首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 125 毫秒
1.
Plutonic rocks in the southern Abukuma Mountains include gabbro and diorite, fine‐grained diorite, hornblende–biotite granodiorite (Ishikawa, Samegawa, main part of Miyamoto and Tabito, Kamikimita and Irishiken Plutons), biotite granodiorite (the main part of Hanawa Pluton and the Torisone Pluton), medium‐ to coarse‐grained biotite granodiorite and leucogranite, based on the lithologies and geological relations. Zircon U–Pb ages of gabbroic rocks are 112.4 ±1.0 Ma (hornblende gabbro, Miyamoto Pluton), 109.0 ±1.1 Ma (hornblende gabbro, the Hanawa Pluton), 102.7 ±0.8 Ma (gabbronorite, Tabito Pluton) and 101.0 ±0.6 Ma (fine‐grained diorite). As for the hornblende–biotite granodiorite, zircon U–Pb ages are 104.2 ±0.7 Ma (Ishikawa Pluton), 112.6 ±1.0 Ma (Tabito Pluton), 105.2 ±0.8 Ma (Kamikimita Pluton) and 105.3±0.8 Ma (Irishiken Pluton). Also for the medium‐ to fine‐grained biotite granodiorite, zircon U–Pb ages are 106.5±0.9 Ma (Miyamoto Pluton), 105.1 ±1.0 Ma (Hanawa Pluton) and the medium‐ to coarse‐grained biotite granodiorite has zircon U–Pb age of 104.5 ±0.8 Ma. In the case of the leucogranite, U–Pb age of zircon is 100.6 ±0.9 Ma. These data indicate that the intrusion ages of gabbroic rocks and surrounding granitic rocks ranges from 113 to 101 Ma. Furthermore, K–Ar ages of biotite and or hornblende in the same rock samples were dated. Accordingly, it is clear that these rocks cooled down rapidly to 300 °C (Ar blocking temperature of biotite for K–Ar system) after their intrusion. These chronological data suggest that the Abukuma plutonic rocks in the southern Abukuma Mountains region uplifted rapidly around 107 to 100 Ma after their intrusion.  相似文献   

2.
To constrain the timing of the tectonothermal events and formation process of a plutonic suite, U–Pb dating was carried out by laser ablation inductively coupled plasma mass spectrometry combined with cathodoluminescence imaging on zircon grains extracted from the Bato pluton, northern Yamizo Mountains, Japan. The Bato pluton consists of gabbro and diorite. Zircon grains separated from a gabbro sample had a unimodal 238U–206Pb age (105.7 ±1.0 Ma). It was interpreted as the solidification age of the gabbro. Cathodoluminescence observation showed that the zircon grains from a diorite sample were characterized by anhedral cores, oscillatory zoned mantles, and dark rims. The 238U–206Pb age of the anhedral cores ranged from 2 165 Ma to 161 Ma, indicating the assimilation of surrounding sedimentary rocks. The 238U–206Pb ages of the oscillatory zoned mantles and dark rims are 109.0 ±1.3 Ma and 107.7 ±1.3 Ma, respectively. Observation under polarizing microscopy suggests that the anhedral cores occurred before plagioclase and hornblende, and the oscillatory zones around the anhedral cores had crystallized at the same time as the crystallization of biotite. Moreover, the dark rims formed at the same time as the crystallization of quartz and K‐feldspar. The formation process of the gabbro‐diorite complex in the Bato pluton was inferred as follows. (i) A mafic initial magma intruded into Mesozoic sedimentary rocks, and the assimilation of these sedimentary rocks led to geochemical variation yielding a dioritic composition. Subsequently, plagioclase and hornblende of the diorite were crystallized before 109.0 ±1.3 Ma. (ii) Biotite crystallized in the middle stage around 109.0 ±1.3 Ma. (iii) Quartz and K‐feldspar of the diorite were crystallized at 107.7 ±1.3 Ma. The gabbroic magma solidified (105.7 ±1.0 Ma) after solidification of the diorite.  相似文献   

