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1.
华南花岗岩物源成因特征与陆壳演化 总被引：15，自引：1，他引：15
华南不同时代的花岗岩按物质来源及成因可划分为:幔源(分异)系列:幔-壳混源(同熔)系列;壳源改造(重熔)系列和幔-壳混源碱性系列等四个不同的系列,简要阐述了划分四类系列的岩石学、地球化学的依据。侧重根据Sm-Nd、Rb-Sr同位素成分,以二元混合模型计算了花岗岩源区物质组成中,上壳和亏损地幔组份各占的比例。在此基础上探讨了不同物源成因系列花岗岩形成与华南陆壳演化的内在联系:东安、雪峰旋回褶皱系阶段花岗岩物源成因类型比较多样,有幔源(分异)系列、不成熟壳源、成熟壳源改造(重熔)系列等;加里东,海西期以陆壳改造(重熔)系列为主;印支、燕山期活化区(地洼)阶段,在地幔活化激发下,中国东部陆缘形成幔—壳混源(同熔)系列花岗岩和相应的火山岩;但沿内部断裂带,在壳下地幔热传输及构造作用下引起陆壳重熔,形成属于壳源改造(重熔)系列花岗岩。同位素的研究证明了:地洼阶段的构造-岩浆活动主要动力是来自壳下地幔的活化。 相似文献
2.
3.
V. M. Savatenkov V. V. Yarmolyuk A. M. Kozlovsky Z. B. Smirnova O. E. Sviridova 《Petrology》2018,26(4):351-367
The Khangai batholith is one of the largest groups of granitoid plutons produced in Central Asia in the Late Permian–Early Triassic, at 270–240 Ma. The batholith occurs in the Khangai collage of Precambrian terranes, which include Early Precambrian crustal blocks (Dzabkhan and Tarbagatai) and Early to Late Neoproterozoic structures of the Songino block in their surroundings. The axial zone of this collage is overprinted by a basin filled with Devonian volcanic–siliceous rocks and Early to Middle Carboniferous terrigenous rocks. The isotopic parameters (Nd and Pb) of granitoids in the Khangai batholith indicate that the melts were derived from compositionally contrasting crustal sources and a single mantle one. The massifs hosted in the Precambrian blocks were produced with the involvement of lower crustal material, with various ages of the origin of the crust and its differentiation into upper and lower ones. The crust of the Tarbagatai and Dzabkhan blocks was produced in the Early Archean and was differentiated at the Archean–Proterozoic boundary. The crust of the Songino block was formed in the Paleoproterozoic and differentiated in the Early Neoproterozoic. According to the Pb and Nd isotopic parameters of granitoids in the Khangai Basin, the regional continental crust was close to the juvenile one, i.e., the continental crust of the Khangai Basin had still not been differentiated by the time when the Khangai batholith was produced. A single mantle source was involved in the origin of the melts of granitoids of the Khangai batholith in various tectonic blocks. The evolution of the Pb isotopic composition of this sources is consistent with the Stacey–Kramers model at µ = 9.5. This source can be identified with the enriched mantle, which has a higher U/Pb ratio than the depleted mantle and lower εNd(T) of 0 to +2. 相似文献
4.
