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By examining the minor and trace element contents of chromites from three intrusions--the Bushveld Complex (South Africa), the Stillwater Complex (USA), and the Great Dyke (Zimbabwe)--and comparing these chromite compositions with those of magmas from which they could have formed (komatiites and picrites) we conclude that (i) the variations in Ti, V, Sc, and Ga contents across stratigraphy and across individual layers do not support the model of magma mixing leading to chromite-only crystallization, (ii) the chromites from the lowest levels of the intrusions could have crystallized from komatiite liquids that were contaminated with continental crust, (iii) the Great Dyke chromites have the highest Cr# and lowest incompatible element contents and formed from a liquid closest to komatiite, (iv) all of the chromites, except those of the Dunite Succession of the Great Dyke have equilibrated with a liquid that also had crystallized pyroxene, (v) the Great Dyke and Stillwater chromites show a narrower range in composition than the Bushveld chromites, and (vi) Chromites from the western limb of the Bushveld Complex contain much higher V contents than all the other chromites. This requires either, that the oxygen fugacity (f[O.sub.2])was lower in the western Bushveld or that the chromites equilibrated with a V-rich magma. We favor a model where chromite and silicate minerals crystallized in cotectic proportions (~2:98). The chromite, silicates, and transporting liquid are emplaced into the magma chamber. During emplacement the chromite and silicate separated due to viscous particle flow to form a massive chromite layer overlain by silicates. Key words: chromite, trace elements, komatiite, picrite, Stillwater, Bushveld, Introduction The composition of chromite is of interest for both petrogenetic and economic reasons. On the petrogenetic side, chromite is one of first minerals to crystallize from primary magmas and [...]

The magmatic evolution of the Early Jurassic Turnagain Alaskan-type intrusion is inextricably linked to terrane accretionary events at the North American continental margin. We have calibrated the multistage assembly of the Turnagain ultramafic-mafic complex using high-precision air and chemical abrasion ID-TIMS U-Pb and [.sup.40]Ar/[.sup.39]Ar geochronology. Our results are the first published geochronological calibration of zoning in an Alaskan-type intrusion. Three of four intrusive phases (Stages 1-4, oldest to youngest) were successfully dated. Cooling ages obtained on Stage 2 dunite-wehrlite include statistically identical [.sup.40]Ar/[.sup.39]Ar plateau dates of 187.4 [+ or -] 1.5 (2[sigma]) Ma on hornblende and 188.6 [+ or -] 1.2 Ma on phlogopite, and a concordant [.sup.206]Pb/[.sup.238]U date of 190.3 [+ or -] 4.6 Ma on titanite. Stage 3 diorite has a [.sup.206]Pb/[.sup.238]U zircon crystallization age of 188.11 [+ or -] 0.13 Ma. Stage 4 wehrlite and leucodiorite yield [.sup.206]Pb/[.sup.238]U zircon crystallization ages of 185.68 [+ or -] 0.19 and 185.30 [+ or -] 0.12 Ma, respectively. The ca. 189 Ma phlogopite date represents a minimum crystallization age for Stage 2, indicating that assembly of the Turnagain intrusion spanned at least ~4 million years from ca. 189 Ma to 185 Ma. Regional progressive contractional deformation in the Early Jurassic is constrained by the geochronological results (>189 to 189 Ma), deformed Stage 1 wehrlite-clinopyroxenite prior to emplacement of Stages 2-4, and subsequently thrust the intrusion and its host rocks onto the ancient North American continental margin ( Key words: Alaskan-type intrusion, geochronology, Early Jurassic, terrane accretion, Yukon-Tanana terrane. L'evolution magmatique de l'intrusion de type alaskien de Turnagain, d'age jurassique precoce, est inextricablement reliee a des episodes d'accretion de terranes a la bordure du continent nord-americain. Nous avons etalonne l'assemblage en plusieurs etapes du complexe ultramafique-mafique de Turnagain en utilisant la geochronologie U-Pb de haute precision par ID-TIMS avec abrasion chimique et a l'air et la geochronologie [.sup.40]Ar/[.sup.39]Ar. Nos resultats constituent le premier etalonnage publie de la zonation geochronologique dans une intrusion de type alaskien. Des ages ont ete obtenus pour trois des quatre phases intrusives (phases 1-4, de la plus vieille a la plus jeune). Les ages de refroidissement de la dunite-wehrlite de la phase 2 comprennent des ages plateaux [.sup.40]Ar/[.sup.39]Ar statistiquement identiques de 187,4 [+ or -] 1,5 (2[sigma]) Ma sur hornblende et 188,6 [+ or -] 1,2 Ma sur phlogopite, et un age [.sup.206]Pb/[.sup.238]U concordant de 190,3 [+ or -] 4,6 Ma sur titanite. La diorite de phase 3 a un age de cristallisation [.sup.206]Pb/[.sup.238]U sur zircon de 188,11 [+ or -] 0,13 Ma. La wehrlite et la leucodiorite de la phase 4 donnent des ages de cristallisation [.sup.206]Pb/[.sup.238]U sur zircon de 185,68 [+ or -] 0,19 et 185,30 [+ or -] 0,12 Ma, respectivement. L'age de ~189 Ma sur phlogopite represente un age minimum de cristallisation pour la phase 2, qui indique que l'assemblage de l'intrusion de Turnagain s'est etale sur au moins ~4 millions d'annees, de ~189 Ma a 185 Ma. Une deformation regionale progressive par contraction au Jurassique precoce delimitee par les resultats geochronologiques (>189 Ma a 189 Ma), la deformation de la wehrlite-clinopyroxenite de phase 1 avant la mise en place des roches des phases 2 a 4 et le chevauchement subsequent de l'intrusion et des roches encaissantes sur l'ancienne bordure continentale nord-americaine ( Mots-cles : intrusion de type alaskien, geochronologie, Jurassique precoce, accretion de terranes, terrane de Yukon-Tanana., Introduction The Early Jurassic Turnagain Alaskan-type intrusion in north-central British Columbia, situated 70 km east of Dease Lake (Fig. 1 inset), belongs to a global class of ultramafic-mafic intrusions emplaced [...]

