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Tectonics and metallogenesis of Proterozoic rocks of theReading Prong

Linda C.S. Gundersen

US Geological Survey, MS 911 National Center, Reston, VA 20194, USA

Abstract

Detailed geologic mapping, petrography, and major and trace-element analyses of Proterozoic rocksfrom the Greenwood Lake Quadrangle, New York are compared with chemical analyses and stratigraphicinformation compiled for the entire Reading Prong. A persistent regional stratigraphy is evident in themapped area whose geochemistry indicates protoliths consistent with a back-arc marginal basin sequence.The proposed marginal basin may have been floored by an older sialic basement and overlain by a basin-fill sequence consisting of a basal tholeiitic basalt, basic to intermediate volcanic or volcaniclastic rocks andcarbonate sediments, a bimodal calc-alkaline volcanic sequence, and finally volcaniclastic, marine, andcontinental sediments. The presence of high-chlorine biotite and scapolite may indicate circulation of brinefluids or the presence of evaporite layers in the sequence. Abundant, stratabound magnetite deposits witha geologic setting very unlike that of cratonic, Proterozoic banded-iron formations are found throughoutthe proposed basin sequence. Associated with many of the magnetite deposits is unusual uranium andrare-earth element mineralization. It is proposed here that these deposits formed in an exhalative, volcano-genic, depositional environment within an extensional back-arc marginal basin. Such a tectonic settingis consistent with interpretations of protoliths in other portions of the Reading Prong, the CentralMetasedimentary Belt of the Canadian Grenville Province, and recent interpretation of the origin of theFranklin lead-zinc deposits, suggesting a more cohesive evolving arc/back-arc tectonic model for the entireProterozoic margin of the north-eastern portion of the North American craton.Published by Elsevier Ltd.

1. Introduction

The Reading Prong is one of the largest exposures of Proterozoic rocks in the eastern UnitedStates. It forms the Hudson Highlands, New Jersey Highlands, and Reading Hills as it crosses

0264-3707/$ - see front matter Published by Elsevier Ltd.

doi:10.1016/j.jog.2004.02.012

Journal of Geodynamics 37 (2004) 361–379

E-mail address: lgundersen@usgs.gov (L.C.S. Gundersen).

New York, New Jersey, and Pennsylvania respectively (Fig. 1). Metamorphosed rocks within thisgranulite facies terrain have been generally dated as Grenville, however, in the last decade, a spateof new ages have been published that supercede much of the dating that occurred in the 1970sand 1980s. Table 1 provides an overview of recent age determinations. As can be seen fromTable 1, ages of the major igneous and meta-igneous plutons and suites range from 1 to 1.3 Ga.Ages of metamorphic rocks described as paragneiss, metavolcanic gneiss, and orthogneiss prob-ably reflect the main Grenville metamorphism and are 1–1.1 Ga, however the cores of suspecteddetrital zircon grains and other zircon cores in paragneiss range in age from 1.1–2.0 Ga. Theyoungest rocks include distinctive pegmatitic masses of various composition and late diorite thatare 960–1008 Ma. Overgrowths on zircons reported for a wide variety of rock types are com-monly 990–1000 Ma.

The origin of the Reading Prong rocks has been controversial over most of the one hundredand fifty years they have been studied. Early geologic studies in the Hudson and New JerseyHighlands focused on descriptions of the magnetite deposits and surrounding host gneisses.Rodgers (1840), Kitchell (1857), and Cook (1868) all agreed that the Highland gneisses were

Fig. 1. General map showing the Proterozoic rocks of the Reading Prong (stippled area) and location of geologic mapsand authors used in compiling data for this study including the stratigraphic sections in Fig. 2.

