Description: Publication date: September 2018 Source: Precambrian Research, Volume 314 Author(s): Maurício Bulhões Simon, Everton Marques Bongiolo, Ciro Alexandre Ávila, Elson Paiva Oliveira, Wilson Teixeira, Rômulo Campos Stohler, Filipe Vidal Soares de Oliveira Field, petrographic and geochemical data combined with in situ zircon U-Pb LA-ICP-MS ages are documented for the São Tiago Batholith (southernmost portion of the São Francisco Craton) to understand its origin and magmatic evolution. The geologic relations indicate that the batholith is composed of granitic to granodioritic orthogneisses (L2) with tonalitic xenoliths (L1) intruded by pegmatite (L3) and metagranite (L4). L1 consists of two facies of tonalitic orthogneiss, one biotite-rich, and the other biotite-poor. The geochemical evidence, including high K 2 O with mantle-like chemical signature, suggests that the Bt-rich tonalitic gneiss (2816 ± 30 Ma) was derived from contamination of mafic magmas by crustal-derived components. The Bt-poor tonalitic gneiss, of TTG affinity, was generated by partial melting of LILE-enriched mafic rocks, possibly from oceanic plateus in a subduction environment. L2 includes two distinct types of rocks: (i) granodioritic orthogneiss, chemically ranging from medium-pressure TTGs to potassic granitoids originated via partial melting of previous TTG crust, including L1 Bt-poor; and (ii) granitic gneiss (2664 ± 4 Ma), geochemically similar to crustal-derived granites, produced by melting of the L1 Bt-rich tonalitic gneiss or mixed TTG/metasedimentary sources. L3 pegmatite (2657 ± 23 Ma) results from melting of L2, whereas L4 metagranite (dikes and stocks) shows petrogenesis similar to that of the L2 granitic gneiss. Related orthogneisses occur near the São Tiago Batholith: (i) a hornblende-bearing tonalitic gneiss, and (ii) a hybrid hornblende-bearing granitic gneiss (2614 ± 13 Ma), whose genesis is linked with interaction of sanukitoid and felsic potassic melts, representing the last Archean magmatic pulse of the region. The Minas strata along the Jeceaba-Bom Sucesso lineament near our study region encircle the São Tiago Archean crust, representing an irregular paleo-coastline or a micro-terrane amalgamation with the São Francisco Proto-craton, with possible subsequent dome-and-keel deformational processes. Our petrological and geochronological data reevaluate nebulous concepts in the literature about the SFC, revealing (i) a chemically and compositionally diverse crustal segment generated at the Late Archean in diverse geodynamic scenarios, and (ii) a more complex lineament than previously thought in terms of the paleogeography of the southern São Francisco Craton.

Description: Publication date: Available online 3 July 2018 Source: Precambrian Research Author(s): Dengfeng He, Yunpeng Dong, Xiaoming Liu, Xiaohu Zhou, Feifei Zhang, Shengsi Sun The Neoproterozoic granitoids in East Kunlun Orogen (EKO) record critical information on the composition and evolution of the early crustal materials, as well as the Precambrian tectonic evolutionary history of the Qaidam Block, northern Tibetan Plateau. Zircon U–Pb geochronology and Hf isotopic compositions of three newly discriminated gneissic granitoids in the Hongshuihe, Dangennaomuru and south Bokalike areas are employed in this study to elucidate the Neoproterozoic tectonics of EKO. Petrological studies reveal that these granitoids are S–type granite with abundant Al–rich minerals like muscovite and garnet in their mineral assemblages. Zircon U–Pb geochronological results show that the Hongshuihe and Dangennaomuru plutons yield weighted mean 206 Pb/ 238 U ages of 914 ± 5 Ma (MSWD = 0.25) and 935 ± 6 Ma (MSWD = 0.56), respectively, indicating their magmatic crystallization ages. And the south Bokalike pluton yields upper and lower intercept ages of 1002 ± 31 Ma and 388 ± 39 Ma (MSWD = 0.63), disclosing its magmatic crystallization age and the time of subsequent Paleozoic metamorphic event, respectively. The ε Hf (t) values of zircons in this study are dominated by negative with minor positive values ranging from −7.18 to +2.76. The two–stage Hf model ages of the zircons having negative ε Hf (t) values are ca. 1.7–2.0 Ga, whereas those with positive ε Hf (t) values are ca. 1.5 Ga. Both the ε Hf (t) values and the two–stage Hf model ages indicate that these granitoids were probably derived from the reworking of ancient continental crustal materials with minor mantle contribution. Combined with available data of the granitoids in EKO, central and north Qaidam Block, we propose that the Neoproterozoic granitoids in Qaidam Block have similar crystallization ages of ca. 0.9–1.0 Ga, as well as same or similar geochemical and zircon Hf isotopic compositions revealing the Qaidam Block was involved into the amalgamation of the supercontinent Rodinia. Graphical abstract

