The Overmaraat-Gol Alkaline Pluton in Northern Mongolia: U–Pb Age and Preliminary Implications for Magma Sources and Tectonic Setting

A new Wenlockian zircon U–Pb age (~426 Ma) of the Overmaraat-Gol nepheline syenite (foyaite, juvite) pluton in the SW Lake Hovsgol area (Northern Mongolia) prompts a long history of alkaline magmatism in the western Central Asian Orogenic Belt, exceeding the duration of the Devonian and Permian–Triassic events. The LILE and HFSE patterns of pluton samples analyzed by X-ray fluorescence (XRF) and inductively coupled plasma (ICP-MS) methods indicate intrusion in a complex tectonic setting during interaction of a mantle plume with accretionary-collisional complexes that previously formed on the active continental margin. As a result, the parent magma had a heterogeneous source with mixed mantle (PREMA and EM) and crustal components. This source composition is consistent with Nd–Sr isotope ratios of the Overmaraat-Gol alkaline rocks, from −0.1 to −1.2 εNd(t) and from ~0.706 to 0.707 87Sr/86Sr(t).


Introduction
Main events of continental and marine alkaline magmatism are often coeval with the activity pulses of mantle plumes [1,2]. This synchronicity is evident in within-plate settings, but is often obscured in orogenic belts where supracrustal contamination masks the true magma sources [3][4][5][6][7][8][9][10][11]. The plume-lithosphere interaction can produce hybrid magmas with high Al 2 O 3 contents and induce the formation of nepheline-enriched plutonic rocks. Constraints on ages and trace-element compositions of alkaline intrusions in fold belts have important implications for their origin.

Geology and Petrography of the Overmaraat-Gol Intrusion
The studied intrusions are located in the SW Hovsgol area, within a fault-bounded block of the Precambrian Tuva-Mongolia terrane in the middle of the Central Asian Orogenic Belt [22]. The terrane has a Neoproterozoic basement of marbles and schists derived from Vendian-Cambrian continental-margin metacarbonate and clastic sediments. The Overmaraat-Gol pluton and related alkaline plutons in the Beltesin-Gol-Udgigin-Gol interfluve follow an N-S backbone fault [13,21]. The intrusions crosscut basement marbles and Early Paleozoic gabbro-diorites and granitoids ( Figure 1b).

Geology and Petrography of the Overmaraat-Gol Intrusion
The studied intrusions are located in the SW Hovsgol area, within a fault-bounded block of the Precambrian Tuva-Mongolia terrane in the middle of the Central Asian Orogenic Belt [22]. The terrane has a Neoproterozoic basement of marbles and schists derived from Vendian-Cambrian continental-margin metacarbonate and clastic sediments. The Overmaraat-Gol pluton and related alkaline plutons in the Beltesin-Gol-Udgigin-Gol interfluve follow an N-S backbone fault [13,21]. The intrusions crosscut basement marbles and Early Paleozoic gabbro-diorites and granitoids (Figure 1b). The Overmaraat-Gol pluton, exposed over 30 km 2 on the erosion surface, has an isometric shape in the map view and consists of several blocks (Figure 1c). The rocks comprise main petrographic varieties of coarse-grained K-Na nepheline syenite (foyaite and juvite for brevity), transient from one to another, with variable amounts of nepheline, feldspars (microcline and albite), and femic minerals (aegirine-salite-hedenbergite, aegirine-augite, sodic and sodic-calcic amphiboles-arfvedsonite and katophorite-hastingsite) [13]. Older subalkaline gabbro and theralites are preserved only as small xenolith-like bodies. Secondary alteration of igneous rocks has produced lepidomelane, muscovite, and cancrinite. Juvites and foyaites are crosscut by Devonian syenite and leucogranite dikes [21], which does not contradict the obtained U-Pb age of alkaline intrusions.

