T11B-0567
Spatial Distribution of Magnetic Susceptibility in the Mt. Barcroft Granodiorite, White Mountains, California: Implications for Arc Magmatic Processes
The petrographic or chemical zonation of plutons has been widely studied and used to constrain petrogenetic processes and emplacement mechanisms. The time involved in modal data collection, as well as the cost of chemical analyses, makes the search for pluton-scale zoning patterns the exception rather than the norm in magmatic arc studies. Yet, the magnetic susceptibility (Km) of plutonic rocks, both magnetite-bearing and magnetite-free, can be an invaluable tool to quickly assess the internal organization of any pluton. New field observations, new magnetic mineral data and reprocessed Km data on the Barcroft granodiorite pluton, White Mountains, California are presented. The average Km of 660 specimens from 76 stations ranges from 140 x 10-6 [SI] to 75000 x 10-6 [SI] with an average at about 16800 x 10-6 [SI]. The distribution of Km is unimodal. The hysteresis parameters of the Barcroft rocks indicate that Km is controlled mainly by multi-domain magnetite. The contribution of mafic silicates (biotite and hornblende) to Km ranges from 0.4 to 99%, with an average at about 1.8%. As in many other ferromagnetic (i.e. magnetite- bearing) plutons, Km variations reflect different amounts of magnetite which itself results from petrographic variations. This is supported by the positive correlation between major oxide variations (e.g., SiO2, FeO) and Km. A new Km map of the Barcroft pluton shows several important features including (a) a low Km zone in the SW corner of the pluton, near areas that exhibit economic mineralization possibly related to hydrothermal fluids; (b) a few isolated anomalies that may be attributed to transformation of normal magnetite into lodestone; (c) a north south high Km ridge that could possibly result from local mingling between the main granodiorite rock type and syn-plutonic mafic dikes; (d) a broad reverse Km zonation, i.e.; higher Km in the centre; and (e) possibly a positive correlation between Km and topographic elevation (between 5000 and 13000 feet), which could be explained by a higher fO2 at a higher structural level in the chamber. These preliminary results suggest that (1) syn-plutonic diking may play a significant role in the geochemical differentiation of granodiorite plutons; (2) the classic dichotomy between ilmenite-series and magnetite-series of granitoids might, at least to some extent, depend on the exposure level if such intrusions are confirmed to be vertically differentiated and, (3) mapping Km in a ferromagnetic pluton can be an efficient tool to constrain its internal organization.
T11B-0568
Shear Wave Splitting Results from the Coast Mountains Batholith, British Columbia
The main goal of the interdisciplinary Batholiths continental dynamics project is to understand how the composition, structure, and fabric of the crust and mantle evolved following the emplacement of the Coast Mountains Batholith (CMB). The major issue related to the CMB is determining whether crustal melting and distillation generated an ultramafic root as in the Sierra Nevada and if so, determining the ultimate fate of this material. A key element of this analysis involves characterizing mantle anisotropy throughout the region. We utilize a dataset of 45 broadband seismometers deployed between June 2005 and September 2006. These stations were deployed along two transects approximately 200 km apart across the Insular superterrane, Coast Shear Zone, CMB, and interior plateau. This study presents new shear wave anisotropy results calculated from teleseismic SKS phases collected across the Batholiths array. At this time we have constraints from the highest quality event, analyzed using the methods from Silver and Chan (1991). Further analysis will include all good quality SKS phases. Previously the only anisotropy measurement is a well constrained null value from permanent station at Bella Bella, BC (BBB) just west of the batholith along the southern seismic transect. Stations to the east of BBB show splits of ~1.5 seconds along an ENE-WSW fast direction. This major increase in SKS anisotropy over a small spatial interval may result from shallow mantle flow beneath the batholith or a frozen fabric within the lithosphere. Stations along the northern transect show a somewhat smaller average splitting time of 1.2 s and a clockwise rotation of the fast direction into roughly E-W. We aim to resolve whether our initial anisotropy measurements, which are generally consistent along each seismic transect but comparatively different between transects, result from a regional flow pattern within the asthenosphere or an already established fabric within the lithosphere, including the crust.
