Ga Grains Research Articles

An imbricate midcrustal suture zone: The Mojave-Yavapai Province boundary in Grand Canyon, Arizona

The Paleoproterozoic Mojave and Yavapai crustal provinces in southwestern Laurentia contain evolved and juvenile crust, respectively, but the nature of the province boundary remains uncertain. 1.78–1.35 Ga crystalline basement rocks of the Mojave Province preserve an evolved isotopic signature reflecting an Archean crustal component in several isotopic systems (Nd, Pb, Hf). However, no Archean rocks have been found, and hence the origin and tectonic significance of this Archean component are also unclear. This paper analyzes the U-Pb age and Hf isotopic composition of zircons from both the oldest granodiorite plutons (1.84–1.71 Ga) and the oldest metasedimentary rocks (1.75 Ga Vishnu Schist) across a 180-km-long cross-strike transect in Grand Canyon. This transect crosses the Crystal shear zone, which has been proposed as the location of a suture separating the provinces. Our results show that the characteristically bimodal population of detrital zircons in the Vishnu Schist (2.5 Ga and 1.8 Ga modes) yields mixed ɛHf(t) values, primarily between +5 to –5, that are uniform across the transect. Another new finding is that the 1.84 Ga Elves Chasm pluton, on which the Vishnu Schist was deposited, yields juvenile ɛHf(t) values of +5 to +12 and was not the dominant source for the ca. 1.85 Ga peak in the 1.75 Ga Vishnu Schist. Instead, the Vishnu Schist was derived from an Archean craton mixed with intermediate to evolved 1.85 Ga crust. Metasediments show no evidence in support of the proposed suture. Paradoxically, plutons east and west of the Crystal shear zone do support models for a crustal suture. Plutons east of the Crystal shear zone dated at 1.74–1.71 Ga yield juvenile ɛHf(t) values of +5 to +12 that are characteristic of the Yavapai Province. Plutons west of the Crystal shear zone show juvenile to evolved Paleoproterozoic grains (ɛHf(t) of –5 to +10) as well as xenocrystic Archean and 1.85 Ga grains (ɛHf(t) of –12 to +10). These data support the proposition that the Crystal shear zone marks a sharp boundary between the Mojave and Yavapai crustal provinces. However, the overlapping Vishnu Schist suggests a more complicated crustal architecture. The depositional setting of the Vishnu Schist remains unclear; however, we interpret the ultimate geometry of the transect to reflect an ∼200-km-wide middle-crustal duplex system in which the 1.75 Ga Vishnu Schist was deposited across both Mojave and Yavapai crust. This system was subsequently imbricated in an accretionary complex. The ultimate architecture is of a distributed boundary with slivers of plutons that carry the isotopic signature of their respective provinces imbricated within metasediments.

Detrital zircon geochronology of Neoproterozoic–Lower Cambrian passive-margin strata of the White-Inyo Range, east-central California: Implications for the Mojave–Snow Lake fault hypothesis

Correlation of lithotectonic packages across major transcurrent structures is critical to understanding the tectonic evolution of the North American continental margin. Detrital zircon geochronology of uppermost Proterozoic to Lower Paleozoic miogeoclinal strata from the White-Inyo Mountains permits evaluation of: (1) the age and provenance of these metasediments and (2) a model for truncation of the passive margin along a postulated large-magnitude Cretaceous dextral shear zone, i.e., the Mojave–Snow Lake fault. U-Pb ages of detrital zircons from the Neoproterozoic Wyman Formation, the uppermost Proterozoic Reed Dolomite (Hines Tongue Member), and clastic strata of the Lower Cambrian Deep Springs, Campito (Montenegro Member), Poleta, and Harkless formations reflect ultimate derivation from the adjacent 1.7–pre-1.8 Ga Mojavia terrane and/or 1.7–1.8 Ga Yavapai continental basement, with subsidiary sources in both the ca. 1.4 Ga Yavapai-Mazatzal anorogenic granitoids and the >2.5 Ga North American craton, and a small proportion of 1.0–1.3 Ga grains, most likely reworked from Grenville clastic wedge deposits. Detrital zircon age spectra from the Lower Cambrian Andrews Mountain Member of the Campito Formation are unique in comparison with the remainder of the studied section, containing a major age peak centered at ca. 1.1 Ga and a subsidiary Lower Cambrian age peak, permitting calculation of a ca. 527 ± 12 (2σ) Ma maximum depositional age. These features, in addition to abundant detrital magnetite-ilmenite grains, reflect a distal source for these rocks, most likely from the ca. 1.1 Ga Pikes Peak batholith and/or the Midcontinent rift and ca. 0.53 Ga bimodal intrusions of the Oklahoma-Colorado aulacogen. In terms of zircon age distribution, allochthonous metamorphic pendants in the Snow Lake terrane of the central Sierra Nevada batholith are most similar to those of stratigraphically equivalent units in the Death Valley region and, to lesser degrees, the White-Inyo section, and the Mojave Desert region. Given the similarity and relative proximity of Death Valley facies assemblages to the Snow Lake terrane, we suggest that the latter was not transported northward from the Mojave Desert region and instead represents footwall assemblages of a late Early to early Middle Jurassic low-angle normal fault system, probably along the outer transform-truncated margin of the Last Chance thrust stack. This model implies a few tens of kilometers of offset, in contrast to the hundreds of kilometers required by the Mojave–Snow Lake fault hypothesis.

