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1.
Migmatites are predominant in the North Qinling (NQ) orogen, but their formation ages are poorly constrained. This paper presents a combined study of cathodoluminescence imaging, U–Pb age, trace element and Hf isotopes of zircon in migmatites from the NQ unit. In the migmatites, most zircon grains occur as new, homogeneous crystals, while some are present as overgrowth rims around inherited cores. Morphological and trace element features suggest that the zircon crystals are metamorphic and formed during partial melting. The inherited cores have oscillatory zoning and yield U–Pb ages of c. 900 Ma, representing their protolith ages. The early Neoproterozoic protoliths probably formed in an active continental margin, being a response to the assembly of the supercontinent Rodinia. The migmatite zircon yields Hf model ages of 1911 ± 20 to 990 ± 22 Ma, indicating that the protoliths were derived from reworking of Palaeoproterozoic to Neoproterozoic crustal materials. The anatexis zircon yields formation ages ranging from 455 ± 5 to 420 ± 4 Ma, with a peak at c. 435 Ma. Combined with previous results, we suggest that the migmatization of the NQ terrane occurred at c. 455–400 Ma. The migmatization was c. 50 Ma later than the c. 490 Ma ultra‐high‐P (UHP) metamorphism, indicating that they occurred in two independent tectonic events. By contrast, the migmatization was coeval with the granulite facies metamorphism and the granitic magmatism in the NQ unit, which collectively argue for their formation due to the northward subduction of the Shangdan Ocean. UHP rocks were distributed mainly along the northern margin and occasionally in the inner part of the NQ unit, indicating that they were exhumed along the northern edge and detached from the basement by the subsequent migmatization process.  相似文献   

2.
Migmatite gneisses are widespread in the Dabie orogen, but their formation ages are poorly constrained. Eight samples of migmatite, including leucosome, melanosome, and banded gneiss, were selected for U–Pb dating and Hf isotope analysis. Most metamorphic zircon occurs as overgrowths around inherited igneous cores or as newly grown grains. Morphological and internal structure features suggest that their growth is associated with partial melting. According to the Hf isotope ratio relationships between metamorphic zircon and inherited cores, three formation mechanisms for metamorphic zircon can be determined, which are dissolution–reprecipitation of pre‐existing zircon, breakdown of Zr‐bearing phase other than zircon in a closed system and crystallization from externally derived Zr‐bearing melt. Four samples contain magmatic zircon cores, yielding upper intercept U–Pb ages of 807 ± 35–768 ± 12 Ma suggesting that the protoliths of the migmatites are Neoproterozoic in age. The migmatite zircon yields weighted mean two‐stage Hf model ages of 2513 ± 97–894 ± 54 Ma, indicating reworking of both juvenile and ancient crustal materials at the time of their protolith formation. The metamorphic zircons give U–Pb ages of 145 ± 2–120 ± 2 Ma. The oldest age indicates that partial melting commenced prior to 145 Ma, which also constrains the onset of extensional tectonism in this region to pre‐145 Ma. The youngest age of 120 Ma was obtained from an undeformed granitic vein, indicating that deformation in this area was complete at this time. Two major episodes of partial melting were dated at 139 ± 1 and 123 ± 1Ma. The first episode of partial melting is obviously older than the timing of post‐collision magmatism, corresponding to regional extension. The second episode of partial melting is coeval with the widespread post‐collision magmatism, indicating the gravitational collapse and delamination of the orogenic lithospheric keel of the Dabie orogen, which were possibly triggered by the uprising of the Cretaceous mid‐Pacific superplume.  相似文献   

3.
《Precambrian Research》2001,105(2-4):315-330
U–Pb isotopic dating has been carried out on titanites and rutiles from the Karelian Protocraton, the Belomorian Mobile Belt and the intervening junction zone. These are some of the principal Archaean crustal units in the Baltic Shield which have undergone regeneration to various degrees during the Palaeoproterozoic. Palaeoproterozoic resetting of U–Pb titanite ages was complete in the Belomorian Belt and almost complete in the junction zone, while it hardly affected the Karelian Protocraton. In the latter, major crustal cooling occurred at 2.71–2.69 Ga after a major igneous event at 2.74–2.72 Ga, but a tectonothermal event at 2.65–2.64 Ga was less comprehensive. In the Belomorian Belt, a northeastern marginal zone immediately underlying the collisional-thrusting suture of the Lapland-Kola orogen has somewhat higher titanite ages of ca. 1.94–1.87 Ga than the central zone where these ages range between 1.87 and 1.82 Ga. Comparison between the titanite and rutile U–Pb ages suggests a postorogenic cooling rate between 2 and 4°/Ma in these parts of the Belt. The Neoarchaean junction zone between the Karelian and Belomorian provinces was a zone of particularly intense tectonic, magmatic and hydrothermal activity during or after the Palaeoproterozoic Lapland-Kola orogeny. Dominant, newly grown titanites in that zone have ages as young as 1.78–1.75 Ga, and the age differences between the titanite and rutile U–Pb ages are substantially smaller than elsewhere.  相似文献   

