SHRIMP Zircon Ages Research Articles

Discovery of early Mesoarchean crust in the eastern Gawler Craton, South Australia

New U–Pb SHRIMP zircon ages and Sm–Nd isotopic results identify early Mesoarchean (∼3150 Ma) granitic crust cropping out in the eastern Gawler Craton, southern Australia. These rocks are more than 500 million years older than the oldest previously dated rocks from the Gawler Craton, and challenge the view that pre-3.0 Ga crust in Australia is restricted to the Yilgarn and Pilbara Cratons of Western Australia. We interpret these new data to indicate the presence of ∼3150 Ma crust at or near the surface throughout a wedge-shaped region covering an area of at least ∼1500 km 2. The region is characterised by high surface heat flow, and contains Paleo- and Mesoproterozoic intrusive rocks with evolved Nd isotopic compositions and high radiogenic heat production values. The newly identified Mesoarchean rocks provide an obvious source for the evolved Nd signatures, and are themselves relatively enriched in heat producing elements, particularly considering their age. We speculate that the presence of relatively radiogenic Mesoarchean crust underlies much of the area covered by the South Australian Heat Flow Anomaly. Identification of Mesoarchean crust near the eastern margin of the Gawler Craton prompts consideration of links with other Australian Cratonic elements. As far as we are aware, no direct age equivalents of these rocks have been identified elsewhere in Australia, although we note that some recent models for Australian Proterozoic geodynamic evolution may be used to predict the presence of equivalent aged crust in the North Australian Craton.

Reclassification of the Meso‐ and Neoproterozoic Chronostratigraphy of North China by SHRIMP Zircon Ages

Abstract:High‐quality zircon U‐Pb ages acquired from Meso‐ and Neoproterozoic strata in North China in recent years has provided a high‐resolution chronostratigraphic framework for dating. A basis of this high‐level chronostratigraphic system provides the foundation for a global Precambrian study and stratigraphic correlation and so recent geological studies have focused attention on systemic SHRIMP zircon dating. A chronology of Meso‐ and Neoproterozoic strata and the time of origin of the overlying Changcheng System is given on the basis of new SHRIMP zircon dating from the Qianxi Complex and diabase of the Chuanlinggou Formation. A new tectonostratigraphy for a Neoproterozoic chronostratigraphic framework in the southeastern margin of the North China continent is underpinned by the new SHRIMP zircon dating of a Neoproterozoic mafic magma diabase in the Jiao‐Liao‐Xu‐Huai Sub‐Province.

Evolução tectono-cronológica da estrutura de interferência de Nossa Senhora de Lourdes, Faixa Sergipana, NE-Brasil

The Nossa Senhora de Lourdes structure is a regional dome-crescent-mushroom pattern in the Macurure domain of the Sergipano belt. It marks the interference of successive folding events that have occurred by the overprinting of three ductile deformation episodes (D 1 -D 3 ), each of them followed or crosscut by distinct granite plutons. D 1 is characterized by south-verging nappes and thrust zones. D 2 is marked by coaxial reactivation of D 1 , has a transpressive character and is the main deformation event in the belt. There are two groups of granites emplaced during D 2 : the pre- to early D 2 Camara tonalite with a U-Pb SHRIMP zircon age of 628±12 Ma which is considered the maximum age for the onset of D 2 . The other group is the syn- to tardi-D 2 granites for which two U-Pb TIMS titanite ages were obtained, namely, 584±10 Ma for the syn-D 2 Angico granite and 571±9 Ma for the tardi-D 2 Pedra Furada granite. Using these ages, D 2 has been in operation in the Sergipano Belt for at least 57 million years. D 3 is the last ductile deformation event in the interference structure and it took place after 571 Ma when the belt experienced a high amount of uplift in response to compression in a brittle to ductile-brittle regime. D 4 ended the Neoproterozoic orogeny in the region and is characterized by brittle structures. D 3 and D 4 were responsible for exhumation of the Nossa Senhora de Lourdes interference structure to shallow crustal levels and for the present crustal architecture in this part of the Sergipano belt.

Evolução tectono-cronológica da estrutura de interferência de Nossa Senhora de Lourdes, Faixa Sergipana, NE-Brasil

The Nossa Senhora de Lourdes structure is a regional dome-crescent-mushroom pattern in the Macururé domain of the Sergipano belt. It marks the interference of successive folding events that have occurred by the overprinting of three ductile deformation episodes (D1-D3), each of them followed or crosscut by distinct granite plutons. D1 is characterized by south-verging nappes and thrust zones. D2 is marked by coaxial reactivation of D1, has a transpressive character and is the main deformation event in the belt. There are two groups of granites emplaced during D2: the pre- to early D2 Camará tonalite with a U-Pb SHRIMP zircon age of 628±12 Ma which is considered the maximum age for the onset of D2. The other group is the syn- to tardi- D2 granites for which two U-Pb TIMS titanite ages were obtained, namely, 584±10 Ma for the syn-D2 Angico granite and 571±9 Ma for the tardi-D2 Pedra Furada granite. Using these ages, D2 has been in operation in the Sergipano Belt for at least 57 million years. D3 is the last ductile deformation event in the interference structure and it took place after 571 Ma when the belt experienced a high amount of uplift in response to compression in a brittle to ductile-brittle regime. D4 ended the Neoproterozoic orogeny in the region and is characterized by brittle structures. D3 and D4 were responsible for exhumation of the Nossa Senhora de Lourdes interference structure to shallow crustal levels and for the present crustal architecture in this part of the Sergipano belt.

