Detrital zircon U-Pb ages of Middle–Late Permian sedimentary rocks from the southwestern margin of the North China Craton: Implications for provenance and tectonic evolution

The southwestern margin of the North China Craton (NCC) is located between the Alxa Terrane to the northwest, the North Qilian Orogen to the west and the North Qinling Orogen to the south. However, the paleogeographic and tectonic evolution for the southwestern part of the NCC in the Late Paleozoic is still poorly constrained. In order to constrain the Late Paleozoic tectonic evolution of the southwestern NCC, we carried out detailed field work and detrital zircon U-Pb geochronological research on Middle–Late Permian sedimentary rocks at the southwestern margin of the NCC. The U-Pb age spectra of detrital zircons from six samples are similar, showing four populations of 2.6–2.4 Ga, 2.0–1.7 Ga, 500–360 Ma and 350–250 Ma. Moreover, on the basis of the weighted-mean age of the youngest detrital zircons (257 ± 4 Ma), combined with the published results and volcanic interlayers, we propose that the Shangshihezi Formation formed during the Middle–Late Permian. Our results and published data indicate that the detrital zircons with age groups of 2.6–2.4 Ga and 2.0–1.7 Ga were likely derived from the Khondalite Belt and Yinshan Block in the northwestern NCC. The junction part between the North Qinling and North Qilian Orogen may provide the 500–360 Ma detrital zircons for the study area. The 350–250 Ma detrital zircons were probably derived from the northwestern part of the NCC. The majority of materials from Shangshihezi Formation within the study area were derived from the northwestern part of the NCC, indicating that the northwestern part of the NCC was strongly uplifted possibly resulting from the progressive subduction and closure of the Paleo-Asian Ocean. A small amount of materials were sourced from southwestern part of the NCC, indicating that the North Qinling Orogen experienced a minor uplift resulting from the northward subduction of the South Qinling terrane.     

Middle Pleistocene bivalves of the İznik lake basin (Eastern Marmara, NW Turkey) and a new paleobiogeographical approach

?znik Lake is a tectonically originated basin mainly controlled by the E–W trending middle strand of the North Anatolian Fault (NAF) system. Pleistocene sediments occurring in front of the faults are well exposed both in the northern and in the southern shorelines of the basin. In this study, two endemic brackish water bivalve species, Didacna subpyramidata Pravoslavkev 1939 and Didacna nov. sp. were found in the oldest terrace of the northern Pleistocene sequence. Having characterized morphology, these species serve as stratigraphic indicators in the regional Pleistocene stratigraphy of the Ponto-Caspian region, and thus are well correlated to the assemblages of the early Khazarian subhorizon (Middle Pleistocene). Hence, these data demonstrate that the early Khazarian brackish water sea covered the study area. Additionally, a model for the formation of the basin is proposed: the ?znik lake basin was a gulf of the former Marmara Sea in the early Khazarian, connecting the Marmara to the Black Sea and the Caspian Sea. The subsequent regional prograding uplifts, main dextral strike-slip fault and many normal faults of the NAF Zone cut off the marine connections to the basin, leading to its present location and topographic level.     

Contamination of seismicity catalogs by quarry blasts: An example from İstanbul and its vicinity,northwestern Turkey

Scientists have proposed two fault systems of different ages in the Sea of Marmara: the Thrace-Eski?ehir Fault Zone of Early Miocene–Early Pliocene age and the North Anatolian Fault Zone of Late Pliocene–Recent age. Different seismicity rates and extensions of these faults onto land near ?stanbul have been suggested. One of the reasons for these differences is the contamination of seismicity catalogs by seismic events from quarries operated in ?stanbul and its vicinity, including Gaziosmanpa?a (Cebeci and Kemerburgaz), Çatalca, Ömerli, Gebze, and Hereke.In this study, we investigated waveforms of 179 seismic events (1.8 < Md < 3.0) from the KOERI, NEMC digital database. We determined differences between earthquakes and quarry blasts based on time- and frequency-domain analyses of their seismograms (amplitude peak ratio, power ratio, and spectral amplitude ratio) and used these differences as discriminants. The results of this study indicate that 15% and 85% of the investigated seismic events are earthquakes and quarry blasts, respectively.     

