Espectroscopía Ultravioleta-Visible (UV-VIS)

Our geochemical data suggests that the Halilbağı unit is formed by different slices of seafloor, including metabasites of different ages and geochemical affinities, metasediments and serpentinites, similar to accretionary complexes of much lesser metamorphic grades (Plunder et al. 2015). It is however unclear if the assemblage of different slices within the Halilbağı unit happened at low-PT in an accretionary wedge or more at depth in the subduction channel. The latter possibility was explored by Davis and Whitney (2006, 2008), where they report three possible slices experiencing lawsonite and epidote stability at different stages, and thus coming from different depths/temperature regimes. They concluded that at least the exhumation story was common within the Halilbağı unit. Our ages can neither refute nor confirm this scenario by lack of precision, the older age of 98 Ma obtained in the florencite cores could hint to a protracted prograde history, such as Fornash et al. (2016) interpret from Ar-Ar ages. These could be recording low P-T metamorphism within the accretionary wedge prior to the entrainment into subduction.

This study showed that the Halilbağı unit can be considered as the eastern equivalent of the Devlez formation as an oceanic complex. There, Plunder et al. (2015) showed that the Devlez fm. had gone to lesser P-T than the coherent sequences underneath (such as the Kocasu fm.). Here in the Sivrihisar massif however, the Halilbağı unit records the highest P-T, similar to the Kocasu fm. in the West.

Figure 3 - 30 Updated Halilbağı unit position on Plunder et al. (2015) figures 10 and 11. P-T estimates for the Halilbağı unit from (1) Çetinkaplan et al. (2008); (2) Davis & Whitney (2008); (3) Davis & Whitney (2006)

This shows that the exhumation dynamics for both parts of the Tavşanlı zone are not identical, with the exhumation of deeper oceanic complex in the Sivrihisar unit. In both cases though, the oceanic complexes that have been subducted to depth represent a much smaller volume than the more continental carbonate platform (see Plunder et al. 2015). In the Tavşanlı zone, we thus see several oceanic complexes with similar petrologic and geochemical characteristics exhumed from a broad range of depth within the subduction zone (Figure 3 - 30), indicating a very dynamic environment of accretion-subduction of material from the top of the down-going plate.

7 Conclusions

Mafic rocks in the Halilbağı unit mostly yield a sediment signature, with sills/flow basalts of OIB and MORB affinity, and lenses of oceanic serpentinites.

Magmatic zircons in a MOR-type leucogabbro were dated at 222 ±5 Ma, and an OIB Fe- Ti gabbro at 123 ±5 Ma.

Detrital zircons in a calcsilicate sediment yield a provenance from both the Gondwana margin (e.g. Arabian-Egyptian shield) and either the Sakarya zone Carboniferous arc or the easternmost Variscan detritus.

The presence of OIB and MORB, serpentinite lenses, as well as Triassic and Jurassic magmatic ages are best explained by a composite origin of the Halilbağı unit. The ages correspond to both the base and the top of the coherent carbonate platform in the coherent Orhaneli units, within the range of radiolarites present in the accretionary complexes. We thus confirm previous studies making the Halilbağı unit the equivalent of the Ovacık complex in the western Tavşanlı zone: a composite unit sampling distal seafloor lithologies, similar to widespread cretaceous accretionary complexes in the Tavşanlı- Ankara region.

SHRIMP metamorphic ages are obtained in the U-Th-Pb system in minerals allanite (retrograde), florencite (prograde) and zircon (multi-phased), which yield 88 ±10 Ma, 98 ±8 Ma and 89.5 ±2.2 Ma respectively. Florencite yields potential for dating the prograde stages of metamorphism in Ca-poor rocks but requires further standard development to investigate matrix bias with respect to monazite and allanite.

Chapter 4

Fluid circulation in subducted crust investigated by in situ

O isotopes and trace elements in zircon, garnet, apatite and

lawsonite (HP-LT Halilbağı unit, Turkey).

1 Introduction

In understanding element cycling in the Earth’s mantle, the processes happening in the subducting slab are key, as the slab is the main locus of element transfer from the crust to the mantle. From their oceanic floor protoliths, subducted lithologies undergo major transformations as they dehydrate and deform on their way to sub-arc depths. Metasomatism (e.g. Bebout and Barton 1989) and mechanical mixing (e.g. Penniston-Dorland et al. 2014) between rock types are two dominant processes in the subduction slab which may have long lasting consequences on element recycling. Fluids are the main agent for chemical transfers in the slab, as they catalyse reactions, enable deformation and seismicity, and transfer mass within the subducting slab, to the plate interface and eventually to the overlying mantle. The resulting major and trace-element chemistry of sub-arc lithologies will control what dehydration fluids and melts will permeate to the mantle wedge, and which elements will remain in the slab. This case study uses the Halilbağı unit in the Tavşanlı zone in Turkey as a natural laboratory for studying the interaction of different subducting plate lithologies through fluid exchanges during their history at the plate interface. It builds on and contributes to work in better understanding the subduction factory, from the sea floor to sub- arc depths.

