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The cumulates of the Bushveld Complex, which form the largest layered intrusion on Earth, are known to have crystallized from several isotopically distinct magma pulses. Here, we present in situ Sr isotope compositions combined with the corresponding mineral chemistry of plagioclase from all lithological zones, covering > 6 km of stratigraphy, to constrain the petrogenesis of the complex. The in situ data coupled with high-resolution elemental maps of individual plagioclase grains reveal complex zonation patterns with respect to mineral chemistry and Sr isotope composition. This suggests that interstitial plagioclase in the Bushveld Complex crystallized from multiple, isotopically distinct influxes of melt percolating through a mafic cumulate framework and displacing the resident melt. Similarly, cumulus plagioclase grains are the result of continual ingress of a distinct magma, which picked up previously formed plagioclase crystals. Sr isotope compositions across the layered sequence seem to be largely decoupled from differentiation indices, such as Mg#, anorthite content or trace element ratios. As these observations cannot be explained by bulk assimilation, we propose that the elevated Sr isotopic signature of the Bushveld cumulates may have resulted from the interaction of the parental magma with a fluid derived from the up to 2 km thick dolomitic footwall, which caused a major shift in Sr isotope composition without significantly affecting the degree of differentiation or trace element signature. The decarbonation and/or assimilation of dolomite during the intrusion of the Bushveld Complex may be of major importance, not only for the petrogenesis but also for the emplacement of the layered intrusion, because devolatilization is directly linked to space creation owing to volume loss, thus producing a lopolith.

In Egypt, mafic-ultramafic complexes have been classified into three major types: incomplete ophiolite sequences; Alaskan-type intrusions, concentrically-zoned bodies formed in a subduction arc environment; and layered intrusions, vertically-zoned bodies intruded in post-collisional tectonic environments and rift-related bodies associated with the opening of the Red Sea. We present new field work, geochemical data, mineral chemistry and interpretations for the late Neoproterozoic Dahanib mafic-ultramafic intrusion in the South Eastern Desert of Egypt (northernmost Arabian–Nubian Shield, ANS). The Dahanib intrusion shows no evidence of metamorphism or deformation, with excellent preservation of intrusive contacts, well-preserved textures and primary mineralogy. Field relations indicate that it is younger than the surrounding metamorphic rocks and syn-tectonic granitoids. The intrusion is composed of a basal suite of ultramafic rocks (dunite, lherzolite, wehrlite and pyroxenite) and an overlying suite of mafic rocks (olivine gabbronorite, gabbronorite and anorthosite). It displays evident layering of modal abundance, visible directly in outcrop, as well as cryptic layering discernible through changes in mineral compositions. The western and eastern lobes of the Dahanib intrusion occur in the form of a lopolith with readily correlated layers, especially in the upper mafic unit. The present-day dip of the layering decreases from the ultramafic units into the mafic sequence. Structural and compositional relations show that the ultramafic units are cumulates from a high-Mg tholeiitic parent magma emplaced at deep crustal levels and evolved via fractional crystallization rather than any kind of residual mantle sequence. Fo content of olivine and Mg# of pyroxenes display a systematic decrease from ultramafic to mafic rocks, well-correlated with whole-rock Mg#. Spinels in ultramafic samples vary from Cr-rich to Al-rich and have Mg# and Fe^(3+)# similar to spinels from typical stratiform complexes and clearly different from those found in ophiolitic and Alaskan-type complexes. Although the mafic and ultramafic units are clearly related and can be derived from common parent magma, they were not emplaced coevally; rather, they represent different pulses of magma. The Dahanib mafic–ultramafic intrusion does not display any features that convincingly identify it as a typical Alaskan-type body, particularly the lack of clinopyroxenite and hornblendite, rarity of primary hornblende, and the notable abundance of orthopyroxene and plagioclase in its rocks. Our results confirm that it is more akin to a layered mafic-ultramafic intrusion with a multistage evolution. It was emplaced into a stable post-orogenic cratonic setting, with a trace element signature indicating contamination of the mantle source by previous subduction events.

A joint magnetic and gravity survey was carried out in a Cenozoic magmatic plug in the northernmost of the Brazilian equatorial margin. The Sao Joao plug (0.5 km 2 ) is located in the southern border of the Potiguar Basin, belonging to the N–S oriented Macau-Queimadas Alignment formed by Macau magmatism, the youngest magmatism of the Borborema Province. This plug is hosted in Acu and Jandaira formations, composed of sandstones and limestones that suffered significant thermal changes. The study area is located in a region characterized by a sequence of magnetic highs and lows. Based on the MaxMin technique, no significant remnant magnetization could be detected in the magmatic body. We measured magnetic susceptibility and density of the igneous rocks and their metamorphic and sedimentary bedrocks. The thermal metamorphism promoted an overall increase of bulk density. Joint magnetic and gravity modeling revealed the complex internal geometry of the Sao Joao plug, formed by a 400-m deep, probably a lopolith like body with a magmatic conduit as deep as 970 m. In addition, 3D modeling allowed mapping other igneous bodies, with no superficial expression, near to the studied plug.

This paper presents a new mathematical model of coupled processes of filtration, deformations of porous skeleton and mass transfer between solid and liquid phases in sedimentary and igneous rocks. A numerical solution is obtained and processes which can be described by the model are analyzed. Examples of such processes are: formation of a fluid layer along metamorphism of sedimentary rocks, formation of a structure and filtration characteristics of the Bazhenov Shale, appearance of intrusive igneous body (lopolith).

The Late Devonian postorogenic Cobaw batholith, in southeastern Australia, is an oval, east-west-orientated, terrane-stitching lopolith that intruded low-grade metaturbidites. The initial intrusion (at 370 Ma) was the small, hypabyssal, S-type Rainy Creek Rhyolite (RCR). At 369 Ma, the foliated S-type Pyalong pluton was emplaced, apparently along an east-west-orientated fracture zone. Around 367 Ma, the main I-type Baynton pluton intruded as numerous shallow-dipping sheets. The last plutonic event was the intrusion of the broad, thin, flat-lying, and crosscutting sheet of the I-type Beauvallet pluton. The Cobaw plutons had independent origins, with magmas derived from contrasting, internally heterogeneous source rocks. For both the RCR and the Pyalong pluton, the sources were old metasedimentary rocks, while the magmas for the Baynton and Beauvallet plutons formed through partial melting of metadacitic to meta-andesitic rocks, probably with an admixture of immature volcaniclastic graywackes. The s...

