Conceptual models in exploration geochemistry: The Canadian Cordillera and Canadian Shield

This volume summarizes the exploration geochemical conditions in the secondary environment, in the Canadian Cordillera and the Canadian Shield. This is achieved by a number of conceptual models which describe the principles and mechanisms of formation of anomalies, which govern the use of exploration geochemistry. These models have been constructed by drawing together information already existing in the literature plus 38 individual case histories contained in this volume.The formation of anomalies is described for: (1) residual overburden, (2) overburden of local origin (e.g. till), and (3) transported overburden of remote origin (e.g. stratified glacial drift and alluvium). Within each of these categories the effect of element mobility, seepage zones, bogs, variation in overburden thickness, rock type change and soil type change are also described.An attempt has been made, not only to summarize both these conditions where geochemistry can be used as a reliable exploration tool, but also to identify areas where the use of geochemistry is unreliable.A summary is also given of the length of anomalous dispersion and contrast in both soil and sediments for all the case histories quoted, both in this volume and in the literature. This summary is divided according to the type of deposit, i.e. porphyry copper, massive sulphide, etc., and provides a guide for planning the optimum sampling interval.     

Pyrite deformation in stratiform lead-zinc deposits of the Canadian Cordillera

Pyrite textures in five stratiform lead-zinc deposits from lower to upper greenschist facies environment of the Canadian Cordillera are described and discussed in terms of deposition/early diagenesis, deformation, metamorphism and hydrothermal alteration processes. Overgrowth is an important process during both diagenesis and deformation. Diagenetic and deformational overgrowths can be distinguished. Diffusive mass transfer, involving pressure solution and oriented overgrowth of pyrite is the main deformation mechanism in pyrite deposits at low metamorphic grades. Although diffusive mass transfer favours fine-grained mineral aggregates, its effect on coarse pyrite grains has also been identified. Ore minerals dissolved by pressure solution may be transported, with the assistance of pore fluids within fractures and grain boundaries, over distances significantly greater than the scale of individual grains to give a range of pressure solution/overgrowth textures. The textural modification of pyritic ores from the early stages of diagnesis, through metamorphism and deformation, to post deformation thermal annealing, has important implications for the distribution of trace elements and isotopic compositions in pyritic ores.     

The molasse facies of the Columbian orogen,Canadian Cordillera

The authors present a tentative model for the relationship between deformation, uplift and molasse sedimentation in the Columbian Orogen of the Canadian Cordillera. Data are mainly drawn from the north-central part of the Columbian Orogen. Two distinct deformational episodes are recognized. The first, probably of mid-Jurassic age, was accompanied by regional metamorphism and resulted in southwesterly directed folds and related faults. This episode of crustal shortening appears to have been the precursor of uplift in the western Columbian Orogen (Omineca Crystalline Belt), causing the progradation of a primogenic non-marine molasse over marine shale and flysch. The second deformation, of probable late Cretaceous to Eocene age, caused overprinting of earlier structures by steep faults in the west and led to the growth of the fold-and-thrust belt of the Rocky Mountains. This episode created a morphogenic drainage system with molasse deposition in intramontane fault basins and along both flanks of the Columbian Orogen. Modal analyses from fluviatile sandstones within the primogenic and morphogenic molasse illustrate the progressive stripping of sedimentary and volcanic units along both sides of the orogenic core zone. Post-orogenic drainage from Miocene to Recent was strongly modified by the development of high-gradient drainage towards the Pacific Ocean.     

Structural evolution of the Southeast Canadian Cordillera: A new hypothesis

The Shuswap metamorphic core complex of the southeast Canadian Cordillera, which is allochthonous with respect to a fixed reference frame beneath the Plains of the North American craton, evolved and became consolidated prior to major development of thrust faulting in the Rocky Mountains.Polyphase deformation of the infrastructure of the complex beneath a weakly compressed suprastructure is probably due to shear strains generated by underthrusting of marginal lithosphere rather than to gravitational upwelling and lateral spreading.Pre-metamorphic to early metamorphic strain of the complex may be related to eastward underthrusting from the west. Late metamorphic strain may be related to westward underthrusting of the North American craton from the east. Continued underthrusting of the craton led to uplift and northeastward translation of the consolidated metamorphic complex together with development of thrust faults in the Rocky Mountains.     

