Early Cretaceous Time Research Articles

Chapter 3.1b Antarctic Peninsula and South Shetland Islands: petrology

Abstract The Antarctic Peninsula contains a record of continental-margin volcanism extending from Jurassic to Recent times. Subduction of the Pacific oceanic lithosphere beneath the continental margin developed after Late Jurassic volcanism in Alexander Island that was related to extension of the continental margin. Mesozoic ocean-floor basalts emplaced within the Alexander Island accretionary complex have compositions derived from Pacific mantle. The Antarctic Peninsula volcanic arc was active from about Early Cretaceous times until the Early Miocene. It was affected by hydrothermal alteration, and by regional and contact metamorphism generally of zeolite to prehnite–pumpellyite facies. Distinct geochemical groups recognized within the volcanic rocks suggest varied magma generation processes related to changes in subduction dynamics. The four groups are: calc-alkaline, high-Mg andesitic, adakitic and high-Zr, the last two being described in this arc for the first time. The dominant calc-alkaline group ranges from primitive mafic magmas to rhyolite, and from low- to high-K in composition, and was generated from a mantle wedge with variable depletion. The high-Mg and adakitic rocks indicate periods of melting of the subducting slab and variable equilibration of the melts with mantle. The high-Zr group is interpreted as peralkaline and may have been related to extension of the arc.

ЛАТЕРАЛЬНАЯ ЗОНАЛЬНОСТЬ АНЮЙСКОГО СЕГМЕНТА ЧУКОТСКОГО ПЛУТОНИЧЕСКОГО ПОЯСА (ЗАПАДНАЯ ЧУКОТКА)

Early Cretaceous granitoids of the Anyui segment of the Chukchi plutonic belt are represented by associations: gabbro-granite, monzodiorite-granite and granite-leucogranite. For the rocks of gabbro-granite and monzonite-granite associations with respect to the primitive mantle, enrichment of Cs, Rb, K was established, with depletion of Nb, Hf, Ti. Granitoids of the granite-leucogranite association are characterized by the most contrasting ratios of enriched and depleted elements, with deep lows of Ba, Nb, Sr, P, Ti against the background of high Cs, Rb, K, Th, U, as well as the absence of a minimum of Hf. Along with the presence of lateral zonality in the placement of granitoids in relation to the southwestern border of the Anyuy Terrain, these data suggest the subductive nature of the formation of gold-bearing gabbro-granite and monzonite-granite associations in the conditions of an active continental margin that arose in the southwestern part of the Chukchi Terrain in Early Cretaceous time due to subduction.

The closure of the Vardar Ocean (the western domain of the northern Neotethys) from the early Middle Jurassic to the Paleocene time, based on the surface geology of eastern Pelagonia and the Vardar zone, biostratigraphy, and seismic-tomographic images of the mantle below the Central Hellenides.

Seismic tomographic images of the mantle below the Hellenides indicate that the Vardar Ocean probably had a composite width of over 3000 km. From surface geology we know that this ocean was initially located between two passive margins: Pelagonian Adria in the west and Serbo-Macedonian-Eurasia in the east. Pelagonia was covered by a carbonate platform that accumulated, during Late Triassic to Early Cretaceous time, where highly diversified carbonate sedimentary environments evolved and reacted to the adjacent, converging Vardar Ocean plate. We conceive that on the east side of the Vardar Ocean, a Cretaceous carbonate platform evolved from the Aptian to the Maastrichtian time in the forearc basin of the Vardar supra-subduction volcanic arc complex. The closure of the Vardar Ocean occurred in one episode of ophiolite obduction and in two episodes of intra-oceanic subduction. 1. During the Middle Jurassic time a 1200-km slab of west Vardar lithosphere subducted beneath the supra-subduction, 'Eohellenic', arc, while a 200-km-wide slab obducted onto Pelagonia between the Callovian and Valanginian times. 2. During the Late Jurassic through to the Cretaceous time a 1700-km-wide slab subducted beneath the evolving east Vardar-zone arc-complex. Pelagonia, the trailing edge of the subducting east-Vardar Ocean slab, crashed and underthrust the Vardar arc complex during the Paleocene time and ultimately crashed with Serbo-Macedonia. Since the late Early Jurassic time, the Hellenides have moved about 3000 km toward the northeast while the Atlantic Ocean spread.

