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1.
ABSTRACT

The Neoproterozoic tectonic evolution of the Jiangnan Orogen is controversial, with one of the issues being whether the ca. 850–820-Ma granitoids were generated by mantle plumes or the collision between the Yangtze and Cathaysia blocks. This paper tackles this problem by examining the age and petrogenesis of one of the granitoids, the Getengling pluton in the central Jiangnan Orogen, and through comparison with a regional geochronological–geochemical database compiled from previous studies. The Getengling pluton is characterized by high A/CNK values (~1.5), slight negative whole-rock εNd(t) values (?2.8 to ?3.4), and positive zircon εHf(t) values (0.7 ± 1.1), suggesting S-type granite affinities with juvenile contributions. Rb/Sr, Rb/Ba, and high CaO/Na2O ratios indicate psammitic sources with both clay-rich and clay-poor characters. These geochemical characteristics are distinct from those of the granitoids (typically of A type) associated with mantle plumes. The zircon laser ablation-inductively coupled plasma-mass spectrometry U–Pb age of 845 ± 4 Ma obtained in this study, together with other ca. 835–820 Ma ages of S-type granites in the Jiangnan Orogen, indicates that the felsic magmatism in the Jiangnan Orogen lasted for ca. 25 Ma, which is longer than typical plume-related felsic magmatism. In addition, the mafic rocks in the Jiangnan Orogen and elsewhere in the South China Block are geochemically distinct from the coeval mantle plume-related ones in Australia and west Laurentia. In geochemical diagrams diagnostic of tectonic settings, the Getengling pluton and other ca. 850–820 Ma intrusions plot in the syn- and post-collisional fields, whereas the pre-850 and post-820-Ma igneous rocks plot in the arc and within-plate settings, respectively. This sequential tectonic evolution from plate subduction through collision to within-plate environments further supports the hypothesis that the ca. 850–820-Ma granitoids in the Jiangnan Orogen resulted from the Yangtze–Cathaysia collision rather than from mantle pluming.  相似文献   

2.
《International Geology Review》2012,54(15):1876-1886
ABSTRACT

The Neoproterozoic glaciations represent a milestone in the Earth evolution due to their influence on atmosphere, biosphere and hydrosphere. Evidence for the Sturtian glaciation, the early stage of Cryogenian, has been recorded worldwide, but the precise timing and synchroneity of its counterpart, the Chang’an glaciation, in South China have been controversial. As such, new zircon U–Pb ages from the pre-Sturtian Gongdong Formation and the overlying the Chang’an Formation in southeastern Yangtze Block were reported. The youngest U–Pb zircon age from a tuff sample of the topmost Gongdong Formation was 716.8 ± 6.8 Ma, and that from a sandstone sample of the lower Chang’an Formation was 725.9 ± 4.4 Ma. The zircon weighted mean age of 716.8 ± 6.8 Ma was interpreted as the maximum depositional age of the termination of the Danzhou Group. This age, along with the ages reported from the bottom of the Danzhou Group, constrains deposition of the Danzhou Group to between ca. 820 Ma and ca. 715 Ma. The age of 716.8 ± 6.8 Ma from the top of the Gongdong Formation is consistent with the SIMS U-Pb age of 715.9 ± 2.8 Ma from the Sibao section, as well as ages from the Banxi Group, Liantuo Group, and Kaijianqiao Formation in the Yangtze Block, which further constrain the onset time of the Sturtian glaciation in South China at ca. 715 Ma. It is also, with uncertainties, consistent with ages from pre-Sturtian strata in Laurentia and Oman, which indicates a global synchroneity and extent for the Sturtian glaciation.  相似文献   

