Origin and Evolution of Gabbroic Pegmatites in the Skaergaard Intrusion, East Greenland

Abstract

The layered tholeiitic gabbros of the Skaergaard Intrusion contain a large variety of gabbroic pegmatites characterized by plagioclase, pyroxene, Fe—Ti oxides, olivine, and apatite, as well as quartz, amphibole, and biotite. The pegmatites of the Layered Series (LS) have a characteristic geometric evolution from the Lower Zone (LZ) through the Middle Zone (MZ) to the Upper Zone (UZ). Pegmatites of the LZ and the lower half of the MZ form irregular podiform bodies 0.2–5 m in diameter, whereas those of the upper half of the MZ and the UZ appear as semi-conformable sheets of 0–2–0–5-metre thickness, extending from 200 to > 1000 m in the plane of bedding. A third variety of gabbroic pegmatites was generated in structural weakness zones, yielding columnar pegmatites intersecting 10 m or more of cumulate stratigraphy. The pegmatites have three structural elements in common: (A) the margin, characterized by laths of plagioclase of 2–15-cm length, grading into (B) the gabbroic zone, consisting of plagioclase, pyroxene, ilmenite, magnetite, apatite, ± olivine; (B) becomes progressively more enriched in amphibole, ± biotite as unit (C) is approached; (C) is the granophyric zone with quartz, orthoclase, plagioclase, fayalite, hastingsite, apatite, ± pyroxene. The zones are spatially arranged with (A) in the margin and bottom of the pegmatites through (B) to (C), which lies in the upper parts of the pegmatites. Fluid inclusion studies show that the upper parts of the gabbroic zone and the granophyric zone coexisted with aqueous solutions with 2–6 mol% CH4 and 17.5–22.8 wt.% dissolved NaCl. The modal zoning in the podiform pegmatites of the LZ is accompanied by a cryptic variation from (A) to (C): from An 45 to An24, from En35 to En25, and from Fo30 to Fo5. Minimum temperatures of crystallization for (A), (B), and (C) are 1006 oC, 859 oC, and 766 oC, respectively. The compositions of the plagioclases and pyroxenes constituting the gabbroic zone of the pegmatites are similar to intercumulus products from the layered gabbros hosting the pegmatites. This relationship shows that the pegmatites crystallized from the intercumulus liquids which were derived from the Skaergaard parental melts. The pegmatitic melts were buoyant owing to a density contrast of −0.4 g/cm3 compared with the cumulates. Diapiric flow was the dominant mechanism of vertical ascent of pegmatitic melts in the lower half of the magma chamber, whereas the melts followed a system of structural weakness zones or fractures in the upper half of the semi-solid magma chamber. The gabbroic pegmatites are essentially late magmatic products derived from the Skaergaard melt and, therefore, provide us with unique objects for the study of terminal crystallization in this classic layered gabbro.