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1.
We present results of geochemical and Sm-Nd isotope studies of high-grade metaterrigenous rocks of the Kitoi and northwestern Irkut terranes of the Sharyzhalgai uplift on the Siberian Platform in comparison with paragneisses of the southeastern Irkut terrane. The metasedimentary rocks of the first region are high-alumina garnet-sillimanite-cordierite-bearing paragneisses; their protoliths were mostly mudstones and pelitic mudstones by major-element composition. The low-alumina biotite gneisses of the Kitoi terrane formed, most likely, from magmatic protoliths similar in petrochemical features to intraplate volcanics. The major factor controlling the composition of the studied high-alumina paragneisses is precipitation of most of incompatible trace elements in the clay fraction of sediments, as evidenced from the positive correlation between trace-element and Al2O3 contents. The Cr and Ni contents, showing a positive correlation with MgO and no correlation with Al2O3, are an indicator of the contribution of the mafic-source material to the formation of high-alumina rocks. The contribution of a mafic source-derived material to the formation of terrigenous rocks increases in passing from Kitoi to northwestern Irkut terrane. The high-alumina and garnet-biotite paragneisses of the southeastern Irkut terrane are similar in trace-element patterns to the analogous rocks of the Kitoi terrane and northwestern part of the Irkut terrane but show higher Th contents and a distinct negative Eu anomaly related to the change in the composition of the felsic source. The participation of felsic potassic igneous rocks in the formation of the southeastern terrigenous sediments is consistent with their deposition after the Neoarchean collision processes (metamorphism and granite magmatism), whereas sedimentation in the Kitoi and northwestern Irkut terranes preceded them. The Sm-Nd isotope characteristics indicate that the latter sediments formed mostly as a result of the erosion of the Paleo-Mesoarchean crust, whereas the metasediments of the southeastern Irkut terrane formed with the participation of Paleoproterozoic juvenile rocks. Thus, the variations in the trace-element and isotope compositions of the high-grade metamorphosed terrigenous rocks reflect recycling and growth of the continental crust of the Sharyzhalgai uplift during the Neoarchean-Pa- leoproterozoic transition.  相似文献   

2.
The Early Precambrian granulite-gneiss complex of the Irkut Block (Sharyzhalgai salient of the Siberian Craton basement) with the protoliths represented by a wide range of magmatic and sedimentary rocks, has a long-term history including several magmatic and metamorphic stages. To estimate the age of sedimentation and metamorphism of the terrigenous deposits, the composition of the garnet-biotite, hyper-sthene-biotite, and cordierite-bearing gneisses has been studied; their isotopic Sm-Nd values have been revealed; and the U-Pb zircon dating has been performed using the SHRIMP II ion microprobe. The protoliths of the terrigenous sediments metamorphosed under conditions of the granulite facies correspond to a rock series from siltstones and graywackes to pelites. The Nd model ages of paragneisses range from 2.4 to 3.1 Ga. Zircons of the cordierite-bearing and hypersthene—biotite gneisses show the presence of cores and rims. The clastic, smoothed, and irregular shape of the cores indicates their detrital character and relicts of oscillatory zoning suggest the magmatic origin of zircon. The rim’s metamorphic genesis is indicated by the lack of zoning and by the lower Th/U ratio compared to that of the cores. The age of the detrital cores (≥2.7, ~2.3, and 1.95—2.0 Ga) and metamorphic rims (1.85–1.86 Ga) defines the time of sedimentation at 1.85–1.95 Ga ago. Potential sources for the Archean detrital zircons were metamagmatic rocks of the granulite—gneiss complexes in the southwestern margin of the Siberian Craton. The age of the dominant detrital cores at 1.95–2.0 Ga ago, together with the minimal TNd(DM) values, indicates the contribution of the juvenile Paleoproterozoic crust to the formation of sediments. The juvenile Paleoproterozoic crust was likely represented by magmatic complexes similar to the volcanic and granitoid associations of the Aldan shield, which were formed 1.99–2.0 Ga ago and showthe model age of 2.0—2.4 Ga. The isotopic Sm-Nd data show that the Late Paleoproterozoic metasedimentary rocks occur not only in the Sharyzhalgai salient but in the Aldan and Anabar shields of the Siberian Craton as well.  相似文献   