3.
Adakitic rocks in continental settings are commonly considered to be formed by partial melting of thickened or delaminated lower crust. Investigations on this kind of rocks can provide important information about crustal evolution complementary to information from other rocks. This paper reports adakitic granodiorite of the Lingxi pluton in the interior of the Cathayisa Block. LA-ICP-MS zircon U-Pb dating shows that it was formed in the late Early Cretaceous(100±1 Ma). The granodiorite has geochemical features of adakitic rocks derived from partial melting of the thickened lower crust, e.g., high SiO2(mainly ranging from 64.4 to 68.9 wt.%) and Sr(624–894 ppm) contents, Sr/Y(49.9–60.8) and La/Yb(23.4–42.8) values, low Y(10.3–17.1 ppm), Ni(5.62–11.8 ppm) and MgO(mostly from 0.86 wt.% to 1.57 wt.%) contents and weak Eu anomaly. It has initial 87Sr/86 Sr ratios of 0.7086–0.7091, εNd(t) values of.6.2 to.5.9 and zircon εHf(t) values mostly of.10.1 to.7.6. Based on the geochemical characteristics and simple modelling, it is suggested that the most likely generation mechanism of the Lingxi granodiorite is partial melting of a thickened Proterozoic lower continental crust at a pressure ≥12 kbar(or crust thickness ≥40km), leaving a garnet-bearing amphibolite residue. Combining our results and previous studies of the tectonic evolution of the Cathaysia Block, we propose that the crust was thickened to over 40 km by a compressive event occurring during the late Early Cretaceous, which is supported by the observation that there is an angular unconformity between the Upper Cretaceous Series and the early Lower Cretaceous or the Jurassic rocks. After this event, the Cathaysia Block experienced a lithospheric extension and thinning probably driven by the high-angle paleo-Pacific subduction. With the attenuation of lithosphere, the lower crust was heated to partial melting by upwelling asthenospheric materials, resulting in generation of the Lingxi granodiorite and other coeval granitoids in the Cathaysia Block. This study provides new information on the crustal evolution of the Cathaysia Block during the Early Cretaceous.  相似文献   

4.
Abstract The Ryoke Belt in the Ikoma Mountains, Nara Prefecture, Japan, is composed mainly of various granitic, intermediate and gabbroic rocks. Igneous activity in this area is divided into two periods, early–middle Jurassic and late Cretaceous, based on isotopic dating. The intermediate plutonic rocks in the Fukihata area are composed of two rock types: Kyuanji quartz diorite and Fukihata tonalite. Rb–Sr whole-rock isochron ages have been determined for both plutonic rocks. Their ages and initial 87Sr/86Sr ratios are as follows: the Kyuanji quartz diorite has an age of 161.0 ± 17.9 Ma with an initial 87Sr/86Sr ratio of 0.70727 ± 0.00007, while the Fukihata tonalite has an age of 121.4 ± 24.6 Ma with an initial 87Sr/86Sr ratio of 0.70753 ± 0.00020. Our chronological results indicate that the Kyuanji quartz diorite belongs to the Jurassic mafic rocks, such as the Ikoma gabbroic mass, while the Fukihata tonalite belongs to the early Cretaceous granitic rocks. Both these intermediate plutonic rocks have different chemical characteristics and were derived from different magmas.  相似文献   

