40Ar-39Ar Dating Research Articles

Geochronological constraints on the Baguamiao gold deposit, West Qinling orogen, central China: Implications for ore genesis and geodynamic setting

The Baguamiao gold deposit, located in the Fengxian-Taibai ore district, is one of the largest gold deposits in the West Qinling orogen, central China. The poor age constraints on the gold mineralization event at the Baguamiao has caused strong debate on ore genesis and associated geodynamic controls. We carried out Sm-Nd and 40Ar-39Ar geochronological studies to determine more precise age constraints on the god deposition. The gold mineralization at Baguamiao deposit was developed during four hydrothermal stages that formed sequentially: (I) NW-trending ankerite-quartz veins, (II) deformed and folded NW-trending sulfide-ankerite-quartz veins, (III) joint-hosted NE-trending sulfide-ankerite-quartz veinlets, and (IV) NW- and NE-trending quartz fissure veins. Stages II and III define the main gold mineralization. The Sm-Nd dating of dolomite, ankerite, and calcite yielded isochron ages of 209.3 ± 4.2 Ma (MSWD = 1.80) and 208.1 ± 3.1 Ma (MSWD = 0.50) for gold mineralization of stages Ⅱ and III, respectively. The 40Ar-39Ar dating of muscovite intergrown with sulfides of stage Ⅱ yielded a plateau age of 209.5 ± 1.4 Ma (MSWD = 0.30). The age data obtained from the two geochronological methods are consistent with each other within uncertainties, indicating clearly the Baguamiao gold deposit formed at ca. 209 Ma. Integrating deposit geology from this and previous studies, geochronological data, and regional tectonics, we classify Baguamiao as an orogenic gold deposit, which was resulted from a number of fluid pulses but during a single ore-forming episode in late Norian-Rhaetian time of the Late Triassic. The ore-forming fluids and metals are suggested to have been derived from both metamorphic and magmatic processes in a post-collisional regime of the West Qinling orogen.

Tephrochronology and chronostratigraphy of the Miocene Chilcatay and Pisco formations (East Pisco Basin, Peru)

Strata of Chilcatay and Pisco formations exposed in the Ica Desert (East Pisco Basin, southern Peru) preserve one of the most complete and rich records of Miocene marine vertebrates of the world. Despite its exceptional importance, the chronostratigraphy of these fossil-bearing deposits has been only sporadically studied in the literature until recently. This work presents a detailed reconstruction of the chronostratigraphic framework, achieved by mapping and measuring of seven sections (totaling a height of almost 1000 m) of the Miocene Chilcatay and Pisco formations along the western side of the Ica River. The Chilcatay Formation consists of two allomembers, namely Ct1 and Ct2, bounded at the base by unconformities CE0.1 and CE0.2, respectively. Similarly, the immediately overlying Pisco Formation is divided into allomembers P0, P1, and P2, bounded at the base by unconformities PE0.0, PE0.1 and PE0.2, respectively. The new 39Ar–40Ar results presented here, combined with ages of previous work, provide precise constraints on the age of several stratigraphically referenced volcanic ash layers intercalated in the studied fossil-bearing succession, placing its vertebrate fossil fauna within a refined temporal framework and laying the solid ground for its detailed regional and global comparison. The ages of the allomembers, and thus their associated faunas, can be reliably estimated by the combination of 39Ar–40Ar dating on tephra layers with diatom biostratigraphy. In the study area, two methods are mutually consistent and constrain the deposition of the Chilcatay Formation between 19.2 and 18.0 Ma, that of P1 between 9.5 and 8.6 Ma, and that of P2 between 8.4 and 6.7 Ma. In the absence of direct dating of the P0 allomember, which lacks both preserved tephra suitable for 39Ar–40Ar dating and microfossils, its age can be constrained to the temporal gap between the youngest age available from the underlying Chilcatay strata (18.0 Ma) and the oldest age available from the overlying P1 strata (9.5 Ma).

