V23C-1548
He diffusion and (U-Th)/He thermochronometry of rutile
Rutile is a primary accessory mineral in many HT and HP metamorphic rocks (e.g., blueschist, eclogite, and granulite) and some plutonic rocks. In this study, we present new experimental data constraining the He diffusion kinetics of rutile. We have undertaken He diffusion experiments on compositionally well-characterized rutile from a variety of metamorphic and plutonic environments. The majority of rutile He diffusion experiments show well- behaved Arrhenius behavior characterized by activation energies of 48-51 kcal/mol and log(D0/a2) of 6.5-7.1 s-1, translating into closure temperature estimates of ~220-235°C (-dT/dt = 10°C/myr). However, several rutile diffusion experiments have shown significant deviation from linear Arrhenius behavior either during early portions of the heating schedule or during higher temperature heating steps above 500°C. Previous studies attribute this low diffusivity during early heating steps to the presence of small near-surface He diffusion domains (e.g, defects, cracks, etc.). Higher temperature deviation from well-behaved volume diffusion is irreversible and we are investigating whether this phenomenon is related to phase instability under in-vacuo heating conditions (low fO{2}). Furthermore, we are also studying potential microstructural effects, such as ilmentite exsolution lamellae and the influence of major (e.g. Sn substitution) and minor (e.g., Nb, Cr, Zr and U) element composition on diffusion kinetics. In order to demonstrate the accuracy of rutile (U-Th)/He dating, we have dated fast cooled rutile from mantle and lower-crustal xenoliths from Tertiary volcanic rocks from several locations in the western United States. These rutile (U-Th)/He ages tend to be in excellent agreement with both 40Ar/39Ar and zircon (U-Th)/He data. Very low U contents in eclogitic rutile, however, commonly require TIMS or MC-ICP-MS analysis to obtain single-grain age precision comparable to zircon (U-Th)/He dating (<10%). In conclusion, rutile is characterized by distinct He diffusion kinetics and a closure temperature of ~220-235°C. Rutile (U-Th)/He dating, therefore, offers the possibility of quantifying an important portion of the thermal history in metamorphic rocks that is often difficult to constrain, if carried out in a well- characterized petrological context.
V23C-1549
Assessment of the rutile (U-Th)/He thermochronometry on the KTB drill hole, Germany
(U-Th)/He dating of rutile, a common accessory mineral in HT and HP metamorphic rocks, such as eclogite, blueschist, and granulite, has to the potential to constrain cooling histories of rocks that have traditionally been problematic to date using established radiometric techniques. This study presents new rutile (U-Th)/He age and diffusion data from the German Continental Deep Drilling Project (KTB) to quantify the position of the rutile He partial retention zone (HePRZ) and thermal history of the KTB borehole. The down-hole rutile (U-Th)/He age profile allows for the empirical determination of He diffusion kinetics in rutile over geological time scales and validation of laboratory-derived He diffusion kinetics. Initial step-heating experiments of rutile have established a closure temperature of ~220-235°C. The KTB borehole is ideally suited to perform in-situ calibrations of rutile as a (U-Th)/He thermochronometer due to its rutile-bearing lithologies, depth and temperature range (up to 260°C), and previous fission track, (U-Th)/He, and 40Ar/39Ar thermochronometry studies, establishing a detailed thermal history for the KTB and adjacent vicinity. The 9 km deep KTB drill hole penetrates a series of tectonically-stacked fault blocks of metapelitic paragneisses and amphibolites along the western margin of the Bohemian Massif. Similar to published titanite (U-Th)/He data, rutile and zircon (U-Th)/He ages systematically vary from ~72 Ma to 0 Ma below the base of the partial retention zones. Low parent nuclide concentrations in rutile (<1 ppm) from KTB amphibolites require TIMS or MC-ICP analysis to obtain reliable age data. In addition, we are experimentally quantifying He diffusion kinetics on selected samples from the KTB drill hole. However, low U and Th concentrations and subsequent 4He ingrowth limit 4He diffusion experiments and require 3He diffusion experiments on proton-irradiated samples. This approach also allows quantification of diffusion kinetics on totally reset samples below the rutile HePRZ. The down-hole rutile (U-Th)/He age profile compliments the laboratory determined He diffusion kinetics of rutile by providing a natural quantification of He-diffusion over geologic time scales. Our new zircon and rutile (U-Th)/He analyses complement preexisting thermochronometric data and further constrain the thermal history of the KTB drill hole and surrounding Variscan tectonic zones.
V23C-1550
Thermal Modeling as a Test for Internal Consistency Between Thermo- and Geochronology and Thermobarometry: Application to the Acadian Orogeny in SE Vermont.
We develop a one-dimensional thermal model integrating U-Pb geochronology, 40Ar/39Ar thermochronology, and thermobarometry to test tectonic models for internal consistency in a classical region of the northern Appalachians. This model is based on the finite-difference method utilizing an implicit formulation with steady state topography (i.e. sedimentation keeps pace with subsidence and erosion keeps pace with uplift). The Chester and Athens domes of Vermont are Acadian structures cored by Proterozoic gneisses, flanked by Cambrian and Ordovician metamorphic rocks, and unconformably overlain by Silurian-Devonian metasedimentary rocks. The polymetamorphic history of these rocks includes the Grenville (>1 Ga), the Taconic (470-440 Ma), and the Acadian (410-370 Ma) orogenies. 40Ar/39Ar cooling ages from core rocks define continuous late Devonian through Carboniferous cooling at ~3.5 oC per m.y. Multiple U-Pb SHRIMP analyses of detrital zircons from Devonian Gile Mtn. fm. quartzites yield an age of 409 ± 5 Ma, interpreted as a maximum age of deposition, thus fixing the earliest time at which tectonic loading rates (>2 mm/yr) can commence. Grt-Ms-Bt-Pl and Amp-Pl thermobarometry from core rocks yields temperatures of 650- 700 oC and pressures of 13-16 kbars defining a geothermal gradient of 12-13 oC per km. In the first model, loading-unloading schedules consistent with detrital zircon ages and 40Ar/39Ar data input for a one-dimensional model simulate the Acadian orogeny. Results of this model fail to reach temperature estimates by ~20% and pressure estimates by ~50%. A second model accommodating the Taconic and Acadian orogenies does reach maximum temperatures of ~650 oC but still fails to reach pressure estimates by ~50%. Multiple iterations of the second model using a wide variation in geophysical parameters suggest minimum pressure estimates can not be reached given the limited time between deposition of cover rocks (~410 Ma) and the end of metamorphism (40Ar/39Ar cooling age of amphibole ~374 Ma). Due to similar time constraints imposed on the Taconic orogeny (e.g. Karabinos et al., 1998, Laird et al., 1984), we conclude that P-T estimates obtained in core rocks of the Chester dome are likely inherited from the Grenville orogeny.
V23C-1551
LA-ICP-MS dating of Archean Metamorphic Zircons From the Kapuskasing Structural Zone, Ontario: Evidence for the Geodynamic Origin and Evolution of the Abitibi–Opatica Terrane
The Late Archean Abitibi greenstone belt, in SE Superior Province, is one of the largest contiguous greenstone belts on Earth. Geochronological data from upper crustal rocks indicate it was formed by volcanism and plutonism that spanned 2760 Ma through 2660 Ma. The classical model for the Abitibi belt is one of tectonic accretion and thrust stacking of magmatic arcs, and possibly plateau fragments, resulting in a thick crust, with a granulitic base. A Proterozoic uplift, the Kapuskasing structural zone, on the western side of the Abitibi belt, is believed to represent the lower to middle crust of the Abitibi. Previous U-Pb zircon dates from the Kapuskasing uplift correspond to a period of granitic plutonism at the end of the Abitibi magmatic history (ca. 2660 Ma), consistent with underthrusting and underplating, during and following tectonic collision. An alternative model for the Abitibi belt is that it represents part of a single, large oceanic plateau, modified by subduction and slab-window magmatism. The Abitibi greenstone belt and the Opatica gneiss belt, to the north, would then represent a single tectonic terrane, called the Abitibi–Opatica terrane. That model requires the existence of a thick crust prior to tectonic accretion, rather than thickening of crust as the result of collision. The plateau model can be tested by dating metamorphic minerals from lower to middle crustal rocks in the Kapuskasing uplift. We have used laser ablation (LA) -ICP-MS, to carry out "in-situ" dating of zircons from a mafic clinopyroxene- garnet granulite and from two hornblende-biotite-garnet metatonalites. Zircons in the rocks are less than 100 microns in size, mostly 40 microns or less. BSE and CL images of the zircons reveal that some are unzoned, some are patchwork zoned, and others are sector zoned. The crystals are devoid of oscillatory zoning and their morphologies are mainly anhedral to subhedral; they are interpreted to be of metamorphic in origin. That interpretation is supported by REE elements patterns that show characteristic HREE depletion, suggesting equilibrium of zircons with garnet. Metamorphic zircons dated 3190 Ma, 2850 Ma, and 2800 Ma, all record pre-Abitibi (pre-2760 Ma) metamorphic events. Thus the amphibolite- to granulite-grade rocks of the Kapuskasing zone contain components of an older high-grade terrane. Younger zircon dates record thermal events in the lower to middle crust that accompanied three known Abitibi magmatic events, including tonalite (2748 Ma), granodiorite (2700 – 2690 Ma), syenite and sanukitoid (2685 – 2675 Ma), and granite (2660 Ma) magmatism. It is interpreted that the metamorphic zircons formed as the result of thermal events that accompanied the magmatic events. The results show that in-situ analyses of zircons, using LA-ICP-MS, is a powerful method for detailed investigations of metamorphic histories. The results also indicate the existence of high-grade metamorphic rocks in a crust 20 km thick (or more), that recorded thermal events before, as well as during, collisional tectonics, supporting the oceanic plateau model for the Abitibi–Opatica terrane. Finally, the results also indicate that the Abitibi–Opatica oceanic plateau was initiated during rifting of an older continental margin, from which the pre- 2760 Ma zircon components were derived.
