Neoarchean crustal evolution in the Madras Block: Geochemical and petrological evidence from felsic gneiss and amphibolite

Neoarchean crustal evolution in western Shandong Province of the North China Craton: The role of 2.7–2.6 Ga magmatism

Geochemical and isotopic constraints on the tectonic and crustal evolution of the Shackleton Range, East Antarctica, and correlation with other Gondwana crustal segments

40Ar/39Ar thermochronology is commonly used to constrain the rates and times of cooling in exhumed metamorphic terranes, with ages usually linked to temperature via Dodson's closure temperature (TC) formulation. Whilst many metamorphic 40Ar/39Ar data are consistent with the timing of crystallisation or cooling within a chronological framework defined by other, higher temperature, chronometers, other 40Ar/39Ar data are more difficult to interpret. We report white mica and biotite single grain fusion and laser ablation 40Ar/39Ar ages from felsic gneisses from the Western Gneiss Region, Norway. The rocks record isothermal decompression from peak eclogite-facies conditions (white mica stable) to amphibolite-facies conditions (biotite stable) at c. 700°C. White mica and biotite yield dispersed single grain fusion dates from 416 to 373Ma and 437 to 360Ma respectively. In-situ laser ablation analyses provide a similar range, with white mica spot ages ranging from 424 to 370Ma and biotite spot ages ranging from 437 to 370Ma. The dates span the duration of the metamorphic cycle suggested by previous studies, and cannot be reconciled with the results of simple models of Ar loss by diffusion during cooling. Samples that show evidence for different physical processes, such as the chemical breakdown of white mica, partial melting, and fluid ingress, generated different age populations to samples that did not experience or record obvious petrological evidence for these processes. Samples that record significant recrystallization and deformation yielded younger white mica (but older biotite) single grain fusion ages than more pristine samples. Amphibolite-facies gneisses that preserve evidence for significant partial melting generated younger biotite ages than samples that recorded evidence for significant hydration. Our data support other reported observations that high-temperature metamorphic mica 40Ar/39Ar dates cannot be assumed to record the timing of cooling through a specific temperature window. Careful assessment of the petrographic context of the dated minerals and consideration of their post-crystallisation history may provide a more robust insight into whether ‘age’ links to ‘stage’ in a temporally meaningful way.

Composition and tectonic evolution of the Chinese continental crust constrained by Poisson's ratio

High-pressure metamorphism and deep-crustal seismicity: evidence from contemporaneous formation of pseudotachylytes and eclogite facies coronas

High Ba–Sr adakitic charnockite suite from the Nagercoil Block, southern India: Vestiges of Paleoproterozoic arc and implications for Columbia to Gondwana

Chapter 3.1 The Early Archean Acasta Gneiss Complex: Geological, Geochronological and Isotopic Studies and Implications for Early Crustal Evolution

The Qinling Group was previously interpreted as the oldest Precambrian basement unit of the North Qinling Terrain, recording its formation and early crustal evolution. The Qinling Group consists predominantly of gneisses, amphibolites, and marbles, which underwent multi-phase deformation and metamorphism. In order to better constrain the provenance and tectonic setting of this group and the evolution of the North Qinling orogenic belt, in situ U–Pb dating and oxygen isotopic analysis of zircons in combination with whole-rock geochemistry and Sr–Nd isotope analysis was performed on the two dominant rock types, amphibolite and felsic gneiss. Felsic gneisses exhibit enrichment of LREEs and LILEs (Rb, Ba, Th, K, Pb), negative Eu anomalies and depletion of HFSEs (Nb, Ta, P, Ti). The rocks have slightly elevated δ18O values (6.5–9.3 ‰) and initial e Nd values of −4.6 corresponding to two-stage Nd model age of 1.99 Ga. Amphibolites are also enriched in LILEs and LREEs and depleted in Nb and Ta and have homogeneous δ18O values (5.0–6.0 ‰), but higher initial e Nd values (2.8–3.3) and younger two-stage Nd model ages (1.29–1.24 Ga) compared to the gneisses. The zircon age record indicates that the gneisses and amphibolites were formed in a ~960 Ma volcanic arc environment rather than in a rift setting as previously suggested. A major metamorphic event took place during the Early Paleozoic. Based on the age spectrum of detrital zircons, the Qinling Group is interpreted as an autonomous geological unit, which was mainly derived mostly from 1,000 to 900 Ma old granitoid rocks. The North Qinling Terrain can be regarded as a remnant of the Grenville orogenic belt with an early Neoproterozoic evolution different from that of the North and South China blocks.

