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1.
The Meso-Cenozoic geodynamic evolution of the eastern Pontides orogenic belt provides a key to evaluate the volcanogenic massive sulfide (VMS) deposits associated with convergent margin tectonics in a Cordilleran-type orogenic belt. Here we present new geological, geochemical and zircon U–Pb geochronological data, and attempt to characterize the metallogeny through a comprehensive overview of the important VMS mineralizations in the belt. The VMS deposits in the northern part of the eastern Pontides orogenic belt occur in two different stratigraphic horizons consisting mainly of felsic volcanic rocks within the late Cretaceous sequence. SHRIMP zircon U–Pb analyses from ore-bearing dacites yield weighted mean 206Pb/238U ages ranging between 91.1 ± 1.3 and 82.6 ± 1 Ma. The felsic rocks of first and second horizons reveal geochemical characteristics of subduction-related calc-alkaline and shoshonitic magmas, respectively, in continental arcs and represent the immature and mature stages of a late Cretaceous magmatic arc. The nature of the late Cretaceous magmatism in the northern part of the eastern Pontides orogenic belt and the various lithological associations including volcaniclastics, mudstones and sedimentary facies indicate a rift-related environment where dacitic volcanism was predominant. The eastern Pontides VMS deposits are located within the caldera-like depressions and are closely associated with dome-like structures of felsic magmas, with their distribution controlled by fracture systems. Based on a detailed analyses of the geological, geophysical and geodynamic information, we propose that the VMS deposits were generated either in intra arc or near arc region of the eastern Pontides orogenic belt during the southward subduction of the Tethys oceanic lithosphere. 相似文献
2.
The composite Meghri–Ordubad and Bargushat plutons of the Zangezur–Ordubad region in the southernmost Lesser Caucasus consist of successive Eocene to Pliocene magmatic pulses, and host two stages of porphyry Cu–Mo deposits. New high-precision TIMS U–Pb zircon ages confirm the magmatic sequence recognized by previous Rb–Sr isochron and whole-rock K–Ar dating. A 44.03 ± 0.02 Ma-old granite and a 48.99 ± 0.07 Ma-old granodiorite belong to an initial Eocene magmatic pulse, which is coeval with the first stage of porphyry Cu–Mo formation at Agarak, Hanqasar, Aygedzor and Dastakert. A subsequent Oligocene magmatic pulse was constrained by U–Pb zircon ages at 31.82 ± 0.02 Ma and 33.49 ± 0.02 Ma for a monzonite and a gabbro, and a late Miocene porphyritic granodioritic and granitic pulse yielded ages between 22.46 ± 0.02 Ma and 22.22 ± 0.01 Ma, respectively. The Oligo-Miocene magmatic evolution broadly coincides with the second porphyry-Cu–Mo ore deposit stage, including the major Kadjaran deposit at 26–27 Ma.Primitive mantle-normalized spider diagrams with negative Nb, Ta and Ti anomalies support a subduction-like nature for all Cenozoic magmatic rocks. Eocene magmatic rocks have a normal arc, calc-alkaline to high-K calc-alkaline composition, early Oligocene magmatic rocks a high-K calc-alkaline to shoshonitic composition, and late Oligocene to Mio-Pliocene rocks are adakitic and have a calc-alkaline to high-K calc-alkaline composition. Radiogenic isotopes reveal a mantle-dominated magmatic source, with the mantle component becoming more predominant during the Neogene. Trace element ratio and concentration patterns (Dy/Yb, Sr/Y, La/Yb, Eu/Eu*, Y contents) correlate with the age of the magmatic rocks. They reveal combined amphibole and plagioclase fractionation during the Eocene and the early Oligocene, and amphibole fractionation in the absence of plagioclase during the late Oligocene and the Mio-Pliocene, consistent with Eocene to Pliocene progressive thickening of the crust or increasing pressure of magma differentiation. Characteristic trace element and isotope systematics (Ba vs. Nb/Y, Th/Yb vs. Ba/La, 206Pb/204Pb vs. Th/Nb, Th/Nb vs. δ18O, REE) indicate that Eocene magmatism was dominated by fluid-mobile components, whereas Oligocene and Mio-Pliocene magmatism was dominated by a depleted mantle, compositionally modified by subducted sediments.A two-stage magmatic and metallogenic evolution is proposed for the Zangezur–Ordubad region. Eocene normal arc, calc-alkaline to high-K calc-alkaline magmatism was coeval with extensive Eocene magmatism in Iran attributed to Neotethys subduction. Eocene subduction resulted in the emplacement of small tonnage porphyry Cu–Mo deposits. Subsequent Oligocene and Miocene high-K calc-alkaline and shoshonitic to adakitic magmatism, and the second porphyry Cu–Mo deposit stage coincided with Arabia–Eurasia collision to post-collision tectonics. Magmatism and ore formation are linked to asthenospheric upwelling along translithospheric, transpressional regional faults between the Gondwana-derived South Armenian block and the Eurasian margin, resulting in decompression melting of lithospheric mantle, metasomatised by sediment components added to the mantle during the previous Eocene subduction event. 相似文献
3.
