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《International Geology Review》2012,54(4):482-495
The Sipingshan gold deposit, located in the eastern part of the Nadanhada Terrane, is hosted within cherts and silicified breccias of the Upper Cretaceous Sipingshan Formation and rhyolites of the Upper Cretaceous Datashanlinchang Formation. The orebodies are composed of gold- and pyrite-bearing cherts, silicified breccias, and quartz veins accompanied by various types of wall rock alteration, including silicification, pyritization, sericitization, chloritization, pyrophyllitization, and carbonatization. LA-ICP-MS U–Pb zircon ages determined for the ore-bearing rhyolites range between 122 ± 1.4 and 135.2 ± 1.9 million years slightly older than the metallogenic age of the Sipingshan gold deposit. The rhyolite has aluminium saturation index values ranging from 0.015 to 1.25 and shows the following features: enrichment in LILE (e.g. Rb, Pb, K, and Th); depletion of Ba, Sm, and Ti; and negative Eu anomalies. These geochemical characteristics indicate that (1) the rhyolite contains features typical of S-type granites; (2) the felsic magma likely originated through partial melting of the continental crust; and (3) plagioclase crystals were present in the partial melt residues in the magma source region, or else magma evolution involved plagioclase fractionation. The host cherts have high Al/(Al + Fe + Mn) ratios (0.23–0.81, averaging 0.60) and low Al2O3 and TiO2 contents. Their North American shale-normalized REE patterns are characterized by flat REE, slightly positive Eu anomalies, no Ce anomalies, and (La/Yb)SN ratios of 1.27–1.38, indicating that these cherts formed in a continental margin environment. In addition, the analysed cherts have low ΣREE (1.56–3.64 ppm) and Zr (9.1–13.5 ppm) contents, suggesting a hydrothermal origin. Fluid inclusions in quartz veins show elliptical to irregular shapes that range from 5 to 12 μm in size and have homogenization temperatures of 118.7–223.4°C, densities of 0.84–0.94 g/cm3, and pressures of 21.2–51.4 MPa, indicating that the hot-spring-type Sipingshan gold deposit is epithermal in origin. 相似文献
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Remnant lake and stream terraces of the Wadi el Hasa (west-central Jordan) are associated with in situ prehistoric sites spanning > 100,000 years. Eighteen radiocarbon dates from cultural and geological deposits on the terraces facilitate the first comprehensive prehistoric landscape chronology for the southern Levant east of the Jordan Rift. In the eastern Hasa basin, the uppermost of three cut and fill surfaces (>20 m) is linked to massive fossil spring deposits and an early Middle Paleolithic occupation (100,000–70,000 B. P.), suggestive of considerably wetter climates. A later Middle Paleolithic occupation may be synchronous with the emergence of Pleistocene Lake Hasa (ca. 70,000 B. P.). Peak lake levels were attained 40,000 years ago. Dates proliferate after 25,000 B. P. and register recession of Lake Hasa (ca. 20,000 B. P.), an intervening erosional phase, and the initiation of complex humid-desiccation cycles for the terminal Pleistocene—Holocene (17,000–9,000 B. P.). The contemporary Wadi el Hasa channel began aggrading its floodplain after 8000 B. P. and was incised to its present depths 1000–500 years ago. The prehistoric landscape history of the Hasa drainage is broadly synchronous with sequences in the Rift Valley and Negev desert and offers baseline chronologies for the Late Quaternary of eastern Jordan and the Arabian peninsula. © 1994 John Wiley & Sons, Inc. 相似文献
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C. Vance Haynes 《Geoarchaeology》1995,10(5):317-388
