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1.
How the earth's crust formed and evolved during the Precambrian times is one of the key questions to decipher the evolution of the early Earth. As one of the few cratons containing well-preserved Eoarchean to Neoarchean basement on Earth, the North China Craton is an ideal natural laboratory to unravel the early crustal evolution. It is controversial whether the Archean tectonothermal events in this area represents reworking or growth of the continental crust. To solve this issue, we have compelled field-based mapping, zircon U–Pb dating by SHRIMP RG and LA–ICP–MS U–Pb, zircon SHRIMP SI oxygen and LA–MC–ICP–MS Hf isotope, and whole-rock Nd–O isotope analyses from the Archean granitoids in northern Liaoning, North China Craton. On the basis of zircon U–Pb isotopic dating and measured geological section investigation, two distinct magmatic suites as enclaves in the Jurassic granites are recognized, viz. a newly discovered 3.0 Ga crustal remnant and a 2.5 Ga granitoid. The Mesoarchean zircons from the 3.0 Ga granodioritic gneisses exhibit heterogeneous Hf isotopic compositions, with the most radiogenic analysis (εHf(t) = +3.8) following the depleted mantle evolution array and the most unradiogenic εHf(t) extending down to −3.4. This implies that both ancient continental crust at least as old as 3.4 Ga and depleted mantle contributed to the magma source of the protoliths of the Mesoarchean gneisses. The εHf(t) values of the Neoarchean zircons from these gneisses overlap the 3.4–3.0 Ga zircon evolution trend, indicating that the ancient crustal materials have been reworked during the late Neoarchean. The Neoarchean zircons from the 2.5 Ga granitoids have a relatively small variation in the Hf isotope and are mainly plotted in the 3.0–2.8 Ga zircon evolution field. However, taking all the εHf(t) values of the Neoarchean zircons into the consideration, we find that the Hf model age of the Neoarchean zircon does not represent the time of crustal growth or reworking but are artifacts of magma mixing. The interaction between the magmas derived from the ancient crustal materials and the depleted mantle is also supported by zircon O isotopic data and Hf–O isotopic modeling of the Neoarchean granitoids. Both Mesoarchean and late Neoarchean tectonothermal events involved synchronous crustal growth and reworking, which may be applicable to other parts of the world.  相似文献   

2.
The eastern part of the Guiana Shield, northern Amazonian Craton, in South America, represents a large orogenic belt developed during the Transamazonian orogenic cycle (2.26–1.95 Ga), which consists of extensive areas of Paleoproterozoic crust and two major Archean terranes: the Imataca Block, in Venezuela, and the here defined Amapá Block, in the north of Brazil.

Pb-evaporation on zircon and Sm–Nd on whole rock dating were provided on magmatic and metamorphic units from southwestern Amapá Block, in the Jari Domain, defining its long-lived evolution, marked by several stages of crustal accretion and crustal reworking. Magmatic activity occurred mainly at the Meso-Neoarchean transition (2.80–2.79 Ga) and during the Neoarchean (2.66–2.60 Ga). The main period of crust formation occurred during a protracted episode at the end of Paleoarchean and along the whole Mesoarchean (3.26–2.83 Ga). Conversely, crustal reworking processes have dominated in Neoarchean times. During the Transamazonian orogenic cycle, the main geodynamic processes were related to reworking of older Archean crust, with minor juvenile accretion at about 2.3 Ga, during an early orogenic phase. Transamazonian magmatism consisted of syn- to late-orogenic granitic pulses, which were dated at 2.22 Ga, 2.18 Ga and 2.05–2.03 Ga. Most of the εNd values and TDM model ages (2.52–2.45 Ga) indicate an origin of the Paleoproterozoic granites by mixing of juvenile Paleoproterozoic magmas with Archean components.

