The campsite dykes: A window into the early post-solidification history of the Skaergaard Intrusion, East Greenland
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The Skaergaard Intrusion of East Greenland is cut by several generations of dykes, the earliest of which is thought to have intruded shortly after solidification of the Skaergaard. Two ~ 6 m wide doleritic dykes from the earliest generation are exposed in the campsite area near Homestead Bay of the Skaergaard Peninsula. One of the dykes (the Campsite Dyke) locally contains abundant xenoliths of troctolitic cumulate. The other (the Plagioclase-phyric Dyke) contains abundant large plagioclase phenocrysts. Cross-cutting relationships between the two dykes are not exposed. The median clinopyroxene–plagioclase–plagioclase dihedral angle, Θcpp, in the Campsite Dyke is 88–89.5°, whereas that of the Plagioclase-phyric Dyke is 79°. Using an empirical relationship between Θcpp and the duration of crystallisation derived from dolerite sills, the observed Θcpp suggests that the Campsite Dyke is the older of the two, intruding the Skaergaard when it had cooled to 920–970 °C. The Plagioclase-phyric Dyke intruded later, once the Skaergaard had cooled below 670 °C. The troctolitic xenoliths divide into two separate groups. Type A xenoliths have microstructures similar to those of the Skaergaard Layered Series although mineral compositions are generally more primitive than those of the exposed cumulates — this type of xenolith is likely to have been derived from either deeper levels in the Skaergaard Intrusion or from a closely-related underlying magma chamber. One Type A xenolith has mineral compositions and Θcpp consistent with an origin in LZb of the Layered Series — this xenolith contains partially inverted pigeonite, suggesting that inversion of low-Ca pyroxene in the lower part of the Layered Series took place after the intrusion had completely solidified. Type B xenoliths are characterized by plagioclase containing large and abundant melt inclusions. Comparison with the microstructures of glassy crystalline nodules from Iceland points to a multi-stage cooling history for Type B xenoliths, consistent with step-wise entrainment of partially crystallised material from a deep chamber. Type B xenoliths are very unlikely to have been derived from deeper levels in the Skaergaard chamber.
We thank Madeleine Humphreys for her assistance in collecting samples from the Campsite area. We are grateful to Monica Price of the Oxford University Natural History Museum for access to samples from the Wager East Greenland collection, and to Christian Tegner and Kent Brooks for loan of the sample from the Campsite Dyke chill zone. John Maclennan loaned us material from Iceland and we both thank him and David Neave for interesting discussions about their microstructures. Insightful and helpful comments from Tony Morse and an anonymous reviewer greatly improved an earlier version of this contribution. QEMSCAN® is a registered trademark of FEI Company. FEI Company sponsored the QEMSCAN® analyses, which were completed by Dr Gavyn Rollinson, at Camborne School of Mines, University of Exeter, UK. This work was supported by the Natural Environment Research Council [grant numbers NE/F020325/1 and NE/J021520/1].
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Vol. 182-183, pp. 134 - 149