John N. Aleinikoff

Deciphering igneous and metamorphic events in high-grade rocks of the Wilmington Complex, Delaware: Morphology, cathodoluminescence and backscattered electron zoning, and SHRIMP U-Pb geochronology of zircon and monazite

High-grade rocks of the Wilmington Complex, northern Delaware and adjacent Maryland and Pennsylvania, contain morphologically complex zircons that formed through both igneous and metamorphic processes during the development of an island-arc complex and suturing of the arc to Laurentia. The arc complex has been divided into several members, the protoliths of which include both intrusive and extrusive rocks. Metasedimentary rocks are interlayered with the complex and are believed to be the infrastructure upon which the arc was built. In the Wilmington Complex rocks, both igneous and metamorphic zircons occur as elongate and equant forms. Chemical zoning, shown by cathodoluminescence (CL), includes both concentric, oscillatory patterns, indicative of igneous origin, and patchwork and sector patterns, suggestive of metamorphic growth. Metamorphic monazites are chemically homogeneous, or show oscillatory or spotted chemical zoning in backscattered electron images. U-Pb geochronology by sensitive high resolution ion microprobe (SHRIMP) was used to date complexly zoned zircon and monazite. All but one member of the Wilmington Complex crystallized in the Ordovician between ca. 475 and 485 Ma; these rocks were intruded by a suite of gabbro-to-granite plutonic rocks at 434 ± 5 Ma. Detrital zircons in metavolcanic and metasedimentary units were derived predominantly from 0.9 to 1.4 Ga (Grenvillian) basement, presumably of Laurentian origin. Amphibolite to granulite facies metamorphism of the Wilmington Complex, recorded by ages of metamorphic zircon (428 ± 4 and 432 ± 6 Ma) and monazite (429 ± 2 and 426 ± 3 Ma), occurred contemporaneously with emplacement of the younger plutonic rocks. On the basis of varying CL zoning patterns and external morphologies, metamorphic zircons formed by different processes (presumably controlled by rock chemistry) at slightly different times and temperatures during prograde metamorphism. In addition, at least three other thermal episodes are recorded by monazite growth at 447 ± 4, 411 ± 3, and 398 ± 3 Ma.

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.

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 zircon geochronology of the Paleoproterozoic Tagragra de Tata inlier and its Neoproterozoic cover, western Anti-Atlas, Morocco

Abstract New U–Pb zircon data obtained by sensitive high resolution ion micro probe (SHRIMP) from the Tagragra de Tata inlier in the western Anti-Atlas, Morocco establish Paleoproterozoic ages for the basement schists, granites, and metadolerites, and a Neoproterozoic age for an ignimbrite of the Ouarzazate Series in the cover sequence. The age of interbedded felsic metatuff in the metasedimentary and metavolcanic sequence of the basement schists is 2072±8 Ma. This date represents: (1) the first reliable age from the metasedimentary and metavolcanic sequence; (2) the oldest reliable age for the basement of the Anti-Atlas; (3) the first date on the timing of deposition of the sediments on the northern edge of the Paleoproterozoic West African craton; (4) a lower age limit on deformation during the Eburnean orogeny; and (5) the first date obtained from the non-granitic Paleoproterozoic basement of Morocco. Ages of 2046±7 Ma (Targant granite) and 2041±6 Ma (Oudad granite) support earlier interpretations of a Paleoproterozoic Eburnean igneous event in the Anti-Atlas. The granites post-date the Eburnean D1 deformation event in the Paleoproterozoic schist sequence, and place a ∼2046 Ma limit on short-lived Eburnean deformation in the area. Cross-cutting metadolerite is 2040±6 Ma; this is the first date from a metadolerite in the western Anti-Atlas. All of the dolerites in the area post-date emplacement of the two granites and the new age constrains the onset of late- or post-Eburnean extension. Ignimbrite of the Ouarzazate Series, immediately above the Paleoproterozoic basement is 565±7 Ma. This Neoproterozoic age agrees with ages of similar volcanic rocks elsewhere from the Ouarzazate Series. The date also agrees with the ages of associated hypabyssal intrusions, and marks the second and final stage of Pan-African orogenic activity in the western Anti-Atlas.

Origin and paleoclimatic significance of late Quaternary loess in Nebraska: Evidence from stratigraphy, chronology, sedimentology, and geochemistry

Muhs, D. R., Bettis III, E. A., Aleinikoff, J. N., McGeehin, J. P., Beann, J., Skipp, G., Marshall, B. D., Roberts, H. M., Johnson, W. C., Benton, R. (2008). Origin and paleoclimatic significance of late Quaternary loess in Nebraska: Evidence from stratigraphy, chronology, sedimentology, and geochemistry. GSA Bulletin, 120(11-12), 1378-1407.

