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.

Lower Continental Crusts Involved in the Alpine Orogeny:New Insight from the Santa Lucia Nappe Metasediments(Corsica,France)

In the Alpine Corsica(France),the Santa Lucia Nappe represents a peculiar unit preserving the unique relicts of Paleozoic lower to medium continental crust.It consists of composite basement affected by Permian granulite facies metamorphic conditions unconformably covered by a Late Cretaceous clastic sequence(Tomboni Conglomerate and Tralonca Flysch)affected by polyphase deformation and low-grade-metamorphism.In this work,we present a new reconstruction of the deformation events registered by the Tralonca Flysch during the Alpine orogeny.The D1 phase was testified by rare isoclinal folds.The D2 phase produced a continuous foliation and a map-scale sheath-fold with a top-to-W sense of shear.The D3 phase produced E-verging non-cylindrical folds and S3 crenulation cleavage that is not associated to metamorphic blastesis.We present the first temperature-pressure-deformation(P-T-d)path for the Tralonca Flysch,demonstrating that the Santa Lucia Nappe underwent accretion and then first stage exhumation in the Alpine wedge during the D1 phase with pressure and temperature peaks both occurred under blueschist metamorphic conditions.The D2 phase occurred at lower pressure-temperature conditions during a second stage exhumation.This pressure-temperaturedeformation path is comparable with those of the Lower Units(i.e.,the subducted continental units of Alpine Corsica)suggesting a common geodynamic history.

Intra-continental Orogeny:Insights from Magnetotelluric Data into the Mesozoic Uplift History of the Eastern Jiangnan Orogen in South China

Despite extensive efforts to understand the tectonic evolution of the Jiangnan Orogen in South China,the orogenic process and its mechanism remain a matter of dispute.Previous geodynamic studies have mostly focused on collisional orogeny,which is commonly invoked to explain the Jiangnan Orogen.However,it is difficult for such hypotheses to reconcile all the geological and geophysical data,especially the absence of ultrahigh-pressure metamorphic rocks.Based on the magnetotelluric data,we present a group of resistivity models produced through the combination of two-dimensional and three-dimensional inversions,revealing the geo-electrical structures of Jiangnan and a typical collisional orogen.In our models,the resistive crust is separated into three parts by a prominent conductive layer with opposite dipping directions on both sides.A special thrust-nappe system,which is different from that developed in a typical collisional process,is revealed in the Jiangnan Orogen.This structure suggests a process different from the simple collisional orogeny.To interpret our observations,an'intra-continental orogeny'is proposed to address the development of the Jiangnan Orogen in the Mesozoic.Furthermore,this'reworked'process may contain at least two stages caused by the decoupling of the lithosphere,which is revealed by an extra conductive layer beneath Jiangnan.

^(40)Ar/^(39)Ar Thermochronological Constraints on the Timing of Collisional Orogeny in the Mian-Lite Collision Belt,Southern Qinling Mountains

Silurian, Devonian and Carboniferous geological bodies in the Mianxian-Lueyang (Mian-Lue) collisional belt (MLB) and its neighbouring areas, southern Qinling Mountains, China, show similar characteristics of having undergone deformation of two stages. The earlier one, which is inferred to be related to collisional orogeny between the Yangtze and Sino-Korean palaeocontinents based on previous geological data, is responsible for large-scale, north-verging recumbent folds and overthrusts, and associated with low greenschist fades metamorphism. 40Ar/39Ar dating of three muscovite samples taken from different localities yields plateau ages of 226.9±0.9 and 219.5±1.4 Ma and an apparent age of 194.5±3.0 Ma. Thus, the late Triassic collision between the Yangtze and Sino-Korean palaeocontinents has been constrained.

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.

Intracontinental Collisional Orogeny During Late Permian-Middle Triassic in South China: Sedimentary Records of the Shiwandashan Basin

Sedimentary response to an orogenic process is important for determining whether South China had compressional or extensional orogeny during the period from the Late Permian to the Middle Triassic besides the tectonic and magmatologic evidence. An intracontinental collision event took place between the Yangtze and Cathaysia blocks in the Late Permian. Beginning at the Late Triassic, the tectonic movement was completely changed in nature and entered a post-collisional extensional orogenic and basin-making process. This paper presents sedimentological evidence from the Late Permian to the Middle Triassic in the Shiwandashan basin at the southwestern end of the junction zone between the Yangtze and Cathaysia blocks.

