Zircon Signatures Research Articles

Paleovolcanology, geochemistry, and zircon U-Pb-Hf isotopes of the volcano-sedimentary sequences from the Ediacaran Campo Alegre-Corupá Basin, southern Brazil: Linking volcanism and tectonics during Western Gondwana consolidation

The Campo Alegre-Corupá Basin (CACB) records a well-documented sequence of volcano-sedimentary episodes associated with specific periods of crustal extension during the late-convergent to post-collisional tectonic stages of the consolidation of Western Gondwana. Two main volcano-sedimentary stages were detailed through stratigraphic and lithofaciological studies, combined with whole-rock geochemistry and zircon U-Pb-Hf analysis, resulting in a more complete understanding of the tectono-magmatic cycles and volcanological evolution of the CACB. During the Basin Stage (605–590 Ma), regional collisional tectonics was responsible for initiating the basin as a syn-orogenic rift, during which the sedimentary cycle progressed from alluvial fans at the northern limit of the basin to braided rivers and into a lake to the south. U-Pb geochronology and Lu-Hf isotope signatures of detrital zircon revealed that the sediments were mainly derived from the Paleoproterozoic basement of the Luis Alves Terrane, which constitutes the direct basement of the CACB, with contributions from the nearby Piên Magmatic Arc and possibly from the Curitiba Terrane to the North. The sedimentary strata were progressively covered by transitional to mildly alkaline OIB-like basalts, occurring mostly as lava flows with structural aspects suggestive of subaqueous to subaerial environments, interbedded with trachydacites and surge-like pyroclastic deposits. Large volumes of widespread and densely-welded to rheomorphic ignimbrite sheets within the CACB attest to a second infilling period corresponding to the Caldera Stage (583–577 Ma). This stage was inaugurated by the outpouring of trachytic and very subordinated IAB-like basaltic lava flows at ∼583 Ma. The caldera eruption seems to have been initiated by the production of pumiceous fallouts during an early Plinian-like period of decompression of shallow magma chambers at ∼580 Ma, during which the rift-related structural framework of the Basin Stage probably controlled the collapse of a graben-like caldera. Compositional and isotopic signatures suggest lithospheric mantle sources for the magmas from both volcanic stages and support a connection between basic-intermediate and silicic rocks from both bimodal associations. They also suggest a cogenetic nature between the silicic rocks from the Caldera Stage and the A-type granitoids from the nearby plutonic Graciosa Province.

Foreland basin response to middle Cretaceous thrust belt evolution, southwestern Montana, USA

Abstract The middle Cretaceous Blackleaf Formation records the first major transgressionregression of the Western Interior Seaway into the southwestern Montana retroforeland basin. Although Blackleaf sedimentology is well documented, sediment provenance and potential linkages with regional tectonics are not. Recent characterization of hinterland tectonics, fold-thrust belt detrital zircon signatures, and advances in high-n detrital zircon analysis allow for significant provenance refinement. We present new detrital zircon ages (n = 5468) from ten samples from the upper Blackleaf Formation (Intervals C and D) in southwestern Montana. Based on maximum depositional ages, sedimentation spanned from 106 to 92 Ma. Jurassic and Cretaceous grains were primarily derived from the older portion of the Cordilleran magmatic arc in western Idaho. Triassic and older grains were recycled from older central Idaho sedimentary strata inboard of the arc. Three depositional stages are identified based on statistical modeling of detrital age distributions. Stage 1 (106–104 Ma) records sourcing from lower Paleozoic strata in central Idaho. Stage 2 (105–101 Ma) records initial unroofing of upper Paleozoic–Triassic strata via propagation of the fold-thrust belt into eastern Idaho, accommodating shortening of Mississippian and younger rocks above the Lemhi Arch. Stage 3 (102–100 Ma) records continued unroofing in central Idaho down to Cambrian stratigraphic levels and distal mixing of sources in the eastern part of the basin. Exhumation in the fold-thrust belt beginning at ca. 105 Ma is coincident with marginwide fault slip-rate increases. We infer that increased sedimentation rates and lowmagnitude flexural loading from shallow thrusting in eastern Idaho drove clastic wedge progradation across the basin.

Composition of continental crust altered by the emergence of land plants

The evolution of land plants during the Palaeozoic era transformed Earth’s biosphere. Because the Earth’s surface and interior are linked by tectonic processes, the linked evolution of the biosphere and sedimentary rocks should be recorded as a near-contemporary shift in the composition of the continental crust. To test this hypothesis, we assessed the isotopic signatures of zircon formed at subduction zones where marine sediments are transported into the mantle, thereby recording interactions between surface environments and the deep Earth. Using oxygen and lutetium–hafnium isotopes of magmatic zircon that respectively track surface weathering (time independent) and radiogenic decay (time dependent), we find a correlation in the composition of continental crust after 430 Myr ago, which is coeval with the onset of enhanced complexity and stability in sedimentary systems related to the evolution of vascular plants. The expansion of terrestrial vegetation brought channelled sand-bed and meandering rivers, muddy floodplains and thicker soils, lengthening the duration of weathering before final marine deposition. Collectively, our results suggest that the evolution of vascular plants coupled the degree of weathering and timescales of sediment routing to depositional basins where they were subsequently subducted and melted. The late Palaeozoic isotopic shift of zircon indicates that the greening of the continents was recorded in the deep Earth. Colonization of continents by plants some 430 Myr ago enhanced the complexity of weathering and sedimentary systems, and altered the composition of continental crust, according to statistical assessment of zircon compositions.