Australasian impact crater buried under the Bolaven volcanic field, Southern Laos

The crater and proximal effects of the largest known young meteorite impact on Earth have eluded discovery for nearly a century. We present 4 lines of evidence that the 0.79-Ma impact crater of the Australasian tektites lies buried beneath lavas of a long-lived, 910-km3 volcanic field in Southern Laos: 1) Tektite geochemistry implies the presence of young, weathered basalts at the site at the time of the impact. 2) Geologic mapping and 40Ar-39Ar dates confirm that both pre- and postimpact basaltic lavas exist at the proposed impact site and that postimpact basalts wholly cover it. 3) A gravity anomaly there may also reflect the presence of a buried ∼17 × 13-km crater. 4) The nature of an outcrop of thick, crudely layered, bouldery sandstone and mudstone breccia 10-20 km from the center of the impact and fractured quartz grains within its boulder clasts support its being part of the proximal ejecta blanket.

The 40Ar–39Ar dating of biotite in ore veins and zircon U–Pb dating of porphyrtic granite dyke in the Nanyangtian tungsten deposit in SE Yunnan, China

The Nanyangtian tungsten deposit is located in the Laojunshan Sn–W polymetallic metallogenic province in SE Yunnan, China. Tectonically it is located in the conjunction of the Cathaysia and Indochina blocks. The orebodies primarily hosted within the stratiform-like skarn and marble in the Paleoproterozoic Nanyangtian formation. Three ore-forming stages have been identified based on mineral assemblages and on the intercalated relationships between the veins. They include skarn-scheelite stage, quartz-scheelite vein stage and carbonate-scheelite vein stage. In this paper, we present 40Ar–39Ar laser incremental heating analyses of hydrothermal biotite intergrowth with scheelite from quartz-scheelite vein with two yielded well-defined 40Ar–39Ar plateau ages of 97.49 ± 0.97 Ma and 99.48 ± 0.97 Ma, and two isochronal ages of 97.01 ± 1.39 Ma and 99.58 ± 0.98 Ma, respectively, representing the age of second mineralization stage of the Nanyangtian tungsten deposit. The ages are well consistent with the emplacement age of the Laojunshan granite (96 Ma to 117 Ma). U–Pb geochronological data for magmatic zircons separated from porphyritic granite dyke in the mine pit indicate that the granite body was emplaced in the post-ore stage (89.88 ± 0.53 Ma). Integrated with previously high resolution diagenetic and metallogenic geochronological data, two significant magmatic-mineralization events are presented in SE Yunnan, China and North Vietnam. The first event took place from 97 Ma to 124 Ma, which is closely related to the tungsten mineralization. The second event occurred from 75 Ma to 95 Ma, which is closely related to the Sn–Cu–Ag–Zn mineralization. According to the exhibited data, we preliminarily conclude that the Late Cretaceous magmatic-mineralization activity in SE Yunnan and North Vietnam is the product of post-collision of the Cathaysia and Indochina blocks and should have formed under a same geodynamic setting.

Structural Connections between the Urals and Western Siberia: A Common Stage of Formation at the Permian–Triassic Boundary

The 40Ar–39Ar dating of mica from schists and blastomylonites, sampled within the limits of a fault that separates the Eastern zone of the Middle Urals, sinking beneath the sedimentary cover of the West Siberian Plate, from the structures located to the west of this plate, in the exposed part of the Urals, has shown that the final phase of deformations was represented by sinistral strike-slip faults trending submeridionally with an age of 251 Ma. The deformations studied virtually exactly coincide in time with the formation of submeridionally oriented grabens in the basement of the West Siberian Plate.

Initiation and development of the Pennine Basal Thrust (Swiss Alps): a structural and geochronological study of an exhumed megathrust

