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1.
Twenty granodioritic rocks and one amphibolitic enclave of the “basement” of the Suomussalmi-Kuhmo Archaean (2.65 Ga) greenstone belts (central-eastern Finland), have been chosen together with one greenstone sample for Rb-Sr and Sm-Nd geochronological and isotopic studies.The granitoïd rocks are subdivided into three groups: two generations of grey gneisses and a post-belt augen gneiss. The Rb-Sr ages of the first and second generation of grey gneisses are 2.86 ± 0.09 and 2.62 ± 0.07 Ga, respectively. These results are corroborated by Sm-Nd data. The post-belt augen gneiss gives an age of 2.51 ± 0.11 Ga. The results show that the two generations of grey gneisses, the greenstone belts and the post-greenstone augen gneiss, were developed over a period > 350 Ma. The two generations of grey gneisses show identical ISr values (0.7023 ± 8 and 0.7024 ± 6) which contrast with that of the augen gneiss (0.7049 ± 8). The low ISr and the near-chondritic ?TCHUR values indicate that the grey gneisses cannot derived from much older continental materials. Trace element studies suggest that these grey gneisses have had a multi-stage development. The augen gneiss with a moderately high ISr is likely to be derived from a granodiorite originated by partial melting of older sialic crust. The more probable parent rock seems to be the first generation grey gneisses. The ISr and average Rb/Sr values preclude the greenstone belt and the second generation of grey gneisses as the protolith. 相似文献
2.
The Archean basement of Sierra Leone is a typical example of granite-greenstone terrains found in ancient continental nucleii. Reconnaissance field mapping showed that the area can be subdivided into old gneiss, which predates the greenstone belts, and young granite which is later than the greenstone belts.New Rb-Sr whole-rock age determinations on two suites of old tonalitic gneiss yield ages of 2786 ± 49 Ma and 2770 ± 137 Ma, which either reflect the time of formation of the original tonalites or their metamorphism. Three new Rb-Sr whole-rock age determination on young granites yield ages of 2786 ± 143 Ma, 2780 ± 79 Ma and 2770 ± 50 Ma, which are interpreted as the time of emplacement. The widespread occurrence of similar young granites, throughout the Archaean of West Africa, suggests that these results date a major event in the evolution of this segment of the crust.A published Pb-Pb age of the old gneiss and the new ages of the young granite bracket the age of the greenstone belts to 3000-2770 Ma. However, if the Rb-Sr ages of the old gneiss reported in this paper reflect the time of their formation, the age of the greenstone belts is tightly bracketed to ca. 2770 Ma. There is no isotopic evidence for rocks substantially older than 3000 Ma in the West African Archaean. 相似文献
3.
鞍山本溪地区太古代变质岩可分为三套,即含铁的表壳岩建造、侵入于铁建造中的花岗质片麻岩和铁架山奥长花岗质-花岗质片麻岩,后者为表壳岩的基底。原划为上鞍山群樱桃园组(齐大山矿带)和山城子组(歪头山-北台矿带)的斜长角闪岩分别获得2729Ma和2724Ma的Sm-Nd等时线年龄。这就为有争议的鞍本地区铁建造属于同一时代提供了依据,并讨论了表壳岩中的变质沉积岩以及铁架山基底片麻岩的同位素年代。 相似文献
4.
In east-central Finland, Archaean terrains present three main lithologic units: a) gneissic basement, emplaced from 2.86 G.a. to 2.62 G.a., b) greenstone belt (2.65 G.a.) and c) calc-alkaline magmatism (2.50 G.a. to 2.40 G.a). Twenty three rocks of the calc-alkaline suite have been chosen for geochronologic and Rb-Sr isotopic studies. These rocks are subdivided into three groups: 1) acid volcanics from Luoma, 2) augen gneiss from Arola, and 3) post kinematik pink leucogranite from Arola. The 2.50±0.10 G.a. age of the Luoma volcanics indicates that they represent the upper part of a greenstone belt composed of a single sequence of volcanic rocks. The ages, initial 87Sr/86Sr (ISr) and major element compositions of the augen gneisses of Arola and Suomussalmi indicate that these rocks are the plutonic equivalents of the Luoma acid volcanics. The Arola pink leucogranite marks the terminal phase of Archaean magmatic activity (from 2.86 G.a. to 2.41 G.a.). This was followed by at least 0.40 G.a. of quiescence. The ISr and major element compositions suggest that the genesis of the calc-alkaline magmatic rocks involved crustal materials, but all their geochemical features cannot be explained without the participation of mafic greenstone belt materials. The first crustal components had low I and low K2O/ Na2O ratios while the younger ones (calc-alkaline magmas) had medium to high ISr and high K2O/Na2O ratios. Thus the petrogenetic processes have changed with time from ensimatic to ensialic, implying major reworking of preexisting crustal materials. This evolution leads to the accretion of the continental crust from the mantle. 相似文献
5.