4.
Absolute ages of migmatization in the polymetamorphic, parautochthonous basement of the Sveconorwegian Province, Sweden, have been determined using U–Pb ion probe analysis of zircon domains that formed in leucosome of migmatitic orthogneisses. Migmatite zircon was formed by recrystallization whereas dissolution–reprecipitation and neocrystallization were subordinate. The recrystallized migmatite zircon was identified by comparison of zircon in mesosomes and leucosomes. It is backscatter electron‐bright, U‐rich (800–4400 ppm) with low Th/U‐ratios (generally 0.01–0.1), unzoned or ‘oscillatory ghost zoned’, and occurs as up to 100 μm‐thick rims with transitional contacts to cores of protolith zircon. Protolith ages of 1686 ± 12 and 1668 ± 11 Ma were obtained from moderately resorbed, igneous zircon crystals (generally Th/U = 0.5–1.5, U < 300 ppm) in mesosomes; protolith zircon is also present as resorbed cores in the leucosomes. Linkage of folding, synchronous migmatization and formation of recrystallized zircon rims allowed direct dating of south‐vergent folding at 976 ± 7 Ma. At a second locality, similar recrystallized zircon rims in leucosome date pre‐Sveconorwegian migmatization at 1425 ± 7 Ma; an upper age bracket of 1394 ± 12 Ma for two overprinting phases of deformation (upright folding along gently SSW‐plunging axes and stretching in ESE) was set by zircon in a folded metagranitic dyke. Lower age brackets for these events were set at 952 ± 7 and 946 ± 8 Ma by zircon in two crosscutting and undeformed granite–pegmatite dykes. Together with previously published data the present results demonstrate: (i) Tectonometamorphic reworking during the Hallandian orogenesis at 1.44–1.42 Ga, resulting in migmatization and formation of a coarse gneissic layering. (ii) Sveconorwegian continent–continent collision at 0.98–0.96 Ga, involving (a) emplacement of an eclogite unit, (b) regional high‐pressure granulite facies metamorphism, (c) southvergent folding, subhorizontal, east–west stretching and migmatization, all of which caused overprint or transposition of older Mesoproterozoic and Sveconorwegian structures. The Sveconorwegian migmatization and folding took place during or shortly after the emplacement of Sveconorwegian eclogite and is interpreted as a result of north–south shortening, synchronous with east–west extension and unroofing during late stages of the continent–continent collision.  相似文献   

5.
Whole‐rock geochemistry, zircon U–Pb and molybdenite Re–Os geochronology, and Sr–Nd–Hf isotopes analyses were performed on ore‐related dacite porphyry and quartz porphyry at the Yongping Cu–Mo deposit in Southeast China. The geochemical results show that these porphyry stocks have similar REE patterns, and primitive mantle‐normalized spectra show LILE‐enrichment (Ba, Rb, K) and HFSE (Th, Nb, Ta, Ti) depletion. The zircon SHRIMP U–Pb geochronologic results show that the ore‐related porphyries were emplaced at 162–156 Ma. Hydrothermal muscovite of the quartz porphyry yields a plateau age of 162.1 ± 1.4 Ma (2σ). Two hydrothermal biotite samples of the dacite porphyry show plateau ages of 164 ± 1.3 and 163.8 ± 1.3 Ma. Two molybdenite samples from quartz+molybdenite veins contained in the quartz porphyry yield Re–Os ages of 156.7 ± 2.8 Ma and 155.7 ± 3.6 Ma. The ages of molybdenite coeval to zircon and biotite and muscovite ages of the porphyries within the errors suggest that the Mo mineralization was genetically related to the magmatic emplacement. The whole rocks Nd–Sr isotopic data obtained from both the dacite and quartz porphyries suggest partial melting of the Meso‐Proterozoic crust in contribution to the magma process. The zircon Hf isotopic data also indicate the crustal component is the dominated during the magma generation.  相似文献   

