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1.
The geochemical composition of metavolcanics (metabasalts and metaandesites) of the Tunka terrane is considered. They are differently enriched in incompatible elements relative to N-MORB. The geochemical features of the basalts (LILE and LREE enrichment, Zr and Hf negative anomalies) point to the suprasubductional nature of synsedimentary volcanism; at the same time, the rocks lack negative Nb, Ta, and Ti anomalies. Comparison of the Tunka metavolcanics with those of modern geodynamic settings shows their analogy to back-arc basin basalts.  相似文献   

2.
Late Paleozoic blueschists present good exposures in the Pateyar metamorphic complex (south of Chupanan, Isfahan Province). They are formed by metamorphism of primitive basaltic lavas. Petrography and microprobe analyses show that the studied rocks consist of glaucophane, actinolite, actinolitic hornblende, plagioclase (albite), sphene, magnetite, quartz and apatite. Secondary minerals are epidote, chlorite, pyrite, hematite and calcite. Mineralogical assemblages are consistent with blueschist facies metamorphism, which is followed by a retrograde metamorphism in greenschist facies. Estimation of the metamorphic conditions suggests 300–400°C and 7–11 kbar. Chemical signatures of the studied metamorphic rocks conclude that they retain main geochemical characteristics of the protoliths, which allow the petrochemical interpretations. Geochemical analyses of these blueschists show that they were originally tholeiitic basalts. Evident negative anomalies of Nb, Ta and Ti relative to Th, La and Ce, in the primitive mantle normalized spider-gram, reveal subduction role in their petrogenesis. The studied metavolcanics exhibit an intermediate chemistry between the N-MORB (normal mid-ocean ridge basalt) and IATB (island arc tholeitic basalt). Enrichment in LREE (light rare earth elements) and LILE (large ion lithophile elements) and relative depletion in HFSE (high field strength elements) suggest a back-arc basin paleotectonic setting for the Chupanan samples. The primitive magma of the analyzed samples possibly have been produced by 8–13% melting of a spinel lherzolite. The field and petrological data propose that the studied Paleozoic metavolcanics were formed in a back-arc basin above the northward subduction of Paleo-Tethys oceanic lithosphere in Central Iran. The chemical criteria of the LILE/HFSE ratio suggests that the subduction zone was young and immature during the volcanism.  相似文献   

3.
Samples of volcanic rocks from Alborán Island, the Alboran Sea floor and from the Gourougou volcanic centre in northern Morocco have been analyzed for major and trace elements and Sr–Nd isotopes to test current theories on the tectonic geodynamic evolution of the Alboran Sea. The Alborán Island samples are low-K tholeiitic basaltic andesites whose depleted contents of HFS elements (0.5×N-MORB), especially Nb (0.2×N-MORB), show marked geochemical parallels with volcanics from immature intra-oceanic arcs and back-arc basins. Several of the submarine samples have similar compositions, one showing low-Ca boninite affinity. 143Nd/144Nd ratios fall in the same range as many island-arc and back-arc basin samples, whereas 87Sr/86Sr ratios (on leached samples) are somewhat more radiogenic. Our data point to active subduction taking place beneath the Alboran region in Miocene times, and imply the presence of an associated back-arc spreading centre. Our sea floor suite includes a few more evolved dacite and rhyolite samples with (87Sr/86Sr)0 up to 0.717 that probably represent varying degrees of crustal melting. The shoshonite and high-K basaltic andesite lavas from Gourougou have comparable normalized incompatible-element enrichment diagrams and Ce/Y ratios to shoshonitic volcanics from oceanic island arcs, though they have less pronounced Nb deficits. They are much less LIL- and LREE-enriched than continental arc analogues and post-collisional shoshonites from Tibet. The magmas probably originated by melting in subcontinental lithospheric mantle that had experienced negligible subduction input. Sr–Nd isotope compositions point to significant crustal contamination which appears to account for the small Nb anomalies.