K Naha R Srinivasan K Gopalan GVC Pantulu M V Subba Rao A B Vrevsky Ye S Bogomolov 《Journal of Earth System Science》1993,102(4):547-565
The Archaean Peninsular Gneiss of southern India is considered by a number of workers to be the basement upon which the Dharwar supracrustal rocks were deposited. However, the Peninsular Gneiss in its present state is a composite gneiss formed by synkinematic migmatization during successive episodes of folding (DhF1, DhF1a and DhF2) that affected the Dharwar supracrustal rocks. An even earlier phase of migmatization and deformation (DhF*) is evident from relict fabrics in small enclaves of gneissic tonalites and amphibolites within the Peninsular Gneiss. We consider these enclaves to represent the original basement for the Dharwar supracrustal rocks. Tonalitic pebbles in conglomerates of the Dharwar Supergroup confirm the inference that the supracrustal rocks were deposited on a gneissic basement. Whole rock Rb-Sr ages of gneisses showing only the DhF1 structures fall in the range of 3100–3200 Ma. Where the later deformation (DhF2) has been associated with considerable recrystallization, the Rb-Sr ages are between 2500 Ma and 2700 Ma. Significantly, a new Rb-Sr analysis of tonalitic gneiss pebbles in the Kaldurga conglomerate of the Dharwar sequence is consistent with an age of ～2500 Ma and not that of 3300 Ma reported earlier by Venkatasubramanian and Narayanaswamy (1974). Pb-Pb ages based on direct evaporation of detrital zircon grains from the metasedimentary rocks of the Dharwar sequence fall into two groups, 3300–3100 Ma, and 2800–3000 Ma. Stratigraphic, structural, textural and geochronologic data, therefore, indicate that the Peninsular Gneiss of the Dharwar craton evolved over a protracted period of time ranging from > 3300 Ma to 2500 Ma. 相似文献
6.
Previous models for the temporal evolution of greenstone belts and surrounding granitoid gneisses in the northern Kaapvaal Craton can be revised on the basis of new single zircon ages, obtained by conventional U---Pb dating and Pb---Pb evaporation. In the Pietersburg greenstone belt, zircons from a metaquartz porphyry of the Ysterberg Formation yielded an age of 2949.7±0.2 Ma, while a granite intruding the greenstones, and deformed together with them, has an age of 2853 + 19/−18 Ma. These data show felsic volcanism in this belt to have been coeval with felsic volcanism in the Murchison belt farther east, and the date of 2853 Ma provides an older age limit for deformation in the region. In contrast, a meta-andesite of the Giyani greenstone belt has a zircon age of 3203.3±0.2 Ma, while a younger and cross-cutting feldspar porphyry has an emplacement age of 2874.1±0.2 Ma. The meta-andesite is intercalated with various mafic and ultramafic rocks and, therefore, the age of 3.2 Ga appears plausible for the bulk of the Giyani greenstones.Granitoid gneisses surrounding the Pietersburg and Giyani belts vary in composition from tonalite to granite and texturally from well-layered to homogeneous but strongly foliated. These rocks yielded zircon ages between 2811 and 3283 Ma. The pre-3.2 Ga gneisses are polydeformed and may have constituted a basement to the Giyani greenstone sequence, while the younger gneisses are intrusive into the older gneiss assemblage and/or into the greenstones. The Giyani and Pietersburg belts probably define two separate crustal entities that were originally close together but were later displaced by strike-slip movement. 相似文献
7.
