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Proceedings of the United States National Museum — THE MINERALOGY AND PETROLOGY OF INTRUSIVE TRI- ASSIC DIABASE AT GOOSE CREEK, LOUDOUN COUNTY, VIRGINIA. By Earl V. Shannon, Assistant Curator of Geology, United States National Museum. INTRODUCTION. The present paper records observations made on several trips to the Goose Creek trap quarry, together with the results of a large amount of office and laboratory study of the specimens collected. The data thus acquired have become somewhat voluminous and, since other duties are forcing attention, it is considered best to present the results thus far attained while they are fresh in mind. My work does not by any means exhaust the locality but merely serves as a start- ing point from which study of the interesting features may be con- tinued. The locality is a pleasant journey from Washington and is recommended to any petrologist or mineralogist seeking an educa- tional day's outing in the field to relieve the monotony of office routine. ACKNOWLEDGMENTS. I have enjoyed an unusual amount of cooperation in preparing this paper and it seems appropriate to make some acknowledgments. For the suggestion that the locahty would be found worthy of study I have to thank Dr. George P. Merrill. One of my visits to the quarry was made in company with the following members of the Mineralogical Society of Washington: W. F. Foshag, W. S. Burbank, Frank L. Hess, Esper S. Larsen, jr,, Clarence S. Ross, Edward Samp- son, Waldemar T. Schaller, Edgar T. Wherry, and Ralph W. G. Wyckoff. Observations and opinions of all of these men and speci- mens collected by them have been at my disposal. In addition to the help of the above scientists, various points have been discussed with Dr. Norman L. Bowen, who has pointed out similarities to his Gowganda Lake area, and with Drs. Herbert E. Merwin and Clarence N. Fenner, who are familiar with the locality. I have especially to thank Drs. Esper S. Larsen and Clarence S. Ross for constant advice No. 2539. Proceedings U. S. National Museum. Vol. 66, Art. 2. 94110—24 1 .^ 2 PROCEEDINGS OF THE NATIONAL, MUSEUM. vol. (jG. and help in interpreting the various phenomena and in the micro- scopic work, Dr. Henry S. Washington for courteously reviewing that part of the discussion which deals with the quantitative classification and Dr. Edgar T. Wherry for carefully reading and editing the manusci-ipt. To Harry Warner and Frank Reid, preparators for the National Museum and Geological Survey, I am indebted for the skillful preparation of the numerous thin sections required. Finally, I would acknowledge as the work of J. S. Olmstead, photographer of the National Museum, the excellent natural-size photographs her© reproduced. The photomicrographs I myself took in the Depart- ment of Geology laboratory. LOCATION. The locality in question is a quarry opened in the diabase for " trap rock" which is crushed and sold for road making, and is alongside the right of way of the Washington & Old Dominion Electric Railway just east of Goose Ci-eek, about 6,400 meters (4 miles) southeast of Leesburg. The quarry is about 800 meters (^mile) northwest of Belmont Park Station. GENERAL RELATIONSHIPS. No attempt was made to work out any areal geology except to casu- ally examine a few outcrops in the general vicinity. The locality is within the Harpers Ferry Quadrangle described by Keith. The out- crop of the diabase as shown on Keith's map is extremely irregular with a maximum width at the south of the quadrangle of about 6,400 meters (4 miles). The section shown as crossing the diabase sheet about 8,000 meters (5 miles) to the south of the quarry indicates an intrusive mass of sill-like form, in general conformable with the bed- ding, having a thickness computed from the section of some 750 meters (2,400 feet) . The quarry here described lies within a few dozen meters of the eastern edge of the outcrop and hence is presumably practically at the base of the sill. The ridge on which the quanT is situated is entirely composed of the trappean rock. The next ridge to the east is seen, where cut through for the railroad, to consist of baked and mottled Triassic shale, while the intervening vale is devoid of expos- ures. Keith gives a brief description of the diabase^ stating that it is intrusive \vith a maximum width of possibly 250 meters (800 feet) If it actually is as thin as this the base must be somewhat flatter than shown on his section. The Triassic rocks of Virginia are now being studied for a report to be published by the Virginia Geological Survey. A preliminary paper on the diabases, including the diabase pegmatites 'Arthur Keith, Harpers Ferry Folio, Folio 10. U. S. Geol. Surv. ART. 2. PETROLOGY AT GOOSE CEEEK SHANNON. 3 of Goose Creok quarry, has been published by Roberts.