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In this study, we undertook zircon U--Pb dating and Hf isotopic and whole-rock geochemical analysis of Mesozoic granites from the Zhalantun and Arxan areas of the Great Xing'an Range, northeastern China. Zircon U--Pb dating of four samples indicates they formed in the Early Cretaceous (ca. 140.7-130.0 Ma). Geochemically, the samples have high silica (74.00-77.29 wt.%) and total alkali (8.52-8.90 wt.%) and low CaO (0.21-0.71 wt.%), MgO (0.06-0.48 wt.%), and [P.sub.2][O.sub.5] (0.01-0.06 wt.%) contents and exhibit the weakly peraluminous (A/CNK = 0.98-1.06) and high-K calc-alkaline characteristics, with highly differentiated I-type granite affinities (differentiation index = 93.19-96.93). The studied granites are enriched in light rare earth and large ion lithophile elements and depleted in heavy rare earth and high field strength elements. Their zircon [[epsilon].sub.Hf](t) values are mainly positive (+2.49 to +11.64) and yield two-stage Hf model ages of 1023-445 Ma. The geochemical and isotopic data indicate they might have been derived from the partial melting of medium- to high-K basaltic crust material during the Mesoproterozoic to Phanerozoic. They underwent fractional crystallization in the magma evolution process, and K-feldspar and plagioclase were the major crystallization phases. Combined with the temporal--spatial migration of the Mesozoic magmatic events in the whole northeastern China, we propose that the generation of Early Cretaceous highly differentiated I-type granites in the central Great Xing'an Range was associated with the delamination effect caused by the roll-back of the Paleo-Pacific plate. Key words: central Great Xing'an Range, Early Cretaceous, highly differentiated I-type granites, zircon U--Pb--Hf isotopes, Paleo-Pacific plate. Nous avons effectue la datation U--Pb sur zircons et l'analyse geochimique des isotopes de Hf et sur roche totale de granites mesozoiques des regions de Zhalantun et d'Arxan de la chaine du Grand Xing'an (nord-est de la Chine). La datation U--Pb sur zircons de quatre echantillons indique qu'ils se sont formes au Cretace precoce (vers 140,7-130,0 Ma). Du point de vue geochimique, les echantillons ont de fortes teneurs en silice (74,00-77,29 % en poids) et en alcalis totaux (8,52-8,90 % en poids) et de faibles teneurs en CaO (0,21-0,71 % en poids), MgO (0,06-0,48 % en poids) et [P.sub.2][O.sub.5] (0,01-0,06 % en poids) et presentent des caracteres legerement hyperalumineux (A/CNK = 0,98-1,06) et calcoalcalins riches en K, ainsi que des affinites avec des granites de type I fortement differencies (indice de differentiation = 93,19-96,93). Les granites etudies sont enrichis en terres rares legeres et en elements lithophiles a grand rayon ionique et appauvris en terres rares lourdes et en elements a forte liaison atomique. Les valeurs de [[epsilon].sub.Hf](t) de leurs zircons sont principalement positives (de +2,49 a +11,64) et donnent des ages de modele Hf en deux etapes de 1023-445 Ma. Les donnees geochimiques et isotopiques indiquent qu'ils pourraient avoir ete derives de la fusion partielle de croute basaltique a teneur en K intermediaire a forte durant l'intervalle du Mesoproterozoique au Phanerozoique. Ils ont subi une cristallisation fractionnee durant le processus d'evolution magmatique, le feldspath K et le plagioclase etant les principaux produits de la cristallisation. Nous proposons que, combinee a la migration dans le temps et l'espace des episodes magmatiques mesozoiques a la grandeur du nord-est de la Chine, la production de granites de type I hautement differencies au Cretace precoce dans la chaine du Grand Xing'an centrale etait associee a l'effet de delamination cause par le recul de la plaque paleo-pacifique. [Traduit par la Redaction] Mots-cles : chaine du Grand Xing'an centrale, Cretace precoce, granites de type I fortement differencies, isotopes de U--Pb--Hf de zircons, plaque paleo-pacifique., Introduction Widespread magmatic events took place in northeastern China (NE China) during the Phanerozoic (Wu et al. 2000, 2002, 2006, 2007, 2011; Ge et al. 2005, 2007; Liu et al. [...]

Mesozoic magmatic rocks are widely distributed in the North China Craton (NCC) and are crucial to understanding the timing, location, and geodynamic mechanisms of lithospheric thinning of the NCC. In this study, we report geochronological, petrogeochemical, and Lu-Hf isotopic data for adakitic granitoids from different parts of Xiuyan pluton in the Liaodong Peninsula, aiming to constrain their magma sources, petrogenesis, and tectonic implications. The adakites are metaluminous to weakly peraluminous and are classified as high-K calc-alkaline I-type granite with Early Cretaceous zircon U-Pb ages of 129-126 Ma. They exhibit adakite-like geochemical characteristics, such as high Sr content and low Yb and Y contents, coupled with high Sr/Y and no pronounced Eu anomalies. They are enriched in Rb, U, and light rare-earth elements and are depleted in Ta, Nb, P, and Ti. The adakites from the eastern part of the pluton have low [[epsilon].sub.Hf](t) values (-8.5 to -4.0) with old [T.sub.DM2] ages (1.57-1.31 Ga), indicating they were derived from the lower crust containing juvenile mantle-derived materials. In contrast, adakites from the northern part of the pluton have lower [[epsilon].sub.Hf](t) values (-19.7 to -16.6) with older [T.sub.DM2] ages (2.21-2.03 Ga), indicating that they were derived mainly from an ancient crust. Our results show that both adakitic magmas were derived from partial melting of delaminated lower crust. Their relatively high MgO and Ni contents and Mg# values indicate that the melts interacted with mantle peridotites. The lower crust delamination beneath the Liaodong Peninsula resulted from paleo-Pacific plate subduction during the Early Cretaceous, which resulted in thinning of Mesozoic crust in the Xiuyan area. Key words: adakite, lower crust delamination, lithospheric thinning, Cretaceous granitoids, Liaodong Peninsula. Des roches magmatiques mesozoiques sont repandues dans le craton de Chine septentrionale (CCS) et sont d'importance cruciale pour la comprehension de la chronologie, de l'emplacement et des mecanismes geodynamiques d'amincissement de la lithosphere dans le CCS. Nous presentons des donnees geochronologiques, petrochimiques et d'isotopes de Lu-Hf pour des granitoides adakitiques de differentes parties du pluton de Xiuyan, dans la peninsule du Liaodong, dans le but d'etablir les sources de leurs magmas, leur petrogenese et leur signification tectonique. Les adakites sont metalumineuses a faiblement hyperalumineuses et tombent dans la categorie des granites calcoalcalins riches en K de type I, donnant des ages U-Pb sur zircons de 129-126 Ma (Cretace precoce). Elles presentent des caracteristiques geochimiques typiques des adakites, comme des teneurs elevees en Sr et faibles en Yb et Y, jumelees a des rapports Sr/Y eleves et l'absence d'anomalies d'Eu prononcees. Elles sont enrichies en Rb, U et terres rares legeres et appauvries en Ta, Nb, P et Ti. Les adakites de la partie est du pluton presentent de faibles [[epsilon].sub.Hf](t) (de -8,5 a-4,0) jumeles a de vieux ages [T.sub.DM2] (1,57-1,31 Ga), ce qui indique qu'elles sont derivees de croute inferieure contenant du materiau derive de manteau juvenile. En comparaison, les adakites de la partie nord du pluton ont de plus faibles [[epsilon].sub.Hf](t) (de -19,7 a -16,6) et des ages [T.sub.DM2] (2,21-2,03 Ga) encore plus vieux, ce qui indique qu'elles sont principalement derivees de croute ancienne. Nos resultats montrent que les deux magmas adakitiques sont derives de la fusion partielle de croute inferieure delaminee. Leurs concentrations en MgO et en Ni et leurs Mg# relativement eleves indiquent que les magmas ont interagi avec des peridotites mantelliques. La delamination de la croute inferieure sous la peninsule du Liaodong est le resultat de la subduction de la plaque paleo-Pacifique au Cretace precoce, qui s'est traduite par l'amincissement de la croute mesozoique dans la region de Xiuyan. [Traduit par la Redaction] Mots-cles : adakite, delamination de la croute inferieure, amincissement de la lithosphere, granitoides cretaces, peninsule du Liaodong., Introduction Adakitic rocks are characterized by low contents of heavy rare-earth elements and high Sr/Y and La/Yb ratios. Several possible origins have been proposed for such rocks, including melting of [...]

