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Recalibrating the history of metamorphism and exhumation in the Adirondack Highlands leads to a major revision of Neoproterozoic global paleogeography

Session: Evolution of Orogenic Belts Through Time: Insights from Sedimentation, Deformation, Magmatism, and Metamorphism, Part I

Presenting Author:

Yiming Zhang

Authors:

Zhang, Yiming¹, Hodgin, Eben Blake², Schmitz, Mark D.³, Schwartz, Darin⁴, Mohr, Michael T.⁵, Swanson-Hysell, Nicholas L.⁶

(1) Department of Earth and Environmental Sciences, University of Minnesota, Twin Cities, Minneapolis, MN, USA, (2) Brown University, Providence, RI, USA, (3) Boise State University, Boise, ID, USA, (4) Boise State University, Boise, ID, USA, (5) Boise State University, Boise, ID, USA, (6) University of Minnesota Twin Cities, Minneaoplis, MN, USA,

Abstract:

The ca. 1090-980 Ma Grenvillian orogeny involves a Himalayan-scale continent-continent collision along Laurentia’s present-day eastern margin. Well-exposed high-grade metamorphic rocks in the Adirondack Highlands in the orogenic hinterland have long been a popular natural laboratory for metamorphic petrology and geochronology research. It has previously been considered that the ca. 1090-1020 Ma Ottawan orogenic phase featured high-pressure, high-temperature metamorphism, leading to crustal over-thickening and subsequent orogenic collapse, whereas the ca. 1010-980 Ma Rigolet orogenic phase involved minor crustal reheating and non-deformational metamorphism in the Adirondack Highlands. New U-Pb zircon, garnet, and titanite ID-TIMS geochronology data from the Marcy Massif reveal that high-grade protracted metamorphism occurred through the Rigolet orogenic phase.

Tandem ID-TIMS and LA-ICP-MS data from rutile and apatite of the Marcy Massif provide constraints on the subsequent cooling history. Grain size-date correlations and depth profiles show that both minerals experienced volume diffusion Pb loss, consistent with them recording cooling ages. However, forward modeling shows that none of the previously proposed thermal histories for the Adirondack Highlands is consistent with the observed rutile and apatite dates. By integrating geologic data, petrologic data, and our new geochronologic data, we conduct a probabilistic inversion to reconstruct the post-Grenville thermal history of the Adirondack Highlands. Similar to previous estimates, our results show that the Adirondack Highlands cooled slowly throughout the Neoproterozoic after protracted Grenvillian metamorphism. However, our inversion results indicate that the overall absolute timing of cooling is younger than previous estimates. As a result, the new thermal history models recalibrate the timing of magnetic remanence acquisition of the Marcy anorthositic rocks by nearly 100 million years. Rather than the supercontinent Rodinia moving rapidly to high southerly latitudes as orogenesis was ongoing, the updated age of the Adirondack Highlands paleomagnetic pole is consistent with a slowly moving supercontinent that drifted from equatorial to higher latitudes through the early Tonian Period of the Neoproterozoic Era.