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1.
The Menderes Massif, exposed in western Anatolia, is a metamorphic complex cropping out in the Alpine orogenic belt. The metamorphic rock succession of the Massif is made up of a Precambrian basement and overlying Paleozoic-early Tertiary cover series. The Pan-African basement is composed of late Proterozoic metasedimentary rocks consisting of partially migmatized paragneisses and conformably overlying medium- to high-grade mica schists, intruded by orthogneisses and metagabbros. Along the southern flank of the southern submassif, we recognized well-preserved primary contact relationship between biotite and leucocratic tourmaline orthogneisses and country rocks as the orthogneisses represent numerous large plutons, stocks and vein rocks intruded into a basement of garnet mica schists. Based on the radiometric data, the primary deposition age of the precursors of the country rocks, garnet mica schist, can be constrained between 600 and 550?Ma (latest Neoproterozoic). The North Africa–Arabian-Nubian Shield in the Mozambique Belt can be suggested as the possible provenance of these metaclastics. The intrusion ages of the leucocratic tourmaline orthogneisses and biotite orthogneisses were dated at 550–540?Ma (latest Neoproterozoic–earliest Cambrian) by zircon U/Pb and Pb/Pb geochronology. These granitoids represent the products of the widespread Pan-African acidic magmatic activity, which can be attributed to the closure of the Mozambique Ocean during the final collision of East and West Gondwana. Detrital zircon ages at about 550?Ma in the Paleozoic muscovite-quartz schists show that these Pan-African granitoids in the basement form the source rocks of the cover series of the Menderes Massif.  相似文献   

2.
This paper presents geochemical, Sr–Nd isotopic, and U–Pb zircon geochronological data on the Alvand plutonic complex in Sanandaj–Sirjan zone (SSZ), Western Iran. The gabbroic rocks show a trend of a calc-alkaline magma suite and are characterized by low initial 87Sr/86Sr ratios (0.7023–0.7037) and positive εNd(t) values (2.9–3.3), which suggest derivation from a moderately depleted mantle source. Geochemical features of the granites illustrate a high-K calc-alkaline magma series, whereas the leucocratic granitoids form part of a low-K series. Granites have intermediate 87Sr/86Sr ratios (0.707–0.719) and negative εNd(t) values (−1.0 to −3.4), while leucocratic granitoids have higher initial 87Sr/86Sr ratio (0.713–0.714) and more negative εNd(t) values (−3.5 to −4.5). Potential basement source lithologies for the granites are Proterozoic granites and orthogneisses, and those for the leucocratic granites are plagioclase-rich sources such as meta-arkoses or tonalites. The U–Pb dating results demonstrate that all granitoids were exclusively emplaced during the Jurassic instead of being Cretaceous or younger in age as suggested previously. The pluton was assembled incrementally over c. 10 Ma. Gabbros formed at 166.5 ± 1.8 Ma, granites between 163.9 ± 0.9 Ma and 161.7 ± 0.6 Ma, and leucocratic granitoids between 154.4 ± 1.3 and 153.3 ± 2.7 Ma. Granites and leucocratic granitoids show some A-type affinity. It is concluded that the Alvand plutonic complex was generated in a continental-arc-related extensional regime during subduction of Neo-Tethyan oceanic crust beneath the SSZ. The U/Pb zircon age data, recently corroborated by similar results in the central and southern SSZ, indicate that Jurassic granitoids are more areally extensive in this belt than previously thought.  相似文献   

