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1.
Located in the middle segment of the Trans-North China Orogen, the Fuping Complex is considered as a critical area in understanding the evolution history of the North China Craton （NCC）. The complex is composed of various high-grade and multiply deformed rocks, including gray gneiss, basic granulite, amphibolite, fine-grained gneiss and marble, metamorphosed to upper amphibolite or granulite facies. It can be divided into four rock units： the Fuping TTG gneisses, Longquanguan augen gneisses, Wanzi supracrustals, and Nanying granitic gneisses. U-Pb age and Hf isotope compositions of about 200 detrital zircons from the Wanzi supracrustals of the Fuping Complex have been analyzed. The data on metamorphic zircon rims give ages of 1.82-1.84 Ga, corresponding to the final amalgamation event of the NCC, whereas the data for igneous zircon cores yield two age populations at -2.10 and -2.51 Ga, with some inherited ages scattering between 2.5 and 2.9 Ga. These results suggest that the Wanzi supracrustals were derived from the Fuping TTG gneisses （-2.5 Ga） and the Nanying granitic gneisses （2.0-2.1 Ga） and deposited between 2.10 and 1.84 Ga. All zircons with -2.51 Ga age have positive initial εHf values from ＋1.4 to ＋10.9, suggesting an important crustal growth event at -2.5 Ga through the addition of juvenile materials from the mantle. The Hf isotope data for the detrital zircons further imply that the 2.8 Ga rocks are important components in the lower crust, which is consistent with a suggestion from Nd isotope data for the Eastern Block. The zircons of 2.10 Ga population have initial εHf values of-4.9 to ＋6.1, interpreted as mixing of crustal re-melt with minor juvenile material contribution at 2.1 Ga. These results are distinct from that for the Western Block, supporting that the Fuping Complex was emplaced in a tectonic active environment at the western margin of the Eastern Block.  相似文献   

2.
《地学前缘(英文版)》2019,10(6):2007-2019
Madagascar,a major fragment of Gondwana,is mainly composed of Precambrian basenent rocks formed by Mesoarchean to Neoproterozoic tectono-thernial events and recording a Pan-African metamorphic overprint.The Ranotsara Shear Zone in southern Madagascar has been correlated with shear zones in southern India and eastern Africa in the reconstruction of the Gondwana supercontinent.Here we present detailed petrology,mineral chemistry,metamorphic P-T constraints using phase equilibrium modelling and zircon U-Pb geochronological data on high-grade metamorphic rocks from Ihosy within the Ranotsara Shear Zone.Garnet-cordierite gneiss from Ihosy experienced two stages of metamorphism.The peak mineral assemblage is interpreted as garnet+sillimanite+cordierite+quartz+plagioclase+Kfeldspar+magnetite+spinel+ilmenite,which is overprinted by a retrograde mineral assemblage of biotite+garnet+cordierite+quartz+plagioclase+K-feldspar+magnetite+spinel+ilmenite.Phase equilibria nodelling in the system Na_2 O-CaO-K_2 O-FeO-MgO-Al_2 O_3-SiO_2-H_2 O-TiO_2-Fe_2 O_3(NCKFMASHTO) indicates peak metamorphic conditions of 850-960 C and 6.9-77 kbar,and retrograde P-Tconditions of 740 C and 4.8 kbar,that define a clockwise P-T path.Near-concordant ages of detrital zircon grains in the garnet-cordierite gneiss dominantly exhibit ages between 2030 Ma and 1784 Ma,indicating dominantly Paleoproterozoic sources.The lower intercept age of 514±33 Ma probably indicates the timing of high-grade metamorphism,which coincides with the assembly of the Gondwana supercontinent.The comparable rock types,zircon ages and metamorphic P-T paths between the Ranotsara Shear Zone and the Achankovil Suture Zone in southern India support an interpretation that the Ranotsara Shear Zone is a continuation of the Achankovil Suture Zone.  相似文献   

