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1.
Nd model ages using depleted mantle (TDM) values for the sedimentary rocks in the Inner Zone of the SW Japan and western area of Tanakura Tectonic Line in the NE Japan allow classification into five categories: 2.6–2.45, 2.3–2.05, 1.9–1.55, 1.45–1.25, and 1.2–0.85 Ga. The provenance of each terrane/belt/district is interpreted on the basis of the TDMs, 147Sm / 144Nd vs. 143Nd / 144Nd relation, Nd isotopic evolution of the source rocks in East China and U–Pb zircon ages. The provenance of 2.6–1.8 Ga rocks, which are reported from Hida–Oki and Renge belts and Kamiaso conglomerates, is inferred to be the Sino–Korean Craton (SKC). The 2.3–1.55 Ga rocks, mostly from Ryoke, Mino and Ashio belts, are originally related with the SKC and/or Yangtze Craton (YC). The provenances of the sedimentary rocks with 1.45–0.85 Ga, from the Suo belt, Higo and some districts in the Mino and Ashio belts, are different from the SKC and YC. Especially, the Higo with 1.2–0.85 Ga is considered as a fragment of collision zone in East China. Akiyoshi belt probably belongs to the youngest age category of 1.2–0.85 Ga.Some metasedimentary rocks from the Ryoke belt have extremely high 147Sm / 144Nd and 143Nd / 144Nd ratios, whose main components are probably derived from mafic igneous rocks within the Ryoke belt itself and from the adjacent Tamba belt.  相似文献   

2.
Detrital zircon U–Pb ages, whole-rock Nd isotopic, and geochemical data of metasedimentary rocks from the Wutai Complex in the Central Zone, North China Craton, have been determined. Compositionally, these rocks are characterized by a narrow variation in SiO2/Al2O3 (2.78–3.96, except sample 2007-1), variable Eu anomalies, spanning a range from significantly negative Eu anomalies to slightly positive anomalies (Eu/Eu* = 0.58–1.12), and positive ε Nd (t) values (0.1–1.97). The 18 detrital zircons of one sample yielded age populations of 2.53 Ga, 2.60 Ga, and 2.70–2.85 Ga. Geochemical data reveal intermediate source weathering, varying degrees of K-metasomatism in the majority of these metasedimentary rocks, whereas other secondary disturbances seem to be negligible. Detailed analysis in detrital zircon U–Pb geochronology, whole-rock Nd isotope, and geochemistry shows that these metasedimentary rocks are derived from a mixed provenance. The predominant derivation is from the late Archean granitoids and metamorphic volcanics in the Wutai Complex, and there is also input of older continental remnants, except TTG gneisses, from the Hengshan and Fuping Complexes. The sediments were probably deposited in fore-arc or/and intra-arc basins within an arc system.  相似文献   

3.
This paper presents the results of Sm-Nd isotopic-geochemical and U-Pb geochronological studies of metamorphic (Ilikan Sequence) and associated igneous rocks from the Ilikan lithotectonic zone (terrane) located in the Dzhugdzhur-Stanovoi Superterrane from the Central Asian Foldbelt. The Nd model age, T Nd(DM), of metamorphic rocks from the Ilikan Sequence is 2.6–3.2 Ga pointing to the likelihood that the lower boundary of their protolith formation probably does not exceed 2.6 Ga. The age of detrital zircons from metasedimentary rocks of the Ilikan Sequence is 2700–2900 Ma, which absolutely agrees with Sm-Nd isotopic-geochemical results. The U-Pb zircon age of metagabbro that intruded the rocks of the Ilikan Sequence and underwent high-temperature amphibolite metamophism with subsequent structural transformations is 2635 ± 4 Ma. The obtained results allow us to conclude that the age of the Ilikan Sequence is 2630–2700 Ma. All this gives grounds to state that the Dzhugdzhur-Stanovoi Superterrane in the Central Asian Foldbelt was formed due to amalgamation of non-Siberian terranes as is assumed for the Argun, Bureya, and Mamynskii terranes of the Amur Superterrane from the Central Asian Foldbelt.  相似文献   