3.
In the Menderes Massif (western Taurides) a Neoproterozoic basement comprising metasediments and intrusive granites is imbricated between Paleozoic platform sediments. U–Pb–Hf zircon analyses of Menderes rock units were performed by us using LA-ICP-MS. The U–Pb detrital zircon signal of the Neoproterozoic metasediments is largely consistent with a NE African (Gondwana) provenance. The oldest unit, a paragneiss, contains significant amounts (~ 30%) of Archean-aged zircons and εHf (t) values of about a half of its Neoproterozoic zircons are negative suggesting contribution from Pan-African terranes dominated by reworking of an old crust. In the overlying, mineralogically-immature Core schist (which is still Neoproterozoic), the majority of the detrital zircons are Neoproterozoic, portraying positive εHf (t) values indicating derivation from a proximal juvenile source, resembling the Arabian–Nubian Shield.The period of sedimentation of the analyzed metasediments, is constrained between 570 and 550 Ma (Late Ediacaran). The Core schist sediments, ~ 9 km thick, accumulated in less than 20 My implying a tectonic-controlled sedimentary basin evolved adjacent to the eroded juvenile terrane. Granites, now orthogneisses, intruded the basin fill at 550 Ma, they exhibit ± 0 εHf (t = 550 Ma) and TDM ages of 1.4 Ga consistent with anatexis of various admixtures of juvenile Neoproterozoic and Late Archean detrital components. Granites in the northern Arabian–Nubian Shield are no younger than 580 Ma and their εHf (t) are usually more positive. This implies that the Menderes does not represent a straightforward continuation of the Arabian–Nubian Shield.The lower part of the pre-Carboniferous silisiclastic cover of the Menderes basement, comprises a yellowish quartzite whose U–Pb–Hf detrital zircon signal resembles that of far-traveled Ordovician sandstones in Jordan (including 0.9–1.1 Ga detrital zircons), supporting pre-Triassic paleorestorations placing the Tauride with Afro-Arabia. The detrital signal of the overlying carbonate-bearing quartzitic sequence indicates contribution from a different source: the majority of its detrital zircons yielded 550 Ma and ± 0 εHf (t = 550 Ma) values identical to that of the underlying granitic gneiss implying exposure of Menderes-like granites in the provenance.260–250 Ma lead-loss and partial resetting of the U–Pb system of certain zircons in both basement and cover units was detected. It is interpreted as a consequence of a Permian–Early Triassic thermal event preceding known Triassic granitoid intrusions.  相似文献   

4.
Pan-African basement rocks and a Paleozoic cover series, which were intruded by the protoliths of leucocratic orthogneisses, have been recognized in the Menderes Massif, located in the western part of the Alpine orogenic belt of Turkey. This geochemical and geochronological study focuses on the evolution of the Menderes Massif at the end of Paleozoic time. Geochemical data suggest that the crustally derived leucocratic orthogneisses have chemical composition typical of calc-alkaline and S-type granite. Zircon grains which are euhedral with typical igneous morphologies were dated by the 207Pb/206Pb evaporation method. Single-zircon dating of three samples yielded mean 207Pb/206Pb ages of 246LJ, 241LJ and 235Lj Ma. These ages are interpreted as the time of protolith emplacement in Triassic. Geological and geochronological data suggest that leucocratic granites were emplaced in a period following a metamorphic event related to the closure of the Paleo-Tethys. The leucocratic granites were metamorphosed during the Alpine orogenesis and transformed into orthogneisses. The similar Triassic magmatic event at 233DŽ Ma was also occurred, using single-zircon evaporation method, from granitic gneisses which rest upon the migmatites with tectonic contacts in Naxos, Cycladic complex. This indicates that the Menderes Massif and Cycladic complex had a common pre-Early Triassic magmatic evolution.  相似文献   

5.
《Gondwana Research》2014,25(3-4):1031-1037
Although the Sibumasu terrane in Asia was previously considered to be composed of Phanerozoic rocks with Cambrian crystalline basement, no reliable or direct radiometric dating evidences of such crystalline basement was ever reported. Our new in-situ zircon U/Pb dating of the Khao Tao orthogneiss yields a concordant age of 501.5 ± 7.5 Ma (2σ), which provides the first robust evidence for the Cambrian crust in Upper Peninsula of Thailand. The zircon εHf(T) values range from + 3.7 to − 6.1 with model ages (TCDM) of 1244–1827 Ma, suggests a mixed crust-mantle source. The chemical similarity and spatial continuity of the Khao Tao orthogneiss with other pre-Neotethys marginal Eurasian and Sibumasu granitoids indicate the linear paleogeographic association under a similar magmatic arc-related regime along the Gondwana India–Australia margin as part of the Pan-African Orogeny system.  相似文献   

