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1.
John N. Aleinikoff Ian S. Williams William Compston John S. Stuckless Ronald G. Worl 《Contributions to Mineralogy and Petrology》1989,101(2):198-206
Gneissic rocks that are basement to the Late Archean granites comprising much of the Wind River Range, west-central Wyoming,
have been dated by the zircon U-Pb method using both conventional and ion microprobe techniques. A foliated hornblende granite
gneiss member from the southern border of the Bridger batholith is 2670±13 Ma. Zircons from a granulite just north of the
Bridger batholith are equant and faceted, a typical morphology for zircon grown under high grade metamorphic conditions. This
granulite, which may be related to a second phase of migmatization in the area, is 2698±8 Ma. South of the Bridger batholith,
zircons from a granulite (charnockite), which is related to an earlier phase of migmatization in the Range, yield a discordia
with intercept ages of about 2.3 and 3.3 Ga. However, ion microprobe analyses of single zircon grains indicate that this rock
contains several populations of zircon, ranging in age from 2.67 to about 3.8 Ga. Based on zircon morphology and regional
geologic relationships, we interpret the data as indicating an age of ≃3.2 Ga for the first granulite metamorphism and migmatization.
Older, possibly xenocrystic zircons give ages of ≃3.35, 3.65 and ≃3.8 Ga. Younger zircons grew at 2.7 and 2.85 Ga in response
to events, including the second granulite metamorphism at 2.7 Ga, that culminated in the intrusion of the Bridger batholith
and migmatization at 2.67 Ga. These data support the field and petrographic evidence for two granulite events and provide
some temporal constraints for the formation of continental crust in the Early and Middle Archean in the Wyoming Province. 相似文献
2.
V. A. Glebovitskii I. S. Sedova N. G. Berezhnaya A. N. Larionov L. M. Samorukova 《Stratigraphy and Geological Correlation》2012,20(2):139-165
This work presents new U-Pb data (SHRIMP-II) for zircons from products of granitization and leucosomes of migmatites from
amphibolite- and granulite-facies zones developed on rocks of the tonalite-trondhjemite group of the unstratified basement
and supracrustal formations of the western part of the Aldan granulite area. The age data obtained were interpreted using
the data available on the U and Th geochemistry. The main geochemical trend of transition from primary zircons, crystallizing
from the melt to the later metamorphic zircons is manifested in increasing U and Th concentrations in zircons. In this case,
the Th/U ratio decreases, as do the values of the Ce anomaly and LuN/LaN ratio. By studying the sequence of autochthonous
and paraautochthonous granite formation in the amphibolite-facies zone the ancient (3222–3226 Ma) metamorphic event in the
Aldan Shield (a manifestation of the ultrametamorphic processes (granitization and migmatization), superimposed on rocks of
an ancient infracomplex (3.3–3.4 Ga) and gneisses and schists of supracrustal formations) was established. The data obtained
indicate the Middle Archean age of both metamorphosed rock complexes. The ancient period of evolutionary development of the
Aldan shield was followed by development of diatectic granitoids with an age of 2450 Ma, which is correlated well with Proterozoic
granitoids from the conjunction zone between the Aldan granulite area and Olekma granite-greenstone terrain. 相似文献
3.
Tsuyoshi Komiya Shinji Yamamoto Shogo Aoki Keiko Koshida Masanori Shimojo Yusuke Sawaki Kazumasa Aoki Shuhei Sakata Takaomi D. Yokoyama Kenshi Maki Akira Ishikawa Takafumi Hirata Kenneth D. Collerson 《地学前缘(英文版)》2017,8(2):355-385
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
4.
