Beschreibung: Anatectic granites from the Fosdick migmatite–granite complex yield U–Pb zircon crystallization ages that range from 115 to 100 Ma, with a dominant grouping at 107–100 Ma, which corresponds to the timing of dome formation during the regional oblique extension that facilitated exhumation of the complex. The occurrence of leucosome-bearing normal-sense shear zones in migmatitic gneisses indicates that suprasolidus conditions in the crust continued into the early stages of doming and exhumation of the complex. The structure allows access to variably oriented granites in networks of dykes at deeper structural levels and subhorizontal sheeted granites at shallower structural levels within the complex. This feature allows an evaluation of the mechanisms that modify the composition of granite melts in their source and of granite magmas during their ascent and emplacement using whole-rock major, trace element and Sr and Nd isotope compositions, zircon Hf and O isotope compositions, and phase equilibria modelling of potential source rocks. Geochemical variability within the granites is attributed to source heterogeneity and blending of melts, which themselves are consistent with derivation from regional metasedimentary and metaplutonic source materials. The granites typically contain coarse blocky K-feldspar and/or plagioclase grains within interstitial quartz, and have low Rb/Sr ratios and large positive Eu anomalies. These features are inconsistent with the composition of primary crustal melts derived from metasedimentary and metaplutonic source materials, but consistent with early fractional crystallization of feldspar and subsequent drainage of the fractionated melt. Processes such as peritectic mineral entrainment and accessory mineral dissolution, entrainment and crystallization did not have any significant influence on the major and trace element composition of the granites. The granites in the networks of dykes are interpreted to represent choking of magma transport channels through the middle crust as the rate of magma flow declined during doming and exhumation, whereas the sheeted granites record collapse of subhorizontal, partially crystallized layers of magma by filter pressing and melt exfiltration during vertical shortening associated with doming and exhumation. These processes separated feldspar-rich residues from evolved melt. Based on the results of this study, caution is urged in estimating melt proportion from the volume of granite retained in migmatitic gneiss domes, as the granites may not represent liquid compositions.

Beschreibung: We report geochemical data from (meta-)sedimentary and igneous rocks that crop out in the Ford Ranges of western Marie Byrd Land and discuss the evolution and reworking of the crust in this region during Paleozoic subduction along the former Gondwanan convergent plate margin. Detrital zircon age spectra from the Swanson Formation, a widespread low-grade metaturbidite sequence, define distinct populations in the late Paleoproterozoic, late Mesoproterozoic, and Neoproterozoic–Cambrian. The late Paleoproterozoic group records magmatism derived from a mixed juvenile and crustal source. By contrast, the late Mesoproterozoic group yields Hf isotope values consistent with derivation from a juvenile Mesoproterozoic source inferred to be an unexposed Grenville-age orogenic belt beneath the East Antarctic ice sheet. For the Neoproterozoic–Cambrian population, Hf isotope values indicate reworking of these older materials during Ross–Delamerian orogenesis. New U-Pb ages from the Devonian–Carboniferous Ford Granodiorite suite across the Ford Ranges reveal an extended period of arc magmatism from 375 to 345 Ma. For four younger samples of Ford Granodiorite, Hf and O isotope values in zircon suggest involvement of a larger (meta-)sedimentary component in the petrogenesis than for two older samples. This contrasts with the secular trend toward more juvenile values documented from Silurian to Permian granite suites in the Tasmanides of eastern Australia and Famennian to Tournasian granite suites in New Zealand, pieces of continental crust that were once contiguous with western Marie Byrd Land along the Gondwana margin. The differences may relate to an along-arc change from the typical extensional accretionary mode in eastern Australia to a neutral or an advancing mode in West Antarctica, and to an across-arc difference in distance from the trench between the New Zealand fragments of Zealandia and western Marie Byrd Land. Upper Devonian anatectic granites in the Ford Ranges most likely record reworking of early Ford Granodiorite suite members during arc magmatism.

