Basaltic rocks in the Rensselaer Plateau and Chatham slices of the Taconic allochthon: Chemistry and tectonic setting

N. M. RATCLIFFE U.S. Geological Survey, Reston, Virginia 22092

ABSTRACT that contains abundant coarse-grained detri- compared the Nassau-Rensselaer sequence to tus of probable eastern North American Mesozoic rocks of the Newark Supergroup and Tholeiitic to transitional alkalic basalt and Proterozoic basement These relations sug- suggested a similar origin. Until recently, how- basaltic tuff form widely separated but dis- gest a fault-bounded submarine basin (Nassau- ever, the basalts associated with the Rensselaer tinctive units within the Nassau Formation of Rensselaer basin) located near the continental Graywacke Member have been interpreted as late Proterozoic or Early Cambrian age, margin that was fed by submarine fan depos- dikes or sills rather than flows, and the associa- within the Rensselaer Plateau and Chatham its. Submarine morphology of the distributing tion of these basalts in time with the deposition slices of the Taconic allochthon in eastern fan complex may have channelized the ba- of the sedimentary rocks has not been entirely New York State and western Massachusetts. salts. The association of the tholeiitic to tran- clear (Balk, 1953; Prindle and Knopf, 1932), Examination of all known occurrences of sitional alkalic basalts with interpreted except for the exposures of pillow basalt at these basaltic rocks and detailed mapping of marine-fan deposits suggests that the volcan- Banker Pond (loc. 2 in Fig. 1) which were rec- the enclosing strata indicate that these basalts ism occurred after Iapetan rifting and ther- ognized as contemporaneous deposits by Dale are submarine lava flows and water-transported mal subsidence over tectonically thinned (1892, p. 310, 311, and 327), by Balk (1953, tuffaceous deposits restricted to the lower sialic crust, perhaps near the time and place of p. 829), and by Potter (1972, p. 12). Ratcliffe third of the stratigraphic section in both eventual separation. (1969,1974,1978) described the basaltic rocks slices. In each slice, the basalts thicken to the from the Chatham slice and concluded that they west along with thickening and coarsening of INTRODUCTION were basaltic lava flows and/or tuffs inter- enclosing graywacke beds (Rensselaer Gray- calated in the Nassau Formation. As part of the wacke Member of the Nassau Formation) Tholeiitic(?) basalt sills, flows, pillow basalt, reconnaissance geologic investigation for the and thin eastward into pillow basalt and and basaltic tuffs are minor but distinctive rocks new bedrock geologic map of Massachusetts tuffaceous basalt that is associated with fine- in or near the base of the Rensselaer Plateau (Zen, 1983), a systematic examination of the grained graywacke and thinly laminated slice and Chatham slice of the Taconic alloch- major areas of basaltic rocks and tuffaceous vol- purple and green turbidites of the Nassau thon in Rensselaer and Columbia Counties, canics in the Taconic Range was conducted in 1 Formation. New York. Basaltic rocks are absent elsewhere 1978. The results of this study (Table A) and new chemical analyses of the basalts are These high-Ti02, low-MgO basalts resem- within the Cambrian and late Proterozoic strata ble very closely late Proterozoic basalts and of the Taconic slices of Vermont, Massachusetts, presented here along with a brief comparison feeder dikes of the Catoctin Formation of the and southeastern New York State. These basalts with similar Iapetan rift-stage basaltic rocks Blue Ridge in Virginia and late Proterozoic are interlayered with coarse graywacke and from the Blue Ridge of Virginia; Hudson High- metadiabase dikes in the northern Reading graywacke conglomerate of the Rensselaer lands, New York; and Vermont. All of these Prong in New York, both of which intrude Graywacke Member of the Nassau Formation basalts have striking similarities to, and distinc- pre-Iapetan, eastern North American base- or with graywacke beds in the Nassau Forma- tive differences from, eastern North America ment. Major-element and rare-earth chemis- tion that have been correlated with the Rensse- Mesozoic rift basalts. Minor occurrences of try and geology also establish a correlation laer (Ratcliffe, 1974). The age of the enclosing basaltic rocks of probable Early Ordovician age between the Taconic metabasalts and meta- sedimentary rocks is thought to be late Protero- in the Taconic allochthons at Stark's Knob basalts preserved in the Tibbit Hill Volcanic zoic^) or Early Cambrian. Bird and Dewey north of Albany, New York, and in the Ham- Member of the Pinnacle Formation that un- (1970) suggested that the Rensselaer Graywacke burg Klippe of eastern Pennsylvania are not conformably overlies middle Proterozoic Member of the Nassau Formation and basaltic basement, in central Vermont. Palinspastic rocks constitute a rift facies associated with the earliest stages of continental extension leading to 'Tables A and B may be secured free of charge by reconstruction places the Chatham slice requesting Supplementary Data 87-29 from the GSA oceanward of the Rensselaer Plateau slice the development of Iapetus. Bird (1975) later documents Secretary.

Additional material for this article (Tables A and B) may be secured free of charge by requesting Supplementary Data 87-29 from the GSA Documents Secretary.

Geological Society of America Bulletin, v. 99, p. 511 -528,2 pis., 11 figs., 1 table, October 1987.

511

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associated with rift rocks. The Pennsylvania tinguished: middle Proterozoic gneiss (Y), Chatham slice (II) within the Nassau Forma- pillow basalts differ markedly in tectonic setting, Cambrian and Early Ordovician shelf sequence tion, except for several localities of tuff and age, and chemistry (Lash, 1985) from the basalts carbonates (OC), Middle Ordovician black tuff(?) reported by Balk (1953, PI. 1) which described here. shale (Ow), late Proterozoic and Early Cam- occur in the Giddings Brook slice(?) (I) north of brian Hoosac Formation (€Zh), and late Prot- Petersburg, New York, east of locality 3, and in STRATIGRAPHY AND STRUCTURAL erozoic, Early Cambrian through Middle the Berlin Mountain slice (IV) west of Williams- SETTING OF VOLCANIC ROCKS Ordovician rocks of the Taconic allochthons town, Massachusetts. Several of the latter tuff IN THE TACONIC ALLOCHTHON (OCt, €Znr, €Z, CZnu). The structural level localities mapped by Balk have been examined (lowest to highest) of the Taconic slices is de- by either Ratcliffe (this study) or by Potter dur- Figure 1 shows the distribution and structural noted by Roman numerals I to V on Figure 1. ing reconnaissance mapping for the bedrock position of major slices of the Taconic alloch- Rocks of the Giddings Brook slice (I) range map of Massachusetts and are not included as thon in eastern New York State and adjacent from late Proterozoic (Nassau Formation) to verifiable volcanic deposits. Localities discussed western Massachusetts [based on Potter, 1972, Early Ordovician in age and overlap ages of in this report are identified on Figure 1. All of 1979; Ratcliffe and others, 1975; and the bed- rocks in slices II through V. Basalts and basaltic the volcanic rocks are metamorphosed to the rock geologic map of the State of Massachusetts volcaniclastic rocks (shown in black) are largely lower greenschist facies and consist of well to (from Zen, 1983)]. Five major units are dis- confined to the Rensselaer Plateau slice (III) and poorly foliated greenstone containing varying

EXPLANATION W Shows inferred relative west to east depositional positions of units

Autochthons -Taconic allochthons - 1 r Eugeosynclinal —. IO rocks of O O e o vWaliom-- Rowe-Hawley Ordovician '.•:>sacwr Giddings Brook Zone SI Formation slice Ow.-.'-'

>0- Rensselaer 0€t Ceu s: « Cambrian Plateau Chatham ®o Hoosac slice slice. <%r Berlin Mtn. Formation Everett Mtn. amphibolite slices i€Zrii (CZha) ij V ! fez- €Znr

Late Greylock ®Ì2 Proterozoic slice <19 u Nassau Formation (-CZn) i P diabase Rensselaer Graywacke Member (€Znr) dike Volcanics €Znv Green Mountain and Berkshire Massif Middle Roman numerals l-V show present stacking Proterozoic sequence of Taconic allochthons from structurally lowest I to structurally c ® U> highest V, numbers indicate localities discussesd in text. io s2 -A- Syn to post metamorphic thrust m faults (Late Taconic and Younger) Premetamorphic thrust faults bounding Taconic allochthons

Figure 1A. Explanation for Figure IB.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/4/511/3998358/i0016-7606-99-4-511.pdf by guest on 25 September 2021 Figure IB. Geologic map showing distribution of volcanic rocks in relationship to major rock units in the Rensselaer Plateau and Chatham slices. Localities 1 through 14 discussed in text.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/4/511/3998358/i0016-7606-99-4-511.pdf by guest on 25 September 2021 Rensselaer Plateau slice Chatham slice

Turner Mountain Banker Mercer Mtn.- Queechy Lake The Knob Berkshire Boys Farm (loc. 5, fig. 1) Pond-Ward Hollow Fog Hill (Ioc. 9, fig. 1) (Ioc. 10, fig. 1) East side (ioc. 2, fig. 1) \'oc. 7, fig. 1) Canaan Valley (Ioc. 11, fig. 1) CZnr CZng- top of section MffiöfHj-CZngr not exposed CZnr CZnr. ! I mixed volcanics basalt CZng CZncj ,CZngr basalt base of 'and sediments CZnr illow pillows METERS FEET -CZnr ü .-. -i>P basalt/tuff Turner Mtn. 0 CZnr CZngr CZnr CZngr-' O-i-O basalt CZnp-toÄ«* basalt CZnp lower basalt CZngr .. . ° .\o CZnr-fc^il 1' CZnr CZng ^rir-^CZnp -1000 "_r_=-CZnp sole of CZnr ~3-CZnr Chatham slice CZn sole of ~.rtft' 9 sole of -2000 CZnp Chatham slice sole of Chatham slice sole of Rensselaer Chatham slice Plateau slice 1000 H 3000 thicknesses from -CZnp Potter, 1972 (Rensselaer Plateau slice) Chatham slice Plate 2, sections l-H Proximal facies Distal fades -4000 CZnb (Bomoseen Graywacke Member of the Nassau Formation) CZngr (gray phyllite, metasiltstone) -5000 CZnp basalt horizon

-6000 sole of 2000- CZng (green lustrous phyllite and Rensselaer quartzose phyllitic quartzite) Plateau slice (purple and green phyllite)

SW NE Generalized facias relationships of Rensselaer Plateau and Chatham slices showing position of basalts and volcaniclastic sediments

Figure 2. Schematic correlation chart showing position of volcanic rocks in relation to major units at localities 5,2,7,9,10, and 11.

