Xenoliths and their implications for the deep geology of the Midland Valley of Scotland and adjacent region

1984 ◽  
Vol 75 (2) ◽  
pp. 65-70 ◽  
Author(s):  
Brian G. J. Upton ◽  
Peder Aspen ◽  
Robert H. Hunter
Keyword(s):  
Upper Mantle ◽  
Lower Crust ◽  
Retrograde Metamorphism ◽  
Widespread Occurrence ◽  
Peridotite Xenoliths ◽  
Igneous Origin ◽  
Ultramafic Xenoliths ◽  
Lower Palaeozoic ◽  
Lower Crustal ◽  
Alkalic Basalts

ABSTRACTLate Palaeozoic alkalic basalts in and around the Midland Valley of Scotland contain a wide variety of ‘plutonic’ xenoliths. Pyroxene-rich ultramark xenoliths (wehrlites, clinopyroxenites and garnet pyroxenites) may be representative of younger components within a dominantly peridotitic upper mantle represented by ubiquitous magnesian peridotite xenoliths. Glimmerites and other biotite-rich ultramafic xenoliths are probable samples of metasomatised upper mantle facies.Xenoliths composed mainly of plagioclase, clinopyroxene ± orthopyroxene ± magnetite are widespread. These pyroxene granulites may typify the lower crustal layers. Garnet granulites are rare; such rocks may formerly have been important with loss of garnet occurring through retrograde metamorphism. Anorthositic xenoliths are relatively common. The lower crust may consist largely of anhydrous rocks, of gabbroic to anorthositic composition, ccurring as stratiform bodies of metacumulates.Other xenoliths of igneous origin include tonalitic and trondhjemitic gneisses. Although these may play some role in the lower crust, they may be more abundant in the mid-crustal domains underlying the deformed upper Precambrian and lower Palaeozoic supracrustal strata. Xenoliths of quartzofeldspathic, granulitic gneisses containing garnet ± sillimanite ± rutile are also of widespread occurrence; many of these are of metasedimentary provenance and are regarded as being derived from the mid-crustal layers beneath the Southern Highlands, Midland Valley and Southern Uplands and their Irish counterparts.

Crustal structures of southern Mongolia from seismic anisotropy and gravity analysis

2021 ◽  
Author(s):  
Alexandra Guy ◽  
Christel Tiberi ◽  
Saandar Mijiddorj
Keyword(s):  
Upper Mantle ◽  
Lower Crust ◽  
Seismic Anisotropy ◽  
Homogeneous Layer ◽  
Low Density ◽  
Gravity Modelling ◽  
Low Velocity ◽  
Seismic Stations ◽  
Lower Crustal ◽  
Density Zone

<p>This study integrates gravity modelling and analysis with seismic constraints through the prism of seismic anisotropy to characterize the structures of southern Mongolia, in particular at the lower crustal but also the upper mantle levels. Recently, gravity signal analysis and forward modelling combined with magmatic geochemistry and thermodynamic modelling demonstrate that relamination of allochtonous felsic to intermediate lower crust played a major role in southern Mongolia structure. Relamination of material induces a homogeneous layer in the lower crust, which contrasts with the highly heterogeneous upper crustal part composed of different lithotectonic domains. The seismic signals of the seven southernmost stations of the MOBAL2003 experiment were analyzed to get the receiver functions. The data treatment was performed following a new protocol, which reduces the noise on the different components. This treatment reveals the variation of the crustal thickness of cca. 10 km along the first 450 km of the profile. In addition, some seismic stations display significant signals related to the occurrence of a low velocity zone (LVZ) at lower crustal and upper mantle levels. The depth of the Moho discontinuity and the dips of the seismic interfaces obtained from the seismic inversions as well as the boundaries of the different tectonic zones constitute the starting points from the 2D forward gravity modelling along the southern part of the MOBAL 2003 profile. Moreover, the density values applied to the different blocks were determined according to the global lithological composition of the different units and the vergences of the tectonic contacts were constrained by the geodynamic studies. The gravity modelling reveals the occurrence of a low density zone in the lower crust beneath the four southernmost seismic stations, which corresponds to the LVZ observed with the receiver function analysis. The combination of the independent methods enhances the occurrence of a low velocity and a low density zone (LVLDZ) at lower crustal level beneath the southernmost part of the MOBAL 2003 seismic profile. These LVLDZ may demonstrate the existence of the relamination of a hydrous material in southern Mongolia.</p>

