Jaana Halla

Archaean granitoids: an overview and significance from a tectonic perspective

Abstract This special publication presents a collection of papers on Archaean igneous rocks which aim to provide evidence of a tectonic scenario that is increasingly accepted by scientists studying the evolution of the early Earth. Papers on diverse igneous rocks from the North Atlantic Craton, as well as Archaean terrains of the Fennoscandian, Indian and Ukrainian shields, have a common focus on crust–mantle interactions and granitoid diversification, especially at the end of the Archaean, accompanied by insights into metamorphic rocks. This volume, together with present research, provides evidence for a change in global tectonic regime close to the Archaean–Proterozoic boundary. After the long-term episodic formation of tonalite–trondhjemite–granodiorite (TTG) suites of oceanic origin, convergent continental margins with abundant batholiths of potassic granitoids appeared for the first time at 3.0–2.5 Ga. The batholiths involve both mantle-derived and recycled crustal material. It seems that the diversification of granitoids was caused by increased crust–mantle interactions, reflecting a significant change in mantle dynamics and plate tectonics during the Neoarchaean.

The diversification of granitoids and plate tectonic implications at the Archaean–Proterozoic boundary in the Bundelkhand Craton, Central India

Abstract The Bundelkhand Craton in Central India holds a large Archaean granitoid complex consisting of cores of TTG (tonalite–trondhjemite–granodiorite) gneisses of island arc or oceanic origin surrounded by abundant younger high-K calc-alkaline granitoids. Major and trace element groupings and ion probe U–Pb zircon datings of the groups show a time gap of 130 Ma between the main formation episodes of the TTGs (3.5/3.3–2.7 Ga) and the emplacement of the first high-K granitoids (2.57–2.54 Ga). Based on their geochemical diversity, the high-K calc-alkaline granitoids can be divided into low-silica high-magnesium (LSHM) granitoids such as sanukitoids and Closepet-type granitoids, and high-silica low-magnesium (HSLM) monzogranites with low-HREE and low-Eu subgroups. The former group points to mantle or mixed mantle and crustal sources, and the latter to pure crustal sources. All the varieties of the high-K granitoids formed within a narrow time span, which indicates large-scale partial melting and fluid activity in the mantle and crust, possibly resulting from a slab breakoff or delamination at the margin of an Archaean TTG continent. Supplementary material: Major and trace element concentrations and U-Pb results of granitoids from the Bundelkhand Craton are available at https://doi.org/10.6084/m9.figshare.c.3576377

Geochronology of Neoarchaean granitoids of the NW eastern Dharwar craton: implications for crust formation

Abstract The Neoarchaean Era is characterized by large preserved record of continental crust formation. Yet the actual mechanism(s) of Neoarchaean crustal growth remains controversial. In the northwestern part of the eastern Dharwar craton (EDC) granitoid magmatism started at 2.68 Ga with gneissic granodiorites showing intermediate character between sanukitoid and tonalite–trondhjemite–granodiorite (TTG). This was followed by intrusion of transitional (large-ion lithophile element-enriched) TTGs at 2.58 Ga. Finally 2.53–2.52 Ga sanukitoid and Closepet-type magmatism and intrusion of K-rich leucogranites mark the cratonization in the area. These granitoids mostly display initial negative ϵNd and Mesoarchaean depleted mantle model ages, suggesting presence of older crust in the area. Available data show that most of the Neoarchaean sodic granitoids in the EDC are transitional TTGs demonstrating the importance of reworking of older crust. It is suggested that the various c. 2.7 Ga greenstone mafic–ultramafic volcanic rocks of EDC formed in oceanic arcs and plateaus which accreted to form continental margin environment. Subsequent 2.7–2.51 Ga granitoid magmatism involved juvenile addition of crust as well as reworking of felsic crust forming transitional TTGs, sanukitoids and K-rich leucogranites. Microcratons were possibly the source of older crustal signatures and their accretion appears to be one of the important processes of Neoarchaean crustal growth globally. Supplementary material: Analytical techniques are available at https://doi.org/10.6084/m9.figshare.c.3470724

Alkaline-rich quartz syenite intrusions of the Western Karelia subprovince

Abstract We have studied a group of granitoids from the Western Karelia subprovince of the Fennoscandian Shield. This group is referred to as quartz syenites, but shows compositional variation from syenites to quartz monzonites, with a small number of monzonites and granites. Compositionally studied rocks are alkali and alkali-calcic, and magnesian, mostly metaluminous. Characteristically, they have a high content of alkaline (Na, K), large ion lithophile elements (LILE) (Ba, Sr), high-field strength elements (HFSE) (TiO2, Zr, Ce), as well as a low content of Mg, Ni and Cr, by which they can be distinguished from sanukitoid and quartz diorite suites of the Karelia Province. These quartz syenites were emplaced between 2.74 and 2.66 Ga, representing late-phase intrusions overlapping in age with the sanukitoids, the quartz diorites and the leucogranitoids. Initial whole-rock ϵNd values of quartz syenites vary from 1.8 to −1.8, and do not indicate a significant contribution of considerably older crust. Oxygen-isotope data for zircon indicate a varying mantle source (δ18O 5.35–7.15‰), with a contribution from source(s) with elevated δ18O values. Our data provide constraints on compositionally diverse Neoarchaean magmatism in the Archaean Karelia Province. The late Archaean evolution of the Western Karelia subprovince resembles that of the Neoarchean domains worldwide with respect to granitoid composition and temporal distribution. Supplementary material: Tables detailing geochemical analyses, analytical data for the five age samples and oxygen-isotope analyses from this study are available at https://doi.org/10.6084/m9.figshare.c.3459771

Recycling of Lead at Neoarchean Continental Margins

A time-fixed Pb-Pb model of 2.7 ± 0.1 Ga mantle-derived granitoids from different Archean cratons suggests that the Pb isotope heterogeneity of Neoarchean granitoids can be explained by sediment recycling and subduction at oceanic and continental margins consisting of different-aged crustal segments. Recycling of crustal Pb to the mantle wedge gave rise to increasingly radiogenic mantle sources for the granitoids as the accretion of oceanic island arcs (OIA) and Mesoarchean microcontinents proceeded, leading to the formation of young ( 3.2 Ga) continental margins (OCM) encompassing fragments of Paleo- and Eoarchean protocrust provided the high- or low-µ Pb isotope signatures, depending on the age and U/Pb ratio of the crustal lead sources.

Crust–Mantle Interactions and Granitoid Diversification: Insights from Archaean Cratons

This Special Publication sheds light on crust formation and tectonic processes in early Earth by focusing on Archaean granitoids and related rocks from West Greenland in the North Atlantic Craton, Karelia Province of the Fennoscandian Shield, Eastern Dharwar and Bundelkhand cratons in the Indian Shield and Bug Complex of the Ukrainian Shield. Resulting from the IGCP-SIDA 599 project ‘The Changing Early Earth’, this compilation of papers provides explanations on the nomenclature of Archaean granitoids and explores the petrology, element and isotope geochemistry, geochronology and metamorphism of granitoids and supracrustal rocks of variable metamorphic grade. This volume provides information on the increase and timing of crust-mantle interactions and granitoid diversification from early Archaean protoliths of island arc origin to the emergence of multi-source high-K calc-alkaline granitoid batholiths at convergent continental margins. The formation of abundant granitoid batholiths suggests a significant change in mantle dynamics and plate tectonics towards the end of the Archaean.