Browsing by Author "Menold, Carrie A."

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    Fluid evolution during burial and exhumation of the Tso Morari UHP complex, NW India: Constraints from mineralogy, geochemistry, and thermodynamic modeling
    (Springer, 2022-12-25) Pan, Ruiguang; Macris, Catherine A.; Menold, Carrie A.; Earth and Environmental Sciences, School of Science
    The Tso Morari terrane within the Himalayan orogenic belt underwent ultrahigh-pressure (UHP) coesite-eclogite metamorphism due to northward subduction of the Indian continent under the Eurasian continent during the early Eocene. The Tso Morari UHP terrane has been intensely studied petrologically, mineralogically, and geochemically over the past several decades. However, the fluid history (e.g., phases and pressure–temperature conditions, fluid compositions and sources, and processes of fluid–rock interactions) and thermal structure during exhumation remain unresolved. To address these issues, we sampled a traverse from the center of an eclogite boudin out into the host orthogneiss. Three major fluid evolution stages (FESs) were identified and characterized using petrography, mineral and bulk-rock chemistry, and thermodynamic modeling. FES 1 constrained mineral dehydration and hydration reactions during prograde metamorphism before reaching peak pressure at 29.0 ± 0.8 kbar and 591 ± 9 °C by modeling garnet growth in the eclogites. FES 2 constrained mineral reactions in the eclogite matrix due to destabilization of internal hydrous minerals. This FES caused the formation of epidote at 22.8 ± 0.6 kbar, amphibole core domains (glaucophane) at 19.0 ± 0.4 kbar, amphibole rim domains (barroisite) at 14.5 ± 1.0 kbar, and symplectite at 9.0 ± 1.0 kbar, during isothermal decompression (600–650 °C). FES 3 caused amphibolization of eclogite at the boudin rim at 625 ± 50 °C and 9.0–14.0 kbar. Metasomatism resulted in increased K2O, CO2, and bulk-rock Fe3+/ΣFe in the amphibolized eclogites. Large ion lithophile elements (LILE) (e.g., K, Rb, Cs, Sr, Ba) and trace element ratios of Ba/Rb and Cs/Rb are also elevated relative to the eclogite core. The fluid most likely originated from dehydrating host orthogneiss and/or metasediments. Thermodynamic modeling also predicts that the Tso Morari complex was exhumed through a low-temperature (< 650 ± 50 °C) regime in the subduction channel
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    Metamorphic P-T Path and Multiple Fluid Events During Burial and Exhumation of the Tso Morari UHP Terrane, NW Himalaya
    (2021-11) Pan, Ruiguang; Macris, Catherine A.; Barth, Andrew P.; Gilhooly, William P. III; Moreno, Max Jacobo; Menold, Carrie A.
    The Tso Morari terrane within the Himalayan orogenic belt underwent ultrahigh-pressure (UHP) coesite-eclogite metamorphism due to northward subduction of the Indian continent under the Eurasian continent during the early Eocene. In this study we optimized a best protocol for thermodynamically modelling pressure-temperature (P-T) paths of high-grade metabasites using the Tso Morari eclogite as a case study through evaluating the effects of employing commonly used thermodynamic modeling techniques (e.g., programs, thermodynamic datasets, a-X relations). A “fishhook” shaped clockwise P-T path was obtained with a peak pressure of ~28.5 kbar at ~563 °C, followed by a peak temperature of ~613 °C at ~24.5 kbar. The peak pressures predicted by modelling protocols are consistent with the conventional thermobarometry results and petrographic observations from the Tso Morari eclogites. Secondly, thermodynamic modelling using P-M(H2O) pseudosections on Tso Morari UHP rocks indicates three distinct fluid events during the prograde and retrograde metamorphism. Fluid Event 1 caused the fluid-assisted homogenization of prograde garnet cores in eclogite at ~18.5 kbar and ~555 °C; Fluid Event 2 is evidenced by the formation of poikiloblastic epidote (~23.5 kbar and ~610 °C, at the expense of lawsonite) and amphibole (from ~19.0 to ~14.5 kbar at ~610 °C, at the expense of omphacite and talc), and symplectite association (~8.7 kbar and ~625 °C) in the eclogite matrix without external fluid supply. Fluid Event 3 was determined through modelling the amphibolitization of eclogites with external fluid infiltration at ~9.0–12.5 kbar and ~608 °C. This fluid phase most likely derived from the mixing of dehydrated host orthogneiss and/or metasediments during exhumation through the amphibolite-facies zone in the subduction channel. This study demonstrates the need for using careful petrographic observations in parallel with thermodynamic modelling to achieve realistic results.
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    Thermodynamic Modeling of High-grade Metabasites: A Case Study Using the Tso Morari UHP Eclogite
    (Springer, 2020) Pan, Ruiguang; Macris, Catherine A.; Menold, Carrie A.; Earth Sciences, School of Science
    Thermodynamic modeling is an important technique to simulate the evolution of metamorphic rocks, particularly the poorly preserved prograde metamorphic reactions. The development of new thermodynamic modeling techniques and availability of updated thermodynamic databases and activity–composition (a–X) relations, call for an evaluation of best practices for modeling pressure–temperature (P–T) paths of metabasites. In this paper, eclogite from the Tso Morari UHP terrane, NW India, is used as a representative metabasite to directly compare the outputs (pseudosections and P–T paths) generated from recent versions of the widely used THERMOCALC and Theriak-Domino programs. We also evaluate the impact of using the most updated thermodynamic database (ds 62, Holland and Powell in J Metamorph Geol 29(3):333–383, 10.1111/j.1525-1314.2010.00923.x, 2011) relative to an older version (ds 55, Holland and Powell in J Metamorph Geol 16(3):309–343, 10.1111/j.1525-1314.1998.00140.x, 1998), and the effect of the user’s choice of mineral a–X relations while considering the effect of garnet fractionation on the rock’s effective bulk composition. The following modeling protocols were assessed: (1) TC33; THERMOCALC version 3.33 with database ds 55 and garnet a–X relations of White et al. (J Metamorph Geol 25(5):511–527, 10.1111/j.1525-1314.2007.00711.x, 2007); (2) TC47; THERMOCALC version 3.47 with database ds 62 and garnet a–X relations of White et al. (J Metamorph Geol 32(3):261–286, 10.1111/jmg.12071, 2014a); (3) TDG; Theriak-Domino with database ds 62 and garnet a–X relations of White et al. (2014a), and (4) TDW; Theriak-Domino with database ds 62 and garnet a–X relations of White et al. (2007). TC47 and TDG modeling yield a similar peak metamorphic P–T of 34 ± 1.5 kbar at 544 ± 15 °C and 551 ± 12 °C, respectively. The results are 5–8 kbar higher in pressure than that determined from TC33 modeling (26 ± 1 kbar at 565 ± 8 °C), and TDW modeling (28.5 ± 1.5 kbar at 563 ± 13 °C). Results indicate that all four modeling protocols generally provide consistent metamorphic phase relations and thermodynamic simulations regarding fractionation of the bulk composition and prograde metamorphism within uncertainty. In all model calculations, the initial bulk composition measured by XRF does not represent the effective bulk composition at the time of garnet nucleation. The choice of garnet a–X relations can affect predictions of peak pressure, regardless of program choice. This study illustrates the importance of careful consideration of which a–X relations one chooses, as well as the need for comparison between modeling predictions and evidence from the geochemistry and petrography of the rock(s) themselves.