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metamorphosed sediments. A sedimentary origin for the magnetite ore and enclosing gneiss wasgenerally accepted, however, Rodgers (1840) believed that the magnetite ore was of igneousorigin. Spencer (1904, 1905, 1909) proposed an igneous plutonic origin for all of the gneisses anddivided them into three formations—the Pochuck Gneiss, Losee Gneiss, and Byram Granite. Thelatter two are now referred to as the Losee Metamorphic Suite and the Byram Intrusive Suite(Drake, 1984). Bayley (1910) concurred with Spencer, but also interpreted parts of the PochuckGneiss as sedimentary. Berkey (1907) interpreted the gneisses in the Hudson Highlands as agranitic intrusive sequence with many similarities to those in the New Jersey Highlands. Colony(1923) interpreted the New York magnetite deposits to be the result of magmatic replacement ofthe host rock. Sims and Leonard (1952), Hotz (1953, 1954), Sims (1958), Hagner et al. (1963),Buddington (1966), Collins (1969), Baker and Buddington (1970), and Foose and McLelland(1995) concluded that the iron-ore districts of the New Jersey and Hudson Highlands did includea minor proportion of metasedimentary rocks, but that the magnetite deposits were of intrusiveorigin and the result of magma driven replacement or hydrothermal processes.

More recent work in the Reading Prong departs significantly from previous interpretations,while a few studies appear to confirm the abundance of igneous intrusive rocks in the sequence.Helenek (1971), Jaffe and Jaffe (1973), Drake (1970, 1984), Murray (1976), Grauch (1978),Kastellic (1979), and Gundersen (1984, 1985) all recognized that the major-element chemistryof quartz-plagioclase gneiss and amphibolitic gneiss in their field areas is similar to that of

Table 1Recent age determinations on rocks from the Reading Prong

Reference

Age Ma Rock Unit and State Method

Aleinikoff et al. (1982)

1023�59–964 Pegmatite (NY) U-Pb

Aleinikoff and Grauch (1990)

1010�6 Canada Hill (NY) U-Pb 1159–1460 Detrital zircon from paragneiss (NY) U 985 Monazite-xenotine gneiss (NY) Pb

Aleinikoff and Walsh (2002)

1311�7 Granite gneiss (CT) SHRIMP U-Pb

1045�8

Augen gneiss (CT) 1049�13 Biotite gneiss (CT) 1057�10 Biotite gneiss restite (CT)

993�8

Amphibolite (CT) 990 Zircon overgrowths (CT)

Drake et al. (1991a)

1020�4 Mt Eve (NY) U-Pb

Drake et al. (1991b)

1088�41 Byram Intrusive Suite (NJ) Pb-Pb Gates et al. (2001) 1008�4 Diorite (NY) SHRIMP U-Pb

1160 to 1230

Metavolcanic gneiss (NY)

1491+8

Zircon core paragneiss (NY) 2041+23 Zircon core paragneiss (NY)

Ratcliffe and Aleinikoff (1990)

1130 Storm King (NY) U-Pb Ratcliffe (1992) 1136�43 Reservoir Gneiss (NY) U-Pb

Ratcliff and Aleinikoff (2001)

1174�15 Canada Hill (NY) SHRIMP U-Pb 1144�13 Canopus pluton (NY) 1000 Zircon overgrowths (NY)

Volkert et al. (2000)

1116�41 Byram Intrusive Suite (NJ) Rb-Sr 1095�9 Lake Hapatacong Intrusive Suite (NJ) Rb Sr

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keratophyre, andesite, basalt, and spilite. Gundersen (1984, 1985, 2000) proposed that many ofthe magnetite deposits originated in an extensional back-arc marginal basin and proposed abimodal, calk-alkaline volcanic origin for portions of the gneiss. Gundersen (2000) and Puffer(2001) acknowledge that there are several types of magnetite deposits, including vein depositscreated by remobilization into faults and fractures and deposits that are related to intrusive rocksand hydrothermal activity. Puffer and Volkert (1990), Drake et al. (1990, 1996), Volkert andDrake (1999), and Ratcliffe and Aleinikoff (1990) all observed trondjhemitic compositions, aswell as rhyolitic through andesitic compositions in the suspected orthogneiss. Drake et al. (1990)believe the Losee Gneiss to be the base of the sequence, and metavolcanic with trondjhemitic andtonalitic intrusions, whereas Puffer and Volkert (1990) believe the Losee Gneiss to be pre-dominantly tonalite that has been partially melted to form masses of trondjhemite. Most recently,Volkert and Drake (1999) have proposed that many of the layered charnokitic rocks thought tobe metavolcanic basalt and keratophyre are actually associated with the tonalite and are theresult of fractionation of a single parent diorite to produce andesite, dacite, tonalite, andtrondhjemite. Further, Volkert (2001) has proposed that the magnetite deposits were depositedprior to metamorphism in an extensional setting. Throughout the Reading Prong, numerous andvariably composed granitic masses are thought to have been emplaced locally within the sequenceduring regional metamorphism and deformation. These granitic masses range in compositionfrom alaskite to hornblende granite, and from syenite to monazite, and enderbite (Ratcliffe, 1971;Drake, 1970, 1984; Drake et al., 1990; Young, 1971, 1978; Helenek and Mose, 1984; Volkert etal., 2000).

