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1.
A zircon U-Pb geochronological study on the volcanic rocks reveals that both of the Zhangjiakou and Yixian Formations, northern Hebei Province, are of the Early Cretaceous, with ages of 135-130 Ma and 129-120 Ma, respectively. It is pointed out that the ages of sedimentary basins and volcanism in the northern Hebei -western Liaoning area become younger from west to east, i. e. the volcanism of the Luanping Basin commenced at c. 135 Ma, the Luotuo Mount area of the Chengde Basin c. 130 Ma, and western Liaoning c. 128 Ma. With a correlation of geochronological stratigraphy and biostratigraphy, we deduce that the Xing'anling Group, which comprises the Great Hinggan Mountains volcanic rock belt in eastern China, is predominantly of the early-middle Early Cretaceous, while the Jiande and Shimaoshan Groups and their equivalents, which form the volcanic rock belt in the southeastern coast area of China, are of the mid-late Early Cretaceous, and both the Jehol and Jiande Biotas are of the Early Cretaceous, not L  相似文献   

2.
A zircon U-Pb geochronological study on the volcanic rocks reveals that both of the Zhangjiakou and Yixian Formations, northern Hebei Province, are of the Early Cretaceous, with ages of 135-130 Ma and 129-120 Ma, respectively. It is pointed out that the ages of sedimentary basins and volcanism in the northern Hebei -western Liaoning area become younger from west to east, i. e. the volcanism of the Luanping Basin commenced at c. 135 Ma, the Luotuo Mount area of the Chengde Basin c. 130 Ma, and western Liaoning c. 128 Ma. With a correlation of geochronological stratigraphy and biostratigraphy, we deduce that the Xing'anling Group, which comprises the Great Hinggan Mountains volcanic rock belt in eastern China, is predominantly of the early-middle Early Cretaceous, while the Jiande and Shimaoshan Groups and their equivalents, which form the volcanic rock belt in the southeastern coast area of China, are of the mid-late Early Cretaceous, and both the Jehol and Jiande Biotas are of the Early Cretaceous, not L  相似文献   

3.
Whole-rock geochemical, zircon U-Pb geochronological and Sr-Nd-Hf isotopic data are presented for the Early Cretaceous volcanic rocks from the northern Da Hinggan Mountains. The volcanic rocks generally display high SiO2(73.19–77.68 wt%) and Na2O+K2O(6.53–8.98 wt%) contents, with enrichment in Rb, Th, U, Pb and LREE, and depletion in Nb, Ta, P and Ti. Three rhyolite samples, one rhyolite porphyry sample, and one volcanic breccia sample yield weighted mean 206Pb/238 U ages of 135.1±1.2 Ma, 116.5±1.1 Ma, 121.9±1.0 Ma, 118.1±0.9 Ma and 116.9±1.4 Ma, respectively. All these rocks have moderate(87Sr/86Sr)i values of 0.704912 to 0.705896, slightly negative εNd(t) values of –1.4 to –0.1, and positive εHf(t) values of 3.7 to 8. Their zircon Hf and whole-rock Nd isotopic model ages range from 594 to 1024 Ma. These results suggest that the Early Cretaceous volcanic rocks were originated from melting of subducted oceanic crust and associated sediments during the closure of the Mongol-Okhotsk Ocean.  相似文献   

4.
This paper deals with the meta-mafic volcanic rocks of the Gongyiming iron deposit at Baotou, Inner Mongolia. The major and trace elements and REE data indicate that the meta-mafic volcanic rocks occurred in the environment similar to a modern continental rift. Sm-Nd and Rb-Sr isotopic studies indicated that the meta-basic rocks were formed during the Early Neoarchean from 2800 Ma to 2900 Ma and reworked during the Late Neoarchean (2500 Ma) by metamorphism. Because of the separation of the North China Craton from the Siberia Craton during the Middle Proterozoic (1600 Ma), the Rb-Sr systematics of the rocks has been changed. The Se‘ ertengshan greenstone seems to occur during the Middle Archean. A stable continental crust may have existed during the Paleoarchean.  相似文献   