Age provinces in the Antarctic craton: Evidence from detrital zircons in Permian strata from the Beardmore Glacier region, Antarctica

The Antarctic Gondwana sequence crops out along the Transantarctic Mountains. Permian strata at Mt. Bowers and Mt. Weeks in the central part of the range were mainly derived from the ice-covered East Antarctic craton, for which the geology is inferred from coastal outcrops and the Miller Range in the Transantarctic Mountains. Analysis of detrital zircons from Permian sandstones at those two localities demonstrate a slowly changing zircon age-probability distribution for four samples from the lower part of the sequence, with an abrupt change for the upper Buckley Formation sandstone. The stratigraphically lowest sample, from a unit informally designated the Mt. Bowers sandstone, is dominated by zircons in the age range 610–540Ma, with minor components at ∼1.0Ga and∼2.7Ga. Upward, the Pagoda Formation sandstone is likewise dominated by zircons of 600–550Ma age together with a significant Grenville-age cluster (1.3–1.0Ga) and a minor 1.5–1.4Ga cluster. The Fairchild Formation sandstone has a preponderance of grains in the range 600–530Ma, with a scatter of older ages. Zircon grains in the lower Buckley Formation sandstone are principally in the range 575–520Ma with another cluster at 470–450Ma and a scattering of older ages. Most zircons in the upper Buckley Formation sandstone are Permian in age, but with a very small component in the range 650–475Ma and a trace of older ages. Archean to early Mesoproterozoic zircon grains are few in number and may have been recycled from distal outcrops. The 1.5–1.4Ga grains suggest an unexposed Mesoproterozoic source within the East Antarctic craton, which could have been linked to Laurentian North America. The Grenville-age source is considered to be, similarly, a sub-glacial belt in the interior of East Antarctica. Zircons with a late Neoproterozoic to Ordovician signature (600–460Ma) were derived from a Ross Orogen foreland sequence, and/or an early and inland extension of the orogen itself. Permian zircons in the upper Buckley sandstone reflect the development of a magmatic arc on the Gondwana Pacific margin.

Detrital zircon U-Pb geochronology and Hf isotope geochemistry of Paleozoic and Triassic passive margin strata of western North America

U-Pb geochronologic and Hf isotopic analyses have been conducted on detrital zircons extracted from 36 samples of Neoproterozoic through Triassic passive margin strata from western North America. The data serve as an improved reference for comparison with inboard strata that accumulated on the North American craton and outboard strata belonging to potentially displaced Cordilleran terranes. As expected, this reference documents significant variations in ages and Hf isotope compositions both north-south and also through time. The data also provide insights into the provenance of Cordilleran passive margin strata. During Neoproterozoic, Cambrian, and Early–Middle Devonian time, most grains were shed from relatively local basement rocks and from Mesoproterozoic clastic strata containing 1.2–1.0 Ga grains that originated in the Grenville orogen. This pattern was interrupted during Ordovician time, when much of the Cordilleran margin was blanketed by detritus shed from the northern Canadian Shield. Beginning in Late Devonian time, and continuing through late Paleozoic and Triassic time, most regions were dominated by locally derived detritus (largely recycled from underlying strata), but also received 0.7–0.4 Ga grains that were shed from the Franklinian, Caledonian, Appalachian, and Ouachita-Marathon orogens. This pattern is complicated in southern transects as a result of mid-Paleozoic emplacement of off-shelf assemblages onto the continental margin (e.g., Antler orogeny) and construction of Permo-Triassic magmatic arcs along the margin. Our data also provide a robust record of the crustal evolution of western North America, with significant production of juvenile crust during late Archean (3.0–2.5 Ga) and Paleoproterozoic (1.78–1.6 Ga) time and phases of mainly crustal reworking at 2.0–1.78, 1.5–1.3, 1.2–1.0, and 0.6–0.2 Ga. This history is somewhat different from that of other continents, with western Laurentia comprising a greater overall proportion of juvenile crust, punctuated by greater degrees of crustal reworking between 2.2 and 1.78 Ga and 0.3–0.2 Ga.

