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1.
Mesozoic volcanic rocks are widespread throughout the Great Xing'an Range of northeastern China. However, there has been limited investigation into the age and petrogenesis of the Mesozoic volcanics in the eastern Great Xing'an Range. According to our research, the volcanic rocks of the Dayangshu Basin, eastern Great Xing'an Range are composed mainly of trachybasalt, basaltic andesite, and basaltic trachyandesite, with minor intermediate–basic pyroclastic rocks. In this study, the geochemistry and geochronology of the Mesozoic volcanic rocks are presented in order to discuss the petrogenesis and tectonic setting of the Ganhe Formation in the Dayangshu Basin. Zircon U–Pb dating by laser ablation inductively coupled plasma–mass spectrometry indicates that the Mesozoic lavas formed during the late Early Cretaceous (114.3–108.8 Ma). This suite of rocks exhibits a range of geochemical signatures indicating subduction‐related genesis, including: (i) calc‐alkaline to high‐K calc‐alkaline major element compositions; (ii) enrichment of large ion lithophile elements (e.g. Rb, Ba, K) and light rare earth elements (LREEs/HREEs =7.33–9.85); and (iii) weak depletion in high field strength elements (e.g. Nb, Ta, Ti). Furthermore, Sr–Nd–Pb isotopic data yield initial 87Sr/86Sr values of 0.70450–0.70463, positive εNd(t) values of +1.8 to +3.3, and a mantle‐derived lead isotope composition. Combined with the regional tectonic evolution, the results of this study suggest that the Ganhe Group lavas are derived from decompression melting of a metasomatized (enriched) lithospheric mantle, related to asthenospheric upwelling, which resulted from lithospheric mantle delamination and produced extension of the continental margin following the subduction of the Paleo‐Pacific Plate.  相似文献   

2.
Although a number of petrographic observations and isotopic data suggest that magma mixing is common in genesis of many granite plutons, it is still controversial whether the mantle-derived magmas were involved in granites. We carried out in this study a systematic analysis of in situ zircon Hf-O isotopes for three early Yanshanian intrusions dated at ca. 160 Ma from the Nanling Range of Southeast China. The Qinghu monzonite has very homogeneous zircon Hf-O isotopic compositions, εHf(t) =11.6±0.3 and δ18O=5...  相似文献   

3.
The Nanling Mountains lying in the southern part of South China are an economically important gran-ite-related multi-metallogenic province. The Nanling Mountains granites can be described as: temporally spanning from Caledonian to Yanshanian and spatially distributed as three EW trending zones: the north one in Zhuguangshan-Qingzhangshan, the middle one in Dadongshan-Guidong, and the south one in Fogang-Xinfengjiang with two neighboring zones’ midline having an interval of ca. latitude …  相似文献   

4.
This paper presents systematic studies on the C—O and Sr—Nd isotopic compositions for Cretaceous Badou carbonatites, Fangcheng basalts, and Jiaodong lamprophyres and Paleozoic Mengyin kimberlites in Shandong Province, China. Paleozoic kimberlites have normal and uniform C—O isotopic compositions with δ13C and δ18O in the range of −4.8‰—−7.6‰ and +9.9‰—+13.2‰, respectively. However, Cretaceous three different types of mantlederived rocks have quite different C—O isotopic compositions, indicating that the mantle sources are probably partially contaminated with organic carbon-bearing crustal materials. These Cretaceous rocks show uniform and EMII-like Sr—Nd isotopic compositions and also indicate that the mantle sources were affected by recycled crustal materials. Comparative studies of C—O and Sr—Nd isotopes reveal that the lithospheric mantle beneath the eastern North China Craton had different isotope characteristics in the Paleozoic, the early Cretaceous, and the Tertiary time. This demonstrates that the lithospheric mantle beneath the region underwent at least twice reconstructions since the Paleozoic. Available data imply that the first reconstruction mainly happened during the Triassic-Jurassic time with gradual changes and the second in the Cretaceous with abrupt changes. Results also show that the early Cretaceous (especially at 120-130 Ma) was perhaps the key period leading to the dramatic change of the Mesozoic geodynamics on the eastern North China Craton.  相似文献   