6.
U–Pb and Rb–Sr dating was undertaken in combination with P–T estimates to (1) constrain the time of ultrahigh-pressure (UHP) eclogite formation in the Stadlandet UHP province of Norway, (2) date later crustal melting–migmatization of the eclogite country gneisses, and (3) temporally trace post-migmatite cooling and retrogression under amphibolite facies metamorphic conditions. In contrast to earlier U–Pb studies which used accessory minerals from the gneisses, we focused on the direct dating of minerals defining the HP assemblage. For the eclogite, rutile and omphacite fractions were analyzed for U–Pb, and from an adjacent migmatite leucosome titanites and K-feldspar. For Rb–Sr dating, phengite was measured for the eclogite, and biotite for two leucosome layers of the migmatite–eclogite complex. A U–Pb age of 389±7 (2σ) Ma is obtained if the full set of 12 rutile and five omphacite analyses is regressed (MSWD: 16), and 389±2 Ma for those nine data which strictly satisfy isochron conditions (MSWD: 0.78). The 389-Ma age is interpreted to date equilibration and freezing of the eclogite paragenesis at maximum temperatures of 770 °C, reached during decompression to 1.8 GPa. Decompression from 2.8 to 1.8 GPa occurred in the partial melting domain of granitic crust, with the migmatites being dated at 375±6 Ma by titanite and K-feldspar from an eclogite-adjacent granitic leucosome. This titanite age also shows that the U–Pb chronometer in rutile is very robust to high temperatures—it remained a closed system for at least 14 million years, at temperatures in excess to 650 °C. After decompression and migmatization, exhumation is accompanied by rapid cooling to reach the 300 °C isograde by 357± 9 Ma, determined by a biotite isochron for a leucosome in a slightly shallower structural level. In considering that the time of maximum pressure is bracketed by early zircon crystallization during subduction and later omphacite–rutile equilibration in the eclogites, an exhumation rate of 5 mm/year is deduced for initial exhumation, occurring between 394 and 389 Ma. For subsequent cooling from 770 to 600 °C, we obtain a rate of 2.3±1.3 mm/year. First stages of exhumation most likely occurred under an overall compressional regime, whereas Devonian basin formation is associated to detachment movements during 389–375 Ma exhumation. This period of extension is followed by a much younger, decoupled thermal phase at 327±5 Ma, occurring under static conditions within very restricted zones, most likely in association with the circulation of fluid phases along old discontinuities. Initial isotopic signatures of Sr and Pb substantiate Paleo- to Meso-Proterozoic crust formation times of the Stadlandet UHP province precursor lithologies.  相似文献   

7.
The Koktokay No. 3 pegmatite is the largest Li–Be–Nb–Ta–Cs pegmatitic rare‐metal deposit of the Chinese Altai orogenic belt, and is famous for its concentric ring zonation pattern (nine internal zones). However, the formation age and evolution time span have been controversial. Here, we present the results of LA‐ICP–MS zircon U–Pb dating and muscovite 40Ar–39Ar dating. Four groups of zircon U–Pb ages (～210 Ma, ～193–198 Ma, ～186–187 Ma and ～172 Ma) for Zones II, V, VI, VII, and VIII, and a weighed mean 206Pb/238U age of 965 ± 11 Ma for Zone IV are identified. Also, Zones II, IV, and VI have muscovite 40Ar–39Ar plateau ages of 179.7 ± 1.1 Ma, 182.1 ± 1.0 Ma, and 181.8 ± 1.1 Ma, respectively. Considering previous U–Pb age studies (Zones I, V, and VII), the ages of emplacement, Li mineralization peak, hydrothermal stage of the No. 3 pegmatite are in ranges of 193–198 Ma, 184–187 Ma and 172–175 Ma, with weighted mean 206Pb–238U ages of 194.8 ± 2.3 Ma, 186.6 ± 1.3 Ma and 173.1 ± 3.9 Ma, respectively. The No. 3 pegmatite formed in the early Jurassic. The results of xenocrysts suggest that there is another pegmatite forming event of around 210 Ma in the mining district and the old zircon U–Pb ages imply that Neoproterozoic crustal rocks pertain to sources of the No. 3 pegmatite. Including the previous muscovite 40Ar–39Ar age studies (Zones I and V), a cooling age range of 177–182 Ma is considered as the time of hydrothermal stage and end of formation. The evolution process of the No. 3 pegmatite lasted 16 Ma. Therein, the magmatic stage continued for 9–11 Myr and the magmatic–hydrothermal transition and hydrothermal stages were sustained at 5–7 Ma. These time spans are long because of huge scale, cupola shape, large formation depth, and complex internal zoning patterns and formation processes. Considering some pegmatite dikes in the Chinese Altai, there is an early Jurassic pegmatite forming event.  相似文献   

