Miocene Age Research Articles

Destructive episodes and morphological rejuvenation during the lifecycles of tectonically active seamounts: Insights from the Gorringe Bank in the NE Atlantic

Seamounts are spectacular bathymetric features common within volcanic and tectonically active continental margins. During their lifecycles, they evolve through stages of construction and destruction. The latter are marked by variable magnitude flank collapses that often interrupt the evolution of seamounts and constitute a major source of hazard. The Southwest Iberian Margin is a tectonically complex region with moderate to high seismicity where numerous seamounts occur. On such a setting, earthquake-triggered collapses on seamount flanks are common, leading to the deposition of Mass-Transport Deposits (MTDs) on the surrounding abyssal plains. Using a wealth of 2D seismic reflection profiles, we investigate the lifecycle of the tectonic Gorringe Bank (GB), the largest submarine seamount offshore European margins, based on the magnitude and recurrence patterns of MTDs along the active thrust flank. Eight MTDs with relevant expression on the seismic data were analysed, four of estimated Miocene age and four on a Pliocene-Quaternary interval. Miocene MTDs are overall larger and correlate with the main uplift stages of the GB structure. Their distribution and relative timing suggest that failure-triggering earthquakes were common along the whole length of the GB. Pliocene to Quaternary MTDs tend to cluster along the northern half of the GB flank and are generally smaller. Overall, the size and magnitude of the mass-waste events is directly relatable to the magnitude of the uplift stages. Given the lack of indicators of large MTDs on the modern GB morphology, we propose that the lifecycle of tectonic seamounts is marked by morphological rejuvenation driven by thrust fault activity in between major collapse events or cycles. Tectonic-driven rejuvenation is thus key to hinder or obliterate evidence of past high-magnitude destructive events on tectonic seamount morphology.

Anatomical characteristics of fossil wood collected from the Manchar Formation (Miocene), Thano Bula Khan, Sindh, Pakistan

The characterization of petrified wood provides valuable information about paleoclimatology and geological history and helps to reconstruct the past forest flora of different parts of the earth. The present study was undertaken to evaluate the anatomical characteristics of fossil wood of the Miocene age collected from the Manchar Formation exposed at Thano Bula Khan, Sindh, Pakistan. In order to carry out a detailed anatomical investigation, three-dimensional sections were prepared using a petrotome. The microscopic analysis allowed us to study vessel size and arrangement, wood parenchyma, fibers, and xylem rays. Based on the comparison between recent and fossil wood, we concluded that the investigated characters are comparable with those of the genus Atalantia Corrêa of the Rutaceae family. Therefore, it was named as Atalantioxylon thanobolensis sp. nov. with reference to the location of Thano Bula Khan from which the fossil wood was collected.

Palaeoentomological study of the lacustrine oil-shales of the lower Miocene San Chils locality (Ribesalbes-Alcora Basin, Castellón province, Spain)

For the first time, the study of the fossil record present in laminated bituminous dolostones from the San Chils locality, lower Miocene in age (ca. 19 Ma), located at the Ribesalbes-Alcora Basin, is addressed. The identification of 11 families and three genera belonging to six insect orders (Orthoptera, Thysanoptera, Hemiptera, Coleoptera, Hymenoptera and Diptera) has been carried out. Furthermore, ephippia of ladoceran crustaceans, one bird feather, vertebrate coprolites and plant remains have been found. The San Chils palaeodiversity has been compared to that of La Rinconada, a similar outcrop in the same basin and depositional unit. Both localities correspond to lacustrine deposits and were mined during the past two centuries. This work has increased the palaeontological richness of the Ribesalbes-Alcora Basin with new fi ndings such as specimens of the subfamily Corixinae (Hemiptera: Corixidae), some ant workers (Hymenoptera: ormicidae), a specimen assigned to the genus Exechia (Diptera: Mycetophilidae), ephippia of Daphnia (Ctenodapnia ) sp. (Branchiopoda: Cladocera), the possible first report of the genus Sciocoris (Hemiptera: Pentatomidae) for the Neogene fossil record and the possible presence of the family Aeolothripidae (Thysanoptera). The current collection of fossil specimens from San Chils is small, but shows that it shared similar palaeoenvironmental conditions to that of La Rinconada, both in the lake waters and in the surroundings.

