PAPER TO THE FIELD TRIP TO NAXOS

PRESENTED BY:

- DJOUGNE FOSSI MICHELE SANDRINE

- KUEGNONG NZEFOKOP CAMILLE ROMEO

TOPIC: Pressure – Temperature – Time Evolution: Naxos

Abstract:

Naxos is the largest island of the cycladic archipelago in the Aegean Sea. Two main successive

Alpine metamorphic events have affected the metamorphic zoning of the middle and lower units within the Naxos Metamorphic ore complex (e.g. Andriessen et al.,1979, Urai et al.,1990). The

earlier high-pressure (HP) – low-temperature (LT) Eocene metamorphism (M1) phase is recorded

by relics of blueschist facies minerals preserved in metamorphic rocks of southern sectors of the

island . In the higher grade metamorphic rocks of Naxos, no relics of an HP–LT metamorphism

have been identified. This blueschist metamorphic phase (M1) was later overprinted by a

widespread Barrovian type (MP–MT) metamorphism (M2) during the Miocene grading from

greenschist facies to amphibolite facies conditions, and reaching partial melting conditions at the

deepest structural levels, as attested by the abundant migmatization in the dome core. This

metamorphic evolution has defined a series of metamorphic zones from the shallowest structural level in the south-eastern part of the island (zone I) toward the deepest migmatitic core (zone

VI). New geothermobarometric data, Rb/Sr dating, the petrological study of mineral assemblages

as well as the fabric and the microstructure analysis of rocks sampled on each metamorphic zone

(structural level) have allowed the reconstruction of Pressure-Temperature-time paths in

Naxos.40Ar/39Ar white mica ages of ca. 45 ± 5 Ma document an Eocene age for blueschist

metamorphism M1. The Minimum pressure associated to M1 calculated from Si content in

Phengite is 12kbar. Toward the center of the dome, metamorphic assemblages record an

increase in peak-temperature and corresponding pressure (from 500 to 700°C, and from 5 to 8

kbar). Based on 40Ar/39Ar mineral ages, the timing of peak Barrovian-type M2 metamorphism

was first estimated to have occurred between 21 and 16 Ma and is consistent with new Rb/Sr

ages for micas and garnets. The P-T-t interpretation suggests that the HP/LT to MP/MT transition

is due to: I) Heating of deeper parts of the dome through magma injection or II) either homogeneous

(75 %) or localized thinning during dome formation.

I-Introduction

The late Mesozoic- Cenozoic evolution of the Mediterranean region was characterised by two major tectono-metamorphic events that affected a pre-alpine basement (Henjes Kunst and Kreuzer 1982…), the outlines of these geological events are well illustrated in Naxos through magmatic, metamorphic and structural features. Actually Naxos is an island situated in the centre of the Aegean Sea. It is a cordillera-type metamorphic core complex built up after the collapse of the alpine orogenic belt of the Hellenides (Dov Avigad; 1998).Jansen and Schuiling (1977) as well as Feenstra (1985) describes the island as a N-S elongated, roughly coinciding thermal and structural dome. The study of this metamorphic evolution of rocks coupled with former geochronological data have allowed Duchêne S, Rhaba A. and Vanderhaeghe O.(2006) to Draw a P-T-t path of this area. This paper will redraw the evolution of the metamorphic parameters (P-T) in Naxos and their evolution with time.

Fig1 Geological and structural map of the Cyclades showing the location of the island of Naxos after Avigad & Garfunkel, 1991) 1) Cycladic blueschist unit overprinted in greenschist and amphibolite facies, 2) eclogite-facies rocks, 3) LP metamorphic Upper Unit, 4) middle-Miocene granitoids, 5) early- Miocene clastic. Sediments of the Upper Unit, 6) ophiolites of the Upper Unit, 7) basal unit possibly belonging to the external Hellenides, 8) low-angle nor-mal fault.