Using detrital zircon ages and Hf isotopes to identify 1.48–1.45 Ga sedimentary basins and fingerprint sources of exotic 1.6–1.5 Ga grains in southwestern Laurentia

Globally rare 1.6–1.5 Ga zircons provide an underutilized correlation tool for Precambrian supercontinent reconstructions. Detrital 1.6–1.5 Ga zircons, long known from the Belt Supergroup, have recently been discovered in multiple metasedimentary successions in southwestern North America. Few igneous or metamorphic sources in this age range are known in Laurentia, implying non-Laurentian provenance. Combined U-Pb and Hf isotopic data offer a robust test of their provenance. New U-Pb detrital zircon ages from quartzite exposed in the Defiance uplift in northeastern Arizona indicate a maximum depositional age of ca. 1476 Ma and a predominance of locally derived ca. 1655 Ma and older detritus. The quartzite contains populations at 1570, 1554, and 1519 Ma that do not have known Laurentian sources and are similar to our new and recently published data from the ca. 1474–1436 Ma Blackjack Formation in south-central Arizona. Based on similarities between age spectra and depositional age constraints, we suggest a correlation between the two stratigraphic sections and postulate that a previously unrecognized, and now largely eroded, 1475–1450 Ma sedimentary basin or set of basins may have extended across much of southwestern Laurentia. We refer to this basin as the Yankee Joe-Defiance basin. Hf isotopic analysis of ca. 1.6–1.5 Ga detrital zircons from both localities yields positive epsilon-Hf (ɛHf) values of +0.2 to +12 indicating a near-juvenile Hf fingerprint with a mean of ∼ +7. This range overlaps with, but is more juvenile than, ɛHf values of +3 to +7.5 from the Belt basin. In terms of possible non-Laurentian source regions, Hf data from both the North and South Australia cratons overlap with, but are also generally less juvenile than, ɛHf values of Laurentian zircons. Only the North Australia craton contains the full range of ages and Hf isotope values recognized in the 1.6–1.5 Ga Yankee Joe-Defiance populations. This finding supports plate reconstructions involving Australia as a source for exotic detritus in western Laurentia ca. 1480–1450 Ma.

Where did the lower Paleozoic rocks of Yucatan come from? A U–Pb, Lu–Hf, and Sm–Nd isotope study

Detrital zircon grains from Lower Paleozoic sedimentary rocks in the Yucatan Peninsula have an age distribution characterized by major probability peaks at ~1.0, ~1.2, and ~1.5Ga (Martens et al., 2010). Here, we present new Lu–Hf data (MC–ICPMS) paired with U–Pb ages (ID-TIMS) for additional zircon grains from the same rocks. This analytical approach yields precise information about the time and geochemical environment of zircon growth, which in turn helps to distinguish between different crustal source regions that just happen to host zircon populations of similar age. In addition, single zircon grains from granitoids that intruded the sedimentary rocks were dated by U–Pb laser ablation MC–ICPMS and ID-TIMS, and their Hf-isotope compositions were determined by solution MC–ICPMS. The zircon data are complemented by Sm–Nd analyses of the sedimentary and igneous whole rocks.The Yucatan Peninsula, which forms part of the Maya block of Central America, includes lower Paleozoic rocks in the Maya Mountains of Belize. The pre-Mesozoic geologic history of the Maya block is related to the evolution of the mid-Proterozoic basement in Mexico (Oaxaquia), other Paleozoic peri-Gondwanan terranes, and the Pan-African–Brasiliano type basement of Florida. The initial 176Hf/177Hf values of ~1.0Ga detrital zircon grains lie on a crustal evolution trajectory similar to that defined by older, ~1.2 to ~1.5Ga grains. This trajectory is consistent with those that would be produced by crustal reservoirs that separated from the depleted mantle between 1.70 and 2.05Ga. However, some grains have significantly less radiogenic 176Hf/177Hf(t), indicating influence from even older cratonic crust. Zircon grains from granitoids that intruded the Early Paleozoic sedimentary rocks of the Yucatan Peninsula yielded Late Silurian to Early Devonian (~415–400Ma) crystallization ages. More radiogenic Hf isotope ratios indicate anatexis of a crustal reservoir that is distinct from that of the Early Paleozoic sedimentary rocks. The Sm–Nd systematics of whole rock samples further support the results from the Hf isotopes in zircon grains. The data suggest a more continental provenance for the sedimentary rocks from Yucatan as compared to typical ca. 1.3 to 1.0Ga outcrops in southern and central Mexico. The results indicate that sediments were shed either from mid-Proterozoic complexes of NW Amazonia or from similar continental sequences that were thrust over Oaxaquia during the Grenville orogeny and subsequently eroded in the early Paleozoic. Integrating the data into a new model for early Paleozoic times, the southern Maya block is inferred to have formed during the opening of the Rheic Ocean along the western margin of Amazonia adjacent to Oaxaquia. The paucity of Ediacaran (Pan African–Brasiliano) signatures implies that before the Silurian, the southern Maya block evolved geographically separated from NW Yucatan and Florida, where Pan African–Brasiliano crystalline rocks have been reported.