3.
The Berere HTHP Complex belt in Maevatanana area of north–central Madagascar formed in the ~ 2.5 Ga orogeny and underwent high temperature (up to 1050oC) and high pressure (up to 11.5 kbar) granulite facies metamorphism. Then a widespread anatexis took place and numerous widely distributed felsic leucosomes formed. The majority of these leucosomes are parallel to the schistosity of the complex or are present as stockworks, as thin layers, or as lenses at different scales in the host rocks. Here, we report new petrographic data, zircon LA-ICP-MS U-Pb ages, and Lu–Hf isotopic data for felsic leucosomes within this complex. Anatexis, as identified by the petrological study of felsic leucosomes in the field and in thin sections, involved initial ternary feldspar exsolving to produce antiperthite and a quartz + plagioclase ± K-feldspar + sericite mineral assemblage around feldspar grain boundaries. Dissolution is apparent along muscovite grain boundaries, and residual sericite is present around the margins of feldspar and quartz, all suggesting that anatexis was driven by reactions involving muscovite. Zircon U–Pb dating indicates that the felsic leucosomes within the complex formed at 2467–2369 Ma. The majority of samples have positive εHf(t) values, although a few have negative values, suggesting their formation from magmas predominantly sourced from the depleted mantle, possibly with the involvement of minor amounts of crustal materials. Two-stage Hf model ages and εHf(t) values for these samples are consistent with those for gneisses of the basement, indicating that the felsic leucosomes were formed by the anatexis of gneisses and both of their protolith formed during the formation of continental crust in Meso-Neoarchean (ca. 3.1–2.7 Ga). As such, the crystallization age of the felsic leucosome (~2.4 Ga) represents the timing of regional anatexis and a change to post-orogenic tectonism. And this anatexis is also corresponds to the thermal event in Dharwar craton in India which has a pronounced similar Precambrian geology with Madagascar, providing an important constraints on the correlation of the two continental fragments  相似文献   

4.
An unusual zircon SHRIMP dating result of a granitic gneiss from the Qinglongshan eclogite-gneiss roadcut section is presented in this paper. The very peculiar and complicated internal structures, as well as the very low Th/U ratios (0.01-0.08) of the zircons indicate that they were formed by metamorphic recrystallization. Strongly in contrast with previously published zircon U-Pb ages of the Dabie-Sulu UHP metamorphic rocks where protolith ages of 600-800 Ma are commonly recorded, only metamorphic age of 218±5 Ma, defined by 18 analytical spots either in rim or in core of zircons, are recorded in this granitic gneiss. This age represents the time of the complete metamorphic recrystallization overprint on primary magmatic zircons. The recrystallization was derived by the UHP metamorphism, and was strengthened by the early stage of retrograde metamorphic fluid activity.  相似文献   

5.
The Archean continental crusts account for ca.20% of the present volume,but the thermal history of the Earths' mantle suggests much more continental crusts were formed in the early Archean.Because the Archean continental crust underwent severe metamorphism,it is important to avoid influence by the later thermal events.We carried out a comprehensive geochronological work of Cathodoluminescence(CL) observation and U-Pb dating of zircons from orthogneisses and supracrustal rocks over the Saglek Block to obtain their protolith ages.The zircons were classified into three domains of core,mantle and rims,and the cores were further classified into three groups of inherited,altered and zoned cores based on the zonation on the CL images.We estimated the protolith ages from Pb-Pb ages of the zoned-cores of zircons with low U contents.We made a detailed sketch of a small outcrop in St.John's Harbour South(SJHS) area,and classified the orthogneisses and mafic enclaves into seven generations based on the geologic occurrence.The first and second generations comprise mafic rocks and lack magmatic zircons.We conducted CL imaging and U-Pb dating of zircons from the third,sixth and seventh generation of the orthogneisses to estimate the protolith ages at 3902 L 25,3892 ± 33 and 3897 ± 33 Ma for each,supporting the presence of the over 3.9 Ca Iqaluk Gneiss.The geological occurrence that the mafic rocks occur as enclaves within the 3.9 Ga Iqaluk Gneiss indicates that they are the oldest supracrustal rocks in the world.Our geochronological and geological studies show the Uivak Gneiss is quite varied in lithology and age from 3.6 to 3.9 Ga,and tentatively classified into six groups based on their ages.The oldest Uivak Gneiss components including the Iqaluk Gneiss are present around the SJHS area,and the orthogneisses become young as it is away.The lines of evidence of overprinting of younger granitoid on older granitoid in small outcrops and geological-map scale as well as presence of inherited zircons even in the oldest suite suggests that crustal reworking played an important role on erasing the ancient crusts.  相似文献   

