Final Amalgamation Research Articles

Aeromagnetic study of the Hengshan–Wutai–Fuping region: Unraveling a crustal profile of the Paleoproterozoic Trans-North China Orogen

An integrated crustal profile of the intervening Trans-North China Orogen (TNCO) is one of the key issues to understanding the tectonic evolution of the North China Craton. However, the existing geological studies focus only on the surface-mapping based petrological, geochemical and structural analysis, but lack subsurface geophysical evidence and thus make the crustal profile interpretations ambiguous. In contrast, the current geophysical data covers a very large-scale lithospheric mantle and fails to image the detailed structural pattern of the orogenic crust. To achieve this goal, we present high-resolution aeromagnetic data for the Hengshan–Wutai–Fuping region, the largest exposure of the central TNCO. The reduced-to-pole magnetic anomaly map firstly verifies the regional tectonic subdivision that the high-grade metamorphic terranes (i.e. Hengshan and Fuping Complexes) are consistent with high-magnetic responses and long-wavelength anomalies, intervened by a low-grade terrane (Wutai Complex) characterized by low-magnetic responses and short-wavelength anomalies. 3D Euler deconvolution reveals that the tendencies of the clustered solutions show large consistence with the major structural pattern of the region which is characterized by a fan-shaped doubly-vergent orogenic wedge. Upward continuation further shows that the northwest part of the orogen yields a thicker crust and is most likely located closer to the paleosubduction zone. The new aeromagnetic data, combined with structural, petrological and metamorphic data indicate that an eastward-dipping subduction zone was most possibly active before the collision of the Western and Eastern Blocks, leading to the formation of the TNCO and the final amalgamation of the North China Craton.

Middle Neoproterozoic (∼845 Ma) continental arc magmatism along the northwest side of the Jiangshan–Shaoxing suture, South China: Geochronology, geochemistry, petrogenesis and tectonic implications

This paper presents the first detailed zircon U–Pb chronology, major and trace element, and Nd–Hf isotope geochemistry of two Neoproterozoic plutons (Shanhou and Jiangshan) along the northwest side of the Jiangshan–Shaoxing suture, Zhejiang province, South China. The Shanhou pluton consists of gabbros, dioritic rocks and monzogranites. SHRIMP and LA-ICP-MS zircon U–Pb dating indicates that the gabbros (∼846Ma), dioritic rocks (∼845Ma) and monzogranites (∼847Ma) were emplaced simultaneously. The Jiangshan pluton consists mainly of dioritic rocks that were also emplaced in the middle Neoproterozoic with a SHRIMP zircon U–Pb age of ca. 842Ma. The Shanhou gabbros are mainly high-K calc-alkaline, and are enriched in LREE and LILE and depleted in HFSE with marked negative Ta–Nb anomalies. They have positive ɛNd (T) (2.0–3.5) and ɛHf (T) (in-situ zircon) (3.3). The Shanhou and Jiangshan dioritic rocks as well as the Shanhou granites are also high-K calc-alkaline, enriched in LREE and LILE and depleted in HFSE with marked negative Ta–Nb anomalies. They also show positive ɛNd (T) (0.1–4.7) and ɛHf (T) (in-situ zircon) (0.8–5.0). Detailed elemental and isotopic data suggest that both plutons were formed in a continental arc setting with gabbros derived from partial melting of subduction-modified mantle wedge. Such gabbroic magmas underwent fractionation crystallization of clinopyroxene+amphibole, forming the less felsic dioritic rocks. Progressive fractionation crystallization of amphibole+plagioclase from the evolved melts with a little crustal assimilation produced the more felsic dioritic rocks and granites. Our new data suggest that final amalgamation between the Yangtze and Cathaysia blocks postdated 842Ma.

P-T-t reconstructions of South Yenisei Ridge metamorphic history (Siberian craton): Petrological consequences and application to the supercontinental cycles

