New detrital zircon U–Pb insights on the palaeogeographic origin of the central Sanandaj–Sirjan zone, Iran

New detrital U–Pb zircon ages from the Sanandaj–Sirjan metamorphic zone in the Zagros orogenic belt allow discussion of models of the late Neoproterozoic to early Palaeozoic plate tectonic evolution and position of the Iranian microcontinent within a global framework. A total of 194 valid age values from 362 zircon grains were obtained from three garnet-micaschist samples. The most abundant detrital zircon population included Ediacaran ages, with the main age peak at 0.60 Ga. Other significant age peaks are at c. 0.64–0.78 Ga, 0.80–0.91 Ga, 0.94–1.1 Ga, 1.8–2.0 Ga and 2.1–2.5 Ga. The various Palaeozoic zircon age peaks could be explained by sediment supply from sources within the Iranian microcontinent. However, Precambrian ages were found, implying a non-Iranian provenance or recycling of upper Ediacaran–Palaeozoic clastic rocks. Trace-element geochemical fingerprints show that most detrital zircons were sourced from continental magmatic settings. In this study, the late Grenvillian age population at c. 0.94–1.1 Ga is used to unravel the palaeogeographic origin of the Sanandaj–Sirjan metamorphic zone. This Grenvillian detrital age population relates to the ‘Gondwana superfan’ sediments, as found in many Gondwana-derived terranes within the European Variscides and Turkish terranes, but also to units further east, e.g. in the South China block. Biogeographic evidence proves that the Iranian microcontinent developed on the same North Gondwana margin extending from the South China block via Iran further to the west.

Paleogene granite magmatism in the north of the Truong Son belt and implication for crustal evolution

Abundant granitoids aged 24.59 Ma to 28.62 Ma were exposed along Phu Hoat high metamorphic zone, northern of the Truong Son belt, termed Na Khoun complex in Northern Laos (NL) and Ban Chieng complex in Western Vietnam (WV). Ten granitic samples were collected from these complexes show geochemical characteristics of high SiO2 and K2O contents, medium peraluminous that belong to S-type granites. Initial 87Sr/86Sr isotopic ratios and εNd(t) are broad values of 0.708507 to 0.74539 and -5.22 to -12.66, respectively, together with high 206Pb/204Pb (18.864-19.392), 207Pb/204Pb (15.736-15.841) and 208Pb/204Pb (39.224-40.080) which indicated crustal origin, we suggest that the NL-WV intrusion was associated with transpression form by the India-Asia collision events during Cenozoic.

Extension and early orogenic inversion along the basal detachment of a hyper-extended rifted margin: an example from the Central Pyrenees (France)

The North Pyrenean Zone (NPZ) inverts remnants of an Aptian-Cenomanian rifting during which subcontinental mantle was exhumed. These remnants contain a syn-rift HT-LP metamorphic domain, the Internal Metamorphic Zone (IMZ). New field and RSCM data and structural cross-sections constrain the structural and metamorphic relationships between the IMZ and the underlying low-grade NPZ. The IMZ is a tectonic nappe that overthrusts the European margin along the 3M Fault. Along this contact, the Tuc de Haurades peridotite is surrounded by tectonic breccia composed of ductilely deformed carbonate and sparse lherzolite clasts that passes upward into foliated marbles. Marbles contain top-to-south ductile shear, recording ongoing extensional deformation that marks the onset of HT metamorphism. During Early Cretaceous rifting, European Mesozoic sedimentary cover metamorphosed and its base brecciated as it slid basinward on Triassic salt onto exhumed mantle. As the exhumed mantle domain closed during early convergence, the detached metamorphosed cover was transported northward and thrust into the distal European margin, sampling lherzolite tectonic lenses. This triggered the first tectonic loading on the European plate. This study highlights the role of the IMZ in the early Pyrenean orogenic phase and gives new insights on the E-W diversity of structural setting of the NPZ peridotites.Table with RSCM temperatures and original and high quality photographs of the samples are available on the GSL Figshare portal https://doi.org/10.6084/m9.figshare.c.5539260.

