Zircon coronas around Fe–Ti oxides: a physical reference frame for metamorphic and metasomatic reactions

Ilmenite in coronitic gabbros from the Bamble and Kongsberg sectors, southern Norway, is surrounded by zircons ranging in diameters from a fraction of a micrometer to 10 μm across. The zircons are inert during subsequent metamorphism (amphibolite- to pumpellyite–prehnite facies) and metasomatism (scapolitization and albitization) and can be found as trails in silicates (phlogopite, talc, chlorite, amphibole, albite, and tourmaline) in the altered rocks. The trails link up to form polygons outlining the former oxide grain boundary. This 3-dimensional framework of zircons is used to (a) recognize metasomatic origin of rocks, (b) quantify the mobility of elements during mineral replacement, (c) establish the growth direction of reaction fronts and to identify the reaction mechanism as dissolution–reprecipitation. Zircon coronas on Fe–Ti oxides have been described from a number of terrains and appear to be common in mafic rocks (gabbros and granulites) providing a tool for a better understanding of metasomatic and metamorphic reactions.     

Fingerprinting the 8.2 ka event climate response in a coupled climate model

Using results from coupled climate model simulations of the 8.2 ka climate event that produced a cold period over Greenland in agreement with the reconstructed cooling from ice cores, we investigate the typical pattern of climate anomalies (fingerprint) to provide a framework for the interpretation of global proxy data for the 8.2 ka climate event. For this purpose we developed an analysis method that isolates the forced temperature response and provides information on spatial variations in magnitude, timing and duration that characterise the detectable climate event in proxy archives. Our analysis shows that delays in the temperature response to the freshwater forcing are present, mostly in the order of decades (30 a over central Greenland). The North Atlantic Ocean initially cools in response to the freshwater perturbation, followed in certain parts by a warm response. This delay, occurring more than 200 a after the freshwater pulse, hints at an overshoot in the recovery from the freshwater perturbation. The South Atlantic and the Southern Ocean show a warm response reflecting the bipolar seesaw effect. The duration of the simulated event varies for different areas, and the highest probability of recording the event in proxy archives is in the North Atlantic Ocean area north of 40° N. Our results may facilitate the interpretation of proxy archives recording the 8.2 ka event, as they show that timing and duration cannot be assumed to correspond with the timing and duration of the event as recorded in Greenland ice cores. Copyright © 2011 John Wiley & Sons, Ltd.     

Non-classical types of loess

The purpose of this contribution is to describe the sequence of physical and chemical processes resulting in the sediment-type named loess, a fine-grained sediment deposit of universal occurrence. Owing to historical causes, loess has been (and still is) implicitly linked to glacial/periglacial environments among most naturalists. However it is known today that most eolian dust is deflated from tropical deserts. Hence, that sequence of processes is more comprehensive than the former narrow cold scenario. Six examples of different “non-classical” cases (from South America and Europe) that fit well to the loess definition are developed: 1) volcanic loess in Ecuador: pyroclastic eruptions/valley wind/mountain praire/silica structuring; 2) tropical loess in northeastern Argentina, Brazil and Uruguay: deflation of river and fan splays/savanna/iron sesquioxide structuring; 3) gypsum loess in northern Spain: destruction of anhydrite/gypsiferous layers in a dry climate/valley wind/Saharian shrub peridesert/gypsum structuring; 4) trade-wind deposits in Venezuela and Brazil: deflation in tidal flats/trade wind into the continent/savanna/iron hydroxide structuring; 5) anticyclonic gray loess in Argentina: continental anticyclone on plains/anti-clockwise winds and whirls/steppe/carbonate structuring. All these non-classical types conform to the accepted loess definitions and they also share the most important field characteristics of loess such as grain size, friability, vertical or sub-vertical slopes in outcrops, subfusion and others. Other cases can probably be recognized when systematically scrutinized.     

