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Petrology of Granophyre in Diabase Near Dillsburg, Pennsylvania

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Petrology of Granophyre in Diabase Near Dillsburg, Pennsylvania BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 64. PP. 676-704. 14 FIGS. JUNE 1963 PETROLOGY OF GRANOPHYRE IN DIABASE NEAR DILLSBURG, PENNSYLVANIA BY PRESTON E. HOTZ ABSTRACT Small bodies of granophyre occur in the upper part of diabase bodies of Triassic age in southeastern Penn- sylvania. One near Harrisburg was penetrated by a diamond-drill. Drill core specimens show a gradation from diabase to granophyre. New data include 10 chemical analyses, spectrographic determinations of trace elements, and the results of petrographic study of specimens from the drill core. The sequence, from diabase to granophyre, includes a chilled zone that represents an original magma of tholeiitic composition, normal diabase, pegmatitic facies of diabase, and granophyric diabase that is intermediate in composition and petrographic characteristics between diabase and granophyre, and finally granophyre. Alkalies and silica increase progressively from diabase to granophyre; iron increases to a maximum in transitional granophyric diabase, then decreases in the granophyre. It is concluded that crystal fractionation in a large sheetlike body of tholeiitic magma yielded a small amount of granophyre. Prior to complete solidification, a residual liquid rich in iron, alkalies, and silica accumulated locally in the upper part of the diabase sheet. In places volatile-rich iron-bearing solutions escaped into the overlying sedimentary rocks and deposited magnetite; the remaining liquid crystallized to fine-grained granophyre. CONTENTS Page Introduction 676 Granophyre 701 Acknowledgments 676 Temperature 702 General geology 676 Summary 702 Petrography 678 References cited 703 Definition of diabase 678 General characteristics 678 Diabase 678 ILLUSTRATIONS Pegmatitic facies 678 Figure Page Granophyre 679 Map showing the distribution of diabase in Diabase-granophyre sequence exposed by Triassic rocks of southeastern Penn- diamond-drilling 680 sylvania 677 Petrography of the drill hole section 682 2. Geologic map of northwestern York General statement 682 County, Pennsylvania 678 Chilled contact facies 682 3. Geologic map of the Dillsburg district, Diabase with pegmatitic network 682 York County, Pennsylvania 680 Normal diabase 682 4. Schematic representation of drill hole 3 ... 681 Pegmatitic facies 683 5. Diabase and fine grained diabase with Transitional granophyric diabase 683 coarse-grained network 682 Granophyre 684 6. Pegmatitic facies 683 Mineralogy 684 7. Transitional granophyric diabase 683 Oli vine 684 8. Granophyre 684 Feldspar 685 9. Triangular diagram showing composition Micropegmatite 685 of the pyroxenes 686 Pyroxene 685 10. Variation in modal minerals of the diabase- Mineral variation 688 granophyre sequence 688 Chemical data 688 11. Variation in chemical constituents of the Composition 688 diabase-granophyre sequence 694 Chemical variation 692 12. Variation of FeO, MgO, and K2O + Petrology 700 Na2O in the diabase-granophyre se- General statement. 700 quence 695 Origin of the diabase-granophyre series. 700 13. Variation of CaO, K2O and Na20 in the General statement 700 diabase-granophyre sequence 696 Mineralization 701 14. Na2O:K2O ratio of 21 analyzed grano- Pegmatitic facies 701 phyres 698 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/64/6/675/3441449/i0016-7606-64-6-675.pdf675 by guest on 26 September 2021 676 P. E. HOTZ—GRANOPHYRE NEAR DILLSBURG, PENNSYLVANIA INTRODUCTION a cover of sedimentary rocks. The upper part of this body contains a zone of pink granophyre Terrestrial and lacustrine sedimentary rocks grading above and below into apparently nor- of Triassic age occupy several troughlike basins mal diabase and affords an unequalled oppor- occurring with some interruptions from Nova tunity to make a detailed petrographic study. Scotia to South Carolina. The basins are nar- row and are elongate north and south; and ACKNOWLEDGMENTS range from 25 to more than 300 miles in length and from 10 to 30 miles in width. The largest The field work and part of the laboratory extends from southeastern New York to north- studies for this report were carried on by the ern Maryland; its northern part is commonly writer in 1945 and 1946 as a phase of the study known as the Newark Basin and the southern of the Triassic magnetite deposits of the Dills- part as the Gettysburg Basin. Toward the end burg district by the U. S. Geological Survey of the Triassic period, basaltic magma invaded (Hotz, 1950). This was supplemented by addi- these basins and formed intrusive sheets and tional independent laboratory study during flows. 