Geoarchaeological investigations at the Clovis type site, Blackwater Locality No. 1, in 1983 and 1984 included core drilling, archaeological test excavations, stratigraphic profiling, sedimentary analyses, and radiocarbon dating. Six lines of core holes transverse to the outlet channel clearly defined the subsurface configuration and stratigraphy of the prehistoric spring run. Pieces of large animal bone from units B, C, D, and E that elsewhere in the site contain Paleoindian artifacts suggest occurrences of additional buried sites along the ancient spring run. Four Paleoindian projectile points recovered during archaeological testing confirm these prospects. The Clovis type site, located in an abandoned gravel pit, is in a natural depression initially occupied by a late Pleistocene lake. After breaching of the depression by overflow or sapping, it became a springhead and was enlarged by slumping and slopewash. Detailed stratigraphic profiling of the south wall of the abandoned gravel pit provided precise stratigraphic control for sediment sampling and radiocarbon dating, and revealed more complex microstratigraphy and facies relationships than heretofore known for the site. The interfingering of dune facies around the depression with lacustrine and spring-laid facies within it aid paleoclimatic interpretation. Deflational contacts within the depression appear to correlate with adjacent wedges of dune sand reflecting relatively arid intervals. Between these arid episodes occur intervals of increased ground water level attended initially by deposition of spring-laid sands of unit B during the late Pleistocene (13,000–11,500 yr B.P.). As the water table rose following a period of severe deflation, slumping and gravity flow deposited clayey sand, Unit C, on the floor of the blowout between 11,500 and 11,000 yr B.P. During this time Clovis people first appeared at the site. After another brief period of deflation, a lake rose causing sand of Unit D0 to be washed in from shore followed by deposition of diatomities, units D1 and D2. These were separated by a brief influx of eolian sand, unit D2z. Between 10,800 and 10,000 yr B.P. outflow from the lake was reduced by accumulation of eolian sand in the outlet while Folsom people and later Agate Basin people arrived to hunt bison during this time. Cody complex people appeared during and after a brief erosional episode that preceded deposition of eolian silt and sand of units E and F from 10,000 to 8000 yr B.P. Eolian deposition during post-Folsom time converted the pond to a wet meadow and eventually, during Cody time, to a grassy swale. Some of these deposits were blown out during the Altithermal arid period (ca. 8000-5000 yr B.P.), a time when prehistoric Archaic peoples excavated wells in the floor of the depression. Subsequent eolian activity has resulted in deflation and dune migration during the late Holocene. The best prospects for Paleoindian finds are along the buried outlet south of the south wall and in early Holocene dune sands on the uplands around the depression. © 1995 John Wiley & Sons, Inc. 相似文献
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新疆阿尔泰康布铁堡组钾质和钠质流纹岩的成因及同位素年代学研究 总被引：10，自引：2，他引：8
阿尔泰南缘康布铁堡组酸性火山岩分布在麦兹、克朗、冲呼尔以及阿舍勒等火山-沉积盆地.本文对其中的钾质流纹岩(TM1,铁木尔特地区)和钠质流纹岩(KK7,可可塔勒地区)进行了SHRIMP锆石U-Pb定年以及地球化学的研究.钾质流纹岩的年龄为400.8±8.4Ma(加权平均年龄为394.8±7.9Ma),钠质流纹岩的年龄为402.2±6Ma(加权平均年龄为396.8±5.1Ma).主元素分析结果证明研究区在时间和空间上紧密共生的高钾流纹岩和高钠流纹岩同属于高碱、高硅、低钙、过铝质的钙碱性岩系.相似的稀土元素分配模式和同样亏损高场强元素(Ti、Nb、Ta)的地球化学特征暗示二者的原始岩浆源于上地壳部分熔融.钾质流纹岩富集Rb,Ba、Pb,钠质流纹岩则富集Sr,这种元素分相富集的特征符合在岩浆液态不混溶条件下微量元素在共轭钠质熔体和钾质硅酸盐熔体中的分配原则.由此推论,本区产出的钾质流纹岩和钠质流纹岩可能是上地壳部分熔融形成的高挥发分酸性岩浆液态不混溶的产物. 相似文献
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At least four volcanic complexes of different age and petrologic character occur in southeastern New England. Each complex contains a variety of fine-grained felsitic rocks, and three of these are known to have been quarried by prehistoric people for material used in the production of stone tools. These volcanics include a Late Proterozoic calcalkaline suite (Lynn-Mattapan) and several alkaline suites of Ordovician to Carboniferous age (Blue Hill, Spencer Hill, and Wamsutta suites). Each suite exhibits unique petrographic and geochemical features that help to constrain sources of felsitic archaeological materials. Distinctive petrographic features are: (1) Lynn/Mattapan: mostly pyroclastic rocks that typically contain broken crystals, volcanic clasts, and relict pumice and glass shards; phenocrysts of plagioclase, quartz, and perthite, commonly in glomeroporphyritic clusters; accessory sphene and prominent late-stage epidote; some varieties exhibit distinct flow-banding. (2) Blue Hill and Spencer Hill rocks: mainly lava flows; abundant perthite and quartz phenocrysts, with minor or no plagioclase; accessory minerals may include fluorite, aegerine, riebeckite, zircon, and allanite. (3) Wamsutta rhyolite: phenocrysts solely of anorthoclase; quartz restricted to late-stage filling