V34A-01
Continental Collision Zones are Primary Sites of net Continental Crustal Growth: Evidence From the Linzizong Volcanic Succession in Southern Tibet
The Linzizong volcanics (ca. 65-45Ma) and the coeval batholiths (ca. 60-40Ma) of andesitic-to-rhyolitic composition are magmatic response to the India-Asia continental collision that began at ca. 70-65Ma and ended at ca. 45-40Ma with convergence continuing to present [1,2]. These syncollisional magmatic rocks are widely distributed along much of the >1500km long Gangdese Belt immediately north of the India-Asia suture (Yarlung-Zangbo) in southern Tibet [2-6]. Our study of the Linzizong volcanics from the Linzhou Basin (near Lhasa) encourages the proposal that syncollisional granitoid magmatism may in fact account for much of the net contribution to continental crust growth. The Linzizong volcanics in the Linzhou Basin show a first-order temporal change from the lower andesitic formation (64.4-60.6Ma), to the middle dacitic formation (ca. 54Ma), and to the upper rhyolitic formation (48.7-43.9Ma). The three formations show no systematic but overlapping Nd-Sr isotope variations. The isotopically depleted samples with εNd(t)>0 (up to + 8) indicate that their primary sources are of mantle origin. The best source candidate in the broad context of Tethyan ocean closing and India- Asia collision is the remaining part of the Tethyan ocean crust [6]. This ocean crust melts when reaching its hydrous solidus during and soon after the collision in the amphibolite facies, producing andesitic melts parental to the Linzizong volcanics (also the coeval batholiths) with inherited mantle isotopic signatures [6]. Ilmenite is abundant in amphibolite [7], and partial melting of amphibolite with ilmenite as a residual phase accounts for the depletion of Nb, Ta and Ti in the melt. The effect of ocean crust alteration plus involvement of mature crustal materials (e.g., recycled terrigeneous sediments) enhances the elevated abundances of Ba, Rb, Th, U, K and Pb in the melt [8,9]. These give the syncolissional Linzizong volcanics characteristic "arc-like" geochemical signature. Residual amphibole that possesses super-chondritic Nb/Ta ratio explains the sub-chondritic Nb/Ta ratio in the melt, typical of continental crust [10]. These observations and reasoning plus the remarkable compositional similarity between the lower andesitic formation and the model bulk continental crust [11] corroborates our proposal [6] that continental collision zones are sites of net crustal growth (juvenile crust) through process of syncollisional granitoid magmatism. While these interpretations are reasonable in terms of petrology, geochemistry and tectonics, they require further testing through detailed studies of samples with a greater spatial coverage along the entire Gangdese belt, which are currently underway. References: [1] Yin, Harrison, Ann. Rev. Earth Planet. Sci., 28, 211-280, 2000; [2] Mo et al., Earth Sci. Front., 10, 135-148, 2003; [3] Mo et al., Acta Geol. Sinica, 79, 66-76, 2005; [4] Mo et al., Geol. Soc. Am. Spec. Pap., 409, 507-530, 2006; [5] Mo, Hou, Niu et al., Lithos, 96, 225-242, 2007; [6] Mo, Niu, Dong et al., Chem. Geol. (submitted); [7] Niu, Lesher, Econ. Geol., 86, 983-1001, 1991; [8] Plank, Langmuir, Chem. Geol., 145, 325-394, 1998; [9] Elliot, Geophys. Monogr., 138, 23-46, 2003; [10] Foley et al., Nature, 417, 837- 840, 2002; [11] Rudnick & Gao, Treatise on Geochemistry, 3, 1-64, 2003.
V34A-02
Continental Growth and the Thermal History of the Earth
First generation thermal history models predicted a Urey ratio (radiogenic mantle heat production divided by heat loss) of 0.7. Geochemical arguments constrain the Urey ratio to a range of 0.15-0.6. To potentially help resolve this discrepancy, we explore the effects of adding continental growth to thermal history models. Placement of radiogenics into the continental crust decreases the amount of radioactive material in the mantle. The thermal insulation effect of the continents affects the heat flow out of the mantle. We incorporated these coupled effects into thermal history models to constrain the continental growth scenarios that can satisfy constraints for present day mantle potential temperature and heat flow while at the same time predicting Urey ratios in accord with geochemical constraints. To further constrain allowable models, we also included a cooling core in the calculations. This provides two added constraints in that allowable models must have a core heat flux sufficient to generate a geodynamo and must predict an inner core size consistent with present observations. Under the assumptions of our modeling approach, these constraints limited the class of allowable continental growth models to those that have a component of progressive growth over geologic time (i.e., either smooth or episodic growth of continents over the Earths history).
