minerals Article 40Ar/39Ar Geochronology of the Malyy (Little) Murun Massif, Aldan Shield of the Siberian Craton: A Simple Story for an Intricate Igneous Complex Alexei V. Ivanov 1,* , Nikolay V. Vladykin 2, Elena I. Demonterova 1, Viktor A. Gorovoy 1 and Emilia Yu. Dokuchits 2 1 Institute of the Earth’s Crust, Siberian Branch of the Russian Academy of Sciences, 664033 Irkutsk, Russia; [email protected] (E.I.D.); [email protected] (V.A.G.) 2 A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, 664033 Irkutsk, Russia; [email protected] (N.V.V.); [email protected] (E.Y.D.) * Correspondence: [email protected]; Tel.: +7-395-242-7000 Received: 17 November 2018; Accepted: 16 December 2018; Published: 19 December 2018 Abstract: The Malyy (Little) Murun massif of the Aldan Shield of the Siberian Craton has long been a kind of Siberian Mecca for geologists. It has attracted thousands of geologists, prospectors, and mineral collectors despite its remote location. It is famous for a dozen new and rare minerals, including the gemstones charoite and dianite (the latter is the market name for strontian potassicrichrerite), as well as for a range of uncommon alkaline igneous rocks. Despite this, the age of the Malyy Murun igneous complex and associated metasomatic and hydrothermal mineral associations has remained poorly constrained until now. In this paper, we provide extensive 40Ar/39Ar geochronological data to reveal its age and temporal history. It appears that, although unique in terms of rocks and constituent minerals, the Malyy Murun is just one of multiple alkaline massifs and lavas emplaced in the Early Cretaceous (~137–128 Ma) within a framework of the extensional setting of the Aldan Shield and nearby Transbaikalian region. The extension took place 40–60 million years after the supposed closure of the Mongolia–Okhotsk Ocean and orogenic peak in the Early–Middle Jurassic. Keywords: Malyy (Little) Murun massif; Aldan Shield; Siberian Craton; 40Ar/39Ar dating 1. Introduction The Malyy (Little) Murun massif of the Aldan Shield of the Siberian Craton (Figure1), though located in remote Siberia, is familiar to several generations of Russian geologists and to the international community due to a specific assemblage of alkaline igneous rocks [1–16] and mineral deposits for U, Th, Au, Pb, Ti, Sr, and Ba, associated with intrusive and post-intrusive stages of this massif [12–19]. At last, but not least, the Malyy Murun massif is famous for the only world occurrence of gemstone charoite [20–24]. In the 1980s, up to several thousand geologists and miners worked at the Malyy Murun massif during the short Siberian summers in search for various metal and non-metal resources. Today, only limited mining operations are conducted there for the gemstone market (charoite, dianite—the market name for strontian potassicrichrerite [24]), and metallurgy (Ba–Sr benstonite carbonatite [12–14]). The Malyy Murun has a high potential for mineralogists and petrologists. It is worth mentioning that a number of new minerals and mineral varieties have been discovered there (charoite [20,25], tinaksite [26,27], tokkoite [27,28], frankamenite [29,30], strontian potassicrichrerite [31,32], tausonite [33,34], murunskite [35,36], vladykinite [37]), and various exotic rocks such as lamproites, carbonatites, alkaline granites, and various syenitic rocks. Minerals 2018, 8, 602; doi:10.3390/min8120602 www.mdpi.com/journal/minerals Minerals 2018, 8, 602 2 of 21 Minerals 2018, 8, x FOR PEER REVIEW 2 of 21 Despite the enormous efforts for understanding the geology of the Malyy Murun massif, Despite the enormous efforts for understanding the geology of the Malyy Murun massif, its its geochronology was constrained to Early Cretaceous by an outdated K-Ar dating of voluminously geochronology was constrained to Early Cretaceous by an outdated K-Ar dating of voluminously predominant syenites [38]. A few attempts were also made recently to limit the timing of the charoite predominant syenites [38]. A few attempts were also made recently to limit the timing of the charoite mineralization by 40Ar/39Ar dating [39,40]. In this study, we provide new 40Ar/39Ar results on mineralization by 40Ar/39Ar dating [39,40]. In this study, we provide new 40Ar/39Ar results on all all major igneous and hydrothermal stages recognized in the Malyy Murun massif. We use this major igneous and hydrothermal stages recognized in the Malyy Murun massif. We use this geochronological information to constrain the duration of magmatic and later hydrothermal activity geochronological information to constrain the duration of magmatic and later hydrothermal activity within the massif and to compare it with the information on alkaline magmatism of the Aldan Shield within the massif