T11B-0569
Whole crustal response to Late Tertiary extension near Prince Rupert, British Columbia
The 1994 wide angle offshore-onshore seismic component of ACCRETE (funded by NSF-Continental Dynamics program) led to exceptional Vp and Vs images of the crust under the Coast Mountains of British Columbia (Morozov et al. 2003). As part of this experiment, some 1700 km of crust penetrating multichannel seismic (MCS) data were collected, and a portion of these data imaged features attributable to Late Tertiary extension in east Dixon Entrance, British Columbia. The data show grabens up to 3 km deep, arches of the Moho with relief up to 3 km (Moho depths 24.8 to 27.5 km), mid to lower crustal sub horizontal reflecting horizons, mid crustal west- dipping reflecting packages interpreted as shear zones, and a highly reflective Moho above the arches. We conclude these features formed during east-west directed extension. Based on dated extension-related features reported for the region, this extension occurred between 40 and 20 Ma and may have been concentrated between 25 and 20 Ma (Dehler et al. 1997). We estimate a minimum amount of thinning due to the extension in east Dixon Entrance to be 30% if the Early Tertiary crustal thickness had been 34 km, which was the thickness of the adjacent crust to the east during the Late Tertiary. The grabens, reflective Moho, lower crustal reflectivity, west dipping lower crustal reflective packages, and arches in Moho imply flow of the whole crust above a stronger mantle lithosphere during the extension. Our data thus confirm and extend the interpretation of crustal extension by Lowe and Dehler (1995) and Dehler et al. (1997) for Hecate Strait and Queen Charlotte Sound. During the Late Tertiary extension, pre-existing zones of structural weakness appear to have controlled the pattern of crustal thinning and of relief on Moho. The offshore-onshore and MCS data of ACCRETE may be the last such data collected by an academic group in coastal British Columbia, using airguns as a sound source, according to the notice of termination of the Batholiths marine seismic experiment (see Public Affairs abstract by Hollister for this meeting).
T11B-0570
Analysis of Local Earthquakes Recorded by the Batholiths Array in Western British Columbia: Pn Velocity and the State of Stress of the Coast Mountains Batholith
The Batholiths Array consisted of 46 broadband seismometers operating for 15 months in 2005-2006. Two 23 station lines with a 12-15 km station spacing operated through Bella Coola (southern line) and through Kitimat (northern line). During the recording period, nine earthquakes with magnitudes ranging from 5.0 to 5.4 and similar epicenters occurred on the offshore Queen Charlotte fault between the ends of the lines approximately 210 km west of the array. Analysis of Pn first arrivals shows significant variations in Pn velocity within each line. In the north Pn velocity changes from 8.0 to 8.1 to 7.9 km/s from SW to NE. In the south Pn velocity changes from 8.0 to 8.2 km/s from W to E. Changes in Pn velocity do not appear to correlate with the surface expression of terrane boundaries. Preliminary earthquake locations for land based earthquake events show a pattern of seismicity following the eastern edge of the Coast Shear Zone and crossing the southern line. The seismicity pattern ceases approximately midway between the northern and southern lines. Focal mechanisms from the largest of these earthquakes will be used to constrain a model for the state of stress in the Coast Mountains Batholith.
T11B-0571
Seismic Constraints on the Structure and Evolution of the Coast Plutonic Complex in Western British Columbia
The teleseismic component of the Batholith project consisted of 44 broadband seismometers deployed for 15 months in two line arrays in western British Columbia. The arrays traversed the Coast Shear Zone (CSZ) which defines the western extent of the Eocene granitic flare-up that formed the Coast Mountain batholith (CMB). Body wave tomography results show 5% peak to peak p-wave velocity variations and 10% s-wave velocity variations in the crust and upper mantle. The velocity variations in the crust correlate well with the different tectonic terranes. The outboard Wrangellia terrane and the inboard Stikina terrane are generally fast while the CMB is slow, consistent with velocity contrasts between mafic and intermediate composition crust. Receiver function images show significant structural differences along strike of the CSZ. On the south line, the crust thickens from 25km in the outboard Wrangellia terrane to 35km under the CMB. This crustal welt probably formed in the late cretaceous during a period of transpression in which Wrangellia underthrust the inboard terrane. In stark contrast, the north line has a relatively flat moho and the crust is 25-30km thick. We attribute this contrast to the differential exhumation history along the CSZ. Massive exhumation exposed the mid-crustal Central Gneiss Complex at the north line in the early Eocene. Initial s-wave velocity maps from diffusive surface wave inversion correlate well with the receiver function images and show a high velocity anomaly below the batholith.