V23C-1552
Unraveling Ages of Discrete Cryptic Events During Polyphase Orogenesis in SE Brazil
The Neoproterozoic assembly of cratons in eastern Brazil is recorded by the successive Brasília, Ribeira and Buzios subduction-to-collision orogenic events in the intervals ca. 660-605 Ma (Reno et al., Frontiers in Mineral Sciences, abstract, 2007), ca. 580-550 Ma (Heilbron & Machado, Precambrian Res., 2003) and ca. 525- 505 Ma (Schmitt et al., Precambrian Res., 2004), respectively. The southern Brasília Belt displays characteristics of a passive margin in the east and an accretionary orogen in the west prior to terminal collision and final emplacement of the ENE-verging nappes. In the southern Brasília Belt, peak temperatures of 800-950 C are recorded, whereas in the Ribeira and Buzios Belts peak temperatures register around 780 C. Cryptic thermal overprinting by the younger events on the older orogenic belts is demonstrated by monazite age data from these Brasiliano-age belts in southeast Brazil. In the southern-most portion of the southern Brasília Belt, a SSE-plunging mineral elongation lineation and the occurrence of sillimanite represent macroscopic structural and petrologic evidence, respectively, of tectonic overprinting imposed during the Ribeira Orogeny. Comprehensive dating of monazite using the electron probe microanalysis method from throughout the southern-most Brasília Belt indicates widespread Ribeira-age (580-530 Ma) monazite growth, even tens of km north of the sillimanite-out isograd. The Buzios Belt lies outboard of the Ribera Belt; monazite from Ribera Belt migmatitic metapelites records Buzios-age growth. Monazite dates from the Ribeira belt determined using the electron probe microanalysis method were culled by chemical zone within individual grains. Exploratory data analysis (see Reno et al., Fall AGU, 2007) reveals distinct Ribeira-age (553-533 Ma) cores and Buzios-age (530-505 Ma) rims in monazite from rocks with no petrologic evidence of overprinting. Monazite grains likely grew during the Ribeira Orogeny, and were either partially recrystallized by dissolution-reprecipitation during overprinting by the Buzios Orogeny, or new monazite overgrew existing grains. In both the southern Brasília Belt and the Ribera Belt, monazite geochronology provides a record of cryptic overprinting by younger thermal events on older metamorphic terranes; this overprinting is neither recorded by the more robust U-Pb system in zircon, nor, in general is there a macroscopic record of new rock- forming mineral growth.
V23C-1553
Data Analysis and Statistical Methods for the Assessment and Interpretation of Geochronologic Data
Ages are traditionally reported as a weighted mean with an uncertainty based on least squares analysis of analytical error on individual dates. This method does not take into account geological uncertainties, and cannot accommodate asymmetries in the data. In most instances, this method will understate uncertainty on a given age, which may lead to over interpretation of age data. Geologic uncertainty is difficult to quantify, but is typically greater than analytical uncertainty. These factors make traditional statistical approaches inadequate to fully evaluate geochronologic data. We propose a protocol to assess populations within multi-event datasets and to calculate age and uncertainty from each population of dates interpreted to represent a single geologic event using robust and resistant statistical methods. To assess whether populations thought to represent different events are statistically separate exploratory data analysis is undertaken using a box plot, where the range of the data is represented by a ‘box' of length given by the interquartile range, divided at the median of the data, with ‘whiskers' that extend to the furthest datapoint that lies within 1.5 times the interquartile range beyond the box. If the boxes representing the populations do not overlap, they are interpreted to represent statistically different sets of dates. Ages are calculated from statistically distinct populations using a robust tool such as the tanh method of Kelsey et al. (2003, CMP, 146, 326-340), which is insensitive to any assumptions about the underlying probability distribution from which the data are drawn. Therefore, this method takes into account the full range of data, and is not drastically affected by outliers. The interquartile range of each population of dates (the interquartile range) gives a first pass at expressing uncertainty, which accommodates asymmetry in the dataset; outliers have a minor affect on the uncertainty. To better quantify the uncertainty, a resistant tool that is insensitive to local misbehavior of data is preferred, such as the normalized median absolute deviations proposed by Powell et al. (2002, Chem Geol, 185, 191-204). We illustrate the method using a dataset of 152 monazite dates determined using EPMA chemical data from a single sample from the Neoproterozoic Brasília Belt, Brazil. Results are compared with ages and uncertainties calculated using traditional methods to demonstrate the differences. The dataset was manually culled into three populations representing discrete compositional domains within chemically-zoned monazite grains. The weighted mean ages and least squares uncertainties for these populations are 633±6 (2σ) Ma for a core domain, 614±5 (2σ) Ma for an intermediate domain and 595±6 (2σ) Ma for a rim domain. Probability distribution plots indicate asymmetric distributions of all populations, which cannot be accounted for with traditional statistical tools. These three domains record distinct ages outside the interquartile range for each population of dates, with the core domain lying in the subrange 642-624 Ma, the intermediate domain 617-609 Ma and the rim domain 606-589 Ma. The tanh estimator yields ages of 631±7 (2σ) for the core domain, 616±7 (2σ) for the intermediate domain and 601±8 (2σ) for the rim domain. Whereas the uncertainties derived using a resistant statistical tool are larger than those derived from traditional statistical tools, the method yields more realistic uncertainties that better address the spread in the dataset and account for asymmetry in the data.
