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• In the Ontario gneiss belt, Nd isotope boundaries can be identified in the field. • These isotope boundaries provide the best constraint on major crustal structures. • Spatial control from Nd isotope mapping complements age control from U-Pb dating. • Detrital zircon Hf model ages rather than U-Pb ages yield crustal extraction ages. With the increasing availability of U-Pb-Hf isotope analysis of zircon suites, does whole-rock Nd isotope analysis still have a role to play in mapping Precambrian gneiss terranes? I answer this question by examining the Ontario Gneiss Belt of the Grenville Province, and show that Nd isotope mapping still has a vital role to play in studies of Precambrian crustal evolution. Although U-Pb-Hf isotope analysis of detrital zircons can provide large suites of crystallization ages and epsilon Hf values, the poor spatial control of these data means that precise geographical control of crustal structure from Nd isotope mapping is still important. I demonstrate this for several regions of the Ontario Gneiss Belt, including northern Algonquin Park, western Lake Nipissing and Georgian Bay. In all these regions, mapping complex geographical patterns of crustal formation ages allows detailed documentation of large-scale crustal structure. Only with such an accurate understanding of crustal structure can accurate models of tectonic evolution be constructed in a complex, deeply exhumed orogenic belt such as the Grenville Province. [ABSTRACT FROM AUTHOR]

• The Sassandra-Cavally domain is located across the Paleoproterozoic and the Archean teranes. • Rocks consist of migmatitic gneisses, metasediments and a granitic intrusion. • U-Pb ages obtained on zircon span from the Mesoarchean to the Paleoproterozoic. • Hf compositions show mixed crustal growth processes at 3328–2812 Ma and 2213–2088 Ma. Crustal evolution in the south West African Craton is dominated by a significant input of juvenile material into the crust at ca. 2.1 Ga and it remains unclear how much of the Paleoproterozoic continental mass was influenced by the presence of pre-existing Archean crustal domains. The Sassandra-Cavally (SASCA, Côte d'Ivoire) domain is strategically located east of the Sassandra shear zone at the transition of the Paleoproterozoic and Archean terranes of the Leo-Man shield. Combined U-Pb and Lu-Hf isotopic analyses by LA-(MC-)ICP-MS were acquired on zircon grains extracted from migmatitic gneisses, metasedimentary rocks and a granitic intrusion. The migmatitic gneisses, yield Archean ages between ca. 3330 and 2810 Ma with ε Hf ranging from −9.4 to + 3.3 and a metamorphic age at 2076 ± 6 Ma. They are tentatively interpreted as orthogneisses extracted from the mantle during the Paleoarchean and reworked substantially in the Mesoarchean. Detrital zircon grains from metasedimentary units adjacent to the Archean migmatitic gneisses yield ages ranging from ca. 2213 to 2088 Ma with ε Hf ranging from + 0.0 to + 5.5, indicating derivation from juvenile Paleoproterozoic source rocks. A granite intrusion was dated within uncertainty of the metamorphic age at 2084 ± 6 Ma. It exhibits a hybrid isotopic signature with ε Hf forming two distinct clusters between −4.9 and −8.5, and between + 2.2 and + 6.5 for inherited zircon grains dated between ca. 2343 to 2100 Ma. The near continuous U-Pb age record from this early Mesoarchean event to the Neoarchean is associated with constant initial 176Hf/177Hf suggesting for an ancient lead loss event at ca. 2800 Ma or a prolonged period of zircon dissolution/precipitation and/or crystallization. This early period is followed by peaks of zircon dates highlighting crustal extraction during the Eoeburnean (ca. 2250 to 2150 Ma) and Eburnean orogenies (ca. 2140 to 2100 Ma). The absence of detrital zircon of the ages similar to those of migmatitic gneisses (∼3200–2800 Ma) in Paleoproterozoic metasediments suggests their deposition distal from the Archean terranes followed by a tectonic assemblage of the SASCA domain during the later stages of the Eburnean orogeny. [ABSTRACT FROM AUTHOR]