Petrogenesis of plio-quaternary intra-plate continental alkaline lavas from the İskenderun Gulf (Southern Turkey): Evidence for metasomatized lithospheric mantle

The Quaternary alkaline volcanic field of Southern Turkey is characterized by intra-continental plate-type magmatic products, exposed to the north of the ?skenderun Gulf along a NE-SW trending East Anatolian Fault, to the west of its intersection with the N–S trending Dead Sea Fault zone. The ?skenderun Gulf alkaline rocks are mostly silica-undersaturated with normative nepheline and olivine and are mostly classified as basanites and alkaline basalts with their low-silica contents ranging between 43 and 48?wt.% SiO₂. They display Ocean Island Basalt (OIB)–type trace element patterns characterized by enrichments in large-ion-lithophile elements (LILE) and light rare earth element (LREE), and have (La/Yb)_N?=?8.8–17.7 and (Hf/Sm)_N?=?0.9–1.6 similar to those of basaltic rocks found in intraplate suites. The basanitic rocks have limited variations Sr-Nd isotopic ratios (⁸⁷Sr/⁸⁶Sr?=?0.70307–0.70324, ¹⁴³Nd/¹⁴⁴Nd?=?0.512918–0.521947), whereas the alkali basalts display more evolved Sr-Nd isotopic ratios (⁸⁷Sr/⁸⁶Sr?=?0.70346-0.70365, ¹⁴³Nd/¹⁴⁴Nd?=?0.512887–0.521896). The ?skenderun Gulf alkaline rocks also display limited Pb isotopic variations with ²⁰⁶Pb/²⁰⁴Pb?=?18.75–19.09 ²⁰⁷Pb/²⁰⁴Pb?=?15.61–15.66 and²⁰⁸Pb/²⁰⁴Pb?=?38.65–39.02, indicating that they originated from an enriched lithospheric mantle source. Calculated fractionation vectors indicate that clinopyroxene and olivine are the main fractionating mineral phases. Similarly, based on Sr-Nd isotopic ratios, the assimilation and fractional crystallization (AFC) modeling shows that the alkali basalts were affected by AFC processes (r?=?0.2) and were slightly contaminated by the upper crustal material.The high TiO₂ contents, enrichments in Ba and Nb, and depletions in Rb can likely be explained by the existence of amphibole in the mantle source, which might, in turn, indicate that the source mantle has been affected by metasomatic processes. The modeling based on relative abundances of trace elements suggests involvement of amphibole-bearing peridotite as the source material. ?skenderun Gulf alkaline rocks can thus be interpreted as the products of variable extent of mixing between melts from both amphibole-bearing peridotite and dry peridotite.     

Detrital zircon geochronology of Devonian quartzite from tectonic mélange in the Mianlue Suture Zone,Central China: provenance and tectonic implications

ABSTRACT

Devonian quartzite occurs as blocks within a phyllite matrix in Puziba area of the Mianlue Suture Zone (MLSZ) in central China. The depositional time of the quartzite is younger than 425 Ma (mainly Early Devonian), constrained by the zircon U–Pb geochronology data from the quartzite, cross-cutting relationships with granite, and palaeontology evidence. The detrital zircons in the quartzite show typical magmatic features with four main age peaks at: 2676–2420 Ma (11.6% of the population), 1791–1606 Ma (4.8%), 997–817 Ma (26.5%), and 597–425 Ma (17.5%). In combination with the zircon εHf(t) values, we propose that the quartzite in the MLSZ was sourced from Neoproterozoic and Palaeozoic magmatic and sedimentary rocks in the South Qinling Block and the South China Block (particularly from the Bikou Terrane), with minor contributions from Archaean and Palaeoproterozoic magmatic units from both of the South and North China blocks. The blocks of quartzite, slate, marble, metasandstone, and chert blocks in the phyllite matrix in the Puziba area show a typical block-in-matrix texture in a tectonic mélange, and provide significant evidence from sedimentary rock blocks rather than ophiolite or volcanic rock for the existence of the MLSZ.     