5.
The Higo metamorphic terrane situated in west-central Kyushu island, southwest Japan, is composed of greenschist- to granulite-facies metamorphic rocks. The southern part of the metamorphic terrane consists mainly of garnet–biotite gneiss and garnet–cordierite–biotite gneiss, and orthopyroxene or cordierite-bearing S-type tonalite with subordinate amounts of hornblende gabbro. Rb–Sr, Sm–Nd and K–Ar isotopic ages for these rocks have been determined here. The garnet–biotite gneiss gives an Sm–Nd age of 227.1 ± 4.9 Ma and a Rb–Sr age of 101.0 ± 1.0 Ma. The hornblende gabbro has an Sm–Nd age of 257.9 ± 2.5 Ma and a K–Ar age of 103.4 ± 1.1 Ma. These age differences of the same samples are due to the difference in the closure temperature for each system and minerals. The garnet-cordierite–biotite gneiss contains coarse-grained garnet with a zonal structure conspicuously distinguished in color difference (core: dark red; rim: pink). Sm–Nd internal isochrons of the garnet core and the rim give ages of 278.8 ± 4.9 Ma (initial 143Nd/144Nd ratio = 0.512311 ± 0.000005) and 226.1 ± 28.4 Ma (0.512277 ± 0.000038), respectively. These ages are close to formation of the garnet core and the rim. It has been previously suggested that the Higo metamorphic terrane belongs to the Ryoke metamorphic belt. But Sr and Nd isotopic features of the rocks from the former are different from those of the Ryoke metamorphic rocks, and are similar to those of the granulite xenoliths contained in the Ryoke younger granite.  相似文献   

6.
Abstract Miyanohara tonalite occurs in the middle part of the Higo metamorphic belt in the central Kyushu, Southwest Japan. This tonalite intrudes into early Permian Ryuhozan metamorphic rocks in the south and is intruded by Cretaceous Shiraishino granodiorite in the north. The Miyanohara tonalite yielded three mineral ages: (i) 110–100 Ma for Sm–Nd and Rb–Sr internal isochrons and for K–Ar hornblende; (ii) 183 Ma for Sm–Nd internal isochron; and (iii) 211 Ma for Sm–Nd internal isochron. The ages of 110–100 Ma may indicate cooling age due to the thermal effect of the Shiraishino granodiorite intrusion. The ages of 183 Ma and 211 Ma are consistent with timing of intrusion of the Miyanohara tonalite based on geologic constraints. The hornblende in the sample which gave 183 Ma shows discontinuous zoning under microscope, whereas the one which gave 211 Ma does not show zonal structure. These mineralogical features suggest that the 183 Ma sample has suffered severely from later tectonothermal effect compared with the 211 Ma sample. Therefore, the age of 211 Ma is regarded as near crystallization age for the Miyanohara tonalite. The magmatic process, geochronology and initial Sr and Nd isotope ratios for the Miyanohara tonalite are similar to those of early Jurassic granites from the Outer Plutonic Zone of the Hida belt that constitutes a marginal part of east Asia before the opening of the Japan Sea. Intrusion of the Miyanohara tonalite is considered to have taken place in the active continental margin during the late Triassic.  相似文献   

7.
The Moshirabad pluton is located southwest of the Sanandaj–Sirjan Metamorphic Belt, Qorveh, western Iran. The pluton is composed of diorite, monzodiorite, quartz diorite, quartz monzodiorite, tonalite, granodiorite, granite, aplite, and pegmatite. In this study 31 samples from various rocks were chosen for whole‐rock analyses and 15 samples from different lithologies were chosen for mineral chemical studies. The compositions of minerals are used to describe the nature of magma and estimate the pressure and temperature at which the Moshirabad pluton was emplaced. Feldspar compositions are near the binary systems in which plagioclase compositions range from An5 to An53 and alkali‐feldspar compositions range from Or91 to Or97. Mafic minerals in the plutonic rocks are biotite and hornblende. Based on the composition of biotites and whole‐rock chemistry, the Moshirabad pluton formed from a calc‐alkaline magma. Amphiboles are calcic amphiboles (magnesio‐hornblende or edenite). Temperatures of crystallization, calculated with the hornblende–plagioclase thermometer, range 550–750°C. These temperatures indicate that plutonic rocks have undergone some retrogressive changes in their mineral compositions. Aluminum‐in‐hornblende geobarometry indicates that the Moshirabad pluton was emplaced at pressures of 2.3–6.0 kbar, equal to depths of 7–20 km, but with consideration of regional geology, lower pressures than the above pressure range are more probable. Alteration of amphiboles can be the reason for some overestimation of pressures.  相似文献   