Small plutons of mafic to intermediate composition are locally associated with the abundant Devonian granitic plutons that intruded the Goldenville and Halifax groups in the eastern Meguma terrane of mainland [...]

Using Electron Probe Microanalysis, the composition of biotite was determined for the Teahan area granitoids (Pollett River Granodiorite and Rat Tail Brook Granodiorite) along with amphibole and chlorite compositions of [...]

The Cape Spencer area is located 15 km southeast of Saint John, New Brunswick. Gold mineralization has been identified mainly in illitized (illite-carbonate [+ or -] quartz [+ or -] [...]

Precise chemical abrasion--thermal ionization mass spectrometry (CA-TIMS) U-Pb zircon ages in combination with detailed field mapping, [sup.40]Ar/[sup.39]Ar thermochronology, and finite difference thermal modeling in the magmatic lobes of the Tuolumne batholith characterize these 10-60 [km.sup.2] bodies as shorter-lived, simpler magmatic systems that represent increments of batholith growth. Lobes provide shorter-term records of internal and external processes that are potentially obliterated in the main body of long-lived, composite batholiths. Zircon ages complemented by thermal modeling indicate that lobe-sized magma chambers were present between ~0.2 and 1 m.y., representing only a small fraction of the total duration of melt presence in the main body. During these shorter intervals, a concentric pattern of normal compositional zoning formed during inward crystallization and widespread zircon recycling in the lobes. Lobes largely evolved as individual magma bodies that did not interact significantly with the main, more complex magma chamber(s). Antecrystic zircons and the range of autocrysts, used to track the extent of interconnected melt, record only a limited range of ages and have contrasting zircon populations to those found in the same units in the main batholith. We consider lobes to either be single batches formed during continuous magma flow or multiple, quickly coalescing pulses that in either case formed separate magma chambers that failed to amalgamate with other compositionally distinct pulses such as those occurring in the central batholith. Zircon age comparisons between all four lobes and the main body imply that growth of the Tuolumne intrusion was not stationary, but that the locus of magmatism shifted both inward and northwestward. doi: 10.1130/B30004.1

A new theory is presented on the processes of granitic magmatism, which examines the migration of granite magma through the earth crust, pluton shapes, melting, segregation, ascent and emplacement. It is posited that these processes may have happened much faster than previously thought, taking only hundreds instead of millions of years.

