Ar Ages Research Articles

Petrogenesis and the evolution of Pliocene Timar basalts in the east of Lake Van, Eastern Anatolia, Turkey: A consequence of the partial melting of a metasomatized spinel–rich lithospheric mantle source

The Eastern Anatolian region provides a good example for collision–related volcanic activity. This activity started during the Middle Miocene and continued through the Pliocene and Quaternary. The new K − Ar ages reveal that the post–collisional Pliocene Timar basalts located in the East Anatolian Accretionary Complex (EAAC) erupted between 4.72 (Zanclean) and 3.29–2.93 Ma (Piacenzian). The Pliocene Timar basalts consist of plagioclase, clinopyroxene (augite), and olivine minerals and have glomeroporphyritic, intergranular, and ophitic textures. They display enrichment in light rare earth elements (LREEs) with (La/Yb)N values that range between 3.19 and 6.66 to 21.63–5.61. 87Sr/86Sr(i) and 143Nd/144Nd(i) isotopic ratios of the Timar basalts are 0.70599–0.70436 and 0.51277–0.51259, respectively. The εNd(t) values of these rocks range between 1.0116 and 1.0121. 87Sr/86Sr(i) and Nd(t) contents indicate crustal contamination during the emplacement of the Timar basalts. According to the Energy Constrained–Assimilation Fractional Crystallization (EC–AFC) model, this contamination rate is between ~1.4 and 4.4%. The melting model shows that the least evolved samples of the Timar basalts were formed by ~0.1–2% partial melting of the lherzolitic source. Primitive melts that constituent Timar basalts were occurred with partial melting of a magma source that has predominantly spinel bearing mineralogy. All data indicate that the origin of the parental melts of the Timar basalts is metasomatized lithospheric mantle source and then these melts reached the surface from the fissures that is related to extensional tectonics in Pliocene.

Metallogeny of the continental collision-related Jiagang W-Mo deposit, Tibet: Evidence from geochronology and petrogenesis

The Jiagang W-Mo deposit is the first greisen-type tungsten deposit recognized in the Lhasa terrane, and thus it has important genetic implications for understanding the regional tungsten mineralization potential. Greisen-type orebodies dominate in the deposit. Ores characterized by veinlets and dissemination are located in intensively altered monzogranite and contact zones between the monzogranites and sedimentary rocks. The minor quartz vein-type orebodies occuring as large quartz veins mainly precipitated in the sedimentary rocks. Intrusions associated with the Jiagang deposit are monzogranites, which can be subdivided into medium-grained monzogranite (MMG), porphyritic monzogranite (PMG), and fine-grained monzogranite (FMG). The zircon U-Pb ages of the PMG and MMG are 18.2 ± 0.2 Ma and 18.6 ± 0.1 Ma, respectively. These ages are consistent with the muscovite 40Ar-39Ar plateau ages (18.7 ± 0.2 Ma, 18.5 ± 0.2 Ma) and the molybdenite Re-Os isochron age (19.0 ± 0.3 Ma) within errors, indicating that the Jiagang W-Mo mineralization took place during the post-collisional stage of the Indo-Asian continental collision. The mineralization is temporally, spatially and genetically associated with the monzogranitic intrusions. The FMGs have the lowest SiO2 contents (70.8–71.5 wt%), lowest differentiation index (DI = 85.7–86.3) and least obviously negative Eu anomalies (Eu/Eu* = 0.22–0.42). The PMGs and MMGs have higher SiO2 contents (72.6–76.3 wt%), higher differentiation index (DI = 89.3–92.7) and more negative Eu anomalies (Eu/Eu* = 0.11–0.24). Almost all of the rocks are strongly peraluminous (A/CNK = 1.02–1.29), enriched in LILEs and depleted in HFSEs. Concentrations of K2O, CaO, FeOT and TiO2 of the three types of monzogranites show negative correlations with SiO2, while P2O5 exhibits the opposite trend. Values of Nb/Ta decrease with SiO2 contents, and those of (La/Yb)N increase with La contents. Whole rock εNd(t) of the FMGs range from −11.3 to −11.1 with two-stage Nd isotopic model ages (TDM2) of 1741–1797 Ma, which are similar to those of the PMGs and MMGs (εNd(t) = −10.9– −10.6; TDM2 = 1709–1816 Ma). Zircon εHf(t) values of the PMG and MMG range from −13.8 to −5.3 with two-stage Hf isotopic model ages of 1439–1979 Ma. These data suggest that the ore-forming monzogranites are strongly peraluminous S-type granites, which should be derived from anatexis of the Proterozoic metagreywackes underneath the central Lhasa subterrane. The PMG and MMG are highly fractionated through various degrees of fractional crystallization of plagioclase, K-feldspar, biotite, monazite and allanite from the parental melts represented by the FMG. Zircon trace elements suggest that oxygen fugacity of the ore-forming magmas is relatively low and decreased as the magmas evolved, which is favorable to remove substantial tungsten from melts into the tungsten-mineralizing hydrothermal fluids. A high degree of fractional crystallization of the crustally-derived magmas with relatively low magmatic oxidation state are the critical factors determining the generation of the tungsten mineralization in the Jiagang deposit. Combined with the existing breakthroughs in prospecting exploration, we propose that the central Lhasa subterrane has great metallogenic potential in W-(Mo) mineralization associated with the Indo-Asian continental collision.