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amounts of epidote, chlorite, stilpnomelane NW SE and/or tremolite-actinolite. Basalts with clearly basalt CZnv recognizable relict igneous texture (diabasic, in- Lower pillowed Upper pillowed tergranular, variolitic, and/or pillow structure) Rensselaer Graywacke Member zone zone of Nassau Formation occur at localities 1 through 10. Volcanic rocks CZnr at localities 11 through 13 are volcaniclastic in well-laminated red argillite part, with admixed quartzose sediment without red argillite clearly recognizible igneous textures. These de- and graywacke posits are interpreted as volcanic sediments and/or basaltic tuffs. No rhyolitic tuffs are recognized at any locality. Stratigraphie columns for several of the important volcanic localities €Znr €Zn\ broken pillows possible internal are presented in Figure 2. Important geologic red argillite 'n plate 1-A,B flow contact relationships of localities 1 through 13 are described below and in Table A. It is important Figure 3. Schematic cross section showing pillow basalt at Banker Pond locality 2 in relation to note that all basalt occurrences are found to sedimentary host rocks. within the late Proterozoic Nassau Formation of the Rensselaer Plateau and Chatham slices. (1972, p. 12) is 2.4 km northeast of locality 1 east or right. The structures suggest deformation Basalt Occurrences in the along the regional strike. A simplified cross sec- of soft pillows. The glassy to globular textured Rensselaer Plateau Slice tion (Fig. 3) shows the main mass of pillow matrix (PI. IB) locally incorporates broken basalt and admixed purple and green slate to be pillows and forms fracture fillings of basaltic Ward Hollow, Locality 1 about 13 m thick. At its base, the basalt rests material, thus indicating primary flow structures. accordantly on a well-laminated, greenish slate A small exposure at the crest of the hill eleva- 1 cm thick that overlies a well-bedded, Stillham, New York, Locality 3 tion 1,280 ft; 3.2 km southwest of Banker Pond quartzitic, purple-slate and graywacke sequence. consists of dark, fine-grained basalt about 5 m The basalt passes upward into chaotic zones of Outcrops of well-laminated, epidote-rich, thick (loc. 1 in Fig. 1; Potter, 1972). The thin pillow basalt with intermixed purple or green silt- dark green slate and interlayered dark gray to basalt is chilled at its upper and lower contact stone (lower pillowed zone). A possible internal black, or dull greenish-brown, stilpnomelane- and is concordant with bedding in graywacke flow-contact is present in the middle of the rich phyllite crop out for a distance of about above and beneath. At the lower contact, a thin basalt, and the upper zone is strongly pillowed. 270 m along strike south of the secondary road 1-cm layer of laminated green phyllite separates A red mudstone overlies the basalt, and this leading to Babcock Lake, 1.6 km north of Still- the basalt and the graywacke. The basalt is passes upward into greenish siltstone and inter- ham, New York. This locality and another strictly concordant to this layer (see Table 1 for mixed graywacke, and the entire sequence is nearby locality were described by Potter (1972) further descriptions of the rocks). overlain by a coarse-grained graywacke marked as "sheared volcanics." The rocks are poorly by an abundance of angular fragments of white- exposed, but distinctive epidote-rich quartz phyl- Banker Pond, Locality 2 weathering oligoclase. A chemical analysis of lites interlayered with more mafic, dark green or basalt from locality A in Figure 3 is given in black layers 0.5 to 1 m thick over a distance of This exceptionally fine exposure of aphanitic Table 1, sample 5. In the photographs (Pis. 1A 45 m across strike are found south of the road. to variolitic pillow basalt and intermixed purple and IB), contorted pillows are flattened and Two samples of the most igneous-looking rocks, phyllite (loc. 2 in Fig. 1) described by Potter convoluted in a down-dip direction to the south- that appear tuffaceous, actually exhibit excellent

TABLE 1. ANHYDROUS MAJOR-ELEMENT ANALYSES OF BASALTIC ROCKS FROM THE TACONIC ALLOCHTON AND HUDSON HIGHLANDS DIKES

1 2 3 4 5 6 7 8 9 10 11 12 Loc . 5 Loc. 5 Loc. 6 Loc. 1 Loc. 2 Loc. 8 Loc. 8 Loc. 9 Loc. 5 Loc . 5 9 16 (see Fig. 1) Basalts Dikes

Si02 49.3 49.6 48.7 49.6 54.7 50.5 49.4 50.0 49.1 51.7 49.8 49.8 ai2o3 12.6 13.8 14.8 13.6 16.1 12.8 14.3 14.8 10.7 12.0 13.2 13.4 f«2°3 3.1 3.6 3.4 5.6 5.8 6.4 3.9 4.3 4.6 1.4 4.0 3.2 FeO 13.4 11.1 13.2 9.5 3.3 9.1 10.3 11.6 12.0 13.4 11.5 11.0 MgO 4.8 5.1 6.2 5.6 4.5 4.0 6.1 5.0 5.5 7.0 5.5 5.7 CaO 8.3 8.9 6.0 7.9 9.9 7.3 10.0 6.6 10.6 7.8 8.2 9.1 Na20 2.2 3.0 2.5 2.1 4.6 3.0 2.4 2.7 2.9 3.0 2.6 2.7 K20 1.4 1.0 1.0 2.4 0.08 1.1 0.26 0.3- 1.2 0.36 1.1 I.I Ti02 4.0 3.0 3.2 3.0 0.66 4.8 2.6 3.4 2.8 2.9 3.3 3.0 p2o5 .57 0.41 0.42 0.39 0.11 0.55 0.36 0.47 0.25 0.31 0.38 0.61 MnO 0.25 0.23 0.22 0.22 0.15 0.25 0.18 0.22 0.25 0.22 0.23 0.22 co2 0.03 0.09 0.31 0.04 0.08 0.09 0.10 0.49 0.00 0.00 0.12 0.31

Anhydrous TOTAL 99.95 99.98 99.90 99.95 99.98 99.89 99.9 99.94 99.9 100.1 99.93 100.2

H20 3.8 3.8 5.4 3.9 1.9 3.6 3.5 4.6 3.0 3.9 3.9 1.6

Location of samples: (1), (2), (9), and (10): Turner Mountain; (3): Glass Lake; (4): Ward Hollow; (5): Banker Pond; (6) and (7): Fog Hill; (8); Queechy Lake; (II): average 9 basalts except 5; (12): average 16 metadiabase dikes from New York and New Jersey Hudson Highlands.

Samples 1 through 8 by rapid rock techniques at U.S. Geological Survey 2. Z. Brown, analyst. Samples 9-10, Turner Mtn., Balk (1953, table 1).

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Plate 1A. Disarticulated and slumped pillows in Banker Pond expo- 0 1 sure locality 2; fine-grained matrix is variolitic basalt and oxidized I I "sideromelane"-rich basalt now consisting of chlorite and fine opaque CM dust. View is to northeast; southeast is to the right downdip. U.S. Plate IB. Photomicrograph of thin section of variolitic, fragmented penny shows scale. basalt in outcrop shown in Plate 1A, showing brittle-fractured basalt and fracture-filling by "glassy" basaltic matrix.

