The Influence of the Closure of the East Paleo-Tethys Ocean on Southern South China:Evidences from Kinematics and^(40)Ar/^(39)Ar Geochronology of the Rongxian Ductile Shear Zone in Southeastern Guangxi

The Triassic was a crucial period in the tectonic evolution of the South China Block.Research on tectonic deformation during this period provides information on intracontinental orogenic mechanisms in South China.In this study,alongside thermochronological analyses,we examine the macroscopic and microscopic structural features of the Rongxian ductile shear zone,located south of the Darongshan granite in the southeastern part of Guangxi Province,on the southern margin of South China.Sinistral shear is indicated by the characteristics of rotatedσ-type feldspar porphyroclasts,stretching lineations defined by elongated quartz grains and the orientations of quartz c-axes.LA-ICP-MS U-Pb dating of zircons from two samples of granitic mylonite and one of granite yielded ages of ca.256 Ma.Furthermore,two samples of granitic mylonite yield muscovite^(40)Ar/^(39)Ar plateau ages of 249-246 Ma.These results indicate that the Rongxian ductile shear zone resulted from Early Triassic deformation of the late Permian Darongshan granite.This deformation was likely related to the closure of the eastern Paleo-Tethys Ocean and the subsequent collision of the South China and Indochina blocks,during the early stage of the Indosinian orogeny.

Geochronology and Petrogenesis of Late Carboniferous to Early Permian Basalts in the Central Lhasa Subterrane, Southern Tibet: Implications for the Evolution of the Sumdo Paleo-Tethys Ocean

Basalts from the Late Carboniferous to Early Permian are extensively developed in the central Lhasa subterrane, southern Tibet. Studying the petrogenesis of these rocks may have implications for the late Paleozoic arc magmatism along the central Lhasa subterrane uncovering more of the evolution of the Sumdo Paleo-Tethys Ocean and its dynamic mechanism. Basalt samples from the Luobadui Formation in the Leqingla area, NW of Linzhou City in the central Lhasa subterrane, southern Tibet exhibit arc-like geochemical signatures in a subduction-zone tectonic setting characterized by high Al_(2)O_(3) and low TiO_(2) contents, fractionated REE patterns with low Nb/La ratios and high LREE concentrations, and negative HFSE anomalies. Based on their higher Th/Ce, Nb/Zr, and lower Ba/Th, Pb/Nd ratios, slightly negative to positive ε_(Nd)(t) values, and the relatively high Sr-Pb isotopic compositions, these samples were probably derived from partial melting of a depleted mantle source of garnet + spinel lherzolite, metasomatized by subducted sediments around 297 Ma. Modeling of the trace elements indicates that these basalts experienced fractional crystallization of olivine, clinopyroxene and minor plagioclase during magma ascent and eruption. It is proposed that these Late Carboniferous–Early Permian basalts are associated with the northward subduction of the Sumdo Paleo-Tethys Ocean seafloor along the southern margin of the central Lhasa subterrane.

Early Triassic Silicic Volcanics of South China:Petrogenesis and Constraints on the Geodynamic Evolution of the Paleo-Tethys Ocean Region

The only occurrence of Lower Triassic silicic volcanic rocks within the South China Block is in the Qinzhou Bay area of Guangxi Province.LA-ICP-MS zircon U-Pb dating reveals that volcanic rocks of the Beisi and Banba formations formed between 248.8±1.6 and 246.5±1.3 Ma,coeval with peraluminous granites of the Qinzhou Bay Granitic Complex.The studied rhyolites and dacites are characterized by high SiO_(2),K_(2)O,and Al_(2)O_(3),and low MgO,CaO,and P_(2)O_(5) contents and are classified as high-K calc-alkaline S-type rocks,with A/CNK=0.98-1.19.The volcanic rocks are depleted in high field strength elements,e.g.,Nb,Ta,Ti,and P,and enriched in large ion lithophile elements,e.g.,Rb,K,Sr,and Ba.Although the analyzed volcanic rocks have extremely enriched zircon Hf isotopic compositions(ε_(Hf)(t)=-29.1 to-6.9),source discrimination indicators and high calculated Ti-in-zircon temperatures(798-835℃)reveal that magma derived from enriched lithospheric mantle not only provided a heat source for anatectic melting of the metasedimentary protoliths but was also an endmember component of the S-type silicic magma.The studied early Triassic volcanics are inferred to have formed immediately before closure of the Paleo-Tethys Ocean in this region,as the associated subduction would have generated an extensional setting in which the mantle-derived upwelling and volcanic activity occurred.

