Maximum Depositional Age Research Articles

Geochronology and detrital zircons sources from the Sierra Albarrana Domain (SW Iberian Massif)

This study compiles the U-Pb and 40Ar/39Ar geochronology of the metasedimentary, migmatitic and granitic rocks of the Sierra Albarrana Domain, based on data recently published by the authors together with other bibliographic sources. Maximum depositional ages (MDA) and magmatic and metamorphic events ages are discussed. MDAs suggest that this domain was deposited during the middle Cambrian (ca. 511) and U-Pb ages of zoned magmatic and metamorphic zircon grains indicate that the main tectono-magmatic event is late Cambrian (granite emplacement; 481 ± 2 Ma) to early Ordovician (migmatization; 478 ± 2 Ma). The 40Ar-39Ar ages evidence a Cambro-Ordovician metamorphic event (ca. 482 Ma) and another related to the Variscan Orogeny (ca. 337-392 Ma). The most important detrital source corresponds to a Cambrian magmatic event (535-515 Ma), probably developed during the initial episodes of the early Paleozoic rifting affecting the N margin of Gondwana. The remaining Mesoproterozoic, Paleoproterozoic and Archean zircon grains would have been contributed by Paleoproterozoic basement and/or older continental crust recycled in the western sections of the Gondwana margin. We suggest that this domain is probably part of the autochthonous section of the Central Iberian Zone (CIZ) and derived from the Saharan Metacraton and/or Tuareg Shield.

Syn-Bushveld “granite sheets” associated with the Molopo Farms Complex intruding into Transvaal Supergroup strata in southern Botswana

Abstract LA-ICPMS U-Pb isotope analyses are presented for zircons from a thin granite sheet intersected in a borehole drilled into the upper Transvaal Supergroup wall rocks to the Molopo Farms Complex in southern Botswana. Many of the zircons have irregular or angular grain margins, and some have rounded cores. Approximately half of the analysed grains yielded concordant 207Pb/206Pb ages ranging between 2 282 ± 29 and 2 113 ± 16 Ma. Assuming that these grains were inherited from the surrounding upper Transvaal Supergroup sedimentary strata, the youngest zircon age provides a maximum depositional age for these sediments. Importantly, these zircon ages coincide with dates from detrital zircons in upper Transvaal Supergroup strata in the eastern Kaapvaal Craton and in the Magondi Supergroup in western Zimbabwe. Hf isotope analyses of the rounded zircons suggest that these grains were sourced from a variable mixture of an old basement and more juvenile material. A younger age of 2 060 ± 12 Ma was obtained from a single subhedral zircon grain from the granite sheet. This is interpreted as the granite’s emplacement age, making it coeval with the adjacent Molopo Farms Complex that forms part of the Bushveld Large Igneous Province.

Exploring the tectonostratigraphy of the Pakistan Himalaya with new detrital zircon geochronology and neodymium isotope data from the Kaghan Valley

In the central Himalaya, the Greater and Lesser Himalayan tectonostratigraphic zones can be distinguished from each other by combining detrital zircon U-Pb ages and whole-rock Nd isotopic data. In the Hazara-Kashmir syntaxis of northern Pakistan, rock units in the Kaghan valley have been variously assigned to the Tethyan, Greater, and Lesser Himalayan zones. To determine which tectonostratigraphic zones are represented, we sampled across the Batal thrust in the northern Kaghan valley because this structure is identified in some research as the Main Central thrust. Samples from the footwall yield Paleo- to Mesoproterozoic maximum depositional ages (∼1.8–1.1 Ga) and an average εNd(0) value of −14.3. Samples from the hanging wall yield Neoproterozoic maximum depositional ages (∼1000–600 Ma) and εNd(0) values of −17.3. The contrasting detrital zircon ages show that the Batal thrust in the Kaghan valley is a major structural division. However, these data lack the characteristic detrital zircon age spectra and εNd(0) values of Greater and Lesser Himalayan rocks north and south of the Main Central thrust in the central Himalaya, respectively. Therefore, either the Batal thrust is not the Main Central thrust, or the Main Central thrust in the Kaghan valley cuts a different structural level and carries rock units not seen outside the Hazara-Kashmir syntaxis.

