engleski

A 3D snapshot from granitic system: Tourmaline nodules and their bearing on the granite evolution

Peraluminous granites generated during the Late Cretaceous evolution of the LP-HT zone in the Adria-Europe plate boundary setting host tourmaline nodules. These tourmaline-bearing bodies from the Moslavačka Gora (MG), Croatia show great similarity with leucogranitic veins and can be described as compact spherical to ovoid aggregates (cm to dm in diameter) with a fine-grained (grain size 1-2 mm) core (slightly alkali-deficient dravite to schorl tourmaline (#Fe 0.40-0.66) + quartz + albite + K-feldspar ± muscovite) enveloped by a leucocratic halo (quartz + K-feldspar + oligoclase An11-21 ± muscovite). When observed in 2D sections solely, the isolated nature of tourmaline nodules can be easily mistaken with similar 2D cross-section of a leucogranitic vein. For that reason tourmaline nodules’ spatial distribution inside the host, shape and internal structure of individual bodies have been reconstructed and visualized through destructive serial sectioning tomography with physical resolution of 3.5 mm (serial cutting) or 0.35 mm (serial lapping) between individual planes. Obtained 3D reconstructions of rock volumes containing tourmaline nodules showed that they are indeed isolated spherical bodies dispersed inside the granitic host and not vein formations. The two structural units of a nodule, core and halo, are clearly distinguishable in 3D and show sharp contacts to each other but also to the granitic host. The morphology, peculiar texture, distribution and origin of tourmaline nodules inside granite can be most suitably explained through the emplacement mechanism and crystallization setting of the host granite at upper crustal level (at MG locality calculated approx. depth of 5-6 km, T=720 °C). During emplacement, decompression and arising immiscibility leads to melt unmixing and production of two different melt phases: “normal” granitic and B-rich one. Prominent depolymerization of B-rich melt, followed by density and viscosity decrease together with lowering of liquidus and solidus temperatures, leads to physical separation of a buoyant B- and fluid-rich phase in form of distinct B-rich bubbles or pockets, which coalesce in order to decrease surface tension. Such isolated volumes now contain necessary concentration of boron and other elements needed for tourmaline growth and become precursors for the future solidified tourmaline nodules.

tourmaline; nodule; granite; 3D visualization; Moslavačka Gora

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