Description: Publication date: Available online 2 July 2018 Source: Precambrian Research Author(s): C.M. Hill, D.W. Davis, P.L. Corcoran Detrital zircon U-Pb ages from one sandstone and one claystone unit in the upper Huronian Supergroup, near Flack Lake, Ontario, Canada, provide new evidence for the maximum depositional age of the two youngest formations and reinterpretation of the depositional history of the upper Huronian Supergroup. The average age of the youngest zircon grains constrains the age of deposition to sometime after 2315 ± 5 Ma, but prior to intrusion of gabbro (Nipissing) dykes approximately 95 m.y. later. Clusters of late Archean age zircon are prominent in both samples, but a considerable addition from post-Archean sources is also present. The sample from the Bar River Formation contains a greater proportion of post-Archean material, which may be a result of grain-size bias. Our results are similar to U-Pb zircon ages determined from stratigraphically lower purported tuff beds in the Gordon Lake Formation, however our field and Scanning Electron Microscope (SEM) analyses demonstrate that the samples in the present study were not part of tuff units.

Description: Publication date: September 2018 Source: Precambrian Research, Volume 314 Author(s): Kai Wang, Zheng-Xiang Li, Shuwen Dong, Jianjun Cui, Baofu Han, Tao Zheng, Yilong Xu We report here new zircon U-Pb age and Hf-isotope as well as geochemical analyses of the recently discovered Archean-Paleoproterozoic Zhongxiang Complex in the northern-central Yangtze Craton, South China, and interpret the early crustal evolution of the Yangtze Craton. Zircon LA-ICP-MS dating yielded magmatic crystallization ages of ca. 2.90–2.87 Ga for two monzogranites, ca. 2.77 Ga for a trondhjemitic gneiss, and ca. 2.67–2.62 Ga for three potassic granites. The trondhjemitic gneiss also contains zircon rims that record a metamorphic event at ca. 2.08 Ga. These results demonstrate that the Zhongxiang Complex is significantly older than previously thought, and it contains the only potential TTG with ages of 2.8–2.7 Ga for the Yangtze Craton. These granitoids vary considerably in zircon Hf isotopic and geochemical characteristics. The ca. 2.90–2.87 Ga monzogranites correspond in composition to alkaline and calc-alkalic peraluminous granites originated from mixing of ancient and juvenile crustal components at relatively shallow depths. In contrast, the ca. 2.77 Ga trondhjemitic gneisses are calc-alkalic and peraluminous with relatively high Sr/Y and (La/Yb) N ratios and slightly evolved zircon Hf signatures, but without significant Eu anomaly that is, typical of a medium-pressure TTG component. Consequently, they are interpreted to be the products of partial melting of a basaltic source with appreciable involvement of pre-existing crustal components (i.e., tonalites), with garnet and amphibole in the residue. The ca. 2.67–2.62 Ga potassic granites are alkaline, faintly peraluminous and ferroan, exhibiting a high-silica A-type granite affinity. They are characterized by low Al 2 O 3 /TiO 2 ratios with dominantly undepleted zircons, suggesting their derivation from melting of meta-sedimentary rocks under high-temperature conditions with plagioclase as the main residual phases. The Zhongxiang Complex thereby differs from the Kongling Complex and other Archean provinces in the Yangtze Craton by preserving distinct magmatic pulses (ca. 2.90–2.87 Ga; ca. 2.77 Ga; ca. 2.67–2.62 Ga) with different nature and sources. A comparative analysis of the crustal evolution of the Zhongxiang Complex and other Archean provinces of the Yangtze Craton indicates a compositionally heterogeneous Archean Yangtze Craton, which likely comprises several Archean terranes that accreted together to form a uniform craton during the late Archean to Paleoproterozoic time.

