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1.
This paper summarizes the geochronological, geochemical and zircon Hf isotopic data for Mesozoic granitoids within the Erguna Massif, NE China, and discusses the spatial-temporal variation of zircon Hf isotopic compositions, with the aim of constraining the accretion and reworking processes of continental crust within the Erguna Massif, and shedding light on the crustal evolution of the eastern segment of the Central Asian Orogenic Belt. Based on the zircon U-Pb dating results, the Mesozoic granitic magmatisms within the Erguna Massif can be subdivided into five stages: Early-Middle Triassic(249–237 Ma), Late Triassic(229–201 Ma), Early-Middle Jurassic(199–171 Ma), Late Jurassic(155–149 Ma), and Early Cretaceous(145–125 Ma).The Triassic to Early-Middle Jurassic granitoids are mainly I-type granites and minor adakitic rocks, whereas the Late Jurassic to Early Cretaceous granitoids are mainly A-type granites. This change in magmatism is consistent with the southward subduction of the Mongol-Okhotsk oceanic plate and subsequent collision and crustal thickening, followed by post-collision extension. Zircon Hf isotopic data indicate that crustal accretion of the Erguna Massif occurred in the Mesoproterozoic and Neoproterozoic. ZirconεHf(t) values increase gradually over time, whereas two-stage model(TDM2) ages decrease throughout the Mesozoic. The latter result indicates a change in the source of granitic magmas from the melting of ancient crust to more juvenile crust. Zircon εHf(t)values also exhibit spatial variations, with values decreasing northwards, whereas TDM2 ages increase. This pattern suggests that,moving from south to north, there is an increasing component of ancient crustal material within the lower continental crust of the Erguna Massif. Even if at the same latitude, the zircon Hf isotopic compositions are also inconsistent. These results reveal lateral and vertical heterogeneities in the lower continental crust of the Erguna Massif during the Mesozoic, which we use as the basis of a structural and tectonic model for this region.  相似文献   

2.
Yanshanian magmatisms are intensive in the southern Anhui Province and can be divided into early(152–137 Ma)and late(136–122 Ma) stages. A Yanshanian granitic zone was found to crop out along Qingshan to Changgai areas in the Tunxi district in Field investigation which has a genetic link with molybdenum multiple metal mineralization. To be a representative syenitic granite in the southern Anhui Province, the Huangshan pluton has not been found so far to have any genetic link with mineralization. Zircon LA-ICP-MS dating indicate that the four granitic bodies from the Qingshan-Changgai zone have concurrent formed ages from 140±4 to 141±2 Ma, belonging to the Yanshanian early stage magmatism. However, the Huangshan granite is dated to be 129±2 Ma, belonging to the Yanshanian late stage magmatism. The Qingshan-Changgai granites show high SiO2 and K2 O contents, low P2O5 contents and middle Al2O3 contents and are high-K calc-alkaline series metaluminum I-type granite. These rocks are characterized by enrichments in the large ion lithophile elements and light rare earth elements(REE), depletions in the high field-strength elements, and middle degree negative anomalies of Eu, geochemical features of arc or continent crustal derived magma affinities. These rocks have 87Sr/86Sr(t) ratios from 0.7120 to 0.7125, εNd(t) values from.7.24 to.4.38 and zircon εHf(t) values of.4.4 to 6.7, similar to that of the coeval ore-bearing granodiorites in the southern Anhui Province. Integrated geochemical studies indicate that the Yanshanian ore-bearing granodiorites were formed by partial melting of the Meso-Neoproterozoic accreted thickened low crust. Meanwhile, the Qingshan-Changgai granites were formed through a AFC process of plagioclase+amphibole+Shangxi Group of magmas that formed the ore-bearing granodiorites. The Huangshan granites are characterized by high SiO2 and K2 O contents, moderate Al2O3 contents, seagull shape REE distributed pattern and distinct Eu negative abnormities. Comparing with the Qingshan-Changgai granites, the Huangshan granites show more Ba, Sr, P,and Ti negative abnormities with no Nb and Ta depletions and are high-K calc-alkaline series metaluminum A-type granite. εHf(t)values of the Huangshan granites are from.6.6 to.1.2, similar to that of the early stage ore-bearing granodiorites, indicating that they were also formed by anatexis of the Meso-Neoproterozoic accreted crust, but their magma sources might be residual granulitic crust which ever underwent Yanshanian early stage I-type intermediate-acid magma extraction. Comparing studies on the two stages granites indicate that the early stage granites derived from a relative thickened low crust under a lower temperature condition. Their magma sources were Meso-Neoproterozoic accreted crust which enriched in ore-forming materials and further became more enriched through processes of magma AFC evolution. However, the late stage A-type granites originated from relative shallow crust under a higher temperature condition. Their magma source was depleted in ore-forming materials due to the early stage magma extraction and thus had weak ore-forming capacity. From early to late stage, the magmatisms tectonic setting translated from post-orogenic to anorogenic and the later corresponded to a back-arc extensional setting as increase of the slab subducted angle of the Paleo-Pacific plate.  相似文献   

