Titanite petrochronology of the Pamir gneiss domes: Implications for middle to deep crust exhumation and titanite closure to Pb and Zr diffusion

Abstract

The Pamir Plateau, a result of the India-Asia collision, contains extensive exposures of Cenozoic middle to lower crust in domes exhumed by north-south crustal extension. Titanite grains from 60 igneous and metamorphic rocks were investigated with U-Pb + trace element petrochronology (including Zr thermometry) to constrain the timing and temperatures of crustal thickening and exhumation. Titanite from the Pamir domes records thickening from~44 to 25 Ma. Retrograde titanite from the Yazgulem, Sarez, and Muskol-Shatput domes records a transition from thickening to exhumation at~20-16 Ma, whereas titanite from the Shakhadara dome records prolonged exhumation from~20 to 8 Ma. The synchronous onset of exhumation may have been initiated by breakoff of the Indian slab and possible convective removal of the Asian lower crust and/or mantle lithosphere. The prolonged exhumation of the Shakhdara and Muztaghata-Kongur Shan domes may have been driven by continued rollback of the Asian lithosphere concurrent with shortening and northwestward translation of the Pamir Plateau.

Protracted thrusting followed by late rapid cooling of the Greater Himalayan Sequence, Annapurna Himalaya, Central Nepal: Insights from titanite petrochronology

“…Experiments (Cherniak, , ) imply diffusion distances of 400 μm for Zr and 1,800 μm for Pb at 800°C for 10 Ma. However, recent studies of natural samples have documented retention of Zr and radiogenic‐Pb at temperatures of ~800°C on geological time‐scales (Gao, Zheng, Chen, & Guo, ; Kohn & Corrie, ; Spencer et al., ; Stearns, Hacker, Ratschbacher, Rutte, & Kylander‐Clark, ). In the present study, Zr concentration steps are preserved between sectors on spatial scales of a few microns, on time‐scales of 5–10 Ma, and at temperatures of ~765°C (Figure , KN14‐51A).…”

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

“…This exhumation was spatially and coevally associated with crustal anatexis throughout the South and Central Pamir (Figure e; for example, Hacker et al., ; Robinson et al., ; Schwab et al., ; Smit et al., ; Stübner et al., ), and in the Chitral region by the intrusion of the Garam Chasma leucogranite within the highest grade rocks (Figure e; for example, Faisal et al., ; Hildebrand et al., ). However, in contrast to the metamorphic domes of the Pamir, where normal‐sense shear zones actively exhumed material in the Miocene (Hacker et al., ; Rutte, Ratschbacher, Khan, et al., ; Rutte, Ratschbacher, Schneider, et al., ; Stearns et al., ; Stübner et al., ), no such structures have been recognized in the Chitral area that could have accommodated rapid exhumation between 28 and 23 Ma. We, therefore, suggest that early exhumation from 28 to 23 Ma was likely related to the reactivation of the Tirich Mir fault, as well as local smaller reverse faults (such as the one partly bounding the Garam Chasma pluton and the western sillimanite‐grade zone; Figure e).…”

“…Final and rapid exhumation/cooling (~50°C/km from 20 to 8 Ma; Faisal et al., ) may have been related to break off of the Indian slab at 25–20 Ma and the subsequent underthrusting of the Indian continental margin (DeCelles, Kapp, Quade, & Gehrels, ; Mahéo et al., ; Replumaz, Negredo, Guillot, & Villaseñor, ; Rolland et al., ; Rutte, Ratschbacher, Khan, et al., ; Rutte, Ratschbacher, Schneider, et al., ; Stearns et al., ; Van Hinsbergen et al., ). While in the Miocene the northern part of the South Pamir and the Central Pamir underwent decompression melting during ~N–S extension (Hacker et al., ; Rutte, Ratschbacher, Khan, et al., ; Rutte, Ratschbacher, Schneider, et al., ; Stearns et al., ), the rest of the South Pamir and the Karakoram experienced ~N–S crustal thickening and anatexis (e.g. Chapman, Robinson, et al., ; Chapman, Scoggin, et al., ; Hacker et al., ; Stearns et al., ).…”

“…Unlike the Tibetan plateau, the Karakoram–Hindu Kush–Pamir belt has, during the Cenozoic, undergone high erosion/tectonic denudation rates in response to collision of the Asian margin with the Indian plate (Robinson et al., ; Rutte, Ratschbacher, Schneider, et al., ; Schmidt et al., ; Searle, ; Stearns, Hacker, Ratschbacher, Rutte, & Kylander‐Clark, ; Stübner et al., ; Zhuang et al., , ). This allows the direct study of the tectonic evolution of metamorphic and magmatic rocks residing at mid‐crustal conditions, and the crustal foundering that affected its root zone during this time (Hacker et al., ; Kooijman, Smit, Ratschbacher, & Kylander‐Clark, ), especially in the Central Pamir and Karakoram blocks (Fraser, Searle, Parrish, & Noble, ; Mahéo, Blichert‐Toft, Pin, Guillot, & Pêcher, ; Palin, Searle, Waters, Horstwood, & Parrish, ; Rolland, Mahéo, Guillot, & Pêcher, ; Rolland, Villa, Guillot, Mahéo, & Pêcher, ).…”

2. Phase Relations, Reaction Sequences and Petrochronology

“…Titanite can be directly dated using U-Pb geochronology, but there is some uncertainty 173 about the closure temperature for Pb, which is generally considered to be around 600°C (Warren 174 et al 2012; Spencer et al 2013;Stearns et al 2015; Kirkland et al 2016), depending on cooling 175 rate. Diffusion profiles of trace elements in titanite can be used to determine the timing and 176 duration of cooling (e.g., geospeedometry) and this is covered in more detail in Kohn (2017, this 177 volume).…”