Late Paleozoic Time Research Articles

Metal endowment and geodynamic evolution of the Late Paleozoic SEDEX deposits in South China: The Yunfu giant iron-sulfide deposit, Yunkai Domain

Metal endowment process of the clustered SEDEX (sedimentary exhalative) deposits in the Yunkai Domain (Cathaysia Block, South China) and their genetic connection with the regional tectonics is still open to debate. As a representative syngenetic deposit hosted in the black shale, the Yunfu iron-sulfide deposit (pyrite: 200 Mt @ 35%) has the potential to evaluate the missing linkage. Herein, pyrite Re–Os age and in-situ trace elements and sulfur isotope of principal ores were employed to constrain the metal accumulation process. The laminated pyrite (Py1), with the Re–Os isochron age of 303.4 ± 6.9 Ma (MSWD = 0.15), is enriched in heterogeneous distributed trace elements and highly negative heavy sulfur composition (δ34SV-CDT, mean = − 19.82‰, n = 29, standard deviation, s.d. = 0.67). This indicates the sulfur and metal precipitation was attributed for the bacterial sulfate reduction (BSR) during rapid deposition in the anoxic-seawater column of open sea at the Late Paleozoic time. The massive pyrite (Py2) hosts evenly distributed trace elements and slightly negative sulfur compositions (δ34SV-CDT, mean = − 2.00‰, n = 5, s.d. = 0.19), interpreted as further refinement and purification from Py1. To further trace its metallogenic tectonic, one hundred thirty-six (136) detrital zircon grains from ores and host rocks were selected for U–Pb and Lu–Hf isotopic analyses. Four samples of ores and ore-enveloping successions obtain the agreed youngest U–Pb ages of ∼ 430 Ma, also containing some inherited zircon grains of ca. 1200–800 Ma and ca. 2500 Ma. They possess a distinctive εHf(t) value of + 28.9 to + 40.5 for Archean, −15 to + 20 for Proterozoic and − 46.6 to − 0.2 for Paleozoic grains, respectively. This records that the episodic magmatism, including the Archean subduction, Proterozoic back-arc basin, and Paleozoic intra-continental orogeny prior to the eventual pyrite deposition, supply with substantial provenance. As a result, these collective data support that the Yunfu deposit was formed during the Late Paleozoic sedimentary exhalative deposition and hydrothermal activity in a local depression basin on the Yunkai passive continental margin without coeval magmatism.

Cordilleran Subduction Initiation: Retroarc Timing and Basinal Response in the Inyo Mountains, Eastern California

Abstract Subduction zones drive plate tectonics on Earth, yet subduction initiation and the related upper plate depositional and structural kinematics remain poorly understood because upper plate records are rare and often strongly overprinted by magmatism and deformation. During the late Paleozoic time, Laurentia’s western margin was truncated by a sinistral strike-slip fault that transformed into a subduction zone. Thick Permian strata in the Inyo Mountains of central-eastern California record this transition. Two basins that were separated by a transpressional antiform contain sedimentary lithofacies that record distinct patterns of shoaling and deepening conditions before and during tectonism associated with subduction initiation. Sandstone petrography and lithofacies analysis show that rocks in a southeastern basin are dominated by carbonate grains derived from adjacent carbonate shelves, whereas sandstones in a northwestern basin are predominantly quartzose with likely derivation from distant ergs or underlying strata. Detrital zircon spectra from all but the youngest strata in both basins are typical of Laurentian continent spectra with prominent peaks that indicate ultimate sources in Appalachia, Grenville, Yavapai/Mazatzal, and the Wyoming or Superior cratons. The first Cordilleran arc-derived detrital zircon grains appear in the uppermost strata of the northwestern basin and record Late Permian (ca. 260 Ma) Cordilleran arc magmatism at this approximate latitude, and a possible source area is suggested by geochemical similarities between these detrital zircons and broadly coeval magmatic zircons in the El Paso Mountains to the southwest. Deformation responsible for basin partitioning is consistent with sinistrally oblique contraction in the earliest Permian time. The data presented from the Inyo Mountains shed more light on the nature of Cordilleran subduction initiation and the upper-crustal response to this transition.

