Origin of quartzo-feldspathic supracrustal rocks from the central part of the Nagssugtoqidian Mobile Belt of West Greenland

Abstract

The Marranguit-Kangilerssua supracrustals constitute a sequence of variable supracrustallithologies intensely folded into surrounding orthogneisses and metamorphosed under granulite facies conditions. No field or petrographic evidence indicates the original nature of the individual supracrustal rock types. Fortynine samples from the quartzo-feldspathic part of the supracrustals have been analysed for major elements and the trace elements Cr, Co, Ni, Cu, Zn, Rb, Sr, Zr and Ba. Ten different geochemical discrimination methods indicate that both metasedimentary and metaigneous material are present. The metaigneous rocks possess typical calc-alkaline differentiation trends and were originally extrusives. Mixing-zones between metasedimentary and metavolcanic material are interpreted as reflecting a pyroclastic nature of some of the metavolcanic rocks. The quartzo-feldspathic part of the M-K supracrustals is dominated by greywacke/lithic arenite with subordinate intercalations of mudstone and single layers or lensoid bodies of subarkose and sublithic arenite. The greywacke/lithic arenite grade into calc-alkaline pyroclastics, which vary in composition from rhyodacite to rhyolite. Associated quartz-andesitic and dacitic extrusives occur throughout the supracrustal formation, which in addition contains thin layers af limestone, basic and ultrabasic rocks. The supracrustals may be either Archaean or Proterozoic in age.