U. Mass Lowell Prof. Nelson Eby Department of Environmental, Earth, & Atmospheric Sciences




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Cortlandt-Beemerville Magmatic Belt

The Cortlandt-Beemerville magmatic belt (Ratcliffe, 1981) extends approximately 100 km from the Peach Lake-Croton Falls area of Westchester County, New York, S80oW to Beemerville, New Jersey, (Figure at right) and transects the structural grain of the Appalachians. The calc-alkalic to alkalic Cortlandt and Rosetown complexes mark the eastern end of the belt and the strongly alkalic Beemerville complex marks the western end, although the belt may extend farther to the west beneath Silurian strata. A 30 km wide belt of abundant mafic and intermediate dikes extends from Cortlandt to Beemverville. At the eastern end of the belt these dikes are weakly alkalic (spessartite, kersantite, andesite, and rhyodacite) while at the western end they are strongly alkalic (phonolite and camptonite). Individual dikes typically have NE or NW strikes. Ratcliffe (1968, 1981) concludes that the Cortlandt complex was emplaced syntectonically relative to Taconic dynamo-thermal metamorphism and the Beemerville complex and associated dikes were emplaced post tectonically relative to folding and slatey cleavage in the Martinsburg. Ratcliffe (1981) suggests that the juncture of two distinct Taconic salients controlled the location of the belt and that the magmatic activity occurred in response to differential stresses that produced local rupturing of the continental crust and mantle. These stresses arose because of different directions or rates of plate consumption and collision during the Ordovician.

The present study is focused on the petrology, geochemistry (including INAA trace elements), mineral chemistry (electron microprobe), and geochronology (fission track) of the Beemerville complex and associated felsic and mafic dikes. Additional geochronological studies (titanite and zircon U-Pb ages) are underway to further characterize the timing of the various magmatic events. Norman Charnley (Oxford) has assisted with the mineral chemistry studies. Undergraduate Honor's projects have been done on the Beemerville complex by Scott Maher and Laurene Poland. Publications resulting from the present study are listed below.



Eby, G. N. (2004) Petrology, geochronology, mineralogy, and geochemistry of the Beemerville alkaline complex, northern New Jersey. In Puffer, J. H. and Volkert, R. A. (eds.) Neoproterozoic, Paleozoic, and Mesozoic Intrusive Rocks of Northern New Jersey and Southeastern New York. Twenty-First Annual Meeting Geological Association of New Jersey, Mahwah, NJ, pp. 52-68.

Eby, G. N., Maher, S. G., and Poland, L. J. (1994) Petrology and geochemistry of the Beemerville nepheline syenite complex, northern New Jersey. Geological Association of Canada-Mineralogical Association of Canada Program with Abstracts 19, A32.

Eby, G. N. and Sclar, C. B. (1993) Geochemistry of magmatic titanite from the Beemerville nepheline syenite, Sussex County, New Jersey. Geological Society of America Abstracts with Programs 25, 6, A-381.

Eby, G. N., Sclar, C. B., and Myers, P. B. (1992) A fission-track date on magmatic titanite from the Beemerville nepheline syenite, Sussex County, N. J. Geological Society of America Abstracts with Programs 24, p. 18.

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Petrology, Geochronology, Mineralogy, and Geochemistry of the Beemerville Alkaline Complex, Northern New Jersey

Eby, G. N.

The Beemerville alkaline complex, New Jersey, occurs at the western end of the Cortlandt-Beemerville magmatic belt. The eastern end of the belt is defined by the Cortlandt complex, New York, which largely consists of ultramafic and mafic rocks. Between these two intrusive centers is a linear almost east-west trending zone of lamprophyre and felsic dikes. The Beemerville complex consists of two large nepheline syenite bodies, several diatremes, and associated lamprophyre and phonolite dikes, all of which intrude the shales and graywackes of the Ordovician Martinsburg Formation. The emplacement of these igneous bodies is post-tectonic and titanite fission-track ages indicate an emplacement age of 420 6 Ma. The mean apatite fission-track age is 156 4 Ma, which represents the time the rocks of the complex were last at temperatures of 60oC to 100oC.

The major rock forming minerals are orthoclase, nepheline, and clinopryroxene. Minor minerals are biotite and titanite. Accessory minerals are sodalite, fluorite, calcite, and opaque oxides. Cancrinite alteration of nepheline is common. Based on electron microprobe data the pyroxenes range in composition from diopside through aegirine-augite to aegirine. The feldspars in the nepheline syenites and phonolites are orthoclase (>Or80), with albite noted in some specimens. Two biotite populations can be identified: one encompasses the nepheline syenites and phonolites and the other the lamprophyre dikes and matrix material from the diatremes, an observation which suggests a genetic linkage between the lamprophyres and the diatremes.

Based on textural, phase equilibria, and chemical criteria, the phases fractionating from the syenitic magmas were aegirine, nepheline, and titanite. The nepheline syenites are essentially crystal cumulates with some trapped interstitial melt. Few, if any, of the phonolites represent liquid compositions, and this is partly due to post-magmatic alteration. Nb/Ta and Zr/Hf ratios are very variable, in part due to the fractionation of titanite and in part due to hydrothermal alteration. Three distinct REE patterns are observed for the phonolites: uniform slope downwards from LREE to HREE, flattening at the HREE end, and U-shaped. These patterns are partly due to the fractionation of titanite, but a more detailed explanation most likely involves the role of fluids in altering these patterns.

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Petrology and Geochemistry of the Beemerville Nepheline Syenite Complex, Northern New Jersey, U.S.A.