4.
扬子地块东南缘沉积岩的Nd同位素研究   总被引：16，自引：1，他引：15  
周泰禧  徐祥 《沉积学报》1995,13(3):39-45
扬子地块东南缘上溪群分布区及其周边沉积岩的Ｎｄ同位素研究结果,支持存在一条苏浙皖古生代裂陷槽（或江南深断裂）的观点。上溪群以北直至长江边所分布的震旦系－古生代的盖层沉积岩,其Ｎｄ模式年龄有两组,表明物源区不同。裂陷槽以北,沉积岩的物源区为Ｎｄ模式年龄约２.０～２.１Ｇａ的扬子物源区;以南的沉积岩表现出明显的幔源物质混染,显示出元古代岩浆活动的影响,而上溪群分布区以南直到江绍断裂附近主要表现上溪物源区的影响,华夏地块古老基底岩石则无显著贡献。  相似文献   

5.
The Borborema Province has three major subprovinces. The northern subprovince lies north of the Patos shear zone and is comprised of Paleoproterozoic cratonic basement with Archean nuclei, plus overlying Neoproterozoic supracrustal rocks and Brasiliano plutonic rocks. The central subprovince occurs between the Patos and Pernambuco shear zones and is mainly comprised of the Zona Transversal. The southern subprovince occurs between the Pernamabuco shear zone and the São Francisco craton and is comprised of a tectonic collage of various blocks, terranes, or domains ranging in age from Archean to Neoproterozoic. This report focuses on the Zona Transversal, especially on Brasiliano rocks for which we have the most new information.Paleoproterozoic gneisses with ages of 2.0–2.2 Ga occur discontinuously throughout the Zona Transversal. The Cariris Velhos suite consists of metavolcanic, metasedimentary, and metaplutonic rocks yielding U–Pb zircon ages of 995–960 Ma. This suite is mainly confined to a 100 km wide belt that extends for more than 700 km within the Alto Pajeú terrane. Sm–Nd model ages in metaigneous rocks cluster about 1.3–1.6 Ga, indicating that older crust was involved in genesis of their magmas. Brasiliano supracrustal rocks dominate the Piancó-Alto Brígida terrane, and they probably also constitute significant parts of the Alto Pajeú and Rio Capibaribe terranes. They are only slightly older than early stages of Brasiliano plutonism, with detrital zircon ages at least as young as 620 Ma; most TDM ages range from 1.2 to 1.6 Ga.Brasiliano plutons range from ca. 640 to 540 Ma, and their TDM ages range from 1.2 to 2.5 Ga. Previous workers have shown significant correlations among U–Pb ages, Sm–Nd model ages, petrology, and geochemistry, and we are able to reinforce and extend these correlations. Stage I plutons formed 640–610 Ma and have TDM ages less than 1.5 Ga. Stage II (610–590 Ma) contains few plutons, but coincides with the peak of compressional deformation, metamorphism, and formation of migmatites. Stage III plutons (590 to ca. 575 Ma) have older TDM ages (ca. 1.8–2.0 Ga), as do Stage IV plutons (575 to ca. 550 Ma; TDM from 1.9 to 2.4 Ga). Stage III plutons formed during the transition from compressional to transcurrent deformation, while Stage IV plutons are mainly post-tectonic. Stage V plutons (550–530 Ma) are commonly undeformed (except along younger shear zones) and have A-type geochemistry. The five stages have distinct geochemical properties, which suggest that the tectonic settings evolved from early, arc-related magma-genesis (Stage I) to within-plate magma-genesis (Stage V), with perhaps some intermediate phases of extensional environments.  相似文献   