Analytical Methods
Major elements in rocks were analyzed by X-ray fluorescence (XRF) on a Thermo Scientific ARL 9900XP spectrometer at the V.S. Sobolev Institute of Geology and Mineralogy (Novosibirsk). Trace-element and REE abundances were measured by mass spectrometry with inductively coupled plasma (ICP-MS) on an Agilent 7500cx spectrometer under standard operation conditions, at the Analytical Center of Geochemistry of Natural Systems at the Tomsk National Research State University (Tomsk, Russia).
Zircon U-Pb ages were determined on a SHRIMP-II ion microprobe at the Center of Isotope Studies of the A.P. Karpinsky Russian Geological Research Institute (St. Petersburg, Russia), following the standard procedure [23]. Cathodoluminescence (CL) images were obtained on an ABT55 scanning electron microscope in the conventional operation mode. Data were processed using SQUID software (Version 1.00) [24]. U/Pb ratios were normalized to those in the TEMORA standard zircon [25]. The errors were within ±1σ in measured isotope ratios and ages, but ±2σ in calculated concordant ages and intersections with concordia. Concordia diagrams were plotted in ISOPLOT/Ex (Version 2.10) [26].
Sm-Nd and Rb-Sr isotope analyses were carried out by the standard technique [27] on the Finnigan MAT-262 and MI 1201-T mass spectrometers at the Geological Institute of the Kola Science Center (Apatity, Russia). The ε Nd and ε Sr values, and primary Nd and Sr isotope ratios were used for reference in calculations of U-Pb zircon ages (see text), assuming modern CHUR 143 Nd/ 144 Nd = 0.512638, 147 Sm/ 144 Nd = 0.1967; UR 87 Sr/ 86 Sr = 0.7045, 87 Rb/ 86 Sr = 0.0827 [28]. The contents of the elements were determined by isotope dilution to an accuracy of 0.5 rel. % for Sm and Nd, and 1 rel. % for Rb and Sr. Measurements for the La Jolla standard sample yielded the average ratio, 143 Nd/ 144 Nd = 0.511851 (N = 20). 87 Sr/ 86 Sr ratios were normalized to the value of 0.710235 of NBS SRM-987.

U-Pb Zircon Dating
The age of the Overmaraat-Gol pluton was determined by U-Pb dating of eight accessory zircons from a juvite sample (OMG 2013, Table 3). They were dipyramid-prismatic crystals with oscillatory zoning, or crystal chips with Th and U contents, which varied notably even within single grains and Th/U ratios from 0.1 to 1.1. The whole zircon population showed a concordant age of 426.5 ± 3.5 Ma (Figure 4), which may correspond to the time of magma emplacement. Some grains had reverse zonation with a ≈ 5-15 Ma difference between core and rim (points 6.1, 6.2, 9.1, and 9.2 in Figure 4), possibly, as a result of lead loss upon hydrothermal leaching of alkaline igneous rocks [45]. The contents of U, Th, and radiogenic 206 Pb in the zoned grains decreased markedly from core to rim. Reverse zonation was also reported for zircons in juvite from the Kurgusul pluton in the Kuznetsk Alatau [12]. A close age of~425-435 Ma was inferred for some granitoids in the Kuznetsk Alatau and Sayan areas [46,47]. Similar Paleozoic alkaline-mafic intrusions in the western CAOB emplaced in discrete events at~500,~400, and~300 Ma, which did not overlap with the U-Pb age of this study [6][7][8]12,14,15].

U-Pb Zircon Dating
The age of the Overmaraat-Gol pluton was determined by U-Pb dating of eight accessory zircons from a juvite sample (OMG 2013, Table 3). They were dipyramid-prismatic crystals with oscillatory zoning, or crystal chips with Th and U contents, which varied notably even within single grains and Th/U ratios from 0.1 to 1.1. The whole zircon population showed a concordant age of 426.5 ± 3.5 Ma (Figure 4), which may correspond to the time of magma emplacement. Some grains had reverse zonation with a ≈ 5-15 Ma difference between core and rim (points 6.1, 6.2, 9.1, and 9.2 in Figure 4), possibly, as a result of lead loss upon hydrothermal leaching of alkaline igneous rocks [45]. The contents of U, Th, and radiogenic 206 Pb in the zoned grains decreased markedly from core to rim. Reverse zonation was also reported for zircons in juvite from the Kurgusul pluton in the Kuznetsk Alatau [12]. A close age of ~ 425-435 Ma was inferred for some granitoids in the Kuznetsk Alatau and Sayan areas [46,47]. Similar Paleozoic alkaline-mafic intrusions in the western CAOB emplaced in discrete events at ~ 500, ~ 400, and ~ 300 Ma, which did not overlap with the U-Pb age of this study [6][7][8]12,14,15].   (Table 3).

