Growth of the Archean sialic crust as revealed by zircon in the TTGs in eastern Finland

U–Pb age determinations on zircon from granitoids in the Archean of eastern Finland were conducted using the SIMS, LA-ICP-MS and TIMS techniques, with an emphasis on low-HREE granitoids. The oldest rocks in the Fennoscandian Shield are 3.4–3.5 Ga. Several samples were collected close to these rocks, but none of the samples were as old, indicating that the oldest rocks are only small, possibly allochthonous fragments in the Neoarchean bedrock. Some tonalite–trondhjemite–granodiorite (TTG) samples yielded homogeneous 2.72–2.73 Ga zircon populations, and in these samples, the initial εNd was also close to the depleted mantle (DM) values. However, several granitoid samples with a main zircon population of 2.7–2.8 Ga had 2.9–3.2 Ga grains or inherited cores, and in some samples, all grains were of 2.9–3.0 Ga. In these samples, the εNd value was also close to zero or slightly negative. These features suggest that apart from the juvenile Neoarchean magmas, the abundance of reworked 2.9 Ga material is considerable in the Archean crust, which developed during successive juvenile magmatic inputs that melted and assimilated the older sialic crust. The low- HREE geochemical character of granitoids has no correlation with their age, with the low-HREE granitoids yielding an age span of 2.72–2.98 Ga.

New tectonic model and division of the Ubendian-Usagaran Belt, Tanzania: A review and in-situ dating of eclogites

ABSTRACT Records of high-pressure/low-temperature (HP-LT) metamorphic interfaces are not common in Precambrian orogens. It should be noted that the association of HP-LT metamorphic interfaces and strongly deformed ocean plate stratigraphy that form accretionary prisms between trenches and magmatic arcs are recognized as hallmark signatures of modern plate tectonics. In East Africa (Tanzania), the Paleoproterozoic Ubendian-Usagaran Belt records a HP-LT metamorphic interface that we consider as a centerpiece in reviewing the description of tectonic units of the Ubendian-Usagaran Belt and defining a new tectonic model. Our new U-Pb zircon age and the interpretations from existing data reveal an age between 1920 and 1890 Ma from the kyanite bearing eclogites. This establishment adds to the information of already known HP-LT metamorphic events at 2000 Ma, 1890–1860 Ma, and 590–520 Ma from the Ubendian-Usagaran Belt. Arc–back-arc signatures from eclogites imply that their mafic protoliths were probably eroded from arc basalt above a subduction zone and were channeled into a subduction zone as mélanges and got metamorphosed. The Ubendian-Usagaran events also record rifting, arc and back-arc magmatism, collisional, and hydrothermal events that preceded or followed HP-LT tectonic events. Our new tectonic subdivision of the Ubendian Belt is described as: (1) the western Ubendian Corridor, mainly composed of two Proterozoic suture zones (subduction at 2000, 1920–1890, Ma and 590–500 Ma) in the Ufipa and Nyika Terranes; (2) the central Ubendian Corridor, predominated by metamorphosed mafic-ultramafic rocks in the Ubende, Mbozi, and Upangwa Terranes that include the 1890–1860 Ma eclogites with mid-ocean ridge basalt affinity in the Ubende Terrane; and (3) the eastern Ubendian Corridor (the Katuma and Lupa Terranes), characterized by reworked Archean crust.

Thermal state and evolving geodynamic regimes of the Meso- to Neoarchean North China Craton

Constraining thickness and geothermal gradient of Archean continental crust are crucial to understanding geodynamic regimes of the early Earth. Archean crust-sourced tonalitic–trondhjemitic–granodioritic gneisses are ideal lithologies for reconstructing the thermal state of early continental crust. Integrating experimental results with petrochemical data from the Eastern Block of the North China Craton allows us to establish temporal–spatial variations in thickness, geothermal gradient and basal heat flow across the block, which we relate to cooling mantle potential temperature and resultant changing geodynamic regimes from vertical tectonics in the late Mesoarchean (~2.9 Ga) to plate tectonics with hot subduction in the early to late Neoarchean (~2.7–2.5 Ga). Here, we show the transition to a plate tectonic regime plays an important role in the rapid cooling of the mantle, and thickening and strengthening of the lithosphere, which in turn prompted stabilization of the cratonic lithosphere at the end of the Archean.