A. Dombrowski M. Okrusch P. Richter F. Henjes-Kunst A. Höhndorf A. Kröner 《International Journal of Earth Sciences》1995,84(2):399-411
The Spessart Crystalline Complex, north-west Bavaria contains two orthogneiss units of granitic to granodioritic composition, known as the Rotgneiss and Haibach gneiss, respectively, which are structurally conformable with associated metasediments. The igneous origin of the Rotgneiss is apparent from field and textural evidence, whereas strong deformation and recrystallization in the Haibach gneiss has obscured most primary textures. New geochemical data as well as zircon morphology prove the Haibach gneiss to be derived from a granitoid precursor, which was chemically similar to the Rotgneiss protolith, thus suggesting a genetic link between those two rock units. Both gneiss types have chemical compositions typical of anatectic two-mica leucogranites. They show characteristics of both I- and S-type granites. Rb-Sr whole rock data on the Haibach gneiss provide an isochron age of 407±14 Ma (IR = 0.7077±0.0007; MSWD 2.2), which is slightly younger than the published date for the Rotgneiss (439±15 Ma; IR=0.7048±0.0026; MSWD 4.9). Single zircon dating of six idiomorphic grains, using the evaporation method, yielded a mean 207Pb/206Pb age of 410±18 Ma for the Haibach gneiss and 418±18 Ma for the Rotgneiss. Both zircon ages are within analytical error of the Rb-Sr isochron dates and are interpreted to reflect the time of protolith emplacement in Silurian times. Three xenocrystic zircon grains from the Rotgneiss yielded 207Pb/206Pb ages of 2278±12, 2490±13 and 2734±10 Ma, respectively, suggesting that late Archaean to early Proterozoic crust was involved in the generation of the granite from which the Rotgneiss is derived. Although it is assumed that the granitic protoliths of the two gneisses were formed through anatexis of older continental crust, the relatively low 87Sr/86Sr initial ratios of both gneisses may also indicate the admixture of a mantle component. The Rotgneiss and the Haibach gneiss thus document granitic magmatism at an active continental margin during late Silurian times. 相似文献
8.
The Vestfold Block, like other Archaean cratons in East Antarctica and elsewhere, consists predominantly of felsic orthogneiss (Mossel and Crooked Lake gneisses), with subordinate mafic granulite (Tryne metavolcanics) and paragneiss (Chelnok supracrustals). Two major periods of continental crust formation are represented. The Mossel gneiss (metamorphosed about 3,000 Ma ago) is mainly of tonalitic composition, and is similar to much of the roughly coeval Napier Complex in Enderby Land. The Crooked Lake gneiss was emplaced under high-grade conditions about 2,450 Ma ago and comprises a high proportion of more potassic rocks (monzodioritic and monzonitic suites), as well as tonalite and minor gabbro and diorite. Both Mossel and Crooked Lake gneisses are depleted in Y and have moderate to high Sr, Ce/Y, and Ti/Y, consistent with melting of a mafic source (?subducted hydrated oceanic crust) leaving major residual hornblende (± garnet). Most Crooked Lake gneisses are more enriched in incompatible elements (P, Sr, La, Ce, and particularly Rb, Ba, and K) than Mossel gneisses, suggesting derivation from a more enriched mafic source. The Vestfold Block contains few orthogneisses derived by melting of older felsic crustal rocks, in marked contrast to the Archaean Napier Complex and, in particular, southern Prince Charles Mountains. Both Mossel and Crooked Lake tonalites are strongly depleted in Rb, K, Th, and U, and have very low Rb/Sr and high K/Rb; more potassic orthogneisses are depleted in Th, U, and, to lesser extents, Rb. Tryne metavolcanics are depleted in Th and Rb, but appear to have been enriched in K (and probably Na), possibly during early low-grade alteration. 相似文献
9.