Studies of gneisses from the Yenisei regional shear zone (YRSZ) provide the first evidence for Mesoproterozoic tectonic events in the geologic history of the South Yenisei Ridge and allowed the recognition of several stages of deformation and metamorphism spanning from Late Paleoproterozoic to Vendian. The first stage (~1.73 Ga), corresponding to the period of granulite-amphibolite metamorphism at P = 5.9 kbar and T = 635 °C, marks the final amalgamation of the Siberian craton to the Paleo-Mesoproterozoic Nuna supercontinent. During the second stage, corresponding to a hypothesized breakup of Nuna as a result of crustal extension, these rocks underwent Mesoproterozoic dynamic metamorphism (P = 7.4 kbar and T = 660 °C) with three peaks at 1.54, 1.38, and 1.25 Ga and the formation of high-pressure blastomylonite rocks in shear zones. Late-stage deformations during the Mesoproterozoic tectonic activity in the region, related to the Grenville-age collision processes and assembly of Rodinia, took place at 1.17-1.03 Ga. The latest pulse of dynamic metamorphism (615–600 Ma) marks the final stage of the Neoproterozoic evolution of the Yenisei Ridge, which is associated with the accretion of island-arc terranes to the western margin of the Siberian craton. The overall duration of identified tectonothermal processes within the South Yenisei Ridge during the Riphean (~650 Ma) is correlated with the duration of geodynamic cycles in the supercontinent evolution. A similar succession and style of tectonothermal events in the history of both the southern and the northern parts of the Yenisei Ridge suggest that they evolved synchronously within a single structure over a prolonged time span (1385–600 Ma). New data on coeavl events identified on the western margin of the Siberian craton contradict the hypothesis of a mantle activity lull (from 1.75 to 0.7 Ga) on the southwestern margins of the Siberian craton during the Precambrian. The synchronous sequence and similar style of tectonic events on the periphery of the large Precambrian Laurentia, Baltica, and Siberia cratons suggest their spatial proximity over a prolonged time span (1550–600 Ma). The above conclusion is consistent with the results of modern paleomagnetic reconstructions suggesting that these cratons represented the cores of Nuna and Rodinia within the above time interval.

Late Neoproterozoic granulite facies metamorphism in the Menderes Massif, Western Anatolia/Turkey: implication for the assembly of Gondwana

The Menderes Massif is a major polymetamorphic complex in Western Turkey. The late Neoproterozoic basement consists of partially migmatized paragneisses and metapelites in association with orthogneiss intrusions. Pelitic granulite, paragneiss and orthopyroxene-bearing orthogneiss (charnockite) of the basement series form the main granulite-facies lithologies. Charnockitic metagranodiorite and metatonalite are magnesian in composition and show calc-alkalic to alkali-calcic affinities. Nd and Sr isotope systematics indicate homogeneous crustal contamination. The zircons in charnockites contain featureless overgrowth and rim textures representing metamorphic growth on magmatic cores and inherited grains. Charnockites yield crytallization age of ~590 Ma for protoliths and they record granulite-facies overprint at ~ 580 Ma. These data indicate that the Menderes Massif records late Neoproterozoic magmatic and granulite-facies metamorphic events. Furthermore, the basement rocks have been overprinted by Eocene Barrovian-type Alpine metamorphism at ~42 Ma. The geochronological data and inferred latest Neoproterozoic–early Cambrian palaeogeographic setting for the Menderes Massif to the north of present-day Arabia indicate that the granulite-facies metamorphism in the Menderes Massif can be attributed to the Kuunga Orogen (600–500 Ma) causing the final amalgamation processes for northern part of the Gondwana.

Late Neoproterozoic gabbro emplacement followed by early Cambrian eclogite-facies metamorphism in the Menderes Massif (W. Turkey): Implications on the final assembly of Gondwana

Numerous (meta-)gabbroic dikes or stocks occur within the latest Neoproterozoic–early Cambrian series of the Menderes Massif (Anatolide–Tauride Block, western Turkey). These well-preserved rocks were locally converted into eclogitic metagabbros and garnet amphibolites along the contacts or shear zones. Both bulk-rock composition and compositions of igneous clinopyroxenes suggest continental tholeiitic affinity. U–Pb dating of igneous zircons from gabbroic rocks yielded a mean age of 563±1Ma (2σ), indicating emplacement during the latest Neoproterozoic (Ediacaran). On the other hand, rims of zircons from eclogitic metagabbro gave 535±3Ma (2σ) (early Cambrian), in addition to 558±3Ma (2σ) obtained from the igneous core of zircons. These ages are interpreted as the time of high-P metamorphism and crystallization age of gabbroic protolith, respectively. Given the estimated paleogeographic position of the Anatolide–Tauride Block during the late Neoproterozoic and early Cambrian, this orogenic event can be spatially and temporally related to the northward continuity of 600–500Ma orogenic event (Malagasy/Kuunga orogeny) extending from western margin of India, Madagascar, via Arabia up to northern margin of Gondwana beneath thick Phanerozoic cover series in Arabian Peninsula. Therefore, the high-P evolution of the basement of the Menderes Massif and associated basic intrusions can be interpreted to mark the latest stages of consumption of the basin/oceanic branches and final amalgamation of the Gondwana during the late Neoproterozoic–early Cambrian around the Arabian region.

Chapter 3 The Archaean and Proterozoic history of Peninsular India: tectonic framework for Precambrian sedimentary basins in India

Abstract The Precambrian geological history of Peninsular India covers nearly 3.0 Ga. The Peninsula is an assembly of five different cratonic nuclei known as the Aravalli–Bundelkhand, Eastern Dharwar, Western Dharwar, Bastar and Singhbhum cratons along with the Southern Granulite Province. Final amalgamation of these elements occurred either by the end of the Archaean (2.5 Ga) or by the end of the Palaeoproterozoic ( c. 1.6 Ga). Each of these nuclei contains one or more sedimentary basins (or metasedimentary basins) of Proterozoic age. This chapter provides an overview of each of the cratons and a brief description of the Precambrian sedimentary basins in India that form the focus of the remainder of this book. In our view, it appears that basin formation and subsequent closure can be grossly constrained to three separate intervals that also broadly correspond to the assembly and disaggregation of the supercontinents Columbia, Rodinia and Gondwana. The oldest Purana-I basins developed during the 2.5–1.6 Ga interval, Purana-II basins formed during the 1.6–1.0 Ga interval and the Purana-III basins formed during the Neoproterozoic–Cambrian interval.