3.
Zircon LA-ICP-MS U-Pb dating reveals that the Baimashan Pluton is composed mainly of late Indosinian (204.5±2.8 Ma-209.2±3.8 Ma) biotite granodiorites/monzonitic granites (LIGs) and early Yanshanian (176.7±1.7 Ma) two-micas monzonitic granites (EYGs), and the coeval (203.2±4.5 Ma-205.1±3.9 Ma) mafic microgranular enclaves (MMEs) are generally found in the former. In addition, the ages of cores within zircons from LIGs and MMEs ranging from 221.4±4.0 Ma to 226.5±4.1Ma provide evidence of multistage magma intrusion during Indosinian in the study area. Measured 3010±20.6 Ma of inherited zircon age suggests that there may be recycling Archaean curstal material in existence in this area. LIGs and EYGs share some similar geochemical features: subalkaline and peraluminous granites, enrichment of Th, U, K, Ta, Zr, Hf and LREE but depletion of Ba, Nb, P, Ti and Eu, low εNd(t) values but high (87Sr/86Sr)i ratios, and old T2DM (ca. 1.9-2.0 Ga). The behaviors of incompatible elements and REE are mainly dominated by fractional crystallization of plagioclase, K-feldspar, ilmenite and apatite, but that of Sr isotope mainly controlled by EC-AFC. They are crust-sourced and derived from partial melting of paleo-Proterozoic metagreywackes and related to biotite dehydration melting. LIGs are formed in post-collisional tectonic setting as crustal local extension and thinning during late Indosinian. But EYGs may be evolved products of congeneric granitic magma with LIGs formed in late Indoinian, which were emplaced again when crust underwent extensive thinning and extension in post-orogenic tectonic setting during Yanshanian in SC after undergoing EC-AFC. MMEs should be cognate enclaves and derived from liquid immiscibility of host magma.  相似文献   

4.
The SHRIMP zircon U-Pb geochronology of three typical samples, including two monzo nitic granites from the Lincang batholith and a rhyolite from the Manghuai Formation are presented in the southern Lancangjiang, western Yunnan Province. The analyses of zircons for the biotite monzonitic granites from the northern (02DX-137) and southern (20JH-10) Lincang batholith show the single and tight clusters on the concordia, and yield the weighted mean 206Pb/238U ages of 229.4 ± 3.0 Ma and 230.4 ± 3.6 Ma, respectively, representing the crystallized ages of these granites. The zircons for the rhyolitic sample (02DX-95) from the Manghuai Formation give a weighted mean 206Pb/238U age of 231.0 ± 5.0 Ma. These data suggest that the igneous rocks from the Lincang granitic batholith and Manghuai Formation have a similar crystallized age. In combination with other data, it is inferred that both were generated at a narrow age span (~230 Ma) and were originated from the postcollisional tectonic regime. An early Proterozoic 206Pb/238U apparent age of 1977±44 Ma is additionally obtained from one zircon from the biotite monzonitic granite (southern Lincang batholith), indicative of devel- opment of the early Proterozoic Yangtze basement in the region. These precisely geochronological data provide important constraints on better understanding the Paleozoic tectonic evolution of the Tethys, western Yunnan Province.  相似文献   