Hielke A. Jelsma Michael L. Vinyu Jan R. Wijbrans E. A. T. Verdurmen P. J. Valbracht G. R. Davies P. J. Valbracht 《Contributions to Mineralogy and Petrology》1996,124(1):55-70
The U-Pb ages of zircons from seven felsic volcanic and plutonic rocks from northern Zimbabwe combined with field data and
Pb-Pb and Sm-Nd whole-rock isotope data, constrain the timespan of development of the Harare-Shamva granite-greenstone terrain
and establish the relative involvement of juvenile mantle-derived and reworked crustal material. Basement-cover field relationships
and isotope and geochemical data demonstrate that the greenstones were deposited onto 3.2–2.8 Ga basement gneisses, in ensialic,
continental basins. Geodynamic models for the generation of the areally extensive bimodal magmatic products and growth of
the pre-existing crustal nucleus consistent with our interpretations are rift-related: (1) intracontinental rifting related
to mantle plume activity or; (2) rifting in a back-arc environment related to a marginal volcanic arc. The data, in conjunction
with field evidence, do not indicate the presence and accretion of an older (ca. 2.70 Ga) and a younger (ca. 2.65 Ga) greenstone
sequence in the Harare part of the greenstone belt, as was recently postulated on the basis of SHRIMP zircon ages. Zircon
ages for basal felsic volcanics (2715±15 Ma) and a subvolcanic porphyry (2672±12 Ma) constrain the initiation and termination
of deposition of the greenstone sequence. The timespan of deposition of the Upper Bulawayan part of the greenstone sequence
corresponds well with radiometric ages for Upper Bulawayan greenstones in the central and southern part of the craton and
supports the concept of craton-wide lithostratigraphic correlations for the late Archaean in Zimbabwe. Zircon ages for a syn-tectonic
gneiss (2667±4 Ma) and a late syn-tectonic intrusive granodiorite (2664±15 Ma) pinpoint the age of deformation of the greenstone
sequence and compare well with a Pb-Pb age of shear zone related gold mineralization (2659±13 Ma) associated with the latter
intrusive phase. The intimate timing relation of greenstone deformation and granitoid emplacement, but also the metamorphic
evidence for a thermal effect of the batholiths on the surrounding greenstone belts, and the structural and strain patterns
in the greenstone sequence around and adjacent to the batholiths, imply that the intrusion of the granitoids had a significant
influence on the tectono-thermal evolution of the greenstone belt. Prolonged magmatic activity is indicated by the zircon
ages of small, post-tectonic plutons intrusive into the greenstone belt, with a mineralized granodiorite dated at 2649±6 Ma
and an unmineralized tonalite at 2618± 6 Ma. The 2601±14 Ma crystallization age of an “external” Chilimanzi-type granite agrees
well with existing radiometric ages for similar granites within the southern part of the craton, demonstrating a craton-wide
event and heralding cratonization. The similarity between U-Pb zircon ages and TDM model ages (2.65–2.62 Ga) and the positive ɛNdT values (+3 to +2) for the late syn-tectonic and post-tectonic intrusive plutons within the greenstone belt indicate magmatism
was derived directly from the mantle or by anatexis of lower crustal sources, with very short crustal residence times, and
minor contamination with older crust. The rather high model μ1 values (8.2–8.6) are unlikely to indicate the involvement of significant amounts of older crust and may be inherited from
a high U/Pb mantle source, as was suggested by previous workers for the Archaean mantle beneath Southern Africa. The older
TDM ages for the felsic volcanics (3.0–2.8 Ga) and the porphyries (2.8–2.7 Ga) suggest that these felsic magmas were derived
by partial melting of a source that was extracted from the mantle ca. 200 Ma prior to volcanism or may indicate interaction
between depleted mantle-derived melts and older crustal material.
Received: 15 August 1995 / Accepted: 12 January 1996 相似文献
10.
