Jane H. Scarrow

The Eocene bimodal Piranshahr massif of the Sanandaj–Sirjan Zone, NW Iran: a marker of the end of the collision in the Zagros orogen

Abstract: The bimodal Piranshahr massif is composed of coeval but geochemically unrelated mafic (40.7 ± 0.2 Ma zircon U–Pb sensitive high-resolution ion microprobe (SHRIMP) age) and A-type felsic rocks (41 ± 0.5 Ma Rb–Sr and 41.3 ± 0.8 Ma zircon U–Pb SHRIMP age). The mafic rocks consist of two geochemical types of gabbros that derived from different magmas. The more abundant gabbros are moderately alkaline, have ratios of large ion lithophile elements to REE and high field strength elements to REE similar to those of intraplate mantle magmas, 87Sr/86Sr41 Ma ≈ 0.7036 and ε(Nd)41 Ma ≈ +7.2. The less abundant gabbros have calc-alkaline affinities, 87Sr/86Sr41 Ma ≈ 0.7043 and ε(Nd)41 Ma ≈ +4.78. Felsic rocks are metaluminous A2-type annite–fayalite–hedenbergite, hypersolvus, leucocratic, coarse-grained agpaitic syenites, pulaskites and granites, with 87Sr/86Sr41 Ma ≈ 0.7048 and ε(Nd)41 Ma ≈ +3.6–4.5. Syenites, pulaskites and granites are genetically related. Pulaskites probably represent alkali-enriched water-rich residual melts from which an F-rich vapour phase was later separated. Granites were probably generated during open-system processes, in which F-rich hydrous alkaline fluids released from the syenites acted upon pre-existing felsic rocks. The c. 41 Ma age of the post-collisional Piranshahr massif indicates that the related collision probably occurred at 50–60 Ma (i.e. Late Palaeocene or Early Eocene), thus resolving a much debated question.

Zircon thermometry and U–Pb ion-microprobe dating of the gabbros and associated migmatites of the Variscan Toledo Anatectic Complex, Central Iberia

In the Central Iberian Zone there are several large thermal domes in which small bodies of ultramafic, mafic and intermediate rocks appear intimately associated with crustal granites and migmatites. The closest spatial association between the ultramafic, mafic and intermediate rocks and migmatites is in the Toledo Anatectic Complex, where field relationships suggest that these rocks are coeval and have an age close to 340 Ma. This, and the recent discovery in the neighbouring Ossa Morena Zone of a large mid-crustal seismic reflector interpreted as a 335–350 Ma mafic sill, reinforce the hypothesis that heat for crustal melting was supplied from early Variscan mantle magmas emplaced in the middle crust. However, precise ion-microprobe U–Pb zircon dating and Ti-in-zircon thermometry in Toledo do not support this idea. Whereas the mean age of four mafic bodies is 307 ± 2 Ma, the migmatites are c. 25 Ma older. The migmatites hosting ultramafic, mafic and intermediate bodies have the same age and Ti-in-zircon temperatures as migmatites far from any mafic intrusion. These data reveal that ultramafic, mafic and intermediate magmas are late Variscan; they were emplaced in already cooling anatectic zones once the extensional collapse was initiated, and their thermal impact on the mid-crustal Variscan anatexis of Central Iberia was negligible.

Ophiolite obduction pulses as a proxy indicator of superplume events

Abstract A major new synthesis of ophiolite geochronology and map of global Phanerozoic distribution indicates that previously noted pulses of ophiolite obduction can be linked to superplume-related tectonism. A marked cyclicity is evident in obduction ages obtained from minerals in ophiolite metamorphic soles and obduction-related minor intrusions. This episodicity is in phase with periods of predominantly uniform polarity of the geomagnetic field, formation of massive carbon-rich deposits, sea-level highstands, and formation of large flood basalt provinces; all generally considered to be superplume proxy indicators. A key to interpreting ophiolite obduction as a further proxy is the mid-Cretaceous, superplume-associated, ocean-margin compressional deformation. During this event, thermal rejuvenation and increased buoyancy of ocean lithosphere caused arc–terrane collision, marginal-basin shoaling, back-arc basin closure and ophiolite obduction. Convergent margins were placed in compression with increased coupling between subducting and overriding plates resulting in major ocean margin deformation. Being directly datable and tectonically and petrologically distinctive makes ophiolite assemblages of considerable use for identifying superplume events.

