Deformation and Alteration of a Granodiorite

Deformation and alteration of a granodiorite | RESEARCH

Deformation and alteration of a granodiorite during low-angle normal faulting (Serifos, Greece)

Cornelius Tschegg1,* and Bernhard Grasemann2 1DEPARTMENT OF LITHOSPHERIC RESEARCH, UNIVERSITY OF VIENNA, ALTHANSTRASSE 14, 1090 VIENNA, AUSTRIA 2DEPARTMENT FOR GEODYNAMICS AND SEDIMENTOLOGY, UNIVERSITY OF VIENNA, ALTHANSTRASSE 14, 1090 VIENNA, AUSTRIA

ABSTRACT

On Serifos (Western Cyclades, Greece), a late Miocene I-type granodiorite pluton intruded a low-angle normal fault (LANF) during extension and exhumation of the middle crust. In the studied structural section, the LANF cuts the granodiorite and, due to the minor displacement (<3 km), the roof of the intrusion was offset onto structurally lower parts of the pluton. As a result, the (micro)structural, petrological, bulk and mineral geochemical development of the LANF, at progressively increasing strains in a relatively homogeneous granodiorite, has been studied. Field data show that a totally bleached granodiorite in the hanging wall, weakly deformed under lower greenschist-facies conditions, has been juxtaposed along the LANF against an upper greenschist-facies mylonitic granodiorite in the footwall. The latter exposes a gradual decrease in deformation intensity toward lower structural levels, into the undeformed granodiorite. Bulk major and trace element compositions of footwall samples showed almost no whole-rock compositional, mass, or volume change with increasing deformation. In ultramylonites, however, the modal mineral contents and associated bulk geochemistries have been slightly altered, as a result of strain- triggered, intrinsic fl uid-assisted, reaction mechanisms. Because the fi eld observations and geochemical data do not show evidence for high-tensile fl uid overpressures within the LANF, we suggest that reduction of the apparent fault strength by weak fault zone material and/ or a change in the deformation mechanisms facilitated movement.

LITHOSPHERE; v. 1; no. 3; p. 139–154. doi: 10.1130/L33.1

INTRODUCTION Changes in element chemistry and stable isotopes are clear evidence for strong fl uid fl ow in many shear zones (Kerrich et al., 1980; Rolland et Low-angle normal faults (LANFs) with fault dips of less than 30° al., 2003; Hürzeler and Abart, 2008). Several studies have demonstrated are an important class of faults that have been described in continental that both brittle faults and ductile shear zones are preferential conduits for crust (Axen et al., 1999; Burchfi el et al., 1992; Wernicke, 1981) and at fl uid fl ow parallel to the fault but may also act as a barrier perpendicular to slow-spreading mid-ocean ridges (Garcés and Gee, 2007; MacLeod et al., the fault or shear zone (McCaig, 1987; Kisters et al., 2000; Sibson, 2000; 2002). In continental extensional settings, LANFs are frequently associ- Micarelli et al., 2003). ated with the exhumation of lower to mid-crustal rocks and syntectonic to In this work, we describe the geochemical, petrological, and struc- late tectonic granitic plutons (Davis and Lister, 1988; Gans et al., 1989; tural development of a major LANF that evolved close to the roof of an Hill et al., 1995; Lister et al., 1984; Wernicke, 1981). Although such upper crustal granodiorite intrusion that is part of the Miocene Serifos continental LANFs have received considerable attention, their forma- metamorphic core complex (Western Cyclades, Greece). In the studied tion remains controversial (see Axen, 2007, and references cited therein) location, the LANF developed exclusively in the granodiorite; the lim- because (1) fault mechanical theory does not predict the formation of ited offset along the fault has led to the special situation that the foot- extensional faults with orientations of less than 30° in the brittle upper wall, consisting of granodiorite with increasing magnitude of ductile crust (Anderson, 1951), and (2) evidence for large earthquakes on LANFs deformation toward the fault, is juxtaposed along a knife-sharp catacla- is rare or absent (Jackson and White, 1989; Wernicke, 1995, Collettini stic fault against strongly “bleached” granodiorite in the hanging wall. and Sibson, 2001). Although some reported continental LANFs may have Therefore, the source rock before deformation was, at the scale of the rotated from steep to shallow dips (Buck, 1988; Koyi and Skelton, 2001), outcrop, a relatively homogeneous granodiorite intrusion with a clearly others clearly originated at low angles, especially where syntectonic igne- defi ned geochemical composition. This makes it ideally suited for study- ous intrusion and associated dikes do not allow large rotations due to geo- ing the processes and effects of deformation on the protolithic micro- metric constraints (Buick, 1991; Faure et al., 1991; Iglseder et al., 2008). structure and geochemistry. LANFs that move at low dip angles require rotation of the stress fi eld around the fault (Yin, 1989; Spencer and Chase, 1989) or low apparent GEOLOGICAL OUTLINE fault friction, either due to weak fault-zone materials (e.g., Collettini and Holdsworth, 2004; Gueydan et al., 2004; Mehl et al., 2005; Niemeijer and Serifos, located ~100 km SSE of Athens, geologically belongs to Spiers, 2007; Numelin et al., 2007) and/or high fl uid pressure within or the Attic-Cycladic massif. The island is dominated by a late Miocene, adjacent to the fault zone, creating tensile fl uid overpressures in the foot- high-level I-type granodiorite pluton (Ballindas, 1906; Marinos, 1951) wall of the LANFs (Reynolds and Lister, 1987). that intruded into orthogneisses, amphibolites, greenschists, and marbles (Fig. 1). Intrusion ages of the main granodiorite body and its associated *[email protected] dikes lie between 11.6 and 9.5 Ma (Iglseder et al., 2008) followed by rapid

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Πλατυ´ς Γιαλο´ς Κα´ βος 24°25′ 24°30′ Platy At Platis Yialos ( ) in the N, at Kavos Kiklopas ( Yialos Quarternary beach Κυ´κλωπας) in the SW, and at Tsilipaki (Τσιλιπα´ κι) in the SE (Fig. 1), sediments Dacitic and/or andesitic excellent exposures of the LANF are preserved. These outcrops have the dikes (~11–9 Ma) following characteristics in common (Grasemann and Petrakakis, 2007): Granodiorite (~11–9 Ma) (1) The footwall below the low-angle shear zone consists of gneisses, Greenschist gneisses and greenschists with marble layers, which record an earlier deformation marbles Low-angle and metamorphic history, probably related to an earlier high-pressure faults event typical for the Attic-Cycladic massif (Salemink and Schuiling, Panorama Fig. 2 1987). The dominant lineation trends ENE-WSW. (2) Toward the low- ′ angle fault, there is an increase in noncoaxial strain with a NNE-SSW–

37°10 trending stretching lineation recording top-to-SSW–directed shear. (3)

Agios Sostis Below the major fault plane, there are several meters of ultrafi ne-grained (<10 μm), ultramylonitic marbles with top-to-SSW shear-sense indicators. (4) The LANF is marked by a knife-sharp plane associated with Livadhi polyphase ultracataclasites recording top-to-SSW–directed shear. (5) The greenschists, gneisses, and marbles above the fault are strongly overprinted by protocataclastic deformation and ankeritized by massive Tsilipaki Kavos fl uid infi ltration. (6) The LANF predates, interacts with, and postdates Kiklopas N sets of WNW-ESE–striking, conjugate high-angle normal faults indicat- 3 km ing a localization of the LANF during ongoing NNE-SSW extension of the crust under subvertical maximum principal stresses. The transition from ductile to brittle deformation mechanism suggests Figure 1. Geologic map of Serifos, in the Western Cyclades in Greece that the low-angle fault operated at the brittle-ductile transition zone. This (strongly simplifi ed after Grasemann and Petrakakis, 2007). is supported by the 12–8 km granodiorite intrusion depth, which was emplaced during extensional movement of the fault (Stouraiti and Mitro- poulos, 1999). Furthermore, quartz grains in the marble mylonites record microstructural evidence for dislocation glide and brittle fracturing but no cooling (Altherr et al., 1982; Henjes-Kunst et al., 1988). Apatite fi ssion- evidence for dislocation creep. This transition is inferred to be at tempera- track ages from the central parts of the granodiorite plot between 6.7 ± 0.8 tures of ~280 °C for rocks deforming at strain rates within the commonly and 5.3 ± 0.6 Ma (Hejl et al., 2002). The granodiorite intruded into upper inferred range of 10−11 s−1 to 10−14 s−1 (Stipp et al., 2002). crustal levels (Stouraiti and Mitropoulos, 1999) during ongoing extension. In the SE part of Serifos, N of Tsilipaki, the LANF cuts through the The main granodiorite body discordantly crosscuts the regional folia- granodiorite pluton at the bay of Aghios Sostis (Αγιος Σω´ στης), recording tion of the host rocks and is essentially undeformed or slightly foliated. progressive deformation of the undeformed granodiorite at lower structural However, the roof of the granodiorite intrusion, associated dikes, and the levels to mylonitically deformed granodiorite within the shear zone. Due country rocks have been deformed by a greenschist-facies, brittle-ductile to the relatively homogeneous lithology of the plutonic body, the geom- to brittle LANF during cooling and exhumation. The LANF arches over etry, kinematics, and associated chemical and mineralogical changes of the the island and consistently records a top-to-SSW–directed shear sense. shear zone, which are the focus of this work, could be studied in detail. The formation of the LANF has been interpreted as the result of Miocene crustal-scale extension and metamorphic core complex formation (Grase- STRUCTURAL RECORDS mann and Petrakakis, 2007; Iglseder et al., 2008), similar, although with opposing kinematics, to other islands in the Cyclades (e.g., Avigad and Structural fi eld observations are shown in Figures 2, 3, and 4. Garfunkel, 1989; Forster and Lister, 1999; Gautier and Brun, 1994; Jolivet Figure 2 shows a panoramic view toward the SE, onto the headland at et al., 2004; Lister et al., 1984). the bay of Aghios Sostis. Here, a complete section of the undeformed or

