Widespread Plutonism Research Articles

Dating initial crystallisation of some Devonian plutons in central Victoria and geological implications

Rather than being inherited as detrital grains, many of the rounded and/or embayed cores in zircon crystals from granitic rocks are probably antecrysts, formed early in the magmatic systems. Using laser-ablation, sector-field ICP-MS, we obtained internally consistent weighted-mean concordia ages of 375.3 ± 2.5 Ma and 376.9 ± 2.6 for early crystallisation of two samples from the previously undated Mount Disappointment pluton in the Melbourne Zone. Within uncertainty, our date of 376.4 ± 2.4 Ma for the Baringhup pluton of the Harcourt batholith is the same as the published and our new date for the Mount Alexander pluton, in the same batholith. Using the same techniques, we produced an improved date of 370.9 ± 6.5 Ma for the I-type Ercildoun Granite in the neighbouring Bendigo Zone of the Lachlan Fold Belt. We also obtained dates that confirm published ages for the Mount Bute Granite in the Stawell Zone, the Tynong pluton of the Tynong batholith in the Melbourne Zone and the Oberon pluton of the Wilsons Promontory batholith in the Bassian Zone. These dates confirm the Late Devonian (Fransian) age of most of this widespread plutonism, as well as the reported Emsian age of the Wilsons Promontory batholith. However, part of what has been mapped as the Mount Wombat pluton of the Strathbogie batholith could be significantly older than the rest. KEY POINTS Many rounded or embayed cores in zircon crystals are antecrysts rather than inherited detrital grains. The Mount Disappointment pluton is confirmed as Late Devonian in age, at 375.2 ± 2.5 Ma. At 378.2 ± 2.0 Ma, the Baringhup pluton of the Harcourt batholith is the same age as the Mount Alexander pluton, in the same batholith. Part of the Mount Wombat pluton in the Strathbogie batholith may be older than the rest of the pluton.

The early-Sveconorwegian orogeny in southern Norway: Tectonic model involving delamination of the sub-continental lithospheric mantle

The Sveconorwegian orogeny is a more than 250 Myr-long multiphase orogenic event that contributed to the amalgamation of Fennoscandia into Rodinia at the end of the Mesoproterozoic. We propose a new lithospheric-scale tectonic model for the pre- to early Sveconorwegian orogenic evolution between 1280 and 1080 Ma. This evolution is recorded in southern Norway by two c. 25 km wide belts of up to granulite-facies rocks, the Bamble and Kongsberg Lithotectonic Units, and a c. 80 km belt in the Telemarkia Lithotectonic Unit, which contains lower-grade rocks and evidence of widespread plutonism. New zircon U–Pb geochronological data from 54 samples confirm magmatic activity between 1575 and 1485 Ma, and 1210 and 1140 Ma. The new data constrain peak amphibolite to granulite-facies metamorphism in Bamble and Kongsberg to between 1145 and 1130 Ma. New geological mapping shows that the Kongsberg Lithotectonic Unit is characterized by a regional, N–S trending highly transposed lithological banding reflecting E–W orthogonal shortening as expressed by tight to isoclinal folds, and a penetrative subvertical foliation bearing a steep peak-metamorphic lineation. Telemarkia is instead characterized by crustal extension-related bimodal magmatism and intermontane basin sedimentation between 1280 and 1080 Ma. Paired and in part coeval extension and compression are interpreted as reflecting asthenospheric upwelling, orogenic plateau development and delamination of the sub-continental lithospheric mantle beneath Telemarkia, followed by foundering (dripping-off) of the lithospheric mantle slab under the Kongsberg and Bamble Lithotectonic Units. This model stresses the pull effect of a foundering mantle slab in an actively shortening orogen to facilitate highly localised compression and granulite-facies metamorphism in the crust. It accounts for the well-known, first-order geological characteristics of the orogen and does not require the accretion of exotic terranes to explain the early-Sveconorwegian orogenic evolution.

