Sheet Intrusions Research Articles

The Derim Derim Dolerite, greater McArthur Basin, Australia: Using subsurface data to characterise a mesoproterozoic magma plumbing system

The application of high-resolution seismic reflection data has spurred major advances in knowledge of the emplacement of sub-volcanic mafic magma plumbing systems in sedimentary basins, highlighting the importance of interconnected sheet intrusions in facilitating lateral magma transport, the links between host rock mechanical properties and emplacement processes, and providing insights into how intrusive activity in basins impacts their resource potential. However, most studies have focused on Mesozoic-Cenozoic mafic magma plumbing systems situated along offshore rifted margins characterised by extensive subsurface datasets. The extensive Mesoproterozoic (c. 1300 Ma) Derim Derim Dolerite, which intrude the greater McArthur Basin in northern Australia, provide a unique opportunity to study the emplacement of a Proterozoic magma plumbing system due to its penetration by numerous hydrocarbon and mineral drillholes, in addition to seismic reflection coverage. Understanding the emplacement of this system is important because of its interactions with prospective unconventional shale reservoirs in the Velkerri and Kyalla Formations, which represent one of the world's oldest known petroleum systems. This paper focuses on characterising the intrusion emplacement and magma plumbing system using an array of subsurface data, to constrain the distribution, morphology, and emplacement mechanisms of the Derim Derim Dolerite.The morphology of the large, strata-concordant intrusions encountered at shallow present-day depths (<2 km) in the greater McArthur Basin is indicative of greater original emplacement depths of >3–4 km, suggesting a significant extent of uplift and erosion. Density-derived porosity values for the Velkerri and Kyalla Formations are anomalously low for their present-day depths, corroborating a greater palaeo-depth at time of emplacement. The extent of alteration of the host rock surrounding the Derim Derim Dolerite is highly variable, with a small number of occurrences of graphitisation of the organic matter adjacent to intrusions. However, this appears to be highly localised and the detrimental impact of the Derim Derim Dolerite on potential reservoir and/or source rocks appears generally minimal.

Intrusion tip velocity controls the emplacement mechanism of sheet intrusions

Space for intruding magma is created by elastic, viscous, and/or plastic deformation of host rocks. Such deformation impacts the geometries of igneous intrusions, particularly sills and dikes. For example, tapered intrusion tips indicate linear-elastic fracturing during emplacement, whereas fluidization of host rocks has been linked to development of elongate magma fingers with rounded tips. Although host rock fluidization has only been observed at the lateral tips of magma fingers, it is assumed to occur at their leading edges (frontal tips) and thereby control their propagation and geometry. Here, we present macro- and microstructural evidence of fluidized sedimentary host rock at the lateral tips of magma fingers emanating from the Shonkin Sag laccolith (Montana, western United States), and we explore whether fluidization could have occurred at their frontal tips. Specifically, we combine heat diffusion modeling and fracture tip velocity estimates to show that: (1) low intrusion tip velocities (≤10−5 m s−1) allow pore fluids ahead of the intrusion to reach temperatures sufficient to cause fluidization, but (2) when tip velocities are high (∼0.01−1 m s−1), which is typical for many sheet intrusions, fluidization ahead of propagating tips is inhibited. Our results suggest that intrusion tip velocity (i.e., strain rate) is a first-order control on how rocks accommodate magma. Spatially and temporally varying velocities of lateral and frontal tips suggest that deformation mechanisms at these sites may be decoupled, meaning magma finger formation may not require host rock fluidization. It is thus critical to consider strain rate and three-dimensional intrusion geometry when inferring dominant magma emplacement mechanisms.