The Archean Amapá Block is limited in at southwest by the Carecuru Domain, a granitoid-greenstone terrane that had a geodynamic evolution mainly during the Paleoproterozoic, related to the Transamazonian orogenic cycle. In this latter domain, a widespread calc-alkaline magmatism occurred at 2.19–2.18 Ga and at 2.15–2.14 Ga, and granitic magmatism was dated at 2.10 Ga. Crustal accretion was recognized at about 2.28 Ga, in agreement with the predominantly Rhyacian crust-forming pattern of the eastern Guiana Shield. Nevertheless, TDM model ages (2.50–2.38 Ga), preferentially interpreted as mixed ages, and εNd < 0, point to some participation of Archean components in the source of the Paleoproterozoic rocks. In addition, the Carecuru Domain contains an oval-shaped Archean granulitic nucleus, named Paru Domain. In this domain, Neoarchean magmatism at about 2.60 Ga was produced by reworking of Mesoarchean crust, as registered in the Amapá Block. Crustal accretion events and calc-alkaline magmatism are recognized at 2.32 Ga and at 2.15 Ga, respectively, as well as charnockitic magmatism at 2.07 Ga.

The lithological association and the available isotopic data registered in the Carecuru Domain suggests a geodynamic evolution model based on the development of a magmatic arc system during the Transamazonian orogenic cycle, which was accreted to the southwestern border of the Archean Amapá Block.  相似文献   


3.
Central Amapá, northern Brazil is located at the boundary between: (a) a northern Paleoproterozoic domain, consisting mainly of granite-greenstones terrains and (b) a southern Archean continental block (Amapá block), including an Archean basement reworked during the Transamazonian orogeny (2.26–1.95 Ga). Field investigations, Pb–Pb zircon and Sm–Nd whole rock geochronology supported by geochemical data on granitoids brought further constraints on Paleoproterozoic crustal growth in the southeastern Guyana Shield. A first magmatic episode, dated at 2.26 Ga, is marked by the crystallization of metaluminous low-K tholeiitic tonalites and quartz-diorites, which geochemical affinity with volcanic arc and association with T-MORB amphibolites suggest that they formed in a back-arc basin – island arc system. This event is coeval to the oceanic stage registered in French Guyana during the Eorhyacian (2.26–2.02 Ga). A second magmatic episode is represented by peraluminous, medium- to high-K calc-alkaline tonalite and granodiorite, which revealed some similarities with Mesorhyacian TTG rocks of French Guyana. For granitoids of both episodes, TDM and εNd values indicate the contribution of some Archean crustal component, probably by assimilation or contamination. This second magmatic episode occurred at 2.10 Ga, indicating that the period of successive calc-alkaline magmatic arcs formation may have extended until the Neorhyacian. Meanwhile, during this time, tectonic accretion by collision of the newly formed continental landmass was the prevailing process in French Guyana. The latter magmatic episode, even though poorly constrained, was registered around 2.08–2.02 Ga in central Amapá. It corresponds to the emplacement and solidification of high-K collisional granitoids, produced by partial melting of the Archean continental crust, as testified by the Archean TDM, inherited Pb–Pb zircon ages and strongly negative εNd values. Our results point toward the existence of a protracted episode of crustal growth during the Neorhyacian in the southeastern Guyana Shield. This episode has been predominantly driven by magmatic arc accretion during, at least, 160 My, along the period of 2.26–2.10 Ga. This cycle ended with diachronic closure of the oceanic basins and arc–continent collision.  相似文献   

4.
The composition and formation of the Earth’s primitive continental crust and mantle differentiation are key issues to understand and reconstruct the geodynamic terrestrial evolution, especially during the Archean. However, the scarcity of exposure to these rocks, the complexity of lithological relationships, and the high degree of superimposed deformation, especially with long-lived magmatism, make it difficult to study ancient rocks. Despite this complexity, exposures of the Archean Mairi Gneiss Complex basement unit in the São Francisco Craton offer important information about the evolution of South America’s primitive crust. Therefore, here we present field relationships, LA-ICP-SFMS zircon U-Pb ages, and LA-ICP-MCMS Lu-Hf isotope data for the recently identified Eoarchean to Neoarchean gneisses of the Mairi Complex. The Complex is composed of massive and banded gneisses with mafic members ranging from dioritic to tonalitic, and felsic members ranging from TTG (Tonalite-Trondhjemite-Granodiorite) to granitic composition. Our new data point to several magmatic episodes in the formation of the Mairi Gneiss Complex: Eoarchean (ca. 3.65–3.60 Ga), early Paleoarchean (ca. 3.55–3.52 Ga), middle-late Paleoarchean (ca. 3.49–3.33 Ga) and Neoarchean (ca. 2.74–2.58 Ga), with no records of Mesoarchean rocks. Lu-Hf data unveiled a progressive evolution of mantle differentiation and crustal recycling over time. In the Eoarchean, rocks are probably formed by the interaction between the pre-existing crust and juvenile contribution from chondritic to weakly depleted mantle sources, whereas mantle depletion played a role in the Paleoarchean, followed by greater differentiation of the crust with thickening and recycling in the middle–late Paleoarchean. A different stage of crustal growth and recycling dominated the Neoarchean, probably owing to the thickening of the continental crust by collision, continental arc growth, and mantle differentiation.  相似文献   