T11B-0572
Compositional Variation in the Crust of the Coast Mountains Batholith From P-S Scattered Seismic Waves
The Coast Mountains Batholith (CMB) of British Columbia provides an excellent locale to study the processes whereby accreted terranes and subduction-related melts combine to form stable continental crust of intermediate to felsic composition. Genetic and spatial relationships between adjacent geologic units, however, are largely obscured by widespread plutonism and poorly understood structural contacts at the surface. Seismic measurements, combined with calculated elastic properties of various CMB rock compositions, provide a window into subsurface lithologies. By comparing the arrival times of earthquake-generated P to S scattered phases and their multiples, we calculate average Vp/Vs and crustal thickness profiles along two orogen perpendicular transects of the CMB. Along the northern transect, which crosses the CMB in the vicinity of Kitimat (54° N), we observe high Vp/Vs (1.91 ± 0.12) in the outboard Insular superterrane, a sharp decrease to 1.71 ± 0.08 in the western Yukon Tanana block, and values fluctuating around 1.8 across the central gneiss complex and western Stikine Terrane. The crust is thinnest (26 ± 3km) beneath the outboard terranes and averages ~30 km east of the Shames River detachment. The southern transect crosses the CMB near Bella Coola (52.3° N), where extension between the Coast Shear Zone and the Shames River detachment is much more limited than along the northern line and there is no surface exposure of the central gneiss complex. Here we find thin crust (27 ± 4km) and high Vp/Vs (1.86 ± 0.12) in the outboard terranes. East of the Coast Shear Zone, we image the Moho at an average depth of 32 ± 1.5 km and Vp/Vs decreases to an average value of 1.77. We combine these data with calculated phase diagrams based on known and inferred compositions at a range of crustal depths to constrain the vertical and lateral extent of major tectonostratigraphic units.
T11B-0573
Constraints on The Northern Cascadia Subduction Zone Structure From 3D Shear-wave Tomographic Velocities
A 3-D first-arrival travel-time tomographic inversion was employed to construct an S-wave minimum structure velocity model for SW British Columbia and NW Washington State to image the Juan de Fuca slab position, and to constrain the structure of the forearc crust along a 300 km north-south stretch of the Northern Cascadia subduction zone. Approximately 28,000 S-wave travel time picks recorded at 91 stations for 2,500 earthquakes were used in the inversion. The velocity model was parameterized in the forward and inverse directions with a node and cell spacing of (2 × 2 ×2) km and (4 × 4 × 2) km, respectively. Initial 1-D S-wave velocity model was constructed from the P-wave velocity model, obtained from a previous tomographic inversion of P-wave first arrival travel-times from earthquake recordings, employing a Vp/Vs ratio of 1.75. The RMS travel-time misfit for the initial and final S-wave travel-time data was 727 ms and 282 ms, respectively. The S wave velocity model along with a previously constructed P-wave velocity model were used to constrain the structure of the forearc crust, and the position of the subducting Juan de Fuca slab. In the forearc crustal section, the Eocene volcanic Crescent Terrane rocks are mapped with high shear-wave velocities in the mid crust and many earthquakes hypocenters fall with in this region. The Olympic Core rocks are imaged with low shear-wave velocity, devoid of earthquakes, and are inferred to underthrust the Crescent Terrane rocks down to approximately 35 km depth. In the Cascadia subduction zone to the west of the volcanic front, there is no P-wave reflection signature of the forearc Moho in deep seismic sections suggesting a small impedance contrast or a gradient boundary. The S-wave velocity shows a good signature of the forearc Moho as a velocity gradient at approximately 35 km depth. The junction of the forearc crust, forearc mantle and the subducting slab is clearly imaged in the S-wave velocity model along the length of the margin. A better estimate of the slab position is necessary to define the location of the locked zone along the subduction interface for better earthquake hazard estimation. The slab position inferred from this study on vertical cross-sections of S-wave velocity model is consistent with the position of the plate mapped earlier using P-wave tomographic velocities. The velocity model does not show the inversion of high shear velocity lower crust over lower velocity upper mantle inferred in a recent study. Also the plate position beneath south-western British Columbia is of much debate and the uncertainty in the plate position between various studies is of the order of ~10 km. Our interpretation of the regional S-wave and P-wave velocity models indicates that the slab is deeper than the position inferred in recent studies, consistent with earlier interpretations.
