Mykonos granite

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The Mykonos granite (also Mykonos laccolith) is a composite, syntectonic granitoid that intruded into the paragneiss and mica schist of the migmatitic basement of the northern Cyclades in the Middle Miocene ( Serravallian ) 13.5 million years ago BP in the form of a laccolite . The cooling of the pluton from 680 to 60 ° C took place over a period of 13 to 9 million years (Serravallium to Tortonium ).

Geologically, it has a share in the dome-like bulging of a metamorphic core complex , the metamorphic core complex of Mykonos . The core complex was relieved by means of a huge shear , the North Cycladic Shear System , in the period 14 to 10 million years BP (Serravallium to Tortonium ) and emerged around 8 kilometers as a result.

Geological framework

The Mykonos granite near Kapari on Mykonos. Delos can be seen in the background .

The Cyclades archipelago is located in the center of the Aegean Sea . After a period of orogenic crustal thickening by nappe during the Eocene beginning in the Oligocene crustal strain in the former back-arc basin one because the subducting northward undercurrent ( Engl. Hellenic slab ) shrank faster now to the south. This stretching phase was accompanied by the formation of several metamorphic core complexes along with their shear horizons. The decompression resulted in the formation of granitic magmas ( anatexis ), which were sucked in in the form of plutons in pressure shadows. Despite the intrusions and the intensive reprocessing through shearing, the original ceiling stack can still be reconstructed and is structured as follows (from hanging wall to lying ):

The upper Cycladic ceiling in the hanging wall consists mainly of ophiolites and does not cause any tertiary paragneiss. The Cycladic blue schists in the middle section of the ceiling pile are composed of metapelites, marbles and metabasites that have undergone complex metamorphic development. Its parts now present as paragneiss indicate an Eocene high-pressure-low-temperature metamorphosis (HP-LT), which was overprinted by a low-pressure-high-temperature metamorphosis (HT-LP) in the course of the Oligocene and Miocene. In the amphibolite facial Cycladic basement , metamorphosed mainly in the Oligocene and Miocene (with only sparse remnants of the Eocene high pressure phase), there was partial melting, responsible for the formation of the Mykonos granite, among other things. Exactly when the transition from compression to elongation tectonics took place is still a matter of dispute, at least the period 35 to 30 million years BP ( Priabonium to Rupelium ) comes into question .

Occurrence

The Mykonos granite underlies large parts of Mykonos and Delos , a smaller deposit is also on Rinia . Geodynamically quite comparable Miocene granitoids intruded at the Attic Laurion , on Serifos , Tinos , Naxos , Keros , Ikaria , Thera , Kos and Samos as well as at Bodrum in Turkey . These intrusions form a geochemical continuum, with granodiorites (basic) in the west, granites in the center, and monzogranites (acidic) in the east.

mineralogy

Mineralogically, the main minerals of Mykonos granite are quartz (18 to 42 percent by volume), plagioclase (35 to 66 percent by volume), orthoclase megacrystals (34 to 64 percent by volume), low-aluminum biotite and subordinate green hornblende . Act as accessory minerals magnetite , sphene , red-brown, pleochroic Allanit , apatite , zircon and Uranothorianit . In fine-grained, mafic inclusions, in addition to biotite and hornblende, clinopyroxene also occurs. The granodiorites contain orthopyroxene .

The plagioclase is an oligoclase or andesine with the composition An 43-19 Ab 55-79 Or 2-1 , the orthoclase is albite- rich and has the composition Or 81-93 Ab 17-6 An 2-1 . The biotite has 3.3 to 4.3 percent by weight TiO 2 with X Mg = 0.46-0.39. The hornblende has an Na / K ratio of 2.6 to 1.3 with X Mg = 0.51-0.39.

Chemical composition

The following table of the main elements is intended to illustrate the breadth of variation in the chemical compositions of the Mykonos granite:

Oxide
wt.%		 Quartz monzonite Delos granodiorite granite Aplit
SiO 2 64.70 68.00 73.40 75.80
TiO 2 0.79 0.57 0.27 0.10
Al 2 O 3 15.80 15.20 13.60 12.80
Fe 2 O 3 0.49 0.61 0.32 0.63
FeO 3.43 2.20 1.01 0.29
MnO 0.08 0.05 0.02 0.02
MgO 1.39 1.24 0.34 0.05
CaO 3.65 3.62 1.85 0.92
Na 2 O 3.26 3.09 2.63 2.74
K 2 O 4.87 4.11 5.35 5.64
P 2 O 5 0.25 0.14 0.07 0.02
H 2 O 0.47 0.65 0.55 0.36
Mg # 0.42 0.47 0.48 0.10
A '/ F -0.14 - 0.08 0.24 1.69

Petrology

Petrologically , the Mykonos granite is a potassium-rich , peralkaline (hypaluminous) to normally aluminous granite of the I-type , in which four different facies can be distinguished:

Some rock samples can already be classified as quartz monzonite in the TAS diagram . In addition, very SiO 2 -rich, hyperaluminous aplit dikes occur. The zoning in the Mykonos granite is inverse, ie the mafic facies such as the pyroxene granodiorite are in the center of the pluton , whereas the bright syenogranite can only be observed at the edge. This suggests a multiphase seating that took place in several magma bursts.

