The Paleozoic-Aged University Foidolite-Gabbro Pluton of the Northeastern Part of the Kuznetsk Alatau Ridge, Siberia: Geochemical Characterization, Geochronology, Petrography and Geophysical Indication of Potential High-Grade Nepheline Ore

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.


Introduction
Alkaline magmatism has been long considered to be typical of platformal settings, and where it occurred within folded regions, and then 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  [34,49,50] with a fragment of the geological map of the northern part of the Kuznetsk Alatau (KA) ridge and (b) after [51] with changes and additions by the authors. Red box locates Figure 2 and Figure 4.

Geology and Petrography of the University Pluton
The Kuznetsk Alatau (KA) terrane is a typical Early Caledonian (Salairian) tectonic terrane, with folding processes completing in the middle-upper Cambrian [52,53]. This area also includes inliers of Precambrian basement, a system of troughs and uplifts of the Salair orogen and superimposed Middle Paleozoic rift basins belonging to the Altai-Sayan Rift/LIP system [43,48].
The University pluton (N55°05′30″, E88°23′30″) is an inlier localized in a small erosion window (0.86 km 2 ) of Early Cambrian carbonate deposits, which are overlapped by Middle Cambrian volcanic rocks. The pluton is poorly exposed and partially overlain by large-blocks of deluvium deposits derived from erosion of the Voskresenskii gabbro-diorite-granodiorite intrusion (presumably Upper Cambrian) from the northern part of the area. The contacts of the pluton are almost everywhere tectonic with gabbroids and plagiogranites of the Voskresenskii intrusion. The pluton shape resembles a stock (2.5 × 0.2-0.6 km, with a total area of 0.53 km 2 ), significantly complicated by faults ( Figure 2). The Ust-Kundat Formation of the Lower Cambrian is composed of limestones with interlayers of clay shales, sandstones, tuffs and metavolcanics of andesite-basaltic composition. Volcanic units consisting of basalts of andesite-basalts, dacites and tuffs of the Middle Cambrian Berikul Formation are widespread in the region, and occur with an angular unconformity with underlying rocks.

Figure 2.
Simplified scheme of the geological structure and a section along the A-A1 line of the University pluton on a scale of 1:5000 according to [54,55] with the additions by the authors. The center of the image is at about N55°05′30″, E88°23′30″.

Materials and Methods
Ground magnetic prospecting at the University pluton site was carried out in 1983 with an M-27 optical-mechanical magnetometer with the measurement of the vertical component of the magnetic field vector in gammas [56]-units of the CGS system: 1 gamma = 10 −5 oersted (Oe). In modern research, magnetic field maps are plotted according to the values of magnetic induction, measured in nT. 1 nT = 10 −9 T are SI units of magnetic induction. 1 Oe is numerically equal to 10 −4 T. In this article, the magnetic survey results are presented in modern concepts based on the calculation that 1 gamma is numerically equal to 1 nT.
The concentrations of petrogenic and rare trace elements were measured by XRF at the Institute of Geology and Mineralogy V.S. Sobolev Siberian Branch of the Russian Academy of Sciences (Novosibirsk, Russia) on spectrometer ARL-9900XP and by ICP-MS at Tomsk State University (Tomsk, Russia) on spectrometer Agilent 7500.
X-ray fluorescence silicate analysis was performed from fused pellets: the analyzed sample was dried at 105 • C for 1.5 h, then annealed at 960 • C for 2.5 h and then mixed with flux (66.67% lithium tetra borate; 32.83% lithium meta borate and 0.5% lithium bromide) in a ratio of 1:9 (the total weight of the mixture was 5 g). The mixture was melted in platinum crucibles in a Lifumat-2.0-Ox induction furnace according to the standard method [57].
To perform mass spectral analysis with inductively coupled plasma, a 0.1 g sample was treated with 10 mL of HF acid with 4-h exposure in an open system at a temperature of 70 • C, after which 2 mL of HNO 3 concentrate was added. The samples were exposed to microwave action in a closed system at a power of 700 W with a gradual increase in temperature to 200 • C. After this, the sample was evaporated to dryness, treated twice with 6.2 M HCl, then evaporated again and treated with concentrated HNO 3 . Then the dry residue was transferred to a solution of 15% HNO 3 . Indium was used as an internal standard. Immediately prior to ICP-MS measurements, the sample was diluted by nitric acid to yield a concentration 3%. The dilution factor was 1000 [58].

