The late Early-Paleozoic granitic magmatism in Northwestern Fujian, China: constraints on intraplate orogeny in the South China block

The Early Paleozoic tectono-thermal event was a significant orogenic activity during the Phanerozoic era,which had a profound impact on the early crust of the South China Block(SCB) and established the foundation for later tectonic activity.The Wuyi-Yunkai orogenic belt in Southeastern China was extensively exposed to Early Paleozoic magmatism,the genetic mechanism of which remains controversial.To shed light on this issue,detailed petrological,geochemical,and zircon U-Pb-Hf isotopic studies were carried out on two granitoids,namely the Yuntongshan pluton and the Gaoqiao pluton,identified in the central Wuyishan.Zircon U-Pb chronology of the Yuntongshan and Gaoqiao bodies yielded ages of437±4 Ma(MSWD=2.2) and 404±2 Ma(MSWD=12),respectively,indicating that they were emplaced during the Early Silurian and Early Devonian periods.These granitoids are primarily composed of biotite-granite and biotite-monzonitic-granites,with high concentrations of S_(i)O_(2)(73.59-75.91 wt%),K_(2)O+Na_(2)O(8.31-8.73wt%),and low contents of MgO,CaO,Cr,Ni.They are classified as high-K calc-alkaline and weakly metaluminous-strongly peraluminous S-type granites.These granitoids are enriched in light rare earth elements(LREEs) and large ion lithophile elements(LILEs) and depleted in heavy rare earth elements(HREEs) and high field strength elements(HFSEs) with arc affinity.The εHf(t) values of-3.3 to-15.4 with two-stage Hf model ages ranging from 2829 to 1644 Ma,combined with the presence of Neoproterozoic inherited zircons,suggest that the primary magma of these granitoids was derived from the partial melting of Neoproterozoic crust with a Paleoproterozoic crustal model age.These findings,combined with the spatio-temporal distribution of regional magmatism,reveal that the late Early-Paleozoic granitoids formed in the intraplate orogenic background originating from the subduction of the proto-Tethys Ocean and proto-Pacific Ocean around the margin of the east Gondwana supercontinent.

Mineralogical and geochemical characterization of the Wadi Natash volcanic field(WNVF),Egypt:Alkaline magmatism in a Late Cretaceous continental rift system

The Wadi Natash volcanic field(WNVF)in the south of the Eastern Desert of Egypt is a typical example of well-preserved intraplate alkaline magmatism during the Late Cretaceous,i.e.,prior to the Oligo-Miocene Red Sea rift.We compiled stratigraphic sections at two sectors;namely East Gabal Nuqra and West Khashm Natash(WKN)where the volcanic flows are intercalated with the Turonian Abu Agag sandstone with occasional paleosols when volcanic activity is intermittent.Peridotite mantle xenoliths are encountered in the first sector whereas flows in the second sector are interrupted by trachyte plugs and ring dykes.On a geochemical basis,the maifc melt originating from the lithospheric mantle beneath the WNVF practiced~5%partial melting of phlogopite-bearing garnet peridotite.Basalts dominate in the two sectors and highly evolved(silicic)rocks are confined to the WKN sector.Rejuvenation of ancient Precambrian fractures following the NW-SE and ENE-WSW trends facilitated the ascend of Late Cretaceous mantle-derived alkaline magma.Structurally,the WNVF developed at the eastern shoulder of the so-called"Kom Ombo-Nuqra-Kharit rift system"that represents a well-defined NW-trending intracontinental rift basin in the southern Eastern Desert.In such a structural setup,the Natash volcanic are confined to half-grabens at the East Gabal Nuqra sector whereas the West Khashm Natash sector is subjected to extensional stresses that propagated eastwards.The WNVF is a typical example of fluvial clastics(Turonian)intercalation with rift-related alkaline volcanic rocks in northeast Africa.