Foreign contemporaries – Unravelling disparate isotopic signatures from Mesoproterozoic Central and Western Australia

Extensive time constrained isotopic (Sm–Nd, Lu–Hf) datasets on samples with well-studied geological context demonstrates that the Musgrave Province of central Australia and the Albany–Fraser Orogen of south Western Australia evolved through temporally similar Mesoproterozoic orogenies – at 1345–1260 and 1215–1140Ma – but on completely different basement. The West and North Australian Cratons amalgamated before the 1345–1260Ma event, which saw the convergence and final amalgamation of those combined cratons with the South Australian Craton. Well-defined Nd- and Hf-isotopic evolution arrays for the Musgrave Province reflect a juvenile basement with distinct 1950–1900, 1600–1550Ma crust formation events. Rare non-radiogenic material is seen only in detritus and in magmas that assimilated this material. This evolved detritus was derived from non-Yilgarn Archean sources, and reworked Archean sources, which were added to sedimentary basins developed over the Musgrave Province basement during and after c. 1400Ma. To the south of the Musgrave Province, the Madura Province has a similarly radiogenic signature to the Musgrave Province and is likely contiguous with it beneath younger cover sequences. It shows 2.0–1.4Ma mantle extraction ages and is dominated by juvenile rocks reflecting oceanic crust consumed to the north of the South Australian Craton and east of the West Australian Craton. Available data support the suggestion that the Musgrave Province is simply modified Madura Province crust situated along the edge of the North Australian Craton. Together, the Musgrave Province and the Madura Province represent Proterozoic Australia's most juvenile crustal remnant. In stark contrast to this juvenile remnant, the isotopic evolution array for the Albany–Fraser Orogen reveals a much more evolved signature, charting progressive juvenile input into an Archean source with strong isotopic and age similarity to the Yilgarn Craton of the West Australian Craton.

Latest Early Permian granitic magmatism in southern Inner Mongolia, China: Implications for the tectonic evolution of the southeastern Central Asian Orogenic Belt

Latest Early Permian (277–275Ma) magmatic episode is one of the most important tectono-magmatic events in southeastern Central Asian Orogenic Belt (CAOB), recording a tectonic transition from accretionary to post-accretionary evolution of the Paleo-Asian Ocean. The latest Early Permian granitoids along the Solonker suture zone in southern Inner Mongolia were emplaced between 275Ma and 277Ma as determined by SHRIMP zircon U–Pb dating. They are characterized by variable alkali contents, a range from calcic to slightly alkalic compositions, and are magnesian I-type granitoids. Their εNd(t) values from −0.4 to +3.1 record predominantly juvenile crustal sources with moderate involvement of old crustal components. Furthermore, these granitoids show positive zircon εHf(t) values of +7.6 to +10.7 that are also indicative of predominantly juvenile nature. Their high zircon δ18O values of 6.29‰ to 8.13‰, suggest significant involvement of supracrustal materials in their generation. These characteristics imply they were derived from an already hybrid andesitic magma in active continental margin. The resultant composite magmas were finally attached (relaminated) to the lower crust, where they produced segregated melt, to rise with minor assimilation of supracrustal materials, and to form these granitic plutons. The outboard migration of continental arc in response to slab roll-back, from the Late Carboniferous to the latest Early Permian, and tectonic switching from this prolonged extension to the short-lived contraction resulting from slab break-off and final amalgamation of the southeastern CAOB in the Middle-Late Permian lead to efficient continental growth in the southeastern CAOB.

Petrogenesis of the 2.1 Ga Lushan garnet-bearing quartz monzonite on the southern margin of the North China Craton and its tectonic implications

