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1.
The Archean eastern Dharwar craton is transacted by at least four major Proterozoic mafic dyke swarms. We present geochemical data for the ~2.21–2.22 Ga N-S to NNW-SSE trending Kunigal mafic dyke swarm of the eastern Dharwar craton to address its petrogenesis and formation of large igneous province as well as spatial link to supercontinent history. It has a strike span of about 200 km; one dyke of this swarm runs ~300 km along the western margin of the Closepet granite. Texture and mineral compositions classify them as dolerite and olivine dolerite. They show compositions of high-iron tholeiites, high-magnesian tholeiites or picrites. Geochemical characteristics of the sampled dykes suggest their co-genetic nature and show variation from primitive (Mg#; as high as ~76) to evolved (differentiated) nature. Although geochemical characteristics indicate possibility of minor crustal contamination, they show their derivation from an uncontaminated mantle melt. These mafic dykes are probably evolved from a sub-alkaline basaltic magma generated by ~20 % batch melting of a depleted lherzolite mantle source and about 15–30 % olivine fractionation. Paleoproterozoic (~2.21–2.22 Ga) mafic magmatism is recognized globally as dyke swarms or gabbroic sill complexes in the Superior, Slave, North Atlantic, Fennoscandian and Pilbara cratons. Possible Paleoproterozoic Dharwar–Superior–North-Atlantic–Slave correlations are constrained with implications for the configuration of supercraton Superia.  相似文献   

2.
A vast tract of ENE–WSW to NE–SW trending mafic dyke swarm transects Archaean basement rocks within the eastern Dharwar craton. Petrographic data reveal their dolerite/olivine dolerite or gabbro/olivine gabbro composition. Geochemical characteristics, particularly HFSEs, indicate that not all these dykes are co-genetic but are probably derived from more than one magma batch and different crystallization trends. In most samples the LaN/LuN ratio is at ∼2, whereas others have a LaN/LuN ratio >2 and show higher concentrations of high-field strength elements (HFSEs) than the former group. As a consequence, we assume that the ENE–WSW to NE–SE trending mafic dykes of the eastern Dharwar craton do not represent one single magmatic event but were emplaced in two different episodes; one of them dated at about 2.37 Ga and another probably at about 1.89 Ga. Trace element modelling also supports this inference: older mafic dykes are derived from a melt generated through ∼25% melting of a depleted mantle, whereas the younger set of dykes shows its derivation through a lower degree of melting (∼15%) of a comparatively enriched mantle source.  相似文献   

3.
The presence of recycled crust in the lithospheric mantle of the Dharwar craton has been investigated using trace element geochemistry of olivine grains from an ENE-trending Paleoproterozoic picrite dyke (associated with the ca. 1.89–1.88 Ga Hampi dyke swarm) emplaced in the western Dharwar craton. Olivine grains are purely magmatic, formed as early phenocrysts in a fractionated basaltic melt. They exhibit enrichment in NiO contents (0.32–0.43 wt%) and depletion in Ca (1366–2105 ppm), Mn (1578–2663 ppm) and 100 1 Mn/Fe (1.28–1.48). Further, the compiled whole-rock geochemical data of the picrite dyke and associated dyke swarm illustrates relatively low CaO/MgO (0.55–1.78), intermediate FeO/MnO (47–54), negative to positive PX# (?0.34 to +1.86), and high values of FC3MS (0.24–0.90) and FCKANTMS (0.19–1.11). These chemical markers are not consistent with the derivation of the primary melt from a pure peridotite or a pyroxenite source; therefore, contribution from a mixed type of source having both peridotite and pyroxenite end members (pyroxene rich and olivine poor lithology) is suggested. The amount of pyroxenite and recycled crust varies from 46% to 86% and 14% to 44%, respectively. The Al-in-olivine based thermometer estimates the maximum crystallization temperature as 1407 °C, which is 137 °C higher than the average temperature of MORB and accordant with several well-established plume-induced large igneous provinces (LIPs) worldwide. Therefore, it is suggested that the studied picrite dyke is derived from a primary melt generated by plume-induced melting of a peridotite-pyroxenite mixed source. The ca. 1.89–1.88 Ga Hampi dyke swarm, being genetically linked with the studied dyke, could also be derived from this same source. Further, the recycled crust in the subcontinental lithospheric mantle of the western Dharwar craton may have generated the pyroxene rich mafic source during the Neoarchean convergence between eastern and western Dharwar craton.  相似文献   

