Geochemistry of the metamorphosed Ordovician Taconian Magmatic Arc, Bronson Hill anticlinorium, western New England

Kurt Hollocher*, Jon Bull, Peter Robinson

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

42 Citations (Scopus)


The Bronson Hill anticlinorium (BHA) in western New England is a north- to northeast-trending belt of gneiss domes containing a metamorphosed stratified sequence of Ordovician, Silurian, and Devonian sedimentary and volcanic rocks. These overlie mostly late Ordovician intrusive igneous rocks. The intrusive rocks, exposed in the dome centers, are dominated by felsic gneisses of tonalitic to granitic compositions. They are the exposed parts of a composite batholith that developed along the axis of the Taconian volcanic arc. The Fourmile Gneiss of the Pelham dome, and the Monson Gneiss of the Monson dome, covered in most detail here, are calc-alkaline, dominantly tonalitic to granodioritic gneisses having 61-78% SiO2. These are divided into two geochemical groups. The most abundant rocks have low Sr (∼50-200 ppm), high Y (∼10-50 ppm), low Al2O3, high FeO1, commonly negative Eu anomalies, and have flat to somewhat concave upward MREE and HREE patterns. These rocks were probably derived from melting of plagioclase-pyroxene-amphibole granulites in the deep crust. Rocks of the subordinate high-Sr group are similar in composition to adakites and have high Sr (∼300-600 ppm), low Y (∼1-13 ppm), higher Al2O3, lower FeO1, commonly no Eu anomalies, and are strongly depleted in HREE. These rocks were probably derived from a pyroxene-amphibole-garnet ± plagioclase source at higher pressure. Amphibolites are low in abundance in the Fourmile and Monson Gneisses. Most are low-Nb type (<6 ppm Nb), compositionally similar to typical calc-alkaline island arc basalts. These were probably derived by melting of spinel lherzolite in the mantle wedge under the arc. Gabbroic anorthosite and ultramafic rocks also occur in the Monson dome and appear to be cumulates from magmas similar to the low-Nb amphibolites. A high-Nb (6-12 ppm) amphibolite group was identified and is unique to the Monson Gneiss. These rocks are transitional between calc-alkaline and alkaline basalts, are strongly LREE-enriched (La is 46-440 times chondrite), are enriched in strongly incompatible elements, and two of seven samples have normative nepheline. A broader survey of available analytical data from BHA felsic gneisses shows that gneiss compositions vary from dominantly tonalitic and granodioritic in the southern BHA to dominantly granitic from the Croydon and Mascoma domes (west-central New Hampshire) northward. This variation suggests that the southern part of the arc exposed in the BHA was based on mafic crust, whereas the northern part was based on intermediate to felsic crust. Examination of radiometric dates of Taconian igneous rocks, and biostratigraphic constraints in the Taconic allochthons and in the Taconian foreland, are consistent with a model in which the BHA gneisses are the youngest igneous components of the Taconian arc proper. Collision of this composite arc with Laurentia reached a conclusion in the latest Caradoc or Ashgill of the late Ordovician or in the very earliest Silurian. Younger plutons continuing into the Silurian (e.g., Highlandcroft in part) may be related to magmas generated during post-collision delamination or detachment of the subducted lapetus oceanic slab.

Original languageEnglish
Pages (from-to)5-45
Number of pages41
JournalPhysics and Chemistry of the Earth
Issue number1-3
Publication statusPublished - 2002
Externally publishedYes


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