II-Geological, structural and metamorphic outlines (settings) in Naxos (accompanied a map)

The pattern of the island have been shaped by two major tectono-metamorphism events: an early alpine compressional phase involving subduction of continental margin material, generation of nappe pile and associated high pressure low temperature (HP-LT) metamorphism (M1) during the Eocene. This is testified by the presence of glaucophane in the outermost part of the island (Dov Avigad 1998). The second event correspond to a lithospheric scale extension belt (Hellenides)(M2) accompanied with back arc basin formation in the Aegean, crustal thinning, increase of the geothermal gradient and subsequent partial melting and thermal dome formation, to name but a few(Urai et al 1990). This second deformation is characterized by medium pressure to medium or high temperature (MP-MT) metamorphism during the early Miocene that overprinted M1 and with an increasingly intensity from the south eastern part of the island toward the migmatitic core. The metamorphic evolutions of the island have resulted in a dome shape structure paired with a concentric evolution of metamorphic features (Jansen and Schuiling). They defined a series of metamorphic zones (Fig2) from the south-eastern part of the island (zone I) toward the migmatitic core (zone VI) on the basis of mineral isograds, this zoning is interpreted as a record of a metamorphic gradient increasing from 400°C, 12 kbar in the shallowest structural level toward 700°C, 7 kbar in the deepest. This metamorphic complex is intruded by magmatic bodies including a 13Ma granodiorite pluton cropping out on the western coast (Duchêne et al 2006) and is later cut by low and high angle normal faults (Lister et al., 1984). Isolated sedimentary basins collect the erosion products of metamorphic and granodiorite rocks

Fig2 Geological, Structural and Metamorphic map of Naxos; showing a

concentric evolution of metamorphic features Jansen and Schuiling [4]. From the SE part of the island (zone I) toward the migmatitic core (zone VI).

III- Pressure-Temperature estimation In recent papers many authors brought contributions in the P-T-t evolution on Naxos (Stephanie Duchene et al. 2006). They use petrological, new thermobarometric and Rb-Sr geochronological data on metasedimentary rocks sampled at different structural levels along a E-W cross section to reconstruct the P-T-t paths for each of these structural levels. The rocks exposed in the SE of Naxos have particularly well preserved the M1 high pressure metamorphism. This is detailed by the work of Avigad (1998), which also reconstruct the P-T -t in this part of the island.

III-A- Petrological analysis and Pressure -temperature determination A.1- Zone I & II The petrographic study of the jadeite-blueschist and metabasic blueschist collected in zone I &II present: a texturally oldest mineral assemblage constituted of garnet, aegirine-jadeite, ferroglaucophane rimmed by magnesoriebeckite, rutile and quartz. Aegirine-jadeite occurs both in the matrix and as inclusions in garnet and has jadeite content up to 60% indicating relatively high pressure. The late mineral assemblage includes albite-hematite symplectites, that formed on the expense of quartz and aegirine-jadeite during the decompression phase (Jadeite (0,6)+quartz =albite). The pressure calculated at 500°c based on this reaction and using Thermocalc augmented by “ax”(activity calculation) program(Powell and Holland,1988) yield a minimum pressure of 12kbar. Higher minimum pressure are even obtained (12-14kbar) from 3,5 Si in phengite. A maximum temperature of up to 500°c are constrained by the presence of M1 diaspore in metabauxites(Feenstra,1985 ,1996) which indicates temperature lower than the univariant reaction line diaspore=corundum+water calculated using thermocalc. The mineral paragenesis augmented by garnet-clinopyrexene thermometry gives a better assessment of temperature that range between 450 and 480°c.

Fig3 P-T paths of high pres-

sure rocks exposed on SE

Naxos M1 correspond to

pre-50 Ma (Wijbrans and

Mcdougall.1988) HP meta-

morphism and. P-T condi-

tions during the M2 over-

print on SE Naxos are more

difficult to assess because

M2 on SE Naxos took place

at relatively low tempera-

tures .Univariant reaction

lines Jadeite + Quartz = Al-

bite, and Diaspore - Corun-

dum + H2O were calculated using Thermocalc (Powell &

Holland, 1988).

