Hercynian Times Research Articles

The superposed orogenesis of the alpine-mediterranean edifice

The circum-Mediterranean chains must be considered as the result of two distinct orogenies. The apparent unity of the present structure is of formal order, due to the latest deformations. Since the Hercynian time there have been two periods of paroxysmal deformation; the younger fits the definition of the alpine orogeny; the older occurred during the Cretaceous and may correspond to the first great convergent relative drift of the Eurasiatic and African blocks. The Cretaceous or Mesogean orogeny is independent from the Alpine orogeny stricto sensu (Oligo-Miocene) and cannot be considered as its prefiguration. Being independent in time, it is independent in space as well. Even if this Mesogean orogeny can appear locally restricted to the ‘’internal’’ parts of the Alpine chains (Central Mediterranean area, Carpathes, Dinarides) this cannot be taken as a rule: towards the west, the Cretaceous deformations cross the axis of the western Alps and extend (new investigations) over Provence to the Betic chains and the Pyrenean area. Towards the east, the deformations of this period cross the Hellenides (new observations) and spread over the ‘’external’’ area in a spectacular way, interesting areas which have never been tectonised again (Cyprus, south-eastern Anatolia, northern Syria, Oman). As a whole, this large Cretaceous orogenic zone is part of a wider domain which extends over central Iran towards the Himalayas and eastern Asia, and has its equivalent on the western side of the Atlantic Ocean, in the Caribbean islands, Mexico and the Americas.

Wealth of P–T–t information in medium-high grade metapelites: Example from the Jubrique Unit of the Betic Cordillera, S Spain

It is demonstrated here that medium-high grade metapsammopelites contain a wealth of information to derive a well-constrained P–T–t path relevant for the understanding of the geodynamics resulting in a collisional orogen. The presented example is related to three samples from the Migmatite gneiss zone of the Jubrique Unit, being part of the Alpujárride Complex half way between Malaga and Cadiz. The rocks were studied to determine their pressure (P)–temperature (T) evolution mainly on the basis of the zonation of mm-sized garnet and contoured P–T pseudosections calculated with PERPLE_X. In addition, monazite was dated with the electron microprobe. Ca-poor garnet cores yielded granulite-facies P–T conditions around 0.7 GPa and 800 °C. This metamorphic event occurred in late Hercynian times [287 ± 9 (2σ) Ma] according to monazite enclosed in such garnet cores. The cores are surrounded by a Ca-richer garnet zone, which points to a high-pressure (HP) event close to 1.3 GPa and temperatures above 650 °C. At this stage, melting occurred as indicated by abundant melt inclusions in the Ca-rich garnet zone. Subsequently, the rocks were exhumed and heated to peak temperatures of 740 °C. The final metamorphic stage is characterised by late sillimanite and an outermost garnet rim which is poor in Ca. The melting event is late Alpine confirmed by age dating on monazite in the rock matrix. Similar conditions of a late Alpine HP event were also reported from other areas of the Alpujárride Complex. As the studied rocks never reached P–T conditions of the diamond stability field, it is concluded that the previously reported nanodiamonds in these rocks either grew metastably or were artificially introduced into rock thin-sections.

Garnet variety and zircon ages in UHP meta-sedimentary rocks from the Jubrique zone (Alpujárride Complex, Betic Cordillera, Spain): evidence for a pre-Alpine emplacement of the Ronda peridotite