Geodynamic evolution in the late Paleozoic is significant for understanding the final amalgamation of the Central Asian Orogenic Belt (CAOB). No consensus has yet been reached regarding the late Paleozoic geodynamic evolution of the northern Great Xing'an Range (GXR) in northeastern China, the eastern CAOB. Furthermore, late Paleozoic syenogranitediabase dyke association is present in the Xiaokele area in northern GXR. It provides an important opportunity to understand the nature of magmatism and the geodynamic evolution during this period. This paper presents new zircon U-Pb ages, zircon Hf isotopic compositions, and geochemical data of whole rocks for Xiaokele syenogranite and diabase. Zircon U-Pb dating suggests that the Xiaokele syenogranite (292.5 [+ or -] 0.9 Ma) and diabase (298.3 [+ or -] 1.5 Ma) were emplaced during the early Permian. The Xiaokele syenogranites have high Si[O.sub.2] contents, low MgO contents, and enriched zircon [[epsilon].sub.Hf](t) values, suggesting that their primary magma was generated by the partial melting of the juvenile crustal material. The Xiaokele diabases have low Si[O.sub.2] contents, high MgO contents, are enriched in large-ion lithophile elements, depleted in high-field-strength elements, and exhibit enriched zircon [[epsilon].sub.Hf](t) values. They derived from a lithospheric mantle source that had previously been metasomatized by slab-derived fluids. Combined with previous research results, we believe that the continent-continent collision between the Xing'an and Songliao blocks occurred during the late early Carboniferous - early late Carboniferous (330-310 Ma), and the two blocks were transformed into a post-collisional extensional setting during the latest Carboniferous - early Permian. Key words: early Permian, syenogranite, diabase, post-collisional extensional setting, Xiaokele, northern Great Xing'an Range. L'evolution geodynamique au Paleozoique tardif est importante pour la comprehension de l'amalgamation finale de la ceinture orogenique d'Asie centrale (COAC). Aucun consensus n'a encore ete etabli quant a l'evolution geodynamique au Paleozoique tardif du Grand Xing'an (GX) septentrional de l'est de la COAC dans le nord-est de la Chine. En outre, une association d'age paleozoique tardif de syenogranite et de dykes de diabase est presente dans la region de Xiaokele du GX septentrional, qui offre une bonne occasion de mieux comprendre la nature du magmatisme et de l'evolution geodynamique durant cette periode. L'article presente de nouveaux ages U-Pb sur zircons, des donnees sur la composition des isotopes de Hf des zircons et des donnees geochimiques sur roche totale pour le syenogranite et la diabase de Xiaokele. La datation U-Pb sur zircons indiquerait que le syenogranite (292,5 [+ or -] 0,9 Ma) et la diabase (298,3 [+ or -] 1,5 Ma) de Xiaokele ont ete mis en place durant le Permien precoce. Les syenogranites de Xiaokele ont des teneurs elevees en Si[O.sub.2] et faibles MgO et presentent des valeurs enrichies de [[epsilon].sub.Hf](t) des zircons, ce qui indiquerait que leur magma primaire est un produit de la fusion partielle de materiau crustal juvenile. Les diabases de Xiaokele ont des teneurs en Si[O.sub.2] faibles et en MgO elevees et sont enrichies en elements lithophiles a grand rayon ionique, appauvries en elements a forte liaison atomique et presentent des valeurs enrichies de [[epsilon].sub.Hf](t) des zircons. Elles proviennent d'une source dans le manteau lithospherique ayant prealablement subi un metasomatisme cause par des fluides issus d'une plaque subductee. A la lumiere de ces resultats et de resultats de travaux anterieurs, nous croyons que la collision continent-continent des blocs de Xing'an et de Songliao s'est produite durant l'intervalle du Carbonifere precoce tardif au Carbonifere tardif precoce (330 Ma-310 Ma) et que les deux blocs se sont transformes en un milieu d'extension post-collision durant l'intervalle de la fin du Carbonifere au Permien precoce. [Traduit par la Redaction] Mots-cles: Permien precoce, syenogranite, diabase, milieu d'extension post-collision, Xiaokele, Grand Xing'an septentrional., Introduction The tectonic evolution of orogenic belts is characterized by changes in the associated magmatic composition (Harris et al. 1986). Post-collisional magmatism, as one of the common characteristics of many [...]

In the aftermath of the Middle Devonian Acadian Orogeny, a thick mafic body is inferred to have been emplaced at the base of the crust during development of the Maritimes Basin complex in eastern Canada and is believed to have sourced regional Late Devonian to Mississippian volcanism. By late Mississippian times, volcanism was limited to central New Brunswick and the Magdalen Islands of eastern Quebec. Whereas Late Devonian to early Mississippian mafic volcanism was mainly characterized by subalkaline lavas produced by the high-degree partial melting of a subduction-enriched lithospheric mantle source, late Mississippian volcanism was dominated by alkali mafic lavas produced by the low-degree partial melting of a sublithospheric mantle source that shows no evidence of subduction imprint. Two distinct upper Mississippian volcanic successions were identified in central New Brunswick and are estimated to be separated by ~7 million years based on U-Pb dates on felsic intervals. They respectively belong to the Cumberland Hill Formation and a new volcanic member of the Hopewell Cape Formation. As they share close geochemical affinities, we interpret both volcanic units as having been derived from the same primary source but from distinct magmatic pulses. Based on the sedimentology of interbedded clastic units as well as available mapping and geophysical data in New Brunswick, we interpret that the upper Mississippian mafic lavas were issued from large NW-SE striking dykes responding to NW-SE compression in a pull-apart basin, whereas less voluminous and more localised felsic rocks were possibly issued from discrete volcanic pipes. Durant la formation du complexe du bassin des Maritimes dans l'est du Canada, suite à l'orogénèse Acadienne (Dévonien moyen), Il est inféré qu'un épais corps mafique a été mis en place à la base de la croûte et aurait été la source d'un volcanisme régional d'âge Dévonien tardif à Mississippien. Au Mississippien tardif, ce volcanisme ne se limitait plus qu'au centre du Nouveau-Brunswick et aux îles de la Madeleine de l'est du Québec. Si le volcanisme mafique du Dévonien tardif au Mississippien précoce était principalement caractérisé par des laves subalcalines produites par de forts degrés de fusion partielle dans une source du manteau lithosphérique enrichie par la subduction, le volcanisme au Mississippien tardif était dominé par des laves mafiques alcalines produites par de faibles degrés de fusion partielle dans une source du manteau sous la lithosphère qui ne montre aucun signe d'une composante associée à la subduction. Deux séquences volcaniques distinctes du Mississippien supérieur ont été cernées dans le centre du Nouveau-Brunswick, l'intervalle estimé entre leurs emplacements étant de ~7 millions d'années selon les âges U-Pb de couches felsiques intercalées. Elles appartiennent respectivement à la Formation de Cumberland Hill et à un nouveau membre volcanique de la Formation de Hopewell Cape. Comme elles ont des affinités géochimiques semblables, nous en concluons que les deux unités volcaniques sont issues de la même source primaire, mais d'impulsions magmatiques distinctes. Sur la base de la sédimentologie d'unités clastiques interlitées ainsi que des données cartographiques et géophysiques disponibles pour le Nouveau-Brunswick, nous concluons que les laves mafiques du Mississippien supérieur atteint la surface à partir coulé de grands dykes d'orientation NO-SE formés par une compression NO-SE dans un bassin de transtension, alors que les roches felsiques moins volumineuses et plus localisées ont probablement été issues de cheminées volcaniques individuelles. [Traduit par la Rédaction], Introduction Following the Middle Devonian Acadian orogeny, eastern Canada evolved as a complex system of strike-slip basins that hosted widespread and locally abundant Upper Devonian to Tournaisian (lower Mississippian) volcanic [...]

