L. Peter Gromet

The 'North American shale composite' - Its compilation, major and trace element characteristics

Abstract The compilation and major element composition of the “North American shale composite” (NASC) are reported for the first time, along with redeterminations for the REE and selected other elements by modern, high precision analytical methods. The NASC is not strictly of North American origin; 5 of the constituent samples are from Africa and Antarctica, and 15 are from unspecified locations. The major element composition of the NASC compares quite closely with other average shale compositions. New analyses of the NASC document that significant portions of the REE and some other trace elements are contained in minor phases (zircon and possibly other minerals) and that their uneven distribution in the NASC powder appears to have resulted in heterogeneity among analyzed aliquants. The results of this study show that the REE distributions of detrital sediments can be dependent to some extent on their minor mineral assemblages and the sedimentological factors that control these assemblages. Consequently, caution should be exercised in the interpretation of the REE distributions of sediment samples as they may be variable and biased relative to average REE distribution of the crustal rocks supplying detritus. These effects appear to be largely averaged out in sediment composites, with the result that their REE distributions are more likely to be representative of their provenances.

Rare earth element distributions among minerals in a granodiorite and their petrogenetic implications

A study of the distribution of lanthanide rare earths in a granodiorite from the eastern Peninsular Ranges batholith, southern California, reveals that a large fraction of the REE in this rock resides in the accessory phases sphene and allanite. The minerals plagioclase, alkali feldspar, biotite, epidote and apatite each contribute approximately 1% or less of each REE to the whole rock, with the exception of Eu for which plagioclase contributes 7%. Sphene and allanite together contain 80% to 95% of each REE. Each of these phases is zoned in REE concentration with substantial decreases from core to margin. Textural observations argue for relatively early saturation and precipitation of sphene and allanite in the magma. REE zoning trends in sphene and allanite, and unexpectedly low REE concentrations in largely later crystallizing minerals such as feldspar, indicate that the precipitation of sphene and allanite significantly reduced REE concentrations in residual melts. These results illustrate the potential that sphene and allanite have for controlling the behavior of REE in granitic magmas. Available information collectively suggest that the sampled granodiorite existed as a complete melt, that the REE contained in the assemblage of phases were derived by direct crystallization from the melt, and that the melt behaved essentially as a closed system once crystallization of the phases now present began. Close correspondences between the major and trace element chemistries of the granodiorite and phenocryst-poor lavas from similar tectonic settings support these conclusions. The REE pattern of the granodiorite melt appears to have originated at depth and is characteristic of its source regions and derivation mechanism. The high liquidus temperature of a granodiorite melt (~ 1000°C) indicates the importance of mantle-derived components within the sources of batholithic magmas in the Peninsular Ranges.

Improved chronostratigraphic reference curve of late Neogene seawater 87Sr/86Sr

We present a reference curve of seawater 87 Sr/ 86 Sr variation through the past 7 m.y. based on 455 samples of planktonic foraminifera from Ocean Drilling Program (ODP) Site 758 in the Indian Ocean. This single-site curve is superior to extant curves because of several features: (1) a continuous sedimentary section free of disturbances; (2) a well-determined and precise chronostratigraphy that is calibrated to numeric age on the basis of continuous magnetostratigraphy and a refined astronomical time scale; (3) a high temporal resolution (sample interval averages 15 ka); (4) an improved analytical reproducibility; and (5) a uniformly high sample quality. The gross structure of the Site 758 curve is generally similar to that observed in previous work, but the increased temporal resolution and decreased analytical scatter of the curve provide improved chronostratigraphic resolution. At the 95% confidence level, uncertainties on age estimates range from ±0.60 m.y. in the lower Pleistocene and upper Miocene to ±2.03 m.y. in the middle Pliocene. Moreover, this detailed record identifies two periods of steplike increases in seawater 87 Sr/ 86 Sr (6.10 to 5.80 Ma and 1.46 to 1.13 Ma) that are characterized by rates of change of ∼150 × 10 −6 /m.y. These features, if verified at other sites, indicate abrupt changes in the Sr mass balance of the ocean and may be used as precise event markers in late Neogene marine sections.

Timing and Duration of the Calc‐Alkaline Arc of the Pampean Orogeny: Implications for the Late Neoproterozoic to Cambrian Evolution of Western Gondwana

The Pampean Orogen in the Eastern Sierras Pampeanas contains two paired magmatic belts, an eastern calc‐alkaline magmatic belt and a western peraluminous granite/high‐grade metasedimentary belt. The relationship between the two belts and their relative timing are constrained through new U‐Pb zircon ages on granodiorites, monzogranites, and associated volcanic rocks from Sierra Norte and the easternmost Sierras de Córdoba. These ages indicate that calc‐alkaline arc magmatism was active over at least a 30‐m.yr. period from 555 to 525 Ma, terminating at the same time that peraluminous magmatism and associated high‐grade metamorphism began in the adjoining metasedimentary belt (525–515 Ma). These temporal relationships and the metamorphic characteristics of the two belts appear to be in conflict with previously proposed models for the Pampean Orogeny as a continental‐collision event, but they are consistent with models that propose eastward‐dipping subduction of oceanic crust initiated at ca. 555 Ma, followed by ridge‐trench collision at ca. 525 Ma. Similar‐aged belts of arc‐related and peraluminous magmatism occur elsewhere along the paleo‐Pacific margin of Gondwana, suggesting that similar processes of subduction and noncollisional peraluminous magmatism occurred along much of the Gondwana margin in late Neoproterozoic to Cambrian time.

