SHRIMP II GEOCHRONOLOGY OF THE ADIRONDACK AMCG SUITE, IV: TIMING AND DEPTH OF EMPLACEMENT OF ANORTHOSITE IN THE NORTHEASTERN ADIRONDACKS

Anorthositic rocks and calc-silicate skarns provide compelling evidence for timing and depth of anorthosite intrusion in the NE Adirondacks, NY. Results indicate a polymetamorphic history of shallow anorthosite intrusion at ca. 1155 Ma, followed by a granulite facies overprint after 1090 Ma.

The Woolen Mill locality, 1 mi. W of Elizabethtown, exposes mutual cross-cutting relations of anorthosite and metagabbro. Outcrops of pegmatitic anorthosite yielded prismatic zircons characterized by sector and oscillatory zonation and a SHRIMP age of 1151 ± 6 Ma (errors 2s). Zircons from the gabbro contain cores with sector and oscillatory zonation whose ages cluster at 1154 ± 9 Ma. Additional anorthositic rocks yielded similar, but low-U, zircons that give comparable but less precise ages. All ages are comparable to ages of anorthosite and related rocks elsewhere in the Adks. (Hamilton et al. 2002, McLelland et al. 2002).

The 25 km long Willsboro – Lewis skarn belt, just N of Woolen Mill, borders the Westport anorthosite dome and provides field and geochemical evidence that skarns were formed in response to a complex shallow hydrothermal system involving anorthosite. Skarns include wollastonite skarn (Wo + Gt + Cpx) and garnetite. Garnets with oscillatory zonation (from garnetite) preserve a record of meteoric and magmatic (anorthosite) fluid interaction. REE patterns in wollastonite skarn minerals indicate interaction of skarn forming fluids with plagioclase-rich rock (anorthosite). The low d¹⁸O nature (to –2‰ for Gt) of the skarns indicates heated meteoric water involvement during formation. Hydrostatically-pressured meteoric water can’t penetrate into the deep, lithostatically-pressured ductile crust, thus low d¹⁸O values indicate shallow (<10km) skarn formation, and therefore shallow anorthosite intrusion. Deformation features and intermineral isotopic equilibration indicate metamorphism after formation.

Fabrics and assemblages of the Hawkeye granite constrain granulite facies metamorphism of the Ottawan orogeny in the Adks. to be younger than 1090 Ma. Garnets from wollastonite skarn yield an age of ca. 1035 Ma (K. Burton, pers. comm.) reflecting the age of metamorphic recrystallization. In comparison, low-U zircon overgrowths from the Woolen Mill metagabbro document a metamorphic event at ca. 1010 Ma.