Evolution Of Lithosphere Research Articles

New identification and significance of Early Cretaceous mafic rocks in the interior South China Block

Early Cretaceous mafic rocks are first reported in the northern Guangxi region from the western Qin-Hang belt in the interior South China Block. A systematic investigation of zircon U–Pb dating, whole-rock geochemistry, Sm–Nd isotopes and zircon Hf–O isotopes for these mafic rocks reveals their petrogenesis and the mantle composition as well as a new window to reconstruct lithospheric evolution in interior South China Block during Late Mesozoic. Zircon U–Pb dating yielded ages of 131 ± 2 Ma to 136 ± 2 Ma for diabase and gabbro from Baotan area, indicating the first data for Early Cretaceous mafic magmatism in the western Qing-Hang belt. These mafic rocks show calc-alkaline compositions, arc-like trace element distribution patterns, low zircon εHf(t) of − 9.45 to − 6.17 and high δ18O values of + 5.72 to + 8.09‰, as well as low whole-rock εNd(t) values of − 14.27 to − 9.53. These data suggest that the studied mafic rocks are derived from an ancient lithospheric mantle source that was metasomatized during Neoproterozoic subduction. Thus, the occurrence of these mafic rocks indicates a reactivation of Neoproterozoic subducted materials during an extension setting at Late Mesozoic in the western Qin-Hang belt, an old suture zone that amalgamates the Yangtze and Cathaysia blocks.

Detrital chromites reveal Slave craton's missing komatiite

Abstract Komatiitic magmatism is a characteristic feature of Archean cratons, diagnostic of the addition of juvenile crust, and a clue to the thermal evolution of early Earth lithosphere. The Slave craton in northwest Canada contains >20 greenstone belts but no identified komatiite. The reason for this dearth of komatiite, when compared to other Archean cratons, remains enigmatic. The Central Slave Cover Group (ca. 2.85 Ga) includes fuchsitic quartzite with relict detrital chromite grains in heavy-mineral laminations. Major and platinum group element systematics indicate that the chromites were derived from Al-undepleted komatiitic dunites. The chromites have low 187Os/188Os ratios relative to chondrite with a narrow range of rhenium depletion ages at 3.19 ± 0.12 Ga. While these ages overlap a documented crust formation event, they identify an unrecognized addition of juvenile crust that is not preserved in the bedrock exposures or the zircon isotopic data. The documentation of komatiitic magmatism via detrital chromites indicates a region of thin lithospheric mantle at ca. 3.2 Ga, either within or at the edge of the protocratonic nucleus. This study demonstrates the applicability of detrital chromites in provenance studies, augmenting the record supplied by detrital zircons.

Petrogenesis of a calc-alkaline lamprophyre (minette) from Thanewasna, Western Bastar Craton, Central India: insights from mineral, bulk rock and in-situ trace element geochemistry

Abstract The lamproites and kimberlites are well known from the Eastern Bastar Craton, Central India. However, a Proterozoic lamprophyre dyke is discussed here, from the Western Bastar Craton (WBC). The field geology, petrographic, mineralogical and whole-rock and in-situ trace element geochemistry of biotite are described to understand the petrogenesis and lithospheric evolution in the WBC. The Thanewasna lamprophyre (TL) is undeformed and unmetamorphosed, intruded into c. 2.5 Ga charnockite and metagabbro but closely associated with c. 1.62 Ga undeformed Mul granite. The TL has a characteristic porphyritic texture, dominated by phenocrysts of biotite, microphenocryst of amphibole, clinopyroxene and a groundmass controlled by feldspar. Mineral chemistry of biotite and amphibole suggest a calc-alkaline (CAL) type, and pyroxene chemistry reveals an orogenic setting. The TL is characterized by high SiO 2 and low TiO 2, MgO, Ni and Cr, consistent with its subcontinental lithospheric origin. The presence of crustal xenolith and ocelli texture followed by observed variations in Th/Yb, Hf/Sm, La/Nb, Ta/La, Nb/Yb, Ba/Nb indicate substantial crustal contamination. Whole-rock and in-situ biotite analysis by laser ablation inductively coupled plasma mass spectrometry show low concentrations of Ni (30–50 ppm) and Cr (70–150 ppm), pointing to the parental magma evolved nature. Enrichment in H 2 O, reflected in magmatic mica dominance, combined with high large ion lithophile element, Th/Yb ratios, and striking negative Nb–Ta anomalies in trace element patterns, is consistent with a source that was metasomatized by hydrous fluids corresponding to those generated by subduction-related processes. Significant Zr–Hf and Ti anomalies in the primitive mantle normalized multi-element plots and the rare earth element pattern of the TL, similar to the global CAL average trend, including Eastern Dharwar Craton lamprophyres. Our findings provide substantial petrological and geochemical constraints on petrogenesis and geodynamics. However, the geodynamic trigger that generated CAL magmatism and its role in Cu–Au metallogeny in the WBC, Central India, is presently indistinct in the absence of isotopic studies. Nevertheless, the lamprophyre dyke is emplaced close to the Cu–(Au) deposit at Thanewasna.

