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1.
Two stages of early Cretaceous post-orogenic granitoids are recognized in the Dabie orogen, eastern China, which recorded processes of extensional collapse of the orogen. The early stage granitoids ( 132 Ma) are foliated hornblende quartz monzonites and porphyritic monzogranites. They are of high-K calc-alkaline series and metaluminous to weakly peraluminous, with high K2O and low MgO contents (Mg# values: 32.0–46.0), they contain high Sr, low Y and heavy rare earth elements (HREE), and have high Sr/Y and (La/Yb)N ratios, without clear negative Eu, Sr and Ti anomalies. The early stage deformed granitoids have adakitic geochemical compositions and are equilibrated with residues rich in garnet and poor in anorthite-rich plagioclase, and thus indicate the existence of an over-thickened (> 50 km) crustal root beneath the orogen at 132 Ma. The later stage granitoids ( 128 Ma) are undeformed fine-grained monzogranites, fine-grained K-feldspar granites and coarse-grained K-feldspar granite-porphyry. They belong to a peraluminous and high-K calc-alkaline to shoshonite series, and display a flat HREE pattern and have strong negative Eu, Sr and Ti anomalies, with low Sr/Y and (La/Yb)N ratios. The late stage granitoids are equilibrated with residues rich in anorthite-rich plagioclase, hornblende, ilmenite/titanite and poor in garnet, indicating that the crust of the Dabie orogen became thinner (< 35 km) at 128 Ma. SHRIMP zircon U–Pb ages and changing compositional trends for these two stages of granitoids indicate that the over-thickened crust formed by the Triassic continental subduction/collision under the Dabie orogen remained until the early Cretaceous, and collapsed quickly in a few million years during the early Cretaceous. 相似文献
2.
Zircon U–Pb ages and trace elements were determined for granulites and gneiss at Huangtuling, which are hosted by ultrahigh-pressure metamorphic rocks in the Dabie Orogen, east-central China. CL images reveal core–rim structure for most zircons in the granulites. The cores show oscillatory zoning, relatively high Th/U ratios, and HREE enriched patterns, consistent with a magmatic origin. They gave a weighted mean 207Pb/206Pb age of 2766 ± 9 Ma, interpreted as dating magma emplacement of the protolith. The rims are characterized by sector or planar zoning, low Th/U ratios, negative Eu anomalies and flat HREE patterns, consistent with their formation under granulite-facies metamorphic conditions. Zircon U–Pb dating yields a weighted mean 207Pb/206Pb age of 2029 ± 13 Ma, which is interpreted to record a metamorphic event, possibly during assembly of the supercontinent Columbia. The gneiss has a protolith age of 1982 ± 14 Ma, which is younger than the zircon age of the granulite-facies metamorphism, suggesting a generally delay between HT metamorphism and the intrusion of post-collisional granites. A few inherited cores with igneous characteristics have 207Pb/206Pb ages of 2.90, 3.28 and 3.53 Ga, suggesting the presence of Mesoarchean to Paleoarchean crustal remnants in the Yangtze Craton. A few Cretaceous metamorphic ages were also obtained, suggesting the influence of post-collisional collapse in response to Cretaceous extension of the Dabie Orogen. It is inferred that the recently discovered Archean basement of the Yangtze Craton occurs as far north as the Dabie Orogen. 相似文献
3.
Y. Agbossoumond R.-P. Mnot J.L. Paquette S. Guillot S. Yssoufou C. Perrache 《Gondwana Research》2007,12(4):476-488
The Palimé–Amlamé Pluton (PAP) in southern Togo, consists of silica-rich to intermediate granitoids including enclaves of mafic igneous rocks and of gneisses. They are commonly called the “anatectic complex of Palimé–Amlamé” and without any convincing data, they were interpreted either as synkinematic Pan-African granitoids or as reworked pre Pan-African plutons. New field and petrological observations, mineral and whole-rock chemical analyses together with U–Pb zircon dating, have been performed to evaluate the geodynamic significance of the PAP within the Pan-African orogenic belt. With regard to these new data, the granitoids and related enclaves probably result from mixing and mingling processes between mafic and silicic magmas from respectively mantle and lower crust sources. They display Mg–calc-alkaline chemical features and present some similarities with Late Archaean granites such as transitional (K-rich) TTGs and sanukitoids.
