NADIA MALASPINA. PLINIUS n. 32, 2006

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1 ULTRAHIGH-PRESSURE METAMORPHISM AND METASOMATISM IN MAFIC-ULTRAMAFIC ROCKS FROM EASTERN CHINA: IMPLICATIONS FOR FLUID RELEASE AND VOLATILE TRANSFER AT SUBDUCTION ZONES NADIA MALASPINA Dipartimento per lo Studio del Territorio e delle sue Risorse, Università di Genova, C.so Europa 26, I Genova Subduction zones are the Earth s environments where fluids or melts released by the slab recycle elements into the mantle wedge, triggering partial melting and arc volcanism. In this recycling process the interactions of slab derived fluids taking place in the deep domains of the mantle wedge are virtually unknown. Relevant information for fluid/melt related processes occurring at sub-arc depth can be gained by the study of ultrahigh-pressure (UHP) continental crust, which contains felsic rocks associated with peridotites. Such an association represents a natural laboratory to study the element exchange processes between crustal and mantle-like systems at depths corresponding to fluid extraction for arc lavas sources. This PhD thesis is focussed on the Dabie-Sulu UHP complex (Eastern China), which represents the largest known slice of deeply subducted continental crust. In this terrane coesite-bearing eclogites, garnet pyroxenites and garnet-peridotites are included in leucocratic gneisses, which experienced common UHP metamorphism. In such occurrences mafic and ultramafic rocks record intense metasomatic exchanges with fluids and/or melts coming from crustal reservoirs. This terrain thus represents a unique situation for the study of element transfer within the slab and from crust to mantle at extreme conditions. A petrologic and geochemical study in this thesis have been addressed on UHP eclogites (s.l.) from the Dabie Mountains. The investigated samples were collected in the North Dabie Complex, where eclogite-facies rocks are significantly overprinted by granulite-facies metamorphism and partial melting. The studied eclogites are included in meta-lherzolitic bodies, which are in turn hosted by leucocratic gneisses. The textural relations among the various rock-forming minerals enabled to identify several re-crystallisation stages. The peak (UHP) paragenesis consists of garnet, clinopyroxene and rutile. UHP garnet and clinopyroxene display oriented inclusions of polycrystalline rods of rutile + ilmenite and of albite, K-Ba-feldspar and quartz, respectively. Garnet and clinopyroxene are both rimmed by an inclusion free zone that formed after the peak, still at highpressure conditions. Such optical zoning does not correspond to a difference in major element concentrations between garnet core and rim. This observation provides evidence that the major element composition of garnet was reset during exhumation, thus preventing thermobarometric determination of peak metamorphic conditions. Further decompression is documented by the formation of limited ilmenite + amphibole and granulite-facies coronas consisting of clinopyroxene, orthopyroxene, plagioclase and amphibole around garnet. In order to investigate the stability of different phases and to better constrain peak metamorphic conditions and compositions of clinopyroxene and garnet, a series of reconnaissance piston cylinder synthesis experiments were carried out in an identical bulk composition. The experimental study indicates that the peak metamorphic paragenesis is stable at P ~ 3.5 GPa and T C. The petrological study, 1