V23C-1554
Unraveling Polyphase High-Grade Metamorphism and Anatexis in the Fosdick Migmatite Dome, West Antarctica, Using Mineral Equilibria Modeling and In Situ Monazite Geochronology
In situ monazite geochronology coupled with mineral equilibria modeling can provide a direct link between microstructure, quantitative pressure-temperature estimates, and age. Application of pseudosection modeling and in situ monazite geochronology using the electron probe microanalyzer provide robust constraints on the polymetamorphic history of the Fosdick migmatite dome in Marie Byrd Land, West Antarctica. High-grade metamorphism and melting leading to dome formation have been attributed to the transition from convergence to extension along the proto-Pacific margin of East Gondwana at 125-95 Ma. However, new geochronological data suggest that rocks within the migmatite dome experienced a polymetamorphic evolution with evidence for high temperature metamorphism and anatexis during the Devonian-Carboniferous and in the Cretaceous. Metasedimentary units in the dome comprise quartz-plagioclase-ilmenite-biotite ± sillimanite ± garnet ± cordierite ± K-feldspar ± magnetite. Microstructural relationships suggest that garnet is produced from a melt-producing biotite breakdown reaction in the presence of sillimanite. Monazite included within the garnet porphyroblasts yields ages ranging from ca. 368 Ma to ca. 348 Ma, interpreted as the timing of prograde to peak metamorphism. Forward mineral equilibria modeling using appropriate protolith compositions constrain peak conditions to 800-860°C and 7-10 kbar during this Devonian-Carboniferous metamorphism. This event corresponds to a well-established period of arc magmatism along the active margin of East Gondwana. Cordierite associated with a later event replaces garnet, or replaces biotite-sillimanite in garnet-absent domains. Monazite associated with cordierite yields ages of ca. 150 Ma. Mineral equilibria modeling using residual paragneiss compositions constrain peak conditions to 820-870°C and 6.5-7.5 kbar during this younger metamorphic event. The growth of cordierite at ca. 150 Ma occurred just prior to and concurrent with Cretaceous arc magmatism in contiguous New Zealand-Marie Byrd Land, and likely records heating of the middle crust at this time. Monazite rim analyses yield an age of ca. 115 Ma, which corresponds to the onset of transtension, granitic magmatism, voluminous anatexis and formation of the Fosdick migmatite dome.
V23C-1555
Preliminary Monazite U-Th-Total Pb Absolute Age Constraints on Crustal Thickening and Siluro-Devonian Dextral Transpression: Central Appalachian Piedmont, SE Pennsylvania
High grade metamorphic rocks in the Central Appalachian piedmont of SE Pa. occur in at least three distinct lithotectonic belts that are bounded by steeply dipping, NE-striking transcurrent or transpressional shear zones. From NW to SE these belts consist of rocks of Laurentian-, uncertain-, and arc-affinity. The central belt is characterized by basement gneiss-cored nappes and associated shallowly to moderately SE dipping S2 foliation. This belt followed a clockwise metamorphic P-T path; maximum temperatures (T ~ 640 °C at P ~ 0.7 GPa) are associated with syn- to post-S2 garnet growth. An increase in pressure of ~ 0.1 GPa is inferred from post- kinematic high-Ca garnet rims. This paper reports preliminary EPMA results on monazite from two samples, WG- 216 and U-154, from the Glenarm Wissahickon, the metasedimentary cover sequence in the central belt. A few monazite grains from WG-216 exhibit small low-Th cores which give latest Cambrian ages (492 +/- 10 Ma, n = 10; n = # of analyses, all precision is reported as 2 s.e.). All grains in WG-216 contain cores with irregular, patchy zoning that yield late Ordovician ages (454 +/- 6 Ma, n = 31). High-Th, low-Y rims, which yield early Devonian ages (410 +/- 11 Ma, n = 12), are present on monazite inclusions in the outer portions of garnet and staurolite, but are absent in monazite inclusions within microlithons characterized by S1 foliation. High-Th, low-Y rims grow along 001 cleavage in S2 foliation-forming micas, so S2 can be no younger than monazite rims. Similar EPMA monazite results were obtained from U-154, also from the Glenarm Wissahickon. At least one Ordovician core is present (455 +/- 4 Ma, n = 8); pre-S2 matrix monazite and one inclusion in staurolite yield late Silurian ages (422 +/- 4 Ma, n = 29); and one inclusion within outermost garnet yields a Devonian age (415 +/- 8 Ma, n = 8). These results constrain the timing of S2 formation and peak metamorphism in the central belt to the early Devonian. The SE-belt is characterized by NW-dipping foliation that predates moderate pressure Devonian-aged metamorphism (T ~ 600 °C at P ~ 0.8 GPa). The right-lateral transpressive Rosemont shear zone separates the central- and SE-belts. Monazite results reported here and previous work in the SE-belt suggest that crustal thickening associated with latest Silurian through early Devonian oblique convergence buried rocks on both sides of the shear zone resulting in the moderate pressure metamorphism and opposing vergence observed at the present level of exposure.
