Thermodynamic modeling is an important technique to interpret metamorphic phase relations and calculate model pressure-temperature (P-T) paths for metamorphic rocks. This study uses representative, coesite-bearing eclogites from the Tso Morari UHP terrane of the NW Himalaya to simulate its prograde metamorphism using multiple modeling programs and thermobarometry.
Our modeling yields a peak metamorphism P-T of ~32-33 kbar and ~560-570 °C by the THERMOCALC345 and Theriak-Domino programs (Green et al., 2016), which is ~5 kbar higher in pressure and ~15 °C lower in temperature than that determined by using THERMOCALC333 (White et al., 2007) (~27.8 kbar and ~580 °C). The significantly higher pressure obtained using the THERMOCALC345 and Theriak-Domino is likely a result of the upgrade of thermodynamic parameters of minerals (i.e. garnet Wpy-gr and agr) in the newer a-x relations. The modeled effective bulk compositions and mineral stabilities along the calculated P-T path show different patterns under the two modeling techniques. Modeling by the Theriak-Domino programs is preferred in this case because the results are more consistent with the measured mineral compositions of our rocks. Multiple thermobarometers by garnet-omphacite-phengite, garnet-omphacite, garnet-phengite on the garnet rim, high-Si phengite and matrix omphacite yield a peak metamorphism of ~ 28.5-29.0 kbar and ~ 650-728 °C, which is generally consistent with the modeled P-T path.
Based on our model calculations, the initial bulk composition measured by XRF does not represent the reactant bulk composition at the time of garnet nucleation, and this compositional discrepancy possibly is caused by the crystallization of pre-garnet minerals (i.e. hematite), reaction overstepping, or partial reequilibration. In summary, by implementing and evaluating multiple modeling strategies and considering the petrography and metamorphic mineralogy of the rocks, this study finds that the eclogite modeling using Theriak-Domino programs in the Tso Morari terrane provide more consistent metamorphic phase relations and more reasonable thermodynamic simulations regarding fractionation of the bulk composition and prograde metamorphism.
References:
Green et al. J Metamorph Geol 34, 845-869 (2016)
White et al. J Metamorph Geol 25, 511-527 (2007)
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Preferred orientations of garnet porphyroblasts reveal previously cryptic templating during nucleation
Abstract Electron back scattered diffraction data of garnet crystals from the Nelson Aureole, British Columbia and the Mosher’s Island formation, Nova Scotia, reveals that 22 garnet crystals are all oriented with one of three crystal directions parallel to the trace of the foliation plane in thin section. Structural models suggest that these relationships are due to preferential garnet nucleation onto muscovite, with the alignment of repeating rows of Al octahedra and Si tetrahedra in each leading to inheritance of garnet orientation from the muscovite. These results highlight that epitaxial nucleation may be a prevalent process by which porphyroblast minerals nucleate during metamorphism and carry implications for the role that non-classic nucleation pathways play in the crystallization of metamorphic minerals, the distribution of porphyroblasts in metamorphic rocks, and, in cases in which nucleation is the rate limiting step for crystallization, the energetics of metamorphic reactions.
