Kärenlampi, K., Heinonen, J. S., Kontinen, A., Hanski, E., & Huhma, H. (2021). Geochemical and thermodynamic modeling of the petrogenesis of A1-type granites and associated intermediate rocks: A case study from the central Fennoscandian Shield. Geochemistry, 81(2), 125734. https://doi.org/10.1016/j.chemer.2020.125734
Geochemical and thermodynamic modeling of the petrogenesis of A1-type granites and associated intermediate rocks : a case study from the central Fennoscandian Shield
|Author:||Kärenlampi, Kimmo1; Heinonen, Jussi S.2; Kontinen, Asko3;|
1Oulu Mining School, P.O. Box 3000, 90014 University of Oulu, Finland
2Department of Geosciences and Geography, P.O. Box 64, 00014 University of Helsinki, Finland
3Geological Survey of Finland, P.O. Box 1237, 70211 Kuopio, Finland
4Geological Survey of Finland, P.O. Box 96, 02151 Espoo, Finland
|Online Access:||PDF Full Text (PDF, 10.2 MB)|
|Persistent link:|| http://urn.fi/urn:nbn:fi-fe2021082744558
|Publish Date:|| 2021-08-27
The origin of ferroan A-type granites in anorogenic tectonic settings remains a long-standing petrological puzzle. The proposed models range from extreme fractional crystallization of mantle-derived magmas to partial melting of crustal rocks, or involve combination of both. In this study, we apply whole-rock chemical and Sm-Nd isotopic compositions and thermodynamically constrained modeling (Magma Chamber Simulator, MCS) to decipher the genesis of a suite of A1-type peralkaline to peraluminous granites and associated intermediate rocks (monzodiorite-monzonite, syenite) from the southwestern margin of the Archean Karelia craton, central Finland, Fennoscandian Shield. These plutonic rocks were emplaced at ca. 2.05 Ga during an early stage of the break-up of the Karelia craton along its western margin and show trace element affinities to ocean island basalt-type magmas. The intermediate rocks show positive εNd(2050 Ma) values (+1.3 to +2.6), which are only slightly lower than the estimated contemporaneous depleted mantle value (+3.4), but much higher than average εNd(2050 Ma) of Archean TTGs (-10) in the surrounding bedrock, indicating that these rocks were essentially derived from a mantle source. The εNd(2050 Ma) values of the peralkaline and peraluminous granite samples overlap (-0.9 to +0.6 and -3.2 to +0.9, respectively) and are somewhat lower than those in the intermediate rocks, suggesting that the mafic magmas parental to granite must have assimilated some amount of older Archean continental crust during their fractionation, which is consistent with the continental crust-like trace element signatures of the granite members. The MCS modeling indicates that fractional crystallization of mantle-derived magmas can explain the major element characteristics of the intermediate rocks. The generation of the granites requires further fractional crystallization of these magmas coupled with assimilation of Archean crust. These processes took place in the middle to upper crust (∼2–4 kbar, ∼7–15 km) and involved crystallization of large amounts of clinopyroxene, plagioclase and olivine. Our results highlight the importance of coupled FC-AFC processes in the petrogenesis of A-type magmas and support the general perception that magmas of A-type ferroan granites become more peraluminous by assimilation of crust. They further suggest that variable fractionation paths of the magmas upon the onset of assimilation may explain the broad variety of A-type felsic and intermediate igneous rocks that is often observed emplaced closely in time and space within the same igneous complex.
|Type of Publication:||
A1 Journal article – refereed
|Field of Science:||
Financial support to Kimmo Kärenlampi was provided by the Advanced Materials Doctoral Programme (ADMA-DP) of the University of Oulu, K.H. Renlund Foundation and Tauno Tönning Foundation, and Jussi S. Heinonen by the Academy of Finland (Grant no. 295129). The field work was supported by Otanmäki Mine Oy (www.otanmaki.fi) and Jouko Jylänki, the CEO of the company, which is gratefully acknowledged.
© 2020 The Author(s). Published by Elsevier GmbH. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).