Age and geochemical evolution of granite magmatism in Olkhon region from Caledonian syncollisional ore-free granite to the rare metal granite and pegmatite of Middle Paleozoic intraplate setting

The detailed description of two granite complexes in the Olkhon subterrane is given.The Early Paleozoic Sharanur complex was formed by granitization of gneisses of the Olkhon series.It includes migmatites,granite-gneisses,granites and pegmatites of normal alkalinity;they belong to the type of syncollisional granites.The Middle Paleozoic Aya granite complex includes mother Aya massif of amazonite-bearing granites and several types of rare-metal pegmatites.They have elevated alkalinity,low of Ba,Sr,and high LILE and HFSE elements contents.The Aya pegmatites lie in northwest cracks of stretching and associated with the rise of the territory under the influence of the North Asian plume.These cracks and pegmatites mark the beginning of a new intraplate geodynamic setting.Two geochemical types are distinguished among the pegmatites of this complex.These are amazonite pegmatites of Li-F type with Ta mineralization and complex type pegmatite with Be-Rb-Nb-Ta and Li-F mineralization(the Ilixin vein).The Tashkiney pegmatite vein is similar to Ilixin,but lies in the gneisses of the Olkhon series.It shows high concentrations of Be,Nb,Ta,as well as W,Sn,but lacks Li and F,due to a greater depth and higher temperature of the melt crystallization of this pegmatite.

A review of geophysical studies on the Mongolian Plateau

The Mongolian Plateau in Central Asia is an intracontinental tectonic system far from active plate boundaries.Despite its distance from these boundaries,the plateau is characterized by intense crustal deformation accompanied by voluminous Cenozoic volcanism and active modern seismicity.However,the intraplate deformation mechanism has long been debated owing to the scarcity of observations and contradictions between different results.In recent years,growing geophysical studies have been conducted on the Mongolian Plateau,providing constraints on its lithospheric structure and dynamics.Here,we review the geophysical research on the Mongolian Plateau over the last decade,including seismological,geodetic,gravity,magnetotelluric,and geodynamic aspects.This review aims to(a)describe crustal and mantle structures based on multiscale seismic images;(b)describe deformation patterns based on seismic anisotropy,focal mechanisms,and global positioning system(GPS)observations;and(c)discuss the mechanisms behind intraplate deformation,volcanism,and seismic activity across the Mongolian Plateau.Seismic images show that the crustal structure of the plateau has significant east-west differences.Several blocks in the western Mongolian Plateau have thick crusts,including the Altai Mountains,Hovsgol Rift,and Hangay Dome.The lithospheric deformation across the Mongolian Plateau has strong lateral variation,with NE-SW shortening in the Altai Mountains and W-E or NW-SE shear deformation in the Hangay Dome region and the eastern part.The varied deformation may result from the superposition of multiple mechanisms,including far-field stress in the Altai Mountains,mantle upwelling,and mantle flow in the Hangay Dome region.However,it is difficult to identify the geodynamics of the formation of the entire Mongolian Plateau because the deformation is too complicated,and the present models are not sufficient and are always partial.Overall,this review encompasses recent advances in seismic observations of the Mongolian Plateau,illuminates the heterogeneities in the crust and mantle structure and deformation of the plateau,and discusses the mechanisms behind the deformation,magmatism,and seismicity.

Relocation of Uppermost Mantle Earthquakes in the Atlas Mountains, Morocco

Upper mantle earthquakes are usually associated with plate boundary tectonics, but rarely occur beneath intracontinental orogenic belts. In the Moroccan Atlas Mountains, earthquakes determined at subcrustal depths are a controversial topic because they are few in number compared to subduction zones and are not related to plate boundary tectonics. A recent increase of broadband stations in Morocco has revealed numerous events below the Atlas belts, thought to occur from the upper mantle. Using additional available stations, these Atlas events were relocated and new epicenter resolutions were acquired following rigorous depth and RMS error criteria. 309 events were reprocessed and epicenter depths obtained were between 31 and 240 km during the last 23 years. Temporal variations of High Atlas events appear to be continually dipping while Anti Atlas events show no temporal variation trends. In addition, a recent strong event M6.8 occurred in September 2023 at the transition crust-uppermost mantle followed by several aftershocks which have been relocated at uppermost mantle depths. These events support delamination model under the High-Middle Atlas which could flow southward beneath the Anti Atlas lithosphere, and explain the large variation observed in lithosphere thickness between the High-Middle Atlas, and the Anti Atlas. Subcrustal events beneath the Atlas may be related to upper mantle earthquakes beneath the neighboring Canary Islands which have experienced recent swarms and eruptions. This possible correlation cannot be excluded since descending and ascending material is necessary for a regional geodynamic balance.

