Skip to Main Content

Sensitive high-resolution ion microprobe (SHRIMP) analyses of zircons from four charnockitic gneisses and a ferrodiorite dike belonging to the anorthosite-mangerite-charnockite-granite (AMCG) suite of the Adirondack Mountains are consistent with prior multigrain U-Pb zircon determinations that indicated an emplacement age of ca. 1150–1160 Ma for other units of the suite. This result differs from recent assertions that Adirondack anorthosites were emplaced at ca. 1050 Ma. Each of the four charnockites discussed in this paper is intimately associated with adjacent massif anorthosite, with which they commonly exhibit mutually crosscutting and usually gradational relationships. Xenoliths of anorthosite in the charnockitic rocks provide a minimum age for the associated anorthosite. The presence of xenocrysts of blue-gray andesine in the charnockites reflects plucking from a coeval, but not wholly consolidated, anorthositic magma.

Ages determined for the four charnockitic rocks are 1176 ± 9 Ma (Minerva), 1154 ± 17 Ma (Gore Mountain), 1174 ± 25 Ma (Snowy Mountain), and 1164 ± 11 Ma (Diana Complex). Results also indicate an age of 1156 ± 7 Ma for a ferrodiorite dike that cross-cuts, and is thought to be comagmatic with, the Oregon Dome anorthosite massif. These five new ages fall within a restricted interval of ca. 1150–1170 Ma that is in close agreement with similar AMCG magmatism in the central and eastern Grenville Province and, as a result, reinforce the proposal that this magmatism took place in response to orogen collapse and delamination following a widespread collisional event at ca. 1200–1160 Ma.

You do not currently have access to this chapter.

Figures & Tables

Contents

GeoRef

References

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal