V33A-2193 INVITED
The fate of aging: Insight into arc maturation processes from the Kohistan Island arc
The temporal evolution of island arcs from juvenile (dominantly tholeitic) to mature (dominantly calc-alkaline) stages is generally considered important to understand the formation of the continental crust. Even though arc maturation is attributed to sub-arc mantle processes the details of arc maturation are essentially unknown. Insights into arc maturation processes can only be gained from high-resolution numerical models integrating high-quality geochemical data that trace the long-lasting thermo-mechanical and petrological evolution of arcs. However, the high-quality data required from the geochemical side to evaluate the plausibility of geodynamical models is mostly lacking. The main reason for this is the scarcity of appropriate sample sets. Present day active arcs generally allow only direct surface observation documenting a time span seldom exceeding few millions of years, a snapshot of arc history considering the potentially long-lived activity of arcs. To overcome these drawbacks we carried out an isotopic study within the Kohistan paleo- island arc, an accreted Mesozoic-Tertiary intraoceanic arc complex exposed in NW Pakistan. We present the results of combined high-precision U-Pb ages and Hf isotopic data from zircons and Pb-Sr-Nd whole rock isotopic data on a variety of widely distributed intrusive units of this paleo-arc. The data set documents ~120 Ma history of the Kohistan arc. The isotopic composition of all the studied mafic to granitic rocks can be explained by a mixture of a depleted upper mantle component and enriched sources. Detailed study of a single intrusion reveals that intra-crustal assimilation, a principal source for the enriched components, has only negligible influence on the isotopic signature. Therefore we conclude that the isotopic composition is derived from the melting region. In agreement with many previous studies we assume that the enriched compositions are derived from the subducting slab. The temporal trends show a surprisingly uniform secular trend indicating increasing contribution of enriched components in the Kohistan arc magmas with time (e.g. decrease in åNd and åHf and an increase in 87/86Sr and 208,207,206/204Pb). Older samples are generally depleted whereas younger ones yield enriched compositions. The trends indicate a long-term metasomatic enrichment of the Kohistan sub-arc mantle source by a slab-derived component. Our results imply that the sub arc-mantle did not reach a chemical steady state even after 120 Ma of magmatic activity. The long- lasting, continuous enrichment of a slab component indicates that the influx from the subducting slab exceeds the degree of depletion of the sub-arc mantle due to partial melting and the influx of fresh "unmetasomatized" material into the wedge due to convection.
V33A-2194
Studies of the Southern Izu-Bonin-Mariana (IBM) Forearc using Shinkai 6500: Watery Glimpses of an In Situ Forearc Ophiolite
Two expeditions with research submersible Shinkai 6500 and R/V Yokosuka (YK06-12 and YK08-08-2) studied the lithospheric structure of the Mariana forearc south of Guam. Igneous crustal and mantle rocks are well exposed along the inner trench wall because of the great depth of the trench, low sediment flux, and recent shearing and extension along N-S and E-W faults. A total of 12 dives studied crust between 6500 and 2000 mbsl along ~500km of the forearc. West Santa Rosa Bank Fault (WSRBF), a major N-S fault at ~144°10'E above a tear in the subducted slab, marks an important lithospheric boundary, with very thin crust to the west and thicker crust to the east. 3 of 4 dives west of WSRBF recovered peridotite and a fourth (#1096) sampled a scarp between 6100 and 5400m depth exposing multiple flows of fresh basalt. This may mark a previously unknown, N-S oriented forearc rift (W. Santa Rosa Terrane, WSRT). The zone of thin crust and shallow peridotite continues west as far as ~143°07'E, as demonstrated by the fact that 3 out of 4 Shinkai dives and 15 out of 18 total bottom samplings in this region recovered peridodite; thicker crust lies west of this. The concentration of shallow (<25km deep) seismicity between 143- 144°E further indicates a broad zone of crustal extension in the SE part of the Mariana Trough encompassing the WSRT. In contrast, peridotite was not recovered from 8 dives east of WSRBF and only recovered in 3 out of 19 total samplings; these peridotites may sample incipient serpentinite mud volcanoes forming along the disrupted outer forearc. Diabase was recovered from 3 out of 8 Shinkai dives east of the WSRBF and 4 out of 19 total samplings, indicating that dikes or sills are exposed at depths accessible to Shinkai, consistent with what is likely to be exposed for Mariana inner forearc crustal thicknesses of ~20-25 km. Gabbro and pyroxenite is reasonably common among samples from west of WSRBF (0 of 4 Shinkai dives; 7 of 26 total samples) and scarcer east of WSRBF (0 of 8 dives; 3 0f 19 samples), consistent with inferences that only upper to mid-crust is sampled in this region. The units sampled by Shinkai and other studies in this region demonstrate that the southern IBM forearc represents an in situ ophiolite, ready to be obducted when buoyant lithophere is subducted beneath it.