6.
B. Seth  S. Jung  B. Gruner   《Lithos》2008,104(1-4):131-146
Three dating techniques for metamorphic minerals using the Sm–Nd, Lu–Hf and Pb isotope systems are combined and interpreted in context with detailed petrologic data from crustal segments in NW Namibia. The combination of isochron ages using these different approaches is a valuable tool to testify for the validity of metamorphic mineral dating. Here, PbSL, Lu–Hf and Sm–Nd garnet ages obtained on low- to medium-grade metasedimentary rocks from the Central Kaoko Zone of the Neoproterozoic Kaoko belt (NW Namibia) indicate that these samples were metamorphosed at around 550–560 Ma. On the other hand, granulite facies metasedimentary rocks from the Western Kaoko Zone underwent two phases of high-grade metamorphism, one at ca. 660–625 Ma and another at ca. 550 Ma providing substantial evidence that the 660–625 Ma-event was indeed a major tectonothermal episode in the Kaoko belt. Our age data suggest that interpreting metamorphic ages by applying a single dating method only is not reliable enough when studying complex metamorphic systems. However, a combination of all three dating techniques used here provides a reliable basis for geochronological age interpretation.  相似文献   

7.
We report quantitative X-ray diffraction, whole-rock geochemical and Sm–Nd isotope data for metabasalts from the Aspiring and Torlesse Terranes in the South Island of New Zealand. These rocks underlie the Mesozoic metasedimentary Otago Schist which is anomalously enriched in gold and host to at least one world-class orogenic gold deposit at Macraes (> 125 t Au). Geochemical and Nd isotopic similarities between the samples point to a common history of the two terranes rather than the amalgamation of one or two allochthonous plate fragments. Furthermore, geochemical and Nd data suggest the metabasalts within both terranes formed in an oceanic but essentially non-subduction-related setting. The origin of the Aspiring and Torlesse basalts can be linked to the formation of an oceanic plateau that had resulted from a (?Permian) mantle plume initiation event proximal to a mid-oceanic rise or triple junction. Given the intrinsically gold-enriched nature of certain oceanic-character mafic rocks, the anomalous gold endowment of the Otago Schist may have been enhanced via the accretion and subduction of a gold-enriched oceanic plateau fragment. The metabasalts are generally enriched in gold (up to 13 ppb) compared to their enclosing metasedimentary rocks (typically ca. 1 ppb), with sulphide-rich metabasaltic rocks having up to 550 ppb Au. However, the relatively small volume of metabasalts in the Otago Schist precludes these rocks as the principal source for Otago Schist orogenic gold, with a primarily metasedimentary source of the gold potentially having a limiting effect on the overall endowment of the Otago Schist. This approach, that employs petrogenetic fingerprinting of potentially fertile source rocks for the assessment of gold endowment, might prove useful in the conceptual exploration targeting of relatively immature and poorly exposed terrains.  相似文献   

8.
 详细的主元素和Sm-Nd同位素体系研究表明,扬子块体南缘元宝山地区四堡群中镁铁质-超镁铁质岩可能来源于Al亏损地幔;而宝坛地区镁铁质-超镁铁质岩的源区则可能包括了A1未亏损和A1亏损两种地幔端元组分,部分样品可能受到围岩混染。镁铁质-超镁铁质岩的Sm-Nd数据构成了一条无地质意义的假等时线,由其斜率获得的年龄约2.2Ga明显偏老。四堡群浅变质沉积岩的Nd模式年龄限定了镁铁质-超镁铁质岩和四堡群的地层年龄应小于1.8Ga.扬子南缘最老的基底四堡群(及相应地层)主要是由地壳存留年龄为1.8-1.9Ga的未成熟陆壳再循环物质组成,明显不同于华南块体(华夏古陆)的早-中元古代变质基底。迄今为止获得的沉积岩和花岗岩的Sm-Nd同位素资料都不支持扬子南缘存在早元古代-晚太古代基底。  相似文献   

