Geology and Geochemistry of the Ausable and Wanakena Fayalite-Augite Syenites,
Adirondack Mts., New York
Fayalite-augite syenites are the latest intrusives in the Adirondack region of the Grenville
Province (1047 ±2.2Ma from McLelland et al, 2001) ; accordingly, their petrogenesis provides insight into crustal conditions late in the Grenville Orogeny. Probable origins of these syenites are: (1) partial melt of Anorthosite-Mangerite-Charnockite-Granite suite rocks (AMCG) (McLelland et al., 2001) (2) a facies of the Lyon Mountain Gneiss (LMG) (Whitney et al., 1993).
Spatial, temporal, and geochemical association link s fayalite-augite syenites and LMG (granitic facies – a minimum temperature melt); however, lack of penetrative foliation in the Ausable syenite as well as the observation of deformed entrained enclaves of LMG requires that syenite intrusion occurred after metamorphism of the LMG.
Comparable major and trace element concentrations indicate that fayalite-augite syenites likely formed in similar crustal source regions under comparable P-T conditions. It has proven difficult to explain why fayalite-augite syenites show such high geochemical homogeneity. High temperatures of crystallization may explain anomalously high Ga contents (27-45 ppm). Whole rock and mineral compositions constrain melt temperatures ranging from 720-780oC. Comparison of pyroxene trace element concentrations with whole rock abundances shows that whole rock content is largely a reflection of the presence of this phase.
The Rb-Sr-Ba ternary system and Harker diagrams of AMCG and LMG data illustrate increasing concentrations in refractory and incompatible elements. Fayalite-augite syenites fall high in these trends and indicate that melts formed from partial melting.
Within AMCG granitic suites, fractional crystallization is the dominant process; however, since fayalite-augite syenites are so homogeneous they are difficult to group into these trends. With limited refractory phases and inclusions, as well as geochemical homogeneity, it is proposed that fayalite-augite syenites formed from high degrees of partial melting of a refractory quartzo- feldpathic source region that was dehydrated and reduced during prior metamorphic and partial melting extraction events.