4.
Mafic rocks of Western Dharwar Craton (WDC) belong to two greenstone cycles of Sargur Group (3.1–3.3 Ga) and Dharwar Supergroup (2.6–2.8 Ga), belonging to different depositional environments. Proterozoic mafic dyke swarms (2.4, 2.0–2.2 and 1.6 Ga) constitute the third important cycle. Mafic rocks of Sargur Group mainly constitute a komatiitic-tholeiite suite, closely associated with layered basic-ultrabasic complexes. They form linear ultramaficmafic belts, and scattered enclaves associated with orthoquartzite-carbonate-pelite-BIF suite. Since the country rocks of Peninsular Gneiss intrude these rocks and dismember them, stratigraphy of Sargur Group is largely conceptual and its tectonic environment speculative. It is believed that the Sargur tholeiites are not fractionated from komatiites, but might have been generated and evolved from a similar mantle source at shallower depths. The layered basic-ultrabasic complexes are believed to be products of fractionation from tholeiitic parent magma. The Dharwar mafic rocks are essentially a bimodal basalt-rhyolite association that is dominated by Fe-rich and normal tholeiites. Calc-alkaline basalts and andesites are nearly absent, but reference to their presence in literature pertains mainly to carbonated, spilitized and altered tholeiitic suites. Geochemical discrimination diagrams of Dharwar lavas favour island arc settings that include fore-, intra- and back-arcs. The Dharwar mafic rocks are possibly derived by partial melting of a lherzolite mantle source and involved in fractionation of olivine and pyroxene followed by plagioclase. Distinctive differences in the petrography and geochemistry of mafic rocks across regional unconformities between Sargur Group and Dharwar Supergroup provide clinching evidences in favour of distinguishing two greenstone cycles in the craton. This has also negated the earlier preliminary attempts to lump together all mafic volcanics into a single contemporaneous suite, leading to erroneous interpretations. After giving allowances for differences in depositional and tectonic settings, the chemical distinction between Sargur and Dharwar mafic suites throws light on secular variations and crustal evolution. Proterozoic mafic dyke swarms of three major periods (2.4, 2.0–2.2 and 1.6 Ga) occur around Tiptur and Hunsur. The dykes also conform to the regional metamorphic gradient, with greenschist facies in the north and granulite facies in the south, resulting from the tilt of the craton towards north, exposing progressively deeper crustal levels towards the south. The low-grade terrain in the north does not have recognizable swarms, but the Tiptur swarm consists essentially of amphibolites and Hunsur swarm mainly of basic granulites, all of them preserving cross-cutting relations with host rocks, chilled margins and relict igneous textures. There are also younger dolerite dykes scattered throughout the craton that are unaffected by this metamorphic zonation. Large-scale geochemical, geochronological and palaeomagnetic data acquisition through state-of-the-art instrumentation is urgently needed in the Dharwar craton to catch up with contemporary advancements in the classical greenstone terrains of the world.  相似文献   

5.
The Indian Shield is cross-cut by a number of distinct Paleoproterozoic mafic dyke swarms. The density of dykes in the Dharwar and Bastar Cratons is amongst the highest on Earth. Globally, boninitic dyke swarms are rare compared to tholeiitic dyke swarms and yet they are common within the Southern Indian Shield. Geochronology and geochemistry are used to constrain the petrogenesis and relationship of the boninitic dykes (SiO2 = 51.5 to 55.7 wt%, MgO = 5.8 to 18.7 wt%, and TiO2 = 0.30 wt% to 0.77 wt%) from the central Bastar Craton (Bhanupratappur) and the NE Dharwar Craton (Karimnagar). A single U-Pb baddeleyite age from a boninitic dyke near Bhanupratappur yielded a weighted-mean 207Pb/206Pb age of 2365.6 ± 0.9 Ma that is within error of boninitic dykes from the Dharwar Craton near Karimnagar (2368.5 ± 2.6 Ma) and farther south near Bangalore (2365.4 ± 1.0 Ma to 2368.6 ± 1.3 Ma). Rhyolite-MELTS modeling indicates that fractional crystallization is the likely cause of major element variability of the boninitic dykes from Bhanupratappur whereas trace element modeling indicates that the primary melt may be derived from a pyroxenite mantle source near the spinel-garnet transition zone. The Nd isotopes (εNd(t) = −6.4 to +4.5) of the Bhanupratappur dykes are more variable than the Karimnagar dykes (εNd(t) = −0.7 to +0.6) but they overlap. The variability of Sr-Nd isotopes may be related to crustal contamination during emplacement or is indicative of an isotopically heterogeneous mantle source. The chemical and temporal similarities of the Bhanupratappur dykes with the dykes of the Dharwar Craton (Karimnagar, Penukonda, Chennekottapalle) indicate they are members of the same giant radiating dyke swarm. Moreover, our results suggest that the Bastar and Dharwar Cratons were adjacent but likely had a different configuration at 2.37 Ga than the present day. It is possible that the 2.37Ga dyke swarm was related to a mantle plume that assisted in the break-up of an unknown or poorly constrained supercontinent.  相似文献   

