Rare earth element geochemistry and strontium isotopic composition of a massif-type anorthositic-charnockitic body: the Hidra Massif (Rogaland,SW Norway) |
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| Authors: | Daniel Demaiffe Jan Hertogen |
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| Institution: | Minéralogie-Pétrologie, Université Libre de Bruxelles, B-1050 Bruxelles, Belgium;Instituut voor Nucleaire Wetenschappen, Rijksuniversiteit Gent, B-9000 Gent, Belgium |
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| Abstract: | The Hidra Massif (Rogaland Complex, SW Norway) mainly consists of plagioclase cumulates (anorthosites and leuconorites), which grade progressively into a fine-grained (200 μm). locally porphyritic, jotunitic rock towards the contact with the granulite facies gneisses. The massif is cross-cut by thin (10 cm up to 1 m) charnockitic dykes.The petrographical and geochemical evolution of the Hidra Massif can be explained by fractional crystallization of a jotunitic parental magma. Major and trace element constraints indicate that mafic phases are underabundant in the exposed levels of the massif, most likely as a result of plagioclase flotation in the early stages of solidification. Partitioning into the cumulate minerals (mainly plagioclase and orthopyroxene) governs the trace element contents of the leuconoritic adcumulates. However, the trace element geochemistry of the apparently early formed anorthositic orthocumulates largely depends upon the amount of a trapped intercumulus liquid. On the basis of trace element abundances (high REE, Rb, Th, U; negative Eu anomalies) the silicic charnockitic dykes can be considered as the residual liquid of the anorthositic fractionation trend. The higher initial ratios (0.7086 ± 0.0006 vs 0.7055 ± 0.0004 for the plagioclase cumulates and jotunites) point to contamination of the charnockitic liquids by surrounding gneissic material. |
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