Facies architecture and provenance of a boulder-conglomerate submarine channel system, Panoche Formation, Great Valley Group: A forearc basin response to middle Cretaceous tectonism in the California convergent margin

Tectonic reorganization induced by a rapid increase in plate motion ­obliquity and rate beginning at ca. 100 Ma affected California’s Andean-style convergent margin, with concomitant changes in the accretionary prism of the Franciscan Complex, the Great Valley forearc basin, and the Sierran continental arc. Using facies analysis and a combined provenance approach, we suggest that this ca. 100 Ma tectonic signal is preserved in a Cenomanian (Upper Cretaceous) boulder-conglomerate outcrop along the San Luis Reservoir (SLR) in the southern Great Valley, which represents the thickest and coarsest deep-water deposit ever described in the Great Valley Group (GVG). We document a 1.8-km-thick by 4-km-long depositional-dip profile of an interpreted SE-directed (axial) submarine channel system that is part of a conglomeratic package that stretches 20 km along the east-central Diablo Range. Our facies analysis of the SLR area documents five facies associations within four aggradational channel complex sets, followed by regional abandonment. Sandstone petrography and mudrock geochemical data suggest a dissected continental Sierra Nevadan arc source. Conglomerate clast counts show abundant ophiolitic-type clasts that may be derived from the Coast Range Ophiolite and/or the Western Sierra Nevada Metamorphic Belt. Detrital-­zircon geochronology data also indicate western and central Sierra Nevadan sources; however, we interpret an anomalous (relative to other Cenomanian localities) 105–95 Ma zircon population to indicate the initial erosional products from the volcanic carapace associated with the Late Cretaceous magmatic flare-up within the eastern Sierran arc. This flare-up has been linked to an increase in arc-parallel plate motion that induced deformation along shear zones in the eastern Sierra Nevada, allowing for widespread plutonism. Our provenance interpretation makes the SLR area the earliest Upper Cretaceous GVG locality to receive significant detritus from the flare-up, effectively linking tectonic plate motion changes and coarse-grained, deep-water forearc sedimentation.

Petrogenesis and tectonic implications of the Early Carboniferous to the Late Permian Barleik plutons in the West Junggar (NW China)

The Paleozoic accretionary orogenesis and continental crustal growth in Central Asia are thought to have close relationship with the evolution of the Paleo-Asian Ocean (PAO). The well-exposed plutons in the northern Barleik Mountains of the West Junggar region, NW China, may provide essential insights into the evolution of the Junggar Ocean, a branch of the PAO, and mechanism of continental crustal growth. Our work on the Barleik plutons indicates an early suite of 324–320Ma diorite and a late suite of 314–259Ma quartz syenite and granitic porphyry. All the plutons are characterized by high-K calc-alkaline to shoshonitic signatures, varying depletion in Nb, Ta, Sr, P, Eu, and Ti, low initial 87Sr/86Sr ratios (0.70241–0.70585), strongly positive εNd(t) values (+5.7–+7.7), and young one-stage Nd model ages (390–761Ma), suggesting that they resulted from different batches of magma that were produced by fractional crystallization of a metasomatized mantle source with minor crustal contamination. The diorite is coeval with the youngest arc magmatic rocks, indicating a subduction-related origin. By contrast, the quartz syenite and granitic porphyry are geochemically similar to A2-type granites, with high Zr, Ga, and FeOT/[FeOT+MgO], and are coeval with the widespread plutons in the West Junggar. This, together with the occurrence of Late Carboniferous fluvial deposits and the lack of <320Ma ophiolitic and subduction-related metamorphic lithologies, definitively indicates a post-collisional setting after the closure of the Junggar Ocean. Slab breakoff accompanied by asthenospheric upwelling and basaltic underplating is a possible geodynamic process that is responsible for the post-collisional magmatism and vertical crustal growth in the region. Thus a tectonic switch from subduction to post-collision started at the end of the Early Carboniferous (~320Ma), probably as a result of the final closure of the Junggar Ocean.