The distribution and morphology of Late Jurassic to Early Cretaceous igneous intrusions in the Northern Carnarvon Basin

The Northern Carnarvon Basin (NCB) contains extensive networks of igneous intrusions emplaced during Late Jurassic and Early Cretaceous rifting that led to the breakup of the Greater India from Australia. We present the first basin-wide study of the distribution and morphology of these igneous intrusions through the interpretation of regionally extensive 3D and closely spaced 2D seismic data across the Exmouth Plateau and Exmouth Sub-basin. We observe three dominant intrusion morphologies: (1) Saucer-shaped intrusions up to ~20 × 40 km, but commonly much smaller, present in Jurassic strata of the southern Exmouth Plateau and central Exmouth Sub-basin; (2) Large, stacked, strata parallel, sheet intrusions, often &gt;100 km in length, dominant in Triassic strata in the Exmouth Plateau and southern Exmouth Sub-basin; and (3) Variably sized, predominantly strata parallel and occasionally fault hosted intrusions (ranging in dimension from ~5 × 8 to ~35 × 65 km) present in Jurassic rocks in the Exmouth Sub-basin, and uppermost Triassic rocks in the Exmouth Plateau. We suggest that the morphologies of intrusions in the NCB are predominantly controlled the mechanical properties of their host rocks.

Emplacement of a lamprophyric sheet swarm under ductile transpression in continental crust. Insights from a Variscan magmatic complex in the West Armorican belt, Western France

We present structural data from a Variscan magmatic complex of dominantly lamprophyric (kersantite) and associated felsic (microgranodiorite) sheets - the West Armorican swarm (WAS) - injected into Paleozoic metasedimentary series at the western extremity of a transpressional system in the Armorican belt, Western France. The comparative structural analysis of nearly 100 individual intrusive sheets and their host-rocks demonstrates that the WAS complex is a syntectonic magmatic structure which intruded during the onset of regional shortening via successive pulses alternating with three strain episodes (D1-D2-D3). The moderate/high dipping attitude of a great majority of intrusions, along with their prominent orientation parallel to the regional cleavage (S1), i.e. orthogonal to the direction of shortening, challenge theoretical predictions about magma-filled fractures in contractional settings. These apparently atypical features are reconciled in a kinematic model which emphasizes the influence of both host-rock planar anisotropies (S1 cleavage and D3 shear patterns) and structural inheritance on magma pathways, and instead hypothesizes the negligible role of the ambient stress field. Comparisons are attempted with syncollisional lamprophyre systems present in the European Variscides, and more especially in the SE Bohemian massif.

Magnetic fabrics reveal three-dimensional flow processes within elongate magma fingers at the margin of the Shonkin Sag laccolith (MT, USA)

Unravelling magma flow in ancient sheet intrusions is critical to understanding how magma pathways develop and feed volcanic eruptions. Analyzing the shape preferred orientation of minerals in intrusive rocks can provide information on magma flow, because crystals may align parallel to the primary flow direction. Anisotropy of magnetic susceptibility (AMS) is an established method to quantify such shape preferred orientations in igneous sheet intrusions with weak or cryptic fabrics. However, use of AMS data to characterize how magma flows within the individual building blocks of sheet intrusions (i.e., magma fingers and segments), hereafter referred to as elements, has received much less attention. Here we use a high spatial resolution sampling strategy to quantify the AMS fabric of the Eocene Shonkin Sag laccolith (Montana, USA) and associated elongate magma fingers. Our results suggest that magnetic fabrics across the main laccolith reflect sub-horizontal magma flow, and inferred flow directions are consistent with an underlying NE-SW striking feeder dyke. Within the magma fingers, we interpret systematic changes in magnetic fabric shape and orientation to reflect the interaction between competing forces occurring during finger-parallel magma flow (i.e., simple shear) and horizontal and vertical inflation (i.e., pure shear flattening). For example, we highlight how local crossflow of magma between coalesced fingers increases the complexity of magma flow kinematics and related fabrics. Despite these complexities, the AMS data in coalesced magma fingers maintain their internal flow- and inflation-related fabrics, which suggests that magma flow within the fingers remains channelized after coalescence. Given that many sheet intrusions consist of amalgamated elements, our findings highlight the need to carefully consider element distribution and sample locations when interpreting magma flow based on AMS measurements.

