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The Bras d'Or terrane of central Cape Breton Island, Nova Scotia, Canada, contains a well-preserved record of the Ediacaran to early Cambrian evolution of Ganderia, a Gondwana-derived terrane in the northern Appalachian orogen. A complex assemblage of low- to high-grade metasedimentary rocks has varied detrital zircon signatures from laser ablation inductively coupled plasma mass spectrometry U-Pb zircon dating, but combining three or more samples yielded representative age spectra that support correlation of the low- and high-grade metasedimentary rocks throughout the Bras d'Or terrane and the corresponding Ganderian Brookville terrane of southern New Brunswick. In quartzite samples from the McMillan Flowage Formation in the northwestern Bras d'Or terrane, the youngest detrital zircons have ages >900 Ma, in contrast to previously studied psammitic and semipelitic samples from correlative units in the eastern Bras d'Or terrane in which youngest detrital ages are 620-600 Ma. Both quartzite and semipelitic samples from the McMillan Flowage Formation contain Neoproterozoic dates from zircon rims, which reflect metamorphic overgrowths during peak metamorphism at ca. 550 Ma, providing a robust age for peak metamorphism in the Bras d'Or terrane that supports similar, albeit sparse, ages reported previously from monazite and titanite samples. This metamorphism is coeval with the emplacement of voluminous dioritic to granitic plutons that occur throughout the Bras d'Or terrane and form in an Andean-type continental margin subduction zone. New U-Pb zircon ages presented here from plutons in the northern Bras d'Or terrane, combined with previously published ages, are consistent with subduction-related magmatism and associated metamorphism between ca. 575 and 540 Ma. Key words: U-Pb geochronology, Appalachian metamorphism, Bras d'Or terrane, Cape Breton Island, McMillan Flowage Formation, detrital zircon, plutonic ages, Introduction The Bras d'Or terrane of central Cape Breton Island, Nova Scotia (Fig. 1), consists of mainly Ediacaran and Cambrian metamorphic and plutonic rocks overlain unconformably by Carboniferous sedimentary rocks [...]

The Morin terrane is an allochthonous crustal block in the southwestern Grenville Province with a relatively poorly constrained metamorphic history. In this part of the Grenville Province, some terranes were part of the ductile middle crust during the 1.09-1.02 Ga collision of Laurentia with the Amazon craton (the Ottawan phase of the Grenvillian orogeny), while other terranes were part of the orogen's superstructure. New U--Pb geochronology suggests that the Morin terrane experienced granulitefacies metamorphism during the accretionary Shawinigan orogeny (1.19-1.14 Ga) and again during the Ottawan. Seven zircon samples from the 1.15 Ga Morin anorthosite suite were dated to confirm earlier age determinations, and Ottawan metamorphic rims (1.08-1.07 Ga) were observed in two samples. U--Pb dating of titanite in nine marble samples surrounding the Morin anorthosite suite yielded mixed ages spanning between the Shawinigan and Ottawan metamorphisms (n = 7), and predominantly Ottawan ages (n = 2). Our results show that Ottawan zircon growth and resetting of titanite ages is spatially heterogeneous in the Morin terrane. Ages with a predominantly Ottawan signature are recognized in the Morin shear zone, which deforms the eastern lobe of the anorthosite, in an overprinted skarn zone on the western side of the massif, and in the Labelle shear zone that marks its western boundary. In the rest of the Morin terrane, titanite with Shawinigan ages appear to have been only partially reset during the Ottawan. Further work is needed to better understand the relationship between the character of Ottawan metamorphism and resetting in different parts of the Morin terrane. Key words: Grenville Province, Morin terrane, geochronology, metamorphism. Le terrane de Morin est un bloc crustal allochtone dans le sud-ouest de la province de Grenville dont l'histoire metamorphique est assez mal delimitee. Dans cette partie de la province de Grenville, certains terranes faisaient partie de la croute intermediaire ductile durant la collision, de 1,09 a 1,02 Ga, de la Laurentie avec le craton amazonien (la phase ottavienne de l'orogenese grenvillienne), alors que d'autres terraines faisaient partie de la superstructure de l'orogene. De nouvelles donnees geochronologiques U--Pb portent a croire que le terrane de Morin a connu un metamorphisme au facies des granulites durant l'orogenese accretionnaire de Shawinigan (1,19-1,14 Ga) et a nouveau durant l'Ottavien. La datation de sept echantillons de zircons provenant de la suite anorthositique de Morin, de 1,15 Ga, a ete effectuee afin de confirmer les ages determines precedemment, et des bordures metamorphiques ottaviennes (1,08-1,07 Ga) ont ete observees dans deux echantillons. La datation U--Pb de titanite dans neuf echantillons de marbres entourant la suite anorthositique de Morin ont donne un melange d'ages entre les metamorphismes shawiniganais et ottavien (n = 7) et des ages a dominance ottavienne (n = 2). Nos resultats font ressortir l'heterogeneite spatiale de la croissance des zircons et de la reinitialisation des ages de titanite durant l'Ottavien dans le terrane de Morin. Des ages de signatures a dominance ottavienne sont releves dans la zone de cisaillement de Morin, qui deforme le lobe est de l'anorthosite, dans une zone de skarn superposee sur le cote ouest du massif et dans la zone de cisaillement de Labelle, qui marque sa limite ouest. Dans le reste du terrane de Morin, la titanite qui donne des ages shawiniganais semble n'avoir ete que partiellement reinitialisee durant l'Ottavien. D'autres travaux sont necessaires pour mieux comprendre la relation entre le charactere du metamorphisme et la reinitialisation a l'Ottavien dans differentes parties du terrane de Morin. [Traduit par la Redaction] Mots-cles : province de Grenville, terrane de Morin, geochronologie, metamorphisme., Introduction The Morin terrane is a granulite-facies sector of the southern Grenville Province and has many similarities with the betterstudied Adirondack Highlands to the south (Fig. 1). It is dominated [...]

The Bancroft terrane and the associated Central Metasedimentary Belt boundary thrust zone represent the northern part of the Central Metasedimentary Belt (CMB) of the Canadian Grenville Province. Only a few direct pressure and temperature calculations based on phase equilibrium petrology methods exist in the central Bancroft terrane, and this study applies thermodynamic approaches such as garnet isopleth geothermobarometry to fill this gap and investigate the metamorphic history of the northern CMB. Four metapelitic rock samples were collected in the vicinity of the enigmatic Bancroft shear zone, which approximates the border between the Bancroft terrane and the Elzevir terrane to the south. Garnet isopleths for these samples only intersect if a certain amount of excess oxygen is added to the bulk rock composition corresponding to a [Fe.sup.3+]/F[e.sub.tot] ratio of 0.33-0.38. The northernmost sample records metamorphic peak conditions of approximately 1 GPa and 780 [degrees]C, whereas the southernmost sample, which is located in the Elzevir terrane, records a peak metamorphic pressure of approximately 0.9 GPa at a temperature of 520 [degrees]C. The latter result contradicts previous pressure estimates of the region and the proposed metamorphic field gradient but is based on a poorly constrained sample in terms of thermodynamic modeling. Hence, we conclude that the metamorphic field gradient in the northern CMB conceals two different P-T trajectories. Such a scenario is commonly observed in crustal thickening models and suggests that the cold upper plate (Elzevir terrane) was thrust over the warm lower plate (Bancroft terrane) in a northwesterly direction. Key words: Bancroft terrane, Central Metasedimentary Belt, Grenville Province, excess oxygen, thermodynamic modeling. Le terrane de Bancroft et la zone de chevauchement bordant la ceinture metasedimentaire centrale associee representent la partie nord de la ceinture metasedimentaire centrale (CMC) de la Province canadienne de Grenville. Seules quelques determinations directes de la pression et de la temperature basees sur des methodes petrologiques d'equilibre des phases existent pour la partie centrale du terrane de Bancroft, et la presente etude applique des approches thermodynamiques comme la geothermobarometrie basee sur les isoplethes de grenat pour combler cette lacune et examiner l'histoire metamorphique de la partie nord de la CMC. Quatre echantillons de metapelite ont ete preleves a proximite de l'enigmatique zone de cisaillement de Bancroft, qui marque l'emplacement approximatif de la limite entre le terrane de Bancroft et le terrane d'Elzevir au sud. Les isoplethes de grenat pour ces echantillons ne se croisent que si une certaine quantite d'oxygene excedentaire est ajoutee a la composition de la roche totale correspondant a un rapport [Fe.sup.3+]/F[e.sub.tot] de 0,33-0,38. L'echantillon preleve le plus au nord donne des conditions du pic du metamorphisme d'environ 1 GPa et 780 [degrees]C, alors que l'echantillon preleve le plus au sud, dans le terrane d'Elzevir, enregistre une pression metamorphique d'environ 0,9 GPa a une temperature de 520 [degrees]C. Ce dernier resultat est en contradiction avec des estimations passees de la pression pour la region et le gradient metamorphique de terrain propose, mais repose sur un echantillon dont le modele thermodynamique est mal contraint. Nous en concluons que le gradient metamorphique de terrain dans la partie nord de la CMC dissimule deux trajectoires P-T differentes, un scenario souvent observe dans des modeles d'epaississement crustal et qui indiquerait que la plaque superieure froide (terrane d'Elzevir) a ete charriee vers le nord-ouest sur la plaque inferieure chaude (terrane de Bancroft). Mots-cles : terrane de Bancroft, ceinture metasedimentaire centrale, Province de Grenville, exces d'oxygene, modelisation thermodynamique., Introduction The Canadian Grenville Province exposes the western roots of an ancient orogenic belt that surrounded the Laurentian and Baltic cratons, forming the supercontinent Rodinia around 1 Ga (e.g., Cawood [...]

