The early Caledonian (Finnmarkian) event reassessed in Finnmark: Ar/Ar cleavage age data from NW Varangerhalvøya, N. Norway

The relative significance of early (Finnmarkian) and late (Scandian) Caledonian deformation in N. Norway is uncertain. Early studies suggested pervasive Finnmarkian deformation whilst later results indicated a restricted Finnmarkian domain. The present work suggests it was more widespread than accepted and that inter Finnmarkian–Scandian deformation occurred. ⁴⁰Ar/³⁹Ar dating of 2–6 and 6–11 μm pelitic fractions from the lower to mid-greenschist facies Tanahorn Nappe (five samples; base Middle Allochthon) and the epizone Løkvikfjellet and Barents Sea Groups (three samples; North Varanger Region) in the north Scandinavian Caledonides show slightly discordant spectra. Most spectra from the Tanahorn Nappe preserve possible evidence of an early Caledonian event in the high temperature steps, with recoil/excess Ar effects in the low temperature steps; no pre-Caledonian relict component has been recorded. The results indicate Finnmarkian deformation continued to 460 Ma, with Scandian reactivation at 425–415 Ma. From the North Varanger Region, a strongly crenulated sample yielded plateau ages (444–442 Ma); means of combined young steps from weakly to uncrenulated samples gave 470–450 Ma, suggesting penetrative strike-slip deformation occurred in the late Finnmarkian to inter-Finnmarkian–Scandian period. No Scandian ages were recorded in the North Varanger Region. Reassessment of published data from the Laksefjord Nappe and Gaissa Thrust Belt suggests they were affected by Finnmarkian deformation.     

The Finnmarkian Orogeny revisited: An isotopic investigation in eastern Finnmark, Arctic Norway

The Scandinavian Caledonides have been viewed as resulting from either a single Silurian (i.e. Scandian) event or from polycyclic orogenies involving several collisions on the margin of Baltica. Early studies of the Kalak Nappe Complex (KNC) in Finnmark, Arctic Norway, led to the hypothesis of an Early Cambrian-Early Ordovician (520-480 Ma) Finnmarkian Orogeny, though the nature of this tectonic event remains enigmatic. In this contribution we have employed in situ UV laser ablation Ar-Ar dating of fine-grained phyllite and schist from the eastern Caledonides of Arctic Norway to investigate the presence of pre-Scandian tectonometamorphic events. U-Th-Pb detrital zircon and whole rock Sm-Nd analyses have been used to test the regional stratigraphic correlations of these metasedimentary rocks. These results indicate that the Berlevåg Formation within the Tanafjord Nappe, previously assumed to be part of the KNC, was deposited after 1872 Ma and prior to a low temperature hydrothermal event at 555 ± 15 Ma. It has a likely provenance on the Baltica continent, lacks any Grenville-Sveconorwegian detrital zircons, and thus cannot be part of the KNC which contains abundant detritus in this age range. Instead the Berlevåg Formation is interpreted as part of the Laksefjord Nappe Complex, which structurally underlies the KNC. Laser-ablation argon-argon dating also shows that late Caledonian (i.e. Scandian) tectonometamorphism affected both the KNC and its immediate footwall at c. 425 ± 15 Ma. This is corroborated by a step-heating argon-argon muscovite age of 424 ± 3 Ma which is interpreted as dating cooling. However, within two samples from the KNC, an earlier (Middle-Late Cambrian) metamorphic event is also recorded. A biotite-grade schist yielded an Ar-Ar inverse isochron age of 506 ± 17 Ma from whole rock surfaces, in which the mineral domains are too fine-grained to date individually. An early generation of muscovite from a coarser-grained amphibolite-facies sample yielded an inverse isochron of 498 ± 13 Ma. Both isochron ages have atmospheric argon intercept values. Previous studies have documented similar Cambrian ages in the Caledonian nappes below the KNC. These results suggest correlative tectonometamorphic events in the eastern KNC and its footwall at c. 500 Ma. This Cambrian event may reflect the arrival of the Kalak Nappe Complex as a previously constructed exotic mobile belt onto the margin of Baltica. Combined with recent studies from the western Kalak Nappe Complex, the results do not support the traditional constraint on the Finnmarkian Orogeny sensu stricto. However they vindicate classic tectonic models involving a Cambrian accretion event.     

Initial tectonothermal evolution within the Scandinavian Caledonide Accretionary Prism: constraints from 40Ar/39Ar mineral ages within the Seve Nappe Complex, Sarek Mountains, Sweden

In the Caledonide orogen of northern Sweden, the Seve Nappe Complex is dominated by rift facies sedimentary and mafic rocks derived from the Late Proterozoic Baltoscandian miogeocline and offshore-continent–Iapetus transition. Metamorphic breaks and structural inversions characterize the nappe complex. Within the Sarek Mountains, the Sarektjåkkå Nappe is composed of c. 600-Ma-old dolerites with subordinate screens of sedimentary rocks. These lithological elements preserve parageneses which record contact metamorphism at shallow crustal levels. The Sarektjåkkå Nappe is situated between eclogite-bearing nappes (Mikka and Tsäkkok nappes) which underwent high-P metamorphism at c. 500 Ma during westward subduction of the Baltoscandian margin. ⁴⁰Ar/³⁹Ar mineral ages of c. 520–500 Ma are recorded by hornblende within variably foliated amphibolite derived from mafic dyke protoliths within the Sarektjåkkå Nappe. Plateau ages of 500 Ma are displayed by muscovite within the basal thrust of the nappe and are consistent with metamorphic evidence which indicates that the nappe escaped crustal depression as a result of detachment at an early stage of subduction. Cooling ages recorded by hornblende from variably retrogressed eclogites in the entire region are in the range of c. 510–490 Ma and suggest that imbrication of the subducting miogeocline was followed by differential exhumation of the various imbricate sheets. Hornblende cooling ages of 470–460 Ma are recorded from massive dyke protoliths within the Sarektjåkkå Nappe. These are similar to ages reported from the Seve Nappe Complex in the central Scandinavian Caledonides. Probably these date imbrication and uplift related to Early Ordovician arrival of outboard terranes (e.g. island-arc sequences represented by structurally lower horizons of the Köli Nappes). Metamorphic contrasts and the distinct grouping of mineral cooling ages suggest that the various Seve structural units are themselves internally imbricated, and were individually tectonically uplifted through argon closure temperatures during assembly of the Seve Nappe Complex. The cooling ages of 520–500 Ma recorded within Seve terranes and along terrane boundaries of the Sarek Mountains provide evidence of significant accretionary activity in the northern Scandinavian Caledonides in the Late Cambrian–Early Ordovician.     

Subduction-flip during Iapetus Ocean closure and Baltica–Laurentia collision, Scandinavian Caledonides

Evidence is presented here from the northern Scandinavian Caledonides for development of an extensional basin of Ashgill to Mid Llandovery age along the Baltoscandian margin immediately prior to Baltica–Laurentia collision. U/Pb multigrain and ion microprobe zircon dating of plagiogranites in the Halti Igneous Complex complement previous baddeleyite and zircon dating of a dolerite dyke, and zircon dating of anatectic granite; they demonstrate that this dunite, troctolite, gabbro, sheeted‐dyke complex ranges in age from c. 445 to 435 Ma. The dolerite dykes intruded and melted arkoses of inferred Neoproterozoic age. This evidence, taken together with previous documentation of ophiolites (Solund–Stavfjord), ophiolite‐like associations (Sulitjelma Igneous Complex) and several other mafic suites (e.g. Råna, Artfjället) of Ashgill to Llandovery age further south in the northern Scandinavian Caledonides, implies that Scandian collisional orogeny along this nearly 2000‐km‐long mountain belt was immediately preceeded by development of short‐lived marginal basins. The latter developed during the final closure of the Iapetus Ocean and are inferred to be of back‐arc origin, some (perhaps all) related to E‐dipping subduction. Collision of the continents at c. 435 Ma is inferred to have induced a flip in subduction polarity, leading to underthrusting of Laurentia by Baltica.     

Early Ordovician U–Pb metamorphic ages of the eclogite-bearing Seve Nappes, Northern Scandinavian Caledonides

Eclogite-grade metamorphism of the Seve Nappe Complex (SNC) in Norrbotten, Sweden, records the attempted subduction of the Baltic continental margin during the early Palaeozoic evolution of the Iapetus Ocean. Metamorphic titanite sampled from several calcsilicate gneisses of the SNC in Norrbotten occurs as part of a prograde, eclogite facies metamorphic mineral assemblage and yields concordant to nearly concordant U/Pb ages of 500–475  Ma. Later structural disruption of these rocks occurred during the Siluro-Devonian Scandian phase of the Caledonide orogeny, but the U/Pb systematics show no evidence of a second generation (metamorphic or recrystallized) of titanite, or of post-Early Ordovician disturbance through Pb loss. Hence the U/Pb ages are believed to record the time of prograde mineral growth during eclogite facies metamorphism of the SNC.
These results support earlier Sm/Nd and ⁴⁰Ar/³⁹Ar studies indicating an Early Ordovician metamorphic age for the eclogitic Norrbotten SNC, and confirm the Early Ordovician destruction of at least this segment of the Palaeozoic passive margin of Baltica. These results indicate that the SNC in the northern Scandinavian Caledonides was subducted and metamorphosed to high grade some 50–70  Myr prior to the high-grade metamorphism of the SNC in the central Scandinavian Caledonides. This result requires significantly different early Palaeozoic tectonic histories for rocks mapped as SNC in the northern Caledonides and those in the central Caledonides, despite a seemingly similar tectonostratigraphic position and broadly similar high-grade metamorphism.     

