Robert D. Tucker

Ages of ultrahigh pressure metamorphism and protolith orthogneisses from the eastern Dabie Shan: U/Pb zircon geochronology

The Dabie Shan contains two rare geologic features. One, the occurrence of ultra-high pressure metamorphic rocks (peak P > 2.7 GPa = UHP) in the east-central part of this belt, and two, some of these UHP rocks are characterized by anomalously 18O-depleted oxygen isotopic compositions. Geochronological data are presented that constrain the age of the UHP metamorphism, the ages of protoliths, and potentially the age of hydrothermal alteration believed to be responsible for the anomalous oxygen isotopic compositions. The age of UHP metamorphism in the eastern Dabie Shan is dated at 218.5 ± 1.7 Ma using U/Pb in zircon separated from host gneisses. This age is identical within uncertainty with a less precise age using U/Pb in zircon separated from UHP eclogite (225.5 + 3/−6 Ma) and with previously published ages from UHP eclogites of the Dabie Shan. Map relationships strongly support an in-situ, as opposed to exotic, orgin of the UHP eclogites in this region. The identical ages of UHP metamorphism in the gneisses and eclogites provides additional support for this interpretation. The age of the protolith gneisses is dated as 772.5 ± 9.5 Ma based on the upper intercept of the zircons which accords well with other dates of granitic gneisses in the Qinling belt. The association of mafic and felsic magmatism is interpreted to reflect rifting along the northern margin of the Yangtze block at this time. These upper intercept ages may date the hydrothermal circulation of 18O-depleted meteoric water which gave rise to the anomalously 18O-depleted UHP protoliths of the Dabie Shan and Sulu region. The coincidence of these protolith ages with the early Sinian glaciation of South China is consonant with the suggestion that the highly anomalous oxygen isotopic compositions in this region date from this time. The upper intercept age of the eclogite is imprecisely determined at 447 +82/−79 Ma but is younger than the host gneisses and is not characterized by anomalous oxygen isotopic compositions.

New constraints on the ages of Neoproterozoic glaciations in south China

The most complete Neoproterozoic successions in south China contain three diamictite intervals in the Changan, Tiesiao, and Nantuo Formations. The youngest and most widespread Nantuo glacial deposit overlies the Datangpo Formation and underlies the fossil- rich Doushantuo Formation. Previous authors have correlated the Nantuo diamictite to either Sturtian or Marinoan glacial deposits elsewhere. Here we report sedimentary and δ13C chemostratigraphic data of the Doushantuo cap dolostone, which overlies the Nantuo Formation. Facies-dependent variation in δ13C is interpreted as evidence for spatial heterogeneity in δ13C and/or temporal diachroneity in the initiation of cap carbonate sedimentation. Sedimentary and chemostratigraphic data are indicative of a Marinoan age for the Nantuo glaciation. This inference is supported by a new U-Pb zircon age of 663 ± 4 Ma from a tuffaceous bed in the Datangpo Formation. The new date and other isotopic ages from south China constrain the age of the Changan and Tiesiao glaciation(s) as between 761 ± 8 Ma and 663 ± 4 Ma, and the Nantuo glaciation as between 663 ± 4 Ma and 599 ± 4 Ma.

New U-Pb zircon ages and the duration and division of Devonian time

Newly determined U‐Pb zircon ages of volcanic ashes closely tied to biostratigraphic zones are used to revise the Devonian time-scale. They are: (1) 417:6 1:0 Ma for an ash within the conodont zone of Icriodus woschmidti=I. w. hesperius (early Lochkovian); (2) 408:3 1:9 Ma for an ash of early Emsian age correlated with the conodont zones of Po. dehiscens‐Lower Po. inversus; (3) 391:4 1:8 Ma for an ash within the Po. c. costatus Zone and probably within the upper half of the zone (Eifelian); and (4) 381:1 1:3 Ma for an ash within the range of the Frasnian conodont Palmatolepis punctata (Pa. punctata Zone to Upper Pa. hassi Zone). U‐Pb zircon ages for two rhyolites bracketing a palyniferous bed of the pusillites‐lepidophyta spore zone, are dated at 363:8 2:2 Ma and 363:4 1:8 Ma, respectively, suggesting an age of363 Ma for a level within the late Famennian Pa. g. expansa Zone. These data, together with other published zircon ages, suggest that the base and top of the Devonian lie close to 418 Ma and 362 Ma, respectively, thus lengthening the period by20% over current estimates. We suggest that the duration of the Middle Devonian (Eifelian and Givetian) is rather brief, perhaps no longer than 11.5 Myr (394 Ma‐382.5 Ma), and that the Emsian and Famennian are the longest stages in the period with estimated durations of15.5 Myr and 14.5 Myr, respectively.