ROLE OF THE CRUSTAL DETACHEMENTS DURING RIFTING, INSIGHTS FROM LABRADOR SEA

The Labrador Sea is part of the Atlantic rift system which opens during Early Cretaceous times in the intracontinental domain. The rifting stage continues and generates an expansion center, with the establishments of an oceanic ridge during Late Cretaceous-Paleocene times. The rifting was asymmetric with a gradual propagation northward. The Cretaceous stretching and thinning of the continental crust led to the development of the transitional domain, with a controversial origin either thinned continental or oceanic. With this study, we bring new evidences for the presence of exhumed upper mantle in the transition domain based on geometry of the detachment faults, typical “S” shape, and we emphasize the role of thedetachments in the rifting of continental crust and its ultimate break-up. We observed the lateral change, from South to North, of the continental to oceanic crust transition, from a step-up to a step-down. Based on, we suggest that it might reflect the changes in the upper versus lower plate position of the Labrador Sea, a change accommodated across the Snorri Fracture Zone.

Dinosaur Tracks from the Caucasian Mineral Waters Country in the Context of the Study of Lower Cretaceous Paleosols of This Region

This article examines the geological and stratigraphic context of several types dinosaur tracks discovered in Lower Cretaceous deposits exposed in the vicinity of city of Kislovodsk (Caucasian Spa Country; Stavropol Krai). The tracks occur on the bedding surfaces of Lower Cretaceous (Valanginian) limestones, dolostones and marls. Two track morphotypes are described. Some ideas are proposed on the paleoclimatic and environmental conditions in the region during Early Cretaceous times. We suggest that the tracks were produced in a coastal plain in the littoral zone of a marine basin (possibly, a brackish lagoon) during a warm, seasonally dry climate.

The Barreirinhas Basin internal architecture and the evidence of transform movements along the Romanche Fracture Zone, Brazilian Equatorial Margin

The Barreirinhas Basin, located in the central part of the Brazilian Equatorial Margin, was originated by Pangea breakup during Early Cretaceous time. The Romanche Fracture Zone played an essential role during the entire tectono-stratigraphic evolution of the Barreirinhas Basin. This fracture zone divided the basin in two sub-basins to the south and north with distinct structural style printed on the basin internal geometry. This study is aimed to investigate how the tectonic and sedimentary evolution in the onshore, shallow and deep-water Barreirinhas Basin, were influenced by the Romanche Fracture Zone. Eight 2D seismic sections, subjected to seismic attributes to enhance the seismic stratigraphic horizons and structural features, and supported by five well profiles, allowed analyzing the basin infill, major faults, and the spatial relationship with the fracture zone. The basin evolution and the tectono-stratigraphic architecture are marked by structures from rift to drift stages. The rift sequences are composed of four seismic units (U1-U4) delimited on top by the horizon R4. The sag and drift sequences are composed of six seismic units (U5-U10) delimited between horizons R4-R10. The structural architectures are dominated by transtensional regimes, such as negative flower faults, tilted blocks, and wipeout zones, whereas transpressional tectonics is also present by reverse faults and positive flowers, which are fingerprints of Romanche Fracture Zone. Especially in the southern sub-basin, the transform tectonic activity deformed the entire basin from Aptian to Holocene. These structures correspond to those in Africa conjugate basins, where recurrent dextral strike-slip plate motions caused transtensional and transpressional deformations in the sedimentary pile since separation of South America and Africa. Asymmetric basin architecture, intensity and asynchrony of tectonic regimes distinguish the Atlantic Equatorial basins.