0 1 2 Plate 1C. Photomicrograph of thin section showing well-bedded, I I I epiclastic tuff and I tuffaceous sediments; well-bedded, chlorite-epidote- CM magnetite greenstones (darker layers), interlayered with siliclastic beds having varying proportions of admixed chlorite-epidote and Plate ID. Photomicrograph of thin section showing laminated na- magnetite. Plagicclase is absent in both layers. ture of interbedded mafic (chlorite-epidote-magnetite beds) and more quartzose interlayers shown in Plate 1C.

igneous flow structure and contain micropheno- aphanitic pillowed basalt and with the hyalo- dark banded rock consists of 10-cm-thick layers crysts of plagioclase 0.15 mm in length set in a clastic(?) matrix seen at Banker Pond. of aphyric basalt, containing microscopic-scale ground mass consisting of stubby flow-oriented layering (PI. 2B). Plagioclase phenocrysts show plagioclase and opaque-rich "altered glass." Vari- Stillham, New York, Locality 4 a decrease in size and exhibit flow layering near ations in grain size and in orientation of plagio- margins of these zones that terminate with dark clase crystals (0.05 to 0.1 mm) define flow On the slopes south of Stillham, New York, a green, chlorite-rich layers lacking in feldspar. layers 1 cm to 1 mm thick with no admixed small exposure of dark green to black well- Concentrations of opaque minerals mark edges sediment present (PI. 2A). No chemical analyses layered volcanic rock is exposed beneath a thick of the layers. Calcite filled ellipsoidal cavities were attempted. Three samples of basalt studied section of deeply pitted, dull, greenish-brown- suggest flattened vesicles. The basalt is only are identical in mineralogy and texture to weathering, stilpnomelane-rich phyllite. The about 10 m thick and is poorly exposed. A layer

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of dark brown-weathering, stilpnomelane-albite- hematite phyllite that overlies the basalt is a chaotic or fragmental deposit that lacks through- going quartzose beds common in phyllites else- where in the Taconics. In over-all appearance, it resembles stilpnomelane-rich volcanic rocks in- terpreted as tuffs or tuffaceous sediments found along the east side of the Canaan Valley (loc. 11) in rocks of the Chatham slice.

Turner Mountain, Locality 5

Two layers of medium- to coarse-grained augite basalt crop out on Turner Mountain and on the hills 1.6 km to the east, at the south end of the Rensselaer Plateau slice. These igneous rocks have been interpreted as dikes by Balk (1953) and by Prindle and Knopf (1932). Map- ping shows that the basalt layers, 60 to 75 m thick, extend parallel to strike of the enclosing graywacke for a mile in the east-west direction before wedging out against the sole thrust to the east, or disappearing beneath cover to the west.

0 1 2 0 1 2 The lower basalt is chilled at its base and con- I I I I I I tains small flattened ovoids filled with chlorite CM CM that possibly are vesicles. Irregular patches that Plate 2B. Finely layered, greenish basalt may represent a glassy mesostasis occur within Plate 2A. A layered, dark variolitic basalt from locality 4 showing disrupted fine- the intergranular matrix of augite and small at locality 3 showing disrupted texture near grained zones alternating with coarser basalt plagioclase (0.1 mm to 0.5 mm in length). base. having intersertal texture, net-veined with About 4.5 m above the base, the rock is coarser- quartz. grained augite basalt that nevertheless has a sug- gestion of a glassy mesostasis. A chemical analysis of basalt taken 4 m above the base of the lower basalt near the old airway beacon on Turner Mountain is given in Table 1 (no. 1). On Turner Mountain, the two basalts are separated y by a thin zone of Rensselaer graywacke 30 m tlmmfc- '„v.'v thick that grades upward into finely laminated gray siltstones with 1-1.5 mm to papery-thin,

* . V blue-gray, tuffaceous(?) beds consisting of 70% - plagioclase, stilpnomelane, and fine quartz. This * \IffiP« ' bed is overlain concordantly by a second chilled

Plate 2C. Disrupted, epidotite-actinolitic greenstone unit at Fog Hill (locality 8), show- ing relict fragmental or globular tuffaceous texture suggestive of disrupted pillow basalt or aquagene tuff. No interbeds of siliciclastic rocks are found. Relict intersertal texture is found in the larger fragments, whereas the matrix is very fine grained chlorite actinolite and epidote. (A chemical analysis of the rock appears in Table 1, no. 7.)

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basalt with glomeroporphyritic clumps of augite this rock is given in Table 1, no. 3. In mineral- base. Although the greenstones appear massive, set in a fine-grained (0.5 to 0.7 mm) matrix of ogy and texture, this basalt resembles closely the large-size thin sections (PI. 2C) reveal complex plagioclase and augite with a texture that varies basalt at Ward Hollow, locality 1. internal structures not visible in outcrop. The from diabasic to a weakly oriented flow struc- structure is disrupted and chaotic with abrupt ture. The basalt coarsens rapidly upward into a Summary of Geologic Relationships of Basalt variations in grain size across irregularly shaped coarse-grained au jite basalt with large (up to in the Rensselaer Plateau Slice domain!;. This distinctive basalt, marked by a 7 mm long) phenocrysts of augite and smaller very coarse grained texture, and dull brown- plagioclase set in a. much finer graned intergran- From the observations presented above, clear weathered surfaces owing to abundant yellow- ular matrix (0.5 mm) charged with tiny needles evidence of basaltic flows is presented for locali- brown stilpnomelane, is present at Fog Hill (loc. of apatite. Large ( 2—5 mm) crystals of ilmeno- ties 2,3, and 4. Coarser-grained basalts at locali- 8). Thus basalt closely resembles the second, magnetite are distributed throughout. Interstitial ties 1, 5, and 6 are concordant bodies and coarse-grained basalt at Turner Mountain (loc. quartz (1%) is present. A chemical analysis of possibly also are flows. No evidence for a dike 5) and the second basalt layers at localities 9,10, this rock is given in Table 1, no. 2. The sample habit has been found. Reconnaissance mapping and 11 to be discussed later. The basalts at comes from the upper third of the basalt. in the southern end of the Rensselaer Plateau Mercer Mountain and Fog Hill are locally con- On the hills to the east of Turner Mountain, a slice suggests that the basalt horizons at Turner cordant but at map scale rest on different units, greater thickness of normal graywacke inter- Mountain could project along strike to the Glass suggesting low-angle truncation of units by the venes between the two basalts. The base of the Lake locality. Attempts to trace the basalts were base of the flows. At locality 8A, basalt and upper basalt here is marked by a zone 3 m thick futile owing to extensive cover of Pleistocene basaltic tuffs grade upward into metasedi- of dark gray, glassy appearing, highly weathered sands in the Dunham Hollow area northwest of mentary rock. Chemical analyses of the lower basalt with large crystals of augite set in a matrix Turner Mountain. Form lines drawn on distinc- epidotic greenstone and the upper stilpnomelane- of glass replaced by yellow-brown stilpnome- tive graywacke and purple slate horizons, how- rich basalt at Fog Hill are given in Table 1, nos. lane. Above this, there is normal augite basalt ever, suggest the general correlation of the two 6 and 7, respectively. The total thickness of ba- similar to that of the second basalt on Turner localities as shown on Figure 1. The similarity of salt ranges from 60 to 120 m. Mountain. Coarse-grained feldspathic gray- the pillow basalt at Banker Pond to the flow- wacke overlies the upper basalt at both locali- layered basalt at Stillham (Iocs. 3 and 4) is quite Canaan Mountain, Localities 9 and 10 ties, but the upper contact of neither basalt is strong and suggests correlation of all of the ba- exposed. The chemical analyses (Table 1, sam- salts (see Fig. 2). Two large areas of basalt occur in the central ples 9 and 10) come from this upper basalt on phyllite ridge on Canaan Mountain, and in the hill 1653 (Balk, 1953, p. 824). Basalt Occurrences in the Chatham Slice Canaan Valley. At Queechy Lake, locality 9, a The map data ¡¡uggest that the two basalt lay- thin, lower, black aphyric basalt 1.5 to 3 m thick ers on Turner Mo untain are concordant features, The most extensive exposures of basaltic is overlain by the coarse-grained Rensselaer either flows or sills, rather than dikes. The dark rocks in the Taconics occur within rocks of the Graywacke Member of the Nassau Formation hyalohyaline zone at the base of the second ba- Chatham slice (II) in the State Line and Canaan, (CZnr) and interlayered epidotic quartz phyllite. salt on hill 1653 suggests a palagonite zone near New York, quadrangles (Ratcliffe, 1974,1978), Overlying this sequence concordantly, there is a the base of a flow, and a flow interpretation is where they are closely associated with sporadic second, dark green to brownish-green, coarse- favored. occurrences of Rensselaer(?) Graywacke Mem- grained., stilpnomelane-rich augite basalt as ber (€Znr). Within the Chatham slice, Rens- much as 60 m thick, but the top of the section is Glass House, New York, Locality 6 selaer(?) Graywacke beds thin eastward, north- not exfiosed. A chemical analysis of one sample eastward (see inset Fig. 2), and southeastward. of the upper basalt taken from 5 m above the A poorly exposed layer of augite basalt 3 to Graywack similar to that of the Rensselaer oc- base of the second basalt is given as sample 8 in 10 m thick concordantly overlies beds of Rens- curs locally in the Everett slice (IV) as 10- to Table 1. selaer Graywacke Member of the Nassau For- 25-m-thick beds that are not traceable for any On The Knob (loc. 10) about 60 m of coarse- mation 600 m east of Glass Lake in the distance (Zen and Hartshorn, 1966; Zen and grained augite basalt overlies Rensselaer Gray- southeastern comer of the Averill Park, New Ratcliffe, 1971; Ratcliffe, 1974). Volcanic rocks wacke Member and green quartzose phyllite of York, quadrangle (Prindle and Knopf, 1932, have not been recognized with those gray- the Nassau Formation. The irregular area of ba- p. 282). The basalt is holocrystalline throughout wackes, although dark green, very minor epi- salt resembles a stock in plan view and has been except for the lower several centimetres of the dotic phyllite is present locally. interpreted as such by Prindle and Knopf basalt, which a insists of glomeroporphyritic (1932). Mapping shows that the basalt occupies plagioclase set in a fine-grained groundmass of Mercer Mountain-Fog Hill Area: the core of a small refolded syncline, and the glass (?) and radiating plagioclase needles 0.2 Localities 7 and 8 basalt overlies the metasediments along a folded mm long. Irregular ovoidal areas up to 2.5 mm contact; thus the stock-like appearance is mis- long filled with chlorite and lacking the opaque At Mercer Mountain (loc. 7) and Fog Hill leading (Ratcliffe, 1978). The contact is well mineral that fills the interstitial matrix appear to (loc. 8), a thick sequence of epidote-actinolite exposed on the east-facing cliff northeast of The be flattened vesicles. About 0.5 m above, the greenstones overlie purple and green phyllite Knob. Here a dark-gray to black, well-laminated augite is commonly 1-2 mm in size, and plagio- (CZnp), and green quartzose phyllite (CZng) of zone -3 m thick marks the lowermost part of clase phenocrysts up to 3 mm are present, but the Nassau Formation in which beds of Rensse- the basalt, and this unit conformably overlies vesicles are still present. A sample 3 m from the laer(?) Graywacke Member (CZnr) occur. The well-bedded, gray, laminated siltstone (CZngr) upper contact iu holocyrstalline augite basalt basalts exhibit relict igneous intersertal texture, similar to the well-laminated epidotic slates with excellent intersertal texture containing in- relict augite, and primary quartz; they are inter- immediately beneath the upper flow at Turner terstitial, primary quartz. A chemical analysis of layered with metasedimentary rocks at their Mountain. The lower 3 m of the basalt, re-