The Ronda peridotite is a group of lherzolite slabs (1.5 to 2 km thick) in southern Spain. Despite clear evidence that pre-Alpine events affected pre-Permo-Triassic units from the Alborán domain (internal zone of the Betic-Rif Cordillera, Spain, and Morocco), numerous papers continue to emphasize Alpine metamorphic and structural evolution. Here, we evaluate the pre-Cenozoic evolution of the Ronda peridotite by reporting new petrographic and U–Pb SHRIMP zircon dating of meta-sedimentary rocks from the Jubrique zone (Alpujárride Complex, Betic Cordillera, Spain) directly overlying the Ronda peridotite. Field inspection and petrographical study revealed generalized migmatitic textures and a gradual transition mainly defined by garnet content (from ~30 to <3 wt.%) and size (from 1.5 cm to <0.5 mm) in the overlying granulite-gneiss sequence, suggesting that most garnet grew as a consequence of the peridotite emplacement. Garnet shows notable variations in composition and inclusion types, which are interpreted as reflecting different stages of garnet growth. Diamond-bearing garnets are only well-preserved in gneisses from the uppermost part of the sequence, whereas the large garnets from rocks overlying the peridotite mainly record later thermal events. SHRIMP zircon dating indicates two age peaks at 330 ± 9 and 265 ± 4 Ma. The oldest age characterizes rims overgrowing detrital cores and reflects an early Hercynian metamorphism; the younger age characterizes zircon with magmatic oscillatory zoning, reflecting anatexis. On the basis of these data and of previous dating of monazite included in the large garnets, we conclude that the peridotite was emplaced either shortly before or during early Hercynian times, ~330 Ma.

Tectonic history of the Irtysh shear zone (NE Kazakhstan): New constraints from zircon U/Pb dating, apatite fission track dating and palaeostress analysis

The Irtysh shear zone (ISZ) is an important structure in the framework of the Central Asian Orogenic Belt (CAOB). It represents the site of final collision of Kazakhstan with Siberia during Hercynian times and records up to 1000 km of lateral displacement during subsequent reorganization in the CAOB edifice. We present new zircon U/Pb, apatite fission track and fault kinematic data along the ISZ and consequently derived its tectonic history with emphasis on its formation and reactivation episodes. Carboniferous (∼340–320 Ma) zircon U/Pb ages were obtained for the syn- and post-collisional Kalba–Narym intrusives, dating their emplacement in the framework of the Siberia–Kazakhstan collision. During this period, the ISZ experienced an ‘early brittle’ left-lateral, mainly transtensional stress regime. Late Carboniferous–Early Permian post-collisional intrusives were emplaced and the stress regime changed to a ‘late brittle’ regime, characterized by more compressional conditions, indicating rheological strengthening as a response to cessation of ductile shearing and cooling of the ISZ crust. Apatite fission track data and thermal history modeling reveal Late Cretaceous (∼100–70 Ma) cooling of the ISZ basement rocks as a response to denudation of a bordering Late Mesozoic Altai orogen. After this denudation event, the tectonic activity ceased during the Late Mesozoic–Early Cenozoic. A final step of cooling (from ∼25 Ma), exhibited by some of the thermal history models, may reflect reactivation of the ISZ and initiation of Cenozoic Altai mountain building. The Late Plio-Pleistocene phase of mountain building coincides with a new change in the Palaeostress field, characterized by minor transpressional, right-lateral shear conditions.

The geological relationships between Sardiniaand Calabria during Alpine and Hercynian times

What were the geological relations between Calabria and Sardinia before the opening of the Tyrrhenian Sea? It would be premature to reach a definitive conclusion, so we consider (without reaching any conclusions) four major open questions that bear on the original geologic relationships between Sardinia and Calabria, and that are closely interconnected: (1) Was Calabria a single terrane during post-Hercynian time (Mesozoic and Cenozoic), or is it a composite terrane formed by Cenozoic collision of two subterranes, northern and southern? (2) What has the history of subduction under Calabria been? One interpretation envisions an early «Alpine», east-dipping subduction followed by a later «Apennine», west-dipping subduction; an alternative view is that only west-dipping subduction has occurred. (3) Is there a recognizable match between the Hercynian and Mesozoic geology of Sardinia and of Calabria? The Hercynian evolution of Sardinia is now generally understood, although with some open questions. That of Calabria is still poorly known, which makes it hard at this point to use a match of the Hercynian patterns to constrain a pre-Tyrrhenian terrane reconstruction, but the character of Mesozoic carbonate sequences may provide an additional constraint. (4) How was Calabria extended during its rifting away from Sardinia?