The unmistakable supra-subduction zone (SSZ) signature shown by our Alboran basalts and basaltic andesite samples refutes geodynamic models that attribute all Neogene volcanism in the Alboran domain to decompression melting of upwelling asthenosphere arising from convective thinning of over-thickened lithosphere. Our data support recent models in which subsidence is caused by westward rollback of an eastward-dipping subduction zone beneath the westernmost Mediterranean. Moreover, severance of the lithosphere at the edges of the rolling-back slab provides opportunities for locally melting lithospheric mantle, providing a possible explanation for the shoshonitic volcanism seen in northern Morocco and more sporadically in SE Spain.  相似文献   


4.
The eclogite gravels, which were found in the Mesozoic Fenghuangtai and Maotanchang formations on the northern margin of the Dabie orogenic belt, are rich in K2O(1.21%),∑REE (278μg/g) ,and LILE(such as Rb, Ba, K, Th, etc.) , with high (La/Yb)N ratios(14.4),on the basis of the analyses of major elements, rare-earth elements (REE) and trace elements. Their enrichment in LILE, notable Nb-Ta depletion through, and depletion in HFSE relative to REE in comparison with the primitive mantle and N-MORB indicate that the protoliths of the eclogite gravels were formed in an island-arc setting. According to the Th-Hf-Ta discrimination diagram, the protoliths of the eclogite gravels are characterized by volcanic arc basalts.Trace element data indicate that the subducted marine sediments were assimilated in the magma chamber, resulting in the enrichment of LILE in the protoliths. Therefore, the protoliths of the eclogite gravels are considered to have been formed in an inland-arc setting, indicating that there had developed a paleo-inland arc before Triassic collision between the North and South China blocks in the Dabie orogenic belt. There is a marked difference between the eclogite gravels and the eclogites developed along the Dabie orngenic belt, solely based on their geochemical data,especially REE. Therefore, the eclogite gravels may not be derived from eclogite terrains preserved in the Dabie orogenic belt.  相似文献   

5.
本文以三江南段景洪大勐龙地区新发现的退变榴辉岩和蓝片岩为研究对象,对其进行了系统的地球化学、原岩性质及锆石U-Pb定年的综合研究。大勐龙地区退变榴辉岩呈透镜状产于白云钠长石英片岩、白云母片岩和斜长角闪(片)岩中,其原岩为亚碱性拉斑玄武岩,球粒陨石标准化稀土元素配分曲线呈轻稀土弱亏损、重稀土平坦型的分布特征,不具有Nb、Ta、Ti的亏损,与典型的N-MORB(正常型洋中脊玄武岩)地球化学特征一致,表明其原岩可能来源于亏损的地幔区,形成于洋中脊环境;变质锆石SHRIMP U-Pb年龄为230.3±1.7Ma,可能代表榴辉岩峰期的变质时代。蓝片岩的原岩为亚碱性拉斑玄武岩,具有较高Ti玄武岩的特征(TiO2含量2.55%~2.88%,均大于2%),稀土元素总量变化范围小,比典型的N-MORB稀土总量偏高,轻稀土元素(LREE)亏损,无明显负Eu异常,地球化学性质与N-MORB类似。结合以往区域研究成果,进一步确定研究区退变榴辉岩和蓝片岩的原岩属于典型N-MORB型,该项研究不仅在昌宁-孟连缝合带的南段发现典型的特提斯洋壳残片,而且为进一步深入探讨滇西地区特提斯洋消减-闭合的动力学背景及其复杂的构造演化过程提供了重要的科学依据。  相似文献   

6.
The Slyudyanka crystalline complex is located within the composite Khamar-Daban metamorphic terrane, the part of the Central Asian fold belt. Geochemical composition of the basic crystalline schists of the Slyudyanka Group (subterrane) metamorphosed under the high-temperature subfacies of the granulite facies suggests that their protoliths were tholeiitic basalts. Their geochemical signatures are intermediate between mid-ocean ridges and island arc basalts, best approximating back-arc basin basalts. The types of the metamorphic rocks of the Slyudyanka Group and their combination in sequences also most correspond to accumulation in back-arc basins. It was concluded that the high-grade metavolcanic rocks retain main geochemical signatures of protoliths, which allows the reconstruction of their paleogeodynamic settings, including back-arc basins.  相似文献   