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sembling tuff, actually is chilled aphyric basalt NW SE and microcyrstalline vesicular basalt closely Rocks of Nassau Formation resembling the layered basalt at Stillham (Iocs. 3 1 METER green phyllite and 4) and the base of the upper basalt at hill J 1653 northeast of Turner Mountain (loc. 5). The yellow-green epidote-actinolite field relationships suggest that both localities 9 ''siliceous chlorite-stilpnomelane and 10 are basaltic lava flows comparable in phyllite greenstone stratigraphic position to those at localities 7 purple and green and 8. gray laminated laminated phyllite siltstone well-laminated Volcanic Rock on East Side of Canaan Valley, green phyllite and Localities 11 and 11A crossbedded Epidote-actinolite-greenstones and stilpnome- tuffaceous lane-rich strongly foliated mafic volcanic rocks greenstone crop out for several kilometres in rocks of the Chatham slice from the Massachusetts Turnpike north to Lebanon, New York. Two major belts interlayered with thin beds of Rensselaer Gray- wacke are separated by a dark gray laminated dark gray, laminated metasiltstone metasiltstone unit (CZngr) near the Berkshire Boys Farm. The lower volcanic rock is epidote CZnp \ gray, laminated rich and pale green in color, and is between 0 CZnv metasiltstone and 15 m thick. The upper volcanic unit is dull stratigraphie tops' greenish brown, rich in stilpnomelane, and is also about 15 m thick. Both units have exposed upper and lower contacts and exhibit gTadational 100 FEET transitions into laminated albitic metasiltstone. n True igneous textures are lacking, and augite is 20 METERS not preserved. A small area of a third basalt is present near the north end of the outcrop belt, Figure 4. Diagrammatic section of albite-epidote-actinolite chlorite greenstone beds in well- but this layer may be structurally repeated. laminated, green and purple phyllite of the Nassau Formation, locality 11 A. In Figure 4, a section exposed in a hillside just north of the Berkshire Spur of New York Thru- way near the state line is shown (loe. 11 A). Here and their concordant contacts with surrounding Berlin Mountain, Stephentown, Locality 13 three layers of epidotic greenstones are exposed, strata in the Nassau metasediments. The repeti- interlayered with green and purple phyllites of tive bedding characteristics suggest repeated Excellent exposures of well-layered, epidote- the Nassau Formation. The layers are 1.5 m, 4.6 deposition of transported basalt debris in nearly and quartz-rich metavolcanic rocks are at the m, and 18 m thick. The base of each layer is undiluted form (little or no admixed quartzose northwest margin of Berlin Mountain (Potter, exposed and is immediately underlain by planar- sediment). The upward-finding cycles in the 1979, p. 185) in rocks here considered part of bedded, extremely well laminated, light gray to fine-grained metasediments beneath each green- the Chatham slice. Locally, pods of epidote-rich dark gray metasiltstone that is concordantly stone suggest upper surfaces of distal turbidites basalt up to 0.5 m in diameter are found near the overlain on absolutely planar surfaces by green- or submarine overbank deposits. The internal upper third of the exposure. These pillow-like stone. The upper surfaces of greenstones 1 and 2 lamination, current cross beds, and scoured features show relict igneous textures; vesicular are exposed and are channeled and scoured with upper surfaces of the metavolcanics suggest that and aphyric to very fine-grained basalt with thin quartzite sands that pass up into papery they were deposited in a hydraulic realm similar crude 1-mm scale layering accentuated by thin, gray to yellowish-gray, well-laminated met- to that of the enclosing metasediments. These changes in concentrations of opaque minerals, asiltstone and argillite (phyllite). The lower observations suggest that relatively distal silici- chlorite, and plagioclase. The basalts are not greenstone is distinctly layered on a 2- to 4-cm clastic turbidites, possibly lower fan or levee continuous and may be dismembered thin flows scale and in the lower third contains cross-beds deposits, are associated with tuffaceous meta- or sedimentary (that is, transported) pillows. or current ripples that top southeast and face volcanic rocks. The excellently laminated greenstones (Pis. 1C down-dip (see inset in Fig. 4). Greenstones 2 and ID) are best interpreted as volcanic-derived and 3 are more massive, although faint lamina- Jiminy Peak Area, Locality 12 sedimentary rocks owing to their high percent- tions or irregular scalloped zones are present in- age of quartz and lack of augite or igneous ternally. No graywacke is interbedded with the Sporadic occurrences of epidote-rich, highly plagioclase. The sedimentary structures and volcanics at this locality, although graywacke is altered greenstone occur near the northeast mar- presence of chlorite and opaques as the original present to the north and south along strike. gin of the Chatham slice. The northernmost ex- "igneous" mineralogy suggest derivation from The relations seen in these and other expo- posures west of Jiminy Peak Ski area are about weathered basaltic detritus and possible epiclas- sures on the east side of the Canaan Valley illus- 4.6 m thick. Igneous textures are not preserved. tic origin. These rocks closely resemble lami- trate well the bedded nature of these greenstones The greenstone is overlain by graywacke. nated phyllites present beneath the upper Turner

