Beschreibung: Publication date: Available online 9 June 2018 Source: Precambrian Research Author(s): Jun-Hui Bi, Wen-Chun Ge, De-He Xing, Hao Yang, Yu Dong, De-Xin Tian, Hui-Jun Chen Whole-rock major and trace element geochemistry, along with zircon U–Pb ages and Lu–Hf isotope data, are reported for Palaeoproterozoic meta-rhyolites and meta-dacites of the Liaohe Group in the northern segment of the Jiao-Liao-Ji Belt, North China Craton. These data are used to constrain the petrogenesis of the rocks and the Palaeoproterozoic tectonic evolution of the Jiao-Liao-Ji Belt. Zircons from these rocks are euhedral–subhedral, display oscillatory growth zoning, and have high Th/U ratios (0.26–1.21), implying a magmatic origin. LA–ICP–MS zircon U–Pb age data indicate that the silicic metavolcanic rocks formed during the Palaeoproterozoic (2184–2127 Ma). The zircons yield ε Hf (t) values of –9.7 to +8.9 and T DM2 model ages of 2205 to 3313 Ma. Geochemically, the silicic metavolcanic rocks are metaluminous to weakly peraluminous and belong to the calc-alkaline I-type series. The rocks are characterized by enrichment in large-ion lithophile elements and depletion in high-field-strength elements (e.g., Nb, Ta, P, and Ti), and have affinities with igneous rocks that form in an active continental margin setting. The geochemical and isotopic data suggest that the rocks were derived mainly from partial melting of Meso-Neoarchaean juvenile lower crust, with a possible contribution from older materials. The present results, combined with the regional geology, further demonstrate that an oceanic plate was subducted northward beneath the Longgang Block during the Palaeoproterozoic.

Beschreibung: Publication date: Available online 6 June 2018 Source: Precambrian Research Author(s): Zhiyuan He, Bo Wang, Linglin Zhong, Xiaoyan Zhu The Central Tianshan Block is one of the major continental constituents of the Central Asian Orogenic Belt (CAOB) and contains vital information for understanding the amalgamation of the Eurasia continent. Its tectonic affinity remains controversial hindering a better reconstruction of the paleogeographic evolution of the Paleo-Asian Ocean and geodynamic processes of the CAOB. In order to put more constraints on the Precambrian basement composition and crustal evolution of the Central Tianshan Block, we conducted field structural investigations and zircon U-Pb dating on the meta-sedimentary and meta-igneous rocks along the Wulasitai - Wulanmoren ductile shear zone (WWSZ), northern Baluntai area (NW China). The WWSZ is geometrically a SE-NW-striking high-strain deformation zone of ∼1-5 km wide. It is lithologically composed of greenschists, paragneiss, amphibolitic metabasalts, migmatitic granites and mylonitic granites. The greenschist to amphibolite facies meta-sandstones and meta-volcanic rocks belong to the Ahebulake Group, in which NE- or SW-dipping steep mylonitic foliations and sub-horizontal lineations are well developed. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U-Pb dating of detrital zircons from four representative meta-sedimentary rocks from the Ahebulake Group yielded several major age groups of 3130-2530 Ma, 2450-2150 Ma, 2000-1700 Ma, 1620-1433 Ma, 1170-655 Ma, ∼450 Ma, ∼425 Ma, ∼400 Ma, ∼360 Ma and ∼312 Ma. Three ductilely deformed and metamorphosed granitic samples show four major zircon U-Pb age peaks at ∼430 Ma, ∼400-390 Ma, ∼360 Ma and ∼330 Ma. On the basis of a careful analysis of the texture of the dated zircons and the structural features of their host rocks, and combined with the available isotopic ages and regional geological data, we propose that: (1) meta-sedimentary rocks from the Ahebulake Group were likely deposited later than about 400 Ma instead of in the Silurian, as previously assumed; (2) the distinct Neoproterozoic detrital zircon age population in these meta-sediments probably reflects the widespread occurrence of coeval in-situ basement rocks as source; (3) the Central Tianshan Block may have close tectonic affinities with both the Yili Block and the Tarim Craton during the Proterozoic; and (4) the ductile shearing along the WWSZ is likely later than ∼330 Ma, probably initiated at ∼312-299 Ma. Our new results provide further arguments for the paleogeographic linkage of the microcontinents within the Paleo-Asian Ocean and tectonic reconstruction of the southwestern CAOB.