Long-lived transpression in the Archean Bird River greenstone belt, western Superior Province, Southeastern Manitoba

The Archean Bird River greenstone belt (BRGB) is located on the southwestern edge of the Superior Province between the 3.2 Ga old Winnipeg River subprovince to the south and the metasedimentary belt of the English River subprovince (ERSP) to the north. This position between two major subprovinces makes the BRGB a primary target for investigating the geodynamic and kinematic evolution of a major structural boundary. New structural and geochronological data have allowed us to present an evolutionary framework for the southern boundary of the North Caribou superterrane. The BRGB underwent 3 main deformation phases. The D1 event took place ca. 2698 Ma and displays a north-side-up shearing. The D2 event, occurring at ca. 2684 Ma in a transpressive context, presents a complex structural pattern mixing vertical tectonics in the BRGB and strike-slip tectonics along the boundaries of the greenstone belt with other subprovinces. Between the BRGB and the ERSP, the 2832–2858 Ma old Maskwa batholith acted as a rigid passive block during the collision and marks the boundary between pure dextral strike-slip tectonics along his northern boundary with the ERSP and vertical south-side-up motion in the BRGB. The BRGB can be considered as a pop-up structure with anastomosed shear zones displaying different horizontal offset according to the orientation of the shear zones. The southern boundary with the Winnipeg River subprovince is represented by a sinistral south-side-up shear zone. The same pattern is found at the regional scale where major shear zones acted as a conjugate set in the horizontal plane. At ca. 2640 Ma, the D3 event occurred in a general dextral transpressive tectonic regime coeval with the emplacement of rare-elements pegmatitic plutons in a still hot (400–500 °C) country rock. The geodynamical and mechanical significance of the partitioning between pure strike-slip tectonics in the English River subprovince and vertical motion in the BRGB can be explained by the rheological behaviour of a hot and weak lithosphere undergoing transpressive strain. The structural framework of the BRGB is the result of strong interactions between hot and weak domains, coeval with widespread plutonism, and a rigid older domain (Maskwa batholith) during the D2 transpressive event.

Review: secular tectonic evolution of Archean continental crust: interplay between horizontal and vertical processes in the formation of the Pilbara Craton, Australia

AbstractThe Archean Pilbara Craton contains five geologically distinct terranes – the East Pilbara, Karratha, Sholl, Regal and Kurrana Terranes – all of which are unconformably overlain by the 3.02‐ to 2.93‐Ga De Grey Superbasin. The 3.53–3.17 Ga East Pilbara Terrane (EP) represents the ancient nucleus of the craton that formed through three distinct mantle plume events at 3.53–3.43, 3.35–3.29 and 3.27–3.24 Ga. Each plume event resulted in eruption of thick dominantly basaltic volcanic successions on older crust to 3.72 Ga, and melting of crust to generate first tonalite‐trondhjemite‐granodiorite (TTG), and then progressively more evolved granitic magmas. In each case, plume magmatism was accompanied by uplift and crustal extension. The combination of conductive heating from below, thermal blanketing from above, and internal heating of buried granitoids during these events led to episodes of partial convective overturn of upper and middle crust. These mantle melting events caused severe depletion of the subcontinental lithospheric mantle, making the EP a stable, buoyant, unsubductable continent by c. 3.2 Ga. Extension accompanying the latest event led to rifting of the protocontinent margins at between 3.2 and 3.17 Ga. After 3.2 Ga, horizontal tectonic forces dominated over vertical forces, as revealed by the geology of the three terranes (Karratha, Sholl and Regal) of the West Pilbara Superterrane. The c. 3.12‐Ga Whundo Group of the Sholl Terrane is a fault bounded, 10‐km‐thick volcanic succession with geochemical characteristics of modern oceanic arcs (including boninites and evidence for flux melting) that indicate steep Archean subduction. At 3.07 Ga, the 3.12‐Ga Sholl Terrane, 3.27‐Ga Karratha Terrane and c. 3.2‐Ga Regal Terrane accreted together and onto the EP during the Prinsep Orogeny. This was followed by development of the De Grey Superbasin – an intracontinental sag basin and widespread plutonism (2.99–2.93 Ga) as a result of orogenic relaxation and slab break off. Craton‐wide compressional deformation at 2.95–2.93 Ga culminated with 2.91‐Ga accretion of the 3.18 Ga Kurrana Terrane with the EP. This compression caused amplification of the dome‐and‐keel structure in the EP. Final cratonization was effected by emplacement of 2.89–2.83 Ga post‐tectonic granites.

