V34C-01
Ductile Zircon Dating of Lithosphere Flow
Ductile flow fabrics in the deep crust are an ubiquitous and valuable record of strain that has often been difficult to correlate with surface structures and other geodynamic records due to an inability to directly measure fabric age. We have combined in situ and single-zircon U-Pb isotope, micro-XRD, EBSD and colour SEM-CL analyses to directly date a crystal-plastic deformation fabric in Archean xenolith samples of the African Moho at 2023+/-15 million years, indicating deep level flow coeval with rebound of the 2020+/-3 Ma giant Vredefort impact crater. The deformed zircons are comparable to those in tectonic settings and are restricted to the main mylonitic fabric whereas undeformed zircons in the same sample occur in the cores of garnet porphyroclasts. LREE enrichment of the deformed and isotopically disturbed ductile zircon zones suggests participation of fluid in the up to 100 percent evacuation of radiogenic Pb from the zircon lattice. Our study demonstrates a new method for strain chronometry of planetary materials while offering a first view into post-impact processes at the unexposed crust- mantle transition of continental lithosphere.
V34C-02
High-Precision Dating of Migmatites in an Exhumed Continental Arc
Migmatites play an important role in the evolution of mountain systems by inducing rheological contrasts and focusing strain. To understand the role of partially molten crust in tectonometamorphic processes, it is important to determine how much of the crust was partially molten for how long, and to link the conditions, timing, and consequences of partial melting to tectonic processes at different crustal levels during construction and collapse of orogens. The Skagit Gneiss, in the high-grade core of the North Cascade range of Washington and BC, contains abundant migmatites generated in a continental magmatic arc. In one outcrop near the E margin of the Skagit Gneiss, leucosomes occur with a variety of textures (e.g., layer-parallel/discordant; fine- grained/pegmatitic). Zircons from different leucosomes were analyzed using the chemical abrasion (CA-TIMS) technique to determine migmatite crystallization ages. Zircon dates range from ca. 53 Ma for a deformed, cross- cutting pegmatitic leucosome that envelops boudins of garnet-bearing metagraywacke, to 47 Ma for a strongly lineated biotite gneiss. A ~ 6 cm thick layer-parallel, coarse-grained leucosome was dated between the pegmatite and biotite gneiss at ca. 51 Ma. Other leucosomes in different textural and structural settings of the Skagit Gneiss yield older zircon crystallization ages (ca. 69-60 Ma), suggesting at least two pulses of migmatite crystallization during the evolution of the magmatic arc. The ca. 50 Ma ages are similar to Ar cooling ages from much of the Skagit Gneiss and to the timing of Eocene extensional basin formation and volcanic activity in the region. Furthermore, the ca. 53-47 Ma dates are at the young end of the age spectrum for the North Cascades and indicate that magmatism, metamorphism, and partial melting were active until the latest stages of orogeny.
V34C-03
Adding t to garnet P-T paths using the Lu-Hf method: Age constraints on thrusting and exhumation events in the hinterland of the Sevier orogen
In orogenic belts, garnets in metamorphic rocks grow in response to changes in pressure and temperature (P-T) conditions that result from burial and exhumation. Thermodynamic modeling may be used to extract P-T paths from garnet growth zoning provided temperature thresholds for cation diffusion are not exceeded and the garnets grew in equilibrium. Garnet dating by the Lu-Hf method provides a powerful method to date garnet growth and thus date tectonic events. Here we apply these techniques to constrain the tectonic history of the hinterland of the Sevier orogen. Previous work from the Albion Range, Idaho, has determined a P-T path from the schist of Mahogany Peaks consisting of an isothermal pressure increase, followed by a pressure decrease during a temperature increase, followed by second isothermal pressure increase. We interpret the isothermal pressure increases to be the result of tectonic burial by thrusting along the Basin-Elba fault and the exhumation event to be the result of combined extensional and erosional exhumation. To place age constraints on these events we have determined a garnet Lu-Hf age for the same sample on which we determined a P-T path: 150 ± 12 Ma (2σ; MSWD = 9.1) based on 3 garnet fractions and a whole rock. The same unit 50 km away in the western Raft River Mountains in Utah yielded an identical but much more precise age of 149.1 ± 1.2 Ma (2σ; MSWD = 1.1), also based on three garnet fractions and a whole rock. Both determinations constrain a Late Jurassic age for these events over a broad area. This is considerably older than the 105 ± 12 (2σ) Ma age recently determined for the oldest pervasive fabric (D1) in the core complex by dating strain fringes using the Ar-Ar laserprobe method, which indicates that development of the fabric occurred much later than major thrusting, consistent with a recent interpretation of this fabric developing by orogen-parallel gravitationally induced extension. The Lu-Hf age constraints provide the first clear evidence for major Late Jurassic thrust burial in the hinterland of the Sevier belt. This helps resolve important questions including the timing of hinterland to foreland thrust progression, and the presence and location of hinterland thrust loads necessary to explain otherwise enigmatic Late Jurassic subsidence histories in foreland sediments.