U-Pb Isotope Geochronology of Syntectonic Granites from Hainan Island,South China:Constraints on Tectonic Evolution of the Eastern Paleo-Tethys Ocean

Since the recognition of the Indosinian orogeny in northern Vietnam,Triassic orogens have been widely identified around the western and southwestern boundaries of the South China block.The paleo-Tethys sutures stretch from west to southeast,from Jinshajiang-Ailaoshan via NE Vietnam to Hainan Island;these sutures exhibit and develop voluminous Permian-early Triassic magmatism and numerous high-strain shear zones.As ophiolites related to the paleo-Tethys are lacking on Hainan Island,the eastward extension of the Indosinian orogeny and subduction and closure time of the paleo-Tethys Ocean on Hainan Island remain controversial.Here,an integrated kinematic and geochronological study has been conducted on two shear zones,called the Xiaomei and Mangsan shear zones.U-Pb zircon dating yields an age of early Triassic(252-251 Myr)for Xiaomei syntectonic granites which formed in the same tectonic setting and presented the similar nanoparticles to the Indosinian granites from Ailaoshan ductile shear zone,including the strawberry-like and flower-like nanoparticles.The NE-trending Mangsan shear zone represented by the gneissic granites with middle Permian ages(264-262 Myr)formed in the same tectonic setting as the Wuzhi-shan granites that were proposed as I-type granites.These middle Permian gneissic granites with arc affinity may represent an arc setting related to the NW subduction of the paleo-Tethys.The analyses reveal that granites with late Triassic ages(235-232 Myr)in the Xiaomei shear zone have the characteristics of A-type granites.The late Triassic extensional events on Hainan Island may be related to the subduction of the paleo-Pacific Plate beneath the East Asian continent.

Subduction and Collision of the Jinsha River Paleo-Tethys: Constraints from Zircon U-Pb Dating and Geochemistry of the Ludian Batholith in the Jiangda–Deqen–Weixi Continental Margin Arc

The Jiangda–Deqen–Weixi continental margin arc(DWCA) developed along the base of the Changdu–Simao Block and was formed as a result of the subduction of the Jinsha River Ocean Slab and the subsequent collision. The Ludian batholith is located in the southern part of the DWCA and is the largest batholith in northwest Yunnan. Granite samples from the Ludian batholith yield an early Middle Permian age of 271.0 ± 2.8 Ma. The geochemical data of the early Middle Permian granitoids show high Si2 O, low P2 O5 and MgO contents that belong to calc-alkaline series and peraluminous I-type rocks. Their εHf(t) values range from-5.01 to +0.58, indicating that they were formed by hybrid magmas related to the subduction of the Jinsha River Tethys Ocean. The monzonite and monzogranite samples yield Late Permian ages of 250.6 ± 1.8 Ma and 252.1 ± 1.3 Ma, respectively. The Late Permian granitoids are high-K calc alkaline and shoshonite series metaluminous I-type rocks. Their εHf(t) values range from-4.12 to-1.68 and from-7.88 to-6.64, respectively. The mixing of crustal and mantle melts formed the parental magma of the Late Permian granitoids. This study, combined with previous work, demonstrates the process from subduction to collision of the Jinsha River Paleo-Tethys Ocean.

Geochemistry and Geochronology of the Jinghong Ophiolites:Implications for the Tectonic Evolution of the Eastern Paleo-Tethys

