Jörg A. Pfänder

MPI‐DING reference glasses for in situ microanalysis: New reference values for element concentrations and isotope ratios

We present new analytical data of major and trace elements for the geological MPI-DING glasses KL2-G, ML3B-G, StHs6/80-G, GOR128-G, GOR132-G, BM90/21-G, T1-G, and ATHO-G. Different analytical methods were used to obtain a large spectrum of major and trace element data, in particular, EPMA, SIMS, LA-ICPMS, and isotope dilution by TIMS and ICPMS. Altogether, more than 60 qualified geochemical laboratories worldwide contributed to the analyses, allowing us to present new reference and information values and their uncertainties (at 95% confidence level) for up to 74 elements. We complied with the recommendations for the certification of geological reference materials by the International Association of Geoanalysts (IAG). The reference values were derived from the results of 16 independent techniques, including definitive (isotope dilution) and comparative bulk (e.g., INAA, ICPMS, SSMS) and microanalytical (e.g., LA-ICPMS, SIMS, EPMA) methods. Agreement between two or more independent methods and the use of definitive methods provided traceability to the fullest extent possible. We also present new and recently published data for the isotopic compositions of H, B, Li, O, Ca, Sr, Nd, Hf, and Pb. The results were mainly obtained by high-precision bulk techniques, such as TIMS and MC-ICPMS. In addition, LA-ICPMS and SIMS isotope data of B, Li, and Pb are presented.

Timing of accretion and collisional deformation in the Central Asian Orogenic Belt: implications of granite geochronology in the Bayankhongor Ophiolite Zone

Abstract Growing evidence suggests that the mechanism of Palaeozoic continental growth in Central Asia was by subduction–accretion with punctuated collisions that produced ophiolitic sutures between accreted blocks. The Bayankhongor ophiolite is the largest ophiolite in Mongolia and possibly all of Central Asia, and is interpreted to mark the collisional suture between the Baidrag and Hangai continental blocks. New 207Pb/206Pb zircon evaporation ages for granite plutons and dykes that intrude the ophiolite and its neighbouring lithotectonic units suggest that the ophiolite was obducted at c. 540 Ma at the beginning of a collisional event that lasted until c. 450 Ma. The new data, combined with that of previous studies, indicate regional correlation of isotopic ages north-westward from Bayankhongor to southern Tuva. These data record oceanic crust formation at c. 570 Ma, followed by approximately 30 million years of subduction–accretion that culminated in obduction of ophiolites, collision related metamorphism, and magmatism in the period c. 540–450 Ma. Correlation of isotopic-age data for the ophiolites of western Mongolia and southern Tuva suggests that the ophiolites define a major collisional suture in the Central Asian Orogenic Belt (CAOB) that defines the southern and western margins of the Hangai continental block.

Source composition and melting temperatures of orogenic granitoids: constraints from CaO/Na2O, Al2O3/TiO2 and accessory mineral saturation thermometry

Granitoid melts, generated experimentally from various materials by fluid-absent melting, show characteristic major-element compositions that can be used to infer source characteristics and melting temperatures. CaO/Na2O ratios distinguish between pelite-derived melts (CaO/Na2O < 0.5) and melts derived from greywackes or igneous sources (CaO/Na2O: 0.3–1.5). Distinctly more mafic melts (granodiorites and quartz diorites) generated by fluid-absent melting of amphibolite can show even higher CaO/Na2O ratios, up to 10, although the majority of the melts have CaO/Na2O ratios between 0.1 and 3. Al2O3/TiO2 ratios reflect the melting temperature, and mathematical formulations are presented that allow using this ratio as a geothermometer for given source compositions. A comparison of temperatures from melting experiments with corresponding Al2O3/TiO2 values indicate a reasonably good correlation ( r 2: 0.70–0.91), demonstrating the usefulness of temperature estimates in granitoid rocks based on Al2O3/TiO2 systematics. Application to well investigated S-type and A-type granites and quartz diorites from the Damara Belt (Namibia) shows different CaO/Na2O and Al2O3/TiO2 ratios for all rock types, supporting their origin from different sources at different temperatures. For the quartz diorites, temperature estimates derived from Al2O3/TiO2 ratios, and those derived from apatite solubility in mafic rocks, agree within ± 20 °C. On the other hand, temperature estimates for A-type and S-type granites derived from Al2O3/TiO2 ratios are systematically higher by 50–150 °C compared with those from accessory mineral saturation, suggesting disequilibrium during partial melting of the lower crust.