Evolution of the Early Proterozoic Colorado province: Constraints from U-Pb geochronology

The Colorado province represents an addition of a belt of rocks more than 500 km wide to the southern margin of the Archean Wyoming craton during the Early Proterozoic, between about 1790 and 1660 Ma. Correspondence in ages between metamorphism, deformation, and plutonism; association of volcanic rocks with comagmatic calc-alkalic plutons; and lack of older basement are all consistent with the interpretation that the rocks of the province are products of arc magmatism and cannibalistic sedimentation along a convergent margin at the southern edge of the craton.

Ages and origins of rocks of the Killingworth Dome, south-central Connecticut: implications for the tectonic evolution of southern New England

The Killingworth dome of south-central Connecticut occurs at the southern end of the Bronson Hill belt. It is composed of tonalitic and trondhjemitic orthogneisses (Killingworth complex) and bimodal metavolcanic rocks (Middletown complex) that display calc-alkaline affinities. Orthogneisses of the Killingworth complex (Boulder Lake gneiss, 456 ± 6 Ma; Pond Meadow gneiss, ∼460 Ma) were emplaced at about the same time as eruption and deposition of volcanic-sedimentary rocks of the Middletown complex (Middletown Formation, 449 ± 4 Ma; Higganum gneiss, 459 ± 4 Ma). Hidden Lake gneiss (339 ± 3 Ma) occurs as a pluton in the core of the Killingworth dome, and, on the basis of geochemical and isotopic data, is included in the Killingworth complex. Pb and Nd isotopic data suggest that the Pond Meadow, Boulder Lake, and Hidden Lake gneisses (Killingworth complex) resulted from mixing of Neoproterozoic Gander terrane sources (high 207Pb/204Pb and intermediate εNd) and less radiogenic (low 207Pb/204Pb and low εNd) components, whereas Middletown Formation and Higganum gneiss (Middletown complex) were derived from mixtures of Gander basement and primitive (low 207Pb/204Pb and high εNd) sources. The less radiogenic component for the Killingworth complex is similar in isotopic composition to material from Laurentian (Grenville) crust. However, because published paleomagnetic and paleontologic data indicate that the Gander terrane is peri-Gondwanan in origin, the isotopic signature of Killingworth complex rocks probably was derived from Gander basement that contained detritus from non-Laurentian sources such as Amazonia, Baltica, or Oaxaquia. We suggest that the Killingworth complex formed above an east-dipping subduction zone on the west margin of the Gander terrane, whereas the Middletown complex formed to the east in a back-arc rift environment. Subsequent shortening, associated with the assembly of Pangea in the Carboniferous, resulted in Gander cover terranes over the Avalon terrane in the west; and in the Middletown complex over the Killingworth complex in the east. Despite similarities of emplacement age, structural setting, and geographic continuity of the Killingworth dome with Oliverian domes in central and northern New England, new and published isotopic data suggest that the Killingworth and Middletown complexes were derived from Gander crust, and are not part of the Bronson Hill arc that was derived from Laurentian crust. The trace of the Ordovician Iapetan suture (the Red Indian line) between rocks of Laurentian and Ganderian origin probably extends from Southwestern New Hampshire west of the Pelham dome of northcentral Massachusetts and is coverd by Mesozoic rocks of the Hartford basin.

Late Quaternary loess in northeastern Colorado: Part II—Pb isotopic evidence for the variability of loess sources

A new application of the Pb isotopic tracer technique has been used to determine the relative importance of different silt sources for late Wisconsin loess in the central Great Plains of eastern Colorado. Samples of the Peoria Loess collected throughout the study area contain K-feldspar derived from two isotopically and genetically distinct sources: (1) glaciogenic material from Early and Middle Proterozoic crystalline rocks of the Colorado province, and (2) volcaniclastic material from the Tertiary White River Group exposed on the northern Great Plains. Pb isotopic compositions of K-feldspar in loess from two dated vertical sections (at Beecher Island and Last Chance, Colorado) vary systematically, implying climatic control of source availability. We propose a model whereby relatively cold conditions promoted the advance of Front Range valley glaciers discharging relatively little glaciogenic silt, but strong winds caused eolian erosion of White River Group silt due to a decrease in vegetation cover. During warmer periods, valley glaciers receded and discharged abundant glaciogenic silt, while surfaces underlain by the White River Group were stabilized by vegetation. Isotopic data from eastern Colorado loess sections record two warm-cold-warm cycles during late Wisconsin time between about 21 000 and 11 000 radiocarbon yr B.P., similar to results from other studies in the United States and Greenland.