T11B-0574
Zircon U-Pb ages for the McKinley Sequence and Associated Plutons, Central Alaska Range
Plutonic rocks in the Central Alaska Range were emplaced during important tectonic events including the end of Late Cretaceous-Paleocene arc magmatism, the terminal phase of terrane accretion, ridge subduction, strike- slip faulting, oroclinal rotation of western Alaska, and resumption of arc magmatism during Eocene time along the proto-Aleutian arc system. This study builds on previous studies of these rocks by providing comprehensive age and geochemical data for plutons in the context of these tectonic events. Here we report the first set of LA- ICPMS zircon U-Pb ages for (1) the McKinley Sequence granites, (2) a set of composite plutons and (3) the Foraker pluton. Four samples dated from the McKinley Sequence include two peraluminous granites that yielded 206Pb/238U ages of 62±1 and 60±1 Ma and two A-type granites of 57±1 and 51±1 Ma. Two granodiorites from the composite plutons and a granite from the Foraker pluton gave ages of 69±1, 67±1 and 37±1 Ma, respectively. Our results indicate that the composite plutons and the McKinley granites have an older age range than previously known and that the McKinley granites can be divided into two magma suites marked with peraluminous and A-type compositions that took place in ca. 62-60 and 57-51 Ma, respectively. These plutonic rocks, excluding the A-type granites, are products of the ending phase of Late Cretaceous-Paleocene arc magmatism. Subsequent generation of the A-type granites may be attributed to ridge subduction, a mechanism that may have also accounted for broadly coeval volcanism in the nearby Cantwell basin and northern Talkeetna Mountains. The Foraker plutons are products of renewed magmatism along the proto-Aleutian arc. Our preliminary Nd and Sr isotope data suggest a temporal change in the magma's isotopic compositions, with the Foraker granites showing high eNd(T) values of about +6 and the composite and McKinley Sequence rocks showing lower and heterogeneous eNd(T) values from +3 to -3. More analyses will further constrain mantle reservoirs beneath central Alaska and the influence of terrane accretion on continental margin magmatism.