6.
The Danubian domain basement of the South Carpathians, Romania, comprises two Neoproterozoic continental crustal fragments, the Dr?g?an and Lainici-P?iu? terranes, which were sutured by the closure of an intervening oceanic domain, the Ti?ovi?a terrane. Magmatic and detrital zircons extracted from an orthogneiss, four granitoid plutons, two metasedimentary units, and a Liassic sandstone were dated by zircon U/Pb LA-ICP-MS. The F?ge?el augen gneiss from the Dr?g?an terrane basement yielded an age of 803.2 ± 4.4 Ma, the oldest well-constrained crystallization age reported from the Romanian Carpathians basement. The Tismana, ?u?i?a, Novaci and Olte? granitoid plutons, which intrude the Lainici-P?iu? terrane basement, yielded ages of 600.5 ± 4.4, 591.0 ± 3.5, 592.7 ± 4.9, and 588 ± 2.9 Ma, respectively. The Tismana granitoid age of 600 Ma and the youngest detrital zircon ages of 637–622 Ma from a metaquartzite within the Lainici-Paiu? terrane, constrain the deposition of the metaquartzite protolith to ca. 620–600 Ma. The 803 Ma age represents an old Pan-African age, whereas the younger Neoproterozoic ages suggest Pan-African/Cadomian thermotectonic events. Detrital and inherited zircon ages within the Dr?g?an and Lainici-Paiu? terranes attest to a peri-Amazonian, Avalonian-type provenance for the Dr?g?an terrane and possibly a Ganderian-type provenance for the Lainici-P?iu? terrane. The Lainici-P?iu? terrane rifted off Gondwana before the Dr?g?an terrane. Both terranes were attached to Moesia during the Early Paleozoic.  相似文献   

7.
The Temaguessine high-level subcircular pluton is intrusive into the LATEA metacraton (Central Hoggar) Eburnian (2 Ga) basement and in the Pan-African (615 Ma) granitic batholiths along a major NW–SE oriented major shear zone. It is dated here (SHRIMP U–Pb on zircon) at 582 ± 5 Ma. Composed of amphibole–biotite granite and biotite syenogranite, it comprises abundant enclaves: mafic magmatic enclaves, country-rock xenoliths and remarkable Fe-cordierite (#Fe = 0.87) orbicules. The orbicules have a core rich in cordierite (40%) and a leucocratic quartz–feldspar rim. They are interpreted as resulting from the incongruent melting of the meta-wacke xenoliths collapsed into the magma: the breakdown of the biotite + quartz assemblage produced the cordierite and a quartz–feldspar minimum melt that is expelled, forming the leucocratic rim. The orbicule generation occurred at T < 850° and P < 0.3 GPa. The Fe-rich character of the cordierite resulted from the Fe-rich protolith (wacke with 4% Fe2O3 for 72% SiO2). Strongly negative εNd (−9.6 to −11.2), Nd TDM model ages between 1.64 and 1.92 Ga, inherited zircons between 1.76 and 2.04 Ga and low to moderately high ISr (0.704–0.710) indicate a Rb-depleted lower continental crust source for the Temaguessine pluton; regional considerations impose however also the participation of asthenospheric material. The Temaguessine pluton, together with other high-level subcircular pluton, is considered as marking the end of the Pan-African magma generation in the LATEA metacraton, resulting from the linear delamination along mega-shear zones, allowing asthenospheric uprise and melting of the lower continental crust. This implies that the younger Taourirt granitic province (535–520 Ma) should be considered as a Cambrian intraplate anorogenic event and not as a very late Pan-African event.  相似文献   

