Forceful Emplacement Research Articles

Extensive soft-sediment deformation and peperite formation at the base of a rhyolite lava: Owyhee Mountains, SW Idaho, USA

In the Northern Owyhee Mountains (SW Idaho), a >200-m-thick flow of the Miocene Jump Creek Rhyolite was erupted on to a sequence of tuffs, lapilli tuffs, breccias and lacustrine siltstones of the Sucker Creek Formation. The rhyolite lava flowed over steep palaeotopography, resulting in the forceful emplacement of lava into poorly consolidated sediments. The lava invaded this sequence, liquefying and mobilising the sediment, propagating sediment subvertically in large metre-scale fluidal diapirs and sediment injectites. The heat and the overlying pressure of the thick Jump Creek Rhyolite extensively liquefied and mobilised the sediment resulting in the homogenization of the Sucker Creek Formation units, and the formation of metre-scale loading structures (simple and pendulous load casts, detached pseudonodules). Density contrasts between the semi-molten rhyolite and liquefied sediment produced highly fluidal Rayleigh-Taylor structures. Local fluidisation formed peperite at the margins of the lava and elutriation structures in the disrupted sediment. The result is a 30–40-m zone beneath the rhyolite lava of extremely deformed stratigraphy. Brittle failure and folding is recorded in more consolidated sediments, indicating a differential response to loading due to the consolidation state of the sediments. The lava-sediment interaction is interpreted as being a function of (1) the poorly consolidated nature of the sediments, (2) the thickness and heat retention of the rhyolite lava, (3) the density contrast between the lava and the sediment and (4) the forceful emplacement of the lava. This study demonstrates how large lava bodies have the potential to extensively disrupt sediments and form significant lateral and vertical discontinuities that complicate volcanic facies architecture.

Forceful emplacement of the Eureka Valley–Joshua Flat–Beer Creek composite pluton into a structural basin in eastern California; internal structure and wall rock deformation

Abstract Anisotropy of Magnetic Susceptibility parameters have been analyzed at 311 locations in the Eureka Valley–Joshua Flat–Beer Creek (EJB) pluton of eastern California. The large amount of data has allowed for the AMS parameters to be contoured using techniques that both reveal map-scale trends and emphasize small-scale differences. The contour maps suggest that magnetic susceptibility is dominantly controlled by composition of the magma but may also be affected by emplacement-related strain as the magma chamber inflated and forced the wall rocks outward. Pluton construction involved two major pulses of different composition magmas that were emplaced sequentially but with overlapping periods of crystallization. The magmas initially intruded as sill-like bodies into a structural basin. The magnetic foliation of the pluton cuts across internal magmatic contacts on the map scale and is parallel to local contacts between the pluton and surrounding metasedimentary wall rocks. The magnetic fabric is similar in orientation and symmetry to intense flattening strains recorded in the aureole rocks. The metasedimentary wall rocks have been shortened between 60 and 70% and this strain magnitude is approximately equal on the west, south, and east margins of the pluton. Strain in the wall rocks is dominantly flattening and concentrated into a narrow (1 km wide) inner aureole. Mapping of bedding/cleavage intersection lineations south of the pluton indicates that the magma made room for itself by translating the wall rocks outward and rotating the already inward dipping wall rocks of the structural basin to sub-vertical. Stretching of the inner aureole around an expanding magma chamber was responsible for the intense shortening. Limited data on the Marble Canyon pluton to the south of the EJB pluton indicates a very similar emplacement process.

The relationship between forceful and passive emplacement: The interplay between tectonic strain and magma supply in the Rosses Granitic Complex, NW Ireland

