Petrological Consequences Research Articles

Conjugate shear model for emplacement of mid-crustal plutons during dextral transpression, southern Nova Scotia, Canada: tectonic and petrological consequences

The South Mountain Batholith (SMB) is a syn-tectonic composite batholith emplaced in the upper crust within the Meguma terrane between 380 and 370 Ma during the later stages of the Neoacadian orogeny. Coeval plutons in southern Nova Scotia are surrounded by mid-crustal (∼4 kbar) andalusite–staurolite aureoles and are discordant to northeast-trending, regional Neoacadian folds. Detailed field studies, combined with published results, indicate emplacement within a dextral transpressional regime during the transition from distributed (D1) to focused heterogeneous strain (D2), which provided vertical conduits that facilitated magma ascent. The Port Mouton Pluton intruded along P-orientated crustal-scale fractures as a series of subvertical granitic sheets, which were progressively rotated and folded with ongoing dextral shearing. By contrast, the Barrington Passage Pluton intruded between crustal-scale, antithetic (sinistral) P-shear fractures and spread laterally between them as pulsed increments to form a layered, subhorizontal, and sill-like complex resembling the laccolithic structure of the SMB. The SMB was emplaced below the Meguma Supergroup, with magma derived from underthrust Avalon terrane and Silurian Rockville Notch Group. The lack of mantle components in the SMB suggests transpressional orogenesis facilitated conductive crustal heating without significant mantle addition, consistent with low p-wave velocities of Meguma lower crust. 400–355 Ma zircons, recorded either as inherited grains in granites or in felsic granulite xenoliths, imply the Neoacadian thermal anomaly extended for 45 Ma, but magmatism represented only ∼40% of that perturbation.

Nd isotope systematics of Late Paleozoic granitoids from the Western Transbaikalia (Russia): Petrological consequences and plume model testing

Late Paleozoic granites of the Angara-Vitim batholith (AVB) occupy an area of 200,000 km2 in Western Transbaikalia (the eastern part of the Central Asian fold belt). Batholith granitoids form a sheet-like body with an average thickness of 7–10 km and a volume of about 1 million km3. The granitoid massifs that make up the batholith are composed of high-potassium calc-alkaline and subalkaline quartz monzonites, quartz syenites, amphibole-biotite granodiorites, and biotite granites of autochthonous and allochthonous facies. An extremely high heterogeneity of the batholith isotopic structure was established, which basically corresponds to the parameters of uneven-aged crustal metaterrigenous protoliths. There are significant variations in εNd(Т) and accordingly T(DM-2) in granitoids of different complexes. It is assumed that the isotopic heterogeneity of AVB was formed due to the melting of a limited protoliths number that are maximally contrasting in isotopic and lithological composition: the Paleoproterozoic continental crust with εNd(Т) ≈ -20 ÷ -22 and T(DM-2) = 2.9–2.5 Ga and Neoproterozoic mafic granulites of increased potassium alkalinity, enriched in the juvenile component (εNd(Т) ≈ -3.0; T(DM-2) = 1.2–1.3 Ga). The latter were the main magmas source of postbatholitic alkali granites. Melts from these contrasting protoliths were only in some cases complementary to the salic melts sources. The main mechanism that determined the isotopic composition of AVB granitoids was the mixing of isotopically contrasting magmas in different proportions. Mingling dikes, mafic inclusions in granitoids, and synplutonic mafic intrusions in the AVB indicate an additional mixing component. It was mafic magma from an enriched mantle reactivated in the Late Paleozoic under the mantle plume influence on the heated plastic crust of a young (Hercynian) orogen.

P-T-t reconstructions of South Yenisei Ridge metamorphic history (Siberian craton): Petrological consequences and application to the supercontinental cycles

