Ga Cores Research Articles

Nanohelix-Induced Optical Activity of Liquid Metal Nanoparticles.

Liquid metals (such as gallium or Ga) exist in liquid states under ambient conditions and are hardly sculpted in chiral structures. Herein, through electron-beam evaporation of Ga, hemispherical achiral Ga nanoparticles (NPs) are randomly immobilized along helical surfaces of SiO2 nanohelices (NHs), functioning as a chiral template. Helical assembly of Ga NPs shows chiroplasmonic optical activity owing to collective plasmon-plasmon interactions, which can be tuned as a function of a helical SiO2 pitch (P) and the amount of Ga evaporated. At a P of ≈150nm, the chiroplasmonic optical activity, evaluated with anisotropic g-factor, can be as large as ≈0.1. Because the SiO2 NHs and Ga NPs have high environmental stability of nanostructures, the chiroplasmonic optical activity shows excellent anti-aging stability, despite slight blue shift and chiroplasmonic degradation for the first 2 weeks. Spontaneous oxidation of the Ga NPs enables the formation of dense Ga2 O3 layers covering Ga cores to prevent further oxidation and thus to stabilize the chiroplasmonic optical activity. This work devises an alternative approach to impose optical activity onto Ga NPs, providing an additional degree of freedom (i.e., chirality) for Ga-based flexible electronic devices to develop advanced applications of 3D display, circular polarizers, bio-imaging, and bio-detection.

New petrographic and U–Pb geochronology data from the Mazagan Escarpment, offshore Morocco: Support for an African origin

Two samples of a granodiorite and a hypersthene granodiorite provided a unique opportunity to investigate the nature of the basement of the Mazagan Escarpment, the northwestern margin of continental Africa (Morocco) on a steep part of the lower continental slope in offshore Morocco. We conducted U–Pb LA-ICPMS geochronology on zircon from the granodiorite, which was acquired from a deep sea drilling program core DSDP544, and on zircon and monazite from a hypersthene granodiorite collected previously during the Cyamaz submersible campaign, to determine the ages of the rocks of the Mazagan Escarpment and interpret their origins. Zircon from the granodiorite yielded a 556 ± 10 Ma crystallization age, abundant inherited zircon cores up to 620 Ma and three ~2.68, ~1.76 and ~1.20 Ga cores. The hypersthene granodiorite yielded ~1950–1750 Ma zircon and ~1820–1640 Ma monazite ages. Some rocks of the closest onshore continental platform have recently been interpreted as belonging to an exotic Avalonian terrane, and correlated with the Mazagan Escarpment. Our data do not provide evidence that the Mazagan Escarpment is part of an exotic terrane. It is more likely that the hypersthene granodiorite formed in an extensional setting after the 2.25–2.07 Ga Eburnean orogeny, at a time of otherwise predominantly mafic magmatism that may have caused magmatic underplating and heating from below. The granodiorite formed during the late Ediacaran, which is characterized by widespread magmatism in Northwest Africa. Therefore, it is likely that the rocks of the Mazagan Escarpment have a Northwest African origin, which implies that the Pangean suture zone lies west of it.

Neoarchean to Paleoproterozoic fragments in the Brunovistulia terrane, S Poland: a component of the Columbia Supercontinent?

The composite terrane of Brunovistulia includes basement of the Upper Silesia Block, southern Poland. In its NE part, the basement is elevated by the Rzeszotary Horst. In the Rzeszotary 2 borehole (Rz2) drilled in the horst, partly migmatized amphibolites, felsic gneisses and granites occur. An Na-plagioclase-phengite-K-feldspar neosome contained zircons that yielded U-Pb SHRIMP ages ~2.75–2.6 Ga (cores and single grains) and ~2.0 Ga (rims and single grains). The older ages are interpreted as the time of origin of the igneous protolith of the migmatized amphibolites. The younger ages recorded metamorphism and migmatization that affected both the magmatic precursor of the amphibolites and accompanying felsic rocks during a contractional tectonic/orogenic event. Migmatization was greatly enhanced by an influx of alkali-bearing fluids which heralded intrusion of late-orogenic unfoliated K-granite in an extensional regime, terminating the 2.0 Ga event. It is proposed that the entire orogenic edifice, of which the Brunovistulian rocks drilled in Rz 2 are a small part, represents fragments of the Columbia Supercontinent that was assembled in the Paleoproterozoic and broken up in the Mesoproterozoic. In Ediacaran times, these fragments became eventually incorporated into the Cadomian orogen in the form of its foreland and contributed to the formation of the composite terrane of Brunovistulia. Such a scenario explains why the U-Pb zircon age spectra in the Rzeszotary terrain differ dramatically from those in the remainder of Brunovistulia, which is thought to be the Cadomian hinterland.

