Small-scale Hf isotopic variability in the Peninsula pluton (South Africa): the processes that control inheritance of source 176Hf/177Hf diversity in S-type granites

The ¹⁷⁶Hf/¹⁷⁷Hf composition of inherited and magmatic zircon in the 538 Ma S-type Peninsula pluton (South Africa) has been determined at different scales. In the smallest rock samples investigated (<0.5 dm³), as well as within individual thin sections, magmatic zircon crystals exhibit the same wide range in ε_Hf(538) as the pluton (8ε units). In addition, across a significant range of bulk-rock compositions, both the range and average of the magmatic zircon Hf isotopic composition do not vary significantly with compositional parameters that are expected to scale with the proportion of mantle-derived magma addition (e.g., Mg# and Ca). At all scales, the ε_Hf variability in the magmatic zircon fraction matches well with that portrayed by the time-evolved inherited zircon population [i.e., with the ε_Hf(538) range of the inherited zircon cores]. This evidence suggests that the ε_Hf heterogeneity of magmatic zircon is directly inherited from the source. However, the analysis of zircon core–rim pairs reveals that the ¹⁷⁶Hf/¹⁷⁷Hf composition of the inherited crystals does not directly transfer to their magmatic overgrowths. Small-scale modeling of zircon dissolution and re-precipitation in a static magma generates sub-mm melt domains having variable Zr content and Hf isotope composition. The composition of these domains is controlled by the size and isotope composition of the nearest dissolving zircon crystals and the cooling rate of the magma. These results suggest that in magma systems with a substantial inherited zircon load, zircon crystals within the same rock should record variable ¹⁷⁶Hf/¹⁷⁷Hf in the magmatic zircon fraction.     

Coupled U–Pb–Hf of detrital zircons of Cambrian sandstones from Morocco and Sardinia: Implications for provenance and Precambrian crustal evolution of North Africa

U–Pb–Hf of detrital zircons from diverse Cambrian units in Morocco and Sardinia were investigated in order to clarify the sandstone provenance and how it evolved with time, to assess whether the detrital spectra mirror basement crustal composition and whether they are a reliable pointer on the ancestry of peri-Gondwanan terranes. Coupled with Hf isotopes, the detrital age spectra allow a unique perspective on crustal growth and recycling in North Africa, much of which is concealed below Phanerozoic sediments.In Morocco, the detrital signal of Lower Cambrian arkose records local crustal evolution dominated by Ediacaran (0.54–0.63 Ga) and Late-Paleoproterozoic (1.9–2.2 Ga; Eburnian) igneous activity. A preponderance of the Neoproterozoic detrital zircons possess positive ε_Hf(t) values and their respective Hf model ages (T_DM) concentrate at 1.15 Ga. In contrast, rather than by Ediacaran, the Neoproterozoic detrital signal from the Moroccan Middle Cambrian quartz-rich sandstone is dominated by Cryogenian-aged detrital zircons peaking at 0.65 Ga alongside a noteworthy early Tonian (0.95 Ga) peak; a few Stenian-age (1.0–1.1 Ga) detrital zircons are also distinguished. The majority of the Neoproterozoic zircons displays negative ε_Hf(t), indicating the provenance migrated onto distal Pan-African terranes dominated by crustal reworking. Terranes such as the Tuareg Shield were a likely provenance. The detrital signal of quartz–arenites from the Lower and Middle Cambrian of SW Sardinia resembles the Moroccan Middle Cambrian, but 1.0–1.1 Ga as well as ~ 2.5 Ga detrital zircons are more common. Therefore, Cambrian Sardinia may have been fed from different sources possibly located farther to the east along the north Gondwana margin. 1.0–1.1 Ga detrital zircons abundant in Sardinia generally display negative ε_Hf(t) values while 0.99–0.95 Ga detrital zircons (abundant in Morocco) possess positive ε_Hf(t), attesting for two petrologically-different Grenvillian sources. A paucity of detrital zircons younger than 0.6 Ga is a remarkable feature of the detrital spectra of the Moroccan and Sardinian quartz-rich sandstones. It indicates that late Cadomian orogens fringing the northern margin of North Africa were low-lying by the time the Cambrian platform was deposited. About a quarter of the Neoproterozoic-aged detrital zircons in the quartz-rich sandstones of Morocco (and a double proportion in Sardinia) display positive ε_Hf(t) values indicating considerable juvenile crust addition in North Africa, likely via island arc magmatism. A substantial fraction of the remaining Neoproterozoic zircons which possess negative ε_Hf(t) values bears evidence for mixing of old crust with juvenile magmas, implying crustal growth in an Andean-type setting was also significant in this region.     