scholarly journals Lithospheric Xenoliths from the Marie Byrd Land Volcanic Province, West Antarctica

2021 ◽  
Author(s):  
◽  
Richard J Wysoczanski
Keyword(s):  
Upper Mantle ◽  
Lower Crust ◽  
Mineral Assemblage ◽  
Geophysical Methods ◽  
Crustal Material ◽  
West Antarctica ◽  
Rock Composition ◽  
Temperature Oxidation ◽  
Crustal Xenoliths ◽  
Lower Crustal

<p>Studies of the Earths lithosphere, and particularly the lower crust, have in the past relied on geophysical methods, and on geochemical studies of granulite terrains exposed at the surface. Geophysical studies can not evaluate the compositions to any large extent. Granulite terrains typically represent ancient rather than present day sections, have invariably suffered retrograde metamorphism, and have been affected by fluids during uplift. More recently, studies of lithospheric xenoliths (fragments of the lithosphere brought to the surface by entraining (typically alkaline) melts) have been used to study the composition of, and processes influencing, the lithosphere. Xenoliths have the advantage of representing relatively unaltered and young fragments of the lithosphere, and together with other studies have added much to our understanding of the Earths composition and processes. The study of the lithosphere in Marie Byrd Land (MBL), West Antarctica, is complicated by the difficult access and harsh climate of the region. Geophysical studies are limited, and deep crustal exposures are entirely absent. In an attempt to study the composition and structure of the MBL lithosphere, xenoliths were collected from various volcanic edifices in MBL, including the volcanoes of the Executive Committee Range (ECR), and the USAS Escarpment in central MBL, and Mount Murphy on the Walgreen coast. The xenolith suite consists of peridotites, pyroxenites and granulites, spanning a vertical section from upper mantle to lower crust, that are in pristine condition, due to the arid Antarctic conditions. The peridotite suite from MBL consists of spinel Iherzolites from Mounts Hampton and Cumming in the ECR, the USAS Escarpment, and Mount Murphy. Cr-diopside rich peridotites also occur at Mounts Hampton and Murphy, indicating a more chemically diverse upper mantle in these regions (e.g. Mg# 75-92 in Cr-diopside rich peridotites compared to Mg# 87-92 in spinel Iherzolites). REE contents of the peridotites vary from LREE-depleted (up to 0.293 (La/Yb)n in USAS Escarpment peridotites) to LREE-enriched (up to 10.015 (La/Yb)n in Mount Hampton peridotites), further indicating the extreme heterogeneity of the MBL upper mantle. Lower crustal xenoliths from Mounts Sidley and Hampton in the ECR, and from Mount Murphy have meta-igneous textures ranging from pyroxenite to gabbro. They consist of varying amounts of olivine, clinopyroxene, orthopyroxene, plagioclase and spinels; garnet is entirely absent. Orthopyroxene is absent in Mount Sidley xenoliths, whereas olivine is rare in Mount Hampton xenoliths. Mineral P-T equilibria indicate crystallisation of Mounts Sidley and Murphy pyroxenites at lower levels (7-11 kb and 6.5-12 kb respectively) than the granulites (3-5.5 kb and 3-9 kb), with Mount Hampton pyroxenites (6-7.5 kb) and granulites (5.5-8.5 kb) crystallising at similar crustal levels. High temperatures of equilibration (> 1000 [degrees] C) are consistent with a rift-like geotherm in the MBL lithosphere. Whole rock composition of the lower crustal xenoliths is controlled by the mineral assemblage, reflecting their origin as mafic cumulate rocks. Elements that partition readily into the xenolith mineral assemblage are present in higher abundances (e.g. up to 1700 ppm Sr in plagioclase rich xenoliths, and 3745 ppm Cr in clinopyroxene rich pyroxenites) than elements that do not (e.g. Rb < 6 ppm in all lower crustal xenoliths). 87Sr/86Sr (0.702861 [plus or minus] 7 to 0.704576 [plus or minus] 15) and 143Nd/144Nd (0.512771 [plus or minus] 6 to 0.512870 [plus or minus] 5) ratios indicate that the melts were primitive magmas, that did not assimilate any isotopically evolved crustal material prior to or during crystallisation. The single-pyroxene mineral assemblage of Mount Sidley (and possibly Mount Murphy) xenoliths crystallised from an alkaline melt, whereas the two-pyroxene assemblage of Mount Hampton xenoliths crystallised from a sub-alkaline melt. Xenoliths from Mount Sidley reveal petrographic and geochemical evidence for melt-fluid interaction at lower crustal depths. This interaction is inferred to be associated with late Cenozoic plume-related volcanism. It is manifested by high-temperature oxidation of olivine, replacement of clinopyroxene by kaersutite, traces of alkaline mafic glass, and the growth of apatite, Fe-Ti oxides and plagioclase. The xenolith suite has been enriched in elements that readily partition into these mineral phases (e.g. Ti, K, P, Sr, Ba), as well as in mobile elements (e.g. LILEs and LREEs). Pb isotopic ratios (e.g. 206Pb/204Pb from 18.005 - 19.589) and REEs define mixing lines between unradiogenic lower crust (206Pb/204Pb = 18.005) and small volume melts (206Pb/204Pb > 19.53) approaching HIMU composition, sourced from the inferred mantle plume. The composition of the infiltrating melts has also evolved, by percolative fractional crystallisation in the lower crust. The chemical heterogeneity detected in the MBL lower crust indicates a lower crustal discontinuity in the ECR, between Mount Sidley and Mount Hampton, here termed the ECR lower crustal discontinuity. Granulites from Mount Sidley are similar in composition to granulites from the Transantarctic Mountains (TM) in the McMurdo Sound region, Mount Ruapehu and Fiordland (New Zealand). Granulites from Mount Hampton are similar in composition to granulites from Mount Murphy, and the Ross Embayment (RE). These groups have been termed the TM Group and the RE Group respectively. The compositional similarity of granulites in each group may indicate the derivation of the lower crust in these regions from similar melts, and possibly indicate their juxtaposition as parts of the Gondwana supercontinent. The mafic cumulate character of the xenolith suite is inferred to represent original oceanic crust, and a model for the growth of the crust since its formation in latest pre-Cambrian - early Cambrian is presented here.</p>