3.
We study migmatized aluminous gneisses in the northwest of the Irkut granulite complex in the southeastern Sharyzhalgai uplift of the Siberian Platform basement. Migmatized gneisses with the mineral assemblage Grt + Sil + Bt + Kfs + Pl + Qz (+ Crd + Opx + Spl) contain a leucosome and widespread cordierite-bearing (+ orthopyroxene, quartz, and spinel) symplectites developed after garnet and sillimanite. Study of the microstructural relationships of minerals and modeling using the PERPLEX 672 software have shown a retrograde P-T path of metamorphism for the metasedimentary gneisses, close to the isothermal decompression (ITD). The parameters of the peak of metamorphism are T = 850-870 °C and P > 7 kbar. The weighted average age of zircon from the metasedimentary gneisses (1856 ± 13 Ma, SHRIMP) corresponds to the time of metamorphism. The decompression type of retrograde metamorphism of the rocks in the northwest of the Irkut block indicates their formation in the crust extension and thinning setting. The presence of domal structures in the section of the Irkut block on the shore of Lake Baikal suggests that the dome tectogenesis was involved in the exhumation processes. The Paleoproterozoic metamorphism and granite formation were associated with the same stage of collision processes, when the compression setting was changed by an extension one (1.88-1.85 Ga).  相似文献   

4.
An elongate belt of mid-Cretaceous, compositionally banded gneisses and granulites is exposed in Cucamonga terrane, in the southeastern foothills of the San Gabriel Mountains of southern California. Banded gneisses include mafic granulites of two geochemical types: type 1 rocks are similar to high Al arc basalts and andesites but have higher HFSE (high-field-strength-element) abundances and extremely variable LILE (largeion-lithophile-element) abundances, while type 2 rocks are relatively low in Al and similar to alkali rich MOR (midocean-ridge) or intraplate basalts. Intercalated with mafic granulites are paragneisses which include felsic granulites, aluminous gneisses, marble, and calc-silicate gneisses. Type 1 mafic granulites and calcic trondhjemitic pegmatites also oceur as cross-cutting, synmetamorphic dikes or small plutons. Small-scale heterogeneity of deep continental crust is indicated by the lithologic and isotopic diversity of intercalated ortho-and paragneisses exposed in Cucamonga terrane. Geochemical and isotopic data indicate that K, Rb, and U depletion and Sm/Nd fractionation were associated with biotite +/- muscovite dehydration reactions in type 1 mafic granulites and aluminous gneisses during high-grade metamorphism. Field relations and model initial isotopic ratios imply a wide range of protolith ages, ranging from Early Proterozoic to Phanerozoic.  相似文献   

5.
Abstract: Sensitive, high-resolution ion microprobe zircon U–Pb ages of Paleoproterozoic, high-grade, metasedimentary rocks from the south-western part of the Siberian Craton are reported. Early Precambrian, high-grade complexes, including garnet–biotite, hypersthene–biotite, and cordierite-bearing gneisses compose the Irkut terrane of the Sharyzhalgay Uplift. Protoliths of studied gneisses correspond to terrigenous sediments, ranging from greywacke to shale. The paragneiss model Nd ages of 2.4–3.1 Ga indicate Archean-to-Paleoproterozoic source provinces. Zircons from gneisses show core-rim textures in cathodoluminescence (CL) image. Round or irregular shaped cores indicate detrital origin. Structureless rims with low Th/U are metamorphic in origin. The three age groups of detrital cores are: ≥2.7, ~2.3, and 1.95–2 Ga. The ages of metamorphic rims range from 1.86 to 1.85 Ga; therefore, the sediments were deposited between 1.95 and 1.86 Ga and derived from Archean and Paleoproterozoic source rocks. It should be noted that Paleoproterozoic metasedimentary rocks of the Irkut Block are not unique. High-grade metaterrigenous sediments, with model Nd ages ranging from 2.3 to 2.5 Ga, are widely distributed within the Aldan and Anabar Shields of the Siberian Craton. The same situation is observed in the North China Craton, where metasedimentary rocks contain detrital igneous zircon grains with ages ranging from 3 to 2.1 Ga (Wan et al., 2006). All of these sedimentary units were subjected to Late Paleoproterozoic metamorphism. In the Siberian Craton, the Paleoproterozoic sedimentary deposits are possibly marked passive margins of the Early Precambrian crustal blocks, and their high-grade metamorphism was related to the consolidation of the Siberian Craton.  相似文献   