The Late Devonian Eagle Lake Granite is an elongate 1.2 km-long hypabyssal stock located in southwestern New Brunswick just south of the Belleisle Fault. Copper, Mo, and Au mineralization is [...]

The Coldbrook Group is a unique suite of Late Ediacaran volcanic and epiclastic rocks that together with related plutons was emplaced in the Avalonian Caledonia terrane of southern New Brunswick. [...]

It is of importance to understand the effect of production blasts on the surrounding rock formations in underground mines. This study presents a numerical procedure to simulate stress waves resulting from nearby production blasts. First, the damping coefficient and peak borehole pressure are calibrated using a dynamic numerical model of a single blast hole. The resulting time-varying particle velocities in the surrounding rock mass at the specified points are calculated. These are then used as input parameters in a three-dimensional mine-wide model, considering a positional relationship between the blast hole and the specified points on the wall rock. The mine-wide model encompasses a fault running parallel to a steeply dipping, tabular ore deposit. Dynamic analysis simulating the effect of production blasts is conducted after the extraction of mining blocks with static analysis. In this study, variations of stresses along the fault due to blast-induced stress waves are examined. Results demonstrate that the developed methodology can reasonably simulate stress changes induced by stress waves on the fault. The methodology considers blast sequences and time-varying blast loads that vary according to the positional relationship between the blast holes and the specified points on the wall rock. Key words: dynamic analysis, geological structure, stress wave, production blast, peak particle velocity, charge-weight scaling law. Il est important de comprendre les effets des sautages de production sur les formations rocheuses environnantes dans les mines souterraines. Cette etude presente une procedure numerique pour simuler les ondes de contraintes causees par les sautages de production a proximite. Premierement, le coefficient d'amortissement et la pression au pic du forage sont calibres a l'aide d'un modele numerique dynamique d'un trou de sautage unique. Les vitesses des particules, variable dans le temps, dans la masse rocheuse encaissante a des points de mesure specifiques, ont ete calculees. Celles-ci sont ensuite utilisees comme parametres d'entree dans un modele en trois dimensions de la mine complete, en considerant une relation de position entre le trou de sautage et les points specifies sur la face rocheuse. Le modele de la mine complete comprend une faille parallele a un gisement mineral tabulaire a pente abrupte. Des analyses dynamiques qui simulent l'effet des sautages de production sont realisees apres l'extraction des blocs miniers avec une analyse statique. Dans cette etude, les variations de contraintes le long de la faille, causees par les ondes de contraintes induites par les sautages, sont examinees. Les resultats demontrent que la methodologie developpee peut simuler raisonnablement les changements de contraintes induits par les ondes de contraintes sur la faille. La methodologie considere les sequences de sautage et les charges de sautages variables dans le temps, qui varient selon la relation de position entre les trous de sautage et les points specifies sur la face rocheuse. [Traduit par la Redaction] Mots-cles: analyse dynamique, structure geologique, onde de contrainte, sautage de production, vitesse au pic des particules, loi d'echelle charge-poids., Introduction A better understanding of stress waves induced by production blasts is of paramount importance in estimating wall rock damage or evaluating the stability of nearby faults. To date, a [...]

The Corvet Est gold deposit is hosted by Archean rocks of the Superior Province in the James Bay region, northern Quebec, Canada. The Marco zone is hosted by amphibolite-grade, strongly foliated volcanic rocks and consists of disseminated gold, with an apparent thickness ranging from 1.8 to 39.5 m and gold grades up to 23 g x [t.sup.-1] over 1 m, that is continuous along strike for ~1.3 km. The lithotectonic sequence comprises footwall basaltic andesite amphibolite overlain by a lenticular unit of metadacite and then by hanging-wall basaltic andesite amphibolite, all intruded by quartz-feldspar porphyry dikes. Dacite, basaltic andesite amphibolite, and quartz-feldspar porphyry show a calc-alkaline to transitional affinity and plot in the plate margin arc basalt field, with typical volcanic arc trace element patterns. Mineralization consists of pyrite, arsenopyrite, pyrrhotite, chalcopyrite, and gold, disseminated in deformed dacite, in andesite amphibolite, and in quartz-feldspar porphyry dikes. Dacite and andesite display weak alteration characterized by silicification. Native gold forms inclusions in metamorphic quartz, garnet, feldspar, arsenopyrite, and pyrite or free grains interstitial to quartz, feldspar, pyrite, chalcopyrite, and arsenopyrite. Free gold in late quartz veins cut the sericitized metamorphic fabric. Inclusion and interstitial native gold within minerals annealed during metamorphism shows that gold mineralization is pre- to syn-metamorphic, with some gold remobilized in later veins. Rhenium-osmium dating of arsenopyrite yields an isochron age of 2663 ± 13 Ma for mineralization and a weighted average model age of 2632 ± 7 Ma for arsenopyrite formed during peak metamorphism. The ~2663 Ma arsenopyrite has a low initial [sup.187]Os/[sup.188]Os of 0.19 ± 0.10, suggesting ajuvenile crust or a mantle Os source. The sulfur isotope composition of Marco zone pyrite and arsenopyrite shows that sulfur could have been leached from its volcanic host rocks or from reduction of Archean seawater. The Corvet Est deposit is interpreted to be an orogenic gold deposit (2663 Ma) deformed and recrystallized during amphibolite-grade metamorphism (2632 Ma). Resume : Le gfte aurifere Corvet Est est encaisse dans des roches archeennes de la Province du Superieur dans la region de la baie James, Nord du Quebec, Canada. La zone Marco est encaissee dans des roches volcaniques, intensement foliees, au grade des amphibolites et elle comprend de l'or dissemine dans une zone d'une epaisseur apparente variant de 1,8 a 39,5 m; la teneur de l'or atteint 23 g/t sur 1 m et la zone est continue le long de la direction sur environ 1,3 km. La sequence lithotectonique comprend une andesite basaltique amphibolitisee dans l'eponte inferieure recouverte d'une unite lenticulaire de metadacite puis d'une andesite basaltique amphibolitisee dans eponte superieure; le tout est recoupe par des dykes porphyriques de quartz-feldspath. La dacite, l'andesite basaltique amphibolitisee et le porphyre quartz-feldspath montrent une affinite calco-alcaline a transitionnelle, se positionnant dans le champ des basaltes d' arc en marge de plaque et presentant des patrons d' elements traces typiques des arcs volcaniques. La mineralisation comprend de la pyrite, de l'arsenopyrite, de la pyrrhotite et de l'or, dissemine dans la dacite deformee, de l'andesite amphibolitisee et des dykes de porphyre quartz-feldspath. La dacite et l'andesite montrent une faible alteration caracterisee par une silicification. L'or natif forme des inclusions dans le quartz, le grenat, le feldspath, l'arsenopyrite et la pyrite metamorphiques ou il se trouve sous forme de grains interstitiels dans le quartz, le feldspath, la pyrite, la chalcopyrite et l'arsenopyrite. L'or libre se trouve dans des veines de quartz tardives qui recoupent la texture metamorphique sericitisee. L'or natif interstitiel en inclusion dans les mineraux recuits durant le metamorphisme montre que la mineralisation est pre- a syn-metamorphique, avec de l'or remobilise dans des veines tardives. L'age de la mineralisation obtenu par datation rhenium-osmium de l'arsenopyrite est de 2663 ± 13 Ma et l'age modele moyen pondere pour l'arsenopyrite forme durant le pic metamorphique est de 2632 ± 7. L'arsenopyrite d'environ2663Maa un faible rapport [sup.187]Os/[sup.188]Os initial de 0,19 ± 0,l0, suggerant que l'osmium provient d'unecroutejuvenileou d'une source mantellique. La composition isotopique du soufre de la pyrite et de l' arsenopyrite de la zone Marco indique que le soufre pourrait avoir ete lessive de ses roches volcaniques encaissantes ou provenir d'une reduction dans l'eau de mer archeenne. Le gfte Corvet- Est est interprete comme etant un gfte aurifere orogenique (2663 Ma) deforme et recristallise durant un metamorphisme au grade des amphiboli tes (2632 Ma). [Traduit par la Redaction], Introduction Since the discovery of the Eleonore gold deposit by Virginia Gold Mines Inc. (Fig. 1; Robertson 2005), the Archean rocks of the Superior Province, in the James Bay region, [...]

