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The Nagssugtoqidian Mobile Belt (NMB) is a 300 km wide, 800 km long orogenic zone in central Greenland. It is part of the >20,000 km long complex of orogenic belts that formed between 1.7 and 2.0 Gya during assembly of Columbia, the first supercontinent. The NMB is composed of several major crustal-scale shear zones that intervene between coherent blocks of high grade metamorphic rocks. By combining detailed field, laboratory and aeromagnetic studies along the most northern of the shear zones (the Nordre Stromfjord Shear Zone - NSSZ), we have been able to document processes indistinguishable from those observed in Phanerozoic collisional orogens:
1. At 1,923 +/- 20 Mya: Emplacement of a calc-alkaline complex within and on ca. 2.8 Gya continental crust. The basal cumualte portion of the complex is well preserved and records invasion of multiple magma pulses. Coeval pillow basalts and cogenetic porphyritic mafics are also present.
2. 1,900 to 1,800 Mya: Tectonic emplacement of mafic and ultramafic rocks under high pressure (>2.5 GPa), eclogite facies conditions. The emplacement of these lenses probably occurred prior to or during thrust stacking associated with continent-continent collision. The age of the high pressure metamorphism makes these the oldest known eclogite facies metamorphic rocks in the world.
3. 1760 to 1720 Mya: Development of the transcurrent NSSZ, with displacements in excess of a hundred kilometers. Profound chemical enrichment of potassium and phosphorus along the entire length of the NSSZ unequivocally demonstrates that the shear zone was the focus of massive fluid movement that chemically modified the crust.
The association of the calc-alkaline complex with high pressure metamorphic rocks reminiscent of dismembered ophiolite sequences are the signature of a subduction complex. Detailed analysis of phase relationships documents a P-T path identical in form to that of Alpine metamorphic rocks, but displaced toward higher temperatures. These observations provide compelling evidence that assembly of this segment of Columbia involved subduction of oceanic crust under a convergent continental margin. Subduction ceased when collision occurred between converging continental blocks, similar to the Alpine-Himalayan system. The convergent movement of the colliding continents was then transposed into left-lateral transverse slip, resulting in the development of a crustal scale shear zone along the collision front. Subducted continental crust heated and dehydrated, releasing a large volume of fluid. That fluid, along with trapped fluids in the subduction complex, escaped along the NSSZ, enriching the rocks of the shear zone in potassium and phosphorus. Similar chemical modification has been observed in other segments of the orogenic zones within Columbia. Calculations demonstrate that the total fluid volume that caused the observed chemical changes exceeded that of the current world ocean.
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