Metasomatized eclogite xenoliths from the central Kaapvaal craton as probes of a seismic mid-lithospheric discontinuity

The central region of the Kaapvaal craton is relatively understudied in terms of its lithospheric mantle architecture, but is commonly believed to be significantly impacted by post-Archean magmatism such as the ca. 2056 Ma Bushveld large igneous event. We investigate a collection of 17 eclogite xenoliths from the Cretaceous Palmietfontein kimberlite at the Western Limb of the Bushveld Complex for their mineralogical compositions (major and trace elements, plus Fe3+ contents), as well as stable oxygen and radiogenic Pb isotopic compositions to gain further insights into the nature and evolution of the central Kaapvaal cratonic mantle lithosphere. New U/Pb age determinations on mantle-derived zircon yield a magma emplacement age of ca. 75 Ma for the Palmietfontein Group-1 kimberlite, which means that the entrained eclogite xenoliths may record a protracted metasomatic history from the Proterozoic through to most of the Phanerozoic eon.Garnet δ18O values of up to 6.9‰ and positive Eu anomalies for the bulk rocks suggest seawater-altered oceanic crustal protoliths for the Palmietfontein eclogite xenolith suite, which is typical for the eclogitic components of the Kaapvaal root and other cratonic mantle sections worldwide. However, several features of the Palmietfontein eclogites are commonly not observed in other mantle-derived eclogite xenolith suites. Firstly, the samples studied yield relatively low equilibration pressures and temperatures between 2.7 and 4.5 GPa and 740–1064 °C, indicating a relatively shallow residence between 90 and 150 km depths. Secondly, euhedral coarse amphibole is present in several eclogite nodules where it is in equilibrium with ‘touching’ garnet, supporting eclogite residence within the amphibole stability field at uppermost lithospheric mantle conditions. Thirdly, primary omphacitic clinopyroxene is often overgrown by diopside, and is significantly enriched in incompatible trace elements. The clinopyroxene is also characterized by elevated 206Pb/204Pb of 17.28–19.20 and 207Pb/204 Pb of 15.51–16.27, and these Pb isotopic compositions overlap with those of Mesozoic Group-2 kimberlites from the Kaapvaal craton.Our results show that eclogites reside at ~85 km depth beneath the central Kaapvaal craton as part of a layer that corresponds to an approximately 50 km thick seismically-detected mid-lithospheric discontinuity. Mid-lithospheric discontinuities have been interpreted as metasomatic fronts formed by focussed crystallization of hydrous mineral phases from enriched volatile-bearing melts, and as such the strongly overprinted amphibole-bearing eclogite xenoliths from Palmietfontein may represent a physical expression of such seismically anomalous metasomatic layer at mid-lithospheric depth. Our Pb isotope data suggest that the focussed metasomatism can be attributed to volatile-rich melts reminiscent of potassic Group-2 kimberlites, which have been invoked in MARID-style metasomatic overprinting of the lower lithospheric mantle beneath the western Kaapvaal craton. However, the relatively low fO2 recorded by the Palmietfontein eclogites (minimum FMQ-4.5) suggests that the metasomatism at mid-lithospheric depth was less protracted compared to the more intensive and oxidizing metasomatism typically observed near the base of cratonic mantle roots. While it is possible that Proterozoic magmatic events were responsible for the focussed mid-lithospheric metasomatism of the Kaapvaal mantle, on the basis of the Pb isotope constraints the Palmietfontein eclogites were most likely overprinted during ca. 120 Ma Group-2 kimberlite magmatism.