A 15-year-Long catalog of seismicity in the Eastern Tennessee Seismic Zone(ETSZ)using matched filter detection

We present a detailed catalog of 13671 earthquakes in the Eastern Tennessee Seismic Zone(ETSZ)that spans January 1,2005 to July 31,2020.We apply a matched filter detection technique on over 15 years of continuous data,resulting in arguably the most complete catalog of seismicity in the ETSZ yet.The magnitudes of newly detected events are determined by computing the amplitude ratio between the detections and templates using a principal component fit.We also compute the b-value for the new catalog and comparatively relocate a subset of newly detected events using XCORLOC and hypoDD,which shows a more defined structure at depth.We find the greatest concentration along and to the east of the New York-Alabama Lineament,as defined by the magnetic anomaly,supporting the argument that this feature likely is related to the generation of seismicity in the ETSZ.We examine seismicity in the vicinity of the Watts Bar Reservoir,which is located about 5 km from the epicenter of the M_(W) 4.4 December 12,2018 Decatur,Tennessee earthquake,and find possible evidence for reservoir modulated seismicity in this region.We also examine seismicity in the entire ETSZ to search for a correlation between shallow earthquakes and seasonal hydrologic changes.Our results show limited evidence for hydrologicallydriven shallow seismicity due to seasonal groundwater levels in the ETSZ,which contradicts previous studies hypothesizing that most intraplate earthquakes are associated with the dynamics of hydrologic cycles.

Petrographical and mineral chemistry evidence to delineate the source/sources of the Central Indian Ocean Basin pumices

We present data pertaining to mineral assemblages and composition of the Central Indian Ocean Basin(CIOB)pumices.Eight groups of pumices were identified considering the presence of phenocrysts of plagioclase,clinopyroxene,orthopyroxene,hornblende and biotite together with the occurrence of quartz and glass.Pigeonite,fayalite and ulvospinelare reported for the first time from these pumices.In the eight groups,the modal percentage of the constituents are phenocrysts 3%to 19%(avg 9.6%),silicic glass 33%to 54%(avg 43%)and the rest is vesicles.Based on the above factors we have identified the possible sources of the CIOB pumices.The mineral compositions of plagioclase,pyroxenes,and biotite of the CIOB pumices were compared with those of Krakatau and Toba.Most of the plagioclase and pyroxene compositions resemble the Haranggoal Dacite Tuff of Toba and Krakatau.Considering the mineral assemblages and compositions,there are pumices which do not correlate to any of the above eruptions and are probably from yet unidentified source/sources.These sources could either be from nearby terrestrial volcanoes or intraplate seamounts present in the CIOB.In a global context,it is viable that petrological characteristics could be used as initial criteria to determine the source of pumices that occur at abyssal depths in the world ocean.

Petrography, Geochemistry and Relative Chronology of Quaternary Volcanic Formations in the Mermoz and Fann Sectors, West Senegal

Detailed work on Quaternary volcanism has been carried out in the Mermoz and Fann sectors of western Senegal. In the Mermoz sector, the main emission zone is a collapsed crater located at the intersection of three major fractures: NE-SW, NW-SE and N-S. The lithological succession in this Mermoz sector comprises, from bottom to top: 1) a substratum with at its base Eocene limestones on which lie Quaternary sands surmounted by stratified tuffs;2) a vesicular ball dolerite which deforms the stratified tuffs;3) a dark early breccia;4) two generations of basanites: the first is vesicular, the second non-vesicular;5) a clear intermediate breccia and finally 6) a late breccia. The Fann sector contains several emission zones, most of which are currently located in the ocean. The lavas may have reached the coast through E-W and NE-SW faults. The lithological succession includes from bottom to top: 1) scoria-rich early volcanic breccias;2) a first generation of non-vesicular mesocrate dolerite (D1);3) a second generation of melanocrate vesicular dolerite (D2);4) basanites and finally 5) a late breccia. The geochemical characteristics of the lavas studied are compatible with a very enriched and very deep magmatic source of the garnet lherzolite type located in the lower mantle. The magma from this source would have risen in the form of mantle plumes through major NE-SW and E-W faults in a continental intraplate context.