Orogeny,crustal hydrogeology and the generation of epigenetic ore deposits in the Canadian Cordillera

Summary The Mesozoic-Cenozoic history of the Canadian Cordillera in British Columbia was composed of five major stages: preaccretion island arc development in the Early Jurassic, micro-plate vs. continent collision in the Middle Jurassic, uplift and extension in the Late Jurassic to Early Cretaceous, renewed collisional tectonics in the Middle Cretaceous, and extension and strike-slip faulting in the Late Cretaceous to Eocene. All of these intervals except the Late Jurassic to Early Cretaceous were associated with extensive plutonism and volcanism. An analysis of the age distribution of epigenetic ores (porphyry Cu ± Mo, Au; mesothermal Au; epithermal Au ± Ag; Ag-Pb-Zn veins), however, indicates that only the Early Jurassic, the Late Jurassic to Early Cretaceous, and the Late Cretaceous to Eocene were characterized by widespread development of epigenetic ores. Especially notable is the absence of significant epigenetic mineralization from the Middle Jurassic and Middle Cretaceous periods, even though magmatism and metamorphism were widespread during these time periods.Studies of synbrogenic regional fluid flow in the Canadian Cordillera show that differences in regional hydrogeology between the compressional-thrust and the extensional intervals are probably the key factor in determining which intervals were extensively mineralized. In compressional-thrust regimes characteristic of the Middle Jurassic and Middle Cretaceous, vertical permeability is disrupted by low permeability thrust zones, which inhibits the influx of surface fluids or venting of subsurface fluids and consequently, limits the ability of the system to concentrate metals. In extensional regimes, vertical fracturing provides regions of high permeability, which provide for the rapid influx and circulation of surface fluids and venting of devolatilizational fluids and consequently, the widespread formation of epigenetic ores.

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Gebirgsbildung, Hydrogeologie der Kruste, und Entstehung von epigenetischen Erzlagerstätten in der kanadischen Cordillera

Zusammenfassung Die mesozoisch-känozoische Geschichte der kanadischen Cordillere in Britisch Kolumbien umfaßt fünf Haupt-Stadien: Entwicklung eines Inselbogens im Unter-Jura, Mikroplatten/Kontinent-Kollision im Mittel-Jura, Hebung and Extension im Oberjura bis Unterkreide, Kollisionstektonik in der Mittelkreide und schließlich Extension und Strike-Slip-Faulting von der Oberkreide bis ins Eozän. Alle diese Phasen, mit Ausnahme der im späten Jura und in der frühen Kreide, waren mit ausgedehnter plutonischer und vulkanischer Aktivität verbunden. Eine Analyse der Altersverteilung epigenetischer Erzlagerstätten (Porphyry Cu + Mo, mesothermales Au, epithermales Au + Ag; AgPb-Zn-Gänge), weist darauf hin, daß nur im Unterjura, und vom Oberjura bis in die Unterkreide, und von der Oberkreide bis ins Eozän die Entstehung epigenetischer Erzlagerstätten zu beobachten war. Besonders bemerkenswert ist das Fehlen signifikanter epigenetischer Mineralisation im Mitteljura und in der Mittelkreide, obwohl Magmatismus und Metamorphose während dieser Zeitabschnitte zu beobachten waren.Untersuchungen von synorogenem, regionalem Fluidverhalten in der kanadischen Kordillere zeigen, daß Unterschiede in der regionalen Hydrogeologie zwischen kompressionalen Überschiebungsphasen und extensionalen Perioden ein wichtiger Faktor in der Entwicklung der Mineralisation sind. In Kompressions/Überschiebungsregimen, die für den Mitteljura und die Mittelkreide charakteristisch sind, war vertikale Permeabilität durch Überschiebungsbahnen von niedriger Permeabilität unterbrochen. Dies wiederum verhinderte den Zufluß von Oberflächen-Fluiden oder das Aufsteigen von Fluiden aus der Tiefe und, in der Folge, die Fähigkeit des Systems, Metalle zu konzentrieren. In Extensions-Bereichen bieten vertikale Bruchstrukturen Bereiche hoher Durchlässigkeit an, die wiederum rasche Zufuhr und Zirkulation von Oberflächenfluiden, das Aufsteigen von Fluiden, sowie die Bildung von Lagerstätten erleichtern.