scholarly journals Lithospheric Xenoliths from the Marie Byrd Land Volcanic Province, West Antarctica

2021 ◽  
Author(s):  
◽  
Richard J Wysoczanski
Keyword(s):  
Upper Mantle ◽  
Lower Crust ◽  
Mineral Assemblage ◽  
Geophysical Methods ◽  
Crustal Material ◽  
West Antarctica ◽  
Rock Composition ◽  
Temperature Oxidation ◽  
Crustal Xenoliths ◽  
Lower Crustal

<p>Studies of the Earths lithosphere, and particularly the lower crust, have in the past relied on geophysical methods, and on geochemical studies of granulite terrains exposed at the surface. Geophysical studies can not evaluate the compositions to any large extent. Granulite terrains typically represent ancient rather than present day sections, have invariably suffered retrograde metamorphism, and have been affected by fluids during uplift. More recently, studies of lithospheric xenoliths (fragments of the lithosphere brought to the surface by entraining (typically alkaline) melts) have been used to study the composition of, and processes influencing, the lithosphere. Xenoliths have the advantage of representing relatively unaltered and young fragments of the lithosphere, and together with other studies have added much to our understanding of the Earths composition and processes. The study of the lithosphere in Marie Byrd Land (MBL), West Antarctica, is complicated by the difficult access and harsh climate of the region. Geophysical studies are limited, and deep crustal exposures are entirely absent. In an attempt to study the composition and structure of the MBL lithosphere, xenoliths were collected from various volcanic edifices in MBL, including the volcanoes of the Executive Committee Range (ECR), and the USAS Escarpment in central MBL, and Mount Murphy on the Walgreen coast. The xenolith suite consists of peridotites, pyroxenites and granulites, spanning a vertical section from upper mantle to lower crust, that are in pristine condition, due to the arid Antarctic conditions. The peridotite suite from MBL consists of spinel Iherzolites from Mounts Hampton and Cumming in the ECR, the USAS Escarpment, and Mount Murphy. Cr-diopside rich peridotites also occur at Mounts Hampton and Murphy, indicating a more chemically diverse upper mantle in these regions (e.g. Mg# 75-92 in Cr-diopside rich peridotites compared to Mg# 87-92 in spinel Iherzolites). REE contents of the peridotites vary from LREE-depleted (up to 0.293 (La/Yb)n in USAS Escarpment peridotites) to LREE-enriched (up to 10.015 (La/Yb)n in Mount Hampton peridotites), further indicating the extreme heterogeneity of the MBL upper mantle. Lower crustal xenoliths from Mounts Sidley and Hampton in the ECR, and from Mount Murphy have meta-igneous textures ranging from pyroxenite to gabbro. They consist of varying amounts of olivine, clinopyroxene, orthopyroxene, plagioclase and spinels; garnet is entirely absent. Orthopyroxene is absent in Mount Sidley xenoliths, whereas olivine is rare in Mount Hampton xenoliths. Mineral P-T equilibria indicate crystallisation of Mounts Sidley and Murphy pyroxenites at lower levels (7-11 kb and 6.5-12 kb respectively) than the granulites (3-5.5 kb and 3-9 kb), with