Origin and Evolution of Gabbroic Pegmatites in the Skaergaard Intrusion, East Greenland
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 e...
| Published in: | Journal of Petrology |
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| Main Authors: | , |
| Format: | Text |
| Language: | English |
| Published: |
Oxford University Press
1994
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| Subjects: | |
| Online Access: | http://petrology.oxfordjournals.org/cgi/content/short/35/6/1651 https://doi.org/10.1093/petrology/35.6.1651 |
| Summary: | 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% CH 4 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 An 24 , from En 35 to En 25 , and from Fo 30 to Fo 5 . 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 ... |
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