Imaging the Deep Crustal Structure of Central Oklahoma Using Stacking and Inversion of Local Earthquake Waveforms

AbstractThe southern Granite‐Rhyolite province (SGRP) contains a comprehensive record of lithospheric evolution in North America. During the last decade, increased seismicity along with improved seismic monitoring in Oklahoma provided a rich catalog of local earthquakes. The source‐receiver geometry of this data set is well posed to illuminate the middle and lower crust through long offset recordings of the Pg phase. We present a 3‐D P‐wave velocity model for central and northern Oklahoma developed through a non‐standard processing scheme applied to local earthquake waveforms recorded from 2010 to 2017, focusing on the deeper crust. We employed common mid‐point sorting, stacking, and inversion of Pg phases which resulted in a set of localized velocity‐depth functions up to depths of 40 km. Using this methodology, we significantly increased the S/N ratio for far offset (∼200 km) local earthquake waveforms which led to the increase in depth of investigation in the final 3‐D velocity model. We find high velocity (>7 km/s) lower crust throughout the investigated area which suggests a mafic lower crust. The high velocities support previously established models which state that the lower crust of the SGRP was derived from melting of older crust. We further relate shallow and middle crustal velocity anomalies to other data sets such as gravimetric and magnetic anomalies, and the spatial distribution of earthquakes. We do not find clear evidence for the existence of the Midcontinent Rift in northern Oklahoma.

An island arc origin of Jurassic plagiogranite in the Shiquanhe ophiolite, western Bangong Suture, Tibet: Zircon U–Pb chronology, geochemistry, and tectonic implications of Bangong Meso‐Tethys

The plagiogranites in ophiolites are minor in volume but can provide crucial information for the origin and tectonic evolution of ancient oceanic lithosphere. This paper presents the geochronology and geochemistry of a newly discovered plagiogranite in the Shiquanhe ophiolite, from the west end of the Shiquanhe‐Jiali ophiolite sub‐belt, Bangong Suture, central Tibet. Zircon U–Pb dating of two samples yields Middle Jurassic ages (167.4 ± 1.2 Ma and 167.5 ± 1.5 Ma). The plagiogranite has positive whole‐rock εNd(t) (4.2–4.9) and zircon εHf(t) (9.6–14.3) values, high Th/Nb ratios (0.6–2.8) but relatively low La/Nb ratios (0.9–9.9), indicating that it was possibly derived from a depleted mantle with the contribution of minor subducted sediments. The LREE‐enrichment but HREE‐flat patterns with negative Eu anomalies and negative Nb‐Ti anomalies resemble those of shear‐type plagiogranites, which mean that this rock was likely formed by partial melting of metabasite. Combined with the plagiogranite which does not exhibit chilled contacts against the Shiquanhe ophiolitic metabasite, suggests that the plagiogranite may have been derived from the associated ophiolitic metabasite. Geochemical calculating and modelling indicate that the plagiogranite was possibly produced by a low degree (<10%) partial melting of metabasite and replenished by minor sediments melts at low temperature (<800 °C) and low pressure (<0.1 GPa) conditions. The Shiquanhe plagiogranite, together with the contemporaneous Lagkorco plagiogranite in the same ophiolite sub‐belt, indicates that an intra‐oceanic island arc system was developed in the Bangong Meso‐Tethys during the Middle Jurassic.

Compositional signatures of dolerite dykes from the Purang ultramafic massif, Tibet: Implications for garnet-bearing components in the Neo-Tethyan mantle

We present geological, whole-rock geochemical, Sr–Nd isotopic and U–Pb geochronological data for the mafic dykes from the Purang ultramafic massif, South Tibet (China). The mafic dykes are composed of dolerite within a mylonitized harzburgite body. These mafic dykes are dated at 138.0–138.5 Ma using zircon and apatite U–Pb methods, and are characterized by enrichments in large-ion lithophile elements (LILEs) and high-field-strength elements (HFSEs), resembling those of typical ocean island basalts (OIB). They have initial 87Sr/86Sr ratios of 0.70612 to 0.70670, initial 143Nd/144Nd ratios of 0.51231 to 0.51233 and high εNd(t) values of −3.1 to −2.6. Geochemical modeling suggests that the parental melts of dolerites were derived by ~50% melting of garnet pyroxenite or eclogite veins in the lithospheric mantle. The formation of the dyke-filling melts can thus be linked to melting of a thin mantle layer containing old low buoyancy plume (LPB)-components in the form of garnet-bearing veins. Our results provide a scenario for the tectono-magmatic evolution of the oceanic lithosphere beneath the Neo-Tethys in the South Tibet. We conclude that the existence of a thermal plume might not be essential to the petrogenesis of the Yarlung-Zangbo Suture Zone (YZSZ) ophiolites.