Slab dehydration and fluid-rock interactions are complex, because of two main factors: first, fluid flow and deformation are tightly intertwined in subduction dynamics (subduction deformation regimes are presented in Figure 4 - 1). At shallow levels in the seismic zone, abundant fluids (e.g. Hacker 2008) are a major player in seismicity and deformation (e.g. Vrolijk 1987; Byrne and Fisher 1990; Breeding and Ague 2002; Bachmann et al. 2009). The modelling of subduction geotherms is complicated at depth by the upward migration of fluids creating heat advection, as well as the depth and extent of slap-mantle coupling below the seismic zone to the maximum decoupling depth MDD (Wada and Wang 2009; Penniston-Dorland and Kohn 2015). Slab-mantle decoupling is subordinate to the presence of hydrated deformable lithologies in the mantle wedge (e.g. Scambelluri and Tonarini 2012), and thus the depth of the MDD, usually around 80 km

(Figure 4 - 1), can change according to the specific subduction zone fluid regime, which in turns determines where the temperature for main dehydration reactions is obtained in the slab lithologies (Syracuse et al. 2010; Schmidt and Poli 2014). The field record of HP-LT rocks is valuable to study how fluid influxes correlate with temperature and deformation changes in different subduction zones.

Figure 4 - 1 Sketch of subduction tectonic environments in the subduction zone (modified from

Hilairet and Reynard 2009)

Second, the subducting slab, made of oceanic mantle, crust and sediments, is heterogeneous in composition and rheology. Although studies have shown that the slab component comprises elements of both metasediments and mafic and ultramafic oceanic crust (Kessel et al. 2005; Spandler and Pirard 2013), the modalities of mechanical and chemical interactions between these two reservoirs are critical to the understanding of sub-arc processes. Both oceanic crust alteration by interaction with seawater and sediments (Miller and Cartwright 2000; Spandler et al. 2004), and the evolution of mélanges at the slab interface - made of mechanical and chemical hybridization of slab lithologies (Marschall and Schumacher 2012) - are thought to play an important role in the geochemical evolution of the slab. The mineralogy and chemistry of these modified and hybridized lithologies are suspected to control the generation and chemical composition of the slab component through dehydration and melting in sub-arc regions. The interaction of sediment-derived fluids with mafic and ultramafic rocks can be traced by fluid- mobile, incompatible trace elements and Sr- and Pb-isotopic studies. However, it is much more difficult to trace the influx of trace element poor fluids from mafic or ultramafic rocks with sediments. This work aims at pinpointing the P-T conditions at which fluid-rock interaction and isotopic mixing occur between different rock types of the subducted slab, and untangling seafloor from subduction hybridization.

Oxygen isotopes are used as monitors for influences of fluids originated in sediments (heavy), or oceanic crust (light) fluids. Indeed, the signatures present in different reservoirs of the subducting slab are diverse. Sediments are rich in the heavy isotope of oxygen (δ18_{O = 15-25 ‰), whereas}

the altered oceanic crust and yield intermediate signatures depending on different temperatures of interaction with seawater (12 ‰ –LT to 3 ‰ – HT). Previous studies have used whole-rock or mineral separate signatures to identify where sediment-dominated fluids (e.g. Bebout and Barton 1989; Miller et al. 2001), mafic-dominated fluids (e.g. Philippot et al. 2007), or no significant modification (Barnicoat and Cartwright 1995) are found in different subducted terranes. Here, as outlined in the thesis Introduction, the novelty of the method used is the in situ analysis of minerals. This study distinguishes mineral growth zones to establish how much and at which stages of the P-T evolution oxygen is exchanged between sedimentary lithologies and the mafic/ultramafic parts of the slab during subduction metamorphism.

In situ oxygen isotopes in garnet growth zones have been used by a few previous studies of subduction processes: in the Monviso eclogites (Rubatto and Angiboust 2015), in the Franciscan (Russell et al. 2013; Errico et al. 2013; Page et al. 2014), and in the high-pressure rocks of Corsica (Martin et al. 2014b). These studies allowed recognizing fluids of varying signatures during the subduction history of garnet-bearing rocks. Here, the aim is combining garnet data with those from other metamorphic minerals to get a record that is as complete as possible for the reconstruction of the fluid history during subduction, in a range of lithologies. A range of target minerals is used in this study. (i) Zircon is a robust mineral used for oxygen in magmatic and metamorphic settings (Peck et al. 2003; Valley 2003; Page et al. 2014). (ii) Apatite is a ubiquitous fluid-reactive mineral (Smith and Yardley 1999; Spear and Pyle 2002; Li and Hermann 2015) used routinely for in situ oxygen analysis in fossils (Rigo and Joachimski 2010; Trotter et al. 2015), and very recently investigated for geological processes (Sun et al. 2016). (iii) Lawsonite is a trace-element- and water- rich mineral thought to play a significant role in slab-mantle transfer processes (Spandler et al. 2003; Dragovic et al. 2012; Martin et al. 2014a; Vitale Brovarone and Beyssac 2014) and which has not yet been used for in situ oxygen; exploratory results in collaboration with Laure Martin are presented. In each mineral, trace element analysis will allow linking fluid circulations to P-T conditions (similar to work done in zircon and garnet: Rubatto 2002; Martin et al. 2011; Gauthiez-Putallaz et al. 2016; Chapter 2), but also identifying changes in fluid-mobile trace-elements along the P-T path (for instance Sr/Pb variations identified in lawsonite by Martin et al. 2014a). These mineral data are then compared to whole rock (WR) analyses to assess the influence of metasomatism on overall signatures.

