Petrology of the Upper Border Series of the Skaergaard Intrusion |
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Authors: | NASLUND H R |
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Institution: | Department of Earth Sciences, Dartmouth College Hanover, N.H. 03755 |
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Abstract: | The Upper Border Series of the Skaergaard intrusion consistsof a 960 m thick sequence of rocks that crystallized againstthe roof of the magma chamber. The texture and composition ofthe unit vary systematically from top to bottom as a resultof changes that occurred in the magma during the solidificationof the intrusion. The order of crystallization of primocrystminerals in the Upper Border Series was: olivine; + plagioclase;+ apatite; + ilmenite; + magnetite; + Ca-rich pyroxene;olivine;+ olivine; + ferrobustamite. The major silicate phases varyfrom high-temperature compositions to low-temperature compositionswith increasing distance from the upper contact. Post-crystallizationre-equilibration has affected the compositions of the oxideminerals and to a lesser extent the compositions of olivineand Ca-rich pyroxene. The Upper Border Series differentiationsequence differs from the Layered Series sequence, in that:(1) apatite appears much earlier; (2) magnetite precipitatedbefore Ca-rich pyroxene rather than after it; (3) orthopyroxeneis much less common; (4) the plagioclase is systematically poorerin K2O; and (5) the rocks are systematically richer in K2O andSiO2. The upper part of the Skaergaard magma appears to havebeen enriched in H2O, K2O, SiO2, and P2O5 relative to the partthat was parental to the Layered Series. |
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