The Central-Sudetic ophiolites (SW Poland): Petrogenetic issues, geochronology and palaeotectonic implications

The Central-Sudetic ophiolites (CSO), including the largest and best-studied Ślęża ophiolite, are relatively well-preserved, complete Palaeozoic ophiolitic sequences at the NE margin of the Bohemian Massif, near the eastern edge of the Variscan Belt in Central Europe. Despite a detailed study and a well-documented MORB-type affinity, the geochemical complexity of these ophiolites has only recently been appreciated, allowing construction of petrogenetic models. We review here petrological and geochemical data indicating a contrast between the plutonic and volcanic members of the Ślęża ophiolite. This contrast was previously interpreted in terms of a model of two-stage incremental melting of a depleted asthenospheric source producing basaltic magmas and subsequent melts crystallizing as gabbros. However, we argue that the geochemical differences between the plutonic and volcanic members can be explained by differentiation and cumulation processes of broadly similar parental magmas. Preliminary interpretation of new SHRIMP zircon age data from plutonic and volcanic members are consistent with earlier U–Pb zircon ages and provide evidence for magmatic crystallization of the Ślęża ophiolite at c. 400 Ma. The CSO, interpreted as traces of the Rheic Ocean, are key for reconstructing the early- to mid-Palaeozoic rifting processes. Distinct similarities exist with time-equivalent ophiolitic units in the Western Variscan segment of the NW-Iberian Massif. The CSO represent c. 400 Ma oceanic-crust fragments, and their structural position delineates likely major Variscan tectonic sutures and tectonic-mosaic domains within the Variscan accretionary prism.

SHRIMP Pb/U zircon ages constrain gabbroic crustal accretion at Atlantis Bank on the ultraslow-spreading Southwest Indian Ridge

Absolute ages of plutonic rocks from mid-ocean ridges provide important constraints on the scale, timing and rates of oceanic crustal accretion, yet few such rocks have been absolutely dated. We present 206Pb/ 238U SHRIMP zircon ages from two ODP Drill Holes and a surface sample from Atlantis Bank on the Southwest Indian Ridge. We report ten new sample ages from 26–1430 m in ODP Hole 735B, and one from 57 m in ODP Hole 1105A. Including a previously published age, eleven samples from Hole 735B yield 206Pb/ 238U zircon crystallization ages that are the same, within error, overlap with the estimated magnetic age and are inferred to date the main period of crustal growth, the average age of analyses is 11.99 ± 0.12 Ma. Any differences in the ages of magmatic series and/or tectonic blocks within Hole 735B are unresolvable and eight well-constrained ages vary from 11.86 ± 0.20 Ma to 12.13 ± 0.21 Ma, a range of 0.27 ± 0.29 Ma, consistent with the duration of crustal accretion observed at the Mid-Atlantic Ridge. An age of 11.87 ± 0.23 Ma from Hole 1105A is within error of ages from Hole 735B and permits previous correlations made between zones of oxide-rich gabbros in each hole. Pb/U zircon ages > 0.5 Ma younger than the magnetic age are recorded in at least three samples from Atlantis Bank, one from Hole 735B and two collected along a fault scarp to the East. These young ages may date one or more off-axis events previously suggested from thermochronologic data and support the interpretation of a complex geological history following crustal accretion at Atlantis Bank. Together with results from the surface of Atlantis Bank, dating has shown that while the majority of Pb/U SHRIMP zircon ages record the short-lived (< 0.5 Ma) phase of crustal accretion on-axis, results from several samples precede and post-date this period by > 1 Ma suggesting a complex and prolonged magmatic/tectonic history for the crust at Atlantis Bank.