<p>High-grade Archean gneiss terranes expose mid to lower crustal rocks and are generally dominated by tonalite-trondhjemite-granodiorite (TTG) gneisses. Occurrences of mafic-ultramafic bodies and garnet-bearing felsic gneisses within these environments have been interpreted as supracrustal or near-surface rocks requiring a tectonic process involving mass transfer from the near-surface to the mid-crust. However, there is significant uncertainty regarding the nature of this mass transfer, with suggestions including a range of uniformitarian and non-uniformitarian scenarios.  One non-uniformitarian scenario, ‘sagduction’, has been proposed as a possible mechanism (Johnson <em>et al.,</em> 2016, and references therein), although the dynamics of sagduction are still relatively unexplored.</p><p>This study focuses on mafic, ultramafic and garnet-bearing felsic gneiss bodies in the central region in the Lewisian Gneiss Complex of northwest Scotland as test cases to investigate the behaviour of possibly supracrustal rocks in a mid-crustal environment. Existing datasets of TTGs (Johnson <em>et al.,</em> 2016), mafic gneisses (Feisel <em>et al.,</em> 2018) and ultramafic gneisses (Guice <em>et al.,</em> 2018) from across the central region were utilised in addition to felsic and mafic gneiss samples obtained in this study from the ~10 km<sup>2</sup> Cnoc an t-Sidhean (CAS) suite. The CAS suite is the largest reported supracrustal in the Lewisian, and dominantly comprises garnet-biotite felsic gneiss assemblages and an associated two-pyroxene mafic gneiss. Field mapping was undertaken to collect samples representative of the observed heterogeneity of the suite, and to assess field associations between possible supracrustals and surrounding TTGs. Phase equilibria modelling was conducted on all lithologies to ascertain peak pressure-temperature (<em>P-T</em>) conditions, and to calculate the density of the modelled rocks at peak conditions.</p><p>The results obtained in this study indicate peak metamorphic conditions of 950 ± 50 °C and 9 ± 1 kbar for the CAS suite, consistent with the central region of the Lewisian Complex (Feisel <em>et al.,</em> 2018). Density contrasts at mid-crustal conditions of 0.12–0.56 gcm<sup>-3</sup> were calculated between TTGs and the other lithologies and used to estimate the buoyancy force that drives density-driven segregation. This allowed us to investigate the rates of vertical motion that result from density contrasts, as a function of the effective viscosity during metamorphism. Independent viscosity estimates were attained using mineral flow-laws and our estimated <em>P-T</em> conditions, and from examination of modern-day regions of crustal flow. We were therefore able to estimate the conditions under which sagduction could have been a viable mechanism for crustal evolution in the Lewisian and similar high-grade metamorphic terranes. We conclude that sagduction was unlikely to have operated in the Lewisian under the dry conditions implied by preserved mineral assemblages.</p><p> </p><p> </p><p>Feisel, Y., et al. 2018. New constraints on granulite facies metamorphism and melt production in the Lewisian Complex, northwest Scotland. Journal of Metamorphic Geology. <strong>36</strong>, 799-819</p><p>Guice, G.L., et al. 2018. Assessing the Validity of Negative High Field Strength-Element Anomalies as a Proxy for Archaean Subduction: Evidence from the Ben Strome Complex, NW Scotland. Geosciences, <strong>8, </strong>338.</p><p>Johnson, T.E., et al. 2016. Subduction or sagduction? Ambiguity in constraining the origin of ultramafic–mafic bodies in the Archean crust of NW Scotland. Precambrian Research, <strong>283</strong>, 89-105.</p>

Crustal xenoliths from Cenozoic volcanic fields of West Germany: Implications for structure and composition of the continental crust

The evolution of terrestrial planets (the Earth, Venus, Mars, Mercury, and Moon) was proved to have proceeded according to similar scenarios. The primordial crusts of the Earth, Moon, and, perhaps, other terrestrial planets started to develop during the solidification of their global magmatic “oceans”, a process that propagated from below upward due to the difference in the adiabatic gradient and the melting point gradient. Consequently, the lowest melting components were “forced” toward the surfaces of the planets in the process of crystallization differentiation. These primordial crusts are preserved within ancient continents and have largely predetermined their inner structure and composition. Early tectono-magmatic activity at terrestrial planets was related to the ascent of mantle plumes of the first generation, which consisted of mantle material depleted during the development of the primordial crusts. Intermediate evolutionary stages of the Earth, Moon, and other terrestrial planets were marked by an irreversible change related to the origin of the liquid essentially iron cores of these planets. This process induced the ascent of mantle superplumes of the second generation (thermochemical), whose material was enriched in Fe, Ti, incompatible elements, and fluid components. The heads of these superplumes spread laterally at shallower depths and triggered significant transformations of the upper shells of the planets and the gradual replacement of their primordial crusts of continental type by secondary basaltic crusts. The change in the character of the tectono-magmatic activity was associated with modifications in the environment at the surface of the Earth, Mars, and Venus. The origin of thermochemical mantle plumes testifies that the tectono-magmatic process involved then material of principally different type, which had been previously “conserved” at deep portions of the planets. This was possible only if (1) the planetary bodies initially had a heterogeneous inner structure (with an iron core and silicate mantle made up of chondritic material); and (2) the planetary bodies were heated from their peripheral toward central portions due to the passage of a “thermal wave”, with the simultaneous cooling of the outer shells. The examples of the Earth and Moon demonstrate that the passage of such a “wave” through the silicate mantles of the planets was associated with the generation of mantle plumes of the first generation. When the “wave” reached the cores, whose composition was close to the low-temperature Fe + FeS eutectic, these cores started to melt and gave rise to superplumes of the second generation. The “waves” are thought to have been induced by the acceleration of the rotation of these newly formed planets due to the decrease of their radii because of the compaction of their material. When this process was completed, the rotation of the planets stabilized, and the planets entered their second evolutionary stage. It is demonstrated that terrestrial planets are spontaneously evolving systems, whose evolution was accompanied by the irreversible changes in their tectono-magmatic processes. The evolution of most of these planets (except the Earth) is now completed, so that they “dead” planetary bodies.

Metamorphic events in the eastern Arunta Inlier, Part 1. Metamorphic petrology

Coupled U-Pb and Rb-Sr laser ablation geochronology trace Archean to Proterozoic crustal evolution in the Dharwar Craton, India

Bimodal magmatism in the Eastern Dharwar Craton, southern India: Implications for Neoarchean crustal evolution

Crustal evolution of the Eastern Block in the North China Craton: Constraints from zircon U–Pb geochronology and Lu–Hf isotopes of the Northern Liaoning Complex