ABSTRACT Prolonged slow cooling (average 1–3 °C/m.y.) of Ottawan phase granulite-facies gneisses (peak temperature ≥850 °C ca. 1090–1080 Ma) through the argon closure temperatures ( T C ) of hornblende ca. 980–920 Ma and biotite ca. 890–820 Ma in the western Grenville Province and in an inlier in the central Appalachians is well established, but its tectonic setting has not been systematically investigated. Here, the case is made that this slow cooling occurred in the suprasolidus cores of large metamorphic core complexes that were exhumed during mid-Ottawan (ca. 1050 Ma) extensional orogenic collapse. The ductile midcrustal metamorphic cores of the large metamorphic core complexes are overlain across gently dipping extensional detachments by a brittle-ductile cover composed of upper orogenic crust, parts of which preserve evidence of relict pre-Ottawan fabrics and peak prograde Ottawan temperatures of <500 °C ( T C of Ar in hornblende), collectively implying thermal, structural, and rheological decoupling across the detachments. Slow average rates of cooling of the orogenic midcrust for >150 m.y. imply an anomalously hot upper mantle and mask short periods of more rapid cooling indicated by analyses of retrograde diffusional mineral zoning patterns. It is suggested that these slow average rates of cooling, coupled with slow average rates of exhumation of ≤0.1 km/m.y. modeled for one data set, were a result of decompression melting of rising asthenosphere and emplacement of voluminous mafic intrusions within or at the base of the crust, which reduced the buoyancy of the residual thinned lithosphere. This process is compatible with either delamination of subcontinental lithospheric mantle or slab rollback. The high-strain extensional detachments of the large metamorphic core complexes are sites of amphibolite-facies retrogression, suggesting a feedback between ingress of hydrous fluid, which was likely derived from beneath the detachment during crystallization of migmatite, and strain. Extensional juxtaposition of the hot midcrust ( T >850 °C) and cooler cover ( T <500 °C) across the detachments led to conductive heating of the base of the cover, locally raising its temperature above 500 °C, as recorded by amphibolite-facies metamorphism and young cooling ages. The slow cooling and exhumation of Grenvillian large metamorphic core complexes contrast with much faster rates in smaller metamorphic core complexes in other settings (e.g., North American Cordillera). The slow rates of these processes in large metamorphic core complexes are attributed to the prolonged high temperature and low viscosity of their metamorphic cores due to proximity of the asthenosphere, and to the intrusion of voluminous asthenospheric mafic magmas that both advected heat and reduced lithospheric buoyancy.

Small deposits of Neoproterozoic ironstone in the New Jersey Highlands are hosted by the Chestnut Hill Formation, a terrestrial sequence of siliciclastic rocks, sparsely preserved felsic and mafic volcanic and tuffaceous rocks, and thin limestone metamorphosed at greenschist-facies conditions. Sediments of the Chestnut Hill Formation were deposited in alluvial, fluvial, and lacustrine environments in a series of fault-bounded subbasins along the Iapetan eastern Laurentian margin. Ironstone occurs mainly in the upper part of the sequence in sandstones, quartzites, fine-grained tuffs, tuffaceous sediments, and carbonate-bearing beds. Ore is massive to banded and contains the assemblage hematite ± magnetite, which is locally associated with tourmaline and Fe silicates + sericite + calcite + chlorite ± quartz. Ironstone alternates with clastic bands, and sedimentary structures in ore bands and clastic bands are consistent with alternating chemical and clastic sedimentation deposited synchronously. Chestnut Hill rocks exhibit geochemical compositions that are dissimilar to typical sedimentary and volcanic rocks. They display evidence for two stages of post-diagenetic alteration. The first stage involved widespread potassium metasomatism, which produced increased values of K, Ba, and Rb that are not correlated with increased Fe or other hydrothermal elements. The metasomatizing fluid may have been basinal water heated during emplacement of Chestnut Hill volcanic rocks. The second stage produced alteration of Chestnut Hill rocks, and also Mesoproterozoic rocks along the footwall contact of the deposits, by hydrothermal fluids likely from a volcanogenic source. The ironstone deposits were formed by hydrothermal processes related to extension during formation of continental rift subbasins in the New Jersey Highlands. Iron was sourced from Fe-rich Mesoproterozoic rocks at depth, where it was leached by hydrothermal fluids that migrated upward along extensional faults. Iron and other metals were precipitated in permeable basin sediments and chemically favorable volcanic rocks, as well as precipitated directly as chemical sediment.