V. P. Kovach V. V. Yarmolyuk V. I. Kovalenko A. M. Kozlovskyi A. B. Kotov L. B. Terent’eva 《Petrology》2011,19(4):399-425
Part II of this paper reports geochemical and Nd isotope characteristics of the volcanogenic and siliceous-terrigenous complexes
of the Lake zone of the Central Asian Caledonides and associating granitoids of various ages. Geological, geochronological,
geochemical, and isotopic data were synthesized with application to the problems of the sources and main mechanisms of continental
crust formation and evolution for the Caledonides of the Central Asian orogenic belt. It was found that the juvenile sialic
crust of the Lake zone was formed during the Vendian-Cambrian (approximately 570–490 Ma) in an environment of intraoceanic
island arcs and oceanic islands from depleted mantle sources with the entrainment of sedimentary crustal materials into subduction
zones and owing to the accretion processes of the amalgamation of paleoceanic and island arc complexes and Precambrian microcontinents,
which terminated by ∼490 Ma. The source of primary melts for the low-Ti basalts, andesites, and dacites of the Lake zone ophiolites
and island arc complexes was mainly the depleted mantle wedge above a subduction zone. In addition, an enriched plume source
contributed to the genesis of the high-Ti basalts and gabbroids of oceanic plateaus. The source of terrigenous rocks associating
with the volcanics was composed of materials similar in composition to the country rocks at a minor and varying role of ancient
crustal materials introduced into the ocean basin owing to the erosion of Precambrian microcontinents. The sedimentary rocks
of the accretionary prism were derived by the erosion of mainly juvenile island arc sources with a minor contribution of rocks
of the mature continental crust. The island arc and accretion stages of the development of the Lake zone (∼540–590 Ma) were
accompanied by the development of high- and low-alumina sodic granitoids through the melting at various depths of depleted
mantle reservoirs (metabasites of a subducted oceanic slab and a mantle wedge) and at the base of the island arc at the subordinate
role of ancient crustal rocks. The melts of the postaccretion granitoids of the Central Asian Caledonides were derived mainly
from the rocks of the juvenile Caledonian crust at an increasing input of an ancient crustal component owing to the tectonic
mixing of the rocks of ophiolitic and island arc complexes and microcontinents. The obtained results indicate that the Vendian-Early
Paleozoic stage of the evolution of the Central Asian orogenic belt was characterized by the extensive growth of juvenile
continental crust and allow us to distinguish a corresponding stage of juvenile crust formation. 相似文献
5.
大洋地幔内部存在广泛的不均一性,其成因可有多种模式,其中俯冲循环作用对地幔组成的变化具有重要影响. 为明确各循环组分对亏损地幔的改造作用及其在富集源区中的相对贡献,系统总结了不同循环组分（远洋沉积物、俯冲洋壳、陆壳）的平均微量元素特征,计算了各循环组分在俯冲过程中经历的化学变化. 基于改造后的循环组分,开展与亏损地幔源区的混合和熔融模拟. 结果表明,HIMU型玄武岩可以由纯俯冲洋壳（≤10%）与亏损地幔（≥90%）混合形成的源区,经较低程度熔融（0.5%~1.5%）形成;而EMI型玄武岩可以由俯冲洋壳（≤10%）、俯冲剥蚀的下陆壳物质（≤3%）、亏损地幔（≥90%）混合形成的源区,经较低程度熔融（1%~2%）形成;EMII型玄武岩可以由俯冲洋壳（≤10%）、GLOSS-II（全球俯冲沉积物）或上陆壳物质（≤0.8%）与亏损地幔（≥90%）混合形成的源区,经较低程度熔融（1%~1.5%）形成. 相似文献
6.
Oliver E. Jagoutz 《Contributions to Mineralogy and Petrology》2010,160(3):359-381
Results of simple model calculations that integrate cumulate compositions from the Kohistan arc terrain are presented in order
to develop a consistent petrogenetic model to explain the Kohistan island arc granitoids. The model allows a quantitative
approximation of the possible relative roles of fractional crystallization and assimilation to explain the silica-rich upper
crust composition of oceanic arcs. Depending in detail on the parental magma composition hydrous moderate-to-high pressure
fractional crystallization in the lower crust/upper mantle is an adequate upper continental crust forming mechanism in terms
of volume and compositions. Accordingly, assimilation and partial melting in the lower crust is not per se a necessary process
to explain island arc granitoids. However, deriving few percent of melts using low degree of dehydration melting is a crucial
process to produce volumetrically important amounts of upper continental crust from silica-poorer parental magmas. Even though
the model can explain the silica-rich upper crustal composition of the Kohistan, the fractionation model does not predict
the accepted composition of the bulk continental crust. This finding supports the idea that additional crustal refining mechanism
(e.g., delamination of lower crustal rocks) and/or non-cogenetic magmatic process were critical to create the bulk continental
crust composition. 相似文献
7.