The Lushan garnet-bearing quartz monzonite was emplaced into the western Taihua Complex at 2134±17Ma on the southern margin of the North China Craton (NCC). The rocks contain SiO2 contents of 56.98–59.05wt.%, with high K2O+Na2O (7.46–9.14wt.%) and Al2O3 (15.87–17.58wt.%), and low CaO (2.38–2.77wt.%) and MgO (1.18–1.50wt.%) contents, and belong to an intermediate shoshonitic series. Magmatic garnet grains in the rocks are chemically homogeneous, with relatively high CaO (6.36–7.92wt.%), but low MnO (1.45–1.94wt.%) and MgO (2.47–3.45wt.%) contents and high Fe/Mn ratios. They contain 61.94–66.39mol.% almandine, 18.60–23.40mol.% grossular, 10.06–15.11mol.% pyrope and 1.09–4.32mol.% spessartine, similar to high-pressure garnets formed from M-type magma but different from those found in S-type granites and metapelites. Phase equilibrium diagrams in combination with trace element distribution coefficients between zircon and garnet suggest whole-rock zircon saturation temperatures varying from 891°C to 951°C and the mineral assemblage of garnet+plagioclase+amphibole or biotite in these rocks is stable at >15kbar and 890–950°C with H2O<3wt.% in the magma. This temperature range permits the breakdown of biotite and Ca-amphibole (T>800°C). All of the rocks are enriched in LILE and LREE with low Nb/La ratios (0.24–0.61), similar to those of lithospheric mantle, which has a Nb/La ratio of ∼0.5. Therefore, the garnet and whole-rock chemical compositions indicate that the Lushan quartz monzonite possibly formed by small degrees of partial melting of a phlogopite-bearing lithospheric mantle. The variable zircon ɛHf (t) values (−3.5 to +4.1) with TDMC from 2.34 Ga to 2.51Ga suggest involvement of crustal materials. The rocks have slightly negative Nb and Ta anomalies, characteristics of island arc basalts (IAB). However, their high Zr (598–926ppm) and Zr/Y ratios (17.1–21.8) suggest an affinity with ocean island basalt (OIB). Hence, the mixed IAB- and OIB-like geochemical signatures are indicative of continental intra-plate rifting, possibly related to extension and thinning of the lithosphere and upwelling of asthenosphere. Our study proposes that an intra-continental rifting event occurred at 2.2–2.0Ga on the southern margin of the NCC after the final amalgamation of NCC.

Investigating mid‐Ediacaran glaciation and final Gondwana amalgamation using coupled sedimentology and 40Ar/ 39Ar detrital muscovite provenance from the Paraguay Belt, Brazil

AbstractUnderstanding the evolution of the northern Paraguay Belt, Brazil, is critical in two current controversies: (i) the number, timing and significance of Ediacaran glaciations; and (ii) the timing of amalgamation of South American Gondwana. The Neoproterozoic Alto Paraguay Group forms much of the northern Paraguay Belt. The Serra Azul Formation, within this Group, contains unequivocal evidence for a glacial influence on sedimentation, including multi‐directional striations on sandstone clasts and striated, polished and bullet‐shaped mudstone clasts. However, the age of the Serra Azul Formation is not well‐constrained. The northern Paraguay Belt also formed after the traditionally accepted time for amalgamation of South American Gondwana. If the orogen represents closure of an ocean, then this traditional view is incorrect. A significant number of single grain 40Ar/39Ar detrital muscovite cooling ages (ca 120) from the Alto Paraguay Group are presented. The three youngest grains from the Serra Azul Formation yield a weighted mean age of 640 ± 15 Myr, providing a robust maximum depositional age for this formation. This age, when considered with other data, suggests that the Serra Azul Formation developed in a mid‐Ediacaran glaciation consistent with that expressed in the Gaskiers Formation of Newfoundland, Canada. Cryogenian 40Ar/39Ar detrital muscovite ages from the Alto Paraguay Group are hard to reconcile with the known geology of Amazonia and are interpreted as being sourced from the evolving orogen to the east – from an arc terrane, possibly the Goiás–Paranapanema Massif. Detrital muscovites in the upper part of the Alto Paraguay Group are as young as 544 ± 7 Myr, consistent with mounting evidence that indicates a Cambrian age for orogenesis within the Paraguay Belt during the final amalgamation of Gondwana. This article suggests that the data best support a model where ocean closure in the region continued until Ediacaran/Cambrian times, with final ocean closure represented by orogenesis in the Paraguay–Araguaia orogen.

Geochemistry and petrogenesis of Early Carboniferous volcanic rocks in East Junggar, North Xinjiang: Implications for post-collisional magmatism and geodynamic process