8.
Central North Sudan, west of the Keraf suture, is part of the Saharan Metacraton whose crystalline basement encompasses migmatite gneisses and granites. Granites intrude migmatites in form of small plutons, veins, lenses and pods, indicating a complex chronology. This study, based on whole rock element concentrations, isotope geochemistry and single mineral geochronology, is aimed to unravel the petrogenesis of these basement rocks.Whole rock geochemistry indicates an I-type potassic calc-alkaline meta- to peraluminous composition. Granite zircon U–Pb and Pb–Pb evaporation analyses yield an identical age range (597 ± 25–602 ± 3.5 Ma). Similar ages (597 ± 8.6–603.8 ± 2 Ma) are obtained for the migmatite gneisses. Titanite U–Pb ages are also similar in both rock types, but are younger or closely conform with zircon ages. Biotite Rb/Sr ages are younger and identical (566 ± 11–570 ± 17 Ma). These age data suggest coeval granitization and migmatization during the Pan-African period and somewhat later cooling of the central North Sudan basement. Older zircon U–Pb ages, ranging from 613 to 1322 Ma, are thought to be signatures of inheritance, while younger ones (336–594 Ma) suggest radiogenic Pb loss. Sr initial ratios (0.70257–0.72102) and εNd values (−2.3 to −8.8), calculated for the zircon crystallization age of ∼600 Ma indicate a crustal signature. Coupled with Nd model ages of 1460–1990 Ma, isotope data indicate that the central North Sudan basement is recycled Middle to Late Proterozoic continental crust.  相似文献   

9.
Late-stage Pan-African granitoids, including monzogranite, syenogranite and alkali granite, were collected from four separate localities in Sinai. They were selected to represent both the calc-alkaline and alkaline suites that have been viewed as forming separate magmatic episodes in the Eastern Desert of Egypt, with the transition to alkali granite at ~ 610 Ma taken to mark the onset of crustal extension. Although intrusive relations were observed in the field, the emplacement ages of the granitoids cannot be distinguished within analytical uncertainty and they all formed within a restricted time span from 579 to 594 Ma. This indicates that the two suites are coeval and that some calc-alkaline rocks were also likely generated during the late extensional phase. These ages are identical to those recently obtained from similar rocks in the North-Eastern Desert, confirming that Sinai is the northern extension of the Eastern Desert Pan-African terrane of Egypt. Rare inherited zircons with ages of ~ 1790 and ~ 740 Ma are present in syenogranite from northeastern Sinai and indicate that older material is present within the basement. A few zircons record younger ages and, although some may reflect later disturbance of the main zircon population, those with ages of ~ 570 and 535 Ma probably reflect thermal events associated with the extensive emplacement of mafic and felsic dykes in both northeastern and southern Sinai.  相似文献   

10.
Metamorphic and magmatic rocks are present in the northwestern part of the Schwaner Mountains of West Kalimantan. This area was previously assigned to SW Borneo (SWB) and interpreted as an Australian-origin block. Predominantly Cretaceous U-Pb zircon ages (c. 80–130 Ma) have been obtained from metapelites and I-type granitoids in the North Schwaner Zone of the SWB but a Triassic metatonalite discovered in West Kalimantan near Pontianak is inconsistent with a SWB origin. The distribution and significance of Triassic rocks was not known so the few exposures in the Pontianak area were sampled and geochemical analyses and zircon U-Pb ages were obtained from two meta-igneous rocks and three granitoids and diorites. Triassic and Jurassic magmatic and metamorphic zircons obtained from the meta-igneous rocks are interpreted to have formed at the Mesozoic Paleo-Pacific margin where there was subduction beneath the Indochina–East Malaya block. Geochemically similar rocks of Triassic age exposed in the Embuoi Complex to the north and the Jagoi Granodiorite in West Sarawak are suggested to have formed part of the southeastern margin of Triassic Sundaland. One granitoid (118.6 ± 1.1 Ma) has an S-type character and contains inherited Carboniferous, Triassic and Jurassic zircons which indicate that it intruded Sundaland basement. Two I-type granitoids and diorites yielded latest Early and Late Cretaceous weighted mean ages of 101.5 ± 0.6 and 81.1 ± 1.1 Ma. All three magmatic rocks are in close proximity to the meta-igneous rocks and are interpreted to record Cretaceous magmatism at the Paleo-Pacific subduction margin. Cretaceous zircons of metamorphic origin indicate recrystallisation at c. 90 Ma possibly related to the collision of the Argo block with Sundaland. Subduction ceased at that time, followed by post-collisional magmatism in the Pueh (77.2 ± 0.8 Ma) and Gading Intrusions (79.7 ± 1.0 Ma) of West Sarawak.  相似文献   

