Mesozoic to Cenozoic tectono‐metamorphic history of the South Pamir–Hindu Kush (Chitral, NW Pakistan): Insights from phase equilibria modelling, and garnet–monazite petrochronology

Abstract

The Karakoram–Hindu Kush–Pamir and adjacent Tibetan plateau belt comprise a series of Gondwana‐derived crustal fragments that successively accreted to the Eurasian margin in the Mesozoic as the result of the progressive Tethys ocean closure. These domains provide unique insights into the thermal and structural history of the Mesozoic to Cenozoic Eurasian plate margin, which are critical to inform the initial boundary conditions (e.g. crustal thickness, structure and thermo‐mechanical properties) for the subsequent development of the large and hot Tibetan–Himalaya orogen, and the associated crustal deformation processes. Using a combination of microstructural analyses, thermobarometry modelling and U–Th–Pb monazite and Lu–Hf garnet geochronology, the study reappraises the metamorphic history of exposed mid‐crustal metapelites in the Chitral region of the South Pamir–Hindu Kush (NW Pakistan). This study also demonstrates that trace elements in monazite (especially Y and Dy), combined with thermodynamical modelling and Lu–Hf garnet dating, provides a powerful integrated toolbox for constraining long‐lived and polyphased tectono‐metamorphic histories in all their spatial and temporal complexity. Rocks from the Chitral region were progressively deformed and metamorphosed at sub‐ and supra‐solidus conditions through at least four distinct episodes from the Mesozoic to the Cenozoic. Rocks were first metamorphosed at ~400–500°C and ~0.3 GPa in the Late Triassic–Early Jurassic (210–185 Ma), likely in response to the accretion of the Karakoram during the Cimmerian orogeny. Pressure and temperature subsequently increased by ~0.3 GPa and 100°C in the Early‐ to Mid Cretaceous (140–80 Ma), coinciding with the intrusion of calcalkaline granitic plutons across the Karakoram and Pamir regions. This event is interpreted as the record of crustal thickening and the development of a proto‐plateau within the Eurasian margin due to a long‐lived episode of slab flattening in an Andean‐type margin. Peak metamorphism was reached in the Late Eocene–Early Oligocene (40–30 Ma) at conditions of 580–600°C and ~0.6 GPa and 700–750°C and 0.7–0.8 GPa for the investigated staurolite schists and sillimanite migmatites respectively. This crustal heating up to moderate anatexis likely resulted in the underthrusting of the Indian plate after a NeoTethyan slab‐break off or to the Tethyan Himalaya–Lhasa microcontinent collision and subsequent oceanic slab flattening. Near‐isothermal decompression/exhumation followed in the Late Oligocene (28–23 Ma) as marked by a pressure decrease in excess of ~0.1 GPa. This event was coeval with the intrusion of the 24 Ma Garam Chasma leucogranite. This rapid exhumation is interpreted to be related to the reactivation of the South Pamir–Karakoram suture zone during the ongoing collision with India. The findings of this study confirm that significant crustal shortening and thickening of the south Eurasian margin occurred during the Mesozoic in an accretionary‐type tectonic setting through successive episodes of...

Quartz‐in‐garnet and Ti‐in‐quartz thermobarometry: Methodology and first application to a quartzofeldspathic gneiss from eastern Papua New Guinea

“…For example, pseudosection modelling uses bulk rock geochemistry and mineral chemical compositions to reveal the ranges of stable P–T conditions where possible mineral assemblage can crystallize at chemical equilibrium. Model results have been frequently combined with QuiG to estimate crystallization P–T conditions (Endo et al, ; Soret et al, ; Taguchi et al, ; Taguchi, Enami, & Kouketsu, ; Viete et al, ). Recently, Castro and Spear () and Wolfe and Spear () combined QuiG with the Zr‐in‐rutile solubility model to estimate crystallization P–T conditions on a QuiG isomeke, an elastic thermobarometry method that requires chemical equilibrium controlled by co‐crystallization of quartz, garnet, rutile, and zircon.…”