Isotopic age of the Black Forest Bed, Petrified Forest Member, Chinle Formation, Arizona: An example of dating a continental sandstone

Zircons from the Black Forest Bed, Petrified Forest Member, Chinle Formation, in Petrified Forest National Park, yield ages that range from Late Triassic to Late Archean. Grains were analyzed by multigrain TIMS (thermal-ionization mass spectrometry), single-crystal TIMS, and SHRIMP (sensitive, high-resolution ion-microprobe). Multiple-grain analysis yielded a discordia trajectory with a lower intercept of 207 6 2 Ma, which because of the nature of multiple-grain sampling of a detrital bed, is not considered conclusive. Analysis of 29 detrital-zircon grains by TIMS yielded UPb ages of 2706 6 6M a to 2066 6 Ma. Eleven of these ages lie between 211 and 216 6 6.8 Ma. Our statistical analysis of these grains indicates that the mean of the ages, 213 6 1.7 Ma, reflects more analytical error than geologic variability in sources of the grains. Grains with ages of ca. 1400 Ma were derived from the widespread plutons of that age exposed throughout the southwestern Cordillera and central United States. Twelve grains analyzed by SHRIMP provide 206 Pb*/ 238 U ages from 214 6 2M a to 200 6 4 Ma. We use these data to infer that cores of inherited material were present in many zircons and that single-crystal analysis provides an accurate estimation of the age of the bed. We further propose that, even if some degree of reworking has occurred, the very strong concentration of ages at ca. 213 Ma provides a maximum age for the Black Forest Bed of 213 6 1.7 Ma. The actual age of the bed may be closer to 209 Ma. Dating continental successions is very difficult when distinct ash beds are not clearly identified, as is the case in the Chinle Formation. Detrital zircons in the Black Forest Bed, however, are dominated by an acicular morphology with preserved delicate terminations. The shape of these crystals and their inferred environment of deposition in slow-water settings suggest that the crystals were not far removed from their site of deposition in space and likely not far in time. Plinian ash clouds derived from explosive eruptions along the early Mesozoic Cordilleran margin provided the crystals to the Chinle basin, where local conditions insured their preservation. In the case of the Black Forest Bed, the products of one major eruption may dominate the volcanic contribution to the unit. Volcanic detritus in the Chinle Formation was derived from multiple, distinct sources. Coarse pebble- to cobble-size material may have originated in eastern California and/or western Arizona, where Triassic plutons are exposed. Fine-grained detritus, in contrast, was carried in ash clouds that derived from caldera eruptions in east-central California or western Nevada.

Age and style of deformation and stratal thinning at the transition, Wasatch Plateau to Great Basin, central Utah

The Salina area of central Utah contains an unusually complete Tertiary stratigraphic record, affording unique opportunities to study details of the Tertiary geologic history. Stratigraphic and structural studies reveal Neogene ages for tectonically significant parts of all major structures such as the 80-km-long Sanpete-Sevier Valley anticline (SSVA) and its flanking structures, the Wasatch monocline and Sevier Valley syncline. Newly identified critical components of the Neogene history in the SSVA include: (1) a protracted Eocene through early Miocene period of quasi-continuous sedimentation and relative structural quiescence that buried an early phase of the anticline; (2) east-trending folds in rocks of the anticlinal core; (3) widespread plutons in the southern part of the core; (4) strike-slip faults; (5) major thinning of strata (vertical structural convergence) between uplifted Jurassic rocks of the anticlinal core and downdropped superjacent Tertiary rocks; and (6) a widespread zone of altered rock at the contact between core rocks and overlying Tertiary rocks. We interpret the thinning and widespread alteration (5 and 6) to be linked through protracted processes of extensional faulting (possibly including detachment faulting), fluid flow, massive dissolution, and dissolution collapse at the contact between the Jurassic core rocks and overlying Tertiary rocks. We suggest that the SSVA, one of several diapir-like elongate uplifts in central Utah, is somewhat analogous to flank ridges that develop at landslide margins by outward and upward flow of mechanically weak clay-rich rock or gouge. By analogy, the SSVA results from outward and upward movement of the relatively incompetent Arapien Shale from beneath the adjacent Sevier Valley. If the structures formed during these newly recognized components of Neogene history are palinspastically restored, much of the evidence for contractional deformation at shallow structural levels is eliminated; this affects resource assessments based on models of that deformation. Also, the association of extensional faulting with non-tectonic dissolution collapse and with lateral motion of mechanically weak rocks implies shallow penetration and non-seismogenic to weak seismogenic behavior of some normal fault systems in central Utah.