6.
http://www.sciencedirect.com/science/article/pii/S1674987114000206   总被引：20，自引：0，他引：20  
The North China Craton（NCC） has a complicated evolutionary history with multi-stage crustal growth,recording nearly all important geological events in the early geotectonic history of the Earth.Our studies propose that the NCC can be divided into six micro-blocks with ＆gt;～3.0-3.8 Ga old continental nuclei that are surrounded by Neoarchean greenstone belts（CRB）.The micro-blocks are also termed as highgrade regions（HGR） and are mainly composed of orthogneisses with minor gabbros and BIF-bearing supracrustal beds or lenses,all of which underwent strong deformation and metamorphism of granulite- to high-grade amphibolite-facies.The micro-blocks are,in turn,from east to west,the Jiaoliao（JL）,Qianhuai（QH）,Ordos（ODS）,Ji’ning（JN） and Alashan（ALS） blocks,and Xuchang（XCH） in the south.Recent studies led to a consensus that the basement of the NCC was composed of different blocks/terranes that were finally amalgamated to form a coherent craton at the end of Neoarchean.Zircon U-Pb data show that TTG gneisses in the HGRs have two prominent age peaks at ca.2.9-2.7 and2.6-2.5 Ga which may correspond to the earliest events of major crustal growth in the NCC.Hafnium isotopic model ages range from ca.3.8 to 2.5 Ga and mostly are in the range of 3.0-2.6 Ga with a peak at2.82 Ga.Recent studies revealed a much larger volume of TTG gneisses in the NCC than previously considered,with a dominant ca.2.7 Ga magmatic zircon ages.Most of the ca.2.7 Ga TTG gneisses underwent metamorphism in 2.6-2.5 Ga as indicated by ubiquitous metamorphic rims around the cores of magmatic zircon in these rocks.Abundant ca.2.6-2.5 Ga orthogneisses have Hf-in-zircon and Nd wholerock model ages mostly around 2.9-2.7 Ga and some around 2.6-2.5 Ga,indicating the timing of protolith formation or extraction of the protolith magma was from the mantle.Therefore,it is suggested that the 2.6-2.5 Ga TTGs probably represent a coherent event of continental accretion and major reworking（crustal melting）.As a distinct characte  相似文献   

7.
The Hengshan complex forms part of the central zone of the North China Craton and consists predominantly of ductilely-deformed late Archaean to Palaeoproterozoic high-grade, partly migmatitic, granitoid orthogneisses, intruded by mafic dykes of gabbroic composition. Many highly strained rocks were previously misinterpreted as supracrustal sequences and represent mylonitized granitoids and sheared dykes. Our single zircon dating documents magmatic granitoid emplacement ages between 2.52 Ga and 2.48 Ga, with rare occurrences of 2.7 Ga gneisses, possibly reflecting an older basement. A few granitic gneisses have emplacement ages between 2.35 and 2.1 Ga and show the same structural features as the older rocks, indicating that the main deformation occurred after -2.1 Ga. Intrusion of gabbroic dykes occurred at -1920 Ma, and all Hengshan rocks underwent granulite-facies metamorphism at 1.88-1.85 Ga, followed by retrogression, sheafing and uplift. We interpret the Hengshan and adjacent Fuping granitoid gneisses as the lower, plutonic, part of a late Archaean to early Palaeoproterozoic Japan-type magmatic arc, with the upper, volcanic part represented by the nearby Wutai complex. Components of this arc may have evolved at a continental margin as indicated by the 2.7 Ga zircons. Major deformation and HP metamorphism occurred in the late Palaeoproterozoic during the Luliang orogeny when the Eastern and Western blocks of the North China Craton collided to form the Trans-North China orogen. Shear zones in the Hengshan are interpreted as major lower crustal discontinuities post-dating the peak of HP metamorphism, and we suggest that they formed during orogenic collapse and uplift of the Hengshan complex in the late Palaeoproterozoic （〈1.85 Ga）.  相似文献   