Late Paleozoic geodynamic evolution of the western North Patagonian Massif and its tectonic context along the southwestern Gondwana margin

In this study, we propose a geodynamic model covering sedimentation, metamorphism, magmatism, and exhumation processes for the western North Patagonian Massif basement. The youngest detrital zircon population ages (ca. 369 ± 8 Ma) obtained in a schist sample constrains the sedimentation stage to the Carboniferous Period. A first prograde metamorphic stage (M1-D1) produced the main foliation (S1) under greenschist conditions (~ 500 °C; < 4.5 Kbars). This stage was possibly linked to the regional Carboniferous magmatism event (330–300 Ma). The Permian magmatism (ca. 290 Ma) likely induced partial melting and migmatization of the deepest metasedimentary suite. This event corresponds to the second prograde metamorphic stage (M2) that reached amphibolite conditions (690 °C and 6.5 Kbars). The beginning of the basement uplift corresponds to the first retrograde metamorphic stage possibly developed during Permian – Triassic times (265–235 Ma). This event was triggered by NE-SW compression (σ1) and developed folds (D2-F2), second foliation (S2), micro-textural quartz deformation, and a retrograde evolution path for the garnet-bearing lithofacies. The final stage of the basement exhumation corresponds to the second retrograde metamorphic stage (D3) developed by NNW-SSE compression and linked to open folds (F3) in the Cushamen Formation. The characteristics of the western North Patagonian Massif geodynamic evolution and the adjacent basement regions suggest a paleotectonic subduction setting for the southwestern Gondwana margin during the late Paleozoic times.

The interplay of dynamic topography and eustasy on continental flooding in the late Paleozoic

Global sea level change can be inferred from sequence stratigraphic and continental flooding data. These methods reconstruct sea level from peri-cratonic and cratonic basins that are assumed to be tectonically stable and sometimes called reference districts, and from spatio-temporal correlations across basins. However, it has been understood that long-wavelength (typically hundreds of km) and low-amplitude (<2 km) vertical displacements of the Earth's surface due to mantle flow, namely dynamic topography, can occur in the absence of crustal deformation. Dynamic topography can drive marine inundation or regional emergence of continents and must be taken into consideration for eustasy estimates. Our analysis indicates that the long-term trend in global-scale maximum flooding over the late Paleozoic generally correlates with global sea level curves. The first-order flooding history of North America correlates with some estimates of eustasy. The Paleozoic inundation of South America does not follow long-term sea level variations. The flooding lows during the Early Carboniferous and high during the Late Carboniferous are at odds with estimates of eustasy and can be explained by dynamic uplift and subsidence, respectively. Our dynamic topography models indicate that the Yangtze Platform of South China experienced significant dynamic subsidence during the transition from Permian to Triassic largely due to proto-Pacific subduction and its northward motion to collide with North China. The reference districts – Western New York, Oklahoma and Kansas, and West Texas in North America – were to some degree affected by dynamic uplift and subsidence associated with long-lived Panthalassa subduction zones, closure of the Rheic Ocean and large-scale upwelling above the African deep-mantle structure during late Paleozoic times. This indicates that some published global sea level curves may include non-eustatic signals such as dynamic uplift or subsidence. The interpretation of stratigraphic data gathered from these reference districts should be treated with caution to estimate global sea level variations.

First assessment of recent tectonics and paleoearthquakes along the Irtysh fault (eastern Kazakhstan)

