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1.
Detrital zircons are important proxies for crustal provenance and have been widely used in tracing source characteristics and continental reconstructions. Southern Peninsular India constituted the central segment of the late Neoproterozoic supercontinent Gondwana and is composed of crustal blocks ranging in age from Mesoarchean to late Neoproterozoic–Cambrian. Here we investigate detrital zircon grains from a suite of quartzites accreted along the southern part of the Madurai Block. Our LA-ICPMS U-Pb dating reveals multiple populations of magmatic zircons, among which the oldest group ranges in age from Mesoarchean to Paleoproterozoic (ca. 2980–1670 Ma, with peaks at 2900–2800 Ma, 2700–2600 Ma, 2500–2300 Ma, 2100–2000 Ma). Zircons in two samples show magmatic zircons with dominantly Neoproterozoic (950–550 Ma) ages. The metamorphic zircons from the quartzites define ages in the range of 580–500 Ma, correlating with the timing of metamorphism reported from the adjacent Trivandrum Block as well as from other adjacent crustal fragments within the Gondwana assembly. The zircon trace element data are mostly characterized by LREE depletion and HREE enrichment, positive Ce, Sm anomalies and negative Eu, Pr, Nd anomalies. The Mesoarchean to Neoproterozoic age range and the contrasting petrogenetic features as indicated from zircon chemistry suggest that the detritus were sourced from multiple provenances involving a range of lithologies of varying ages. Since the exposed basement of the southern Madurai Block is largely composed of Neoproterozoic orthogneisses, the data presented in our study indicate derivation of the detritus from distal source regions implying an open ocean environment. Samples carrying exclusive Neoproterozoic detrital zircon population in the absence of older zircons suggest proximal sources in the southern Madurai Block. Our results suggest that a branch of the Mozambique ocean might have separated the southern Madurai Block to the north and the Nagercoil Block to the south, with the metasediments of the khondalite belt in Trivandrum Block marking the zone of ocean closure, part of which were accreted onto the southern Madurai Block during the collisional amalgamation of the Gondwana supercontinent in latest Neoproterozoic–Cambrian.  相似文献   

2.
Earlier geological work in the Istanbul zone, western Pontide tectonic belt, has revealed the presence of extensive basement outcrops exposed underneath Palaeozoic and Mesozoic to Tertiary cover sequences. The basement of suspected Neoproterozoic age plays an important role in understanding the crustal accretion process in NW Turkey. We report the first results of a detailed Pb-Pb and U-Pb zircon study complemented by Nd-Sr whole rock and mineral data from basement rocks exposed in the Karadere valley, Safranbolu area. Five samples were selected for this study, comprising three metagranitoids and two metasediments. Zircon geochronology indicates that the metagranitoids were formed during Late Proterozoic pan-African magmatic events between 590 and 560 Ma. The rocks are of tonalitic and granitic composition and have low Nb/Y ratios and Ti contents, consistent with those of arc rocks. A continental arc setting is supported by their Sr and Nd isotope data that indicate a contribution of a mantle source as well as crustal assimilation during magma genesis. The metasediments can clearly be distinguished from the metagranitoids by their higher 87Sr/86Sr ratios and lower )Nd-values at 580 Ma, which supports the suggestion that the arc was underlain by mature continental crust. Zircons from the metasediments yield a range of Pb-Pb ages between 1,860 and 710 Ma. Thirty per cent of them fall between 890 and 710 Ma, possibly suggesting a derivation from Gondwana (Afro-Arabian) regions. A Sm-Nd garnet-whole rock analysis obtained on a metagranite gives an age of 559NJ Ma, which either reflects pre-metamorphic magmatic growth of garnet in a felsic melt or a syntectonic high-temperature metamorphic event. Uplift and cooling of the basement is further constrained by Rb-Sr biotite ages of 548-545 Ma. These lower Cambrian mineral ages demonstrate that the Istanbul zone was not thermally reactivated during the Hercynian, Cimmerian or Alpine orogeny, in contrast to its neighbouring tectonic zones, confirming its role as a suspect terrane in the modern western Pontide tectonic belt.  相似文献   