T11B-0575
The Pootlass High Strain Zone, Bella Coola BC: Timing, Kinematics and Significance
The Pootlass High Strain Zone (PHSZ) is a corridor of brittle and ductile deformation, at least 2 km wide and up to 30 km long, located in the Coast Belt just northeast of the boundary between the Intermontane and Insular superterranes, which is demarked by the Coast Shear Zone at this latitude (Rusmore et al., 2001). In this paper, we report recent observations from field mapping, and new geochronological and petrological data, from which we place the PHSZ into a regional tectonic framework. The PHSZ is comprised of volcanic and sedimentary rocks of the lower to mid-Jurassic Hazelton Formation, and Jurassic to Eocene plutonic rocks varying in composition from granodiorite to diorite and tonalite. The field area has undergone at least three phases of deformation; a folding event that resulted in tight, southwest-verging folds correlated with the regional Late Cretaceous Coast Belt thrust and fold system, intense ductile, sinistral shear, and brittle dextral shear. The PHSZ is defined by a steep foliation, with a very well developed subhorizontal stretching lineation, and southwest-verging folds with the fold axes parallel to the stretching lineation. Many of the intrusive rocks occur as syn-kinematic mafic and felsic sheeted intrusions that are tightly folded and display sinistral kinematics. L-tectonites within the deformed plutonic rocks attest to the weakness of the rocks during deformation and support syn-kinematic magmatism. Zones of highest strain are found within the mafic and felsic sheeted intrusions. Predominantly sinistral ductile shear in the PHSZ is overprinted by brittle, dextral faults. Along strike to the northwest, the high strain zones displays both dextral and sinistral ductile shear components. Metasedimentary rocks in the PHSZ reached a maximum of amphibolite facies metamorphism as indicated by the abundance of hornblende. Greenschist facies metamorphism is dominant however, with rocks composed mainly of quartz, chlorite, muscovite and garnet. Ar/Ar, analyses of hornblende and biotite from a folded and sinistrally sheared mafic sheeted intrusion within the PHSZ provide cooling ages of 62.39 ± 0.40 Ma and 63.04± 0.49 Ma, respectively. U/Pb analysis of zircons from syn-kinematic, folded and sinistrally sheared felsic sills within the PHSZ returned crystallization ages of 113.8 ± 2.0 Ma and 160.9 ± 4.4 Ma. Additional petrological, microstructural and geochronological studies are in progress. The PHSZ may be kinematically related to similar transpressive structures in the western Canadian Cordillera; namely the Grenville, Kitkatla and Principe-Laredo sinistral shear zones located to the northwest, that exhibit steep to vertical gneissic foliation in plutonic rock and intense isoclinal folding in supracrustal rocks of lower greenschist to amphibolite metamorphic grade, with estimated temperatures of deformation between 300 and 500°C (Chardon et al., 1999). These shear zones are interpreted to have been active from 110 to 87 Ma, during the intrusion of voluminous batholiths (Chardon et al., 1999).Chardon et al., 1999; Tectonics, Volume 18, No. 2, p. 278-292. Rusmore, M.E. et al., 2001; GSA Bulletin, Volume 113, No. 8, p. 961-975.
T11B-0576
Thermal Properties of Granite from Korea
Thermal and physical properties were measured on 206 Jurassic granite samples obtained from three boreholes in the central part of Korea. Thermal conductivity (λ), thermal diffusivity (α), and specific heat (Cp) were measured in a laboratory; the average values are λ=2.813 W/mK (1.962- 3.867 W/mK), α=1.296 mm2/sec (0.93-1.787 mm2/sec), Cp=0.816 J/gK (0.750-0.877 J/gK), respectively. In addition, porosity (φ), and dry and saturated density(ρ) were measured; the average values are φ =0.01 (0.002-0.041), ρ (dry)=2.662 g/cm3 (2.540-2.760 g/cm3), and ρ (saturated)=2.67 g/cm3 (2.547-2.771 g/cm3), respectively. Experimental data show that thermal conductivity increases with increasing thermal diffusivity and decreases with increasing porosity. However, thermal conductivity does not show good correlation with density. Thermal diffusivities of 10 granite samples were measured with increasing temperature from 25° C to 200° C. In this study, we found that thermal diffusivity at 200° C is about 30% lower than thermal diffusivity at 25° C. Using mixing law models, thermal conductivity of a rock sample can be estimated from its mineral contents. The estimated average thermal conductivity values using a geometric and a square-root mean models are 3.685 W/mK and 4.030 W/mK with average relative errors of 22.7% and 33.9%, respectively. Granite samples mainly consist of quartz (~38 Vol%, 7.69 W/mK) and albite (~30 Vol%, 2.14 W/mK). Two main mineral compositions (quartz and albite) strongly influence thermal conductivity of granite. A linear equation including two main components was derived from multiple linear regression (λ=-0.014·VQuartz- 0.047·VAlbite+5.0, where V is Vol%). Specific gravities were measured on 10 granite samples in the laboratory. The measured specific gravity ranges from 2.603 to 2.669. The specific gravity depends on chemical compositions of granite. Therefore, specific gravity can be estimated by the felsic-mafic index (F) that is calculated from chemical contents. The estimated specific gravity ranges from 2.643 to 2.658. The average relative error between measured and estimated specific gravities is 0.677%.