The Western Sierras Pampeanas: Protracted Grenville-age history (1330–1030 Ma) of intra-oceanic arcs, subduction–accretion at continental-edge and AMCG intraplate magmatism

New U–Pb SHRIMP zircon ages combined with geochemical and isotope investigation in the Sierra de Maz and Sierra de Pie de Palo and a xenolith of the Precordillera basement (Ullun), provides insight into the identification of major Grenville-age tectonomagmatic events and their timing in the Western Sierras Pampeanas. The study reveals two contrasting scenarios that evolved separately during the 300 Ma long history: Sierra de Maz, which was always part of a continental crust, and the juvenile oceanic arc and back-arc sector of Sierra de Pie de Palo and Ullun. The oldest rocks are the Andino-type granitic orthogneisses of Sierra de Maz (1330–1260 Ma) and associated subalkaline basic rocks, that were part of an active continental margin developed in a Paleoproterozoic crust. Amphibolite facies metamorphism affected the orthogneisses at ca. 1175 Ma, while granulite facies was attained in neighbouring meta-sediments and basic granulites. Interruption of continental-edge magmatism and high-grade metamorphism is interpreted as related to an arc–continental collision dated by zircon overgrowths at 1170–1230 Ma. The next event consisted of massif-type anorthosites and related meta-jotunites, meta-mangerites (1092 ± 6 Ma) and meta-granites (1086 ± 10 Ma) that define an AMCG complex in Sierra de Maz. The emplacement of these mantle-derived magmas during an extensional episode produced a widespread thermal overprint at ca. 1095 Ma in neighbouring country rocks. In constrast, juvenile oceanic arc and back-arc complexes dominated the Sierra de Pie de Palo–Ullun sector, that was fully developed ca. 1200 Ma (1196 ± 8 Ma metagabbro). A new episode of oceanic arc magmatism at 1165 Ma was roughly coeval with the amphibolite high-grade metamorphism of Sierra de Maz, indicating that these two sectors underwent independent geodynamic scenarios at this age. Two more episodes of arc subduction are registered in the Pie de Palo–Ullun sector: (i) 1110 ± 10 Ma orthogneisses and basic amphibolites with geochemical fingerprints of emplacement in a more mature crust, and (ii) a 1027 ± 17 Ma TTG juvenile suite, which is the youngest Grenville-age magmatic event registered in the Western Sierras Pampeanas. The geodynamic history in both study areas reveals a complex orogenic evolution, dominated by convergent tectonics and accretion of juvenile oceanic arcs to the continent.

Age and early metamorphic history of the Sanbagawa belt: Lu–Hf and P–T constraints from the Western Iratsu eclogite

AbstractTwo distinct age estimates for eclogite‐facies metamorphism in the Sanbagawa belt have been proposed: (i) c. 120–110 Ma based on a zircon SHRIMP age for the Western Iratsu unit and (ii) c. 88–89 Ma based on a garnet–omphacite Lu–Hf isochron age from the Seba and Kotsu eclogite units. Despite the contrasting estimates of formation ages, petrological studies suggest the formation conditions of the Western Iratsu unit are indistinguishable from those of the other two units—all ∼20 kbar and 600–650 °C. Studies of the associated geological structures suggest the Seba and Western Iratsu units are parts of a larger semi‐continuous eclogite unit. A combination of geochronological and petrological studies for the Western Iratsu eclogite offers a resolution to this discrepancy in age estimates. New Lu–Hf dating for the Western Iratsu eclogite yields an age of 115.9 ± 0.5 Ma that is compatible with the zircon SHRIMP age. However, petrological studies show that there was significant garnet growth in the Western Iratsu eclogite before eclogite facies metamorphism, and the early core growth is associated with a strong concentration of Lu. Pre‐eclogite facies garnet (Grt1) includes epidote–amphibolite facies parageneses equilibrated at 550–650 °C and ∼10 kbar, and this is overgrown by prograde eclogite facies garnet (Grt2). The Lu–Hf age of c. 116 Ma is strongly skewed to the isotopic composition of Grt1 and is interpreted to reflect the age of the pre‐eclogite phase. The considerable time gap (c. 27 Myr) between the two Lu–Hf ages suggests they may be related to separate tectonic events or distinct phases in the evolution of the Sanbagawa subduction zone.

Devonian to Permian plate tectonic cycle of the Paleo-Tethys Orogen in southwest China (II): Insights from zircon ages of ophiolites, arc/back-arc assemblages and within-plate igneous rocks and generation of the Emeishan CFB province

