T14B-01
Constraints on the Depth of Generation and Emplacement of Magma in the Coast Plutonic Complex, British Columbia, From Magmatic Epidote Textures
We have examined magmatic epidote-bearing plutons in the Coast Plutonic Complex to understand the depth at which they were generated and emplaced. Mt. Gamsby, located east of the Coast shear zone between Kitimat and Bella Coola, British Columbia, contains a magmatic epidote-bearing tonalite-quartz diorite pluton. This pluton had a crystallization sequence of pyroxene, amphibole, epidote, plagioclase, and biotite. The lack of garnet is consistent with trace element data, which indicate that garnet was not present in the source region and, therefore, that the magma was sourced at pressures less than ca. 1.4 GPa. Epidote inclusions in plagioclase constrain the pressure during early crystallization of the epidote to greater than 1 GPa. P-T estimates from the pluton and surrounding country rock suggest that the magma migrated to shallower levels and was emplaced at depths of ca. 0.8-1.0 GPa. Magmatic epidote also occurs in the Ecstall, Butedale, and Captain Cove plutons west of the Coast shear zone south of Prince Rupert. In the Captain Cove pluton, textures show that plagioclase crystallized before epidote, and pluton crystallization pressures are estimated to be 0.65-0.75 GPa. These data are consistent with experimental work that indicates that the crystallization of plagioclase before epidote occurs at a lower pressure than epidote before plagioclase. The magmatic epidote in the Ecstall and Butedale plutons commonly occurs as large vermicular grains intergrown with quartzofeldspathic minerals. We interpret this texture to indicate late crystallization of epidote, consistent with prior studies. Pressure estimates for the Ecstall pluton are between ca. 0.5 and 0.8 GPa and vary along strike. The pressure of emplacement of the Butedale pluton is not well constrained, but Al-in-hornblende pressures of 0.6 and 0.8 GPa have been obtained from a portion of the pluton. The pressures in the Butedale and Ecstall plutons are lower than pressures at Mt. Gamsby, which shows early crystallization of epidote. Our observations indicate that textures of magmatic epidote, in relationship to other magmatic phases and combined with quantitative P-T estimates, provide a powerful tool for understanding the path followed by magma through the crust.
T14B-02
Crystal-melt separation and the development of isotopic heterogeneities in hybrid magmas.
If a magma is a hybrid of two (or more) isotopically distinct end-members, at least one of which is partially crystalline, separation of melt and crystals after hybridization and homogenization will lead to the development of isotopic heterogeneities in the magma as long as some of the pre-existing crystalline material (antecrysts) retains any of its original isotopic composition. This holds true whether the hybridization event is magma mixing as traditionally construed, bulk assimilation, or melt assimilation. Once a magma-scale isotopic heterogeneity is formed by crystal-melt separation, it is essentially permanent, persisting regardless of subsequent crystallization, mixing, or equilibration events. The magnitude of the isotopic variability resulting from crystal-melt separation can be as large as that resulting from differential contamination/magma mixing or the presence of multiple isotopically distinct sources. In one model, a redistribution of one-third of the antecryst cargo yielded a crystal-enriched sample with 87Sr/86Sr of 0.7058, while the complementary crystal-poor sample has 87Sr/86Sr of 0.7068. In other models, crystal-rich samples are enriched in radiogenic Sr. Isotopic heterogeneities can be either continuous – controlled by the modal distribution of relict antecrysts – or discontinuous – when antecrysts are preserved as cores in isotopically zoned crystals. The first case may be exemplified by some isotopically zoned large volume rhyolites, formed by the eruptive inversion of an isotopically zoned magma chamber. In the latter case, the isotopic composition of the bulk crystal fraction will be distinct from that of any remnant or interstitial liquid. This may, for example, explain the presence of isotopically distinct late stage aplites in plutons. Crystal-melt separation provides an additional option for the interpretation of isotopically zoned/heterogeneous magmas. This option is particularly attractive for systems whose chemical variation is otherwise explicable by fractionation-dominated processes. It goes without saying that non-isotopic chemical heterogeneities can also develop in this fashion.