of vesicles, and to planar, subparallel fractures in-filled as lithophysae; lava flows with devitrified glass matrix. Major element geochemistry is of limited use in distinguishing the volcanic groups, but trace element signatures are distinct and provide excellent criteria to discriminate rocks from each suite. Compared to the Lynn/Mattapan suite, the Blue Hill and Spencer Hill rocks exhibit higher concentrations of Rb, Y, Zr, Nb, La, Ce and Zn, and lower concentrations of Sr and Ba. Wamsutta rhyolite is intermediate in composition, but distinct. Examination of material from six prehistoric quarries, and debitage collected at seven archaeological sites, demonstrates that most samples can be assigned to one of the above volcanic suites based on combined petrographic and geochemical attributes. These geologic attributes add a significant element of quantification to archaeological sourcing problems that lead to improved identification of materials compared to hand sample characterization alone. The volcanics from source areas proximal to the Boston basin were important to Early and Middle Archaic period populations across most of southeastern Massachusetts. During the Late/Terminal Archaic period, these materials were being transported extensively throughout eastern and southeastern Massachusetts, the Narragansett Bay area, and parts of Rhode Island. Thus, routinely applied geological methods can provide useful approaches to constrain the sources of felsites from southeastern New England found in archaeological contexts. © 1997 John Wiley & Sons, Inc. 相似文献
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We introduce the use of Nd isotopes as a high‐precision technique that can be used in conjunction with visual, petrographic, and trace element analyses to source indistinct felsitic debitage. This approach allows sourcing of debitage from southeastern New England, including samples that previously could not be constrained to one source. Combined with trace element data, isotopic data also provide the possibility of sourcing debitage to a particular quarry site within a volcanic complex, and even to a particular ash flow within a quarry site. Determining the origin of debitage so precisely is important for understanding acquisition, trade, and exchange networks in southeastern New England, where distances separating quarry sites within an individual volcanic complex (Lynn–Mattapan) are greater than distances between different volcanic complexes (Lynn–Mattapan, Blue Hill, and Wamsutta). The sourcing of debitage to Mattapan quarries suggests that Middle Archaic populations in the northwest part of the Boston Basin obtained nonlocal lithic material primarily from sources south of the basin. © 2000 John Wiley & Sons, Inc. 相似文献
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Black dimensional stones: geology, technical properties and deposit characterization of the dolerites from Uruguay 总被引：1，自引：1，他引：0
Manuela Morales Demarco Pedro Oyhantçabal Karl-Jochen Stein Siegfried Siegesmund 《Environmental Earth Sciences》2011,63(7-8):1879-1909
Dimensional stones with a black color occupy a prominent place on the international market. Uruguayan dolerite dikes of andesitic and andesitic–basaltic composition are mined for commercial blocks of black dimensional stones. A total of 16 dikes of both compositions were studied and samples collected for geochemical and petrographical analysis. Color measurements were performed on different black dimensional stones in order to compare them with the Uruguayan dolerites. Samples of the two commercial varieties (Absolute Black and Moderate Black) were obtained for petrophysical analysis (e.g. density, porosity, uniaxial compressive strength, tensile strength, etc.). Detailed structural analyses were performed in several quarries. Geochemistry and petrography determines the intensity of the black color. The Uruguayan dolerite Absolute Black is the darkest black dimensional stone analyzed in this study. The petrophysical properties of Uruguayan dolerites make them one of the highest quality black dimensional stones. Structural analyses show that five joint sets have been recognized: two sub-vertical joints, one horizontal and two diagonal. These joint sets are one of the most important factors that control the deposits, since they control the block size distribution and the amount of waste material. 相似文献