Detrital Zircon Geochronology of the Volyn-Orsha Sedimentary Basin in Western Ukraine: Implications for the Meso-Neoproterozoic History of Baltica and Possible Link to Amazonia and the Grenvillian—Sveconorwegian—Sunsas Orogenic Belts

We used LA-ICP-MS U-Pb data for detrital zircon to constrain the Maximum Depositional Age (MDA) and provenance of clastic sedimentary rocks of the Volyn-Orsha sedimentary basin, which filled an elongated (~625 × 250 km) depression in SW Baltica and attained ~900 m in thickness. Eighty-six zircons out of one hundred and three yielded concordant dates, with most of them (86%) falling in the time interval between 1655 ± 3 and 1044 ± 16 Ma and clustering in two peaks at ca. 1630 and 1230 Ma. The remaining zircons yielded dates older than 1800 Ma. The MDA is defined by a tight group of three zircons with a weighted mean age of 1079 ± 8 Ma. This age corresponds to the time of a ~90° clockwise rotation of Baltica and the formation of the Grenvillian—Sveconorwegian—Sunsas orogenic belts. Subsidence was facilitated by the presence of eclogites derived from subducted oceanic crust. The sediments of the Orsha sub-basin in the northeastern part of the basin were derived from the local crystalline basement, whereas the sediments in the Volyn sub-basin, extending to the margin of Baltica, were transported from the orogen between Laurentia, Baltica and Amazonia.

Depositional and metamorphic events recorded in voluminous metasedimentary rocks from northwestern Vietnam: Tectonic implications for eastern Paleo-Tethys evolution, Tibetan Plateau

Metasedimentary rocks in the Son La area of northwestern Vietnam provide unique geochronological and metamorphic evidence for constraining Paleo-Tethys evolution. In this study, detrital zircons from the metasedimentary rocks in the Nam Co Complex were dated and yield ages between 2787 and 618 Ma. Their maximum depositional ages are constrained to ca. 870 Ma and ca. 835 Ma for the lower and upper formations, respectively. Similarly, the ages of detrital zircons from metasedimentary rocks in the Ta Khoa antiform cluster from 3280 to 749 Ma. The depositional ages of the metasandstone and gneisses are constrained at ca. 800 Ma and ca. 760 Ma, respectively. The age patterns and Hf isotope compositions of the Nam Co Complex and the Ta Khoa antiform are similar to those of the western Yangtze Block, suggesting that the metasedimentary rocks in northwestern Vietnam represent the southeastern extent of the western Yangtze Block.Metamorphic zircon and monazite from metasedimentary rocks in northwestern Vietnam constrain Triassic tectonic events at 247–240 Ma. The P-T conditions of the metasedimentary rocks from the Ta Khoa antiform were defined at P = ca. 7.0–7.5 kbar and T = ca. 685 °C. Additionally, mineral chemical analyses indicate that phengites occur in Grt-Ph schists from the Nam Co Complex. The phengites have high Si contents (up to 3.542p.f.u.), indicating that the Grt-Ph schist experienced a high-pressure metamorphic event. Previous studies have also reported that eclogites and granulites in the Jinshajiang, Ailaoshan, and Song Ma zones record Triassic high-pressure metamorphism with a clockwise P-T-t path. These results suggest that the Jinshajiang-Ailaoshan-Song Ma suture zone (JASSZ) is a Paleo-Tethyan ocean-continent subduction-collision orogenic belt. The spatial distribution and tectonic evolution of the metasedimentary rocks in the Son La area support the hypothesis of west-dipping subduction of the Yangtze Block beneath the Indochina Block along the JASSZ.

First Evidence of the Precambrian Diamond Source of the Siberian Platform: U–Pb LA-ICP-MS Geochronological Studies of Detrital Zircon from Sedimentary Rocks of the Moti Group

The U–Pb LA-ICP-MS age is analyzed for detrital zircons from diamondiferous sandstones of the Khuzhir Formation of the Moti Group of a sedimentary cover of the south Siberian Platform. Similar ages are determined for two samples with the most representative maxima at 620, 780, 1860, and 2550 Ma and single data within the ranges of 985–1000, 1400–1500, 1150–1190, 2700–2880, and 3225–3230 Ma. The maximum age of sedimentation is 605 ± 6 Ma, which corresponds to the Ediacaran and is close to a period of carbonatite and alkali-silicate magmatism of the region. The Neoproterozoic ages correspond to the age of rocks of the Tuva–Mongolian Terrane of the Central Asian Orogenic Belt, and the most ancient maxima are typical of the basement rock complexes of the south Siberian Platform. Our result allows the first substantiation of the Precambrian age of diamondiferous sedimentary rocks of the Khuzhir Formation and therefore indicates the presence of Precambrian diamondiferous rocks within the Siberian Platform.