5.
Zircon SHRIMP dating of granites from Dulan,east segment of North Qaidam UHP belt shows that they are 406.6±3.5 Ma for Yematan-E,407.3±4.3 and 397±6 Ma for Balijiehatan-W,404.5±4.0 and 397.0±3.7 Ma for Shuiwenzhan-N,380.5±5.0 Ma for Shuiwenzhan-S,382.5±3.6 and 372.5±2.8 Ma for Chachagongma.These granites from Dulan represent the products of the third and fourth periods of Paleozoic magmatism in North Qaidam.Geochemically,the granitoids with metalumious to weak peratuminous are quartz diorite,granodiorite,and granite in composition and mainly belong to calc-alkaline series,a few samples to calc or alkali-calc series.The third period of granites is a rock association of granodiorite+granite,with initial 87Sr/86Sr ratios from 0.7082 to 0.7110 and T2DM model ages from 1.41–1.90 Ga;and the fourth period of granites is a rock association of quartz diorite+granodiorite+granite,with initial 87Sr/86Sr ratios from 0.7072 to 0.7091 and T2DM model ages from 1.07–1.38 Ga.Therefore,the third period of granites has higher initial 87Sr/86Sr ratios and T2DM model ages.On the contrary,the fourth period of granites has Nd(t)values from 0.6 to-3.0,higher than that of the third granite with Nd(t)values-3.2 to-9.3.Thus,the data comparison indicates that the third granites may derive from Paleo-proterzoic continental crust with mantle material whereas the fourth granites may derive from the Meso-proterzoic basalt crust with continental material.Combined with regional geology,we thought that the third granites were formed relative to plate exhumation and the fourth granites to delamination of the lithospheric mantle.  相似文献   

6.
In order to constrain whether the Lhasa–Qiangtang collision contributed to an early crustal thickening of the central Tibetan Plateau prior to the India–Asia collision,we present zircon LA–ICP–MS U–Pb ages,wholerock geochemistry,and zircon Hf isotopic compositions of the newly discovered rhyolitic crystal tuffs from the Chuduoqu area in the eastern Qiangtang subterrane,central Tibet.Zircon U–Pb dating suggests that the Chuduoqu rhyolitic crystal tuffs were emplaced at ca.68 Ma.The Chuoduoqu rhyolitic crystal tuffs display high SiO2 and K2 O,and low MgO,Cr,and Ni.Combined with their zircon Hf isotopic data,we suggest that they were derived from partial melting of the juvenile lower crust,and the magma underwent fractional crystallization and limited upper continental crustal assimilation during its evolution prior to eruption.They should be formed in a post-collisional environment related to lithospheric mantle delamination.The Chuduoqu rhyolitic crystal tuffs could provide important constraints on the Late Cretaceous crustal thickening of the central Tibetan Plateau caused by the Lhasa–Qiangtang collision.  相似文献   

7.
A geochronological study of zircon U-Pb on the volcanic rocks from the stratotype section of the Qingshan Group within the Jiaozhou Basin, eastern Shandong Province, is presented. The zircons were analyzed using the method of in situ ablation of a 193 nm excimer laser system coupled with an up to date ICP-MS system. Among the three formations of the Qingshan Group, zircons recovered from the lowest part of the Houkuang Fm. were dated at 106±2 Ma (95% confidence, the same below), whereas those from the lower and upper parts of the Shiqianzhuang Fm. were given ages of 105±4 Ma and 98±1 Ma, respectively. A spatially decreasing trend for the Mesozoic magmatic timing from west to east in the province is observed through comparing the data of this study with those by previous works on the Qingshan volcanic lavas occurring at western Shandong and within the Yishu fault zone. The Qingshan volcanic rocks are constituent of the 'Shoshonite Province' in East China. Exposed at most provinces of central East China along the Tan-Lu fault and the Yangtze fault zones, these volcanic suites are characterized by shoshonite and high-K calcalkalic rocks in lithology and thought to be correlated with the partial melting of continental mantle in genesis. It is also shown that the Qingshan potassic volcanic suite from eastern Shandong basins is distinctly younger than those from other ar-eas of the shoshonite province. By contrary, ages of the Mesozoic to Cenozoic alkaline basalts, sourced by asthenospheric mantle, from both northern Huaiyan basin and northern Dabie belt along the Tan-Lu fault zone and from the Ningwu, Lishui and Luzong basins along the Yangtze fault zone are observably older than those occurring within eastern Shandong. The revealed temporal and spatial patterns in magmatism for the two types of volcanic suites make an important geochronological con-straint on the Mesozoic to Cenozoic dynamic evolution model of the subcontinental lithosphere in East China.  相似文献   