3.
The North Qilian Orogenic Belt (NQOB), which consists of ophiolitic mélange and island-arc assemblages containing many granites, blueschists, and eclogites, lies between the Alax and Qilian terranes in northwestern China. The Minleyaogou and Niuxinshan granitoids occur at the northern and southern margins, respectively, in the middle segment of the NQOB. The Minleyaogou pluton is granodiorite in composition, whereas the Niuxinshan pluton consists mainly of red granite with minor grey quartz diorite. Geochemically, the Minleyaogou granite differs from the Niuxinshan granite in that it contains a smaller range in SiO2, has lower total alkalis, and is more peraluminous. Both granitoids are magnesian but the Niuxinshan granite is alkali-calcic, whereas the Minleyaogou granodiorite is calcic. Both granitoids have similar chondrite-normalized rare earth element patterns with light rare earth element enrichment and negative Eu anomalies. They have pronounced negative Ba, Nb, Sr, P, and Ti anomalies indicating that they have an affinity to island-arc or active continental margin magmatism. SHRIMP U–Pb dating of zircons from the granitoids yields a formation age of 477 Ma for the Niuxinshan granite and 463 Ma for the Minleyaogou granodiorite. These ages, combined with the geochemistry and locations of the plutons, suggest that they formed by the double subduction of the North Qilian oceanic plate during early Palaeozoic time. Formation of the Niuxinshan granite may be related to southward subduction under the Qilian terrane at 477 Ma, whereas the Minleyaogou granodiorite was formed by northward subduction at 463 Ma under the Alax terrane.  相似文献   

4.
Detrital zircon from two basement blocks (Kubor and Bena Bena) in the central Highlands of Papua New Guinea has an age signature that strongly suggests a northern Australian provenance. Samples of the Omung Metamorphics, southeastern Kubor Block, together yield principal zircon populations with ages of ca 1.8 Ga (～10% of the total), ca 1.55 Ga (～10%), 470–440 Ma (～15%), ca 340 Ma (～10%) and 290–260 Ma (～40%).Two tonalite stocks of the Kubor Intrusive Complex, which intrude the Omung Metamorphics, yield indistinguishable ages of 244.8 ± 4.9 Ma and 239.1 ± 4.2 Ma.Therefore, the deposition and subsequent deformation of the Omung Metamorphics is Late Permian to Early Triassic. A sample of Goroka Formation (Bena Bena Block) contains detrital zircon of similar ages to the Omung Metamorphics, ca 1.8 Ga (5%), ca 1.55 Ga (～45%), ca 430 Ma (～5%) and ca 310 Ma (～40%), suggesting that the Goroka Formation has a similar provenance and might be correlative. In contrast, a metapsammite from the Bena Bena Formation yielded only ages of 290–280 Ma (85%) and ca 240 Ma (15%). A tuff interbedded in the Bena Bena Formation yielded only igneous zircon with a Late Triassic age of 221 ± 3 Ma. Contrary to previous interpretations, the Bena Bena Formation is probably younger than the Goroka Formation. Ages of New Guinea detrital zircon closely match those of igneous and detrital zircon from the Coen Inlier, northeastern Queensland, but contrast with the ages of zircon from terranes further south, east and west. The Kubor and Bena Bena Blocks are not suspect terranes, but rather form part of the Australian craton. The craton margin, modified by rifting during the Mesozoic, was re‐inverted during Cenozoic compression. The Australian craton, in the eastern Highlands of Papua New Guinea, extends at least as far north as the Markham Valley, the northern edge of the Bena Bena terrane.  相似文献   

5.
Northwestern Fujian contains abundant well-studied Precambrian basement, and was a composite terrane in Cathaysia during the Neoproterozoic; however, its magmatic activity, petrogenesis, and tectonic evolution remain controversial. This article focuses on the geochronology and geochemistry of the Neoproterozoic Group in order to resolve the above problems. We provide new SHRIMP U-Pb zircon dating for the Mamianshan Group: 851.9 ± 9.2 to 825.5 ± 9.8 Ma for the Longbeixi Formation, 796.5 ± 9.3 Ma for the Dongyan Formation, and 756.2 ± 7.2 Ma for the Daling Formation. These ages document the existence of Neoproterozoic magmatism in the northwestern Cathaysia Block. Dongyan Nd-Sr isotopic data show that mafic amphibolite schists, mafic greenschists, and quartzofeldspathic schists were derived from a more depleted mantle (initial εNd ? +5.5 and 87Sr/86Sr ratio 0.703409643), a mixture of depleted mantle and crustal components (initial εNd ? ?1 and 87Sr/86Sr ratio 0.702045282–0.704147714), and late Palaeoproterozoic continental crustal materials (initial εNd < ?1 and 87Sr/86Sr ratio 0.71083603), respectively. These new data, together with previous studies, suggest a bi-subduction-collision orogenic model for the Neoproterozoic evolution of the Yangtze and Cathaysia blocks. Our plate tectonic scenario involves earlier NW-dipping subduction during 1.0 Ga–860 Ma along the southeastern margin of the Yangtze Block and later NW-dipping subduction near the northwestern margin of the Cathaysia Block starting at ca. 850 Ma. The 796.5 ± 9.3 Ma age of the volcanic Dongyan Formation suggests that the final assembly of the Yangtze and Cathaysia blocks probably occurred after ca. 800 Ma. The 756.2 ± 7.2 Ma age of the Daling Formation indicates that post-orogenic extensional magmatism took place after 800 Ma along the northwestern margin of Cathaysia.  相似文献   