W. R. Van Schmus B. B. de Brito Neves P. Hackspacher M. Babinski 《Journal of South American Earth Sciences》1995,8(3-4)
The Borborema Province of NE Brasil comprises the central part of a wide Pan-African-Brasiliano orogenetic belt that formed as a consequence of late Neoproterozoic convergence and collision of the São Luis-West Africa craton and the São Francisco-Congo-Kasai cratons. New Sm/Nd and U/Pb results from the eastern part of this province help to define the basic internal architecture and pre-collisional history of this province, with particular emphasis on delineating older cratonic terranes, their fragmentation during the Mesoproterozoic, and their assembly into West Gondwana during the Pan African-Brasiliano orogeny at ca. 600 Ma.The region can be divided into three major geotectonic domains: a) Rio Piranhas-Caldas Brandão massif, with overlying Paleoproterozoic to Neoproterozoic supracrustal rocks, north of the Patos Lineament; b) the Archean to Paleoproterozoic São Francisco craton (SFC) to the south; and c) a complex domain of Paleoproterozoic to Archean basement blocks with several intervening Mesoproterozoic to Neoproterozoic fold belts in the center (south of Patos Lineament and north of SFC). The northern and central domains comprise the Borborema Province.Archean basement gneiss and Transamazonian granulite of northern SFC are exposed in the southern part of the central domain, underlying southern parts of the Sergipano fold belt. Basement in the Rio Piranhas massif appears to consist mostly of Transamazonian (2.1 to 2.2 Ga) gneissic rocks; Nd model ages (TDM) of ca. 2.6 Ga for 2.15 Ga gneisses indicate a substantial Archean component in the protoliths to these gneisses. The Caldas Brandão massif to the east yields both Transamazonian and Archean U/Pb zircon and Nd (TDM) ages, indicating a complex architecture. Metasedimentary rocks of the Jucurutu Formation yield detrital zircons with original crystallization ages as young as 1.8 Ga, indicating that these rocks may be late Paleoproterozoic and correlate with other ca. 1.8 Ga cratonic supracrustal rocks in Brazil such as the Roraima Group and Espinhaço Group.Most metavolcanic and pre-Brasiliano granitic units of the Sergipano (SDS), Pajeú-Paraíba (SPP), Riacho Pontal (SRP), and Piancó-Alto Brígida (SPAB) fold belts in the central domain formed ˜ 1.0 ± 0.1 Ga, based on U/Pb ages of zircons. Nd model ages (TDM) for these same rocks, as well as Brasiliano granites intruded into them and large parts of the Pernambuco-Alagoas massif, are commonly 1.3–1.7 Ga, indicating that rocks of the fold belts were not wholly derived from either older (> 2.1 Ga) or juvenile (ca. 1.0 Ga) crust, but include mixtures of both components. A simple interpretation of Brasiliano granite genesis and the Nd data implies that there is no Transamazonian or Archean basement underlying large parts of these fold belts or of the Pernambuco-Alagoas massif. An exception is a belt of syenitic Brasiliano plutons (Syenitoid Line) and host gneisses between SPAB and SPP that clearly has a Transamazonian (or older) source. In addition, there are several smaller blocks of Archean to Transamazonian gneiss that can be defined within and among these fold belts. These blocks do not appear to constitute a continuous basement complex, but appear to be isolated older crustal fragments.Our data support a model in which ca. 1.0 Ga rifting was an important tectonic and crust-forming event along the northern edge of the São Francisco craton. Our data also show that significant parts of the Borborema Province are not remobilized Transamazonian to Archean crust, but that Mesoproterozoic crust is a major feature of the Province. There are several small remnants of older crust within the area dominated by Mesoproterozoic crust, suggesting that the rifting event created several small continental fragments that were later incorporated into the Brasiliano collisional orogen. We cannot at present determine if the Rio Piranhas-Caldas Brandão massifs and the older crustal blocks of the central domain were originally part of the São Francisco craton or whether some (or all) of them came from more exotic parts of the Proterozoic Earth. Finally, our data have not yet revealed any juvenile terranes of either Transamazonian or Brasiliano age. 相似文献
5.
甘肃阿克塞县安南坝地区镁铁质麻粒岩呈脉状或透镜状赋存于新太古代米兰岩群和TTG片麻岩中。岩石主要由斜方辉石（Opx）+单斜辉石（Cpx）+角闪石（Amp）+斜长石（Pl）+磁铁矿（Mt）等组成,具有典型中-低压麻粒岩相岩石的矿物组合特征。LA-ICP-MS锆石U-Pb测年结果显示镁铁质麻粒岩原岩形成时代可能为2 561±29 Ma,与塔里木东南缘2.6～2.5 Ga岩浆作用时代基本一致,说明新太古代晚期是塔里木东南缘重要的陆壳增生期。此外,通过测年还获得了～1.95 Ga、～1.86 Ga两期变质年龄。其中,～1.95 Ga的峰期变质年龄是塔里木克拉通东南缘古元古代晚期强烈构造—热事件的地质记录。～1.86 Ga的变质年龄可能与该地区古元古代末期构造—岩浆事件密切相关。镁铁质麻粒锆石176Hf/177Hf比值均＜0.281 621,εHf(t) 值为-14.08～3.12,两阶段模式年龄（TDM2）主要集中在～2.65 Ga,指示其原岩岩浆起源于新太古代富集岩石圈地幔。 相似文献
6.