《Precambrian Research》2001,105(2-4):115-128
The Aasivik terrane is a ∼1500 km2 complex of gneisses dominated by ∼3600 Ma components, which has been discovered in the Archaean craton of West Greenland, ∼20–50 km south of the Paleoproterozoic Nagssugtoqidian orogen. The Aasivik terrain comprises granulite facies tonalitic to granitic gneisses with bands of mafic granulite, which include disrupted mafic dykes. Four gneiss samples of the Aasivik terrain have given imprecise SHRIMP U–Pb zircon ages of 3550–3780 Ma with strong loss of radiogenic lead and new growth of zircon probably associated with a granulite facies metamorphic event(s) at ∼2800–2700 Ma. To the Southeast, the Aasivik terrane is in tectonic contact with a late Archaean complex of granitic and metapelitic gneisses with apparently randomly distributed mafic and ultramafic units, here named the Ukaleq gneiss complex. Two granitic samples from the Ukaleq gneiss complex have U–Pb zircon ages of 2817 ± 10 and 2820 ± 12 Ma and tzircon εNd values of 2.3–5.4. Given their composition and positive εNd values, they probably represent melts of only slightly older juvenile crust. A reconnaissance SHRIMP U–Pb study of a sample of metasedimentary rock from the Ukaleq gneiss complex found ∼2750–2900 Ma zircons of probable detrital origin and that two or more generations of 2700–2500 Ma metamorphic zircons are present. This gneiss complex is provisionally interpreted as a late Archaean accretionary wedge. A sample of banded granulite facies gneiss from a complex of banded gneisses south of the Aasivik terrain here named the Tasersiaq gneiss complex has yielded two zircon populations of 3212 ± 11 and 3127 ± 12 Ma. Contacts between the three gneiss complexes are mylonites which are locally cut by late-post-kinematic granite veins with SHRIMP U–Pb zircon ages of ∼2700 Ma. The isotopic character and the relationships between the lithologies from the different gneiss complexes suggest the assembly of unrelated rocks along shear zones between 2800 and 2700 Ma. The collage of Archaean gneiss complexes were intruded by A-type granites, here named the Umiatsiaasat granites, at ∼2700 Ma, later than the tectonic intercalation of the gneiss complexes. 相似文献
11.
Amphibolite facies early Archaean Amîtsoq gneisses envelop and intrude the c. 3,800 Ma Isua supracrustal belt, Isukasia area, southern West Greenland. Most of these gneisses are strongly deformed, but in a c. 75 km2 augen of lower deformation, the Amîtsoq gneisses are seen to comprise predominantly 3,750–3,700 Ma tonalitic grey gneisses that were intruded first by thin bodies of mafic to dioritic composition, known collectively as the Inaluk dykes, and then by c. 3,600 Ma white gneisses and finally by sporadic c. 3,400 Ma pegmatitic gneiss sheets. The grey gneisses could have formed by partial melting of crust consisting predominantly of basic rocks. The Inaluk dykes are interpreted as strongly fractionated basic melts of mantle origin, contaminated by crustal material. The white gneisses consist mostly of medium grained granite and occur as lenses and anastomosing sheets throughout their host of grey gneisses with subordinate inclusions of supracrustal rocks. The white gneisses have chemistry compatible with formation by partial melting at depth of a source dominated by grey gneisses. The igneous chemistry, including REE abundances, of the grey gneisses and white gneisses has been modified to varying degrees by metasomatism and assimilation reactions during the crystallisation of the white gneisses and also during subsequent tectonometamorphic events. The white gneisses are evidence for considerable reworking by anatexis of sialic crust in the early Archaean, 150 to 100 Ma after its formation. The white gneisses and the pegmatitic gneisses show that granitic rocks s.s. were important in the earliest Archaean, and are further evidence of the diversity of the oldest-known sial.Previously at and the Geological Survey of Greenland, Øster Voldgade 10, 1350 Copenhagen K, Denmark 相似文献
12.
Detailed mineralogical and petrochemical studies show that the Laoniugou gneiss of the Jiapigou gold mine is composed mainly of plagioclase gneiss and irregular to lentiform plagioclase amphibolite melanic enclaves.The major element contents show an obvious bimodal and trondhjemitic series evolutional trend.This situation is significantly different from that encountered in bimodal calc-alkalic volcanic rocks in the rift-type Archaean greenstone belt.The contents of Rb,Sr and Ba are 7-21 ppm,153-363ppm and 201-1451 ppm respectively ,close to those of common Archaean grey gneisses.All the samples of plagioclase gneisses show positive Eu anomalies (even up to 4.6).The protoliths of the plagioclase gneiss are high-Al2O3 trondhjemitic series rocks,belonging to typical TTG of Archaean high-grade metamorphic terrain .The gneiss is quite similar to the B-type Amitsoq gneiss of W.Greenland .The authors believe that the plagioclase amphibolite enclaves are the relics of ancient oceanic crust while the plagioclase gneiss is the TTG ancient intrusive rock resulting from partial melting of the oceanic crust. 相似文献
13.