5.
The tectonic setting of the late mesozoic of South China is in a debate between two schools of thought: an intra‐continental rift zone along a passive continental margin or active rifting associated with subduction of the paleo‐Pacific Plate. In this study, we present new sensitive high‐resolution ion microprobe (SHRIMP) U‐Pb zircon ages, along with geochemical data of three basic dikes that cross‐cut the Dexing porphyry copper deposit. The deposit is the largest of its kind in eastern China and part of large scale mineralization associated with Mesozoic magmatic activity in the area. Our results indicate that the dikes were emplaced in the Late Jurassic with an average U‐Pb age of 153.5 ± 2.4 Ma. The intrusions have bulk εNd(t) of ca +0.7 and zircon εHf(t) value of +1.54 to +6.92. Based on relatively enriched light rare earth elements (LREE) and depleted high‐field‐strength elements (HFSE) abundances with pronounced negative Ta–Nb, Hf–Zr and Ti anomalies in multi‐element diagrams, we propose that these dikes were derived from a subduction‐modified lithospheric mantle source. The variability in Hf isotopes identifies some degree of crustal contaminations. Our data support a scenario with a back‐arc extensional setting or an intra‐arc rift environment associated with the westward subduction of the paleo‐Pacific Plate at or prior to the late Jurassic as the most likely cause for these subduction signatures.  相似文献   

6.
Major- and rare-earth-element (REE) concentrations and UThPb, SmNd, and RbSr isotope systematics are reported for Cenozoic volcanic rocks from northeastern and eastern China. These volcanic rocks, characteristically lacking the calc-alkaline suite of orogenic belts, were emplaced in a rift system which formed in response to the subduction of the western Pacific plate beneath the eastern Asiatic continental margin. The rocks sampled range from basanite and alkali olivine basalt, through olivine tholeiite and quartz tholeiite, to potassic basalts, alkali trachytes, pantellerite, and limburgite. These rock suites represent the volcanic centers of Datong, Hanobar, Kuandian, Changbaishan and Wudalianchi in northeastern China, and Mingxi in the Fujian Province of eastern China.The major-element and REE geochemistry is characteristic of each volcanic suite broadly evolving through cogenetic magmatic processes. Some of the outstanding features of the isotopic correlation arrays are as follows: (1) NdSr shows an anticorrelation within the field of ocean island basalts, extending from the MORB end-member to an enriched, time-averaged high Rb/Sr and Nd/Sr end-member (EM1), (2) SrPb also shows an anticorrelation, similar to that of Hawaiian and walvis Ridge basalts, (3) NdPb shows a positive correlation, and (4) the 207Pb/204Pb vs 206Pb/204Pb plot shows linear arrays parallel to the general trend (NHRL) for MORB on both sides of the geochron, although in the 208Pb/204Pb vs 206Pb/204Pb plot the linear array is significantly displaced above the NHRL in a pattern similar to that of the oceanic island basalts that show the Dupal signatures. In all isotope correlation patterns, the data arrays define two different mantle components—a MORB-like component and an enriched mantle component. The isotopic data presented here clearly demonstrate the existence of Dupal compositions in the sources of the continental volcanic rocks of eastern China. We suggest that the subcontinental mantle beneath eastern China served as the reservoir for the EMI component, and that the MORB component was either introduced by subduction of the Kula-Pacific Ridge beneath the Asiatic plate in the Late Cretaceous, as proposed by Uyeda and Miyashiro, or by upwellings in the subcontinental asthenosphere due to subduction.  相似文献   

7.
The Lhasa Terrane in southern Tibet has long been accepted as the last geological block accreted to Eurasia before its collision with the northward drifting Indian continent in the Cenozoic, but its lithospheric architecture, drift and growth histories and the nature of its northern suture with Eurasia via the Qiangtang Terrane remain enigmatic. Using zircon in situ U–Pb and Lu–Hf isotopic and bulk-rock geochemical data of Mesozoic–Early Tertiary magmatic rocks sampled along four north–south traverses across the Lhasa Terrane, we show that the Lhasa Terrane has ancient basement rocks of Proterozoic and Archean ages (up to 2870 Ma) in its centre with younger and juvenile crust (Phanerozoic) accreted towards its both northern and southern edges. This finding proves that the central Lhasa subterrane was once a microcontinent. This continent has survived from its long journey across the Paleo-Tethyan Ocean basins and has grown at the edges through magmatism resulting from oceanic lithosphere subduction towards beneath it during its journey and subsequent collisions with the Qiangtang Terrane to the north and with the Indian continent to the south. Zircon Hf isotope data indicate significant mantle source contributions to the generation of these granitoid rocks (e.g., ~ 50–90%, 0–70%, and 30–100% to the Mesozoic magmatism in the southern, central, and northern Lhasa subterranes, respectively). We suggest that much of the Mesozoic magmatism in the Lhasa Terrane may be associated with the southward Bangong–Nujiang Tethyan seafloor subduction beneath the Lhasa Terrane, which likely began in the Middle Permian (or earlier) and ceased in the late Early Cretaceous, and that the significant changes of zircon εHf(t) at ~ 113 and ~ 52 Ma record tectonomagmatic activities as a result of slab break-off and related mantle melting events following the Qiangtang–Lhasa amalgamation and India–Lhasa amalgamation, respectively. These results manifest the efficacy of zircons as a chronometer (U–Pb dating) and a geochemical tracer (Hf isotopes) in understanding the origin and histories of lithospheric plates and in revealing the tectonic evolution of old orogenies in the context of plate tectonics.  相似文献   

