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1.
柴北缘夹榴辉岩的片麻岩（片岩）地球化学、Sm-Nd和U-Pb同位素研究——深俯冲的前寒武纪变质基底？ 总被引：18，自引：1，他引：18
在柴北缘的鱼卡-锡铁山-沙柳河一带,出露夹有榴辉岩透镜体的花岗质片麻岩(正片麻岩)和副片麻岩(片岩)。地球化学和Sm-Nd同位素数据显示副片麻岩(片岩)与正片麻岩具有类似的地球化学成分和一致的Nd模式年龄(1.88～2.18Ga),结合副片麻岩(片岩)局部包在正片麻岩中的野外关系,正片麻岩可能为副片麻岩(片岩)原地熔融作用的产物。U-Pb锆石测定表明熔融作用产生的正片麻岩的岩浆形成时代为952Ma。另外,这些夹榴辉岩的片麻岩(片岩)也与柴北缘北侧不夹榴辉岩的深变质基底片麻岩和中南祁连地块的变质基底片麻岩有相似Sm-Nd同位素特征和近一致Nd模式年龄(1.87～2.26Ga)。表明它们具有明显的亲缘关系,可能来源于具有古元古代晚期地壳形成年龄的同一变质基底。然而,与柴北缘北 侧和祁连地块的深变质基底岩石不同的是,这套含榴辉岩的片麻岩(片岩)明显遭受了早古生代变质作用的影响,正片麻岩锆石U-Pb测定获得的下交点年龄为478±44Ma,与柴北缘地区榴辉岩的变质锆石的年龄在误差范围内一致;而已在都兰地区副片麻岩锆石中柯石英包体的发现也证明了含榴辉岩的片麻岩(片岩)与榴辉岩一样同样经历了UHP变质作用。因此,我们认为柴北缘含榴辉岩的片麻岩虽然具有与相邻变质基底相似的早期演化历史,但在早古生代又与所夹的榴辉岩 相似文献
2.
Many granitic bodies intrude the basement gneisses in Meghalaya Plateau, Northeast India. Rb-Sr whole-rock isotopic ages of the granitoids range from 881 to 479 Ma while the ages of the basement orthogneisses vary from 1714 to 1150 Ma. All the plutons are dominantly metaluminous and show geochemical variation. Oxygen isotopic compositions in the granitoids and gneisses are concordant (d18O: + 5.78% to + 8.70%). However, the gneisses from high-grade terrain have low d18O value of +2.52% to +5.31%. Initial 87Sr/86Sr (ISr) ratios of the plutons vary from 0.70459 to 0.71487 and tend to increase with progressive younging in age. The geochemical characters suggest derivation of the granites from lower crustal source. The fractionated rare earth patterns observed in the granitoids can be obtained by partial melting of gneisses or diorites. Some gneiss samples have experienced interaction with hydrothermal fluids resulting in lowering d18O. The isotopic ages of granite plutonism in Meghalaya are similar to the plutonic and tectonothermal events in other parts of India, southwestern Australia and document final amalgamation events of the Gondwana Supercontinent. 相似文献
3.
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
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
4.
Andrew P. Barth J. L. Wooden D. J. May 《Contributions to Mineralogy and Petrology》1992,109(3):394-407
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.
Rubidium-strontium isotopic measurements are reported for total-rockand minerals from igneous and high-grade metamorphic rocks fromthe Willyama Complex, Broken Hill, Australia. The results ofmeasurements on total-rocks and some minerals from the high-gradegneisses indicate that nearly complete strontium isotopic redistributionoccurred within individual rock units 164040 m. y. ago. Thisage is interpreted as that of the high-grade regional metamorphismwhich recrystallized the Willyama rocks to gneisses in the BrokenHill area. Analyses of total-rocks and some minerals from intrusivemuscovite granites and pegmatites give consistent ages of 154O5Om. y. indicating that these rocks were emplaced soon after thehigh-grade regional metamorphism. Rubidium-strontium isotopic analyses of all biotites, and ofmuscovites from pegmatites concordant in schists at Thackaringareveal a 500 m. y. metamorphic episode of lesser intensity accompaniedby pegmatite emplacement. This metamorphism was not of sufficientstrength to open the total-rock systems significantly to rubidiumand strontium isotopes. Biotites, however, appear to have losttheir radiogenic strontium almost completely at this time andit is probable that this event accounts for the observed disturbanceof the potash-feldspar rubidium-strontium systems in most gneissand muscovite granite samples investigated. There is a close similarity between the rubidium-strontium ageresults and the Broken Hill model lead ages. This supports thehypothesis of two-stage lead development and, with the strontiumisotope evidence, suggests that the region has evolved largelyas a closed chemical system since at least the high-grade metamorphism. 相似文献
6.