5.
The vast expanse of Mesozoic igneous rocks in Hong Kong contain important geological records of late Mesozoic magmatic events and tectonic processes from the coastal region of Southeast China. Of these,the Ping Chau Formation in the northwestern New Territories is the youngest known stratum. We perform a detailed study of the volcanic rocks of the Ping Chau Formation utilizing zircon U-Pb dating,with major and trace elements geochemistry. LA-ICP-MS zircon U-Pb data reveal Early Cretaceous age from two volcanic rock samples, with zircon crystallization from magmas at 140.3 ± 0.8 Ma and 139.3 ± 0.9 Ma,respectively. These rocks have high contents of total alkalis(Na_2O + K_2O = 5.58-9.45 wt.%), high-field-strength elements and light rare earth elements, conspicuous negative Eu anomalies, and depletions in Nb, Ta, Ti, Sr, Ba and P. Using this data, in combination with previous studies on the late Mesozoic volcanic belt in Southeast China, we propose that the volcanic rocks of the Ping Chau Formation probably originated from deep melting of the crust in a back-arc extensional setting induced by the subduction of the paleo-Pacific Plate. This formation represents the final stages of Early Cretaceous volcanic activity in Hong Kong, as associated with large-scale lithospheric extension, thinning and magmatism. Our results provide new information that can be used in evaluating the significance of Early Cretaceous volcanism and tectonics in Southeast China.  相似文献   

6.
The meta-basic volcanic rocks in the Tengtiaohe Zone yield zircon U–Pb ages of 258.8±2.5 Ma and 259.2±1.8 Ma, respectively which agree with the ages of flood basalts of ELIP and are similar to the basaltic rocks and komatiites from the Song Da Zone in northern Vietnam. The results suggest that the age of meta-basic volcanic rocks is Late Permian, rather than the Early Permian or Early Carboniferous ages as previously inferred. Most meta-basic volcanic rocks are strongly enriched in LREEs relative to HREEs and display trace element patterns similar to the ELIP high-Ti basalts, and are enriched in LILEs with negative Sr anomalies. Their initial ~(87)Sr/~(86) Sr ratios range from 0.705974 to 0.706188 and εNd(t) from-0.82 to-2.11. Their magmas were derived from an enriched and deep mantle source without significant crustal contamination. These meta-basic volcanic rocks formed in ELIP. Therefore, the Tengtiaohe Zone is not an ophiolite zone and can link to the Song Da Zone in northern Vietnam.  相似文献   

7.
The Xilin Group, composed of the Chenming, Laodaomiaogou, Qianshan and Wuxingzhen formations, is one of the Early Paleozoic terranes in the eastern Songliao Massif, mainly consisting of thick layers of fine clastic and carbonate rocks. This study presents LA-ICP-MS zircon U-Pb geochronological data for the Laodaomiaogou and Qianshan formations, further constraining their provenance and the Early Paleozoic tectonic evolution of the Songliao Massif on the eastern Central Asian Orogenic Belt. Most zircons from the Laodaomiaogou and Qianshan formations show magmatic oscillatory zoning and high Th/U ratios (0.26–2.41). Zircon U-Pb dating results indicate that the detrital zircons from the silty mudstone of the Laodaomiaogou Formation yield peak ages of 634 Ma, 775 Ma, 820 Ma, 880 Ma and 927 Ma, as well as multi-episodic Archean to Paleoproterozoic and Mesoproterozoic ages (1405–643 Ma), implying its deposition time is younger than ~634 Ma. Furthermore, the occurrence of Early Cambrian fossils indicates that the Laodaomiaogou Formation was deposited during the late stage of the Early Cambrian (~514 Ma). The zircons from the K-bentonite of the Qianshan Formation show four peak ages of 444 Ma, 471 Ma, 489 Ma and 518 Ma and the youngest age peak of 444 ± 4 Ma (n = 6) indicates that the Qianshan Formation was deposited during the Late Ordovician. In addition, the peak ages of the detrital zircons in the silty mudstone of the Qianshan Formation are 472 Ma and 498 Ma, as well as two other concordant points with 207Pb/206Pb apparent ages of 1824 Ma and 1985 Ma. The dating results in this study, together with published data, indicate the absence of Pan-African magmatic events in the Songliao Massif prior to the initial deposition of the Xilin Group, in contrast to those distributed widely in the Jiamusi Massif. Taken together, we conclude that the depositional provenance of the Laodaomiaogou and Qianshan formations was derived from the Songliao Massif. Furthermore, the characteristics of the detrital zircon age composition and rock associations indicate that the Laodaomiaogou Formation formed in a passive continental margin environment, in contrast to the Qianshan Formation, which formed in an active continental margin environment. The above results also imply that the Songliao and Jiamusi massifs might not have collided before the Late Ordovician.  相似文献   

