SHRIMP U-PB GEOCHRONOLOGY OF ZIRCON AND SPHENE FROM THE NEW MILFORD QUADRANGLE, EASTERN HUDSON HIGHLANDS, CONNECTICUT

Recent mapping in the New Milford 7.5-minute quadrangle subdivides the Mesoproterozoic crystalline rocks of the easternmost Hudson Highlands massif into several units. Layered quartz-feldspar-biotite gneiss and amphibolite is interpreted as a metavolcanic and metasedimentary sequence, and massive quartz-feldspar-biotite gneiss is presumably of igneous origin. These are intruded by orthogneisses that include granite gneiss and granitic microcline augen gneiss. As a result of Proterozoic and Paleozoic metamorphic and igneous events, zircon and sphene from all samples contain multiple age components, necessitating the use of the sensitive high resolution ion microprobe (SHRIMP) for in situ analysis of ~25-micron areas. Zircons interpreted as igneous in origin yield protolith ages for the orthogneisses, including: 1311 ± 7 Ma (granite gneiss) and 1045 ± 8 Ma (augen gneiss). The granite gneiss is the oldest reliably dated unit in the Hudson Highlands and may link these rocks to similar age rocks in the Green Mountain massif of Vermont. Protolith ages from other gneisses include 1049 ± 13 Ma (massive biotite gneiss) and 1057 ± 10 Ma (biotite gneiss as restite within migmatite). These ca. 1045 - 1060 Ma ages correlate with an Ottawan igneous event. Both the granite gneiss and the augen gneiss contain mappable xenoliths of layered biotite gneiss and amphibolite indicating that the oldest rocks in the area are as yet undated.

    Overgrowths on igneous zircon in all of these units formed at about 990 Ma, in agreement with the age of metamorphic zircon in amphibolite (993 ± 8 Ma). This late Mesoproterozoic age of Grenvillian metamorphism has previously been recognized in the Hudson Highlands. The gneisses are crosscut by foliated gray granite of the Candlewood pluton (443 ± 7 Ma). Migmatization by injection is associated with the Candlewood pluton; the migmatite is composed of pink granite leucosome (444 ± 6 Ma) within layered biotite gneiss indicating that the migmatization is Paleozoic, not Proterozoic. Ages on dark brown sphene from augen gneiss and biotite gneiss are about 430 Ma; light brown sphene from migmatite leucosome, plus overgrowths on brown sphene, are about 400 Ma. As yet, no sphene of Grenville age is found in the basement, suggesting that post-Taconian heating was sufficient to completely reset old sphene in the massif gneisses.