The Rosses Granitic Complex, NW Ireland, part of the late Caledonian (c. 400 Ma) Donegal Batholith, provides the opportunity to study the interplay between relative tectonic strain and magma supply rates in the wall of a major tectonic shear zone. Anisotropy of magnetic susceptibility (AMS) measurements, structural analysis and examination of key intrusive relationships were used to assess the accommodation of magma, associated deformation and magma flow pathways in this complex. In this case the varying emplacement styles, switching from forceful to passive, indicate that relative tectonic strain and magma supply rates were not constant. In the earliest component (a suite of microgranite sheets), AMS reveals subtle fabrics discordant to the sheet margins and is interpreted as post-emplacement deformation fabrics. The concordance of these fabrics to the next component of the complex, the main pluton (G1 and G2 monzogranite), indicates that this deformation was caused by the forceful emplacement of the pluton. AMS fabrics in G1 and G2 reveal a dome shaped foliation with an east-west lineation, indicating an east-west oriented magma transport direction. Outcrops of small stocks or cupolas similar to G2, east of the main pluton, link it to similar lithologies in the Main Donegal Granite further to the east via a partially exposed lateral feeder. This suggests east to west emplacement. The next component, a suite of subvertical porphyritic felsite dykes, is shown (from AMS and visible shearing fabrics) to have been emplaced passively under east-west tension. The final component comprises G3 and G4 of the main pluton, which passively cut all earlier components and contains significant amounts of aplite, pegmatite and greisen. These are interpreted to be cupolas or stocks emanating from an unexposed sheet probably similar in composition and mode of emplacement to G1 and G2. Thus, a general model is put forward where initially forceful subhorizontal sheets extending laterally from the nearby Main Donegal Granite shear zone, gave way to passive emplacement where the roofs of these and subjacent sheets failed. The control on siting of the complex can be related to lower crustal structures, which control upper crustal shear zones. The reason for the switch in emplacement style is suggested to be due to a waning magma supply, in a tectonic strain field where the extensional component was oriented east-west. The waning magma supply then possibly due to switching of the site of emplacement of magma, supplied by the Main Donegal Granite shear zone, to another member of the Donegal Batholith – the Trawenagh Bay Granite.

Forceful Emplacement of Granitic Plutons in an Extensional Tectonic Setting: Syn‐kinematic Plutons in the Yagan‐Onch Hayrhan Metamorphic Core Complex

Abstract It is generally considered that granitic plutons are forcefully emplaced in a compressional setting and permissively emplaced in an extensional setting. This paper, however, shows that syn‐kinematic (extensional) elliptic granitic plutons in the Yagan‐Onch Hayrhan metamorphic core complex (MCC) have relatively strong forceful emplacement, which are indicated by (1) concentric distribution of the rock units, (2) a strain pattern with strong strains on the margins and low strains at the centre of a pluton, and particularly (3) syn‐emplacement shortening of the host rocks within the aureole. The strain analysis for the host rocks shows that the host‐rock ductile shortening, i.e. forceful emplacement, provides about 16–24% of the emplacement space for the present plutons. All these suggest that forceful emplacement occurs not only in a compressional tectonic setting, but also in an extensional setting. This study further demonstrates the significance of the multiple emplacement of granitic plutons and provides new information about the causality between granitic magmatism and the formation of the MCC and its dynamics.

The Role of Fractional Crystallization and Assimilationin the Evolution of the Zoned Mukhattata Pluton, Eastern Desert, Egypt

Abstract The zoned Mukhattata pluton, covering an area of about 25 km 2, is composed of three zones becoming more felsic towards the centre of the pluton (normal zoning). The most outer zone consists of gabbro/diorite composition changing into tonalite (second zone) and granodiorite in the core (third zone). The tonalite is characterized by abundant gabbroid xenoliths, their number and size decreases towards the granodiorite in the core. The Mukhattata pluton exhibits geochemical features of I-type calc-alkaline rocks emplaced in a volcanic arc environment. The geochemical variations of major, trace and REE of gabbro/diorite, tonalite and granodiorite provide evidence that the different intrusions result from a progressive fractional crystallization of a single parental magma. Low and variable Mg#(33–48) of the gabbro/diorite outer zone and transitional contacts between them suggest that they were derived by fractional crystallization. A tonalitic magma intruded the gabbro/diorite producing the outer zone of gabbro/diorite composition. The presence of gabbroid xenoliths in the tonalite suggests that the assimilation process played some role during the formation of tonalitic rocks. Finally, the tonalite was intruded by granodioritic magma producing the core of the Mukhattata pluton. The petrological and geochemical results for the Mukhattata pluton reveal a transition from an early passive style of emplacement involving stoping of the tonalite by gabbro/diorite, to forceful emplacement of later granodiorite in the core.