The paleotectonic evolution of the western Mediterranean: provenance insights from the internal Betics, southern Spain

Since the early Cenozoic, the closure of the Alpine Tethys in the Western Mediterranean has been accomplished by protracted subduction, followed by collision and orogenic collapse. The internal zones of the Betic Cordillera (southern Spain) and Rif (northern Morocco) experienced subduction metamorphism and subsequent exhumation due to the westward migration of the orogenic system. The detrital provenance of these meta-sedimentary units contains crucial insights into their pre-subduction stratigraphic arrangement, which is essential to constrain the pre-Cenozoic paleogeography and tectonic evolution of the Western Mediterranean. This study focuses on the Nevado-Filábride Complex (NFC) and the Eastern Alpujárride Complex in the Internal Betic Cordillera. New depth-profile zircon U-Pb LA-ICP-MS data from the NFC (N = 72) and Alpujárride (N = 21) and in situ apatite U-Pb data from a metabasite within the NFC allow us to establish the pre-subduction stratigraphy, sedimentary provenance, and paleotectonic configuration of the Iberian rifted margin during the Paleozoic–Early Mesozoic. Our dataset demonstrates that the NFC represents an intact Devonian to Early Jurassic stratigraphic sequence that records the evolution of the Western Mediterranean from the Variscan orogeny to rifting and opening of the Alpine Tethys. Detrital zircon U-Pb age modes of the NFC remain remarkably similar for over 200 Myr with only the differences being the progressive addition of new zircon modes related to depositional ages. Additionally, a comparison between the provenance record of the NFC and Alpujárride reveals notably similar zircon signatures of Carboniferous and Permian strata. This study reports the first data from the Triassic–Jurassic strata that record the opening of the Alpine Tethys. Additional evidence for rifting is present in the metabasites from the Veleta unit yielding in situ apatite U-Pb ages of 197 Ma, recording CAMP magmatism and Early Jurassic rifting in the Alpine Tethys. The data demonstrate a palinspastic connection between the NFC and the Alpujárride. Based on these extensive new provenance data, we propose a pre-subduction tectonic configuration in which the NFC and Alpujárride both represent adjacent attenuated continental fragments that are separated from the southern Iberian rifted margin by a narrow oceanic domain of the Alpine Tethys in the early Mesozoic.

Geochronology and geochemistry of zircons from fertile and barren intrusions in the Sangan mining area (NE Iran): Implications for tectonic setting and mineral exploration

The Sangan mining area is the largest Cenozoic skarn iron ore district in the far eastern part of the Alborz Magmatic Arc in Iran. In this study, we evaluate the zircon composition as an important exploration tool and pathfinder for skarn mineralization to elucidate distinctive zircon signatures diagnostic of metallogenic fertility of the parent magma. Granitoid intrusions in the Sangan mining area are subdivided into the fertile Sarnowsar intrusion composed of syenite, syenogranite, and granite and the barren Sarkhar and Bermani intrusions composited of monzogranite and syenogranite. The Eocene Sarnowsar intrusion was produced by the flare-up magmatism and is an oxidized I-type granitoid suite. Fourteen iron ore bodies occur along the contact of this intrusion with the carbonate country rocks. In the barren Sarkhar and Bermani intrusions, monzogranites have zircon U-Pb ages of 41.7 ± 0.6 Ma and 41.9 ± 0.3 Ma, whereas syenogranites have ages of 37.8 ± 0.3 Ma, 37.9 ± 1.7 Ma, and 37.4 ± 1.8 Ma. These ages span the same time interval as the Eocene flare-up occurrences throughout Iran.

Detrital zircon age signatures of the basal Cambrian sandstone unit in North China: implications for drainage divides during global Sauk transgression and separation from Gondwanaland