Late Eocene magmatism of the eastern Qiangtang block (eastern Tibetan Plateau) and its geodynamic implications

Post-collisional magmatism offers significant clues to reveal the deep dynamic process and surface uplift of plateaus. New zircon U-Pb ages, biotite Ar-Ar ages, zircon trace elements, and whole-rock geochemistry, integrated with published data of igneous rocks from the eastern Qiangtang block (eastern Tibetan Plateau) were determined to unravel the geodynamics and constrain the uplift history. These igneous rocks consist of few mafic and many intermediate-felsic series. Geochronological studies reveal that the magmatism mainly occurred in ~43–33 Ma. The late Eocene igneous rocks are characterized by high potassium, enrichment in large ion lithophile elements (LILE, e.g., Rb, Ba, Th, U), depleted in high field strength elements (HFSE, e.g., Nb, Ta, Ti), negligible Eu anomalies, with a wide range of highly incompatible element ratios. The parental magmas of these rocks were most probably generated by different degrees of partial melting of the lithospheric mantle, which were heterogeneously metasomatized by subduction related multiphase fluids/melts. Whole-rock geochemistry, zircon trace elements, and thermodynamic modeling results suggest that the parental magmas experienced complicated and different differentiation processes. Mantle metasomatism, fractional crystallization, and magmatic recharge made contributions to the enrichment of incompatible elements and the wide range ratios between highly incompatible elements. In combination with the estimated Eocene crustal thickness, we suggest that the southward continental subduction of the Songpan-Ganzi block triggered the magmatism, thickened the crust, and induced the surface uplift to no more than ~3 km in the late Eocene.

Leninakan Ignimbrite as Ejecta of Aragats Volcano, Armenia

This paper is devoted to the age, stratigraphic position, and structure of the ignimbrite that occurs in NW Armenia, which is locally named “Leninakan tuff.” The massive structure of the tuff with no internal discontinuities, similar chemical composition and K–Ar ages of all the samples let the authors to deduce that the ignimbrite was formed due to a huge single explosive eruption of the Aragats volcano. The minimum area of distribution of the ignimbrite to the NW of the Aragats volcano within the Shirak Basin was assessed. Numerical age estimations and chemical correlation of the studied ignimbrite and Aragats summit lavas revealed that the formation of both volcanic rocks took place at the end of the III stage of the volcano’s activity. (0.65–0.7 Ma).

Miocene Sn polymetallic mineralization in the Tethyan Himalaya, southeastern Tibet: A case study of the Cuonadong deposit

The Cuonadong deposit is located in the Cuonadong gneiss dome of the eastern Tethyan Himalaya and is characterized as a large- to superlarge-scale Sn polymetallic deposit in the Himalaya orogen. The mineralization pattern includes skarn W-Sn mineralization in the strong shear zone of the dome mantle, hydrothermal Sn-W veins controlled by north-south-trending rifts (NSTRs), and Be-Rb mineralization in pegmatitic leucogranite. The results of this study show that the zircon U-Pb age of undeformed (postkinematic) leucogranites is 15.5 ± 0.1 Ma (n = 28, MSWD = 1.4), the cassiterite U-Pb age of the vein-type Sn-W orebody is 14.3 ± 0.5 Ma (n = 40, MSWD = 3.2), and the muscovite Ar-Ar age of beryl-bearing pegmatitic leucogranite is 15.2 ± 0.2 Ma (MSWD = 1.5). The isotopic dating results demonstrate that the Cuonadong Sn-W-Be-Rb mineralization has a close genetic relationship with the Miocene (ca. 16–14 Ma) leucogranite magmatic hydrothermal activity in the Cuonadong dome. The deformed (synkinematic) two-mica granite (18 Ma) was cut by the Sn-W ore veins (14 Ma) faulted by NSTRs, indicating that the E-W-trending extension of the NSTRs initiated at ca. 18–14 Ma. The Himalaya leucogranite belongs to a highly fractionated granite with high Sn content (>15 ppm). In conclusion, the Himalaya leucogranite is considered to be tin granite in this study, and therefore, the Himalaya presents high potential for Sn polymetallic mineralization prospecting.