8.
In this paper, we report an integrated study of U-Pb age and Hf isotope compositions of zircons from biotite plagioclase gneiss at Lianghe in western Yunnan. The zircons preserved inherited core and rim texture. Igneous zircon grains and rims yielded a weighted mean 206Pb/238U age of 120.4±1.7 Ma, their εHf (120 Ma) values were mainly negative ranging from –13.9 to –10.7, with Hf model ages between 1.9 Ga and 2.0 Ga, some zircons had positive εHf (120 Ma) values ranging from 0.2 to 2.1. The inherited cores showed the wide variations in the U-Pb age of 375–1315 Ma. One of them showed the εHf (506 Ma) value of –4.2, it was similar to the gray gneiss of old crust, which εHf (500 Ma) values were negative ranging from –4.5 to –3.3. Combining geological feature and geochemical data, we concluded that the protolith of biotite plagioclase gneiss was old crust-derived tonalitic magma during the early Cretaceous.  相似文献   

9.
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   

10.
Zircon U-Pb age, trace elements, and Hf isotopes were determined for granulite and gneiss at Huaugtuling (黄土岭), which is hosted by ultrahigh-pressure metamorphic rocks in the Dabie(大别) orogen, east-central China. Cathodolumineseence (CL) images reveal core-rim structure for most zircons in the granulite. The cores show oscillatory zoning, relatively high Th/U and 176 Lu/177 Hf ratios, and high rare earth element (HREE)-enriched pattern, consistent with magmatic origin. They gave a weighted mean 207 Pb/206 Pb age of (2 766±9) Ma, dating magma emplacement of protolith. The rims are characterized by sector ur planar zoning, low Th/U and 176 Lu/177 Hf ratios, negative Euanomalies and flat HREE patterns, consistent with their formation under granulite-facies metamorphicconditions. Zircon U-Pb dating yields an age of (2 029±13) Ma, which is interpreted as a record ofmetamorphic event during the assembly of the supercontinent Columbia. The gneiss has a protolith ageof (1982±14) Ma, which is similar to the zircon U-Pb age for the granulite-facies metamorphism,suggesting complementary processes to granulite-facies metamorphism and partial melting. A fewinherited cores with igneous characteristics have 207 pb/206 Pb ages of approximately 3.53, 3.24, and 2.90Ga, respectively, suggesting the presence of Mesoarchean to Paleoarchean crustal remnants. A fewTriassic and Cretaceous metamorphic ages were obtained, suggesting the influences by the Triassiccontinental collision and postcollisional collapse in response to the Cretaceous extension. Comparingwith abundant occurrence of Triassic metamorphic zircons in ultrahigh-pressure eclogite and granitehydrous melt is evident for zircon growth in theHuangtuling granulite and gneiss during thecontinental collision. The magmatic protolithzircons from the granulite show a large variationin 176 Hf/177 Hf ratios from 0.280 809 to 0.281 289,corresponding to era(t) values of-7.3 to 6.3 andHf model ages of 2.74 to 3.34 Ga. The 2.90 Gainherited zircons show the similar Hf isotope features. These indicate that both growth of juvenile crust and reworking of ancient crust took place at the time of zircon formation. It is inferred that the Archean basement of the Yangtze block occurs in thenorth as the Dabie orogen, with ca. 2.90-2.95 Ga and 2.75-2.80 Ga as two major episodes of crustalformation.  相似文献   