The Magnetic Field of the University Pluton Site
As noted above, the University pluton is poorly exposed, and in order to properly delineate it, both geological mapping (boreholes, pits and ditches) and a ground magnetic survey were used [56]. The magnetic map (based on the ground survey; Figure 4) revealed both the University pluton and crosscutting N-trending dike rocks by elevated values which range from +15 to +150 nT. The zone is characterized by a complex structure, but it stands out quite well against the background values of −10 to −50 nT, created by non-magnetic host sedimentary rocks of the Ust-Kundat and Berikul formations. Values of the magnetic field up to +30 nT are observed over subalkaline gabbroids and gabbro-dolerites that are part of the University pluton. The N-trending dikes of ultrabasic and basic foidolites occur as linear anomalies with amplitudes of +30-+80 nT, and in some cases up to +80-+150 nT. A high-intensity +50-+200 nT anomaly located northwest of the University pluton is associated with gabbro-diorites and plagiogranites of the Voskresenskii intrusion ( Figure 4). The submeridional linear anomaly with amplitude of more than +200 nT on the western flank of the University pluton is of interest as deposits of nepheline ores, and we gave it the name, Boloto intrusion ( Figure 4). In its shape, it is similar to the anomalies from the N-S dikes cutting the University pluton, but typically with almost twice the amplitude.

Main Petrographic Varieties of the University Intrusion Site
The petrographic description is provided only for the main units of the University pluton for which geochemical and isotope-geochronological studies were carried out.
Subalkaline melanocratic gabbro (N55°05′48″, E88°23′47″) is widespread in the eastern and northeastern parts of the pluton (Figure 2). It is a gray to dark gray rock with a medium grain structure and taxite texture (Figures 3a and 5a). The thin section contains hypidiomorphic-grained, poikilitic and poikilophyte structures, of which titanoaugite grains (Fs14) had a higher degree of Values of the magnetic field up to +30 nT are observed over subalkaline gabbroids and gabbro-dolerites that are part of the University pluton. The N-trending dikes of ultrabasic and basic foidolites occur as linear anomalies with amplitudes of +30-+80 nT, and in some cases up to +80-+150 nT. A high-intensity +50-+200 nT anomaly located northwest of the University pluton is associated with gabbro-diorites and plagiogranites of the Voskresenskii intrusion ( Figure 4). The submeridional linear anomaly with amplitude of more than +200 nT on the western flank of the University pluton is of interest as deposits of nepheline ores, and we gave it the name, Boloto intrusion ( Figure 4). In its shape, it is similar to the anomalies from the N-S dikes cutting the University pluton, but typically with almost twice the amplitude.

Main Petrographic Varieties of the University Intrusion Site
The petrographic description is provided only for the main units of the University pluton for which geochemical and isotope-geochronological studies were carried out.
In the central-western area of the University pluton, the drill hole material from approximately 100 m depth shows the presence of urtite-porphyry dikes N-trending (up to 7 cm in size) in the core (N55 • 04 60 , E88 • 23 06 ; the location of the sample C-46 can be seen in Figure 4; Figure 6a). These are independent dikes that cut the ijolite-porphyries, and volcanic strata of the Berikul Formation and provide proof of the existence of a direct genetic relationship between ijolites and urtites, and the formation of urtites as a result of crystallization differentiation from an ijolite melt [45].
The intrusive nature of the relationship of gabbroids with host volcanic rocks can be observed from the same deluvium clastic material, in which acute-angled xenoliths of basaltic clastic rock are clearly recorded as xenoliths in gabbroids (N55 • 05 17 , E88 • 24 24 ) of the pluton (Figure 6b).
In the central-western area of the University pluton, the drill hole material from approximately 100 m depth shows the presence of urtite-porphyry dikes N-trending (up to 7 cm in size) in the core (N55°04′60″, E88°23′06″; the location of the sample С-46 can be seen in Figure 4; Figure 6a). These are independent dikes that cut the ijolite-porphyries, and volcanic strata of the Berikul Formation and provide proof of the existence of a direct genetic relationship between ijolites and urtites, and the formation of urtites as a result of crystallization differentiation from an ijolite melt [45].  In some dikes, nepheline syenites (N55 • 04 45 , E88 • 23 14 ) and microsyenites are noted, which cut thin veins of ijolite plagioclase (Figure 6c).