Metamafic dyke and sill swarms in the Dom Feliciano Belt: Insights for post-collisional strike-slip tectonics and fluid-assisted metamorphism

The oblique collision of the Congo, Kalahari, and Rio de la Plata cratons resulted in the formation of the Dom Feliciano Belt that evolved from rifting, drifting, and amalgamation between ca. 1.0 and 0.5 Ga. The post-collisional stage of the orogenic event (ca. 600 – 530 Ma) is marked by lithospheric scale shear-zones, voluminous magmatism, and volcano-sedimentary sequences. However, the tectonic regimes, metamorphism and the development of space-forming structures are still debated. We present a comprehensive petrographic, mineralogical, and geochemical study of Ediacaran metamafic dyke and sill swarms from the easternmost portion of the São Gabriel Terrane (central Dom Feliciano Belt, southern Brazil). Mafic swarms and lamprophyres from two different areas in the syntectonic Caçapava do Sul aureole show heterogeneous microstructures, metamorphic degree and whole-rock composition due to heterogeneous metamorphism, deformation, and metasomatism. We aimed at deciphering the origin of the sheeted intrusions and the metamorphic processes that led to such heterogeneities. Trace-element composition indicates that the melts were formed at high depths and went through significant crustal assimilation during the ascent through the crust either in a continental arc or an island arc setting. Greenschist facies metamorphism and upper-crust brittle deformation partially preserved subvolcanic textures in metamafic dykes at the outer portion of the aureole. In opposition, amphibolite facies metamorphism with a significant fluid flow component resulted in highly modified and heterogeneous metamafic sills at the inner portion of the aureole and at high strain domains of the Caçapava do Sul Shear Zone. Heterogeneities in different areas are caused by the distance to batholith and to high-strain domains of the shear zone while variations within the same area result from heterogeneous and localized fluid flow during metamorphism. We suggest that the mafic swarms represent markers of the initial stage of the development of the shear zone and were metamorphosed due the subsequent syntectonic emplacement of Caçapava do Sul Granitic Complex. Remarkably, we note that heterogeneities in coeval units that often hinder the understanding of the Precambrian stratigraphy can be explained by heterogeneous fluid-assisted metamorphism, especially near shear zones and syntectonic intrusions.

Developments in the study of volcanic and igneous plumbing systems: outstanding problems and new opportunities

Prior to and during eruptions, magma is stored and transported within volcanic and igneous plumbing systems (VIPS) that comprise a network of magma reservoirs and sheet intrusions. The study of these VIPS requires the combination of knowledge from the fields of igneous petrology, geochemistry, thermodynamic modelling, structural geology, volcano geodesy, and geophysics, which express the physical, chemical, and thermal complexity of the processes involved, and how these processes change spatially and temporally. In this contribution, we review the development of the discipline of plumbing system studies in the past two decades considering three angles: (1) the conceptual models of VIPS and paradigm changes, (2) methodological advances, and (3) the diversity of the scientific community involved in VIPS research. We also discuss future opportunities and challenges related to these three topics.

Emplacement of a felsic dyke swarm during progressive heterogeneous deformation, Eastern Elba Dyke Complex (Island of Elba, Italy)

Magmatic and sub-solidus fabrics in intrusive rocks are frequently used to infer the relative timing of deformation with respect to magma emplacement and cooling. Here, we describe the relationships between strain and fabric development in leucogranite sheets (pegmatite, aplite) emplaced into shear zones that localized post-thermal peak deformation in the contact aureole of an upper crustal pluton (<0.2 GPa) on the Island of Elba, Italy. The leucogranite sheets present igneous, mylonitic, and cataclastic fabrics. Detailed meso- and microscopic structural analysis suggests that the dykes emplaced in the shear zones behaved as competent, rigid bodies during mylonitic deformation of the host rocks. Thermal modelling indicates that emplacement and cooling of the sheets occurred very rapidly (a few days to years) compared to typical tectonic strain rates and strain accumulation timescales in the host rocks. Such a fast cooling does not allow melt or magma-induced thermal softening in the host rocks during deformation. The development of mylonitic and cataclastic fabrics in the dykes was controlled by the localized activation of fluid-controlled reaction softening mechanisms (mylonitic fabric) and embrittlement during cooling in sites of high-strain (cataclastic fabric). We show that a broad spectrum of fabrics can form in igneous sheet intrusions emplaced at the same time and crustal level. The coexistence of isotropic (non-foliated igneous) versus anisotropic (mylonitic and cataclastic) fabrics in igneous sheet intrusions should therefore be evaluated in terms of tectonic strain rates, cooling rates, thermal state of the host, distribution of heterogeneous strain, and activation of strain softening mechanisms. Our observations highlight that the concepts of pre-, syn-, late- and post-tectonic fabrics in intrusive igneous rocks should be used with caution when interpreting relative timing relationships between deformation and magmatism.

