C. Mark Fanning

CONTINUATION OF THE MOZAMBIQUE BELT INTO EAST ANTARCTICA : GRENVILLE-AGE METAMORPHISM AND POLYPHASE PAN-AFRICAN HIGH-GRADE EVENTS IN CENTRAL DRONNING MAUD LAND

The about 500 km long coastal stretch of central Dronning Maud Land (DML), East Antarctica, is critical for understanding both Gondwana and Rodinia assembly. In common Gondwana reconstructions central DML lies at the potential southern extension of the Mozambique Belt. We report the first extensive geochronological study of magmatic and metamorphic rocks from the area. These new U‐Pb SHRIMP zircon and Sm‐Nd‐data of rocks sampled during the German international GeoMaud 1995/96 expedition indicate that the oldest rocks in central DML are Mesoproterozoic in age. The crystallization ages of metavolcanic rocks were determined at c. 1130 Ma. Syn‐tectonic granite sheets and plutons give ages of c. 1080 Ma, contemporaneous with metamorphic zircon growth at granulite facies conditions. An anorthosite intrusion and a charnockite are dated at c. 600 Ma. Subsequent metamorphism is recorded for at least two different episodes at c. 570–550 Ma and between 530 to 515 Ma. The latter metamorphic event reached granulite facies and is associated with the syn‐tectonic intrusion of a granodiorite body at Conradgebirge. Initial εNd,t‐values of the U‐Pb dated rocks with crystallization ages around 1.1 Ga range from c. +7 to –4. These values suggest that their magmatic precursors represent variable mixtures of a primitive mantle‐derived and continental crust component generated within a mature island arc. Initial Nd isotope data of Cambrian meta‐igneous rocks are indistinguishable from the Grenville‐age rocks, probably representing partial melts of the Grenville‐age basement. The occurrence of Pan‐African syn‐tectonic granitoids is unique in DML. The structure and shape of this body indicates that the main structural ENE‐WSW trend of the region is Pan‐African in age and not older, as previously assumed. Some major late ductile sinistral shear zones occuring in the study area fit well in the overall sinistral transpressional setting of the Mozambique Belt. Thus, central DML very probably represents the southern continuation of the Mozambique Belt into East Antarctica.

U-Pb SHRIMP ages of Neoproterozoic (Sturtian) glaciogenic Pocatello Formation, southeastern Idaho

Three stratigraphically well defined rocks from the glaciogenic Scout Mountain Member, Neoproterozoic Pocatello Formation, southern Idaho, yielded sensitive, high-resolution ion-microprobe (SHRIMP) U-Pb zircon ages that constrain the age of the upper diamictite and its cap carbonate to between ca. 710 and 667 Ma. (1) Zircons from an epiclastic plagioclase-phyric tuff breccia immediately below glaciogenic Scout Mountain Member diamictite on Oxford Mountain, just north of the Utah border, yield a SHRIMP U-Pb concordia age of 709 ′ 5 Ma. (2) A porphyritic rhyolite clast from the upper Scout Mountain Member diamictite at Portneuf Narrows, south of Pocatello, yields a concordia age of 717 ′ 4 Ma. (3) The simple igneous zircon population from a reworked fallout tuff bed in the uppermost Scout Mountain Member, 20 m above the upper diamictite and its cap carbonate and immediately below a second cap-like carbonate, has a concordia age of 667 ′ 5 Ma. These data support previous interpretations that the Scout Mountain Member glaciation scoured nearby volcanic highlands composed of the bimodal Bannock Volcanic Member and suggest that the volcanism was 717 ′ 4 Ma. This age is close to, but distinctly older than, ca. 685 Ma U-Pb SHRIMP ages from the lithostratigraphically correlative Edwardsburg Formation in central Idaho. These data imply that the major rifting phase in this part of western Laurentia spanned 717-685 Ma rather than 800-750 Ma, as previously suggested. Further, because the Scout Mountain succession has been correlated with the Sturtian glacial phase on the basis of lithostratigraphy plus C and Sr isotope values in the carbonates, these data suggest that the Sturtian glacial epoch may have lasted until 670 Ma.