Studies of gneisses from the Yenisei regional shear zone (YRSZ) provide the first evidence for Mesoproterozoic tectonic events in the geologic history of the South Yenisei Ridge and allowed the recognition of several stages of deformation and metamorphism spanning from Late Paleoproterozoic to Vendian. The first stage (~1.73 Ga), corresponding to the period of granulite-amphibolite metamorphism at P = 5.9 kbar and T = 635 °C, marks the final amalgamation of the Siberian craton to the Paleo-Mesoproterozoic Nuna supercontinent. During the second stage, corresponding to a hypothesized breakup of Nuna as a result of crustal extension, these rocks underwent Mesoproterozoic dynamic metamorphism (P = 7.4 kbar and T = 660 °C) with three peaks at 1.54, 1.38, and 1.25 Ga and the formation of high-pressure blastomylonite rocks in shear zones. Late-stage deformations during the Mesoproterozoic tectonic activity in the region, related to the Grenville-age collision processes and assembly of Rodinia, took place at 1.17-1.03 Ga. The latest pulse of dynamic metamorphism (615–600 Ma) marks the final stage of the Neoproterozoic evolution of the Yenisei Ridge, which is associated with the accretion of island-arc terranes to the western margin of the Siberian craton. The overall duration of identified tectonothermal processes within the South Yenisei Ridge during the Riphean (~650 Ma) is correlated with the duration of geodynamic cycles in the supercontinent evolution. A similar succession and style of tectonothermal events in the history of both the southern and the northern parts of the Yenisei Ridge suggest that they evolved synchronously within a single structure over a prolonged time span (1385–600 Ma). New data on coeavl events identified on the western margin of the Siberian craton contradict the hypothesis of a mantle activity lull (from 1.75 to 0.7 Ga) on the southwestern margins of the Siberian craton during the Precambrian. The synchronous sequence and similar style of tectonic events on the periphery of the large Precambrian Laurentia, Baltica, and Siberia cratons suggest their spatial proximity over a prolonged time span (1550–600 Ma). The above conclusion is consistent with the results of modern paleomagnetic reconstructions suggesting that these cratons represented the cores of Nuna and Rodinia within the above time interval.

Petrological consequences of variations in metamorphic reaction affinity

The extent to which kinetic barriers to nucleation and growth delay the onset of prograde metamorphic reaction, commonly known as overstepping, is related to the macroscopic driving force for reaction, termed reaction affinity. Reaction affinity is defined in the context of overstepping as the Gibbs free-energy difference between the thermodynamically stable, but not-yet-crystallized, products and the metastable reactants. Mineral reactions which release large quantities of H2O, such as chlorite-consuming reactions, have a higher entropy/volume change, and therefore a higher reaction affinity per unit of temperature/pressure overstep, than those which release little or no H2O. The former are expected to be overstepped in temperature or pressure less than the latter. Different methods of calculating reaction affinity are discussed. Reaction affinity ‘maps’ are calculated that graphically portray variations in reaction affinity on equilibrium phase diagrams, allowing predictions to be made about expected degrees of overstepping. Petrological consequences of variations in reaction affinity include: (i) metamorphic reaction intervals may be discrete rather than continuous, especially in broad multivariant domains across which reaction affinity builds slowly; (ii) reaction intervals may not correspond in a simple way to reaction boundaries and domains in an equilibrium phase diagram, and may involve metastable reactions; (iii) overstepping can lead to a ‘cascade effect’, in which several stable and metastable reactions involving the same reactant phases proceed simultaneously; (iv) fluid generation, and possibly fluid presence in general, may be episodic rather than continuous, corresponding to discrete intervals of reaction; (v) overstepping related to slowly building reaction affinity in multivariant reaction intervals may account for the commonly abrupt development in the field of certain index mineral isograds; and (vi) P–T estimation based on combined use of phase diagram sections and mineral modes/compositions on the one hand, and classical thermobarometry methods on the other, may not agree even if the same thermodynamic data are used. Natural examples of the above, both contact and regional, are provided. The success of the metamorphic facies principle suggests that these kinetic effects are second-order features that operate within a broadly equilibrium approach to metamorphism. However, it may be that the close approach to equilibrium occurs primarily at the boundaries between the metamorphic facies, corresponding to discrete intervals of high entropy, dehydration reaction involving consumption of hydrous phases like chlorite (greenschist–amphibolite facies boundary) and mica (amphibolite–granulite facies boundary), and less so within the facies themselves. The results of this study suggest that it is important to consider the possibility of reactions removed from equilibrium when inferring the P–T–t evolution of metamorphic rocks.