8.
The cratonization history of the North China Craton(NCC) and the nature of tectonothermal events are still highly controversial. Tonalite-trondhjemite-granodiorite(TTG) gneisses, as the dominant lithological assemblages in Archean metamorphic terranes, can provide significant clues to the magmatic and metamorphic evolution of Precambrian crust. This study presents zircon laser-ablation inductively-coupled-plasma mass spectrometry U–Pb ages, trace-element, and in-situ LA-MC-ICPMS zircon Hf isotope data for the TTG gneisses from the Bengbu-Wuhe area on the southeastern margin of the NCC.Cathodoluminescence images and trace elements indicated that magmatic zircons display the characteristics of euhedral-subhedral crystals with oscillatory growth zoning structures, high RREE contents, marked Ce positive anomalies, and Pr–Eu negative anomalies. The metamorphic zircons display the spherical-oval crystals with distinct core-rim structures, high and homogeneous luminescent intensity, lower RREE, Nb, Ta, Hf contents,relative flat REE patterns, weak Ce positive anomalies, and Pr-Eu negative anomalies. The Ti–in–zircon geothermometer data indicate that the crystallization temperature of the TTG gneiss ranged from 754 to 868 ℃. Zircon U–Pb ages indicate that the TTG gneisses formed at 2.79–2.77 Ga and 2.50 Ga and underwent metamorphism at 2.57–2.52 Ga. The Hf isotopic data indicate that the magmatic zircons exhibit high, positive eHf(t) values close to those of the coeval depleted mantle, whereas the metamorphic zircons exhibit negative or nil eHf(t) values. This implies that the TTG gneisses were derived from the partial melting of the ～ 2.9–2.6 Ga juvenile crustal sources mixed with ～ 3.0–2.8 Ga ancient crustal materials.Combined with the regional tectonic evolution, we propose that the metamorphic basement at the southeastern margin of the NCC underwent episodic crustal growth at ～ 2.7 and ～ 2.5 Ga and subsequently underwent crustal reworking or re-melting of the ancient crust during the Neoarchean. The Neoarchean TTG gneisses might have been derived from the partial melting of lower crustal materials related to plate subduction.  相似文献   

9.
The deep carbon cycle, which plays a critical role in mantle evolution and Earth habitability, is closely linked to the recycling of carbonate-bearing rocks through subduction. Marine carbonates are subducted to different depths during the closure of oceanic basins, thus carry important signatures of the disappeared oceanic basins. Petrological and geochemical features of the Hannuoba carbonatites in the northern North China Craton indicate that they were formed by melting of limestone subducted to mantle depths. Here, we show that detrital zircons carried by these carbonatites have a broad spectrum of U-Pb ages from Precambrian to Phanerozoic. Precambrian age peaks are at ~2.5 Ga, 2.1–2.3 Ga, 1.8–2.0 Ga, ~1.65 Ga, 1.3–1.4 Ga, ~1.1 Ga,0.91–0.94 Ga, 0.74–0.81 Ga, and 0.62–0.63 Ga, respectively. The recorded age peaks are different from those in the northern North China Craton and thus preclude an origin of crustal contamination. Nevertheless, the Precambrian age spectra are compatible with those of the Xingmeng Orogen in the southeastern Central Asian Orogenic Belt. Furthermore, the significantly positive εHf(t) values of 7.7–13.5 for the 300–373 Ma zircons are similar to those in the Xingmeng Orogen but different from those in the northern North China Craton. All these features suggest that the limestone precursor for the Hannuoba carbonatites was originated from the Paleo-Asian Ocean, and its deposition time was not earlier than 300 Ma. This indicates that the PaleoAsian Ocean still existed in the late Carboniferous to early Permian. The widespread distribution of metamorphic carbonates in the Central Asian Orogenic Belt indicates that there may have been widespread sedimentary carbonates in the Paleo-Asian Ocean. A large amount of sedimentary carbonates was probably carried into mantle during subduction of the Paleo-Asian oceanic slab, which significantly modified the chemical and physical properties of the lithospheric mantle.  相似文献   

10.
A large number of Eocene-Oligocene alkaline/alkali-rich igneous rocks were developed in the Tuotuohe region of the Qinghai-Tibetan Plateau.In this study,we present zircon U-Pb ages,Hf isotope data,and major and trace element compositions of the Cenozoic alkaline rocks from the Tuotuohe region in order to constraint the petrogenesis and tectonic evolution history of Qiangtang Block.Zircon U-Pb ages were measured via LA-ICP-MS to be39.6,37.6 and 32.0 Ma.The 39.6 Ma trachyte was characterized by low SiO2 and high K2O and MgO contents.The 37.6 and 32.0 Ma orthophyres show enrichment in SiO2 and K2O,but deficient in MgO.All of the samples belong to the alkaline rocks.These rocks display enrichment in REE,LREE,and LILE,depletion in HFSE,and no obvious Eu anomalies.Geological and geochemical features suggest that there were two possible mechanisms for the origin of the alkaline rocks in the Tuotuohe region:(1)the removed mafic lower crust dropped into the asthenosphere,forming the mix magma(Nariniya trachyte);(2)the upwelling asthenosphere triggered the crustal melting(Nariniya and Zamaqu orthophyre).The Eocene-Oligocene alkaline rocks in the study and adjacent areas are likely to be the result of the same tectonic-magmatic event of deep lithospheric evolution that is the crustal material melting triggered by lithospheric delamination.This conclusion extends the influence scope of lithospheric delamination eastward to the Tuotuohe region(*92°E)from Banda Co(*82°E).  相似文献   

