MI1 MI1 MI2 MI2 MI3 MI3 lit 2000 bulk rock Na2O 2.14 1.52 0.84 0.63 4.32 3.23 4.48 3.43 Al2O3 14.91 15.13 14.93 15.18 13.03 13.24 15.00 14.56 TiO2 0.09 0.09 0.11 0.11 0.33 0.33 0.10 0.68 Fe2O3 n.d. n.d. n.d. n.d. n.d. n.d. n.d. 7.55 FeO 2.34 2.34 2.85 2.85 2.59 2.59 1.56 n.d. K2O 5.68 5.13 5.41 4.91 4.50 4.08 5.82 2.53 SiO2 64.24 66.12 66.87 68.94 69.02 71.16 72.59 65.67 MgO 0.12 0.12 0.17 0.17 0.09 0.09 0.01 0.79 CaO 2.43 2.43 4.49 4.49 0.55 0.55 0.44 3.55 P2O5 n.d. n.d. n.d. n.d. n.d. n.d. n.d. 0.24 Total 91.95 92.88 95.68 97.27 94.42 95.25 92.53 99.00 an 12 21 3 2 16 Q 23 29 23 22 28 Or 34 32 27 34 15 Ab 18 7 37 38 29 Hy 4 5 4 3 2 Di 0 1 0 0 0 C 1 0 0 1 0 ASI 1.06 0.97 1.00 1.04 0.98 Fe+MgO 0.036 0.044 0.038 0.022 0.020 1.8 2.0 2.2 2.4 2.6 800 900 1000 P (GPa) T (°C) 2.8 3.0 Coesite Quartz 1 2 3 4 5 - Phase assemblage: Qtz+Pl+Kfs+Bt±Ep±Ap - Size ≤150 µm - Negative crystal shape mostly in inclusions <30 µm in size (e.g. Figs 3a,d) - Abundant decrepitation cracks (Figs 3b,f) - Epidote (Ps=0.25-0.3) occur within garnet in association with Partial melting of granitoids under eclogite-facies conditions: nanogranites in felsic granulites, Orlica–Śnieżnik Dome (Bohemian Massif) Ferrero S. 1 , O´Brien P. 1 , Walczack K. 2 , Wunder B. 3 , Hecht L. 4 , Ziemann M. 1 European Geosciences Union General Assembly Vienna, Austria 2014 Anatectic melt inclusions (MI) in migmatites represents a powerful tool to retrieve the original composi- tion of the anatectic melt (Ferrero et al., 2012; Bartoli et al., 2013). Nanogranites (Cesare et al., 2009) are crystallized anatectic MI, and they were identified in peritectic garnet within high pressure felsic granulites from Orlica–Śnieżnik Dome, NE Bohemian Massif (Walczak, 2011). These rocks origi- nated from partial melting of granit- oids during the Variscan Orogeny (Anczkiewicz et al. 2007). Garnet Porphyroblasts, <2mm in size, occur in a leucocratic matrix (Fig. 2a). Nanogranites are a common oc- currence in garnet, where they form clusters (Figs 2b,c). Mineral inclu- sions are generally rare, mainly Ky, Msp and Qtz (Figs 2d,e). Symplectites of fine-grained am- phibole (Hastingsite) +plagioclase + quartz ±biotite (Fig. 2f) in the matrix are interpreted as pseudomorphs after omphacite. 3) Nanogranites and further evidence for the former presence of melt nanogranites (Fig. 3e), locally in- denting the host (Fig. 3f), and rarely in pseudomorphs after Omp. - Fine-grained intergrowth of Pl+Kfs and nanoporosity are common (Figs. 3b,d,e,f). - Plagioclase shows a bimodal distribution (Fig.4), with subhe- dral, Ca-richer crystals (Pl 1 ) and as anhedral, more albitic (Pl 2 ) within Pl+Kfs intergrowth. Figure 2: Microstructural features of nanogranites-bearing leucogranulites. a) Grt porphyroblasts in a leucocratic matrix; b) and c) garnet with nanogranites clusters (red dashed ovals); d) Ky and nanogranites in Grt; e) ternary feldspar in Grt; f) symplectite of Amp+Pl+Qtz±Bt . 