The extensive P--T stability and the high chemical variability of tourmaline (Tur) together with its common occurrence in metasediments proves its high potential for petrological and (isotope) geochemical studies on fluid--rock interaction in subduction-and collision-related rocks. This paper reviews the occurrence, major element chemistry and boron isotopic composition of Tur in high- and ultrahigh-pressure metamorphic (UHPM) rocks. In addition, it presents a new discovery of coesite-bearing Tur (schorl) from the Erzgebirge (Germany), as well as Tur (dravite) related to the retrograde history of coesite-and diamond-bearing rocks from the Erzgebirge and the Kokchetav Massif (Kazakhstan). The scarce data on worldwide occurrences of (U)HPM Tur reveal a high occupation of the crystallographic X-site (dominated by Na) and the possible presence of excess B, with little further distinctiveness in its major element composition when compared with Tur from medium-grade rocks. High [K.sub.2]O contents in Tur are probably not related to UHP growth or equilibration. The B isotopic composition of (U)HPM Tur ranges in [delta].sup.11] B from -16 to +1 [per thousand], with many samples in or below the range of continental crust. In contrast, Tur formed during retrograde fluid influx typically shows high [[delta]sup.11] B values (up to +28 [per thousand]), suggesting heavy-B fluids infiltrating the exhuming (U)HPM units. Coesite inclusions in Tur, characterized by Raman spectroscopy, are regarded as the best indicator for its UHP stability. Supplementary material: Analytical methods, tourmaline compositions and boron isotope values are available at http://www.geolsoc.org.uk/SUP18354.

The ages obtained by the ([sup.40]Ar-[sup.39]Ar encapsulation technique (retention and total gas ages) on < 2 [micro]m fractions of five metapelites from the Eastern Andean Metamorphic Complex and two from the Chonos Metamorphic Complex allow discussion of the latest recorded metamorphic event in each zone. The Kubler Index (KI) of illite/muscovite (principal component of the metapelites) varies between 0.15[degrees] and 0.45[degrees] [DELTA][degrees]2[theta], indicating regional variation from diagenetic to epizonal metamorphic grade. The [sup.40]Ar-[sup.39]Ar encapsulation analyses reveal [sup.39]Ar loss varying between 21 and 25%, which shows a limited positive correlation with KI values. The obtained retention and total gas metapelite ages reflect distinct metamorphic conditions. Retention ages most probably indicate burial or regional metamorphic events without plutonic influence in the southern Eastern Andean Metamorphic Complex. Total gas ages reflect contact metamorphic ages for metapelites close to intrusions in the northern and southern Eastern Andean Metamorphic Complex and in the Chonos Metamorphic Complex. The thermal overprinting of metapelites occurred in Early Cretaceous times at 130 Ma and 145 Ma and is related to the contact metamorphism of an emplacement pulse of the North Patagonian Batholith. Total gas metapelite ages obtained from the western belt of the Chonos Metamorphic Complex suggest a thermal event related to a distinct pulse of the North Patagonian Batholith. Keywords: [sup.40]Ar-[sup.39]Ar encapsulation, metapelite, metamorphic complexes of Aysen, Chile.

Two types of metamorphic phengites are known: one is linked to high pressure and is 3T; the other is 2[M.sub.1], and its composition is linked to rock-compositional constraints. This work investigates the octahedral sheet crystal-chemical differences between the two phengite types. Seven dioctahedral micas were studied: (1) one 3Tphengite from an ultrahigh-pressure metagranitoid in the Dora Maira massif, Italy (P ~ 4.3 GPa, T ~ 730 [degrees]C); (2) five 2[M.sub.1] phengites from medium-P orthogneisses in the Eastern Alps metamorphic basement, Italy (P [less than or equal to] 0.7 GPa, T ~500-600 [degrees]C); and (3) one 2[M.sub.1] ferroan muscovite from pegmatite in Antarctica (P [less than or equal to] 0.2 GPa, T ~500 [degrees]C). All micas display significant extents of celadonite substitution. In particular, the 2[M.sub.1] -phengite formulae (calculated on the basis of 11 O) have 0.68 < [sup.IV] Al < 0.82 atoms per formula unit (apfu); octahedral atoms are dominated by Al (1.6-1.8 apfu), with minor and variable Fe (0.20-0.35 apfu) and Mg (0.05-0.17 apfu), and very minor Ti, Mn, and Cr. Total octahedral occupancies are slightly above 2.00 apfu, i.e., there seems to be partial occupancy of the third M site. For all micas, we recorded XAFS spectra on mosaics of carefully separated flakes oriented flat on a plastic support that could be rotated so as to account for the polarization of the synchrotron radiation beam, and we processed them on the basis of the AXANES theory. Spectra show angle-dependent absorption variations for Al and Fe, which can be deconvoluted and fitted by dichroic effects. Pre-edges consistently show most Fe to be [Fe.sup.3+] and little angle-dependent intensity variations. The 2[M.sub.1]-ferroan muscovite from Antarctica displays the same AXANES behavior as 2[M.sub.1]-phengites. By contrast, the ultrahigh-pressure 3T-phengite from Dora Maira (having [sup.IV]Al = 0.42 apfu, and Al and Mg as the dominant octahedral constituents) has XAFS spectra that differ significantly. Not only is the [sup.IV]Al feature strongly reduced, in agreement with the increased Si content, but also Fe XAFS spectra show one broad feature only, indicating that all Fe is [Fe.sup.2+] in a fully disordered distribution with no angle-dependent variations. We conclude that this 3 T-phengite is actually contaminated by exsolved Fe-bearing pyrope platelets, which cannot be resolved under SEM examination; by contrast, the 2[M.sub.1]-phengites, unrelated to high-pressure, suggest Al/[Fe.sup.3+] order over the M1 and (M2, M3) sites, as also does the 2[M.sub.1] pegmatitic muscovite. Keywords: Muscovite, aluminoceladonite, celadonite, white micas, dioetahedral micas, XAFS, XANES, geothermobarometry

Fourier transform infrared (FTIR) microspectroscopy of igneous and metamorphic fluorapatites shows measurable carbonate in all specimens. All specimens show the presence of the type A1 andA2 (carbonate for columnar anion, Fleet et al. 2004) and the type B (carbonate for phosphate) substitutions in the carbonate [v.sub.3] domain (1400-1600 [cm.sup.-1]). This is the first report of the type A2 substitution in naturally occurring apatites. Spectra also show [v.sub.3] carbonate peaks that have not been unambiguously assigned. In some spectra, two doublets can be resolved, indicating two different B-type substitutions. The presence of multiple B-type substitutions in infrared spectra is in accord with the recognition of different mechanisms of substitution of the carbonate for the phosphate tetrahedron and explains the scatter observed in published data. Consideration of crystal structural details shows site symmetry of the carbonate group to be [C.sub.s] rather than [C.sub.2v] as has been previously assumed. Decent of [C.sub.s] symmetry from [D.sub.3h] through [C.sub.2v] lifts degeneracy and preserves orthogonality of [v.sub.3] doublets. Extra type B peaks are not due to lower site symmetry, and Davydov splitting can be ruled out. Results demonstrate that carbonate contents of apatites from pegmatites and cumulate marie magmas are well within analytical capability, opening another line of inquiry into C[O.sub.2] behavior in igneous and metamorphic rocks. The type A1 substitution shows the most promise for thermodynamic calculations that would allow extraction of C[O.sub.2] activities. Analyses show similar ratios of water to carbon dioxide activities recorded in pegmatite apatites. Keywords: IR spectroscopy, pegmatites, analysis, chemical (mineral), thermodynamics, crystal structure