6.
The collision between the North and South China cratons in Middle Triassic time (240–225 Ma) created the world’s largest belt of ultrahigh-pressure (UHP) metamorphism. U–Pb ages, Hf isotope systematics and trace element compositions of zircons from the Xugou, Yangkou and Hujialing peridotites in the Sulu UHP terrane mainly record a ~470 Ma tectonothermal event, coeval with the Early Paleozoic kimberlite eruptions within the North China craton. This event is interpreted as the result of metasomatism by fluids/melts derived from multiple sources including a subducting continental slab. The peridotites also contain zircons with ages of ~3.1 Ga, and Hf isotope data imply a component ≥3.2 Ga old. Most zircon Hf depleted mantle model ages are ~1.3 Ga, suggesting that the deep subcontinental lithospheric mantle beneath the southeastern margin of the North China craton experienced a intense mid-Mesoproterozoic metasomatism by asthenospheric components, similar to the case for the eastern part of this craton. Integrating data from peridotites along the southern margin of the craton, we argue that the deep lithosphere of the cratonic margin (≥3.2 Ga old), from which the Xugou, Yangkou and Hujialing peridotites were derived, experienced Proterozoic metasomatic modification, followed by a strong Early Paleozoic (~470 Ma) tectonothermal event and the Early Mesozoic (~230 Ma) collision and northward subduction of the Yangtze craton. The Phanerozoic decratonization of the eastern North China craton, especially along its southern margin, was not earlier than the Triassic continental collision. This work also demonstrates that although zircons are rare in peridotitic rocks, they can be used to unravel the history of specific lithospheric domains and thus contribute to our understanding of the evolution of continental cratons and their margins.  相似文献   

7.
The Roshtkhar area is located in the Khaf-Kashmar-Bardaskan volcano-plutonic belt to the northeastern Iran along the regional E–W trending Dorouneh Fault, northeastern of the Lut Block. There are several outcrops of subvolcanic rocks occurring mainly as dikes in the area, which intruded into Cenozoic intrusive rocks. We present U–Pb dating of zircons from a diabase dike and syenite rock using LA-ICP-MS that yielded an age of 1778 ± 10 Ma for the dike, indicating this Cenozoic dike has zircon xenocrysts inherited from deeper sources; and 38.0 ± 0.5 Ma, indicating an Late Eocene crystallization age for the syenite. Geochemically, the dikes typical of high-K calc-alkaline to shoshonitic magmas. Petrographic observations and major and trace element variations suggest that diabase melts underwent variable fractionation of clinopyroxene, olivine, and Fe-Ti oxides and minor crustal contamination during the differentiation process. Primitive mantle-normalized multi-element diagrams display enrichment in LILE, such as Rb, Ba, Th, U, and Sr compared to HFSE, as well as negative anomalies of Nb, Ta, P, and Ti, suggesting derivation from subduction-modified mantle. Chondrite-normalized REE plots show moderately LREE enriched patterns (<3.83 LaN/YbN <8.27), and no significant Eu anomalies. Geochemical modelling using Sm/Yb versus La/Yb and La/Sm ratios suggests a low-degree of batch melting (~1–3%) of a phlogopite-spinel peridotite source to generate the mafic dikes. The geochemical signatures suggest that the Roshtkhar mafic dikes cannot be related directly to subduction and likely resulted from melting of upper mantle in an extensional setting where the heat flow was provided from deeper levels. These dikes presumably derived the zircon xenocrysts from the assimilation of upper crust of Gondwanian basement. Processes responsible for partial melting of metasomatized lithospheric mantle and post-collision magmatism in NE Iran was triggered by heating due to asthenospheric upwelling in an extensional setting.  相似文献   