Elemental and Sr–Nd–Pb isotopic compositions, and K–Ar ages of transitional and alkaline plateau basalts from the eastern edge of the West Cameroon Highlands (Cameroon Volcanic Line)

This study presents the first geochronological and geochemical data from volcanic rocks on the eastern edge of the West Cameroon Highlands, which is the southern continental sector of the Cameroon Volcanic Line. Whole-rock petrography and geochemistry indicate that the rocks are transitional and alkaline basalts. KAr ages show that the transitional lavas are older (36.67 ± 1.88 Ma) than the alkaline lavas (14.86 ± 1.1 to 8.89 ± 0.7 Ma). The Sr–Nd–Pb isotopic ratios of the transitional lavas (87Sr/86Sri = 0.70380–0.70384, 143Nd/144Ndi = 0.51268–0.51277, 206Pb/204Pb = 18.06–18.65, 207Pb/204Pb = 15.52–15.62, and 208Pb/204Pb = 38.15–39.07) are different to those of the alkaline lavas (87Sr/86Sri = 0.70300–0.70330, 143Nd/144Ndi = 0.51287–0.51298, 206Pb/204Pb = 19.67–19.89, 207Pb/204Pb = 15.63–15.66, and 208Pb/204Pb = 39.35–39.59). Chondrite-normalized rare earth element (REE) and primitive-mantle-normalized trace element patterns of the lavas from the study area are similar to those of oceanic islands basalts (OIB). The transitional lavas have Nb/Y ratios of 0.67–1.18, (La/Yb)N ratios of 4.96–5.84, and Mg# = 29–39, and the alkaline lavas have Nb/Y ratios of 1.65–3.55, (La/Yb)N ratios of 7.76–20.13, and Mg# = 35–58. Both the transitional and alkaline lavas are enriched in Nb, Ta, Ti, and Ba, and depleted in K and Pb on normalized incompatible element diagrams. The geochemical differences may be due to distinct source compositions. The trace elements and REE patterns suggest that garnet and amphibole played a significant role in the generation of the lavas. Batch partial melting modelling suggests that the transitional lavas were formed by higher degrees of partial melting (3%–5%) than the alkaline lavas (0.6%–2.0%). The geochemical and geochronological data imply that the transitional and alkaline volcanism was related to exchange between the upwelling asthenosphere and the lithospheric mantle which might have triggered lithospheric extension during the Cenozoic.

A minimum Thanetian (Paleocene) age for the African Surface in the Eritrean highlands, Northeast Africa

Supergene alunite from a supergene leached capping zone in the Daero Paulos area near Asmara, Eritrea yields Thanetian (late Paleocene) K–Ar ages of 58.6 ± 2.3 and 59.4 ± 2.3 Ma. The leached capping is exposed in an erosional window through a laterite profile, with the supergene alunite forming by oxidation of pyrite contained in quartz veinlet stockworks hosted by porphyry copper-related, felsic porphyry and adjacent volcano-sedimentary country rocks. Geologic relationships show that the leached capping and contained supergene alunite formed simultaneously with the lateritic weathering event that accompanied regional planation of equatorial and subequatorial Afro-Arabia to form the African Surface. Landscape modification of the central Ethiopian-Eritrean plateau by incision of transverse valleys and canyons commenced in the Oligocene, with a principal exhumation event occurring in the Miocene in response to swell-induced rift-flank uplift during Red Sea opening.

Time of Termination of Ultramafic Magmatism in the Ural Platinum Belt

A comprehensive petrological study and age dating of a young clinopyroxenite–gabbro–hornblendite series intruded deformed dunites, clinopyroxenites, and tilaites of the Kytlym mafic–ultramafic massif have been carried out. The convergent 40Ar–39Ar age of amphibole from clinopyroxenite and U–Pb isotope data of zircons from melanocratic gabbro correspond to 415 Ma and indicate that the termination of ultramafic magmatism in the Ural Platinum Belt took place in the Early Devonian.

Gold in the lithosphere of the western South China Block, SW China: Insights from quartz porphyries from the giant Zhenyuan gold deposit