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- NSF-PAR ID:
- 10262184
- Date Published:
- Journal Name:
- Scientific Reports
- Volume:
- 11
- Issue:
- 1
- ISSN:
- 2045-2322
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Metamorphic rocks from the Connecticut Valley Trough (CVT), Vermont, and Massachusetts, have been examined using quartz‐in‐garnet (QuiG) and conventional thermobarometry, thermodynamic reaction modelling, diffusion modelling, and40Ar/39Ar thermochronology to constrain their
P–T–t paths during Acadian metamorphism and subsequent exhumation. Numerous samples, collected in the vicinity of the Acadian domes, contain garnet porphyroblasts that display cloudy zones characterized by numerous fluid inclusions and modified garnet compositions associated with the replacement of the original garnet by biotite±muscovite±plagioclase±quartz±lowX grs/enrichedX sps. QuiG and conventional thermobarometry constrain both the conditions of garnet nucleation and peakP–T conditions to have occurred at ~0.85–1.05 GPa, ~550–600°C. Most notably, QuiG barometry was performed on inclusions adjacent to these reaction zones in conjunction with Gibbs method reaction modelling to reveal that these dissolution–reprecipitation reactions occurred during nearly isothermal decompression from the peakP–T conditions to around ~0.3 GPa, 550°C. Diffusion modelling reveals that the Mn zoning profiles created during garnet resorption that accompanied decompression formed in less thanc . 3 Ma, which constrains the tectonic exhumation to have occurred at 8–10 mm/year. Subsequent cooling to 500°C occurred rapidly at a rate of 100°C/Ma, followed by slower cooling reaching 1.7°C /Ma by the mid Carboniferous. This is the first reported example of QuiG barometry revealing a multi‐stage metamorphic history and highlights the utility of this method for unravelling complex metamorphic terranes. -
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Abstract The southern US and northern Mexican Cordillera experienced crustal melting during the Laramide orogeny (c. 80–40 Ma). The metamorphic sources of melt are not exposed at the surface; however, anatectic granites are present throughout the region, providing an opportunity to investigate the metamorphic processes associated with this orogeny. A detailed geochemical and petrochronological analysis of the Pan Tak Granite from the Coyote Mountains core complex in southern Arizona suggests that prograde metamorphism, melting, and melt crystallization occurred here from 62 to 42 Ma. Ti-in-zircon temperatures (TTi-zr) correlate with changes in zircon rare earth elements (REE) concentrations, and indicate prograde heating, mineral breakdown, and melt generation took place from 62 to 53 Ma. TTi-zr increases from ~650 to 850 °C during this interval. A prominent gap in zircon ages is observed from 53 to 51 Ma and is interpreted to reflect the timing of peak metamorphism and melting, which caused zircon dissolution. The age gap is an inflection point in several geochemical-temporal trends that suggest crystallization and cooling dominated afterward, from 51 to 42 Ma. Supporting this interpretation is an increase in zircon U/Th and Hf, a decrease in TTi-zr, increasing zircon (Dy/Yb)n, and textural evidence for coupled dissolution–reprecipitation processes that resulted in zircon (re)crystallization. In addition, whole rock REE, large ion lithophile elements, and major elements suggest that the Pan Tak Granite experienced advanced fractional crystallization during this time. High-silica, muscovite± garnet leucogranite dikes that crosscut two-mica granite represent more evolved residual melt compositions. The Pan Tak Granite was formed by fluid-deficient melting and biotite dehydration melting of meta-igneous protoliths, including Jurassic arc rocks and the Proterozoic Oracle Granite. The most likely causes of melting are interpreted to be a combination of (1) radiogenic heating and relaxation of isotherms associated with crustal thickening under a plateau environment, (2) heat and fluid transfer related to the Laramide continental arc, and (3) shear and viscous heating related to the deformation of the deep lithosphere. The characteristics and petrologic processes that created the Pan Tak Granite are strikingly similar to intrusive suites in the Himalayan leucogranite belt and further support the association between the North American Cordilleran anatectic belt and a major orogenic and thermal event during the Laramide orogeny.
