Igor M. Villa

Dating the Indian continental subduction and collisional thickening in the northwest Himalaya: Multichronology of the Tso Morari eclogites

Multichronometric studies of the low-temperature eclogitic Tso Morari unit (Ladakh, India) place timing constraints on the early evolution of the northwest Himalayan belt. Several isotopic systems have been used to date the eclogitization and the exhumation of the Tso Morari unit: Lu-Hf, Sm-Nd, Rb-Sr, and Ar-Ar. A ca. 55 Ma age for the eclogitization has been obtained by Lu-Hf on garnet, omphacite, and whole rock from mafic eclogite and by Sm-Nd on garnet, glaucophane, and whole rock from high-pressure metapelites. These results agree with a previously reported U-Pb age on allanite, and together these ages constrain the subduction of the Indian continental margin at the Paleocene-Eocene boundary. During exhumation, the Tso Morari rocks underwent thermal relaxation at about 9 ± 3 kbar, characterized by partial recrystallization under amphibolite facies conditions ca. 47 Ma, as dated by Sm-Nd on garnet, calcic amphibole, and whole rock from metabasalt, Rb-Sr on phengite, apatite, and whole rock, and Ar-Ar on medium-Si phengite from metapelites. Ar-Ar analyses of biotite and low-Si muscovite from metapelites, which recrystallized at <5 kbar toward the end of the exhumation, show that the Tso Morari unit was at upper crustal levels ca. 30 Ma. These results indicate variable exhumation rates for the Tso Morari unit, beginning with rapid exhumation while the Indian margin subduction was still active, and later proceeding at a slower pace during the crustal thickening associated with the Himalayan collision.

Age of the Corsica–Sardinia rotation and Liguro–Provençal Basin spreading: new paleomagnetic and Ar/Ar evidence

Abstract The age of spreading of the Liguro–Provencal Basin is still poorly constrained due to the lack of boreholes penetrating the whole sedimentary sequence above the oceanic crust and the lack of a clear magnetic anomaly pattern. In the past, a consensus developed over a fast (20.5–19 Ma) spreading event, relying on old paleomagnetic data from Oligo–Miocene Sardinian volcanics showing a drift-related 30° counterclockwise (CCW) rotation. Here we report new paleomagnetic data from a 10-m-thick lower–middle Miocene marine sedimentary sequence from southwestern Sardinia. Ar/Ar dating of two volcanoclastic levels in the lower part of the sequence yields ages of 18.94±0.13 and 19.20±0.12 Ma (lower–mid Burdigalian). Sedimentary strata below the upper volcanic level document a 23.3±4.6° CCW rotation with respect to Europe, while younger strata rapidly evolve to null rotation values. A recent magnetic overprint can be excluded by several lines of evidence, particularly by the significant difference between the in situ paleomagnetic and geocentric axial dipole (GAD) field directions. In both the rotated and unrotated part of the section, only normal polarity directions were obtained. As the global magnetic polarity time scale (MPTS) documents several geomagnetic reversals in the Burdigalian, a continuous sedimentary record would imply that (unrealistically) the whole documented rotation occurred in few thousands years only. We conclude that the section contains one (or more) hiatus(es), and that the minimum age of the unrotated sediments above the volcanic levels is unconstrained. Typical back-arc basin spreading rates translate to a duration ≥3 Ma for the opening of the Liguro–Provencal Basin. Thus, spreading and rotation of Corsica–Sardinia ended no earlier than 16 Ma (early Langhian). A 16–19 Ma, spreading is corroborated by other evidences, such as the age of the breakup unconformity in Sardinia, the age of igneous rocks dredged west of Corsica, the heat flow in the Liguro–Provencal Basin, and recent paleomagnetic data from Sardinian sediments and volcanics. Since Corsica was still rotating/drifting eastward at 16 Ma, it presumably induced significant shortening to the east, in the Apennine belt. Therefore, the lower Miocene extensional basins in the northern Tyrrhenian Sea and margins can be interpreted as synorogenic “intra-wedge” basins due to the thickening and collapse of the northern Apennine wedge.