The Asian plate interiors are known to have host strong earthquakes with magnitudes up to M ≈ 8 in recent history, especially around the border area between Mongolia, Kazakhstan, China and Russia (e.g., M7.3 Chuya earthquake, 2003). Their recurrence times seem to be long, because of the relative low slip rates (<1 mm/y) of the faults which caused them.In this study, we focus on a large inherited fault zone (namely the Irtysh Fault Zone) with no historical earthquakes larger than M6, to test whether this structure could have generated surface-rupturing events during the last thousands of years, like some other historically silent similar faults of cratonic southern Kazakhstan did in pre-historic times. To do so, we use tectonic-morphological analyses of satellite images and trenching across several fault portions to detect potential paleoearthquakes.The Irtysh Fault Zone (IFZ) is a 250+ km long basement set of faults that marks a major tectonic block boundary between different units with Paleozoic magmatic rocks and thick deposits of Late Paleozoic age. The formation of the IFZ dates back into the Late Paleozoic times during collision between Siberia-Kazakh cratons and it was repeatedly reactivated in later times (e.g., Oligo-Miocene). The Quaternary activity of the Irtysh Fault Zone has never been addressed in modern studies and, in this study, we document for the first time that the IFZ is an active fault and is a potential source of large earthquakes in easternmost Kazakhstan. Our analyses actually revealed, at different spots along the IFZ, the occurrence of a set of lineaments offsetting lithologies or deflecting streams and other landscape features. Trenching sites across those lineaments eventually confirmed that they were active fault strands and in some trenches, 14C-dated Holocene-Late Pleistocene deposits are clearly faulted. Those recent deposits include organic soils, loess layers and colluvium directly overlying the Paleozoic rocks. Our findings lead to the conclusions that the IFZ is clearly active along several strands of its trace in bedrock. During the Holocene, it hosted earthquakes with surface rupture individual displacements of up to 1.5 m, suggesting events with a magnitude around M ≈ 7.However, our investigations do not allow to confident estimation of slip rates: we propose for one fault section parallel to the main fault a preliminary estimation of ~0.22 mm/y vertical displacement rate, based on a single trench. Based on regional morphoclimatic arguments and landscape features offsets, we also propose a first assessment of the lateral slip rate along the major fault between 0.3 and 0.5 mm/y.Additional investigations should be undertaken to refine surface fault mapping, the paleoearthquake calendar, the slip rate, and potential along-strike evolution, in order to constrain future seismic hazard analyses with fault models in this region of Eastern Kazakhstan.

Mid-Carboniferous shallow-water conodonts from northwest Mexico

Conodont associations found in a carbonate succession cropping out in the Sierra AguaVerde (central Sonora, northwestern Mexico) indicate that the studied section spans the mid-Carboniferous Boundary. Declinognathodus noduliferus s.l., the index conodont for the base of the Pennsylvanian, is here reported for the first time in the studied area. The conodont fauna found in the lower part of the section is dominated by Taphrognathus varians, Gnathodus texanus, and Bispathodus n. sp. A., and also contains shallow-water conodonts such as Cloghergnathus sp., Hindeodus cristula, Cavusgnathus unicornis, Rhachistognathus prolixus, and Rhachistognathus muricatus. Pennsylvanian conodont fauna contains both cavusgnathids and gnathodonthids, including Adetognathus spathus, Adetognathus lautus, Adetognathus inflexus, and species of the Declinognathodus noduliferus group. Conodont associations found in Sonora are similar to those reported in other areas of North America, indicating that the shallow water rocks exposed in northwestern Mexico were part of a carbonate shelf connected to the epicontinental sea that covered most of the Laurentian craton during late Paleozoic times. In addition, some aspects on mid-Carboniferous conodont systematics and biofacies also are reviewed herein.

Neoproterozoic magmatic flare-up along the N. margin of Gondwana: The Taknar complex, NE Iran

Magmatic “flare-ups” are common in continental arcs. The best-studied examples of such flare-ups are from Cretaceous and younger continental arcs, but a more ancient example is preserved in Late Ediacaran–Cambrian or Cadomian arcs that formed along the northern margin of Gondwana. In this paper, we report new trace-element, isotopic and geochronological data on ∼550 Ma magmatic rocks from the Taknar complex, NE Iran, and use this information to better understand episodes of flare-up, crustal thickening and magmatic periodicity in the Cadomian arcs of Iran and Anatolia. Igneous rocks in the Taknar complex include gabbros, diorites, and granitoids, which grade upward into a sequence of metamorphosed volcano-sedimentary rocks with interlayered rhyolites. Granodioritic dikes crosscut the Taknar gabbros and diorites. Gabbros are the oldest units and have zircon U–Pb ages of ca 556 Ma. Granites are younger and have U–Pb zircon ages of ca 552–547 Ma. Rhyolites are coeval with the granites, with U–Pb zircon ages of ∼551 Ma. Granodioritic dikes show two U–Pb zircon ages; ca 531 and 548 Ma. Geochemically, the Taknar igneous rocks have calc-alkaline signatures typical of continental arcs. Whole-rock Nd and zircon O–Hf isotopic data show that from Taknar igneous rocks were generated via mixing of juvenile magmas with older continental crust components at an active continental margin. Compiled geochronological and geochemical data from Iran and Anatolia allow identification of a Cadomian flare-up along northern Gondwana. The compiled U–Pb results from both magmatic and detrital zircons indicate the flare-up started ∼572 Ma and ended ∼528 Ma. The Cadomian flare-up was linked to strong crustal extension above a S-dipping subduction zone beneath northern Gondwana. The Iran–Anatolian Cadomian arc represents a site of crustal differentiation and stratification and involved older (Archean?) continental lower–middle crust, which has yet to be identified in situ, to form the continental nuclei of Anatolia and Iran. The Cadomian crust of Anatolia and Iran formed a single block “Cimmeria” that rifted away from northern Gondwana and was accreted to southern Eurasia in late Paleozoic time.