Folco Giacomini Rosa Maria Bomparola Claudio Ghezzo Hans Guldbransen 《Contributions to Mineralogy and Petrology》2006,152(1):19-42
The high-grade metamorphic complex of northern Sardinia consists of a strongly deformed sequence of migmatitic ortho- and paragneisses interlayered with minor amphibolites preserving relic eclogite parageneses. The protolith ages and geochemical characteristics of selected gneiss samples were determined, providing new constraints for reconstructing the Palaeozoic geodynamic evolution of this sector of the Variscan chain. The orthogneisses are metaluminous to peraluminous calcalkaline granitoids with crustal Sr and Nd isotopic signatures. One orthogneiss from the high-grade zone and one metavolcanite from the volcanic belt in southern Sardinia were dated by LAM-ICPMS (and SHRIMP) zircon geochronology. The inferred emplacement ages of the two samples are 469 ± 3.7 and 464 ± 1 Ma, respectively. The analysed paragneisses are mainly metawackes with subordinate metapelites and rare metamarls. Three paragneiss samples were dated: zircon ages scatter between 3 Ga and about 320 Ma, with a first main cluster from 480 to 450 Ma, and a second one from about 650 to 550. Variscan zircon ages are rare and mostly limited to thin rims and overgrowths on older grains. These data indicate that the high-grade complex principally consists of middle Ordovician orthogneisses associated with a thick metasedimentary sequence characterised by a maximum age of deposition between 480 and 450 Ma. The association of nearly coeval felsic-mafic magmatic rocks with immature siliciclastic sedimentary sequences points to a back-arc setting in the north Gondwana margin during the Early Palaeozoic. The Variscan metamorphic evolution recorded by the high-grade gneisses (Ky-bearing felsic gneisses and mafic eclogites) testifies to the transformation of the Late Ordovician–Devonian passive continental margin into an active margin in the Devonian–Early Carboniferous.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users. 相似文献
7.
D.D. Van Reenen R. Boshoff C.A. Smit L.L. Perchuk J.D. Kramers S. McCourt R.A. Armstrong 《Gondwana Research》2008,14(4):644
The integration of new and published geochronologic data with structural, magmatic/anatectic and pressure–temperature (P–T) process information allow the recognition of high-grade polymetamorphic granulites and associated high-grade shear zones in the Central Zone (CZ) of the Limpopo high-grade terrain in South Africa. Together, these two important features reflect a major high-grade D3/M3 event at ~ 2.02 Ga that overprinted the > 2.63 Ga high-grade Neoarchaean D2/M2 event, characterized by SW-plunging sheath folds. These major D2/M2 folds developed before ~ 2.63 Ga based on U–Pb zircon age data for precursors to leucocratic anatectic gneisses that cut the high-grade gneissic fabric. The D3/M3 shear event is accurately dated by U–Pb monazite (2017.1 ± 2.8 Ma) and PbSL garnet (2023 ± 11 Ma) age data obtained from syntectonic anatectic material, and from sheared metapelitic gneisses that were completely reworked during the high-grade shear event. The shear event was preceded by isobaric heating (P = ~ 6 kbar and T = ~ 670–780 °C), which resulted in the widespread formation of polymetamorphic granulites. Many efforts to date high-grade gneisses from the CZ using PbSL garnet dating resulted in a large spread of ages (~ 2.0–2.6 Ga) that reflect the polymetamorphic nature of these complexly deformed high-grade rocks. 相似文献
8.