The Pennine Basal Thrust (PBT) is an exhumed megathrust developed during continental collision from late Eocene to Miocene. To trace its evolution, five samples, with indications for up to three microstructurally diachronous white-mica generations, were investigated by laser in-situ and step-heating 40Ar–39Ar dating. Three deformation-related crystallization ages can be distinguished: (1) D1, characterized in the PBT hanging wall by an S1 foliation defined by white mica + chloritoid, began at or before ∼38.0 Ma; (2) D2 formed a pervasive S2 cleavage and synchronous white-mica rich veins dated at ∼27 Ma; (3) D3 produced an S3 crenulation cleavage and chlorite + white-mica veins dated at ∼23 Ma. Older ages of ∼96 Ma (footwall) and ∼115 Ma (hanging wall) are interpreted as minimum ages for the detrital component. Finally, discrete faulting produced fault gouge, with an illite K–Ar age of ∼19 Ma. A simplified back-restored reconstruction provides a tectonic context for the dated structures. In this framework, D1 occurred during middle to late Eocene tectonic accretion. After late Eocene initiation of continental collision, D2 reflects Oligocene top-to-NW shearing, with both in- and out-of-sequence thrusting. D3 then developed from 23 to 19 Ma during the progressive deactivation of the PBT.

Petrology and 40Ar-39Ar dating of paragneisses from the Devrekani Massif (Central Pontides, Northern Turkey): Implications for the Jurassic high-T metamorphism in an extensional tectonic environment

The Devrekani Massif in the northern part of the Central Pontides (Northern Turkey) provides important clues to the regional tectonics and geodynamic processes associated with Jurassic high grade metamorphic conditions. This study reports new paragenetic assemblages, mineral compositions, whole-rock geochemistry and 40Ar-39Ar geochronological data from the paragneisses in the massif, and, discusses the P–T conditions and geodynamic implications of the Jurassic metamorphism during continental extension in the Central Pontides. Upper amphibolite to lower granulite facies paragneisses form one of the main lithological units in the massif. Within these, there are five different mineral parageneses with diagnostic mineral assemblages of: quartz, K-feldspar (An0-1Ab4-26Or73-96), plagioclase (An18-35), biotite [XPhl: 0.28–0.57; Mg/(Mg + Fe2+): 0.33–0.61], sillimanite, cordierite [Mg/(Mg + Fe2+): 0.48–0.71] and garnet (Alm43-80Grs0-18Prp5-23And0-4Sps10-33) with minor hercynite. Based on Na-in-Crd thermometry and GASP barometry results, the peak metamorphic conditions are 775 ± 25 °C and 6 ± 1 kbar in the massif. The field relations, petrography and bulk chemical data suggest that the paragneisses, derived from shale-wackestone and pelitic sedimentary protoliths, are typical rock lithologies of an active continental margin. They display enrichments in LILE over HFSE, coupled with negative Nb and Ti anomalies, which are geochemical signatures of subduction-related sources. It is likely that the peak metamorphism took place during the Middle–Upper Jurassic period (ca. 174–156 Ma), suggesting that the metamorphic rocks cooled to 300–350 °C at ca. 156 Ma. The mineral assemblages reveal that the prograde history passed from sillimanite zone conditions up to the cordierite-garnet-K-feldspar zone. The petrological and geochronological data indicate that the protoliths are related to multiple sources such as volcano-sedimentary successions. We conclude that the Devrekani Massif represents the products of pre-Jurassic sedimentation, and Permo-Carboniferous continental arc magmatism, overprinted by Jurassic metamorphism.

Epigenetic Graphitization in the Basement of the Siberian Craton as Evidence of the Migration of Hydrocarbon-enriched Fluids in the Paleoproterozoic

This paper reports on diagnostic and structural studies that were first carried out for carbonaceous material of quartz–muscovite dynamoschists from the schistosity zone in biotite migmatites, pegmatites, and diabases of the southern part of the Baikal ledge of the Siberian craton. The carbonaceous material is represented by phanerocrystalline and microcrystalline graphite with residual hydrocarbon radicals. Native Ni, Sn, zincous Cu, Fe–Ni compounds, sulfides of Cu, rutile, monazite, and zircon were revealed in the intergrowths with carbonaceous material. The carbon isotopic composition ranges from –29.19‰ to –31.58‰, except for carbonaceous material from the schistocity diabase, where δ13C = –24.93‰. 40Ar–39Ar dating of muscovite gave an age of 1947 ± 7.8 Ma pointing to the relation between dynamic metamorphism with accretion of the Akitkan fold system and the ancient complexes of the craton. It was concluded that the deposition of native carbon and metals was caused by migration of essentially hydrocarbonate fluid in the formations of the upper crust (Fig. 4, Table 1).