Lithospheric Structure and Flat‐Slab Subduction in the Northern Appalachians: Evidence From Rayleigh Wave Tomography

AbstractA billion years of tectonic history makes southeastern Canada and the northeastern United States an exciting area to investigate the evolution of continental lithosphere. Our study area comprises terranes with either Laurentian or Gondwanan provenance that accreted to eastern North America at different times. With the aim of resolving the isotropic velocity variations across the northern Phanerozoic Appalachians and the southeastern Proterozoic Grenville Province, we adopted a Rayleigh wave tomography technique that takes multipathing, scattering, and finite frequency effects into account. Our data sets include records of teleseismic earthquakes recorded by 71 broadband seismic stations over a 2‐year period. Our high‐resolution tomography models indicate significant (±3.5%) variations in shear wave velocity across different lithospheric domains, enabling us to discuss tectonic implications. In contrast to the Peri‐Laurentian zones, seismic signatures in the Peri‐Gondwanan domains are more complex and variable. Although systematic variations of seismic velocities across different tectonic zones are observed, we find no simple relation between the lithospheric thicknesses of different tectonic zones and their age. We interpret the lithosphere‐asthenosphere boundary in our study area, located at 70–120 km depth, with the thickest and fastest lithosphere beneath New Brunswick. We suggest that this relatively thicker lithosphere is due to a slab stacking process that occurred after flat subduction of a younger domain in the Late Silurian. Occurrence of flat subduction in the Late Silurian in the northern Appalachians is also supported by geochemical and paleomagnetic evidence.

On Archean craton growth and stabilisation: Insights from lithospheric resistivity structure of the Superior Province

The nature of lithospheric evolution and style of the driving tectonic processes during the growth and stabilisation of continental crust in the Archean remain enigmatic. The hotter, rheologically weak Archean crust would be unable to support thick orogens. Thus, gravitational collapse likely occurred when continental fragments became overthickened and/or when far-field stresses were relieved during the terminal stages of orogenesis. 3D magnetotelluric resistivity models of the Archean Superior Province, reveal the presence of low resistivity zones in the mid-lower crust that reflect a protracted history of magmatic-hydrothermal activity contemporaneous with construction and collapse. These include sub-vertical zones of low resistivity in the mid-upper crust, inferred to represent corridors of paleo-fluid flow along crustal-scale structural networks developed in response to terrane amalgamations. Subsequent orogenic collapse resulted in widespread lateral flow within the lower crust accommodated by sub-horizontal shear zones and included magmatic refertilisation. Thus, the preserved low-resistivity anomalies in the mid-lower crust represent an amalgamation of magmatic-hydrothermal and deformational processes that occurred during construction, peak orogenesis, and collapse in the Archean.

Early Cretaceous extension in South China: constraints from east–west-trending A-type granite belts and growth strata in terrigenous basins

ABSTRACT The late Mesozoic tectonic domain of Southeast China transitioned from compression to extension because of the Paleo-Pacific plate rollback. However, the precise timing of the initiation of extensional tectonics remains unclear. This work focuses on the E–W-trending granite belt and the growth strata of Early Cretaceous red beds in Southeast China to establish the extensional tectonic model in Southeast China. The petrogenesis of four granitic plutons from the E–W-trending granite belt in NE Jiangxi is traced on the basis of zircon U–Pb ages and whole-rock major and trace element abundances to elucidate lithosphere evolution in the study area. U–Pb zircon dating reveals the granite emplacement at 130–135 Ma. Geochemical results demonstrate that all granite intrusions can be identified as A-type granites derived from crustal and a few mantle sources respectively through lithospheric thinning and magma upwelling under the extension background. Field observation shows the enlargement of the sedimentary area of the early Cretaceous Huizhou Formation (Fm.) in the Huangshan Basin closely after (or simultaneously) 139 Ma Shiling volcanic activities. The dip angles of the lower part of Huizhou Fm. vary from 49° to 25° towards the basin centre, and the thickness exhibited a slightly increasing trend towards the lower part within the same bed. These phenomena are consistent with the shape of the growth strata. A model, in which the NE–SW-oriented structure lines transfer to the E–W-trending tectono-magmatic belts by meeting and inheriting the previous E–W-trending tectonic axis, is established in the present work. Lithospheric thinning, magma upwelling, and crustal remelting occur within an oblique extension stress field since ca. 140 Ma. Consequently, these phenomena are influenced by the Paleo-Pacific plate rollback and the inheritance of a structural framework from the regional basement series.