Large-scale Migration of Fluids toward Foreland Basins during Collisional Orogeny: Evidence from Triassic Anhydrock Sequences and Regional Alteration in the Middle-Lower Yangtze Area

The middle-lower Yangtze area underwent a series of complex tectonic evolution, such as Hercynian extensional rifting, Indosinian foreland basining, and Yanshanian transpression-transtension, resulting in a large distinctive Cu-Fe-Au metallogenic belt. In the tectonic evolution, large-scale migration and convergence of fluids toward foreland basins induced during the collisional orogeny of the Yangtze and North China continental blocks were of vital importance for the formation of the metallogenic belt. Through geological surveys of the middle-lower Yangtze area, three lines of evidence of large-scale fluid migration are proposed: (1) The extensive dolomitic and silicic alteration penetrating Cambrian-Triassic strata generally occurs in a region sandwiched between the metallogenic belt along the Yangtze River and the Dabie orogenic belt, and in the alteration domain alternately strong and weak alteration zones extend in a NW direction and are controlled by the fault system of the Dabie orogenic belt; it might record the locus of the activities of long-distance migrating fluids. (2) The textures and structures of very thick Middle-Lower Triassic anhydrock sequences in restricted basins along the river reveal the important contribution of the convergence of regional hot brine in restricted basins and the chemical deposition or their formation. (3) Early-Middle Triassic syndepositional iron carbonate sequences and Fe-Cu-Pb-Zn massive sulfide deposits alternate with anhydrock sequences or are separated from the latter, but all of them occur in the same stratigraphic horizon and are intimately associated with each other, being the product of syndeposition of high-salinity hot brine. According to the geological surveys, combined with previous data, the authors propose a conceptual model of fluid migration-convergence and mineralization during the Dabie collisional orogeny.

Zircon geochronology reveals polyphase magmatism and crustal anatexis in the Buchan Block,NE Scotland:Implications for the Grampian Orogeny

The type locality for high-temperature,low-pressure regional metamorphism,the Buchan Block in NE Scotland,exhibits profound differences to the rest of the Grampian Terrane.These differences have led some to regard the Buchan Block as an exotic crustal fragment comprising Precambrian basement gneisses and cover rocks thrust into their current position during Grampian orogenesis.Although rocks of the Buchan Block are now generally correlated with Dalradian strata elsewhere,the origin of the gneisses and the cause of the high heat flow and associated magmatism is debated.We report SIMS U-Pb and LA-ICPMS Hf isotopic data in zircon from high-grade rocks from the northeast(Inzie Head Gneiss)and northwest(Portsoy)corners of the Buchan Block.Around Inzie Head,upper amphibolite to granulite facies metasedimentary gneisses coexist with diorite sheets that were emplaced contemporaneously with partial melting of their host rocks,at least locally.U-Pb geochronology indicates a crystallisation age for the diorite of 486±9 Ma.Highly-deformed diorites within the Portsoy Gabbro have a crystallisation age of 493±8 Ma.Ages of ca.490 Ma for magmatism and high-grade metamorphism,which are broadly contemporaneous with ophiolite obduction and the onset of orogenesis,are significantly older than the established peak of Grampian metamorphism(ca.470 Ma).We propose a new model for the Grampian Orogeny involving punctuated tectonothermal activity due to tectonic switching during accretionary orogenesis.Rollback of a NW-dipping subduction zone at ca.490 Ma produced a back-arc environment(the Buchan Block)with associated arc magmatism and high dT/dP metamorphism.Arrival of an outboard arc resulted in shortening(the initial phase of the Grampian Orogeny)at ca.488 Ma.Rollback of a NW-dipping subduction zone to the SE of the ca.488 Ma suture began at 473 Ma and led to lithospheric-scale extension,decompression melting and advective heating of the middle crust,producing the widespread ca.470 Ma Grampian(classic Barrovian and Buchan)regional metamorphism.Resumed hinge advance and the final phase of shortening cut off the heat supply at ca.465 Ma,marking the end of the Grampian Orogeny.