8.
Plutons of quartz diorite to adamellite composition give Rb-Sr total-rock and biotite calculated ages of 12 to 176 m.y. along a transect of the Chilean Andes between 45° and 46° S. Miocene biotite dates of quartz diorite from the Puerto Aisen area are interpreted as minimum ages related to probable local structural deformation (faulting). Along a north-south traverse of the Argentine Andes between 40° and 44° S, plutons of similar composition to the Chilean Andes give KAr and RbSr total-rock and mineral dates which range from late Paleozoic to Cretaceous. No systematic unidirectional migration of the radiometric ages is observed.  相似文献   

9.
Zircons separated from Cretaceous granitoids are dated from a south‐central transect of the Abukuma metamorphic and granitic terrane. The zircon ages do not follow ‘older’ and ‘younger’ granitoid ages that are used conventionally. In the western part of the study area (Zones I, II and III) where the Takanuki and Gosaisho metamorphic rocks are exposed, the Iritono quartz dioritic stock intruding the greenschist facies rocks in Zone III exhibits the oldest age of 121 Ma in the studied region. Quartz diorite located northward shows 112 Ma, but the other four granitoids intruding into the Takanuki and Gosaisho metamorphic rocks are younger and 103–99 Ma. Two‐mica and biotite granites belong to the youngest age group of 99 Ma. The granitic activities of both the Abukuma and Ryoke belts were initiated by intrusion of quartz dioritic magmas and were ended by two‐mica granite activity. The ages of the eastern two batholiths vary from 110 to 106 Ma (four samples), and show no age common to the Kitakami granitoids farther to the north. Throughout the Japanese Islands arc, Cretaceous granitic activities became younger toward the marginal sea side from the Kitakami Mountains, to the Abukuma Highland, and the Ryoke Belt, then to the Sanin belt of the Inner Zone of Southwest Japan.  相似文献   

10.
Abstract Volcanism in the back-arc side region of Central Luzon, Philippines, with respect to the Manila Trench is characterized by fewer and smaller volume volcanic centers compared to the adjacent forearc side-main volcanic arc igneous rocks. The back-arc side volcanic rocks which include basalts, basaltic andesites, andesites and dacites also contain more hydrous minerals (ie, hornblende and biotite). Adakite-like geochemical characteristics of these back-arc lavas, including elevated Sr, depleted heavy rare earth elements and high Sr/Y ratios, are unlikely to have formed by slab melting, be related to incipient subduction, slab window magmatism or plagioclase accumulation. Field and geochemical evidence show that these adakitic lavas were most probably formed by the partial melting of a garnet-bearing amphibolitic lower crust. Adakitic lavas are not necessarily arc–trench gap region slab melts.  相似文献   

11.
Takeshi Ikeda 《Island Arc》2002,11(3):185-192
Abstract   The present paper is reporting on the regional occurrence of orthopyroxene-bearing basic rocks from the Ryoke Metamorphic Belt in the Yanai district, southwest Japan. Their localities are confined to the highest-grade zone of the area (i.e. the garnet–cordierite zone, where garnet coexists with cordierite, K-feldspar and biotite in pelitic rocks). Orthopyroxene coexists with quartz and hydrous minerals such as biotite, cummingtonite and hornblende, and in some cases with clinopyroxene, suggesting that the highest grade of the Ryoke metamorphism reached a low-temperature subfacies of the granulite facies, contrary to the upper amphibolite facies as previously asserted.  相似文献   

12.
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   

13.
The Khalkhab–Neshveh (KN) pluton is a part of Urumieh–Dokhtar Magmatic Arc and was intruded into a covering of basalt and andesite of Eocene to early Miocene age. It is a medium to high‐K, metaluminous and I‐type pluton ranging in composition from quartz monzogabbro, through quartz monzodiorite, granodiorite, and granite. The KN rocks show subtle differentiation trends strongly controlled by clinopyroxene, plagioclase, hornblende, apatite, and titanite, where most major elements (except K2O) are negatively correlated with SiO2; and Al2O3, Na2O, Sr, Eu, and Y define curvilinear trends. Considering three processes of magmatic differentiation including mixing and/or mingling between basaltic and dacitic magmas, gravitational fractional crystallization and in situ crystallization revealed that the latter is the most likely process for the evolution of KN magma. This is supported by the occurrence of all rock types at the same level, the lack of mafic enclaves in the granitoid rocks, the curvilinear trends of Na2O, Sr, and Eu, and the constant ratios of (87Sr/86Sr)i from quartz monzodiorite to granite (0.70475 and 0.70471, respectively). In situ crystallization took place via accumulation of plagioclase and clinopyroxene phenocrysts and concentration of these phases in the quartz monzogabbro and quartz monzodiorite at the margins of the intrusion at T ≥ 1050°C, and by filter pressing and fractionation of hornblende, plagioclase, and later biotite in the granitoids at T = ～880°C.  相似文献   

