Udo Zimmermann

Dating the Oldest Greenstone in India: A 3.51-Ga Precise U-Pb SHRIMP Zircon Age for Dacitic Lava of the Southern Iron Ore Group, Singhbhum Craton

This article reports a precise 3506.8 ± 2.3-Ma U-Pb SHRIMP zircon age for dacitic lava in a well-preserved low-grade metamorphic and low-strained greenstone belt succession of the southern Iron Ore Group, Singhbhum craton, India. This age makes the succession the oldest-known greenstone belt succession in India and one of the oldest low-strain greenstone successions in the world after the 3.51-Ga Coonterunah Group of the Pilbara craton, Western Australia, and the moderately deformed 3.54-Ga Theespruit Formation of the Barberton Greenstone Belt, Kaapvaal craton, South Africa. The geochemical composition of the dacitic lava and related volcanic rocks suggests that they formed in a volcanic arc setting. The succession also contains a major ∼120-m-thick oxide facies banded iron formation that distinguishes it from the slightly older successions of the Pilbara and Kaapvaal cratons. This banded iron formation may well be one of the oldest and most well preserved, and together with associated volcanics, it may have immediate implications for understanding >3.5-Ga surface and tectonic processes on Earth.

Provenance studies of very low- to low-grade metasedimentary rocks of the Puncoviscana complex, northwest Argentina

Abstract A provenance study of Neoproterozoic to Lower Cambrian rocks for the entire Puncoviscana Basin was conducted, using 119 samples from 15 different outcrops. Petrographic data (Qt60–80, F15–35, L5–20, P/F 0.2–0.4, Lv/L = 0) show a composition comparable to foreland-basin successions. Lithoclasts are of metamorphic and metasedimentary origin. Volcanic debris is detected only in the form of sanidine, and volcanic lithoclasts were probably decomposed to form pseudo-matrix. Framework clasts are sub-angular to sub-rounded, and the rocks are poorly sorted. Major element geochemistry shows a moderate to high Chemical Index of Alteration (56–77) and failed to provide coherent provenance and rock classification. Trace element geochemistry suggests a rhyodacitic composition overall. Rare earth element patterns are comparable to those of model upper continental crust (UCC), as are concentrations of Nb, Ta, Ti, Th-Sc and Eu/Eu* (0.45–0.87; 95% between 0.4 and 0.7); reworking signatures are not detected. The uniform mineralogical and geochemical composition reflects supra-crustal source(s) for the entire basin, including significant metamorphic rock debris. The Puncoviscana complex is interpreted as a peripheral Pampean foreland basin, fed mainly from an eastern fold-thrust belt, but includes relicts of pre- and syn-collisional magmatic activity as well. A source area of UCC composition to the west is represented by the Arequipa block.

Crustal Evolution along the Early Ordovician Proto‐Andean Margin of Gondwana: Trace Element and Isotope Evidence from the Complejo Igneo Pocitos (Northwest Argentina)

The Paleozoic geodynamic evolution of the proto‐Andean margin of Gondwana was characterized by extensive subduction processes associated with the docking of several terranes. The major and trace element as well as Sr‐Nd‐Pb isotope composition of plutonic rocks from the Complejo Igneo Pocitos (CIP) are used here to constrain the processes of magma genesis in the Early Ordovician continental arc along the proto‐Andean margin of Gondwana in northern Chile and northwestern Argentina. U‐Pb sphene ages of \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

0.3 byr of drainage stability along the Palaeozoic palaeo-Pacific Gondwana margin; a detrital zircon study

476\pm 2

Archean seismites of the Ventersdorp Supergroup, South Africa

\end{document} Ma for the CIP date the time of intrusion. The CIP comprises two magmatic suites, each with a distinct and homogeneous isotope composition that was acquired by magmatic processes in the deep crust or mantle. Both suites are strongly enriched in incompatible trace elements and display a Sr‐Nd isotope composition intermediate between mid‐ocean ridge basalt (MORB) and Andean continental crust. The petrological, geochemical, and isotopic characteristics are best modeled by generation of the CIP melts in a subduction setting. Multicomponent modeling shows that the trace element and isotope inventory of group 1 samples can be generated by admixture of ∼0.5% melts from subducted sediments and 1%–2% of MORB‐derived fluids to the mantle source. Constant initial Sr and Nd isotope compositions at varying SiO2 contents for both suites exclude significant modification of the melts by assimilation and fractional crystallization processes in upper‐crustal levels. Assimilation of group 1 parental magmas in the deep crustal MASH (melting, assimilation, storage, and homogenization) zone with 20%–25% crustal material, however, produced the group 2 parental magmas. The CIP can be considered to be the northern continuation of the Early Ordovician Famatinian arc known from central Argentina. As indicated by the Nd‐Pb isotope composition of the CIP, this arc was built on continental crust of Middle Proterozoic age. Similar protolith ages have been reported for the Famatinian arc in the southern Andes and for the Late Precambrian–Early Paleozoic metamorphic basement of the central Andes. The identification of MASH processes requires a thick continental crust in order to enable melting and assimilation at the base of the crust. Thickening of the crust can be related to the Pampean orogeny at ∼500 Ma. Magmatic rocks from the Famatinian arc in the southern Andes display a more crustlike Sr‐Nd isotope signature compared with the CIP, suggesting that crustal thicknesses in the Early Ordovician continental arc decreased from south to north.