Extraordinary transport and mixing of sediment across Himalayan central Gondwana during the Cambrian-Ordovician

Detrital zircon samples from Cambrian and Lower to Middle Ordovician strata were taken across and along the strike of the Hima- laya from Pakistan to Bhutan (~2000 km). By sampling rocks from one time interval for nearly the entire length of an orogen, and by covering a range of lithotectonic units, we minimize time as a signifi cant source of vari- ance in detrital age spectra, and thus obtain direct assessment of the spatial variability in sediment provenance. This approach was applied to the Tethyan margin of the Hima- laya, which during the Cambrian occupied a central depositional position between two major mountain belts that formed during the amalgamation of Gondwana, the inter- nal East African orogen and the external Ross-Delamerian orogen of East Gondwana. Detrital age spectra from our Lesser and Tethyan Himalayan samples show that well- mixed sediment was dispersed across at least 2000 km of the northern Indian margin. The detrital zircon age spectra for our samples are consistent with sources for most grains from areas outside the Indian craton that record Pan-African events, such as the Ross- Delamerian orogen; East African orogen, in- cluding the juvenile Arabian-Nubian Shield; and Kuunga-Pinjarra orogen. The great dis- tances of sediment transport and high degree of mixing of detrital zircon ages are extraor- dinary, and they may be attributed to a com- bination of widespread orogenesis associated with the assembly of Gondwana, the equa- torial position of continents, potent chemical weathering, and sediment dispersal across a nonvegetated landscape.

SHRIMP U-Pb geochronology of Neoproterozoic Windermere Supergroup, central Idaho: Implications for rifting of western Laurentia and synchroneity of Sturtian glacial deposits

In central Idaho roof pendants, a northwest-trending belt of metamorphosed strata, correlative with the Windermere Supergroup, links northern and southern segments of the western Laurentia Neoproterozoic rift belt. Nine newly named formations within the Gospel Peaks sequence-A through Gospel Peaks sequence-D record Cryogenian preglacial, rift-glacial, and postglacial events as well as Neoproterozoic III glacial and rift events. The Edwardsburg Formation of Gospel Peaks sequence B includes interfingered bimodal rift-related volcanic and glaciogenic diamictite strata. Zircons from a rhyodacite flow in the lower Edwardsburg Formation and from a rhyolite flow at its top, dated by using the sensitive high-resolution ion microprobe (SHRIMP), yielded a weighted average of 685 ′ 7 Ma and 684 ′ 4 Ma. Reevaluation of geochronology and correlations indicates that Cryogenian rifting may have been (1) protracted between 780 and 685 Ma, (2) diachronous along the Cordillera, and/or (3) stepwise with a Cordilleran-wide event at ca. 685 Ma that initiated the formation of the Cordilleran miogeocline and set its geometry. Reevaluation of the Cryogenian glacial record indicates that (1) two associated ca. 685 Ma glacial intervals in the Edwardsburg Formation correlate with the Rapitan glaciation, (2) the Sturtian snowball Earth event must be reevaluated on the basis of revision of Rapitan glaciation from 750-700 Ma to ca. 685 Ma, and (3) there were older Cryogenian glaciations or Cryogenian glaciations were not globally synchronous. New dates and correlations significantly impact the number and synchroneity of possible snowball Earth events and the paleolatitudes of Cryogenian glaciations. Western Laurentian events at ca. 685 Ma particularily affect Neoproterozoic paleocontinental reconstructions by indicating diachronous and multi step breakup of supercontinent Rodinia.

Relationships between crustal partial melting, plutonism, orogeny, and exhumation: Idaho–Bitterroot batholith

Abstract The northern Idaho–Bitterroot batholith region is an exhumed, mid-crustal, plutonic–metamorphic complex that formed during crustal thickening and subsequent extension in the hinterland of the Cordilleran orogen. The relative timing of metamorphism, partial melting, intrusion, and deformation in this area may provide an analogue for magmatic and deformation processes active at mid-crustal depths in modern orogenic belts. Crustal thickening in this part of the Cordilleran began before 100 Ma, but the early stages of this process are poorly constrained. U–Pb zircon dates indicate that high-grade metamorphism was coincident with intrusion of syntectonic quartz diorite plutons at ca. 75–80 Ma, in the Bitterroot metamorphic core complex, where the deepest crustal levels are exposed. Metamorphic conditions reached ∼0.65–0.75 GPa and ∼600–750 °C in the northeastern part of the core complex. Upper amphibolite facies conditions caused widespread partial melting in quartzofeldspathic gneiss facilitated by muscovite breakdown in underlying semi-pelitic schist. New and previously published U–Pb zircon ages indicate that major partial melting of the lower and middle crust occurred between ca. 65 and 53 Ma, leading to the intrusion of the voluminous “main-phase” granitic plutons as thick (3–4 km) sills. Intrusion was accompanied by renewed upper amphibolite facies metamorphism and partial melting forming migmatites at ∼0.65 GPa pressure. The youngest mid-crustal granitic intrusions are about the same age as initial collapse of the orogen and extension at ca. 52–50 Ma. Extension was accommodated mainly on the Bitterroot mylonite zone that deforms the younger intrusions as well as the older high-grade rocks. Therefore, large-scale partial melting of the middle and lower crust followed crustal thickening by as much as 15–35 Ma, but pre-dated extension and exhumation by only 1–3 Ma. Collapse in this sector of the Cordilleran orogen appears to have been focussed where partial melting and plutonism were most intense and long-lived. Exhumation is revealed by the transition from amphibolite facies mylonitization, to greenschist-facies shearing, to brittle faulting, to inactivity of the shear zone that progressed from shallower crustal levels in the west to deeper crustal levels in the east from ca. 51–38 Ma, based on U–Pb and Ar–Ar results. Alkali-feldspar granites were emplaced during the onset of exhumation, but were intruded only into the shallowest Eocene crustal levels. Their generation may have been linked to decompression of the lithospheric column during crustal thinning.

