30. The K-replacement origin for the megacrystal Hermon-type granites in the Grenville Lowlands, northwestern Adirondack Mountains, New York, USA
Lorence G. Collins
December 2, 1998
Geochemical and field studies identify nine different megacrystal Hermon-type granites in the Grenville Lowlands northwest of the Adirondack Mountains, New York (Carl and deLorraine, 1997); Fig. 1). These include the Gray's School, Sherman Lake, Trout Lake, Hermon Village, Fowler, Fullerville, Ore Bed Road, Mott Creek, and Shiner Pond bodies. Although surface exposures of the Fowler granite do not show K-feldspar megacrysts, drill cores have intersected megacrysts in this body. Because the microcline megacrysts, 2 to 40 cm long, are considered by most investigators to be phenocrysts, the granites have always been assumed to be totally magmatic in origin. The granites range in silica content from 51.5% SiO2, typical of syenite in the Gray's School granite, to 74.5% SiO2 in the Shiner Pond and Mott Creek granites, more typical of granite (Fig. 2). Because of the wide distribution of these granites (Fig. 1) and the great variation in MgO and SiO2-contents, the granites are not considered to have a common magmatic source, but rather to have formed separately, supposedly by some magmatic differentiation process. Previous investigators, however, have not studied the transition from the granites to their wall rocks. This article reports the results of field, thin section, and chemical studies of the Fowler, Fullerville, and Gray's School granites and their relationships to the wall rocks and other granites.Table 1.
Field studies of the megacrystal Hermon-type granites show that they have gradational contacts with the more mafic wall rocks. The Fowler and Fullerville granites, which have relatively-low MgO- and high SiO2-contents (Fig. 2), is gradational to biotite-rich gneisses or amphibolites. (See, also, article number 28 in this web site for a discussion of the Popple Hill gneiss.) Gradational contacts of the Trout Lake megacrystal granite also occur with the Popple Hill gneiss in dynamited road outcrops along Trout Lake Road (northwest of Edwards, adjacent to the southeast end of Trout Lake), and gradational contacts of the Ore Bed Road granite body also occur at its northwest end with Popple Hill gneiss along the road from Balmat to Edwards. Therefore, although possible gradational transitions from granites to wall rocks have not been sought for the other four megacrystal granites, such transitions are presumed to exist.
All Hermon-type granites, plotted on Fig. 2, are myrmekite-bearing. Myrmekite also occurs in deformed portions of the adjacent wall rocks. Maximum sizes of the quartz vermicules in the myrmekite range from relatively coarse in the Gray's School granite (Fig. 3) to relatively fine in the Fullerville and Fowler granites (Fig. 4). These maximum sizes correlate with what is expected for oligoclase-andesine in the amphibolite wall rocks of the Gray's School granite and for albite-oligoclase in the Popple Hill gneiss wall rocks of the Fowler and Fullerville granites (Collins, 1988).
Type of granite also correlates with the type of wall rock. Where the megacrystal granite is hornblende-bearing, the adjacent wall rock is an amphibolite or hornblende diorite. Where the megacrystal granite is rich in sphene, the adjacent wall rock is also rich in sphene. And, where the megacrystal granite contains no hornblende and only biotite, the adjacent wall rock is richer in biotite and lacks hornblende.
Gradational contacts of the Gray's School granite with its wall rocks can be observed in many places. West of Dekalb, New York, dynamited outcrops of megacrystal granite (Fig. 5) occur along Maple Ridge Road (intersecting Route 17; Fig. 6). This granite becomes increasingly richer in ferromagnesian silicates westward where only stringers of myrmekite-bearing granite remain in a deformed amphibolite (Fig. 7). Eventually the granite disappears in outcrops still farther west. At the north end of the granite body only remnants of amphibolite occur, but here the rock is strongly mylonitized and altered. Exposures in this area are so poor that gradational relationships between amphibolite and granite are obscure. On the east side of the granite body along the Old River Road are many excellent outcrops of the megacrystal granite (Fig. 8) that contain remnants (enclaves) of amphibolite (Fig. 9). The amphibolite that forms the east wall rock of the granite can be found along a "truck road" that intersects the Old River Road, 1.4 miles (2 km) south of Maple Ridge Road. Here a dynamited outcrop of amphibolite (sample 64; Fig. 6) contains stringers and masses of myrmekite-bearing granite.