T14B-03
Coast Mountains Batholith: The Anatomy Of A Cordilleran Arc
The Coast Mountains Batholith is one of the largest individual segments of any continental (Andean) magmatic arcs on the planet. Here we focus on a segment located between latitudes 52 and 54 N, and on the Cretaceous and younger plutons, since some of the earlier intrusives may have originated elsewhere before docking to the North American continent as out-of-place terranes. The batholith is the sum of numerous individual plutons and volcanic equivalents that were emplaced on the continental margin during the prolonged subduction of various segments of the Pacific realm under North America between the Jurassic and Eocene. The tempo of the batholith was uneven, with several 10 m.y. long high-flux episodes separated by magmatic lulls. The western (Cretaceous) arc was emplaced successively as sheets of calc-alkaline tonalites during times of convective overturn of the mid- to lower crust. Its postulated surface equivalent would have been similar to the modern chain of andesitic volcanoes of the Andean Western Cordillera (the volcanic front). As a result of crustal thickening prior to the last and most voluminous high-flux event (Paleocene-Eocene), the arc migrated inland. This event produced mostly granodiorites and related rocks that were emplaced as sills in the mid-crust and had more irregular shapes as hypabyssal plutons. The wide spatial distribution of this event suggest that it was a result of primarily crustal melting of thick crust, similar to the Miocene to modern magmatic flare-up of the Altiplano-Puna plateau, within the orogenic core of the Andes. Extensional collapse during and immediately after the Eocene flare-up rapidly brought up the ductile mid-crust of the orogenic core, the Central Gneiss Complex, closer to the surface. A subsequent, more cryptic event, during the early Miocene (20 Ma) led to the changes in isotopic characteristics of the underlying mantle, as indicated by isotopic tracers on post batholith mafic dikes. We interpret this to reflect to loss of the arc root via foundering. Overall, we suggest the batholith is the equivalent of a collapsed equivalent to the central Andes, which now exposes the mid-crust of the former arc (average of 25 km). http://www.geo.arizona.edu/tectonics/Ducea/Batholiths/index.html
T14B-04
Along Strike Variation in Response to Tectonic Processes Within a Long Lived Accretionary Orogen, Coast Mountains, British Columbia
Jurassic to Tertiary tectonic interactions between two large superterranes of the Canadian Cordillera are preserved within the Atnarko complex. The complex is located within the Coast plutonic complex, a belt of plutonic and metamorphic rock that occurs near the western margin of the present North American margin. Pre-Early Jurassic metavolcanic and metasedimentary rocks are structurally interleaved with Late Triassic to Early Cretaceous orthogneiss and record several phases of deformation that include: poorly characterized Jurassic deformation; Early to mid-Cretaceous southwest to west directed compression; mid-Cretaceous north to northeast directed compression; mid- to Late Cretaceous dextral and sinistral ductile/brittle shearing; and post latest Cretaceous brittle faulting. Along strike of the Atnarko Complex, deformation is partitioned into compressive and translational regions with the Atnarko complex area defining the transition between the two types of crustal deformation. To the northwest of the Atnarko complex post mid-Cretaceous deformation is expressed as southwest-directed compression while to the southeast, large dextral strike-slip faults formed. It is proposed that compression in the northwest led to increased crustal thickness and partial melting of lower and middle crust in the Tertiary. In contrast the cessation of compression in the southeast lead to a more stable crustal lithosphere (i.e. cooler). A change in relative plate motions in the early Tertiary triggered full-scale orogen- perpendicular collapse in the northwest, facilitated by decoupling between the middle and lower crusts along thermally weakened layers. Localized orogen-parallel extension occurred in the southeast, which was kinematically linked to large dextral strike-slip faults where the upper crust remained coupled to the middle and lower crust.
T14B-05
Magmatic arc construction: Constraints from the structure of the Coast plutonic complex
The Coast plutonic complex of British Columbia exposes the middle crust of a vigorous magmatic arc formed in the late Cretaceous and early Cenozoic. The arc is divided into two parts by the crustal scale Coast shear zone. West of the Coast shear zone between Prince Rupert and Douglas Channel plutons were emplaced at pressures between 500 and 850 MPa, are elongate with steeply dipping contacts and have tadpole shaped terminations. The steeply dipping sides of the plutons are concordant with country rock foliations that grade into mylonitic foliations in the cores of transpressive shear zones which strike NW parallel to the arc axis. Field relationships demonstrate that pluton emplacement occurred during displacement on the shear zones, folding of adjacent country rocks and pluton enhanced anatexis of host rocks. Pressure differences in single plutons and across shear zones attests to major vertical displacements during pluton construction. The data indicate that the plutons record emplacement during major transpressive deformation and crustal thickening. The tabular shape of the plutons, their steep dips and concordance with country rock fabric show that pluton emplacement and plate scale deformation were synchronous between ~120 and 85 Ma. East of plutons that intrude the Coast shear zone a sequence of upper amphibolite to granulite facies gneiss and migmatite (central gneiss complex) occurs that host gently dipping tonalite and granodiorite sills. Mineral assemblages in country rocks of the sills track uplift and exhumation from pressures of ~800 MPa to 200 MPa during sill emplacement between ~75 and 50 Ma. The gently to moderately dipping fabrics that host the sills are the result of transposition of an earlier steeply dipping foliation. Sill emplacement occurred during both contractional and extensional deformation as indicated by a reversal in shear from reverse to normal sense observed at a regional scale. The sills are concordant with host rock foliations and mutually crosscutting relationships show that sill emplacement occurred during foliation transposition. Geometric analysis of foliations, lineations and kinematic indicators, demonstrate that sill emplacement occurred as the host rocks were flattened. Individual sills range in scale from meter to kilometers in thickness, with the largest sills traceable for tens of kilometers along strike. Host rock gneiss in many regions are dominated by orthogneiss suggesting much of the crustal column within the central gneiss complex has plutonic protoliths, particularly south of Douglas Channel. This indicates that much of the crust was constructed by repeated episodes of sill injection. Overall, the changing patterns of magmatism as tracked by changing pluton geometry and deformation style show that pluton emplacement in the Coast plutonic complex was facilitated and controlled by regional deformation patterns. Heat transported by the plutons and the presence of partially molten rock interacted with regional deformation by enhanced localization of strain during pluton construction. The change in geometry of the plutons from steep sided tabular bodies to gently dipping sills coincides with a switch from transpressional/contractional deformation to transtensional/extensional deformation. Thus, the geometry and kinematics of pluton emplacement can provide constraints on regional deformation processes in arcs.