Middle-proterozoic anorthosite–rapakivi granite complexes: An example of within-plate magmatism in abnormally thick crust: Evidence from the East European Craton

Large, bimodal, and multistage anorthosite–rapakivi granite complexes (ARGCs) of Middle-Proterozoic age (1.8–1.5 Ga) are distinct magmatic assemblages in the western part of the present-day European Craton. ARGCs formation commenced after stabilization of the Svecofennian orogen and relics of its abnormally thick crust occur in many places. In the eastern part of the Kola-Karelian domain and the Volga-Urals domain, where crustal thickness is normal ∼40 km, ARGCs are practically absent. Geological data evidence that melting processes occurred both in the crust and in the mantle during formation of the complexes. Geochemical peculiarities of ARGC rocks include enrichment in alkalis (mainly in K), Ti, Zn, Pb, Zr, Be, Sn, In, Y, Nb, Rb, F, Cu, W, Mo, sometimes Li and U. The negative ɛNd values and relatively high Th and Zn concentrations that are most frequently observed in the rocks imply that parental magmas were considerably contaminated by crustal components. According to the gravity and seismic profiles, ARGCs represent upper parts of large transcrustal magmatic systems, composed of alternating basic and silicic rocks layers. They are located over mantle uplift structures 10–20 km high, interpreted to represent former mantle plume heads. Origin of transcrustal systems was probably linked with penetrating sill-like bodies of Fe–Ti basalts, which after intruding into sialic crust, caused large-scale re-melting of the granitic material above them, forming two-layer magma chambers. Solidification of such chambers occurred in two stages: lower basic layers solidified first while heated from below, whereas the upper silicic layer crystallized later. Co-existing neighboring chambers led to gravitational instability, overturn, and mass redistribution in large volumes of crust. Furthermore, multistage formation of the ARGCs led to important petrological consequences: contamination of basic magmas by crustally derived SiO 2 and Al 2O 3, causing preferred precipitation of plagioclase and formation of plagioclase cumulates (anorthosites). Diffusion of Na into the basic melt and K into the silicic melt led to the appearance of potassic rapakivi granites. ARGCs on all Precambrian shields were developed in places with stabilized Paleoproterozoic orogens with atypical unusually thick crust compared with the Phanerozoic, and associated with large Mesoproterozoic belts of within-plate felsic volcanism. Under these conditions, the majority of mantle plume-derived mafic magmas were not able to reach the surface and were lost within the crust in the form of sill-like intrusions with zones of intense melting of a sialic roof. Evidently, ARGCs illustrate structure and processes that are not repeated in the Phanerozoic.

Buoyancy, bending, and seismic visibility in deep slab stagnation

The petrological consequences of deep subhorizontal deflection (“stagnation”) of subducting slabs should affect both apparent seismic velocity structures and slab morphology. We construct kinematic thermal models of stagnant slabs and perform thermodynamic modeling of the consequent perturbation of high-pressure phase transitions in mantle minerals, focusing upon Japan as our study area. We calculate associated thermo-petrological buoyancy forces and bending moments which (along with other factors such as viscosity variations and rollback dynamics) may contribute to slab deformation. We consider effects of variations in depth of stagnation, post-stagnation dip angle, phase transition sharpness, transition triplication due to multiple intersection of geotherms with phase boundaries, and potential persistence of metastable phases due to kinetic hindrance. We also estimate seismic velocity anomalies, as might be imaged by seismic tomography, and corresponding seismic velocity gradients, as might be imaged by receiver-function analysis. We find that buoyant bending moment gradients of petrological origin at the base of the transition zone may contribute to slab stagnation. Such buoyancy forces vary with the depth at which stagnation occurs, so that slabs may seek an equilibrium slab stagnation depth. Metastable phase bending moment gradients further enhance slab stagnation, but they thermally decay after ∱/4600•700km of horizontal travel, potentially allowing stagnant slabs to descend into the lower mantle. Stagnant slabs superimpose zones of negative velocity gradient onto a depressed 660-km seismic discontinuity, affecting the seismological visibility of such features. Seismologically resolvable details should depend upon both stagnation depth and the nature of the imaging technique (travel-time tomography vs. boundary-interaction phases). While seismic tomography appears to yield images of stagnant slabs, discontinuity topography beneath Japan resolved by migrated receiver functions appears to be consistent with slab penetration of the transition zone. However, model slabs which bottom around ∱/4780•810km and then bend upwards by a few degrees can match both the tomographic and receiver-function images.

