The origin of brown hornblende in the Artfjället gabbro and dolerites

Brown hornblende occurs in minor amounts in the Artfjället gabbro and dolerites, except in quartz-dolerites where a pale green hornblende occurs. In the gabbro, brown hornblende is mostly Ti-bearing pargasite or kaersutite. It occurs along veins of orthopyroxene, as rims around and blebs in pyroxenes, with orthopyroxene in coronas between olivine and plagioclase and in coronas between ilmenite and plagioclase. In the olivine-dolerites and orthopyroxene-dolerites brown hornblende is ferroan titanian pargasite or ferroan kaersutite. The pale green hornblende in the quartz-dolerites is a magnesio-hornblende. The hornblendes in the dolerites are interstitial or granular, in some dolerites occurring as coarse oikocrysts. It is proposed that under certain conditions the Ti content of hornblende can be used as a thermometer, derived from experimental data of Helz (1973). Microstructures, compositions and formation temperatures (< 1,040° C) show that the brown hornblende in the gabbro is not magmatic, but of subsolidus origin. Probably it formed as a result of the introduction of water into the gabbro during a deformation event that occurred early in the cooling history of the gabbro. Least-squares modelling of hornblende formation indicates that all magmatic minerals must have participated in the reaction and that the reaction probably was not isochemical. Microstructures, compositions and formation temperatures (1,030-965° C) of brown hornblende in the dolerites are consistent with late-stage crystallization from the magma. For the pale green hornblende in the quartz-dolerites a magmatic origin is likely, but cannot be proven.