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1.
We report here geochemical data, U-Pb zircon ages, and Hf isotopes for the high-Mg diorites (HMDs), Nb-enriched basaltic porphyrys (NEBPs) and plagiogranites (PLAGs) in the Pingshui segment of the Jiangshan-Shaoxing suture zone. The HMDs are characterized by high Mg# (>60), high Na and LREE contents, depletion of HREE and HFSE, and pronounced positive εNd(t) values of 7.0 to 7.7, similar to some adakitic high-Mg andesites. The NEBPs are relatively Na-rich (Na2O/K2O>6) and display high abundances of P2O5 (∼1.00%), TiO2 (∼3.08%) and HFSE (e.g., Nb=9.53–10.27 ppm). Their Nd isotopic compositions (εNd(t)=6.8–8.0) are comparable to those of the HMDs. The PLAGs are metaluminous (A/CNK=0.84–0.89) and sodic (Na2O/K2O>10). Their depletion in HFSE (e.g., Nb, Ta) is consistent with “SSZ-type” plagiogranite. Zircon LA-ICP-MS U-Pb dating yields an age of 932±7 Ma for the HMD, 916±6 Ma for the NEBP, and 902±5 Ma for the PLAG, respectively, indicating that they were products of early Neoproterozoic magmatism. The PLAGs exhibit relatively high zircon Hf isotopes and positive εHf(t) values of 11.0 to 16.2, consistent with their Nd isotopic data (εNd(t)=7.5–8.4). Such features are similar to those of oceanic plagiogranites in ophiolites and distinct from those of crust-derived granites. The PLAGs were most likely derived from partial melting of subducted oceanic crust in an active continental margin. Considering these results in the context of the regional geology, we suggest that a slab window in the subducting oceanic crust between the Yangtze Block and Cathaysia Block was possibly the principal cause for the unique arc magmatism in the area. The upwelling asthenosphere below the slab window may have provided significant thermodynamic conditions. Supported by China Geological Survey (Grant No. 1212010610611) and the Ministry of Land and Resources (Grant No. 200811015)  相似文献   

2.
Bimodal tholeiitic and mildly alkalic basalts occur near Bhir, in the central part of Deccan Volcanic Province (DVP). Major and trace element concentrations show that, of the ten flows, nine are tholeiitic and one is an alkalic basalt. The Bhir basalts have a wide range of chemical composition. Geochemical variations in the stratigraphic section define three distinct phases of evolution (zones 1 to 3). Crystal fractionation of plagioclase, clinopyroxene, olivine and Fe–Ti oxide expanded the compositional range. Low Mg#s (39–55), low concentrations of Ni and Cr and high Zr suggest the evolved nature of the Bhir basalts. Fractionation modeling suggests about 42% fractional crystallization.In spite of the dominant role of fractional crystallization in the evolution of Bhir basalts, some other processes must be sought to explain the chemical variations. Crustal contamination, magma mixing and degree of partial melting are suggested to explain the observed chemical variations. Resorption, reverse zoning and compositional bimodality in plagioclase phenocrysts indicate magma mixing. Samples of flows one and four suspected of being contaminated all have enriched SiO2 and LILE (K, Rb, and Ba) contents and depletion in Ti and P, believed to be due to ‘granitic’ crustal contamination.As compared to tholeiitic basalts, the alkalic basalts are characterized by low SiO2 and high TiO2, Na2O, K2O and P2O5. Alkalic basalts are richer in LILE (Rb and Ba), HFSE (Nb, Zr, and Y) and REE than the tholeiitic basalts. The alkalic basalt occurrence is important from a petrogenetic point of view and also suggests that the sources of alkalic basalt magmas may be of variable ages under different parts of the DVP. Based on major, trace and rare earth element distributions it is suggested that asthenospheric mantle having affinities with the source of OIB was the source material of the magmas and the range in the composition of tholeiitic and alkalic basalts was probably controlled by different degrees of melting and/or inhomogeneities in the mantle source.  相似文献   