Beschreibung: Publication date: Available online 6 June 2018 Source: Precambrian Research Author(s): Yuejun Wang, Chengshi Gan, Qingli Tan, Yuzhi Zhang, Huiyin He, Qian Xin, Yanhua Zhang It has been generally accepted that the Neoproterozoic amalgamation of the Yangtze and Cathaysia Blocks along the Jiangnan orogenic zone created the united South China Block (SCB). However, its final oceanic consumption time and subsequent assemblage process remains controversial. This paper presents a set of new geochronological, geochemical and Sr-Nd-Hf-O isotopic data for the early Neoproterozoic mafic to felsic igneous rocks in the previously-mapped Chencai Group and the associated equivalents. Our data show that the compositions of these rocks plot into the fields of the subalkaline and alkaline series, and they can be geochemically subdivided into five groups. Group 1 has SiO 2 =56.68-63.97 wt% and MgO=2.31-8.90 wt% with Mg-number of 42-69, resembling those in the high-Mg intermediate rocks. Their 87 Sr/ 86 Sr (t) ratios range from 0.7022 to 0.7043, εNd(t) from +3.0 to +5.5, εHf(t) from +4.9 to +7.7 and δ 18 O from 4.78 to 5.57, most likely showing their pretrogenetic relevance to the interaction of young slab-derived fluid with the overlying refractory mantle. Group 2 with SiO 2 of 63.68-74.89 wt.% show high Sr and Ba, Sr/Y and (La/Yb)cn, but low Y and Yb, resembling high-silica adakites. Its arc-like elemental signatures and highly positive ε Nd (t)- εHf(t) values, along with δ 18 O vaules of 4.95-5.39 suggest its derivation from the recently-ponded mafic lower crust with a mantle wedge source similar to Group 1. Group 3 is characterized by the arc-like basaltic and andesitic rocks with SiO 2 =45.06-63.53 wt%, MgO=3.38-12.99 wt%, 87 Sr/ 86 Sr (t)= 0.7032-0.7047, εNd(t)=+2.0 to +5.9, εHf(t)=+3.2∼+7.8, originating from the MORB-like source newly introduced by slab-derived fluid. Group 4 has SiO 2 =44.55 -52.46 wt % and MgO=3.34 -8.07 wt% has Nb of 18.2-55.0 ppm and Nb/La of 1.01-1.76. These rocks can be classified as the high-Nb OIB-like alkaline mafic rocks, and interpreted as the product of the enriched component- modified asthenosphere. Group 5 has similar elemental signatures to the Nb-enriched basalt, with SiO 2 =44.52- 48.80 wt%, Nb/La=0.38-0.81 and Nb=8.2-17.4 ppm, and 87 Sr/ 86 Sr(t)=0.7032-0.7044 and εNd(t)=+2.7∼+5.5, suggestive of an origin from a mantle wedge source recently modified by both slab-derived and OIB-like components. Our zircon U-Pb geochronological results for the representative samples of five groups indicate that all the five groups formed at 830-859 Ma with the cluster age of ∼830-845 Ma in early Neoproterozoic, identical to the deposited time of the Lengjiaxi Group and its equivalents. The coexistence of the 830-860 Ma high-Mg intermediate (Group 1) and high-Si (Group 2) adakitic rocks, arc-like basaltic and andesitic (Group 3) rocks, and high-Nb (Group 4) and Nb-enriched (Group 5) mafic rocks in the Chencai Complex suggests an early Neoproterozoic arc-trench setting and the ongoing of subduction until ∼830 Ma. The initial assemblage of the Yangtze with Cathaysia Blocks along the whole Jiangnan orogen might be roughly synchronous (∼820-830 Ma). An early Neoproterozoic slab window model at subduction termination (∼830 Ma) is proposed, representing an improved understanding of the petrogenesis of the early Neoproterozoic Chencai igneous rocks.