Numerous Au-rich deposits in the western South China Block, SW China, provide insights into the gold enrichment process at a lithospheric scale. Zhenyuan is a giant gold deposit in the western South China Block, but its genesis remains controversial with competing hypotheses of magmatic–hydrothermal and metamorphic–hydrothermal development. Here, we present new petrographic, major and trace element, and zircon U–Pb age data for quartz porphyry from the Zhenyuan deposit to evaluate the specific gold enrichment process. Zircon SHRIMP and LA–ICP–MS results demonstrate that the Zhenyuan quartz porphyries crystallized at ca. 255–247 Ma. This age is broadly consistent with the Re–Os age of gold-bearing pyrite measured in previous studies. The porphyries are characterized by high-K calc-alkaline compositions and exhibit significant enrichment in LILEs and LREEs, and strong depletion in HFSEs. They share the same source (Au-rich lower crust) with Cenozoic Au-bearing intrusions. The initial melts had a weak oxidation state (ΔFMQ = –0.15–0.55), which favored Au accumulation and migration. However, the gold was precipitated with the crystallization of Ti–Fe oxides in the middle crust, resulting in the emplacement of Au-poor magma in the upper crust. The gold orebodies in the Zhenyuan deposit formed due to Cenozoic hydrothermal processes, as evidenced by our observations of ore-bearing pyrite and Ar–Ar ages of phlogopite measured in previous studies. Combined with regional mineralization data, we propose lithosphere-scale processes for gold accumulation, migration, and precipitation from the Neoproterozoic to Cenozoic. Specifically, we predict an Au-rich middle crust beneath the western South China Block.

Alkali basalt from the Seifu Seamount in the Sea of Japan: post-spreading magmatism in a back-arc setting

Abstract. We present geochemical and 40Ar∕39Ar age data for a peridotite xenolith-bearing basalt dredged from the Seifu Seamount (SSM basalt) in the northeast Tsushima Basin, southwest Sea of Japan. An 40Ar∕39Ar plateau age of 8.33±0.15 Ma (2σ) was obtained for the SSM basalt, indicating that it erupted shortly after the termination of back-arc spreading in the Sea of Japan. The SSM basalt is a high-K to shoshonitic alkali basalt that is characterized by light rare earth element enrichment. The trace element features of the basalt are similar to those of ocean island basalt, although the Yb content is much higher, indicating formation by the low-degree partial melting of spinel peridotite. The Nd, Sr, and Pb isotopic compositions of the SSM basalt differ from those of back-arc basin basalts in the Sea of Japan. The Sr–Nd isotopic composition of the SSM basalt suggests its source was depleted mid-ocean ridge mantle containing an enriched mantle (EM1) component. The SSM basalt was formed in a post-back-arc extension setting by the low-degree partial melting of an upwelling asthenosphere that had previously been associated with the main phase of back-arc magmatism.

U-Th-Pb Shrimp dating of hydrothermal monazite from the Trairão Gold Deposit - Alta Floresta Gold Province (Amazon Craton)

Abstract Alta Floresta Gold Province occurs in the center-south portion of the Amazon Craton. Trairao gold deposit, which is located in the Alta Floresta Gold Province, is hosted by 1878 to 1854 Ma arc-related granites. Two important Au deposits take place in the region, the Trairao and Chumbo Grosso, which are structurally controlled by N80°W/S80°E trending lineament and associated with quartz veins and disseminated sulfides in a strong phyllic alteration zone of the host granite. Hydrothermal monazite grains formed during the Au mineralization event occur as fine anhedral crystals filling fractures or as isolated grains associated with Ag, Au, molybdenite, barite, pyrite, galena, and sphalerite. The hydrothermal monazite grains contain very low U, relatively low Th, and moderate Nd and La contents. SHRIMP U-Th-Pb dating of these crystals yielded an age of 1798 ± 12 Ma for Trairao and 1805 ± 22 Ma for Chumbo Grosso Au-deposits, whereas magmatic zircon grains of the granitic host rocks yielded an age of 1854 ± 8 Ma. The ages obtained in this paper are similar to those reported by Assis (2015), who studied Pe Quente (Re-Os in molybdenite, 1792 ± 9 Ma and 1784 ± 11) and Francisco deposits (40Ar-39Ar ages from sericitic halo: 1779 ± 6.2 and 1777 ± 6.3 Ma). Serrato et al. (2014) acquired Re-Os dating in pyrite and molybdenite from Juruena gold deposit. The results show isochronous ages at 1786 ± 1 Ma, with a model age at 1787 ± 3.2 Ma, suggesting a major Statherian gold metallogenic event at the Alta Floresta Gold Province region. Regional fluids-flow has a close relationship with the generation and concentration of several important economic deposits in the Eastern border of Alta Floresta Gold Province, including Pe Quente, Francisco, Juruena, Chumbo Grosso, and Trairao Au-deposits.

