Granulite xenoliths entrained within Quaternary rift basalts in northern Tanzania document the composition, equilibration conditions, age and petrogenesis of the present-day lower crust beneath the eastern margin of the Tanzanian Craton (Labait) and the adjacent Mozambique Belt (Lashaine and Naibor Soito). Mafic to intermediate Archean lithologies dominate throughout the suites ($2•66 Ga based on U^Pb zircon ages), demonstrating that deep-seated Archean lithosphere extends far to the east of the margin of the Tanzanian Craton within the Mozambique Belt. There is no evidence for significant additions to the crust via magmatic underplating since that time. Many of the lower crustal xenoliths share compositional similarities with lavas from the greenstone belts of theTanzanian Craton, suggesting that they crystallized from similar magmas. Extreme depletions of highly incompatible elements (e.g. Cs, Rb, Th and U) in the granulites, relative to the lavas, coupled with unradiogenic 143 Nd/ 144 Nd (0•5114^0•5122) and 87 Sr/ 86 Sr (0•7040^0•7051), suggest that these depletions occurred coincident with or shortly after the rocks crystallized, possibly through partial melting associated with metamorphism.These samples may thus represent high-grade lower crustal complements to the greenstone belt lavas. Compared with the craton-margin samples, granulite xenoliths from within the Mozambique Belt record very high peak equilibration pressures at moderate temperatures (4 1•2 to 41•7 GPa, 7509 608C, based on pseudosections), documenting their equilibration deep within thickened continental crust during the East African Orogeny, c. 560 Myr ago. These samples therefore offer an unusual window into the deepest reaches of the crust in a continental orogen. Despite the fact that the Mozambique Belt experienced significant crustal thickening followed by post-orogenic collapse, there is no evidence for loss of the deep lithosphere associated with these processes.
Mesoproterozoic marble in the New Jersey Highlands hosts small magnetite deposits that occur in discrete groups along linear trends and are concentrated mainly in the western part of the region. Magnetite also forms a layer structurally beneath the Zn-Fe-Mn orebody at the Franklin mine. Marble host rocks are interlayered at Fe and Zn-Fe-Mn deposits with bimodal metavolcanic rocks (amphibolite and rhyolitic gneiss) that were deposited synchronously in a back-arc basin developed on the eastern margin of Laurentia between 1.3 and 1.25 Ga. Magnetite in the deposits ranges from massive to disseminated and is characterized by low Ti, variable Mn, and locally high S. Concentrations of As, V, Sb, and Zn are enriched in magnetite compared to marble host rocks and associated metavolcanic rocks. Carbon and oxygen isotope ratios of the marbles are consistent with water-rock interaction between the marble protolith and a mixture of igneous fluids and seawater. Regional geology, petrology, and isotope data indicate that Fe was introduced into the carbonate protolith as low-temperature Fe oxides and hydroxides via hydrothermal fluids discharged along basinal fracture zones on the sea floor. Related marble-hosted Zn-Fe-Mn deposits at Franklin and Sterling Hill contain the assemblage willemite + zincite + franklinite. Stable isotope compositions of the Zn deposits are consistent with alteration of host rocks by water-rich fluids, preserving protolith carbon isotope ratios. Stable isotope modeling suggests that Zn silicate + oxides or hydroxides represent an appropriate protolith for the high-grade Zn ores. Alteration of metavolcanic rocks at the Fe deposits, combined with the isotope data and other petrologic and geochemical evidence, document the presence of an extensive hydrothermal system of Grenville age centered in the western Highlands. Metals precipitating from this hydrothermal system were responsible for Fe in the marble-hosted magnetite deposits and Zn in the deposits at Franklin and Sterling Hill from the same or a related hydrothermal system. Underplating of the back arc by mafic magma provided the heat to melt the lower crust and drive the hydrothermal system, resulting in carbonate-hosted Fe and Zn ore deposits and associated bimodal volcanism.
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