A.2 - Zone III This sample contains a HP/LT assemblage displayed by phengitic white mica, plagioclase, garnet and. This paragenesis is partly replaced by a muscovite/biotite quartz association (Fig4). According to (Massone and Chreyer 1987) the Si+4 content of phengitic white mica can be used for pressure estimation. In fact a Si+4 content of 3,4 allow them to estimate a minimum pressure of 10kbar at 500°c. The temperature estimation is obtained from phengite/garnet thermometry. The temperture fall in the range of 475-485°c for gar-net/phengite in inclusion and growing in equilibrium with pagioclase. Garnet in the matrix is replaced by biotite at its rims and rim-rim calculations have given temperature falling within 470-570°c.

Fig4 HP Mineral Assemblage A fine-grained association of muscovite and biotite corresponds to the destabilization products of garnet crystals.

A.3- Zone IV The mineralogical assemblage consists of quartz, muscovite, biotite, garnet, kyanite, Fe-staurolite and tourmaline. Staurolite (appearing at rims or between garnet and kyanite) appears as the result of the reaction (fig5): Garnet+kyanite=staurolite +quartz in KFMASH (K2O-SEO-NGO-AL2O3-SIO2-H2O) system. The presence of kyanite indicates a minimum pressure of 4-6kbar at 500-600°c. The garnet /biotite thermometric calculations, at 5kbar give temperature ranging from 580-630°c, mineral equilibrium; Staurolite-kyanite-almadine-Quartz-water and biotite-garnet-kyanite suggest temperature of 600°c at 5kbar. Therefore the peak temperature for this zone ranges between 580-630°c for a minimum pressure of 5kbar.

Fig5 Photomicrographs showing the presence of late staurolite and biotite separating garnet and kyanite crystals.

A.4 - Zone V This sample consists of quartz, biotite, plagioclase, kyanite, rare garnet and rare muscovite (fig6). Plagioclase is albite-rich and there is no obvious growth zoning in the garnet rather a retrograde Fe-Mg zoning is observed at the contact with biotite. In these circumstances the core composition of garnet has been used to calculate garnet/biotite equilibration temperature. These range between around 700°c with a maximum up to 750°c. This seems to be unlikely since no partial melting is observed in this area. The presence of kyanite suggests a minimum pressure of 8,5kbar. However lower Temperatures (600-620°c) and lower pressures (4,5-5.7kbar) are suggested from inclusions of biotite and plagioclase in garnet.

Fig6 Plagioclase, Quartz, and Biotite inclusions in Garnet

A.6 - Zone VI This zone correspond a migmatite, composed of ca. 50 % leucosome, 50 % melanosome. The leucosome consists of veins aligned with the foliation of the melanosome. The migmatite as a whole consists of quartz, biotite, garnet, plagioclase, sillimanite, K-Feldspar and muscovite. Garnets occur either as clusters in the leucosome fraction, or as inclusion-rich crystals in the melanosome without growth zoning. The biotite/garnet equilibrium and the stability of sillimanite allow the determination of a maximum pressure of 6 kbar for temperatures of 600-650°C. The sillimanite/plagioclase/garnet/biotite equilibrium corresponds to a pressure of ca. 4 kbar and a temperature of 628°C (lower than in zone V). This sample therefore does not belong to the same metamorphic gradient. The pressure- temperature conditions in the migmatic core record later stage during cooling and exhumation.

III-B- Timing

1-Methods: geochronological measurements have been made on minerals from different structural levels, based on Rb/Sr and K/Ar on micas (Andriessen et al 1979) 40Ar/39Ar on hornblende (Wijbrans and Mcdougall 1988) and U/Pb on zircon. The results Show that zoning in the age pattern fits with metamorphic zoning defined previously by Jansen and Schuiling. An Eocene age of 45+- 5Ma is suggested for the M1 metamorphism, which is well preserved in the SE (zone I to zone 3), where the M2 overprint is less or not recorded. If the age zonation is not systematique, or younger ages (20-23Ma) are recorded in the outer part of the dome (from zone I toward zone III), this may be attributed to either a rejuvenation of M1 mica ages or the mixing of mica of different generations (M1 and M2). M2 peak is dated at 15-20Ma.