8.
The North China Craton(NCC) experienced strong destruction(i.e., decratonization) during the Mesozoic, which triggered intensive magmatism, tectonism and thermal events and formed large-scale gold and other metal deposits in the eastern part of the craton. However, how the decratonization controls the formation and distribution of large-scale of gold and other metal deposits is not very clear. Based on a large number of published data and new results, this paper systematically summarizes all the data for the rock assemblages, chronology, geochemistry and petrogenesis of Mesozoic magmatic rocks, as well as for the mineralizing ages of gold and other metal deposits and the evolution of the Mesozoic basins in the eastern NCC. The results are used to restore the extensional rates of Mesozoic to Cenozoic basins and the strike-slip distance of the Tanlu Fault, to ascertain the location of the Paleo-Pacific plate subduction zones during the Mesozoic to Cenozoic, and to reconstruct the temporal and spatial distribution of Mesozoic gold and other metal deposits and magmatic rocks in the eastern NCC. It is obtained that the magmatism and mineralization in the eastern NCC westward migrate from east to west during the Early to Middle Jurassic, but they eastward migrate from west to east during the Early Cretaceous. The metallogenesis of these deposits is genetically related to magmatism, and the magmas provided some ore-forming materials and fluids for the generation of metal deposits. The geodynamic mechanism of decratonization and related magmatism and mineralization is proposed, i.e., the westward low-angle subduction of the Paleo-Pacific slab beneath the NCC formed continental magmatic arc with plenty of porphyry Cu-Mo-Au deposits in the Jurassic, similar to the Andean continental arc in South America. The mantle wedge was metasomatized by the fluids/melts derived from the subducting slab, laying a material foundation for hydrothermal mineralization in the Early Cretaceous. While the rollback of the subducting slab with gradually increasing subduction angle and the retreat of the subduction zones during the Early Cretaceous induced strong destruction of the craton and the formation of extensive magmatic rocks and large-scale gold and other metal deposits.  相似文献   

9.
Trace elements and isotopic compositions of whole rocks and mineral separates are reported for 15 spinel-bearing harzburgite and lherzolite xenoliths from southeastern Australia. These samples have an exceedingly large range in isotopic compositions, with87Sr/86Sr ranging from 0.70248 to 0.70834 and εNd values ranging from +12.7 to −6.3. This range in isotopic compositions can be found in xenoliths from a single locality. The isotopic compositions of clinopyroxene separates and their whole rocks were found to be different in some xenoliths. Samples containing small glass pockets, which replace pre-existing hydrous minerals, generally show only small differences in isotopic composition between clinopyroxene and whole rock. In a modally metasomatized peridotite, significant differences in the Sr and Nd isotopic compositions of a coexisting phlogopite-clinopyroxene pair are present. Coexisting clinopyroxenes and orthopyroxenes from an anhydrous lherzolite have Sr isotopic compositions that are significantly different (0.70248 versus 0.70314), and yield an apparent age of 625 Ma, similar to that found previously by Dasch and Green [1]. However, the Nd isotopic compositions of the clinopyroxene and orthopyroxene are identical indicating recent (within 40 Ma) re-equilibration of Nd.Sr and Nd concentrations in the whole rocks and clinopyroxenes show an excellent positive correlation, and have an average Sr/Nd ratio of 15. This ratio is similar to the primitive mantle value, as well as that found in primitive MORBs and OIBs, but is much lower than that measured in island arc basalts and what might be predicted for a subduction zone-derived fluid. This indicates that a significant proportion of the Sr and Nd in these peridotites is introduced as a basaltic melt with intraplate chemical characteristics.The isotopic compositions of the peridotites reflect long-term, small-scale heterogeneities in the continental lithospheric mantle, and are in marked contrast to the near uniform isotopic compositions of the host alkali basalts (87Sr/86Sr= 0.7038–0.7041andεNd = +3.6 to +2.9). A minimum of three evolutionary stages are identified in the growth of the continental lithospheric mantle: an early basalt depletion event, recording the initial development and stabilization of the lithospheric mantle, followed by at least two enrichment episodes. These observations are consistent with continental lithospheric mantle growth involving the underplating of refractory peridotite diapirs.  相似文献   