Early Carboniferous volcanic rocks in the Batamayineishan Formation overlie unconformably the molasse deposits and the ophiolitic mélanges and are restricted in narrow zones along both sides of the Kalamaili orogenic belt in North Xinjiang, southern Central Asian Orogenic Belt. These rocks demonstrate the post-collisional setting in East Junggar commenced in Tournaisian and also mark an important transitional period from the final amalgamation to late Paleozoic voluminous juvenile granitoids in East Junggar. The volcanic rocks are composed of basalt, basaltic andesite, andesite, trachyte and rhyolite. Both mafic and felsic rocks are characterized by enrichments in large ion lithophile elements, light rare earth elements and depletion in Nb and Ta, low initial 87Sr/86Sr and high, positive ɛNd(t). Three groups of mafic rocks have been identified: Shoshonitic group 1 has the highest MgO, CaO, Ni and Cr and the lowest Na2O, Al2O3, La, Ba, La/Yb and Ba/Th with primary magma features; group 2 calc-alkaline and high-K calc-alkaline mafic rocks have the lowest K2O, P2O5, Th and Th/Nb, and the highest TiO2; and group 3 (shoshonitic to potassic alkaline) has the highest K2O, P2O5, La, Ba, La/Yb and Th/Nb, and the lowest TiO2. The A-type-like felsic rocks were derived from the differentiation of the mafic magma. Geological and geochemical evidences indicate that the Batamayineishan Formation was generated from the process of slab breakoff (detachment). Group 1 samples are produced by decompressional melting of the upwelling asthenosphere mainly composed of spinel and garnet (50:50) lherzolite which has been enriched by overlying metasomatized lithosphere during ascent. Group 2 is derived from 5–10% partial melting of shallower spinel-bearing lithospheric mantle induced by the hot rising asthenosphere, where the contribution of slab-derived fluid is predominant. Low partial melting (3–5%) of the mantle wedge and/or thickened lithospheric mantle enriched by slab-derived components generates group 3. Slab breakoff as an important geodynamic process accounts for the post-collisional magmatism between 343.5Ma–330Ma, providing a model for post-collisional crust–mantle interaction in the CAOB.

Patagonia: where does it come from?

Based on the recent finding of archeocyathids in molassic middle Cambrian to Early Ordovician age-sequences of northern Patagonia the relationships between this southern part of South America and East Antarctica need to be re-examined. The early Cambrian age of the archeocyathids, and their derivation from the Shackleton Limestones, open several alternatives that are evaluated based on the lithology and the U-Pb zircon ages of the different metamorphic sequences of Patagonia and the Transantarctic Mountains. Based on these data, it is proposed that the Somuncurá Massif of northern Patagonia is the conjugate margin of the Pensacola Mountains in East Antarctica. The main episodes of deformation within the Cambrian-Ordovician Ross Orogeny are correlated, as well as the passive margin setting during the Silurian-Devonian, which indicate that the lower section of the Beacon Supergroup of Antarctica corresponds to the Sierra Grande Formation in Patagonia. These facts show that the Patagonian terrane may have been situated as the conjugate margin of the Transantarctic Mountains from Southern Victoria Land to the Pensacola Mountains. The rifting of Patagonia from Antarctica and the beginning of subduc­tion along western Patagonia, are correlated among different terranes, showing a robust coherent evolution through early Paleozoic times among these blocks. The final amalgamation of Patagonia with Western Gondwana occurred in late Paleozoic times, but is not analyzed in the present contribution.

Correcting for inclination shallowing of early Carboniferous sedimentary rocks from Kyrgyzstan—indication of stable subtropical position of the North Tianshan Zone in the mid-late Palaeozoic

SUMMARY High-quality palaeomagnetic data for the early Carboniferous of Central Asia are scarce and the palaeogeographic evolution of this area prior to final amalgamation of the region east of the Ural mountains is still rather obscure. Here, we present palaeomagnetic data for early Carboniferous deposits from two areas in the Kyrgyz North Tianshan (NTS). Detailed rockmagnetic analysis indicates the presence of magnetite and haematite as magnetic carriers in these red sediments. In the Kazakh basin section (KEL), we identify a high-temperature component (HTC) of magnetization during stepwise thermal demagnetization at temperatures of up to ∼680 ◦ C yielding a site mean direction of D = 176.2 ◦ , I =− 36.4 ◦ , k = 57.4 and α95 = 8.9 ◦ after tilt correction. Two HTCs of magnetization were identified in samples from the Sonkul Basin (DUN) with maximum blocking temperatures of ∼600 ◦ C (magnetite) and ∼680 ◦ C (haematite). The magnetite component was also identified with alternating field demagnetization. The resulting site mean directions for these two components identified in 16 and 14 sites, respectively, are D = 149.3 ◦ , I =− 50.3 ◦ , k = 73.6 and α95 = 4.3 ◦ for the magnetite and D = 139.6 ◦ , I =− 35.1 ◦ , k = 71.6 and α95 = 4.7 ◦ for the haematite component. All three mean directions show a significant increase of the precision parameter k after tilt correction indicating acquisition of the high-temperature magnetization prior to the main folding event in the Jurassic. We explain the difference of the two components of DUN by a process of inclination bias due to compaction to which the platy haematite particles are more susceptible. Applying the elongation-inclination (E/I) method to directional data from over 100 individual samples from location DUN results in a negligible correction for the magnetite component (<5 ◦ ), whereas the inclination of the haematite component corrects from −35.0 ◦ to −50.3 ◦ (f = 0.6, error interval −41.4 ◦ to −57.9 ◦ ), which is then equal to the uncorrected magnetite inclination. The small number of samples from section KEL does not allow application of the E/I technique and inclination correction based on high field anisotropy of isothermal remanent magnetization was applied, yielding a corrected inclination of −75.2 ◦ ± 4 ◦ . Assuming comparable degrees of compaction for both study areas and applying the flattening factor obtained in DUN on samples from KEL, however, would result in comparable inclinations. The identification of inclination shallowing at both sections indicates that the age of magnetization is close to the deposition age. Assuming a reversed polarity of the directions from both areas results in palaeolatitudes of ∼30 ◦ N for section DUN and ∼60 ◦ N for the anisotropy-based correction of section KEL. The large difference, however, is geologically very unlikely. The inclination of the magnetite component of DUN (unaffected by inclination shallowing) favours a palaeoposition of ∼30 ◦ N. This is supported by the inclination shallowing corrected haematite component of DUN yielding a comparable inclination. Therefore, our results indicate that the NTS domain was situated at ∼30 ◦ Ni n the early Carboniferous. Furthermore, the NTS zone was probably not connected to Baltica or Siberia prior to the late Palaeozoic.