New SHRIMP zircon ages for ophiolites, arc/back-arc assemblages and within-plate igneous rocks constrain the evolution of the Paleo-Tethys Orogen of southwest China. Ophiolites represent remnants of the Paleo-Tethys ocean, which was divided by the Simao terrane into two tracts, the main ocean in the west (current position) and a branch in the east. The main ocean has been inferred to originate from Gondwana break-up in the Early Devonian, and the heritage is preserved as the Changning-Menglian ophiolite (SSZ-type). The oceanic branch is represented by the Ailaoshan (NMORB-type)–Jinshajinag (EMORB-type) ophiolites. Zircons from a diabase and a plagiogranite of the Ailaoshan ophiolite yielded 206Pb/238U ages of 382.9±3.9 Ma and 375.9±4.2 Ma. The age of a cumulate gabbro from the Jinshajiang ophiolite is 343.5±2.7 Ma. A metagabbro of the Changning-Menglian ophiolite yielded a crystallization age of 267.1±3.1 Ma. We interpret the ages of the Ailaoshan and Jinshajiang ophiolites to represent sea-floor spreading in the branch ocean, whereas the age of the Changning-Menglian ophiolite reflects closure of the main ocean. Arc/back-arc assemblages (zoned mafic–ultramafic complexes and arc volcanic rocks) are records of convergent plate boundary magmatic activity. Rocks of the Gicha complex (back-arc rift) yielded zircon ages of 306.2±4.6 Ma for a tonalite, 301.0±2.9 Ma for a microgabbro, 297.1±2.0 Ma for a plagioclase hornblendite, and 281.3±1.7 Ma for a diabase. A gabbro and a tonalite of the Banpo complex (plutonic arc) yielded identical ages of 285.6±1.7 Ma and 285.8±2.0 Ma. A basalt of the Yaxianqiao volcanic arc was dated at 266.2±2.2 Ma. Within-plate igneous rocks were also dated. Two amphbolite xenoliths in the Jinshajiang ophiolite mélange, derived from low-Ti continental flood basalts, have zircon ages of 439.3±3.5 Ma and 403.7±3.1 Ma, which we interpret to represent the inception of continental rifting prior to the opening of the branch ocean. The small Dalongkai differentiated mafic–ultramafic intrusion reflects post-orogenic extension and is characterized by superimposed arc and within-plate geochemical signatures. Zircons from a plagioclase pyroxenite of this intrusion yielded an age of 245.6±1.4 Ma.These new results, combined with geological and geochemical data, allow the reconstruction of a Devonian to Permian plate tectonic cycle in which generation of the Emeishan CFB province correlates with Paleo-Tethys orogeny in time. This study provides evidence for cyclic continent–ocean–continent development via multiple orogenesis, as well as temporal constraints on the dynamic relationship between plate tectonics and generation of a large igneous province.

Paleozoic tectonism on the East Gondwana margin: Evidence from SHRIMP U–Pb zircon geochronology of a migmatite–granite complex in West Antarctica

The Fosdick Mountains migmatite–granite complex in West Antarctica records episodes of crustal melting and plutonism in Devonian–Carboniferous time that acted to transform transitional crust, dominated by immature oceanic turbidites of the accretionary margin of East Gondwana, into stable continental crust. West Antarctica, New Zealand and Australia originated as contiguous parts of this margin, according to plate reconstructions, however, detailed correlations are uncertain due to a lack of isotopic and geochronological data. Our study of the mid-crustal exposures of the Fosdick range uses U–Pb SHRIMP zircon geochronology to examine the tectonic environment and timing for Paleozoic magmatism in West Antarctica, and to assess a correlation with the better known Lachlan Orogen of eastern Australia and Western Province of New Zealand. NNE–SSW to NE–SW contraction occurred in West Antarctica in early Paleozoic time, and is expressed by km-scale folds developed both in lower crustal metasedimentary migmatite gneisses of the Fosdick Mountains and in low greenschist-grade turbidite successions of the upper crust, present in neighboring ranges. The metasedimentary rocks and structures were intruded by calc-alkaline, I-type plutons attributed to arc magmatism along the convergent East Gondwana margin. Within the Fosdick Mountains, the intrusions form a layered plutonic complex at lower structural levels and discrete plutons at upper levels. Dilational structures that host anatectic granite overprint plutonic layering and migmatitic foliation. They exhibit systematic geometries indicative of NNE–SSW stretching, parallel to a first-generation mineral lineation. New U–Pb SHRIMP zircon ages for granodiorite and porphyritic monzogranite plutons, and for leucogranites that occupy shear bands and other mesoscopic-scale structural sites, define an interval of 370 to 355 Ma for plutonism and migmatization. Paleozoic plutonism in West Antarctica postdates magmatism in the western Lachlan Orogen of Australia, but it coincides with that in the central part of the Lachlan Orogen and with the rapid main phase of emplacement of the Karamea Batholith of the Western Province, New Zealand. Emplaced within a 15 to 20 million year interval, the Paleozoic granitoids of the Fosdick Mountains are a product of subduction-related plutonism associated with high temperature metamorphism and crustal melting. The presence of anatectic granites within extensional structures is a possible indication of alternating strain states (‘tectonic switching’) in a supra-subduction zone setting characterized by thin crust and high heat flow along the Devonian–Carboniferous accretionary margin of East Gondwana.

