Metasedimentary Host Research Articles

Cretaceous to Cenozoic evolution of the northern Lhasa Terrane and the Early Paleogene development of peneplains at Nam Co, Tibetan Plateau

Highly elevated and well-preserved peneplains are characteristic geomorphic features of the Tibetan plateau in the northern Lhasa Terrane, north–northwest of Nam Co. The peneplains were carved in granitoids and in their metasedimentary host formations. We use multi-method geochronology (zircon U–Pb and [U–Th]/He dating and apatite fission track and [U–Th]/He dating) to constrain the post-emplacement thermal history of the granitoids and the timing and rate of final exhumation of the peneplain areas. LA-ICP-MS U–Pb geochronology of zircons yields two narrow age groups for the intrusions at around 118Ma and 85Ma, and a third group records Paleocene volcanic activity (63–58Ma) in the Nam Co area. The low-temperature thermochronometers indicate common age groups for the entire Nam Co area: zircon (U–Th)/He ages cluster around 75Ma, apatite fission track ages around 60Ma and apatite (U–Th)/He ages around 50Ma. Modelling of the thermochronological data indicates that exhumation of the basement blocks took place in latest Cretaceous to earliest Paleogene time. By Middle Eocene time the relief was already flat, documented by a thin alluvial sediment sequence covering a part of the planated area. The present-day horst and graben structure of the peneplains is a Late Cenozoic feature triggered by E–W extension of the Tibetan Plateau. The new thermochronological data precisely bracket the age of the planation to Early Eocene, i.e. between ca. 55 and 45Ma. The erosional base level can be deduced from the presence of Early Cretaceous zircon grains in Eocene strata of Bengal Basin. The sediment generated during exhumation of the Nam Co area was transported by an Early Cenozoic river system into the ocean, suggesting that planation occurred at low elevation.

Fractionation and incipient self-granulitization during deep-crust emplacement of Lower Ordovician Valle Fértil batholith at the Gondwana active margin of South America

Large granite batholiths were emplaced at the Gondwana active margin during Lower Ordovician in South America. These have contributed to crustal growth by net addition of silicic rocks to the continental crust. New U–Pb SHRIMP zircon age determinations, together with thermobarometric and geochemical data, yield that batholith magma intrusion is the responsible of heating and self-granulitization of early gabbro pulses. Partially molten granulitic gabbros, which appear as either early intruded into the metasedimentary host or as large inclusions within the batholith-forming Qtz-diorites, contain Opx-bearing, trondhjemitic leucosomes surrounding Hbl+Opx+Pl mafic mesosomes forming typical agmatitic structures. Hornblende–Plagioclase equilibria, applied to mineral pairs of granoblastic aggregates in textural equilibrium of metagabbro mesosomes, yield temperatures in the range 850–910°C for core-to-core pairs and in the range 1000–1075°C for rim-to rim pairs, at pressures of about 0.7GPa. SHRIMP zircon age revealed that the whole batholith was emplaced over a narrow time interval of 20Ma from 465 to 485Ma, with most ages clustered at about 470Ma. The age of metagabbros is 473±7Ma for older zircons and 454±4 for younger zircons. These ages are almost coincident within error with the age of host migmatites (477±5Ma) and those of batholith intrusion of 476±9Ma and 475±3Ma for Qtz-diorites and 475±5Ma for granites. Zircon overgrowths of these intrusive rocks yield sages clustered around 450Ma, revealing a protracted thermal history, more complex than previously believed. The geochemical study reveals that Qtz-diorites, tonalites and granodiorites form a continuous trend produced by magmatic fractionation from a parental dioritic magma. A weak adakitic tendency, with Sr/Y>15 in several samples, implies the presence of Grt in the source or magma chamber at a minimum pressure of 1.0GPa, higher than the depth of emplacement at 0.7GPa. The high temperature of magma emplacement, which induced the incipient self-granulitization of early magmatic pulses, together with the cotectic-like fractionation linking coeval Qtz-diorites, tonalites and granodiorites, is compatible with fractionation at the lower crust of a deep-generated, infracrustal, (sublithospheric?) intermediate magma.