Time of formation and peak of Variscan HP-HT metamorphism of quartz-feldspar rocks in the central Erzgebirge,Saxony, Germany

The Variscan Erzgebirge represents an antiform with a core of gneisses and mica schists, surrounded by a phyllitic mantle. The Gneiss-Eclogite Unit (GEU), in the central part, is a composite tectonometamorphic assemblage characterized by a HP-HT imprint and comprises migmatitic para- and orthogneisses, HT mylonites, HP granulites, eclogites and garnet peridotites. It is tectonically sandwiched between two major units with distinctly lower PT histories. The GEU experienced a characteristic “kinked” retrograde PT path after HP-HT equilibration with: (1) strong near-isothermal decompression at high temperatures; (2) extensive re-equilibration at medium pressures, followed (3) by rapid cooling during continued uplift. We dated zircons (Pb-Pb evaporation) from granitoid orthogneisses and metapelites of the GEU. The orthogneisses contain euhedral, long-prismatic zircons of igneous origin that provided protolith ages between 470 and 524 Ma. Metapelites retain well-preserved granulite-facies mineral assemblages and contain spherical, multifaceted metamorphic zircons that grew near the peak of HP/HT metamorphism. Inclusions of prograde HP phengite (∼15 kbar) and rutile are included in one such zircon. Metamorphic zircons of three samples from different localities yielded identical ²⁰⁷Pb/²⁰⁶Pb ages of 340.5 ± 0.7 Ma, 341.2 ± 0.5 Ma and 341.6 ± 0.5 Ma respectively. Consideration of these zircon ages with published ³⁹Ar/⁴⁰Ar white mica ages suggests fast cooling and uplift rates in excess of 50 °C/Ma and 4 km/Ma. This is typical for large-scale extensional tectonic unroofing of the ultra-deep part of a fossil, thickened Variscan continental crust (>60 km) during continuing continental collision and orogenic collapse. Received: 5 June 1997 / Accepted: 7 January 1998     

40Ar/39Ar record of late Pan–African exhumation of a granulite facies terrain, central Dronning Maud Land, East Antarctica

⁴⁰Ar/³⁹Ar geochronological data on hornblende, biotite and K-feldspar provide constraints on the cooling path experienced by a high-grade metamorphic complex from the Mühlig–Hofmannfjella and Filchnerfjella (6–8°E), central Dronning Maud Land, Antarctica, during the late Neoproterozoic-early Palaeozoic Pan–African orogeny. Hornblende ages yield c. 481 Ma, biotite ages range from c. 466 Ma to c. 435 Ma, whereas K-feldspar ages of the gneisses are c. 437 Ma. The ⁴⁰Ar/³⁹Ar data suggest initial cooling at a rate of ~10 °C/Myr between 481 and 465 Ma, followed by a lower cooling rate of ~6 °C/Myr during the subsequent c. 30 million years. The K-feldspar ⁴⁰Ar/³⁹Ar ages place a lower time limit on the duration of the exhumation, by the time of thermal relaxation to a stable continental geotherm. The ⁴⁰Ar/³⁹Ar data reflecting cooling indicate tectonic exhumation related to orogenic collapse during a later phase of the Pan–African orogeny.     

Ar/Ar dating of hydrothermal activity, biota and gold mineralization in the Rhynie hot-spring system, Aberdeenshire, Scotland

This study presents a new high-precision ⁴⁰Ar/³⁹Ar age for the Devonian hot-spring system at Rhynie. Hydrothermal K-feldspar sampled from two veins that represent feeder conduits and a hydrothermally altered andesite wall rock, date the hydrothermal activity, the fossilised biota, and syn - K-feldspar gold mineralization at 403.9 ± 2.1 Ma (2σ). Oxygen isotope data for the parent fluid (−4‰ to 2‰) show that the K-feldspar was precipitated from a dominantly meteoric fluid, which mixed with magmatic fluids from a degassing magma chamber.The ⁴⁰Ar/³⁹Ar age (403.9 ± 2.1 Ma [2σ]) when recalculated (407.1 ± 2.2 Ma [2σ]) with respect to the astronomically tuned age for Fish Canyon sanidine (28.201 ± 0.023 Ma [1σ]), also provides a robust marker for the polygonalis-emsiensis Spore Assemblage Biozone within the Pragian-?earliest Emsian. Furthermore, the age identifies the Devonian pull-apart volcano-sedimentary basins of the British and Irish Caledonides (and their root zones), as specific targets for future gold exploration.     