Jurassic Tectonics of North China:A Synthetic View

This paper gives a synthetic view on the Jurassic tectonics of North China, with an attempt to propose a framework for the stepwise tectonic evolution history. Jurassic sedimentation, deformation and magmatism in North China have been divided into three stages. The earliest Jurassic is marked by a period of magmatism quiescence （in 205-190 Ma） and regional uplift, which are considered to be the continuation of the “Indosinian movement” characterized by continent-continent collision between the North and South China blocks. The Early to Middle Jurassic （in 190-170 Ma） was predominated by weak lithospheric extension expressed by mantle-derived plutonism and volcanism along the Yanshan belt and alongside the Tan-Lu fault zone, normal faulting and graben formation along the Yinshan- Yanshan tectonic belt, depression and resuming of coal-bearing sedimentation in vast regions of the North China block （NCB）. The Middle to Late Jurassic stage started at 165y.5 Ma and ended up before 136 Ma; it was dominated by intensive intraplate deformation resulting from multi-directional compressions. Two major deformation events have been identified. One is marked by stratigraphic unconformity beneath the thick Upper Jurassic molasic series in the foreland zones of the western Ordos thrust-fold belt and along the Yinshan-Yanshan belt; it was predated 160 Ma. The other one is indicated by stratigraphic unconformity at the base of the Lower Cretaceous and predated 135 Ma. During this last stage, two latitudinal tectonic belts, the Yinshan-Yanshan belt in the north and the Qinling-Dabie belt in the south, and the western margin of the Ordos basin were all activated by thrusting; the NCB itself was deformed by the NE to NNE-trending structural system involving thrusting, associated folding and sinistral strike-slip faulting, which were spatially partitioned. Foliated S-type granitic plutons aged 160-150 Ma were massively emplaced in the Jiao-Liao massif east of the Tan-Lu fault zone and indicate important crustal thickening in this part of the NCB. The Jurassic deformation patterns, different tectonic systems and multi-directional contractions in North China recorded far-field effects of synchronous convergences, toward the East Asian continent, of three different plates, the Siberian plate in the north, the paleo-Pacific oceanic plate in the east and the Lhasa block in the southwest. This Middle to Late Jurassic intraplate orogenesis and pervasive shortening deformation preceded lithospheric attenuation and thinning in East China, which most possibly started by the Early Cretaceous around 135 Ma.

Mesozoic-Cenozoic Multi-Stage Intraplate Deformation Events in the Langshan Region and their Tectonic Implications

In the Langshan region, northwestern China, marked multi-stage intraplate deformation events have occurred since the Mesozoic, including（1） northeast-striking ductile left-lateral strike slip during the Middle-Late Triassic, which is closely related to the collision between the North China and the Yangtze plates;（2） top-to-the-southeast thrust with northwest-southeast trending maximum compression during the Late Jurassic;（3） nearly eastward detachment during the Early Cretaceous;（4） top-to-the-northwest thrust with northwest-southeast trending maximum compression during the Late Cretaceous and Early Cenozoic;（5） northeast-striking brittle left-lateral strike slip with nearly north-south trending maximum compression; and（6） northwest-southeast extension during the Middle-Late Cenozoic. All these deformation events belong to the intraplate deformation across the entire Central Asian region and respond to the tectonic events along the plate boundaries or deep tectonics. The structures developed in early events in the crust were the most important factors controlling the later deformation styles, and few new structures have later developed. Based on previous research and our results, the paleostress inversion in the Langshan region shows that the Mesozoic intraplate deformations in the study region mainly resulted from the tectonic events from the Paleo-Pacific region and have no or a weak relation to the Tethys region. During the Late Jurassic, the maximum compression from the Mongolia-Okhotsk region cannot be excluded. The Langshan region is the bridge between southern Mongolia and the western Ordos tectonic belt and is thus important to understand the nature and relationship between both regions.