The Sierra Norte-Ambargasta batholith: Late Ediacaran–Early Cambrian magmatism associated with Pampean transpressional tectonics

The Sierra Norte-Ambargasta batholith is one of the largest plutonic expressions of the Pampean orogeny in western Argentina. A thorough petrographic, geochemical, isotopic (Sr and Nd) and geochronological (UePb SHRIMP) study is reported. The batholith comprises granitoid rocks that may be subdivided into those affected by Pampean D2 dextral shearing and mylonization and those emplaced after deformation had ceased; representative samples gave UePb zircon ages of 537 ±4 Ma and 530 ±4 Ma respectively. The earlier, dominant, groupwere derived largely frommetaluminous calc-alkaline subduction-related magmas, whereas the late granites are peraluminous. However, all have relatively high initial 87Sr/86Sr ratios (0.706 to at least 0.710, strongly negative eNdt values (-1.7 to -5.9) and, in some cases inherited 600 Ma and 970 Ma zircon, similar to the isotopic and zircon provenance seen in the metamorphic host rocks. A high degree of contamination of the magmas, possibly anatexis in the case of the post-mylonite granite, is related to emplacement during the latestage transpressional docking of the Pampean terrane against the Rio de la Plata craton. The absence of detrital zircon derived from the craton in either the Pampean metasedimentary host rocks or the batholith supports this collisional model for the Pampean orogen.

Late Variscan magmatism in the Nevado-Filábride Complex: U-Pb geochronologic evidence for the pre-Mesozoic nature of the deepest Betic complex (SE Spain)

Orthogneisses crop out in different lithostratigraphic positions throughout the Nevado-Filábride metamorphic Complex (Betic Cordillera, southern Spain). Their abundance offers the opportunity to establish its origin and to date the ages of acid magmatism and of early deposition ages of metasedimentary host rocks of the orthogneisses, in this complex metamorphosed and deformed during the Alpine Orogeny. Petrography and bulk-rock chemistry indicate that the orthoderivates samples are peraluminous tourmaline leucogranites and strongly peraluminous biotite gneisses. Compositional similarity in different outcrops points to common genetic relationships. Although the studied gneisses are affected by metamorphic and deformational processes their compositional variations may be explained by differing degrees of crustal melting, source heterogeneity or minor feldspar fractionation. Variation in REE contents is attributed to accessory minerals fractionation. The geochemical data show that the protoliths originated by low degree of partial melting of biotite-rich, muscovite-plagioclase-bearing metasediments. U-Pb SHRIMP dating of magmatic zircons from six samples yielded ages between ~282±5 and 295±3Ma. Their ages and chemical composition indicate that these intrusive bodies strongly resemble the peraluminous, post-tectonic, leucogranites widespread in the Variscan Orogen, particularly those produced by crustal partial melting of the Central Iberian Zone. Zircons in host pelitic metasediments provide older ages ranging from ~336 to 2.050Ma. They testify to the recycling of Early Carboniferous and older granites, which are found in the Central Iberian zones of the Iberian Massif. The new geochronological data combined with detailed field observations allow drafting a new chronostratigraphy of the Nevado-Filábride Complex. Traditionally proposed Triassic ages for the metasedimentary upper series of the complex are not consistent with the ages reported here, which suggests that the entire complex consists of Paleozoic or older rocks. This conclusion has profound implications on the validity of the tectonic subdivisions proposed so far. Metamorphic zircon rims in two gneisses and a pelitic metasediment yield Miocene (17.3±0.4Ma) ages in line with what previously proposed for the high-pressure metamorphic peak in the Nevado-Filábride Complex. The new radiometric data moreover confirm independent metamorphic and tectonic evolutions for the Nevado-Filábride and the Alpujárride complexes of the Betic Cordillera.

U/Pb and Sm/Nd dating on ophiolitic rocks of the Song Ma suture zone (northern Vietnam): Evidence for upper paleozoic paleotethyan lithospheric remnants

The Sm/Nd isochron mineral dating technique, applied on lenses of ophiolitic rocks of the Song Ma suture zone, reveals crystallization ages of 387–313Ma for titanites extracted from the mafic components (metagabbros, metabasalts) of the ophiolite suite. These ages correspond to a large time interval within the Carboniferous period. Such results mean that these blocks are lithospheric relics of an eastern branch of the Paleotethys. They however do not exclude that an older early Paleozoic ocean has previously existed in the area. The overprinted metamorphism that affect these rocks and the metasedimentary host rocks, including locally HP granulite and eclogite facies conditions (Nakano et al., 2006, 2008, 2010), took place during the Triassic Indosinian orogeny after closure of the ocean, continental subduction and collision, leading to the suturing of the Indochina and South China blocks. U/Pb and Ar/Ar data reveal that peak metamorphic conditions were attained 266–265Ma ago, being then followed by cooling at 250–245Ma.