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This paper was presented at the IGCP 291 Project Symposium Metamorphic Fluids and Mineral Deposits, ETH Zürich, March 21–23, 1991.     

Geochronology of carbonatites from the Canadian and Baltic Shields, and the Canadian Cordillera: clues to mantle evolution

Carbonatites are known to range in age from the Archean through to the Recent, with an increasing abundance towards more recent times. Of the known 500 or so carbonatites, however, few have been precisely dated. In this paper, some new, precise U/Pb, Th/Pb as well as Pb/Pb dates from several carbonatite complexes from Europe and North America are presented and used to establish important reference points in defining mantle perturbations. Events at 2617, 1964, 583 and 382 Ma are now firmly established for the Fennoscandian Shield, while several distinct events of 2680 Ma, and between 1897–1881 and 1164–1083 Ma are documented for the Canadian Shield. Other, less well-defined, events in Canada include magmatism at 1770 Ma from Argor, 570 Ma from Manitou Islands, and an event at 360 Ma from the Canadian Cordillera. One carbonatite from the Ukrainian Shield (Chernigovka) has been precisely dated at 2074 Ma. Events from the Fennoscandian Shield also include carbonatitic magmatism at 1792 Ma from Halpanen, and between 386 and 377 Ma from Kola. Most of these events from quite distinct Archean cratons can be correlated with LIPs and associated mafic magmatism, such as flood basalts and regional dyke swarms, generally considered to be the result of plume magmatism and associated continental fragmentation. Few mafic events are associated with the post-orogenic shoshonitic magmatism at ca. 1800 Ma for the Fennoscandian Shield and the Devonian continental margin extension in the Canadian Cordillera.     

Petrogenesis of Peraluminous Granites, Monashee Mountains, Southeastern Canadian Cordillera

Two-mica granites that locally contain garnet and sillimaniteoccur as dikes, sills, and sheets up to 50 m thick within thesillimanite zonc of the Monashee Mountains in the southeasternCanadian Cordillera of British Columbia. Syn-kinematic and post-kinematicgranites are recognized. U-Pb dating of zircon demonstrates that the syn-kinematic granitesare 100.4?0.3 Ma old, based on duplicate concordant single abradedzircon analyses. Other zircons have slightly older Pb/Pb dates,indicating small amounts of inherited zircons. Monazites are99?10 Ma old. Post-kinematic granites have 62.5?0.2 Ma zirconages and 634+0.1 Ma monazite ages. High initial 87 ratios (0.71492–0.74181)and evidence of Precambrian Pb inheritance indicates that bothsyn- and post-kinematic granites were derived from a crustalsource. Geobarometric estimates suggest that both generationsof granites equilibrated at 6–8 kb (22–30 km). Zirconand monazite saturation temperatures range from 660–824?Cand indicate that these minerals were liquidus phases earlyin the crystallization history of the granites. Because monazitesaturation temperatures generally exceed those of zircon, itis possible that some monazites may be inherited. Apatite saturationtemperatures in excess of 900?C suggest that both generationsof granites contain source inherited apatite. Syn- and post-kinematic granites have essentially identicalmajor and trace element chemistries. Syn-kinematic graniteshave steep light rare earth element (LREE) enriched patternswith pronounced negative Eu anomalies. The REE patterns of post-kinematicgranites range from steep LREE enriched patterns with negativeNd and Eu anomalies to flat patterns with low LREE contents,negative Nd anomalies, and both positive and negative Eu anomalies.Modelling of REE, Rb, Sr, and Ba contents demonstrates thatsyn-kinematic gramtes could have been generated by a low degreeof partial melting (with 10–25% feldspar fractionationof the melt) of Late Proterozoic Horsethief Creek Group metapelitesleaving a monazite-bearing upper amphibolite facies residue.Post-kinematic granites were produced by partial melting ofa geochemically and isotopically similar metapelitic source.The suite of post-kinematic granites can be related by a smallamount (up to 0.1%) of monazite crystal fractionation.     