3.
Mesozoic and Cenozoic volcanic rocks are widely distributed in the circum-Pacific area of eastern China. These rocks have long been genetically linked to westward subduction of the paleo-Pacific oceanic plate to the eastern Asia continent[1,2]. Research in re-cent years[3―6] has attained conclusions that a simple paleo-Pacific subduction model does not work well in interpreting all the volcanic rocks in eastern China, although some of them could be attributed to circum-Pacific interaction …  相似文献   

4.
Basalt and diabase from the Cretaceous Dumisseau Formation, southern Haiti have Mg-numbers of 43–63, TiO2 contents of 1.6–3.9% and La abundances of 3.6–15.3 ppm.La/Ta ratios average 10, and indicate that the basalts are oceanic in character, distinct from the arc associations forming the northern part of Haiti. Oldest lavas have low TiO2 (1.6%) and are LREE-depleted, similar to N-MORBs, whereas overlying lavas have higher TiO2 (2–3.9%) and are LREE-enriched, similar to E-MORBs or hotspot basalts.87Sr86Sr ratios vary from 0.70280 to 0.70316,143Nd144Nd from 0.512929 to 0.513121, and206Pb204Pb from 19.00 to 19.27. LREE-depleted lavas have high143Nd144Nd (0.51309–0.51310) typical of MORBs, whereas143Nd144Nd in the LREE-enriched lavas varies widely (0.512929–0.513121).Chemical features of the Dumisseau basalts are equivalent to those of Caribbean seafloor basalts recovered on DSDP Leg 15, and support the contention that the Dumisseau is an uplifted section of Caribbean Sea crust. Oldest lavas are analogous to MORB-like basalts cored at Leg 15 Sites 146, 150, 152 and 153, and the overlying lavas are analogous to incompatible-element-enriched basalts cored at Site 151 on the Beata Ridge. Isotopic compositions of the Dumisseau basalts overlap with those of the eastern Pacific Galapagos and Easter Island hotspots. However, the presence of N-MORB basalts in the lower part of the Dumisseau and at the majority of Leg 15 Sites indicates that the anomalously thick Caribbean crust probably did not originate as a hotspot-related basaltic plateau, but may have been generated by on-ridge or near-ridge hotspot magmatism.  相似文献   

5.
Extensive Upper Cretaceous volcanism in southern Madagascar was fed in part by mantle sources resembling those expressed today in the Indian Ocean at Marion and Prince Edward islands and on the central Southwest Indian Ridge. In addition, very low εNd(T) (to −17.4), high(87Sr/86Sr)T (to 0.72126) tholeiites in southwestern Madagascar were variably but highly contaminated by ancient continental material broadly like that affecting the Bushe and Poladpur Formations of the later Deccan Traps in India. Alkalic dikes in southwestern Madagascar have a rough analogue in the Mahabaleshwar Formation of the Deccan, in that they document the influence of a low 206Pb/204Pb, negative εNd, relatively low 87Sr/86Sr reservoir. A very similar reservoir is manifested at present in mid-ocean ridge basalts on the central Southwest Indian Ridge near 40°E. The original location of this end-member appears likely to have been in the Madagascan lithospheric mantle, a portion of which may have been removed in the Middle Cretaceous by the action of the Marion hotspot or the rifting of Indo-Madagascar. An origin within the hotspot itself also may be possible; however, recent products of the hotspot appear to lack completely the necessary low 206Pb/204Pb, low εNd signatures.  相似文献   