Timing of deformation in the Sarandí del Yí Shear Zone, Uruguay: implications for the amalgamation of Western Gondwana during the Neoproterozoic Brasiliano–Pan‐African Orogeny

U-Pb and Hf zircon (sensitive high-resolution ion microprobe -SHRIMP- and laser ablation-inductively coupled plasma-mass spectrometry -LA-ICP-MS-), Ar/Ar hornblende and muscovite, and Rb-Sr whole rock-muscovite isochron data from the mylonites of the Sarandi del Yi Shear Zone, Uruguay, were obtained in order to assess the tectonothermal evolution of this crustal-scale structure. Integration of these results with available kinematic, structural, and microstructural data of the shear zone as well as with geochronological data from the adjacent blocks allowed to constrain the onset of deformation along the shear zone at 630–625 Ma during the collision of the Nico Perez Terrane and the Rio de la Plata Craton. The shear zone underwent dextral shearing up to 596 Ma under upper to middle amphibolite facies conditions, which was succeeded by sinistral shearing under lower amphibolite to upper greenschist facies conditions until at least 584 Ma. After emplacement of the Cerro Caperuza granite at 570 Ma, the shear zone underwent only cataclastic deformation between the late Ediacaran and the Cambrian. The Sarandi del Yi Shear Zone is thus related to the syncollisional to postcollisional evolution of the amalgamation of the Rio de la Plata Craton and the Nico Perez Terrane. Furthermore, the obtained data reveal that strain partitioning and localization with time, magmatism emplacement, and fluid circulation are key processes affecting the isotopic systems in mylonitic belts, revealing the complexity in assessing the age of deformation of long-lived shear zones.

Building the Pamir‐Tibetan Plateau—Crustal stacking, extensional collapse, and lateral extrusion in the Central Pamir: 2. Timing and rates

Geothermochronologic data outline the temperature-deformation-time evolution of the Muskol and Shatput gneiss domes and their hanging walls in the Central Pamir. Prograde metamorphism started before ~35 Ma and peaked at ~23–20 Ma, reflecting top-to- ~N thrust-sheet and fold-nappe emplacement that tripled the thickness of the upper ~7–10 km of the Asian crust. Multimethod thermochronology traces cooling through ~700–100°C between ~22 and 12 Ma due to exhumation along dome-bounding normal-sense shear zones. Synkinematic minerals date normal sense shear-zone deformation at ~22–17 Ma. Age-versus-elevation relationships and paleoisotherm spacing imply exhumation at ≥3 km/Myr. South of the domes, Mesozoic granitoids record slow cooling and/or constant temperature throughout the Paleogene and enhanced cooling (7–31°C/Myr) starting between ~23 and 12 Ma and continuing today. Integrating the Central Pamir data with those of the East (Chinese) Pamir Kongur Shan and Muztaghata domes, and with the South Pamir Shakhdara dome, implies (i) regionally distributed, Paleogene crustal thickening; (ii) Pamir-wide gravitational collapse of thickened crust starting at ~23–21 Ma during ongoing India-Asia convergence; and (iii) termination of doming and resumption of shortening following northward propagating underthrusting of the Indian cratonic lithosphere at ≥12 Ma. Westward lateral extrusion of Pamir Plateau crust into the Hindu Kush and the Tajik depression accompanied all stages. Deep-seated processes, e.g., slab breakoff, crustal foundering, and underthrusting of buoyant lithosphere, governed transitional phases in the Pamir, and likely the Tibet crust.

High‐resolution 40Ar/39Ar dating using a mechanical sample transfer system combined with a high‐temperature cell for step heating experiments and a multicollector ARGUS noble gas mass spectrometer

40Ar/39Ar dating of young ( 5 × 10−16 mol 36Ar is better than 0.5‰–1.0‰ (1σ, n = 4–8). We illustrate the system performance by 40Ar/39Ar dating of whole-rock samples and mineral separates from the Oman ophiolite as well as from the Siebengebirge, Heldburg, and Rhon volcanic provinces in Central Germany.