Mecanismo de emplazamiento del Plutón Sierra de Varas Norte, Región de Antofagasta, Chile

The Sierra de Varas Norte pluton is a tonalitic intrusive body with an elliptical shape on the horizontal, it has a 12 km long axis by a recostructed 8 km width. It has a funnel shape in vertical section and is calcalkaline in composition. It was emplaced during the Late Carboniferous into a stratigraphic succession of Pennsylvanian to Permian? age which belongs to the Sierra de Varas Formation. The pluton contains a concentric, steeply inclined magmatic foliation enhanced by concordant 'schlieren', strangled synplutonic microdioritic dykes and microdioritic enclaves. The enclaves are more abundant towards the pluton margin and they are probably related to the dykes. Ellipticity measurements on the enclaves yield an oblated strain ellipsoid with Flinn parameter k=0,5, characterized by an X axis longer than the Y axes, with the XY plane contained in the magmatic foliation. In the deformed country rock an oblate strain ellipsoid also with Flinn parameter k=0,5 was obtained from deformed oncolites. The folding, axial plane cleavage and contact metamorphism in the country rock are more intense towards the contact with the pluton. This strongly suggests a mostly lateral flattening. The pluton and the country rock fabrics are interpreted to be concurrent and derived from a ballooning forceful emplacement of the pluton in shallow levels of the continental crust. The stretched oblate form of the strain ellipsoid (X longer than Y), and also the plunge towards the centre of the pluton of the stretching lineation in the country rock , suggest that the pluton-country rock system retained the memory of a diapiric ascent.

An elastic wedge model for the development of coeval normal and thrust faulting in the Mauna Loa‐Kilauea rift system in Hawaii

A long‐standing enigma of the Mauna Loa‐Kilauea rift system in Hawaii is the coeval development of normal and thrust faults that are vertically partitioned. To address this question, we developed a simple elastic wedge model that explores plausible boundary conditions in terms of tractions for generating such a fault pattern. Analytical solutions that best simulate the observed faulting style and geodetically determined strain at the surface require that (1) the pore fluid pressure ratio within the wedge (λ) and along the basal decollement (λb,) must be exceedingly high (i.e., λ = λb= 0.90–0.95) and (2) a tensile stress of the order of 10–30 MPa must have existed in the very top part of the rift zone at the back side of the wedge‐shaped rift flank. The high pore fluid pressure within the rift flank may be induced by pumping of fluids during emplacement of magma, whereas the high pore fluid pressure along the basal decollement may be caused by compaction of water‐saturated sediments between the volcanic pile above and the oceanic floor below. Although the predicted tensile stress in the rift zone could be related to the presence of a relatively steep topographic slope, our results show that this is not a prerequisite. Therefore we attribute occurrence of tensile stress to either upward bending of the Hawaiian volcanic pile due to emplacement of magma, or inflation of a shallow magma chamber several kilometers beneath the surface. In any case, the results of our model indicate that magma emplacement in the shallow part of the rift zone may be a passive process, while the deep rift zone experiences forceful emplacement (i.e., active rifting via magma push).

Translation and the resolution of the pluton space problem

The Late Cretaceous Mono Creek granite has a pronounced NW–SE elongate shape, 60 km long by 10 km wide, characteristic of plutons from the eastern Sierra Nevada batholith. An 8 km-wide bulge exists on the NE side of this pluton, which exhibits evidence of forceful emplacement (or in-situ ballooning), such as deflection of metamorphic wallrock and igneous foliation, and the orientation of fracture patterns. Three-dimensional strain analysis indicates that wallrock strains do not provide enough volume to accommodate the emplacement of the bulge, a recurring problem in studies of plutonic terranes. We suggest that emplacement of the Mono Creek bulge was accommodated by all components of the three-dimensional displacement field—including translation, rotation, and pure strain (shape change)—of the surrounding units. Classical strain analysis only addresses the rotation and pure strain components, and is incapable of quantifying the translation component. However, our analysis suggests that translation plays the dominant role in the emplacement process. A shell model of translation of the surrounding igneous and metamorphic units is proposed for the Mono Creek bulge, which suggests that the translation component decreases dramatically away from the intrusion, consistent with the observed geology and finite strain analysis. We propose that translation is the solution to the recurring pluton `space' problem, either through tectonically controlled (passive) or magmatically controlled (active) movement of the wallrocks. Translation is generally the neglected component of the displacement field, but it may often be evaluated through judicious use of finite strain analysis and tectonic reconstruction.