New igneous zircon Pb/Pb and metamorphic Rb/Sr ages in the Yaounde Group (Cameroon, Central Africa): implications for the Central African fold belt evolution close to the Congo Craton

Three meta-igneous bodies from the Yaounde Group have been analyzed for their petrography, geochemistry, and 207Pb/206Pb zircon ages. According to their geochemical patterns, they represent meta-diorites. The meta-plutonites yielded identical zircon ages with a mean of 624 ± 2 Ma interpreted as their intrusion age. This age is in agreement with previously published zircon ages of meta-diorites from the Yaounde Group. The meta-diorites derived mainly from crustal rocks with minor contribution from mantle material. The 87Rb/86Sr isochron ages of one meta-diorite sample and three meta-sedimentary host rocks are significantly younger than the obtained intrusion age. Therefore, they are not related to igneous processes. 87Rb/86Sr isochron ages differ from sample to sample (599 ± 3, 572 ± 4, 554 ± 5, 540 ± 5 Ma) yielding the oldest Neoproterozoic age (~600 Ma) for a paragneiss sample at a more northern location. The youngest Rb/Sr isochron age (~540 Ma) was obtained for a mica schist sample at a more southern location closer to the border of the Congo Craton. The 87Rb/86Sr whole rock-biotite ages are interpreted as cooling ages related to transpressional processes during exhumation. Therefore, several discrete metamorphic events related to the exhumation of the Yaounde Group were dated. It could be shown by Rb/Sr dating for the first time that these late tectonic processes occurred earlier at more distant northern locations of the Yaounde Group and lasted at least until early Cambrian (~540 Ma) more closely to the border of the Congo Craton.

Neoproterozoic pegmatite from Skoddefjellet, Wedel Jarlsberg Land, Spitsbergen: Additional evidence for c. 640 Ma tectonothermal event in the Caledonides of Svalbard

Neoproterozoic pegmatite from Skoddefjellet, Wedel Jarlsberg Land, Spitsbergen: Additional evidence forc. 640 Ma tectonothermal event in the Caledonides of SvalbardNeoproterozoic (c. 640 Ma) amphibolite facies metamorphism and deformation have been shown recently to have affected the Isbjørnhamna and Eimfjellet Complex of Wedel Jarlsberg Land in southwestern Spitsbergen. New SHRIMP zircon U-Pb andin situelectron microprobe monazite and uraninite U-Th-total Pb ages are presented here on a pegmatite occurring within the Isbjørnhamna metasedimentary rocks. Although the dated zircons are full of inclusions, have high-U contents and are metamict and hence have experienced notable Pb-loss, the new Cryogenian ages are consistent with the age of regional metamorphism of the host metasediments, providing additional evidence for a clear distinction of the Southwestern Province from the other parts of the Svalbard Caledonides.

Quantitative kinematic analysis within the Khlong Marui shear zone, southern Thailand

The NNE trending Khlong Marui shear zone has a strong geomorphic signal with marked fault-strike parallel topographic ridges. The lithologies within the strike-slip zone mainly consist of vertical layers of mylonitic meta-sedimentary rocks associated with orthogneisses, mylonitic granites, and pegmatitic veins. The pegmatitic veins concordantly intrude the mylonitic foliation but were sheared at the rims indicating syn-kinematic emplacement. Microstructures and mineral assemblages suggest that the rocks in the area have been metamorphosed at amphibolite facies and low to medium greenschist facies by the first deformation. The Khlong Marui shear zone was deformed under dextral simple shear flow with a small finite strain. The ductile-to-brittle deformation involves a period of exhumation of lenses of higher grade rocks together with low grade fault rocks probably associated with positive flower structures. The final stage brittle deformation is reflected by normal faulting and formation of proto-cataclasites to cataclasites of the original mylonitic meta-sedimentary host rock. Although clear age-constraints are still missing, we use regional relationships to speculate that earlier dextral strike-slip displacement of the Khlong Marui shear zone was related to the West Burma and Shan-Thai collision and subduction along the Sunda Trench in the Late Cretaceous, while the major exhumation period of the ductile lens was tectonically influenced by the early India-Asia collision. The changing stress field has responded by switching from dextral strike-slip to normal faulting in the Khlong Marui shear zone, and is associated with “escape tectonics” arising from the overall India-Asia collision.

