Abstract F ew of the nearly 400 iron mines in the New Jersey Highlands share the historical significance of the Andover mine that assumed an important role during the American Revolutionary War. In 1778, possession of both the mine and the Andover furnace were taken by the Continental Congress to supply American troops with iron and steel (Bayley, 1910). The Sulfur Hill mine did not open until between 1855 and 1860 and thus played no part in this conflict. Individual production figures for these two mines are lacking for all but a few years; however, the combined total production is estimated at about 363,000 metric tonnes of ore (Bayley, 1910). The mines at Andover and Sulfur Hill are unique among the Highlands iron mines in affording an opportunity to examine two entirely different genetic types of iron mineralization. These deposits share few similarities beyond their close spatial association (ca. 200 m apart) and the fact that both deposits are shallow, having been worked mainly from open cuts .26 m deep and underground to a limited extent. Some of the principal differences between the two deposits include: (1) a Neoproterozoic age for ore and host rocks at Andover vs. a Mesoproterozoic age (pre- to syn- Ottawan orogeny) for ore and host rocks at Sulfur Hill; (2) mainly hematite ore at Andover vs. magnetite only at Sulfur Hill; (3) separate, discontinuous deposits having different geometries; (4) low-grade metamorphism of ore at Andover vs. high-grade (?) at Sulfur Hill; (5) higher iron content of ore

Abstract The Losee Metamorphic Suite, granites of the Vernon Supersuite, amphibolites, the Franklin Marble, and the Chestnut Hill Formation each contain magnetite or hematite deposits that were mined in the New Jersey Highlands during the last half of the 20th century and the first half of the 21st century. How-ever, not all of these Proterozoic magnetite deposits are the same. Grenvillian Losee-hosted magnetite de-posits occur as concordant veins and disseminations that precipitated from fluids released during the prograde granulite-facies breakdown of biotite, coeval with subduction of its dacitic arc protolith. The Vernon Supersuite-hosted magnetite deposits are chemically and structurally similar to the Losee de-posits and precipitated in a similar deep crustal environment during early or pre-Ottawan orogenic intrusive activity at ca. 1100 Ma. The Vernon Supersuite deposits may have precipitated out of late-mag-matic or deuteric iron-rich fluids that were generated during the local conversion of granite to iron-depleted alaskite. Although magnetite in most of the granite- and gneiss-hosted deposits contains 1 to 3 wt% TiO2, magnetite in some Grenvillian amphibolite-hosted deposits has up to 10 wt % TiO 2 , consistent with the higher temperature magmatic history of their basaltic protolith. The TiO 2 content of magnetite in the Grenvillian carbonate-hosted deposits is less than 0.2 wt % with high MnO contents consistent with distal volcanogenic marine deposition. Chestnut Hill-hosted deposits share some of the char-acteristics of banded iron formation.

Complex swarms of mafic dikes extend across New England and Atlantic Canada. Radiometric dates, distributions and structural patterns, petrologic correlations, and geochemical analyses of the postmetamorphic dikes and related plutonic complexes are used to distinguish at least four petrogenetic groups, or igneous provinces, that span the Mesozoic. These groups are as follows. (1) The Early Cretaceous New England—Quebec igneous province, which contains thousands of lamprophyric dikes and at least 20 associated plutonic complexes in northern New England and southern Quebec. (2) The Early to Middle Jurassic White Mountain magma series in central and northern New Hampshire, which contains numerous alkalic diabase and lamprophyre dikes associated with large syenitoid-granitoid plutons. (3) Quartz tholeiitic dolerites of the Early Jurassic Eastern North America dike province, occur as scattered, large dikes that also fed flood basalts, remnants of which are found in the Mesozoic basins. (4) Olivine diabase dikes are present along southeastern coastal New England (CNE groups), for which K-Ar dates and an association with the Agamenticus complex in Maine indicate Triassic ages. The CNE dikes may be correlated with Triassic intrusions in parts of Atlantic Canada (AC groups) as well as with undated dikes in southern New England.