Petrology, geochemistry, and geochronology of trondhjemites from the Qori Complex, Neyriz, Iran

Metamorphism, magmatism, and thrusting were the result of subduction of Neotethys beneath the continental-margin arc of the Sanandaj–Sirjan shear zone (SSSZ) during the Mesozoic. The Qori metamorphic complex is a part of the southern SSSZ. Leuco-granitic (trondhjemitic) rocks crop out in the Qori metamorphic complex and are rare rock types in the SSSZ. These rocks have intruded into the marbles and garnet amphibolites, the highest grade metamorphic rocks of the Qori metamorphic complex, and in some outcrops, a transitional boundary between the amphibolites and the granitoids can be distinguished. The granitoids are granular in texture and consist of plagioclase (albite-oligoclase), quartz ± K-feldspar ±muscovite and subordinate garnet, spinel, rutile, and apatite which primarily occur as inclusions in the main phases. The peraluminous trondhjemitic rocks are enriched in Na 2O and SiO 2 and depleted in FeO, MgO, and CaO. Similarities with some trondhjemitic liquids produced through partial melting of amphibolites or hydrous basalts (i.e., low-Al 2O 3 content, less than 15 wt.%; low Ba, Sr, TiO 2, and Eu content, all with negative anomalies; moderately enriched LREEs and Y, and flat HREE patterns) suggest that the evolution of the parental magma was controlled by residual plagioclases during partial melting of a garnet amphibolite source. Concentrations of ferromagnesian elements, Mg, Fe, and Mn, are low, suggesting that the granitic rocks were not produced by high degrees of partial melting. Furthermore, they display low amounts of ferromagnesian components from the protolith (garnet amphibolite). This is supported by consideration of compatible elements, especially Cr, Ni and Ti (and the less robust HREE), which respectively show very high and high bulk partition coefficients for relatively small degrees (< 20%) of partial melting of the source. The partial melting of the garnet amphibolites occurred at pressures and temperatures between 7.5 and 9.5 kbar (at a depth of 25 to 32 km) and 680 and 720 °C, respectively, based on the Grt–Hbl and Hbl–Pl thermometers and a Grt–Hbl–Pl–Qtz barometer. Precise U–Th zircon SHRIMP ages of the trondhjemite show magmatic ages of 147.4 ± 0.76 Ma, Volgian, Late Jurassic, and suggest that Neotethys began to subduct beneath the continental-margin arc of the SSSZ. As a result of this process, an arc-related metamorphism occurred, leading to the development of the garnet amphibolites in the Qori metamorphic complex.

Conflicting structural and geochronological data from the Ibituruna quartz-syenite (SE Brazil): Effect of protracted “hot” orogeny and slow cooling rate?

The Ibituruna quartz-syenite was emplaced as a sill in the Ribeira-Araçuaí Neoproterozoic belt (Southeastern Brazil) during the last stages of the Gondwana supercontinent amalgamation. We have measured the Anisotropy of Magnetic Susceptibility (AMS) in samples from the Ibituruna sill to unravel its magnetic fabric that is regarded as a proxy for its magmatic fabric. A large magnetic anisotropy, dominantly due to magnetite, and a consistent magnetic fabric have been determined over the entire Ibituruna massif. The magmatic foliation and lineation are strikingly parallel to the solid-state mylonitic foliation and lineation measured in the country-rock. Altogether, these observations suggest that the Ibituruna sill was emplaced during the high temperature (~ 750 °C) regional deformation and was deformed before full solidification coherently with its country-rock. Unexpectedly, geochronological data suggest a rather different conclusion. LA-ICP-MS and SHRIMP ages of zircons from the Ibituruna quartz-syenite are in the range 530–535 Ma and LA-ICP-MS ages of zircons and monazites from synkinematic leucocratic veins in the country-rocks suggest a crystallization at ~ 570–580 Ma, i.e., an H T deformation > 35My older than the emplacement of the Ibituruna quartz-syenite. Conclusions from the structural and the geochronological studies are therefore conflicting. A possible explanation arises from 40Ar– 39Ar thermochronology. We have dated amphiboles from the quartz-syenite, and amphiboles and biotites from the country-rock. Together with the ages of monazites and zircons in the country-rock, 40Ar– 39Ar mineral ages suggest a very low cooling rate: < 3 °C/My between 570 and ~ 500 Ma and ~ 5 °C/My between 500 and 460 Ma. Assuming a protracted regional deformation consistent over tens of My, under such stable thermal conditions the fabric and microstructure of deformed rocks may remain almost unchanged even if they underwent and recorded strain pulses separated by long periods of time. This may be a characteristic of slow cooling “hot orogens” that rocks deformed at significantly different periods during the orogeny, but under roughly unchanged temperature conditions, may display almost indiscernible microstructure and fabric.