ABSTRACT The basal Cambrian sandstone unit in the North China craton (NCC) is an example of globally widespread siliciclastic succession that resides on the Great Unconformity and deposited on a hypothesized low-lying peneplain during the Cambrian global eustatic sea-level rise. Detrital zircon age signatures from this distinct sequence enable recognition of the ancient drainage system of the NCC in deep time and track its potential linkage with the Gondwana landmass. LA-ICP-MS U–Pb dating of the fossil-calibrated basal Cambrian (Series 2) detrital zircon samples from seven measured sections reveal marked spatial changes in their age signatures that can be divided into three distinct types. The first is generally characterized by the bimodal age populations with broad peaks at ~1.85 Ga and ~2.5 Ga that correlate with the Archean to Paleoproterozoic basement inboard of the NCC. The second is featured by multimodal distribution with diagnostic Neoproterozoic peaks that correspond to subregional magmatic record. The third also shows multiple-zircon age populations, but yields significant crystallization ages close to the early Cambrian age. Comparing our new data with existing age spectra for the Cambrian strata across the NCC and the northern Gondwana demonstrates that separate drainage systems did exist in the peneplained basement during global Cambrian transgression and the basal Cambrian unit in the NCC was not a part of the far-travelled sand sheet across the northern margin of Gondwana. The most suitable source for Cambrian-aged grains constrained by paleogeographic restoration is the arc terrane developed along the northern margin of the NCC as a result of subduction of the Paleo-Asian oceanic plate. Our new continental-scale detrital zircon provenance signatures in the basal Cambrian unit suggest that the NCC should be considered a discrete continental block separated by the Proto-Tethys Ocean in the Cambrian, rather than an integral part of the northern Gondwana.

Detrital zircon geochronology of the Permian Lower Shihezi Formation, northern Ordos Basin, China: time constraints for closing of the Palaeo-Asian Ocean

AbstractTemporal constraints on the closure of the eastern segment of the Palaeo-Asian Ocean along the northern margin of the North China Craton (NCC) remain unclear. As a part of the NCC, the sedimentation and tectonic evolution of the Late Palaeozoic Ordos Basin were closely related to the opening and closing of the Palaeo-Asian Ocean. We use petrology, quantitative mineralogical analysis, U–Pb geochronology and trace element signatures of detrital zircons of the Lower Shihezi Formation from two sections in the eastern north Ordos Basin and two sections in the western north Ordos Basin to reconstruct the sedimentary provenance and tectonic background of the northern Ordos Basin. The results show that the sediments of the western sections were mainly derived from the Yinshan orogenic belt and Alxa block, and that those in the eastern sections only came from the Yinshan orogenic belt. The trace element ratios in detrital zircons from the Late Palaeozoic sandstones indicate that the source areas were mainly subduction-related continental arcs, closely related to the continued subduction of the Palaeo-Asian Ocean in the Late Palaeozoic. Since the main Late Palaeozoic magmatic periods vary on the east and west sides of the northern margin of the Ordos Basin, two main collisions related to Palaeo-Asian Ocean closure are recorded. The collision on the west side occurred significantly earlier than that in the east. This study implies that the Palaeo-Asian Ocean began to subduct beneath the NCC in the Carboniferous and gradually closed from west to east thereafter.

Depositional Controls on Detrital Zircon Provenance: An Example From Upper Cretaceous Strata, Southern Patagonia

Understanding how depositional environments within a sedimentary system redistribute and sequester sediment is critical for interpreting basin-scale provenance trends. However, sedimentary source-to-sink models commonly examine temporal changes and do not consider how variation in sedimentation processes across a dispersal pathway may result in contrasting provenance signatures. In this paper, we demonstrate a down-paleoslope shift in detrital zircon provenance signatures correlated with shallow-marine lithofacies patterns from the Upper Cretaceous La Anita Formation and underlying continental slope lithofacies of the Alta Vista Formation (Magallanes-Austral Basin, southern Patagonia). New stratigraphic, sedimentologic, and lithofacies analysis results from the La Anita Formation suggest an upward shoaling succession, from a 1) storm-influenced shoreface, 2) fluvially dominated, wave-influenced delta, and a 3) high-energy, gravelly foreshore. Stratigraphic sections are paired with U-Pb detrital zircon sandstone samples (N = 20; n = 5,219), which provide both maximum depositional ages and provenance characteristics. While all samples contain abundant zircon derived from the Andean volcanic arc (ca. 145–75 Ma), the amount from both Jurassic distal volcanic massifs (ca. 188–162 Ma) and recycled orogenic sources exhumed during the advance of the Cretaceous fold-and-thrust belt (>200 Ma; 157–142 Ma) vary with changes in depositional environment. We argue that down-paleoslope, systematic enriching of local fold-and-thrust belt material within the La Anita Formation is reflective of progressive mixing of grains transported via shallow-marine processes, while distally enriched fluvio-deltaic transported zircons were sourced from large, regional catchments. This suggests that competition between transport processes across a shallow and marginal marine sequence of rocks affects the resulting provenance signatures recorded within a single stratigraphic succession. These data also detail the degree of sediment pathway connectivity between shallow-marine sources and deep-marine sinks. Detrital zircon results from muddy continental slope facies of the Alta Vista Formation are made up entirely locally derived material, while zircon results from deep-water, sand-rich channel facies of the Formation are indistinguishable from coeval fluvio-deltaic zircon signatures. This implies that continental shelf-to-slope connectivity in a sediment dispersal system, via submarine canyons or shelf-edge delta progradation, is necessary for detrital zircon distributions from the shallow-marine realm to propagate into the deeper marine.

