Melting Of Crustal Rocks Research Articles

Reply to the comments on “Xenoliths in ultrapotassic volcanic rocks in the Lhasa block: direct evidence for crust–mantle mixing and metamorphism in the deep crust”

Stepanov et al. (Contrib Mineral Petrol, 2017) question our conclusion that the UPVs in southern Tibet were derived by partial melting of an old, metasomatized subcontinental lithospheric mantle (SCLM) of the subducted Indian plate. Instead, they propose that these ultrapotassic volcanic rocks (UPVs) are shoshonitic and were generated in two steps: direct melting of crustal rocks first, and then the melts interacted with mantle peridotite. However, the trace element, isotopic, thermal, structural, and seismic evidence is consistent with the xenolith evidence (Wang et al in Contrib Mineral Petrol 172:62, 2016) for hybridisation of ascending Indian subcontinental lithospheric mantle-derived UPV magmas with the deep, isotopically unevolved, Tibetan crust. This necessitates a model whereby partial melting of subducting Indian SCLM generates the UPV suite of southern Tibet.

LA-ICP-MS zircon U-Pb dating, geochemistry and tectonic implications of the Neoproterozoic Xiaoxigong granite at Dunhuang block, northeastern Tarim, NW China

We present new zircon U-Pb ages, Hf isotopes, and whole-rock geochemistry of the Xiaoxigong granite pluton in the Dunhuang block, northeastern Tarim Craton. LA-ICPMS zircon U-Pb dating yields a weighted mean 206Pb/238U age of 841.3 ± 8.1 Ma (MSWD = 4.4), interpreted as the emplacement age of the Xiaoxigong granite. Zircons have eHf(t) values ranging from +4.53 to +8.63 and TDM2 model ages from 1.16 to 1.38 Ga. Geochemical characteristics show that the granite belongs to the shoshonite series and has weakly peraluminous-metaluminous features. It is enriched in Si, K, Na, Rb, Th and REE, and has elevated Ga/Al ratios and high (Zr + Nb + Ce + Y) abundance, and is depleted in Mg, Ca, Mn, Ba, Sr, P and Ti. The REE composition shows significant negative Eu anomalies (Eu/Eu* = 0.18–0.31) and low (La/Yb)N (4.54–7.33). It is typical aluminous A-type granite. The geochemical and Hf isotopic signatures suggest that the Neoproterozoic Xiaoxigong granite was generated by partial melting of crustal rocks during the the post-orogenic period.

Evolution of continental crust of the Aravalli craton, NW India, during the Neoarchaean–Palaeoproterozoic: evidence from geochemistry of granitoids

Neoarchaean–Palaeoproterozoic granitoids of the Aravalli craton, represented by four plutons with different ages, viz. Gingla (2.6–2.4 Ga), Ahar River (2562 Ma), Untala (2505 Ma), and Berach (2440 Ma) granitoids, are classified into three suites: TTG-like, Sanukitoid, and High-K Granitoid suite, all exhibiting negative Nb and Ti anomalies. The TTG-like suite is characterized by high contents of SiO2, Na2O, and LREEs, high (La/Yb)N, low contents of K2O, MgO, Cr, and Ni, and low (Dy/Yb)N, suggesting that this suite formed by partial melting of a subducted basaltic slab without interacting with a mantle wedge. In contrast, the calc-alkaline Sanukitoid suite is marked by a high content of LILEs and mantle-compatible elements, which indicate that this suite formed by partial melting of a slab-fluid metasomatized mantle wedge in a subduction-related arc environment. On the other hand, the High-K Granitoid suite is characterized by high contents of SiO2 and K2O, and low contents of Na2O, MgO, Cr, and Ni with variable Eu anomaly, along with high (La/Sm)N and (La/Yb)N, and low (Dy/Yb)N and Nb/Th. Some high-K granitoids also exhibit A-type characteristics. These features indicate that the High-K Granitoid suite formed by melting of crustal rocks. Early Neoarchaean continental crust formation reflected a slab-melting-dominated magmatic process as evidenced by the TTG-like suite, whereas Palaeoproterozoic petrogenesis was governed by the interaction of slab melt with mantle wedge as demonstrated by the Sanukitoid suite. The High-K Granitoid suite formed during the waning stages of subduction. This study reveals that granitic rocks of the Aravalli craton evolved from slab melting in the Neoarchaean to melting of mantle wedge in the Palaeoproterozoic. Melting of older crust led to the formation of the High-K Granitoid suite.