In eastern Canada, two periods of magmatism were associated with early Mesozoic continental rifting. Triassic dikes in southwest Nova Scotia and the Northumberland Strait F-25 well geochemically resemble alkaline dikes of the coastal New England (CNE) igneous province. Widespread Hettangian tholeiitic magmatism of the eastern North America (ENA) province is represented in eastern Canada by extensive multiple basalt flows around the Bay of Fundy and on the Scotian Shelf and Grand Banks and by several dikes hundreds of kilometers long. The total volume of observed Mesozoic magmatic products on the eastern Canadian margin is small. They were probably emplaced over relatively short time intervals, indicating the importance of tectonic pathways in permitting rise of magma to the surface. Samples from the CNE province and the Glooscap well on the west Scotian Shelf have more highly radiogenic Pb isotope compositions. Samples from southwestern Nova Scotia are enriched in LILE, and have higher Hf/Lu and lower Y/Nb compared with samples from Newfoundland and northern New Brunswick. Such geochemical trends are interpreted to result from the Permian-Triassic hotspot responsible for resetting of K-Ar dates and plutonic activity in New England. The progression from alkalic to tholeiitic magmatism through time is related to this plume. Other early Mesozoic basaltic rocks in eastern Canada have isotopic composition (low ɛ Nd and high 207 Pb/ 204 Pb) and trace element features (such as high La/Ta and low Ti/V) that reflect incorporation of crustal-type material in subcontinental mantle, perhaps by previous subduction. Geochemical comparison with Carboniferous tholeiites suggests that the early Mesozoic tholeiites have not been generally contaminated by continental crust, except for some local fluid phase interaction. The magmas resulted from adiabatic decompression of asthenosphere as a result of continental extension and subsequent plagioclase and pyroxene fractionation in mid-crustal reservoirs. Three major tectonic and igneous phases are distinguished: (1) Anisian to early Hettangian rifting, accompanied by minor alkalic dikes (CNE province), when basins formed by extension and were filled by thick terrigenous clastics and evaporites; (2) a postrifting phase (late Hettangian to Bajocian), separated by a postrift unconformity from underlying strata. This Hettangian unconformity is not a breakup unconformity. ENA magmatic activity (tholeiitic plateau basalts and linear composite dikes, up to 400 km landward of the hinge line) was localized along major deeply penetrating faults. The change in tectonic regime resulted in accelerated lithospheric attenuation accompanied by uplift landward of the hinge zone and an increase in subsidence seaward. (3) The final separation of continental crust and onset of oceanic rifting took place in the Bajocian and was accompanied by only limited igneous activity near the ocean-continent boundary.

Mesozoic dikes in southern Maine occur within a 15–20-km-wide northeast-trending coast-parallel swarm ~150 km in length. The swarm is best exposed from Kittery to Ogunquit, but extends northeast into the Casco Bay area and southwest into New Hampshire. The dikes are dominantly mafic (dolerites and lamprophyres) but syenitic and granitic varieties are also present. Composite and multiple intrusive relations are common. Average dike width is 1.12 m and maximum dike width is ~25 m. Maximum extension values of 23% and dike intensities of 176 dikes/kilometer are found along the interpreted swarm axis. Dike orientations are dominantly northeast-trending and steeply dipping with apparent maxima at N60°E, N45°E, and N35°E, as well as minor northwest trends. These trends reflect a strong structural control on intrusion by the N60°E vertical bedding and a N45°E vertical cleavage in the host Kittery Formation. The horizontal component for dike dilation is dominantly N55°W-S55°E, which results in many sinistral-oblique opening directions and left-stepping en echelon offsets for the more structurally controlled dikes. Most of the mafic dikes were intruded between the syenite-alkaline granite phase and the later biotite granite phase of the Triassic Agamenticus alkaline intrusive complex. Dike intrusion was also contemporaneous with the emplacement of Triassic explosive igneous breccias at Gerrish Island, but prior to the intrusion of the Late Cretaceous Cape Neddick gabbro complex. A linear dike swarm: central intrusive complex model based on the Tertiary igneous province of northwest Scotland is adopted for this phase of early Mesozoic magmatism. The linear coast-parallel dike swarm and associated Triassic Agamenticus alkaline intrusive complex in southern Maine are part of the coastal New England igneous province of McHone and Butler; a 500-km-long, north-northeast-trending zone of crustal extension that includes the early Mesozoic dikes of eastern Massachusetts and Rhode Island to the southwest developed during Triassic rifting.

The northeast-trending composite swarm of Mesozoic mafic dikes in eastern Massachusetts consists of tholeiitic olivine dolerites, transitional-alkalic dolerites, alkaline dolerites, and alkaline lamprophyres. Chemical subtypes thus far identified among the dolerites include dikes relatively high (>9.00 wt%) or low in MgO as well as dikes having relatively low TiO 2 (<3.00 wt%) and low P 2 O 5 (<1.00 wt%) contents. The low-MgO dolerites also include high-TiO 2 and high-P 2 O 5 varieties. The major and trace element compositions indicate that the Eastern Massachusetts dolerites differ markedly from Eastern North America (ENA) dikes and are a part of the Coastal New England (CNE) igneous province. Plotting on a variety of chemical-tectonic discriminant diagrams suggests that the dike compositions are compatable with continental rifting. The more alkaline affinity of the composite swarm relative to ENA dolerites and CNE dolerites to the west and south may be a function of the swarm’s more central location on the horst adjacent to the Connecticut rift basin.

