Longevity of metamorphism in metamorphic soles
implicatins for subduction initiation
Kalijn Peters(1), Douwe van Hinsbergen(1), Matthijs Smit(2), Herman van Roermund(1), Fraukje Brouwer(3), Martyn Drury(1), Alexis Plunder(4), Carl Guilmette(5)
(1) Department of Earth sciences, Utrecht University, The Netherlands, e-mail: M.K.Peters@uu.nl, (2) Department of Earth, Ocean and Atmosphere, The University of British Columbia, Canada, (3) Faculty of Science, VU Amsterdam, The Netherlands, (4) Institute des Sciences de la Terre de Paris, Université Pierre et Marie Curie, Paris, France, (5) French Geological Survey, Orléans, France
Metamorphic soles - do they form at the same moment as the overlying oceanic crust?
The initiation of subduction is key to the formation and recycling of tectonic plates. The best geological archive to study subduction initiation can be found underneath many supra-subduction zone (SSZ) ophiolites as a metamorphic sole. Metamorphic soles are interpreted to represent the upper part of a
downgoing slab at the onset of subduction, when the mantle wedge has not yet cooled down. They consist of metamorphosed oceanic crust and pelagic sediments that are accreted to the mantle section of supra-subduction zone (SSZ) ophiolites.
Metamorphic soles typically show an inverted metamorphic field gradient, ranging from greenschist facies at the bottom to granulite facies at the top with peak metamorphic conditions up to ~850°C and 10-15 kbar (Jamieson, 1986; Dilek and Whitney, 1997; Hacker and Gnos, 1997; Wakabayashi and Dilek,
2000; Guilmette et al., 2008; Myhill, 2011).
The metamorphic history of the soles is typically investigated using 40Ar/39Ar hornblende chronology These age data consistently overlap with the crys- tallization age of their overlying SSZ ophiolites. This synchronicity could indicate that subduction started spontaneously (Stern, 2004). Nevertheless,
40Ar/39Ar data may in fact date cooling after a history that is not yet known.
Results demonstrate that the highest grade rocks from the Pιnarbaşι metamorphic sole (Turkey) started to undergo metamorphism around 104 Ma
(Lu-Hf garnet chronology), earlier than our c. 94 Ma U-Pb zircon ages and published 40Ar/39 Ar cooling ages (90-94 Ma). The Lu-Hf in garnet age reflects the timing for burial and decoupling of the sole from the downgoing slab, and can be interpreted as the earliest documented timing of subduction initia- tion. The U-Pb in zircon age most likely indicates zircon crystalization from a partial melt within the sole at peak temperatures (Guilmette et al., 2018).
From that time on, the metamorphic sole started to exhume, as indicated by published 90-94 Ma Ar-Ar in hornblende ages from metamorphic soles in the Tauride ophiolites (Parlak, 2016). Around 90 Ma, extension in the upper plate led regionally to the formation of SSZ ophiolitic crust (Van Hinsbergen et al., 2016).
The ~10 Myr age difference between the Lu-Hf in garnet ages, and the U-Pb in zircon and Ar-Ar in hornblende ages demonstrates that there is a sig- nificant time lag between the formation of the sole and cooling of the sole and upper plate extension in Pιnarbaşι. Such a time lag provides evidence for far-field forced subduction initiation.
Figure 2. Tectonostratigraphic column and field photographs of the Pınarbaşı metamorphic sole, with serpentinised peridotite at the structural top and tectonic mélange underneath the metamor- phic sole. Structural position of samples containing garnet and zircon indicated with a red star.
Lu-Hf in garnet: prograde metamorphism
U-Pb in zircon: peak metamorphism
Figure 4. (A) Concordia diagram displaying U/Pb ages for zircon grains from the Pınarbaşı metamorphic sole. Ellipses indicate the 2σ uncertainty. MSWD = mean square of weighted deviates.
(B) CL images of zircon grains used for dating; grainsize 90-120 μm.
What is the time difference between formation and cooling of the metamorphic sole?
