EGU2020-11109
https://doi.org/10.5194/egusphere-egu2020-11109
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Self-cannibalisation of an active cumulate system (Blumone complex, Adamello, Italy): Geochemical and experimental evidence

Manuel Pimenta Silva1, Peter Ulmer1, and Othmar Müntener2
Manuel Pimenta Silva et al.
  • 1ETH Zürich, Institute of Geochemistry and Petrology, Department of Earth Sciences, Switzerland (manuel.dossantos@erdw.ethz.ch)
  • 2Institute of Earth Sciences, University of Lausanne, Lausanne, Switzerland (othmar.muntener@unil.ch)

In the southern part of the Adamello Batholith (43-33 Ma; Schaltegger et al., 2019) in Northern Italy (Re di Castello superunit), we identified a multi-generational dyke suite with “exotic” chemical compositions intruding quartz-dioritic units surrounding a gabbroic complex. These dykes are characterised by SiO2 contents between 43 and 46 wt.%, high Al2O3 (20-21 wt.%), and low MgO and Ni (below 6.5 wt.% and 40 μg/g, respectively), displaying a nepheline-normative character. Furthermore, they exhibit positive Sr and Ba anomalies. These chemical features exclude a possible primitive character or derivation from a typical calc-alkaline liquid line of descent, as identified for the Adamello Massif (Ulmer et al, 2018). The primocrystic cargo of these dikes (clinopyroxene, anorthitic plagioclase, and low-Si, high-Na pargasitic amphibole) displays striking similarities with cumulate crystals of the contiguous Blumone amphibole gabbroic cumulate, inferring mechanical interaction of these exotic liquids with and/or derivation from the cumulate complex. Amphibole-plagioclase equilibration temperatures of the dikes (875 to 775ºC) are consistent with thermal equilibration with the surrounding quartz-dioritic mush. Sharp contacts and dyke fragmentation are also observed and are thermally congruent with the ductile-brittle transition of a quartz-dioritic to tonalitic mush (Marxer & Ulmer, 2019).

Simple mass balance calculations modelling of the peritectic melting of pargasitic amphibole and high-An plagioclase (major mineral phases of the contiguous amphibole gabbroic cumulates) with simultaneous crystallisation of low-Al clinopyroxene reveal that melt compositions similar to these dykes can be achieved with amphibole-plagioclase proportions ranging between 65:35 and 50:50. To verify if peritectic cumulate remelting represents a possible generation mechanism of these dykes we performed  experiments at 0.2 GPa.

Established phase equilibria of these dyke compositions reveal a lack of near-liquidus olivine, which is a rare phase in gabbroic complex. This is consistent with preliminary experimental results on cumulate melting, where olivine is also absent at high temperatures (> 1075ºC). These observations further disprove the petrogenesis of these liquids via a calc-alkaline liquid line of descent, where mafic magmas would be early saturated in olivine at low pressure further supporting their generation by local remelting of amphibole-plagioclase dominated mafic cumulates.Geochemical as well as experimental results both strongly point towards the petrogenesis of these nepheline-normative, high-Al, low-Mg picrobasalts by low pressure peritectic melting of a pargasite-anorthite cumulate assemblage in an active magmatic system.

 

Marxer, F. & Ulmer, P. Contrib Mineral Petr. 174(10), 84 (2019).

Schaltegger, U. et al. J Petrol. 60(4), 701-722 (2019).

Ulmer, P. et al. J. Petrol. 59(1), 11-58 (2018).

How to cite: Pimenta Silva, M., Ulmer, P., and Müntener, O.: Self-cannibalisation of an active cumulate system (Blumone complex, Adamello, Italy): Geochemical and experimental evidence , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11109, https://doi.org/10.5194/egusphere-egu2020-11109, 2020

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