From orogenesis to passive margin—the cooling history of the Bemarivo Belt (N Madagascar), a multi-thermochronometer approach

Geodynamic processes, like amalgamation and continental dispersal, control the thermobarometric history of the Earth's crust. Due to its central position, Madagascar is the ideal location for studying important aspects of the amalgamation and breakup of the Gondwana supercontinent. Using geochronological methods (e. g. U–Pb zircon and monazite as well as titanite and apatite fission-track dating), a detailed cooling path for the Bemarivo Belt of northern Madagascar has been deciphered. New SHRIMP zircon ages of ca. 540 Ma and published monazite ages of ca. 515 Ma allow to estimate the duration of the high/ultrahigh-temperature metamorphism related to the early Palaeozoic continental collision during Gondwana formation. The breakup of this supercontinent was associated with crustal thinning, brittle deformation and denudation along the evolving sedimentary basins recorded by the titanite and apatite fission-track ages ranging between ca. 320 Ma and 160 Ma.

Paleozoic multiple subduction-accretion processes of the southern Altaids

The formation and development of the southern Altaids is controversial with regard to its accretionary orogenesis and continental growth. The Altay-East Junggar orogenic collage of North Xinjiang, China, offers a special natural laboratory to resolve this puzzle. Three tectonic units were juxtaposed, roughly from North to South, in the study area. The northern part (Chinese Altay), composed of variably deformed and metamorphosed Paleozoic sedimentary, volcanic, and granitic rocks, is interpreted as a Japan-type island arc of Paleozoic to Carboniferous-Permian age. The central part (Erqis), which consists of ophiolitic melanges and coherent assemblages, is a Paleozoic accretionary complex. The southern part (East Junggar), characterized by imbricated ophiolitic melanges, Nb-enriched basalts, adakitic rocks and volcanic rocks, is regarded as a Devonian-Carboniferous intra-oceanic island arc with some Paleozoic ophiolites, superimposed by Permian arc volcanism. A plagiogranite from an imbricated ophiolitic melange (Armantai) in the East Junggar yields a new SHRIMP zircon age of 503 ± 7 Ma. Using published age constraints, we propose the presence of multiple subduction systems in this part of the Paloasian Ocean in the Paleozoic. The intraoceanic arcs became accreted to the southern active margin of the Siberian craton in the middle Carboniferous-Permian. During the long accretionary processes, in addition to large-scale southward-directed thrusting, large-scale, orogen-parallel, strike-slip movements (for example, Erqis fault) in the Permian translated fragments of these intraoceanic arcs and associated accretionary wedges. This new tectonic model has broad implications for the architecture and crustal growth of Central Asia and for other ancient orogens.

Neoproterozoic diamictite in the Eastern Desert of Egypt and Northern Saudi Arabia: evidence of ~750 Ma glaciation in the Arabian–Nubian Shield?

The Neoproterozoic Atud diamictite in Wadi Kareim and Wadi Mobarak in the Eastern Desert of Egypt and the Nuwaybah formation in NW Saudi Arabia consist of poorly sorted, polymictic breccia, with clasts up to 1 m of granitoid, quartz porphyry, quartzite, basalt, greywacke, marble, arkose, and microconglomerate in fine-grained matrix. Stratigraphic relations indicate that the diamictite was deposited in a marine environment. Integrated field investigation, petrographic study and U–Pb SHRIMP zircon ages demonstrate that the Atud and Nuwaybah are correlative. The distribution of zircon ages indicate that ~750 Ma ages are dominant with a significant component of older materials, characterized by minor Mesoproterozoic and more abundant Paleoproterozoic and Neoarchean ages. Some matrix and metasedimentary clast zircons yield ages that are a few 10s of Ma younger than the age of the youngest clast (754 ± 15 Ma), suggesting Atud/Nuwaybah diamictite deposition ~750 Ma or slightly later, broadly consistent with being deposited during the Sturtian glaciation (740–660 Ma). The Paleoproterozoic and Neoarchean clasts have no source within the ensimatic Arabian–Nubian Shield. The distribution of the pre-Neoproterozoic ages are similar to the distribution of the pre-Neoproterozoic ages in Yemen and Saharan Metacraton, suggesting that these clasts have been transported hundreds of kilometers, maybe by ice-rafting. The Atud diamictite may represent important evidence for Cryogenian “Snowball Earth” in the Arabian–Nubian Shield.

U-Pb zircon SHRIMP ages, geochemical and Sr-Nd isotopic compositions of the Early Cretaceous I-type Sarıosman pluton, Eastern Pontides, NE Turkey