Crystal-size and spatial distributions of minerals in metamorphic rocks provide insight into their nucleation, growth environment, and the metamorphic evolution of their host rocks. Episodic nucleation and growth histories for garnets in eclogites from the DaNe orogen are revealed by variations in crystal size, geochemical zoning, and mineralogy of inclusions in garnets. The studied garnets show pseudo-lognormal crystal-size distributions (CSDs), prograde chemical zoning patterns indexed by mineral inclusions, and are mainly distributed at random. Constant growth rate for each episode is proposed based on the mineralogy of inclusions in garnets and the chemical zoning patterns. The CSD shapes were evaluated in terms of a size-dependent proportionate growth model and a thermally accelerated, diffusion-controlled model but neither is consistent with the geochemical data and the concordant occurrence of mineral inclusions in garnet. Initial increasing nucleation rate followed by a subsequent, medial stage of nearly constant growth and finally declining nucleation rate is inferred from the CSD data. This study suggests that linking chemical analysis with textural analysis is crucial to avoid misleading interpretation solely through CSD shapes of minerals in metamorphic rocks. Keywords: Garnet, crystal size, zoning, growth, DaNe

The geochemistry of kyanite and orthopyroxene eclogites (7 samples) indicate that they are gabbroic cumulates. Incompatible trace elements in these rocks occur at low concentrations compared to regionally associated eclogites that are compositionally similar to basaltic magmas (11 samples). Eclogites with cumulate protoliths commonly contain 100 ppm Zr, >4 ppm Sm, and >0.4 ppm Lu for basaltic eclogites. Because of low Zr concentrations, igneous and metamorphic zircons are rare or absent in these eclogites. Samarium and Lu concentrations are also low in the kyanite and orthopyroxene eclogites, but they have parent/daughter Sm/Nd ratios of 0.23-0.51 and Lu/Hf ratios of 0.22-0.60, higher than most associated basaltic eclogites at 0.22-0.38 and 0.11-0.18, respectively. These results suggest that kyanite and orthopyroxene eclogites are poor targets for zircon geochronologic work, but are good targets for Sm/Nd and Lu/Hf mineral/whole rock geochronology because of their high parent/daughter ratios. Keywords: Norway, kyanite eclogite, geochronology, zircon, gabbro, cumulate, orthopyroxene eclogite, samarium-neodymium, lutetium-hafnium

On the basis of new bulk major and trace element (including REE) as well as Sm-Nd and Rb-Sr isotope data, used in conjunction with available geochronological data, a post-tectonic mafic igneous province and four groups of pre- to syntectonic amphibolite are distinguished in the polymetamorphic Maud Belt of western Dronning Maud Land, East Antarctica. Protoliths of the Group 1 amphibolites are interpreted as volcanic arc mafic intrusions with Archaean to Palaeoproterozoic Nd model ages and depletion in Nb and Ta. Isotopic and lithogeochemical characteristics of this earliest group of amphibolite indicate that the Maud Belt was once an active continental volcanic arc. The most likely position of this arc, for which a late Mesoproterozoic age (c. 1140 Ma) is indicated by available U-Pb single-zircon age data, was on the southeastern margin of the Kaapvaal-Grunehogna Craton. The protoliths of Group 2 amphibolites are attributed to the 1110 Ma Borgmassivet-Umkondo thermal event on the basis of comparable Nd model ages and trace element distributions. Group 3 amphibolite protoliths are characterized by mid-ocean ridge basalttype REE patterns and low Th/Yb ratios, and they are related to Neoproterozoic extension. Group 4 amphibolite protoliths are distinguished by high Dy/Yb ratios and are attributed to a phase of syntectonic Pan-African magmatism as indicated by Rb-Sr isotope data.

Structural and metamorphic data from the island of Amorgos (central Aegean Sea) show evidence for the existence of two distinct high-pressure units, the Metabasite Unit and the Basal Conglomerate Unit. These are exposed at the base of a thick marble sequence and overlying flysch deposits. The Metabasite Unit is characterized by a mineral assemblage of blue amphibole, garnet and clinopyroxene, indicating P-T conditions of 500-600 [degrees]C and >13 kbar. It is juxtaposed below carpholite-bearing metaconglomerates and quartz-rich micaschists of the Basal Conglomerate Unit, for which metamorphic conditions of 300-450 [degrees]C and 10-14 kbar are estimated. The contact between the two units is interpreted as a low-angle detachment fault that accommodated top-to-the-NW sense of motion. The Amorgos succession above the Basal Conglomerate Unit collectively resembles the stratigraphy of external units in the Hellenides and could possibly be correlated with the so-called 'Basal Unit', which crops out in a number of tectonic windows throughout the Aegean Sea. This means that the Metabasite Unit in Amorgos could possibly represent the lowermost structural unit in the central Aegean Sea.

Exhumation of high- and ultrahigh-pressure metamorphic rocks in collisional orogens may be explained by upward extrusion of these rocks, erosion of their overburden, or extensional thinning of the overburden. Some high-pressure terranes, such as the Adula nappe in the Central Alps, fit none of these scenarios. We propose an additional way in which part of the overburden may be removed: it may sink off into the deeper mantle (slab extraction). Structural and metamorphic relationships in and around the Adula nappe indicate that the emplacement of this Alpine high- to ultrahigh-pressure nappe (to 3.2 GPa) in a pile of lower-pressure nappes resulted from the interaction of two subduction zones that accommodated the closure of two ocean basins, ultimately leading to the extraction of the intervening slab. In terms of mechanics, the cause of the exhumation is, in this case, not the buoyancy of the high-pressure rocks, but the negative buoyancy of the extracted slab. Keywords: tectonics, Alps, Adula nappe, eclogite, ultrahigh-pressure metamorphism, exhumation.

The Pan-African (572 Ma) Puttetti syenite (A-type granitoid)-pyroxenite association intrudes the high-grade metamorphic terrain of the Trivandrum Block, South India. Field evidence indicates the contemporaneous nature of syenitic and pyroxenitic liquids. The occurrence of a mixed-rock (~70% syenite and 30% pyroxenite) with an emulsion-like texture, and the occurrence of pyroxenite globules in syenite, is interpreted as relics of immiscible magmas. Both syenite and pyroxenite show similar mineral assemblage (with major minerals having overlapping compositions), but the relative proportions differ. Major element and trace element partitioning trends, parallel REE patterns, and similar Sr initial isotope compositions are in accord with behavior either predicted or measured for immiscible melts in experimental and/or natural systems. The more pronounced Eu anomalies and LREE/HREE ratios of syenite and pyroxenite (relative to the mixed-rock) is related to fractionation caused by immiscible separation. The proposed origin of the Puttetti pluton involves the intrusion of a magma whose bulk composition is that of the mixed-rock. This melt behaved immiscibly and split into two fractions, which produced the syenite and pyroxenite magmas.

The age of Proterozoic granulite facies metamorphism and deformation in the Strangways Metamorphic Complex (SMC) of central Australia is determined on zircon grown in syn-metamorphic and syn-deformational orthopyroxene-bearing, enderbitic, veins. SHRIMP zircon studies suggest that [M.sub.1]-[M.sub.2] and the correlated periods of intense deformation ([D.sub.1][D.sub.2]) are part of a single tectonothermal event between 1,717 [+ or -] 2 and 1,732 [+ or -] 7 Ma. It is considered unlikely that the two metamorphic phases ([M.sub.1], [M.sub.2]) suggested by earlier work represent separate events occurring within 10-25 Ma of each other. Previous higher estimates for the age of [M.sub.1] granulite metamorphism in the SMC (Early Strangways event at ca. 1,770 Ma) based on U-Pb zircon dating of granitic, intrusive rocks, are not believed to relate to the metamorphism, but to represent pre-metamorphic intrusion ages. Conventional multigrain U-Pb monazite analyses on high-grade metasediments from three widely spaced localities in the western SMC yield [sup.207]Pb/[sup.235]U ages between 1,7281 [+ or -] 11 and 1,712 [+ or -] 2 Ma. The age range of the monazites corresponds to the SHRIMP zircon ages in the granulitic veins and is interpreted to record monazite growth (prograde in the metasedimentary rocks). The data imply a maximum time-span of 30 Ma for high-grade metamorphism and deformation in the SMC. There is, thus, no evidence for an extremely long period of continuous high-temperature conditions from 1,770 to ca. 1,720 Ma as previously proposed. The results firmly establish that the SMC has a very different high-grade metamorphic history than the neighbouring Harts Range, where upper amphibolite facies metamorphism in the Palaeozoic caused widespread growth or recrystallization of monazite.