Chemical analyses, radiometric data, petrographic data, and physical features define three different rock types (and associated provinces) in the dikes of southeastern New Hampshire: (1) an alkali basalt group (coastal New England) that ranges in thickness from 2 cm to 5 m, is common throughout the area and tends to follow structural trends of the metamorphic host rock; (2) a tholeiitic dike (eastern North America) classified as high-TiO 2 quartz-normative (HTQ) that ranges in thickness from 18 to 30 m, cuts across the trend of the foliation of the host rock, and can be traced along a north-northeast line for 24 km; and (3) a felsite group (New England–Quebec) that includes andesite, dacite, and rhyolite. Whereas most of the felsitic rocks occur in dikes oriented N45°E to east-west, the rhyolite crops out in a massive stock or laccolith(?) hundreds of meters across. K-Ar whole-rock age determinations yield 222 Ma for an alkali basalt, 177 Ma for a tholeiitic basalt, and 118 Ma for a rhyolite. Although all three rock types traditionally have been considered part of the White Mountain Magma Series in southeastern New Hampshire, the 100 m.y. spread in their emplacement ages suggests that the three rock types are products of different and unrelated igneous events. The mafic rocks were emplaced before or during rifting events that opened the North Atlantic Ocean. The HTQ tholeiitic Onway dike is most likely a northeastern extension of the Higganum dike in Connecticut and Massachusetts. The felsite group appears to be associated with hotspot activity that produced a series of plutons extending from the Monteregian Hills in Quebec, through southeastern New Hampshire, to the New England Seamounts.

The eastern North America (ENA) Mesozoic flood basalt province, particularly the part of the province north of central Virginia, is dominated by quartz-normative tholeiites. High-titanium quartz-normative (HTQ) basalts are the first and most widespread Mesozoic extrusions that flowed across several North Atlantic margin rift basins, including the Fundy, Hartford, Newark, and Culpeper basins of North America and the Argana basin of Morocco, Africa. These initial flows are chemically uniform and appear to be largely unchanged by fractionation from their mantle source. Isotopic data for 87 Sr/ 86 Sr and 143 Nd/ 144 Nd are consistent with a lithospheric subcontinental mantle source that was either undepleted or slightly enriched through previous subduction events. The mantle source presumably underwent adiabatic decompressive melting during Mesozoic North Atlantic rifting. The HTQ flood basalts are chemically the same as the dominant members of flood-basalt provinces that were penecontemporaneously extruding from rifts along the South Atlantic margin (the Lesotho basalts of South Africa and the adjacent Patagonian basalts of Argentina), and from the Siberian platform of Asia. This world-wide distribution of virtually identical basalts associated with Pangean rifting implies the uniform operation of some fundamental petrogenetic controls and a diminished likelihood that localized processes unique to any single basin were dominant controlling factors of the regional HTQ composition. The HTQ flood basalts also chemically resemble some of the least-evolved flows of several other continental flood-basalt provinces. Most world-wide continental provinces, however, are dominated by basalts that are relatively enriched in incompatible elements, as if they were contaminated or fractionated to varying degrees. Perhaps the Pangean rifting event responsible for the flood basalts of the North and South Atlantic margins and the Siberian platform was uniquely powerful enough to allow for the rapid delivery of large quantities of unfractionated and uncontaminated magma directly from shallow undepleted or enriched mantle sources after a prolonged period of insulation and thermal accumulation under the Pangean “supercontinent.” The HTQ flows are chemically distinct from other quartz-normative tholeiites of the ENA Mesozoic flood-basalt province that quickly followed them. Flows overlying the HTQ flows are a chemically diverse array including several varieties of high-iron quartz-normative, low-titanium quartz-normative, and possibly olivine-normative types that were probably generated at mantle sources chemically modified by previous HTQ partial melting or from mantle sources independent of the HTQ source. The diverse array of extrusive basalts that followed the HTQ flows seems to have undergone the varying degrees of contamination and fractionation typical of other continental flood-basalt provinces.