Detrital zircon U-Pb geochronology of Paleozoic strata in the Grand Canyon, Arizona

We determined U-Pb ages for detrital zircons from 26 samples of Paleozoic sandstone from the Grand Canyon. Cambrian strata yield mainly ca. 1.44 and 1.7–1.8 Ga ages that indicate derivation from nearby basement rocks of the Yavapai Province. Devonian strata contain zircons of 1.6–1.8 Ga, 1.34–1.40 Ga, and ca. 520 Ma, suggesting derivation from the Mazatzal and Yavapai Provinces, midcontinent region, and the Amarillo-Wichita uplift, respectively. Mississippian strata record a major change in provenance, with predominantly 415–475 Ma and 1030–1190 Ma grains interpreted to have been shed from the central Appalachian orogen. Pennsylvanian strata contain subequal proportions of 1.4–1.8 Ga grains derived from basement rocks exposed in the Ancestral Rocky Mountains and 409–464 and ca. 1070 Ma grains derived from the Appalachians. Permian strata contain abundant Appalachian zircons, including 270–380 Ma grains, and a lesser proportion of grains derived from the Ancestral Rocky Mountains. Transcontinental transport during Mississippian through Permian time is interpreted to have occurred in large river systems, facilitated by northeasterly trade winds during low sea level and by coastal currents. A compilation of young ages from all Upper Paleozoic strata yields age peaks of 270–365 Ma, 395–475 Ma, and 515–640 Ma, an excellent match for Alleghanian, Acadian, Taconic, and Neoproterozoic (peri-Gondwanan) episodes of magmatism along the Appalachian margin. Lag times of the youngest grains in these Upper Paleozoic strata average ∼25 m.y., suggesting relatively rapid exhumation and erosion of Appalachian source regions.

Late Paleoproterozoic to early Mesoproterozoic Dongchuan Group in Yunnan, SW China: Implications for tectonic evolution of the Yangtze Block

The Dongchuan Group including the Yinmin, Luoxue, Etouchang, and Luzhijiang Formations from the base upward has extensive outcrops in the western Yangtze Block. It was previously thought to be the lower part of the late Mesoproterozoic to early Neoproterozoic Kunyang Group, but there was a lack of precise age determination. The whole sequence is more than 4 km in thickness and consists of conglomerate, sandstone, slate, dolostone, carbonaceous slate, and minor tuffaceous volcanic rocks. The basal unit, the Yinmin Formation, is a continental red-bed deposit thought to have been formed in an intra-continental rift basin. Zircons from the sandstone and volcanic rocks of the Yinmin Formation are analyzed for U–Pb and Lu–Hf isotopes in this study. Detrital zircons from the sandstone have U–Pb age populations at 1.8–1.9 Ga, 2.25–2.35 Ga, ∼2.5 Ga, and 2.7–2.9 Ga with the oldest age of ∼3.7 Ga. Most zircons have high Th/U ratios (>0.1) and were probably derived from igneous rocks, whereas a few 2.05–1.95 Ga grains with low Th/U ratios (<0.1) may have a metamorphic origin. The age groups are roughly consistent with those of known tectonothermal events in the northern Yangtze Block. The youngest detrital zircons of the Yinmin Formation have 207Pb/ 206Pb ages of ∼1780 Ma and dolerite dykes intruding the formation have a zircon 207Pb/ 206Pb age of 1690 ± 32 Ma (2 σ). A zircon 207Pb/ 206Pb age of 1742 ± 13 Ma (2 σ) was obtained from a tuff sample of the Yinmin Formation and is considered to represent the deposition age. In combination with other age data of the Dongchuan Group, the newly obtained U–Pb age data thus constrain the deposition of the Dongchuan Group to the period between ∼1.7 Ga and 1.5 Ga, significantly older than previously thought. Presence of late Paleoproterozoic continental rift in the western Yangtze Block is contemporaneous with the break-up of the Columbia supercontinent, and provide further evidence to support the idea that the Yangtze Block was likely part of the supercontinent. Detrital zircons of the Yinmin Formation mostly have Hf continental modal ages ( T DM C) between 3.5 Ga and 2.6 Ga with the oldest T DM C of 3.9 Ga, indicating that the western Yangtze Block experienced significant continental crustal growth during the Paleo- to Neo-archean and contains Eoarchean crustal rocks. In combination with data from others, our results suggest that the Yangtze Block have a widespread Archean basement. The Archean zircons have both positive and negative ɛ Hf( t) values (−10.1 to +6.5) suggesting both juvenile crustal growth and reworking of older materials. In contrast, the Paleoproterozoic zircons have highly variable but mostly negative ɛ Hf( t) values implying episodic crustal reworking of Archean crustal rocks. Unlike the Paleoproterozoic detrital zircons, igneous zircons from the tuff and dolerite dyke samples mostly have positive ɛ Hf( t) values (−0.8 to +10.1), indicating contributions of juvenile materials from a depleted mantle source due to continental rifting.