8.
Jilin Province in NE China lies on the eastern edge of the Xing–Meng Orogenic Belt. Mineral exploration in this area has resulted in the discovery of numerous large, medium, and small sized Cu, Mo, Au, and Co deposits. To better understand the formation and distribution of both the porphyry and skarn types Cu deposits of the region, we examined the geological characteristics of the deposits and applied zircon U–Pb and molybdenite Re–Os isotope dating to constrain the age of the mineralization. The Binghugou Cu deposit yields a zircon U–Pb age for quartz diorite of 128.1 ± 1.6 Ma; the Chang'anpu Cu deposit yields a zircon U–Pb age for granite porphyry of 117.0 ± 1.4 Ma; the Ermi Cu deposit yields a zircon U–Pb age for granite porphyry of 96.8 ± 1.1 Ma; the Tongshan Cu deposit yields molybdenite Re–Os model ages of 128.7 to 130.2 Ma, an isochron age of 129.0 ± 1.6 Ma, and a weighted mean model age of 129.2 ± 0.7 Ma; and the Tianhexing Cu deposit yields molybdenite Re–Os model ages of 113.9 to 115.2 Ma, an isochron age of 114.7 ± 1.2 Ma, and a weighted mean model age of 114.7 ± 0.7 Ma. The new ages, combined with existing geochronology data, show that intense porphyry and skarn types Cu mineralization was coeval with Cretaceous magmatism. The geotectonic processes responsible for the genesis of the Cu mineralization were probably related to lithospheric thinning. By analyzing the accumulated molybdenite Re–Os, zircon U–Pb, and Ar–Ar ages for NE China, it is concluded that the Cu deposits formed during multiple events coinciding with periods of magmatic activity. We have identified five phases of mineralization: early Paleozoic (~476 Ma), late Paleozoic (286.5–273.6 Ma), early Mesozoic (~228.7 Ma), Jurassic (194.8–137.1 Ma), and Cretaceous (131.2–96.8 Ma). Although Cu deposits formed during each phase, most of the Cu mineralization occurred during the Cretaceous.  相似文献   

9.
This paper addresses the relationships between relic amphibole-eclogite facies (AE) eclogites and their host units, Archaean amphibolites, enveloped by Archaean tonalite–trondhjemite–granodiorite (TTG) gneisses, in the Kuru-Vaara study area in the northern Belomorian Province. According to observational constraints, the crystallization of the relic peak omphacite + Mg-garnet ± kyanite assemblage and the subsequent replacement of omphacite by clinopyroxene–plagioclase symplectite occurred before the earliest deformational, metamorphic, and migmatization events that are recorded in the amphibolites. The amphibolites and their TTG hosts have a shared deformational and metamorphic history that is composed of the Archaean and Palaeoproterozoic periods. This history favours the conclusion that the AE metamorphism recorded in the relic eclogites within the amphibolites occurred during the Mesoarchaean to Neoarchaean periods. The deformation and metamorphism of the amphibolite facies of the second period resulted from the Lapland–Kola collisional orogeny at 1.91–1.93 Ga, which led to eclogite–high-pressure granulite (E–HPG) facies conditions in the lowermost portions of the over-thickened crust in Belomorian Province (the southwestern foreland of the Lapland–Kola collisional orogen). The Palaeoproterozoic E–HPG overprint was reported from the Palaeoproterozoic Gridino mafic dikes. Although the ages of the oldest low Th/U zircons are close to the time of the Lapland–Kola collision, the low Th/U 1.9–1.8 Ga zircons reflect a zircon response to regional fluid infiltration in the eclogites during slow exhumation following the Lapland–Kola orogeny and do not record any metamorphic event. Contrary to the Palaeoproterozoic E–HPG overprint, the areal occurrence of the 2.7–2.8 Ga AE eclogites with mid-ocean ridge basalt-like chemistry and their paragenetic link with the TTG gneisses suggest a tectonic regime that involves subduction. This research favours concepts suggesting that the modern-style plate tectonics has operated in some places, at least since the late Mesoarchaean.  相似文献   

10.
桐柏造山带深熔作用:混合岩LA-ICPMS锆石U-Pb年代学证据   总被引：2，自引：1，他引：1  
刘小驰  吴元保  彭敏  汪晶  王浩  彭德才 《岩石学报》2011,27(4):1163-1171
桐柏造山带是研究秦岭-桐柏-大别-苏鲁变质带演化的关键地区。由于桐柏高级变质杂岩深熔作用发生时间还缺乏准确的限定,这一区域的构造演化仍存在较大的争议。本文对桐柏杂岩中的一个混合岩的中色体和两个混合岩浅色体样品中的锆石进行了LA-ICPMS年代学测定。中色体中锆石分析点获得的上下交点分别为859±73Ma和135±250Ma。接近上交点的6个谐和分析点给出的206Pb/238U加权平均结果为828±7Ma (MSWD=0.57)。这一年龄结果同上交点在误差范围内一致,代表混合岩原岩结晶年龄,对应扬子板块北缘出现的中-新元古代的岩浆事件。另一方面,混合岩浅色体中的新生锆石具有面状分带或是弱的振荡环带,低的Th/U比值,锆石形态学和内部结构也表明新生锆石结晶于与深熔作用有关的熔体中,它们的206Pb/238U加权平均值分别为135±4Ma和131±3Ma。这一年龄范围代表桐柏高级变质地体发生深熔作用时间,区域上与桐柏-大别变质带广泛出现的碰撞后岩浆事件的时代相同。桐柏造山带出现造山后伸展的时间应不晚于135Ma。  相似文献   