14.
The protoliths of mafic-ultramafic plutons in the northern Dabie Mts. (NDM) (Hubei) include pyroxenite and gabbro. The zircon U-Pb dating for a gabbro suggests that emplacement of mafic magma took place in the post-collisional setting at the age of 122.9(0.6 Ma. It is difficult to obtain a reliable Sm-Nd isochron age, due to disequilibrium of the Sm-Nd isotopic system. Two hornblende 40Ar/39Ar ages of 116.1(1.1 Ma and 106.6(0.8 Ma may record cooling of metamorphism in the mafic-ultramafic plutons in Hubei below 500(C. The hornblende 40Ar/39Ar ages for the mafic-ultramafic rocks in Hubei are evidently 15-25 Ma younger than those for the same rocks in Anhui, indicating that there is a diversity of the cooling rates for the mafic-ultramafic rocks in Hubei and Anhui. The difference in their cooling rates may be controlled by the north-dipping normal faults in the NDM. The intense metamorphism occurring in the mafic-ultramafic rocks in Hubei may result from the Yanshanian magmatic reheating and thermal fluid action induced by the Cretaceous migmatization. The geochemical similarity of these mafic-ultramafic rocks wherever in Hubei and Anhui may be attributed to the same tectonic setting via an identical genetic mechanism.  相似文献   

15.
Granodiorite and diorite plutons of the Rosetown complex, N.Y., which are associated with the nearby Cortlandt complex, have chemical and textural characteristics indicating that large-scale liquid immiscibility played a major role in their petrogenesis. Rare earth element, zirconium, niobium and phosphorus abundances are much greater in the diorite precluding the possibility that the Rosetown diorite and granodiorite are related by fractional crystallization. The trace element data also eliminate the possibility that the granodiorite represents: (1) a partial melt of crustal rocks including basalt; (2) a granitic cumulate; or (3) a residue from an aqueous fluid derived either from a silicate melt or crustal rocks. Liquid immiscibility appears to be viable model for the origin of the Rosetown granodiorite and iron-rich diorite. This model is supported by the following: (1) the major element compositions occur in a two-liquid field on a Greig diagram; (2) both bodies have similar Sr isotope compositions; (3) common phases in the two rock types have overlapping compositions; (4) the major and trace element data of the diorite and granodiorite are similar to the experimentally determined partition data of immiscible liquid pairs; and (5) possible ocelli of iron-rich diorite are found in the granodiorite.  相似文献   

16.
Researches over the last 20 years show that the orogenic belt remains rather active after plate colli-sion[1,2]. A complete orogenic cycle in the last period of the Wilson cycle can be defined by three stages of development[3]: (1) horizontal contraction and crustal thickening due to collision, as well as formation of topography and the crustal and lithospheric root; (2) eclogite facies metamorphism of the crustal root; and (3) delamination of the crustal root or lithospheric mantle, extension…  相似文献   

