Eocene age of eclogite metamorphism in Pakistan Himalaya: implications for India‐Eurasia collision

Abstract

A geochronological investigation of two rocks with an eclogitic assemblage (omphacite-garnet-quartz-rutile) from the High Himalaya using the Sm/Nd, Rb/Sr, U/Pb and AdAr methods is presented here. The first three methods outline a cooling history from the time of peak metamorphism at 49k6 Ma recorded by S d N d in garnet-clinopyroxene to the closure of Rb/Sr in phengite at 4 3 f l Ma and U/Pb in rutile at 39-40 Ma. The Sm/Nd isotopic system was fully equilibrated during eclogitization and has not been disturbed since; its mineral ages may date the peak metamorphic conditions (650250°C at 13-18 kbar: Pognante and Spencer, 1991). The AriAr data reveal the presence of substantial amounts of excess 40Ar in hornblende, and yield a statistically acceptable but geologically meaningless phengite plateau age of 81.4k0.2 Ma, inconsistent with Sm/Nd, Rb/Sr and U/Pb. This questions the use of such a chronometer for the dating of high-pressure assemblages. The results imply a Late Palaeocene or Early Eocene subduction of the northern Indian plate margin in NW Himalaya. The fact that eclogites are restricted to NW Himalaya may be the result of a peculiar p-T-t path associated with a high convergence rate during the first indentation, in contrast to the later and slow subduction in Central and Eastern Himalaya.

Detrital zircon U-Pb-He double dating: A method of quantifying long- and short-term exhumation rates in collisional orogens

“…The range of the U/Pb ages indicates a protracted episode as long as 25 Myr for regional metamorphism of the host rocks at deep crust, i.e., a process from continental collision to postcollisional reworking. The upper U/Pb age limit yielded by the zircon rims in this study is consistent with the U/Pb ages of 44±3 Ma for the rim of zircon with a Permian core, a garnet-omphacite Sm-Nd isochron age of 49 ± 6 Ma, a phengite Rb-Sr isochron age of 43 ± 1 Ma for the Kaghan eclogite in Pakistan (Tonarini et al, 1993) as well as a U/Pb age of 50 Ma for overgrown quartz-bearing rims and a U/Pb age of 46 Ma for coesite-bearing rim (Kaneko et al, 2003;Parrish et al, 2006). The long episode of 40-50 Myr is common for high-pressure (HP) to ultrahigh-pressure (UHP) metamorphism in continent-continent collisional orogens such as the Dabei-Sulu in China and the Western Gneiss Region in Norway, whereas the short episode of 5-10 Myr is prominent in arc-continent collisional orogens such as the Himalayas and Alps (Zheng, 2012;Zheng et al, 2013).…”

Multichronometric Evidence for an In Situ Origin of the Ultrahigh‐Pressure Metamorphic Terrane of Dabieshan, China

“…One possible interpretation is the incorporation of excess Ar. Several studies on highpressure white micas indicated that Ar-Ar ages are geologically meaningless due to excess Ar (Tilton et al 1991;Tonarini et al 1993;Li et al 1994;Arnaud and Kelley 1995;Hacker and Wang 1995). Li et al (1994) and Boundy et al (1997) suggested that excess Ar in Phe is mainly observed in Bt-free UHP-HP rocks.…”

Paleozoic ages and excess 40Ar in garnets from the Bixiling eclogite in Dabieshan, China: New insights from 40Ar/39Ar dating by stepwise crushing

“…The purpose of this study was to investigate Ar isotope systematics of garnet and fluid inclusions in garnet, with the specific aim of studying excess 40 Ar by the 40 Ar/ 39 Ar crushing technique given that age scatter attributed to excess 40 Ar is ubiquitous in the (U)HP metamorphic rocks of Dabieshan , the Seward Peninsula of Alaska (Hannula and McWilliams, 1995), the western Alps (Scaillet et al, 1992;Arnaud and Kelley, 1995;Ruffet et al, 1995Ruffet et al, , 1997Scaillet, 1996), the Holsnoy Island of western Norway (Boundy et al, 1997), the Pakistan Himalaya (Tonarini et al, 1993), and the Tavsanli Zone of NW Turkey (Sherlock and Arnaud, 1999;Sherlock and Kelley, 2002).…”