The aim of this work is Sm–Nd study of metamorphic alteration of massive gabbronorites from the Monchetundra pluton. The mafic rocks of the pluton are unevenly metamorphosed up to amphibolite facies with the formation of garnet, hornblende, and other minerals. The presence of garnet is a distinctive feature of this pluton in comparison with other Paleoproterozoic PGE-bearing plutons of the eastern part of the Baltic shield: Mt. General’skaya, Fedorovo–Pana, Imandra lopolith, and Monchepluton [9]. The degree of metamorphic alteration of rocks increases near the faults, the largest of which is Monchetundra fault separating the studied pluton from the Monchepluton. This fault was formed about 1.9–2.0 Ga ago according to Sm–Nd and Rb–Sr mineral isochrons for minerals from metagabbro–anorthosites of the M1 well.

The nickeliferous Madziwa Igneous Complex consists of several lenticular, mafic-ultramafic enclaves within late Archean gneissic terrain. The lenses of igneous rock are locally deformed and metamorphosed to lower greenschist grade. They are here interpreted to be remnants of a large, composite, magmatic structure with two main components: (1) a set of narrow (10s–100s of meters wide) dikes intruding the gneissic foliation, and (2) a large lopolith made up of leuconorite. Some dikes contain a differentiated and vertically oriented layered suite comprising a central pyroxenite layer, plus norite as both a continuous marginal layer and intermittent layers within the pyroxenite. The pyroxenite-norite suite has a preliminary U-Pb zircon age of 2684 Ma. Other dikes are made up of diorite and/or ferrodiorite; in places, the dioritic rocks also intrude the pyroxenite-norite suite. The leuconorite lopolith transects both the dikes and the gneissic country rocks, its basal contacts with the pyroxenite-norite suite varying locally from intrusive to gradational to (magmatic) erosional. Although modified in places by secondary mobilization, disseminated Ni-Cu sulfides are primarily hosted within the central (ortho- to mesocumulate) pyroxenite adjacent to internal norite layers. Whole-rock geochemical data establish the comagmatic origin of the principal rock types and indicate an Archean, D-type, basaltic source magma with ca. 8% MgO, the two dioritic rock types representing late, immiscible, silica- and Fe-Ti-P-rich derivatives of pyroxenite-norite(-leuconorite) crystallization. In addition to the major rock types, peridotites occur in several outlying dikes and may represent fractionates of a komatiitic precursor to the basalt. Geologic and geochemical evidence points to bulk assimilation of country rock gneiss by the Madziwa magma and sulfide segregation triggered by felsic contamination. These processes did not occur locally within the dikes but rather in the conduit system prior to the emplacement of the magma charged with sulfide droplets. The unusual vertical layering of the pyroxenite-norite sequence and the localization of the sulfide ores are attributed to the strong outward, cooling gradient across such narrow dikes and to large-scale, lateral movement of sulfide droplets through the solidifying, pyroxene crystal framework ahead of an advancing postcumulus plagioclase crystallization front.

International audience; Modes of magma emplacement may provide key information on geodynamic-tectonic settings of magmatism, when the threshold of magma volume and duration of emplacement process is crossed, brittle/ductile crustal deformation behavior, and volumes and rates of magma transfer in the crust. The majority of upper-crustal granitic intrusions are incrementally emplaced as laccoliths or sills, while midcrustal or deeper-level granitic intrusions appear to be predominantly emplaced as sills or lopoliths, but relatively few examples have been studied. We present a case study on the Jurassic Qitianling pluton from South China, which was constructed in the upper part of the middle crust (at a depth of ~10-15 km). To characterize its emplacement mechanism and evolution, we have combined field and microscopic observations, Anisotropy of Magnetic Susceptibility measurements and Bouguer gravity anomaly modeling. The main findings are (1) the generally undeformed granite-country rock contacts with a narrow thermal aureole, (2) irregularly wavy and gradational contacts between the main granite facies, (3) dominant subhorizontal magnetic fabrics in the granites with a scattered magnetic foliation, (4) rare occurrence of dykes, and (5) a subcircular surface and lopolith-like pluton geometry. Accordingly, we suggest that the Qitianling pluton forms an ~25-km wide and ~5-km thick lopolith that was incrementally emplaced during a period of tectonic quiescence, most likely by under-accretion of multiple magma sheets deflected from magma pulses that ascended through pipe-like channels. The mineralogical and compositional variation of the pluton may be the result of in situ fractionation, fractionation at deeper crustal level, or variation in source-derived melt compositions.

The Permian Hulu intrusion is one of several sulphide-bearing Permian mafic–ultramafic intrusions in the eastern part of the eastern Tianshan located at the southern margin of the Central Asian Orogenic Belt (CAOB) in Xinjiang, NW China. The intrusion is composed of lherzolite, olivine websterite, gabbro, and gabbro-diorite. Disseminated and net-textured Ni-Cu sulphide ores are located at the bottom of the lopolith complex. Negative Zr, Hf, Nb, and Ta anomalies, whole-rock eNd(t) values of +5.7 to +8.8, and variable (Th/Nb)PM values (from 1.06 to 8.13) suggest that the source of the Hulu complexes is depleted mantle metasomatized by subducted slab-derived fluid and/or melt (~5% global subducted sediment and 15% slab fluid) that has experienced approximately 3% lower crustal and 10% upper crustal contamination. The Hulu intrusion is characterized by low PGE abundances i.e. 0.03–1.08 ppb Ir, 0.04–0.69 ppb Ru, 0.02–2.15 ppb Rh, 0.30–48.71 ppb Pt, and 0.21–344 ppb Pd. Our calculations indicate that if the Pd,...

The 1300–1140 Ma Gardar period in South Greenland involved continental rifting, sedimentation and alkaline magmatism. The latest magmatism was located along two parallel rift zones, Isortoq–Nunarsuit in the north and the Tuttutooq–Ilimmaasaq–Narsarsuaq zone in the south addressed here. The intrusive rocks crystallised at a depth of

The Xade Mafic Complex, situated in central Botswana, was identified during the first regional aeromagnetic survey of the country in 1975–77. It is covered by sedimentary rocks of the Kalahari Group and Karoo Supergroup, including Karoo lavas, with a combined thickness varying from 220 m to 1000 m, extending to greater depths in the north beneath the Meso-proterozoic Passarge Basin. What has generally been viewed as the Xade Complex is evidenced by a high amplitude kidney-shaped zoned magnetic anomaly with two semi-linear anomalies extending to the northwest and northeast in a Y-shaped form. The complex is also marked by a coincident Bouguer gravity anomaly. Historical work has been limited, with only three diamond drill holes having been drilled, separately intersecting gabbroic rocks, weathered basalt grading into dolerite, and amygdaloidal lavas followed by Waterberg Group shales. An U-Pb zircon age of 1109.0 ± 1.3 Ma, which is coeval with the Umkondo Igneous episode, has been published for the gabbroic unit. This study has shown, for the first time, that the Xade Complex comprises two lobes of potentially different magmatic character, i.e. a Southern Lobe, which is the historically identified kidney-shaped zoned magnetic anomaly, as well as a hitherto unrecognized large Northern Lobe. The latter is mostly deeply buried in the north and northwest beneath the Passarge Basin, as evidenced by deep magnetic and gravity sources, but its southern and eastern margins partially suboutcrop beneath Karoo sediments, forming the Y-shaped anomalies north of the Southern Lobe. Forward modelling, of both magnetic and gravity data, indicates that the Southern Lobe is a lopolith with a depth extent of approximately 4 km, whereas the southeastern margin of the Northern Lobe has westerly dips consistent with deepening to the northwest. In addition to the historic boreholes, a recent borehole drilled into the Southern Lobe shows that it comprises a mafic volcanic sequence with subordinate gabbro. A further two boreholes drilled into the Northern Lobe margins indicate that it comprises a texturally heterogeneous and magmatically differentiated sequence of gabbroic rocks, with minor dioritic and monzonitic rocks. The present interpretation has also identified a dyke system associated with the Northern Lobe, which may represent either feeder or exit magmatic conduits. The interpretation further shows that the complex is located in a craton margin setting. The combined extent of both lobes is approximately one-third the size of the Bushveld Complex, making the Xade Mafic Complex the largest Meso-Proterozoic magmatic system in southern Africa. The craton margin setting, size, Meso-Proterozoic age and variability of plutonic and volcanic rocks hallmark the Xade Mafic Complex as being a very large dynamic magmatic system, with potential for Ni-Cu-PGE mineralization.

