The Effects of High-Grade Metamorphism on Cr-Spinel from the Archean Sittampundi Complex, South India

We investigated the crystal and structural behavior of Cr-bearing spinels from the Archean chromitites of Sittampundi (India), which had been subjected to very high-grade metamorphism. The structural data show that their oxygen positional parameters are among the highest ever recorded for Cr-bearing spinels with similar Cr# and Mg# and very similar to those found for other Archean occurrences. The general agreement between electron microprobe and Mössbauer data indicates that the analyzed spinels are stoichiometric. It is therefore most likely that the PH2O and Ptotal values as well as both the oxygen fugacity and the temperature reached during high-grade metamorphism inhibited the possibility of the non-stoichiometry of chromites, contrary to what can happen in ophiolites, where non-stoichiometry has recently been documented.

Progressive Low-Grade Metamorphism Reconstructed from the Raman Spectroscopy of Carbonaceous Material and an EBSD Analysis of Quartz in the Sanbagawa Metamorphic Event, Central Japan

Low-grade metamorphic temperature conditions associated with the Sanbagawa metamorphic event were estimated by the Raman spectroscopy of carbonaceous material (RSCM) in pelitic rocks and an electron backscatter diffraction (EBSD) analysis of the quartz in siliceous rocks. Analytical samples were collected from the Sanbagawa metamorphic complex, the Mikabu greenstones, and the Chichibu accretionary complex in the eastern Kanto Mountains, central Japan. Previously, low-grade Sanbagawa metamorphism was only broadly recognized as pumpellyite–actinolite facies assigned to the chlorite zone. The RSCM results indicate metamorphic temperatures of 358 °C and 368 °C for the chlorite zone and 387 °C for the garnet zone of the Sanbagawa metamorphic complex, 315 °C for the Mikabu greenstones, and 234–266 °C for the Chichibu accretionary complex. From the EBSD analyses, the diameter of the quartz grains calculated by the root mean square (RMS) approximation ranges from 55.9 to 69.0 μm for the Sanbagawa metamorphic complex, 9.5 to 23.5 μm for the Mikabu greenstones, and 2.9 to 7.3 μm for the Chichibu accretionary complex. The opening angles of the c-axis fabric approximate 40–50°, presenting temperatures of 324–393 °C for the Sanbagawa metamorphic complex and the Mikabu greenstones. The temperature conditions show a continuous increase with no apparent gaps from these low-grade metamorphosed rocks. In addition, there exists an empirical exponential relationship between the estimated metamorphic temperatures and the RMS values of the quartz grains. In this study, integrated analyses of multiple rock types provided valuable information on progressive low-grade metamorphism and a similar approach may be applied to study other metamorphic complexes.

Retrograded Eclogite From Chicheng, North China Craton: New Insights Into the Fidelity of U-Pb-Hf-O Isotopes in Zircon During High-grade Metamorphism

Zircon is the most abundantly used mineral for dating igneous and metamorphic events and for tracing source characteristics. Understanding the geochemical behavior of the U-Pb-Hf-O isotope systems during high-grade metamorphism is therefore important for accurate interpretation of the isotopic information. We report zircon U-Pb-Hf-O isotopes and trace elements of retrograded eclogites and host gneisses from Chicheng, North China Craton, with the aim to obtain new insights into the fidelity of U-Pb-Hf-O isotopes in zircon as recorders of high-grade metamorphism. U-Pb dating suggested that the Chicheng mélange experienced eclogite facies metamorphism at ~1.84 Ga, and then exhumed to amphibolite facies at 320–300 Ma. Zircons with Paleoproterozoic ages formed in metamorphic melts-derived from the gneiss during the eclogite facies metamorphism. Zircons with ages of 300–320 Ma formed by recrystallization of peak metamorphic zircons during fluid-assisted amphibolite-facies retrograde metamorphism. This process led to the near-complete resetting not only of U-Pb ages but also of Hf-O isotopic compositions of the peak metamorphic zircons, while preserve REE patterns. These results contrast with the sluggish Hf diffusion rate predicted from experimental studies, and support findings that isotopic data from metamorphic zircons in retrograded high-grade metamorphic rocks need not be faithful recorders of their sources.

Strain localization mechanism of graphitic carbon-bearing rocks: Constraints from microstructure, texture and graphite geothermometry

<p><strong>Abstracts:</strong></p><p>Graphitic carbon-bearing rocks can occur in low- to high-grade metamorphic units. In low-grade matamorphic rocks, graphitic carbon is often associated with brittle fault gouge whereas in middle- to high-grade metamorphic rocks, graphitic carbon commonly occurs in marble, schist or paragneiss. Previous studies showed that carbonaceous material gradually ordered from the amorphous stage, e.g. graphitization, is mainly controlled by increasing thermal metamorphism and has a good correlation with the metamorphic temperature. Besides, this ordered process is irreversible and the resulting structure is not affected by late metamorphism. Subsequently, the degree of graphitization is believed to be a reliable indicator of peak temperature conditions in the metamorphic rock. In this contribution, based on detailed field observations, the variably deformed and metamorphosed graphitic gneisses to phyllites, located within the footwall and hanging-walls unit of the Cenozoic Ailaoshan-Red River strike-slip shear zone are studied. According to lithological features and temperature determined by Raman spectra of carbonaceous material, these graphitic rocks and deformation fabrics are divided into three types. Type I is represented by medium–grade metamorphism and strongly deformed rocks with an average temperature of 509 °C and a maximum temperature of 604 °C. Type II is affected by low-grade metamorphism and deformed rocks with an average temperature of 420 °C. Type III is affected by lower–grade metamorphism and occurs in weakly deformed/undeformed rocks with an average temperature of 350 °C. Slip–localized micro–shear zone and laterally continuous or discontinuous slip planes constituted by graphitic carbon aggregates are developed in Types I and II. The electron back–scattered diffraction (EBSD) lattice preferred orientation (LPO) patterns of graphitic carbon grains were firstly observed in comparison with LPO patterns of quartz and switch from basal <a>, rhomb <a> to prism <a> slip systems, which indicate increasing deformation temperatures. According to the graphitic slip–planes, micro–shear zones and mylonitic foliation constituted by graphitic carbon minerals, we also propose that the development of fine–grained amorphous carbon plays an important role in rheological weakening of the whole rock during progressive ductile shearing.</p><p><strong>Key Words:</strong> graphitic carbon, strain localization, graphitic thermometry, slip–localized micro–shear zone, rheological weakening</p>

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

<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>