11.
Suizhou-Zaoyang area is located in the southern Qinling-Tongbai-Dabie Orogen, China. A combined research on U-Pb ages and Lu-Hf isotopes for detrital zircons from three meta-sedimentary rocks in the Wudang Group is reported. The upper Wudang Group has a major age population of ca. 750 Ma and a sub-major of ca. 860 Ma. But the lower part only yields one age peak at ca. 2.03 Ga. In situ Lu-Hf analyses of the young age group of ca. 750 Ma for zircons from the upper Wudang Group yield an average εHf(t) value of ?8.6 and two-stage Hf model ages(TDM2) from 1837 to 2230 Ma, respectively. However, zircons from the lower Wudang Group give an average εHf(t) value of 4.5 and TDM1 of 2220±22 Ma, close to the timing of zircon crystallization. Thus, it is suggested that, in the study area, the continental crust grew during the middle Paleoproterozoic and reworked during the middle Neoproterozoic, which shows the affinity to the Dabie Orogen. In addition, in the lower Wudang Group, two metamorphic zircon ages of 1992±91 and 1999±61 Ma are consistent with that of the middle Paleoproterozoic metamorphism event in the northern Yangtze Block, which may represent the assemblage of the Columbia Supercontinent. On the basis of the U-Pb ages and Hf isotopes, it is proposed that Suizhou-Zaoyang area was involved in the subduction-collision event in the middle Paleoproterozoic and the Yangtze Block was one of the components of the Paleo-Mesoproterozoic supercontinent.  相似文献   

12.
Many orogenic belts in the world exhibit accretionary and collisional orogenic phases to varying extents. How accretion evolves into collision of the Central Asian Orogenic Belt(CAOB), the largest Phanerozoic accretionary orogenic belt,is an intriguing question. In this paper, we present new U-Pb age, geochemical and isotopic data for Permian-Triassic granitoids from middle Inner Mongolia, Northern China in the southeastern CAOB, and delineate the magmatic transition from subduction to(soft) collision. The magmatic record of soft collision is identified and characterized by thickened lower crust-derived high Sr/Y granitoids with a sub-linear distribution along the Solonker suture zone. Granitoids from Early Permian to Late Permian became more enriched in whole-rock Nd and zircon Hf isotopic compositions(ε_(Nd)(t) values from 2.4 to-19.5, ε_(Hf)(t) values from 11.6 to-33.7), indicating increasing incorporation of old crust. The change in peak timing of magmatism from west(ca. 264 Ma)to east(ca. 251 Ma) along the Solonker suture zone implies "scissor-like" closure of the Paleo-Asian Ocean. Integrated with previous studies, a three-stage tectonic model from the Permian to Triassic by accretion leading to collision on the south-eastern margin of CAOB is proposed.(1) Early Permian( ca. 285 Ma): Juvenile magmatism on an active continental margin with double-sided subduction of the Paleo-Asian Ocean;(2) Middle Permian to Middle Triassic(ca. 285–235 Ma): Magma source transition from juvenile to old crust induced by a tectonic switch from arc to "scissor-like" closure and subsequent intracontinental orogenic contraction;(3) Late Triassic( ca. 235 Ma): A-type and alkaline magmatism in response to post-collisional extension.  相似文献   