Upper Mantle Anisotropic Shear Velocity Structure at the Equatorial Mid‐Atlantic Ridge Constrained by Rayleigh Wave Group Velocity Analysis From the PI‐LAB Experiment

AbstractThe evolution of ocean lithosphere and asthenosphere are fundamental to plate tectonics, yet high resolution imaging is rare. We present shear wave velocity and azimuthal anisotropy models for the upper mantle from Rayleigh wave group velocities from local earthquake and ambient noise at 15–40‐s period recorded by the Passive Imaging of the Lithosphere Asthenosphere Boundary experiment at the equatorial Mid‐Atlantic Ridge covering 0–80‐Myr‐old seafloor. We find slow velocities along the ridge, with faster velocities beneath older seafloor. We image a fast lid (25–30‐km thick) beneath the ridge that increases to 50–60 km beneath older seafloor. Punctuated, ∼100 km wide low velocity anomalies exist off‐axis. There are multiple layers of azimuthal anisotropy, including (i) a lithosphere (20–40 km depth) characterized by strong anisotropy (4.0%–7.0 %) with fast‐axes that rotate from ridge subparallel toward the absolute plate motion/spreading direction at distances >60 km from the ridge, and (ii) weak anisotropy (1.0%–2.0%) at >40 km depth. Our results are consistent with conductive cooling of lithosphere, although with some complexities. Thickened lithosphere beneath the ridge supports lateral conductive cooling beneath slow‐spreading centers. Undulations in lithospheric thickness and slow asthenospheric velocities are consistent with small scale convection and/or partial melt. Lithospheric anisotropy can be explained by vertical flow and a contribution from either fluid or mineral filled cracks organized melt beneath the ridge and plate motion induced strain off axis. Deep azimuthal anisotropy is suggestive of upwelling beneath the ridge and three‐dimensional flow possibly caused by small scale convection off‐axis.

Late Mesoproterozoic low-P/T–type metamorphism in the North Wulan terrane: Implications for the assembly of Rodinia

AbstractRegional metamorphism provides critical constraints for unravelling lithosphere evolution and geodynamic settings, especially in an orogenic system. Recently, there has been a debate on the Rodinia-forming Tarimian orogeny within the Greater Tarim block in NW China. The North Wulan terrane, involved in the Paleozoic Qilian orogen, was once part of the Greater Tarim block. This investigation of petrography, whole-rock and mineral geochemistry, phase equilibrium modeling, and in situ monazite U-Pb dating of garnetite, pelitic gneiss, and quartz schist samples from the Statherian–Calymmian unit of the North Wulan terrane provides new constraints on the evolutionary history of the Greater Tarim block at the end of the Mesoproterozoic during the assembly of Rodinia. The studied samples yielded three monazite U-Pb age groups of ca. 1.32 Ga, 1.1 Ga, and 0.45 Ga that are interpreted to be metamorphic in origin. The tectonic significance of the early ca. 1.32 Ga metamorphism is uncertain and may indicate an extensional setting associated with the final breakup of Columbia. The ca. 1.1 Ga low-pressure, high-temperature (low-P/T)–type granulite-facies metamorphism is well preserved and characterized by a clockwise P-T path with a minimum estimation of ∼840–900 °C and ∼7–11 kbar for peak metamorphism, followed by postpeak decompression and cooling. A tectonothermal disturbance occurred at ca. 0.45 Ga, but with limited influence on the preexisting mineral compositions of the studied samples. The characteristics of the metamorphism indicate an arc–back-arc environment with ongoing subduction of oceanic lithosphere at ca. 1.1 Ga. Combined with previous studies, we suggest that the Greater Tarim block probably experienced a prolonged subduction-to-collision process at ca. 1.1–0.9 Ga during the assembly of Rodinia, with a position between western Laurentia and India–East Antarctica.