8.
The depositional and metamorphic ages and provenances of the Ailaoshan(ALS) Group in the Ailaoshan-Red River(ALS-RR) shear zone, southwestern South China Block(SCB), were investigated to constrain the tectonic history of the southwestern SCB. In this study, we use petrology, geochemical analysis, zircon cathodoluminescence imaging and UPb geochronology to analyse samples of quartzite, garnet-bearing two-mica schist and metapelite. The age spectra of detrital zircon grains from these metasediments show two dominant age peaks at 550–424 Ma and 876–730 Ma and two subordinate peaks at 970–955 Ma and ～2450 Ma. The youngest peak, corresponding to the early Palaeozoic, accounts for more than 20% of the total dates and constrains the deposition of the ALS Group to the Palaeozoic rather than the Palaeoproterozoic as traditionally thought. Moreover, two peaks of metamorphic ages corresponding to the Permo-Triassic and Cenozoic were also identified, and these ages document the tectonothermal events associated with the Indosinian collision between the Indochina Block and the SCB and the Himalayan collision between the Indian and Asian plates. Geochemical data suggest that the provenances of the ALS Group were dominated by continental arc and recycled metasedimentary rocks. The comparison of probability density distribution plots of the detrital zircon U-Pb age data indicates that the Neoproterozoic detritus in the ALS Group was probably derived from the arc-related Neoproterozoic intrusive bodies in the northwestern and southwestern SCB. Furthermore, the early Palaeozoic detritus might have been sourced from eroded early Palaeozoic strata and magmatic plutons in Cathaysia and volcanic rocks in the western Indochina Block.  相似文献   

9.
Turbidites from the Shiquanhe–Namco Ophiolite Mélange Zone(SNMZ) record critical information about the tectonic affinity of the SNMZ and the evolutionary history of the Meso-Tethys Ocean in Tibet.This paper reports sedimentologic,sandstone petrographic,zircon U-Pb geochronologic,and clastic rocks geochemical data of newly identified turbidites(Asa Formation) in the Asa Ophiolite Mélange.The youngest ages of detrital zircon from the turbiditic sandstone samples,together with ～115 Ma U-Pb concordant age from the tuff intercalation within the Asa Formation indicate an Early Cretaceous age.The sandstone mineral modal composition data show that the main component is quartz grains and the minor components are sedimentary and volcanic fragments,suggesting that the turbidites were mainly derived from a recycled orogen provenance with a minor addition of volcanic arc materials.The detrital U-Pb zircon ages of turbiditic sandstones yield main age populations of170–120 Ma,300–220 Ma,600–500 Ma,1000–700 Ma,1900–1500 Ma,and ～2500 Ma,similar to the ages of the Qiangtang Terrane(age peak of 600–500 Ma,1000–900 Ma,～1850 Ma and ～2500 Ma) and the accretionary complex in the Bangong–Nujiang Ophiolite Zone(BNMZ) rather than the age of the Central Lhasa Terrane(age peak of ～300 Ma,～550 Ma and ～1150 Ma).The mineral modal compositions,detrital U-Pb zircon ages,and geochemical data of clastic rocks suggest that the Asa Formation is composed of sediments primarily recycled from the Jurassic accretionary complex within the BNMZ with the secondary addition of intermediate-felsic island arc materials from the South Qiangtang Terrane.Based on our new results and previous studies,we infer that the SNMZ represents a part of the Meso-Tethys Suture Zone,rather than a southward tectonic klippe of the BNMZ or an isolated ophiolitic mélange zone within the Lhasa Terrane.The Meso-Tethys Suture Zone records the continuous evolutionary history of the northward subduction,accretion,arc-Lhasa collision,and Lhasa-Qiangtang collision of the Meso-Tethys Ocean from the Early Jurassic to the Early Cretaceous.  相似文献   