Late Paleozoic assembly of the Alexander-Wrangellia-Peninsular composite terrane, Canadian and Alaskan Cordillera

Late Paleozoic assembly of the Alexander-Wrangellia-Peninsular composite terrane is recorded by two phases of regional deformation, metamorphism, and magmatism within basement complexes of the Alexander (Craig and Admiralty subterranes), Wrangellia, and Peninsular terranes in the Canadian and Alaskan Cordillera. New secondary ion mass spectrometry (SIMS) and chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) zircon U-Pb ages, whole-rock major- and trace-element and Nd-Sr isotope geochemical compositions, and geological field observations of late Paleozoic igneous rocks were used to identify the precise timing and significance of this tectonism in the Saint Elias Mountains region of southwestern Yukon and eastern Alaska. Middle to Late Pennsylvanian (301–307 Ma) igneous rocks, herein assigned to the Barnard Glacier suite, were preferentially emplaced along the Wrangellia-Craig subterrane boundary and mainly comprise syenitic plutons that intrude Paleozoic country rocks with evidence of Pennsylvanian or older (D1) deformation. We propose that Barnard Glacier suite magmatism was produced by a slab breakoff event after the consumption of a narrow backarc ocean basin and early Pennsylvanian collision between the Wrangellia-Peninsular arc and Craig subterrane passive margin. Early Permian (284–291 Ma) dioritic to granodioritic rocks, herein assigned to the Donjek Glacier suite, comprise the vestiges of an extensive magmatic system within the Craig subterrane of southwestern Yukon and southeastern Alaska. The available data suggest that the Donjek Glacier suite represents part of a short-lived, Early Permian arc that initiated along the outboard margin of the Craig subterrane–Wrangellia–Peninsular block after Pennsylvanian collision and slab breakoff. At two field localities in southwestern Yukon, Paleozoic country rocks with D1 fabrics are also intruded by sills and dikes of the Donjek Glacier suite that show evidence of ca. 285 Ma regional deformation and metamorphism (D2). Field evidence for Early Permian tectonism in the Saint Elias Mountains implies direct connections with coeval deformation and metamorphism in the Admiralty subterrane, a microcontinent in the Admiralty Island region of southeastern Alaska that developed separately from the Craig subterrane prior to the Early Permian. D2 tectonism was likely related to the entry of the Admiralty subterrane margin into the Early Permian subduction zone, which resulted in collision and final amalgamation of the Alexander-Wrangellia-Peninsular composite terrane. Our tectonic scenarios require the currently accepted configuration of the Alexander terrane (composite of the Craig and Admiralty subterranes) to have only existed after the Early Permian collision between the Admiralty subterrane and the previously assembled Craig subterrane–Wrangellia–Peninsular terrane. Biogeographic and other geological data suggest that the two-part assembly of the Alexander-Wrangellia-Peninsular composite terrane took place along a convergent margin to the north of the Cordilleran pericratonic arc terranes (Yukon-Tanana, Quesnellia, and others), in between the paleo–Pacific Ocean and paleo–Arctic Ocean realms, to the northwest of the supercontinent Pangea. The assembly of the Alexander-Wrangellia-Peninsular composite terrane might have been associated with the Early to Middle Permian subduction polarity flip recognized in the Cordilleran pericratonic realm, which led to the closure of a backarc ocean basin and Late Permian arc-continent collision along the western margin of North America.

Neoarchean continental crust formation and Paleoproterozoic deformation of the central Rae craton, Committee Bay belt, Nunavut