A long-lasting Cenozoic record ([approximately]50 m.y.) of alkaline igneous rocks characterizes northern Victoria Land, Antarctica. Landscape analysis allows distinction between older volcanic and intrusive rocks with well-developed alpine topography sculptured by wet-based glaciers, and younger volcanic cones lacking these features. Many K-Ar and Rb-Sr dates testify that the erosion that formed the alpine landscape ceased between ca. 8.2 and 7.5 Ma. Since ca. 8 Ma, morphological evolution has been driven by cold-based glaciers; warm-based glaciers were no longer active. That this change affected a 300-km-long coastal area suggests a persistent cause of global significance. Glacier dynamics control landscape shaping as a function of ice thickness and temperature, which are driven by climatic conditions. In this view, a significant climatic change occurred in northern Victoria Land between 8.2 and 7.5 Ma. The perfectly preserved serrated alpine ridges, with their delicate spires, testify that no warm-based ice sheets overrode the region after 7.5 Ma and that polar conditions held sway in the Pliocene and Pleistocene Epochs.

Scaled experiments were carried out to study the deformation of a volcanic edifice by forcible intrusion of a cryptodome. As the magma analogue is injected vertically into a sand cone, asymmetric deformation is caused by the formation of a curved major shear fault, which dips inward from one side of the cone to the opposite edge of the intrusion. The path of the ascending silicone deviates to follow the trajectory of the fault, and a lateral bulge grows slowly from the footwall of the fault. The oblique push makes the bulge migrate outward, causing extension upslope to form an asymmetric graben in the hanging wall of the major shear fault. We suggest that the pattern of internal deformation within Mount St. Helens prior to the May 18, 1980, eruption was similar to that observed in our scaled models.

A high-resolution seismic profile over the Layered Series of the Bjerkreim-Sokndal layered intrusion provides a high-quality seismic image of one of the rock types frequently cited as a source of dense lower crustal reflectivity. The data interpretation is controlled by a detailed comparison with exposed geology directly up-plunge and has implications for reaching a better understanding of deep crustal reflectivity. Reflections are produced by boundaries between major ([greater than or equal to]100 m) lithologic units of moderately differing average composition, and small-scale modal layering (centimeters to several meters) contributes in a minor way. The seismic image provides a standard useful for identifying layered mafic intrusions within the upper and middle crust and for understanding their internal structure. The Bjerkreim data also imply that areas of highly reflective lower crust where underplating is believed to have occurred can result from internal lithologic layering resulting from fractionation and repeated influxes of magma to a closed system. The data show that highly developed modal layering can appear seismically transparent due to destructive interference.

Many granodiorite to diorite plutons in the Great Basin of western North America are surrounded by rim monoclines or anticlines that suggest relative downward movement of the plutons while wall rocks were hot and ductile. We propose that such plutons rise to a level of approximately neutral buoyancy and then founder as their densities increase [approximately equal to]10% during crystallization. Late-stage sinking of intermediate to mafic pluton should be common when wall rocks are rich in weak, low-density minerals such as quartz and calcite. Structures related to sinking will overprint those related to initial pluton emplacement and may be mistaken for regional tectonic structures.

Petrographic, chemical and isotopic analyses of related igneous rocks from the Tertiary East Greenland macrodikes were undertaken to investigate the chemical interaction between magmatic intrusions and host rocks in the dike complex. Results indicate that the composition of the rocks analyzed could not have been brought about by simple mechanical mixing. Rather, rock composition suggests that selective interdiffusion between the intrusion and the host crust took place to produce hybrid magmatic rocks.

Granitoid plutons in contrasting lithospheric blocks in western Idaho were analyzed to determine the strontium and oxygen isotopic and elemental geochemistry of the area under study. Results indicate an abrupt geochemical boundary ranging from tonalite to granite in four plutonic complexes. This is the result of the boundary between accreted terranes from oceanic arcs and the continental margin. The change in crustal characteristics, documented in the elemental composition of the intrusions, is deep indicating a vertical boundary.