3.
The Achankovil Zone of southern India, a NW–SE trending lineament of 8–10 km in width and > 100 km length, is a kinematically debated crustal feature, considered to mark the boundary between the Madurai Granulite Block in the north and the Trivandrum Granulite Block in the south. Both these crustal blocks show evidence for ultrahigh-temperature metamorphism during the Pan-African orogeny, although the exhumation styles are markedly different. The Achankovil Zone is characterized by discontinuous strands of cordierite-bearing gneiss with an assemblage of cordierite + garnet + quartz + plagioclase + spinel + ilmenite + magnetite ± orthopyroxene ± biotite ± K-feldspar ± sillimanite. The lithology preserves several peak and post-peak metamorphic assemblages including: (1) orthopyroxene + garnet, (2) perthite and/or anti-perthite, (3) cordierite ± orthopyroxene corona around garnet, and (4) cordierite + quartz symplectite after garnet. We estimate the peak metamorphic conditions of these rocks using orthopyroxene-bearing geothermobarometers and feldspar solvus which yield 8.5–9.5 kbar and 940–1040 °C, the highest P–T conditions so far recorded from the Achankovil Zone. The retrograde conditions were obtained from cordierite-bearing geothermobarometers at 3.5–4.5 kbar and 720 ± 60 °C. From orthopyroxene chemistry, we record a multistage exhumation history for these rocks, which is closely comparable with those reported in recent studies from the Madurai Granulite Block, but different from those documented from the Trivandrum Granulite Block. An evaluation of the petrologic and geochronologic data, together with the nature of exhumation paths leads us to propose that the Achankovil Zone is probably the southern flank of the Madurai Granulite Block, and not a unit of the Trivandrum Granulite Block as presently believed. Post-tectonic alkali granites that form an array of “suturing plutons” along the margin of the Madurai Granulite Block and within the Achankovil Zone, but are absent in the Trivandrum Granulite Block, suggest that the boundary between the Madurai Granulite Block and the Trivandrum Granulite Block might lie along the Tenmalai shear zone at the southern extremity of the Achankovil Zone.  相似文献   

4.
《Gondwana Research》2014,25(1):190-203
Peninsular India forms a keystone in Gondwana, linking the East African and Malagasy orogens with Ediacaran–Cambrian orogenic belts in Sri Lanka and the Lützow Holm Bay region of Antarctica with similar aged belts in Mozambique, Malawi and Zambia. Ediacaran–Cambrian metamorphism and deformation in the Southern Granulite Terrane (SGT) reflect the past tectonic setting of this region as the leading vertex of Neoproterozoic India as it collided with Azania, the Congo–Tanzania–Bangweulu Block and Kalahari on one side and the Australia/Mawson continent on the other. The high-grade terranes of southern India are made up of four main tectonic units; from north to south these are a) the Salem Block, b) the Madurai Block, c) the Trivandrum Block, and d) the Nagercoil Block. The Salem Block is essentially the metamorphosed Dharwar craton and is bound to the south by the Palghat-Cauvery shear system — here interpreted as a terrane boundary and the Mozambique Ocean suture. The Madurai Block is interpreted as a continuation of the Antananarivo Block (and overlying Palaeoproterozoic sedimentary sequence — the Itremo Group) of Madagascar and a part of the Neoproterozoic microcontinent Azania. The boundary between this and the Trivandrum Block is the Achankovil Zone, that here is not interpreted as a terrane boundary, but may represent an Ediacaran rift zone reactivated in latest Ediacaran–Cambrian times.  相似文献   