Mount Hampton pyroxenites (6-7.5 kb) and granulites (5.5-8.5 kb) crystallising at similar crustal levels. High temperatures of equilibration (> 1000 [degrees] C) are consistent with a rift-like geotherm in the MBL lithosphere. Whole rock composition of the lower crustal xenoliths is controlled by the mineral assemblage, reflecting their origin as mafic cumulate rocks. Elements that partition readily into the xenolith mineral assemblage are present in higher abundances (e.g. up to 1700 ppm Sr in plagioclase rich xenoliths, and 3745 ppm Cr in clinopyroxene rich pyroxenites) than elements that do not (e.g. Rb < 6 ppm in all lower crustal xenoliths). 87Sr/86Sr (0.702861 [plus or minus] 7 to 0.704576 [plus or minus] 15) and 143Nd/144Nd (0.512771 [plus or minus] 6 to 0.512870 [plus or minus] 5) ratios indicate that the melts were primitive magmas, that did not assimilate any isotopically evolved crustal material prior to or during crystallisation. The single-pyroxene mineral assemblage of Mount Sidley (and possibly Mount Murphy) xenoliths crystallised from an alkaline melt, whereas the two-pyroxene assemblage of Mount Hampton xenoliths crystallised from a sub-alkaline melt. Xenoliths from Mount Sidley reveal petrographic and geochemical evidence for melt-fluid interaction at lower crustal depths. This interaction is inferred to be associated with late Cenozoic plume-related volcanism. It is manifested by high-temperature oxidation of olivine, replacement of clinopyroxene by kaersutite, traces of alkaline mafic glass, and the growth of apatite, Fe-Ti oxides and plagioclase. The xenolith suite has been enriched in elements that readily partition into these mineral phases (e.g. Ti, K, P, Sr, Ba), as well as in mobile elements (e.g. LILEs and LREEs). Pb isotopic ratios (e.g. 206Pb/204Pb from 18.005 - 19.589) and REEs define mixing lines between unradiogenic lower crust (206Pb/204Pb = 18.005) and small volume melts (206Pb/204Pb > 19.53) approaching HIMU composition, sourced from the inferred mantle plume. The composition of the infiltrating melts has also evolved, by percolative fractional crystallisation in the lower crust. The chemical heterogeneity detected in the MBL lower crust indicates a lower crustal discontinuity in the ECR, between Mount Sidley and Mount Hampton, here termed the ECR lower crustal discontinuity. Granulites from Mount Sidley are similar in composition to granulites from the Transantarctic Mountains (TM) in the McMurdo Sound region, Mount Ruapehu and Fiordland (New Zealand). Granulites from Mount Hampton are similar in composition to granulites from Mount Murphy, and the Ross Embayment (RE). These groups have been termed the TM Group and the RE Group respectively. The compositional similarity of granulites in each group may indicate the derivation of the lower crust in these regions from similar melts, and possibly indicate their juxtaposition as parts of the Gondwana supercontinent. The mafic cumulate character of the xenolith suite is inferred to represent original oceanic crust, and a model for the growth of the crust since its formation in latest pre-Cambrian - early Cambrian is presented here.</p>