Chronostratigraphic constraints on the genesis of Archean greenstone belts, northwestern Superior Province, Ontario, Canada

The paper reports new U–Pb zircon ages for supracrustal assemblages of the Red Lake, McInnes Lake, Hornby Lake, North Spirit Lake and Favourable Lake greenstone belts of northwestern Superior Province and evaluates their significance within the regional stratigraphic and tectonic context. The new data confirm and refine the existing picture of a multistage evolution lasting ca 300 m.y., initiated at ca 3000 Ma by periods of coeval calc-alkalic, tholeiitic and komatiitic magmatic activity, followed by the eventual accretion of a microcontinental nucleus by ca 2900–2800 Ma, and concluded by an intense period of extensive tholeiitic and calc-alkalic volcanism, and by widespread plutonism and tectonism between 2750 and 2680 Ma. Volcanic sequences in the McInnes Lake and Hornby Lake greenstone belts of the Berens River Subprovince yield ages of 2974±2, 2969±3, 2928±2 and 2901±2 Ma, showing a correlation with the early stages of greenstone belt development in the region. Similarly, a sedimentary unit in the northern Red Lake greenstone belt was probably deposited at ca 2900 Ma, as indicated by a range of detrital zircon ages between 2989 and 2916 Ma, and the absence of younger components. By contrast, a volcanic unit in the center of the Red Lake greenstone belt, previously thought to be 2830 Ma, yields an age of 2745+7/−4 Ma, which correlates with some of the latest volcanic episodes in the Uchi Subprovince. A detrital zircon population in a sedimentary unit of the western Favourable Lake greenstone belt of the Sachigo Subprovince displays single grain ages ranging from 2843 to 2727 Ma, the youngest age corresponding to that of an unconformably underlying tonalite gneiss. The geological and geochronological relationships suggest that the North Spirit Lake and Favourable Lake greenstone belts represent back-arc basins that evolved between ca 2750 and 2730 Ma in a transtensional setting within the older North Caribou terrane; sedimentation was initiated during opening and deepening of the basins and accompanied most of the concluding compression and imbrication stages. The period between 2750 and 2680 Ma was also characterized by widespread granitoid plutonism that enveloped the McInnes Lake and Hornby Lake greenstone belts and older tonalites and formed the bulk of the Berens River Subprovince.

Age constraints on gold mineralization and Paleoproterozoic crustal evolution in the Ashanti belt of southern Ghana

U Pb isotopic dating in the region of the Ashanti belt in Ghana has been carried out to improve understanding of crustal evolution and gold mineralization in this Paleoproterozoic terrane. A U Pb zircon age of ca 2174 ± 2 Ma for a belt-type granitoid from Sekondi accords with previous ages on belt-type plutons in Ghana, but metamorphic titanite within this granitoid gives an age of 2092 ± 3 Ma, coeval with intrusion of the late basin-type granitoids. The within-belt granitoid at Banso village has an age close to 2097 ± 3 Ma, which falls into the age range typical for basin-type granitoids (2116-2088 Ma). These results indicate that widespread plutonism in the Birimian sedimentary basins also produced magmatic and metamorphic effects within the volcanic belts. Most dated detrital zircons from Birimian metasediment samples at the Ashanti gold mine agree in age at 2155 ± 2 Ma, with others that are slightly older. This gives a maximum estimate for the depositional age of the sediments and for gold mineralization at the Ashanti gold mine. Hydrothermally altered and auriferous granitoid plutons along the western flank of the Ashanti belt, at Anyankyerim, Nhyiaso, Yaomensakrom and Esuajah (Ayanfuri), all have intrusion ages within error of 2105 ± 2 Ma. This is also the age of metamorphic titanite in an unmineralized basin-type granitoid at Kwapia. Rutiles from the hydrothermally altered Yaomensakrom and the mineralized Esuajah (Ayanfuri) granitoid were dated at 2098 ± 7 and 2086 ± 4 Ma, respectively, and probably provide minimum ages on gold mineralization. These data indicate a close temporal association among auriferous granitoid plutons near the western margin of the Ashanti belt at 2105 Ma. Gold mineralization may be only slightly younger than magmatic emplacement of the plutons. Granitic plutonism in the sedimentary basins, metamorphism, and hydrothermal gold mineralization in southern Ghana are all part of a major crust formation event, the Eburnean orogeny, that can be constrained to the time range 2120-2080 Ma.