Eby, G. Nelson, Maher, Scott G., and Poland, Laurene J.

The 422 Ma Beemerville complex is comprised of two bodies of medium- to coarse-grained nepheline syenite, a brecciated carbonate-rich intrusive (Rutan Hill) and associated felsic and mafic dikes. The various intrusives were emplaced into the Ordovician shales and graywackes of the Martinsburg formation. Geophysical data indicate that the main Beemerville bodies continue at depth but are offset to the east. Related intrusives may occur to the west where they are overlain by the Upper Silurian Shawangunk Conglomerate.

In the medium to coarse-grained syenites euhedral nephelines occur within large feldspars and as a separate phase. The dominant mafic is aegirine which occurs late in the crystallization sequence and generally has an acicular and/or radiating habit. Large euhedral titanites are an important accessory mineral. Primary fluorite is found in association with aegirine. F-rich (up to 1.2 wt. %) biotite is a minor accessory. Cancrinite is ubiquitous as an alteration product (of nepheline). In terms of major and trace element chemistry the felsic and mafic dikes form two discrete groups which are apparently not related by fractional crystallization. Biotites from the mafic dikes are richer in Al and Mn, at the same Mg/(Mg+Fe+Mn) ratio, than those from the felsic dikes. The mafic dikes are enriched in K2O relative to Na2O, have high (Ce/Yb)N ratios, and show an affinity with the carbonate-rich breccias. The felsic dikes have lower (Ce/Yb)N ratios and often show convex downwards REE patterns. Both groups, however, have low Y/Nb and Yb/Ta ratios (<0.3) indicating that they were derived from an OIB-like source.

The syenitic melts were anhydrous, but relatively fluid due to their high F content and temperature. In the case of the felsic dikes, magmatic evolution was largely controlled by the crystallization of nepheline. The medium- to coarse-grained syenites are essentially feldspar-nepheline cumulates, with intercumulus aegirine.

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Geochemistry of Magmatic Titanite from the Beemerville Nepheline Syenite, Sussex County, New Jersey

Eby, G. N. and Sclar, C. B.

The outer 15 m. of the southernmost part of the Beemerville nepheline syenite pluton contains 3-6% pale pinkish brown euhedral crystals of magmatic titanite ranging in size from 2-5 mm. Titanite was analyzed with the electron microprobe, by ICP spectroscopy, and by INAA. It contains 4430 ppm Zr, 3300 ppm Nb, 3000 ppm total REE, and only 14 ppm Hf, 17 ppm Th, and 5 ppm U. The titanite has a La/Lu ratio of 474 and a chondrite-normalized REE plot shows that it is strongly enriched in light REE.

Although the nepheline syenite contains 450-1200 ppm of Zr, it contains no detectable zircon. It appears that, because of the low activity of silica in the nepheline syenite magma (46-48% SiO2), Zr4+ was admitted dominantly into the structure of titanite and subordinately into the structure of the Mn-rich titanomagnetite. Benoit and Sclar (1991) discovered minute ZrO2 (baddeleyite) inclusions in the titanomagnetite which appear to be exsolution bodies in the pyrophanite-ilmenite solid solution [(MnTiO3)38(FeTiO3)62] exsolved from the titanomagnetite. The high-temperature homogenous igneous spinel which admitted the Zr4+ had the composition [(FeFe2O4)78(Fe2TiO4)13(Mn2TiO4)9]. The high Zr/Hf ratio (308) indicates that the titanite preferentially incorporated zirconium relative to hafnium. This suggests that fractional crystallization of titanite might explain the occurrence of alkalic rocks with low Zr/Hf ratios. The relative concentrations of the REE in this titanite are in accord with the systematics of REE in titanite from other alkalic igneous rocks (Fleischer, 1978).

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A Fission-track Date on Magmatic Titanite from the Bemerville Nepheline Syenite, Sussex County, N.J.

Eby, G. N., Sclar, C. B., and Myers, P. B.

The Beemerville nepheline syenite is a coarse-grained pluton that intruded the Late Ordovician Martinsburg Formation  and was apparently overlain unconformably by the Upper Ordovician(?) to Middle Silurian Shawangunk Formation. To the north, northwest, and east of the nepheline syenite are small plugs of alnoite and carbonatite. Zartman et al (1967) obtained a K-Ar date of 443 20 Ma (corrected to the new K-Ar decay constants) on the biotite phenocrysts in the alnoite. Diamond-drill cores obtained from the southern contact of the nepheline syenite went through hornfels developed from the Martinsburg Formation and reveal that the outer 50 feet of the syenite contains 3-6% of pale pinkish-brown rare earth- and niobium-bearing euhedral magmatic titanite ranging in size from 2-5 mm. The uranium content of the titanite is only 2.5 ppm. The titanite was separated using heavy liquids and magnetic separation, and the age of each of two discrete groups of titanite grains was determined by fission-track methods. One group gave a date of 426 48 Ma; the other gave a date of 417 48 Ma. A combination of analytical uncertainties plus uncertainties in the zeta calibration yield a final age estimate of 422 14 Ma which corresponds to Middle Silurian (Niagaran) time.

The intrusion of the Beemerville nepheline syenite  and the alnoite-carbonatite plugs marks the end of the Taconic orogeny in this area. Therefore, either the Shawangunk Formation may be younger than Albion in this area or the nepehline syenite indeed intruded the Shawangunk Formation in which case the Shawangunk Formation in this area is Albion and the nepheline syenite is Niagaran.

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