The Hercynian collision in the Armorican Massif : evidence of different lithospheric domains inferred from seismic tomography and anisotropy

Abstract The Hercynian belt is a continental collision orogen extending from south-west Iberia to the Bohemian Massif in Czech Republic. The successive stages of its formation are dated from 400 to 260 Ma.The Armorican Massif is a preserved segment of this orogen. It presents structures oriented NW-SE, parallel to the general Hercynian trend in this region. The massif is divided into three domains (North, Central, and South-Armorican domains) separated by two main shearzones, the North- and South-Armorican shear zones. As the Armorican Massif escaped from any important tectonic or thermal event since the end of Hercynian times, it is particularly suited for the study of an old collision orogen. Thus, in the framework of the GéoFrance3D-ARMOR2 project, two passive seismological experiments were conducted in 1997 and 1999 in the Armorican Massif. The main goals concerned the characterization of the deep geometry of both shear zones, the understanding of their geodynamic bearing on the long term evolution of the Hercynian belt, the study of the lithospheric deformation, and the 3D imaging of the Champtoceaux nappes.The data allow to model seismic anisotropy and to build a 3D P-wave velocity model beneath the Armorican Massif. Crustal images do not evidence any deep rooting of the Champtoceaux nappes in the lower crust. However, the upper mantle images show a clear signal interpreted as the relic of the northward subduction which lasted until Devonian (≈350 Ma). The results also show that the North-Armorican Shear Zone is limited at depth to the crust and topmost mantle, while the South-Armorican Shear Zone can be traced over the whole lithosphere.The strong velocity contrasts are associated to probable relic thermal anomalies but are also significantly related to chemical anomalies.

COMMENT TO: “VOLCANIC ACTIVITY FROM THE NEOGENE TO THE PRESENT EVOLUTION OF THE WESTERN MEDITERRANEAN AREA. A REVIEW” BY M. LUSTRINO

In the Discussion and Conclusions of the paper “Volcanic activity from Neogene to the present evolution of the Western Mediterranean sea. A review” (Ofioliti, 25 (2): 87- 101), by Michele Lustrino, mainly deals with the Neogene to present volcanic activity and it presents a “continuous” model from Hercynian time to present: the paper, particularly because it wants to be a review, must take in better and more complete consideration the present-day knowledge on bibliography, data and interpretations, about the Hercynian, Late Hercynian and post-Hercynian geodynamic evolution in Western and Southern Europe. The Discussion and Conclusions chapter reports, in particular, some considerations about the Hercynian and post- Hercynian geodynamic evolution that do not take in consideration present interpretations developed by recent geological, structural and petrological-geochemical investigations carried out by several authors. The presentation of some main topics of this evolution is particularly inadequate for an up-to-date review published on an international bulletin.

Reply to: Comment to: "volcanic activity from the Neogene to the present evolution of the Western Mediterranean area. a review" by Michele Lustrino