Early Precambrian Crustal Evolution in the Irkut Block (Sharyzhalgai Uplift, Southwestern Siberian Craton): Synthesis of U–Pb, Lu–Hf and Sm–Nd Isotope Data

Abstract —The paper presents a synthesis of zircon U–Pb and Lu–Hf and whole-rock Sm–Nd isotope data from main early Precambrian (3.40 to 1.85 Ga) metamorphic and magmatic units of the Irkut block (Sharyzhalgai uplift, southwestern Siberian craton). The Archean complexes consist of relict Paleoarchean (3.4 Ga) melanocratic granulites and predominant Neoarchean mafic and felsic granulites (2.70–2.66 Ga), paragneisses (≤2.75 Ga), and gneissic granites (2.54 Ga). The Paleoproterozoic complexes include paragneisses (1.95–1.85 Ga), granitoids and charnockites (1.86–1.84 Ga), as well as mafic intrusions and dikes (1.86 Ga). Few detrital zircons with Hf model ages of ≥3.6 Ga mark the Eoarchean onset of crustal growth in the Irkut block. Isotopic data record two major stages of crustal growth in early Precambrian evolution of the Irkut block: Paleoarchean (3.6–3.4 Ga) and Neoarchean (~2.7–2.66 Ga). The Paleoarchean crustal growth was most likely associated with plume magmatism fed from depleted and primitive mantle sources. The spatial distribution of Paleoarchean crust is traceable in isotopic signatures of magmatic and detrital zircons from most of Mesoarchean to Paleoproterozoic units. The Neoarchean crustal growth from a depleted mantle source was due to subduction magmatism. Moderate crustal growth occurred in the Paleoproterozoic from 2.30 to 1.85 Ga. At the turn of 1.86–1.85 Ga, mafic magmas and products of their fractionation formed from both depleted and enriched sources under postcollisional extension; the latter sources were the subcontinental lithospheric mantle formed during Neoarchean subduction. Three major stages of crustal recycling have been established: Mesoarchean (~3.0 Ga), Neoarchean (~2.55 Ga), and Paleoproterozoic (1.86–1.85 Ga), which are characterized by near-coeval intracrustal melting and metamorphism. The recycling during the ~2.55 Ga and 1.86–1.85 Ga events apparently occurred in a collisional setting. The 2.7 Ga subduction-related felsic magmas also formed through the recycling of the Paleo-Mesoarchean crust. The hypothesized scenario for the geological evolution of the Irkut block is the dominant vertical growth and crustal recycling for about two billion years. Available isotope data record similar major crustal growth in the Paleoarchean and growth combined with recycling during the Neoarchean and Paleoproterozoic events in both the southwestern and northern and central parts of the Siberian craton. The Irkut block in the southwest differed in a long and continuous recycling during the Mesoarchean and pronounced Neoarchean crustal growth.

Multiple sediment incorporation events in a continental magmatic arc: Insight from the metasedimentary rocks of the northern North Cascades, Washington (USA)

Abstract The rheology and composition of arc crust and the overall evolution of continental magmatic arcs can be affected by sediment incorporation events. The exhumed Cretaceous–Eocene North Cascades arc exposes abundant metasedimentary rocks that were incorporated into the arc during multiple events. This study uses field relationships, detrital zircon geochronology, bulk rock geochemistry, geothermometry, and quartz-in-garnet geobarometry to distinguish approximate contacts and emplacement depths for different metasedimentary units to better understand their protolith incorporation history and impact on the arc. The Skagit Gneiss Complex is one of the main deep crustal units of the North Cascades arc. It includes metasedimentary rocks with distinct detrital zircon signatures: Proterozoic–Cretaceous (Group 1) or Triassic–Cretaceous (Group 2) zircon populations. Both metasedimentary groups achieved near-peak metamorphic conditions of 640–800 °C and 5.5–7.9 kbar; several Group 2 samples reveal the higher pressures. A third group of metasedimentary rocks, which was previously interpreted as metamorphosed equivalents of backarc sediments (Group 3), exhibited unimodal Triassic or bimodal Late Jurassic–Early Cretaceous detrital zircon signatures and achieved near-peak conditions of 570–700 °C and 8.7–10.5 kbar. The combined field and analytical data indicate that protoliths of Group 1 and Group 2 metasedimentary rocks were successively deposited in a forearc basin and underthrusted into the arc as a relatively coherent body. Group 3 backarc sediments were incorporated into the arc along a transpressional step-over zone. The incorporation of both forearc and backarc sediments was likely facilitated by arc magmatism that weakened arc crust in combination with regional transpression.

