Hypersthene Syenite and Related Rocks of the Blue Ridge Region, Virginia1

BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA

V o l. 27, pp. 193-234 June 1, 1916

HYPERSTHENE SYENITE AND RELATED ROCKS OF THE
BLUE RIDGE REGION, VIRGINIA1

BY THOMAS L. WATSON AND JUSTUS H. CLINE

(Presented before the Society December 29, 191k)

CONTENTS

Page

Introduction.................................................................................................................. 194

Previous geologic work............................................................................................. 196 Quartz-bearing hypersthene-andesine syenite...................................................... 197
Distribution.......................................................................................................... 197 Megascopic character......................................................................................... 198 Microscopic character........................................................................................ 199 Chemical composition and classification...................................................... 202 Comparison with quartz monzonite.............................................................. 204
Origin and application of name............................................................. 204 Chemical composition................................................................................ 205
Comparison with akerite.................................................................................. 206 Comparison with syenite (andesine anorthosite) of Nelson County, Virginia.............................................................................................................. 209 Comparison with pyroxene syenite of the Adirondack«.......................... 212 Comparison with charnockite.......................................................................... 218
Unakite type................................................................................................................ 220
Origin of name.................................................................................................... 220 Distribution and characteristics of unakite.............................................. 220 Origin of the unakite........................................................................................ 222 Zirconiferous epidosite...................................................................................... 223
Granite........................................................................................................................... 223 Norite............................................................................................................................. 225
General discussion of characteristics and distribution..................... ..... 225

Megascopic character......................................................................................... 226 Microscopic character........................................................................................ 227 Ilmenite-apatite gabbro..................................................................................... 228 Chemical composition and classification...................................................... 229
Pyroxenite..................................................................................................................... 231
Distribution......................................:.................................................................. 231 Microscopic character........................................................................................ 231 Chemical composition and classification...................................................... 232
Age relations................................................................................................................ 233

1Manuscript received by the Secretary of the Society December 6, 1915.

XV—Bu l l . Ge o l . So c . Am., Vo l . 27, 1915

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194 WATSON AND CLINE---- ROCKS OF THE BLUE RIDGE REGION

I n t r o d u c t i o n

The Blue Ridge, which forms the extreme eastern member of the Appalachian Mountains, constitutes one of the principal topographic divisions of the Appalachian ranges. In Virginia the Blue Eidge Mountains form a fairly continuous and well defined ridge extending from Harpers Ferry southwestward entirely across the State. At Harpers Ferry the Blue Eidge Mountains are narrow, and in elevation are less than 1,000 feet above sealevel; but southwestward through Virginia the ridge becomes broader and higher, and attains its greatest width in North Carolina. Heights of more than 4,000 feet above the sea are reached at several points in Virginia.
The Blue Eidge is composed of a central core of igneous rocks, flanked on the northwest side by the folded sedimentary series of Cambro-Ordovician rocks of the Great Valley province. The basal member of this series is a quartzite (Weverton), which extends for much of the distance as a range of hills along the west flank of the main ridge at an altitude equal in some cases to that of the Blue Eidge. Eemnants of the Cambrian series of sediments are also preserved in places along the southeast slope of the Blue Eidge and at several points in the vicinity of James Eiver Gap. The sediments are arched in anticlinal fashion entirely over the ridge, completely concealing for short distances the central core of igneous rocks. The southeast slope merges into the Piedmont Plateau, along which, in places, are groups of outlying low ridges that have been isolated by erosion from the main ridge, but exhibiting as a rule similar rock types.
In the middle and northern parts of the Blue Eidge and the adjacent portions of the Piedmont Plateau in Virginia one of the dominant igneous rocks of granitoid type is a quartz-bearing pyroxene syenite. The igneous complex, of which pyroxene syenite is the chief type, may represent a Precambrian batholithic intrusion, exposed at intervals for a distance of 150 miles in a belt up to 20 miles or more in width. Differentiation of the syenite magma has given rise to a variety of related rocks, some of which are of particular interest.
Studies of the igneous complex forming the central core of the Blue
Eidge in middle and northern Virginia are sufficiently advanced to indicate that the rock types exhibit certain kinships which mark them as differentiates from a common magma, and that this igneous complex, designated by the writers as the Blue Eidge petrographic province, shows certain important differences in mineralogy and chemistry from the igneous rocks which enter into the composition of the Piedmont Plateau to the east.

MAP

S H O W IN G

LO CA TIO N

OF

THE

R O C K S

196 WATSON AND CLINE---- ROCKS OF THE BLUE RIDGE REGION

The results presented in this paper are based on field and laboratory investigations of the igneous complex which forms the central core of the northern and middle Blue Ridge region and the adjacent portions of the extreme western margin of the Piedmont Plateau, undertaken at brief intervals during the past ten years. The scope of the paper is limited to a petrographic study of this igneous complex, since a complete discussion of the Blue Ridge igneous complex can not be attempted in advance of a thorough investigation of the field relationships. The principal claim for this study, therefore, is as a contribution to the petrography of the Blue Ridge geology.