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Mountain flow, beneath basalt at The Knob, of certainty. Reconnaissance mapping in the The association of basaltic hyaloclastic depos- and interlayered at the tops of the basaltic rocks southern part of the Rensselaer plateau, how- its with sedimentary rocks and turbidite deposits east of Canaan Valley. ever, suggests that the Turner Mountain basalts is not well understood and is rarely reported in do occupy the same structural position as the the literature (Lajoie, 1979). The variations in Summary of Relationships of the Chatham Slice Glass House basalt. If the stratigraphic position the Taconic basalts are closely paralleled by of each of the localities shown on Figure 2 is changes in the enclosing sediments from the Basaltic rocks in the Chatham slice change independently determined from the data of Pot- more proximal to the more distal turbidite fa- from thick, coarse-grained augite basalt in the ter (1972, Plate 2, sees. I and H), Ratcliffe des. The separation of the basalts from the west and southwes t to thin layers of fine-grained (1974, 1978), and data presented here for graywacke and the lack of mixing in many local- basalt(?), and basaltic volcaniclastic rock inter- Turner Mountain, a plausible correlation of all ities (Fig. 4) indicate deposition from separate layered eastward and northeastward with meta- of the basalts is established. In each column, the but spatially related turbidites and flows. The sediments. Although the volcanic rocks are single or double basalt layers occur stratigraph- scouring present at the upper surface of the tuf- closely associated with lenses of Rensselaer-type ically above (as determined from primary tops faceous beds (Fig. 4) attests to the unconsoli- graywacke, both the volcanics and graywacke indicators) a thick section of green and purple dated, nonwelded character of the tuffs, as do thin and become finer grained in the same direc- slate phyllites (CZnp) -600 m above the sole of the internal sedimentary structures. tion. Exposures of laminated tuffaceous green- the Rensselaer or Chatham slices, except for the The relationships recorded here suggest the stones in fine-grained sediments of the Nassau Turner Mountain area where an 1,800-m thick- possibility that basalt pillow lava complexes (see Fig. 4) are best interpreted as water- ness of coarser-grained graywacke and purple formed in the upper part of submarine channel transported, low-density palagonite-rich deposits and green slate may underlie the basalt. As the systems that may have had sufficient slope to formed as turbidiiic deposits in the same hy- basalts thin or die out east-northeastward, beds allow stripping or separation of the upper hyalo- draulic realm as the enclosing sediments. of graywacke decrease in abundance, thickness, clastic zones of the pillow sequences from the and in grain size, suggesting a change from more lavas. These low-density, hot, vesiculated and proximal to more distal sedimentation and a Volcanic Pebbles in the Rensselaer gaseous hyaloclastic materials may have moved clear association in provenance between the Graywacke Member (Locality 14) downslope at velocities lower than the associ- basalts and the sedimentary rocks. Although and Provenance ated siliciclastic turbidites. This explanation plausible, the correlation suggested in Figure 2 would account for the remarkable lack of cannot be uniquely demonstrated because of the Balk (1953, PI. 8, fig. 3) noted that clasts of mixing (lahar-like materials are not present) and physical break between the Chatham and Rens- epidote-rich basalt occur in the Rensselaer for the observation that the base of the aquagene selaer slices and the lack of a coherent internal Graywacke Memljer of the Nassau Formation tuffs and basalts commonly rests on the ultrafine- stratigraphy in the Rensselaer Plateau slice. west of Austerlitz, New York. White- or gray- grained, well-laminated siltstone or argillite at weathering, fine-grained metafelsite (14B) peb- The geologic data suggest, therefore, that the the top of graywacke sequences. bles are fairly common in coarse-grained Rens- Rensselaer-associated basalts emanated from the selaer graywacke of the Rensselaer Plateau slice same side of the sedimentary basin from which CHEMISTRY OF THE BASALTS both above and below the basalts at Turner the cratonic source material in the Nassau was Mountain and Gliiss Lake. At other localities in derived. Moreover, the submarine basalt flows Major-element chemical analyses for eight the Chatham slice, greenish, fine-grained basalt themselves thin northeastward and become fine new samples of the Taconic basaltic rocks are clasts (localities 14A, 14B in Fig. 1) are recog- grained, thus indicating a generally eastward given in Table 1 with two previously analyzed nized. Clearly, basaltic rocks and possibly more northeasterly dipping paleoslope (utilizing pres- rocks from Balk (1953, Table 1). The basalts are felsic igneous rods existed in the provenance of ent geographic coordinates) for both slices. The characterized by distinctly high FeO (total) and the graywacke. Flock clasts in the graywacke geologic relationships suggest that basalt of the Ti02 values and low values of MgO. The water consist of phlogopitic calc-silicate rock, micro- Nassau Formation was derived from the con- content of these greenschist-facies rocks is highly cline mesoperthite, garnet-bearing granitic rocks, tinental side of the marginal basin rather than variable, and even unweathered, fresh rocks, and garnet-bearing quartzite typical of middle deep-ocean basin basalts intercalated with such as No. 3 collected from the lowest grade Proterozoic rocks of the Green Mountains and cratonically derived debris. area, contain high amounts of water. The Berkshire massifs. Oligoclase, biotite, and garnet The basaltic rocks in the Taconic Range ap- CaO/MgO ratio is also highly variable, ranging are important demtal minerals in the coarsely pear to be thick- to thin-flow rocks (Iocs. 1, 5, from .7 to 2.2 and shows a strong negative corre- sorted graywacke. Bird (1963) described Bouma and 6), mixed-flow basalts, pillow basalt, and lation with percentage of total water. K2O and cycles from typiciil exposures of the Rensselaer hyaloclasites (or aquagene tuffs, Iocs. 2, 3,4, 7, Na20 are reasonably consistent through the and attributed these features to turbidite deposits 8, 9, and 10), and aquagene tuff and epiclastic analyses; total alkali ranges from 2.6 to 4.7. derived from a western sialic continental source. tuffs at localities 11,12, and 13. Carlisle (1963) SiC>2 is quite consistent, between 49% and 52%. The pillow basalts, sedimentary-volcanic rocks, reported the deposition of pillow basalt and pil- Given the wide range of water contents and the and clasts of volcanics in the graywacke attest to low breccia with a tuffaceous matrix in the metamorphosed and well-foliated nature of the general contemporaneity of the basalt vol- upper parts of deep, quiet-water eugeoclinal many of the samples, it seems unlikely that canism and deposition of the Nassau Formation. deposits from British Columbia. The tuffaceous mobile elements such as Na20, K2O, SiC>2, and In Figure 2, the basaltic rocks in each of the matrix and laminated tuffs occur within zones of MgO reflect accurately the original chemistry of general areas discussed are interpreted as the broken pillow basalt in the upper parts of nor- the rocks. Ti and P, and Fe and A1 appear to be same general stratigraphic horizon. Field data mal pillow basalt sequences and are ascribed to relatively stable. from the Chatham slice indicate clearly that all globulation and fragmentation by rapid cooling The oxidation ratio Fe203/Fe0 varies from basalt and tuffaceous occurrences within the of submarine basalts. Flow-laminated tuffs con- .104 to .70, and Na20 is quite variable; there- Chatham slice oixur at the same stratigraphic taining broken and distended pillows indicate fore, calculation of normative mineralogy may position and may be correlated (Ratcliffe, 1978, flowage or movement down slope to be an im- be misleading. The least oxidized basalt from 1974). Within the Rensselaer Plateau slice, this portant element in formation of broken pillow this collection (1) is a quartz normative basalt assertion cannot be made with the same degree zones (Carlisle, 1963). containing 4% normative quartz. Using a Stan-

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dard Fe203/Fe0 ratio of .189, the basalts are anomaly. The lower greenstone at Fog Hill tiated quartz tholeiite, such as in the Palisades all quartz normative except for No. 2, No. 4, and (sample 7), and the metabasalt at Glass Lake sill. No. 5, which are olivine normative. Primary (sample 3), Ward Hollow (sample 4), and In Figures 6A, 6B, and 6C, the Taconic ba- minerals in the basalt include quartz and quartz- Turner Mountain (sample 2) all have similar salts are compared to greenstones from the plagioclase intergrowths and herringbone-twin- and less-elevated rare-earth abundance patterns. Camels Hump Group of Vermont and to meta- ned monoclinic pyroxene. Mauve-brown cores The upper basalt at each locality is the most diabase dikes from the Hudson Highlands (Rat- in the pyroxene and the exsolution lamellae TiC>2 enriched and presumably the more highly cliffe, in press). In Ti02 versus La+Sm+Yb, suggest primary pigeonite. These petrographic fractionated basalt. Although LRE are some- La+Sm+Yb versus La/Yb, and Ti02 versus Sc, data suggest quartz tholeiite, although the high what enriched in these samples, given the mod- the Taconic basalts are shown to closely resem- Ti02 and P2O5 suggest slightly alkalic basalts erately low MgO values, this enrichment is not ble samples of the Tibbit Hill and Hunting- and serve to distinguish the basalts from MORB markedly higher than for moderately differen- ton occurrences in Vermont (Coish and others, as a class. In Table 1, the average of nine samples is A) Metabasalt from Rensselaer Plateau slice compared with the average composition of 16 pre-Taconian metadiabase dikes that intrude the 1) Turner Mtn. middle Proterozoic rocks of the Hudson High- 2) Turner Mtn. lands in New York and New Jersey (Ratcliffe, 1983 and unpub. data), the closest and best pre- 3) Glass Lake served rift dike rocks to the Taconic basalts. The 4) Ward Hollow Hudson Highlands dikes are interpreted to be 5) pillow basalt, late Proterozoic in age and of the same general Banker Pond composition as quartz and olivine tholeiites of the Catoctin Formation of the central Appala- chians (Ratcliffe, 1983), and are identical in major-element chemistry and rare-earth-element content to the Taconic basalts described here. The analysis of the variolitic basalt from Banker Pond (loc. 2; Table 1, sample 5) stands out as markedly different from all other samples and is a highly altered sample, as it is veined by epidote and quartz and contains admixed sediment.

Rare-Earth and Trace-Element Geochemistry Dy Ho Er Tm Yb

Instrumental neutron activation analyses of rare-earth elements and trace elements for eight samples of metabasalt in Table 1 are given in Table B.2 Analytical uncertainties, based on counting statistics for replicate analyses ex- Metabasalt from Chatham slice pressed at the 1 sigma level are given in percent- ages in Table B. USGS standards W-2, BHVO-1, AGV-1, ARHCO-1, and DNC-1 were analyzed 6) Fog Hill as controls. Chondrite normalized abundance 7) Fog Hill ratios use the CI chondrite values of Anders and 8) Queechy Lake Ebihara (1982). The metabasalts from the Rensselaer Plateau and Chatham slices, with the exception of the pillow basalt (5) from Banker Pond, have mod- erate light rare-earth enrichment (La = 91 to 53 x chondrite and Yb = 19.8 to 12 x chondrite). The range of rare-earth abundance patterns are nearly identical from the two slices (Fig. 5) and are nearly identical with those of metabasalts at Tibbit Hill but unlike those of the LRE-en- riched Huntington basalt, both members of Coish and others' (1985) Group A rocks. The upper basalts at Turner Mountain (sample 1) and at Fog Hill (sample 6), and at Queechy Lake (sample 8) have elevated rare-earth abun- dance patterns and exhibit a slight negative Eu Figure 5. Rare-earth-element abundance patterns for basalts from the Taconic allochthon. Rare-earth values are normalized to CI chondrite values of Anders and Ebihara (1982). 2See footnote 1. Numbers correspond to samples in Table 1.

