北淮阳变质单元中“浅变质”带——佛子岭群主期变质条件和碎屑锆石年代学探究   总被引：2，自引：1，他引：1  

赵俊先  石永红  唐虎  李秋立  侯振辉  周甜 《岩石学报》2019,35(7):2219-2236

佛子岭群为北淮阳变质单元的重要组成部分,其位于大别碰撞造山带最北缘,处于扬子和华北板块的结合部位,构造位置极为关键。长期以来,佛子岭群一直被视为扬子板块北缘沉积产物,并以加积楔形式产出。然而,随着年代学和构造地质学的研究的深入,对其构造归属和形成动力学机制产生了差异性认识,究其原因主要是对佛子岭群变质属性缺乏准确的判定所致。为此,本研究对佛子岭群展开了详细的野外观测和区域变质岩石学研究,以及岩相学、热力学和年代学分析。研究表明佛子岭群构成较为复杂,主体由细粒的云母石英片岩、石英云母片岩和石榴云母片岩构成。主期变质PT条件评价显示,其温压范围为610～620℃和0. 9～1. 0GPa,普遍经历了中-高压角闪岩相变质,并非为"浅"变质产物,并卷入了较深层次的俯冲-折返过程。锆石U-Pb年代学研究表明,佛子岭群年龄跨度较大,范围为3101±48Ma～413±9Ma,可分为5组年龄,年龄峰值分别为2537Ma、1567Ma、940Ma、749Ma和440Ma,显示了华北和扬子板块混合物源特征,最年轻的年龄峰值暗示其形成于早古生代晚期。  相似文献   

Geochemistry of Archean Metavolcanic Rocks from Kadiri Schist Belt, Andhra Pradesh, India     

K. Satyanarayana  J. Siddilingam  Jagannath Jetty 《Gondwana Research》2000,3(2):235

The basic volcanic group exposed in the Kadiri schist belt includes high Mg-basalt, basalt, basaltic andestite and dacite. The basalts are tholeiitic in composition and high Mg-basalts, basaltic andesites and dacites show calc-alkaline affinity. Major and trace element characteristics suggest that the volcanic suite has been derived from an initial tholeiitic magma which has given rise to an early basaltic type and a later calc-alkaline type of rocks. An island arc and active continental margin tectonic setting was inferred for these rocks.  相似文献   

Occurrence and significance of a quartz–amphibole schist xenolith within a mafic microgranular enclave in the Xiangshan volcanic-intrusive complex,SE China     

《International Geology Review》2012,54(7):894-903

The Xiangshan volcanic-intrusive complex is composed of rhyolitic crystal tuffs, welded tuffs, rhyodacite, porphyroclastic rhyolitic lava, subvolcanic rocks such as granite porphyry, and late quartz monzonitic porphyry and lamprophyre dikes. We report the first occurrence of a quartz–amphibole schist (QAS) xenolith enclosed within a mafic microgranular enclave (MME) in the Xiangshan volcanic-intrusive complex. The mineralogy of this xenolith consists of amphibole, biotite, quartz, and minor plagioclase. Petrographic and mineral composition studies indicate that the protolith of this xenolith likely originated from the metamorphic basement beneath Xiangshan. The amphibole (actinolite and magnesiohorblende) has been partially replaced by orthopyroxene at 800–1000°C and by diopside at <700°C, according to mineral thermometers; this replacement process may have taken place after the xenolith was trapped by the mafic magma host (now an MME). Studies of the QAS xenolith provide new information on the emplacement history of the mafic magma. The peak metamorphic temperature for amphibole replaced by pyroxene is higher than the crystallization temperature of the subvolcanic magma, which indicates that the heat of pyroxene formation must have been provided by the engulfing mafic melt. This magma must have emplaced to crustal level and trapped the QAS as a xenolith and then injected into the felsic magma. We suggested that the hybridization processes for the major elements of the pristine mafic magma may have been contaminated by crustal rocks to form its present composition of MME before mafic magma injection. However, the hybridization process appears not to have been formed via a single-stage process because various types of MMEs are presented in the Mesozoic magmatic rocks of SE China.  相似文献   