11.
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   

12.
Detailed geochemical, isotope, and geochronological studies were carried out for the granitoids of the Chuya and Kutima complexes in the Baikal marginal salient of the Siberian craton basement. The obtained results indicate that the granitoids of both complexes are confined to the same tectonic structure (Akitkan fold belt) and are of similar absolute age. U–Pb zircon dating of the Kutima granites yielded an age of 2019±16 Ma, which nearly coincides with the age of 2020±12 Ma obtained earlier for the granitoids of the Chuya complex. Despite the close ages, the granitoids of these complexes differ considerably in geochemical characteristics. The granitoids of the Chuya complex correspond in composition to calcic and calc-alkalic peraluminous trondhjemites, and the granites of the Kutima complex, to calc-alkalic and alkali-calcic peraluminous granites. The granites of the Chuya complex are similar to rocks of the tonalite–trondhjemite–granodiorite (TTG) series and are close in CaO, Sr, and Ba contents to I-type granites. The granites of the Kutima complex are similar in contents of major oxides to oxidized A-type granites. Study of the Nd isotope composition of the Chuya and Kutima granitoids showed their close positive values of εNd(T) (+ 1.9 to + 3.5), which indicates that both rocks formed from sources with a short crustal history. Based on petrogeochemical data, it has been established that the Chuya granitoids might have been formed through the melting of a metabasitic source, whereas the Kutima granites, through the melting of a crustal source of quartz–feldspathic composition. Estimation of the PT-conditions of granitoid melt crystallization shows that the Chuya granitoids formed at 735–776 °C (zircon saturation temperature) and > 10 kbar and the Kutima granites, at 819–920 °C and > 10 kbar. It is assumed that the granitoids of both complexes formed in thickened continental crust within an accretionary orogen.  相似文献   

13.
The in-situ “chemical” Th–U–Pb dating of monazite with the electron microprobe is used to unravel the Neoproterozoic tectono-thermal history of the “Erinpura Granite” terrane in the foreland of the Delhi Fold Belt (DFB) in the NW Indian craton. These granitoids are variably deformed and show effects of shearing activity. Monazites from the Erinpura granitoids recorded two main events; (1) protolith crystallization at 863 ± 23 Ma and (2) recrystallization and formation of new Th-poor monazite at 775 ± 26 Ma during shear overprint. Some components of the Erinpura granitoids, such as the Siyawa Granite and granites exposed near Sirohi town, show evidence of migmatization. This migmatization event is documented by anatexis and associated monazite crystallization at 779 ± 16 Ma. The age data indicate an overlap in timing between anatectic event and ductile shear deformation. The end of the tectono-thermal event in the Sirohi area is constrained by a 736 ± 6 Ma Ar–Ar muscovite age data from the ductile shear zone.  相似文献   