55 million years of continuous anatexis in Central Iberia: single-zircon dating of the Peña Negra Complex

Single-zircon evaporation and ion-microprobe dating of migmatites and anatectic granites in the Peña Negra Complex of Central Iberia reveal that the Variscan anatexis occurred continuously from 352 to 297 Ma, with a maximum at 335–305 Ma. Anatexis began coeval with the main collision of continental masses. A limited melting event, probably related to syncollision crustal-scale shear zones, produced a population of zircons with ages of c. 350 Ma. The production of new zircons decreased to a minimum at c. 343 Ma but then increased swiftly as the internal thermal evolution of the thickened Central Iberian crust led to widespread anatexis in the Peña Negra region at 332 Ma. Shortly after this, the melt resident in the migmatites was locally segregated into small bodies that crystallized as cordierite leucogranites at 321 Ma. Simultaneously, extensional subhorizontal shear zones were preferentially developed over layers of the migmatite series that, owing to their elevated heat production and fertility, had the highest melt fraction. Shearing provoked further anatexis and contributed significantly to the in situ production of high melt-fraction granodiorites and adamellites from the migmatites. This process occurred from 325 to 305 Ma, with a maximum at 309 Ma marking the peak of the Variscan extensional collapse in Central Iberia. After c. 305 Ma the melt fraction decreased quickly, so that the production of new zircons was insignificant at 300 Ma and had stopped completely by 297 Ma.

A precise late Permian 40AR/39Ar age for Central Iberian camptonitic lamprophyres

The Avila batholith of central Spain is composed, predominantly, of crustal-melt peraluminous granites cut by small-scale mafic alkaline bodies. Dating of the Gredos sector mafic camptonitic lamprophyre dykes was undertaken to constrain the Late Variscan tectonomagmatic evolution of the region. A well constrained late Permian, Capitanian, age of 264.5 ± 0.9 Ma was obtained by 40 Ar/ 39 Ar geochronology using amphibole separates. This new age clearly distinguishes the dykes from other episodes of alkaline mafic magmatism in the region. We suggest that the lamprophyre dykes were emplaced into already solidified granitoids after the tectonic control on magma generation changed from purely extensional to transtensional. 40

Insights into orogenesis: getting to the root of a continent–ocean–continent collision, Southern Urals, Russia

The Ural mountains preserve a late Palaeozoic collision that forms a 2500 km suture in the worlds largest landmass, Eurasia. Several features of the mountain belt, in particular a well-preserved crustal root, are uncharacteristic of other Palaeozoic orogens such as the Appalachians and Caledonides. Previous interpretations of the Southern Uralian root suggested that it is composed of East European Craton crust derived from the west. A new potential field data model, considered in conjunction with published seismic, heat-flow and geological data, indicates that the root is composed mainly of mafic granulite, which we interpret as oceanic arc crust originally accreted from the east, subducted eastward, and metamorphosed. A load caused by crustal lateral density variations, combined with topography, isostatically compensates root buoyancy and is thus the main cause of its preservation.

Tracking arc-continent collision subduction zone processes from high-pressure rocks in the southern Urals

Subduction-related high-pressure metamorphic rocks in the southern Urals confirm material flux was active within the subduction zone. Suprasubduction zone ophiolitic material was tectonically eroded in an intraoceanic setting, metamorphosed under mantle conditions and exhumed. With the entrance of the East European Craton into the subduction zone, the upper continental crust was removed at a shallow level, whereas middle crustal material was subducted to mantle depths, metamorphosed under eclogite facies conditions and interacted with the mantle wedge. Eclogite-bearing rocks were exhumed to an upper mantle level and juxtaposed against material that was tectonically eroded from the fore-arc region of the upper plate.