Обоснование альтернативной конструкции скважин (на примере Среднеботуобинского нефтегазоконденсатного месторождения)

The circulation loss in well construction is one of the most frequent troubles at the Srednebotuobinsky oil and gas condensate field (SBOGCF). Such problems occur in the two basic intervals of the geological section, trapp intrusion and the Botuobinsky productive horizon. The intervals of salts, endogenic fracturing spread in the intrusive sheet, and gas-bearing collectors with abnormally high formation pressure, available in the SBOGCF, are currently considered incompatible drilling conditions. The emergency time, spent for fighting these issues within the trap intrusion interval, may reach 42 days per well. However, the loss of working time is not the only trouble; costs of drill fluids and cement grouts as well as of concomitant emergencies (striking of the drill column bottom arrangements) result in the increasing costs of well construction as well as to the lost advantage due to the late launching of the well. One of the ways to decrease the well construction cycle is reconsidering the existing design and adapting a brand new approach to drilling and well construction, which would permit to use the alternative well design for solving the abovementioned problems.

Dyke Architecture, Mineral Layering, and Magmatic Convection; New Perspectives From the Younger Giant Dyke Complex, S Greenland

AbstractIgneous sheet intrusions are a fundamental component of volcano plumbing systems. Identifying how sheet intrusion emplacement and geometry controls later magmatic processes is critical to understanding the distribution of volcanic eruptions and magma‐related ore deposits. Using the Younger Giant Dyke Complex (YGDC), a Mesoproterozoic suite of large (&lt;800 m wide) mafic dykes in southern Greenland, we assess the influence sheet of emplacement and geometry on subsequent magma flow and mush evolution. Through structural mapping, petrographic observations, and anisotropy of magnetic susceptibility fabric analyses, we show that the YGDC was emplaced as a series of individual dyke segments, which following coalescence into a sheet intrusion remained largely isolated during their magmatic evolution. Through petrographic evidence for liquid‐rich growth of cumulus phases, concentric magnetic fabrics, and the detailed study layered zones within the YGDC, we infer magma convection occurred within the cores of each dyke element. We particularly relate layering to hydrodynamic sorting processes at a magma‐mush boundary toward the base of each convection cell. Overall, our work demonstrates that the initial geometry of sheet intrusions can constrain magma flow patterns and affect the distribution of crystallization regimes.

The building blocks of igneous sheet intrusions: Insights from 3-D seismic reflection data

Abstract The propagating margins of igneous sills (and other sheet intrusions) may divide into laterally and/or vertically separated sections, which later inflate and coalesce. These components elongate parallel to and thus record the magma flow direction, and they can form either due to fracture segmentation (i.e., “segments”) or brittle and/or non-brittle deformation of the host rock (i.e., “magma fingers”). Seismic reflection data can image entire sills or sill-complexes in 3-D, and their resolution is often sufficient to allow us to identify these distinct elongate components and thereby map magma flow patterns over entire intrusion networks. However, seismic resolution is limited, so we typically cannot discern the centimeter- to meter-scale host rock deformation structures that would allow the origin of these components to be interpreted. Here, we introduce a new term that defines the components (i.e., “elements”) of sheet-like igneous intrusions without linking their description to emplacement mechanisms. Using 3-D seismic reflection data from offshore NW Australia, we quantify the 3-D geometry of these elements and their connectors within two sills and discuss how their shape may relate to emplacement processes. Based on seismic attribute analyses and our measurements of their 3-D geometry, we conclude that the mapped elements likely formed through non-elastic-brittle and/or non-brittle deformation ahead of the advancing sill tip, which implies they are magma fingers. We show that thickness varies across sills, and across distinct elements, which we infer to represent flow localization and subsequent thickening of restricted areas. The quantification of element geometries is useful for comparisons between different subsurface and field-based data sets that span a range of host rock types and tectonic settings. This, in turn, facilitates the testing of magma emplacement mechanisms and predictions from numerical and physical analogue experiments.