11.
Both oceanic and continental HP rocks are juxtaposed in the Huwan shear zone in the western Dabie orogen, and thus provide a window for testing the buoyancy‐driven exhumation of dense oceanic HP rocks. The HP metamorphic age of the continental rocks in this zone has not been well constrained, and hence it is not known if they are of the same age as the exhumation of the HP oceanic rocks. In situ laser ablation (multiple collector) inductively coupled plasma mass spectrometry (LA‐(MC‐)ICP‐MS), U–Pb, trace element and Hf isotope analyses were made on zircon in a granitic gneiss and two eclogites from the Huwan shear zone. U–Pb age and trace element analysis of residual magmatic zircon in an eclogite constrain its protolith formation at 411 ± 4 Ma. The zircon in this sample displays εHf (t) values of +6.1 to +14.4. The positive εHf (t) values up to +14.4 suggest that the protolith was derived from a relatively depleted mantle source, most likely Palaeotethyan oceanic crust. A granitic gneiss and the other eclogite yield protolith U–Pb ages of 738 ± 6 and 700 ± 14 Ma, respectively, which are both the Neoproterozoic basement rocks of the Yangtze Block. The zircon in the granitic gneiss has low εHf (t) values of ?14.2 to ?10.5 and old TDM2 ages of 2528–2298 Ma, suggesting reworking of Palaeoproterozoic crust during the Neoproterozoic. The zircon in the eclogite has εHf (t) values of ?1.0 to +7.4 and TDM1 ages of 1294–966 Ma, implying prompt reworking of juvenile crust during its protolith formation. Metamorphic zircon in both eclogite samples displays low Th/U ratios, trace element concentrations, relatively flat heavy rare earth element patterns, weak negative Eu anomalies and low 176Lu/177Hf ratios. All these features suggest that the metamorphic zircon formed in the presence of garnet but in the absence of feldspar, and thus under eclogite facies conditions. The metamorphic zircon yields U–Pb ages of 310 ± 3 and 306 ± 7 Ma. Therefore, both the oceanic‐ and continental‐type eclogites share the same episode of Carboniferous eclogite facies metamorphism. This suggests that high‐pressure continental‐type metamorphic rocks might have played a key role in the exhumation and preservation of oceanic‐type eclogites through buoyancy‐driven uplift.  相似文献   

12.
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   

13.
Thin layers and lenses of granitic leucosome are widely distributed within amphibolites, paragneisses and orthogneisses of the Sulu UHP terrane. They are parallel to, or cross‐cut, foliations in the host rocks at different scales and show evidence of coalescence and migration to form centimetre‐ to decimetre‐scale segregations. Variously migmatized rocks extend at least 350 km from SW Sulu (Maobei) to NE Sulu (Weihai), in a band at least 50 km wide. A combined study of mineral inclusions, cathoduluminescence (CL) images, U–Pb LA‐ICP‐MS dates, and in‐situ trace element compositions of zircon provide clear evidence on the nature and timing of partial melting in these UHP rocks. Most zircon from the granitic leucosomes occurs as distinct overgrowths around inherited (igneous or metamorphic) cores or as new, euhedral crystals. The overgrowths and new crystals commonly show perfectly euhedral shapes, have pronounced oscillatory zoning and contain felsic mineral inclusions, such as Kfs + Pl + Qtz ± Ilm ± monazite (Mon). In contrast, the inherited igneous or metamorphic cores are rounded or irregular, contain low‐P or UHP mineral inclusions and show clear dissolution textures. These data suggest that the new zircon is anatectic in origin and that it grew during partial melting of the UHP rocks. The REE patterns of the anatectic zircon show steep slopes from the HREE to LREE with strongly to moderately negative Eu anomalies (Eu/Eu* = 0.31–0.72) and pronounced positive Ce anomalies (Ce/Ce* = 6.8–26.5). Abundant U–Pb spot analyses of the anatectic zircon reveal two discrete and meaningful ages of partial melting within the Sulu UHP terrane. Anatectic zircon from 12 granitic leucosomes within amphibolites, paragneisses, and orthogneisses from Sulu UHP slices II and III yields consistent mean U–Pb ages of 219.0 ± 1.2 to 218.3 ± 1.6 Ma, 218.8 ± 2.0 to 217.3 ± 1.7 Ma and 218.2 ± 1.4 to 215.0 ± 1.5 Ma, respectively. In contrast, anatectic zircon from six granitic leucosomes within paragneisses and orthogneisses from Sulu UHP slice III records younger mean U–Pb ages of 151.9 ± 1.3 to 151.1 ± 1.8 Ma and 155.9 ± 1.8 to 153.7 ± 1.7 Ma, respectively. These data imply that the Sulu UHP terrane experienced two Mesozoic partial melting events. The first partial melting event (219–215 Ma) was probably associated with a Late Triassic granulite facies stage of ‘hot’ exhumation, whereas the second (156–151 Ma) is interpreted as the result of Middle‐Late Jurassic extension and thinning of the previously thickened crust of the Sulu UHP terrane. Both partial melting events induced extensive retrograde metamorphism of the eclogites and their country rocks.  相似文献   