7.
New geochemical data on volcanic rocks and the first U-Pb zircon ages for the Kalikorva structure made it possible to determine the time and conditions of their formation and constrain geodynamic models. The lower sequences of the Kalikorva structure is dominated by metatholeiites with high MgO, Cr, and Ni contents, high Mg#, and REE distribution patterns close to the mantle level. They contain rare komatiite interlayers and lenses of pyroxenites and peridotites and can be considered as products of the deep melting of mantle material. At the same time, the tholeiitic metabasalts bear island-arc signatures and are intercalated with metagraywackes and metadacites (adakites). This rock association could be formed under spreading conditions at the beginning of an island-arc regime. The upper sequence is dominated by metagraywackes and contains diverse rocks with both MORB (tholeiitic and komatiitic basalts) and island-arc (calc-alkaline andesite and dacites, subalkaline basalts, and picritic basalts) affinity, which is typical of back-arc basins. The U-Pb dating of zircons from the metadacites and detrital zircons from the metagraywackes of the Kalikorva structure yielded similar ages of 2785 ± 13 and 2766 ± 21 Ma, respectively. They coincide with the age of the late volcanic complex of the Hisovaara Group of the Hisovaara structure (2780 Ma). Both complexes include island-arc associations with subduction signatures and contain adakites, Nb-Ti basalts, and basaltic andesites. The metagraywackes and metadacites of the Chupa sequence of the Belomorian mobile belt are older than the similar rocks of the Kalikorva complex and have an age of 2870 ± 30 Ma. Ages of 2735 ± 20 Ma and 2720 ± 4 Ma were previously obtained for the metaandesites of the Kichany volcanogenic complex, which could be an even younger volcanic arc.  相似文献   

8.
. We report major-element, trace-element and isotopic data of volcanic rocks from the late-Neoproterozoic (570 Ma) Agardagh Tes-Chem ophiolite in Central Asia, south-west of Lake Baikal (50.5°N, 95°E). The majority of samples are high-alumina basalts and basaltic andesites having island-arc affinities. They were derived from an evolved parental magma (Mg#̾.60, Cr~180 ppm, Ni~95 ppm) by predominantly clinopyroxene fractionation. The parental magma developed from a primary mantle melt by fractionation of about 12% of an olivine+spinel assemblage. The island-arc rocks have high abundances of incompatible trace elements (light rare-earth element abundances up to 100 times chondritic, chondrite-normalised (La/Yb)n=14.6-5.1) and negative Nb anomalies (Nb/La=0.37-0.62), but low Zr/Nb ratios (7-14). Initial )Nd values are around +5.5, initial Pb isotopic compositions are 206Pb/204Pb=17.39-18.45, 207Pb/204Pb=15.49-15.61, 208Pb/204Pb=37.06-38.05. Enrichment of large-ion lithophile elements within this group is significant (Ba/La=11-130). Another group of samples consists of back-arc basin-related volcanic rocks. They are most likely derived from the same depleted mantle source as the island-arc rocks, but underwent higher degrees of melting (8-15%) and are not influenced by slab components. They have lower abundances of incompatible trace elements, flat rare-earth element patterns [(La/Yb)n=0.6-2.4] and higher )Nd values (+7.8 to +8.5). Negative Nb anomalies are absent (Nb/La=0.81-1.30), but Zr/Nb is high (21-48). At least three components are necessary to explain the geochemical evolution of the volcanic rocks: (1) an enriched (ocean island-like) component characterised by a high Nb concentration (up to 30 ppm), an absent negative Nb anomaly, a low Zr/Nb ratio (~6.5), a low )Nd value (around 0), and radiogenic 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb; (2) a back-arc basin component similar to N-MORB with a flat rare-earth element pattern and a high )Nd value (around +8.5); and (3) an island-arc component from a mantle source which was modified by the downgoing slab. Crystal fractionation superimposed on mixing and source contamination by subducted sediments is suitable to explain the observed geochemical data. The most likely geodynamic environment to produce these characteristics is a young, intra-oceanic island-arc system and an associated back-arc basin.  相似文献   

