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1.
The Permian–Triassic high pressure metamorphism and potassic magmatism in central Korea attest to the extension of the Dabie‐Sulu collision belt in central‐eastern China towards the Korean Peninsula and possibly the Japanese Islands. We present major and trace element and Sr–Nd isotope data for a ca. 230 Ma monzodiorite pluton emplaced in the Goesan area, central Okcheon belt, Korea. This pluton shows geochemical features comparable with those of the coeval monzonite–syenite–gabbro–mangerite suite documented recently in the Gyeonggi massif. The metaluminous and alkali–calcic signatures of the Goesan intrusives correspond to the Caledonian‐type post‐orogenic granitoids. The K2O/Na2O ratios of all analyzed samples are greater than 1, and are not correlative with their SiO2 contents. The enrichment of both large‐ion‐lithophile elements and highly compatible elements in the Goesan pluton is probably indicative of metasomatized mantle origin. The elemental fractionation in the source region must have occurred in the distant past, possibly the Paleoproterozoic, to generate significantly negative εNd(t) values (< –16). Chondrite‐normalized rare earth element patterns as well as Rb/Sr and Ba/Rb ranges suggest that the source consists of amphibole‐bearing rocks. Progressive decreases in negative Eu anomaly and Ba, Sr, Ni, Cr and V contents with increasing SiO2 contents reflect an important role of plagioclase, biotite and hornblende for the fractionation process. Zr is undersaturated in the potassic, metaluminous melt. The initial Sr–Nd isotopic compositions of the samples are correlated with their SiO2 contents, substantiating a role of crustal assimilation during the magmatic differentiation. The Sr–Nd elemental and isotopic modeling suggests that the Goesan pluton was initially slightly heterogeneous in its isotopic composition, and underwent concurrent assimilation and fractional crystallization. The occurrence of the Goesan pluton provides further evidence corroborating the amalgamation of allochthonous terranes within the Okcheon belt during the Permian–Triassic collisional orogeny.  相似文献   

2.
Cheong-Bin  Kim  V. J. Rajesh    M. Santosh 《Island Arc》2008,17(1):26-40
Abstract Geochemical and Sr–Nd–Pb isotope characteristics, as well as K–Ar geochronology of a massive pitchstone (volcanic glass) stock erupted into Late Cretaceous lapilli tuff and rhyolite in the Gohado area, southwestern Okcheon Belt, South Korea, are reported. The pitchstones are highly evolved with SiO2 contents ranging from ～72 to 73 wt%, K2O/Na2O ratios of 1.04–1.23 and low MgO/FeOt values (0.17–0.20). The pitchstones are weakly peraluminous and the ASI (molar Al2O3/Na2O + K2O + CaO) values are significantly lower than 1.1. The pitchstones also display a general calc‐alkaline nature with significant alkali contents. The rare earth elements (REE) compositions show moderately fractionated nature with (La/Yb)N ranging from 11 to 16. Chondrite normalized REE patterns show relative enrichment of light REE over heavy REE and moderate Eu anomaly (Eu/Eu* ratio varies from 0.53 to 0.57). A distinct negative Nb anomaly is observed for all pitchstones on a primitive mantle normalized trace element diagram, typical of subduction‐related magmatism and crustal‐derived granites. All these features are characteristic of I‐type granites derived from a continental arc. The pitchstones have Zr contents of 98.5–103.5 ppm with zircon thermometry yielding temperatures of 749–755°C (mean 752°C). The K–Ar analyses of representative pitchstone samples yielded ages of 58.7 ± 2.3 and 62.4 ± 2.1 Ma with a mean age of 61 Ma. The rocks show nearly uniform initial 87Sr/86Sr isotopic ratios of 0.7104–0.7106 and identical 143Nd/144Nd initial ratio of 0.5120. The rocks display negative εNd (61 Ma) values of ?12. The depleted mantle model ages (TDM) range from 1.54 Ga to 1.57 Ga. The Pb isotope ratios are 206Pb/204Pb = 18.522–18.552, 207Pb/204Pb = 15.642–15.680 and 208Pb/204Pb = 38.794–38.923. These ratios suggest that the Gohado pitchstones were formed in a continental arc environment by partial melting of a 1.54 Ga to 1.57 Ga parental sources of lower crustal rocks probably of mafic or intermediate compositions.  相似文献   