The evolution of the Grenville Province in eastern Labrador, Canada

Abstract Rifting at c. 1.71 Ga separated the Mealy Mountains terrane from pre-Labradorian Laurentia and resulted in a short-lived (backarc?) basin, followed by southward subduction giving rise to 1.68-1.66 Ga Labradorian calc-alkaline arcs, partly built on older crust in the Mealy Mountains terrane. The arcs and subduction were also short lived, terminating when accretion against the Makkovikian and Eastern Churchill provinces occurred at 1.65 Ga. The Trans-Labrador batholith developed as a result of crustal thickening during suturing of the 1.68-1.66 Ga arc-related rocks to pre-Labradorian Laurentia. Further bimodal magmatism, in response to crustal thickening, continued until c. 1.62 Ga, followed by waning granitic magmatism until 1.60 Ga. The Labrador Orogen and older rocks in the Mealy Mountains terrane are flanked to the south by Pinwarian (1.51-1.45 Ga) granitoid rocks, possibly the inboard manifestation of a northward-subducting continental-margin arc at the southern margin of Laurentia. Extension, mostly coincident with the 1.71 Ga zone of weakness, started at c. 1.45 Ga. Although rifting was successful further west, it is only represented by basaltic magmatism (Michael and Shabogamo gabbros) along a linear zone in Labrador. Similar tectonism characterized the same zone periodically for at least the next 150 million years during which time the 1.27 Ga Harp-Nutak dykes were emplaced and slightly younger rocks of the Seal Lake Group formed north of the Grenville Province. After c. 1.23 Ga tectonic conditions changed, partly in response to Elzevirian accretion of arc terranes preserved in the southwest Grenville Province, that are speculated to have once also existed south of the exposed eastern Grenville Province. Alternating compressional and extensional conditions terminated at c. 1.0 Ga during collisional orogenesis associated with the final stages of development of the Grenville Orogen. Grenvillian deformation in Labrador (1.08-0.97 Ga) resulted in northwesterly thrusting, the effects of which are focused in the Exterior Thrust Belt, and widespread plutonism in the Interior Magmatic Belt. Thrusting in the Exterior Thrust Belt is sited along the zone of 1.71 and 1.43 Ga rifting. Uplift and cooling continued until 0.90 Ga as crustal stability was gradually achieved.

Ion microprobe dating of Paleozoic granitoids: Devonian magmatism in New Zealand and correlations with Australia and Antarctica

Precise ion microprobe UPb zircon ages have been obtained from a representative set of Paleozoic igneous rocks from the Western Province of the South Island, New Zealand. Granitoid rocks forming the Karamea Batholith and related plutons in the Buller terrane all yield crystallisation ages that are indistinguishable within a ± 5 Ma uncertainty at 375 Ma (Middle-Late Devonian). Previous workers have suggested that the batholith was emplaced over a long time interval and comprises rocks ranging in age from 370 to 310 Ma, with the bulk in the Early Carboniferous. Granitoid rocks with Carboniferous ages (∼ 330 Ma) do occur further to the west and younger Cretaceous granitoids occur within the Karamea Batholith, but these are not volumetrically significant. A sample from the ultramafic-mafic Riwaka Complex in the adjacent Takaka terrane gave a crystallisation age of 376.9 ± 5.6 Ma (2σ), indicating emplacement coeval with the Karamea Batholith. The Paleozoic granitoids contain a large amount of inherited zircon, with distinct 390-, 500–600- and 1000-Ma components. The 390-Ma age corresponds to widespread plutonism in the Lachlan Fold Belt in SE Australia. The 500–600-Ma (Ross-Delamerian age) and 1000-Ma (Grenville age) age components have also been observed in granitoids from the Lachlan Fold Belt and in Ordovician metasedimentary rocks from SE Australia and New Zealand. The inherited zircons in the Karamea Batholith could be derived from continental basement at depth, from the incorporation of upper-crustal material in the granitoid magmas or both. The Devonian granitoids in New Zealand can now be correlated with rocks of similar age in northern Victoria Land (Admiralty Intrusives) and Marie Byrd Land (Ford Granodiorite) in West Antarctica, in NE Tasmania, and in the central part of the Lachlan Fold Belt in SE Australia. On a reconstruction of the SW Pacific margin of Gondwana, prior to later break-up, it is possible to trace out a semi-continuous magmatic belt in excess of 2000 km in length.

