Contribution Of Juvenile Material Research Articles

Stratigraphy, geochronology, geochemistry and Nd isotopes of the Ouarzazate Group, Anti-Atlas, Morocco: Evidence of a Late Neoproterozoic LIP in the northwestern part of the West African Craton

In the eastern and central Anti-Atlas of Morocco, the Late Neoproterozoic Ouarzazate Group (OG) forms a ∼ 2 km-thick section of volcano-sedimentary rocks deposited during intermittent magmatic activity, spanning a time period between 590 and 540 Ma. In the eastern Anti-Atlas, the stratigraphy of the OG, termed IMS (Imiter Mine succession), includes four units that reflect a gradual change from fluvial to lacustrine depositional sedimentary environments. In the central Anti-Atlas, the deposition of the OG occurs within a caldera environment, consisting of five units referred to as BMS (Bou Azzer Mine succession). UPb dating on zircon from the rhyolite lava flow at the top of the IMS constrains the latest lava flow of the IMS to 570.7 ± 2.1 Ma. In the Bou Azzer inlier, the stratigraphically correlated basal andesite and top dacite lava flows were emplaced at 590.6 ± 4.2 Ma and 555.9 ± 20.4 Ma, respectively, suggesting that the OG was constructed through successive pulses within a long-lived magmatic episode. The magmatic rocks display geochemical signatures typical of continental arc magmas, including high-K calc-alkaline to shoshonite composition, enrichment in LILE, and negative Nb, Sr, Ti and P anomalies. Nd isotopes indicate that magmas supplying the plumbing systems of the IMS and BMS constitute transitional products resulting from a combination of orogenic and intraplate volcanism, involving deep crustal recycling. The compositions of these magmas evolved through processes of crustal contamination, bulk assimilation, and minor fractional crystallization of the parent melt. High εNd values of the BMS rocks argue for a juvenile origin, whereas lower εNdt recorded in the IMS likely reflect old crustal protoliths and an overall less contribution of juvenile material in their source. The magmatic rocks of the OG are interpreted as products of a large igneous province (LIP) developed by a post-collisional delamination model that initiated tens of millions of years after the subduction-collision process stage of the Pan-African orogeny in the northwestern part of the West African Craton (WAC).

The Rio Piranhas-Seridó Domain, Borborema Province, Northeastern Brazil: Review of geological-geochronological data and implications for stratigraphy and crustal evolution

This paper presents a review of available lithostratigraphic and isotopic data, in addition to other information collected from projects of the Geological Survey of Brazil (SGB-CPRM), for the Rio Piranhas-Serido Domain (PSD). This domain is one of the tectonostratigraphic compartments of the central-northeast portion of the Borborema Province (Northeast Brazil), bound by the Picui-Joao Câmara, Patos and Portalegre shear zones. More recent isotopic determinations (U-Pb dating of zircon) and geological maps (usually on a 1:100000 scale) have clearly detected and/or redefined several units, e.g., the Amarante, Campo Grande, Granjeiro and Saquinho complexes, in addition to the Serra do Inga body (Archean), Arabia Complex (Siderian), Caico Complex and Serra da Formiga Suite (Rhyacian), Poco da Cruz Suite (Orosirian-Statherian), Serido Group (Ediacaran) and Ediacaran to Early-Cambrian magmatic suites, in addition to pegmatites and granitic pegmatites. With respect to the supracrustal rocks of the Serido Group, the deposition in the Ediacaran is based on field relations and on the ages of detrital zircons, which showed a greater contribution from Mesoarchean to Neoproterozoic sources, the vast majority of which may come from the PSD itself. Current records indicate that the evolution of the PSD started in the Eoarchean, and peaked in the Neoproterozoic with the Brasiliano event, which is characterized by sedimentation of the Serido Group, intrusion of plutonic bodies, deformation and metamorphism (especially at the regional level), in addition to the formation or reactivation of important shear zones. Such evolution is acknowledged on the basis of the accretion of oceanic arcs during the Paleoproterozoic (containing Archean fragments), including the generation of juvenile magmas and a protocrust (Caico Complex). The Neoproterozoic record, associated with the Brasiliano event, is considered by several authors as reworking with no or little contribution of juvenile material, with deposition of the Serido Group, emplacement of several Ediacaran to Early-Cambrian magmatic suites, accompanied by the formation of shear zones, predominantly in the southern and eastern portion of the domain.

Isotope Lu–Hf composition of detrital zircon from paragneisses of the Sharyzhalgai uplift: evidence for the Paleoproterozoic crustal growth

We present results of study of the trace-element and Lu–Hf isotope compositions of zircons from Paleoproterozoic high-grade metasedimentary rocks (paragneisses) of the southwestern margin of the Siberian craton (Irkut terrane of the Sharyzhalgai uplift). Metamorphic zircons are represented by rims and multifaceted crystals dated at ~1.85 Ga. They are depleted in either LREE or HREE as a result of subsolidus recrystallization and/or synchronous formation with REE-concentrating garnet or monazite. In contrast to the metamorphic zircons, the detrital cores are enriched in HREE and have high (Lu/Gd)n ratios, which is typical of igneous zircon. The weak positive correlation between 176Lu/177Hf and 176Hf/177Hf in the zircon cores evidences that their Hf isotope composition evolved through radioactive decay in Hf = the closed system. Therefore, the isotope parameters of these zircons can give an insight into the provenance of metasedimentary rocks. The Paleoproterozoic detrital zircon cores from paragneisses, dated at ~2.3–2.4 and 2.0–1.95 Ga, are characterized by a wide range of εHf values (from +9.8 to –3.3) and model age TC 2.8–2.0 Ga. The provenance of these detrital zircons included both rocks with juvenile isotope Hf parameters and rocks resulted from the recycling of the Archean crust with a varying contribution of juvenile material. Zircons with high positive εHf values were derived from the juvenile Paleoproterozoic crustal sources, whereas the lower εHf and higher TC values for zircons suggest the contribution of the Archean crustal source to the formation of their magmatic precursors. Thus, at the Paleoproterozoic stage of evolution of the southwestern margin of the Siberian craton, both crustal recycling and crustal growth through the contribution of juvenile material took place. On the southwestern margin of the Siberian craton, detrital zircons with ages of ~2.3–2.4 and 1.95–2.0 Ga are widespread in Paleoproterozoic paragneisses of the Irkut and Angara–Kan terranes and in terrigenous rocks of the Urik–Iya graben, which argues for their common and, most likely, proximal provenances. In the time of metamorphism (1.88–1.85 Ga), the age of Paleoproterozoic detrital zircons (2.4–2.0 Ga), and their Lu–Hf isotope composition (εHf values ranging from positive to negative values) the paragneisses of the southwestern margin of the Siberian craton are similar to the metasedimentary rocks of the Paleoproterozoic orogenic belts of the North China Craton. In the above two regions, the sources of detrital zircons formed by both the reworking of the Archean crust and the contribution of juvenile material, which is evidence for the crustal growth in the period 2.4–2.0 Ga.