Serial interaction of primitive magmas with felsic and mafic crust recorded by gabbroic dikes from the Antarctic extension of the Karoo large igneous province

Two subvertical gabbroic dikes with widths of ~ 350 m (East-Muren) and ≥ 500 m (West-Muren) crosscut continental flood basalts in the Antarctic extension of the ~ 180 Ma Karoo large igneous province (LIP) in Vestfjella, western Dronning Maud Land. The dikes exhibit unusual geochemical profiles; most significantly, initial (at 180 Ma) εNd values increase from the dike interiors towards the hornfelsed wallrock basalts (from − 15.3 to − 7.8 in East-Muren and more gradually from − 9.0 to − 5.5 in West-Muren). In this study, we utilize models of partial melting and energy-constrained assimilation‒fractional crystallization in deciphering the magmatic evolution of the dikes and their contact aureoles. The modeling indicates that both gabbroic dikes acquired the distinctly negative εNd values recorded by their central parts by varying degrees of assimilation of Archean crust at depth. This first phase of deep contamination was followed by a second event at or close to the emplacement level and is related to the interaction of the magmas with the wallrock basalts. These basalts belong to a distinct Karoo LIP magma type having initial εNd from − 2.1 to + 2.5, which provides a stark contrast to the εNd composition of the dike parental magmas (− 15.3 for East-Muren, − 9.0 for West-Muren) previously contaminated by Archean crust. For East-Muren, the distal hornfelses represent partially melted wallrock basalts and the proximal contact zones represent hybrids of such residues with differentiated melts from the intrusion; the magmas that were contaminated by the partial melts of the wallrock basalts were likely transported away from the currently exposed parts of the conduit before the magma–wallrock contact was sealed and further assimilation prevented. In contrast, for West-Muren, the assimilation of the wallrock basalt partial melts is recorded by the gradually increasing εNd of the presently exposed gabbroic rocks towards the roof contact with the basalts. Our study shows that primitive LIP magmas release enough sensible and latent heat to partially melt and potentially assimilate wallrocks in multiple stages. This type of multi-stage assimilation is difficult to detect in general, especially if the associated wallrocks show broad compositional similarity with the intruding magmas. Notably, trace element and isotopic heterogeneity in LIP magmas can be homogenized by such processes (basaltic cannibalism). If similar processes work at larger scales, they may affect the geochemical evolution of the crust and influence the generation of, for example, massif-type anorthosites and “ghost plagioclase” geochemical signature.

How did the Archean crust evolve? Insights from the structure and petrology of the Lewisian of Scotland

<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>

Archean continental crust formed by magma hybridization and voluminous partial melting

Archean (4.0–2.5 Ga) tonalite–trondhjemite–granodiorite (TTG) terranes represent fragments of Earth’s first continents that formed via high-grade metamorphism and partial melting of hydrated basaltic crust. While a range of geodynamic regimes can explain the production of TTG magmas, the processes by which they separated from their source and acquired distinctive geochemical signatures remain uncertain. This limits our understanding of how the continental crust internally differentiates, which in turn controls its potential for long-term stabilization as cratonic nuclei. Here, we show via petrological modeling that hydrous Archean mafic crust metamorphosed in a non-plate tectonic regime produces individual pulses of magma with major-, minor-, and trace-element signatures resembling—but not always matching—natural Archean TTGs. Critically, magma hybridization due to co-mingling and accumulation of multiple melt fractions during ascent through the overlying crust eliminates geochemical discrepancies identified when assuming that TTGs formed via crystallization of discrete melt pulses. We posit that much Archean continental crust is made of hybrid magmas that represent up to ~ 40 vol% of partial melts produced along thermal gradients of 50–100 °C/kbar, characteristic of overthickened mafic Archean crust at the head of a mantle plume, crustal overturns, or lithospheric peels.