The NE Fennoscandian Shield comprises the Northern Belt in Finland and the Southern Belt in Karelia. They host mafic-ultramafic layered Cu-Ni-Cr and Pt-Pd-bearing intrusions. Precise U-Pb and Sm-Nd analyses indicate the 130-Ma evolution of these intrusions, with major events at 2.53, 2.50, 2.45, and 2.40 Ga. Barren phases were dated at 2.53 Ga for orthopyroxenites and olivine gabbro in the Fedorovo-Pansky massif. PGE-bearing phases of gabbronorites (Pechenga, Fedorovo-Pansky, Monchetundra massifs) and norites (Monchepluton) are 2.50 Ga old. Anorthosites of Mt. Generalskaya (Pechenga), the Fedorovo-Pansky, and Monchetundra massifs occurred at 2.45 Ga. This event produced layered PGE-bearing intrusions in Finland (Penikat, Kemi, Koitelainen) and mafic intrusions in Karelia. The Imandra lopolith dikes occurred at the final phase (2.40 Ga). Slightly negative &epsilon, Nd and ISr values (0.703&ndash, 0.704) suggest that intrusions originated from an enriched mantle reservoir. Low 3He/4He ratios in accessory minerals (ilmenite and magnetite) indicate an upper mantle source. Large-scale correlations link the Fennoscandian Shield with the Superior and Wyoming cratons.

The Molopo Farms Complex is an extremely poorly exposed, major, ultramafic-mafic layered intrusion straddling the southern border of Botswana with South Africa. It lies within the south-western part of the ~2.0 Ga large igneous province of southern and central Africa that includes the better known Bushveld Complex. Integrated interpretation of regional gravity data and new high-resolution airborne magnetic data have constrained the geometry of the Molopo Farms Complex in southern Botswana as a strongly faulted, polyphase intrusion compartmentalised by regional ductile shear zones. Previous models showing that the Complex was emplaced in at least two discrete stages are supported. Ultramafic rocks were initially emplaced as a semi-coherent lopolithic sheet up to about 4 km in thickness cutting across Transvaal Supergroup strata that had already been folded into open eastwest trending dome and basin structures with wavelengths of about 4 km. Steeply dipping, dyke-like ultramafic bodies adjacent to, and within major shears are inferred to be solidified feeders to the main lopolithic part of the MFC. It is likely that the initial ultramafic sheet was emplaced at a high crustal level (

The three-dimensional shapes of mafic layered intrusions have to be inferred from surface outcrops, in some cases aided by drilling and/or geophysical data. However, geophysical models are often equivocal. For the 2.06 Ga Bushveld Complex of South Africa, early geological models proposed a shape of a single, gently inward dipping lopolith. Subsequent resistivity and gravity data were interpreted to suggest that the eastern and western limbs were discrete, dipping wedge-shaped intrusions separated by ~150 km. A more recent gravity model that takes crustal flexure into account allows continuity and the reversal to the original model. Distinguishing between these possibilities is difficult from surface-based studies because the central regions of the Complex are obscured by large volumes of younger granites and sedimentary/volcanic cover rocks. Here, we describe xenoliths from the Cretaceous Palmietgat kimberlite pipe, located mid-way between the exposed western and eastern lobes of the Complex. They are chromite-bearing feldspathic pyroxenites considered equivalent to those of the typical outcropping Critical Zone of the Bushveld Complex. This result provides strong support for a regionally interconnected Bushveld Complex, implying its emplacement as a single sill-like body. Confirming the continuity of the Bushveld Complex greatly expands exploration opportunities and implies that other layered mafic intrusions could have similar geometry.

71 Kingash ore field with the deposit of the same name is located within the Archean Kan block of Eastern Sayan and does not go beyond the boundary of the new nickel and platinum bearing province with numerous ore bearing massifs [1]. In plan the Kingash massif forms a lens (Fig. 1) comprising two “megalayers”: lower peridotite–pyroxenite–serpentinite and overly ing gabbroid. In section it presents lopolith with a geo physically reflected gravitational stage approximated with a multichamber incurrent canal [3]. Ultrabasic rocks of the massif contain Pechenga type sulfide ore of chalcopyrite–pentlandite–pyrrhotite composition. Geochemical Model of Genesis of the Kingash Platinoid–Copper–Nickel Deposit O. M. Glazunov and T. A. Radomskaya Presented by Academician A.A. Marakushev April 1, 2009

There are two 300-500-km long belts of Palaeoproterozoic layered intrusions: the Northern (Kola) Belt and the Southern (Fenno-Karelian) Belt in the Baltic (Fennoscandian) Shield. New U-Pb (TIMS) ages and radiogenic isotopic (Nd-Sr-He) data have been determined for mafic-ultramafic Cu-Ni-Ti-Cr and PGEbearing layered intrusions of the Kola Belt. U-Pb ages on zircon and baddeleyite for gabbronorite and anorthosite from the Fedorovo-Pansky, Monchepluton and Main Ridge (Monchetundra and Chunatundra), Mt. Generalskaya intrusions and gabbronorite and dykes from the Imandra lopolith of the Kola Belt define a time interval of more than 130 million years, from ca. 2.52 Ga to 2.39 Ga. At least four intrusive phases have been distinguished: three PGE-bearing, and one barren. This spread of ages is wider than that for intrusions of the Fenno-Karelian Belt which clusters at 2.44 Ga. Nd isotopic values for the Northern Belt range from - 1.1 to -2.4, implying an enriched mantle “EM-1 type” reservoir for these layered intrusions. Initial Sr isotopic data for the rocks of the intrusions are radiogenic relative to bulk mantle, with ISr values from 0.703 to 0.704. Geochemical data and 4He /3He isotopic ratios of the minerals reflect a significant contribution from a mantle source rather than the influence of crustal processes during emplacement. The geological and geochronological data indicate that in the eastern part of the Baltic Shield, mafic – ultramafic intrusive magmatism was active over a protracted period and was related to plume magmatism associated with continental breakup that also involved the Superior and Wyoming provinces.