Improving the chronostratigraphic framework of the Transvaal Supergroup (South Africa) through in-situ and high-precision U-Pb geochronology

The Transvaal Supergroup, on the Kaapvaal Craton in South Africa, is widely accepted as one of the best-preserved sedimentary archives to constrain planetary-scale environmental changes during the late Archean and early Proterozoic, yet the sedimentation age for certain stratigraphic intervals remains poorly constrained. To improve the temporal control on some of the first-order global changes recorded in these rocks, we carried out U-Pb analyses of detrital zircon populations from several clastic and volcano-clastic sedimentary units of the Transvaal Supergroup. We applied the Chemical Abrasion-Isotope Dilution-Thermal Ionization Mass Spectrometry (CA-ID-TIMS) technique on detrital and volcanic zircon populations that had been previously screened using the Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) technique. We report new maximum depositional age estimates for the Pannetjie (2456.6 ± 7.0 Ma), the Heynskop (2451.5 ± 2.5 Ma), the Makganyene (2423.1 ± 1.0 Ma) and the Hekpoort formations (2248.0 ± 1.1 Ma).A ca. 2.25 Ga-old cluster of LA-ICP-MS analyses in the Makganyene Formation was identified to be spurious, since it was completely removed during the chemical abrasion. Thus, we speculate that circulation of hydrothermal fluids and associated Pb-loss from a radiation-damaged lattice during the emplacement of the much younger Hekpoort Formation or possibly the Ophthalmia Orogeny, recorded in Western Australia, may have reset the U-Pb system of this zircon population. This implies that the accurate maximum depositional age of the Makganyene Formation is ca. 2.42 Ga instead, which denotes the age of the oldest glacial event of global extent during the Paleoproterozoic. Therefore, we suggest that the combination of both dating techniques is essential to ensure accurate maximum depositional age constraints for ancient detrital sedimentary rocks. Additionally, our data provides temporal constraints on a period characterized by major fluctuations in atmospheric oxygen. Finally, and supporting the complex nature of the Paleoproterozoic, linkages between widespread glaciations and atmospheric oxygen fluctuations remain to be explored.

Using multiple methods to better constrain the depositional age of a meta-sedimentary succession: An example of the Laoling Group from the northeastern North China Craton

The youngest detrital zircon (DZ) or groups of zircons age is usually interpreted as the maximum depositional age (MDA) and the oldest metamorphic zircon since deposition is used to resolve the minimum depositional age of a meta-sedimentary succession. However, DZ grains with multi-cycle or fingerprinting proximal sources may cause the MDAs older than the true depositional age, whereas post-depositional resetting has the opposite effect. Here, we present DZ U-Pb, Lu-Hf isotopic and trace elemental data, supplemented by whole-rock geochemistry, from the greenschist-facies metamorphosed Laoling Group consisting of clastic and carbonate rocks in the northern margin of the Jiao-Liao-Ji belt (JLJB) in the North China Craton to investigate the use of multiple methods to constrain its true depositional age. The dominant late Neoarchean and middle Paleoproterozoic DZ age peaks from the meta-sandstones, meta-siltstone, meta-mudstone, and meta-carbonate rocks are interpreted to be sourced from the Southern Jilin Complex and the JLJB, respectively. The low Th/U ratios (<0.18) and Ce/Nd ratios (<2.5) of the 1.96–1.77 Ga metamorphic zircons, which is consistent with the monazite ages of 1.95–1.76 Ga, are used to filter the possibly altered detrital magmatic zircons and the source-normalized α-dose metrics bigger than ca. 0.8 imply a predominant first-cycle origin, therefore the true depositional age of the group is constrained at 2.08–2.03 Ga. Additionally, in consideration of the strike of the basin parallel to the boundary between the JLJB and the Longgang Block, two periods of regional extensional tectonism, bimodal provenance, and depositional age coeval with the second magmatic event in the JLJB, we suggest that the Laoling Group most likely deposited in a back-arc basin in the northern margin of the JLJB. Finally, the temporal changes of zircon Hf isotope and Eu/Eu* in zircon crustal thickness proxy, especially the coeval excursion towards more radiogenic Hf signals and crustal thinning at 2.17–2.13 Ga, is consistent with the back-arc basin opening and closure model rather than the intra-continental rift model.