T11B-0577
Heat Flow and Heat Production in Korea
The mean heat flow in Korea estimated in 365 locations (mines, coal fields, groundwater, hot spring wells) is 60 ± 11 mW/m2; mean geothermal gradients is 25.1 °C/km. High heat flow values appear in the southeastern part, the central western part, and the northeastern part of Korea. In the tectonic provinces, heat flow is 66 mW/m2 for Gyeonggi Massif, 65 mW/m2 for Okcheon Fold Belt, 60 mW/m2 for Yeongnam Massif, 72 mW/m2 for Gyeongsang Basin, and 75 mW/m2 for Yeonil Basin. In the aspect of the lithology, heat flow is 71 ± 2 mW/m2 for the sedimentary rock area, 68 ± 2 mW/m2 for the sedimentary/volcanic rock area, 67 ± 1 mW/m2 for the plutonic rock area, and 62 ± 2 mW/m2 for the metamorphic rock area. In the geological time sequence, heat flow is 78 ± 5 mW/m2 for the Cenozoic strata, 68 ± 1 mW/m2 for Mesozoic, 65 ± 3 mW/m2 for the Paleozoic strata, 67 ± 8 mW/m2 for the Proterozoic strata, and 62 ± 2 mW/m2 for the Archean strata. From the relationship between heat flow and Moho depth, we found that the shallow Moho depth area is likely to have higher heat flow values than the area of the thick crust. Heat production was measured by chemical analysis and gamma-ray logs on basement rocks (86 granite and 37 gneiss samples). The mean heat production is 2.15 μW/m3 for granite, and 2.22 μW/m3 for gneiss. The mean heat production is 2.52 μW/m3 for Gyeonggi Massif, 2.16 μW/m3 for Okcheon Fold Belt, 2.35 μW/m3 for Yeongnam Massif, and 2.01 μW/m3 for Gyeongsang Basin. The tectonic provinces with high mean heat flow tend to have low mean heat production. Therefore, heat production is unlikely to have a first-order importance in determining surface heat flow distribution in Korea. The analysis of 12 heat production and heat flow data sets from granite area in Gyeongsang Basin shows a linear relationship between heat production and surface heat flow, which is known as q=qr+AD, where q is surface heat flow, qr is reduced heat flow, A is heat production, and D is characteristic depth. We found q=47.06+12.29A for Gyeongsang Basin.
T11B-0578
New Geochronologic, Stratigraphic and Structural Data Constraining the Cretaceous Evolution of Southwestern Mexico
The Mesozoic volcano-sedimentary successions of southwestern Mexico have been often interpreted to represent the record of a single or multi-arc system (Guerrero Terrane), built on oceanic crust far away from the continent, which was subsequently accreted to the continental margin of the North American plate in Late Cretaceous time after a prolonged period of W-directed subduction. An alternative model proposes that the Guerrero island arc was built partly on oceanic crust and partly on continental crust and later collided with the continent (Tardy et al., 1994). However, no solid data exist to support either a true accretionary process or the collision of two continental masses in southern Mexico during Laramide deformation. Furthermore, the occurrence of Paleozoic and Precambrian terrigenous sediments at various stratigraphic levels of the Cretaceous sedimentary succession of the southern Guerrero Terrane indicates that it formed relatively close to a continent. This is also supported by the Grenvillian inherited component recognized in zircons of the ~120 Ma Placeres del Oro pluton, which resulted from partial assimilation of either a lower crustal continental basement or of upper crustal detrital zircons recycled from Grenvillian basement. Models considering the accretion of intraoceanic insular arcs to nuclear Mexico during Late Cretaceous time cannot explain such continental influence and need to be revisited. We suggest that the Cretaceous volcano-sedimentary successions were deposited directly on the thinned continental margin of the North American plate, and were deformed in Late Cretaceous time to form a thrust-and- fold belt. The apparent litho-stratigrahic incompatibilities of the Mesozoic successions can be completely explained in terms of spatial and temporal variations of facies on a heterogeneous crust with a migrating continental arc, and its back-arc and fore-arc basins. The arc was related to eastward subduction of the Farallon oceanic plate. In this way, the Laramide deformation of southern Mexico is definitively unrelated to accretion of allochtonous terranes, and the intervention of another driving force is required to explain the Late Cretaceous- Early Tertiary shortening of the southern margin of the North American plate.
