Prolonged Paleozoic Magmatism in the East Greenland Caledonides: Some Constraints from U-Pb Ages and Hf Isotopes

Abstract

New U, Pb, and Hf isotopic data from thermal ionization mass spectrometry and inductively coupled plasma mass spectrometry from intermediate calc-alkaline rocks in the Caledonides of central East Greenland provide new evidence of a prolonged period of Paleozoic subduction-related magmatism. The new data point toward two groups of plutons: one older generation crystallizing at ca. 450 Ma and a younger generation at ca. 423 Ma. There is additionally a titanite-forming event at ca. 440 Ma. The Hf isotopes of the younger group have a larger variation and less juvenile character than those of the older group, but data indicate a mixed source with both crustal and mantle components for both groups. These new data combined with already published geochronological data testify to a prolonged and complex magmatic history starting markedly earlier in the southernmost segment of the East Greenland Caledonides than farther north. The varied geochemical compositions of the plutons and the prolonged intrusive activity are in contrast to the short-lived homogeneous S-type leucogranitic magmatism found elsewhere in the East Greenland Caledonides and bear more similarities to arc-related terranes now found in the Irish, Scottish, and Scandinavian Caledonides. It is speculated that the earlier plutons in East Greenland represent the northernmost expression of continental arcs formed in a west-directed subduction system along the full length of the Laurentian margin in the Paleozoic. The later plutons formed concomitant with leucogranite intrusions within a collisional setting.