V34A-03
Lithium Isotopic Composition of Mantle Xenoliths From the Western U.S. - Implications for Metasomatic and Delamination Events of the North American Lithosphere
We report Li isotopic composition of mantle xenoliths from the central Sierra Nevada to the Colorado Plateau to assess compositional variation of the lithospheric mantle in association with the Cenozoic tectonic events of the western U.S. Xenoliths from the central Sierra Nevada (8.3 Ma Big Creek host lava) are highly enriched in fluid mobile elements (Lee, 2005) with up to 22.6 ppm Li and variably low δ7Li in spinel- (1.4 to 3.8‰) and garnet-bearing peridotites (-2.3 and 0.9‰). These characteristics imply that the lithospheric mantle beneath the Sierra has been altered by hydrous fluids. The light isotopic composition (relative to MORB) suggests domination of sediment-derived component in the metasomatic fluids. Spinel peridotite xenoliths from the eastern Sierra (<1 Ma Oak Creek host lava) are not highly enriched in Li (2.7 to 3.8 ppm) and other fluid mobile elements, consistent with little subduction influence. However, clinopyroxenes in Oak creek xenoliths have exceptionally high Li contents (11 to 24 ppm) and are in elemental disequilibrium with coexisting olivine (Dol/cpx < 0.3). Bulk xenoliths are characterized by light Li isotopic composition (-6.7 and -2.9‰) that possibly reflects isotopic fractionation associated with Li diffusion from the lava into clinopyroxene grains. In the Basin and Range Province, Cima volcanic field mantle xenoliths are not enriched in incompatible elements. Either previous enrichment was stripped away by lithosphere and asthenosphere melting associated with the Basin and Range extension, or the xenoliths sample unmodified asthnospheric mantle. Li, being more compatible, appears to preserve the relatively light isotopic values of -1.5 to 2.6‰. Colorado Plateau xenoliths are characterized by high Th/Nd in addition to a smaller enrichment of fluid mobile elements. Li is variably enriched (2 to 113 ppm) and has normal mantle δ7Li (2.6 to 5‰). The isotopic composition correlates with Th/Nd and is consistent with addition of silicate melt. Other xenolith samples (olivine and pyroxene analyses) across the western US (Dish Hill, Lunar Crater, San Carlos, Vulcan's Throne, and Kilbourne Hole) follow the general trend of increasing δ7Li from east to west. These Li isotope data support a model of hydration of the Sierran lithosphere during the flat subduction of the Farallon plate and melt metasomatism toward the west as proposed by Lee (2005). We have also analyzed olivine separates from Pliocene lamprophyric basalts in the Central Sierra Nevada (from Farmer et al., 2002). They have 10 to 22 ppm Li and δ7Li varies from -11 to 1‰. These light isotopic compositions could be derived from a dehydrated subducted slab with modification by diffusion. Our data support a strongly metasomatized mantle source for the potassic volcanism in the Pliocene which may have been triggered by delamination of the lithosphere. References: Lee, C.-T.A., 2005. J. Geology 113, 673-685. Farmer, G.L., Glazner, A.F., Manley, C.R., 2002. GSA Bulletin 114, 754-768.