华北克拉通南缘太华杂岩组成及演化 总被引：1，自引：4，他引：1
太华杂岩位于华北克拉通南部,其组成复杂,记录了几乎所有早前寒武纪各阶段重要的地质事件;此外,由于其所处特殊地理位置,研究太华杂岩对于华北克拉通早前寒武纪地壳形成和演化、构造单元划分和基底拼合等都具有举足轻重的科学价值。本文综合已有的岩石学、变质作用、地球化学以及同位素年代学等诸多研究工作,得到以下阶段性结论和认识:1)将鲁山地区太华划分为以深成侵入岩为主的片麻岩系和以变质沉积-火山岩为主的表壳岩系;前者形成于中太古代晚期-新太古代早期,后者形成于古元古代晚期。而小秦岭地区太华杂岩中变质深成侵入岩形成时间跨度较大,为中太古代晚期-古元古代早期;而其上覆的火山-沉积岩可与鲁山太华杂岩的表壳岩类比,形成时间亦为古元古代晚期。2)中太古代-新太古代(2.91～2.50Ga)为华北克拉通南部大陆最主要的地壳形成时期。提出太华杂岩在太古宙经历了两期明显的地壳生长时间,一期发生在2.85～2.70Ga,以鲁山太华片麻岩系中的深成侵入岩和斜长角闪岩为代表;另一期发生在～2.50Ga,以小秦岭华山和崤山地区太华杂岩中各类花岗质岩石为代表。3)太华杂岩在所谓的全球陆壳生长"沉寂期(2.45～2.20Ga)"岩浆活动异常发育,推测这一时期的岩石形成于古元古代俯冲-汇聚环境,可能是与华北克拉通南部太古宙陆块和其他陆块汇聚-碰撞相关。4)太华杂岩在古元古代晚期普遍遭受了强烈的变质和变形,其变质程度主体为高角闪岩相,局部可达麻粒岩相,且记录了包含近等温降压退变质片段的顺时针变质作用P-T轨迹,经历了一个漫长的变质演化过程(1.97～1.80Ga),变质作用的时限跨度可达150Myr。5)提出华北克拉通南部曾经为一个统一基底,称之为"南部太古宙地块",此地块形成时间为新太古代末期(～2.5Ga)。该古老陆块经历了如下5个构造-演化阶段:(1)冥古宙-始太古代初始陆核形成;(2)中太古代-新太古代陆壳快速生长;(3)古元古代早期(～2.3Ga)岩浆活动异常活跃;(4)古元古代(2.30～1.97Ga)陆内拉伸-破裂;和(5)古元古代末期(1.97～1.80Ga)陆块最终拼合。 相似文献
7.
采用地质调查和显微镜下观察方法,研究了辽南小黑山区太古宙岩石组成和构造变形特征。小黑山区太古宙岩石包括上壳岩、古老片麻岩和变基性岩脉,它们在小黑山变质岩体中呈包体出现。上壳岩由黑云变粒岩、条带状闪石磁铁石英岩组成;古老片麻岩为条带状角闪黑云斜长片麻岩、条带状角闪斜长片麻岩,原岩为英云闪长岩;变基性岩脉为斜长角闪岩和角闪石岩。上壳岩堆积之后有英云闪长岩侵位,基性脉侵位于上壳岩和英云闪长岩（古老片麻岩）。小黑山区太古宙岩石经历了2幕变形：D1幕变形主要表现为褶皱构造（DF1）、与褶皱轴面平行的面理（DS1）、矿物线理（DL1）;D2幕变形在叠加褶皱作用下形成斜歪倾伏褶皱（DF2）,面理和线理不发育。小黑山区太古宙变质岩中发育的变形序列、构造特征、变形特征、变质条件表明,这2幕构造形迹群属于中部构造相。D1幕变形形成逆冲推覆构造,D2幕变形形成第Ⅲ型叠加褶皱,它们都是在同方向的水平挤压应力作用下的产物。 相似文献
8.