T14B-06
Magmatism and Crustal Structure of the North Cascades Arc, NW U.S.A.
The North Cascades crystalline core (Cascades core) is part of an episodic, and at times high flux continental arc system that records a close interplay between magmatism and regional deformation. This thick (> 55 km), 96- to 45 Ma arc is the southern continuation of the Coast Batholith. The magmatic peak was at 96 to 89 Ma, when average fluxes were a minimum of 1.2x10-5 km3/yr/km of arc length. This magmatism was recorded throughout the arc, but is most voluminous in the southern part of the core, which preserves a crustal section extending from 5 to 40 km paleodepth. Tonalites dominate at all levels of the section, and plutonic rocks increase systematically from shallow to middle to deep levels; from ~ 37% to 55% to 65% of the total rock volume. The magmatic peak was coincident with major regional shortening and burial of large tracts of supracrustal rocks to depths of 8 to 11 kbar. Shortening was driven in part by underthrusting of a cool outboard terrane (Wrangellia), which resulted in a low geothermal gradient for an arc and may have led to near cessation of magmatism at 88 Ma. After a magmatic lull, plutonism was restricted to the NE half of the arc. A small peak (2.6x10-6 km3/yr/km of arc length) in magmatism occurred at ca. 78 to 71 Ma. The end of this interval broadly coincided with underthrusting of Late Cretaceous sediments to depths of 30-40 km beneath the arc, and the probable translation of deep- crustal magmatic roots. Plutonism, amphibolite-facies metamorphism, and migmatization continued from ca. 68 to 45 Ma, and spanned the transition from transpression to dextral transtension. Latest arc magmatism in the Eocene was broadly coeval with ridge subduction and rapid exhumation, including nearly isothermal decompression from 10 to < 5 kbar of the Skagit Gneiss Complex. Magmatism was also associated with sub- horizontal, arc-sub-parallel, southward-directed deep-crustal flow from the region of peak metamorphism. Eocene magmatism was extremely variable, ranging from mafic dike swarms and primitive mafic layered complexes, to voluminous leucotonalites and trondhjemitic pegmatites, to calc-alkaline granodiorites, to A-type granites. Nd isotopic values are also more variable than for older components of the arc. The Skagit Gneiss Complex was the focus of Paleogene magmatism and metamorphism, and the volume of leucocratic tonalite orthogneiss suggests that much of the crust in the NE half of the arc was reconstituted at this time. Overall, magmatism and deformation in the arc are markedly varied in space and time. Major structural events, including initial underthrusting of Wrangellia and then Late Cretaceous sediments, later deep-crustal flow associated with exhumation, and ridge subduction significantly modified the deep crust of the arc. These events are probably in part reflected by episodes of higher magmatic fluxes that coincided with periods of major deformation during different tectonic regimes and were separated by lulls during which there is little recorded deformation.