10.
Six new high precision U-Pb zircon ID-TIMS ages plus thirteen in situ high spatial resolution U-Pb zircon LA-MC-ICPMS ages are reported from Jurassic plutonic(metaluminous to weakly peraluminous biotite granites)and Jurassic to Cretaceous hypabyssal(dacites)rocks from Macao.Despite its relatively small area(~30 km^2),the new ages tightly constrain the Macao granitic magmatism to two periods ranging from 164.5±0.6 Ma to 162.9±0.7 Ma and 156.6±0.2 Ma to 155.5±0.8 Ma,separated by ca.6 Ma.Inherited zircons point to the existence of a basement with ages up to Paleo-Proterozoic and late Archean in the region.In addition,younger dacitic rocks were dated at 150.6±0.6 Ma and<120 Ma.U-Pb zircon ages and whole-rock REE data of Macao granites indicate that the first pulse is also represented in Hong Kong and Southeast(SE)China,while magmatism with the chemical characteristics of the second pulse seems to not be represented outside Macao.The two granitic magmatic pulses have distinct mineralogical and geochemical features that support their discrete nature rather than a continuum of comagmatic activity and suggest that the Macao granitic suite was incrementally assembled during a period of ca.9 Ma,a hypothesis also extendable to the neighboring Hong Kong region for a time lapse of ca.24 Ma.In Macao,the transition from granitic magmatism(Middle to Upper Jurassic)to the younger dacite dykes(Upper Jurassic to Lower Cretaceous)most likely corresponds to a change in the regional tectonic setting,from an extensional regime related with foundering of the subducting paleoPacific plate during the Early Yanshanian period to the reestablishment of a normal subduction system in SE China during the Late Yanshanian period.  相似文献   

11.
Precise in situ zircon U-Pb dating and Lu–Hf isotopic measurement using an LA-ICP-MS system, whole-rock major and trace element geochemistry and Sr–Nd isotope geochemistry were conducted on the volcanic host rocks of the Tongyu copper deposit on the basis of further understanding of its geological characteristics. Three zircon samples from the volcanic host rocks yielded 206Pb/238 U weighted average ages ranging from 436±4 Ma to 440±5 Ma, which are statistically indistinguishable and coeval with the ca. 440 Ma northward subduction event of the Paleo-Qinling oceanic slab. The volcanic host rocks were products of magmatic differentiation that evolved from basalt to andesite to dacite to rhyolite, forming an integrated tholeiitic island arc volcanic rock suite. The primitive mantle-normalized trace element patterns for most samples show characteristics of island arc volcanic rocks, such as relative enrichment of LILE(e.g. Th, U, Pb and La) and depletion of HFSE(e.g. Nb, Ta, Ti, Zr and Hf). Discrimination diagrams of Ta/Yb vs Th/Yb, Ta vs Th, Yb vs Th/Ta, Ta/Hf vs Th/Hf, Hf/3 vs Th vs Nb/16, La vs La/Nb and Nb vs Nb/Th all suggest that both the volcanic host rocks from the Tongyu copper deposit and the volcanic rocks from the regional Xieyuguan Group were formed in an island arc environment related to subduction of an oceanic slab. Values of ISr(0.703457 to 0.708218) and εNd(t)(-2 to 5.8) indicate that the source materials of volcanic rocks from the Tongyu copper deposit and the Xieyuguan Group originated from the metasomatised mantle wedge with possible crustal material assimilation. Most of the volcanic rock samples show good agreement with the values of typical island arc volcanic rocks in the ISr-εNd(t) diagram. The involvement of crustal-derived material in the magma of the volcanic rocks from the Tongyu copper deposit was also reflected in the zircon εHf(t) values, which range from-3.08 to 10.7, and the existence of inherited ancient xenocrystic zircon cores(2616±39 Ma and 1297±22 Ma). The mineralization of the Tongyu copper deposit shows syn-volcanic characteristics such as layered orebodies interbedded with the volcanic rock strata, thus, the zircon U-Pb age of the volcanic host rocks can approximately represent the mineralization age of the Tongyu copper deposit. Both the Meigou pluton and the volcanic host rocks were formed during the ca. 440 Ma northward subduction of the Paleo-Qinling Ocean when high oxygen fugacity aqueous hydrothermal fluid released by dehydration of the slab and the overlying sediments fluxed into the mantle wedge, triggered partial melting of the mantle wedge, and activated and extracted Cu and other ore-forming elements. The magma and ore-bearing fluid upwelled and erupted, and consequently formed the island arc volcanic rock suite and the Tongyu VHMS-type copper deposit.  相似文献   

