Resetting of the Rb–Sr whole-rock isotope system of an Ordovician microgranite during Devonian low-grade metamorphism

1989 ◽  
Vol 126 (6) ◽  
pp. 675-679 ◽  
Author(s):  
J. A. Evans
Keyword(s):  
Low Grade ◽  
Sedimentary Sequence ◽  
Sr Isotope ◽  
Grade Metamorphism ◽  
Volcanic Group ◽  
North Wales ◽  
Isotope System

AbstractThe Bwlch y Cywion microgranite intrudes an Ordovician sedimentary sequence in northeast Snowdonia, North Wales. Its age is recorded by hornfelses within its metamorphic aureole which give an Ordovician age of 454 ± 20 Ma. The whole-rock Rb–Sr isotope systems of the intrusion, however, and of an associated ash-flow tuff and a rhyolite of the Llewelyn Volcanic Group (Caradoc), were reset during low-grade metamorphism and give Devonian ages of 392±11 Ma, 392±5 Ma, and 405 ± 6 Ma, respectively.

Palaeoenvironmental signals from stromatolites of the Mesoproterozoic Stoer Group, N.W. Scotland

2021 ◽  
Author(s):  
Paul Dennis ◽  
Bridie Duchesne ◽  
Alina Marca
Keyword(s):  
Large Scale ◽  
Sedimentary Environment ◽  
Lacustrine Sediments ◽  
Prima Facie ◽  
Global Ocean ◽  
Low Grade ◽  
Terrestrial Environment ◽  
Climate Conditions ◽  
Grade Metamorphism ◽  
Isotope System

<p>The sedimentary environment and notably the climate conditions that pertained during deposition of the Mesoproterozoic (~1200 Ma) Torridonian Stoer Group have been subject to debate for some time. On one hand it has been proposed that, despite the low palaeolatitude, the Group is largely represented by fluvio-lacustrine sediments deposited under cold, possibly glacial conditions.<span>  </span>On the other hand, evidence and arguments have been put forward in favour of either a marine, or arid to semi-arid terrestrial environment. Contributing to this debate, in this study we focus on thin calcitic layers within the Clachtoll formation and younger Poll a’ Mhuilt member that may represent stromatolites, or stromatolite like deposits. Whilst recent work has cast doubt on the biogenic origin of these calcite layers, suggesting they may be either evaporitic or detrital in origin, we believe that much of the petrographic and isotope evidence is equivocal. Focusing on large scale morphology, sedimentary structures, micro-fabrics and mineralogy we<span>  </span>present new evidence for the biogenicity of these deposits. A key difficulty is resolving both diagentic (pressure solution, stylolite development and neomorphism) and later grain growth fabrics associated with low grade metamorphism from unaltered fabrics and grains. In combination with bulk (δ<sup>13</sup>C and δ<sup>18</sup>O) and clumped isotope (Δ<sub>47</sub>) studies we find that whilst the Stoer Group has undergone low grade metamorphism with maximum temperatures of ca. 120<sup>o</sup>C the isotope system has remained closed with respect to exchange with diagenetic and metamorphic fluids. The implication is that the very depleted δ<sup>18</sup>O values for the calcites of -18‰<sub>VPDB</sub> are characteristic of the original environmental conditions. Meteoric water values would need to be as low as -15 to -20‰<sub>VSMOW</sub> for precipitation of the calcite at ambient Earth surface temperatures. This is <em>prima facie</em> evidence that the deposits are terrestrial and not marine and at face value also implies cold conditions with isotopically depleted rainfall. We cannot rule out, however, that precipitation sourced from a global ocean that is significantly depleted in <sup>18</sup>O as suggested by some models may contribute to explaining the very depleted isotope signatures and apparent low temperatures.</p>

Low-grade metamorphism and accretion tectonics: Southern Uplands terrain, Scotland

1985 ◽  
Vol 49 (352) ◽  
pp. 335-344 ◽  
Author(s):  
A. E. S. Kemp ◽  
G. H. J. Oliver ◽  
J. R. Baldwin
Keyword(s):  
Subduction Zones ◽  
Structural Data ◽  
Low Grade ◽  
Structural Level ◽  
Concomitant Increase ◽  
Illite Crystallinity ◽  
Grade Metamorphism ◽  
Trench Slope ◽  
Reflectance Data ◽  
Trajectory Characteristic

AbstractPrevious studies of low-grade metamorphism in the Southern Uplands accretionary terrain indicated prehnite-pumpellyite facies/anchizone conditions developed throughout the area, except for local preservation of trench-slope sediments and an accreted seamount at zeolite facies/advanced diagenetic grade. New graptolite reflectance data are presented that show a general northward increase in temperature in the Southern Uplands. The results from two cross-strike traverses in the southern and central belts in contemporaneous sequences, using illite crystallinity, illite lateral spacing (bo) , and graptolite reflectance, indicate the development of systematic accretion-related low-grade metamorphism. Well-developed and constant anchizone conditions occur throughout the NE (Langholm) traverse, associated with common, F1 accretion-related folding and a regionally penetrative S1 cleavage. In the SW (Kirkcudbright) traverse, however, the youngest, last accreted packets are preserved at a transitional diagenetic stage and lack a penetrative S1 cleavage. Illite crystallinity, graptolite reflectance, and bo increase systematically northward through earlier accreted packets, reaching values of the NE traverse only at the northern end. The concomitant increase of bo with illite crystallinity suggests the relatively high P-low T trajectory characteristic of subduction zones. Integration of metamorphic and structural data relates increasing intensity of aceretion-related F1 folding, developmertt of S1 fabric, and onset of later fold phases to grade of metamorphism and structural level within the accretionary pile.

Neodymium and strontium isotopic dating of diagenesis and low-grade metamorphism of argillaceous sediments

1994 ◽  
Vol 58 (5) ◽  
pp. 1471-1481 ◽  
Author(s):  
Urs Schaltegger ◽  
Peter Stille ◽  
Naoual Rais ◽  
Alain Piqué ◽  
Norbert Clauer
Keyword(s):  
Low Grade ◽  
Isotopic Dating ◽  
Grade Metamorphism
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