CELLULAR PLAGIOCLASE INTERGROWTHS AS A RESULT OF CRYSTAL-MAGMA MIXINGIN THE PROTEROZOIC ALAND RAPAKIVI BATHOLITH, SW FINLAND

Finely cellular plagioclase intergrowths have been studied in xenocrys tic andesine (An32) and andesine mantled K-feldspars within mafic magm atic enclaves in a quartz-feldspar porphyry from the Proterozoic subvo lcanic Hammarudda complex, Aland rapakivi batholith, SW Finland. The c ellular intergrowths usually occur as 0.2-2.0 mm mantles around xenocr ysts but also as entirely cellular grains, and are built up of a netwo rk of two distinct phases: one relatively Na-rich (An31) and one relat ively Ca-rich (An50). The grains are also covered by a thin (0.08-0.12 mm), continuous, normally zoned rim outside the cellular mantle. Smal l inclusions (0.0 1-0.05 mm) of Fe - Mg minerals are concentrated in t he Ca-rich part of the network. Compositionally, the Na-rich phase of the network is close to the inner non-cellular andesine of the xenocry sts. However, it has a lower Or- and a slightly lower An-content. The Ca-rich phase has the same composition as the inner part of the normal ly zoned rim, which outwards grades into lower An-contents that overla p the An-content of the matrix plagioclases. The cellular network was developed after the andesine xenocrysts (or andesine mantled K-feldspa rs) were engulfed in mafic magmatic enclaves during a mixing event. Th e xenocrysts became heated to a temperature just below the liquidus of the mafic magma. Dissolution of the xenocrysts developed a spongy cel lular texture which was penetrated by enclave magma. Ca-rich plagiocla se crystallized in the cells in equilibrium with the enclave magma, tr apping Fe - Mg-rich melt. As the enclaves cooled the outermost thin ri m and matrix plagioclases crystallized from the mafic melt. These proc esses operated in fairly large enclaves, as the one studied here, whic h has a diameter of 70 cm. Smaller enclaves, on the other hand, were c ooled more rapidly to temperatures close to the solidus of the enclave magma, and consequently had no time to dissolve the xenocrysts.