Empirical tests of oxygen exchange rate in zircon crystals from amphibolite- and granulite-facies metamorphic rocks of the Grenville Province demonstrate preservation of igneous [delta][sup.18]O through protracted igneous and metamorphic histories, forming the basis of quantitative estimates of diffusion rate. Granitic orthogneisses, which cooled slowly after granulite-facies metamorphism, show no consistent relationship between zircon size and [delta][sup.18]O, indicating slow oxygen diffusion. Detrital zircon crystals from granulite-facies quartzites are out of equilibrium with their host rocks, and no consistent correlation is seen between [delta][sup.18]O and grain size in high-precision analyses by laser fluorination of multiple grains, sieved for size. In a single sample, individual detrital zircon crystals preserve grain-to-grain variability in [delta][sup.18]O (determined by ion microprobe), ranging from 5.0 to 9.5 [per thousand]. The inherited cores of some zircon crystals are up to 5.6 [per thousand] lower than igneous overgrowths, showing that gradients of 5.6 [per thousand] can be preserved over 50 [micro]m even at magmatic conditions. All of these lines of evidence show that oxygen diffusion in zircon in these rocks was slow both during metamorphism and during slow cooling of 1-3 [degrees]/m.y. Calculations based on the measurements indicate that the oxygen diffusion rate in zircon (D) must be [greater than or equal to] [10.sup.-22] [cm.sup.2]/s at 600 [degrees]C to explain [delta][sup.18]O(zircon) values measured from Grenville quartzite and orthogneiss. This value is consistent with the experimentally determined value of D = 2 x [10.sup.-27] [cm.sup.2]/s for dry diffusion experiments extrapolated to 600 [degrees]C (Watson and Chemiak 1997). These results indicate that oxygen-isotope analysis of zircon may be used to see through granulite-facies metamorphism and anatexis, and to unravel crustal recycling processes in igneous rocks.

[1] The Koolen metamorphic dome of Chukotka Peninsula has been interpreted as a mid-Cretaceous extensional core complex. On the NW flank of the dome, three lithotectonic units are exposed: (1) High-Grade unit, composed of sillimanite-grade rocks; (2) Tanatap unit, composed of polydeformed greenschist-grade phyllites and marbles; (3) Chegitun unit composed of unmetamorphosed early Paleozoic limestones. Four Early Cretaceous to Tertiary deformational events are documented. [D.sub.1] is attributed to the collisional closure of the South Anyui suture. The [sup.40]Ar/[sup.39]Ar ages from the Tanatap unit constrain this event at 117-124 Ma. Relict kyanite-bearing assemblages in the High-Grade unit probably formed during this event under moderately high pressure conditions. [D.sub.2] is related to extensional deformation in the Koolen dome. In the High-Grade unit, sillimanite-grade metamorphism was followed by rapid cooling and exhumation at 109-104 Ma. In the Tanatap unit, early fabrics underwent isoclinal folding, and a new cleavage ([S.sub.2t]) was developed. The kinematics of the High-Grade unit indicates ductile deformation by top-to-the-south shear, as is the case in the dome core. This suggests that the Koolen dome is a unidirectional core complex formed by south directed extension. We believe collapse of overthickened crust caused extension in the Koolen dome. The crust was thickened during Early Cretaceous collision and during subsequent oroclinal bending of the structures in the Bering Strait region. The youngest structures in the Chegitun Valley are NW striking normal faults kinematically linked to NE striking right-lateral strike-slip faults. They are probably the continuation of structures found offshore in the Hope Basin. INDEX TERMS: 8109 Tectonophysics: Continental tectonics--extensional (0905); 8025 Structural Geology: Mesoscopic fabrics; 1035 Geochemistry: Geochronology; 8005 Structural Geology: Folds and folding; KEYWORDS: Koolen Dome, extension, Arctic, thermochronology, core complex. Citation: Toro, J., J. M. Amato, and B. Natal'in, Cretaceous deformation, Chegitun River area, Chukotka Peninsula, Russia: Implications for the tectonic evolution of the Bering Strait region, Tectonics, 22(3), 1021, doi: 10.1029/2001TC001333, 2003.

[1] In eastern China, the Sulu area is recognized as the eastern extension of the Qinling-Dabie Belt, which is famous for its ultrahigh-pressure (UHP) metamorphism. Although numerous petrologic and geo-chemical works are available, structural data are still rare. This paper provides the first extensive study of bulk geometry and kinematic analysis of the Shandong Peninsula. The study area is divided into three tectonic areas by Cretaceous faults, namely, a southern UHP belt or Sulu area, a northern migmatite area, and an eastern eclogite and migmatite area or Weihai area. Conversely to the deeply entrenched idea that the later area belongs to the North China Belt, and the two others to the South China Block (SCB), we argue that all three areas are parts of the SCB. Structural, petrologic, [sup.40]Ar/[sup.39]Ar, and U/Pb data comply with this new interpretation. In the North Shandong area, mafic granulites enclosed as blocks within gneissic migmatites do not significantly differ from the Sulu and Weihai eclogites which also experienced a granulite facies overprint before migmatization. The circa 210-200 Ma age of the main ductile deformation is related to an extensional event during the Triassic (or Indosinian) orogeny. This date corresponds to the temperature climax, but the time of the pressure peak, i.e., the real age of the UHP metamorphism is discussed. INDEX TERMS: 8109 Tectonophysics: Continental tectonics--extensional (0905); 1035 Geochemistry: Geochronology; 3660 Mineralogy and Petrology: Metamorphic petrology; 9609 Information Related to Geologic Time: Mesozoic; 9320 Information Related to Geographic Region: Asia; KEYWORDS: ultrahigh-pressure metamorphism, granulite, migmatite, [sup.40]Ar/[sup.39]Ar dating, ductile deformation, east China. Citation: Faure, M., W. Lin, P. Monie, N. Le Breton, S. Poussineau, D. Panis, and E. Deloule, Exhumation tectonics of the ultrahigh-pressure metamorphic rocks in the Qinling orogen in east China: New petrological-structural-radiometric insights from the Shandong Peninsula, Tectonics, 22(3), 1018, doi:10.1029/ 2002TC001450, 2003.

The conditions at which monazite and allanite were produced and destroyed during prograde metamorphism of pelitic rocks were determined in a Buchan and a Barrovian regional terrain and in a contact aureole, all from northern New England, USA. Pelites from the chlorite zone of each area contain monazite that has an inclusion-free core surrounded by a highly irregular, inclusion-rich rim. Textures and [sup.208]Pb/[sup.232]Th dates of these monazites in the Buchan terrain, obtained by ion microprobe, suggest that they are composite grains with detrital cores and very low-grade metamorphic overgrowths. At exactly the biotite isograd in the regional terrains, composite monazite disappears from most rocks and is replaced by euhedral metamorphic allanite. At precisely the andalusite or kyanite isograd in all three areas, allanite, in turn, disappears from most rocks and is replaced by subhedral, chemically unzoned monazite neoblasts. Allanite failed to develop at the biotite isograd in pelites with lower than normal Ca and/ or A1 contents, and composite monazite survived at higher grades in these rocks with modified texture, chemical composition, and Th-Pb age. Pelites with elevated Ca and/or Al contents retained allanite in the andalusite or kyanite zone. The best estimate of the time of peak metamorphism at the andalusite or kyanite isograd is the mean Th-Pb age of metamorphic monazite neoblasts that have not been affected by retrograde metamorphism: 364.3 [+ or -] 3.5 Ma in the Buchan terrain, 352.9 [+ or -] 8.9 Ma in the Barrovian terrain, and 403.4 [+ or -] 5.9 Ma in the contact aureole. Some metamorphic monazites from the Buchan terrain have ages partially to completely reset during an episode of retrograde metamorphism at 343.1 [+ or -] 9.1 Ma. Interpretation of Th-Pb ages of individual composite monazite grains is complicated by the occurrence of subgrain domains of detrital material intergrown with domains of material formed or recrystallized during prograde and retrograde metamorphism.