Chemical variation among French ultramafic xenoliths—evidence for a heterogeneous upper mantle

1975 ◽  
Vol 40 (310) ◽  
pp. 153-170 ◽  
Author(s):  
R. Hutchison ◽  
A. L. Chambers ◽  
D. K. Paul ◽  
P. G. Harris
Keyword(s):  
Partial Melting ◽  
Upper Mantle ◽  
Bulk Composition ◽  
Kimberlite Pipe ◽  
Chemical Variation ◽  
Peridotite Xenoliths ◽  
Ultramafic Xenoliths ◽  
Host Rocks

SummarySome 200 ultramafic xenoliths and their basaltic hosts from five French localities were studied. New analyses are presented, which show the five host-rocks to be nepheline- and olivine-normative. Seven bulk analyses of xenoliths from four localities, together with analyses of their constituent diopsides and, for six, of their orthopyroxenes, are also presented. Xenoliths from four occurrences appear to have equilibrated at pressures between about 8 to 18 kb at sub-basaltic solidus temperatures. Suites of xenoliths are chemically different. Histograms were used to determine compositions of depleted and ‘undepleted’ upper mantle. A suite of peridotite xenoliths from the Bult-fontein kimberlite pipe is no less depleted in fusible oxides than xenoliths from two French localities. ‘Undepleted’ upper mantle is very similar to ‘pyrolite’ in composition, except that the latter has much higher TiO2, Na2O, and K2O contents. No xenolith encountered in this work has a bulk composition that could yield more than 12% oceanic tholeiite on partial melting.

Structural evolution in the Moine of northwest Scotland: a Caledonian linked thrust system?

1986 ◽  
Vol 123 (1) ◽  
pp. 1-11 ◽  
Author(s):  
Robert W. H. Butler
Keyword(s):  
Cross Sections ◽  
Lower Crust ◽  
Structural Evolution ◽  
Amphibolite Facies ◽  
Metamorphic Basement ◽  
Thrust System ◽  
Lower Crustal

AbstractA model is proposed whereby the Caledonian metamorphic basement-cover complex of northwest Scotland (the Moine) is considered as a linked thrust system. This system lies between the Moine thrust at its base and the Naver–Sgurr Beag slide at its top. Ductile fold and thrust zones, which developed at mid crustal levels at metamorphic grades from greenschist to amphibolite facies, are interpreted as decoupling from a detachment presently situated at relatively shallow depths. This model is illustrated by two preliminary balanced cross-sections. These imply shortening across the northwest Scottish Caledonides in excess of 130 km and probably over 200 km. When these structures are restored onto a crustal template a considerable quantity of lower crust is found to be required at depth. The most likely location for the lower crustal wedge is beneath the Grampian Highlands.