Comparative studying on zircon, cassiterite and coltan U-Pb dating and <sup>40</sup>Ar/<sup>39</sup>Ar dating of muscovite rare-metal granitic pegmatites: A case study of the northern Tugeman lithium-beryllium deposit in the middle of Altyn Tagh

稀有金属花岗伟晶岩年代学是花岗伟晶岩型稀有金属矿床研究的重要内容。目前测年方法较多，但不同方法测试结果的对比研究亟待开展。我们选择吐格曼北花岗伟晶岩型锂铍矿床3条含矿伟晶岩开展锆石、锡石、铌钽铁矿及白云母四种矿物不同方法的测年对比研究。结果显示：1）ρ31白云母-锡石伟晶岩中锡石²³⁸U/²⁰⁶Pb-²⁰⁷Pb/²⁰⁶Pb谐和年龄为468±8.7Ma（MSWD=1.1，N=39）、白云母-钠长石-锂辉石伟晶岩中锆石²⁰⁶Pb/²³⁸U谐和年龄为458.7±2.3Ma（MSWD=7.2，N=16）；2）ρ38白云母-钠长石-锂辉石伟晶岩中锆石²⁰⁶Pb/²³⁸U谐和年龄为454.7±4.0Ma（MSWD=8.0，N=10）、白云母⁴⁰Ar/³⁹Ar坪年龄为350.2±1.6Ma（MSWD=4.7）；3）ρ87含铌钽铁矿-白云母-石英伟晶岩中铌钽铁矿²⁰⁶Pb/²³⁸U谐和年龄为464.1±2.7Ma（MSWD=5.2，N=39）。可以看出，铌钽铁矿与锡石的U-Pb年龄在误差范围内一致，可能代表花岗伟晶岩岩浆结晶的年龄；2件样品的蜕晶化锆石U-Pb年龄也可以对比，可能代表岩浆锆石蜕晶化后经流体交代作用及重结晶作用导致U-Pb同位素系统重置的时间。白云母Ar-Ar年龄明显晚于铌钽铁矿、锡石和锆石的U-Pb年龄，鉴于ρ38白云母-钠长石-锂辉石伟晶岩脉中白云母与锂辉石发生了强烈变形与蚀变，认为变形的白云母记录的是叠加变形与热液蚀变的时间。由此推断吐格曼北锂铍花岗伟晶岩形成于468~454Ma，这意味着阿尔金山地区中-晚奥陶世可能存在持续时间较长的稀有金属成矿事件。基于花岗伟晶岩矿物成因与吐格曼北锂铍花岗伟晶岩不同方法测年对比结果，可以得出以下结论：铌钽铁矿与锡石的U-Pb年龄可代表伟晶岩岩浆结晶的年龄，蜕晶化的锆石U-Pb年龄记录的是岩浆锆石蜕晶化后经流体交代作用及重结晶作用导致U-Pb同位素系统重置的时间，含钾矿物的⁴⁰Ar/³⁹Ar年龄能够约束伟晶岩的变形与蚀变年龄；多种定年方法的联合约束可以更好地限定稀有金属花岗伟晶岩的各个阶段成矿事件时间。;Geochronological dating is a vital part of the study of granitic pegmatite type rare-metal deposits. There are many dating methods; however, comparative study on dating results obtained by different methods hasn't been performed. In this study, we report new dating results of zircon, cassiterite, coltan (short for columbite-tantalite) and muscovite from three pegmatites of the northern Tugman lithium-beryllium deposit. The results show that: 1) the ρ31 muscovite-cassiterite pegmatite has a concordant U-Pb age of 468±8.7Ma (MSWD=1.1, N=39) of cassiterite, whereas the muscovite-albite-spodumene pegmatite has a concordant U-Pb age of 458.7±2.3Ma (MSWD=7.2, N=16) of zircon; 2) the deformed ρ38 muscovite-albite-spodumene pegmatite has a concordant U-Pb age of 454.7±4.0Ma (MSWD=8.0, N=10) of zircon and ⁴⁰Ar/³⁹Ar plateau age of 350.2±1.6Ma (MSWD=4.7) of muscovite; and 3) the ρ87 coltan-bearing muscovite-quartz pegmatite has a concordant U-Pb age of 464.1±2.7Ma (MSWD=5.2, N=39) of coltan. The U-Pb ages of coltan and cassiterite are consistent within the error range, they may represent the crystallization age of granitic pegmatite at the magmatic stage. The U-Pb ages of the metamict zircons of the two samples can also be compared, they may represent the reset time of U-Pb isotope system caused by fluid metasomatism and recrystallization of magmatic zircon after metamictization. The ⁴⁰Ar/³⁹Ar plateau age of muscovite is obviously later than the U-Pb age of coltan, cassiterite, and zircon. Considering the strong deformation and alteration of muscovite and spodumene in the ρ38 muscovite-albite-spodumene pegmatite, it is suggested that the ⁴⁰Ar/³⁹Ar age of deformed muscovite may decipher the superposition deformation and alteration time. It is concluded that the lithium-beryllium granitic pegmatites in the northern Tugman area were formed in a period of 468~454Ma. Therefore, we consider that an intensive rare metal mineralization during the Middle-Late Ordovician might occur in the Altyn Tagh area. Moreover, it is also suggested that the U-Pb ages of coltan and cassiterite might represent the crystallization time of a pegmatite during the magmatic stage, whereas the U-Pb age of metamict zircon could record a reset time of U-Pb isotope system by fluid metasomatism and recrystallization, while the ⁴⁰Ar/³⁹Ar age of deformed potassium-bearing minerals might represent the deformation and alteration time of the pegmatites. The combination of different dating methods may constrain the integrated formation epoch of the rare-metal pegmatite deposit.