The chosen field area – the Tavşanlı zone in Turkey – contains some of the best preserved lawsonite eclogites, an invaluable record of cold subduction processes. The main focus is the Halilbağı unit, in the Sivrihisar massif (see maps and context in Chapter 3). Previous studies have focused on the petrography and the low-T, high- P trajectory of the Halilbağı metabasites (Gautier 1984; Whitney and Davis, 2006; Davis and Whitney 2006; Çetinkaplan et al. 2008; Davis and Whitney 2008; Whitney et al., 2014). These studies estimated the maximum P-T conditions as 23-25 kbar, 550°C (see summary of P-T determinations in Chapter 3). Most unusual is the cold gradient of exhumation, as most Halilbağı rocks escape overprinting and retrogression and contain pristine lawsonite, a rare occurrence worldwide (Tsujimori et al. 2006; Whitney and Davis 2006). The paleogeography of the Halilbağı unit is less investigated, although insights are provided by Çetinkaplan et al. (2008), as well as broader scale studies (Okay and Whitney 2010; Plunder et al. 2013; Plunder et al. 2015; Pourteau et al. 2016). In Chapter 3, new geochonological and geochemical data are presented to provide a clearer geodynamic background to the Halilbağı unit. The main conclusions are that the Halilbağı unit is an oceanic complex similar to accretionary units in the western part of the Tavşanlı zone (e.g. Plunder et al. 2015) formed by a juxtaposition of contrasted distal (cherts, MORB and OIB basalts and gabbros, serpentinites)and continental margin lithologies (detrital and carbonate-rich sediments) that was subducted and exhumed along with other oceanic complexes, close to the plate boundary. This area thus provides the opportunity to study the interaction between different lithologies of the subducting slab, down to the maximum decoupling depth (around 80 km).

The timing of subduction is assumed to be Cretaceous, with most ages related to the peak of metamorphism around 90 Ma (e.g. Mulcahy et al. 2014; Fornash et al. 2016, this study, full summary of available data in Chapter 3). Retrograde blueschist assemblages are dated at ca 85 Ma, and a recent study of prograde assemblages concludes that samples have come from different P-T-t trajectories, with prograde ages around 93 Ma (Pourteau, pers. comm.). Such conclusions have also been drawn by Davis and Whitney (2008), on the base of P-T and deformation only. Ar-Ar studies hint to a more protracted history with earliest ages around 125 Ma (Fornash et al. 2016), similar ages in earlier studies were questioned on the basis of the possible presence of excess argon (Sherlock and Kelley 2002). Fornash et al. (2016) assign phengite recrystallization to phases of deformation and fluid flow in the Halilbağı unit. That work ties into deformation studies of the Sivrihisar massif (Gautier 1984; Teyssier et al. 2010; Whitney et al. 2014) that show an increase of deformation intensity towards the northern contact of the Halilbağı unit to the low- grade peridotitic unit, assigned to exhumation-related deformation (Whitney et al. 2014).

Geochemical studies of the Sivrihisar/ Halilbağı unit are scarce. Cr zoning in lawsonite has been related to fluid influxes (Sherlock 1999), and another study has reported halogen concentrations in amphibole, lawsonite, phengite and apatite and concluded that the rocks were depleted in Cl at shallow subduction levels but that F is retained (Pagé et al. 2016). A first part of the dataset on lawsonite trace-elements presented in this chapter was published in Martin et al. (2014a), paper provided in Appendix A4. In this thesis, Chapter 3 provided additional data for impure quartzite SHB046, and petrography and trace-element data acquired on sample SV03-103 provided by D. Whitney (Davis and Whitney 2008). The preliminary work on sample SV03-103 highlighted the fractal heterogeneity of some metabasitic lithologies, thus impairing the ability for distinguishing small scale vs large scale equilibration processes. In this work, the focus is thus on typical examples of the lithologies (marble, impure quartzite, metachert, serpentinite, metabasites: lawsonite eclogite and lawsonite blueschist), on samples that are as homogenous as possible at the thin section to hand specimen scale.

The current study aims at shedding more light on fluid circulation between different rock types in the subducting slab, which are well preserved in the Tavşanlı zone, by combining in situ mineral δ18_{O analysis with in situ trace-element analysis, and integrating it with WR signatures.}

2 Methods

Mineral mounting and imaging methods are presented in Chapter 3.