Petrology of ultramafic xenoliths from Rayfield River, south-central British Columbia

1987 ◽  
Vol 24 (8) ◽  
pp. 1679-1687 ◽  
Author(s):  
Dante Canil ◽  
Mark Brearley ◽  
Christopher M. Scarfe
Keyword(s):  
British Columbia ◽  
Phase Equilibrium ◽  
Partial Melting ◽  
Upper Mantle ◽  
Mantle Xenoliths ◽  
Spinel Lherzolite ◽  
Equilibrium Constraints ◽  
Host Magma ◽  
South Central ◽  
Ultramafic Xenoliths

One hundred mantle xenoliths were collected from a hawaiite flow of Miocene–Pliocene age near Rayfield River, south-central British Columbia. The massive host hawaiite contains subrounded xenoliths that range in size from 1 to 10 cm and show protogranular textures. Both Cr-diopside-bearing and Al-augite-bearing xenoliths are represented. The Cr-diopside-bearing xenolith suite consists of spinel lherzolite (64%), dunite (12%), websterite (12%), harzburgite (9%), and olivine websterite (3%). Banding and veining on a centimetre scale are present in four xenoliths. Partial melting at the grain boundaries of clinopyroxene is common and may be due to natural partial melting in the upper mantle, heating by the host magma during transport, or decompression during ascent.Microprobe analyses of the constituent minerals show that most of the xenoliths are well equilibrated. Olivine is Fo89 to Fo92, orthopyroxene is En90, and Cr diopside is Wo47En48Fs5. More Fe-rich pyroxene compositions are present in some of the websterite xenoliths. The Mg/(Mg + Fe2+) and Cr/(Cr + Al + Fe3+) ratios in spinel are uniform in individual xenoliths, but they vary from xenolith to xenolith. Equilibration temperatures for the xenoliths are 860–980 °C using the Wells geothermometer. The depth of equilibration estimated for the xenoliths using geophysical and phase equilibrium constraints is 30–40 km.

Melt Percolation, Melt-Rock Reaction and Oxygen Fugacity in Supra-Subduction Zone Mantle and Lower Crust from the Leka Ophiolite Complex, Norway

2021 ◽  
Author(s):  
Brian O’Driscoll ◽  
Julien Leuthold ◽  
Davide Lenaz ◽  
Henrik Skogby ◽  
James M D Day ◽  
...  
Keyword(s):  
Single Crystal ◽  
Subduction Zone ◽  
Oxygen Fugacity ◽  
Lower Crust ◽  
Oceanic Lithosphere ◽  
Ophiolite Complex ◽  
Global Comparison ◽  
Oceanic Spreading ◽  
Melt Percolation ◽  
Lower Crustal