TEXTURE AND COMPOSITION OF GARNET AS A GUIDE TO CONTAMINATION OF GRANITOID PLUTONS: AN EXAMPLE FROM THE GOVERNOR LAKE AREA, MEGUMA TERRANE, NOVA SCOTIA

The Governor Lake area, Meguma terrane, Nova Scotia, is underlain mainly by ca. 373 Ma granitoid rocks of the Trafalgar plutonic suite that intruded metasedimentary rocks of the Goldenville and Halifax groups. Garnet is abundant in the Beaverbank Formation of the Goldenville Group and its enclaves in the granitoid rocks. Single crystals of garnet, locally up to 3 cm in diameter, are also present in the Twin Lakes granodiorite and Bog Island Lake tonalite. On the basis of petrographic examination and electron-microprobe data, we have identified four different types of garnet. Type-1 garnet, generally associated with coticule xenoliths, forms small spessartine-rich (Sps 19–74 ) grains concentrated in irregular to planar aggregates. Crystals range from homogeneous (type 1A) to zoned toward a Mn-enriched rim (type 1B). Type-2 garnet is restricted to metasedimentary host rocks, where it forms small, spessartine-rich (Sps 22–71 ) grains zoned from a Mn-rich core to a Mn-poor rim. Type-3 garnet contains abundant metamorphic inclusions ( e.g. , sillimanite) and commonly has an inclusion-rich core with an inclusion-poor rim. Type-3A crystals have a Mn-rich core (Sps 21–52 ), whereas type-3B crystals have a Mn-rich rim (Sps 15–21 ). Type-4 garnet forms large, subhedral to euhedral crystals with abundant inclusions ( e.g. , apatite, plagioclase) of probable igneous origin. Type-4A garnet is spessartine-poor (Sps 4–11 ) and weakly zoned, whereas zoning in type-4B crystals suggests that a Mn-poor core (Sps 8–16 ) was partially resorbed and overgrown by a Mn-rich rim (Sps 11–26 ). Of the various types of garnet identified in the granitic samples, types 1A and 3B are interpreted as orthoxenocrysts, type 1B, as paraxenocrysts, type 3A, as having an orthoxenocrystic core overgrown by an orthomagmatic or paraxenocrystic rim, and types 4A and 4B, as orthomagmatic. The presence of Mn-rich xenocrystic garnet suggests that the Twin Lakes and Bog Island Lake plutons were contaminated by manganiferous rocks, probably derived from the Beaverbank Formation. Incorporation and assimilation of Mn-rich material may also have led to crystallization of orthomagmatic garnet in the granitic magmas.

Petrotectonics of lawsonite eclogite exhumation: Insights from the Sivrihisar massif, Turkey

The Sivrihisar massif of the Tavşanlı Zone of Turkey is 1 of less than 10 known lawsonite eclogite localities worldwide. Rocks of the Sivrihisar massif consist of eclogite and blueschist in contact with metasedimentary host rocks and record decreasing maximum pressure conditions across three WNW‐ESE striking belts from 16–24 kbar in the northern Halilbağı belt to 14–16 kbar in the Karacaören belt and 8–10 kbar in the Kertek belt. Where present, sodic‐amphibole, phengite, chlorite, and quartz define a pervasive S0/S1 foliation; garnet, omphacite, and lawsonite define stretching lineations and kinematic indicators. D1 and D2 structures are similar in the Halilbağı and Karacaören belts but differ to those in the Kertek belt. D3 structures are uniform across the massif including fibrous calcite that occurs parallel to F3 fold axes. Shear sense indicators from field observations and asymmetric type‐I cross girdle of quartz c axes obtained from electron backscatter diffraction (EBSD) show top to the south thrusting throughout much of the massif. D1 and D2 structures are interpreted to have formed during exhumation by extrusion along a ∼5°C/km gradient. The Halilbağı and Karacaören belts were juxtaposed possibly as deep as 45 km within the subduction channel and exhumed by the arrival of the Anatolide microcontinent at approximately 70 Ma. Homogeneity of F3 axes and calcite fibers across the massif suggests that assembly occurred at blueschist conditions before exhumation through the aragonite‐calcite transition (∼350°C) above 8 kbar.

Assessing Bulk Assimilation in Cordierite-bearing Granitoids from the Central System Batholith, Spain; Experimental, Geochemical and Geochronological Constraints