14.
Garnet granulite and pyroxenite xenoliths from the Grib kimberlite pipe (Arkhangelsk, NW Russia) represent the lower crust beneath Russian platform in close vicinity to the cratonic region of the north-eastern Baltic (Fennoscandian) Shield. Many of the xenoliths have experienced strong interaction with the kimberlite host, but in others some primary granulite-facies minerals are preserved. Calculated bulk compositions for the granulites suggest that their protoliths were basic to intermediate igneous rocks; pyroxenites were ultrabasic to basic cumulates. A few samples are probably metasedimentary in origin. Zircons are abundant in the xenoliths; they exhibit complex zoning in cathodoluminescence with relic cores and various metamorphic rims. Cores include oscillatory zircon crystallized in magmatic protoliths, and metamorphic and magmatic sector-zoned zircons. Recrystallization of older zircons led to the formation of bright homogeneous rims. In some samples, homogeneous shells are surrounded by darker convoluted overgrowths that were formed by subsolidus growth when a change in mineral association occurred. The source of Zr was a phase consumed during a reaction, which produced garnet. Late-generation zircons in all xenoliths show concordant U–Pb ages of 1.81–1.84 Ga (1,826 ± 11 Ma), interpreted as the age of last granulite-facies metamorphism. This event completely resets most zircon cores. An earlier metamorphic event at 1.96–1.94 Ga is recorded by some rare cores, and a few magmatic oscillatory zircons have retained a Neoarchaean age of 2,719 ± 14 Ma. The assemblage of metaigneous and metasedimentary rocks was probably formed before the event at 1.96 Ga. Inherited magmatic zircons indicate the existence of continental crust by the time of intrusion of magmatic protoliths in the Late Archaean. The U–Pb zircon ages correspond to major events recorded in upper crustal rocks of the region: collisional metamorphism and magmatism 2.7 Ga ago and reworking of Archaean rocks at around 1.95–1.75 Ga. However, formation of the granulitic paragenesis in lower crustal rocks occurred significantly later than the last granulite-facies event seen in the upper crust and correlates instead with retrograde metamorphism and small-volume magmatism in the upper crust.  相似文献   

15.
An integrated study of U–Pb ages and trace elements was carried out for titanite and zircon from ultrahigh‐pressure (UHP) metagranites in the Sulu orogen, east‐central China. The results provide constraints on the composition of metamorphic fluids during the exhumation of deeply subducted continental crust. Titanite has two domain types based on REE patterns and trace element variations, Ttn‐I and Ttn‐II respectively. These two domains show indistinguishable U–Pb ages of 232 ± 14 to 220 ± 8 Ma, in general agreement with anatectic zircon U–Pb ages of 223 ± 4 to 219 ± 2 Ma for the partial melting event during early exhumation. The Ttn‐I domains have significantly higher REE, Th, Ta and Sr, and higher Th/U ratios than the Ttn‐II domains, indicating that the two domains have grown from metamorphic fluids with different compositions. For the Ttn‐I domains, Zr‐in‐titanite thermometry yields high temperatures of 773–851 °C at 2.5 GPa, and petrographic observations reveal the presence of melt pseudomorphs. Thus, they are interpreted to have grown from hydrous melts in the early exhumation stage. In contrast, the Ttn‐II domains were texturally equilibrated with amphibolite facies minerals such as biotite and plagioclase and contain inclusions of plagioclase and quartz. The Zr‐in‐titanite thermometry yields lower temperatures of 627–685 °C at 1.0 GPa. In combination with their REE patterns, they are interpreted to have grown from aqueous solutions at amphibolite facies metamorphic conditions during further exhumation. The differences in Th and Sr contents are prominent between the Ttn‐I and Ttn‐II domains, signifying the compositional difference between the hydrous melts and aqueous solutions. Therefore, the polygenetic titanite in the UHP metamorphic rocks provides insights into the geochemical property of metamorphic fluids during the continental subduction‐zone processes.  相似文献   