Differential Phanerozoic evolution of cratonic and non-cratonic lithosphere from a thermochronological perspective: São Francisco Craton and marginal orogens (Brazil)

The São Francisco Craton (SFC) and its marginal Araçuaí and Brasília orogens exhibit a significant diversity in their lithospheric architecture. These orogens were shaped during the Neoproterozoic–Cambrian amalgamation of West Gondwana. The rigid cratonic lithosphere of the SFC and the relatively weak lithosphere of the Araçuaí Orogen were disrupted during the Cretaceous opening of the South Atlantic Ocean, whereas the Brasília Orogen remained in the continental hinterland. In earlier research, the thermal effects of the Phanerozoic reactivations in the shallow crust of the Araçuaí Orogen have been revealed by low-temperature thermochronology, mainly by apatite fission track (AFT) analysis. However, analyses from the continental interior are scarce. Here we present new AFT data from forty-three samples from the Brasília Orogen, the SFC and the Araçuaí Orogen, far from the passive margin of the Atlantic coast (~150 to 800 km). Three main periods of basement exhumation were identified: (i) Paleozoic, recorded both by samples from the SFC and Brasília Orogen; (ii) Early Cretaceous to Cenomanian, recorded by samples from the Araçuaí Orogen; and (iii) Late Cretaceous to Paleocene, inferred in samples from all domains. We compare the differential exhumation pattern of the different geotectonic provinces with their lithospheric strengths. We suggest that the SFC likely concentrated the Meso-Cenozoic reactivations in narrow weak zones while the Araçuaí Orogen displayed a far-reaching Meso-Cenozoic deformation. The Brasília Orogen seems to be an example of a stronger orogenic lithosphere, inhibiting reworking, confirmed by our new AFT data. Understanding the role of the lithosphere rigidity may be decisive to comprehend the processes of differential denudation and the tectonic–morphological evolution over Phanerozoic events.

Global variation of seismic energy release with oceanic lithosphere age

Variations in Mid Ocean Ridge seismicity with age provide a new tool to understand the thermal evolution of the oceanic lithosphere. The sum of seismic energy released by earthquakes during a time, and for an area, is proportional to its lithospheric age. Asthenospheric temperatures emerge on ridge centers with new crust resulting in high seismic activity; thus, the energy released sum is highest on the young lithosphere and decreases with age. We propose a general model that relates the systematic variation of seismic energy released with the lithospheric age. Our analysis evaluates the main physical factors involved in the changes of energy released sum with the oceanic lithosphere age in MOR systems of different spreading rates. These observations are substantiated based on three cross-sections of the East Pacific Rise, six sections in the Mid Atlantic Ridge, and three profiles in the Central Indian Ridge. Our global model provides an additional tool for understanding tectonic processes, including the effects of seismicity and mid-plate volcanism, and a better understanding of the thermal evolution for the young oceanic lithosphere.

Cenozoic mountain building and topographic evolution in Western Europe: impact of billions of years of lithosphere evolution and plate kinematics

The architecture and tectono-magmatic evolution of the lithosphere of Europe are the result of a succession of subduction, rifting and inputs from plumes that have modified the lithospheric mantle since the Neoproterozoic (750–500 Ma). These events gave birth to contrasting crust-mantle and lithosphere-asthenosphere mechanical coupling between strong, viscous, thick, cold, depleted mantle of the Archean lithosphere of the West African Craton and the East European Craton, and the weak, low viscous, thin, hot and less depleted mantle of the Phanerozoic lithosphere of Central Europe. These differences were long-lived and explain the first-order present-day stresses and topography as well as the styles of orogenic deformation. The lack of thermal relaxation needed to maintain rheological contrasts over several hundreds of millions of years requires high mantle heat flux below Central Europe since at least the last 300 Ma. A combination of edge-driven convection on craton margins and asthenospheric flow triggered by rift propagation during the Atlantic and Tethys rifting is suggested to be the main source of heat. The topography of Central Europe remained in part dynamically supported during most of the Mesozoic thinning in line with the long-term stability of thermal-mechanical structure of the lithosphere. Timing and rates of exhumation recorded across Western Europe during convergence indicate that an additional control by the architecture of Mesozoic rifted margins is required. By 50 Ma the acceleration of orogenic exhumation, from the High Atlas to the Pyrenees, occurred synchronously with the onset of extension and magmatism in the West European Rift. Extension marks the onset of distinct orogenic evolution between Western Europe (Iberia) and the Alps (Adria) in the east, heralding the opening of the Western Mediterranean. A major kinematic re-organisation occurred triggering the involvement of more buoyant and thicker portions of rifted margins resulting in widespread orogenic growth. We conclude that conceptual models of collision require to better account for the thermo-magmatic evolution of the continental lithosphere, especially the original architecture and composition of its mantle, as well as the precise knowledge of the architecture of the rifted margins to explain the timing and rates of orogenic topography.