Quantifying the influence of sill intrusion on the thermal evolution of organic‐rich sedimentary rocks in nonvolcanic passive margins: an example from ODP 210‐1276, offshore Newfoundland,Canada

Intrusive magmatism is an integral and understudied component in both volcanic and nonvolcanic passive margins. Here, we investigate the thermal effects of widespread (ca. 20 000 km²) intrusive magmatism on the thermal evolution of organic‐rich sedimentary rocks on the nonvolcanic Newfoundland passive margin. ODP 210‐1276 (45.41°N, 44.79°W) intersects two sills: an older, upper sill and a younger, lower sill that are believed to correspond to the high amplitude ‘U‐reflector’ observed across the Newfoundland Basin. A compilation of previous work collectively provides; (1) emplacement depth constraints, (2) vitrinite reflectance data and (3) ⁴⁰Ar/³⁹Ar dates. Collectively, these data sets provide a unique opportunity to model the conductive cooling of the sills and how they affect thermal maturity of the sedimentary sequence. A finite differences method was used to model the cooling of the sills, with the model outputs then being entered into the EASY%R_o vitrinite reflectance model. The modelled maturation profile for ODP 210‐1276 shows a significant but localized effect on sediment maturity as a result of the intrusions. Our results suggest that even on nonvolcanic margins, intrusive magmatism can significantly influence the thermal evolution in the vicinity of igneous intrusions. In addition, the presence of widespread sills on nonvolcanic passive margins such as offshore Newfoundland may be indicative of regional‐scale thermal perturbations that should be considered in source rock maturation studies.     

Re-interpreting the Devonian ophiolites involved in the Variscan suture: U–Pb and Lu–Hf zircon data of the Moeche Ophiolite (Cabo Ortegal Complex,NW Iberia)

New U–Pb zircon data of a mylonitic greenschist from the Moeche Ophiolite, one of the mafic units involved in the Variscan suture in the Cabo Ortegal Complex (NW of the Iberian Massif), yielded an age of 400 ± 3 Ma. Consequently, this unit can be considered one of the Devonian ophiolites, the most extended group of oceanic units in the Variscan belt. The mafic rocks show transitional compositions between N-MORB and island-arc tholeiites, although Lu–Hf isotope signatures of its zircons clearly indicate contribution from an old continental source. εHf values in the analysed zircons are negative (generally below εHf = ?5), and thence, they are not compatible with their generation from a juvenile mantle source. Accordingly, the igneous protoliths were generated in a setting where juvenile mafic magmas interacted with an old continental crust. The Devonian ophiolites from the Variscan suture have been repeatedly interpreted as remnants of the Rheic Ocean. However, the presence of a continental source in the origin of the mafic rocks of the Moeche Ophiolite allows discarding an intraoceanic setting for their generation, at least for the NW Iberian counterparts. The tectonic setting for the Devonian ophiolites of NW Iberia is very likely represented by an ephemeral oceanic basin opened within a continental realm. Herein, the real Rheic Ocean suture could only be located west of the terrane represented by the upper units of the allochthonous complexes. Apparently that suture is not represented in NW Iberia.     

Reworking of Earth's first crust: Constraints from Hf isotopes in Archean zircons from Mt. Narryer,Australia