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Eric T. Karlstrom 《Geoarchaeology》2005,20(1):1-28
Two approximately 5‐ to 6‐km drainage segments on Black Mesa preserve unusually complete sequences of late Quaternary alluvium and soils. Radiocarbon‐ and tree‐ring‐dated alluvial and soil stratigraphy suggests entrenched paleoarroyos were beginning to aggrade at about >24,260, 11,070, 9660, 8800, 7060, 3500, 2140, and 1870 14C yr B.P. Using the quantity of sediment removal from post‐A.D. 1900 arroyos as analogue, at least 77–200% of total valley alluvium has been removed and replaced by younger sediments during an estimated 11 late Pleistocene and Holocene erosion epicycles. Given that most (59%) of the 150 recorded prehistoric sites in the two study areas occur on valley floors where only about 3% of surface alluvium predates Lolomai phase Basketmaker II occupation (˜1900–1600 yr B.P.), it may be inferred that pre‐Lolomai phase Basketmaker II sites which may have been located along washes have been removed or buried by fluvial erosion. Identification of five buried hearths in alluvial sections, including White Dog and Lolomai phase Basketmaker II sites (dating about 3500 and 1870 14C yr B.P., respectively) and one possible Early Archaic site, supports this conclusion. © 2005 Wiley Periodicals, Inc. 相似文献
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西天山阿吾拉勒地区A型流纹斑岩的初步研究 总被引：1，自引：0，他引：1
新疆阿吾拉勒山是中亚造山带的重要组成部分。通过对区内火山岩的主量元素和微量元素地球化学研究,在群吉萨伊-塔尔得套一带下二叠统乌朗组中厘定出了一套铝质A型流纹斑岩。群吉萨伊和塔尔得套的流纹斑岩具有富硅(SiO2≥69.9%)和富碱(Na2O+K2O≥7.06%)的特征。其中,塔尔得套流纹斑岩的总碱含量低于群吉萨伊流纹斑岩,但其K2O含量则明显高于后者,属于高钾火山岩。二者均具有明显的Ba、Sr、Eu和Ti亏损,稀土总量在90.7×10-6～264×10-6之间。主量元素和微量元素特征显示,群吉萨伊和塔尔得套的A型流纹斑岩的源区均与硬质砂岩相似,属于A2型花岗岩,同时具有较高的锆饱和温度(TZr约900°C),其成因可能与后俯冲阶段的岩石圈拆沉过程有关。 相似文献
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E. V. Bibikova S. Claesson A. A. Fedotova L. M. Stepanyuk L. V. Shumlyansky T. I. Kirnozova M. M. Fugzan L. S. Il’insky 《Geochemistry International》2013,51(2):87-108
Zircons from the oldest magmatic and metasedimentary rocks of the Podolia domain of the Ukrainian shield were studied and dated by U-Pb method on a NORDSIM secondary-ion mass spectrometer. The age of zircon cores in the enderbite gneisses taken in the Kazachii Yar and Odessa quarries on the opposite banks of the Yuzhnyi Bug River reaches 3790 Ma. Cores of the terrigenous zircons in the quartzites from the Odessa quarry as well as in the garnet gneisses from the Zaval’e graphite quarry have an age within 3650–3750 Ma. Zircon rims record two metamorphic events at 2750–2850 Ma and around 1900–2000 Ma. Extremely low U content in the zircons of the second age group indicates conditions of the granulite-facies metamorphism in the Paleoproterozoic within the Podolia domain. Obtained data on the orthorocks (enderbite-gneiss) and metasedimentary rocks unambiguously suggest the existence of ancient Paleoarchean crust in the Podolia (Dniester-Bug) domain of the Ukrainian shield. They contribute in our knowledge of scales of the formation and geochemical features of the primordial crust. 相似文献
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Results of intensive archaeological surveys conducted within two environmentally contrastive settings in the Nashville Basin of Tennessee demonstrate that prehistoric human activity was much more extensive in the inner Basin where the Duck River floodplain was flanked by patchy upland vegetation relative to the outer Basin where upland vegetation was more homogeneous. Vertebrate remains from caves and rockshelters show that the inner Basin supported such patchy upland vegetation throughout the known period of hunter-gatherer occupation of the area. There is close correspondance between periods when forest openings were most prevalent (i.e., Late Pleistocene/Early Holocene and Mid-Holocene) and times when prehistoric human occupation of the inner Basin was most intensive (Paleoindian and Middle Archaic). Comparisons of major artifact categories from systematic collections of various landform surfaces show that the T2 terrace was the most extensively utilized by prehistoric hunter-gatherers. the T3/valley slope, the uplands, and the T1 produced progressively lower artifact densities. Results of systematic backhoe trenching indicate that substantial Holocene aggradation accounts for the seemingly minor use of the T1. 相似文献