The petrogenesis and U-Pb SHRIMP zircon ages of the Late Cretaceous Sarıosman pluton in the Eastern Pontides is investigated by means of whole-rock Sr-Nd isotope data with field, petrographic and whole-rock geochemical studies. The bulk of the I-type Sarıosman pluton consists of biotite-hornblende monzogranite, with minor quantities of porphyritic hornblende-biotite monzogranite. The biotite-hornblende monzogranite contains a number of mafic microgranular enclaves (MMEs) of quartz monzodiorite composition. U-Pb zircon sensitive high-resolution ion microprobe dating (SHRIMP) dates the magma emplacement age of the biotite-hornblende monzogranite at 82.7±1.5 Ma. The rocks of the pluton show high-K calc-alkaline, metaluminous to slightly peraluminous characteristics, and are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE), thus displaying features of arc-related granitoids. Chondrite-normalised rare earth-element (REE) patterns have concave upward shapes (La/_{cn}/Lu_{cn}= 10.1-17.4) with pronounced negative Eu anomalies (Eu/Eu*= 0.61-0.80). Initial \epsilon_{Nd} values vary between -3.0 and -4.1 and initial ^{87}Sr/^{86}Sr values between 0.7062 and 0.707. The MMEs are characterised by higher Mg-numbers (27-29) and lower values of both SiO_2 (56-58 wt percent) and aluminium saturation index (0.9-1.0), compared to the monzogranites. Fractionation of plagioclase, hornblende and Fe-Ti oxides played an important role in the evolution of the Sarıosman pluton. The crystallisation temperatures of the melts ranged from 700 to 800 °C and a relatively shallow intrusion depth (~2 to 7 km) is estimated from the Al-in-hornblende geobarometry. The geochemical and isotopic compositions of the Sarıosman pluton suggest an origin through dehydration melting of mafic lower crustal source rocks.

Polyphase deformation of the Fuping Complex, Trans-North China Orogen: Structures, SHRIMP U–Pb zircon ages and tectonic implications

The Fuping Complex is located in the central part of the Trans-North China Orogen (TNCO), a Paleoproterozoic collisional belt along which the Eastern and Western Blocks were assembled to form the North China Craton. Three stages of deformation (D 1–D 3) have been identified in the Fuping Complex. D 1 formed tight to isoclinal folds F 1, penetrative foliation S 1 and mineral lineation L 1, which resulted from the early NW–SE-orientated thrusting and shearing. D 2 progressively overprinted D 1 and was dominated by mostly NW–SE shortening and top-to-the-SE(E) thrusting under a compressive environment. D 1 and D 2 occurred in association with the peak metamorphism (M 1). D 3 occurred during the exhumation after the crustal thickening, producing regional-scale NWW–SEE-trending open folds (F 3) and associated low-angle detachment faults, and was responsible for the isothermal decompression (M 2) metamorphism of the complex. Following the D 3 deformation, the complex was exhumed to shallow crustal levels where the rocks underwent the retrogressive cooling (M 3) metamorphism. Syn-tectonic leucocratic dykes are widespread in the Fuping Complex. Two leucocratic dykes that did not experience D 1 but underwent D 2 yield SHRIMP zircon ages of 1843 ± 12 Ma and 1844 ± 18 Ma, which indicate that D 1 must have occurred at some time earlier than ∼1844 Ma, whereas D 2 must have occurred at some time later than this age. Two weakly deformed post-D 2 leucocratic dykes yield SHRIMP zircon ages of 1817 ± 14 Ma and 1815 ± 45 Ma, suggesting that the D 2 deformation occurred in the period 1843–1815 Ma. The new SHRIMP data indicate that the collision-related deformation of the Fuping Complex must have occurred in the Paleoproterozoic, not in the late Archean. Structural and geochronological data presented in this study, in association with previous lithological and geochemical data, suggest that the Fuping Complex may have been located in the hinterland of an eastward subduction zone and underwent intensive deformation and metamorphism involved in the final collision between the Western and Eastern Blocks at ∼1.85 Ga.

Ordovician passive continental margin magmatism in the Central-European Variscides: U–Pb zircon data from the SE part of the Karkonosze-Izera Massif, Sudetes, SW Poland

Approximately 500-Ma-old orthogneisses are widespread in the eastern part of the Variscan belt and are commonly interpreted to have intruded mica-schist series of assumed Neoproterozoic age. New SHRIMP zircon ages of quartzofeldspathic metavolcanogenic rocks of the mica schist series in the eastern part of the Karkonosze-Izera Massif (SW Poland) indicate that they are late Cambrian/early Ordovician rather than Neoproterozoic in age, based on the zircon age spectra distributed mainly between ca. 500 and 660 Ma (with a few Proterozoic inherited minimum ages of ca. 970 and 1,825 Ma). Younger zircon dates, dispersed between ca. 412 and 464 Ma, are interpreted as a result of Pb-loss likely caused by subsequent metamorphism. Consequently, the felsic metavolcanogenic rocks appear to be roughly contemporaneous with the intrusion of ca. 500-Ma-old orthogneiss protoliths (with the pooled concordia age of 487 ± 8 Ma interpreted as the best approximation of the protolith intrusive age). Field relationships, petrological and geochemical features of the felsic and mafic rocks studied support a model in which the accompanying mica schist series are not the original country rocks to the ca. 500 Ma granite intrusions, and indicate that their recent close proximity is the result of tectonic juxtaposition. However, both the mica schists enclosing the bimodal metavolcanic rocks, and the orthogneisses, are interpreted to represent a Cambro-Ordovician passive continental margin sequence being part of the Saxothuringian domain. They are tectonically overlain to the east by HP/T metamorphic units, comprising MORB-type metaigneous rocks, and delineating a tectonic suture separating the Saxothuringian block in the west from an assumed continental block (Tepla-Barrandian) to the south-east.