The Signature of Amphibole in Mafic Alkaline Lavas, a Study in the Northern Canadian Cordillera

Despite evidence for its involvement, the importance of amphibolein controlling the compositions of mafic alkaline magmas remainsunder-appreciated. Relatively small variations in large ionlithophile elements (LILE) with respect to other incompatibleelements, such as light rare-earth elements (LREE) or Th, requirethat amphibole was an important residual phase during the productionof Late Tertiary to Recent olivine nephelinite (Ol-NEPH) magmasbeneath the northern Canadian Cordillera. The erupted maficmagma types define a continuous array from Ol-NEPH to hypersthene-normativeolivine basalt (Hy-NORM AOB). The overall compositional arrayhas a sense of curvature which is counter to binary mixing,but can be modeled by two distinct linear melting trends: onefrom Ol-NEPH to basanite (BASAN) compositions, during whichamphibole controlled the composition of the melt, and the ratiosof LILE/LREE change significantly, but the ratios of high fieldstrength elements (HFSE) remain relatively constant; the otherfrom BASAN to Hy-NORM AOB corresponding to the melting of alherzolite assemblage, following the exhaustion of amphibole,across which the ratios of LILE/LREE remain relatively constant,but the ratios of HFSE change significantly. Other intraplate alkaline suites, such as those of the HawaiianIslands, show similar evidence for the involvement of residualamphibole in the genesis of Ol-NEPH to BASAN magmas. The meltingof any amphibole-bearing mantle assemblage is likely to be atwo-step process, regardless of whether the amphibole is segregatedas veins or distributed interstitially. In a water-undersaturatedenvironment, the first stage of melting is controlled by thebreakdown of amphibole, which produces silica-saturated liquidsbelow 12 kbar and silica-undersaturated liquids at greater depths,with little contribution from other mineral phases. In the secondstage, following the exhaustion of amphibole, the major elementcompositions of subsequent melts change rapidly to equilibratewith a lherzolite mineralogy, but the incompatible trace-elementcharacteristics of the former amphibole persist. KEY WORDS: amphibole; mafic alkaline magmas; northern Canadian Cordillera; trace elements ^*Corresponding author     

Xenolith evidence for lithospheric melting above anomalously hot mantle under the northern Canadian Cordillera

A comparison of mantle xenolith suites along the northern Canadian Cordillera reveals that the xenoliths from three suites exhibit bimodal populations whereas the xenoliths from the other four suites display unimodal populations. The bimodal suites contain both fertile lherzolite and refractory harzburgite, while the unimodal suites are dominated by fertile lherzolite xenoliths. The location of the three bimodal xenolith suites correlates with a newly discovered P-wave slowness anomaly in the upper mantle that is 200 km in width and extends to depths of 400–500 km (Frederiksen AW, Bostock MG, Van Decar JC, Cassidy J, submitted to Tectonophysics). This correlation suggests that the bimodal xenolith suites may either contain fragments of the anomalously hot asthenospheric mantle or that the lithospheric upper mantle has been affected by the anomalously hot mantle. The lherzolite xenoliths in the bimodal suites display similar major element compositions and trace element patterns to the lherzolite xenoliths in the unimodal suites, suggesting that the lherzolites represent the regional lithospheric upper mantle. In contrast, the harzburgite xenoliths are highly depleted in terms of major element composition, but their clinopyroxenes [Cpx] have much higher incompatible trace element contents than those in the lherzolite xenoliths. The major element and mildly incompatible trace element systematics of the harzburgite and lherzolite xenoliths indicate that they could be related by a partial melting process. The lack of textural and geochemical evidence for the former existence of garnet argues against the harzburgite xenoliths representing actual fragments of the deeper anomalous asthenospheric mantle. Furthermore, the calculated P-wave velocity difference between harzburgite and lherzolite end-members is only 0.8%, with the harzburgites having higher P-wave velocities. Therefore the 3% P-wave velocity difference detected teleseismically cannot be produced by the compositional difference between the lherzolite and harzburgite xenoliths. If temperature is responsible for the observed 3% P-wave velocity perturbation, the anomalous mantle is likely to be at least 200 °C higher than the surrounding mantle. Taken together these data indicate that the refractory harzburgite xenoliths represent the residue of 20–25% partial melting of a lherzolite lithospheric mantle. The incompatible trace element enrichment of the harzburgites suggests that this melting was accompanied by the ingress of fluids. The association of the bimodal xenolith suites with the mantle anomaly detected teleseismically suggests that anomalously hot asthenospheric mantle provided both the heat and volatiles responsible for the localized melting and enrichment of the lithospheric mantle. Received: 16 May 1997 / Accepted: 25 October 1997     