Case study: petrology and multi-mineral chronology of Pınarbaşı metamorphic sole - Turkey
Figure 1. Distribution of Neotethyan ophiolites in the Eastern Mediter- ranean region, after Parlak (2006). 40Ar/39Ar ages for the metamorphic soles from Parlak (2016) and references therein.
References
- Dilek, Y., and D. L. Whitney (1997), Counterclockwise P-T-t trajectory from the metamorphic sole of a Neo-Tethyan ophiolite (Turkey), Tectonophysics, 280, 295–310.
- Guilmette, C., R. Hébert, C. Dupuis, C. Wang, and Z. Li (2008), Metamorphic history and geodynamic significance of high-grade metabasites from the ophiolitic m�elange beneath the Yarlung Zangbo ophiolites, Xigaze area, Tibet, J. Asian Earth Sci., 2008, 423–437.
- Guilmette, C., Smit, M. A., Van Hinsbergen, D.J.J., Grer, M.D.,Corfu, F., Charette, B., Maffione, M., Rabeau, O., Savard, D., (2018), Forced subduction initiation recorded in the sole and crust of the Semail Ophiolite of Oman, Nature Geoscience, 11, 688-695
- Hacker, B. R., and E. Gnos (1997), The conundrum of Samail: Explaining the metamorphic history, Tectonophysics, 279, 215–226.
- Jamieson, R. A. (1986), P-T paths from high-temperature shear zones beneath ophiolites, J. Metamorph. Geol., 4, 3–22.
- Maffione, M., and van Hinsbergen, D.J.J., in press, Reconstructing ridges and trenches in the Jurassic Neo-Tethys from the East and West Vardar Ophiolites (Greece, Serbia), Tectonics
Figure 3. Ca and Mn element maps of garnet grains, and Lu-Hf garnet age of granulite facies garnet-amphi- bolite (A and B, see also figure 2), of the Pınarbaşı metamorphic sole.
- Maffione, M., D. J. J. van Hinsbergen, G. I. N. O. de Gelder, F. C. van der Goes, and A. Morris (2017), Kinematics of Late Cretaceous subduction initiation in the Neo-Tethys Ocean reconstructed from ophiolites of Turkey, Cyprus, and Syria, J. Geophys. Res. Solid Earth, 122
- Myhill, R. (2011), Constraints on the evolution of the Mesohellenic Ophiolite from subophiolitic metamorphic rocks, Geol. Soc. Am. Spec. Pap., 480, 75–94.
- Parlak, O., Yılmaz, H., Boztuğ, D., 2006. Geochemistry and tectonic setting of the metamorphic sole rocks and isolated dykes from the Divriği ophiolite (Sivas, Turkey): Evidence for melt generation within an asthenospheric window prior to ophiolite Emplacement. Turkish Journal of Earth Sciences, 15: 25–45
- Parlak, O., 2016. Tauride Ophiolites in Anatolia (Turkey): A Review. Journal of Earth Science, 27(6): 901–934.
- Stern, R. J. (2004), Subduction initiation: spontaneous and induced. Earth Planet. Sci. Lett. 226, 275–292
- Wakabayashi, J., and Y. Dilek (2000), Spatial and temporal relationships between ophiolites and their metamorphic soles: A test of models of forearc ophio- lite genesis, Geol. Soc. Am. Spec. Pap., 349, 53–64.
With thanks for help in the lab to:
Roel van Elsas, Jamie Cutts, Tilly Bouten, Sergei Matveev,
Fernando Corfu and Derya Gürer.