We recently published a review of geochemical data on volcanic products emplaced during the last 30 Ma in the western Mediterranean area (Lustrino, 2000a). In this paper, the most recent petrogenetic models proposed for various igneous provinces were compared each other and tentatively fitted in a geodynamic scenario of evolution. The aim of that paper was to furnish an updated review of the petrologic models proposed to explain the origin of the main volcanic outcrops of Spain, France, Morocco and Italy, with special emphasis given to the Plio-Pleistocene volcanic rocks of Sardinia. The choice to focus on the recent volcanic activity of Sardinia was thought because of its geographic position (Sardinia represents a lithospheric slice in between two oceanic areas reworked during Hercynian and Alpine orogeneses), its peculiar trace element composition (Plio- Pleistocene volcanic rocks of Sardinia show the highest Ba/Nb and one of the lowest Ce/Pb and Nb/U among the entire Cenozoic European Volcanic Province anorogenic products) and its unique Sr-Nd-Pb isotopic signature (87Sr/86Sr close to bulk Earth values, eNd ~ -5, 206Pb/204Pb down to ~ 17; Lustrino et al., 2000). The comments of Elter seem to not take in consideration the true subject of our paper (geochemistry of 30-0 Ma volcanic rocks) and are entirely focused on the Hercynian geodynamic evolution of Western and Southern Europe. According to Elter, we presented a “continuous” model from Hercynian time to the present, but this is not the case. We don’t present any continuous model: on trace element and Sr-Nd-Pb isotopic grounds (data published elsewhere; Lustrino, 2000c; Lustrino et al., 2000) we simply suppose a relationship between processes responsible for the geochemical signature of mantle sources of Plio-Pleistocene volcanic rocks of Sardinia and those occurred in late Paleozoic, mainly during post-collisional extensional movements. Now we are going to reply to his doubts about the scientific valence of our paper (Lustrino, 2000a); in particular, we will follow its own scheme.

Correlation between the Palaeozoic structures from West Iberian and Grand Banks margins using inversion of magnetic anomalies

The Ibero-Armorican Arc (IAA) is a huge geological structure of Pre-Cambrian origin, tightened during hercynian times and deeply affected by the opening of the Atlantic Ocean and the Bay of Biscay. Its remnants now lie in Iberia, north-western France and the Canadian Grand Banks margins. The qualitative correlation between these three blocks has been attempted by several authors (e.g. Lefort, J.P., 1980. Un ‘Fit’ structural de l'Atlantique Nord: arguments geologiques pour correler les marqueurs geophysiques reconnus sur les deux marges. Mar. Geol. 37, 355–369; Lefort, J.P., 1983. A new geophysical criterion to correlate the Acadian and Hercynian orogenies of Western Europe and Eastern America. Mem. Geol. Soc. Am. 158, 3–18; Galdeano, A., Miranda, J.M., Matte, P., Mouge, P., Rossignol, C., 1990. Aeromagnetic data: A tool for studying the Variscan arc of Western Europe and its correlation with transatlantic structures. Tectonophysics 177, 293–305) using magnetic anomalies, mainly because they seem to preserve the hercynian zonation, in spite of the strong thermal and mechanical processes that took place during rifting and ocean spreading. In this paper, we present a new contribution to the study of the IAA structure based on the processing of a compilation of magnetic data from Iberia and Grand Banks margins. To interpret the magnetic signature, a Fourier-domain-based inversion technique was applied, considering a layer with a constant thickness of 10 km, and taking into account only the induced field. The digital terrain model was derived from ETOPO5 (ETOPO5, 1986. Relief map of the earth's surface. EOS 67, 121) and TerrainBase (TerrainBase, 1995. In: Row III, L.W., Hastings, D.A., Dunbar, P.K. (Eds.), Worldwide Digital Terrain Data, Documentation Manual, CD-ROM Release 1.0. GEODAS-NGDC Key to Geophysical Records. Documentation N. 30, April) databases. The pseudo-susceptibility distribution obtained was repositioned for the 156.5 Ma epoch, using the Srivastava and Verhoef [Srivastava, S.P., Verhoef, J., 1992. Evolution of Mesozoic sedimentary basins around the North Central Atlantic: a preliminary plate kinematic solution. In: Parnell, J. (Ed.), Basins on the Atlantic Seaboard: Petroleum Geology Sedimentology and Basin Evolution, Geological Society Special Publication No. 62, pp. 397–420] pole. Using this coherent magnetic framework, we can verify that the continuity between adjacent blocks is quite good, in terms of the amplitude, wavenumber and magnetic susceptibility pattern. If we accept that the magnetic properties can be taken as a marker of the hercynian zonation, as was verified in previous studies (Miranda, J.M., Galdeano, A., Rossignol, J.C., Mendes-Victor, L.A., 1989. Aeromagnetic anomalies in mainland Portugal and their tectonic implications. Earth Planet. Sci. Lett. 95, 161–177; Galdeano, A., Miranda, J.M., Matte, P., Mouge, P., Rossignol, C., 1990. Aeromagnetic data: A tool for studying the Variscan arc of Western Europe and its correlation with transatlantic structures. Tectonophysics 177, 293–305; Socias, I., 1994. Estudios de los Elementos del Campo Magnético en la España Peninsular a partir de Datos Aeromagmanéticos. Ph.D. thesis, University of Madrid), we can conclude that (1) the characteristic magnetic signature of Ossa Morena Zone is absent on the Iberian Margin and west of it; (2) no eastward continuation of the Collector Anomaly is found in Iberia; (3) only the inner zones of the Variscan Belt can be followed towards NW France; (4) there is a major (left lateral ?) strike-slip fault along the northern Portuguese shoreline that cuts the IAA and significantly displaces the once-contiguous variscan units.