40Ar-39Ar dating and petrology of monzonite ejecta in tephra from Quaternary Gölcük volcano (Isparta, SW Turkey): tear-related contrasting metasomatic symptoms in extensional mantle-derived magmas

Eruption of (74-26 ka old) pyroclastic (pumice fall) flow deposits of the Quaternary Gölcük volcano, Isparta, SW Turkey, carried a cargo of Western Anatolian’s youngest monzonitic-syenitic ejecta, 40Ar-39Ar hornblende ages range 313+64−268+43 ka, and indicate that a shallow level felsic plutonic complex existed underneath the crater. These ejecta represent some of the youngest plutonic rocks known. The cooling time of this complex was coeval with the earlier main pyroclastic flow deposits (440 - 148 ka) of the multi-cycle eruptive center which also had a long-lived magma chamber. The monzosyenite ejecta contain plagioclase, K-feldspar, clinopyroxene, amphibole, mica, and accessory apatite, titanite and magnetite. In terms of Nb-Y-3Ga and Nb-Y-Ce, the ejecta, with strong shoshonitic affinity, plot in the field of A1 subtype granites, and represent A1-type magma generation in a back-arc extensional setting. Low Sr and high Nd isotopic ratios of monzosyenites exhibit isotopic similarity to enriched mantle-derived Gölcük-Isparta shoshonitic-ultrapotassic magmas. REE patterns show no significant Eu anomaly, but a strong enrichment of the LREE over the HREE. In multi-element diagrams, however, all samples have negative anomalies of Nb_Ta, Zr_Hf, P and Ti and strong enrichments of Ba, Sr, U, and Th. Moreover, variable (Hf/Sm)N and low (Ta/La)N ratios reflect subduction-related metasomatism in their mantle source. Gölcük monzosyenites and shoshonitic-ultrapotassic rocks overlap with mantle-derived A1-type alkali syenites in trace element abundances. This observation suggests that not only subduction–related fluids/melts, but also intraplate sublithospheric metasomatic agents played a crucial role on their mantle source. It is contended that, due to the percolation of both sublithospheric alkali silicate- and subduction-released-melts/fluids, the metasomatized mantle-derived parental magmas may illustrate a chemical paradox, carrying geochemical imprints of genetically distinct types of mantle sources. Such a metasomatic component in continental lithosphere defined in the genesis of Gölcük monzonitic/syenitic magmas can be explained by tearing of the African oceanic slab during Miocene, where slab-derived and intraplate melts/fluids concurrently modified the lithospheric mantle. Metasomatism of wall-rock peridotites occurred prior to the generation of Gölcük magmas during Miocene slab-tearing. Plio-Quaternary Gölcük potassic magmas were possibly derived from partial melting of carbonated mantle metasomes under an extensional tectonic regime.

Ar-Ar dating of mafic dykes from the Xiazhuang uranium ore field in northern Guangdong, South China: A reevaluation of the role of mafic dyke in uranium mineralization

Ar-Ar dating of mafic dykes from the Xiazhuang uranium ore field in northern Guangdong, South China: A reevaluation of the role of mafic dyke in uranium mineralization

Geological structure of the Pre-jurassic basement of the Yugansk-Koltogorsk zone of the West Siberia

Subject of the study.We have carried out an additional exploration on petrography, petrochemistry, geochemistry, geochronology and biostratigraphy of rock complexes of the Yugansk-Koltogorsk (central part of West Siberia).Materials and methods.The materials are core samples from wells that penetrated the Pre-Jurassic basement rock complexes The used methods are as follows: X-ray spectrum microanalysis (Cameca SX 100, JEOL-733 Superprobe); the silicate analysis of the rocks (СРМ-18 and EDX-100); mass-spectroscopy with inductively coupled plasma (ICP-MS) (ELAN-9000 and Element2); zircons were analysed with a high resolution ionic microprobe SHRIMP-II (U-Pb); Ar-Ar dating was carried out by Micromass 5400; biostratigraphy, petrochemestry.Results.A geological map of Yugan-Koltogor zone of the central part of West-Siberian plate in a scale 1:500 000 was created. The map is a set of informational geological, geophysical and other layers. It was established that age of granites is Early Permian, the basalts of the rift zone began to form 268.4 ± 7.5 Ma ago (Ar-Ar). Conclusions. Volcanism in the axial rift zones of the West-Siberian megabasin basement began earlier than it is supposed before our study and significantly earlier of that of trap basalts of Siberian platform.