U–Pb ages of detrital zircons from Paleozoic metasandstones of the Gelnica Terrane (Southern Gemeric Unit, Western Carpathians, Slovakia): evidence for Avalonian–Amazonian provenance

U–Pb (SHRIMP) determinations on detrital zircons from the Early Paleozoic Gelnica Terrane metasandstones and their Permian overlap sediments of the Inner Western Carpathian Southern Gemeric Unit define five age populations based on age-probability plots. The metasandstones were sampled for detrital zircons from six stratigraphic levels, four of them in the Late Cambrian/Ordovician Gelnica Terrane metasandstones and the two in Permian envelope sequence. The data set includes 84 U–Pb ages for individual detrital zircons. These ages are combined with the previously dated inherited zircons from the associated metavolcanites (n?=?31). The majority of the pre-Permian detrital and inherited zircons (95%) belong to the three main populations: population A—the Paleoproterozoic/Neoarchean ages ranging from 1.75 to 2.6?Ga; population B—the Mesoproterozoic ages with the range of 0.9 to 1.1?Ga; population C—the Neoproterozoic ages, ranging from 560 to 807?Ma. The detrital zircon age spectrum from the basal Permian sediments reflects the strong recycling from the underlying Gelnica Terrane, with the presence of the dominant Precambrian C and B populations (94% of total), including the minor populations A. The range of the detrital zircon ages from the Late Permian sandstones is wider, with additional population D, ranging from 497 to 450?Ma and population E with a time span from 369 to 301?Ma. Within the Late Permian detrital zircon assemblage, the Proterozoic population A?+?B?+?C form only 25% of total. The detrital zircon data suggest that the Gelnica Terrane belongs to the peri-Gondwanan terrane with a source area located on the northwestern margin of Gondwana close to Amazonia. This terrane should have travelled a long distance in the Phanerozoic times.     

Garnet variety and zircon ages in UHP meta-sedimentary rocks from the Jubrique zone (Alpujárride Complex,Betic Cordillera,Spain): evidence for a pre-Alpine emplacement of the Ronda peridotite

The Ronda peridotite is a group of lherzolite slabs (1.5 to 2 km thick) in southern Spain. Despite clear evidence that pre-Alpine events affected pre-Permo-Triassic units from the Alborán domain (internal zone of the Betic-Rif Cordillera, Spain, and Morocco), numerous papers continue to emphasize Alpine metamorphic and structural evolution. Here, we evaluate the pre-Cenozoic evolution of the Ronda peridotite by reporting new petrographic and U–Pb SHRIMP zircon dating of meta-sedimentary rocks from the Jubrique zone (Alpujárride Complex, Betic Cordillera, Spain) directly overlying the Ronda peridotite. Field inspection and petrographical study revealed generalized migmatitic textures and a gradual transition mainly defined by garnet content (from ~30 to <3 wt.%) and size (from 1.5 cm to <0.5 mm) in the overlying granulite-gneiss sequence, suggesting that most garnet grew as a consequence of the peridotite emplacement. Garnet shows notable variations in composition and inclusion types, which are interpreted as reflecting different stages of garnet growth. Diamond-bearing garnets are only well-preserved in gneisses from the uppermost part of the sequence, whereas the large garnets from rocks overlying the peridotite mainly record later thermal events. SHRIMP zircon dating indicates two age peaks at 330 ± 9 and 265 ± 4 Ma. The oldest age characterizes rims overgrowing detrital cores and reflects an early Hercynian metamorphism; the younger age characterizes zircon with magmatic oscillatory zoning, reflecting anatexis. On the basis of these data and of previous dating of monazite included in the large garnets, we conclude that the peridotite was emplaced either shortly before or during early Hercynian times, ~330 Ma.     

Detrital and volcanic zircon U–Pb ages from southern Mendoza (Argentina): An insight on the source regions in the northern part of the Neuquén Basin