胶北地体多期变质事件的P-T-t轨迹及其对胶-辽-吉带形成与演化的制约 总被引：2，自引：2，他引：0
胶北地体位于华北克拉通东部陆块胶-辽-吉带南端,主要由闪长质-TTG-花岗质片麻岩、变质表壳岩系和变质镁铁-超镁铁质岩所组成。本文通过对胶北早前寒武纪变质岩系的岩石学、矿物化学、变质反应结构和序列、变质温度和压力估算与同位素年代学资料的综合研究和总结,得出以下重要结论:(1)与华北克拉通东部陆块其它地区太古宙变质基底类似,本区也存在~2500Ma区域性新太古代变质事件,且与本区2550~2500Ma岩浆作用在时间上非常接近,其变质作用发生的时间比岩浆作用要晚10~50Myr,指示本区~2500Ma区域性变质事件可能与大规模的幔源岩浆底侵作用存在密切的成因关系。(2)胶北还存在1950~1850Ma区域性古元古代变质事件,并导致了大量高压基性和泥质麻粒岩的形成,高压基性麻粒岩主要以不规则透镜体、变形岩墙群或岩脉群的形式赋存于闪长质-TTG-花岗质片麻岩之中,并集中分布在安丘-平度-莱西-莱阳-栖霞一带,大致沿北东-南西向断续带状分布,构成了一条长约300km的古元古代高压麻粒岩相变质带。(3)本区古元古代高压麻粒岩以记录近等温减压(ITD)及随后近等压降温(IBC)的顺时针P-T-t轨迹为特征,指示本区变质杂岩在古元古代晚期曾强烈地卷入了与俯冲-拼贴-碰撞造山有关的构造过程,并可能经历了如下复杂的构造演化:(I)在古元古代晚期2000~1950Ma,随着有限大洋地壳的持续俯冲作用,本区各类变质岩的原岩开始经历一次构造增厚事件,并导致了它们的原岩经历了早期绿片岩相-角闪岩相进变质作用;(II)1950~1870Ma,大洋地壳俯冲作用结束,本区开始发生弧-陆拼贴和陆-陆碰撞作用,大陆地壳持续缩短和加厚,在加厚下地壳或岛弧根部带约50km的深度,发生了区域性高压麻粒岩相变质作用,并导致了本区变基性岩和变泥质岩分别形成了石榴石+单斜辉石+斜长石±角闪石±石英±铁-钛氧化物和石榴石+蓝晶石+钾长石+斜长石+黑云母+石英+铁-钛氧化物+熔体的高压麻粒岩相矿物组合。(III)1870~1800Ma,在同碰撞峰期变质结束之后,本区造山作用进入了后碰撞构造折返-伸展演化阶段,先后经历了早期快速构造折返和晚期缓慢冷却降温两个构造热演化阶段。其中,在早期快速构造折返阶段,高压麻粒岩经历了峰后近等温或略微增温减压退变质作用的叠加,高压基性麻粒岩表现为沿石榴石边部形成了含斜方辉石的后成合晶。与此同时,早期快速构造折返阶段还伴随着热松弛和伸展作用,出现一系列的幔源基性岩浆活动,不仅导致了本区大量未经历高压麻粒岩相变质的变基性岩群的形成,同时也诱发了区内大规模的地壳深熔作用的发生。自温度高峰期之后,本区地壳岩石还经历了一个近等压冷却降温过程,并发生了区域性角闪岩相退变质作用,高压基性麻粒岩表现为石榴石和斜方辉石边部常出现含角闪石的退变边或后成合晶。最终,在1800Ma左右,本区含电气石花岗伟晶质岩脉的大量出现,则标志着胶北地体古元古代晚期(2000~1800Ma)俯冲-拼贴-碰撞造山作用的最终结束。 相似文献
9.
曹庄片麻杂岩包括35亿年的表壳岩及三个不同时期的复期正片麻岩。水厂片麻杂岩包括水厂表壳岩及侵入其中的淡花岗岩和紫苏花岗岩,后者年龄为26.5亿年。 相似文献
10.
11.
本文通过某些矿物之间的转化及成分的变化，研究变质反应的状态。将区内麻粒岩相变质作用划分为四个阶段，并依据变质作用演化过程中的温度、压力的变化推定了ＰＴ＿ｔ轨迹。 相似文献
12.
W.R. Van Schmus M. Kozuch B.B. de Brito Neves 《Journal of South American Earth Sciences》2011,31(2-3):227-252
The Borborema Province has three major subprovinces. The northern subprovince lies north of the Patos shear zone and is comprised of Paleoproterozoic cratonic basement with Archean nuclei, plus overlying Neoproterozoic supracrustal rocks and Brasiliano plutonic rocks. The central subprovince occurs between the Patos and Pernambuco shear zones and is mainly comprised of the Zona Transversal. The southern subprovince occurs between the Pernamabuco shear zone and the São Francisco craton and is comprised of a tectonic collage of various blocks, terranes, or domains ranging in age from Archean to Neoproterozoic. This report focuses on the Zona Transversal, especially on Brasiliano rocks for which we have the most new information.Paleoproterozoic gneisses with ages of 2.0–2.2 Ga occur discontinuously throughout the Zona Transversal. The Cariris Velhos suite consists of metavolcanic, metasedimentary, and metaplutonic rocks yielding U–Pb zircon ages of 995–960 Ma. This suite is mainly confined to a 100 km wide belt that extends for more than 700 km within the Alto Pajeú terrane. Sm–Nd model ages in metaigneous rocks cluster about 1.3–1.6 Ga, indicating that older crust was involved in genesis of their magmas. Brasiliano supracrustal rocks dominate the Piancó-Alto Brígida terrane, and they probably also constitute significant parts of the Alto Pajeú and Rio Capibaribe terranes. They are only slightly older than early stages of Brasiliano plutonism, with detrital zircon ages at least as young as 620 Ma; most TDM ages range from 1.2 to 1.6 Ga.Brasiliano plutons range from ca. 640 to 540 Ma, and their TDM ages range from 1.2 to 2.5 Ga. Previous workers have shown significant correlations among U–Pb ages, Sm–Nd model ages, petrology, and geochemistry, and we are able to reinforce and extend these correlations. Stage I plutons formed 640–610 Ma and have TDM ages less than 1.5 Ga. Stage II (610–590 Ma) contains few plutons, but coincides with the peak of compressional deformation, metamorphism, and formation of migmatites. Stage III plutons (590 to ca. 575 Ma) have older TDM ages (ca. 1.8–2.0 Ga), as do Stage IV plutons (575 to ca. 550 Ma; TDM from 1.9 to 2.4 Ga). Stage III plutons formed during the transition from compressional to transcurrent deformation, while Stage IV plutons are mainly post-tectonic. Stage V plutons (550–530 Ma) are commonly undeformed (except along younger shear zones) and have A-type geochemistry. The five stages have distinct geochemical properties, which suggest that the tectonic settings evolved from early, arc-related magma-genesis (Stage I) to within-plate magma-genesis (Stage V), with perhaps some intermediate phases of extensional environments. 相似文献
13.
