Summary
The Salmi batholith is situated on the eastern edge of the EW-trending anorthositerapakivi granite belt of the Fennoscandian shield, at the contact between Proterozoic and Archean crustal domains. The tectonic setting and high K, Rb, Nb, Y, Zr, REE (except Eu), F, Sn, Be, and Li contents of.the Salmi batholith indicate that it represents typical subalkaline A-type and within plate granites. Gabbro-anorthosites of the batholith demonstrate a concordant U-Pb apatite age of 1563 ± 9 (2σ) Ma and a Sm-Nd internal isochron age of 1552 ± 69 Ma. Zircons from amphibole-biotite granites have an upper concordia intercept U-Pb age of 1543 ± 8 Ma. An older inherited zircon component with elevated Th/U ratio is found in zircons separated from K-feldspar ovoids. Rb-Sr internal errorchron for the granites yields an age of 1455 ± 17 Ma, probably the time of completion of postmagmatic processes within the batholith. The gabbro-anorthosites and granites show similar initial Nd, Sr, and feldspar Pb isotope compositions (ɛ Nd = - 6.5 to - 8.2;μ 2 = 8.6 to 8.9;κ 2 = 3.9 to 4.0; ISr = 0.7052 to 0.7057 for the basic rocks, andɛ Nd = -6.2 to -8.9;μ 2 = 8.1 to 9.2;κ 2 = 4.0 to 4.4; ISr = 0.7050 to 0.7072 for the granites). Two-stage neodymium TDM model ages for both assemblages range from 2.60 to 2.80 Ga. Old LREE-enriched sources with low time-integrated U/Pb and Rb/Sr and elevated Th/U ratios were involved in the formation of both the gabbroanorthosites and the granites. Bulk contamination with crustal materials cannot explain the data for the basic rocks. Selective incorporation of Pb, Sr, and Nd from Archean lower crust is needed, or else, the gabbro-anorthosites may have been derived from an isotopically anomalous subcontinental mantle source. The ascent of a mantle diapir resulted in anatexis of the lower crust and formation of the parent magma for the rapakivi granites.
Zusammenfassung
Der Salmi-Batholith ist am östlichen Rand des Ost-West streichenden Anorthosit Rapakivi-Granitgürtels des fennoskandischen Schildes am Kontakt zwischen proterozoischen und archaischen Krustenbereichen gelegen. Die tektonische Position und hohe K, Rb, Nb, Y, Zr, REE (mit Ausnahme von Eu), F, Sn, Be und Li-Gehalte des Salmi-Batholithen weisen darauf hin, daß es sich hier um einen typischen subalkalischen A-Typ und “within plate” Granit handelt. Gabbro-Anorthosite des Batholithen zeigen ein konkordantes U-Pb Apatit alter von 1563 ± 9 (2σ) Ma und ein internes Sm-Nd Isochronenalter von 1552 ± 69 Ma. Zirkone aus den Amphibolit-Biotitgraniten haben ein oberes Concordia U-Pb Alter von 1543 ± Ma. Ein ältere, ererbte Zirkonkom ponente mit erhöhtem Th/U Verhältnis kommt in Zirkonen, die von K-Feldspat Ovoi den abgetrennt wurden, vor. Eine interne Rb-Sr Errorchrone für die Granite ergibtein Alter von 1455 ± 7 Ma. Dies repräsentiert wahrscheinlich die Zeit des Abschlusses postmagmatischer Prozesse innerhalb des Batholithen. Die Gabbro-Anorthosite und Granite zeigen ähnliche Nd, Sr und Feldspat Pb Isotopenzusammensetzungen (ε Nd = −6.5 to 8.2;μ 2 = 8.6 to 8.9; K2 = 3.9 to 4.0; ISr = 0.7052 to 0.7057 für die mafischen Gesteine, undɛ Nd = −6.2 to -8.9;μ 2 = 8.1 to 9.2; K2 = 4.0 to 4.4; ISr = 0.7050 to 0.7072 für die Granite). Zweistufige Neodymium TDM Modellalter für beide Assoziationen liegen zwischen 2.60 und 2.80 Ga. Alte LREE-angereicherte Quellen mit niedrigen, zeitintegrierten U/Pb und Rb/Sr und erhöhten Th/U Verhältnissen waren bei der Bildung der Gabbro-Anorthosite und Granite involviert. Kontamination mit Krus tenmaterial kann die Daten für die basischen Gesteine nicht erklären. Entwederist dazu eine selektive Inkorporation von Pb, Sr und Nd aus der tieferen archaischen Kruste erforderlich oder man muß annehmen, daß die Gabbro-Anorthosite von einer isotopisch anomalen subkontinentalen Mantelquelle stammen. Der Aufstieg eines Mantel-Diapirs führt zu Anatexis der tieferen Kruste und zur Bildung der Ausgangsmagmen für die Rapakivi Granite.
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Neymark, L.A., Yu. Amelin, V. & Larin, A.M. Pb-Nd-Sr isotopic and geochemical constraints on the origin of the 1.54–1.56 Ga Salmi rapakivi granite—Anorthosite batholith (Karelia, Russia). Mineralogy and Petrology 50, 173–193 (1994). https://doi.org/10.1007/BF01160146
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DOI: https://doi.org/10.1007/BF01160146