Christoph Breitkreuz

Magmatic flare-up at the Carboniferous/Permian boundary in the NE German Basin revealed by SHRIMP zircon ages

SHRIMP ages of 12 volcanic samples indicate intense magmatic activity to have occurred at the Carboniferous/Permian boundary throughout much of the NE German Basin. Rhyolitic crystal-rich samples have been taken from quarries in the Halle Volcanic Complex (HVC) and in the Flechtingen Ignimbrite, and from drill cores of the Kotzen, Mirow, Friedland and Penkun areas. Ten samples yielded 206Pb/238U ages between 302 and 297 Ma (±3 Ma, respectively) which indicate that the magmatic activity took place concentrated in a relatively short time span throughout much of the NE German Basin. Two HVC samples have ages of 307 and 294 Ma. This remarkably synchronous magmatic activity occurred during the initial phase of the basin development. Considering published volume estimates (48,000 km3), extrusion rates during this time period probably were in the order of 0.01 km3 a−1, implying magma production rates of 0.1 km3 a−1. The calc-alkaline SiO2-rich volcanic rocks originated from magmas that presumably formed during anatexis and subordinate magma mixing in an intra-continental transtensional setting. The basaltic magma which must have provided the thermal input into the lower crust probably formed during decompressional melting of lithospheric mantle fertilised by previous magmatic processes. The dated volcanic rocks occur in three geotectonic provinces, namely the (a) Mid German Crystalline Rise, which forms the northern part of the Internal Variscides, the (b) External Variscides and the Variscan foreland which is considered as (c) Eastern Avalonia. Many of the old zircons found in the HVC samples reflect the magmatic activity of the Mid German Crystalline Rise (325–400 Ma). However, Cadomian (500–650 Ma) and older Gondwanian elements (1674–2373 Ma) are also present. Although the Flechtingen and Kotzen areas are located within the External Variscides, it seems more likely that the middle to lower crust, that experienced anatexis, formed part of the Eastern Avalonia Plate overridden by the Variscan Front. Thus the old zircons found in Flechtingen (350 and 538 Ma) and Kotzen (345 Ma), but certainly those present in the Penkun drill cores (1483 Ma) represent Eastern Avalonia tectonic history. The anatectic magmas in the Friedland area probably formed partly from Baltic crust thrusted below Eastern Avalonia during the Caledonian Orogeny. Among the old zircons from the Friedland samples, in addition to Proterozoic ages (1456 Ma), we found testimony of Caledonian (443 Ma) and of clearly post-Caledonian (387 Ma) magmatism.

Heat flow in the Lesser Antilles island arc and adjacent back arc Grenada basin

Using temperature gradients measured in 10 holes at 6 sites, we generate the first high fidelity heat flow measurements from Integrated Ocean Drilling Program drill holes across the northern and central Lesser Antilles arc and back arc Grenada basin. The implied heat flow, after correcting for bathymetry and sedimentation effects, ranges from about 0.1 W/m2 on the crest of the arc, midway between the volcanic islands of Montserrat and Guadeloupe, to 15 km from the crest in the back arc direction. Combined with previous measurements, we find that the magnitude and spatial pattern of heat flow are similar to those at continental arcs. The heat flow in the Grenada basin to the west of the active arc is 0.06 W/m2, a factor of 2 lower than that found in the previous and most recent study. There is no thermal evidence for significant shallow fluid advection at any of these sites. Present-day volcanism is confined to the region with the highest heat flow.

Paleozoic evolution of active margin basins in the southern Central Andes (northwestern Argentina and northern Chile)