V34A-04
Continental mantle in the Rio Grande Rift: Chemistry and Fabric Studies of Xenoliths from Kilbourne Hole, New Mexico
Kilbourne Hole maar, Potrillo volcanic field (PVF), erupted basanite approximately 10 ka ago and thus the melt composition represents essentially the present-day thermal and compositional character of this segment of the continental mantle. Mantle xenoliths are lherzolite (cpx mode 10-25 percent; Cr number of spinel approx. 0.1) and more depleted harzburgite and dunite (cpx mode 1-8 percent; Cr number of spinel 0.2-0.5). Clinopyroxenite (cpx mode 90-95 percent; Mg number olivine 0.8-0.85; Cr number spinel less than 0.01) occurs as veins crosscutting lherzolite. Bulk rock ICPMS analyses reveal that lherzolite is moderately melt-depleted. Harzburgite and dunite have lower abundances of moderately incompatible elements and higher abundances of strongly incompatible elements. Harzburgite and dunite compositions are successfully modeled by infiltration of melts, which have similar composition to PVF basanites. Pyroxenites are modeled as high pressure cumulate of the same basanitic melts. Lherzolite includes both fine-grained equigranular and porphyroclastic textures. Harzburgite and dunite demonstrate coarse-grained porphyroclastic texture. Fine-grained lherzolite has the lowest temperatures (approx. 980 C), porphyroclastic lherzolite intermediate temperature (1020-1044 C), and harzburgite and dunite the highest temperature (1050-1135 C). No petrographic evidence exists for remnant garnet, implying all samples originated in the spinel stability field. Thus, the Moho, known from current geophysical studies to vary from 30 - 35 km, currently experiences temperatures of approx. 1000 C. This temperature, which is shared by other North American Cenozoic xenolith localities, is a substantial deviation from a steady state (i.e. thermal equilibrium) geotherm and requires either advective or convective heat transfer into this portion of the continental mantle. This conclusion is consistent with tomographic images that show western NA, and particularly areas with Cenozoic volcanism, have slow seismic velocities. J-index shows a positive correlation with temperature and varies from 5 to 10 for the low temperature lherzolite to 20 for the harzburgite and dunite. J-index of 20 is an unusually high value for peridotite. Shear wave-splitting studies from LA RISTRA (a 950 km line from west Texas to Colorado) show fast polarization directions subparallel to NA absolute plate motion and no deviation in direction or delay time as the line crosses the Rio Grande rift. We conclude that the high J-index values for the deepest and hottest xenoliths, which also have experienced basanite melt infiltration, represent deformation at the lithosphere-asthenosphere boundary.
V34A-05
Seismic Expression of Flood-Basalt Lithospheric Refertilization
Analysis of S and P receiver functions from the Southern African Seismic experiment (SASE) for lithospheric discontinuities beneath the Kalahari craton reveals an abrupt reduction in seismic velocity (4%) at approximately 150 km depth, which we term the K-discontinuity. This feature is restricted to the northern half of the array, extending from the Zimbabwe craton south to the Thabazimbi-Murchison lineament, and from Botswana in the west to the edge of the Kalahari craton in the east. It thus spans several Archean sutures and is thus unlikely to be related to Archean tectonics. It does, however, appear to be related to subsequent magmatic episodes. In particular, the strongest expression of the anomaly is coincident with the most intense locale of volcanism associated with the Mesozoic Karoo flood basalt (at the intersection of 4 giant dyke swarms), and it extends to the northern edge of the Bushveld intrusion. From mantle xenoliths and xenocrysts, the lithosphere in this region appears to have experienced a long-term infiltration of basaltic melt and metasomatic fluids. We propose that this seismic discontinuity reflects the influence of this melt/metasomatic infiltration, which, over time, has intruded and refertilized the Archean Lithosphere. Based on kimberlites pipes that show obvious signs of melt metasomatism and likely Karoo influence, such as Lesotho, the observed reduction in seismic velocity is plausibly consistent with the observed major-element and trace-element enrichment at about 150 km depth in this kimberlite. We speculate that this depth represents a permeability barrier to the porous flow of basaltic magma and corresponds to the intersection of the volatile-rich basalt liquidus with the lithospheric geotherm.
V34A-06
The Effects of Melt Depletion on the Density Structure of Cratonic Mantle, and its Implication for the Post-Laramide Convective Thinning of the Wyoming Craton
To explain the stability of cratons, Jordan [1979] suggested that cratonic mantle is neutrally buoyant with respect to hotter asthenospheric mantle. This study hypothesized that extraction of partial melt-- which removes the incompatible elements and modifies the mineral proportions from that of fertile peridotite--reduces the density of the rock enough to counteract the effects of temperature. Many studies have elaborated on this phenomenon, and it is still generally considered that cratonic mantle is either neutrally or positively buoyant with respect to hotter advecting fertile mantle. We have evaluated the melting relations of peridotite to quantify the density effects of melt removal, and find that above about 110 km depth no amount of melt depletion can counteract the effects of temperature. This suggests that cratonic mantle from the Moho to ~110 km depth is negatively buoyant. To confirm this observation, we have examined density variations in more than 100 Kaapvaal craton xenoliths using principle component analysis to separate the effects of melt depletion from other geochemical variations such as silica enrichment. Similar to the isobaric melting results, we find a density cross-over at around 110 km. This implies that uppermost cratonic mantle is stabilized by its high viscosity with respect to asthenosphere, and not its density structure. A test of this hypothesis would be to see if reducing the viscosity of this uppermost mantle would cause it to convectively destabilize. This may have happened when Laramide aged subduction brought the Farallon slab to shallow depths beneath much of the western U.S. Hydration from the slab could have reduced the viscosity of the Wyoming craton, causing it to drip away. Noteably, teleseismic P-wave tomography finds a high velocity "drip" under the Wind River Range in western Wyoming. This feature is within a region of maximal Laramide foreshortening which would have promoted nucleation of the instability.