SB8-07 Bleached, less deformed granodiorite Low-angle normal fault (LANF) 080/20

SB6-07 SB4-07 High-angle normal SE fault 028/62 Mylonitic granodiorite SB1-07 Foliated granodiorite SB2-07 SB3-07

Figure 2. Panorama of Aghios Sostis looking toward SE (location is given in Fig. 1). The undeformed granodiorite (W of the beach) is foliated to strongly deformed below the low-angle normal fault (LANF). Above the LANF, an intensely bleached but less deformed granodiorite is exposed. This bleached granodiorite is downthrown along a high-angle normal fault at the NNE part of the headland. The structural distance of the investigated samples from the footwall to the LANF decreases in the following order: SB1-07, SB4-07, SB6-07, SB2-07, and SB3-07; sample SB8-07 is located in the hanging wall of the LANF.

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A B Strongly foliated sjf3 301/82 granodiorite (128/32)

Chlorite-filled joint szf 124/34 Bleached halo

Deflection of foliation

W NE SW

Mylonitized C D granodiorite sjf2 212/85 Aplitic dike

sjf2 219/64 40°

SSW NNE NNE

E Weakly deformed F bleached granodiorite

Weakly deformed bleached granodiorite

sfp 230/79

Knife-sharp LANF Strongly foliated to (s 080/20, l 200/05) mylonitic granodiorite

Strongly deformed mylonitic granodiorite SSE NNW W E

Figure 3. (A) Unfoliated granodiorite with W-E–striking, chlorite-ﬁ lled joints (sjf3) with cm-thick, symmetrically developed, bleached halo. (B) Shear zone (szf), which localized along a brittle precursor fracture that developed within the foliated granodiorite. Note the deﬂ ection of the foliated grano-

diorite along the shear zone. (C) WNW-ESE–striking aplitic dikes and bleached joints (sjf2) in the unfoliated granodiorite. (D) SSW–dipping bleached

joints (sjf2) crosscut the mylonitized granodiorite. (E) Knife-sharp, low-angle normal fault (LANF) separating the bleached but less deformed granodiorite above from the unbleached mylonitic granodiorite below. (F) NW-SE–striking, high-angle brittle faults (sfp) offset the LANF and juxtapose the bleached granodiorite against the mylonitic granodiorite.

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A erally dips toward SE (Fig. 3A). The LANF indicated in Figure 2 contin- ues to the point where the panorama picture was taken. The structural dis- lm tance between the investigated footwall samples and the LANF decreases Shear zones FW (7), szf szf with the order given in the tables, while at the same time the intensity of Shear zones HW (7), szh the foliation increases. Sample SB8-07 is located in the hanging wall of the LANF (Fig. 2). Foliation (25), sm In the following sections, the main structural characteristics of the Lineation (24), lm rocks are described, starting from the undeformed granodiorite in the sm Lineation on ssz FW (24), lsz szh footwall, passing up into the mylonitic granodiorite, and thence to the bleached granodiorite at the top, in the hanging wall. The protogranodiorite SB1-07 at the lowermost structural level is macroscopically essentially undeformed; locally a vague foliation deﬁ ned by preferred orientation of biotite can be discerned (Fig. 3A). The rocks

are cut by three different joint systems (Fig. 4B). The oldest system (sjf1) B sjh2 consists of isolated joints that dip moderately steeply toward SE. These joints are extremely difﬁ cult to recognize in the undeformed granodiorite

sjf2 and have been only occasionally observed. No bleaching or any evidence sjf1 of diffusive mass transfer has been observed along these joints. Because sjf occurs in the undeformed granodiorite but locally also in the foliated Joints and mode II quartz 1 veins HW (22), sjh granodiorite, where the joints were reactivated as ductile shear zones, this 1, 2 sjh1 Joints FW (7), sjf joint system formed during shearing along the LANF and cooling and 1 exhumation of the pluton. Two, almost vertical, joint systems (sjf and sjf ) Bleached joints FW (37), sjf 2 3 1, 2 are striking roughly WNW-ESE and NE-SW, respectively. Both systems sjf form dense networks of a few cm- to several m-spaced joints, which are 3 partly ﬁ lled with chlorite. The joints are almost invariably surrounded by bleached haloes, generally only a few centimeters thick and symmetrically developed to either side of the joint. In the outcrop section, they are segmented, with lengths of several tens of centimeters up to meters. The C sd segments have en echelon geometry with a slight overlap at their termina- tions (Fig. 3B), but no systematic left- or right-stepping pattern has been LANF observed (Segall and Pollard, 1980). Toward higher structural levels, there is a progressive transition from sfp “undeformed” granodiorite to macroscopically, weakly foliated granodio- Faults with pseudotachylites, (15), sfp rite SB4-07 and into a clearly foliated granodiorite SB6-07 (Fig. 2). With Dikes (8), sd increasing strain toward higher structural levels, isolated ductile shear zones (szf), localized on brittle precursor joints sjf1 (Mancktelow and Pen- nacchioni, 2005), formed a network of shear zones separating mylonitic granodiorite from less strongly deformed granodiorite (Fig. 3B; compare also Figs. 4A and 4B). Note, however, that locally observed deﬂ ection

and/or offset of sm along sjf1 suggest that localization of strain along sjf1 occurred after the formation of an initial main foliation (sm). The initial shear zones, recording an offset of a few millimeters to centimeters, are Figure 4. Stereoplot (equal area projection, lower hemisphere) of structural data: (A) main foliation (sm) and stretching lineation (lm) of the sharp planes with extremely localized shear deformation. However, with deformed granodiorite. Localized shear zones in the deformed granodio- increasing offset, the shear zones broadened, forming an intense mylonitic rite (FW—footwall, szf) and bleached granodiorite (HW—hanging wall, foliation and transform toward higher structural levels into homogeneously szh). (B) Joints and mode II quartz veins in the bleached granodiorite deformed (ultra)mylonites without any evidences of further localization of (HW, sjh). Joints (sjf ) and bleached joints (sjf ) in the deformed grano- 1 2–3 deformation. Along sections on the NE side of Aghios Sostis, the transi- diorite (FW). (C) High-angle faults with frequent pseudotachylites (sfp). tion from weakly deformed to mylonitic granodiorite is more gradational, Dikes within the undeformed granodiorite. Mean orientation of the low- angle normal fault (LANF) and the stretching lineation on the fault. and no strain localization along shear zones has been observed. Two joint systems sjf2 and sjf3 crosscut the main foliation (sm) and clearly postdate the ductile (mylonitic) deformation. Neither system was reactivated by ductile shear, and both record the same characteristics (orientation, en weakly foliated granodiorite (W of the bay), the strongly foliated grano- echelon geometry, spacing, and bleaching; Fig. 3D) as the corresponding diorite (lower structural levels, E of the bay) to the mylonitized granodio- systems in the undeformed granodiorite. rite (central part of the picture), has been exposed. Above the mylonitic The mylonitic granodiorite SB2-07 and the ultramylonite SB3-07, granodiorite, which represents the LANF, the precursor granodiorite is which gradually form the highest structural level below the bleached completely metasomatized by inﬁ ltrating ﬂ uids above a knife-sharp brittle granodiorite of the hanging wall, have a structural thickness of several fault (eastern end and topographic top of the headland). Following the meters, with a strong foliation and stretching lineation. Similar to the less designation of Salemink (1985), this altered type of rock will be called deformed and undeformed granodiorite below, the rocks are cut by joint

bleached granodiorite in the subsequent text. The main foliation (sm) gen- systems sjf2 and sjf3, both with bleached halos.