Geochemical signature and reservoir conditions of Early Jurassic calc-alkaline volcanic rocks from Lonco Trapial Formation, Central Patagonia

Central Patagonia is traversed by a belt of Early to Middle Jurassic calc-alkaline intermediate volcanic rocks interspersed with more felsic volcanic rocks which are associated with the widespread magmatism that took place during Gondwana break-up times. This work uses KAr and ArAr dating and whole-rock and phenocryst (plagioclase, amphibole, clinopyroxene and titanomagnetite) compositional data to refine the age, geochemical signature and reservoir conditions of these volcanic rocks, which are known as Lonco Trapial Formation. The andesites, dacites and trachydacites which were the object of this study have either amphibole or clinopyroxene as the main mafic phenocryst (amphibole-bearing and clinopyroxene-bearing volcanic rocks, respectively), though amphibole is the main mafic phase. Despite the calc-alkaline signature a mild alkaline affinity emerges from some whole-rock trace elements content and from mineral chemistry (amphibole, clinopyroxene and titanomagnetite compositions). The magmatic evolution of the Lonco Trapial andesites, dacites and trachydacites was governed by fractionation of amphibole, clinopyroxene, plagioclase, titanite, titanomagnetite and apatite. Amphibole phenocrysts show an overall normal chemical zoning. The cores of the amphiboles crystallized over a temperature range of 869–916 °C, whereas the rims crystallized over a temperature range of 826–867 °C. Shallow to intermediate depths (2–8 kbar, ∼7–26 km) were inferred from geobarometric calculations. Crystallization temperatures are slightly higher in the clinopyroxene-bearing volcanic rocks, consistent with their more primitive character. The geobarometric estimations of this work are coherent with the lack of marine ingressions and with geophysical estimations which suggest that the Early Jurassic Moho depth would have been ≥35 km The combination of whole-rock and mineral geochemistry is consistent with an extensional affinity for this paleo-volcanic belt.

Geochemical and geochronological evidence for a Middle Permian oceanic plateau fragment in the Paleo-Tethyan suture zone of NE Iran

The mafic–ultramafic Fariman complex in northeastern Iran has been interpreted as a Paleo-Tethyan ophiolitic fragment with subduction- and plume-related characteristics as well as a basin deposit on an active continental margin. Contributing to this issue, we present geochemical, geochronological, and mineralogical data for transitional and tholeiitic basalts. Thermodynamic modeling suggests picritic parental magmas with 16–21 wt% MgO formed at plume-like mantle potential temperatures of ca. 1460–1600 °C. Rare pyroxene spinifex textures and skeletal to feather-like clinopyroxene attest to crystallization from undercooled magma and high cooling rates. Chromium numbers and TiO2 concentrations in spinel are similar to those in intraplate basalts. 40Ar–39Ar dating of magmatic hornblende yielded a plateau age of 276 ± 4 Ma (2σ). Transitional basalt with OIB-like trace element characteristics is the predominant rock-type; less frequent are tholeiitic basalts with mildly LREE depleted patterns and picrites with intermediate trace element characteristics. All samples show MORB-OIB like Pb/Ce, Th/La, and Th/Nb ratios which preclude subduction-modified mantle sources and felsic crustal material. Tholeiitic basalts and related olivine cumulate rocks show MORB-like initial eNd values of + 9.4 to + 6.2 which define a mixing line with the data for the transitional basalts (eNd ca. + 2.6). Initial 187Os/188Os ratios of 0.124–0.293 support mixed sources with a high proportion of recycled mafic crust in the transitional basalts. High concentrations of highly siderophile elements are in agreement with the high mantle potential temperatures and inferred high-melting degrees. It is argued that the Fariman complex originated by melting of a mantle plume component as represented by the OIB-like transitional basalt and entrained asthenosphere predominant in the MORB-like tholeiites. Two lines of evidence such as association of the Fariman complex with pelagic to neritic sedimentary rocks and the tectonic position at the boundary of two continental blocks defined by ophiolites and accretionary complexes of different ages suggest formation in an oceanic domain. Thus, we interpret it as a fragment of an oceanic plateau, which escaped subduction and was accreted as exotic block in the Paleo-Tethyan suture zone.