Carbonate platform evolution of the Tirgan formation during Early Cretaceous (Urgonian) in the eastern Kopet-Dagh Basin, northeast Iran: depositional environment and sequence stratigraphic significance

Tirgan formation of the Kopet-Dagh Basin (northeast Iran) represents one of the Urgonian carbonate platforms that were deposited during the Early Cretaceous time in the northern Alpine Tethys and deformed during the Alpine orogeny. In this study, six stratigraphic sections of the shallow-water platform sediments (Tirgan formation) were measured based on microfacies and fauna abundance. Detail study of petrography, fossil content, and sedimentary structures led to the identification of fifteen microfacies belonging to four facies belts including open marine, shoal, protected lagoon, and tidal flat. The sediments of the Tirgan formation exhibit calcareous green algae, abundant ooids, oysters, bryozoans, and crinoids in inner and middle platform ramp facies and planktonic bivalves and sponge spicules in outer-platform facies. Furthermore, the absence of basinal deposits and lack of evaporate evidence point to deposition under warm-temperate and humid climate conditions. Sequence stratigraphy analysis of Tirgan formation led to distinguish a single depositional sequence in all of the sections which are composed of transgressive and highstand systems tracts with sequence boundaries of type II (SB2). The lowermost lower Aptian Tirgan sequence in the study area relatively shows a similar trend in comparison with the global curve. This basin was deepened over time so that shaly and marly sediments of Sarcheshmeh formation were placed over Tirgan conformably and may suggest a drowning event that was likely related to unusual global warming. Last, this study contributes to the better understanding of the high distribution of facies assemblages in the Urgonian carbonate platforms.

Subduction of a rifted passive continental margin: the Pohorje case of Eastern Alps\u2013constraints from geochronology and geochemistry

This study presents geochronological and geochemical data from newly dated Permian granitic orthogneisses associated with the Eclogite-Gneiss unit (EGU) from the southernmost part of the Austroalpine nappe stack, exposed within the Pohorje Mountains (Slovenia). LA-ICP-MS zircon U–Pb ages of two samples of the augen-gneisses are 255 ± 2.2 Ma and 260 ± 0.81 Ma, which are interpreted as the age of magmatic crystallization of zircon. In contrast, all round zircons from leucogneisses give Cretaceous ages (89.3 ± 0.7 Ma and 90.8 ± 1.2 Ma), considered as the age of UHP/HP metamorphism. The round zircons overgrew older euhedral zircons of Permian and rare older ages tentatively indicating that these rocks are of latest Permian age, too. Zircon εHf(t) values of the four orthogneiss samples are between − 13.7 and − 1.7 with an initial 176Hf/177Hf ratio ranging from 0.282201 to 0.282562; TDMC is Proterozoic. The augen-gneisses show geochemical features, e.g. high (La/Lu)N ratios and strong negative Eu anomalies, of an evolved granitic magma derived from continental crust. The leucogneisses are more heterogeneously composed and are granitic to granodioritic in composition and associated with eclogites and ultramafic cumulates of oceanic affinity. We argue that the Permian granitic orthogneisses might be derived from partial melting of lower crust in a rift zone. We consider, therefore, that segment of the EGU is part of the distal Late Permian rift zone, which finally led to the opening of the Meliata Ocean during Middle Triassic times. If true, the new data also imply that the Permian stretched continental crust was potentially not much wider than ca. 100 km, was subducted and then rapidly exhumed during early Late Cretaceous times.

Late Jurassic and Early Cretaceous sedimentation in the Mandawa Basin, coastal Tanzania

This paper concerns the sedimentary successions deposited in the Mandawa Basin after the separation of East and West Gondwana during the subsequent southwards drift of Madagascar in the Late Jurassic to Early Cretaceous times. The aim of this study was to provide more specific details on the Late Jurassic and Early Cretaceous sedimentation and to report mineralogical and petrographical characteristics on the less well documented successions, namely the Kipatimu, Mitole and Nalwehe formations, in the central and northern parts of the Mandawa Basin.The Late Jurassic and Early Cretaceous depositional environments are reviewed, based on the sedimentological, mineralogical and petrographical results presented here, supplemented by previously published work. The Late Jurassic to Early Cretaceous depositional setting mainly reflects a shallow, tidally influenced, mixed carbonate-silisiclastic coastal ramp. The succession displays cyclical sedimentation best described as a series of transgressive-regressive sequences with limestones overlain by siliciclastics. During transgressions microbial activity was high and sedimentation rates low, resulting in micro-oncoid deposition in the late Kimmeridgian – early Tithonian (Mitole Limestone Member), and oncoids and stromatolites in the Early Cretaceous (Nalwehe Limestone Member). During the regressive phases siliciclastic marginal marine sediments were deposited over the limestones. The regressive sandstones are mainly unfossiliferous and display evidence of being deposited in a tide-dominated, marginal marine environment.