李曙光 《中国地球化学学报》1992,11(4):314-328
A new approach to the investigation of the Sm/Nd evolution of the upper mantle directly from the data on lherzolite xenoliths is described in this paper.It is demonstrated that the model age TCHUR of an unmetasomatic iherzolite zenolith ca represent the mean depletion age of its mantle source, thus presenting a correlation trend between f^Sm/Nd and the mean depletion age of the upper mantle from the data on xenoliths.This correlation trend can also be derived from the data on river suspended loads as well as from granitoids.Based on the correlation trend mentioned above and mean depletion ages of the upper mantle at various geological times, an evolution curve for the mean f^Sm/Nd value of the upper mantle through geological time has been established.It is suggested that the upwilling of lower mantle material into the upper mantle and the recycling of continental crust material during the Archean were more active ,thus maintaining fairly constantf^Sm/Nd and εNd values during this time period. Similarly ,an evolution curve for the mean f^Sm/Nd value of the continental crust through geological time has also been established from the data of continental crust material.In the light of both evolution curves for the upper mantle and continental crust ,a growth curve for the continental crust has been worked out ,suggesting that :(1)about 30%(in volume )of the present crust was present as the continental crust at 3.8 Ga ago ;(2)the growth rate was much lower during the Archean ;and (3)the Proterozoic is another major period of time during which the continental crust wsa built up . 相似文献
8.
青藏高原Pb同位素地球化学及其意义 总被引：12，自引：1，他引：11
根据青藏高原不同构造单元基底片麻岩、花岗岩类和火山岩等不同类型岩石的486套Pb同位素数据的整理和分析,发现青藏高原岩石圈存在3种主要类型,即亏损Pb同位素的特提斯洋地幔域端元、富集Pb同位素的喜马拉雅成熟大陆地壳端元和青藏高原北部的过渡型Pb同位素的地幔端元。这3类地球化学端元与前人通过Sr-Nd同位素研究获得的3类端元一致。拉萨地块内部不同类型岩石的Pb同位素地球化学特征指示出两类岩浆作用,一类是特提斯洋岩石圈俯冲消减再循环和亏损地幔物质注入导致的亲特提斯洋型岩浆作用,另一类是与类似于喜马拉雅大陆地壳物质加入导致的富集地幔源区有关的超钾质岩浆作用。岩浆作用的Pb同位素地球化学记录了特提斯洋俯冲消减作用和随后发生的印度大陆向北拼合、碰撞和俯冲过程,也记录了大规模的壳幔相互作用对高原岩石圈演化与隆升的贡献。 相似文献
9.
V. A. Guryanov A. N. Didenko A. Yu. Peskov G. V. Roganov V. A. Dymovich 《Russian Journal of Pacific Geology》2016,10(3):168-188
New data on the age, composition, sources, and formation conditions of the Early Precambrian granitoids of the Batomga inlier of the southeastern Siberian Platform basement are discussed. Geochronological SRHIMP II U–Pb study of the zircons reveals that the calc-alkaline granitoids of the Khoyunda Complex are 2056–2057 Ma in age and their formation was related to the Early Proterozoic stage in the development of the Batomga granite–greenstone domain. It is established that the primary melts for these rocks formed in subduction settings through melting of the depleted mantle source with some contribution of ancient crustal material. In terms of temperature, partial melting followed by crystallization of the granitoids under peak metamorphic conditions corresponds to the transition between amphibolite and granulite facies at elevated pressure; high temperature and high-grade metamorphism are subduction-related phenomena reflected in the back-arc settings of the active continental margin. The protoliths of calc-alkaline metavolcanics of the Batomga Group are found to be chronologically and compositionally analogous to the subduction granitoids of the Khoyunda and Dzhagdakan complexes; i.e., these granitoids are coeval with the Batomga island arc. The lower age limit of the Batomga Group is estimated at 2.2 Ga and its upper age limit is defined by the age of the intruded Khoyunda granitoids. The formation of the rocks of the Batomga Group and associated granitoids of the Khoyunda and Dzhagdakan complexes reflects the formation of the continental crust at the Early Paleoproterozoic stage of the evolution of the Batomga lithosphere block (2.2–2.0 Ga ago). 相似文献
10.