The 2127 ± 2 Ma age obtained from a precise U/Pb concordia on zircon, points out a Paleoproterozoic age for the magma crystallization and a lower intercept at 625 ± 29 Ma interpreted as rejuvenation during Pan-African tectonics and metamorphism. Based on these results, a Pan-African syn to late orogenic setting for the PAP, i.e. the so-called “anatectic complex of Palimé–Amlamé”, can be definitively ruled out. Moreover according to its location within the nappe pile and its relationships with the suture zone, the PAP probably represents a fragment of the West African Craton reactivated during the Pan-African collision. 相似文献
4.
Ultrahigh-pressure eclogite transformed from mafic granulite in the Dabie orogen, east-central China 总被引：1，自引：0，他引：1
Although ultrahigh‐pressure (UHP) metamorphic rocks are present in many collisional orogenic belts, almost all exposed UHP metamorphic rocks are subducted upper or felsic lower continental crust with minor mafic boudins. Eclogites formed by subduction of mafic lower continental crust have not been identified yet. Here an eclogite occurrence that formed during subduction of the mafic lower continental crust in the Dabie orogen, east‐central China is reported. At least four generations of metamorphic mineral assemblages can be discerned: (i) hypersthene + plagioclase ± garnet; (ii) omphacite + garnet + rutile + quartz; (iii) symplectite stage of garnet + diopside + hypersthene + ilmenite + plagioclase; (iv) amphibole + plagioclase + magnetite, which correspond to four metamorphic stages: (a) an early granulite facies, (b) eclogite facies, (c) retrograde metamorphism of high‐pressure granulite facies and (d) retrograde metamorphism of amphibolite facies. Mineral inclusion assemblages and cathodoluminescence images show that zircon is characterized by distinctive domains of core and a thin overgrowth rim. The zircon core domains are classified into two types: the first is igneous with clear oscillatory zonation ± apatite and quartz inclusions; and the second is metamorphic containing a granulite facies mineral assemblage of garnet, hypersthene and plagioclase (andesine). The zircon rims contain garnet, omphacite and rutile inclusions, indicating a metamorphic overgrowth at eclogite facies. The almost identical ages of the two types of core domains (magmatic = 791 ± 9 Ma and granulite facies metamorphic zircon = 794 ± 10 Ma), and the Triassic age (212 ± 10 Ma) of eclogitic facies metamorphic overgrowth zircon rim are interpreted as indicating that the protolith of the eclogite is mafic granulite that originated from underplating of mantle‐derived magma onto the base of continental crust during the Neoproterozoic (c. 800 Ma) and then subducted during the Triassic, experiencing UHP eclogite facies metamorphism at mantle depths. The new finding has two‐fold significance: (i) voluminous mafic lower continental crust can increase the average density of subducted continental lithosphere, thus promoting its deep subduction; (ii) because of the current absence of mafic lower continental crust in the Dabie orogen, delamination or recycling of subducted mafic lower continental crust can be inferred as the geochemical cause for the mantle heterogeneity and the unusually evolved crustal composition. 相似文献
5.
俯冲到地幔深度的地壳物质不可避免地在板片-地幔界面与地幔楔发生相互作用,由此形成的超镁铁质交代岩就是造山带镁铁质火成岩的地幔源区.因此,造山带镁铁质火成岩为研究俯冲地壳物质再循环和壳-幔相互作用提供了重要研究对象.为了揭示俯冲陆壳物质再循环的机制和过程,对大别造山带碰撞后安山质火山岩开展了元素和同位素地球化学研究.这些安山质火山岩的SIMS锆石U-Pb年龄为124±3~130±2 Ma,表明其形成于早白垩世.此外,残留锆石的U-Pb年龄为中新元古代和三叠纪,分别对应于大别-苏鲁造山带超高压变火成岩的原岩年龄和变质年龄.它们具有岛弧型微量元素特征、富集的Sr-Nd-Hf同位素组成,以及变化的且大多不同于正常地幔的锆石δ18O值.这些元素和同位素特征指示,这些安山质火山岩是交代富集的造山带岩石圈地幔部分熔融的产物.在三叠纪华南陆块俯冲于华北陆块之下的过程中,俯冲华南陆壳来源的长英质熔体交代了上覆华北岩石圈地幔楔橄榄岩,大陆俯冲隧道内的熔体-橄榄岩反应产生了富沃、富集的镁铁质地幔交代岩.这种地幔交代岩在早白垩世发生部分熔融,就形成了所观察到的安山质火山岩.因此,碰撞造山带镁铁质岩浆岩的地幔源区是通过大陆俯冲隧道内板片-地幔相互作用形成的,而加入地幔楔中长英质熔体的比例决定了这些镁铁质岩浆岩的岩石化学和地球化学成分. 相似文献
6.