Using age data and structural considerations, Drake et al. (1990), Ratcliffe and Aleinikoff(1990), Ratcliffe (1992), and Volkert and Drake (1999) propose that the trondjhemitic part of thesequence represents an older sialic basement. A comprehensive stratigraphic sequence linking thedifferent sub-provinces of the Reading Prong has not been proposed, nor has a common tectonichistory been developed. This paper will attempt to resolve the conflicting stratigraphic and tec-tonic interpretations of the New York and New Jersey Highlands in the non-plutonic part of thesequence, stratigraphically above units defined as older sialic basement. The geochemistry of themajor rock units and the petrology and geochemistry of the metal deposits are used as key data inresolving the stratigraphic and tectonic problems.

2. Geology of the Reading Prong

2.1. Stratigraphy and petrology of the non-plutonic rocks

Correlation of geologic units within the Reading Prong has been attempted locally in theHudson Highlands (Helenek, 1971; Dallmeyer, 1972; Ratcliffe, 1992; Gates et al., 2001) and inthe southern New Jersey Highlands and eastern Pennsylvania (Drake, 1984, 1990; Volkert andDrake, 1999). Comparison of the known geology, exclusive of presumed meta-igneous plutonicrocks, reveals a striking similarity between rock units and their succession, especially in NewYork and New Jersey (Gundersen, 1985). The Greenwood Lake area of the southern HudsonHighlands and Northern New Jersey Highlands is relatively free of the major granitoids andmeta-igneous plutonic rocks found in the northern Hudson Highlands, the extensive granitoid

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rocks of the Byram and Lake Hapatacong Suites (Drake et al., 1996), and the trondjhemiticportions of the Losee Gneiss found in the New Jersey Highlands. Based on a detailed strati-graphic analysis of the Greenwood Lake Quadrangle and comparisons with other metasedimen-tary and metavolcanic sequences in the Reading Prong, four major stratigraphic units areproposed: (A) calc-silicate gneiss, consisting of a basal hornblende amphibolite, quartz-feldspar-pyroxene gneiss with minor interlayers of quartz-feldspar-gneiss, pyroxene amphibolite, andmarble; apatite, titanite, allanite, magnetite, and scapolite are common accessories or form thinlayers (B) quartz-plagioclase gneiss with minor hornblende and pyroxene amphibolite; titanite and

Fig. 2. Bedrock geologic map of part of the Greenwood Lake Quadrangle, southeast of Warwick, New York.

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magnetite are common accessories; (C) hornblende gneiss with minor hornblende and pyroxeneamphibolite; titanite and magnetite are common accessories and (D) metasedimentary sequence,some portions of which are rusty weathering, consisting of graphitic quartz-plagioclase gneiss,graphitic biotite gneiss, biotite-garnet gneiss and minor hornblende-feldspar gneiss, marble, andpyroxene amphibolite; monazite, xenotine, titanite, and magnetite are common accessories. Theunit designations of A, B, C, D will be used in this text to refer to the sequence just described.

Generalized stratigraphic sections of the Reading Prong are shown in Fig. 2, which also showsthe stratigraphic association of Fe-U and REE deposits within the sequence. These stratigraphicsections are based on the author’s re-interpretation of the geology, and were derived from themaps of Hotz (1953, 1954), Sims (1958), Dodd (1962), Baker and Buddington (1970), Helenek(1971), Drake (1967a,b) Drake et al. (1985, 1996), Dallmeyer (1972) Murray (1976), Kastellic(1979), Volkert et al. (1989), Gundersen (1986, 1991), Agard and Gundersen (1991), and Ratcliffe(1992). The location of this source material and the respective authors are plotted on Fig. 1. Thethickness of units shown in the stratigraphic sections is the structural thickness; no attempt hasbeen made to restore the units to a pre-deformation thickness. In general, the metavolcanic andmetasedimentary package is tightly folded and interlayered. Many of the authors cited aboveacknowledge that the protoliths for this sequence may be sedimentary and volcanic rocks. Gateset al. (2002) have used the stratigraphy described above as an aid in mapping portions of thewestern Hudson Highlands.

