Zircon and monazite geochronology in the Palmer zone of transpression, south-central New England, USA: Constraints on timing of deformation, high-grade metamorphism, and lithospheric foundering during late Paleozoic oblique collision in the Northern Appalachian orogen

D. P. Moecher, J. K. McCulla, M. A. Massey

Research output: Contribution to journalArticlepeer-review

2 Scopus citations

Abstract

Middle to late Paleozoic high-angle (45°) oblique convergence between Laurentia and composite Avalon terranes resulted in crustal shortening of -5:1 across the Central Maine and Bronson Hill zones of the southern New England Appalachians (USA). The Palmer zone of transpression illustrates the midcrustal expression of magmatism, metamorphism, and ductile deformation that developed in response to oblique convergence in the apparent absence of subduction. Secondary ion mass spectrometry zircon U-Pb and monazite Th-Pb ages, supplemented by zircon laser-ablation-inductively coupled plasma-single-collector mass spectrometry U-Pb ages, expand the geochronology constraining the evolution of the Palmer zone of transpression system and potential models for transcurrent collisional tectonics. Ordovician, Silurian, and Early Devonian plutons, and regional high-grade metapelitic country rocks that comprise the pre-transpressional crustal infrastructure are the same age as lithologic equivalents to the north and northeast along the orogen that did not experience high-angle oblique convergence. All plutons within the Palmer zone of transpression and adjoining areas were deformed (flattened, boudinaged, attenuated, folded, and/or duc-tilely sheared) by the regional transpression system. The most widespread magmatic event in the study area, which is not observed elsewhere in the Northern Appalachians, is intrusion of a diorite-tonalite suite at 370360 Ma, the oldest rocks of which contain granulite-facies mineral assemblages and all of which are deformed. Leucopegma-tites that intrude pelitic paragneisses, which likely formed in response to high-grade metamorphism and which are all deformed, are 370-355 Ma in age. All metapelitic paragneisses contain foliations and linea-tions that are synkinematic with retrograde garnet + K-feldspar sillimanite + biotite assemblages and fabrics that formed during vertical and lateral crustal escape in response to extreme shortening. Contemporaneous diorite-tonalite magmatism and regional high-grade metamorphism are interpreted to reflect regional-scale heating of the crust preceding inception of the transpressional system. Transpressional deformation of both plutons and paragneisses indicates shortening commenced after crustal thermal conditions peaked (ca. 355 Ma). Monazite in paragneiss, the formation of which in most samples is linked texturally to formation of transpressional fabrics, yielded a continuum of Th-Pb ages of ca. 360-330 Ma, indicating transpression continued after peak thermal conditions. The extreme crustal shortening of the Bronson Hill and Central Maine zones from Maine to southern New England, pre-transpressional magmatism, and high-grade metamorphism overprint an Acadian (ca. 400-370 Ma) magmatic-metamorphic infrastructure that occurred in the absence of subduction. The transpressional system exhibits all predicted thermal, magmatic, deformation, metamorphic, and exhumation/erosion characteristics of mantle lithospheric foundering (delamination, detachment, or drip) in response to extreme lithospheric shortening and vertical stretching at ca. 375-370 Ma, leading to advection of heat to the lower con- tinental crust and attendant magmatic and metamorphic responses over the time span 370-355 Ma.

Original languageEnglish
Pages (from-to)1021-1038
Number of pages18
JournalBulletin of the Geological Society of America
Volume133
Issue number5-6
DOIs
StatePublished - May 2021

Bibliographical note

Funding Information:
This work was supported by National Science Foundation grants EAR-1322047 and EAR-1551342; U.S. Geological Survey EDMAP grants 06HQAG0066, 08HQAG0036, EAR-1624663, G10AC00259, and G13AC00100; and the Ferm and Brown-McFarlan funds of the University of Kentucky Department of Earth and Environmental Sciences. We thank Axel Schmitt, Ming-Chang Liu, and Matt Wielicki in the University of California-Los Angeles Keck Secondary Ion Mass Spectrometry (SIMS) laboratory for patient instruction on use of the CAMECA ims1270 and ims1290 instruments; Mark Pecha and Nicky Geisler of the University of Arizona LaserChron laboratory for guidance during zircon U-Pb laser-ablation-induc-tively coupled plasma-mass spectrometry (LA-ICP-MS) analysis; and Mike Williams and Bob Wintsch for insightful manuscript reviews.

Funding Information:
This work was supported by National Science Foundation grants EAR-1322047 and EAR-1551342; U.S. Geological Survey EDMAP grants 06HQAG0066, 08HQAG0036, EAR-1624663, G10AC00259, and G13AC00100; and the Ferm and Brown-McFarlan funds of the University of Kentucky Department of Earth and Environmental Sciences. We thank Axel Schmitt, Ming-Chang Liu, and Matt Wielicki in the University of California–Los Angeles Keck Secondary Ion Mass Spectrometry (SIMS) laboratory for patient instruction on use of the CAMECA ims1270 and ims1290 instruments; Mark Pecha and Nicky Geisler of the University of Arizona LaserChron laboratory for guidance during zircon U-Pb laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analysis; and Mike Williams and Bob Wintsch for insightful manuscript reviews.

Publisher Copyright:
© 2021 Geological Society of America. All Rights Reserved.

ASJC Scopus subject areas

  • Geology

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