Robert Frei Minik T Rosing Tod E WaightDavid G Ulfbeck 《Geochimica et cosmochimica acta》2002,66(3):467-486
Despite superimposed metamorphic overprinting and metasomatic alterations, primary volcanic features remain preserved in low-strain domains of mafic volcanic sequences in the western Isua supracrustal belt (ISB, West Greenland). These basaltic successions represent the hitherto oldest known fragments of oceanic crust on Earth. Early Archean metasomatic fluids, rich in light rare earth elements (LREE), Th, U, Pb, Ba, and alkalies, invaded the supracrustal package and distinctively altered the basaltic sequences. Field relationships, source characteristics traced by Pb isotopes, and geochronological results provide indications that these fluids were genetically related to the emplacement of tonalite sheets into the ISB between 3.81 and 3.74 Ga ago. Subsequent early Archean metamorphism homogenized the mixed primary and metasomatic mineral parageneses of these metavolcanic rocks. Allanite occurs as the most characteristic and critical secondary metasomatic-metamorphic phase and is developed in macroscopically discernible zones of increased metsomatic alteration, even in domains of low strain. Because of its high concentration of LREE, Th, and U, this secondary mineral accounts for much of the disturbances recorded by the Sm-Nd and Th-U-Pb isotope systematics of the pillowed metabasalts.The supracrustal sequences were tectono-metamorphically affected to varying degrees during a late Archean, ∼2.6- to 2.8-Ga-old event, also recognized in the adjacent gneiss terranes of the Isuakasia area. The degree to which bulk rocks were isotopically reequilibrated is directly dependent on the different relative contributions of allanite-hosted parent-daughter elements to the overall whole-rock mass budget of the respective isotope systems. Although low-strained (initially only weakly metasomatized) pillow basalts remained more or less closed with respect to the U-Pb and Rb-Sr systems since ∼3.74 Ga, the Sm-Nd system appears to have been partially opened on a whole-rock scale during the late Archean event. This diversified behavior of the whole-rock isotope systems with respect to late Archean overprinting is explained by the combination of mass budget contributions of the respective elements added during metasomatism and the partial opening of metasomatic macroenvironments during late Archean recrystallization processes with associated renewed fluid flow. In reactivated zones of high strain, where primary metasomatic alteration is most prominently developed, late Archean partial resetting also of the U-Pb isotope system on a whole-rock scale occurred. This is consistent with an apparent late Archean age of kyanite, which initially crystallized during the early Archean metamorphism. Its age is controlled by the U-Pb systematics of allanite inclusions, which have exchanged their isotopic properties during the tectono-metamorphic event that overprinted the oceanic crustal sequence at Isua more than 1000 myr later.These results underline the need for care in the interpretation of whole-rock geochemical data from polymetamorphic rocks in general, and from the Isua oceanic crustal sequences in particular, to constrain isotopic models of early Earth’s evolution. Likewise, this study cautions against the indiscriminate use of geochemical data of metavolcanic rocks from Isua to infer models for geotectonic settings relevant for their formation. 相似文献
14.
