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Petrology and Geochemistry of the Beemerville Carbonatite-Alkalic Rock Complex, New Jersey

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Petrology and Geochemistry of the Beemerville Carbonatite-Alkalic Rock Complex, New Jersey Petrology and geochemistry of the Beemerville carbonatite-alkalic rock complex, New Jersey LAWRENCE R. MAXEY* Department of Geosciences, Rider College, Lawrenceville, New jersey 08648 ABSTRACT Folded miogeosynclinal sedimentary rocks bound the Precam- brian rocks to the west (Fig, 1). The Hardyston Formation of Early The Beemerville carbonatite-alkalic rock complex of Late Or- Cambrian age, about 35 m thick and composed of conglomerate, dovician age consists of two stocklike bodies of nepheline syenite sandstone, and sandy and shaly dolomite, overlies the Precambrian and dikes or sills of phonolite, tinguaite, lamprophyre micromalig- rocks nonconformably. The Kittatinny Formation of Late Cam- nite, lamprophyre micromelteigite, and carbonatite. The complex brian to Early Ordovician age, about 825 m thick, conformably also includes several lamprophyric diatremes with xenoliths of overlies the Hardyston Formation. The Kittatinny Formation, de- sedimentary rock and gneiss and autoliths of carbonatite, potassic posited in a supratidal to neritic environment, is composed mostly syenite, and lamprophyre micromelteigite. The largest diatreme of bedded to massive dolomite. The Jacksonburg Limestone of also contains a small pluglike body of nepheline syenite; fenite after Middle Ordovician age disconformably overlies the Kittatinny graywacke occurs adjacent to one of the diatremes. Intense hy- Formation and is about 80 m thick. The Jacksonburg Limestone is drothermal alteration is particularly evident in phonolite, lam- composed of calcarinite and calcilutite and was deposited when the prophyre, diatreme autoliths, and diatreme matrix rock. area was changing from a carbonate shelf to a flysch basin. The Field and bulk chemical evidence suggests that parental magma Upper Ordovician Martinsburg Formation, composed of slate and was of either highly carbonated melteigitic or slightly carbonated shale interlayered with graywacke siltstone and graywacke, con- malignitic composition. A petrogenetic model based on each of formably overlies the Jacksonburg Limestone. The Martinsburg these possibilities is explored. The model that begins with highly Formation, about 3,000 m thick, locally shows evidence of deposi- carbonated melteigite magma involves immiscibility relations be- tion by turbidity currents. The Lower Silurian Shawangunk Con- tween melteigite and carbonatite magmas as well as fractional crys- glomerate, mostly quartzite conglomerate and quartzite, overlies tallization processes. The model that begins with slightly carbon- the Martinsburg Formation and is about 500 m thick. It is sepa- ated malignite magma includes, in addition, immiscibility relations rated from the Martinsburg Formation by an angular uncon- between carbonated melteigite and syenite magmas. formity and records molasse-type sedimentation following the Taconic orogeny. INTRODUCTION More detailed descriptions of the regional geology are by Hague and others (1956), Baker and Buddington (1970), and Smith The Beemerville complex is an isolated occurrence of highly un- (1969) for the Precambrian rocks and Spink (1967), Drake (1969), dersaturated to nearly silica saturated alkalic rocks and carbona- and Epstein and Epstein (1969) for the Cambrian to Silurian rocks. tite. Rock types in addition to carbonatite are nepheline syenite, phonolite, potash syenite, mafic phonolite, tinguaite, lamprophyre micromalignite, lamprophyre micromelteigite, and fenite. It is the ALKALIC ROCKS AND CARBONATITE only alkalic rock complex in the eastern United States in which carbonatite has been recognized. Since the alkalic rocks form Nepheline Syenite Plutons. The major occurrence of nepheline stocklike bodies, diatremes, dikes, and possibly sills in folded and syenite in the complex is as two stocklike bodies at the western faulted country rock, the complex is clearly a posttectonic feature. extremity (Fig. 1). Nepheline syenite also forms two small pluglike Zartman and others (1967) obtained a radiometric age for Beemer- bodies, one in and one immediately adjacent to a diatreme, which ville biotite of 435 ± 20 m.y., which, considered with the observed will be considered in a following section dealing with the dia- intrusive contacts of the alkalic rocks with country rocks ranging in tremes. age from Precambrian to Late Ordovician, indicates that the intru- The nepheline syenite plutons are bounded on the west by the sive activity followed soon after the Taconic orogeny. Shawangunk Conglomerate and on the east by the Martinsburg Formation baked into a hornfels near nepheline syenite contacts COUNTRY ROCKS that are not, however, exposed. Emerson (1882) and Kemp (1892) believed that the nepheline syenite intruded along the The Precambrian rocks in the eastern part of the complex (Fig. 1) Shawangunk—Martinsburg Formation contact and is, con- underlie the Reading Prong, a northeast-trending physiographic sequently, at least as young as Early Silurian. Vogel (1970) believed and geologic province, and consist of a wide variety of high-grade that the nepheline syenite intruded the Martinsburg Formation, quartzofeldspathic orthogneiss and paragneiss, amphibolite, and which was eroded to a level exposing the nepheline syenite before marble that were plastically deformed and intruded by syntectonic the Shawangunk Conglomerate was deposited. Evidence bearing granite and syenite about 1,150 m.y. ago. The Precambrian rocks on this problem, which is not easily resolved due to the lack of are cut by numerous northeast-trending high-angle faults and lo- exposed nepheline syenite—Shawangunk Conglomerate contacts, is cally contain within them belts of infaulted Cambrian to Silurian an exposure of nepheline syenite southwest of the southernmost sedimentary rocks. pluton. This exposure consists of two blocks of nepheline syenite that are stratigraphically higher than basal Shawangunk (Fig. 1). If " Present address: Direktorat Geologi, Jalan Diponegoro 57, Bandung, Indonesia. in place, they define a dike offshoot into the Shawangunk Con- Geological Society of America Bulletin, v. 87, p. 1551-1559, 3 figs., November 1976, Doc. no. 61103. 1551 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/11/1551/3428996/i0016-7606-87-11-1551.pdf by guest on 29 September 2021 D J RUTAN HILL (TRENO INFERRED) P| P/,1I P ' I P "I BEEMERVILLE /PA 0 DO P/ Vp 0 o o PA/ DO A/I I> I P '/ ,/T LOCATION DIAGRAM NEW if) YORK I EWARKI CITY Ojb. BRANCHVILLE • Figure 1. (Inset is on facing page.) Geologic map of the Beemerville complex. Precambrian ge- ologic Map of the Franklin Furnace Quadrangle (1908). Alkalic rock locations compiled from the ology generalized from geologic maps by Hague and others (1956) and Baker and Buddington above sources and geologic maps by Wilkerson (1952), Davidson (1948), and Spink (1967). (1970). Sedimentary rock geology taken from the Geologic Map of New lersey (1950) and the Ge- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/11/1551/3428996/i0016-7606-87-11-1551.pdf by guest on 29 September 2021 BEEMERVILLE COMPLEX, NEW JERSEY 1553 Figure 1. (Continued). EXPLANATION CAMBRIAN-SILURIAN SEDIMENTARY ROCKS Ssg Shawangunk Conglomerate Omb Martinsburg Formation Ojb Jacksonburg Limestone €0k Kittotinny Formation •eh Hardyston Formation PRECAMBRI AN IGNEOUS AND METAMORPHIC ROCKS p-Cm Marble pCg Silicic rockS) includes various types of quartzo-feldspathic gneisses, amphibolite, and granite LATE ORDOVICIA N IGNEOUS ROCKS NS Nepheline syenite P Highly-altered phonolite dikes/ sills, PA where pronounced albi- tization is evident in thin section BP Basic phonolite dikes L Extremely fine-grained and/or highly-altered lamprophyre dikes LMA Lamprophyre micromalignite dikes LME Lamprophyre micromelteigite dike T Tlngualte dikes/sills C Silicocarbonatite dike/sill D Diatremes augite to aegerine. The distinction between the latter two minerals in this study is based on optic sign, negative for aegerine and posi- tive for aegerine-augite. Zoned clinopyroxene has cores of diopside-hedenbergite (X-.c = 38°) and rims of aegerine-augite or cores of aegerine-augite and rims of aegerine. Biotite occurs in discrete grains and also replaces pyroxene. Cancrinite and sodalite partly replace nepheline, and trace fluorite partly replaces pyroxene. Veinlets of sodalite are common; veinlets of albite are extremely rare. Table 1 gives bulk chemical and modal data for 5 nepheline syenite samples from the two plutons. Data for 13 other nepheline syenite samples are in Appendix l.1 The chemical analyses were made by atomic absorption spectroscopy using the fusion method of Medlin and others (1969) for sample preparation. U.S. Geologi- glomerate from the nepheline syenite pluton immediately north- cal Survey rock standards were used for calibration. Acid-soluble east. C02 values were obtained using the method of Kolthoff and others Iddings (1898a) and Wilkerson (1946) gave detailed pétro- (1969, p. 1105). graphie descriptions of the nepheline syenite. The texture is Phonolite Bodies. Phonolite forms dikes in the southernmost medium to coarse grained and ranges from hypautomorphic- nepheline syenite pluton, dikes that crosscut diatreme materials, granular to porphyritic with phenocrysts of either nepheline or and dikes and possibly sills in the Martinsburg Formation. The orthoclase. Other primary minerals are clinopyroxene, biotite, phonolite dikes in nepheline syenite vary in width from a few mil- sphene, melanite, magnetite, apatite, and traces of pyrite and zir- limetres to 25
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