Abstract The geodynamic evolution of the Paleozoic continental margin of Gondwana in the region of the southern Central Andes is characterized by the westward progression of orogenic basin formation through time. The Ordovician basin in the northwest Argentinian Cordillera Oriental and Puna originated as an Early Ordovician back-arc basin. The contemporaneous magmatic arc of an east-dipping subduction zone was presumably located in northern Chile. In the back-arc basin, a ca. 3500 meter, fining-up volcaniclastic apron connected to the arc formed during the Arenigian. Increased subsidence in the late Arenigian allowed for the accomodation of large volumes of volcaniclastic turbidites during the Middle Ordovician. Subsidence and sedimentation were caused by the onset of collision between the para-autochthonous Arequipa Massif Terrane (AMT) and the South American margin at the Arenigian-Llanvirnian transition. This led to eastward thrusting of the arc complex over its back-arc basin and, consequently, to its transformation into a marine foreland basin. As a result of thrusting in the west, a flexural bulge formed in the east, leading to uplift and emergence of the Cordillera Oriental shelf during the Guandacol Event at the Arenigian-Llanvirnian transition. The basin fill was folded during the terminal collision of the AMT during the Ocloyic Orogeny (Ashgillian). The folded strata were intruded post-tectonically by the presumably Silurian granitoids of the “Faja Eruptiva de la Puna Oriental.” The orogeny led to the formation of the positive area of the Arco Puneno. West of the Arco Puneno, a further marine basin developed during the Early Devonian, the eastern shelf of which occupied the area of the Cordillera Occidental, Depresion Preandina, and Precordillera. The corresponding deep marine turbidite basin was located in the region of the Cordillera de la Costa. Deposition continued until the basin fill was folded in the early Late Carboniferous Toco Orogeny. The basin originated as an extensional structure at the continental margin of Gondwana. Independent lines of evidence imply that basin evolution was not connected to subduction. Thus, the basin could not have been in a fore-arc position as previously postulated. Above the folded Devonian-Early Carboniferous strata, a continental volcanic arc developed from the Late Carboniferous to the Middle Triassic. It represents the link between the Choiyoi Province in central Chile and Argentina, and the Mitu Group rift in southern Peru. The volcanic arc succession is characterized by the prevalence of silicic lavas and tuffs and volcaniclastic sedimentary rocks. During the latest Carboniferous, a thick ostracod-bearing lacustrine unit formed in an extended lake in the area of the Depresion Preandina. This lake basin originated in an intra-arc tensional setting. During the Early Permian, marine limestones were deposited on a marine platform west and east of the volcanic arc, connected to the depositional area of the Copacabana Formation in southern Peru.

Paleozoic volcanic events in the Central Andes

Abstract Paleozoic volcanic rocks of northern Chile (21–27°S) can be subdivided into Early Ordovician, Early Carboniferous, and Late Carboniferous to Triassic episodes. At the western and eastern margin of the Alta Cordillera and in the Chilean Precordillera, isolated outcrops of Early Ordovician submarine basic lavas and siliceous volcanic apron deposits occur (submarine deposition level

The North-Chilean Coast Range — an example for the development of an active continental margin

ZusammenfassungDie Analyse ausgedehnter Plutone in der Küstenkordillere Nordchiles zwischen Lat. 25°30′ und 26°35′ S ergab eine komplexe magmatisch-strukturelle Entwicklung zwischen dem jüngeren Paläozoikum und dem Tertiär.Der Aufstieg der Intrusivkörper wird von einer tiefgreifenden vertikalen Schollentektonik diktiert. Allgemein wechselt der Chemismus von S-Typ-Magmen im Paläozoikum zu I-Typ-Magmen im Mesozoikum und Känozoikum.Dieser Wechsel wird von einer strukturgeologischen Veränderung des Kontinentalrandes begleitet, die wir in sechs hypothetischen Phasen (Devon-Tertiär) darstellen.Wir stehen erst am Beginn einer umfassenden Synthese über die Genese des Anden-Orogens.AbstractAnalysis of extended plutons in the Coast Range of North Chile between 25°30′ and 26°35′ led to the recognition of a complex magmatic and structural evolution from the Upper Paleozoic to the Tertiary.The ascension of the intrusive bodies is dictated by deep-seated block tectonics. Generally the chemistry changes from S-type magmas in the Paleozoic to I-type magmas in the Mesozoic and Cainozoic.This is accompanied by a change in the structural geology of the continental margin which we present in six hypothetic phases (Devonian-Tertiary). We are only at the beginning of an encompassing synthesis of the genesis of the Andean orogen.ResumenEl análisis de extensos plutones en la Cordillera de la Costa/Norte de Chile entre lat. 25°30′ y 26°35′ S ha revelado un complejo desarrollo magmático-estructural entre el Paleozoico Superior y el Terciario.La ascensión de los cuerpos intrusivos está determinado por una profunda tectónica vertical de bloques. En general el quimismo cambia de magmas de tipo “S” en el Paleozoico hacia magmas de tipo “I” en el Mesozoico y Cenozoico.Este cambio está acompañado por una modificación de la estructura geológica del borde continental, que describimos en seis fases hipotéticas (Devónico-Terciario). Nos encontramos tan sólo en el comienzo de una amplia síntesis de la génesis del orógeno de los Andes.Краткое содержаниеВ прибрежных Кордиль ерах северного Чиле м ежду 25°30′ и 26°35′ южной широты был исследова н комплекс магматиче ских пород, возраст которых от позднего палеозоя до третичного периода включительно. Поднятие крупных инт рузивных тел произош ло в результате глубинно й вертикально направл енной тектоники глыб. Причем имело место и смена химизма: магмы типа-S в п алеозое преобразовы ваются в магмы типа I в мезозое и кайнозое. Эту смену сопровожда ло и структурное изме нение континентального кр ая. Авторы, на основании н овейших исследовани й, подразделяют его на 6 г ипотетических фаз (от девона до трети чного периода). Эта работа является п ервой попыткой созда ния теории о генезисе Орогена Анд.