Le pointement de péridotite à grenat-spinelle de La Croix-Valmer (Maures centrales): un cumulat d'affinité océanique impliqué dans la subduction éohercynienne ?

A garnet-spinel metaperidotite body has been found in the leptyno-amphibolitic complex of the central pan of the Maures Massif (Hercynian Belt, south eastern France). Based on bulk rock and mineral chemistry, a mantle tectonite origin can be ruled out for this rock. Instead, it appears to be an igneous cumulate that crystallized at low P, in the olivine-plagioclase stability field, and was later brought down to mantle depths (P = 16–18 kbar, T = 850–860 °C). Although Sm-Nd isotopes preclude a strictly comagmatic origin, the clear oceanic affinity of neighbouring metagabbros (ε Nd 500 + 7.7 and + 8.7) suggests a very evolved extensional tectonic setting, having led to the break-up of the continental lithosphere and the formation of a rifted margin. The high pressure metamorphic overprint recorded by the peridotite is interpreted as reflecting the subduction of a passive margin, at the final stage of consumption of an oceanic domain. This event probably occurred in Early Hercynian times, as documented elsewhere in the west european Hercynides.

Upper mantle anisotropy beneath central Europe from SKS wave splitting: Effects of absolute plate motion and lithosphere-asthenosphere boundary topography?

Fast polarization directions α of split SKS waves in Central Europe change from NE/ENE in the western part to dominatingly E/ESE orientation towards north and east. This coincides strikingly well with the dominating trend of Hercynian deformational crustal features. It hints to frozen anisotropy related to paleo-crustal fabric. But when considering plausible anisotropy values of about 2–3% then only a small fraction ( δt < 0.3 s) of the rather large observed average delay-times ( δt = 0.83 ± 0.31 s ) between the two split waves could be attributed to structural anisotropy in the relatively thin Central European crust. Therefore, the main “anisotropy signal” has to be associated with lattice-preferred orientation (LPO) of olivine below the crust. It may be either frozen in the subcrustal lithosphere since Hercynian times or have developed more recently in the asthenosphere. The thickness of the lithosphere varies significantly beneath Europe and the depth contours show systematic changes in trend. The latter varies from dominatingly NE in the southwest to SE in the north and east. The polarization directions α of the fast split SKS waves observed at seismic stations in proximity to the southern and northeastern boundaries of Central Europe are subparallel to the trends of these strong anomalies in lithosphere topography. A causal relationship is assumed and a new model proposed to explain the observations in α and δt. It takes into account the possible effects of paleo-deformational events. They may have produced both anisotropic crustal fabric and probably still preserved and similarly trending frozen LPO in the subcrustal lithosphere. The model also considers the influence of recent absolute motion of the West European lithospheric plate towards NE and the effect of its pronounced lower boundary topography on the formation and trend of LPO in the asthenosphere. Accordingly, the effects of anisotropy of different nature and age at different depth levels but with similar trend may superimpose constructively. This could explain the rather large delay-times observed at Central European stations which are too large to be attributed to frozen anisotropy in the lithosphere alone. The model would even permit the total effect observed to be attributed to asthenosphere flow controlled by absolute plate motion direction and lithosphere-asthenosphere boundary topography.