The Eocene-Oligocene Nanchititla dike swarm, eastern Michoacán, México

ABSTRACT We present a geologic map for the Eocene-Oligocene Nanchititla mafic dike swarm consisting of over 140 NNW-SSE trending dike segments; whose emplacement was influenced by the left-lateral transtensional deformation regime prevailing in southern México during the Cenozoic. The hosting rock is a mechanically heterogeneous continental sedimentary sequence interbedded with volcanic rocks. Interaction among magma and host rock promoted the formation of dike-sill systems, segmentation and sinuous intrusions. Dike thickness observed is between 1 and 45 m with a peak in the range from 1 to 5 m. Six giant dikes of more than 30 m thick were documented. The 80% of the dikes have porphyritic textures rich in plagioclase phenocrysts with size ranging from 0.5 to 2.5 cm. New and compiled 40Ar-39Ar ages constrain the swarm emplacement between 30 and 34.5 Ma, whereas plagioclase phenocrysts entrained by the magma have older ages between 35 and 42 Ma.

P-T and Ar–Ar age constraints on low- to high-grade metabasites from the Buhwa Greenstone Belt, southern Africa: Implications for Neoarchean to Paleoproterozoic thermal evolution along the Limpopo Complex–Zimbabwe Craton boundary

This study addresses the Neoarchean to Paleoproterozoic thermal evolution from the Buhwa Greenstone Belt (BGB) along the contact zone between the Archean Zimbabwe Craton (ZC) and a granulite terrane of the Northern Marginal Zone (NMZ) of the Limpopo Complex. Available tectonic models suggest a thrusting of the hot NMZ over and onto the cold ZC, which possibly gave rise to local heating of the margin of the craton by a “hot-iron effect”. Petrographic, pressure–temperature data, along with hornblende Ar–Ar ages for metabasites from the BGB were coupled with a discussion of the tectonics of this region. The metabasites typically occur interlayered with metasediments. Four types of metabasites occur, namely, calcic amphibole–epidote schist, amphibolite (garnet-bearing and garnet-free), calcic amphibole–clinopyroxene schist, and mafic granulite, indicating a varying metamorphic grade from epidote–amphibolite facies to granulite facies. The lower-grade calcic amphibole–epidote schists occur in the northwestern margin of the belt, whereas the higher-grade amphibolites and mafic granulites are present along the southeastern margin of the BGB adjacent to the granulite terrane. The increase in metamorphic grade towards the NMZ of the Limpopo Complex is supported by temperature estimates of <500 °C to >700 °C from northwest to south east. The results of our study are consistent with the pervailing tectonic model which considers the NMZ–ZC boundary as a major thrust-sense shear zone dipping southward, and the hot NMZ thrusted over the cold ZC. New hornblende Ar–Ar ages indicated two discrete thermal events recorded in the BGB rocks: 2525 ± 19 Ma from mafic granulite and 2020 ± 8 Ma to 1968 ± 10 Ma from amphibolites. The Neoarchean age probably corresponds to a cooling age after the peak metamorphism, indicating that the NMZ thrusting onto the ZC took place at >2525 Ma. The age of the thrusting of the NMZ is younger than that of the Southern Marginal Zone (SMZ) of the Limpopo Complex onto the Kaapvaal Craton (KC) (ca. 2.72–2.69 Ga). Therefore, the formation of the Limpopo Complex is not because of a simple collision of the KC and the ZC, but the SMZ-KC collision is more than 100 Myr earlier than the NMZ–ZC collision, suggesting multiple collision for the formation of the Limpopo Complex. In contrast, the younger Paleoproterozoic thermal event coincidental with the last high-grade metamorphic event within the Limpopo Complex might correspond to a thermal reactivation along the NMZ–ZC boundary possibly caused by the global ca. 2.0 Ga orogenic events.