V23C-1556
P-T Evolution and Ages of Granulite Facies Rocks from the Winding Stair Gap, Eastern Blue Ridge
The Winding Stair Gap, NW of Franklin, North Carolina, exposes schists, gneisses, granofelses, stromatic metatexites, and minor diatexites representing mostly pelitic and semipelitic rocks affected by granulite facies conditions. Characteristic mineral assemblages in mesosomes and melanosomes include: garnet (Gt) + biotite (Bt) + sillimanite (Sill) + plagioclase (Plg) + quartz (Qz), Gt+ hornblende (Hb) + Bt + Plg + Qz, and orthopyroxene (Opx) + Bt + Qz. Leucosomes are predominated by Plg + Qz, and contain Gt, kyanite (Ky), Sill, and retrograde muscovite (Musc). Moecher et al. (2004) concluded that these rocks followed a counterclockwise P-T path with peak conditions at 850-900°C, 8-9 kbar. However, the occurrence of relict Ky and staurolite overgrown by Gt and Sill, rutile included in Gt or rimmed by ilmenite, and texturally early Ky in leucosomes relative to intergranular fibrolite, all suggest that these rocks followed a clockwise P-T path in which dehydration melting of muscovite began in the stability field of Ky. Rinds of Plg + Qz + Musc separating Gt from Bt, and Bt inclusions in Opx suggest the onset of dehydration melting of Bt. Calculations using THERMOCALC and compositions of Gt and Bt separated by Qz or Plg (XAn = 0.35) suggest equilibration at 750 to 830°C, 7 to 9 kbar. This was followed by a stage of near isobaric cooling during which K- feldspar back reacted to Musc, and Sill + Bt replaced Gt. Two monazite crystals from a Bt + Gt + Sill + Qz + Plg schist, display patchy zoning for Th, Y, U, and Ca. Statistical analysis of microprobe age profiles across these grains yields three age populations of 353 ± 11 Ma, 450 ± 8 Ma, and 488 ± 8 Ma, with the oldest ages restricted to the grain interiors. A third monazite associated with Sill and Bt truncating Gt is almost homogeneous, and yields ages of ~348 ± 8 Ma. These data suggest that peak T metamorphism and partial melting in the eastern Blue Ridge was Taconic (~450 Ma) during which monazite crystals that record an earlier (~490 Ma) event, were partially resorbed and overgrown by new rims. The younger ages of ~350 Ma reflect monazite growth during near isobaric cooling probably effected by some melt extraction.
V23C-1557
A Reconnaissance Study of Rutile from the Barberton Greenstone Belt, South Africa: Implications for a Possible new Archean Impact Layer
The 3.2 to 3.5 Ga Barberton greenstone belt (BGB) is composed of a remarkably well-preserved assemblage of volcanic and sedimentary rocks deposited over a 300-myr period. Rutile is a common minor phase produced by diagenesis and or metamorphism of the ultramafic and mafic volcanic rocks of the BGB. We examined rutile from 6 units in the greenstone belt to determine their potential as either petrogenetic or geochronologic indicators. In all but one unit the rutile occurs as relatively fine-grained acicular clusters with high SiO2 (0.5 wt%) and low Zr (< 0.03 wt%). They have low concentrations of Pb, dominated by Pb 204, which do not yield a meaningful age determination. This rutile appears to have formed under diagenetic or lower greenschist facies conditions in lava flows or primary pyroclastic deposits of basaltic and komatiitic composition. The one sample where meaningful geochronologic information was obtained is a tuffaceous sandstone in the 3.3 Ga Mendon Formation that we informally call the "Green Sand" unit. This 3-4m thick sandstone lies about 1m above a previously unrecognized impact layer and it may represent detrital material derived from the impact crater or its subsequent uplift. This unit contains a remarkably diverse population of zircons, many of them rounded detrital grains, ranging in age from 3.3 to over 3.7 Ga. Rutile within this unit display ages that range from 3.2 to 3.4 Ga and have compositions that also distinguish them from those found in other BGB units. They are large yellow-orange to red single grains with distinctive polysynthetic twinning. They have high Zr, up to 9000 ppm, suggestive of formation temperatures above 1000C -- well above temperatures associated with known metamorphism of the BGB or surrounding granitoids, and possibly another indication of an impact origin. The rutile has high Cr concentrations, indicative of derivation from mafic and ultramafic protoliths. Zr/Nb ratios correlate positively with Pb 207/206 measurement error. Zr/Nb ratios of <1 yield triple digit age errors (up to 300 Ma), while ratios of >1 yielded single and low double digit errors. We believe the Zr/Nb ratio could be one criterion for identifying rutile suitable for Pb 207/206 geochronology. Development of a geochronometer that could be used with mafic and ultramafic rocks would be invaluable to greenstone belt studies.