Ian H. Campbell 《Geochimica et cosmochimica acta》2002,66(9):1651-1661
The Nb/U and Th/U of the primitive mantle are 34 and 4.04 respectively, which compare with 9.7 and 3.96 for the continental crust. Extraction of continental crust from the mantle therefore has a profound influence on its Nb/U but little influence on its Th/U. Conversely, extraction of midocean ridge-type basalts lowers the Th/U of the mantle residue but has little influence on its Nb/U. As a consequence, variations in Th/U and Nb/U with Sm/Nd can be used to evaluate the relative importance of continental and basaltic crust extraction in the formation of the depleted (Sm/Nd enriched) mantle reservoir.This study evaluates Nb/U, Th/U, and Sm/Nd variations in suites of komatiites, picrites, and their associated basalts, of various ages, to determine whether basalt and/or continental crust have been extracted from their source region. Emphasis is placed on komatiites and picrites because they formed at high degrees of partial melting and are expected to have Nb/U, Th/U, and Sm/Nd that are essentially the same as the mantle that melted to produce them. The results show that all of the studied suites, with the exception of the Barberton, have had both continental crust and basaltic crust extracted from their mantle source region. The high Sm/Nd of the Gorgona and Munro komatiites require the elevated ratios seen in these suites to be due primarily to extraction of basaltic crust from their source regions, whereas basaltic and continental crust extraction are of subequal importance in the source regions of the Yilgarn and Belingwe komatiites. The Sm/Nd of modern midocean ridge basalts lies above the crustal extraction curve on a plot of Sm/Nd against Nb/U, which requires the upper mantle to have had both basaltic and continental crust extracted from it.It is suggested that the extraction of the basaltic reservoir from the mantle occurs at midocean ridges and that the basaltic crust, together with its complementary depleted mantle residue, is subducted to the core-mantle boundary. When the two components reach thermal equilibrium with their surroundings, the lighter depleted component separates from the denser basaltic component. Both are eventually returned to the upper mantle, but the lighter depleted component has a shorter residence time in the lower mantle than the denser basaltic component. If the difference in the recycling times for the basaltic and depleted components is ∼1.0 to 1.5 Ga, a basaltic reservoir is created in the lower mantle, equivalent to the amount of basalt that is subducted in 1.0 to 1.5 Ga, and that reservoir is isolated from the upper mantle. It is this reservoir that is responsible for the Sm/Nd ratio of the upper mantle lying above the trend predicted by extraction of continental crust on the plot of Sm/Nd against Nb/U. 相似文献
11.
V. F. Polin S. I. Dril A. I. Khanchuk T. A. Velivetskaya T. A. Vladimirova N. N. Il’ina 《Doklady Earth Sciences》2016,468(2):611-615
The Pb isotope composition of polyformational Mesozoic igneous rocks of the Ketkap–Yuna igneous province (KYIP) and lower crustal metamorphic rocks of the Batomga granite–greenstone area (the complex of the KYIP basement) of the Aldan Shield was studied for the first time. Based on the data obtained, several types of material sources participating in petrogenetic processes were distinguished. The mantle source identified as PREMA is registered in most of the igneous formations and predominates in mafic alkaline rocks. According to the isotope characteristics, the upper crustal source corresponds to a source of the “Orogen” type by the model of “plumbotectonics” or to the average composition of the continental crust by the Stacey–Kramers model. The lower crust is the third material source; however, the type of lower crustal protolith involved in the igneous process is still not defined, which makes difficult to estimate its role in the petrogenetic processes. 相似文献
12.
Mafic basaltic-andesitic volcanic rocks from the Andean Southern Volcanic Zone (SVZ) exhibit a northward increase in crustal components in primitive arc magmas from the Central through the Transitional and Northern SVZ segments. New elemental and Sr–Nd-high-precision Pb isotope data from the Quaternary arc volcanic centres of Maipo (NSVZ) and Infernillo and Laguna del Maule (TSVZ) are argued to reflect mainly their mantle source and its melting. For the C-T-NSVZ, we identify two types of source enrichment: one, represented by Antuco in CSVZ, but also present northward along the arc, was dominated by fluids which enriched a pre-metasomatic South Atlantic depleted MORB mantle type asthenosphere. The second enrichment was by melts having the characteristics of upper continental crust (UCC), distinctly different from Chile trench sediments. We suggest that granitic rocks entered the source mantle by means of subduction erosion in response to the northward increasingly strong coupling of the converging plates. Both types of enrichment had the same Pb isotope composition in the TSVZ with no significant component derived from the subducting oceanic crust. Pb–Sr–Nd isotopes indicate a major crustal compositional change at the southern end of the NSVZ. Modelling suggests addition of around 2 % UCC for Infernillo and 5 % for Maipo. 相似文献
13.