5.
The formation and evolution of continental crust in the Early Earth are of fundamental importance in understanding the emergence of continents, their assembly into supercontinents and evolution of life and environment. The Dharwar Craton in southern India is among the major Archean cratons of the world, where recent studies have shown that the craton formation involved the assembly of several micro-continents during Meso- to Neoarchean through subduction-accretion-collision processes. Here we report U-Pb-Hf isotope data from detrital zircons in a suite of metasediments (including quartz mica schist, fuchsite quartzite and metapelite) from the southern domain of the Chitradurga suture zone that marks the boundary between the Western and Central Dharwar Craton. Morphology and internal structure of the zircon grains suggest that the dominant population was derived from proximal granitic (felsic) sources. Zircon U-Pb data are grouped into Paleo-Mesoarchean and Neoarchean to Paleoproterozoic with peaks at 3227 Ma and 2575 Ma. The age spectra of detrital zircon grains, in combination with the Lu-Hf isotopic analyses indicate sediment provenance from magmatic sources with model ages in the range of ca. 3.67 to 2.75 Ga. A transition from dominantly juvenile to a mixture of juvenile and recycled crustal components indicate progressive crustal maturity. The results from this study suggest major crustal growth events during ca. 3.2 Ga and 2.6 Ga in Dharwar. Our study provides insights into continental emergence, weathering and detrital input through river drainage systems into the trench during Eoarchean to Mesoarchean.  相似文献   

6.
华北克拉通存在3.8 Ga以上的形成演化历史,存在4.0~4.1 Ga锆石年龄记录.本文对华北克拉通太古宙变质基底构造热事件进行了综述,重点是事件时限.最古老变质锆石年龄记录为3.71~3.75 Ga和4.0 Ga,为北秦岭造山带西段古生代变质火山-沉积岩中的碎屑或外来锆石.古太古代（~3.3 Ga）构造热事件在鞍山地区广泛存在,导致条带状奥长花岗岩形成.新太古代早期-中太古代晚期变质锆石年龄可进一步划分为两期:2.65~2.85 Ga和~2.6 Ga.2.65~2.85 Ga变质锆石年龄记录存在于胶东、鲁西、鲁山地区.由于后期构造热事件影响,在很多情况下难以确定变质锆石的准确年龄,但>2.65 Ga构造热事件在华北克拉通无疑存在.与2.65~2.85 Ga构造热事件相比,~2.6 Ga构造热事件更为发育,除在鲁西地区广泛存在外,在胶东、鲁山等地也存在.可把2.6 Ga作为华北克拉通新太古代早期和晚期的年龄界线.华北克拉通最重要的太古宙构造热事件出现在新太古代晚期（2.49~2.53 Ga）.所有太古宙岩石分布区,都遭受了这一构造热事件影响.总体上,与华北克拉通南部地区相比,华北克拉通北部地区普遍记录了更高级别变质作用,可能与地壳剥蚀深度不同有关.华北克拉通太古宙变质作用强度和范围随时间演化不断增大,在新太古代晚期达到高潮.与太古宙大陆壳厚度和规模随时间演化不断增大的演化趋势一致.一些地区存在古元古代最早期（2.40~2.47 Ga）甚至更年轻的变质锆石年龄记录,并不意味着构造热事件的真实存在,而是古元古代晚期构造热事件强烈叠加改造的缘故.   相似文献   