Geochronology of the northern Idaho batholith and the Bitterroot metamorphic core complex: Magmatism preceding and contemporaneous with extension

Granitic plutonism and extension are broadly contemporaneous in many metamorphic core complexes. However, the relationship between magmatism and extension is rarely unambiguous. The northern Idaho batholith (Idaho-Bitterroot batholith), Montana and Idaho, composes the footwall for most of the Bitterroot metamorphic core complex and thus is an ideal area for assessing the relationships between magmatism and extension. We analyzed zircon from six samples of granitic rock from the Idaho-Bitterroot batholith using the SHRIMP (II) ion microprobe. Three samples of mylonitic granite from the Bear Creek pluton, Lost Horse Canyon, give a weighted mean 206 Pb/ 238 U age of 54.3 ± 0.7 Ma. A protomylonitic granite from the central part of the Bitterroot core complex (also Bear Creek pluton) gives a similar 206 Pb/ 238 U age of 54.6 ± 0.8 Ma. Mylonitic megacrystic granite from Sweathouse Canyon yields an age of 63.6 ± 0.6 Ma. A granite sample from the Lochsa Canyon, in the central Idaho-Bitterroot batholith, gives an age of 56.7 ± 1.0 Ma. Inherited zircon from the granitoids ranges in age from 800 to 1820 Ma, but the majority of grains have formation ages of 1750–1800 Ma. This suggests that Paleoproterozoic crust dominates the source region of the Idaho-Bitterroot batholith. Hornblende 40 Ar- 39 Ar age spectra for mafic dikes intruded during late-stage crystallization of main-phase granite in the central Idaho-Bitterroot batholith suggest crystallization of the main-phase plutons in this area at ca. 57 Ma. New and previously published 40 Ar- 39 Ar and K-Ar apparent ages of biotite and muscovite from the Lochsa River area and the western and central Bitterroot core complex are 50 to 47 Ma. Younger mica ages (46–43 Ma) are restricted to the vicinity of the Bitterroot mylonite zone. These results indicate that the cessation of main-phase magmatism within the Bitterroot metamorphic core complex migrated east with time, and that most of the plutons in the core complex were intruded during the Paleocene and early Eocene. When the regional tectonic setting changed from compression to extension at ca. 50 Ma, the late stages of mid-crustal, peraluminous plutonism appear to have been localized within the Bitterroot core complex. The presence of the youngest mid-crustal plutons in this area may have focused extensional deformation leading to the thick mylonite zone, as a consequence of rheological contrasts with cooler areas to the east and west. A progression of K-Ar and 40 Ar- 39 Ar cooling ages from west to east within the core complex part of the batholith is consistent with top-to-the-east shear indicators in the mylonite zone. Thermochronology indicates that the western part of the Bitterroot metamorphic core complex was below ≈350°C at the same time as the last stage of granite emplacement and metamorphism in the east. Therefore, the transition from mylonitization to brittle deformation to inactivity of the shear zone was progressive from west to east across the core complex from ca. 50 to 44 Ma. These features offer an explanation for the previously enigmatic occurrence of amphibolite facies ductile deformation in the eastern part of the core complex coincident with emplacement of epizonal, alkali-feldspar granite plutons along the western side of the complex.