Y. J. Bhaskar Rao T. R. K. Chetty A. S. Janardhan K. Gopalan 《Contributions to Mineralogy and Petrology》1996,125(2-3):237-250
Sittampundi and Bhavani Archean layered meta-anorthosite complexes occur as tectonic lenses within the Cauvery shear zone
(CSZ), a crustal scale shear dividing the Precambrian granulite crust of south India into late Archean (> 2.5 Ga) and Proterozoic (c. 0.55 Ga) blocks. They and their host supracrustal-gneiss rocks record at least two stages of
tectonometamorphic history. The first is seen as regional scale refolded isoclinal folds and granulite metamorphism (D1-M1) while the second stage is associated with dominantly E–W dextral transcurrent shearing and metamorphic recrystallisation
(D2-MCSZ). Whole rock Sm-Nd isochrons for several comagmatic rocks of the layered complexes yield concordant ages: Sittampundi – 2935±60
Ma, ɛNd + 1.85±0.16 and Bhavani – 2899±28 Ma, ɛNd + 2.18±0.14 (2σ errors). Our Sm-Nd results suggest that: (1) the magmatic protoliths of the Sittampundi and Bhavani layered
complexes were extracted from similar uniform and LREE depleted mantle sources; (2) M1 metamorphism occurred soon after emplacement at c.3.0 Ga ago. P-T estimates on garnet granulites from the Sittampundi complex characterise the MCSZ as a high-P event with metamorphic peak conditions of c. 11.8 kbar and 830°C (minimum). The MCSZ is associated with significant isothermal decompression of the order of 4.5–3.5 kbar followed by static high-temperature
rehydration and retrogression around 600°C. The timing of MCSZ is inferred to be Neoproterozoic at c. 730 Ma based on a whole rock-garnet-plagioclase-hornblende Sm-Nd isochron age for
a garnet granulite from the Sittampundi complex and its agreement with the 800–600 Ma published age data on post-kinematic
plutonic rocks within the CSZ. These results demonstrate that the Cauvery shear zone is a zone of Neoproterozoic reworking
of Archean crust broadly similar to the interface between the Napier and Rayner complexes of the East Antarctic shield in
a model Proterozoic Gondwana supercontinent.
Received: 5 December 1995 / Accepted: 3 May 1996 相似文献
15.
S. Siegesmund H. Jelsma J. Becker G. Davies P. Layer E. van Dijk L. Kater M. Vinyu 《International Journal of Earth Sciences》2002,91(1):20-34
In order to constrain the temporal relationship between granite (sensu lato) emplacement and metamorphism, isotope work was carried out on the minerals zircon and apatite (U-Pb), garnet (Pb-Pb) and hornblende (Ar-Ar) from wall rock samples in the Shamva area in Zimbabwe. The area, encompassing parts of the Chinamora and Murehwa batholiths and a wedge-shaped greenstone belt segment in between, is commonly quoted in the literature as an example illustrating pluton emplacement processes and deformational models for the Archean. New U-Pb dating of apatite from a boudinaged pegmatite within mafic schists in the batholith-greenstone contact zone has yielded an age of 2619 +28/-24 Ma. This age is interpreted as the best estimation of the intrusion age of this unit, depending on the assumed closure temperature, and provides an upper age limit for the syntectonic emplacement of the now gneissic granites. Pb-Pb dating of late kinematic garnets in cordierite-bearing rocks within the greenstone belt wall rocks gives an age of 2623NJ Ma. Together, this timing of relatively late, syntectonic plutonism and metamorphic mineral growth at ca. 2.62 Ga compares well with existing zircon crystallization ages for felsic volcanics (2645dž Ma, 2643NJ Ma) and post-tectonic porphyritic monzogranites (2601ᆢ Ma). Ar-Ar hornblende ages for mafic schists from different areas within the greenstone belt wall rocks range between 2621 and 2498 Ma and have been interpreted to indicate mixing between metamorphic ages and cooling ages. The data support a geological model whereby volcanism and sedimentation are associated with an early phase of regional deformation at ca. 2.64 Ga, which may have started earlier and lasted longer, and evolves into the voluminous emplacement of granites (now gneissic granites) in the batholiths at approximately 2.62 Ga. Emplacement of post-tectonic tabular monzogranites takes place at ca. 2.60 Ga. 相似文献
16.