The Rogaland complex covers 1000 km in southwestern Norway and consists mainly of anorthosite massifs and the layered Bjerkreim±Sokndal lopolith (BSL). These rocks intrude charnockitic migmatites containing intercalated marbles and garnetiferous migmatites. High-temperature mineral isograds (pigeonite, osumilite and orthopyroxene) in the metamorphic basement are subparallel to and increase in grade towards the intrusive complex. P±T estimates from the country rocks show a roughly linear increase in temperature towards the BSL consistent with the distribution of isograds. The peak P±T conditions at 20 and 2 5 km from the contact at 5 kbar range from 700 to 41000 C. Field relations and age determinations link the high-T metamorphism and the magmatism. The two-dimensional thermal modelling indicates that heat from a single magmatic cooling unit is not sufficient to produce the array of isograds and the peak metamorphic temperatures. Two magmatic episodes separated by 3Myr, however, can account for the high-temperature metamorphism. In this model, the emplacement and crystallization of the anorthosite produces a regional thermal gradient (from 750 to 600 C). After a brief hiatus a second, smaller body (BSL) provides an additional thermal input that results in an array of high-temperature isograds and country-rock temperatures 41000 C.

Granites in both crystalline terranes and continental magmatic arcs tend to be circular to elliptical in map view and vary in width from about 3 to 100 km. Available gravity and structural data suggests that many of these plutons are tabular in shape with an average thickness of about 3 km. Ductile structures observed around mesozonal granites indicate that space is created by a combination of lateral and vertical displacements of wall rocks, whereas contact relationships of epizonal plutons imply that only vertical displacements are involved during emplacement. In both settings magma arrives at the emplacement site via one or more vertical feeder zones and flows laterally. With the exception of very high-level epizonal plutons, structural studies suggest that space for many tabular intrusions must be provided mainly by floor-depression (lopolith emplacement) rather than roof-lifting (laccolith emplacement). An emplacement model for this type of tabular granite is proposed which involves progressive depression of the floor of an initially horizontal chamber as it is filled by one or more vertical conduits. A crustal-scale balance in the rates of melt extraction, magma ascent and pluton-filling is required by the model, and transfer of material from the source to the pluton is accommodated by broadly distributed deformation of low strain magnitude. The process is evaluated with end-member cantilever and piston sinking mechanisms. The models predict that large (10–100 km wide), tabular plutons (≤3 km thick) can be emplaced quickly (100 a to 1 Ma) with floor-depression and related wall-rock strain rates similar those expected during tectonic deformation (10 – 10 to 10 – 15 s – 1 ). Bulk strains in the intervening crustal column rarely exceed a strain ratio of 1.5, which is likely to remain undetected in the geological record unless the required deformation is accommodated on discrete structures such as normal faults or shear zones at the base of the pluton.

In NW Scotland, several alkaline intrusive complexes of Silurian age intrude the Caledonian orogenic front. The most northerly is the Loch Loyal Syenite Complex, which is divided into three separate intrusions (Ben Loyal, Beinn Stumanadh and Cnoc nan Cuilean). Mapping of the Cnoc nan Cuilean intrusion shows two main zones: a Mixed Syenite Zone (MZ) and a Massive Leucosyenite Zone (LZ), with a gradational contact. The MZ forms a lopolith, with multiple syenitic lithologies, including early basic melasyenites and later felsic leucosyenites. Leucosyenite melts mixed and mingled with melasyenites, resulting in extreme heterogeneity within the MZ. Continued felsic magmatism resulted in formation of the relatively homogeneous LZ, invading western parts of the MZ and now forming the topographically highest terrane. The identification of pegmatites, microgranitic veins and unusual biotite-magnetite veins demonstrates the intrusion's complex petrogenesis. Cross-sections have been used to create a novel 3D GoCad™ model contributing to our understanding of the intrusion. The Loch Loyal Syenite Complex is known to have relatively high concentrations of rare earth elements (REEs), and thus the area has potential economic and strategic value. At Cnoc nan Cuilean, abundant REE-bearing allanite is present within melasyenites of the MZ. Extensive hydrothermal alteration of melasyenites here formed steeply dipping biotite-magnetite veins, most enriched in allanite and other REE-bearing accessories. This study has thus identified the area of greatest importance for further study of REE enrichment processes in the Cnoc nan Cuilean intrusion.

A post-tectonic Caledonian granite in southern Ny Friesland has been fully mapped and the following new names are proposed: the Chydeniusbreen granitoid suite, consisting of the Raudberget granitoid body in the north; the Newtontoppen granitoid body in the middle; and the Ekkoknausane granitoid body in the south The contact relationships, internal structures and distribution of various rock types infer an asymmetric lopolith or a harpolith-like body, a large sickle-shaped intrusion stretched in the direction of general tectonic transport, for the Newtontoppen granitoid body. Seven rock types are described in the Newtontoppen granitoid and four emplacement stages are recognised. The major rock types seem to have an alkali-calcic to alkalic bulk rock chemistry and show a transition between I- and S-type granite derived from anatectic melting of various protoliths under relatively high temperature conditions. Possible later K20 introduction modified the earlier formed rock types. A Rb-Sr whole rock age of 432 ± 10 Ma has been obtained by a seven point isochron with MSDW = 2.59 and an initial Sr isotope ratio = 0.715. This age is approximately 30 Ma older than the previously obtained K-Ar whole rock and Rb-Sr biotite ages, ca. 400 Ma, which represents the period of cooling. The high initial Sr isotope ratio supports the interpretation of an anatectic origin.

Paleoproterozoic layered mafic-ultramafic intrusions are widespread in the eastern Baltic (Fennoscandian) Shield and mark the initial stages of continental rifting and outpouring of voluminous flood basalts. UPb zircon/baddeleyite ages have been determined for seven layered mafic intrusions and a subvolcanic felsic unit associated with two Paleoproterozoic volcanic-sedimentary belts: the Pechenga-Imandra-Varzuga belt in the Kola Peninsula, and the Sumi-Sariola belt in Karelia. The layered intrusions in the Sumi-Sariola belt show an age progression from 2449.0 ± 1.1 Ma for the Burakovka complex, located at the southeastern edge of the belt, to 2442.1 ± 1.4 Ma and 2441.3 ± 1.2 Ma for the Lukkulaisvaara and Tsipringa intrusions of the Olanga complex in northern Karelia (northeastern part of the belt). These age results indicate a total duration of mafic magmatism along the ca. 800 km belt in Karelia to be less than 10 m.y. The Pechenga-Imandra-Varzuga belt, Kola Peninsula, contains two distinct age groups of layered mafic intrusions and related rocks. The younger of these overlaps with the ages obtained for the Olanga intrusions and includes the 2441.0 ± 1.6 Ma Imandra lopolith and a spatially associated felsic subvolcanic unit dated at 2442.2 ± 1.7 Ma. The older group of mafic layered intrusions in the Kola Peninsula includes the 2501.5 ± 1.7 Ma Pana tundra intrusion, the 2504.4 ± 1.5 Ma Monche pluton and the 2505.1 ± 1.6 Ma General'skaya mountain intrusion. The temporal and spatial distribution of the 2.50-2.44 Ga rift-related igneous activity in the Baltic Shield is consistent both with the model of starting mantle plume, and with the model of passive rifting controlled by pre-existing lithospheric weaknesses.