澜沧江西侧的临沧花岗岩带和怒江以西的腾冲花岗岩带是滇西最主要的两个花岗岩带,它们形成于不同构造环境。Pb、Sr 同位素研究表明,腾冲花岗岩主要来源于1200-2000Ma 的上地壳(或未分异的地壳)物质,同位素组成变化范围较小,说明源区物质组分较均一。临沧带 Pb 同位素组成变化较大,其物源是上地壳、造山带和上地幔不均匀的混合物,基底时代为800-1600Ma。因此,临沧地区和腾冲地区的基底时代和性质不同,应视为两个不同的基底地体。 相似文献
14.
兴蒙造山带正ε(Nd,t)值花岗岩的成因和大陆地壳生长 总被引：29，自引：3，他引：26
大陆地壳的生长速率和地壳生长的位置均是地球科学中的最基本的问题。现有的许多大陆地壳生长模式认为 ,90 %的大陆地壳生长于 18亿年以前 ,显生宙以来的地壳生长不到整个地壳的 10 % ,主要位于活动大陆边缘。近年来在兴蒙造山带发现大量具有新生地壳来源性质的花岗岩产生于 50 0～ 10 0Ma ,对上述传统看法提出了挑战。现有的Nd同位素资料表明 ,兴蒙造山带的显生宙花岗岩 ,不论形成于什么时代和什么构造背景 ,也不论属于何种成因类型 ,几乎都具有正ε(Nd ,t)值和年轻的Nd模式年龄tDM 。从西往东 ,随着时代逐渐变新ε(Nd ,t)值有逐渐降低的趋势。花岗岩的tDM同由蛇绿岩和岛弧杂岩记录的古亚洲洋扩张的时间基本一致。只有一些在新元古代微陆块上的花岗岩才显示负ε(Nd ,t)值和较老的tDM,反映了其源岩包括前寒武纪地壳同地幔来源物质的不同程度混合。兴蒙造山带的花岗岩具有地幔来源的ε(Nd ,t)值 ,说明这些花岗岩中有一部分 (例如加里东期和海西早期 )可能同板块俯冲作用有关 ,花岗岩的来源是被交代的地幔楔。而大面积的晚古生代—中生代花岗岩则可能是由 80 0～6 0 0Ma前俯冲的洋壳形成的新生大陆地壳在拉伸体制下部分熔融而成。如果情况是这样 ,显生宙就曾发生过大规模的地壳生长。板内岩浆活动 ,特别是 相似文献
15.
中亚巴尔喀什成矿带晚古生代岩浆活动与斑岩铜矿成矿时代 总被引：3，自引：0，他引：3
巴尔喀什成矿带是中亚成矿域重要的晚古生代斑岩铜钼成矿带。通过该成矿带科翁腊德、博尔雷和阿克斗卡地区与斑岩铜成矿作用密切相关的花岗斑岩类岩体锆石SHRIMP U-Pb定年,主量、稀土和微量元素地球化学,Sr、Nd同位素示踪分析,进一步厘定了斑岩铜成矿作用的时代,并推测了板块构造环境。斑岩铜成矿时代分为两期：早期约为327 Ma,形成科翁腊德和阿克斗卡超大型斑岩铜矿床;晚期约为316 Ma,形成博尔雷大型斑岩铜矿床。与成矿有关的斑岩类主要为高钾钙碱性系列花岗岩,可能为火山岛弧环境,部分具有埃达克岩特征和经典岛弧花岗岩类特征。斑岩类εSr(t)和εNd(t)的变化范围分别为-6.35~34.03和-0.46~5.53。其中,科翁腊德-博尔雷地区斑岩类来源于亏损地幔与大陆地壳表层物质（老地壳物质）的显著混染作用,而阿克斗卡地区斑岩类直接来自于亏损地幔。将巴尔喀什成矿带与我国西准噶尔成矿带进行了对比,认为可能属于同一个晚古生代斑岩铜钼成矿带。 相似文献
16.