Very high rates of cooling and uplift in the Alpine belt of the Betic Cordilleras, southern Spain

Cooling-rate estimates of 150-350 °C/m.y., suggesting magnitudes of uplift and exhumation of ∼15-20 km at rates of 5-10 km/m.y., are proposed for the final stage of the orogenic development in a major part of the Betic Cordilleras. The results are based on a combination of radiometric and paleontological dates. Six isotopic chronometers (muscovite and biotite whole-rock Rb-Sr, muscovite and biotite 40 Ar/ 39 Ar, biotite K-Ar, and whole-rock Rb-Sr) yield analytical ages between 23 and 18.5 Ma for rocks from Alpine nappe complexes. Nappe-sealing marine sedimentary rocks contain early Miocene foraminifera and nannoplankton indicating minimum ages of 18-15.5 Ma. The very high estimates of cooling and uplift-exhumation rates suggest tectonic unroofing, which is tentatively connected geodynamically to lithospheric slab detachment and concomitant diapirism in the upper mantle, and extensional tectonics in the crustal section.

Geochronology of the Hout River Shear Zone and the metamorphism in the Southern Marginal Zone of the Limpopo Belt, Southern Africa

Abstract In this paper monazite U–Pb and zircon evaporation dates, stepleaching Pb/Pb results on garnet, staurolite and kyanite, and hornblende Ar/Ar data are presented which constrain the timing of granulite facies metamorphism in the Southern Marginal Zone of the Limpopo Belt and its thrusting onto the Kaapvaal Craton. The Southern Marginal Zone of the Limpopo Belt is considered to be a lower crustal equivalent of the northern Kaapvaal Craton. Granulite exhumation is associated with southward thrusting along the Hout River Shear Zone which is a set of thrust and strike slip shear zones. Zircon ages for the Matok Intrusive Complex which was emplaced within the zone during this thrusting (charno-enderbites: 2671±4 Ma; granodiorites: between 2667 and 2664 Ma) have previously been interpreted as evidence for rapid exhumation of the Southern Marginal Zone within only ∼7 Ma. We have obtained a U/Pb date of 2691±7 Ma for monazite from the Bandelierkop Quarry in the Southern Marginal Zone, interpreted as the age of high grade metamorphism. A single zircon evaporation Pb/Pb date of 2643±1 Ma from a leucosome band at the same locality may indicate longer lasting metamorphism or decompression melting during exhumation. Anatexis of metapelitic xenoliths within the Matok Intrusive Complex was dated at 2663±4 Ma by U/Pb on monazite, indistinguishable from existing zircon ages for this complex. Pb/Pb step leaching dates obtained on synkinematically grown garnet (2691±20 Ma), staurolite (2712±37 Ma) and kyanite (2672±51 Ma) from the Khavagari Hills in the Giyani Greenstone Belt, in the immediate footwall of the Hout River Shear Zone, indicate that early thrusting was contemporaneous with peak metamorphism in the Southern Marginal Zone. Ar/Ar dating and geochemistry on syntectonic hornblende separates from the same shear zone system yielded disturbed spectra and indicated multiple populations, probably reflecting repeated or continuous tectonic activity of the Hout River Shear Zone up to about 2600 Ma.

The rift-to-drift transition in the North Atlantic: A stuttering start of the MORB machine?

We report U-Pb and 39Ar-40Ar measurements on plutonic rocks recovered from the Ocean Drilling Program (ODP) Legs 173 and 210. Drilling revealed continental crust (Sites 1067 and 1069) and exhumed mantle (Sites 1070 and 1068) along the Iberia margin and exhumed mantle (Site 1277) on the conjugate Newfoundland margin. Our data record a complex igneous and thermal history related to the transition from rifting to seafloor spreading. The results show that the rift-to-drift transition is marked by a stuttering start of MORB-type magmatic activity. Subsequent to initial alkaline magmatism, localized mid-oceanic ridge basalts (MORB) magmatism was again replaced by basin-wide alkaline events, caused by a low degree of decompression melting due to tectonic delocalization of deformation. Such “off-axis” magmatism might be a common process in (ultra-) slow oceanic spreading systems, where “magmatic” and “tectonic” spreading varies in both space and time.