V33A-2195 INVITED
Vertical Stratification of Composition, Density, and Inferred Magmatic Processes in Exposed Arc Crustal Sections
Comparison of exposed arc crustal sections from four ancient magmatic arcs reveals a pattern of depth- specific processes that may be typical of arcs worldwide. These processes include (1) fractionation of mafic and ultramafic cumulates from a mafic parental magma in the uppermost mantle and lowermost crust, (2) subsolidus transformation of mafic plutonic rocks into dense garnet-bearing assemblages in the region of the Moho in thick arc sections (>30 km depth) by isobaric cooling and/or partial melting, (3) dehydration melting of amphibolitized basalt and gabbro (including pre-existing oceanic basement) in the deep to mid crust to produce felsic tonalite (15-25 km depth), (4) mingling/mixing/ homogenization of these felsic crustal melts with mafic mantle-derived magmas in the mid crust (15-25 km depth), (5) increasing homogenization of magmas in mid to upper crustal levels (<20 km depth) to produce intermediate composition magmas. Individual arcs will vary given their unique tectonic settings and petrogenesis. However, these processes may be a general framework that can be used as an overlay for interpretation of geophysical observations in active arcs and for petrogenetic studies of arc volcanic rocks. The four sections used in the study (correlated to observed processes as listed in the first paragraph) include the Jurassic Talkeetna arc exposed in Alaska (processes 1,2,4,5), the Cretaceous Kohistan arc of northern Pakistan (processes 1,2,3,4), the Jurassic Bonanza arc exposed on the west coast of Vancouver Island (processes 3,4,5), and Cretaceous arc basement of the North Cascades crystalline core (southern Coast Plutonic Complex) (processes 3,4,5). Comparable levels of each of these arcs are strikingly similar, both physically and geochemically. The broader issues of density and geochemical stratification of arcs, as well as their bulk composition and accretion, are critical to understanding the growth and compositional evolution of continental crust. Geochemical evidence for delamination is apparent in the accreted Talkeetna arc, where the paleo-Moho is an abrupt transition between gabbroic rocks (at the garnet-in isograd) and ultramafic rocks. Delamination such as this can result in a more felsic residuum comparable to continental crust
V33A-2196
"Anisotropy Architecture" and its Importance for Crustal Strength and Seismic Imaging in Tilted arc Crustal Sections
It is well known that rock anisotropies play an important role in controlling rock behavior and when attempting to use geophysical techniques to image the subsurface architecture of the lithosphere. To date most laboratory studies of rock anisotropies focus on the hand sample to mineral scale whereas geophysical imaging focuses on anisotropies at the 0.5 km or larger scale, the two scales being linked by assumptions of uniformity of anisotropies at the latter scale. These assumptions of uniformity of anisotropies at the regional scale are also common in models of lithospheric behavior during orogenesis. However an examination of tilted crustal sections, such as those presented in this session, clearly establish that all crustal levels display heterogeneous anisotropies and rock strengths at all spatial scales. Thus it is not the presence or absence of anisotropies that controls imaging and rheological behavior but the geometrical arrangements or "architecture" of these anisotropies. To illustrate and begin to quantify this issue various anisotropies encountered in crustal sections (layering, mineral fabrics in metamorphic host rock, pluton shapes, mineral fabrics in plutons, regional structures commonly incorporating the above) can be represented as ellipsoids with various shapes, sizes, and orientations and then combined at various scales using directed or Voight tensor averaging. If a km3 of crust is considered, a number of likely scenarios exist. If strong anisotropies exist in small domains (ellipsoids with large axial ratios) but have highly variable orientations at the km3 scale (ellipsoids have variable orientations) such as in the case of intense metamorphic fabrics but with variable orientations due to folding, then the overall km-scale anisotropy (after directed averaging) remains weak. Alternatively, anisotropies may have similar orientations but vary dramatically in intensity and spatial dimensions (ellipsoids with different ratios and volumes), such as the case of strongly foliated metamorphic rocks intruded by weakly to non-foliated plutonic rocks, then the overall anisotropy also remains weak and heterogeneous. The most dramatic km-scale anisotropies, and thus strongest reflectors and potentially mechanically weakest zones, likely occur when all elements (layering, mineral fabrics, plutons, structures) are anisotropic and aligned. Natural examples of the architecture of anisotropies will be examined from tilted arc crustal sections in the Cascades core, Washington, Sierra Nevada and Joshua Tree areas, California and the Gobi Tienshan Intrusive Complex, Mongolia. However, in general a consideration of the architecture of anisotropies in natural systems indicates that km scale anisotropies will generally be more isotropic in comparison to those in small domains implying that the larger rock volumes may be stronger and appear more isotropic when geophysically imaged.