Partitioning of trace elements between carbonate-silicate melts and mantle minerals: Experiment and petrological consequences

The partitioning of a number of trace elements (Ba, Nb, Zr, Y, REE, etc.) between orthopyroxene, garnet, and carbonate-silicate melt was experimentally studied using a belt apparatus at pressures of 3.5–4.2 GPa and temperatures of 1300–1500°C. The experimental products were investigated by electron microprobe analysis and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The experimental melts varied from carbonatitic (∼5 wt % SiO2) at low temperatures (1300–1350°C) to kimberlitic compositions (30 wt % SiO2) at high temperatures (1500°C). The partition coefficients of most elements between orthopyroxene and melt (DiOpx/L) and garnet and melt (DiGrt/L) were almost independent of melt composition (temperature). The DiOpx/L values ranged from <0.01 for the most incompatible Ba and light REE to 0.02–0.08 for moderately incompatible Zr, Y, and heavy REE. The DiGrt/L values were approximately an order of magnitude higher, ∼0.07 for light REE, 0.7 for Y, and 1.5 for Yb. The character of DiGrt/L variations in the systems studied is in general similar to that established for silicate melts without volatile components. However, the differences in the behavior of moderately incompatible and compatible elements (e.g., light and heavy REE) in the experimental systems are less pronounced compared with CO2-free systems. Considering carbonate-silicate and silicate melts as possible agents of mantle metasomatism, it can be concluded that the former can efficiently transport heavy REE, and the latter have a greater affinity for Nb, Ba, and light REE. A characteristic feature of mantle rocks enriched by carbonate-silicate melts is high Ba/La ratio coupled with relatively weakly fractionated REE distribution patterns. It was shown that the high degrees of enrichment observed in natural kimberlites can be explained by a two-stage scenario, including a preliminary invasion of carbonate-silicate melt into depleted harzburgites in the lower parts of the lithosphere and subsequent very low degree melting.

Magmatic anhydrite in granitic rocks: First occurrence and potential petrologic consequences

The Cajon Pass Deep Scientific Drillhole penetrated a minimum of 1 km of Late Cretaceous (?) intermediate plutonic rocks containing magmatic anhydrite, which occurs as both matrix grains and relic microphenocrysts within poikilitic hornblende, plagioclase, and sphene. Phase equilibria of coexisting silicates, oxides, and apatite support petrographic evidence of hypersolidus anhydrite, indicating that intermediate rocks from a mafic-felsic calc-alkalic suite (or suites) crystallized from about 800 to 700 °C at Ptot ~6 kb from hydrous and oxidized, sulfate-saturated andesitic to dacitic magma. The occurrence of anhydrite as inclusions within early crystallizing phases and its association with sulfate-enriched apatite indicates that, despite the potentially rapid destruction of matrix anhydrite by subareal weathering, petrologic evidence of the nature of such volatile-rich magma systems can be retrieved. If found to be widespread by future work, such evidence could be useful in understanding the mechanisms of volatile enrichment in explosive sulfur-rich volcanic systems and the potential relative roles of metasomatized lithospheric mantle sources and shallow-level mixing/ assimilation processes in Cordilleran arc magma systems.

Synchrotron powder diffraction study of phengite 3T from the Dora-Maira massif: P - V - T equation of state and petrological consequences

X-ray powder diffraction measurements have been carried out at ESRF (Grenoble, France) on the ID30 beamline to study the equation of state of 3T phengite (Dora-Maira massif, Italian western Alps) by a large volume cell up to P = 50 kbar and T = 1000 K. Several equations of state (EOS) models (the Vinet EOS, the Birch-Murnaghan EOS and its variants, a VT-polynomial expansion) have been used to interpolate the experimental data and discussed in the light of the results achieved. The thermoelastic properties of 3T phengite (bulk modulus, its derivatives versus pressure or temperature, bulk thermal expansion) have been obtained and an isochoric curve with slope P/T = 0.02 kbar/K has been calculated by means of the Vinet EOS. This slope value supports either the occurrence at the peak conditions (about 30 kbar and 1000 K) of an originally Mg/Si-richer and stiffer phengite or a non-isochoric P-T retrograde path.

