Alain Cocherie

Geochemical investigations of the origin of the Manaslu leucogranite (Himalaya, Nepal)

The Himalayan orogeny is characterized by an intraplate subduction zone. The thrusting of the Tibetan Slab over the foreland is probably responsible for the origin of the Manaslu-type leucogranites. This geochemical study tries to examine the crustal origin of the granite, to verify whether the paragneisses of the Tibetan Slab are the most probable parent material and to determine the subsequent petrogenetic processes. The Rb-Sr systematics suggest that 1. (1) the scatter of data points in the 87Sr86Srvs. 87Rb86Sr diagram may be due to the heterogeneity of the initial 87Sr86Sr ratios. 2. (2) the initial isotopic ratios are very high (0.730 → 0.760) and are in agreement with those of the gneisses of the Tibetan Slab at the time of the melting. Pb isotopic ratios are homogeneous. They are also consistent with a crustal prehistory. The REE and Th contents are unexpectedly low in comparison with the much higher contents of the parent material. Monazite left in the residue of melting or more probably extracted in an early stage of the magmatic history may explain the observed REE and Th abundances. Alternatively, field observations show that the emplacement of the granite has been accompanied by a very strong hydrothermal activity. In consequence, the low REE and Th contents of the granite could also be due to the loss by solution during prolonged fluid circulation throughout the granitic melt and the Sr and Pb isotope distribution to the effect of late to post magmatic fluids the isotopic character of which was determined by the underlying Tibetan Slab.

Geochronology of polygenetic monazites constrained by in situ electron microprobe Th-U-total lead determination: implications for lead behaviour in monazite

Abstract Monazite grains are generally concordant and, with an electron microprobe, give Th-U-total Pb isochron ages in agreement with conventional U-Pb data. Monazites from four samples of post-Archean migmatite from the Ivory Coast record two main ages: an Archean age at 2.80 Ga partly overprinted by a late migmatization event at 2.03 Ga. The in situ electron microprobe determination has also shown a third event that systematically appears at 2.72 Ga for all the monazites from two of the four studied rocks. The three events at 2.80, 2.72, and 2.03 Ga recorded by the Th-U-total Pb system in monazite were each obtained on independent homogeneous parts of a single grain with no sign of Pb diffusion. Thus the oldest monazite remained closed during a high-grade resetting (migmatization) at a temperature estimated at around 700°C, and also remained closed during the growth of secondary monazite. Conventional U-Pb data (isotope dilution) on monazite from the same rocks did not allow precise age determinations because the representative points were highly (up to 83%) discordant and did not fit well along a chord in the Concordia diagram. The U-Pb isotope dilution method gave a lower intercept age of 2029 ± 25 Ma in agreement with the electron microprobe age, although with an unusually high uncertainty due to a poor fit of the data. The Pb-evaporation method gave an old Archean age at 2830 ± 7 Ma and Proterozoic ages ranging from 2417 ± 10 to 2074 ± 7 Ma. Thus, although the oldest Archean age obtained with the Pb-evaporation and electron microprobe methods is similar within the analytical error, the Proterozoic ages obtained by the Pb-evaporation method are significantly older than the Th-U-total Pb Proterozoic age obtained with the electron microprobe. This discrepancy is presumed to be due to the influence, in the Pb-evaporation method, of radiogenic lead contained within Archean zones of the monazite grains; in other words, it means that migmatization took place later than 2074 ± 7 Ma, and probably around 2030 Ma. The second Archean event at 2.72 Ga revealed by high spatial resolution of the electron microprobe could not be determined by other methods. The Th-U-total Pb method appears to be an inexpensive alternative method for dating simple monazites with a precision close to 20 Ma, even if the U-Pb isotopic dilution technique remains the reference method. Moreover, owing to the very high spatial resolution (1 μm) of the electron microprobe, it appears to have no competitor for dating complex polygenetic monazites, especially when more than two events are recorded. Homogeneous zones of different ages coexist in a single grain with no major indication of lead diffusion. A procedure for age and error calculations is defined.