Temperature and magnetic field dependence of the voltagein GaAs films with superconducting Ga grains

We have studied granular films consisting of nanoscale Ga droplets formed on GaAs films via a method of vacuum annealing to promote As evaporation. For temperatures and magnetic fields below the bulk Ga critical parameters, the samples are very sensitive towards external microwave radiation when two point voltage measurements are performed. Together with the observation of an oscillating magnetic field dependence of the voltage, a scenario in which the samples consist of Josephson-coupled loops seems to be the most likely one for explaining the obtained results.

The genetic relationship of the deformed peridotites and garnet megacrysts from kimberlites with asthenospheric melts

New data on geochemistry of incompatible trace elements (Nb, Zr, Hf, Ti, Y, REE) in garnets (Ga) and clinopyroxenes (Cpx) from deformed high-temperature garnet lherzolites, harzburgites, and garnet megacrysts from the Udachnaya pipe are presented. According to textures and petrographic composition, the deformed Ga-bearing peridotites are subdivided into coarse- and fine-porphyroclastic. Rocks of the first type (Ga-lherzolites) are enriched in Ga and Cpx (up to 30% each); among them, there are xenoliths with deformed Ga, Cpx, and Ol megacrysts. Deformed fine-porphyroclastic peridotites (Ga-lherzolites and Ga-harzburgites) are depleted in Ga (≤10%) and Cpx (≤3–5%). Four Ga megacrysts are typical of a Cr-poor association (0.33–1.63% Cr 2O 3; 0.56–1.05% TiO2; Mg# = 0.67–0.86). Garnets from deformed high-temperature coarse-porphyroclastic Ga-lherzolites are similar in geochemistry to Ga megacrysts: Their chondrite-normalized REE curves rise smoothly from La to Yb, and their spidergrams show peaks of Nb, Zr + Hf, and Ti. The narrow marginal zones of Ga grains from Ga-lherzolites are enriched in LREE. The peaks of HFSE in the Ga curves are smoothed out. The element patterns of garnet from fine-porphyroclastic Ga-lherzolites and Ga-harzburgites show a sinusoidal distribution of REE and Zr + Hf and Ti minima on the spidergrams. The element patterns of Cpx from both types of rocks are nearly the same: The curves are convex in the field of LREE, drastically drop from Nd to Yb, and have minima of Nb, Zr + Hf, and Ti. The curves of melts (L) equilibrated with Ga (calculated from partition coefficients) show a drastic enrichment in LREE and peaks of Nb, Zr + Hf, and Ti for garnet from deformed coarse-porphyroclastic lherzolites and megacrysts. The L curves of fine-porphyroclastic lherzolites and harzburgites have distinct minima of Zr + Hf and more sharply slope from Nd to Yb. The L curves of Cpx have minima of Nb, Zr + Hf, and Ti and lie near the field of basaltoid kimberlites from the Udachnaya pipe. All these data are consistent with the model of percolative fractional crystallization. The matter of mantle plume arrived from the mantle transition zone might have been the source of asthenospheric liquids inducing specific magmatic metasomatism at the continental lithosphere–asthenosphere boundary. The source material was enriched in majorite and silicate-perovskite or in ancient oceanic crustal matter.

Constraints on Hadean zircon protoliths from oxygen isotopes, Ti‐thermometry, and rare earth elements

We report zircon oxygen isotope ratios and reconnaissance Ti‐in‐zircon concentrations, guided by cathodoluminescence image studies, for detrital zircons up to 4.34 Ga from the Narryer Gneiss Complex of Western Australia. Zircon oxygen isotope results bolster the view that some Hadean (&gt;3.85 Ga) zircon source melts were enriched in heavy oxygen, a sensitive proxy for melt contamination by sediments altered in liquid water. Zircon crystallization temperatures calculated from Ti concentration in pre‐3.8 Ga zircons yield values around 680°C in all cases except for one lower value in a 4.0 Ga grain. Elevated zircon δ18O values reported here and elsewhere, combined with low minimum‐melt crystallization temperatures, and analysis of zircon/melt partitioning of rare earth elements (REEs) provide mutually consistent lines of evidence that the Hadean Earth supported an evolved rock cycle which included formation of granitic water‐saturated melts, extensive continental crust, hydrosphere‐lithosphere interactions, and sediment recycling within the first 150 million years of planet formation.