3.
Precambrian basement rocks have been affected by Caledonian thermal metamorphism. Caledonian‐aged zircon grains from Precambrian basement rocks may have resulted from thermal metamorphism. However, Hercynian ages are rarely recorded. Zircon U–Pb Sensitive High Resolution Ion Microprobe (SHRIMP) dating reveals that zircon ages from the Huyan, Lingdou, and Pengkou granitic plutons can be divided into two groups: one group with ages of 398.9 ±5.3 Ma, 399 ±5 Ma, and 410.2 ±5.4 Ma; and a second group with ages of 354 ±11 Ma, 364.6 ±6.7 Ma, and 368 ±14 Ma. The group of zircon U–Pb ages dated at 410–400 Ma represent Caledonian magmatism, whereas the 368–354 Ma ages represent the age of deformation, which produced gneissosity. The three plutons share geochemical characteristics with S‐type granites and belong to the high‐K calc‐alkaline series of peraluminous rocks. They have (87Sr/86Sr)i ratios of 0.710 45–0.724 68 and εNd(t) values of ?7.33 to ?10.74, with two‐stage Nd model ages (TDM2) ranging from 1.84 Ga to 2.10 Ga. Magmatic zircon εHf(t) values range from ?3.79 to ?8.44, and have TDMC ages of 1.65–1.93 Ga. The data suggest that these granites formed by partial melting of Paleoproterozoic to Mesoproterozoic continental crust. A collision occurred between the Wuyi and Minyue microcontinents within the Cathaysia Block and formed S‐type granite in the southwest Fujian province. The ca 360 Ma zircon U–Pb ages can represent a newly recognized period of deformation which coincided with the formation of the unified Cathaysia Block.  相似文献   

4.
To constrain the depositional age of the lowermost Nakdong Formation in the Early Cretaceous Gyeongsang Basin, SHRIMP U–Pb age determination was carried out on zircon separates. The U–Pb compositions of detrital zircons from the Nakdong Formation yield a wide range of ages from the Archean to the Cretaceous but show a marked contrast in age distribution according to the geographical locations within the basin. The provenance of the southern Nakdong Formation is dominantly the surrounding Yeongnam Massif, which is composed of Paleoproterozoic metamorphic rocks and Triassic to Jurassic plutonic rocks, whereas the central to northern Nakdong Formation records significant sediment derivation from the Okcheon Metamorphic Belt, which is distributed to the northwest, in addition to the contribution from the Yeongnam Massif. It is suggested that the maximum depositional age of the Nakdong Formation is ca 127 Ma, based on its youngest detrital zircon age population. The onset of its deposition at 127 Ma coincided with the tectonic inversion in East Asia from a compressional to an extensional geodynamic setting, probably due to the contemporaneous change in the drift direction of the Izanagi Plate and its subsequent oblique subduction.  相似文献   

5.
Geochemical and Pb-Sr-Nd isotopic compositions of five Indosinian granitoid intrusions from the western Qinling belt provide insights into basement nature and tectonic affinity. The results show that the western Qinling granitoids incline towards basic in their bulk chemical composition. The granitoids belong to high-K to shoshonitic series with K2O/Na2O=1.04-1.86 and are dominantly metaluminous with A/CNK=0.90-1.05 (most samples have A/CNK of <1.0). They have similar trace elemental compo- sitional patterns. In Sr-Nd isotopic compositions, they display some extent heterogeneity with Isr=0.70682-0.70845, εNd(t)=?4.85 to ?9.17 and TDM=1.26-1.66 Ga. They are characterized by high ra- diogenic Pb isotopic compositions. Their initial Pb isotopic ratios are 206Pb/204Pb=17.996-18.468, 207Pb/204Pb=15.565-15.677 and 208Pb/204Pb=38.082-38.587. Geochemical and Sr-Nd isotopic composi- tions reveal that magma for the granitoids was derived from partial melting of high-K (Rb) basaltic rocks, which might be formed in 900-1400 Ma. It is suggested that a large amount of the Proterozoic high-K (Rb) basaltic rocks, which underlie the Phanerozoic sedimentary cover, constitute the crustal basement of the western Qinling belt. Pb-Sr-Nd isotopic compositional comparison between the east- ern Qinling and the western Qinling Indosinian granitoids indicates that the crustal basement of the western Qinling is distinct from that of the eastern Qinling. The Baoji-Chengdu railway close to south-north orientation can be taken as an approximate boundary between both basements. The Pb-Nd isotopic compositional characteristics of the western Qinling granitoids suggest that the basement of the western Qinling belt has an affinity with the Yangtze block.  相似文献   

