Shrimp U-Pb evidence for a Late Silurian age of metasedimentary rocks in the Merrimack and Putnam-Nashoba terranes, eastern New England

Abstract

Zircon and monazite were extracted from ϳ5 to 10 kg rock samples by routine mineral separation processes, including crushing, pulverizing, Wilfley table concentration of heavy minerals, and purification of mineral fractions using magnetic separation and methylene iodide. Detrital zircons were sprinkled onto double-stick tape to obtain relatively unbiased samples. Individual grains of monazite and titanite were handpicked under a binocular microscope. All samples were mounted in epoxy, ground to nearly half-thickness, and polished using 6 m and 1 m diamond suspension. All grains were digitally imaged in both transmitted and reflected light to identify areas free from inclusions, cracks, and other imperfections. Using the scanning electron microscope, zircons were imaged by cathodoluminescence (CL) to distinguish cores and overgrowths; monazites and titanites were imaged by back-scattered electrons (BSE).U-Pb ages of zircon and monazite were determined using the USGS/Stanford sensitive high resolution ion microprobe-reverse geometry (SHRIMP-RG) at Stanford University, following the methods described in Williams (1998); titanite was analyzed on SHRIMP II at the Research School of Earth Sciences, Australian National University. The area excavated by the primary oxygen ion beam was about 25 to 30 m in diameter for zircon and titanite, and about 15 to 20 m for monazite. 206 Pb/ 238 U ages were corrected for instrument fractionation using zircon standard R33 (419 Ϯ 1 Ma; Black and others, 2004), monazite standard 44069 (424 Ϯ 1 Ma; Aleinikoff and others, 2006) or titanite standard BLR-1 (1047 Ϯ 1 Ma, Aleinikoff and others, 2007). Uranium concentrations are believed to be accurate to Ϯ 20 percent. Raw data were reduced using Squid 1 (Ludwig, 2001) and plotted using Isoplot 3 (Ludwig, 2003). For detrital zircons from metasedimentary rocks, U-Pb data are plotted on Tera-Wasserburg concordia diagrams to display degree of discordance. Analyses of detrital zircons that are less than 10 percent discordant (table 1) are used to construct relative probability age plots. Specifically, we use the 206 Pb/ 238 U age for zircons younger than about 1200 Ma, and 207 Pb/ 206 Pb ages older than about 1200 Ma. A well-known phenomenon first documented by Williams and Hergt (2000) is that SHRIMP analyses of zircons containing Ͼϳ2500 ppm uranium can result in 206 Pb/ 238 U ages that are too old, presumably due to a change in instrument fractionation (that is, a calibration shift). Data from such high-U concentration, reversely discordant metamorphic zircons with 206

Insights into the Acadian orogeny, New England Appalachians: a provenance study of the Carrabassett and Kittery formations, Maine

“…The earliest sediments deposited in the Central Maine Basin were derived from Laurentia whereas the uppermost formations preserve paleocurrent indicators from the east, suggesting an outboard source thought to be Avalonia Hanson et al 1993;Bradley and Hanson 2002). Because sediments carry the major and trace element, isotopic, and detrital zircon signatures of their source rocks (O'Nions et al 1983;Allegre and Rousseau 1984;Michard et al 1985;McLennan et al 1990;Gleason et al 1994;Anderson and Sampson 1995;Bock et al 1998;Wintsch et al 2007), we undertook this study to determine whether or not the bulk-rock geochemical characteristics and detrital zircon geochronology of easterly derived sediments, i.e., the Carrabassett Formation of Maine, would identify the outboard terrane that collided with Laurentia. We compare these geochemical and geochronological parameters with those of the older formations of the Central Maine Basin in order to make comparisons between the source regions of sediments shed from Laurentia and outboard sources.…”

“…It was originally defined by Katz (1917) to consist of the Kittery, Eliot, and Berwick formations that constitute a thick sequence of calcareous turbidites in the New Hampshire and Maine (Katz 1917;Billings 1956;Bothner et al 1984;Bothner and Hussey 1999). Recently, Wintsch et al (2007) concluded from detrital zircon geochronology that the Berwick Formation was Silurian and Laurentia-derived, whereas the Kittery Formation (middle Ordovician or younger), with its paleocurrent indicators of an eastern source and different detrital zircon signatures, is peri-Gondwanan. They interpreted the fault that separates the Berwick Formation from the Kittery and Eliot formations as a thrust fault that placed the Laurentiaderived Berwick on top of the peri-Gondwana-derived Kittery and Eliot.…”

“…10c have some similarities to the Carrabassett Formation ages, but are of vastly different proportions. The Kittery Formation of the Merrimack Trough has a relative probability plot that is also different from the Carrabassett Formation, even though Wintsch et al (2007) considered the Kittery Formation to be derived from Gondwana. The Kittery Formation has a prominent peak at ~ 1000 Ma whereas the most prominent peak in the Carrabassett Formation is at ~ 1200 Ma.…”

“…Provenance and tectonic setting of the Kittery Formation Wintsch et al (2007) noted that the Kittery Formation contains primarily Mesoproterozoic detrital zircons; only two of their analyses ware younger. Paleocurrent directions from the east and the absence of a significant Ordovician age population in the formation led Wintsch and coworkers to suggest that the formation was derived from peri-Gondwanan sources and deposited in the Fredericton Sea, farther east of the Miramichi arch (Tucker et al 2001).…”

Alleghanian deformation of Cambrian metasedimentary rocks on Avalonia in south-central Rhode Island, USA

“…New England is composed of tectonic blocks, microcontinents, and/or terranes accreted to eastern Laurentia primarily during the Silurian-Devonian Salinic, Acadian, and Neoacadian orogenies, and the PennsylvanianPermian Alleghanian orogeny (O'Hara and Gromet 1985;Goldstein 1989;Skehan and Rast 1990;Wintsch et al 1992Wintsch et al , 2007Rast and Skehan 1993;Hepburn et al 1995;van Staal 2005;Hibbard et al 2006Hibbard et al , 2007. Over the last three decades, the number and extent of these terranes and the timing of their accretion and juxtaposition have been debated.…”