Rocks composing an early Mesozoic basaltic suite (MBS) occur throughout the Circum-Atlantic region as dikes, flows, and sheets. Numerous classification schemes have been proposed for rocks from specific geographic areas. All of these rocks, however, can be broadly classified into one of two groups—enriched or depleted—based on TiO 2 -MgO relations. The enriched group, for comparable MgO contents, has TiO 2 concentrations elevated above those of the depleted group. Most of the classification schemes proposed for this group of rocks can be viewed in the context of this more general classification. Over most of the Circum-Atlantic the enriched rocks dominate. However, despite the widespread occurrence of the enriched rocks, chemical compositions of this group are remarkably uniform. This is in contrast to the geographically restricted depleted rocks, which are quite heterogeneous with respect to incompatible-element content. Several factors have been described, any of which could be responsible for generating the chemical differences observed between the enriched and depleted MBS rocks. These include enriched subcontinental mantle, open- versus closed-system magmatism, depth and/or density-controlled partial melting, and fracture-zone or transform influence. All of these processes are consistent with our conceptual framework for MBS magmatism, the depleted rocks being derived from deeper, rift-flank source areas and the enriched rocks being derived from shallower, rift-axial source regions. It is highly possible that all of the factors discussed may have contributed to some extent to the generation of the observed chemical differences.

Flood basalts erupted at three distinct times during formation of the Hartford basin. Such tripartite activity is believed to be inherent to most continental rift-associated magmatism. In the Hartford basin, lavas erupted from three northeast-trending dikes. First, the Talcott basalt erupted from the eastern Higganum dike; 138 ka later the Holyoke basalt erupted from the central Buttress dike; and 345 ka later still, the Hampden basalt erupted from the western Bridgeport dike. The time between eruptions is estimated from Milankovitch-type cycles in lake sediments between the flows. It is proposed that magmas were generated by decompression melting of the adiabatically rising mantle beneath the basin. All three basalts rose through a common conduit in the lower lithosphere, but the upper part of this dike was repeatedly beheaded and shifted eastward by crustal extension that occurred along an eastward-dipping detachment surface that passed beneath the basin. Extension rates determined from dike displacements range from 4 to 9 cm/yr, being greatest where the basin is widest. The compositional changes and decreases in eruption temperature of successive basalts are thought to result from adiabatic partial melting of the source region as it continued to rise beneath the basin. The 18 °C decrease in eruption temperature (experimentally determined) between the Talcott and Holyoke basalts can be attributed to the Holyoke magma being formed by ~4% partial melting of the mantle. Similarly, the Hampden basalt would be formed by 2.2% partial melting to give a 9 °C decrease. If 1% melt must remain in the mantle, the fractions of partial melt that formed these two basalts are in the same proportions as the estimated erupted volumes of these basalts. Each of the basalts has a calculated magma density that is significantly less than average crustal densities. Consequently, magma would not have ponded at depth but would have risen rapidly toward the surface. Because the magmas were dry, the partial melting that began in the mantle continued all the way to the surface. During ascent, therefore, the decreasing crystal content of the magma caused the viscosity to decrease exponentially and the bulk density to decrease linearly. Moreover, the volume expansion on melting added to the buoyant force causing the magma to rise. These factors combined to give high magmatic fluxes, which account for the enormous volumes of single eruptive units. Because of the rapid ascent of the magmas, little intratelluric differentiation could occur. The major compositional differences between successive basalts must therefore reflect differences in the proportions of melt to solid in the magmas rising from the source or to phase changes resulting from decompression of the ascending source region. Resorption of phenocrysts and filter pressing during ascent modified magma compositions sufficiently to produce rocks that plot near multiple saturation boundaries. The magmas were also contaminated by low-melting fractions of crustal rocks during turbulent emplacement through wide dikes.

The early Mesozoic North Mountain basalts of Nova Scotia are quartz-normative tholeiites with compositions comparable to other Northern Appalachian Triassic-Jurassic tholeiitic lavas, dikes, and sills related to the initial stages of the opening of the Atlantic. The subaerial basalts form a northeast-southwest-trending belt, ~200 km long, with thickness decreasing northeastward from about 400 m to 275 m. Distinct but constant initial Sr isotopic ratios indicate that the magma was not contaminated by the crust during its ascent and emplacement, although other evidence suggests that it underwent fractional crystallization dominated by the separation of pyroxenes and plagioclase. Bands of mafic pegmatite and rhyolite in the thick flows are the results of “in situ” differentiation of the host basalts. The trace element traits, including the relative depletion of Nb and enrichment of K, Rb, Ba, light rare earth elements, and Th as well as the “enriched” isotopic characteristics of the lavas, suggest a subcontinental lithospheric mantle source for the North Mountain basalts.