The Genesis and Time Distribution of Two Distinctive Proterozoic Stromatolite Microstructures

Stromatolites are commonly viewed as sedimentary proxies for microbial communities. In consequence, secular variation in stromatolite form has been attributed to evolutionary change in mat organisms and/or their interactions with global environments. This interpretation requires that one be able to identify features of stromatolite macrostructure or microstructure that are under direct biological influence and use them to document the sedimentological or petrological consequences of microbial evolution. Well-preserved stromatolites in Proterozoic carbonates of the Siberian Platform contain two distinctive types of microstructure that enable us to address such issues. Preserved microstructures in Baicalia lacera and Tungussia confusa from lower Neoproterozoic (lower Upper Riphean) platform carbonates of several widely separated regions clearly reflect the biology of underlying mat communities; mm-scale laminae of densely interwoven calcified filaments alternate with filament-poor microspar. Comparable carbonate microstructures are known from a number of other Neoproterozoic stromatolites, but are as yet unreported from older successions. Filamentous cyanobacteria formed mats throughout the Proterozoic Eon; thus, the temporal distribution of this microstructure appears principally to reflect secular and environmental variations in carbonate cementation and diagenesis. Omachtenia omachtensis is the characteristic stromatolite of Mesoproterozoic (Lower and, occasionally, Middle Riphean) successions of the Siberian Platform and elsewhere. Its distinctive microstructure consists of mechanically deposited event laminae separated by thin organic films that served as nucleation sites for micritic and, less commonly, fibro-radiate carbonate precipitates. Microbial mats may have stabilized sediments between events, but there is little evidence that mats played an active role in the trapping and binding or precipitation of laminae. Thus, physico-chemical factors must also be responsible for the distribution of these stromatolites in time and space. Analysis of these two microstructures suggests that, in general, secular trends in stromatolite microstructure may encrypt important information about environmental change through the Proterozoic Eon. Microbial evolution may also play a role in determining the stratigraphic distributions of particular Proterozoic stromatolites, but this remains to be demonstrated. Evolution may be most important in driving the progressive environmental restriction of stromatolite-forming microbial communities through time.

Divergent double subduction:Tectonic and petrologic consequences: Comment and Reply

Divergent double subduction:Tectonic and petrologic consequences: Comment and Reply

Divergent double subduction: Tectonic and petrologic consequences

Divergent double subduction, involving removal of an oceanic basin from both sides and collision, has distinct fingerprints of magmatism and significant implications for the tectono-magmatic evolution of orogenic belts. The complete evolution of a divergent double subduction system involves four stages: (1) initial interaction between the two divergent subduction zones on both sides of a single oceanic plate; (2) closure of the oceanic basin when the two overriding plates meet, followed by (3) detachment of the oceanic lithosphere from the overlying crust and sedimentary sections; (4) sinking and disappearance of the oceanic slab into the mantle. Detachment of the oceanic slab leads to intense decompressional melting of mantle and triggers melting of accretionary-wedge strata and lower crust, resulting in large-scale magmatic and volcanic activity. The main consequences of a divergent double subduction system are that it produces opposed thrust systems, extensive long-lived granitoid magmatism with mantle isotopic signature, and volcanism that evolves toward bimodalism. This model can be applied to the western half of the Paleozoic Lachlan fold belt (southeastern Australia) to explain the wide-scale Silurian to early Carboniferous granitoid magmatism, its spatial and temporal relationships, and late bimodal magmatism. Closure of the oceanic basin is thought to have occurred during the Early to Middle Devonian. Detachment of the oceanic slab led to felsic to intermediate-composition magmatism. Bimodal volcanism involving local basaltic flows is a reflection of subsequent sinking of the slab.

Baddeleyite (ZrO2) and zircon (ZrSiO4) from anorthositic rocks of the Laramie anorthosite complex, Wyoming; petrologic consequences and U-Pb ages

Baddeleyite (ZrO₂) and zircon (ZrSiO₄) from anorthositic rocks of the Laramie anorthosite complex, Wyoming; petrologic consequences and U-Pb ages