SHRIMP Ages of Zircon in the Uppermost Tuff in Chattisgarh Basin in Central India Require ∼500-Ma Adjustment in Indian Proterozoic Stratigraphy: A Discussion

Previous articleNext article No AccessDiscussion and ReplySHRIMP Ages of Zircon in the Uppermost Tuff in Chattisgarh Basin in Central India Require ∼500-Ma Adjustment in Indian Proterozoic Stratigraphy: A DiscussionA. Mukherjee and R. K. RayA. MukherjeeCentral Ground Water Board, North Central Chattisgarh Region, Raipur, India(e-mail: [email protected]) Search for more articles by this author and R. K. RayCentral Ground Water Board, North Central Chattisgarh Region, Raipur, India(e-mail: [email protected])1Second Floor, Reena Apartment, Pachpedi Naka, Dhamtari Road, Raipur, Chattisgarh, India. Search for more articles by this author Central Ground Water Board, North Central Chattisgarh Region, Raipur, India(e-mail: [email protected])PDFPDF PLUSFull Text Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 116, Number 5September 2008 Article DOIhttps://doi.org/10.1086/590126 Views: 15Total views on this site Citations: 8Citations are reported from Crossref HistoryReceived 20 March 2008Accepted 13 May 2008 © 2008 by The University of Chicago. All rights reserved.PDF download Crossref reports the following articles citing this article:A. K. Jain, D. M. Banerjee, Vivek S. Kale Proterozoic ‘Purana’ Basins, (Apr 2020): 235–294.https://doi.org/10.1007/978-3-030-42845-7_5Piyali Sengupta, Arijit Ray, Sayantani Pramanik Mineralogical and chemical characteristics of newer dolerite dyke around Keonjhar, Orissa: Implication for hydrothermal activity in subduction zone setting, Journal of Earth System Science 123, no.44 (Jun 2014): 887–904.https://doi.org/10.1007/s12040-014-0440-1Arunangshu Mukherjee, Ranjan Kumar Ray, Dinesh Tewari, Vijay Kumar Ingle, Bikram Kumar Sahoo, M W Y Khan Revisiting the stratigraphy of the Mesoproterozoic Chhattisgarh Supergroup, Bastar craton, India based on subsurface lithoinformation, Journal of Earth System Science 123, no.33 (May 2014): 617–632.https://doi.org/10.1007/s12040-014-0418-zD.K. Sinha, S.K. Jain, Kusum P. Naganath Tectonic Significance and Age of Doleritic Sill Near Bandhalimal in the Singhora Protobasin of Chhattisgarh Basin, Central India, (Jan 2011): 167–187.https://doi.org/10.1007/978-3-642-12496-9_10A. Mukherjee and R. K. Ray An Alternate View on the Stratigraphic Position of the ∼1‐Ga Sukhda Tuff vis‐à‐vis Chronostratigraphy of the Precambrians of the Central Indian Craton A. Mukherjee and R. K. Ray, The Journal of Geology 118, no.33 (Jul 2015): 325–332.https://doi.org/10.1086/651502Partha Pratim Chakraborty, Ashish Sarkar, Kaushik Das, Priyabrata Das Alluvial fan to storm-dominated shelf transition in the Mesoproterozoic Singhora Group, Chattisgarh Supergroup, Central India, Precambrian Research 170, no.1-21-2 (Apr 2009): 88–106.https://doi.org/10.1016/j.precamres.2008.12.002Abhijit Basu, Sarbani Patranabis-Deb, Juergen Schieber, Pratap C. Dhang Stratigraphic position of the ∼1000Ma Sukhda Tuff (Chhattisgarh Supergroup, India) and the 500Ma question, Precambrian Research 167, no.3-43-4 (Dec 2008): 383–388.https://doi.org/10.1016/j.precamres.2008.09.004 S. Patranabis-Deb , M. E. Bickford , B. Hill , A. K. Chaudhuri , and A. Basu SHRIMP Ages of Zircon in the Uppermost Tuff in Chattisgarh Basin in Central India Require ~500–Ma Adjustment in Indian Proterozoic Stratigraphy: A Reply, The Journal of Geology 116, no.55 (Jul 2015): 540–542.https://doi.org/10.1086/590127

SHRIMP Ages of Zircon in the Uppermost Tuff in Chattisgarh Basin in Central India Require ∼500-Ma Adjustment in Indian Proterozoic Stratigraphy: A Reply

SHRIMP Ages of Zircon in the Uppermost Tuff in Chattisgarh Basin in Central India Require ∼500-Ma Adjustment in Indian Proterozoic Stratigraphy: A Reply