V22A-01 INVITED
Distillation of continental crust from above and below
Bulk continental crust (CC) is evolved (Mg# approx 0.5) compared to mafic magmas derived from the mantle (Mg# approx 0.7) and mafic cumulates (Mg#>0.7). CC is a mixture of rocks, so one can infer that mafic rocks are under-represented in the mix. Most of the mix originally formed via arc magmatism. Here we ask, if the average is evolved compared to mantle-derived arc magmas, where did the more mafic material go? We review proposed explanations in the light of recent observations on arc crustal sections and calculations of physical properties as a function of rock composition. Ringwood and Green, Tectonophys 66, proposed that mafic material at the base of the crust becomes denser than the underlying mantle, driving viscous "foundering" of the dense material, leaving a less mafic, buoyant crust. There is evidence that foundering does happen. However, based on two well studied and complete arc crustal sections, Talkeetna and Kohistan, it is not clear that foundering alone could produce CC. Even after some foundering, arc lower crust is compositionally different from and - at Moho PT - denser than continental lower crust (CLC). Mafic rocks in CLC only become denser than mantle and hot enough to founder in garnet granulite conditions, not higher in the crust. IF all arc crust passes through garnet granulite facies metamorphic conditions AND IF foundering efficiently removes most dense lithologies, the result might approach CC. Is that really what happened? A related idea is that the "mantle" below CC includes a lot of mafic igneous rock, in the form of dense garnet granulite and/or pyroxenite that is seismically fast enough to masquerade as peridotite. This may be consistent with data on some arcs where sub-Moho Vp is approx 7.6 km/s, but is not so likely for CC with sub- Moho Vp of approx 8.0. Mechanical weathering and subduction of sediments may play a role. Arc volcanics are dominantly mafic, whereas mid-crustal plutonic rocks are generally "granitic". Rapid weathering of lava flows and volcanoclastic sediments plus slow weathering of underlying granitic plutons may ensure subduction of relatively mafic compositions in "immature" volcanic arcs. Chemical erosion might preferentially remove Mg, but what about Na and K, which are abundant in CC? An intriguing and relatively new idea is that compositionally buoyant, viscous diapirs arise from subducting plates. Buoyant rocks can viscously detach from their denser neighbors above approx 700°C provided layer thicknesses exceed a few 100 meters. Such processes play a role in exhumation of UHP metamorphic terranes. Where they are felsic and buoyant, fore arc sediments removed by subduction erosion may be added to arc lower crust rather than subducted into the mantle. A bit deeper, generation of metasedimentary diapirs, followed by decompression melting as they pass upward in the mantle, may account for recycling of the "sediment component" in arcs. Still deeper, during arc-arc or continent-continent collision, entire granitic batholiths carried into the mantle should be sufficiently buoyant to rise toward the surface.
V22A-02
Seismic Structure and Geodynamic Evolution of the Lithosphere and Upper Mantle in the Pannonian - Carpathian Region
The Pannonian Basin is the largest of a group of Miocene-age extensional basins within the arc of the Alpine-Carpathian Mountain Ranges. These basins are extensional in origin, but the surrounding Carpathians result from sustained convergence during and since the period of active extension. A significant part of the mantle lithosphere here has been replaced, as gravitational instability caused an overturn of the upper mantle. The Carpathian Basins Project (CBP) is a major international broadband seismology experiment, supported by geodynamical modelling and designed to improve our understanding of the structure and evolution of the lithosphere and upper mantle beneath the Pannonian and Vienna Basins. Between 2005 and 2007 we deployed 56 portable broadband seismic stations in Austria, Hungary and Serbia, spanning the Vienna Basin and the western part of the Pannonian Basin. Arrival time residuals from teleseismic earthquakes are delayed by about 0.8 sec in the Vienna Basin and early by a similar amount in southwest Hungary. Tomographic inversion of the travel time residuals shows relatively fast P-wave velocities in the upper mantle beneath the western Pannonian Basin and slow P-wave velocities beneath the West Carpathians. Seismic anisotropy (SKS) measurements reveal an intriguing pattern of lithospheric anisotropy: in the north-west the fast direction is generally elongated EW, perpendicular to the shortening direction across the Alps. Across the Vienna Basin the fast direction is NW-SE, perpendicular to the major bounding fault systems. Across the Pannonian Basin the dominant fast direction is EW, but in several locations the vectors are rotated toward NW-SE. The Mid-Hungarian Line, a major strike-slip structure already clearly identified in the gravity field, also is associated with abrupt changes in the azimuth of lithospheric anisotropy, and crustal receiver function signature. The object of these investigations is to use the seismic data to discriminate between different models for how this orogenic system evolved. In support of this aim we are developing 2D and 3D mechanical models of lithospheric deformation driven by boundary stresses and gravitational instability of the mantle lithosphere.