V33A-2197 INVITED
Tectonic setting of magmatism in an arc and retroarc crustal section in southern California
The Transverse Ranges expose a tilted crustal section through the Cordilleran Mesozoic continental arc, from the shallow retroarc to arc paleodepths >30 km (with blocks reconstructed using models for San Andreas and Eastern California shear zone evolution). Arc magmas transited a two-sided compressional orogen with a mid-crustal high strain zone (possible decollement) that separates the upper crust from coeval, structurally deeper plutons. At high crustal levels, comparatively homogeneous granodioritic and granitic plutons are largely discordant to gneissic fabrics in Proterozoic wallrocks, with which they experienced little compositional interaction. At mid-crustal depths (18-22 km) wallrock is conspicuously less abundant and plutonic rocks are subhorizontally sheeted, heterogeneous at meter to decimeter scale, and retain both magmatic and locally penetrative subsolidus fabrics. The sheeted mid-crust appears to give way downward (24-30 km) to comparatively more homogeneous and less deformed, generally more mafic plutonic rocks with greater preserved wallrock, although this transition is cut out by the San Andreas fault in areas mapped to date. Mantle-derived mass and heat input and intra-crustal melting are apparent in deeper-level quartz diorite and the presence of mafic enclaves and granites at all crustal levels. If the mid-crustal high strain zone is viewed as a migmatite complex (Matti et al., 1994), upper crustal plutons may have formed above a compacting, partially molten middle crustal channel, as proposed for hotter convergent orogens. Such an origin may reflect construction of the arc in thicker, more radiogenic felsic crust compared to the contemporaneous, yet apparently more vertically integrated Sierra Nevada and Peninsular Ranges batholiths. Alternatively, if the mid-crustal high strain zone is primarily a little-deformed intrusive complex, it may represent an inflated mid-crustal zone of emplacement of mafic and granitic magmas exiting the lower crust and trapped at a mid-crustal transition in rock properties and/or environmental conditions. Reheating and magma mixing in this intrusive zone may have formed more buoyant melts able to ascend and construct upper crustal granodiorite plutons.
V33A-2198
Temporal evolution and paleodepth variations in arc granitic rocks from the Transverse Ranges tilted crustal section, southern California
The Transverse Ranges tilted crustal section exposes an Andean-style plutonic arc comprised of overlapping Triassic through Late Cretaceous arc segments. Alkali-calcic to calc-alkalic arc plutons emplaced from hypabyssal to mid-crustal paleodepths up to 30 km show variably enriched isotopic signatures that are ascribed to crustal contamination or an enriched mantle lithospheric source. We compared a series of Triassic, Middle Jurassic, Late Jurassic and Late Cretaceous plutonic rocks to evaluate the role of the crust in arc plutonism through depth and with time. Permo-Triassic plutonic suites range from low K to shoshonitic over a comparatively limited compositional range of 56-68% SiO2, Middle Jurassic suites are high K calc- alkalic, Late Jurassic plutons are alkalic or medium K calc-alkalic, and Late Cretaceous plutonic suites are calc-alkalic. Secular variation is expressed in increasing Rb/Sr, La/Sm, La/Yb, and Sri and by decreasing εNd(t), recorded in plutons at both mid-crustal and upper crustal levels. Middle crustal intrusive rocks are characterized by Sri (and εNd) values of about 0.705 (-2) in Triassic time, increasing to 0.707-0.710 (-11 to -12) by Cretaceous time. Comparable values are 0.708-0.712 (-10 to -16) in upper crustal plutons. Lying structurally between the middle and upper crustal plutons is a lithologically heterogeneous sheeted plutonic complex comprised of tonalitic to granitic rocks with Sri (and εNd) values of 0.708-0.714 (-11 to -16.5). Components of the sheeted mid-crustal complex show significant overlap in Sri and εNd(t) with upper crustal plutonic rocks, suggesting that magmas feeding upper crustal plutons originated by mixing of tonalite and granite in the mid-crust. Data plot along mixing trends indicative of an enriched mantle source for mafic rocks and minor but persistent assimilation of crustal rocks during ascent of mafic magmas through the crust.
V33A-2199
Growth of a Large Composite Magma System: the EJB Pluton, Eastern California.
