Abstract
Geological, geochemical and ground magnetic techniques are used to characterize the University alkaline-gabbroid pluton and crosscutting N-S trending alkaline dikes, located northeast of the Kuznetsk Alatau ridge, Siberia. Trace element concentrations and isotopic compositions of the igneous units were determined by XRF, ICP-MS and isotope analysis. The Sm-Nd age of subalkaline (melanogabbro, leucogabbro 494–491 Ma) intrusive phases and crosscutting alkaline dikes (plagioclase ijolite, analcime syenite 392–389 Ma) suggests two stages of activity, likely representing separate events. The subalkaline and alkaline rocks are characterized by low silicic acidity (SiO2 = 41–49 wt %), wide variations in alkalinity (Na2O + K2O = 3–19 wt %; Na2O/K2O = 1.2–7.2 wt %), high alumina content (Al2O3 = 15–28 wt %) and low titanium content (TiO2 = 0.07–1.59 wt %). The new trace element data for subalkaline rocks (∑REE 69–280 ppm; La/Yb 3.7–10.2) of the University pluton and also the crosscutting younger (390 Ma) alkaline dikes (∑REE 10–1567 ppm; La/Yb 0.7–17.8 ppm) both reflect an intermediate position between oceanic island basalts (OIBs) and island arc basalts (IABs). The presence of a negative Nb–Ta anomaly and the relative enrichment in Rb, Ba, Sr, and U indicate a probable interaction of mantle plume material with the lithospheric mantle beneath previously formed accretion complexes of subduction zones. The isotopic signatures of strontium (εSr(T) +3.13–+28.31) and neodymium (εNd(T) +3.2–+8.7) demonstrate the evolution of parental magmas from a plume source from moderately depleted PREMA mantle, whose derivatives underwent selective crustal contamination.
Highlights
The crosscutting younger (390 Ma) alkaline dikes ( REE 10–1567 ppm; La/Yb 0.7–17.8 ppm) both reflect an intermediate position between oceanic island basalts (OIBs) and island arc basalts (IABs)
Alkaline magmatism has been long considered to be typical of platformal settings, and where it occurred within folded regions, and it was given secondary importance
Drilling of the University pluton did not yield the expected nepheline ore deposits, but urtite xenoliths were found in trenches to the west of the bodies of subalkaline gabbroids, and in drill-hole number C-46, an urtite-porphyry dike N-trending was found breaking through the Berikul formation [33,56]
Summary
Alkaline magmatism has been long considered to be typical of platformal settings, and where it occurred within folded regions, and it was given secondary importance. Various origins have been considered: (1) under conditions of a quiet tectonic regime (platform, postorogenic), differentiation of mafic magmas resulting in the formation of small volumes of residual alkaline melts [1,2,3]; (2) an Minerals 2020, 10, 1128; doi:10.3390/min10121128 www.mdpi.com/journal/minerals differentiation of mafic magmas resulting in the formation of small volumes of residual alkaline melts [1,2,3]; (2) an association with extensional processes and with rifting events [4,5] and (3) a link with plume activity [6,7]. 1128 CAOB, the large Altai-Sayan orogenic system [43] frames the southern part 2 of of the Siberian craton. This system includes several smaller terranes, such as the Kuznetsk Alatau (KA), Western and Eastern Sayan, Tuva-Sangilen and Tuva-Mongolian [44,45]. The KA is a Caledonian association with extensional processes and with rifting events to [4,5]
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