THE FIRST EVIDENCE OF PRECAMBRIAN DIAMOND SOURCES IN THE SOUTH OF THE SIBERIAN PLATFORM: RESULTS OF U–Pb (LA-ICP-MS) GEOCHRONOLOGICAL INVESTIGATIONS OF DETRITE ZIRCON FROM SEDIMENTARY ROCKS OF THE MOTSKAYA SERIES

U–Pb (LA-ICP-MS) geochronological studies of detrital zircons from diamond-bearing sandstones of the Khuzhir Formation of the Motskaya series of the sedimentary cover of the south of the Siberian platform were carried out. Similar spectra were obtained for two samples with the most representative peaks at 620, 780, 1860 and 2550 Ma and single data in the intervals 985–1000, 1400–1500, 1150–1190, 2700–2880 and 3225–3230 Ma. The maximum age of sedimentation is 605 ± 6 Ma, which corresponds to the Ediacaran and close to the time of the carbonatite and alkali-silicate magmatism in the region. Neoproterozoic data reflect the ages of the rocks of the Tuva-Mongolian terrane of the Central Asian Orogenic Belt, and the most ancient peaks are typical for the basement complexes of the southern Siberian platform. The obtained result supports the Ediacaran age the sediments studied and could be consider as first evidence of the presence of Precambrian diamond-bearing rocks within the Siberian platform.

In search of the Rheic suture: detrital zircon geochronology of Neoproterozoic to Lower Paleozoic metasedimentary units in the Balkan fold-and-thrust belt in Bulgaria

The Rheic Ocean opened between Avalonia and Gondwana in the late Cambrian to early Ordovician, triggering break-up of other terranes from the former Avalonian–Cadomian belt. In turn, the Gondwana-derived terranes recorded contrasting drift histories and were variably reworked within the younger orogens, tracing the Rheic suture thus remains challenging. In this paper, we attempt to locate the enigmatic easterly trace of the suture along the boundary of the Avalonian-type Moesia microplate attached to the southeastern margin of Baltica and the Balkan fold-and-thrust belt in Bulgaria, which contains Neoproterozoic to Early Paleozoic basement slivers of unknown provenance. We present new U–Pb detrital zircon ages from several pre-Darriwillian low-grade units (Lakavitsa, Berkovitsa, Bilo, Diabase–Phyllitoid Complex) but also from the overlying Darriwillian Grohoten and Famennian–Frasnian Katina formations. The age spectra and the maximum depositional ages of the pre-Darriwillian units suggest deposition in an accretionary wedge/forearc basin setting, or in an incipient rift superposed on the Cadomian forearc/arc region, from the latest Ediacaran to late Cambrian. The basins were sourced from a magmatic arc with a limited input of cratonic detritus, presumably adjacent to the Trans-Saharan belt or Saharan metacraton. In contrast, samples from the overlying formations indicate a sustained delivery of the cratonic detritus, perhaps first into passive-margin basins of the ‘Gondwana super-fan’ system and then into a Variscan foreland basin. Our statistical analysis shows that the sampled units exhibit age spectra compatible with the Cadomian terranes and that they differ significantly from those reported from Moesia. The detrital zircon ages thus possibly define the Rheic suture and also suggest that the easterly Cadomian terranes may have remained attached to Gondwana during the Early Paleozoic. It cannot be excluded, however, that the Rheic suture even did not reach that far east and terminated at an intraoceanic transform plate boundary.

Redefinition of the Petersburg batholith and implications for crustal inheritance in the Dinwiddie terrane, Virginia, USA