Extensional tectonics in the central Iberian Peninsula during the Variscan to Alpine transition

The passage from the Variscan cycle to the Early Alpine framework in the central part of the Iberian Peninsula can be explained in terms of a transitional process involving four clearly differentiated tectonic episodes. 1. (1) A first Variscan compressional stage (VI, Middle Devonian to Early Carboniferous) dominated by compressional conditions leading to the building-up of the orogenic edifice. The stress regime was relevant to what might be called “Variscan-type” compression (E-W-oriented). This stage was characterized by major Himalayan-type tectonics with frontal nappes, thrusts, overturned folds, lateral transcurrent ramps, and localized anatectic magmatism. Minor synorogenic extension and plutonism was also recorded during this stage in the Tormes Granitic Dome. 2. (2) A second Variscan stage (V2, Early to Middle Carboniferous) was characterized by increasing extensional conditions leading to widespread plutonism (adamellites, granodiorites). Wanning compressional conditions were restricted to the eastern and southern realms of central Iberia (the eastern part of the Spanish Central System, and the Toledo Mountains). 3. (3) A third stage, here defined as Late Variscan (LV), developed from Middle Carboniferous to Early Permian, as a result of N-S late-orogenic extension. This episode is relevant to detachment tectonics and the gravitational collapse of the Variscan orogenic edifice under combined simple/ pure-shear conditions. Plutonism (granites and leucogranites) was still of major importance. Early Permian andesitic to dacitic volcanism and sedimentary basins developed within the eastern part of the Spanish Central System. 4. (4) A fourth stage, here defined as Early Alpine (EA, Early Permian to Triassic) marks the onset of the Alpine framework. This stage was characterized by what might be called an “Early Alpine-type” regional stress regime i.e. E-W extension and N-S compression, within a simple-shear model, and resulted in the configuration of the Iberian Peninsula into two contrasted realms: a western inherited Variscan block, and an eastern Alpine block subjected to post-orogenic extension. Elements developed during this event include N-S high-angle normal faults, NW-SE and NE-SW conjugate strike-slip faulting, and asymmetric rifting involving listric low-angle detachments.

Field relationships and geochemistry of pre-collisional (India-Asia) granitoid magmatism in the central Karakoram, northern Pakistan

In the central Karakoram Range, northern Pakistan, seven major granitic units which were emplaced prior to the Eocene (ca. 50 Ma) India-Asia collision have been mapped and analysed geochemically. The mid-Cretaceous Hunza plutonic complex dominates the Karakoram batholith in the west and is composed of quartz diorite-granodiorite plutons which have been deformed in the south by later collision-related thrusting. The Sost and Khunjerab plutons intrude the Palaeozoic-early Mesozoic sediments north of the batholith. In the east pre-collision magmatic units occur both north of (K2 gneiss, Broad Peak quartz diorite, Muztagh Tower unit) and south of (Hushe gneiss) the batholith which is dominated by a Miocene monzogranite-leucogranite pluton (Baltoro unit). The plutonic units to the north of the batholith are mid- to late Cretaceous with an age range of ca. 115-80 Ma whereas the Hushe gneiss to the south is dominantly Jurassic spanning the age range 200-145 Ma. All the pre-collision magmatic units display similar chemical and isotopic characteristics. Major, trace and REE variations in the Hunza plutonic complex are controlled by high-level fractionation of clinopyroxene, hornblende, plagioclase, biotite, ilmenite and allanite. High LILE/HFSE and LREE/HREE ratios, together with negative Nb, P and Ti anomalies suggest an ultimate source in the mantle wedge above the subducting slab. 87Sr/ 86Sr initial ratios (0.7055-0.7157) in the Hunza plutonic complex can best be explained by contamination of a mantle-derived magma by a crustal component with highly radiogenic Sr. All the units except the Hushe gneiss are probably related to subduction during closure of a back-arc basin between the Karakoram to the north and the Kohistan-Dras arc to the south during the mid- to late Cretaceous, and correlate with widespread plutonism from the same period in Kohistan, Ladakh and southern Tibet along the southern margin of Asia. The Hushe gneiss is probably related to an earlier northward-directed subduction phase during the Jurassic.