Mesoproterozoic Earthquake Events and Breakup of the Sino-Korean Plate

In the Mesoproterozoic time, the northern part of the Sino-Korean Plate experienced a period of intensive tectonic extension and breakup. 1. An abundance of sedimentary earthquake records is preserved in the Chuanlinggou, Tuanshanzi and Gaoyuzhuang formations in the Mesoproterozoic Changcheng System （1800-1400 Ma） and in the Mesoproterozoic Wumishan Formation of the Jixian System （1400-1000 Ma）. These earthquake records are characterized by various liquefied sand-veins, carbonate microspar and coarser spar veins, limestone dikes, liquefied breccia and various forms of liquefied contorted bedding. This deformation is always associated with synsedimentary faults and igneous activity. 2. Three liquefaction models for soft carbonate sediments are recognized, including liquefaction in laminated carbonate rocks, liquefaction in thin-bedded carbonates and large-scale liquefaction along huge carbonate dikes. 3. Based on the record of earthquake and volcanic activities, the Sino-Korean Plate experienced at least twice intraplate breakups. One occurred between 1800-1400 Ma, and the other occurred at about 1200 Ma. The last breakup resulted in formation of the Yan-Liao aulacogen, a tectonic zone characterized by deeper material vibrancy, active faults, major igneous activity and frequent earthquakes.

Temporal-Spatial Structure of Intraplate Uplift in the Qinghai-Tibet Plateau

The intraplate uplift of the Qinghai-Tibet Plateau took place on the basis of breakup and assembly of the Precambrian supercontinent, and southward ocean-continent transition of the Proto-, Paleo-, Meso- and Neo-Tethys during the Caledonian, Indosinian, Yanshanian and Early Himalayan movements. The intraplate tectonic evolution of the Qinghai-Tibet Plateau underwent the early stage of intraplate orogeny characterized by migrational tectonic uplift, horizontal movement and geological processes during 180-7 Ma, and the late stage of isostatic mountain building characterized by pulsative rapid uplift, vertical movement and geographical processes since 3.6 Ma. The spatial-temporal evolution of the intraplate orogeny within the Qinghai-Tibet Plateau shows a regular transition from the northern part through the central part to the southern part during 180-120 Ma, 65-35 Ma, and 25-7 Ma respectively, with extensive intraplate faulting, folding, block movement, magmatism and metallogenesis. Simultaneous intraplate orogeny and basin formation resulted from crustal rheological stratification and basin-orogen coupling that was induced by lateral viscous flow in the lower crust. This continental dynamic process was controlled by lateral flow of hot and soft materials within the lower crust because of slab dehydration and melted mantle upwelling above the subducted plates during the southward Tethyan ocean-continent transition processes or asthenosphere diapirism. Intraplate orogeny and basin formation were irrelevant to plate collision. The Qinghai-Tibet Plateau as a whole was actually formed by the isostatic mountain building processes since 3.6 Ma that were characterized by crust-scale vertical movement, and integral rapid uplift of the plateau, accompanied by isostatic subsidence of peripheral basins and depressions, and great changes in topography and environment. A series of pulsative mountain building events, associated with gravity equilibrium and isostatic adjustment of crustal materials, at 3.6 Ma, 2.5 Ma, 1.8-1.2 Ma, 0.9-0.8 Ma and 0.15-0.12 Ma led to the formation of a composite orogenic belt by unifying the originally relatively independent Himalayas, Gangdise, Tanghla, Longmenshan, Kunlun, Altyn Tagh, and Qilian mountains, and the formation of the complete Qinghai-Tibet Plateau with a unified mountain root after Miocene uplift of the plateau as a whole.

The Yanshanian Orogeny and Two Kinds of Yanshanides in Eastern-Central China

The Tan-Lu Fault was once a transform fault in the Paleotethys, west of which was the Qinling-Dabie Ocean separating the Yangtze Craton from the North China Craton, and east of which was the Su-Lu Ocean separating the Su-Wan Block from the Jiao-Liao Craton. The Qinling-Dabie Ocean closed in the Indosinian orogeny, which created the China-Southeast Asia Subcontinent, with the Tan-Lu Fault becoming a marginal shear zone along the newly-formed amalgamated subcontinent. The Su-Lu Ocean subducted partly in the Indosinian.orogeny, but not closed. In the Jurassic and Early Cretaceous, the Su-Wan Block drifted northwards with subduction of the Su-Lu Ocean and moved westwards to converge the subcontinent by sinistral sheafing of the ENE-striking fractures. The Su-Lu Ocean finally closed and the Su-Wan Block collided with the Jiao-Liao Craton in the Early Cretaceous, which constituted a part of the magnificent interplate Yanshanides. The interplate orogeny rejuvenated the fossil sutures and deep fractures, as well as the Indosinian orogen, and the intraplate （intracontinental） Yanshanian orogeny occurred in the subcontinent. The East Asia Yanshanides, consisting of the interplate orogens in the outer side and the intraplate orogens in the inner side, collapsed quickly in the latest Early Cretaceous and Late Cretaceous. The eastern China area entered a tensile period from the Eogene, and the tectonic differentiation between the central and eastern China areas since the Jurassic was further strengthened.