Tectonic evolution of the Zagros foreland basin since Early Cretaceous, SW Iran: Regional tectonic implications from subsidence analysis

• Basement palaeo-highs exert major impact on the basin's subsidence history. • Architecture of the Zagros foreland dramatically changed during the Cretaceous. • NE-SW deep-seated basement faults were intensively re-activated during the Turonian . • N-S deep seated basement faults were re-activated during the last 20 Ma. Subsidence analysis of 42 exploration wells in the Zagros foreland basin indicates the tectonic evolution of the basin since Early Cretaceous. The impact of inherited basement structures on the basin evolution is documented in this study. Subsidence maps show that the architecture of the basin dramatically changed during the Cretaceous. In the Central Zagros, the Hercynian palaeo-high (i.e. Wadi Al Batin palaeo-high), which affected sedimentation until the Early Cretaceous time gradually disappeared and the South Pars palaeo-high within the central Persian Gulf started to show more impact on sedimentation. The palaeo-highs in the Zagros basement have different geometrical characteristics and origin. The Hercynian and South Pars are wide-wavelength palaeo-highs with a regional-scale impact on subsidence. In contrast, the Hendijan, Kharg-Mish and Kazerun palaeo-highs with different orientations and quite smaller wavelengths show different impact on the subsidence pattern. These kinds of palaeo-highs could be related to re-activation of pre-existing basement faults. Whereas South Pars and Hercynian paleo highs are possibly related to thermal expansion or contraction of the crust. The Hendijan and Kharg-Mish palaeo-highs with the NE-SW orientation were intensively re-activated during Turonian time. On the contrary, the Kazerun Fault with the N-S orientation became more active and considerably influenced sedimentation during the last 20 Ma. Selective re-activation of deep-seated faults might have taken place as a result of changes in the orientation of the far-field stress in the region. Understanding the complexities of inherited structures and their impact on reservoir characteristics can support future exploration activities in the Zagros foreland basin.

Magmatic and tectonic fabrics in the Upper Jurassic La Hoya Pluton, North Patagonian Batholith (~43°S) as a record of the early stages of the Andean deformation

The magmatic record of the North Patagonian Batholith starts in Middle Jurassic times and rapidly grew in Early Cretaceous times, where it experienced its main building phase. Slightly after, in the Late Cretaceous, the North Patagonian fold and thrust belt experienced its first contractional deformation pulse. The La Hoya Pluton is an Upper Jurassic shallow intrusive body emplaced in the North Patagonian Andes in the proximities of the city of Esquel in Argentina. Its excellent exposures, which bear magmatic and high- and low-temperature solid-state deformation structures, are studied in this work through field, microstructural, and AMS studies. The initial shape of the pluton could be envisaged as having a geometry compatible with coalescent laccoliths, given by the presence of subhorizontal magmatic to solid-state foliations with associated E-W or N–S trending magmatic to solid-state lineations. These magmatic structures, which bear a mild solid-state overprint, were probably formed during progressive pluton cooling. Moreover, the nature of these structures suggests that the pluton cooled fast at shallow crustal levels, indicating that they are probably related to its emplacement processes that started, at least, in Late Jurassic times. Contrastingly, NW-SE and NE-SW trending high-angle subvertical fractures and low-temperature mylonitic zones with steeply plunging lineations, which are compatible with an E-W shortening direction, cut the previously formed subhorizontal magmatic structures. These compressive structures found in the La Hoya Pluton were probably developed during the Late Cretaceous main stage of contractional deformation. The contractional deformation is constrained to have happened before the intrusion of undeformed basaltic dikes crosscutting the mylonitized areas of the La Hoya Pluton. These dikes have an 40Ar-39Ar age in plagioclase of 42.15 ± 0.40 Ma and geochemical features typical of volcanic arcs above a subduction zone. Their geochemical signature is sensitive to the process of arc resumption after the Paleogene waning of arc activity, as it is transitional between the alkaline-like signature of Pilcaniyeu Belt (~57.8–42; Paleocene-Eocene) and the predominantly slab-derived magmas of El Maitén Belt (~37–19; Late Eocene-Early Miocene). These dikes probably formed part of the early stages of the El Maitén Belt.