Geologic structure of the Mukodek gold field (northern Transbaikalia) and sources of matter (Pb-Pb and Sm-Nd data)

The Mukodek gold field is located in the Yana zone of the Baikal-Muya fold belt (northern Transbaikalia). Its orebodies are gold-bearing metasomatites confined to the fault of NE strike. It is shown that pre-ore faults played a crucial role in the spatial localization of the orebodies, as they favored the concentration of ore-bearing fluids. Results of geological and structural studies, along with available geochronological data (40Ar-39Ar dates for micas in gold-bearing pyrite), testify to ore-forming processes in the ore field during two Late Paleozoic stages. Early gold-bearing metasomatites of sericite-chlorite-albite-ankerite-quartz composition formed at 320 Ma, and late gold-bearing vein metasomatites of ankerite(calcite)-quartz composition originated at 285 Ma. A Pb-Pb isotope study of gold mineralization has shown that ore lead was supplied into the mineral-forming system mainly from a mantle source, most likely, regional Neoproterozoic igneous rocks. According to Sm-Nd isotope data, these rocks formed with the participation of a material of mantle genesis.

Fluid charging and hydrocarbon accumulation in the Cambrian Longwangmiao Formation of Moxi Structure, Sichuan Basin, SW China

Abstract The multi-stage minerals filled in pore space were sequenced, and the charging stages of fluid and hydrocarbon were reconstructed based on the observation of drilling cores and thin sections, homogeneous temperature testing of fluid inclusions, Laser Raman composition analysis and isotope geochemical analysis. The Cambrian Longwangmiao Formation in the study area went through 5 stages of fluid charging, in which 3 stages, mid-late Triassic, early-mid Jurassic and early-mid Cretaceous, were related to oil and gas charging. Especially the oil and gas charging event in early-mid Cretaceous was the critical period of gas accumulation in the study area, and was recorded by methane gas inclusions in the late stage quartz fillings. The 40Ar-39Ar dating of the 3rd stage methane inclusions shows that the natural gas charging of this stage was from 125.8±8.2 Ma. Analysis of Si, O isotopes and 87Sr/86Sr of the late stage quartz indicates that the fluid source of the quartz was formation water coming from long term evolution and concentration of meteoric water, but not from deep part or other sources, this also reflects that, in the critical charging period of natural gas, the Cambrian Longwangmiao Formation in Moxi structure had favorable conservation conditions for hydrocarbon accumulation, which was favorable for the formation of the Longwangmiao large natural gas pool.

Dating Jurassic volcanic rocks in the Western Hills of Beijing, North China: Implications for the initiation of the Yanshanian tectonism and subsequent thermal events

The “Yanshanian movement” (tectonism) was first recognized in the Western Hills of Beijing and represents the most intense Mesozoic tectonic event in eastern China. Although many researches have been conducted since its identification in 1927, the timing for initiation of the Yanshanian tectonism has not been well constrained. The Jurassic sedimentary succession in the Western Hills of Beijing consists of two volcanic units and interbedded coal-bearing strata, which is similar to those in other regions of the Yanshan Fold-and-Thrust Belt (YFTB). As the earliest Mesozoic volcanism in the Western Hills of Beijing, the Nandaling Formation contains the earliest volcanic and sedimentary records that are associated with the initiation of the Yanshanian tectonism. Previous researches considered the Nandaling volcanic rocks as Early Jurassic in age based on the floral fossil assemblage of Coniopteris–Phoenicopsis in the Yaopo Formation that conformably overlies the Nandaling Formation. In this study we perform a systematic geological survey and present the results of zircon U–Pb dating of the Nandaling volcanic rocks to demonstrate that both the Nandaling volcanic rocks and the overlying floral fossil assemblage are Middle Jurassic in age. LA–ICP–MS zircon U–Pb dating of the Nandaling volcanic rocks in the northern part of the basin yields eruption ages of 175 ± 1 to 168 ± 2 Ma. Dioritic porphyry sills are intruded concordantly into folded Mesoproterozoic–lower Paleozoic strata of the Qingbaikou anticline, and yield a zircon U–Pb age of 177 ± 1 Ma. These ages indicate that the initiation of the Yanshanian basins, and therefore the initiation of the Yanshanian tectonism, occurred no earlier than the Middle Jurassic. Our 40Ar–39Ar dating of the Nandaling volcanic rocks yields younger ages of 138–151 Ma that show a southward-younging trend, indicating that the Nandaling volcanic rocks in the Western Hills of Beijing were overprinted by a tectono-thermal event that peaked during the Early Cretaceous.