Enigmatic millimetre-scale micro-concretions with pseudocrystal faces and dominated by green clay minerals occur in unfossiliferous siliciclastic mudstone of the Lower Ordovician (479.0-466.0 Ma) Tonggao Formation, South China. The fossil-free mudstone unit is associated with local biodiversity decline. The mineralogy and mineral chemistry of these concretions were unknown previously, and this study comprises a preliminary investigation. The concretions are dominated by Fe-rich phyllosilicate minerals including glauconite and clinochlore, with minor quartz and traces of magnetite. The textural relations between the micro-concretions and the surrounding matrix, and the preservation of original mudstone laminations within the concretions, point to an origin during early diagenesis. The mineralogy and chemistry of these concretions are consistent with an origin in a restricted, hypersaline, relatively deep-water environment, in accordance with stratigraphical and paleonotological data. These micro-concretions provide clues for a stressed environment with poor water circulation and anomalies of salinity and oxygen. Des micro-concretions enigmatiques d'echelle millimetrique, avec des faces pseudocristallines et dominees par des mineraux argileux verts, se trouvent dans des mudstones siliclastiques non fossiliferes de la Formation de Tonggao de l'Ordovicien inferieur (479,0-166,0 Ma), dans le sud de la Chine. L'unite mudstone sans fossile est associee a un declin de la biodiversite locale. La mineralogie et la chimie des mineraux de ces concretions n' etaient pas connues anterieurement et la presente etude comprend une investigation preliminaire. Les concretions sont dominees par des mineraux phyllosilicates riches en Fe, incluant la glauconite et le clinochlore, avec un peu de quartz et des traces de magnetite. Les relations texturales entre les micro-concretions et la matrice environnante ainsi que la preservation des laminations originales du mudstone a l'interieur des concretions signalent une origine au debut d'une diagenese. La mineralogie et la chimie de ces concretions correspondent a une origine dans un environnement aqueux limite, hypersalin et relativement profond, ce qui concorde avec les donnees stratigraphiques et paleontologiques. Ces micro-concretions fournissent des indices qu'il s'agissait d'un environnement perturbe avec une mauvaise circulation d' eau et des teneurs anormales en sel et en oxygene. [Traduit par la Redaction], Introduction The Lower Ordovician (479.0-466.0 Ma) Tonggao Formation in Guizhou, South China (Fig. 1), contains abundant microconcretions of clay minerals, which occur in the largely unfossiliferous units of siliciclastic deposits [...]

The coexistence of pseudotachylyte, a brittle fault rock representing seismic slip, and mylonite, a plastic or semiplastic fault rock associated with aseismic slip, has been debated for its origin in earthquake-generating faults. Deformation experiments on halite gouge at seismic slip rates show that mylonite forms simultaneously with frictional melt (pseudotachylyte) in the brittle regime, equivalent to upper crustal conditions. The plastic deformation of halite, resulting in mylonite, is facilitated by the conduction of heat from a slip-concentrated molten layer formed by frictional heating at seismic slip rates. Thus mylonites, which are (1) kinematically consistent and (2) just at the contact with pseudotachylyte, might have formed together with the pseudotachylyte in the brittle regime during seismic slip. doi: 10.1130/G31593.1

Zircon U-Pb dating, whole-rock major oxide, trace element and Sr-Nd isotopic data are presented for the Late Mesozoic mafic intrusive rocks from Yiwulushan of western Liaoning along the eastern segment of the Yanshan belt, North China craton, with two episodes of magmatism documented. Middle Jurassic hornblende-rich gabbros show enrichment of large ion lithophile elements and light REE, and prominent depletion in high field strength elements, and possess moderately enriched isotopic compositions with [([sup.87]Sr/[sup.86]Sr).sub.i] ranging from 0.7056 to 0.7065 and [[epsilon].sub.Nd](t) from -5.0 to -7.1. These features suggest that the gabbros were derived from an amphibole-bearing harzburgitic lithospheric mantle source metasomatized recently by slab-derived fluids. By contrast, Early Cretaceous mafic dykes are gabbroic dioritic to dioritic in composition, with comparable trace element characteristics to continental crust and depleted isotopic signatures ([([sup.87]Sr/[sup.86]Sr).sub.i] = 0.7048-0.7055, [[epsilon].sub.Nd](t) = 0 to -3.0). They probably originated from partial melting of a relatively fertile asthenospheric mantle in the spinel stability field, with subsequent lower crustal assimilation and fractional crystallization. These two contrasting mafic intrusive suites, together with multiple Mesozoic mafic volcanic rocks from western Liaoning, documented a localized lithospheric thinning process, mainly through prolonged hydro-weakening or melt rock interaction and triggered by gravitational collapse, possibly within an evolved post-collisional to within-plate extensional regime. Keywords: mafic intrusions, Late Mesozoic, petrogenesis, lithospheric thinning, North China craton. doi: 10.1017/S0016756810000373

We report the occurrence and characteristics of diopsidite dykes and veins from the Palghat-Cauvery Suture Zone (PCSZ) marking the boundary between the Archaean Dharwar craton to the north and the Proterozoic Madurai Block to the south, which is considered as a trace of the Cambrian Gondwana suture zone in southern India. The diopsidites are composed predominantly of coarse crystals of diopside [Mg no. (100 Mg/(Mg+[Fe.sup.101])) up to 89] surrounded by retrograde calcic amphibole, plagioclase and phlogopite with accessory titanite and calcite. The major, trace and rare earth element characteristics of the diopside crystals suggest their formation in a subduction zone setting. We correlate the petrogenesis of the diopsidites with the tectonics associated with the subduction and closure of the Neoproterozoic Mozambique Ocean prior to the final collisional assembly of the Gondwana supercontinent in Cambrian. Keywords: diopsidite, subduction, suture zone, southern India, Gondwana. doi: 10.1017/S0016756810000154