6.
Abstract Whole‐rock chemical and Sr and Nd isotope data are presented for gabbroic and dioritic rocks from a Cretaceous‐Paleogene granitic terrain in Southwest Japan. Age data indicate that they were emplaced in the late Cretaceous during the early stages of a voluminous intermediate‐felsic magmatic episode in Southwest Japan. Although these gabbroic and dioritic rocks have similar major and trace element chemistry, they show regional variations in terms of initial Sr and Nd isotope ratios. Samples from the South Zone have high initial 87Sr/86Sr (0.7063–0.7076) and low initial Nd isotope ratios (?Nd, ?2.5 to ?5.3); whereas those from the North Zone have lower initial 87Sr/86Sr (usually less than 0.7060) and higher Nd isotope ratios (?Nd, ?0.8 to + 3.3). Regional variations in Sr and Nd isotope ratios are similar to those observed in granitic rocks, although gabbroic and dioritic rocks tend to have slightly lower Sr and higher Nd isotope ratios than granitic rocks in the respective zones. Limited variations in Sr and Nd isotope ratios among samples from individual zones may be attributed partly to a combination of upper crustal contamination and heterogeneity of the magma source. Contamination of magmas by upper crustal material cannot, however, explain the observed Sr and Nd isotope variations between samples from the North and South Zones. Between‐zone variations would reflect geochemical difference in magma sources. The gabbroic and dioritic rocks are enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE), showing similar normal‐type mid‐ocean ridge basalt (N‐MORB) normalized patterns to arc magmas. Geochronological and isotopic data may suggest that some gabbroic and dioritic rocks are genetically related to high magnesian andesite. Alternatively, mantle‐derived mafic or intermediate rocks which were underplated beneath the crust may be also plausible sources for gabbroic and dioritic rocks. The magma sources (the mantle wedge and lower crust) were isotopically more enriched beneath the South Zone than the North Zone during the Cretaceous‐Paleogene. Sr and Nd isotope ratios of the lower crustal source of the granitic rocks was isotopically affected by mantle‐derived magmas, resulting in similar initial Sr and Nd isotope ratios for gabbroic, dioritic and granitic rocks in each zone.  相似文献   

7.
Zircons from two samples of the Sukeng pluton in the southwest Fujian Province, China, were analyzed by LA–ICP–MS with the aim of determining the timing of formation. The zircons from the two samples yield similar U–Pb ages of 100.47 ± 0.42 and 102.46 ± 0.69 Ma, indicating that the Sufeng pluton is contemporaneous with the Sifang and Luoboling plutons, all of which are also related to Cu–Au–Pb–Zn–Mo mineralization within the study area. All three plutons have geochemical features of I‐type granites, are high‐ to mid‐K calc‐alkaline metaluminous rocks, and have average molar Al2O3/ (CaO+Na2O+K2O) values of 0.95, initial 87Sr/86Sr ratios of 0.70465–0.70841, εNd(t) values at 101 Ma from –1.72 to –7.26, and two‐stage Nd model ages (T2DM) from 1.16 to 1.60 Ga. Zircons within these plutons have εHf(t) values at 101 Ma from –3.5 to 6.25 and T2DM ages from 0.74 to 1.46 Ga, suggesting these I‐type granites formed from magmas generated by partial melting of Mesoproterozoic to Neoproterozoic continental crust that mixed with mantle‐derived magmas. The magmatism was associated with thickening of the lower crust caused by collisions between microcontinents in the Cathaysian Block, which were driven by Early Cretaceous subduction of the Pacific Plate.  相似文献   