Two stages of granitic magmatism occurred during the Pan-African evolution of the Kerala Khondalite Belt (KKB) in southern India. Granitic gneisses were derived from porphyritic granites, which intruded prior to the main stage of deformation and peak-metamorphism. Subsequently, leucogranites and leucotonalites formed during fluid-absent melting and intruded the gneiss sequences. Monazites from granitic gneisses, leucogranites and a leucotonalite were investigated by conventional U-Pb and electron microprobe dating in order to distinguish the different stages of magma emplacement. U-Pb monazite dating yielded a wide range of ages between 590–520 Ma which are interpreted to date high-grade metamorphism rather than magma emplacement. The results of this study indicate that the KKB experienced protracted heating (>50 Ma) at temperatures above 750–800 °C during the Pan-African orogeny. The tectonometamorphic evolution of the study area is comparable to southern Madagascar which underwent a similar sequence of events earlier than the KKB. The results of this study further substantiate previous assertions that the timing of high-grade metamorphism in East Gondwana shifted from west to east during the Late Proterozoic. 相似文献
9.
U–Pb sensitive high resolution ion microprobe (SHRIMP) dating of zircons from charnockitic and garnet–biotite gneisses from the central portion of the Mozambique belt, central Tanzania indicate that the protolith granitoids were emplaced in a late Archaean, ca. 2.7 Ga, magmatic event. These ages are similar to other U–Pb and Pb–Pb ages obtained for other gneisses in this part of the belt. Zircon xenocrysts dated between 2.8 and 3.0 Ga indicate the presence of an older basement. Major and trace element geochemistry of these high-grade gneisses suggests that the granitoid protoliths may have formed in an active continental margin environment. Metamorphic zircon rims and multifaceted metamorphic zircons are dated at ca. 2.6 Ga indicating that these rocks were metamorphosed some 50–100 my after their emplacement. Pressure and temperature estimates on the charnockitic and garnet–biotite gneisses were obscured by post-peak metamorphic compositional homogenisation; however, these estimates combined with mineral textures suggest that these rocks underwent isobaric cooling to 800–850 °C at 12–14 kbar. It is considered likely that the granulite facies mineral assemblage developed during the ca. 2.6 Ga event, but it must be considered that it might instead represent a pervasive Neoproterozoic, Pan African, granulite facies overprint, similar to the ubiquitous eastern granulites further to the east. 相似文献
10.
北大别片麻岩中锆石U—Pb年龄离子探针初步测定 总被引：24，自引：5，他引：19
对北大别鹿吐石和道士冲二个片麻岩中锆石进行了离子探针Ｕ－Ｐｂ年龄测定。二个样品１３个颗粒的２０个有效测定点的年龄表明,鹿吐石片麻岩主要分布在２３３￣３９４Ｍ道士冲片麻岩在３０７￣７１８Ｍａ之间。道十冲片麻岩年龄的一致性程度要高于鹿叶石,两个样品均无低于２２０Ｍａ的年龄信息。由于存在年龄的不一致性,这些年龄可能代表了由古老岩浆锆石和三叠纪的超高压变质锆石或者早白垩纪岩浆锆石所代表的混合、增生锆石。Ｈ 相似文献
11.
应用云母Rb Sr等时线法对大别山高级变质岩的后期改造历史进行了初步的探讨。采用重液变温技术将云母按比重的区别 ,分成若干组分 ,然后用常规的Rb Sr方法测定。应用这种技术对大别山黄土岭太古代麻粒岩相岩石中的黑云母和产于超高压变质带内的石马含石榴石片麻岩的白云母进行了Rb Sr等时线年龄测定。黄土岭太古代麻粒岩相片麻岩中的黑云母 全岩Rb Sr等时线年龄为 12 7± 9Ma ,与该区片麻岩中的角闪石的K Ar和Ar Ar年龄 ,及燕山期花岗岩的Rb Sr年龄一致 ,说明这一太古代下地壳岩石受到了燕山期深熔 岩浆事件的影响 ;大别山东南部石马含石榴石片麻岩 (榴辉岩相 )中的白云母Rb Sr等时线年龄为 191± 10Ma ,反映了印支 早燕山期区域性低级变质事件的时代。研究表明 ,大别山区的不同构造单元中产出的不同类型的高级变质岩自中生代以来有不同的后期演化历史。 相似文献
12.