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Above the mylonitic granodiorite, a knife-sharp, brittle, low-angle An60 Ab39 Or1 to An37 Ab61 Or2, has been preserved. Biotite clasts do not fault dips gently eastwards and separates the footwall mylonites from indicate chemical alteration. weakly foliated, totally bleached, hanging-wall granodiorite SB8- Structurally upwards, the foliated granodiorite shows further notice- 07 (Figs. 2 and 3E). This fault corresponds and links with the LANF able grain-size reduction (Fig. 5C). The foliation consists of ﬁ ne-grained, mapped to the S, on Tsilipaki (Fig. 1). Although dm-thick lenses of non- elongated, dynamically recrystallized quartz grains deformed by subgrain cohesive ultracataclasites and fault gouge have been observed along the rotation (Regime 2 of Hirth and Tullis, 1992) and elongated aggregates of fault, the contact is in places a sharp, brittle fault. Isolated, several tens of recrystallized K-feldspar showing no compositional variations. Relic large meters long, and up to several centimeters thick localized ductile shear perthitic K-feldspar do not record any preferred orientation of Ab lamellae zones (szh) with consistent SW-directed kinematics have been observed, nor do they show chemical alteration (see Table 1). Fragmented, fractured, mainly on the S side of the headland of Aghios Sostis (Fig. 4A). These tilted, and boudinaged plagioclase grains show interstitial recrystalliza- shear zones clearly have a different orientation than the localized shear tion of K-feldspar and biotite, with preferred growth of these minerals zones in the granodiorite (szf). The bleached granodiorite and the shear in the strain shadow of plagioclase (see Fig. 5G). Plagioclase porphyro- zones are cut by a decimeter- to meter-spaced, subvertical joint system clasts have preserved a magmatic zonation but indicate subgrain rotation

striking roughly both N-S (sjh1) and W-E (sjh2; Fig. 4B). This joint sys- and dynamic recrystallization to andesine (An44 Ab54 Or2; see Table 2). tem is restricted to the bleached granodiorite in the hanging wall and has Recrystallized biotites indicate an increasing amount of FeO, from 16.7 not been observed in the footwall granodiorite. wt% in biotite grains in the protogranodiorite to 17.3 wt% in recrystallized

All lithologies and structures are cut by brittle normal faults asso- biotite along the foliation, with a simultaneous decrease of TiO2 from 4.35 ciated with slickensides and pseudotachylites (Figs. 3F and 4C). The wt% to 3.14 wt% (see Table 3). This grain downsizing and lowering of Ti faults have a normal offset in the order of several meters and dip steeply in biotites at increasing deformation has been described elsewhere (e.g., to the NE or SW (Fig. 4C). A major, high-angle normal fault, which Kerrich et al., 1980). separates the NNE part of the headland from the rest of Aghios Sostis In thin sections, the mylonitic granodiorite indicates strongly elongated (see Fig. 2) dips NNE and juxtaposes bleached hanging-wall granodio- quartz layers, residual perthitic K-feldspar phenocrysts in thin recrystal- rites to the NNE against mylonitic footwall granodiorites to the SSW. lized K-feldspar layers and plagioclase porphyroclasts, connected with As a result, the LANF that is exposed in the footwall at a maximum interstitially growing K-feldspar, albite, biotite, and ore minerals, together altitude of 20 m above sea level has been downthrown below sea level forming the foliation (Fig. 5D). The mineral composition of K-feldspar in the hanging wall. clasts has not changed compared to that in the protogranodiorite; recrystallized K-feldspar grains also do not show any chemical alteration MICROSTRUCTURES AND MINERAL CHEMISTRY (Table 1). Although plagioclases apparently lost the magmatic zoning, mainly through grain size reduction, the compositions do not alter. Strain Microstructural features of all investigated samples are summarized in shadows of plagioclase clasts or necks of boudinaged plagioclases con- Figure 5, and representative mineral chemistries of potassium feldspars, sist of dynamically recrystallized K-feldspar (Fig. 5G) or albitic subgrains plagioclases, as well as biotites, are listed in Tables 1–3. of the porphyroclast, often intergrown with recrystallized K-feldspar and In thin sections, the lowest granodioritic sample collected from biotite (see Fig. 5H and Tables 1–3). the footwall shows a typical protogranular microstructure with quartz At the top of the footwall, the ultramylonitic sample consists of quartz grains, perthitic K-feldspars, and plagioclases >500 µm (Fig. 4A). ribbons, layers with extremely ﬁ ne recrystallized albitic plagioclase (<10 Accessorial mineral phases are represented by biotite (>400 µm) and µm) intergrown with quartz, K-feldspar, and biotite, as well as layers con- hornblende as well as mostly idiomorphic magnetite, Ti-magnetite, sisting of mainly K-feldspar with minor amounts of biotite (Fig. 5E). In apatite, titanite, magmatic epidote, and zircon. K-feldspars have ortho- the bright layers in Figure 5E, consisting mainly of recrystallized K-feld-

clase composition (Or94) with BaO contents of 0.45 wt%, often showing spar and biotite, the minerals have similar compositions to those in the less sutured grain boundaries with plagioclases (see Table 1). The plagio- deformed granodiorite (see Tables 1 and 3). The dark layers in Figure 5E

clases show compositional magmatic zonation from anorthitic (An61 are mainly composed of recrystallized andesine (An41 Ab58 Or1), quartz,

Ab38 Or1) cores to albitic (An41 Ab58 Or1) rims (labradorite to andesine, and biotite neoblast ﬂ akes. Intergrown K-feldspar blasts that are related to

respectively; Table 2). Biotite clasts in SB1-07 are characterized by Mg# this reaction matrix have orthoclase compositions (Or95) with exceedingly

of 0.45 and TiO2 contents of 4.35 wt% (see Table 3); amphiboles are typ- high BaO contents (0.53 wt%) compared to the other ribbon or intersti-

ical magnesio-hornblende (TiO2: 1.22 wt%, Al2O3: 7.3 wt%, Fe2O3: 3.3 tially recrystallized grains of K-feldspar (see Table 1). Plagioclase phe- wt%, FeO: 11.4 wt%, MgO: 13.6 wt%, CaO: 11.8 wt%; see Appendix A nocrysts up to 200 µm in size are homogeneously dispersed. A plethora for analytical techniques). Generally, the quartz grains are strain free or of unequivocal shear-sense criteria (Fig. 5E) such as shear bands, S-C-C have a patchy undulatory extinction. Feldspars are locally fractured, and fabrics, σ- and δ-Fsp clasts, and shape- and lattice-preferred orientations biotite packages are frequently kinked. of quartz (see Passchier and Trouw, 2005 and references cited therein) The microstructures of the foliated to mylonitized granodiorite record suggests a high component of noncoaxial deformation. the following characteristics. In the slightly foliated granodiorite, a con- Microscopically, the bleached granodiorite in the hanging wall shows siderable grain-size reduction of all phases has been observed (Fig. 5B). a weak foliation deﬁ ned by an interconnected weak layer of dynamically Dynamically recrystallized quartz grains partly form elongated ribbons recrystallized quartz (Fig. 5F). A few old, larger grains have patchy undu- that deﬁ ne the foliation. The recrystallized grains record a strong shape latory extinction and irregular grain boundaries. New grains form aggre- and lattice preferred orientation consistent with the overall SSW-directed gates of small, equally sized grains and developed at the expense of old

shear. Igneous perthitic K-feldspar relics remain as large clasts, whereas grains by low-temperature grain boundary migration. Oligoclase (An22

orthoclase partly recrystallized as interstitial grains within the main folia- Ab75 Or3) grains have been strongly altered through ﬂ uid interaction and tion. Neither clasts nor newly formed orthoclase show any compositional sericitized. They have lost any magmatic zonation and commonly show changes compared to the protorock (see Table 1). Fractured plagioclase extremely ﬁ ne grained (<10 μm) subgrain formation at mineral boundar-

grains rotated during noncoaxial ﬂ ow, but their magmatic zonation, from ies or along intracrystalline fractures. K-feldspars (Or93) show ﬂ ame-like

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(A) SB1-07 (B) SB4-07 Kfs Pl Qtz Qtz Qtz Bt Pl Bt Pl

Qtz Bt Qtz Kfs Qtz Bt Bt Qtz Bt Kfs Qtz Pl Pl Pl Pl Bt 2 mm 2 mm

Qtz Qtz Pl Pl Bt Pl Bt Pl Bt Qtz Qtz Pl Kfs Pl Pl Pl Pl Bt Qtz

2 mm 2 mm

(E) SB3-07 (F) SB8-07 Pl Qtz

Pl Kfs

Kfs Pl

Pl Pl Kfs

Qtz Kfs Qtz Kfs

Qtz 2 mm 2 mm

(G) SB2-07 (H) SB2-07 Qtz Qtz

Pl Pl Bt Kfs Kfs Bt Qtz

Qtz Kfs

Qtz

200 µm 200 µm

Figure 5 (on this and following page).

