Abstract

The Precambrian metasediments of the Grenville series, or rocks with kindred features, have been identified from Georgian Bay, southeastern Ontario, northeastward as far as Labrador and southward into the Adirondack Mountains of New York, the Green Mountains of Vermont, and the Precambrian highlands of northern New Jersey, southern New York, and southeastern Pennsylvania. This region also includes a distinctive series of intrusive rocks ranging from anorthosite to granite and may be referred to as the Grenville subprovince. The thickness of the Grenville series, or its inferred equivalents, is the subject of much speculation. Its maximum thickness is probably in southeastern Ontario and in the northwest Adirondacks, where the exposed sequences probably total about 20,000 feet. Eastward, in the vicinity of the type locality at Grenville village, Argenteuil County, Quebec, the exposed Grenville series may be only 5000 or at most 10,000 feet thick; northeast of the type locality, near Shawinigan Falls, the known Grenville series may be less than 5000 feet thick. Few data are available on thickness of the Grenville series in other parts of the subprovince.

Most of the Grenville series is moderately to highly deformed, and injected. In the Adirondacks and in parts of Ontario and Quebec, the anorthosites appear to be the oldest rocks intrusive into the Grenville series, followed successively by gabbros, diorites, syenites, and a granitic complex. In some parts of western Quebec and elsewhere, a post-Grenville pre-anorthosite epoch of granitic intrusion may have occurred. Throughout the subprovince, however, an igneous series of anorthosite, gabbro, syenite, and granite appear to be genetically related, either as successive fractions from a deep-seated magma source or as differentiates evolved from a magma of intermediate composition during its penetration of the Grenville series.

All the intrusive magmas have interacted with the Grenville series to produce hybrid rocks and skarns. The widespread chemical alterations and dynamothermal metamorphism of the Grenville series largely are related, however, to the emplacement of granites which post-date both anorthosite and gabbro. Some granitic masses exhibit well-defined contact-metamorphic aureoles. Elsewhere areas of granite are not clearly separable from granitized sediment and migmatite. Nests and irregular masses of skarn and contact minerals also are interspersed with relatively unaltered metasediment. Paragneisses are injected lit-par-lit and partially granitized. Some marbles are converted into hornblende-andesine amphibolites, and others, through interaction with granitic and aqueous fluids, are altered to or replaced by pyrite, magnetite, diopside, tremolite, phlogopite, scapolite, serpentine, talc, and many other minerals.

A gross relationship exists between the most thoroughly deformed and sheared-out metasedimentsand areas of abundant igneous rock. Two of the best-documented examples of regional variations in degree of dynamothermal metamorphism that can be related to large igneous massifs are those in the Haliburton-Bancroft area of Ontario and in the Adirondack Mountains of New York. Even those metasediments farthest from igneous massifs are completely recrystallized and reconstituted, however, commonly into the amphibolite or a higher-rank metamorphic fades. In the clastic rocks almandite, biotite, and either oligoclase or potash feldspar are dominant minerals, and many segments of these rocks contain appreciable sillimanite. Carbonate rocks are dominated by diopside, tremolite, hornblende, and andesine with occasional nests of wollastonite formed along and near the contacts of various intrusive rocks.

Most of the Grenville series, and the Grenville type metasediments, are highly folded, commonly along northeast-trending axes. Marked divergences from this regional trend are especially prominent in the vicinity of large anorthosite and granitic massifs. At least one region of more open folds and moderate deformation of the Grenville series appears between the Morin anorthosite mass and the Saguenay River In the areas of more profound deformation, all the metasediments have yielded not only by cataclassis but by profound solid flow. Flowage features are especially prominent, however, in the carbonate zones. Some of these carbonate units have pulled or flowed apart in areas of constriction or stretching, commonly along the short flanks of major folds, and are enormously thickened elsewhere, usually in apical areas of folds. Differences in rate or amount of flow in different lithologic layers have induced markedly disharmonic folded forms, pseudo-unconformities, and tectonic breccias. In general, however, rock motions have followed the sedimentary patterns, and the gross patterns of flow surfaces are rudely accordant with relict bedding.