ZHOU Jiyuan CUI Bingfang CHEN Shizhong Nanjing Institute of Geology Mineral Resources Chineese Academy of Geological Sciences Zhongshan East R Nanjing. Jiangsu Fei Zhenbi 《《地质学报》英文版》1999,73(1):19-29
The Hongshan porphyry-cryptoexplosive breccia type copper deposit occurs in a metamorphic rockseries of the Mesoproterozoic Zhongcun Group. Orebodies are distributed inside and outside porphyry-cryptoexplosive breccia pipes. The deposit involves five ore-forming types, Le the porphyry type, crytoexplosivebreccia type. contact-zone veinlet-disseminated type, in-pipe fracture-zone filling-replacement type and out-of-pipetracture-zone filling-replacement type, forming an ore-forming system of "five ore-forming types within a singlerock body" Fluid inclusion and isotope geochemical studies indicate the following: S, Pb, O and Sr were derivedfrom the lower crust, Nd was derived from the continental crust or depleted mantle and rare earth elements (REE)and trace elements have the crustal source characters; fluids consist dominantly of formation water, metamorphic wa-ter and meteoric water with a part of magmatic mater; heat came from porphyry while the latter originated from par-tial melting caused by shear heating in the lower crust and upper mantle. According to its origin the deposit is classi-fied as the hypabyssal and near-surface, meso-and hypothermal copper deposit ussociated with the late Yanshanianporphyry-cryptoexplosive breccia. 相似文献
17.
《Gondwana Research》2013,24(4):1554-1566
The paradox of the Earth's continental crust is that although this reservoir is generally regarded as having differentiated from the mantle, it has an andesitic bulk composition that contrasts with the intrinsic basaltic composition of mantle-derived melts. Classical models for new crust generation from the mantle in two-stage processes fail to account for two fundamental facts: the absence of ultramafic residues in the lower crust and the hot temperature of batholith magma generation. Other models based on the arrival of already-fractionated silicic magmas to the crust have not received the necessary attention. Addition of new crust by relamination from below of subducted materials has been formulated as a process complementary to delamination of mafic residues. Here we show important support to relamination from below the lithosphere as an important mechanism for new crust generation in magmatic arcs of active continental margins and mature intraoceanic arcs. The new support is based on three independent lines: (1) thermo-mechanical modeling of subduction zones, (2) experimental phase relations and melt compositions of subducted materials and (3) geochemical relations between mafic granulites (lower crust) and batholiths (upper crust). The mineral assemblage and bulk geochemistry of lower crust rocks are compared with solid residues left after granite melt segregation. The implication is that an andesite magma precursor is responsible for the generation of new continental crust at active continental margins and mature oceanic arcs. According to our numerical and laboratory experiments, melting and eventual reaction with the mantle of subducted oceanic crust and sediments produce the andesite magmas. These ascend in the form of mantle wedge diapirs and are finally attached (relaminated) to the continental crust, where they crystallize partially and produce the separation of the solid fraction to form mafic granulites (lower crust) and granitic (sl) liquids to form the batholiths (upper crust). 相似文献
18.
北秦岭灰池子花岗岩基成岩物质来源探讨 总被引：3，自引：0，他引：3
本文主要通过Ｎｄ，Ｓｒ和Ｏ同位素示踪研究，分析了灰池子花岗岩基的成岩物质来源，初步查明该岩体的岩浆源区由６９％的地幔物质和３１％的陆壳物质混合而成，属壳幔型花岗岩。 相似文献
19.
Among the Phanerozoic granitoids of East Asia, the most prevailing Cenozoic–Mesozoic rocks are reviewed with respect to gabbro/granite ratio, bulk composition of granitoids, redox state, and O- and Sr-isotopic ratios. Quaternary volcanic rocks, ranging from basalt to rhyolite, but typically felsic andesite in terms of bulk composition in island arcs, are oxidized type, possibly due to oxidants from subducting oceanic crust into the source regions. Miocene plutonic rocks in the back-arc of Japan could be a root zone for such volcanism but are more felsic in composition. Cenozoic–Mesozoic plutonic zones are classified by (1) the redox state (magnetite/ilmenite series), and (2) average bulk composition (granodiorite/granite). The granodioritic magnetite series occur with fairly abundant gabbro and diorite in the back-arc of island arcs (Greentuff Belt) and intercontinental rapture zones (Yangtze Block). These rocks are mostly juvenile in terms of the 87Sr/86SrI and δ18O values.The granitic magnetite series with some gabbroids occur in rapture zones along the continental coast (Gyeongsang Basin, Fujian Coast) and the back-arc of island arc (Sanin Belt). They were generated mostly in felsic continental crust, with the help of heat and magmas from upper mantle. The generated granitic magmas had little interaction with C- and S-bearing reducing materials, due probably to extensional tectonic settings. The δ18O value gives narrow ranges but the 87Sr/86SrI ratio varies greatly depending upon the age and composition of the continental crust. Granitic ilmenite-series are characterized by high δ18O values, implying much contribution of sediments. The 87Sr/86SrI ratios are low in island arcs but very high in continental interior settings. Amount of mafic magmas from the upper mantle seems a key to control the composition of granitoid series in island arc settings, while original composition of the protolith may be the key to control granitoid composition in continental interiors. 相似文献
20.