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(I) SB6-07 Qtz (J) SB3-07 Pl

Pl Kfs

Kfs Pl Kfs Pl

Qtz Qtz Kfs

Qtz

200 µm 200 µm

Figure 5. Backscattered–electron probe microanalyzer (EPMA) images (5 mm wide) of investigated granitoid rocks from Serifos showing the textures at increasing strain: (A) Protogranodiorite, (B) slightly foliated granodiorite, (C) foliated granodiorite, (D) mylonitic granodiorite, (E) ultramylonitic granodiorite, (F) bleached granodiorite. (G) and (H) illustrate details from the mylonitic state, (I) from the slightly foliated, and (J) from the ultramylonitic state of deformation. Abbreviations: Bt—biotite; Kfs—K-feldspar; Pl—plagioclase; Qtz—quartz.

TABLE 1. REPRESENTATIVE K-FELDSPAR ANALYSES (IN WT%) SB1-07 SB4-07 SB6-07 SB2-07 SB3-07 SB8-07 Clast Clast Recryst. Clast Recryst. Clast Recryst. Clast Recryst. Matrix Clast

SiO2 64.5 64.7 64.6 64.9 65.1 64.6 64.8 64.9 64.7 64.5 64.6

Al2O3 18.7 18.8 18.7 18.8 18.6 18.7 18.8 18.8 18.8 18.9 18.9 CaO 0.03 <0.02 <0.02 <0.02 <0.02 0.02 <0.02 0.03 <0.02 0.03 <0.02

Na2O 1.02 1.03 0.95 1.09 0.96 1.07 1.12 1.13 1.04 0.74 1.07

K2O 15.2 15.3 15.3 15.3 15.4 15.3 15.1 15.1 15.5 15.5 15.2 BaO 0.45 0.39 0.35 0.36 0.26 0.31 0.33 0.23 0.35 0.53 <0.03 Total 99.9 100.2 100.0 100.4 100.3 100.0 100.2 100.1 100.3 100.2 99.8 Numbers of ions on the basis of eight oxides Si 2.984 2.982 2.985 2.988 2.995 2.984 2.984 2.983 2.981 2.983 2.981 Al (IV) 1.018 1.020 1.019 1.019 1.006 1.017 1.021 1.019 1.021 1.030 1.028 Na 0.092 0.093 0.085 0.097 0.086 0.095 0.100 0.109 0.093 0.042 0.096 Ca 0.001 0.000 0.000 0.000 0.000 0.001 0.000 0.001 0.000 0.001 0.000 K 0.901 0.899 0.903 0.898 0.904 0.898 0.889 0.884 0.909 0.915 0.895 Ba 0.008 0.005 0.006 0.006 0.005 0.005 0.006 0.004 0.006 0.010 0.000

An 0.2 0.0 0.0 0.0 0.0 0.1 0.0 0.2 0.0 0.2 0.0 Ab 6.3 6.3 5.8 6.7 5.9 6.5 6.9 7.0 6.3 4.5 6.6 Or 93.5 93.7 94.2 93.3 94.1 93.3 93.1 92.8 93.7 95.3 93.4 Recryst.—recrystallized.

TABLE 2. REPRESENTATIVE PLAGIOCLASE ANALYSES (IN WT%) SB1-07 SB4-07 SB6-07 SB2-07 SB3-07 SB8-07 Core Rim Core Rim Core Rim Subgrain Core Strain Core Matrix Core shadow

SiO2 56.7 61.8 57.0 62.5 56.5 61.5 61.0 56.2 60.3 56.4 61.0 65.2

Al2O3 27.5 24.1 27.5 23.8 27.6 24.8 24.9 27.8 25.4 27.4 24.3 22.1 FeO* 0.18 0.15 0.13 0.19 0.15 0.07 0.19 0.17 0.23 0.19 0.14 0.09 CaO 9.54 5.68 9.46 5.03 10.3 5.94 6.30 10.8 6.76 9.24 5.97 2.81

Na2O 5.91 8.08 6.04 8.33 5.52 8.15 7.83 5.41 7.74 5.98 8.41 9.76

K2O 0.18 0.17 0.21 0.35 0.18 0.11 0.26 0.17 0.30 0.22 0.15 0.38 BaO 0.05 <0.03 <0.03 <0.03 <0.03 <0.03 0.04 0.07 0.04 0.05 <0.03 <0.03 Total 100.0 100.0 100.4 100.2 100.2 100.6 100.5 100.5 100.7 99.5 100.0 100.3 Numbers of ions on the basis of eight oxides Si 2.544 2.738 2.549 2.761 2.5342.714 2.701 2.517 2.669 2.551 2.716 2.861 Al (IV) 1.453 1.262 1.450 1.242 1.458 1.290 1.296 1.465 1.325 1.445 1.273 1.143 Fe3+ 0.007 0.006 0.005 0.007 0.0060.003 0.007 0.006 0.009 0.007 0.005 0.003 Na 0.514 0.695 0.523 0.714 0.4800.697 0.672 0.470 0.664 0.525 0.725 0.830 Ca 0.459 0.270 0.453 0.238 0.4920.281 0.299 0.517 0.321 0.448 0.285 0.132 K 0.010 0.010 0.012 0.020 0.0100.006 0.015 0.010 0.017 0.013 0.009 0.021 Ba 0.001 0.000 0.000 0.000 0.0000.000 0.001 0.001 0.001 0.001 0.000 0.000

An 61.1 40.8 60.2 36.7 64.3 41.8 43.8 65.9 45.7 59.9 41.1 21.7 Ab 37.8 58.0 38.5 60.8 34.6 57.4 54.4 33.1 52.3 38.7 57.9 75.3 Or 1.1 1.2 1.3 2.5 1.1 0.8 1.8 1.0 2.0 1.4 1.0 2.9 *Total iron shown as FeO.

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TABLE 3. REPRESENTATIVE BIOTITE ANALYSES (IN WT%) ﬁ ed as granodiorites, except for the ultramylonitic sample SB3-07 and the SB1-07 SB4-07 SB6-07 SB2-07 SB3-07 bleached granodiorite SB8-07, which, due to an increased potassium and Clast Clast Recrystallized Recrystallized Recrystallized decreased calcium concentration, have a granite composition. The same SiO 37.0 36.7 37.8 38.0 38.0 2 classiﬁ cation has been obtained from mass-balance calculations that were TiO2 4.35 4.32 3.14 3.21 3.40

Al2O3 13.9 14.0 14.1 14.3 14.4 used to estimate the mineral proportions of the samples (see Table 5). FeO* 16.7 16.6 17.3 17.2 17.1 Table 4 and Figure 6 indicate that the variations of major element MnO 0.37 0.46 0.30 0.23 0.18 MgO 13.5 13.9 13.1 12.8 12.6 contents along the sampled traverse through the shear zone, from the CaO <0.02 0.03 0.04 <0.02 0.06 protogranodiorite to the ultramylonite, are minor for SiO2, Al2O3, TiO2, Na2O 0.14 0.10 0.08 0.09 0.12 and Fe2O3, while the CaO, Na2O, and K2O amounts show signifi cant K2O 9.42 9.38 9.47 9.44 9.80 Total 95.4 95.5 95.3 95.3 95.7 differences. Figure 6A illustrates the similarities of the samples to conti- Mg# 0.45 0.45 0.43 0.43 0.42 nental arc granitoids. The hanging-wall sample refl ects a highly altered Numbers of ions on the basis of 11 oxides Si 2.794 2.77 2.855 2.861 2.859 composition with respect to the footwall samples with a much higher Al (IV) 1.206 1.230 1.145 1.139 1.141 content of SiO2, compensated by lower Al2O3 and CaO. This refl ects Al (VI) 0.028 0.016 0.110 0.136 0.139 the lack of Fe- and Ti-bearing and mafi c minerals as well as the modal Ti 0.247 0.245 0.178 0.182 0.192 Fe2+ 1.052 1.051 1.092 1.086 1.072 decrease of plagioclase. To investigate sequential changes in major Mn 0.024 0.029 0.019 0.015 0.011 element geochemistry during progressive deformation, the analyses Mg 1.524 1.559 1.475 1.440 1.411 were normalized by the precursor granodiorite composition (SB1-07; Ca 0.000 0.002 0.003 0.001 0.005 Na 0.020 0.015 0.012 0.013 0.017 Fig. 6C). For MgO, this shows an essentially constant relative depletion K 0.908 0.903 0.913 0.908 0.940 of −30% to −40% for all samples. A gradual depletion with progres- *Total iron shown as FeO. sive strain has been observed only for CaO. For the composition of the − − ultramylonite, the relative losses of Fe2O3 ( 38%), CaO ( 59%), and − Na2O ( 22%) are remarkable, as is the more than counterbalancing gain