5.
In a comprehensive U–Pb electron microprobe study of zircon and monazite from the khondalite belt of Trivandrum Block in southern Kerala, we present age data on five key metapelite locations (Nedumpara, Oottukuzhi, Kulappara, Poolanthara and Paranthal). The rocks here, characterized by the assemblage of garnet–sillimanite–spinel–cordierite–biotite–K–feldsapr–plagiocalse–quartz–graphite, have been subjected to granulite facies metamorphism under extreme thermal conditions as indicated by the stability of spinel + quartz and the presence of mesoperthites that equilibrated at ultrahigh-temperature (ca. 1000 °C) conditions. The oldest spot age of 3534 Ma comes from the core of a detrital zircon at Nedumpara and is by far the oldest age reported from this supracrustal belt. Regression of age data from several spot analyses in single zircons shows “isochrons” ranging from 3193 ± 72 to 2148 ± 94 Ma, indicating heterogeneous population of zircons derived from multiple provenance. However, majority of zircons from the various localities shows Neoproterozoic apparent ages with sharply defined peaks in individual localities, ranging between 644–746 Ma. The youngest zircon age of 483 Ma was obtained from the outermost rim of a grain that incorporates a relict core displaying ages in the range of 2061–2543 Ma.The cores of monazites also show apparent older ages of Palaeo-Mesoproterozoic range, which are mantled by late Neoproterozoic/Cambrian rims. The oldest monazite core has an apparent age of 2057 Ma. Extensive growth of new monazite during latest Neoproterozoic to Cambrian–Ordovician times is also displayed by grain cores with apparent ages up to 622 Ma. The homogeneous core of a sub-rounded monazite grain yielded a maximum age of 569 Ma, markedly younger than the 610 Ma age reported in a previous study from homogenous and rounded zircon core from a metapelite in Trivandrum Block. These younger ages from abraded grains that have undergone fluvial transport are interpreted to indicate that deposition within the khondalite belt was as young as, or later than, this range. Probability density plots indicate that majority of the monazite grain population belong to Late Proterozoic/Cambrian age (ca. 560–520 Ma) with major peaks defining sharp spikes in individual localities.The age data presented in this study indicate that the metasediments of the Trivandrum Block sourced from Archaean and Paleo-Mesoproterozoic crustal fragments that were probably assembled in older supercontinents like Ur and Columbia. The largest age population of zircons belong to the Neoproterozoic, and are obviously related to orogenies during the pre-assembly phase of Gondwana, possibly from terrains belonging to the East African Orogen. Several prominent age spikes within the broad late Neoproterozoic–Cambrian age range displayed by monazites denote the dynamic conditions and extreme thermal perturbations attending the birth of Gondwana. Our study further establishes the coherent link between India and Madagascar within the East Gondwana ensemble prior to the final assembly of the Gondwana supercontinent.  相似文献   

6.
We report here U–Pb electron microprobe ages from zircon and monazite associated with corundum- and sapphirine-bearing granulite facies rocks of Lachmanapatti, Sengal, Sakkarakkottai and Mettanganam in the Palghat–Cauvery shear zone system and Ganguvarpatti in the northern Madurai Block of southern India. Mineral assemblages and petrologic characteristics of granulite facies assemblages in all these localities indicate extreme crustal metamorphism under ultrahigh-temperature (UHT) conditions. Zircon cores from Lachmanapatti range from 3200 to 2300 Ma with a peak at 2420 Ma, while those from Mettanganam show 2300 Ma peak. Younger zircons with peak ages of 2100 and 830 Ma are displayed by the UHT granulites of Sengal and Ganguvarpatti, although detrital grains with 2000 Ma ages are also present. The Late Archaean-aged cores are mantled by variable rims of Palaeo- to Mesoproterozoic ages in most cases. Zircon cores from Ganguvarpatti range from 2279 to 749 Ma and are interpreted to reflect multiple age sources. The oldest cores are surrounded by Palaeoproterozoic and Mesoproterozoic rims, and finally mantled by Neoproterozoic overgrowths. In contrast, monazites from these localities define peak ages of between 550 and 520 Ma, with an exception of a peak at 590 Ma for the Lachmanapatti rocks. The outermost rims of monazite grains show spot ages in the range of 510–450 Ma.While the zircon populations in these rocks suggest multiple sources of Archaean and Palaeoproterozoic age, the monazite data are interpreted to date the timing of ultrahigh-temperature metamorphism in southern India as latest Neoproterozoic to Cambrian in both the Palghat–Cauvery shear zone system and the northern Madurai Block. The data illustrate the extent of Neoproterozoic/Cambrian metamorphism as India joined the Gondwana amalgam at the dawn of the Cambrian.  相似文献   

