Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy最新文献

Granulite terranes from Central Spain are Hercynian anatectic areas of mid-crustal levels (P-T conditions: 750–800° C and 4–6 kb). The granulite xenolith suite of the same sector comes from the lower crust as deduced from their P-T estimates (850–950° C and 8–11 kb). Chemical differences between granulites from terranes and those from xenoliths are less marked in Central Spain than in other places as this xenolith suite lack mafic lithologies. Granulitic xenoliths show U and Rb depletion, and occasionally a subtle enrichment in Ba, Fe, Mg, Ti, (V) and HREE. Contents in Zr, Th, Y, and REE in both granulite types might be explained as a consequence of the minor role of accessory phases in controlling the geochemistry of the residual rocks in the lower crust. Migmatitic leucosomes and other felsic anatectic melts show the same trend of Rb and K enrichment and Fe, Mg, Ti, Cr, Ni, V and Th, Y, Zr, Nb, REE impoverishment (compared to protoliths) than granitic plutons. Although plutonic granites show higher Ca (Na), Fe and Mg contents a genetic link with anatectic melts is suggested. Isotopic differences (Sr, O) between granitic plutons and anatectic melts might be a reflection of their derivation from a different crustal level in an isotopically stratified crust.

Metapelitic xenoliths enclosed in the Crd-Grt-bearing lavas of the Neogene Volcanic Province of SE Spain retain evidence of partial melting and relevant information on the mechanisms and P-T conditions of crustal anatexis, preserved by rapid exhumation and cooling during eruption. Both at El Joyazo and Mazarrón, microstructures show that anatexis was accompanied by foliation development, implying that the xenoliths represent portions of a deforming crystalline basement, partially molten before being enclosed in the dacite.

At El Joyazo, the xenoliths have a marked restitic composition, are made of Bt-Pl-Sil-Grt-graphite (±Ilm, Crd, Her, Qtz), and contain abundant leucogranitic glass. Primary glass inclusions in all minerals indicate that the whole restite assemblage crystallised in the presence of melt, which is only possible by a disequilibrium melting mechanism due to very rapid heating rates. Variable degrees of graphite crystallinity point to syn-anatectic crystallisation of graphite, implying that the main stage of anatexis took place under fluid-present conditions. Further melting of biotite to hercynitic spinel was probably fluid-absent. Mass balance calculations among glasses, xenoliths and probable metapelitic protoliths from the basement of the Betic Cordillera indicate degrees of melting in the range of 35–60 wt. %.

Crustal anatexis took place at 5–7 kbar, 850 ± 50 °C, and was followed by a further melting stage at T>900°C, probably when the xenoliths were already incorporated into the dacite. Calculated pressures approximate the actual Moho depth in the region (ca. 21 km), and suggest that partial melting of the xenoliths occurred close to the crust-mantle boundary. The very high temperatures, the absence of HP relicts, and the syn-anatectic pseudomorphs of sillimanite after andalusite observed in many xenoliths at Mazarrón, are difficult to reconcile with a model of decompression melting, and rather suggest regional scale (isobaric) heating related to emplacement at shallow depth of asthenospheric mantle and/or mantle derived magmas.

Melt extraction in migmatites occurs via melt channels of increasing width from source areas (often recognizable by the location of incongruent phases) via concordant leucosomes to discordant melt conduits. To test the contribution of migmatites to granite genesis, melt production and melt loss need to be quantified at specimen and outcrop scale.

Melt production during dehydration melting can be modelled by calculating the volume ratio of melt and incongruent phases from balanced melting reactions. Melt loss can be quantified by (i) comparing these predicted volume ratios with ratios derived in outcrop; and (ii) modelling strain patterns near melt loss structures. A field test in SW Finland shows a reasonable correspondence between melt loss estimates from leucosome/garnet volume ratios and from melt loss structures, if the original shape of the melt patch is assumed to have been linear.