T14B-07
Evolution of a long-lived magmatic center in Idaho
The Idaho batholith/Challis magmatic system is characterized by a history of magmatism roughly 75 million years in length, marked by major shifts in the locus and composition of magma in an evolving tectonic setting, making it a key but poorly understood element in the Cordilleran batholith system of western North America. Integration of new in situ U-Pb geochronology and Hf isotopic analysis of zircon with existing data is beginning to elucidate the story told by this long-lived magmatic center. Magmatism in this region began around 125 Ma, during the terminal stages of the accretion of the Wallowa- Seven Devils terrane to North America. Small, deformed tonalite, trondhjemite, and quartz diorite plutons range in age from 125 Ma to about 100 Ma and intrude the suture zone formed by this collision. These early plutons are characterized by primitive epsilon Hf values (~ +10) indicative of a dominantly mantle source. These were followed around 90 Ma by a more voluminous pulse of tonalitic magmatism just east of the suture zone and the emplacement of a discontinuous belt of metaluminous hornblende-bearing plutons approximately 70 km to the east, all with more unradiogenic Hf and Nd (~ -5 to -10) isotopic signatures. The main pulse of peraluminous magmatism which produced the biotite granodiorite and muscovite-biotite granites of the Atlanta lobe of the Idaho batholith lasted roughly from 85 to 70 Ma. This was distinct from a later episode of magmatism that formed the Bitterroot lobe to the north, which was focused between 62 and 54 Ma. While similar to the Atlanta lobe in terms of petrography and major and trace element geochemistry, the Bitterroot lobe shows a noticeably less radiogenic Hf isotopic signature with epsilon values ranging from -15 to -21, compared to a range of -8 to -12 for the Atlanta lobe. These isotopic signatures, coupled with the ubiquitous presence of inherited Paleoproterozoic zircon cores with epsilon Hf values ranging from -30 to -50, suggest that the bulk of the batholith is largely the product crustal recycling with a relatively small mantle component. Space, time, and compositional patterns suggest that the mass contribution of primitive arc magmas never was comparable to Cordilleran batholiths, and decreased through time as the crust was progressively thickened. Only during the Eocene relaxation of the lithosphere, corresponding to the bimodal Challis magmatic flare-up and a regional southward sweep in magmatism, did mantle melts reestablish a significant direct role in the modification of the lithosphere in this sector of the Cordillera.
T14B-08
Continental-Arc Geotherms, How Much do we Understand Their Shape?
The distribution of temperature with depth in orogenic belts controls deformation, metamorphism and plutonism. Investigating the shape of geotherms in mountain belts as well as the major parameters that modify them is very important in order to understand the tectonics and geodynamics of orogens. Common observations in orogenic belts are: 1) the presence of rocks that show a wide range of pressures but similar high temperatures; 2) the appearance of rocks which crossed the andalusite-sillimanite transition, which indicate a very hot geothermal gradient in the upper middle crust; 3) the abundance of plutonic rocks and migmatites at all levels of the crust, which indicate that melting processes and pervasive migration of melt play a major roll in shaping the geotherm; 4) the predominance of granites over more mafic compositions for the plutonic bodies, indicating partial melting of the crust is widespread. In this work we modeled the shape of geotherms to explain the above observations in orogens using a 1-D advection-diffusion-melting equation. The initial condition is a conductive geotherm for the crust and asthenospheric temperatures below the Moho. Melt is produced when the temperature of the rocks reaches their solidus temperature. We use amphibolite as the dominant composition for the lower/middle crust, based on studies which show amphibolite as the likely source rock for calc-alkaline plutons. Advection is allowed only within the crust by moving melts generated by anatexis after they reach a threshold of 10 vol.%. The models do not account for changes in crustal thickness due to erosion, thickening or extension. The modeled geotherms represent a continuous thermal evolution of an orogen with constant crustal thickness and heat input from the mantle. Different models evaluate the effect of the heat input into the deep crust on the geotherms. By doing this we found that when the Moho heat flux is 120 mW/m2 or higher, the quasi-steady-state is obtained after 35 Ma. When the Moho heat flux is less than 120 mW/m2 as is common for mountain belts, the steady-state geotherm is not obtained in geologically realistic time spans. By introducing the process of melt focusing through the modeled profile, a hot geotherm that fits the field observations in orogens can be obtained. Melt focusing is the process of lateral migration of melt from adjacent regions at the base of the crust which then rises through the column. If the melt is focused by a factor of two, a quasi-steady state is predicted to occur after 25.5 Ma for a mantle heat flux of 47.5 mW/m2. The modeled melt-enhanced quasi-steady-state geotherm has very steep geothermal gradients of more than 50 ° C/km in the upper crust consistent with the generation of andalusite-sillimanite metamorphic terranes. The middle and lower crust are nearly-isothermal because the temperature is buffered by the amphibolite solidus curve. This thermal profile for the crust produces conditions that would favor orogenic instability, and may be a necessary precondition for late orogenic extensional collapse. For terranes where collapse occurs, exhumation would follow the pressure peak and be nearly-isothermal because the entire middle and lower crust is hot. Thus exceptionally fast exhumation rates are not necessary for nearly-isothermal decompression. For terranes that do not collapse, isobaric heating and cooling paths would be followed, with long lived slow cooling.