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1985, Group A greenstones), although the Sc as Pennsylvania, Maryland, and Virginia Catoc- versus MgO fields for dikes from the New York contents (Fig. 6C) are higher. In addition they tin continental metabasalt and metadiabase; as Hudson Highlands and the Taconic basalts over- are similar to late Proterozoic continental meta- continental metadiabase dikes from the Hudson lap and are distinctly richer in FeO than are diabase dikes in the Hudson Highlands of New Highlands, New York and New Jersey, and ENA Mesozoic dikes or basalts. York that intrude l,000-m.y.-old middle Prot- dikes and flows from Labrador and western In a Ti02 versus P2O5 diagram (Fig. 8), ba- erozoic basement. Newfoundland—all believed to be late Prot- saltic lava flows of the Taconics (this report) are erozoic in age and interpreted to be associated compared to lava flows and dikes of the Catoc- Comparison with Other Eastern North with Iapetan rifting of sialic crust in eastern tin Formation and to metadiabase dikes of the America Late Proterozoic and Early North America. Compared with eastern North Hudson Highlands. For comparison, the general Mesozoic Diabase and Basalt Associations America (ENA) Mesozoic tholeiitic diabase and field for MORB is shown. ENA Mesozoic ba- Mafic index versus Ti02 (Fig. 7) values for basalt, the Iapetan rift basalts are markedly salts, including fractionated Palisades diabase the Taconic basalts plot in the same general field richer in P2O5, ÜO2, and total iron. Total FeO magma (not shown), plot in the same general

5.0 iQ \ * \ 100 4.5 Taconic v* allochthon p.,,A. basalts- / 1 ; / ' / 80 / 1 / / 1 [ aE a Coish et al (1985) Coish et al (1985) / ct-^sti Group A £ 60 Group A /

/''/* E CO / / Taconic allochthon I I t 40 basalts - / I I >Coish et al (1985) S 1.5 - / t Group B

20

O 56--3B Coish et al (1985) B O 56-3B Group B 1 1 L_ 20 40 60 80 100 6 8 10 12 La/Yb La + Sm + Yb (ppm) 6i-

~ 5 U) 3 3 O t Taconic allochthon basalts

O Basalts in Rensselaer Plateau and Chatham slices Coish et al (1985) ^ Group A \ A Metadiabase dikes, ^^ Coish et al (1985) Hudson Highlands, N.Y. " Group B O 56-3B

J 20 30 40 50 Sc (ppm)

Figure 6. Diagrams illustrating similarities between Group A greenstones in the Camels Hump Group (Coish and others, 1985), late Proterozoic continental dikes from the Hudson Highlands of New York and New Jersey (Ratclifie, 1983) and basalts from the Taconics. A. TiC>2 versus La + Sm + Yb diagram; B. La + Sm + Yb versus La/Yb diagram; C. Ti02 versus Sc diagram.

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field as the Catoctin dikes and Taconic basalts. Lower Cambrian Cheshire Quartzite (Doll and ment on the east flank of Lincoln Mountain in The late Proterozoic dikes and flows, however, others, 1961). Volcanic rocks of the Tibbit Hill central Vermont (Cady, 1956) and is overlain are distinctly tholeiitic from primary mineralogy Volcanic Member of the Pinnacle Formation by the Lower Cambrian Cheshire Quartzite and have geologic settings indicating that they form the base of the Camels Hump Group in (Dowling and others, 1987). The late Protero- were generated beneath continental crust or Quebec where they are overlain by graywackes zoic rock sequence in central and northern Ver- adjacent to it and cannot be ocean-floor basalts of the Pinnacle Formation that thin eastward mont resembles closely and is correlated with or arc-related volcanics strictly on geologic into finer-grained metapelites resembling the the metabasalt and graywacke section in the grounds. Underhill Formation (Doolan, 1984). To the Taconics described in this paper. Basaltic volcanic rocks, tuffaceous metasedi- south, volcanic rocks of the Tibbit Hill appear to Coish and others (1985) show recently pub- mentary rocks, and tuff breccias are abundant occur at a higher stratigraphic level where they lished chemcial data from metabasaltic tuffs and locally within the Camels Hump Group (Cady, occur at the Pinnacle-Underhill contact. Pin- metabasalts within the Pinnacle-Underhill and 1956) of northern Vermont, a complex rock nacle graywacke beneath the volcanic rocks Pinney Hollow Formations of the Vermont se- succession that lies stratigraphically beneath the unconformably overlies middle Proterozoic base- quence. Their data indicate that basaltic rocks

T B highly differentiated Palisades (Data for Catoctin basalt .90 .90 /and York Haven magmas — and dikes from Reed and Morgan, 1971, \Q ck Espenshade, 1986, ^ • and Blackburn, in press) .80 .80 _ E. N. A. low GL Ti qtz. o © « field for norm. N. A. high /^m^,1 Catoctin basalt VFe203 qtz. gfl/« basalts and / / i norm, basalt A^m 7 .70 diabase I .70 / £ dikes / : ¡a' ® btt ^ f \i>\high Ti metadiabase of v~' Hudson Highlands .60 metadiabase dikes from Hudson Highlands, New York ' ° J&-.9— E. N. A. high Ti qtz. norm, basalt

''Mi—E. N. A. ol. norm, basalt .50 _a i L_J i i i i 1.0 2.0 3.0 4.0 5.0 ' 0.0 1.0 2.0 3.0 4.0 5.0

Ti02%

T T T

— Late Proterozoic Labrador and Newfoundland d> -90 IN flows and dikes and basalts of a> Taconic allochthon

S .80 -

.70 -

Nassau Formation basalt field .60 (Strong and Williams 1978) (Strong 1974)

.50 0.0 1.0 2.0 3.0 4.0 5.0 2.0 3.0 Ti02% Ti02%

Figure 7. Mafic index versus T1O2 diagrams. A. Taconic basalts in (samples numbered as in Table 1) comparison to metadiabase dikes (solid dots) from the Hudson Highlands, New York, and field for Catoctin basalt from Virginia and Pennsylvania. B. Comparison of Taconic basalts (half circles); Hudson Highlands dikes (solid dots) to mqjor eastern North America Mesozoic basalt and diabase (open circles and fields, data for Mesozoic rocks from Walker, 1969 and unpub. data; from Palisades sill of Ratcliffe and others (1970) and Smith and others (1975).

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semble flows, silk, and volcaniclastic beds associated with the Pinnacle Formation, and specifically the Tibbit Hill subset of Group A greenstones of Coish and others (1985) as pre- dicted by the geology. The Ti02, P2O5. and MnO contents of the Taconic basalts are nearly identical to Coish and others' (1985) (type A) greenstones from the Pinnacle Formation as a whole. When plotted on the tectonomagmatic discrimination diagram of Wood (1980), the seven Taconic basalts (exclusive of no. 5) cluster •o tightly in the within-plate basalt field (Fig. 9), with a Ta/Hf ratio of about four. These basalts are nearly identical to the 16 metadiabase dikes from the Hudson Highlands of New York. In Th, Hf, and Ta content, both the Hudson High- + ^ land dikes and the Taconic basalts are less , -P O EXPLANATION _ alkalic than greenstones from the Tibbit Hill ++ • O subset (Group A of Coish and others) and • Hudson Highlands overlap in part with five metadiabase dikes from D o ^ , Metadiabase dikes the Catoctin Formation. By this measure, the Catoctin dikes M. O. R. B. * dA Taconic basalts are not alkalic despite the field (Espenshade 1986) _ a pronounced Ti02 and P2O55 enrichment. • O Catoctin dikes and O greenstones (Reed On a TiOj, MnO x 10, P 0 x 10 diagram A- \f and Morgan 1971) 2 5 / ° Metabasalt Taconic (not shown), basalts from the Taconic alloch- / V + allochthon N.Y. _ thons, except for sample 5, plot within the same / o area of the diagram as do type A rocks of Coish + + A Tibbit Hill Formation and others (1985). According to the classifica- greenstone Vt. (Coish tion of Mullen (1983), Tibbit Hill rocks, the / and others 1985) D + Dikes and flows Labrador — Taconic basalts, and late Proterozoic dikes of the -1 / Hudson Highlands, New York (Ratcliffe, 1983) Qo / and Newfoundland o\ / (Strong and Williams 1972) plot in the field of within-plate oceanic tholeiites and ocean-island basalts. This, combined with the elevated REE abundance level and moder- 0 1 2 3 4 5 6 7 ately fractionated REE patterns (principally from the Huntington body) suggested to Coish Ti02% and others (1985) that the Tibbit Hill volcanics Figure 8. P2O5 versus TÌO2 diagram for Taconic basalts in relation to Catoctin and metadia- probably represent within-plate tholeiitic to base dikes from the Hudson Highlands, New York. transitional alkalic basalts, generated at early stages of rifting, which this paper in part sup- ports. The similarity of the Tibbit Hill metaba- included in the section may be placed in two Coish and others (1985, 1986) has shown that salts to the Taconic rocks (this paper), to groups: Group A includes the Tibbit Hill and basaltic volcanic rocks of east-central Vermont continental metadiabase dikes from the Hudson Huntington greenstone bodies that are high- are geochemically complex. Coish and others Highlands (Ratcliffe, 1983) and from the Catoc- tin Formation in the Blue Ridge of Virginia sug- FeO, high-Ti02, high-P205 metabasalts asso- (1985) showed that flows and tuffaceous volcan- ciated with the Pinancle Formation and which iclastic rocks (Doolan, 1984) within the Pinna- gests a similar continental-rift regime for all of these rocks. are closely comparable to basalts associated with cle Formation are high-Ti02, -P2O5, -Zr, -Y the Rensselaer Graywacke reported here; Group tholeiitic to alkalic basalts which have high B includes metabiisalt interbedded with the Un- general abundances of REE and LREE and are CONCLUSIONS derhill and Pinney Hollow Formations that are moderately fractionated (La = 50 to 200 x chon- poorer in Ti02, P2O5, exhibit lower REE abun- drite and Yb = 11 to 30 * chondrite), but the Tholeiitic to transitional alkalic basalt flows, dance patterns than do Group A rocks, and are more alkalic basalts come exclusively from the sills(?), and volcanogenic metasedimentary rocks judged to be more MORB-like. Huntington body. Greenstones from the Under- form minor but distinctive rocks within the Basaltic rocks in the Rensselaer Plateau and hill and Pinney Hollow Formations are in part lower one-third of the Rensselaer plateau and Chatham slices of the Taconic allochthon distinctly different. They are predominantly Chatham slices of the Taconic allochthon in closely resemble volcanic rocks of the Tibbit lower in Ti02, P2O5, Zr, Y, and rare earths eastern New York State and western Massachu- Hill Volcanic Member of the Pinnacle Forma- (Figs. 6, 7, and 8) than the Tibbit Hill and are setts. The basalts are locally known to be sub- tion of north-cen tral Vermont and specifically only slightly enriched in LREE (10 to 33 x marine lava flows and occur in a particular part the main mass at Tibbit Hill proper. Geochemi- chondrite for La) relative to HREE (8 to 12 x of the stratigraphic section closely associated cal study of the Tibbit Hill (Pieratti, 1976) and chondrite for Yb) (Coish and others, 1985). with the Rensselaer Graywacke Member. Two of greenstones from the Pinnacle, Underhill, Rare-earth-element data available for the Ta- major basalt flows, or zones of aquagene tuff Pinney Hollow, and Stowe Formations by conic basalts show that these rocks closely re- and volcanogenic sediment, are present at many

