MAJOR objectives of this study have been to define the character of the igneous rocks present; to determine the number of separate igneous complexes, their relative ages, their variations, the relationships of these variations to structure, and the interpretation of their origin; to elucidate the mechanics of intrusion; and to discriminate primary magmatic structures from postconsolidation metamorphic features and to describe and interpret the mineralogic facies as a result of metamorphism.
The oldest rocks of the region are crystalline schists, gneisses, granulose rocks, marbles, quartzites, skarns, and amphibolites of the Grenville series.
A concept widely held is that the Grenville series was invaded by granite during an early, if not the earliest, period of magmatic intrusion in the Adirondacks, but the present conclusion is that no really satisfactory unequivocal proof for the existence of this older granite intrusion in the northern and northwest Adirondacks, at least, has yet been presented, though it may well be present.
The anorthositic series of rocks constitutes the earliest intrusives proven to be developed on a large scale. Both the main mass and the outlying intrusive rocks show a similar development of finer-grained granular border facies more mafic than the core, of segregations of more mafic rock containing included blocks of anorthosite, and locally a series of intrusions with similar sequence of composition in the order: anorthosite, gabbroic anorthosite, gabbro, and very mafic gabbros rich in apatite and ilmenite-magnetite as conformable segregations, sheets, and dikes. All the rocks of the anorthositic series belong to the “saturated” class. The foliation of the main anorthosite body indicates several domical surfaces in the roof like that of a laccolithic group, but no positive direct evidence for a base has been found. Several anorthositic masses, one of considerable size, have the form of approximately conformable sheets or sills in the Grenville formations. The main anorthosite body created considerable disturbance of structure in its immediate vicinity and developed a primary banding and flowage structure, most obvious in the border facies. The primary magma yielding the anorthositic complexes is assumed to have had the composition of a gabbroic anorthosite or kenningite. By analogy with stratiform sheets, a primary bytownite anorthosite horizon in the earth’s crust is postulated, which on partial melting yielded the required gabbroic andesine anorthositic magma. The general restriction of com-parable anorthosite bodies to the pre-Cambrian the world over is ascribed to higher isogeotherms in the earth’s crust at that time.
The “basic gabbros” of the older geologists, interpreted by them as younger than the syenite-granite series, are here described as younger than the anorthositic rocks but older than the syenitic and granitic complexes. Fine-grained, or apparently fine-grained, border zones of the gabbro masses against the alkalic-siliceous intrusives, supposed on the older hypothesis to signify “chill zones,” are here interpreted to be “pseudo-chill” zones, the result of contact metamorphic recrystallization consequent upon the intrusion of the younger felsic rocks. The gabbro masses in the Grenville series, where in large bodies, occur in large part as conformable sheets intruded in gently disturbed beds and subsequently folded. There are also many small gabbro lenses. In the anorthosite the gabbro forms both crosscutting dikes and sheets conformable with the foliation. The gabbros uniformly have the composition of olivine gabbro and are “undersaturated.” They occur extensively throughout the whole Adirondacks, though relatively subordinate in volume to anorthosite, quartz syenitic rocks, and younger granites. The primary variations in composition are in general within a narrow range and are attributed in part to differentiation of the type found in gravity-stratified sheets and in part to other origin. Locally in metagabbro masses adjacent to anorthosite bodies there are conformable bands of mafic metagabbro, in part very rich in ilemite-magnetite.
There is a series of dioritic rocks (Rossie series) intruded largely as sills or laccoliths younger than the gabbros and older than the Diana syenitic complex. These rocks are quite local in the Grenville belt. They range from pyroxene diorite to biotite-quartz diorite and may in part be modifications of the gabbros by thermal solutions accompanying younger intrusions.
The next magmatic epoch is characterized by the intrusion of a primary magma thought to have had generally the composition of a pyroxene-quartz syenite. In part this magma yielded by differentiation rocks varying from pyroxene syenite to biotite granite, in part perhaps by assimilation of amphibolite formed a mafic syenite, and in part solidified as quartz syenite. The last forms a major element throughout the entire Adirondacks. Three syenitic complexes, with rocks of variable composition, are described: Diana, Santa Clara, and Tupper-Saranac.