Hong-Fei Zhang Randall Parrish Li Zhang Wang-Chun Xu Hong-Lin Yuan Shan Gao Quentin G. Crowley 《Lithos》2007,97(3-4):323-335
The Songpan–Garze fold belt covers a huge triangular area (> 200,000 km2), confined by the South China (Yangtze), North China and Tibetan Plateau continental blocks. In the Songpan–Garze fold belt, Triassic adakitic granitoids have been identified. However, whether there are Triassic A-type granites is unclear. Here, we report our first finding of an A-type granite (Nianbaoyeche), which occurs in the central part of the Songpan–Garze fold belt. The Nianbaoyeche granite ( 820 km2) is characterized by arfvedsonite in its mineral assemblage. Using both LA-ICPMS and TIMS U–Pb zircon dating methods, we obtain a magma crystallization age of 211 ± 1 Ma, which is slightly younger than Triassic adakitic granitoids (216–221 Ma) in the Songpan–Garze fold belt. The Nianbaoyeche granite is enriched in Si, K, Na, Rb, REE, HFSE (Nb, Ta, Zr, Hf), with elevated FeOtot/(FeOtot + MgO) and Ga/Al ratios, but is depleted in Al, Mg, Ca, Ba and Sr. The REE compositions show moderately fractionated patterns with (La/Yb)N = 2.67–7.54 and Eu/Eu = 0.09–0.34. These geochemical characteristics indicate that the Nianbaoyeche granite has an A-type affinity. Geochemical data and U–Pb zircon age, combined with regional studies, show that the Nianbaoyeche granite formed in a post-collisional tectonic setting. Sr–Nd isotopic data for the granite exhibit ISr = 0.7090–0.7123 and εNd(t) = − 2.72 to − 4.26 with TDM = 1.15–1.51 Ga, suggesting that the magma has a dominantly crustal source, though a minor contribution from the mantle cannot be ruled out. Melting to produce an A-type granite may have resulted from Triassic lithospheric delamination after Triassic crustal thickening of the Songpan–Garze fold belt due to convergence between the Yangtze, North China and North Tibet continental blocks. The lithospheric delamination model also helps to explain the Triassic adakitic magma generation in the Songpan–Garze belt. We conclude that association of A-type granite and adakitic granitoids in post-collisional environment could be a useful indicator of lithospheric delamination. 相似文献
7.
MO Xuanxue DONG Guochen ZHAO Zhidan GUO Tieying WANG Liangliang CHEN Tao 《《地质学报》英文版》2005,79(1):66-76
Abstract Abundant mafic microgranular enclaves (MMEs) extensively distribute in granitoids in the Gangdisê giant magmatic belt, within which the Qüxü batholith is the most typical MME‐bearing pluton. Systematic sampling for granodioritic host rock, mafic microgranular enclaves and gabbro nearby at two locations in the Qüxü batholith, and subsequent zircon SHRIMP II U‐Pb dating have been conducted. Two sets of isotopic ages for granodioritic host rock, mafic microgranular enclaves and gabbro are 50.4±1.3 Ma, 51.2±1.1 Ma, 47.0±1 Ma and 49.3±1.7 Ma, 48.9±1.1 Ma, 49.9±1.7 Ma, respectively. It thus rules out the possibilities of mafic microgranular enclaves being refractory residues after partial melting of magma source region, or being xenoliths of country rocks or later intrusions. Therefore, it is believed that the three types of rocks mentioned above likely formed in the same magmatic event, i.e., they formed by magma mixing in the Eocene (c. 50 Ma). Compositionally, granitoid host rocks incline towards acidic end member involved in magma mixing, gabbros are akin to basic end member and mafic microgranular enclaves are the incompletely mixed basic magma clots trapped in acidic magma. The isotopic dating also suggested that huge‐scale magma mixing in the Gangdisê belt took place 15–20 million years after the initiation of the India‐Asia continental collision, genetically related to the underplating of subduction‐collision‐induced basic magma at the base of the continental crust. Underplating and magma mixing were likely the main process of mass‐energy exchange between the mantle and the crust during the continental collision, and greatly contributed to the accretion of the continental crust, the evolution of the lithosphere and related mineralization beneath the portion of the Tibetan Plateau to the north of the collision zone. 相似文献
8.
