Paleozoic magmatism in the northeastern Fennoscandian Shield: Relations between tholeiitic and alkaline magmatism
Arzamastsev A. A.
Geological Institute of the Kola Science Centre RAS, Apatity, Russia
One of the most important manifestations of the processes of plume–lithosphere interaction are large intrusions of mantle-derived melts with a predominance of tholeiite magma into the upper crust. There are high volumes of basalt magma, forming large igneous provinces (LIPs) on the platforms, as well as in ancient cratonized areas. In these areas, the spatial and temporary connection of the continental plateau basalts with alkaline magmatism is observed. As an example, there are many large alkaline provinces, such as Maimecha-Kotui, whose formation is close to the age of formation of the effusions of Siberian traps; the South African one, formed during an unified cycle of magmatic activity together with effusions of the Etendeka plateau basalts; the West Indian one, connected with the Reunion Plume, caused the formation of the Deccan plateau basalts; and other alkaline provinces. Based on empirical data, some researchers have offered hypothesis to justify the genetic relationship between alkaline and tholeiite magmas formed during the processes of plume–lithosphere interaction (Comin-Chiaramonti et al., 1997, Wilson. Patterson, 2001, Frey et al., 2002, Kogarko et al., 2008).
The Kola alkaline province, formed within the period of 380–360 Ma in the northeastern part of the Fennoscandian Shield (Kramm et al., 1993) is characterized by features, which give logical reasons to consider them as the typical manifestations of plume magmatism. Provinces include large plutons, made up of agpaitic syenites, carbonatite intrusions, and numerous dikes of alkaline rocks, as well as remnants of subalkaline and alkaline extrusives. Manifestations of tholeiite magmatism in the region have not yet been identified definitively. On the basis of a geochronological study, the dolerite dikes and sills, regarded as possible Paleozoic analogs, were proved to be Proterozoic formations.
Spatial Distribution of Paleozoic Dolerites. Manifestations of tholeiite magmatism in the region are represented by dolerite dikes forming three swarms: Pechenga, the Barents Sea, and East Kola. The Pechenga swarm includes a series of large vertical dikes of submeridional trend (NE 10°), some of which have been traced to a distance of more than 80 km on the Sredny and Rybachy peninsulas. The largest dykes are 10–40 m thick; they consist of individual segments with the strike within 10°–15°. The Barents Sea swarm includes numerous dikes found on the coast between the Voron’ya River and Svyatoi Nos Cape. In Ivanovskaya Bay large vertical dolerite dikes with a thickness of 10–35 m have a northeastern strike. The East Kola swarm includes several dozen large submeridional dolerite dikes, stretching up to 100 km and crossing all the Precambrian structures between the Terskii coast and the Murmansk coast.
Geochronological Data. For the sample, picked up in a dike in the area of Liinakhamari, a Sm–Nd mineral isochron was obtained, corresponding to the age of 362 ± 40 Ma. Based on 40Ar/39Ar dating, the biotite fraction from another dyke of the Pechenga swarm yielded an age of 381 ± 6 Ma. These data are the same as those obtained with the 40Ar/39Ar method for dolerite dikes in the Varanger Peninsula (Guise, Roberts, 2002), as well as with our age datings for one of the dikes in the Barents Sea swarm. Dolerite dykes of the East Kola swarm yielded only two approximate Sm–Nd age datings. According to the regression equations, the age dating of a dike of the northeastern trend, located the area of the Serpovidny Ridge, the Western Keivy, is 322 ± 68 Ma; the age dating of a submeridional dike, exposed in the Pattilem River mouth, Eastern Keivy, is 295 ± 78 Ma.