Liquid Gallium Nanospheres Emitting White Light

AbstractGallium (Ga) is not only a metal possessing surface plasmon resonance in the ultraviolet region but also a phase‐change material with very low melting point. Spherical Ga nanoparticles composed of liquid Ga cores and solid Ga2O3 shells, which exhibit surface plasmonic resonances spanning the ultraviolet to near‐infrared spectral region, are fabricated by using femtosecond laser ablation. Under the excitation of femtosecond laser light, only second harmonic generation is observed in the Ga nanospheres placed on a glass substrate. In sharp contrast, white light emission can be generated in such Ga nanospheres when they are placed on a thin silver film. It is revealed numerically and confirmed experimentally that a Fano resonance originating from the interference between the mirror‐image‐induced magnetic dipole mode and the gap plasmon mode is formed in the backward scattering spectra of the Ga nanospheres. Efficient white light emission can be achieved by resonantly exciting either the magnetic dipole resonance or the Fano resonance. Based on spectral analysis, the white light emission is identified as the hot‐electron intraband luminescence. The fabricated core–shell Ga nanospheres provide an ideal platform on which the optical properties and practical applications of liquid Ga nanoparticles can be systematically investigated.

U-Pb zircon and titanite ages from granulites of the Koraput area – Evidence for Columbia, Rodinia and Gondwana from the Eastern Ghats Province, India

The Eastern Ghats Province (EGP) of India, characterized by ∼1 Ga granulite metamorphism (M1), collided with the Archaean Indian craton in either Grenvillian (∼1 Ga) or Pan-African (∼0.6–0.5 Ga) time. At ∼850 Ma, the Koraput Alkaline Complex (KAC) intruded into M1 EGP granulites, and was metamorphosed during a second granulite event (M2) in the EGP. U-Pb zircon dates from gneisses in Koraput yield magmatic ages ∼1500 Ma, with overgrowths at 1100–900 Ma and 800–700 Ma. Cratonic zircons from the EGP contact zone have Archaean 2.8–2.6 Ga cores with overgrowths at ∼700 Ma. Titanites from a calc-silicate gneiss within the EGP define a U-Pb isochron around 700 Ma, whereas titanites from nepheline syenites within the KAC yielded a range of dates from ∼622 to 520 Ma. The c. 1500 Ma age in the EGP either implies Columbia break-up or proximity to a Columbia margin. The ∼700 Ma age is interpreted as the age of M2 granulite metamorphism that followed intracontinental thrusting of the EGP over ∼2.8 Ga granulites of the Jeypore Province. The ∼520 Ma titanite ages are correlated with final juxtaposition of the EGP-Jeypore Province amalgam with the Indian craton during Gondwana formation.

Synthesis, molecular structures and EPR spectra of the paramagnetic cuboidal clusters with Mo3S4Ga cores

The paramagnetic cuboidal clusters [Mo3(GaBr)(μ3-S)4(diphos)3Br3] (diphos = dppe, dmpe) were synthesized by the reduction of triangular clusters [Mo3S4(diphos)3Br3]Br with elemental Ga.