8.
Coupled paleomagnetic and geochronologic data derived from mafic dykes provide valuable records of continental movement. To reconstruct the Proterozoic paleogeographic history of Peninsular India, we report paleomagnetic directions and U-Pb zircon ages from twenty-nine mafic dykes in the Eastern Dharwar Craton near Hyderabad. Paleomagnetic analysis yielded clusters of directional data that correspond to dyke swarms at 2.37 Ga, 2.22 Ga, 2.08 Ga, 1.89–1.86 Ga, 1.79 Ga, and a previously undated dual polarity magnetization. We report new positive baked contact tests for the 2.08 Ga swarm and the 1.89–1.86 Ga swarm(s), and a new inverse baked contact test for the 2.08 Ga swarm. Our results promote the 2.08 Ga Dharwar Craton paleomagnetic pole (43.1° N, 184.5° E; A95 = 4.3°) to a reliability score of R = 7 and suggest a position for the Dharwar Craton at 1.79 Ga based on a virtual geomagnetic pole (VGP) at 33.0° N, 347.5° E (a95 = 16.9°, k = 221, N = 2). The new VGP for the Dharwar Craton provides support for the union of the Dharwar, Singhbhum, and Bastar Cratons in the Southern India Block by at least 1.79 Ga. Combined new and published northeast-southwest moderate-steep dual polarity directions from Dharwar Craton dykes define a new paleomagnetic pole at 20.6° N, 233.1° E (A95 = 9.2°, N = 18; R = 5). Two dykes from this group yielded 1.05–1.01 Ga 207Pb/206Pb zircon ages and this range is taken as the age of the new paleomagnetic pole. A comparison of the previously published poles with our new 1.05–1.01 Ga pole shows India shifting from equatorial to higher (southerly) latitudes from 1.08 Ga to 1.01 Ga as a component of Rodinia.  相似文献   

9.
This study provides the first evidence for the occurrence of ultrahigh-temperature (UHT) granulite-facies metamorphism in the Yenisei Ridge (Angara–Kan block). UHT metamorphism is documented in Fe-Al-rich metapelites on the basis of the garnet–hypersthene–sillimanite–cordierite–plagioclase–biotite–spinel–quartz–K-feldspar assemblage. Microtextural relationships and compositional data for paragneisses of the Kan complex attest to three distinct metamorphic episodes: (M1) pre-peak prograde (820?900°C/5.5–7 kbar), (M2) peak UHT (920–1000°C/7–9 kbar), and (M3) post-peak retrograde (770?900°C/5.5–7.5 kbar). The observed counterclockwise P–T evolution at a high geothermal gradient (dT/dP = 100–200°C/kbar) suggests that UHT metamorphic assemblages were formed in an overall extensional tectonic setting accompanied by underplating of mantle-derived mafic magmas, which may be sourced from ~1750 Ma giant radiating dike swarms linked to the Vilyuy mantle plume as part of the Trans-Siberian LIP. The broad synchroneity of UHT metamorphism (1744 ± 26 Ma; monazite–zircon isochron age) and rift-related endogenic activity in the region can provide an additional line of evidence for the two-stage evolution of granulite-facies metamorphism in the Angara–Kan block. The Aldan–Stanovoy, Anabar, and Baikal basement inliers of high-grade metamorphic rocks within the Siberian craton record two Paleoproterozoic peaks (1.9 and 1.75 Ga) of granulite-facies metamorphism. The synchronous sequence of tectonothermal events at the periphery of the large Precambrian Laurentian, Baltica, and Siberian cratons provide convincing evidence for their spatial proximity over a wide time interval, which is consistent with the most recent paleomagnetic reconstructions of the Proterozoic supercontinent Nuna.  相似文献   

10.
A brief geological and petrographic characterization of the Early Precambrian dike complexes of the Kola region is given along with data on new estimates of dike age and analysis of their distribution over the entire Fennoscandian Shield. The emplacement of dikes in the Archean core of the shield continued after consolidation of the sialic crust 2.74?C1.76 Ga ago. After the Svecofennian Orogeny, dikes continued to form in the west in the area of newly formed crust, while the amagmatic period began in the Archean domain. The intense formation of dikes in the Svecofennian domain lasted approximately for 1 Ga (1.8?C0.84 Ga). The younger igneous rocks in the crustal domains of different age are less abundant and localized at their margins. A similar distribution of dikes is characteristic of other shields in different continents. This implies that the formation of the sialic crust in the shields is not completed by its consolidation and formation of the craton. For 1 Ga after completion of this process, the crust is underplated by mantle-derived magmas. This process is reflected at the Earth??s surface in the development of mantle-derived mafic and anorogenic granitoid magmatism. The process of crust formation is ended as the subcratonic lithosphere cools and the amagmatic period of the craton history is started. Beginning from this moment, the manifestations of cratonic magmatism were related either to the superposed tectonomagmatic reactivation of the cold craton under the effect of crust formation in the adjacent mobile belts or to the ascent of mantle plumes.  相似文献   