14.
A combined study of zircon U–Pb ages and Lu–Hf isotopes, mineral O isotopes, whole-rock elements and Sr–Nd isotopes was carried out for Mesozoic granitoids from the Shandong Peninsula in east-central China, which tectonically corresponds to the eastern part of the Sulu orogen that formed by the Triassic continental collision between the South and North China Blocks. Four plutons were investigated in this region, with the Linglong and Guojialing plutons from the northwestern part (Jiaobei) and the Kunyushan and Sanfoshan plutons from the southeastern part (Jiaodong). The results show that these granitoids mostly have high Sr, low Yb and Y contents, high (La/Yb)N and Sr/Y ratios with negligible to positive Eu anomalies (Eu/Eu* = 0.69–1.58), which are similar to common adakites. On the other hand, they have relatively low MgO, Cr, Ni contents and thus low Mg#. Zircon U–Pb dating yields Late Jurassic ages of 141 ± 3 to 157 ± 2 Ma for the Linglong and Kunyushan plutons, but Early Cretaceous ages of 111 ± 2 to 133 ± 3 Ma for the Guojialing and Sanfoshan plutons. Some zircon cores from the Linglong and Kunyushan granitoids have Neoproterozoic U–Pb ages. All the granitoids have variably negative zircon εHf(t) values of ?39.6 to ?5.4, with Mesoproterozoic to Paleoproterozoic Hf model ages of 1515 ± 66 to 2511 ± 97 Ma for the Sanfoshan pluton, but Paleoproterozoic to Paleoarchean Hf model ages of 2125 ± 124 to 3310 ± 96 Ma for the other three plutons. These indicate that the Mesozoic granitoids formed in the postcollisional stage and were derived mainly from partial melting of the subducted South China Block that is characterized by Paleoproterozoic juvenile crust and Neoproterozoic magmatic rocks along its northern edge. However, there are some differences between the Jiaobei and Jiaodong plutons. Compared to the Jiaodong granitoids, the Jiaobei granitoids have very old zircon Hf model ages of 3310 ± 96 Ma suggesting the possible involvement of a Paleoarchean crust that may be derived from the North China Block. Therefore, the continental collision between the two blocks would bring crustal materials from both sides into the subduction zone in the Triassic, yielding subduction-thickened crust as the magma source for the adakite-like granitoids. While lithospheric extension and orogenic collapse are considered a major cause for postcollisional magmatism, anatexis of the subducted mafic crust is proposed as a mechanism for chemical differentiation of the continental crust towards felsic composition.  相似文献   

15.
The enigmatic Arequipa Massif of southwestern Peru is an outcrop of Andean basement that underwent Grenville-age metamorphism, and as such it is important for the better constraint of Laurentia–Amazonia ties in Rodinia reconstruction models. U–Pb SHRIMP zircon dating has yielded new evidence on the evolution of the Massif between Middle Paleoproterozoic and Early Paleozoic. The oldest rock-forming events occurred in major orogenic events between ca. 1.79 and 2.1 Ga (Orosirian to Rhyacian), involving early magmatism (1.89–2.1 Ga, presumably emplaced through partly Archaean continental crust), sedimentation of a thick sequence of terrigenous sediments, UHT metamorphism at ca. 1.87 Ga, and late felsic magmatism at ca. 1.79 Ga. The Atico sedimentary basin developed in the Late-Mesoproterozoic and detrital zircons were fed from a source area similar to the high-grade Paleoproterozoic basement, but also from an unknown source that provided Mesoproterozoic zircons of 1200–1600 Ma. The Grenville-age metamorphism was of low-P type; it both reworked the Paleoproterozoic rocks and also affected the Atico sedimentary rocks. Metamorphism was diachronous: ca. 1040 Ma in the Quilca and Camaná areas and in the San Juán Marcona domain, 940 ± 6 Ma in the Mollendo area, and between 1000 and 850 Ma in the Atico domain. These metamorphic domains are probably tectonically juxtaposed. Comparison with coeval Grenvillian processes in Laurentia and in southern Amazonia raises the possibility that Grenvillian metamorphism in the Arequipa Massif resulted from extension and not from collision. The Arequipa Massif experienced Ordovician–Silurian magmatism at ca. 465 Ma, including anorthosites formerly considered to be Grenvillian, and high-T metamorphism deep within the magmatic arc. Focused retrogression along shear zones or unconformities took place between 430 and 440 Ma.  相似文献   