An aluminous gneissic rock associated with high-pressure mafic and felsic granulites in the Palghat-Cauvery Shear Zone of southern India provides a classic example of quartz-absent muscovite melting. The anatectic gneiss shows a conspicuous migmatitic structure defined by closely spaced centimeter to decimeter sized, corundum-bearing leucosomes developed in a weakly foliated mesosome of plagioclase ([An.sub.21][Ab.sub.77][Or.sub.2]) and biotite (4.9 wt% Ti[O.sub.2], [X.sub.Mg] = 0.51-0.47). The boundaries between leucosome and mesosome domains are sharp, and no melanosome selvages are developed at the interface. Corundum occurs as euhedral crystals up to 2 cm in diameter, typically centered in the leucosome matrix of coarse-grained perthitic alkali feldspar (integrated composition: [An.sub.2][Ab.sub.35][Or.sub.63]), minor relict biotite (4.2-5.1 wt% Ti[O.sub.2], [X.sub.Mg] = 0.48-0.46) and plagioclase ([An.sub.21][Ab.sub.78][Or.sub.1]). In some domains, the mesosomes also contain elongate clusters of similarly oriented smaller corundum plates that are intergrown with perthitic alkali feldspar, presumably replacing former kyanite blades. The textural and mineralogical characteristics and petrogenetic grid considerations indicate breakdown of muscovite through two successive dehydration-melting reactions: (1) formation of corundum+K-feldspar-clusters via the reaction muscovite+aluminosilicate [right arrow] corundum+liquid at the sites of kyanite/sillimanite, and (2) development of corundum-bearing leucosomes through the reaction muscovite [right arrow] corundum+K-feldspar+liquid, focused around the sites of nucleation and growth of peritectic corundum. P-T pseudosection modeling in the [Na.sub.2]O-CaO-[K.sub.2]O-FeO-MgO- [Al.sub.2][O.sub.3]-Si[O.sub.2]-[H.sub.2]O-Ti[O.sub.2] system locates the onset and completion of the muscovite-melting reaction 2 in the steep narrow quadrivariant field Ms+Bt+Pl+Kfs+Crn+Liq, which extends from ~6 khar, 720[degrees]C to higher pressures. Biotite remained stable and was not involved in the melting reactions. Two-feldspar thermometry gives peak-temperatures of 800 [+ or -] 50[degrees]C. Combined with P-T estimates for metapelitic granulites in the area, these P-T constraints appear to be consistent with a clockwise P-T evolution of the eastern Palghat Cauvery shear zone with peak P-T conditions not exceeding ca. 800[degrees]C and 10-12 kbar. The timing of partial melting and HT-metamorphism is constrained at ~529 Ma by U-Pb spot dating of oscillatory zoned individual grains and overgrowths on detrital zircon cores included in peritectic corundum of leucosome domains. The zircon cores indicate a Paleoproterozoic (2.5-2.0 Ga) provenance of the sedimentary protolith. Keywords: Aluminous migmatite, silica-undersaturated muscovite melting, corundum-beating leucosomes, Palghat-Cauvery shear zone system DOI: 10.2138/am.2010.3399

The Jiuhuashan-Qingyang complex is one of the Mesozoic granite complexes in the NE Yangzte Block, China. New petrographical and petrochemical data show that the complex comprises a dominant granodiorite-monzogranite, the Qingyang body, which was intruded by the Jiuhuashan granite body. The two are characterized by distinct mineral components and trace element patterns. Compared to the Qingyang granodiorite and monzogranite, the Jiuhuashan granite is enriched in Rb, Th, U, Nb, Ta, Hf, Yb and Eu, and depleted in Ba, Sr, Nd, Sm, Eu, Gd and Ti, which are ascribable to the separation of plagioclase and biotite, and crystallization of thorite and fergusonite during the magrnatism. New LA-ICPMS zircon U-Pb dating suggests that the crystallization age of the Qingyang body is 139-133 Ma, and the Jiuhuashan granite followed at 127 Ma. Moreover, the new zircon U-Pb dates reveal that Archaean materials were involved in the formation of these magmas, and that a sodium-rich metasomatic event occurred at about 100 Ma. The CHIME monazite and zircon ages studied for the Jiuhuashan body agree well with the LA-ICPMS zircon ages. Integrating this information with previous studies for granites in the NE Yangtze Block and in the coastal area of SE China, we believe that all of these Late Mesozoic granites were produced under the tectonic regime of palaeo-Pacific plate subduction towards the SE China continent in a NW direction, but the granites in the NE Yangtze Block are basically derived by crustal melting with limited mixing of juvenile material during the magma generation. Keywords: granite complex, petrochemistry, zircon U-Pb age, CHIME age, SE China. doi: 10.1017/S0016756809990367

Mesozoic to Palaeogene intrusive igneous rocks in West Greenland range from a large, coast-parallel dyke swarm to small, poorly defined dyke swarms or single intrusions. New age and geochemical data indicate that intrusion forms and melt compositions changed with time, dependent on changing stress fields and increasing lithospheric attenuation. During the period c. 220-150 Ma (Late Triassic to Late Jurassic) incipient stretching is reflected in the production of highly alkaline, volatile-rich melts formed in small volumes in the deep lithosphere. Around 150 Ma (Kimmeridgian), increased extension took place and melts were intruded in a 60 km long swarm of scattered alkaline dykes. In the Early Cretaceous, 140-133 Ma, the regional stress field was intense, upwelling asthenospheric mantle started to melt, and alkali basaltic magmas were emplaced in a 400 km long coastal dyke swarm parallel to large linear faults offshore. In the Palaeocene, continental break-up took place and flood basalts (62-60 Ma) were extruded in the Nuussuaq Basin. Large basalt sills and dykes extend the region with Palaeocene activity 150 km southwards and form a link between the Nuussuaq Basin and the Sisimiut Basin offshore. Dykes with ages of 57-51 Ma indicate widespread younger volcanic activity. Supplementary material: Sample details, Ar/Ar data and plots, and Rb-Sr isochrons are available at http://www.geolsoc.org.uk/SUP 18374. doi: 10.1144/0016-76492009-038.

We document an example of serpentinization of olivine and orthopyroxene that produced virtually no magnetite, but instead relatively Fe-rich yellow-colored lizardite ([X.sub.Fe] = 0.08 to 0.17), and the native Fe-Ni-Co metals, awaruite and wairauite. Lizardite's identity was confirmed by micro-Raman spectroscopy, although peaks are broad. Electron microprobe analyses of the lizardite yield a continuous compositional trend of formula contents suggestive of the progressive uptake of [Fe.sup.3+] exclusively on M sites, where it is charge balanced by vacancies. Although these observations are unusual, this secondary mineral assemblage can be explained in terms of the likely intensive variables [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.], and [MATHEMATICAL EXPRESION NOT REPRODUCIBLE IN ASCII.] attending the alteration. The absence of magnetite in serpentinization does not signify a lack of oxidation. By forming the hydrated phase-component ferri-lizardite instead of magnetite from the fayalite and ferrosilite components, the yield of hydrogen is reduced by two-thirds. The usual inverse correlation of rock density with magnetic susceptibility is unlikely to be the case in this kind of serpentinization. Keywords: Serpentinite, ferrian lizardite, olivine-websterite, micro-Raman, hydrogen, magnetic susceptibility DOI: 10.2138/am.2009.3301