Single mineral dating by the PbPb step-leaching method: Assessing the mechanisms

Stepwise PbPb leaching (PbSL) has been successfully used to date rock-forming silicates directly linked to metamorphic reactions defining a PT path. The two features of PbSL are an increase of precision and a control on accuracy: the former, by enhancing the 206Pb/204Pb and 207Pb/204Pb ratios, and the latter, by revealing heterochemical inclusions via the 208Pb/206Pb ratio and checking isotopic equilibrium with the host. The question of the need for inclusions as a prerequisite enabling PbSL dating was investigated on a centimenter-sized single crystal of museum-quality titanite. We obtained petrographic (optical microscope, SEM, electron microprobe, proton microprobe), chemical (ICP-MS), and isotopic (TIMS) data on pristine and increasingly leached splits of different grain sizes, as well as on leach solutions. The PbSL age of 1.00 Ga is identical to the concordant conventional UPb age. By use of isotopic and elemental correlation diagrams, we were able to resolve three isotopically distinct sources of Pb. Visible inclusions of K-feldspars as micro-crack fillings may contribute to the first 50% of common Pb (204Pb) release, and visible 5 μm zircons account, at least in part, for the residue. An additional effect noticed during the first leach steps is related to surface hydrolysis and consequent weakening of metal cation-oxygen bonds, which releases structurally bound common Pb substituting for Ca in the titanite lattice. The radiogenic Pb released during most leach steps has uniform 208Pb/206Pb ratios, suggesting that it is derived from a single mineral phase. Further, as different titanite grain sizes produce a different leach trajectory in Pb isotopic space, it can be concluded that this radiogenic Pb is actually derived from the titanite itself. Therefore, PbSL is indeed capable of discriminating common and radiogenic Pb from a single phase. During leaching, a well defined reaction front is seen to advance into the grains. Inside this front, the titanite is unaltered, and outside it, a silica-gel-like rim is formed, in which electron and proton microprobe data show retention of high field strength (HFS) elements. We propose that radiogenic Pb occurs in the tetravalent state (due to recoil stripping) so that it behaves as an HFS element and is preferentially retained in the gel rim during leaching, relative to common divalent Pb. The mechanism by which a spread in Pb isotope data is obtained during PbSL is, therefore, explained by two processes controlling the release of cations from a mineral grain into a leach solution: (1) an effective surface dependent hydrolysis of metal cations at the inward migrating reaction front whose influence decreases as leaching proceeds and (2) an increasingly dominant remobilization of HFS-elements and radiogenic Pb from the micro-environment of the leached gel-like layer which acts as a selective HFS cation absorber. In conclusion, Pb/Pb dating by stepwise leaching can be effective as a dating tool, even in minerals free from micro-inclusions.

Geochronology of the Larderello geothermal field: new data and the “closure temperature” issue

The Larderello geothermal field is generally accepted to have been produced by a granite intrusion at 4–9 km depth. Hydrothermal parageneses and fluid inclusions always formed at temperatures greater than or equal to the current ones, which implies that the field has always undergone a roughly monotonic cooling history (fluctuations < 40 K) since intrusion of the granite at 4 Ma. The heat required to maintain the thermal anomaly over such a long period is supplied by a seismically anomalous body of ≈ 32000 km3 rooted in the mantle. Borehole minerals from Larderello are thus a unique well-calibrated natural example of thermally induced Ar and Sr loss under geological conditions and time spans. The observations (biotites retain Ar above 450°C) agree well with other, albeit less precise, geological determinations, but contrast with laboratory determinations of diffusivity from the literature. We therefore performed a hydrothermal experiment on two Larderello biotites and derived a diffusivity DLab(370°C)=5.3·10-18 cm2s-1, in agreement with published estimates of diffusivity in annite. From DLab and the rejuvenation of the K/Ar ages we calculate maximum survival times at the present in-hole temperatures. They trend smoothly over almost two orders of magnitude from 352 ka to 5.3 ka, anticorrelating with depth: laboratory diffusivities are inconsistent not only with geological facts, but also among themselves. From the geologically constrained lifetime of the thermal anomaly we derive a diffusivity DG(370°C)=3.81·1021 cm2s-1, 3±1 orders of magnitude lower than DLab. The cause of these discrepancies must be sought among various laboratory artefacts: overstepping of a critical temperature T*; enhanced diffusivities in “wet” experiments; presence of fast pathway (dislocation and pipe) diffusion, and of dissolution/reprecipitation reactions, which we imaged by scanning electron microscopy. These phenomena are minor in geological settings: in the absence of mineral transformation reactions, complete or near-complete resetting is achieved only by volume diffusion. Therefore, laboratory determinations will necessarily result in apparent diffusivities that are too high compared to those actually effecting the resetting of natural geochronometers.