Piecemeal Delamination of Thickened Lithosphere Triggered Pulsed Magmatism and Mineralization during Late Mesozoic Intracontinental Orogeny in East Asia

The East Asia continent is characterized by a mosaic architecture with various composing blocks,such as the North and South China blocks,which had been collaged in Late Permian to Triassic in response to the break-up of Pangea.In the Late Mesozoic.

Inversion of two-phase extensional basin systems during subduction of the Paleo-Pacific Plate in the SW Korean Peninsula:Implication for the Mesozoic “Laramide-style” orogeny along East Asian continental margin

During subduction, continental margins experience shortening along with inversion of extensional sedimentary basins. Here we explore a tectonic scenario for the inversion of two-phase extensional basin systems, where the Early-Middle Jurassic intra-arc volcano-sedimentary Oseosan Volcanic Complex was developed on top of the Late Triassic-Early Jurassic post-collisional sequences, namely the Chungnam Basin. The basin shortening was accommodated mostly by contractional faults and related folds. In the basement, regional high-angle reverse faults as well as low-angle thrusts accommodate the overall shortening, and are compatible with those preserved in the cover. This suggests that their spatial and temporal development is strongly dependent on the initial basin geometry and inherited structures.Changes in transport direction observed along the basement-sedimentary cover interface is a characteristic structural feature, reflecting sequential kinematic evolution during basin inversion. Propagation of basement faults also enhanced shortening of the overlying sedimentary cover sequences. We constrain timing of the Late Jurassic-Early Cretaceous(ca. 158-110 Ma) inversion from altered K-feldspar 40 Ar/39 Ar ages in stacked thrust sheets and K-Ar illite ages of fault gouges, along with previously reported geochronological data from the area. This "non-magmatic phase" of the Daebo Orogeny is contemporaneous with the timing of magmatic quiescence across the Korean Peninsula. We propose the role of flat/low-angle subduction of the Paleo-Pacific Plate for the development of the "Laramide-style" basement-involved orogenic event along East Asian continental margin.

How Central Asian Orogeny Evolves:New Insights from End-Permian to Middle Triassic Magmatic Record along the Solonker Suture Zone

Objective The modem Earth is characterized by two types of orogens： collisional orogen and accretionary orogen. It is widely accepted that the Central Asian Orogenic Belt （CAOB） is made up of widespread multiple ancient archipelagos. It has been recognized as a non-collisional orogen, contrasting with the archetypical Alpine- Himalayan-type collisional orogens. Although the CAOB is traditionally compared to subduction-accretion systems that have been well studied, all accretionary orogens ultimately passed into a collisional phase at the end of the orogenic Wilson cycle due to ocean closure and termination of subduction, which will lead to subsequent crustal shortening, thickening and reworking. Therefore, the archipelago-type CAOB may also preserve a terminal collision record, although this is not a classic continental collision at all.

Zircon U-Pb age of Fengxian acid pyroclastic rocks and its enlightenment to the existence of Pan-African orogeny in the West Qinling Orogenic Belt, China

1.Objective The formation of East Gondwana and its combination with West Gondwana is a result of a series of complex orogenic events,known as Pan-African orogeny.The Northern Qinling Mountain is a microcontinent that formed on the ancient oceanic crust in the northern of Yangtze craton before Neoproterozoic.It is likely that the North Qinling,even the Yangtze craton and its surrounding oceanic crust belong to the ancient Gondwana tectonic domain and be a part of East Gondwana supercontinent(Chen Q,2007).

ANALYSIS ON GOLD AND COPPER ORE-FORMING SYSTEM WITH COLLISION OROGENY OF EASTERN TIANSHAN

According to tectono stratigraphical division principle, different units in Kangguertage Huangshan collision zone, Eastern Tianshan, are divided into order and disorder two types of stratum rock associations, which belong to two kinds of different tectono volcanic activity belts. The collision ororeny and ductile shear zone have a strong space time coupling. Based on the island arc bearing volcanic rock on both sides of the collision zone, time of ore forming and rock forming, characteristics of collision granit, geochemical province, special multistage collision orogeny and intracontinental orogeny basin forming developed features have been found. Gold and copper deposits, with the relation to the collision orogeny, are divided into seven genetic types. The ten metallogenic zones are classified into two kinds of ore forming system of paleo continental margin. Metallogenesis of gold deposits can be classified into five stages. Gold and copper deposits are distributed in belts with the relation to the development of the collision orogeny. The distribution of main large scale copper deposits in the north part of the collision zone and most large middle scale gold deposits in the south part of the collision zone can direct the prospecting for gold and copper deposits. The actual targets are put forward.