In other places gradational contacts are absent and instead the wall rocks are interlayered with the granite. For example, at the southwest end of the Gray's School granite body and extending northward along the western third of the body, the granite is interlayered with biotite-rich gneiss, amphibolite, and coarse diorite. Dynamited outcrops of amphibolite (sample 89) with veins of granite can be observed along the dirt road in the southwestern part of the granite (Fig. 6). Biotite-rich gneisses (samples 85 and 82) and granite (sample 84) occur in outcrops along the road that extends south from the dirt road trending east-west along the south end of the pluton. Likewise, at the southeast end of the Gray's School granite body, both biotite- and quartz-rich Popple Hill gneiss and biotite-hornblende amphibolite (samples 77A and 97) are interlayered with the Gray's School Hermon-type granite (sample 77B, Fig. 6).
Thin section studies of the amphibolites and biotite-rich gneiss that are the wall rocks for the Gray's School granite show cataclasis of normal zoned plagioclase crystals, but coexisting microcline megacrysts are unbroken. This relationship give evidence that the microcline is later than the cataclasis. Plagioclase in the amphibolite (or diorite) has remnant normal zoning with relatively calcic cores. In some places, however, microcline penetrates the plagioclase in veins and partially replaces interiors of deformed plagioclase crystals (Fig. 10). In the microcline, island of unreplaced plagioclase are in parallel optical continuity with the larger plagioclase crystal outside the microcline (Fig. 10).
In other places biotite has quartz sieve textures where quartz has replaced the biotite. Hornblende also locally shows replacement by quartz blebs in the interiors of some crystals. The same textural and mineralogical replacement patterns are also found in granite along the Old River Road outcrops where enclaves of both biotite-plagioclase and biotite-hornblende-plagioclase layers can be found. Microcline megacrysts locally replace plagioclase crystals in these enclaves (Fig. 9).
Although many contacts between amphibolite and the Gray's School granite appear to be sharp in the field, thin sections across the contacts commonly show early stages of K-feldspar replacement of interiors of plagioclase crystals and the formation of myrmekite in the amphibolite adjacent to the contact.
Carl and deLorraine (1997) also reported the presence of zoned K-feldspar in one locality with inclusions of biotite and plagioclase aligned parallel to possible crystal faces of a growing K-feldspar crystal. This suggests that a facies of the original primary diorite contained zoned orthoclase. The lack of such zoning in all other K-feldspar megacrysts in other parts of the Gray's School granite lends support to the hypothesis that most of the K-feldspar has formed by replacement processes.
Chemical analyses of the Gray's School granite, amphibolite, and biotite gneiss (Table 1 and Table 2) give support to the K- and Si-replacement model. Relative to related amphibolite and biotite gneiss samples, the granite is enriched in SiO2, K2O, and Pb and depleted in Fe2O3, MgO, MnO, CaO, Sr, Co, Ni, Cu, Zn, and V. Other oxides and trace elements give mixed results. Fig. 11 shows the trend of data for MgO wt. % versus SiO2 wt. % for the parent biotite-rich gneisses (samples 82 and 85) relative to their associated granite (sample 84) and for the parent amphibolites (samples 64, 77A, 89, and 97) relative to their associated granites (sample 77B and samples analyzed by Carl and deLorraine, 1997). Losses of MgO are relatively large where the parent rocks are replaced by granite.
The Fowler granite is a lenticular body enclosed in Popple Hill gneiss in the western limb of a tight isoclinal fold whose core is the Upper Marble unit that contains zinc sulfide concentrations (Fig. 12). The Ore Bed Road granite is probably a continuation of the Fullerville granite. Both the Fowler and Fullerville granites are similar to the Gray's School granite in physical appearance but differ in kinds of wall rocks. Around the Gray's School granite the plagioclase in the wall rock gneisses and amphibolites tends to be more calcic (oligoclase or andesine) and coexisting with hornblende whereas around the Fowler and Fullerville granites the plagioclase in the Popple Hill gneiss is more sodic (albite or oligoclase) and does not coexist with hornblende. Unlike wall rocks of the Gray's School granite, however, the Popple Hill gneiss along highway 58 west of the city of Fowler near Popple Hill and west of the Fowler granite body contain granite dikes that cut the foliation with sharp contacts.
