Yu. A. Balashov

Contrasting geochemistry of magmatic and secondary zircons

The intensity of the redistribution of trace elements in zircons significantly varies depending on the types of secondary processes affecting the magmatic rocks. The Neoarchean alkaline granites of the Keivy structure in the Kola Peninsula are employed as an illustrative example of differences in the variation dynamics of the proportions of certain elements (REE, Th, U, Hf, and others) with the transition from the magmatic to metamorphic crystallization of zircons during Proterozoic amphibolite-facies metamorphism over-printed onto the rocks. Changes are detected in the proportions of LREE and HREE, in the Ce4+/Ce3+ and Th/U ratios, and in other incompatible elements. The data obtained by geochemically comparing the redistribution of certain elements and their pairs in zircons during amphibolite-facies metamorphism and Phanerozoic hydrothermal alteration (literature data) are used to gain insight into the genesis of detrital Hadean zircons. Certain similarities and remarkable differences are detected in the effects of Hadean processes and Phanerozoic-Precambrian magmatism and secondary recrystallization on the behavior of chemical elements.

Cycles of alkaline magmatism

Geochronological data (∼1800 dates) have been analyzed by the probabilistic statistical analysis of samplings of different subalkaline and alkaline rocks through the whole of geological time. The distribution of five groups of subalkaline and alkaline rocks within the Late Archean-Phanerozoic are strictly controlled by mantle cycles, which were distinguished from data on the upper mantle magmatic rocks. Since high alkali rocks are plume related, their universal participation in each of the revealed mantle cycles emphasizes the importance of this magmatism in the evolution of the crustal-mantle system. The initial Sr and Nd isotope ratios are subdivided into two groups: with mantle and crustal signatures. Mantle isotope ratios are typically observed throughout the entire geological interval of dated rocks, while the role of crustal isotope signatures increases from the Archean to Phanerozoic, reflecting the increasing the role of fluids and crustal rocks in the magmatic processes during the generation of mantle magmas and their consolidation in the crust. Since alkaline magmatic sources are formed during mantle metasomatism, which enriched the magma generation zones in incompatible elements, the repeated occurrence of this process in separate mantle zones may have lead to the anomalous accumulation of these elements, which should be reflected in the alkaline magmas.

Endogenic Cycles and the Problem of Crustal Growth

The statistical analysis of geochronological data (more than 14200 dates) has been carried out by using the method of the probabilistic description of summary information on the crust and mantle and separately on both of the Earth’s upper shells considered together over the whole geological history. Various lines of evidence are presented for the necessity of using the whole set of geochronological methods to reveal any systematic pattern in the evolution of crust formation and to demonstrate the uselessness of utilizing selected data obtained by any one of the methods because of the limited analytical capability of each of them. These constraints, together with compositional variations of the dated rocks and the variable amount of the initial information, lead to uncertainty in estimation of megacyclicity as a sum of contrasting dynamics of endogenic events occurring in the crust and the mantle. It has been shown that mantle processes become more intense during periods of the synchronous activation of endogenic events in both shells; mantle activity sharply decreases in the epochs when endogenic processes in these shells are waning. This difference may serve as an objective criterion for estimating the maximum duration of cycles of mantle activity, which is distinct in the Early-Middle Archean, Late Archean-Proterozoic, and Phanerozoic. These conclusions are supported by examples of geochronological systematics for cratons of northeastern Labrador, western Greenland, and western Australia.

Time of formation and genesis of yttrium-zirconium mineralization in the Sakharjok massif, Kola Peninsula

The Kola geotectonic province in the northeastern Fennoscandian Shield accommodates a significant number of alkaline rock massifs differing in age. They are of mantle and mantle-crustal origin (alkali and nepheline syenites, carbonatites) and related to crustal sources (Neoarchean alkali granites). Among them, the Neoarchean Sakharjok nepheline syenite massif is related to the oldest intrusions of this kind bearing yttrium-zirconium mineralization. The crystallization of alkali syenite pertaining to the first intrusive phase of the intrusive Sakharjok massif is dated to 2645 ± 7 Ma, and this implies that this syenite postdated alkali granites (2.66–2.67 Ga). To date the yttrium-zirconium ore, we applied the local U-Pb method to zircon crystals occurring in the mineralized block hosted in nepheline syenite. The earliest fragments of zircon crystallized 1832 ± 7 Ma ago; the age of metamorphism is estimated at 1784 ± 13 Ma. These dates indicate the Paleoproterozoic age of the yttrium-zirconium mineralization, which was formed as a product of fluid reworking of the Neoarchean nepheline syenite of the Sakharjok massif.

Geochemistry and U–Pb age of zircons from the Vurechuaivench massif, Monchegorsk complex, Kola region

The paper presents data on the geochemical and geochronological characteristics of zircons from mafic rocks of part of the Monchegorsk layered complex represented by the Vurechuaivench massif. Ages of zircons (SHRIMP-II) from samples V-l-09 (anorthosite) and V-2-09 (gabbronorite) are dated back to 2508 ± 7 and 2504 ± 8 Ma, respectively. The chondrite-normalized REE patterns confirm the magmatic nature of zircons. The data unequivocally indicate that the U–Pb age of zircon from both gabbronorite and anorthosite corresponds to the age of melt crystallization in a magmatic chamber. The mantle origin of gabbroic rocks of the Vurechuaivench massif is confirmed by the REE patterns of three zircon generations with different crystallization sequences. The wide range of the Ce/Ce* ratio (9.96–105.24) established for zircons from gabbroic rocks of the Vurechuaivench massif indicates sharply oxidative conditions of zircon crystallization. For deepseated mantle rocks, these data can only be explained by significant contamination of the melt with country rock material.

Development of a heterogeneity in the lithosphere: Geochemical evidence

Systematization of information on multivalent trace elements in peridotite xenoliths made it possible to reveal differences in the distribution of these elements in the subcontinental and suboceanic segments of the lithosphere, which reflects the development of a geochemical heterogeneity in the lithosphere during the early (Hadean) stage of its evolution. The vast extent of trace-element differentiation in Hadean peridotite xenoliths is most probably explained by the appearance of appreciable masses of condensed water and, consequently, active mantle metasomatism in the hydrated lithosphere. The latter formed the upper depleted (oceanic) zone underlain by an “undifferentiated” zone enriched in trace elements. The removal of trace elements from both zones, a process that does not rule out the participation of earlier accretion in it, gave rise to a crust strongly enriched in these elements. The existence of long-lived extensive lithosphere heterogeneity calls for revision of the concept of multistage crustal growth with a general tendency toward an increase in its bulk volume.