Crustal development in relation to granitic magmatism in regard to D/H partition between coexisting hornblende and biotite in the Svecofennian belt

Bulletin of the Geological Society of Finland

关键词: granites;    rapakivi;    hornblende;    geochemistry;    D/H;    igneous activity;    fractional crystallization;    Svecofennides;    Proterozoic;    Finland;    Sweden;   

DOI  :  10.17741/bgsf/65.2.003

来源: DOAJ

【 摘 要 】

Tie-lines in the σD-XFe diagram of hornblende and biotite in Japanese island arc granites give commonly a similar slope to that derived from the equation of Suzuoki-Epstein's experimental work (1976). This indicates that in the process of granite intrusion in the island arc environment, the water/rock ratio is high enough to allow the presence of molecular water within and around the rock mass crystallizing from the magma. However, the tie-lines of the anorogenic rapakivi granites of Finland (age 1.65-1.54 Ga) are quite different from those of island arc granites, i.e. the σD of hornblende is much lower than that of biotite and the XFe values of both of the minerals are almost the same and high (0.8-0.9). Thus, the tie-lines are nearly vertical. Moreover, the OH contents of hornblende and biotite are very low and this suggests the absence of molecular water in the ascending and crystallizing granitic magma. This probably suggests that the D/H fractionation factor between the hydrous silicates and hydroxyl (OH) in the magma without molecular water is different from that of the island arc granites with plenty of molecular water. For the Svecofennian granitic rocks which are older than the rapakivi, e.g. the Turku, Uppsala, Vänge and Revsund masses, the σD-XFe relationships of hornblende and biotite change systematically following the chronological order of the intrusion. The σD-XFe relationships of the granitic rocks around Turku are close to those of the island arc granites, those of the Uppsala and Vänge granites show the intermediate patterns between the Turku and Revsund, and those of the Revsund granites are rather similar to rapakivi. It is considered that the geological settings of granitic magmatism in the Svecofennian belt of the Baltic shield developed from a more mobile to a more continental one.

【 授权许可】

Unknown