The Eastern Pontides Orogenic Belt represents one of the best examples of fossil convergent margins in the eastern Mediterranean region. However, the origin and geodynamic setting of the late Mesozoic–Cenozoic magmatism in this belt remain controversial due to lack of systematic geological, geochemical and chronological data. The general consensus is that the late Mesozoic–Cenozoic igneous activity is related to northward subduction of oceanic lithosphere in the late Mesozoic and following collision between Tauride and Pontide blocks in the early Cenozoic. Here we present a comprehensive study focusing on the origin and geodynamic setting of gabbro bodies exposed along a narrow zone, parallel to the southeastern coast of the eastern Black Sea basin, in the Northern Zone of the Eastern Pontides Orogenic Belt.The studied gabbro bodies are hosted within late Cretaceous basaltic, andesitic, and dacitic volcanics including pyroclastic rocks and interbedded sedimentary rocks. The gabbro bodies range in size from 0.1 km2 to 1.5 km2, and outcrop patterns vary from round or elliptical to markedly elongate with sharp and discordant contact with the host rocks. Their mineral assemblage includes mainly clinopyroxene, plagioclase, minor olivine, amphibole, magnetite and rarely orthopyroxene, biotite, zircon and titanite. The occurrence of sutured grain boundaries on clinopyroxene and plagioclase, and the presence of reverse compositional zoning in clinopyroxene and olivine suggest mixing between magmas of contrasting compositions during mineral growth. Thermobarometric computations indicate that the temperature at the beginning of crystallization was ~ 1250 °C and crystallization was polybaric. Zircon and titanite U–Pb ages indicate that these small intrusions were emplaced into crustal rocks of the Eastern Pontides Orogenic Belt during Lutetian (45 ± 2 Ma). The depletion of HFSE is consistent with the involvement of an arc-related source in the petrogenesis of these rocks, and low to moderate enrichment Ce, Rb, Ba, K, Pb, Sr and Th suggests that involvement of subducted oceanic sediment was modest. The low Th content and low Th/Yb indicate that the role of sediment addition was nevertheless limited. The Nd, Sr and Pb isotopic data are consistent with the interpretation that the dominant source component in these gabbros is a depleted, peridotitic mantle, and that crustal contamination is relatively unimportant. We suggest that mafic magmas that produced the gabbroic intrusions were derived from melting of a depleted mantle source under the forearc region of the Eastern Pontides Orogenic Belt during southward subduction of two oceanic plates separated by a mid-ocean ridge, leading to the formation of a slab window. We also infer fractional crystallization and assimilation during both magma storage in the crust–mantle transition zone and transfer into the overlying arc crust. 相似文献
4.
Yongfeng Gao Zhusen Yang Zengqian Hou Ruihua Wei Xiangjin Meng Shihong Tian 《Lithos》2010,117(1-4):20-32
In the Yangbajing area, southern Tibet, several monogenic volcanoes were conformably superimposed on the Linzizong calc-alkaline volcanic successions. According to their petrologic and geochemical characteristics, these monogenic volcanoes are composed of three rock varieties: tephritic phonolitic plugs and shoshonitic and trachytic lavas. Their geochemical systematics reveals that low-pressure evolutionary processes in the large voluminous Linzizong calc-alkaline magmas were not responsible for the generation of these potassic–ultrapotassic rocks, but the significant change in petrologic and geochemical characteristics from the Linzizong calc-alkaline to potassic–ultrapotassic magma is likely accounted for the change of metasomatic agents in the southern Tibetan lithospheric mantle source during the Paleocene to Eocene. The tephritic phonolites containing both leucite and plagioclase show primary ultrapotassic character similar to that of Mediterranean plagioleucititic magmas. Radiogenic Sr increases with SiO2 in the xenolith-bearing trachytes strongly suggesting significant crustal assimilation in the shoshonitic magmas. The Yangbajing ultrapotassic rocks have high K2O and Al2O3, and show depletion of high field strength elements (HFSEs) with respect to large ion lithophile elements. In primitive mantle-normalized element diagrams, all samples are characterized by positive spikes at Th (U) and Pb with negative anomalies at Ba, Nb–Ta and Ti, reflecting the orogenic nature of the ultrapotassic rocks. They are characterized by highly radiogenic 87Sr/86Sr(i) ratios (0.7061–0.7063) and unradiogenic 143Nd/144Nd(i) (0.5125), and Pb isotopic compositions (206Pb/204Pb = 18.688–18.733, 207Pb/204Pb = 15.613–15.637, and 208Pb/204Pb = 38.861–38.930) similar to the global subducting sediment. Strong enrichment of incompatible trace elements and high Th fractionation from the other HFSEs (such as Nb and U) clearly indicate that the Th-enriched sedimentary component in a network veined mantle source was mainly introduced by sediment-derived melts. In addition, the ultrapotassic rocks have significant Ce (Ce/Ce* = 0.77–0.84) and Eu (Eu/Eu* = 0.72–0.75) anomalies, suggesting a subduction sediment input into the southern Tibetan lithospheric mantle source. In contrast, high U/Th (> 0.20) and Ba/Th (> 32) and low Th/La (< 0.3) in the shoshonites indicate that the Eocene potassic magma originated from partial melting of the surrounding peridotite mantle pervasively affected by slab-related fluid addition from the dehydration of either the subducting oceanic crust or the sediment. Thus, at least two different subduction-related metasomatic agents re-fertilized the upper mantle. According to the radiometric ages and spatial distribution, the Gangdese magmatic association shows a temporal succession from the Linzizong calc-alkaline to ultrapotassic magmas. This indicates a late arrival of recycled sediments within the Tibetan lithospheric mantle wedge. The most diagnostic signatures for the involvement of continent-derived materials are the super-chondritic Zr/Hf (45.5–49.2) and elevated Hf/Sm values (0.81–0.91) in the ultrapotassic rocks. Therefore, the occurrence of orogenic magmatism in the Gangdese belt likely represents the volcanic expression of the onset of the India–Asia collision, preceding the 10 Ma Neo-Tethyan slab break-off process at 42–40 Ma. The absence of residual garnet in the mantle source for the ultrapotassic volcanism seems to imply that the southern Tibetan lithosphere was not been remarkably thickened until the Eocene (～ 50 Ma). 相似文献
5.