Beschreibung: In the Wet Mountains of central Colorado, we document evidence for increasing metamorphic grade and associated higher amounts of partial melting along a transect from northwest to southeast. Field observations of structural orientation and style, qualitative assessment of strain intensity, analysis of metamorphic mineral assemblages, and macroscopic identification of leucosomes and migmatites are complemented by the use of melt microstructures to carefully document the presence and locations of former partial melt and to identify melt-producing reactions. In the northwest Wet Mountains, migmatitic foliation is moderately well developed, and partial melting occurred via granite wet melting and muscovite-dehydration melting, with rare melt pseudomorphs remaining. At Dawson Mountain in the central part of the range, inferred former melt channels are preserved along grain and subgrain boundaries, deformation appears more intense, and anatexis occurred through biotite-dehydration melting. Farthest to the south, the highest intensity of strain is inferred, with very closely spaced foliations, abundant dynamic recrystallization, and local mylonitization occurring in rocks of granitic composition, and partial melting occurring via granite wet melting. Metapelitic rocks experienced biotite-dehydration melting and contain garnet with Mn-rich rims and Mn-poor cores mantled by plagioclase, decussate biotite, and quartz, textures indicating back-reaction between melt and garnet. These textures indicate there was abundant melt within these highest-grade rocks and extensive melt-rock interaction. Throughout the Wet Mountains, deformation is concentrated in areas where melt-producing reactions occurred, and melt appears to be localized along deformation-related features, suggesting a correlation between partial melting and deformation. The northern Wet Mountains contain few plutons, whereas the central and southern portions of the Wet Mountains contain more pervasive dikes and sills and may contain more former melt as a result of both higher metamorphic grade and widespread thermal insulation. The Wet Mountains represent an exhumed section of formerly molten middle crust located at the transition between upper and lower crust and provide insight into processes ongoing at depth in modern orogenic belts. The microstructures indicative of former partial melt, textures associated with melt-rock interaction, and melting reactions we have identified in the Wet Mountains will greatly facilitate the recognition of other such exhumed sections.

Beschreibung: Detrital zircon provenance analysis is used to resolve the age of sandstone injectites together with source sandstones that form fault-bounded, tabular bodies within Mesoproterozoic crystalline rocks of the Colorado Front Range. Named Tava sandstone (informal), the unit is a product of liquefaction and remobilization of mature quartz sediment within source bodies having volumes ≥1 x 10 6 m 3 into dikes up to 6 m in width. To surmount the indeterminate age of emplacement, we obtained new U-Pb detrital zircon age data for two source sandstones, three dikes and one sill, for comparison to four Paleozoic arenites. Tava age distributions feature a dominant 1.33–0.97 Ga broad age group and narrow ca. 1.11, 1.44, and 1.70 Ga groups, with several smaller age groups 〉1.5 Ga. The Tava detrital zircon results are dissimilar to Paleozoic sandstones but closely resemble published detrital zircon reference data for Grenville orogen–derived siliciclastic units of the western United States. The similarity in age distributions is borne out by statistical comparisons among Tava sandstone, Paleozoic samples, and Neoproterozoic strata that reveal a high probability of correlation of Tava sandstone to ca. 800–680 Ma strata deposited during intracontinental extension. We conclude that Tava sandstone is Neoproterozoic in age and provides a new avenue to investigation of Rodinia’s terrestrial paleoenvironment.

Beschreibung: The ability of models to elucidate climate and ice sheet dynamics at the Eocene-Oligocene climate transition (34 Ma) is limited by a reliance on present-day topography as a boundary condition. We present a reconstruction of the Antarctic palaeotopography at the E-O boundary that restores sediment eroded from the continent. Estimates of sediment volume surrounding Antarctica constrain our restoration. Using data from coring and seismic imaging and allowing for a moderate biogenic fraction, weathering reactions and sediment porosity, a source volume of 5-13 million cubic km is thought to have been removed from an area of ca. 13 million square km. Changes to the East Antarctic landscape by local, regional and continental-scale ice have been estimated using an ice sheet and erosion model. Material is restored in response to basal conditions under a range of modelled ice-sheet configurations. These models can restore 3-4 million cubic km to East Antarctica. In West Antarctica, factors including the variable position of the grounding line make it impractical to use quantitative erosion models. Here we link geological evidence for known or suspected remnants of Eocene topography with our understanding of processes and patterns of erosion and deposition to drive construction of potential surfaces. There are several options for geologically reasonable surfaces that imply 5-10 million cubic km of eroded volume. The uncertainty in eroded volume is muted by the transformation to palaeo-elevation because isostatic compensation generally limits the change in average regional elevation to 15-20% of the thickness eroded.