Emplacement of a Lamprophyric Intrusive Sheet Swarm in a Ductilly Contracted Continental Crust. Insights from a Variscan Magmatic Complex in the West Armorican Belt, Western France

Emplacement of a Lamprophyric Intrusive Sheet Swarm in a Ductilly Contracted Continental Crust. Insights from a Variscan Magmatic Complex in the West Armorican Belt, Western France

Relationship Between Dike Injection and b‐Value for Volcanic Earthquake Swarms

AbstractDike swarms are the fossil remains of regions of the crust that have undergone repeated magma injections. Volcanic earthquake swarms and geodetic measurements are, at least in part, a record of active injection of fluids (water, gas, or magma) into fractures. Here, we link these two ways of observing magmatic systems by noting that dike thicknesses and earthquake magnitudes share similar scaling parameters. In the Jurassic Independence dike swarm of eastern California median dike thickness is ∼1 m, similar to other swarms worldwide, but glacially polished exposures reveal that a typical dike comprises a number of dikelets that are lognormally distributed in thickness with a mean of ∼0.1 m. Assuming that dikes fill penny‐shaped cracks of a given aspect ratio, the geodetic moment and earthquake magnitude of a diking event can be estimated. A Monte Carlo simulation of dike‐induced earthquakes based on observed dike thickness variations yields a frequency‐magnitude distribution remarkably like observed volcanic earthquake swarms, with a b‐value of ∼1.7. We suggest that swarms of dikes composed of dikelets, as well as plutons built incrementally by sheet intrusions, are physical complements to volcanic seismic swarms, and that at least some earthquake swarms are a palpable expression of incremental magma emplacement.

Segment tip geometry of sheet intrusions, II: Field observations of tip geometries and a model for evolving emplacement mechanisms

Igneous sheet intrusions are segmented across several orders of magnitude, with segment tip geometry commonly considered indicative of the propagation mechanism (brittle or non-brittle). Proposed propagation mechanisms are inferred to represent host rock mechanical properties during initial magma emplacement; typically, these models do not account for segment sets that show a range of tip geometries within the same lithology. We present a detailed structural characterization of basaltic sill segments and their associated host rock deformation from the Little Minch Sill Complex, Isle of Skye, UK, and a broader comparison with segment geometries in three additional intrusive suites (Utah, USA; and Mull and Orkney, UK). Each separate host lithology shows multiple tip geometries and styles of host rock deformation, from elastic-brittle fracture, to viscous indentation and fluidisation. We attribute this range of host rock deformations to evolving conditions that occur at the tips both during sheet growth and arrest.

Segment tip geometry of sheet intrusions, I: Theory and numerical models for the role of tip shape in controlling propagation pathways

Inferences about sheet intrusion emplacement mechanisms have been built largely on field observations of intrusion tip zones: magmatic systems that did not grow beyond their observed state. Here we use finite element simulation of elliptical to superelliptical crack tips, representing observed natural sill segments, to show the effect of sill tip shape in controlling local stress concentrations, and the potential propagation pathways. Stress concentration magnitude and distribution is strongly affected by the position and magnitude of maximum tip curvature κmax. Elliptical tips concentrate stress in-plane with the sill, promoting coplanar growth. Superelliptical tips concentrate maximum tensile stress (σmax) and shear stress out-of-plane of the sill, which may promote non-coplanar growth, vertical thickening, or coplanar viscous indentation. We find that σmax = Pe(1+ 2(√[aκmax]), where Pe is magma excess pressure and a is sill half length. At short length-scales, blunted tips locally generate large tensile stresses; at longer length-scales, elliptical-tipped sills become more efficient at concentrating stress than blunt sills.

