The Romanesque

Paper by Alexandra Wilhelmsen and Heri Bert Bartsch

Petrology and geochronology of "muscovite age standard" B4M

Muscovite B4M, distributed in 1961 as an age standard, was ground under ethanol. Five grain size fractions were obtained and characterized by X-ray diffraction. They display a mixing trend between a phengitic (enriched in the fraction 20 µm). High-pressure phengite is preserved as a relict in retrograde muscovite. Electron microprobe analyses of the distributed mineral separate reveal at least four white mica populations based on Si, Al, Mg, Na, Fe and F. Rb/K ratios vary by one order of magnitude. Rb–Sr analyses link the mineralogical heterogeneity to variable Rb/Sr and 87Sr/86Sr ratios. The grain size fractions define no internal isochron. Relict fine-grained phengite gives older ages than coarse-grained retrograde greenschist facies muscovite. The inverse grain size–age relationship also characterizes 39Ar/40Ar analyses. Cl/K anticorrelates with step ages: Cl-rich coarse muscovite is younger than Cl-poor fine relict phengite. Sr and Ar preserve a similar isotopic inheritance despite peak metamorphism reaching 635±20 °C. A suitable mineral standard requires that its petrological equilibrium first be demonstrated. Relicts and retrograde reaction textures are a guarantee of isotopic disequilibrium and heterogeneous ages within single crystal at the micrometre scale

Geochemistry, geochronology and isotope geochemistry of eocene dykes intruding the Ladakh batholith

Eocene dykes intruding the Ladakh batholith were sampled along the southern margin of the Trans-himalayan plutonic arc in Ladakh, NW-India. Approximately 30 dykes were encountered in the 40 km trail between Leh and Hemis Shugpachan. The dykes in the east of the field are trending E to NE and those in the west trending N to NW, exhibiting sub-parallel orientations within each area. Eighteen dykes were sampled (two of them multiple times) and subjected to petrographic, geochemical and isotopic analyses. They exhibit various degrees of differentiation from basaltic to rhyolitic compositions and are mainly composed of plagioclase, quartz, hornblende (s.l.) and/or biotite and magnetite. Furthermore, dykes in the eastern part of the field area contain quartz xenocrysts resulting from crustal assimilation, while no relict quartz was found in the west. The dykes exhibit alteration phases and features suggesting that they underwent autometamorphism, i.e. hydration reactions due to igneous cooling. Whereas the dykes in the east of the field area record low-T alteration, the mineral parageneses in the west are typical for alteration at elevated temperatures typical for greenschist metamorphic facies. Al-in-hornblende barometry performed on Magnesio-hornblende and Tschermakitic-hornblende phenocrysts of the least altered dyke indicates formation in upper-amphibolite metamorphic facies conditions and pressures of about 6 kbar corresponding to an intrusion depth of approximately 20 km. Major and trace element analyses and Rb-Sr and Sm-Nd isotope analyses revealed a stunning variability in geochemistry and isotopic composition amongst the coeval dykes. All dykes exhibit LREE enrichment and HREE depletion as well as negative Tb and Nb anomalies characteristic for subduction-related intrusives and extrusives. Their REE patterns support a clear subdivision into chemically distinct groups. The group hypothesis was further tested and found valid using statistical tools designed to assess similarity/dissimilarity amongst individuals of a group with a common ancestor, such as hierarchical cluster analysis and multidimensional scaling. The dykes are cogenetic, but clearly not consanguineous, i.e. have not formed from one, progressively differentiating magma chamber. The variability observed in Sr-Nd isotopes can be explained by the dykes having undergone differing degrees of crustal assimilation. In particular the dykes in the east containing quartz xenocrysts show negative iiNd) and positive N(Sr) values caused by crustal assimilation, whereas the dykes in the west with no quartz xenocrysts exhibit positive qqNd) and N(Sr) near zero. 39Ar-40Ar dating by incremental heating of several hornblende-bearing dykes revealed crystallization ages between 50 and 54 Ma, whereas two biotite-bearing dykes gave ages of 45 and 37 Ma, likely to be cooling or recrystallisation ages. The combination of structural field evidence with petrographic, petrologic, geochemical, isotopic and geochronological analyses demonstrates that the dykes, although sharing a common origin, i.e. having formed in the same tectonic setting at roughly the same time, have undergone further geological processes leading to an unexpected diversification of the dykes. These findings provide ample scope for further in-depth and breadth investigations on “late-magmatic dykes” in the future.published_or_final_versionEarth SciencesDoctoralDoctor of Philosoph

Petrology and geochronology of ‘muscovite age standard’ B4M

<p>Muscovite B4M, distributed in 1961 as an age standard, was ground under ethanol. Five grain size fractions were obtained and characterized by X-ray diffraction. They display a mixing trend between a phengitic (enriched in the fraction <0.2 µm) and a muscovitic component (predominant in the fraction >20 µm). High-pressure phengite is preserved as a relict in retrograde muscovite. Electron microprobe analyses of the distributed mineral separate reveal at least four white mica populations based on Si, Al, Mg, Na, Fe and F. Rb/K ratios vary by one order of magnitude. Rb–Sr analyses link the mineralogical heterogeneity to variable Rb/Sr and ⁸⁷Sr/⁸⁶Sr ratios. The grain size fractions define no internal isochron. Relict fine-grained phengite gives older ages than coarse-grained retrograde greenschist facies muscovite. The inverse grain size–age relationship also characterizes ³⁹Ar/⁴⁰Ar analyses. Cl/K anticorrelates with step ages: Cl-rich coarse muscovite is younger than Cl-poor fine relict phengite. Sr and Ar preserve a similar isotopic inheritance despite peak metamorphism reaching 635±20 °C. A suitable mineral standard requires that its petrological equilibrium first be demonstrated. Relicts and retrograde reaction textures are a guarantee of isotopic disequilibrium and heterogeneous ages within single crystal at the micrometre scale. </p