Major-and Trace-Element Compositions of Subalkaline and Alkaline rocks
The igneous rocks of the University pluton are characterized by low silicic acidity (SiO 2 = 41-49 wt %), wide variation in alkalinity (Na 2 O + K 2 O = 3-19 wt %; Na 2 O/K 2 O = 1.2-7.2 wt %), low titanium content (TiO 2 = 0.07-1.59 wt %) and high alumina content (Al 2 O 3 = 15-28 wt %), which corresponds to K-Na derivatives of the basic alkaline formation (Figure 7a; Table S1). The average compositions of the main varieties are plotted on the APF diagram (Figure 7b). The rocks of the pluton were divided into four groups according to their mineralogical composition: subalkaline gabbroids (ca. 490 Ma), foidolites, syenites (N-S dikes of ca. 390 Ma) and leucotheralites (there was no data on age, but we linked their formation with N-S dikes of ca. 390 Ma), which occupy an intermediate position in composition between gabbroids and foidolites.
Minerals 2020, 10, x FOR PEER REVIEW 10 of 23 The intrusive nature of the relationship of gabbroids with host volcanic rocks can be observed from the same deluvium clastic material, in which acute-angled xenoliths of basaltic clastic rock are clearly recorded as xenoliths in gabbroids (N55°05′17″, E88°24′24″) of the pluton (Figure 6b).

Major-and Trace-Element Compositions of Subalkaline and Alkaline rocks
The igneous rocks of the University pluton are characterized by low silicic acidity (SiO2 = 41-49 wt %), wide variation in alkalinity (Na2O + K2O = 3-19 wt %; Na2O/K2O = 1.2-7.2 wt %), low titanium content (TiO2 = 0.07-1.59 wt %) and high alumina content (Al2O3 = 15-28 wt %), which corresponds to K-Na derivatives of the basic alkaline formation (Figure 7a; Table S1). The average compositions of the main varieties are plotted on the APF diagram (Figure 7b). The rocks of the pluton were divided into four groups according to their mineralogical composition: subalkaline gabbroids (ca. 490 Ma), foidolites, syenites (N-S dikes of ca. 390 Ma) and leucotheralites (there was no data on age, but we linked their formation with N-S dikes of ca. 390 Ma), which occupy an intermediate position in composition between gabbroids and foidolites.  Table S1). The enrichment of light rare earth elements (LREE) relative to heavy rare earth elements (HREE) was characteristic of all samples (Figure 8).  Table S1). The enrichment of light rare earth elements (LREE) relative to heavy rare earth elements (HREE) was characteristic of all samples (Figure 8).

Nd-Sr Isotope Systematics
Subalkaline, alkaline rocks of the University pluton had common values of primary isotopes of neodymium 143 Nd/ 144 Nd(t) = 0.512223-0.512358 and ε Nd (T) ranging from +3.2 to +8.7 and, possibly, originated from the moderately depleted mantle type (PREMA) with crustal enrichment of the primary strontium isotope ratio 87 Sr/ 86 Sr(t) = 0.704834-0.706037 and ε Sr (T) from +12.93 to 28.31 ( Figure 9, Table 1). Enrichment in radiogenic 87 Sr was found for many Paleozoic-Mesozoic alkaline and carbonatite complexes in the northern part of the Kuznetsk Alatau, Southeastern Tuva, and the southeastern part of the Russian Altai [31]. Figure 8. Chondrite-normalized rare earth elements (REE) and primitive mantle normalized high-field-strength elements (HFSE) [69] igneous rocks of the University pluton. The oceanic island basalt (OIB) spectrum line is given in [69], island arc basalt (IAB) in [70]. Subalkaline gabbro ca. 490 Ma; leucotheralites (there is no data on age, but we link their formation with N-S dikes of ca. 390 Ma); alkaline syenites and foidolites ca. 390 Ma.
Subalkaline, alkaline rocks of the University pluton had common values of primary isotopes of neodymium 143 Nd/ 144 Nd(t) = 0.512223-0.512358 and εNd(T) ranging from +3.2 to +8.7 and, possibly, originated from the moderately depleted mantle type (PREMA) with crustal enrichment of the primary strontium isotope ratio 87 Sr/ 86 Sr(t) = 0.704834-0.706037 and εSr(T) from +12.93 to 28.31 ( Figure 9, Table 1). Enrichment in radiogenic 87 Sr was found for many Paleozoic-Mesozoic alkaline and carbonatite complexes in the northern part of the Kuznetsk Alatau, Southeastern Tuva, and the southeastern part of the Russian Altai [31].     Figure 10). These approximate ages suggest that the University pluton belongs ca. 500 Ma event, which is widespread in Mongolia (Figure 6 in [20,47]), while the crosscutting N-S dikes belong to the ca. 400 Ma Altay-Sayan Rift/LIP event [43,48].  Table 1.