The Jinghong mafic-ultramafic complex,exposed in the eastern margin of the Lancangjiang tectonic belt,is related to the subduction of the Paleo-Tethys Ocean.Its petrogenesis plays a key role in constraining the tectonic evolution of the eastern Paleo-Tethys Ocean in southwestern China.In this study,we present petrological,geochemical and geochronological results of the Jinghong complex rocks,in order to decipher their origin and tectonic significance.The Jinghong mafic-ultramafic complex was composed of peridotite,gabbro,basalt and minor plagiogranite.Whole-rock geochemical data of the mafic rocks indicate that they have both MORB and IAB affinities and plot in the back-arc basin basalt(BABB)field in the FeO^(*)/MgO vs.TiO_(2) diagram.Combined with their trace element characteristics,it can be concluded that the Jinghong mafic-ultramafic complex represents an ophiolite suite that was formed in a back-arc ocean basin.Precise LA-ICP-MS zircon U-Pb dating yielded weighted mean ^(206)Pb/^(238)U ages of 298.4±1.7 Ma,294.3±1.6 Ma,and 292.8±2.0 Ma for gabbroic rocks from this complex,which indicates that the Jinghong ophiolites were formed during the early Permian(298-293 Ma).We propose that during subduction of the main Paleo-Tethys Ocean,a back-arc ocean basin was formed at the east of the Lancangjiang tectonic belt.

New Zircon U-Pb Age of Late Devonian Tuff in Guangxi, South China and the Significance for the Paleo-Tethys Branch Ocean

Objective The eastern extending range of the Paleo-Tethys Ocean in southwest China has always been focused and controversial. At the beginning of this century some scholars proposed that the Paleo-Tethys Branch Ocean may extend to the Babu-Pingxiang and Cengxi areas along the stouthwestern margin of the South China Block （Cai and Zhang, 2009）. However, the pivotal proof of Late Paleozoic ophoilite and magmatic rocks was absent or sporadical in the Guangxi Zhuang Autonomous Region, resulting in insufficient evidence to support this viewpoint. For the first time this work reported the Late Devonian tuff in the Qinzhou area in the south of the Guangxi Zhuang Autonomous Region and conducted zircon U-Pb dating on it in order to constrain the extension of the Paleo-Tethys Ocean in the South China Block.

Late Devonian OIB Alkaline Gabbro in the Yarlung Zangpo Suture Zone:Remnants of the Paleo-Tethys?

The Yarlung Zangpo Suture Zone(YZSZ) is believed to be composed of material largely derived from the destruction of the Neo-Tethys ocean.We report here geochronological and petrological data obtained for newly discovered alkaline gabbro blocks embedded in a m(?)lange zone of the YZSZ,in Zhongba area, southwestern Tibet.The gabbros are Late Devonian in age and of intra-oceanic Ocean Island Basalt(OIB) affinity.Single-zircon U-Pb analyses from one representative gabbro sample by SIMS(Secondary

Arc-trench System of the Paleo-Tethys Ocean: Inferred from Ophiolite in the Southern Lancangjiang Belt, SW China

The Paleo-Tethys Ocean is usually interpreted as a Paleozoic ocean basin located between the Gondwana and Laurasia supercontinents. The Paleo-Tethyan orogenic record is well preserved in the Sanjiang area of SW China. However, ophiolites are commonly dismembered in orogenic belt, and complete ophiolite sequences are rare in the Sanjiang area. The southern Lancangjiang belt is the most complicated tectonic complex of the Sanjiang Paleo-Tethyan orogen, SW China, and is key to understanding the evolution of the orogen. In this study, we focused on mafic–ultramafic rocks in the Yakou and Banpo areas of the southern Lancangjiang belt, of which newly discovered Yakou rocks show a complete ophiolite sequence. These rocks are composed of serpentinized peridotite, isotropic and cumulate gabbros, massive and pillow basalts, and plagiogranite. Whole-rock geochemical data indicate that these rocks were formed in an oceanic ridge setting, and they show depletions in Nb, Ta and Ti, and enrichment in Pb, suggesting a supra-subduction zone affinity of a back-arc setting. Furthermore, positive εNd(t)(+4.5 to +6.7) and zircon εHf(t) values(+12.4 to +14.3), as well as mantle-like δ18O values(~5.5‰), suggest that these rocks were derived from a long-term depleted mantle source. All of these features suggest that the Yakou mafic-ultramafic complex represents an ophiolite suite, making it the first complete ophiolite sequence to be discovered in the southern Lancangjiang orogenic belt. The Banpo complex gabbroic rocks have similar whole-rock geochemical and Sr-Nd isotopic, and zircon O-Hf isotopic compositions to those of the Yakou complex, suggesting an N-MORB affinity. Thus, maficultramafic rocks from the Banpo and Jinghong areas are most likely dismembered ophiolite suites. Considering these various characteristics, we consider that the Yakou, Banpo, and Jinghong mafic-ultramafic complexes represent an ophiolite belt but not a magmatic arc belt. SHRIMP zircon U-Pb dating yield weighted mean ages of 305±3 Ma, 310±2 Ma, and 313±6 Ma. Therefore, we suggest that the Banpo-Jinghong mafic-ultramafic complex represents a Late Carboniferous(313–305 Ma) ophiolite belt in the Sanjiang Paleo-Tethyan orogen of SW China. Finally, we propose that an arc-trench system could have developed in the Sanjiang Paleo-Tethyan orogenic belt of SW China during the Late Carboniferous.