Detrital zircon ages, provenance and tectonic evolution in the early Paleozoic of Tasmania and Waratah Bay, Victoria

The provenance of the upper Cambrian to Upper Ordovician sedimentary rocks of Tasmania and Waratah Bay in southern Victoria provides information about the complex and dynamic tectonic environment present during their deposition. This paper uses U–Pb detrital zircon data to constrain stratigraphic comparisons and tectonic reconstructions of these rock sequences. Multivariate statistics are used to investigate the similarity between the U–Pb ages and quantify the disparity among different samples from various locations. In western and central Tasmania, the Tyennan region supplied most detrital zircons during the late Cambrian and Early Ordovician. The overlying Middle Ordovician Pioneer Sandstone records a switch in provenance with zircons derived from the Mount Read Volcanics (MRV) mixed with zircons similar to those from continental-derived Paleozoic sedimentary rocks deposited throughout east Gondwana. The Middle to Upper Ordovician Gordon Group in western and central Tasmania lacks detrital zircons younger than 1.2 Ga, which indicates a return to a local provenance from Precambrian rocks. In southern Tasmania, the switch to zircons derived from the MRV and east Gondwana-like sources occurred earlier within the Cambrian Deadmans Bay Formation, which is dominated by the east Gondwana Paleozoic zircon age signature. In the East Tasmania Terrane, Ordovician sedimentary rocks from Lefroy have detrital zircon populations dominated by Neoproterozoic and earliest Paleozoic sources similar to the Ordovician sedimentary rocks in the Lachlan Orogen. In southern Victoria, the Bear Gully Chert from Waratah Bay exhibit both Tyennan and distal Gondwana detrital sources. The switching of detrital zircon sources in the west Tasmanian sedimentary sequences implies the docking of Tasmania with mainland Australia during the Cambrian Tyennan Orogeny. The arrival of the distal zircons into these basins occurred at different times in the different areas, reflecting a complex local topography and paleogeography. Key Points Paleozoic sedimentary rocks in Tasmanian exhibit multisource detrital U–Pb age signatures that change over time, implying tectonic activity during their deposition. Paleozoic sedimentary rocks in northeastern Tasmania show Gondwana-wide detrital signatures similar to Lachlan Orogen. The Ordovician Bear Gully Chert at Waratah Bay in southern Victoria shows mixed Tasmanian and distal Gondwana detrital populations. The change in detrital zircon signature in western Tasmania suggests that VanDieland docked with the Australian continent during the Cambrian Tyennan Orogeny.

Evolution of the Late Cretaceous Nanaimo Basin, British Columbia, Canada: Definitive provenance links to northern latitudes

AbstractAccurate reconstruction of the Late Cretaceous paleogeography and tectonic evolution of the western North American Cordilleran margin is required to resolve the long-standing debate over proposed large-scale, orogen-parallel terrane translation. The Nanaimo Basin (British Columbia, Canada) contains a high-fidelity record of orogenic exhumation and basin subsidence in the southwestern Canadian Cordillera that constrains the tectonic evolution of the region. Integration of detrital zircon U-Pb geochronology, conglomerate clast U-Pb geochronology, detrital muscovite 40Ar/39Ar thermochronology, and Lu-Hf isotopic analysis of detrital zircon defines a multidisciplinary provenance signature that provides a definitive linkage with sediment source regions north of the Sierra Nevada arc system (western United States).Analysis of spatial and temporal provenance variations within Nanaimo Group strata documents a bimodal sediment supply with a local source derived from the adjacent magmatic arc in the southern Coast Mountains batholith and an extra-regional source from the Mesoproterozoic Belt Supergroup and the Late Cretaceous Atlanta lobe of the Idaho batholith. Particularly robust linkages include: (1) juvenile (εHf >+10) Late Cretaceous zircon derived from the southern Coast Mountains batholith; (2) a bimodal Proterozoic detrital zircon signature consistent with derivation from Belt Supergroup (1700–1720 Ma) and ca. 1380 Ma plutonic rocks intruding the Lemhi subbasin of central Idaho (northwestern United States); (3) quartzite clasts that are statistical matches for Mesoproterozoic and Cambrian strata in Montana and Idaho (northwestern United States) and southern British Columbia; and (4) syndepositional evolved (εHf >−10) Late Cretaceous zircon and muscovite derived from the Atlanta lobe of the Idaho batholith. These provenance constraints support a tectonic restoration of the Nanaimo Basin, the southern Coast Mountains batholith, and Wrangellia to a position outboard of the Idaho batholith in Late Cretaceous time, consistent with proposed minimal- fault- offset models (<~1000 km).