New U-Pb zircon dates for the Neoproterozoic Ghubrah glaciation and for the top of the Huqf Supergroup, Oman

The Huqf Supergroup, Oman, contains volcanic horizons with the potential to calibrate Neoproterozoic events and chemostratigraphy using U-Pb zircon geochronology. A tuffaceous bed near the base, within the lower (Ghubrah) diamictite and beneath a lower cap carbonate, provides the first U-Pb zircon date obtained from within a Neoproterozoic glacial deposit. This date of 723 +16/−10 Ma suggests a Sturtian age. Diamictites about 1 km higher in the section are overlain by the Hadash cap dolomite, which compares with cap dolomites above Marinoan glacial units elsewhere. A U-Pb zircon age of 544.5 ± 3.3 Ma from ignimbrites in the Fara Formation, near the top of the supergroup, is just beneath that of the Precambrian-Cambrian boundary (ca. 543 Ma), consistent with the presence of the Neoproterozoic skeletal-fossil Cloudina in correlative subsurface rocks. This revised chronology confirms the existence of four negative δ13C excursions between ca. 723 and 543 Ma, the lower two of which are clearly associated with glaciations.

U‐Pb Geochronology and Isotope Geochemistry of the Archean and Proterozoic Rocks of North‐Central Madagascar

New U‐Pb zircon ages and Sm‐Nd and Rb‐Sr isotopic data are presented for orthogneisses from north‐central Madagascar, including Île Sainte Marie, Alaotra‐Beforona, Maevatanana, and Ambatolampy‐Ambatomarina. A migmatite tonalite gneiss from Île Sainte Marie is dated precisely at \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 ages of granitoid rocks in the north Dabie Complex, eastern Dabie Shan, China

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Slow cooling of deep crustal granulites and Pb-loss in zircon

\end{document} Ma and has a Sm‐Nd model age (TDM) of 3204 Ma, thereby establishing a Middle Archean age for the oldest, juvenile gneisses in northeast Madagascar. Dated orthogneisses, intrusive into the schist/paragneiss sequences, range in age between 2522 and 2494 Ma and have Sm‐Nd model ages (TDM) between 3207 Ma and 2541 Ma. These data establish a Late Archean or older age for two of the schist/paragneiss sequences of Madagascar and suggest that the cratonal regions of north‐central Madagascar and south India were once contiguous. Strontium and neodymium isotopic data from the Late Archean rocks are interpreted to reflect mixing between depleted mantle magmas and evolving Middle Archean crust. U‐Pb geochronology of other plutonic igneous rocks demonstrates that the Middle Neoproterozoic (800–640 Ma) represents an important period of igneous activity throughout north‐central Madagascar. In addition, a latest Neoproterozoic–Early Cambrian (580–520 Ma) period of high‐grade metamorphism and intrusive igneous activity is recorded in western and central parts of north Madagascar. We attribute this later activity to the effects of continental collision between East and West Gondwana.

U–Pb geochronology of Seychelles granitoids: a Neoproterozoic continental arc fragment

Published geologic maps, regional geological relations, and new U-Pb ages for intrusive igneous rocks in west-central Madagascar define a 450-km-long belt of rocks emplaced in middle Neoproterozoic time. We report precise U-Pb zircon and baddeleyite ages for 11 coeval gabbro and granitoid plutons from the Itremo region, located in the southern part of this belt. The ages for all gabbroic and granitic plutons and deformed equivalents define an ∼25 m.y. period of magmatic activity between 804 and 779 Ma (at maximum uncertainty). Granitoids intrusive into the Quartzo-Schisto-Calcaire series provide a minimum depositional age of 791 Ma for this Mesoproterozoic platformal sedimentary sequence. Our results, combined with other recent U-Pb age determinations, define a belt of plutonic rocks in west-central Madagascar emplaced between 804 and 776 Ma. We propose that these middle Neoproterozoic rocks constitute the root of a continental magmatic arc emplaced at the time of, or slightly preceding, the breakup of the Rodinian supercontinent. Neoproterozoic plate reconstructions place Madagascar on the putative margin of Rodinia, and therefore the plutonic belt in west-central Madagascar provides important constraints on the timing and extent of middle Neoproterozoic tectonic events in Rodinia and the critical period of Rodinia9s transformation into Gondwana.