Retrograde Evolution Research Articles

Role of fluids in migmatites: CO2‐H2O fluid inclusions in leucosomes from the Deep Freeze Range migmatites (Terra Nova Bay, Antarctica)

ABSTRACT The metasedimentary sequence of the Deep Freeze Range (northern Victoria Land, Antarctica) experienced high‐T/low‐F metamorphism during the Cambro‐Ordovician Ross orogeny. The reaction Bt + Sil + Qtz = Grt + Crd + Kfs + melt was responsible for the formation of migmatites. Peak conditions were c. 700–750° C, c. 3.5–5 kbar and xH2Oc. 0.5).Distribution of fluid inclusions is controlled by host rock type: (1) CO2‐H2O fluid inclusions occur only in graphite‐free leucosomes; (2) CO2–CH4± H2O fluid inclusions are the most common type in leucosomes, and in graphite‐bearing mesosomes and gneiss; and (3) CO2–N2–CH4 fluid inclusions are observed only in the gneiss, and subordinately in mesosomes.CO2–H2O mixtures (41% CO2, 58% H2O, 1% Nad mol.%) are interpreted as remnants of a synmig‐matization fluid; their composition and density are compatible P–T–aH2O conditions of migmatization (c. 750° C, c. 4 kbar, xH2Oc. 0.5). CO2‐H2O fluid in graphite‐free leucosomes cannot originate via partial melting of graphite‐bearing mesosomes in a closed system; this would have produced a mixed CO2–CH4 fluid in the leucosomes by a reaction such as Bt + Sil + Qtz + C ± H2O = Grt + Crd + Kfs + L + CO2+ CH4. We conclude that an externally derived oxidizing CO2‐H2O fluid was present in the middle crust and initiated anatexis.High‐density CO2‐rich fluid with traces of CH4 characterizes the retrograde evolution of these rocks at high temperatures and support isobaric cooling (P–T anticlockwise path). In unmigmatized gneiss, mixed CO2–N2–CH4 fluid yields isochores compatible with peak metamorphic conditions (c. 700–750° C, c. 4–4.5 kbar); they may represent a peak metamorphic fluid that pre‐dated the migmatization.

Shallow Gas Reservoir in a Pleistocene Transgressive Sand Sheet Developed During the Drowning of Retrograde Delta Lobes, Louisiana Continental Shelf: ABSTRACT

A strongly negative seismic amplitude discovered in our 3-D data set at 450 ms has been tied to the interface marking the top of a thick sand section ca. -1300 ft sub-sea. Sequence stratigraphic and stacking pattern analysis of SP and GR logs point to a Late Pleistocene low-stand delta bar origin for the thick, blocky sands in the lower part of the section. Resistivity data shows the delta bar sands are wet with salt water while an uppermost, thin sand member, capped by shale constituting a notable flooding surface, and potential vertical seal, exhibited a high resistivity signature. With other evidence, this is considered to reflect the presence of free gas in the sand's pore spaces. An amplitude extraction made to evaluate the reservoir potential of the gas-charged sand member revealed a pattern consistent with three, deltaic lobes aligned along a former drainage axis. The mapped features are considered the result of retrograde delta migration and geomorphic evolution in response to rising sea levels late in the low stand. The upper, gas-charged sand member was interpreted, based on modern analogs, as a transgressive sand sheet containing a combination of facies related to the sub-environments of delta lobe destruction andmore » flooding during rapid marine transgression, e.g.: re-worked barrier island; marine sand shoal; and, inner neuritic shelf sands. The Chandeleur Islands and Ship Shoal are modern examples of these features. Because of the relatively thin but widespread character and good sand quality expected for a transgressive sand sheet, this prospect was selected as a low-risk, low-cost candidate for horizontal drilling and completion.« less

On the levels of enzymatic substrate specificity: Implications for the early evolution of metabolic pathways

The most frequently invoked explanation for the origin of metabolic pathways is the retrograde evolution hypothesis. In contrast, according to the so-called “patchwork” theory, metabolism evolved by the recruitment of relatively inefficient small enzymes of broad specificity that could react with a wide range of chemically related substrates. In this paper it is argued that both sequence comparisons and experimental results on enzyme substrate specificity support the patchwork assembly theory. The available evidence supports previous suggestions that gene duplication events followed by a gradual neoDarwinian accumulation of mutations and other minute genetic changes lead to the narrowing and modification of enzyme function in at least some primordial metabolic pathways.