Geochemistry, Nd Isotopic Characteristics of Metamorphic Complexes in Northern Hebei： Implications for Crustal Accretion 总被引：4，自引：0，他引：4
K. H. PARK 《《地质学报》英文版》2006,80(6):807-818
The middle segment of the northern margin of the North China Craton （NCC） consists mainly of metamorphosed Archean Dantazi Complex, Paleoproterozoic Hongqiyingzi Complex and unmetamorphosed gabbro-anorthosite-meta-alkaline granite, as well as metamorphosed Late Paleozoic mafic to granitoid rocks in the Damiao-Changshaoying area. The -2.49 Ga Dantazi Complex comprises dioritic-trondhjemitic-granodoritic-monzogranitic gneisses metamorphosed in amphibolite to granulite facies. Petrochemical characteristics reveal that most of the rocks belong to a medium- to high-potassium calc-alkaline series, and display Mg^# less than 40, right-declined REE patterns with no to obviously positive Eu anomalies, evidently negative Th, Nb, Ta and Ti anomalies in primitive mantlenormalized spider diagrams, εNd（t）=＋0.65 to -0.03, and depleted mantle model ages TDM=2.78-2.71 Ga. Study in petrogenesis indicates that the rocks were formed from magmatic mixing between mafic magma from the depleted mantle and granitoid magma from partial melting of recycled crustal mafic rocks in a continental margin setting. The 2.44-2.41 Ga Hongqiyingzi Complex is dominated by metamorphic mafic-granodioritic-monzogranitic gneisses, displaying similar petrochemical features to the Dantazi Complex, namely medium to high potassium calc-alkaline series, and the mafic rocks show evident change in LILEs, negative Th, Nb, Ta, Zr anomalies and positive P anomalies. And the other granitiod samples also exhibit negative Th, Nb, Ta, P and Ti anomalies. All rocks in the Hongqiyingzi Complex show right-declined REE patterns without Eu anomaly. The metamorphic mafic rocks with εNd（t） = -1.64 may not be an identical magmatic evolution series with granitoids that have εNd（t） values of ＋3.19 to ＋1.94 and TDM ages of 2.55-2.52 Ga. These granitic rocks originated from hybrid between mafic magma from the depleted mantle and magma from partial melting of juvenile crustal mafic rocks in an island arc setting. All the -311 Ma Late Paleozoic metamorphic mafic rocks and related granitic rocks show a medium-potassium calc-alkaline magmatic evolution series, characterized by high Mg^#, obviously negative Th, Nb, Ta anomalies and positive Sr anomalies, from no to strongly negative Ti anomalies and flat REE patterns with εNd（t） = ＋8.42, implying that the maflc magma was derived from the depleted mantle. However the other granitic rocks are characterized by right-declined REE patterns with no to evidently positive Eu anomalies, significantly low εNd（t） = -13.37 to -14.04, and TDM=1.97-1.96 Ga, revealing that the granitoid magma was derived from hybrid between maflc magma that came from -311 Ma depleted mantle and granitoid magma from Archean to Early Paleoproterozoic ancient crustal recycling. The geochemistry and Nd isotopic characteristics as well as the above geological and geochronological results indicate that the middle segment of the northern margin of the NCC mainly experienced four crustal growth episodes from Archean to Late Paleozoic, which were dominated by three continental marginal arc accretions （-2.49, -2.44 and 311 Ma）, except the 1.76-1.68 Ga episode related to post-collisional extension, revealing that the crustal accretion of this segment was chiefly generated from arc accretion and amalgamation to the NCC continental block. 相似文献
17.
We report field relationships, petrography and isotopic ages from two superposed basement units of the Kabul Block, the so called Lower Sherdarwaza and Upper Welayati formations. The Sherdarwaza Formation is represented mostly by migmatites and gneisses that are derived from pelitic and psammitic lithologies with lenses and layers of mafic and carbonate rocks. Several bodies of orthogneisses are also exposed in the Sherdarwaza Formation. The Upper Welayati Formation is characterized by micaschist, quartzite and amphibolites. SHRIMP U–Pb data on zircon from the orthogneiss in the Sherdarwaza Formation indicates a Neoarchean age of ca 2.5–2.8 Ga for their magmatic crystallization. The rocks exhibit granulite facies conditions of 5–7 kbar and 800 °C that are documented by the presence of orthopyroxene and Ti-rich biotite in the orthogneiss and by olivine and phlogopite in some calc-silicate rocks at contact with marble. A Paleoproterozoic age of ca. 1.85–1.80 Ga for this metamorphism was obtained using U-Pb SHRIMP dating on zircon and U-Th dating on monazite. Mineral textural relations also show a younger amphibolite facies metamorphism that is documented in both the Sherdarwaza and Welayati formations. This metamorphism occurred at relatively higher pressure conditions of up to 9 kbar at ca. 650 °C, compared to the granulite facies event. A Neoproterozoic age of ca 0.85–0.9 Ga, for this metamorphism is confirmed by Ar-Ar data on biotite and white mica as well as by U-Th data on monazite. By combining the presented results on the metamorphic petrology, geochronology and geochemistry, we conclude that: (1) The Kabul basement is a fragment of an Archean block (craton); (2) the ca. 1.85–1.8 and 0.9–0.85 Ga metamorphism marks an important orogenic events for the basement rocks of the Kabul Block which was stabilized during the early Precambrian; (3) the two metamorphic ages correlate well with global-scale orogenies related to the assembly of the Paleoproterozoic Columbia and Neoproterozoic Rodinia supercontinents; (4) based on metamorphic characteristics and ages, the Kabul basement rocks show an affinity to the Neoarchean rocks of the Tarim and/or South China cratons. 相似文献
18.