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Interpretation of geochronological and petrological data from partially-melted granulite is challenging. However, integration of multiple chronometers and mineral assemblage diagrams (MAD) can be used to estimate the nature and duration of processes. Excellent lower-crustal exposures of garnet granulite from the Malaspina Pluton, Fiordland New Zealand provide an ideal place to employ this kitchen sink approach. We use zircon U-Pb ages from LA-ICPMS, SHRIMP-RG, and CA-TIMS, garnet Lu-Hf and Sm-Nd ages, and MAD in order to evaluate local partial melting vs. melt injection, equilibrium volumes, P-T conditions, and the duration of lower crustal thermal events. Host diorite (H), garnet-clinopyroxene reaction zones (GRZ), coarse garnet selvages, and tonalite veins provide a record of intrusion and granulite facies partial melting. Zircon U-Pb ages range from 123 to 107 Ma (all); LA-ICPMS ages contain the entire range; CA-TIMS ages range from 118.30±0.13 to 115.7±0.18 Ma; and SHRIMP-RG ages range from 121.4±2 to 109.8±1.8 Ma. The latter two techniques are interpreted to indicate primary igneous crystallization from ~119 to ~116 Ma and the youngest ~110 Ma ages are interpreted as metamorphic zircon growth. Garnet ages for ~1 cm grains are ~113 Ma (Lu-Hf & Sm-Nd) and record metamorphic growth, and <0.3 mm grains with Sm-Nd ages from 113 to 104 Ma reflect high temperature intracrystalline diffusion and isotopic closure during cooling to amphibolite facies. Zircon trace-element compositions indicate 2 distinct crystallization trends reflecting evolution of primary magma batches. MAD indicate that garnet was not in equilibrium with sampled rock compositions. Instead, garnet shows apparent equilibrium with a modeled mixture of the GRZ and the H and grew in equilibrium with an effective bulk composition that shifted toward the leucosome. This would produce the observed increase in garnet grossular content. We conclude that: Malaspina rocks from Crooked Arm preserve evidence for 2 igneous layers which evolved as discrete magmas, igneous crystallization lasted 2 to 3 m.y., granulite metamorphism peaked ~ 3 m.y. after intrusion, metamorphism lasted ≥3 m.y., cooling occurred at ~20°C/m.y., and granulite minerals equilibrated with a mixture of solid phases and melt at ~14 kbar and 920°C (based on garnet compositions and MAD).more » « less
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Abstract Mineral inclusions are ubiquitous in metamorphic rocks and elastic models for host‐inclusion pairs have become frequently used tools for investigating pressure–temperature (
P–T ) conditions of mineral entrapment. Inclusions can retain remnant pressures () that are relatable to their entrapment P–T conditions using an isotropic elastic model andP–T–V equations of state for host and inclusion minerals. Elastic models are used to constrainP–T curves, known as isomekes, which represent the possible inclusion entrapment conditions. However, isomekes require a temperature estimate for use as a thermobarometer. Previous studies obtained temperature estimates from thermometric methods external of the host‐inclusion system. In this study, we present the firstP–T estimates of quartz inclusion entrapment by integrating the quartz‐in‐garnet elastic model with titanium concentration measurements of inclusions and a Ti‐in‐quartz solubility model (QuiG‐TiQ). QuiG‐TiQ was used to determine entrapmentP–T conditions of quartz inclusions in garnet from a quartzofeldspathic gneiss from Goodenough Island, part of the (ultra)high‐pressure terrane of Papua New Guinea. Raman spectroscopic measurements of the 128, 206, and 464 cm−1bands of quartz were used to calculate inclusion pressures using hydrostatic pressure calibrations (), a volume strain calculation ( ), and elastic tensor calculation ( ), that account for deviatoric stress. values calculated from the 128, 206, and 464 cm−1bands’ hydrostatic calibrations are significantly different from one another with values of 1.8 ± 0.1, 2.0 ± 0.1, and 2.5 ± 0.1 kbar, respectively. We quantified elastic anisotropy using the 128, 206 and 464 cm−1Raman band frequencies of quartz inclusions and stRAinMAN software (Angel, Murri, Mihailova, & Alvaro, 2019, 234 :129–140). The amount of elastic anisotropy in quartz inclusions varied by ~230%. A subset of inclusions with nearly isotropic strains gives an averageand of 2.5 ± 0.2 and 2.6 ± 0.2 kbar, respectively. Depending on the sign and magnitude, inclusions with large anisotropic strains respectively overestimate or underestimate inclusion pressures and are significantly different (<3.8 kbar) from the inclusions that have nearly isotropic strains. Titanium concentrations were measured in quartz inclusions exposed at the surface of the garnet. The average Ti‐in‐quartz isopleth (19 ± 1 ppm [2 σ ]) intersects the average QuiG isomeke at 10.2 ± 0.3 kbar and 601 ± 6°C, which are interpreted as theP–T conditions of quartzofeldspathic gneiss garnet growth and entrapment of quartz inclusions. TheP–T intersection point of QuiG and Ti‐in‐quartz univariant curves represents mechanical and chemical equilibrium during crystallization of garnet, quartz, and rutile. These three minerals are common in many bulk rock compositions that crystallize over a wide range ofP–T conditions thus permitting application of QuiG‐TiQ to many metamorphic rocks.
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1038/s41598-021-85525-7