6.
《International Geology Review》2012,54(12):1492-1509
ABSTRACT

The Biarjmand granitoids and granitic gneisses in northeast Iran are part of the Torud–Biarjmand metamorphic complex, where previous zircon U–Pb geochronology show ages of ca. 554–530 Ma for orthogneissic rocks. Our new U–Pb zircon ages confirm a Cadomian age and show that the granitic gneiss is ~30 million years older (561.3 ± 4.7 Ma) than intruding granitoids (522.3 ± 4.2 Ma; 537.7 ± 4.7 Ma). Cadomian magmatism in Iran was part of an approximately 100-million-year-long episode of subduction-related arc and back-arc magmatism, which dominated the whole northern Gondwana margin, from Iberia to Turkey and Iran. Major REE and trace element data show that these granitoids have calc-alkaline signatures. Their zircon O (δ18O = 6.2–8.9‰) and Hf (–7.9 to +5.5; one point with εHf ~ –17.4) as well as bulk rock Nd isotopes (εNd(t) = –3 to –6.2) show that these magmas were generated via mixing of juvenile magmas with an older crust and/or melting of middle continental crust. Whole-rock Nd and zircon Hf model ages (1.3–1.6 Ga) suggest that this older continental crust was likely to have been Mesoproterozoic or even older. Our results, including variable zircon εHf(t) values, inheritance of old zircons and lack of evidence for juvenile Cadomian igneous rocks anywhere in Iran, suggest that the geotectonic setting during late Ediacaran and early Cambrian time was a continental magmatic arc rather than back-arc for the evolution of northeast Iran Cadomian igneous rocks.  相似文献   

7.
New zircon LA-ICP-MS U–Pb age, zircon Hf isotope, and whole-rock major and trace elemental data of the Late Cretaceous Ageledaban complex in the Karakorum Terrane (KKT), northwest Tibet, provide new constraints on the tectonic processes of the collision and thickening of the terrane between the Lhasa and Qiangtang terranes. The granitoids from the Ageledaban complex have a variable SiO2 content, from 62.83 to 73.35 wt.% and A/CNK<1.1 (except for YM61-2). They have rare earth element and trace element patterns that are enriched in light rare earth elements, Rb, Pb, Th, and U, and are depleted in Ba, P, Sr, Ti, and Nb, indicative of weakly peraluminous-metaluminous I-type affinity. Zircon U–Pb dating reveals that the Ageledaban complex was emplaced at ca. 80 Ma. Zircons from the monzogranite and monzonite samples with concordant 206Pb/238U ages about 80 Ma have a zircon εHf(t) of ?6.6 to ?1.1, corresponding to the Mesoproterozoic Hf crustal model ages (TDMC = 1.2–1.6 Ga); the remaining inherited zircons from the monzonite with concordant 206Pb/238U ages of about 108.1 Ma have εHf(t) values that range from ?8.3 to ?5.0, corresponding to the Mesoproterozoic Hf crustal model ages with an average of 1.6 Ga. These signatures indicate that the Ageledaban granitoids may have been derived from the partial melting of a mixed mantle-crust source. Together with the age and geochemical data in the literature, we propose that the collisional event in the KKT in northwestern Tibet would postdate the northern Lhasa–southern Qiangtang collision, which occurred first in the Amdo in the east and later in the Shiquanhe in central Tibet. Our results support the previous view that the collision of the Bangong–Nujiang suture zone (BNSZ) may be diachronous.  相似文献   