9.
Mafic alkaline lavas from the Venetian Volcanic Province (NE Italy) contain orange–brown zircon megacrysts up to 15 mm long, subhedral to subrounded and showing equant morphology, with width-to-length ratios of 1:2–1:2.5. U–Pb ages of zircon (51.1 ± 1.5 to 30.5 ± 0.51 Ma) fit the stratigraphic age of the host lava (Middle Eocene and Oligocene) and their oxygen isotope composition (δ18O = 5.31–5.51‰) is similar to that of zircon formed in the upper mantle. Cathodoluminescence images and crystal chemical features, e.g. depletion of incompatible elements such as REE, Y, U and Th at constant Hf content, indicate that centre-to-edge zircon zoning is not consistent with evolution of the melt by fractional crystallization. All the above features, together with the fact that zircon and host basalts are coeval, indicate that the studied Zr megacrysts crystallised from a primitive alkaline mafic magma, which later evolved to the less alkaline host magma. 相似文献
10.
Shi-Yong Liao Yao-Hui Jiang Shao-Yong Jiang Wan-Zhi Yang Qing Zhou Guo-Dong Jin Peng Zhao 《Mineralogy and Petrology》2010,100(1-2):55-74
This paper presents detailed SHRIMP zircon U–Pb chronology, mineral chemistry, major and trace element, and Sr–Nd–Hf isotope geochemistry of the Datong pluton and its quenched enclaves from the western Kunlun orogen, northwest China, in an attempt to achieve a better understanding on the origin of diverse arc magmas. The Datong host granitoids are intermediate to acid in composition (SiO2?=?57.5?~?73.1 wt.%), and exhibit high-K calc-alkaline to shoshonitic affinities. The quenched enclaves are silica-rich ultrapotassic rocks. Detailed SHRIMP zircon U–Pb dating indicates that the Datong pluton was emplaced in Ordovician time (473.4–447.7 Ma), which places the Datong pluton in an active continental margin setting, rather than a syn-collision setting of Early Silurian age. The Datong host granitoids were derived by partial melting of subducted sediments, with the subsequent melt interacting with the overlying mantle wedge during its ascent. Partial melting of the veined mantle wedge hybridized by sediment-derived melts generated the silica-rich ultrapotassic magma, which was injected into the Datong granitoid magma chamber and quenched, resulting in enclaves hosted by granitoids. This contribution provides evidence that arc magmas can be derived directly by partial melting of subducted sediments, which is helpful to further understand the origin of diverse arc magmas. 相似文献
11.