7.
Several bodies of granulites comprising charnockite, charno-enderbite, pelitic and calc-silicate rocks occur within an assemblage of granite gneiss/granitoid, amphibolite and metasediments (henceforth described as banded gneisses) in the central part of the Aravalli Mountains, northwestern India. The combined rock assemblage was thought to constitute an Archaean basement (BGC-II) onto which the successive Proterozoic cover rocks were deposited. Recent field studies reveal the occurrence of several bodies of late-Palaeoproterozoic (1725 and 1621 Ma) granulites within the banded gneisses, which locally show evidence of migmatization at c. 1900 Ma coeval with the Aravalli Orogeny. We report single zircon ‘evaporation’ ages together with information from LA-ICP-MS U-Pb zircon datings to confirm an Archaean (2905 — ca. 2500 Ma) age for the banded gneisses hosting the granulites. The new geochronological data, therefore, suggest a polycyclic evolution for the BGC-II terrane for which the new term Sandmata Complex is proposed. The zircon ages suggest that the different rock formations in the Sandmata Complex are neither entirely Palaeoproterozoic in age, as claimed in some studies nor are they exclusively Archaean as was initially thought. Apart from distinct differences in the age of rocks, tectono-metamorphic breaks are observed in the field between the Archaean banded gneisses and the Palaeoproterozoic granulites. Collating the data on granulite ages with the known tectono-stratigraphic framework of the Aravalli Mountains, we conclude that the evolution and exhumation of granulites in the Sandmata Complex occurred during a tectono-magmatic/metamorphic event, which cannot be linked to known orogenic cycles that shaped this ancient mountain belt. We present some field and geochronologic evidence to elucidate the exhumation history and tectonic emplacement of the late Palaeoproterozoic, high P-T granulites into the Archaean banded gneisses. The granulite-facies metamorphism has been correlated with the thermal perturbation during the asymmetric opening of Delhi basins at around 1700 Ma.  相似文献   

8.
The Khetri region forms a late Palaeoproterozoic igneous–metamorphic complex in NE Rajasthan, India. Seven granitoid plutons of the Khetri complex have been studied for zircon U–Pb and Pb–Pb dating along with whole-rock and Nd–Sr isotope geochemistry to provide new constraints on the Palaeoproterozoic magmatic activity in the Aravalli orogen of northwestern India. Most intrusives show evidence of moderate to extreme albitisation forming microcline–albite granite and albite granite, respectively. The rocks are metaluminous to weakly peraluminous, largely ferroan and intraplate A-type granites. The U–Pb zircon ages for four plutons cover a time span of 1732–1682 Ma, whereas Pb–Pb zircon evaporation data for three intrusives indicate minimum emplacement ages between 1671 and 1537 Ma. The Nd–Sr isotopic systematics suggest the involvement of Neoarchaean to Palaeoproterozoic crustal components in the petrogenesis of these granitoids. A regional survey of late Palaeoproterozoic ages in the Aravalli orogen provides evidence for a geographically widespread extension-related event in the northwestern Indian shield about 1720–1700 Ma ago. The record of comparable ages and the magmatic history reported in parts of North America and the North China Craton may indicate the significance of this event for the rift tectonics of the hypothetical supercontinent Columbia.  相似文献   

9.
This work provides five new U–Pb zircon dating and the corresponding Nd isotope data for felsic granulites from the south Itabuna-Salvador-Curaçá Block (ISCB), in the São Francisco Craton, Brazil. Three major sets of felsic granulites can be recognised. The oldest set is tonalitic in composition and of TTG affinity. It is Archaean in age with magmatic zircon cores dated at 2675 ± 11 Ma by LA-ICPMS and up to ca 2.7–2.9 Ga by SHRIMP on an other sample. It exhibits epsilon Nd values between ?8 and ?11 at 2.1 Ga. This Nd signature is similar to that of granulites found in the western Archaean Jequié Block. Cartographically, this set of Archaean terrains represents at least 50% of the granulites in the studied area. The second set corresponds to a Palaeoproterozoic calc-alkaline tonalitic suite with zircon ages from 2019 ± 19 Ma to 2191 ± 10 Ma and epsilon Nd values between ?3 and ?4 at 2.1 Ga, corresponding partially to a newly formed crust. The third set of granulites is also Palaeoproterozoic. It is shoshonitic to monzonitic in composition and synchronous with the high grade metamorphism dated by metamorphic zircons at 2086 ± 7 Ma (average of five samples). The Nd isotope signature for this alkaline set is similar to that of the Palaeoproterozoic calc-alkaline one. Nd isotopes appear to be a very efficient tool to distinguish Archaean from Palaeoproterozoic felsic protoliths in granulitic suites of the Itabuna-Salvador-Curaçá Block (ISCB). Finally, the southern part of the ISCB is composed of a mixture of Archaean and Palaeoproterozoic protoliths, in similar amounts, suggesting that it was probably an active margin between 2.1 and 2.2 Ga located on the eastern border of the Archaean Jequié Block. A major crustal thickening process occurred at ca 2.09 Ga in the ISCB and seems significantly younger towards the west, in the Jequié granulites, where an average of 2056 ± 9 Ma is determined for the high grade event.  相似文献   