6.
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   

7.
The TiO2 contents of rutile-saturated melts ranging from basalt to rhyodacite have been investigated at P = 8–30 kbar and T = 1000–1300°C under hydrous, CO2-saturated, and volatile-absent conditions. Dissolved TiO2 is positively correlated with T and not strongly dependent on Ptotal. For fixed P and T, TiO2 content decreases markedly as the melts become more felsic. The distribution of TiO2 between rutile and liquid, expressed as a wt.% concentration ratio, D (rut/liq), is given by: In D = −3.16 + (9373T) + 0.026P − 0.152FM where T is in Kelvins, P in kbar and FM is a melt composition parameter, FM = [Na+K+ 2(Ca+Fe+Mg)]/Al· 1/Si in which the chemical symbols represent cation fractions. The first term expresses the competition of aluminate and titanate anions for charge-compensating cations, and the second term expresses the inverse dependence of dissolved TiO2 on SiO2 content. There is no apparent dependence of rutile solubility on water content.For ranges of probable solidus conditions, rutile saturation in basaltic, andesitic, and dacitic liquids requires 7–9, 5–7, and 1–3 wt.% TiO2, respectively. These concentrations are well in excess of those found in the respective rock types, so depletion in Nb, Ta, and Ti and reduced Nb/U and Nb/Th ratios in volcanic rocks erupted at convergent plate margins cannot be attributed to residual rutile in their source regions. Thus, Nb, Ta and Ti depletion must be an inherent property of the source region.We suggest that the island-arc source region has been depleted in Nb and Ta by a previous episode of melt extraction (MORB), zoning refining, or equilibration with a percolating melt or fluid. Such a process markedly depletes the LILE and HFSE element concentrations in the residuum, but ratios such as Nb/U, Nb/Th and U/Th remain relatively constant due to similar solid-melt partition coefficients. The depletion of Nb relative to Th in the source regions of island-arc magmas occurs during hybridization of the source by rutile-saturated (Nb/Ta-depleted) melts or aqueous fluids. If the hybridizing agent is a melt, a relatively felsic composition, produced under low T (900°) hydrous conditions, is required.  相似文献   

8.
We describe the mode of occurrence and geochemical characteristics of basalts, in the Khangai–Khentei belt in Mongolia, overlain by Middle Paleozoic radiolarian chert in an extensive accretionary complex. These basalts are greatly enriched in K, Ti, Fe, P, Rb, Ba, Th, and Nb in comparison to the composition of the mid‐ocean ridge basalts, indicative of within‐plate alkaline type. Ti/Y vs Nb/Y and MnO/TiO2/P2O5 ratios of the basalts also suggest within‐plate affinities. Considering the geochemical characteristics as well as the conformable relationship with the overlying radiolarian chert, the alkaline basalts were clearly not continental but formed a pelagic oceanic island. The mode of occurrence and geochemistry of the basalts show that the alkaline basaltic volcanic activity had taken place to form an oceanic island in the Paleozoic pelagic region sufficiently far from continents to allow radiolarian ooze accumulation.  相似文献   

9.
The Mugouriwang Cenozoic volcanic rocks exposed in the north Qiangtang Block of Tibetan Plateau are mainly composed of basalt and andesitic-basalt,both characterized by the lower SiO2 (51%―54%),high refractory elements (i.e. Mg,Cr,Ni) as well as the moderate enrichment in light rare earth elements (LREE) relative to a slight depleted in Eu and high strength field elements (HFSE,i.e. Nb,Ta,Ti). Be-sides,the fairly low Sm/Yb value (3.07―4.35) could signify that the rocks should be derived directly from partial melting of the spinel lherzolite at the upper part of the asthenosphere. These rocks have radiogenic Sr and Pb (87Sr/86Sr = 0.705339 to 0.705667; 208Pb/204Pb = 38.8192 to 38.8937; 207Pb/204Pb = 15.6093 to 15.6245; 206Pb/204Pb = 18.6246 to 18.6383),and non-radiogenic Nd (143Nd/144Nd = 0.512604 to 0.512639; εNd = 0.02 to -0.66) in agreement with those values of the BSE mantle reservoir. The DUPAL anomaly of the rocks can be evidently attested by the △8/4Pb = 66.82 to 74.53 ,△7/4Pb = 9.88 to 11.42,△Sr>50,implying that the Mugouriwang volcanic rock is likely to be generated by partial melting of a Gondwana-bearing asthenospheric mantle ever matasomatised by the fluid from subduction zone. Depending on the previous study on the high-K calc-alkaline intermediate-felsic volcanics in the study area,this paper proposed that the fluids derived from the subducted Lhasa Block metasomatised the asthenosphere beneath the Qiangtang Block,and induced its partial melting,and then the melt under-plated the thickened Qiangtang lithosphere and caused the generation of the Cenozoic adakite-like felsic magmas in the Qiangtang region.  相似文献   

