Contrasting tourmaline types from peraluminous granites: a case study from Moslavačka Gora (Croatia)

• Published in: Mineralogy and petrology 2011-10, Vol.102 (1-4), p.117-134
• Main Authors: Balen, Dražen, Petrinec, Zorica
• Format: Article
• Language: English
• Subjects: Case studies; Chemical interactions; Earth and Environmental Science; Earth Sciences; Geochemistry; Granite; Inorganic Chemistry; Mineralogy; Special Issue Accessory Minerals; Stone
• ISSN: 0930-0708; 1438-1168
• DOI: 10.1007/s00710-011-0164-8

Two texturally and chemically distinct types of tourmaline are found inside peraluminous granites of the Moslavačka Gora, Croatia: nodular tourmaline in the two-mica granite and disseminated tourmaline in the cross-cutting leucogranite dykes. Both tourmaline types belong to alkali tourmaline group, nodular tourmaline being dravite to schorl and disseminated tourmaline corresponding to schorl. Comparison of characteristic parameters of nodular (Nt) and disseminated tourmaline (Dt) shows significant differences in #Fe (0.40–0.65 for Nt vs. 0.74–0.85 in Dt) along with variations in the calculated X-site vacancy (0.22–0.37 pfu in Nt and 0.33–0.44 in Dt) and □/(□+Na) ratio (0.23–0.40 in Nt and 0.34–0.45 in Dt). Disseminated tourmaline from the MG leucogranites is regarded as an early crystallized magmatic phase, while the interstitial tourmaline from the cores of tourmaline nodules originated from more complex mineralogical and chemical interactions inside the two-mica granite melt. Major element gain (Mg) and loss (Fe, Ca, Na, K) for the “idealized nodule” (34 vol. % core + 66 vol. % halo) when compared to the host granite shows that the nodule’s volume is not a completely independent and closed system. Based on the observed characteristics, nodule’s halo can be considered as a “transitional zone” between the tourmaline-bearing core and the host granite, texturally and mineralogically related to the host two-mica granite, chemically being an integral part of the nodule’s volume at the same time.