The infill of the Neuquén Basin recorded the Meso-Cenozoic geological and tectonic evolution of the southern Central Andes being an excellent site to investigate how the pattern of detrital zircon ages varies trough time. In this work we analyze the U–Pb (LA–MC–ICP–MS) zircon ages from sedimentary and volcanic rocks related to synrift and retroarc stages of the northern part of the Neuquén Basin. These data define the crystallization age of the synrift volcanism at 223 ± 2 Ma (Cerro Negro Andesite) and the maximum depositional age of the original synrift sediments at ca. 204 Ma (El Freno Formation). Two different pulses of rifting could be recognized according to the absolute ages, the oldest developed during the Norian and the younger during the Rhaetian–Sinemurian. The source regions of the El Freno Formation show that the Choiyoi magmatic province was the main source rock of sediment supply. An important amount of detrital zircons with Triassic ages was identified and interpreted as a source area related to the synrift magmatism. The maximum depositional age calculated for the Tordillo Formation in the Atuel-La Valenciana depocenter is at ca. 149 Ma; as well as in other places of the Neuquén Basin, the U–Pb ages calculated in the Late Jurassic Tordillo Formation do not agree with the absolute age of the Kimmeridgian–Tithonian boundary (ca. 152 Ma). The main source region of sediment in the Tordillo Formation was the Andean magmatic arc. Basement regions were also present with age peaks at the Carboniferous, Neoproterozoic, and Mesoproterozoic; these regions were probably located to the east in the San Rafael Block. The pattern of zircon ages summarized for the Late Jurassic Tordillo and Lagunillas formations were interpreted as a record of the magmatic activity during the Triassic and Jurassic in the southern Central Andes. A waning of the magmatism is inferred to have happened during the Triassic. The evident lack of ages observed around ca. 200 Ma suggests cessation of the synrift magmatism. The later increase in magmatic activity during the Early Jurassic is attributed to the onset of Andean subduction, with maximum peaks at ca. 191 and 179 Ma. The trough at ca. 165 Ma and the later increase in the Late Jurassic could be explained by changes in the relative convergence rate in the Andean subduction regime, or by the shift to a more mafic composition of the magmatism with minor zircon fertility.     

S,O and Pb isotopic evidence on the origin of the İnkaya (Simav-Kütahya) Cu–Pb–Zn–(Ag) Prospect,NW Turkey

The İnkaya Cu–Pb–Zn–(Ag) prospect is a typical example of the hydrothermal mineralization occurring in the Menderes Massif, which crop out in Western Anatolia. The prospect located approximately 20 km west of Simav (Kütahya-Turkey) in northern part of the Menderes Massif have been characterized through the detailed examinations involving geological, mineralogical, whole-rock geochemistry, fluid inclusion, stable isotope and lead isotope.The İnkaya Cu–Pb–Zn–(Ag) prospect is located along an E–W-trending fault in the Cambrian Simav Metamorphics, which consist of quartz–muscovite schist, quartz–biotite schist, muscovite schist, biotite schist and the Arıkayası Formation, which is composed of marbles. Galena, sphalerite, chalcopyrite, pyrite and fahlore are the main minerals, and they are accompanied by small amounts of cerussite, anglesite, digenite, enargite, chalcocite, covellite, bornite, and Fe-oxides with gangue quartz. In addition to Pb, Zn, Cu, Ag, the ore samples contain substantial quantities of As, Cd and Bi and small amount of Au. Average contents of Cu, Pb, Zn and Ag are 77,400 ppm, 102,600 ppm, 6843 ppm and 203 ppm, respectively.The δ³⁴S values for galena, chalcopyrite and pyrite formed in the same stage vary in the range from − 1.7 to − 2.1‰ (average − 2.0), 0.1 to 0.3‰ (average 0.2) and − 1.5 to 2.6‰ (average + 1.5), respectively.δ³⁴S values for H₂S, representing the composition of the fluids responsible for the sulfide mineral formations and calculated from the δ³⁴S value are between − 2.77 and 1.33‰; it is consistent with the sulfur in sulfide minerals. δ¹⁸O_quartz values range from 11.3 to 16.4‰ and estimated δ¹⁸O_fluid values range from 5.4 to 10.6‰.Pyrite–galena and pyrite–chalcopyrite pairs calculated to determine equilibrium isotope temperatures based on δ³⁴S values are between 254.6 and 277.4 °C for pyrite–galena and 274.7 °C for pyrite–chalcopyrite. Sulfur and oxygen isotope values similar to the values for fluid equilibrated with an felsic magmatic source.Fluid inclusion studies on quartz of the same silicification stage coexisting with galena, sphalerite and chalcopyrite collected from the mineralized vein indicate that the temperature range of the fluids is 235 °C to 340 °C and that the salinities are 0.7 to 4.49 wt.% NaCl equivalent. The wide range of homogenization temperatures and relatively lower salinities of the fluid inclusions indicate that at least two different fluid generations were trapped in the quartz from only one fluid type. Also, lower salinities of fluid inclusion probably indicate mixing of meteoric water and magmatic fluid.The galena has ²⁰⁶Pb/²⁰⁴Pb values of 18.862–18.865, ²⁰⁷Pb/²⁰⁴Pb values of 15.707–15.711, and ²⁰⁸Pb/²⁰⁴Pb values of 39.033–39.042. The lead isotope values show a similarity with upper crustal values.     