The western part of the central belt of the Qilian orogenic belt, northeastern Tibetan Plateau, includes a compositionally diverse range of Cambrian to late Silurian felsic intrusions that reflect the changing tectonic process that molded this Paleozoic convergent margin. The Hf-isotopic compositional range of zircon from these rocks shows a significant role for Proterozoic crust – likely as microcontinents – rather than simply a history through oceanic arc accretion. Felsic magmatism includes shoshonitic magmas, and rarer shoshonite-OIB associations, dated from c. 465 to 445 Ma, which together form at least 30% of the presently exposed Paleozoic granitic crust of this region. Accepting a typically shoshonite petrogenesis for these magmas, involving asthenospheric upwelling and consequent remobilization of subduction-modified lithosphere, would infer a post-collisional setting at this stage. This could, perhaps, reflect slab-detachment, convective thinning of the lithosphere or orogenic collapse, resulting from collision and subduction between the Quanji block and the Central Qilian block. However, this requires the onset of a post-collisional setting at least 25 Ma before previously thought, and at the same time as intra-oceanic subduction is thought to have been active in other parts of the Qilian belt. These findings either require a reappraisal of the evidence for c. 490–440 Ma intra-oceanic subduction, or the formation of voluminous high-K and shoshonite magmatism in a pre-collisional setting, perhaps related to a period of unusually strong syn-arc rifting. 相似文献
6.
Yener Eyuboglu M.Santosh Francis O.Dudas Enver Akaryali Sun-Lin Chung Kemal Akdag Osman Bektas 《地学前缘(英文版)》2013,4(4):353-375
The eastern Pontides orogenic belt provides a window into continental arc magmatism in the Alpine-Himalayan belt.The late Mesozoic-Cenozoic geodynamic evolution of this belt remains controversial.Here we focus on the nature of the transition from the adakitic to non-adakitic magmatism in the Kale area of Gumushane region in NE Turkey where this transition is best preserved.The adakitic lithologies comprise porphyries and hyaloclastites.The porphyries are represented by biotite-rich andesites,hornblende-rich andesite and dacite.The hayaloclastites represent the final stage of adakitic activity and they were generated by eruption/intrusion of adakitic andesitic magma into soft carbonate mud.The non-adakitic lithologies include basaltic-andesitic volcanic and associated pyroclastic rocks. Both rock groups are cutting by basaltic dikes representing the final stage of the Cenozoic magmatism in the study area.We report zircon U-Pb ages of 48.71±0.74 Ma for the adakitic rocks,and 44.68±0.84 Ma for the non-adakitic type,suggesting that there is no significant time gap during the transition from adakitic to non-adakitic magmatism.We evaluate the origin,magma processes and tectonic setting of the magmatism in the southern part of the eastern Pontides orogenic belt.Our results have important bearing on the late Mesozoic-Cenozoic geodynamic evolution of the eastern Mediterranean region. 相似文献
7.
The Palaeozoic to Mesozoic igneous and metamorphic basement rocks exposed in the Mérida Andes of Venezuela and the Santander Massif of Colombia are generally considered to define allochthonous terranes that accreted to the margin of Gondwana during the Ordovician and the Carboniferous. However, terrane sutures have not been identified and there are no published isotopic data that support the existence of separate crustal domains. A general paucity of geochronological data led to published tectonic reconstructions for the evolution of the northwestern corner of Gondwana that do not account for the magmatic and metamorphic histories of the basement rocks of the Mérida Andes and the Santander Massif. We present new zircon U–Pb (ICP-MS) data from 52 igneous and metamorphic rocks, which we combine with whole rock geochemical and Pb isotopic data to constrain the tectonic history of the Precambrian to Mesozoic basement of the Mérida Andes and the Santander Massif. These data show that the basement rocks of these massifs are autochthonous to Gondwana and share a similar tectono-magmatic history with the Gondwanan margin of Peru, Chile and Argentina, which evolved during the subduction of oceanic lithosphere of the Iapetus Ocean. The oldest Palaeozoic arc magmatism is recorded at ~ 500 Ma, and was followed shortly by Barrovian metamorphism. Peak metamorphic conditions at upper amphibolite facies are recorded by anatexis at ~ 477 Ma and the intrusion of synkinematic granitoids until ~ 472 Ma. Subsequent retrogression resulted from localised back-arc or intra-arc extension at ~ 453 Ma, when volcanic tuffs and interfingered sedimentary rocks were deposited over the amphibolite facies basement. Continental arc magmatism dwindled after ~ 430 Ma and terminated at ~ 415 Ma, coevally with most of the western margin of Gondwana. After Pangaea amalgamation in the Late Carboniferous to Early Permian, a magmatic arc developed on its western margin at ~ 294 Ma as a result of subduction of oceanic crust of the palaeo-Pacific ocean. Intermittent arc magmatism recorded between ~ 294 and ~ 225 Ma was followed by the onset of the Andean subduction cycle at ~ 213 Ma, in an extensional regime. Extension was accompanied by slab roll-back which led to the migration of the arc axis into the Central Cordillera of Colombia in the Early Jurassic. 相似文献
8.