The first representative of the trap-jaw ant genus Anochetus Mayr, 1861 in Neogene amber from Sumatra(Hymenoptera: Formicidae)

The ponerine ant Anochetus miosumatrensis Ngô-Muller, Garrouste & Nel, n. sp. is described from a fossil alate female preserved in amber of Sumatra which is reputedly of Miocene age. On the basis of the general morphology, the fossil could be attributed to the extant Sumatran species group risii Brown, 1978. By comparing with the living environment of the extant species, this ant probably lived in a warm humid forest where it was trapped in dipterocarpacean resin during nuptial flight. Until now, the known Cenozoic distribution of the genus Anochetus was restricted to the Neotropical region. Thus A. miosumatrensis Ngô-Muller, Garrouste & Nel, n. sp. brings the first record of the genus from Indomalaya biogeographic region.

First Miocene rodents from Kutch, western India

ABSTRACT Following recent work on the Miocene terrestrial vertebrate fauna of Kutch, we focused on bulk wet screening for microfossils from beds exposed near Tapar (=Tappar) and Pasuda villages in central Kutch (Gujarat), western India. Processing >1500 kg of bulk samples, led to the recovery of diverse rodent remains (nine species, mostly sciurids, rhizomyids, cricetids, and murids) as well as a vespertillionid bat and a possible lorisid for the first time from the Miocene succession of Kutch. This complements the previously described large and megamammal fossils of rhinos, anthracotheres, proboscideans, and ruminants from the same sections. Isolated rodent teeth represent taxa of generally short time ranges and wide distribution that assume considerable biostratigraphic significance. The rodents are similar to contemporaneous species to the north in India and to the west in Pakistan, including the Potwar Plateau. Based on the rodent assemblage, early Late Miocene (>10 Ma) age is assigned to the Tapar and Pasuda localities. The Kutch sample provides a new reference point for detailed comparisons with the better studied Manchar and Siwalik faunas of South Asia but also with broadly contemporaneous faunas from the Murree, Dagshai/Kasauli, and Dharamsala beds of the Outer Himalaya, and the Kargil molasse of Ladakh.

Early Miocene small mammals from MWQ1/2001 Turtle Joint (Mokrá-Quarry, South Moravia, Czech Republic): biostratigraphical and palaeoecological considerations

Mokra-Quarry (South Moravia, Czech Republic) represents a unique fossil site with an extraordinary abundance and diversity of vertebrate fossil remains. Most research on Mokra-Quarry localities was focused on herpetofauna. Despite its relevance, the mammal fauna has not been yet studied in detail. In this work, the small mammals from one of the karstic fissures, Mokra´-Western Quarry (1/2001 Turtle Joint), are thoroughly described for the first time, including eight different taxa: Prolagus schnaitheimensis, Prolagus cf. vasconinensis, Rhinolophus cf. cluzeli, Rhinolophus cf. grivensis, Galerix sp., Aliveria aff. luteyni, Megacricetodon sp., and Melissiodon dominans. The Megacricetodon finds represent one of the first appearances of this cricetid in Central Europe and sheds light on the early evolution of the genus. Moreover, the small mammal assemblage confirms an early Miocene age (Burdigalian, MN4) for MWQ1/2001. The paleoenvironment inferred shows a dry karst landscape, with patches of woodlands and open steppe, together with marshy areas.

Trace element composition and U-Pb ages of cassiterite from the Bolivian tin belt