11.
Geological mapping and zircon U–Pb laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) dating has identified a Mesoarchean (2857 ± 17 Ma) geological unit in the Luanjiajie area of the northern margin of the North China Craton, within the northern part of Liaoning Province, China. This unit is dominated by tonalitic and trondhjemite gneisses that form part of a typical tonalite–trondhjemite–granodiorite (TTG) rock assemblage. These Mesoarchean gneisses are enriched in Na and depleted in K, yield K2O/Na2O ratios of 0.34–0.50, have Rittmann index (σ) values of 1.54–3.04, and are calc-alkaline. They have EuN/EuN* values of 0.77–1.20 (average of 1.03), indicating that these samples have negligible Eu anomalies, and yield high LaN/YbN values (4.92–23.12). These characteristics indicate that these Mesoarchean gneisses have fractionated rare earth element (REE) compositions that are enriched in the light REE (LREE) and depleted in the heavy REE (HREE), with steeply dipping chondrite-normalized REE patterns. These gneisses are also enriched in Rb, Th, K, Zr, and Hf, and are relatively depleted in Ta, Nb, P, and Ti. In summary, the magma that formed these tonalitic and trondhjemite gneisses was most likely derived from the partial melting of lower-crustal basaltic rocks during subduction. The timing of formation (2.85 Ga) of the Luanjiajie tonalite and trondhjemite gneisses probably represents the timing of initiation of plate tectonics within the LongGang Block during a SE-directed subduction event. The presence of inherited zircons with ages of >3.0 Ga within the Luanjiajie gneisses suggests that this area may contain as yet undiscovered rocks that formed before 3.0 Ga.  相似文献   

12.
This paper presents results of U–Pb dating (SHRIMP-II) and Lu–Hf (LA–ICP MS) isotope study of zircon from Paleoarchean plagiogneisses and plagiogranitoids of the Onot and Bulun blocks of the Sharyzhalgai uplift. Magmatic zircons from the Onot plagiogneiss and Bulun gneissic trondhjemite are dated at 3388±11 and 3311±16 Ma, respectively. Magmatic zircons from plagiogneisses and plagiogranitoids of the studied tonalite–trondhjemite–granodiorite (TTG) complexes are characterized mainly by positive values of εHf indicating that felsic melts were generated mainly from juvenile (mafic) sources, which are derived from a depleted mantle reservoir. The variable Hf isotope composition in magmatic zircons and the lower average εHf values in comparison with the depleted mantle values suggest the contributions of both mafic and more ancient crustal sources to magma formation. Metamorphic zircons from the gneissic plagiogranite and migmatized plagiogneiss either inherited the Hf isotope composition from magmatic zircon or are enriched in radiogenic Hf. The more radiogenic Hf isotope composition of metamorphic zircons from the migmatized plagiogneisses is due to their interaction with melt during partial melting. Variations in the Lu–Hf isotope composition of zircon from the Bulun rocks in the period 3.33–3.20 Ga are due to the successive melting of mafic crust or the growing contribution of crustal material to their genesis. Correlation between the Lu–Hf isotope characteristics of zircon and the Sm–Nd parameters of the Onot plagiogneisses points to the contribution of ancient crustal material to their formation. The bimodal distribution of the model Hf ages of zircons reflects two stages of crustal growth in the Paleoarchean: 3.45–3.60 and ~ 3.35 Ga. The isotope characteristics of zircon and rocks of the TTG complexes, pointing to recycling of crustal material, argue for the formation of plagiogneisses and plagiogranitoids as a result of melting of heterogeneous (mafic and more ancient crustal) sources in the thickened crust.  相似文献   

13.
The Storø greenstone belt, southern West Greenland, consists of thrust-imbricated slices of Mesoarchean (>3060 Ma) and Neoarchean (ca. 2800 Ma) mafic to ultramafic volcanic rocks, volcaniclastic sediments, and gabbro–anorthosite associations. The belt underwent polyphase metamorphism at upper amphibolite facies conditions between 2650 and 2600 Ma. The contacts between the Mesoarchean and Neoarchean volcanic rocks, and surrounding Eoarchean to Neoarchean tonalite–trondhjemite–granodiorite (TTG) gneisses are tectonic and typically bounded by high-grade mylonites. Regardless of age, the volcanic rocks are dominated by mafic amphibolites with a tholeiitic basalt composition, near-flat to slightly enriched light rare earth element (LREE) patterns (La/Smcn = 0.91–1.48), relatively flat to slightly depleted heavy-REE (HREE) (Gd/Ybcn = 1.0–1.28), and pronounced negative Nb–Ta anomalies (Nb/Nb* = 0.34–0.73) on chondrite- and primitive mantle-normalized diagrams. These geochemical characteristics are consistent with subduction zone geochemical signatures and partial melting of a shallow (<80 km) mantle source free of residual garnet. There is no geochemical evidence for contamination by older continental crust. The overall field and geochemical characteristics suggest that the thrust-imbricated basaltic rocks were erupted in intra-oceanic subduction zone settings. Sedimentary rocks are represented by garnet–biotite and quartzitic gneisses. They are characterized by relatively high contents of transition metal (Ni = 10–154 ppm; Cr = 7–166 ppm) and enriched LREE patterns (La/Smcn = 1.38–3.79). These geochemical characteristics suggest that the sedimentary rocks were derived from erosion of felsic to mafic igneous source rocks. Collectively, the structural and lithogeochemical characteristics of the Storø greenstone belt are consistent with collision (accretion) of unrelated Archean volcanic rocks formed in supra-subduction zone geodynamic settings. Accordingly, the Mesoarchean and Neoarchean rock record of the Storø greenstone belt may well be explained in terms of modern-style plate tectonic processes.  相似文献   