8.
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   

9.
Feng  Guo  Weiming  Fan  Yuejun  Wang  Chaowen  Li 《Island Arc》2005,14(2):69-90
Abstract Early Cretaceous high‐K calc‐alkaline volcanism occurring in the Laiyang Basin north of the Sulu high‐pressure to ultrahigh‐pressure (HP‐UHP) Metamorphic Belt, eastern China, comprises a wide spectrum of rock types, ranging from trachybasalts to trachydacites. The basaltic–andesitic rocks erupted at 107–105 Ma, spanning an SiO2 range of 50.1–59.6% and an MgO range of 2.6–7.2%, and are characterized by large ion lithophile element (LILE; e.g. Ba and K) and light rare earth element (LREE) enrichment, high field strength element (HFSE) depletion and highly radiogenic Sr but non‐radiogenic Nd isotopic compositions (87Sr/86Sr(i) = 0.70750–0.70931; ?Nd(t) = ?17.9 ? ?15.6). The geochemical similarities between these rocks and the earlier Sulu Belt lamprophyres suggest that both types of mafic rocks were derived from similar mantle sources with LILE and LREE enrichment. Thus, the Wulian–Qingdao–Yantai Fault that separates the two terranes at the surface should not be considered as a lithospheric boundary between the North China and Yangtze blocks. The felsic lavas erupted at 93–91 Ma, spanning an SiO2 range of 61.6–67.0% and an MgO range of 1.1–2.6%, and show a trace element geochemistry similar to the basaltic rocks, but with higher radiogenic Sr and even lower Nd isotopic compositions (87Sr/86Sr(i) = 0.70957–0.71109; ?Nd(t) = ?19.1 ? ?17.5), similar to I‐type granitoids in the Sulu Belt. A crustal origin was proposed to explain their compositions (which are comparable to those of experimental slab melts), the >10 Ma eruption interval and the compositional gaps in some elements (e.g. P, Ti and Sr) between them and the older basaltic–andesitic rocks. These melts were derived from predominant metaigneous protoliths containing mafic accumulative counterparts of the basaltic–andesitic and/or lamprophyric magmas. The extensive extrusion of Early Cretaceous high‐K calc‐alkaline rocks in the Laiyang Basin favored an extensional regime in response to the progressive attenuation of the thickened lithosphere and orogenic collapse, as reflected in the development of the basin from a foreland basin (before the end of the Jurassic period) to a fault basin (since the Early Cretaceous period).  相似文献   

10.
Seven LA-ICP-MS zircon U-Pb datings from granitoids in the southern basement of the Songliao basin were done in order to constrain the ages of the basin basement. The cathodoluminescence (CL) images of the zircons from seven granitoids indicate that they are euhedral-subhedral ones with striped ab-sorption and obvious oscillatory zoning rims. The dating results show that a weighted mean 206Pb/238U age is 236±3 Ma for quartz diorite (sample No.T6-1) located in the western slope of the basin,that weighted mean 206Pb/238U ages are 319±1 Ma (2126 m) and 361±2 Ma (1994 m) for diorite (sample No.YC1-1) and granite (sample No.YC1-2) located in northern part of southeastern uplift of the basin,respectively,and that weighted mean 206Pb/238U ages are 161±5 Ma,165±2 Ma,165±1 Ma and 161±4 Ma for samples Q2-1,SN121,SN122,and SN72 granitoids located in southern part of southeastern uplift of the basin,respectively. The statistical results of ages suggest that the middle Jurassic granitoids con-stitute the main part of basement granitoids,and that the Hercynian and Indo-Sino magmatisms also occur in the basin basement. It is implied that the Songliao basin should be a rift one formed in the intracontinent or active continental margin settings in the late Mesozoic after the Middle Jurassic orogeny took place.  相似文献   

11.
This paper reports geochemical and Pb-Sr-Nd isotopic compositions of the Indosinian Yangba (215 Ma),Nanyili (225 Ma) and Mopi granitoids from the Bikou block of the northwestern margin of the Yangtze plate. These granitoids are enriched in Al (Al2O3:14.56%―16.48%) and Sr (352 μg/g―1047 μg/g),and depleted in Y (<16 μg/g) and HREE (e.g. Yb<1.61 μg/g),resulting in high Sr/Y (36.3―150) and (La/Yb)N (7.8―36.3) ratios and strongly fractionationed REE patterns. The Indosinian granotoids show initial Sr isotopic ratios (ISr) from 0.70419 to 70752,εNd(t) values from-3.1 to -8.5,and initial Pb isotopic ratios 206Pb/204Pb=17.891-18.250,207Pb/204Pb=15.494-15.575,and 208Pb/204Pb=37.788-38.335. Their geochemi-cal signatures indicate that the granitoids are adakitic. However,they are distinct from some adakites,generated by partial melting of subducted oceanic slab and/or underplated basaltic lower crust,be-cause they have high K (K2O: 1.49%―3.84%) and evolved Nd isotopic compositions,with older Nd iso-topic model ages (TDM=1.06―1.83 Ga). Geochemical and Sr-Nd isotopic compositions suggest that the magmas of the Insoninian adakitic rocks in the Bikou block were derived from partial melting of thick-ened basaltic lower crust. Combined with regional analyses,a lithospheric delamination model after collision between the North China and South China plates can account for the Indosinian adakitic magma generation. On the other hand,based on the Pb-Sr-Nd isotopic probing to the magma sources of the adakitic rocks,it is suggested that there is an unexposed continent-type basement under the exposed Bikou Group volcanic rocks. This can constrain on the Bikou Group volcanic rocks not to be MORB-or OIB-type.  相似文献   

