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Trondhjemite and metamorphosed quartz keratophyre tuff of the Ammonoosuc volcanics (Ordovician), western New Hampshire and adjacent Vermont and Massachusetts.

Geological Society of America Bulletin

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Abstract

These volcanic rocks consist of a lower, mainly mafic unit of hornblende-plagioclase amphibolite and an upper, mainly felsic metamorphosed quartz keratophyre tuff. They are intruded by sills, dykes and plugs of trondhjemite; which is highly silicic (SiO2, 73-81%), low in Al2O3 (11.3-13.5%) and generally contains <1% K2O. Both trondhjemite and volcanics are calc-alkaline. The major- and minor-element geochemistry of the trondhjemites is closely similar to that of the quartz keratophyre tuff. These rocks were probably produced by partial melting of basaltic source rocks, rather than by fractional crystallization, in view of the virtually bimodal nature of the Ammonoosuc assemblage. The generation of the felsic rocks occurred at deeper levels along a subduction zone dipping eastward.-L.C.H.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Trondhjemite and metamorphosed quartz keratophyre tuff of the Ammonoosuc volcanics (Ordovician), western New Hampshire and adjacent Vermont and Massachusetts.
Series title:
Geological Society of America Bulletin
Volume
96
Issue:
12
Year Published:
1985
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Geological Society of America Bulletin
First page:
1493
Last page:
1507