Growth, preservation of Paleoproterozoic-shear-zone-hosted monazite, north of the Western Dharwar Craton (India), and implications for Gondwanaland assembly

We examine the conditions and processes of growth and preservation of multiaged monazite in micaceous matrix and in garnet porphyroblasts in staurolite–kyanite mica schists hosted in a hitherto-undiscovered shear zone that limits the northern extent of the Western Dharwar Craton (WDC), India. Garnet in the footwall schists grew during mid-crustal (600 ± 40 °C, 7.3 ± 1.2 kbar) loading and cooling as a consequence of the northward transport of the WDC lithologies. U–Th–Pb (total) ages in monazites in the matrix and in post-tectonic garnets yield well-defined peaks at 2.5, 2.2 and 1.9 Ga. In garnet, 2.5 and 2.2 Ga monazite grains, and 2.2 Ga monazites with 2.5 Ga cores are commonly occluded, but monazites with 1.9 Ga mantles around older cores are rare. By contrast, in the matrix, 1.9 Ga monazite grains and monazite with 1.9 Ga mantles around older cores are prominent, but the peak age frequencies of the two older populations are significantly lower than for monazites hosted as inclusions in garnet. Both in the matrix and garnet, the low-Th, high-Y domains in monazites yield the two older peak ages, while the 1.9 Ga ages correspond to the high-Th, low-Y domains. The preponderance of older ages in monazite hosted as inclusions in garnet relative to matrix monazites is because garnets formed between 2.2 and 1.9 Ga shielded the older monazites from dissolution–precipitation at 1.9 Ga. A few 1.9 Ga monazites hosted as inclusions in the garnet rims suggest renewed garnet growth at post-1.9 Ga. Multiple Pb–Pb age populations (2.5, 2.25, 2.1 and 1.8 Ga) in detrital zircon in the Sahanataha Group north of the Paleoarchean Antongil-Masora block (NE Madagascar) are identical to the multiple monazites ages north of the WDC, inferred to share a similar history and to be contiguous with the Antongil-Masora block in pre-Jurassic reconstructions of the Gondwanaland. We suggest the newly discovered Paleoproterozoic tectonic zone continued westward into Madagascar north of the Antongil-Masora block and constituted the hitherto-unexplained basement for the multiaged detrital zircons in the Sahanataha quartzites (337).

Discussion on the Formation and Bonding in Three [Ga8R10] Compounds: The Diversity in Classical GaGa Bonds

Herein we describe three Ga(8) compounds that feature gallium atoms in an oxidation state of 1.25 with normal valent 2e-2c bonding. Their structures and the influence of their ligands (phosphorus and nitrogen atoms directly bonded to the Ga(8) moieties) are discussed on the basis of DFT calculations, providing an insight into a probable mechanism for insertion reactions between GaX and GaX(3) species that lead to a reaction cascade via halides like Ga(2)X(4) and Ga(5)X(7) to Ga(8)X(10) and Ga(8)X(12) (2-), respectively. Finally, the Ga(8) cores of the three title compounds were compared with the topology of carbon atoms in C(8) alkanes.

Paleocene–Eocene high-grade metamorphism, anatexis, and deformation in the Thor–Odin dome, Monashee complex, southeastern British Columbia

The Thor–Odin dome of the Monashee complex, in the southeastern Canadian Cordillera, comprises Paleoproterozoic basement gneiss with infolds of unconformably overlying rocks of a supracrustal cover sequence and is the deepest exposed structural level in the Omineca belt. Orthogneiss and paragneiss of the basement are migmatitic and contain ubiquitous stromatic leucosome and discrete phenocrystic and pegmatitic vein-type leucosome, which are all interpreted to have formed as a result of in situ melting. The stromatic leucosome is infolded with the country rock (F2), contains a weakly developed foliation, and has a biotite-rich melanosome. The phenocrystic and pegmatitic vein-type leucosome crosscut the stromatic leucosome and the transposition foliation (S2). Evidence to support an igneous and anatectic source for the leucosome includes (i) petrography, (ii) major and trace element chemistry, (iii) zircon morphology, and (iv) peak pressure–temperature (P–T) conditions. Sensitive high-resolution ion microprobe (SHRIMP) 206Pb/238U zircon dates range from ca. 56 to 54 Ma and are interpreted to represent the age of leucosome crystallization. Zircon commonly contains discrete ca. 2.6-1.8 Ga cores that are interpreted as detrital grains inherited from the host paragneiss. Anatexis was ongoing by ca. 56 Ma, as a result of regional prograde metamorphism, and was coincident, at least in part, with the formation of the penetrative S2 transposition foliation and large recumbent F2 tight to isoclinal folds. Anatexis continued during F3 and F4 folding. Melting may have continued until ca. 51 Ma, driven by decompression reactions, and was concomitant with the D5 extensional deformation.