O Plúton Cupira está localizado no Domínio Sul da Província Borborema, possui área aflorante de aproximadamente 300 km2 e forma ovalada com leve alongamento na direção E-W. Com objetivo de determinar o modelo de alojamento do plúton e os alinhamentos magnético-estruturais, foi realizada a integração inédita de estudos de anisotropia de suscetibilidade magnética (ASM) e magnetometria. Os dados de ASM mostram que a susceptibilidade total apresenta variação de 0,105x10-3 SI a 34,618x10-3 SI, indicando caráter ferromagnético. O grau de anisotropia varia de 1,06 a 1,272 e apresenta correlação positiva com a susceptibilidade total. O parâmetro de forma varia de -0,802 a 0,897, com preponderância de elipsoides oblatos sobre prolatos, refletindo trama magnética planar. A maioria das lineações magnéticas possuem direção NE-SW, com caimentos suaves a fortes para NE e para SW. A foliação magnética varia de NE a NNE, com mergulhos moderados a fortes (30° a 86°). A porção central do corpo tem foliações magnéticas e magmáticas essencialmente concêntricas, com mergulhos fortes para o centro do plúton, associadas à lineações com caimento forte, sugerindo ser a zona de alimentação magmática. Os alinhamentos magnéticos reforçam as direções estruturais evidenciadas pela ASM. Estes dados favorecem a proposição de modelo de alojamento do Plúton Cupira a partir de uma zona de alimentação magmática central, com fluxo do magma distribuído de forma concêntrica, com geometria de intrusão similar a um lopólito.

The Bucknalla Complex is a small (10 km2) layered, tholeiitic, mafic‐ultramafic intrusion located 50 km southwest of Rockhampton that was emplaced into a regional continental margin accretionary environment in the Late Permian and hosts minor Cu‐Pd‐Au‐Pt mineralization. It is a saucer‐shaped lopolith (2200 m × 6 km at maximum stratigraphic intersection) and consists of over 15 laterally discontinuous igneous units ranging in thickness from 1–50 m. Samples from two traverses perpendicular to layering reveal cryptic variation in the primary phases (olivine, plagioclase and clinopyroxene) which is not a simple function of stratigraphic height. The mineralization consists of chalcopyrite and bornite and their alteration products digenite and covellite, electrum (Au‐Ag alloy), Pd‐As, Pd‐Sb, Pd‐S, michenerite (PdBiTe2) and sperrylite (PtAs2) and is considered to be primary magmatic because of: (i) the association of the platinum‐group elements (PGE) with primary magmatic‐textured Cu‐sulphides; (ii) the absence ...

A long history preceded the publication of two papers (Alvarez et al. 1980; Smit and Hertogen 1980) on a hypothesis that was to change the way we think about mass extinctions. The idea of a ma− jor impact ending the reign of the dinosaurs had been launched earlier, albeit without a scrap of evidence. Many other hypotheses had been put forward and had fared no better by lack of supporting data. De Laubenfels (1956) introduced “one more hypothesis”, a meteorite impact, and suggested that the heat flash had killed off the dinosaurs. The problem with this and other hypotheses (i.e., diseases, egg predation, pituitary gland anomalies, oversize, over− specialisation, magnetic reversal, sea level changes, etc.) is that they account for only small group of (generally) terrestrial verte− brates, but that they do not explain the simultaneous extinction of marine life. De Laubenfels’s impact idea was inspired by the proximity in 1937 and 1941 of the 1km−sized asteroid Hermes, at only 1,6 times the Earth−Moon distance. He even estimated the frequency of large, 10 km−sized planetesimal collisions correctly: about 2–3 such impacts during the Phanerozoic, a number subsequently con− firmed by better observations and statistics (Dachille 1977; Grieve et al. 1995). Nobel laureate Harold Urey suggested in 1973 that geologists should be on the lookout for (micro)tektites at geological bound− aries, especially at the Cretaceous–Paleogene (K/Pg) boundary, but failed to find them (Urey 1973). Christensen et al. (1973) ana− lysed in detail the Stevns Klint K/Pg boundary clay by Atomic Absorption Spectroscopy (AAS) and found anomalously high concentrations of Cr and Ni. However, further than a comparison with anaerobic black shale enrichments they did not go, although the enrichments were much greater than those in black shales. In the 1960s we learned from Professor H.A. Brouwer (per− sonal communication 1969) that the craters on the moon might be volcanic. In our petrology classes we were taught that the Sudbury igneous body was a volcanic lopolith (Lowman 1992) and O’Keefe (1976) believed that tektites could be produced by lunar volcanoes. The Apollo lunar missions changed all that and generated detailed research of impacts and meteorites. Urey (1957), Shoemaker (1963) and others convincingly demon− strated that both the Moon and the Earth were saturated with im− pact craters, but that those on the Earth were largely eroded away. Impacts slowly came to be seen as “a matter of fact” in the geological record. In the early 1960s, in particular in the Chicago Fermi labora− tories, the analytical technique of Neutron Activation (INAA) (developed by George de Hevesy) was used for the detection of many trace elements, in particular iridium. Barker and Anders (1968) and Crocket and Kuo (1979) used this technique to esti− mate the accretion rate of cosmic matter. They could apply this estimate because it had previously been discovered that iridium in cosmic matter (meteorites) was orders of magnitude more abundant than in terrestrial crustal materials (~500 versus 0.02 ng/g). Iridium is a special element in INAA. It has a relatively large “neutron capture cross section”, which means that Ir ab− sorbs easily a neutron to become the radioactive isotope Ir. This Ir decays with a half life of 74 days emitting two easily identifiable gamma rays of 316 and 468 kev energy. Therefore, among the Platinum Group Elements (PGE), it is by far the easi− est to identify.