12.
Nd and Sr isotope determinations on late Precambrian to early Palaeozoic igneous and sedimentary rocks from the Arabian Shield are used to investigate the proportion of reworked “older” crust, and the rate at which new crust was generated during the Pan African event. Eight Rb/Sr whole rock isochrons on igneous suites yield ages in the range 770?590 Ma and initial 87Sr/86Sr ratios of 0.7038?0.7023. These data confirm that magmatism in this area was largely restricted to the period 850-550 Ma, and the initial ratios are sufficiently low to preclude significant contributions from a long-lived upper crustal source. The initial 143Nd/144Nd ratios of a variety of lithologies, including several samples of possible “basement”, are all higher than the contemporaneous values for CHUR (εNd = +1.6 to +6.9), suggesting that many were derived directly from the upper mantle, and that any inferred crustal source regions for the remainder could not have separated from likely LREE-depleted mantle reservoirs before 1200 Ma. The Arabian Shield therefore provides an example of rapid crustal growth during the Late Proterozoic, and contrasts with the Damara intracratonic belt of Namibia where Nd and Sr isotopes provide strong evidence for extensive reworking of older continental crust during the same period.  相似文献   

13.
Abstract Rb–Sr and Sm–Nd isochron ages were determined for whole rocks and mineral separates of hornblende‐gabbros and related metadiabases and quartz‐diorite from Shodoshima, Awashima and Kajishima islands in the Ryoke plutono‐metamorphic belt of the Setouchi area, Southwest Japan. The Rb–Sr and Sm–Nd whole‐rock‐mineral isochron ages for six samples range from 75 to 110 Ma and 200–220 Ma, respectively. The former ages are comparable with the Rb–Sr whole‐rock isochron ages reported from neighboring Ryoke granitic rocks and are thus due to thermal metamorphism caused by the granitic intrusions. On the contrary, the older ages suggest the time of formation of the gabbroic and related rocks. The initial 87Sr/86Sr and 143Nd/144Nd ratios of the gabbroic rocks (0.7070–0.7078 and 0.51217–0.51231 at 210 Ma, respectively) are comparable with those of neighboring late Cretaceous granites and lower crustal granulite xenoliths from Cenozoic andesites in this region. Because the gabbroic rocks are considered to be fragments of the lower crustal materials interlayered in the granulitic lower crust, their isotopic signature has been inherited from an enriched mantle source or, less likely, acquired through interaction with the lower crustal materials. The Sr and Nd isotopic and petrologic evidence leads to a plausible conclusion that the gabbroic rocks have formed as cumulates from hydrous mafic magmas of light rare earth element‐rich (Sm/Nd < 0.233) and enriched isotopic (?Sr > 0 and ?Nd < 0) signature, which possibly generated around 220–200 Ma by partial melting of an upper mantle. We further conclude that they are fragments of refractory material from the lower crust caught up as xenoblocks by granitic magmas, the latter having been generated by partial melting of granulitic lower crustal material around 100 Ma.  相似文献   

14.
Late Miocene (7–9 Ma) basaltic rocks from the Monbetsu‐Kamishihoro graben in northeast Hokkaido have chemical affinities to certain back‐arc basin basalts (referred to herein as Hokkaido BABB). Pb‐, Nd‐ and Sr‐isotopic compositions of the Hokkaido BABB and arc‐type volcanic rocks (11–13 Ma and 4–4.5 Ma) from the nearby region indicate mixing between the depleted mantle and an EM II‐like enriched component (e.g. subducted pelagic sediment) in the magma generation. At a given 87Sr/86Sr, Hokkaido BABB have slightly lower 143Nd/144Nd and slightly less radiogenic 206Pb/204Pb compared with associated arc‐type lavas, but both these suites are difficult to distinguish solely on the basis of isotopic compositions. These isotopic data indicate that while generation of the Hokkaido BABB involves smaller amounts of the EM II‐like enriched component than do associated arc lavas, Hokkaido BABB are isotopically distinct from basalts produced at normal back‐arc basin spreading centers. Instead, northeast Hokkaido BABB are more similar to basalts erupted during the initial rifting stage of back‐arc basins. The Monbetsu‐Kamishihoro graben may have developed in association with extension that formed the Kurile Basin, suggesting that opening of the basin continued until late Miocene (7–9 Ma).  相似文献   