Muscovite 40Ar−39Ar age and its geological significance in Zhuxi W(Cu) deposit, northeastern Jiangxi

The Zhuxi W(Cu) skarn deposit, the world’s largest tungsten deposit is newly discovered in Jingdezhen city, northeastern Jiangxi province, China. It mainly occurs near the contact zone between the Yanshanian granites and the Late Paleozoic carbonate rocks. Three types of mineralization including skarn type, altered granite type and quartz vein-veinlet type orebodies have been observed. In this study, the 40Ar−39Ar age of hydrothermal muscovite coexisting with copper mineralization in the altered granite type orebody formed near the unconformity interface is determined by step-heating technology using CO2 laser. The plateau age, isochron age, and inverse isochron age of muscovite are (147.39±0.94) Ma, (147.2±1.5) Ma, and (147.1±1.5) Ma, respectively. These ages are almost identical to the ages of ore-related granite and other mineralization types in the Zhuxi W(Cu) deposit, indicating that the Cu mineralizations occurred at the shallow depth and near the unconformity interface are contemporaneous during the Late Jurassic. This further suggested that the acompanied W and Cu mineralization in the Zhuxi W(Cu) deposit which may be controlled by the magma source is enriched in both W and Cu.

Age, composition and sources of rocks and ores of the Okunevskoe fluorite-leucophanite deposit, Western Sayan: assessment of the contribution of magmatism to ore mineralization

The paper presents data on the structure of the Okunevskoe fluorite-Leucophanite deposit, located within the Early Paleozoic rare-metal East Sayan rare metal metallogenic zone. The deposit is controlled by alkali granitoids, with ore mineralization concentrated at the contact of granitoids and host carbonates. It is represented by leucophanite-fluorite and pyroxene-fluorite ore types. The Ar-Ar age of granitoids is established at ~485Ma. Geochemical characteristics of igneous rocks, ores, and host carbonates are determined. It is shown that in geochemical parameters leucophanite-fluorite ores are close to alkaline granites, while pyroxene-fluorite ores are close to alkaline syenites. The characteristics of the Nd isotop composition in rocks and ores of the deposit are given. Igneous rocks (granitoids and basite dikes) are characterized byNd (t) values from +4 to +5.5. TheNd (t) values in ores range from +1.2 to+4.2, in skarns itis +4.8. The host carbonates have abruptly contrasting valuesNd (t) = 4.2. Based on these data, which demonstrate a high compositional similarity between granitoids and ores, aconclusion is drawn about the leading contribution from magmatic processes to the ore mineralization of the Okunevskoe deposit.

Cretaceous ultrapotassic magmatism from the Sava-Vardar Zone of the Balkans

Late Cretaceous global plate reorganization associated with the inception of counterclockwise rotation of Africa relative to Europe initiated in the Balkan region small-volume magmatism of diverse geochemical signature along the enigmatic Sava-Vardar Zone. We study a Late Cretaceous lamprophyric sill in Ripanj village near Belgrade to constrain this magmatic episode. The lamprophyre is characterized by high contents of Na, P, Fe and Al, and low contents of K, Ca and Mg. Its original nature (Na, K, Ca and Mg) is concealed by intense alteration (albitization of feldspar and partial chloritization of phlogopite) that erased the ultrapotassic affinity of the rocks and resulted in extremely low K/Na ratios. The recalculated chemical composition demonstrates that the rocks are ultrapotassic, with K2O and MgO >3 wt % and K2O/Na2O > 2, and belong to the durbachite-vaugnerite series, i. e., the plutonic equivalents of minettes and kersantites.Two phlogopite concentrates gave Ar–Ar ages of 86.80 ± 0.5 Ma and 86.90 ± 0.5 Ma. Our combined elemental and Sr–Nd–Pb isotope data (87Sr/86Sr 0.70667–0.70677, 143Nd/144Nd 0.512426–0.512429, 206Pb/204Pb 18.82–19.13, 207Pb/204Pb 15.67–15.68, 208Pb/204Pb 38.92–39.19) for representative lamprophyric samples suggests magma derivation from a light rare earth elements (LREE) and K enriched, metasomatized mantle source. The content of LREE of the rocks is enriched, whereas heavy rare earth elements (HREE) is depleted. Rare earth elements (REE) of the whole rock and REE of diopside all indicate that garnet was present in their source.There are two viable and mutually-excluding geodynamic scenarios for the Late Cretaceous magmatism in the Balkans: (i) If the Sava-Vardar ocean still existed in the Late Cretaceous and was subducted under the European plate with arc volcanism along the Apuseni-Banat-Timok-Panagyurishte-Srednjogorje belt, coeval magmatism in the Sava-Vardar Zone occurred in a fore-arc setting, and may be related to ridge subduction; (ii) If the Mesozoic ocean closed already during the Upper Jurassic or Lower Cretaceous, the Late Cretaceous volcanism within the Sava-Vardar Zone represents intracontinental volcanism associated with transtensional tectonics.