Fig. 3 is a bedrock geologic map of part of the Greenwood Lake Quadrangle showing locationsof the magnetite deposits, original geology mapped by the author, and selected strikes and dips oflayering. Generally, layering in the bedrock strikes between due north to N45E and dips steeplyto the east. Three periods of folding have been recognized in the Hudson Highlands (Dallmeyer,1972; Helenek and Mose, 1984; Ratcliffe and Burton, 1990). It is thought that initial F1 foldingwas isoclinal and recumbent, with axial surfaces and lineations plunging south. The Greenwood

Fig. 3. Generalized stratigraphic sections for the proposed metasedimentary-metavolcanic sequence in the ReadingProng, compiled by the author and going from southwest to northeast. (1) southern New Jersey Highlands andPennsylvania; (2) central and northern New Jersey Highlands and southernmost Hudson Highlands; (3) south eastern

Hudson Highlands; (4) central and northern Hudson Highlands; (5) eastern most Hudson Highlands.

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Lake area however is dominated by F2 folds that are upright, tight to isoclinal, and trendnortheast. Magnetite deposits of the field area occur in the nose of F2 folds or high along theeastern limb of folds. Axial surfaces of F2 folds and mineral lineations plunge 30–60 degrees tothe north. Late, open, F3 folding is seen trending northwest-southeast and gently curves theoverall layering of the units into open S shapes on map scale.

2.2. Major and trace element chemistry of non-plutonic rocks

Major and trace element chemistry was determined by whole rock X-ray fluorescence spectro-metry (XRF) and inductively-coupled plasma mass spectrometry (ICP-MS) (Paul Briggs, USGeological Survey unpublished) for samples of gneiss and magnetite ore from Greenwood Lake.Major element chemistry for forty-nine samples of suspected metavolcanic gneiss from theGreenwood Lake Quadrangle and previously published data from several localities in the Read-ing Prong (Table 2) were plotted (Figs. 4 and 5) using the methods of Irvine and Baragar (1971)

Table 2

Data showing average major element chemistry for quartz plagioclase gneiss (Qtz-plag.) amphibolite (Amph.) horn-blende gneiss (Hbl) and calc-silicate gneiss (Calc-silicate). Data were compiled from Sims (1958), Heleneck (1971),Young (1971), Murray (1976), Drake (1984) and unpublished data by the author. The number given in parentheses is

the number of samples compiled from each author and used in the plots for Fig. 4A–D

Drake (3)Qtz-plag.

Young (5)Qtz-plag.

Sims (3)Qtz-plag.

Gundersen (11)Qtz-plag.

Gundersen (14)Qtz-plag.

Murray (2)Qtz-plag.

NJ, PA

NJ NJ NJ, NY NJ, NY NY

SiO2

68.57 72.67 71.43 63.7 68.53 62.4 Al2O3 15.3 14.26 15.79 17.2 14.02 18.5 Fe2OT 4.06 2.05 1.34 3.26 3.41 4.38 MgO 0.85 0.49 0.74 0.76 1.02 1.75 CaO 2.6 2.02 0.99 3.49 3.44 5.5 Na2O 6.13 5.29 6.99 5.53 3.32 5.05 K2O 0.69 1.52 1.60 2.27 5.8 0.65 TiO2 0.41 0.56 0.53 0.7 0.26 0.95 P2O5 0.34 – 0.03 0.035 0.12 – MnO 0.023 0.058 – 0.025 0.02 0.1

Drake (7)Amph.

Gundersen (6)Amph.

Heleneck (11)Amph

Murray (9)Amph.

Gundersen (7)Hbl.