6.
中国东北~500Ma泛非期孔兹岩带的确定及其意义   总被引：14，自引：7，他引：7  
周建波  张兴洲  郑常青 《岩石学报》2011,27(4):1235-1245
东北地区的变质基底,如佳木斯地块的麻山群、兴安地块的兴华渡口群等岩石组合是以(含石墨)大理岩、夕线石榴片麻岩、斜长角闪岩等为主要标志的孔兹岩系。在额尔古纳、兴安和佳木斯-兴凯地块的夕线石榴片麻岩分别得到锆石U-Pb年龄,证明这些高级变质岩的原岩年龄以新元古代(600~850Ma)为主,变质年龄为~500Ma,因此东北地区的变质基底记录了从Rodinia 到 Gondwana大陆的聚合与离散的过程。泛非期高级变质岩及其同期岩浆岩在额尔古纳、兴安、松辽、佳木斯 和兴凯地块等断续分布,总体出露范围>1300km,并沿虎头、鸡西、萝北、兴华渡口和漠河一线总体北西向沿黑龙江断续分布,因此我们命名为"中国东北泛非期孔兹岩带"。 "中国东北泛非期孔兹岩带"的提出和进一步研究, 对深入探讨东北地区各地块基底组成的异同性以及陆块聚合的过程,以及东北地区的构造演化历史及其在Gondwana大陆重建中的位置都具有重要的科学意义。  相似文献   

7.
Major and trace element compositions of the Paleoproterozoic metaterrigenous rocks (Neroi Group) formed in a large sedimentation basin in the southwestern Siberian Craton (Biryusa Block) were determined to reconstruct the protoliths of metasediments, degree of their recycling, and maturity of source rocks. Primary rocks from the lower part of the sequence (Alkhadyr Formation) are represented by both petrogenic (“first cycle”) and recycled sediments of the graywacke to siltstone and aluminous pelite series. Protoliths of the micaceous and carbonaceous schists from the upper part of the sequence (Tumanshet Formation) correspond to silty pelites and pelites. As the micaceous schists of the Alkhadyr Formation, these rocks have K2O/Al2O3 < 0.3 and elevated Th concentrations, indicating the contribution of recycling in the formation of the fine-grained rocks. Distribution of trace and rare earth elements (REE) in metaterrigenous rocks of the Neroi Group testifies to the predominance of felsic rocks in the source area, while the prominent Eu minimum indicates the presence of granitoids—the products of crustal melting. Rocks of the Alkhadyr Formation also show elevated contents of Cr, Co, Ni, Sc, and Fe, indicating the development of mafic rocks in the source area. Comparison of the trace element contents and their ratios in rocks of the Neroi Group with those in the Archean (3.5–2.5 Ga) and Paleoproterozoic (2.5–1.6 Ga) upper continental crust made it possible to establish that metasedimentary rocks of the Neroi Group were formed by the erosion of sufficiently mature (geochemically differentiated) protoliths, which are similar to the Paleoproterozic crust. Judging from the Sm-Nd isotope data, one of the components of source areas for the terrigenous rocks of the Neroi Group were Archean rocks similar to basement rocks of the Biryusa block with the Nd model ages within 2.8–2.6 Ga. The second component in the source area could be juvenile Paleoproterozoic crust (Nd model age ∼1.9 Ga), which was probably represented by the metavolcanic associations of grabens surrounding the Biryusa block. The minimum Nd model ages for metaterrigenous rocks of the Neroi Group define the lowermost sedimentation boundary at 1.9 Ga.  相似文献   

8.
Chemical and isotopic compositions of Proterozoic metaterrigenous rocks of the Kan Block (Central and Idar terranes) of the Eastern Sayan are studied. The results of the reconstruction of their provenances and sedimentation conditions are presented. The rocks under investigation correspond by their petrogeochemical composition to graywackes of island arcs. The combination of geochemical and isotope data shows that sediments of the Central terrane had a local provenance represented by Early Proterozoic subduction magmatic complexes, whereas sediments of the Idar terrane formed probably as the result of mixing of terrigenous material related to the destruction of rocks of Meso-Neoproterozoic oceanic and more ancient continental crust.  相似文献   

