The Montana metasedimentary terrane in the northern Wyoming Province provides valuable insight into crustal formation and reworking processes along the cratonic margin and offers a unique opportunity to decipher the complex Neoarchean−Paleoproterozoic terrane assembly in southwestern Laurentia. We report new zircon U-Pb dates and Hf isotopes from seven metaigneous samples in the northwestern Montana metasedimentary terrane. The internal textures of zircon in this study are complex; some lack inherited cores and metamorphic overgrowths, while others exhibit core-rim relationships. Based on the cathodoluminescence (CL) features, we interpret these grains to be magmatic populations. These data demonstrate discrete igneous pulses at 2.7 Ga, 2.4 Ga, and 1.7 Ga, which indicate significant crustal formation intervals in the Montana metasedimentary terrane. Zircons at 2.7 Ga have positive εHf values (+2.4 to +0.9) that indicate a depleted mantle source. Most 2.4 Ga and 1.7 Ga samples have negative εHf values (−1.6 to −15.5), which indicate significant contributions from preexisting crust. Two 1.7 Ga samples, however, have near-chondritic εHf values (+0.4 to +0.3) that indicate larger juvenile contributions. The time-integrated Hf isotope trend suggests that the Paleoproterozoic zircons were produced from a mixture of older crust and juvenile mantle inputs. Additionally, the isotopic age fingerprint of the Montana metasedimentary terrane suggests that it differs from northern-bounding terranes. Viewed more broadly, the 2.7 Ga and 1.7 Ga age peaks that the Montana metasedimentary terrane shares with the global zircon age spectrum suggest that the drivers of these events in the Montana metasedimentary terrane were common throughout the Earth and may be associated with the assembly of supercontinents Kenorland and Nuna.
Orogenic to postorogenic (1.20-1.15 Ga) magmatism in the Adirondack Lowlands and Frontenac terrane, southern Grenville Province, USA and Canada
Magmatism in the southern Grenville Province records a collisional and postcollisional history during the period 1.20–1.15 Ga in the Adirondack Lowlands (New York State, USA) and the Frontenac terrane (Ontario, Canada). The 1.20 Ga bimodal Antwerp-Rossie suite of the Adirondack Lowlands was produced by subduction in the Trans-Adirondack backarc basin. This was followed by intrusion of the 1.18 Ga alkalic to calc-alkalic Hermon granite, which may have been generated by melting of metasomatized mantle during collision of the Adirondack Lowlands and Frontenac terrane during the Shawinigan orogeny. The Hyde School gneiss plutons intruded the Adirondack Lowlands at 1.17 Ga, and Rockport granite intruded into the Adirondack Lowlands and Frontenac terrane, stitching the Black Lake shear zone, which marks the boundary between these terranes. Subsequent extensional collapse and lithospheric delamination caused voluminous anorthosite-mangerite-charnockite-granite plutonism. In the Frontenac terrane, this event is represented by the 1.18–1.15 Ga Frontenac suite, which is composed predominately of ferroan granitoids produced from melting of the lower crust by underplating mafic magmas. The Edwardsville, Honey Hill, and Beaver Creek plutons are newly recognized members of this suite in the Adirondack Lowlands. High oxygen isotope ratios of this suite in the central Frontenac terrane and western Adirondack Lowlands point to the presence of underthrust altered oceanic rocks in the lower crust. Oxygen isotopes of the Frontenac suite in both terranes preclude its derivation from mantle melts alone.
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- Award ID(s):
- 0942447
- NSF-PAR ID:
- 10539198
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- Geosphere
- Volume:
- 9
- Issue:
- 6
- ISSN:
- 1553-040X
- Page Range / eLocation ID:
- 1637 to 1663
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Ganderia and the Southeastern New England Avalon terrane are both terranes that rifted from Gondwana and accreted to North America in the early to mid-Paleozoic. Accretion of the Avalon terrane was accompanied by plutonism, deformation, and metamorphism including partial melting within the Nashoba terrane, the trailing edge of Ganderia, and may be interpreted as indicators for mid- to lower-crustal channel flow. Channel flow describes the flow of weak, partially molten material between more competent crust as a result of pressure gradients in the mid- to lower crustal levels. Such flow should typically result in seismic anisotropy due to the crystallographic preferred orientations of minerals and shape preferred orientations at various scales. Here, we present first results for the crustal anisotropic structure beneath the Nashoba terrane that were produced with a newly developed approach from currently collected data in the region. To investigate the hypothesis of crustal flow during the orogenic history of Southeastern New England, we deployed a dense profile of 6 broadband seismic stations crossing the Nashoba terrane. We analyze the harmonic variation of amplitudes in teleseismic P-Receiver Functions (RFs) to identify interfaces of isotropic and anisotropic contrasts within the crust. In the case of particularly prominent anisotropic features that have significantly larger amplitudes than other signals, it is feasible to derive quantitative constraints on the strength and orientation of the anisotropy. However, with growing complexity, a classical forward modelling or grid search approach becomes unfeasible. These difficulties can be mitigated by applying Bayesian inversion, which infers values of model parameters from a probabilistic perspective. Applying a Bayesian inversion to the harmonically decomposed RFs has the potential to infer complex anisotropic seismic structures. We find evidence for two crustal anisotropic layers with confined properties to the geologic units of the Ganderia, Nashoba and Avalon terranes that might be related to episodes of lower crustal flow. In addition, we identify anisotropy at shallow mantle levels beneath the Nashoba and Avalon Terranes possibly indicating the upper interface of a shallow asthenosphere in the region.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1130/GES00879.1