Timing of collision initiation and location of the Scandian orogenic suture in the Scandinavian Caledonides

The Scandinavian Caledonides represent a classical example of a deeply eroded Himalayan‐style orogen formed during Baltica–Laurentia continent collision. We propose that initial contact along continental‐margin promontories led to a drop in convergence rate, resulting in increased slab rollback along parts of the margin still undergoing oceanic subduction. Slab rollback caused extension of the overlying lithosphere with orogen‐wide emplacement of mafic layered intrusions, ophiolite formation and bimodal magmatism at 438–434 Ma, in what immediately thereafter became the upper plate (Laurentia) in the Scandian continent–continent collision. A compilation of magmatic ages provides evidence of long‐lived, Ordovician arc magmatism in units above the suture, which is essentially absent below the suture. This model provides a tight constraint on the timing of collision initiation, and provides a framework by which tectonic units comprising the Scandinavian Caledonides can be assigned a Baltican or more exotic heritage.     

A Late Ordovician U–Pb age for the Tromsø Nappe eclogites,Uppermost Allochthon of the Scandinavian Caledonides

The Scandinavian Caledonides contain the record of several high-pressure events reflecting distinct episodes of collision and subduction in the course of the global Caledonian plate reorganization process. In this study, the timing and speed of one of these events in the Tromsø Nappe of the Uppermost Allochthon are detailed using multiple U–Pb geochronometers. This unit contains eclogites, the largest of which forms a whole mountain top, whereas many others occur as smaller lenses enclosed within a metamorphosed supracrustal sequence. A minimum age for the sedimentation is provided by a zircon age of 493 +5/-2 Ma for an eclogitized felsic intrusion. Formation of the eclogite, at pressures reaching 2.8 GPa, occurred at 452.1±1.7 Ma as evidenced by U–Pb in eclogitic zircon. Similar ages of 451–450 Ma are also provided by high-Al titanite in eclogite and titanite in leucosome veins, the latter of which was formed by partial melting during the exhumation of the eclogite. An age of 449 Ma for a rutile porphyroblast in another vein further confirms the rapidity of this high-pressure process. Matrix rutiles in two other eclogites yielded ages of 436 Ma and younger, probably indicating partial resetting during a subsequent metamorphic overprint. Lead isotopic compositions with high ²⁰⁷Pb/ ²⁰⁴Pb ratios are indicative of old crustal sources, thus supporting the previously proposed notion that the Uppermost Allochthon was derived from the Neoproterozoic margin of Laurentia.     

Two–stage Cretaceous Exhumation of Hengshan Complex in Hunan Province,SE China: Constraints Arising from ~(40)Ar–~(39)Ar Geochronology and Cretaceous Tectonic Implications

The Hengshan complex is located in the central part of SE China, which underwent rapid tectonic uplift in the Cretaceous just like many other complexes on the continent. (40)~Ar–(39)~Ar geochronological data from the Hengshan complex suggest that two episodes of crustal cooling/extension took place in this part of the continent during the Cretaceous time. The first stage of exhumation was active during ca. 136–125 Ma, with a cooling rate of 10 °C/Ma. The second stage of exhumation happened at ca. 98–93 Ma, with a cooling rate of 10 °C/Ma. Considering the folding in the Lower Cretaceous sedimentary rocks and the regional unconformity underneath the Upper Cretaceous red beds, it is believed that the Cretaceous crustal extension in SE China was interrupted by a compressional event. The reversion to extension, shortly after this middle Cretaceous compression, led to the rapid cooling/exhumation of the Hengshan complex at ca. 98–93 Ma. The Cretaceous tectonic processes in the hinterland of SE China could be controlled by interactions between the continental margin and the Paleo–pacific plate.     

Diachronous Palaeoproterozoic deformation and metamorphism in the Committee Bay belt,Rae Province,Nunavut: insights from 40Ar–39Ar cooling ages and thermal modelling