Electron-microprobe dating as a tool for determining the closure of Th-U-Pb systems in migmatitic monazites

Abstract High spatial resolution dating of monazite by the electron-probe microanalyzer (EPMA) enables systematic and detailed studies of small minerals. Like zircon, monazite records the complex history undergone by the host rocks. Recent improvements in the statistical treatment of many in situ data now make it possible to decipher the related thermal events and so obtain reliable and precise ages. Our work shows that a significant number of individual spot analyses is required to reach such precise information (i.e., more than 30.40 data). Using the examples of monazites from three migmatites and one granite, we show how to select the most efficient method of age calculation according to the U and Th geochemistry of the grains, or grain domains, that we are trying to date. Three situations may be met: (1) monazites exhibiting significant Th/U ratio variation, (2) monazites exhibiting a fairly constant Th/U ratio, but significant U + Th heterogeneity, and (3) monazites of constant U and Th concentrations. For the first case, a precise mean age can be calculated using a method of data reduction in the Th/Pb = f(U/Pb) diagram, whereby a precision of ±5−10 Ma (2σ) is commonly achieved. For the second case, an isochron age can be calculated according to the Pb = f(Th*) method, with a common precision of around 20 Ma (2σ), whereas for the third case, a simple weighted average age can be calculated. Using these approaches, coupled with a back-scattered electron image study, we demonstrate that inheritance is probably as common for monazite as for zircon. In addition, the combination of high spatial resolution and precise age determination show the limited extent of Pb diffusion in monazite. Finally, an example from a migmatite from southern French Guiana demonstrates the especially robust behavior of the Th-U-Pb system in monazite. This system remains closed during late migmatization and during the subsequent zircon crystallization and zircon overgrowth of protolith zircons. The monazite yielded exactly the same age as the protolith zircons.

Radiometric dating of granitic rocks from the Central Bohemian Plutonic Complex (Czech Republic): constraints on the chronology of thermal and tectonic events along the Moldanubian-Barrandian boundary

abstract Single-zircon dating by step-wise evaporation has established that successive granitic intrusions were emplaced in the Central Bohemian Plutonic Complex (CBPC) during a short time span of about 10 Ma. In agreement with field data, the Požary trondhjemite, emplaced early at 351 ±11 Ma and subcontemporaneously with the Sazava granodiorite dated at 349 ±12 Ma, was followed by the Blatna granodiorite at 346 ±10 Ma. The magnesium-potassium-rich units (durbachites) indicate younger ages both for the Certovo Břemeno melagranite at 343 ±6 Ma (within the CPBC) and for durbachite from the Třebic Massif (south-east of the CPBC) at 340 ±8 Ma. These data provide evidence that the sequence of intrusion and the age of the emplacement of the CBPC are comparable with those of other western Variscan batholiths (i.e. the Vosges or the French Massif Central) in similar structural environment.

Systematic use of trace element distribution patterns in log-log diagrams for plutonic suites

Trace element modelling has been widely used for petrogenetic interpretation of basaltic systems. This paper indicates how to select trace element pairs having very different bulk distribution coefficients (D) which when plotted on simple log-log diagrams permit the identification of the main magmatic process (magma mixing, partial melting, fractional crystallization) involved in the genesis of plutonic rocks. Fractional crystallization gives a straight line on such a diagram with a strong decrease of the compatible element whereas the concentrations of the element with D ⪡ 1 increase slowly. A similar evolution of the solids in equilibrium is observed and when data of at least one of the cumulates are directly available, it is possible to calculate the D and F parameters of the sequence of fractional crystallization. An example of this procedure is shown for a French Hercynian plutonic suite: the basic suite of Variscan Corsica.