Crustal Melting in the Cordilleran Interior: The Mid-Cretaceous White Creek Batholith in the Southern Canadian Cordillera

The mid-Cretaceous White Creek batholith in southeast BritishColumbia is a zoned pluton ranging from quartz monzodioriteon the margin, to hornblende-and biotite-bearing granodioritetowards the interior of the batholith, which are in turn crosscutby two-mica granite. This range in rock type is similar to therange displayed by Mesozoic granitoid suites found in the Cordilleraninterior of western North America. The lithological zones inthe White Creek batholith correlate with distinet jumps in majorelement, trace element, and isotopic compositions, and indicatethat several pulses of magma were emplaced within the WhiteCreek magma chamber. The hornblende-and biotite-bearing granitoidsare metaluminous to weakly peraluminous, have strong light rareearth element (LREE) enrichment, and small negative Eu anomalies.These granitoids have initial _Sr ranging from +32 to +84 (⁸⁷Sr/⁸⁶SrTfrom 0.7069 to 0.7106), initial _Nd ranging from –5 to–10, and initial ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pbranging from 18.3 to 18.7, 15.58 to 15.65, and 38.3 to 39.0,respectively. The two-mica granites and associated aplites arestrongly peraluminous, and show only moderate LREE enrichmentand strong negative Eu anomalies. These granites have _Sr rangingfrom +174 to + 436 (⁸⁷Sr/⁸⁶Sr_T from 0.7169 to 0.7354), _Nd rangingfrom –12 to –16, and more radiogenic initial Pbisotope ratios than the hornblende-and biotite-bearing granitoids. Oxygen, Sr, Pb, and Nd isotopes, REE modelling, and phase equilibriumconstraints are consistent with crustal anatexis of Precambrianbasement gneisses and Proterozoic metapelites exposed in southeastBritish Columbia, the product being the hornblende-biotite granitoidsand two-mica granites, respectively. The sequence of intrusionin the White Creek batholith constrains the melting sequence.A zone of anatexis proceeded upwards through the crust, firstmelting basement gneisses then melting overlying metapelites.A model for basaltic magmatic underplating as a primary causeof anatexis of the crust during the mid-Cretaceous magmaticepisode is difficult to reconcile with the absence of earlyCretaceous basalt in the southern Canadian Cordillera. A muchmore likely petrogenetic model is that crustal anatexis wasprobably a response to crustal thickening in association withterrane accretion and collision along the western margin ofthe North American continent.     

加拿大岩石圈探测计划(LITHOPROBE)

加拿大岩石圈探测计划(LITHOPROBE), 是加拿大政府为了全面了解北美大陆演化过程而设立的一个国家级地球科学研究合作项目, 主要科学目标和组织机构都是围绕对10个断面或者研究区进行研究的基础上而建立, LITHOPROBE被认为是国际上目前为止最成功的国家地学项目之一, 运用了地球物理(地震多道反射、地震折射和电磁等)、地球化学、地质年代学等各种地质方法。该计划在深部基础地质研究、矿产资源勘探、地质灾害预警、人才培养、深部探测仪器研制、重大科技专项的管理等方面均取得了很好的成果。LITHOPROBE也为世界其他国家的类似科研项目树立了模范。本文通过对LITHOPROBE项目立项过程、取得的成果、产生的社会效益和存在的问题等进行分析和研究, 希望对我国的深部探测起到借鉴作用。     

The environment in and around ice-dammed lakes in the moderately high relief setting of the southern Canadian Cordillera