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.

Geochronology and Tectonic Evolution of Karimnagar and Bhopalpatnam Granulite Belts, Central India

The Karimnagar Granulite Belt (KGB) and the Bhopalpatnam Granulite Belt (BGB) occur along both flanks of the Pranhita-Godavari (PG) rift basin. We present a state-of-the-art overview on the geochronological and tectonic aspects of these belts and surrounding geologic domains, and report new age data on zircon, monazite and uraninite recovered from granulite facies assemblages from KGB and BGB based on electron microprobe analyses (EPMA). Zircons from KGB charnockites show core ages of up to 3.1 Ga mantled by rims of 2.6 Ga. Zircons from BGB have 1.9 Ga cores mantled by 1.7 Ga rims. Zircons with core ages of 1.6 to 1.7 Ga in BGB rocks suggest new growth at this time. Monazites and uranitite from KGB show clear peaks with well-defined ages in the narrow range between 2.42±0.08 Ga and 2.47°0.03 Ga. Rims of monazite show mean age of 2.21±0.08 Ga. Monazites from BGB define sharp linear trend in PbO vs. ThO 2* diagram delineating a clear isochron with age of 1.59±0.03 Ga. Age data from KGB and BGB presented in this report negate current models linking these terrains to "Godavari Granulite Belt" and considering them as single and contemporaneous entity. The mid-Archaean to early Palaeoproterozoic signature recognized from KGB is totally missing in BGB. On the other hand, KGB rocks do not record any evidence for major Mesoproterozoic thermal regime. The two granulite belts shouldering the PG rift basin have therefore evolved in different times under distinct P-T conditions and thermal regimes. Our results have important implications in evaluating models of supercontinent assemblies, particularly the older assemblies of Ur, Columbia and Rodinia. While tectonothermal events in KGB broadly match with those of East Dharwar, we propose that BGB represents a 1.6 Ga collisional mobile belt between the Bastar and the Dharwar cratons. The 1.6 Ga collisional mobile belt at the southern margin of the Bastar craton was superposed by rift activity along the PG basin at 1.5 Ga. This sequence of events goes against the existence of a 3.0 Ga old contiguous assembly of Ur but closely matches with the history of accretion and break-up of the Columbia. Further, parts of the PG basin located away from the influence of the Eastern Ghats Mobile Belt, neither recorded any Grenville ages (1.0 Ga) corresponding to the Rodinia accretion nor late Pan-African ages (ca. 550 Ma) relating to the Gondwana amalgamation, indicating that the region did not witness any of these younger tectonic events.

A rare earth element study of complex zircons from early Archaean Amı̂tsoq gneisses, Godthåbsfjord, south-west Greenland

We present high spatial resolution ion-microprobe rare earth element (REE) data for discrete growth phases of complex polyphase zircons from early Archaean Amı̂tsoq gneisses, outer Godthåbsfjord, SW Greenland. In Matsuda diagrams, the two major growth phases, >3.8 Ga cores and ca. 3.65 Ga rims, have steep positive slopes from La to Lu, prominent positive Ce anomalies and negative Eu anomalies that are consistent with growth in a melt. Exceptions to this are non-cathodoluminescent zircon developed between the cores and rims, sometimes truncating zoning in the cores, and late Archaean prismatic tip overgrowths, both of which exhibit flatter light REE (LREE) patterns and have small or no Eu anomaly, which we interpret as the result of metamorphism and/or small-degree, isolated partial melting. Our data support previous interpretations that the ca. 3.65 Ga zircon phase was generated in a melt, with the >3.8 Ga phase representing either original protolith zircons in a large degree partial melt or inherited zircons in an introduced magma. Regardless which of these two interpretations is correct for these, and similar, rocks in the outer Godthåbsfjord, the 3.65 Ga event will have profoundly affected isotopic systems and obscured beyond recognition any earlier igneous features such as cross-cutting relationships, which may only be assigned a minimum 3.65 Ga age.