Description: Publication date: September 2018 Source: Precambrian Research, Volume 314 Author(s): Jinwei Guo, Jianping Zheng, Xianquan Ping, Yusheng Wan, Yihe Li, Yuanbao Wu, Junhong Zhao, Wei Wang Granites generated by anatexis are ideal crustal probes to decipher the differentiation processes within deep continental crust. In this study, the successive emplacement of granites, including the porphyries and the coarse-grained granites, was found in the nucleus (the Kongling Complex) of the Yangtze Craton. The porphyries, intruding at 1840 ± 17 Ma (SHRIMP ages), occur as veins and contain phenocrysts of K-feldspar (∼15%), quartz (∼5%), plagioclase (∼4%) and garnet (∼1%) setting in a fine-grained groundmass (∼75%) of K-feldspar, plagioclase, quartz and biotite. The coarse-grained granites, emplacing at 1821 ± 8 Ma (SHRIMP ages), are outcropped as pluton and comprise K-feldspar (∼60%), quartz (∼30%), biotite (∼5%), plagioclase (∼5%). They are similar to A-type granites with relatively high SiO 2 (69.2–71.5 wt%, 72.6–74.5 wt%), high Fe 2 O 3 T /MgO (21.5–27.7, 5.9–19.5), 10 4 *Ga/Al ratios (3.6–4.3, 2.7–3.5) respectively and enriched HFSEs (Th, U, Pb, Ga). The porphyries contain higher total trace element contents and zirconium saturation temperature (mean 892 °C) than those of the coarse-grained granites (mean 803 °C). Magmatic zircons from the porphyries and coarse-grained granites have ε Hf (t) values of −17.6 to −11.7 and −21.0 to −14.8, which yield two-stage Hf model ages of 3.6–3.3 Ga and 3.8–3.4 Ga respectively, both falling into the evolutionary trend of the 2.9–2.8 Ga Kongling TTGs. It implies both granites are anatectic (crustal reworking) derivatives from partial melting of the Archean TTGs under sustaining extension in Paleoproterozoic. Applying trace element modelling, we further identify a unique tonalitic source for both granites which is analogous to the exposed natural Kongling tonalite, and find the porphyries slightly affected by the juvenile crust. Based on the intrusion of porphyry veins into granitic gneisses, we envisage the crustal structure beneath the craton is layered from older (∼2.94 Ga) tonalites upward to younger (∼2.87 Ga) granitic gneisses. Such hierarchical structure could shed new light on the imperceptible vertical differentiation process as cratonization.

Description: Publication date: September 2018 Source: Precambrian Research, Volume 314 Author(s): Guangyan Zhou, Yuanbao Wu, Long Li, Wenxiang Zhang, Jianping Zheng, Hao Wang, Saihong Yang In this contribution, we carry out an integrated study of petrology, whole-rock geochemistry, as well as zircon morphology, trace element, U–Pb and Lu–Hf isotopes for granitic gneisses and leucogranites from the Yudongzi complex, a poorly studied Archean outcrop in the northwestern Yangtze Block. The granitic gneisses have the TTG affinity characterized by relatively high Na 2 O/K 2 O and Sr/Y ratios, and fractionated REE pattern with relatively high LREE but low HREE concentrations. Zircon grains in the granitic gneisses show magmatic cores and metamorphic rims. U–Pb dating on zircon core of four granitic gneiss samples yielded their formation ages as 2660–2632 Ma with a weighted average of 2650 ± 21 Ma (MSWD = 1.6; n = 4). Their average ε Hf (t) values range from −0.6 to 0.5 and two-stage model ages from 3.20 to 3.10 Ga, suggesting that the granitic gneisses were likely generated by remelting of thickened Mesoarchean proto-mafic crust. The metamorphic zircon rims have relatively high Th/U ratios and identical Hf isotope compositions to their corresponding cores, indicating that the rims were formed via solid-state recrystallization process. The metamorphic zircons recorded a metamorphic event at ca. 2.5 Ga. U-Pb dating and Lu-Hf isotopes on magmatic zircons from a leucogranite sample revealed a magmatic event at 2477 ± 18 Ma. The recognization of the 2.65 Ga TTG magmatism and the 2.48 Ga intercrustal remelting event in the Yudongzi complex demonstrates the widespread Neoarchean magmatism in the Yangtze Block. Combined with the age spectra of previous detrital zircons, we show that the Yangtze Block has experienced a major tectonothermal event in the late Archean. The Yudongzi complex has a distinct crustal evolution history compare with the Paleoarchean continent nucleus in the eastern part of the Yangtze Block, suggesting that the continent nucleus might have subjected to lateral growth by accretion processes.