10.
The Gangdese magmatic belt is located in the southern margin of the Lhasa terrane, south Tibet. Here zircon U–Pb ages and Hf isotopic data, as well as whole‐rock geochemistry and Sr–Nd isotopes on andesites from the Bima Formation with a view to evaluating the history of the Gangdese magmatism and the evolution of the Neotethys Ocean. Zircon U–Pb dating yields an age of ca 170 Ma from six samples, representing the eruptive time of these volcanic rocks. Zircon Hf isotopes show highly positive εHf(t) values of +13 to +16 with a mean of +15.2. Whole‐rock geochemical and Sr–Nd isotopic results suggest that the magma source of these andesites was controlled by partial melting of a depleted mantle source with addition of continental‐derived sediments, similar to those in the southern arcs of the Lesser Antilles arc belt. In combination with published data, the volcanic rocks of the Bima Formation are proposed to have been generated in an intra‐oceanic arc system, closely associated with northward subduction of the Neotethyan oceanic lithosphere.  相似文献   

11.
The North China Craton (NCC) witnessed Mesozoic vigorous tectono-thermal activities and transition in the nature of deep lithosphere. These processes took place in three periods: (1) Late Paleozoic to Early Jurassic (～170 Ma); (2) Middle Jurassic to Early Cretaceous (160–140 Ma); (3) Early Cretaceous to Cenozoic (140 Ma to present). The last two stages saw the lithospheric mantle replacement and coupled basin-mountain response within the North China Craton due to subduction and retreating of the Paleo-Pacific plate, and is the emphasis in this paper. In the first period, the subduction and closure of the Paleo- Asian Ocean triggered the back-arc extension, syn-collisional compression and then post-collisional extension accompanied by ubiquitous magmatism along the northern margin of the NCC. Similar processes happened in the southern margin of the craton as the subduction of the Paleo-Tethys ocean and collision with the South China Block. These processes had caused the chemical modification and mechanical destruction of the cratonic margins. The margins could serve as conduits for the asthenosphere upwelling and had the priority for magmatism and deformation. The second period saw the closure of the Mongol-Okhotsk ocean and the shear deformation and magmatism induced by the drifting of the Paleo-Pacific slab. The former led to two pulse of N-S trending compression (Episodes A and B of the Yanshan Movement) and thus the pre-existing continental marginal basins were disintegrated into sporadically basin and range province by the Mesozoic magmatic plutons and NE-SW trending faults. With the anticlockwise rotation of the Paleo-Pacific moving direction, the subduction-related magmatism migrated into the inner part of the craton and the Tanlu fault became normal fault from a sinistral one. The NCC thus turned into a back-arc extension setting at the end of this period. In the third period, the refractory subcontinental lithospheric mantle (SCLM) was firstly remarkably eroded and thinned by the subduction-induced asthenospheric upwelling, especially those beneath the weak zones (i.e., cratonic margins and the lithospheric Tanlu fault zone). Then a slightly lithospheric thickening occurred when the upwelled asthenosphere got cool and transformed to be lithospheric mantle accreted (～125 Ma) beneath the thinned SCLM. Besides, the magmatism continuously moved southeastward and the extensional deformations preferentially developed in weak zones, which include the Early Cenozoic normal fault transformed from the Jurassic thrust in the Trans-North Orogenic Belt, the crustal detachment and the subsidence of Bohai basin caused by the continuous normal strike slip of the Tanlu fault, the Cenozoic graben basins originated from the fault depression in the Trans-North Orogenic Belt, the Bohai Basin and the Sulu Orogenic belt. With small block size, inner lithospheric weak zones and the surrounding subductions/collisions, the Mesozoic NCC was characterized by (1) lithospheric thinning and crustal detachment triggered by the subduction-induced asthenospheric upwelling. Local crustal contraction and orogenesis appeared in the Trans-North Orogenic Belt coupled with the crustal detachment; (2) then upwelled asthenosphere got cool to be newly-accreted lithospheric mantle and crustal grabens and basin subsidence happened, as a result of the subduction zone retreating. Therefore, the subduction and retreating of the western Pacific plate is the outside dynamics which resulted in mantle replacement and coupled basin-mountain respond within the North China Craton. We consider that the Mesozoic decratonization of the North China Craton, or the Yanshan Movement, is a comprehensive consequence of complex geological processes proceeding surrounding and within craton, involving both the deep lithospheric mantle and shallow continental crust.  相似文献   