8.
The Palaeoproterozoic Hekou Group, an outcrop along the SW-margin of the Yangtze Block, consists of volcanic and sedimentary rocks that experienced greenschist facies metamorphism and was intruded by gabbroic and granitic plutons. The sedimentary rocks consist of coarse to fine-grained siliciclastic and carbonate rocks including quartzite, mica schists, polymictic meta-conglomerates and marble, whereas volcanic rocks consist of sodic lava and pyroclastic rocks including albitites, interbeded metatuffs, and metabasalts. Metatuffs from five layers have zircon U–Pb age of 1710 ± 18 Ma (MSWD = 1.6), 1637 ± 7 Ma (MSWD = 0.65), 1601 ± 15 Ma (MSWD = 0.94), 1661 ± 7 Ma (MSWD = 1.4), and 1718 ± 11 Ma (MSWD = 0.3) and these ages show that the Hekou Group deposited at ~1.7–1.6 Ga. The high content of light rare earth element (LREE), the low content of highrare earth element (HREE) and negative Ti anomalies, relatively high content of incompatible fluid-insoluble elements (Nb, Ta, and Th), and the high varied εNd(t) values (?6.0 to +4.6) of the metavolcanic rocks show that these rocks are formed in back-arc basin. Our study also implies that the Yangtze Block also underwent subduction-related, continental margin accretion on its SW-margin during the growth of the Nuna supercontinent at ~1.7–1.6 Ga.  相似文献   

9.
The Batouri gold mining area in southeastern Cameroon is part of the Adamawa–Yadé Domain of the Central African Fold Belt (Pan-African). It is underlain by a variety of granitic rocks, including alkali-feldspar granite, syeno-monzogranite, granodiorite, and tonalite. Geochemical data suggest that these rocks formed by differentiation of I-type tonalitic magma under oxidizing conditions in a continental volcanic arc setting. U–Pb dating of zircons from gold-associated monzogranite-granodiorite at Kambélé gave concordant ages of 619 ± 2 and 624 ± 2 Ma, while Ar–Ar dating of alkali-feldspar granite yielded a non-plateau maximum age of 640–620 Ma. These ages imply that the Batouri granitoids were emplaced during the collision of the West African Craton and the Congo Craton.

The geochemical characteristics of the Batouri granitoids as well as their oxidized state (magnetite series) are typical of gold-associated felsic rocks in subduction settings elsewhere. The similarities in age, composition, and geochemical affinities of these granitoids with those reported from other localities in the Adamawa–Yadé Domain reinforce the earlier assumption that the granitic rocks of this domain represent parts of a regional-scale batholith, with commonly small-scale, high-grade auriferous quartz veins in structurally favourable sites. The spatial and temporal association of gold mineralization and the Batouri granitoids may suggest potential for regional-scale, high-tonnage, granite-related gold ore.  相似文献   

10.
张喜  高俊  董连慧  李继磊  江拓  钱青  苏文 《岩石学报》2011,27(6):1637-1648
新疆中天山乔霍特铜矿位于中天山南缘,毗邻南天山缝合带。矿区南侧出露有1个花岗闪长岩岩体,该岩体与包裹于赋矿火山岩中的钾长花岗岩均属钙碱性弱过铝质I型花岗岩,具有相似的地球化学特征,富集LILE、亏损HFSE,具显著的Eu、Ta、Nb、Ti负异常,其形成可能与南天山洋的北向俯冲密切相关。LA-ICP-MS锆石U-Pb定年获得花岗闪长岩年龄为450.4±1.1Ma,钾长花岗岩年龄为430.8±4.1Ma,指示晚奥陶世时期,乔霍特地区存在南天山洋向中天山复合弧地体之下的俯冲;早志留世晚期,俯冲作用依然持续,此时,天山地区岩浆活动强烈。乔霍特铜矿赋矿火山岩的形成时代晚于431Ma,矿区南侧出露的花岗闪长岩早于赋矿火山岩形成,成矿作用可能与花岗闪长岩的侵位无直接关系。  相似文献   