Regional‐scale ⁴⁰Ar–³⁹Ar data presented in this paper reveal significant across‐strike and along‐strike age differences in the Committee Bay belt (CBb), Rae Province, Nunavut, Canada, that complement variations in observed monazite ages. ⁴⁰Ar–³⁹Ar hornblende ages are c. 1795, 1775, and 1750 Ma in the western, eastern and central parts of the Prince Albert Group (PAG) domain respectively. The migmatite domain and Walker Lake intrusive complex are characterized by c. 1750–1730 ⁴⁰Ar–³⁹Ar hornblende ages without significant along‐strike variation. The ⁴⁰Ar–³⁹Ar data provide important constraints on the cooling history and on thermal modelling that elucidates the controls on diachroneity and metamorphic patterns within the belt. In the western CBb, prograde monazite growth occurred 26 ± 10 Myr earlier in the migmatite domain (1864 ± 9 Ma; peak P–T = 5 kbar?700 °C) than in the PAG domain (1838 ± 5 Ma; peak P–T = 5 kbar?580 °C). Calculations indicate that this earlier monazite growth results from tectonic thickening of higher heat productivity Archean lithologies in the migmatite domain, which undergoes more rapid prograde heating than the less radiogenetic and lower grade rocks of the PAG domain. Granite generation via biotite dehydration melting at 800 °C and 20 km depth is predicted to occur c. 1835 Ma, in agreement with geochronological constraints. The tectonic burial of crustal domains with contrasting radiogenic properties also explains the general congruence of lower to upper amphibolite facies metamorphic zones generated during the two main orogenic cycles (i.e. M₂–D₁ and M₃–D₂). The modelled timing of prograde monazite growth in the migmatite domain suggests that D₂ tectonic thickening began at 1872 ± 9 Ma, some 8 ± 3 Myr before monzazite growth, coeval with the inferred time of collision of the Meta Incognita terrane with the southern Rae Province. Along‐strike diachroneity, reflected in 25 Myr younger monazite and ⁴⁰Ar–³⁹Ar hornblende ages in the eastern relative to the western PAG domain, cannot be accounted for by heat productivity contrasts along the belt. Instead the younger deformation and metamorphism in the eastern CBb was driven by its proximity to the eastern promontory of the Superior Province which collided with the Rae Province at c. 1820 Ma. The ⁴⁰Ar–³⁹Ar data presented here support the interpretation that the youngest monazite in the CBb crystallized at c. 1790 Ma in the central CBb when this part of the belt was downfolded into a gentle synformal structure while the western part of the belt cooled through ⁴⁰Ar–³⁹Ar hornblende closure. The results of this study illustrate the important influence of contrasting rock properties on the thermal evolution of orogenic belts and on the temporal record of this evolution.     

New Age and Geochemical Data from the Southern Colville and Kermadec Ridges,SW Pacific: Insights into the recent geological history and petrogenesis of the Proto-Kermadec (Vitiaz) Arc

The intra-oceanic Kermadec arc system extends ~1300 km between New Zealand and Fiji and comprises at least 30 arc front volcanoes, the Havre Trough back-arc and the remnant Colville and Kermadec Ridges. To date, most research has focussed on the Kermadec arc front volcanoes leaving the Colville and Kermadec Ridges virtually unexplored. Here, we present seven ⁴⁰Ar/³⁹Ar ages together with a comprehensive major and trace element and Sr-, Nd-, and Pb-isotope dataset from the Colville and Kermadec Ridges to better understand the evolution, petrogenesis and splitting of the former proto-Kermadec (Vitiaz) Arc to form these two remnant arc ridges. Our ⁴⁰Ar/³⁹Ar ages range from ~7.5–2.6 Ma, which suggests that arc volcanism at the Colville Ridge occurred continuously and longer than previously thought. Recovered Colville and Kermadec Ridge lavas range from mafic picro-basalts (MgO = ~8 wt%) to dacites. The lavas have arc-type normalised incompatible element patterns and Sr and Pb isotopic compositions intermediate between Pacific MORB and subducted lithosphere (including sediments, altered oceanic crust and serpentinised uppermost mantle). Geochemically diverse lavas, including ocean island basalt-like and potassic lavas with high Ce/Yb, Th/Zr, intermediate ²⁰⁶Pb/²⁰⁴Pb and low ¹⁴³Nd/¹⁴⁴Nd ratios were recovered from the Oligocene South Fiji Basin (and Eocene Three Kings Ridge) located west of the Colville Ridge. If largely trench-perpendicular mantle flow was operating during the Miocene, this geochemical heterogeneity was likely preserved in the Colville and Kermadec sub arc mantle. Between 4.41 ± 0.35 and 3.40 ± 0.24 Ma some Kermadec Ridge lavas record a shift from Colville Ridge- to Kermadec arc front-like, suggesting the proto-Kermadec (Vitiaz-) arc split post 4.41 ± 0.35 Ma. The Colville and Kermadec Ridge data therefore place new constraints on the regional tectonic evolution and highlight the complex interplay between pre-existing mantle heterogeneities and material fluxes from the subducting Pacific Plate. The new data allow us to present a holistic (yet simplified) picture of the tectonic evolution of the late Vitiaz Arc and northern Zealandia since the Miocene and how this tectonism influences volcanic activity along the Kermadec arc at the present.     

