Magmatism in southern Uruguay and the early rifting of the South Atlantic

A suite of early Cretaceous vo1canic rocks (Puerto GOmez and Arequita FIm.) are preserved within the N60° E trending Santa Lucia Basin, southern Uruguay which lies at the southern margin of the Parana - Etendeka continental flood basalt province. New Ar- Ar ages of the basalts range from 134 to 130 Ma while ages from the rhyolites range from 130 to 124 Ma. This magamtism was contemporaneous with the main flood basalt event, although rhyolite activity continued after rifting (127 - 126 Ma). The province therefore contains unique information about melting conditions at the periphery of the influence of the Tristan da Cunha plume. The volcanic rocks of southern Uruguay are bimodal in silica, and the majority of basalts of the Puerto G6mez Fm., herein termed the Treinte Y Tres magma type, have major-, trace- element and initial isotope ratios similar to the low -Ti/Y Gramado-Tafelberg magma types of the Paranl-Etendeka. There are also a number of unique basalt samples termed the Santa Lucia magma type. which have low La/Nb, and are considered to have been generated by mixing between lithosphere - and asthenosphere - derived melts. These magmas represent the first sampling of true plume material in this CFB province. The rhyolites of the Arequita Fm. are relatively evolved with variably sized euhedral to anbedral quartz phenocrysts, and ignimbritic textures that are the first described from this province. The rhyolites have lower magmatic temperatures (850 - 950°C) than those of the Parana-Etendeka, and are divided into two geochemical series, the Lascano Series and the Aigtia Series. The rhyolites of the Lascano and Aigua Series are not related to the Puerto Gomez Fm. basaks, but rather they originated from separate sources in the mid to lower crust. where melting was facilitated by mid-crustal level intrusions of basaJtic material as recognised from a large gravity anomaly. Melt production rates in southern Uruguay were low (0.01 km3 yr-1) similar to the rates ca1cuIated for the waning stages of magmatism on the Serra Geral escarpment, southern Brazil. These rates are consistent with the notion that by this time the principal melt production was located in the newly forming ocean with Uruguay at the margins of the influence of the plume

Erosion during extreme flood events dominates Holocene canyon evolution in northeast Iceland

Extreme flood events have the potential to cause catastrophic landscape change in short periods of time (10(0) to 10(3) h). However, their impacts are rarely considered in studies of long-term landscape evolution (>10(3) y), because the mechanisms of erosion during such floods are poorly constrained. Here we use topographic analysis and cosmogenic (3)He surface exposure dating of fluvially sculpted surfaces to determine the impact of extreme flood events within the Jökulsárgljúfur canyon (northeast Iceland) and to constrain the mechanisms of bedrock erosion during these events. Surface exposure ages allow identification of three periods of intense canyon cutting about 9 ka ago, 5 ka ago, and 2 ka ago during which multiple large knickpoints retreated large distances (>2 km). During these events, a threshold flow depth was exceeded, leading to the toppling and transportation of basalt lava columns. Despite continuing and comparatively large-scale (500 m(3)/s) discharge of sediment-rich glacial meltwater, there is no evidence for a transition to an abrasion-dominated erosion regime since the last erosive event because the vertical knickpoints have not diffused over time. We provide a model for the evolution of the Jökulsárgljúfur canyon through the reconstruction of the river profile and canyon morphology at different stages over the last 9 ka and highlight the dominant role played by extreme flood events in the shaping of this landscape during the Holocene

The Weaklaw Vent, SE Scotland:Metasomatism of eruptive products by carbo-hydro-fluids of probable mantle origin

This is the author accepted manuscript. The final version is available from CUP via the DOI in this record The Weaklaw vent in SE Scotland (East Lothian coast), inferred to be Namurian, produced lava spatter and volcanic bombs. The latter commonly contained ultramafic xenoliths. All were metasomatised by carbonic fluids rich in incompatible elements. The lavas and xenoliths are inferred to have been basanites and lherzolites prior to metasomatism. The abundance and size of (carbonated) peridotite xenoliths at Weaklaw denotes unusual rapidity of magma ascent and high-energy eruption making Weaklaw exceptional in the British Isles. The lavas and xenoliths were altered subsequently by low-temperature (<200°C) carbo-hydrous fluids to carbonate, clay and quartz assemblages. A small irregular tuffisite 'dyke' that transects the ejecta is also composed dominantly of carbonates and clays. The peridotitic xenoliths are typically foliated, interpreted as originating as pre-entrainment mantle shear-planes. Analyses of the relic spinels shows them to be compositionally similar to spinels in local unaltered lherzolites from near-by basanitic occurrences. Chromium showed neither significant loss nor gain but was concentrated in a di-octahedral smectite allied to volkonskoite. It is in the complex association of smectite with other clays, chlorite and possibly fuchsite that the diverse incompatible elements are concentrated. We conclude that late Palaeozoic trans-tensional fault movement caused mantle shearing. Rapid ascent of basanite magma entrained large quantities of sheared lithospheric mantle. This was followed by ascent of an aggressive carbonate-/ hydroxyl-rich fluid causing pervasive metasomatism. The vent is unique in several ways: in its remarkable clay mineralogy and in displaying such high Cr-clays in a continental intra-plate setting; in being more productive in terms of its 'cargo' of peridotite xenoliths; in presenting an essentially un-eroded sequence of Namurian extrusives; and, not least, for giving evidence for post-eruptive, surface release of small-melt, deep-source fluids

