Deciphering high-pressure metamorphism in collisional context using microprobe mapping methods: Application to the Stak eclogitic massif (northwest Himalaya)

Abstract

The Stak massif, northern Pakistan, is a newly recognized occurrence of eclogite formed by the subduction of the northern margin of the Indian continent in the northwest Himalaya. Although this unit was extensively retrogressed during the Himalayan collision, records of the high-pressure (HP) event as well as a continuous pressure-temperature (P-T) path were assessed from a single thin section using a new multiequilibrium method. This method uses microprobe X-ray compositional maps of garnet and omphacitic pyroxene followed by calculations of ~200,000 P-T estimates using appropriate thermobarometers. The Stak eclogite underwent prograde metamorphism, increasing from 650 °C and 2.4 GPa to the peak conditions of 750 °C and 2.5 GPa, then retrogressed to 700-650 °C and 1.6-0.9 GPa under amphibolite-facies conditions. The estimated peak metamorphic conditions and P-T path are similar to those of the Kaghan and Tso Morari high-to ultrahigh-pressure (HP-UHP) massifs. We propose that these three massifs defi ne a large HP to UHP province in the northwest Himalaya, comparable to the Dabie-Sulu province in China and the Western Gneiss Region in Norway.

Metamorphic history of riebeckite- and aegirine-augite-bearing high-pressure–low-temperature blocks within the Siuna Serpentinite Mélange, northeastern Nicaragua

“…It then identifies mineral phases, calculates the structural formula for each pixel in the mapped area, and evaluates pressure and temperature using empirical and semiempirical thermobarometers (e.g. Lanari et al 2013). Note that the version we have used estimates ferric iron contents for at least clinoyproxene and garnet based on the method detailed above.…”

Deeply subducted continental fragments – Part 1: Fracturing, dissolution–precipitation, and diffusion processes recorded by garnet textures of the central Sesia Zone (western Italian Alps)

“…For compositional mapping, the procedure of Lanari et al (2013) was used. Spot analyses were measured for each mineral phase present in the area of the maps before the maps were acquired.…”

“…The quantitative micro-mapping strategy employed in this study has wellestablished advantages (e.g. Marmo et al, 2002;Lanari et al, 2013;Ortolano et al, 2014b;Angiboust et al, 2017) over traditional spot analyses: (1) it allows key relationships, such as the successive growth zones, to be identified and relevant compositions to be constrained; (2) it permits testing if chemical zoning patterns are consistent over several grains, which helps support (or refute) the assumption of grain boundary equilibrium (e.g. ; (3) it can be used to approximate local reactive bulk composition by accounting for (non-reactive) mineral relics.…”

Crustal-scale structure of South Tien Shan: implications for subduction polarity and Cenozoic reactivation

“…However, the temperatures are slightly lower, ranging between 450 and 4808C for metasediments and 505-5358C for mafic eclogites. Following Lanari et al (2013), we can assume an error linked to the microprobe uncertainty of +208C on temperature. The real uncertainty, when also considering the error on the thermodynamic data, is probably higher at +508C.…”