Climate variability in the SE Alps of Italy over the past 17 000 years reconstructed from a stalagmite record

Stalagmite SV1 from Grotta Savi, located at the SE margin of the European Alps (Italy), is the first Alpine speleothem that continuously spans the past c. 17kyr. Extension rate and δ18Oc record for the Lateglacial probably reflect a combination of temperature and rainfall, with rainfall exerting the dominant effect. Low speleothem calcite δ18 Oc values were recorded from c. 14.5 and 12.35 kyr, during GI-1 (Bolling— Allerod) interstadial, which in our interpretation, was warm and wet. The GS-1 (Younger Dryas) was characterized by a shift to heavier δ18 Oc, coinciding with δ13Cc enrichment and extremely low extension rate (<8 μm/year). These characteristics indicate that GS-1 climate was cool and dry in the SE Alps. Calibration using historical data revealed that there is a positive δ18Oc/dT relationship. A 1°C rise in mean annual temperature should correspond to c. 2.85% increase of SV-1 δc18Oc. We reconstructed a slow and steady temperature rise of c. 0.5°C since 10 kyr BP, in broad agreement with reconstructions from pollen data for SE Europe. Stalagmite SV1 indicates that climate variability in the SE Alps has been influenced by the Mediterranean Sea for the past c. 17 kyr.

Direct measurement of Ar diffusion profiles in a gem-quality Madagascar K-feldspar using the ultra-violet laser ablation microprobe (UVLAMP)

Abstract Controversy surrounding the mechanisms and controls on argon diffusion in K-feldspar has led us to undertake direct diffusion measurements on a crystal with simple microtextures, over a range of temperatures. Measurements of argon diffusion profiles in a gem-quality iron-rich orthoclase heated in a cold seal apparatus, have been undertaken in situ using an ultra-violet laser ablation microprobe (UVLAMP) technique. The results agree very closely with the previously determined bulk values for Benson Mines orthoclase (activation energy (E)=43.8±1 kcal mol−1) and vacuum furnace cycle-heating studies of K-feldspars (E=46±6 kcal mol−1). However, instead of defining a single activation energy (E) and diffusion coefficient (Do), the data yield two sets of parameters: a low-temperature (550–720°C) array with an E of 47.2±2.5 kcal mol−1 (198.2±10.5 kJ mol−1) and a Do of 0.0374+0.1123−0.0281 cm2 s−1, and a high-temperature (725–1019°C) array with an E of 63.8±3.4 kcal mol−1 (268.0±14.3 kJ mol−1) and a Do of 55.0+225.5−44.2 cm2 s−1. The new results closely reproduce two sets of apparent activation energies previously measured in cycle-heating studies of Madagascar K-feldspar (40±3 and 57±3 kcal mol−1). Previous interpretations of the two arrays have included multiple domains with variable activation energies and fast track diffusion. However, the UV depth profile analyses indicate simple diffusion to the grain surface and importantly, diffusion radii calculated by combining the UVLAMP and cycle-heating data, are the same as the physical grain sizes used in the experiments, around 1 mm. Vacuum furnace stepped heating experiments on slowly cooled K-feldspars have been interpreted as showing diffusion radii of around 6 μm and indicate complex populations of sub-grains. This study indicates that Madagascar K-feldspar and thus probably all gem-quality K-feldspars act as single diffusion domains and that short-circuit (or pipe) diffusion was not an important loss mechanism. An apparent diffusion compensation relationship in the stepped heating data for Madagascar K-feldspar implies that similar relationships seen in other K-feldspars are a result of a range of diffusion mechanisms.

Multichronometric Evidence for an In Situ Origin of the Ultrahigh‐Pressure Metamorphic Terrane of Dabieshan, China

U‐Pb zircon data on gneisses indicate a Late Proterozoic protolith at ∼700 Ma at Bixiling and a Late Archean to Early Proterozoic for the Shuanghe sheet within the ultrahigh‐pressure metamorphic terrane of Dabieshan (China). U‐Pb zircon and Sm‐Nd dates on the gneisses of the Shuanghe sheet constrain a metamorphic peak at ∼230 Ma. Rb‐Sr and 40Ar‐39Ar analyses of phengite from five gneisses in the Bixiling Complex yield ages at 198–212 Ma, overlapping the 190–220‐Ma age on eclogites. The host gneisses record a similar metamorphic evolution as the coesite‐bearing eclogites since both the peak and the retrograde evolution were contemporaneous. This argues for an in situ tectonic relationship.