Reconstructing orogenic systems made up dominantly by sediments accreted in trenches is challenging because of the incomplete lithological record of the subducted oceanic domain and its attached ...passive continental margin thrusted by collisional processes. In this respect, the remarkable ~600 km long continuity of sediments exposed in the Eastern Carpathian thin‐skinned thrust and fold belt and the availability of quantitative reconstructions for adjacent continental units provide excellent conditions for a paleogeographical study by provenance and sedimentological techniques constraining sediment routing and depositional systems. These sediments were deposited in the Ceahlău‐Severin branch of the Alpine Tethys Ocean and over its European passive continental margin. We report sedimentological, paleomagnetic, petrographic, and detrital zircon U‐Pb data of Lower Cretaceous sediments from several thin‐skinned tectonic units presumably deposited in the Moldavides domain of the Eastern Carpathians. Sedimentological observations in the innermost studied unit demonstrate that deposition took place in a deepwater basin floor sheets to sandy turbidite system. Detrital zircon age data demonstrate sourcing from internal Carpathian basement units. The sediment routing changes in more external units, where black shales basin floor sheets to sandy mud turbidites were sourced from an external, European continental area. Although some degree of mixing between sources located on both margins of the ocean occurred, constraining a relatively narrow width of the deep oceanic basin, these results demonstrate that the internal‐most studied unit was deposited near an Early Cretaceous accretionary wedge, located on the opposite internal side relative to the passive continental margin domain of other Moldavides units.
Key Points
Quantitative reconstruction of sediment routing constrained the paleogeography in the key studied areas of the Eastern Carpathians
The Lower Cretaceous basin accommodated pelagic and turbidite sediments supplied by its both active and passive continental margins
The partial closure of the ocean activated internal source areas along the continental margin of the Dacia tectonic mega‐unit
We combine detrital zircons (DZ) and epsilon neodymium (ɛNd) signatures with field mapping in the Paro Formation in western Bhutan. DZ age spectra are strongly variable and display signatures that ...have been used to uniquely identify both Greater Himalayan (GH) and Lesser Himalayan (LH) strata. DZ age peaks from six quartzite samples require sources for ∼0.5, 0.8, 1.2, 1.4, 1.7, 1.8, and 2.5 Ga zircons in the Paro Formation. The youngest (∼0.5 Ga) zircons argue for a Cambrian maximum deposition age. Two samples have a youngest 1.8 Ga peak typically attributed to Paleoproterozoic LH rocks. A ∼450 Ma crystallization age from two granite samples constrains the minimum deposition age as Ordovician. New ɛNd signatures from six detrital samples from the Paro Formation show significant variation with lithology. Schists have ɛNd(0) values between −12.0 and −16.9, while quartzite values vary between −18.8 and −24.5. These data imply that the Paro Formation was derived from both young and old sources, with DZ and ɛNd values obtained from the same quartzite samples requiring old detritus while the ɛNd values obtained from interbedded schist require younger detritus. Using published isotopic and chronologic definitions of Himalayan strata, schist‐rich layers would be considered GH, while the interbedded quartzite would be LH. Thus, the Paro Formation refutes the generally accepted notion that different Himalayan tectonostratigraphic zones have unique DZ and ɛNd signatures. Our data recommend caution in the use of DZ and ɛNd signatures for tectonic interpretation, especially when making correlations with studies that extend 1000s of km along strike.
Recent analytical developments in the field of mass spectrometry have made possible accurate measurements of “non-traditional” isotopic ratios of elements such as Fe, Cu, Ag, Sn, Sb and Hg. The ...stable isotopes of these elements do not have any radioactive parents, but their ratios undergo limited fractionation from various causes, most of them mass-dependent. These effects can lead to variation in isotopic ratios of natural materials (minerals, rocks, ores, etc.) and in archaeological artifacts derived from them. Research since 2010 has investigated whether variation in these isotopic ratios can be used to infer the geological provenance of archaeological materials, including bronze and glass. Here we review recent research on these isotopic systems in archaeology, their principal applications, as well as expected future developments in their use. We conclude that none of these isotopic systems are likely to be very useful for provenance, mostly because of limited ranges of isotopic ratios and/or extensive overlap between the isotopic ratios of most geological sources. Copper isotope ratios are however a reliable method for inferring the type of ore (supergene versus hypogene) smelted to produce copper, and recent studies indicate that silver isotope ratios can also be applied to this effect.