F. Finger I. Broska B. Haunschmid L. Hrasko M. Kohút E. Krenn I. Petrík G. Riegler P. Uher 《International Journal of Earth Sciences》2003,92(1):86-98
Accessory monazites from 35 granitoid samples from the Western Carpathian basement have been analysed with the electron microprobe in an attempt to broadly constrain their formation ages, on the basis of their Th, U and Pb contents. The sample set includes representative granite types from the Tatric, Veporic and Gemeric tectonic units. In most cases Lower Carboniferous (Variscan) ages have been obtained. However, a much younger mid-Permian age has been recorded for the specialised S-type granites of the Gemeric Unit, and several small A- and S-type granite bodies in the Veporic Unit and the southern Tatric Unit. This distinct Permian plutonic activity in the southern part of the Western Carpathians is an important, although previously little considered geological feature. It appears to be not related to the Variscan orogeny and is interpreted here to reflect the onset of the Alpine orogenic cycle, with magma generation in response to continental rifting. The voluminous Carboniferous granitoid bodies in the Tatric and Veporic units comprise S- and I-type variants which document crustal anatexis accompanying the collapse of a compressional Variscan orogen sector. The Variscan magmas were most likely produced through the remelting of a subducted Precambrian volcanic arc-type crust which included both igneous and sedimentary reworked volcanic-arc material. Although the 2C errors of the applied dating method are quite large and typically ᆞ-20 Ma for single samples, it would appear from the data that the Variscan S-type granitoids (333-367 Ma) are systematically older than the Variscan I-type granitoids (308-345 Ma). This feature is interpreted in terms of a prograde temperature evolution in the deeper parts of the post-collisional Variscan crust. In accordance with recently published zircon ages, this study shows that the Western Carpathian basement must be viewed as a distinct "eastern" tectonomagmatic province in the Variscan collision zone, where the post-collisional crustal melting processes occurred ~20 Ma earlier than in the central sector (South Bohemian Batholith, Hohe Tauern Batholith). 相似文献
12.
南阿尔金造山带是目前报道的具有最深俯冲记录的大陆超高压变质带,其内出露有高压-超高温麻粒岩,它们对深入理解大陆地壳岩石超深俯冲与折返过程具有重要意义.介绍了对南阿尔金巴什瓦克地区长英质麻粒岩和基性麻粒岩的岩相学、矿物化学、相平衡模拟及锆石U-Pb年代学研究成果.其中基性麻粒岩主要记录了深俯冲大陆地壳折返过程的变质演化:包括高压榴辉岩相、高压-超高温麻粒岩相、低压-超高温麻粒岩相及随后的近等压降温演化阶段;长英质麻粒岩除了记录与基性麻粒岩相似的折返过程外,还记录了从角闪岩相到超高压榴辉岩相的进变质演化过程.结合已有研究资料,确定超高压榴辉岩阶段峰期条件> 7~9 GPa和>1 000℃,可达到斯石英稳定域.锆石年代学显示两种岩石类型的原岩和变质年龄均分别在900 Ma和500 Ma左右.变质作用与年代学研究表明,南阿尔金大陆地壳岩石在早古生代发生超深俯冲至200~300 km后,折返至加厚地壳底部发生高压-超高温变质作用,随后被快速抬升至地壳浅部发生低压-超高温变质作用并经历迅速冷却. 相似文献
13.
We constrain the origin and tectonic setting of the giant Duolong porphyry–epithermal Cu–Au deposit in the South Qiangtang Terrane of northern Tibet, based on new zircon U–Pb ages and Hf isotopic data, as well as whole-rock major and trace element data from poorly studied ore-associated intrusions in the Duolong area. The LA–ICP–MS zircon U–Pb dating indicates that the ore-associated rocks formed between 121 and 126 Ma. These ore-associated rocks are geochemically similar to low-K tholeiitic M-type granitoids and to mid- to high-K, calc-alkaline I-type granitoids. They have variable and predominantly positive zircon εHf(t) values (− 1.4 to + 15.6) and variable crustal model ages (TCDM(Hf); 176–1122 Ma). Taking into account previous data and the regional geology of the study area, we propose that the ore-associated rocks originated from fractional crystallization of mantle-derived mafic melts and magma mixing of mantle-derived mafic and hybrid lower crust-derived felsic melts, and the hybrid lower crust included a mix of juvenile and older continental material. The Duolong porphyry–epithermal Cu–Au deposit formed within an ‘ensialic forearc’ of an active continental margin as a result of the northwards subduction of the Bangong–Nujiang Ocean crust beneath the South Qiangtang Terrane. 相似文献
14.
Tina van de Flierdt Stephan Hoernes Stefan Jung Peter Masberg Edgar Hoffer Urs Schaltegger Hans Friedrichsen 《Lithos》2003,67(3-4):205-226
The Bandombaai Complex (southern Kaoko Belt, Namibia) consists of three main intrusive rock types including metaluminous hornblende- and sphene-bearing quartz diorites, allanite-bearing granodiorites and granites, and peraluminous garnet- and muscovite-bearing leucogranites. Intrusion of the quartz diorites is constrained by a U–Pb zircon age of 540±3 Ma.