Gundersen (11)Calc-silicate

Murray (5)Calc-silicate

NJ, PA

NJ, NY NY NY NY NY NY

SiO2

49.28 51.1 49.73 47.57 61.7 56.24 54.14 Al2O3 15.24 14.94 16.92 15.6 15.7 15.96 16.64 Fe2OT 10.69 10.83 9.86 9.95 4.77 5.86 8.66 MgO 6.63 4.3 5.86 8.64 1.8 2.2 6.84 CaO 8.32 7.1 9.93 9.38 4.23 6.38 6.02 Na2O 3.04 4.29 3.73 3.02 3.75 4.0 4.08 K2O 1.89 1.89 1.04 0.91 4.47 4.4 0.52 TiO2 1.58 2.07 0.87 2.4 0.71 1.02 1.16 P2O5 0.47 0.83 0.1 – 0.53 0.46 – MnO 0.19 0.13 0.15 0.21 0.04 0.06 0.24

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Fig. 4. (A) The AFM projection for subalkaline rocks. The curved line separates calc-alkaline compositions (below)

from tholeiitic compositions (above). Suspected metavolcanic rocks are plotted on the left and suspected metasedi-mentary rocks are plotted on the right. (B) The An–Ab–Or projection. The curved lines separate sodic-rich (far left),average (middle), and potassic-rich (far right) subalkaline rocks. Ol0–Ne0–Q projection. (B) The Ol0 and Ne0 are thenormalized calculations of olivine and neptheline from the major oxide data. The line separates the alkaline field from

the subalkaline field (right).

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to examine the chemical trends existing in these rocks. These samples were interpreted (by theauthors indicated in Tables 2) to be metavolcanic on the basis of their major element chemistry,stratigraphic position, and lateral extent. The rock types plotted are from rock units A, B, and C,and include hornblende gneiss, amphibolite, quartz-plagioclase gneiss, and pyroxene-quartz-feldspar gneiss. In the Greenwood Lake area, there are two groups of quartz-plagioclase gneissthat are chemically, slightly different from one another. One quartz-plagioclase gneiss has higherAl and Na and the other is higher in Si and K and appears to be related to the presence ofK-feldspar in the rock. In addition, 62 rocks of metasedimentary origin from the Greenwood LakeQuadrangle were plotted using the same methods. These samples are from rock units A and Dand include graphite, biotite, and garnet bearing plagioclase gneisses, allanite, scapolite and pyr-oxene bearing gneisses, hornblende-feldspar gneiss, and pyroxene amphibolites. The suspectedvolcanic rocks are plotted on the left side of each figure and the metasedimentary rocks areplotted on the right side. Rocks of volcanic origin should plot within distinct fields and shoulddefine trends reflecting their chemical evolution, whereas sedimentary rocks should be random,especially if a chemical overprint caused by Grenville metamorphism does not exist. As can beseen in Fig. 4A–C, the rocks selected as volcanic in origin define a consistent calc-alkaline trend

Fig. 5. Normative plagioclase composition versus normative color index. The lines divide the differentiation series into

basalt, andesite, dacite, and rhyolite. Al2O3 versus normative plagioclase showing the separation of the calc-alkalineand tholeiitic fields.

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with a minor tholeiitic component. Amphibolite samples from the base of unit A are tholeiitic.Amphibolite from unit B and quartz-plagioclase gneiss, hornblende gneiss, and pyroxene-bearingquartz-plagioclase gneiss from units A and C are calc-alkaline. The rocks defined as sedimentaryare randomly scattered in each diagram and show no consistent chemical trends. A notable pat-tern is seen in Fig. 4B where quartz plagioclase gneiss, hornblende gneiss, and most amphiboliteplot in the sodic rich field, while calc-silicate gneiss and some amphibolite plot in the morepotassic field. Calc-alkaline trends have been documented in the Hudson Highlands by Ratcliffe(1992) and in the New Jersey Highlands by Volkert and Drake (1999) in leucocratic and char-nockitic gneiss of varying mineralogy. Fig. 6 shows the outline of their data plotted with theGreenwood Lake data and the similarity of trends among these rocks.