In this paper we present new zircon U–Pb ages, Hf isotope data, and whole-rock major and trace element data for Early Mesozoic intrusive rocks in the Erguna Massif of NE China, and we use these data to constrain the history of southward subduction of the Mongol–Okhotsk oceanic plate, and its influence on NE China as a whole. The zircon U–Pb dating indicates that Early Mesozoic magmatic activity in the Erguna Massif can be subdivided into four stages at ~ 246 Ma, ~ 225 Ma, ~ 205 Ma, and ~ 185 Ma. The ~ 246 Ma intrusive rocks comprise a suite of high-K calc-alkaline diorites, quartz diorites, granodiorites, monzogranites, and syenogranites, with I-type affinities. The ~ 225 Ma intrusive rocks consist of gabbro–diorites and granitoids, and they constitute a bimodal igneous association. The ~ 205 Ma intrusive rocks are dominated by calc-alkaline I-type granitoids that are accompanied by subordinate intermediate–mafic rocks. The ~ 185 Ma intrusive rocks are dominated by I-type granitoids, accompanied by minor amounts of A-types. These Early Mesozoic granitoids mainly originated by partial melting of a depleted and heterogeneous lower crust, whereas the coeval mafic rocks were probably derived from partial melting of a depleted mantle modified by subduction-related fluids. The rock associations and their geochemical features indicate that the ~ 246 Ma, ~ 205 Ma, and ~ 185 Ma intrusive rocks formed in an active continental margin setting related to the southward subduction of the Mongol–Okhotsk oceanic plate. The ~ 225 Ma bimodal igneous rock association formed within an extensional environment in a pause during the subduction process of the Mongol–Okhotsk oceanic plate. Every magmatic stage has its own corresponding set of porphyry deposits in the southeast of the Mongol–Okhotsk suture belt. Taking all this into account, we conclude the following: (1) during the Early Mesozoic, the Mongol–Okhotsk oceanic plate was subducted towards the south beneath the Erguna Massif, but with a pause in subduction at ~ 225 Ma; and (2) the southward subduction of the Mongol–Okhotsk oceanic plate not only caused the intense magmatic activity, but was also favorable to the formation of porphyry deposits. 相似文献
9.
Orhan Karsli Abdurrahman Dokuz İbrahim Uysal Faruk Aydin Raif Kandemir Jan Wijbrans 《Lithos》2010,114(1-2):109-120
Early Cenozoic (48–50 Ma) adakitic volcanic rocks from the Eastern Pontides, NE Turkey, consist of calc-alkaline and high-K calc-alkaline andesite and dacite, with SiO2 contents ranging from 56.01 to 65.44 wt.%. This is the first time that Early Eocene volcanism and adakites have been reported from the region. The rocks are composed of plagioclase, amphibole, quartz, and Mg-rich biotite. They have high and low-Mg# values ranging from 55 to 62 and 13 to 42, respectively. High-Mg# rocks have higher Ni and Co contents than low-Mg# samples. The rocks exhibit enrichments in large ion lithophile elements including the light rare earth elements, depletions in Nb, Ta and Ti and have high La/Yb and Sr/Y ratios. Their relative high ISr (0.70474–0.70640) and low εNd (50 Ma) values (? 2.3 to 0.8) are inconsistent with an origin as partial melts of a subducted oceanic slab. Combined major- and trace element and Sr–Nd isotope data suggest that the adakitic magmas are related to the unique tectonic setting of this region, where a transition from a collision to an extension stage has created thickening and delamination of the Pontide mafic lower crust at 50 Ma. The high-Mg adakitic magmas resulted from partial melting of the delaminated eclogitic mafic lower crust that sank into the relatively hot subcrustal mantle, and its subsequent interaction with the mantle peridotite during upward transport, leaving garnet as the residual phase, elevates the MgO content and Mg# of the magmas, whereas low-Mg# magmas formed by the melting of newly exposed lower crustal rocks caused by asthenospheric upwelling, which supplies heat flux to the lower crust. The data also suggest that the mafic lower continental crust beneath the region was thickened between the Late Cretaceous and the Late Paleocene and delaminated during Late Paleocene to Early Eocene time, which coincides with the initial stage of crustal thinning caused by crustal extensional events in the Eastern Pontides and rules out the possibility of an extensional regime before ~ 50 Ma in the region during the Late Mesozoic to Early Cenozoic. 相似文献
10.
Subduction of active spreading ridges most likely occurs throughout Earth's history. Interaction or collision between spreading center and trench, with the active spreading ridge downgoing and shallowly being buried in subduction zone, results in low-pressure but high-temperature near-trench magmatism in the forearc and accretionary prism setting. The Central Asian region, a complex orogenic belt created during the evolution and closure of the Paleo-Asian Ocean (PAO) at ~ 1000–300 Ma, provides an ideal place to study the subduction of PAO spreading ridges beneath ancient continental margins. It had been suggested that the low-pressure and high-temperature mafic and intermediate to felsic magmas from the Karamaili ophiolite (KO) in the NE corner of the Junggar basin (NW China) in Central Asia were likely produced by ridge subduction (Liu et al., 2007). In this paper, we combine our new geochemical data with previous results to show that the geochemical characteristics of the bulk of KO mafic rocks range from arc basalt-like to mid-ocean ridge basalt-like and ocean island basalt-like. Their trace element patterns range from depleted to enriched in highly incompatible elements, but depleted in Nb and Ta, indicating a subduction-influenced origin. The KO intermediate to felsic rocks are calc-alkaline and boninitic in composition and have trace element signatures similar to the associated mafic rocks. The low Nb/Ta ratios of some of the mafic rocks and boninitic character of some of the intermediate to felsic rocks reflect a highly depleted source, perhaps due to prior backarc magmatism. Major and trace element models indicate complex fractional crystallization histories of parental KO magmas to generate both the mafic and intermediate to felsic rocks, but in general, crystal fractionation occurred at 1000 to 1200 °C and moderate to low (0.5 kbar to 10 kbar) pressure or < 23 km depth. We conclude that the KO was formed in a forearc region of a subduction system that experienced ridge subduction. 相似文献
11.