Proterozoic–Mesozoic history of the Central Asian orogenic belt in the Tajik and southwestern Kyrgyz Tian Shan: U-Pb, 40Ar/39Ar, and fission-track geochronology and geochemistry of granitoids

Multimethod geochronology (U-Pb zircon; 40 Ar/ 39 Ar hornblende, biotite, feldspar; apatite fission track) on granitoids, gneisses, and Cenozoic intramontane basin clastics of the Gissar-Alai ranges, South Tian Shan collisional belt, west of the Talas-Fergana fault, elucidates a history of Neoproterozoic magmatism, late Paleozoic magmatism and metamorphism, and Mesozoic−Cenozoic thermal reactivation. Zircon-core and grain-interior U-Pb ages of ca. 2.7−2.4, 2.2−1.7, 1.1−0.85, and 0.85−0.74 Ga tie the early evolution of the Gissar-Alai ranges to that of the Tarim craton. At least part of the Gissar range crystalline basement—the Garm massif—shows U-Pb zircon crystallization ages of ca. 661‒552 Ma (median ca. 609 Ma), again suggesting a Tarim craton connection. Tarim collided with the Middle Tian Shan block at ca. 310‒305 Ma, completing the protracted formation of the South Tian Shan collisional belt. The massive Gissar range granitoids intruded later (ca. 305‒270 Ma), contemporaneous with peak Barrovian-type metamorphism in the Garm massif rocks. Major- and trace-element compositions suggest that the Gissar granitoid melts have continental arc affinity. Zircon e Hf and whole-rock e Nd values of −2.1 to −6.9 and −2.7 to −7.2, respectively. and Hf-isotope crustal model and Nd-isotope depleted mantle model ages of ca. 1.0‒1.2 and ca. 1.1‒2.2 Ga, respectively, suggest significant input of Precambrian crust in the Gissar granitoid and Garm orthogneiss melts, consistent with the U-Pb ages of inherited and detrital zircons. The distinct ca. 661‒552 Ma Garm gneiss crystallization ages and the ca. 1.0−2.2 Ga model ages (and the lack of 2.4−3.4 Ga model ages) tie the Garm gneisses and the reworked crust of the Gissar range to the northern rim—the Kuqa and Kolar sections—of the Tarim craton, suggesting a united Karakum-Tarim craton. Although about contemporaneous with widespread postcollisional magmatism in the entire Tian Shan, the large volume and short duration of the Gissar range magmatism, including crustal thickening and prograde metamorphism during Tarim craton‒Middle Tian Shan block collision, and formation and closure of an oceanic back-arc basin (the Gissar basin), indicate its origin in a distinct setting. Combined, this likely resulted in midcrustal melting and upper-crustal batholith emplacement. Mafic dikes and pipes intruded at ca. 256−238 Ma (median ca. 241 Ma); the source region of the parental melts was within the asthenospheric mantle. The simplest interpretation for these basanites is that they were part of the Tarim flood basalt province; this would extend this province westward from the Tarim craton into the southwestern Tian Shan and imply that the relatively short-lived flood basalt event (ca. 290‒270 Ma) was followed by much less voluminous but longer-lasting hotspot magmatism. The 40 Ar/ 39 Ar and detrital apatite fission-track dates outline post−Gissar-Alai range granitoid emplacement cooling, Cimmerian collision events at the southern margin of Asia, Late Cretaceous crustal extension and local magmatism, and early Cenozoic shortening and burial in the far field of the India-Asia collision.

Building the Pamir–Tibet Plateau—Crustal stacking, extensional collapse, and lateral extrusion in the Pamir: 3. Thermobarometry and Petrochronology of Deep Asian Crust

