The generation of melt in ultrahigh-temperature metagranitic rocks - the Saxon Granulite Massif

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1 The generation of melt in ultrahigh-temperature metagranitic rocks - the Saxon Granulite Massif Oliver Frei Institute for Geology, TU Bergakademie Freiberg, Germany Abstract. Near-isothermal decompression followed by increased fluid activity during retrogression lead to partial melting in the Saxon Granulite Massif. Channelized fluid flow of an upper and lower shear zone, associated with the lateral extrusion of the Granulite Complex, produced granitic melt that crystallized in-situ or escaped through fissures. Melt segregation was triggered by shearing. Therefore, granitic mobilisates occur within and above the granulites. The character of the fluids is likely to be an extensive mixture of mantle- and crustderived components. Cordierite-garnet gneisses, located between the Granulite Complex and the Schist Mantle above, represent restitic rocks, as indicated by geochemical data and microstructures. Introduction In the Saxon Granulite Massif (SGM) occur synmetamorphic granitic mobilisates, derived from the migmatization of granulites. The granites are present within the granulites as well as in the Schist Mantle above. They form discrete veins, partly pegmatitic or aplitic, or form a blurred contact to the granulite, assimilating granulite xenoliths. There are two possible tectonopetrologic scenarios for the melting of the granulites. If temperature conditions are sufficiently high, the process of partial melt formation begins. This can be achieved by fast exhumation or additional heat influx, both resulting in temperature exceeding pressure influence. On the other hand, hydration of rocks can also initiate melt generation.

2 Oliver Frei Regional Geology The Saxon Granulite Massif (SGM) is an exotic high pressure, ultrahigh temperature rock assemblage within the Saxothuringian Basement on the northwestern margin of the

2 2 Oliver Frei Regional Geology The Saxon Granulite Massif (SGM) is an exotic high pressure, ultrahigh temperature rock assemblage within the Saxothuringian Basement on the northwestern margin of the Bohemian Massif (Fig.1). The rocks can be divided in the Granulite Core massif and the Schist Mantle, which itself can be subdivided in several parts and is of lower metamorphic grade. Although the Granulite Core complex mainly consists of felsic kyanite-garnet granulites, interbedded mafic granulites occur, comprising garnet and pyroxene. Additionally there are lenses of Fig. 1. Geological map of the Saxon Granulite Massif (Reinhardt & Kleemann 1994, modified after Mathé 1969 and Rötzler 1992) (ultra-)mafic rocks, often serpentinized, that are genetically not directly linked to the granulites but reveal a mantle influence in the evolution of the Granulite

3 The generation of melt in ultrahigh-temperature metagranitic rocks - the Saxon Granulite Massif 3 Complex. Mantle involvement is further assumed due to the presence of deep mantle (>400 km) megacrysts in metaperidotites (Massonne and Bautsch 2002). The protoliths of the mafic granulites are either olivine tholeiites or MOR basalts, whereas the felsic granulites developed from a more evolved magmatic succession. Despite the fact that the majority of the felsic granulites is of magmatic origin, sparse occurrence of boron-rich silicates such as kornerupine indicate some sediment involvement. The Granulite Core is separated from the above Schist Cover by a shear zone, to which the intense retrograde overprint is restricted. The affected rocks are referred to as the internal domain of the Inner Schist Mantle, mainly comprising cordierite and garnet bearing gneisses and often presenting migmatitic features. Migmatites show a complex fabric, partly cross cutting veins, implicating subsequent, syntectonic melt formation. Augengneisses formed due to intense shearing activity. Massive growth of biotite and fibrolithic sillimannite indicate high fluid activity within the shear zone. Shearing was accompanied by intense chemical mobility (Kroner 1995). The external domain of the Inner Schist Mantle is characterized by mafic rock assemblages that represent metamorphosed ophiolites, comprising gabbro, serpentinite, gneisses and mica schists. The outer Schist Mantle consists of low metamorphic mica schists and phyllites, derived from sedimentary rocks of the older Paleozoic. The metamorphic grade of the Schist Cover increases towards the Granulite Core. Although the SGM is the locus typicus for the granulite, the peak metamorphism is located in the eclogite facies (O Brien 2006) followed by a diaphtorithic transformation in the granulite facies. Peak metamorphism at 340 Ma is characterized by a maximum of 23 kbar and C for the felsic granulites and 22 kbar and C for the mafic granulites (Rötzler 2004). The following drastic pressure decrease under high temperature conditions is nearisothermal. Extensive cooling occurred at low pressures (Fig.2).