内蒙古呼和浩特北二道凹岩群黑云角闪片岩SHRIMP锆石U-Pb定年     

张玉清  张婷  宝音乌力吉  银海  曾庆荣 《地质论评》2017,63(5):1328-1336

二道凹岩群是20世纪70年代初1∶200000区域地质调查时在呼和浩特市北二道凹地区命名的,其岩石组合为变质砾岩、各种片岩、大理岩等。依据最新1∶250000区调成果,自下而上分了三个岩组,即红山口岩组、哈拉更沟岩组、东梁岩组。为进一步研究二道凹岩群的地质时代,本文对东梁岩组的黑云角闪片岩中锆石进行了高精度SHRIMP U-Pb测年研究,其时代为1890±8 Ma,认为该年龄值系黑云角闪片岩原岩的形成时代,即代表了二道凹岩群上部中基性火山岩的形成年龄,结合已有的年龄数据,进一步佐证了二道凹岩群形成于古元古代(1800~2400Ma)。  相似文献   

Zircon U–Pb and biotite 40Ar/39Ar geochronology from the Anzan emerald deposit in Zambia     

《Ore Geology Reviews》2017

The Kafubu Emerald Area in Zambia is an important producer of gemstone-quality emeralds. The country rocks include carbonatization altered rock and emerald-hosting biotite chlorite schist from the Anzan emerald deposit in the Kafubu area, Zambia. The technique of LA-MC-ICP-MS is used to perform chronology measurements of the country rock and emerald-hosting rock which belong to Muva Supergroup, yielding zircon U-Pb concordia ages of 1966 ± 12 Ma in carbonatization altered rock and 1853 ± 58 Ma and 1344 ± 30 Ma in biotite chlorite schist. Meanwhile, dating of biotite chlorite schist using the biotite ⁴⁰Ar–³⁹Ar method has obtained the plateau age of t = 578.3 ± 2.6 Ma, isochron age of 577.5 ± 3.0 Ma and reverse isochron age of 577.4 ± 3.0 Ma. Thus, we have redefined the age of Muva Supergroup in the Copperbelt Province in Zambia to be older than or equal to 1966 ± 12 Ma, and found that the ore bodies in the Anzan emerald deposit underwent three phases of metamorphism at 1853 ± 58 Ma, 1344 Ma ± 30 and 578.3 ± 2.6 Ma and finally accomplished the emerald mineralization. The age of the Anzan emerald deposit is earlier than the Kagem (452.1 ± 16 Ma) and the Kamakanga emerald deposits (447 ± 8.6 Ma).  相似文献   

Neoproterozoic Tectonic Setting of Southeast China: New Constraints from SHRIMP U-Pb Zircon Ages and Petrographic Studies on the Mamianshan Group     

WU Ganguo  YU Xinqi  DI Yongjun  ZHANG Da 《《地质学报》英文版》2010,84(2):333-344

<正>Precambrian tectonic history of Zhejiang,Fujian,and Jiangxi provinces of south China is important for understanding the tectonic evolution of South China but its magmatic activity, petrogenesis,stratigraphic sequence of the Mamianshan Group is still strongly controversial.Here we present new sensitive high resolution ion micro-probe(SHRIMP) U-Pb zircon geochronological data for the Mamianshan Group and petrographical data to constrain the tectonic framework of the regions.Our results showed that the SHRIMP U-Pb zircon age of green schists of the Dongyan Formation is 796.5±9.3 Ma,the Daling Formation is 756.2±7.2 Ma,and mica-quartz schist of the Longbeixi Formation is 825.5±9.8 Ma.These data indicate that the Mamianshan Group was formed not in the Mesoproterozoic,but in the Neoproterozoic and its stratigraphic sequences should be composed of Longbeixi,Dongyan,and Daling Formations from the bottom to the top.Rocks from this Group, from Zhejiang,Fujian and Jiangxi provinces,constituted the upper basement of the Cathaysia Block that overlay the lower basement of the Mayuan Group.Detailed petrographic studies demonstrate that the amphibole schists of the Dongyan Formation in the Mamianshan Group were formed within an intra-arc rift setting rather than a continental rift as previously suggested.Rather,this island-arc type formation was developed by collision and/or subduction between various blocks resulting from the breakup of the supercontinent Rodinia at c.850-750 Ma.The Zhuzhou conglomerate,distributed near Dikou Town,Jian'ou City,Fujian Province and previously considered as evidence of the Mesoproterozoic Dikou movement,is shown here not to be the basal conglomerate above the angular unconformity between the upper and lower basements.Our conclusions have important implications for understanding the Precambrian tectonics of South China.  相似文献   