Magma origin

Low initial values of ϵ Nd (I) from - 7.43 to -9.18, high initial values ​​of 87 Sr / 86 Sr (I) from 0.709829 to 0.711015 and δ 18 O values between 10.2 and 11 .5 ‰ suggest a crustal origin of the magmas . Neither the isotope nor the geochemical values ​​require a juvenile mantle component as an explanation. Nevertheless, the melting with dehydration of intensely weathered metasediments (e.g. metapelite or aluminum-rich greywacke) contradicts the found high molecular quotients of CaO / (MgO + FeO dead ), to the rather high Na 2 O values, to the low molecular quotients K 2 O / Na 2 O and to the low Rb / Sr ratios (1.046 to 2.701). The magma source is therefore likely to be found in metamorphic igneous rocks and / or in metamorphic, aluminum-poor Grauwacken. For Stouraiti and colleagues (2010) the magma was created by partial melting of a biotite-bearing gneiss, the metasedimentary protolith of which resembles the metagrauwacken Rinias.

Spatial organization

The north coast of Delos. in the foreground the migmatitic paragneiss of the basement, in the background the overlying Mykonos granite

The Mykonos granite is an asymmetrical intrusion resembling a Lakkolite with a rather flat and concordant underside. It generally has a well-developed foliation trending north , which only dips slightly to the east. The lineations in the foliation level (stretching linear ) also dip only moderately to the east, but in the eastern part of Mykonos they show a successive turn in the northeast direction. The interior of the Lakkolite is built up by numerous layered intrusions. Its root zone is outside the main body on Rinia and Delos. The elliptical, apron-like main body with N 070 stroking longitudinal axis, which adjoins it in the east-northeast direction, takes up the majority of Mykonos. The seating took place on the border between the Cycladic basement and the Cycladic blue slates and then penetrated into the blue slate, recognizable by mica slate layers near the basal contact as well as mica slate xenolites in the laccolith itself. The pluton roof reached the upper Cycladic ceiling , with growth of the pluton was probably caused by successive inflation. The pulse-like growth can be seen in the main body by means of a mineralogical zoning with sharp borders and in the layer complex of Delos on the southwestern edge of the Pluton. Inclusions of acidic granites in more basic ones suggest that the basic magmas appeared later. Since basic magmas also form the bottom of the laccolith, they must also have flowed in there later. The total thickness of the pluton cannot be determined directly, but based on true-to-scale profiles, two to three kilometers can be assumed. Its total volume is estimated at 150 cubic kilometers. According to de Saint Blanquat and colleagues (2010), the seating did not take more than 10,000 years.

structure

Leukogranite near the new port of Mykonos

The Mykonos Lakcolith has a primary, very distinct igneous structure that is protomylonitic and mylonitic overprinted. As the shear system in the roof of the pluton approaches, the mylonitic deformation increases noticeably. The North Cycladic Shear System has split into the lower ductile Livada Shear and the brittle Mykonos Shear just above it . The sense of movement of the hanging wall on the two cut-offs is clearly to the northeast.

Stretch linear in granite indicate a counterclockwise rotation, for example they point to the east on the Apollonia Peninsula (N 090) and then turn via N 060 to N 030 near the shear system along the northeast coast. This contrasts with other Cycladic islands, whose stretch lines are mainly oriented N 020 to N 030 and can be explained with a block rotation of Mykonos compared to the regional tension in the north-northeast direction.

The Livada Shear has reprocessed the contact area between the Mykonos granite and the greenschist facial metabasites of the Upper Cycladic Nappe.

About 5 to 10 meters thick cataclasites follow above the Mykonos shear and above, inserted on a shallow fault , syntectonic sediments ( breccias ) of the Neogene (late Miocene).

Mineralization

The shear system in the roof area of ​​the pluton is linked to previously mined hydrothermal mineralization , which can be divided into two basic types:

The gold mineralization affected the cataclasites and is accompanied by hydrothermal quartz, barite and primary sulfides such as silver-bearing galena . The barite mineralization occurred in northwest-southeast trending fissures and veins that extend into the granite. In addition to barite, they contain base metal sulfides, which at higher altitudes have been oxidized to form iron hydroxides and oxides as well as copper minerals such as angelsite and cerussite .

On the basis of hydrothermal chlorites , the period of mineralization could be determined to be 11 to 10 million years BP (beginning tortonium), with temperatures between 280 and 200 ° C.