R.D. Beckinsale N.H. Gale R.J. Pankhurst A. Macfarlane M.J. Crow J.W. Arthurs A.F. Wilkinson 《Precambrian Research》1980,13(1):63-76
Field mapping and structural studies in northern Sierra Leone by an I.G.S. team have established a stratigraphic sequence in this part of the Archaean of the West African Craton. An older “Leonian” granite-greenstone terrain is identified which experienced a tectonic-metamorphic event before the formation of the granite-greenstone terrain which ended with the Liberian tectonic-metamorphic event. Granite gneisses in the Fadugu district with Leonian structures yield statistically acceptable but discordant Pb-Pb and Rb-Sr whole-rock isochron ages of 2959±50 Ma and 2753±61 Ma, respectively (2 σ errors). These ages may be correlated with radiometric ages for the Leonian and Liberian structures elsewhere in Sierra Leone, and it is concluded that the Fadugu Rb-Sr whole-rock isochron has been reset by the Liberian event. The Pb-Pb whole-rock isochron for the Fadugu gneisses and a previously determined (but recomputed and partially checked) Rb-Sr whole-rock isochron age of 2980+80 Ma for granite gneisses from southeastern Sierra Leone provide a definitive age for the Leonian tectonic-metamorphic event at about 2970 Ma. Both the initial 87Sr/86Sr ratios and present-day first-stage model 238U/204Pb value for the Leonian granitoids are indistinguishable from mantle values, but do not preclude the possibility that these granitoids were derived from parental material with a short history in the crust or lower crust. The Rb-Sr whole-rock isochron age of 2753+61 Ma for the Fadugu granite gneiss provides a definitive age for the Liberian event in northern Sierra Leone. A succession of rocks older than the Leonian (i.e., older than 2970 Ma) has been identified in the field but not yet dated. 相似文献
14.
The western border of the Archaean province of the Baltic shield: evidence from northern Sweden 总被引：1，自引：0，他引：1
Björn Öhlander Torbjörn Skiöld P. Joseph Hamilton Lars -Åke Claesson 《Contributions to Mineralogy and Petrology》1987,95(4):437-450
Restricted areas of acid to intermediate gneisses in northernmost Sweden are known to be Archaean and several other gneissic granodioritic to tonalitic massifs have been suggested to be of this age. To delimit the western border of the Archaean province of the Baltic Shield, and to characterize the Swedish Archaean, we have studied the 2.83 Ga old Soppero gneiss and 8 of the proposed Archaean massifs in northern Sweden by Sm-Nd isotopic analyses, U-Pb zircon dating (in two cases), and geochemistry.Of the Archaean gneisses proposed thus far, only those from the Soppero and Kukkola areas have Archaean ages (2.83 and 2.67 Ga, respectively). These gneisses have geochemical characteristics such as LREE-enrichment, low U-contents, and low K/Na-ratios, which are typical of Archaean TTG (tonalite-trondhjemite-granodiorite) rocks worldwide. The REE results indicate that the Soppero and the Kukkola gneisses were generated by partial melting of basic rocks, presumably amphibolites. According to the Sm-Nd isotopic data for the Soppero gneiss, this process occurred only a short time after initial segregation from the mantle (
Nd values between +0.9 and +3.5). The Kukkola gneiss, however, has longer crustal residence time as indicated by negative
Nd values (–1.0 and –2.5) at the U-Pb zircon age of 2.67 Ga. Evidence in support of an important event at that time is given by secondary overgrowth on zircon cores in the 2.83 Ga old Soppero gneiss.The Sm-Nd isotope results show that the Proterozoic granitoids in northern Sweden have a decreasing involvement of Archaean source material with increasing distance from the Archaean areas. Before the Proterozoic reworking, the rather small Archaean areas in northern Sweden presumably belonged to a large continuous Archaean craton. The western palaeoboundary of this craton in Sweden probably follows approximately a line extrapolated from the western limit of the Archaean of central Finland, passing west of Kukkola, and then continues to the Lofoten area in northern Norway. 相似文献
15.