10.
The potential genetic link between granites and their host sediments can be assessed using zircon age inheritance patterns. In the Lachlan fold belt, southeastern Australia, granites and associated high-grade metasedimentary rocks intrude low-grade Ordovician country rock. This relationship is well-exposed in the Tallangatta region, northeast Victoria (part of the Wagga-Omeo Metamorphic Complex). In this region granites (two I-types and two S-types) have intruded during the mid-late Silurian between approximately 410–430 Ma based on the ages of magmatic zircons. The age spectra for inherited zircons from the granites have been compared with those of detrital zircons from the enclosing low- and high-grade metasediments. In broad terms, both for detrital zircons in all four sediments and for inherited zircons in three of the four granites, the dominant ages are early Paleozoic and Late Precambrian, with sporadic older Precambrian ages extending up to 3.5 Ga. The ages of the youngest detrital zircons from the low-grade Lockhart and Talgarno terranes limit the time of sedimentation to ca. 466 Ma or younger. The youngest detrital zircons from two samples of the high-grade Gundowring terrane are 473 Ma, making these sediments Ordovician or younger, not Cambrian as originally suggested. However, the individual age spectra for the four selected metasediments are not well matched when closely examined. The age spectra of the inherited zircons in the granites also do not adequately match those in any of the metasediments. Thus, the metasediments might not be representative of the actual source rocks of the granites. While the exact source of the granites cannot be identified from the analysed samples, the existence of a large population of ca. 495 Ma inherited zircon grains in the S-type granites requires that the granite source contains a significant proportion of Cambrian or younger material. This does not preclude the existence of a Precambrian basement to the Lachlan fold belt but indicates that at the level of S-type magma generation, a Cambrian and/or younger protolith is required. Received: 28 August 1998 / Accepted: 7 July 1999  相似文献   

11.
An ion-microprobe (SIMS) U-Pb zircon dating study on four samples of Precambrian metasediments from the high-grade Bamble Sector, southern Norway, gives the first information on the timing of discrete crust-forming events in the SW part of the Baltic Shield. Recent Nd and Pb studies have indicated that the sources of the clastic metasediments in this area have crustal histories extending back to 1.7 to 2.1 Ga, although there is no record of rocks older than 1.6 Ga in southern Norway. The analysed metasediments are from a sequence of intercalated, centimetre to 10-metre wide units of quartzites, semi-metapelites, metapelites and mafic granulites. The zircons can be grouped in two morphological populations: (1) long prismatic; (2) rounded, often flattened. The BSE images reveal that both populations consist of oscillatory zoned, rounded and corroded cores (detrital grains of magmatic origin), surrounded by homogeneous rims (metamorphic overgrowths). The detrital zircons have 207Pb/206Pb ages between 1367 and 1939 Ma, with frequency maxima in the range 1.85 to 1.70 Ga and 1.60 to 1.50 Ga. There is no correlation between crystal habit and age of the zircon. One resorbed, inner zircon core in a detrital grain is strongly discordant and gives a composite inner core-magmatic outer core 207Pb/206Pb age of 2383 Ma. Two discrete, unzoned zircons have 207Pb/206Pb ages of 1122 and 1133 Ma, representing zircon growth during the Sveconorwegian high-grade metamorphism. Also the μm wide overgrowths, embayments in the detrital cores and apparent “inner cores” which represent secondary metamorphic zircon growth in deep embayments in detrital grains, are of Sveconorwegian age. The composite-detrital-metamorphic zircon analyses give generally discordant 206Pb/238U versus 207Pb/235U ratios and maximum 207Pb/206Pb ages of 1438 Ma. These data demonstrate the existence of a protocrust of 1.7 to 2.0 Ga in the southwestern part of the Baltic Shield, implying a break in the overall westward younging trend of the Precambrian crust, inferred from the southeastern part of the Baltic Shield. Received: 8 April 1997 / Accepted: 14 July 1997  相似文献   