9.
Summary The rocks of the crystalline basement of the East European Craton in southern Estonia show effects of partial melting under granulite facies conditions. Zircons extracted from partial melting products (tonalite from the Tapa Zone – 1824 ± 26, tonalite from the South Estonian Zone – 1788 ± 16 Ma and charnockite from the Tapa Zone – 1761 ± 11 Ma) yield U–Pb crystallisation ages that span over approximately 80 Ma, suggesting a prolonged high-grade metamorphism or several separate events. U–Pb zircon age of one sample of charnockite is concordant with the Nd model age of partial melting of its host mafic granulite facies gneiss (intercept at 1.76 Ga). Linear geochemical trends and similar initial Nd isotopic compositions of mafic granulites and charnockites suggest their possible genetic relationship. From our new and previously published data it follows that the peak granulite metamorphic conditions and formation of tonalites and charnockites (850 °C and 6 kbar) in the Estonian basement occurred at 1788–1778 Ma. Then, the rocks cooled down, passing through the garnet closure temperature of approximately 650–700 °C at 1728 ± 24 Ma. The age of metamorphism of the Estonian granulites is lower than the metamorphic ages known from southern Finland, but it is similar to the age of metamorphism reported from the Belarus-Baltic Granulite Belt in Latvia.  相似文献   

10.

Nd isotope studies of the oldest metasedimentary rocks from the Wonominta Block, western New South Wales reveal that these samples have a model age (TDM) of 1780–2010 Ma, slightly younger than that of low‐grade Willyama Supergroup metasediments (1920–2160 Ma), and significantly younger than those ages previously reported from high‐grade rocks of the Broken Hill Block (2200–2300 Ma). These differences have important implications for tectonic reconstruction in this region and support a model of transitional tectonics from the Broken Hill to Wonominta Blocks, as suggested by earlier geochemical studies of mafic rocks. Those studies revealed that the mafic rocks from the basal sequence of the Wonominta Block may have formed in a back‐arc basin, developed from a propagating rifting, an environment contiguous to that in which Willyama Supergroup was deposited. These results also carry significant implications for tectonic reconstruction of eastern Australia.  相似文献   

11.
The results of Sm–Nd studies of metasedimentary rocks from the Tukuringra terrane, one of the largest units in the structure of the eastern part of the Mongolian–Okhotsk mobile belt, are reported. Metasedimentary rocks of the Algainskaya, Garmakanskaya, and Teploklyuchevskaya formations are characterized by similar model ages tNd(DM) = 1.5–1.1 Ga. This shows that the major protoliths of metasedimentary rocks of the terrane are characterized by Mesoproterozoic estimates of the Nd model ages. The results obtained allow us to assume that introduction of terrigenous material into the sedimentary basin mostly occurred from the continental massifs united into the Amur superterrane, since their magmatic and sedimentary complexes are characterized by identical values of the Nd model ages.  相似文献   

12.
The Central Hebei Basin （CHB） is one of the largest sedimentary basins in the North China Craton, extending in a northeast-southwest direction with an area of 〉350 km2. We carried out SHRIMP zircon dating, Hf-in-zircon isotopic analysis and a whole-rock geochemical study on igneous and metasedi- mentary rocks recovered from drill holes that penetrated into the basement of the CHB, Two samples of gneissic granodiorite （XG1-1） and gneissic quartz diorite 048-1） have magmatic ages of 2500 and 2496 Ma, respectively. Their zircons also record metamorphic ages of 2.41-2.51 and ～2.5 Ga, respec- tively. Compared with the gneissic granodiorite, the gneissic quartz diorite has higher REE contents and lower Eu/Eu＊ and （La/Yb）n values. Two metasedimentary samples （MG1, H5） mainly contain ～2,5 Ga detrital zircons as well as late Paleoproterozoic metamorphic grains. The zircons of different origins have eHf （2.5 Ga） values and Hf crustal model ages ranging from 0 to 5 and 2.7 to 2,9 Ga, respectively, Therefore, ～2.5 Ga magmatic and Paleoproterozoic metasedimentary rocks and late Neoarchean to early Paleoproterozoic and late Paleoproterozoic tectono-thermal events have been identified in the basement beneath the CHB. Based on regional comparisons, we conclude that the early Precambrian basement beneath the CHB is part of the North China Craton.  相似文献   