Layered anorthosite, chromiferous pyroxenite, and spatially associated mafic and felsic rocks of the 2.9 Ga Sittampundi layered complex (SLC), South India, underwent high-grade metamorphism at ca. 2.5 Ga and were subjected to amphibolite-facies metamorphism accompanied by intrusion of granitoid plutons during late-Neoproterozoic tectonothermal activity (0.72-0.45 Ga). During the latter event, anorthosite developed millimeter- to centimeter-thick compositional layers rich in clinopyroxene+amphibole+clinozoisite+chlorite [+ or -] chromite and corundum+spinel+chlorite. Tiny grains of hogbomite replaced only the grains of corundum and spinel and are in textural equilibrium with chlorite. The studied hogbomite contains appreciable Ti[O.sub.2] (>4.4 wt%) but insignificant NiO and ZnO. [Cr.sub.2][O.sub.3] content reaches up to 0.35 wt% only in chromite-bearing samples. Systematic partitioning of Fe and Mg between hogbomite and associated Fe-Mg minerals demonstrate attainment of chemical equilibrium among these phases. Integrating textural relations and algebraic analyses of the phase compositions, several reactions were constructed involving the associated oxide phases (spinel, corundum, hogbomite), calcite, and silicates (amphibole, chlorite, anorthite, clinozoisite). Interpretation of the reaction reveals that (1) [Mg.sup.2+] and [Ti.sup.4+] were mobile for more than 2 cm during the formation of hogbomite and chlorite, and (2) chloritization of amphibole in the clinopyroxene+amphibole-bearing layers released Ti that was transported to the spinel+corundum-bearing layers to develop hogbomite. Stability fields of some critical mineral assemblages in [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] space combined with geothermobarometry in the associated rocks tightly constrain the growth of hogbomite in the presence of aqueous fluids ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] < 0.15) to the P- Trange of 7 [+ or -] 1 kbar, 650 [+ or -] 50[degrees]C. These aqueous fluids, presumably derived from the Pan-African granitoid batholiths, chloritized amphibole grains, and transported the released [Ti.sup.4+] to the spinel+corundum-bearing layers to develop hogbomite. Topological relations in isothermal-isobaric fugacity diagrams ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) in the system FeO-[A1.sub.2][O.sub.3]Ti[O.sub.2]-[O.sub.2]-[S.sub.2]-[H.sub.2]0-C[O.s (+MgO, [Cr.sub.2][O.sub.3]) indicate that the stability and compositional characteristics of natural h6gbomite are strongly influenced by [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.], and concentrations of other soluble species (Ti, Mg, Cr, etc.) in the metamorphic fluids. Keywords: Hogbomite, corundum, spinel, anorthosite, Ti-mobility, Sittampundi, South India DOI: 10.2138/am.2009.3121

A seiles of X-ray absorption spectroscopy (XAS) experiments were made to determine the structure and stability of aqueous REE (La, Nd, Gd, and Yb) chloride complexes to 500[degrees]C and 520 MPa. The [REE.sup.3+] ions exhibit inner-sphere chloroaqua complexation with a steady increase of chloride coordination with increasing temperature in the 150 to 500[degrees]C range. Furthermore, the degree of chloride coordination of [REE.sup.3+] inner-sphere chloroaqua complexes decreases significantly flora light to heavy REE. These results indicate that steric hindrance drives the reduction of chloride coordination of [REE.sup.3+] inner-sphere chloroaqua complexes flora light to heavy REE. This results in greater stability and preferential transport of light [REE.sup.3+] over heavy [REE.sup.3+] ions in saline hydrothermal fluids. Accordingly, the preferential mobility of light REE directly influences the relative abundance of REE in rocks and minerals and thus needs to be considered in geochemical modeling ofpetrogenetic and ore-forming processes affected by chloride-bearing hydrothermal fluids. Keywords: Petrogenetic indicators, rare earth elements, hydrothermal fluids, synchrotron radiation

This work presents a comparative study of Roman and modern slags, which represent the same type of mining waste but which were produced at vastly different times. The natural laboratory in which both materials are found is Sao Domingos, one of the most emblematic of Portuguese mining districts in the Iberian Pyrite Belt (IPB). The methodoiogy included: (1) detailed studies of the mineralogy and geochemistry of both materials with a reflected-light optical microscope, scanning electron microscope (SEM), electron microprobe analyses (EMPA), and bulk-rock analyses; (2) MELTS thermodynamic software to quantitatively test fractional erystallization of both glassy matrix types; (3) TWQ-v. 2.32 software for performing internally consistent thermometric calculations; and (4) a modified Community Bureau of Reference (BCR)-sequential extraction procedure applied to the chemical speciation of potentially toxic elements. The combination of petrologic studies and sequential extraction leaching may be used to evaluate and predict pollution impact on the environment. We conclude that a 2000 year time gap has not produced an important modification to the base-metal extraction system as reflected by the petrologic similarity of both slags and that, considering the total mass of modern slags, the transfer rate of metals to the environment over the next 2000 years will be ~6.7 t/year of Fe, 1 t/year of S, 81.1 kg/year of Zn, 55.5 kg/year of Pb, 6.5 kg/year of Cu, 0.7 kg/year of As, 31.2 g/year of Sb, and 18.3 g/year of Cr. The results demonstrate the pollutant potential of the slags within this IPB mining district as revealed by the spoiled state of the fluvial courses in the region. Keywords: Environmental impact, petrogenesis, prediction, slag, sequential extraction

Using global correlations of effusive eruption style with convergence rates and surface heat flux at volcanic arcs, this study identifies several arcs with excess surface heat flux, related to the presence of unusually hot and [H.sub.2]O-poor magmas. Geophysical evidence suggests that these melts are linked to discontinuities in the subducting slab, which would facilitate upwelling of hot sub-slab asthenospheric mantle. It is demonstrated that excess heat flux arcs are sites where Sr-rich adakitic compositions ate generated, and that the arc proportion erupting Sr-rich adakitic magmas is directly proportional to the degree of observed heat flux excess. A model is presented in which the formation of Sr-rich adakitic compositions, which require garnet as a residual phase, is facilitated at excess heat flux arcs as a result of a decrease in [H.sub.2]O content through contributions from sub-slab asthenospheric mantle, leading to an expansion of the garnet stability field. Future geochemical, geophysical and experimental work required to test this model is addressed.

We report 11 new U-Pb zircon ages obtained by sensitive high-resolution ion microprobe (SHRIMP) and laser ablation--inductively coupled plasma--mass spectrometry (LA-ICP--MS) for a large province of Early Cretaceous Comei igneous rocks consisting of basaltic lavas, mafic sills and dikes, and gabbroic intrusions together with subordinate layered ultramafic intrusions and silicic volcanic rocks exposed in the Tethyan Himalaya, southeastern Tibet. Available zircon U-Pb ages obtained from various rocks in this province, which has an areal extent of ~40,000 [km.sup.2] (~270 km x 150 km), indicate that the magmatism occurred ca. 132 Ma ago, coeval with the Bunbury Basalt in southwestern Australia. Such a striking similarity in emplacement age, in combination with the tectonic reconstruction of eastern Gondwana ca. 132 Ma ago, allows us to propose that the extensive Comei igneous rocks in southeastern Tibet and the Bunbury Basalts in southwestern Australia may represent the erosional and/or deformational remnants of a large igneous province, which we call the Comei-Bunbury LIP. We argue that this newly identified LIP was likely caused by the Kerguelen mantle plume, which started in the Early Cretaceous and may have played a role in the breakup of eastern Gondwana and the development of the 132 Ma old Weissert oceanic anoxic event.