Magma and Rock Sources
The evolution of alkaline and carbonatite magmatism is often attributed to the activity of mantle plumes which drain HIMU/FOZO [44] reservoirs and interact with EM 1 material [48]. Products of nephelinite volcanism may differ in Nd and Sr systematics even in coeval and spatially proximal volcanic centers, as it was shown for the East African rift [48]. The Nd isotope composition, with −1.2 to −0.1 ε Nd (t), indicated that the parent melts of the Overmaraat-Gol rocks originated at mantle depths and contained a PREMA plume component and a major contribution of EM-type enriched lithospheric mantle material (Figure 3). Isotope heterogeneity results from differences in the relative percentages of material from different reservoirs more or less strongly mixed in the magma source. Like the case of volcanic rocks from Italy [49], the isotope geochemistry of the Overmaraat-Gol igneous rocks may correlate with melt fraction in moderately depleted mantle mixed with the material of an ITEM-like mantle source containing 87 Sr markedly above the OIB level. On the other hand, continental crust inputs to the sublithospheric upper mantle may have contributed to the origin of such a mantle domain.
Although bearing signatures of mantle origin, the rocks had quite high ratios of 87 Sr/ 86 Sr (≈0.706-0.707) and δ 18 O (≈8-11‰) [3], corresponding to supracrustal material. 87 Sr may come from brines that were preserved in sediments and mobilized by the hot intrusions [3,6]. Crustal contamination may account for the lack of correlation between the Nd and Sr isotope compositions and for the magma evolution within the mantle array ( Figure 3). Similar signatures of interaction were reported for many alkaline and carbonatite plutonic complexes of different ages in the western CAOB ( Figure 3). Simultaneous involvement of EM-type and mature continental crust material was inferred for Mesozoic intrusions in areas of thick lithosphere, such as Western Transbaikalia, Southern Mongolia, and Russian Altai [4,40,41], but not in the southwestern Hovsgol area and the Sangilen Plateau in southeastern Tuva (Korgeredaba pluton). Therefore, magma sources may differ even in adjacent areas.
General similarity in the isotope evolution of alkaline magmatism in the western CAOB suggests a genetic relationship of magma sources and plume-lithosphere interaction in the same tectonic setting. Given that the history of magmatism comprised several events of different ages, it is reasonable to hypothesize that the igneous rocks inherited isotope signatures from remolten lower lithosphere material metasomatized by the initial plume [12]. The predominant PREMA component in mafic magmas was noted previously in the context of the Paleozoic history of the North-Asian superplume [18].