This paper describes in detail an example of a calc-alkaline batholith, located in the Gredos massif (Spanish Central system batholith, Iberian Variscan massif), intruded into pelitic metasediments at shallow crustal levels. The igneous rocks of the study area are divided into three main groups according to their petrographic and geochemical (major, trace and rare earth element) characteristics: calc-alkalic (Qtz diorites and tonalites), transitional (Bt granodiorites), and alkali-calcic series (typically composed of Crd monzogranites). A paradoxical feature of the Crd monzogranites is the presence of euhedral Crd crystals in strong disequilibrium within a non-anatectic monzogranite. The field relations and geochemical data strongly suggest a process of bulk assimilation of the host metasediments by the intruding Bt granodiorites to generate the Crd monzogranites. Geochemical trends defined by the Crd monzogranites clearly depart from cotectic-like evolutions of liquid composition, and point to the composition of the metapelitic host-rocks. Laboratory experiments have been performed to check this hypothesis, and the results show that cordierite forms in a local domain where the reactants of the peritectic reaction Bt + Qtz + Pl + Als → Crd + Kfs + Melt are located. The process includes the isolation of metapelitic xenoliths in the granodiorite magma (a feature observed in the field), partial melting of these xenoliths with generation of cordierite as a peritectic phase in local equilibrium with the melt inside the entrapped xenolith, and the survival of these crystals because the residual melt of the granodiorite magma converges compositionally toward the peraluminous leucogranite generated in the relic xenolith. In turn, the granodiorite gains K released from the xenoliths, inducing the crystallization of K-feldspar and shifting to a Crd monzogranitic composition. Inherited zircons are present in the Crd monzogranites, similar to those found in the host metasediments; these are virtually absent in the intrusive Bt granodiorites. Mass-balance calculations and estimates based on the amount of Crd present have been applied to quantify the extent of assimilation in the petrogenesis of the granitoids; the volume of assimilated material can be high, depending on the proximity to the contacts with the xenoliths and the amount of pelitic material in the metasediments. © The Author 2010. Published by Oxford University Press. All rights reserved.

Hydrothermal origin of manganese in the high-pressure ophiolite metasediments of Praborna ore deposit (Aosta Valley, Western Alps)

The manganese ore deposit of Praborna crops out in the Zermatt-Saas unit of the Western Alps, in the St. Marcel valley. It represents a Jurassic ophiolitic sedimentary cover subducted at high-pressure conditions during the Alpine orogeny. Major- and trace-element analyses of representative samples of the ore and the host metasediments were collected to geochemically characterise the deposit. Selected phases (piemontite, braunite, garnet, clinopyroxene, white mica and manganiandrosite) were investigated with ion and electron microprobes to link the mineral chemistry to the bulk-rock chemistry. Compared to shales, Praborna is enriched in Mn (up to 38.7 wt% Mn 2 O 3 ) and in many trace elements (Sc, Co, Ni, Cu, Ge, As, Sr, Ag, Sb, Te, Ba, Tl, Pb and Bi). The bulk-rock REE pattern suggests 20 % hydrogenous and 80 % hydrothermal inputs in the proto-ore. Compared to the shale, the hanging-wall Mn-poor schists share with the Mn ore body the enrichment in Sc, Mn, Co, Sr and Te, suggesting a common enrichment process involving these elements. The REE pattern suggests a sedimentary origin for these schists, which were probably composed of clay mixed with components of volcanic origin. In order to confirm the hydrothermal origin of the Praborna Mn ore deposit, we built up a database of more than 5000 data of modern hydrogenous and hydrothermal oceanic Mn deposits worldwide, adding data of oceanic Mn-rich sediments and of the Ligurian Mn ore deposits, which are thought to be the unmetamorphosed geological equivalent of Praborna. The classic ternary Mn–Fe–(Cu + Co + Ni) diagram, the agglomerative hierarchical clustering and the principal-component analysis, which takes into account a larger set of elements, strongly support the hypothesis of an oceanic hydrothermal origin for manganese in the Praborna and in the Ligurian ore deposits.

Fluid inclusions in quartz related to subsequent stages of foliation development during a single metamorphic cycle (Schneeberg Fault Zone, Eastern Alps, Austria)

Macro- and micro-scale structures, fluid inclusions, quartz rheology and host mineral chemistry were used to establish the P– T– d evolutionary path of monometamorphic metapelites of the Schneeberg Fault Zone. Four selected quartz generations indicate formation conditions during stages of prograde and retrograde metamorphism. Earliest foliation development S1 is outlined by quartz foam microstructures that are armored within foliation-parallel garnet layers which reflect individual growth characteristics during the prograde and early retrograde stage. The prograde path is linked with garnet growth at low differential stress up to peak pressure conditions of ca. 10 kbar. Final garnet growth during increasing temperatures and WNW-directed shearing up to ≤ 600 °C predates exhumation along an isothermal decompressional path and increasing differential stress. Exhumation was accompanied by the formation of younger quartz generations and the formation of a second foliation S2 that was incorporated into intense folding. Minimum pressure condition for S2 is about 4.5 kbar resulting from the gradient of fluid inclusion density isochores estimated from S2-related pressure shadows in combination with the temperature range for dislocation creep in quartz aggregates between 500 and 600 °C. The dominance of carbonic fluid inclusions indicates decarbonation associated with H 2O leakage as a main process during Alpine monometamorphism in the metasedimentary host rocks. Aqueous fluid inclusions are arranged along late fluid planes and reflect a wide range in densities with variable salinities. This study corroborates the successful approach of combining mineral growth, fluid inclusions and macro- to micro-scale deformation stages to constrain a prograde to retrograde growth history of metamorphic minerals like garnet and quartz even though these underwent polyphase deformation and upper amphibolite facies metamorphism. It supports previous arguments about low differential stress at high-pressure conditions and allows speculations about the location of the metasediments of the Schneeberg Fault Zone nearby the low strength plate interface during late stage of Cretaceous subduction and subsequent early exhumation.