Field relations as well as geochemical and petrologic studies of meta-igneous rocks assigned to the Pennsylvanian–Permian Petersburg batholith identify at least two distinct rock types: foliated metagranitoid gneiss and massive to porphyritic granite. Foliated metagranitoid gneiss of mostly granodioritic composition is geochemically distinct from associated massive and porphyritic granitic rocks. These gneissic rocks yield radiometric ages from ca. 425 Ma to ca. 403 Ma and document that many of the rocks assigned to the late Paleozoic Petersburg batholith are 100 m.y. older than the youngest portions of the composite batholith and are part of an earlier infrastructural terrane. Two samples of massive equigranular granite southwest of Petersburg, Virginia, yield ages of ca. 321 Ma and ca. 317 Ma, which are 15–20 m.y. older than ca. 300 Ma ages for porphyritic granite, massive granite, and monzodiorite near Richmond, Virginia. Geologic mapping shows that the Early Pennsylvanian granite southwest of Petersburg is separated from Late Pennsylvanian to early Permian granite near Richmond by a map-scale septum of Silurian–Devonian foliated metagranitoid gneiss, referred to herein as the informal Pocoshock Creek gneiss. Laser ablation–inductively coupled plasma–mass spectrometry data from one sample of a quartz-muscovite felsic schist xenolith show a peak age mode of ca. 529 Ma that we interpret to be the maximum depositional age. Inherited zircons from foliated metagranitoid gneiss and massive equigranular granite range from ca. 631 Ma to ca. 376 Ma, but many are Cambrian. Neoproterozoic–Cambrian quartz-muscovite felsic schist and amphibolite, Silurian–Devonian Pocoshock Creek gneiss, and Pennsylvanian–Permian granite comprise a fault-bounded terrane referred to herein as the Dinwiddie terrane. Ages of inherited cores in zircon from igneous rocks and limited detrital zircon geochronology suggest the terrane is of peri-Gondwanan affinity. U/Pb ages of healed fractures in zircon grains from foliated metagranitoid gneiss indicate low-grade deformation of the gneiss at ca. 378–376 Ma, while ca. 320–280 Ma rims on many grains record intrusion of late Paleozoic granite. The temperature-time-deformation history of the Dinwiddie terrane is distinct from the adjacent Goochland and Roanoke Rapids terranes. Orogen-scale dextral transpression likely translated the Dinwiddie terrane southward during the Alleghanian orogeny, at which time they were intruded by Pennsylvanian to Permian granite.

Late Paleoproterozoic deposition and Mesoproterozoic metamorphism of detrital material in the southernmost Baltic Sea region (Gdańsk IG1 borehole): monazite versus zircon and chemical versus isotopic age record

ABSTRACT The SHRIMP ion microprobe zircon and monazite U–Th–Pb isotope geochronology of a biotite-sillimanite paragneiss derived from the Gdańsk IG1 borehole that penetrated the Precambrian basement in the southernmost Baltic Sea area, northern Poland, has constrained the ages of detrital zircon cores and metamorphic overgrowths. In addition, monazites have been dated by electron beam chemical technique either as a grain separate or in thin sections. The metasediments show a specific provenance pattern with a dominant input from the late Paleoproterozoic sources. The ages range from 2749 to 1643 Ma with peaks at 1779, 1815, 1840 and 1870 Ma. The maximum depositional age, defined by the youngest detrital zircons, was determined at 1643 Ma. Zircon rims and monazite isotope analyses record a metamorphic event at 1485, and 1498 Ma, respectively, whereas Th–U–Pb chemical dating on monazite revealed an age of 1494 Ma. These ages correspond with the time of the AMCG magmatism in the basement of northern Poland. The metamorphic monazite predates the age of the zircon rims by over 10 m.y. This discrepancy is likely caused by the different behavior of the two accessories since the zircon metamorphic crystallization requires fluid/melt-related support. Thus, zircon is not expected to record the time of peak metamorphism. In consequence, monazite tends to be a more sensitive metamorphic geochronometer. Moreover, the EPMA method provides greater spatial resolution. Additional older fragments, with ages of 1539, 1596, 1600 and 1620 Ma, were identified within monazites reflecting earlier stages of metamorphic evolution.

Redetermination of the Depositional Age of the Haerdaban Group in the Northern Margin of the Yili Block, Western Tianshan, NW China: Implications for Regional Tectonics and Pb‐Zn Mineralization

AbstractThe Yili Block in the Western Tianshan orogen is a key area for understanding the early crustal formation and evolution of the Central Asian orogenic belt, due to the widely‐distributed Precambrian rocks. Also, it hosts a lot of medium– to large‐scale sedimentary exhalative (SEDEX) Pb‐Zn deposits that mainly occur in Proterozoic metamorphosed clastic‐carbonate rocks. In this study, LA‐ICP‐MS U‐Pb analyses were carried out on detrital zircons in siltstones of the Precambrian Haerdaban Group in the Haerdaban Pb‐Zn deposit and magmatic zircons in the diorite dyke that cuts through the strata and orebodies. The maximum depositional age of the siltstones was determined to be about 604 Ma, the diorite having formed at approximately 500 Ma. As such, the Haerdaban Group was most likely formed in the Neoproterozoic Sinian, rather than the previously considered Mesoproterozoic Changchengian. Detrital materials of the Haerdaban Group were mostly derived from the Nanhua–Sinian mafic dykes and granitic rocks around Lake Sayram at the northern margin of the Yili Block. It is proposed that the Yili Block, together with the Kazakhstan and Central Tianshan blocks and the Tarim Craton, might all pertain to the same Rodinia supercontinent, which has great potential for targeting large to super‐large SEDEX Pb‐Zn deposits.