The inner zones IV and V have ages ranging from 10 to 16.4Ma. Finally the 40Ar/39Ar sug-gest the lowest ages (6-12Ma) inside the migmatitic core (zone VI). All these ages may be summarized in three groups: – A first group of ages, at 40-50 Ma, is thought to represent HP/LT metamorphism. Most of them are mica Rb/Sr, K/Ar, and 40Ar/39Ar ages which may be considered as cooling ages and thereby constitute minimum ages for the metamorphic event. -Ages at ca. 20-23 Ma represent the youngest ages obtained in the outer part of the dome (zones I to III), and have been interpreted as early M2 muscovite re-crystallization (Andries-sen ,1991) or as mixing ages between micas of different generations (Wijbrans, J.R. and McDougall,1986). – The third group of ages (ca. 16-21 Ma) represents the main re-crystallization event related to M2 greenschist to amphibolite facies metamorphism, as well as partial melting (ca. 17-21 Ma) in the migmatitic dome.

III-C-Pressure-Temperature-time path construction Based on geochronological results (Tab1 and fig7), ages have been placed along the P-T path and a pressure-temperature-time path have been drawn

-An Eocene age of 45+- 5Ma for the HP-LT M1 metamorphism -A 29,3Ma have been interpreted for the zone III as the age of the phengitic white mica

recrystallisation. This stage may represent a retrogressive evolution from the peak HP/LT stage (M1) testified in this zone by the presence of glaucophane (Buick, 1988) -The M2 recrystallisation is dated from 19,8-22,7 Ma using K/Ar and 39Ar/40Ar on

muscovite. However Rb/Sr and K/Ar ages obtained on biotite range from 12.5 to 14 Ma (Andriessen,& al 1991) .This younger ages is interpreted as cooling age.

-In zone IV, the oldest ages (15.2-17.9 Ma) are given by K/Ar and 39Ar/40Ar dating of horn-blende (Wijbrans, J.R. and McDougall, 1986). They are interpreted by these authors as cool-ing ages postdating M2 metamorphism.

-In zone VI, the age of migmatization is constrained at 16.8- 20.7 by U/Pb ages on zircons from the migmatite obtained by (Keay et al 2001). Some K/Ar ages on horblende indicate cooling below ~500°C between 15 and 18.8 Ma (Andriessen, & al1979). K/Ar on muscovite (11.3-12.1 Ma Wijbrans, J.R. and McDougall, 1986) and Rb/Sr and K-Ar on biotite (10.0-12.7 Ma Wijbrans, J.R. and McDougall, 1986) indicate further cooling below 300-350°C.

1979

1988 1988

1979 1988

1979

1988 1979

1986 1979

1988 1988 1988

1979 1979

1991

1991

1991 1991 1988 1991 1988 1979 1991 1988 1979

1991 1988

1998

2001

1988 1991

1988

1988

1991

1979 1988

1991 1991

1991

1979

Fig7 Pressure-

Temperature path of

Naxos based on geo-chronogical data (also

indicated in the table).

Average geothermal gradients are reported

as dashed lines. PT paths reported in dot-

ted lines correspond to

the exhumation; Note the isothermal exhuma-

tion path from HP/LT

toward MP/MT condi-

tions and the exhuma-tion accompanied with

heating. (Berman, R.G, 1988)

IV-Relation between microstructures and P-T

Beside the petrological analysis we think that another way of assessing Pressure-Temperature

evolution is the study of structural evolution and the relationship between deformation and

metamorphic evolution. We concede that alone it is nether the optimal way of reconstructing P-T

path nor it gives absolute ages for geological processes. However combined with methods

discussed previously, it may be for great help. In fact microfabric and textural framework gives

indices about the metamorphic regimes that prevailed during their formation as well as the time

relationship. From the work of (Urai et al1990), that emphasise on the deformation and the

description of the structures) this has been resume in the table below (Tab2)