Quartz diorites, granodiorites and granites display heterogeneous initial Nd- and O isotope compositions (Nd (540 Ma)=−6.3 to −19.8; δ18O=9.0–11.6‰) but rather low and uniform initial Sr isotope compositions (87Sr/86Srinitial=0.70794–0.70982). Two leucogranites and one aplite have higher initial 87Sr/86Sr ratios (0.70828–0.71559), but similar initial Nd (−11.9 to −15.8) and oxygen isotope values (10.5–12.9‰). The geochemical and isotopic characteristics of the Bandombaai Complex are distinct from other granitoids of the Kaoko Belt and the Central Zone of the Damara orogen. Our study suggests that the quartz diorites of the Bandombaai Complex are generated by melting of heterogeneous mafic lower crust. Based on a comparison with results from amphibolite-dehydration melting experiments, a lower crustal garnet- and amphibole-bearing metabasalt, probably enriched in K2O, is a likely source rock for the quartz diorites. The granodiorites/granites show low Rb/Sr (<0.6) ratios and are probably generated by partial melting of meta-igneous (intermediate) lower crustal sources by amphibole-dehydration melting. Most of the leucogranites display higher Rb/Sr ratios (>1) and are most likely generated by biotite-dehydration melting of heterogeneous felsic lower crust. All segments of the lower crust underwent partial melting during the Pan-African orogeny at a time (540 Ma) when the middle crust of the central Damara orogen also underwent high T, medium P regional metamorphism and melting. Geochemical and isotope data from the Bandombaai Complex suggest that the Pan-African orogeny in this part of the orogen was not a major crust-forming episode. Instead, even the most primitive rock types of the region, the quartz diorites, represent recycled lower crustal material. 相似文献
15.
《International Geology Review》2012,54(2):208-226
The origin of microgranitoid enclaves in granitic plutons has long been debated (hybrid magma blobs vs. refractory restites or cognate fragments). This article presents detailed petrography, SHRIMP zircon U–Pb chronology, bulk-rock major and trace element analyses, and Sr–Nd isotope and in situ zircon Hf isotopic geochemistry for microgranitoid enclaves within two Late Triassic granitic plutons in the Qinling orogen. Zircon U–Pb dating shows that the enclaves formed during the Carnian (222.5 ± 2.1 to 220.7 ± 1.9 Ma) coeval with their host granitoids (220.0 ± 2.0 to 218.7 ± 2.4 Ma). Field and petrological observations (e.g. double enclaves, xenocrysts, acicular apatite, and poikilitic K-feldspar or quartz) suggest that the enclaves are globules of a mantle-derived more mafic magma that was injected into and mingled with the host magma. The enclaves are mainly ultrapotassic, distinct from the host granitoids that have high-K calc-alkaline bulk-rock compositions. Although the enclaves have closely similar bulk-rock Sr–Nd isotope [initial 87Sr/86Sr?=?0.7046–0.7056, ?Nd (T)?=?–0.3 to –5.0] and in situ zircon Hf isotope [?Hf (T)?=?–1.5 to?+2.9] ratios as the granitoids [initial 87Sr/86Sr?=?0.7042–0.7059, ?Nd (T)?=?–0.6 to –6.3, ?Hf (T)?=?–2.2 to?+1.6], chemical relationships including very different bulk-rock compositions at a given SiO2 content lead us to interpret the isotopic similarities as reflecting similar but separate isotopic source rocks. Detailed elemental and isotopic data suggest that the enclaves and the host granitoids were emplaced in a continental arc environment coupled with northward subduction of the Palaeo-Tethyan oceanic crust. Partial melting of subducted sediments triggered by dehydration of the underlying igneous oceanic crust, with melts interacting with the overlying mantle wedge, formed high-K calc-alkaline granitic magmas, whereas partial melting of diapiric phlogopite-pyroxenites, solidified products of the same subducting sediment-derived melts, generated ultrapotassic magmas of the microgranitoid enclaves. Our new data further confirm that in the Late Triassic time the Qinling terrane was an active continental margin rather than a post-collisional regime, giving new insights into the tectonic evolution of this orogen. 相似文献
16.