4) Partial melting conditions 5) Re-homogenization experiments by Piston cylinder and results 6) Discussion and conclusions (a) The present study represents the first attempt to re-homogenize nanogranites formed at P>2.0 GPa. They are product of the crystallization of a hydrous melt produced by ana- texis at ~875°C and 2.7 ≤P≤ 3.0 GPa within a overthickened crust. (b) Similar microstructures were reported in UHP rocks from Dabie Shan by Gao et al. (2012), Penglei Liu et al. (2014) and Hermann et al. (2013). (c) Our preliminary results show a syenitic-granitic magma with Na<<K. The water content is <8 wt%, roughly consistent with experi- ments on hydrous melt stability at Figure 3: Petrographic features of nanogranites, BSE images. Red arrow=decrepitation cracks. Figure 4: Large nano- granite inclusion and bimodal distribution of plagioclase. Red dashed line= intergrowth of Pl 2 +Kfs. Further evidence for the former presence of melt such as pseudomorphs after melt-filled pores (Holness & Sawyer, 2008), visible in figs 5a,b,c, and euhedral garnet (Fig. 5d) occur in the leucogranulites. Figure 5: microstructural evidence for the former presence of melt. a) film of Pl around Rt+Ilm; b) although commonly interpreted as evidence for former coesite (see e.g. Bakun-Czubarow, 1992), this microstructure actually consists of Qtz+Pl, with melt pseudomorph of Qtz; c) melt pseudomorph formed by cuspate Kfs; d) MI-bearing Grt with developed crystal faces. Red arrow= melt pseudomorphs. PT estimates for the formation of peritectic garnet, i.e. partial melting, available in literature show roughly similar temperature, 900-1100°C, but a wide range of P values, from 1.8 GPa (Stipská et al., 2004) to 2.8-3.0 GPa (e.g. Klemd & Bröcker, 1999). Garnet composition: Alm 56-58 Pyr 10-15 Sps 1 Grs 28-31 - No compositional difference among MI-bearing and MI-free Grt - Mesoperthite in Grt: An 3 Ab 40 Or 57 - No coesite or quartz pseudo- morphs after coesite → P <3.0 GPa at 900°C. Estimated PT conditions for anatexis, classic geothermo- barometry T min = 875°C/2.4-3.0 GPa 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 600 700 800 900 1000 1100 Kyanite Sillimanite Wet solidus P (GPa) T (°C) FL88 KN88 H01 2.8 3.0 P estimates GASP KN88 H01 Koziol & Newton (1988) Holdaway (2001) with activity models from Berman & Aranovich (1996) and Furnham & Lindsley (1988) Minimum T estimates FL88 Fuhrman & Lindsley 1988 on Msp in Grt Granitic system wet solidus (Holtz et al., 2001) Coesite Quartz - Nanogranites must be re- homogenized before proceeding with in situ analyses to obtain the composition of the anatectic melt. - Starting material: Grt chip with preserved nanogranites. - Experiments were performed at T= 875-900°C and variable P (Fig. 8), for 2 days, under dry conditions. - The aim is to re-homogenize completely the inclusions, avoiding a) MI decrepitation (internal P>> confining P), and b) formation of new phases (e.g. new Grt) or gas bubbles. 1 2 Glass Pl Qtz Qtz Glass Glass 3 4 4 5 15 µm Shrinkage bubble Glass Glass New Omp Glass Shrinkage bubble Figure 8: Experimental runs condi- tions and relative products. Boxes= experimental runs; Red arrows= de- crepitation cracks; Orange arrows=new garnet. The new Grt in runs 2 and 3 has >Mg and <Ca than the original phase. Figure 7: PT conditions of partial melting based on mineral-mineral equilibria. 