Stable isotope geochemistry of silicon in granitoid zircon

Zircon is often used to study granites (sensu lato) and continental crust because it is very resistant and can be analyzed for various isotope systems that provide time and source information about their parental melts. Granites with different petrological histories have distinct bulk-rock silicon isotope compositions but it is unclear if these differences are also detectable in zircon because of superimposed fractionation effects (e.g., related to temperature, silica content, magmatic processes). The present study explores the Si isotope signatures of zircon from various granite types to constrain their isotope fractionation behavior and uses them as igneous petrogenetic tools, and possibly as granite source discriminators when zircon is found in detrital sediments. Our results show that although Si isotope compositions in zircon can be modified by secondary (post-crystallization) processes such as alteration/weathering and metamorphism, they are primarily controlled by zircon-melt isotope fractionation, which depends on both zircon crystallization temperature and magma silica content. Once these fractionation effects are understood and filtered out, a pattern emerges between Si isotope signatures of zircons from different granite types that is consistent with theoretical and experimental results as well as with known Si isotope differences at the bulk-rock scale. Silicon isotope ratios in zircon can track magma evolution (e.g., temperature and SiO2 changes) and, hence, reveal complex processes that involved magma mingling, fractional crystallization, and/or multiple sources. This study, therefore, illustrates that Si isotopes in zircon can be used to investigate magma evolution and represents a useful complement to existing techniques in granite studies involving zircon (e.g., U-Th-Pb, Lu-Hf and O isotopes) provided that it is not used as a stand-alone technique.

The Variscan basement in the western shoulder of the Lusitanian Basin (West Iberian Margin): insights from detrital-zircon geochronology of Jurassic strata

There is no consensus about the geological nature of the westernmost portion of the Iberian Massif. In the present research, the detrital zircon U–Pb signatures of Jurassic strata of the Lusitanian Basin, known to be west-sourced, are combined with published U–Pb data for the Precambrian-Palaeozoic basement and other Lusitanian Basin units to better understand this poorly exposed portion of the Iberian Massif. Cryogenian to Ediacaran ages prevail in a northern Upper Jurassic unit, while Lower and Upper Jurassic rocks in southern locations yield mostly Carboniferous to upper Permian zircons. These age results, coupled with their respective U/Th ratios, suggest that the basin covers two distinct terranes of the Iberian Massif. Another noteworthy feature of west-derived deposits is the abundance of < 310 Ma ages. It is proposed that a combination of crustal thinning in the West Iberian Margin with regional eastward basement tilt, favoured the enrichment of relatively young zircon in the western shoulder of the basin relative to its eastern margin. The detrital zircon age signatures also reveal a middle to late Permian thermal event in restricted areas, which is probably associated with the oldest stages of Alpine extension in West Iberia.

Model versus measured detrital zircon age signatures of the early Earth

The scarcity of Archean (4.0–2.5 Ga) rocks on Earth limits our ability to analyse when plate tectonics began and what tectonic mode may have preceded it. Proposed times for the initiation of plate tectonics range from the Hadean eon (>4 Ga) to the Neoproterozoic era (ca 1.0–0.5 Ga), before which the Earth may have been dominated by stagnant-lid regimes that cooled through volcanic advection, partial crustal convection, and/or conduction. Here, we use various 1D numerical simulations of potential geodynamic states of the early Earth to predict detrital zircon age patterns. We find that different Archean tectonic modes yield contrasting temporal differences between zircon crystallization ages and respective maximum depositional ages of the sediments from which the detrital zircons were sourced (we have termed such age differences ‘ΔtDZ’). We compare these predictions with a global compilation of detrital zircon age data from >400 Archean (meta)sedimentary units, including new data from six samples from the Pilbara Craton, Western Australia. Combined, the data show significantly increased contributions from high ΔtDZ detrital zircons after ca 3.4 Ga. For the Pilbara Craton, 90% of the detrital zircons in each of two >3.4 billion-year-old metasedimentary samples have crystallization ages that are ≤110 million years older than the maximum age of deposition (we use ‘ΔtDZ90’ to denote such age differences hereafter). The ΔtDZ90 of Pilbara strata formed at ca 3.29–3.27, 3.24–3.18 and ca 2.95 Ga show successively larger ΔtDZ90 values of ca 270, 290 and 570 million years, respectively. Globally, pre-3.4 billion-year-old samples show a low mean ΔtDZ90 of around 70 million years, but a mean of around 320 million years for 3.40–2.80 billion-year-old samples. Samples with >90% of zircons featuring >250 million year ΔtDZ values only occur at or after ca 3.23 Ga. These results are consistent with a significant slow-down in the rate of volcanic resurfacing, and increasing contributions of detritus from basins associated with exhumation and/or volcanic quiescence after ca 3.4 Ga. Compared with detrital zircon age predictions, our findings are best explained by a transition from volcanism-dominated stagnant-lid modes to one increasingly dominated by subduction over the period 3.4–3.2 Ga, with the emergence of plate tectonics within this period or at some time thereafter.