15.
The Sindong Group was deposited in the north–south trending half‐graben Nakdong Trough, southern Korean peninsula. The occurrence of detrital chromian spinels from the Jinju Formation of the Sindong Group in the Gyeongsang Basin means that the mafic to ultramafic rocks were exposed in its provenance. The chromian spinels from the Jinju Formation are characterized by extremely low TiO2 and Fe3+. Moreover, their range of Cr# is from 0.45 to 0.80 and makes a single trend with Mg#. The chemistry of chromian spinels implies that the source rocks for chromian spinels were peridotites or serpentinites, which originated in the mantle wedge. To more narrowly constrain their source rocks, the Ulsan and Andong serpentinites exposed in the Gyeongsang Basin were examined petrographically. Chromian spinels in the Andong serpentinite differ from those of the Jinju Formation and those in the Ulsan serpentinite partly resemble them. Furthermore, the Jinju chromian spinel suite is similar to the detrital chromian spinels from the Mesozoic sediments in the Circum‐Hida Tectonic zone, which includes the Nagato Tectonic zone in Southwest Japan and the Joetsu Belt in Northeast Japan. This suggests that the basement rocks, which were located along the main fault bounding the eastern edge of the Nakdong Trough, had exposures of peridotite or serpentinite. It is possible that the Nakdong Trough was directly adjacent to the Circum‐Hida Tectonic zone before the opening of the Sea of Japan (East Sea).  相似文献   

16.
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   

17.
Abstract The Ogcheon fold belt and the Ryeongnam massif in the Korean Peninsula are made up of Precambrian igneous and sedimentary rocks that have been metamorphosed, tectonically deformed and extensively intruded by mafic to felsic plutonic rocks of Permian to Jurassic age. In the present study, we report seven new U–Pb zircon ages and Sr‐Nd‐Pb isotopic data for Permian to Jurassic plutons in the Ogcheon belt and the Ryeongnam massif. In the Ogcheon belt, these are: the Cheongsan porphyritic granite (217 ± 3.1 My), the Baegrog foliated granodiorite (206.4 ± 3.6 My), the Sani granite (178.8 ± 2.9 My) and the Yeonggwang foliated granite (173.0 ± 1.7 My). For the Ryeongnam massif, we report on the Yeongdeog foliated granodiorite (252.2 ± 2.9 My), the Sancheong gabbro (203.8 ± 3.3 My) and the Baegseogri foliated granodiorite (177.8 ± 2.4 My). All of these ages are lower concordia intercepts; the upper concordia intercepts indicate derivation from a Precambrian protolith. Sr, Nd and Pb isotopes also reveal that much of the Permian–Jurassic (252–173 Ma) plutonism in Korea was generated by recycling of Precambrian rocks. These new ages, together with other published zircon ages indicate that the plutonism in the Ogcheon fold belt is coeval with that in the Ryeongnam massif, but based on the Sr‐Nd‐Pb isotopic evidence, they are not cogenetic. In addition, zircon ages provide information on the movement along the Honam shear zone, which cuts across the whole Korean Peninsula and along most of its length provides the boundary between the Ogcheon fold belt and the Ryeongnam massif. It has a prolonged history of movement and deformation and appears to have been active from the Precambrian through to the Mesozoic, from before 1924 Ma to at least 180 Ma. The Permian–Jurassic igneous and tectonic activity in Korea is a manifestation of the more extensive orogenic activities that affected the East Asian continent at that time. In China, ultra high‐pressure rocks of the Qinling–Dabie belt formed between 210 and 230 Ma as result of the collision between the South China block and the North China block. In central Japan, corresponding plutonic activity is dated as 175 to 231 Ma. The absence of ultra high‐pressure rocks in Korea and Japan precludes a simple extension of the Qinling–Dabie belt eastwards; however, the effects of the continental collision eastwards are apparent from the igneous and tectonic activity.  相似文献   

