Mark W. Martin

Extinction of Cloudina and Namacalathus at the Precambrian-Cambrian boundary in Oman

Biostratigraphic, carbon isotope, and U-Pb zircon geochronological data from the Ara Group of Oman indicate an abrupt last appearance of Cloudina and Namacalathus coincident with a large-magnitude, but short-lived negative excursion in the carbon isotope composition of seawater that is globally coincident with the Precambrian-Cambrian boundary. U-Pb zircon age data from an intercalated ash bed directly define this negative excursion to be at 542.0 ± 0.3 Ma, consistent with previous age constraints from Siberia and Namibia. Combined with the global biostratigraphic record, these new data strengthen hypotheses invoking mass extinction within terminal Proterozoic ecosystems at or near the Precambrian-Cambrian boundary.

Timing of recovery from the end-Permian extinction: Geochronologic and biostratigraphic constraints from south China

Four volcanic-ash beds bracket the Early-Middle Triassic boundary, as defined by conodont biostratigraphy, in a stratigraphic section in south China. High-precision U-Pb dates of single zircons allow us to place the Early to Middle Triassic (Olenekian-Anisian) boundary at 247.2 Ma. Magnetic-reversal stratigraphy allows global correlation. The new dates constrain the Early Triassic interval characterized by delayed biotic recovery and carbon-cycle instability to ∼5 m.y. This time constraint must be considered in any model for the end-Permian extinction and subsequent recovery.

U-Pb zircon dating and Sr isotope systematics of the Vindhyan Supergroup, India

The Vindhyan Supergroup of central India, the focus of many paleontological studies, has been reported to contain Cambrian small shelly fossils, Ediacaran fossils, trace fossils, and Proterozoic microfossils and carbonaceous megafossils. New U-Pb zircon and 87 Sr/ 86 Sr isotopic data from the Lower Vindhyan Supergroup require that the rocks are latest Paleoproterozoic to earliest Mesoproterozoic in age. Two rhyolitic volcanic horizons from the Deonar Formation, between the Kajrahat and Rohtasgarh Limestones and below the unit containing trace fossils, yield U-Pb zircon ages of 1631 ± 5 Ma and 1631 ± 1 Ma. The Kajrahat and Rohtasgarh Limestones of the Semri Group that are below and above the reported Mesoproterozoic trace fossils have 87 Sr/ 86 Sr ratios of 0.70460 and 0.70479, respectively. The Bhander Limestone from the Upper Vindhyan Supergroup has an 87 Sr/ 86 Sr ratio of 0.70599, consistent with a Neoproterozoic age for this formation. These results indicate that the Kajrahat Limestone is of latest Paleoproterozoic age and the Rohtasgarh Limestone is of probable Mesoproterozoic age. These findings are in conflict with the report of Cambrian small shelly fossils and fossils of articulate brachiopods in the Rohtasgarh Limestone and argue for a Mesoproterozoic age for the formation that contains the alleged trace fossils. Reports of an Ediacaran fossil Spriggina (?) from the Lower Vindhyan Supergroup from the northern margin of the Vindhyan Basin suggest either incorrect stratigraphic correlation of units or misidentification of this fossil.

Metamorphism, melting, and extension: Age constraints from the High Himalayan Slab of southeast Zanskar and northwest Lahaul

We present evidence for two distinct stages of Tertiary metamorphism (M1 and M2) in the High Himalayan Slab of southeast Zanskar and northwest Lahaul, as well as evidence for an older, pre‐Himalayan metamorphism (pre‐M1). The M1 was a regional Barrovian‐type event related to crustal shortening and thickening of the Indian plate margin, while M2 was associated with crustal melting and the emplacement of the Gumburanjon leucogranite into the Zanskar Shear Zone at the top of the slab. U‐Pb dating of metamorphic and magmatic accessory phases constrains the timing of M1 between 30 and 37 Ma and the crystallization and emplacement age of the Gumburanjon leucogranite at 21–22 Ma. Inherited accessory phases in metamorphic and magmatic samples suggest that the protoliths of the slab are at least Lower Paleozoic in age and that they experienced a major pre‐M1 thermal perturbation at ca. 450–500 Ma. Whether this was associated with a regional Barrovian‐type metamorphism or whether it was a thermal event related to the intrusion of large Cambro‐Ordovician granites remains uncertain. The 40Ar/39Ar cooling ages of muscovites from metamorphic schists range from \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

U-Pb zircon age for the Umkondo dolerites, eastern Zimbabwe: 1.1 Ga large igneous province in southern Africa–East Antarctica and possible Rodinia correlations

22.0\pm 0.6

Syncontractional extension and exhumation of deep crustal rocks in the east Greenland Caledonides

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U-Pb and 40Ar/39Ar constraints on the Fjord Region Detachment Zone : a long-lived extensional fault in the central East Greenland Caledonides

20.4\pm 0.6