Felsic-intermediate magmatism and brittle deformation in Sierra del Tzirate (Michoacán-Guanajuato Volcanic Field)

The Tzirate volcanic complex (TVC) is located in the central part of the Michoacán-Guanajuato volcanic field (MGVF), central Mexico. It is characterized by a series of andesitic-dacitic domes and by active faulting of the Morelia-Acambay fault system. Its age and the relationship between the magmatic processes that generated the TVC rocks and the evolution of the local stress field were determined with field work, structural analysis, 40Ar-39Ar dating, geochemistry and petrography of the lavas. The dacitic volcanism of the TVC occurred during the Pleistocene (1 Ma) with the eruption of lavas that formed a series of domes (SiO2 = 63–69 wt.%) aligned in NNE-SSW and NW-SE directions. The area is covered by andesitic-basaltic cinder-lava cones and andesitic lava cones of Pleistocene-Holocene age (2.5 Ma – 0.011 Ma) produced by monogenetic volcanism of the MGVF that surrounds the TVC.The TVC magmas produced rocks that can be classified in four mineral groups. One group contains olivine and pyroxene phenocrysts. The second biotite and amphibole euhedral phenocryst, and the other two contain biotite euhedral phenocryst, amphibole microphenocrysts and biotite subhedral crystals surrounded by a rim of plagioclase microcrysts. The diversity of textures in biotite crystals and the linear trends of major and trace elements between La Huacana granite and the less evolved lavas of the TVC suggest that assimilation of a granitic body in the crust might have influenced the evolution of the TVC rocks.The results of the structural analysis show that when the minimum principal stress was vertically oriented the rise of magma to the surface is hindered, favoring the entrapment of magma in shallow magma chambers. The magmatic assimilation process possibly occurred during the same phase, forming the felsic magmas. Subsequently, the local stress field became extensional, and the vertical principal maximum stress developed normal or extensional faults that affected the magma chamber and served as plumbing system for the rise of felsic lava.

40Ar-39Ar dating of Archean iron oxide Cu-Au and Paleoproterozoic granite-related Cu-Au deposits in the Carajás Mineral Province, Brazil: implications for genetic models