V23C-1558
Constraints on Mesozoic and Tertiary Brittle Faulting in the Southern East Gobi Fault Zone, Southeastern Mongolia
The East Gobi Fault Zone (EGFZ) is a NE trending structural corridor in southeastern Mongolia. Following the Paleozoic amalgamation of Central Asia from numerous arc terranes and microcontinents, the EGFZ records a protracted history of Mesozoic-Cenozoic intracontinental deformation and basin development. Several workers have proposed a common Mesozoic and/or Cenozoic strike-slip history for the EGFZ and the Altyn Tagh fault; however this correlation has remained largely untested due to a lack of field-based studies in southeastern Mongolia. This study presents the preliminary results of ongoing work that examines the structural history of the EGFZ. Analysis of fault-slip data collected during field studies in the southern EGFZ identified multiple generations of brittle faults. Faults were broken down into subsets on the basis of crosscutting/overprinting and kinematic relationships. Subsets I and II were the oldest identified but presently their relative timing is unknown. Both subsets crosscut Upper Permian to Lower Triassic(?) strata. Faults in subset I are locally crosscut/overprinted by normal faults inferred to have originated during previously documented Late Jurassic-Early Cretaceous NW-SE extension. (1) Subset I mainly consists of NE-ENE striking sinistral faults and NW-WNW striking dextral faults. Inversion of this subset suggests it was produced during regional N-S directed shortening and E-W directed extension. (2) Subset II contains SSW dipping thrust faults with a top-to-the-NE sense of shear, consistent with regional NE-SW directed shortening. (3) Subset III consists of NE striking dextral faults, NW striking sinistral faults, and E or W dipping thrust faults. Faults within this subset were observed crosscutting Lower Cretaceous strata and overprinting normal faults inferred to be related to Late Jurassic-Early Cretaceous extension. Inversion of this subset indicates that it is associated with E-W directed shortening. (4) Subset IV consists of N-NE striking sinistral faults and W-NW striking dextral faults. Faults within this subset were observed crosscutting Lower Cretaceous strata. Inversion of this subset indicates formation during regional N-S directed shortening and E-W directed extension. Subset I confirms the presence of Mesozoic NE-striking sinistral faults in the southern EGFZ. Future work will explore any links between this subset and early Mesozoic sinistral shear documented in the northern EGFZ. The timing, orientation, and shear-sense of the faults within subset II are similar to pre-extensional fabrics observed nearby at the Yagan-Onch Hayrhan metamorphic core complex, suggesting this deformation may be related to the emplacement of large carbonate klippe in the region. Future work will investigate any correlations between this phase of deformation and Middle-Late Jurassic thrusting documented in the Noyon Uul and Beishan regions. Subset III is interpreted as the southern continuation of deformation associated with middle-Late Cretaceous transpression in the EGFZ. Subset IV represents the continuation of the Tertiary NE-striking sinistral faults previously documented in the northern EGFZ. This extends the trace of Tertiary sinistral faulting in the EGFZ to the southern border of Mongolia, in close proximity to the Alxa fault zone.
V23C-1559
Insights Into the Evolution of the SW Laurentian Margin: Geochronological Constraints From the Paleoproterozoic Farmington Canyon Complex, Utah
The Farmington Canyon Complex (FCC) of northeastern Utah is located near the southwestern margin of the Archean Wyoming Province. It constitutes the largest exposure of Early Paleoproterozoic crust along the southwestern Laurentian margin and is comprised of quartzofeldspathic gneisses and migmatites intercalated with amphibolites and metasedimentary rocks. Previous work, based partly on Archean Sm-Nd model ages, suggested an Archean age for the FCC. While new whole rock common Pb data confirm the predominantly Archean source of the metasedimentary rocks, the presence of ca. 2.4 Ga detrital zircons constrains the depositional age of the protoliths to the Early Paleoproterozoic. Zircons from an orthogneiss yield a U-Pb crystallization age of ~2.44 Ga and suggest an age of metamorphism at ~1.8 Ga. These data suggest the rocks exposed in the Farmington Canyon Complex comprise a Paleoproterozoic rift assemblage that probably formed on extended Archean crust along the margin of the Wyoming Province. Igneous rocks produced during this extension were overlain by detritus predominantly, although not solely derived, from the Wyoming Province. The igneous rocks and metasedimentary protoliths were then deformed together during the ~1.8 Ga metamorphic event resulting from accretion of Proterozoic and Archean blocks now to the west and southwest. By constraining the primary depositional and crystallization ages of these rocks in conjunction with their metamorphic and cooling histories, comparisons can be made with the tectonic histories of other Paleoproterozoic belts along the southwestern margin of Laurentia, like the Great Falls Tectonic Zone. These constraints are necessary to test models of possible Neoproterozoic conjugates, and to establish the piercing points critical to a more accurate reconstruction of Neoproterozoic global paleogeography, and the supercontinent Rodinia.