Discoveries of >4 Ga old zircon grains in the northwest Yilgarn of Western Australia led to the conclusion that evolved crust formed on the Earth within the first few 100 Ma after accretion. Little is known, however, about the fate of the first crust that shaped early Earth's surface. Here we report combined solution and laser-ablation Lu–Hf–U–Pb isotope analyses of early Archean and Hadean detrital zircon grains from different rocks of the Narryer Gneiss Complex (NGC), Yilgarn Craton, Western Australia. The zircons show two distinct groups with separate evolutionary trends in their Hf isotopes. The majority of the zircon grains point to separation from a depleted mantle reservoir at ∼3.8–3.9 Ga. The second Hf isotope trend implies reworking of older Hadean zircon grains. The major trend starting at 3.8–3.9 Ga defined by the Hf isotopes corresponds to a Lu/Hf that is characteristic for felsic crust and consequently, the primary sources for these zircons presumably had a chemical composition characteristic of continental crust. Reworked Hadean crust appears to have evolved with a similar low Lu/Hf, such that the early crust was probably evolved with respect to Lu–Hf distributions. The co-variation of Hf isotopes vs. age in zircon grains from Mt. Narryer and Jack Hills zircon grains implies a similar crustal source for both sediments in a single, major crustal domain. Age spectra and associated Hf isotopes in the zircon grains strongly argue for ongoing magmatic reworking over hundreds of millions of years of the felsic crustal domain in which the zircon grains formed. Late-stage metamorphic zircon grains from the Meeberrie Gneiss unit yield a mean U–Pb age of 3294.5 ± 3.2 Ma with initial Hf isotopes that correspond to the evolutionary trend defined by older NGC zircon grains and overlap with other detrital zircon grains, proving their genetic relationship. This ‘Meeberrie event’ is interpret here as the last reworking event in the precursor domain before final deposition. The continuous magmatic activity in one crustal domain during the Archean is recorded by the U–Pb ages and Hf isotope systematics of zircon grains and implies reworking of existing crust. We suspect that the most likely driving force for such reworking of crustal material is ongoing crustal collision and subduction. A comparison of Hf isotope signatures of zircon grains from other Archean terranes shows that similar trends are recognised within all sampled Archean domains. This implies either a global trend in crustal growth and reworking, or a genetic connection of Archean terranes in close paleo-proximity to each other. Notably, the Archean Acasta gneiss (Canada) shows a similar reworking patterns to the Yilgarn Craton of Hadean samples implying either a common Hadean source or amalgamation at the Hadean–Archean transition.     

Multiple accretion at the eastern margin of the Rio de la Plata craton: the prolonged Brasiliano orogeny in southernmost Brazil

The Neoproterozoic-Eoplalaeozoic Brasiliano orogeny at the eastern margin of the Rio de la Plata craton in southernmost Brazil and Uruguay comprises a complex tectonic history over 300?million years. The southern Brazilian Shield consists of a number of tectono-stratigraphic units and terranes. The S?o Gabriel block in the west is characterized by c.760?C690?Ma supracrustal rocks and calc-alkaline orthogneisses including relics of older, c. 880?Ma old igneous rocks. Both igneous and metasedimentary rocks have positive ??Nd(t) values and Neoproterozoic TDM model ages; they formed in magmatic arc settings with only minor input of older crustal sources. A trondhjemite from the S?o Gabriel block intruding dioritc and tonalitic gneisses during the late stages of deformation (D₃) yield an U?CPb zircon age (LA-ICP-MS) of 701?±?10?Ma giving the approximate minimum age of the S?o Gabriel accretionary event. The Encantadas block further east, containing the supracrustal Porongos belt and the Pelotas batholith, is in contrast characterized by reworking of Neoarchean to Palaeoproterozoic crust. The 789?±?7?Ma zircon age of a metarhyolite intercalated with the metasedimentary succession of the Porongos belt provides a time marker for the basin formation. Zircons of a sample from tonalitic gneisses, constituting the Palaeoproterozoic basement of the Porongos belt, form a cluster at 2,234?±?28?Ma, interpreted as the tonalite crystallization age. Zircon rims show ages of 2,100?C2,000?Ma interpreted as related to a Palaeoproterozoic metamorphic event. The Porongos basin formed on thinned continental crust in an extensional or transtensional regime between c. 800?C700?Ma. The absence of input from Neoproterozoic juvenile sources into the Porongos basin strongly indicates that the Encantadas and S?o Gabriel blocks were separated terranes that became juxtaposed next to each other during the Brasiliano accretional events. The tectonic evolution comprises two episodes of magmatic arc accretion to the eastern margin of the Rio de la Plata craton, (i) accretion of an intra-oceanic arc at c. 880?Ma (Passinho event) and (ii) accretion of the 760?C700?Ma Cambaí/Vila Nova magmatic arc (S?o Gabriel event). The latter event also includes the collision of the Encantadas block with the Rio de la Plata craton to the west. Collision and crustal thickening was followed by sinistral shear along SW?CNE-trending orogen-parallel crustal-scale shear zones that can be traced from southern Brazil to Uruguay and have been active between 660 and 590?Ma. Voluminous granitic magmatism in the Pelotas batholith spatially related to shear zones is interpreted as late- to post-orogenic magmatism, possibly assisted by lithospheric delamination. It marks the transition to the post-orogenic molasse stage. Localized deformation by reactivation of preexisting shear zones continued until c. 530?Ma and can be assigned to final stages of the amalgamation of West Gondwana.