 |
| Titel |
Crustal melting during subduction at mantle depth: anatomy of near-UHP nanogranites (Orlica-Śnieżnik Dome, Bohemian Massif) |
| VerfasserIn |
Silvio Ferrero, Martin Ziemann, Katarzyna Walczak, Bernd Wunder, Patrick J. O'Brien, Lutz Hecht |
| Konferenz |
EGU General Assembly 2015
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250105774
|
| Publikation (Nr.) |
 EGU/EGU2015-5340.pdf |
|
|
|
|
|
| Zusammenfassung |
| Small volumes (≤ 50µm) of hydrous melt were trapped as primary inclusions in peritectic garnets during partial melting of metagranitoids from the Orlica-Śnieżnik Dome (Bohemian Massif) at mantle depth [1]. Detailed microstructural/microchemical investigation confirmed the occurrence of a granitic assemblage (biotite+feldspars+quartz) in every investigated inclusion, i.e they are nanogranites [2].
MicroRaman mapping of unexposed inclusions showed the occurrence of residual, H2O-rich glass in interstitial position. Despite the oddity of this finding within a classic regional HP/HT terrain, an incomplete crystallization of the melt inclusions (MI) is consistent with the (relatively) rapid exhumation of the Orlica-Śnieżnik Dome proposed by some authors [e.g. 3]. Moreover glassy and partially crystallized MI have been already reported in lower-P (<1 GPa) migmatites [4]. MicroRaman investigation also showed the possible presence of kumdykolite, a high-temperature polymorph of albite reported in UHP rocks from the Kokchetav Massif as well as the Bohemian massif ([5] and references therein).
Experimental re-homogenization of nanogranites was achieved using a piston cylinder apparatus at 2.7 GPa and 875°C under dry conditions, in order to investigate melt composition and H2O content with in situ techniques. The trapped melt is granitic, hydrous (6 wt% H2O) and metaluminous (ASI=1.03), and it is similar to those produced experimentally from crustal lithologies at mantle conditions.
Re-homogenization conditions are consistent with the results of geothermobarometric calculations on the host rock, suggesting that no H2O loss occurred during exhumation - this would have caused a shift of the inclusion melting T toward higher values. Coupled with the absence of H2O-loss microstructural evidence, e.g. decrepitation cracks and/or vesciculation [4] in re-homogenized nanogranites, this evidence suggests that the nanogranites still preserves the original H2O content of the melt. Our study supports therefore the hypothesis that H2O re-equilibration via diffusion of MI in garnet cannot be implicitly inferred, as already proposed by [5] for lower-P nanogranites, even in case of near-UHP inclusions.
In conclusions, the combined petrological-experimental investigation of near-UHP nanogranites is a novel and fruitful approach, which unlocks the access to deep melt in natural eclogite-facies crustal rocks, improving our understanding of deep melting processes in collisional settings.
References
[1] Walczak, K. (2011), Ph.D. thesis, Krakow, Poland.
[2] Cesare, B. et al. (2009), Geology, 37, 627–630.
[3] Anczkiewicz, R. et al. (2007), Lithos, 95, 363–380.
[4] Ferrero, S. et al. (2012), JMG, 30, 303–322.
[5] Kotková, J. et al. (2014), Am. Min., 99, 1798-1801.
[6] Bartoli, O. et al. (2014), EPSL, 395, 281–290. |
|
|
|
|
|
|