Abstract

The major gneissic metasediment within the Grenville series of the northwest Adirondacks extends northeastward across the Grenville Lowlands for about 35 miles, though areas of different degrees of metamorphism and igneous intrusion. Within this region, the gneiss is interpreted to be grossly monoclinal, but overturned to form the southeast flank of a regional anticlinorium. The opposite flank of this structure probably lies just northwest of the St. Lawrence River in the Brockville-Mallorytown-Kingston areas, Ontario.

The gneiss is interstratified with thick zones of siliceous magnesian marble, thin quartzites, and schists. Two siliceous marble zones stratigraphically below the gneiss have a total thickness of about 8000 feet. A siliceous dolomite zone immediately above the gneiss (southeast of it) is at least 4000 feet thick and, with an overlying feldspathic quartzite, adjoins the central Adirondack igneous-rich massif. The gneiss itself is about 3000 feet thick and, except for thin interlayers of amphibolite and marble, the layers interpreted as relict beds are monotonous in texture and composition. These least-altered layers of gneiss are granoblastic, faintly foliated, quartz-greenish biotite-oligoclase gneiss in which the ratio of Na2O/K2O is approximately 1.3:1. Consequently the rock is far more sodic than typical shales. Layers of this composition are widely although sporadically distributed through the complex, and are injected lit-par-lit, replaced by and transitional into biotitic quartz-microcline-oligoclase granite, pegmatite, and alaskitic granite. Variants from these fades include garnetiferous and sillimanitic gneiss and migmatite, and tourmaline-bearing pegmatites.

The simple assemblage quartz-greenish biotite-oligoclase, found principally in those areas farthest from large bodies of igneous granite, undoubtedly represents not only the least-altered metasediment but moreover the lowest-rank metamorphic facies into which the parent sediment is reconstituted. This rock type is abundant northwest of Hyatt, New York, in that segment of the gneiss farthest from the central Adirondack igneous complex.

The alteration of the gneiss into various rock types reflecting increasing degree or rank of metamorphism as well as increasing interaction with magmatic fluids may be observed as the gneiss is traced northeastward toward Edwards, New York, which lies adjacent to the igneous massif. Alteration of the quartz-greenish biotite-oligoclase gneiss by granitic fluids commonly is accompanied by successive and marked decreases in biotite and in both the amount of plagioclase and in the per cent anorthite molecule of the plagioclase, as well as by a decrease in quartz content. Marked increases in potash feldspar content, particularly, serve as a useful index against which other mineralogical and textural changes may be plotted. As these changes appear in the gneiss, the greenish-brown biotite commonly gives way to a reddish-brown, probably more magnesian and titaniferous variety. A corollary to these successive mineralogical changes of gneiss toward granitic types are the chemical modifications involving, especially, a diminution in total iron, lime, and magnesia, and an increase in alkalis, particularly K2O, which becomes dominant over Na2O in many granitized facies. Intermediate types of this alkali-rich, potassic derivative of the gneiss most nearly approximate the composition of well-sorted shales and are possibly those considered by some earlier workers as representative of the sedimentary parent.

Textural changes include the development not only of migmatite, pegmatite, and equigranular granite, but also of widespread augen gneisses and “porphyritic” granite, both of which contain large porphyroblasts and possibly some phenocrysts, chiefly microcline.

The processes of injection and granitization seem to involve both magma and fluids from magma, which permeated the gneiss, interacting with and replacing it. Where the interaction of granitic fluids and gneisses occurred near, along, and in the central igneous-rich complex of the Adirondacks, in an environment of higher temperatures, the reddish-brown biotite appears almost or quite to the exclusion of greenish varieties. In addition, almandite garnet and sillimanite are formed, in that order, as products of the interaction. Almandite and some orthoclase also have formed as new phases presumably reflecting increasing rank of regional metamorphism without significant chemical modification of the parent sediment. Some of the earliest magmatic fluids appear to have included iron-rich derivatives, which by metasomatism formed scattered ferriferous biotite and hornblendic layers in and along the gneiss. These are largely granitized by the subsequent and dominant incursions of potassic, low-iron magma and associated fluids. Considerations of volume relations, compositional features, and paragenesis in the gneiss complex indicate that the potash in the “injecting” fluids had its source outside the gneiss; that is, it could not have been derived from the gneiss itself by secondary leaching, differential anatexis, or related processes. Nor seemingly could the relatively high soda content of the least-altered gneiss represent an earlier, but secondary, inoculation of the gneiss or parent sediment after diagenesis. Accordingly, the soda to potash ratio which approaches 1.3:1 in the quartz-biotite-oligoclase layers is regarded as either inherent in the sedimentary particles, or added essentially at the interface of sedimentation.