J.-J. PEUCAT R. CAPDEVILA A. DRARENI P. CHOUKROUNE C. M. FANNING J. BERNARD-GRIFFITHS S. FOURCADE 《Journal of Metamorphic Geology》1996,14(6):667-692
The In Ouzzal granulitic massif is composed mainly of various meta-igneous rocks which, in spite of Rb, U, Th, Cs and some K and Sr mobility, can be dated and generally classified according to their chemical composition as follows. Basic and ultrabasic granulites interlayered with the metasediments correspond to (1) ultrabasic cumulates from dislocated tholeiitic bodies, (2) ancient komatiitic to high-Mg tholeiitic basalts similar to the suites found in Archaean greenstone belts and (3) calcalkaline protoliths of high-K andesitic composition. No geochronological constraints are available apart from the depositional age of some associated sediments which is younger than 2.70 Ga detrital zircons, and the Nd model age of the andesitic granulites of c. 3.4 Ga. In spite of the high-grade metamorphism, the acidic magmatic precursors of the charnockites can be divided in three groups. (1) The most juvenile acid orthogneisses are trondhjemitic or tonalitic in composition, being similar to the TTG suites which are classically considered to be formed by partial melting of mantle-derived protoliths. The 3.3–3.2 Ga TDM indicates a possible age of separation from the mantle reservoir while the plutons may have been emplaced between 3.3 and 2.7 Ga (U–Pb zircon & Nd ages). (2) A group of alkaline granitic gneisses, similar in composition to rift-related-granites, were emplaced at 2650±10 Ma (U–Pb & Rb–Sr ages) in a thick continental crust. (3) Calcalkaline granodioritic and monzogranitic suites derived from the partial melting of continental precursors (3.5–3.3 Ga), in lower to middle levels of the continental crust. They were emplaced close to 2.5 Ga during crustal thickening. The very high-temperature metamorphism occurred at 2002±7 Ma from the age of synfoliation intrusions and was probably related to major overthrusting. Retrogressive metamorphism is dated at 1.95 Ga from garnet-Nd ages. In spite of the very high-temperature conditions, partial melting during granulite facies metamorphism may be restricted to scarce cordierite-bearing monzogranitic gneisses. The 2.0 Ga VHT metamorphism could be related to overthrusting, extensional or underplating processes. 相似文献
16.
We report single grain and grain-domain U–Pb zircon ages for the Tojottamanselkä tonalitic gneiss previously investigated by the whole-rock Rb–Sr, Pb–Pb and Sm–Nd methods, by conventional U–Pb zircon density/size fraction analysis and by Hf-isotopes (Kröner et al. 1981; Patchett et al. 1981; Jahn et al. 1984) and established as one of the oldest known rocks of the Baltic shield. Our data confirm the intrusive age as 3115±29 Ma (standard error), but we also found slightly older xenocrystic zircon cores with 207Pb/206Pb ages between 3161±19 and 3248±10 Ma that may either be derived from earlier phases of the tonalite melt or from pre-tonalite sialic crust. New magmatic zircon growth, probably during a metamorphic event that led to migmatization, is recorded by an age of 2836±30 Ma and may be coeval with widespread tonalite emplacement elsewhere in the northern Baltic shield at about this time. 相似文献
17.
曹庄片麻杂岩包括35亿年的表壳岩及三个不同时期的复期正片麻岩。水厂片麻杂岩包括水厂表壳岩及侵入其中的淡花岗岩和紫苏花岗岩,后者年龄为26.5亿年。 相似文献
18.
Four suites of granitoids intruded the supracrustal greenstone sequence in the Murchison Province of the Archaean Yilgarn Craton during a 300 million year period. The earliest granitoid suite intruded the base of the developing greenstone sequence as a series of thin subhorizontal tabular plutons of monzogranite and granodiorite at 2.9Ga. This suite has been deformed and metamorphosed, and is now a pegmatite-banded gneiss. At about 2.7 Ga, thick, subhorizontal, tabular plutons of monzogranite intruded the base of the greenstone sequence. This suite, which now forms much of the regions between greenstone belts, was folded and recrystallized during regional deformation and metamorphism. Two distinct but contemporaneous suites of post-folding granitoids intruded the greenstone belts at 2.6 Ga, largely post-dating regional metamorphism. One suite of post-folding granitoids comprises tonalite, trondhjemite, granodiorite and monzogranite plutons, confined mainly to the north of the Province. The other suite comprises quartz-rich monzogranite and syenogranite plutons, confined mainly to the south of the Province.Pegmatite-banded gneiss, recrystallized monzogranite, and the northern suite of post-folding granitoids were all derived by partial-melting of mafic crustal rocks. Most post-folding granitoids from the southern suite were derived by partial-melting of siliceous crustal material at least as old as basal greenstones. The modes and sites of intrusion of all granitoid plutons were controlled by active tectonic processes or by structural features of the crust. Widespread 2.6 Ga Rb---Sr ages of pegmatite-banded gneiss and recrystallized monzogranite reflect post-metamorphic cooling which was contemporaneous with intrusion of post-folding granitoids. 相似文献
19.