11.
The Jurassic ophiolites in the South Apuseni Mountains represent remnants of the Neotethys Ocean and belong to the East Vardar ophiolites that contain ophiolite fragments as well as granitoids and volcanics with island-arc affinity. New U–Pb zircon ages, and Sr and Nd isotope ratios give insights into their tectono-magmatic history. The ophiolite lithologies show tholeiitic MOR-type affinities, but are occasionally slightly enriched in Th and U, and depleted in Nb, which indicates that they probably formed in a marginal or back-arc basin. These ophiolites are associated with calc-alkaline granitoids and volcanics, which show trace element signatures characteristic for subduction-enrichment (high LILE, low HFSE). Low 87Sr/86Sr ratios (0.703836–0.704550) and high 143Nd/144Nd ratios (0.512599–0.512616) of the calc-alkaline series overlap with the ratios measured in the ophiolitic rocks (0.703863–0.704303 and 0.512496–0.512673), and hence show no contamination with continental crust. This excludes a collisional to post-collisional origin of the granitoids and is consistent with the previously proposed intra-oceanic island arc setting. The new U–Pb ages of the ophiolite lithologies (158.9–155.9 Ma, Oxfordian to Early Kimmeridgian) and granitoids (158.6–152.9 Ma, latest Oxfordian to Late Kimmeridgian) indicate that the two distinct magmatic series evolved within a narrow time range. It is proposed that the ophiolites and island arc granitoids formed above a long-lived NE-dipping subduction zone. A sudden flip in subduction polarity led to collision between island arc and continental margin, immediately followed by obduction of the ophiolites and granitoids on top of the continental margin of the Dacia Mega-Unit. Since the granitoids lack crustal input, they must have intruded the Apuseni ophiolites before both magmatic sequences were obducted onto the continental margin. The age of the youngest granitoid (~153 Ma, Late Kimmeridgian) yields an estimate for the maximum age of emplacement of the South Apuseni ophiolites and associated granitoids onto the Dacia Mega-Unit.  相似文献   

12.
扬子克拉通保存的独特拉伸纪晚期碎屑岩建造为研究新元古代中期演化提供了绝佳素材,内部丰富的凝灰岩夹层和同位素年龄也为扬子克拉通新元古代中期演化提供有效的年代学约束。然而扬子克拉通拉伸系莲沱组内部演化缺少年代学约束,制约了拉伸纪晚期扬子克拉通的沉积演化研究。本研究通过野外地质调查,采用LA-ICP-MS技术获取莲沱组底界(神农架)凝灰岩锆石U-Pb谐和年龄为763.1±6.2 Ma,莲沱组下部陆相地层中获得(鹤峰)的凝灰岩锆石U-Pb谐和年龄761.8±7.1 Ma,莲沱组中下部海陆过渡段获得通山(764.1±3.5 Ma)、长阳(751.5±6.3 Ma)、神农架(752.1±6.5 Ma)三组凝灰岩锆石U-Pb谐和年龄。莲沱组顶部海相地层获得729.6±9.2 Ma(皖南休宁组)、722.4±4.5 Ma(神农架)凝灰岩锆石U-Pb谐和年龄,以及城口龙潭河组凝灰岩锆石U-Pb谐和年龄(712.4±6.4 Ma)。这些凝灰岩锆石年龄数据和地层序列表明：820～770 Ma扬子克拉通普遍处于暴露剥蚀环境,770～750 Ma开始沉积陆相-海陆过渡相莲沱组,750 Ma之后扬子克拉通过渡到海...  相似文献   

13.
On the basis of U-Pb, Rb-Sr and K-Ar isotope analyses of Proterozoic rocks and minerals, a chronology has been established for the tectonic, intrusive and metamorphic evolution of the Svecokarelian orogeny 1750–1950 Ma ago in the Uppsala Region, Eastern Sweden. It is suggested that when synkinematic granitoids intruded the orogenic belt, at a stage of general subsidence and at medium metamorphic conditions (600°C and 3.5–4 kbar), the U-Pb isotope system in zircons closed earlier than the Rb-Sr whole-rock system. The zircon age (1886 Ma) reflects the intrusion and crystallization of the rock melt and the Rb-Sr whole-rock age (1830 Ma) the time when the temperature had decreased below the threshold for 87Sr migration. The Rb-Sr whole rock age (1898 Ma) determined for metaandesites and metadacites reflects a recrystallization related to the intrusion of the granitoids. On the contrary, the more silicic metarhyodacitic volcanic rocks have a Rb-Sr whole rock age (1830 Ma) reflecting the cessation of the synkinematic metamorphism. The difference in the way the Rb-Sr isotope system responds in subsilicic or silicic metavolcanics is probably dependent on the amount of radiogenic 87Sr and on the fixation of 87Sr in Ca-rich minerals. Subsequent, late-kinematic, low amphibolite facies metamorphism has not altered the Rb-Sr ages of the granitoids and the recrystallized metavolcanics.  相似文献   