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localities, and these rocks may represent a re- At the latitude of the Chatham and Rensse- Palinspastic Positions of Slices stricted period of basaltic volcanic activity asso- laer Plateau slices, dikes or flows similar to the and Sedimentary-Volcanic Model ciated with rifting of the North American craton Tibbit Hill-Pinnacle or to Nassau-Rensselaer are during development of Iapetus in late Protero- rare in rocks east of the Berkshire massif. Minor In Figure 10, the distribution of volcanic zoic time. hornblende-plagioclase-epidote-greenstones in the rocks and coarse-grained graywacke in the Hoosac Formation in the Blandford area (CZha Rensselaer Plateau and Chatham slices is shown. Grain-Size Distributions and Source Direction south of 42°15'N and north of 42°30'N) above The slices are shown in restored relative position for Rensselaer Graywacke the Middlefield thrust (Fig. 1) petrographically with the underlying Chatham slice restored to a resemble the basaltic rocks of the Taconic position east of the Rensselaer Plateau slice Balk (1953) concluded that the coarsest con- Range and may represent remnants of the same based on the observed structural overlap of the glomerates existed along the west and southwest volcanic episode preserved in even more prox- Rensselaer Plateau slice over the Chatham slice margins of the plateau, and that the coarsest imal sedimentary rocks of the same general late of at least 25 km. The present structural overlap rocks were present in the Austerlitz outlier Proterozoic age as the Rensselaer-related volcan- and stacking sequence, with the Rensselaer Pla- (Chatham slice of this report), in agreement with ics (see Fig. 1). Minor post-"Grenville" mafic teau slice above the Chatham slice, would ap- my observations. Ondrick and Griffiths (1969) dikes exist in the Green Mountain and Berkshire pear to require that the restored position of the reported that grain size of quartz grains in the basement massifs, but these dikes contain biotite Rensselaer Plateau slice be east of the Chatham coarsest and finest graywacke from 66 sample and plagioclase phenocrysts rather than the py- slice. The model for structural evolution of the sites across the plateau show maximum values in roxene present in epidote-actinolite greenstones Taconic allochthons, however, involves diver- the north, west, and southwest parts of the pla- (Cullen, 1979). ticulation (Stanley and Ratcliffe, 1985), in teau. From these data, they concluded that the Volcanic rocks and coarse graywacke are ab- which more easterly and deeper water slices graywacke was deposited from several direc- sent from the Everett and Greylock slices that were emplaced first and the more proximal mate- tions; north, west, and south. When the general are believed to have been deposited west of the rials emplaced last. This westward-progressing synformal structure of the plateau is considered, paleodepositional site of the Rensselaer and thrusting has produced the reversal in relative it is seen that the coarsest conglomerates and the Chatham slices (Fig. 10). The restored positions position of the allocthons. The 25-km overlap coarsest quartz grain size are restricted to the of the slices of the Taconic allochthon relative to shown in Figure 10 shows the minimum spacing same general stratigraphic level, that is, in the the Berkshire massif and the Hoosac Formation between the two slices. Volcanic rocks and lower one-third of the graywacke sequence are shown in Figure 11. graywacke are common to both slices, and they rimming the north, west, and south parts of the Rensselaer Plateau. This supports a general EXPLANATION southwest-to-northeast thinning and fining Fields direction. A. N— type M.O.R.B. • 7 Taconic basalts A similar fining direction was determined for the Chatham slice. Ratcliffe and others (1975) presented data that showed the Rensselaer Graywacke Member of the Chatham slice thins both to the northeast and southwest, suggesting that an intrabasinal source might be required. This pattern might also be produced by lobate, channelized turbidite deposits that entered the basin from the west as proposed here. The Chatham slice contains fewer of these gray- wackes than the Rensselaer slice but the coarsest grained graywacke exposed in either slice is found near Austerlitz (Balk, 1953) and is con- temporaneous with the basaltic rocks, because it contains clasts of basalt. The observations re- garding grain size could be accounted for by a series of submarine-fan deposits that flowed generally east. The Chatham slice is regarded as more basinal, with only the coarse turbidite fan complex, associated with the volcanic rocks, ex- tending into the deeper water, and the Rensse- laer plateau slice, the more proximal. The data currently available are consistent with a tectono-sedimentologic model in which the Rensselaer and Rensselaer-associated basalts formed as rift-stage volcanics on a structurally Figure 9. Samples of Taconic basalts exclusive of sample 5 plotted on Hf/3, Th, Ta discrimi- evolving cratonic margin in Late Proterozoic nation diagram of Wood (1980) showing field of 16 Hudson Highlands metadiabase dikes time. The restriction of basalt to the lower one- (enclosed by solid line), and field of five metadiabase dikes of Espenshade (1980) from the third of the Taconic sequence suggests that the Catoctin Formation. Group A metabasalts and Group B metabasalts refer to the basalts from rift stage was short-lived at the paleodepositional the Pinnacle and Underbill Formations of Vermont from Coish and others (1985). Dash lines site of these slices. represent areas of overlap between tectonomagmatic fields of Wood (1980).

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Rensselaer Plateau Rensselaer Plateau i slice

Zone A coarsest graywacke thickest basalt

Zone B finer graywacke pillow basalt and basaltic tuff

Zone C MBCWt laminite fine graywacke no basalt

N A

Chatham slice

Zone' B,

t

0 25 0 25 L / 1 1 KM ( KM

Figure 10A. Geologic map showing zonation in Rensselaer Plateau slice Figure 10B. Schematic palinspastic diagram showing relative location of basalt/graywacke (coarse stipple) and Chatham slice (fine stipple). facies in the Rensselaer Plateau and Chatham slices. Polarity for coarse graywacke sediments and basalts in each slice was from the craton.

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Diagramatic Representation of late Precambrian (Late Proterozoic) Pre-shelf Nassau depositional basins

Proximal turbidites and Distal turbidites and thick flows volcanogenic sediments

Figure 11. Schematic cross sec- Hoosac-Greylock Rensselear Plateau tion showing tectonic setting of the Everett slices slice pre-shelf part of the Rensselaer Sea level Plateau and Chatham slices of the Taconic allochthon and inferred submarine rift basins. Both intrusive and extrusive basalts are shown.