A detailed study of the geochemical and isotopic characteristics of the volcanic rocks of the Hidden and Louis formations, which make up the hanging wall to the volcanogenic massive sulphide deposits at Flin Flon, Manitoba, was carried out on a stratigraphically controlled set of samples. The stratigraphy consists of the lowermost, dominantly basaltic, Hidden formation, and the overlying, dominantly basaltic, Louis formation. Of importance petrogenetically, is the 1920 unit a basaltic andesite with Nb/[Th.sub.mn] = 0.54-0.62, ζ[Nd.sub.(1.9Ga)] = +3.6-+5.9, ζ[Hf.sub.(1.9Ga)] = +8.5-+9.6, and [sup.204]Pb/[sup.206]Pb = 23.9. The basaltic flows that dominate the Hidden formation have Nb/[Th.sub.mn] = 0.16-0.29, ζ[Nd.sub.(1.9Ga)] = +1.7-+4.4, ζ[Hf.sub.(1.9Ga)] = +7.0-+11.8 and [sup.204]Pb/[sup.206]Pb = 16.9-18.6). The Carlisle Lake basaltic-andesite (top of Hidden formation) is characterized by Nb/[Th.sub.mn] = 0.16-0.14, and [sup.204]Pb/[sup.206]Pb = 21.4. The rhyodacitic Tower member (bottom of Louis formation) has Nb/[Th.sub.mn] = 0.23, ζ[Nd.sub.1.9Ga] = +4.6, ζ[Hf.sub.1.9Ga] = +9.3, and [sup.204]Pb/[sup.206]Pb = 22.2. The basaltic flows that dominate the Louis formation have Nb/[Th.sub.mn] = 0.18-0.25, ζ[Nd.sub.(1.9Ga)] = +3.6-+4.2, ζ[Hf.sub.(1.9Ga)] = +8.4-+11.3 and [sup.204]Pb/[sup.206]Pb = 17.9. The Hidden and Louis formations show dominantly transitional arc tholeiite signatures, with the 1920 unit having arc tholeiite characteristics. It is interpreted to have formed through extensive fractional crystallization of differentiated magmas at shallow levels in oceanic crust. Given the geological, geochemical, and isotopic characteristics of the Hidden and Louis formations, they are interpreted to represent subducted slab metasomatism with minor contamination from subducted sediments. Une etude detaillee des caracteristiques geochimiques et isotopiques des roches volcaniques des formations de Hidden et de Louis, lesquelles forment l'eponte superieure des gisements de sulfures volcanogenes massifs a Flin Flon, Manitoba, a ete effectuee sur un ensemble d'echantillons controles par la stratigraphie. La stratigraphie comprend, tout a la base, la formation de Hidden, principalement basaltique et la formation de Louis sus-jacente, principalement basaltique. L'unite 1920, une andesite basaltique, est importante d'un point de vue petrogenetique; elle possede les valeurs suivantes : Nb/[Th.sub.mn] = 0.,54-0,62, ζ[Nd.sub.(1.9Ga)] = +3,6 a +5,9, ζ[Hf.sub.(1.9Ga)] = +8,5 a +9,6, et [sup.204]Pb/[sup.206]Pb = 23.9. Les coulees basaltiques qui dominent la formation de Hidden ont les valeurs suivantes : Nb/[Th.sub.mn] = 0,16-0.29, ζ[Nd.sub.(1.9Ga)] = +1,7 a +4,4, ζ[Hf.sub.(1.9Ga)] = +7,0 a +11,8 et [sup.204]Pb/[sup.206]Pb = 16,9-18,6). L'andesite basaltique Carlisle Lake (sommet de la formation de Hidden) est caracterisee par Nb/[Th.sub.mn] = 0,16-0,14 et [sup.204]Pb/[sup.206]Pb = 21,4. Le membre Tower (bas de la formation de Louis) a les valeurs suivantes : Nb/[Th.sub.mn] = 0,23, ζ[Nd.sub.1.9Ga] = +4,6, ζ[Hf.sub.1.9Ga] = +9,3 et [sup.204]Pb/[sup.206]Pb = 22,2. Les coulees basaltiques qui dominent la formation de Louis ont les valeurs suivantes : Nb/[Th.sub.mn] = 0,18-0,25, ζ[Nd.sub.(1.9Ga)] = +3,6 a +4,2, ζ[Hf.sub.(1.9Ga)] = +8,4 a +11,3 et [sup.204]Pb/[sup.206]Pb = 17,9. Les formations de Hidden et de Louis presentent des signatures surtout d'arc tholeiitique de transition ou l'unite 1920 a des caracteristiques d'arc tholeiitique. Cette unite se serait formee par la cristallisation fractionnee a grande echelle de magmas differencies a de niveaux de faible profondeur dans la croute oceanique. E tant donne les caracte ristiques geologiques, geochimiques et isotopiques des formations de Hidden et de Louis, elles representeraient un metasomatisme de plaque subductee avec une contamination minime par les sediments subductes. [Traduit par la Redaction], Introduction Volcanogenic massive sulphide (VMS) deposits occur in extensional environments within a variety of tectonic settings, including oceanic, fore-arc, arc, back-arc, continental-margin, or continental settings (Franklin et al. 1981, 2005; [...]

The [approximately equal to]3,450-million-year-old Strelley Pool Formation in Western Australia contains a reef-like assembly of laminated sedimentary accretion structures (stromatolites) that have macroscale characteristics suggestive of biological influence. However, direct microscale evidence of biology--namely, organic microbial remains or biosedimentary fabrics--has to date eluded discovery in the extensively-recrystallized rocks. Recently-identified outcrops with relatively good textural preservation record microscale evidence of primary sedimentary processes, including some that indicate probable microbial mat formation. Furthermore, we find relict fabrics and organic layers that covary with stromatolite morphology, linking morphologic diversity to changes in sedimentation, seafloor mineral precipitation, and inferred microbial mat development. Thus, the most direct and compelling signatures of life in the Strelley Pool Formation are those observed at the microscopic scale. By examining spatiotemporal changes in microscale characteristics it is possible not only to recognize the presence of probable microbial mats during stromatolite development, but also to infer aspects of the biological inputs to stromatolite morphogenesis. The persistence of an inferred biological signal through changing environmental circumstances and stromatolite types indicates that benthic microbial populations adapted to shifting environmental conditions in early oceans. microbe | paleontology | biosignature | carbonate | reef

Integration of detailed field, petrological, structural, and geochemical observations of the Buck Creek mafic-ultramafic suite, which is among the larger southern Appalachian ultramafic exposures, points to an igneous origin as a mid-ocean-ridge cumulate massif, with subsequent emplacement in a deep subduction-zone setting. These results help to clarify the unresolved role of 'alpine-type' ultramafic bodies in the southern Appalachians, a problem complicated by their generally small and fragmented nature and by significant tectonic overprinting. As an ophiolite fragment, the Buck Creek rocks are unusual in their preservation of relatively high-pressure, anhydrous conditions, and thus they provide constraints on mechanisms of ophiolite emplacement. Field support for a cumulate origin for the Buck Creek suite includes centimeter- to meter-scale interlayering of dunite, troctolite, anorthosite, and spinel-rich layers, and local gradation between Buck Creek troctolitic rocks and surrounding amphibolites. Relict anorthitic plagioclase, forsteritic olivine, and augitic clinopyroxene define a 'cumulate triangle' on an MgO versus [Al.sub.2][O.sub.3] diagram: meta-troctolite and meta-dunite compositions define a linear array between olivine and plagioclase compositions, and many amphibolites fall within this triangle, reflecting gabbroic protoliths. The relatively abundant troctolite cumulates, scarce pyroxene in the ultramafic rocks, and the high [Al.sub.2][O.sub.3]/Ti[O.sub.2] and low Cr numbers in spinel best match LOT- or L-type (lherzolite) ophiolites, consistent with crystallization in a slow-spreading mid-ocean-ridge setting. Sapphirine-bearing, spinel symplecrites in the meta-troctolites suggest that Buck Creek rocks remained anhydrous to ~800[degrees]C and ~0.9-1.1 GPa, representing conditions atypical of ophiolite emplacement. Comparisons to the Zermatt-Saas ophiolite and Bergen eclogite suggest rapid subduction of Buck Creek rocks to depths of ~30 km, where partial hydration, perhaps facilitated by prograde dehydration reactions in surrounding rocks, caused strain softening that permitted ductile disaggregation and aided in their emplacement into the overlying accretionary complex. Alternatively, emplacement might have resulted from a switch in subduction polarity, with hydration following emplacement. Uplift, shortening, and hydration within the accretionary sequences are responsible for the dominant structural features. Keywords: Blue Ridge Province, ophiolite, ultramafic, petrogenesis, troctolite, high pressure.