The dominant metasedimentary rock types within the Grenville series are marbles, quartzites, amphibolites, and biotitic, garnetiferous, and sillimanitic paragneisses. Siliceous and magnesian marbles are especially typical along and west of the Ottawa River, in southeastern Ontario, in the Adirondacks, Vermont, and in the highlands of northern New Jersey. Eastward beyond Lake St. John (St. Jean), Quebec, the correlation of the “Grenville series” is based largely upon amphibolites, quartzites, and “paragneisses” which include both biotitic and hornblendic quartz-feldspar gneisses.

The southwesterly, carbonate-rich fades are in large part unequivocally correlative with the type section described by Logan near Grenville village, Quebec. In the Adirondacks and northwest toward the Haliburton-Bancroft area of Ontario, this segment of the Grenville series includes at least two magnesian carbonate units, each a mile or more thick before metamorphism. These appear to thin northeastward from the type locality and southeast across the Adirondacks. In northern New Jersey, however, the Grenville-type marble of the Franklin formation appears to be at least 2000 feet thick.

The magnesia content of the marbles may be in part primary or syngenetic, although in the Haliburton-Bancroft area of Ontario, in parts of southwestern Quebec, and in northern New Jersey, dolomitization of an initially calcitic marble is reportedly related to the intrusion of igneousrocks. In the Northwest Adirondacks, some of the oldest marble layers are dolomite. Some of the magnesia in these has been abstracted during metamorphism, moved appreciable distances, and concentrated in deposits of tremolite, anthophyllite, serpentine, and talc.

Much of the silica in the marbles of the Grenville series undoubtedly is primary although probably a fourth to a half of the present total is introduced. Other substances in the marble which may be at least in part sedimentary include pyrite, graphite, gypsum, anhydrite, halite, and members of the paraffin series. In the northwest Adirondacks, anhydrite and methane occur in well-defined interbeds, and the existing data favor a sedimentary origin for these components.

Quartz-feldspar paragneisses, either appreciably sillimanitic and garnetiferous or highly biotitic, appear almost everywhere that Grenville-like rocks are found in the subprovince. In areas of appreciable marble paragneisses commonly rank second to marbles in abundance. In the northwest Adirondacks, for example, the largest gneissic unit is about 3000 feet thick. Northeastward from the type locality, as the marbles thin or disappear, paragneisses tend to dominate the Grenville series. Most of the thicker gneissic units are either quartz-sillimanite-feldspar or quartz-biotite-feldspar types, in general much injected by granite. In the Adirondacks, in parts of southeastern Ontario, east of the Saguenay River in Quebec, and in New Jersey, the least-altered gneisses have a bulk composition approaching that of many graywackes, dacitic tuffs, or sodic shales. Along the Ottawa River, however, and in parts of southwestern Quebec, the paragneisses seem to approach more closely the composition of Clarke's average shale.

Quartzites appear as narrow interlayers and as zones throughout the known and inferred extent of the Grenville series. Among the quartzites are almost pure quartz types, as well as types intermediate between quartzite and gneiss, marble, and amphibolite. Many of the siliceous interlayers, especially in the marble, are either laminated or lenticular to nodular, and some if not most of these rocks presumably were deposited as microcrystalline cherts or novaculite-like types.

Amphibolites also are ubiquitous. Most are hornblende-andesine rocks with associated augite, diopside, apatite, calcite, and other minerals. Many amphibolites clearly are derived from gabbroic and basaltic rocks, and others as surely represent skarnlike replacements of marbles. In addition, numerous amphibolites are inferred to represent reconstituted tuffs and marly interbeds. In many areas of the Grenville subprovince, however, neither the rock types transitional between the parent and the resulting amphibolite, nor the parent rock are preserved, and the origin of the evolved amphibolite cannot be stated with any assurance. An obvious corollary is that problems of stratigraphic and structural synthesis are greatly complicated where the section includes an appreciable thickness of enigmatic amphibolites.