The Bolivian tin belt is a metallogenic province in the Eastern Cordillera of the Andes known for its Sn, W, Ag, and base metal deposits. Cassiterite, which is a major constituent in many magmatic-hydrothermal ore deposits from the Bolivian tin belt, can incorporate dozens of elements within its crystal lattice, making it a useful geological tracer mineral and also a potential host of critical elements. New U-Pb dating of cassiterite yields Late Triassic (Kellhuani deposit) and Late Oligocene to earliest Miocene (Viloco, Huanuni, and Llallagua deposits) ages. These ages confirm that Sn mineralization in the Bolivian tin belt occurred at least in two separate events during two major magmatic episodes apparently triggered by mantle upwelling, decompression melting, and basalt production promoting high heat flow into the overlying crust. The composition of studied hydrothermal cassiterite yields some geochemical trends that are attributed to its distance to the causative intrusion and/or level of emplacement. For example, cassiterite is generally enriched in Nb and Ta and yields higher Ti/Zr and Ti/Sc ratios in samples from xenothermal ore deposits located adjacent to intrusive complexes relative to shallow xenothermal and epithermal ore deposits. Therefore, these geochemical trends in cassiterite are useful tracers pointing to magmatic-hydrothermal centers. REE distribution in cassiterite was likely influenced by boiling processes, which resulted in tetrad-type irregularities. Cassiterite from the Bolivian tin belt is unattractive as a source for Nb (interquartile range [IQR] 4.84–0.037 ppm), Ta (IQR 0.0924–0.0126 ppm), and Ge (IQR 3.92–0.776 ppm). Some deposits, however, contain cassiterite relatively enriched in In (IQR 96.9–9.78 ppm, up to 1414 ppm) and Ga (IQR 92.1–3.03, up to 7437 ppm), that could constitute an attractive supplementary source for these elements in addition to sulfide minerals in the same deposits.

Chapter 5.3a Mount Early and Sheridan Bluff: volcanology

Abstract Two small monogenetic volcanoes are exposed at Mount Early and Sheridan Bluff, in the upper reaches of Scott Glacier. In addition, the presence of abundant fresh volcanic detritus in moraines at two other localities suggests further associated volcanism, now obscured by the modern Antarctic ice sheet. One of those occurrences has been attributed to a small subglacial volcano only c. 200 km from South Pole, making it the southernmost volcano in the world. All of the volcanic outcrops in the Scott Glacier region are grouped in a newly defined Upper Scott Glacier Volcanic Field, which is part of the McMurdo Volcanic Group (Western Ross Supergroup). The volcanism is early Miocene in age ( c. 25–16 Ma), and the combination of tholeiitic and alkaline mafic compositions differs from the more voluminous alkaline volcanism in the West Antarctic Rift System. The Mount Early volcano was erupted subglacially, when the contemporary ice was considerably thicker than present. By contrast, lithologies associated with the southernmost volcano, currently covered by 1.5 km of modern ice, indicate that it was erupted when any associated ice was either much thinner or absent. The eruptive setting for Sheridan Bluff is uncertain and is still being investigated.

Biostratigraphy and paleoenvironments of the Pelotas Basin, southernmost Brazil, using Miocene dinoflagellate cysts

Dinoflagellate cysts assigned to the subfamily Protoperidinioideae are frequently recorded in Miocene coastal deposits, and their relative abundance is directly related to primary productivity and favorable post depositional conditions. However, these microfossils are poorly studied in Brazilian Miocene sequences. This paper presents the results of a palynological study on 18 samples obtained from the core CBM001-ST-RS drilled in the coastal plain of southernmost Brazil, related to the Cidreira Formation, Pelotas Basin. Forty-nine taxa are identified, 46 dinoflagellate cysts, two genera of chlorophyte algae, and one acritarch species. Overall, the samples are characterized by large numbers of Spiniferites spp. and Brigantedinium spp., as well as a significant richness of the subfamily Protoperidinioideae. Several species of dinoflagellate cysts indicate a Miocene age, but the record of Sumatradinium hamulatum and Quadrina? condita restricts the age of the interval analyzed to the middle Miocene. Both the PMI and the P/G palynological indices indicate a neritic marine environment and moderate primary productivity, respectively.

Tight oil exploration in the Onnagawa formation, Miocene biosiliceous rocks of Japan: what we have learned and its implication for Monterey-like siliceous rocks in the Circum-North Pacific

We re-discovered tight/shale oil accumulations through a successful acid treatment in the Onnagawa Formation, the main petroleum source rock in Japan, which is Monterey-like biosiliceous shale of the Middle Miocene age. Its productivity is fairly high, comparable with that of the Bakken shale, and is supported by matrix pores of quartz-porcelanite and carbonate-cemented natural fractures which is stimulated effectively through acidizing. The tight oil horizon corresponds to the best source and the best reservoir rocks. It is the most siliceous part of the formation, which is reasonably understood as a result of an increase in primary productivity with diatom blooms in the Miocene age. We think the tight oil is generated in situ and a part of continuous accumulation, while it is in a structural trap so far. The above exploration concept will be applicable to the Monterey Formation of California and the Dae-Khurie Formation of Sakhalin, which are siliceous rocks of same type, and re-evaluation of non-commercial hydrocarbon flows or shows in exploratory wells is recommended.