Early Palaeoproterozoic volcanism of the Karelian Craton: age, sources, and geodynamic setting

A combined study of major and trace elements, Nd isotopes, and U-Pb systematics has been conducted for the early Palaeoproterozoic (Sumian) volcanic rocks and granites localized in different portions of the Karelian Craton. SHRIMP dating of zircons from the Sumian basalts indicates an emplacement age of 2423 ± 31 Ma, which constrains the lower age boundary of the early Palaeoproterozoic sequence at the Karelian Craton. The early Palaeoproterozoic mafic volcanic rocks of the Karelian Craton show practically no lateral geochemical and isotope-geochemical variations. The rocks bear signs of crustal contamination, in particular Nb and Ti negative anomalies, light rare earth element (LREE) enrichment, and nonradiogenic Nd isotope composition. However, some correlations between incompatible element ratios suggest that the crustal signatures were mainly inherited from mantle sources metasomatized during a previous subduction event. En route to the surface, melts presumably experienced only insignificant contamination by crustal material. Felsic rocks do not define common trends with mafic rocks and were formed independently. They exhibit higher REE contents, large-ion lithophile element (LILE) enrichment, and extremely wide variations in Nd isotope composition, which clearly demonstrates a considerable contribution of heterogeneous basement to their formation. Geochemically, the felsic rocks of the Karelian Craton correspond to A2-type granites and were formed by melting of crustal rocks in an anorogenic setting. Their possible sources are Archaean sanukitoid-type granitoids and Archaean granite gneisses. The high Yb content and pronounced Eu anomaly imply that they were generated from a garnet-free pyroxene – plagioclase source at shallow depths. By the Palaeoproterozoic, the older Vodlozero block was colder than the Central Domain, which facilitated the development of the brittle deformations and faulting and, correspondingly, rapid magma ascent to the surface without melting of crustal rocks. This resulted in the absence of felsic rocks and the formation of more primitive basalts in this area.

Genesis and evolution of subduction-zone andesites: evidence from melt inclusions

Andesite magmatism plays a major role in continental crustal growth, but its subduction-zone origin and evolution is still a hotly debated topic. Compared with whole-rock analyses, melt inclusions (MIs) can provide important direct information on the processes of magma evolution. In this article, we synthesize data for melt inclusions hosted by phenocrysts in andesites, extracted from the GEOROC global compilation. These data show that melt inclusions entrapped by different phenocrysts have distinct compositions: olivine-hosted melt inclusions have basalt and basaltic andesite compositions, whereas melt inclusions in clinopyroxene and othopyroxene are mainly dacitic to rhyolitic. Hornblende-hosted melt inclusions have rhyolite composition. The compositions of melt inclusions entrapped by plagioclase are scattered, spanning from andesite to rhyolite. On the basis of the compositional data, we propose a mixing model for the genesis of the andesite, and a two-chamber mechanism to account for the evolution of the andesite. First, andesite melt is generated in the lower chamber by mixing of a basaltic melt derived from the mantle and emplaced in the lower crust with a felsic melt resulting from partial melting of crustal rocks. Olivine and minor plagioclase likely crystallize in the lower magma chamber. Secondly, the andesite melt ascends into the upper chamber where other phenocrysts crystallize. According to SiO2-MgO diagrams of the MIs, evolution of the andesite in the upper chamber can be subdivided into two distinct stages. The early stage (I) is characterized by a phenocrystal assemblage of clinopyroxene + othopyroxene + plagioclase, whereas the late stage (II) is dominated by crystallization of plagioclase + hornblende.

