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| Titel |
Comparative chronology of Archean HT/UHT crustal metamorphism |
| VerfasserIn |
Mark Caddick, Besim Dragovic, Victor Guevara |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250147383
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| Publikation (Nr.) |
 EGU/EGU2017-11541.pdf |
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| Zusammenfassung |
| Attainment of high crustal heat fluxes and consequent partial melting is critical to the
stabilization of continental roots. Understanding the processes and timescales behind partial
melting of continental crust in the Archean is thus paramount for understanding
Archean tectonic modes and how stable cratons formed. High-temperature (HT) to
ultrahigh-temperature (UHT) metamorphic rocks can record evidence for dynamic processes
that result in advective heat fluxes and a substantial deviation from normal crustal geothermal
gradients. Examination of the pressure-temperature conditions and timescales of HT/UHT
metamorphism is thus essential to understanding the tectonic processes behind extreme
crust heat fluxes and the formation of stable cratonic crust. Here, utilizing both
traditional and nontraditional petrologic and geochronologic techniques, we compare the
pressure-temperature-time paths of two Neoarchean terranes: the eastern Beartooth
Mountains of the Wyoming Craton and the Pikwitonei Granulite Domain of the Superior
Province.
The Beartooth Mountains of Montana, USA, expose Archean rocks of the Wyoming Craton
that are dominated by an ∼2.8 Ga calc-alkaline granitoid batholith known as the
Long Lake Magmatic Complex (LLMC). The LLMC contains widespread, up to
km-scale metasedimentary roof pendants, with ID-TIMS Sm-Nd garnet geochronology
and laser ablation split stream (LASS) monazite geochronology suggesting that
metamorphism occurred almost 100 Ma after entrainment by the LLMC [1]. Phase equilibria
modeling and Zr-in-rutile thermometry constrain peak pressures and temperatures of
∼6-7 kbar and ∼780–800˚ C. Major element diffusion modeling of garnet suggest
that granulite-facies temperatures were only maintained for a short duration, < 2
Ma.
In contrast, the Pikwitonei Granulite Domain consists of >150,000 km2 of high-grade
metamorphic rocks situated in the NW Superior Province. Phase equilibria modeling and
trace element thermometry constrain peak temperatures in the southernmost part of the
PGD to ∼760˚ C, while across the vast central and western parts of the PGD, peak
temperatures range from 900-1000ºC. LASS monazite and zircon ages, combined
with ID-TIMS zircon and Sm-Nd garnet ages range from ∼2720 Ma to ∼2600
Ma, and combined with the thermometry, suggest that temperatures of >700˚ C
were maintained region-wide for over 100 Ma, and that this was punctuated by
thermal perturbations exceeding 900-950˚ C and occurring over substantially shorter
timescales.
The depths, temperatures and timescales inferred here suggest that although these regions
were experiencing metamorphism within ∼100 Ma of each other, the primary driver for this
metamorphism was different in each case. Timescale of metamorphism might be the most
important constrained parameter here, highlighting the benefit of high resolution isotopic and
geospeedometry approaches.
[1] Dragovic et al., 2016. Precamb. Res., 283, 24-49.
[2] Guevara et al., 2016. AGU abstracts with programs. |
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