Formation Mechanism of "Drag Depressions" and Irregular Boundaries in Intraplate Deformation

Almost all intraplate caprocks experienced strong deformation during the convergence of microplates, and then disintegrated into many secondary geologic units with the special characters, such as irregular boundaries and particular structural assemblages. In order to understand the formation mechanism of these special phenomena, a rheological experiment on the structural scenery of the Tongling area is carried out. The result shows that the primary regular and uniform boundaries of the Tongling area becomes irregular because of the enclosing and confinement of surrounding geological units in the process of 'compression-shearing-rotation-drag'; simultaneously, two specific 'drag depressions' developed at two opposite corners of the block. The former and the later phenomena can be regarded as a typical regional-scale rheological effect and necessary outcome of intraplate deformation respectively.

Detrital-zircon geochronology of the Jurassic coal-bearing strata in the western Ordos Basin, North China: Evidences for multi-cycle sedimentation

The western Ordos Basin(WOB), situated in a tectonic transition zone in the North China Craton, acts as an excellent example for studying the Mesozoic intraplate sedimentation and deformation in Asia. In this study, U-Pb ages for 1203 detrital zircons of 14 sandstone samples collected from 11 sections are presented to unravel the sediment source locations and paleogeographic environments of the Early-Middle Jurassic coal-bearing Yan'an Formation in the WOB. Data show that there are five prominent age groups in the detrital zircons of the Yan'an Formation, peaking at ca. 282 Ma, 426 Ma, 924 Ma, 1847 Ma, and2468 Ma. Samples from the northern, middle, and southern parts of the WOB contain these five age categories in various proportions. In the northern region, the Yan'an Formation exclusively contains Early Permian detrital zircons with a single age group peaking at 282 Ma, matching well with the crystallizing ages of the widespread Early Permian granites in the Yinshan Belt to the north and the Alxa Block to the northwest. While in the southern region, the Yan'an Formation mainly contains three groups of detrital zircons, with age peaks at 213 Ma, 426 Ma, and 924 Ma. These zircon ages resemble those of the igneous rocks in the Qilian-Qinling Orogenic Belt to the south-southwest. Samples in the middle region, characterized by a mixture age spectrum with peaks at 282 Ma, 426 Ma, 924 Ma, 1847 Ma and 2468 Ma, are previously thought to have mixed derivations from surrounding ranges. However, by referring to the detrital-zircon age compositions of the pre-Jurassic sedimentary successions and combining with paleontological and petrographic analysis, we firstly propose that the sediments of the Yan'an Formation in the middle region were partly recycled from the Triassic and Paleozoic sedimentary strata in the WOB.The occurrence of recycled sedimentation suggests that the Late Triassic-Early Jurassic intraplate compressional deformation was very intense in the WOB, especially for regions in front of the Qilian Orogenic Belt.

Deep structure and origin of active volcanoes in China

We synthesize significant recent results on the deep structure and origin of the active volcanoes in China's Mainland. Magmatism in the western Pacific arc and back-arc areas is caused by dehydration of the subducting slab and by corner flow in the mantle wedge, whereas the intraplate magmatism in China has different origins. The active volcanoes in Northeast China （such as the Changbai and Wudalianchi） are caused by hot upwelling in the big mantle wedge （BMW） above the stagnant slab in the mantle transition zone and deep slab dehydration as well. The Tengchong volcano in Southwest China is caused by a similar process in the BMW above the subducting Burma microplate （or Indian plate）. The Hainan volcano in southernmost China is a hotspot fed by a lower-mantle plume which may be associated with the Pacific and Philippine Sea slabs＇ deep subduction in the east and the Indian slab＇s deep subduction in the west down to the lower mantle. The stagnant slab finally collapses down to the bottom of the mantle, which can trigger the upwelling of hot mantle materials from the lower mantle to the shallow mantle beneath the subducting slabs and may cause the slab-plume interactions.