New UPb zircon ages and SmNd data are presented for layered basic-ultrabasic intrusions of the Kola Peninsula and northern Karelia in the northeastern part of the Baltic Shield. All ages are in the range between 2.5 and 2.4 Ga, but there appear to exist three phases of intrusion. Thus the Monchegorsk Intrusion and the Fedorovy-Pansky Massif have ages between 2493 and 2470 Ma, the Kivakka and Lukkulaisvaara massifs of the Olanga group of intrusions between 2445 and 2435 Ma, while the Imandra Lopolith is 2396±7 Ma old. A narrow range of negative ϵNd(T) values between −2.3 and −1.2 suggests formation by partial melting of a semi-homogeneous enriched mantle source.

Fluid inclusions and mineral associations were studied in late-stage charnockitic granites from the Bjerkreim-Sokndal lopolith (Rogaland anorthosite province). Because the magmatic and tectonic evolutions of this complex appear to be relatively simple, these rocks are a suitable case for investigation of the origin and evolution of “granulitic fluids”. Fluid inclusions, primarily contained in quartz, can be divided into four types: carbonic (type I), N2-bearing (type II), CO2+H2O (type III) and aqueous inclusions (type IV). For each type, the role of leakage and fluid mixing are discussed from microthermometric and Raman spectrometric data. The most striking features of CO2-rich inclusions (the predominant fluid) is the presence of graphite in numerous, trail-bound inclusions (Ib) and its absence in a few isolated, very dense (d=1.16), pure CO2 inclusions (Ia) and in the late carbonic inclusions (Ic). Fluid chronology and mineral assemblages suggest that carbonic Ia inclusions represent the first fluid (pure CO2) trapped at or close to magmatic conditions (T=780–830° C, fO2=10-15 atm and P=7.4±1 kb), outside the graphite stability field. In contrast, type Ib inclusions enclosed graphite particles from a channelized fluid during retrograde rock evolution (P=3–4 kb and T=600° C). Decreases in T-fO2 could explain a progressive evolution from a CO2-rich fluid to an H2O-rich fluid in a closed C−O−H system. However, graphite destabilization observed in type Ic inclusions implies some late introduction of external water during the last stage of retrogression. The main results of this study are the following: (1) a carbonic fluid was present in an early stage of rock evolution (probably in the charnockitic magma) and (2) this granulite occurrence offers good evidence of crossing the graphite stability field during post-magmatic evolution.

We present centrifuge experiments to study the surface deformation induced by shallow pluton emplacement in a rheologically stratified crust. Sand simulates the topmost brittle crust; plastilina and denser silicone represent more and less competent crustal portions, respectively; lighter silicone simulates a buoyant intrusion. In the models, density differences affect the rate of intrusion but not their evolution or shape, whereas viscosity and strength stratifications control both the shape and rate of the intrusions. With a higher viscosity contrast (10 2–4 ) between the intrusion and the embedding media, the rise of the lighter silicone induces a laccolith-like intrusion, responsible for doming and thinning of the overburden; an apical depression may form, inducing silicone extrusion. Conversely, with a lower (10 1 ) viscosity contrast, the overburden and the intrusion exhibit a lens-shaped form, with a broad central depression bordered by an upward flexure towards the periphery. A sag in the floor of the intrusion is commonly observed; no silicone extrusion occurred. The intrusion is a hybrid between a laccolith and a lopolith. The comparison with nature (1) confirms roof uplift as an important means of accommodating space during pluton emplacement and (2) suggests that, where roof uplift plays a major role, pluton emplacement can induce a well-correlated sequence of events at surface: doming, the development of a depression and extrusion.

A detailed re-examination of some of the more fractionated portions of the Bjerkrem-Sogndal lopolith (BSL) has provided evidence for widespread mixing between quartz mangeritic and monzonoritic (Eia-Rekefjord) magmas. The higher-temperature monzonoritic liquids are preserved as chilled pillows within quartz mangerite. Chemical compositions of these and other fine-grained monzonorites suggest that they are chilled residual liquids related to the BSL, and that they were in equilibrium with the more fractionated cumulates of the BSL. The chilled monzonoritic liquids appear to define a fractionation trend of increasing SiO2 (up to 60 wt. %). The quartz mangerites appear to represent separate partial melts derived from the country rock.

The East Bull Lake intrusion is a layered basic lopolith of early Aphebian age that intrudes Archean basement rocks at the southern margin of the Superior Province. It contains at least two magmatic cycles. Cumulate olivine, plagioclase, and clinopyroxene form anorthosite, troctolite, and olivine gabbro–norite in the lower portion of each cycle. These rocks are overlain by two pyroxene gabbros–norites in which orthopyroxene (or pigeonite) joins augite and plagioclase as the major cumulate minerals. Molecular proportion ratio (MPR) variation diagrams support this interpretation of the petrology of the intrusion and show that the major cumulate rock types represent magmatic liquids formed sequentially by fractionation of the above-mentioned cumulate phases.Small-scale phase layering is common in the intrusion. Size-graded layers are absent, and cross-bedded and igneous lamination features are not common. We conclude that this scale of layering formed in a boundary layer along the floor of the intrusion as a result of variable nucleation of the cumulate minerals. Much thicker isomodal layers of rock form the main mass of the intrusion. They may have formed by a double diffusion process that caused the formation of a density-stratified magma chamber.

Charnockitic acidic rocks related to anorthosites are well represented in the Rogaland anorthositic province. Quartz mangerites constitute the upper level of the Bjerkrem-Sogndal layered lopolith and are related to anorthosites through a complete sequence of differentiated products which includes leuconorites, norites and mangerites. In the Hidra body, charnockitic rocks occur as a stockwork of dykes cutting across anorthosite and leuconorite. The southwest end of the magmatic province consists of two distinct bodies of acidic rocks: the Farsund charnockite and the Lyngdal hornblende granodiorite. They are considered to be related to the neighboring anorthosite massifs. Some trace and major elements (REE, Rb, Sr, U, Th, Hf, Zr, K 2 O, SiO 2 ) of these rocks as well as the strontium and oxygen isotopic ratios are investigated. Quantitative modeling of the behavior of trace elements using the Rayleigh equation indicates in agreement with the isotopic data that direct derivation of the acidic rocks by fractional crystallization of a parental magma is a possible mechanism in Hidra and in the Lyngdal granodiorite. In Bjerkrem-Sogndal, however, the REE evolution in the acidic rocks and the high value of the 87 Sr 86 Sr initial ratio (0.7105) compared to that of the anorthosites (0.705–0.706) cannot be explained by a simple fractional crystallization process. Contamination by supracrustal material relatively poor in REE and with a high Sr isotopic ratio must also be invoked. In the Farsund charnockite REE distribution and Sr and O isotopic ratios suggest an origin by anatexis of gneisses of sedimentary origin.

The Tehilla complex is a shallow-level, post-tectonic, ring structure of basic and acid intrusions of possible Lower Palaeozoic age. The basic rocks comprise an early phase of troctolites and norites emplaced in the form of a lopolith and sill bodies respectively. The acid rocks (mainly granites) have been emplaced later as high-angle cone-sheets. The complex has a general alkaline character and seems to have evolved during a relatively short time by a process of differentiation, probably from two contrasted magmas.