4 4 Oliver Frei Fig.2 Clockwise p-t-path for the granulites (Rötzler 2004) Structural evolution of the Saxon Granulite Massif During the Variscan orogeny the protoliths of the Saxon granulites and associated rocks have been subducted into depths of about km and extensively heated. Delamination of the subducted slab resulted in its buoyant upward movement followed by crustal flow due to a pressure gradient (Kroner et al. 2007). This lateral extrusion postulates a related lower shear zone. The presence of a lower shear zone is crucial for the further understanding of fluid activity and related migmatization, wether it occurred by the means of additional fluid influx or temperature conditions exceeding the dry solidus. The fast exhumation rate of 9-

5 The generation of melt in ultrahigh-temperature metagranitic rocks - the Saxon Granulite Massif 5 18 mm/a subsequently decreases to <2 mm/a. Inasmuch decreases the slow cooling rate of C/Ma to 6 C/Ma (Rötzler and Romer 2001b). Fig.2 Exhumation of the Saxon granulite rocks by lateral extrusion tectonics. (modified after Kroner et al. 2007) Migmatization and partial melting Migmatites can be formed either by subsolidus reactions or partial melting. The latter can be generated by sufficient temperature under certain pressure conditions, referred to as dry melting. In contrast to that is the so called fluid induced or wet melting, where the solidus of a particular rock chemistry is shifted to lower temperatures and pressures due to fluid presence. Since the dry solidus for the Saxon granulites is not reached at peak metamorphism, partial melt formation must be caused by increased fluid acivity. Major and trace element data as well as field studies have shown a continuous relationship between granulites, retrograde overprinted metagranulites and granites. Further, strong similarity between granulites and granites is indicated by δ 18 O and δ 15 N/C N data (Müller et al. 1987). Thus, it can be assumed, that the granitic melts are extracted mobilisates during

6 6 Oliver Frei migmatization of the granulites. The few discrepancies in trace element concentrations are likely to reflect metasomatism due to the fluid influx. It is generally accepted that the formation of partial melts in the SGM was decisively generated by retrograde fluid activity, accompanied by a rapid, near isothermal decompression (Rötzler and Romer 2001a). The entire granulite complex is surrounded by a contact aureole, assuming the granulite complex itself functioned as a major heat source. The fast exhumation rate after extensive subduction-related heating causes the convective heat transport to upper crustal levels. In the late stages of the SGM formation, the lateral extrusion of the granulites probably includes significant shear heating, accompanied by the necessary fluid influx, providing advective heat transport. Partial melting in the Granulite Complex At about 800 C the breakdown of biotite, due to fluid presence, produces melt, as shown in the following equation: Bt + Sill/Ky + Pl + Qz + Fl Grt/Crd + M (+ Kfsp) (1) Thereby, the amount of melt increases not only with larger quantities of fluid and higher temperatures, but there is also a strong dependency on the mode of the rock (Douce and Johnston 1991). Although dehydration melting of biotite during prograde metamorphism should have occurred, it is structurally not evident (Rötzler and Romer 2001a). As especially diatexites rarely show premigmatization structures, which is also true for the chaotic fabrics of the SGM migmatites (Kroner 1995), prograde melt generation can not be ruled out. Nonetheless, Equation (1) is considered to be responsible for partial melt formation during retrogression (Reinhardt and Kleemann 1994). This process does not result in voluminous mobile magma. However, small restite-rich syntectonic magma bodies can be produced. Melt extraction is triggered by shearing. Oxygen isotope data suggest low fluid activities during peak and early retrograde metamorphism (Hagen et al. 2008). Retrograde biotite yields crystallization