Palaeomagnetic studies in Morocco: tectonic implications for the Meseta and Anti-Atlas since the Permian

Abstract Most Palaeozoic rocks studied in Morocco show evidence of partial or complete remagnetization that occurred towards the end of Hercynian times (late Carboniferous and Permian). The age of remagnetization can be established by direct comparisons with other Permian palaeomagnetic directions. The late Carboniferous and Permian rocks, and the remagnetizations of this age, show essentially identical directions throughout the Meseta and Anti-Atlas regions of Morocco and suggest that there have been few significant rotations of blocks within the Meseta domain since Permian times. This implies that most later motions in this region have not involved significant rotations about vertical axes or large scale changes in the latitude.

Massif Central (France): new constraints on the geodynamical evolution from teleseismic tomography

The Massif Central, the most significant geomorphological unit of the Hercynian belt in France, is characterized by graben structures which are part of the European Cenozoic Rift System (ECRIS) and also by distinct volcanic episodes, the most recent dated at 20 Ma to 4000 years BP. In order to study the lithosphere-asthenosphere system beneath this volcanic area, we performed a teleseismic field experiment. During a six-month period, a joint French-German team operated a network of 79 mobile short-period seismic stations in addition to the 14 permanent stations. Inversion of P-wave traveltime residuals of teleseismic events recorded by this dense array yielded a detailed image of the 3-D velocity structure beneath the Massif Central down to 180 km depth. The upper 60 km of the lithosphere displays strong lateral heterogeneities and shows a remarkable correlation between the volcanic provinces and the negative velocity perturbations. The 3-D model reveals two channels of low velocities, interpreted as the remaining thermal signature of magma ascent following large lithospheric fractures inherited from Hercynian time and reactivated during Oligocene times. The teleseismic inversion model yields no indication of a low-velocity zone in the mantle associated with the graben structures proper. The observation of smaller velocity perturbations and a change in the shape of the velocity pattern in the 60–100 km depth range indicates a smooth transition from the lithosphere to the asthenosphere, thus giving an idea of the lithosphere thickness. A broad volume of low velocities having a diameter of about 200 km from 100 km depth to the bottom of the model is present beneath the Massif Central. This body is likely to be the source responsible for the volcanism. It could be interpreted as the top of a plume-type structure which is now in its cooling phase.

Origin of cordierite-bearing granites by assimilation in the Central Iberian Massif (CIM), Spain

The Central Iberian Massif (CIM) is characterized by the great abundance and variety of granitoids emplaced during Hercynian times. One of the most conspicuous is cordierite-bearing biotite granite (CG), which in the western part of the CIM is associated with other granites lacking cordierite [amphibole-bearing biotite granite (AG) and biotite granite (BG)] and with anatectic host-rocks [cordierite-rich nebulites (NB)]. Major-, trace- and rare-earth element data for AG and BG show that these granites have chemical affinities, and parallel or overlapping differentiation trends, suggesting two different granite magmas, with a common or similar deep-seated source. CG show two extreme facies: a marginal cordierite-rich facies (MCG) in direct contact with host-rocks (schists, migmatites), and an internal cordierite-poor facies (ICG) farther away from the host-rocks. Gradual transitions between MCG and ICG are commonly observed, as well as between ICG and BG. An anatectic hypothesis for CG is difficult to accept because: (a) CG have higher CaO contents relative to NB or other country rocks, a smaller Eu/Eu∗ ratio and similar Sr contents; and (b) the similar REE absolute contents and patterns in CG, BG and NB, and the absence of REE fractionation with respect to country rocks. Simple mixing equations are computed to test assimilation processes involving BG as a primary magma and NB as the contaminant in producing CG. CaO and MgO data have been used for this purpose since they are the most discriminating elements between BG and NB. The results of the computation indicate that: (a) assimilation is possible for nearly the whole CaO range in NB if CaO in the BG ranges between 1.91% and 2.86%, and MgO is between 0.93% and 1.18% (these extreme values are comprised within the observed range for CaO and MgO in BG); (b) CaO and MgO data for MCG are compatible with assimilation for nearly the whole range of data observed; and (c) the percentage of assimilation is < 30% if all data are considered, but 92% of the solutions can be explained by assimilation percentages of < 18% NB. These data make the assimilation model the one preferred by the authors.