7.
华北克拉通具有3.8Ga以上的演化历史,TTG是其地质记录的最重要载体。华北克拉通太古宙(特别是中太古代以前)地质演化在很大程度上与TTG岩石密切相关。在华北克拉通,始太古代(3.6~4.0Ga)TTG岩石仅在鞍本地区被发现,但冀东地区已在多种变质碎屑沉积岩中发现大量3.6~3.88Ga碎屑锆石;古太古代(3.2~3.6Ga)TTG岩石在鞍本、冀东、信阳地区被识别出来;中太古代(2.8~3.2Ga)TTG岩石在鞍本、冀东、胶东、鲁山等地存在;可把新太古代(2.5~2.8Ga)进一步划分为早期和晚期两个阶段:新太古代早期(2.6~2.8Ga)TTG岩石已在10余个地区被发现,新太古代晚期(2.5~2.6Ga)TTG岩石几乎在每一个太古宙基底岩石出露区都存在。野外地质、锆石定年、元素地球化学和Nd-Hf同位素组成研究表明,中太古代以前TTG岩石局部存在,主要分布于Wan et al.(2015)所划分的三个古陆块中;新太古代TTG岩石广泛分布,是陆壳增生最重要时期岩浆作用的产物。TTG岩石类型随时代变化,3.1~3.8Ga和2.7~2.9Ga TTG岩石分别主要为奥长花岗岩和英云闪长岩;2.5~2.6Ga期间花岗闪长岩大规模出现,并有壳源花岗岩广泛分布,表明这时陆壳已有相当的成熟度。奥长花岗岩轻重稀土分异程度从弱到强的时间出现在~3.3Ga;2.5~3.3Ga的TTG岩石轻重稀土分异程度变化很大,表明其形成条件存在很大差异。TTG岩石主要为新生地壳,但也有相当部分为壳内再循环产物或形成过程中受到陆壳物质影响。华北克拉通中太古代以前的主要构造机制是板底垫托或地幔翻转作用,新太古代晚期板块构造体制可能已起作用。  相似文献   

8.
《China Geology》2018,1(1):109-136
The mainland of China is composed of the North China Craton, the South China Craton, the Tarim Craton and other young orogenic belts. Amongst the three cratons, the North China Craton has been studied most and noted for its widely-distributed Archean basement rocks. In this paper, we assess and compare the geology, rock types, formation age and geochemical composition features of the Archean basements of the three cratons. They have some common characteristics, including the fact that the crustal rocks prior to the Paleoarchean and the supracrustal rocks of the Neoarchean were preserved, and Tonalite-Trondhjemtite-Granodiorite (TTG) magmatism and tectono-magmatism occurred at about 2.7 Ga and about 2.5 Ga respectively. The Tarim Craton and the North China Craton show more similarities in their early Precambrian crustal evolution. Significant findings on the Archean basement of the North China Craton are concluded to be: (1) the tectonic regime in the early stage (>3.1 Ga) is distinct from modern plate tectonics; (2) the continental crust accretion occurred mostly from the late Mesoarchean to the early Neoarchean period; (3) a huge linear tectonic belt already existed in the late Neoarchean period, suggesting the beginning of plate tectonics; and (4) the preliminary cratonization had already been completed by about 2.5 Ga. Hadean detrital zircons were found at a total of nine locations within China. Most of them show clear oscillatory zoning, sharing similar textures with magmatic zircons from intermediate-felsic magmatic rocks. This indicates that a fair quantity of continental material had already developed on Earth at that time.  相似文献   