13.
Whole-rock geochemical analyses using major and trace elements in combination with the Sm–Nd and Pb–Pb isotope systems, together with SHRIMP age dating on metasedimentary rocks from the Sierras de Chepes, the Sierras de Córdoba, the Sierra Norte and the San Luis Formation in the Sierra de San Luis, have been carried out to unravel the provenance and the geodynamic history of the Eastern Sierras Pampeanas, Central Argentina. The geochemical and the Sm–Nd data point to a slightly stronger mafic and less-fractionated material in the provenance area of the Sierras de Córdoba when compared to the other units. The TDM model ages from the Sierras de Chepes (~1.82 Ga) and the Sierra Norte (~1.79 Ga) are significantly older than the data from the Sierras de Córdoba (1.67 Ga). The Pb data are homogeneous for the different units. Only the 208Pb/204Pb ratios of some samples from the Sierras de Córdoba are higher. A late Pampean detrital zircon peak around 520 Ma from the Sierras de Chepes is in accordance with the new data from the San Luis Formation. This is similar to the literature data from the Famatina Belt located to the northwest of the Sierras de Chepes and also fits the detrital zircon peaks in the Mesón group. These maximum depositional ages were also reported from some locations in the Puncoviscana Formation but are absent in the Sierras de Córdoba. An improved model for the development of the Eastern Sierras Pampeanas in the area between the Sierras de Córdoba and the Puncoviscana Formation is provided. This gives new insights into the late Pampean development of the Sierra de San Luis and the complex development of the Eastern Sierras Pampeanas. This new model explains the younger detrital ages in the Puncoviscana Formation compared with the older ages of the Sierras de Córdoba. Another model of the Sierra de San Luis explains the younger depositional ages of the Pringles Metamorphic Complex and the San Luis Formation when compared to the Nogolí Metamorphic Complex and the Conlara Metamorphic Complex. Additionally, the rather fast change of the high-grade metamorphic conditions in the Pringles Metamorphic Complex and the low-grade metamorphic conditions in the San Luis Formation is explained by extension, the ascent of (ultra) mafic material and later folding and erosion.  相似文献   

14.
The Peramora Mélange is part of an accretionary complex between the South Portuguese Zone (a fragment of Laurussia) and the Ossa Morena Zone (para-autochthonous Gondwana) and is an expression of the Pangean suture zone in southwestern Iberia. The suture zone is characterized by fault-bounded units of metasedimentary rocks, mélanges, and mafic complexes. Detailed geologic mapping of the Peramora Mélange reveals a complex pattern of imbricated schists and mafic block-in-matrix mélanges. Geochemical signatures of the Pulo do Lobo schist (PDL) are consistent with derivation from both mafic and continental sources. The mafic block-in-matrix mélange displays normal mid-ocean ridge basalt (NMORB) geochemical signature, juvenile Sm–Nd isotopic compositions, and a range of zircon ages similar to those observed in the PDL, suggesting a sedimentary component. Taken together, these data suggest a complex tectonic history characterized by erosion of a NMORB source, mélange formation, and imbrication during underplating occurring during the final stages of continent–continent collision.  相似文献   