11.
A newly recognized remnant of a Paleoproterozoic Large Igneous Province has been identified in the southern Bastar craton and nearby Cuddapah basin from the adjacent Dharwar craton, India. High precision U–Pb dates of 1891.1 ± 0.9 Ma (baddeleyite) and 1883.0 ± 1.4 Ma (baddeleyite and zircon) for two SE-trending mafic dykes from the BD2 dyke swarm, southern Bastar craton, and 1885.4 ± 3.1 Ma (baddeleyite) for a mafic sill from the Cuddapah basin, indicate the existence of 1891–1883 Ma mafic magmatism that spans an area of at least 90,000 km2 in the south Indian shield.This record of 1.9 Ga mafic/ultramafic magmatism associated with concomitant intracontinental rifting and basin development preserved along much of the south-eastern margin of the south Indian shield is a widespread geologic phenomenon on Earth. Similar periods of intraplate mafic/ultramafic magmatism occur along the margin of the Superior craton in North America (1.88 Ga Molson large igneous province) and in southern Africa along the northern margin of the Kaapvaal craton (1.88–1.87 Ga dolerite sills intruding the Waterberg Group). Existing paleomagnetic data for the Molson and Waterberg 1.88 Ga large igneous provinces indicate that the Superior and Kalahari cratons were at similar paleolatitudes at 1.88 Ga but a paleocontinental reconstruction at this time involving these cratons is impeded by the lack of a robust geological pin such as a Limpopo-like 2.0 Ga deformation zone in the Superior Province. The widespread occurrence of 1.88 Ga intraplate and plate margin mafic magmatism and basin development in numerous Archean cratons worldwide likely reflects a period of global-scale mantle upwelling or enhanced mantle plume activity at this time.  相似文献   

12.
造山后脉岩组合与内生成矿作用   总被引：14，自引：2，他引：12  
造山带大规模花岗质岩浆活动之后往往有一期区域性脉岩产出,被称为岩基后岩墙群。这类脉岩具有近同时形成、宽成分谱系和小体积的特点。根据太行山、燕山、东昆仑山、天山等造山带的观察,这类脉岩可以划分成煌斑岩质、玄武质、闪长质(安山质)、花岗闪长质(英安质)和花岗质(流纹质)等5组。前人大多偏重于研究其中基性部分,因而常常将其与大陆裂解相关基性岩墙群混为一谈。岩石地球化学分析表明,虽然同组脉岩不同样品之间可能存在演化关系,不同脉岩组之间很难相互演化。结合近年来有关岩浆过程速率的研究成果,推测这些脉岩是原生或近原生岩浆固结的产物。这意味着区域地温曲线在不同深度同时穿过所有相应原岩的固相线。基于岩浆起源热体制和区域岩石圈岩石学结构分析,笔者曾经指出,这样的岩浆产生条件要求造山带岩石圈拆沉作用。因此,这类岩墙群的形成是区域构造应力场由挤压向伸展转换阶段的产物,可以用来标定造山过程的结束,因而称其为造山后脉岩组合。进一步对比分析表明,这类脉岩组合分布非常普遍,是地球上业已发现的三类区域性岩墙群之一。尽管如此,基于热传递速率的分析,造山后脉岩组合的形成还应当伴随大规模流体活动。由于深部流体中成矿元素的浓度强烈依赖于压力,新的岩石成因模型意味着造山后脉岩组合与成矿作用相伴生。野外检验表明,可以基于露头观察识别成矿流体的通道和成矿元素大规模堆积的场所。因此,造山后脉岩组合不仅可以用来标定区域造山过程结束的时间,也是区域找矿预测的有效标志。  相似文献   

13.
《International Geology Review》2012,54(13):1772-1790
The Quanji Massif (QM), in the northeast part of Tibet, consists of Palaeoproterozoic metamorphic rocks, granitoids, and mafic dikes. U–Pb dating of a diorite gneiss and a mafic dike in the QM yielded a crystallization age of 2272 ± 15 Ma and a metamorphic age of 1928 ± 11 Ma, respectively. Although some post-emplacement alteration has occurred, the mafic dikes display a sub-alkaline signature with slight light rare earth element-enrichment, depletion in Th, Nb, Ta, and Ti, and have a rare earth element pattern consistent with volcanic arc basalts. Based on the geochronology and field relationships, we conclude that the mafic dikes formed in an extensional setting within either a fore-arc or back-arc environment. We argue that the metamorphism that affected the dikes occurred shortly after intrusion. Our diorite gneiss and monzodiorite samples are characterized by relatively high Mg# (47–56) and Sr contents (367–1070 ppm), low-to-moderate Sr/Y (10–90), low Rb/Sr (0.03–0.28) and high K/Rb (179–775). These felsic melts likely originated from partial melting of a mafic source. Our new data, combined with results from previous studies, indicate that the QM formed between 2.50 and 2.30 Ga and underwent metamorphism around 1.95–1.75 Ga that may relate to the dispersal of Neoarchaean ‘Kenorland’ and the formation of the Columbia supercontinent. The similarity between the Palaeoproterozoic events in the Tiekelik, North Altyn–Dunhuang, Alashan blocks, and QM suggests that QM was part of either the Tarim or the North China Craton in the late Archaean and Palaeoproterozoic. If the model is correct, then there was a single ‘North China–Quanji–Tarim Craton’ that was later disrupted by Neoproterozoic to Phanerozoic tectonic events.  相似文献   