7 The generation of melt in ultrahigh-temperature metagranitic rocks - the Saxon Granulite Massif 7 temperatures of C, assuming increased fluid activity at about 800 C, since its formation can be described by: Grt + Sill + Kfsp + H 2 O Bt + Pl + Qtz (2) (Rötzler and Romer 2001a). The restite problem The cordierite-bearing gneisses, occuring between the Granulite complex and the Schist Mantle above, show a varying garnet content of up to 45 %. Their restitic character is supported by the occurrence of garnet and cordierite, according to Equation (1), as well as low Na, Si and K contents (Reinhardt and Kleemann 1994). Furthermore there are relictic structures of the granulite within the cordierite-garnet gneisses. Evidence are zoned garnet crystals, whose cores correlate with garnets of the granulite (Kroner 1995). Vinogradov and Pokrovsky (1987) concluded a independent origin of the gneisses by the means of Rb-Sr and oxygen isotopic studies. They favorized a volcanic-sedimentary sequence as the protolith. However, it should be considered that retrograde fluids caused a complex geochemical and isotopic pattern of the cordierite-garnet gneisses. These gneisses are most likely the voluminous restite for the rather small mobilisates, which is in conjunction to the migmatization style. Fluid recycling or mantle influx? During prograde metamorphism below the solidus, dehydration reactions set free fluids that are heterogenously distributed and locally accumulated (Brown 2001). In part, these fluids become available after peak metamorphism during retrograde processes (Kohn et al. 1997). Additionally, remaining hydrous minerals can react with present melt to generate anhydrous alkalifeldspar-quartz granulite with small amounts of Al-Fe-Mg silicates and free H 2 O (Thompson 2001). Supra-solidus reactions of biotite require only small amounts of fluid to produce melt. Due to the

8 8 Oliver Frei cyclic coupling of these reactions a small fraction of fluids subsequently contributes to the partial melting of the surrounding rocks. With respect to the SGM, however, geochemical data as well as shear zone mechanics suggest a further influx rather than merely recycled fluids. This addition of volatiles was likely to be accompanied by Th, U, Zr, Pb, Ta, Nb, K and Ba transport (Gerstenberger et al. 1987). Some of these elements are enriched in mantlederived fluids, whereas K and Ba are elements typically representing the crust. The nature of external fluids is generally hard to determine (Thompson 2001). Due to subduction involvement, prograde dehydration and shearing it is likeli that the final fluid was an extensive mixture of crustal and mantle components. Because of biotite and subordinate amphibole occurences, an aqueous fluid is preferred. This is supported by fluid inlcusion studies (Behr 1980), yielding H 2 O- inclusions at the margins of the Granulite Complex. On the other hand, these studies revealed CO 2 -inclusions in the lower parts of the granulites. As there are no indications for dry melting, this is consistent with the existence of a lower shear zone, as suggested by the lateral extrusion model and the fluid influx thereby. Granitic mobilisates within the granulite, therefore, are most likely to be generated by this lower shear zone. Conclusions The retrograde process of cracking, fluid influx, melting and mobilisate emplacement appears to be iterative. Closure of pathways by granite crystallization results in increasing fluid pressure until next crack is produced. Partial melt formation occurred under temperature conditions of about 800 C. Fluids subsequently contribute to voluminous migmatization, eventually leading to granitic mobilisates. These processes yield a complex pattern of cross cutting veins. The cyclic fluid activity and resulting melt generation is likely to be in conjunction with shear zone activity. Channelized flow of aqueous fluid in lower and upper shear zone explains granitic mobilisate occurences within and above the granulite. The fluids were derived from prograde dehydration and an additional