16.
Sm–Nd (garnet), U–Pb (monazite) and Rb–Sr (biotite) ages from a composite migmatite sample (Damara orogen, Namibia) constrain the time of high‐grade regional metamorphism and the duration of regional metamorphic events. Sm–Nd garnet whole‐rock ages for a strongly restitic melanosome and an adjacent intrusive leucosome yield ages of 534±5, 528±11 and 539±8 Ma. These results provide substantial evidence for pre‐500 Ma Pan‐African regional metamorphism and melting for this segment of the orogen. Other parts of the migmatite yield younger Sm–Nd ages of 488±9 Ma for melanosome and 496±10, 492±5 and 511±16 Ma for the corresponding leucosomes. Garnet from one xenolith from the leucosomes yields an age of 497±2 Ma. Major element compostions of garnet are different in terms of absolute abundances of pyrope and spessartine components, but the flat shape of the elemental patterns suggests late‐stage retrograde equilibration. Rare earth element compositions of the garnet from the different layers are similar except for garnet from the intrusive leucosome suggesting that they grew in different environments. Monazite from the leucosomes is reversely discordant and records 207Pb/235U ages between 536 and 529 Ma, indicating that this monazite represents incorporated residual material from the first melting event. Monazite from the mesosome MES 2 and the melanosome MEL 3 gives 207Pb/235U ages of 523 and 526 Ma, and 529 and 531 Ma, respectively, which probably indicates another thermal event. Previously published 207Pb/235U monazite data give ages between 525 and 521 Ma for composite migmatites, and 521 and 518 Ma for monazite from neosomes. Monazite from granitic to granodioritic veins indicates another thermal event at 507–505 Ma. These ages are also recorded in 207Pb/235U monazite data of 508 Ma from the metasediment MET 1 from the migmatite and also in the Sm–Nd garnet ages obtained in this study. Taken together, these ages indicate that high‐grade metamorphism started at c. 535 Ma (or earlier) and was followed by thermal events at c. 520 Ma and c. 505 Ma. The latter event is probably connected with the intrusion of a large igneous body (Donkerhoek granite) for which so far only imprecise Rb–Sr whole‐rock data of 520±15 Ma are available. Rb–Sr biotite ages from the different layers of the migmatite are 488, 469 and 473 Ma. These different ages indicate late‐stage disturbance of the Rb–Sr isotopic system on the sub‐sample scale. Nevertheless, these ages are close to the youngest Sm–Nd garnet ages, indicating rapid cooling rates between 13 and 20°C Ma?1 and fast uplift of this segment of the crust. Similar Sm–Nd garnet and U–Pb monazite ages suggest that the closure temperatures for both isotopic systems are not very different in this case and are probably similar or higher than the previously estimated peak metamorphic temperatures of 730±30°C. The preservation of restitic monazite in leucosomes indicates that dissolution of monazite in felsic water‐undersaturated peraluminous melts can be sluggish. This study shows that geochronological data from migmatites can record polymetamorphic episodes in high‐grade terranes that often contain cryptic evidence for the nature and timing of early metamorphic events.  相似文献   

17.
Abstract A major episode of continental crust formation, associated with granulite facies metamorphism, occurred at 2.55–2.51 Ga and was related to accretional processes of juvenile crust. Dating of tonalitic–trondhjemitic, granitic gneisses and charnockites from the Krishnagiri area of South India indicates that magmatic protoliths are 2550–2530 ± 5 Ma, as shown by both U–Pb and 207Pb/206Pb single zircon methods. Monazite ages indicate high temperatures of cooling corresponding to conditions close to granulite facies metamorphism at 2510 ± 10 Ma. These data provide precise time constraints and Sr–Nd isotopes confirm the existence of late tonalitic–granodioritic juvenile gneisses at 2550 Ma. Pb single zircon ages from the older Peninsular gneisses (Gorur–Hassan area) are in agreement with some previous Sr ages and range between 3200 ± 20 and 3328 ± 10 Ma. These gneisses were derived from a 3.3–3.5-Ga mantle source as indicated from Nd isotopes. They did not participate significantly in the genesis of the 2.55-Ga juvenile magmas. All these data, together with previous work, suggest that the 2.51-Ga granulite facies metamorphism occurred near the contact of the ancient Peninsular gneisses and the 2.55–2.52-Ga ‘juvenile’tonalitic–trondhjemitic terranes during synaccretional processes (subduction, mantle plume?). Rb–Sr biotite ages between 2060 and 2340 Ma indicate late cooling probably related to the dextral major east–west shearing which displaced the 2.5-Ga juvenile terranes toward the west.  相似文献   