Magnetic Anomalies of the University Pluton Site and N-S Trending Crosscutting Younger Swarm of Alkaline Dikes
The magnetic data clearly show northwest trending faults bounding the University intrusion. North trending linear positive magnetic anomalies mark dikes cutting the University pluton. In addition to direct geological observations in the form of open mines (ditches and pits) [73], ground magnetic data confirm two intense but narrow (1 km wide) dike belts (swarms) of N-S trends (see note in Figure 4 Foidolite dike).
Drilling of the University pluton (the maximum borehole depth was 160 m) 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 (see note in Figure 4 sample C-46 from drill-hole and Figure 6) [33,56]. We interpreted that the distinct linear anomaly in the western part of the pluton was caused by bodies of urtites, which we discovered for the first time (Boloto intrusion). To what event (ca. 490 Ma or ca. 390 Ma) the Boloto intrusion should be attributed to remained unclear.

Petrographic Synthesis of the University Pluton
According to geological and petrographic observations, three separate associations can be distinguished among igneous rocks in the University pluton area: (1) subalkaline gabbroids ca. 490 Ma; (2) dikes (N-trending) of basic foidolites, with subordinate amounts of feldspar ca. 390 Ma and (3) dikes (N-trending) of ultrabasic foidolites, characterized by the presence of only nepheline as a salic component ca. 390 Ma. There can be transitional boundaries between these types.
According to geological and petrographic observations in other alkaline-gabbroid plutons of the Kuznetsk Alatau (Figure 1b), in the Belogorskii [8] and Upper Petropavlovka [9] plutons, feldspar ijolites are independent intrusive phases and cut subalkaline gabbroids. At the Kiya-Shaltyrskii deposit [21], urtite bodies are also an independent and later phase. Separated from gabbroids, basic foidolites are represented as separate bodies within the Goryachegorskii pluton (both gradual transitions and intrusive relationships between plagioclase ijolites and feldspar urtites are observed here) [31].
Thus, the alkaline dikes of the N-S trending of the ijolite-porphyry with urtite xenoliths and the dikes of the urtite-porphyry belonging to the University pluton in their petrographic, petrochemical and geochemical composition are identical with the urtites of the Kiya-Shaltyrskii deposit ca. 400 Ma (deposits of rich nepheline ores) [21].

Magma Sources for the University Pluton and Crosscutting Dikes
The seven alkaline-gabbroid plutons of the northern part of the KA ridge region (Figure 9; as described above) and six of their varieties of igneous rocks (subalkaline gabbroids, ultramafic, basic foidolites, theralites, syenites and carbonatites) were analyzed for Sr and Nd isotopic compositions. The samples show slight variations in radiogenic ε Nd (T) (from +1.74 to +8.7), but with respect to the radiogenic ε Sr (T) (from +3.43 to +36.6), the values exhibited a wide range, and a shift was observed in the early intrusive phases of gabbroids to late alkaline dike rocks (Figure 9), from more mantle compositions toward a crustal component and this may indicate active crustal contamination of magma. One of the possible mechanisms of selective crustal contamination during the emplacement of magmas is the thermal mobilization of Sr-rich brines conserved in the Cambrian sedimentary strata of the KA [74,75]. Due to the strong contamination of all igneous rocks by crustal strontium, we assumed that the primary source of the magmatic melt was the plume component of the primitive mantle (PREMA) and these data are consistent with a number of studies [8,9,20,27,74,76]. V.V. Yarmolyuk and V.I. Kovalenko [77] in their work showed that the development of the Early Middle Paleozoic basic magmatism in the northwestern part of the CAOB occurred under the influence of a North-Asian superplume on the lithosphere, which was dominated by PREMA material.
This North-Asian superplume ca. 500 Ma affected the northern part of the KA; Gornaya Shoriya; Batenevskii range; Gorny Altai; Eastern and Southeastern Tuva; Eastern Sayan, Southern Pribaikal'e; Yenisei range; Priolkhon'e; Zabaikal'e, Prikhubsugul'e and Western Mongolia during the period from the Early Cambrian to the Middle Ordovician, producing large volumes of granites and various types of mantle magmatism [20,47,78]. The University pluton, with 494-491 Ma subalkaline gabbroids was part of this superplume event during the accretion of the KA terrane. The next stage was the introduction of an extensive swarm of N-S trending dikes of both alkaline and subalkaline composition 394-389 Ma. Recent studies [43,48] have recognized the large Altai-Sayan rift system (ASRS) and associated Altai-Sayan LIP, which also extend into the KA terrane. The crosscutting N-S striking dikes that cut the University pluton are likely related to the ASRS plume event. For a more detailed determination of the cause-and-effect relationships with certain regional magmatic events, we planned to conduct additional isotopic (Rb-Sr; U-Pb) studies of subalkaline and alkaline intrusions of the University pluton and crosscutting dikes.