^ In this paper this mass of intrusive diabase is named the Belmont Stock. GENERAL DESCRIPTION. The quarry exposes no contacts of the intrusive -rocks with the nclosing sediments. The material exposed consists in greatest part of the igneous rock, described below as normal diabase. Next irs abundance comes a rock which does not differ greatly in composition from the normal diabase but which is very coarse grained, the mate- rial of the maximum coarseness containing augite blades 25 cm. (10 inches) in length and 3 cm. in width. A third type comprises other masses, usually of small size, with a similar coarse texture but con- sisting in the main of albite and quartz, the titaniferous augite which is characteristic of the first two types being more or less completely replaced by diopside. This type is considered to be in part an enid differentiate of the coarse second type and in part a hydrothermal alteration product. The masses of the third type contain small miarolitic cavities lined with contemporary crystals of quartz, albite, titanite, and diopside, usually also with a later series of minerals con- sisting of fine fibrous hornblende, epidote, chalcopyrite, and chlorite. The fourth rock type consists also mainly of quartz and albite, and occurs in narrow dikes of aplitic habit filling persistent narrow cracks in the normal rock. These dikes are considered to be essentially of the same origin as the magmatic third rock type, the principal dif- ference being the textural change caused by their intrusion into narrow cracks. All of the rock types considered are cut by numerous joints and fis- sures. Adjacent to these there is hydrothermal alteration of the inclosing rocks of various types which are discussed below^, and the cracks and open spaces themselves are filled with minerals deposited from solution. These minerals, which are described individually in detail in the following pages, consist of several varieties each of horn- blende and chlorite epidote, ; albite; the sulphides, galena, chalcopyrite, pyrite, and possibly pyrrhotite; diopside, axinite, datohte, prehnite' apophyllite, quartz, calcite, laumontite, chabazite, stilbite, etc. There appears to be represented in the limits of the quarry a gradation from the original crystallization of the normal diabase through a series of magmatic differentiates into high temperature hydrothermal deposits, represented by the hornblende in cracks and the minerals in the miarolitic cavities. The latter overlap a sequence found in the veins which grades into the series of minerals characteristic of typical zeohtic deposits. 2 Joseph K. Roberts, Jurassic Intrusives of Virginia. Pan-American Geologist, vol. 39,-t't'pp. 289-296 MayJ-, 1923. 4 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.66. DETAILED DESCRIPTION. OUTLINE. The following detailed description may be represented by the fol- lowing outline although the treatment does not strictly adhere to the outline^in its entirety. 1. Jointing and fissuring. 2. Normal diabase. a.^Macroscopic features. h. Texture. c. Minerals. (1). Feldspar. (2). Pyroxene. (3). Accessories, (a) Iron ore. (6) Biotite. (c) Quartz and micropegmatite. (d) Apatite. 3. Diabase Pegmatite. Definition. a. Macroscopic features. &. Texture. c. Minerals, (1) Feldspar. (2) Pyroxene. (3) Accessories. (a) Micropegmatite. (&) Ilmenite, Iron ore. (c) Biotite. (d) Apatite. (e) Alteration products. 4. Albitic pegmatites—definition. a. Macroscopic features. h. Texture. c. Minerals. (1) Feldspar. (2) Micropegmatite. (3) Pyroxenes. (4) Titanite. (5) Apatite. d. Composition. e. Origin and comparison. 5. Aplitic albite rocks. a. Occurrence, size. h. Macroscopic features. c. Microscopic features, texture. ART. 2. PETROLOGY AT GOOSE CREEK—SHANNON. 5 5. Aplitic albite rocks— Continued. d. Minerals. (1) Quartz. (2) Feldspars. (3) Diopside. (4) Epidote. (5) Alteration products. e. Discussion, Origin. 6. Miarolitic cavities. a. Occurrence, size. h. Original minerals. (1) Quartz. (2) Albite. (3) Diopside. (4) Titanite. c. Second generation minerals. (1) Plornblende (byssolitic) (2) Chalcopyrite. (3) Epidote. (4) Chlorite. d. Discussion of origin. 7. Hydrothermal alteration along seams and fissures. a. Diopside-filled cracks accompanied by diopsidization of the adjacent diabase. 6. Chlorite seams accompanied by hornblendization of the normal rock. c. Hornblende-filled cracks without alteration of the adjacent diabase. d. Bluish hornblende coatings on fracture surfaces. e. Alteration of normal diabase adjacent to zeolite-bearing veins. /. "Diabantite varnish" on slickensided Joints. g. Alteration of diabase pegmatite where intersected by diop- side seams. li. Hornblendization of normal diabase pegmatite along cracks and seams. i. Hornblendization of diopside of albite-diopside pegmatite rocks. 8. Hydrothermal joint and cavity fillings. a. Occurrence. 6. Minerals. (1) Albite. (2) Chlorites. (3) Hornblende (byssolitic). (4) Epidote. PROCEEDINGS OF THE NATIONAL MUSEUM. vol. G6. 8. Hydrothermal joint and cavity fillings- Continued h. Minerals. ^—Continued. j,.RT. 2. PETROLOGY AT GOOSE CREEK—SHANNON. 7 southwest corner of the quarry was seen to containdiabantite-coated fragments with interstitial apophylHte, datoHte, and calcite; and one m the northeast corner furnished calcite, datolite, prehnite, laumontite, and stilbite. The rock adjacent to these joints is for the most part fresh and free from indications of high temperature hydrothermal alteration.
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