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• Some Fe-oxide grains crystallized from silicate melt were incorporated into downward percolated sulfide liquid. • Fe-oxide in massive ores crystallized from the sulfide liquid pool mingled with the downward percolated sulfide liquid. • Downward percolation of sulfide liquid is crucial in the formation of the net-textured and massive ores. The mineralogy and chemical composition of magnetite and chromite series (simplified as magnetite) can provide valuable insights into the formation processes of magmatic Ni-Cu-(PGE) sulfide deposits. Previous studies mainly focus on magnetite in disseminated ores and massive ores, but very little is known about the magnetite in net-textured ores. In this study, magnetite in net-textured ores (type 1 magnetite) and massive ores (type 2 magnetite) from the Zhdanov Ni-Cu-(PGE) deposit in the Pechenga ore field, were recognized and analyzed using scanning electron microscope (SEM), electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Both the type 1 and type 2 magnetite grains show zoned textures with Cr concentration decreasing from the core to the rim. Type 1 magnetite has higher Al and Mg concentrations than type 2 magnetite. These observations indicate that type 1 and type 2 magnetite crystallized from and record the solidification process of silicate melt and sulfide liquid, respectively. Type 1 magnetite within sulfides of net-textured ores has identical chemical composition to that within silicates, which suggests that the chemical composition of magnetite is not significantly affected by sub-solidus re-equilibrium with sulfide minerals. It also suggests that the type 1 magnetite within sulfides was incorporated into sulfide liquid later during the downward percolation of sulfide liquid through the magnetite-bearing olivine crystal mush. On the other hand, type 2 magnetite has comparable concentrations of W, Sc, Ga, Cr, Mo, and Mn to the type 1 magnetite. Such compositional similarity can be explained by the crystallization of type 2 magnetite from the sulfide liquid pool that have been mingled with the downward percolated sulfide liquid. Accordingly, the downward percolation of sulfide liquid plays an important role in the formation of net-textured ores and massive ores in the Zhdanov deposit. [ABSTRACT FROM AUTHOR]

Ediacaran and Permian‒Triassic stages of plagiogranite magmatism are distinguished for the Ust-Bel'sky and Algansky terranes of the West Koryak fold belt, NE Russia. The U‒Pb ages of Ediacaran plagiogranites are 556 ± 3 Ma (SIMS data) and 538 ± 7 Ma (LA–ICP–MS data), respectively, and 235 ± 2 Ma (SIMS data) for the Permian‒Triassic plagiogranites. The Ediacaran and Permian‒Triassic plagiogranites are low-K igneous rocks. Sr‒Nd isotopy and REE patterns of both plagiogranite suites allow suggesting their formation via partial melting of primary mantle material or fractional crystallization of basaltic magma. Ediacaran plagiogranites were formed in the oceanic arc setting contemporaneously with the volcanic-sedimentary complex of Otrozhnaya slice, Ust-Bel'sky terrane. Tuffaceous conglomerates in the lower part of this complex contain Ediacaran zircons with a peak age of 571 Ma. Formation of the Permian‒Triassic plagiogranites took place within the Ust-Bel'sky segment of the Koni‒Taigonos arc via partial melting of melanocratic ophiolite material in the accretionary structure of this arc or fractional crystallization of basaltic magma, melted from such substrate. [ABSTRACT FROM AUTHOR]

The lower continental crust, representing up to 50% of the continental mass, is largely inaccessible, making its composition difficult to constrain. Previous composite models based on geophysical evidence and geochemical data of granulite terrains and xenoliths have proposed varying results, from a mafic, relatively refractory lower crust to an intermediate-felsic, more enriched composition. Here, we investigated the mineralogy and geochemistry of predominantly mafic granulite xenoliths from eastern Australia and the Kola Peninsula, Russia, using an in situ analytical approach that minimises host magma contamination. The resulting xenolith compositions are variably and often strongly depleted in most highly incompatible trace elements, including the heat-producing elements. These xenoliths represent an extremely refractory component of the lower continental crust, likely formed after high degrees of partial melting or crystallisation from a depleted source. A lower crust composed solely of this refractory endmember would be too exhausted in heat-producing elements to satisfy heat-flow constraints. However, a volumetrically significant component of the lower crust is this mafic and refractory material, combined with undifferentiated material and a felsic or metapelitic portion. Using geophysical constraints on proportions of refractory (55%), undepleted (38%) and enriched (7%) components, a new estimate for average lower continental crust that satisfies heat flow limits was calculated, including for elements such as Be, B, Cs, W and Tl, where previous estimates relied on very few data. Finally, we show that because much of the lower continental crust is so refractory and depleted in incompatible elements, it is unlikely to be a reservoir that can balance radiogenic isotope (unradiogenic Pb) and trace element ratios (e.g. Rb/Cs, Nb/Ta) for which bulk silicate Earth departs from chondritic ratios. • Granulite xenoliths are investigated to constrain the composition of the deep crust. • Most of the lower crust is extremely depleted in highly incompatible trace elements. • An updated compositional estimate for the lower crust is presented. • Deep crust has limited potential as a "hidden" reservoir for mass balance issues. • Only ancient delaminated deep crust can contribute significantly to the bulk Earth. [ABSTRACT FROM AUTHOR]