Mineralogy, geochemistry, and origin of felsic dike swarms in Aqaba complex (Wadi Al-Yutum), South Jordan

The present work deals with the geologic setting and mineralogical and geochemical study of Late Precambrian magmatic rocks especially the felsic dike swarms of northeast Aqaba complex in South Jordan. The northeast Aqaba complex represents one of the most significant regions in the South Jordan basement. The basement rocks in the studied region are composed of schists, gneisses, migmatites, met gabbros, diorite, and granite. It is invaded by post-orogenic dike swarms. The post-orogenic dike swarms have been recognized as felsic dikes of dacite, rhyodacite, and rhyolite composition. They are composed of plagioclase, K-feldspar, quartz, biotite, and hornblenble with a porphyritic texture. Chemically, they are enriched in compatible elements especially in the large ion lithophile elements such as K, Rb, and Ba. The values of A/NK are more than A/CNK, indicating that dacitic dike swarms have metaluminous nature. Meanwhile, the values of A/NK are less than A/CNK in rhyodacitic and rhyolitic dike swarms of prealuminous nature. This felsic dike can be related to an intercontinental setting that was accompanied by a chemical evolution of the extensional movements and is formed by partial melting of crustal rocks, which are already known from other areas in the northeast portion of the Arabian-Nubian Shield.

Melting of crustal rocks as a possible origin for Middle Miocene to Quaternary rhyolites of northeast Hokkaido, Japan: Constraints from Sr and Nd isotopes and major- and trace-element chemistry

Felsic volcanic rocks (mainly rhyolites) and basalts found in northeast Hokkido, Japan, result from intense volcanism during the Middle Miocene (14–9Ma), Late Miocene (8–6Ma) and Pliocene to Quaternary (5–2Ma). Rhyolites were examined to determine any genetic relationship to coeval basalts, high magnesian andesite (HMA), lower crustal rocks and mantle peridotite.Rhyolites have initial Sr and Nd isotopic ratios (SrI and NdI) which overlap with coeval north Hokkaido basaltic rocks and HMA. Previously published Sr and Nd isotopic data show that most Middle Miocene to Quaternary (14–0Ma) basaltic rocks from north Hokkaido have a relatively narrow SrI- and NdI- range (SrI 0.70299 to 0.70400, and NdI 0.51281 to 0.51311), with no temporal variation in either SrI or NdI. Basalt and HMA from three locations are, however, more undepleted in terms of both SrI and NdI than other north Hokkaido basaltic rocks. Some of rhyolites (termed undepleted rhyolites here) have similar SrI and NdI to some north Hokkaido basalts and HMA. Rhyolites with similar SrI and NdI to other north Hokkaido basaltic rocks are termed depleted rhyolites.Although the similarity of SrI and NdI between rhyolites and coeval basalts and HMA can be accounted for by fractional crystallization, this process is inconsistent with the REE chemistry of basalts, HMA and rhyolites, and with the results of fractional crystallization modeling. However, a few rhyolites may result from the fractional crystallization of basaltic and HMA magmas with assimilation of some metasedimentary rocks. Small degrees of partial melting of a metazomatized mantle peridotite is an unlikely mechanism to explain the genesis of rhyolites according to REE chemistry and partial melt modeling of an amphibole bearing spinel lherzolite source.Gabbros of the Hidaka metamorphic belt are a possible source for isotopically depleted rhyolites, as both the rhyolites and gabbros have similar SrI and NdI. I-type tonalite and some gabbros in the Hidaka metamorphic belt are possible source rocks for isotopically undepleted rhyolites based on similarity in SrI and NdI. These hypotheses are supported by partial melt modeling of olivine gabbro and I-type tonalite respectively.A possible tectono-magmatic model for the production of post-Middle Miocene rhyolites from NE Hokkaido involves upwelling of the asthenosphere during the Middle Miocene, associated with the spreading of the Kurile back-arc basin and Japan Sea back-arc basin. This would have resulted in thinning of the overlying lithosphere beneath north Hokkaido, and the production of asthenosphere-derived basaltic rocks with low SrI and high NdI throughout north Hokkaido since the Middle Miocene, but less common production of lithosphere-derived basaltic rocks and HMA (with high SrI and low NdI). Basaltic magmas formed since the Middle Miocene either erupted, or caused melting of the crust, resulting in the generation of rhyolitic magma.

