The emergence of the Eoarchaean proto-arc: Evolution of a c. 3700 Ma convergent plate boundary at Isua, southern West Greenland

Eoarchaean juvenile crust formed as 'proto-arcs'. The northern side of the Isua supracrustal belt is an archetypal proto-arc, with ≥3720 Ma boninites, c. 3720 Ma basalts and gabbros, 3720-3710 Ma andesites, diorites and mafic tonalites, 3710-3700 Ma intermediate-felsic volcanic and sedimentary rocks and 3700-3690 Ma chemical sedimentary rocks. On its northern side there is an extensive body of 3700-3690 Ma tonalite. During its evolution, the c. 3700 Ma Isua volcanic-sedimentary assemblage was partitioned into tectonic slices, with intercalation of mantle dunites with pillow basalts, prior to intrusion of c. 3710 Ma quartz diorites. Partitioning also occurred at 3690-3660 Ma, when the 30-20 million years life of the c. 3700 Ma Isua proto-arc was terminated by juxtaposition with the c. 3800 Ma terrane that occurs along the south of the Isua supracrustal belt. The trace element chemistry for all the ≥3720-3700 Ma mafic to intermediate volcanic rocks indicates fluid-fluxing mantle melting. The c. 3690 Ma tonalites have signatures showing melting of garnet-bearing mafic (eclogite) sources. The Isua c. 3700 Ma assemblage developed at an intra-oceanic convergent plate boundary, and it has a life-cycle broadly analogous to (but not identical to) an oceanic island arc eventually accreted against older crust.