Complex geochemical structure, age and petrogenesis of the A-type Mesoproterozoic Karlshamn pluton, South Sweden

Abstract

The Karlshamn pluton which intruded at ca. 1.45 Ga in southern Fennoscandia contains two major coeval suites. Both of these have A-type affinity and consist of metaluminous, ferroan, alkali-calcic, and very-high K (shoshonitic) granitic rocks. The two suites are geographically separated within the pluton, and form (semi)parallel trends on Harker-type diagrams. The Eastern suite comprises quartz monzodiorite, quartz monzonite and granite while the Western suite mainly consists of monzogranites. Both suites exhibit similar chondrite-normalized REE patterns with relatively steep LREE and flat HREE. Aluminium-in-hornblende barometric and zircon-saturation thermometric estimates show that both suites were crystallized at relatively high temperatures of 850 - 900 °C and low pressures of 0.33– 0.43 GPa.

Ion microprobe zircon dating from a monzonite of the Eastern suite yielded concordant to nearly concordant U-Pb isotopic compositions. The weighted average ²⁰⁷Pb/²⁰⁶Pb age is 1445 ± 11 Ma (2σ, MSWD 1.2), interpreted to date magmatic crystallization. The complex zircons from a monzogranite in the Western suite showed somewhat more scatter and slightly younger ages, however, there are no signs of older inherited materials and no significant age differences between the core-like domains and the overgrowths. The cores yielded a weighted average ²⁰⁷Pb/²⁰⁶Pb age of 1431 ± 20 Ma (2σ, MSWD 1.5) and the rims an age of 1424 ± 19 Ma (2σ, MSWD 4.2). The overall age of the zircons from the Western suite is 1426 ± 11 Ma (2σ, MSWD 2.6).

The two major suites as well as related leucogranites and mafic enclaves that are present mainly in the Western suite have indistinguishable initial ε_Nd values of −2.2 to −2.7.The Rb- Sr system shows some disturbances, however, initial ⁸⁷Sr/⁸⁶Sr ratios of undisturbed samples from the two major suites vary between 0.703 and 0.706. Both suites originated either by partial melting of pre-existing mafic rocks within the lower crust or by mixing of mantle-derived magma with crustal melts derived from partial melting of older country rocks.

The mafic enclaves are interpreted as cumulates formed along the walls of the feeder conduits by fractional crystallization caused by the temperature gradient across the feeder walls. The presence of these mafic enclaves in the Western but not the Eastern suite may indicate that they were formed from the Eastern suite magma and subsequently were picked up and engulfed in the later Western suite magma, using the same magma conduits.

Major element geochemical modeling indicate that the Eastern suite intruded first (in accordance with age determinations) and that the remaining magma experienced around 20 % of plagioclase fractionation before it intruded to form the Western suite. The vicinity of the Karlskrona deformation zone may have influenced the intrusion of the pluton.