Precambrian Rocks of the Lake Hopatcong Area, New Jersey
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DAVIS A. YOUNG Department of Geology, Washington Square College of Arts and Science, New University, New Yor{, New Yor\ 10003 Precambrian Rocks of the Lake Hopatcong Area, New Jersey ABSTRACT Precambrian rocks near Lake Hopatcong, New Jersey, form a part of the intensely deformed and metamorphosed Reading Prong. The Lake Hopatcong area is divisible into several northeast-trending fault blocks, each of which contains a mappable stratigraphic sequence of paragneisses and granitic or syenitic rocks. The paragneisses generally are well foliated and well layered. They consist chiefly of biotite- feldspar-quartz gneisses and quartz-oligoclase Figure 1. Geographic setting of the Reading Prong. leucogneisses that are interpreted as meta- Stippled area represents Precambrian crystalline rocks of morphosed potassium-rich sandstones and the Prong. Dashed area locates the Lake Hopatcong quartz keratophyres, respectively. A thin well- region of New Jersey. foliated unit of biotite-plagioclase gneiss is line rocks that extends from the Connecticut- thought to be a metamorphosed sill of gabbroic New York boundary to the vicinity of Reading, anorthosite. Pennsylvania (Fig. 1). To the northwest of the The granitic and syenitic rocks generally Prong are complexly refolded lower Paleozoic form thick, regionally concordant sheets. They sedimentary rocks (Drake, 1969; Epstein and are typically foliated and are composed chiefly Epstein, 1969), and to the southeast are chiefly of microcline microperthite and plagioclase (or Triassic sedimentary rocks of the Newark mesoperthite), quartz, and iron-rich hornblende Group, basalts, and diabase sheets and dikes. and clinopyroxene. These foliated granitic and Comparatively little is known of the geology syenitic rocks are viewed as syntectonic of the Prong, although several recent studies magmatic intrusives. One regionally discordant, have commented on the gross structure and unfoliated sheet of clinopyroxene quartz major rock types of various parts of the prov- syenite is probably a late tectonic magmatic ince. The work of Hague and others (1956), intrusive. Sims (1958), Buddington and Baker (1961), Mineral assemblages in Lake Hopatcong Drake (1969), and Young (1969) in New paragneisses may be assigned to the hornblende Jersey, of Buckwalter (1959, 1962) and Drake granulite subfacies of metamorphism. The (1969) in Pennsylvania, and of Hotz (1953), presence of Ca-bearing mesoperthite in biotite- Dodd (1965), Helenek (1965), Omeld (1967), feldspar-quartz gneiss indicates that meta- and Murray (1968) in New York has demon- morphic temperatures exceeded 700° C, and strated that the Prong consists of several the assemblage garnet-sillimanite-quartz with- parallel, northeast-trending fault blocks, each out cordierite indicates that load pressure was of which contains a stratigraphic sequence of greater than 2.5 kb. The rocks have thus prob- gneissic units that characteristically have been ably been buried to depths in excess of 10 km. deformed into northeast-plunging isoclinal to open folds. The gneissic sequences commonly INTRODUCTION include thick, homogeneous sheets of foliated The New Jersey Highlands form a part of the granitic and syenitic gneiss of probable in- Reading Prong, a belt of Precambrian crystal- trusive origin. The general geology of the Geological Society of America Bulletin, v. 82, p. 143-158, 7 figs., January 1971 143 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/1/143/3428234/i0016-7606-82-1-143.pdf by guest on 26 September 2021 144 D. A. YOUNG—PRECAMBRIAN ROCKS, LAKE HOPATCONG, NJ. northern part of the New Jersey Highlands well-foliated, well-lineated, and generally lay- recently has been summarized by Smith (1969). ered feldspar-quartz paragneisses and horn- The present paper presents new field, blende- or biotite-plagioclase gneisses that form chemical, and mineralogical data from the Lake open folds plunging about 22° N., 36° E. The Hopatcong, New Jersey, region and evaluates gneissic sequence overlies hornblende-bearing metamorphism and magmatic activity in the granitic gneiss and is partly transected by a Reading Prong from these data. broadly folded sheet of clinopyroxene quartz syenite. GEOLOGIC SUMMARY OF THE LAKE Although numerous stratigraphic units con- HOPATCONG AREA tinue along strike for several miles within each The Lake Hopatcong area of New Jersey block, it has not yet been possible to correlate (Fig. 1) is located about 40 mi west-northwest units from one block to another or to determine of New York City. It is situated just west of the top and bottom in any one block (Fig. 3). Green Pond syncline, a strip of Silurian and Devonian sedimentary rocks that