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MPN-12 Sederholm symposium on high-grade metamorphism, crustal melting, migmatites and granites - Part 2

Diatexite-granite connection: The origin of the 1.88 Ga Vaasa granite, Fennoscandian Shield

Hannu Mäkitie, Geological Survey of Finland (Finland)
Pekka Sipilä, Geological Survey of Finland (Finland)
Matti I Lehtonen, Geological Survey of Finland (Finland)
Hannu Kujala, Geological Survey of Finland (Finland)
Raimo Lahtinen, Geological Survey of Finland (Finland)
Petri Virransalo, Geological Survey of Finland (Finland)
 

The 1.88 Ga Vaasa granite in western Finland is a large peraluminous (∼5000 km²) granitoid area, which has gradational contacts with Svecofennian pelitic diatexites and garnet-cordierite metatexites that have been metamorphosed in upper amphibolite-facies conditions. It is composed of even-grained granodiorites (granites) (Bt content: 10-25%), porphyritic granodiorites (granites) (Bt: 15-25%) and orthopyroxene tonalites (granodiorites) (Bt: 15-25%), all containing metasedimentary relicts. Garnet and monazite are accessory minerals but hornblende and sphene are absent in the granitoids. The diatexites (Bt: 15-30%) form 0.01-3 km wide irregular zones between the granitoids and metatexites (Bt: 20-35%).
In the southern part of the Vaasa granite, the diatexites have slightly higher K₂O and Rb, but lower FeO_T, MgO and CaO contents than the metatexites. The even-grained and porphyritic granitoids resemble chemically each others. Compared to the diatexites, they have slightly elevated K₂O (3.9 wt%), Rb (165 ppm) and F (0.11 wt%) contents, but are little depleted in FeO_T (4.5 wt%), MgO (1.8 wt%), Ni (29 ppm) and Cr (66 ppm). The orthopyroxene granitoids are enriched, particularly, in TiO₂, FeO_T, MgO and CaO, but little depleted in SiO₂ (62.5 wt%) relative to the other rocks. The REE patterns of studied rocks are generally similar, with negative Eu anomalies. The metatexites are characterized by low variation in their REE contents; other rocks display greater variations. Orthopyroxene in the Vaasa granitoids has elevated Al₂O₃ value (2.5 wt%) with Fe/(Fe+Mg) ratio of 0.53, and biotite has a high mg-number (48).
The diatexites form a continuous geochemical, mineralogical and textural link between the metatexites and the Vaasa granite. This indicates that the latter is derived by partial melting; obviously biotite, plagioclase and quartz produced garnet, orthopyroxene, K-feldspar and melt, but without complete breakdown of biotite. Chemically, biotite corresponds to residual mineral in the granitoids. The granite variations of the granitoids may represent melt-enrichment portions and the orthopyroxene tonalites a residuum-enrichment portion or slightly different source rocks. Crystallization of the melt is reflected in coarser grain size and in the disappearance of cordierite and in igneous textures within the granitoids where it is close to the diatexites. The absence of intrusive leucogranites also suggests that the formation of the Vaasa granite was (par)autochthonous.