9.
王盟  钱加慧  张进江  张波 《地学前缘》2019,26(3):171-182
恒山五台阜平地区地处华北克拉通中部造山带的中段,早前寒武纪地体出露较好,是解析华北克拉通早前寒武纪演化过程的最佳地段。龙泉关剪切带位于五台杂岩和阜平杂岩的交接部位,其主要岩石类型为眼球状花岗质片麻岩、变粒岩和斜长角闪岩。本文利用LA-ICP-MS方法对龙泉关剪切带中的眼球状花岗质片麻岩进行了锆石U-Pb年龄和Lu-Hf同位素的测试。结果显示,龙泉关眼球状花岗质片麻岩原岩结晶年龄为(2 547±7) Ma,其εHf(t)值介于+2.7~+9.2,表明其源区主要为相对年轻的地壳物质,与五台地区新太古代花岗岩非常相似。龙泉关花岗质眼球状片麻岩中锆石两阶段Hf模式年龄(TDM2)为2 477~2 872 Ma,具有2.58 Ga、2.71 Ga和2.81 Ga三个峰值,记录了华北克拉通新太古代初始地壳生长事件。  相似文献   

10.
Despite representing one of the largest cratons on Earth, the early geological evolution of the Amazonia Craton remains poorly known due to relatively poor exposure and because younger metamorphic and tectonic events have obscured initial information. In this study, we investigated the sedimentary archives of the Carajás Basin to unravel the early geological evolution of the southeastern Amazonia Craton. The Carajás Basin contains sedimentary rocks that were deposited throughout a long period spanning more than one billion years from the Mesoarchean to the Paleoproterozoic. The oldest archives preserved in this basin consist of a few ca. 3.6 Ga detrital zircon grains showing that the geological roots of the Amazonia Craton were already formed by the Eoarchean. During the Paleoarchean or the early Mesoarchean (<3.1 Ga), the Carajás Basin was large and rigid enough to sustain the formation and preservation of the Rio Novo Group greenstone belt. Later, during the Neoarchean, at ca. 2.7 Ga, the southeastern Amazonia Craton witnessed the emplacement of the Parauapebas Large Igneous Province (LIP) that probably covered a large part of the craton and was associated with the deposition of some of the world largest iron formations. The emplacement of this LIP immediately preceded a period of continental extension that formed a rift infilled first by iron formations followed by terrigenous sediments. This major change of sedimentary regime might have been controlled by the regional tectonic evolution of the Amazonia Craton and its emergence above sea-level. During the Paleoproterozoic, at ca. 2.1 Ga, the Rio Fresco Group, consisting of terrigenous sediments from the interior of the Amazonia Craton, was deposited in the Carajás Basin. At that time, the Amazonian lithosphere could have either underwent thermal subsidence forming a large intracratonic basin or could have been deformed by long wavelength flexures that induced the formation of basins and swells throughout the craton under the influence of the growing Transamazonian mountain belt.  相似文献   

11.
《International Geology Review》2012,54(12):1134-1142
The Rio Maria area (southeastern Carajás metallogenic province, Brazil) consists of a typical Archean Au-bearing granite-greenstone association intruded by Paleoproterozoic anorogenic granites. U-Pb analyses were performed on single zircon grains from the main Archean units of this area in order to elucidate the history of formation and evolution of the continental crust in this part of the southeastern Amazonian Craton. The Arco Verde metatonalite yielded one of the oldest U-Pb zircon ages from the Amazonian Craton (2957 +25/?21 Ma). Zircons from a meta-rhyodacite from the greenstone belt of the Lagoa Seca Group (Andorinhas Supergroup) are dated at 2904 +29/?21 Ma. This sequence is crosscut by Archean granitoids, such as the Rio Maria granodiorite, emplaced at 2874 +9/?10 Ma. These results suggest a short time interval (～100 Ma) for crustal evolution of the Rio Maria area.  相似文献   