The characteristic assemblage of marble, paragneiss, and quartzites which constitutes the typical Grenville series has been described repeatedly as representing “normal marine limestones, shales and sandstones of the well sorted type.” Other workers in the same or near-by areas have described interlayers and zones n i the Grenville series as arkoses, tuffs, and graywackes. Some of the same parts of the Grenville series also are depicted as either including, or being intimately associated with, conglomerates, pillow lavas, and iron formations. The implication is clearly that of a highly anomalous association of rock types which do not fit into contemporary, genetic classifications, or readily inferred environments, of sediments.

Many seeming contradictions disappear, however, if all citations of original sedimentary fades are excluded from discussions except those that are reasonably established in origin and recognizable as probable parts of the Grenville series. Some real enigmas remain, nevertheless, which cannot be ascribed to inaccurate identifications or uncertain correlations.

All the examples of sedimentary iron formation ascribed to the Grenville series appear to be most hypothetical, if not spurious. Arkoses are equally suspect, especially in the carbonate-rich segments of the series. Feldspathic quartzites or paragneisses are common enough as integral parts of the sequence, but instead of representing arkoses these are more reasonably interpreted as secondarily feldspathized quartzites or as reconstituted, illitic sandstones.

The term graywacke is applied widely and loosely in descriptions of the Grenville series to many gray to dark-green gneissic and schistose interlayers which must have had divergent origins. In Hastings County, Ontario, however, and southwestward along this margin of the Grenville subprovince, unequivocal conglomerates, pillow lavas, and probably agglomerates either underlie, or overlie, and possibly form interlayers in carbonate-rich segments of the Grenville series. These suggest that the Grenville series of this area was deposited on an unstable area of the earth's crust in which positive as well as markedly negative segments evolved during sedimentation. Graywackes may have formed although documentary data on their existence are lacking. Eastward from this area, the conglomerates and lavas disappear, although part of this region includes not only marble and quartzites, but the paragneisses with the composition and many lithologic features of graywackes, tuffs, or aberrantly sodic shales. There are in addition the paragneisses which may have evolved from typical shales. The extreme thickness of the section, the abundance of siliceous magnesian carbonate rocks, and the probable occurrence of sedimentary anhydrite and halite suggest that this segment of the Grenville series was deposited in a large persistently negative basin, either periodically or locally cut off from the open ocean.

The basin appears to have extended from Lake Nipissing southeastward across the Adirondacks, possibly into New Jersey. Its northeast margin must have been well beyond the type locality in western Quebec.

The environment of deposition of the Grenville-type sediments northeast of the Saguenay will remain most speculative until much more is known of the composition and geologic features of the metasediments. They undoubtedly are derived from clastic rocks, especially sandstones and finer fractions. Marbles are uncommon, or relatively unimportant, although some of the amphibolites could represent skarnlike replacements of carbonate zones. Whether arkoses or graywackes are represented also is of particular interest in proposing an environment of deposition. The numerous reports of abundant sodic oligoclase in many of the paragneisses suggest that some clastic zones could have been graywackes, or tuffs of intermediate composition, or sodic shales.

Throughout most of the Grenville subprovince, the Grenville series are the oldest decipherable rocks. Analyses of lead and uranium in granitic and pegmatitic minerals, and of helium in magnetite ores in the Grenville subprovince give ages ranging from 1.0 x 109 to 1.3 x 109 years. All these minerals were emplaced at about the culmination of metamorphism of the Grenville series. Earlier epochs as yet undated include the premetamorphic intrusions of anorthosites, gabbros, and syenites and the period of Grenville sedimentation. Presumably the inception of this sedimentation occurred some 1.2 x 109 or more years ago.

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