Substrate and inhibitor specificities of the thermostable alcohol dehydrogenase allozymes ADH-71k and ADH-FCh.D. ofDrosophila melanogaster

Purified thermostable alcohol dehydrogenase allozymes ADH-71k and ADH-FCh.D. of Drosophila melanogaster have been compared with the two common enzyme forms ADH-F and ADH-S. Enzyme kinetic parameters for various primary and secondary alcohols were determined under standard conditions used previously. Both ADH-71k and ADH-FCh.D. show ADH-S-like reaction kinetics and Km values, due to retrograde evolution at site 214, Pro-->Ser. Inhibition studies with alcohol dehydrogenase inhibitors pyrazole, 4-methylpyrazole, and cibacron blue 3GA were also performed. Activity measurements on crude extracts of larvae and flies from isogenic lines of ADH-FCh.D. revealed a consistently higher activity than in ADH-71k-containing strains, in contrast to the original strains.

Orthopyroxenes from granulite rocks of the Wilson Terrane (Victoria Land, Antarctica): crystal chemistry and cooling history

The compositions of 17 orthopyroxenes from granulites facies rocks of the Wilson Terrane (Ross Orogen), determined by microprobe analysis, are consistent with a metamorphic origin. Three groups may be distinguished : a) orthopyroxenes with high Al 2 O 3 content (2.85-5.33 weight%) and X Mg =0.46-0.60, from garnetcordierite granulites; b) orthopyroxenes with low Al 2 O 3 (less than 0.9 wt%) and X Mg =0.49-0.62, from enderbites; c) orthopyroxenes with very low Al 2 O 3 (less than 0.4 wt%) and X Mg =0.70, from two-pyroxene mafic granulites. These differences do not reflect different metamorphic conditions, but are due to local mineral assemblages. Zoning occurs in few orthopyroxenes from garnet-cordierite and mafic granulites, in both cases doe to a retrograde evolution. The crystal chemistry and site populations of two Al-rich and one Al-poor specimen were determined by X-ray single crystal diffraction. The most relevant structural differences are the decrease in M1 and M2 polyhedral dimensions, and an increase in TB, both induced by Al. The high degree of Fe 2+ -Mg order (k D between 0.017 and 0.023) is coherent with slow cooling. Closure temperatures of the ordering process are between 214 ° and 243 ° C, with cooling rates in the range expected for a regional metamorphic terrane

Petrological constraints and PT path of Devonian collapse tectonics within the Scandian mountain belt (Western Gneiss Region, Norway)

In Norway, post-Caledonian extension controlled the formation and sedimentation of Middle Devonian sedimentary basins. These ‘collapse basins’ were deposited along a low-angle extensional detachment fault on top of Caledonian high-grade metamorphic rocks of the Western Gneiss Region. This paper concerns the petrological analysis of eclogite and high-pressure micaschist collected within the extensional shear zone. Study of gneisses and micaschists surrounding these high-pressure rocks allows description of the complete PT path which characterized this area. Application of appropriate geothermometers and geobarometers confirms that the retrograde evolution following high-pressure conditions (ll-15kbar at 500-600°C) was characterized by nearly isothermal decompression followed by amphibolite facies metamorphism. The shape of the PT path is compared with theoretical models of thermal evolution of continental orogenic crust during uplift. We can observe an increase in temperature during the first stages of decompression smaller than predicted by the models. Considering the evidence for extensional tectonic in the study area, we propose to explain such a small increase in temperature during the decompression by the intervention of extension which accelerates the collapse process and reduces the late-orogenic heating of the Caledonian thickened crust.

Retrograde evolution in the Central Vosges mountains (northeastern France): implications for the metamorphic history of high-grade rocks during the Variscan orogeny

Metamorphism in the Vosges massif is characterized by rocks formed under a wide range of pressures and temperatures: high-pressure rocks (garnet peridotites and eclogites facies rocks) are found together with low-pressure rocks (garnetcordierite stability field). Hitherto, metamorphism has been interpreted within the framework of an identical P- T evolution for all rock types corresponding to either two separate geotectonic events or a unique P- T evolution from high-pressure to low-pressure conditions. We studied gneisses from the Vosges massif in order to constrain the P- T- t path they followed. P- T estimation indicates that gneisses equilibrated between 5 and 7 kbar during peak metamorphism time. Rocks later experienced cooling from temperatures above 650°C down to 550°C. It is possible that the entire P- T evolution of the gneisses took place within the lower part of the amphibolite facies; which is inconsistent with the P- T evolution of the high-pressure rocks. The noteworthy absence of muscovite + staurolite assemblages is attributed to conditions of low H 2O and low O 2 activity. The consequences for the geodynamics in the Vosges massif are that at least two separate geotectonic events are required in order to explain the juxtaposition of high-pressure rocks with low-pressure amphibolite facies rocks.