17.
Abstract   Thick Middle (–Upper) Miocene turbiditic deposits filled very deep and narrow foredeep basins formed in the western margin of the Hidaka collision zone in central Hokkaido. Cobble- to boulder-sized clasts of eight monzogranites and a single granodiorite in the Kawabata Formation in the Yubari Mountains area yielded biotite K–Ar ages of 44.4 ± 1.0 to 45.4 ± 1.0 Ma and 42.8 ± 1.1 Ma, respectively. Major elemental compositions of the clasts all fall in the field of S-type granite on an NK/A (Na2O + K2O/Al2O3 in molecule) versus A/CNK (Al2O3/CaO + Na2O + K2O in molecule) diagram, verifying their peraluminous granite character (aluminium saturation index (ASI): 1.12–1.19). These geochronological and petrographical features indicate that the granitoid clasts in the Kawabata Formation correlate with Eocene granitic plutons in the northeastern Hidaka Belt, specifically the Uttsudake (43 Ma) and Monbetsu (42 Ma) plutons. Foredeep basins are flexural depressions developed at the frontal side of thickened thrust wedges. The results presented here suggest that deposition of the Middle Miocene turbidites was coeval with rapid westward up-thrusting and exhumation of the Hidaka Belt. This early mountain building may have occurred in response to thrusting in the Tertiary fold-and-thrust system of central Hokkaido.  相似文献   

18.
Abstract Zircon U–Pb sensitive high mass-resolution ion microprobe dating was carried out on three types of granitic rock (gneissose biotite granodiorite, biotite granite and two-mica granite) from the Cretaceous Ryoke belt of the Kinki district, Southwest Japan. The results give the ages of granitic magmatism in the Shigi-san area of between 87 and 78 Ma and suggest extensive melting of the Cretaceous Ryoke granitic crust to form the two-mica granite, probably at ca 80 Ma. Discrimination into older and younger granites based on development of gneissosity does not appear to represent the sequence of magma generation, although there is some scope in the interpretation of the zircon U–Pb data that would allow all three granites to form at 83 Ma. Compilation of chemical Th-U-total Pb isochron dating method ages, whole rock Rb–Sr isotope ages and U–Pb isotope ages indicates that most Ryoke plutonism occurred from ca 70 Ma to ca 100 Ma. Younger (85 Ma–70 Ma) plutonism with the formation of two-mica granite occurred only in the eastern sector of the Ryoke belt, including the Kinki District.  相似文献   

19.
Abundant dunite and harzbugite xenoliths are preserved in Early Cretaceous high-Mg# [63–67, where Mg# = molar 100 × Mg/(Mg + Fetot)] diorite intrusions from western Shandong in the North China Craton (NCC). Dunite and some harzburgite xenoliths typically preserve areas of orthopyroxenite (sometimes accompanied by phlogopite) either as veins or as zones surrounding chromite grains. Harzburgite is chiefly composed of olivine, orthopyroxene, minor clinopyroxene and chromian-spinel. High Mg#'s (averaging 91.4) and depletions in Al2O3 and CaO (averaging 0.52 wt.% and 0.29 wt.%, respectively) in harzburgite and dunite xenoliths suggest that they are residues formed by large degrees of polybaric melting. However, olivines and orthopyroxenes from dunite xenoliths spatially associated with orthopyroxenite display lower Mg#'s (i.e., 82–87 and 83–89, respectively), suggesting that an adakitic melt–peridotite reaction has taken place. This is consistent with the production of veined orthopyroxene or orthopyroxene + phlogopite in dunite and some harzburgite xenoliths in response to the introduction of adakitic melt into the previously depleted lithospheric mantle (i.e., harzburgite and dunite xenoliths). The presence of orthopyroxene in veins or as a zones surrounding chromite in peridotite xenoliths is thought to be representative of adakitic melt metasomatism. The dunite and harzbugite xenoliths are relatively rich in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), poor in heavy rare earth elements (HREEs) and high field strength elements (HFSEs), and lack Eu anomalies on chondrite normalized trace element diagrams. The initial 87Sr/86Sr ratios and εNd(t) values for the xenoliths range from 0.7058 to 0.7212 and + 0.18 to ? 19.59, respectively. Taken together, these features, combined with the strong depletion in HFSE and the existence of Archean inherited zircons in the host rocks, suggest that the adakitic melt was derived from the partial melting of early Mesozoic delaminated lower continental crust. The interaction of the adakitic melt with peridotite is responsible for the high-Mg# character of the early Cretaceous diorites in western Shandong.  相似文献   

20.
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   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号