Early Jurassic intraplate extension in west-central Argentina constrained by U-Pb SHRIMP dating: Implications for the opening of the Neuquén basin

The early Mesozoic extensional processes recognized in SW Gondwana have been related to a supercontinent precursor break-up phase, the gravitational collapse of a late Paleozoic orogen, changes in subduction dynamics or the impact of a thermal anomaly. However, intraplate extension is still lacking accurate temporal constraints for the area devoid of the influence of subduction-related extensional processes that affected its margin between mid-Permian and Early Cretaceous times. Evidence from this intraplate extensional phase is found in the Cara Cura – Reyes depocenter of the Neuquén basin located in west-central Argentina. Field and seismic evidence allowed identifying synextensional growth geometries, and zircon U-Pb SHRIMP dating of volcanic rocks revealed ages of 198.8 ± 2.0 Ma and 195.4 ± 1.0 Ma for the oldest and youngest exposed synrift deposits. These allowed limiting the mechanical subsidence phase to the Sinemurian, which was followed by a Pliensbachian – Toarcian hiatus separating it from the postrift deposits. These deposits are characterized by a series of marine transgressive-regressive cycles separated by sedimentation gaps, constrained in this sector to the Bathonian – Callovian and the Kimmeridgian. When inserted into the broader realm of the Neuquén basin, a diachronic development for both the synrift and postrift deposits is evident, although the synrift-postrift transition occurred at ca. 190 Ma. The correlation of these events along SW Gondwana supports a common tectonic evolution which, at the same time, allows correlating the hiatus detected in postrift sedimentation of the Cara Cura – Reyes depocenter with Jurassic intraplate contractional events. The shift in the intraplate tectonic regime coincides with a radical change in Gondwana's absolute motion, being northwards during extension, and southwards during contraction, after the opening of the Central Atlantic Ocean. Therefore, upper-plate dynamics seems to have played a first-order role during the opening of the Neuquén basin.

Petrogenesis of Dehsard felsic rocks in the southwest of Kerman, Iran: Inference for the evolution of Sanandaj-Sirjan zone

The Upper Triassic Dehsard felsic rocks (DFR), leucocratic to hololeucocratic granitic rocks, are exposed in the southernmost of the Phanerozoic Sanandaj-Sirjan zone in the Zagros orogenic belt of south-Central Iran. They intruded into the Paleozoic Khabr and Rutchun metamorphic complexes that consist of marble, greenschist, calcschist and micaschist rocks. The DFR are cut through by monzonitic and dioritic dikes. Comprehensive petrological and geochemical studies disclosed DFR as a plagiogranite. They are calc-alkaline, metaluminous to slightly peraluminous I-type granitoids and exhibit enrichment in large ion lithophile elements (LILEs; e.g. Na, K, Rb, Ba) in contrast to the depletion of high field strength elements (HFSEs; e.g. Nb, Ta, P, Ti). Their chondrite normalized REE patterns show LREE enrichment [(La/Yb)N = 4.41–23.77] and unfractionated HREE [(Gd/Yb)N = 0.60 to 2.23] that suggest the lack of garnet during partial melting processes. The positive Eu anomalies advocate plagioclase accumulation. The low Nb, Ta, P2O5, TiO2 content, isotopic data, LREEs enrichment, various tectono-magmatic discrimination diagrams may point out the formation of DFR in a mature island arc environment through partial melting of metabasic source with contribution of some greywacke sediments and subduction-related volatiles. The partial melting occurred above garnet stability field. The involvement of variable amounts of oceanic sedimentary rocks especially greywackes in the melting processes in a mature island arc setting, as characterised by εNdt results ranging from +0.93 to −5.19 at 220 Ma, could also cause the DFR inclination toward active continental margin environment. It seems that the development of an island arc in an intra-oceanic setting must be happened prior to the Late Jurassic–Early Cretaceous time, while northward subduction of Neotethys oceanic crust was in progress under southern margin of the Central Iranian Microcontinents. Subsequent collision of the island arc with SSZ led to the tectonic proximity of the DFR into SSZ constituents. This approach gives us new intuition for the renovation of the geodynamic history of Sanandaj–Sirjan zone.