The composite EJB pluton crops out in the White-Inyo Mountains of eastern California, and comprises the Eureka Valley monzonite (EVM), the Joshua Flat quartz monzonite (JFQM), the Beer Creek granite (BCG), and an unnamed diorite. While sometimes equivocal, field relationships suggest that the EVM was emplaced first, followed by the JFQM, and finally the BCG; the diorite predates the BCG. Sylvester and others (GSAB, 1978) reported zircon U-Pb ages of 179±2 Ma for the EVM and 174±5 Ma for the JFQM. Coleman and others (GSAB, 2003) determined a U-Pb age of 179±3 Ma (via Pb-loss trajectory) for the BCG. Because of the uncertainty in the ages and ambiguous field relations, the sequence and duration of EJB magmatism remain unclear. To understand more fully the timing of EJB magmatism, we separated zircons from 12 samples collected from each of the main EJB units. These samples were characterized using light microscopy, SEM and CL techniques. U-Pb ages were determined from individual zircons by LA-ICP-MS following the method of Chang and others (G3, 2006). For the ages reported below, the reported uncertainties are based on factors within the analysis, but do not include external factors such as sample/standard bias or other matrix effects. Overall uncertainty in LA-ICPMS U-Pb geochronology is hard to assess, but we estimate that all ages reported below are subject to a minimum 2% uncertainty. We determined a concordant U-Pb age of 180±2 Ma for the EVM, which agrees with the results of Sylvester and others (1978). The unnamed diorite produced a concordant U-Pb age of 177±3 Ma. Concordant U-Pb ages of 172±2, 172±3, 173±2, 174±2, and 175±2 Ma were determined for individual samples of the JFQM and agree with the age reported by Sylvester and others (1978) of 174±5 Ma. Concordant U-Pb ages of 168±4, 168±3, 169±1, 172±2, and 172±2 Ma were determined for individual BCG samples. Within the reported error, there is no difference in age between individual samples of the BCG, but taken as absolute, the ages tantalizingly decrease from NW to SE within the exposed area of the BCG. No such pattern is suggested within the JFQM. Collectively, these new LA-ICP-MS zircon age data support the observed field relationships and suggest that the EJB magma system was periodically active for as long as 10-12 million years. This time scale agrees well with current models of incremental growth of plutons and has important implications for strain accumulation in mid-crustal arc environments.
V33A-2200
Geology and Architecture of a Continental Arc in Flare-up Mode: Observations and Insights From a Crustal Cross-Section in Sierras Valle Fertil-La Huerta, Argentina.
New understanding of the mechanisms and rates by which magmatic arcs are constructed requires geological control from crustal cross-sections. Previously characterized arc sections are generally from island arcs, and may reflect relatively low-flux conditions potentially quite different than those associated with the large scale volcanism and rapid formation of pluton-flooded, mid-to-upper crust such as occurs in the Puna-Altiplano region today. However the deepest levels of the early Ordovician Famatinian arc crust provides a window into the 'plumbing' of a continental arc in flare-up mode and is exposed in an approximately 140 km by 40 km cross-section in the Sierras Valle Fertil-La Huerta in northwestern Argentina. The Valle Fertil portion of the arc grades continuously from regional-scale calc-alkaline metaluminous I-type granitoids (granodiorite into deeper heterogeneous tonalities) downward into a complex arrangement of large mafic intrusions (both cumulate and non-cumulate) and partially melted supracrustal packages composed largely of siliciclastic sediments. The region is characterized by granulite grade conditions formed broadly synchronously with igneous plutonic activity spanning up to 20 Ma, with a portion of the section recording 5.2-7.1 kbar and temperatures of 770-840°C. Peak metamorphic conditions produced thermal gradients of around 36° per km and there was little pressure change upon cooling producing a very flat ccw P-T loop. No resolvable differences in peak metamorphic conditions from localities along strike 70 km apart were detected, indicating that the arc is exposed at the same paleo-depth and the average crustal thickness was more than 20-25 km. It has also been proposed that this orogen formed in a margin without continent-continent collision and hence no terrane accretion. Thus the lower crustal section exposed in the Sierras Valle Fertil-La Huerta may be an analog for the Cenozoic high plateau of the Central Andes. This is an especially attractive idea as the large mafic bodies and heterogeneous tonalities might be an analog for the attenuating mid-crustal low velocity layer imaged in Puna-Altiplano region today. Our presentation will present work in progress on our efforts to refine the regional architecture of this arc with a special emphasis on applications to continental arcs in flare- up mode.