Y. J. Bhaskar Rao Anil Kumar A. B. Vrevsky R. Srinivasan G. V. Anantha Iyer 《Journal of Earth System Science》2000,109(1):57-65
Whole-rock Sm-Nd isochron ages are reported for two stratiform meta-anorthosite complexes emplaced into the Archean supracrustal-gneiss
association in the amphibolite facies terrain around Holenarsipur, in the Dharwar craton, South India. While these metaperidotite-pyroxenite-gabbro-anorthosite
complexes are petrologically and geochemically similar, they differ in the intensity of tectonic fabric developed during the
late Archean (c. 2.5 Ga) deformation. They also differ in their whole-rock Sm-Nd isochron ages and initial Nd isotopic compositions:
3.285 ± 0.17 Ga,ɛNd0.82 ± 0.78 for the Honnavalli metaanorthosite complex from a supracrustal enclave in the low-strain zone, and 2.495 ± 0.033
Ga, ɛNd = -2.2 ± 0.3 for the Dodkadnur meta-anorthosites from the high-strain southern arm of the Holenarsipur Supracrustal
Belt (HSB). We interpret these results as indicating that the magmatic protoliths of both meta-anorthosite complexes were
derived from a marginally depleted mantle at c. 3.29 Ga but only the Dodkadnur rocks were isotopically reequilibrated on a
cm-scale about 800 Ma later presumably due to the development of strong penetrative fabrics in them during Late Archean thermotectonic
event around 2.5 Ga. Our results set a younger age limit at c. 3.29 Ga for the supracrustal rocks of the HSB in the Dharwar
craton. 相似文献
19.
Peter A. Nielsen C. Willem Langenberg H. Baadsgaard John D. Godfrey 《Precambrian Research》1981,16(3):171-193
A complex of Precambrian polymetamorphic gneisses and granitoids of the Churchill structural province, northeastern Alberta, Canada has been examined structurally, petrographically, chemically and geochronologically. An Archean basement gneiss complex is indicated by Rb-Sr dating of pegmatites which cut both gneisses and granitoids (2470 ± 26 Ma with an initial 87Sr/86Sr ratio of 0.7030 ± 0.0008). A high-pressure granulite facies (M1) mineral assemblage and older structures (D1) are assigned to the Archean. A moderate-pressure granulite facies (M2.1), a low-pressure amphibolite facies (M2.2), a greenschist facies (M2.3), and younger structures (D2) are of Aphebian age. Formation of granitoids by anatexis of the pre-existing Archean basement complex during M2.1 is indicated by their Aphebian ages (ca. 1900 Ma) and high initial 87Sr/86Sr ratios (0.7100 ± 0.0018). The path of retrograde metamorphism is linked with relatively slow rates of uplift and cooling. Late Aphebian sediments attained low-grade greenschist facies metamorphism only and are younger than the other metamorphic rocks. The tectonic evolution of this Precambrian mobile belt during the Aphebian contrasts with the stable Archean cratonic block in the Slave province to the north. 相似文献
20.
冀西北地区怀安杂岩由变质表壳岩和变质深成岩组成,其中变质表壳岩的形成时代、怀安杂岩的构造背景以及其与孔兹岩带间的关系一直存在较大争议.本文对怀安杂岩的几处代表性露头进行了详细野外考察,对4件样品进行了岩石学、锆石SHRIMP U-Pb定年、同位素和元素地球化学分析.所有样品都给出了1.86~1.81 Ga的变质锆石年龄,进一步支持怀安杂岩广泛遭受到古元古代晚期变质作用改造的认识.侵入/包裹含BIF表壳岩组合的变质辉长岩（HB1425）和片麻状英云闪长岩（HB1426）分别给出了~2.5 Ga和2.55 Ga的形成年龄,限制表壳岩形成时代老于2.55 Ga,推测为新太古代表壳岩.浅粒岩（HB1431）和紫苏石榴黑云斜长片麻岩（HB1435）中最老的碎屑锆石分别为2.46 Ga和2.51 Ga,可能还存在古元古代的碎屑锆石,表明它们都为古元古代表壳岩.上述结果进一步确定了怀安杂岩中发育两期表壳岩组合.变质辉长岩和片麻状英云闪长岩的全岩εNd（t）、TDM1和TDM2分别为+2.19~+3.06、2.67~2.75 Ga和2.67~2.69 Ga,表明其物源区不存在较大规模的古老陆壳物质,新太古代是怀安地区主要陆壳生长期.变质辉长岩中~1.82 Ga变质锆石中较多具有正的εHf（t）值,最高可达11.1,最可能的解释是古元古代变质过程存在地幔添加作用.锆石的O同位素分析显示区域上可能存在低δ18O的岩石,在古元古代变质过程中,可能存在低δ18O流体对锆石的改造.怀安杂岩和西部孔兹岩带中不同类型岩石的比例明显不同,但两者都同样发育新太古代和古元古代的双层地壳结构,怀安杂岩或许代表孔兹岩带剥蚀更深而出露的深部地壳部分. 相似文献