Geochemical and Stratigraphic Analysis of the Chisana Formation, Wrangellia Terrane, Eastern Alaska: Insights Into Early Cretaceous Magmatism and Tectonics Along the Northern Cordilleran Margin

AbstractThe Chisana Formation consists of Lower Cretaceous volcanic rocks that occur in the Nutzotin Mountains of eastern Alaska. New stratigraphic analysis indicates that the volcanic succession is >2 km thick at the Bonanza Creek type section. We present stratigraphic, geochemical, Sr‐Nd‐Pb isotope, and U‐Pb age data from samples collected from various stratigraphic levels of the Chisana Formation. We demonstrate that the Chisana Formation can be divided into a lower subaqueous unit, a middle transitional unit, and an upper subaerial unit. Chisana Formation lavas range from transitional to subalkaline basalts through andesites. Trace element geochemistry shows high field strength element depletions relative to large ion lithophile elements and hydrous mineral assemblages with calc‐alkaline to tholeiitic chemistries, all consistent with a magmatic arc origin. Chisana lavas yield geochemical compositions and isotope characteristics that overlap with magmas from volcanic suites formed within juvenile continental crust and immature island arcs. Volcanism occurred between ~131 and 117 Ma judging from previously reported lava ages and new U‐Pb ages of detrital zircons recovered from sandstones that conformably underlie the lowermost Chisana Formation lavas. Our results support existing tectonic models in which an east dipping subduction zone existed beneath Wrangellia during Early Cretaceous time. The upsection shift from marine to terrestrial depositional conditions in the Chisana Formation and the overlying ~117–93 Ma Beaver Lake Formation was coincident with regional shortening. Together, the geologic evidence for shortening and terrestrial deposition are interpreted to reflect accretion/suturing of Wrangellia against inboard terranes.

Crustal deformation in the Rio de la Plata estuarine (South Atlantic passive margin) and the associated seismicity as a consequence of an isostatic readjustment

The Río de la Plata estuarine has been the epicenter of the 30 November 2018 intra-crustal earthquake with a magnitude and focal depth of 3.7 and 19 km (±6 km), respectively, which alerted the Buenos Aires city population. This area has also been the locus of a series of earthquakes with magnitudes ranging from 3.7 to 5, and particularly the 1888 earthquake, probably with an epicenter near the axis of the estuarine. These earthquakes occur in the Salado basin area, an aulacogen basin associated with the early stages of the South Atlantic spreading in late Early Cretaceous times, that subsided all through the Cenozoic. Previous gravimetric analyses revealed that this basin is characterized by an important crustal attenuation zone. From a new gravimetric analysis, using satellite gravity data, we find isostatic anomalies associated with this section of the Atlantic passive margin. Therefore, we propose that the origin of these earthquakes is the isostatic readjustment of this aulacogen type basin, a process that could have reactivated shallow crustal structures. From a preliminary evaluation, we estimate that the Salado basin would subsidize other ~2 km to compensate for the measured thick anti-root, constituting a crucial seismogenic source that should be considered due to the demographical and economic importance of the region.