9.
An elongate belt of mid-Cretaceous, compositionally banded gneisses and granulites is exposed in Cucamonga terrane, in the southeastern foothills of the San Gabriel Mountains of southern California. Banded gneisses include mafic granulites of two geochemical types: type 1 rocks are similar to high Al arc basalts and andesites but have higher HFSE (high-field-strength-element) abundances and extremely variable LILE (largeion-lithophile-element) abundances, while type 2 rocks are relatively low in Al and similar to alkali rich MOR (midocean-ridge) or intraplate basalts. Intercalated with mafic granulites are paragneisses which include felsic granulites, aluminous gneisses, marble, and calc-silicate gneisses. Type 1 mafic granulites and calcic trondhjemitic pegmatites also oceur as cross-cutting, synmetamorphic dikes or small plutons. Small-scale heterogeneity of deep continental crust is indicated by the lithologic and isotopic diversity of intercalated ortho-and paragneisses exposed in Cucamonga terrane. Geochemical and isotopic data indicate that K, Rb, and U depletion and Sm/Nd fractionation were associated with biotite +/- muscovite dehydration reactions in type 1 mafic granulites and aluminous gneisses during high-grade metamorphism. Field relations and model initial isotopic ratios imply a wide range of protolith ages, ranging from Early Proterozoic to Phanerozoic.  相似文献   

10.
The Aluchin ophiolites represent a tectonomagmatic complex, the upper crustal part of which is made up of two dike series. One series includes diabases and gabbrodiabases, which are exposed in the Late Triassic Atamanov Massif (226 Ma) and subdivided into low-potassium, low and moderate-titanium varieties. In terms of rare-earth element (REE) distribution pattern, these rocks correspond to the mid-ocean ridge basalts (N-MORB). At the same time, trace element composition of some samples indicates the enrichment in subduction component (Ba, Th), as well as variable depletion in Ta, Nb and other high-field strength elements (HFSE), which leads us to conclude that these rocks were formed from melts similar to back-arc basin basalts (BABB) at a sufficiently mature stage of back-arc spreading. The diabases of other series form separate dike bodies (dike swarms) that cut across a mantle ultrabasic body in the northern part of the Aluchin Massif. The most part of these rocks reveal prominent island-arc signatures, primarily, REE and trace-element distribution patterns. At the same time, they are characterized by slightly elevated contents of titanium, nickel, and chromium, and low content of aluminum. On the basis of these data, the diabases of the Aluchin Massif can be regarded as BABB basalts with distinct island-arc characteristics, which are usually termed as arc-like member. The joint geochemical evolution of the two diabase series is well consistent with that of the rocks from different structures of the Mariana trough, which, together with compositional data, indicate that the studied dike series mark the initial and mature stages of the opening of the Late Triassic suprasubduction basin.  相似文献   

11.
内蒙朝克山蛇绿岩地球化学: 洋内弧后盆地的产物？   总被引：8，自引：6，他引：2  
王树庆  许继峰  刘希军  侯青叶 《岩石学报》2008,24(12):2869-2879
朝克山蛇绿岩是内蒙贺根山地区出露最好的蛇绿岩之一,可能形成于中晚石炭世。朝克山蛇绿岩中的基性岩具有LREE亏损、类似N-MORB的稀土配分模式,而相对N-MORB富集大离子亲石元素,亏损Nb、Ta等高场强元素又类似岛弧火山岩的成分特征,因此,我们认为朝克山蛇绿岩应形成于弧后盆地。将朝克山蛇绿岩的基性岩与现代Mariana洋内弧后盆地和Okinawa陆缘弧后盆地的玄武岩及同属中亚造山带的、形成于洋内弧后盆地的新疆库尔提蛇绿岩对比,朝克山蛇绿岩更类似于Mariana玄武岩和库尔提蛇绿岩,因此其很可能形成于洋内弧后盆地而不是大陆边缘弧后盆地环境。  相似文献   

12.
We conducted a geochemical study of eclogites (40 samples) from a boudin of the Lower Unit of the Maksyutov Complex in the South Urals in order to determine their protolith nature. The eclogites have major element compositions corresponding to quartz-bearing hypersthene basalts. Trace-element characteristics of the eclogites further suggest that they resemble enriched-type of tholeiites such as E-MORB. The compositional variation of eclogites was likely caused by fractional crystallization of parental melt under hypabyssal conditions, during its intrusion in thinned continental crust shortly before subduction. The high-pressure metamorphism has not affected significantly the major- and trace-element signatures of the protoliths. The compositions of co-existing minerals from the distinguished rock groups do not show significant distinctions. The considerable scatter of P–T estimates of metamorphic conditions does not depend on whole-rock composition. Therefore, the eclogitization was preceded by a chemical differentiation of an initial magmatic source, which is responsible for co-existence of rocks of variable composition in the same boudin. Dikes or sills of tholeiite basalts having geochemical characteristics of E-MORB could be the protoliths for the Maksyutov eclogites.  相似文献   