•Cu isotopes are an established method for inferring ore type.•Recent studies suggest that Ag isotopes can also be applied to infer ore type.•Sn isotopes are not sufficient to provenance tin and bronze metal alone.•Fe isotopes are not suitable to provenance ferrous and non-ferrous metals.•It is too soon to judge whether Sb or Hg isotopes can become useful methods.
Double-dating using the apatite U-Pb and fission-track systems is becoming an increasingly popular method for resolving mid- to upper- crustal cooling. However, these thermochronometers constrain ...dates that are often difficult to link through geological time due to the large difference in temperature window between the two systems (typically >250 °C). In this study, we apply apatite U-Pb, fission-track, and apatite and whole rock geochemistry to fourteen samples from four tectonic domains common in Cordilleran orogenic systems: (1) basement-cored uplifts, (2) plutons intruded through a thick crustal column, (3) metamorphic core complexes and associated detachment faults, and (4) rapid, extrusive volcanic cooling, in order to provide a link between in situ geochemical signatures and cooling mechanisms. Comparisons of trace element partitioning between apatite and whole rock provide insights into initial apatite-forming processes and/or subsequent modification. Apatite trace element geochemistry and the Th/U and La/LuN ratios provide tools to determine if an apatite is primary and representative of its parent melt or if it has undergone geochemical perturbation(s) after crystallization. Further, we demonstrate that by using a combined apatite U-Pb, FT, trace element, and whole rock geochemistry approach it is possible to determine if a rock has undergone monotonic cooling since crystallization, protracted residence in the middle crust, and provide unique structural information such as the history of detachment faulting. Insights provided herein offer new applications for apatite thermochronology.
•A global survey of age – chemistry of zircons shows similar features of granitic magmatism over time.•Changes in temperature and chemistry do exist over time in the global database.•3.2 Ga is a ...moment of great change in continental evolution.•A Neoproterozoic spike in crustal thickness and metamorphism is notable in the database.
Temporal trends in granitoid chemistry and thermometry constrain major global changes in magmatism, tectonism or crustal thickness in the continents. Our study relies on zircon geochronology and trace element geochemistry on four new detrital rocks (two modern sediments and two Archean metasedimentary rocks) and a global compilation of published single zircon detrital chronology and trace chemistry data acquired on 5587 individual grains. Zircons of all ages from 4.4 Ga to present exist in this archive. Ti-in-zircon thermometry indicates that more than 98% of the grains with concordant U-Pb ages formed at temperatures exceeding 650°C. The great majority of these zircons formed in the 650–850°C range consistent with growth in intermediate to silicic magmas. Magmatic temperatures increased over time for the first 1.2 Ga of Earth's history after which they stayed constant before decreasing during the more recent past. U/Th<5 values in the overwhelming majority of grains are consistent with a magmatic origin. La/Yb, Sm/Yb and Eu/Eu* values are relatively constant throughout the history of the Earth suggesting that most granitoids formed at, or evolved from magmatic reservoirs located at depths of 35–45 km in the presence of amphibole, garnet and limited plagioclase. Such reservoirs are common today in hot deep crustal environments beneath some of the thicker island arcs and all continental arcs along subduction zones. Processes other than modern day style subduction may have contributed to the formation of granitoids in the early Earth but temperatures, depths and the presence of water arbitrated by the presence of amphibole were similar. These results also suggest that the thickness of continental crust in areas that produced granitoids was similar to today's global average throughout the 4.4 Ga time period covered by the zircon archive. There is no correlation between zircon chemistry over time and the assembly of supercontinents.
