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Index map. (1) Paleozoic alkaline intrusions; (2) Neoproterozoic sedimentary rocks; (3) Paleoproterozoic granitic intrusions ; (4) Paleoproterozoic volcanic–sedimentary rocks; (5) Neoarchean intrusions of alkali granite; (6) supracrustal volcanic– sedimentary rocks of Keivy structural unit; (7) Neo-and Mesoarchean intrusive and volcanic–sedimentary rocks; (8) location of Sakharjok massif. Im-V, Imandra–Varzuga paleorift; Ke, Keivy province.
Source publication
Zircon is an accessory mineral in alkali and nepheline syenites of the Neoarchean Sakharjok intrusion. Zircon in association with britholite and pyrochlore forms orebodies in nepheline syenite of this massif. Zircon crystals reveal an inhomogeneous zonal, occasionally mosaic structure comprising fragments and zones related to magmatic, hydrothermal...
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Citations
... Chondrite values were obtained from[30]. Values of magmatic zircon and metamorphic zircon obtained from[34].(a): sample PR; (b): sample S2; (c): sample J6; (d): sample J3; (e): sample RSN10; (f): sample RSR2, (g): sample PM-3 ...
Zircon grains are reasonably ubiquitous in river banks and beach placers as detrital minerals, including along the ~1700 km long Indian east coast, from Odisha state to the state of Kerala. Zircons from beach placers and river banks located along the eastern part of Odisha, India, were studied using LA-ICP-MS in order to delineate their geochemical characteristics. Hf (mean = 11270 ppm) and Y (mean = 1064 ppm) were the two most abundant trace elements found within zircon grains as compared to other trace elements. The abundance of uranium was observed to be 2–4 times larger than that of thorium. Zircon overgrowths formed in equilibrium with a partial melt and were similar to magmatic zircon in terms of the high Y, Hf and P content, steep heavy-enriched REE pattern, positive Ce anomaly and negative Eu anomaly. The average low Th/U ratio of the studied zircon grains distinguished them from the magmatic ones. The REE present in zircon grains was restricted to high-grade metamorphic events. The result of the present study would be useful for delineating the source region and the efficacy of resource potential and indigenous export.
... Other mechanisms such as initial formation of plagioclase-rich melt and its progressive extraction from the arc-root with plagioclase-free partial melt residue or deep level cumulates left behind (Ducea, 2001;Tang et al., 2019) could have also contributed to the increase in Eu anomalies over time. Interestingly, zircons also exhibit large positive Ce-anomalies (Ce/Ce*); the typical Ce-anomaly of igneous zircon is about ∼105 (Vetrin and Skublov, 2015;Rubatto, 2002) and the anomalies calculated for studied zircons are up to 200. Similar to Eu, the behavior of Ce also depends on the redox conditions during zircon crystallization; however, unlike Eu-anomalies, Ceanomalies increase with the increase in oxygen activity (e.g. ...
... Similar to Eu, the behavior of Ce also depends on the redox conditions during zircon crystallization; however, unlike Eu-anomalies, Ceanomalies increase with the increase in oxygen activity (e.g. Vetrin and Skublov, 2015). The increase in oxidation state of arc magmas is usually attributed to either (1) addition of volatile-rich oxidizing components derived from subducted sediments and oceanic crust to the sub-arc mantle (e.g. ...
... Low Th/U values correlate with a reduction in Ce/Ce* (4-50) values for overgrowths. Scattered Ce/Ce* values in detrital cores are typical of igneous zircon from a wide variety of source plutons, as typical Ce/Ce* for igneous rocks range from 1 to >300 (Vetrin and Skublov, 2016). The reduction in Ce/Ce* values from the cores to the overgrowths, which correlates to the oxidation state of magmatic or metamorphic conditions, is consistent with other metamorphic zircons, suggesting overgrowth development occurred under relatively reduced metamorphic conditions compared to the initial igneous crystallization of detrital cores. ...
Recent geologic mapping in the northern Plomosa Mountains (west-central Arizona, USA) documented a newly recognized exposure of the Laramide Orocopia Schist subduction complex in the footwall of the Miocene Plomosa detachment fault. Zircon U-Pb ages from this schist strongly resemble those from other Orocopia Schist exposures, and record dominantly Mesozoic age peaks at 85-75 Ma, ca. 100 Ma, ca. 120 Ma, ca. 165 Ma, and 245-210 Ma, and minor Proterozoic peaks at 1.2 Ga, 1.4 Ga, and 1.7 Ga. Thin ( < 10 μm) zircon overgrowths on detrital cores, resolved using laser ablation-inductively coupled plasma-mass spectrometry depth profiling of unpolished grains, range from 75 to 47 Ma and have low Th/U ( < 0.1) compared to the cores. These zircon grains record the transition from deposition in the paleotrench to subduction-related metamorphism between 77 and 68 Ma for individual samples, and an average depositional age of ca. 73 Ma. This well-defined depositional age of the Orocopia Schist protolith is older than previously thought but similar to reported ages for the outboard Pelona Schist. The timing of sediment subduction appears to be contemporaneous with extinction of magmatism in the Mojave region, suggesting the two events are linked. Zircon xenocrysts within Miocene plutonic rocks yield age spectra similar to those for the Orocopia Schist, indicating assimilation of the schist during core complex magmatism. Together, these data show the Orocopia Schist of the northern Plomosa Mountains was deposited off the California coast, subducted at ca. 73 Ma, and metamorphosed during Laramide subduction underplating and Paleogene exhumation from 73 to 47 Ma, before assimilation into Miocene plutons and final exhumation during core complex extension.