Geological relationships in high‐grade gneiss of the Brattstrand Bluffs coastline, Prydz Bay, East Antarctica

The Brattstrand Bluffs coastline of Prydz Bay, east Antarctica consists of Proterozoic granulite and subordinate late intrusions. Two gneissic associations have been identified, one comprising interlayered felsic and mafic orthogneiss, and the other comprising metapelite‐dominated paragneiss. The orthogneiss exhibits fabrics (S1) and migmatitic textures developed in metamorphic events not experienced by the paragneiss, which is interpreted as a cover sequence deposited onto the orthogneiss after these early events. Cover and basement were tectonically interleaved (D2), and then metamorphosed during a Proterozoic granulite event (∼ 1000 Ma). Geothermobarometry of garnet‐orthopyroxene‐plagioclase‐quartz semi‐pelite yields peak pressure‐temperature estimates of 0.6 GPa and 850°C for this event. Peak metamorphism was associated with development of a penetrative foliation (S3), and generation and extraction of leucocratic melts from metapelite. Textural studies of metapelitic migmatite indicate that partial melting proceeded through incongruent biotite melting reactions such as: biotite + sillimanite + quartz = garnet + cordierite + K‐feldspar + melt, which proceed in the absence of a volatile fluid phase. Retrograde mineral textures, such as spinel‐cordierite symplectites after garnet and sillimanite, indicate decompression from peak pressures to 0.3–0.4 GPa, which was partly synchronous with localized development of flat‐lying folds and shear fabrics (D4). This combination of decompression and subhorizontal shear is attributed to extensional collapse of overthickened crust. Further retrograde evolution was characterized by cooling, emplacement of a granite pluton and dykes, and D5 shortening to produce a regional antiform under amphibolite to granulite conditions. A pegmatite suite was intruded at ∼ 770 Ma, and mylonite development, further granite intrusion and a greenschist overprint occurred at 500 Ma.

From granulites to eclogites in the Sesia zone (Italian Western Alps): a record of the opening and closure of the Piedmont ocean

The Sesia zone (Italian Western Alps) offers one of the best preserved examples of pre‐Alpine basement reactivated, under eclogite facies conditions, during the Alpine orogenesis. A detailed mineralogical study of eclogitized acid and basic granulites, and related amphibolites, is presented. In these rare weak to undeformed rocks microstructural investigations allow three main metamorphic stages to be distinguished.(a) A medium‐ to low‐Pgranulite stage giving rise to the development of orthopyroxene + garnet + plagioclase + brown amphibole + ilmenite ± biotite in basic granulites and garnet + K‐feldspar + plagioclase + cordierite + sillimanite + biotite + ilmenite in acid granulites.(b) A post‐granulite re‐equilibration, associated with the development of shear zones, producing discrete amphibolitization of the basic granulites and widespread development of biotite + sillimanite + cordierite + spinel in the acid rocks.(c) An eo‐Alpine eclogite stage giving rise to the crystallization of high‐Pand low‐Tassemblages.In an effort to quantify this evolution, independent well‐calibrated thermobarometers were applied to basic and acid rocks. For the granulite event,P‐Testimates are 7–9 kbar and 700–800° C, and for subsequent retrograde evolution,P‐Twas 4–5 kbar and 600° C. For the eo‐Alpine eclogite metamorphism, pressure and temperature conditions were 14–16 kbar and 550° C.The inferredP‐Tpath is consistent with an uplift of continental crust produced by crustal thinning prior to the subduction of the continental rocks. In the light of the available geochronological constraints we propose to relate the pre‐Alpine granulite and post‐granulite retrograde evolution to the Permo‐Jurassic extensional regime. The complex granulite‐eclogite transition is thus regarded as a record of the opening and of the closure of the Piedmont ocean.

Le socle vosgien / The vosgian basement

The Vosges basement — This synthesis about the Vosges basement is based on the assessment of the geological results of the two last decades. The geologic units, their structures and their geodynamic evolution are described, first for the northern Vosges (Saxo-Thuringian Vosges), then for the middle and southern Vosges (Moldanubian Vosges). The history of these two areas which display a contrasted evolution during the Lower Palaeozoic can be studied as part of the geodynamics of the Variscan Central Europe. The metamorphism of the pre-Devonian basement shows a progressive retrograde evolution after an eclogitic episode at the base of the Palaeozoic. The magmatism is contemporaneous with the paroxysmal structuration of the variscan belt and displays a jerky evolution which leads to products resulting from an important crustal contamination. The Devono-Dinantian volcano-sedimentary cover gives evidence of marine sedimentary basins which evolve to emersion. Afterwards, this cover is affected by thrust tectonics which produces migmatization at the bottom of the crystalline nappes. The important Visean shortening generates the nappe structuration with vergences on both sides of the Lalaye-Lubine dislocation. At last, comparisons with other massives of the Variscan belt are outlined.