Oceanic lithosphere heterogeneity in the eastern Paleo-Tethys revealed by PGE and Re-Os isotopes of mantle peridotites in the Jinshajiang ophiolite

Platinum group elements(PGE)and Re-Os isotopes of mantle peridotites in the Jinshajiang ophiolite(SW China)were investigated in this study,in order to constrain the evolution of the lithospheric mantle beneath the Jinshajiang-Ailaoshan Ocean,which was a branch of the eastern Paleo-Tethys.The Jinshajiang peridotites have whole-rock compositions(e.g.,MgO=32.7-38.1 wt.%;Al_(2)O_(3)=0.67-1.30 wt.%)and spinels with moderate Cr#values(0.4-0.6)similar to those of abyssal peridotites,which indicate moderate degrees of partial melting(15%-20%).These peridotites exhibit U-shaped chondrite-normalized REE patterns that could be caused by hydrothermal alteration or melt-rock interaction after mantle melting.In addition,Pd concentrations and(Pd/Ir)_(N)ratios of the Jinshajiang peridotites increases with decreasing Al_(2)O_(3) concentrations.These negative correlations cannot be explained by simple partial melting but record a melt-rock reaction event after mantle melting.This study therefore demonstrates the efficiency of PGE in detecting the melt-rock reaction process relative to whole-rock major and trace elements.The suprachondritic^(187)Os/^(188)Os ratios(0.1272-0.1374)further indicate that the later percolating melt derived from a mantle domain with distinct^(187)Os-enriched isotopic compositions.In comparison with peridotites in the Ailaoshan ophiolite belt,which were not significantly affected by melt percolation,this study further highlights that the lithospheric mantle compositions beneath different segments of the same ocean basin are highly variable and might be controlled by distinct mantle processes in response to different rifting mechanisms.

Detrital Zircon of Devonian Sandstones in ChangningMenglian Suture Zone, Yunnan, SW China: Implications for the Early Evolution of Paleo-Tethys

The Changning-Menglian suture zone is a critical tectonic belt pivotal to the evolution of the Paleo-Tethys.The Wenquan Formation,characterized as turbidite deposited on the western side of the Changning-Menglian suture zone as well as the eastern edge of the Baoshan Block.Analysis of detrital zircons from the Wenquan Formation reveals two significant age peaks approximately at 440 and 980 Ma,with additional age populations around 600,780,and 2500 Ma.The major age peak at about 440 Ma is come from the andesitic-dominant volcanic arc within the Lancang Block and the western Simao Block,corroborated by the presence of andesite fragments in thin section studies.Meanwhile,the zircons that form the secondary age peak at~980 Ma and other older age groups probably originate from the Baoshan Block.Deposited on the western side of the Paleo-Tethyan ophiolites,the Wenquan Formation received detrital materials from the continental margin on the opposite side.Thus,the main Paleo-Tethyan Ocean basin was not sufficiently broad enough to cut off the transference of detrital materials.It was a relatively narrow basin in the Early Devonian.

Paleozoic Multi-Stage Magmatic Events Related to Proto-Tethys and Paleo-Tethys Evolution:Insights from Intrusive Rocks in the Eastern Altyn Orogen,NW China