12.
A great deal of early-to-mid Early Cretaceous magmatic activities have been recorded in the Zhalantun area of Inner Mongolia,while the late Early to Late Cretaceous magmatic rocks have been barely reported(Guo et al.,2018;Zhang Xiangxin et al.,2017).At present,only a few Late Cretaceous magmatic activities were reported in the Arongqi area,such as volcanic rocks of the Gushanzhen Formation.However,the Gushanzhen Formation lacks accurate isotopic age,and contemporaneous intrusive rocks has not been reported yet.In this work,we collected the volcanic rocks from the Gushanzhen Formation and contemporaneous intrusive rocks in the Zhanlantun and nearby,and aim to figure out the formation ages of volcanic rocks of the Gushanzhen Formation and accompanied intrusive rocks by analyzing zircon U-Pb isotopes(Fig.1).  相似文献   

13.
The Solonker suture zone has long been considered to mark the location of the final disappearance of the PaleoAsian Ocean in the eastern Central Asian Orogenic Belt(CAOB). However, the time of final suturing is still controversial with two main different proposals of late Permian to early Triassic, and late Devonian. This study reports integrated wholerock geochemistry and LA-ICP-MS zircon U-Pb ages of sedimentary rocks from the Silurian Xuniwusu Formation, the Devonian Xilingol Complex and the Permian Zhesi Formation in the Hegenshan-Xilinhot-Linxi area in central Inner Mongolia, China. The depositional environment, provenance and tectonic setting of the Silurian-Devonian and the Permian sediments are compared to constrain the tectonic evolution of the Solonker suture zone and its neighboring zones. The protoliths of the silty slates from the Xuniwusu Formation in the Baolidao zone belong to wacke and were derived from felsic igneous rocks with steady-state weathering, poor sorting and compositional immaturity. The protoliths of metasedimentary rocks from the Xilingol Complex were wackes and litharenites and were sourced from predominantly felsic igneous rocks with variable weathering conditions and moderate sorting. The Xuniwusu Formation and Xilingol Complex samples both have two groups of detrital zircon that peak at ca. 0.9–1.0 Ga and ca. 420–440 Ma, with maximum deposition ages of late Silurian and middle Devonian age, respectively. Considering the ca. 484–383 Ma volcanic arc in the Baolidao zone, the Xuxiniwu Formation represents an oceanic trench sediment and is covered by the sedimentary rocks in the Xilingol Complex that represents a continental slope sediment in front of the arc. The middle Permian Zhesi Formation metasandstones were derived from predominantly felsic igneous rocks and are texturally immature with very low degrees of rounding and sorting, indicating short transport and rapid burial. The Zhesi Formation in the Hegenshan zone has a main zircon age peak of 302 Ma and a subordinate peak of 423 Ma and was deposited in a back-arc basin with an early marine transgression during extension and a late marine regression during contraction. The formation also crops out locally in the Baolidao zone with a main zircon age peak of 467 Ma and a minor peak of 359 Ma, and suggests it formed as a marine transgression sedimentary sequence in a restricted extensional basin and followed by a marine regressive event. Two obvious zircon age peaks of 444 Ma and 280 Ma in the Solonker zone and 435 Ma and 274 Ma in Ondor Sum are retrieved from the Zhesi Formation. This suggests as a result of the gradual closure of the Paleo-Asian Ocean a narrow ocean sedimentary environment with marine regressive sedimentary sequences occupied the Solonker and Ondor Sum zones during the middle Permian. A restricted ocean is suggested by the Permian strata in the Bainaimiao zone. Early Paleozoic subduction until ca. 381 Ma and renewed subduction during ca. 310–254 Ma accompanied by the opening and closure of a back-arc basin during ca. 298–269 Ma occurred in the northern accretionary zone. In contrast, the southern accretionary zone documented early Paleozoic subduction until ca. 400 Ma and a renewed subduction during ca. 298–246 Ma. The final closure of the Paleo-Asian ocean therefore lasted at least until the early Triassic and ended with the formation of the Solonker suture zone.  相似文献   