16.
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   

17.
The medium- to high-grade polymetamorphic basement rocks of the Peloritani Mountains, northern Sicily, include large volumes of augen gneiss of controversial age and origin. By means of a geochemical and SHRIMP zircon study of representative samples, the emplacement age of the original granitoid protoliths of the augen gneisses and the most likely processes and sources involved in that granitoid magmatism have been determined. U–Pb dating of three samples from widely spaced localities in the Peloritani Mountains yielded igneous protolith ages of 565 ± 5, 545 ± 4 and 545 ± 4 Ma, respectively. These late Ediacaran/early Cambrian ages are much older than was previously assumed on geological grounds, and are typical of the peri-Gondwanan terranes involved in the geodynamic evolution of the northern Gondwana margin at the end of the Avalonian–Cadomian orogeny. Major and trace element compositions and Sr–Nd isotopic data, in combination with zircon inheritance age patterns, suggest that the granitoid protoliths of the Sicilian and coeval Calabrian augen gneisses were generated by different degrees of mixing between sediment- and mantle-derived magmas. The magmas forming the ca. 545 Ma inheritance-rich granitoids appear to have had a significant contribution from partial melting of paragneiss that is the dominant rock type in the medium- to high-grade Peloritanian basement. The closeness of the inferred deposition age of the greywacke protoliths of the paragneisses with the intrusion age of the granitoids indicates rapid latest Precambrian crustal recycling involving erosion, burial, metamorphism to partial melting conditions, and extensive granitoid magmatism in less than ca. 10 Ma.  相似文献   

18.
A 40Ar/39Ar geochronological study was performed on amphibole and biotite from some representative units of distinct tectonic domains of the southeastern Guiana Shield, north of the Amazonian Craton, the Amapá Block and the Carecuru Domain. In the Amapá Block, an Archean continental block involved in the Transamazonian orogenesis (2.26–1.95 Ga), the investigated minerals, from rocks of the Archean high-grade basement assemblage, give only Paleoproterozoic ages, indicating their complete resetting during the Transamazonian orogenic event. Amphibole ages vary from 2087 ± 3 to 2047 ± 20 Ma, and biotite ages spread mainly between 2079 ± 18 and 2033 ± 13 Ma. In the Carecuru Domain, in which the geodynamic evolution is related to Paleoproterozoic magmatic arc setting during the Transamazonian event, calc-alkaline granitoids yield amphibole age of 2074 ± 17 Ma, and biotite ages of 1928 ± 19 Ma and 1833 ± 13 Ma.These data reinforce the importance of the Transamazonian orogenic cycle in the investigated area, and indicate that the rocks were not significantly affected by post-Transamazonian events. When coupled with available U–Th–Pb monazite and Pb–Pb zircon geochronological records and petro-structural observations, the new 40Ar/39Ar data delineate contrasting cooling and exhumation histories for the tectonic domains. In the Amapá Block, the data suggest nearly vertical T–t paths that reflect fast cooling rates, which indicate tectonically controlled exhumation, related to collisional stages of the Transamazonian event, between 2.10 and 2.08 Ga. Conversely, in the Carecuru Domain, low cooling rates suggest that the arc-related granitoids underwent slow and monotonous cooling since their emplacement until reaching the biotite isotopic closure temperature.  相似文献   