12.
 Single zircon U–Pb dating combined with 207Pb/206Pb ages obtained by the evaporation method constrains the emplacement of tonalitic, trondhjemitic, and granodioritic orthogneisses of the Moldanubian zone in the Black Forest between 500 and 510 Ma. Two detrital zircon populations of 1.9 and 1.6 Ga indicate Early-Middle Proterozoic material in the former setting of the basement. The initial eNd values range from –0.1 to –3.4 and mean crustal residence ages of 1.0–1.4 Ga are consistent with involvement of Early-Middle Proterozoic crust, and a subordinate juvenile component probably originating from subduction-related melting of the mantle. The orthogneisses have fractionated REE patterns and slightly higher K2O/Na2O ratios than typical low-K tonalite–trondhjemite–granite suites. The chemical data are interpreted as evidence for melting of amphibolite and contributions from evolved crust. The emplacement of the orthogneisses was superceded by a high-temperature metamorphic event at ∼480 Ma which we interpret as a result of lithospheric thinning in a marginal basin behind a Cambrian magmatic arc. Received: 29 March 1999 / Accepted: 25 August 1999  相似文献   

13.
邹雷  刘平华  刘利双  王伟  田忠华 《地球科学》2020,45(9):3313-3329
叠布斯格杂岩作为阿拉善地块变质程度最高、形成时代最老的变质岩系,是进一步深入认识阿拉善地块形成、演化与构造亲缘性的最佳对象.本文对叠布斯格杂岩中黑云斜长片麻岩开展了系统的野外地质调查、岩相学观察和锆石U-Pb年代学研究.碎屑锆石U-Pb定年和野外地质调查表明,叠布斯格杂岩中黑云斜长片麻岩碎屑锆石年龄介于2 177~2 010 Ma,最小年龄峰值为2 050 Ma,结合变质锆石年龄数据,限定其沉积时代在2 050~1 969 Ma之间.区域对比研究表明,阿拉善地块叠布斯格杂岩中黑云斜长片麻岩与西阿拉善地块龙首山杂岩、华北克拉通孔兹岩带（尤其是孔兹岩带西部）中的变沉积岩系在沉积时代与源区特征上具有高度的一致性.此外,变质锆石U-Pb定年和微量元素分析表明,叠布斯格杂岩中黑云斜长片麻岩记录了古元古代晚期1 969~1 811 Ma连续的变质年龄.对比分析阿拉善地块与孔兹岩带变沉积岩系的碎屑锆石年龄谱系-变质时代表明,阿拉善地块与孔兹岩带在古元古代可能具有相同的沉积-变质演化历史,在古元古代期间阿拉善地块应是华北克拉通孔兹岩带的西延部分.   相似文献   

14.
Understanding Neoproterozoic crustal evolution is fundamental to reconstructing the Gondwana supercontinent, which was assembled at this time. Here we report evidence of Cryogenian crustal reworking in the Madurai Block of the Southern Granulite Terrane of India. The study focuses on a garnet-bearing granite–charnockite suite, where the granite shows in situ dehydration into patches and veins of incipient charnockite along the contact with charnockite. The granite also carries dismembered layers of Mg–Al-rich granulite. Micro-textural evidence for dehydration of granite in the presence of CO2-rich fluids includes the formation of orthopyroxene by the breakdown of biotite, neoblastic zircon growth in the dehydration zone, at around 870°C and 8 kbar. The zircon U–Pb ages suggest formation of the granite, charnockite, and incipient charnockite at 836 ± 73, 831 ± 31, and 772 ± 49 Ma, respectively. Negative zircon εHf (t) (?5 to ?20) values suggest that these rocks were derived from a reworked Palaeoproterozoic crustal source. Zircon grains in the Mg–Al-rich granulite record a spectrum of ages from ca. 2300 to ca. 500 Ma, suggesting multiple provenances ranging from Palaeoproterozoic to mid-Neoproterozoic, with neoblastic zircon growth during high-temperature metamorphism in the Cambrian. We propose that the garnet-bearing granite and charnockite reflect the crustal reworking of aluminous crustal material indicated by the presence of biotite + quartz + aluminosilicate inclusions in the garnet within the granite. This crustal source can be the Mg–Al-rich layers carried by the granite itself, which later experienced high-temperature regional metamorphism at ca. 550 Ma. Our model also envisages that the CO2 which dehydrated the garnet-bearing granite generating incipient charnockite was sourced from the proximal massive charnockite through advection. These Cryogenian crustal reworking events are related to prolonged tectonic activities prior to the final assembly of the Gondwana supercontinent.  相似文献   