Description: Publication date: September 2018 Source: Precambrian Research, Volume 314 Author(s): Wen Zhang, Fulai Liu, Jia Cai, Chaohui Liu, Jianhui Liu, Pinghua Liu, Lishuang Liu, Fang Wang, Hong Yang The Jiao-Liao-Ji Belt is located between the Longgang and Langrim blocks in the Eastern Block of the North China Craton (NCC). The belt comprises Palaeoproterozoic volcanic–sedimentary successions that are divided into the Ji’an, South Liaohe, and Jingshan groups in the south and the Laoling, North Liaohe and Fenzishan groups in the north. The tectonic setting and depositional environments of the Liao-Ji area are debated. This paper presents geochemical and detrital zircon U-Pb data for the boron-bearing, graphite-bearing, aluminum-rich Ji’an Group and the clastic sediments and carbonates of the Laoling Group in southern Jilin Province, in order to constrain their depositional ages, provenance and tectonic setting. Zircon geochronology for both the Ji’an and Laoling groups yields age peaks of 2.57–2.41, 2.23–2.09, and 2.04–2.02 Ga, indicating that the sediments were derived from the Longgang and/or Liaonan block(s), the 2.2–2.1 Ga Liaoji granitoids and ∼2.0 Ga igneous rocks, respectively. On the basis of the youngest detrital zircons and the oldest metamorphic zircon peaks, depositional ages can be constrained to the period of 2.13–1.94, 2.10–1.94, and 1.95–1.90 Ga for the Mayihe, Huangchagou, and Dadongcha formations of the Ji’an Group, and 2.18–1.90, 2.08–1.91, 1.99–1.90, 1.96–1.90, and 1.94–1.90 Ga for the Dataishan, Zhenzhumen, Huashan, Linjiang, and Dalizi formations of the Laoling Group, respectively. The zircon age spectra of each formation indicate that deposition of the Ji’an and Laoling groups was related to the opening and closing of a back-arc during the period 2.2–1.9 Ga. The Dataishan Formation (Laoling Group) was deposited in the rift environment during opening of the back-arc. During the early stage, the Mayihe and Huangchagou formations (Ji’an Group) were deposited on the side of the magmatic arc, and the Zhenzhumen Formation (Laoling Group) was deposited on the passive-margin side. During the late stage, both Ji’an and Laoling groups were deposited in a pre-orogenic setting, including the Dadongcha, Huashan, Linjiang and Dalizi formations. Detrital zircon age spectra of the Ji’an Group are similar to those of the South Liaohe Group (with the exception of the Gaojiayu Formation), indicating that the two groups had a similar provenance.

Description: Publication date: September 2018 Source: Precambrian Research, Volume 314 Author(s): Xiaodong Shang, Małgorzata Moczydłowska, Pengju Liu, Lei Liu We studied microfossils and their embedding matrix minerals from chert nodules in the Ediacaran Doushantuo Formation of the Yangtze Gorges area in South China using Raman spectroscopy as well as transmitted- and polarized light microscopy. The microfossils are composed of organic carbonaceous material with weak structural organization/ordering, which indicates a low degree of thermal maturation and alteration. Raman spectral disparity of carbonaceous material among different portions (vesicle/inclusion) of microfossils reveals some differences in organic composition and structure of their precursor biological matters. The mineral phases of silica matrix that entombs the microfossils are recognized as opal-CT, cryptocrystalline quartz, and their transitional phases. Overall, Raman spectral analyses and petrographic observations document a complete and continuous sequence of silica phase transformation from opal-CT to quartz. When reconstructing the diagenetic history of sediments and mineralization process of microfossils in a localized microenvironment, the silica in pore waters precipitated preferentially at the periphery of microorganisms in the form of opal, gradually decreasing in abundance as it transformed into more stable cryptocrystalline quartz. The recrystallization took place in cryptocrystalline quartz to form grains. Authigenic fluorapatite readily precipitated in the vicinity of some microfossils and within their degraded organic fragments. While apatite was crystallizing around organisms and their biostructures, silica precipitated in the surrounding voids, inhibiting the space of potential apatite crystallization. The silicification and phosphatization of microfossils occurred in the microbial sulfate reduction (MSR) zone of the sediment column, and both were driven by the localized pH change resulting from MSR and pyrite precipitation in the microenvironment, thus facilitating the exceptional fossil preservation in the Doushantuo chert nodules.