19.
The Pan-African Lufilian belt (Zambia and Democratic Republic of Congo) is known for its world-class copper and cobalt deposits. In addition, the Lufilian Copperbelt hosts several uranium occurrences concentrated within deformed siliciclastic rocks of the basal Neoproterozoic Katanga Supergroup. We report LA-ICPMS and EMP analyses of the rare earth element (REE) and yttrium (Y) abundances (designated as the REY signatures) of uranium oxides from two uranium mineralizing events of the Lufilian belt previously dated at 652 ± 8 Ma and 530 ± 6 Ma by the U–Pb method on uraninite. Uranium oxides dated at ca. 650 Ma from the External fold-and-thrust belt are characterized by (i) bell shape REE patterns centered on middle REE (MREE), (ii) positive europium (Eu) anomalies and (iii) relatively low Y contents. In contrast, uranium oxides dated at ca. 530 Ma from the Domes region are characterized by (i) REE patterns but with a less pronounced light REE (LREE) fractionation, (ii) negative Eu anomalies and (iii) higher Y contents. Moreover, the External fold-and-thrust belt also contains uranium mineralization dated at ca. 530 Ma having the same characteristics as the ca. 530 Ma uranium oxides from the Domes region (a moderately fractionated REE pattern and a negative Eu anomaly).As REY signatures are known to reflect mineralizing processes, the distinct geochemical signatures of the two uranium oxide generations (ca. 650 Ma and ca. 530 Ma) provide meaningful information about the uranium cycle during the Pan-African orogeny. Compared to the REY signatures of the known worldwide uranium deposit types, the REY signature of uranium oxides dated at ca. 650 Ma of the External fold-and-thrust belt is similar to the REE patterns from unconformity-related U deposits (Athabasca in Canada and Kombolgie in Australia). Uranium oxides of the Domes region and some of the External fold-and-thrust belt display similar characteristics to syn-metamorphic U deposit (Mistamisk in Canada). Accordingly, we propose that the two stages of uranium oxide crystallizations within the Lufilian belt, at ca. 650 and ca. 530 Ma, occurred under distinct physico-chemical conditions. The first stage, at ca. 650 Ma, may be related to late diagenesis hydrothermal processes, at the basement/cover interface, with the circulation of highly saline basinal brines linked to evaporites of the Roan Group. This Pan-African unconformity-related uranium deposit is the youngest of this type described to date. The second stage may be connected to metamorphic fluid circulations, at about 530 Ma, during the Lufilian orogeny in the Domes region and also in the External fold-and-thrust belt.  相似文献   

20.
The Sergipano Belt is the outcome of collision between the Pernambuco–Alagoas Massif and the São Francisco Craton during Neoproterozoic assembly of West Gondwana. Field relationships and U–Pb geochronology of granites intruded in garnet micaschists of the Macururé Domain are used to constrain the main collisional event (D2) in the belt. The granites are divided into two groups, the pre-collisional granites (pre- to early-D2) and the syn-collisional granites (syn- to tardi-D2), the latter were emplaced as sheets along the S2 axial plane foliation or they were collected at the hinge zones of F2 folds. A U–Pb SHRIMP zircon age of 628 ± 12 Ma was obtained for the pre-collisional Camará tonalite. Two U–Pb TIMS titanite ages were obtained for the syn-collisional granites, 584 ± 10 Ma for the Angico granite and 571 ± 9 Ma for the Pedra Furada granite, and these ages are close to the garnet-whole rock Sm–Nd isochron of 570 Ma found for the peak of metamorphism in the Sergipano Belt. The ages of the Camará tonalite (628 Ma) and the Pedra Furada granite (571 Ma) mark respectively the maximum age for beginning of the D2 event and minimum age for the end in the Macururé Domain. Using these ages, the main Neoproterozoic D2 collisional event has been in operation in the Sergipano Belt for at least 57 million years. Correlation with coeval granitoids farther north in the Borborema Province indicate that while in the Sergipano Belt the syn-D2 granites (ca. 590–570 Ma) were emplaced under compression, in the Borborema Province they emplaced under extensional conditions related to regional strike-slip shear zones. These contrasting emplacement settings for contemporaneous Neoproterozoic granitoids are explained by a combination of continent–continent collision and extrusion tectonics.  相似文献   

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