2 combined with bulk-rock and mineral trace element analyses, provides evidence of intense metasomatism affecting these eclogites. The bulk-rock major and trace element compositions indicate that the eclogites derive from basaltic protoliths with MORB and E-MORB affinity. Compared with such basalts, the studied rocks show strong depletion in SiO 2 and alkalis and enrichment in MgO and FeO. These features likely derive from element exchange with ultramafic rocks prior to subduction, possibly related with the influx of Si-depleted and Mg-enriched fluids produced during the serpentinisation of the associated lherzolitic rocks. On the other hand, the trace element bulk-rock compositions show strong enrichment in Cs, Ba and Pb. The same characteristic enrichment and fractionation is recorded by peak metamorphic clinopyroxene but not in retrograde amphibole. Therefore, the bulk rock and mineral trace element patterns indicate influx of crustal fluids during subduction. Since the UHP clinopyroxene shows K-Ba-feldspar segregations and incompatible element enriched signature, and since retrograde amphibole does not show such features, the crustal metasomatism must have occurred prior to or during UHP metamorphism. The observed features in the studied eclogites provide evidence for two stages of metasomatism prior to UHP crystallisation. The first stage features the income of Si-undersaturated and Mg-rich fluids, a process that likely occurred under low-grade metamorphic conditions, and possibly related with the serpentinisation of the associated lherzolitic rocks. A second stage of metasomatism was accompanied by the influx of crustal fluids transporting LILE and light elements. This stage likely record the tectonic coupling at HP to UHP with the associated crustal rock units and provides evidence of LILE mobility from crustal to mafic-ultramafic slab components. The central part of the Dabie belt experienced UHP metamorphism at GPa, C (Liou & Zhang, 1998; Zhang et al., 2000). In this setting the Bixiling and Maowu ultramafic complexes are hosted by coesite-bearing gneisses and the rock samples studied here experienced intense metasomatic effects. Textural and geochemical data demonstrate that most of the garnetorthopyroxenites and websterites within these ultramafic complexes derive from harzburgite by the addition of a SiO 2, Al 2 O 3 and LREE-rich agent, most likely a hydrous granitic melt, sourced from the associated crustal rocks. The pyroxenites are characterised by a peak assemblage of millimetre-sized orthopyroxene (Opx 2 ), inclusion-rich garnet, and minor clinopyroxene, associated with discontinuous layers of Ti-clinohumite. Rounded blebs of relict olivine and orthopyroxene (Opx 1 ) are included in poikiloblastic Opx 2 : in such occurrences Opx 2 displays clear replacive textures after olivine same as those described in the oceanic mantle as the result of reactive flow of silicate melts (Piccardo et al., 2004). The high Mg # of bulk rocks (from 89 to 91) and of single minerals (93-94), together with the high pyrope contents of garnet (70-73%), indicate that the orthopyroxenites likely derive from precursor mantle peridotites. Moreover the Ni contents of Opx 2 ( ppm) are high and clearly resemble those of precursor olivine, thus reinforcing our interpretation of pristine mantle peridotite precursors for the opx-rich rocks. Presence of olivine and of Opx 1 relics, suggest that harzburgites (± garnet) were the starting mantle materials, affected by olivine dissolution and orthopyroxene precipitation during percolation of siliceous agents at UHP conditions. Chondrite normalised rare earth element (REE) patterns show that these pyroxenites are very similar to mantle rocks for what concerns the MREE and HREE. On the other hand, they are strongly enriched in LREE. They also show a spike in Ba and the clinopyroxene-bearing websterite shows a selective fractionation in large ion lithophile elements (LILE). Low CaO ( wt.%) and Al 2 O 3 (< 0.1 wt.%) in orthopyroxene coexisting with clinopyroxene and garnet agree with the previous P-T estimates of 750 C and

3 GPa. Laser Ablation ICP-MS analyses show that Opx 2 is enriched in LREE with respect to Opx 1. The garnet REE pattern also shows a relative slight enrichment in LREE. This indicates that garnet and Opx 2 growth was induced by the influx of a SiO 2 and Al 2 O 3 rich melt characterised by high LREE content. This metasomatic melt is likely produced by the associated crustal rocks, which are above the wet solidus at the peak pressure and temperature conditions (Hermann & Green, 2001). Porphyroblastic garnets are zoned and display core clusters of primary polyphase solid microincluisons. The inclusions display negative crystal shapes and constant volume proportions of infilling opaque minerals (10-20 vol.%), chlorite, amphibole, talc and apatite (Fig. 1a). These feature indicate that the microinclusions crystallised from trapped fluid and/or melt phases. Separated inclusion-rich garnets were run in a piston cylinder experiment at 900 C and 3.5 GPa. After the run, the primary inclusions all appeared homogeneised and contained porous quench indicating that garnet originally trapped soluterich aqueous fluids (Fig. 1b). The trace element patterns of the multiphase inclusions and of the experimentally rehomogenised inclusions are enriched in LILE and LREE with respect to the host garnet. They also show positive spikes of Cs, Ba, and Pb relative to Rb and K and high U/Th ratios (Fig. 2). The Central Dabie garnet-pyroxenites represent an excellent natural laboratory to study the trace element transfer from subducted crustal rocks to the mantle wedge at sub-arc depths. The observed textures and chemical characteristics provide evidence for the infiltration of a hydrous felsic melt into a peridotite, similarly to what is expected when sediment-derived melts interact Fig. 1 - Photomicrographs of solid inclusions from Maowu ultramafic orthopyroxenites. A: transmitted light image of polyphase solid inclusions in porphyroblastic garnet core; B: transmitted light image of re-homogenised inclusion in garnet after the piston cylinder run. Fig. 2 - Primitive Mantle (PM) normalised trace element concentrations of polyphase inclusions (Incs + Grt) and inclusion-free domain in their host garnet (Host Grt). 3