15.
The paper presents geological, geochemical, and isotopic data on metamorphic rocks in the Anuy block (dome) in the Northern Sikhote-Alin and the surrounding sedimentary rocks of the Samarka accretionary prism. The geochemistry and isotopic composition of the amphibolite-facies metamorphic rocks (variably migmatized gneisses and crystalline schists) in the Anuy block and unmetamorphosed Jurassic-Cretaceous sediments surrounding the block are proved to be similar. All of them corresponded to the erosion products of the transitional-type crust (mature island arcs and active continental margins), have similar major- and trace-element compositions, and Nd model ages of 1.25–1.4 Ga. The geochemistry and isotopic parameters of metapelites in the Anuy block are principally different from those of analogous rocks in the Khanka Massif (the latter rocks are erosion products of the mature crust and have a Nd model age of 1.7–1.9 Ga). The metabasites, which are found as beds and lenses in gneisses and crystalline schists in the Anuy block and among sedimentary rocks surrounding the block, have a composition corresponding to oceanic basalts of the N- and E-MORB types. Based on the synthesis of geological, geochemical and isotopic data it was suggested that the Anuy block could be not a fragment of the basement of an ancient continent (as was believed previously) but rather a complex of the Early Cretaceous granite-metamorphic core of the Cordilleran type.  相似文献   

16.
The Orós belt is a metamorphosed and deformed supracrustal sequence whose deposition started at ca. 1.8 Ga. The volcanic rocks form an essentially bimodal association with a predominance of felsic volcanics. Mafic volcanics show geochemical and Nd isotopic differences which point to separate origins for each type. The mafic rocks are either chemically similar to EMORBs or are transalkaline types, enriched in LILE and LREE and relatively depleted in HFSE but with different isotopic signatures. One mafic type is associated with dominant andesites in a suite which could have evolved by an AFC process involving a Transamazonian-age source rock which later produced some of the overlying felsic volcanics. The rhyolites have variable geochemical signatures, all typical of anatectic products derived from continental crustal rocks. Sedimentary rocks are dominated by pelitic types of different provenances, accompanied by minor arenites and lesser carbonate rocks and calc-silicates. The depositional environment of the supracrustal sequence was continental, and a number of lines of evidence suggest that a rift environment probably developed during relaxation following the Transamazonian orogeny. The most abundant igneous rocks are felsic plutonics, emplaced nearly 100 Ma after the volcanic activity, but whose geochemical signature is similar to that of most of the rhyolites. These are cut by more alkaline anorogenic intrusives. The volcano-sedimentary sequence was intruded at ca. 0.9 Ga by a mafic-ultramafic sill. All deformation and metamorphism occurred during the Brasiliano orogeny, which was accompanied by the intrusion of syn-tectonic granites.The Orós belt, therefore, represents an intracontinental ensialic late Paleoproterozoic volcano-sedimentary basin initially related to strain relaxation of the crust. With subsequent collapse and development of faults, the anorogenic alkaline granites intruded at ca. 1.7 Ga. Based on the data available on this province, the Orós belt forms a part of a series of supracrustal belts of different ages, which were consolidated and welded in the Brasiliano orogeny.Within the Borborema Province, supracrustal sequences with ages of ≈ 2.0-1.9 Ga, ≈ 1.8-1.7 Ga (Orós), ≈ 1.1-1.0 Ga, and ≈ 0.6 Ga are presently known. All were aglutinated or amalgamated to neighboring blocks during the Brasiliano orogenic cycle.  相似文献   