P r e v i o u s g e o l o g i c W o r k

Probably the first reference to the syenites of the Blue Ridge in Virginia was by Prof. William Barton Rogers2 in his annual reports on the Geology of the Virginias from 1835 to 1841. He refers to the occurrence of syenitic rocks at various places in the Blue Ridge, notably in the James River and Tye River gaps, and makes special reference to the pronounced porphyritic texture of the syenite in Tye River Gap.
During his early work in Virginia Prof. William M. Fontaine devoted considerable time in the field to study of the Blue Ridge syenites. Only occasional reference is made to their occurrence in his publications, yet the degree to which they attracted his attention is shown by the large collection of the rocks which he made from different parts of the Blue Ridge and adjacent portions of the Piedmont Plateau. These collections are preserved in Brooks Museum at the University of Virginia and have been freely used in this study.
In 1884, Professor Fontaine sent to the United States National Museum specimens of unakite from Milams Gap, in Page and Madison counties, Virginia, and later, in 1913, specimens of the associated syenite This led to the first petrographic description of these rocks in 1904 by Mr. W. C. Phalen,3 who designated the syenite of the Page-Madison counties area as hypersthene akerite because of its similarity to the akerites of Norway described by Brogger.
In 1894, Mr. Arthur Keith4 mapped the syenite occurring southwest of
Front Royal along the extreme western side of the crystalline belt as granite, six varieties of which he distinguished and described in the Blue Ridge region. Keith’s description of one of the varieties follows:

2See a reprint of annual reports and other papers on the geology of the Virginias, by William Barton Rogers, 1884, 832 pages.
3W. C. Phalen: A new occurrence of unakite. Smithsonian Miscellaneous Collections, vol. 45, 1904, pp. 306-316.
4Arthur Keith: Geology of the Catoctin Belt. Fourteenth Ann. Kept. U. S. Geol. Survey, 1894, part 11, pp. 285-395.

197

PREVIOUS GEOLOGIC WORK

“Granite from one mile northwest of Browntown, Virginia, shows quartz, orthoelase, plagioclase, hornblende, a little biotite, magnetite, and, along the feldspar cleavage, chlorite. Another specimen from the same locality shows the same minerals in larger crystals. A third specimen contains a large amount of garnet in rude crystals and fragments, a little apatite and pyrite, and but very little orthoelase” (page 300).

In 1906, Weed and Watson5 described the coarse-grained hypersthene syenite traced by them at irregular intervals along the west side of the Blue Eidge from Dickeys Hill, south of Front Royal, in Warren County, southward to Hightop and beyond in Greene County. Farther southwest, at Stony Man, they stated that the field relations indicated the syenite to be of later age than the Catoctin schist, which is regarded as Algonkian.
In 1907, Watson8 published a brief discussion of hypersthene syenite and associated hornblende norite from a locality in the northern part of Floyd County, Yirginia. Attention was called to the close similarity of the syenite from Floyd County to the unakite-bearing syenite at Milams Gap, in Page and Madison counties, more than 100 miles to the northeast, and to similar syenites of Norway.
In 1913, Watson and Taber7published a detailed discussion, including descriptions of a syenite-norite-nelsonite series of rutile-bearing rocks in Amherst and Nelson counties, Yirginia, and of the hypersthene syenite occurring on the west slope of the Blue Ridge farther southwest in Roanoke County. The Amherst-Nelson counties rock series shows striking relationships in mineralogy and chemistry to the granite-syenite-norite series of the Blue Ridge igneous complex of the larger Blue Ridge province.

Qu a r t z -b e a r i n g H y p e r s t h e n e -A n d e s i n e S y e n i t e

DISTRIBUTION

Quartz-bearing hypersthene syenite is. one of the most abundantly occurring granitoid rocks in the central Blue Ridge region. It has been observed in the Blue Ridge and adjacent portions of the Piedmont Plateau in numerous exposures distributed over a distance of about 150 miles in a northeast-southwest direction (see map, figure 1). The maximum distance between extreme occurrences along a direction (northwest-southeast) normal to the axis of the Blue Eidge will probably not exceed 30

EW. H. Weed and T. L. Watson: The Virginia copper deposits. Economic Geology, vol. i, 1906, pp. 309-330 ; see especially pp. 318-319. s Thomas L. Watson: The occurrence of nickel in Virginia. Trans. Amer. Inst. Mng. Engrs., 1907, pp. 306-316; Mineral Resources of Virginia, 1907, pp. 31-33, 580-582.
7Thomas L. Watson and Stephen Taber: Geology of the titanium and apatite deposits of Virginia. Bull. III-A, Virginia Geol. Survey, 1913, 308 pages; see also Bull. 430, U. S. Geol. Survey, 1910, pp. 200-213.

