Post-collisional potassic-ultrapotassic magmatism of the variscan orogen: Implications for mantle metasomatism during continental subduction
Mantle-derived potassic to ultrapotassic magmatism is a typical feature of collisional orogens. Potassium-rich magmas recurrently formed across the European Variscides during a period of 50 Myr, following the peak of the orogeny at 340 Ma. Lamprophyre dykes are part of this magmatism and have crust-...
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| Main Authors: | , |
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| Format: | Article (Journal) |
| Language: | English |
| Published: |
06 June 2018
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| In: |
Journal of petrology
Year: 2018, Volume: 59, Pages: 1007-1034 |
| ISSN: | 1460-2415 |
| DOI: | 10.1093/petrology/egy053 |
| Online Access: | Verlag, Volltext: https://doi.org/10.1093/petrology/egy053 Verlag: http://adsabs.harvard.edu/abs/2018JPet...59.1007S Verlag, Volltext: https://academic.oup.com/petrology/article/59/6/1007/5033660 
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| Author Notes: | Christian G. Soder and Rolf L. Romer |
| Summary: | Mantle-derived potassic to ultrapotassic magmatism is a typical feature of collisional orogens. Potassium-rich magmas recurrently formed across the European Variscides during a period of 50 Myr, following the peak of the orogeny at 340 Ma. Lamprophyre dykes are part of this magmatism and have crust-like trace element patterns, as well as elevated initial 87Sr/86Sr and 207Pb/204Pb and low 143Nd/144Nd along with high mantle-compatible trace element concentrations. This hybrid nature requires at least two source components: subducted continental crustal material and mantle peridotite. Sampling of dyke rocks across tectonic zones of contrasting development reveals two groups of K-rich mantle-derived rocks with distinct trace element patterns and isotopic compositions. The dataset covers a wide range of magma compositions, reflecting their multi-stage petrogenesis. Many of the geochemical characteristics of ultrapotassic magmas, such as very high K2O/Na2O, are already features of the high-pressure crustal melts. Whether the trace element signature is transferred unchanged into these liquids or not largely depends on the behaviour of accessory phases. For instance, high Th/La is related to residual allanite during partial melting of subducted felsic crust. The silica-rich liquids migrate from the slab into the overlying lithospheric mantle. Reaction with peridotitic wall-rocks during channelized flow crystallizes orthopyroxene ± garnet at the expense of olivine, resulting in depletion in Al2O3 and garnet-compatible trace elements in the coexisting melt. Progressive wall-rock interaction causes enrichment in incompatible trace elements and may produce peralkaline melt compositions. These metasomatic agents eventually freeze in the lithospheric mantle, forming non-peridotitic lithologies rich in hydrous minerals such as phlogopite. Variable degrees of melting during post-collisional and later lithospheric extension preferentially affect the heterogeneously metasomatized mantle domains, which results in a broad range of lamprophyre compositions, including amphibole lamprophyres, mica lamprophyres and peralkaline lamproites. |
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| Item Description: | Gesehen am 17.12.2019 |
| Physical Description: | Online Resource |
| ISSN: | 1460-2415 |
| DOI: | 10.1093/petrology/egy053 |