Myr
~10
104.5 ± 1.3 Ma
93.51 ± 0.36 Ma
0.282 0.284 0.286
0.0 0.5 1.0 1.5 2.0
176Hf/177Hf
176Lu/177Hf
0.282 0.287 0.292 0.297
0.0 2.0 4.0 6.0 8.0
176Hf/177Hf
176Lu/177Hf
high low
Ca
low high
Ca
Mn
Ca
Mn
high
high
low
low
Garnet + Clinopyroxene + Hornblende + Pla-
gioclase ± Quartz ± Zircon ± Titanite ± Ilmenite ± Hematite ± Apatite
Hornblende + Pla- gioclase ± Epidote ± Quartz ± Titanite
± Apatite ± Hematite
Calcite + Plagioclase + Chlorite ± Epidote
± Quartz ± Mica
± Amphibole
* A
B
A
B
0.0138 0.0142 0.0146 0.0150 0.0154 0.0158
0.08 0.09 0.10 0.11 0.12 0.13 0.14
206 Pb/238 U
207Pb/235U
90 92
94 96
98 100
102
PB3.11A:grt-cpx-amphibolite mean concordant 206Pb/238U (2 pts)
93.51 ± 0.36 Ma MSWD = 1.16 mean 206Pb/238U (4 pts (+)) 92.2±1.9 Ma & 1680±500 Ma
MSWD = 1.2
mean 206Pb/238U (4 pts ( )) 93.12±0.63 & 3321±730 Ma
MSWD = 1.3
(A)
206Pb/238U (4 pts (+)) 92.2±1.9 Ma & 1680±500Ma
MSWD = 1.16 206Pb/238U (4 pts ( )) 93.12±0.63 Ma & 3321±730Ma
MSWD = 1.3
(B)
27 This paper (1) presents the major- and trace-element
chemistry of the metamorphic sole and isolated dyke rocks intruding both the metamorphic sole and the mantle tectonites, (2) investigates possible protoliths of the material accreted to the base of mantle tectonites during intraoceanic subduction, and (3) presents the evidence for late-stage dyke intrusions fed by melts that originated within an asthenospheric window due to slab break-off, shortly before the emplacement of the Divri¤i ophiolite onto the Tauride platform in the Late Cretaceous.
Geological Setting
The Divri¤i region in east-central Anatolia comprises the Tauride platform unit, ophiolitic mélange, ophiolite- related metamorphic rocks, ophiolitic rocks, a volcano- sedimentary unit, granitoid rocks and Tertiary cover sediments (Figure 2). Detailed (1:25000-scale) geological mapping of the internal stratigraphy of the ophiolitic units of the Divri¤i region was first carried out by Y›lmaz
et al. (2001). The structurally lowest unit in the study area is the Munzur Limestone. The Munzur Limestone is present in the Mesozoic carbonate sequence of most of the autochthonous and allochthonous units of the Tauride belt (Özgül & Turflucu 1984). The base of the Munzur limestone is not exposed in the study area, and this unit is tectonically overlain by the Yefliltaflyayla ophiolitic mélange and above that, the metamorphic sole and Divri¤i ophiolite (Figures 2 & 3). The Munzur Limestone comprises, from bottom to top, algal limestone, oolitic limestone, algal and foraminiferal limestone, cherty limestone, neritic limestone, rudistic limestone and pelagic limestone (Özgül & Turflucu 1984). The type locality of the Munzur Limestone has yielded an Early Triassic–Campanian age (Özgül & Turflucu 1984);
however, the fossil content of this unit in the study area indicates an Early Carboniferous–Campanian age (Öztürk
& Öztunal› 1993; Y›lmaz et al. 2001).
The Yefliltaflyayla mélange tectonically overlies the Munzur Limestone east of Ekinbafl› and Maltepe villages, and is tectonically overlain by either metamorphic-sole
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Km 0 100
Guleman ophiolite Munzur
Küre ophiolite Çangaldağ
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Massif Moesian Platform
EURASIA
Mirdita Ophiolite
PO Hellenides
Dinarides
Adriatic Sea
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Varda r Zon
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Black Sea
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Pontides
Ankara Melange
Kızıldağ ophiolite
20°E 24°E 28°E 32°E 36°E 40°E
44°N
42°N
40°N
36°N
34°N Bitlis-Zagros
Suture Zone
Pınarbaşı
Ar-Ar in hornblende ages:
90-94 Ma