Plate interior polyphase fault systems and sedimentary basin evolution: A case study of the East Gobi Basin and East Gobi Fault Zone, southeastern Mongolia

Structural interpretation of 2-D seismic reflection data and subsurface-outcrop correlations reveal six distinct phases of deformation recorded in the Paleozoic basement rocks and Mesozoic-Cenozoic basin fill of the East Gobi Basin (EGB), southeastern Mongolia. These phases include arc accretion and arc-continent collision in late Paleozoic time, Late Triassic sinistral shear-zone development, Early Jurassic fold and thrust belt style shortening, Middle Jurassic-Lower Cretaceous extension and rift basin development, middle Cretaceous shortening with basin inversion and regional unconformity development, and Late Cretaceous-Oligocene thermal subsidence with renewed Paleogene left-lateral strike slip faulting across the fault zone. The five post-amalgamation deformation phases are localized along the East Gobi Fault Zone, suggesting that preexisting structures and boundary conditions exert fundamental controls on the long term evolution of intracontinental basins such as the EGB. Subsurface geophysical data and outcrop correlations demonstrate that the main subbasins of the EGB contain major differences in basement metamorphic and structural fabrics, basin fill patterns, and distinct Mesozoic-Cenozoic fault generations. Potential causes related to far-field boundary conditions and other driving factors are suggested for each of the major deformation phases.

Phanerozoic magmatic tempos of North China

Detrital zircons from northeast China record cyclic magmatism along the northern and eastern margins of the North China block during late Paleozoic time and Mesozoic time, respectively. The late Paleozoic zircons record three magmatic flare-ups with a period of ∼60 m.y. that occurred within a magmatic arc constructed along the Paleoasian (northern) margin of North China, and are accompanied by negative εHf(t) excursions representing shortening and increased crustal melting over the duration of each flare-up. The intervening magmatic lulls are accompanied by rapid positive εHf(t) excursions signifying influxes of juvenile magma into the arc, probably during extension and foundering of underlying melt residua. The lack of similar isotopic patterns in zircons derived from contemporaneous intrusions into older continental settings inboard of the arc indicate that this process was restricted to the arc itself. Mesozoic magmatism in North China occurred along the Paleo-Pacific margin following closure of the Paleoasian Ocean, and exhibits a ∼50 m.y. periodicity that is out-of-phase with that of the Paleozoic arc. Although the tectonic setting of North China during Mesozoic time is complex and still controversial, it is possible that this younger periodicity is governed by similar processes as those that dominated the Paleozoic arc. This is a testable hypothesis that warrants further attention. Crustal shortening was widespread in North China during Mesozoic time, and documented lithosphere removal events in eastern North China occurred during the Mesozoic magmatic lulls. Lithospheric thickening/foundering cyclicity, well-documented in Cordilleran arc systems, may be a common process in continental arcs through space and time.