Beschreibung: Publication date: Available online 6 June 2018 Source: Precambrian Research Author(s): Yun-Chuan Zeng, Qin Chen, Ji-Feng Xu, Jian-Lin Chen, Huang Feng, Hong-Xia Yu, Pei-Pei Zhao The Precambrian origin and evolution of the Lhasa Terrane remain unclear. Here, we present new zircon U–Pb isotopic and whole-rock geochemical data for the RenTso gabbros of the central Lhasa Terrane. These data indicate that these gabbros formed during the early Neoproterozoic (∼925 Ma) and have affinities with high-Fe–Ti basaltic rocks, given that they contain low concentrations of SiO 2 (39.8–49.0 wt. %) and high concentrations of FeO T (11.3–23.5 wt. %), TiO 2 (2.19–5.70 wt. %), and V (333–1204 ppm). These gabbros also have N-MORB-like trace element features such as low (La/Sm) N ratios (0.50–0.57) and the absence of negative Nb–Ta anomalies. Combining these features with the high whole-rock ε Nd (t) (+5.04 to +7.55) and zircon ε Hf (t) (+7.8 to +13.0) values for these samples indicate that the RenTso gabbros formed by the partial melting of depleted asthenospheric mantle. The Fe–Ti enrichment in these gabbros reflects conditions of low oxygen fugacity during magma fractionation. These gabbros are spatially associated with troctolite and peridotite units, indicating that together these units form a typical ophiolitic lithological assemblage. These geochemical and field observations suggest that the study area contains a subduction-unrelated-type ophiolite that most likely represents the embryonic crust of the Mozambique Ocean following continental rifting and breakup. The results therefore indicate that the Lhasa Terrane has a Precambrian affinity with the East African orogen and occupied a different position to the Qiangtang Terrane within the Rodinia supercontinent.

Beschreibung: Publication date: Available online 6 June 2018 Source: Precambrian Research Author(s): C.L. Kirkland, C. Yakymchuk, J. Hollis, H. Heide-Jørgensen, M. Danišík Recent geochronological and petrographic studies in the Nuuk region, West Greenland, have established that modern-style tectonic processes were active in the Neoarchean. However, more limited work has addressed Neoarchean processes elsewhere in West Greenland. In particular, conflicting models have been proposed for the tectonothermal history of the Akia terrane, to the north of the Nuuk region. In this study, the depositional and metamorphic history of supracrustal rocks in the Akia terrane have been investigated using field relationships, petrography, phase equilibria modelling, and zircon and apatite U-Pb geochronology. The Akia terrane is a Palaeo- to Mesoarchean tonalite-trondhjemite-granodiorite (TTG) gneiss terrane with subordinate supracrustal rocks, mafic to ultramafic intrusions – all metamorphosed at amphibolite to granulite facies – Proterozoic mafic dyke suites, Neoproterozoic and Jurassic kimberlitic rocks, and a Mesoarchean and a Jurassic carbonatite. Some of the supracrustal rocks are known to be Mesoarchean, whilst others lack age constraint. New detrital and metamorphic zircon ages show that the metasedimentary rocks studied – the Kangerluarsuk Supracrustal Belt – were sourced from TTG gneisses within the host Akia terrane and deposited in the period ≤2877 Ma to ≥2857 Ma; this requires exhumation of the Akia terrane after c. 3010–2970 Ma magmatism and low-pressure granulite facies metamorphism. After exhumation, the Kangerluarsuk Supracrustal Belt was buried and underwent prolonged high-temperature metamorphism, reaching ∼820–850°C and 8–10 kbar during regional ductile deformation in the period 2857–2700 Ma. The same rocks grew metamorphic zircon and neoblastic apatite at c. 2630 Ma and >450 to 〈850°C, based on U-Pb geochronology and Pb diffusion modelling respectively. This evidence of Neoarchean metamorphism of the Akia terrane shows that a large part of West Greenland was in tectonic communication and experienced common high temperature conditions in the latest Mesoarchean to early Neoarchean and by c. 2700 Ma. The recognition of prolonged Neoarchean high-temperature metamorphism and partial melting during regional ductile deformation raises questions about the veracity of textural evidence for a giant bolide impact at ≥3000 Ma in the same region as such evidence would likely have been obliterated through such pervasive metamorphic and tectonic reworking.