Geological features and genesis of the Bayan Obo REE ore deposit, Inner Mongolia, China

Geotectonically, the Bayan Obo deposit is located on the northern margin of the North China platform. When the Proterozoic Bayan Obo Group was formed, the area was probably a rift tectonic environment. Dolomite, the main wall rock (or host rock) of the orebody of the Bayan Obo REE deposit, shows a distinct stratified structure and is remarkably similar to REE-rich and Nb-rich carbonatite rocks elsewhere in the world in mineral constituents, petrochemical composition and incompatible element composition. K-feldspar rock, another host rock of the orebody, may have been formed as a result of volcanic exhalation. The Sm Nd whole rock (whole ore) isochron age of the Main orebody of the Bayan Obo deposit is 1.58 Ga, and the Sm Nd model age is 1.61 Ga, with ε Nd(t) being +6.1. The data suggest that the ore deposit was formed in the Middle Proterozoic, and that the ore-forming material came from the mantle. Middle Proterozoic seems to be the time when the Bayan Obo Group was deposited; the REE metallization occurred together with the deposition of the strata. The isochron age is close to the model age, implying that the ore-forming material was precipitated to form the ore deposit immediately after its separation from the mantle, without much contamination by crustal substances. Beside a Middle Proterozoic metallogenic age, Caledonian and Hercynian ages have also been recorded: for the former time interval, the Rb Sr isochron age of the Kuangou carbonatite vein determined by the authors is 433 Ma; for the latter time interval, the Th Pb age of aeschynite from the ore deposit and the Rb Sr isochron age of biotite granite from the southern part of the ore district were already determined by previous workers. These data indicate that the ore-forming process was polyphasic. In addition, δ 18O magnetite values of massive REE-Fe ore are −2.99 to +3.55%, and δ 18O magnetite values of riebeckite REE-Fe ore are +0.76 to +1.92%. These values, together with the S isotope data obtained by previous workers, also suggest that the ore-forming material for the ore deposit was derived from deep sources. The ore-forming process of the Bayan Obo deposit was, as indicated above, polyphasic, but the major metallization took place in the Middle Proterozoic. The ore-forming material was brought upward in the form of volcanic exhalations from the mantle, and then through sedimentation formed an ore deposit which was genetically of alkalic rock-carbonatite type. In the Caledonian time interval, with the intrusion of carbonatite veins, small amounts of ore-forming material might have been brought upward from depths, leading to extensive metasomatism. In the Hercynian time interval, with the intrusion of granite in the south, the ore deposit was transformed and the metasomatism further developed; nevertheless, little REE ore-forming material was brought into the orebody from depths or from the outside at that time.

Single-zircon dating by step-wise Pb evaporation: Comparison with other geochronological techniques applied to the Hercynian granites of Corsica, France

The Corsican Batholith provides an opportunity to test the single-zircon evaporation technique developed by B. Kober for Hercynian times. In this paper we attempt to clarify the method of calculating207Pb*/206Pb* ages and errors. The significance of the207Pb*/206Pb* ages are discussed with particular reference to the determination of the crystallization age of plutons. Zircon morphology is clearly an important criterion in selecting grains for analysis. Geological relations in the example that we are using are firmly established, enabling discussion of the207Pb*/206Pb* model age in the light of previous age determinations using other methods (RbSr,40Ar/39Ar and conventional UPb).We interpret the well-defined322±12-Ma207Pb*/206Pb* age as the time of the crystallization of the U1, MgK rocks. This is in good agreement with RbSr ages of the younger units U2a and U2b (312±9and290±6Ma, respectively).Lead model ages on single-zircon crystals can be very precise and can give geologically more meaningful ages than those given by other methods, including the bulk-fraction conventional UPb method. The problem of episodic loss of radiogenic Pb can be avoided, making the grain concordant, and inherited grains or cores are easily detected.