Mesozoic tectono-magmatic evolution of Mexico: An overview

The Geological Framework of Mexico is formed by a mosaic of tectono-stratigraphic terranes that were assembled during the Paleozoic and Mesozoic as the result of the complex interaction between Laurentia, Gondwana, and the paleo-Pacific plate. Even though the Mexican tectono-magmatic evolution is still object of study, some main representative stages can be recognized. Determining the time span and regional distribution of magmatism throughout the geological history of Mexico is crucial for the exploration of mineral resources. This paper contains a brief description of the terranes and is focused on summarizing the main Mesozoic tectono-magmatic events, from the assembling of Pangea in Late Paleozoic time to the end of the Mesozoic. These main events are: 1) Permo-Triassic continental (submarine) arc that developed along eastern Oaxaquia and was related to a subduction zone developed along the paleo-pacific margin. 2) Magmatic activity ceased in central and southern Mexico, during Middle to Late Triassic time, and a wide submarine fan was formed along the western margin of Pangea, which apparently acted like a passive margin. Evidence of a contemporaneous intra-oceanic arc is found at Baja California. 3) Inland Early Jurassic volcanism is found in northeast and central Mexico. Whether this magmatic event evolved in a rift or arc (Nazas) tectonic setting is still undetermined. 4) By Late Jurassic, magmatism was widespread. It is the time of the breakup of Pangea, leading to the opening of the Gulf of Mexico and the Arperos Basin. Therefore, volcanism was mostly related to subduction and supra-subduction rifting. 5) The Gulf of Mexico and Arperos basins were formed by Early Cretaceous time. Igneous rocks of that age have a wide range of composition (arc, continental rift, strike slip, and oceanic rift, within plate), and are geographically widespread. 6) At the end of Early Cretaceous, volcanism moved to a more localized belt along the paleo Pacific margin; it is also the time of formation of major carbonate platforms along east Mexico. 7) Coeval to Sevier-Laramide orogeny of North America, a large fold and thrust belt developed throughout Mexico, during Late Cretaceous time. It was accompanied by arc magmatism along the pacific margin of Mexico (Tarahumara Arc). It is the first continental arc that evolved after accretion and consolidation of Guerrero and Alisitos terranes to Continental Mexico. These Mesozoic tectonic processes originated westward-eastward migrations of arc-related and rift-related volcanism, which in turn lead to the present distribution of Mesozoic mineral deposits.

Early Paleozoic tectonic reconstruction of Iran: Tales from detrital zircon geochronology

In this study we use detrital zircons to probe the Early Paleozoic history of NE Iran and evaluate the link between sediment sources and Gondwanan pre-Cadomian, Cadomian and younger events. U–Pb zircon ages and Hf isotopic compositions are reported for detrital zircons from Ordovician and Early Devonian sedimentary rocks from NE Iran. These clastic rocks are dominated by zircons with major age populations at ~2.5Ga, ~0.8–0.6Ga, 0.5Ga and ~0.5–0.4Ga as well as a minor broad peak at ~1.0Ga.The source of 2.5Ga detrital zircons is enigmatic; they may have been supplied from the Saharan Metacraton (or West African Craton) to the southwest or Afghanistan–Tarim to the east. The detrital zircons with age populations at 0.8–0.6Ga probably originated from Cryogenian–Ediacaran juvenile igneous rocks of the Arabian–Nubian Shield; this inference is supported by their juvenile Hf isotopes, although some negative εHf (t) values suggest that other sources (such as the West African Craton) were also involved. The age peak at ca 0.5Ga correlates with Cadomian magmatism reported from Iranian basement and elsewhere in north Gondwana. The variable εHf (t) values of Cadomian detrital zircons, resembling the εHf (t) values of zircons in magmatic Cadomian rocks from Iran and Taurides (Turkey), suggest an Andean-type margin and the involvement of reworked older crust in the generation of the magmatic rocks. The youngest age population at 0.5–0.4Ga is interpreted to represent Gondwana rifting and the opening of Paleotethys, which probably started in Late Cambrian–Ordovician time. A combination of U–Pb dating and Hf-isotope data from Iran, Turkey and North Gondwana confirms that Iran and Turkey were parts of Gondwana at least until late Paleozoic time.

The Neotethyan Sanandaj‐Sirjan zone of Iran as an archetype for passive margin‐arc transitions