Fault-controlling safe slope design procedure in an open-pit mine case study: Tunçbilek-Kütahya (Turkey) coal field

Miocene age coaly Tuncbilek basin (Kutahya/Turkey), which is named as “Tuncbilek Series”, is located at nearly the top level of the continental sequence. Lignite coal vein is bedded nearly at the base of the M2 unit consisting of claystone-marl interlayers. In the basin, the lignite coal vein with low dip angle (10‑30°) has been operated through open-pit and underground mining operations since the 1920s. At Y2-panel where the coal will be operated with the open-pit mining technique, firstly overburden material is removed by establishing benches, the coal bed is opened up, and then the coal is taken from the panel. Different panel geometries in the field are prepared for coal production. This research aims to predict further possible debris slides which may develop due to existing faults with high dip before excavation works in open-pit mine sites and to prevent land instabilities by changing the preplanned positions of the slope portions with the help of kinematic analyses. In order to safely design the overall slopes of the Y2 panel of Tuncbilek coal field, the relationships between the slopes and existing faults which may cause debris slides and those between the slopes and bedding planes and joints were kinematically investigated using the stereographic projection techniques before the excavations. Debris slides were determined in the slopes of A, C, H and L, wedge-type failures were determined in the slopes of B and C, and toppling failure was determined in the combined slope of E and F. The results obtained from these analyses suggest that the preplanned slope positions against probable slope instabilities should be changed to N5E/45NW for slope B, N36W/40SW for slope C, N80E/45SE for slope D, N20E/45SE for slopes E and F, N40W/45NE for slope G, N50W/45NE for slope H and N40W/45NE for slope L during the excavation phase.

Chemo-stratigraphy, petrology and U-Pb geochronology of South-eastern part of the Yuntdağ volcano (Karakılıçlı – Manisa) in Western Anatolia

There are a number of volcanic eruption centres that form large caldera structures, and possibly feeding from relatively large magma chambers during the Miocene period in Western Anatolia. One of them is Yuntdağ volcano, and its products have formed a thick volcanic succession covering large areas. In this study, an area where the volcanic sequence is seen well in the south-eastern part of the volcano was selected. Geochemistry and geochronology analyses were carried out along a line between Kalpakkaya and Çamlık Hills in order to better understand the petrological evolution of melting, assimilation, storage and homogenisation processes in the magma chamber (and/or chambers) feeding the volcano within a limited time period (~4 Ma). Samples acquired from the bottom, middle and upper portions of these sections display Early-Middle Miocene ages of 17.64 ± 0.20, 17.22 ± 0.15, 16.16 ± 0.17 and 16.36 ± 0.13, 15.79 ± 0.71 and 13.61 ± 0.20 Ma, respectively. Although, felsic crystal melts, magma mixing and multiple mafic inputs were the dominant processes, fractional crystallisation was less efficient. Also, geochemistry data also indicate that the studied volcanic rocks generated by the melting of the lithospheric mantle, by slab retreat and roll-back of the Hellenic – Cyprus slab, and evolved in shallow magma chambers during the Early – Middle Miocene.

Comment on ‘A Miocene age for the Molo Formation, Norwegian Sea shelf off Vestfjorden, based on marine palynology’

Comment on ‘A Miocene age for the Molo Formation, Norwegian Sea shelf off Vestfjorden, based on marine palynology’

Planktonic foraminiferal biostratigraphy of the Limboto Limestone, Gorontalo Province, Indonesia