New insights into the petrogenesis of the Puerto Vallarta Batholith, Mexico: Evidence from petrology, zircon petrochronology, and phase equilibrium modeling

At subduction zones, continental magmatic arcs are produced by differentiation processes. The Puerto Vallarta Batholith, at the Mexican Pacific margin, is the final product of the subduction of oceanic Farallon plate beneath the North American plate during the Cretaceous. Although this Cordilleran Batholith has been subject to different studies, its petrogenetic evolution remains unclear. To better constrain the magmatic processes that gave origin to the granitic rocks of the Puerto Vallarta Batholith, we combined detailed fieldwork, zircon petrochronology, and conventional thermobarometry coupled with phase equilibrium modeling. We present the crystallization and emplacement evolution, and its relationship to magmatic processes of the Puerto Vallarta Batholith. In the granitic rocks, we observed calc-alkaline and metaluminous signatures, as well as enrichment in light-rare earth elements (LREE) relative to the heavy-rare earth elements (HREE). The early crystallization stages of the Puerto Vallarta Batholith started at ~729–776 °C and ~3.5–5.3 kbar, with the last melt portion crystallized at ~675–685 °C. Fieldwork observations and Zr saturation thermometry reveal mixing of melts during the last crystallization stages. Geochemical signatures of zircons from granitic rocks suggest crystallization after minor plagioclase fractionation with likely assimilation of crustal components. Our results from geochemical whole-rock data further indicate partial melting of a heterogenous source in the mantle wedge and the differentiation of those primary melts as the principal processes that gave origin to the Puerto Vallarta Batholith. We show that the magmatic processes that originated the granitic rocks of this Cordilleran Batholith occurred in the middle crust during the Late Cretaceous. Our results provide new evidence to better constrain the origin of this magmatic system.

Evidence from the U–Pb–Hf signatures of detrital zircons for a Baltican provenance for basal Old Red Sandstone successions, northern Scottish Caledonides

The provenance of Devonian Old Red Sandstone (ORS) continental successions in the northern Scottish Caledonides is poorly known: were they derived locally or from more distal sources? The integration of U–Pb and Hf isotope analyses in detrital zircon crystals reduces potential ambiguities arising from non-unique age populations and yields information on the crustal evolution of the source terranes. Samples of basal ORS successions yield zircon U–Pb age groupings of c. 1800–1500 and c. 1200–900 Ma, with minor Neoarchean, Tonian, Ediacaran and Ordovician contributions. SW Baltica provides the best match for detrital zircon ages and Hf isotopic signatures and much of the &gt;900 Ma zircon population was probably recycled from Neoproterozoic successions. ε Hf ( t ) values in c. 1800–1000 Ma grains reflect the assembly of Nuna, the development of a long-lived retreating subduction system along its margin and Grenville collisional orogenesis. These basal ORS successions were probably deposited within the same regional fluvial system as coeval sedimentary rocks in the Midland Valley, draining an area of positive relief in SW Baltica where continental convergence continued through the Early Devonian. Supplementary material : Isotopic data, statistical comparison of selected samples and TIMA heavy mineral data are available at https://doi.org/10.6084/m9.figshare.c.5301780

Fossil oceanic core complexes in the Alps. New field, geochemical and isotopic constraints from the Tethyan Aiguilles Rouges Ophiolite (Val d\u2019H\xe9rens, Western Alps, Switzerland)

Exhumation of basement rocks on the seafloor is a worldwide feature along passive continental margins and (ultra-) slow-spreading environments, documented by dredging, drilling or direct observations by diving expeditions. Complementary observations from exhumed ophiolites in the Alps allow for a better understanding of the underlying processes. The Aiguilles Rouges ophiolitic units (Val d’Hérens, Switzerland) are composed of kilometre-scale remnants of laterally segmented oceanic lithosphere only weakly affected by Alpine metamorphism (greenschist facies, Raman thermometry on graphite: 370–380 °C) and deformation. Geometries and basement-cover sequences comparable to the ones recognized in actual (ultra-) slow-spreading environments were observed, involving exhumed serpentinized and carbonatized peridotites, gabbros, pillow basalts and tectono-sedimentary cover rocks. One remarkable feature is the presence of a kilometric gabbroic complex displaying preserved magmatic minerals, textures and crosscutting relationships between the host gabbro and intruding diabase, hornblende-bearing dikelets or plagiogranite. The bulk major and trace element chemistry of mafic rocks is typical of N-MORB magmatism (CeN/YbN: 0.42–1.15). This is supported by in-situ isotopic signatures of magmatic zircons (εHf = + 13 ± 0.6) and apatites (εNd = + 8.5 ± 0.8), determined for gabbros and plagiogranites. In-situ U–Pb dating was performed on zircons by laser ablation-ICP-MS, providing ages of 154.9 ± 2.6 Ma and 155.5 ± 2.8 Ma, which are among the youngest for oceanic gabbros in the Alps. Our study suggests that the former Aiguilles Rouges domain was characterized by tectonism and magmatism resembling present-day (ultra-) slow-spreading seafloor. It also suggests that the Tethyan lithosphere is laterally segmented, with punctuated magmatism such as the Aiguilles Rouges gabbros and carbonated ultramafic seafloor covered by basalts and Jurassic tectono-sedimentary deposits.