Destruction of the North China Craton and its influence on surface geology and terrestrial biotas

The North China Craton (NCC), a major geological element of East Asia, had hehaved as a stable craton untile late Mesozoic. The NCC experiencd multiple contraction and extension in late Mesozoic, evidened by three phases of shortening and two phases of stretching. The three shortening phases occurred in the late Middle Jurassic, earliest Early Cretaceous, and latest Early Cretaceous, respectively. Crustal stretching happened in Late Jurassic and Early Cretaceous times, and was marked by large-scale magmatism and development of rift basins and metamorphic core complexes. Widespread vigorous magmatism and extensive rifting indicate the loss of the NCC stability in the Early Cretaceous. Alternating crustal contraction and extension of the NCC are attributed to the change in subduction angles of the paleo-Pacific plate, with low-angle subduction resulting in shortening and high-angle subduction leading to stretching. However, magmatism, deformation and lithospheric thinning are merely the various expressions of the NCC destruction. It is shown that deep thermal processes must have strongly altered the properties of overlying lithosphere, resulting in a change from the continental into oceanic mantle. It was the transformation of physico-chemical properties of the lithospheric mantle that destabilized the NCC. Also noticeable are occurrences of the Yanliao and Jehol biotas that evolved in the Middle-Late Jurassic and Early Cretaceous, respectively, and just coincided with the initial and peak stages of the NCC destruction. Flourishing and vanishing of the biotas appeared closely related to volcanic intensity and rift basin development in space and time, implicating a profound control of deep dynamic processes on surface paleogeography and ecosystems. Implicitly, life should have been evolving in accord with surrounding environments. Volcanism might have exerted a strong influence on prosperity and disappearance of the Yanliao and Jehol biotas, with ash fallout leading to perfect preservation of a variety of precious fossils. Also noteworthy is that biotas thrived in the period when rift basins were developing, whereas biotas died out when basins were inverted. It is still a challenge to explore the linkage of terrestrial life evolution and tectonic processes. Recent developments in geodynamic studies reveal that subduction of the western paleo-Pacific plate might have produced a unique mantle profile in late Mesozoic, with the stagnant plates in mantle transition zone being sandwiched between the lower mantle and the big mantle wedge. Extensive dehydration and decarbonization of the stagnant plates resulted in melting and metasomatism of the upper and lower parts of mantle transition zone. These processes should have played a crucial role in altering the nature of lithospheric mantle and in triggering large-scale magmatism in the crust. A big mantle wedge is thought to have developed beneath the East Asia continent during the Early Cretaceous as a result of the slab rollback and the stagnation in the mantle transition zone. The combined tectonic processes must have exerted a profound influence on both the NCC destruction and Yanliao/Jehol biotal evolution. Interdisciplinary researches will help us understand the complicated interaction of deep and surface processes and comprehend the past, present and future of our habitable planet.

Estimation of the ages of Devonian and Cretaceous stage boundaries in the Geologic Time Scale

Estimation of the ages of period and stage boundaries of the Geologic Time Scale (GTS) has a long history that commenced over a century ago with the pioneering work of Arthur Holmes. Frequencies, precision and accuracy of radiogenic isotope age determinations used for time scale construction continue to increase steadily. Later stage boundary age estimates are accompanied by error bars based on 2-sigma age dating errors with incorporation of stratigraphic uncertainty. Most GTS2004 and GTS2012 results involved spline-curve fitting. In GTS2012, Milankovitch-type orbital climate cyclicity was used to tune the Neogene geologic time scale while seafloor spreading was combined with sedimentary cycle scaling to construct the Paleogene time scale, and it also contributed to the construction of the Cretaceous and Jurassic time scales. Geomathematical procedures continue to be refined for the next GTS which is in Gradstein et al. (2020). In this study smoothing splines are used to construct Devonian and Late Jurassic - Early Cretaceous time scales. This methodology and its results are described and some estimates are refined by incorporating Milankovitch cycle durations.

Slab break-off origin of 105 Ma A-type porphyritic granites in the Asa area of Tibet