Granite intrusion by externally induced growth and deformation of the magma reservoir, the example of the Plasenzuela pluton, Spain

The Plasenzuela pluton in the Central Extremadura batholith in the southern Iberian Massif, is an example of permissive emplacement in relation to the tectonic development of extensional fractures in the upper continental crust. Paradoxically, this pluton has a concordant structural pattern which is classically attributed to diapirism or ballooning. This pattern consists of the following elements: (a) nearly elliptical shape in the horizontal section; (b) conformity of the pre-existing aureole structures to the shape of the pluton contacts; and (c) development of a crenulation cleavage, parallel to the contacts, in the vicinity of the pluton walls. All these features have been interpreted in many plutons as resulting from the pushing-aside of the country rock structures due to the expansion of the pluton. However, the detailed structural relationships in the aureole do not favour a forceful emplacement mechanism. By contrast, these relationships constitute prime evidence of permissive intrusion in extensional fractures. According to this interpretation, the concordant shape of the pluton was acquired by syn-plutonic opening of a mixed tensional-shear fracture, parallel to the main foliation in the host rocks, and by folding of the fracture walls together with the previous anisotropy of the country rocks. This is a growth–deformation process that can operate at local conditions in the upper continental crust giving rise to concordant syn-tectonic plutons.

Emplacement-related deformation of granitoid magmas, southern Adamello Massif, Italy

The Lago della Vacca Suite in the Tertiary Adamello Massif, Italian Alps, is a small (∼10 km 2 ), crudely zoned granitoid pluton emplaced at a pressure of ∼3.5 kbar into folded Mesozoic sediments and precursor intrusive rocks. Field relations and detailed mapping reveal a temporal evolution in the style of emplacement from early, passive stoping by the mafic to intermediate composition marginal units, to later forceful emplacement of the more silicic core. Marginal units of the pluton have highly discordant, stoped outer contacts against folded Triassic carbonates and precursor mafic/ultramafic rocks. In contrast, the core units have steeply inward-dipping contacts against these marginal units. Magmatic foliations defined by the planar alignment of plagioclase, amphibole, and biotite, and by mafic enclaves, are well developed in all units. They describe steeply inward-dipping concentric trajectories that approximately parallel unit contacts and in general increase in intensity toward the pluton margin. Lineations are rarely present. The most intensely foliated rocks occur in the marginal units, as anastomosing, sinuous bands that wrap around lenses of less intensely foliated granitoid. Three-dimensional strain analysis using mafic enclaves from throughout the pluton indicates that these bands represent zones of very high strain. Strain ellipsoids are weakly prolate within the core of the pluton, becoming increasingly oblate as strain increases toward the pluton margin. The short axis (Z) of the strain ellipsoid is uniformly subnormal to the foliation, suggestive of radial flattening. Textures of the marginal rocks, which record high strain, coupled with amphibole-plagioclase thermometry, indicate that deformation occurred in the magmatic or submagmatic state (∼650-780 °C) and was accompanied by significant migration of interstitial melt. The documented transition from passive to forceful emplacement, and final geometry of the suite, resulted from temporal changes in the rheology of both the magma and adjacent country rocks. Pulses of low-density hydrous magma were initially emplaced by stoping denser country rocks. On cooling and solidification, these magmas contracted by 15%-20%, creating an unpressured volume, which allowed subsequent magma pulses to be emplaced into the pluton core. These pulses distended and flattened their partially molten carapace and those parts of the aureole that had been thermally softened by intrusion of earlier magmas. Emplacement of the Lago della Vacca Suite was therefore the product of several internal or magma-related processes, an interaction between stoping, radial expansion, and volume contraction on solidification of the hydrous magmas. The heterogeneous distribution of strain in the marginal units reflects local variations in temperature and, most importantly, in the distribution of interstitial melt during emplacement.