Diversity of igneous rocks from the Isachsen Dome, Ellef Ringnes Island, Canadian High Arctic

The sedimentary deposits of the central Sverdrup Basin on Ellef Ringnes Island were affected by both salt diapirism and igneous activity of the Cretaceous High Arctic Large Igneous Province (HALIP). Occurrences of mafic igneous rocks on the centre of salt domes are usually assigned to the Cretaceous magmatism. Petrographic, geochemical, Sr–Nd isotope, and geochronological (Ar–Ar dating) studies on six rock fragments collected on the top of Isachsen Dome reveal a larger variability in the composition and the age compared to HALIP-related rocks from elsewhere on Ellef Ringnes Island. A blocky fragment of tholeiitic hornblende dolerite with an Ar–Ar age of 121 Ma is interpreted to originate from an in-situ intrusion and was uplifted by evaporite diapirism. The other samples are small, sub-rounded pebbles, which represent three different parent rock types. A kaersutite-bearing alkali basalt was possibly formed during the Triassic or Permian. A leucocratic, albitised rock fragment has experienced contact metasomatism at 102 Ma, and three very similar tholeiitic basalt fragments probably derived from the margin of a dyke were auto-hydrothermally affected at 69 Ma. The inferred source area of these exotic pebbles is the segment of the Arctic continental margin in the northeast that was repeatedly affected by igneous events between the Carboniferous and latest Cretaceous and that was uplifted and eroded during the Eocene. The rock fragments were probably deposited as Palaeogene sediments above the rising Isachsen diapir shortly before or as the head of the salt dome has pierced the Earth’s surface. Glacial transport up the top of Isachsen Dome during the Late Wisconsinan glaciation is not very likely due to the contrast between the north to northwest directed ice-flow direction reported in the literature and the assumed northeastern source area of the pebbles; however, it cannot be completely excluded.

K‐Ar Dating of Fossil Seismogenic Thrusts in the Shimanto Accretionary Complex, Southwest Japan

AbstractK‐Ar ages of clay‐sized mineral grains are used to determine the timing of activity on fossil seismogenic faults within the Cretaceous‐Paleogene Shimanto accretionary complex, southwest Japan. Samples were collected from three regional faults that separate hanging wall coherent rocks from footwall subduction mélange: the Goshikinohama Fault that caps the Yokonami mélange, the roof thrust of the Okitsu mélange, and the Nobeoka Thrust that caps the Hyuga mélange. The K‐Ar ages of fault rocks decrease with decreasing 2M1 illite polytype component, indicating a mixture of 1Md and 2M1 illite polytypes. Based on illite dating analysis, the extrapolated ages of the pure 2M1 illite polytype from the Goshikinohama Fault, the roof thrust of the Okitsu mélange, and the Nobeoka Thrust are 79.3 ± 5.0, 66.1 ± 8.1, and 46.7 ± 8.2 Ma, respectively, similar to the depositional age of each host rock. Lower intercepts of regression lines, which correspond to samples containing 100% authigenic illite, are calculated as 50.7 ± 1.4, 18.4 ± 1.2, and 24.4 ± 1.4 Ma, respectively. These ages are significantly younger than both the depositional ages and the timing of accretion. These results indicate that authigenic illite associated with fault slip is not related to underthrusting along the subduction interface but rather formed during out‐of‐sequence thrusting in the upper plate. Early Miocene slip along faults of the northern Shimanto belt is coincident with major tectonic events along the convergent margin, including collision with elements of the Izu‐Bonin volcanic arc‐backarc system, and opening of the Japan Sea.

A late Caledonian tectono-thermal event in the Gaissa Nappe Complex, Arctic Norway: evidence from fine-fraction K‒Ar dating and illite crystallinity from the Digermulen Peninsula

ABSTRACTFine-fraction K‒Ar dating and illite crystallinity determination were applied on a peculiar pale olive green shale sample from the upper Ediacaran Indreelva Member (Stáhpogieddi Formation, Vestertana Group, Gaissa Nappe Complex) of the Digermulen Peninsula in Finnmark, Arctic Norway, to constrain the age and metamorphic conditions of tectono-thermal overprint. The <2 and <0.2 µm grain-size fractions are almost purely illite and yielded an illite crystallinity (expressed as the Kübler index) of 0.215 Δ° 2θ and 0.228 ∆° 2θ and K‒Ar ages of 403.9 ± 4.2 and 391.5 ± 4.0 Ma, respectively. The K‒Ar ages are interpreted to present late-stage thermal overprint under low epizonal conditions along a localised shear zone, likely post-dating the peak of metamorphism and cleavage generation on the Digermulen Peninsula. Thus, a later tectono-metamorphic event related to the late stage of the Scandian orogeny is locally recorded in the Gaissa Nappe Complex of the Caledonides of Finnmark. This late Scandian event was probably caused by orogenic extensional collapse and appears to have extended at least into Mid-Devonian time.