13.
位于北山中带的月牙山-洗肠井蛇绿岩带是北山地区出露最好的蛇绿岩带之一,枕状玄武岩和堆晶辉长岩表现出轻稀土元素亏损-平坦的分配模式,(La/Yb)N=0.47～1.62,类似N-MORB;而相对于N-MORB则又富集大离子亲石元素,亏损Nb、Ta等高场强元素,与典型岛弧火山岩相似;即基性岩类同时具有类似岛弧火山岩和洋中脊...  相似文献   

14.
滇东南建水地区位于师宗-弥勒构造带的南段,区内出露一套玄武岩–安山岩–英安岩–流纹岩组合。研究这套火山岩的形成时间和形成环境对认识滇东南构造格局有着重要意义。本文首次对建水火山岩进行LA-ICP-MS锆石U-Pb测年,获得两个样品的年龄分别为261.9±2.2 Ma(MSWD=0.80)、264.8±1.7 Ma(MSWD=1.12),属中二叠世晚期,代表建水火山岩的形成时间。建水玄武岩与安山岩具有低TiO_2含量(0.50%~0.88%)、高Mg~#(52.0~64.5)、弱富集LREE((La/Yb)_N=1.42~3.44)、富集LILE(Rb,Th,U,Pb)、轻微亏损HFSE(Nb,Ta,Ti)的特点,具典型岛弧玄武岩的特征;英安岩和流纹岩高ΣREE含量(139.5×10~(–6)~313.6×10~(–6))、富集LREE((La/Yb)N=4.16~9.78)和LILE(Rb,Ba,Th,U)、亏损HFSE(Nb,Ta)、强亏损Sr、Ti、Y等元素的特点与典型的岛弧流纹岩相似,但高钾(K_2O含量平均7.73%)、钙碱性(δ=0.93~2.94)、强过铝质(A/CNK=1.13~2.10)的特点使之区别于一般岛弧酸性火山岩,而具有一些上地壳部分熔融形成的S型花岗岩的特点。综合建水火山岩岩石组合、地球化学和区域地质背景,认为建水火山岩形成于活动大陆边缘的弧后盆地伸展环境,是地幔部分熔融形成的玄武质岩浆结晶分异与上地壳混染作用的共同结果,与滇桂交界处岛弧火山岩、两广交界处岛弧玄武岩等同为哀牢山洋向北俯冲的产物。  相似文献   

15.
New geochemical and Sr–Nd isotopic data for the Iratsu eclogite and surrounding metamorphic rocks of the Sanbagawa belt, Japan, show that, while the protoliths of the metamorphic rocks formed in a variety of tectonic settings, the Iratsu body represents a deeply subducted and accreted island arc. The igneous protoliths of eclogites and garnet amphibolites were probably generated from a mantle source that had components of both a depleted mantle modified by slab-released fluid (as seen in a negative Nb anomaly) and an enriched mantle, similar to that of ocean island basalts (OIB). Fractional crystallization modeling indicates that the protoliths of some garnet clinopyroxenites from the Iratsu body are cumulates from a basaltic magma that crystallized under high O2 and H2O fugacities in the middle to lower crust. The source characteristics and crystallization conditions suggest that the protoliths of the Iratsu rocks formed in an oceanic island arc. Quartz eclogites from the marginal zone of the Iratsu body have geochemical signatures similar to turbidites from the Izu–Bonin island arc (as seen in a negative Nb anomaly and a concave REE pattern). The protoliths might be volcaniclastic turbidites that formed in a setting proximal to the oceanic island arc. Geochemical and isotopic signatures of the surrounding mafic schists are similar to normal (N-) and enriched (E-) mid-ocean-ridge basalt (MORB), and distinct from the rocks from the Iratsu body. The protoliths of the mafic schists likely formed in a plume-influenced mid-ocean ridge or back-arc basin. Pelitic schists from the surrounding rocks and pelitic gneisses from the marginal zone of the Iratsu body have evolved, continental geochemical signatures (as seen in a negative εNd(t) value (~?5)), consistent with their origin as continent-derived trench-fill turbidites.  相似文献   