New U-Pb sensitive high-resolution ion microprobe (SHRIMP) ages from zircon and monazite document a 350 m.y. geologic evolution for the New Jersey Highlands. Two pulses of calc-alkaline magmatism that include the Wanaque tonalitic gneiss (1366 ± 9 Ma and 1363 ± 17 Ma) and Losee Suite tonalitic gneiss (1282 ± 7 Ma), dacitic gneiss (1254 ± 5 Ma), and dioritic gneiss (1248 ± 12 Ma) represent the southern continuation of eastern Laurentian margin arc activity. Supracrustal paragneisses, marble, and cogenetic metavolcanic rocks were deposited in a backarc basin inboard of the Losee arc. Ages of 1299 ± 8 Ma to 1240 ± 17 Ma for rhyolitic gneisses provide lower and upper limits, respectively, for the age of the supracrustal succession. Inherited cores in zircon grains from supracrustal rhyolitic gneiss and from Losee Suite rocks yield overlapping ages of 1.39–1.30 Ga and indicate proximity to an older arc source temporally equivalent to the Wanaque tonalitic gneiss. Location of the backarc inboard of the Losee arc implies a northwest-dipping subduction zone at this time beneath the eastern Laurentian margin. A-type granite magmatism of the Byram and Lake Hopatcong intrusive suites at 1188 ± 6 Ma to 1182 ± 11 Ma followed termination of arc and backarc magmatism and documents a change to decompression melting of delaminated lithospheric mantle by upwelling asthenospheric mantle. Waning stages of A-type granite magmatism include clinopyroxene granite (1027 ± 6 Ma) and postorogenic Mount Eve Granite (1019 ± 4 Ma). Overgrowths on zircon and monazite give ages of 1045–1024 Ma, fixing the timing of granulite-facies metamorphism in the New Jersey Highlands; other overgrowth ages of 996–989 Ma reflect the thermal effects of postorogenic felsic magmatism and hydrothermal activity associated with regional U–Th–rare earth element (REE) mineralization.

The Mount Eve granite suite is a postorogenic, A-type granitoid suite that consists of several small plutonic bodies occurring in the northwestern New Jersey–Hudson Highlands. Mount Eve granite suite rocks are equigranular, medium- to coarse-grained, quartz monzonite to granite, consisting of quartz, microperthite, and oligo-clase, with minor hornblende, biotite, and accessory zircon, apatite, titanite, magnetite, and ilmenomagnetite. Whole-rock analyses indicate that Mount Eve granite is meta-luminous to slightly peraluminous (ASI or aluminum saturation index, A/CNK or Al 2 O 3 /(CaO + Na 2 O + K 2 O) = 0.62 to 1.12) and has A-type compositional affinity defined by high K 2 O/Na 2 O (1.4 to 2.8), Ba/Sr (3 to 12), FeO t /(FeO t +MgO) (0.77 to 0.87), Ba (400 to 3000 ppm), Zr (200 to 1000 ppm), Y (30 to 130 ppm), Ta (2.5 to 6 ppm), total rare earth elements or REE (300 to 1000 ppm), low MgO (<1 wt%), Cr and Ni (both <5 ppm); and relatively low Sr (200 to 700 ppm). Variably negative Eu anomalies (Eu/Eu* = 0.13 to 0.72, where Eu/Eu* is the chondrite-normalized ratio of measured Eu divided by the hypothetical Eu concentration required to produce REE pattern with no Eu anomaly) and systematic decreases in Sr, Ba, Zr, Hf, Nb, and Ta, with constant total REE content and increasing Ce/Yb and SiO 2 contents, suggest crystallization of feldspars + zircon + titanite ± apatite. Possible modes of origin include dry melting of charnockitic gneisses or Fe-rich mafic to intermediate diorites within the Mesoproterozoic basement. Two possible tectonic mechanisms for generation of Mount Eve granite include (1) residual thermal input from a major lithospheric delamination event during or immediately after peak Ottawan orogenesis (1090–1030 Ma) or (2) broad orogenic relaxation between peak Ottawan and a late (1020–1000 Ma) high-grade, right-lateral transpressional event.

The New Jersey Highlands preserve a diverse assemblage of Mesoproterozoic rocks, whose geologic evolution correlates in part to other Grenville terranes in eastern North America, mainly north of the Blue Ridge. Voluminous undated—but postulated to be >1200 Ma—calc-alkaline metaplutonic and metavolcanic rocks of the Losee Metamorphic Suite were formed in a continental-margin magmatic arc, and metamorphosed supracrustal rocks >1174 Ma were formed along eastern Laurentia in a marginal back-arc basin. They include terrigenous to shallow marine metasand-stones, a shelf sequence of sand-dominated metaclastic rocks, stromatolitic marble, locally pillowed mafic volcanics, and deep-water volcanogenic metagraywacke. Sub-duction and calc-alkaline magmatism had ceased by 1176 Ma, but timing of accretion of the arc complex to Laurentia is uncertain. Between 1160 and 1130 Ma, supracrustal rocks were intruded by thin sheets and dikes of meta-anorthosite and megacrystic amphibolites likely coeval with anorthosite, mangerite, charnockite, granite (AMCG) magmatism in the Adirondack Highlands. Meta-anorthosite and megacrystic amphibolites in New Jersey were intruded by Atype granite of the 1110 ± 25 Ma Byram and 1095 ± 9 Ma Lake Hopatcong Intrusive Suites. Emplacement of these suites was followed by upper amphibolite- to granulite-facies metamorphism at 1090–1030 Ma, during the Ottawan orogeny. Sheets of late synorogenic microperthite alaskite were emplaced during the latter part of Ottawan orogenesis. Postorogenic 1020 ± 4 Ma Mount Eve Granite and undeformed 1029 ± 1 Ma trondhjemite date the close of the Ottawan orogeny in the New Jersey Highlands. Undeformed discordant pegmatites and small granite bodies were emplaced between 1004 and 989 Ma.