The limestone research within the Limboto Basin of Gorontalo Province becomes a new challenge, particularly for the study of planktonic foraminiferal biostratigraphy. This study uses the data obtained from the measured section in the north-western part of Limboto Lake. The purpose of this study is to determine the planktonic foraminiferal biozonation and the relative age of Limboto limestones. The analyzed planktonic foraminiferal fossils can be classified as well to moderately preserved of various species, in the context of abundance, categorized as frequent to abundant. There are three recognized planktonic foraminiferal biozones, i.e., two biozones for Miocene age (M13b and M14) and one biozone for Pliocene age (PL1). The Miocene biozones are named as Globorotalia plesiotumida partial range zone (M13b) and Pulleniatina primalis-Globoquadrina dehiscens concurrent range zone (M14), while the name of Pliocene biozone is Globorotalia acostaensis partial range zone (PL1). The results of this study can be a reference to propose an age of Limboto Limestone Formation. Identification and demarcation of the Limboto Limestone Formation are based on the time interval and relative age of the formation based on planktonic foraminifera.

When parasites persist: tapeworms survive host extinction and reveal waves of dispersal across Beringia.

Investigations of intercontinental dispersal between Asia and North America reveal complex patterns of geographic expansion, retraction and isolation, yet historical reconstructions are largely limited by the depth of the record that is retained in patterns of extant diversity. Parasites offer a tool for recovering deep historical insights about the biosphere, improving the resolution of past community-level interactions. We explored biogeographic hypotheses regarding the history of dispersal across Beringia, the region intermittently linking Asia and North America, through large-scale multi-locus phylogenetic analyses of the genus Schizorchis, an assemblage of host-specific cestodes in pikas (Lagomorpha: Ochotonidae). Our genetic data support palaeontological evidence for two separate geographic expansions into North America by Ochotona in the late Tertiary, a history that genomic evidence from extant pikas does not record. Pikas descending from the first colonization of Miocene age persisted into the Pliocene, subsequently coming into contact with a second wave of Nearctic colonists from Eurasia before going extinct. Spatial and temporal overlap of historically independent pika populations provided a window for host colonization, allowing persistence of an early parasite lineage in the contemporary fauna following the extinction of its ancestral hosts. Empirical evidence for ancient 'ghost assemblages' of hosts and parasites demonstrates how complex mosaic faunas are assembled in the biosphere through episodes of faunal mixing encompassing parasite lineages across deep and shallow time.

Age and associated stress field of middle Miocene back‐arc basalt magmatism in Northeast Japan

AbstractA large volume of middle Miocene basaltic rocks is widely distributed across the back‐arc region of Northeast Japan, including around the Dewa Mountains. Petrological research has shown that basaltic rocks of the Aosawa Formation around the Dewa Mountains were generated as a result of the opening of the Sea of Japan. To determine the precise ages of the middle Miocene basaltic magmatism, we conducted U–Pb and fission‐track (FT) dating of a rhyolite lava that constitutes the uppermost part of the Aosawa Formation. In addition, we estimated the paleostress field of the volcanism using data from a basaltic dike swarm in the same formation. The rhyolite lava yields a U–Pb age of 10.73 ±0.22 Ma (2σ) and a FT age of 10.6 ±1.6 Ma (2σ), and the paleostress analysis suggests a normal‐faulting stress regime with a NW–SE‐trending σ3‐axis, a relatively high stress ratio, and a relatively high magma pressure. Our results show that the late Aosawa magmatism occurred under NW–SE extensional stress and ended at ~ 11 Ma.

Limestone Facies Change of Jonggrangan To Sentolo Formation in The Western Part of Yogyakarta-Central Java Basin

The research location is in the western of Yogyakarta-Central Java Basin, where limestone of Jonggrangan and Sentolo Formation are found with the same age but have different depositional facies characteristics. These differences will be interesting to study the historical process of its formation. The research was conducted through a series of methods. Starting from geological mapping to determine the order of rock stratigraphy, the depiction of outcrop profiles, and limestone sampling for petrographic analysis. Eventually, the depositional environment of the two rock formations was discovered. Field observation results and laboratory analysis were further integrated to determine the rock facies that make up the two formations. The rock units in the study area from old to young are the pyroxene andesite breccia unit of Upper Oligocene age and limestone. The bioclastic limestone and layered limestone units are deposited by interfingering, unconformity over the pyroxene andesite breccia unit. The Middle Miocene bioclastic limestone unit is equivalent to Jonggrangan Formation in the inner shelf environment (Restricted Lagoon and Open Lagoon facies environment). Whereas layered limestone of Middle Miocene age is equivalent to the Sentolo Formation in the lower slope environment (Restricted Lagoon, Open Lagoon, and Open Sea Shelf) facies.

