The origin of Alpine-Himalayan K-rich orogenic lavas: an integrated experimental and geochemical approach
Final Report Abstract
In this project, we simulate the melting processes within the mantle source of orogenic magmas aiming to resolve the perplexing issues of mutual occurrence of different K-rich lavas and their contrasting geochemical signals. K-rich mantle-derived magmatism occurring within Alpine-Himalayan orogenic belt (AHOB) is characterized by the close association of different petrological members including high-K calk-alkaline, shoshonitic and lamproitic lavas. Rather than homogeneous peridotite, their source is heterogeneously metasomatised with the metasomatic assemblages – metasomes - situated in the veins within peridotitic wall-rock, originated through the melt-mantle interaction at the plate boundaries during the previous subduction. The project was focused on two major processes: i) the melting of the metasomes in the presence of peridotite; here we study physico-chemical characteristics of the resulting melts including their melting mode. This part entails 1-5GPa experiments (metasome-peridotite experiments), where we simulate the melting processes within the mantle source of orogenic melts, but also cratonic mantle-derived magmas. We combine phlogopite-clinopyroxenites with either harzburgite or lherzolite in which these rock types make up two halves of each capsule, allowing metasome-melt to infiltrate and react with the peridotite simulating melt:rock reaction; ii) the ultimate formation of the metasomes (crust- peridotite experiments). The main findings of our research can be summarized as follows: 1. In all our metasome-peridotite experiments two distinctive melt compositions are produced: the metasome-melt with lower SiO2, higher MgO and similar K2O contents compared with the infiltration-melt within the peridotitic part, that reacts with the peridotite and assimilates Opx and crystallize Ol, resulting in increases of the Ol/Opx ratio. 2. Metasome-peridotite experiments demonstrate a substantial role of the peridotite type (lherzolite vs. harzburgite) on the composition of the resulting melt: when lherzolite is involved, the resulting infiltration-melts are silica-rich (up to 55 wt.% SiO2), with high Al2O3 (up to 17 wt.%), resembling orogenic high-K calk-alkaline and shoshonitic lavas; when harzburgite is involved, the resulting infiltration-melts are still silica-rich (up to 55 wt.% SiO2), but with lower Al2O3 (around 10 wt.%) and CaO, resembling orogenic lamproites. 3. The metasome-peridotite experiments with the kaersutite-rich metasome and peridotite, demonstrate that at low temperature the infiltration melts are silica-rich (up to 50 wt.% SiO2), with low Na2O/K2O (~1) resembling Alpine-Himalayan orogenic belt high-K calk-alkaline as well as shoshonitic lavas. However, with increased T the infiltration melts resemble intraplate lavas not only in major elements having considerably higher Na2O/K2O and lower SiO2 (>>1) but also in trace element patterns. 4. Metasome-peridotite experiments also demonstrate that the trace element content of infiltrating melt is fully controlled by the composition of the metasome, no matter if the peridotite is lherzolitic or harzburgitic. 5. Our metasome-peridotite experiments illustrate that melting of mixed source regions is not simply a question of producing and mixing two melt types; rather, the melt produced in the rock with the lower melting temperature reacts with the second rock, changing its mineralogy and taking up some of its components. 6. In our crust-peridotite experiments, we observed the separation of elements and a layered arrangement of metasomatic phases, with layers consisting of orthopyroxene, mica-pyroxenite, and clinopyroxenite at the sediment-peridotite interface as well as within the peridotite. The selective incorporation of elements in these metasomatic layers closely resembles chemical patterns found in K-rich magmas.
Publications
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Reaction Experiments of Glimmerite + Harzburgite at 1-2 GPa and Genesis of Orogenic Ultrapotassic Magmas, EMPG XV, Zurich, 06/2016
Förster, M. W., Prelević, D., Buhre, S., Schmück, H. R., Veter, M., Mertz-Kraus, R., Foley, S. F., Jacob, D. E.
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Reaction Experiments of Glimmerite + Harzburgite at 3-5 GPa and Genesis of low-SiO2 Ultrapotassic Magmas, EMPG XV, Zurich, 06/2016
Förster, M. W., Prelević, D., Buhre, S., Schmück, H. R., Veter, M., Foley, S. F., Jacob, D. E.
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Partitioning of nitrogen during partial melting of phlogopite-rich metasomes. Goldschmidt2017, Paris, France, 08/2017
Förster, M. W., Prelević, D., Buhre, S., Foley, S. F.
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(2018). The origin of Alpine-Himalayan orogenic K- rich lavas: an integrated experimental and geochemical approach. EMAW 2018., 3rd European mantle workshop, Pavia - Università Centrale
Prelević, D., Buhre, S., Förster, M.
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(2018). The origin of Alpine-Himalayan orogenic K- rich lavas: an integrated experimental and geochemical approach. Goldschmidt 2018, abstracts
Prelević, D., Buhre, S., Förster, M.
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Nitrogen partitioning in subduction zone processes. IMA2018, Melbourne, 08/2018
Förster, M. W., Prelević, D., Buhre, S., Foley, S. F.