Nature and timing of the Solonker suture of the Central Asian Orogenic Belt: insights from geochronology and geochemistry of basic intrusions in the Xilin Gol Complex,Inner Mongolia,China

The Solonker suture zone of the Central Asian Orogenic Belt (CAOB) records the final closure of the Paleo-Asian Ocean. The nature and timing of final collision along the Solonker suture has long been controversial, partly because of an incomplete record of isotopic ages and differing interpretations of the geological environments of key tectonic units. The Xilin Gol Complex, consisting of strongly deformed gneisses, schists and amphibolites, is such a key tectonic unit within the CAOB. Lenticular or quasi-lamellar amphibolites are dispersed throughout the complex, intercalated with biotite–plagioclase gneiss. Both rock types experienced amphibolite-facies metamorphism. The protolith of the amphibolite is a basic rock that intruded into the biotite–plagioclase gneiss at 319 ± 4 Ma based on LA-ICPMS zircon U–Pb dating. The basic intrusion was sourced from a modified magma that experienced crystal fractionation and was admixed with slab-derived fluids. The slab-derived fluids, which formed during Early Paleozoic oceanic subduction along the north-dipping Sonidzuoqi–Xilinhot subduction zone, mixed with the magma source and produced subduction-related geochemical signatures superimposed on volcanic arc chemistry. After Early Paleozoic oceanic subduction and arc-continent collision, a transient stage of extension occurred between 313 and 280 Ma in the Sonidzuoqi–Xilinhot area. Deformation and recrystallization during the switch from compression to extension and reheating by the later magmatic intrusions reset the isotope systems of minerals in the Xilin Gol Complex, recorded by a 312.2 ± 1.5 Ma biotite ⁴⁰Ar/³⁹Ar age from biotite–plagioclase gneiss, a 309 ± 12 Ma zircon intercept age and a 307.5 ± 3.5 Ma hornblende ⁴⁰Ar/³⁹Ar age from amphibolites in the complex. There was an arc/forearc-related marine basin at the southern margin of the Xilin Gol Complex during the Permian. The closure of the oceanic basin led to Late Paleozoic–Middle Triassic north-dipping subduction beneath the Xilin Gol Complex and induced the amphibolite-facies metamorphism of the complex. The final suturing of the Solonker zone occurred from 269 to 231 Ma. This latest amphibolite-facies metamorphism with pressures of 0.31–0.39 GPa and temperatures of 620–660 °C was recorded at 263.4 ± 1.4 Ma to the Xilin Gol Complex, as indicated by the hornblende ⁴⁰Ar/³⁹Ar age from the amphibolites, as well as several zircon ages of 260 ± 3–231 ± 3 Ma. The Xilin Gol Complex documented the progressive accretion of a single, long-lived subduction system at the southern margin of the south Mongolian microcontinent from the Early Paleozoic (~452 Ma) to Middle Triassic (~231 Ma). The CAOB shows protracted collision prior to final suturing.     

40Ar/39Ar geochronology constraints on the Middle Tertiary basement extensional exhumation,and its relation to ore-forming and magmatic processes in the Eastern Rhodope (Bulgaria)

The interaction of distinct geologic processes involved during late orogenic extensional exhumation history of the metamorphic units in the Eastern Rhodope is refined by new and reviewing ⁴⁰Ar/³⁹Ar geochronological and structural data. Minerals with different closure temperatures from metamorphic rocks investigated in this study are combined with those from magmatic and ore-forming hydrothermal rocks in two late stage metamorphic domes – the Kesebir-Kardamos and the Biala reka-Kehros domes. The 38-37 Ma muscovite and biotite cooling ages below 350°-300°C characterize basement metamorphic rocks that typified core of the Kesebir-Kardamos dome, constraining their exhumation at shallow crustal levels in the footwall of detachment. These ages are interpreted as reflecting last stage of ductile activity on shear zone below detachment, which continued to operate under low-temperature conditions within the semi-ductile to brittle field. They are close to and overlap with existing cooling ages in southern Bulgaria and northern Greece, indicating supportively that the basement rocks regionally cooled between 42-36 Ma below temperatures 350°-300°C. The spatial distribution of ages shows a southward gradual increase up structural section, suggesting an asymmetrical mode of extension, cooling and exhumation from south to the north at latitude of the Kesebir-Kardamos dome. The slightly younger 36.5-35 Ma crystallization ages of adularia in altered rocks from the ore deposits in the immediate hanging-wall of detachments are attributed to brittle deformation on high-angle normal faults, which further contributed to upper crustal extension, and thus constraining the time when alteration took place and deformation continued at brittle crustal levels. Silicic dykes yielded ages between 32-33 Ma, typically coinciding with the main phase of Palaeogene magmatic activity, which started in Eastern Rhodope region in Late Eocene (Priabonian) times. The ⁴⁰Ar/³⁹Ar plateau ages from the above distinct rock types span time interval lasting approximately ca. 6 Ma. Consequently, our geochronologic results consistently indicate that extensional tectonics and related exhumation and doming, epithermal mineralizations and volcanic activity are closely spaced in time. These new ⁴⁰Ar/³⁹Ar age results further contribute to temporal constraints on the timing of tectonic, relative to ore-forming and magmatic events, suggesting in addition that all above mentioned processes interfered during the late orogenic extensional collapse in the Eastern Rhodope region.     