Abundant mafic-felsic intrusions distributed in the Altyn Orogen record orogenic histories related to Proto-Tethys and Paleo-Tethys evolution.Zircon U-Pb dating of the intrusive rocks in the eastern Altyn Orogen identifies at least three major tectono-magmatic episodes,yielding ages of∼426,∼376–373 and∼269–254 Ma.The first two emplacement episodes correspond to the post-collisional magmatism in the Altyn Orogen.The∼426 Ma granitoids possess adakitic characteristics coupled with enriched isotopes,suggesting that they originated from partial melting of thickened lower continental crust induced by upwelling asthenospheric mantle after slab break-off of the South Altyn Ocean Plate.Next,the∼376–373 Ma mafic-intermediate rocks and coeval granitoids represent a large thermal event that involved mantle melting with induced new juvenile lower continental crust melting in a post-collisional extensional setting.Finally,the∼254 Ma diabase dykes intruded into the∼269 Ma granitoids,which were related to the widespread Late Paleozoic magmatism resulting from Paleo-Tethys Ocean subduction.Post-collisional magmatism in the Altyn Orogen significantly enhances understanding of the tectono-magmatic evolution in the northern Tibetan Plateau.The penetrative influence of Paleo-Tethys Ocean subduction was more extensive than previously thought.

Frontier of the Paleo-Tethys Ocean in the western Mediterranean:Isotopic(Sm-Nd)constraints on sources of Devonian units from Menorca Island

The c.1000-m-thick pre-orogenic Devonian mainly metapelitic sequence of North Menorca Island shows a fairly complete stratigraphic succession.The rocks of this sequence indicate gradually increasing deeper marine conditions of sedimentation towards its uppermost levels.Furthermore,the obtained sedimentary characteristics resemble those related to a deep and narrow basin-associated deposit.Thin sills of Ti-augite-bearing alkaline gabbros occur within the Devonian sequence.The intensity of Variscan deformation increases downwards through the structure.According to the characteristics of the Devonian sequence and its location within the Variscan Orogen,a correlation with similar units located in the southern sectors of the Central Iberian Zone(Iberian Massif)is suggested.The Devonian metapelitic rocks have geochemical characteristics suggesting that they represent moderately recycled mature siliciclastic sediments,generated from erosion of distant source areas belonging to an upper continental crust.The relatively narrow range of variation observed in initial^(143)Nd/^(144)Nd ratios supports a similar source for the Menorcan slates(0.51165–0.51182).However,a marked trend is observed in these isotope ratios,from lower values at the base of the stratigraphic column(minimum value of 0.511941)to higher values at the top(maximum value of 0.512131).The^(147)Sm/^(144)Nd ratios vary between 0.1074 and 0.1238,within the range defined for siliciclastic rocks with felsic crustal provenance.The Nd model ages define a narrow range between 1496 Ma and 1754 Ma(Late Paleoproterozoic–Early Mesoproterozoic),and they are consistently younger up-section.These data rule out a provenance from the erosion of the West Africa Craton,as they are more compatible with a provenance from regions located in the Trans-Saharan Belt or Sahara Metacraton.The characteristics of the Menorcan Devonian sequence are compatible with its deposition in a narrow and deep peri-Gondwanan transtensional basin,generated to the south of an advancing Variscan orogenic wedge.Systematic variations in the Nd isotopic composition indicate the progressive and continuous denudation of increasingly more easterly North African sectors in a collisional context between Laurussia and Gondwana with a marked dextral component.These data must be interpreted in the sense that there was not a large oceanic domain during Devonian times to the south of Iberia,able to block the arrival of detrital material from North Africa.A large tract of the Paleo-Tethys Ocean would therefore not have existed during the Devonian south of Iberia.This ocean must therefore have had limited extent in this period towards the westernmost sectors.The Devonian peri-Gondwanan shelf was apparently continuous around Iberia.This platform was progressively affected by Variscan deformation advancing from north to south and incorporated into the Variscan orogenic wedge with the same vergence.

Geochronology and geochemistry of lithium-rich tuffs in the Sichuan basin,western Yangtze:Implication for the magmatic origin and final closure of eastern Paleo-Tethys