Generation and preservation of continental crust in the Grenville Orogeny

Detrital zircons from modern sediments display an episodic temporal distribution of U-Pb crystallization ages forming a series of ＇peaks＇ and ＇troughs＇. The peaks are interpreted to represent either periods of enhanced generation of granitic magma perhaps associated with mantle overturn and superplume events, or preferential preservation of continental crust during global collisional orogenesis. The close association of those peaks with the assembly of supercontinents implies a causal relationship between collisional orogenesis and the presence of zircon age peaks. Here these two end-member models （episodic periodicity of increased magmatism versus selective preservation during collisional orogenesis） are assessed using U-Pb, Hf, and 0 analysis of detrital zircons from sedimentary successions deposited during the - 1.3-1.1 Ga accretionary, -1.1-0.9 Ga collisional, and 〈 0.9 Ga extensional collapse phases of the Grenville orogenic cycle in Labrador and Scotland. The pre-collisional, accretionary stage provides a baseline of continental crust present prior to orogenesis and is dominated by Archean and Paleoproterozoic age peaks associated with pre-1300 Ma Laurentian geology. Strata deposited during the Grenville Orogeny display similar Archean and Paleoproterozoic detrital populations along with a series of broad muted peaks from - 1500 to 1100 Ma. However, post-collisional sedimentary successions display a dominant age peak between 1085 and 985 Ma, similar to that observed in modern North American river sediments. Zircons within the post-orogenic sedimentary successions have progressively lower EHf and higher -lSO values from - 1800 to - 1200 Ma whereupon they have higher EHf and -3180 within the dominant 1085-985 Ma age peak. Furthermore, the Lu-Hf isotopic profile of the Grenville-related age peak is consistent with significant assimilation and contamination by older crustal material, The timing of this dominant age peak coincides with the peak of metamorphism and magmatism associated with the Grenville Orogeny, which is a typical collisional orogenic belt. The change from broad muted age peaks in the syn-orogenic strata to a single peak in the post-orogenic sedimentary successions and in the modern river sediments implies a significant shift in provenance following continental collision. This temporal change in provenance highlights that the source（s）, from which detrital zircons within syn-orogenic strata were derived, was no longer available during the later stages of the accretionary and collisional stages of the orogenic cycle. This may reflect some combination of tectonic burial, erosion, or possibly recycling into the mantle by tectonic erosion of the source（s）. During continental collision, the incorporated continental crust is isolated from crustal recycling processes operative at subduction margins. This tectonic isolation combined with sedimentary recycling likely controls the presence of the isotopic signature associated with the Grenville Orogeny in the modern Mississippi and Appalachian river sed- iments. These results imply that zircon age peaks, which developed in conjunction with supercontinents, are the product of selective crustal preservation resulting from collisional orogenesis.

How many subductions in the Variscan orogeny?Insights from numerical models

We developed a 2 D numerical model to simulate the evolution of two superposed ocean-continent-ocean subduction cycles with opposite vergence,both followed by continental collision,aiming to better understand the evolution of the Variscan belt.Three models with different velocities of the first oceanic subduction have been implemented.Striking differences in the thermo-mechanical evolution between the first subduction,which activates in an unperturbed system,and the second subduction,characterised by an opposite vergence,have been enlighten,in particular regarding the temperature in the mantle wedge and in the interior of the slab.Pressure and temperature(P-T) conditions predicted by one cycle and two cycles models have been compared with natural P-T estimates of the Variscan metamorphism from the Alps and from the French Massif Central(FMC).The comparative analysis supports that a slow and hot subduction well reproduces the P-T conditions compatible with data from the FMC,while P-T conditions compatible with data of Variscan metamorphism from the Alps can be reproduced by either a cold or hot oceanic subduction models.Analysing the agreement of both double and single subduction models with natural P-T estimates,we observed that polycyclic models better describe the evolution of the Variscan orogeny.