Emplacement and Segment Geometry of Large, High-Viscosity Magmatic Sheets

Understanding magma transport in sheet intrusions is crucial to interpreting volcanic unrest. Studies of dyke emplacement and geometry focus predominantly on low-viscosity, mafic dykes. Here, we present an in-depth study of two high-viscosity dykes (106 Pa·s) in the Chachahuén volcano, Argentina, the Great Dyke and the Sosa Dyke. To quantify dyke geometries, magma flow indicators, and magma viscosity, we combine photogrammetry, microstructural analysis, igneous petrology, Fourier-Transform-Infrared-Spectroscopy, and Anisotropy of Magnetic Susceptibility (AMS). Our results show that the dykes consist of 3 to 8 mappable segments up to 2 km long. Segments often end in a bifurcation, and segment tips are predominantly oval, but elliptical tips occur in the outermost segments of the Great Dyke. Furthermore, variations in host rocks have no observable impact on dyke geometry. AMS fabrics and other flow indicators in the Sosa Dyke show lateral magma flow in contrast to the vertical flow suggested by the segment geometries. A comparison with segment geometries of low-viscosity dykes shows that our high-viscosity dykes follow the same geometrical trend. In fact, the data compilation supports that dyke segment and tip geometries reflect different stages in dyke emplacement, questioning the current usage for final sheet geometries as proxies for emplacement mechanism.

Sheeted intrusion of granitic magmas in the upper crust – Emplacement and thermal evolution of the Guandacolinos pluton, NW Argentina

The Lower Carboniferous Guandacolinos pluton of northwestern Argentina (Western Sierras Pampeanas) preserves field, structural, and petrological evidence of sheet-like transport and assembly of granitic magmas in the upper crust. The pluton is a relatively small (~24 km2) subduction-related granitic body, elongated in map view, and hosted in Neoproterozoic metamorphic rocks. Exceptional exposure records a subparallel array of steep NNE-SSW trending structures, including steep contacts partly concordant with host rock structure, numerous sheets of granite separated by host rock rafts, abundant xenoliths, and magmatic and solid-state foliations. Along the eastern half of the pluton, the granite is massive and host rock inclusions are less abundant. Regional markers of the host rock are deflected along a concordant bulged contact in the northeastern region of the pluton. Field relations indicate emplacement by multiple material transfer processes including fracture propagation, magma wedging, stoping, and lateral shortening. Contrasting mechanisms imply a changing mechanical response of host rock and multiple stages of intrusion. Emplacement began with dominant brittle fracturing and intrusion of sheets influenced by host rock anisotropies, followed by a viscoelastic phase were larger batches of magma caused downward transfer of stoped blocks, lateral expansion, and minor ductile deformation of the host rock. Thermal modelling indicates that the construction of the pluton required lateral accretion rates in the order of dm/years and less than a few tens of thousands of years to form. This case study documents the ability of incrementally assembled sheeted intrusions to efficiently heat rocks of the upper crust and trigger conditions favourable for transfer and storage of magma.

Pressure-Driven Opening and Filling of a Volcanic Hydrofracture Recorded by Tuffisite at Húsafell, Iceland: A Potential Seismic Source