Bulk chemical control on metamorphic monazite growth in pelitic schists and implications for U–Pb age data     

I. C. W. FITZSIMONS  P. D. KINNY  S. WETHERLEY  D. A. HOLLINGSWORTH 《Journal of Metamorphic Geology》2005,23(4):261-277

Several petrographic studies have linked accessory monazite growth in pelitic schist to metamorphic reactions involving major rock‐forming minerals, but little attention has been paid to the control that bulk composition might have on these reactions. In this study we use chemographic projections and pseudosections to argue that discrepant monazite ages from the Mount Barren Group of the Albany–Fraser Orogen, Western Australia, reflect differing bulk compositions. A new Sensitive High‐mass Resolution Ion Microprobe (SHRIMP) U–Pb monazite age of 1027 ± 8 Ma for pelitic schist from the Mount Barren Group contrasts markedly with previously published SHRIMP U–Pb monazite and xenotime ages of c. 1200 Ma for the same area. All dated samples experienced identical metamorphic conditions, but preserve different mineral assemblages due to variable bulk composition. Monazite grains dated at c. 1200 Ma are from relatively magnesian rocks dominated by biotite, kyanite and/or staurolite, whilst c. 1027 Ma grains are from a ferroan rock dominated by garnet and staurolite. The latter monazite population is likely to have grown when staurolite was produced at the expense of garnet and chlorite, but this reaction was not intersected by more magnesian compositions, which are instead dominated by monazite that grew during an earlier, greenschist facies metamorphic event. These results imply that monazite ages from pelitic schist can vary depending on the bulk composition of the host rock. Samples containing both garnet and staurolite are the most likely to yield monazite ages that approximate the timing of peak metamorphism in amphibolite facies terranes. Samples too magnesian to ever grow garnet, or too iron‐rich to undergo garnet breakdown, are likely to yield older monazite, and the age difference can be significant in terranes with a polymetamorphic history.  相似文献   

High-pressure–low-temperature metamorphism of metasedimentary rocks, southern Menderes Massif, western Turkey     

Donna L. Whitney  Christian Teyssier  Seth C. Kruckenberg  Valerie L. Morgan  Lindsay J. Iredale 《Lithos》2008,101(3-4):218-232

Kilometer-scale lenses of quartz-rich metasedimentary rocks crop out in a discontinuous belt along the southern margin of the Menderes Massif, Turkey, and preserve evidence for high-pressure–low-temperature (HP–LT) metamorphism related to subduction of a continental margin during Alpine orogeny. Kyanite schist, quartzite, and quartz veins contain kyanite + phengite + Mg-chlorite, and the veins also contain magnesiocarpholite. A deformed carbonate metaconglomerate juxtaposed with the quartzite-dominated unit does not contain HP index minerals, and likely represents the tectonized boundary of the siliceous rocks with adjacent marble. The HP–LT rocks (10–12 kbar, 470–570 °C) record different pressure conditions than the adjacent, apparently lower pressure Menderes metasedimentary sequence. Despite this difference there is disagreement as to whether these HP–LT rocks are part of the Menderes sequence or are related to the tectonically overlying Cycladic blueschist unit. If the former, the entire southern Menderes Massif experienced HP–LT metamorphism but the evidence has been obliterated from most rocks; if the latter, rocks recording different metamorphic-kinematic conditions experienced different tectonic histories and were tectonically juxtaposed during thrusting. Based on observations and data in this study, the second model better accounts for the differences in P–T-deformation histories of the southern Menderes Massif rocks, and suggests that the HP–LT rocks are not part of the Menderes cover sequence.  相似文献   