During decay of the Cordilleran Ice Sheet, ˜13 000–10000 cal. yr BP, numerous ice-dammed, ribbon-shaped lakes developed within the moderately deep valleys of the Interior Plateau of British Columbia. We describe the pattern and characteristics of lake sediments within the Thompson Valley, propose a palaeoenvironmental model for glacial lakes Thompson and Deadman and explore their implications for the palaeogeography of Cordilleran Ice Sheet decay. Seventeen glaciolacustrine lithofacies are identified within deltas, subaqueous fans and lake-bottom beds. Sediments accumulated at high rates and by a diversity of sediment dispersal and depositional processes: hyperpycnal and surge-type turbidity currents, grain flows and debris flows. Megascale subaqueous failures (tens of metres thick) were facilitated by high sedimentation rates. The palaeoenvironmental model highlights: (i) high rates of basin infilling; (ii) the dominant role of tributary rivers, rather than valley-occupying ice, in delivering water and sediment to lakes; and (iii) the role of melt cycles, jökulhlaups and hyperpycnal flows in sediment delivery. These conditions, in combination with a lack of organics and a fining upward sequence in lake sediments, suggest that glacial lakes Thompson and Deadman were coeval with dwindling plateau ice.     

U–Pb isotopic constraints on diachronous metamorphism in the northern Monashee complex, southern Canadian Cordillera

U–Pb isotopic data from the northern Monashee complex, one of the deepest structural exposures in the southern Canadian Cordillera, indicate that the age of metamorphism varies according to structural position in a 6 km thick section. This metamorphism resulted in an unusual sequence in which rocks with the lowest-grade mineral assemblage (kyanite–sillimanite–staurolite–muscovite) are underlain and overlain by higher-grade rocks. Xenotime and monazite U–Pb dates vary progressively from 64 Ma in the structurally highest rocks to 49 Ma in the deepest rocks. Discordant U–Pb ages from Proterozoic and Cretaceous monazite and titanite are used to interpret the thermal significance of the early Tertiary dates. The discordant analyses define linear arrays with lower intercepts that broadly overlap with early Tertiary, and the amount of discordance varies with structural level; it is least in the deeper rocks and greatest in higher rocks. Electron microprobe work showed that the monazite discordance in the deeper rocks resulted from Tertiary mineral overgrowth and recrystallization rather than Pb diffusion. We use previous studies of Pb diffusion and the fact that Proterozoic monazite and titanite suffered only negligible to moderate amounts of diffusive Pb loss to contend that elevated temperatures (c. 600–650 °C are inferred from pelitic mineral assemblages) existed in the deeper rocks for a short duration, perhaps a few million years. The downwards younging 64–49 Ma U–Pb dates are interpreted as closely reflecting xenotime and monazite growth ages rather than cooling ages or substantially reset ages based on the lack of Pb diffusion in monazite and the previously obtained ⁴⁰Ar/³⁹Ar data which suggest that rapid cooling occurred immediately after the U–Pb dates. In addition, growth ages are interpreted as thermal peak ages based on U–Pb dates from coeval kyanite-bearing leucosomes, the consistent nature of the U–Pb dates throughout the study area, and petrographic relationships which suggest that monazite grew before or during development of the syn-metamorphic foliation. These interpretations lead us to conclude that metamorphism was diachronous according to structural level, with higher rocks attaining peak temperatures and cooling rapidly while deeper rocks were heating towards a thermal peak that was attained a few million years later. This thermal scenario requires that higher rocks cannot have been the heat source for the deeper metamorphism, as was previously proposed.     

The Lithospheric Mantle beneath Continental Margins: Melting and Melt-Rock Reaction in Canadian Cordillera Xenoliths

Seven alkali basalt centers in the southern Canadian Cordilleracontain mantle xenolith suites that comprise spinel Cr-diopsideperidotites, spinel augite-bearing wehrlites and orthopyroxene-poorlherzolites, and minor pyroxenites. The Cr-diopside peridotitesappear to be residues of the extraction of Mg-rich basalts byup to 15% partial melting (median 5–10%) of a pyrolite-likesource in the spinel stability field. The xenoliths are similarto other mantle xenolith suites derived from beneath convergentcontinental margins, but are less depleted, less oxidized, andhave lower spinel mg-number than peridotites found in fore-arcsettings. Their dominant high field strength element depletedcharacter, however, is typical of arc lavas, and may suggestthat fluids or melts circulating through the Canadian Cordilleralithosphere were subduction related. Modeling using MELTS isconsistent with the augite-bearing xenoliths being formed byinteraction between crystallizing alkaline melts and peridotite.Assimilation–fractional crystallization modeling suggeststhat the trace element patterns of liquids in equilibrium withthe augite xenoliths may represent the initial melts that reactedwith the peridotite. Moreover, the compositions of these meltsare similar to those of some glasses observed in the mantlexenoliths. Melt–rock interaction may thus be a viablemechanism for the formation of Si- and alkali-rich glass inperidotites. KEY WORDS: Canadian Cordillera; mantle xenolith; peridotite; wehrlite; melt–rock reaction     

