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

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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 1 Manuscript received by the Secretary of the Society December 6, 1915. XV—Bull. Geol. Soc. Am., Vol. 27, 1915 (193) 194 WATSON AND CLINE---- ROCKS OF THE BLUE RIDGE REGION I ntroduction The Blue Ridge, which forms the extreme eastern member of the Appa­ lachian 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-Ordo- vician 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 Cam­ brian 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 indi­ cate 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 SHOWING LOCATION OF THE ROCKS F i g u r e 1 .— Outline Map of Virginia Showing location of hypersthene-andesine syenite in black and the Amherst-Nelson counties comagmatic area in parallel ruled lines 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 io u s geologic W ork Probably the first reference to the syenites of the Blue Ridge in Vir­ ginia 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 Mu­ seum 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: 2 See a reprint of annual reports and other papers on the geology of the Virginias, by William Barton Rogers, 1884, 832 pages. 3 W. C. Phalen: A new occurrence of unakite. Smithsonian Miscellaneous Collections, vol. 45, 1904, pp. 306-316. 4 Arthur K eith: Geology of the Catoctin Belt. Fourteenth Ann. Kept. U. S. Geol. Survey, 1894,
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