V33A-2201
Plutonism at Different Crustal Levels of an Arc: Insights From the 5 to 40 km (Paleodepth) North Cascades Crustal Section, Washington
The crystalline core of the North Cascades preserves a Cretaceous crustal section that facilitates evaluation of pluton construction, emplacement, geometry, composition, and deformation at widely variable crustal levels (~5 to 40 km paleodepth) in a thick (> 55 km) continental magmatic arc. The oldest and largest pulse of plutonism was focused between 96-89 Ma when fluxes were a minimum of 3.9x10-6km3/yr/km of arc length, but the coincidence with regional crustal thickening and underthrusting of a cool outboard terrane resulted in relatively low mid- to deep-crustal temperatures for an arc. A second, smaller peak of magmatism at 78-71 Ma (minimum of 8.2x10-7km3/yr/km of arc length) occurred during regional transpression. Tonalite dominates at all levels of the section. Intrusions range from large plutons to thin (< 50 m) dispersed sheets encased in metamorphic rocks that record less focused magmatism. The percentage of igneous rocks increases systematically from shallow to middle to deep levels; from approximately 37% to 55% to 65% of the total rock volume. Unfocused magmas comprise much higher percentages (approximately 19%) of the total plutonic rock at deep- and mid-crustal depths, but only 1% at shallower levels, whereas the largest intrusions were emplaced into shallow crust. Plutons have a range of shapes, including: asymmetric wedges to funnels; subhorizontal tabular sheets; steep-sided, blade-shaped bodies with high aspect ratios in map view; and steep-sided, vertically extensive (> 8 km) bodies shaped like thick disks and/or hockey pucks. Sheeted intrusions and gently dipping tabular bodies are more common with depth. Some of these plutons fit the model that most intrusions are subhorizontal and tabular, but many do not, reflecting the complex changes in lithology and rheology in arc crust undergoing regional shortening. The steep sheeted plutons partly represent magma transfer zones that fed the large shallow plutons, which were sites of intermittent magma accumulation for up to 5.5 m.y. Downward movement of host rocks by multiple processes occurred at all crustal levels during pluton emplacement. Ductile flow and accompanying rigid rotation were the dominant processes; stoping played an important secondary role, and magma wedging and regional deformation also aided emplacement. Overall, there are some striking changes with increasing depth, but many features and processes in the arc are similar throughout the crustal section, probably reflecting the relatively small differences in peak temperatures between the mid- and deep crust. Such patterns may be representative of thick continental magmatic arcs constructed during regional shortening.
V33A-2202
Inverted Regional Metamorphism in the Coaxially Refolded Tonga Formation: Evidence for Cretaceous Accretional Tectonics in the Cascades Crystalline Core
The Tonga Formation, on the westernmost boundary of the Cascades crystalline core, records Cretaceous plutonism, contact to regional metamorphism, and multiple episodes of folding related to intense east-west contractional deformation. The Tonga Formation is exposed in a fault-bounded, north-south elongate tectonic domain that comprises pelite-psammite metasediments, which increase from greenschist to amphibolite grade (south to north). This metamorphic gradient is inverted relative to a major westward verging and downward facing fold system that dominates the internal architecture of the formation. Sedimentary structures are remarkably well-preserved in the Tonga Formation, which allowed for the determination of younging directions. Using these and bedding-cleavage relationships, detailed field mapping indicates a stratigraphically overturned section that forms a large-scale antiformal syncline (exposed in the northern and eastern domain) and related synformal anticline (southern and western domain). The overturned nature of the strata and the geometry of gently north-plunging folds imply upsection a pre-existing tight, recumbent anticline refolded into a co-axial (type III) fold interference pattern. The core of this early anticline, exposed in the northern domain, corresponds with the higher metamorphic conditions of the inverted metamorphic gradient and early Cascades regional metamorphism ('M1') rarely decipherable in the adjacent Chiwaukum Schist. The co-axial, superposed folding in the Tonga Formation and the overall N-S arrangement of the component fold generations suggests a strong component of east-west shortening in the foreland of the Cascades core. Fold geometries account for the inverted metamorphic zonation as well as control the localization of plutons, which also elongate parallel to the regional fold axes. The central and southern and portion of Tonga Formation records subsequent contact ('M2') and regional ('M3') metamorphism, and appears to be a lower-grade equivalent of the Chiwaukum Schist, as protoliths of each unit are also similar. Exposure of the Evergreen fault is limited, but the observed structural and metamorphic relationships suggest that it is a late, east-dipping reverse fault that places the deeper, higher-grade Chiwaukum Schist structurally adjacent to the lower-grade Tonga Formation. Though the Tonga formation exposes, at different crustal levels, multiple stages of deformation and metamorphism, a single foliation dominates the meso-scale rock fabric. It is inferred that this fabric accumulates over the entire strain history, departing from a strong ~layer-parallel anisotropy/fabric related to the initial isoclinal folding episode.
V33A-2203
Mafic Magmatism in the Coast Mountains, Western British Columbia: Evidence for Slab Removal and Convective Thinning of an Ultramafic Root?
We present isotopic (Sr-Nd-Pb) data to determine the geochemical nature of the mantle beneath the Coast Mountains Batholith (CMB). Nineteen mafic (42-52% SiO2) dike and cinder cone samples, ranging in age from Eocene to Holocene, suggest that a transition from a hydrated mantle wedge, to dry, upwelling asthenosphere took place sometime after 10 Ma. Holocene magmas have higher average ε-Nd (8.2) and lower initial 87Sr/86Sr (0.7029) than the Eocene-late Miocene magmas from the same region (εNd = 3.5, 87Sr/86Sr = 0.7043). Geophysical evidence includes lack of an orogenic root and a sharp, shallow Moho beneath the Coast Mountains at the present day. The Holocene magmas may have been produced and emplaced during the time that dense residues from batholith formation detached from the lower crust and foundered into the mantle. Our delamination model suggests piecemeal removal of batholith residues persisted over a ~50 Ma period, rather than in a single removal event. This model also accounts for the regional extension and exhumation that have been documented in the orogen.