Prograde reactions and garnet zoning reversals in staurolite schist, British Columbia: significance for thermobarometric interpretations     

D. L. WHITNEY  E. D. GHENT 《Journal of Metamorphic Geology》1993,11(6):779-788

Abstract An outcrop of staurolite-bearing pelitic schist from the Solitude Range in the south-western Rocky Mountains, British Columbia, was examined in order to determine the nature of prograde garnet- and staurolite-producing reactions using information from garnet zoning and inclusion mineralogy. Although not present as a matrix phase, chloritoid is present as inclusions in garnet and is interpreted to have participated in the simultaneous growth of garnet and staurolite by a reaction such as chloritoid + quartz = garnet + staurolite + H₂O.
A garnet zoning trend reversal, which is most pronounced with respect to almandine and grossular components, is present in the outer core of garnets. The location of the zoning reversal corresponds to the outer limit of chloritoid inclusions in garnet. As there is no evidence for polymetamorphism, the zoning reversal is interpreted to indicate continued garnet growth by prograde reaction(s) during a single metamorphic event after the exhaustion of chloritoid as a matrix phase.
Metamorphic conditions recorded by mineral rim compositions are 550–600° C at 6–7 kbar. Because there is no evidence for partial resorption of garnet during production of staurolite, we interpret these results to represent peak conditions.  相似文献   

The Nigerian manganese-rich iron-formations and their host rocks—from sedimentation to metamorphism     

Arno Mücke 《Journal of African Earth Sciences》2005,41(5):407-436

This investigation deals with the Nigerian iron-formations and their host rocks and is based on about 560 mineral analyses (electron-microprobe) and 93 whole-rock analyses (64 iron-formations and 29 host rocks). The manganese-rich and Al-bearing iron-formations occurring in various schist belts of the northern and southern part of West-Nigeria consist of the magnetite-free silicate, the magnetite–silicate and the quartz-rich hematite facies.Iron-formations and host rocks originated from submarine-volcanogenic exhalations enriched in Fe, Mn and CO₂ and from Al₂O₃, SiO₂ and alkali (K₂O and Na₂O)-rich continental-derived pelitic to psammitic material. From these sources and their interaction and controlled by the volcanogenic activity, differently composed protoliths were deposited in the marine basin during the Birimian time. Subsequent metamorphism of greenschist to low amphibolite facies conditions during the Eburnian time led to the formation of the metaprotoliths of the magnetite–silicate (consisting of predominantly magnetite and quartz and subordinate of garnet and amphibole), the silicate facies (consisting of garnet, amphibole and rarely Mn-bearing ilmenite and quartz) and the metasediment phyllite. Garnets are predominantly almandine–spessartine solid solutions, whereas amphiboles are Mn and Ca-bearing grunerite–cummingtonite solid solutions. In the course of a second tectono-metamorphic event of Pan-African age, the magnetite–silicate facies iron-formation/phyllite association was transformed into the hematite facies and muscovite/biotite schists, whereas the silicate facies is characterized by extensive silicification features. The hematite facies and the silicified silicate facies are restricted to southern Nigeria where the second and heterogeneous tectono-metamorphic event is more pronounced (amphibolite facies conditions) than in northern Nigeria.The genesis, summarized as the metamorphic model, shows that the carbonate-rich (siderite, rhodochrosite and subordinate magnesite and calcite) protoliths were metamorphically transformed into the silicate and magnetite–silicate facies. The separation of Mn and Fe, leading to manganese-bearing iron-formations and iron-bearing manganese-formations was explained by varying pH-conditions, under which siderite (pH: 6.8–9.4) and rhodochrosite (pH: 9–11) precipitated.Similar to the Gunfit and Biwabik iron-formations of Minnesota, USA, the iron-formation of Bingi (Maru schist belt), now present in the form of the fayalite bearing silicate facies, was overprinted by contact metamorphism caused by a gabbro intrusion.  相似文献