Isotope Lu–Hf composition of detrital zircon from paragneisses of the Sharyzhalgai uplift: evidence for the Paleoproterozoic crustal growth

We present results of study of the trace-element and Lu–Hf isotope compositions of zircons from Paleoproterozoic high-grade metasedimentary rocks (paragneisses) of the southwestern margin of the Siberian craton (Irkut terrane of the Sharyzhalgai uplift). Metamorphic zircons are represented by rims and multifaceted crystals dated at ~ 1.85 Ga. They are depleted in either LREE or HREE as a result of subsolidus recrystallization and/or synchronous formation with REE-concentrating garnet or monazite. In contrast to the metamorphic zircons, the detrital cores are enriched in HREE and have high (Lu/Gd)_n ratios, which is typical of igneous zircon. The weak positive correlation between ¹⁷⁶Lu/¹⁷⁷Hf and ¹⁷⁶Hf/¹⁷⁷Hf in the zircon cores evidences that their Hf isotope composition evolved through radioactive decay in Hf = the closed system. Therefore, the isotope parameters of these zircons can give an insight into the provenance of metasedimentary rocks. The Paleoproterozoic detrital zircon cores from paragneisses, dated at ~ 2.3–2.4 and 2.0–1.95 Ga, are characterized by a wide range of ε_Hf values (from + 9.8 to –3.3) and model age T ^C 2.8–2.0 Ga. The provenance of these detrital zircons included both rocks with juvenile isotope Hf parameters and rocks resulted from the recycling of the Archean crust with a varying contribution of juvenile material. Zircons with high positive ε_Hf values were derived from the juvenile Paleoproterozoic crustal sources, whereas the lower ε_Hf and higher T ^C values for zircons suggest the contribution of the Archean crustal source to the formation of their magmatic precursors. Thus, at the Paleoproterozoic stage of evolution of the southwestern margin of the Siberian craton, both crustal recycling and crustal growth through the contribution of juvenile material took place. On the southwestern margin of the Siberian craton, detrital zircons with ages of ~ 2.3–2.4 and 1.95–2.0 Ga are widespread in Paleoproterozoic paragneisses of the Irkut and Angara–Kan terranes and in terrigenous rocks of the Urik–Iya graben, which argues for their common and, most likely, proximal provenances. In the time of metamorphism (1.88–1.85 Ga), the age of Paleoproterozoic detrital zircons (2.4–2.0 Ga), and their Lu–Hf isotope composition (ε_Hf values ranging from positive to negative values) the paragneisses of the southwestern margin of the Siberian craton are similar to the metasedimentary rocks of the Paleoproterozoic orogenic belts of the North China Craton. In the above two regions, the sources of detrital zircons formed by both the reworking of the Archean crust and the contribution of juvenile material, which is evidence for the crustal growth in the period 2.4–2.0 Ga.     

U–Pb zircon ages and structural development of metagranitoids of the Teplá crystalline complex: evidence for pervasive Cambrian plutonism within the Bohemian massif (Czech Republic)

U–Pb zircon dating of three metagranitoids, situated within a tilted crustal section at the northwestern border of the Teplá Barrandian unit (Teplá crystalline complex, TCC), yields similar Cambrian ages. The U–Pb data of zircons of the Teplá orthogneiss define an upper intercept age of 513 +7/–6?Ma. The ²⁰⁷Pb/²⁰⁶Pb ages of 516±10 and 511±10?Ma of nearly concordant zircons of the Hanov orthogneiss and the Lestkov granite are interpreted to be close to the formation age of the granitoid protolith. Similar to the Cambrian granitoids of the southwestern part of the Teplá Barrandian unit (Doma?lice crystalline complex, DCC) the Middle Cambrian emplacement of the TCC granitoids postdates Cadomian deformation and metamorphism of the Upper Proterozoic country rocks, but predates Variscan tectonometamorphic imprints. Structural data as well as sedimentological criteria suggest a dextral transtensional setting during the Cambrian plutonism, related to the Early Paleozoic break-up of northern Gondwana. Due to strong Variscan crustal tilting, the degree of Variscan tectonometamorphic overprint is strikingly different in the dated granitoids. It is lowest in the weakly or undeformed Lestkov granite, located in the greenschist-facies domain. The Teplá orthogneiss in the north underwent pervasive top-to-NW mylonitic shearing under amphibolite-facies conditions. There is no indication for a resetting of the U–Pb isotopic system of the Teplá orthogneiss zircons that could be attributed to this imprint. Radiation damages accumulated until recent have probably caused lead loss.     