and to compare it with the information on alkaline magmatism of the Aldan Shield of the Siberian Craton and the Transbaikalian rifted region [41–53] (Figure1). of the Siberian Craton and the Transbaikalian rifted region [41–53] (Figure 1). Figure 1. LocationLocation ofof thethe MalyyMalyyMurun Murun and and some some other other alkaline alkaline complexes complexes in in the the tectonic tectonic framework framework of theof the Siberian Siberian Craton Craton and and the Mongolia–Okhotskthe Mongolia–Okhotsk belt (modifiedbelt (modified after [39after]). The[39]). latter The islatter a Jurassic is a Jurassic orogen formedorogen dueformed to the due closure to the of anclosure ocean of thean sameocean name. of the Aldan same Shield name. is theAldan cratonic Shield basement is the intrudedcratonic bybasement numerous intruded Cretaceous by numerous small volume Cretaceous alkaline small intrusions volume [38 alkaline]. TB (hatched) intrusions is a [38]. Transbaikalian TB (hatched) region is a riftedTransbaikalian in Cretaceous region with rifted the in formation Cretaceous of basinswith the and formation associated of alkalinebasins and basaltic associated volcanism alkaline [45]. Acronyms:basaltic volcanism MM—Malyy [45]. Acronyms: Murun, I—Inagly, MM—Maly R—Ryabinovy,y Murun, I— O—Oshurkov.Inagly, R—Ryabinovy, O—Oshurkov. 2. Materials and Methods According toto [15[15],], the the Malyy Malyy Murun Murun massif massif can can be subdividedbe subdivided into fourinto igneousfour igneous phases phases or stages or (earlystages intrusive,(early intrusive, main intrusive, main intrusive, volcanic, volcanic and late, and intrusive). late intrusive). The relation The relation between between different different igneous stagesigneous was stages not alwayswas not clear always because clear the because rocks classifiedthe rocks toclassified one or another to one or of theanother four stagesof the four are usually stages separatedare usually by separated faults, and by no faults direct, and contacts no direct can contacts be observed can forbe observed all rock types for all to rock establish types the to completeestablish schemethe complete based scheme purely onbased geological purely relationships. on geological Thus, relationships. it is not surprising Thus, it is that not other surprising subdivisions that other can besubdivisions found in the can literature be found (e.g., in the [22 literature]). Here we (e.g. follow, [22]). the Here subdivision we follow of [the15], subdivision though we slightlyof [15], though modify itwe as slightly indicated modify below. it as indicated below. Rocks of the early intrusive stage are predominantlypredominantly located in the northeastern part of the massif ( (FigureFigure 2),2), where where they they were were recovered recovered through through drilling drilling and andmining mining of an exploratory of an exploratory gallery. gallery.Biotite-pyroxenites, Biotite-pyroxenites, K-ijolites, K-ijolites, olivine olivinelamproites, lamproites, feldspar feldspar-shonkinites,-shonkinites, leucitic leucitic shonkinites shonkinites,, and andcumulative cumulative olivine olivine-spinel-spinel and and olivine olivine-pyroxene-phlogopite-monticellite-pyroxene-phlogopite-monticellite rocks rocks form form a a layered complex. For For the the geochronological purpose, mica was separated from biotite-pyroxenite,biotite-pyroxenite, olivine lamproite,lamproite, and olivine-pyroxene-phlogopite-monticelliteolivine-pyroxene-phlogopite-monticellite rockrock (Appendix(AppendixA A).). Rocks of the main intrusive stage composed the central part of the massif (Figure 2), which consists of different types of syenites (leucitic, feldspar–calcilitic, biotite–pyroxene–K-feldspar). For Minerals 2018, 8, x FOR PEER REVIEW 3 of 21 40MineralsAr/39Ar2018 dating,, 8, 602 we separated mica from abundant biotite-pyroxene-K-feldspar syenite (referred3 of to 21 as alkaline syenite by Russian geologists). Figure 2. AA schematic schematic map map of the the Malyy Murun massif with the location of the studied samples. TheRocks volcanic of the main stage intrusive was recognized stage composed by the thelava central of leucitic part oflamproites the massif and (Figure melaphonolites2), which consists and volcanicof different breccia. types The of syenites latter includes (leucitic, clasts feldspar–calcilitic, of rocks of the biotite–pyroxene–K-feldspar). main intrusive stage. Dykes Forof tinguaites,40Ar/39Ar richteritedating, we–sanidine separated lamproites, mica from abundanttrachyte–porphyries, biotite-pyroxene-K-feldspar syenite–porphyries syenite, and (referred (eudialyte to as-bearing alkaline) lujavritessyenite by we Russianre included geologists). into the volcanic stage. For 40Ar/39Ar dating,