Integrated mapping, structural analysis, and U–Pb geochronology of the Committee Bay area, Nunavut, establish a record of Neoarchean crustal growth followed by penetrative Paleoproterozoic deformation. Supracrustal rocks include a lower ca. 2.73 Ga mafic–ultramafic volcanic-dominated sequence, a middle, economically significant 2.71 Ga intermediate volcanic-bearing sequence with intercalated sulphidized, gold-bearing iron formation, and an upper &lt;2.69 Ga clastic ± komatiite–quartzite sequence. Following an 80 million year hiatus, this succession was intruded by voluminous ca. 2.61–2.57 Ga granodiorite–tonalite–granite ± diorite, which do not appear to have thermally or tectonically affected the supracrustal belt. Instead, three generations of structures record polyphase Paleoproterozoic deformation of the region. D1 structures are consistent with a doubly vergent structural fan developed at ca. 2.35 Ga in response to the Arrowsmith orogeny that affected the western Rae margin. Penetrative D2 structures dominate the map pattern and include northeast-trending, southeast-dipping folds and fabrics within which gold is localized. The general southeast dips of S2 and inclined, northwest-vergent attitude of F2 reflect northwest-directed shortening at 1.84–1.82 Ga. The absence of syn-D2 plutonic rocks in the west and central Committee Bay belt support amphibolite-facies metamorphism as a response to crustal thickening, which, in turn, led to syn-D2 crustal melting in the east. Regionally extensive upright to northwest-vergent D2 structures and associated ca. 1.85–1.82 Ga tectonometamorphism across the Rae craton are attributed to an early stage of the Hudsonian orogeny involving microcontinent collision(s) with its southeastern margin. D3 folds and dextral shearing at ca. 1780 Ma accommodated localized, late-stage compressional strain during final amalgamation of Laurentia.

Geochemistry and detrital zircon U–Pb and Hf isotopes of the paragneiss suite from the Quanji massif, SE Tarim Craton: Implications for Paleoproterozoic tectonics in NW China

The Delingha paragneiss suite in the Quanji massif, southeastern Tarim Craton, is composed of mica schist, paragneiss, leptynite and quartzite, similar to the ‘khondalite suites’ described from elsewhere in the world. The mica schist is rich in Al2O3 (up to ∼26wt%) and contains graphite and diagnostic minerals including sillimanite and garnet, with metamorphism under amphibolite-facies to locally granulite-facies conditions as manifested by association with amphibolite and granulite. The detrital zircon U–Pb ages and geochemical data indicate that the protolith materials of the Delingha paragneiss suite were mainly sourced from 2.20 to 2.45Ga granites, felsic volcanic rocks and TTG, and were deposited at 2.17–1.92Ga. The detrital zircon Hf and whole-rock Nd isotopes document important crustal growth at ∼2.5–2.7Ga. The detrital zircon age spectra, the whole rock Nd and zircon Hf model ages, the low-maturity of the protolith, and short-distance transportation suggest that the detritus were derived from the underlying Delingha Complex and the lower Dakendaban sub-Group. The timing of magmatic activities in the source region, the depositional age and metamorphic histories of the Delingha paragneiss suite are all comparable to those recorded in the khondalite belt along northern margin of the Ordos Block in the North China Craton. Our study shows that the 2.2–2.45Ga magmatic rocks were generated in arc or active continental margin settings, suggesting a prolonged subduction and accretion history prior to final amalgamation (∼2.5–1.8Ga) to form the unified North China Craton and the assembly of the Tarim Craton in NW China.

Origin and pre-Cenozoic evolution of the south Qiangtang basement, Central Tibet

The Qiangtang terrane in Central Tibet, with its high-pressure rocks, is a key area to unravel the evolution of the Paleo-Tethys. We present a new 1:50,000 map of the Rongma area in the central Qiangtang terrane, of which relatively little is known so far. We show that the Qiangtang metamorphic belt can be separated in a Paleozoic autochthonous basement and an overlying allochthonous thrust stack of subduction mélange that contains high-pressure rocks and Permian sediments. Detrital zircon ages (youngest peak at 591 Ma) and an ~ 470 Ma granite intrusion age constrain the age of the Qiangtang Basement to be between the Late Precambrian to Middle Ordovician. This is in agreement with the observed unconformity between basement and overlying Mid–Late Ordovician strata. This unconformity and the zircon age spectra are comparable to the Himalaya area. The Qiangtang terrane must thus have been part of the Gondwana supercontinent during the Early Paleozoic. The occurrence of the Late Triassic eclogite and glaucophane-bearing schists in the Central Qiangtang terrane indicates the existence of a suture zone between the North and South Qiangtang terranes before the Late Triassic. This suture zone resulted from closure of the Paleo-Tethys between the North and South Qiangtang terranes. Late Triassic syn-collisional granites and rapidly exhumed high-pressure rocks resulted from the closure of this ocean and final amalgamation of the re-united Qiangtang terranes with Eurasia. Collision of the Lhasa and Qiangtang terranes is correlated with north dipping reverse faults in the south Qiangtang terrane, which were probably related to exhumation of the Qiangtang basement and formation of the Qiangtang Culmination.