Submarine record of volcanic island construction and collapse in the Lesser Antilles arc: First scientific drilling of submarine volcanic island landslides by IODP Expedition 340

IODP Expedition 340 successfully drilled a series of sites offshore Montserrat, Martinique and Dominica in the Lesser Antilles from March to April 2012. These are among the few drill sites gathered around volcanic islands, and the first scientific drilling of large and likely tsunamigenic volcanic island-arc landslide deposits. These cores provide evidence and tests of previous hypotheses for the composition and origin of those deposits. Sites U1394, U1399, and U1400 that penetrated landslide deposits recovered exclusively seafloor sediment, comprising mainly turbidites and hemipelagic deposits, and lacked debris avalanche deposits. This supports the concepts that i/ volcanic debris avalanches tend to stop at the slope break, and ii/ widespread and voluminous failures of preexisting low-gradient seafloor sediment can be triggered by initial emplacement of material from the volcano. Offshore Martinique (U1399 and 1400), the landslide deposits comprised blocks of parallel strata that were tilted or microfaulted, sometimes separated by intervals of homogenized sediment (intense shearing), while Site U1394 offshore Montserrat penetrated a flat-lying block of intact strata. The most likely mechanism for generating these large-scale seafloor sediment failures appears to be propagation of a decollement from proximal areas loaded and incised by a volcanic debris avalanche. These results have implications for the magnitude of tsunami generation. Under some conditions, volcanic island landslide deposits composed of mainly seafloor sediment will tend to form smaller magnitude tsunamis than equivalent volumes of subaerial block-rich mass flows rapidly entering water. Expedition 340 also successfully drilled sites to access the undisturbed record of eruption fallout layers intercalated with marine sediment which provide an outstanding high-resolution data set to analyze eruption and landslides cycles, improve understanding of magmatic evolution as well as offshore sedimentation processes.

The Late Carboniferous to Triassic Volcanic Belt in Northern Chile

A prominent feature of the pre-Jurassic Andean basement are outcrops of late Palaeozoic to Triassic, mainly siliceous volcanoplutonic complexes. Published models assume a magmatic trend to have developed from a “normal” subduction setting with calc-alkaline magmatism during Carboniferous times towards an ensialic marginal rift during the Permo-Triassic, which gave rise to (calc-)alkaline and peralkaline magmas. In the Precordillera and Cordillera Occidental of northern Chile (20°–25°S), widespread outcrops of late Carboniferous to Triassic volcanosedimentary successions occur (Peine Group). Caldera eruption seems to have been important in this region. The volcanic rocks are of calc-alkaline to slightly alkaline, predominantly siliceous composition (SiO2 range 50–80%). The geochemical features of the volcanic rocks display a clear volcanic arc affiliation. In parts, a diagenesis/anchimetamorphism of the volcanic rocks caused considerable variations of mobile elements (e.g. K, Na, Rb, Sr, Cu, Si). HFS and RE elements display patterns of a non-tensional subduction-controUed setting of the magmatism at a continental margin of moderate thickness. In the Salar de Atacama area, a conspicuous alluvial-limnic volcanosedimentary intercalation occurs. The terrestrial basin which accommodated this sequence formed during the latest Carboniferous-?early Permian on top of the deposits of the pre-existing late Carboniferous composite volcanism. The lower part of the intercalation consists of green limnic and multicoloured alluvial fan deposits and the upper part is made up of red alluvial sedimentary rocks. Throughout the sequence, minor basic volcanic rocks occur. The considerable size of the basin, the limnic-alluvial fan facies association and the accompanying intrabasinal basic volcanism indicate a basin formation controlled by extensional arc tectonics. With regard to the north Chilean Peine Group, this arc-graben represents the only support to the aforementioned model of a tensional regime during the Permo-Triassic.