Thermal and petrological consequences of melt migration within mantle plumes

The high temperatures and high degrees of melting expected in the core of mantle plumes have virtually no expression in the eruption temperatures of hotspot lavas, nor in the composition of their glasses, which is restricted in the basaltic field. A solution to this paradox is looked for in the melt migration processes within the melting region of mantle plumes. Three dimensional convective calculations at Rayleigh number of 10 6 allow estimates of the possible temperature, melt fraction and stress fields within a plume. Two regions with different melt migration patterns can be distinguished. A lower zone ranging in depth from the base of the melting region (150 km) to around 80—100 km where the first melt fraction is redistributed in a sub-horizontal vein network and convects in response to the steep horizontal temperature gradient. This process is able to homogenize the temperature within the melting region very efficiently. The high (300 °C) temperature contrast between the centre of the plume and the surrounding mantle can be reduced to a few tens of degrees at the top of this zone. Fractional crystallization of high pressure phases will strongly modify the composition of the melt as it circulates toward the periphery of the melting region. A second upper zone, where the sub-vertical vein orientation will make possible rapid melt migration toward the surface, extends to the base of the lithosphere. Due to the buffering of the plume temperature around a value close to the mean upper mantle temperature, the degree of adiabatic melting within this upper zone will not greatly exceed that beneath normal spreading centres, even in the case of on-ridge hotspots. The lavas erupted at hotspots are likely to result from the mixing in various proportions of these low pressure melts (basalts) with the highly evolved liquids (possibly with kimberlitic to alkalic affinities) resulting from fractional crystallization of the high-pressure melt fractions produced at the base of the melting region. This scenario could account for the low eruption temperatures and Mg contents of hotspot lavas, in spite of a complex high pressure, and thus high temperature, history evidenced by some geochemical trends.

Amphibolite dehydration-melting: sorting out the solidus

Abstract Amphibolite melts under vapour-absent conditions by dehydration-melting, during which H 2 O released from hornblende is transferred directly into H 2 O-undersaturated silicate liquid. Five independent 1991 studies on amphibolite dehydration-melting differ from each other and from earlier conceptual treatments. Five solidi between 7 and 10 kb vary from 975°C to 740°C. Two solidi above 10 kb are near-vertical, near 975°C and 850°C. Some differences are due to different amphibolite compositions and mineralogy. We use the results of earlier experiments involving garnet, supported by our own new results, to construct a closed-system phase diagram for a simple, fully-hydrated amphibolite (hornblende + plagioclase) showing the solidus and the melting interval for (hornblende + garnet + other minerals + liquid). The amphibolite vapour-absent solidus is defined by the beginnings of two multivariant reactions: (1) a near-vertical curve (large positive dP/dT) where the formation of augite dominates; and (2) a near-horizontal curve at higher pressures where the formation of garnet dominates. The solidus curve bends back (estimated near 900°C, 9 kb) with slope changing to low positive dP/dT. The new phase diagram expands the field for liquid generation with garnet-amphibolite residues to much lower temperatures and pressures than the other recent experimental results, increasing the range from which small amounts of hydrous granitoid melts may be segregated by compaction. The reversed, near-horizontal slope of the solidus near 10 kb has interesting petrological consequences.

Modification of antiphase domain sizes in omphacite by dislocation glide and creep mechanisms and its petrological consequences

AbstractA T.E.M. study of omphacites from the western Italian Alps (Sesia Lanzo Zone and Monviso eclogites) has revealed a bimodal size distribution of antiphase domains: (a) 250–350 Å, (b) ≥ 2500 Å. In addition observed dislocation substructures and ‘large-scale’ antiphase domains are intimately interconnected.A model is presented that can explain modification of the antiphase domain sizes by the interplay of cooling/growth and dislocation glide and/or creep mechanisms. Subsequent coarsening of the modified antiphase domains is inferred to be the result of surface free-energy processes. The model clearly illustrates that only the ‘relatively undeformed’ areas containing the small-scale antiphase domains can be used for thermometric methods.

Experimental petrology: Quantitative boundaries for petrogenesis

The paper is an overview of the current status on experimental petrology — its objectives and its major role in solving problems related to various earth processes. It describes how investigations related to solid-solid transitions, dehydration or decarbonation reactions, and melting studies have helped to formulate petrological models for the earth’s internal structure. It also describes how measurements of physical properties of minerals, melts and vapour under extreme conditions have provided vital information of fluid dynamics of magmatic systems. The paper narrates the role of experimental petrology in calibrating geophysical processes with petrological consequences. Model P-T-X (SiO2)-fluid systems are considered to emphasize the role of various gas species in shifting the solidus in a P-T space, in degrees of melting and composition of the melt. Synthetic models and study of whole rock systems are considered to discuss the zonation and metasomatic processes in the mantle of the earth. The paper is also concerned with mantle convection and the uprise of thermal plumes, particularly, in the oceanic environment. It discusses the petrological structures associated with the plume and shows how static petrological maps are modified by the dynamics of the plumes.