V34A-07
A Mantle Cross-Section Through Western And Central Nevada From Young Basaltic Magmas In The Sierra Nevada And Western Great Basin
The geochemistry of basaltic magmas erupted in the Basin and Range province of the western USA has demonstrated that at least two mantle sources exist, one with a subduction signature and another with an "ocean island basalt" (OIB) signature. Here we investigate the distribution of these two sources during the Pleistocene and Holocene in a 250 km-long transect from the eastern Sierra Nevada near Reno, NV, into central Nevada. Samples were collected from young, dated mafic lava flows from the Carson Range (2.5 to 1.4 Ma), Steamboat Hills (2.6 Ma), Virginia City and Chalk Hills (1.5 to 1.44 Ma), east of Carson City (1.36 Ma), Rattlesnake Hill (1.2 to 0.9 Ma), Buffalo Valley (1.1 to 0.95 Ma), Upsal Hogback (0.6 Ma), and Soda Lake (Holocene). With the exception of Carson Range andesites, all of the lavas are alkaline basalts and basaltic trachyandesites with K2O/Na2O > 0.4. Incompatible element abundances, incompatible element ratios, and radiogenic isotope ratios vary widely between locations. Many key incompatible element ratios, such as Ce/Pb, Sr/P, Ba/Nb, and Nb/La, and isotopic ratios vary as a function of age and longitude. Lavas less than 1 Ma in age have low Ba/Nb, Sr/P, 87Sr/86Sr, 206Pb/204Pb, and high Ce/Pb and Nd/La compared to lavas greater than 1Ma in age. These ratios vary more strongly as a function of longitude, from high Ba/Nb, Sr/P, 206Pb/204Pb, 87Sr/86Sr and low Ce/Pb and Nb/La (subduction signature mantle) lavas in the Sierra Nevada margin to lavas with the opposite characteristics (OIB signature mantle) in central Nevada. La/Sm does not vary with either age or longitude. The relationship with longitude indicates that two mantle sources currently exist beneath western Nevada, subduction-modified mantle to the west and OIB-type mantle to the east, and that these two mantle types probably taper in thickness towards one another. The termination of subduction beneath the Reno area at 5-3 Ma, in conjunction with 87Sr/86Sr greater than modern Cascade arc lavas, suggests that the subduction-modified source resides primarily in the lithospheric mantle and that melting occurs due to extension of the lithosphere. Either an asthenospheric or lithospheric source of OIB-type lavas is possible. The results for this cross-section mirror those reported for a transect across southern and central Nevada, with the exceptions that a) the age range considered here is shorter and closer to the present day, and b) isotopic compositions from Buffalo Valley at the east end of the section do not as closely approach depleted upper mantle values such as those reported for the Lunar Crater volcanic field to the south.
V34A-08
Characteristics of mantle fabrics beneath the southern-central United States: Constraints from shear-wave splitting measurements
New shear-wave splitting measurements at permanent broadband seismic stations in the southern-central United States reveal the direction and strength of mantle fabrics, and provide constraints on models of the formation of these fabrics. For stations on the stable North American craton, correspondence between observed fast directions and the trend of Proterozoic and Paleozoic structures associated with rifts and orogenic belts implies a lithospheric origin of the observed anisotropy. The largest splitting times (up to 1.7 s) are observed at stations located in the ocean-continent transitional zone, in which the fast directions are parallel to the Gulf of Mexico continental margin. The parallelism and the geometry of the keel of the craton beneath the study area suggest that either asthenospheric flow around the keel of the North American craton, lithospheric fabrics developed during Mesozoic rifting, or a combination of the two are responsible for the observed anisotropy above the transitional crust.