14.
The Miyun area of Beijing is located in the northern part of the North China Craton(NCC)and includes a variety of Archean granitoids and metamorphic rocks.Magmatic domains in zircon from a tonalite reveal Early Neoarchean(2752±7 Ma) ages show a small range in ε_(Hf)(t) from 3.1 to 7.4and t_(DM1)(Hf) from 2742 to 2823 Ma,similar to their U-Pb ages,indicating derivation from a depleted mantle source only a short time prior to crystallization.SHRIMP zircon ages of granite,gneiss,amphibolite and hornblendite in the Miyun area reveal restricted emplacement ages from 2594 to2496 Ma.They also record metamorphic events at ca.2.50 Ga,2.44 Ga and 1.82 Ga,showing a similar evolutionary history to the widely distributed Late Neoarchean rocks in the NCC.Positive ε_(Hf)(t) values of 1.5 to 5.9,with model ages younger than 3.0 Ga for magmatic zircon domains from these Late Neoarchean intrusive rocks indicate that they are predominantly derived from juvenile crustal sources and suggest that significant crustal growth occurred in the northern NCC during the Neoarchean.Late Paleoproterozoic metamorphism developed widely in the NCC,not only in the Trans-North China Orogen,but also in areas of Eastern and Western Blocks,which suggest that the late Paleoproterozoic was the assembly of different micro-continents,which resulted in the final consolidation to form the NCC,and related to the development of the Paleo-Mesoproterozoic Columbia or Nuna supercontinent.  相似文献   

15.
http://www.sciencedirect.com/science/article/pii/S1674987112001569   总被引：1，自引：0，他引：1  
The southeastern Anatolia comprises numbers of tectono-magmatic/stratigraphic units such as the metamorphic massifs,the ophiolites,the volcanic arc units and the granitoid rocks.All of them play important role for the late Cretaceous evolution of the southern Neotethys.The spatial and temporal relations of these units suggest the progressive development of coeval magmatism and thrusting during the late Cretaceous northward subduction/accretion.Our new U-Pb zircon data from the rhyolitic rocks of the wide-spread volcanic arc unit show ages of(83.1±2.2)-(74.6±4.4) Ma. Comparison of the ophiolites,the volcanic arc units and the granitoids suggest following late Cretaceous geological evolution.The ophiolites formed in a suprasubduction zone(SSZ) setting as a result of northward intra-oceanic subduction.A wide-spread island-arc tholeiitic volcanic unit developed on the top of the SSZ-type crust during 83-75 Ma.Related to regional plate convergence, northward under-thrusting of SSZ-type ophiolites and volcanic arc units was initiated beneath the Tauride platform(Malatya-Keban) and followed by the intrusion of l-type calc-alkaline volcanic arc granitoids during 84-82 Ma.New U-Pb ages from the arc-related volcanic-sedimentary unit and granitoids indicate that under-thrusting of ophiolites together with the arc-related units beneath the Malatya-Keban platform took place soon after the initiation of the volcanic arc on the top of the SSZtype crust.Then the arc-related volcanic-sedimentary unit continued its development and lasted at～75 Ma until the deposition of the late Campanian—Maastrichtian shallow marine limestone.The subduction trench eventually collided with the Bitlis-Ptrge massif giving rise to HP-IT metamorphism of the Bitlis massif.Although the development of the volcanic arc units and the granitoids were coeval at the initial stage of the subduction/accretion both tectono-magmatic units were genetically different from each other.  相似文献   