T11B-0579
Petrology of sapphirine-bearing gedrite-cordierite gneiss, Okanogan dome, Washington USA, and implications for gneiss dome tectonics
The northern Cordilleran migmatite domes (Thor-Odin, Valhalla, Okanogan) contain Mg-Al-rich (Si-poor) orthoamphibole-cordierite gneiss as layers and lenses surrounded by quartzofeldspathic migmatite. The Mg-Al- rich rocks contain assemblages and reaction textures that provide information about metamorphic conditions during the tectonic evolution of the migmatite dome. In the Okanogan dome, gedrite + anorthite + cordierite + spinel + sapphirine +/- kyanite +/- hornblende assemblages of the Tunk Creek Amphibolite indicate T > 700- 800 degrees C. This Okanogan unit structurally overlies a migmatite domain dominated by diatexite. In contrast to gedrite-cordierite gneiss in other northern Cordilleran domes, the Okanogan rocks occur in a discontinuous kilometer-scale unit rather than small pods; are more calcic; and lack garnet. In addition, kyanite did not transform to sillimanite, and spinel occurs most commonly as a blocky matrix phase rather than as vermicules in symplectite. The differences in textures of gedrite-cordierite gneiss in the Cordilleran domes may in part be related to differences in bulk composition, but are also likely related to differences in tectonic evolution. The Okanogan dome is located at a greater distance from the Rocky Mountain foreland-hinterland boundary, a major crustal boundary that localized and therefore maximized vertical flow (decompression) in domes adjacent to it. The Okanogan dome, ~200 km west of the boundary, may have experienced less isothermal decompression during dome emplacement compared to domes located closer to the boundary, and therefore contains relict kyanite and only minor corona/symplectite development.
T11B-0580
Geochemical Characteristics of Plutons Emplaced in a Contracting Arc and Evidence for the Development of Gravitationally Unstable Residual Arc Rocks, Coast Mountains, B.C.
New geochemical, Nd, and Sr data from plutonic rocks of the Coast Mountains Batholith show that these rocks were derived from both primitive and somewhat compositionally evolved sources. Preliminary results indicate that arc plutons ranging in age from 9 to 188 Ma contain varying amounts of slightly evolved or older primitive terrane material with initial εNd ranging from +1 to +7 and initial 87Sr/86Sr ratios mainly in the range of 0.703 - 0.705. A minimum in initial εNd ranging from +1 to +4 at 85 - 95 Ma is interpreted to reflect crustal thickening and increased lower crustal assimilation into magmas as terranes were progressively thrust eastward. Similarly at 50 - 55 Ma very thick hot crust during a magmatic flare up event promoted the assimilation of lower crustal material. Maximum εNd of +5 to +7 from 68 - 82 Ma occurs due to decreased lower crustal assimilation into magmas during a period when magmatism occurred in a more restricted zone. Initial 87Sr/86Sr ratios corroborate Nd data and also indicate a cyclic pattern of assimilating slightly more compositionally evolved materials. The observed cycle of initial εNd and 87Sr/86Sr in these rocks reflects fundamental changes in the arc thermal/petrologic processes through time. It is known that to generate large quantities of intermediate composition crust requires the production of a complimentary ultramafic residue. Geochemical characteristics, specifically La/Yb ratios which steadily increase from 180 - 50 Ma, of these rocks indicate thickening crust and varying amounts of garnet as a residual phase in what would be a dense granulitic - eclogitic residual root susceptible to gravitational instabilities. From preliminary geochemical data it is apparent that the incorporation of slightly evolved or possibly older primitive terrane materials may be an important process during magmatic flare-ups, promoting the formation of garnet rich residues. The fate of these dense residues has important implications for our understanding of fundamental tectonic processes which occur in continental arcs.