Evolution of a tourmaline-bearing lawsonite eclogite from the Elekdağ area (Central Pontides,N Turkey): evidence for infiltration of slab-derived B-rich fluids during exhumation

An undated high-pressure low-temperature tectonic mélange in the Elekda area (central Pontides, N Turkey) comprises blocks of MORB-derived lawsonite eclogite within a sheared serpentinite matrix. In their outer shells, some of the eclogite blocks contain large (up to 6 cm) tourmaline crystals. Prograde inclusions in poikiloblastic garnet from a well-preserved eclogite block are lawsonite, epidote/clinozoisite, omphacite, rutile, glaucophane, chlorite, Ba-bearing phengite, minor actinolite, winchite and quartz. In addition, glaucophane, lawsonite and rutile occur as inclusions in omphacite. These inclusion assemblages document the transition from a garnet-lawsonite-epidote-bearing blueschist to a lawsonite eclogite with the peak assemblage garnet + omphacite I + lawsonite + rutile. Peak metamorphic conditions are not well-constrained but are estimated approximately 400–430°C and >1.35 GPa, based on Fe–Mg exchange between garnet and omphacite and the coexistence of lawsonite + omphacite + rutile. During exhumation of the eclogite–serpentinite mélange in the hanging wall of a subduction system, infiltration of B-rich aqueous fluids into the rims of eclogite blocks caused retrogressive formation of abundant chlorite, titanite and albite, followed by growth of tourmaline at the expense of chlorite. At the same time, omphacite I (X_Jd=0.24–0.44) became unstable and partially replaced by omphacite II characterized by higher X_Jd (0.35–0.48), suggesting a relatively low silica activity in the infiltrating fluid. Apart from Fe-rich rims developed at the contact to chlorite, tourmaline crystals are nearly homogeneous. Their compositions correspond to Na-rich dravite, perhaps with a small amount of excess (tetrahedral) boron (~5.90 Si and 3.10 B cations per 31 anions). ¹¹ B values range from –2.2 to +1.7. The infiltrating fluids were most probably derived from subducting altered oceanic crust and sediments.     

New zircon ages on the Cambrian–Ordovician volcanism of the Southern Gemericum basement (Western Carpathians,Slovakia): SHRIMP dating,geochemistry and provenance

International Journal of Earth Sciences - The Southern Gemericum basement in the Inner Western Carpathians, composed of low-grade volcano-sedimentary rock complexes, constitutes a record of the...     

Ediacaran to Lower Ordovician age for rocks ascribed to the Schist–Graywacke Complex (Iberian Massif,Spain): Evidence from detrital zircon SHRIMP U–Pb geochronology

New SHRIMP U–Pb ages of detrital zircon obtained from eight samples of Neoproterozoic to Lower Paleozoic graywackes, schists, microconglomerates and shales provide the maximum depositional age and a new zircon age pattern for the Schist–Graywacke Complex (SGC) from the Iberian Massif (SW Europe). The ages of the youngest zircon grains found in four samples provide a maximum depositional age of latest Ediacaran–Lower Cambrian for the complex. Lower-Middle Cambrian fossiliferous formations on top of the lithologies correctly attributed to the SGC constrain its minimum depositional age. Unexpectedly, two samples attributed to the SGC yielded Cambro-Ordovician zircon populations. These must belong to younger Lower Ordovician sedimentary successions that, up to now, have not been differentiated from those of the SGC. The new age patterns are mainly composed of Neoproterozoic (73%) and Paleoproterozoic (15%) ages, with minor Neoarchean (7%), Mesoarchean (2%), Mesoproterozoic (3%) and Cambrian (1%) ages for the latest Ediacaran–Lower Cambrian successions, and Neoproterozoic (46%) and Cambro-Ordovician (46%) ages, with minor Neoarchean (1%), Mesoarchean (0.5%), Paleoproterozoic (6%), Mesoproterozoic (0.5%) and Carboniferous (1%) ages for the Lower Ordovician successions. The presence of Mesoproterozoic zircon points to the Saharan Metacraton as a contributing source for these sediments. Cadomian granitoids could have been a local Neoproterozoic source. The Cambro-Ordovician zircons may also indicate that Cambro-Ordovician magmatism contributed as a source. Cambro-Ordovician volcanism, the most probable source of the Cambro-Ordovician zircons, would have been coeval with the deposition of the Lower Ordovician successions.     

