The pre-Atlantic Hf isotope evolution of the east Laurentian continental margin: Insights from zircon in basement rocks and glacial tillites from northern New Jersey and southeastern New York
This paper presents laser ablation U-Pb age and Hf isotope data for zircons from basement rocks and glacial deposits in northern New Jersey and southeastern New York. The purpose is to understand the eastern Laurentian continental margin's Hf isotope record in relation to its geologic evolution prior to the opening of the Atlantic Ocean. The basement samples encompass a Meso- to Neoproterozoic continental margin arc, an anatectic magmatic suite, as well as a Late Ordovician alkaline igneous suite emplaced during post-orogenic melting of the lithospheric mantle. Additional samples were collected from terminal moraines of two Quaternary continental ice sheets. Across the entire dataset, zircons with ages corresponding to the timing of continental margin arc magmatism (~1.4Ga to ~1.2Ga) have positive ε Hf(initial) values that define the more radiogenic end of a crustal evolution array. This array progresses towards more unradiogenic ε Hf(initial) values along a series of low 176 Lu/ 177 Hf (0.022 to 0.005) trajectories during subsequent anatectic magmatism (~1.2Ga to ~1.0Ga) and later metamorphic and metasomatic re-working (~1.0Ga to ~0.8Ga) of the continental margin arc crust. In contrast, nearly chondritic ε Hf(initial) values from the Late Ordovician alkaline magmas indicate that the Laurentian margin was underlain by a re-fertilized mantle source. Such a source may have developed by subduction enrichment of the mantle wedge beneath the continental margin during the Mesoproterozoic. Additionally, preliminary data from a metasedimentary unit of unknown provenance hints at the possibility that some of the sediments occupying this portion of the Laurentian margin prior to the Ordovician were sourced from crust older than ~1.9Ga.
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