1) Introduction and geological setting - Complete re-homogenization of nano- granites <10 µm achieved at 875°C and 2.7 GPa of confining P (Red box in Fig. 8). - Their glassy nature is supported by Raman analyses (Fig. 9a), which also shows the presence of water in the glass (Fig. 9b). - After alkali-loss correction, the first EMP analyses (M1 & MI2 in table1) show a granitic/syenitic composition with Na<<K and fluid content (=100 - EMP total) 4.3-8. Partially decrepitated MI (MI3 in table1) show granitic composition, with Na≈K and fluid content 5-6 %. Table1: EMP analyses and bulk rock composition. Nanogranite MI3 show cracks connecting to larger MI, where glass coexists with quartz. Red box= analyses on fully re-homogenized MI; italic= original analyses before alkali-loss correction; Lit 2000= melt resulting from partial melting of leuco- granulites produced at 950°C/2.5 GPa by Litvinovsky et al. (2000). 0 500 1000 1500 2000 2500 3000 3500 4000 4500 2 5 00 2700 2900 3100 3300 3 5 00 3700 3900 4100 4300 Water band Re-homogenized nanogranite (MI2) Host garnet Characteristic glass bands MI2 MI3 Standard glass 6.1 Wt% water Standard glass 3.1 Wt% water a b Figure 9 (below): Raman spectra. a) host garnet and nanogranite after re-homogenization; b) detail of the water band region. Signals of hydrous standard glasses are re- ported for comparison. 2) Petrographic features of the HP leucogranulites No melt visible similar PT conditions (Hermann & Spandler, 2008). (d) Ep was most likely involved as a reactant in the partial melting reac- tion. No other OH-bearing residual phase is still visible in Grt. Bt occurs in Grt only as crystallization product within nanogranites. Omphacite, now altered, likely belonged to the peak assemblage with Ky and Msp. (e) Nanogranites investigation allows us to gain invaluable insights on anatexis at mantle depths of gran- itoids, a rock type widespread throughout the whole Bohemian Massif (Janousek et al., 2004). 1 Institut für Erd- und Umweltwissenschaften, Universität Potsdam, Potsdam-Golm, Deutschland 2 Institute of Geological Sciences, Polish Academy of Sciences, 31-002 Krakow, Poland 3 Helmholtz-Zentrum Potsdam, GFZ, D-14473 Potsdam, Deutschland 4 Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodi- versitätsforschung, 10115 Berlin, Deutschland Berlin Prague 100 km Coesite/Diamond, P> 4GPa Moldanubian Zone Moravo-Sileasian Zone Sample locality (Orlica–Śnieżnik Dome) Anczkiewicz, R. et al., (2007). Lithos 95, 363–380. Bakun-Czubarow, N. (1992). Archeological Mineralogy 48, 3–25. Bartoli, O. et al. (2013). Geology 41, 115–118. Cesare, B. et al. (2009). Geology 37, 627- 630. Ferrero, S. et al. (2012). Journal of Metamor- phic Geology 30, 303–322. Gao, X.Y. et al. (2012). Journal of Metamor- phic Geology 30, 193–212 Hermann, J. et al. (2013). Elements 9, 281- 287. Hermann, J. & Spandler, C. (2008). Journal of Petrology 49, 717–740. Holness, M.B. & Sawyer, E.W. (2008). Journal of Petrology 49, 1343–1363. Klemd, R. & Bröcker, M. (1999). Contributions to Mineralogy and Petrology 136, 358–373. Janousek, V. et al. (2004). Transactions of the Royal Society of Edinburgh: Earth Sciences 95, 141–159. Litvinovsky, B.A. et al. (2000). Journal of Pe- trology 41, 717–747. Penglei Liu, Y.W. et al. (2014). Lithos 192-195, 86–101. Štipská, P. et al. (2004). Journal of Metamor- phic Geology 22, 179–198. Walczak, K. et al. (2011). Doctoral dissertation, Institute of Geological Sciences, Polish Acad- emy of Sciences, Poland. References