14.
The subduction polarity and related arc–magmatic evolutional history of the Bangong–Nujiang Ocean, which separated the South Qiangtang terrane to the north from the North Lhasa terrane to the south during the Mesozoic, remain debated. This study tries to reconstruct the subduction and evolution of the Bangong–Nujiang Ocean on the basis of U–Pb and Hf isotopic analyses of detrital zircons in samples from sedimentary rocks of the middle-western section of the Bangong–Nujiang suture zone in Gerze County, central Tibet. The Middle Jurassic Muggargangri Group in the Bangong–Nujiang suture zone was deposited in a deep-sea basin setting on an active continental margin. The Late Jurassic strata, such as the Sewa Formation, are widely distributed in the South Qiangtang terrane and represent deposition on a shelf. The Early Cretaceous Shamuluo Formation in the Bangong–Nujiang suture zone unconformably overlies the Muggargangri Group and was probably deposited in a residual marine basin setting. The detrital zircons of the Muggargangri Group contain seven U–Pb age populations: 2.6–2.4 Ga, 1.95–1.75 Ga, 950–900 Ma, 850–800 Ma, 650–550 Ma, 480–420 Ma, and 350–250 Ma, which is similar to the age populations in sedimentary rocks of the South Qiangtang terrane. In addition, the age spectra of the Shamuluo Formation are similar to those of the Muggargangri Group, indicating that both had a northern terrane provenance, which is conformed by the north-to-south palaeocurrent. This provenance indicates northward subduction of the Bangong–Nujiang oceanic crust. In contrast, two samples from the Sewa Formation yield variable age distributions: the lower sample has age populations similar to those of the South Qiangtang terrane, whereas the upper possesses only one age cluster with a peak at ca. 156 Ma. Moreover, the majority of the late Mesozoic detrital zircons are characterized by weakly positive εHf(t) values that are similar to those of magmatic zircons from arc magmatic rocks in the South Qiangtang terrane. The findings, together with information from the record of magmatism, indicate that the earliest prevalent arc magmatism occurred during the Early Jurassic (ca. 185 Ma) and that the principal arc–magmatic stage occurred during the Middle–Late Jurassic (ca. 170–150 Ma). The magmatic gap and scarcity of detrital zircons at ca. 140–130 Ma likely indicate collision between the Qiangtang and Lhasa terranes. The late Early Cretaceous (ca. 125–100 Ma) magmatism on both sides of the Bangong–Nujiang suture zone was probably related to slab break-off or lithospheric delamination after closure of the Bangong–Nujiang Ocean.  相似文献   

15.
This paper presents data on the geological position, geochemistry, age, and isotopic characteristics of the granitoids of the southern part of the Voznesenka terrane, Southern Primorye (Muraviev–Amursky Peninsula and its vicinities). All of the studied granitoids were formed in three stages: the Ordovician, Silurian, and Permian. The Silurian and Permian ages of the granitoid intrusions have been previously determined (Ostrovorussky Massif, 432–422 Ma, and 250 ± 4 Ma, early and late associations, respectively; Sedanka massif, 261 ± 3 Ma). The granites of the Artem and Nadezhdinsky massifs define an U–Pb zircon age of 481 ± 6 and 452 ± 4 Ma, respectively. The geochemical and isotope data show mainly the crustal nature of the granitoids. Their formation was related to melting of relatively immature rocks of the continental crust (mafic–intermediate volcanic rocks). The Nd isotope composition of the granitods (TNd(DM–2) = 1.3 Ga) indicates the absence of the mature ancient crust at the basement of the southern Voznesenka terrane. The maximum contribution of mantle sources to the granite formation is recorded in the Permian associations. A comparison of the peaks of intrusive magmatism in the southern part of the Voznesenka terrane and adjacent territories suggests that the formation of the granitoids of the Muraviev–Amursky Peninsula and its vicinities was caused by the interaction of continental blocks with two oceanic basins: the Paleoasian (and its fragments) and Paleopacific ones.  相似文献   

