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Determining Anatectic Melt Composition of Nanogranite Inclusions in High-Grade Metapelites from the Ivrea-Verbano Zone (Italy)

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Determining Anatectic Melt Composition of Nanogranite Inclusions in High-Grade Metapelites from the Ivrea-Verbano Zone (Italy) Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2011-2012 DETERMINING ANATECTIC MELT COMPOSITION OF NANOGRANITE INCLUSIONS IN HIGH-GRADE METAPELITES FROM THE IVREA-VERBANO ZONE (ITALY). Ph.D. candidate: ALICE TURINA, I course Tutor: Prof. BERNARDO CESARE Co-Tutor: Prof. STEFANO POLI Cycle: XXVII Abstract Nanogranites are tiny polycrystalline inclusions representing the anatectic melt that was trapped during incongruent melting reactions in minerals of high-grade metamorphic rocks. Their characterisation therefore allows to determine the chemistry of primary anatectic melts, which is otherwise one of the biggest unknowns in crustal petrology. I am analysing the composition and the microstructures of nanogranites in amphibolite- to granulite-facies metapelites from the Ivrea-Verbano Zone. These results, together with those from classical petrology and thermodynamic modelling, will shed light on the metamorphic history of the rocks. To analyse the composition of the melt nanogranites were re-melted using a piston-cylinder apparatus. Experiments were performed at 800, 850 and 900 °C and 8-12 Kbar. The inclusions did not totally homogenise at 800 °C but show evidences of over-heating at 900 °C, while at 850 °C the results are still unclear. Further experiments will constrain the temperature of melting. Introduction Melt inclusions are drops of silicic liquid trapped in “host” minerals during growth (Sorby, 1858). They have been extensively investigated in igneous petrology because they can preserve some chemical and physical features of the original magmatic system and thus provide information that otherwise would be lost (Lowestern, 1995; Frezzotti, 2001; Bodnar and Student, 2006). Recently, anatectic melt inclusions, called nanogranites, have been discovered in high-grade metamorphic rocks that experienced partial melting (Cesare et al., 2009). These inclusions can be trapped in peritectic minerals formed during incongruent melting reactions and thus, if they remain a closed system, their composition is representative of that of primary anatectic melts (Cesare et al., 2011; Ferrero et al., 2012). In crustal petrology the composition of the very first melt that forms during anatexis of various source rocks can be either inferred from: i) the composition of leucosomes in migmatites, that can be modified as a consequence of fractional crystallization or segregation (Sawyer, 1987; Solar and Brown, 2001) or ii) experimental works, that cannot totally reproduce natural conditions, especially at low melting degrees (Vielzeuf and Schmidt, 2001). Both methods suffer from limitations that could be overcome through the in situ analysis of natural anatectic melt inclusions; therefore it is important to develop new techniques to determine the composition of the melt trapped as nanogranites (Bartoli et al., in press). The aim of my Ph.D. project is to characterise nanogranites trapped in peritectic garnet of metapelites from the Ivrea-Verbano Zone. This area is of great interest because it is a widely studied example of an exhumed crustal section that underwent regional metamorphism and partial melting of Varisican age (Handy and Zingg, 1991; Schnetger, 1994; Handy et al., 1999). The sequence evolves from amphibolite-facies rocks of shallower crustal levels (locally called Kinzigites) to deeper granulites (Stronalites) through well-recognized mineral isogrades and P-T conditions (Schmid and Wood, 1976; Zingg, 1980; Henk et al., 1997). During this project I am analysing the composition of melt inclusions both in amphibolitic and granulitic metapelites to gain more information on the composition and the volatile contents of primary anatectic melts and possibly to understand the formation and evolution processes of the melt. In order to achieve a complete petrologic characterisation I will accomplish a petrographic and microstructural study of the inclusions and mineral assemblages, to shed light on the P-T conditions of formation and trapping. Moreover, nanogranites from the Kaligandaki valley in Central Nepal (Himalaya) will also be characterised to compare melt compositions in different geological and geodynamic settings. Methods • Petrographic and microstructural observations are carried out through optical microscopy (both in reflected and transmitted light) and scanning electron microscopy EDS spectroscopy, BSE imaging 1 Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2011-2012 and mapping. The minerals that trap nanogranites are polished to uncover the tiny cryptocrystalline