Partial melting and decompression of the Thor-Odin dome, Shuswap metamorphic core complex, Canadian Cordillera

The Thor-Odin dome region of the Shuswap metamorphic core complex, British Columbia, contains migmatitic rocks exhumed from the deep mid-crust of the Cordilleran orogen. Extensive partial melting occurred during decompression of the structurally deepest rocks, and this decompression path is particularly well recorded by mafic boudins of silica-undersaturated, aluminous rocks. These mafic boudins contain the high-temperature assemblages gedrite+cordierite+spinel+corundum+kyanite/sillimanite±sapphirine±högbomite and gedrite+cordierite+spinel+corundum+kyanite/sillimanite+garnet±staurolite (relict)±anorthite. The boudins are interlayered with migmatitic metapelitic gneiss and orthogneiss in this region.

The mineral assemblages and reaction textures in these rocks record decompression from the kyanite zone (P>8–10 kbar) to the sillimanite–cordierite zone (P<5 kbar) at T750 °C, with maximum recorded temperatures of 800 °C. Evidence for high-temperature decompression includes the partial replacement of garnet by cordierite, the partial to complete replacement of kyanite by corundum+cordierite+spinel (hercynite)±sapphirine±högbomite symplectite, and the replacement of some kyanite grains by sillimanite. Kyanite partially replaced by sillimanite, and sillimanite with coronas of cordierite±spinel are also observed in the associated metapelitic rocks.

Partial melt from the surrounding migmatitic gneisses has invaded the mafic boudins. Cordierite reaction rims occur where minerals in the boudins interacted with leucocratic melt. When combined with existing structural and geochronologic data from migmatites and leucogranites in the region, these petrologic constraints suggest that high-temperature decompression was coeval with partial melting in the Thor-Odin dome. These data are used to evaluate the relationship between partial melting of the mid-crust and localized exhumation of deep, hot rocks by extensional and diapiric processes.     

Deformation at the eastern margin of the Northern Canadian Cordillera: Potentially related to opening of the North Atlantic

The Northern Canadian Cordillera (NCC) comprises the Mackenzie Mountains, which are characterized by earthquakes occurring ~1,000 km east of the western North American margin. However, no recognized convergence has occurred in this inboard region since the Mesozoic to early Cenozoic formation of the Cordillera. This lack of an obvious driver for the modern NCC deformation has generated considerable debate and various geodynamic models. We show here thermal histories derived from (U‐Th‐Sm)/He data that are interpreted to indicate reactivated deformation and formation of the eastern deformation front beginning ~30 Ma. At that time the western margin of North America was mainly a transform boundary, which typically transmits very limited amount of stress into the continent. Along the northeastern margin of North America, however, the North Atlantic was opening and may have caused horizontal forces that drove deformation far to the west where the rigid craton encountered the weak Cordillera.     

Diachronous deformation and a strain gradient beneath the Selkirk allochthon,northern Monashee complex,southeastern Canadian Cordillera

Structural relationships of granitoid rocks dated by the U–Pb method indicate that deformation was diachronous and a strain gradient exists in a 6-km-thick section beneath the Selkirk allochthon, in the northern Monashee complex, one of the deepest structural exposures in the southern Canadian Cordillera. At high structural levels, immediately beneath a crustal-scale thrust zone that transported the allochthon eastward, a metasedimentary-dominated cover sequence was strongly affected by kilometre-scale east-verging isoclinal folds (F₁) and outcrop-scale folds (F₂) that are associated with the dominant foliation and lineation. The F₂ folding occurred, at least in part, after 58 Ma and ceased by 55 Ma. In deeper levels of the cover sequence and the underlying orthogneiss-dominated basement, F₂ folding occurred, at least in part, after 52 Ma and ceased by 49 Ma. Proterozoic dykes in the basement were locally weakly affected by D₂. These new findings require that: (i) D₂ compression youngs structurally downward, synchronous with the thermal peak of metamorphism; (ii) D₂ in deeper levels is synchronous with extension above the complex that was partly responsible for its exhumation; and (iii) a D₂ strain gradient lies between strongly deformed cover rocks and weakly D₂-deformed basement rocks. We propose a model in which rocks that were tectonised at different places and times within the orogen were juxtaposed, likely during east-verging kilometre-scale F₁ folding and shearing along the isocline limbs (a similar model was previously proposed to explain a pattern of downward younging thermal peak ages and an inverted metamorphic sequence in higher rocks). The rapid downward decrease in deformation intensity suggests that the lower limit of significant Cordilleran strain lies in the exposed basement. Cessation of deformation at this level is attributed to the fact that the basement attained elevated temperatures and began straining when the Cordilleran tectonic regime changed from compressional to extensional.     