U–Pb geochronology of zircon and polygenetic titanite from the Glastonbury Complex, Connecticut, USA: an integrated SEM, EMPA, TIMS, and SHRIMP study

Abstract U–Pb ages for zircon and titanite from a granodioritic gneiss in the Glastonbury Complex, Connecticut, have been determined using both isotope dilution thermal ionization mass spectrometry (TIMS) and the sensitive high resolution ion microprobe (SHRIMP). Zircons occur in three morphologic populations: (1) equant to stubby, multifaceted, colorless, (2) prismatic, dark brown, with numerous cracks, and (3) elongate, prismatic, light tan to colorless. Cathodoluminescence (CL) imaging of the three populations shows simple concentric oscillatory zoning. The zircon TIMS age [weighted average of 207Pb/206Pb ages from Group 3 grains—450.5±1.6 Ma (MSWD=1.11)] and SHRIMP age [composite of 206Pb/238 U age data from all three groups—448.2±2.7 Ma (MSWD=1.3)], are interpreted to suggest a relatively simple crystallization history. Titanite from the granodioritic gneiss occurs as both brown and colorless varieties. Scanning electron microscope backscatter (BSE) images of brown grains show multiple cross-cutting oscillatory zones of variable brightness and dark overgrowths. Colorless grains are unzoned or contain subtle wispy or very faint oscillatory zoning. Electron microprobe analysis (EMPA) clearly distinguishes the two populations. Brown grains contain relatively high concentrations of Fe2O3, Ce2O3 (up to ∼1.5 wt.%), Nb2O5, and Zr. Cerium concentration is positively correlated with total REE+Y concentration, which together can exceed 3.5 wt.%. Oscillatory zoning in brown titanite is correlated with variations in REE concentrations. In contrast, colorless titanite (both as discrete grains and overgrowths on brown titanite) contains lower concentrations of Y, REE, Fe2O3, and Zr, but somewhat higher Al2O3 and Nb2O5. Uranium concentrations and Th/U discriminate between brown grains (typically 200–400 ppm U; all analyses but one have Th/U between about 0.8 and 2) and colorless grains (10–60 ppm U; Th/U of 0–0.17). In contrast to the zircon U–Pb age results, SHRIMP U–Pb data from titanite indicate multiple growth episodes. In brown grains, oscillatory zoned cores formed at 443±6 Ma, whereas white (in BSE) cross-cutting zones are 425±9 Ma. Colorless grains and overgrowths on brown grains yield an age of 265±8 Ma (using the Total Pb method) or 265±5 Ma (using the weighted average of the 206Pb/238U ages). However, EMPA chemical data identify zoning that suggests that this colorless titanite may preserve three growth events. Oscillatory zoned portions of brown titanite grains are igneous in origin; white cross-cutting zones probably formed during a previously unrecognized event that caused partial dissolution of earlier titanite and reprecipitation of a slightly younger generation of brown titanite. Colorless titanite replaced and grew over the magmatic titanite during the Permian Alleghanian orogeny. These isotopic data indicate that titanite, like zircon, can contain multiple age components. Coupling SHRIMP microanalysis with EMPA and SEM results on dated zones as presented in this study is an efficient and effective technique to extract additional chronologic data to reveal the complexities of igneous crystallization and metamorphic growth.

U–Pb evidence of ∼1.7 Ga crustal tectonism during the Nimrod Orogeny in the Transantarctic Mountains, Antarctica: implications for Proterozoic plate reconstructions

Abstract The Pacific margin of East Antarctica records a long tectonic history of crustal growth and breakup, culminating in the early Paleozoic Ross Orogeny associated with Gondwanaland amalgamation. Periods of older tectonism have been proposed (e.g. Precambrian Nimrod and Beardmore Orogenies), but the veracity of these events is difficult to document because of poor petrologic preservation, geochronologic uncertainty due to isotopic resetting, and debated geological field relationships. Of these, the Nimrod Orogeny was originally proposed as a period of Neoproterozoic metamorphism and deformation within crystalline basement rocks of the Nimrod Group, based on ∼1000 Ma K–Ar mineral ages. Later structural and thermochronologic study attributed major deformation features in the Nimrod Group to Ross-age basement reactivation. Yet, new SHRIMP ion microprobe U–Pb zircon age data for gneissic and metaigneous rocks of the Nimrod Group indicate a period of deep-crustal metamorphism and magmatism between ∼1730–1720 Ma. Igneous zircons from gneissic Archean protoliths show metamorphic overgrowths of ∼1730–1720 Ma, and an eclogitic block preserved within the gneisses contains zircons yielding an average metamorphic crystallization age of ∼1720 Ma. Deformed granodiorite that intrudes the gneisses and associated metasedimentary rocks yields a concordant zircon crystallization age of ∼1730 Ma. Despite scant petrologic evidence for these metamorphic and igneous events, the zircon ages from these diverse rock types indicate major crustal thickening, possibly due to collision, in the late Paleoproterozoic. We therefore recommend revival of the term Nimrod Orogeny to describe Paleoproterozoic tectonic events in rocks of the East Antarctic shield. Similarities in the ages of igneous and metamorphic events in the Nimrod Group and geologic units elsewhere in present-day East Antarctica, southern Australia and southwestern North America suggest they may have played a role in early supercontinent assembly. In particular, similarity with the Laurentian Mojave province is consistent with Proterozoic plate reconstructions joining ancestral East Antarctica with western Laurentia.

