Geochemistry and tectonic implications of Paleozoic metavolcanics from eastern Vermont

Conference · Mon Mar 01 04:00:00 EST 1993 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5688243

Stoll, H M; Karabinos, P ^[1]; Hepburn, J C ^[2]

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1. Williams Coll., Williamstown, MA (United States). Dept. of Geology
2. Boston Coll., Chestnut Hill, MA (United States). Dept. Geology and Geophysics

The Barnard Volcanic and overlying Cram Hill Members of the Missisquoi Fm. (Doll et al., 1961) comprise a thick and varied sequence of metamorphosed mafic to felsic flows, tuffs, sills, and admixed sedimentary rocks in eastern Vermont. U-Pb and evaporation ages on zircon from felsic layers reveal the presence of both Early Ordovician and Silurian volcanics (Karabinos and Aleinikoff, 1991). Geochemistry of 32 samples indicates that the igneous rocks are all subalkalic; they range in composition from basalt through andesite, rhyodacite, and rhyolite according to alkalies vs. SiO[sub 2] diagrams and those using only immobile element ratios, such as Zr/TiO[sub 2] vs. Nb/Y. Al[sub 2]O[sub 3] values of basaltic samples range from 13.3% to 20.8%; many can be classified as high alumina basalts. Zr/Hf ratios of basalts range from 33 to 56 and the average is 39. REE abundances of the basalts show enrichment in the LREE (La[sub N]/Yb[sub N] = 3--6) in some samples, but the majority have La[sub N]/Yb[sub N] ratios near 1 and La[sub N]/Sm[sub N] between 0.6 and 0.8. The intermediate and felsic samples all show LREE enrichment, and appear to be related to the basaltic samples by fractionation. Tectonic discrimination diagrams indicate that the rocks include both tholeiitic and calc-alkaline basalts whose geochemistry is consistent with an origin at a destructive plate margin or in a back arc basin setting. This is supported by MORB-normalized spidergrams'' which show Nb and Ta depletions in all but 2 samples. Felsic rocks likewise plot in the volcanic arc fields on Rb vs. Y + Nb and Nb vs. Y diagrams (Pearce, 1984). Thus the authors interpret these rocks to have formed in an arc or back-arc setting. Existing tectonic models account for Ordovician arc-related volcanism, however the presence of Silurian volcanics suggests post-Taconian tectonic activity along the margin of Laurentia.

• OSTI ID: 5688243
• Report Number(s): CONF-9303211--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 25:2
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
AGE ESTIMATION
CHEMISTRY
DEVELOPED COUNTRIES
GEOCHEMISTRY
GEOLOGIC AGES
GEOLOGIC FORMATIONS
GEOLOGIC MODELS
GEOLOGY
IGNEOUS ROCKS
ISOTOPE DATING
MINERALS
NORTH AMERICA
ORDOVICIAN PERIOD
PALEOZOIC ERA
PETROGENESIS
PETROLOGY
ROCKS
SILICATE MINERALS
SILURIAN PERIOD
USA
VERMONT
VOLCANIC ROCKS
ZIRCON