10.
The South China, including Yangzi Craton and the Cathaysian Block, belongs to the southern part of East Asia continent. It borders Pacific plate on the east side and Qinling-Dabie Orogen on the north side. During the middle-late Cretaceous, a number of downfaulted red basins (the terrestrial sedimentary basins in Fig. 1) and volcanic- sedimentary basins had been developed in South China[1,2], in which the con-temporaneous basaltic rocks were generally distributed (Fig. 1). Although the …  相似文献   

11.
Abstract Pliocene–Quaternary adakites and spatially and temporally associated niobium-enriched basalts (the latter thought to be derived by melting of slab melt-metasomatized mantle) from the Philippine island arcs have been selected for analysis of high field strength elements (HFSE). All these samples have nearly constant and chondritic Zr/Hf ratio (36.5) and slightly infrachondritic Nb/Ta ratio (14.7). We interpret adakitic magmas to be derived from the partial melting at approximately 900°C of subducted mid-ocean ridge basalts (MORB) crust, with rutile and/or ilmenite as residual minerals. Melting calculations show that, under these conditions, slab melts should have suprachondritic Nb/Ta ratios. The obvious discrepancy with our data is attributed to insufficient knowledge of rutile-melt partition coefficients for HFSE. Consequently, abnormal Nb/Ta or Zr/Hf ratios cannot be considered as potential markers of slab melting processes in island arcs.  相似文献   

12.
Magmatism in SE China was dormant during 204–180 Ma, but was reactivated in 180–170 Ma (early J2), and then became more and more intensive towards the end of early Cretaceous. The small-scale early J2 magmatism is the incipience to long-term and large-scale magmatism in this region. A near east-west (EW) trend volcanic belt was distributed across south Hunan, south Jiangxi and southwest Fujian was formed during early J2 time. Along this belt from the inland toward the coast, the lithology of basalts changes from alkali into tholeiite, and the amount of erupted volcanic rocks and the proportions of rhyolites coexisting with the basalts increase. On the basis of geochemical characteristics of these basalts, we infer that the melting degree of source rocks and the extent of fractional crystallization and crustal contamination all increased whereas the depth of mantle source decreased from the inland to the coast, which led to the variations of geological characteristics of the volcanic belt. In early J2, the western spreading Pacific plate began to subduct underneath SE China continental block, reactivating near EW trend deep fault that was originally formed during the Indosinian event. The stress of the western spreading Pacific plate and the extent of asthenosphere upwelling increased from the inland to the coast, which is consistent with the generation and evolution of early J2 basalts.  相似文献   

13.
Basalts and a mafic dyke collected from the city of Panzhihua show characteristics of high-Ti and low-Ti Emeishan basalts respectively. The dyke yielded a SHRIMP zircon U–Pb mean age of 261 ± 5 Ma making it contemporaneous with the eruption of Emeishan basalts. The basalts have ISr ranging from 0.7059 to 0.7062 with εNd(T) ranging from ?1.1 to + 0.7 whereas the dyke has ISr ranging from 0.7056 to 0.7064 with εNd(T) ranging from + 0.3 to + 0.5. Trace element modeling shows that the two rock types can be generated by different degrees of partial melting from the same garnet-bearing source. Assimilation of crustal material is required in order to produce the depletion of some trace elements (e.g. Nb and Ta) of the dyke however crustal assimilation is not required to produce the basalts. Trace element modeling and isotopic data of the Emeishan basalts suggest that, in general, the high- and low-Ti basaltic rocks are likely derived from the same source and represent different degrees of partial melting with or without crustal assimilation. The location and geological relationships of the ‘high-Ti’ basalts indicate they erupted relatively early and within the central part of the ELIP, casting doubt on the previous spatial–compositional distribution of the Emeishan basalts.  相似文献   

