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171.
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
172.
Reconnaissance mapping and 40Ar/39Ar age determinations establish an eruptive chronology for Koniuji Island in the central Aleutian island arc. Koniuji is a tiny 0.95 km2 island that rises only 896 ft above the Bering Sea. Previous accounts describe Koniuji as a mostly submerged, deeply eroded, dormant stratovolcano. However, new 40Ar/39Ar ages constrain the duration of subaerial eruptive activity from 15.2 to 3.1 ka. Furnace incremental heating experiments on replicate groundmass separates from two samples of a 30–50 m thick basaltic andesite flow at the southernmost point of the island gave a weighted mean 40Ar/39Ar age of 15.2 ± 5.0 (2σ). The next phase of eruptive activity includes a series of 5.8–4.6 ka basaltic andesitic to andesitic lava flows preserved along the western shoreline. The basal lavas contain numerous mafic enclaves and dioritic cumulates suggesting a major disturbance in the plumbing system during the initial stages of emplacement. The 5.8–4.6 ka lavas are truncated by an andesitic dome complex that includes hornblende-bearing domes, flows and pyroclastics which extruded into the center of the island and comprise the majority of the subaerial eruptive volume. An angular block from within the dome complex yielded 40Ar/39Ar age of 3.1 ± 1.9 ka, thereby making it one of the youngest island arc volcanics to be dated using the 40Ar/39Ar method. Overall, the 40Ar/39Ar data indicate that Koniuji is a nascent stratovolcano that has only recently emerged above sea level, not a glacially-eroded, long-lived volcanic complex like those found on many other central Aleutian Islands. 相似文献
173.
Adakitic lavas in the Central Luzon back‐arc region,Philippines: lower crust partial melting products? 总被引:6,自引:0,他引:6
Graciano P. Yumul JR Carla Dimalanta Herve Bellon Decibel V. Faustino Joel V. De Jesus Rodolfo A. Tamayo JR Ferdinand T. Jumawan 《Island Arc》2000,9(4):499-512
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
174.
The Akan‐Shiretoko volcanic chain, situated in the Southwestern Kurile arc, consists mainly of nine subaerial andesitic stratovolcanoes and three calderas. The chain extends in a SW–NE direction for 200 km, situated oblique to the Kurile trench at an angle of 25 degrees. Thirty‐seven new K–Ar ages, plus previous data, suggest that volcanic activity along the Akan‐Shiretoko volcanic chain began at ca 4 Ma at Akan, at the southwestern end of the chain, and systematically progressed northeastward, resulting in the southwest‐northeast‐trending volcanic chain. This spatial and temporal distribution of volcanoes can be explained by anticline development advancing northeastward from the Akan area, accompanied by magma rising through northeast‐trending fractures that developed along the anticlinal axis. The northeastward development of the anticline caused uplifting of the Akan‐Shiretoko area and changed the area from submarine to subaerial conditions. Anticline formation was likely due to deformation of the southwestern Kurile arc, with southwestward migration of the Kurile forearc sliver caused by oblique subduction of the Pacific plate. The echelon topographic arrangement of the Shiretoko, Kunashiri, Etorofu and Urup was formed at ca 1 Ma. 相似文献
175.
MULTIPLE ISLAND ARC-BASIN SYSTEM AND ITS EVOLUTION IN GANGDISE TECTONIC BELT,TIBET 相似文献
176.
The Central Tianshan belt in northwestern China is a small Precambrian block located in the southern part of the Central Asia Orogenic Belt (CAOB), which is considered as "the most voluminous block of young continental crust in the world" that comprises numerous small continental blocks separated by Paleozoic magmatic arcs. The Precambrian basement of the central Tianshan Mountains is composed of volcanic rocks and associated volcano-sedimentary rocks that were intruded by granitic plutons. Geochemical analyses demonstrate that the granitic plutons and volcanic rocks were generated in the Andean-type active continental arc environment like today's Chile, and the zircon U-Pb SHRIMP dating indicates that they were developed at about 956 Ma, possibly corresponding to the subduction of the inferred Mozambique Ocean under the Baltic-African super-continent. 相似文献
177.
依吉林省中,新生代陆缘弧发展演化特征,将其划分为三个发展演化阶段,即陆缘弧隆断阶段(T3-K1)陆缘弧隆拗阶段(K2-K2)和陆缘弧裂解阶段(E-Q)。根据陆缘弧发展演化过程中的性状变化特点,陆缘弧类型可相应地划分在挤压型陆缘弧,盆岭型陆缘弧和裂解型陆缘弧。 相似文献
178.
179.
180.
东天山古生代板块构造特点及其演化模式 总被引:20,自引:0,他引:20
东天山的古板块构造格局主要由塔里木陆壳板块、西伯利亚陆壳板块和哈萨克斯坦洋壳板块在古生代的活动所奠定的。在古生代,东天山的板块构造格局主要表现为多列岛弧及其间弧间盆地和弧后盆地的形式。自北而南依次发育:博格达-哈尔里克泥盆-石炭纪岛弧,吐哈弧间盆地,觉罗塔格泥盆-石炭纪岛弧,吐哈弧间盆地,觉罗塔格泥盆-石炭纪岛弧,中天山志留-石炭纪岛弧,南天山-红柳河弧后盆地和北山陆缘裂谷带。其主要成因是由于准噶尔洋壳板块向塔里木陆壳板块下俯冲,俯冲带不断后退所形成的。奥陶纪中后期,中天山由塔里木北缘分出,形成具有古老陆块基底的类似于现今日本列岛的中天山岛弧。在其后形成南天山-红柳河弧后盆地和北山陆缘裂谷带。泥盆纪早期,俯冲带后退至觉罗塔格北侧形成觉罗塔格岛弧。泥盆纪晚期,俯冲带后退至博格达-哈尔里克北缘,形成博格达-哈尔里克岛弧。中石炭世至早二叠世,博格达同准噶尔陆块碰撞造山,哈尔里克同麦钦乌拉岛弧碰撞造山。与此同时,南天山-红柳河弧后盆地和北山裂谷带也相继闭合,而吐哈弧间盆地则成为未被消减完的弧间盆地残留下来。东天山古生代板块演化可与现今印度尼西亚地区的板块演化相类比。 相似文献