12.
The Lower Pliocene volcanic rocks occurring in the Gölcük area of SW Turkey exhibit alkaline major element trends with a general potassic character. The development of volcanism can be divided into 2 major stages such as trachytic ancient lavas/domes and tephriphonolitic, trachyandesitic to trachytic Gölcük eruptions (ignimbrites, lava/dome extrusions, phreatomagmatic deposits, and finally, young domes). Volcanic rocks consist primarily of plagioclase, clinopyroxene (which ranges in composition from diopside to augite and are commonly zoned), biotite, and phlogopite. Amphibole phenocrysts are restricted to the pyroclastic deposits. Pseudoleucites are also seen only in the lava/dome extrusions. Oxides and apatites are common accessory phenocryst phases. As would be expected from their potassic–alkaline nature, the volcanic rocks of the Gölcük area contain high amounts of LILE (Ba, Sr, Rb and K), LREE, and Zr. Concentrations of compatible elements such as Cr, Ni and V are very low, possibly indicating fractionation of olivine and clinopyroxene. Correlation of SiO2, Rb/Sr and MgO with 87Sr/86Sr (0.703506–0.704142) exhibit an increasing trend in the direction of crustal contamination. However, the isotopic compositions of Sr are not as high to indicate a high level of crustal contamination. Geochemical data are consistent with the derivation of Gölcük volcanic rocks from a metasomatized and/or enriched lithospheric mantle source during crustal extension in the area. This metasomatism was probably occurred by fluids released from the northward subduction between African and Eurasian plates during Tertiary, as the Gölcük volcanic rocks display features of island-arc magmas with having high Ba/Nb (>28) ratios, and Nb and Ti depletions. Lower Pliocene volcanism in the Gölcük was response to extensional tectonics.  相似文献   

13.
Abstract This paper presents systematic studies on the C-O and Sr-Nd isotopic compositions for Cretaceous Badou carbonatites, Fangcheng basalts, and Jiaodong lamprophyres and Paleozoic Mengyin kimberlites in Shandong Province, China. Paleozoic kimberlites have normal and uniform C-O isotopic compositions with δ13C and δ18O in the range of-4.8‰-7.6‰ and +9.9‰-+13.2‰, respectively. However, Cretaceous three different types of mantle-derived rocks have quite different C-O isotopic compositions, indicating that the mantle sources are probably partially contaminated with organic carbon-bearing crustal materials. These Cretaceous rocks show uniform and EMII-like Sr-Nd isotopic compositions and also indicate that the mantle sources were affected by recycled crustal materials. Comparative studies of C-O and Sr-Nd isotopes reveal that the lithospheric mantle beneath the eastern North China Craton had different isotope characteristics in the Paleozoic, the early Cretaceous, and the Tertiary time. This demon  相似文献   

14.
Zircon U-Pb results of basalt from the Dashizhai Town in Inner Mongolia, NE China, shows that the basaltic lava was erupted at 439±3 Ma, much older than the “Permian basalts” as previously thought. These rocks show arc-type trace element patterns (i.e., Nb-Ta depletion and light REE and large ion lithophile element enrichment) and unradiogenic Sr and highly radiogenic Nd and Hf isotope compositions. They can be subdivided into two petrogenetic groups: Group 1 basalts have relatively high TiO2, MgO and compatible elements and low Sr and Th, characterized by mid-oceanic ridge basalt (MORB)-type Sr-Nd-Hf isotope compositions (87Sr/86Sr(i)=0.7028−0.7032, εNd(t)=+9.8−+11.2, εHf(t)=+16.1−+18.4). Group 2 has lower TiO2, MgO and compatible elements and higher Sr and Th, and relatively evolved Sr-Nd-Hf isotope compositions (87Sr/86Sr(i)=0.7037−0.7038, εNd(t)=+5.7−+7.3, εHf(t)=+12.6−+13.0). Both groups were interpreted as melts derived from a metasomatized mantle wedge formed during the subduction of Paleo-Asian Ocean. The mantle source for Group 1 was probably a highly isotopically depleted oceanic mantle modified by predominant slab fluids; whereas subducted sediments had an important contribution to the melting source for Group 2. The petrogenesis of the Dashizhai basalts provides clear evidence for early Paleozoic subduction of the Paleo-Asian Ocean, and the highly radiogenic Nd and Hf compositions in these rocks suggest that these lavas and their possible intrusive counterparts were one of the important components for Phanerozoic crustal growth. Our and previous studies on the “Dashizhai Formation” volcanic rocks yield an unrealistic eruption range of 440-270 Ma for different rock types, we thus advise to disassemble the previously defined “Dashizhai Formation” into multiple lithologic units and to reinterpret the spatial and temporal distributions of different volcano-sedimentary associations. Supported by National Basic Research Program of China (Grant No. 2006CB403504)  相似文献   