Microprobe analyses of biotite from throughout the Greenwood Lake section and from similarunits from the Reading Prong in the Monroe Quadrangle of New York and the BoyertownQuadrangle in Pennsylvania show high concentrations of Cl ranging from 0.7–3%, (Swope et al.,1991). Such high concentrations may be indicative of evaporites in the sequence and/or thecirculation of saline fluids during formation. This is further substantiated by the common occur-rence of Cl-rich scapolite that forms distinct layers in unit A, the calc-silicate portion of thesequence. Mora and Valley (1989) demonstrated that halogen-rich scapolite and biotite in dis-crete layers could have been created by the internal buffering of fluids and presence of halite inthe sequence. Further analysis of the scapolite and biotite chemistry in the Greenwood Lake areais needed to determine the full nature of the halogen enrichment.

Fig. 6. AFM projection for subalkaline rocks showing the data trends from this study, Ratcliffe (1992) and Volkertand Drake (1999).

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2.3. Magnetite and uranium deposits

A dominant feature of the geology of the Reading Prong is the ubiquitous strataboundmagnetite deposits, which have a geologic setting unlike cratonic banded iron formations butare similar to many other uncategorized Proterozoic iron occurrences that are associated withuranium (U) and rare earth elements (REE) as described by Hauck and Kendall (1984) and byHitzman et al. (1992). Geologic and petrologic data gathered by the author from the majormagnetite deposits depicted on the map in Fig. 3 reveal a diverse geometry and petrology of theore. Magnetite is concentrated in elongate lenses that are generally concordant with surround-ing rock units and foliation, and plunge parallel to major fold axes. Although the magnetite isusually massive, it also forms sequences in which thin layers of magnetite alternate with thinlayers of quartz and pyroxene. Discordant lenses of magnetite, pyrite, and pyrrhotite, are com-monly found in zones of brecciation, cataclasis, and mylonitization. Magnetite ores in theReading Prong range from several meters to as much as 600 m thick, and several tens of metersto more than 16 km long. In the Greenwood Lake Quadrangle, four major historical mineareas were mapped and several smaller magnetite bodies were found. They ranged from severalmeters thick to up to over 1 km long. Pegmatites of variable composition are common adjacentto the ore and may be the result of local anatexis caused by the lowering of the granite-meltingcurve in proximity to the magnetite. Pegmatites commonly occur in tight rootless folds or aselongate bodies in the nose of folds. Pyrite, pyrrhotite, and minor chalcopyrite are disseminatedthroughout the magnetite lenses. Molybdenite, sphalerite, galena, covellite, and bornite are lesscommon. The most abundant gangue minerals are apatite, quartz, feldspar, pyroxene, allanite,and titanite, and occur as disseminated crystals and as thin layers in the magnetite. Biotite,chlorite, actinolite, albite, epidote, calcite, rutile, ilmenite, scapolite, pumpellyite, sappherine, andtourmaline are found in variable amounts. Magnetite forms deposits in units A, B, C, and thebase of D.

Uranium mineralization in the Reading Prong has been documented most recently by Klemicet al. (1959), McCarly (1961), Offield (1967), Grauch and Zarinsky (1976), Grauch (1978),Gundersen (1984, 1986, 1991, 2000), and Muessig (1989). Uraninite and other U-bearingminerals form layers and disseminations in several kinds of host rocks, including the magnetitedeposits, pegmatites, intrusive granitic rocks, marble, veins, mylonites, and biotite-garnet gneisswith layers of monazite and xenotine. Uranium mineralization is most commonly associated withmagnetite. Like the magnetite deposits, most uranium mineralization is conformable with thecompositional layering and appears stratabound, but does occur remobilized in veins, shear zones,and as clots of minerals in pegmatites. The Pb–Pb age determinations (Grauch and Ludwig,1980) on uranium-bearing samples from four uranium occurrences in the Reading Prong indicateuranium concentration and/or recrystallization between 975 and 950 m.y.

A particularly well exposed section, about nine meters thick, on the south side of WarwickMountain (Fig. 3) shows important vertical and lateral relationships of layered magnetitedeposits and the other rock types. The magnetite forms continuous thin beds alternating withdiopside and quartz (Fig. 7). Towards the top of this section, feldspar-rich layers increase asdiopside layers decrease. Farther up section, to the west of this exposure, calc-silicate gneisswhich hosts stratabound, unbanded masses of magnetite and uranium, grades into pyroxene-bearing quartz-feldspar gneiss and eventually to quartz-plagioclase gneiss and amphibolite.