15.
对德令哈市石底泉地区宗务隆构造带内花岗闪长岩进行了锆石U-Pb测年和岩石地球化学分析。岩石地球化学分析结果显示,其具有岛弧或活动大陆边缘花岗岩的属性,原岩可能为新元古代早期(870.0±4.5Ma左右)硅铝地壳或地壳物质熔融的产物,它们的形成与板块的俯冲作用有关LA-ICP-MS锆石U-Pb测年显示2组年龄,分别为870.0±4.5Ma和443.3±2.3Ma。其中870.0±4.5Ma代表源区继承锆石的年龄,反映了新元古代时期的构造岩浆事件;443.3±2.3Ma代表花岗闪长岩岩浆锆石的结晶年龄,即花岗闪长岩侵位于晚奥陶世。综合分析可以确定,宗务隆构造带内花岗闪长岩与柴达木盆地北缘加里东期的板块汇聚碰撞造山作用有关,证实了宗务隆构造带内岩体形成时代并非全部是海西期一印支期。  相似文献   

16.
Several types of growth morphologies and alteration mechanisms of zircon crystals in the high-grade metamorphic Ivrea Zone (IZ) are distinguished and attributed to magmatic, metamorphic and fluid-related events. Anatexis of pelitic metasediments in the IZ produced prograde zircon overgrowths on detrital cores in the restites and new crystallization of magmatic zircons in the associated leucosomes. The primary morphology and Th-U chemistry of the zircon overgrowth in the restites show a systematic variation apparently corresponding to the metamorphic grade: prismatic (prism-blocked) low-Th/U types in the upper amphibolite facies, stubby (fir-tree zoned) medium-Th/U types in the transitional facies and isometric (roundly zoned) high-Th/U types in the granulite facies. The primary crystallization ages of prograde zircons in the restites and magmatic zircons in the leucosomes cannot be resolved from each other, indicating that anatexis in large parts of the IZ was a single and short lived event at 299 ± 5 Ma (95% c. l.). Identical U/Pb ages of magmatic zircons from a metagabbro (293 ± 6 Ma) and a metaperidotite (300 ± 6 Ma) from the Mafic Formation confirm the genetic context of magmatic underplating and granulite facies anatexis in the IZ. The U-Pb age of 299 ± 5 Ma from prograde zircon overgrowths in the metasediments also shows that high-grade metamorphic (anatectic) conditions in the IZ did not start earlier than 20 Ma after the Variscan amphibolite facies metamorphism in the adjacent Strona–Ceneri Zone (SCZ). This makes it clear that the SCZ cannot represent the middle to upper crustal continuation of the IZ. Most parts of zircon crystals that have grown during the granulite facies metamorphism became affected by alteration and Pb-loss. Two types of alteration and Pb-loss mechanisms can be distinguished by cathodoluminescence imaging: zoning-controlled alteration (ZCA) and surface-controlled alteration (SCA). The ZCA is attributed to thermal and/or decompression pulses during extensional unroofing in the Permian, at or earlier than 249 ± 7 Ma. The SCA is attributed to the ingression of fluids at 210 ± 12 Ma, related to hydrothermal activity during the breakup of the Pangaea supercontinent in the Upper Triassic/Lower Jurassic. Received: 7 July 1998 / Accepted: 4 November 1998  相似文献   

17.
阿拉善地块西部的北大山地区识别出两期韧性变形构造,早期的韧性变形以从南(南东)向北(北西)逆冲为特征,晚期的韧性变形以近东西向左行剪切为特征。利用LA-ICP-MS锆石U-Pb测年方法,在记录早期韧性变形的片麻岩中获得锆石的变质年龄为422±1 Ma,代表晚志留世变质事件;在记录晚期韧性变形的2件二长花岗岩中获得锆石结晶年龄分别为326.2±1.2 Ma和323.8±2.6 Ma,代表早石炭世岩浆活动。结合前人研究成果,发现阿拉善地块西部北大山地区的两期韧性变形特征、变形时代与阿拉善地块东部巴彦乌拉山-狼山地区相似,同时阿拉善地块东、西部晚奥陶世-石炭纪多期岩浆活动的岩石类型、期次、地化及Nd同位素等特征也非常相似。以上特征表明,阿拉善地块东、西部经历了相同的构造演化历史,形成一条发育在早前寒武纪变质基底之上、呈近东西向弧形展布的晚奥陶世-石炭纪构造-岩浆岩带(西起桃花拉山,经北大山、诺尔公-巴彦乌拉山,东到狼山),其成因与古亚洲洋的俯冲造山事件密切相关。  相似文献   

