The trace element geochemistry of clinopyroxenes from pyroxenites in the Lewisian of NW Scotland: insights into light rare earth element mobility during granulite facies metamorphism |
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| Authors: | Hugh Rollinson Philip Gravestock |
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| Institution: | (1) School of Science, University of Derby, Kedleston Road, Derby, DE22 1GB, UK;(2) University of Gloucestershire, Francis Close Hall, Swindon Road, Cheltenham, GL50 4AZ, UK |
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| Abstract: | Clinopyroxenes from layered pyroxenites and from pyroxenite pods in felsic gneisses of the Lewisian granulite complex, NW
Scotland, have distinctive chemistries suggestive of different origins. Clinopyroxenes in the layered pyroxenites crystallised
from mafic melts in a magma chamber located in the middle to shallow crust, whereas clinopyroxenes in pods in the felsic gneisses
crystallised from the tonalitic protolith to the felsic gneisses. In detail clinopyroxenes in the layered pyroxenites are
variably enriched in the light REE. Inversion modelling shows that this is not a primary feature inherited from their parent
magmas. Rather selective light rare earth element enrichment took place through reaction with a felsic melt generated by the
localised partial melting of the hornblende pyroxenites during granulite facies metamorphism. Published isotopic evidence
suggests that the light REE mobilisation took place at ca 2.7 Ga, about 200 Ma after the time of crust formation. This observation
provides an explanation for the scattered pattern of whole-rock isochron ages from the Lewsian granulites. |
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