12.
By the analysis of the published zircon U-Pb ages and Hf isotope data, this paper firstly presents a comprehensive review about the staggered growth and reworking of early Precambrian continental crust in Alxa Block. The results show that the ancient crustal remnants of Alxa Block was formed in Meso-Paleo Archean, which was recorded by 3.0~3.6 Ga detrital zircons and Hf model ages. The early crustal growth of Alxa Block could be traced back to early Paleo-archean. Currently, the oldest zircon U-Pb age is about 3.6 Ga. Analogous to the other places of North China Craton, the Alxa Block underwent two-stage crustal growth at 2.7~2.9 Ga and 2.5~2.6 Ga respectively, and the former might be wider. The ~2.5 Ga (TTG) tectono-magmatic event, which represents the North China continent’s cratonization, also existed in Alxa Block. The corresponding zircon Hf isotope data indicate that the TTGs were mainly derived by melting of 2.7~2.9 Ga juvenile crust, possibly by mixing with a certain ancient crust, and a small portion was produced by instant reworking of 2.5~2.6 Ga juvenile crust. Proceeding to Paleo-proterozoic, the Alxa Block underwent multi-stage tectono-magmatic events, approximately peaked at 2.30~2.35 Ga, 2.15~2.17 Ga, 2.00~2.10 Ga, 1.95~1.98 Ga and ~1.90 Ga. The continental crust was mainly formed by reworking of 2.7~2.9 Ga and 2.5~2.6 Ga juvenile crust, simultaneously by a fraction of ~2.1 Ga juvenile crust. In Paleo-proterozoic, not only the Archean crustal reworking but also the juvenile crustal growth existed in Alxa Block.  相似文献   

13.
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   

14.
Discoveries of >4 Ga old zircon grains in the northwest Yilgarn of Western Australia led to the conclusion that evolved crust formed on the Earth within the first few 100 Ma after accretion. Little is known, however, about the fate of the first crust that shaped early Earth's surface. Here we report combined solution and laser-ablation Lu–Hf–U–Pb isotope analyses of early Archean and Hadean detrital zircon grains from different rocks of the Narryer Gneiss Complex (NGC), Yilgarn Craton, Western Australia. The zircons show two distinct groups with separate evolutionary trends in their Hf isotopes. The majority of the zircon grains point to separation from a depleted mantle reservoir at ∼3.8–3.9 Ga. The second Hf isotope trend implies reworking of older Hadean zircon grains. The major trend starting at 3.8–3.9 Ga defined by the Hf isotopes corresponds to a Lu/Hf that is characteristic for felsic crust and consequently, the primary sources for these zircons presumably had a chemical composition characteristic of continental crust. Reworked Hadean crust appears to have evolved with a similar low Lu/Hf, such that the early crust was probably evolved with respect to Lu–Hf distributions. The co-variation of Hf isotopes vs. age in zircon grains from Mt. Narryer and Jack Hills zircon grains implies a similar crustal source for both sediments in a single, major crustal domain. Age spectra and associated Hf isotopes in the zircon grains strongly argue for ongoing magmatic reworking over hundreds of millions of years of the felsic crustal domain in which the zircon grains formed. Late-stage metamorphic zircon grains from the Meeberrie Gneiss unit yield a mean U–Pb age of 3294.5 ± 3.2 Ma with initial Hf isotopes that correspond to the evolutionary trend defined by older NGC zircon grains and overlap with other detrital zircon grains, proving their genetic relationship. This ‘Meeberrie event’ is interpret here as the last reworking event in the precursor domain before final deposition. The continuous magmatic activity in one crustal domain during the Archean is recorded by the U–Pb ages and Hf isotope systematics of zircon grains and implies reworking of existing crust. We suspect that the most likely driving force for such reworking of crustal material is ongoing crustal collision and subduction. A comparison of Hf isotope signatures of zircon grains from other Archean terranes shows that similar trends are recognised within all sampled Archean domains. This implies either a global trend in crustal growth and reworking, or a genetic connection of Archean terranes in close paleo-proximity to each other. Notably, the Archean Acasta gneiss (Canada) shows a similar reworking patterns to the Yilgarn Craton of Hadean samples implying either a common Hadean source or amalgamation at the Hadean–Archean transition.  相似文献   

