engleski

Tourmaline nodules - product of devolatilization during final stage of granite melt evolution?

Tourmaline nodules have been found in some evolved granitic rocks of various age, origin and occurrence. Usually they are interpreted as a product of final stage of granite melt crystallization. The aim of this contribution is to rouse the discussion about genesis of tourmaline nodules and its importance for evolution of granite, based on example from Moslava&egrave ; ; ka Gora (MG) granite in Croatia (Pannonian Basin crystalline basement). Tourmaline nodules (usually 1 to 10 cm in diameter) at MG are typically spherical aggregates of tourmaline crystals, consisting of dravite-enriched (Fe# 0.43-0.58) and slightly Na-defficient tourmaline with plagioclase and quartz. Leucocratic rim envelopes that core and represents integral part of nodule comprising quartz, plagioclase (An11-21) &plusmn ; ; muscovite. The nodules are placed inside the Cretaceous peraluminous granite (ASI=1.2) characterized with relatively flat REE chondrite normalized patterns, slight enrichment in the LREE and strongly negative Eu anomaly. The mineral assemblage of host granite is represented by quartz, plagioclase, biotite (annite), muscovite and K-feldspar locally forming megacrysts up to 10 cm in size. MG granite comprises metapelitic xenoliths (biotite, quartz, feldspar, sillimanite &plusmn ; ; andalusite). The nodules are often found in the inhomogeneous randomly distributed swarms aligned along streamlines defined by chaotic magmatic flow field. Voids and/or miarolitic cavities are occasionally present inside the nodule cores. Textural features like rounded shape, physical separation from the groundmass, differences in grain size and texture, lack of (micro)vein network connecting the nodules corroborate a magmatic origin and indicate that the nodules once formed, behaved as an almost closed system in the course of their crystallization. A plausible genetic model can be summarized as follows: magma is intruded from source to shallow emplacement level as inferred from presence of magmatic andalusite and miarolitic cavities ; growth of K-feldspar megacrysts is controlled by a low nucleation to growth ratio in environment during increasing of undercooling ; decompression along path promotes exsolution of fluid phase from melt and its mixing with fluids from the roof part ; crystallization of volatile-free minerals further leads to a rise of volatile concentration (B, H2O, ...) in melt and to formation of ?boiling? bubbles. Exsolved low density B-rich bubbles start to rise in magmatic column and to accrete into a sphere to decrease surface tension. By reaching of solidus temperature crystallization of bubble, which did not manage to escape from coexisting host granite magma, while it was still behaving as a melt, produces tourmaline nodule with characteristic morphology.

tourmaline; nodule; devolatilization; granite; melt evolution

nije evidentirano