Beschreibung: Publication date: September 2018 Source: Precambrian Research, Volume 314 Author(s): Xiao-Fei Xu, Long-Long Gou, Xiao-Ping Long, Yun-Peng Dong, Xiao-Ming Liu, Jian-Wei Zi, Zheng-Hui Li, Cheng-Li Zhang, Liang Liu, Jiao Zhao The Helanshan complex in the Khondalite Belt, North China Craton (NCC) is characterized by the occurrence of both medium-pressure (MP) and high-pressure (HP) pelitic granulites. This paper presents new results from phase equilibrium modelling and SHRIMP zircon U–Pb dating on the MP garnet-sillimanite granulites, in order to better understand the relationship between the MP and HP pelitic granulites, the tectonic implications of the MP pelitic granulites for the formation of the Khondalite Belt, and their geodynamic implications for the Paleoproterozoic orogenesis. Based on petrographic observations, the metamorphic evolution can be divided into four stages (M 1 –M 4 ). The M 1 –M 3 stages are suprasolidus as reflected by the migmatitic structure on the outcrop. The peak metamorphic stage (M 1 ) is represented by the rim of garnet porphyroblasts along with inclusions of sillimanite + quartz ± biotite, and rutile relicts + plagioclase + K-feldspar in the matrix; the M 2 is defined by the ilmenite that have partially replaced rutile; and the M 3 is characterized by the replacement of garnet by cordierite. The subsolidus mineral assemblage is garnet + biotite + plagioclase + K-feldspar + sillimanite + cordierite + ilmenite + quartz (M 4 ). Combined with the phase equilibrium modelling in the Na 2 O–CaO–K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 –O 2 chemical system, the MP pelitic granulites record a clockwise P–T path, which is characterized by near-isothermal decompression (ITD) from ∼10.1 kbar at ∼830 °C to ∼5.8 kbar at ∼800 °C and subsequent close-to-isobaric cooling (IBC). SHRIMP zircon U–Pb dating yielded a metamorphic age of 1931 ± 21 Ma, interpreted as the time of the retrograde cooling. Integrated with previously reported results, we propose that slab break-off subsequent to Yinshan-Ordos collision at ca. 1.95 Ga led to uplift of the MP and HP granulites in the Khondalite to the middle crustal level in an extensional regime as indicated by their ITD evolution, which was followed by slow cooling at ca. 1.93 Ga as evidenced by their nearly IBC evolution.

Beschreibung: Publication date: August 2018 Source: Precambrian Research, Volume 313

Beschreibung: Publication date: Available online 4 June 2018 Source: Precambrian Research Author(s): M.B. Klausen, M.K.M. Nilsson Following upon previous work on the 1630 Ma Melville Bugt Dyke Swarm (MBDS) along the NW coast of Greenland, this paper confirms its proposed continuation in SE Greenland, into an extraordinary > 2000 km long trans-Greenlandic dyke swarm. This correlation is not only based on the swarm’s continuous pattern, but now also by a matching 1630 ± 4 Ma U – Pb baddeleyite age determination and similar bulk rock compositions, as well as distinctive outcrop characteristics. A greater number of thinner dykes across SE Greenland supports a previous notion of MBDS being emplaced laterally from a more southerly located magma chamber; whereas, a doubling of both individual and cumulative dyke thicknesses towards the north may relate to how the swarm’s blade-shaped dyke geometries are exposed in greater abundance farther from its source. Remarkably homogenous yet differentiated, transitional to trachybasaltic compositions indicate a single magma chamber source that was buffered by between 4 – 7 wt% MgO, through magma replenishments and cotectic plagioclase-olivine fractionation at lower crustal depths for >13 ± 6 Myrs (published age range). Geochemical signatures – of particularly high Ba and other LILE’s, negative Nb-Ta spikes and otherwise slightly enriched HFSE patterns – bear a conspicuous resemblance to >100 Myr-older late- to post-Ketilidian appinites (orogenic lamprophyres), which are both thought to have been derived from a common highly metasomatised, young sub-continental lithospheric mantle. The dry mineralogy of the MBDS and a geologically based Nuna supercontinent reconstruction is more consistent with a breakup setting, which together with neighbouring coeval AMCG-complexes may arguably combine into an unusual LIP. However, such a relatively short-lived LIP-setting may be questioned if the plagioclase-phyric MBDS was derived from a similar deep crustal magma chamber source, as proposed for four Mesoproterozoic cycles of AMCG-emplacements during a >500 Myr Nuna –Rodinia supercontinental transition. Consequently, arguing for a more long-lived active continental margin, as suggested by palaeomagnetically based Nuna supercontinent reconstructions.