There is ongoing debate as to the subduction direction of the Bangong–Nujiang Ocean during the Mesozoic (northward, southward or bidirectional subduction). Arc-related intermediate to felsic intrusions could mark the location of the subduction zone and, more importantly, elucidate the dominant geodynamic processes. We report whole rock geochemical and zircon U–Pb and Hf isotopic data for granitoids from the west central Lhasa subterrane (E80° to E86°). All rocks show metaluminous to peraluminous, calc-alkaline signatures, with strong depletion of Nb, Ta and Ti, enrichment of large ion lithophile elements (e.g., Cs, Rb, K), a negative correlation between SiO2 and P2O5, and a positive correlation between Rb and Th. All these features are indicative of I-type arc magmatism. New zircon U–Pb results, together with data from the literature, indicate continuous magmatism from the Late Jurassic to the Early Cretaceous (160 to 130 Ma). Zircon U–Pb ages for samples from the northern part of the west central Lhasa subterrane (E80° to E82°30′) yielded formation ages of 165 to 150 Ma, whereas ages of 142 to 130 Ma were obtained on samples from the south. This suggests flat or low-angle subduction of the Bangong–Nujiang Ocean, consistent with a slight southward decrease in zircon εHf(t) values for Late Jurassic rocks. Considering the crustal shortening, the distance from the Bangong–Nujiang suture zone, and a typical subduction zone melting depth of ~ 100 km, the subduction angle was less than 14° for Late Jurassic magmatism in the central Lhasa interior, consistent with flat or low-angle subduction. Compared with Late Jurassic rocks (main εHf(t) values of − 16 to − 7), Early Cretaceous rocks (145 to 130 Ma) show markedly higher εHf(t) values (mainly − 8 to 0), possibly indicating slab roll-back, likely caused by slab foundering or break-off. Combined with previously published works on arc magmatism in the central Lhasa and west part of the southern Qiangtang subterranes, our results support the bidirectional subduction of the Bangong–Nujiang Ocean along the Bangong–Nujiang Suture Zone, and indicates flat or low-angle southward subduction (165 to 145 Ma) followed by slab roll-back (145 to 130 Ma). 相似文献
12.
Ore-forming porphyries and barren granitoids from porphyry Cu deposits differ in many ways, particularly with respect to their adakitic affinity and calc-alkaline characteristics. In this study, zircon U–Pb and molybdenite Re–Os dating, whole rock geochemistry, whole rock Sr–Nd–Pb and zircon O–Hf isotopic analyses were carried out on the ore-forming granitoids from the Kounrad, Borly and Sayak deposits, and also on pre-ore and post-ore granitoids in adjacent regions of Central Kazakhstan. Geochronology results indicate that pre-ore magmatism occurred in the Late Devonian to Early Carboniferous (361.3–339.4 Ma), followed by large scale Cu mineralization (325.0–327.3 Ma at Kounrad, 311.4–315.2 Ma at Borly and 309.5–311.4 Ma at Sayak), and finally, emplacement of the Late Carboniferous post-ore barren granitoids (305.0 Ma). The geochemistry of these rocks is consistent with calc-alkaline arc magmatism characterized by strong depletions in Nb, Ta and Ti and enrichments in light rare earth elements and large ion lithophile elements, suggesting a supra-subduction zone setting. However, the ore-forming rocks at Kounrad and Sayak show adakitic characteristics with high Sr (517.5–785.3 ppm), Sr/Y (50.60–79.26), (La/Yb)N (9.37–19.62) but low Y (6.94–11.54 ppm) and Yb (0.57–1.07 ppm), whereas ore-forming rocks at Borly and barren rocks from northwest of Borly and Sayak have normal arc magma geochemical features. The Sr–Nd–Hf–O isotopic compositions show three different signatures: (1) Sayak granitoids have very young juvenile lower crust-derived compositions ((87Sr/86Sr)i = 0.70384 to 0.70451, ɛNd (t) = + 4.9 to + 6.0; TDM2 (Nd) = 580 to 670 Ma, ɛHf (t) = + 11.3 to + 15.5; TDMC (Hf) = 330 to 600 Ma, δ18O = 6.0 to 8.1‰), and were probably generated from depleted mantle-derived magma with 5–15% sediment melt addition in the magma source; (2) the Kt-1 granite from northwest of Sayak shows extremely enriched Sr–Nd isotopic compositions ((87Sr/86Sr)i = 0.71050, ɛNd (t) = − 7.8, TDM2 (Nd) = 1700 Ma), likely derived from partial melting of ancient continental crust; (3) other granitoids have transitional Sr–Nd compositions between the Sayak and Kt-1 samples, indicating a juvenile lower crust source with the addition of 10–30% of ancient crustal material. The pre-ore magmatism was probably related to partial melting of juvenile lower crust due to northward subduction of the Junggar–Balkhash Ocean, whereas the ore-forming adakitic rocks at Aktogai, Kounrad and Sayak formed by partial melting of thickened lower crust which subsequently delaminated. The ore-forming rocks at Borly, and the later post-ore barren granites, formed by partial melting of juvenile lower crust with normal thickness. This tectonic setting supports the existence of an Andean-type magmatic arc in the Devonian to the Late Carboniferous, resulting from the subduction of the Junggar–Balkhash oceanic plate. The link between whole rock geochemistry and scale of mineralization suggests a higher metallogenic potential for adakitic rocks than for normal arc magmatism. 相似文献
13.