A U–Pb zircon Paleoproterozoic age for the metasedimentary host rocks and gold mineralization of the Crixás greenstone belt, Goiás, Central Brazil

The Crixás greenstone belt, Central Brazil, consists of a greenschist to low amphibolite facies metamorphic volcanosedimentary sequence that contains the sixth largest gold reserve (∼ 70 t Au, in the average 12 g/t) of Brazil and the largest gold-only deposit of Central Brazil. The gold orebodies are hosted by low-angle thrust faults within the stratigraphically upper metasedimentary rocks and are crosscut by a set of post-mineralization mafic dykes. For decades the sequence has been interpreted as a typical Archean greenstone belt of the Brazilian Shield and the gold mineralization considered either Archean or Neoproterozoic. This study shows the results of in situ U–Pb SHRIMP and LA-ICP-MS dating of detrital zircon grains from graywacke and mafic dyke samples, as well as Pb and Nd isotopic signatures of the ores. The oldest detrital zircon grains of the graywacke samples yielded U–Pb ages of 3354 ± 40 Ma and the youngest between 2209 ± 28 Ma and 2222 ± 30 Ma, which indicates a mixture of Archean and Paleoproterozoic (Rhyacian) source area and that the maximum depositional age is of ca. 2.2 Ga. Magmatic zircon of the post-mineralization mafic dykes yielded the U–Pb age of 2170 ± 17 Ma, which suggests that the basin stage, its deformation, metamorphism and mineralization were restricted to the Rhyacian and took place during a time-span of about 30–40 m.y. The paper thus provides a new insight into the age of the Crixás greenstone belt rocks and of its gold-only deposits, and reveals that Mid-Paleoproterozoic was a more widespread metallogenetic epoch related to greenstone belts of the Brazilian Precambrian Platform then previously interpreted. The observed Pb and Nd isotopic signatures of the ores suggest that metabasalts, carbonaceous schist and metagraywackes could provide the adequate sources for the ore-bodies and that the initiation of fluid circulation may have resulted by the emplacement of a high-level granite.

Age of alkaline rocks in the Seiland Igneous Province, Northern Norway

Abstract: Spatially associated carbonatite and syenite complexes at Breivikbotn on the island of Sørøy, in the Caledonian nappes of northern Norway, have been dated by isotope dilution thermal ionization mass spectrometry U–Pb analysis on zircon and titanite. Zircon crystals in malignite and silico-carbonatite contain many inclusions of Th- and U-rich minerals and have been locally affected by dissolution–reprecipitation processes, all features that tend to cause disturbed U–Pb systems. Some alkalic gneisses also exhibit evidence of new zircon growth during the Caledonian metamorphism. The carbonatite was emplaced into the Kalak Nappe Complex at 574 ± 5 Ma, and various alkaline gneisses and nepheline syenite dykes intruded the neighbouring Breivikbotn Gabbro and its metasedimentary host between 580 and 560 Ma. The alkaline rocks were coeval with, and probably genetically related to major mafic and ultramafic magmatism throughout the Seiland Igneous Province, preceding the emplacement of nepheline syenite pegmatites at 530–520 Ma. The ages of the Seiland Igneous Province indicate formation in an extensional setting, the multistage history of the host Kalak Nappe Complex pointing to a potential origin as an Avalonian-type peri-Gondwanan terrane.

Geochemical, Sm–Nd isotopic characteristics and petrogenesis of Paleoproterozoic mafic rocks from the Georgetown Inlier, north Queensland: Implications for relationship with the Broken Hill and Mount Isa Eastern Succession