Crust and mantle contributions to granite genesis — An example from the Variscan batholith of Corsica, France, studied by trace-element and NdSrO-isotope systematics

The Corsican batholith, formed during the Variscan orogeny, was studied with particular emphasis on the respective roles played by crustal- and mantle-derived material in granite genesis. Major- and trace-element data identify: the various types of magma and their genetic relationships; the magmatic processes that gave rise to the observed rocks; and the primitive liquids. OSrNd-isotope systematics constrain the origin of primitive liquids. Trace-element models provide data on the various source materials and their possible geodynamic setting. The evolution of two main calc-alkaline plutonic associations is compared: an older high-MgK association (U1) with both ultrapotassic mafic and silicic rocks, the latter ranging from monzonite to leucosyenogranite, and a younger calc-alkaline composite association (U2) involving a mafic cumulate sequence and granodiorite to leucomonzo-granite. Both types of granite are accompanied by mafic units that were non-cogenetic with surrounding granitic melts. Trace-element calculations indicate that a primitive liquid of monzodioritic composition gave rise to the U1 MgK granite by fractional crystallization, amphibole and titanite playing a major role, which, with increasing crystallization, gave decreasing REE contents without a strong negative Eu anomaly. The U2 calc-alkaline suite was formed by fractional crystallization involving feldspar and LREE-rich minerals, i.e. monazite, from a liquid of monzogranitic composition. Field and petrographical data identify magma mingling in both U1 and U2, between mafic and silicic rocks, but geochemical data only indicate how magma-mixing processes led to U2 granodiorite. Geochemical modelling shows that a single protolith of calculated graywacke composition yielded the two associations under different melting conditions: the high-MgK monzodiorite melt was formed after partial melting of a 30% non-modal batch of granulite-facies metamorphic protolith (low PH2O), but the calc-alkaline monzogranite was formed by the same process in an amphibolite-facies source of similar composition (higher pH2O). The primitive granite magmas of both associations show the same crustal characteristics, i.e.: Sri = 0.706–0.707; ϵNd (t) = −4.3 to −2.2 and δ18O = +7 to +8‰. This provides evidence that composition of both U1 and U2 granite melts probably was controlled by physico-chemical fusion rather than protolith-composition parameters. The U1 ultrapotassic mafic rock is thought to be of mantle origin, i.e. a deep source containing phlogopite (±garnet). Zone refining led to a significant increase of incompatible trace-element contents during ascent of the magma. Mantle-crust interaction lowered the La/Yb and 143Nd144Nd ratios, but the 87Sr86Sr ratio increased. Nevertheless, interaction with associated U1 MgK granite was not important and the crustal component is thought to be different from the associated granitic magma. In U2, the mafic cumulate sequence, clearly of mantle origin, has E-MORB characteristics and seems the result of 10% non-modal partial melting of a heterogeneous mantle of spinel or amphibole peridotite without garnet. Some interaction occurred with the associated granitic melt. The E-MORB character of the mafic rock indicates that a large part of the batholith, and at least the U2 magmatic association, was generated under post-collisional extensional conditions.