16.
U–Pb zircon analyses from three meta-igneous and two metasedimentary rocks from the Siviez-Mischabel nappe in the western Swiss Alps are presented, and are used to derive an evolutionary history spanning from Paleoarchean crustal growth to Permian magmatism. The oldest components are preserved in zircons from metasedimentary albitic schists. The oldest zircon core in these schists is 3.4 Ga old. Detrital zircons reveal episodes of crustal growth in the Neoarchean (2.7–2.5 Ga), Paleoproterozoic (2.2–1.9 Ma) and Neoproterozoic (800–550 Ma, Pan-African event). The maximum age of deposition for the metasedimentary rocks is given by the youngest detrital zircons within both metasedimentary samples dated at ~490 Ma (Cambrian-Ordovician boundary). This is in the age range of two granitoid samples dated at 505 ± 4 and 482 ± 7 Ma, and indicates sedimentation and magmatism in an extensional setting preceding an Ordovician orogeny. The third felsic meta-igneous rock gives a Permian age of intrusion, and is part of a long-lasting Variscan to post-Variscan magmatic activity. The zircons record only minor disturbance of the U–Pb system during the Alpine orogeny.  相似文献   

17.
Determining an age framework for Precambrian crystalline rocks and associated granulite-facies metamorphism of the inner blocks in the North China Craton (NCC) is important for determining the tectonic setting and evolution of the craton during the Neoarchaean–Palaeoproterozoic. The Eastern Hebei terrane (EHT), located in the Eastern Block of the NCC, is composed of tonalitic-trondhjemitic-granodioritic (TTG) gneisses and potassium-rich granitoids, along with rafts of supracrustal rocks that are intruded by basic dikes. TTG gneisses in the EHT yield crystallization ages of 2516–2527 Ma. The oldest age of inherited zircons from a mylonitic TTG gneiss is ~2918 Ma. Granulite-facies supracrustal metamorphic rocks in the Zunhua high-grade meta-greenstone belt indicate an andesitic/basaltic protolith that was formed at ~2498 Ma. A syn-deformational granite in the Jinchangyu greenschist-facies shear zone yields a crystallization age of ~2474 Ma. Metamorphism of the supracrustal rocks and mylonitic greenschist took place at ~2461 and ~2475 Ma, respectively. Rare earth elements (REE) patterns and slightly negative Nb and Ta anomalies indicate that the magmatic precursors of the supracrustal rocks might be derived from partial melting of a sub-arc mantle wedge and metasomatized by fluids derived from a subducting slab. These rocks plot in the island arc basalts (IAB) field on a La/Nb vs. La diagram, further supporting this interpretation. The microstructures of a garnet–two-pyroxene granulite indicate an approximately clockwise P-T path. The crystallization ages of the TTG gneisses represent periods of the major crustal growth in the NCC, and the granulite- and greenschist-facies metamorphism indicates an orogenic event that involved crustal thickening at ~2.47 Ga.  相似文献   

18.
《International Geology Review》2012,54(11):1382-1397
ABSTRACT

The number of migmatization events in the Xolapa Complex and their absolute age are controversial. U–Pb dating by laser ablation–inductively coupled plasma–mass spectrometry was performed on zircon grains from migmatites to investigate the age of different textural domains. Rare-earth element (REE) partition coefficients between zircon and garnet were compared with those established for different temperatures in order to test for equilibrium growth. Two age domains were identified. In one sample where zircon and garnet coexist, the outer zircon overgrowths yield a mean age of 54.16 ± 0.29 Ma (mean square weighted deviation (MSWD) = 3.5), whereas intermediate zones, between the core and outer overgrowths, yield an age of 122.7 ± 1.8 Ma (MSWD = 2.5). Partition coefficients were calculated for REEs between coexisting garnet (two different populations) and zircon using (1) the composition of ca. 54 Ma zircon overgrowths and garnet rims and (2) zircon intermediate zones together with garnet cores. The cores of small garnet grains (garnet A) may have grown in equilibrium with zircon domains of ca. 122 Ma. Both garnet cores and rims of the larger porphyroblasts (garnet B) seem to be in equilibrium with ca. 54 Ma zircon overgrowths. Petrographic observations suggest that crystallization of garnet A occurred during partial melting, placing equilibrium growth and therefore a first migmatitic event during the Early Cretaceous at ca. 122 Ma. This migmatitic event may be related to the collision of the Chortís Block with western Mexico. A second migmatitic event of ca. 54 Ma is suggested by equilibrium growth of large garnets (group B) and the outer zircon overgrowths. The high geothermal gradient necessary for this second migmatitic event might be related to the exhumation of the Xolapa Complex, as a result of the transpression and tectonic transport of the Chortís Block to the southeast from the end of the Mesozoic to most of the Cenozoic.  相似文献   