Structure of the Variscan belt beneath the British and Armorican overstep sequences

Interpretation of gravity and magnetic data from southern England and of deep seismic data from the English Channel, Celtic Sea, and Paris basin provides a new picture of the buried Hercynian belt between France and England. The main structure corresponds to a mobile belt of crustal thickening that developed between the Midlands block, a northern block that had detached from the Armorican block prior to Hercynian time, and the Armorican block. The frontal collision between the two blocks recorded during the late Carboniferous was responsible for a pronounced narrowing of the belt between the Isle of Wight and Normandy. During collision, structural wedges were ejected in the channel and possibly also in the Hercynian basement beneath the Paris basin.

Is there an Archean Crust beneath Chesapeake Bay?

A crescent‐shaped terrane of continental crust centered on Chesapeake Bay is interpreted as being a collisional relic of Archean crust that was probably accreted during late Hercynian times. Arcuate gravity and magnetic anomalies, which represent the possible suture, separate Avalonian‐Carolinian terranes from what seems to be African‐like rocks seen in boreholes in the east Chesapeake Bay region. Seismic and heat flow data also suggest an important lithological variation in basement type across the suture.

Sources of Hercynian granitoids from the French Massif Central: Inferences from Nd isotopes and consequences for crustal evolution

The French Massif Central is characterized by a great abundance and variety of granitoids emplaced during Hercynian times (360-280 Ma). Distinct ϵ Nd(i) and ( 87 Sr 86 Sr) i are not associated with particular petrographic types; however, lower ϵ Nd(i)-values (−8.5) occur among the younger leucogranites from the western part of the massif. Intermediate ϵ Nd( i) (−6) correlate with aluminous monzogranites and higher values (−6 to −3) with calc-alkaline granodiorites. In the eastern Massif Central ϵ Nd(i) are homogeneous at the regional scale. In the western Massif Central, the leucogranites and the monzogranite-granodiorites display different trends of ϵ Nd( i) with emplacement ages. From these results, extreme models involving a depleted mantle, or an old (Archean or Early Proterozoic) source may be ruled out. The trend of increasing ϵ Nd( i) with decreasing age observed in the monzogranite-granodiorite group suggests either an increasing contribution of a mantle component, or the progressive melting of more mafic, more refractory crustal sources, or both. The reverse trend displayed by the leucogranites might be explained in terms of the upward migration of partial melting across an isotopically zoned metasedimentary pile. The large amount of granites may be the result of favourable processes during the collision tectonics, combined with the unusual abundance of fertile materials, enriched in water and heat-producing elements, in the pre-Carboniferous crust.

Detachment faulting and late Paleozoic epithermal Ag-base-metal mineralization in the Spanish central system

Hydrothermal activity during late Hercynian time resulted in epithermal silver-base-metal (Pb-Zn-Cu) vein formation in the eastern part of the Spanish central system. During the Hercynian orogeny, the central Iberian crust was thickened by compressional tectonics, heated, weakened, and subsequently overthickened by massive late Hercynian granitic intrusions. Subsequently, the central Iberian crustal welt underwent extensional collapse through lithosphere-scale, low-angle detachment faulting. The detachment systems evolved through tectonic denudation, isostatic rebound, and upward arching to define an extensional province much like the U.S. Basin and Range. Andesitic volcanism and hydrothermal activity occurred during extension, inducing epithermal-type hydrothermal convecting systems that leached, transported, and precipitated silver and base metals along fractures crosscutting the Hiendelaencina metamorphic core complex.