Geochemical comparison between Himalayan and Hercynian leucogranites

Abstract Isotopic (Sr, Pb, Nd, O), REE and trace element data from three Himalayan (Nepal) and six Hercynian (Brittany, France) leucogranites are compared. For the Himalayan granites—Makulu, Mustang and especially Manaslu—the 87Sr/86Sr versus 87Rb/86Sr scatter diagrams, and hence variability of the initial 87Sr/86Sr ratios, reflect heterogeneity of the source materials. These ratios are very high (0.730- > 0.770). For Manaslu, the very high 207Pb/204Pb ratios (∼ 15.8) at the corresponding 206Pb/204Pb ratio of ∼ 18.8, the very low 143Nd/144Nd ratios (ϵi Nd ∼ −13 to −17), and the very high 18O/16O ratios (∼ 12‰) confirm the crustal origin with the paragneisses of the Tibetan Slab Formation I as the probable parent source. For the six Hercynian granites with an individual RbSr isochron established for many of them, the initial 87Sr/86Sr ratios range from 0.706 to 0.717. The lead, neodymium and oxygen isotope compositions confirm their crustal source, of presumably Palaeozoic age but not specifically identified. The rare earth patterns and thorium contents are comparable to those for Himalayan granites. The similarities of the geochemical signatures (isotopes and trace elements) of the Himalayan and Hercynian leucogranites suggest that the petrogenetic processes were comparable and that the Himalayan continental collision model can be applied to the Hercynian. The depletion in REE and Th implies either interaction of the granitic liquids with fluid or, more probably, early precipitation of monazite. The increase in Sm/Nd in the granites compared to their parent materials implies that the neodymium model ages of primary mantle extraction are maximum rather than minimum ages. In contrast to the Hercynian leucogranites, the extremely radiogenic Sr of the Himalayan leucogranites reflects the very long crustal residence time before the melting event. The apparent uniformity of the initial 87Sr/86Sr ratios of the individual Hercynian granites may be related to an “aging effect” where the straight line fitting improves with increase in age of the pluton.

Genesis of a Variscan batholith: Field, petrological and mineralogical evidence from the Corsica-Sardinia batholith

Abstract The genesis of the Corsica-Sardinia batholith (CSB) can be explained in a continental collision setting. During the time of construction of the batholith, which is dated at about 70 Ma, three magmatic associations were emplaced: Mg-K calc-alkaline (U1), composite (mafic-silicic) calc-alkaline (U2) and post-collisional alkaline (U3). In addition, the level of intrusion became progressively shallower (from 5–6 kbar for U1 to less than Ikbar for U3) in a large-scale uplift. Associations U1 and U2 exhibit non-cogenetic mafic and felsic components. The processes and products of interaction between mantle- and crustal-derived melts are however different, and may be related to the thickness of the crust which controls both the parameters T, pCO2, pH2O andPtot, and the time of mingling of the products of anatexis and basaltic melts. Both U1 and U2 granitoids were formed by crystallization of a magma derived by fusion of a source of greywacke composition. The melting occurred successively under granulitic (U1) and amphibolite facies conditions (U2). In association U1, basaltic melts displaying potassic lamprophyre-like composition (up to 8% K2O) are of uncertain affinity. Furthermore, crustal interaction with the basic magma precludes precise definition of the source. In the U2 composite association, the primary tholeiitic character of the mantle-derived basic rocks is constrained by both mineralogical and geochemical data. “Dry” and “hydrated” subunits are distinguished among the basic rocks associated with the granitoids. No hybridization processes have been identified in the core of the mafic complexes of the dry subunits, but they are recognized at the level of emplacement at the margins at the contact with the granitoids. Contamination in the mafic hydrated subunit is due to the increase in water content of crustal origin in the magma during the ascent and emplacement of the basic complexes, leading to the development of abundant green amphibole. Extensive hybridization is restricted to the base of the crust where it led to incomplete mixing, yielding granodiorites and enclaves. The genesis and emplacement of the composite calc-alkaline U2 association took place in an extensional setting in an uplifting basement postdating a collisional event. During the Devonian, an eastern Austro-Alpine block was thrust onto the western “Ebro-Balearic” continent. The U1 Mg-K granitoids resulted from anatexis under conditions of pCO2 >pH2O; this took place at the base of the Austro-Alpine crust which was undergoing fusion due to its adiabatic ascent. The U2 calc-alkaline granitoid rocks were derived, in a hydrated environment, by fusion of the same Austro-Alpine crust in Corsica and of the Ebro-Balearic crust on Sardinia and in the Pyrenees. The conditions of interaction between mafic and felsic magmas may serve as a geodynamic marker providing information on the thickness of the crust in time and space and on the genesis of continental crust. The persistence during the Palaeozoic of dominantly tholeiitic mafic magmatism suggests that hot spot activity occurred in this area of southwestern Europe during pre-Variscan and Variscan times up to the time of opening of the Liguro-Piedmontese ocean, birthplace of the Alpine orogen.