The felsic volcanogenic tuffs named"green-bean rocks"(GBRs),characterized by a green or yellowish green color,are widely distributed in the western Yangtze platform and have a high lithium content(286-957 ppm).This paper studies the ages,origin and tectonic setting of the GBRs in the Sichuan basin on the western margin of the Yangtze platform through the whole-rock geochemistry and zircon trace elements by using U-Pb dating and Hf-O isotopes.The GBR samples from the Quxian and Beibei sections yielded zircon U-Pb ages of 245.5±1.8 Ma and 244.8±2.2 Ma.These samples can be used as the isochronous stratigraphic marker of the Early-Middle Triassic boundary(EMTB)for regional correlation.The whole-rock and zircon geochemistry,and zircon Hf-O isotopes exhibited S-type geochemical affinities with high positiveδ^(18)0 values(9.28‰-11.98‰),low negativeε_(Hf)(t)values(-13.87 to-6.79),and T_(DM)^(2)ages of 2150-1703 Ma,indicating that the lithium-rich GBRs were generated by the remelting of the pre-existing ancient Paleoproterozoic layer without mantle source contamination in the arcrelated/orogenic tectonic setting.The results of this study demonstrate that the lithium-rich GBRs in the western Yangtze platform were derived from arc volcanic eruptions along the Sanjiang orogen,triggered by the closure of the eastern Paleo-Tethys Ocean and the syn-collision between the continental Indochina and Yangtze blocks at ca.247 Ma.This was marked by a major shift from I-type magmas with intermediateε_(Hf)(t)values to S-type magmas with low negativeε_(Hf)(t)values.Collectively,our results provide new insights into the origin of the GBRs and decodes the closure of the eastern Paleo-Tethys.

Devonian-Carboniferous Pillow OIB-Type Basalts in the Youjiang Basin,SW China:Implications for the Eastern Extension of the Paleo-Tethys Branch Ocean

Late Paleozoic pillow basalts are well preserved in the Youjiang Basin of SW China.In this study,we conducted geochemical and Sr-Nd isotopic study on the Late Devonian and Late Carboniferous basalts in Napo area,Youjiang Basin.The analyzed samples comprise moderate SiO2,TiO2,Mg#,and REE contents,with slightly positive or negative Eu anomalies,similar to the chemical nature of OIB(Oceanic island basalt).All basalts show low(^(143)Nd/^(144)Nd)iand(^(87)Sr/^(86)Sr)ivalues,close to EMI-type(I-type Enriched Mantle)magma source.Late Carboniferous basalts display higher(^(87)Sr/^(86)Sr)i,Nb/Th,and lowerε_(Nd)(t),Zr/Nb values than those of the Late Devonian basalts,implying progressively decreased enrichment components in the magma source processes.Such evolution trend reflects a continuous lithospheric thinning event,supporting a progressive rifting setting of the Youjiang Basin along the southwestern margin of the Yangtze Block in Late Paleozoic.Combined with regional geological background,the Youjiang Basin is viewed as a continental rift basin related to the opening of the Paleo-Tethys Ocean in Late Devonian and became a passive continental margin basin in Carboniferous.The Paleo-Tethys branch ocean probably extended further to the southern Youjiang Basin,likely corresponding to the Song Ma suture zone.

The fragment of Paleo-Tethys ophiolite from central Qiangtang, Tibet:Geochemical evidence of meta-basites in Guoganjianian

N-MORB-type metabasites are discovered in the Guoganjianian area, central Qiangtang, Tibet, which are mainly metagabbro with cumulate structure and metabasalt. The rocks are distributed nearly from west to east unconformably underlying the Wanghuling Group of upper Triassic. On the basis of geo- chemical analysis, we find that the content of SiO2 is 43.03%―53.42%, and TiO2 1%―2.67%, Al2O3 16.75%―21.52%, CaO 7.03%―11.13%, K2O 0.05%―0.38%; the REE pattern is slight depletion or flat, and the trace spider diagram is like that of N-MORB, so we consider that the metabasite was formed under the setting of mid-ocean ridge or adult back-arc basin, and it is the fragment of Paleo-Tethys ophiolite.

Characteristics of rare earth elements and cerium anomalies in cherts from the Paleo-Tethys in Changning-Menglian belt in western Yunnan, China

Based on research on rare earth elements (REEs) and cerium anomalies in cherts from the Paleo-Tethys, their depositional environments are shown and the evolution of the Paleo-Tethys is traced. The amount of terrigenous, biogenic and hydrothermal inputs are the important factors that affect the REEs composition of the cherts.