Provenance of Triassic Xiazijie Fan-Delta in Junggar Basin, Northwestern China: Insights from U-Pb Dating of Detrital Zircons

Abundant hydrocarbon resources were discovered in the Xiazijie fan-delta in the Triassic Baikouquan Formation in Mahu sag, Junggar Basin. However, the maximum depositional age of Baikouquan Formation and provenance of this fan-delta are still unclear, which would be unfavourable for further hydrocarbon exploration. In this study, we used detrital zircon U-Pb dating and composition statistics of conglomerate clast and sandstone grain from Baikouquan Formation to constrain the maximum depositional age and provenance of the Xiazijie fan-delta. The results showed that (1) the conglomerate clast compositions of Xiazijie fan-delta mainly consisted of tuff and intermediate-felsic magmatic rocks, and sandstone samples could be classified as litharenite type with the lithic fragments were almost entirely volcanic lithic fragments; (2) the average Qt:F:L values of sandstone samples (M152-S1 and M152-S2) were 26:7:67 and 21:8:71, respectively, and they plotted in the magmatic arc domain in the Qt-F-L ternary diagram, indicating the tectonic setting of source area of Xiazijie fan-delta was magmatic arc; (3) M152-S1 yielded U-Pb ages ranging from 417 Ma to 253 Ma, with a dominant age peak at 313 Ma and two secondary age peaks at 411 Ma and 268 Ma, respectively, while M152-S2 yielded U-Pb ages ranging from 467 Ma to 256 Ma, with a dominant age peak at 307 Ma and two secondary date peaks at 405 Ma and 262 Ma; (4) the mean age of youngest two zircon grains of M152-S1 was 254.8 ± 4.7 Ma, while that of M152-S2 was 257.6 ± 3.8 Ma, suggesting the Baikouquan Formation might be deposited after the Changhsingian to Olenekian; (5) the magmatic rock ages of central West Junggar were distributed mostly between 450–260 Ma, with a dominant age peak at 307 Ma. The ages distribution between magmatic rock of central WJ and detrital zircons of M152-S1 and M152-S2 were similar, indicating the central WJ domain should be the major source area of the Xiazijie fan-delta; (6) the magmatic rock of Hakedun–Hongguleleng area in the Central WJ was characterized by a peak age at 305 Ma, which was consistent with the peak ages of M152-S1 and M152-S2, indicating the Hakedun-Hongguleleng area was likely to be their major source area; and (7) one minor peak age at 411 Ma and another at 405 Ma were obtained from M152-S1 and M152-S2, respectively, and a zircon grain with Middle Ordovician age at 467 Ma was obtained from M152-S2, indicating Late Silurian–Early Devonian Chagankule pluton in the Saier Mountain and Ordovician Honggleleng ophiolite mélange in the Sharburt mountain were the minor source areas. This research has significant implications for stratigraphic correlation in Junggar Basin and hydrocarbon exploration in the Xiazijie fan-delta conglomerate reservoir.

Cadomian to Cenerian accretionary orogenic processes in the Alpine basement: the detrital zircon archive

New whole-rock geochemical and detrital zircon U–Pb and Lu–Hf data of metasedimentary sequences of the Silvretta Nappe, Orobic Alps, Strona-Ceneri Zone, Gotthard Massif and Venediger Nappe are presented. These units seem to share a common early to middle Paleozoic geological record, which has alternatively been interpreted as the result of intraplate or orogenic processes. Detrital zircon data mainly indicate late Ediacaran to early Ordovician maximum sedimentation ages for the studied sequences, suggesting that they were intimately related to Cadomian and Cenerian orogenic processes along the northwestern Gondwana margin. The common presence of late Ediacaran to Cambrian Cadomian ages associated with variable subchondritic to suprachondritic Lu–Hf compositions points to recycling processes of Cadomian sequences, further supported by geochemical data indicating a relatively low to moderate maturity of sedimentary protoliths. The occurrence of Cenerian arc-related intrusions in Austroalpine and South Alpine basement unit points to an arc/back-arc position in the early Paleozoic Cenerian orogen, except for the Strona-Ceneri Zone, which was likely located closer to the forearc region, as indicated by the presence of high-pressure metamorphism. Younger sequences, such as the Landeck Quartz-phyllite, document post-Cenerian sedimentation, whereas those of the Venediger Nappe more likely record the early stages of Variscan subduction, as indicated by Devonian maximum deposition ages.