Kinematics of large syn-orogenic intrusions: example of the Lower Proterozoic Saraya batholith (Eastern Senegal)

The large Saraya batholith intruded the Lower Proterozoic sequences of Eastern Senegal. Structural analysis indicates that this large granitic batholith corresponds to a composite body consisting of several coalescent plutons and interfering diapirs. Interpretation of the structural fabric patterns in these plutons and in the host-rocks constitutes the basis of a hypothetical model of initiation, ascent and emplacement as follows: (1.) development of a deep N-S granitic ridge controled by a major crustal fracture generated by an earlier extensional phase re-activated by a strike-slip fault which, probably, would facilitate segregation, collection and ascent of magmas; (2.) intrusion and splitting up of the ridge into several diapirs which ascended simultaneously and separately in a setting of a N-S sinistral transcurrent deformation inducing a transverse NW-SE shortening and forceful emplacement of plutons and (3.) final emplacement characterized by interference between regional deformation and pluton ‘ballooning’, the latter leading to the coalescence of intrusions and to the formation of a larger, apparently homogeneous body.

Geologic, fluid inclusion, and stable isotope studies of the gold-bearing breccia pipe at Kidston, Queensland, Australia

The Kidston mine, which is situated approximately 280 km west-northwest of Townsville, North Queensland, is currently Australia's second largest gold producer. The mineralization is hosted within a trapezoid-shaped breccia pipe with surface dimensions of 1,100 X 900 m. Brecciation and gold mineralization are spatially and temporally related to a swarm of Permo-Carboniferous rhyolite dikes which intrude middle Proterozoic metamorphic and Siluro-De-vonian granitoid host rocks. The rhyolite dikes are interpreted as being underlain by a Permo-Carboniferous batholith.Three phases of brecciation have been distinguished within the breccia pipe, all being associated with magmatic and or magmatic hydrothermal processes. Fluid inclusion data indicate that the present level of exposure was approximately 3,500 m below the contemporary land surface at the time of mineralization. The lack of any significant input of meteoric fluid into the hydrothermal system suggests that the breccia pipe, the bulk of which formed by collapse, failed to breach the contemporary land surface. Volatile-rich fluids, envisaged as being an integral part of breccia pipe formation, escaped fractures now occupied by breccia dikes.Prebreccia mineralization is uneconomic, consisting of stockwork vein mineralization in the carapaces of small prebreccia rhyolite stocks. Stockwork veining and localized brecciation resulted from the multiple buildup and escape of a high-temperature (>500 degrees C), highly saline (>40 wt % NaCl equiv) magmatic fluid associated with a number of crystallizing rhyolite stocks. This exsolved magmatic fluid comprised a liquid with a delta 18 O value of between 9.4 and 9.8 per mil. The pressure drop associated with fracture propagation and brecciation produced a vapor phase with a salinity of 3 to 12 wt percent NaCl equiv, which condensed at temperatures between 380 degrees and 460 degrees C due to adiabatic expansion to produce a liquid of similar salinity.Postbreccia mineralization was dominated by magmatic fluids (calculated delta 18 O value of 3-8ppm and delta D value of -50 to -20ppm), which in turn were dominated by a liquid with a salinity of 2 to 10 wt percent NaCl equiv. This liquid resulted from condensation of a vapor produced by the boiling of a highly saline magmatic fluid at a deeper level within the breccia pipe. Fluid inclusion studies indicate trapping temperatures in the ranges of 400 degrees to 540 degrees C for early-stage cavity infilling to as low as 170 degrees to 300 degrees C during the deposition of the sulfides and carbonate in the late-stage quartz veins and cavities. The economic-grade gold mineralization was deposited during late-stage mineralization. The lateral and vertical decrease in the grade of gold mineralization within the late-stage sheeted veins and cavities is associated with an increase in ratio of pyrrhotite to pyrite, which is interpreted as reflecting increasing temperature.The structural control on the distribution of postbreccia mineralization was an inverted funnel-shaped zone of enhanced permeability produced by the forceful emplacement of the postbreccia rhyolite into the lower portion of the breccia pipe. The persistence of this zone throughout the postbreccia mineralizing event is further evidence of the close genetic relationship between rhyolite magmatism and gold mineralization at Kidston.