V23C-1560
Thermochronology of early Mesozoic shear in the East Gobi Fault Zone, Mongolia
Tectonic studies of the East Gobi Fault Zone in SE Mongolia reveal distinct intracontinental deformation events postdating Late Paleozoic arc accretion and continental amalgamation. Significantly, metamorphic tectonites of the Tsagan Subarga and Tavan Har blocks, previously mapped as Precambrian basement, comprise a shear zone dominated by steeply-dipping foliations and shallowly-plunging stretching lineations. Kinematic indicators such as sigma and delta-type objects, asymmetric boudinage, and S-C fabrics indicate sinistral shear sense. 40Ar/39Ar step-heating analyses of minerals from pre-, syn-, and late- to post-kinematic lithologies bracket the timing of ductile deformation as Middle to Late Triassic. The NE-trending shear zone has been documented over 250 km along strike and has been reactivated by at least three subsequent brittle deformation events in the Mesozoic and Cenozoic. The oldest 40Ar/39Ar age from the metamorphic rocks, 240.3 +/- 2.2 Ma (plateau ages and 2 sigma errors are reported), was obtained on amphibole from a migmatite and provides a minimum age for a partial melting event prior to activation of the shear zone. The timing of sinistral shear at amphibolite-facies conditions is constrained at ca. 225 Ma by 40Ar/39Ar dating of amphibole that defines the stretching lineation in amphibolite-facies gneiss (224.5 +/- 1.5 Ma), syn-kinematic white mica in garnet-grade metapelite (226.5 +/- 1.4 Ma), and amphibole from a syn-kinematic intrusion (223.3 +/- 1.7 Ma). Waning of ductile deformation, at least locally, is constrained by a 211.8 +/- 1.1 Ma 40Ar/39Ar age obtained on biotite from a dike that cross-cuts the foliation. While the dike appears undeformed in outcrop, quartz and feldspar microstructures indicate minor deformation at the microscopic scale. Cooling below ~300 deg C was achieved by the Early Jurassic. All apparent age spectra are complicated by argon loss profiles that, in most cases, based on geologic context and microstructural evidence can be related to Mesozoic and Cenozoic brittle deformation; minimum ages range from 191 to 47 Ma.
V23C-1561
Monazite geochronology of the Bodonch metamorphic complex, southwestern Mongolia
The Bodonch metamorphic complex in southwestern Mongolia is a steeply dipping assemblage of low to high grade metamorphic rocks exposed along a ~30 km stretch of the Bodonch River which has been variably regarded as either a microcontinental block, due to presence of gneissic foliation; a metamorphic core complex; or an accretionary complex in the Central Asian Orogenic System. Metamorphic grade generally increases from subgreenschist facies at the southern margin of the complex to amphibolite facies in the central section, and decreases again to subgreenschist facies further to the north. Foliation is approximately E-W striking and predominately dips northward, although isoclinal folding is pervasive at all scales. Lithologically, the complex consists primarily of metapelites, with minor interbedded metavolcanics and metacherts. Carbonates and quartzites are generally absent and are observed only in thin depositional lenses. The entire complex is intruded by granitic dikes ranging from ~1 m in thickness to several tens of meters which postdate the metamorphism. The most striking feature of the Bodonch metamorphic complex is the occurrence of garnet bearing schist in the central section which contains zones of almost pure garnet mineralization. Along its southern margin, the assemblage is thrust over a sequence of unmetamorphosed to low grade Devonian-Carboniferous volcanics and sediments which are classified as belonging to an oceanic island arc terrane. Based on the observed lithologies, lack of crystalline basement, and style of deformation we favor the accretionary complex model for the tectonic origin of the Bodonch metamorphic complex. We present new Th-Pb ages of monazite inclusions in garnets obtained using in situ ion-microprobe techniques, which are interpreted to correspond to the timing of accretion related metamorphism.
V23C-1562
Early Mesozoic overprinting of Paleozoic protoliths during shear zone formation in the southeast Gobi, Mongolia
The geology of southeastern Mongolia records continental growth via accretion of Paleozoic volcanic arcs and possibly fragments of continental crust. Locally, geologic maps imply major outcrops of Precambrian basement within the southern and southeast Gobi, the basis of the hypothesized South Gobi Microcontinent. However, results of tectonic studies of the East Gobi Fault Zone (EGFZ) indicate that the presence of bonafide Precambrian basement is suspect. We present field, structural and petrographic evidence that the Precambrian basement rocks mapped in the Tsagan Subarga and Tavan Har blocks of the EGFZ instead comprise Paleozoic sedimentary and volcanic protoliths overprinted by early Mesozoic metamorphism and deformation. NW-SE transects across the structural grain of the EGFZ were completed at Tsagan Subarga and Tavan Har. The Tsagan Subarga transect reveals a progressive increase in metamorphic grade from incipiently metamorphosed Carboniferous sedimentary and volcanic rocks in the north into greenschist-facies and amphibolite-facies metasedimentary and metavolcanic lithologies to the south. Synkinematic intrusives were observed in southernmost Tsagan Subarga. Despite the fact that relationships are complicated due to subsequent brittle deformation events that include Cenozoic strike-slip faulting, the lithologies, ductile deformation and metamorphism observed at Tsagan Subarga appear to correlate with that observed at Tavan Har. We present comparative analyses of mineral assemblages observed in thin sections of the metamorphic tectonites at Tsagan Subarga and Tavan Har with those of the Paleozoic protoliths. Microstructural analyses investigate the relationship between metamorphism and deformation. The results of this research bear on the tectonic evolution of Asia as well as mechanisms related to formation, growth and subsequent modification of continental crust.