9.
The paper presents data on high-grade silicate–carbonate rocks (calciphyres) from the Irkut block (Sharyzhalgai uplift, southwestern Siberian craton). Their origin and age were determined from the rock characteristics, U–Pb dating, REE content, and Hf isotope composition of zircon. The calciphyres occur both as independent section fragments and as interbeds within Paleoproterozoic garnet-bearing and high-alumina (cordierite- and sillimanite-bearing) gneisses. They were produced by metamorphism of terrigenous-carbonate sediments. The terrigenous sediments range in maturity from arenites and wackes to argillaceous rocks; this is consistent with the reconstruction of the sedimentary protoliths of paragneisses, which are predominant in the metasedimentary rocks. The petrogeochemical features of the calciphyres, their LREE enrichment relative to “pure” carbonate rocks, and a distinct Eu anomaly were inherited from the terrigenous component of calc-silicate sediments. The Nd model age (2.4–2.7 Ga) of the calciphyres and the value THf(DM-2st) = 2.5–3.0 Ga for zircon from these rocks indicate that carbonate accumulation was accompanied by the supply of terrigenous material, which formed during the erosion of Archean and Paleoproterozoic crust. Zircon from the calciphyres is similar to metamorphic zircon in REE patterns and Th/U ratios. It might have been of detrital origin and then recrystallized during high-temperature metamorphism. Terrigenous-silicate rocks were metamorphosed at ca. 1.87 Ga. This is close to the previous age estimates for the terrigenous rocks metamorphism (1.85–1.86 Ga) and the age of baddeleyite from apocarbonate metasomatic rocks (1.86 Ga).  相似文献   

10.
The transition zone between Archean low- and high-grade rocks in southern India represents eroded crustal levels representative of 15–20 km. It is comprised chiefly of tonalitic gneisses with some varieties showing incipient charnockitization and of minor amounts of granitic gneiss and charnockite, both of which appear to have developed from the tonalitic gneisses.Tonalitic gneisses and charnockites are similar in major and trace elements composition while granitic gneisses are relatively enriched in Rb, K, Th, Ba and light rare earth element (REE) and depleted in Cr and Sc. All three rock types exhibit enriched light REE patterns with variable positive Eu anomalies. Total REE content decreases with increasing Eu/Eu and SiO2 and with decreasing Fe2O3T and MgO in the tonalitic gneisses and charnockites.An internally consistent model for the production of the tonalitic gneisses involves partial melting of an enriched mafic source with variable ratios of hornblende to clinopyroxene. This source, in turn, is derived from an ultramafic mantle relatively enriched in incompatible elements. Granitic gneisses form from tonalitic gneisses by alkali metasomatism from chloride-bearing fluids with high H2O/CO2 ratios purged from the lower crust by CO2, and charnockites are produced from tonalitic gneisses (and granitic gneisses) by ischochemical CO2 metamorphism following the alkali metasomatism.  相似文献   

11.
The Irkut Block with dominant rocks of the Sharyzhalgai Series and Kitoi Block with prevalence of the Kitoi Series rocks are the main structures of the Presayanian basement elevation of the Siberian Craton. Two stages of metamorphism, Neoarchean (2.6–2.7 Ga) and Paleoproterozoic (1.85–1.87 Ga), were established for the granulitic complexes of these blocks. The rocks of the Kitoi sillimanite schist deposit composed of sillimanite, andalusite-sillimanite, and garnet-sillimanite schists and gneisses underwent by intense ultrametamorphic transformations which led to the formation of sillimanite- and garnet-bearing plagioclase and feldspar migmatites, and also granites, sienites, and granite-pegmatites. The geochronological study of melanocratic schists and leucocratic plagiogneisses-the typical rocks of the Kitoi deposit-showed the manifestation of metamorphism only at the Archean and Proterozoic boundary (2450–2550 Ma).  相似文献   