Petrology and tectonic significance of augen gneiss from a belt of Mississippian granitoids in the Yukon-Tanana terrane, east-central Alaska

An approximately east-west-trending belt of porphyritic peraluminous granitoids, metamorphosed and deformed to augen gneiss, is exposed for 400 km across the Yukon-Tanana terrane. This belt consists of compositionally, texturally, and isotopically similar, concordant intrusions of augen gneiss in the Big Delta, Eagle, and Tanacross quadrangles of east-central Alaska, in the Fiftymile batholith of west-central Yukon Territory, and in two batholiths in the offset part of the Yukon-Tanana terrane northeast of the Tintina fault in southeastern Yukon Territory. Chemical analyses of augen gneiss from widely separated localities within the Big Delta intrusion suggest that the various samples are related by crystal fractionation. Comparison of major- and trace-element data from concordant layers of augen-poor gneiss and aplitic gneiss with similar data from adjacent augen gneiss suggests that the layers represent co-magmatic sills of more highly differentiated filter-pressed melt. Similarities between elemental abundances, particularly those of rare-earth elements, in augen gneiss from the large bodies in the Big Delta, Eagle, and Tanacross quadrangles, Alaska, and southeastern Yukon Territory suggest a common origin for these bodies. The peraluminous composition of the gneiss, its high initial 87 Sr/ 86 Sr ratio and high Th concentration indicate that the augen gneiss protolith contains a large component of crustal material. Geochronologic studies of augen gneiss in east-central Alaska and southeastern Yukon Territory indicate a Mississippian intrusive age and an early Proterozoic age for the crustal component. The augen gneiss bodies were intruded late in a middle Paleozoic volcanic-plutonic episode, and they may represent a deeply eroded continental magmatic arc. Regional amphibolite-facies metamorphism and mylonitization may have occurred late during the tectonic episode that resulted in intrusion of the porphyritic granitoid protoliths.

Geochronology of augen gneiss and related rocks, Yukon-Tanana terrane, east-central Alaska

Using several isotopic techniques, we have determined the ages of selected metamorphic rocks in the Yukon-Tanana terrane (YTT) of east-central Alaska. U-Pb zircon data from an augen gneiss body in the Big Delta quadrangle indicate that the granitoid protolith of the gneiss was intruded 341 ± 3 m.y. ago (lower intercept age). An upper intercept age of 2,136 ± 31 m.y. indicates an inherited early Proterozoic component in these zircons. This inheritance age is substantiated by a Sm-Nd whole-rock model age of 2.09 ± 0.08 b.y. from the Big Delta augen gneiss body. Detrital zircons from quartzitic wall rocks to this body were also derived from an early Proterozoic (∼2.1 to 2.3 b.y. old) crustal source(s). Zircons from three other augen gneisses occurring in an east-west belt which extends into the southern Yukon Territory, Canada, have similar Mississippian and early Proterozoic intercept ages. A Rb-Sr whole-rock isochron from widely separated bodies of augen gneiss has an age of 333 ± 26 m.y. and an initial 87 Sr/ 86 Sr ratio of 0.728 ± 0.002, confirming the Mississippian intrusive age for the protolith. The high initial 87 Sr/ 86 Sr ratio further indicates an old crustal component in these rocks. A Rb-Sr mineral isochron (115 ± 4 m.y.), K-Ar data from hornblende and micas (128 to 107 m.y.), and U-Pb data from sphene (134. m.y.) from augen gneiss and related rocks are similar to many K-Ar ages in this region and confirm the occurrence of an early Cretaceous thermal event. U-Pb ages of zircons from three metavolcanic units in the YTT suggest that extrusion of the protoliths of these rocks occurred 360–380 m.y. ago. Scatter in the data is caused by ubiquitous inheritance and multiple lead-loss events. Significant Devonian-Mississippian igneous activity (380-340 m.y.) took place in the YTT, followed by one, or possibly two, metamorphic episodes (Mississippian? and Cretaceous). As yet, no early Proterozoic source rocks for the Paleozoic magmas have been identified in the YTT, but rocks of similar early Proterozoic ages occur to the east and south in the Northwest Territories and British Columbia, Canada.