A role for icebergs in the 8.2 ka climate event

We investigate the potential role of icebergs in the 8.2 ka climate event, using a coupled climate model equipped with an iceberg component. First, we evaluate the effect of a large iceberg discharge originating from the decaying Laurentide ice sheet on ocean circulation, compared to a release of an identical volume of freshwater alone. Our results show that, on top of the freshwater effect, a large iceberg discharge facilitates sea-ice growth as a result of lower sea-surface temperatures induced by latent heat of melting. This causes an 8% increased sea-ice cover, 5% stronger reduction in North Atlantic Deep Water production and 1°C lower temperature in Greenland. Second, we use the model to investigate the effect of a hypothetical two-stage lake drainage, which is suggested by several investigators to have triggered the 8.2 ka climate event. To account for the final collapse of the ice-dam holding the Laurentide Lakes we accompany the secondary freshwater pulse in one scenario with a fast 5-year iceberg discharge and in a second scenario with a slow 100-year iceberg discharge. Our experiments show that a two-stage lake drainage accompanied by the collapsing ice-dam could explain the anomalies observed around the 8.2 ka climate event in various climate records. In addition, they advocate a potential role for icebergs in the 8.2 ka climate event and illustrate the importance of latent heat of melting in the simulation of climate events that involve icebergs. Our two-stage lake drainage experiments provide a framework in the discussion of two-stage lake drainage and ice sheet collapse.     

Middle Miocene peralkaline ignimbrites in the Hermosillo region (Sonora, Mexico): Geodynamic implications

Scattered ignimbritic mesas crop out in the Hermosillo region (Sonora, Mexico). These rocks that have been dated at 12.5 Ma (Middle Miocene) have the petrography and chemical characteristics of comendites. Such a flare-up of peralkaline acidic volcanism, after a long period of subduction-related arc volcanism, emphasises an important change in the source of volcanism. It corresponds to the latest stage of continental extension prior to the marine invasion and the development of spreading centres in the Gulf of California. To cite this article: J. Vidal Solano et al., C. R. Geoscience 337 (2005).     

The Aguablanca Ni–(Cu) sulfide deposit, SW Spain: geologic and geochemical controls and the relationship with a midcrustal layered mafic complex

The Aguablanca Ni–(Cu) sulfide deposit is hosted by a breccia pipe within a gabbro–diorite pluton. The deposit probably formed due to the disruption of a partially crystallized layered mafic complex at about 12–19 km depth and the subsequent emplacement of melts and breccias at shallow levels (<2 km). The ore-hosting breccias are interpreted as fragments of an ultramafic cumulate, which were transported to the near surface along with a molten sulfide melt. Phlogopite Ar–Ar ages are 341–332 Ma in the breccia pipe, and 338–334 Ma in the layered mafic complex, and are similar to recently reported U–Pb ages of the host Aguablanca Stock and other nearby calc-alkaline metaluminous intrusions (ca. 350–330 Ma). Ore deposition resulted from the combination of two critical factors, the emplacement of a layered mafic complex deep in the continental crust and the development of small dilational structures along transcrustal strike-slip faults that triggered the forceful intrusion of magmas to shallow levels. The emplacement of basaltic magmas in the lower middle crust was accompanied by major interaction with the host rocks, immiscibility of a sulfide melt, and the formation of a magma chamber with ultramafic cumulates and sulfide melt at the bottom and a vertically zoned mafic to intermediate magmas above. Dismembered bodies of mafic/ultramafic rocks thought to be parts of the complex crop out about 50 km southwest of the deposit in a tectonically uplifted block (Cortegana Igneous Complex, Aracena Massif). Reactivation of Variscan structures that merged at the depth of the mafic complex led to sequential extraction of melts, cumulates, and sulfide magma. Lithogeochemistry and Sr and Nd isotope data of the Aguablanca Stock reflect the mixing from two distinct reservoirs, i.e., an evolved siliciclastic middle-upper continental crust and a primitive tholeiitic melt. Crustal contamination in the deep magma chamber was so intense that orthopyroxene replaced olivine as the main mineral phase controlling the early fractional crystallization of the melt. Geochemical evidence includes enrichment in SiO₂ and incompatible elements, and Sr and Nd isotope compositions (⁸⁷Sr/⁸⁶Sr_i 0.708–0.710; ¹⁴³Nd/¹⁴⁴Nd_i 0.512–0.513). However, rocks of the Cortegana Igneous Complex have low initial ⁸⁷Sr/⁸⁶Sr and high initial ¹⁴³Nd/¹⁴⁴Nd values suggesting contamination by lower crustal rocks. Comparison of the geochemical and geological features of igneous rocks in the Aguablanca deposit and the Cortegana Igneous Complex indicates that, although probably part of the same magmatic system, they are rather different and the rocks of the Cortegana Igneous Complex were not the direct source of the Aguablanca deposit. Crust–magma interaction was a complex process, and the generation of orebodies was controlled by local but highly variable factors. The model for the formation of the Aguablanca deposit presented in this study implies that dense sulfide melts can effectively travel long distances through the continental crust and that dilational zones within compressional belts can effectively focus such melt transport into shallow environments.Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Stratigraphy and Mesozoic–Cenozoic tectonic history of northern Sierra Los Ajos and adjacent areas,Sonora, Mexico