Cambrian extensional tectonics and magmatism within the Southern Oklahoma aulacogen

The tectonic evolution of the Southern Oklahoma aulacogen is partially constrained by the petrological consequences of a bimodal igneous suite associated with the Cambrian rift. Mineralogical and compositional layers and erosional surfaces are recognized as initially subplanar, subhorizontal markers. The progressive rotation of these horizons can be explained by uniform-sense normal faulting. Magmatism, confined to the aulacogen trend, elevated the thermal gradient producing a crustal strength-anisotropy. This ensured the localization of the extension throughout the Cambrian rifting event. The presence of a substantial volume of mafic igneous rocks within the crust along the aulacogen's trend suggests that crustal attenuation was compensated for by the addition of mantle derived material during extension.

Laboratory modeling of convection in magma chambers: Crystallization against sloping floors

Crystallization against inclined floors in magma chambers commonly releases a light depleted fluid which drives compositional convection. We describe the fundamental fluid mechanics of this process and derive equations describing the buoyant compositional boundary layer adjacent to the top of the crystal pile. The question of whether magma chambers become zoned as a result of compositional convection from sloping floors, as has been previously suggested, is addressed in a laboratory study. Because crystal sizes in magma chambers are usually smaller than the thickness of the compositional boundary layer, an appropriate laboratory model is thermal convection from a smooth, sloping heat exchanger. We investigate the circumstances under which this configuration leads to thermal stratification and find that only when the experimental tank has an extension with an unheated floor does a filling box process lead to significant stratification. This geometry, however, is geologically unrealistic, requiring that crystallization be confined to only part of the floor. We conclude that compositional convection from inclined floors will not lead to compositional stratification in magma chambers. Experiments investigating the same problem by cooling and crystallizing aqueous solutions are misleading partly because the size of crystals grown from aqueous solutions is extremely large compared with the thickness of the compositional boundary layer, whereas this is not usually true in magma chambers. This difference has a major influence on the nature of convection and we suggest that caution should be applied in extending the results of aqueous solution experiments involving crystallization to magma chambers. The petrological consequences of this work are (1) the fluid mechanics of crystallization at an inclined floor and horizontal floor are similar and suggestions, made elsewhere, concerning the geological implications in the latter case apply equally to the sloping floor case, (2) large volumes of homogeneous magma can exist in a magma chamber when crystallization is taking place against an inclined floor, and (3) models describing the crystallization of layered intrusions which involve the interaction of crystallization against an inclined floor with compositionally stratified magma require the stratification to be established by an external process.

Cocorp and the continental crust

This is a speculative paper on the continental crust, a major frontier of modern earth science. Seismic reflection profiling data from the COCORP project, coupled with other kinds of geophysical and geological observations, suggest some fresh perspectives on the crust. For example, there is evidence for great differences within the basement at a single site and from one COCORP site to another. A simple layered model is surely not an adequate representation of the crust in most areas. There is, however, the puzzling and rather common occurrence of limited zones of parallel, though not always flat, layered features at various depths in certain parts of the crust. Are the corresponding rocks metasedimentary? Or igneous? Or both? There are transparent zones from which little or no coherent reflected seismic energy is detected. Are the corresponding rocks intrusions? Or highly deformed metasediments? Can degree of deformation suggest whether rocks were deformed in the hanging wall or footwall of a thrust? There is evidence for large‐scale thrusting and related features that emphasizes the great importance of the Wilson cycle to continental tectonics and that raises the possibility that water and rocks of sedimentary origin are trapped deep in the crust by the thrusting process and remain there for long periods with a variety of possible rheological and petrological consequences. In some places the modern Moho and adjacent rocks may correspond to parts of former oceanic or continental margin sections that have been emplaced at depth as a result of thrusting. The widely held concept of the Moho as a consistent boundary that is everywhere similar and that always separates the same kinds of rocks may be a major barrier to further progress in understanding the continental crust.