The P-T (pressure-temperature) paths taken by the high-grade metamorphic rocks during orogenesis govern which melting and crystallisation reactions are encountered and hence the location and amount of melt. Small differences in rock fertility, water amount and migmatite deformability influence the amount and distribution of anatectic partial melt on an outcrop scale. Layer-scale migration of H₂O in response to gradients in μH₂O controls when melting in one layer occurs while partial melt in a nearby layer crystallises. Suprasolidus decompression - dehydration reactions (SDDR) can occur patchily at mid-crustal depths generating feldspathic segregations with alumino-silicates, and releasing H₂O. Quite different mineralogies and textures are diagnostic of the possible crystallisation reactions at different crustal depths. Local mineralogical variations in anatectites can reveal whether melting occurred in response to decompression through a melting reaction or to access of H₂O. This is particularly important to help decide whether anatexis required a localised heat supply or an influx of H₂O.

Most published interpretations infer that the Cooma Granodiorite (southeastern Australia) was formed by more or less in situ melting of metasedimentary rocks of the Cooma Complex. Detailed work has shown that melting of metapelites, which occurred by biotite breakdown during D3 (after muscovite had disappeared), produced relatively immobile, plagioclase-poor or plagioclase-free leucosomes that are compositionally unsuitable as the source magma for the granodiorite. However, melting of feldspathic metapsammitic rocks, which occurred during D5, as P-T conditions followed an anticlockwise path, produced mobile, plagioclase-rich leucosomes that are more appropriate for the granodiorite source magma. Though gradations from metapsammite-derived migmatite to Cooma Granodiorite are present, accumulation of magma derived locally from metapsammite melting appears to be unable to account for all of the exposed body of Cooma Granodiorite, implying some ascent of similar magma from deeper levels of the source rocks.

An interdisciplinary approach is used to quantify partial melt fractions and to infer the origin and distribution (melt structure) of melts located in the crust beneath the Central Andes and the Tibetan plateau. In these areas field observations of Low Velocity Zones (LVZ) and High Conductivity Zones (HCZ), which are commonly attributed to partial melting, are used to quantify melt fractions. Additional information is obtained from ν_P/ν_S ratios, seismic attenuation data, and heat flow density and gravity anomalies. These data accompanied by thermal modelling suggest that melts of mainly crustal origin are interconnected through dykes and veins. Experimental results and model calculations indicate that the minimum fraction of melt necessary to describe the LVZs and HCZs in the Central Andes and the Tibetan plateau is approximately 20 vol.%, and the melt has a non-ideal interconnectivity.

Geophysical and field-based studies indicate that granitic plutons occur as either tabular (disk) or wedge (funnel) shapes whose length (L) to thickness (T) ratio is controlled by the empirical power law, T = 0.6(±0.15)L^0.6(±0.1). The dimensions of plutons are self-similar to other natural subsidence phenomena (calderas, ice cauldrons, sinkholes, ice pits) and it is proposed that they grow in a similar fashion by withdrawal of material (melt) from an underlying source, which is then transferred to the growing pluton within the crust. Experimental studies show that growth of subsidence structures occurs by vertical inflation ⪢ horizontal elongation of an initial depression with L ≈ width of the source region. If pluton growth is modelled in the same way, the empirical power law relating T and L defines limits for pluton growth that are imposed by the width, thickness and degree of partial melting from a lower crustal source. Several growth modes that predict testable internal structural patterns are identified for plutons, depending on whether they are tabular or wedge-shaped, grow by continuous or pulsed magma delivery and whether magma is accreted from bottom to top, or vice versa. Rates of pluton growth are geologically fast (hundreds to hundreds of thousands of years) if magma supply is effectively continuous, but can also take millions of years if the time between magma delivery events is much longer than magma injection events. Plutons formed by melt extraction from an area directly beneath require large degrees of partial melting and or very thick sources. Lower degrees of partial melting and thinner sources are permitted when melt extraction occurs over a larger region, which can lead to the formation of spaced plutons. Tabular pluton growth will tend to favour widely spaced plutons, unless degrees of partial melting in the source are high. Wedge-shaped plutons can form much closer together and require lower degrees of partial melting. These results are in general agreement with current geophysical, petrological and experimental estimates of partial melting in the lower continental crust.