Detrital zircon (DZ) ages, augmented with εNd(0) and δ13C isotopic values from 18 new and 22 published samples collected from Lesser Himalayan (LH), Greater Himalayan (GH) and Tethyan Himalayan (TH) ...rocks in Bhutan, support deposition of >7km of sedimentary rock in late Cambrian–Ordovician time and provide a stratigraphic framework for the pre-collisional Indian margin. Youngest GH DZ grains become younger upsection from 900Ma to 477Ma. Youngest DZ grains in TH samples are ~490–460Ma. Both the LH Jaishidanda Formation (Fm), and the LH Baxa Group overlie Paleoproterozoic LH rocks. The Jaishidanda Fm exhibits distinct populations of youngest DZ peaks, 475–550Ma, and 800–1000Ma. The Baxa Group (Manas, Pangsari, and Phuntsholing formations) contains youngest DZ peaks at both 500–525Ma and 0.9–1.0Ga. However, most samples from the Baxa Group in western Bhutan contain no grains younger than 1.8Ga. Samples from the LH Paro Fm, which sits directly under the MCT in western Bhutan, have youngest DZ peaks at 0.5, 0.8, 1.0, 1.7, 1.8Ga. εNd values generally match DZ spectra, with samples that contain old, youngest grain populations corresponding to more negative εNd signatures. The Paro Fm is an exception where εNd (0) values from quartzite samples are quite negative (−19 to −24) whereas the εNd (0) values from interbedded schist contain younger detritus (−12 to −17). δ13C values from the Jaishidanda, Paro and Manas formations have δ13C values (−1.8 to +6) suggestive of deposition over late Neoproterozoic to Ordovician time. δ13C values from the Pangsari Fm vary from −2.8 to +1.8, compatible with deposition in the early- to middle Neoproterozoic. The young, latest Cambrian–Ordovician grains preserved in TH, GH and LH rocks suggest that the late Cambrian–Ordovician orogeny, documented in GH rocks throughout the orogen, served as a significant sediment source in Bhutan.
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► Neoproterozoic–Ordovician are key times of deposition along the NE Indian margin. ► Cambro-Ordovician DZ grains in TH and GH rocks suggest retro-arc foreland basin. ► The pre-Himalayan northern Indian margin was complicated but continuous.
Most arcs show systematic temporal and spatial variations in magmatism with clear shifts in igneous rock compositions between those of the magmatic front (MF) and those in the backarc (BA). It is ...unclear if similar magmatic polarity is seen for extensional continental arcs. Herein, we use geochemical and isotopic characteristics coupled with zircon U‐Pb geochronology to identify the different magmatic style of the Iran convergent margin, an extensional system that evolved over 100 Myr. Our new and compiled U‐Pb ages indicate that major magmatic episodes for the NE Iran BA occurred at 110–80, 75–50, 50–35, 35–20, and 15–10 Ma. In contrast to NE Iran BA magmatic episodes, compiled data from MF display two main magmatic episodes at 95–75 and 55–5 Ma, indicating more continuous magmatism for the MF than for the BA. We show that Paleogene Iran serves as a useful example of a continental arc under extension. Our data also suggest that there is not a clear relationship between the subduction velocity of Neotethyan Ocean beneath Iran and magmatic activity in Iran. Our results imply that the isotopic compositions of Iran BA igneous rocks do not directly correspond to the changes in tectonic processes or geodynamics, but other parameters such as the composition of lithosphere and melt source(s) should be considered. In addition, changes in subduction zone dynamics and contractional versus extensional tectonic regimes influenced the composition of MF and BA magmatic rocks. These controls diminished the geochemical and isotopic variations between the magmatic front and backarc.
Plain Language Summary
Most arcs show systematic temporal and spatial variations in magmatism with a clear shift in the composition of igneous products between those of the magmatic front and those in the backarc (BA). Our U‐Pb ages for the NE Iran BA identify five magmatic episodes for the NE Iran backarc, which occurred at 110–80 million years ago (Ma), 75–50, 50–35, 35–20, and 15–10 Ma. The 110–80 Ma magmatic episode was a time of strong regional extension due to subduction initiation along the Zagros suture zone, while the younger episodes reflect maturation of the magmatic arc. The first magmatic episode is represented by magmas dominated by inputs from the underlying mantle, as does the next episode (75–50 Ma). The third pulse (50–35 Ma) shows increasing contributions from the underlying crust. The fourth magmatic pulse occurred at 35–20 Ma, and the volume of these magmas was less than other magmatic pulses. The final pulse in the NE Iran BA (15–10 Ma) suggests that there was a change in magmatic architecture beneath the BA. Magmatic pulses at 110–80 and 50–35 Ma accompanied extension, while others accompanied compression due to collision with Arabia. Contribution of continental crust components was highest for the 50–35 Ma magmatic episode.