18.
《Gondwana Research》2015,28(4):1392-1406
The Ider Complex of the Tarbagatai Block in northwestern Mongolia is part of a Precambrian microcontinental terrane in the Central Asian Orogen Belt and has experienced a polymetamorphic tectono-metamorphic evolution. We have investigated an enderbitic gneiss, derived from a quartz diorite and a charnockite, derived from a leucogranite, and zircon SHRIMP data reveal late Archaean protolith ages of 2520–2546 Ma for these rocks. Metamorphic overgrowth on these zircons as well as newly-formed metamorphic zircons document a high-temperature metamorphic event (T = 930–950 °C) at about 1855–1860 Ma. Nd whole-rock isotopic systematics show these and other gneisses of the Ider Complex straddling the CHUR-line in a Nd isotope evolution diagram, suggesting both crustal reworking and input of some juvenile material, with Nd model ages ranging between ca. 2.5 and 3.1 Ga. Hf-in-zircon isotopic data provide a similar pattern and also yielded Archaean Hf crustal model ages. The metamorphic zircons seem to have inherited their Hf isotopic composition from the igneous grains, suggesting a complex process of dissolution, transportation, and re-precipitation involving a fluid phase during high-grade metamorphism. The zircon age patterns do not make it possible to unambiguously assign the Tarbagatai Block to any of the cratons bordering the Central Asian Orogenic Belt, since age peaks at ca. 2520–2550 and ca. 1860 Ma are common in the Siberian, North China and Tarim cratons.  相似文献   

19.
The Baoshan block of the Tethyan Yunnan, southwestern China, is considered as northern part of the Sibumasu microcontinent. Basement of this block that comprises presumably greenschist-facies Neoproterozoic metamorphic rocks is covered by Paleozoic to Mesozoic low-grade metamorphic sedimentary rocks. This study presents zircon ages and Nd–Hf isotopic composition of granites generated from crustal reworking to reveal geochemical feature of the underlying basement. Dating results obtained using the single zircon U–Pb isotopic dilution method show that granites exposed in the study area formed in early Paleozoic (about 470 Ma; Pingdajie granite) and in late Yanshanian (about 78–61 Ma, Late Cretaceous to Early Tertiary; Huataolin granite). The early Paleozoic granite contains Archean to Mesoproterozoic inherited zircons and the late Yanshanian granite contains late Proterozoic to early Paleozoic zircon cores. Both granites have similar geochemical and Nd–Hf isotopic charateristics, indicating similar magma sources. They have whole-rock T DM(Nd) values of around 2,000 Ma and zircon T DM(Hf) values clustering around 1,900–1,800 and 1,600–1,400 Ma. The Nd–Hf isotopic data imply Paleoproterozoic to Mesoproterozoic crustal material as the major components of the underlying basement, being consistent with a derivation from Archean and Paleoproterozoic terrains of India or NW Australia. Both granites formed in two different tectonic events similarly originated from intra-crustal reworking. Temporally, the late Yanshanian magmatism is probably related to the closure of the Neotethys ocean. The early Paleozoic magmatism traced in the Baoshan block indicates a comparable history of the basements during early Paleozoic between the SE Asia and the western Tethyan belt, such as the basement outcrops in the Alpine belt and probably in the European Variscides that are considered as continental blocks drifting from Gondwana prior to or simultaneously with those of the SE Asia.  相似文献   

20.
The Songpan-Garze fold belt, located in the eastern part of the Tibetan Plateau, covers a huge triangular area bounded by the Yangtze (South China), the North China and the Tibetan Plateau blocks. In the northeastern part of the Songpan-Garze fold belt, the Yanggon and Maoergai granitoids provide insights into regional tectono-magmatic events, basement nature and tectonic evolution. U–Pb zircon SHRIMP dating shows that the Yanggon and Maoergai granitoids have magmatic crystallization ages of 221 ± 3.8 Ma and 216 ± 5.7 Ma, respectively. Both the granitoids display adakitic geochemical signatures, suggesting that their magma was derived from partial melting of thickened lower crust. Pb–Sr–Nd isotopic compositions for granitoids reveal that there is an unexposed Proterozoic basement in the Songpan-Garze belt, which has an affinity with the Yangtze block. During development of the Paleo-Tethys ocean, the basement of the Songpan-Garze belt would be a peninsula approaching the Paleo-Tethys ocean from the Yangtze block.  相似文献   

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