花こう岩の変形構造と貫入テクトニクス

Emplacement mechanism of granitic bodies is classified into forceful emplacement and permitted emplacement. Forceful emplacement is caused by buoyancy of granitic magma relative to the country rocks. This mechanism is called diapirism. Permitted emplacement is promoted by regional tectonic deformation or fault activities which provided space for their emplacement. In many cases, emplacement mechanism and tectonic setting of each granitic body can be clarified through its deformation features. The basic methods for structural studies on granites and some representative emplacement meachanisms are described. Future problems of deformation structures, emplacement mechanism, and tectonic setting of emplacement for Japanese granitic bodies are discussed.

The influence of large ductile shear zones on the emplacement and deformation of the Wyangala Batholith, SE Australia

The Wyangala batholith consists of many small S- and I-type plutons, most of which are elongate in a N-S direction. Our studies of this batholith indicate that N-S-striking magmatic foliations exist in some of the individual plutons but are not strongly developed. A few individual plutons show evidence of forceful emplacement, but, in general, most do not. Intense post-emplacement deformation occurs in a series of E-directed thrusts that border the eastern edge of individual plutons, or the batholith as a whole. This thrusting formed gneissic layering, mylonites and secondary foliations in the batholith, and complex polyphase folding and cleavage development in the wall rocks. We suggest that the batholith was emplaced as sheet-like bodies, possibly along steep reverse faults. Data presently available indicate that the pre-emplacement deformation of the wall rocks is Middle Silurian, that the batholith was emplaced in the late Silurian, and that post-emplacement thrusting is late Silurian to early Carboniferous. Finally, the close association of batholiths and regions of complex deformation in the Lachlan fold belt suggest to us that these elements may represent shallow-level equivalents of Hollister & Crawford's “surge tectonics” ( Geology 14, 558–561, 1986).

Structural geology of felsic plutonic rocks in the Arabian Shield; styles, modes and levels of emplacement

Previous attempts to relate Arabian granite types to tectonic setting, depth and time of emplacement relative to orogeny are simplistic. They give little importance to the role of lineaments in magma genesis and ascent, do not consider the style of emplacement, either forceful or passive, and over-generalize the relationship between pluton emplacement and regional deformation. Whereas a great deal is known of the geochemistry and geochronology of these rocks, very little has been reported of their structural geology. Depth, style and time of emplacement provide an additional frame of reference through which to view these felsic plutonic rocks. Although plutons with features of forceful catazonal emplacement are reported in the southern Arabian Shield, they were predominantly emplaced transitionally between the catazone and mesozone. True catazonal plutons are rare, but parts of the Shield, notably the northern and eastern areas, contain country rocks of sufficiently high metamorphic grade to indicate that detailed mapping may eventually identify more. Diapiric plutons in the central and eastern Shield are examples of forceful, post-tectonic emplacement in the mesozone and, transitionally, between the mesozone and epizone. Passive, mesozonal emplacement involves the stoping and assimilation of the envelope. Epizonal plutons occur throughout the Shield and include many ring complexes, bell-jar shaped and nested plutons typical of passive emplacement, as well as some examples of forceful emplacement by piercement, diapiric deformation of the envelope and gas coring. Many of the plutons described are post-tectonic but one is syn-tectonic relative to a shear zone, and some are pre-tectonic. Others show evidence of a combination of forceful and passive emplacement. The localization of magmas due to control by major structural lineaments is common within the Shield, but only one instance, a shear zone in the northwestern Hijaz, has been studied in detail.

Horizontal cleavage in southeastern Sinai: the case for a coaxial strain history

A horizontal cleavage and associated lineation are developed in a low-pressure igneous-related metamorphic terrain in southestern Sinai. The cleavage is axial planar to recumbent folds, which are never large, and varies from a smooth slaty cleavage to a discrete crenulation cleavage. Structural evidence on the macro, meso and microscales suggests that cleavage and lineation were formed during irrotational extension and not by simple shear. Estimates of strain using the March method indicate 50–70% vertical shortening. This structural evidence when combined with metamorphic and age data strongly suggests that the cleavage was formed by the forceful emplacement of a pluton at depth.