15.
Tholeiitic basalts and associated intrusives are the major component of the Karoo igneous province. They are of Mesozoic age and constitute one of the world's classic continental flood basalt (CFB) provinces. It has been argued that most Karoo basalts have not undergone significant contamination with continental crust and that their lithospheric mantle source areas were enriched in incompatible minor and trace elements during the Proterozoic. The only exceptions to this are late-stage MORB-like dolerites near the present-day continental margins which are considered to be of asthenospheric origin.When data for the “southern” Karoo basalts are plotted on many of the geochemical discriminant diagrams which have been used to infer tectonic setting, essentially all of them would be classified as calc-alkali basalts (CAB's) or low-K tholeiites. Virtually none of them plot in the compositional fields designated as characteristic of “within-plate” basalts. There is little likelihood that the compositions of the Karoo basalts can be controlled by active subduction at the time of their eruption and no convincing evidence that a “subduction component” has been added to the subcontinental lithospheric mantle under the entire area in which the basalts crop out. It must be concluded that the mantle source areas for CAB's and the southern Karoo basalts have marked similarities.In contrast, the data for “northern” Karoo basalts largely plot in the “within-plate” field on geochemical discriminant diagrams. Available data suggest that the source composition and/or the restite mineralogy and degree of partial melting are different for southern and northern Karoo basalts. There is no evidence for any difference in tectonic setting between the southern and northern Karoo basalts at the time they were erupted. This appears to be clear evidence that specific mantle source characteristics and/or magmatic processes can vary within a single CFB province to an extent that renders at least some geochemical discriminant diagrams most unreliable for classifying tectonic environment with respect to continental volcanic rocks.  相似文献   

16.
Initial87Sr/86Sr ratios have been determined for 34 plutonic and volcanic rocks covering the entire age span of magmatic events associated with the Andean orogeny between latitudes 26° and 29° south. The igneous rocks, the majority dated by K/Ar mineral techniques, range in age from Lower Jurassic (190 m.y.) to Quaternary (0.89 m.y.). In addition, initial ratios were determined for three granitoid plutons and one metasediment from the pre-Mesozoic basement which underlies the entire Andean orogen in this transect at shallow depth. The compositions vary from basalt to rhyolite, and from quartz diorite to granodiorite or trondjemite, for the extrusives and intrusives, respectively.Mid-Cretaceous to Quaternary rocks exhibit a systematic west to east increase in mean strontium isotope ratio from 0.7022 to 0.7077, whereas the initial ratios of Jurassic plutons vary from 0.7043 to 0.7059, and do not correlate with age.The existence of unusually low initial ratios (e.g. 0.7022, 0.7023) for several Mesozoic plutonic rocks strongly implies a sub-crustal source for at least some of the Andean magmas. The time-dependent post-Jurassic increase in initial ratio is considered to reflect a systematic change in the composition of partial melts generated in response to the progressive subduction of a lithospheric slab. It is suggested that a systematic change in the locus of melting takes place from along or close to the upper surface of the subduction slab into hanging-wall mantle peridotite as subduction continues.  相似文献   