“…For example, Raman measurements can be performed on a quartz inclusion in garnet, and the same inclusion can be subsequently exposed and polished for Ti-in-quartz measurements. Most previous applications of elastic thermobarometry have generally used a temperature estimate obtained from an external method or an assumed constraint to determine the P-T of mineral entrapment conditions on an isomeke (Anzolini et al, 2019;Ashley et al, 2014Ashley et al, , 2016Barkoff et al, 2017;Bayet, John, Agard, Gao, & Li, 2018;Castro & Spear, 2017;Enami et al, 2007;Endo et al, 2012;Kouketsu, Hattori, Guillot, & Rayner, 2016;Kouketsu, Enami, & Mizukami, 2010;Nestola et al, 2018;Soret et al, 2019;Spear, Thomas, & Hallett, 2014;Taguchi, Enami, & Kouketsu, 2016Wolfe & Spear, 2018;Zhong et al, 2019). For example, pseudosection modelling uses bulk rock geochemistry and mineral chemical compositions to reveal the ranges of stable P-T conditions where possible mineral assemblage can crystallize at chemical equilibrium.…”

The Greater Himalayan Thrust Belt: Insight Into the Assembly of the Exhumed Himalayan Metamorphic Core, Modi Khola Valley, Central Nepal

“…Garnet cores, which have almost homogenous profiles for major elements, are characterized by high HREE and Y concentrations that gradually decrease toward the rims (Figures 4c–4e). Whereas HREE‐enriched cores with low (Gd/Yb) N values that gradually change into low HREE and high (Gd/Yb) N values toward the rims may reflect Rayleigh fractionation of these elements during garnet growth (Otamendi et al, 2002), the sharp increases in HREE and Y at the outermost rims of some of the grains in these specimens (Figures 4d and 4e) are interpreted to reflect late‐stage subsolidus dissolution/resorption of garnet and/or breakdown of HREE‐bearing minerals across the solidus (Corrie & Kohn, 2008; Soret et al, 2019).…”

“…Trace element compositions of minerals have been widely used to infer the growth and breakdown of coexisting mineral phases in metamorphic systems (e.g., Foster et al, 2000, 2002, 2004; Hermann & Rubatto, 2003; Larson et al, 2018; Pyle & Spear, 1999; Regis et al, 2014; Rubatto et al, 2006; Shrestha et al, 2019). In particular, trace element partitioning between major silicate minerals such as garnet and plagioclase and accessory minerals such as monazite (LREE‐phosphate), xenotime (Y‐rich phosphate), allanite (REE silicate), and apatite (MREE‐rich phosphate) has been used extensively to inform the relative/absolute timing of growth and/or breakdown of those minerals (e.g., Godet et al, 2020; Hermann & Rubatto, 2003; Larson et al, 2018; Rubatto et al, 2006; Rubatto & Chakraborty, 2013; Shrestha et al, 2019; Soret et al, 2019). Moreover, because some trace elements are less prone to high‐temperature diffusion than major elements (Chernoff & Carlson, 1999; Otamendi et al, 2002), they may be able to provide useful information on the metamorphic history in specimens where major elements may have been homogenized.…”

Mesozoic Tectonic Evolution in the Kurgovat‐Vanch Complex, NW Pamir

“…The Rushan‐Pshart suture zone may also have been reactivated during the late Early Cretaceous coeval with a period of fast cooling from ∼110 to 100 Ma recorded by zircon U‐Pb and mica 40 Ar/ 39 Ar geo‐thermo‐chronometers (Schwab et al., 2004). The southernmost Karakoram terrane documents amphibolite facies regional metamorphism and crustal shortening‐exhumation from ∼135 to 115 Ma coeval with ∼130‐80 Ma arc magmatism (Crawford & Searle, 1992; Faisal et al., 2016; Gaetani et al., 1993; Hildebrand et al., 2001; Searle et al., 1990; Soret et al., 2019).…”