AbstractThe Sanandaj‐Sirjan zone of Iran is a northwest trending orogenic belt immediately north of the Zagros suture, which represents the former position of the Neotethys Ocean. The zone contains the most extensive, best preserved record of key events in the formation and evolution of the Neotethys, from its birth in Late Paleozoic time through its demise during the mid‐Tertiary collision of Arabia with Eurasia. The record includes rifting of continental fragments off of the northern margin of Gondwanaland, formation of facing passive continental margins, initiation of subduction along the northern margin, and progressive development of a continental magmatic arc. The latter two of these events are critical phases of the Wilson Cycle that, elsewhere in the world, are poorly preserved in the geologic record because of superimposed events. Our new synthesis reaffirms the similarity between this zone and various terranes to the north in Central Iran. Late Paleozoic rifting, preserved as A‐type granites and accelerated subsidence, was followed by a phase of pronounced subsidence and shallow marine sedimentation in Permian through Triassic time, marking the formation and evolution of passive margins on both sides of the suture. Subduction and arc magmatism began in latest Triassic/Early Jurassic time, culminating at ~170 Ma. The extinction of arc magmatism in this zone, and its shift northeastward to form the subparallel Urumieh‐Dokhtar arc, occurred diachronously along strike, in Late Cretaceous or Paleogene time. Post‐Cretaceous uplift transformed the zone from a primarily marine borderland into a marine archipelago that persisted until mid‐Tertiary time.

Fore arc tectonothermal evolution of the El Oro metamorphic province (Ecuador) during the Mesozoic

The El Oro metamorphic province of SW Ecuador is a composite massif made of juxtaposed terranes of both continental and oceanic affinity that has been located in a fore-arc position since Late Paleozoic times. Various geochemical, geochronological, and metamorphic studies have been undertaken on the El Oro metamorphic province, providing an understanding of the origin and age of the distinct units. However, the internal structures and geodynamic evolution of this area remain poorly understood. Our structural analysis and thermal modeling in the El Oro metamorphic province show that this fore-arc zone underwent four main geological events. (1) During Triassic times (230–225 Ma), the emplacement of the Piedras gabbroic unit at crustal-root level (~9 kbar) triggered partial melting of the metasedimentary sequence under an E-W extensional regime at pressure-temperature conditions ranging from 4.5 to 8.5 kbar and from 650 to 900°C for the migmatitic unit. (2) At 226 Ma, the tectonic underplating of the Arenillas-Panupali oceanic unit (9 kbar and 300°C) thermally sealed the fore-arc region. (3) Around the Jurassic-Cretaceous boundary, the shift from trench-normal to trench-parallel subduction triggered the exhumation and underplating of the high-pressure, oceanic Raspas Ophiolitic Complex (18 kbar and 600°C) beneath the El Oro Group (130–120 Ma). This was followed by the opening of a NE-SW pull-apart basin, which tilted the massif along an E-W subhorizontal axis (110 Ma). (4) In Late Cretaceous times, an N-S compressional event generated heterogeneous deformation due to the presence of the Cretaceous Celica volcanic arc, which acted as a buttress and predominantly affected the central and eastern part of the massif.

Patagonia: where does it come from?

Based on the recent finding of archeocyathids in molassic middle Cambrian to Early Ordovician age-sequences of northern Patagonia the relationships between this southern part of South America and East Antarctica need to be re-examined. The early Cambrian age of the archeocyathids, and their derivation from the Shackleton Limestones, open several alternatives that are evaluated based on the lithology and the U-Pb zircon ages of the different metamorphic sequences of Patagonia and the Transantarctic Mountains. Based on these data, it is proposed that the Somuncurá Massif of northern Patagonia is the conjugate margin of the Pensacola Mountains in East Antarctica. The main episodes of deformation within the Cambrian-Ordovician Ross Orogeny are correlated, as well as the passive margin setting during the Silurian-Devonian, which indicate that the lower section of the Beacon Supergroup of Antarctica corresponds to the Sierra Grande Formation in Patagonia. These facts show that the Patagonian terrane may have been situated as the conjugate margin of the Transantarctic Mountains from Southern Victoria Land to the Pensacola Mountains. The rifting of Patagonia from Antarctica and the beginning of subduc­tion along western Patagonia, are correlated among different terranes, showing a robust coherent evolution through early Paleozoic times among these blocks. The final amalgamation of Patagonia with Western Gondwana occurred in late Paleozoic times, but is not analyzed in the present contribution.

Geocronología U-Pb e isótopos de Hf-O en circones del batolito de la Costa Pensilvaniana, Chile.