新太古代早期是全球地质历史上一个重要的地壳生长时期,世界众多克拉通中广泛存在2. 7Ga左右的岩浆年龄记录。华北克拉通最主要的岩浆事件发生在新太古代晚期,这与世界其他克拉通广泛存在～2. 7Ga的构造热事件明显不同。但全岩Nd和锆石Hf同位素研究表明,华北克拉通～2. 5Ga的岩石主体来自于中太古代晚期-新太古代早期大陆物质的重熔或再造。因此,厘定～2. 7Ga地质事件在华北克拉通的空间分布对深入理解新太古代地壳形成与演化具有重要科学意义。华北克拉通已识别出的～2. 7Ga的花岗质岩石主要分布在胶东、鲁西、武川、赞皇和太华等少数杂岩区,中部带的恒山、阜平和中条杂岩中亦有零星出露。左权变质杂岩位于中部带中南段,赞皇杂岩西南,初步地球化学和锆石年代学研究表明,该地区有多种岩石类型记录了～2. 7Ga的年龄信息,包括副片麻岩、长英质浅色体、磁铁矿角闪片麻岩和TTG片麻岩。其中,TTG片麻岩的原岩为英云闪长岩,锆石发育明显的核边结构,核部具有清晰的岩浆环带,两个不同LA-ICP-MS实验室获得的不一致线上交点年龄分别为2727±14Ma和2731±12Ma,代表了TTG岩浆岩的结晶年龄。同时,左权变质岩石中较好地保存了新太古代晚期的岩浆和变质年龄记录,推测其所代表的构造热事件与华北克拉通～2. 5Ga所经历的大规模幔源岩浆的底侵作用有关。 相似文献
20.
In an attempt to elucidate the pre-Variscan evolution history of the various geological units in the Austrian part of the Bohemian Massif, we have analysed zircons from 12 rocks (mainly orthogneisses) by means of SHRIMP, conventional multi-grain and single-grain U–Pb isotope-dilution/mass-spectrometry. Two of the orthogneisses studied represent Cadomian metagranitoids that formed at ca. 610 Ma (Spitz gneiss) and ca. 580 Ma (Bittesch gneiss). A metagranite from the Thaya batholith also gave a Cadomian zircon age (567±5 Ma). Traces of Neoproterozoic zircon growth were also identified in several other samples, underlining the great importance of the Cadomian orogeny for the evolution of crust in the southern Bohemian Massif. However, important magmatic events also occurred in the Early Palaeozoic. A sample of the Gföhl gneiss was recognised as a 488±6 Ma-old granite. A tonalite gneiss from the realm of the South Bohemian batholith was dated at 456±3 Ma, and zircon cores in a Moldanubian metagranitic granulite gave similar ages of 440–450 Ma. This Ordovician phase of magmatism in the Moldanubian unit is tentatively interpreted as related to the rifting and drift of South Armorica from the African Gondwana margin. The oldest inherited zircons, in a migmatite from the South Bohemian batholith, yielded an age of ca. 2.6 Ga, and many zircon cores in both Moravian and Moldanubian meta-granitoid rocks gave ages around 2.0 Ga. However, rocks from the Moldanubian unit show a striking lack of zircon ages between 1.8 and 1.0 Ga, reflecting an ancestry from Armorica and the North African part of Gondwana, respectively, whereas the Moravian Bittesch gneiss contains many inherited zircons with Mesoproterozoic and Early Palaeoproterozoic ages of ca. 1.2, 1.5 and 1.65–1.8 Ga, indicating a derivation from the South American part of Gondwana. 相似文献