V22A-03
Mantle-derived Calc-alkalic vs. Crust-derived Tholeiitic Magmas: A Radical Veiw of Andesite Genesis
Two distinctive differentiation trends, tholeiitic and calc-alkalic, are recognized in Zao volcano, which is located immediately behind the volcanic front of the NE Japan arc. The genetic relation between these two magma series is critical to for a better understanding of andesite genesis, because in arc volcanoes they often coexist, in close spatial and temporal proximity in arc volcanoes. Petrographic features indicative of 'disequilibrium', such as reversely zoned pyroxene phenocrysts, the wide and bimodal compositional distribution in Ca/(Ca+Na) of plagioclase phenocrysts, honeycomb textures and dusty zones that these plagioclase phenocrysts often exhibit, and the presence of olivine-pyroxene pairs with different Mg/Fe, are observed exclusively in calc-alkalic rocks. In tholeiitic rocks the Sr isotopic ratios of plagioclase phenocrysts, determined by both micromilling combined with thermal ionization mass spectrometryscopy, and laser- ablation inductively coupled plasma mass spectrometry ICP-MS techniques, are constant at 0.7042-0.7044 in tholeiitic rocks. On the other hand, those in calc-alkalic rocks (0.7033-0.7042) show more complex characteristics, which can be best understood if at least three end-member components,; a calc-alkalic basaltic melt, a tholeiitic basaltic melt and a tholeiitic felsic melt, contribute to the production of mixed calc- alkalic magmas. 87Sr/86Sr and trace element compositions of the least-differentiated basalt magmas, which are inferred from the composition of the calcic plagioclase (Ca/(Ca+Na) >0.9), suggest that two types of basaltic magma, calc-alkalic and tholeiitic, exist beneath the volcano. The tholeiitic basalt magma possesses higher 87Sr/86Sr than the calc-alkalic magma (0.7042 vs. 0.7038) and shows characteristic trace element signatures consistent with the presence of plagioclase and amphibole as melting residues. This suggests that the tholeiitic magmas are produced via anatexis of amphibolitic crust caused by underplating and/or intrusion of mantle-derived calc-alkalic basalt magmas into the sub-Zao crust. The mantle-derived calc-alkalic basalt magma mixes with crust-derived tholeiitic melts to form calc-alkalic andesite magmas.
V22A-04
Magma processing in the lower crust as recorded in mafic inclusions from Mt. Shasta, CA
Here we present petrologic analyses and major and trace element compositional data on a set of relatively rare (although ubiquitous in time) 10-100 cm sized quenched magmatic inclusions from Mt. Shasta, CA. They preserve evidence of significant lower crustal fractionation as well as deep and early mixing of primitive mantle melts that produced the evolved Shasta andesite and dacite lavas. These inclusions are the most magnesian rocks erupted from the main Shasta edifice, and texturally show very little equilibration with the host lava (crenulated margins and the occasional host phenocryst). These inclusions lie on experimentally determined liquid lines of decent of primitive magmas erupted at satellite vents in the Mt. Shasta area. Along with experimental evidence, this study supports the case that magma mixing at Shasta occurs over a range of depths beginning at mid to lower crustal levels, and even possibly the crust – mantle interface. The inclusions are typically aphanitic and contain plag and pargasite microlites set in a glassy matrix. The inclusions are typically phenocryst poor containing < 1 to 2 % phenos. Common groundmass assemblages in the inclusions are Parg+Plag, Parg+Plag+Oliv±apatite, and Opx+Cpx+plag±olivine (+glass +oxide for all), which are distinct from host lavas which are porphyritic (> 20 % phenos) and contain several generations of resorbed plag, and pyroxenes. Xenocrysts of pargasite rimmed olivine and opx are consistent with fractionation of these phases in the lower crust. High magnesium number pargasites, as well as olivine-pargasite reaction textures are only produced in experiments with high water contents (> 10 wt.% H2O). High water contents imply lower viscosity and lower density. Both factors favor thorough mixing. Over the past ~300 ka Mt. Shasta, CA has erupted lavas that are homogeneous, yet complex mixtures of several magmatic end member components. The Mt. Shasta lavas are relatively far removed in composition from the primitive flank eruptions in the Mt. Shasta region. This unique suite of inclusions allows for the identification of multiple mantle inputs and differentiation processes.