The Popple Hill gneiss is variable in composition but commonly is relatively felsic, consisting mostly of quartz, plagioclase, and small amounts of biotite (Carl, 1998; Engel and Engel, 1958). In some places, however, bands rich in biotite (30 vol. % or more) occur. Disoriented enclaves of this biotite-rich gneiss occur in the contorted Fullerville granite in the western limb of the drag fold as seen along the road towards Edwards, 0.5-2 km north of the Four Corners in Balmat (Fig. 12). Here, the granite is fine-grained, lacking megacrysts, and contains little to no plagioclase, the feldspar being mostly microcline. Farther northeast along the drag fold in the nose of the fold and its eastern limb where the rock is less deformed (southeast and east of the pond on the Earl Murphy property), the biotite-plagioclase-quartz Popple Hill gneiss interfingers with layers of Fullerville granite containing microcline megacrysts. Mineral compositions and groundmass textures of gneiss and granite are quite similar except that less biotite and plagioclase and more microcline occur in the granite.
Quartz vermicules in myrmekite in the Popple Hill gneiss (Fig. 13) are the same size as quartz vermicules in myrmekite on borders of megacrysts in the adjacent Fullerville granite (Fig. 14). This relationship is consistent with the hypothesis that the granite is derived from the gneiss simply by replacing some of the plagioclase by microcline megacrysts.
As in the Gray's School granite, in both the Fowler and Fullerville granites,microcline veins replace interiors of plagioclase crystals, and islands of unreplaced plagioclase, remaining in the microcline, occur in optical parallel continuity with the larger unreplaced plagioclase grain outside the microcline (Fig. 15).
In some places the microcline megacrysts in the Fullerville granite are poorly zoned, containing remnant, oriented plagioclase grains with albite-twinning that is aligned parallel to possible faces of the growing K-feldspar crystal (Fig. 16). This zonation might result where random plagioclase grains in the groundmass happen to have lattices that are oriented parallel to the K-feldspar lattice and, thereby, are more stable and less prone to replacement than those whose deformed lattices are inclined to the K-feldspar lattice. The lack of well developed faces on these plagioclase inclusions, the embayments of K-feldspar into the plagioclase, and the equality in size of the plagioclase inclusions with the groundmass plagioclase grains lend support to this incomplete replacement hypothesis.
Thin sections of the leucocratic dikes that cut the Popple Hill gneiss near Popple Hill along route 58 west of Fowler show that these dikes consist dominantly of microcline and quartz with occasional myrmekite adjacent to remnant plagioclase. In a few places thin sections of the Popple gneiss adjacent to these dikes show microcline (1 cm long) replacing plagioclase, and some of the microcline is bordered by myrmekite which looks like the myrmekite in the dikes. Therefore, although the dikes give the physical appearance of intrusions of magma from the nearby Fowler granite, they likely represent strong replacements of crushed rocks along fractures cutting the foliation in which most of the biotite is replaced by quartz and most of the plagioclase is replaced by microcline.
Chemical analyses of the Fowler and Fullerville granites and the Popple Hill gneiss (Table 1 and Table 2) also give support to the K- and Si-replacement model. Relative to the gneiss samples, the granites are enriched in K2O, Rb, and Pb and depleted in Fe2O3, MgO, MnO, CaO, Sr, Co, Ni, Cu, Zn, and V. The SiO2 content is about the same because the gneisses are quartz rich. Some granite samples have less SiO2 than the gneiss. The megacrystal granite sample 123 shows a slight increase in CaO over the gneiss sample 121, and this may be because of the occurrence of epidote in the granite. Other oxides and trace elements give mixed results. Fig. 17 shows the trend of MgO wt. % versus SiO2 wt. % for the parent Popple Hill gneisses (samples 98, 99, 114, and 121) relative to their associated granites (samples 98A, 113, 118, and 123, and samples analyzed by Carl and deLorraine, 1997). Losses of MgO are relatively large where the parent rocks are replaced by granite.