Large domes of crystalline, mid–deep crustal rocks of Asian provenance make the Pamir a unique part of the India–Asia collision. Combined major-element and trace-element thermobarometry, pseudosections, garnet-zoning deconstruction, and geochronology are used to assess the burial and exhumation history of five of these domes. All domes were buried and heated sufficiently to initiate garnet growth at depths of 15–20 km at 37–27 Ma. The Central Pamir was then heated at ~10–20°C/Myr and buried at 1–2 km/Myr to 600–675°C at depths of 25–35 km by 22–19 Ma. The Shakhdara Dome in the South Pamir was heated at ~20°C/Myr and buried at 2–8 km/Myr to reach 750–800°C at depths of ≥50 km by ~20 Ma. All domes were exhumed at >3 km/Myr to 5–10 km depths and ~300°C by 17–15 Ma. The pressures, temperatures, burial rates, and heating rates are typical of continental collision. Decompression during exhumation outpaced cooling, compatible with tectonic unroofing along mapped large-scale, normal-sense shear zones, and with advection of near- or suprasolidus temperatures into the upper crust, triggering exhumation-related magmatism. The Shakhdara Dome was exhumed from greater depth than the Central Pamir domes perhaps due to its position farther in the hinterland of the Paleogene retrowedge and to higher heat input following Indian slab breakoff. The large-scale thickening and coincident ~20 Ma switch to extension throughout a huge area encompassing the Pamir and Karakorum strengthens the idea that the evolution of orogenic plateaux is governed by catastrophic plate-scale events.

Anomalously old biotite 40Ar/39Ar ages in the NW Himalaya

Biotite 40 Ar/ 39 Ar ages older than corresponding muscovite 40 Ar/ 39 Ar ages, contrary to the diffusion properties of these minerals, are common in the Himalaya and other metamorphic regions. In these cases, biotite 40 Ar/ 39 Ar ages are commonly dismissed as “too old” on account of “excess Ar.” We present 32 step-heating 40 Ar/ 39 Ar ages from 17 samples from central Himachal Pradesh Himalaya, India. In almost all cases, the biotite ages are older than predicted from cooling histories. We document host-rock lithology and chemical composition, mica microstructures, biotite chemical composition, and chlorite and muscovite components of biotite separates to demonstrate that these factors do not offer an explanation for the anomalously old biotite 40 Ar/ 39 Ar ages. We discuss possible mechanisms that may account for extraneous Ar (inherited or excess Ar) in these samples. The most likely cause for “too-old” biotite is excess Ar, i.e., 40 Ar that is separated from its parent K. We suggest that this contamination resulted from one or several of the following mechanisms: (1) 40 Ar was released during Cenozoic prograde metamorphism; (2) 40 Ar transport was restricted due to a temporarily dry intergranular medium; (3) 40 Ar was released from melt into a hydrous fluid phase during melt crystallization. Samples from the Main Central Thrust shear zone may be affected by a different mechanism of excess-Ar accumulation, possibly linked to later-stage fluid circulation within the shear zone and chloritization. Different Ar diffusivities and/or solubilities in biotite and muscovite may explain why biotite is more commonly affected by excess Ar than muscovite.

Radial fast-neutron fluence gradients during rotating 40Ar/39Ar sample irradiation recorded with metallic fluence monitors and geological age standards

Characterizing the neutron-irradiation parameter J is one of the major uncertainties in 40Ar/39Ar dating. The associated uncertainty of the individual J-value for a sample of unknown age depends on the accuracy of the age of the geological standards, the fast-neutron fluence distribution in the reactor, and the distances between standards and samples during irradiation. While it is generally assumed that rotating irradiation evens out radial neutron fluence gradients, we observed axial and radial variations of the J-values in sample irradiations in the rotating channels of two reactors. To quantify them, we included three-dimensionally distributed metallic fast (Ni) and thermal- (Co) neutron fluence monitors in three irradiations and geological age standards in three more. Two irradiations were carried out under Cd shielding in the FRG1 reactor in Geesthacht, Germany, and four without Cd shielding in the LVR-15 reactor in Řež, Czech Republic. The 58Ni(nf,p)58Co activation reaction and γ-spectrometry of the 811 keV peak associated with the subsequent decay of 58Co to 58Fe allow one to calculate the fast-neutron fluence. The fast-neutron fluences at known positions in the irradiation container correlate with the J-values determined by mass-spectrometric 40Ar/39Ar measurements of the geological age standards. Radial neutron fluence gradients are up to 1.8 %/cm in FRG1 and up to 2.2 %/cm in LVR-15; the corresponding axial gradients are up to 5.9 and 2.1 %/cm. We conclude that sample rotation might not always suffice to meet the needs of high-precision dating and gradient monitoring can be crucial.