16.
A zircon U-Pb geochronological study on the volcanic rocks reveals that both of the Zhangjiakou and Yixian Formations, northern Hebei Province, are of the Early Cretaceous, with ages of 135-130 Ma and 129-120 Ma, respectively. It is pointed out that the ages of sedimentary basins and volcanism in the northern Hebei -western Liaoning area become younger from west to east, i. e. the volcanism of the Luanping Basin commenced at c. 135 Ma, the Luotuo Mount area of the Chengde Basin c. 130 Ma, and western Liaoning c. 128 Ma. With a correlation of geochronological stratigraphy and biostratigraphy, we deduce that the Xing‘anling Group, which comprises the Great Hinggan Mountains volcanic rock belt in eastern China, is predominantly of the early-middle Early Cretaceous, while the Jiande and Shimaoshan Groups and their equivalents, which form the volcanic rock belt in the southeastern coast area of China, are of the mid-late Early Cretaceous, and both the Jehol and Jiande Biotas are of the Early Cretaceous, not Late Jurassic or Late Jurassic-Early Cretaceous. Combining the characteristics of the volcanic rocks and, in a large area, hiatus in the strata of the Late Jurassic or Late Jurassic-early Early Cretaceous between the formations mentioned above and the underlying sequences, we can make the conclusion that, in the Late Jurassic-early Early Cretaceous, the eastern China region was of high relief or plateau, where widespread post-orogenic volcanic series of the Early Cretaceous obviously became younger from inland in the west to continental margin in the east. This is not the result of an oceanward accretion of the subduction belt between the Paleo-Pacific ocean plate and the Asian continent, but rather reflects the extension feature, i.e. after the closure of the Paleo-Pacific ocean, the Paleo-Pacific ancient continent collided with the Asian continent and reached the peak of orogenesis, and then the compression waned and resulted in the retreating of the post-orogenic extension from outer orogenic zone to inner part (or collision zone). The determination of the eruption age of the volcanics of the Zhangjiakou Formation definitely constrains the switch period, which began in the Indosinian and finished in the Yanshanian, that is, 140-135 Ma. The switch is concretely the change from the approximate E-W Paleo-Asian tectonic system to the NE to NNE Pacific system, and the period is also the apex of a continent-continent collision and orogenesis of subduction, being consumed and eventually disappearing of the Paleo-Pacific ancient continent, and all the processes commenced in the Indosinian. While the following post-orogenic large-scale eruption in the Early Cretaceous marks the final completeness of the Paleo-Pacific structure dynamics system.  相似文献   

17.
The North Tianshan Orogenic Belt contains the youngest ophiolites in the Tianshan and provides some information on timing of the last closure of the Junggar-Balkhash Ocean. LA-ICP-MS zircon U-Pb dating was conducted to define the formation age of the Arbasay Formation in the Shichang Region of North Tianshan, which is exposed near the suture zone but its age remains debated. The Arbasay Formation is mainly composed of volcanic and volcaniclastic rocks with tuff interlayers. The zircons from the tuffs yield two age populations of 315 ± 3 Ma and 304± 2 Ma, constraining the commencement and demise timings of volcanism, respectively. Furthermore, zircon U-Pb age spectra of the tuffaceous sandstones display the youngest peak age at 308 Ma, indicating a 308 Ma age for the depositional age of volcaniclastic rocks. The volcaniclastic rocks therefore were likely to deposit together with the syn-sedimentary volcanism during Late Carboniferous. This means that the Arbasay Formation in Shichang Region should be re-assigned to Late Carboniferous in age. Given that the Arbasay Formation was likely to be formed during the tectonic transition from compression to extension, the Junggar-Balkhash Ocean possibly closed during Late Carboniferous.  相似文献   