Structure,Timing, and Mechanism of the Pliocene and Late Miocene Uplift Process of the Ailao Shan-Diancang Shan,SE Tibet,China

The uplift of the Ailao Shan-Diancang Shan(ASDS) along the Ailao Shan-Red River(ASRR) shear zone is an important geological event in the southeastern margin of Qinghai-Tibet Plateau tectonic domain in the Late Cenozoic,and it preserves important information on the structures,exhumational history and tectonic evolution of the ASRR shear zone.The uplift structural mode and uplift timing of the ASDS is currently an important scientific topic for understanding the ASDS formation and late stage movements and evolution of the ASRR shear zone.The formation of the ASDS has been widely considered to be the consequence of the strike-slip movements of the ASRR shear zone.However,the shaping of geomorphic units is generally direct results of the latest tectonic activities.In this study,we investigated the timing and uplift structural mechanism of the ASDS and provided the following lines of supportive evidence.Firstly,the primary tectonic foliation of the ASDS shows significant characteristic variations,with steeply dipping tectonic foliation developed on the east side of the ASDS and the relatively horizontal foliation on the west side.Secondly,from northeast to southwest direction,the deformation and metamorphism gradually weakened and this zone can be further divided into three different metamorphic degree belts.Thirdly,the contact relationship between the ASDS and the Chuxiong basin-Erhai lake is a normal fault contact which can be found on the east side of the ASDS.40Ar/39 Argeochronology suggests that the Diancang Shan had experienced a fast cooling event during 3–4 Ma.The apatite fission track testing method gives the age of 6.6–10.7 Ma in the Diancang Shan and 4.6–8.4 Ma in the Ailao Shan,respectively.Therefore the uplift of the ASDS can be explained by tilted block mode in which the east side was uplifted much higher than the west side,and it is not main reason of the shearing movements of the ASRR shear zone.The most recent uplift stages of the ASDS happened in the Pliocene(3–4 Ma) and Late Miocene(6–10 Ma).     

Subduction and collision processes between the North China and South China blocks constrained by the geochronology and geochemistry of the Wuguan Complex in the Qinling orogen,China

ABSTRACT

The Qinling orogen is a key area for understanding the processes of subduction and collision between the South China Block (SCB) and North China Block (NCB). The Wuguan Complex, distributed along the southern margin of the Shangdan suture zone, can provide important constraints on the age of collision between NCB and SCB and the tectonic evolution of the Qinling orogen in Late Paleozoic. Detrital zircons from meta-sedimentary rocks of the Wuguan Complex in the Danfeng-Shangnan area have an age spectrum with two main peaks at ~448 Ma and ~819 Ma, and two subordinate peaks at ~938 and ~1440 Ma, respectively, and are interpreted to have been derived from the North Qinling terrane (NQT). The petrographic and geochemical characteristics of the meta-sedimentary rocks indicate that they were deposited in a fore-arc basin along the southern margin of the NQT. The youngest detrital zircons yield a weighted mean age of 378 ± 3 Ma, indicating that the fore-arc deposition was continuing at least to this time, which implies that the Paleo-Qinling Ocean between the NCB and SCB was not finally closed until at least the late Devonian. In combination with regional data, we propose that sedimentary rocks of the Wuguan Complex might once have been a sequence of late Ordovician to late Devonian strata with intercalated mafic rocks, which has been dismembered by the later tectonic activity. It was metamorphosed during northward subduction of the Paleo-Qinling Ocean at ca. 320 Ma, and slowly cooled through ca. 350°C at ca. 247 Ma (muscovite ⁴⁰Ar/³⁹Ar age). It has recorded the detailed processes of subduction and collision between the NCB and SCB.     

Direct 40Ar/39Ar dating of Late Ordovician and Silurian brittle faulting in the southwestern Norwegian Caledonides

Structural data as well as U–Pb zircon and ⁴⁰Ar/³⁹Ar biotite and muscovite ages were collected from the Rolvsnes granodiorite in western Norway. The granodiorite intruded at c. 466 Ma, cooled quickly and escaped later viscous deformation. Brittle top‐to‐the‐NNW thrust faults (Set I) and WNW–ESE striking dextral strike‐slip faults (Set II) formed in a NNW–SSE transpressional regime. ⁴⁰Ar/³⁹Ar dating of synkinematic mica from both sets reveals a c. 450 Ma (Late Ordovician) age of faulting, which constrains early‐Caledonian brittle deformation. Set I and II faults are overprinted by a set of lower‐grade, variably oriented chlorite‐ and epidote‐coated faults (Set III) constraining WNW–ESE shortening. A lamprophyric dyke oriented compatibly with this stress field intruded at c. 435 Ma (Silurian), indicating that Set III formed at the onset of the Scandian Baltica–Laurentia collision. The preservation of Caledonian brittle structures indicates that the Rolvsnes granodiorite occupied a high tectonic level throughout the Caledonian orogeny.     