18.
Geochemical and isotopic analyses (Sr–Nd–Pb) of late Miocene to Quaternary plateau lavas from the Pali Aike and Morro Chico areas (52°S) were undertaken to constrain the melting processes and mantle sources that contributed to magma generation and the geodynamic evolution of southernmost Patagonia, South America. The Pali Aike and Morro Chico lavas are alkaline (Pali Aike, 45–49 wt.% SiO2; 4.3–5.9 wt.% Na2O+K2O) and subalkaline (Morro Chico, 50.5–50.8 wt.% SiO2; 4.0–4.4 wt.% Na2O+K2O), relatively primitive (Pali Aike, 9.5–13.7 wt.% MgO; Morro Chico, 7.6–8.8 wt.% MgO) mafic volcanic rocks that have typical intraplate ocean island basalt‐like signatures. Incompatible trace element ratios and isotopic ratios of the Pali Aike and Morro Chico lavas differ from those of the majority of Neogene southern Patagonian slab window lavas in showing more enriched characteristics and are similar to high‐μ (HIMU)‐like basalts. The rare earth element (REE) modeling to constrain mantle melting percentages suggests that these lavas were produced by low degrees of partial melting (1.0–2.0% for Pali Aike lavas and about 2.6–2.7% for Morro Chico lavas) of a garnet lherzolite mantle source. The major systematic variations of Sr–Nd–Pb isotopes in southern Patagonian lavas are related to geographic location. The Pali Aike and Morro Chico lavas from the southernmost part of Patagonia have lower 87Sr/86Sr and higher 143Nd/144Nd and 206Pb/204Pb ratios, relative to most of the southern Patagonian lavas erupted north of 49.5°S, pointing to a HIMU‐like signature. An isotopically depleted and HIMU‐like asthenospheric domain may have been the main source of magmas in the southernmost part of Patagonia (e.g. Pali Aike, Morro Chico, and Camusu Aike volcanic field), suggesting the presence of a major discontinuity in the isotopic composition of the asthenosphere in southern Patagonia. On the basis of geochemical and isotope data and the available geological and geotectonic reconstructions, a link between the HIMU asthenospheric mantle domain beneath southernmost Patagonia and the HIMU mega‐province of the southwestern Pacific Ocean is proposed.  相似文献   

19.
Abstract Rb–Sr and K–Ar chronological studies were carried out on granitic and metamorphic rocks in the Ina, Awaji Island and eastern Sanuki districts, Southwest Japan to investigate the timing of intrusion of the granitoids in the Ryoke belt. Intrusions of 'younger' Ryoke granitic magmas took place in the Ina district between 120 Ma and 70 Ma, and cooling began immediately after the emplacement of the youngest granitic bodies. Igneous activity in Awaji Island was initiated at 100 Ma and continued to 75 Ma. Along-arc variations of Rb–Sr whole-rock isochron ages suggest that magmatism began everywhere in the Ryoke and San-yo belts at almost the same time ( ca 120 Ma). The last magmatism took place in the eastern part of both belts. Rb–Sr and K–Ar mineral ages for the granitoids young eastwards. The age data suggest that the Ryoke belt was uplifted just after the termination of igneous activity. Initial Sr and Nd isotopic ratios for the Ryoke granitoids indicate that most were derived from magmas produced in the lower crust and/or upper mantle with uniform Sr and Nd isotopic compositions. Several granitoids, however, exhibit evidence of assimilation of Ryoke metamorphic rocks or older Precambrian crustal rocks beneath the Ryoke belt.  相似文献   

20.
The geological relationship between the Okcheon and Taebaeksan basins of the Okcheon belt on the Korean peninsula is a key issue in reconstructing the tectonic evolution of the peninsula. The boundary between the two basin sequences has been variously interpreted as a conformable, unconformable, or thrust contact, without clear evidence being provided for any of these hypotheses. Detailed examination of structures and microfabrics of deformed rocks adjacent to the contact in the Bonghwajae area suggests that the boundary between the two basin sequences is a thrust. Based on the U–Pb ages of detrital zircons from metasedimentary rocks and pre‐existing geologic data from the Okcheon belt, the thrust is a relay structure between two segments of a continental transform fault along which the Okcheon Basin was juxtaposed against the Taebaeksan Basin during the Permian–Triassic suturing of the North and South China Cratons.  相似文献   

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