Tourmaline-out isograd formed by the breakdown of tourmaline is defined in the upper amphibolite-facies metapelites in the Yanai area, Ryoke metamorphic belt, SW Japan. The rim composition of tourmaline progressively becomes aluminous with ascending metamorphic grade, and the chemical zoning of tourmaline is controlled by [sup.X][square]Al [Na.sub.-1] [Mg.sub.-1] and Mg[Ti.sup.Y-] [Al.sub.-2] vectors in low- to medium-grade zones where muscovite is stable, whereas it is controlled by Mg[(OH).sup.Y] [Al.sub.-1] [O.sub.-1], CaMg[O.sup.X][[square].sub.-1.sup.Y][Al.sub.-1][(OH).sub.-1] and Mg[Ti.sup.Y][Al.sub.-2] vectors in further higher-grade, muscovite-unstable zones. The size of tourmaline increases drastically where breakdown of muscovite + quartz takes place, probably due to the growth of tourmaline during breakdown of muscovite. On the high-temperature side of the tourmaline-out isograd, depletion of whole-rock boron is observed. Escape of boron-bearing melt or the fluid evolved from the melt during its crystallization probably caused this depletion, although locally trapped, boron-bearing melt or fluid formed irregularly shaped tourmaline and dumortierite during retrograde metamorphism.

A diverse suite of ultrahigh-temperature (UHT) granulite xenoliths from kimberlites of the central Kaapvaal craton (South Africa) record the deep crustal manifestation of Neoarchean Ventersdorp extensional tectonics and magmatism. U-Pb geochronological data for texturally characterized metamorphic zircon and monazite from a number of metasedimentary (garnet + quartz [+ or -] sapphirine [+ or -] sillimanite [+ or -] antiperthite) UHT granulites are combined with published geothermobarometry to elucidate the pressure-temperature-time (P-T-t) path of metamorphism. Prograde metamorphic growth of zircon and monazite indicates ramping of lower crustal temperatures [greater than or equal to] 25 Ma prior to peak UHT metamorphic conditions at ca. 2720-2715 Ma. The subsequent cooling path is established by dating of 2690 Ma zircon in retrograde kyanite-rich leucosomes, indicating initial cooling rates of [greater than or equal to] 10[degrees]C/Ma. Preservation of peak and prograde monazite dates through the UHT conditions support a very high volume diffusion closure temperature for this mineral, similar to that of zircon (>1000[degrees]C). The 5-10 Ma duration of UHT metamorphism in the lower crust is directly correlative with the rapid eruption of the Ventersdorp flood basalts through the central Kaapvaal craton at 2714 [+ or -] 8 Ma, and associated crustal melting, plutonism, and widespread extensional tectonics. UHT metamorphism is postulated to have occurred in response to nonuniform intracratonic lithospheric thinning and superimposed magmatic heat advection. The links between upper and lower crustal manifestations of 2.7 Ga magmatism and extension support the hypothesis that the present-day flat seismic Moho of the central and western Kaapvaal craton is a lower crustal rheological response to Ventersdorp tectonomagmatism. The lithospheric thermal gradient necessitated by the UHT lower crustal granulites demands a transient removal of the thermal lithosphere localized along the north-northeast-trending axis of the Ventersdorp rift at 2.72 Ga. However, contemporaneous maintenance of a coherent mantle keel with 'cratonic' thermal gradients 200 km distal to the rift is required in order to preserve ancient (3.2-2.9 Ga) diamondiferous lithosphere. The contrast between the craton-wide geography of crustal extension and the more restricted localization of thermal effects in the lithospheric mantle embodies a profound crust-mantle decoupling in the Kaapvaal craton during intracratonic rifting, and bears upon our understanding of the generation and preservation of cratonic lithospheric mantle keels. Keywords: U-Pb geochronology, zircon, monazite, UHT metamorphism, sapphirine, granulite, lower crust, Ventersdorp Supergroup, Kaapvaal craton, rifting.

The Middle Jurassic Barcroft mafic granodiorite and Late Cretaceous, ternary-minimum McAfee Creek Granite are important components of the igneous arc sited along the SW North American margin. Bulk-rock analyses of 11 samples of the metaluminous, I-type Barcroft comagmatic suite have an average [delta][sup.18]O value of 7.4 [+ or -] 0.6[per thousand] (all values [+ or -] 1[sigma]). Four Barcroft specimens average [[epsilon].sub.Nd] = -3.6 [+ or -] 1.8, [sup.87]Sr/[sup.86]Sr = 0.707 [+ or -] 0.001. The pluton consists of petrochemically gradational, Ca-amphibole-rich gabbro/diorite, granodiorite, metadiorite, and rare alaskite-aplite; for most of the pluton, oxygen isotope exchange of quartz, feldspar(s), biotite, and Ca-amphibole accompanied local deuteric alteration. Eight specimens of slightly peraluminous granitic rocks of the muscovite-bearing McAfee Creek series have an average [delta][sup.18]O of 8.6 [+ or -] 0.5[per thousand]. Four McAfee-type samples average [[epsilon].sub.ND] = -7.8 [+ or -] 1.7, [sup.87]Sr/[sup.86] Sr = 0.711 [+ or -] 0.004. For both plutons, bulk-rock evidence of exchange with near-surface water is lacking, suggesting ~5-10 km cooling depths. Barcroft minerals exhibit regular oxygen isotopic partitioning from high to low [delta][sup.18]O in the sequence quartz > plagioclase > K-feldspar > > amphibole [greater than or equal to] biotite. Along the SE margin of the pluton, quartz and biotite in Lower Cambrian quartzites are higher in [delta][sup.18]O, and show slightly larger fractionations than igneous analogues. Exchange with fluids derived from these heated, contact-metamorphosed country rocks increased bulk [sup.18]O/[sup.16]O ratios of Barcroft border rocks (and constituent plagioclase + subsolidus tremolite-actinolite), especially of granitic dikes transecting the wall rocks. Oxygen isotope thermometry for seven Barcroft pluton quartz-amphibole and six quartz-biotite pairs indicate apparent subsolidus temperatures averaging 519 [+ or -] 49[degrees]C. Quartz-plagioclase pairs from two Barcroft granodiorites yield values of 519 and 515[degrees]C. A quartz-biotite pair from a quartzite adjacent to the Barcroft pluton yields an apparent temperature of 511[degrees]C, in agreement with estimates based on contact metamorphic parageneses. Except for its SE margin, Barcroft pluton silicates evidently exchanged oxygen isotopes under local deuteric conditions. Compatible with Ca-amphibole thermobarometric analyses, areal distributions for quartz-plagioclase, quartz-amphibole, and quartz-biotite pairs reveal that putative annealing temperatures are lowest in NE-trending axial portions of the Barcroft body, so it simply cooled inwards. Intrusion ~70 million years later by the McAfee Creek Granite had no discernable effect on [delta][sup.18]O values of Barcroft minerals and bulk rocks.

