Dating deformation: multichronometric examples from the Western Alps, Naxos, and the Garhwal Himalaya

In the eclogite facies shear zone of Bottarello, Monte Rosa, Western Alps [1], fault recrystallization around 600 °C gives concordant Lu-Hf (garnet) and ³⁹Ar-⁴⁰Ar (white mica, WM) 47 Ma ages whereas <100 m from the fault the unsheared rock at the same T preserves Mesozoic inheritance. The Ar retentivity of WM is not accurately predicted by hydrothermal laboratory experiments, because the latter are plagued by massive dissolution artefacts [2]. Independent field observations confirm that WM only starts losing Ar in dry rocks above 600 °C [3-8], but when retrograde reactions occur, WM can recrystallize and be totally reset below 230 °C [9]. The Bottarello fault obliterated all relict WM from the protolith; the neoformed WM records its own formation age.

The island of Naxos (Cyclades, Greece) is the classic example of multiple, coexisting WM generations [10]: relict pre-eclogitic basement WM, and eclogitic phengite retrograded to muscovite. Electron microprobe element maps demonstrate intergrowths at a scale <5 µm, which makes laser microprobe dating useless. Bulk mica dissolution for Rb-Sr gives Eocene ages [11], which agree with bulk K-Ar ages. This is paradoxical, as Ar diffusivity is c. 4 orders of magnitude higher than that of Sr [12]; the only explanation is that both chronometric systems record formation ages around 500-600 °C. The WM generations can be unravelled by their Ca/Cl/K signatures; coarse and fine sieve fractions are never isomineralic. Ages of pure mica generations are obtained by extrapolating Ca/Cl/K-vs-age trends.

The in-sequence thrusts of the Garhwal Himalaya add one complication: thrusting was long-lived. Microstructures combined with chemical microanalysis distinguish three monazite generations (dated by U-Pb) and three WM generations: relicts in microlithons, foliation-defining mica, and static coronas. As in the previous examples, intergrowths are <<10 µm and only combining Ca/Cl/K systematics with the observed differences in structural breakdown temperatures can assign the different WM ages in the same sample to chemically distinct generations [13]. WM formation ages overlap with Mnz ages and date the onset of faulting, the kinematic peak, and the post-faulting corona formation.

There is no free lunch: dating deformation is extremely labor-intensive and requires, always, establishing the context between microtextural, microchemical, petrological and multichronometric analyses. Whenever one of these four is missing, the tectonic reconstruction is invariably faulty [14].

 

[1] Villa &al, J Petrol 55 (2014) 803-830

[2] Villa, Geol Soc London Spec Pub 332 (2010) 1-15

[3] Di Vincenzo &al, J Petrol 45 (2004) 1013-1043

[4] Itaya &al, Island Arc 18 (2009) 293-305

[5] Heri &al, Geol Soc London Spec Pub 378 (2014) 69-78

[6] Laurent &al, Lithos, 272-273 (2017) 315-335

[7] Airaghi &al, J Metam Geol 36 (2018) 933-958

[8] Imayama &al, Geol Soc London Spec Pub 481 (2019) 147-173

[9] Maineri &al, Mineralium Deposita 38 (2003) 67-86

[10 Wijbrans & McDougall, Contrib Min Petr 93 (1986) 187-194

[11] Peillod &al, J Metam Geol 35 (2017) 805-830

[12] Cherniak & Watson, EPSL 113 (1992) 411-425

[13] Montemagni &al, Geol Soc London Spec Pub 481 (2019) 127-146

[14] Bosse & Villa, Gondwana Res 71 (2019) 76-90