14.
The Turpan-Hami basin (as the Tu-Ha basin here-after) and the Santanghu basin, as the late Paleozoic– Mesozoic-Cenozoic reworked and superimposed sedi-mentary basins with the similar evolution history 1, 2), are located in between the Tianshan and the Altay moun-tains in northeastern Xinjiang. As the major oil-and gas-bearing basins in Xinjiang, study of both the ba-sins through their complicated tectonic evolution his-tory is scientifically significant for exploring conti-nental geology …  相似文献   

15.
~~Characteristics of the mantle source region of sodium lamprophyres and petrogenetic tectonic setting in northeastern Hunan,China~~  相似文献   

16.
Cretaceous volcanic rocks (SCV) are widely developed in the northern part of the Sanandaj-Sirjan Zone, northwest Iran. Based on the mineralogy, texture and geochemical composition these rocks are divided in two main groups, the first and main one situated in the central part of the study area and the second one in the northeast. The former is dominantly basalts, andesitic basalts, and andesites and the latter comprises andesite, trachy-andesite to acidic variants, with porphyritic to microlithic porphyry and vitrophyric textures. Beside the differences between these two groups, the chemical compositions all of these rocks show a calc-alkaline affinity and enrichment in LIL elements (Rb, Ba, Th, U, and Pb) and depletion in Nb, Ti, and Zr, as evident in spider diagrams normalized to primitive mantle. The rocks are particularly enriched in Rb and depleted in Nb and Ti, as well as displaying high Rb/Sr and Rb/Ba ratios and low ratios of incompatible elements such as Nb/U (<10; range, 0.6–9), Th/U (<2), and Ba/Rb (<20). The significant U enrichment relative to neighbouring Nb and Th in the mantle-normalized variation diagram is mainly a result of source enrichment by slab-derived fluids. Significantly lower Nb/U ratios are observed in arc volcanics. These low values are generally ascribed to the strong capacity of LILE and the inability to transfer significant amounts of HFSE via slab-derived hydrous fluid. The results of geochemical modelling suggest a mantle lithospheric source that was metasomatized by fluids derived from a Neo-Tethyan subducted slab during the Middle to Late Cretaceous in the northern part Sanandaj-Sirjan Zone.  相似文献   

17.
Erciyes stratovolcano, culminating at 3917 m, is located in the Cappadocian region of central Anatolia. During its evolution, this Quaternary volcano produced pyroclastic deposits and lava flows. The great majority of these products are calc-alkaline in character and they constitute Kocdag and Erciyes sequences by repeated activities. Alkaline activity is mainly observed in the first stages of Kocdag and approximately first-middle stages of Erciyes sequences. Generally, Kocdag and Erciyes stages terminate by pyroclastic activities. The composition of lavas ranges from basalt to rhyolite (48.4–70.5 wt.% SiO2). Calc-alkaline rocks are represented mostly by andesites and dacites. Some compositional differences between alkaline basaltic, basaltic and andesitic rocks were found; while the composition of dacites remain unchanged. All these volcanics are generally enriched in LIL and HFS elements relative to the orogenic values except Rb, Ba, Nb depleted alkaline basalt. 87Sr/86Sr and 143Nd/144Nd isotopic composition of the volcanics range between 0.703344–0.703964, 0.512920–0.512780 for alkaline basalts and change between 0.704322–0.705088, 0.512731–0.512630 for alkaline basaltic rocks whereas calc-alkaline rocks have relatively high Sr and Nd isotopic ratios (0.703434–0.705468, 0.512942–0.512600). Low Rb, Ba, Nb content with high Zr/Nb, low Ba/Nb, La/Yb ratio and low Sr isotopic composition suggest an depleted source component, while high Ba, Rb, Nb content with high La/Yb, Ba/Nb, low Zr/Nb and low 87Sr/86Sr ratios indicate an OIB-like mantle source for the generation of Erciyes alkaline magma. These elemental and ratio variations also indicate that the different mantle sources have undergone different degree of partial melting episodes. The depletion in Ba, Rb, Nb content may be explained by the removal of these elements from the source by slab-derived fluids which were released from pre-collisional subduction, modified the asthenospheric mantle. The chemically different mantle sources interacted with crustal materials to produce calc-alkaline magma. The Ba/Nb increase of calc-alkaline samples indicates the increasing input of crustal components to Erciyes volcanics. Sr and Nd isotopic compositions and elevated LIL and HFS element content imply that calc-alkaline magma may be derived from mixing of an OIB-like mantle melts with a subduction-modified asthenospheric mantle and involvement of crustal materials in intraplate environments.  相似文献   