Geodynamic controls on the contamination of Cenozoic arc magmas in the southern Central Andes: insights from the O and Hf isotopic composition of zircon

AbstractSubduction zones, such as the Andean convergent margin of South America, are sites of active continental growth and crustal recycling. The composition of arc magmas, and therefore new continental crust, reflects variable contributions from mantle, crustal and subducted reservoirs. Temporal (Ma) and spatial (km) variations in these contributions to southern Central Andean arc magmas are investigated in relation to the changing plate geometry and geodynamic setting of the southern Central Andes (28–32°S) during the Cenozoic. The in-situ analysis of O and Hf isotopes in zircon, from both intrusive (granitoids) and extrusive (basaltic andesites to rhyolites) Late Cretaceous – Late Miocene arc magmatic rocks, combined with high resolution U–Pb dating, demonstrates distinct across-arc variations. Mantle-like δ18O(zircon) values (+5.4‰ to +5.7‰ (±0.4 (2σ))) and juvenile initial εHf(zircon) values (+8.3 (±0.8 (2σ)) to +10.0 (±0.9 (2σ))), combined with a lack of zircon inheritance suggests that the Late Cretaceous (∼73Ma) to Eocene (∼39Ma) granitoids emplaced in the Principal Cordillera of Chile formed from mantle-derived melts with very limited interaction with continental crustal material, therefore representing a sustained period of upper crustal growth. Late Eocene (∼36Ma) to Early Miocene (∼17Ma) volcanic arc rocks present in the Frontal Cordillera have ‘mantle-like’ δ18O(zircon) values (+4.8‰ (±0.2 (2σ) to +5.8‰ (±0.5 (2σ))), but less radiogenic initial εHf(zircon) values (+1.0 (±1.1 (2σ)) to +4.0 (±0.6 (2σ))) providing evidence for mixing of mantle-derived melts with the Late Paleozoic – Early Mesozoic basement (up to ∼20%). The assimilation of both Late Paleozoic – Early Mesozoic Andean crust and a Grenville-aged basement is required to produce the higher than ‘mantle-like’ δ18O(zircon) values (+5.5‰ (±0.6 (2σ) to +7.2‰ (±0.4 (2σ))) and unradiogenic, initial εHf(zircon) values (−3.9 (±1.0 (2σ)) to +1.6 (±4.4 (2σ))), obtained for the Late Oligocene (∼23Ma) to Late Miocene (∼9Ma) magmatic rocks located in the Argentinean Precordillera, and the Late Miocene (∼6Ma) volcanic rocks present in the Frontal Cordillera. The observed isotopic variability demonstrates that the assimilation of pre-existing continental crust, which varies in both age and composition over the Andean Cordillera, plays a dominant role in modifying the isotopic composition of Late Eocene to Late Miocene mantle-derived magmas, implying significant crustal recycling. The interaction of arc magmas with distinct basement terranes is controlled by the migration of the magmatic arc due to the changing geodynamic setting, as well as by the tectonic shortening and thickening of the Central Andean crust over the latter part of the Cenozoic

Spatio-temporal trends in normal-fault segmentation recorded by low-temperature thermochronology: Livingstone fault scarp, Malawi Rift, East African Rift System

The evolution of through-going normal-fault arrays from initial nucleation to growth and subsequent interaction and mechanical linkage is well documented in many extensional provinces. Over time, these processes lead to predictable spatial and temporal variations in the amount and rate of displacement accumulated along strike of individual fault segments, which should be manifested in the patterns of footwall exhumation. Here, we investigate the along-strike and vertical distribution of low-temperature apatite (U–Th)/He (AHe) cooling ages along the bounding fault system, the Livingstone fault, of the Karonga Basin of the northern Malawi Rift. The fault evolution and linkage from rift initiation to the present day has been previously constrained through investigations of the hanging wall basin fill. The new cooling ages from the footwall of the Livingstone fault can be related to the adjacent depocentre evolution and across a relay zone between two palaeo-fault segments. Our data are complimented by published apatite fission-track (AFT) data and reveal significant variation in rock cooling history along-strike: the centre of the footwall yields younger cooling ages than the former tips of earlier fault segments that are now linked. This suggests that low-temperature thermochronology can detect fault interactions along strike. That these former segment boundaries are preserved within exhumed footwall rocks is a function of the relatively recent linkage of the system. Our study highlights that changes in AHe (and potentially AFT) ages associated with the along-strike displacement profile can occur over relatively short horizontal distances (of a few kilometres). This is fundamentally important in the assessment of the vertical cooling history of footwalls in extensional systems: temporal differences in the rate of tectonically driven exhumation at a given location along fault strike may be of greater importance in controlling changes in rates of vertical exhumation than commonly invoked climatic fluctuations