Geochemical Development of Proterozoic Granites in the SW Baltic Shield

University dissertation from SMC, Box 839, SE-981 28 Kiruna, Sweden

Abstract: The plutonic rocks in the Western Segment of the Southwestern Swedish Gneiss Complex show a distinct geochemical evolution. The 1.6 Ga Åmål granitoids and Slottsbron migmatites are a quartz dioritic to granodioritic, calc-alkaline rock suite with smooth, but somewhat fractionated, REE and other trace-element patterns. The younger microcline granites of the 1.5 Ga and 1.3 Ga generations are evolved rocks having fractionated REE patterns with deep negative Eu anomalies. The 1.5 Ga granites vary from calc-alkalic, metaluminous granodiorites to evolved alkali-calcic peraluminous leucogranites. The REE and other trace-element patterns are less evolved than those of the 1.3 Ga rocks. The 1.3 Ga granites are the most evolved intrusions in the area. Granites with a bimodal geochemical character occur. All rocks can be ascribed to one of three types: i. The HUS1 type is homogeneous, alkali-calcic with high SiO2 content and is often leucocratic. ii. The SS2 type has a large geochemical variation, but shows regular inter-element relationships. It is alkali-calcic and has the most evolved REE patterns. iii. In the TYP3 type, calc-alkaline compositions are common. They are the least evolved rocks of this generation. The differences within the 1.3 Ga generation itself are partly due to a variety of source rocks, but also to different melting conditions. The chemical variation suggests that the SS2 type formed at deeper levels than the HUS1 and 1.5 Ga granites. The degree of melting was probably also higher than for the two latter types. The TYP3 rocks are likely to have formed at shallow levels, but with a higher degree of melting than the HUS1 rocks. A model based on isotopic, major- and trace-element data is consistent the theory that rocks similar to the Åmål granitoids are the source rock for the HUS1 type but not for the SS2 type. The epsilonNd(1.3) values for one of the HUS1-type rocks are, however, outside the known range of the time integrated epsilonNd(1.3) for the Åmål granitoids and could have a more primitive origin. Rocks from the Eastern Segment differ from those of the WS. The 1.7 Ga Mårdaklev granite has an evolved appearance similar to the HUS1 type, but no coeval rocks in the WS have this characteristic. The compositions of the 1.4 TTK intrusions vary between monzonite and granite - a trend not found in the Western Segment. The differences in geochemical character reflect differences in geological settings. The TTK intrusions of the Eastern Segment are interpreted to have formed at deep levels in a thicker and more evolved continental crust than the roughly coeval granites in the Western segment. The calc-alkaline character of the 1.6 Ga generation rocks suggests a less evolved, subduction related setting. The younger granites are derived from ensialic crust and variations in composition are interpreted to reflect different source rocks and different levels of magma generation. This investigation supports earlier suggestions that the evolutions of the Eastern and Western Segments were separate before Sveconorwegian time.

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