1947 and 1948. I am indebted to Princeton In the Gettysburg Basin and southern part University for office facilities and for a research of the Newark Basin are many intrusions of grant to cover the cost of 10 chemical analyses, diabase (Fig. 1). Many have a ring-shaped out- and to Professors A. F. Buddington and H. H. crop pattern due to their warped sheetlike habit Hess for their interest and helpful suggestions as described elsewhere (Hotz, 1952). Magnetite throughout the course of the investigation. deposits are associated with bodies of diabase in southeastern Pennsylvania, the most impor- GENERAL GEOLOGY tant and best known of which is at Cornwall, The areal geology of the Dillsburg district Lebanon County. has been more or less completely described. The undifferentiated magma that invaded The magnetite mines were first described by these sedimentary basins represents a world- Frazer (1877). A later detailed report was made wide type, the tholeiitic magma type of Ken- by Spencer (1908, p. 74-96). An important nedy (1933, p. 240-242). The thick sill that discussion of the origin and somewhat different forms the Palisades in New Jersey and New interpretation of the structure of the ore de- York cooled slowly enough to permit a con- posits was made by Harder (1910). A map and siderable degree of differentiation. Lewis (1908, report on the geology of York County, includ- p. 155-162) and Walker (1940, p. 1059-1105) ing the Dillsburg district, was prepared by have studied the petrography, and have de- Stose and Jonas (1939). Two recent articles on scribed the differentiation process that formed the results of diamond drilling exploration of the well-known olivine-rich layer near the bot- the magnetite deposits have been prepared by tom of the sill. Stose and Lewis (1916, p. 623- Neumann (1947) and the writer (Hotz, 1950). 643) described the diabase in the vicinity of The oldest rocks in the area are early Paleo- Gettysburg, and Stose and Jonas (1939, p. 126- zoic and occur northwest of Dillsburg, where 130) described the Triassic intrusions in York they are in fault contact with Triassic rocks County, where they recognized small isolated (Fig. 2). masses of "pink diabase or diabase pegmatite" The Triassic sedimentary rocks are predomi- as differentiates of the diabase magma. nantly red shale and sandstone of the Gettys- Dillsburg is a small village in northeastern burg shale. This formation is characterized by York County, Pennsylvania, about 15 miles lenticular beds of limestone conglomerate southwest of Harrisburg (Fig. 1). From about whose constituent pebbles have been derived 1855 to the early 1900's, a small amount of from the near-by Paleozoic rocks. The magne- magnetite was produced in the district from tite deposits are replacement bodies in the underground workings and shallow open cuts. limestone conglomerate. Within the formation In 1945, during further exploration of the de- is also a belt of coarse arkosic sandstone and posits, drilling located a body of diabase beneath quartzose f anglomerate, which has been mapped Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/64/6/675/3441449/i0016-7606-64-6-675.pdf by guest on 26 September 2021 o H Ow ot-l o o FIGURE 1. MAP SHOWING THE DISTRIBUTION or DIABASE IN TRIASSIC ROCKS op SOUTHEASTERN PENNSYLVANIA Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/64/6/675/3441449/i0016-7606-64-6-675.pdf by guest on 26 September 2021 678 P. E. HOTZ—GRANOPHYRE NEAR DILLSBURG, PENNSYLVANIA and described by Stose and Jonas as the Heid- used here, diabase includes hypabyssal rock of lersburg member of the Gettysburg shale. basaltic composition and ophitic or subophitic Adjacent to the bodies of diabase, the normally texture, which is commonly called dolerite by red sedimentary rocks are bleached and meta- writers outside North America. Gettysburg shale Red sandstone, shale,and limestone conglomerate len«i.Contains Heid- lersburg member of orkosic land- Hone and quortzose fcnglomerote, l^gh.'Contoct-metomorphoted'rockt neor diabase shown by tone. "Contact °- Dashed where approximately located Fault Dai fit d where approximately located FIGURE 2. GEOLOGIC MAP OF NORTHWESTERN YORK COUNTY, PENNSYLVANIA morphosed to gray and buff hornfels and General Characteristics quartzite, and the limestone conglomerate is Diabase.—The diabase of the Triassic basins recrystallized to marble or replaced by silicates. of the eastern United States is exceptionally In the Dillsburg region (Fig. 2), the main uniform. It is commonly gray, medium-grained body of diabase is continuous to the south with and of uniform texture. Fresh white feldspar the long, apparently conformable Gettysburg laths intergrown with dark grayish-green to sill (Stose and Bascom, 1929, p. 11). Westward, black pyroxene, and scattered black metallic this large intrusive mass forms a ring that granules of ilmenite magnetite can be identified almost completely encloses
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