Permian–Triassic Highly-Fractionated I-Type Granites from the Southwestern Qaidam Basin(NW China)：Implications for the Evolution of the Paleo-Tethys in the Eastern Kunlun Orogenic Belt

Late Paleozoic to Early Mesozoic granites are widespread in the southern Qaidam Basin, northern margin of the eastern Kunlun orogenic belt. Their petrogenesis can provide us insights into the tectonic evolution and crustal growth process in the Qaidam Basin. This paper reports Permian–Triassic granites from the Kunbei area, southwestern Qaidam Basin. Detailed zircon LA-ICP MS U-Pb dating reveals that the granites from the four drilling cores（q404, q406, q1612-8, q1613-8） have identical ages of 251±3, 256±4, 247±2, and 251±6 Ma, respectively, these ages are identical with the Permian–Triassic granites from the eastern Qaidam Basin. Detailed geochemical analyses indicate that these granites display typical affinities of highly-fractionated I-type granites：（1） they have high SiO_2（up to 76.5 wt.%）, Na_2O＋K_2O（7.91 wt.% to 9.48 wt.%） contents and high FeO^T/MgO values of 4.7 to 9.3, suggesting significant fractional crystallization;（2） their low A/CNK values of 0.54 to 1.03, no normative Al-rich minerals, inconsistent with the per-aluminous S-type granites;（3） their low Ga（14.5 ppm to 20.7 ppm） and 10 000×Ga/Al（2.23 to 3.03, most of them 〈2.6） values are inconsistent with the A-type granites;（4） the high Rb（191 ppm to 406 ppm） contents and Rb/Sr（2.1 to 13.4） ratios, as well as the significant negative Eu anomalies（0.10 to 0.42） also indicate significant fractional crystallization of feldspars;（5） their low P_2O_5 contents（0.02 wt.% to 0.10 wt.%） suggest the limited solubility of phosphorus in primitive metaluminous melts. In combination with the geological background, we propose that the Permian–Triassic highly-fractionated I-type granites resulted from partial melting of intra-crustal mafic rocks, and the primitive I-type granitic melts underwent significant fractional crystallization of feldspars. The occurrence of highly-fractionated I-type granites in the southwestern Qaidam Basin suggests a Permian–Triassic active continental margin in the northern margin of the East Kunlun orogenic belt.

Zircon and Monazite Ages Constraints on Devonian Magmatism and Granulite-Facies Metamorphism in the Southern Qaidam Block:Implications for Evolution of Proto-and Paleo-Tethys in East Asia

High-temperature magma generation process and granulite-facies metamorphism can provide important information about mantle-crustal interaction and tectonic evolution. The strongly peralu- minous monzonite pluton, the Jinshuikou cordierite granite on the southern margin of the Qaidam Block, can provide important information about the mantle-crustal interaction and constraints on tectonic tran- sition from Proto-Tethys to Paleo-Tethys. This pluton develops enclaves of mafic granulite, amphibolite and quartzofeldspathic rocks, and is cut by massive monzonitic leuco-granite veins. Zircon and monazite U-Pb dating for the cordierite granite, the granulite enclaves and a massive monzonitic leuco-granite vein reveal that the cordierite granitic magma was generated from Mesoproterozoic continental crust with protolith derived from a provenanee that was composed of 〉2.8 Ga old recycled crustal materials and re- corded a -1.7 Ga magmatic event. The continental crust underwent low-pressure granulite-facies metamorphism at -380 Ma ago, whereas the cordierite granite magmas was generated and emplaced during 380 Ma, followed by intrusion of the massive monzonitic leuco-granite vein at circa 370-330 Ma. These data suggest that after the final closure of Proto-Tethys Ocean spreading along the southern Qaidam Block at -420 Ma, break-off of the subducted slab or delamination of the lower crustal base and upwelling of the asthenospheric mantle beneath the southern Qaidam Block occurred before the Mid-Devonian, and that the initiation of the Paleo-Tethys tectonics might initiate near the end of Early-Carboniferous in the East Kunlun-Qaidam region, East Asia.

The Late Paleozoic Radiolarian Cherts in Southern Guangxi and Preliminary Exploration on Paleo-Tethys in Guangxi

The existence of the Paleo-Tethys in western Yunnan has generally beenrecognized as the progress in geological studies there during the last decade.Recently, Japanese geologists proposed that the Carboniferous-Permian volcanic arcand seamount rock associations in Japan originated from the low-latitudeProto-Pacific realm, and their northward migration seems to have started in or afterthe Triassic times. Then the very interesting problems are facing us: how does