Genetic Nexus of Alkaline-Basic Intrusions in the Kuznetsk Alatau Terrane
For clarity, we presented the age dates of seven complexly differentiated alkaline-basic intrusions of the KA terrane, which were discussed in our studies earlier (Figure 9 and Section 5.3) and this will help to divide the magmatic events of their formation into separate stages of formation (Table 2). Consequently, it will be possible to compare the magmatic events that took place at the University pluton with the events of other alkaline-basic intrusions KA, which will help to reveal the conditions of the formation of the University pluton. Accordingly, as a result of the data from isotope-geochronological studies (Sm-Nd, Rb-Sr, U-Pb and Ar-Ar), alkaline intrusions can be divided into three age groups, corresponding to the Cambrian and Early Ordovician (510-480 Ma), Early and Middle Devonian (410-390 Ma) and Late Permian (265 Ma) [8][9][10][11][12]18,21,27,28,[31][32][33]45,46,48,72].
Events in the Middle-Upper Cambrian, which formed the Upper Petropavlovka alkaline-basic pluton [9] and the subalkaline gabbro University pluton, we associated with the manifestation of the North Asian superplume [23] in the western territory of the CAOB. As a result, this intrusive magmatism of intraplate specificity was widely developed at the initial stage of the accretionary stage of the development of the terrane KA crust, when numerous plutons of alkaline and subalkaline rocks of the region, and picrate and picrodolerite magmatism in the north part Mongolia ca. 500 Ma [23,47,78].
In the KA terrane, the second stage of magmatism consists of Early and Middle Devonian alkaline-basic magmatism forming the Belogorskii [8], Kiya-Shaltyrskii, Dedovogorskii, Kurgusuyulskii plutons [21] and dikes of the N-S trending cutting the University pluton. We associate this event with the emergence of the Altai-Sayan rift system/LIP ca. 400 Ma [43,48].
Additionally, recent U-Pb dating of the foyaite phase of the Goryachegorskii pluton, which cuts only Devonian volcanic sediments, in contrast to other alkaline-basic massifs where they interact with both carbonate and volcanogenic-sedimentary strata, showed its belonging attachment to the late Paleozoic era (Upper Permian Epoch) [21,31]. Apparently, the Goryachegorskii pluton is a product of the bimodal basalt-comendite and basalt-pantellerite volcanic associations, which controlled the distribution of numerous plutons of alkaline granites and syenites in the rift system of Central Asia during the closure of the Paleo-Asian Ocean and collision of the Siberian and North China continents [23].

