Precambrian geology and magnetite deposits of the New Jersey highlands in Warren County, New Jersey
1980; United States Department of the Interior; Linguagem: Inglês
10.3133/ofr80789
ISSN2332-4899
Autores Tópico(s)Geology and Paleoclimatology Research
ResumoThe Precambrian rocks of the New Jersey Highlands in the Belvidere and Washington quadrangles consist of marble, amphibolite, quartzo-feldspathic gneiss, and syntectonic granite which developed during regional metamorphism under granulite-facies conditions.Field relationships, petrographic data, and bulk •/ chemical analyses support the view that the quartzpotassium feldspar gneiss, quartz-epidote gneiss, marble, and garnet-biotite gneiss are of raetasedimentary origin.The quartz-potassium feldspar gneiss and quartz-epidote gneiss were probably derived from arkosic sediments which contained intercalated limestones.The compositional layering in the metamorphic rocks appears to reflect original sedimentary bedding.Chemical analyses indicate that the amphibolite *• was derived from sedimentary rocks.Chemical analyses "< 9 support the conclusion that the premetamorphic parent of the oligoclase-quartz gneiss was probably an ash flow or keratophyric tuff.; Perthitic granite bodies do not show either intrusive contacts with the country rock or chilled margins.Small bodies of granite are commonly enclosed by gneiss and may have developed by partial fusion (in situ anatexis) of metasedimentary quartz-potassium feldspar gneiss.Syenite bodies are interpreted as feldspar-rich residues resulting from filter pressing in areas of tight isoclinal folding during anatexis.Sodium-rich granitic rocks apparently developed by anatexis of oligoclase-quartz gneiss.* The magnetite deposits in the study area are massive granular mixtures of magnetite and quartz with minor pyrite.The classical hypothesis for the origin of these deposits is that the unique"and distinctive pyroxenite associated with the gneiss-enclosed deposits is a contact-metasomatic skarn developed by fluids emanating from alaskite intrusions and that the ores are, therefore, of pneumatolytic origin ^.ar they are spatially and genetically associated with these pyroxene-rich skarns.However, the occurrence of magnetite concentrations in sharply bounded tabular-'f 9 shaped bodies, the absence of large igneous intrusions which could have been the source of residual ironbearing fluids, the absence of minerals which are characteristic of contact-metasomatic skarns, and the high Mg/Fe and relatively high Na content of the clinopyroxene of the pyroxenite are not consistent with the pneumatolytic hypothesis.These observations, however, are compatible with the hypothesis that the iron deposits are of metasedimentary origin.If the latter is correct, then the original mineralogical expression of the iron could have been siliceous siderite layers in a sedimentary sequence of impure carbonate rocks.During granulito-facies metamorphism, magnetite would-have Foxwedl by decarbonation of siderite, and the pyroxenite could have fo <<* by dedolomitization of a siliceous dolomitic marl.Granite pegmatite along the footwall and the hanging wall of the magnetite orebody at the Washington mine separates the magnetite ore from the adjoining quartz-potassium feldspar gneiss.This pegmatite was probably not, as earlier supposed, the ore bringer.Rather, it probably £o dL by interface melting of the quartz-potassium feldspar gneiss during granulite-facies metamorphism where the quartz-potassium feldspar gneiss v/as in'contact with the ore, because the effect of iron is to lower markedly the temperature of fusion of the low-melting compositions in the system quartz-orthoclase-albite.* * ." Pochuck gneiss which includes the mafic rocks.The Byrara and Losee gneisses v/ere considered to be metamorphosed igneous intrusions, v/hereas the Pochuck gneiss and Franklin limestone were considered to be
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