12.
大青山-乌拉山变质杂岩立甲子基性麻粒岩主要由角闪二辉麻粒岩、含榴角闪二辉麻粒岩和黑云角闪二辉麻粒岩所组成,并以变形岩墙和不规则透镜体形式赋存于富铝片麻岩和花岗质片麻岩之中.立甲子基性麻粒岩中变质锆石含有单斜辉石(Cpx)+角闪石(Amp)+斜长石(Pl)+磷灰石(Ap)的包体矿物,与寄主岩石——基性麻粒岩矿物组合及其化学成分十分一致,相应的207 pb/206 Pb表面年龄变化于1933±39Ma ～ 1834±40Ma,加权平均年龄为1892±7Ma(MSWD =0.50,n=46),应代表立甲子基性麻粒岩原岩经历中低压麻粒岩相的变质时代.在变质过程中,以大离子亲石元素(K、Na、Sr、Rb)为代表的活动元素发生了显著的改变;而高场强元素(Nb、Zr、Ti)和稀土元素基本无变化,保持稳定.立甲子基性麻粒岩原岩属于拉斑玄武质岩石系列,其SiO2集中变化于45.58％ ～51.40％,Mg#值集中介于41 ～54之间;在球粒陨石标准化稀土配分图中,立甲子基性麻粒岩具有平坦型的稀土配分曲线特征((La/Yb)cN=1.30～1.51),Eu异常不明显(Eu/Eu*=0.93～1.04).与显生宙岛孤拉斑玄武岩类似,立甲子基性麻粒岩所有样品皆具有Nb、Zr、Ti负异常特征.综合分析认为,立甲子基性麻粒岩原岩形成于2450～1930Ma,并于～1900Ma经历中低压麻粒岩相变质作用的改造,其主量元素和微量元素组成具有岛弧拉斑玄武质岩石的地球化学特征,其原岩可能是板块汇聚动力学背景下,岛弧构造环境中形成的辉长岩或辉绿岩.  相似文献   

13.
早奥陶世和早志留世是北祁连加里东造山带构造演化和盆地转变的关键时期。在造山带东段景泰地区,下奥陶统阴沟组和下志留统肮脏沟组两套砂岩的微量元素和稀土元素特征显示,阴沟组杂砂岩样品（Cj1和Cj3）具有最小的Eu/Eu*及最大的Th/Sc和REE,肮脏沟组杂砂岩具有较小的Eu/Eu*和较大的Th/Sc及REE;阴沟组岩屑砂岩样品（Cj13、Cj15和Cj18）具有最大的Eu/Eu*及最小的Th/Sc、REE和La/Yb。多个物源、构造背景判别图解和多元素蛛网图分析表明,阴沟组杂砂岩样品具大陆边缘的构造背景,主要物源为大陆上地壳再旋回沉积物和长英质岩石;岩屑砂岩样品为岛弧构造背景,以中基性安山质岩石为主要物源,可能受陆源物质的微弱影响。肮脏沟组杂砂岩构造背景复杂,表现出大陆岛弧、活动陆缘和被动陆缘三种环境共存的特点,受中基性火山弧物质、长英质岩石和再旋回沉积岩的混合物源的影响。两套砂岩的元素特征表明二者可能具有相似的源区。阴沟组杂砂岩源区可能为阿拉善地块南缘海原群变沉积岩或其他相似的陆源再旋回沉积物,砂岩碎屑以来自初始火山弧物质为主,以石灰沟岛弧型中基性火山岩作为其源岩最合适。阴沟组形成于初始弧后盆地环境,是岛弧活动的直接记录。肮脏沟组可能的源岩为阿拉善地块南缘海原群变沉积岩和中高等成熟度的石灰沟岛弧型火山岩及海原群岛弧型变火山岩,沉积于弧后前陆盆地,对构造环境的反映存在滞后性。  相似文献   

14.
Metabasites exposed in far-eastern Nepal provide an important insight into the metamorphic evolution of the Himalayan orogen independent from data obtained on metapelites. The P–T conditions and formation process of mafic granulite intercalated within Early Oligocene migmatites and two amphibolites surrounded by Early Miocene metapelites were inferred from pseudosection modeling and conventional geothermobarometry combined with the occurrences of field and microstructures. A mafic granulite in the Higher Himalaya Crystalline Sequence (HHCS) yields P–T conditions of 6.5–8 kbar, 730–750 °C. The similar peak P–T condition and retrograde path with low P/T gradient of mafic granulite and surrounding migmatite indicate that both rocks were simultaneously metamorphosed and exhumed together along the tectonic discontinuities in the HHCS. In contrast, the P–T conditions (2–5 kbar, 500–600 °C) of highly-deformed amphibolite block above the Main Central Thrust (MCT) records significantly lower pressure than garnet-mica gneisses in the country rock, suggesting that the amphibolite block derived from upper unit of the MCT zone and became tectonically mixed with the gneisses of hanging wall near the surface. An amphibolite lense below the MCT preserves the prograde P–T conditions (6–7.5 kbar, 550–590 °C) of Early Miocene syn-tectonic metamorphism that occurred in the MCT zone. This study indicates the top-to-the south movement of the MCT zone results in the tectonic assembly of rocks with different P–T–t conditions near the MCT.  相似文献   