DEEP GEODYNAMICS OF THE HIMALAYA OROGENY

The Himalaya is a frontal part of an intracontinental collision system connected with an approach of Asia and Hindustan. This approach takes place in the “cold” sector of the Earth (global seismic tomography data) expressed by the geoid surface’s depression. This depression is limited from east and west by lineaments (linear slopes the geoid surface) penetrated up to the core\|mantle boundary. It consists of an Asian low step and the Indian ocean minimum of the geoid surface stipulated by gigantic “cold” crust\|mantle blocks. The Indian ocean crust\|mantle block has 1500km vertical sizes and moves northward, and displaces the Tibet—Himalaya fragment of the Mediterranean mobile belt to the same direction more than 1000km. The Himalaya is on the contact of the Asian and the Indian ocean crust\|mantle blocks. The lithospheric plates’ displacements add the approach of gigantic “cold” crust\|mantle blocks.

The Gondwana Orogeny in northern North Patagonian Massif:Evidences from the Caita Có granite, La Se?a and Pangaré mylonites,Argentina

Structural analyses in the northern part of the North Patagonia Massif, in the foliated Caita Co granite and in La Sefia and Pangare mylonites, indicate that the pluton was intruded as a sheet-like body into an opening pull-apart structure during the Gondwana Orogeny. Geochronological studies in the massif indicate a first, lower to middle Permian stage of regional deformation, related to movements during indentation tectonics, with emplacement of foliated granites in the western and central areas of the North Patagonian Massif. Between the upper Permian and lower Triassic, evidence indicates emplacement of undeformed granitic bodies in the central part of the North Patagonian Massif. A second pulse of deformation between the middle and upper Triassic is related to the emplacement of the Caita CO granite, the development of mylonitic belts, and the opening of the Los Menucos Basin. During this pulse of deformation, compression direction was from the eastern quadrant.

Changes in Relative Plate Motion during the Isan Orogeny(1670-1500Ma) and Implications for Pre-Rodinia Reconstructions

Foliation inflexion/intersection axes （FIAs） preserved within porphyroblasts that grew throughout Isan orogenesis reveal significant anticlockwise changes in the direction of bulk horizontal shortening between 1670 and 1500 Ma from NE-SW, N-S, E-W to NW-SE. This implies an anticlockwise shift in relative plate motion with time during the Isan orogeny. Dating monazite grains amongst the axial planar foliations defining three of the four FIAs enabled an age for the periods of relative plate motion that produced these structures to be determined. Averaging the ages from monazite grains defining each FIA set revealed 1649^-12 Ma for NE-SW shortening, 1645±7 Ma for N-S shortening, and 1591±10 Ma for that directed E-W. Inclusion trail asymmetries indicate shear senses of top to the SW for NW-SE FIAs and dominantly top to the N for E-W FIAs, reflecting thrusting towards the SW and N. No evidence for tectonism related to early NE-SW bulk horizontal shortening has previously been detected in the Mount Isa Inlier. Amalgamation of the Broken Hill and possibly the Gawler provinces with the Mount Isa province may have taken place during these periods of NE-SW and N-S-directed thrusting as the ages of tectonism are similar. Overlapping dates, tectonic, metamorphic, and metallogenic similarities between eastern Australia （Mount Isa and Broken Hill terranes） and the southwest part of Laurentia imply a most probable connection between both continental masses. Putting Australia in such position with respect to North America during the Late-Paleo-to-Mesoproterozoic time is consistent with the AUSWUS model of the Rodinia supercontinent.