17.
We report U-Pb crystallization ages from four metavolcanic rocks and two granitic gneiss samples as well as whole-rock chemical analyses and Sm-Nd isotopic ratios from 25 metaigneous and metasedimentary rocks from the Chopawamsic and Milton terranes, southern Appalachian Orogen. A metarhyolite sample from the Chopawamsic Formation and a metabasalt sample from the Ta River Formation in the Chopawamsic terrane have indistinguishable U-Pb crystallization ages of 471.4+/-1.3 Ma and 470.0+1.3/-1.5 Ma, respectively. A sample from the Prospect granite that intruded metavolcanic rocks of the Ta River Formation yields a younger U-Pb date of 458.0+/-1 Ma. Metarhyolite and granitic gneiss samples from the northern part of the Milton terrane yield U-Pb dates of 458.5+3.8/-1.0 Ma and 450+/-1.8 Ma, respectively. Metavolcanic and metaplutonic rocks from both terranes span a range in major element composition from basalt to rhyolite. Trace element concentrations in these samples show enrichment in large-ion lithophile elements K, Ba, and Rb and depletion in high field strength elements Ti and Nb, similar to those from island arc volcanic rocks. Initial epsilon(Nd) values and T(DM) ages of the metaigneous and metasedimentary samples range from 0.2 to -7.2 and from 1200 to 1700 Ma for the Chopawamsic terrane and from 3.7 to -7.2 and from 850 to 1650 Ma for the Milton terrane. The crystallization ages for the metavolcanic and metaplutonic samples from both terranes indicate that Ordovician magmatism occurred in both. Similar epsilon(Nd) values from representative samples from both terranes suggest that both were generated from an isotopically similar source. Xenocrystic zircons from metavolcanic rocks in the Chopawamsic terrane have predominately Mesoproterozoic (207)Pb/(206)Pb ages (600-1300 Ma), but a single Archean (2.56 Ga) core was also present. The xenocrystic zircons and the generally negative epsilon(Nd) values indicate that both terranes are composed of isotopically evolved continental crust.  相似文献   

18.
The Borborema Province in northeastern South America is a typical Brasiliano-Pan-African branching system of Neoproterozoic orogens that forms part of the Western Gondwana assembly. The province is positioned between the São Luis-West Africa craton to the north and the São Francisco (Congo-Kasai) craton to the south. For this province the main characteristics are (a) its subdivision into five major tectonic domains, bounded mostly by long shear zones, as follows: Médio Coreaú, Ceará Central, Rio Grande do Norte, Transversal, and Southern; (b) the alternation of supracrustal belts with reworked basement inliers (Archean nuclei + Paleoproterozoic belts); and (c) the diversity of granitic plutonism, from Neoproterozoic to Early Cambrian ages, that affect supracrustal rocks as well as basement inliers. Recently, orogenic rock assemblages of early Tonian (1000–920 Ma) orogenic evolution have been recognized, which are restricted to the Transversal and Southern domains of the Province.Within the Transversal Zone, the Alto Pajeú terrane locally includes some remnants of oceanic crust along with island arc and continental arc rock assemblages, but the dominant supracrustal rocks are mature and immature pelitic metasedimentary and metavolcaniclastic rocks. Contiguous and parallel to the Alto Pajeú terrane, the Riacho Gravatá subterrane consists mainly of low-grade metamorphic successions of metarhythmites, some of which are clearly turbiditic in origin, metaconglomerates, and sporadic marbles, along with interbedded metarhyolitic and metadacitic volcanic or metavolcaniclastic rocks. Both terrane and subterrane are cut by syn-contractional intrusive sheets of dominantly peraluminous high-K calc-alkaline, granititic to granodioritic metaplutonic rocks. The geochemical patterns of both supracrustal and intrusive rocks show similarities with associations of mature continental arc volcano-sedimentary sequences, but some subordinate intra-plate characteristics are also found.In both the Alto Pajeú and Riacho Gravatá terranes, TIMS and SHRIMP U–Pb isotopic data from zircons from both metavolcanic and metaplutonic rocks yield ages between 1.0 and 0.92 Ga, which define the time span for an event of orogenic character, the Cariris Velhos event. Less extensive occurrences of rocks of Cariris Velhos age are recognized mainly in the southernmost domains of the Province, as for example in the Poço Redondo-Marancó terrane, where arc-affinity migmatite-granitic and meta-volcano-sedimentary rocks show U–Pb ages (SHRIMP data) around 0.98–0.97 Ga. For all these domains, Sm–Nd data exhibit TDM model ages between 1.9 and 1.1 Ga with corresponding slightly negative to slightly positive εNd(t) values. These domains, along with the Borborema Province as a whole, were significantly affected by tectonic and magmatic events of the Brasiliano Cycle (0.7–0.5 Ga), so that it is possible that there are some other early Tonian rock assemblages which were completely masked and hidden by these later Brasiliano events.Cariris Velhos processes are younger than the majority of orogenic systems at the end of Mesoproterozoic Era and beginning of Neoproterozoic throughout the world, e.g. Irumide belt, Kibaride belt and Namaqua-Natal belt, and considerably younger than those of the youngest orogenic process (Ottawan) in the Grenvillian System. Therefore, they were probably not associated with the proposed assembly of Rodinia. We suggest, instead, that Cariris Velhos magmatism and tectonism could have been related to a continental margin magmatic arc, with possible back-arc associations, and that this margin may have been a short-lived (<100 m.y.) leading edge of the newly assembled Rodinia supercontinent.  相似文献   

