The Maz Metasedimentary Series (Western Sierras Pampeanas, Argentina). A relict basin of the Columbia supercontinent?

The Maz Metasedimentary Series is part of the Maz Complex that crops out in the sierras of Maz and Espinal (Western Sierras Pampeanas) and in the Sierra de Umango (Andean Frontal Cordillera), northwestern Argentina. The Maz Complex is found within a thrust stack of Silurian age, which later underwent open folding. The Maz Metasedimentary Series mainly consists of medium-grade garnet–staurolite–kyanite–sillimanite schists and quartzites, with minor amounts of marble and calc-silicate rocks. Transposed metadacite dykes have been recognized along with amphibolites, metagabbros, metadiorites and orthogneisses. Schist, quartzite and metadacite samples were analysed for SHRIMP U–Pb zircon dating. The Maz Metasedimentary Series is polymetamorphic and records probably three metamorphic events during the Grenvillian orogeny, at c. 1235, 1155 and 1035 Ma, and a younger metamorphism at c. 440–420 Ma resulting from reactivation during the Famatinian orogeny. The sedimentary protoliths were deposited between 1.86 and 1.33–1.26 Ga (the age of the Andean-type Grenvillian magmatism recorded in the Maz Complex), and probably before 1.75 Ga. The main source areas correspond to Palaeoproterozoic and, to a lesser magnitude, Meso-Neoarchaean rocks. The probable depositional age and the detrital zircon age pattern suggest that the Maz Metasedimentary Series was laid down in a basin of the Columbia supercontinent, mainly accreted between 2.1 and 1.8 Ga. The sedimentary sources were diverse, and we hypothesize that deposition took place before Columbia broke up. The Rio Apa block, and the Río de la Plata, Amazonia and proto-Kalahari cratons, which have nearby locations in the palaeogeographic reconstructions, were probably the main blocks that supplied sediments to this basin.

Geochemistry and Sr-Nd isotopic studies of Palaeoproterozoic (ca. 2.3 Ga) meta-lamprophyre from Rapuru area, Nellore Schist Belt, Southern India: Implication for back-arc basin magmatism and its relevance to the Columbia Supercontinent assembly

Present study reports two lamprophyre dykes from the Rapuru area along the margin of Eastern Dharwar Craton (EDC) and Nellore Schist Belt (NSB). The Rapuru lamprophyre (RL) dykes are situated along southern extension of the Prakassam Alkaline Province (PAP). The Rapuru lamprophyre (RL) dykes are deformed yet preserves the porphyritic-panidiomorphic texture with mica phenocrysts and amphibole and feldspars in the groundmass. The geochemically RL have low Mg# (0.28-0.37), Ni (30 ppm - 60 ppm) and Cr (119 ppm - 228 ppm) concentration indicates their evolved nature like other reported lamprophyre from PAP and EDC. Which is a further supported Sr-Nd isotopic ratios shows affinity towards MORB-OIB like signature and juvenile magmatic nature. The RL seems to have inherited by two major influences, namely, primary source region: which is geochemically juvenile similar to the compositional field of enriched-MORB, and the continental lithosphere. Such magmas are known to have formed in the back-arc-basin environment. The initial 87 Sr/ 86 Sr ratio (∼0.7012-0.7045) and initial εNd (3.13 to 7.93) ratios are in line with the Back-Arc Basin Basalts (BABB) recorded in other parts of the world. The field observation and bulk-rock Sr-Nd radiogenic isotope in the present study support Palaeoproterozoic nature of RL. This concurrence of juvenile radiogenic isotopes and fluid-related trace element compositions apparently suggest dehydration of a subducted slab triggered metasomatism of the overlying mantle wedge in subduction related geodynamic setting. Such intrusive lamprophyre rocks of the older ages are limited in India as well as other part of the world. The rocks of 2.1 and 1.8 Ga, are widely considered to be the ages of initial accretion and final breakup of an erstwhile Columbia Supercontinent assembly. We argue that the RLs were formed in Palaeoproterozoic period during the waxing stages of the Columbia Supercontinent Assembly in BAB environment; most probably due to low degree of partial melting of asthenosphere and lithospheric interaction by the introduction of influx of subduction component into arc-back arc basin system.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5598266

Petrogenesis of (meta-) basalts from the North Qilian Orogenic Belt, NW China: implications for the Palaeoproterozoic–Mesoproterozoic tectonic evolution of the North Qilian Block

The North Qilian Orogenic Belt is surrounded by the Tarim Craton to the NW and the North China Craton to the NE. The Precambrian continental crust remnants that are distributed in the North Qilian Orogenic Belt are termed the North Qilian Block (NQB), and their tectonic evolution has profound implications for the evolution of the Columbia Supercontinent. Here we present major- and trace-element and Sr–Nd–Hf isotope data for (meta-) basalts from the Beidahe Group (BDHG) and Zhulongguan Group (ZLGG) in the western North Qilian Orogenic Belt, to investigate the tectonic evolution of the NQB during the Proterozoic Eon. The protoliths of Palaeoproterozoic amphibole gneisses and plagioclase amphibolites from the BDHG are calc-alkaline series basalts. These metabasalts show island-arc-basalt affinities with variable Nd and Hf isotopes (ϵNd(t) = −5.0–0.6 and 2.7–4.3; ϵHf(t) = −14.2–2.0 and 6.9–8.8) and were generated by partial melting of the asthenospheric mantle that was metasomatized by aqueous fluid and sediment melt in a continental-arc setting. The early Mesoproterozoic ZLGG basalts show features of shoshonite-series basalts and are geochemically similar to ocean-island basalts. These basalts show variable (87Sr/86Sr)i, ϵNd(t) and ϵHf(t) values of 0.70464–0.70699, −1–2.6 and −1.5–5.7, and are products of mantle plume magmatism that participated with subducted oceanic crust in an intracontinental rift setting. This study suggests that the NQB underwent tectonic evolution from palaeo-oceanic subduction to intracontinental rifting during the Palaeoproterozoic–Mesoproterozoic eras. Furthermore, the above tectonomagmatic events were in response to convergence–splitting events of the Columbia Supercontinent during the Palaeoproterozoic–Mesoproterozoic eras.