Evolution of the volcanic plumbing systemof Alicudi (Aeolian Islands - Italy): evidence from fluid and melt inclusionsin quartz xenoliths

Quartz-rich xenoliths in lavas (basalts to andesites; 90-30 ka) from Alicudi contain abundant melt and fluid inclusions. Two generations of CO2-rich fluid inclusions are present in quartz-rich xenolith grains: early (Type I) inclusions related to partial melting of the host xenoliths, and late Type II inclusions related to the fluid trapping during xenolith ascent. Homogenisation temperatures of fluid inclusions correspond to two density intervals: 0.93-0.68 g/cm3 (Type I) and 0.47-0.26 g/cm3 (Type II). Early Type I fluid inclusions indicate trapping pressures around 6 kbar, which are representative for the levels of partial melting of crustal rocks and xenolith formation. Late Type II fluid inclusions show lower trapping pressures, between 1.7 kbar and 0.2 kbar, indicative for shallow magma rest and accumulation during ascent to the surface. Data suggest the presence of two magma reservoirs: the first is located at lower crustal depths (about 24 km), site of fractional crystallization, mixing with source derived magma, and various degrees of crustal assimilation. The second magma reservoir is located at shallow crustal depths (about 6 km), the site where magma rested for a short time before erupting.

Armouring effect on Sr-Nd isotopes during disequilibrium crustal melting: the case study of frozen migmatites from El Hoyazo and Mazarron, SE Spain

Crustal melting is responsible for the production of large volumes of rhyolitic melt and therefore is central to understand the rheology of the crust and the mechanisms of crustal differentiation. The attainment of isotopic equilibrium during melting of crustal rocks is implicitly assumed in most isotopic dating and tracing studies. This assumption considers the melting event as an instantaneous process and does not take into account the duration of anatexis. To assess the critical role of the timescale of crustal melting, we have studied the unique occurrence of erupted migmatites enclosed as xenoliths in the El Hoyazo and Mazarron dacites of the Neogene Volcanic Province of SE Spain. These xenoliths represent the residue after some 30-60 % rhyolitic melt extraction at P-T conditions of 5-7 kbar and ∼850 °C, and consist of biotite, plagioclase, sillimanite, garnet, cordierite, graphite and abundant glass inclusions (i.e., not extracted rhyolitic melt) within each mineral phase. The timescale of melt extraction was ∼3 Myr and <0.8 Myr at El Hoyazo and Mazarron, respectively, resembling the duration of melting events during rapid anatexis caused by basalt underplating and crustal assimilation processes. In both localities, the minerals and glass inclusions of erupted migmatites preserve a significant Sr and minor Nd isotope disequilibrium. At Mazarron the isotopic disequilibrium is most marked owing to the shorter residence time of the melt within the source. The isotopic disequilibrium is not caused by the major xenolith-forming minerals but rather by the accessory phosphate inclusions (apatite ± monazite ± xenotime) hosted in garnet and biotite. The preservation of isotopic disequilibrium in these accessory phases has been facilitated by both their intrinsically low Sr and Nd diffusion coefficients and the armouring effect caused by their occurrence within biotite and garnet crystals, which acted as chemical barriers to Sr and Nd diffusion. This result implies that modelling of radiogenic isotope equilibration in natural systems should consider elemental diffusion in a composite medium with a resistance at the interface, i.e. different partition coefficients between adjacent mineral phases.