V34C-04
Evidence from U-Pb geochronology, mineral rare-earth element chemistry and trace element thermometry for short-lived ultrahigh-temperature metamorphism in the Anápolis-Itau\c{c}u Complex, Southern Brasília Belt
Integrated petrologic, chemical, and geochronologic studies of zircon, garnet, and rutile from ultrahigh- temperature (UHT) granulites in the Anápolis-Itau\c{c}u Complex, central Brazil, are used to constrain the significance of U-Pb zircon dates. REE analysis of zircon cores indicate positive-sloping HREE patterns, commonly inferred to be magmatic, whereas unambiguous metamorphic grains and overgrowths have flat to slightly negatively-sloping HREE patterns. DREE(zrc/grt) partition coefficients in two samples investigated indicate equilibrium of zircon with cores of garnet, and progressive enrichment of garnet in HREE towards the rim. Ti-in-zircon crystallization temperatures are consistent with growth during both the prograde and post-peak evolution, and not at UHT conditions. Maximum Zr-in-rutile crystallization temperatures of inclusions armored in garnet are around 980°C, indicating growth of this phase near peak conditions indicated by independent calculated phase equilibria. Other populations of rutile linked to reaction microstructures developed during retrograde metamorphism record crystallization temperatures of 850-820°C. Rutile that is intergrown with ilmenite included within orthopyroxene and associated with exsolved zircon records temperatures around 760°C, indicating equilibrium with exsolved zircon. ID-TIMS U-Pb geochronological data from two of the four UHT granulite samples investigated define upper intercept ages of 641.3 ± 8.4 Ma (MSWD = 0.91) and 638.8 ± 2.5 Ma (MSWD = 1.03) that correlate with distinct periods of zircon growth near peak to immediately post-peak. Individual zircon U-Pb dates retrieved from all samples range from 649 to 634 Ma, indicating a maximum duration of approximately 15 Myr for the prograde to post-peak evolution of this terrain. This event is interpreted to record modest thickening of a backarc located behind the Arenópolis Arc at the western edge of the São Francisco Craton consequent upon terminal collision of the Parána Block with the arc during the amalgamation of Gondwana.