16.
锡林浩特岩群是内蒙古中东部锡林郭勒微地块内原锡林郭勒杂岩的重要组成部分,由一套片麻岩夹层状斜长角闪岩、磁铁石英岩和变粒岩等变质岩组成,形成于中元古代［1］。岩群中片麻岩样品显示轻稀土元素富集的中等分异特征((La/Yb)N=4.71~7.97),斜长角闪岩具有平坦型((La/Yb)N=0.74~0.95)和轻稀土元素微弱富集型((La/Yb)N=1.06~3.63)两种稀土元素配分模式。在微量元素N MORB标准化图解中,斜长角闪岩显示出洋中脊玄武岩和岛弧玄武岩的过渡类型特征。同时结合岩相学和岩石地球化学图解分析,推断片麻岩原岩为富含泥质的亚杂砂岩,而斜长角闪岩原岩为一套具有拉斑质到钙碱性过渡型特征的玄武岩;变质沉积岩形成于活动大陆边缘大陆岛弧环境;变质基性火山岩形成于岛弧弧后盆地。二者指示锡林浩特岩群形成于活动大陆边缘体系。  相似文献   

17.
We present new regional petrologic, geochemical, Sr–Nd isotopic, and U–Pb geochronological data on the Turonian–Campanian mafic igneous rocks of Central Hispaniola that provide important clues on the development of the Caribbean island-arc. Central Hispaniola is made up of three main tectonic blocks—Jicomé, Jarabacoa and Bonao—that include four broad geochemical groups of Late Cretaceous mafic igneous rocks: group I, tholeiitic to calc-alkaline basalts and andesites; group II, low-Ti high-Mg andesites and basalts; group III, tholeiitic basalts and gabbros/dolerites; and group IV, tholeiitic to transitional and alkalic basalts. These igneous rocks show significant differences in time and space, from arc-like to non-arc-like characteristics, suggesting that they were derived from different mantle sources. We interpret these groups as the record of Caribbean arc-rifting and back-arc basin development in the Late Cretaceous. The> 90 Ma group I volcanic rocks and associated cumulate complexes preserved in the Jicomé and Jarabacoa blocks represent the Albian to Cenomanian Caribbean island-arc material. The arc rift stage magmatism in these blocks took place during the deposition of the Restauración Formation from the Turonian–Coniacian transition (~ 90 Ma) to Santonian/Lower Campanian, particularly in its lower part with extrusion at 90–88 Ma of group II low-Ti, high-Mg andesites/basalts. During this time or slightly afterwards adakitic rhyolites erupted in the Jarabacoa block. Group III tholeiitic lavas represent the initiation of Coniacian–Lower Campanian back-arc spreading. In the Bonao block, this stage is represented by back-arc basin-like basalts, gabbros and dolerite/diorite dykes intruded into the Loma Caribe peridotite, as well as the Peralvillo Sur Formation basalts, capped by tuffs, shales and Campanian cherts. This dismembered ophiolitic stratigraphy indicates that the Bonao block is a fragment of an ensimatic back-arc basin. In the Jicomé and Jarabacoa blocks, the mainly Campanian group IV basalts of the Peña Blanca, Siete Cabezas and Pelona–Pico Duarte Formation, represent the subsequent stage of back-arc spreading and off-axis non-arc-like magmatism, caused by migration of the arc toward the northeast. These basalts have geochemical affinities with the mantle domain influenced by the Caribbean plume, suggesting that mantle was flowing toward the NE, beneath the extended Caribbean island-arc, in response to rollback of the subducting proto-Caribbean slab.  相似文献   