Why is amazonitic K-feldspar an earmark of NYF-type granitic pegmatites? Clues from hybrid pegmatites in Madagascar

The presence of amazonitic K-rich feldspar is considered an earmark of evolved granitic pegmatites ofNYF type. Elements of an explanation of the pegmatite-amazonite connection emerged in a study of the classic Anjanabonoina hybrid granitic pegmatite in Madagascar. The NYF assemblages there and at the Sakavalana and Anjahamiary pegmatites contain magmatic U- and Th-rich accessory phases and an amazonitic microcline perthite in miarolitic cavities. The early miarolitic minerals are overgrown by a Li- and Cs-enriched assemblage, depleted in HFSE, containing pale-gray microcline perthite. The amazonitic microcline perthite at Anjanabonoina has a δ 18 O value above 15%o, and that at Sakavalana, close to 13%o, indicative of formation of the NYF-pegmatite-forming magma by the melting of crustal rocks. Locally, the source rocks had become slightly alkaline and enriched in HFSE during periods of metasomatism of the hot granulitic crust accompanying episodes of distension over the course of the protracted Pan-African orogeny. The fertilized crust underwent local anatexis after the culmination of Gondwana re-assembly. The proximity of pyrochlore and other U- and Th-bearing accessory phases in the NYF assemblage provided the ionizing radiation required to interact with H 2 O trapped in the vacancy next to the Pb ions in the structure. The presence of an amazonitic K-feldspar ultimately signals net additions of Pb, U, Th, and the alkalis to the source prior to anatexis during a period of distension after a major orogenic disturbance. Plumbian K-feldspar also occurs in granitic pegmatite bodies formed by anatexis in the vicinity of a galena-bearing orebody (e.g., at Broken Hill, Australia); in this case, the resulting granitic pegmatite is neither of NYF type nor enriched in rare elements, however.

TTG magmatism in the Congo craton; a view from major and trace element geochemistry, Rb–Sr and Sm–Nd systematics: case of the Sangmelima region, Ntem complex, southern Cameroon

TTG rocks of the Sangmelima region (Ntem complex, Congo craton) in southern Cameroon range in modal composition from rocks of the charnockitic suite, tonalites to granodiorites. They display a wide spectrum of SiO 2 composition (54–76 wt%), including metaluminous intermediate rocks (0.56 < A/CNK < 1) and rocks that are slightly peraluminous (1 < A/CNK < 1.08). Charnockites and tonalites define a trondhjemitic suite while granodiorites portray a slight potassic calc-alkaline affinity. The primordial mantle-normalized spidergrams are characterized by negative Nb–Ta anomalies, suggesting a subduction related signature or melting of crustal rocks that were formed along subduction zones. More siliceous members of the rock suites are enriched in LILE. REE fractionation is shown by 2 < La N/Yb N < 28 in the charnockitic suite while higher ratios of 12 < La N/Yb N < 114 in the tonalitic suite and 18 < La N/Yb N < 99 in granodiorites indicate melting of source rocks with garnet as a restite phase. Samples with overall lowest abundance in REE show a positive Eu anomaly and represent small amounts of magmas derived from a source with much feldspar contribution to the melt. Sr and Nd systematics coupled with Mg#–SiO 2 compositions are consistent with derivation by partial melting in a subduction setting or merely from a thickened Archaean crust of eclogite facies basaltic proto-crust with mantle material input. From Rb–Sr and Sm–Nd whole-rock data, intrusion timing is defined around 2900 Ma for rocks of the charnockitic suite. The Rb–Sr system in biotite portrays post-emplacement reheating during the Eburnean orogeny. TTG intrusions in the Sangmelima region represent a major Archaean accretion event in the Congo craton.

The rates and extent of textural equilibration in high-temperature fluid-bearing systems

The geometry of fluid-filled pores in texturally equilibrated aggregates can, in theory, be uniquely determined given the porosity and the equilibrium dihedral angle for all possible orientations and combinations of the constituent solid phases. While it is useful to be able to do this, one should also ask whether, and to what extent, textural equilibrium is actually attained during fluid-present intervals in high-temperature rocks. In fact, textural equilibrium may be only rarely attained during melting of crustal rocks, and equilibration under mid-ocean ridges is possibly incomplete to depths as great as the garnet stability zone. There is very little published information on the rates of attainment of textural equilibrium in fluid-bearing geological environments. The available experimental data for the rates of growth of equilibrated domains are consistent with a power-law relationship between domain size and equilibration time with an exponent between 2 and 3. A simplified theoretical treatment of pore shape change governed by the Gibbs–Thompson relationship demonstrates that an exponent of 3 is to be expected for equilibration by diffusion within the fluid phase, with an effective diffusivity consistent with the available data.