V34C-05
U/Pb Rutile Dating in Granulite-Facies Rocks by LA-ICP-MS
Rutile is a common accessory phase in several metamorphic rock types (mafic, pelitic and felsic protoliths; common at blueschist-, eclogite- and granulite-facies conditions) as well as one of the most stable minerals in sedimentary environments. Although it is virtually omnipresent, it has yet not attracted the attention in geochronology that it could have, mostly due to a frustrating high variability in U concentrations (<0.01 to >100 ppm). Especially rutiles in eclogites (where rutile is one of the few potentially datable minerals) are notoriously low in U. To overcome these obstacles, we have chosen a simple, pragmatic approach, namely scanning a thin section for high-U rutiles by short (10 sec) analyses of trace element concentrations (besides U also V, Cr, Zr, Nb, Sn, Hf, Ta, W, Pb and Th) with a 10 μm laser spot (New Wave 213 nm Nd:YAG laser) coupled with an Argilent 7500ce ICP-MS. The programmable stage control allows easy reanalysis of the highest U rutiles for U/Pb dating (conducted with a 40-90 μm laser spot for 30 sec). A high degree of accuracy is reached by standardizing against a natural homogeneous rutile with high U concentrations (40 ppm) and a TIMS age of 1095 Ma. Results of this study show that U concentrations are roughly positively correlated with Zr (as a proxy for temperature; Zack et al., 2004), potentially due to the fact that increasing temperature makes one phase less favourable for element incorporation over another (e.g. monazite and zircon). In general, granulite-facies rocks appear to be good samples for U/Pb rutile dating as they commonly contain rutiles with >20 ppm U. From an analytical point of view, rutile is a particularly attractive phase for laser ablation ICP-MS dating for two reasons: 1) it forms often large (as compared to e.g. zircon), homogeneous single grains of >200 microns, therefore allowing larger spots sizes and hence better precision and 2) it mostly contains virtually no Th (Th/U ratios commonly <0.001), hence allowing for common lead correction via 208Pb. The second point is especially relevant as 208Pb is ca. 40 times more abundant than 204Pb and has no interferences with isotopes in the argon gas (e.g. 204Hg). With the analytical strategies outlined above, U/Pb rutile analysis offers new aspects of dating granulites. Single grain analysis of regular thin sections allows controlled dating both texturally and chemically. We are therefore confident that we can address the discrepancy of closure temperature estimates between nature (ca 450oC; e.g. Mezger et al. 1989) and experiment (ca 600oC; Cherniak 2000) by examining age differences of inclusion vs matrix rutiles (as for Zr in rutile; Zack et al. 2004) in various granulites.
V34C-06
Constraints on the Extent of Phanerozoic Sedimentary Sequences in the Western Canadian Shield From Radiation Damage Control on (U-Th)/He Apatite Dates
Dynamic topography and plate margin tectonism, in combination with global sea level rise, have been invoked to explain the distribution of Phanerozoic sedimentary sequences extending deep into the cratonic interior of North America. This interpretation suggests a more dynamic history of vertical motions than once believed in regions traditionally considered "stable". Constraints on the timing and magnitude of continental interior flooding are required to resolve low amplitude episodes of burial and unroofing linked to such epeirogenic events. (U-Th)/He apatite thermochronometry provides a means to establish the thermal imprint of strata for which geological evidence has been erased by subsequent unroofing. Recognition of radiation damage control on apatite He retentivity now allows additional insights into thermal histories by investigation of apatites with a range of closure temperatures from the same crustal level in these low relief cratonic landscapes. The East Lake Athabasca region in northern Saskatchewan, now devoid of Phanerozoic cover, is located within the cratonic western Canadian Shield. Previously published (U-Th)/He apatite dates from this area range from 0.95 to 0.55 Ga. Low [eU] apatites were targeted for additional analysis following recognition of the radiation damage effect on apatite dates. The new dates are Phanerozoic in age and significantly younger than previously published Proterozoic dates for moderate to high [eU] apatites from the same area (and even the same outcrop). The distributions of dates correlated with [eU] can be explained by application of the radiation damage trapping model, and using a thermal history characterized by Phanerozoic reburial of this region by > 1 km of strata that have since been denuded. Apatite fission-track dates for the same samples are Proterozoic regardless of [eU], and similarly appear to require a Phanerozoic phase of unroofing. Together these results point toward significant deposition of Phanerozoic strata substantially further eastward in the Canadian cratonic interior than previously recognized, most likely during inundation of the continent during the Devonian or Cretaceous sea level high- stands.