V22A-05 INVITED
Numerical Modeling of the Heat and Mass Transfer in Arcs: Unifying Geological and Geophysical Observations with a Multi-scale approach
Understanding the specific processes of two-way vertical transport of mass and enthalpy at the crust-mantle boundary and upper plate remains a grand challange in understanding the dynamics of convergent margins and volcanic arcs. The interplay of diverse melting processes within the mantle, delamination of cumulates, and the quantity of supracrustal material introduced from subduction erosion, has been difficult to constrain from geological examples. In addition the rates of magmatism, suchas normal subduction or high-flux flare-up conditions, and the water content of primary magmas can produce very diferent large scale compositional and rheological domains in the crust. Numerical modeling can provide a technology for illuminating how important degrees of freedom: basaltic magma flux and composition, country rock melting systematics, tectonic state and geotherm whether transpression or static arc, yields end members that are in accord with natural examples. However these models must be dynamic, with the capacity for melt migration and mixing during coupled intrusion, fractionation and country rock melting. I will exemplify two modeling approaches: one that uses switching functions to toggle various rheological states, melting conditions and melt migration, and one that is hierachical where large-scale features are modeled directly and small scale features are modeled in a sub-grid fashion. Limitations and verisimilitude of both approaches will be evaluated in light of three known crustal sections, Ivrea Zone, Fiordland and the Famatinian arc.
V22A-06
CERRO GALAN IGNIMBRITE: TRACE ELEMENT, ISOTOPIC AND 40AR/39AR AGE CONSTRAINTS ON THE EVOLUTION OF THE CENTRAL ANDEAN LITHOSPHERE
The giant ignimbrites erupted from the Cerro Galán caldera complex in the southern Puna of the high Andean plateau are considered to be linked to crustal and mantle melting as a consequence of Pliocene delamination of gravitationally unstable thickened crust and mantle lithosphere over a steepening subduction zone. Ongoing imaging in the PUNA08 seismic array shows very low velocity anomalies in the mantle wedge and in the crust under the Cerro Galan caldera supporting recent delamination and crustal melting. Sixty new major and trace element analyses show the Galan ignimbrites (68-71 percent SiO2) are characterized by steep heavy REE patterns (Sm/Yb, 4.2-6.5) with low Eu/Eu* (0.45-0.70) ratios indicating both equilibration with garnet, and plagioclase fractionation. They have generally intraplate-like signatures with little arc influence (La/Ta, 14-28; Zr/Nb, 9-12; Ba/Th,18-26; Ba/La, 9-16). AFC modeling and fractionation corrected delta 18O values from quartz (+7.63-8.85 ‰) are consistent with the ignimbrite magmas being near 50:50 mixtures of enriched mantle (87Sr/86Sr ~ 0.7055) and crustal (87Sr/86Sr near 0.715-0.735) melts. Mantle enrichment is attributed to Neogene addition of continental lithosphere in association with delamination as well as forearc subduction erosion signaled by 50 km of late Neogene frontal arc migration. Hybrid melts produced by mixing and fractionation of intruding mafic magmas and melted thick lower crust are argued to separate and ascend to mid crustal depths that correspond to low velocity zones on seismic images, where they fractionate before erupting. Somewhat more arc-like characteristics in the older ignimbrites support a diminishing slab influence in a thickening mantle wedge under a delaminating lithosphere. 40Ar/39Ar single crystal sanidine weighted mean plateau ages from pumices that are in excellent agreement with isochron ages show that the youngest Cerro Galan eruptions include a 2.126±0.017 Ma flow within the caldera and extracaldera flows to the west at 2.096±0.016 Ma and to the north at 2.068±0.019 Ma. Biotite ages in the same flows are consistently older at 2.683±0.063 Ma, 2.317±0.036 Ma and 2.467±0.038 Ma respectively.