A New Age Interpretation For The Meluhu Formation In Toronipa Peninsula, The Southeast Arm Of Sulawesi, Indonesia

The offshore area in the northeast of Kendari city, the southeast arm of Sulawesi, is an area with favourable hydrocarbon prospectivity shown by numerous oil and gas seeps in the surrounding coastal area. It is a frontier basin in eastern Indonesia, known as the Manui Basin. An exploration well named Abuki-1 was drilled in 1990 suggested a Miocene transgressive sequence as a potential reservoir and source rock at this basin. However, this unit has no analogous exposure in the onshore area resulting in the lack of study and knowledge of this potential Miocene unit. Therefore, we revisit the sedimentary rocks exposure nearby Abuki-1 well in the Toronipa peninsula to study about its sedimentary facies and palynological contents. These outcrops by previous researchers were included in Toronipa Member of Meluhu Formation, and a Triassic age was suggested for this unit. By contrast, our result shows that these exposures are Middle to Late Miocene in age as indicated by the occurrence of the Florschuetzia group pollens (Florschuetzia trilobata, F. levipoli, and F. meridionalis). The absence of Plio-Pleistocene pollen and spores index fossils (Stenochlaena milnei group, Dacrycarpus imbricatus, and Phyllocladus) supports the Middle to Late Miocene age interpretation. A wave-dominated estuary depositional model is proposed based on the presence of river-dominated, mixed-energy, and wave-dominated facies associations. We suggest that the studied sediments are the outcrop analogues for the Middle to Late Miocene transgressive sequence found in Abuki-1 well. Furthermore, we recommend that these Miocene estuary-fill complexes should have excellent hydrocarbon potential. The reservoir potential is the sand deposits of the fluvial, tidal, washover, and shoreface facies with moderately to well-sorted characteristics. The source rocks candidate is the mud of lagoon, tidal flat, floodplain, and marine offshore facies. Moreover, the Manui Basin, with its Miocene estuarine deposits, requires further study to reveal its hydrocarbon accumulation potential.

Geology, geochemistry and geochronology of Lindero porphyry gold deposit in the Southern Puna plateau, Argentina