Sedimentary Provenance of the Neoarchean Ventersdorp Supergroup, Southern Africa: Shedding Light on the Evolution of the Kaapvaal Craton during the Neoarchean

High-precision SHRIMP U–Pb zircon dating yields a late Palaeoarchaean age (3290 ± 8.6 Ma) for a large, unmetamorphosed, weakly peraluminous TTG body (the Keonjhargarh–Bhaunra pluton) in the Singhbhum craton of eastern India. One inherited subhedral zircon grain gave a concordant age of 3495.9 ± 5.3 Ma and Nd isotope characteristics show a juvenile trend with eNd t ~ 0 and T DM 3395–3453 Ma. The data support a model of typical Archaean crustal evolution until late Palaeoarchaean times for the Singhbhum craton, which is in contrast to the more southerly Bastar craton where Palaeoarchaean non-TTG granites have been identified. These data demonstrate the diachronous development of continental crustal blocks now forming the basement of the eastern and central peninsular of India.

Evaluation of porosity change during chemo-mechanical compaction in flooding experiments on Liège outcrop chalk

The palaeo-Pacific margin of Gondwana in the present-day south–central Andes is marked by tectonic activity related to subduction and terrane accretion. We present detrital zircon U–Pb data encompassing the Palaeozoic era in northern Chile and northwestern Argentina. Cathodoluminescence images reveal dominantly magmatic zircon barely affected by abrasion and displaying only one growth phase. The main age clusters for these zircon grains are Ediacaran to Palaeozoic with an additional peak at 1.3–0.9 Ga and they can be correlated with ‘Grenvillian’ age, and the Brasiliano, Pampean, and Famatinian orogenies. The zircon data reveal main transport from the nearby Ordovician Famatinian arc and related rocks. The Silurian sandstone units are more comparable with Cambrian units, with Brasiliano and Transamazonian ages (2.2–1.9 Ga) being more common, because the Silurian deposits were situated within or east of the (extinct) Famatinian arc. Hence, the arc acted as a transport barrier throughout Palaeozoic time. The complete suite of zircon ages does not record the accretions of exotic terranes or the Palaeozoic glacial periods. We conclude that the transport system along the palaeo-Pacific margin of Gondwana remained stable for c. 0.3 byr and that provenance data do not necessarily reflect the interior of a continent. Hence, inherited geomorphological features must be taken into account when detrital mineral ages are interpreted. Supplementary material: U–Pb data, CL images, and detailed geological maps are available at www.geolsoc.org.uk/SUP18796.

Comparative Study of Five Outcrop Chalks Flooded at Reservoir Conditions: Chemo-mechanical Behaviour and Profiles of Compositional Alteration

Outcrop chalk of late Campanian age (Gulpen Formation) from Liege (Belgium) was flooded with in a triaxial cell for 516 days under reservoir conditions to understand how the non-equilibrium nature of the fluids altered the chalks. The study is motivated by enhanced oil recovery (EOR) processes because dissolution and precipitation change the way in which oils are trapped in chalk reservoirs. Relative to initial composition, the first centimeter of the flooded chalk sample shows an increase in MgO by approximately 100, from a weight percent of 0.33% to 33.03% and a corresponding depletion of CaO by more than 70% from 52.22 to 14.43 wt.%. Except for Sr, other major or trace elements do not show a significant change in concentration. Magnesite was identified as the major newly grown mineral phase. At the same time, porosity was reduced by approximately 20%. The amount of in the effluent brine remained unchanged, whereas was depleted and enriched. The loss of and gain in are attributed to precipitation of new minerals and leaching the tested core by approximately 20%, respectively. Dramatic mineralogical and geochemical changes are observed with scanning electron microscopy–energy-dispersive x-ray spectroscopy, nano secondary ion mass spectrometry, x-ray diffraction, and whole-rock geochemistry techniques. The understanding of how fluids interact with rocks is important to, for example, EOR, because textural changes in the pore space affect how water will imbibe and expel oil from the rock. The mechanisms of dissolution and mineralization of fine-grained chalk can be described and quantified and, when understood, offer numerous possibilities in the engineering of carbonate reservoirs.