A series of boreholes in Horton Quarry, northwest Yorkshire (Horton-in-Ribblesdale Inlier) penetrated mudstones and slates belonging to the Austwick Formation (Windermere Supergroup) overlying laminated mudstones of the Ingleton Group. Illite (IC) and chlorite (ChC) crystallinity measurements indicate a metamorphic inversion between the two groups of mudrocks. The Windermere Supergroup mudrocks are mostly in the high anchizone or epizone, whereas the Ingletonian samples are lower grade in terms of IC, and are mostly deep diagenetic zone or low anchizone. Hence younger strata at higher grades rest on older strata at lower grades, creating a metamorphic inversion. Ingletonian slates exposed at Pecca Falls on the River Twiss show epizonal and anchizonal IC values, and greywacke samples from Ingleton Quarry contain pumpellyite. This suggests that grade in the Ingletonian may increase to the NW from the Horton to Ingleton inliers. K-white mica b cell dimensions show further differences between the Ingleton Group and the Windermere Supergroup. The Ingletonian samples are characterized by low b cell values (8.989-9.035, mean 9.007 [Angstrom]), whereas the Windermere Supergroup has higher values in the range 9.022-9.034, mean 9.027 [Angstrom]. The Windermere Supergroup values are similar to those recorded from the Windermere Supergroup of the southern Lake District, and Lower Palaeozoic rocks from the Scottish Southern Uplands, and are consistent with metamorphism in a low heat flow, convergent geotectonic setting. The Ingletonian b cell values suggest metamorphism in a higher heat flow setting, most likely an extensional basin. The metamorphic inversion at Horton and differences in K-white mica b cell dimensions suggest that the Ingleton Group and Windermere Supergroup strata evolved in different geotectonic settings and record two separate metamorphic events. The discovery of the metamorphic inversion at Horton provides further evidence in favour of an Ordovician rather than Neoproterozoic depositional age for the controversial Ingleton Group. Keywords: Ingleton Group, Windermere Supergroup, low-grade metamorphism, clay minerals, illite crystallinity, metamorphic inversion.

Zircon U-Pb age, whole-rock elemental and Sr-Nd isotopic data are presented for the Rushan intrusive complex from the northern Sulu orogen, China. The intrusion, emplaced at c. 111 Ma, consists mainly of biotite-bearing gabbro and pyroxene-bearing diorite. The rocks are high-K calcalkaline in major elements, and enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), depleted in high field strength elements (HFSE), and possess narrow ranges of initial [sup.87]Sr/[sup.86]Sr (0.70746-0.70827) and eNd(t) (-12.37 to -8.26). The complex is interpreted to originate from decompression melting of spinel-facies cratonic mantle that was metasomatized by the subducted Yangtze crustal materials and the melts experienced fractionation of olivine, clinopyroxene, apatite and Fe-Ti oxides. The Rushan complex is similar in composition to other Early Cretaceous mafic-intermediate complexes from the southern margin of the North China Craton, such as the Fangcheng basalts and Yinan gabbros, implying the mantle source of the Rushan intrusion is tectonically affiliated to the southern wedge of the Craton. We infer that the Rushan complex formed in an extensional regime corresponding to the asthenosphere upwelling during gradual erosion and replacement of the cratonic mantle by the fertile lithosphere beneath the eastern North China Craton. Keywords: gabbro-diorite, zircon U-Pb ages, geochemistry, petrogenesis, northern Sulu orogen.

Iron formations are typically thinly bedded or laminated sedimentary rocks containing 15% or more of iron and a large proportion of silica (commonly >40%). In the ca. 2590-2460 Ma Campbellrand-Kuruman Complex, Transvaal Supergroup, South Africa, iron formation occurs as a sediment-starved deepwater facies distal to carbonates and shales. Iron minerals, primarily siderite, define the lamination. The silica primarily occurs as thin beds and nodules of diagenetic chert (now microcrystalline quartz), filling pore space and replacing iron formation minerals and co-occurring deepwater lithologies. Mechanisms proposed to explain precipitation of the iron component of iron formation include photosynthetic oxygen production, anoxygenic photosynthesis, abiotic photochemistry, and chemoautotrophy using Fe(II) as an electron donor. The origin and mechanism of silica precipitation in these deposits have received less attention. Here we present a conceptual model of iron formation that offers insight into the deposition of silica. The model hinges on the proclivity of dissolved silica to adsorb onto the hydrous surfaces of ferric oxides. Soluble ferrous iron is oxidized in the surface ocean to form ferric hydroxides, which precipitate. Fe[(OH).sub.3] binds silica and sinks from the surface ocean along with organic matter, shuttling silica to basinal waters and sediments. Fe(III) respiration in the sediments returns the majority of iron to the water column but also generates considerable alkalinity in pore waters, driving precipitation of siderite from [Fe.sup.2+] and respiration-influenced C[O.sub.2]. Silica liberated during iron reduction becomes concentrated in pore fluids and is ultimately precipitated as diagenetic mineral phases. This model explains many of the mineralogical, textural, and environmental features of Late Archean and earliest Paleoproterozoic iron formation. Keywords: South Africa, iron cycle, banded iron formation, chert, Transvaal, Kuruman.

East of the Patagonian Andes, mafic volcanic rocks (mainly lava flows and scoriae) are exposed in the Sierra de San Bernardo fold belt and neighbouring areas (central Patagonia; 44.5-46[degrees] S, 69 71 [degrees] W). They were erupted over a wide interval of time (late Eocen--Pleistocene; 14 new K-Ar ages), and show systematic chemical and Sr-Nd-Pb isotopic variations in time. The alkaline lavas (Mg number 57-66) erupted during the late Eocene and early Miocene, have an intraplate geochemical affinity, and have the highest [sup.143]Nd/[sup.144]Nd and [sup.206]Pb/[sup.204]Pb and the lowest [sup.87]Sr/[sup.86]Sr ratios of the dataset. Their compositions indicate that their depth of equilibration in the mantle was greater than that of subsequent lavas. In contrast, the Plio-Pleistocene alkaline lavas (Mg number 58-71) are the most enriched in incompatible elements, still showing an intra-plate signature, and have the lowest [sup.143]Nd/[sup.144]Nd and [sup.206]pb/[sup.204]Pb and the highest [sup.87] Sr/[sup.86] Sr ratios. A distinctive group of early Miocene subalkaline lavas is characterized by slightly more evolved compositions (Mg number 56-59), coupled with very low incompatible element contents, flat LREE and fractionated HREE patterns ('kinked' pattern), and intermediate Sr-Nd-Pb isotope compositions. The Pleistocene basanites (Mg number 71-72) from the Cerro Ante monogenetic cone, on the easternmost slopes of the Patagonian Andes, have a marked orogenic geochemical signature and Sr-Nd-Pb isotope ratios that overlap with those of volcanic rocks from the adjacent active Andean arc. They originated in a mantle source extensively modified by the addition of materials from the subducting Pacific oceanic plates. We suggest that the wide chemical and isotopic variability of the Sierra de San Bernardo lavas reflects the upwelling of asthenospheric mantle beneath the study area, which induced lithospheric erosion and progressive involvement of enriched mantle domains in the genesis of magmas. In this context, late Eocene and early Miocene alkaline magmatism was dominantly sourced from the asthenospheric mantle, whereas Plio-Pleistocene alkaline magmas contain the largest proportion of an enriched lithospheric component. The peculiar compositional features of the early Miocene subalkaline lavas are interpreted in terms of high-degree mantle melting followed by melt-lithospheric mantle reaction processes. Based on current knowledge about the relative movement and decoupling between lithosphere and asthenosphere, we propose that the asthenosphere below the study area rose up to compensate for the westward drift of the mantle wedge coupled with the South American lithosphere. Keywords: Patagonia, basaltic rocks, Cenozoic, petrogenesis, geodynamics, isotope geochemistry.