L.C.S. Gundersen / Journal of Geodynamics 37 (2004) 361–379 371

Westward the section is dominated by biotite, garnet, and graphite-bearing gneisses with minorquartz-plagioclase gneiss of possible volcaniclastic origin.

2.4. Rare earth element deposits and data for non-plutonic rocks

Samples from the Greenwood Lake Quadrangle were analyzed for REE concentration by ICP-MS (Paul Briggs, US Geological Survey unpub.). They display an overall enrichment of REE inmost rock types. Rare earth elements are concentrated within biotite-garnet gneiss as xenotimeand monazite layers in unit D. Within the calc-silicate gneiss of unit A, and in most magnetitedeposits, the REE are concentrated in allanite. Concentration of allanite appears to increase ingneiss with proximity to the magnetite deposits. Chondrite-normalized plots of REE from severalof the major rock types in the Greenwood Lake Quadrangle (Fig. 8) show an overall unusuallyhigh rare earth enrichment that is not typical for a marginal basin sequences. It is not wellunderstood what the mineralizing processes were for the REE abundance. Fluid inclusions ofCO2 are abundant in calc-silicate gneisses where light REE enrichment is most pronounced, andH2O inclusions are abundant in quartz-plagioclase gneiss where light REE is much less pro-nounced or unmeasurable. Of twelve samples of quartz-plagioclase gneiss from unit B, only onehad rare earth elements within the range of the other rock types. The remaining samples had rareearth concentrations below detection limits. This relationship suggests that CO2-rich fluids mayhave played a role in concentrating the REE in the calc-silicate gneiss.

Fig. 7. Photograph of banded iron-formation, Warwick Mountain, Miles Standish Mine, Greenwood Lake Quadrangle,

New York.

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Fig. 8. Chondrite-normalized rare-earth element plots of data from samples from the Greenwood Lake Quadranglecollected by the author.

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3. Discussion

The data presented above suggest that the non-plutonic rocks of the Reading Prong are sedi-mentary and volcanogenic in origin. It is proposed here that they possibly formed in an exten-sional back-arc marginal basin setting. Comparison of published geologic data with thestratigraphy determined in the Greenwood Lake Quadrangle demonstrates a similarity of thelithologic sequence in different portions of the Reading Prong and potentially a similar evolution.This succession, as well as the occurrence of hydrothermal, sedimentary, and exhalative Fe, Zn,and Cu-sulphide deposits in the sequence is predicted for an evolving back-arc basin setting (Caryand Sigurdsson, 1984; Mitchell and Garson, 1981). The metasedimentary rocks and their positionin the sequence are also consistent with a back-arc basin setting (for instance, Karig and Moore,1975). Geochemical data (Figs. 4 and 5) indicate that the suspected volcanic rocks are dominatedby a bi-modal calc-alkaline trend (units B and C) that follows initial extrusion of a tholeiiticbasalt (base of unit A). Carbonate accumulations probably formed around volcanic islands orremnant arc in the basin. The presence of scapolite suggests the presence of evaporite or salinebrines, which could be formed by the evaporation of shallow water over, carbonate platforms andatolls. These sediments and vocaniclastic debris were washed into the basin forming unit A. Thisis followed by extrusion or possibly ash fall deposition of volcanogenic units B and C. In Fig. 4bthe plagioclase gneiss and amphibolites from units B and C tend to be very sodic, perhaps indi-cating seawater alteration and that these units may more appropriately be interpreted as spiliteand keratophyre. The final stage of basin deposition, unit D, has sediments with a high aluminumcontent, suggesting a continental influence. Stratabound monazite and xenotime deposits in unitD suggest former sedimentary heavy mineral layers.