The characters of known stratiform igneous sheets (sills, lopoliths, laccoliths) are described, their characteristic features are set forth, and criteria are developed which may be taken as indicative of gravity stratification. The characters and relationships of the various facies in the Diana and Santa Clara complexes are viewed in the light of these features and criteria, and the tentative interpretation is drawn that, giving the independent structural evidence due consideration, these complexes are closely folded or overturned isoclinally folded gravity-stratified sheets, which vary from mafic pyroxene syenite near the base to horn-blende or biotite granite at the top. The mafic syenite facies developed at the base of the Tupper-Saranac complex, adjacent to the anorthosite massif and locally in each of the other bodies, however, is interpreted as in part the product of modification and assimilation in contact zones between quartz syenitic magmas and metagabbro or mafic Grenville skarn. The syenitic sheets are interpreted as essentially pre-kinematic or at most early kinematic.
At some time after the emplacement, differentiation, and consolidation of the quartz syenitic complexes, there followed a period of orogeny during which the Grenville series and their intruded igneous sheets were strongly folded. This is indicated by the fact that the limbs of the folds of the Diana complex where they strike NE are cut by a series of hypersthene metadiabase dikes striking NW, but that where the limbs of the fold strike NW the metadiabase dikes are parallel to the banding and foliation which indicate the folds. This orogeny is considered to represent the first major period of folding, but it is thought that the major epoch of metamorphism, comprising crushing and recrystallization, occurred later. The hypersthene metadiabase dikes are the next younger igneous rocks after the intrusion of the quartz syenitic complexes and the period of folding.
The folded Diana, Santa Clara, and Tupper-Saranac complexes are intruded by a younger granite (Lowville-St. Regis). This predicates a major time break between all the older intrusives and the granite intrusions of this epoch. It is here shown that, when mapped and viewed on a large scale, the younger granite forms a mass of batholithic dimensions and in considerable part shows a pronounced transgression across the banding and structure of the older Diana and Santa Clara syenite-granite complexes and in subordinate part exhibits a tendency toward a phacolithic development along the foliation planes of the older complexes.
The granitic intrusives in the Grenville belt of metamorphic formations are compared with the batholithic granite of the Adirondack highlands, and it is found that in the Grenville there are, in general, three types of granite. The Hermon type is in general an augen gneiss, coarsely porphyritic and less siliceous than the granite of the batholith; it occurs as folded phacolithic sheets, emplaced in large part within what were originally folded aluminous sediments or in metagabbro, and most intimately involved with them. In part the granite is the result of replacement and modification of these schists by thermal solutions. A syenitic facies is found as a local development of the Hermon type and is attributed to modification consequent upon emplacement in limestone or amphibolite. The Antwerp type of granite is a medium-grained equigranular type of composition similar to or somewhat more siliceous than that of the Lowville-St. Regis batholith. It occurs as a facies associated with the Hermon type and as independent lenses. The Alexandria type is alaskitic, medium- to fine-grained, and more siliceous than the batholithic granite. It occurs almost exclusively as anticlinal phacoliths, many of which have been subsequently strikingly deformed. All the granites are interpreted as outlying intrusives in the Grenville belt, genetically related to the Lowville-St. Regis batholith of the northwestern part of the Adirondack highlands.
The hypersthene metadiabase dikes, and locally the granites on a large scale, have both been strongly deformed. There is no certainty whether this is to be attributed to a single orogeny with two major epochs—one post-metadiabase and previous to the emplacement of the granites at their present horizon, the other post-granite emplacement—or whether there were two distinct orogenies. There is evidence that a period of profound metamorphism preceded the emplacement of the younger granites at their present horizon. An hypothesis that the intrusion of the younger granites was contemporaneous with the latter part of a single orogeny seems consistent with the present data. The younger granites are interpreted as syn-kinematic.
The Grenville series, the anorthositic and gabbroic rocks, the syenite-granite series of the Diana, Santa Clara, and Tupper-Saranac complexes, hypersthene metadiabase dikes and the Lowville-St. Regis batholithic granite and related intrusives are all believed to be of Archaean age.
The youngest intrusives in the area are basaltic diabase dikes, possibly of Keweenawan age, which have undergone practically no dynamic metamorphism. They are variable in composition, ranging through olivine diabase, augite diabase, augite-biotite diabase, and quartz diabase with locally monzonitic and granophyric types. The phenomena of orthoclase replacement of part of the plagioclase to form a “complex igneous rock” is found in some of these dikes. They are definitely post-kinematic.
