Paper No. 8
Presentation Time: 10:55 AM
1.3 GA CONTINENTAL-MARGIN MAGMATIC ARC AND BACK ARC IN THE NEW JERSEY HIGHLANDS AND IMPLICATIONS FOR THE ORIGIN OF ZINC + IRON DEPOSITS
New SHRIMP U-Pb zircon data confirm that calc-alkaline and tholeiitic metaplutonic (tonalite-diorite-gabbro) and metavolcanic (basalt-andesite-dacite-rhyolite) rocks of the Losee Suite are correlative with magmatic arcs that formed along the convergent eastern Laurentian margin at ca. 1.3 Ga. Geochemical characteristics of the suite, and the dominance of felsic to intermediate rocks, are consistent with arc development on continental crust. Felsic rocks are low Al (<15 %) and high Al (>15 %) types. Low-Al rocks formed through plagioclase-controlled fractionation of a mafic source at a depth <35 km. High-Al rocks formed through partial melting of the subducting slab at >35 km leaving residual garnet and/or amphibole. Nd isotopic data (åNd = +0.87 to +4.6) support the derivation of the Losee Suite from a juvenile lower crustal source. Variants of the Losee Suite overlap temporally and display geochemical and isotopic similarities, suggesting they are parts of the same magmatic arc. Geochronologic data indicate that arc magmatism was coeval with sedimentation and bimodal volcanism that occurred in an extensional back-arc basin inboard of the Losee arc. This implies a northwest-dipping subduction zone beneath the Laurentian margin. Thin marble layers in the eastern Highlands and intercalated graphitic gneisses represent deeper-water marine deposits. Carbon isotope data and stromatolites in the Franklin Marble in the western Highlands reflect a shallow-marine environment on the continental margin side of the back arc, providing an important constraint on the depositional setting of Zn mineralization. SHRIMP U-Pb ages of felsic volcanic rocks intercalated with Franklin Marble constrain the timing of Zn mineralization in the carbonate protolith to 1.25 to 1.29 Ga. Hydrothermal seafloor vents in the back arc likely provided Fe+Mn in magnetite deposits hosted by supracrustal gneisses and marble. Felsic volcanic rocks have an A-type granite affinity, similar to VMS-related rhyolites from continental back-arcs. They represent crustal melts formed in a shallow magma chamber that supplied heat to the seafloor hydrothermal system. Closure of the arc and back arc, granulite-facies metamorphism and conversion of Zn+Fe+Mn to the present high-grade ore assemblages occurred during the Ottawan orogeny.