4 with mantle wedge peridotites. The SiO 2 and Al 2 O 3 component of the hydrous melt reacted with olivine to form orthopyroxene and garnet. The newly formed garnet and orthopyroxene are able to accommodate some of the LREE, however, the H 2 O and LILE component of the melt were partitioned into a residual aqueous fluid phase. Remnants of such an aqueous fluid were trapped within the garnet and formed the polyphase inclusions. The trace element pattern of such inclusions is very similar to the incompatible element enrichment observed in arc lavas and attributed to the subduction component. The residual aqueous fluid is in equilibrium with mantle mineralogy and hence is able to transport a crustal trace element signature to the locus of partial melting within the mantle wedge. This has been confirmed by the geochemical study of a garnet peridotite from a drill core in the Sulu UHP belt. Previous works have demonstrated that these peridotites represent slices of mantle wedge tectonically sampled by the subducted crust (Zhang et al., 2000). Polyphase inclusions have been found also in porphyroblastic garnet of this mantle-derived peridotite and look identical to the ones studied in Maowu orthopyroxenites. Moreover, the Laser Ablation analyses performed on these inclusions show the same LILE enrichment with spikes in Cs, Ba, K, Pb and Sr. The metasomatised ultramafic rocks studied in this thesis might therefore represent a proxy for the trace element transfer from subducted crustal rocks to the mantle wedge at sub-arc depths. Once a silicate-rich hydrous melt (or superctitical fluid with intermediate composition similar to pelitic melts) is produced by subducted sediments, it will react with mantle wedge peridotites. The products of this reaction is a metasomatic LREE-enriched garnetorthopyroxenite layer and a nonaggressive residual aqueous fluid phase which concentrates the LILE and transports a crustal signature to the locus of partial melting within the mantle wedge (Fig. 3). If subducted metasediments with residual phengite are the primary reservoirs for the incompatible elements, a W-type LILE signature characterises the evolved fluid phase produced (spikes in Cs, Ba, Pb, and negative anomalies in Rb, K). The conservative behaviour of such incompatible elements during the overall exchange process enables the aqueous fluid to transfer the W- shaped signature to the mantle wedge. Fig. 3 - Schematic cartoon showing the melt/fluidmediated element exchange between the subducting slab and the overlying mantle (modified after Scambelluri et al., 2006). 4

5 REFERENCES: Hermann, J. & Green, D.H. (2001): Experimental constraints on high pressure melting in subducted crust. Earth Planet. Sci. Letters, 188, Liou, J.G. & Zhang, R.Y. (1998): Petrogenesis of an ultrahigh-pressure garnet-bearing ultramafic body from Maowu, Dabie Mountains, east-central China. Isl. Arc, 7, Piccardo, G.B., Müntener, O., Zanetti, A., Romairone, A., Bruzzone, S., Poggi, E., Spagnolo, G. (2004): The Lanzo South peridotite: melt/peridotite interaction in the mantle lithosphere of the Jurassic Ligurian Tethys. Ofioliti, 29, Scambelluri, M., Hermann, J., Morten, L., Rampone, B. (2006): Melt versus fluid induced metasomatism in spinel to garnet wedge peridotites (Ulten Zone, Eastern Italian Alps): clues from trace elements and Li abundances. Contrib. Mineral. Petrol., 151, Zhang, R.Y., Liou, J.G., Yang, J.S., Yui, T.F. (2000): Petrochemical constraints for dual origin of garnet peridotites from the Dabie-Sulu UHP terrane, eastern-central China. J. Metam. Geol., 18,