Archaean seafloors shallowed with age due to radiogenic heating in the mantle

Given the scarcity of geological data, knowledge of Earth’s landscape during the Archaean eon is limited. Although the continental crust may have been as massive as present by 4 Gyr ago, the extent to which it was submerged or exposed is unclear. One key component in understanding the amount of exposed landmasses in the early Earth is the evolution of the oceanic lithosphere. Whereas the present-day oceanic lithosphere subsides as it ages, based on numerical models of mantle convection we find that higher internal heating due to a larger concentration of radioactive isotopes in the Archaean mantle halted subsidence, possibly inducing seafloor shallowing before 2.5 Gyr ago. In such a scenario, exposed landmasses in the form of volcanic islands and resurfaced seamounts or oceanic plateaus can remain subaerial for extended periods of time, and may have provided the only stable patches of dryland in the Archaean. Our results therefore permit a re-evaluation of possible locations for the origin of life, as they provide support to an existing hypothesis that suggests that life had its origins on land rather than in an oceanic environment. In contrast to present-day seafloor subsidence with age, there may have been Archaean seafloor shallowing and landmass exposure due to high internal heating in the mantle that halted subsidence, according to numerical models of mantle convection.

Research progress of Yarlung Zangbo ophiolite

Ophiolite is a relic of the paleo oceanic crust preserved on the earth continent. It preserves a lot of important evidences in the process of crust-mantle interaction and records information about oceanic lithospheric dynamics and geotectonics. It has always been a research hotspot in the field of geology. Especially in recent years, the ophiolite has made a lot of important progress in the global lithospheric evolution, ocean-continent transition history in different geological periods and geodynamics, which has aroused great research enthusiasm in the field of geology. The Yarlung Zangbo ophiolite is the most important ophiolite in China.However, the tectonic setting, rock composition and genesis of the Yarlung Zangbo ophiolite are still controversial. This paper expounds the Yarlung Zangbo ophiolite from three aspects: brief history, characteristics and existing problems, so as to promote the Yarlung Zangbo ophiolite to the international academic circles.

Synthesis of KAlSiO4 by Hydrothermal Processing on Biotite Syenite and Dissolution Reaction Kinetics

To make potassium from K-bearing rocks accessible to agriculture, processing on biotite syenite powder under mild alkaline hydrothermal conditions was carried out, in which two types of KAlSiO4 were obtained successfully. The dissolution-precipitation process of silicate rocks is a significant process in lithospheric evolution. Its effective utilization will be of importance for realizing the comprehensiveness of aluminosilicate minerals in nature. Two kinds of KAlSiO4 were precipitated in sequence during the dissolution process of biotite syenite. The crystal structures of two kinds of KAlSiO4 were compared by Rietveld structure refinements. The kinetics model derived from geochemical research was adopted to describe the dissolution behavior. The reaction order and apparent activation energy at the temperature range of 240–300 °C were 2.992 and 97.41 kJ/mol, respectively. The higher dissolution reaction rate of K-feldspar mainly relies on the alkaline solution, which gives rise to higher reaction order. During the dissolution-precipitation process of K-feldspar, two types of KAlSiO4 with different crystal structure were precipitated. This study provides novel green chemical routes for the comprehensive utilization of potassium-rich silicates.

Thermochemical lithosphere differentiation and the origin of cratonic mantle.