Banded and massive magnetite deposits are found in units A, B, C, and the basal portions ofunit D. The mineralogy of the banded magnetite deposits is similar to rocks studied by Klein(1973), who presents chemical equations and conditions that could produce alternating magne-tite, quartz, and pyroxene from metamorphosed banded iron formation. Kastellic (1979) sug-gested a chemical sedimentary banded iron formation environment for the particular deposits heexamined in the Reading Prong, (similar to an Algoma or Superior type deposit). Johnson andSkinner (2003) measured oxygen isotopes in the minerals of the Furnace magnetite bed of theFranklin marble and found values within the ranges characteristic of carbonates and iron oxidesin banded iron formation. Grauch (1978) suggested that the Camp Smith, New York, depositformed as a distal-oxide facies of an exhalative, volcanogenic-massive-sulphide environment.However, magnetite is far more abundant than massive sulphides, and sulphide minerals arelimited to dominantly pyrrhotite and pyrite. The predominance of volcanogenic rocks with themagnetite and the sometimes banded nature of the ore are more consistent with an exhalativebanded iron formation environment of deposition. The banded iron sequence exposed at War-wick Mountain illustrates the transition from an exhalative sediment (magnetite, quartz, anddiopside which were formerly iron, chert, and carbonate) to an increasing mixture of volcani-clastic sediment (more feldspar grading to a quartz-feldspar gneiss) and eventually returning to acarbonate sequence with massive, unbanded magnetite. Magmatic-hydrothermal activity fromvents may account for the banding as well as the large unbanded masses of magnetite anduranium, depositing them as lenticular layers with sediment at the water-sediment interface.A deposit of this type is also consistent with deposition in a back-arc basin.

374 L.C.S. Gundersen / Journal of Geodynamics 37 (2004) 361–379

The magnitude of REE enrichment seen in rocks of Greenwood Lake is higher than whatwould be expected in a marginal basin sequence of rocks. Preferential concentration of REE withproximity to the magnetite deposits may indicate syn- or post depositional enrichment. Thisenrichment may have also resulted from the circulation of CO2-rich fluids during Grenville-agemetamorphism and crystallization of allanite. Seawater alteration in an exhalative environmentmay have also redistributed and concentrated available rare-earth elements prior to, or duringmetamorphism. Alternatively, the light REE enrichment seen could be indicative of the kind ofhydrothermal chemical setting envisioned by Barton and Johnson (1996) where hot, sulfur-poorbrines generated by interaction with evaporitic materials facilitate the enrichment of siderophileand lithophile elements and sodic alteration. Johnson et al. (1990) and Johnson and Skinner(2003) also invoke hydrothermal, saline, sulfur-poor fluids in the formation of the Franklinmarble zinc deposits. Both of these models require an extensional tectonic setting.

4. Conclusion

The model proposed here for the tectonic and stratigraphic evolution of the Reading Prong isillustrated in Fig. 9. Stage 1 is extension in a volcanic arc, along the Proterozoic margin of the

Fig. 9. Schematic tectonic and stratigraphic model illustrating the three-stage development of the Reading Prong.

L.C.S. Gundersen / Journal of Geodynamics 37 (2004) 361–379 375

craton, creating a back arc basin with initial deposition of a tholeiitic basalt (base of unit A).Stage 2 shows basin sedimentation and arc volcanism. Initial sedimentation dominated by arc-derived sediments and volcanic debris of unit A is followed by volcanic extrusives and possibleash fall tuffs of units B and C. The latter stage of basin filling (unit D) reflects increasing con-tinental influence as fluvial-deltaic sediments (molasse) prograde over the basin. Throughoutmost of stage 2 exhalative processes produce localized masses of iron and uranium. The GrenvilleOrogeny takes place in stage 3, accompanied by granulite facies metamorphism, folding, andsyn-orogenic granitoid intrusion; resulting in the complex high grade terrain with tonaliticand trondjhemitic gneisses thrusted and folded with the younger basin, shelf, and arc derivedmetasedimentary and metavolcanic rocks.

Acknowledgements

Discussion about the Reading Prong with N. Ratcliffe, A. Drake, R. Grauch, D. Frishman, J.Munoz, and J. Smoot has helped to clarify some of the ideas of the author expressed here.Thanks is especially given to R. Grauch for directing me towards research on the magnetitedeposits of the Reading Prong and to N. Ratcliffe for allowing me access to unpublished mappingin my quest to examine all the magnetite deposits of Hudson Highlands. Reviews by A. Hills, A.Gates, H. Helenek, and M. Gorring have greatly improved the manuscript.

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