The exhumation history of the Shuswap Metamorphic Core Complex (Shuswap MCC) is deciphered from structural analysis and [sup.40]Ar/[sup.39]Ar thermochronology. With the exception of a few samples contaminated by excess argon, the analytical results indicate a consistent range of early Tertiary [sup.40]Ar/[sup.39]Ar ages throughout the area. Hornblende yields ages ranging from ~59-54 Ma; muscovite and biotite ages are clustered between 49.5 and 48 Ma, and are consistent with the timing of closure of K-feldspars, typically ranging from 50 to 43 Ma, except for samples in the immediate footwall of the Columbia River fault that yield ages as young as 26 Ma. These [sup.40]Ar/[sup.39]Ar ages, combined with existing U/Pb ages on zircons and monazites, constrain the cooling history at several localities in the Shuswap MCC between Trans Canada Highway 1 and the Thor-Odin dome. The migmatitic core of the complex is affected by a rapid cooling event (~700 [degrees]C to ~300 [degrees]C), from crystallization of zircons through closure of argon diffusion in hornblende and micas, between ~56 and ~48 Ma. Based on our previous structural analysis, we propose that this first period of rapid cooling follows an exhumation event associated with the formation of the Shuswap MCC by activation of a low-angle detachment and ductile thinning of a previously thickened and partially molten crust. Multidiffusion-domain thermal modeling of K-feldspar constrains the lower temperature history from ~350 [degrees]C to ~150 [degrees]C and indicates a period of thermal stability from ~48 to ~45 Ma, and a cooling event at ~45 Ma, except for the K-feldspars sampled in the immediate footwall of the Columbia River fault, which indicate rapid cooling at ~33 Ma. These cooling events are attributed to exhumation accommodated by local activation of high-angle normal faults. Therefore, we distinguish two periods of exhumation in the Canadian Cordillera: (1) Exhumation in Paleocene time related to crustal thinning and tectonic denudation by activation of detachment zones at ~60-50 Ma, which corresponds to gravitational collapse of the thickened and partially molten Cordilleran crust; and (2) Exhumation related to Eocene regional extension, Basin-and-Range style, at ~45 Ma, with reactivation of the eastern boundary fault of the core complex, the Columbia River fault, in Oligocene time at ~33 Ma. Keywords: Argon thermochronology, Canadian Cordillera, exhumation, metamorphic core complex, Shuswap, tectonics.

A recently proposed model for intraplate orogenesis couples long-term self-heating of basement rocks by radioactive decay with thermal blanketing by overlying sedimentary deposits. This model has been tested in one of the type areas in central Australia, the Proterozoic Musgrave Complex, which was reworked in an Early Cambrian orogeny. We have determined the source of the preorogenic and postorogenic sediments in the Amadeus basin immediately to the north of the reworked basement, including the fan deposits associated with uplift, comprising Uluru (Ayers Rock) and Kata Tjuta (Olgas). Detrital-zircon age populations indicate that all basin sediments were derived from the Musgrave Complex, which was therefore emergent rather than covered by sediments as required by the model. In the basement, the preservation of Mesoproterozoic mica ages during transpressive burial to depths of ~40 km ca. 550 Ma indicates that the associated thermal pulse was short-lived, not long-lived as envisaged in the model. We conclude therefore that the thermal-blanketing model is inconsistent with the isotopic data and that the localization of deformation in intracratonic settings is associated with regions of contrasting strengths; in central Australia, these are along the margins of the Amadeus basin with the Musgrave Complex and Arunta inlier. Keywords: tectonics, provenance, metamorphism, thermal disequilibrium.

We investigated the macroscale fluid flow and oxygen isotope exchange in contact--metamorphic aureoles by using two-dimensional finite-element modeling. The model parameters are assigned according to geologic observations in the Notch Peak metamorphic aureole, Utah. The results show that fluid flow in contact aureoles depends strongly on time and space, unlike the commonly assumed uniform and unidirectional fluid flow in previous studies. In an aureole with homogeneous permeability, a simple convection cell develops in the inner aureole, causing [delta.sup.18]O values to decrease in the inner aureole and increase in the upper outer aureole. However, when the horizontally layered permeability structure of the Notch Peak aureole is represented in the model, the resulting fluid flow is concentrated within high-permeability aquifers, and local convection cells develop near the contact within each aquifer. Sub-horizontal down-temperature flow and up-temperature flow can coexist in the same aquifer. The up-temperature flow in the lower part of each aquifer mainly causes depletion of [sup.18]O in the inner aureole. Release of magmatic fluid significantly enhances the depletion of [sup.18]O in the inner aureole at early stages, although sub-horizontal up-temperature flow at later stages tends to reduce the effects of the early-stage exchange. The isotopic features of the Notch Peak aureole are best explained by (1) infiltration of magmatic fluid having a [delta.sup.18]O value of 8[per thousand], (2) ~10% of the wall rocks involved in the oxygen isotope exchange with fluids, (3) no isotope exchange below 250[degrees]C and during retrograde cooling, and (4) enhanced permeability in the inner aureole in addition to the lithologically layered permeability structure. Keywords: fluid-rock interactions, metamorphic aureoles, Notch Peak, numerical models, oxygen isotopes.

Metamorphic mineral compositions and textures are integrated with microstructures to test tectonic models for the construction and exhumation of a mid-crustal terrane in western Turkey. The southern Menderes massif, part of the Alpine orogen, is composed of a tilted sequence of metasedimentary rocks that structurally overlies orthogneiss. Garnet--biotite equilibria for schists collected along north-south traverses consistently indicate temperatures of 430 [degrees]C for the southernmost, structurally highest garnet-bearing rocks, ~500 [degrees]C for structurally intermediate rocks, and ~550 [degrees]C for the structurally deepest rocks near the contact with the orthogneiss. There are no detectable discontinuities in metamorphic grade from north to south within the schist unit, or between the schist and overlying lower-grade and underlying higher-grade rocks. Pressure estimates are less certain; geobarometric results for the schists yield P [approximately equal to] 8 kbar, but mineral assemblages and structural data are consistent with lower pressures ([less than or equal to] 6 kbar). Metamorphic textures and garnet zoning in the Menderes schists indicate metamorphism during a single thermal event accompanying development of the main penetrative foliation. Top-to-north fabrics formed during folding/thrusting and were synkinematic with garnet growth. Top-to-south chloritic shear bands truncate the earlier foliation and overprint peak-metamorphic assemblages and textures. We propose that these metamorphic features and structures indicate synmetamorphic shortening followed by extension during Alpine orogenesis. Keywords: Alpine orogeny, geobarometry, geothermometry, Menderes massif, Turkey.

The metamorphic sole along the base of the Semail ophiolite in Oman records the earliest thrust slice subducted and accreted to the base of the ophiolite mantle sequence. In the Bani Hamid area (United Arab Emirates) a c. 870 m thick thrust slice of granulite facies rocks includes garnet + diopside amphibolites, enstatite + cordierite + sillimanite + spinel [+ or -] sapphirine quartzites, alkaline mafic granulites (meta-jacupirangites) quartzo-feldspathic gneisses and calc-silicates. The latter contain garnet + diopside + scapolite + plagioclase [+ or -] wollastonite. P-T conditions of granulite facies metamorphism are in the range 800-860[degrees]C and 10.5 [+ or -] 1.1 kbar to 14.7 [+ or -] 2.8 kbar. Garnet + clinopyroxene + hornblende + plagioclase amphibolites from the metamorphic sole record peak P-T conditions of 840 [+ or -] 70[degrees]C and 11.6 [+ or -] 1.6 kbar (THERMOCALC average P-T mode) and 840-870[degrees]C and 13.9-11.8 kbar (conventional thermobarometry) with low degrees of partial melting producing very small melt segregations of tonalitic material. Pressure estimates are equivalent to depths of 57-46 km beneath oceanic crust, much deeper than can be accounted for by the thickness of the ophiolite. [sup.40]Ar/[sup.39]Ar hornblende ages from the amphibolites range from 95-93 Ma, synchronous with formation of the plagiogranites in the ophiolite crustal sequence (95 Ma), eruption of the Lasail (V2) volcanic sequence and deposition of Cenomanian-Turonian radiolaria in metalliferous sediments between the Geotimes (V1) and Lasail (V2) lavas. Protoliths of the metamorphic sole were Triassic-Jurassic and early Cretaceous Haybi volcanic rocks, Exotic limestones and quartzites and were clearly not equivalent to the Semail ophiolite rocks, showing that initiation of subduction could not have occurred at the ridge axis. Heat for metamorphism was derived from the mantle sequence harzburgites and dunites which were at or around 1100-1500[degrees]C. All data from the sub-ophiolite metamorphic sole in Oman and the United Arab Emirates indicate that the ophiolite was formed in a Supra-Subduction zone setting and that obduction occurred along a NE-dipping high-temperature subduction zone during Late Cretaceous times. Keywords: Oman, ophiolite, subduction, mid-ocean ridges, metamorphism.

Using electron microprobe analysis, researchers evaluate the crystal field stabilization energy of rock-forming minerals, focusing on the partitioning of Fe(2+). Research indicates that the Fe(2+) partitioning behavior is more highly affected by geometrical factors rather than crystal field stabilization energy.