Contrasting geochemical patterns in the 3.7–3.8 Ga pillow basalt cores and rims, Isua greenstone belt, Southwest Greenland: implications for postmagmatic alteration processes

Pillow basalts from the early Archean (3.7 to 3.8 Ga) Isua greenstone belt, West Greenland, are characterized by well-preserved rims and concentric core structures. The pillow rims and cores have different mineral assemblages, and chemical and isotopic compositions. The rims have systematically higher contents of Fe 2O 3, MgO, MnO, K 2O, Rb, Ba, Ga, Y, and transition metals than the cores. In contrast, the cores possess higher concentrations of SiO 2, Na 2O, P 2O 5, Sr, Pb, U, Nb, and the light rare earth elements (REEs than the rims). These compositional variations in the rims and cores are likely to reflect the mobility of these elements during posteruption alteration. Variations of many major and trace element concentrations between the rims and cores of the Isua pillow basalts are comparable to those of modern pillow basalts undergoing seafloor hydrothermal alteration. Al 2O 3, TiO 2, Th, Zr, and the heavy REEs display similar values in both rims and cores, suggesting that these elements were relatively immobile during postemplacement alteration. In addition, the rims and cores have distinctive Sm-Nd and Rb-Sr isotopic compositions in that the rims are characterized by higher 143Nd/ 144Nd and 87Sr/ 86Sr ratios than the cores. The pillow basalts yield 2569 ± 170 Ma and 1604 ± 170 Ma errorchron ages on 143Nd/ 144Nd vs. 147Sm/ 144Nd and 87Sr/ 86Sr vs. 87Rb/ 86Sr diagrams, respectively. The Sm-Nd errorchron age may correspond, within errors, to a late Archean tectonothermal metamorphic event recorded in the region. The Sm-Nd errorchron may have resulted from a combination of isotopic homogenization and preferential loss of Nd, relative to Sm, during late Archean metamorphism. Although the Rb-Sr errorchron age overlaps with the timing of an early to mid-Proterozoic tectonothermal metamorphic event recorded in the region, because of a considerably large mean square of weighted deviates value and scatter in 86Sr/ 87Sr and 87Rb/ 86Sr ratios, this age may not have a precise geological significance. The 1.6 Ga Rb-Sr errorchron is likely to have resulted from the loss of radiogenic 87Sr. Collectively, the Sm-Nd and Rb-Sr data obtained from the 3.7–3.8 Ga Isua pillow basalt rims and cores are consistent with disturbances of the Sm-Nd and Rb-Sr systems by tectonothermal metamorphic events long after their eruption. In contrast to the Sm-Nd and Rb-Sr systems, the Lu-Hf system appears to be largely undisturbed by metamorphism. Five core samples and three rim samples yield a 3935 ± 350 Ma age, within error of the approximate age of eruption (3.7 to 3.8 Ga). Two rim samples that have gained Lu give an age of 1707 ± 140 Ma, within error of the Rb-Sr errorchron age. Initial 176Hf/ 177Hf ratios of the undisturbed samples at 3.75 Ga lie within ±1 ε-unit of the chondritic value, suggesting no long-term depletion in the mantle source of the basalts.

2.9, 2.36, and 1.96 Ga zircons in orthogneiss south of the Red River shear zone in Viet Nam: evidence from SHRIMP U–Pb dating and tectonothermal implications