Zircon formation and modification during magmatic crystallization and high-grade metamorphism are explored using TIMS and LA-ICP-MS U–Pb geochronology, Lu–Hf isotope chemistry, trace element analysis and textural clues on zircons from the Koraput alkaline intrusion, Eastern Ghats Belt (EGB), India. The zircon host-rock is a granulite-facies nepheline syenite gneiss with an exceptionally low Zr concentration, prohibiting early magmatic Zr saturation. With zircon formation occurring at a late stage of advanced magmatic cooling, significant amounts of Zr were incorporated into biotite, nearly the only other Zr-bearing phase in the nepheline syenite gneisses. Investigated zircons experienced a multi-stage history of magmatic and metamorphic zircon growth with repeated solid-state recrystallization and partial dissolution–precipitation. These processes are recorded by complex patterns of internal zircon structures and a wide range of apparently concordant U–Pb ages between 869 ± 7 Ma and 690 ± 1 Ma. The oldest ages are interpreted to represent the timing of the emplacement of the Koraput alkaline complex, which significantly postdates the intrusion ages of most of the alkaline intrusion in the western EGB. However, Hf model ages of TDM = 1.5 to 1.0 Ga suggest an earlier separation of the nepheline syenite magma from its depleted mantle source, overlapping with the widespread Mesoproterozoic, rift-related alkaline magmatism in the EGB. Zircons yielding ages younger than 860 Ma have most probably experienced partial resetting of their U–Pb ages during repeated and variable recrystallization events. Consistent youngest LA-ICP-MS and CA-TIMS U–Pb ages of 700–690 Ma reflect a final pulse of high-grade metamorphism in the Koraput area and underline the recurrence of considerable orogenic activity in the western EGB during the Neoproterozoic. Within the nepheline syenite gneisses this final high-grade metamorphic event caused biotite breakdown, releasing sufficient Zr for local saturation and new subsolidus zircon growth along the biotite grain boundaries. 相似文献
13.
《Gondwana Research》2009,15(4):644-662
The integration of new and published geochronologic data with structural, magmatic/anatectic and pressure–temperature (P–T) process information allow the recognition of high-grade polymetamorphic granulites and associated high-grade shear zones in the Central Zone (CZ) of the Limpopo high-grade terrain in South Africa. Together, these two important features reflect a major high-grade D3/M3 event at ~ 2.02 Ga that overprinted the > 2.63 Ga high-grade Neoarchaean D2/M2 event, characterized by SW-plunging sheath folds. These major D2/M2 folds developed before ~ 2.63 Ga based on U–Pb zircon age data for precursors to leucocratic anatectic gneisses that cut the high-grade gneissic fabric. The D3/M3 shear event is accurately dated by U–Pb monazite (2017.1 ± 2.8 Ma) and PbSL garnet (2023 ± 11 Ma) age data obtained from syntectonic anatectic material, and from sheared metapelitic gneisses that were completely reworked during the high-grade shear event. The shear event was preceded by isobaric heating (P = ~ 6 kbar and T = ~ 670–780 °C), which resulted in the widespread formation of polymetamorphic granulites. Many efforts to date high-grade gneisses from the CZ using PbSL garnet dating resulted in a large spread of ages (~ 2.0–2.6 Ga) that reflect the polymetamorphic nature of these complexly deformed high-grade rocks. 相似文献
14.