17.
Ar–Ar dating, major and trace element analyses, and Sr–Nd–Pb isotope results of two groups of Lower Cretaceous (erupted at 126 and 119 Ma, respectively) intermediate–felsic lava from the northeastern North China Block (NCB) suggest their derivation from melting of mixtures between the heterogeneous lower crust and underplated basalts. Both groups exhibit high‐K calc‐alkaline to shoshonitic affinities, characterized by light rare earth element (LREE) and large ion lithophile element (LILE) enrichment and variable high field strength element (HFSE, e.g. Nb, Ta and Ti) depletion, and moderately radiogenic Sr and unradiogenic Nd and Pb isotopic compositions. Compared with Group 2, Group 1 rocks have relatively higher K2O and Al2O3/(CaO + K2O + Na2O) in molar ratio, higher HFSE concentrations and lower Nb/Ta ratios, and higher Sr–Nd–Pb isotope ratios. Group 1 rocks were derived from a mixture of an enriched mantle‐derived magma and a lower crust that has developed radiogenic Sr and unradiogenic Nd and Pb isotopic compositions, whereas the Group 2 magmas were melts of another mixture between the same mantle‐derived component and another type of lower crust having even lower Sr, Nd, and Pb isotopic ratios. Shift in source region from Group 1 to Group 2 coincided with a change in melting conditions: hydrous melting of both the underplated basalt and the lower crust produced the earlier high‐Nb and low‐Nb/Ta melts with little or no residual Ti‐rich phases; while the younger low‐Nb and high‐Nb/Ta magmas were melted under a water‐deficient system, in which Ti‐rich phases were retained in the source. Generation of the two groups of intermediate–felsic volcanic rocks was genetically linked with the contemporaneous magma underplating event as a result of lithospheric thinning in the eastern NCB.  相似文献   

18.
Subduction‐related volcanic rocks are widespread in the Central Pontides of Turkey, and represented by the Hamsaros volcanic succession in the Sinop area to the north. The volcanic rocks display high‐K calc‐alkaline, shoshonitic and ultra‐K affinities. 40Ar/39Ar age data indicate that the rocks occurred during the Late Cretaceous (ca 82 Ma), and the volcanic suites were coeval. Primitive mantle‐normalized trace element patterns of all the lavas are characterized by strong enrichments in large ion lithophile elements (LILE) (Rb, Ba, K, and Sr), Th, U, Pb, and light rare earth elements (LREE; La, Ce) and prominent negative Nb, Ta, and Ti anomalies, all typical of subduction‐related lavas. There is a systematic increase in the enrichment of incompatible trace elements from the high‐K calc‐alkaline lavas through the shoshonitic to the ultra‐K lavas. In addition, the shoshonitic and ultra‐K lavas have significantly higher 87Sr/86Sr (0.70666–0.70834) and lower 143Nd/144Nd (0.51227–0.51236) initial ratios than coexisting high‐K calc‐alkaline lavas (87Sr/86Sr 0.70576–0.70613, 143Nd/144Nd 0.51245–0.51253). Geochemical and isotopic data show that the shoshonitic and ultra‐K rocks cannot be derived from the high‐K calc‐alkaline suite by any shallow level differentiation process, and point to a derivation from distinct mantle sources. The shoshonitic and ultra‐K rocks were derived from metasomatic veins related to melting of recycled subducted sediments, but the high‐K calc‐alkaline rocks from a lithospheric source metasomatized by fluids from subduction zone.  相似文献   