T11B-0581
Evidence for rapid recycling of subduction erosion forearc material into Cordilleran TTG batholiths: insight from the Peninsular Ranges of southern and Baja California
Studies at oceanic convergent margins show unambiguously that large volumes of continental material are recycled into the mantle by sediment subduction and forearc subduction erosion. The fate of this subducted continental material is poorly known; mass balance consideration suggest that at least 95 percent of this material is recycled to deep mantle circulation. The Late Cretaceous La Posta-type TTG suite of the eastern Peninsular Ranges batholith however displays evidence that accretionary prism sediments were subducted and magmatically recycled into the Late Cretaceous the on a very short (<10 Ma) timescale. The La Posta suite represents a tremendous flare-up of deep (garnet-involved) partial melting preceded by an episode of crustal shortening and presumed crustal thickening from ca. 98-92 Ma. Evidence from the Catalina Schist subduction complex terrane directly outboard of the PRB demonstrates that these rocks were being underthrust and metamorphosed synchronously with La Posta magma generation. Elevated δ18O values from La Posta plutons indicate substantial contributions of isotopically unevolved (initial Sr < 0.706) intermediate to mafic composition supracrustal materials into the melt souce regions consistent with contributions from the Catalina Schist accretionary complex. Further, inherited zircon in the La Posta rocks are dominated by a 120-100 Ma component that closely matches the age of the western PRB as well as the dominant detrital zircon component in Catalina Schist. These results are consistent with "conveyor belt-type" recycling of western PRB material into the forearc and then downward via underthrusting into the magma source region of La Posta melts.
T11B-0582
Crustal Structure And Magmatism, Coast Mountains Orogen, Latitude 52-53 degrees North, British Columbia, Canada
New geologic data and U-Pb ages reveal complex history of arc accretion, crustal thickening and migration of magmatic fronts during deformation. Plutonic ages define distinct western and eastern Jurassic – mid Cretaceous arcs that share a common history after ~90 Ma. Juxtaposition of these arcs occurred during mid- Cretaceous crustal shortening in a dominantly SW-vergent crustal-scale thrust belt. Significant crustal thickening buried 151 Ma granitic clasts to pressures > 6 kb, and mid-Cretaceous plutons were emplaced at this depth along the axis of the orogen. Thrusting continued after establishment of the 90 Ma arc; a regional SW-verging thrust emplaced high-grade metamorphic rocks of the Yukon-Tanana terrane and deep-seated plutons over low- grade rocks of the Alexander and Wrangellia terranes. The shear zone is coincident with the western boundary of 82-89 Ma plutons and a regionally extensive, late-kinematic, sill-like pluton. Dextral shear zones preserved on the flanks of the orogen suggest a component of Late Cretaceous transpression. By 75 Ma, metamorphism, deformation, and magmatism had migrated central portions of the orogen and there is no evidence of ductile deformation and syn-kinematic metarmorphism younger than ~70 – 65 along the western flank of the orogen. The Coast shear zone localized 62-58 Ma synkinematic plutons during NE-side up displacement, creating a sharp western magmatic front. Sparse cooling ages suggest plutons and metamorphic rocks adjacent to the CSZ cooled through 500-600 deg between 54-58 Ma during exhumation along the shear zone. Voluminous granitic plutons were emplaced from ~55-50 Ma, but significant crustal extension that affected the eastern side of the orogen farther north is not evident along this transect. This history supports previous models of crustal subcretion and the generation of arc magmas in thickened crust. Definition of two pre-90 Ma arcs negates models calling for simple Andean-style orogen prior to mid-Cretaceous times. Late Cretaceous crustal shortening involved dextral transpression. The pattern of Eocene magmatism and apparent lack of significant coeval crustal extension suggest that widespread Eocene magmatism was related to gradual eastward migration of the magmatic arc, rather than extension-related processes.