Geology of the High Grade Proterozoic Terrains of Sri Lanka, and the Assembly of Gondwana: an Update on Recent Developments 总被引：1，自引：0，他引：1
Nd model ages determined for the high-grade rocks of Sri Lanka delineate three crustal units, viz., the Highland Complex (HC), the Wanni Complex WC), and the Vijayan Complex (VC). The distribution of these three units differs considerably from the three geological divisions demarcated previously on the basis of geological mapping. The centrally located HC comprises mainly granulite grade charnockitic rocks, and metasediments characterized by older Nd model ages (2.0–3.4 Ga). The Highland sedimentary pile was thickened by intermittent granitoid intrusions, most of which are now charnockitic gneiss, and granulites, and basaltic sills, and dikes. All these metaigneous rocks now occur as conformable bands or layers due to intense polyphase deformation. The HC is bounded on the east by the amphibolite grade VC, composed mainly of granitic gneisses, basic gneisses, and migmatites, and they have ‘younger’ Nd model ages (1.1–1.8 Ga). The isotopic, and geochemical characteristics identify the precursors to the Vijayan rocks as I-type calc-alkaline granitoids originated at an ‘arc’-related tectonic environment. Thus, the earlier interpretation that the Vijayan rocks represent reworked HC was rejected. The granulite inliers within the VC, earlier considered as “resisters” to re-working, are now shown as overthrust klippen or rotated rafts of the HC. The WC, demarcated on the basis of Nd model ages (1.1–1.8 Ga) similar to those of the VC, lies west of the HC. It consists mainly of granitic gneisses, charnockitic gneisses, and migmatites, and the metamorphic grade ranges from amphibolite to granulite.Comprehensive geothermobarometric surveys constrain the P-T evolution of the three crustal units, and indicate that both the HC, and WC underwent near isobaric cooling, followed by a decompression with decreasing temperature. Extensive isotopic studies (U-Pb, Pb-Pb, Sm-Nd, Rb-Sr) have established a new geochronological framework for these high-grade rocks of Sri Lanka. The new framework has bracketed the age of high grade metamorphism in the three crustal units at 550–600 Ma.The recent advances in knowledge of the geology of Sri Lanka favour a strong geological correlation of the HC, and the VC of Sri Lanka, respectively, with the Lutzöw-Holm Complex, and the Yatmato-Belgica Complex in the East Antarctica. The geology of the WC suggests a possible correlation with Madagascar, and East Africa. The amalgamation of the three crustal units of Sri Lanka, is apparently related to the two distinct orogenic events that resulted in the assembly of the Gondwana supercontinent. 相似文献
15.
Polymetamorphic evolution of the Lewisian complex,NW Scotland,as recorded by U-Pb isotopic compositions of zircon,titanite and rutile 总被引：3，自引：1，他引：2
U-Pb isotopic relations in zircon and titanite of granulite and amphibolite gneisses in the Lewisian complex and bordering Laxford Front reveal complex discordance patterns indicating multiple Late Archean and Early Proterozoic crystallization, overgrowth and Pb-loss events. The earliest stages in the evolution of the complex remain poorly resolved. Zircon ages of 2710 Ma date high-grade metamorphism and magmatism probably related to tectonic and magmatic accretion in a continental arc setting. A distinct event at 2490–2480 Ma, possibly initiated by metamorphism and deformation at high-grade conditions, caused amphibolitization of the granulites and emplacement of granitic pegmatites. This event can be correlated with development of Inverian shear zones and formation of granitoid layers along the Laxford Front. The emplacement of a younger generation of granitoid sheets during the Laxfordian event fromed hydrothermal titanite at 1754 Ma in gneisses south of the Laxford Bridge and partially reset older titanite at Scourie. Growth of secondary titanite and rutile also occurred during subsequent low-grade metamorphism at 1690–1670 Ma. 相似文献
16.
Ling Wenli Gao Shan Cheng Jianping Zhang Benren Faculty of Earth Sciences China University of Geosciences Wuhan Zheng Haifei Department of Geology Peking University Beijing 《中国地质大学学报(英文版)》1998,9(2)
ＴｈｅＹａｎｇｔｚｅａｎｄＮｏｒｔｈＣｈｉｎａｃｒａｔｏｎｓ（ｓｍＹＣａｎｄＮＣＣ）ａｒｅｔｗｏｏｆｔｈｅｍａｉｎｃｏｎｓｔｉｔｕｅｎｔｐａｒｔｓｏｆｔｈｅｃｏｎｔｉｎｅｎｔｉｎＣｈｉ－ｎａ,ａｎｄｃｏｎｎｅｃｔｗｉｔｈｔｈｅｆａｍｏｕｓＱｉｎｌｉｎ... 相似文献
17.