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The first systematic geoarchaeological study of a rockshelter in Kansas yielded information about rockshelter formation, Holocene paleoenvironments, archaeological site formation processes, and prehistoric human habitation and site use. The rockshelter was carved by groundwater sapping; colluvium, alluvium, and roof‐fall debris subsequently accumulated on its floor. An assemblage of Late Archaic (∼2000–4000 14C years before present [yr B.P.]) cultural deposits (lithic material, bone, and charcoal) is preserved in a 2‐m thick package of colluvium and alluvium in front of the shelter. Chipped stone analysis indicates that Late Archaic occupants of the shelter were using mostly local materials and resharpening their stone tools at the shelter. Stable carbon isotope and phytolith analyses indicate that a mixed C3 and C4 plant community existed in the study area and that it changed in composition through the late Holocene. Based on the phytolith assemblage from an unlined fire pit at the front of the shelter, Late Archaic people used local hackberry trees for fuel and/or food. 相似文献
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Felsic magmatism associated with ocean–ocean and ocean–continent subduction processes provide important evidence for distinct episodes of crust-generation and continental lithospheric evolution. Rhyolites constitute an integral component of the tholeiitic to calc-alkaline basalt–andesite–dacite–rhyolite (BADR) association and contribute to crustal growth processes at convergent plate margins. The evolution of the Dharwar Craton of southern peninsular India during Meso- to Neoarchean times was marked by extensive development of greenstone belts. These granite-greenstone terranes have distinct volcano-sedimentary associations consistent with their geodynamic setting. The present study deals with geochemistry of rhyolites from the Chitradurga-Shimoga greenstone belts of western (WDC) and the Gadwal-Kadiri greenstone belts of eastern (EDC) sectors of Dharwar Craton to compare and evaluate their petrogenesis and geodynamic setting and their control on the continental lithospheric evolution of the Dharwar Craton. At a similar range of SiO2, Al2O3, Fe2O3, the rhyolites of WDC are more potassic, whereas the EDC rhyolites are more sodic and less magnesian with slight increase in TiO2. Minor increase in MgO content of WDC rhyolites reflects their ferromagnesian trace elements which are comparatively lower in the rhyolites of EDC. The relative enrichment in LILE (K, Rb) and depletion in HFSE (Nb, Ta, Zr, Hf) marked by negative Nb–Ta, Zr–Hf and Ti anomalies endorse the convergent margin processes for the generation of rhyolites of both the sectors of Dharwar Craton. The high silica potassic rhyolites of Shimoga and Chitradurga greenstone belts of WDC showing prominent negative Eu and Ti anomalies, flat HREE patterns correspond to Type 3 rhyolites and clearly point towards their generation and emplacement in an active continental margin environment. The geochemical characteristics of Gadwal and Kadiri rhyolites from eastern Dharwar Craton marked by aluminous compositions with low and fractionated HREE patterns and minor negative Eu anomalies are in conformity with Type 1 rhyolites and suggest that they were erupted in an intraoceanic island arc system. The overall geochemical systematics of the rhyolites from both the sectors of Dharwar Craton suggest a change in the geodynamic conditions from intraoceanic island arc of eastern Dharwar Craton and an active continental margin of western Dharwar marked by ocean–ocean subduction and migration of oceanic arc towards a continent followed by arc-continent collision that contributed for the evolution of continental lithosphere in the Dharwar Craton. 相似文献
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本文对吉林省中部大口钦地区营城组流纹岩进行了岩石学和地球化学研究,结果表明:流纹岩以富硅、贫钙、贫镁、贫铁为特征,在地球化学上具有典型A型流纹岩特征。在痕量元素组成上,富集LREE和LILE元素(如K、Rb、Th、U),亏损HREE和HFSE元素(如P、Ti),具强烈的负Eu异常。其原始岩浆来源于下地壳的部分熔融,形成于滨太平洋板块俯冲引起的加厚岩石圈拆沉后的伸展环境。 相似文献