Mantle heterogeneity,plume-lithosphere interaction at rift controlled ocean-continent transition zone:Evidence from trace-PGE geochemistry of Vempalle flows,Cuddapah Basin,India

This study reports major, trace, rare earth and platinum group element compositions of lava flows from the Vempalle Formation of Cuddapah Basin through an integrated petrological and geochemical approach to address mantle conditions, magma generation processes and tectonic regimes involved in their formation. Six flows have been identified on the basis of morphological features and systematic three-tier arrangement of vesicular-entablature-colonnade zones. Petrographically, the studied flows are porphyritic basalts with plagioclase and clinopyroxene representing dominant phenocrystal phases.Major and trace element characteristics reflect moderate magmatic differentiation and fractional crystallization of tholeiitic magmas. Chondrite-normalized REE patterns corroborate pronounced LREE/HREE fractionation with LREE enrichment over MREE and HREE. Primitive mantle normalized trace element abundances are marked by LILE-LREE enrichment with relative HFSE depletion collectively conforming to intraplate magmatism with contributions from sub-continental lithospheric mantle(SCLM) and extensive melt-crust interaction. PGE compositions of Vempalle lavas attest to early sulphur-saturated nature of magmas with pronounced sulphide fractionation, while PPGE enrichment over IPGE and higher Pd/Ir ratios accord to the role of a metasomatized lithospheric mantle in the genesis of the lava flows. HFSEREE-PGE systematics invoke heterogeneous mantle sources comprising depleted asthenospheric MORB type components combined with plume type melts. HFSE-REE variations account for polybaric melting at variable depths ranging from garnet to spinel lherzolite compositional domains of mantle. Intraplate tectonic setting for the Vempalle flows with P-MORB affinity is further substantiated by(i) their origin from a rising mantle plume trapping depleted asthenospheric MORB mantle during ascent,(ii) interaction between plume-derived melts and SCLM,(iii) their rift-controlled intrabasinal emplacement through Archeane Proterozoic cratonic blocks in a subduction-unrelated ocean-continent transition zone(OCTZ). The present study is significant in light of the evolution of Cuddapah basin in the global tectonic framework in terms of its association with Antarctica, plume incubation, lithospheric melting and thinning, asthenospheric infiltration collectively affecting the rifted margin of eastern Dharwar Craton and serving as precursors to supercontinent disintegration.

Multistage Deformation in the Northeastern Segment of the Jiangshao Fault (Suture) Belt: Constraints for the Relationship between the Yangtze Plate and the Cathaysia Old Land

Multistage deformation events have occurred in the northeastern Jiangshao Fault （Suture） Belt. The earliest two are ductile deformation events. The first is the ca. 820 Ma top-to-the-northwest ductile thrusting, which directly resulted from the collision between the Cathaysia Old Land and the Chencai Arc （？） during the Late Neoproterozoic, and the Jiangnan Orogenic Belt that formed as the ocean closed between the Yangtze Plate and the jointed Cathaysia Old Land and the Chencai Arc due to continuous compression. The second is the ductile left-lateral strike-slipping that occurred in the latest Early Paleozoic. Since the Jinning period, all deformation events represent the reactivation or inversion of intraplate structures due to the collisions between the North China and Yangtze plates during the Triassic and between the Philippine Sea and Eurasian plates during the Cenozoic. In the Triassic, brittle right-lateral strike-slipping and subsequent top-to-the south thrusting occurred along the whole northeastern Jiangshao Fault Zone because of the collision between the North China and Yangtze plates. In the Late Mesozoic, regional extension took place across southeastern China. In the Cenozoic, the collision between the Philippine Sea and Eurasian plates resulted in brittle thrusts along the whole Jiangnan Old land in the Miocene. The Jiangshao Fault Belt is a weak zone in the crust with long history, and its reactivation is one of important characteristics of the deformation in South China; however, late-stage deformation events did not occur beyond the Jiangnan Old Land and most of them are parallel to the strike of the Old Land, which is similar to the Cenozoic deformation in Central Asia. In addition, the Jiangnan old Land is not a collisional boundary between the Yangtze Plate and Cathaysia Old Land in the Triassic.