Tectonic Setting of the Overmaraat-Gol Intrusion and Its Place in the History of Alkaline Magmatism in the Western CAOB
The patterns of trace elements from the Overmaraat-Gol pluton record heterogeneous sources and a complex tectonic setting of alkaline magmatism. Although REE in the igneous rocks show similar fractionation degrees (La/Yb N~7 -10), most HFSE have contents commensurate with the average values for IAB, which are consistent with higher element concentrations in the Middle Cambrian-Devonian foidic intrusions from the Kuznetsk Alatau (Figure 2c,d). Relatively high contents of Rb and Ba, as well as Th and U, may record an OIB contribution associated with a mantle plume. The positive Eu-anomaly (Eu/Eu* = 1.2-1.3) provides implicit evidence for an originally large depth of magma generation. The behavior of HFSE corresponds to magma evolution in an active continental margin setting. The inheritance of geochemical signatures from earlier subduction magmatism was discussed previously for alkaline rocks, as well as for Early Paleozoic granitic and gabbro-monzonitic rocks intruding the accretionary-collisional complexes of the Cambrian Kuznetsk-Altai island arc in the western CAOB [6,7,46,50]. Likewise, the alkaline intrusions of Northern Mongolia may have formed during migration of a mantle plume in the ocean-to-continent transition zone.
The heterogeneity of material was further confirmed by variations in the Th/Yb-Ta/Yb, Th N -Nb N , and Nb/Y-Zr/Y ratios, which either corresponded to the plume/non-plume discrimination line for magma sources (Figure 5c), or converged with the fields of within-plate and continental-margin basaltic rocks (Figure 5a,b,d). This similarity was not fortuitous and may have resulted from the interaction of plume material with older accretionary-collisional complexes on the active margin of the Paleoasian ocean. The contribution of mature continental crust to the magma sources was consistent with the probable age, geochemistry, and isotope systematics of the Overmaraat-Gol rocks. basaltic rocks (Figure 5a,b,d). This similarity was not fortuitous and may have resulted from the interaction of plume material with older accretionary-collisional complexes on the active margin of the Paleoasian ocean. The contribution of mature continental crust to the magma sources was consistent with the probable age, geochemistry, and isotope systematics of the Overmaraat-Gol rocks.  [52]. OIB = ocean island basalts, ACM = active continental margin, WPVZ = within-plate volcanic zone, WPB = within-plate basalts, E-MORB = "enriched-type" mid-ocean ridge basalts; (b) Th N -Nb N diagram [53]. AB = alkali basalt, BAB = back-arc basin basalt; N-MORB-normalized Th and Nb [31]; (c) Nb/Y-Zr/Y diagram [54]: ARC = island arc basalt, OPB = oceanic plateau basalt, N-MORB = "normal-type" mid-ocean ridge basalt, IAB = island arc basalt. Crosses and white star in panels (b,c), respectively, mark average compositions of oceanic basalts [31,32]; (d) Rb-(Y + Nb) diagram [55]. syn-COLG = collision granites, VAG = volcanic arc granites, WPG = within-plate granites.
Alkaline magmatism with such signatures apparently evolved in a setting of active continental-margin distributed rifting, like the Basin and Range Province in California. Repeated formation of mantle magma centers during the early CAOB history supports the idea of periodic plume-related activity during the Paleozoic [56]. The synchronicity of the Cambrian-Early Ordovician, Early-Middle Devonian, Late Carboniferous-Permian, and (partly) Early Triassic events of high-alkali magmatism in the western CAOB over the~520-260 Ma time span with periods of plume activity ( Figure 6) may be evidence of cyclic mantle processes. According to the new U-Pb data, the Overmaraat-Gol pluton in Northern Mongolia resulted from an Early Silurian (Wenlock,~426 Ma) event of alkaline magmatism which was the final phase ("last echo") of the North-Asian plume. events of high-alkali magmatism in the western CAOB over the ~520-260 Ma time span with periods of plume activity ( Figure 6) may be evidence of cyclic mantle processes. According to the new U-Pb data, the Overmaraat-Gol pluton in Northern Mongolia resulted from an Early Silurian (Wenlock, ~426 Ma) event of alkaline magmatism which was the final phase ("last echo") of the North-Asian plume. Figure 6. Correlation between plume activity events and pluton ages: Plutons of the alkaline provinces are shown according to published evidence [5,8,12,[14][15][16]33,38,[57][58][59][60] and our unpublished data. Igneous provinces are shown by different colors: Red for Northern Mongolia; green for Kuznetsk Alatau; orange for Baikal; blue for Russian Altai; purple for SE Tuva.

Concluding Remarks
The obtained Wenlock isotope age of the Overmaraat-Gol pluton indicates that alkaline magmatism in the western CAOB had a long history. The earliest intrusions, along with Devonian and Permian-Triassic events, may have been associated with the activity of the Early Paleozoic North-Asian mantle plume. The isotope systematics and trace-element chemistry of the Overmaraat-Gol rocks suggest a multi-component source of their parent alkaline-mafic magma, which comprised mixed components of depleted and enriched mantle, as well as an inhomogeneous substrate of continental crust. As in the case of some other derivatives of Early Paleozoic alkaline magmatism in the CAOB, magma may have emplaced during interaction of a mantle plume with accretionary-collisional complexes that formed previously on the active margin of the Paleoasian ocean.   [5,8,12,[14][15][16]33,38,[57][58][59][60] and our unpublished data. Igneous provinces are shown by different colors: Red for Northern Mongolia; green for Kuznetsk Alatau; orange for Baikal; blue for Russian Altai; purple for SE Tuva.

Concluding Remarks
The obtained Wenlock isotope age of the Overmaraat-Gol pluton indicates that alkaline magmatism in the western CAOB had a long history. The earliest intrusions, along with Devonian and Permian-Triassic events, may have been associated with the activity of the Early Paleozoic North-Asian mantle plume. The isotope systematics and trace-element chemistry of the Overmaraat-Gol rocks suggest a multi-component source of their parent alkaline-mafic magma, which comprised mixed components of depleted and enriched mantle, as well as an inhomogeneous substrate of continental crust. As in the case of some other derivatives of Early Paleozoic alkaline magmatism in the CAOB, magma may have emplaced during interaction of a mantle plume with accretionary-collisional complexes that formed previously on the active margin of the Paleoasian ocean.