13.
We report U–Pb dating of zircon,as well as geochemical and Hf isotope data,in order to constrain the formation time,magma source,and tectonic setting of granite porphyry dykes in the Xicha gold–(silver) district in southern Jilin Province,Northeast China.The zircon grains are euhedral–subhedral,display oscillatory growth zoning and have Th/U ratios varying between 0.11 and 0.78,which together imply a magmatic origin.The dating results indicate the porphyry formed in the Early Cretaceous (122±1 Ma)and it contains SiO_2=70.64–72.31 wt%,Al_2O_(3-)=13.99–14.64 wt%,K_2O+Na_2O=6.96–7.81 wt%K_2O/Na_2O=1.24–2.10,and A/CNK=1.11–1.41.Chemically,the porphyry belongs to a high-K calc-alkaline S-type granite.Chondrite-normalized rare earth elements (REE)patterns show LREE enrichment,light rare earth elements(LREE)/heavy rare earth elements (HREE)=9.93–11.97(La/Yb)_N=11.08–15.16,and d Eu=0.69–0.95.On the trace element spider diagram,large ion lithophile elements such as Rb,Ba,K,Th,and U are enriched,whereas the high field strength elements Ti and P are depleted.The e Hf(t) values of zircon from the granite porphyry vary between-17.1 and-13.2,and their Hf two-stage model ages vary from 2.01 to 2.26 Ga,implying that the magma was derived from partial melting of old lower crust.The granite porphyry dykes and many A-type granites in the region formed at the same time,suggesting an extensiona environment.The combination of the occurrence of strong magmatism,large-scale mineralization,and extensiona tectonics throughout much of Eastern China indicate that the Early Cretaceous was a period of significant lithospheric thinning.The southern Jilin Province,therefore,experienced lithospheric thinning during the Early Cretaceous.  相似文献   

14.
A combined study using LA-ICP-MS U-Pb dating, Hf isotopes, trace elements and the Ti-in-zircon geo-thermometer was carried out on zircons from the metamorphosed basic-ultrabasic rocks in the meta-morphic basement of the Cathaysia Block, southwestern Zhejiang Province. The formation and meta-morphic ages of the rocks from the metamorphic basement of the Cathaysia Block were determined based on zircon U-Pb geochronology. The age for the magmatic crystalline zircons from the protolith is about 1.85 Ga. The εHf(t) values of the older zircons were from ?7 to ?3, with two-stage model Hf ages (TDM2LC) of about 2.9 to 3.4 Ga, indicating that the source material was derived from anatexis and recy-cling of the Archean crust. The newly formed metamorphic zircons yielded U-Pb ages of 260―230 Ma. The metamorphic temperature calculated using the Ti-in-zircon geothermometer ranged from 610 to 720℃, consistent with the results from petrographic observations, indicating that the Cathaysia Block experienced an amphibolite facies metamorphism during the Indosinian. Results from this study pro-vided an important timeframe for the tectonic evolution in South China and the Southeast Asia during the Late Permian and Early Triassic times.  相似文献   

15.
South China is famous for the extensive magmatism and polymetallic mineralization that took place there in the Mesozoic. Shilu is a large porphyry–skarn Cu–Mo deposit in the Yangchun Basin, South China. The lithology of the Shilu intrusion is granodiorite and quartz diorite, both of which are high-K calc-alkaline series, with high Sr([400 ppm) content along with low Y and Yb contents. Most of the samples have characteristics of adakite except for a few samples that have slightly higher Y and Yb contents, which may be plausibly explained by crustal contamination. Laser Ablation Inductively Coupled Plasma Mass Spectrometry zircon U–Pb dating revealed ages between 106.6 ± 1.3 and 103.9 ± 0.5 Ma, with multiple magmatic pulses. Molybdenite Re–Os isochron age of 102.2 ± 2.9 Ma(MSWD = 9.4) was determined, which is identical to the youngest zircon U–Pb age(103.9 ± 0.5 Ma) within error.The Shilu intrusion has high oxygen fugacity as indicated by high zircon Ce~(4+)/Ce~(3+) and Eu_N/Eu_N* ratios. Considering the geochemical characteristics(high Sr, and low Y and Yb contents), high oxygen fugacity, and copper mineralization of the Shilu intrusion, it was most likely formed by partial melting of a subducted young oceanic slab. Whole-rock Sr–Nd isotope-, zircon Hf isotope-, and whole-rock trace element analyses show that Shilu adakitic magmas may have interacted with type II enriched mantle and/or crustal materials during ascent. South China was affected by the Pacific tectonic regime to the east and the Neo-Tethys tectonic regime to the south in the Cretaceous. Based on the Pacific Plate drifting and rotation history, it is hard to explain how the Pacific Plate would have subducted and melted, forming adakitic rocks in the Shilu region. Considering the tectonic history of Southeast Asia and the South China Sea, the Neo-Tethys trench should have been much closer to the South China Block in the Cretaceous, and thus have had a greater impact on the South China Block. Based on the subduction direction, time of subduction,and distance between the Neo-Tethys subduction zone and the Shilu deposit, subduction of the Neo-Tethys ridge is the best mechanism for explaining the Shilu adakitic rocks and Cu–Mo mineralization.  相似文献   