Abstract T his P ortion of the field trip features visits to the worldfamous Sterling Hill and Franklin mines, in northwestern New Jersey, and their respective mining museums. Although both mines are no longer in operation, important geologic features can still be observed. At the Sterling Hill mine, we will have the opportunity to see zinc ore in place, both in an underground drift and in the Passaic open pit; at Franklin the pit is filled with water and only disseminated mineralization is still visible. The Furnace magnetite bed will also be seen adjacent to the Franklin open pit, as well as a large postore minette dike. If time permits we will visit a surface exposure of the Zero Fault in Franklin, which is an important regional structure that cuts the Sterling Hill orebody.

Abstract The New Jersey Highlands are underlain principally by Mesoproterozoic rocks that were metamorphosed at upper amphibolite to granulite facies between 1080 and1030 Ma during the Ottowan Orogeny. The oldest rocks are inferred to be metamorphosed ca. 1300 Ma dacite, tonalite, and trondhjemite and associated charnockitic plutonic and metavolcanic rocks of the Losee Metamorphic Suite. These originated in a continental margin magmatic arc setting dominated by calc-alkaline and tholeiitic magmatism. Rocks of the Losee record a compressional event in the north-central Appalachians that was likely coeval with the 1300 to 1200 Ma Elzevirian orogeny in the northern Appalachians and Grenville Province. The Losee is unconformably overlain by a sequence of supracrustal rocks older than 1100 Ma that were deposited in an extensional tectonic setting. These include quartzofeldspathic gneisses, metaquartzite, calc-silicate rocks, marble, and amphibolite. The earliest recognized part of this succession consists of metamorphosed arkosic and quartzose sandstones that were deposited in a continental rift basin. These are spatially associated with a bimodal assemblage of metarhyolite and metabasalt. Overlying sanddominated clastic rocks (quartzofeldspathic and siliceous calc-silicate gneisses) and marble are interpreted as a platformal, shallow-marine succession based on lithologic associations, stromatolite occurrences in marble, and stable isotope data. This succession grades into overlying supracrustal rocks that reflect an arc-derived sediment source, and that contain a higher proportion of mafic volcanic rocks, including locally pillowed metabasalts of MORB affinity. Losee and supracrustal rocks were intruded by voluminous, widespread synorogenic A-type granitoids of the Vernon Supersuite that consist of the 1116 ± 41 Ma hastingsite and biotite-bearing Byram Intrusive Suite and the 1095 ± 9 Ma hedenbergite-bearing Lake Hopatcong Intrusive Suite. The postorogenic 1020 ± 4 Ma Mount Eve Granite and associated 1004 ± 3 Ma pegmatites, which straddle the boundary between New Jersey and New York, provide a minimum age for the Ottawan orogeny in the Highlands. Discordant postorogenic pegmatites were emplaced at 998 to 989 Ma and 965 Ma. Mesoproterozoic rocks of the Highlands host economic deposits of iron, zinc, and graphite. Deposits of high Ti and low Ti magnetite ± hematite are widely distributed throughout the region and occur within virtually every Mesoproterozoic rock type. Small U and REE deposits are associated with a few of the Fe deposits. Neoproterozoic greenschist-facies felsic volcanic and terrestrial sedimentary rift basin rocks host small hematite deposits that have characteristics of iron-formation. Two large marble-hosted zinc (zincite, willemite, franklinite) deposits occur in the northwestern Highlands at Franklin and Sterling Hill, and marble-hosted sphalerite occurs at the Raub mine in the southwest Highlands. Zinc deposits at the Franklin and Raub mines directly overlie Mn-bearing Fe oxide deposits, implying a genetic relationship. Graphite deposits are confined to the eastern Highlands where they are hosted by sulfidic biotite-quartz-feldspar gneiss and metaquartzite. Available geologic evidence suggests that metals in the Zn and most of the Mesoproterozoic Fe deposits were introduced prior to ca. 1080 to 1030 Ma granulite facies metamorphism.

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

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