Paleozoic deformation in the Sierras de Cordoba and Sierra de Las Minas, eastern Sierras Pampeanas, Argentina

Basement orthogneisses, paragneisses, and migmatites in the Sierras de Cordoba of the eastern Sierras Pampeanas of central Argentina represent remnants of a Cambrian arc and accretionary prism that initially formed above a subduction zone along the early Cambrian margin of Gondwana. These basement rocks contain many high strain zones that record major events in the tectonic evolution of the western margin of Gondwana during the latest Proterozoic - middle Paleozoic. Initial orthogonal shortening and chevron folding of the accretionary prism rocks occurred prior to a high temperature/low pressure, relatively static metamorphism and migmatization event during 509- 525 Ma, simultaneous with the Pampean orogeny. Localized deformation along a narrow zone of dextral transpression occurred late in the Pampean cycle. After peak metamorphism, the gneisses and migmatites deformed on east-dipping, greenschist-grade, mylonite-, ultramylonite-, and pseudotachylyte-bearing zones. Kinematic data from a selection of these shear zones include field data, microstructural shear sense indicators, and quartz C-axis fabric asymmetry. Almost all show west-directed, dip-slip reverse fault movement, but most do not achieve major crustal shortening. One major ultramylonite zone in western Sierras de Cordoba may represent a major tectonic boundary with the adjacent Sierra de San Luis rocks. This high strain zone is intruded by the Devonian-aged Achala batholith. Other age constraints include a pseudotachylyte vein that has been dated by 40 Ar/ 39 Ar method as mid-Silurian and Ordovician I-type plutonic rocks in the Sierra de Las Minas to the west that are deformed into greenschist-grade protomylonite and pseudotachylyte-bearing zones. Our data are consistent with late Ordovician to mid-Devonian orthogonal deformation throughout the Sierras de Cordoba, related to the emplacement of the Precordillera and Chilenia terranes to the west. q 2002 Elsevier Science B.V. All rights reserved.

U-Pb dating of prograde and retrograde titanite growth during the Scandian orogeny

U/Pb ages of metamorphic titanite from high-grade gneisses of the Nasafjallet window in the north-central Scandinavian Caledonides record growth over an ∼20 m.y. interval during the Late Silurian to Early Devonian. Biotite-amphibolite gneisses located near the basement-cover boundary contain metamorphic titanite in two microtextural settings, one as lenticular grains distributed throughout a recrystallized matrix of metamorphic phases, and one as plate-like grains enclosed within biotite grains. These distinct morphological populations exhibit core-overgrowth relationships with compositionally similar cores and overgrowths, but the plate-like titanite grains have a much higher proportion of overgrowth and give slightly younger ages. Cores and overgrowths in both types are inferred to have formed along the prograde (ca. 413 Ma) and retrograde (ca. 395 Ma) paths, respectively, of a single metamorphic cycle associated with the burial and exhumation of a slice of Baltic basement during the Caledonide orogeny (Baltica-Laurentia collision). The existence of polygenic U/Pb systematics in metamorphic titanite demonstrates the potential usefulness of titanite to reveal multiple metamorphic and deformational episodes, and to constrain the duration of metamorphic events.

Analysis of the Wallowa-Baker terrane boundary: Implications for tectonic accretion in the Blue Mountains province, northeastern Oregon

The Baker terrane, exposed in the Blue Mountains province of northeastern Oregon, is a long-lived, ancient (late Paleozoic–early Mesozoic) accretionary complex with an associated forearc. This composite terrane lies between the partially coeval Wallowa and Olds Ferry island-arc terranes. The northern margin of the Baker terrane is a broad zone (>25 km wide) of fault-bounded, imbricated slabs and slices of meta-igneous and metasedimentary rocks faulted into chert-argillite melange of the Elkhorn Ridge Argillite. Metaplutonic rocks within tectonic units in this zone crystallized between 231 and 226 Ma and have low initial 87 Sr/ 86 Sr ratios (0.7033–0.7034) and positive initial e Nd values (+7.7 to +8.5). In contrast, siliceous argillites from the chert-argillite melange have initial 87 Sr/ 86 Sr values ranging from 0.7073 to 0.7094 and initial e Nd values between −4.7 and −7.8. We interpret this broad, imbricate fault zone as a fundamental tectonic boundary that separates the distal, Wallowa island-arc terrane from the Baker accretionary-complex terrane. We propose that this terrane boundary is an example of a broad zone of imbrication made up of slabs and slices of arc crust tectonically mixed within an accretionary complex, providing an on-land, ancient analogue to the actualistic arc-arc collisional zone developed along the margins of the Molucca Sea of the central equatorial Indo-Pacific region.