Early Devonian boninites from the Magnitogorsk Arc, Southern Urals (Russia): Implications for early development of a collisional orogen

In the Paleozoic rocks of the Uralian orogen, boninites are well exposed in the Baimak-Buribai area. They are part of the arc complex of the Devonian Magnitogorsk zone that collided with the East European craton during the Uralian orogeny. The boninite lavas and dikes and shallow, high-Mg diorite intrusions compose the lowest stratigraphic level of a sequence that progresses to boninitic andesite lavas and then to explosive tholeiitic and calc-alkalic products (lavas and tuffs). Relict petrographic characteristics serve to identify the boninites: glassy texture, abundant olivine and Cr-rich spinel, and the absence of early-crystallized plagioclase. Compositional similarities to Tertiary Western Pacific boninites-particularly to the intermediate- and high-Ca boninites from the Izu-Bonin forearc region drilled during ODP (Ocean Drilling Program) Leg 125 and the high-Ca boninites from the northern Tonga Ridge-suggest that the Baimak-Buribai boninites and related rocks were erupted in the forearc region early in the development of an island arc. They give paleogeodynamic information about timing and location of the initiation of the subduction in the southern pre-Uralian ocean. The geologic setting of the lower Baimak-Buribai Complex was an extensional, non-accretionary forearc, but by the time the tholeiitic and calc-alkalic rocks erupted, the setting had changed to an established accretionary forearc. Comparison of an Emsian stratigraphic age for the boninites with radiometric dates from collisional metamorphic rocks indicates an interval of 14 to 40 m.y. between initiation of oceanic subduction and arc-continent collision. The inferred duration for arc development during the Paleozoic Uralian orogeny is similar to, or shorter than, the duration of present-day examples of ongoing orogeny in the Western Pacific region.

Four decades of geochronological work in the southern and middle Urals: a review

The Uralide Orogen, the geographic and geologic divide between Europe and Asia, has been the subject of geochronological study for more than 40 years. This compilation summarizes age data from the Southern and Middle Urals beginning with Archean to Proterozoic dates from the East European Craton in the west and advancing eastward where progressively younger geological events are recorded. Archean to Proterozoic basement rocks crop out throughout the East European Craton. Neoproterozoic gneisses provide evidence of a Pre-Uralian orogeny that affected large parts of the eastern margin of the East European Craton. Early Paleozoic magmatism related to rifting of the East European Craton in the Middle Urals is recorded by geographically restricted nepheline syenite massifs. The most westerly, and oldest, material accreted during the Paleozoic orogeny is Middle Ordovician to Late Devonian ophiolites and ultramafic/mafic massifs generally associated with the principal suture of the orogen, the Main Uralian fault. Closure of the Uralian paleo-ocean basin led to eastward subduction of the leading edge of the thinned East European Craton generating Late Devonian high pressure complexes now exposed in the Main Uralian fault footwall. East of the Main Uralian fault, Silurian to Early Carboniferous ocean volcanogenic complexes crop out. These accreted island arc terranes are intruded by abundant Late Devonian to Late Carboniferous plutonic complexes with subduction-related characterisitcs. Further to the east, Permian granitoids were generated by melting of orogenically thickened crust. In general, post-Paleozoic magmatism is sparse throughout the Urals.

Shoshonites, vaugnerites and potassic lamprophyres: similarities and differences between ‘ultra’-high-K rocks

A comparative study of three main igneous rock associations that plot in the K 2 O–SiO 2 diagram shoshonite field: shoshonite series absarokites–shoshonites–banakites (henceforth referred to as shoshonites s.l.), vaugnerites, and potassic lamprophyres, reveals that similarities between the associations are superficial. Vaugnerites and lamprophyres are more magnesian, richer in large ion lithophile and high field strength elements and have higher light rare earth/heavy rare earth ratios than shoshonites. Furthermore, shoshonites have low radiogenic heat production, typical of subduction-related rocks, but most vaugnerites and some lamprophyres are highly radioactive. Relative to bulk-Earth, shoshonites have depleted, asthenospheric mantle-like Sr and Nd isotope signatures, whereas vaugnerites and potassic lamprophyres have enriched, crust or lithospheric mantle-like compositions. Though vaugnerites and some lamprophyres show evidence of crustal contamination, the contaminated magma was not originally shoshonitic. Their composition is consistent with derivation from a metasomatised upper mantle source enriched long before melting, thus precluding an active subduction setting. In conclusion, the term shoshonite, implying late-stage arc magmas, cannot be applied to a rock series simply because it plots into the K 2 O–SiO 2 diagram shoshonite field. Shoshonites with a subduction-related source may, however, be identified by discriminant function analysis.