V. V. Yarmolyuk V. I. Kovalenko A. M. Kozlovsky V. P. Kovach E. B. Sal’nikova D. V. Kovalenko A. B. Kotov E. A. Kudryashova V. I. Lebedev G. Eenzhin 《Petrology》2008,16(7):679-709
The paper reports data on the evolutionary history of magmatism, its conditions, and sources in the process of the development
of the Southern Mongolian Hercynides during the pre-accretion, continental-margin, and rifting stages within the time span
from the Silurian to Early Permian. The Hercynian continental crust in the southern Mongolian segment of the Central Asian
Foldbelt (CAFB) was determined to have grown in the environment of ensimatic island arcs, backarc basins, spreading centers,
and oceanic islands or plateaus, with material coming from the depleted and, perhaps, also enriched mantle sources in the
open ocean that surrounded the Siberian paleocontinent on the side of the Caledonian margin. This made it possible to recognize
the Early-Middle Paleozoic epoch of juvenile crustal growth in CAFB and the corresponding isotopic crustal province with a
total area of more than 200 thousand km2. The principal differences between the composition and structure of the blocks surrounding the Hercynian regions (Caledonides
in the Gobi Altai and Grenwillides in the South Gobi microcontinent) testify that the southern margin of the Caledonian Siberian
continent and the Grenvillides of the South Gobi microcontinent had different geological histories and were spatially separated.
The structural complex of the Paleoasian ocean, including the terranes of the South Gobi microcontinent, were transformed
into a continental block in the latest Devonian-earliest Carboniferous, in relation with accretion processes, folding, metamorphism,
and tectonic delamination along the boundaries of structurally heterogeneous domains. The subsequent recycling of the crust
by magmatic processes was related to the development of an active continental margin (ACM). The development of an ACM in the
Hercynides resulted from and was a continuation of the motions of the continental and oceanic lithospheric plates, i.e., processes
that brought about the Hercynian accretion. The evolution history of the ACM was subdivided into two stages: early (a continental-margin
stage proper) and late (rifting stage). The rocks of the early stage were produced at 350–330 Ma and compose a differentiated
basalt-andesite-rhyodacite complex and related massifs of the granodiorite-plagiogranite and banatite (diorite-monzonite-granodiorite)
associations. During the rifting stage at 320–290 Ma, a bimodal basalt-comendite-trachyrhyolite association was formed, along
with accompanying alkali granite massifs. In the southern Mongolian segment of the Hercynides, the rocks of the rifting stage
compose two subparallel rift zones: Gobi-Tien Shan, which extends along the boundaries of the South Gobi microcontinent, and
the Main Mongolian lineament, which marks the boundaries between the Hercynides and Caledonides in the CAFB. The rift structures
are made up of alkali granitoids and normal-alkalinity granitoids, which are atypical of rift zones. Their genesis is thought
to have been related to crustal anatexis, a process that was triggered by rift-related magmas at an unusual combination of
rifting and ACM tectonic setting. The basic rocks of the rift associations have geochemical signatures atypical of continental
rifting. They show Ta and Nb minima and K and Pb maxima, as is typical of rocks generated at convergent plate boundaries.
Nevertheless, the broad variations in the concentrations and ratios of some major and incompatible trace elements and in the
Sr, Nd, and O isotopic composition of the rift basaltoids allowed us to distinguish their high-and low-Ti varieties, which
were produced with the participation of three mantle sources: depleted mantle similar to the source of basalts in midoceanic
ridges, enriched mantle like the source of basalts in oceanic islands, and the mantle material of the metasomatized mantle
wedge. The origin of andesites in the rift zones is explained by the contamination of mantle basaltoid melts with sialic (predominantly
sedimentary) material of the continental crust or the assimilation of anatectic partial granite melts. 相似文献