Crust‐mantle relationships in the French Variscan chain: the example of the Southern Monts du Lyonnais unit (eastern French Massif Central)

Garnet lherzolite from the Lyonnais area (eastern French Massif Central) occurs as several lenses elongated within the regional foliation of garnet‐biotite‐sillimanite gneisses. Within the peridotites a mylonitic foliation can be observed which clearly is oblique to the regional foliation of the surrounding gneisses. Petrological and thermobarometric studies emphasize a tectonometamorphic re‐equilibration for both crustal and mantle rocks characterized by a prograde metamorphic stage followed by retrograde evolution. During the burial stage, interpreted as lithospheric subduction, the peridotites underwent their mylonitic deformation, under high‐pressure conditions (23–30 kbar). In contrast, the paragneisses have suffered their deformation during the retromorphic evolution under mesozonal conditions (6–8 kbar, 700°C). Our thermobarometric investigations allow us to interpret the granulitic/ultramafic association from the Monts du Lyonnais area as a lithospheric section buried into a Palaeozoic subduction zone, laminated during continental collision and uplifted by erosion processes.

Mineralogy, petrology and P–T–t path of Ca–Na amphibole assemblages, Saint‐Martin des Noyers formation, Vendée, France

Abstract The Saint‐Martin des Noyers Formation is interpreted as a slice of an island‐arc system of Lower to Middle Palaeozoic age, located in the internal part of the Variscan orogen in Vendée (Armorican Massif, France). Metamorphosed igneous rocks range in composition from ultramafic to rhyolitic. The regular increase in the FeO/(FeO+MgO) ratio, from mafic to silicic samples, results in a systematic variability in the nature and composition of the metamorphic phases. In basaltic samples, the occurrence of relict garnet‐barroisite assemblages suggests relatively high‐pressure conditions for the peak of metamorphism. During a subsequent retrograde evolution, the primary barroisitic hornblendes recrystallized to texturally complex mixtures of actinolite and hornblende. Despite this complication, it is possible to decipher a P–T‐t path based on amphibole chemistry. The P–T trajectory deduced is dominated by the effect of pressure and consistent with early underthrusting and subsequent tectonic uplift of the ancient arc of Saint‐Martin des Noyers.

A case of obduction-related high-pressure, low-temperature metamorphism in upper crustal nappes, Arabian continental margin, Oman: P-T paths and kinematic interpretation

In the southeastern Oman Mountains, the Saih Hatat tectonic window reveals Arabian continental material beneath the oceanic (sensu. lato) nappes i.e., the Hawasina sedimentary units and the huge Sumail ophiolite. The northeastern (internal) half of the window suffered an intense deformation associated with a high-pressure, low-temperature ( HP- LT) metamorphism, approximately dated at 80-75 Ma according to the available stratigraphie data. After the deformation, due to the absence of collision, the obduction-inherited structural and metamorphic pattern was virtually unmodified. A metamorphic map is established based on mineral assemblages in metapelites and metabasites. Low-grade assemblages occur in the highest units (Muscat nappes, Quryat unit). Characteristic minerals are Fe Mg carpholite (± lawsonite) associated with kaolinite or pyrophyllite and locally sudoite in the metapelites while lawsonite and blue amphiboles are found in the metabasites. High-grade rocks outcrop in the deepest and easternmost units (As Sifah antiform) and are typified by an epidote-glaucophane-bearing eclogite assemblage. Intermediate units (Hulw) show assemblages of intermediate grade (chloritoid schists and glaucophane-bearing metabasites). Higher grade assemblages such as talc-chloritoid were not found. Inverted gradients are observed locally in the Muscat nappes (pyrophyllitebearing units above kaolinite-quartz-bearing units) and these are ascribed to late metamorphic thrust displacements which are demonstrated by microstructures. A complex P- T evolution is fairly well documented by the mineral assemblages. Prograde evolution in the subophiolitic (continental rocks) began with a rather “hot” gradient of about 30°–35°C/km (with chloritoid growth in the higher units). Thereafter pressure increased with the temperature remaining constant or even decreasing (chloritoid replaced by kaolinite + oxides and carpholite growth) in synkinematic conditions. The peak P- T conditions are close to 8 kbar, 270°C for the Muscat nappes and 11 kbar, 400°C for the As Sifah unit. The corresponding surface-related gradient is low (10°–15°C/km). During the retrograde evolution, pressure and temperature generally decreased concomitantly, but in some of the upper units temperature remained constant or increased slightly for some time, at which point sudoite growth from carpholite took place. The metamorphic pattern and mineral evolution are interpreted in the framework of the Late Cretaceous obduction. The already cold oceanic lithosphere was thrust upon a relatively hot thinned passive margin (with coeval basaltic volcanism). Pressure progressively increased in the passive margin due to the wedge shape of the advancing ophiolite, the maximum thickness of which was about 25 km in the As Sifah area. In this area, the overburden was also attributable to the accumulation of continental material (up to 5 km thick) by means of a complex set of synmetamorphic, often conjugated folds and thrusts. The basal peridotites of the obducted slab were probably not hotter than 300° C during its thrusting upon the continental margin. The temperature of the tectonic pile below the ophiolite remained low because it was thrust above more external and still colder continental areas. Late thrusting in the same southerly direction controlled the retrograde evolution, together with erosional and gravity-driven tectonic unloading.