Allard stock, La Plata Mountains, Colorado—an alkaline rock-hosted porphyry copper – precious metal deposit

The Allard stock, which hosts a porphyry Cu–Ag–Au–Pt metals deposit, is an epizonal Laramide pluton at the southwest end of the Colorado mineral belt. The stock and associated mineralization evolved by a complex interaction of igneous and metasomatic processes as follows. Stage 1: forceful emplacement of syenite plutons. Stage 2: intense hydrothermal argillization of stage 1 syenites. Stage 3: intrusion of mafic syenite plutons, some of which are layered, along shear zones. Stage 4: violent escape of volatiles, producing intrusive breccia pipes. Stage 5: intense K-metasomatic alteration of the older stock units with production of a fenitelike alteration halo extending 1000 m beyond the stock and simultaneous intrusion of carbonate-bearing alkali feldspar dikes. Stage 6: emplacement of trachyte, syenite pegmatite, and mafic pegmatite dikes. Stage 7: deposition of metallic minerals, sanidine, quartz, calcite, and fluorite as veinlets and disseminated replacement masses from hydrothermal solutions.The stock shows enrichment of volatiles (CO2, F, and S), high concentrations of Ba and Sr, low concentrations of Rb, high concentrations of Cu, Ag, Au, Bi, Te, Pt, and Pd, with little or no enrichment in Nb, U, Y, and Zr.The Allard stock and its associated mineralization are unlike the calc-alkaline porphyry copper deposits of the western United States and Canada. Although the Allard syenites are silica saturated (quartz-normative), some aspects of Allard alteration and mineralization show affinities with copper and (or) precious metal deposits hosted by alkaline silica-undersaturated (nepheline- or leucite-normative) plutons and carbonatite complexes including the "alkaline suite" porphyry copper deposits of British Columbia, the Palabora, South Africa, copper deposit, and the Cripple Creek, Colorado, gold telluride deposit. Computer-assisted evaluation of major-element analyses of igneous rocks suggests that Allard-type Cu – precious metal deposits may occur in other North American alkaline rock provinces. Epithermal gold telluride deposits in subvolcanic alkaline rocks may grade downward into alkaline porphyry Cu – precious metal deposits.

Reverse zoning in the resurgent intrusions of the Grizzly Peak cauldron, Sawatch Range, Colorado

The deeply eroded Grizzly Peak cauldron, 17 by 23 km and source of the 34-m.y.-old Grizzly Peak Tuff, is located in the Sawatch Range in west-central Colorado. The Lincoln Gulch composite stock is exposed in the core of a resurgent dome in the northern part of the cauldron and consists of three units. Their forceful emplacement was a cause of cauldron resurgence. The first intrusion lifted its roof along a vertical fault that surrounds the stock. The second resurgent intrusion, a complex body with broad sill-like overhangs, was emplaced within the first intrusion by further piston uplift of the roof. The third intrusion, emplaced in the second, is a small funnel-shaped body. Normal faults in surrounding inter-layered intracaldera tuff and caldera-collapse breccia are radial to the intrusion-cored piston block. Most of the tuff dips away from the resurgent intrusions. Pyroclastic dikes cutting tuff that roofs the stock may have been emplaced by venting of the intrusions. The three resurgent intrusions are concentrically zoned, with the most differentiated material at the margins. From margin to core, compositions range from biotite granodiorite to hornblende-bearing biotite quartz monzodiorite. The intrusions are porphyroaphanitic throughout, but both phenocrysts and groundmass become coarser toward the cores. The compositional patterns are termed reverse zonations because most documented concentrically zoned intrusions become more silicic toward their cores. Crystal settling and marginal accretion of crystalline phases can be eliminated as means of forming reverse zoning because mafic-coreward gradients in element and mineral content are developed inward from contacts of all attitudes. Diffusion-based mechanisms of in situ differentiation are too slow, given the short solidification times for these sill-like bodies. We interpret reverse zonation to result from rearrangement of a vertically graded magma column during emplacement of the intrusions. Progressively more mafic magma from successively deeper levels in a subjacent chamber rose into the cores of the intrusions, displacing more silicic magma toward the margins. This emplacement pattern was preserved by rapid solidification, perhaps due to volatile loss on venting.