18.
The recently discovered Taolaituo porphyry Mo deposit and Aobaotu hydrothermal vein Pb–Zn deposit are both located in the Great Xing’an Range, Northeast China. Here we present new zircon U–Pb ages, whole-rock geochemical and Pb isotopic data, and molybdenite Re–Os ages for these two deposits. The Mo mineralization in the Taolaituo area occurred in quartz porphyry, which yields zircon U–Pb ages ranging from 138.5 ± 0.8 to 139.1 ± 0.5 Ma. Fine-grained granite representing pre-mineralization magmatic activity was formed at 145.2 ± 0.5 Ma. Molybdenite Re–Os dating indicates that Mo mineralization occurred at 133.8 ± 1.2 Ma. In the Aobaotu deposit, the ore-related granodioritic porphyry has a zircon U–Pb age of 140.0 ± 0.4 Ma. These geochronological data indicate that these magmatic and hydrothermal activities occurred during the Early Cretaceous. The mineralogical and geochemical features of the Taolaituo and Aobaotu granitoids suggest they can be classified as A1-type within-plate anorogenic granites and I-type granites, respectively. The Pb isotopic compositions suggest a mixed crust–mantle origin of the granitoids in these two deposits. The Taolaituo granitoids were formed by the partial melting of lower crust and crust–mantle interaction, with subsequent fractionation of apatite, feldspar, Ti-bearing phases and allanite or monazite. In contrast, the Aobaotu granites were derived primarily from lithospheric mantle that had been transformed or affected by the addition of subduction-related components. Combined with the regional geology, tectonic evolution and available age data from the literature, our results suggest that the Early Cretaceous (140–100 Ma) was likely to be the most important peak period for metallogenic mineralization in Northeast China. The Taolaituo and Aobaotu deposits formed under an extensional environment at an active continental margin in response to subduction of the Palaeo-Pacific oceanic plate.  相似文献   

19.
This paper presents data on the geological position, geochemistry, age, and isotopic characteristics of the granitoids of the southern part of the Voznesenka terrane, Southern Primorye (Muraviev–Amursky Peninsula and its vicinities). All of the studied granitoids were formed in three stages: the Ordovician, Silurian, and Permian. The Silurian and Permian ages of the granitoid intrusions have been previously determined (Ostrovorussky Massif, 432–422 Ma, and 250 ± 4 Ma, early and late associations, respectively; Sedanka massif, 261 ± 3 Ma). The granites of the Artem and Nadezhdinsky massifs define an U–Pb zircon age of 481 ± 6 and 452 ± 4 Ma, respectively. The geochemical and isotope data show mainly the crustal nature of the granitoids. Their formation was related to melting of relatively immature rocks of the continental crust (mafic–intermediate volcanic rocks). The Nd isotope composition of the granitods (TNd(DM–2) = 1.3 Ga) indicates the absence of the mature ancient crust at the basement of the southern Voznesenka terrane. The maximum contribution of mantle sources to the granite formation is recorded in the Permian associations. A comparison of the peaks of intrusive magmatism in the southern part of the Voznesenka terrane and adjacent territories suggests that the formation of the granitoids of the Muraviev–Amursky Peninsula and its vicinities was caused by the interaction of continental blocks with two oceanic basins: the Paleoasian (and its fragments) and Paleopacific ones.  相似文献   

20.
The Yunmengshan Geopark in northern Beijing is located within the Yanshan range. It contains the Yunmengshan batholith, which is dominated by two plutons: the Yunmengshan gneissic granite and the Shicheng gneissic diorite. Four samples of the Yunmengshan gneissic granite give SHRIMP zircon U–Pb ages from 145 to 141 Ma, whereas four samples of the Shicheng gneissic diorite have ages from 159 Ma to 151 Ma. Dikes that cut the Yunmengshan diorite record SHRIMP zircon U–Pb age of 162±2 and 156±4 Ma. The cumulative plots of zircons from the diorites show a peak age of 155 Ma, without inherited zircon cores, and the peak age of 142 Ma for granite is interpreted as the emplacement age of the Yunmengshan granitic pluton, whose igneous zircons contain inherited zircon cores. The data presented here show that there were two pulses of magmatism: early diorites, followed c13 Ma later by true granites, which incorporated material from an older continental crust.  相似文献   

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