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Geoarchaeological and chronological evidence from the remote Gilf Kebir Plateau in southwest Egypt suggests a new model for the influence of early and mid‐Holocene precipitation regimes on land‐use strategies of prehistoric settlers in what is now the center of the largest hyperarid area on earth. We hypothesize that the quantitatively higher, daytime, monsoon summer rainfall characteristic of the early Holocene (9300–5400 14C yr B.P./8400–4300 yr B.C.) resulted in less grass growth on the plateau compared to the winter rains that presumably fell in the cool nights during the terminal phase of the Holocene pluvial (5400–4500 yr B.P./4300–3300 yr B.C.). The unparalleled climatic transition at 5400 yr B.P. (4300 yr B.C.) caused a fundamental environmental change that resulted in different patterns of human behavior, economy, and land use in the canyon‐like valleys and on the plains surrounding the plateau. The model emphasizes the crucial impact of seasonal rainfall distribution on cultural landscapes in arid regions and the lower significance of annual precipitation rates, with implications for future numeric climate models. It also serves as an example of how past climate changes have affected human societies. © 2004 Wiley Periodicals, Inc. 相似文献
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周家志 《华东地质学院学报》1998,21(3):213-219
本文介绍的火山岩带,有层状流纹岩及穹丘状、岩颈状等非层状流纹岩,分别属溢流相、侵出相及火山管道相,三种流纹岩均受火山岩带核中断裂及晚期火山构造控制,复盖或穿发寿昌组一段地层,又被寿昌组一段之后的次火山岩及隐爆岩所穿切,局部被横山组不整合复盖。该类流纺岩同位素年龄值较小,多数在１３０Ｍａ左右,处于晚侏罗世与早白垩世过渡期。根据其层序、穿切关系及同位素年龄等,认为三种流纹岩均属火山岩带晚期不同岩相的产 相似文献
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An array of samples from the eastern Upper Basin Member of the Plateau Rhyolite (EUBM) in the Yellowstone Plateau, Wyoming, were collected and analyzed to evaluate styles of deposition, geochemical variation, and plausible sources for low δ18O rhyolites. Similar depositional styles and geochemistry suggest that the Tuff of Sulphur Creek and Tuff of Uncle Tom’s Trail were both deposited from pyroclastic density currents and are most likely part of the same unit. The middle unit of the EUBM, the Canyon flow, may be composed of multiple flows based on a wide range of Pb isotopic ratios (e.g., 206Pb/204Pb ranges from 17.54 to 17.86). The youngest EUBM, the Dunraven Road flow, appears to be a ring fracture dome and contains isotopic ratios and sparse phenocrysts that are similar to extra-caldera rhyolites of the younger Roaring Mountain Member. Petrologic textures, more radiogenic 87Sr/86Sr in plagioclase phenocrysts (0.7134–0.7185) than groundmass and whole-rock ratios (0.7099–0.7161), and δ18O depletions on the order of 5‰ found in the Tuff of Sulphur Creek and Canyon flow indicate at least a two-stage petrogenesis involving an initial source rock formed by assimilation and fractional crystallization processes, which cooled and was hydrothermally altered. The source rock was then lowered to melting depth by caldera collapse and remelted and erupted. The presence of a low δ18O extra-caldera rhyolite indicates that country rock may have been hydrothermally altered at depth and then assimilated to form the Dunraven Road flow. 相似文献
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The Miocene Kitami rhyolite, consisting of orthopyroxene and plagioclase-phyric lavas and dikes, occurs on the back-arc side of the Kuril arc with coeval basalts and Fe-rich andesites. Temperatures estimated from orthopyroxene–ilmenite pairs exceed 900°C. Although the whole rock compositions of the Kitami rhyolite correspond to S-type granites (i.e., high K, Al, large ion lithophile elements, and low Ca and Sr), Sr–Nd isotope compositions are remarkably primitive, and similar to those of the coeval basalts and andesites. They are distinct from those of lower crustal metamorphic rocks exposed in the area. Comparison of chondrite-normalized rare earth element (REE) patterns between the rhyolite and the basalts and andesites show that the rhyolite is more light REE enriched, but has similar heavy REE contents than the basalts. All rhyolites show negative Eu anomalies. The geochemical data suggest that did not formed by simple dehydration melting of basaltic rocks or fractional crystallization of basaltic magmas. The features of slab-derived fluids expected from recent high pressure experimental studies indicates that mantle wedge is partly metasomatized with “rhyolitic” materials from subducted slabs; it is more likely that very low degree partial melting of the metasomatized mantle wedge formed the rhyolite magma. 相似文献