V33A-2204
Elemental and Isotopic Evidence for Positive and Negative Feedback Mechanisms Governing Magmatic Flux in the Coast Mountains Batholith, British Columbia
New REE data from 80 plutonic rocks from the southern Coast Mountains Batholith (CMB) ranging from 198 - 50 Ma form distinct populations of steeply ((La/Yb)N = 30 - 65) and shallowly dipping((La/Yb)N < 10) chondrite-normalized patterns. Nd and Sr isotopes have overall primitive characteristics with initial ε Nd from -2 to +8 and initial 87Sr/86Sr of 0.7030 - 0.7065. Periods of high magmatic flux (160-140 Ma, 120-80 Ma, and 60-50 Ma) when high (La/Yb)N was accompanied by less primitive Nd and Sr are preceded by low magmatic flux periods (200-160 Ma, 140- 120 Ma and 80-60 Ma) when (La/Yb)N was low and Nd and Sr appear more mantle-like. Magmatic flux in the CMB appears have been governed by shortening and thickening in the upper plate, which caused an increase in lower crustal melting. Continued crustal shortening eventually led to a negative feedback in CMB magmatic flux, as the combination of an over thickened crust and crowding of the wedge beneath the arc with igneous residues and lithospheric mantle inhibited underthrusting and the supply of fertile lithologies to the lower crust. Delamination or foundering of igneous residues and mantle lithosphere during low magmatic flux periods alleviates the space problem in the sub-arc wedge and allows renewed underthrusting to initiate the next period of high magmatic flux. This explains why CMB granitoids were generated deeper in the crust with garnet-rich residues (high (La/Yb)N) and less primitive initial ε Nd and 87Sr/86Sr during periods of high magmatic flux, while during periods of low magmatic flux granitoids were generated at a shallower depth with opposite signatures. Recognition of delamination events in the CMB has important implications for how continental arcs evolve through time.
V33A-2205
Styles of pluton emplacement during contraction or extension, Coast Mountains, British Columbia
The Coast Plutonic Complex, British Columbia, is an ~ 1800 km belt of plutonic rocks formed in the Mesozoic and Early Cenozoic. We describe the results of strain, kinematic, and geometric analysis across an east-west transect across the Coast Mountains from the Quottoon pluton, which is part of the great tonalite sill, with nearly vertical contacts to the ~ 6 km thick Chief Matthew's Pluton, with shallow dipping contacts. In both cases, the country rocks were at upper amphibolite facies of metamorhism when the plutons intruded. We applied the autocorrelation function (ACF) to estimate finite strain in oriented thin sections of both the country rocks and plutons, cut parallel to the lineation and perpendicular to foliation. The ACF is a Fast Fourier Transform of the image, which is a function of grain size, shape and orientation as well as proportion of mafic and felsic minerals. A minimum estimate of the finite strain can be calculated by taking the ratio of the maximum and minimum axes of the ellipse representing the 96th gray-level contour in the ACF image. The country rocks of the Chief Matthew's pluton consistently record a weak ACF, reflecting a random distribution of grain long axes. This is consistent with the variation in orientation of mineral lineation measured in the field, which, in stereographic projection, defines a girdle parallel to the moderately (~ 20°) southwest dipping foliation. These features suggest a flattening strain field, with steeply inclined shortening. Kinematic indicators consistently indicate normal shearing during pluton emplacement. In contrast, rocks around the Quottoon pluton contain vertical foliations and steeply inclined mineral lineations. Kinematic indicators and northeast vergent folds with steeply dipping axial surfaces are consistent with top to the northeast reverse shear. The measured ACF ellipticity for the Quottoon pluton and its country rock is consistently ~ 0.7 independent of the grain size, indicating fabric development during penetrative ductile deformation. The ACF ellipses define a clear strain gradient across the field area, from high strains near the Quottoon pluton (west), to lower strains around the Chief Matthew's pluton (east). These results show that the Quottoon pluton was emplaced during contraction. The Chief Mathew's pluton intruded during extensional deformation, producing moderate to gently dipping sills. In both cases the plutons have tabular shapes perpendicular to the inferred shortening direction suggesting that pluton geometry is controlled by strain accumulation during ductile deformation.