Low P-T Caledonian resetting of U-rich Paleoproterozoic zircons, central Sweden

Abstract Uranium-rich zircons from a Paleoproterozoic, high-grade deformation zone in the Fennoscandian Shield, central Sweden, show an almost complete resetting of the U-Pb system in early Phanerozoic time. A mylonitic gneiss in the deformation zone contains two types of highly discordant (>70%), U-rich zircons: large, brown, cloudy prisms, and small milky-white irregularly shaped grains. The gneiss also contains mostly clear prismatic zircon of lower U content with mildly discordant to concordant U-Pb ages. Laser Raman spectroscopy reveals that the dark cathodoluminescent areas in brown zircons have a highly metamict crystal structure, whereas the structures of both the dark cathodoluminescent milky-white grains and the bright cathodoluminescent clear prisms have higher degrees of crystallinity. Age dates obtained by U-Pb SIMS analysis of 40 zircons of the three types described above range continuously from concordant at 1871 ± 11 Ma to 98% discordant at 384 ± 15 Ma. The strongly discordant zircons clearly have suffered severe disturbance at about the time of the Caledonian orogeny. However, Caledonian metamorphic temperatures and pressures in this region did not exceed 150-200 °C and 1-3 kbar, too low to strongly disturb the U-Pb systematics in nonmetamict zircon by thermal means alone. Independent evidence indicates that saline fluids were circulating in the Paleoproterozoic basement rocks at this time, possibly driven by hydrological gradients generated in front of the encroaching Caledonian orogenic wedge. These low-temperature saline fluids are inferred to be responsible for causing both strong Pb loss in the mostly metamict brown zircons via a diffusive process, and the formation of small milky-white zircon via a lowtemperature recrystallization or dissolution/re-precipitation process.

Applications of single-grain zircon evaporation analyses to detrital grain studies and age discrimination in igneous suites

Abstract 207 Pb 206 Pb ages determined by evaporation of single zircon grains from rocks of the New England Appalachians in the USA permit an assessment of the applicability of this method to rocks that have been strongly disturbed by orogenesis. Tests conducted on highly metamorphosed and deformed orthogneisses of known protolith age, whose U-Pb upper intercept zircon ages obtained through isotope dilution methods show profound post-crystallization discordance, demonstrate that the evaporation method is successful in recovering protolith ages. Uncertainties are ±10–15 My for rocks of Paleozoic age or older. Preliminary results suggest that the method is particularly useful for provenance studies of sedimentary rocks and in the discrimination among groups of rocks that are known to differ in age but are otherwise difficult to separate on the basis of field or petrographic characteristics. 207 Pb 206 Pb ages of detrital zircon grains from two metasedimentary units from opposite sides of the New England Appalachians reveal the distinct provenances of these rocks. The Missisquoi Formation in southeastern Vermont, USA, is comprised, in part, of detritus derived from the 1.0–1.2 Ga Grenvillian province of Laurentia; whereas this component is conspicuously absent from the Plainfield Formation in easternmost Connecticut. The detrital zircon age data suggest that the Missisquoi Formation accumulated peripheral to the Laurentian craton during the early Paleozoic, whereas the Plainfield Formation detritus was supplied by non-Laurentian, ossibly West African, sources during the Late Proterozoic or early Paleozoic. In southern Vermont, the evaporation method successfully distinguishes between two groups of augen gneisses with ages of ~960 Ma and ~ 1120 Ma, which were first recognized through isotope dilution methods. Collectively, these results indicate that the dating of single zircon grains by the evaporation method can be widely exploited in the study of complexly deformed and strongly metamorphosed rocks.

An evaluation of the age of high-grade metamorphism in the Caledonides of Biskayerhalv⊘ya, NW Svalbard

Abstract New geochronological data on eclogite-bearing gneisses of the Richarddalen Complex from Biskayerhalv⊘ya in the Caledonides of NW Svalbard indicate these rocks were strongly recrystallized during a major Ordovician (c. 455 Ma) metamorphic and deformational event of high amphibolite or higher grade. This event was not recognized in previous studies, which identified igneous protoliths of the Richarddalen Complex as Middle and latest Proterozoic in age and provided evidence suggesting that high-pressure eclogitic metamorphism was latest Proterozoic in age. New zircon 207Pb/206Pb ages indicate that the c. 650 Ma igneous crystallization ages previously identified in felsic dikes (agmatite) associated with the eclogites are widespread in mafic and felsic gneisses of the Richarddalen Complex. The results of this study do not rule out the possibility of a latest Proterozoic to Cambrian high P and T metamorphism, previously inferred from a zircon age from an eclogite. However, the more widespread occurren...