V22A-07 INVITED
What can seismic velocity structure tell us about the composition of island arc lower crust? An example from the central Aleutians
The P-wave seismic velocity structure of island arcs derived from wide-angle seismic reflection/refraction studies provides one of the few constraints on the bulk composition of arc middle and lower crust. However, a wide range of rock compositions have similar P-wave velocities, rendering inferences about composition based on P-wave velocity alone non-unique. Here we explore additional methods to glean more information on deep arc composition from seismic velocities, using an arc-parallel line in the central Aleutians (Line A2) as an example. The central Aleutians is a good test case, because P-wave velocity models created from both controlled-source wide-angle seismic data and from local earthquakes are available. Results from both datasets show that the bulk composition of the central Aleutian arc is more mafic than average continental crust, and that the crust is thicker here than many other island arcs worldwide. One of the most notable results of modeling along Line A2 is the P-wave velocity structure of the lower crust; velocities of 7.3-7.7 km/s are observed over depths of ~20-35 km and have been interpreted as ultramafic-mafic cumulates and/or garnet granulites. These lower crustal velocities are among the highest velocities associated with the lower crust of any island arc studied to date and are important for 1) constraining magmatic processes within arcs, 2) assessing the stability of arc lower crust, and 3) testing models for building continental crust from island arcs. To learn more about the possible composition of the Aleutian lower crust, we explore two different approaches. First, we will present initial results of forward modeling S-wave arrivals from Line A2. S-wave arrivals can be observed on some instruments at source-receiver offsets as great as ~150 km, and thus provide constraints on deep S-wave structure of the central Aleutian arc, which in conjunction with P-wave structure will provide tighter compositional constraints than either dataset alone. Secondly, we consider variations in velocity with depth predicted for different simple magma evolution paths in the Aleutians. Although the velocities of individual layers may be highly non-unique in isolation, the spatial distribution of velocities in the middle and lower crust and upper mantle may be more diagnostic of magmatic processes and the resulting crustal composition. The highest lower-crustal velocities (and presumably most mafic material) observed on Line A2 are detected in the middle of a segment in the overriding plate, implying that arc segmentation may exert an influence over crustal composition. Furthermore, there appears to be a first- order anti-correlation between lower-crustal velocity and the composition of volcanics at the surface, such that higher lower-crustal velocities are associated with more felsic volcanism. This implies that more mafic cumulates are emplaced at depth to create more felsic magmas at shallower levels. Building on this observation and complementary efforts in the IBM arc, we will explore the effects on seismic velocity of primary magma composition, temperature-pressure paths of crystallizing magmas and water content by modeling the solid mineral assemblages resulting from fractional crystallization and the variations in velocity associated with those solids.
V22A-08 INVITED
Lithosphere Erosion and Crustal Growth in Subduction Zones: Insights from Initiation of the Nascent East Philippine Arc
The Philippine Trench marks a nascent plate margin where subduction initiation is propagating from north to south. Magma compositions in the East Philippine Arc record thinning of arc lithosphere as it is eroded from below. The southern part of the arc is dominated by a continuous spectrum of compositions from high-Mg andesite to adakitic rhyolite. Isotopic ratios and most incompatible trace element ratios are indistinguishable from contemporaneous calc-alkaline magmatism. These similarities point to derivation from similar sources i.e. metasomatised mantle wedge. Rare earth elements, notably Dy/Yb ratios, demonstrate that high-Mg andesitic and adakitic compositions developed through differentiation of hydrous basaltic magma at more than 30km depth, where garnet is stable. In the Philippines, where arc crust is thin, this represents sub-Moho depths. This is interpreted as the result of relatively thick lithosphere beneath the younger, southern part of the arc causing basaltic magma to stall and fractionate garnet at high pressure. In the mature, northern section basaltic magma differentiates at shallower levels, at pressures where garnet is not stable. The northern, more mature part of the arc has produced lavas that are dominantly calc-alkaline. This reflects the more typical architecture of active arcs where the lithosphere has been thinned from below by ablation. Rare occurrences of rocks with mildly adakitic chemistry indicate that local variations in lithosphere thickness exist and that lithospheric thinning is rapid and may be piecemeal. Fluctuations in arc lithosphere thickness throughout the history of this margin appear to control spatial and temporal variations in magma fluxes into the arc crust. Varying fractionation depths of hydrous basalt may help explain the andesitic composition of bulk continental crust.