SIGNIFICANCE OF KUMDYKOLITE, KOKCHETAVITE AND CRISTOBALITE CRYSTALLIZED FROM MELT INCLUSIONS IN FELSIC GRANULITES, ORLICA-SNIEZNIK DOME (BOHEMIAN MASSIF)

Small volumes (≤ 50µm) of hydrous melt were trapped as primary melt inclusions (MI) in garnets under near-UHP conditions. Both phases resulted from partial melting at mantle depth of metagranitoids in the Orlica-Śnieżnik Dome (Bohemian Massif) [1]. These inclusions, now crystallized as “nanogranites” [2][3], contain a unique assemblage including kumdykolite, kokchetavite – polymorphs of albite and K-feldspar respectively-, cristobalite, micas and calcite. A residual, H₂O-rich glass is often present in interstitial position, as already reported in lower-P (<1 GPa) migmatites [3].

Experimental re-homogenization of nanogranites was achieved using a piston cylinder apparatus at 2.7 GPa and 875°C. The trapped melt has similar composition to those produced experimentally from crustal lithologies at mantle conditions [1]. Re-homogenization conditions are consistent with geothermobarometric calculations, suggesting that no H₂O loss occurred during exhumation [see also 4].

Both kumdykolite and kokchetavite have been reported mainly in natural rocks equilibrated in the diamond stability field [5] [6] [7]. The precise calculation of the PT path of the MI on cooling and the comparison with previous studies suggests however that pressure is not influential to their formation, ruling out the possible interpretation of kumdykolite and kokchetavite as indicators of ultra-high, and possibly even high, pressure conditions.

Our results support the hypothesis that the presence of such phases should be instead regarded as direct mineralogical criterion to identify former melt inclusions with preserved composition, including H₂O and CO₂ contents, and to infer rapid cooling of the host rocks. Thus the present study provides novel criteria for the interpretation of fluid/melt inclusions in natural rocks, and allows a more rigorous characterization of the nature of metamorphic fluids during deep subduction and their behavior on exhumation.

References

[1] Ferrero, S. et al. (2015), Geology, doi:10.1130/G36534.1.

[2] Cesare, B. et al. (2009), Geology, 37, 627–630.

[3] Ferrero, S. et al. (2012), JMG, 30, 303–322.

[4] Bartoli, O. et al. (2014), EPSL, 395, 281–290.

[5] Hwang, S-L. et al. (2004), EJM, 21, 1325–1334.

[6] Hwang, S-L. et al., (2009), CMP, 148, 380–389.

[7] Kotková, J. et al. (2014), Am. Min., 99, 1798-1801.