is infolded PETROLOGY OF THE GNEISSES and infaulted into the Precambrian Highlands and divides them into two roughly equal parts. Biotite-Feldspar-Quartz Gneisses Major geological features are portrayed in The Lake Hopatcong area contains several Figure 2, and stratigraphic columns from lithologically heterogeneous units that consist each of the four major fault blocks in the area chiefly of well-layered, distinctly foliated are presented in Figure 3. biotite-feldspar-quartz gneiss interlayered with Block A contains a group of paragneisses lenses of amphibolite, hornblende-quartz- forming isoclinal folds that are overturned to feldspar gneiss, and biotite-quartz-plagioclase the southwest and plunge about 33° S., 64° E. gneiss. The units range from 100 to 1500 ft in Interlayered with these rocks are thick, homo- thickness, and individual layers typically are geneous, concordant sheets of hornblende- 0.5 to 4 in. thick. The dominant rock in these bearing granitic gneiss and hornblende-bearing units is xenoblastic or lepidoblastic gneiss that syenitic gneiss. contains, in rough order of decreasing abun- Block B contains two units of the eastern dance, quartz, microcline microperthite, plagi- limb of the Beaver Lake antiform that was oclase, and biotite. Common variations of this mapped in detail to the north by Buddington mineral assemblage and typical modal analyses and Baker (1961). The antiform, a large are listed in Table 1. isoclinal fold that plunges 25° N., 35° E. and is About 5 percent of well-oriented biotite slightly overturned toward the northwest, is flakes are randomly distributed throughout defined in the Franklin and Hamburg quad- biotite-feldspar-quartz gneiss, although locally rangles by a series of quartz-microcline and the biotite, as well as garnet, blocky sillimanite, quartz-plagioclase gneisses that is regionally and graphite, is concentrated into 1-mm-thick conformable with sheets of hornblende-bearing seams parallel to compositional layering and granitic gneiss and clinopyroxene-bearing sy- mineral foliation. Mesoperthite, with host enitic gneiss. In the Lake Hopatcong area, a oligoclase (AnirAn22), is quite common, but body of indistinctly foliated quartz-oligoclase discrete grains of oligoclase and highly perthitic leucogneiss that may exceed 2000 ft in thick- microcline coexist in most rocks. Minor ilmenite ness is exposed in the apparent core of the fold. is present locally, and zircon forms exceedingly Overlying the leucogneiss unit, and locally small (0.01-0.05 mm), nearly spherical grains. transecting foliation in it at a low angle, is a The pronounced layering, wide lithologic sheet of clinopyroxene-bearing syenitic gneiss. variations, high quartz content, abundance of Block C consists of clinopyroxene-bearing potassium feldspar, low mafic mineral content, amphibolite and biotite-feldspar-quartz gneiss and local occurrence of sillimanite and garnet units interlayered with thick sheets of clino- in the predominant gneisses of the units similar pyroxene-bearing granitic gneiss. The rocks to A-2 and A-3 (Table 1) generally are sug- are cast into a series of northeast-trending gestive of derivation from potassium-rich sand- isoclinal folds that alternately plunge to the stone parent rocks. Such biotite-feldspar-quartz northeast and southwest and are overturned gneisses comprise about 40 percent of the to the northwest. Compositional layering in all paragneisses of the Lake Hopatcong area. units is transected by axial-plane foliation near Similar rocks are very common elsewhere the noses of minor folds. throughout the Reading Prong. Field and Block D consists of more than 5000 ft of petrographic data suggest that microcline Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/1/143/3428234/i0016-7606-82-1-143.pdf by guest on 26 September 2021 4I'00' / hg /'hqo/ hg / bfq y^r LEGEND RWAGNEISSES DioptidB-scapolte-feldapar-quartz bfq Biotite-feldipar- quartz gntie Biotite-plagioctot* gntitt am Amphibolit* hqo | Hornblende- qjartz-otifoclas* 'qo "I Quartz-oligoclas* lBucogn«l«t boqp Bkitite-orthopyroxena-quartz-plagiodaM gn*it» i u Undlfftrcntiatad gn*iss«t GRANITIC ANDSYENITIC ROCKS Hornbltnd* cycnitic gntiss > / pg Clinowraxtnt granitic gnttu / P19SJS. i / / X P'Xl Oinopyroxcnc syenitic gneiss ~ — "-H pq> ainopyroxtnt quartz ty«nit« x-.-'' E3./ ^ x X^'T Gneisilc foliation \ - -/" X.X Lineation Overturned plunging anti form Overturned plunging synform Plunging •ynform Figure 2. Geologic map of the Lake Hopatcong area. Numbered crosses indicate locations of samples discussed in text. YOUNG, FIGURE 2 Geological Society of America Bulletin, v. 82, no. 1 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/1/143/3428234/i0016-7606-82-1-143.pdf by guest on 26 September 2021 — «— hg 800 am (basalt) dsfq 1200 (calc