Key Points
Late Mesozoic‐Cenozoic active continental margin in Iran was characterized by vigorous magmatism in magmatic front and backarc
Iran back‐arc flare‐up occurred in two pulses at 110–80 and 50–35 Ma., while other pulses at 75–50, 35–20, and 15–10 Ma show magmatic lulls
In contract to NE Iran BA magmatic episodes, the magmatic front indicates a continuous magmatism from 95 to 5 Ma
The Moroccan Anti-Atlas orogenic belt encloses several Precambrian inliers comprising two major Neoproterozoic ophiolitic complexes: the Sirwa and Bou Azzer ophiolites. These ophiolites expose ...crustal and mantle units, thrusting over fragments of a long-lived intra-oceanic arc system. We present a detailed geochronological and petro-geochemical study of three mafic/ultramafic units of these two ophiolites: the Khzama sequence (Sirwa ophiolite) and the Northern and Southern Aït Ahmane sequences (Bou Azzer ophiolite). The crystallization of layered metagabbros from the Bou Azzer ophiolite (North Aït Ahmane sequence) has been dated here at 759 ± 2 Ma (U-Pb on zircons). This new age for the Bou Azzer ophiolite is similar to the formation of the Sirwa ophiolite (762 Ma) and suggests that both units formed during the same spreading event. Metabasalts of the three units show tholeiitic signature but with variable subduction-related imprints marked by LILE enrichments, HFSE depletions and variable Ti contents, similar to modern back-arc basin basalts (BABB). Their back-arc origin is also supported by the geochemical signature of ultramafic units showing very low contents in major and trace incompatible elements (Al2O3: 0.12–1.53 wt%, Ti: 3.5–64.2 ppm and Nb: 0.004–0.10 ppm), attesting of a highly refractory protolith. This is in agreement with the high Cr# (0.44–0.81) and low to intermediate Mg# (0.25–0.73) of their constitutive Cr-spinels. Dynamic melting models suggest that these serpentinites experienced intense and polyphased hydrous melting events, strongly influenced by supra-subduction zone SSZ-fluid influx and subduction-related melt percolation. Being particularly affected by these SSZ-melt/rock interactions and closer to arc units to the south, the Sirwa ophiolite and the South Aït Ahmane unit of the Bou Azzer ophiolite likely represent an early stage of the arc-back-arc system, which has been more influenced by the magmatic products of the arc activity compared to the North Aït Ahmane unit of the Bou Azzer ophiolite.
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•Magmatic age of the Bou Azzer ophiolite (Moroccan Anti-Atlas) is 759 ± 2 Ma.•Sirwa and Bou Azzer ophiolites originate from a same intra-oceanic back-arc system.•Both ophiolites recorded multiple arc-back-arc evolution stages.
New eclogite localities and new 40Ar/39Ar ages within the Western Gneiss Region of Norway define three discrete ultrahigh‐pressure (UHP) domains that are separated by distinctly lower pressure, ...eclogite facies rocks. The sizes of the UHP domains range from c. 2500 to 100 km2; if the UHP culminations are part of a continuous sheet at depth, the Western Gneiss Region UHP terrane has minimum dimensions of c. 165 × 50 × 5 km. 40Ar/39Ar mica and K‐feldspar ages show that this outcrop pattern is the result of gentle regional‐scale folding younger than 380 Ma, and possibly 335 Ma. The UHP and intervening high‐pressure (HP) domains are composed of eclogite‐bearing orthogneiss basement overlain by eclogite‐bearing allochthons. The allochthons are dominated by garnet amphibolite and pelitic schist with minor quartzite, carbonate, calc‐silicate, peridotite, and eclogite. Sm/Nd core and rim ages of 992 and 894 Ma from a 15‐cm garnet indicate local preservation of Precambrian metamorphism within the allochthons. Metapelites within the allochthons indicate near‐isothermal decompression following (U)HP metamorphism: they record upper amphibolite facies recrystallization at 12–17 kbar and c. 750 °C during exhumation from mantle depths, followed by a low‐pressure sillimanite + cordierite overprint at c. 5 kbar and c. 750 °C. New 40Ar/39Ar hornblende ages of 402 Ma document that this decompression from eclogite‐facies conditions at 410–405 Ma to mid‐crustal depths occurred in a few million years. The short timescale and consistently high temperatures imply adiabatic exhumation of a UHP body with minimum dimensions of 20–30 km. 40Ar/39Ar muscovite ages of 397–380 Ma show that this extreme heat advection was followed by rapid cooling (c. 30 °C Myr−1), perhaps because of continued tectonic unroofing.
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