The Coastal Batholith of south-central Chile between latitudes 33° and 40°S is composed of calc-alkaline granitoids emplaced in a relatively restricted time period. New SHRIMP U-Pb zircon ages on eight quartzdioritic to granitic rocks collected over a distance of 800 km yielded ages between 300 and 320 Ma, Pennsylvanian (late Carboniferous). Lu-Hf isotopic analyses on the same zircon grains have initial eHf(i) values from +1.67 to -5.64. The δ18O ratios for that grains range from 6.4 to 8.6‰. These new isotopic data point to a relative homogeneous source with prominent components of the continental crust. The calculated Mesoproterozoic Depleted Mantel model ages, in addition to the short span of intrusive ages give insights to the position of the proto-Gondwana margin and the changing subduction mechanism at the end of late Paleozoic time.

Large-scale flat-lying isoclinal folding in extending lithosphere: Santa María de la Alameda dome (Central Iberian Massif, Spain)

The exhumation mechanisms of deep-seated continental crust can be constrained by analyzing the structural and metamorphic imprints left in lithological ensembles. The Santa Maria de la Alameda dome formed during the collision of Gondwana and Laurussia in late Paleozoic time and is located in the Central Iberian Zone of the Iberian Massif (Spain). Rocks of the dome are part of the autochthonous Gondwanan sections of the Variscan belt, and they occur in the Variscan hinterland. The lithostratigraphy of the dome consists of metasedimentary rocks alternating with orthogneiss massifs showing irregular and sinuous structure. The metamorphic record indicates peak pressures indicative of lower-crust depths followed by isothermal decompression to middle-upper-crust levels. Exhumation resulted in the exposure of different crustal levels (represented by subsolidus vs. supersolidus mineral assemblages). The exhumation was accompanied by initial layer-parallel stretching and subsequent large-scale isoclinal folding developed in a heterogeneous, flat-lying shear zone with top-to-the-SE kinematics. SE-directed shearing and lateral extensional flow occurred in response to thermomechanical disequilibrium of previously thickened orogenic crust, probably assisted by coeval accretion of tectonic slices and lithospheric bending about a vertical axis. Positive feedback among partial melting, exhumation, and crustal attenuation resulted in the formation of a NE-SW–trending, migmatite-cored dome, and in refolding of early isoclinal folds and an associated axial surface regional foliation. The dome formed beneath a set of extensional detachments and was reshaped by WNW-ESE upright folds during later convergent deformation. The latter event brought in further instabilities throughout the belt, triggering in this region the development of a late extensional detachment under low-grade metamorphic conditions (top-to-the-S kinematics). The development of a regional train of flat-lying isoclinal folds is presented here as the macrostructural expression of the combination of vertical and lateral extensional flow, both of which are particularly common in orogens worldwide.

Evolution history of the crust underlying Cerro Pampa, Argentine Patagonia: Constraint from LA-ICPMS U-Pb ages for exotic zircons in the Mid-Miocene adakite

This paper newly reports results of LA-ICPMS U‐Pb dating for 282 zircon crystals separated from a Middle Miocene adakite in Cerro Pampa, southern Argentine Patagonia. With the exception of one spot age, 174 of the U‐Pb concordia ages are markedly older (>94 Ma) than the cooling ages of the adakite magma (ca. 12 Ma). The presence of numerous exotic zircon crystals indicates that the adakitic magma carries up information related to the crustal components during its ascent through the entire crust underneath Cerro Pampa. The obtained concordia ages of exotic zircons, 94‐1335 Ma, are divisible into five groups having distinctive peaks on a population diagram. The first (94‐125 Ma) and second age groups (125‐145 Ma) correspond to the age of plutonic activities that formed the main body of the South Patagonian Batholith. The third to fifth groups respectively correspond to activities of the El Qumado-Ibanez volcanic complex (145‐170 Ma), plutonic rocks scarcely exposed in Central Patagonia (170‐200 Ma), and the Eastern Andean metamorphic complex of Late Paleozoic to Early Mesozoic ages (200‐380 Ma). Our data suggest that the crust underneath Cerro Pampa was formed mostly after 380 Ma, the majority forming during the Early Cretaceous to Middle Jurassic. The processes of crustal development ceased for ca. 80 m.y. until the activity of the Cerro Pampa adakite in ca. 12 Ma. In contrast to the existence of numerous Archaean‐Palaeoproterozoic exotic zircons in Mesozoic plutonic rocks distributed in Andean Cordillera at around 46°S, no evidence was found for Archaean‐Paleoproterozoic crust on the Cerro Pampa region at 48°S. This evidence suggests that two crusts must have aggregated along a boundary between 46°S and 48°S with the continental margin of Gondwana during Late Paleozoic times, as part of the amalgamation of Pangea.