Reinterpretation of the Ordovician rotations in NW Argentina and Northern Chile: a consequence of the Precordillera collision?

Early Paleozoic paleomagnetic data from NW Argentina and Northern Chile have shown large systematic rotations within two domains: one composed of the Western Puna that yields very large (up to 80°) counter-clockwise rotations, and the other formed by the Famatina Ranges and the Eastern Puna that shows (~40°) clockwise rotations around vertical axes. In several locations, lack of significant rotations in younger rocks constrains this kinematic pattern to have occurred during the Paleozoic. Previous tectonic models have explained these rotations as indicative of rigid-body rotations of large para-autochthonous crustal blocks or terranes. A different but simple tectonic model that accounts for this pattern is presented in which rotations are associated to crustal shortening and tectonic escape due to the collision of the allochthonous terrane of Precordillera in the Late Ordovician. This collision should have generated dextral shear zones in the back arc region of the convergent SW Gondwana margin, where systematic domino-like clockwise rotations of small crustal blocks accommodate crustal shortening. The Western Puna block, bordering the Precordillera terrane to the north, might have rotated counterclockwise as an independent microplate due to tectonic escape processes, in a fashion similar to the present-day relationship between the Anatolia block and the Arabian microplate.     

The tectono-magmatic evolution of the West Junggar terrane (NW China) unravelled by U–Pb ages of detrital zircons in modern river sands

ABSTRACT

The West Junggar terrane (WJT) is an outstanding laboratory for studying the tectonic evolution of the Junggar–Balkhash Ocean, because it contains widespread Paleozoic magmatism in different tectonic settings. We attempt to reconstruct the tectono-magmatic evolution of WJT through U–pb analysis of detrital zircons from three modern river sand samples from the Harabura, Baibuxie, and Aletengyemule rivers in the Barleik Mountains of the central WJT. A total of 232 concordant spots show Th/U ratios of 0.14–1.69, typical of igneous origin, and they contain abundant Paleozoic (96%) and few Precambrian (4%) ages, with major age populations at 450–530, 400–430, 320–380, and 265–320 Ma. The first two groups may be derived from the early subduction- and accretion-related magmatic rocks of the WJT, whereas the third group is congruent with magmatic activities related to the final subduction and basin-filling processes within a framework of the remnant Junggar–Balkhash Ocean. By combining with the regional data, the last group of magmatic events is referred to as post-subduction magmatism. The missing Mesozoic–Cenozoic magmatism clearly indicates a pre-Permian closure for the Junggar–Balkhash Ocean, nearly coeval with the closure of other oceans in the southwestern Palaeo-Asian Ocean.     

Ion microprobe UPb ages on detrital zircon grains from the Ghanzi Group: implications for the identification of a Kibaran-age crust in northwest Botswana

Ion microprobe UPb ages of 1104 ± 16 Ma, 1363 ± 11 Ma and 1748 ± 13 Ma have been obtained on detrital zircons from the siliciclastic rocks of the Ghanzi Group intersected in borehole CKP4 in northern Botswana. Available data show that the time of deposition of the Ghanzi Group is bracketed between 1106 ± 2 and 627 ± 6 Ma. These ages permit constraints to be placed on the source of the sediments. It is shown that the main component of the sediments is sourced locally, from the Kibaran-age Kgwebe igneous rocks. These data are taken to indicate that the Kibaran orogenic event affected northwest Botswana. The Kibaran-age lithological units of northern Botswana represent a link between the Kibaran-age belts of central-eastern Africa and southern Africa. The other two populations of zircons are probably exotic. One is linked to the ca. 1400-1300 Ma rocks known within the Kibaran orogenic system in central and southwest Africa, possibly the Choma-Kalomo Block in Zambia, whereas the ca. 1750 Ma detrital zircons were probably sourced from Palæproterozoic granitoids exposed in northern Namibia and southern Angola.