Beschreibung: Publication date: September 2018 Source: Precambrian Research, Volume 314 Author(s): Sukanta Dey, Sayantan Pal, S. Balakrishnan, Jaana Halla, Matti Kurhila, Esa Heilimo Several profound changes, including those involving formation of the continental crust, occurred on Earth during the Neoarchaean Era. However, the tectonic settings associated with Neoarchaean crustal growth are not well understood and vigorously debated. The Neoarchaean Veligallu greenstone belt, eastern Dharwar craton hosts a variety of ultramafic, mafic and felsic volcanic rocks. Whole-rock elemental and Nd isotope data along with zircon U-Pb dating on these rocks provide significant insights into the origin and tectonic setting of Neoarchaean crust formation. The volcanism in the Veligallu belt started with ∼2.67 Ga tholeiitic basalts derived from shallow melting of a slightly depleted mantle (ε Ndt  = +0.6 to +1.1). Moderate negative Nb anomalies, slightly elevated Th/Yb and LREE, and an absence of evidence for crustal contamination are consistent with extraction of these basalts from a mantle source weakly metasomatized by subducted slab-derived fluids in an incipient oceanic arc setting. As the arc matured, clastic sediments started forming with concurrent emplacement of komatiites, komatiitic basalts and ferropicrites showing strong signatures of contamination with continental crust (negative Nb and Ti anomalies, LREE enrichment and negative ε Ndt ). In the final stage (∼2.58 Ga), a variety of felsic volcanic rocks (sodic trachyandesite, high Mg# andesite, rhyolite, calc-alkaline andesite) formed. The rock association and distinct geochemical signatures (enrichment of LILE, negative Nb and Ti anomalies, Mesoarchaean Nd model ages and inherited older zircons) suggest a continental margin arc environment which contained older crust. The evolutionary history of the Veligallu belt implies that both the arc- and plume-related processes, and their interplay contributed significantly to the growth of Neoarchaean crust.

Beschreibung: Publication date: Available online 2 June 2018 Source: Precambrian Research Author(s): Hang Liu, Jun-Hong Zhao, Peter A. Cawood, Wei Wang The South Qinling Belt, incorporating the northern margin of the Yangtze Block, South China Craton, records a series of Neoproterozoic to Triassic cycles of ocean opening and closing. The Neoproterozoic Suixian Group provides a record of breakup of the Rodinia supercontinent. New zircon U-Pb ages, Hf isotopes and whole-rock elemental data from the group constrain the broader tectonic evolution of the South Qinling Belt and the position of South China within Rodinia. The Suixian Group comprises sandstone and siltstone associated with minor interlayered volcanic rocks. Detrital zircons show three U-Pb age populations at ∼2450 Ma, ∼1990 Ma and ∼750 Ma, with a minimum age of ∼700 Ma. Their εHf(t) values range from -22.5 to +11.1. Zircon U-Pb and Hf isotopic data are similar to the age range and character of magmatic zircons from the Precambrian igneous rocks in the South Qinling Belt, which in combination with the euhedral to subhedral form of the detrital zircons, suggests local derivation from rock units within the belt. The sedimentary rocks from the Suixian Group have high La/Sc (0.91-7.28), Th/Sc (0.56-2.16) and low Cr/Th ratios (0.94-5.28), implying derivation from felsic volcanic rocks. Their elemental contents and ratios are similar to those accumulating in a continental arc environment. Similarly, the detrital zircons from the late Neoproterozoic sedimentary rocks along the margins of the Yangtze Block show a dominant U-Pb age peak of 770 Ma to 710 Ma, suggesting significant input from an approximately syn-depositional magmatic source. The Neoproterozoic detrital zircons show similar age distributions and εHf(t) values to those of the Neoproterozoic igneous rocks around of the Yangtze Block, suggesting that these sedimentary rocks were sourced from a magmatic arc. The Yangtze Block and northern India preserve similar Neoproterozoic sedimentary successions and detrital age populations indicating that they were closely connected and located along either the margin of Rodinia or on a plate separate from Rodinia.