perthitic morphologies. The mylonitized rocks within the localized shear of K2O. The loss of CaO and Na2O, as well as the gain of K2O, respec- zones record an extreme grain size reduction, mainly by fracturing of the tively, is reﬂ ected by the modal mineral proportions in the ultramylonite, feldspars. Quartz was deformed by dislocation glide and bulging of grain which indicate increased K-feldspar contents from 14% to 40%, with boundaries (Regime 1 of Hirth and Tullis, 1992). Shear-sense criteria decreased plagioclase contents from 40% to 21%, compared to the pro- σ δ - and -feldspar clasts, and shape- and lattice-preferred orientations of torock (see Table 5). The loss of Fe2O3 is consistent with decreasing quartz, indicate top-to-SW–directed kinematics. amounts of modal accessorial Fe-bearing minerals. Despite increasing deformation from the protogranodiorite to the ultramylonitic state, the mineral chemistry of K-feldspar, even in a recrys- Trace and Rare Earth Elements tallized nonperthitic state, has not systematically changed (see Table 1 and Fig. 6G). Only BaO shows a slight but progressive depletion in the Upper continental crust (UCC; McLennan, 2001) normalized REE porphyroclasts at increasing strains. Despite having the lowest BaO val- patterns and protogranodiorite normalized patterns are illustrated in Fig- ues in K-feldspar clasts, the ultramylonite shows the highest concentra- ures 6B and 5E, respectively. The analyzed REEs show patterns very tions of barium in K-feldspar grains within the Pl-Qtz-Kfs-Bt aggregates. similar to the composition of the UCC. Both light rare-earth elements The composition of magmatic plagioclase has also not been signiﬁ cantly (LREEs; La-Sm) and heavy rare-earth elements (HREEs; Eu-Lu) show

altered (see Table 2 and Fig. 6H). Subgrains of recrystallized plagioclase relatively fl at trends (Lan/Smn for SB3-07 is 1.26; Gdn/Ybn for SB3-07 is in the strain shadow of plagioclase porphyroclasts or in necks of boudi- 1.14). The HREE patterns, however, indicate a slight enrichment (~1.2 naged plagioclases consistently show andesine composition, very similar × UCC for Lu) for SB1-07 to SB6-07, whereas the mylonitic samples to the rim composition of magmatic plagioclases. Biotite recrystalliza- SB2-07 and SB3-07 show a slight depletion (~0.8 × UCC for Lu) with tion into fi ne fl akes is obvious from SB6-07 to SB3-07 reducing the Mg# respect to the UCC composition. The REE pattern of SB8-07 is gener-

from 0.45 to 0.42–0.43 wt% and TiO2 from 4.32–4.35 wt% to 3.14–3.40 ally depleted compared to the UCC but mainly in the LREE (~0.3 ×

wt% (see Fig. 5F). Except along the bleached haloes adjacent to sjf2 and UCC for La, 0.5 × UCC for Lu). The Eu anomaly, however, points to a

sjf3, which clearly formed under brittle conditions after ductile mylonitic modal depletion of plagioclase. Comparing the deformed rock sequence deformation, no evidence for chloritization of biotite or sericitization of to the REE composition of the protogranodiorite (Fig. 6E), the LREE feldspars has been observed. patterns are hard to differentiate, whereas the HREEs show two clearly separate patterns, with a slight enrichment in SB4-07 and SB6-07 (aver- WHOLE-ROCK GEOCHEMISTRY age enrichment of 4.5% from Eu to Lu) and a depletion in SB2-07 and SB3-07 (average depletion of 26% from Eu to Lu). La to Nd contents Major, trace, and rare-earth element (REE) compositions are summa- of the ultramylonite SB3-07 suffered a signiﬁ cant enrichment of these rized in Table 4, arranged by increasing state of deformation. The data elements in contrast to the depleted HREE content. are illustrated in Figures 6A and 6B. Bulk compositional comparisons of In the normalized trace-element plot (Fig. 6D), the relative gain of the essentially undeformed magmatic protogranodiorite with the progres- large ion lithophile elements (LILE) Rb (+77%) and Ba (+147%) in sively strained rock sequence are shown in Figures 6C–6E. the ultramylonite is associated with the marked increase of potassium, whereas the trends of other LILEs, Cs, Th, and U, are variable, and Sr is Major Elements quite stable. In general, the transition trace elements (TTE) Sc–Zn show

a tendency of depletion with increasing deformation, similar to Fe2O3, Major element compositions indicate that all the samples are subalka- MnO, and MgO. The differences, however, are not that signiﬁ cant. The line (calc-alkaline) granitoid rocks (cf. Iglseder et al., 2008). Despite the amounts of high ﬁ eld strength elements (HFSE) Y–Nb are relatively con- increasing deformation, the rocks hosting the shear zone have been classi- stant during progressing strain.

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3 10 Whole-rock composition Whole-rock composition SB1-07 SB4-07 SB6-07 SB2-07 IAG 2 SB3-07 1 SB8-07

CAG CCG

Al/(Na+K) Metaluminous Peraluminous 1 Samples/UCC .1 SB1-07 Peralkaline SB4-07 SB6-07 SB2-07 SB3-07 A SB8-07 B 0 .01 0.5 1.0 1.5 2.0 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Al/(Ca+Na+K)

1.5 2.5 Whole-rock composition Whole-rock composition SB4-07 SB4-07 SB6-07 2.0 SB6-07 1.0 SB2-07 STRAIN SB2-07 STRAIN SB3-07 SB3-07 1.5

0.5 1.0

0.5 0.0

0.0

-0.5 Gain/loss relative to SB1-07 -0.5 C D -1.0 -1.0 SiO2 TiO2 Al23 O Fe2 O3 MnO MgO CaO Na2 O KO2 PO25 Li Be Sc V Co Ni Zn Ga Rb Sr Y Zr Nb Cs Ba Th U

0.6 1.0 Whole-rock composition Biotite composition SB4-07 clast 0.4 SB6-07 recryst. SB2-07 recryst. STRAIN 0.5 SB3-07 recryst. 0.2

0.0 0.0

-0.2 SB4-07 -0.5 SB6-07 -0.4 SB2-07 STRAIN SB3-07 E F -0.6 -1.0

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu SiO2 TiO2 Al23 O FeO MnO MgO Na2 O KO2

1.0 1.0 K-feldspar clast composition Plagioclase clast composition SB4-07 SB4-07 SB6-07 SB6-07 SB2-07 STRAIN SB2-07 STRAIN 0.5 SB3-07 0.5 SB3-07

0.0 0.0

-0.5 -0.5 Gain/loss relative to SB1-07 Gain/loss relative to SB1-07 Gain/loss relative to SB1-07 Gain/loss relative to SB1-07 Gain/loss relative to SB1-07 G H -1.0 -1.0

SiO2 Al23 O Na2 O KO2 BaO SiO2 Al23 O FeO CaO Na2 O KO2

Figure 6. Diagrams illustrating bulk major and trace element compositions as well as mineral compositions of analyzed granitoid rocks from Serifos: (A) Shand’s diagram (Maniar and Piccoli, 1989) showing the geochemical composition and the tectonic discrimination of the study samples and (B) upper continental crust (UCC, McLennan, 2001) normalized bulk rare-earth element (REE) patterns. (C–E) show relative gains and losses of bulk major, trace, and REE elements of deformed granitoids compared to the composition of the unaltered protogranodiorite SB1-07. (F–H) indicate relative gains and losses of representative mineral compositions of the investigated rocks with increasing strain compared to representative mineral compositions of the undeformed protogranodiorite. CAG—continental arc granitoids; CCG—continental collision granitoids; IAG—island arc granitoids.