Early Mesoproterozoic evolution of midcontinental Laurentia: Defining the geon 14 Baraboo orogeny

New geochronologic data from midcontinental Laurentia demonstrate that emplacement of the 1476-1470 Ma Wolf River granitic batholith was not an isolated igneous event,but was accompanied by regional metamorphism,deformation,and sedimentation.Evidence for such metamorphism and deformation is best seen in siliciclastic sedimentary rocks of the Baraboo Interval,which were deposited closely following the 1.65-1.63 Ga Mazatzal orogeny.In Baraboo Interval strata,muscovite parallel to slatey cleavage,in hydrothermal veins,in quartzite breccia,and in metamorphosed paleosol yielded ^(40)Ar/^(39)Ar plateau ages of 1493-1465 Ma.In addition,U-Th-total Pb dating of neoblastic overgrowths on detrital monazite gave an age of 1488±20 Ma,and recrystallized hematite in folded metapelite gave a mean U/Th-He age of 1411±39 Ma.Post-Baraboo,arkosic polymictic conglomerate,which contains detrital zircon with a minimum peak age of 1493 Ma,was intruded by a 1470 Ma granite porphyry at the northeastern margin of the Wolf River batholith.This episode of magmatism,regional deformation and metamorphism,and sedimentation,which is designated herein as the Baraboo orogeny,provides a midcontinental link between the Picuris orogeny to the southwest and the Pinware orogeny to the northeast,completing the extent of early Mesoproterozoic(Calymmian)orogenesis for 5000 km along the southern margin of Laurentia.This transcontinental orogen is unique among Precambrian orogenies for its great width(-1600 km),the predominance of ferroan granites derived from partial melting of lower continental crust,and the prevalence of regional high T-P metamorphism related to advective heating by granitic magmas emplaced in the middle to upper crust.

Geotectonic Subdivision and Paleozoic Subduction and Collision Orogeny of East Qinling and Its Adjacent Regions:Evidence from Geochemical Research

The regional lithospheric chemical heterogeneity in-ers that the East Qinling and its adjacent cratonic re-ions , as suggested by some authors , belong to two eotectonic units,the North China subdomain including he North China Craton and its southern continental largin (the North Qinling Belt), and the Yangtzean ubdomain comprising the Yangtze Craton and its torthern continental margin (the South Qinh'ng Belt). In the North Qinling Belt the metamorphosed olcanic rocks and graywackes of the Early Paleozoic Oanfeng Group south of the Early Proterozoic Qinling Froup show geochemical characteristics resembling hose of the arc volcanics and arc graywackes -espectively. The Early Paleozoic granites intruding in he Qinling Group also show similar geochemical features and similar compositional polarities to the arc-type granites . The Erlangping Group north of the Qinling Group is a volcanic-sedimentary sequence produced in an Early Paleozoic back - arc basin based on geochemical evidence . It is therefore believed that the North Qinling Belt comprised the active continental margin of the North China Craton , beneath which the ancient Qinling oceanic plate underthrusted and was consumed from 480 to 380Ma ago . The South Qinling Belt is generally considered to be a passive continental margin of the Yangtze Craton on which developed the thick Sinian and Paleozoic sediments of continental shelf and continental slope fades . The source of fine -grained clastic sedimentary rocks of various geological periods has been geochemically studied.The result demonstrates that the terrigenous elastics of the South Qinling Belt came only from the Yangtze Craton prior to the Silurian , and since then began to be fed by both the Yangtze Craton and the North Qinling . The Devonian sediments display a clear two - component mixing model in their source material . The change in the source materi-al strongly suggests that the Yangtze passive continental margin approached the active continental margin of the North China Craton and finally came into contact with it during the Silurian and the Early Devonian . On both sides of the Shangdan Fault Zone , the lithospheric megasuture of the Qinh'ng orogen , are distributed the Late Paleozoic (323-262 Ma ) granites which intruded in the Danfeng Group and the Qinling Group to the north , and in the Devonian strata to the south of the suture . The older granites of that time interval are comparable in geological and geochemical characteristics to the syn - collision granites from other continent-continent collision zones . The younger calc - alkaline granites which were em placed at about 260 Ma ago have been classified as the late - collision granites by various geochemical discrimination methods . On these grounds it may be deduced that the continent - continent collision orogeny did not begin to act until the late Early Carboniferous epoch and that its main episode was the Late Paleozoic , although it might have continued to the Early Mesozoic .The collision orogeny was separated from the subduction orogeny by an interval of about 60 Ma and the subduction of the oceanic crust was accompanied by the northward shifting of the Yangtze Craton and its passive continental margin . However , the ocean basin still remained in the South Qinling Belt for a long time after the disappearance of the ocean .