Storage, migration, and eruption of magma at Kilauea volcano, Hawaii, 1971–1972

The magmatic plumbing system of Kilauea Volcano consists of a broad region of magma generation in the upper mantle, a steeply inclined zone through which magma rises to an intravolcano reservoir located about 2 to 6 km beneath the summit of the volcano, and a network of conduits that carry magma from this reservoir to sites of eruption within the caldera and along east and southwest rift zones. The functioning of most parts of this system was illustrated by activity during 1971 and 1972. When a 29-month-long eruption at Mauna Ulu on the east rift zone began to wane in 1971, the summit region of the volcano began to inflate rapidly; apparently, blockage of the feeder conduit to Mauna Ulu diverted a continuing supply of mantle-derived magma to prolonged storage in the summit reservoir. Rapid inflation of the summit area persisted at a nearly constant rate from June 1971 to February 1972, when a conduit to Mauna Ulu was reopened. The cadence of inflation was twice interrupted briefly, first by a 10-hour eruption in Kilauea Caldera on 14 August, and later by an eruption that began in the caldera and migrated 12 km down the southwest rift zone between 24 and 29 September. The 14 August and 24–29 September eruptions added about 10 7 m 3 and 8 × 10 6 m 3, respectively, of new lava to the surface of Kilauea. These volumes, combined with the volume increase represented by inflation of the volcanic edifice itself, account for an approximately 6 × 10 6 m 3/month rate of growth between June 1971 and January 1972, essentially the same rate at which mantle-derived magma was supplied to Kilauea between 1952 and the end of the Mauna Ulu eruption in 1971. The August and September 1971 lavas are tholeiitic basalts of similar major-element chemical composition. The compositions can be reproduced by mixing various proportions of chemically distinct variants of lava that erupted during the preceding activity at Mauna Ulu. Thus, part of the magma rising from the mantle to feed the Mauna Ulu eruption may have been stored within the summit reservoir from 4 to 20 months before it was erupted in the summit caldera and along the southwest rift zone in August and September. The September 1971 activity was only the fourth eruption on the southwest rift zone during Kilauea's 200 years of recorded history, in contrast to more than 20 eruptions on the east rift zone. Order-of-magnitude differences in topographic and geophysical expression indicate greatly disparate eruption rates for far more than historic time and thus suggest a considerably larger dike swarm within the east rift zone than within the southwest rift zone. Characteristics of the historic eruptions on the southwest rift zone suggest that magma may be fed directly from active lava lakes in Kilauea Caldera or from shallow cupolas at the top of the summit magma reservoir, through fissures that propagate down rift from the caldera itself at the onset of eruption. Moreover, emplacement of this magma into the southwest rift zone may be possible only when compressive stress across the rift is reduced by some unknown critical amount owing either to seaward displacement of the terrane south-southeast of the rift zone or to a deflated condition of Mauna Loa Volcano adjacent to the northwest, or both. The former condition arises when the forceful emplacement of dikes into the east rift zone wedges the south flank of Kilauea seaward. Such controls on the potential for eruption along the southwest rift zone may be related to the topographic and geophysical constrasts between the two rift zones.

Structural effects on the Proterozoic Ullensvang Group (West Norway) relatable to forceful emplacement of expanding plutons

An isolated synclinorium, comprising the up to 5 km thick Ullensvang Group of metavolcanic and metasedimentary rocks, is surrounded by granitoid plutons and has no recognizable older basement. From an earlier interpretation based on regional data this area is inferred to be part of a Proterozoic, cordilleran-type volcanic-plutonic belt. Major structures in the synclinorium are interpreted to have evolved as follows: 1) Early folds were produced by regional compression; 2) within a part of the synclinorium, such folds were tightened and reoriented during a subsequent deformation phase caused by forceful emplacement of the Kvinnherad batholith, flanking the synclinorium to the southwest; 3) this deformation was accompanied by thrust faulting towards the northeast, away from the batholith; 4) forceful emplacement of plutons belonging to the Eidfjord-Kinsarvik batholith, on the northeastern flank of the synclinorium, produced southwest-vergent overfolding of the earlier structures in the synclinorium. In addition, local deformation around a pluton intruding the northern part of the synclinorium near Utne produced areally restricted deformation structures, approximately corresponding in time with the second deformation event (2) above.