Cratons record the early history of continental lithosphere formation, yet how they became the most enduring part of the lithosphere on Earth remains unknown1. Here we propose a mechanism for the formation of large volumes of melt-depleted cratonic lithospheric mantle (CLM) and its evolution to stable cratons. Numerical models show large decompression melting of a hot, early Earth mantle beneath a stretching lithosphere, where melt extraction leaves large volumes of depleted mantle at depth. The dehydrated, stiffer mantle resists further deformation, forcing strain migration and cooling, thereby assimilating depleted mantle into the lithosphere. The negative feedback between strain localization and stiffening sustains long-term diffused extension and emplacement of large amounts of depleted CLM. The formation of CLM at low pressure and its deeper re-equilibration reproduces the evolution of Archaean lithosphere constrained by depth-temperature conditions1,2, whereas large degrees of depletion3,4 and melt volumes5 in Archaean cratons are best matched by models with lower lithospheric strength. Under these conditions, which are otherwise viable for plate tectonics6,7, thermochemical differentiation effectively prevents yielding and formation of margins: rifting and lithosphere subduction are short lived and embedded in the cooling CLM as relict structures, reproducing the recycling and reworking environments that are found in Archaean cratons8,9. Although they undergo major melting and extensive recycling during an early stage lasting approximately 500million years, the modelled lithospheres progressively differentiate and stabilize, and then recycling and reworking become episodic. Early major melting and recycling events explain the production and loss of primordial Hadean lithosphere and crust10, whereas later stabilization and episodic reworking provides a context for the creation of continental cratons in the Archaean era4,8.

Tectono-magmatic evolution of Tethyan oceanic lithosphere in supra subduction zone fore arc regime: Geochemical fingerprints from crust-mantle sections of Naga Hills Ophiolite

The Naga Hills Ophiolite (NHO) belt in the Indo-Myanmar range (IMR) represents a segment of Tethyan oceanic crust and upper mantle that was involved in an eastward convergence and collision of the Indian Plate with the Burmese Plate during the Late Cretaceous-Eocene. Here, we present a detailed petrological and geochemical account for the mantle and crustal sections of NHO, northeastern India to address (i) the mantle processes and tectonic regimes involved in their genesis and (ii) their coherence in terms of the thermo-tectonic evolution of Tethyan oceanic crust and upper mantle. The NHO suite comprises well preserved crustal and mantle sections discretely exposed at Moki, Ziphu, Molen, Washelo and Lacham areas. The ultramafic-mafic lithologies of NHO are mineralogically composed of variable proportions of olivine, orthopyroxene, clinopyroxene and plagioclase. The primary igneous textures for the mantle peridotites have been overprinted by extensive serpentinisation whereas the crustal section rocks reflect crystal cumulation in a magma chamber. Chondrite normalised REE profiles for the cumulate peridotite-olivine gabbro-gabbro assemblage constituting the crustal section of NHO show flat to depleted LREE patterns consistent with their generation from depleted MORB-type precursor melt in an extensional tectonic setting, while the mantle peridotites depict U-shaped REE patterns marked by relative enrichment of LREE and HREE over MREE. These features collectively imply a dual role of depleted MORB-type and enriched arc-type mantle components for their genesis with imprints of melt-rock and fluid-rock interactions. Tectonically, studied lithologies from NHO correspond to a boninitic to slab-proximal Island Arc Tholeiite affinity thereby conforming to an intraoceanic supra subduction zone (SSZ) fore-arc regime coherent with the subduction initiation process. The geochemical attributes for the crustal and mantle sections of NHO as mirrored by Zr/Hf, Zr/Sm, Nb/Ta, Zr/Nb, Nb/U, Ba/Nb, Ba/Th, Ba/La and Nd/Hf ratios propound a two-stage petrogenetic process: (i) a depleted fore arc basalt (FAB) type tholeiitic melt parental to the crustal lithologies was extracted from the upwelling asthenospheric mantle at SSZ fore-arc extensional regime thereby rendering a refractory residual upper mantle; (ii) the crust and upper mantle of the SSZ fore arc were progressively refertilised by boninitic melts generated in response to subduction initiation and slab-dehydration. The vestiges of Tethyan oceanic lithosphere preserved in NHO represent an accreted intra-oceanic fore arc crust and upper mantle section which records a transitional geodynamic evolution in a SSZ regime marked by subduction initiation, fore arc extension and arc-continent accretion.