V33A-2206
Tracking Paleostrain in Arcs Using Magmatic Fabrics in Well-dated Plutons: An Example From the Tuolumne Batholith and Surrounding Plutons, Central Sierra Nevada, California
Magmatic fabrics in plutons are often interpreted as records of magma flow rather than as records of regional strain in arcs. However, our field studies in the Sierra Nevada, California and elsewhere in Cordilleran arcs, provide examples of the benefits of studying plutons to constrain short increments of regional strain histories and inferred paleostresses. These plutons preserve alignments of magmatic minerals, which reflect increments of strain that are sometimes caused by magma flow during chamber growth, but more commonly record the regional strain in the arc that was imprinted on magma chambers. In the latter case, these 'regional strain increments' during pluton crystallization represent shorter time spans than what can be deciphered in metamorphic host rocks because of the longer histories recorded in metamorphic rocks and because recent improvements in U/Pb zircon geochronology allow documentation of the time magmatic fabrics lock in with high precision (<0.5 m yr). Our studies in the Cretaceous Tuolumne batholith, where we now have >4000 measurements of magmatic fabrics, has led to the recognition of 4 different magmatic fabrics that are, from oldest to youngest: 1) due to localized magma flow in tubes and troughs in an existing chamber, 2) margin parallel fabrics related to boundary effects, 3) an older, NW-SE striking magmatic foliation, and 4) a slightly younger, ENE-WSW striking foliation, both reflecting regional strain. Magmatic lineations are mostly steep and shared by all 4 different foliations. Mapping in and dating of other Mesozoic plutons (165 Ma to 85 Ma) surrounding the Tuolumne batholith documented similar orientations of magmatic foliations and lineations and the same temporal change in the magmatic foliation strikes, suggesting that regional strain related fabrics systemically shifted through time from NW to WNW in this part of the arc. We interpret this change in orientation as likely evidence for a temporally changing paleostress field. The findings of four magmatic fabrics in the Tuolumne batholith and consistent fabric patterns in surrounding plutons emphasize that 1) magmatic fabrics can have diverse causes and do not always record magma flow processes, especially in long lived and complex magmatic bodies such as the Tuolumne; and 2) magmatic fabric studies combined with high precision geochronology are a powerful technique to document paleostrain and inferred paleostress fields in arcs, which can then be used to evaluate changes in plate motions and/or other processes influencing these strain/stress fields.
V33A-2207
Paleo- Stress and Strain Rates in an Intra-Arc Strike-Slip Fault, Sierra Nevada, California
Structures and microstructures of the Proto-Kern Canyon fault (PKCF), a 130-km-long dextral strike-slip shear zone of the southern Sierra Nevada batholith, provide constraints on displacement, flow stress, and strain rate during arc formation. Shear strain analyses of S-C mylonites indicate ~5 km of ductile dextral slip along the PKCF. But field mapping and measurements of individual plutons and metamorphic pendants show these bodies have much more elongated aspect ratios, of up to 1:17, within the shear zone than outside of it. This suggests significantly higher strain and dextral slip of up to 15 km along the highest-strain zone of the PKCF. Petrographic observations of high-strain igneous rocks near Lake Isabella indicate that deformation started at temperatures of 400-450° C and continued through cooling to ~300° C. Based on 40Ar/39Ar dating of hornblende, mica, and K-feldspar, early cooling (~20° C/m.y.) from 88-70 Ma was followed by very slow cooling (~1° C/m.y.). These data, combined with cross-cutting relationships, suggest that dextral ductile shear was active from 90-86 Ma. Grain sizes of dynamically recrystallized pure quartz mylonites in this part of the shear zone were used to estimate flow stresses of 20-40 MPa. Applying mylonitization temperature estimates of 400-350° C and lithostatic pressures of 350-400 MPa (from Al-in-hbl barometry) yields paleo-strain rates along the PKCF of ~10- 13-10-15/s. Additional quartzite piezometry, as well as calcite piezometry on marble mylonites, should provide further constraints on stress and strain rates along the length and depth exposures of this intrabatholithic shear zone.
