Date of Award

2008

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Earth and Environmental Sciences

Keywords

Earth sciences, Greenland, Ivisaartoq belt, Mesoarchean, Neoarchean, Supracrustal

Supervisor

Ali Polat

Supervisor

Brian J. Fryer

Rights

info:eu-repo/semantics/openAccess

Abstract

This study presents new structural observations and high precision major and trace element data for metamorphosed volcanic and sedimentary rocks in the Ivisaartoq (ca. 3075 Ma), Ujarassuit (ca. 3070 Ma), and Storø (2800-2840 Ma) greenstone belts, Nuuk region, southwestern Greenland. The new data are used to investigate postdepositional alteration, petrogenesis of volcanic rocks, provenance of sedimentary rocks, and geodynamic setting.

The Mesoarchean Ivisaartoq belt underwent two stages of calc-silicate alteration. Stage-I alteration appears to have resulted from sea-floor hydrothermal alteration. Stage-II alteration was developed during the regional metamorphism. Both stages of alteration caused mobilization of major elements, large ion lithophile elements (LILE: Rb, Cs, Sr, Ba, Pb), and light rare earth elements (LREE). Heavy rare earth elements (HREE) and high field strength elements (HFSE: Th, Nb, Ta, Zr, Ti) remained relatively immobile during stage-I alteration but were variably disturbed during the stage-II metasomatism. Transition metals (e.g., Ni, V, Co, Cr, and Sc) were immobile during both metasomatic events.

The geochemical characteristics of metavolcanic rocks in the Mesoarchean Ujarassuit and Ivisaartoq greenstone belts suggest that these belts include island arc tholeiites (IAT), andesites, boninites, subduction-related picrites, and normal-mid-ocean ridge basalts (N-MORB). Metasedimentary rocks have low chemical indexes of alteration values (CIA = 46 to 62), enriched LREE patterns, and high contents of transition metals, indicating that they have been derived from poorly weathered felsic to mafic source rocks. Accordingly, the Ujarassuit and Ivisaartoq greenstone belts are interpreted to represent dismembered fragments of Mesoarchean supra-subduction zone oceanic crust formed either in a forearc or back-arc tectonic setting.

The Neoarchean Storø greenstone exhibit a tectonic contact with the surrounding tonalite-throndjemite-granodite (TTG) gneisses. Metavolcanic rocks in this belt possess near-flat to slightly enriched LREE patterns and pronounced negative Nb-Ta anomalies indicating a subduction zone geochemical signature. Metasedimentary rocks are characterized by low chemical indexes of alteration values (CIA = 50 to 71), high contents of transition metal, and enriched LREE patterns. These characteristics suggest a mixed-provenance consisting of poorly weathered felsic to mafic igneous source rocks. Collectively, the lithogeochemical characteristics of the Storø greenstone belt are consistent with a supra-subduction zone geodynamic setting.

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