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

Abstract

The Salt Range Formation is an extensive evaporite sequence 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 Mine, where it accommodates approximately 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 laser and furnace methods. 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. 254 and 277 ∘C for a cooling rate of 10 ∘C Myr−1. 40Ar/39Ar age results for both samples did not return any reliable crystallisation 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 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 measured step ages and represent a deformation event that partially reset the K/Ar system based on oldest significant ages between ca. 381 and 415 Ma. We interpret the youngest measured step ages, between ca. 286 and 292 Ma, to represent the maximum age of deformation-induced recrystallisation. Both the youngest and oldest measured 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. Otherwise, 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.