Basement rocks of Distended sialic crust Berkshire massif

EXPLANATION 10 MILES €Zr —Nassau Formation CZnv—Basalt of Nassau Formation CZnr—Rensselear Graywacke Member 15 20 KILOMETERS of the Nassau Formation

thin northeastward or eastward into fine-grained derived. These basaltic rocks, like those exposed The basement rock containing mafic dikes and graywacke and laminated gTeen phyllite. The in Labrador and as dikes in the Long Range the hypothetical flood basalts are not preserved base of both sequences consists of purple and (Strong and Williams, 1972; Strong, 1974), at the latitude of Massachusetts but may be green laminated slate or phyllite (CZnp). Over- could have served as source rocks of the highly found in the Tibbit Hill-Pinnacle sequence of all, the sediments of the Chatham slice are more chloritic, iron-rich metasediments in the Nassau Vermont and as dikes and flows in the Reading distal than rocks of the Rensselaer Plateau slice, Formation as a whole. The basement rocks con- Prong and Blue Ridge to the south. Locally fis- but basaltic rocks are present as thick flows or taining abundant dikes or lava flows, analogous sure flows erupted into the adjacent water and fairly continuous units of volcanogenic graywacke. to those exposed in the Blue Ridge of Virginia, flowed down the geomorphic surface of the The geologic data indicate that the source of are no longer preserved at the latitude of the graywacke fans to be deposited on the supra- or the basalt and of the coarse conglomerates was central Taconic Mountains, as mafic dikes are lower-fan deposits. Downslope and oceanward, to the west-southwest for each slice and that the rare in the Berkshire massif. these volcanics thinned to toes of pillow basalt basalts were erupted onto well-laminated, fine- and volcaniclastic debris that were deposited as grained sediments (CZnp orCZgr) in a subma- distal turbidites. Farther up slope, fissure flows Paleoenvironment and Tectonic Setting rine environment. The close association of basalt erupted into proximal and slightly younger and turbiditic beds of graywacke is clear, but a rocks of the Hoosac Formation, implying that cause for the association with submarine-fan and Reconstruction is inhibited by the lack of fos- the volcanicity began earlier in the east. lower-slope deposits is less clear. One possible sils or other clear indications of marine deposi- Pelite and graywacke of the Nassau Forma- interpretation is that the basalt originated from tion. It is clear from the well-rounded, large tion in the Rensselaer and Chatham slices differ fissure flows that capped the exposed Protero- boulders of gneiss, quartzite, marble, and auto- little from the Nassau of other slices, except for zoic basement east of any preserved basement in clastic debris (up to 2 m in length) in the gray- the unusually thick and coarse-grained nature of the Berkshire massif. Locally these flows may wacke that a mixed provenance consisting of the Rensselaer Graywacke Member itself, which have been channelized to flow down the subma- nearby alluvial deposits existed to the "west" of is largely, but not altogether, absent from other rine alluvial-fan surfaces in a general easterly the basin. Finer-grained siltstone and laminated slices. Commonly the Rensselaer is regarded as direction, finally to discordantly overlie several slates of the Nassau Formation closely resemble rift sediments older than the more marine beds slope-fan facies before thinning to pillow basalts the fine-grained sediments commonly attributed in the Nassau proper and, together with the Pin- and tuffaceous turbidites. To the east, westerly to deposition on the lower slope after drifting nacle, constitutes a pre-shelf assemblage. Recent derived volcanogenic metasedimentary rocks had begun (Keith and Friedman, 1977). It is mapping, however, has shown that the gray- may have been deposited in deeper water important to note that fine-grained laminites of wackes of the Rensselaer type are interbedded beyond the limits of the fan morphology. The the Nassau are also abundant in the Giddings throughout the Nassau and are not confined to similarity of the basalt chemistry to volcanic Brook slice, where they grade upward into ma- the base of the Taconic sequence (Ratcliffe, rocks of late Proterozoic age, known to intrude rine sediments containing Early Cambrian 1974; Ratcliffe and others, 1975). Moreover, and rest on North American cratonic rock, (see trilobites. graywackes are interbedded and pass into distal previous sections) further supports the general The model preferred for the Rensselaer basin turbidites and well-laminated pelites, suggesting conclusion proposed here. Detrital clasts of ba- is a fault-controlled continental margin with that the entire sequence was deposited in marine salt within the coarse proximal facies of the high relief and a narrow shoreline (Fig. 11). Ba- conditions in a setting similar to younger slope- graywacke indicate that volcanic-flow rocks saltic rocks, probably continental flood basalts, rise sediments of the Giddings Brook slice. The were present in the source area of the unconsoli- capped the basement rocks, contributing high available data therefore suggest that the volcanic dated sediment from which the graywacke was Fe-Mg to the chloritic sediments of the Nassau. rocks of the Rensselaer and Chatham slices were

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erupted as very lale stage rift volcanics above ACKNOWLEDGMENTS Petrology, v. 47, no. 3, p. 1220-1241. Klitgord, K. D., and Behrendt, J. C., 1979, Basin structure of the U.S. Atlantic highly distended and subsided continental crust margin: Tulsa, Oklahoma, Geology and Geophysical Investigations of Continental Margins, p. 85-112. at or near the margin of the continent. The William Blackburn kindly permitted me to Lajoie, Jean, 1979, Facies models 17, volcanidastic rocks, in Walker, R. G., Rensselaer graywackes and volcanics therefore plot Ti0 -M I values for Catoctin basalt from eel, Facies models: Geoscience Canada, Reprint Series 1, p. 191-200. 2 Lash, G. G., 1986, Sedimentologic and geochemical evidence for Middle may represent late-rift, marine sediments, per- his unpublished manuscript. His data are in- Ordovician near-trench volcanism in the central Appalachian orogen: Journal of Geology, v. 94, p. 91-107. haps bridging the gap in space and time between cluded in Figure 5, as are Ti02 and P2O5 values LEG 104 Scientific Party, 1986, Dipping reflectors in the Norwegian Sea— rifting and drifting at the initiation of Iapetus. from five metadiabase dikes from the Catoctin ODP Leg 104 drilling results: Geological Society of London Journal, v. 143, p. 911-912. In major-element and rare-earth chemistry, Formation of Virginia from Gil Espenshade's Mullen, E. D, 1983, Mn0/Ti02/P.0s: A minor element discriminant for basaltic rocks of oceanic environments and its implications for petro- metabasalt in the Rensselaer Plateau and Cha- study of the Marshall, Virginia, area (Espen- genesis: Earth and Planetary Science Letten, v. 63, no. 1, p. 152-162. shade, 1986). Reviews by Rolfe Stanley, Barry Mutter, J. C„ Talwani, Marik, and Stoib, P. L„ 1982, Origin of seaward- tham slices closely resembles greenstones from dipping reflectors in oceanic crust off the Norwegian margin by "sub- the base of the Camels Hump Group of Ver- Doolan, William Blackburn, Norm Hatch, Peter aerial sea-floor spreading": Geology, v. 10, p. 353-357. Ondrick, C. W., and Griffiths, J. C., 1969, Trend surface analysis applied to the mont, and specifically greenstones at Tibbit Hill Lyttle, Jack Bird, and Chris Hepburn are all Rensselaer Graywacke and its implications to the Taconics: Journal of greatly appreciated. Sedimentary Petrology, v. 39, no. 1, p. 176-186. (a subset of Group A of Coish and others, Pieratti, D. P., 1976, The origin and significance of the Tibbit Hill melavol- 1985). Coish and others interpreted their Group canics, northwestern Vermont [M.A. thesis]: Burlington, University of Vermont, 136 p. A and Group B greenstones as different varieties REFERENCES CITED Potter, D. 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C., 1981, Regional plate tectonics and the quadrangle, Massachusetts, Connecticut, and New York: U.S. Geologi- generated above highly distended and therefore evolution of Australia's passive continental margins: Bureau of Mineral cal Sttivey Geologic Quadrangle Map GQ-507. subsided continental crust near the site of even- Resources, Journal of Australian Geology and Geophysics, v. 6, PI. 29. Zen-E-An, and Ratdiffe, N. M., 1966, A possible breccia in southwestern Fisher, D. L., 1984, Bedrock geologic map of the Glens Falls-Whitehall area. Massachusetts and adjoining areas, and its bearing on the existence of tual separation. Recent drilling of these features New York: New York State Museum Map and Chart Series, no. 35, the Tannic allochthon, in Geological Survey Research, 1966, p. 36-44, scale 1:48,000. Chap. D: U.S. Geological Survey Professional Paper 550-D, 265 p. (LEG Scientific Party, 1986) off the coast of Herz, N. L., 1958, Bedrock geology of the Cheshire quadrangle, Massachusetts: 1971, Bedrock geologic map of the Egremont quadrangle and adjoining Norway, albeit on a far grander scale than that U.S. Geological Survey Quadrangle Map GQ-108. areas, Berkshire County, Massachusetts, and Columbia County, New Hinz, Z., 1981, A hypothesis on terrestrial catastrophies. Wedges of very thick York: U.S. Geological Survey Miscellaneous Investigations Map 1-628. discussed here, has confirmed the existence of oceanward-dipping layets beneath passive continental margins—Their origin and paleoenvironmental significance: Geologisches Jahrbuch, significant pre-shell" volcanic rocks in the passive Reihe E. Geophysik, v. 22, p. 3-28. MANUSCRIPT RECEIVED BY THE SOCIETY JUNE 4,1986 continental margin of the Norwegian Sea. Keith, B. D., and Friedman, G. M., 1977, A slope-fon-basin-plain model, REVISED MANUSCRIPT RECEIVED MARCH 25, 1987 Taconic sequence. New York and Vermont Journal of Sedimentary MANUSCRIPT ACCEPTED APRIL 2, 1987

Printed in U.S.A.

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