16.
Late Mesozoic granitic magmatism(158–112 Ma) are widespread in the southern margin of the North China Craton(NCC), contemporary with many world-class Mo-Au-Ag-Pb-Zn polymetallic deposits. There are abrupt changes in the elements and isotopic compositions of these granites at about 127 Ma. The early stage(158–128 Ma) granites show slightly or no negative Eu anomalies, large ion lithophile elements enriched and heavy REE depleted(such as Y and Yb), belonging to typical I-type granite. The late stage(126–112 Ma) granites are characterized by A-type and/or highly fractionated I-type granite, with higher contents of SiO2, K2 O, Y, Yb and Rb/Sr ratio and lower contents of Sr, δEu value and Sr/Y ratio than that of the early-stage granites.Moreover, the whole rock Nd and Hf isotopic compositions of the granites younger than 127 Ma show more depleted than those of the older one. The two stages of Late Mesozoic granites were derived from a source region of the ancient basement of the southern margin of the NCC incorporated the mantle material. The late stage(126–112 Ma) granites contain more fractions of mantle material with depleted isotopic composition than the early ones. The granites record evidence for a strong crust-mantle interaction. They formed in an intracontinental extensional setting which was related to lithospheric thinning and asthenospheric upwelling in this region, which was possibly caused by westward subduction of the Paleo-Pacific plate. 127 Ma is an critical period of the transformation of the tectonic regime.  相似文献   

17.
The Dongco ophiolite occurred in the middle-western segment of the Bangong-Nujiang suture zone. The thickness of the ophiolite suite is more than 5 km, which is composed, from bottom to top, of the mantle peridotite, mafic-ultramafic cumulates, basic sills (dykes) and basic lava and tectoni- cally emplaced in Jurassic strata (Mugagongru Group). The Dongco cumulates consist of dunite- troctolite-olivine-gabbro, being a part of DTG series of mafic-ultramafic cumulates. The basic lavas are characterized by being rich in alkali (Na2O K2O), TiO2, P2O5 and a LREE-rich type pattern dip- ping right with [La/Yb]=6.94―16.6 as well as a trace elements spider-diagram with normal anomaly of Th, Nb, Ta, Hf. Therefore, the Dongco basic lavas belong to ocean-island basalt (OIB) and dis- tinctly differ from mid-ocean ridge basalt (MORB) and island-arc basalt (IAB) formed in the plate convergence margin. The basic lavas have higher 87Sr/86Sr (0.704363―0.705007), lower 143Nd/144Nd (0.512708―0.512887) and εNd(t ) from 2.7― 5.8, indicating that they derive from a two-components mixing mantle source of depleted mantle (DM) and enriched mantle (EMI). From above it is ready to see that the Dongco ophiolite forms in oceanic island (OIB) where the mantle source is replaced by a large amount of enriched material, therefore it distinctly differs from these ophiolites formed in island-arc and mid-oecan ridge. Newly obtained SHRIMP U-Pb dating for zircon of the cumulate troctolite is 132 ± 3 Ma and whole-rock dating of ~(39)Ar/~(40)Ar for the basalt is 173.4 ± 2.7 Ma and 140.9 ± 2.8 Ma, indicating that the Dongco ophiolite formed at Early Cretaceous and the middle-western segment of the Bangong-Nujiang oceanic basin was still in the developing and evolving period at Early Cretaceous.  相似文献   

18.
A map of major Precambrian mafic dyke swarms and related units in the North China Craton is compiled, and the features and geological implications of these swarms are demonstrated. The Archean dyke swarms are available to portray the early crustal growth and cratonization. The middle Paleoproterozoic(2200–1850 Ma) swarms and related magmatic series could constrain the tectonic evolution: They approve that the craton was amalgamated by two sub-cratons. The late Paleoproterozoic(1800–1600 Ma), Mesoproterozoic(1400–1200 Ma) and Neoproterozoic(1000–800 Ma) series swarms are important in paleogeographic reconstruction: they indicate that North China might have connected with some of the North European and North American cratons during Proterozoic. Dyke swarms are not only geological timescales and tectonic markers but also evolution indicators of lithospheric mantle: they imply a rejuvenation of the sub-continental lithospheric mantle of North China at 1780–1730 Ma. These swarms occurred with several rifts, including the Hengling(2200–1970 Ma), Xuwujia(1970–1880 Ma), Xiong'er(1800–1600 Ma), Yan-Liao(1730–1200 Ma), and Xu-Huai(1000–800 Ma). Among them, the Xuwujia rift was possibly continental arc associated; whereas the others were intra-continental. In addition, the Xiong'er and Xu-Huai rifts were possibly triple junctions along the present southern and southeastern margins of the Craton, respectively. Different tectonic settings of these rifts and dyke swarms would result in diversified series of ore deposits.  相似文献   

