Preprints
https://doi.org/10.5194/gchron-2021-16
https://doi.org/10.5194/gchron-2021-16

  28 May 2021

28 May 2021

Review status: this preprint is currently under review for the journal GChron.

Deformation recorded in polyhalite from evaporite detachments revealed by 40Ar/39Ar dating

Lachlan Richards1, Fred Jourdan2, Alan Stephen Collins1, and Rosalind Clare King1 Lachlan Richards et al.
  • 1Centre for Tectonics, Resources and Exploration (TRaX), Department of Earth & Environmental Sciences, The University of Adelaide, SA 5005, Australia
  • 2School of Earth and Planetary Sciences & JDL Centre, Curtin University, Perth WA 6845, Australia

Abstract. The Salt Range Formation is an extensive evaporite formation in northern Pakistan that has acted as the primary detachment accommodating Himalayan orogenic deformation from the north. This rheologically weak formation forms a mylonite in the Khewra mines, where it accommodates approximate 40 km displacement and is comprised of intercalated halite and potash salts and gypsiferous marls. Polyhalite [K2Ca2Mg(SO4)4•2H2O] grains taken from potash marl and crystalline halite samples are used as geochronometers to date the formation and identify the closure temperature of the mineral polyhalite using the 40Ar/39Ar step heating method. The diffusion characteristics measured for two samples of polyhalite are diffusivity (D0), activation energy (Ea), and %39Ar. These values correspond to a closure temperature of ca. 281 and 296 °C for a cooling rate of 10 °C/Ma. 40Ar/39Ar age results for both samples did not return any reliable crystallization age. This is not unexpected as polyhalite is prone to 40Ar* diffusion loss and the evaporites have experienced numerous phases of deformation resetting the closed K/Ar system. An oldest minimum heating step age of 514 ± 3 Ma from sample 06-3.1 corresponds relatively well to the established early Cambrian age of the formation. Samples 05-P2 and 05-W2 have apparent step ages and represent a deformation event that partially reset the K/Ar system based on oldest significant ages between ca. 381 Ma and 415 Ma. We interpret the youngest apparent step ages, between ca. 286 Ma and 292 Ma, to represent the maximum age of deformation-induced recrystallisation. Both the youngest and oldest apparent step ages for Samples 05-P2 and 05-W2 occur within the time of a major unconformity in the area. These dates may reflect partial resetting of the K/Ar system from meteoric water infiltration and recrystallisation during this non-depositional time. Else, they may result from mixing of Ar derived by radiogenic decay after Cambrian precipitation with partially reset Ar from pervasive Cenozoic deformation and physical recrystallisation.

Lachlan Richards et al.

Status: open (until 17 Jul 2021)

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Lachlan Richards et al.

Lachlan Richards et al.

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Short summary
This research is part of a Ph.D thesis examining evaporite detachments characteristics. 40Ar/39Ar geochronology is employed to constrain the timing of formation and deformation events. A diagenetic age of 514 ± 3 Ma is interpreted from the oldest significant step age. Other step ages may represent a Cambrian-Permian deformation event or a complex mixing age of diagenetic Ar with partially reset Ar during the Cenozoic. We report the first closure temperature for polyhalite between 281 and 296 °C.