《Gondwana Research》2014,25(1):170-189
The Lhasa terrane in southern Tibet is composed of Precambrian crystalline basement, Paleozoic to Mesozoic sedimentary strata and Paleozoic to Cenozoic magmatic rocks. This terrane has long been accepted as the last crustal block to be accreted with Eurasia prior to its collision with the northward drifting Indian continent in the Cenozoic. Thus, the Lhasa terrane is the key for revealing the origin and evolutionary history of the Himalayan–Tibetan orogen. Although previous models on the tectonic development of the orogen have much evidence from the Lhasa terrane, the metamorphic history of this terrane was rarely considered. This paper provides an overview of the temporal and spatial characteristics of metamorphism in the Lhasa terrane based mostly on the recent results from our group, and evaluates the geodynamic settings and tectonic significance. The Lhasa terrane experienced multistage metamorphism, including the Neoproterozoic and Late Paleozoic HP metamorphism in the oceanic subduction realm, the Early Paleozoic and Early Mesozoic MP metamorphism in the continent–continent collisional zone, the Late Cretaceous HT/MP metamorphism in the mid-oceanic ridge subduction zone, and two stages of Cenozoic MP metamorphism in the thickened crust above the continental subduction zone. These metamorphic and associated magmatic events reveal that the Lhasa terrane experienced a complex tectonic evolution from the Neoproterozoic to Cenozoic. The main conclusions arising from our synthesis are as follows: (1) The Lhasa block consists of the North and South Lhasa terranes, separated by the Paleo-Tethys Ocean and the subsequent Late Paleozoic suture zone. (2) The crystalline basement of the North Lhasa terrane includes Neoproterozoic oceanic crustal rocks, representing probably the remnants of the Mozambique Ocean derived from the break-up of the Rodinia supercontinent. (3) The oceanic crustal basement of North Lhasa witnessed a Late Cryogenian (~ 650 Ma) HP metamorphism and an Early Paleozoic (~ 485 Ma) MP metamorphism in the subduction realm associated with the closure of the Mozambique Ocean and the final amalgamation of Eastern and Western Gondwana, suggesting that the North Lhasa terrane might have been partly derived from the northern segment of the East African Orogen. (4) The northern margin of Indian continent, including the North and South Lhasa, and Qiangtang terranes, experienced Early Paleozoic magmatism, indicating an Andean-type orogeny that resulted from the subduction of the Proto-Tethys Ocean after the final amalgamation of Gondwana. (5) The Lhasa and Qiangtang terranes witnessed Middle Paleozoic (~ 360 Ma) magmatism, suggesting an Andean-type orogeny derived from the subduction of the Paleo-Tethys Ocean. (6) The closure of Paleo-Tethys Ocean between the North and South Lhasa terranes and subsequent terrane collision resulted in the formation of Late Permian (~ 260 Ma) HP metamorphic belt and Triassic (220 Ma) MP metamorphic belt. (7) The South Lhasa terrane experienced Late Cretaceous (~ 90 Ma) Andean-type orogeny, characterized by the regional HT/MP metamorphism and coeval intrusion of the voluminous Gangdese batholith during the northward subduction of the Neo-Tethyan Ocean. (8) During the Early Cenozoic (55–45 Ma), the continent–continent collisional orogeny has led to the thickened crust of the South Lhasa terrane experiencing MP amphibolite-facies metamorphism and syn-collisional magmatism. (9) Following the continuous continent convergence, the South Lhasa terrane also experienced MP metamorphism during Late Eocene (40–30 Ma). (10) During Mesozoic and Cenozoic, two different stages of paired metamorphic belts were formed in the oceanic or continental subduction zones and the middle and lower crust of the hanging wall of the subduction zone. The tectonic imprints from the Lhasa terrane provide excellent examples for understanding metamorphic processes and geodynamics at convergent plate boundaries. 相似文献
14.
Cenozoic volcanism on the Tibetan plateau, which shows systematic variations in space and time, is the volcanic response to the India–Asia continental collision. The volcanism gradually changed from Na-rich + K-rich to potassic–ultrapotassic + adakitic compositions along with the India–Asia collision shifting from contact-collision (i.e. “soft collision” or “syn-collision”) to all-sided collision (i.e. “hard collision”). The sodium-rich and potasium-rich lavas with ages of 65–40 Ma distribute mainly in the Lhasa terrane of southern Tibet and subordinately in the Qiangtang terrane of central Tibet. The widespread potassic–ultrapotassic lavas and subordinate adakites were generated from ~ 45 to 26 Ma in the Qiangtang terrane of central Tibet. Subsequent post-collisional volcanism migrated southwards, producing ultrapotassic and adakitic lavas coevally between ~ 26 and 8 Ma in the Lhasa terrane. Then potassic and minor adakitic volcanism was renewed to the north and has become extensive and semicontinuous since ~ 20 Ma in the western Qiangtang and Songpan–Ganze terranes. Such spatial–temporal variations provide important constraints on the geodynamic processes that evolved at depth to form the Tibetan plateau. These processes involve roll-back and break-off of the subducted Neo-Tethyan slab followed by removal of the thickened Lhasa lithospheric root, and consequently northward underthrusting of the Indian lithosphere. The Tibetan plateau is suggested to have risen diachronously from south to north. Whereas the southern part of the plateau may have been created and maintained since the late-Oligocene, the northern plateau would have not attained its present-day elevation and size until the mid-Miocene when the lower part of the western Qiangtang and Songpan–Ganze lithospheres began to founder and detach owing to the persistently northward push of the underthrust Indian lithosphere. 相似文献
15.