15.
New conventional and sensitive high-resolution ion microprobe zircon U-Pb dating has led to a new understanding of the subdivision and evolution of the Amazon Craton during Precambrian time, with major improvements and changes made to the previous Rb-Sr based model. The interpretation of U-Pb and Sm-Nd isotopic data identifies eight main Precambrian tectonic provinces in the Craton, with ages ranging from 3.1 to 0.99 Ga. Some of the provinces were generated by accretional, arc-related processes (Carajás, Transamazonic, Tapajós-Parima and Rondônia-Juruena) and others by recycling of continental crust (Central Amazon, Rio Negro and Sunsas). The exposed Archean crust is restricted to the east (Carajás and south Amapá in Brazil) and north (Imataca in Venezuela) of the craton, indicating that the Amazon Craton is largely a Proterozoic crust. The Carajás-Imataca (3.10–2.53 Ga) and Transamazonian (2.25–2.00 Ga) Provinces are composed predominantly of granite-greenstone terranes. The Tapajós-Parima (2.10–1.87 Ga) and Rondônia-Juruena (1.75–1.47 Ga) Provinces represent new crust added as orogenic belts, while the Rio Negro (1.86–1.52 Ga) and Sunsas (1.33–0.99 Ga) Provinces originated mainly by magmatic-tectonic recycling of the above two orogenic belts. The only zone with a prominent northeast trend is the poorly known K'Mudku Shear Belt, characterized by a 1.20 Ga shear zone which deforms the rocks of at least three different provinces (Rio Negro, Tapajós-Parima and Transamazonic). The Central Amazon Province comprises mostly Orosirian volcano-plutonic rocks (Uatumã Magmatism) and is a terrane in which the exposed crustal structure and deformation are pluton-related. The Sm-Nd TDM model ages and Nd suggest that the Central Amazon Province was generated by the partial melting of Archean continental crust (Carajás Province?), perhaps related to underplating that began at the end of the Tapajós-Parima Orogeny (1.88–1.86 Ga).  相似文献   

16.
目前塔里木地块东南缘早前寒武纪岩石形成时代和成因机制研究较为薄弱,制约了人们对塔里木大陆地壳形成和早期演化的认识.岩石地球化学及锆石Hf同位素组成表明塔里木地块东南缘安南坝石英闪长片麻岩原岩岩浆为下地壳变玄武岩部分熔融形成,并有少量幔源物质的加入.LA-ICP-MS锆石U-Pb测年结果显示石英闪长片麻岩成岩年龄为2 662±12 Ma、2 676±15 Ma,结合已有资料综合说明塔里木东南缘太古宙岩石形成时代主要集中在2.55~2.70 Ga,变质年龄分别为1 980±30 Ma、1 828±20 Ma~2 087±29 Ma,是古元古代中晚期与造山作用有关的岩浆-变质事件的地质记录.石英闪长片麻岩中锆石两阶段模式年龄TDM2为2 954~3 742 Ma,峰值为~3.24 Ga,据此认为~3.2 Ga是塔里木东南缘大陆地壳生长的主要时期,并于2.55~2.70 Ga发生地壳再造,古老地壳再循环可能是该地区新太古代中晚期大陆地壳演化的重要方式.   相似文献   

17.
肖玲玲  刘福来  张健 《岩石学报》2019,35(2):325-348
新太古代早期是全球地质历史上一个重要的地壳生长时期,世界众多克拉通中广泛存在2. 7Ga左右的岩浆年龄记录。华北克拉通最主要的岩浆事件发生在新太古代晚期,这与世界其他克拉通广泛存在～2. 7Ga的构造热事件明显不同。但全岩Nd和锆石Hf同位素研究表明,华北克拉通～2. 5Ga的岩石主体来自于中太古代晚期-新太古代早期大陆物质的重熔或再造。因此,厘定～2. 7Ga地质事件在华北克拉通的空间分布对深入理解新太古代地壳形成与演化具有重要科学意义。华北克拉通已识别出的～2. 7Ga的花岗质岩石主要分布在胶东、鲁西、武川、赞皇和太华等少数杂岩区,中部带的恒山、阜平和中条杂岩中亦有零星出露。左权变质杂岩位于中部带中南段,赞皇杂岩西南,初步地球化学和锆石年代学研究表明,该地区有多种岩石类型记录了～2. 7Ga的年龄信息,包括副片麻岩、长英质浅色体、磁铁矿角闪片麻岩和TTG片麻岩。其中,TTG片麻岩的原岩为英云闪长岩,锆石发育明显的核边结构,核部具有清晰的岩浆环带,两个不同LA-ICP-MS实验室获得的不一致线上交点年龄分别为2727±14Ma和2731±12Ma,代表了TTG岩浆岩的结晶年龄。同时,左权变质岩石中较好地保存了新太古代晚期的岩浆和变质年龄记录,推测其所代表的构造热事件与华北克拉通～2. 5Ga所经历的大规模幔源岩浆的底侵作用有关。  相似文献   