Zircon isotope evidence for ≥3.5 Ga continental crust in the Yangtze craton of China

The presence or absence of Paleoarchean crust in different continents provides an important basis for a comparison of their growth histories. Since the finding of ca. 3.8 Ga detrital zircon in the North China craton by means of SHRIMP U–Pb dating, it has been a question whether the Yangtze craton, one of the two major blocks constituting the basement of China continent, also contain Paleoarchean crustal relicts. It is generally accepted that the basement of the Yangtze craton mainly formed in the Paleoproterozoic, with only minor Archean rocks in ages of 2.5–3.3 Ga. A combined study of zircon U–Pb dating and Hf isotope analyses for the Kongling migmatite and gneiss from this craton reveals the existence of ca. 3.5 Ga rocks. The zircon U–Pb ages are divided into three groups: ∼3.2 Ga in cores, ∼2.9 Ga in most grains and ∼2.0 Ga in rims. Both ∼3.2 Ga cores and ∼2.9 Ga grains have negative ɛ Hf( t) values of −1.6 and −4.0, respectively. Mantle-like δ 18O values of 5.6–5.8‰ were obtained for the zircons, precluding a significant input by sedimentary source component for the magmas that are responsible for protolith of the Kongling metamorphic rocks. From the Hf model ages of these zircons, about 3.5 Ga crustal relicts are speculated to occur in the Kongling rocks. This provides the first demonstration for the existence of Paleoarchean rocks in the Yangtze craton. Thus the early evolutionary history of the Yangtze craton may resemble the North China craton that has crustal relicts with the minimum ages of 3.5 Ga. These Paleoarchean rocks resided in the crust of the Yangtze craton for at least several hundreds of million years before remelting, probably leaving some clues to be traced and verified. Paleoproterozoic U–Pb ages of 1.8–2.0 Ga are identified in magmatic and metamorphic rocks in the Yangtze craton, providing a likely connection of this craton with the North China craton with respect to assembly and breakup of the supercontinent Columbia during the late Paleoproterozoic.

Nanoimprint Mold Repair by Ga+ Focused-Ion-Beam Direct Etching

Nanoimprint lithography (NIL) Si molds with protrusion- and hollow-defects were repaired by Ga+ focused-ion-beam (FIB) direct etching and chemical-vapor-deposition (CVD). The usability of the repaired mold was confirmed by carrying out NIL on poly(methylmethacrylate) (PMMA). After the NIL process, Ga grains appeared on the etching-repaired mold, which was confirmed by scanning electron microscopy-energy dispersive spectrometry (SEM-EDS). The thresholds of ion dose and annealing temperature for Ga grain appearance are studied.

The Archean Murmac Bay Group: evidence for a giant Archean rift in the Rae Province, Canada

The lower amphibolite to-granulite facies Archean Murmac Bay Group of the southwestern Rae Province, Canada, formed during continental extension (rifting) of an underlying granitoid basement complex that includes the 3060±40Ma Lodge Bay Granite and the 2999±7Ma Cornwall Bay Granite. The contact between the Murmac Bay Group and the basement complex is an unconformity that is locally marked by polymictic conglomerate. Overlying the conglomerate is basal quartzite of the Murmac Bay Group that contains a laterally extensive (>10km) intraformational oligomictic quartzite-pebble conglomerate that may have formed by uplift during rifting. Distinctive medium- to coarse-grained gabbro dykes and sills that cut both the basement complex and the basal quartzite are interpreted as feeders to stratigraphically overlying tholeiitic basalt flows that have geochemical signatures consistent with formation in a continental rift setting. The quartzite-basalt contact exhibits rare stratigraphic interlayering, and is a horizon along which oxide-and silicate-facies iron formation, dolostone, and pelite locally occur. Ultramafic rocks with komatiitic compositions also intrude these lower stratigraphic units. The upper Murmac Bay Group is dominated by pelitic to psammitic rocks that may represent deepening of the sedimentary basin during continued extension. Ancient approximately 3.62 and >3.7Ga grains dominate the detrital zircon population of the Murmac Bay Group sediments, but deposition occurred after approximately 2.77Ga, the youngest detrital zircon in our study. The age of the Murmac Bay Group is thus constrained as younger than approximately 2.77Ga and older than the crosscutting 2631±12Ma Dead Man and Donaldson Lake Granites.The gross lithological and stratigraphic make-up of the Murmac Bay Group closely resembles that of the Prince Albert, Woodburn Lake and Mary River groups of the northern Rae Province, all three of which have been interpreted as approximately 2.7Ga continental rift successions. We propose that the Murmac Bay Group is correlative with these successions. If correct, the total length of this Neoarchean rift is at least 2000km, extending from Baffin Island to northwestern Saskatchewan, Canada. The supracrustal rocks of the rift record the break-up of a large Paleo- to early Neoarchean continent (Nunavutia), which experienced magmatic events at approximately 3.0, 3.62, and >3.7Ga.