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TABLE 4. BULK MAJOR AND TRACE ELEMENT COMPOSITIONS MASS BALANCE AND VOLUME CHANGE Protogranodiorite → ultramylonite SB1-07 SB4-07 SB6-07 SB2-07 SB3-07 SB8-07 To compare the whole-rock data of the progressively deformed rocks to the initial nondeformed granodiorite composition, and to study the X-ray fl uorescence analyses (wt%) compositional changes that resulted due to the shearing, isocon dia- SiO 69.4 70.1 70.1 70.7 70.8 77.2 2 grams were plotted (Gresens, 1967; Grant, 1986, 2005). In addition TiO2 0.58 0.63 0.65 0.56 0.51 0.11 Al O 14.6 14.7 14.4 14.7 14.1 12.3 to the analyzed rock densities, changes of single elements as well as 2 3 whole-rock mass and volume changes were calculated. Elements that Fe2O3* 3.16 2.91 3.28 2.71 1.97 0.27 MnO 0.05 0.04 0.05 0.03 0.02 0.01 remained immobile during deformation were identifi ed mathematically MgO 1.11 0.67 0.70 0.78 0.70 0.06 and graphically following the procedure of Grant (2005). SiO2 and Al2O3 CaO 3.46 3.18 3.09 2.90 1.41 0.29 varied least during deformation, refl ecting their immobile behavior and Na O 3.35 3.50 3.32 3.29 2.63 2.82 2 were, therefore, chosen as the isocon in all plots. In addition to the K O 2.96 3.20 3.22 2.87 6.52 5.70 2 immobility of Al, which has been confi rmed elsewhere, Ti, P, and Zr, P2O5 0.11 0.13 0.13 0.11 0.10 0.02 Total 98.8 99.1 98.9 98.6 98.7 98.8 which have been shown to be immobile in shear zones, show good linear Loss on ignition 0.41 0.81 0.64 1.02 0.58 0.78 correlations with the isocons (Kerrich et al., 1980; Hippertt, 1998, and Density† 2.58 2.54 2.54 2.49 2.48 2.46 references therein; Sturm and Steyrer, 2003; Rolland et al., 2003, and X-ray fl uorescence analyses (ppm) references therein). Because many elements and oxides have concentra- Sc 14 16 16 13 11 4 tions in the same range, the gained data were rescaled to plot clearly V 44403533343 (Grant, 2005). Whole-rock mass changes, as well as whole-rock volume Ni963462changes were calculated taking the considerations of Coelho (2006) into Zn 29 29 35 29 24 13 account. Figure 7 illustrates the isocon diagrams, comparing the com- Ga 23 23 21 23 22 19 position of the protolith (C0) to the chemical composition of the four Rb 101 120 106 98 179 139 D Sr 277 281 255 277 341 76 progressing states of deformation (C ). Elements plotting above these Y 343739313926 defi ned reference lines were therefore gained during the deformation, Zr 332 302 311 332 331 77 whereas elements plotting below it indicate depletion. Nb 21 22 22 19 21 20 The tight scattering of elements around the defi ned isocon in Fig- Ba 707 705 744 767 1749 366 ures 7A and 7B reveals relatively immobile elements participated in the Inductively coupled plasma mass–spectrometry analyses (ppm) deformation process, as was also shown in Figures 6C–6E. Rare-earth ele- Li 16.1 7.38 9.73 11.2 9.29 14.1 ments, which are considered to be mobile in shear zones (Rolland et al., Be 4.80 4.22 2.82 3.16 1.93 1.84 2003), also show perfect correlations with the defi ned isocon. Lines of Co 5.13 7.09 6.10 3.01 2.04 0.84 constant mass and constant volume hardly show differences in slope to the Cs 1.29 2.53 1.93 1.00 1.01 0.75 isocons, pinpointing a more or less constant mass and volume deforma- La 27.3 28.7 33.5 27.7 38.2 7.71 Ce 59.9 66.8 79.3 65.0 84.3 17.6 tion at this state. This comparability of isocon-based element gains and/or Pr 7.19 7.44 7.92 6.59 7.87 1.78 losses and relative abundances of elements under assumed constant mass Nd 28.4 30.8 32.3 30.3 32.6 8.33 and volume substantiates the signifi cance of the element patterns in Fig- Eu 1.07 1.02 1.06 0.92 0.81 0.19 ure 6. Whole-rock mass losses of 1% and volume gains of 0.5% were Sm 5.70 5.87 5.80 4.98 4.56 1.64 assessed for SB4-07 and SB6-07. Gd 4.95 5.22 5.21 4.30 3.91 1.43 In the mylonitic sample (SB2-07), the scattering of elements is still Tb 0.83 0.88 0.85 0.67 0.60 0.26 Dy 5.18 5.65 5.42 4.05 3.78 1.85 minor, but a linear correlation and systematic slight depletion of REE has Ho 1.07 1.18 1.14 0.81 0.79 0.41 been observed, with the exception of La, Ce, and Nd, which indicate slight Er 2.82 3.08 2.96 2.02 2.10 1.14 enrichment (Fig. 7C). CaO and MgO indicate depletion, which has also Tm 0.46 0.50 0.47 0.30 0.33 0.19 been observed in all other states of deformation. A whole-rock mass loss Yb 2.74 2.91 2.76 1.73 1.99 1.25 of 1.8% and volume gain of 1.7%, respectively, have been calculated for Lu 0.38 0.40 0.37 0.22 0.27 0.17 this sample. In the ultramylonitic state (Fig. 7D), a fractionation of LREE Th 9.27 15.6 11.2 10.4 10.4 19.1 U 2.34 3.00 1.40 1.88 3.16 2.99 and HREE has clearly occurred, in which mainly La and Ce, lying on a linear correlation through the origin, have been considerably enriched. Pr *Total amount of iron is shown as Fe2O3. †Density in g/cm3. and Nd also plot above the isocon, whereas for the rare-earth elements Sm to Lu, depletion is evident. Under assumed constant mass and volume conditions, K, Rb, Ba, and U indicate major gains in the ultramylonite, whereas Fe, Mn, Ca, Na, and the transition metals are refl ected by systematic losses. A whole-rock mass loss of 1.9% as well as a volume change of TABLE 5. MINERAL PROPORTION ESTIMATES (IN %) +1.9% can be assessed. Protogranodiorite → ultramylonite The isocons are all characterized by a slope very close to one, indi- SB1-07 SB4-07 SB6-07 SB2-07 SB3-07 SB8-07 cating that the chosen immobile elements as well as all other elements Quartz 35.2 35.1 35.9 35.9 31.3 37.4 K-feldspar 15.1 16.3 17.1 16.6 39.9 37.7 plotting along this reference frame kept almost constant concentrations Plagioclase 40.0 39.5 37.3 38.2 21.2 23.8 despite the increasing fi nite strain. Therefore, the graphically obtained Biotite 5.6 5.3 5.6 5.4 4.7 gains and losses of elements are nearly equal to the calculated ones under Accessory 4.1 3.9 4.1 3.9 2.9 1.1 minerals assumed constant mass and volume conditions. Nevertheless, the samples Note: Mineral contents calculated with MONA (Metzner and Grimmeisen, reveal a slight decrease of mass with increasing strain at simultaneous 1990). slight increase of volume.

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100 100

PO25 Isocon PO25 Isocon

Constant mass Na2 O Constant mass SiO2 Fe O SiO2 Al O 23 Constant volume 23 Constant volume TiO Al23 O 80 80 2 TiO2 Na2 O CaO Ce Fe O CaO Ce Sr 23 Zn MnO Th La Y Nd Y 60 Nd Zr 60 Zr La MnO Sr Rb Nb Zn V Th ScNb Co KOSc Ga Rb MgO 2 MgO Pr Ga V D D 40 40 Co KO2 C (SB6-07) C (SB4-07) Dy Ba Ba Cs Pr Sm Gd Sm Er Dy Cs Gd U Ni Er Li Yb Be 20 Li 20 Yb Eu Eu Be Ho WRMC: -1.0 % Ho WRMC: -1.0 % Tb Tb U Ni Tm WRVC: +0.5 % A Tm WRVC: +0.5 % B Lu Lu 0 0 0 204060801000 20406080100 C0 (SB1-07) C0 (SB1-07)

100 100

Isocon Isocon Ba Constant mass Constant mass SiO2 SiO2 Al O KO2 Constant volume 23 Constant volume Ce Al23 O 80 PO 80 25 Na2 O Sr CaO Rb Ce TiO La PO25 2 Fe O Sr Zr 23 Nd Zr TiO Y 2 Na2 O 60 Nd 60 La Zn MgO MgO Th Y Th Nb Fe O Ga Ga 23 Ba Pr Zn V D Rb D 40 Nb V MnO 40 C (SB2-07) Sc C (SB3-07) KO 2 CaO Pr Sm Sc Sm Li MnO Gd U Dy Ni Li 20 U 20 Gd Dy Be Co Er Er Ni CsYb WRMC: -2.0 % CsEu Yb WRMC: -1.8 % Eu Co Ho Be TbHo WRVC: +1.7 % C Tb WRVC: +2.0% D Tm Tm 0 Lu 0 Lu 0 204060801000 20406080100 C0 (SB1-07) C0 (SB1-07) Figure 7. Isocon diagrams of four granitoid rocks with increasing state of deformation from Serifos showing gains and losses of single-element concentrations as well as whole-rock mass and volume changes compared to the undeformed protogranodiorite. Protogranodiorite versus (A) slightly foliated granodiorite SB4-07, (B) foliated granodiorite SB6-07, (C) mylonite SB2-07, and (D) ultramylonite SB3-07. Dot-dashed lines in (C and D) indicate linear correlations of heavy rare-earth elements (HREEs) and light rare-earth elements (LREEs). WRMC—whole-rock mass change; WRVC—whole-rock volume change.