V34C-07
Th-U-total Pb Geochronology of Authigenic Monazite Near the top of the Sturtian-Marinoan Interglacial, Adelaide Rift Complex, South Australia
The Adelaide Rift Complex in South Australia contains the type sections for Sturtian and Marinoan glacial deposits. The litho- and chemostratigraphy of these deposits play a central role in evaluating global Neoproterozoic ice age hypotheses ("snowball Earth") and Rodinia supercontinent reconstructions, but precise ages on igneous units do not yet exist. We report preliminary results of in situ Th-U-total Pb electron microprobe dating of monazite in sandstones within the Holowilena Ironstone ("older" Sturtian glacial at Enorama Creek) and at the top of the Enorama Shale (youngest pre-Marinoan, interglacial clastics at Elatina Creek). Several distinct populations are recognized. First, rounded cores with high Th, U, and Y + HREE abundances are interpreted as igneous or metamorphic detrital grains and yield ca. 1590 Ma, ca. 1280-1300 Ma, and ca. 1040 Ma dates related to well-known orogenic events in surrounding cratonic regions. A second group also occurs as "cores" but contains significantly lower U and Y + HREE, characteristics that may be indicative of an authigenic origin. Some rounded domains may represent "recycled" authigenic grains and yield dates of ca. 880 Ma and ca. 760 Ma. However, a subset observed in the Enorama sample occurs as very small (~2 x 10 microns), euhedral lathes that are unlikely to have survived a detrital history and yield a date of 680 +/-23 Ma. The youngest population forms very low Th and U, inclusion-rich overgrowths with ca. 500 Ma dates (Delamerian Orogeny) that probably grew hydrothermally. The recognition of "recycled" authigenic monazite further emphasizes the detail in textural and petrological documentation that is required for accurate geochronological interpretations. The date of 680 +/-23 Ma (1) provides an estimate for the age of the base of the Trezona carbon isotopic anomaly just beneath the Marinoan glacial deposits, (2) provides an absolute minimum age constraint on the underlying Sturtian glacial deposits, and (3) is confirming of proposed correlations between type Marinoan deposits and precisely dated glacial deposits in Namibia and China.
V34C-08
Tracking the Petrogenetic and Rheological Evolution of Continental Lower Crust Through Time With Monazite
Monazite (Mnz) is a common accessory phase in one of Earth's largest exposures of exhumed continental lower crust: the Athabasca granulite terrane, western Canadian Shield. The terrane includes three shear zone-bounded domains in northern Saskatchewan interpreted to have experienced lower crustal residence near the Moho for at least 650 m.y. prior to exhumation. In situ high-resolution X-ray mapping and precise trace element electron probe microanalysis of Mnz reveal fundamental links between Mnz composition, texture, Th-U-total Pb geochronology, and evolution of continental lower crust. Monazite is used to contrast two variations of weak, lower crustal flow in the Neoarchean with a Paleoproterozoic record of partitioned sub-vertical fabric development and strain-hardening. Early, sub-horizontal fabrics in granodioritic orthogneisses contain Ca-rich Grt that preferentially nucleated on the Na-rich, dynamically- recrystallized mantles of calcic Pl-porphyroclasts. Syn-kinematic Mnz domains are depleted in Y, Sm, and Gd relative to early Y-rich domains and are interpreted to directly date the onset of Grt-growth concurrent with solid- state lower crustal flow at ca. 2.6-2.55. Melt-enhanced lower crustal flow is implicated elsewhere in felsic granulites that record peritectic Grt-growth. In general, sealed Mnz inclusions in Grt consist of two domains: high- Th cores depleted in Y and Sm that are linked to crystallization in the presence of melt + Grt at ca. 2.62-2.6 Ga and low-Th rims depleted in Ca and enriched in Eu that are linked to melt-absent growth of Grs-rich Grt and Pl-loss during crustal thickening at ca. 2.58-2.55 Ga. Following a protracted period of isobaric-cooling and strengthening of lower crust, syn-kinematic Mnz domains throughout the terrane constrain the onset of sub-horizontal shortening and intra-continental dextral shear strain at ca. 1.9 Ga, immediately prior to the onset of terrane exhumation. Nearly all Mnz grains analyzed in this study are marked by positive Eu-anomalies relative to CI chondrite (1.2 to 14.5), in stark contrast to virtually all published examples. A direct link is implied between Y, Sm, Eu, and Gd in Mnz and two major phases in continental lower crust: garnet and plagioclase. Eu-anomalies in lower crustal Mnz associated with modally-abundant Grt + Pl appear directly related to depletions of Y, Sm, and Gd during Grt- growth, and loss (or removal) of Pl. These links permit tight constraints on the evolution of continental lower crust at depth, e.g. during melting, crustal flow, isobaric-cooling, and strain-partitioning.