Paleoproterozoic crustal evolution of the Hengshan–Wutai–Fuping region, North China Craton

An arguable point regarding the Neoarchean and Paleoproterozoic crustal evolution of the North China Craton (NCC) is whether the tectonic setting in the central belt during the mid-Paleoproterozoic (2.35–2.0 Ga) was dominated by an extensional regime or an oceanic subduction–arc regime. A review of the mid-Paleoproterozoic magmatism and sedimentation for the Hengshan–Wutai–Fuping region suggests that a back-arc extension regime was dominant in this region. This conclusion is consistent with the observation that the 2.35–2.0 Ga magmatism shows a typical bimodal distribution where the mafic rocks mostly have arc affinities and the acidic rocks mainly comprise highly-fractioned calc-alkaline to alkaline (or A-type) granites, and that this magmatism was coeval with development of extensional basins characteristic of transgressive sequences with volcanic interlayers such as in the Hutuo Group. Although the final amalgamation of the NCC was believed to occur at ∼1.85 Ga, recent zircon U–Pb age dating for mica schist in the Wutai Group suggests a collisional event may have occurred at ∼1.95 Ga. The metamorphic ages of ∼1.85 Ga, obtained mostly from the high-grade rocks using the zircon U–Pb approach, most probably indicate uplifting and cooling of these high-grade terranes. This is because (i) phase modeling suggests that newly-grown zircon grains in high-grade rocks with a melt phase cannot date the age of peak pressure and temperature stages, but the age of melt crystallization in cooling stages; (ii) the metamorphic P–T paths with isobaric cooling under 6–7 kb for the Hengshan and Fuping granulites suggest their prolonged stay in the middle–lower crust; and (iii) the obtained metamorphic age data show a continuous distribution from 1.95 to 1.80 Ga. Thus, an alternative tectonic scenario for the Hengshan–Wutai–Fuping region involves: (i) formation of a proto-NCC at ∼2.5 Ga; (ii) back-arc extension during 2.35–2.0 Ga resulting in bimodal magmatism and sedimentation in rifting basins on an Archean basement; (iii) a crustal thickening event in the extended region resulting in a kyanite-type metamorphism at ∼1.95 Ga, and (iv) uplifting and cooling of the thickened crust from 1.93 to 1.80 Ga.

Imprints of Archean to Neoproterozoic crustal processes in the Madurai Block, Southern India

Madurai Block, the largest crustal block in the Southern Granulite Terrane (SGT) of Peninsular India, preserves the imprints of multistage tectonic evolution. Here, we present U–Pb and Hf isotope data on zircons from a charnockite–granite suite in the north-western part of this block. The oscillatory zoning, and the LREE to HREE enriched patterns of the zircons with positive Ce and negative Eu anomalies suggest that the zircon cores are of magmatic origin, with ages in the range of 2634–2435Ma implying Neoarchean–Paleoproterozoic magmatism followed by subsequent metamorphism and protocontinent formation in the north-western part of the Madurai Block. A regional 550–500Ma metamorphic overprint is also preserved in the zircons coinciding with the final amalgamation of the Gondwana supercontinent. The Hf isotopic data suggest that the granite and charnockite were derived from isotopically heterogeneous juvenile crustal domains and the charnockites show a significant contribution of mantle-derived components. Therefore, the Hf isotopic data reflect mixing of crustal and mantle-derived sources for the generation of Neoarchean crust in the north-western Madurai Block, possibly in a suprasubduction zone setting during continent building processes.

Petrography and geochemistry of lower carboniferous greywacke and mudstones in Northeast Junggar, China: Implications for provenance, source weathering, and tectonic setting

Northeast Junggar occupies an important position that links East Junggar and Chinese Altai. Numerous magmatic and sedimentary rocks of the Paleozoic in this area recorded the final amalgamation processes of East Junggar and Chinese Altai. This study analyzes the petrological and geochemical characteristics of sandstones and mudstones from the Early Carboniferous Nanmingshui formation in Northeast Junggar. The provenance and tectonic setting of these clastic rocks are discussed. Petrography indicates that the composition and texture maturity of the sandstones are low. The components of the sandstones are mainly volcanic fragments (61–87%), feldspars (9–30%), and monocrystalline quartz (2–18%), with a few polycrystalline quartz and other minerals. Slice observation indicates that the majority of the volcanic fragments of sandstones are basic-intermediate volcanic rocks with a few dacite and felsic plutonic fragments. The detrital modes of the sandstones reflect that these sandstones are derived from undissected arcs. A low to moderate chemical index of alteration and the Al2O3–CaO*+Na2O–K2O diagram reflect a low to moderate weathering degree in the source area. Trace and rare earth element (e.g., La, Th, Hf, Sc, Cr, Co, and Eu) contents and their ratios suggest that the source rocks of the clastic rocks are intermediate-basic rocks with some felsic rocks. Compared with sandstones, the source rocks for mudstones are more felsic. The petrography and geochemistry characteristics of the clastic rocks suggest that the proximal Dulate arc should be the primary source area. Mixing calculations based on REE data suggest that approximately two-thirds of the sandstone fragments are intermediate-basic volcanic rocks. The contents of the major and trace elements, as well as the stratum features, of the clastic rocks manifest that these clastic rocks resemble sedimentary rocks in a back-arc basin. The formation of this back-arc basin is caused by the southward subduction of the Zaysan–Erqis Ocean.