engleski

Tourmaline nodules – 3D insight into late-stage boron separation from granite melt

Spatial distribution, shape and internal structure of tourmaline nodules found in the peripheral parts of the Cretaceous Moslavačka Gora (Croatia) peraluminous granite pluton have been visualized and reconstructed using BLOB3D software. Data acquisition included serial cutting and lapping i.e. serial sectioning tomography. The “data brick” obtained from investigated granite consisted of a series of grayscale images with physical resolution of 3.5 mm (cutting) or 0.35 mm (lapping) between individual slices. The origin of tourmaline nodules and their peculiar textures has been related to the separation of a late-stage boron-rich volatile fluid phase from the granite magma. Based on the field, mineralogical and textural observations, it is evident that tourmaline nodules formed during the final stage of granite evolution, when an undersaturated granite magma intruded shallow crustal horizons, becoming saturated and exsolving the fluid phase from the residual melt in the form of buoyant bubbles or pockets. Immiscibility of aluminosilicate and waterrich melts seems like a plausible mechanism which could have produced such spherical segregations. It also produced adequate concentrations of elements essential for the formation of the tourmaline nodules, especially boron, a crucial component enabling the physical formation of bubbles or pockets, followed by tourmaline crystallisation. Tourmaline nodules (usually 1 to 10 cm in diameter) that formed from these bubbles show a complex texture comprising two clearly distinct units: tourmaline-bearing core and leucocratic envelope (halo). The core, a spherical aggregate of tourmaline crystals, consists of dravite-enriched (Fe# 0.43-0.58) and slightly Na-deficient tourmaline, together with quartz + albite + K-feldspar ± muscovite. The nodule's halo, consisting of quartz + feldspar + muscovite, represents an integral part of the nodule and envelopes the tourmaline-bearing core.

tourmaline; nodule; 3D; serial sectioning; tomography; boron; granite; melt

nije evidentirano