14.
A Middle Paleozoic tectonothermal event in the eastern Siberian craton was especially active in the area of the Vilyui rift, where it produced a system of rift basins filled with Devonian–Early Carboniferous volcanics and sediments, as well as long swarms of mafic dikes on the rift shoulders. Basalts occur mostly among Middle Devonian sediments and are much less spread in Early Carboniferous formations. The dolerite dikes of the Vilyui–Markha swarm in the northwestern rift border coexist with the Mirnyi and Nakyn fields of diamond-bearing kimberlites. The voluminous dikes and sills intruded before the emplacement of kimberlites. The Mir kimberlite crosscuts a dolerite sill and a dike in the Mirnyi field, while a complex dolerite dike (monzonite porphyry) cuts through the Nyurba kimberlite in the Nakyn field. Thus, the kimberlites correspond to a longer span of Middle Paleozoic basaltic magmatism. The basalts in Middle Paleozoic sediments have faunal age constraints, but the age of dolerite dikes remains uncertain. The monzonite porphyry dike in the Nyurba kimberlite has been dated by the 40Ar/39Ar method, and the obtained age must be the upper bound of the dike emplacement. The space and time relations between basaltic and kimberlitic magmatism were controlled by Devonian plume–lithosphere interaction.  相似文献   

15.
Three Paleoproterozoic A-type rapakivi granite suites (Jamon, Serra dos Carajás, and Velho Guilherme) are found in the Carajás metallogenic province, eastern Amazonian craton. Liquidus temperatures in the 900–870 °C range characterize the Jamon suite, those for Serra dos Carajás and Velho Guilherme are somewhat lower. Pressures of emplacement decrease from Jamon (3.2±0.7 kbar) through Serra dos Carajás (2.0±1.0 kbar) to Velho Guilherme (1.0±0.5 kbar). Oxidizing conditions (NNO+0.5) characterized the crystallization of the Jamon magma, the Velho Guilherme magmas were reducing (marginally below FMQ), and the Serra dos Carajás magmas were intermediate between the two in this respect. The three granite suites have Archean TDM model ages and strongly negative Nd values (−12 to −8 at 1880 Ma), and they were derived from Archean crust. The Jamon granite suite may have been derived from a quartz dioritic source, and the Velho Guilherme granites from K-feldspar-bearing granitoid rocks with some sedimentary input. The Serra dos Carajás granites either had a somewhat more mafic source than Velho Guilherme or were derived by a larger degree of melting. Underplating of mafic magma was probably the heat source for the melting. The petrological and geochemical characteristics of the Carajás granite suites imply considerable compositional variation in the Archean of the eastern Amazonian craton. The oxidized Jamon suite granites are similar to the Mesoproterozoic magnetite-series granites of Laurentia, and they were derived from Archean igneous sources that were more oxidized than the sources of the Fennoscandian rapakivi granites. The Serra dos Carajás and Velho Guilherme granites approach the classic reduced rapakivi series of Fennoscandia and Laurentia. No counterparts of the Mesoproterozoic two-mica granites of Laurentia have been found, however. Following the model of Hoffman [Hoffman, P., 1989. Speculations on Laurentia's first gigayear (2.0 to 1.0 Ga). Geology 17, 135–138], the origin of the 1.88 Ga Carajás granites is related to a mantle superswell beneath the Trans-Amazonian supercontinent. This caused breakup of the continent and was associated with magmatic underplating and resultant crustal melting and generation of A-type granite magmas. The Paleoproterozoic continent that included the Archean and Trans-Amazonian domains of the Amazonian craton was assembled at 2.0 Ga; its disruption was initiated at 1.88 Ga, at least 200 Ma earlier than in Laurentia and Fennoscandia. The Carajás granites were related to the breakup of the supercontinent, not to subduction processes.  相似文献   