18.
Detailed geological mapping, structural, petrological and chronological investigation allow us to place new constraints on the tectono‐thermal evolution of the North Qilian high pressure/low temperature (HP/LT) metamorphic belt. The North Qilian HP/LT metamorphic belt manly consists of eclogite, blueschist, metasedimentary rocks and serpentinite. Most of eclogites and mafic blueschists occur as lenses within metasedimentary rocks, and minor eclogites within serpentinite. Petrological and geochemistical data indicate that the protoliths of eclogite and mafic blueschist includes E‐, N‐MORB, OIB and arc basalt. Geochronology and Lu‐Hf isotope of detrital zircons from metasedimentary rocks indicate the detritus materials are derived from Qilian block and likely deposit in continental margin or fore‐arc basin. Zircon U‐Pb datings show that the protolith ages of eclogites vary between 500 Ma and 530 Ma, and the metamorphic age of eclogite between 460 and 489 Ma. The detrital zircon ages of metasedimentary rocks distribute between 532 and 2700 Ma. The structural data show that the deformation related to the subduction during prograde is recorded in eclogite blocks. In contrast, the dominant deformation structures are characterized by tight fold, sheath fold and penetrative foliation and lineation, which are recorded in various rocks, reflecting a top‐to‐the‐south shear sense and representing the deformation related to the exhumation. The petrological data suggest that the different rocks in the North Qilian HP/LT metamorphic belt equilibrated at different peak metamorphic conditions and recorded different P‐T path. Synthesizing the structural, petrological, geochemical and geochronological data suggest a subduction channel model related to oceanic subduction during Paleozoic in the North Qilian Mountains. The different HP/LT metamorphic rocks formed in different settings with various protolith ages were carried by the subducted oceanic crust into different depth in subduction channel, and experienced independent tectono‐thermal evolution inside subduction channel. The North Qilian HP/LT mélange reflects a fossil oceanic subduction channel.  相似文献   

19.
The discovery of glaucophane relicts in the high-pressure tectonites of the Yenisei suture zone of the Yenisei Ridge suggests the manifestation of the “Chilean-type” convergent margin on the western Siberian Craton, which was controlled by subduction of oceanic crust beneath the continental margin. These rocks are restricted to the tectonic suture between the craton and the Isakovka ocean-island terrane and experienced two metamorphic stages. Petrogeochemical characteristics of the mafic tectonites indicate that their protoliths are N-MORB and E-MORB basalts. More primitive N-MORB basalts were formed at the initial spreading stages through melting of the upper depleted mantle. Higher Ti basalts were formed by melting of enriched mantle protolith at the later spreading stages. U–Pb zircon age of 701.6 ± 8.4 Ma of the metamorphosed analogues of normal basalts marks the initiation of oceanic crust in the region. Revealed sequence of spreading, subduction (640–620 Ma), and shear deformations (~600 Ma) records the early stages in the evolution of the Paleoasian ocean in its junction zone with the western margin of the Siberian craton: from formation of fragments of oceanic crust to the completion of accretionary–subduction events.  相似文献   

20.
New geochronological, trace element and Sr–Nd isotope data for metabasalts, dolerites and amphibolites from the Río Verde Complex, Central Hispaniola, are integrated with existing geochemical data for mafic volcanic rocks and metamorphic derivatives from the Los Ranchos, Amina and Maimón Formations, giving new insights into magma petrogenesis and paleotectonic reconstruction of the Lower Cretaceous Caribbean island-arc–back arc system. U–Pb and 40Ar/39Ar age data show that the Río Verde Complex protoliths were in part coeval with volcanic rocks of the Los Ranchos Formation (Upper Aptian to Lower Albian). The geochemical data establish the existence of gradients in trace element parameters (Nb/Yb, Th/Yb, Zr/Yb, Zr/Ba, and normalized Ti, Sm, Y and Yb abundances) and Nd isotope compositions from throughout Hispaniola, which reflect differences in the degree of mantle wedge depletion and contributions from the subducting slab. The Río Verde Complex mafic rocks and some mafic sills and dykes intruding in the Loma Caribe Peridotite, have a transitional IAT to N-MORB geochemistry and a weak subduction-related signature, and are interpreted to form in a rifted arc or evolving back-arc basin setting. The Los Ranchos, Amina and Maimón Formations volcanic rocks have arc-like characteristics and represent magmatism in the volcanic front. Trace element and Nd isotope modeling reproduce observed data trends from arc to back-arc and suggest that the variations in several geochemical parameters observed in a SW direction across the Caribbean subduction system can be explained from the progressively lower subduction flux into a progressively less depleted mantle source. The low Nb contents and high (εNd)i values in both arc and back-arc mafic rocks imply, however, the absence of a significant Lower Cretaceous plume enriched component. In order to explain these observations, a model of proto-Caribbean oceanic lithosphere subducting to the SW at least in the 120–110 Ma interval, is proposed to cause the observed magmatic variations in the Lower Cretaceous Caribbean island-arc–back-arc system. In this context, arc rifting and initial sea-floor spreading to form the Río Verde Complex protoliths occurred in the back-arc setting of this primitive island-arc, built on the NE edge of the Caribbean plate.  相似文献   

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