The gradational changes in mineralogy between wall rocks and adjacent granites and the direct correlation between these mineral changes and the degree of deformation of the parent rocks have not been suspected by other investigators and, therefore, such relationships have not been looked for. In the drag fold on the eastern limb of the isoclinal fold north of the Four Corners in Balmat (Fig. 12), the great abundance of microcline and nearly complete absence of plagioclase results from the nearly complete replacement of the former plagioclase by microcline in the strongly deformed Popple Hill gneiss once occupying this space. The progressive and continuous deformation, as the drag fold was formed, would have caused mineral grains in the limbs of the drag fold to be sheared repeatedly, thereby keeping the grain size small (no megacrysts formed) and enabling most of the broken plagioclase grains to be replaced by the microcline. Recrystallization would eliminate most of the former cataclastic texture.
The above relationships between deformation and feldspar compositions are characteristic throughout the megacrystal Hermon-type granites in the Grenville Lowlands. In those places that were affected by the strongest deformation, the granite has the highest percentage of microcline and the least plagioclase, although much of the evidence for this deformation may be eliminated by the replacement and recrystallization. For example, structurally, there is a correlation between deformation and creation of the Fowler and Fullerville granites. These granites are not connected around the nose of the isoclinal fold (Fig. 12). The reason for this is because the nose of this fold is structurally the tightest and the place where the least deformation would occur. Granite was not created here by replacement processes because few (if any) openings where produced through which fluids could move. In contrast, in the limbs of the fold the most sliding of adjacent layers past each other would occur as the rock layers adjusted to the creation of the isoclinal folding. Therefore, rock layers in the limbs underwent the greatest degrees of cataclasis, and these places would have been the most open for through-going K-replacement fluids to form the fine-grained and/or megacrystal K-feldspar in the granite.
Similarly, the Gray's School granite (Fig. 6) occupies a space which was affected by strong deformation. As the marble unit (south of the now-granite body) was thrust (rolled) against the biotite-rich gneiss and diorite (amphibolite) layers, these layers slid differentially past each other (like a sliding deck of cards) to make space for the movement of the marble. In that process, the plagioclase crystals would have been deformed and granulated so that K released from replaced biotite could convert the plagioclase into microcline, thereby changing these rocks into a megacrystal granite. Supporting this hypothesis are the facts that (1) the greatest conversion of the biotite-rich gneiss and diorite to granite occurs along strike opposite the blunt end of the southern part of the the granite body against the marble where deformation was the strongest, and (2) the degree of replacement dies out westward where the marble unit has not shoved as far northward and where the gneiss and diorite layers have not been as strongly deformed. Replacements also die out northward where the body tapers to a point. Evidence for the strong deformation in the southern part of the pluton has been largely eliminated by the recrystallization that occurred during the K-metasomatism.
The formation of megacrystal Hermon-type granite gneisses by K-replacement of older Popple Hill gneiss, biotite-rich gneisses, and amphibolites (diorites) in the Grenville Lowlands on the northwest side of the Adirondack massif in New York is similar to that found in the megacrystal granite on the eastern side of the Waldoboro complex (web site article number 6), in the megacrystal quartz monzonite at Twentynine Palms, California (web site article number 9), in the megacrystal parts of the outer Ardara granite in Ireland (web site article number 10), in the augen Ponaganset gneiss in Rhode Island (web site article number 22), and in the megacrystal portions of the Kavala pluton in northern Greece (web site article number 29). All of these megacrystal granites have common K-replacement characteristics in that they are in terranes exhibiting strong deformation resulting from tectonism in fault zones or thrust belts or from upward movements of solidified diapiric plutons adjacent to their walls.
I wish to thank Cathy Shrady and John Bursnall for leading me to the Gray's School granite area and providing a reprint of an article. Jim Carl also sent me reprints describing the area. Bill deLorraine spent a half-day in the field with me and provided some geologic maps. All of this assistance has been of great help. Finally, my wife, Barbara, spent many hours suggesting editorial changes that have remarkably improved its presentation.
Dr. Lorence G. Collins California State University Northridge Department of Geological Sciences 18111 Nordhoff Street Northridge, California 91330-8266 FAX 818-677-2820