiric expression of these negative drips. Transitional TTG with elevated K mark a second evolutionary stage, and are blends of subsided and remelted older TTG forming K-rich melts and new TTG melts. Ascending TTG-derived melts from asymmetric drips interact with the asthenospheric mantle to form hot, high-Mg sanukitoid. These melts are small in volume, predominantly underplated, and their heat triggered melting of lower crustal successions to form higher-K granites. Importantly, this evolution operates as a disseminated process in space and time over hundreds of millions of years (greater than 200 Ma) in all cratons. This focused ageing of the crust implies that compiled geochemical data can only broadly reflect geodynamic changes on a global or even craton-wide scale. The observed change in crustal chemistry does mark the lead up to but not the initiation of modern-style subduction.This article is part of a discussion meeting issue ‘Earth dynamics and the development of plate tectonics’.

On the origin of oldest Iron-Oxide-Copper-Gold (IOCG) deposits at the transition from Archean drip to plate tectonics

“…1) shows that vertical tectonics played an important role during granitoid emplacement from 3.01 to 2.83 Ga. In the absence of Archean "modern-style" subduction [1,2,4], crustal tectonics could be driven by forces related to mantle convection under a cold lithospheric lid [34]. Numerical models indicate that this process creates lithospheric drips where mantle convection cells downwell and is an e cient way to drag crustal rocks deep into the mantle, where they can melt at high pressures [34].…”

“…In contrast, Phanerozoic IOCG deposits are preferentially located in extensional or transtensional zones of arcs [13,14]. The existence of "modernstyle" 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 [4].…”

“…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 [2]. 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-trondhjemitegranodiorite (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 ma c 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 ma c 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 [9].…”

“…Contemporaneously with the transitional TTG, a speci c and restricted type of granitoid, known as sanukitoid [10] began to appear in nearly all cratons, where they are late additions accompanied in some cases by K-rich granitoids [3]. 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-eld 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 [11].…”