The Lindero gold deposit is located in the Southern Puna plateau, northwest Argentina. The deposit is centered in a cluster of six subvolcanic intrusions emplaced at the margin of the Arizaro Basin. Three alteration types were recognized: (i) Ca–Na silicate (clinopyroxene + magnetite + K-feldspar + quartz + calcite ± plagioclase), (ii) K-silicate (K-feldspar + quartz + magnetite ± biotite ± anhydrite) and (iii) chlorite-calcite alteration. The highest ore grades are linked to the K-silicate alteration. The proven plus probable reserves of Lindero are 84,226 t with average grades of 0.63 g/t Au and 0.11% Cu. A previous study assigned Lindero to the iron oxide copper-gold (IOCG) deposit type but many features of Lindero suggest that it is a porphyry gold deposit, including (i) the temporal and spatial link between alteration and the intrusive bodies, (ii) the alteration distribution pattern, particularly the small volume of rock affected by Ca–Na silicate alteration, (iii) the Au-rich and Cu-poor mineralization style.The magmatic complex at Lindero comprises an early-mineral unit (FPD), four inter-mineral units (CPD1, PBFD, CPD2 and DDP) and one post-mineral unit (PMI). In-situ U/Pb SIMS dating of the oldest (FPD), an intermediate (PBFD) and the youngest (DDP) intrusive units, confirms a middle Miocene age. The weighted mean ages of the oldest and youngest units are indistinguishable, with 15.36 ± 0.13 Ma (n = 21) and 15.47 ± 0.11 Ma (n = 16), respectively. Individual ages from each unit range by ~1 m.y. and the overall spread of zircon ages is 15.92 ± 0.23 to 14.44 ± 0.33 Ma. We suggest that emplacement of the subvolcanic stocks took place within this span time, likely at the lower end of this range (15.0–14.4 Ma). Two 40Ar/39Ar ages of hydrothermal biotites from the K-silicate alteration (14.99 ± 0.16 Ma and 14.93 ± 0.12 Ma) indicate that hydrothermal alteration began practically simultaneously with the emplacement of the porphyry units.All of the intrusive units are similar compositionally. They show a fine- to medium-grained porphyric texture (1–4 mm) comprising plagioclase, amphibole, clinopyroxene and scarce quartz phenocrysts (40–55 vol % of phenocrysts) in a K-feldspar ± quartz microcrystalline (0.02–0.07 mm) groundmass, except in the post-mineral unit which has a cryptocrystalline groundmass. Whole-rock analyses reveal a narrow range of dioritic composition (58.6–61.9 wt % SiO2) and high-K calc-alkaline character for all units. Trace element features (low Ba/Nb ratios, high Nb) of the Lindero magmas indicate a back-arc affinity, similar to those from the Southern Puna and distinct from the Central Volcanic Zone (CVZ) frontal arc. The Sr and Nd isotope ratios (87Sr/86Sr = 0.706042 to 0.706607; 143Nd/144Nd = 0.512501 to 0.512582) from Lindero intrusives are also similar to Southern Puna back-arc volcanic rocks. The Pb isotope ratios of Lindero (206Pb/204Pb = 18.79 to 18.83; 207Pb/204Pb = 15.60 to 15.63; 208Pb/204Pb = 38.66 to 38.74) overlap with both back-arc and arc magmas in the CVZ. The narrow age range, spatial association and uniform chemical and isotopic composition of Lindero porphyry units suggest that were derived from a common magma source, which underwent fractionation and/or crustal assimilation before emplacement as suggest by the low concentrations of Mg, Cr, Ni and Sr. The Lindero porphyry units show chemical and isotopic similarities with those from porphyry gold deposits in the Maricunga belt, Chile, and with the porphyry copper deposits of Argentina located in a back-arc setting; however, they differ from porphyry copper deposits in the frontal-arc setting of Chile, notably by the lack of an adakite-like signature (high Sr/Y ratio).

Early Miocene arc volcanism in the Mexico City Basin: Inception of the Trans-Mexican Volcanic Belt

In central Mexico, remnants of the early volcanic activity in the Trans-Mexican Volcanic Belt (TMVB) are exposed at Sierra de Guadalupe in the northern part of the Mexico City Basin and to the south in the Tepoztlán, Malinalco, Tenancingo, and Chiltepec areas. A few published studies indicate a general early to middle Miocene age for some of these volcanic centers. We present new geologic, geochronologic and geochemical data that refine the timing of this early volcanic activity and its geochemical character, shedding light on the geodynamic evolution during this period. The Sierra de Guadalupe is a volcanic complex with effusive and explosive activity that generated andesitic lavas with plagioclase, ortho- and clinopyroxene plus amphibole phenocrysts, as well as dacitic lava flows and domes with plagioclase, orthopyroxene, amphibole and/or biotite phenocrysts. Pyroclastic and epiclastic (lahars) deposits are also observed in the northern slopes of this volcanic complex. The chemical composition of the Sierra de Guadalupe rocks is calc-alkaline andesitic to dacitic. New 40Ar/39Ar and U-Pb zircon ages demonstrate that volcanic activity at Sierra de Guadalupe lasted ca. 5.0 Ma from ca. 20.1 Ma to 14.8 Ma. Remnants of volcanic centers found in the Tepoztlán, Malinalco, Tenancingo, and Chiltepec areas ~80 km to the south show similar compositions and eruptive styles, with 40Ar/39Ar ages ranging between 21 and 16 Ma. Rocks with similar ages and lithologies were also described in deep wells drilled in the Mexico City Basin. As a whole, the age distribution shows that early to middle Miocene volcanism, making up the initial activity of the TMVB, migrated toward the north from Tepoztlán (310 km from the trench) up to Pachuca (445 km from the trench), and is likely associated with a shallowing the subducting Cocos slab underneath this region. Our results confirm that magmatism in the TMVB began in the early Miocene by dehydration of the Cocos slab as attested by high Ba/La and Th/Yb ratios.