Geochemical features. The normative mineral composition of all of the Paleozoic dolerites are characterized by the occurrence of hypersthene (6–21%). In the Pechenga swarm, there are olivine-normative (1–8% Ol) and quartz-normative (1–3% Q) dolerite varieties. In the Barents Sea and East Kola swarms, there are only rocks with a content of normative quartz in the range of 1–10%. Kola dolerites are represented by the low-titanium type (TiO2 = 0.8–1.8 wt %) in the Pechenga swarm and the high titanium type (TiO2 = 1.5–3.6% wt %) in the Barents Sea and East Kola swarms. Low Mg# values, varying within the range of 22–49, as well as Ni and Cr contents not exceeding 150 and 350 ppm, respectively, indicate that only some dolerite dikes of the Pechenga swarm can be considered as the primary melts of the tholeiite series. A positive correlation between MgO with Ni and Cr indicates that olivine and pyroxene control existed during crystallization of melts. The contents of high-field strength elements (HFSE) and large-ion lithophile elements, as well as their relationships, suggest that the Paleozoic dolerites belong to a group of continental plateau basalts. The dolerites are characterized by a negative niobium anomaly, mainly manifested in the low titanium dolerites of the Pechenga swarm. All the dolerites studied have average contents of Ba (180 ppm) and Sr (295 ppm), which, in combination with the isotopic characteristics, suggests that crustal components took part in their formation. A common feature of REE geochemistry of all dolerites is the absence of an Eu anomaly (Eu/Eu* = 0.89–1.04). The relatively low ratios of (La/Yb)N = 3.5 ± 1.1, and (Tb/Yb)N = 0.34 ± 0.08, as well as higher ratios of Lu/Hf = 0.15 ± 0.05, indicate the absence of garnet as a residual phase in the zone of melting of tholeiite melts. Taking into account the ratios of Ce/Y and Zr/Nb, calculated for different degrees of melting of garnet lherzolites and spinel lherzolites, one can suggest that the tholeiite magmas were formed under spinel facies conditions with a significant degree of melting of the mantle substrate. Isotopic characteristics of dolerites show wide εSr(T) variations, which is probably connected with the contamination of dolerites by crustal material. In the Sm–Nd system, not significantly subjected to contamination influence, the εNd(T) values are within the limits of +0.9...+5.4. These values are close to those for alkaline rocks of the igneous province (Kramm et al., 1993; Kramm, Kogarko, 1994). They testify that the tholeiite melts originated from sources with a significant share of a depleted mantle component (DM).
Discussion. The finding of the Devonian tholeiites, spread significantly in northeastern Fennoscandia, leads to the need to define their position in the general evolution model of Paleozoic magmatism, and, first of all, to define their relationships with alkaline melts. As was earlier shown on the basis of geochronological data (Arzamastsev et al., 2003), the first manifestations of plume magmatism, which occurred in the period 410–390 Ma were represented by subalkaline volcanics, preserved in the Lovozero Caldera and the Kontozero Caldera. The main phase of magmatism, during which the formation of the Khibiny and Lovozero massifs and carbonatite intrusions occurred, could be defined, according to (Kramm et al., 1993; Kramm, Kogarko, 1994), by an age interval of 376–363 Ma. In the final phase (369–360 Ma), swarms of alkaline dikes and diatremes were formed. The above mentioned data of isotopic dating of dolerites make it possible to refer them to the early stage (405–380 Ma) of Paleozoic endogenous activity in the region. Analysis of the spatial distribution of the Paleozoic dolerite dykes shows that they are located on the periphery of the area of predominant development of alkaline intrusions and swarms of dikes of alkaline lamprophyres. Beyond this area there is only the Ivanovskii complex of alkaline and volcanic rocks. As shown in (Arzamastsev, Mitrofanov, 2009), the initial melts of the Paleozoic alkaline intrusions corresponded to olivine melanephelinite, which was generated under the conditions of garnet lherzolite facies. According to the above mentioned materials, one can reach the conclusion that the tholeiites correspond to the initial phase of the Paleozoic process of plume-lithosphere interaction, during which the occurrence of tholeiite melts in the peripheral zone of the plume was the result of partial melting of the mantle substrate under the conditions of spinel lherzolite facies. During the subsequent development of the process of plume–lithosphere interaction, the deep mantle areas corresponding to the conditions of garnet lherzolite facies were in the zone of melting. The partial melting of these lherzolites under the conditions of mantle metasomatism led to the formation of melanephelinite melts, the basis of formation of the Paleozoic Kola alkaline province.
This work was supported by programs of the Department of Earth Sciences, Russian Academy of Sciences, no. 8 and no. 6.
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