Dynamic model of dehydration melting motivated by a natural analogue: applications to the Ivrea–Verbano zone, northern Italy

ABSTRACT:Dehydration melting of crustal rocks may commonly occur in response to the intrusion of mafic magma in the mid- or lower crust. However, the relative importance of melt buoyancy, shear or dyking in melt generation and extraction under geologically relevant conditions is not well understood. A numerical model of the partial melting of a metapelite is presented and the model results are compared with the Ivrea-Verbano Zone in northern Italy. The numerical model uses the mixture theory approach to modelling simultaneous convection and phase change and includes special ramping and switching functions to accommodate the rheology of crystal-melt mixtures in accordance with the results of deformation experiments. The model explicitly includes both porous media flow and thermally and compositionally driven bulk convection of a restitecharged melt mass. A range of melt viscosity and critical melt fraction models is considered. General agreement was found between predicted positions of isopleths and those from the Ivrea-Verbano Zone. Maximum melt velocities in the region of porous flow are found to be 1 × 10−7 and 1 × 10−1m per year in the region of viscous flow. The results indicate that melt buoyancy alone may not be a sufficient agent for melt extraction and that extensive, vigorous convection of partially molten rocks above mafic bodies is unlikely, in accord with direct geological examples.

Metamorphism of the mandatoriccio unit in the context of the hercynian and pre-hercynian evolution of the calabrian-peloritan arc (southern italy).

The Mandatoriccio Unit represents one of the structural elements of the Hercynian Calabrian-Peloritan Chain and is constituted by unknown age metamorphic rocks considered to be part of the pre-Cambro-Ordovician basement of the Calabrian-Peloritan Arc. In the Sila this unit tectonically overlie the Bocchigliero Paleozoic sequence, which represents the original cover of the pre-Cambro-Ordovician basement, metamorphosed during the Hercynian orogeny (330 Ma). Four metamorphic events are recognized in the rocks of the Mandatoriccio Unit. The two oldest greenschists facies events are marked by penetrative deformations, the third low to medium-low temperature event lacks them, and the fourth greenschists facies event bears weak penetrative deformation characteristics. Geochronological studies demonstrate that the last metamorphic event has an age of 326 ± 6 Ma, which coincides perfectly with the age of the Bocchigliero Unit Paleozoic metamorphic rocks; also the environment in which the metamorphism originated is analogous. The presence of the Alpine metamorphism is excluded; the mica ages ranging from 250 to 43 Ma are interpreted as partial openings of the systems caused mainly by the Alpine tectonics. The thermal metamorphism, which is certainly older than 3 30 Ma, is considered to be Ordovician and linked to the tensional phenomena responsible for the melting of crustal rocks and for volcanism in the Paleozoic basins. The two oldest metamorphic events of dynamic type are related to the pre-Cambro-Ordovician orogeny. It is suggested that the pre-Cambro-Ordovician basement of the Calabrian-Peloritan Arc has a metamorphic history similar to that recorded in the rocks of the Mandatoriccio Unit. In particular, the last greenschists facies metamorphic effects appear in most cases to be of the Hercynian age. The variable and younger than 270 Ma ages may rather represent the effects of Alpine tectonics, fluid circulation or weathering. The presence of Caledonian metamorphism in the Arc is excluded.