Metamorphic pressure-temperature conditions of the Lützow-Holm Complex of East Antarctica deduced from Zr-in-rutile geothermometer and Al2SiO5 minerals enclosed in garnet

<p>The Zr content of rutile coexisting with zircon and quartz is mainly a function of the temperature condition and is calibrated as Zr-in-rutile geothermometers. Because of their robustness under high-temperature conditions, they have been applied to granulite facies rocks instead of the conventional Fe-Mg exchange type geothermometers to estimate more reliable temperature conditions. However, it is recently pointed out that in order for rutile to retain the primary Zr content, rutile must be chemically isolated from zircon and quartz during cooling. In this context, inclusion rutile separately enclosed in garnet can be considered to retain the primary Zr content at the time of entrapment, only if rutile, zircon, and quartz are all enclosed in a contemporaneous domain of the garnet.</p><p>In this study, we re-examined the pressure-temperature (<em>P-T</em>) conditions of high-grade pelitic gneisses from selected regions (Akarui Point, Skarvsnes, Skallen, and Rundvågshetta) of the Lützow-Holm Complex (LHC), East Antarctica. The LHC has been divided into the upper-amphibolite facies zone, the transitional zone, and the granulite facies zone, based on matrix mineral assemblages of mafic- to intermediate gneisses. Akarui Point is located in the transitional zone and others in the granulite facies zone.</p><p>While previous studies commonly applied the conventional Fe-Mg exchange type geothermometers, we applied the Zr-in-rutile geothermometer of Tomkins et al. (2007) to rutile grains enclosed in garnet that also encloses zircon, quartz, and Al<sub>2</sub>SiO<sub>5</sub> minerals. By utilizing the phosphorus zoning in garnet, we defined contemporaneous domains of the garnet and identified coexisting inclusion minerals in each domain. In this way, coexisting Al<sub>2</sub>SiO<sub>5</sub> minerals and rutile grains were utilized to constrain the <em>P-T</em> condition of each domain of the garnet.</p><p>As a result, samples from Akarui Point, Skarvsnes, and Skallen were shown to have experienced almost the same <em>P-T</em> conditions around the kyanite/sillimanite transition boundary (~ 830-850 °C/~ 11 kbar). This is significantly higher than the previously estimated peak condition of 770-790 °C/7.7-9.8 kbar based on the conventional garnet-biotite geothermometer in the case of Akarui Point. From Rundvågshetta, where ultrahigh-<em>T</em> metamorphism is reported by previous studies, higher-<em>T</em> condition (850 ± 15 °C/0.1 kbar to 927 ± 16 °C/12.5 kbar) than those of other three regions was confirmed from inclusion rutile in garnet enclosing sillimanite. Therefore, the traditional metamorphic zone mapping, which classified Akarui Point as belonging to the transitional zone, does not reflect the highest metamorphic grade attained. It should be noted that the regional <em>P-T</em> conditions estimated from inclusion minerals in this study is that of earlier higher-<em>P</em> metamorphic stage than the regional <em>P-T</em> conditions determined by the metamorphic zone mapping utilizing matrix mineral assemblages. This result indicates that the Zr-in-rutile geothermometer is a powerful tool to reveal the <em>P-T</em> evolution of high-grade metamorphic terrains, when combined with detailed microstructural observations focusing on the relationship between rutile, zircon, and quartz.</p>

Geochemistry, mineral chemistry and P-T evaluation of metasediments of Bahram-Gur complex, ES Sanandaj-Sirjan zone, Iran

The Bahram-Gur area in the southeastern part of the Sanandaj – Sirjan metamorphic zone, contains metabasites and metasediments. The metasedimentary rocks are mainly garnet schists and garnet-staurolite schists that were metamorphosed under amphibolite facies conditions. The rocks consist of garnet ± staurolite, biotite, muscovite, chlorite and quartz. The geochemistry of the Bahram-Gur metasediments classifies them as quartziferous sedimentary rocks. The protoliths of the metasedimentary rocks were close to greywackes from an ensialic arc basin depositional setting, with a source comprising mostly mixture of acid and intermediate magmatic rocks in the upper continental crust. The metamorphic conditions of formation of the Bahram-Gur metasedimentary are investigated by geothermobarometric methods. The results show that the metasedimentary rocks formed at temperatures of 600-750°C and pressures of 5-7.5 kbar.

Science Studies and the Metamorphic Multiple Earth: Bruno Latour’s Risky Diplomacy

This essay focuses on Bruno Latour’s recent attempts to study the metamorphic zone of terrestrial life, within the current climate crisis and environmental “cosmocolossus.” I explore his proposal for a risky diplomacy in the Anthropocene “end times,” as a way of dealing with the increasingly tense relations between polarized and weaponized perspectives. I show how his work continues to seek a form of scientific practice that involves the invention of equipment (apparatus) that make perceptible (principally to humans) the existence of nonhuman agencies, thereby expanding the opportunities for alliances and a pluralist ecology, and ultimately assembling another more-than-human political body. I also discuss critiques and concerns regarding the specifics of this proposal.