The complete nucleotide sequence of the ilvGMEDA cluster of Escherichia coli K-12

The ilvGMEDA gene cluster of Escherichia coli K-12 has been the focus of intensive genetic and biochemical analysis for the past 30 years. Genetic regulation of the ilvGMEDA cluster involves attenuation, internal promoters, internal Rho-dependent termination sites, a site of polarity in the ilvG pseudogene of the wild-type organism, and autoregulation by the ilvA gene product, the biosynthetic l-threonine deaminase. We have now completed the nucleotide sequence of the 6600-bp cluster and have analyzed it, along with the ilvYC, ilvBN, and ilvIH genes, for codon frequencies and possible evolutionary relationships. The isoleucine content of each of the gene products of the ilvGMEDA cluster is quite similar (less than a two-fold variation), thus excluding one possible interpretation of the isoleucine-specific downstream amplification phenomenon. There is no evidence for retrograde evolution in the cluster since no significant homologies are detectable among genes that catalyze sequential reactions of the pathway. A highly significant homology does exist, however, between the threonine deaminases of yeast mitochondria and E. coli. The sequence at the boundary of the ilvA and ilvD genes is TAATAATG, so that the second TAA stop codon of ilvD overlaps the ATG initiation codon of ilvA.

The eclogites of the Lato hills, south Togo, West Africa: Relics from the early tectonometamorphic evolution of the Pan-African orogeny

The eclogites of the Lato hills outcrop within the Pan-African orogenic belt of south Togoland (Dahomeyides) are located within the western border of the Bénin plain structural unit (ex Dahomeyen). They are closely associated with epi- to mesozonal metasediments, which are similar to the Pan-African cover formations of the Atacora unit. Two groups of eclogites have been distinguished according to field relations and to the nature of the country rocks; but they display a similar metamorphic history. The mineralogical and textural transformations of eclogites indicate: (1) a progressive increase of the P— T regime; (2) a mylonitization event close to the culmination of eclogitic metamorphism; and (3) a retrograde evolution towards amphibolite-facies conditions. Likewise, quartzites, closely associated with eclogites, exhibit high- P parageneses compatible with that of the eclogite formation. These observations allow us to consider the eclogitisation processes as a crustal transformation of mafic igneous rocks, and to assign a Pan-African age to these recrystallizations. Therefore, the Mount Lato eclogites represent rare witnesses from the early tectonometamorphic evolution of the Dahomeyides range.

Enzyme recruitment in evolution of new function.

Ancient cells must have possessed small gene content. Primitive enzymes may have possessed broad specificity and undeveloped regulation mechanisms. The considerable substrate ambiguity of these enzymes resulted in the formation of minor amounts of erroneous products. Fortuitous formation of metabolites offered ancient cells maximum biochemical flexibility with minimal gene content. Gene duplication provided the opportunity for increased gene content and increased specialization of the diverging enzymes, the substrate specialization being further reinforced by the development of regualtory mechanisms. Recuritment of enzymes for new pathways did not necessarily require the sequential and backwardly evolving progression of evolutionary steps required by the hypothesis of retrograde evolution of biochemical pathways. Substrate ambiguity remains a conspicuous feature of many contemporary proteins, and evolutionary exploitation of substrate ambiguity in a variety of organisms is still apparent.