14.
The hornblende-biotite gneisses of Central Kerala which cover approximately 490km2 exhibit schlieric and nebulitic structures, tending towards a homophanous nature and are classified here as diatexites. Mafic protoliths and restite biotite, each representing the refractory residuum of two independent partial melting episodes are widely present in the gneisses. The general mineral assemblage of the gneisses comprise quartz, K-feldspar, oligoclase, biotite and hornblende. Chemically, they are dominantly adamellitic and the behaviour of major and trace elements is consistent with a magmatic parentage. Based on petrochemical criteria, a two-stage evolution model is proposed here, which involves (i) partial melting of mafic granulites under high Archean geothermal gradients and generation of tonalite/trondhjemite through amphibole and plagioclase fractionation and (ii) partial melting and subsequent quartz-alkali feldspar fractionation of the tonalite/trondhjemite under amphibolite facies conditions with synchronous K-enrichment resulting in the diatectic adamellites.  相似文献   

15.
High-K granitoids are among the most abundant rock types in many Archean cratons. Late Neoarchean monzogranitic to syenogranitic gneisses with high-K affinities are widely distributed in the Anshan, Suizhong, Qinhuangdao, and Aolaishan areas on the northeastern margin of the North China Craton (NCC). In this contribution, we present an integrated study of zircon U–Pb–Hf–O isotopic compositions and whole-rock elemental compositions of amphibolites and trondhjemitic and monzo–syenogranitic gneisses of the Jinzhou area in the metamorphic basement of eastern Hebei–western Liaoning, with the aim of constraining their petrogenesis and geodynamic setting. Emplacement ages of the amphibolites and trondhjemitic–monzogranitic gneisses are 2543 ± 27, 2532 ± 19, and 2513 ± 7 Ma, respectively. The amphibolites are tholeiitic in composition with SiO2 contents of 49.7–50.8 wt%, variable degrees of light rare-earth-element (LREE) enrichment and high-field-strength element (HFSE) depletion, and high zircon εHf(t) values of +2.6 to +6.3, suggesting a depleted lithospheric mantle origin. The major- and trace-element compositions of the trondhjemitic gneisses are similar to those of the low-pressure tonalite–trondhjemite–granodiorite (TTG) suite. The zircon Hf (εHf(t) = +1.6 to +3.9) and O (δ18O = +3.76‰ to +5.73‰) isotopic compositions of the trondhjemitic gneisses indicate a juvenile basaltic source at the base of a thickened magmatic arc. The monzogranitic gneisses differ from their TTG counterparts in that they have lower SiO2 and higher MgO, K2O, and incompatible-element (especially Ba, Th, Sr, P, and LREE) contents. They also have slightly evolved zircon εHf(t) values (+0.6 to +3.8) and higher δ18O values (+4.69‰ to +6.13‰). These features suggest that the monzogranitic gneisses represent sanukitoid-type rocks, with a mantle source modified by crust-derived melts. The weakly deformed syenogranitic gneisses are characterized by high SiO2 and K2O, and very low MgO, Cr, and Ni contents, suggesting that they were formed by partial melting of local TTG rocks. Our results, together with those of previous investigations, suggest that the 2554–2513 Ma low- to high-K magmatism in the Jinzhou area most likely originated in an arc–back-arc tectonic setting on the northern margin of the NCC. The large volumes of high-K granitoids in eastern Hebei–western Liaoning are related to extensive mantle–crust interactions and crustal reworking in such a setting.  相似文献   