16.
The NNW-trending Nova Lacerda tholeiitic dike swarm in Mato Grosso State, Central Brazil, intrudes the Nova Lacerda granite (1.46 Ga) and the Jauru granite-greenstone terrain (ca. 1.79–1.77 Ga). The swarm comprises diabases I and II and amphibolites emplaced at ca. 1.38 Ga. Geochemical data indicate that these are evolved tholeiites characterized by high LILE/HSFE and LREE/HSFE ratios. Isotopic modelling yields positive ?Nd(T) values (+0.86 to?+2.65), whereas values for ?Sr(T) range from positive to negative (+1.96 to -5.56). Crustal contamination did not play a significant petrogenetic role, as indicated by a comparison of isotopic data (Sr–Nd) from both dikes and country rocks, and by the relationship between isotopic and geochemical parameters (SiO2, K2O, Rb/Sr, and La/Yb) of the dikes. We attribute the origin of these tholeiites to fractional crystallization of evolved melts derived from a heterogeneous mantle source. Comparison of the geochemical and isotopic data of the studied swarm and other tholeiitic Mesoproterozoic mafic intrusions of the SW Amazonian Craton – the Serra da Providência, Colorado, and Nova Brasilândia bimodal suites – indicates that parental melts of the Nova Lacerda swarm were derived from the most enriched mantle source. This enrichment was probably caused by the stronger influence of the EMI component on the DMM end-member. These data, coupled with trace element bulk-rock geochemistry of the country rocks, and comparisons with the Colorado Complex of similar age, suggest a continental-margin arc setting for the emplacement of the Nova Lacerda dikes.  相似文献   

17.
Geological studies on saturated to oversaturated and subsolvus aegirine-riebeckite syenite bodies of the Pulikonda alkaline complex and Dancherla alkaline complex were carried out. The REE distribution of the Dancherla syenite shows a high fractionation between LREE and HREE. The absence of Eu anomaly suggests source from garnet peridotite. The Pulikonda syenite shows moderate fractionation between LREE and HREE as reflected by enrichment of HREE and moderate enrichment of LREE. The negative Eu anomaly indicates role of plagioclase fractionation.Three distinct co-eval primary magmas i.e. mafic syenite-, felsic syenite- and alkali basalt magmas — all derived from low-degrees of partial melting of mantle differentiates and enriched metasomatised lower crust played a major role in the genesis and emplacement of the syenites into overlying crust along deep seated regional scale trans-lithospheric strike-slip faults and shear zones following immediately after late-Archaean calc-alkaline arc magmatism at different time-space episodes i.e. initially at craton margin and later on into the thickened interior of the Eastern Dharwar craton. The ductile sheared and folded Pulikonda alkaline complex was evolved dominantly from the magmas derived from partial melting of lower crust and minor juvenile magmas from mantle. Differentiation and fractionation by liquid immiscibility of mafic magma and commingling-mixing of intermediate and felsic magmas followed by fractionational crystallisation under extensional tectonics during waning stages of calc-alkaline arc magmatism nearer to the craton margin were attributed as the main processes for the genesis of Pulikonda syenite complex. Commingling and limited mixing of independent mantle derived mafic and felsic syenitic magmas and accompanying fractionation resulting into soda rich and potash rich syenite variants was tentatively deduced mechanism for the origin of Dancherla, Danduvaripalle, Reddypalle syenites and other bodies belonging to Dancherla alkaline complex at the craton interior. The Peddavaduguru syenite was formed by differentiation of alkali mafic magma (gabbro to diorite) and it’s simultaneous mingling with fractionated felsic syenitic magma under incipient rift. Vannedoddi and Yeguvapalli syenites were derived due to desilicification and accompanying alkali feldspar mestasomatism of younger potash rich granites along Guntakal-Gooty fault and along Singanamala shear zone respectively.  相似文献   