The alkaline Meidob volcanic field (Late Cenozoic, northwest Sudan)

The Meidob volcanic field (MVF) forms part of the Darfur Volcanic Province and developed from 7 Ma to 5 ka as indicated by K/Ar, thermoluminescence and 14C ages. It is situated in an uplifted high of the Pan-African basement, which consists of greenstones, high-grade gneisses and granites, and which is covered by Cretaceous sandstone. The MVF basaltic lavas, which originated from more than 300 scoria cones, formed a lava plateau of 50×100 km and up to 400 m thickness in the time between 7 and < 0.3 Ma. Young phonolitic mesa flows, together with rare trachyticbenmoreitic lava flows, trachytic pumice fallout deposits, ignimbrites and maars, form the central part of the field. The total amount of volcanic rocks is between 1400 and 1800 km3, with 98 vol.% being basaltic rocks, which results in an integrated magma output rate of ∼ 0.0002 km3 a−1. A combination of age data of the lavas with erosional features yields uplift rates for the Darfur Dome of ∼30 m Ma-1 in the MVF area. Magma was generated by 3–5% melting of predominantly asthenospheric mantle with a HIMU contribution. Fractionation of olivine, pyroxene, An-poor plagioclaseanorthoclase, magnetite and apatite leads to a differentiation from basanite to phonolite. Assimilation of crustal rocks near the top of the phonolitic upper crustal magma chambers - facilitated by volatile enrichment - produced magmas which gave way to benmoreitic and trachytic lavas, as well as to trachytic ignimbrites and pumice fallout deposits. Ultramafic cumulate xenoliths indicate the existence of major magma reservoirs at the crust-mantle boundary during MVF activity. Magma ascent occurred in a tensional regime, which changed its orientation at around 1 Ma. Early during MVF development, west-east and subordinately northeastsouthwest trending lineaments were active whereas volcanic activity younger than 1 Ma took place along northwest-southeast and northeast-southwest trending systems. The Central African Fault Zone, a transcontinental, lithospheric shear zone, played an important role for the rise of magmas in the Darfur Dome.

UPb geochronology and significance of Late Permian ignimbrites in Northern Chile

Abstract U Pb zircon ages of four ignimbrite samples from a Late Paleozoic volcaniclastic succession in northern Chile are presented. The 2 km thick succession consists of the Peine Fm. and the overlying Cas Fm., which crop out at the eastern margin of the Salar de Atacama near the village of Peine. One sample was collected from the oldest ignimbrite of the succession (Lower Member of the Peine Fm.); the other three samples were collected from the Middle Member of the Cas Fm. and its assumed equivalents. The 206 Pb 238 U ages of all samples overlap within their analytical uncertainties, which results in a best estimate of 248 ± 3 Ma (Late Permian: Harland, 1990) for deposition of the entire succession. The age for the stratigraphically older sample is statistically indistinguishable from the other three samples, so that the depositional interval could have been less than 1 million years; maximum errors in the data allow that the Peine and Cas Fms. could have formed during a time span of 6 Ma. The data provide tight constraints on the age of these units, part of which formed in the Lanquir Caldera Complex. They also have implications for the stratigraphy and paleoclimatic models of the Southern Central Andes during the Late Permian. Although the zircon fractions analyzed here document Late Permian ages, they show varying amounts of inheritance of early Paleoproterozoic (to Late Archean?) zircons and also indicate the incorporation of ca. 2.5 Ga initial common lead to varying degrees. This and results from other studies in the area lead us to infer the existence, at least during the Paleozoic, of Precam-brian basement located below the Salar de Atacama area.

Late Carboniferous intra-arc sediments in the north Chilean Andes: Stratigraphy, paleogeography and paleoclimate

SummaryThick terrestrial Late Carboniferous to Triassic volcanosedimentary successions, a prominent feature of the Chilean and Argentinian High Andes, were formed on the active continental margin of Gondwanaland. Their stratigraphic position and the paleogeographic and paleoclimatic relations to neighbouring successions are poorly defined. A more precise age has been obtained for alluviolimnic intra-arc sediments (Miembro Medio), which are intercalated in the Late Carboniferous-Triassic volcano-sedimentary successions in the Salar de Atacama area of northern Chile.The ostracodesCarbonita cf.pungens andParaparchites sp., which occur in the lower part of the Miembro Medio, are of Late Carboniferous, probably Westphalian age. The diverse taphoflora, which occurs in a higher stratigraphic level than the ostracodes, includes sphenophytes, ferns, gymnosperms and pteridophylls, for which we assume a late Westphalian-Early Permian age. Considering radiometric data of under-and overlying volcanic rocks, a Westphalian-Stephanian (to? Early Permian) age is inferred for the Miembro Medio.Fauna and flora indicate that warm-humid and seasonal climatic conditions existed during the deposition of the lower fossiliferous part of the Miembro Medio. This coincides with the sedimentary paleoclimatic indicators of the Miembro Medio and the climate which was assumed to have predominated in wide parts of the Central and Southern Andes during the Latest Carboniferous.