NEW INSIGHTS INTO THE TECTONIC HISTORY OF THE MESOPROTEROZOIC BASEMENT OF THE NORTHERN APPALACHIANS: ONE COHERENT TERRANE?

Outliers of basement in the northern Appalachians from New York to Vermont, all contain similar Early to Mid-Mesoproterozoic paragneiss sequences consisting of biotite-quartz- plagioclase gneiss interbedded with quartzite, aluminous quartz-ribbed schist, magnetite quartzite, calc-silicate gneiss and marble, intruded by equally abundant ultramafic, mafic, and tonalitic to granitic gneisses. Recent TIMS and SHRIMP U/Pb zircon ages point to similarities as well as differences of tectonic history among outliers. In Vermont, a distinct east-west belt of pre-Grenville trondhjemitic, tonalitic and dioritic gneisses that include minor enclaves of older paragneiss were intruded continuously from 1426±19 to 1331±6 Ma, in an Andean setting. The youngest age of the paragneiss is given by cross cutting 1313±7 Ma kyanite-bearing granodiorite stocks. Deformation and high-grade metamorphism (YF1) occurred before widespread Elzevirian intrusion of late syntectonic (YF2) 1244±8 and 1221±4 Ma granites. Megacrystic 1172±7 Ma granite sills crosscut gneissic granites and all rocks were deformed again in the Ottawan (YF3) prior to intrusion of sheets of post-tectonic 962±1 Ma rapakivi granite. Albitization, magnetite mineralization, and intrusion of 1036±6 Ma muscovite-albite pegmatites in the northern half of the Green Mountain massif suggest Ottawan retrogression; notably syntectonic Ottawan granite intrusives are absent.

In the Hudson Highlands of New York and New Jersey and Manhattan Prong, SHRIMP ages define widespread post-YF1 intrusion of hornblende granite into paragneisses at 1174±15 Ma, and YF2 pre-Ottawan regional hornblende granulite metamorphism and deformation prior to the 1144±13 Ma intrusion of ferro- and monzodiorite during right-lateral transpressive folding. Ottawan (YF3) compression accompanied intrusion of minor anatectic 1010±16 Ma sillimanite granite in troughs of still tightening north-south synforms. Abundant granite of the Tyringham Gneiss in the Berkshire massif, is interpreted as syn-Ottawan based on an older upper intercept age of 1040-1080 Ma. Whereas Vermont basement is correlated directly with the Adirondacks, great diversity north to south within Grenville and pre-Grenville basement of the Appalachians belies a uniform characterization.