The TTG-Amphibolite Terrains of Arctic Fennoscandia: Infinite Networks of Amphibolite Metatexite-Diatexite Transitions

Jaana Halla

doi:10.3389/feart.2020.00252

Abstract

In the Archaean, the Earth’s early basaltic crust episodically converted into felsic TTG (tonalite-trondhjemite-granodiorite) crust by unknown tectonic processes. To contribute to the debate on the possible tectonic settings of TTGs, this paper illustrates and explains migmatite structures of TTG-amphibolite terrains in Arctic Fennoscandia. The Lake Inari and Rommaeno complexes in northern Finland and West Troms Complex in northern Norway consist of folded and banded TTG gneisses with abundant amphibolite enclaves. The terrains show migmatite structures generated by in-situ and in-source melting of amphibolites and repeated metatexite-diatexite transitions that form infinite and boundless interconnected networks over vast areas. The aim of this paper is to show that the TTGs of these terrains represent coalesced in-situ and in-source neosomes of amphibolite protoliths and are not similar to granitoids sensu stricto generated by modern-style plate tectonics. The best-fit explanation for the origin of the TTG-amphibolite associations of Arctic Fennoscandia is intracrustal differentiation by in-situ and in-source partial melting of metabasalts in deep parts of a thick oceanic plateau. Preliminary geochemical results on bimodal TTG-amphibolite sample sets of the Lake Inari Complex as well as some other recent TTG research support this conclusion. Successful further research on the formation of the Earth’s earliest crust requires a three-fold strategy of parallel investigations of TTGs and associated basalts as well as migmatite structures.

Highlights

In the Archean, the Earth’s early basaltic crust was episodically converted into a thickening continental tonalite-trondhjemite-granodiorite (TTG) crust
Instead of clear pluton emplacement structures, the TTG-amphibolite terrains of Arctic Fennoscandia show field evidence for syn-anatectic in situ and in source melting, metatexite and diatexite transitions, and melt migration pathways
The TTG-amphibolite terrains may have formed by migmatization in the lower part of a thick basaltic oceanic plateau, but further microstructural, geochemical and geochronological studies of the terrains are needed to sustain the origin of the TTG-amphibolite terrains of Arctic Fennoscandia

Summary

INTRODUCTION

In the Archean, the Earth’s early basaltic crust was episodically converted into a thickening continental tonalite-trondhjemite-granodiorite (TTG) crust. Instead of clear pluton emplacement structures, the TTG-amphibolite terrains of Arctic Fennoscandia show field evidence for syn-anatectic in situ and in source melting, metatexite and diatexite transitions, and melt migration pathways. Increasing-Strain Regime Increasing syn-anatectic strain causes the melt fraction to migrate to dilatant structures in the migmatite creating a morphology termed dilation-structured metatexite In this structure, leucosomes are located in low-pressure (dilatant) structural sites such as spaces between boudins, in pressure shadows, or fractures (Figure 6). With increasing amounts of melting, diatexites that are texturally more homogeneous start to form They have a massive appearance with more or less abundant vestiges of disrupted amphibolite metatexites showing further melting (Figure 13). The most advanced stage of migmatization forms large volumes of flow-banded diatexite with schlieren and occasional vestiges of amphibolite paleosome, metatexite, and residuum

DISCUSSION

CONCLUSION