Accretion and subduction mass transfer processes: Zircon SHRIMP and geochemical insights from the Carboniferous Western Series, Central Chile

ABSTRACT In the region of Pichilemu (Central Chile), the exhumed roots of the Carboniferous paleo-accretionary wedge developed during subduction of an oceanic realm underneath the western Gondwana margin are exposed. We focus on the areas of Infiernillo beach and Punta de Lobos: the former representing a well-characterized transitional blueschist-facies coherent stack of metabasitic lithologies interlayered at the centimetre- to metre-scale within metasedimentary rocks and glaucophanites, whereas the latter corresponds to a massive, up to hundreds of metres thick, strained metapillow lavas edifice surrounded (not interlayered) by metasedimentary rocks. We provide new field, geochemical, Raman thermometry and zircon SHRIMP U-Th-Pb geochronology on the lithologies forming the Infiernillo-Punta de Lobos complex aiming at characterizing the sedimentary sources and the extent of mechanical mixing. Field observations suggest that the Infiernillo-Punta de Lobos complex can be considered as part of the same successions, subducted and basally accreted almost coherently at high pressure-low temperature (HP-LT) conditions. The geochemistry signal in metasedimentary rocks and maximum deposition ages point to a marked forearc source for all the metasedimentary and glaucophanite lithologies, whereas massive metapillow lavas and greenschists might represent volcanic products and oceanic roughnesses erupted near the trench when the oceanic material approached the Gondwana margin at c. 329 Ma. It is suggested that the interlayering now observed in the field is mostly the consequence of sedimentary and/or volcanic processes close to the trench environment, with minor tectonic reworking of the pre-subduction structures (e.g. lithological contacts). Our results suggest that the Punta de Lobos edifice is closely related to the subduction of large volumes of continent-derived, trench-filling sediments towards HP-LT conditions. Thus, in line with previous experimental and numerical simulations, we emphasize the potential role of oceanic roughness as stress perturbations that could contribute to the subduction of thick sedimentary sequences, and discuss their implications for seismogenesis.

Current knowledge of the Ordovician System in Antarctica

Abstract Evidence of Early Ordovician deposition and intrusion in East Antarctica is best known from the Ross Orogen, postdating the 495–489 Ma Ross Orogeny. Here, c. 490–475 Ma granites (with related dykes and sills) of the Granite Harbour Intrusives represent roots of a continental margin arc. Detrital zircon grains in the upper Byrd Group (Central Transantarctic Mountains) are of comparable Early Ordovician age. Contemporaneous fossils are rare. In northern Victoria Land they include latest Cambrian to earliest Ordovician conodonts and microbrachiopods in allochthonous limestones of the Handler Formation (Robertson Bay Group) in the Robertson Bay Terrane, and probable Early Ordovician trace fossils in the Camp Ridge Quartzite of the Leap Year Group in the Bowers Terrane. In the Shackleton Range of Coats Land, West Antarctica, the Blaiklock Glacier Group contains a diverse ichnofossil fauna of probable Ordovician age associated with undescribed bivalved arthropods and segmented crustacea. The Swanson Formation of the Ross Province in Marie Byrd Land (correlated with the Robertson Bay Group of the Ross Orogen) is a turbiditic unit dominated by quartz-rich sandstones. Its Ordovician age is based on a post-depositional whole rock K–Ar metamorphic age of 448–444 Ma, with detrital zircon grains indicating a late Cambrian maximum depositional age.

Provenance of the conglomerate and siliciclastic rocks from the Gadag Greenstone Belt, Western Dharwar Craton, India: Implications for understanding Neoarchean basin margin sedimentation