A garnet-two-mica granite, Coyote Mountains, southern Arizona: Geologic setting, uranium-lead isotopic systematics of zircon, and nature of the granite source region

The 58-m.y.-old Pan Tak Granite, considered representative of the widespread early Tertiary garnet-two-mica granites of south-central Arizona, consists of biotite and muscovite-biotite granite intruded by garnet-muscovite and garnet-biotite granite and associated pegmatite. Textural evidence indicates that muscovite is a primary, magmatic mineral. The granite was intruded into Jurassic dioritic to granitic rocks and Paleozoic metasedimentary rocks during a latest Cretaceous and early Tertiary regional metamorphic episode. Part of the pluton was subsequently mylonitized during an early or middle Tertiary deformational episode of “metamorphic-core-complex” aspect. The fabrics produced by these two metamorphic episodes differ in distribution, style, orientation, and, probably, age. The zircon population of the Pan Tak Granite consists of a mixture of small euhedral crystals and larger, rounded crystals with or without overgrowths. Uranium-lead isotopic ratios of five size fractions of discordant zircon define a chord with a lower concordia intercept age of 58 ± 2 m.y. and a late Precambrian upper intercept. 207 Pb*/ 206 Pb* ages range from 214 m.y. for the finest fraction to 434 m.y. for the coarsest fraction. The Pan Tak discordia is interpreted as a mixing line between 58-m.y.-old magmatic zircon and an older zircon component largely or entirely of late Precambrian age. Geologic and isotopic evidence indicates that this older zircon component was not assimilated from the country rocks but, rather, was inherited from the source material of the granite. The presence of this inherited zircon indicates that the Pan Tak magma was produced by melting of crustal rocks containing late Precambrian zircon. This source material was probably the 1.7- to 1.4-b.y.-old crystalline basement of south-central Arizona. Generation of the Pan Tak and related granites by crustal anatexis was one aspect of a latest Cretaceous to early Tertiary orogenic episode in south-central Arizona.

Investigation of the Manicouagan Impact Crater, Quebec: An introduction

A heavily cratered surface is a feature common to the early evolution of all the terrestrial planetary bodies. However, because the multiple impact history and inaccessibility of other bodies provide such poor geologic control, it is considered that the integrated effects of these impacts are best detailed and constrained through the intensive study of terrestrial impact structures. A survey of terrestrial structures indicates that the 65‐km‐diameter Manicouagan ring structure in Quebec, Canada, has by virtue of its size, exposure, and relatively well known geologic and geophysical characteristics the potential to provide details of impact processes, in particular those concerned with impact melting. This paper serves as an introduction to several detailed companion papers that resulted from a recent multi‐disciplinary study of Manicouagan. The major conclusions are as follows: The structure is 214 m.y. old. The melt rocks are texturally inhomogeneous but chemically homogeneous, can be modeled as a mixture of target lithologies, and have a 87Sr/86Sr ratio compatible with the melting of crustal rocks. Grain size and inclusion content of melt rocks vary inversely, grain size increasing upward. The melt had a two‐stage cooling history with clast‐melt interactions indicative of mixing superheated silicate liquid with cold clasts. Extensive digestion and reaction of clasts in the melt during the first stage of cooling resulted in a more refractory clast population than would be provided directly from the target rocks. However, a stronger bias toward refractory clasts in the interior zones of the sheet suggests a higher equilibration temperature and a lower initial content of clasts in these zones. Thermal equilibration of clasts and melt during the first stage of cooling required tens of seconds to a few minutes, whereas completion of crystallization during the second stage required about 1600 more years. Geophysical data suggest the presence of uplifted mafic rocks in the center of the structure and are compatible with an initial transient cavity with a radius of 15 to 22 km and a modeled depth of 6 to 9 km.

On the geochemistry of strontium in tertiary igneous rocks of the Tokaj Mountains

The distribution of strontium in 26 igneous rocks from the Tokaj Mountains, ranging from basalt to rhyolite has been determined by using stable isotope dilution techniques. The comparison with main component analytical data allows a classification of the different rock types (basalts, andesites, dacites, rhyolites) into only two distinctly separate genetic groups, which can be described schematically by independent liquid-solid fractionation processes. It is shown that alkaline metasomatism can produce Sr/Ca relationships similar to those developed in a liquid-solid fractionation process. Contamination by partial melting of crustal rocks, as well as magma contamination by transvaporizational mechanisms, results in similar changes in the Sr/Ca ratio, which therefore can be misleading in defining various stages of magma development.