大别山是由华南板块在245～210 Ma向华北板块之下俯冲并发生陆陆碰撞形成的。随着南、北板块的汇聚继续,地壳持续加厚。然而,加厚的下地壳岩石（特别是镁铁质下地壳岩石）在重力作用下密度增大、稳定性降低,在145～130 Ma 时发生深熔作用;130 Ma 左右加厚下地壳拆沉,引发软流圈上涌,产生了130～110 Ma的大规模镁铁质和花岗质岩浆作用以及北大别发生强烈的混合岩化作用。其中,北大别混合岩中不同类型浅色体（至少可以分为4种）和碰撞后变质闪长岩的甄别及其岩石地球化学和同位素年代学方面系统研究为大别山印支期深俯冲陆壳的折返以及燕山期镁铁质下地壳岩石拆沉和山根垮塌所引发的多期深熔作用提供了新的关键证据。山根垮塌诱发的地幔对流导致～145 Ma时岩石圈开始减薄,进而导致加厚镁铁质下地壳温度和地壳中下部地热增温率升高,并使其发生部分熔融;加厚下地壳的部分熔融导致造山带下地壳持续弱化,加剧其重力不平衡,从而引发深部俯冲的镁铁质下地壳岩石的大规模拆沉和山根垮塌。 相似文献
17.
哈尔里克山西段早志留世二长花岗岩和正长花岗岩呈北西西向带状展布,侵入奥陶系塔水组（O1-2t）,LA-ICP-MS锆石U-Pb年龄为438.8±2.3~435.8±3.1 Ma。岩石高硅（SiO2含量73.0%~77.8%）、富钾（K2O含量3.31%~4.26%）、低镁（MgO含量0.03%~0.59%）,铝饱和指数A/CNK值1.02~1.08,属高钾钙碱性弱过铝质岩石。二长花岗岩轻重稀土分馏显著,Eu异常中等,亏损Nb、Ta、Ti、P,富集Rb、Ba、K,表现为分异的Ⅰ型花岗岩特征,源区为基性下地壳;正长花岗岩强烈亏损Eu、P、Ti、Sr,不同程度富集Rb、K、Zr、Hf,表现为A型花岗岩特征,其源区为缺水的浅部长英质地壳。结合区域地层不整合资料,认为东准噶尔地区早志留世为后碰撞环境而非岛弧带,后碰撞软流圈上涌带来的热熔融准噶尔年轻地壳形成了岩性丰富的东准噶尔志留纪后碰撞岩浆岩组合。 相似文献
18.
Yuejun Wang Weiming Fan Min Sun Xinquan Liang Yanhua Zhang Touping Peng 《Lithos》2007,96(3-4):475-502
The Indosinian granites in the South China Block (SCB) have important tectonic significance for the evolution of East Asia. Samples collected from Hunan Province can be geochemically classified into two groups. Group 1 is strongly peraluminous (A/CNK > 1.1), similar to S-type granites, and Group 2 has A/CNK = 1.0–1.1, with an affinity to I-type granites. Group 1 has lower FeOt, Al2O3, MgO, CaO, TiO2 and εNd(t) values but higher K2O + Na2O, Rb/Sr, Rb/Ba and 87Sr/86Sr(t) than those of Group 2. Samples of both groups have similar LREE enriched pattern, with (Eu/Eu) = 0.19–0.69, and strongly negative Ba, Sr, Nb, P and Ti anomalies. Geothermobarometry study indicates that the precursor magmas were emplaced at high-level depth with relatively low temperature (734–827 °C). Geochemical data suggest that Group 1 was originated from a source dominated by pelitic composition and Group 2 was from a mixing source of pelitic and basaltic rocks with insignificant addition of newly mantle-derived magma. Eight granitic samples in Hunan Province are dated at the cluster of 243–235 and 218–210 Ma by zircon U–Pb geochronology. Together with recent zircon U–Pb ages for other areas in the SCB, two age-clusters, including 243–228 Ma just after peak-metamorphism ( 246–252 Ma) and 220–206 Ma shortly after magma underplating event (224 Ma), are observed. It is proposed that in-situ radiogenic heating from the over-thickened crust induced dehydrated reaction of muscovite and epidote/zoisite to form the early Indosinian granites in response to the isostatic readjustments of tectonically thickened crust. Conductive heating from the underplating magma in the postcollisional setting triggered the formation of late Indosinian granites. Such a consideration is supported by the results from FLAC numerical simulation. 相似文献
19.