《Chemie der Erde / Geochemistry》2015,75(2):237-260
The northeastward subduction of the Neo-Tethyan oceanic lithosphere beneath the Iranian block produced vast volcanic and plutonic rocks that now outcrop in central (Urumieh–Dokhtar magmatic assemblage) and north–northeastern Iran (Alborz Magmatic Belt), with peak magmatism occurring during the Eocene. The Karaj Dam basement sill (KDBS), situated in the Alborz Magmatic Belt, comprises gabbro, monzogabbro, monzodiorite, and monzonite with a shoshonitic affinity. These plutonic rocks are intruded into the Karaj Formation, which comprise pyroclastic rocks dating to the lower–upper Eocene. The geochemical and isotopic signatures of the KDBS rocks indicate that they are cogenetic and evolved through fractional crystallization. They are characterized by an enrichment in LREEs relative to HREEs, with negative Nb–Ta anomalies. Geochemical modeling using Sm/Yb versus La/Yb and La/Sm ratios suggests a low-degree of partial melting of a phlogopite–spinel peridotite source to generate the KDBS rocks. Their low ISr = 0.70453–0.70535, ɛNd (37.2 Ma) = 1.54–1.9, and TDM ages ranging from 0.65 to 0.86 Ga are consistent with the melting of a Cadomian enriched lithospheric mantle source, metasomatized by fluids derived from the subducted slab or sediments during magma generation. These interpretations are consistent with high ratios of 206Pb/204Pb = 18.43–18.67, 207Pb/204Pb = 15.59, and 208Pb/204Pb = 38.42–38.71, indicating the involvement of subducted sediments or continental crust. The sill is considered to have been emplaced in an environment of lithospheric extension due to the slab rollback in the lower Eocene. This extension led to localized upwelling of the asthenosphere, providing the heat required for partial melting of the subduction-contaminated subcontinental lithospheric mantle beneath the Alborz magmatic belt. Then, the shoshonitic melt generates the entire spectrum of KDBS rocks through assimilation and fractional crystallization during the ascent of the magma. 相似文献
16.
《Gondwana Research》2013,24(4):1378-1401
The Qilian Orogen at the northern margin of the Tibetan Plateau is a type suture zone that recorded a complete history from continental breakup to ocean basin evolution, and to the ultimate continental collision in the time period from the Neoproterozoic to the Paleozoic. The Qilian Ocean, often interpreted as representing the “Proto-Tethyan Ocean”, may actually be an eastern branch of the worldwide “Iapetus Ocean” between the two continents of Baltica and Laurentia, opened at ≥ 710 Ma as a consequence of breakup of supercontinent Rodinia.Initiation of the subduction in the Qilian Ocean probably occurred at ~ 520 Ma with the development of an Andean-type active continental margin represented by infant arc magmatism of ~ 517–490 Ma. In the beginning of Ordovician (~ 490 Ma), part of the active margin was split from the continental Alashan block and the Andean-type active margin had thus evolved to western Pacific-type trench–arc–back-arc system represented by the MORB-like crust (i.e., SSZ-type ophiolite belt) formed in a back-arc basin setting in the time period of ~ 490–445 Ma. During this time, the subducting oceanic lithosphere underwent LT-HP metamorphism along a cold geotherm of ~ 6–7 °C/km.The Qilian Ocean was closed at the end of the Ordovician (~ 445 Ma). Continental blocks started to collide and the northern edge of the Qilian–Qaidam block was underthrust/dragged beneath the Alashan block by the downgoing oceanic lithosphere to depths of ~ 100–200 km at about 435–420 Ma. Intensive orogenic activities occurred in the late Silurian and early Devonian in response to the exhumation of the subducted crustal materials.Briefly, the Qilian Orogen is conceptually a type example of the workings of plate tectonics from continental breakup to the development and evolution of an ocean basin, to the initiation of oceanic subduction and formation of arc and back-arc system, and to the final continental collision/subduction and exhumation. 相似文献
17.
In situ zircon U–Pb ages and Hf isotope data, major and trace elements and Sr–Nd–Pb isotopic compositions are reported for coeval syenite–granodiorites–dacite association in South China. The shoshonitic syenites are characterized by high K2O contents (5.9–6.1 wt.%) and K2O/Na2O ratios (1.1–1.2), negative Eu anomalies (Eu/Eu* = 0.65 to 0.77), enrichments of Rb, K, Nb, Ta, Zr and Hf, but depletion of Sr, P and Ti. The adakitic granodiorite and granodiorite porphyry intrusions are characterized by high Al2O3 contents (15.0–16.8 wt.%), enrichment in light rare earth elements (LREEs), strongly fractionated LREEs (light rare earth elements) to HREEs (heavy rare earth elements), high Sr (438–629 ppm), Sr/Y (29.2–53.6), and low Y (11.7–16.8 ppm) and HREE contents (e.g., Yb = 1.29–1.64 ppm). The calc-alkaline dacites are characterized by LREE enrichment, absence of negative Eu anomalies, and enrichment of LILEs such as Rb, Ba, Th, U and Pb, and depletion of HFSEs such as Nb, Ta, P and Ti.Geochemical and Sr–Nd–Hf isotopic compositions of the syenites suggest that the shoshonitic magmas were differentiated from parental shoshonitic melts by fractional crystallization of olivine, clinopyroxene and feldspar. The parent magmas may have originated from partial melting of the lithospheric mantle with small amount contribution from crustal materials. The adakitic granodiorite and granodiorite porphyry have Sr–Nd–Pb isotopic compositions that are comparable to that of the mafic lower crust. They have low Mg# and MgO, Ni and Cr contents, abundant inherited zircons, low εNd(t) and εHf(t) values as well as old whole-rock Nd and zircon Hf model ages. These granodiorites were likely generated by partial melting of Triassic underplated mafic lower crust. The Hf isotopic compositions of the dacites are relatively more depleted than the Cathaysia enriched mantle, suggesting those magmas were derived from the partial melting of subduction-modified mantle sources. The coeval shoshonitic, high-K calc-alkaline and calc-alkaline rocks in Middle to Late Jurassic appear to be associated with an Andean-type subduction. This subduction could have resulted in the upwelling of the asthenosphere beneath the Cathaysia Block, which induced partial melting of the mantle as well as the mafic lower crust, and formed an arc regime in the coastal South China during Middle to Late Jurassic. 相似文献
18.