DISCUSSION and Knipe, 1990). This transfer caused a spatially limited geochemical redistribution of mineral phases and associated elements within the ultra- Compositional Alteration and Mass and Volume Change in the mylonite that resulted in small-scale geochemical anomalies. Dissolu- Deformed Granodiorite tion, transfer, and precipitation mechanisms, mainly affecting feldspars, occurred in reaction domains of quartz, plagioclase, K-feldspar, and bio- In spite of the increasing ﬁ nite ductile strain toward higher structural tite that have only been observed in the ultramylonite. Element redistribu- levels, samples from the undeformed to the mylonitic granodiorite in the tion was, therefore, caused by breakdown reactions of K-feldspar clasts footwall of the LANF remain compositionally relatively homogeneous. and plagioclases. In the ultramylonitic sample, however, some major and trace elements Lateral transport and accumulation of K-feldspar constituents in the indicate bulk chemical alteration with respect to the protorock. The most ultramylonite resulted in a spatially heterogeneous recrystallization giving

signiﬁ cant changes are the increasing K2O, Rb, and Ba at simultaneous the present-day altered assemblage. Similar strain-triggered, mass-transfer

decreasing CaO and Na2O, as well as selective gains of LREEs and losses processes involving feldspars were described by Ishii et al. (2007) and of HREEs (Table 4; Figs. 6C–6E and 7D). Mass-transfer mechanisms Menegon et al. (2008). Further, a non–mass-conservative exchange of Na causing such heterogeneities are likely controlled by deformation and/or and Ca with K in granitoid rocks was mentioned by Hippertt (1998). The the fl ow of both externally derived infi ltrating and internal fl uids. How- increase in modal K-feldspar from 14% in the protorock to almost 40% in ever, due to the absence of vein development in the shear zone, nonperva- the ultramylonite, together with the loss of modal plagioclase from 40% sive fl uid fl ow can be a priori excluded (McCaig and Knipe, 1990). to 21%, as well as the loss of biotite and Fe-bearing accessorial minerals The highest deformation, in the ultramylonite, served as the driving (Table 5), was, therefore, a local ultramylonitic deformation-induced het- force for the observed lateral mass transfer within the shear zone (McCaig erogenization. K-feldspar compositions from the reaction matrix indicate

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that it was not a primary recrystallization product, as at lower strains, but clinic microstructures (i.e., shear-sense criteria) with increasing defor- grew from additional Ba and K, forming a second generation of recrystal- mation clearly link this foliation to SSW-directed shearing along the lized K-feldspars that potentially increased the bulk mode. The apparent LANF. However, isolated and localized shear zones also developed in the fractionated REE patterns were caused mainly by ﬂ uid-composition– strongly foliated granodiorite, with a heterogeneous distribution. These dependent alteration, dissolution, and precipitation of magmatic REE- shear zones nucleated on brittle precursor fractures that have the same

bearing minerals (Rolland et al., 2003), also induced by local fl uid fl ow orientation as the joint system sjf1 in the undeformed granodiorite. With during ultramylonitic deformation. increasing strain, these shear zones offset the foliated granodiorite with Despite change in the mineralogy and associated compositional a strong displacement gradient, resulting in a defl ection of the foliation changes, the samples only record marginal mass and volume changes (Fig. 3B) and change of normal and reverse drag from the center to the from protogranodiorite to ultramylonite (see Fig. 7D). The minor bulk tip of the shear zone (Grasemann et al., 2005). This is an important obser- geochemical changes are refl ected in microstructural and mineral chemi- vation because the brittle fractures cannot be related to cooling of the cal observations. Plagioclase clasts maintained their initial labradorite pluton or thermal contraction and differential solidifi cation of magmatic core composition until ultramylonitization, despite intense fracturing bodies (Bergbauer and Martel, 1999) since they offset the strongly foli- and dismantling. Dynamically recrystallized plagioclases have the same ated granodiorite and therefore formed during solid-state deformation. chemistry as the andesine rim composition of magmatically zoned miner- Brittle deformation followed by ductile fl ow has been observed not only als (see Table 2 and Figures 5G–5I and 6H). Apart from a slight systematic under low-grade metamorphic conditions but also under amphibolite-, decrease of BaO, K-feldspars remained compositionally homogeneous; granulite-, and eclogite-facies conditions (e.g., Austrheim et al., 1996; the recrystallized equivalents that appear fi rst in the slightly foliated Kisters et al., 2000; Mancktelow and Pennacchioni, 2005). The transition granodiorite do not indicate any signifi cant variations (see Table 1 and from brittle to viscous behavior in rocks can be achieved by an increase in Fig. 5G). Recrystallized K-feldspar that grew within the reaction matrix of temperature or a change in confi ning and fl uid pressure (Tullis and Yund, the ultramylonite appears to vary slightly geochemically in terms of com- 1977; Kisters et al., 2000; Kolb et al., 2004). However, the Serifos pluton position and accounts for the modal enrichment of K-feldspar compared was intruded into shallow crustal levels (Stouraiti and Mitropoulos, 1999) to the protogranodiorite. Biotite grains recrystallized with progressive and experienced rapid cooling (Altherr et al., 1982; Henjes-Kunst et al., deformation and show an expected Ti decrease (Table 3 and Fig. 6F). 1988; Hejl et al., 2002; Iglseder et al., 2008). Therefore, cooling followed In summary, the results suggest that, although increasing strains led to by reheating can be excluded. Furthermore, our geochemical investiga- progressive microstructural changes, these did not infl uence the bulk geo- tions exclude a major component of rock-fl uid interaction and/or mass and chemical compositions. In the ultramylonitic granodiorite, however, iso- volume change during deformation in the footwall of the LANF. Note that choric chemical mass transfer caused changes in the reaction mechanisms, fl uid-rock interactions played a major role in skarn mineralization (Sale- resulting in the observed heterogeneity in the mineral mode, although mink and Schuiling, 1987), and the hanging wall of the LANF on Serifos, these did not greatly affect either the bulk mass or volume. like the bleached granodiorite, has been strongly altered by hydrothermal fl uid fl ow. Additionally, protocataclastic marbles and calc-rich schists Localization of Ductile Strain in the Granodiorite above the LANFs at Platis Yialos and Kavos Kiklopas were strongly altered by ankeritic fl uids. However, ductile shearing of the granodiorite Since ductile shear zones contain important information about the and localization of shear zones on brittle precursors occurred with almost deformation history of an area, numerous studies have addressed the no bulk geochemical changes. This has been also described from other question of how strain became localized in essentially homogeneous and ductilely deformed plutons (Pennacchioni, 2005). The absence of fl uid isotropic materials. Localization has been successfully modeled, either by fl ow into the ductile shear zones in the footwall of the LANF and the applying damage rheologies (e.g., Sleep, 2002; Simakin and Ghassemi, strong evidence for a massive fl uid-driven alteration of brittle and catacla- 2005) or by using rheologies that combine the effects of shear heating, stic faulting cutting the hanging wall and the footwall after movement of elasticity, diffusion creep, dislocation creep, and low-temperature plastic- the LANF probably directly refl ects the fundamental difference between ity (e.g., Regenauer-Lieb and Yuen, 2003; Kaus and Podladchikov, 2006). brittle and ductile shear zones. The mean stress in brittle fault zones is Furthermore, rheologically controlled localization of shear zones has been lower compared to the surrounding rocks, but the pressure in viscous shear modeled to occur at preexisting discontinuities (e.g., Mancktelow, 2002; zones is higher than in the surrounding material (Mancktelow, 2006). Mandal et al., 2004). Thus, although some authors suggest that shear Neither an increase in temperature nor a change in confi ning and fl uid zones are unrelated to preexisting heterogeneities and have propagated pressure is supported by the geological data as mechanisms to explain the into the undeformed host rock from discrete localization sites (Poirier, brittle fractures that formed during ductile shearing of the Serifos pluton. 1980; White et al., 1980; Ingles et al., 1999), other studies present clear Alternative mechanisms, such as stress inhomogeneities and/or a strong fi eld evidence for nucleation of ductile shear zones on preexisting brittle control of the strain rates including a more general rheology of the ductile joints or fractures (Segall and Pollard, 1983; Takagi et al., 2000; Manck- crust combining elastic, viscous, and plastic mechanisms (Bürgmann and telow and Pennacchioni, 2005; Pennacchioni et al., 2006; Pennacchioni Pollard, 1994; Regenauer-Lieb and Yuen, 2003; Mancktelow, 2006), must and Mancktelow, 2007). be considered in order to explain the ductile localization of shear zone on Structural fi eld work at Aghios Sostis showed a diffuse deforma- brittle fractures. tion gradient, with the intensity of the bulk foliation increasing from an essentially undeformed granodiorite to a penetratively foliated mylonite Structural Evolution of the LANF in the Granodiorite directly below the LANF. Sibson (1977) suggested that a gradual decrease of strain and foliation into the footwall block is typical for normal faults, The structural evolution of the LANF in the granodiorite at and this has been described from many crustal-scale extensional faults Aghios Sostis is illustrated in Figures 8A and 8B. For clarity, later doming (e.g., Herren, 1987; Selverstone, 1988; Mancktelow, 1992). On Serifos, of the whole island (Grasemann and Petrakakis, 2007) has been removed this zone of diffuse increase in foliation intensity toward the LANF has by rotating the block diagram by 20° around a rotation axis parallel to the a structural thickness of ~20–30 m. Progressive development of mono- NNE-SSW–striking stretching lineation (see also Figs. 4A and 4C).