TiO2 activity in metamorphosed pelitic and basic rocks: principles and applications to metamorphism in southeastern Canadian Cordillera

The activity of TiO₂ can be precisely defined as a function of pressure, temperature and activities of other components for common mineral assemblages in metapelites (ilmenite-quartz-garnet-plagioclase-Al₂SiO₅) and in metabasites (plagioclase-sphene-ilmenite-quartzgarnet). These mineral assemblages can be modelled by the equilibria: 1) 3ilmenite+Al₂SiO₅+2quartz=almandine+3TiO₂ 2) anorthite + 2sphene = grossular + 2TiO₂ + quartz 3) 3anorthite+3quartz+6ilmenite = grossular+ 6TiO₂+2almandine. These mineral assemblages can be used at (rutile saturation) and a given T to get maximum pressure limits of some metapelites and metabasites. When electron microprobe analyses of mineral grains adjacent to Ti-bearing phases are made, these data give maximum pressure estimates in reasonable agreement with other geobarometers. The activity of TiO₂ in many metapelites is very near rutile saturation, but for metabasites the activity of TiO₂ in some sillimanite zone rocks is as low as 0.6. The solubility of TiO₂ in biotite, hornblende and garnet is a complex function of T, P, the activities of components in coexisting minerals and crystal chemical constraints in these minerals. At a given P and T the solubility of TiO₂ in biotite and hornblende does not appear to be strongly dependent upon for sphene and ilmenite versus rutile-bearing assemblages.     

Northeast-trending folds in the western Skeena Fold Belt,northern Canadian Cordillera: a record of Early Cretaceous sinistral plate convergence

The western portion of the Skeena Fold Belt, northern Canadian Cordillera, contains northeast-trending folds that are highly oblique to northwest-trending folds in the eastern portion of the fold belt, and to most Mesozoic contractional structures in the northern Cordillera. The northeast-trending folds locally interfere with the northwest-trending folds, and one region includes transected folds. Geometric relationships within and between the two fold sets are not easily reconciled by notions of the northeast-trending folds resulting from vertical axis rotation of blocks, influence of basement features, or lateral variations in magnitude of shortening. The northeast-trending folds are inferred to result from sinistral plate convergence early in the history of the fold belt (Early Cretaceous).Northeast-trending folds in the Skeena Fold Belt are the most conspicuous elements of a seldom-studied group of similarly oriented contractional structures, which collectively define a belt at least 1700 km long, within and bordering the Coast Belt. The extent of Early Cretaceous structures potentially related to sinistral convergence supports them having originated in response to the relative plate motion rather than local controls (e.g. indentors). This agrees with relative plate motion studies based on ocean floor reconstructions, which suggest a mid-Cretaceous change from sinistral to dextral convergence.     

Paleosol sequences in the Eastern Cordillera of Colombia

This paper presents paleosol chronosequences from the Eastern Cordillera of Colombia, covering the last 35,000 yr. The changes in the morphology of the soils which developed in successive layers of volcanic ash reflect climatic changes and are correlated to the palynological studies of Van Der Hammen. Middle Pleniglacial soils are regarded as gleyed (Polylepis) forest soils formed under low evapotranspiration, and soils of the Middle/Upper Pleniglacial transition as humiferous (grassland) Paramo soils. An intervening arid erosion phase was followed by a humid and warmer period (Holocene) during which humiferous soils developed partly under forest, partly under grassland, but throughout under conditions of higher evapotranspiration.