Caledonian eclogite-facies metamorphism of Early Proterozoic protoliths from the North-East Greenland Eclogite Province

High-pressure metamorphic assemblages occur in mafic, ultramafic and a few intermediate rocks in a gneiss complex that covers an area of approximately 400 × 100 km in the North-East Greenland Caledonides. Detailed petrologic and geochronologic studies were carried out on three samples in order to clarify the P-T-t evolution of this eclogite province. Geothermobarometry yields temperature estimates of 700–800 °C and pressure estimates of at least 1.5 GPa from an eclogite sensu stricto and as high as 2.35 GPa for a garnet websterite. The eclogite defines a garnet-clinopyroxene-amphibole-whole rock Sm-Nd isochron age of 405 ± 24 Ma (MSWD 0.9). Isofacial garnet websterites define garnet-clinopyroxene-orthopyroxene-amphibole-whole rock-(biotite) ages of 439 ± 8 Ma (MSWD =2.1) for a coarse-grained sample and 370 ± 12 Ma (MSWD=0.6) for a finer-grained variety. Overgrowths on zircons from the fine-grained pyroxenite and the eclogite give a pooled ²⁰⁶Pb/²³⁸U SHRIMP age of 377 ± 7 Ma (n=4). Significantly younger Rb-Sr biotite ages of 357 ± 8, 330 ± 6 and 326 ± 6 agree with young Rb-Sr, K-Ar and ⁴⁰Ar/³⁹Ar mineral ages from the gneiss complex and indicate slow cooling of the eclogitic rocks. High-pressure metamorphism may have been at least 439 Ma old (Siluro-Ordovician) with cooling through amphibolite-facies conditions in the Devonian and continued crustal thinning and exhumation well into the Carboniferous. Sm-Nd whole rock model ages indicate the eclogite protoliths are Early Proterozoic in age, while ²⁰⁷Pb/²⁰⁶Pb SHRIMP ages of 1889 ± 18 and 1981 ± 8 from anhedral zircon cores probably reflect Proterozoic metasomatism. The samples have negative ɛ_Nd values (−5 to −16) and elevated ⁸⁷Sr/⁸⁶Sr ratios (0.708–0.715), consistent with field evidence that the eclogite protoliths were an integral part of the continental crust long before Caledonian metamorphism. The presence of a large Caledonian eclogite terrane in Greenland requires modification of current tectonic models that postulate subduction of Baltica beneath Laurentia during the Caledonian orogeny. Received: 9 October 1996 / Accepted: 7 July 1997     

40Ar/39Ar Thermochronology west of the Main Uralian fault, southern Urals, Russia

West of the Main Uralian fault, the main suture in the southern Urals, ⁴⁰Ar/³⁹Ar apparent ages of amphibole, muscovite and potassium feldspar are interpreted as cooling ages. A fast exhumation of the metamorphic complex of Kurtinsky during Upper Carboniferous time is indicated by the small age difference (15 Ma) between cogenetic amphibole and muscovite. Differentiated movement in the footwall of the Main Uralian fault along strike is indicated by the age difference of 70 Ma between the metamorphic complexes of Kurtinsky (north) and Maksyutov (south). No Upper Paleozoic (Uralian) medium- to high-temperature event is recorded in ⁴⁰Ar/³⁹Ar data from the metamorphic complex of Beloretzk (MCB). An amphibole age of 718±5 Ma and the occurrence of mafic intrusions might signal the break-up of Rodinia and therefore indicate the rifting period followed by the separate movement of the "Beloretzk terrane". Muscovite ages of approximately 550±5 Ma, the unique pre-Ordovician tectonometamorphic evolution of the MCB and the Late Vendian sedimentary history of the western Bashkirian Megaanticlinorium (BMA) imply the existence of a Neoproterozoic orogeny at the eastern margin of Baltica. This orogeny might have been initiated by the accretion of the "Beloretzk terrane". The metamorphic grade of the overlain Silurian shales and the K/Ar microcline ages from the "Beloretzk terrane" give evidence for a new thermal event at approximately 370 Ma. A microcline age of 530–550 Ma obtained for the Vendian conglomerate in the western BMA suggests that a maximum temperature of approximately 200°C was reached in Cambrian or Vendian times. An orthoclase age (590–630 Ma) of the Vendian Zigan flysch deposits might be inherited from the eastern source area, the Cadomian orogen. An orthoclase age (910–950 Ma) from the Riphean Zilmerdak conglomerate coincides with a documented decrease in the subsidence rate of the Upper Riphean basin.