Petrographic, whole‐rock geochemical and geochronological analysis of polymictic conglomerate, associated siltstone and greywacke in the Neoarchean Gadag Greenstone Belt in the Western Dharwar Craton, India, provides new insights into sedimentary provenance and tectonic setting. The polymictic conglomerate contains granitic, gneissic, felsic and mafic volcanic, quartzite and banded ferruginous chert clasts within a quartzo‐feldspathic matrix. Framework grains in the siltstone and greywacke are quartz, feldspar with rock fragments, embedded in a chlorite‐dominant matrix. The chemical index of alteration values (~70) and A‐CN‐K plot for the rocks suggest a low‐to‐moderate degree of weathering. Rare earth element (REE) patterns show moderate fractionation (LaN/YbN ~ 12.31) with a weak negative europium anomaly (0.82). Enrichment in transition elements (Ni, Cr, Co, Sc) and depletion in high‐field‐strength elements, as well as relatively low La/Co (0.62), moderate to high Th/Yb (3.06), La/Th (5.16), La/Yb (17.5), and La/Sc (~2.37) ratios, suggest a mixed felsic‐mafic provenance. U–Pb dating of detrital zircon from the conglomerate and greywacke revealed major age peaks at ca. (3.2, 2.9, 2.8, 2.6, 2.5 Ga) and a maximum age of deposition of 2,508 ± 26 Ma, 2,493 ± 20 Ma, respectively. The above data suggest low to moderately weathered proximal sources for both conglomerate and greywacke, indicating that these two rock types were not deposited simultaneously, but rather separately. The association of these conglomerates and greywacke with continental arc‐related volcanics indicates the closing of an arc at the time of sedimentation. Complex provenance consisting of recycled and magmatic arc during basin closure confirm the notion of modern style plate tectonics operating during Neoarchaean.

Early Cretaceous displacement on the Tanymas thrust fault, Northern Pamir, Tajikistan, and regional tectonic implications

An ongoing question in understanding the evolution of the Himalayan-Tibetan orogeny is how much of the observed upper crustal shortening and crustal thickness is related to the Cenozoic collision between India and Asia vs earlier tectonic events along the southern margin of Asia. While the Pamir Mountains located at the western end of the orogen have been proposed to have experienced significant Cenozoic shortening, recent studies have interpreted upper crustal shortening to be primarily mid- to Late Cretaceous. To further understand the timing of upper crustal deformation in the Pamir, we investigated synorogenic clastic deposits within the footwall of the north-dipping Tanymas thrust fault along the suture between the Northern and Central Pamir terranes. Sandstones from these deposits were analyzed by detrital zircon U-Pb, zircon fission track, and muscovite 40Ar/39Ar analyses to assess the age and source of the detritus. Results show the deposits were sourced from the Northern Pamir (hanging wall of the Tanymas thrust) and provide an Early Cretaceous maximum deposition age of ∼130–120 Ma, interpreted to constrain their age and date motion on the Tanymas thrust fault as Early Cretaceous. Our results, integrated with previous studies, show Cretaceous deformation in the Pamir began in the Northern Pamir (∼140–110 Ma) before sweeping into the Southern Pamir in the mid- to Late Cretaceous (∼110–75 Ma). These results are consistent with previous interpretations of an Early Cretaceous phase of shallow- or flat-slab northward subduction followed by slab rollback and southward migration of deformation and magmatism in the mid- Cretaceous.

Protracted Thermal Evolution of a Migmatitic Terrane as Revealed by Multiple Geochronometers From the Retro‐Arc of the Early Paleozoic Famatinian Orogen in NW Argentina

AbstractU‐Pb dates of zircon, monazite, and titanite combined with trace element composition, allows characterization of the thermal evolution of the migmatitic Agua del Sapo complex. This complex comprises Al‐rich and Ca‐rich metasedimentary rocks with a detrital zircon maximum depositional age of 550 Ma. The rocks record two consecutive early Paleozoic orogenies. During subduction associated with the 550–510 Ma Pampean orogeny, the complex was in the fore‐arc region and was intruded by 550–520 Ma granites indicative of anomalous heating possibly related to ridge subduction. During the subsequent 500–440 Ma Famatinian orogeny, the arc migrated trenchwards and the region became part of a retro‐arc that underwent melting at upper‐amphibolite facies. This event was recorded differently by each of the accessory phases. Detrital zircon cores were overgrown by rare Famatinian rims that range from 500 to 420 Ma, while monazite records only Famatinian dates with a growth peak at ∼457 Ma that extends to 410 Ma, possibly due to coupled dissolution‐precipitation. Published titanite dates define a 120 Myr thermal history, starting at ∼500 Ma with temperatures of ∼750°C ± 25°C, ending at 380 Ma and ∼700°C ± 25°C. Cooling was accompanied by a decrease in titanite light rare earth element contents in response to increased abundance of allanite/epidote. Thus, the complementary time‐compositional record of the accessory phases reveals continued high heat flow, associated with deformation, to 380 Ma. This prolonged event blurs the boundary between the Famatinian and the subsequent Achalian/Chanic orogenies and extends the Silurian Rinconada tectonic phase of the Famatinian orogeny to the east into the Eastern Sierras Pampeanas.