Orthogneissic rocks coexisting with migmatites and containing small amphibolite lenses are exposed in the center of the metamorphic belt which runs parallel to the Day Nui Con Voi–Red River shear zone in northern Viet Nam. The orthogneiss complex has given some radiogenic dates of Early Proterozoic and Late Archean, which are the oldest ages ever registered for the Southeast Asian continent. Zircon grains separated from three samples of the orthogneiss complex have been dated to establish the protolith age and the timing of high-grade tectonothermal events in the complex. Sixty-five SHRIMP U–Th–Pb analyses of these zircons define three age groups of 2.84–2.91, 2.36, and 1.96 Ga. The age groups correspond to three periods of zircon generation. The oldest ∼2.9 Ga cores indicate a minimum age for the protolith of the orthogneiss complex. Two younger generations (including ∼2.36 Ga outer-cores and ∼1.96 Ga rims) probably grew during later high-grade tectono-metamorphic events, which were previously suggested by K–Ar and 40Ar/ 39Ar cooling ages of ∼2.0 Ga for synkinematic hornblendes. An early thermal history of the orthogneiss complex has been constrained, including a primary magma-crystallization stage starting at ∼2.9 Ga, followed by two Early Proterozoic (∼2.36 and ∼1.96 Ga) high-grade tectonothermal events. The ca. 2.9 Ga protolith age of the orthogneiss complex documented in this study provides new convincing evidence for the presence of Archean rocks in Indochina, and clearly indicates that the crustal evolution of northern Viet Nam started as early as Late Archean time.

Age and zircon inheritance of eastern Blue Ridge plutons, southwestern North Carolina and northeastern Georgia, with implications for magma history and evolution of the Southern Appalachian Orogen

High-resolution ion microprobe analysis of zircon has provided ages for previously undated plutons of the high-grade eastern Blue Ridge of northeastern Georgia and southwestern North Carolina. These data, together with backscattered electron imaging, reveal the presence of nearly ubiquitous inherited cores of highly variable age and magmatic rims that have experienced variable Pb loss, thus making interpretation of conventional U-Pb analyses very difficult. Ion probe rim analyses indicate that the plutons were emplaced during both the mid-Ordovician (Taconian orogeny; Whiteside pluton, 465 Ma; Persim- mon Creek Gneiss, 480 Ma) and mid-Devonian (Acadian orogeny; Rabun pluton, 375 Ma; Pink Beds pluton, 390 Ma; Looking Glass pluton, 380 Ma). A large trondhjemite dike north of Asheville is less confidently assigned an age of 415 Ma. Zircons from all intrusions have predominantly 1.0 to 1.25 Ga cores (Grenvillian). In addition, both Devonian and Ordovician plutons have smaller populations of Late Proterozoic-early Paleozoic (0.5-0.75 Ga), Middle Proterozoic (1.4 Ga), and Late Archean (2.6-2.9 Ga) cores. The ubiquitous, round cores and thick magmatic rims suggest significant resorption and then protracted growth within the melts. Zircon saturation tem- peratures based on whole-rock (melt) Zr concentrations are lower than ex- pected for magma generation (710°-760°C). Zirconium concentrations may not reflect saturation at maximum temperature, if melting was very rapid (10 5 yrs), or if zircon cores represent grains that were shielded from melt inside host grains for much of the magmatic history. The Late Proterozoic-early Paleozoic and Grenvillian inheritance is similar to documented ages of basement exposed in the eastern Blue Ridge. No expo- sures of 1.4 Ga rocks are known in the southern Appalachians, but detrital and inherited zircons of this age have been reported, and 1.4 Ga granites are widespread in the craton to the northwest. However, the combination of absence of Early Proterozoic zircon and presence of Late Archean zircon is inconsistent with the known distribution of basement rocks in southeastern North America. No Archean rocks or inherited zircons have been reported from the southern and central Appalachians; the nearest Archean exposures are 1000 km north, across a dominantly Early Proterozoic terrane. This suggests either that the Laurentian configuration of Archean basement was very complex or that the crust that underlay the plutons at the time of their emplacement included a far-travelled terrane (emplaced during Grenvillian or early Paleozoic orogeny?). Ages of magmatic and inherited zones of zircon from the plutons demon- strate that similar crust underlay the eastern Blue Ridge during both Taconian and Acadian orogenies, that there was no single episode of voluminous magma- tism, and that metamorphism and deformation began before 470 Ma and contin- ued after 370 Ma. These plutons do not constitute a significant convergence- related arc, though it is possible that they represent a displaced part of an arc that lies primarily to the east (in the Inner Piedmont?). Previous geochemical studies have demonstrated that the eastern Blue Ridge intrusions include both a very primitive component, either mafic magma or relatively young mafic source rock, and a component derived from more mature