Conditions for the Formation of the University Pluton in the Kuznetsk Alatau Terrane
Subalkaline gabbro's have LREE patterns (Figure 8) between the oceanic island basalt (OIB) [69] and island arc basalt (IAB) [70] types [69], and HREE with the exception of (Tm ≈ 18.43; Yb ≈ 17.66; Lu ≈ 14.88 ppm), which in turn were more enriched than oceanic islands basalts (Tm ≈ 13.73; Yb ≈ 12.71; Lu ≈ 11.81 ppm). In terms of total REE ( REE ≈ 226.38 ppm), leucotheralites were more differentiated than subalkaline gabbro the foidolites (of the pluton) and show a similar pattern to OIB, but with a noticeable enrichment in HREE relative to OIB. Foidolites of the pluton were identical in their pattern with subalkaline gabbro, except for a few samples (K55/6; K55/2; U6), which had the lowest REE concentrations ( REE = 59.91; 95.54; 150.37 ppm; Table S1), less than the IAB [70] standard. The nepheline and alkaline syenites did not differ much from the subalkaline and alkaline units of the pluton, but in two samples (15B; SH-394/4; Figure 6c The behavior of REE and HFSE in the rocks of the University pluton suggests that the magma sources were heterogeneous. Despite the different degrees of melt differentiation (La/Yb(n) = 0.53-12.72), the geochemical parameters reflected the joint involvement of both OIB and IAB components in melt genesis. Mixing of these components is also noted for a number of other Early and Middle Paleozoic alkaline-basic intrusions of the region, the origin of which is associated with the processes of plume-lithospheric interaction and the inheritance of geochemical signatures of subduction in the products of mantle diapirism [8,9,18,27]. Negative (Nb, Ta, Zr and Hf) anomalies and the enrichment of mobile elements of fluids (Rb, Sr, Ba and U) in the rocks of the studied association indicate a probable interaction of plume melts with lithospheric mantle that was metasomatized during prior subduction events associated with the formation of accretion complexes.
The heterogeneity of the data is confirmed by a number of trace element discrimination diagrams (Th/Yb-Ta/Yb; Th/Ta-La/Yb; Nb/Y-Zr/Y; Tb/Ta(n)-Th/Ta(n); Zr/Nb-Nb/Th and Th/Nb-Ba/La; Figure 11). Subalkaline and alkaline plutonic rocks of the University pluton had within-plate character suggesting a plume source, but occurred in a region with an active continental margin (Figure 11a-d). These processes can be the interaction of plume material with more ancient accretion-collisional complexes on the active margin of the Paleo-Asian Ocean [8,10,20,22,27,31].
Taking into account the increased alumina content and low titanium content of subalkaline and alkaline rocks of the University pluton, and the observed ratios of highly charged elements, with a relative enrichment in rubidium, strontium and uranium, and with a noticeable depletion in niobium and tantalum, these geochemical features collectively indicate a complex geodynamic paleo environment of formation, juxtaposing convergence features of island arc, continental margin and with intraplate magmatism (Figure 11e,f). An example of such a complex combination of tectonic regimes initiating magmatic activity can be the modern active continental margin of the Californian type [79].
The CAOB records convergence and interactions between various types of orogenic components, including arc systems of the Japanese, Mariana and Alaskan-Aleutian types, and active continental margins of the Siberian Craton, which imply wide accretionary complexes and accreted arcs and terranes [25,26,50,89]. In the KA terrane, there can be spatially combined subduction and plume magmatism, recognized by geochemical components from heterogeneous sources [27].

Conclusions
Magnetic mapping of the poorly exposed University pluton of the Kuznetsk Alatau ridge, Siberia exhibited positive anomalies of intrusive units of the University pluton against the background of negatively magnetized host rocks (sedimentary rocks of the Ust-Kundat and Berikul formations). The high-intensity positive anomaly in the western part of the study area is probably associated with nepheline mineralization, which may be an analogue of the nearby economically important Kiya-Shaltyrskii nepheline ore deposit in addition, linear (N-trending) positive anomalies are associated with younger crosscutting N-S trending alkaline dikes.
Based on the Sm-Nd age dates presented in this manuscript, the University pluton was likely emplaced at about 490 Ma, which indicates its likely membership in a widespread intraplate event that extends into Mongolia [47]. The University pluton is cut by 390 Ma alkaline N-S trending dikes that likely belong to the regional ca. 400 Ma Altay-Sayan Rift System/LIP [43,48]. As is known, magmatism in the western part of the CAOB had a long history and some events, as in the KA terrane, are associated with the activity of mantle plumes. The broad signature (ε Nd (T) from +1.74 to +8.7 and ε Sr (T) from +3.43 to +36.6) of the isotopic composition of the alkaline-gabbroid association indicates the generation of initial magmas from a plume source of the moderately depleted PREMA mantle, whose derivatives experienced a selective crustal contamination likely through interaction with lithosphere metasomatized during final orogenic assembly of the CAOB.

Funding:
Geochemical study reported was funded by RFBR (project number 19-35-90030). Isotope-geochronological studies by the Sm-Nd method were carried out at the expense of the Russian Science Foundation (project number 18-17-00240) and the Rb-Sr method at the expense of a mega-grant in accordance with the Decree of the Government of the Russian Federation (Agreement number 14.Y26.31.0012). Clarification of the geological structure of the University pluton was carried out at the expense of the State Task of the Ministry of Science and Higher Education of the Russian Federation (project number 0721-2020-0041).