New geological observations, recent published data and U–Pb SHRIMP zircon dating from the Karakoram Mountains along the Nubra and Shyok Rivers reveal that the initial subduction of the Tethyan oceanic lithosphere took place ~ 110 Ma beneath the Paleozoic–Mesozoic platform of the southern edge of the Asian Plate. This has produced the I-type plutons within the Karakoram Batholith Complex, well before the juxtaposition of the Asian Plate along the Karakoram Shear Zone. Within this shear zone, U–Pb zircon crystallisation ages of ~ 75 Ma from mylonitised granitoids and 68 Ma from undeformed Tirit granodiorite constrain the timing of suturing of the Karakoram terrain with the Trans-Himalaya between 75 and 68 Ma. Post-shearing leucogranite was episodically generated within frontal migmatised Karakoram Metamorphic Belt and emplaced between 20 and 13 Ma within the shear zone. Presence of a low resistivity zone as a possible indication of mid-crustal partial molten crust underneath the Higher Himalaya–Ladakh–Karakoram terrains manifests the impingement of the Indian Plate along the Main Himalayan Thrust at depth.
Physical continuity of the Baltoro granite belt into the Karakoram Batholith is established as well as the continuity of the Shyok suture as the Shiquanhe Suture Zone in western Tibet through the Chushul–Dungti sector. The Karakoram Shear Zone, therefore, displays a complex geological history of movements since ~ 75 Ma and plays a very significant role in the overall India–Asia convergence, rather than merely being a strike-slip fault for eastward extrusion of a segment of Asia in Tibet. 相似文献
20.
北祁连东段冷龙岭地区毛藏寺岩体和黄羊河岩体的岩石成因及其构造意义 总被引：5，自引：0，他引：5
文中研究了北祁连东段冷龙岭地区毛藏寺岩体和黄羊河岩体的年代学、地球化学和Sr-Nd同位素组成。毛藏寺岩体主要岩石类型为花岗闪长岩。锆石U Pb定年获得花岗闪长岩岩浆结晶年龄为（424±4） Ma。花岗闪长岩具有高的Mg#（约55）,K2O/Na2O=0.77~0.91,A/CNK=0.92~0.94,表明岩石属准铝质。在微量元素组成上,花岗闪长岩富集LILE、亏损HFSE,轻重稀土分异明显［(La/Yb)N=16.9~19.5］,具有弱的Eu负异常（Eu/Eu*=0.75~0.83）;花岗闪长岩具有ISr=0.706 3~0.706 5,εNd(t) =-1.5~-1.1,TDM=1.10~1.16 Ga。这些地球化学特征和Sr Nd同位素组成表明,花岗闪长岩岩浆源区为基性下地壳变玄武质岩石,但在成岩过程中有少量幔源物质的加入。黄羊河岩体主要由钾长花岗岩组成,其岩浆结晶年龄为（402±4） Ma。岩石富碱（K2O+Na2O=6.91‰~7.66%）,K2O/Na2O>1,A/CNK=0.97~1.05。钾长花岗岩富集LILE及HFSE,轻重稀土元素分馏中等［(La/Yb)N =10.6~17.8］,并具有明显的负Eu异常（Eu/Eu*=0.43~0.68）,表明钾长花岗岩具有铝质A型花岗岩的地球化学特征。钾长花岗岩具有ISr=0.710 3~0.711 3,εNd(t)=-6.7~-6.0,TDM=1.46~1.55 Ga,反映岩浆主要来自地壳中长英质物质的部分熔融。冷龙岭地区花岗岩类的岩石成因及其岩浆演化揭示了北祁连山造山带从加里东早期的挤压构造体制向加里东晚期的伸展构造体制的演化。这些花岗岩类形成于碰撞后构造背景,岩浆的产生可能与俯冲的北祁连洋板片的断离作用有密切联系。 相似文献