Beschreibung: ANTscape is a project of the Antarctic Climate Evolution (ACE) Research Program to develop a series of maps to show changes in Antarctic paleotopography over the last ~100 million years. The reconstructions will provide a base for summarising a range of paleoenvironmental data, and for use as inputs for the next generation of ice sheet-ice shelf models. The present-day bedrock topography from the SCAR BEDMAP project will be used as a starting point for reconstructing past paleotopography, moving to BEDMAP 2 when it becomes available. Six maps, one for each significant climatic regime or shift, are planned: 4, 14, 34, 50, 70 and 92 Ma. Work is well advanced on the map for 34 Ma (Wilson and Luyendyk, 2009, Geophysical Research Letters). This is a time that is far enough back for there to be a significantly different topography, but not so far back that reconstruction is seriously unconstrained. It is also of great interest to paleoclimatologists as the largely ice-free landscape on which the first continental ice-sheet formed. The maps prepared by ANTscape will depend not only on restoration of Antarctic continental geography by reversing tectonic movements and elevation changes, but also the restoration of sediment eroded from the continent and deposited around and beyond the Antarctic margin. This will require modeling changes to the Antarctic landscape from erosion (Jamieson et al., 2010, Earth & Planetary Science Letters) and estimates of sediment volumes through the Circum-Antarctic Stratigraphy and Paleobathymetry Project (CASP). For further information see www.ANTscape.aq

Beschreibung: ANTscape is an ACE project to develop over the next three years a series of maps to show the changes in Antarctic paleotopography over the last ~100 million years. The reconstructions will provide a base for summarising a range of paleoenvironmental data, and be useful both as inputs for the next generation of ice sheet-ice shelf models, and for credible and realistic visualization of past landscapes to promote wider appreciation of past changes in the Antarctic environment. The first meeting of the group in April 2009 in Leeds agreed that for younger periods (Cenozoic) the present-day bedrock topography from the SCAR BEDMAP project would be a useful starting point for reconstructing past paleotopography, moving to BEDMAP 2 when it became available. However for older periods researchers would have to draw more on current knowledge of plate movements, tectonic deformation, thermal evolution and personal geological experience. Because of the scarcity of geological data, it was recognised that the reconstructions would entail considerable geological interpretation. However it was acknowledged that even poorly constrained reconstructions would be a significant improvement on the current practice of using present day topography for models of past ice sheets, when we know past topography was different.The following six time slices, each representing a significant climatic regime or shift, were proposed for a map: 4, 14, 34, 50, 70 and 92 Ma, with work beginning first on a map for 34 Ma. This is a time that is far enough back for there to be a significantly different topography, but not so far back that reconstruction is seriously unconstrained. It is also of great interest to paleoclimatologists as the largely ice-free landscape on which the first continental ice-sheet formed. The group leader for this time slice is Doug Wilson. The group decided the maps could most conveniently be developed by considering the Antarctic as comprising three large regions: 1) West Antarctica: Marie Byrd Land, Antarctic Peninsula, Ellsworth Mountains, West Antarctic rift system, Weddell Sea and Ross Sea, 2) East Antarctica including Transantarctic Mountains, and 3) the Antarctic margin, comprising the continental shelf and slope as far as the continent- ocean transition. A number of procedural issues are under discussion by ANTscape members, and input is sought from ACE 2009 participants in order that the issues be resolved in 2009. These include the primary geospatial tools to be used, spatial resolution of the primary product, the organizational scheme for gridding the data, and data/document storage and access. For the moment a report on the Leeds workshop along with abstracts and many of the presentations given there can be found at http://groups.google.com/group/antscape?hl=en The maps prepared by ANTscape will depend not only restoration of Antarctic continental geography by reversing tectonic movements and elevation changes, but also the restoration of sediment eroded from the continent and deposited around and beyond the Antarctic margin. This will require close collaboration with ROSSmap and the Circum-Antarctic Stratigraphy and Paleobaythmetry Project (CASP).