Early Archean crust in the northern Wyoming province: Evidence from U–Pb ages of detrital zircons

U–Pb ages of individual detrital and metamorphic zircons from 12 Archean metasedimentary rocks, including quartzites, from the Beartooth, Ruby, and Tobacco Root uplifts of the northern Wyoming province indicate that they were deposited between 2.7 and 3.2 Ga. Younger, metamorphic zircons are found as overgrowths and new grains in some samples, and yield ages between 2.7 and 1.9 Ga. They are, however, much less abundant than detrital grains, which constitute >75% of the 355 grains analyzed. The majority of the detrital grains have ages between 3.2 and 3.4 Ga; none are younger than 2.9 Ga. Grains with 207Pb/ 206Pb ages between 3.4 and 4.0 Ga constituted approximately 15% of all grains with analyses within 10% of concordia, but are concentrated in samples from the eastern Beartooth Mountains. Comparison of the average of the Pb–Pb ages of individual zircons within 10% of concordia with previously published Lu–Hf chondritic model ages for some individual samples suggests that the age distribution recorded by the U–Pb system in these zircons has not been significantly disturbed by pre- or post-depositional Pb-loss. Collectively, these data suggest that the individual metasedimentary rocks did not completely share a common provenance and that a major crust-forming cycle occurred 3.2 to 3.4 Ga. In conjunction with previously published U–Th–Pb whole-rock data, these results suggest that rocks with a relatively high proportion of >3.4 Ga grains may have had crust of comparable age in their provenance.

Detrital zircon geochronology of the Alexander terrane, southeastern Alaska

U-Pb analyses of 101 detrital zircon grains from Paleozoic and Triassic clastic strata of the Alexander terrane indicate that most detritus in the terrane was derived from intraterrane igneous rocks, although the occurrence of 1.0–3.0 Ga grains in a Lower Devonian sandstone indicates proximity to a continental region during the Silurian–Devonian Klakas orogeny. Because these Precambrian grains are not the same age as grains that were accumulating along the western margin of North America, it appears unlikely that the terrane was in proximity to the Cordilleran margin during Early Devonian time. Continental regions that could have shed grains of the appropriate ages, and that also record Silurian–Devonian orogenic activity, include the paleo-Pacific margin of Australia and the Scandinavian portion of Baltica. Existing paleomagnetic data are consistent with either paleoposition, whereas previous biogeographic studies suggest closer ties with Baltica.

U-Pb dating of detrital zircons from the Sangun metamorphic rocks, Kyushu, Southwest Japan: An evidence for 1.9-2.0 Ga granite emplacement in the provenance.

U-Pb and Rb-Sr radiometric dating has been done on zircon samples and mineral concentrates, respectively, separated from psammitic schists of the Sangun metamorphic rocks in central Kyushu, Japan. Based on their shapes and colors, zircon grains were grouped into six. Rounded and short prismatic zircons define, independently of their colors, a common discordia with an upper intercept at 1.91 ± 0.04 Ga. Elongated zircons define another discordia with an upper intercept at 0.93 ± 0.06 Ga. A Rb-Sr internal isochron defined by the mineral concentrates gives an age of 197.7 ± 6.5 Ma, which represents the time of the Sangun metamorphism in central Kyushu. The U-Pb age of 1.91 Ga coincides with that of zircons from the Oki metamorphic rocks (1.96 ± 0.04 Ga: Yamashita and Yanagi, 1994). This and the great abundance of 1.91 Ga grains in the zircon populations seem to indicate a prominent plutonic activity at 1.9–2.0 Ga in the provenance common for the Sangun and Oki metamorphic rocks. The U-Pb ages of zircon from the Sangun metamorphic rocks indicate the episodic growth at 1.91 and 0.93 Ga of the East Asian continent close to the Japanese Islands.

Provenance and U-Pb geochronology of the Mesoproterozoic Belt Supergroup (northwestern United States): implications for age of deposition and pre-Panthalassa plate reconstructions