16.
Two natural, low K2O/Na2O, TTG tonalitic gneisses (one hornblende-bearing and the other biotite-bearing) were partially melted at 0.8–1.2 GPa (fluid-absent). The chief melting reactions involve the breakdown of the biotite and hornblende. The hornblende tonalite is slightly less fertile than the biotite tonalite, but melt volumes reach around 30% at 1,000°C. This contrasts with results of most previous work on more potassic TTGs, which generally showed much lower fertility, though commonly producing more potassic melts. Garnet is formed in biotite-bearing tonalitic protoliths at P > 0.8 GPa and at > 1.0 GPa in hornblende-bearing tonalitic protoliths. All fluid-absent experiments produced peraluminous granitic to granodioritic melts, typically with SiO2 > 70 wt.%. For the biotite tonalite, increasing T formed progressively more melt with progressively lower K2O/Na2O. However, the compositions of melts from the hornblende tonalite do not vary significantly with T. With increasing P, melts from the biotite tonalite become less potassic, due to the increasing thermal stability of biotite. For the hornblende tonalite, again there is no consistent trend. Fluid-absent melting of sodic TTGs produces melts with insufficient K2O to model the magmas that formed the voluminous, late, potassic granites that are common in Archaean terranes. Reconnaissance fluid-present experiments at 0.6 GPa imply that H2O-saturated partial melting of TTGs is also not a viable process for producing magmas that formed these granites. The protoliths for these must have been more potassic and less silicic. Nevertheless, at granulite-facies conditions, sodic TTGs will produce significant quantities of broadly leucogranodioritic melt that will be more potassic than the protoliths. Upward abstraction of this melt would result in some LILE depletion of the terrane. Younger K-rich magmatism is unlikely to represent recycling of TTG crust on its own, and it seems most likely that evolved crustal rocks and/or highly enriched mantle must be involved. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

17.
Tonalite–trondhjemite–granodiorite gneisses (TTG) and K-rich granites are extensively exposed in the Mesoarchean to Paleoproterozoic Bundelkhand craton of central India. The TTGs rocks are coarse- grained with biotite, plagioclase feldspar, K-feldspar and amphibole as major constituent phases. The major minerals constituting the K-rich granites are K-feldspar, plagioclase feldspar and biotite. They are also medium to coarse grained. Mineral chemical studies show that the amphiboles of TTG are calcic amphibole hastingsite, plagioclase feldspars are mostly of oligoclase composition, K-feldspars are near pure end members and biotites are solid solutions between annite and siderophyllite components. The K-rich granites have biotites of siderophyllite–annite composition similar to those of TTGs, plagioclase feldspars are oligoclase in composition, potassic feldspars have \(\hbox {X}_{\mathrm{K}}\) ranging from 0.97 to 0.99 and are devoid of any amphibole. The tonalite–trondhjemite–granodiorite gneiss samples have high \(\hbox {SiO}_{2}\) (64.17–74.52 wt%), \(\hbox {Na}_{2}\hbox {O}\) (3.11–5.90 wt%), low Mg# (30–47) and HREE contents, with moderate \((\hbox {La/Yb})_{\mathrm{CN}}\) values (14.7–33.50) and Sr/Y ratios (4.85–98.7). These geochemical characteristics suggest formation of the TTG by partial melting of the hydrous basaltic crust at pressures and depths where garnet and amphibole were stable phases in the Paleo-Mesoarchean. The K-rich granite samples show high \(\hbox {SiO}_{2}\) (64.72–76.73 wt%), \(\hbox {K}_{2}\hbox {O}\) (4.31–5.42), low \(\hbox {Na}_{2}\hbox {O}\) (2.75–3.31 wt%), Mg# (24–40) and HREE contents, with moderate to high \((\hbox {La/Yb})_{\mathrm{CN}}\) values (9.26–29.75) and Sr/Y ratios (1.52–24). They differ from their TTG in having elevated concentrations of incompatible elements like K, Zr, Th, and REE. These geochemical features indicate formation of the K-granites by anhydrous partial melting of the Paleo-Mesoarchean TTG or mafic crustal materials in an extensional regime. Combined with previous studies it is interpreted that two stages of continental accretion (at 3.59–3.33 and 3.2–3.0 Ga) and reworking (at 2.5–1.9 Ga) occurred in the Bundelkhand craton from Archaean to Paleoproterozoic.  相似文献   