V23C-1563
Implications for Mesozoic to Modern Tectonic Deformation in Sub-basins Along the East Gobi Fault Zone, Southern Mongolia
This study presents preliminary results of ongoing sedimentary basin analysis for late Mesozoic rift sub-basins (Unegt and Zuunbayan) along the East Gobi Fault Zone (EGFZ), southeastern Mongolia. A better understanding of deformational events recorded in the pre-, syn-, and post-rift sedimentary units of these sub-basins is required to place the Mesozoic and Cenozoic (the focus of this study) histories of the region within the larger context of Central Asian tectonics. Fault generation maps produced from proprietary 2-D seismic reflection surveys, coupled with recent field studies, suggest localized partitioning of deformation throughout the basins with time, often focusing along basin-bounding faults. Reinterpretation of available seismic data highlights areas north (e.g., Har Hotol locality) of the main North Zuunbayan strike-slip fault (the NZB fault), which show a strikingly different deformational character than those south of the NZB fault (e.g., Tavan Har locality). The presence of preexisting thrust-faults in Paleozoic basement blocks beneath the Unegt sub-basin suggests that basement architecture may be the main control in regional Mesozoic-Recent deformation as well as rift basin architecture. For example, the Unegt sub-basin is broad and relatively shallow, while the Zuunbayan is half as wide but considerably deeper along the zones of greatest subsidence. North of the main NZB fault, deformation above the mid-Cretaceous unconformity is apparently focused along a NE-trending, basin-bounding thrust-fault that places Paleozoic basement on top of Upper Cretaceous post-rift strata and potentially Tertiary units.. In contrast, south of the main NZB fault, fault activity above the mid-Cretaceous unconformity is more common and is suggested here to represent the main zone of deformation following Late Cretaceous sediment deposition. Furthermore, Paleozoic basement features may influence and localize continued (modern) deformation along the main axis of the EGFZ. Ongoing deformation in the EGFZ is evidenced by recent fault scarps that cut and deform Upper Cretaceous strata of the Bayanshire Formation, the overlying Tertiary (?) strata, as well as modern alluvium. Recent fault activity was identified in this study at multiple localities along > 200 km of the EGFZ Many of these structures have been correlated to subsurface faults that offset the mid-Cretaceous unconformity in the seismic data. Further study of such young features is crucial for making accurate estimates of total Mesozoic and Cenozoic sinistral off- set along the EGFZ.
V23C-1564
Magmatism in the Tsagaandelger, Eastern Mongolian Volcanic belt: Petrological, Geochemical and Isotopic Constraints on Mesozoic Geodynamic Setting
Mesozoic alkaline to calc-alkaline igneous suites have been wide spread in Central and Eastern Mongolia. The Mesozoic igneous suites passes upwards from alkaline series trachytic rocks and overlain by tuffaceous sediments. Those are intruded by calc alkaline leucocratic granite and latter covered by Late Mesozoic calc alkaline bimodal volcanic rocks. Rb-Sr mineral isochron age dating result shows that alkaline series volcanic sequences were erupted in Early-Middle Triassic with 241 Ma, composed of trachydacite, trachyrhyolite, trachyandesite and tuff. They are characterized by relatively homogeneous high Al2O3 (up to 17.51%), LILE, LREE enrichment and significant Nb-Ta depletion, as well as slightly enriched Nd and weakly enriched Sr isotopic ratios (initial 87Sr86Sr=0.70481 to 0.70628 and \varepsilonNd(t)=+0.74 to +4.06). Whole- rock Rb-Sr isochron age dating result for granitoids is 231 Ma, composed of monzogranite, granodiorite and aplite. They are high-K series calc-alkaline and I-type. Granites show enrichment in LILE and LREE with slight negative Eu anomalies and depletion in Ba, Nb, Sr, Ti and HREE, whereas strongly enriched Sr and weakly enriched Nd isotopic ratios (initial 87Sr86Sr =0.710582±0.002733 and \varepsilonNd( t)=+0.77 to +2.64). Further Cretaceous volcanic sequences are consisting of plagioclase-olivine-phyric basalt, aphiric olivine basalt, basaltic andesite and rhyolite. Those are high-K series, calc-alkaline and have lower contents of LILE and higher contents of HFS and REE, comparing with Triassic volcanic sequences. Arc fingerprints of Early Cretaceous are disappearing gradually comparing with Triassic strong depletion, might suggesting that subduction effect is gradually decreasing. They are characterized by little depleted to slightly enriched Nd and weakly enriched Sr isotopic ratios (initial 87Sr86Sr =0.704785 to 0.711808 and \varepsilonNd(t)=-0.70 to +1.56). Geochemical characteristics and evolved affinities of volcanic rocks imply that beneath the Tsagaandelger area there existed many crustal magma reservoirs throughout the collision episodes and active continental margin settings were proposed for the origin of Triassic volcanic and plutonic rocks, and subduction associated post-collisional extension is proposed for the origin of the Cretaceous volcanic rocks. Thus, the compression regime with Triassic volcanic and plutonic rocks should be related with the westward closure of Mongol-Okhotsk Sea, amalgamation of North China-Mongolian block and Siberian plate, and the following extensional regime could be started with Cretaceous bimodal volcanism.