18.
在对一些重点地区新太古代早期—中太古代晚期（2.6~3.0 Ga）岩石的空间分布、岩石类型和形成时代作简要介绍基础上,文章总结了华北克拉通这一时代花岗质岩石的年龄分布模式、地球化学和Nd-Hf-O同位素组成特征。新太古代早期—中太古代晚期变质基底具有如下特征:①新太古代早期—中太古代晚期岩浆作用在华北克拉通几乎连续分布,峰期为2.70~2.75 Ga;②新太古代早期—中太古代晚期岩石在华北克拉通广泛存在,主要分布在东部古陆块、中部古陆块和南部古陆块中;③新太古代早期—中太古代晚期侵入岩以英云闪长岩为主,存在奥长花岗岩和花岗闪长岩及其他类型岩石;④新太古代早期—中太古代晚期表壳岩规模很小,零星分布于花岗质岩石中,岩石类型主要为变玄武质岩石,一些地区存在变质科马提岩、变质安山质?英安质火山岩和变质碎屑沉积岩;⑤2.6 Ga可作为华北克拉通新太古代早期和晚期的界线;⑥TTG岩石的Sr/Y和La/Yb比值存在很大变化,在Sr/Y-Y和La/Yb-Yb图中位于高压、中压和低压TTG分布区;除少量富钾花岗岩外,华北克拉通新太古代早期—中太古代晚期岩石大都具有亏损Nd-Hf同位素组成特征;岩浆锆石O同位素组成与全球太古宙岩浆锆石类似;⑦许多地区都具有类似地质特征,但一些地区显示出较大的独特性。新的研究进一步支持了这样的认识:与全球其他许多典型克拉通类似,新太古代早期—中太古代晚期是华北克拉通最重要的陆壳增生时期,主要区别是华北克拉通叠加了强烈的新太古代晚期岩浆构造热事件。   相似文献   

19.
The geological setting, geochemistry, and Nd isotopic systematics of tonalite-trondhjemite-granodiortite (TTG) series in ancient cratons are considered. It is shown that the TTG series were formed from ∼4.2 to 2.6 Ga ago in the oldest continental cores; many TTG series do not reveal chronological links to greenstone belts. This follows from the evolution of the Slave Craton in the Canadian Shield, the Vodlozero Craton in the Baltic Shield, and the Pilbara and Yilgarn cratons in the Australian Shield, where greenstone associations postdated TTG series. As has been established at the Baltic Shield, the primary melts of the Mesoarchean TTG associations were formed at a shallower depth (P < 15 kbar) compared to the Neoarchean TTG, likely, due to the increasing thickness of the continental crust beneath the Baltic Shield over time.  相似文献   

20.
扬子陆核的生长和再造：锆石U-Pb年龄和Hf同位素研究   总被引：5，自引：2，他引：5  
张少兵  郑永飞 《岩石学报》2007,23(2):393-402
对宜昌三峡附近崆岭杂岩中混合岩、片麻岩和变沉积岩以及莲沱砂岩进行了锆石U-Pb和Hf-O同位素研究,研究结果深化了我们对扬子陆核生长和再造的认识。在莲沱砂岩中发现了老达3.8Ga的碎屑锆石,说明扬子陆块可能存在这个年龄的地壳物质;其Hf同位素组成指示初生地壳生长出现在4.0Ga。崆岭杂岩中混合岩和片麻岩的U-Pb年龄表明,在3.2～3.3Ga和2.9～3.0Ga有两期重要的岩浆活动,指示扬子陆核可能于中太古代就开始形成。锆石Hf同位素研究则指示,其原岩至少从3.5Ga就开始从亏损地幔分异出来。混合岩和变沉积岩中所记录的1.9～2.0Ga变质事件,是扬子陆核再造并发生克拉通化的主要时期。而广泛分布于扬子陆块周边的新元古代岩浆活动不仅导致了许多太古宙和古元古宙地壳重熔,而且引起了初生地壳的快速再造。  相似文献   

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