The subduction polarity of Tethyan oceanic lithosphere during Jurassic is a controversial topic in relation to the geodynamic evolution of the Alpine–Himalayan system. We present new geological, geochemical and zircon U–Pb data from four different regions of the Eastern Pontides Orogenic Belt, a key area of the Alpine–Himalayan system. We discuss the origin of the magmatism and also the existence of an ocean in the eastern Mediterranean region during the Jurassic period. Jurassic intrusions, predominantly gabbro, tonalite and minor diorite, are well exposed in the southern and axial zones of the orogenic belt. Thermobarometry indicates that high-pressure (6–10 kb) crystallization of these intrusions occurred at temperatures of 1183–1250 °C. Zircon U–Pb dating from 10 samples show ages between 195 and 165 Ma, indicating that magmatism occurred between Sinemurian and Callovian time. We characterize the intrusions from electron microprobe, zircon geochronology, and whole rock and Sr, Nd, and Pb isotopes. Our data show that the studied intrusions are broadly tholeiitic, except for two calc-alkaline bodies, and formed in an arc-related setting with minimal involvement of older crust or sediment.The most widely accepted model proposes that the ultramafic–mafic rocks exposed between the Pontide arc and the Tauride belt are remnants of a Jurassic Penrose-type and/or suprasubduction zone ophiolite. However, new zircon U–Pb age data from mafic lithologies cutting the Kop ultramafic massif do not support this model and clearly indicate that the ultramafic lithologies are Paleozoic or older in age and are not remnants of a Jurassic ocean that known as ‘’Northern Branch of Neotehtys”. 相似文献
19.
《Gondwana Research》2014,25(2):797-819
A suite of Paleozoic granitoids in Central Tianshan was studied for both geochemistry and geochronology in an effort to constrain their origin and tectonic setting. We combined LA-ICP-MS dating of zircon, standard geochemical analyses and Hf-isotopic studies of zircon to develop our tectonic model. Based on our analysis, the granitoids formed in three distinctive stages: ~ 450–400 Ma, ~ 370–350 Ma and ca. 340 Ma. The first stage (450–400 Ma) granitoids exhibit metaluminous, magnesian, high-K to shoshonitic characteristics of I-type granitoids (arc-setting), that are enriched in LREE relative to HREE with high (La/Yb)CN values, show negative Eu anomaly and are depleted in Nb, Ta and Ti. This phase of granitoid emplacement was most likely related to the southward subduction of the Paleo-Tianshan Ocean beneath the Tarim block and the subsequent Central Tianshan arc. In contrast, the second stage granitoids (370–350 Ma) are distinctly different and are classified as calc-alkaline or shoshonitic plutons with a weak positive Eu anomaly. Within the second stage granitoids, it appears that the earlier (~ 365 Ma) granitoids fit within the A-type field whereas the younger (~ 352 Ma) granitoids plot within the post-collisional potassic field. These granitoids formed during collisions between Central Tianshan and the Tuha terrane that occurred along the northern margin of Central Tianshan. Lastly, the ca. 340 Ma granitoids are typical of volcanic arc granitoids again that probably formed during the northward subduction of the South Tianshan Ocean beneath the Central Tianshan landmass or the subsequent southward subduction of the residual Paleo-Tianshan Ocean.The Hf isotopic data of zircons from all the studied granitoids were pooled and yielded three prominent Hf TDMC model age populations: ca. 2400 Ma, ca. 1400 Ma and ca. 1100 Ma. The Hf-data shows a significant input of juvenile crust in addition to crustal recycling. We interpret these three phases of juvenile crustal addition to phases of global growth of continental crust (~ 2400 Ma), the addition of juvenile crust during the breakup of the Columbia supercontinent (~ 1400 Ma) and the assembly of Rodinia (~ 1100 Ma). 相似文献
20.
We present new U–Pb isotopic age data for detrital zircons from 16 deformed sandstones of the Ross Supergroup in north Victoria Land, Antarctica. Zircon U/Th ratios primarily point to dominantly igneous parent rocks with subordinate contributions from metamorphic sources. Comparative analysis of detrital zircon age populations indicates that inboard stratigraphic successions (Wilson Terrane) and those located outboard of the East Antarctic craton (the Bowers and Robertson Bay terranes) have similar ~ 1200–950 Ma (Mesoproterozoic–Neoproterozoic) and ~ 700–490 Ma (late Neoproterozoic–Cambrian, Furongian) age populations. The affinity of the age populations of the sandstones to each other, as well as Gondwana sources and Pacific-Gondwana marginal stratigraphic belts, challenges the notion that the outboard successions form exotic terranes that docked with Gondwana during the Ross orogeny and instead places the terranes in proximity to each other and within the peri-Gondwana realm during the late Neoproterozoic to Cambrian. The cumulative zircon age suite from north Victoria Land yields a polymodal age spectra with a younger, primary 700–480 Ma age population that peaks at ~ 580 Ma. Cumulative analysis of zircons with elevated U/Th ratios (> 20) indicating metamorphic heritage yield ~ 657–532 Ma age probability peaks, which overlap with the younger dominantly igneous zircon population. The data are interpreted to give important new evidence that is consistent with ongoing convergent arc magmatism by ~ 626 Ma, which provided the dominant zircon-rich igneous rocks and subordinate metamorphic rocks. Maximum depositional ages as young as ~ 493–481 Ma yielded by deformed sequences in the outboard Bowers and Robertson Bay terrane samples provide new support for late Cambrian to Ordovician deformation in outboard sectors of the orogen, consistent with tectonic models that call for cyclic phases of contraction along the north Victoria Land sector of the Ross–Delamerian orogen. 相似文献