19.
The recently discovered Taolaituo porphyry Mo deposit and Aobaotu hydrothermal vein Pb–Zn deposit are both located in the Great Xing’an Range, Northeast China. Here we present new zircon U–Pb ages, whole-rock geochemical and Pb isotopic data, and molybdenite Re–Os ages for these two deposits. The Mo mineralization in the Taolaituo area occurred in quartz porphyry, which yields zircon U–Pb ages ranging from 138.5 ± 0.8 to 139.1 ± 0.5 Ma. Fine-grained granite representing pre-mineralization magmatic activity was formed at 145.2 ± 0.5 Ma. Molybdenite Re–Os dating indicates that Mo mineralization occurred at 133.8 ± 1.2 Ma. In the Aobaotu deposit, the ore-related granodioritic porphyry has a zircon U–Pb age of 140.0 ± 0.4 Ma. These geochronological data indicate that these magmatic and hydrothermal activities occurred during the Early Cretaceous. The mineralogical and geochemical features of the Taolaituo and Aobaotu granitoids suggest they can be classified as A1-type within-plate anorogenic granites and I-type granites, respectively. The Pb isotopic compositions suggest a mixed crust–mantle origin of the granitoids in these two deposits. The Taolaituo granitoids were formed by the partial melting of lower crust and crust–mantle interaction, with subsequent fractionation of apatite, feldspar, Ti-bearing phases and allanite or monazite. In contrast, the Aobaotu granites were derived primarily from lithospheric mantle that had been transformed or affected by the addition of subduction-related components. Combined with the regional geology, tectonic evolution and available age data from the literature, our results suggest that the Early Cretaceous (140–100 Ma) was likely to be the most important peak period for metallogenic mineralization in Northeast China. The Taolaituo and Aobaotu deposits formed under an extensional environment at an active continental margin in response to subduction of the Palaeo-Pacific oceanic plate.  相似文献   

20.
In northwestern California, the Franciscan subduction complex has been subdivided into seven major tectonostratigraphic units. We report U-Pb ages of ≈2400 detrital zircon grains from 26 sandstone samples from 5 of these units. Here, we tabulate each unit’s interpreted predominant sediment source areas and depositional age range, ordered from the oldest to the youngest unit. (1) Yolla Bolly terrane: nearby Sierra Nevada batholith (SNB); ca. 118 to 98 Ma. Rare fossils had indicated that this unit was mostly 151–137 Ma, but it is mostly much younger. (2) Central Belt: SNB; ca. 103 to 53 Ma (but poorly constrained), again mostly younger than previously thought. (3) Yager terrane: distant Idaho batholith (IB); ca. 52 to 50 Ma. Much of the Yager’s detritus was shed during major core complex extension and erosion in Idaho that started 53 Ma. An Eocene Princeton River–Princeton submarine canyon system transported this detritus to the Great Valley forearc basin and thence to the Franciscan trench. (4) Coastal terrane: mostly IB, ±SNB, ±nearby Cascade arc, ±Nevada Cenozoic ignimbrite belt; 52 to <32 Ma. (5) King Range terrane: dominated by IB and SNB zircons; parts 16–14 Ma based on microfossils. Overall, some Franciscan units are younger than previously thought, making them more compatible with models for the growth of subduction complexes by progressive accretion. From ca. 118 to 70 Ma, Franciscan sediments were sourced mainly from the nearby Sierra Nevada region and were isolated from southwestern US and Mexican sources. From 53 to 49 Ma, the Franciscan was sourced from both Idaho and the Sierra Nevada. By 37–32 Ma, input from Idaho had ceased. The influx from Idaho probably reflects major tectonism in Idaho, Oregon, and Washington, plus development of a through-going Princeton River to California, rather than radical changes in the subduction system at the Franciscan trench itself.  相似文献   

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