Distribution and characteristics of metamorphic belts in the south‐eastern Alaska part of the North American Cordillera

The Cordilleran orogen in south‐eastern Alaska includes 14 distinct metamorphic belts that make up three major metamorphic complexes, from east to west: the Coast plutonic–metamorphic complex in the Coast Mountains; the Glacier Bay–Chichagof plutonic–metamorphic complex in the central part of the Alexander Archipelago; and the Chugach plutonic–metamorphic complex in the northern outer islands. Each of these complexes is related to a major subduction event. The metamorphic history of the Coast plutonic–metamorphic complex is lengthy and is related to the Late Cretaceous collision of the Alexander and Wrangellia terranes and the Gravina overlap assemblage to the west against the Stikine terrane to the east. The metamorphic history of the Glacier Bay–Chichagof plutonic–metamorphic complex is relatively simple and is related to the roots of a Late Jurassic to late Early Cretaceous island arc. The metamorphic history of the Chugach plutonic–metamorphic complex is complicated and developed during and after the Late Cretaceous collision of the Chugach terrane with the Wrangellia and Alexander terranes.The Coast plutonic–metamorphic complex records both dynamothermal and regional contact metamorphic events related to widespread plutonism within several juxtaposed terranes. Widespread moderate‐P/T dynamothermal metamorphism affected most of this complex during the early Late Cretaceous, and local high‐P/T metamorphism affected some parts during the middle Late Cretaceous. These events were contemporaneous with low‐ to moderate‐P, high‐T metamorphism elsewhere in the complex. Finally, widespread high‐P–T conditions affected most of the western part of the complex in a culminating late Late Cretaceous event. The eastern part of the complex contains an older, pre‐Late Triassic metamorphic belt that has been locally overprinted by a widespread middle Tertiary thermal event.The Glacier Bay–Chichagof plutonic–metamorphic complex records dominantly regional contact‐metamorphic events that affected rocks of the Alexander and Wrangellia terranes. Widespread low‐P, high‐T assemblages occur adjacent to regionally extensive foliated granitic, dioritic and gabbroic rocks. Two closely related plutonic events are recognized, one of Late Jurassic age and another of late Early and early Late Cretaceous age; the associated metamorphic events are indistinguishable. A small Late Devonian or Early Mississippian dynamothermal belt occurs just north‐east of the complex. Two older low‐grade regional metamorphic belts on strike with the complex to the south are related to a Cambrian to Ordovician orogeny and to a widespread Middle Silurian to Early Devonian orogeny.The Chugach plutonic–metamorphic complex records a widespread late Late Cretaceous low‐ to medium/high‐P, moderate‐ T metamorphic event and a local transitional or superposed early Tertiary low‐P, high‐T regional metamorphic event associated with mesozonal granitic intrusions that affected regionally deformed and metamorphosed rocks of the Chugach terrane. The Chugach complex also includes a post‐Late Triassic to pre‐Late Jurassic belt with uncertain relations to the younger belts.

Devonian Plutonism in South-Central British Columbia

An apparently tabular body of granitoid gneiss, 3 to 5 km wide and more than 70 km long, that lies along the western margin of the Shuswap Metamorphic Complex between Shuswap and Admas Lakes, shows intrusive relationships with Palaeozoic and older rocks and has yielded zircons whose minimum age is 372 Ma. This intrusion, together with other granitoid plutons in the area that appear to be related to it, provide evidence of widespread plutonism during Middle Devonian time near the western edge of the Paleozoic Cordillera geosyncline and necessitate significant revisions in the interpretation of the crustal history of this region.