18.
Magma type and tectonic setting discrimination using immobile elements   总被引：1，自引：0，他引：1  
Five minor and trace elements have been variously combined to produce a set of binary diagrams in addition to total alkali-SiO2 diagrams, that discriminate between fresh tholeiitic and alkali basalts. These diagrams are TiO2-Zr, TiO2-Y/Nb, P2O5-Zr, TiO2-Zr/P2O5, and Nb/Y-Zr/P2O5. A clear discrimination between alkaline and tholeiitic basalts can be obtained, although no meaningful separation can be made between the broad groups of oceanic and continental basalts, of either magma type, on the diagrams. As these elements (Ti, P, Zr, Y and Nb) are generally considered immobile during alteration processes, it should be possible to distinguish the magma type of ancient basic volcanics that have been subjected to submarine weathering, spilitization and low-grade metamorphism.  相似文献   

19.
Field observation showed that there are many irregular leucocratic intrusive rocks in pillow lavas in the Danfeng Group in the Xiaowangjian area, north Qinling orogenic belt. Photomicrographs indicated that the protoliths of those altered leucocratic intrusive rocks are dioritic rocks. Geochemical analyses showed that pillow lavas have a range of SiO2 from 47.35% to 51.20%, low abundance of TiO2 from 0.97% to 1.72%, and percentages of MgO (MgO#=41―49). Chondrite-normalized REE patterns of pillow lavas are even, indicative of a weak differentiation between LREE and HREE (La/YbN=1.52―0.99). N-MORB-normalized trace element abundances showed that pillow lavas are enriched in incompatible elements (e.g., K, Rb, and Ba). Leucocratic intrusive rocks in pillow lavas have a wide range of SiO2 from 53.85%―67.20%, low abundances of TiO2 from 0.51%―1.10%, and MgO (MgO#=40―51), and higher percentages of Al2O3 (13.32%―16.62%) and concentration of Sr (342-539 μg/g), ratios of Na2O/K2O (2―7) and Sr/Y (17―28). Chondrite-normalized REE patterns of leucocratic intrusive rocks showed highly differentiation between LREE and HREE (La/YbN=12.26―19.41). N-MORB-normalized trace element abundances showed that leucocratic intrusive rocks are enriched in incompatible elements (e.g., K, Rb, and Ba), and significantly depleted in HFSE (e.g., Nb, Ta, Zr and Ti), indicative of a relationship to subduction. Isotopically, leucocratic intrusive rocks have a similar εNd(t) ( 7.45― 13.14) to that of MORB ( 8.8― 9.7), which indicates that those leucocratic intrusive rocks sourced from depleted mantle most likely. SHRIMP U-Pb analyses for zircon showed that those leucocratic intrusive rocks were formed at 442±7 Ma, yielding an age of subduction in the early Paleozoic in the north Qinling orogenic belt.  相似文献   

20.
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   

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