Zircon growth in high-grade metamorphic rocks may be triggered by net transfer reactions involving the breakdown of a phase bearing zirconium (Zr). We have measured the concentration of Zr in the major minerals in granulite-facies rocks of differing bulk composition. Both garnet and hornblende contain tens of parts per million Zr, and no other major phase contains significant Zr. Simple calculations show that reaction of either garnet or hornblende to form non-Zr-bearing phases will release sufficient Zr to account for at least some new zircon growth. U-Pb ages from new zircon, grown as a result of either hornblende or garnet breakdown, are not expected to record the time of peak metamorphism, but rather will record the time of particular metamorphic reactions, allowing direct correlation of zircon ages with petrologically derived pressure-temperature-time paths. This approach offers the potential for more rigorous interpretation of the metamorphic significance of zircon ages than has previously been possible.

Low-grade metamorphic rocks from the Littleton Formation, New Hampshire, and the Carrabassett Formation, Maine, show large compositional differences at the centimeter, meter, and kilometer scales. Quartzite rocks show greater variation than slates. Most of the rocks contain mainly clay, quartz, and chlorite-mica. High-grade rocks show similar compositional differences. These compositional differences have been inherited from the sedimentary protoliths with few changes. However, elements have moved a few centimeters during conversion of sedimentary rocks into metamorphic rocks.

The Point Lake orogenic belt in the Archean Slave province of Canada contains deformed metasedimentary rocks ranging from greenschist to granulite facies. At low grade, combinations of simple shear, pure shear, and initial lamina shape form a wide geometric variety of strained cross-laminae. At higher metamorphic grades, deformed sedimentary structures are recrystallized and in places included in porphyroblasts as internal foliations or inclusion trails. Different manifestations of deformed sedimentary layering produce textures resembling early-differentiated tectonic foliations. Recognition of the reorientation of sedimentary structures and their progressive incorporation into porphyroblasts greatly simplifies the structural history of the Slave province; previous models invoked as many as ten structural events to account for observed fabrics and inclusion trails, whereas the model presented herein requires only two.

A detailed survey was conducted of the Nd-io\solope compositions of Precambrian metamorphic rocks both within and adjacent to the Cheyenne belt, a metamorphosed crustal segment in southeastern Wyoming that defines the Proterozoic suture zone between the Archean Wyoming craton and accreted Proterozoic island arc(s). The results of the study indicate that Proterozoic miogeoclinal sedimentary rocks exposed north of the Cheyenne belt and deposited at he southern margin of the Wyoming craton contain only Archean crustderived material and no detritus from proterozoic island arcs approaching from the south. Similarly, Proterozoic crust exposed south of the Cheyenne belt contains no crustal material derived from Archean sources. However, within the Cheyenne belt, paragneiss of the Barber Lake block has 2.0 to 2.4 Ga Nd model ages and is interpreted as metamorphosed intermixed sedimentary detritus from both Archean and Proterozoic sources. The paragneiss is thought to represent remnants of oceanic trench or foredeep sediments deposited in close proximity to both the archean craton and the Proterozoic island arcs, similar to sedimentary rocks with 2.0 to 2.4 Ga Nd model ages exposed in the Penokean orogen in Wisconsin. The overall lack of 2.0 to 2.4 Ga modal age crustal material in the vicinity of the Cheyenne belt may reflect the fact that much of this crustal material was overthrust onto the Wyoming craton during accretion and then eroded during postorogenic rebound of the continental margin.

Mafic meta-igneous supracrustal rocks in the east-central part of the western Triassic and Paleozoic belt are interlayered with and appear to predominantly overlie fine-grained clastic and cherty metasedimentary strata. This complex constitutes a lithostratigraphic terrane exposed in the vicinity of Sawyers Bar, California. Basaltic flows, dikes, and sills are informally referred to as the 'Yellow Dog greenstones.' Dark-colored, amygdaloidal flow breccias rich in titanium, iron, phosphorus, and light rare-earth elements (LREE's) constitute the lowest, mildly alkalic members of the mafic flow series, intimately interlayered with fine-grained terrigenous detritus. Element proportions suggest that the dark, Ti + [Fe.sup.*] + P-rich basaltic lavas may have been extruded in an oceanic intraplate setting. Interstratification with distal turbiditic strata indicates concurrent submarine volcanism and deep-sea sedimentary deposition. The main mass of the stratigraphically higher extrusive sequence is a light-colored, massive tholeiite series, poorer in Ti + [Fe.sup.*] + P + LREE's. Hypabyssal rocks belong exclusively to this second igneous suite. Compositions of the pale-colored, overlying basaltic/diabasic mass of the Yellow Dog greenstone section are consistent with eruption in an immature magmatic arc. Dikes presumably comagmatic with terminal-stage Yellow Dog volcanism cut the overlying, more eastern Stuart Fork terrane. The Yellow Dog greenstones contain relict magmatic clinopyroxene and/or pargasitic hornblende. Both melt series include Mg-rich units ([ave.sub.17]: MgO = 14.4 [+ or -] 1.9 wt. %; Cr = 650 [+ or -] 200 ppm; Ni = 270 [+ or -] 100 ppm). Sea-floor alteration apparently affected these mafic rocks in the marine environment following eruption, as reflected by [[delta].sup.18.O], MgO, and CaO/[Al.sub.2.O.sub.3] values and by minor alkali and silica metasomatism. Yellow Dog igneous activity was extinguished during mid-Jurassic suturing of the oceanward Sawyers Bar terrane against and beneath the pre-existing landward Stuart Fork blueschist complex of Late Triassic metamorphic age. Subsequently, the terrane amalgam was invaded by Middle/Late Jurassic granitoids. Minor chemical alteration, including [sup.18.O] enrichment, also probably accompanied later regional, and especially contact, metamorphism. Nevertheless, many major-, minor-, and rare-earth-element variations in the Yellow Dog greenstones are compatible with igneous fractional crystallization trends but exclude the assimilation of significant amounts of sialic crust.

Metamorphic pressure-temperature (P-T) paths from the upper and lower plates of the Fall Mountain nappe, southwest New Hampshire, reveal different thermal histories in the two structural levels. Upper-plate rocks experienced early low-P, high-T (contact?)) metamorphism to peak P-T conditions of 700-750 [degrees]C, 3-4.5 kbar. These P-T conditions were followed by loading to approximately 5-6 kbar, and then by nearly isobaric cooling to approximately 500 [degrees]C, 4.5 kbar. Upper-plate rocks then experienced minor heating with unloading and finally cooling. The lower-plate rocks also experienced early low-P, high-T (contact?) metamorphism but only to P-T conditions of 480-510 [degrees]C, 2-3.5 kbar. Nappe emplacement resulted in nearly isothermal loading to 500 [degrees]C, 5-6 kbar, which was followed by either heating or cooling, depending on the proximity of the sample to the upper plate. The early pressures recorded in both the upper- and lower-plate rocks suggest a depth of approximately 10 km for origination of the nappe; the post-nappe pressures (5-6 kbar) imply a total tectonic thickness of approximately 20 km or a doubling of the overburden thickness. Diffusion calculations suggest that the emplacement of the nappe took less than approximately 10 m.y., and U/Pb chronology on zircon suggests that it occurred between 400 and 410 [Ma.sup.40.Ar/.sup.39] Ar cooling ages on micas indicate that the terrane was cooled to approximately 300 [degrees]C by 340 Ma.

A previously unrecognized pattern of garnet zoning has been preserved in garnets from the upper amphibolite facies Brit domain, Ontario Grenville province. Some large garnets contain discrete regions of high grossular content as well as variations in the Mg/(Mg+Fe) and Mn nearly parallel to grain boundaries. The latter are superimposed on and cut across regions of high grossular content. Such crosscutting zoning patterns have not been previously recognized in garnets. The Ca zoning involves changes of 4 to 10 mol% grossular and is interpreted to represent growth zoning, whereas the decreasing Mg/(Mg+Fe) and increasing Mn from core to rim are believed to be a diffusional retrograde effect. The Ca zoning indicates a pressure drop of ca. 5 kbar at 700 [+ or -] 50[degrees]C (< 5[degrees]C/km). Preservation of grossular-rick areas also suggests that the diffusion rate of Ca is slower than those of Fe, Mn, and Mg in garnet.

A proposed model for the homogenization of oxygen isotopes in the Thor-Odin metamorphic core complex in British Columbia is questioned on the basis that the model failed to include other investigations documenting hydrothermal fluid flow in the complex. The mineralization in the area proposed by the investigators is also raised as a point of contention. The reply of the proponents of the model is presented.