The Kiglapait basic layered intrusion in northern Labrador previously has been interpreted, on geological grounds, to be a lopolith with a maximum thickness of 8.7 km. It is associated with a large (~45 mGal) positive gravity anomaly that for the most part is very similar to a theoretical anomaly computed for the proposed lopolith model, except over the southeastern part of the exposure where the theoretical anomaly is significantly more positive. Model studies of the gravity data suggest that the form and dimensions of the lopolithic model predicted on geological criteria are essentially valid and that a granitic intrusion is present in the southeastern region of the lopolith causing the discrepancy between observed and theoretical anomalies. The latter intrusion is believed to be genetically related to the Manvers granite, which occurs throughout the southeastern region as dykes and small stocks. The presence of another buried granitic mass south of the Kiglapait intrusion is also suggested by the gravity data.

The Transition Zone between the anorthositic-noritic rhythmic units and the mangeritic unit of the Bjerkreim-Sokndal layered lopolith (southwest Norway) is characterized by thick layers of ultramafic rock (UML), interleaved with norites in the lower part and mangerites in the upper part. In the latter rocks large mesoperthite crystals occur in a fine-grained groundmass (porphyritic texture). Rapid variations in chemical composition are shown by the mafic minerals, especially by olivine (Fos0 to Fo19 ) and by Ti-magnetite (10-19% TiO~). The whole-rock composition of the UML does not change in the upper part of the Transition Zone, and the Fe/(Fe + Mg) ratio of the mangerites remains constant (Bowen trend). Formation of the UML through liquid immiscibility is rejected because: (1) the liquidus minerals are not always identical in the Fe-rich and associated silica-rich rocks; (2) the distribution of P205, REE and Ba between associated rocks are not in agreement with experimental data; (3) the major elements vary along parallel trends in the Creigh diagram. Cumulative processes and adcumulus growth can best explain the UML, oscillatory nucleation can account for the norites, and flotation of mesoperthite is plausible in the mangerites, where accumulation of feldspar is evidenced by a strong positive Eu anomaly and porphyritic textures. The mangeritic unit of the lopolith is interpreted as a flotation cumulate at the roof of the magma chamber. The residual liquids of the Bjerkreim-Sokndal differentiation were either trapped in the mangerites, or expelled in the form of dykes, or were mixed with quartz mangeritic liquids above the intrusion.

Rare-earth elements (REE) were investigated by neutron-activation analysis in nine apatites and seven coexisting plagioclase cumulate minerals at three different levels of the Bjerkrem-Sogndal layered lopolith (Rogaland, S.W. Norway), a massif which displays the complete andesine anorthosite-charnockite series of rocks. The REE (except Eu) behave as hygromagmatophile elements (as defined by Treuil) in the course of the fractional crystallization process which has given rise to the massif. The concentration is indeed inversely proportional to F, the fraction of residual liquid. This characteristic permits placing an upper limit to the value of the partition coefficients of REE between apatite and magma. They must approximately be equal or less than 5. The apatite-plagioclase distribution coefficients show little variation during differentiation. This can be explained by an open system crystallization of the intercumulus liquid and permits calculating the plagioclasemagma partition coefficients. Values similar to those of Paster et al. (1974) are obtained. A range of possible D apat Eu is calculated, together with the corresponding D plag Eu at three different levels of the intrusion. Drake's (1975) geobarometer based on Eu in plagioclase is used to estimate the fugacity of oxygen. There is good agreement with the values obtained with the Buddington and Lindsley geobarometer. A continuous decrease of f O2 throughout the whole sequence cannot be invalidated. REE were also determined in apatite from five Fe-Ti oxide ore-bodies occurring in the same magmatic province. The consistency between the trace element behaviour in the oxides and the REE in the apatite permits identifying fractional crystallization as controlling the generation of the ore-bodies.

The East Bull Lake anorthosite-gabbro, a late Archean (2472 ± 24Ma) magmatically layered lopolith at the southern margin of the Superior Province, has experienced regional retrograde metamorphism, and is transected by the Folson Lake fault zone, a structure of regional extent. The fault zone displays repeated episodes of ductile-brittle deformation and vein development. It has been extensively metasomatized in domains marked by the sequential development of quartz, adularia, epidote, chlorite, laumontite, calcite, and clay-Fe oxyhydroxide. Downhole geochemical profiles through the fault zone show coherent patterns for Zr-Hf, Zr-Th, Ta-Nb and Cr-Co-Ni, which are interpreted to reflect variations in primary igneous compositions by alteration-insensitive elements. Metasomatism is marked by peaks of SiO2, K2O or Na2O with Al2O3, Fe, Mn and Mg, and Ca plus volatiles, corresponding to domains of quartz, adularia or albite, chlorite, and laumontite plus calcite, respectively. The analogue elements Th and U are present at uniformly low levels averaging 1.1 and 0.4 ppm, respectively. Thorium covaries with Zr, such that variations of abundance reflect primary differences, whereas Th/U ratios scatter between 1.5 to 10, indicating minor remobilization of U. Strontium has been extensively redistributed within the fault zone, and Cs covaries with K. Retrograded areas of the pluton have whole-rock δ18 values ranging between 4.5 and 7.2 per mil, both below and above primary values of 5.7 per mil, indicating open system behaviour during regional retrograde metamorphism. Within the fault zone, hydrothermal reservoirs involved in later K-metasomatism were at temperatures of 150–250°C, where δ18 O H2O= −2.6 to +1.7 per mil, likely representing isotopically and chemically evolved deep-pore fluids expelled during crustal activity related to tectonic processes in the nearby Grenville Province. Calcite and laumontite developed by the advection through the fault zone of Ca-rich thermal brines at temperatures around 50–200°C, where δ18OH2O= −15 to 0 per mil; such fluids were evolved formation brines with a major component of meteoric H2O, expelled in the lower Paleozoic. The multiple deformation-metasomatism events are interpreted in terms of episodic seismic pumping on a fault that was intermittently active over a time scale in excess of 1 Ga.

Layered and massive gabbros of the Humboldt Lopolith and extrusive equivalents in west-central Nevada yield well-grouped paleomagnetic data on a site basis. With corrections for attitudes of compositional layering in gabbros and flow contacts in lavas (kcorr/kin situ = 5.5), the mean of 15 site means is: D = 307°, I = 21°, α95 = 9°, k = 17. While K-Ar data imply a Late Jurassic age, we prefer an older, probably Early Jurassic, age for the lopolith and its magnetization based upon magnetization data and relations of these rocks with host strata. Compared with Early Jurassic expected directions, the data reveal a discordancy in declination of −37° (+/− 14-22°) which implies a significant counterclockwise rotation of the lopolith. Local rotation of Fence-maker allochthon units most likely occurred when southeasterly-directed thrust sheets obliquely contacted a generally north-south trending shelf margin. Mesozoic deformation will be more accurately estimated with better age control and measurement of possible Cenozoic crustal rotation.