Geologic mapping in the northern Sierra Los Ajos reveals new stratigraphic and structural data relevant to deciphering the Mesozoic–Cenozoic tectonic evolution of the range. The northern Sierra Los Ajos is cored by Proterozoic, Cambrian, Devonian, Mississippian, and Pennsylvanian strata, equivalent respectively to the Pinal Schist, Bolsa Quartzite and Abrigo Limestone, Martin Formation, Escabrosa Limestone, and Horquilla Limestone. The Proterozoic–Paleozoic sequence is mantled by Upper Cretaceous rocks partly equivalent to the Fort Crittenden and Salero Formations in Arizona, and the Cabullona Group in Sonora, Mexico.Absence of the Upper Jurassic–Lower Cretaceous Bisbee Group below the Upper Cretaceous rocks and above the Proterozoic–Paleozoic rocks indicates that the Sierra Los Ajos was part of the Cananea high, a topographic highland during the Late Jurassic and Early Cretaceous. Deposition of Upper Cretaceous rocks directly on Paleozoic and Proterozoic rocks indicates that the Sierra Los Ajos area had subsided as part of the Laramide Cabullona basin during Late Cretaceous time. Basal beds of the Upper Cretaceous sequence are clast-supported conglomerate composed locally of basement (Paleozoic) clasts. The conglomerate represents erosion of Paleozoic basement in the Sierra Los Ajos area coincident with development of the Cabullona basin.The present-day Sierra Los Ajos reaches elevations of greater than 2600 m, and was uplifted during Tertiary basin-and-range extension. Upper Cretaceous rocks are exposed at higher elevations in the northern Sierra Los Ajos and represent an uplifted part of the inverted Cabullona basin. Tertiary uplift of the Sierra Los Ajos was largely accommodated by vertical movement along the north-to-northwest-striking Sierra Los Ajos fault zone flanking the west side of the range. This fault zone structurally controls the configuration of the headwaters of the San Pedro River basin, an important bi-national water resource in the US-Mexico border region.     

Quaternary deformation around the Palo Negro area,Pampa Norte,Argentina

The Pampa Norte region is a great plain characterized by low slopes and accumulation of hundreds of meters of thick loose sediments. A high morphostructure denominated San Guillermo block stands out in the central plain, the Tostado–Selva scarp forming its western boundary. It is located in an intraplate setting characterized by low tectonic activity. However recent uplift can be inferred by means of terrain analysis and the sedimentology of the Palo Negro Fm. Pond deposits (Palo Negro Fm.) observed in the scarp suggest topographic inversion during the Late Quaternary in the Palo Negro area. The morphology indicates that the deformation was widely distributed, forming a gentle (ca. 5 m amplitude and 13 km wavelength) asymmetric fold. Low sinuosity lineaments located in the base of the scarp, coincident with knick points in the topographic profile, can be interpreted as the projection of tip-lines by high-angle fault-propagation. This geometry is compatible with reverse kinematics on blind faults.A deformation style with reactivation of pre-existing faults is consistent with structural observations. Seismic reflectors suggest the presence of Cretaceous high-angle normal faults, and the orientation of lineaments is similar to the orientation of the graben systems and transverse accommodation zones originated during the opening of the South Atlantic Ocean.One OSL dating of 67.4 ± 5.1 kyr B.P (from Palo Negro Fm. supposed as deposited on a flat plain floor) and a height difference of 9.5 m measured in the Tostado–Selva scarp account for an averaged uplift rate of 0.14 mm/year.