18.
The dominantly high-K, moderate to high SiO2 containing, variably fractionated, volcanic-arc granitoids (± sheared) from parts of Bundelkhand craton, northcentral India are observed to contain molybdenite (Mo) in widely separated 23 locations in the form of specks, pockets, clots and stringers along with quartz ± pyrite ± arsenopyrite ± chalcopyrite ± bornite ± covellite ± galena ± sphalerite and in invisible form as well. The molybdenite mineralization is predominantly associated with Bundelkhand Tectonic Zone, Raksa Shear Zone, and localized shear zones. The incidence of molybdenite is also observed within sheared quartz and tonalite–trondhjemite–granodiorite (TTG) gneisses. The fluid inclusion data show the presence of bi-phase (H2O–CO2), hypersaline and moderate temperature (100°–300°C) primary stretched fluid inclusions suggesting a possible hydrothermal origin for the Mo-bearing quartz occurring within variably deformed different granitoids variants of Archean Bundelkhand craton.  相似文献   

19.
河南嵩山地区位于华北克拉通南缘,是我国记录前寒武纪地质的典型地区之一。该区广泛出露新太古代TTG质片麻岩套,主要以英云闪长岩类为主,闪长岩类次之,测得的锆石SHRIMP年龄在2600~2500Ma。该套TTG质片麻岩富Na2O(3%~7%)、SiO2(>67%)、贫铁、镁,高的Na2O/K2O比值（多在1.5~5.2）,Al2O3=13.72%~16.37%,A/CNK=0.97~1.21,属英云闪长岩-奥长花岗岩系列,显示新太古代富铝型TTG岩石特征。岩石富Sr (平均433×10-6),Rb/Sr比值(<0.5) 较低,Sr/Y比值高(平均165),Nb、Ta和Ti负异常,∑REE偏低,强烈分异((La/Yb) N=27~150),基本无Eu异常,低的Nb/Ta(14左右)、La/Nb (平均约为7)比值及其它微量元素特征表明其与岛弧或大陆边缘弧玄武质岩石特征相似。全岩Nd同位素和锆石Hf同位素数据显示岩石源区是来自亏损地幔的约2.66Ga的初生地壳;Mg#值变化较大反映存在地幔楔不同程度的混染。地球化学特征指示该岩浆是在较高温度和压力(约700~1000℃,>1.5Gpa)下由俯冲的含水玄武质洋壳部分熔融形成,残留相中有石榴石和角闪石而不含斜长石。嵩山地区TTG片麻岩的这种成因机制表明当时陆壳以水平方式增生,也说明在随后的2.5Ga左右微陆块碰撞拼合事件之前不同陆块之间很可能被一个古大洋所分隔。  相似文献   

20.
The Trans-North China Orogen (TNCO) along the central part of the North China Craton (NCC) is considered as a Paleoproterozoic suture along which the Eastern and Western Blocks of the NCC were amalgamated. Here we investigate the Precambrian crustal evolution history in the Fuping segment of the TNCO and the subsequent reactivation associated with extensive craton destruction during Mesozoic. We present zircon LA-ICP-MS U–Pb and Lu–Hf data on TTG (tonalite–trondhjemite–granodiorite) gneiss, felsic orthogneiss, amphibolite and granite from the Paleoproterozoic suite which show magmatic ages in the range of 2450–1900 Ma suggesting a long-lived convergent margin. The εHf(t) values of these zircons range from −11.9 to 12 and their model ages suggest magma derivation from both juvenile components and reworked Archean crust. The Mesozoic magmatic units in the Fuping area includes granite, diorite and mafic microgranular enclaves, the zircons from which define a tight range of 120–130 Ma ages suggesting a prominent Early Cretaceous magmatic event. However, the εHf(t) values of these zircons show wide a range from −30.3 to 0.2, indicating that the magmatic activity involved extensive rejuvenation of the older continental crust.  相似文献   

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