If the composition of the quartz-biotite-oligoclase gneiss near Hyatt was inherent in the clastic particles deposited in Grenville seas, graywacke type sediments or tuffs are the most logical parent rocks. If soda was added to the clastic particles at the interface of deposition, or penecontemporaneously, the pre-existing sediment may have been one of the more common clay-weathering products. Unfortunately neither alternative is particularly compelling or more readily demonstrated than the others. The seemingly intimate and concordant intercalation of the gneiss with thick marbles and pure quartzites suggests that the gneiss was derived from shales or argillaceous sandstones which were products of marked residual weathering and good sorting. Graywackes, on the other hand, are presumed to imply a minimum of chemical weathering with rapid erosion, transportation, and deposition in an unstable crustal environment. Thick, monotonous tuffs are as anomalous within this Grenville association of sediments as are graywackes and hardly represent a logical parent to the gneiss.

To assume soda-rich clays as the parent sediment poses problems equally great. The uniform addition of soda to great volumes of the commoner clayey products of residual weathering is almost unknown as a normal marine process. Commonly potash, rather than soda, is added to the accumulating clays, forming illitic shales. In addition to potash, both calcium and magnesium have greater replacing power than sodium and thus would replace it in requisite marine environments. The principal justification for assuming that soda could have been added to Grenville clays lies in indications that parts of the Grenville seas were, periodically at least, abnormally saline and soda rich. The great thickness of the carbonate sequence, its content of magnesia, silica, anhydrite, and halite indicate this, although most or all of these substances except the silica could be of secondary origin.

If seas of abnormal salinity were involved, several possibilities suggest themselves. Some water-laid tuffs, altered tuffs, and clays are known to alter to zeolites and zeolitelike minerals. Thus, clinoptilolite, analcime, and apophyllite with more or less admixed clay are found as marine beds evolved through interaction of tuffaceous material and saline water. Some zeolites even form in open seas. The zeolitic rocks occur as both intermediate and end-stage products in the weathering of tuffs and probably of other rocks and form widespread uniform beds. As such they suggest a possible parent to the gneiss.

Another alternative is that during sedimentation sodic shales are formed, by base exchange, from clay products of marked residual weathering. Base exchange of this type is credible if the seas were abnormally warm, rich in soda, and depleted in other bases which have greater energies of exchange than soda. In this environment, common clay minerals such as montmorillonite are known to take up soda in exchange for a previously held base, especially calcium, magnesium, or hydrogen.

Lateral extensions of the Adirondack type gneiss should appear in the near-by parts of Canada where most gneisses in the Grenville series are described as “normal shales.” It is hardly reasonable to infer that the lateral transition from a sodic to potassic sediment coincides with the international boundary. The data, though fragmentary, suggest that gneiss of the Adirondack type occurs in the Brockville-Mallorytown-Kingston areas and possibly for many miles beyond. Along and northeast of the Ottawa River, however, most gneisses described to date appear to represent derivatives of more normal shales or argillaceous sandstones. Accordingly the Adirondack gneiss may be inferred to pinch out to the northeast or grade laterally into metasediment probably derived from much less sodic parent elastics. In the northern and northeastern parts of the subprovince, however, paragneisses which seem to approach the Adirondack type are reported in numerous areas. The relations of these rocks to those in the southwest part of the subprovince are, unfortunately, a major enigma.

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