engleski

Tracing evolution of peraluminous granitoids through contrasting tourmaline types

The peripheral parts of the Cretaceous Moslavačka Gora (Croatia) peraluminous granites host two contrasting tourmaline types having separated origin: slightly alkali-deficient nodular schorl to dravite (Fe# 0.43-0.58) and disseminated schorl (Fe# 0.75-0.85). Origin of the nodular tourmaline has been attributed to the interaction of a fluid phase from the residual granitic melt with a fluid derived from the wall rock environment, which resulted in increasing the MgO content in the nodular tourmaline compared to the disseminated one. Observed textures suggest that nodular tourmaline crystallized as the last mineral in a quasi-closed system inside host granite. The peculiar texture and distribution of tourmaline nodules can be most suitably explained through an emplacement setting of the studied granite intrusion. In a situation when magma rapidly reached shallow crustal levels, resulting decompression and cooling lead to the separation of a boron-rich melt in form of distinct volatile-rich bubbles ± crystals. If these bubbles were unable to escape the system owing to impermeable host and wall rock, they cause the formation of tourmaline nodules in almost completely solidified granite. The Moslavačka Gora two-mica granite that hosts tourmaline nodules can thus be regarded as an occurrence of magma that escaped from the plutonic levels and stalled in a low pressure crustal setting (ca. 720 °C, 70 – 270 MPa, H2Omelt up to 4.2 wt. %). Decreasing pressure and related cooling were major factors controlling the melt differentiation, liquid immiscibility, fluid behaviour, partitioning of elements and crystallization path of the two-mica granite. Disseminated tourmaline from leucogranite host, on the other hand, can be considered as a typical magmatic product and common accessory phase.

evolution; peraluminous; granite; tourmaline; nodule

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