Tab2 Schematic diagram

illustrating the relationship

between deformation and

metamorphism. data on timing are mainly based on jansen &

shuiling1976 and Wijbrans, J.R.

and McDougall 1988 .the two

main deformation phases and

their relation to the fold

generations B1;B2 and B3 are

generate by compressional

regime for the fold B1 and B2

corresponding in time to M1

50Ma with a T°of 480°c and a

and extensional regime for the

fold B3 where B1 and B2 has

been refolded with a

temperature of 700°c

corresponding in time to 25-

13Ma At the microstructural level (Shuyum Cao et al 2013) resume his work by the three following groups. Group 1: These rocks represent variable overprint on Eocene M1 and Early Miocene M2 metamorphic fabrics according to metamorphic isogrades proposed by Jansen and Schuiling (1976). Group 2: By comparison, a second group set is represented rocks with a high-temperature migmatitic fabric within a thick aplitic dyke exposed at the northwestern margin of the migmatite dome. The vast majority of the deformation in the migmatitic core is characterized by melt-present deformation conditions and microstructures show recrystallization at high-temperature. The presence of melanosome layers or patches (e.g., biotite selvages) provides the best evidence of local melt formation, and the presence of leucosome (rich in non-ferromagnesian minerals generally quartz and feldspar), where the melt collected. Near internal migmatitic structure from the central portion of the Naxos Microstructures show strong plastic deformation of the main mineral phases in samples of the sheared aplite dyke. The most common microstructure here (Group 2) is the quartz ribbon. Elongated quartz grains compose quartz banding in a matrix of sheared feldspar grains. Such quartz ribbons form strong linear fabrics on the hand specimen scale. Feldspar grains have irregular forms and sub-granular shapes. They were elongated and have intragranular deformation. These evidences suggest that the rocks were deformed at high-or medium-temperature solid-state plastic deformation.

Group 3: These rocks underwent high-pressure metamorphism these are mainly exposed in southern Naxos and exhibit high-pressure microstructures (see fig: (Avigad, 1998) of Eocene age. In this high-pressure belt (Group 3), most quartz grains have typically polygonal aggregate shapes and are parallel or subparallel to the major foliation and lineation of the rocks (Fig. 7h). Some quartz ribbons or aggregates with serrated grain boundaries occur in pressure shadows on both sides of the feldspar grains. White mica is overgrown by

dynamically recrystallized quartz grains. They also occur in ribbons of elongate rectangular grains or sigma-type aggregates aligned parallel to the macroscopic foliation in the mylonites

Fig8 Quartz ribbons or aggregates with serrated or sutured grain boundaries.

VII-Discussion and conclusion In orogenic belt, the thermal-tectonic evolution from HP-LT subduction setting to MP-MT during the exhumation is controlled by the thermal diffusivity, the heat by radioactive decay in the crust, the advection of heat related to exhumation or to magmatic influx [2]. But since the composition of the crust in naxos was constant over the whole metamorphic complex, the difference between isothermal decompression in the outer part of the dome and decompression accompanied by heating in its central part may be explained by different advective fluxe resulting either from different exhumation rates (advection of the crust), or from different magmatic supplies (advection of material through the crust). But the exhumation rate is nearly identical over the metamorphic core complex (1 mm yr-1). If the rocks from the core of Naxos (zones IV to VI) underwent heating during the exhumation whereas rocks from the outer part (zones I to III) did follow isothermal path, then it must have been because of local heat influx through magma injection. Furthermore rocks of SE Naxos even cooled during decompression( from 450-480°C during M1 to 420°CduringM2)It is thus more probable that the cooling observed on SE Naxos reflects rapid removal of the overburden from above the rocks, possibly as a consequence of extensional unroofing (Rupple et al, 1988). It is thus suggested that the preservation of contrasting P-T paths on Naxos reflects two competing geodynamic phenomena: heat added from below the section (from as yet unspecified source) against cooling caused by unroofing and denudation of the section from above. Furthermore there is confusion on timing in literature concerning the deformation episodes responsible of structures and microstructures that display the metamorphic regimes of either HP-LT or MP-MT

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