15.
赵祖斌  高山 《地学前缘》2000,7(2):431-439
分析了华北克拉通新太古代—三叠纪 16个碎屑沉积岩组合样品。与Taylor和McLennan等提出的太古宙—元古宙界线前后沉积岩及上地壳化学组成变化不同 ,新太古代五台群沉积岩具明显负Eu异常 ,相容元素含量很低 ,不相容元素含量较高 ,与典型后太古宙沉积物组成类似。而古元古代沉积岩比五台群显示出异常高的Eu/Eu 值 ,w(Sc) /w(Th) ,w(Cr) /w (Th)比值。青白口纪、寒武纪、石炭纪和二叠纪沉积岩显示正常的后太古宙沉积岩特征。三叠纪沉积岩的Eu/Eu 值 ,w (Sc) /w (Th) ,w(Cr) /w(Th)比值再次显著升高 ,推测与华北和扬子克拉通最终的陆陆碰撞作用有关。因此 ,太古宙—元古宙界线并不一致对应于上地壳演化程度迅速增高。大陆上地壳并非总是向着分异程度提高的方向演化 ,而是部分时期可出现演化程度降低的异常现象。  相似文献   

16.
Geochemical compositions of the Lower Cambrian Niutitang Formation shales in the southeastern Yangtze Platform margin were investigated for provenance, tectonic setting, and depositional environment. The shale samples are characterized by higher abundances of large ion lithophile elements (Cs, Ba, and Pb), lower abundances of high field strength elements (Cr, Sc, and Co) and transition elements (Th, Zr, Hf, Nb, and Ta) relative to average shale. North American shale composition (NASC) -normalized rare earth element (REE) patterns are observed, with negative Ce anomalies, negative Eu anomalies, and positive Y anomalies. The chemical index of alteration (CIA) varies from 68.67–74.93. Alkali and alkaline element contents and CIA values suggest that the source rocks have undergone moderate weathering. The index of compositional variability (ICV), Zr/Sc and Th/Sc ratios vary from 0.53 to 1.07, 5.31 to 8.18 and 0.52–1.02, respectively. ICV values and relationships between Zr/Sc and Th/Sc ratios indicate negligible sedimentary recycling. The Al2O3/TiO2 (14–26) and TiO2/Zr (56–77) ratios imply that the source rocks of the investigated shales had intermediate igneous compositions. However, Cr/V ratios and a La/Th–Hf discrimination diagram suggest that the intermediate compositional signal of the source rocks was derived from a mixture of 75% mafic and 25% felsic igneous rocks rather than intermediate igneous rocks. The major source was the Jiangnan continental island arc with bimodal igneous rocks, lying to the south of the study area, together with a contribution from granites and gneisses uplifted and eroded in the Yangtze Block. Discrimination of tectonic setting using major and trace elements indicates that the source rocks originated in a transitional setting from active continental to passive margin, consistent with the failed intracontinental rift model for the evolution of the South China plate. The Niutitang Formation shales were deposited in a rift basin setting under conditions of anoxic bottom water in a redox-stratified water column, with organic-rich shales prospective for shale-gas production being found in deep-water downslope and basin environments rather than the shallow-water shelf.  相似文献   

17.
A suite of metapelites, charnockites, calc-silicate rocks, quartzo-feldspathic gneisses and mafic granulites is exposed at Garbham, a part of the Eastern Ghats granulite belt of India. Reaction textures and mineral compositional data have been used to determine the P–T–X evolutionary history of the granulites. In metapelites and charnockites, dehydration melting reactions involving biotite produced quartzofeldspathic segregations during peak metamorphism. However, migration of melt from the site of generation was limited. Subsequent to peak metamorphism at c . 860° C and 8 kbar, the complex evolved through nearly isothermal decompression to 530–650° C and 4–5 kbar. During this phase, coronal garnet grew in the calc-silicates, while garnet in the presence of quartz broke down in charnockite and mafic granulite. Fluid activities during metamorphism were internally buffered in different lithologies in the presence of a melt phase. The P–T path of the granulites at Garbham contrasts sharply with the other parts of the Eastern Ghats granulite belt where the rocks show dominantly near-isobaric cooling subsequent to peak metamorphism.  相似文献   