Electron-microprobe dating as a tool for determining the closure of Th-U-Pb systems in migmatitic monazites

Abstract High spatial resolution dating of monazite by the electron-probe microanalyzer (EPMA) enables systematic and detailed studies of small minerals. Like zircon, monazite records the complex history undergone by the host rocks. Recent improvements in the statistical treatment of many in situ data now make it possible to decipher the related thermal events and so obtain reliable and precise ages. Our work shows that a significant number of individual spot analyses is required to reach such precise information (i.e., more than 30.40 data). Using the examples of monazites from three migmatites and one granite, we show how to select the most efficient method of age calculation according to the U and Th geochemistry of the grains, or grain domains, that we are trying to date. Three situations may be met: (1) monazites exhibiting significant Th/U ratio variation, (2) monazites exhibiting a fairly constant Th/U ratio, but significant U + Th heterogeneity, and (3) monazites of constant U and Th concentrations. For the first case, a precise mean age can be calculated using a method of data reduction in the Th/Pb = f(U/Pb) diagram, whereby a precision of ±5−10 Ma (2σ) is commonly achieved. For the second case, an isochron age can be calculated according to the Pb = f(Th*) method, with a common precision of around 20 Ma (2σ), whereas for the third case, a simple weighted average age can be calculated. Using these approaches, coupled with a back-scattered electron image study, we demonstrate that inheritance is probably as common for monazite as for zircon. In addition, the combination of high spatial resolution and precise age determination show the limited extent of Pb diffusion in monazite. Finally, an example from a migmatite from southern French Guiana demonstrates the especially robust behavior of the Th-U-Pb system in monazite. This system remains closed during late migmatization and during the subsequent zircon crystallization and zircon overgrowth of protolith zircons. The monazite yielded exactly the same age as the protolith zircons.

Zircon geochronology of Archaean felsic sequences in the Zimbabwe craton: a revision of greenstone stratigraphy and a model for crustal growth

Abstract U-Pb ion-microprobe (SHRIMP) work on zircon populations from 13 Zimbabwean Archaean felsic rocks are presented and interpreted. Samples were extracted from felsic volcanic sequences from most of the major greenstone belts and represent the first zircon geochronological data from within the greenstone belts themselves. The data demonstrate a Late Archaean volcanicity spanning 250 Ma which began at least 2900 Ma ago and ended at 2650 Ma. The intrusion of extensive granitoid sills of the Chilimanzi suite at c. 2.6 Ga marks the widespread stabilization of the craton. Based on the new zircon data and a re-evaluation of published mapping, a new stratigraphic subdivision is presented for the Late (<2.9 Ga) Archaean of Zimbabwe. A feature of the stratigraphic model is the cyclicity of magmatism which begins with ultramafic-mafic rocks, progresses through felsic volcanism and ends with a granitoid event. These cycles are repeated at least three or four times in the 250 Ma time span. An important characteristic of the felsic volcanic rocks is that the bulk of the material examined contains inherited, xenocrystic zircons whose ages range from 1000 Ma to 20 Ma older than the host rocks. The oldest xenocrystic zircons (c. 3.6 Ga) are restricted to volcanic rocks which erupted through the Tokwe segment; itself the only known > 3.3 Ga fragment of Archaean crust in Zimbabwe. These data suggest that the Early Archaean crust is restricted to the Tokwe segment in the south of the country. Since even the oldest of the felsic volcanics (2.90 Ga, Lower Belingwean) have zircons which are 50 Ma older, it is suggested that remnants of earlier cyclic greenstone-granitoid events must underlie the present craton and that all of the currently exposed greenstone belts of Zimbabwe were developed on sialic crust.