aggregates they contain, then analysed with a Field Emission Scanning Electron Microscope (FE- SEM) using EDS maps to determine the constituent mineral phases. Quantitative analyses are not possible due to the small dimensions of crystals that range from some nanometres up to 5 micrometres. The polishing process is a major limit in the work because it often results in mechanical removal of the inclusion; we are now developing new methods to solve this problem. • To obtain the composition of the primary melts, nanogranites need to be fully re-homogenised at the temperature of trapping. Therefore re-melting experiments are performed using a single-stage piston cylinder at the Department of Earth Sciences, University of Milan. Fragments of garnet that contain nanogranites are extracted from the rock and sealed in a golden capsule with silica powder, then loaded in the apparatus. At the end of the experiments the capsules are mounted in epoxy and uncovered through polishing. Successfully re-melted inclusions are then analysed using EMP for major elements, with correction for Na and K loss, following the method explained in Morgan and London (2005). • X-ray micro-tomography has been used to perform a non-destructive microstructural investigation for the analysis of inclusions distribution in garnets. Activities of the first year During the first year of Ph.D. new samples were collected in two different periods of fieldwork; the first one (28-30 May 2012, in collaboration with Prof. Siegesmund) was aimed at the investigation of Valle Strona (northern Ivrea Zone), where the metamorphic evolution is better constrained and continuously mapped. 28 samples of amphibolites and granulites were collected for thin section analyses. In the second one (10-12 October 2012, in collaboration with Prof. Boriani) I went to Valle d’Ossola and Valle Sesia (southern Ivrea Zone) to obtain samples (38) from different areas. Some thin sections from Valle Fiorina (northern Ivrea Zone, provided from Prof. Mazzucchelli) were also observed to detect the presence of nanogranites. Up to now I have performed three experiments, one of which was accomplished during my master’s degree and the other two during this Ph.D. first year, on samples from the Ivrea-Verbano Zone (provided by Dr. Tanya Ewing) and from the Kaligandaki valley (provided by Prof. Dario Visonà). These experiments were conducted at 900, 850 and 800 °C and 8-12 Kbar confining pressure. Some of the activities of the first year regarded the improvement in sample preparation. For the first re- melting experiments we obtained garnet fragments through milling of rocks and handpicking or through separation of “chips” from thick sections. Now I am testing the possibility of drilling cores of garnet containing inclusions using a hollow-core drill. This will permit a major improvement in the efficiency of the experiments. To better understand the 3D distribution of inclusions and the trapping mechanism, three X-ray computed micro-tomography datasets were collected and inclusions were studied through image-analysis and 3D calculations to obtain the volume and position with respect to the garnet core. For this preliminary analysis we decided to investigate inclusions in garnets from El Hoyazo (Spain) because, due to their formation process (Cesare et al., 2007; Acosta-Vigil et al., 2007), these inclusions are not polycrystalline, a feature that could be a limitation in the processing of data. Results The preliminary microscopic observations have permitted the identification of nanogranite inclusions in the majority of amphibolite-facies samples from Valle Strona while in granulite-facies metapelites these inclusions are much less widespread. Nanogranites are also present in most of the garnets from migmatitic paragneisses of the Kaligandaki valley. Usually, inclusions from the Ivrea Zone are smaller (10-15 µm) than those from Himalaya (10-20 µm). Petrographic observations confirm that the transition from amphibolites to granulites in Valle Strona is marked by the decrease of biotite and increase of garnet, as defined by Schmid and Wood (1976). Not all the samples collected represent metapelitic rocks sensu stricto, which contain the mineral assemblage: Qtz+Sil+Bt+Grt+Kfs+Pl±Gr±Ms (abbreviations after Kretz, 1983), since more arenaceous protoliths can produce assemblages that also contain Opx. 2 Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2011-2012 The spatial distribution of inclusions in thin sections suggests a preferentially ring-shaped arrangement localized between the core and the rim of garnets. Since this characteristic feature is reproduced in different samples we decided to investigate the 3D distribution through micro-tomography. The results show that in El Hoyazo garnets the distribution of melt inclusions is bell-shaped, with a maximum
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