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ABsjh1, 2

sjf1, 2 LANF N szh

3km

sjf1 N sm szf

sfp

Figure 8. Synoptic block diagram of the structures associated with the syn-postplutonic deformation. (A) Intrusion of the granodiorite into the low-

angle normal fault (LANF). The undeformed granodiorite and the foliated (sm) granodiorite are cut by brittle joints (sjf1) that are reactivated within

the LANF into shear zones (szf). The hanging wall above the LANF is cut by a conjugate joint system (sjh1, 2). Localized shear zones develop in the granodiorite above the LANF (szh). (B) After a maximum dip-slip component of ~4 km, the undeformed but strongly bleached granodiorite is juxta-

posed against the mylonitic but unaltered granodiorite. A new joint system developed (sjf1, 2). All structures including the LANF are cut and offset by high-angle normal faults with abundant generation of pseudotachylites (sfp).

Between ~11.5 and 9.5 Ma, the granodioritic pluton and associated as veins or alteration of the fractures and/or shear zones to suggest perva- dikes intruded into a SSW-dipping LANF that accommodated upper sive fl uid activity during ductile deformation in the footwall of the LANF. crustal extension by top-to-SSW–directed shearing (Iglseder et al., 2008). This observation is in good agreement with the geochemical data and with At Tsilipaki, Platis Yialos, and Kavos Kiklopas, the LANF was active similar studies of shear zones in plutons, where ductile fl ow was shown at the ductile-brittle transition, juxtaposing calcitic marbles deformed at to deform rocks essentially isochemically (Pennacchioni et al., 2006; see lower greenschist-facies conditions in the footwall against cataclasites in discussion above). the hanging wall. This observation agrees with the suggested intrusion The hanging-wall granodiorite above the LANF records a completely depth of 12–8 km (Stouraiti and Mitropoulos, 1999). However, disloca- different metamorphic, structural, and geochemical evolution. This totally tion climb recrystallization in K-feldspar and isochemical nucleation and bleached granodiorite was strongly altered by pervasive fl uid fl ow and, growth of new grains of K-feldspar in the strongly deformed granodiorite as a result, essentially lacks any mafi c minerals. An orthogonal bleached

in the footwall of the LANF at Aghios Sostis suggest deformation tem- fracture set (sjh1, 2) is associated with en echelon quartz veins. Intense peratures above 450–500 °C at geologically relevant strain rates (Pass- metasomatic-hydrothermal activity along the roof and the margins of the chier and Trouw, 2005, and references cited therein). Since these clearly pluton, associated with leaching of the maﬁ c components (mainly Fe, higher temperatures during deformation along the LANF were restricted Mg, and Mn) and deposition of the same components as ore bodies in to the shear zones within the granodiorite, we suggest that locally high the immediate host rocks, was investigated in detail by Salemink (1985). paleogeothermal gradients were related to advection of heat by the intru- Fracturing of feldspar grains, dislocation glide, and low-temperature sive body and to the residual heat of the pluton. In accord with models of grain boundary migration (bulging) in quartz suggest temperatures dur- the spatial and temporal evolution of normal faults (Sibson, 1977), the ing deformation of <350 °C (for a review, see Stipp et al., 2002). There- intensity of deformation increased from the undeformed granodiorite in fore, a minimum of 100 °C difference in deformation temperature has the footwall toward the ultramylonites below the LANF. Ductile deforma- been recorded by the granodiorite above and below the LANF. Using a tion was closely associated with brittle fracturing (see discussion above). simple analytical solution of the heat diffusion equation (Jaeger, 1964) and

In the undeformed granodiorites, fracture segments sjf1 do not show any modeling a cylindrical intrusion (6 km radius) into an infi nite halfspace macroscopic offset (i.e., joints) or alteration and/or bleaching and are (10−6m2s−1 thermal diffusivity; for a detailed description of the parame- therefore diffi cult to recognize in the fi eld. However, the same orienta- ters, see Salemink, 1985), a magma with a temperature between 700 and tion of brittle fractures developed in the strongly foliated granodiorite, 800 °C intruding into host rocks at ~200–300 °C would need ~500,000 where ongoing deformation eventually reactivated these precursor joints years to cool below 300–400 °C. Assuming that the distance from Aghios generating heterogeneous ductile shear zones with displacement gradi- Sostis to the extrapolated margin of the pluton toward NNE is ~2–3 km ents and defl ections of the preexisting foliation (e.g., Pennacchioni and would result in maximum displacement rates in the order of 5 mm/a, if Mancktelow, 2007). Note, however, that there is no fi eld evidence such the total offset has been accommodated by ductile shearing. However, in

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order to explain the temperature difference of ~100 °C across the LANF, a (6) WNW-ESE–striking, conjugate, high-angle normal faults, which signifi cant displacement distance must have been accomplished by brittle crosscut all other structures, indicate ongoing brittle crustal extension after deformation. Given that in such tectonic settings the geothermal gradient cessation of the movement along the LANF. In contrast to the LANF, the can be >>50 °C/km (Vanderhaeghe et al., 2003), even reaching values of high-angle faults are typically characterized by the formation of pseudo- ~100 °C/km, the slip in the brittle regime along a LANF dipping between tachylites. 20°–30° must exceed ~1500 m, reducing the calculated ductile slip rates to less than 2.5 mm/a. APPENDIX A The whole island of Serifos, including the granodiorite and the LANF, is cut by WNW-ESE–striking, conjugate, high-angle normal faults, which Analytical Techniques are typically associated with the formation of pseudotachylites (sfp in A Cameca SX-100 electron probe microanalyzer (EPMA) equipped with four Figs. 4C and 8), suggesting ongoing NNE-SSW extension after cessation wavelength-dispersive spectrometers (Department of Lithospheric Research, of movement along the LANF. Interestingly, no pseudotachylites have University of Vienna) at working conditions of 15 kV acceleration voltage and 20 been found along the LANF, probably suggesting that the LANF slipped nA beam current was used to determine mineral compositions. All analyses were by aseismic creep. made against natural standards with ZAF corrections. For feldspar, mica, and amphibole, a defocused (3 µm) beam technique was applied. All measurements Based on geometric arguments constrained by the extent of the were carried out in multiples to check the reproducibility of the results. High- granodiorite pluton, the possibility that the LANF on Serifos rotated resolution, backscattered-electron (BSE) images with dimensions of 4 × 3 mm from high to low angles during slip can be ruled out. Conjugate high- were acquired from a matrix of 10 × 10 pictures automatically stitched using angle normal faults, suggesting a vertical maximum principal stress instrument integrated software. direction, both predate and postdate the formation of the LANF on Seri- For whole-rock, major element compositions, fused beads were made from a 1:5 mixture of fi red specimens and fl ux; for trace element compositions, pressed fos (Grasemann and Petrakakis, 2007), and therefore models suggesting powder pellets were prepared. The analyses were made on a Phillips PW 2400 rotation of the stress fi eld around the LANF are also inapplicable. Since sequential, wavelength-dispersive, X-ray fl uorescence (XRF) spectrometer our fi eld observations and geochemical data do not indicate high fl uid equipped with an Rh-excitation source (Department of Lithospheric Research, pressures either within or adjacent to the fault zone attaining tensile fl uid University of Vienna). Loss on ignition (LOI) was determined by heating to 850 °C. Analyses of further trace elements and REEs were performed on a Per- overpressures in the footwall of the LANFs, we suggest reduction of the kin Elmer ELAN 6100 DRC inductively coupled plasma mass-spectrometer apparent fault strength by weak fault zone material and/or a change in (ICP-MS) (Department of Lithospheric Research, University of Vienna), follow- the deformation mechanisms (Rutter et al., 2001) that facilitated move- ing the digestion method and the analytical procedure described in Tschegg et ment, which will be subjects for further studies. al. (2008). Densities were determined using the method of Hutchison (1974), with a Mettler P2010N. CONCLUSIONS ACKNOWLEDGMENTS Structural fi eldwork, together with petrological and geochemical investigations, constrain the following conditions for a LANF cutting the This work was supported by the Austrian Science Fund (FWF) grant num- roof of a ca. 11.5–9.5 Ma granodiorite pluton: bers P18823-N19 (Project ACCEL) and P18908-N19. Fruitful discussions (1) A brittle, knife-sharp, low-angle fault plane separates mylonitic with Kostas Petrakakis, Theo Ntafl os, Gerlinde Habler, and Hugh Rice are granodiorite, deformed under upper greenschist-facies conditions in the gratefully acknowledged. S. Hrabe, C. Beybel, and L. Slawek are thanked footwall, from a weakly foliated granodiorite deformed under lower for high-quality thin-section preparation. We want to express our sincere greenschist-facies conditions in the hanging wall. gratitude to Hugh Rice for revision of the manuscript, to Jafar Hadizadeh (2) Although strongly deformed under upper greenschist-facies con- and an anonymous reviewer for their constructive reviews, as well as to ditions, the bulk geochemistry of the progressively deformed footwall James Evans for editorial handling. granodiorite recorded almost no whole-rock compositional, mass, or volume changes up to the mylonitic deformation state. 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