19.
The North China Craton (NCC) has been thinned from >200 km to <100 km in its eastern part. The ancient subcontinental lithospheric mantle (SCLM) has been replaced by the juvenile SCLM in the Meoszoic. During this period, the NCC was destructed as indicated by extensive magmatism in the Early Cretaceous. While there is a consensus on the thinning and destruction of cratonic lithosphere in North China, it has been hotly debated about the mechanism of cartonic destruction. This study attempts to provide a resolution to current debates in the view of Mesozoic mafic magmatism in North China. We made a compilation of geochemical data available for Mesozoic mafic igneous rocks in the NCC. The results indicate that these mafic igneous rocks can be categorized into two series, manifesting a dramatic change in the nature of mantle sources at ~121 Ma. Mafic igneous rocks emplaced at this age start to show both oceanic island basalts (OIB)-like trace element distribution patterns and depleted to weakly enriched Sr-Nd isotope compositions. In contrast, mafic igneous rocks emplaced before and after this age exhibit both island arc basalts (IAB)-like trace element distribution patterns and enriched Sr-Nd isotope compositions. This difference indicates a geochemical mutation in the SCLM of North China at ~121 Ma. Although mafic magmatism also took place in the Late Triassic, it was related to exhumation of the deeply subducted South China continental crust because the subduction of Paleo-Pacific slab was not operated at that time. Paleo-Pacific slab started to subduct beneath the eastern margin of Eruasian continent since the Jurrasic. The subducting slab and its overlying SCLM wedge were coupled in the Jurassic, and slab dehydration resulted in hydration and weakening of the cratonic mantle. The mantle sources of ancient IAB-like mafic igneous rocks are a kind of ultramafic metasomatites that were generated by reaction of the cratonic mantle wedge peridotite not only with aqueous solutions derived from dehydration of the subducting Paleo-Pacific oceanic crust in the Jurassic but also with hydrous melts derived from partial melting of the subducting South China continental crust in the Triassic. On the other hand, the mantle sources of juvenile OIB-like mafic igneous rocks are also a kind of ultramafic metasomatites that were generated by reaction of the asthenospheric mantle underneath the North China lithosphere with hydrous felsic melts derived from partial melting of the subducting Paleo-Pacific oceanic crust. The subducting Paleo-Pacific slab became rollback at ~144 Ma. Afterwards the SCLM base was heated by laterally filled asthenospheric mantle, leading to thinning of the hydrated and weakened cratonic mantle. There was extensive bimodal magmatism at 130 to 120 Ma, marking intensive destruction of the cratonic lithosphere. Not only the ultramafic metasomatites in the lower part of the cratonic mantle wedge underwent partial melting to produce mafic igneous rocks showing negative εNd(t) values, depletion in Nb and Ta but enrichment in Pb, but also the lower continent crust overlying the cratonic mantle wedge was heated for extensive felsic magmatism. At the same time, the rollback slab surface was heated by the laterally filled asthenospheric mantle, resulting in partial melting of the previously dehydrated rocks beyond rutile stability on the slab surface. This produce still hydrous felsic melts, which metasomatized the overlying asthenospheric mantle peridotite to generate the ultramafic metasomatites that show positive εNd(t) values, no depletion or even enrichment in Nb and Ta but depletion in Pb. Partial melting of such metasomatites started at ~121 Ma, giving rise to the mafic igneous rocks with juvenile OIB-like geochemical signatures. In this context, the age of ~121 Ma may terminate replacement of the ancient SCLM by the juvenile SCLM in North China. Paleo-Pacific slab was not subducted to the mantle transition zone in the Mesozoic as revealed by modern seismic tomography, and it was subducted at a low angle since the Jurassic, like the subduction of Nazca Plate beneath American continent. This flat subduction would not only chemically metasomatize the cratonic mantle but also physically erode the cratonic mantle. Therefore, the interaction between Paleo-Pacific slab and the cratonic mantle is the first-order geodynamic mechanism for the thinning and destruction of cratonic lithosphere in North China.  相似文献   

20.
himu, em i andem ii are three of the main geochemical mantle components that give rise to oceanic island basalts [1]. They represent the end members that produce the extreme isotopic compositions measured on intraplate volcanics. In French Polynesia, all three mantle components are represented in volcanic rocks. The characteristichimu signature is found in Tubuai, Mangaia and Rurutu,em i is present in the source of Rarotonga and Pitcairn volcanics andem ii dominates the composition of most Society Islands. Intermediate values between the three end members are found on most islands.We suggest that the three components are not independent but are physically related in the mantle. Thehimu component is thought to be recycled oceanic crust that lost part of its Pb through hydrothermal processes prior to and during subduction.em i andem ii are believed to acquire their isotopic and trace element characteristics through entrainment of sediments that were subducted together with the oceanic crust.The trace element pattern and the isotopic composition ofhimu lavas can be quantitatively modelled using a mixture of 25% old recycledmorb crust and 75% mantle peridotite. The extreme Pb composition is modelled assuming that Pb was lost from oceanic crust when hydrothermal alteration at the ridge leached Pb from the basalt to redeposit it as sulphides on top of and throughout the crust, followed by preferential dissolution of sulphides during dehydration in the subduction zone. These processes led to a drastic increase of theU/Pb ratio of the subducted material which evolved over 2 Ga to very radiogenic Pb isotopic compositions. Pb isotopic compositions similar to those ofem i andem ii are modelled assuming that sediments with average crustal Pb isotopic compositions were subducted and recycled into the mantle together with the underlyingmorb oceanic crust. Pelagic sediments (μ 5 andκ 6) account for the Pb isotopic composition ofem i whereas terrigenous sediments (μ 10 andκ 4.5) evolve towards theem ii end member. A few percent of sediment in the recycled crust-sediment mixture will destroy the characteristic Pb isotopic signature of thehimu component. This, together with the low probability of isolating oceanic crust in the mantle for 2 Ga, explains why the extremehimu composition, as seen on Tubuai and St Helena, is sampled so rarely by oceanic volcanism.  相似文献   

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