GEOCHRONOLOGY OF ANORTHOSITE IN THE NEW JERSEY HIGHLANDS AND HONEY BROOK UPLAND (PA)

The timing and nature of anorthosite-suite magmatism is an essential part of understanding Mesoproterozoic plate tectonics, especially in the Grenville Province, where anorthosite-suite rocks make up a large percentage of currently exposed rocks. We present new zircon U-Pb ages of anorthosite and granitic rocks in the New Jersey Highlands (NJH) and the Honey Brook Upland, PA (HBU), Grenville inliers in the Appalachians interpreted to be correlative to rocks in the Adirondacks and the Quebec Grenville Province.

NJH anorthosites crop out in two nearby locations in the central part of the inlier. The 2.5 x 0.5 km Minnisink Road anorthosite is made up of Hb + Pl ± Cpx ± Ap ± Mag, and the 7 km x 60 m Mase Mountain anorthosite is Bt + Pl ± Hb ± Cpx ± Grt. Both bodies display a penetrative metamorphic foliation and are deformed by folds that plunge to the northeast, parallel to regional folds. The Minnisink Road anorthosite and the Mase Mountain anorthosite have crystallization ages of ca. 1150 Ma and ca. 1180 Ma, respectively, and they also record Ottawan and Rigolet metamorphic ages. The Minnisink Road anorthosite is cross-cut by an undeformed quartz-syenite and is adjacent to a mapped pyroxene monzonite unit, both of which are late- to post-tectonic (ca. 1010 Ma). The Honey Brook anorthosite contains Hb + Pl ± Mag ± Ap ± Bt ± Grt and is a foliated 10 x 6 km pluton located in eastern PA. It has an emplacement age of ca. 1070 Ma and is cross-cut by a younger, post-orogenic pegmatite (ca. 1000 Ma). Another nearby granitic body previously thought to be related to the anorthosite is older (ca. 1250 Ma), with metamorphic dates of ca. 1010 Ma and 1055 Ma.

Anorthosite magmatism in Grenville inliers of the northern Appalachians is coeval with that in the Adirondacks and Quebec. The NJH anorthosites share the 1175–1140 Ma ages of the Marcy massif, other anorthosites in the Adirondacks, and the Morin and Lac Saint-Jean anorthosites. The Honey Brook anorthosite’s age is similar to the 1060–1050 Ma Saint-Urbain and Lac Allard anorthosites. None of the granitic bodies dated in this study have the same age as nearby anorthosite. These new data confirm geologic links and perhaps tectonic setting between the NJH and HBU and the Adirondacks and Quebec Grenville Province during Geon 10 and 11.