Age significance of U–Th–Pb zircon data from early Archaean rocks of west Greenland—a reassessment based on combined ion-microprobe and imaging studies

The geochronological evolution of early Archaean Amı̂tsoq gneisses from southern West Greenland is reassessed here using the well-established cathodoluminescence imaging method both to reveal previously undocumented complex zircon growth histories and to control positioning of ion-microprobe U–Th–Pb analyses. Several of the gneisses typically contain zircon grains with ≥3.8 Ga cores of igneous origin, mostly completely mantled by growth banded magmatic zircon at ca. 3.65 Ga and in turn partly mantled by late Archaean metamorphic overgrowths at ca. 2.7 Ga. The ≥3.8 Ga cores are regarded as inherited, with apparent dates ranging down to ca. 3.65 Ga reflecting differential Pb loss. This contrasts with published views interpreting ≥3.8 Ga dates as emplacement ages of the host rock to the zircons, with all younger ages representing later Pb loss and/or metamorphic recrystallisation. Two of the analysed gneisses have discordant relationships to metasedimentary enclaves containing accessory apatite with graphite inclusions whose C-isotope ratios have been considered as biogenic in origin. The present data show that the minimum age of the enclaves is 3.65 Ga and not 3.85 Ga as previously claimed. This has important implications for the current debate on the possibility of temporal overlap of earliest life with a bolide impact scenario terminating at ≥3.8 Ga (as on the moon). We also discuss the implication of the discovery of highly complex internal structures in Amı̂tsoq gneiss zircons for the interpretation of published, zircon-derived Lu–Hf information.

Ion microprobe U-Pb zircon geochronology and correlation of Archaean gneisses from the Lewisian Complex of Gruinard Bay, northwestern Scotland

We present ion microprobe U-Pb zircon geochronology for three samples from the late Archaean Lewisian gneiss complex at Gruinard Bay, northwestern Scotland, in an attempt to resolve the discrepancy between Sm-Nd and Pb-Pb isochron ages for trondhjemite and hornblendite components. Whole-rock samples imply ca. 2.8 Ga. ages for both components (Whitehouse et al., 1996), but separated minerals have yielded ca. 2.4–2.5 Ga for trondhjemite and 3.3 Ga for hornblendite (Burton et. al., 1994). Zircons separated from three representative lithologies at Gruinard Bay (trondhjemite, hornblendite, and banded tonalite) yield concordant or near concordant U-Pb data with 207Pb206Pb ages mostly between ca. 2.85 Ga and ca. 2.75 Ga, and a few outliers at ca. 2.65 Ga. Backscattered electron images of zircons from the trondhjemite allow differentiation between ca. 2.85 Ga cores and ca. 2.75 Ga overgrowths; the latter are interpreted as igneous on the basis of field relationships of the trondhjemite to the other samples. Morphology of the hornblendite zircons is more consistent with crystallisation under metamorphic rather than igneous conditions at ca. 2.8 Ga, although the previous arguments of Whitehouse et al. (1996) for a protolith age not greatly in excess of this remain valid. The new zircon data suggest that magmatism at Gruinard Bay occurred over a relatively short time interval, in accord with the previous observations from whole-rock isotopic data. There is no evidence from the analysed zircons for ca. 2.4–2.5 Ga or 3.3 Ga magmatic ages in the trondhjemite and hornblendite, respectively. The mineral isochron ages for the trondhjemite probably represent metamorphic resetting related to the regional ca. 2.5 Ga high-grade event recorded in the central region. For the hornblendite, the mineral Sm-Nd ages are confirmed as spurious, resulting from post-crystallisation open-system isotope behaviour on the mineral scale. Regional comparison of ages within the Lewisian Complex suggests that the ≤5 ca. 2.85 Ga Gruinard Bay TTG gneisses might have been amalgamated with ≥ 2.96 Ga gneisses of the central region before or during a ca. 2.76 Ga event and subsequently have experienced a similar geological evolution.