V33A-2208
Mafic-Felsic Magma Interactions in an Enclave Mega-Plume, Gobi-Tienshan Intrusive Complex, Southern Mongolia
The Gobi-Tienshan Intrusive Complex (GTIC), Southern Mongolia, is a tilted 313-292Ma continental arc cross-section in the southernmost magmatic belt of the Central Asian Orogenic Belt. The complex contains a range of magma compositions (granodiorite, granite and syenogranite) and extensive diking (of andesite to quartz diorite). Magma mixing and mingling is common throughout the complex, including the disaggregation of dikes into microgranitoid enclaves and as the hybridization of dikes with granodiorite and granite. An enclave-rich mega-plume, defined by a sharp increase of enclave percentages from ~5 to 30-50% and exposed aerially over a ~15 km2, intrudes a large body of granodiorite in the central portion of the complex. Enclaves found within the mega-plume show a complex variation in texture as well as structure. Hornblende textural variation includes acicular groundmass crystals, clusters of crystals, and large euhedral crystals extracted from the host granodiorite. Plagioclase variations include fine-grained crystals in groundmass, large crystals in enclaves similar in morphology to those in the host rock, and crystals with rapakivi texture. Enclaves can have chilled margins and display a wide range of textures, from uniformly fine- grained enclaves, to enclaves with strong hybridization and diffuse margins. There is no systematic change in enclave texture; all textures are found within a single outcrop. Enclaves are commonly elongate, and their size is inconsistent throughout the plume, with long axes ranging from centimeters to tens of centimeters with average long axes of 20-25cm and short axes of 5-10cm. Enclave foliations are inconsistent with mineral foliation in the host-rock, and in some places enclave orientation changes dramatically from subhorizontal to subvertical within meters to tens of meters. Granodiorite, granite, and enclave geochemistry show curvilinear trends of major oxides versus SiO2 and MgO. However, trace elements show that each group falls into a distinct field on co-variation plots of Rb, Sr, and Ba. One group of enclaves has lower total concentration of REE and pronounced heavy REE depletion. These are similar to disaggregating dikes, potential source-rocks identified in a structurally lower portion of the complex. A second group of enclaves are more enriched in all REE with a distinctive negative Eu anomaly. Trace element variation patterns on spider diagrams overlap for enclaves and hypothesized source-rocks. Enclaves with the most mixed appearance share chemical characteristics with local host granodiorites. Al-in-Hbl thermobarometry was conducted by microprobe transects on hornblende and plagioclase crystals in SiO2 saturated samples. Granodiorites yielded a pressure of 2.68±.10kbar, assuming that the most sodic plagioclases were in equilibrium with the final crystallizing assemblage. The standard deviation for all analyzed grains was .20, indicating very homogeneous plagioclase and hornblende compositions. Enclaves yielded pressures of 2.84±.17 and 3.20±.56kbar, also using the most sodic plagioclases. The standard deviation for all analyzed grains was .59-.60, indicating some compositional complexity. Occasional plagioclase crystals in the enclave samples contained very calcic cores with resorption textures. These calcic plagioclase cores indicate complex crystallization history. Trace elements suggest that enclaves may have fractionated from compositions of feeder systems and that subsequent physical mixing processes integrated host granodiorite crystals. Juxtaposition of many enclave types is evidence for a dynamic intrusion environment of the enclave plume.
V33A-2209
4d Architectural Variation in the Gobi-Tienshan Intrusive Complex, Southern Mongolia
The Gobi-Tienshan Intrusive Complex is a recently characterized tilted section through the upper 15 km of a Carboniferous aged continental margin arc in the Gobi desert, southern Mongolia. Preliminary geochronology indicates that the GTIC, a 2,500 km2 batholith, intruded over a 21 m.y. timespan. This high- flux magmatic event is reflected in field relationships identified through detailed mapping of a depth-transect through the crustal section, which revealed dramatic changes in magmatic architecture at different crustal levels. The deepest exposed levels (12-15 km) are complex mingling zones comprised of 10s to 100s of meters scale intrusions of granite to quartz diorite intruded by mafic dikes that display ubiquitous mixing and mingling structures. These dikes disaggregate into enclaves and hybridize with host magmas to generate magmas that re-intrude and re-mix with the host magmas. Even though these dikes are observed over a huge area, as well as intruding a huge range in host magma compositions, their mechanical behavior when intruding host crystal mushes is identical. At shallower levels (6-10 km), multi-km scale granodioritic plutons host literally hundreds of millions of microgranitoid enclaves, which can be texturally and geochemically linked to mafic source materials deeper in the complex. At this crustal level, large granodioritic plutons host granitic intrusions that are organized into sets of 100 meter wide sheets. These sheets contain granites with a wide variety of textural and geochemical characteristics, including many high K granites. Contact relationships indicate that granodiorites were partially molten at the time of granitic intrusion, but major mixing and mingling between these units did not occur. The shallowest levels of the complex show more evolved granites and syenogranites that contain sheets of quartz dioritic magma that is observed to have hybridized with units in the subvolcanic level. Finally, all units are cross-cut by an additional dike swarm of basaltic to rhyolitic composition, which does not cross into the youngest subvolcanic intrusions. The injection of mafic magmas and their hybridization with host magmas are a ubiquitous and time-transgressive feature in the batholith. Meanwhile, the magmatic architecture of the GTIC transitions with depth from complex mingling and mixing to large homogeneous bodies to subvolcanic intrusions with mingling structures. We also observe architectural transitions through time, from the building of magma bodies, internal processes occurring in plutons, and the final cooling stages of the magmatic system that may provide clues to the evolution of rheological behavior during these stages and with depth in the arc.