18.
The East Tianshan contains many late Paleozoic magmatic and polymetallic deposits. Recent studies demonstrate that the early Paleozoic volcanic rocks discovered in the northern region of East Tianshan can be subdivided into the Daliugou, Hongliuxia and Kalatag formations. Here, we report zircon U-Pb ages of volcanic rocks and quartz diorite porphyry, together with whole-rock geochemical and Sr-Nd isotope data of early Paleozoic volcanic rocks, in order to investigate their petrogenesis and geody...  相似文献   

19.
The Zhalantun terrane from the Xing'an massif, northeast China, was used to be considered as Proterozoic basements. However, amounts of detrital zircon ages from the meta-sedimentary rocks deny the existence of Precambrian basements recently. Notably, magmatic rocks were barely reported to limit the exact ages of the Zhalantun basements. In this study, we collected rhyolite, gabbro and quartz diorite for zircon in-situ U-Pb isotopic dating, which yield crystallization ages of ～505 Ma, ～447 Ma and ～125 Ma, respectively. Muscovite schist and siltstone define maximum depositional ages of ～499 Ma and ～489 Ma, respectively. Additionally, these dated supracrustal rocks and plutons also yield ancient detrital/xenocryst zircon ages of ～600–1000 Ma, ～1600–2220 Ma, ～2400 Ma, ～2600–2860 Ma. Based on the whole-rock major and trace element compositions, the ～505 Ma rhyolites display high SiO_2 and alkaline contents, low Fe_2O_3T, TiO_2 and Al_2O_3, and relatively high Mg O and Mg#, which exhibit calc-alkaline characteristics. These rhyolites yield fractionated REE patterns and negative Nb, Ta, Ti, Sr, P and Eu anomalies and positive Zr anomalies. The geochemistry, petrology and Lu-Hf isotopes imply that rhyolites were derived from the partial melting of continental basalt induced by upwelling of sub-arc mantle magmas, and then experienced fractional crystallization of plagioclase, which points to a continental arc regime. The ～447 Ma gabbros exhibit low Si O2 and alkaline contents, high Fe2 O3 T, Ti O2, Mg O and Mg#. They show minor depletions of La and Ce, flat MREE and HREE patterns, and negative Nb, Ta, Zr and Hf anomalies. Both sub-arc mantle and N-MORB-like mantle were involved in the formation of the gabbros, indicative of a probable back-arc basin tectonic setting. Given that, the previously believed Proterozoic supracrustal rocks and several plutons from the Zhalantun Precambrian basements were proved to be Paleozoic to Mesozoic rocks, among which these Paleozoic magmatic rocks were generally related to subduction regime. So far, none Proterozoic rocks have been identified from the Zhalantun Precambrian basement, though some ～600–3210 Ma ancient detrital/xenocryst zircons were reported. Combined with ancient zircon ages and newly reported ～2.5 Ga and ～1.8 Ga granites from the south of the Zhalantun, therefore, the Precambrian rocks probably once exposed in the Zhalantun while they were re-worked and consumed during later long tectonic evolutionary history, resulting in absence of Precambrian rocks in the Zhalantun.  相似文献   

20.
Studies of the Pb, Sr and Nd isotopic composition of Mesozoic intrusive rocks indicate that the basement of the copper-gold metallogenic belt of the middle and lower reaches of the Yangtze River has "two-layer structure" and partly has "multi-layered structure", and is inhomogeneous and shows the distinct feature of E-W provincialism. The calculated model lead ages (t1) are mostly greater than 2600 Ma, and the model neodymium ages (TDM) vary from 953 to 2276 Ma and concentrate in two time intervals: 1800-2000 Ma and 1200-1600 Ma. It is concluded that the basement of the MBYR is composed of the Late Archaeozoic to Middle Proterozoic metamorphic series and that the crust was initiated in the Archaean and continued to grow in the Early and Middle Proterozoic, and the proportion of new crust formed by mantle differentiation during the Late Proterozoic is low.  相似文献   

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