Provenance and paleogeography of Archean Fig Tree siliciclastic rocks in the East-Central Barberton Greenstone Belt, South Africa

The 3.6–3.2 Ga Barberton Greenstone Belt, South Africa, is a complex terrain divided into multiple structural blocks. The structural deformation and poor geochronological constraints often make correlation among blocks difficult. To overcome structural complexities, provenance proxies including sandstone petrography, shale geochemistry, and detrital zircon geochronology are here used to compare source terrain signatures among multiple structural blocks in the East-Central Domain of the Barberton Greenstone Belt. This will expand our understanding of Paleoarchean paleogeography and the nature of crustal uplift in the southeastern part of the Barberton Greenstone Belt.The Manzimnyama and Mlumati Synclines are composed of 600 m-thick sections of the Fig Tree Group, Mapepe Formation. These synclines have high sedimentary and volcanic lithic contents in sandstones, an upward transition from felsic to mafic signatures in shales, and a detrital zircon age distribution transition from a basal member with a main peak at 3.28 Ga to members with a peak at about 3.45 Ga. There are three other East-Central Domain, fault-bounded blocks made up mostly of Mapepe Formation strata. The Eastern Barite Valley lies northwest of the Manzimnyama Syncline. From older to younger strata, the Eastern Barite Valley progresses from high lithics to more quartz and feldspar in sandstones, from more mafic to felsic signatures in shales, and from a unimodal 3.45 Ga detrital zircon signature at the base to complexly mixed age distributions with a strong 3.24 Ga peak at the top. The Paulus Syncline, southeast of the Manzimnyama Syncline, has a high chert content, mafic to ultramafic geochemistry signatures, and a main detrital zircon peak at 3.28–3.30 Ga with a minor peak at 3.45 Ga. The Emlembe Syncline, southeast of the Paulus Syncline, has a high monocrystalline quartz content, mixed geochemistry signatures, and unimodal 3.28–3.30 Ga detrital zircon ages. The proximity and sedimentology of these structural blocks could suggest deposition in a single basin; however, contrasts in provenance suggests sediments were deposited in different basins, at different times, or in one basin with provenance partitioning. The blocks likely had locally different sources, implying the sequences were not part of a single large source, transport, and depositional fairway.

Nature and (in-)coherent metamorphic evolution of subducted continental crust in the Neoproterozoic accretionary collage of SW Mongolia

In continental subduction complexes minor volumes of high-pressure mafic rocks (eclogites) often co-exist with much more abundant felsic (granitic) and metasedimentary rocks, which are vital for resolving the origin and metamorphic evolution of subducted continental crust. In SW Mongolia, the Alag Khadny eclogite-bearing accretionary complex (AKC) is assumed to represent either a remnant of oceanic slab, or a continental margin, subducted in the Early Cambrian. Here we present geochronological, geochemical and petrological evidence of subduction records for the three major types of lithologies that host mafic eclogites, including Mesoproterozoic and Neoproterozoic granitic basement and overlying Neoproterozoic continental-margin sediments. Variably deformed, ferroan and peraluminous metagranitoids compose a major part of AKC and are interlayered with eclogites in its southern and eastern margins. They have geochemical features of post-collisional/intraplate high-K calc-alkaline granites. LA-ICP-MS U-Pb zircon geochronology of three distinct metagranite samples show uniform protolith crystallization ages of ca. 0.96 Ga and uncertain re-crystallization in the Late Neoproterozoic or Early Paleozoic metamorphic event, whereas abundant zircon inheritance indicates older, Mesoproterozoic to Paleoproterozoic crustal substrate during granite generation. The existence of Mesoproterozoic crust is highlighted by finding of distinct metagranitoids with the U-Pb zircon crystallization age of ca. 1.6 Ga. Hafnium isotope signatures (TDMC 2.88–1.85 Ga) of zircons from all lithologies preserved the evidence of reworked Neoarchean to Paleoproterozoic crust, similar to that of the Baidrag block (southern Mongolia), for both Mesoproterozoic and Neoproterozoic rocks. Regardless of the specific lithology, the rocks display indicators of high-pressure metamorphic re-equilibration, including garnet (XCa up to 0.65) + epidote + phengite (Si p.f.u. up to 3.56) ± rutile assemblage in metagranitoids, garnet + phengite (Si p.f.u. up to 3.42) in quartz-rich semi-pelites and garnet + phengite (Si p.f.u. up to 3.39) + medium-Mg chloritoid (XMg up to 0.25) + kyanite + rutile in metapelites. Corresponding P-T conditions recovered from different lithologies reveal incoherent subduction of rocks, which could be shallow for granitic basement (1.1–1.4 GPa and 600–670 °C) and clastic metasediments (1.4–1.6 GPa, 570–620 °C), but deeper for metapelites (2.1–2.3 GPa, 500–570 °C). consistent with that of eclogites, The combined data show that the Alag Khadny complex represents a remnant of a rifted Mesoproterozoic to Neoproterozoic (ca. 1.6–0.96 Ga) continental margin consequently metamorphosed under HP conditions during Late Neoproterozoic–Early Cambrian evolution of the southern Central Asian Orogenic Belt. Acquired P-T estimates imply that high-pressure metagranitoids and metasedimentary rocks equilibrated at different depths, but most likely shared a common subduction-related metamorphic evolution.