Description: Publication date: September 2018 Source: Precambrian Research, Volume 314 Author(s): S.M. Mahbubul Ameen, Simon A. Wilde In the Yalgoo area of the north-central Yilgarn Craton, three small granitoid bodies, named the Lady Lydia (LLG), Yalgoo east (YEG) and Yalgoo north granitoids (YNG) intruded ∼2.8 Ga mafic supracrustal rocks. They form two suites: i) granodiorites (from both the LLG and YEG) with strongly fractionated REE patterns, high LREE contents, low HREE contents, weakly positive Eu anomalies, and of Archean TTG affinity, and ii) biotite monzogranites (from the YNG) with very high LREE contents and prominent negative Eu anomalies. All the granitoids are calc-alkaline, with the granodiorites recording ages of 2763–2752 Ma, and the biotite monzogranites recording ages of 2636–2634 Ma. The REE and HFSE characteristics of the granodiorites favor an Archean subduction environment. Abundant xenocrystic zircons (3.00–2.94 Ga) in the granodiorites and their Lu-Hf isotopic characteristics imply a long history of crustal recycling in the north-central Yilgarn. The Lu-Hf isotopes indicate that the ∼2.76 Ga granodiorites and xenocrysts were formed from a composite source ranging in age from 3.5 to 3.0 Ga. Increased crustal thickening due to convergence (after cessation of subduction) caused melting in the upper/mid-crustal levels, with generation of parental melt at ∼2.64 Ga to source the biotite monzogranites. The latter are characterized by strongly −ve εHf(t) (−10 to −12) and largely Eoarchean T DM2 (Hf) model ages (∼3.85–3.6 Ga). Significantly, they provide evidence of Eoarchean crustal material in this part of the Yilgarn Craton. Graphical abstract

Description: Publication date: Available online 11 June 2018 Source: Precambrian Research Author(s): Naomi M. Tucker, Martin Hand, David E. Kelsey, Richard Taylor, Chris Clark, Justin L. Payne We present LA–ICP–MS U–Pb monazite and zircon geochronology, trace element chemistry and phase equilibria forward modelling to constrain the P–T–t evolution of the Bunger Hills, East Antarctica. Metasedimentary rocks in the Bunger Hills record evidence for a protracted metamorphic history during the Mesoproterozoic. Taken in isolation, zircon and monazite ages suggest an extremely long duration of high-temperature conditions (ca. 200 Myr). Calculated P–T models indicate metamorphism involved medium pressures of 5.5–7.1 kbar and high to ultrahigh temperatures of 800–960 °C, and that the P–T path likely tracked along a down-pressure to isobaric cooling trajectory. Integrating trace element data from zircon, monazite and garnet indicates that, despite the spread in U–Pb ages, peak metamorphism essentially occurred over the interval ca. 1220–1180 Ma. The age and conditions of Mesoproterozoic metamorphism are consistent with the high-grade metamorphic evolution proposed previously for Stage-2 of the Albany–Fraser Orogeny in southwestern Australia. The P–T–t conditions are interpreted to reflect extension, potentially associated with unloading and exhumation of a collisional orogen following Stage-1 of the Albany–Fraser Orogeny. This is the first study to integrate geochronology, trace element chemistry and P–T modelling to constrain the metamorphic evolution of the Bunger Hills and to interpret these constraints within the context of the now separate terranes of the Musgrave–Albany–Fraser Orogen. The three-way approach adopted in this study demonstrates that zircon and monazite may grow and modify through a number of processes. An integrated petrochronologic approach is therefore essential for investigations on high-grade terranes.