Diamonds occurs in ultrahigh pressure metamorphic rocks from Dabie Shan, Anhui Province, eastern China. Diamond-bearing rocks include eclogite, garnet-pyroxenite, and jadeitite. Diamonds occurs in a mineral assemblage with coesite and jadeite. The diamonds and diamondiferous rocks of Dabie Shan are interpreted to be the products of ultrahigh pressure metamorphism in the underthrust basement of the Yangtze continental plate during the early Mesozoic, at greater than 4.0 gigapascals and 900 ° C. This interpretation is based on the distribution of rock units, the stability field of diamond, the isotopic data indicating a crustal origin for he rocks. Most diamonds occur as euhedral inclusions in garnets and are 10 to 60 micrometers across, although some are up to 700 micrometers across., Formation of diamond requires high pressures and temperatures; thus, its presence in crustal metamorphic rocks has important implications of tectonic models of metamorphism in that burial of crustal rocks to [...]

Feldspars are a common component in igneous and metamorphic rocks. Most feldspars exhibit luminescence, and this has proved useful in a number of mineralogical applications. In this paper, we concentrate on the thermoluminescence (TL) properties of feldspar, or the luminescence produced when a sample of feldspar is heated. We determined the induced TL properties of four feldspars of various compositions in their natural states, and after heating, and we compared the TL data with structural changes as determined by X-ray diffraction. The major TL peak at 120-240 [degrees] C in the TL glow curve, a plot of light intensity against temperature, varies significantly among feldspar-bearing samples. Meteorites and lunar samples with slow cooling histories of ~10 [degrees] C/My as determined by independent methods, have induced TL peak temperatures of ~120 [degrees] C, while samples with fast cooling histories (~100 [degrees] C/My) have induced TL peak temperatures of ~220 [degrees] C. This variation in TL peak temperature can be reproduced by heating the present feldspar samples, meteorites and lunar samples prior to the TL measurement. Most of the present samples in their natural state had TL peak temperatures of ~120 [degrees] C. Heating below 750 [degrees] C in the laboratory caused no change in TL peak temperatures or the structural disorder of the feldspar, while heating [is greater than] 750 [degrees] C caused TL peak temperatures to move to ~220 [degrees] C and disordered the feldspar structure. We suggest that induced TL peak temperature in feldspar is influenced by the degree of Al-Si ordering in the feldspar. Thus, induced TL peak temperature can be used as an indicator of cooling rate for igneous and metamorphic rocks.

Abundant exsolved quartz rods occur in matrix clinopyroxene of eclogite from the Kokchetav massif, Kazakhstan. These rocks are diamond-grade, ultrahigh-pressure (UHP) metamorphic rocks that recrystallized at P [is greater than] 6 GPa and T [is greater than] 1000 [degrees] C. Zircon is an excellent container, which effectively protects peak UHP metamorphic phases from retrogression. Therefore, to ascertain the pre-exsolution composition of the clinopyroxene, we analyzed clinopyroxene inclusions in zircon of the eclogite and a diamond-bearing biotite gneiss. Clinopyroxene in zircon has an excess of [Fe.sup.3+] + [sup.VI]Al over [sup.IV]Al + Na + K, and calculated cation totals significantly less than 4.0 per six O atoms. The stoichiometry of these pyroxenes can be reconciled if the Ca-Eskola end-member ([Ca.sub.0.5] [[].sub.0.5]Al[Si.sub.2][O.sub.6]) is considered. The zircon-hosted clinopyroxene in the eclogite contains up to 9.6 mol% of the Ca-Eskola component, and in the biotite gneiss up to 18.2 mol%, whereas the matrix clinopyroxene contains much less (1.3 mol%, on average). Recalculation of the composition of the matrix clinopyroxene prior to exsolution of quartz rods yields 6.8 mol% Ca-Eskola component, which is consistent with the composition of the clinopyroxene inclusions in zircon. We conclude that the Ca-Eskola component in the peak metamorphic clinopyroxenes broke down by a retrograde reaction: 2 [Ca.sub.0.5] [[].sub.0.5]Al[Si.sub.2][O.sub.6] [right arrow] Ca[Al.sub.2]Si[O.sub.6] + 3 Si[O.sub.2] resulting in exsolution of the quartz rods in the matrix clinopyroxene. Subducted crustal and supracrustal rocks form the Ca-Eskola clinopyroxene at high pressures and temperatures. The vacancy-containing clinopyroxene may have an important bearing on the physico-chemical properties of the subducted slab at upper mantle depth.

Kinoshitalite, ideally Ba[(Mg).sub.3]([Al.sub.2][Si.sub.2])[O.sub.10][(OH,F).sub.2], was studied in a granulite-facies manganese ore associated with calcium-bearing magnesian tephroite, hausmannite, calcite and manganoan clinochlore, and in a second assemblage together with hausmannite, altered tephroite, manganoan diopside, and calcite. The metamorphic rocks occur enclosed in peridotites of the Semail Ophiolite, Sultanate of Oman and were metamorphosed during ophiolite obduction. Single-crystal X-ray data, collected on an inclusion-free kinoshitalite of [Ba.sub.0.99][K.sub.0.06][Na.sub.0.01] ([Mg.sub.2.64][Mn.sub.0.31][Al.sub.2.01][Si.sub.2.03]) [O.sub.10]([OH.sub.1.61][F.sub.0.37][Cl.sub.0.02]) composition, yielded a C-centered lattice of monoclinic symmetry with a = 5.316(1), b = 9.230(2), c =10.197(2) [Angstrom], [Beta] = 100.06(1) [degrees], V= 492.6 [[Angstrom].sup.3], and Z = 1 characteristic of the 1 M polytype. The structure was refined in two models assuming complete Si, Al ordering in the space groups C2 and C[bar] 1 allowing for additional twinning. The data clearly suggest that in spite of the Si/Al ratio of 1 assumption of complete Si, Al ordering can be rejected. Two models, both in agreement with space group C2/m, causing Si, Al disorder, are discussed. Three-dimensional Si, Al disorder would lead to violation of the Al avoidance rule and substantial Ba displacement. The rather well defined Ba position capped by two six-membered rings of [Si.sub.3][Al.sub.3][O.sub.18] composition suggests one dimensional disorder where completely Si, Al ordered layers exist parallel to (001). The disorder occurs perpendicular to (001) and may be interpreted as random stacking faults.

Spatial correlation functions, which quantify spatial relationships among porphyroblasts over a range of length scales, can be used in combination with other techniques of quantitative textural analysis to constrain crystallization mechanisms in metamorphic rocks. The utility, reliability, and robustness of these functions, however, depend critically upon correct methods of calculation and application to geological samples. Application of the L'-function, Pair Correlation Function, and Mark Correlation Function (Stoyan and Stoyan, 1994) to artificial arrangements of crystals yields results consistent with their predetermined ordering and clustering qualities. These results serve as a foundation for the interpretation of more complex simulated and natural crystal arrays. Analysis of artificial and simulated crystal arrays in which ordering signals are obscured in various ways (displacing crystals in an ordered array by increasing amounts, reducing the number of crystals, and increasing the sample's aspect ratio) demonstrates that these scale-dependent functions are robust indicators of effects diagnostic of certain crystallization mechanisms, even in complex circumstances. The effects of clustering of nucleation sites, however, can strongly obscure any underlying signal that might reveal crystallization mechanisms. The L'-function and the Pair Correlation Function are sensitive to short-range ordering of crystals, which may reflect suppression of nucleation in the vicinity of growing porphyroblasts. The Mark Correlation Function is sensitive to size-isolation correlations, which may reflect retardation of growth among crystals competing for nutrients. Interpretation of these functions, however, requires careful attention to proper calculation of Monte Carlo simulations, which are used to identify values of the functions that constitute a null-hypothesis region for comparison to samples with unknown ordering and clustering characteristics. To yield functional values commensurate with those calculated for a particular natural rock specimen, each simulation must match as closely as possible several critical features of the natural rock, including the set of crystal radii, limitations on the observability of crystals, and the shape and size of the bounding surface of the sample. Crystallization mechanisms in seven previously studied gametiferous rocks from three localities (Carlson et al. 1995; Denison and Carlson 1997) have been reassessed using both scale-dependent correlation functions and single-valued spatial statistics, both evaluated by comparison to rigorously computed null-hypothesis regions. The results confirm previous inferences that the nucleation and growth rates of the garnet porphyroblasts in these specimens were governed by rates of diffusion through the intergranular medium.