Seismogenic Structures of the 2006 ML4.0 Dangan Island Earthquake Offshore Hong Kong

The northern margin of the South China Sea, as a typical extensional continental margin, has relatively strong intraplate seismicity. Compared with the active zones of Nanao Island, Yangjiang, and Heyuan, seismicity in the Pearl River Estuary is relatively low. However, a ML4.0 earthquake in 2006 occurred near Dangan Island(DI) offshore Hong Kong, and this site was adjacent to the source of the historical M5.8 earthquake in 1874. To reveal the seismogenic mechanism of intraplate earthquakes in DI, we systematically analyzed the structural characteristics in the source area of the 2006 DI earthquake using integrated 24-channel seismic profiles, onshore–offshore wide-angle seismic tomography, and natural earthquake parameters. We ascertained the locations of NW-and NE-trending faults in the DI sea and found that the NE-trending DI fault mainly dipped southeast at a high angle and cut through the crust with an obvious low-velocity anomaly. The NW-trending fault dipped southwest with a similar high angle. The 2006 DI earthquake was adjacent to the intersection of the NE-and NW-trending faults, which suggested that the intersection of the two faults with different strikes could provide a favorable condition for the generation and triggering of intraplate earthquakes. Crustal velocity model showed that the high-velocity anomaly was imaged in the west of DI, but a distinct entity with low-velocity anomaly in the upper crust and high-velocity anomaly in the lower crust was found in the south of DI. Both the 1874 and 2006 DI earthquakes occurred along the edge of the distinct entity. Two vertical cross-sections nearly perpendicular to the strikes of the intersecting faults revealed good spatial correlations between the 2006 DI earthquake and the low to high speed transition in the distinct entity. This result indicated that the transitional zone might be a weakly structural body that can store strain energy and release it as a brittle failure, resulting in an earthquake-prone area.

Opaque Mineralogy as a Tracer of Magmatic History of Volcanic Rocks: an Example from the Neogene-Quaternary Harrat Rahat Intercontinental Volcanic Field, North Western Saudi Arabia

The Neogene-Quaternary Harrat Rahat volcanic field is part of the major intercontinental Harrat fields in western Saudi Arabia. It comprises lava flows of olivine basalt and hawaiite, in addition to mugearite, benmorite, and trachyte that occur mainly as domes, tuff cones and lava flows. Based on opaque mineralogy and mineral chemistry, the Harrat Rahat volcanic varieties are distinguished into Group I （olivine basalt and hawaiite） and Group II （mugearite, benmorite and trachyte）. The maximum forsterite content （-85） is encountered in zoned forsteritic olivine of Group I, whereas olivine of Group II is characterized by intermediate （Fo=50）, fayalitic （Fo=25） and pure fayalite in the mugearite, benmorite and trachyte, respectively. The more evolved varieties of Group II contain minerals that show enrichment of Fe2＋, Mn2~ and Na~ that indicates normal fractional crystallization. The common occurrence of indicates that pS＋ becomes saturated in this coarse apatite with titanomagnetite in the benmorite rock variety and drops again in trachyte. Cr-spinel is recorded in Group I varieties only and the Cr# （0.5） suggests lherzolite as a possible restite of the Harrat Rahat volcanics. The plots of Cr# vs. the forsterite content （Fo） suggest two distinct trends, which are typical of mixing of two basaltic magmas of different sources and different degrees of partial melting. The bimodality of Harrat Rahat Cr-spinel suggests possible derivation from recycled MORB slab in the mantle as indicated by the presence of high-Al spinel. It is believed that the subcontinental lithospheric mantle was modified by pervious subduction process and played the leading role in the genesis of the Harrat Rahat intraplate volcanics. The trachytes of the Harrat Rahat volcanic field were formed most probably by melting of a lower crust at the mantle-crust boundary. The increase in fo2 causes a decrease in Cr203, and A1203, and a strong increase in the proportion of Fe3＋ and Mg# of spinel crystallizing from the basaltic melt at T -1200~C. The olivine-pyroxene and olivine-spinel geothermometers yielded equilibrium temperature in the range of 935-1025~C, whereas the range of 〈500-850~C from single-pyroxene thermometry indicates either post crystallization re- equilibrium of the clinopyroxene, or the mineral is xenocrystic and re-equilibrated in a cooling basaltic magma.