• IN-SIMS zircon analysis yields a ca. 2.1 Ga age for Chipman and Kazan dike swarms. • Geochemical data suggests an asthenospheric-source for the STZ LIP swarms. • The STZ LIP underwent contamination by Neoarchean metasomatized lithosphere. • New data indicate that STZ dike swarms may be 2.1 Ga, and extend for > 800 km. • The STZ LIP formed during incipient rifting of a coherent Rae-Hearne boundary. • The STZ was reactivated during Hudsonian orogenesis from 1.98 to 1.80 Ga. IN-SIMS (in-situ secondary ionization mass spectrometry) micro-zircon geochronology resolves the age of the Chipman and Kazan mafic dike swarms exposed along the central segment of the Snowbird Tectonic Zone (STZ), the geophysically defined feature separating the Rae and Hearne subprovinces of the western Churchill Province. Long-considered syn -kinematic with respect to Paleoproterozoic transpressional deformation, the primitive composition of the STZ Large Igneous Province (LIP) and granulite grade metamorphic overprint has made establishing crystallization ages difficult. In-situ U-Pb isotopic analysis of micro-zircon yielded scattered and discordant results, except for four zircon crystals, which yielded a weighted mean of 2113 Ma +/- 22 (n = 8 analyses; MSWD: 0.46). This age is similar to other mafic complexes in the western Churchill Province including the Griffin Gabbro sills interpreted to have formed during opening of the Manikewan Ocean. Nd isotopic analyses of Kazan dikes paired with geochemical data suggest that the Chipman and Kazan dikes were asthenospherically-derived but contaminated by a metasomatized lithospheric root. At ca. 2.1 Ga, the STZ was the site of intracratonic extension within a coherent Rae-Hearne crustal block (Snowbird LIP Phase I). Existing data suggests that subsequent tectonism led to focused dextral transpression along the trace of the dike swarm and attests to a long-lived interplay of preexisting weaknesses and deformation. [ABSTRACT FROM AUTHOR]

[Display omitted] • Zircon U-Pb data constrain emplacement of mafic–ultramafic suite at ca. 3.2 Ga. • Magmatism within a subduction metasomatised mantle wedge. • Mesoarchean subduction-accretion and amalgamation of ancient continental nuclei. Archean mafic–ultramafic rocks provide important clues on the tectonic and crust building processes in the early Earth. The Western Dharwar Craton (WDC) and the Karwar Block (KB) in southern Peninsular India preserve some of the oldest records of craton building processes in the Archean Earth. The tectonic affinity of the Karwar Block that lies at the western periphery of the WDC remains uncertain. Here we present the results from an integrated study of a mafic–ultramafic rock suite from this block using petrology, whole-rock geochemistry, and zircon U–Pb and Lu-Hf data. These rocks occur as deformed and dismembered units within basement granitic gneiss. The primary igneous minerals such as pyroxene and plagioclase show variable degrees of alteration. The rocks show negative Nb anomalies and prominent Pb enrichment. The uniform enrichment of Th and U over Nb-Ta indicates the influence of metasomatic fluids in the mantle wedge. The geochemical data indicate that these rocks were derived from basaltic magmas generated within a subduction metasomatised mantle wedge in an intra-oceanic arc setting without any significant crustal contamination. U-Pb isotope analysis of zircon grains from the metapyroxenite shows upper intercept ages of 3172 to 3213 Ma and weighted mean ages of 3170 to 3203 Ma. A metagabbro sample yielded upper intercept age of 2792 Ma and the 207Pb/206Pb spot ages in another sample range up to 3202 Ma. Zircon U-Pb data suggest that the mafic–ultramafic suite formed at ca. 3.2 Ga and were overprinted by 2.7 Ga thermal event. The εHf(t) values are all positive and range between 0.73 and 6.02, suggesting that the magma was derived dominantly from Mesoarchean juvenile (depleted mantle) sources. We propose a tectonic model that involves a subduction-accretion stage followed by suturing of the ancient continental nuclei. [ABSTRACT FROM AUTHOR]

[Display omitted] • Zircon xenocrysts have low-δ18O and high-εHf(t) values. • Zircon xenocrysts were generated from high temperature altered gabbroic rocks of subducted oceanic slab. • This paper provides new evidence for Paleoproterozoic oceanic slab subduction in the Trans-North China Orogen. The Trans-North China Orogen (TNCO) is suggested to form under a transition from subduction to collision from 2500 Ma to ∼ 1850 Ma, and the Taihua TTG gneiss in the Xiong'ershan district is thought to have formed in an Andean-type continental arc setting or island arc setting during this geological event. However, to date, the geochemical record of the subducted oceanic slab is still lacking. This study focuses on Hf-O isotopic analyses of zircon xenocrysts from the Qi189 hornblende monzogranite, Qiyugou Au orefield, TNCO. The results reveal that zircon xenocrysts with ages from 2304 Ma to 1934 Ma generally have low-δ18O values from 2.66‰ to 3.83‰ and high-εHf(t) values from 2.7 to 7.8. These zircon grains were generated from remelting of high-temperature altered gabbroic rocks of the subducted oceanic slab during the transition from compression to extension. In this paper, we present new evidence for Paleoproterozoic oceanic slab subduction in the TNCO. [ABSTRACT FROM AUTHOR]