The Mesoproterozoic Belt-Purcell Supergroup is a thick succession of siliciclastic and carbonate sedimentary rocks in the northwestern United States and adjacent Canada. Recent stratigraphic studies provide strong evidence for an enclosed basin setting with a tectonically active western margin. The age of the western (present coordinates) landmass is unknown, but has been inferred to be different from North American basement that borders the Belt basin on the basis of whole rock Sm-Nd studies. We report U-Pb isotopic analyses of individual detrital zircon, monazite, and xenotime grains separated from westerly derived clastic units in the Belt Supergroup as well as Sm-Nd isotopic data for the same grains where appropriate. These data provide new constraints on the age and isotopic character of the western landmass. Most U-Pb data are concordant or slightly discordant; exceptions are noted below. The Revett Formation (Ravalli Group, lower Belt Supergroup) contains grains with concordant ages of 1590–1600 Ma and a grain with a discordant 207Pb/ 206Pb age of 1780 Ma: the ca. 1600 Ma age is very unusual in North America and effectively dates “Belt Island”, long postulated to be the Revett source in what are now terranes of southeast Washington and northeast Oregon that were accreted in the Mesozoic. Detrital zircons from the Missoula Group (Bonner and Mount Shields formations; upper Belt Supergroup) form a distinct group at 1670–1859 Ma with two ca. 2.6 Ga grains. U-Pb ages from monazite range from 1642 to 1786 Ma, and together with Nd isotopic data ( T DM model ages of 2.05–3.36 Ga) suggest variable interaction with previously differentiated crust. The Buffalo Hump Formation in northeast Washington may correlate with the Bonner Formation or may lie unconformably on the Belt Supergroup; it contains detrital zircons of ca. 1840 Ma and, surprisingly young, 1070–1244 Ma (several grains). Monazites have U-Pb ages of 1753–1774 Ma and Nd signatures similar to the Bonner Formation data. Several major conclusions can be drawn from these data. The ages of 1590–1600 Ma have no known possible source within western North America (although 1576 Ma augen gneiss is present in the Priest River Complex northwest of the Belt basin), and the 1642–1786 Ma ages are uncommon to rare in Precambrian rocks to the immediate south and east; together these data support previous Sm-Nd whole rock and stratigraphic conclusions of a dominantly western source for the Belt basin. The ages in the Buffalo Hump Formation can be interpreted in two ways. Either the Buffalo Hump correlates with the Bonner Formation and therefore the top of the Belt Supergroup is substantially younger (< 1070 Ma) than previously inferred (1250 Ma) and may overlap with Grenville-age tectonic activity. Alternatively, the Buffalo Hump Formation may represent a new unit that is younger than the Belt. Irrespective of this issue, these data help to constrain proposed pre-Paleozoic plate reconstructions; from the 1070–1244, 1590–1600 and 1642–1786 Ma source ages and related Nd data, we suggest that the basement blocks of south-central Australia (Gawler, Musgrave, Willyama, Arunta and possibly Mount Isa) were joined to the western side of Laurentia, adjacent to the Belt basin, prior to Neoproterozoic formation of the Panthalassa Ocean.

An initial investigation of the microstructure of Ti/Pd/Au ohmic contact structures for gaas microwave devices applications

A preliminary investigation of both as-deposited and annealed titanium (75 nm), palladium (75 nm), gold (400 nm), ohmic contacts to thinp +-GaAs layers, was carried out using a combination of transmission electron microscopy, energy dispersive x-ray analysis, secondary ion mass spectroscopy and electrical measurements. The annealed contacts showed limited interaction between the metallization and the semiconductor with a metal penetration depth of only 2 nm for a 4 minute anneal at 380° C. The contacts were found to remain layered after annealing. The layers consisted of a uniform upper layer of large a Au(Ga) grains, a central, non-uniform layer containing small Pd-rich grains and a lower uniform layer of almost pure Ti. Preliminary SIMS studies suggested Zn dopant outdiffusion from the epilayer into the metal layer and this may have important implications for the electrical properties of these contacts.

U–Pb geochronology of detrital zircons from a continental margin assemblage in the northern Coast Mountains, southeastern Alaska

Metamorphic rocks within and west of the northern Coast Mountains in southeastern Alaska consist of an Upper Proterozoic(?) to upper Paleozoic continental margin assemblage that we interpret to belong to the Yukon-Tanana terrane. U–Pb geochronologic analyses of single detrital zircon grains from four samples of quartzite suggest that the zircons were shed from source regions containing rocks of ~495 Ma, ~750 Ma, 1.05–1.40 Ga, 1.75–2.00 Ga, ~2.3 Ga, 2.5–2.7 Ga, and ~3.0 Ga. Multigrain fractions from two samples yield upper intercepts between 2.0 and 2.3 Ga, but the scarcity of single grains of similar age suggests that these fractions comprise a mixture of &lt; 2.0 and &gt; 2.3 Ga grains. Zircons in these rocks generally overlap in age with (i) detrital zircons in metasedimentary rocks of the Yukon–Tanana terrane in eastern Alaska and Yukon, (ii) detrital zircons in strata of the Cordilleran miogeocline, and (iii) plutonic and gneissic rocks that intrude or are overlain by miogeoclinal strata. In addition, the pre-1.7 Ga grains overlap in age with dated crystalline rocks of the western Canadian Shield. These similarities raise the possibility that metaclastic rocks in the northern Coast Mountains accumulated in proximity to western North America. The younger zircon populations were likely shed from mid-Proterozoic to early Paleozoic igneous rocks that now occur locally (but may have been widespread) along the Cordilleran margin. Recognition of a continental margin assemblage of possible North American affinity in the Coast Mountains raises the possibility that some arc-type and oceanic terranes inboard of the Coast Mountains may be large klippen that have been thrust over the North American margin.