9 The generation of melt in ultrahigh-temperature metagranitic rocks - the Saxon Granulite Massif 9 mantle component. Dry melting is unlikely to have occurred, however can not be completely ruled out for the whole granulite complex, especially in greater depths. Appendix Phase Abbreviations Qz quartz, Kfsp alkali feldspar, Pl plagioclase, Bt biotite, Sill sillimannite, Ky kyanite, Grt garnet, Crd cordierite, Fl fluid, M - melt References Brown, M. (2001): Orogeny, migmatites and leucogranites: A review. Proc. Indian Acad. Sci. (Earth Planet. Sci.) 110, No. 4: Behr, H.-J. (1980): Polyphase shear zones in the granulite belts along the margins of the Bohemian Massif. Journal of Structural Geology 2: Douce, A.E.P.; Johnston, A.D. (1991): Phase equilibria and melt productivity in the pelitic system: implications for the origin of peraluminous granitoids and aluminous granulites. Contributions to Mineralogy and Petrology 107: Gerstenberger, H.; Stiehl, G.; Vinogradov, V.I.; Müller, A.; Wand, U. (1987): Isotope Geochronological, Isotope and Elemental Geochemical Investigations of the Sächsisches Granulitgebirge: A Synopsis of the Results. ZFI - Mitteilungen 133: Hagen, B.; Hoernes, S.; Rötzler, J. (2008): Geothermometry of the ultrahigh-temperature Saxon granulites revisited. Part II: Thermal peak conditions and cooling rates inferred from oxygen-isotope fractionations. European Journal of Mineralogy 20: Kohn, M.J.; Spear, F.S.; Valley, J.W. (1997): Dehydration-Melting and Fluid Recycling during Metamorphism: Rangeley Formation, New Hampshire, USA. Journal of Petrology Vol 18: Kroner, U. (1995) : Postkollisionale Extension am Nordrand der Böhmischen Masse - die Exhumierung des Sächsischen Granulitgebirges: Freiberger Forschungshefte C 457 Kroner, U.; Hahn, T.; Romer, R.L.; Linnemann, U. (2007): The Variscan orogeny in the Saxo-Thuringian zone Heterogenous overprint of Cadomian/Paleozoic Peri- Gondwana crust. Geological Society of America, Special Paper 423: Massonne, H.-J.; Bautsch, H.-J. (2002): An unusual garnet pyroxenite from the Granulitgebirge, Germany: Origin in the transition zone (>400 km depths) or in a shallower upper mantle region? International Geology Review 44:

10 10 Oliver Frei Müller, A.; Stiehl, G.; Böttger, T.; Bothe, H.K.; Gebhardt, O.; Geisler, M.; Handel, D.; Nitzsche, H.M.; Schmädicke, E.; Gerstenberger, H. (1987): Geochemical, stable isotope and petrographic investigations of granulites, pyriclasites and metagranulitic rocks of the Sächsisches Granulitgebirge: ZFI - Mitteilungen 133: Nurse, R.E.G. (1994): Paragenetische und thermobarometrische Untersuchungen an strukturgebundenen Mineralisationen im Sächsischen Granulitgebirge. Dissertation TU Bergakademie Freiberg O Brien, P.J. (2006): Type-locality granulites: high-pressure rocks formed at eclogite-facies conditions. Mineralogy and Petrology Vol 86: Reinhardt, J.; Kleemann, U. (1994): Extensional unroofing of granulitic lower crust and related low-pressure, high-temperature metamorphism in the Saxonian Granulite Massif, Germany, Tectonophysics 238: Rötzler, J.; Romer, R.L. (2001)a: P-T-t Evolution of Ultrahigh-Temperature Granulites from the Saxon Granulite Massif, Germany. Part I: Petrology. Journal of Petrology 42: Rötzler, J.; Romer, R.L. (2001)b: P-T-t Evolution of Ultrahigh-Temperature Granulites from the Saxon Granulite Massif, Germany. Part II: Geochronology. Journal of Petrology 42: Rötzler, J.; Romer, R.L.; Budzinski, H.; Oberhänsli, R. (2004): Ultrahigh temperature highpressure granulites from Trischheim, Saxon Granulite Massif, Germany: P-T-t path and geotectonic implications. European Journal of Mineralogy Vol 16: Simmons, Wm.B.; Webber, K.L. (2008): Pegmatite genesis: state of the art. European Journal of Mineralogy 20: Thompson, A.B. (2001): Clockwise P T paths for crustal melting and H 2 O recycling in granite source regions and migmatite terrains. Lithos 56: Vinogradov, V.I.; Pokrovsky, B.G. (1987): O-18 and Rb-Sr systematics in metamorphic rocks of the Saxonian granulite massif. ZFI - Mitteilungen 133: 73-87

Rocks of the Saxonian Granulite Massif (Sächsisches Granulitgebirge)

Rocks of the Saxonian Granulite Massif (Sächsisches Granulitgebirge) Karsten Reiter TU Bergakademie Freiberg, Germany Abstract. The granulite massif is an exotic element of the Saxo-Thuringian zone. The