19.
Granulite rocks are exposed in eastern Minas Gerais, Brazil. Its early neoproterozoic evolution is characterised by a history of an active continental margin, including the accretion of suspect terranes. The Manhuaçu Terrane is one of those which is represented by a granitic continental plutonic arc and terrigeneous metasediments reflecting a continental margin. A metasedimentary gneiss belt at this margin with shallow to deep marine clastic lithologies as well as metavolcanic and metaplutonic mafic rocks was interpreted as an extensive tectonic segment with suspect development in a back-arc setting. Fragments of a volcanic arc are identified and interpreted as an evidence for a probable island-arc domain. The granulites occur as massive rocks as well as high-grade gneisses and show lithological, structural and metamorphic attributes consistent with their host belt type. In the western portion granulites derived from sedimentary protoliths, have been deposited, deformed and metamorphosed together with the mafic intrusions and as well as with their crystallization. Regional uplift exposed these rocks probably immediately after the metamorphism. In these belts the metamorphic grade is not uniform, especially where uplifting has exposed oblique cross sections over the granulitic rocks. Geothermobarometric calculations indicate that the granulites has been generated under T conditions between 800 and 990 ± 50 C and from medium (4.8 kb) to relatively high (10.0 kb) pressures.  相似文献   

20.
Abstract

The diverse geological and geophysical data sets compiled, interrogated and interpreted for the largely undercover southern Thomson Orogen region reveal a Paleozoic terrane dominated by deformed metasedimentary rocks intruded by S- and I-type granites. An interpretive basement geology map and synthesis of geochronological constraints allow definition of several stratigraphic packages. The oldest and most widespread comprises upper Cambrian to Lower Ordovician metasedimentary rocks deposited during the vast extensional Larapinta Event with maximum depositional ages of ca 520 to ca 496 Ma. These units correlate with elements of the northern Thomson Orogen, Warburton Basin and Amadeus Basin. The degree of deformation and metamorphism of these rocks varies across the region. A second major package includes Lower to Middle Devonian volcanic and sedimentary units, some of which correlate with components of the Lachlan Orogen. The region also includes a Middle to Upper Ordovician package of metasedimentary rocks and a Devonian or younger package of intermediate volcaniclastic rocks of restricted extent. Intrusive units range from diatremes and relatively small layered mafic bodies to batholithic-scale suites of granite and granodiorite. S-type and I-type intrusions are both present, and ages range from Ordovician to Triassic, but late Silurian intrusions are the most abundant. Two broad belts of intrusions are recognised. In the east, the Scalby Belt comprises relatively young (Upper Devonian) intrusions, while in the west, the Ella Belt is dominated by intrusions of late Silurian age within a curvilinear, broadly east–west trend. The stratigraphic distributions, characteristics and constraints defined by this interpretive basement mapping provide a basic framework for ongoing research and mineral exploration.  相似文献   

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