Assimilation and modification in contact zones between magma and country rock is a constant but subordinate feature of all the major bodies and kinds of igneous rocks. Several detailed descriptions are given. It is found that, where intruded in limestone, the quartz syenite and the younger granite both develop locally a syenitic facies, without any coordinate increase in mafic minerals. Mafic quartz-poor or quartz-free syenites are locally developed where quartz syenite or the younger granites are involved with metagabbro. The anorthosite commonly develops a mafic dioritic facies in contact zones with Grenville but is locally mafic-poor and quartzose.
Contact metamorphic phenomena for each of the major kinds of magma are discussed, and it is concluded that in general the younger granites show a type and spread of mineralization indicative of high volatile content in the magma; the quartz syenitic magma a quite localized contact metamorphism characterized by some minerals indicative of a higher temperature than that accompanying the granites and indicative of relatively low volatile content; and the anorthositic magma, an intensive metamorphism characterized by the development of garnet, hedenbergitic pyroxene, and wollastonite in limestone. Mafic gneisses or skarns of gabbroic composition are developed from layers of limestone included in the quartz syenite, granite, and anorthosite. In the granite hornblende is commonly a major mineral in the intensely metamorphosed facies, whereas in the anorthosite, where present, it is largely secondary after pyroxene and developed under conditions of later dynamothermal metamorphism. Diorites are a common local development as a result of granitization of the gabbros by the younger granite magmas.
The large-scale structure of the Adirondacks as a whole is reviewed. It is found that they consist of a great central core, primarily of igneous rock, flanked on the northwest and on the south by broad belts composed largely of the Grenville series. The foliation dips predominantly to the northwest on the northwest and predominantly to the south on the south. This suggests, but does not fully prove, an anticlinal structure. The axial planes of the folds in the northwest Grenville belt are arranged as in an asymmetrical fan, between an anticlinorial granite body on the northwest and the igneous core of the Adirondack highlands on the southeast. All the igneous rocks in the Grenville belt are conformable with its structure. The maximum intensity of folding and deformation occurs on the borders of this wedge of the Grenville. The eastern Adirondacks are characterized by a belt striking north-northeast, athwart the general structure in which occurs the main anorthosite massif and numerous smaller anorthosite bodies.
Probably three-fourths of the igneous rocks of the Adirondacks show crush structures, and most of them show evidences of recrystallization and reconstitution. As an explanation for this, the alternative concepts of plastic flow with dynamothermal metamorphism of solid rock and of magmatic flowage are considered; the former is adopted.
This is based upon the following relationships. On a regional scale the trend lines of the foliation are arranged in a fashion indicative of control by tangential orogenic stresses, and on a small scale the orientation of the foliation for rocks of similar physical properties is in part independent of their surfaces of contact. The size of grain of the secondary material (mylonitic or crushed and recrystallized grains) has a systematic regional variation, and, concomitantly, there is a consistent variation in the mineralogic facies developed by reconstitution in each type of rocks within the different regions. If the interpretation of the Diana and Santa Clara complexes as gravity-stratified sheets is correct, the deformation of these rocks must have occurred after their consolidation, for thorough-going crush structures are not known in sills undeformed by orogenic stress, and, in the Adirondacks, sills of quartz syenite and granite, preserved in limestone which has acted as a cushion, are undeformed even in areas where crushing and recrystallization are normally intense. The trend lines of the foliation and the linear structure, the different physical facies, and the different mineralogic facies all have a systematic regional development consistent with an interpretation of origin in terms of dynamothermal metamorphism, in terms of variation in depth, temperature, and stress. Four belts of metamorphism are defined: (1) Cataclasis predominant; (2) crushed and recrystallized without production of garnet; (3) crushed and recrystallized with formation of garnet; and (4) two large cores of gneissoid relatively uncrushed, unrecrystallized rock. All the igneous rocks, with the probable exception of part of the younger granites, are thought to have been wholly consolidated at the time of the metamorphism, and their foliation and linear structure, as now found, are products of plastic flow of solids. The foliation of the granites is in part primary gneissoid, in part protoclastic, and in part due to crushing and recrystallization in the solid state. The relative importance of these factors is widely variable.