We present field and petrological observations on the nature of layer-parallel leucosomes in late syntectonic migmatites in the contact aureole of the Onawa pluton, central Maine, USA, and use the observations to constrain a model for the formation of these leucosomes by deformation-controlled melt segregation during anatexis. Observations include: 1) the preferred position of lit-par-lit leucosomes near or at contacts between graded sedimentary beds; 2) the presence of symmetric melanosomes adjacent to leucosome layers; 3) evidence for asymmetric inflation of leucosomes by melt; 4) wide variability in leucosome modal mineralogy; 5) the similarity of plagioclase compositions and zoning trends in leucosomes and adjacent mesosomes; 6) quartz and plagioclase clusters in mesosome aligned parallel to the axial surface of regular folds of the layers; 7) symmetric alkali element depletion trends in mesosomes adjacent to lit-par-lit leucosomes; 8) textural evidence indicating that melt was involved in leucosome formation. Based on these observations, we propose a model for lit-par-lit leucosome formation by differential stress-driven melt segregation from less competent, micarich layers, or portions of graded layers, to more competent quartzo-felspathic layers, or portions of layers, leading to layer inflation by melt inflow, rather than segregation of melt in shear or tensile structures. Syn-anatectic contractional folds in part controlled the patterns of melt migration to the leucosomes. Variable proportions of unmelted material initially present in the leucosome layers, of melt added to these layers and minerals crystallized from this melt, and of melt lost from the layers to external sinks explain the variability in lit-par-lit leucosome compositions in these rocks.

The thermal and rheological structure of orogens determines their mechanical behaviour. Collosional orogens are characterized by a clockwise P-T evolution, which means that in the core, where temperatures exceed the wet solidus for common crustal rocks, melt may be present during orogenesis. Field observations of eroded orogens show that middle crust is migmatitic, and geophysical observations have been interpreted to suggest the presence of melt in active orogens. Indeed, the vol. % melt in some active orogens has been estimated by conductivity modelling, assuming that melt is the cause of the anomalies recorded in the data and based on laboratory experiments to calibrate the models. A consequence of these results is that orogenic collapse in mature orogens may be controlled by a partially-molten layer that decouples weak crust from subducting lithosphere, and such a weak layer may enable exhumation of deeply buried crust. Field observations in ancient orogens show that melt segregation and extraction are syntectonic processes, and that melt migration pathways commonly relate to rock fabrics. These processes are being investigated using analog and numerical models. Leucosomes in depleted migmatites record the remnant permeability network, but evolution of permeability networks and amplification of anomalies are poorly understood. Melt segregation and extraction may be cyclic or continuous, depending on the level of applied differential stress and rate of melt pressure buildup. During the clockwise P-T evolution, H₂O is transferred from protolith to melt as rocks cross dehydration melting reactions, and H₂O may be evolved at low P by crossing supra-solidus decompression—dehydration reactions if micas remain in the depleted protolith. The presence of crystallizing melt or H₂O may enable reaction during cooling. However, metasomatism in the evolution of the crust remains a contentious issue. Processes in the lowermost crust may be inferred from studies of xenolith suites brought to the surface in lavas. Using geochemical data, statistical methods and modeling may be applied to evaluate whether migmatites are sources or magma transfer zones for granites, or simply segregated melt that was stagnant in residue, and to compare xenoliths of inferred lower crust with exposed deep crust. Upper crustal granites are a necessary complement to melt-depleted granulites common in the lower crust, but the role of mafic magma in crustal melting remains uncertain. Plutons occur at various depths above and below the brittle-to-viscous transition in the crust and have a variety of 3-D shapes that may vary systematically with depth. The switch from ascent to emplacement may be caused by amplification of instabilities within (permeability, magma flow rate) or surrounding (strength or state of stress) the ascent column, or by the ascending magma intersecting some discontinuity in the crust. Pluton empl