Hot collisional orogens are characterized by abundant syn-kinematic granitic magmatism that profoundly affects their tectono-thermal evolutions. Voluminous granitic magmas, emplaced between 360 and 270 Ma, played a visibly important role in the evolution of the Variscan Orogen. In the Limousin region (western Massif Central, France), syntectonic granite plutons are spatially associated with major strike–slip shear zones that merge to the northwest with the South Armorican Shear Zone. This region allowed us to assess the role of magmatism in a hot transpressional orogen. Microstructural data and U/Pb zircon and monazite ages from a mylonitic leucogranite indicate synkinematic emplacement in a dextral transpressional shear zone at 313 ± 4 Ma. Leucogranites are coeval with cordierite-bearing migmatitic gneisses and vertical lenses of leucosome in strike–slip shear zones. We interpret U/Pb monazite ages of 315 ± 4 Ma for the gneisses and 316 ± 2 Ma for the leucosomes as the minimum age of high-grade metamorphism and migmatization respectively. These data suggest a spatial and temporal relationship between transpression, crustal melting, rapid exhumation and magma ascent, and cooling of high-grade metamorphic rocks.Some granites emplaced in the strike–slip shear zone are bounded at their roof by low dip normal faults that strike N–S, perpendicular to the E–W trend of the belt. The abundant crustal magmatism provided a low-viscosity zone that enhanced Variscan orogenic collapse during continued transpression, inducing the development of normal faults in the transpression zone and thrust faults at the front of the collapsed orogen. 相似文献
18.
波罗斯坦庙杂岩是东阿拉善地块典型的中-高级变质杂岩之一,它的深入研究对进一步认识阿拉善地块起源、形成与演化过程具有十分重要的科学意义.通过对波罗斯坦庙杂岩中石英闪长质片麻岩、斜长角闪岩与花岗伟晶岩的野外观察、岩石学与锆石U-Pb定年发现,波罗斯坦庙杂岩中石英闪长质片麻岩、斜长角闪岩与花岗伟晶岩的岩浆锆石加权平均年龄分别为284±2 Ma、278±3 Ma,276±2 Ma、271±3 Ma与242±7 Ma,该组年龄被解释为其原岩成岩时代.石英闪长质片麻岩与斜长角闪岩中变质锆石加权平均年龄分别为274±6 Ma、272±5 Ma与269±3 Ma、268±2 Ma,代表它们遭受了晚古生代变质作用的时间.综合以上分析与前人研究资料,初步认为以往曾被认为是新太古代-古元古代的波罗斯坦庙杂岩,实际上可能是一个古元古代-晚古生代中-高级变质杂岩,并遭受了280~260 Ma角闪岩相变质作用,指示它们曾被卷入到中亚造山带晚古生代造山作用过程. 相似文献
19.
A single zircon geochronological study of gneisses from the Obudu Plateau of southeastern Nigeria, using the evaporation technique, indicates that zircons recorded several Precambrian high-grade metamorphic events (Eburnean and Pan-African). Igneous and multifaceted metamorphic zircons yielded 207Pb/206Pb ages of 2062.4 ± 0.4 Ma, 1803.8 ± 0.4 Ma and 574 ± 10 Ma, respectively and confirm for the first time that granulite-facies metamorphism affected the basement of southeastern Nigeria, resulting in the formation of charnockites and granulitic gneisses. The Pan-African high-grade event was coeval with the formation of granulites in Cameroon, Togo and Ghana and resulted from collisional processes during continental amalgamation to form the Gondwana supercontinent. The sources of the sediments, which were deposited at ≈605 Ma and metamorphosed at 574 Ma, comprise older igneous and metamorphic protoliths (including inherited xenocrystic zircons up to 2.5 Ga in age). The Palaeoproterozoic zircons seem to have survived Pan-African melting. 相似文献
20.
Kent C. Condie Philip Allen B. L. Narayana 《Contributions to Mineralogy and Petrology》1982,81(3):157-167
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