Abstract

The progressive metamorphism and partial granitization of a belt of quartz-mica feldspar-garnet paragneiss is considered in detail. This paragneiss is traced and sampled along a belt 35 miles long that extends across the Grenville Lowlands into the central massif of the Adirondack Mountains, New York. Geologic thermometers indicate tern peratures of metamorphism of about 500° C. at the southwest end of the belt and about 600° C. near the perimeter of the massif.

Minimum temperatures of metamorphism in the gneiss arc determined largely from solid solutions of magnesite in dolomite, FeS in sphalerite, paragonite in muscovitc, and TiO2 in magnetite. Maximum temperatures of metamorphism are inferred principally from the absence of wollastonite in closely associated siliceous marbles. The gradient in T is checked by the δ O18 in quartz and coexisting magnetite in the gneiss.

The composition of the paragneiss and its constituent minerals is determined from 75 new chemical analyses, 50 partial chemical analyses, 400 analyses of trace elements, and modal analyses of approximately 400 rocks.

At the lower-temperature end of the belt the least altered gneiss is a quartz-biotile-oligoclase-muscovite gneiss averaging (weight per cent) 70.25 SiO2, 0.67 TiO2, 14.14 Al2O3, 0.55 Fe2O3, 3.83 FeO, 2.20 CaO, 1.76 MgO, 0.05 MnO, 3.43 Na2O, 2.40 K2O, and (in ppm) B, 10; Ba, 600; Co, 8; Cr, 35; Cu, 16; Ga, 11; Ni, 15; Pb, 12; Sc, 12; Sr, 300; V, 56; Y, 50; Yb, 3; Zr, 170. This is inferred to approximate the bulk composition of the parent sedimentary rock.

With increasing temperature of metamorphism of the least altered gneiss, the mineral composition changes as follows: muscovite disappears, garnet appears, plagioclase increases in abundance, and average An content and quartz decrease. Complementary changes in chemical composition include an increase in Al, Fe++, total Fe, Mg, Ca, Cr, Ga, Ni, and V. Amounts of K, Si, Fe+++, H2O, and Ba decrease.

This “degranitization” or “basification” of the gneiss appears to be a metamorphic process that begins at about 550° C. and is well defined at 600° C. The mobilized Si, K, and H2O appear to be partly liberated and partly trapped as a venitic migmatite.

Granitization of parts of the gneiss is accompanied by an increase in K feldspar and Ab content of plagioclase and by a decrease in biotite, plagioclase, and quartz. Chemical changes in major elements include an increase in K and a decrease in Ti, Fe+++, Fe++, Mg, Ca, H2O, and in Na–K ratio. Changes in the amounts of minor elements in granitized parts of the gneiss include increases in Ba and Pb and a decrease in Co, Cr, Ni, Sc, Sr, Ti, V, and Y. All granitizing substances in the gneiss in areas of lower-temperature metamorphism appear to be introduced either laterally or from below. Those in areas of highest-temperature metamorphism are partly introduced, partly derived locally from the gneiss. The implied basic front evolved during granitization of the gneiss may have been large, for the introduced granitizing substances replace one-third of the sedimentary rock throughout a zone over ½ mile thick, 75 miles wide, and more than 40 miles long.

Calculations of the chemical composition of the more even-textured gneiss, from modal analyses and mineral analyses, show about the same deviation from the actual chemical analyses as exists in the 34 analyses of G-1 and W-1 as reported by Fairbairn et al, (1951).

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