Iron-rich (Fs:En ∼0.8) calcic pyroxenes that have been subjected to granulite-facies metamorphism contain up to seven generations of exsolution lamellae. They can be grouped into four stages. In stage 1 pigeonite exsolved parallel to ‘001’ and ‘100’ (where ‘hkl’ signifies ∼ (hkl)) and mostly inverted later to orthopyroxene. During stage 2 orthopyroxene exsolved parallel to (100), while during stage 3 orthopyroxene was quickly followed by metastable ‘001’ pigeonite. The stage 3 precipitates clearly grew and thickened together for some time. During stage 4 a ‘100’ pigeonite was exsolved. The stage 3 and 4 precipitates show evidence of reheating, dissolution and later, renewed growth. Sometimes orthopyroxenes of stage 3 have crossed a ‘001’ pigeonite lamella and caused it to invert by a shear mechanism.Chemical analysis shows no rotation of the tie lines between Ca-rich and Ca-poor phases, in contrast to previous studies of Skaergaard and Bushveld pyroxenes. The geothermometers of Wood and Banno (1973) and Wells (1977) indicate solidus temperatures of about 850°C and 900°C respectively, but the geothermometers were found to be unsuitable for subsolidus conditions. We estimate the pressure to have been about 9 kbar during solidification. Estimates of nucleation temperatures obtained from the orientations of the exsolved lamellae (Robinson et al., 1977) were 850–700°C for stage 1, and 600–400°C for stage 3. We believe this geothermometer to be unreliable for the low temperatures involved in stage 4.

The East Bull Lake Pluton, a layered gabbro–anorthosite intrusion 90 km west of Sudbury, Ontario, is in an inward-dipping lopolith and is 780 m thick in the centre and elliptical in plan view (13.5 km × 3.5 km). It intruded Archean metavolcanic and metaplutonic rocks of the Superior Province during the Early Proterozoic (2480 Ma).The intrusion consistes of a basal anorthositic unit, succeeded by rhythmic-layered gabbro, troctolite, layered gabbro, and massive and dendritic gabbro units. It is offset by the west-northwest-striking, subvertical Folson Lake fault. South of this fault, only anorthosite and massive and dendritic gabbro are exposed. North of the fault, subsurface lithologies intersected by me boreholes correlate with surface lithologies. Troctolite, the most distinctive marker that can be correlated between boreholes and surface exposures, confirms the general attitude and shape of the layers and lopolith.Chemical composition of the intrusion ranges from high-Mg tholeiite to calc-alkaline high-alumina basalts.Fractures occur in several preferred orientations, the most common being northwesterly, subparallel to the Folson Lake fault, numerous mafic dykes, and topographic lineaments. Complex fracture-filling and alteration mineralogies formed under a wide range of P–T conditions representative of epidote-amphibolite – greenschist facies, pumpellyite–prehnite facies, zeolite facies, and low-temperature rock–water interaction processes.The last movement on the Folson Lake fault was a dextral strike slip of up to 3.0 km that postdates most mafic dykes.

The occurrence and origin of marialitic scapolite in the Humboldt lopolith was investigated in the field and in the laboratory using petrographic and experimental techniques. Scapolite occurs in three modes: as a pervasive replacement of plagioclase and other minerals in gabbro, diorite and extrusive rocks; as a poikiloblastic mineral in scapolitite dikes; and as a fracture-filling mineral with analcime, albite and sphene in scapolite veins. Additional secondary minerals associated with scapolite include epidote, prehnite, hornblende and diopside-salite clinopyroxene. Relations with these minerals suggest that most marialitic scapolite grew at temperatures around 400° C. Scapolite composition varies from EqAn12 to EqAn37, containing from 72 to 96 atomic% Cl in the R position. Experiments on systems of similar compositions indicate that NaCl-H2O fluid having more than 40 mol% NaCl is needed to stabilize the scapolite. Variation in scapolite compositions is due to thermal and fluid compositional gradients normal to conduits of hydrothermal fluids, and occurs on a scale up to 100 m. The likely source of Na and Cl is pre-existing evaporites or evaporitic brine derived from the wallrocks. Salinity could have been increased to a level sufficient to stabilize scapolite by hydration of an originally dry magma, possibly aided by hydrothermal boiling. Results may be applied to hydrothermal alteration in areas of rifting or back-arc spreading, and in mid-ocean ridge hydrothermal systems.

By using seismic and detailed geological data from a region of the KenyaRift where a complete Caenozoic rock sequence is exposed, it has been possible to interpret the gravity data and produce density models of a somewhat greater degree of uniqueness than has previously been possible. The configuration of the dense rocks responsible for the ‘axial high’ gravity anomalies is still not well defined but they seem to form a broad (20–35 km) zone within the basement, with strike along the Rift axis, and of limited average vertical extent (4–6 km). Such a model may imply a lopolith type of basic intrusion which would fit in with the small estimate of crustal extension across the Rift at this latitude which has been proposed. However, it is still possible to devise a dyke intrusion interpretation.

Zircons separated from monzonoritic rocks and quartz mangerites (igneous and metamorphic) have been studied for various mineralogical, chemical and physical properties (colour, zoning, presence of core, U distribution, morphology, lattice parameters, thermoluminescence) to determine the relationships with chemical composition of the host rocks and geological setting. The zircons grade from nearly perfectly crystalline to slightly metamict types. The crystal morphology shows prismatic habits and a striking lack of well-developed pyramidal faces, possibly indicating growth in a dry formation environment. The zircons can be divided into two groups. The first one comprises zircons from monzonoritic rocks. They are unzoned, devoid of brownish core, with a uniform U distribution, and low U content (except in Hidra). The second group comprises zircons from igneous and metamorphic quartz mangerites; they contain brownish, zoned, U-rich cores. In rocks where temperature of crystallization can be determined, Watson and Harrison's (1983) experimental data indicate strong zircon saturation. The cores are considered as inherited, a fact that can be accounted for either by dry anatexis or bulk contamination. In the Bjerkreim-Sokndal layered lopolith, previous isotopic data are confirmed, which indicate that the acidic upper part of the massif is not comagmatic with the anorthosito-noritic lower part and the monzonoritic rocks.

The Duluth gabbro complex is a large sill-like, composite intrusive consisting of mafic igneous rocks. It crops out within the city of Duluth, Minnesota, and extends for about 150 miles in a northeasterly direction west of Lake Superior. The Duluth gabbro was intruded along the base of the North Shore volcanic group of lava flows, tuffs, and interflow sediments that comprise part of the western limb of the Keweenawan syncline. The dip of the Duluth gabbro and the North Shore volcanic group is to the southeast under Lake Superior.