Geodynamics

Until the onset of hydrothermal mineralization around 11 million years BP, the Mykonos granite was exposed to a stress field dominated by pure expansion in the north-northeast direction (σ 1 = vertical, σ 2 = N 120, σ 3 = N 030), which can be explained by the receding southward Subduction current. When it left the ductile range at around 280 ° C, the stress field assumed an increasingly compressive character. Above the Mykonos shear level, due to a main stress now oriented east-southeast (σ 1 = N 120), first northwest-southeast trending lateral shifts formed, which reactivated some of the veins, and then minor faults. The stretching in the north-northeast direction still persisted, since there was only a rotation (and thus an interchange) of the two stress components σ 1 and σ 2 . The emergence of the compression component σ 1 in an easterly direction around 10 million years BP is very likely due to the beginning westward drift of the Anatolia block, which from 6 million years ago slid along BP ( Messinian ) at its northern boundary, the right- shifting North Anatolian fault , and over the Dardanelles advanced into the Aegean region and narrowed it. The east-west compression in the north Aegean still exists today, as evidenced by GPS data .

Individual evidence

  1. ↑ Brichau, S. et al .: Timing, slip rate, displacement and cooling history of the Mykonos detachment footwall, Cyclades, Greece, and implications for the opening of the Aegean Sea basin . In: Journal of the Geological Society of London . tape 165 , 2008, p. 263-277 .
  2. ↑ Lecomte, E. et al .: Geometry and kinematics of Mykonos detachment (Cyclades, Greece): evidence for slip at shallow dip . In: Tectonics . tape 29 , 2010, p. 22 .
  3. ↑ Vanderhaeghe, O. et al: Penrose conference - extending a continent - Naxos Field guide . In: G. Lister, M. Forster and U. Ring, Inside the Aegean Metamorphic Core Complexes (Eds.): Journal of the Virtual Explorer . tape 27 , 2007, doi : 10.3809 / jvirtex.2007.00175 .
  4. ↑ Mehl, C. et al .: Structural evolution of Andros island (Cyclades, Greece): a key to the behavior of a flat detachment within an extending continental crust . In: T. Taymaz, Y. Dilek and Y. Ylmaz, The Geodynamics of the Aegean and Anatolia (Eds.): Geol. Soc. Lond. Spec. Publ. Volume 291 , 2007, p. 41-73 .
  5. ↑ Jolivet, L. et al .: Progressive strain localization, boudinage and extensional metamorphic complexes, the Aegean Sea case . In: DL Whitney, C. Teyssier and CS Siddoway, Gneiss Domes in Orogeny (Eds.): Geological Society of America Special Paper . tape 380 , 2004, p. 185-210 .
  6. ↑ Trotet, F. et al .: Tectono-metamorphic evolution of Syros and Sifnos islands (Cyclades, Greece) . In: Tectonophysics . tape 338 , 2001, p. 179-206 .
  7. ↑ Altherr, R. et al: A Late Oligocene / Early Miocene high temperature belt in the anticycladic crystalline complex (SE Pelagonian, Greece) . In: Geological Yearbook . tape 23 , 1982, pp. 97-164 .
  8. ↑ a b c Altherr, R. and Siebel, W .: I-type plutonism in a continental back-arc setting: Miocene granitoids and monzonites from the central Aegean Sea, Greece . In: Contrib. Mineral. Petrol. tape 143 , 2002, p. 397-415 , doi : 10.1007 / s00410-002-0352-y .
  9. ↑ Lucas, I .: Le pluton de Mykonos-Delos-Rhenee (Cyclades, Grèce): un exemple de mise en place synchrone de l'extension crustale (doctoral thesis) . Orléans 1999, p. 491 .
  10. ↑ Stouraiti, C. et al .: Geochemistry and petrogenesis of late Miocene granitoids, Cyclades, southern Aegean: Nature of source components . In: Lithos . tape 114 , 2010, pp. 337-352 , doi : 10.1016 / j.lithos.2009.09.010 .
  11. ↑ Roman-Berdiehl, T. et al .: Analogue models of laccolith formation . In: Journal of Structural Geology . tape 17 , 1995, p. 1337-1346 .
  12. ↑ Menand, T .: The mechanics and dynamics of sills in layered elastic rocks and their implications for the growth of laccoliths and other igneous complexes . In: EPSL . tape 267 , 2008, p. 93-99 .
  13. ↑ de Saint Blanquat, M. et al .: Multiscale magmatic cyclicity, duration of pluton construction, and the paradoxical relationship between tectonism and plutonism in continental arcs . In: Tectonophysics . 2010, doi : 10.1016 / j.tecto.2009.12.009 .
  14. ↑ a b Denèle, Y. et al .: Granite intrusion in a metamorphic core complex: The example of the Mykonos laccolith (Cyclades, Greece) . In: Tectonophysics . tape 501 , 2011, p. 52 -70 , doi : 10.1016 / j.tecto.2011.1001.1013 .
  15. ↑ Menant, A. et al .: The North Cycladic Detachment Sdxdsystem and associated mineralization, Mykonos, Greece: Insights in the evolution of the Aegean domain . In: Tectonics . tape 32 , 2013, p. 433-452 .
  16. ↑ Le Pichon, X. and Kreemer, C .: The Miocene-to-present kinematic evolution of the Eastern Mediterranean and Middle East and its implications for dynamics . In: Annu. Rev. Earth Planet. Sci. tape 38 , 2010, p. 323-351 , doi : 10.1146 / annurev-earth-040809-152419 .