Fe-rich tholeiites intruded the lower Etheridge Group of the Georgetown Inlier, north Queensland during the late-Paleoproterozoic (ca. 1685–1640 Ma). Emplacement of the tholeiites occurred during a prolonged period of rift-related sedimentation associated with extension. The tholeiites comprise two lithologically distinct units: the extrusive Dead Horse Metabasalt (DHM), a conformable 1000 m-thick series of fine- to coarse-grained basalts and pillow lavas, and the intrusive Cobbold Metadolerite (CMD), a multitude of sills and lesser dykes that range up to 500 m thick. The DHM and CMD are comagmatic, and have compositions typical of relatively evolved, low-K, Fe-rich continental (rift) tholeiites. Immobile major and trace element patterns suggest that the tholeiites were derived from a depleted upper mantle source (initial ɛ Nd values range between +2.6 and +5.3). The parental melts evolved along a trend of increasing Fe-enrichment (Fe 2O 3* 6.81–21.11 wt%) via closed-system fractionation, with little or no replenishment. Several samples show distinct REE patterns, consistent with limited crustal contamination. Trace element and isotopic data show that the crustal contaminants were derived from the host metasediments of the lower Etheridge Group. The age and geochemical characteristics of the Etheridge Group Fe-rich tholeiites show striking similarities to similar mafic sills in the southeastern Curnamona Province around Broken Hill in western New South Wales, and to basaltic lavas and dolerites of the Soldiers Gap Group in the eastern edge of the Mt Isa Block (some 400 km west of the Georgetown Inlier), suggesting a spatial link between these terranes during the Paleoproterozoic. They may have formed along a more than 2000 km segment of a ca. 1685–1640 Ma volcanic passive margin, tectonically and magmatically akin to the modern East Greenland margin.

Geodynamic evolution of the SW Variscides: Orogenic collapse shown by new tectonometamorphic and isotopic data from western Ossa‐Morena Zone, SW Iberia

The pre‐Mesozoic geodynamic evolution of SW Iberia has been investigated on the basis of detailed structural analysis, isotope dating, and petrologic study of high‐pressure (HP) rocks, revealing the superposition of several tectonometamorphic events: (1) An HP event older than circa 358 Ma is recorded in basic rocks preserved inside marbles, which suggests subduction of a continental margin. The deformation associated with this stage is recorded by a refractory graphite fabric and noncoaxial mesoscopic structures found within the host metasediments. The sense of shear is top to south, revealing thrusting synthetic with subduction (underthrusting) to the north. (2) Recrystallization before circa 358 Ma is due to a regional‐scale thermal episode and magmatism. (3) Noncoaxial deformation with top to north sense of shear in northward dipping large‐scale shear zones is associated with pervasive hydration and metamorphic retrogression under mostly greenschist facies. This indicates exhumation by normal faulting in a detachment zone confined to the top to north and north dipping shear zones during postorogenic collapse soon after 358 Ma ago (inversion of earlier top to south thrusts). (4) Static recrystallization at circa 318 Ma is due to regional‐scale granitic intrusions.

Detrital Modes in a Late Miocene Wedge-Top Basin, Northeastern Calabria, Italy: Compositional Record of Wedge-Top Partitioning

Abstract The composition and stratigraphic relations of sandstones and associated conglomerates derived from erosion of the northern Calabrian terranes provide important constraints for paleotectonic and paleogeographic models of the Neogene wedge-top depozone evolution of the southern Italy foreland basin system. After onset of continental collision in the southern Italy orogenic belt during late Oligocene to early Miocene, the Calabrian Arc drifted northeastward, accreting over the Adria lithosphere during mid to late Miocene time. Since the Tortonian, the northern Calabrian terranes represented an uplifted thrust belt in which along its leading edge tectonic subsidence was responsible for accumulation of a thick Tortonian to Quaternary succession, dominantly siliciclastic (up to 3300 meters thick), deposited in the wedge-top depozones (Rossano and Crotone basins). Sandstone assemblages have a homogeneous quartzofeldspathic composition in the Rossano and Crotone basins, characterized by a compositional trend, through time in response to regional tectonics. Evolution of the basins fill occurred during (1) Serravallian?–Tortonian, (2) late Tortonian to early Messinian, and (3) late Messinian. Detritus shed into those basins was derived mainly from terranes within the strongly deformed Sila Mountains allochthon, which was one of the principal structural elements of the nearby northern Calabrian terranes, consisting of Hercynian plutons intruding Paleozoic (Cambrian to Carboniferous) metasedimentary host rocks, and locally having a sedimentary cover of Mesozoic (Longobucco Group) or early Miocene (Paludi unit) clastics. Low- to medium-grade metamorphic and minor plutonic grain types (metamorphiclastic petrofacies) represent the main detritus in the earliest (Serravallian?–Tortonian) sedimentation that accompanied progressive flexural bending of the western margin of the foreland-basin system. Tortonian to early Messinian time was characterized by isolation of the Mediterranean region from the global marine realm, leading to evaporite sedimentation in the foreland in addition to intense regional and local tectonics. Compositional response to those events was a marked increase in sedimentary detritus (sedimenticlastic petrofacies) related to cannibalization of early Messinian evaporite successions; after the evaporitic events within the basins and the gradual re-establishment to the normal marine conditions, sandstone detrital modes are characterized mainly by a return of the Serravallian?–Tortonian compositional modes (i.e., metamorphic–plutoniclastic petrofacies). The post-evaporitic sedimentation corresponds to a major tectonic rearrangement on both Rossano and Crotone basins related to extrusion and inversion of the basal portion of the Serravallian?–Tortonian sequences in the intermediate zone to form an intervening Ciro structural high. In the Crotone Basin during late Messinian, deposition of the Carvane Conglomerate records an abrupt sudden arrival of distinctive sedimentary containing clasts of detritus of reddish limestone, marl, quartz arenite, siltstone and chert derived from Mesozoic to Tertiary basinal sequences of the southern Apennines domains (i.e., Sicilide Complex terranes). Detrital-mode evolution of the inner portions of the late Miocene southern Italy foreland-basin system was in response to abrupt tectonic partitioning due to continuation of accretionary processes, rapid uplift of mid-crustal blocks, and the superposition of wrench tectonics.

