DIRECT DATING OF ADIRONDACK ANORTHOSITE BY U-PB SHRIMP TECHNIQUES: IMPLICATIONS FOR AMCG GENESIS

Proterozoic massif anorthosites are commonly associated with granitoids forming anorthosite-mangerite-charnockite-granite (AMCG) suites. Observations suggest they are coeval, but dating of anorthosites suffers from paucity of igneous zircons. SHRIMP dating has resolved this problem and made the small populations of zircons adequate for determination of U-Pb zircon ages. In the Adirondacks, eight anorthosite samples, one ferrodiorite, and one olivine gabbro yield an average age of 1155 ± 6 Ma (MSWD=0.14, Prob=0.999). Fourteen associated granitoids have been dated (SHRIMP-8, TIMS-6) and yield an average age of 1158 ± 5 Ma (MSWD=0.86, Prob.=0.6), thus establishing the coeval nature of the suite and having significant genetic and tectonic implications. It is generally agreed that anorthosites evolve from gabbros by fractionation at P ~10-14 kb where intermediate plagioclase crystallizes and floats while olivine and mafic minerals sink. An appropriate site for this process is the mantle-crust boundary where heat of crystallization and elevated isotherms melt the lower crust producing granitoid, and perhaps Al-rich gabbroic magmas. Such sites should also be devoid of 1) rapid rifting causing pre-differentiation ascent of parental gabbros, or 2) horizontal contraction that might cause hybridization and termination of fractionation. Recently, zircon dating in the Grenville Province has indicated that AMCG suites and contractional orogenesis are not mutually exclusive; instead, the magmas are late- to post- orogenic. In the case of the Adirondack, Morin, and Lac St-Jean AMCG complexes, emplacement followed orogeny at ca 1200-1150 Ma. We propose that this timing reflects delamination of an overthickened orogen resulting in emplacement of hot, new athenosphere to the base of the crust accompanied by dominance of buoyancy forces and orogen uplift. This provides the quiescence required for evolution of plagioclase-rich crystal mush and lower crustal melts that ascend to form coeval AMCG suites. Upon ascent, anorthositic magmas interact (AFC) with lower crustal restites to acquire appropriate Sr, O, Sm-Nd, and Re-Os geochemical signatures. Slightly older AMCG granitoid ages suggest a coeval, but not comagmatic, relationship with anorthosites.