40Ar-39Ar dating of biotite from IOCG and granite-related Cu-Au deposits in the Carajas Mineral Province provides evidence for the timing of mineralization and constraints on genetic models of ore formation. Ages of biotite from greisen and quartz-rich vein and breccia deposits, Alvo 118—1885 ± 4 Ma, Breves—1886 ± 5 Ma, Estrela—1896 ± 7 Ma, and Gameleira—1908 ± 7 Ma, demonstrate the close temporal relationship between Cu-Au mineralization and subjacent A-type granites. Mineralization is hosted within granite cupolas (Breves) or in vein/breccia systems emanating from the cupolas (Estrela and Gameleira), consistent with a genetic relationship of mineralization to the B-Li-F-rich granites. Plateau and minimum ages of biotite from IOCG deposits, including Igarape Bahia, Cristalino, Corta Goela, and GT34, range from 2537 ± 6 Ma to 2193 ± 4 Ma. The 40Ar-39Ar age of biotite from Igarape Bahia (2537 ± 6 Ma) is similar to a previous SHRIMP 207Pb-206Pb age for monazite of 2575 ± 12 Ma when the uncertainties in the respective analyses and standards are taken into account. The age spectrum for biotite from Cristalino shows increasing ages for successive steps, consistent with post-crystallization Ar loss, and the age of 2388 ± 5 Ma for the last three steps is considered a minimum age for Cu-Au mineralization. The age of biotite from the GT34 prospect (2512 ± 7 Ma) coincides with a previously identified period of basement reactivation and may indicate the formation of Cu-Au mineralization at this time or resetting of biotite from an older mineralization event at this time. At Corta Goela, within the Canaa Shear Zone, the biotite age of 2193 ± 4 Ma lies between the ages of IOCG (2.57–2.76 Ga) and granite-related Cu-Au (~ 1.88 Ga) deposits elsewhere in the Carajas district but is similar to previously reported 40Ar-39Ar ages for amphibole from Sossego, possibly indicating that mineralization at both Sossego and Corta Goela was affected by a thermal event at this time. The Paleoproterozoic Cu-Au deposits are commonly hosted within Neoarchean IOCG alteration systems and the common occurrence of potassic alteration (especially biotite) in both types of deposits means that special care is required in interpreting the paragenesis of alteration in both types of deposits. The Paleoproterozoic Cu-Au deposits are reduced, and sulfur- and quartz-rich deposits lacking in major amounts of iron oxides and are therefore unlike IOCG deposits. Instead, they share many characteristics in common with widespread Paleoproterozoic Sn-W deposits in the Amazon Craton, including close spatial and temporal relationships with reduced A-type B-Li-F granites, and the occurrence of greisen and quartz-rich vein/breccia systems within and above granite cupolas. The occurrence of sericitic alteration in the Paleoproterozoic Cu-Au deposits is not evidence for an upward transition to sericitic alteration in IOCG deposits in the Carajas Mineral Province.

The Hydrothermal Breccia of Berglia-Glassberget, Trøndelag, Norway: Snapshot of a Triassic Earthquake

The quartz-K-feldspar-cemented breccia of Berglia-Glassberget in the Lierne municipality in central Norway forms an ellipsoid structure 250 m × 500 m in size. The hydrothermal breccia is barren in terms of economic commodities but famous among mineral collectors for being a large and rich site of crystal quartz of various colours and habits. Despite being a famous collector site, the mineralization is rather unique in respect to its geological setting. It occurs within Late Palaeoproterozoic metarhyolites of the Lower Allochthon of the Norwegian Caledonides regionally isolated from any other contemporaneous hydrothermal or magmatic event. In order to understand better the formation of the Berglia-Glassberget breccia, the chemistry, fluid inclusion petrography and age of the breccia cement were determined. Structural features indicate that the Berglia-Glassberget is a fault-related, fluid-assisted, hydraulic breccia which formed by single pulse stress released by a seismic event. 40Ar-39Ar dating of K-feldspar cement revealed a middle Triassic age (240.3 ± 0.4 Ma) for this event. The influx into the fault zone of an aqueous CO2-bearing fluid triggered the sudden fault movement. The high percentage of open space in the breccia fractures with cavities up 3 m × 3 m × 4 m in size, fluid inclusion microthermometry, and trace element chemistry of quartz suggests that the breccia was formed at depths between 4 and 0.5 km (1.1 to 0.1 kbar). The origin of the breccia-cementing, CO2-bearing Na-HCO3-SO4 fluid may have been predominantly of metamorphic origin due to decarbonation reactions (T > 200 °C) of limestones of the underlying Olden Nappe. The decarbonation reactions were initiated by deeply derived, hot fluids channelled to sub-surface levels by a major fault zone, implying that the breccia is situated on a deep-seated structure. Regionally, the Berglia-Glassberget occurs at a supposed triple junction of long-lived fault zones belonging to the Møre-Trøndelag, Lærdal-Gjende and the Kollstraumen fault complexes. These fault systems and the associated Berglia-Glassberget earthquake are the expression of rifting and faulting in northern Europe during the middle/late Triassic.

Elmar K. Jessberger (1943–2017)

Elmar K. Jessberger (1943–2017)