AbstractThe study of the petrogenesis of some magmatic rocks with special geochemical attributes provides effective information for us to explore the deep geodynamic background of their formation. A series of granitic porphyry dykes have been found in the mélange zone of the Asa region in southern Tibet, whose genesis may be closely related to the evolution of the Meso-Tethyan Ocean. Regional geodynamic evolution is investigated by whole-rock geochemical analysis, zircon U–Pb dating and Lu–Hf isotopic analysis of two porphyritic granites. The Asa porphyritic granites have high SiO2(74.29–78.65 wt %) and alkalis (Na2O + K2O = 6.51–9.35 wt %) contents, and low Al2O3(11.60–14.51 wt %), CaO (0.04–0.19 wt MgO (0.01–0.10 wt %) contents. They are enriched in Zr, Nb, Ce, Y and Hf and depleted in Ti, Ba, Sr and P, consistent with A-type granites. The samples are relatively rich in LREEs, with LREE/HREE ratios of 1.73–3.04. They display negative Eu anomalies (Eu/Eu* = 0.24–0.28) and obvious Ce anomalies in some samples. Zircon U–Pb analyses show that the porphyritic granites formed in late Early Cretaceous time, 107.4 to 105.5 Ma. Zircon εHf(t) values are in the range of 6.9 to 12.0. These data indicate that the porphyritic granites were sourced from interaction between mantle-derived and juvenile lower crust-derived melts, with the addition of oceanic sediment-derived melts. This occurred when the subducting Bangong–Nujiang oceanic crust split to create a slab window. Rising asthenosphere triggered re-melting of lower crust basalts, resulting in the formation of the late Early Cretaceous A-type granites around Asa.

Early Cretaceous subduction initiation beneath southern Tibet caused the northward flight of India

Collision between the Indian and Eurasian plates formed the ~2500 ​km long Yarlung Zangbo Suture Zone and produced the Himalaya mountains and Tibetan plateau. Here we offer a new explanation for tectonic events leading to this collision: that the northward flight of India was caused by an Early Cretaceous episode of subduction initiation on the southern margin of Tibet. Compiled data for ophiolites along the Yarlung Zangbo Suture Zone show restricted ages between 120 ​Ma and 130 ​Ma, and their supra-subduction zone affinities are best explained by seafloor spreading in what became the forearc of a north-dipping subduction zone on the southern margin of Tibet. The subsequent evolution of this new subduction zone is revealed by integrating data for arc-related igneous rocks of the Lhasa terrane and Xigaze forearc basin deposits. Strong slab pull from this new subduction zone triggered the rifting of India from East Gondwana in Early Cretaceous time and pulled it northward to collide with Tibet in Early Paleogene time.

Mesozoic Acid Magmatites of Southeastern Transbaikalia: Petrogeochemistry and Relationship with Metasomatism and Ore Formation

The characteristic features of the manifestations of acid magmatism and metasomatic hydrothermal alteration processes in Southeastern Transbaikalia, an important mining region in Russia, are discussed. The zoned or beltlike distribution of the ores of various metals, uranium in particular, and other mineral deposits is due to the general evolutionary trends of the geodynamic regimes of the respective blocks of the territory and the subsequent magmatic differentiation trends. The highly differentiated and fluorine-rich granitoids of Southeastern Transbaikalia are located only in the zones of the consolidated crust, which are characterized by low gravity and negative quiet magnetic field values. In other zones, i.e., volcanic belts, volcanogenic troughs, and the regions dominated by the intrusive massifs of the Shakhtama complex, rare metal manifestations are absent. It was demonstrated that there is no correlation between the localization of large uranium deposits and leucocratic rare metal granite domains and that these deposits are concentrated in large volcanic calderas or volcanic troughs, controlled by long-lived fault systems. In addition to uranium, these volcanogenic structures control base metal, molybdenum, gold, fluorite, and some other mineral deposits. The results of a comparative analysis of the geochemical features of the acid volcanics that host the uranium mineralization and are classified as the last differentiation products of the contrasting latite volcanism, on the one hand, and the subvolcanic and hypabyssal derivatives of the rare metal granites, on the other, are presented. The analysis revealed the signatures of their fundamental difference from each other and that they belong to different branches of magmatic differentiation. A comparative analysis of the metasomatic aureoles of the altered wallrocks in the Late Mesozoic volcanics and the Shakhtama granitoid domains, on the one hand, and the leucocratic rare metal granite domain, on the other, is given. The data presented attest to the existence of a mixed geodynamic regime within the study area of the Central Asian mobile belt during the Jurassic and the earliest Cretaceous time: residual collisions with strong deformations in a compressional setting, accompanied by the development of late acid differentiation products of the magmatic latite series, and the incipient rifting associated with the intrusion of the leucogranites of the Kukul’bei complex in a more quiet extensional setting.