Anomalously old biotite⁴⁰Ar/³⁹Ar ages in the NW Himalaya

Abstract

Biotite 40 Ar/ 39 Ar ages older than corresponding muscovite 40 Ar/ 39 Ar ages, contrary to the diffusion properties of these minerals, are common in the Himalaya and other metamorphic regions. In these cases, biotite 40 Ar/ 39 Ar ages are commonly dismissed as "too old" on account of "excess Ar. " We present 32 step-heating 40 Ar/ 39 Ar ages from 17 samples from central Himachal Pradesh Himalaya, India. In almost all cases, the biotite ages are older than predicted from cooling histories. We document host-rock lithology and chemical composition, mica microstructures, biotite chemical composition, and chlorite and muscovite components of biotite separates to demonstrate that these factors do not offer an explanation for the anomalously old biotite 40 Ar/ 39 Ar ages. We discuss possible mechanisms that may account for extraneous Ar (inherited or excess Ar) in these samples. The most likely cause for "too-old" biotite is excess Ar, i.e., 40 Ar that is separated from its parent K. We suggest that this contamination resulted from one or several of the following mechanisms: (1) 40 Ar was released during Cenozoic prograde metamorphism; (2) 40 Ar transport was restricted due to a temporarily dry intergranular medium; (3) 40 Ar was released from melt into a hydrous fluid phase during melt crystallization. Samples from the Main Central Thrust shear zone may be affected by a different mechanism of excess-Ar accumulation, possibly linked to later-stage fluid circulation within the shear zone and chloritization. Different Ar diffusivities and/or solubilities in biotite and muscovite may explain why biotite is more commonly affected by excess Ar than muscovite.

Segmentation of the Main Himalayan Thrust Revealed by Low‐Temperature Thermochronometry in the Western Indian Himalaya

“…Overview of published and new thermochronometry data from the study area (rectangle) and adjacent regions. Data sources: Kumar et al (), Dèzes et al (), Lal et al (), Jain et al (), Schlup (); Thiede et al (), Vannay et al (), Thiede et al (), Thiede et al (), Adams et al (), Adlakha et al () ; Stübner et al (); Stübner et al (); Thiede et al (); Stübner et al (). Closure temperatures (Tc) after Wolf et al (), and Farley () for apatite U/Th‐He (AHe), Gleadow and Duddy () for apatite fission track (AFT), Reiners et al () for zircon U/Th‐He (ZHe), Brandon et al () for zircon fission track (ZFT), and Hodges () for muscovite 40 Ar/ 39 Ar.…”

“…Transverse profiles across (a) the Dhauladar range, (b) the Beas and Chandra valleys, and (c) the Sutlej Valley. In each transect, the upper panel shows the respective thermochronometer cooling ages (Adams et al, ; Dèzes et al, ; Jain et al, ; Kumar et al, ; Lal et al, ; Schlup, ; Stübner et al, , ; Stübner et al, ; Thiede et al, , , ; Thiede et al, ; Vannay et al, ). The lower panel shows inferred subsurface geometry; beneath the Dhauladar Range a steeply dipping frontal ramp is proposed to cause rapid exhumation at the MBT (Thiede et al, ), where no evidence for STD (2) activity exists.…”

Influence of dissolution/reprecipitation reactions on metamorphic greenschist to amphibolite facies mica 40Ar/39Ar ages in the Longmen Shan (eastern Tibet)

“…Biotite in amphibolite facies rocks in the Himalayas also commonly records ages consistent with the timing of prograde growth (e.g. Stübner et al., ), although the older ages are commonly interpreted as being due to the presence of excess 40 Ar. The Ar inherited from the first biotite that grew during the prograde to peak metamorphic path in our samples may have contributed to the old ages.…”

Mid‐Miocene initiation of E‐W extension and recoupling of the Himalaya

“…Moreover, biotite from the Himalaya commonly contains excess Ar (e.g. Stübner et al, ). Complete, inclusion‐free muscovite from two specimens of medium‐grained, undeformed Qtz + Afs + Ms + Tur + Pl leucogranite, GG10 and GG12 (Figure ), were analysed.…”