18.
The paper summarizes data on the geochemistry of metaterrigenous rocks from 26 reference Archean territories: the Pilbara and Yilgarn blocks; Isua and Akilia complexes; Wittwatersrand, Swaziland, Pongola, and Yellowknife supergroups; Khapchanskaya and Gimol’skaya groups; Kan, Sharyzhalgai, Chupa, Slyudyanka, and Onot complexes; etc. The general sets of data points and the calculated median values of the concentrations of trace elements and their ratios are compared to those of Archean and post-Archean shales. In Ce/Cr-Co/Hf, Eu/Eu*-GdN/YbN, Ce/Cr-Th/Sc, Th/Sc-Sc, Th-La, La/Sm-Sc/Th, Yb-GdN/YbN, Th/Sc-Cr, Ni-Cr, and some other diagrams, the fields in which the most data points of Archean metaterrigenous rocks group are outlined. The results of this research indicate that there are no values of geochemical parameters that are inherent only in Archean or only in post-Archean fine-grained terrigenous rocks. Within 80–85% confidence levels, most individual compositions of Archean metaterrigenous rocks are characterized by the following geochemical parameters: (1) Th/Sc < 0.6–0.7, (2) Ce/Cr < 0.6, and (3) Eu/Eu* > 0.70–0.75. If the median values are used, these ranges can be further constrained to (i) Th/Sc < 0.55, (ii) Ce/Cr < 0.4, (iii) Cr/Th > 25, and (iv) Th < 12 ppm. Compared to PAAS, Archean metaterrigenous rocks are characterized by higher median concentrations of Cr and Ni and the Eu/Eu*, Sc/Th, Cr/Th, and Co/Hf ratios, whereas the Nb, La, Ce, Yb, Hf, Th, and U concentrations and the La/Sm and Ce/Cr ratios of PAAS are, conversely, lower. The median values of the LaN/YbN ratios of reference Archean terranes can be either higher or lower than in PAAS, likely depending on the proportions of various rock types in the sources of the terrigenous material. The medians of the GdN/YbN ratios of ～60% of the reference Archean metaterrigenous terranes in our databank are slightly higher than the GdN/YbN ratios of PAAS. The median values of the LaN/SmN ratios of Archean terrigenous rocks are mostly slightly lower than the typical PAAS ratios.  相似文献   

19.
In the Strangways Range a broad tract of lower Proterozoic mafic and silicic granulites with δ 18O = 0.1 to 7.3% is depleted in 18O on average by 2–47% compared with high-grade gneisses and granitoids of Canada and other shields. The Fraser Range mafic granulites (δ18O ~ 7.2%) are enriched with respect to unaltered sea-floor basalts (~5.7%).In some rocks depletion in 18O could be related to dehydration during granulite-facies metamorphism and removal of the resultant products of partial melting. In other rocks pre-granulite reaction between heated seawater and hot basic intrusives seems to be a plausible mechanism of depletion in 18O. A direct correlation between depletion in 18O and the abundance of brown granulite hornblende suggests that 18O-depleted water was present in certain mafic rocks before the onset of granulite metamorphism, whereas in others brown hornblende was introduced during a phase of the granulite facies metamorphism itself.  相似文献   

20.
The granulite‐facies rocks in the Tomkinson Ranges of central Australia are dominated by layered felsic (quartzofeldspathic) gneisses with minor interbanded mafic, calcareous, ferruginous, and quartzitic granulites. They are regarded as representing a middle Proterozoic metasedimentary and/or metavolcanic sequence which has undergone anhydrous granulite‐facies metamorphism approximately 1200 m.y. ago. Conditions of metamorphism have been derived from a petrogenetic grid based on several experimentally determined reactions and give estimates of 10–11 kb pressure and 950–1000°C. Such metamorphism could take place close to the base of the crust with a moderate geothermal gradient of 25–30°C/km.  相似文献   

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