• Cuxiú Formation sandstone of Aquiri region was deposited between 2705 and 2665 Ma. • Hinterland consisted of Neo- to Paleoarchaean (2705–3600 Ma) granitoids. • Hf model ages indicate reworking of late Hadean to Mesoarchean (4.0–3.0 Ga) crust. • Zircon ages and shapes suggest erosion of heterogeneous basement and short transport distance. • Correlation of zircon size vs. roundness results from fluvial transport. Provenance studies are commonly based on in situ U–Pb dating and Hf isotope analyses of detrital zircon grains, while leaving shape parameters unconsidered, even though these can provide valuable information about the hinterland and detritus transport. This is demonstrated herein by presenting a comprehensive data set derived from approximately 950 detrital zircon grains from three quartzite samples of the Cuxiú Formation of the Aquiri region, which is part of the eastern realm of the Amazon Craton. The data set combines results of zircon U–Th–Pb–Hf isotope analyses with zircon shape, and some physical parameters, including length, width, aspect ratio, roundness, typology, alpha-decay dose and density. Zircon U–Pb ages and field relationships indicate that pebbly sandstones of the Cuxiú Formation were deposited during the Neoarchaean between 2705 and 2665 Ma, likely in a fluvial or near-shoreline environment of a rift basin, with zircon grains supplied from igneous rocks of the surrounding Amazon Craton. The detrital zircon grains record crystallisation ages of 3100–2800 Ma (74%), and subordinately of 2800–2700 Ma (13%) and 3600–3100 Ma (3%). Hafnium isotope analyses further suggest crust formation in the hinterland at 4.01–3.65 Ga (14%), 3.60–3.10 Ga (85%) and 2.95 Ga (1%). Well preserved euhedral shapes of more than 50% of the detrital zircon grains, independent of U–Pb ages, suggest fast, direct supply from heterogeneous Archaean basement sources to the sedimentary basin, with a short-distance transport, plausibly in a fluvial setting. Correlation of zircon roundness with width in the size range from 50 to 150 µm indicates more intense abrasion of larger zircon grains compared to smaller ones, reflecting difference in transport mechanisms – i.e., abrasive-rolling transport vs. non-abrasive suspension transport. Similar average values for length (~120 µm), width (~66 µm), aspect ratio (1.9) and roundness (~2.5), obtained from zircon grains of very different ages (3600–2700 Ma), as well as log-normal distribution of the width parameter, suggest thorough hydrodynamic sorting during transport. Zircon typology analysis reveals a wide range of combinations of {1 0 0}:{1 1 0} prism faces with{1 0 1}:{3 0 1}:{2 1 1} pyramids, pointing to zircon crystallisation from 900 to 600 °C (average 755 °C). Uranium–Th contents indicate low α-decay doses of ≪1 × 10−15 decay events/mg for all zircon grains by the time of deposition, providing an additional explanation for the excellent zircon preservation. [ABSTRACT FROM AUTHOR]

• Early Neoproterozoic magmatic arc may exist in the interior of the Yangtze Block. • The Xiaoxikou metasedimentary rocks of the Miaowan Complex were deposited in a fore-arc setting. • The ca 850 Ma leucocratic veins intruding the Xiaoxikou metasedimentary rocks were formed in a post-tectonic setting. • The construction of the current unified Yangtze Block most likely occurred between 900 and 860 Ma. The present Yangtze Block was proposed to be formed as a unified block in the Mesoproterozoic-Neoproterozoic transition, but the detailed timing and process are poorly constrained. Here we report integrated in situ U–Pb and Hf–O isotopic analyses of detrital zircons from the Xiaoxikou metamorphosed sedimentary rocks of the Miaowan Complex in the Yangtze Block, as well as U–Th–Pb and Nd–O isotopic analyses of magmatic monazites from an undeformed and unmetamorphosed leucocratic vein intruding these metasedimentary rocks. U–Pb dating results for detrital zircons from the metasedimentary rocks display two main age peaks at ca 970 and 900 Ma. The 970 Ma zircons are dominantly characterized by mantle-like δ18O zir values (4.5–5.8‰) and highly depleted ε Hf (t) values (+5 to + 14). The 900 Ma zircons mostly have O and Hf isotopic features similar to the 970 Ma zircons, except relative small proportion of grains with high δ18O values (6.0–9.2‰) and variable ε Hf (t) values (–33 to + 7). Monazites from the vein are dated at ca 850 Ma and have uniform δ18O values (5.4–6.6‰), but significantly enriched ε Nd (t) values (–14.5 to –12.6). Based on the new isotopic results and immature lithology, the Xiaoxikou metasedimentary rocks are interpreted to be deposited between 900 and 850 Ma in a fore-arc basin, and clastic materials derived from ca 970–900 Ma arc rocks were related to the northward subduction (present-day coordinates) of the Miaowan oceanic plate. The 850 Ma leucocratic vein shows geological and isotopic characteristics consistent with local 860–840 Ma post-tectonic granitic plutons, and may represent a fractionated product of local coeval granitic magmas. We propose that the closure of the Miaowan oceanic basin and formation of the current unified Yangtze Block occurred between 900 and 860 Ma, i.e., later than the deposition of the fore-arc metasedimentary rocks while earlier than the emplacement of the post-tectonic plutons. [ABSTRACT FROM AUTHOR]