198 WATSON AND CLINE---- ROCKS OP THE BLUE RIDGE REGION

miles. The most southwesterly known exposure of the syenite is in Floycl County and the most northeasterly one is in Warren County, a short distance south of Front Royal. Exposures of the rock have been observed in every county of the Blue Ridge region between Floyd and Warren counties.
The syenite frequently occurs in the northwest slope of the Blue Ridge, where it may form the basement on which the Lower Cambrian sediments rest. Usually it is more abundantly exposed on the southeast slope of the ridge, since in this position the Cambrian sediments have been removed to a large extent by erosion. It is not restricted, however, in distribution to the slopes of the Blue Ridge, but is noted in many localities in the most elevated portions of the ridge. Along the western margin of the Piedmont Plateau hypersthene syenite is the chief rock, forming Tobacco Row Mountain in Amherst County, and is also noted in similar positions in Madison and Greene counties. Hypersthene syenite forms the Peaks of Otter, and it is very abundant in James River Gap and vicinity, where it makes up large portions of the central core of the Blue Ridge.

MEGASCOPIC CHARACTER

Normally the hypersthene syenite is of greenish color, which varies in shade from a moderately light greenish to a dark greenish gray rock. It shows granitic texture, which varies from medium fine to coarse granular, and having usually a pronounced greasy or waxy appearance. In places the rock is porphyritic in texture, with phenocrysts of orthoelase, rarely of pyroxene. Variation in color of the fresh rock is noted, dependent on the relative proportions of the felsic and mafic minerals, the former varying in amount from 75 per cent to 92 per cent. In structure the rock is usually massive, but in places indistinct to pronounced foliation is shown, due to the effects of pressure metamorphism. Weathered surfaces are usually deeply pitted from inequality of resistance of the component minerals, and the rock yields on complete decay a reddish brown soil which is quite fertile.

The syenite is composed essentially of orthoelase (microcline), plagioclase, and pyroxene, each of which may be recognized in hand specimens. Feldspar is the most abundant constituent of the rock and is usually of dark greenish gray color, with glistening cleavage surfaces, but decidedly waxy on fracture surfaces. Quartz is nearly always present, but in varying amount, some specimens showing very little, while others contain considerable. Biotite occurs in places and in several localities it is present in sufficient amount to be designated a characterizing accessory. There are facies of the rock in which hornblende is an important con-

QUARTZ-BEARING HYPERSTHENE-ANDESTNE SYENITE

199

stituent. Garnet of red color is developed in grains and crystals of the syenite of certain localities in Roanoke County and of those in Greene and adjoining counties in the northern Blue Ridge.

MICROSCOPIC CHARACTER

Normative feldspar ranges in amount from 56 per cent to 66 per cent.
The varieties of orthoclase, microcline, albite, and calcic andesine make up the feldspar content of the rock. Of these andesine is the chief feldspar, although orthoclase or microcline may equal or exceed it in amount in some thin sections. Like the rocks of other areas which it most
-closely resembles and with which it is compared in subsequent pages of this paper, a noteworthy feature of the soda-lime feldspars in the rock is the frequent absence of twinning lamellas, which might readily be mistaken for orthoclase. Microcline is the dominant variety of feldspar in some thin sections. Albite occurs in most thin-sections studied, not as separate individuals, but intergrown with the’potash varieties as mocroperthite. Andesine is developed in anhedral forms and is frequently twinned, both on the albite and pericline laws. Its composition, determined by Larsen 011 four specimens of the rock collected from different localities, by measurements on the rhombic section and of the index of refraction, is as follows:

III
........................................................................................ Ab1An1

......... AbgjAn^

III ........................................................................................ Ab^An^ IV ........................................................................................ AbjoAn*, V

....................................................................................... Ab^Aiijj
I. Specimen of syenite collected near the southeast foot of the Blue Ridge, in Browns Gap, Albemarle County, Virginia.
II. Specimen of syenite collected near the southeast foot of Tobacco Row
Mountain, in the vicinity of Elon, Amherst County, Virginia.
III. Specimen of syenite collected on northwest slope of the northern Blue
Ridge, near the boundary between Madison and Greene counties, Vir- ginia.
IV. Specimen of syenite collected from the base of northeast slope of the
Peaks of Otter, Bedford County, Virginia.
V. Specimen of syenite collected 1y2 miles east of Vinton, Roanoke County,
Virginia.