The opening of magmatic hydraulic fractures is an integral part of magma ascent, the triggering of volcano seismicity, and defusing the explosivity of ongoing eruptions via outgassing magmatic volatiles. If filled with pyroclastic particles, these fractures can be recorded as tuffisites. Tuffisites are therefore thought to play a key role in both initiating eruptions and controlling their dynamics, and yet their genesis remains poorly understood. Here we characterise the processes, pressures and timescales involved in tuffisite evolution within the country rock through analysis of the sedimentary facies and structures of a large sub-horizontal tuffisite vein, 0.9 m thick and minimum 40 m in length, at the dissected Húsafell volcano, western Iceland. The vein occurs where a propagating rhyolitic sheet intrusion stalled at a depth of ∼500 m beneath a relatively strong layer of welded ignimbrite. Laminations, cross-stratification, channels, and internal injections indicate erosion and deposition in multiple fluid pulses, controlled by fluctuations in local fluid pressure and changes in fluid-particle concentration. The field evidence suggests that this tuffisite was emplaced by as many as twenty pulses, depositing sedimentary units with varying characteristics. Assuming that each sedimentary unit (∼0.1 m thick and minimum 40 m in length) is emplaced by a single fluid pulse, we estimate fluid overpressures of ∼1.9–3.3 MPa would be required to emplace each unit. The Húsafell tuffisite records the repeated injection of an ash-laden fluid within an extensive subhorizontal fracture, and may therefore represent the fossil record of a low-frequency seismic swarm associated with fracture propagation and reactivation. The particles within the tuffisite cool and compact through time, causing the rheology of the tuffisite fill to evolve and influencing the nature of the structures being formed as new material is injected during subsequent fluid pulses. As this new material is emplaced, the deformation style of the surrounding tuffisite is strongly dependent on its evolving rheology, which will also control the evolution of pressure and the system permeability. Interpreting tuffisites as the fossil record of fluid-driven hydrofracture opening and evolution can place new constraints on the cycles of pressurisation and outgassing that accompany the opening of magmatic pathways, key to improving interpretations of volcanic unrest and hazard forecasting.

The Generation of Arc Andesites and Dacites in the Lower Crust of a Cordilleran Arc, Fiordland, New Zealand

Abstract We present microbeam major- and trace-element data from 14 monzodiorites collected from the Malaspina Pluton (Fiordland, New Zealand) with the goal of evaluating processes involved in the production of andesites in lower arc crust. We focus on relict igneous assemblages consisting of plagioclase and amphibole with lesser amounts of clinopyroxene, orthopyroxene, biotite and quartz. These relict igneous assemblages are heterogeneously preserved in the lower crust within sheeted intrusions that display hypersolidus fabrics defined by alignment of unstrained plagioclase and amphibole. Trace-element data from relict igneous amphiboles in these rocks reveal two distinct groups: one relatively enriched in high field strength element concentrations and one relatively depleted. The enriched amphibole group has Zr values in the range of ∼25–110 ppm, Nb values of ∼5–32 ppm, and Th values up to 2·4 ppm. The depleted group, in contrast, shows Zr values &amp;lt;35 ppm and Nb values &amp;lt;0·25 ppm, and Th is generally below the level of detection. Amphibole crystallization temperatures calculated from major elements range from ∼960 to 830 °C for all samples in the pluton; however, we do not observe significant differences in the range of crystallization temperatures between enriched (∼960–840 °C) and depleted groups (∼940–830 °C). Bulk-rock Sr and Nd isotopes are also remarkably homogeneous and show no apparent difference between enriched (εNdi = 0·1 to –0·1; 87Sr/86Sri = 0·70420–0·70413) and depleted groups (εNdi = 0·3 to –0·4; 87Sr/86Sri = 0·70424–0·70411). Calculated amphibole-equilibrium melt compositions using chemometric equations indicate that melts were highly fractionated (molar Mg# &amp;lt;50), andesitic to dacitic in composition, and were much more evolved than bulk lower continental crust or primitive basalts and andesites predicted to have formed from hydrous melting of mantle-wedge peridotite beneath an arc. We suggest that melts originated from a common, isotopically homogeneous source beneath the Malaspina Pluton, and differences between enriched and depleted trace-element groups reflect varying contributions from subducted sediment-derived melt and sediment-derived fluid, respectively. Our data demonstrate that andesites and dacites were the dominant melts that intruded the lower crust, and their compositions mirror middle and upper bulk-continental crust estimates. Continental crust-like geochemical signatures were acquired in the source region from interaction between hydrous mantle-wedge melts and recycled subducted sediment rather than assimilation and/or remelting of pre-existing lower continental crust.

Paleostress analysis of Miocene sheet intrusions in northern Amakusa-Shimoshima, Kyushu, Japan

Paleostress analysis of Miocene sheet intrusions in northern Amakusa-Shimoshima, Kyushu, Japan