Reply to: “The mountains of North-East Greenland are not remnants of the Caledonian topography. A comment on Pedersen et al. (2012)”

In this reply, we comment on the issues raised by Japsen, Chalmers, and Green [Japsen et al. (2013). The mountains of North-East Greenland are not remnants of the Caledonian topography. A comment on Pedersen et al. (2012): Tectonophysics]. We find that new vitrinite reflectance measurements from one location in the region may suggest a more complicated post-Caledonian evolution than proposed by Pedersen et al. (2012) for the southernmost part of the studied area. Overall, however, we cannot reconcile our AFT data set with a complete obliteration of the topography in late Paleozoic times, followed by episodic post-Caledonian exhumation and re-burial, as suggested by Japsen et al. (2013).

중부 옥천대의 지구조 발달과정

The tectonic evolution of the Central Ogcheon Belt has been newly analyzed in this paper from the detailed geological maps by lithofacies classification, the development processes of geological structures, microstructures, and the time-relationship between deformation and metamorphism in the Ogcheon, Cheongsan, Mungyeong Buunnyeong, Busan areas, Korea and the fossil and radiometric age data of the Ogcheon Supergroup(OSG). The 1st tectonic phase() is marked by the rifting of the original Gyeonggi Massif into North Gyeonggi Massif(present Gyeonggi Massif) and South Gyeonggi Massif (Bakdallyeong and Busan gneiss complexes). The Joseon Supergroup(JSG) and the lower unit(quartzose psammitic, pelitic, calcareous and basic rocks) of OSG were deposited in the Ogcheon rift basin during Early Paleozoic time, and the Pyeongan Supergroup(PSG) and its upper unit(conglomerate and pelitic rocks and acidic rocks) appeared in Late Paleozoic time. The 2nd tectonic phase(Ogcheon-Cheongsan phase/Songnim orogeny: D1), which occurred during Late Permian-Middle Triassic age, is characterized by the closing of Ogcheon rift basin(

Petrogenesis of the Kekesai composite intrusion, western Tianshan, NW China: Implications for tectonic evolution during late Paleozoic time

The late Carboniferous to Permian is a critical period for final amalgamation of the Central Asian Orogenic Belt (CAOB), which is characterized by voluminous igneous rocks, particularly granitoids. The Kekesai composite granitoid porphyry intrusion, situated in the Chinese western Tianshan (southwest part of CAOB) includes two intrusive phases, a monzogranite phase, intruded by a granodiorite phase. LA–ICPMS U–Pb zircon analyses suggest that the monzogranitic rocks were formed at 305.5±1.1Ma, with a wide age range of inherited zircons (358–488Ma and 1208–1391Ma), whereas the granodioritic rocks formed at 288.7±1.5Ma. The monzogranitic and granodioritic phases have similar geochemical features and Sr–Nd–Hf isotopic compositions. They exhibit high and variable SiO2 (66–71wt.%) and MgO (0.41–2.14wt.%) contents with some arc-like geochemical characteristics (e.g., enrichment of large ion lithophile elements and negative anomalies of Nb, Ta and Ti) and relatively high initial 87Sr/86Sr ratios (ISr=0.7055–0.7059), low positive εNd(t) (+0.84 to +1.03) as well as a large variation in Hf isotopic compositions with εHf(t) between +3.43 and +14.8, implying both of them were derived from similar source materials. These geochemical characteristics suggest that they might be mainly derived from the partial melting of arc-derived Mesoproterozoic mafic lower crust with involvement of a mantle-derived component in variable proportions by mantle-derived magma underplating. The presence of late-Ordovician to earliest early Carboniferous inherited zircons and the Hf isotopic compositions in the monzogranitic sample, similar to that of the widespread juvenile arc rocks, indicates some crust contamination during magma emplacement. Our new data, combined with previous studies, imply that extensive post-collisional magmatism due to underplating of mantle-derived magma, could plausibly be explained by slab break-off regime.