Abstract Samples of peridotites and pyroxenites from the mantle and lower crustal sections of the Leka Ophiolite Complex (LOC; Norway) are examined to investigate the effects of melt-rock reaction and oxygen fugacity variations in the sub-arc oceanic lithosphere. The LOC is considered to represent supra-subduction zone (SSZ) oceanic lithosphere, but also preserves evidence of pre-SSZ magmatic processes. Here we combine field and microstructural observations with mineral chemical and structural analyses of different minerals from the major lithologies of the LOC. Wehrlite and websterite bodies in both the mantle and lower crust contain clinopyroxene likely formed at a pre-SSZ stage, characterised by high Al, high Cr, low Mg crystal cores. These clinopyroxenes also exhibit low Al, low Cr, high Mg outer rims and intracrystalline dissolution surfaces, indicative of reactive melt percolation during intrusion and disruption of these lithologies by later, SSZ-related, dunite-forming magmas. Chromian-spinel compositional variations correlate with lithology; dunite-chromitite Cr-spinels are characterised by relatively uniform and high TiO2 and Al2O3, indicating formation by melt-rock reaction associated with SSZ processes. Harzburgite Cr-spinel compositions are more variable but preserve a relatively high Al2O3, low TiO2 endmember that may reflect crystallisation in a pre-SSZ oceanic spreading centre setting. An important finding of this study is that the LOC potentially preserves the petrological signature of a transition between oceanic spreading centre processes and subsequent supra-subduction zone magmatism. Single crystal Cr-spinel Fe3+/ΣFe ratios calculated on the basis of stoichiometry (from electron microprobe [EPMA] and crystal structural [X-ray diffraction; XRD] measurements) correlate variably with those calculated by point-source (single crystal) Mössbauer spectroscopy. Average sample EPMA Fe3+/ΣFe ratios overestimate or underestimate the Mössbauer-derived values for harzburgites, and always overestimate the Mössbauer Fe3+/ΣFe ratios for dunites and chromitites. The highest Fe3+/ΣFe ratios, irrespective of method of measurement, are therefore generally associated with dunites and chromitites, and yield calculated log(fO2)FMQ values of up to ~+1.8. While this lends support to the formation of the dunites and chromitites during SSZ-related melt percolation in the lower part of the LOC, it also suggests that these melts were not highly oxidised, compared to typical arc basalts (fO2FMQ of &gt;+2). This may in turn reflect the early (forearc) stage of subduction zone activity preserved by the LOC and implies that some of the arc tholeiitic and boninitic lava compositions preserved in the upper portion of the ophiolite are not genetically related to the mantle and lower crustal rocks, against which they exhibit tectonic contacts. Our new data also have implications for the use of ophiolite chromitites as recorders of mantle oxidation state through time; a global comparison suggests that the Fe3+/ΣFe signatures of ophiolite chromitites are likely to have more to do with local environmental petrogenetic conditions in sub-arc systems than large length-scale mantle chemical evolution.

Plumbing and architecture of a crustal mineralizing system in the Archean Superior Province, Canada

2021 ◽  
Author(s):  
Eric Roots ◽  
Graham Hill ◽  
Ben M. Frieman ◽  
James A. Craven ◽  
Richard S. Smith ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Spatial Association ◽  
Three Dimensional ◽  
3D Models ◽  
Superior Province ◽  
Seismic Reflection Data ◽  
Reflection Data ◽  
Abitibi Subprovince ◽  
Lower Crustal

&lt;p&gt;The role of melts and magmatic/metamorphic fluids in mineralization processes is well established. However, the role of crustal architecture in defining source and sink zones in the middle to lower crust remains enigmatic. Integration of three dimensional magnetotelluric (MT) modelling and seismic reflection data across the Archean Abitibi greenstone belt of the Superior Province, Canada, reveals a &amp;#8216;whole-of-crust&amp;#8217; mineralizing system and highlights the controls by crustal architecture on metallogenetic processes. Electrically conductive conduits in an otherwise resistive upper crust are coincident with truncations and offsets of seismic reflections that are mostly interpreted as major brittle-ductile fault zones. The spatial association between these features and low resistivity zones imaged in the 3D models suggest that these zones acted as pathways through which fluids and melts ascended toward the surface. At mid-crustal levels, these &amp;#8216;conduit&amp;#8217; zones connect to ~50 km long, north-south striking conductors, and are inferred to represent graphite and/or sulphide deposited from cooling fluids. At upper mantle to lower crustal depths, east-west trending conductive zones dominate and display shallow dips. The upper mantle features are broadly coincident with the surface traces of the major deformation zones with which a large proportion of the gold endowment is associated. We suggest that these deep conductors represent interconnected graphitic zones perhaps augmented by sulphides that are relicts from metamorphic fluid and melt emplacement associated primarily with the later stages of regional deformation. &amp;#160;Thus, from the combined MT and seismic data, we develop a crustal-scale architectural model that is consistent with existing geological and deformational models, providing constraints on the sources for and signatures of fluid and magma emplacement that resulted in widespread metallogenesis in the Abitibi Subprovince.&lt;/p&gt;

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