19.
The late Permian–Triassic granites in southeastern China have important tectonic significance for the evolution of South China. Here, we present the detailed geochronological, geochemical and petrological analyses for the Jinlongyan(JLY) granite in northwest Fujian Province, southeast China. LA–ICP–MS zircon U–Pb dating yielded a weighted average ~(206)Pb/~(238)U age of 224.1 ±3.3 Ma. The granite is mainly comprised of K-feldspar,plagioclase, quartz, biotite and minor amphibole. It is characterized by enrichments in Rb, Th, REEs(total REE = 295.1–694.3 ppm), and HFSEs(e.g., Zr = 289–520 ppm, Hf = 9.3–15.0 ppm, Y = 36.2–68.2 ppm) but depletions in Ba, Sr, Eu and Ti. The granite is metaluminous to weakly peraluminous and show a clear A-type granite geochemical signature with high SiO_2(70.89 wt%–75.76 wt%), total alkalis(Na_2O + K_2O = 7.51 wt%–8.72 wt%), Ga/Al ratios(10000 Ga/Al = 2.72–3.43). Insitu zircon Hf isotope analysis shows their eHf(t) values ranging from-7.2 to-3.2, with Mesoproterozoic T2DM ages(1308–1525 Ma). Whole-rock Nd isotope data show their eNd(t) values in the range of-9.5 to-9.1 and yield paleoproterozoic TDMages(1606–1985 Ma). These characteristics indicate that the JLY A-type granite magma was formed by the partial melting of Meso-Paleoproterozoic crust rocks in the Cathaysia Block. Our study of the JLY A-type granite, together with other Triassic A-type granitesin South China, defines an extensional environment in the late Triassic which probably was caused by the collision of the South China Block with Indochina Block.  相似文献   

20.
It is well known that the destruction of the North China Carton(NCC) is closely related to subduction of the PaleoPacific slab, but materials recording such subduction has not been identified at the peak time of decratonization. This paper presents data of whole-rock major and trace elements and Sr-Nd-Hf isotopes and zircon U-Pb ages and Hf-O isotopes for Mesozoic volcanic rocks from the Liaodong-Jinan region in the northeastern NCC, in order to trace the subduction-related materials in their source and origin. The Mesozoic volcanic rocks in the Liaodong-Jinan region are mainly composed of two series of rocks, including alkaline basaltic trachyandesite, trachyandesite and trachyte, and subalkaline trachyandesite and andesite. Zircon U-Pb dating yields eruption ages of 129–124 Ma for these rocks. The Early Cretaceous volcanic rocks are all enriched in LILEs(such as Rb, Sr, Ba and Th) and LREEs, depleted in HFSEs(such as Nb, Ta and Ti), indicating that they were originated from mantle sources that had been modified by subducted crustal materials. However, they have relatively heterogeneous and variable isotopic compositions. The alkaline basaltic trachyandesite, trachyandesite and trachyte have enriched whole-rock Sr-Nd-Hf and zircon Hf isotopic compositions and mantle-like δ~(18)O values, suggesting that they were derived from low-degree partial melting of an isotopically enriched lithospheric mantle source. In contrast, the subalkaline trachyandesite and andesite have relatively depleted isotopic compositions with zircon ε_(Hf)(t) values up to +5.2 and heavy zircon O isotopic compositions with δ~(18)O values of +8.1‰ to +9.0‰, indicating that they were originated from a lithospheric mantle source that had been metasomatized by melts/fluids derived from the recycled low-T altered oceanic basalt. All of these geochemical features suggest that the Early Cretaceous volcanic rocks in the Liaodong-Jinan region would result from mixing of mafic magmas with different compositions. Such magmas were originated from the enriched lithospheric mantle and the young metasomatized mantle, respectively, with variable extents of enrichment and depletion in trace elements, radiogenic isotopes and O isotopes. Importantly, the identification of the low-T altered oceanic crust component in the origin of Early Cretaceous volcanic rocks by the zircon Hf-O isotopes provides affirmative isotopic evidence and direct material records for Mesozoic subduction of the Paleo-Pacific slab that induced decratonization of the North China Craton.  相似文献   

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