Petrogenesis of albite-rich mid- to late Proterozoic tephra-fall deposits (‘brown beds’)

AbstractThe discovery of volcanogenic ‘brown beds’ in Dalradian and Torridonian (Sleat Group and Diabaig Group) sequences in Scotland has raised questions about their origin and provenance. New discoveries of a grey facies cognate to the ‘brown beds’ have led to an hypothesis which proposes that these beds originated as tephra-fall deposits of intermediate to mafic composition. Subsequent prehnite–pumpellyite- and greenschist-facies metamorphism generated an albite–chlorite–muscovite–quartz–calcite assemblage. Recent sub-aerial weathering selectively dissolved interstitial calcite and oxidized iron, which left an albite-rich ‘brown bed’ with a porous saccharoidal texture. Field relationships and mineralogy support their origin as tephra-fall deposits and their widespread occurrence suggests they are not localized phenomena. Crucially, the weathered ‘brown beds’ point to the existence of otherwise cryptic grey metamorphosed tuffs which tend to blend in with their host metasediments.

Timing of high‐pressure metamorphism and exhumation of the eclogite type‐locality (Kupplerbrunn–Prickler Halt, Saualpe, south‐eastern Austria): constraints from correlations of the Sm–Nd, Lu–Hf, U–Pb and Rb–Sr isotopic systems

AbstractSm–Nd, Lu–Hf, Rb–Sr and SIMS U–Pb data are presented for meta‐gabbroic eclogites from the eclogite type‐locality (Haüy, 1822) Kupplerbrunn–Prickler Halt and other areas of the Saualpe (SE Austria) and Pohorje Mountains (Slovenia). Mg‐rich eclogites derived from early gabbroic cumulates are kyanite‐ and zoisite rich, whereas eclogites with lower Mg contents contain clinozoisite ± kyanite. Calculated P–T conditions at the final stages of high‐pressure metamorphism are 2.2 ± 0.2 GPa at 630–740 °C.Kyanite‐rich eclogites did not yield geologically meaningful Sm–Nd ages due to incomplete Nd isotope equilibration, whereas Sm–Nd multifraction garnet–omphacite regression for a low‐Mg eclogite from Kupplerbrunn yields an age of 91.1 ± 1.3 Ma. The Sm–Nd age of 94.1 ± 0.8 Ma obtained from the Fe‐rich core fraction of this garnet dates the initial stages of garnet growth. Zircon that also crystallized at eclogite facies conditions gives a weighted mean U–Pb SIMS age of 88.4 ± 8.1 Ma. Lu–Hf isotope analysis of a kyanite–eclogite from Kupplerbrunn yields 88.4 ± 4.7 Ma for the garnet–omphacite pair. Two low‐Mg eclogites from the Gertrusk locality of the Saualpe yield a multimineral Sm–Nd age of 90.6 ± 1.0 Ma. A low‐Mg eclogite from the Pohorje Mountains (70 km to the SE) gives a garnet–whole‐rock Lu–Hf age of 93.3 ± 2.8 Ma.These new age data and published Sm–Nd ages of metasedimentary host rocks constrain the final stages of the eo‐Alpine high‐pressure event in the Saualpe–Pohorje part of the south‐easternmost Austroalpine nappe system suggesting that garnet growth in the high‐pressure assemblages started at c. 95–94 Ma and ceased at c. 90–88 Ma, probably at the final pressure peak. Zircon and amphibole crystallization was still possible during incipient isothermal decompression. Rapid exhumation of the high‐pressure rocks was induced by collision of the northern Apulian plate with parts of the Austroalpine microplate, following Jurassic closure of the Permo‐Triassic Meliata back‐arc basin.