Devonian geodynamic evolution of the Variscan Belt, insights from the French Massif Central and Massif Armoricain

The Paleozoic French Variscan Belt in Massif Central and Massif Armoricain is a collision belt that provides a good example of a suture zone where ophiolites are rare, and the frontal (i.e., the magmatic arc) part of the upper plate is not present. In the lower plate (or Gondwana), the continental rocks are subdivided into an Upper Gneiss Unit (UGU) and a Lower Gneiss Unit (LGU). The UGU experienced a high-pressure (and likely ultra-high-pressure) metamorphism followed by crustal melting during their exhumation. New chemical U-Th-Pb monazite ages and ion-probe U-Pb zircon ages on migmatites allow us to constrain the P-T-t paths followed by the UGU and LGU. By comparison with thermomechanical experiments, a possible geodynamic evolution scenario can be proposed for the Variscan convergence. The high-compression regime of continental subduction developed during the initial subduction of the northern margin of Gondwana under Armorica in Silurian times. This induced the formation of a new subduction zone in the back-arc basin, which is the youngest, hottest, and thus mechanically the weakest part of the overriding plate. As a result, the arc-back-arc basin domain has been almost totally subducted below Armorica. Only a limited part of the back-arc basin rocks remains exposed in the Devonian St-Georges-sur-Loire Unit. Subsequently, the continental subduction of Gondwana resumed with a steeper dip associated with low-compression regime that in turn allowed the high-pressure rocks to be exhumed and partly melted in Late Devonian times. Such a scheme depicts quite well the complexity of the Variscan Belt.

ASSESSMENT OF THE PRECISION AND ACCURACY OF LEAD-ISOTOPE RATIOS MEASURED BY TIMS FOR GEOCHEMICAL APPLICATIONS : EXAMPLE OF MASSIVE SULPHIDE DEPOSITS ( RIO TINTO, SPAIN)

The true reproducibility of lead-isotope measurements by thermal-ionization mass-spectrometer (TIMS) was assessed on both the international standard (NBS 982) and sulphide samples from the South Iberian Pyrite Belt (SIPB). Lead-isotope analyses were made on 21 pyrite and galena samples from Rio Tinto sulphide orebodies in the Spanish part of the SIPB. Independent lead-isotope analyses were made in the UM2 laboratory (Montpellier), using a VG Sector 54 mass spectrometer, and at BRGM (Orleans) using a Finnigan Mat 262 mass spectrometer. Internal precision and reproducibility of the isotope measurements were calculated for lead-isotope standards (NBS 982), and for pyrite and galena samples. The reproducibility (2σ) is 0.12% for 206Pb/204Pb, 0.16% for 207Pb/204Pb and 0.22% for 208Pb/204Pb while the internal precision is on average 0.01% for each isotope ratio (2σm). This indicates that the usually adopted errors of between 0.10% and 0.25% are appropriate for geological samples. TIMS has been the most common method for measuring U/Pb ratios by isotopic dilution, but since a few years ICP–MS has also been used for such determinations. We thus checked the reproducibility of U/Pb-ratio measurements by ICP–MS, in order to apply this method to sulphide samples. Independent measurements (2 to 7) for each of 20 analysed samples showed that the measured average reproducibility for U/Pb ratios is better than 5%. This method is thus suitable for determination of U/Pb ratios in sulphide samples and most other geological materials. Lead analyses for the Rio Tinto deposit were made of the pyritic and orebodies, and the stringers. This deposit, one of the biggest massive sulphides in the world, is remarkably homogeneous from a lead-isotope viewpoint, and no difference can be seen between pyritic and polymetallic orebodies. The isotope composition of the deposit can be considered as the average composition of the South Iberian crust during the Devonian–Early Carboniferous period of crustal fusion.