18.
《地学前缘(英文版)》2020,11(6):2127-2139
The Dharwar Craton in Peninsular India was intruded by a series of mafic dykes during the Paleoproterozoic and these mafic magmatic events have important implications on continental rifting and LIPs. Here we report ten precise Pb–Pb TE-TIMS age determinations on baddeleyite grains separated from seven mafic dykes and three sills, intruding into Archean basement rocks and Proterozoic sedimentary formations of the Eastern Dharwar Craton respectively. The crystallization age of the baddeleyite shows 2366.3 ​± ​1.1 ​Ma, and 2369.2 ​± ​0.8 ​Ma for the NE–SW trending dykes, 2368.1 ​± ​0.6 ​Ma, 2366.4 ​± ​0.8 ​Ma, 2207.2 ​± ​0.7 ​Ma and 1887.3 ​± ​1.0 ​Ma for the ENE–WNW to E–W striking dykes, 1880.6 ​± ​1.0 ​Ma, 1864.3 ​± ​0.6 ​Ma and 1863.6 ​± ​0.9 ​Ma for Cuddapah sills, and 1861.8 ​± ​1.4 ​Ma for the N–S trending dyke. Our results in conjunction with those from previous studies identify eight distinct stages of widespread Paleoproterozoic magmatism in the Dharwar craton. The mantle plume centres of the four radiating dyke swarms with ages of ~2367 ​Ma, ~2210 ​Ma, ~2082 ​Ma, and ~1886 ​Ma were traced to establish their proximity to the EDC kimberlite province. Though the ~2367 ​Ma and ~1886 ​Ma plume centres are inferred to be located to the west and east of the present day Dharwar craton respectively away from the kimberlite province, location of plume heads of the other two swarms with ages of ~2207 ​Ma and ~2082 ​Ma are in close proximity. In spite of the ubiquitous occurrence of dyke intrusions of all the seven generations in the kimberlite province, only few of these kimberlites are diamondiferous. Kimberlite occurrences elsewhere in the vicinity of older Large Igneous Provinces (LIPs) like the Mackenzie, Karoo, Parana-Etendeka and Yakutsk-Vilui are also non-diamondiferous. This has been attributed to the destruction of the lithospheric mantle keel (that hosts the diamonds) by the respective mantle plumes. The diamondiferous nature of the EDC kimberlites therefore suggests that plume activity does not always result in the destruction of the mantle keel.  相似文献   

19.
华北克拉通1.75Ga基性岩墙群特征及其研究进展   总被引：2，自引：1，他引：2  
基性岩墙群是地壳伸展背景下,来自地幔的基性岩浆侵入体。华北克拉通同世界上其它克拉通一样,广泛发育前寒武纪基性岩墙群。它们在不同时代均有产生,其中1.75Ga前后的规模最大,分布范围最广,几乎遍布整个克拉通,对其进行深入研究,可以揭示华北克拉通该期构造演化过程。华北克拉通1.75Ga前后的岩墙几何形态多变,直立或近直立,走向主要为NNW向和近EW向。岩石以拉斑玄武质岩类占绝对优势(>80%),主要造岩矿物为单斜辉石和斜长石。根据岩墙走向、岩浆分异程度和岩石地球化学特征可将其分五组:低分异LT组、低分异HT组、高分异NW组、高分异EW组,以及具明显差异的高铁系列。同位素和微量元素研究显示,岩浆源区主要与富集Ⅰ型地幔(EMⅠ)、弱亏损的常规地幔(DM-PREMA)以及陆下岩石圈地幔有关。目前对华北克拉通1.75Ga基性岩墙群产出的构造环境在认识上有分歧,其中地幔柱观点和碰撞后伸展观点最为人们所关注。  相似文献   

20.
The widespread records of mafic intrusives (both sills and dykes) are reported from the Proterozoic sedimentary basins of the Indian Shield. Amongst them, the Bijawar basin is also intruded by Paleoproterozoic (ca. 1.98−1.97 Ga) mafic sills. We provide first hand information on petrological and geochemical characteristics of these mafic sills together with a few NW-trending mafic dykes belong to the Jhansi swarm emplaced within the Bundelkhand craton, adjacent to the Bijawar basin. These Paleoproterzoic mafic intrusive rocks, i.e. sills and dykes, are believed to be integral parts of the Jhansi LIP, identified in the Bundelkhand craton. The studied mafic magmatic samples are medium- to coarse-grained and contain doleritic mineral compositions and textures. Geochemically, the mafic sill samples of the Bijawar basin, which belong to the Darguwan-Surjapura mafic sills (DSMS), are sub-alkaline basaltic-andesite to andesite in character. They are co-genetic in nature and show close geochemical similarities with a set of NW-trending mafic dykes (low-Ti) emplaced in the Bundelkhand craton. On the other hand, another set of NW-trending mafic dykes (high-Ti) of the Bundelkhand craton have distinct geochemical nature; likely to have different genetic history. The rare-earth element contents and trace-element modeling suggest that the DSMS and low-Ti dyke samples are likely to be derived from a melt generated ≥20 % melting of a shallower mantle source (spinel stability field), whereas the high-Ti dyke samples show their derivation from a melt generated through ≤15 % melting of the similar mantle source but at deeper level (garnet or garnet-spinel transition stability field); with a substantiate percentage of olivine fractionation of melts before crystallization. Their emplacement in an intracratonic tectonic regime and role of plume in the genesis of these rocks are suggested. The geochemical signature also indicates the role of an ancient (Archean) subduction event that has metasomatized the mantle before the cratonization. Their spatiotemporal correlation with other similar magmatic events of the globe indicate that the Bundelkhand craton was closer to the Karelia-Kola craton (Baltica Shield), North China craton and northern Superior craton, which could be part of the Columbia supercontinent, during its assembly.  相似文献   

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