Jurassic quartz-normative tholeiite dikes from Anticosti Island in the Gulf of St. Lawrence are generally similar to the low-TiO 2 , quartz-normative tholeiites (Weigand and Ragland, 1970) of the eastern North American province, but have TiO 2 contents intermediate between low- and high-TiO 2 types. The Anticosti dikes contain olivine microphenocryst pseudomorphs, sparse plagioclase phenocrysts, groundmass plagioclase laths, ophitic pigeonite and augite, ± interstitial Fe-Ti oxides, ± hornblende, ± biotite, and variable proportions (0%–20%) of interstitial devitrified glass. High K, light rare earth element, and Ba contents suggest some crustal contamination, but the low Zr/Y ratio of plausible model contaminants into normal mid-ocean ridge basalt implies that the assimilate was poor in Zr.

Petrographic, structural, and whole-rock geochemical data from eight diabase sheets and one associated dike in the central Newark basin indicate a common petrogenesis. The petrogenetic model also can be applied to the Palisades sill in the northern Newark basin, that together with some of the central Newark basin diabases make up a single “megasheet” extending for ~150 km. Regional chill-margin compositions are extremely constant and are high-titanium, quartz-normative tholeiites (HTQ) typical of the Mesozoic eastern North America (ENA) magmatic province. On the basis of their compositional uniformity and their close similarity to many other ENA-HTQ basalts, the HTQ chills are believed to best approximate the typical diabase parental magma composition. A few coarse-grained samples may have been derived from an ENA low-titanium, quartz-normative tholeiite (LTQ) basalt. Mass-balance models show that the early fractionation of the HTQ parent was dominated by clinopyroxene with lesser amounts of orthopyroxene and plagioclase; accumulations of these phases in parental magmas produced MgO-rich compositions. Plagioclase fractionation became dominant after ~20%–25% crystallization; Fe-Ti oxide and apatite fractionation increased dramatically at the latter stages of differentiation. No in situ olivine fractionation is required throughout the entire diabase differentiation sequence. Residual granophyric compositions were produced by ~70%–80% total crystallization and they exhibit no geochemical evidence for being the result of large-scale crustal anatexis. However, a local, selective contamination by Triassic sedimentary wall rocks or xenoliths resulted in alkali enrichment and calcium depletion in a few diabase samples and the Quarry dike. Typically, diabase sheets with MgO-rich rocks lack abundant granophyres; conversely, granophyre-rich sheets usually lack any significant amounts of MgO-rich rocks. Granophyre-rich zones are consistently found at higher structural levels than exposures with MgO-rich units. This distribution is consistent with a dynamic fractionation model where low-density residual liquids are displaced both laterally and up-dip along a sheet. The injection of multiple magma pulses is recognized in at least one sheet exposure, and this process, along with cumulate compaction, could have been the driving force for the migration of residual liquids.

The Upper Nyack (UN) section of the Palisades sill is comprised principally of pigeonite-augite and augite diabases with an interstitial (remnant) glassy mesostasis. Major- and trace-metal analyses of 110 diabases, 20 being analyzed for rare-earth elements (REE), provide data regarding the source of chemical variations within the UN section and suggest a unifying theory, the propsed C-T-D Model, for differentiation within the Palisades sheet. Preliminary work establishes that the chemostratigraphic horizons present in the type section at Englewood Cliffs (ENG)are broadly continuous from south to north and that important petrographic facies have undergone substantial mineralogic reorganization. The three most significant S → N transitions include: (1) holocrystalline subophitic chilled diabase (ENG) → hyalosiderite chilled diabase (Piermont) → glomeroporphyritic to intersertal chilled diabase (UN); (2) olivine zone (ENG) → (augite diabase) [UN]) → bronzite diabase (Nyack Beach State Park); and (3) fayalite granophyre and related “sandwich” horizon facies (ENG) → altered (porphyritic) augite ferrodiabase (UN). Bulk compositional and trace-metal plots establish that south to north petrologic tiers comprise a single differentiation series. Differentiation series data are consistent with three-stage model that involves early olivine fractionation followed by pyroxene- and plagioclase-dominated sequences. The cumulus horizons are modeled principally in terms of the accumulation of pyroxene-dominated fractionation products by varying proportions of olivine fractionated or later liquid. It is proposed that crystals collect in convection cells or like environments at deep levels within the sheet are partially remelted, resorted, and/or recrystallized during epizonal emplacement due to: magma decompression, shallow level flow differentiation, and magma–wall rock interactions. The Cumulus-Transport-Decompression (C-T-D) Model, based on these observations, is advanced to integrate multiple differentiation factors within a single interpretive framework for Palisades sill evolution.