Precambrian sedimentology and stratigraphy: Problems and prospects     

Rajat Mazumder   《Gondwana Research》2007,12(4):548-549

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Fluid-controlled crustal metasomatism within a high-pressure subducted mélange (Mt. Hochwart, Eastern Italian Alps)     

Simone Tumiati  Gaston Godard  Silvana Martin  Urs Kltzli  Damiano Monticelli 《Lithos》2007,94(1-4):148-167

The Nonsberg–Ultental Region of northern Italy contains a Palaeozoic mélange that was partially subducted during the Variscan orogeny. This mélange is constituted mainly by metapelites characterized by shale-type REE-patterns, displaying partial melting which began under high-pressure conditions. The resulting migmatites enclose minor slivers of mantle-wedge peridotites that have been incorporated into the mélange during subduction. Peridotites display important large ion lithophile elements (LILE) enrichment consequent to amphibole recrystallization contemporaneously with metapelite migmatization at P ≈ 2.7 GPa and T ≈ 850 °C in the garnet–peridotite field. Crustal and mantle (ultramafic) rocks of the mélange display the same Sm–Nd ages of about 330 ± 6 Ma, which dates both the metamorphic peak and the migmatization event. The zircon U–Pb age of the metasomatic amphibolitic contact between garnet peridotite and migmatite is identical (333.3 ± 2.4 Ma) within analytical errors. Therefore, metasomatism, migmatization and peak metamorphism are constrained to the same event. The presence of Cl-rich apatite and ferrokinoshitalite in the contact amphibolite, together with the trace-element patterns of peridotites, suggest that metasomatism was driven by Cl- and LILE-rich fluids derived from ocean water transported into the subduction zone by sediments and crustal rocks. These fluids interacted with the crust, prompting partial melting under water oversaturated conditions and partitioning LILE from the crust itself. Peridotites, which were well below their wet solidus temperature, could not melt but they recrystallized in the crustal mélange under garnet-facies conditions. Crustal fluids caused extensive hydration and LILE-enrichment in peridotites and severe Sm–Nd isotope disequilibrium between minerals, especially in the recrystallized peridotites. The proposed scenario suggests massive entrapment of crustal aqueous fluids at high-pressure conditions within subduction zones.  相似文献   

金厂沟梁—二道沟金矿田内花岗岩类侵入体锆石的离子探针U—Pb年代及意义   总被引：13，自引：5，他引：13  

苗来成  范蔚茗  翟明国  Qiu Y M  McNaughton N J  Groves D I 《岩石学报》2003,19(1):71-80

金厂沟梁-二道沟金矿田内花岗岩类侵入体SHRIMP锆石U-Ph定年结果表明,该区中生代以来到少经历了三次中酸性岩浆侵入作用,它们分别以西台子二长花岗岩、娄上含辉石石英闪长岩和西对面沟花岗闪长岩及闪长玢岩脉在218±4Ma(印支期)、161±1Ma(燕山早期)和126±1Ma(燕山晚期)的侵位为标志。花岗岩类的地球化学资料表明,这些花岗岩类侵入体形成与造山作用有关,为造山后或陆内拉张作用的产物。二道沟矿区成矿前或成矿期闪长玢岩脉的年龄126±1Ma限定了该区金矿化的最大年龄;结合前人有关年代学资料,认为本区金矿化应发生在126～118Ma。这说明金厂沟梁-二道沟金矿田的金矿床与我国胶东、小秦岭和辽东等地区的金矿床是同时形成的,再次证明燕山晚期是中国最重要而广泛的金成矿期。此外,在这些主要的金矿区内,金矿化时间上均具有与各自区内最晚一次花岗质岩浆作用同时或稍晚的特征,这可能指示金矿床是造山作用或区域构造-岩浆演化最晚期的产物。  相似文献   

Timing of granite emplacement and cooling in the Songpan–Garzê Fold Belt (eastern Tibetan Plateau) with tectonic implications     

F. Roger  J. Malavieille  Ph. H. Leloup  S. Calassou  Z. Xu 《Journal of Asian Earth Sciences》2004,22(5):465

New U–Pb and Rb–Sr geochronology on syn- and post-orogenic granites provide constraints on the timing of major tectonic events in the Songpan–Garzê fold belt, west Sichuan, China. The Ma Nai granite was probably syn-kinematic with the main deformation and yields an age of 197±6 Ma that is interpreted as an upper age limit of the Indosinian event. Zircons and apatites from the post-kinematic Rilonguan granite also yield Jurassic ages (195±6 and 181±4 Ma). The post-orogenic Markam massif gives two ages of 188±1 and 153±3 Ma. Both granites are undeformed and cut structures in the Triassic sedimentary rocks. These results demonstrate that the major deformation and décollement tectonics in the Songpan–Garzê fold belt occurred prior to the Early Jurassic. The wide range of ages obtained for post-kinematic granites (from Early Jurassic to Late Jurassic) suggests that, locally, magmatic activity persisted for a long time (at least 50 Ma) after the Indosinian compressional tectonism. No Tertiary ages have been obtained, suggesting that these granites were not affected strongly by the India–Asia collision.  相似文献   

The Las Chacras-Potrerillos batholith (Pampean Ranges,Argentina): structural evidences,emplacement and timing of the intrusion     

Siegesmund  S.  Steenken  A.  López de Luchi  M. G.  Wemmer  K.  Hoffmann  A.  Mosch  S. 《International Journal of Earth Sciences》2004,93(1):23-43

Within the southern part of the Sierra Pampeanas (the Sierra de San Luis, Argentina), a series of extensive intrusive bodies are regarded to post-date the Famatinian cycle but were emplaced during the Achalian, a period of heterogeneous deformation along crustal scale fault zones. The largest of those is the Las Chacras-Potrerillos batholith that is situated at the northern end of the transpressive, sinistral Guzmán shear zone. This composite pluton exhibits three sub-domains that comprise two granitoid sub-units each: The southern Potrerillos stock (muscovite-bearing red granite and biotite-bearing red granite) and the central (biotite porphyritic granite and giant porphyritic granite) and northern domain (equigranular granite and porphyritic granite) of the Las Chacras stock. The crystallisation ages of the biotite porphyritic granite is around 381 Ma (U/Pb on zircons and Pb/Pb on sphene), while the host rock was already cooled below 350 °C at 420 Ma. Thermal modelling approaches favour a pulsed intrusion with a duration of 1.5 Ma. The emplacement was followed by rapid cooling below the muscovite cooling temperature. Biotite cooling ages in different sub-units reflect either a long-lasting cooling history of approximately 30 Ma (which is supported by the modelling) or a reheating effect at around 350 Ma. Devonian-age determinations on the fault rocks and granitoids point to a syn-tectonic emplacement of the batholith. The pluton is interpreted to be positioned at the crossover of sinistral shear zones. The origin of this NNE directed extensional setting in a transpressive regime seems to be related to the transfer of displacement along a secondary set of NNW-trending sinistral faults. The final emplacement is due to a subsequent ballooning of the batholith following the direction of space creation. This model is based on the relative timing of the emplacement sequence and macroscopically visible planar fabrics in the field as well as magnetic fabric data. Our results indicate that the emplacement is syn-kinematic with respect to the Achalian deformation event.  相似文献   

Re–Os geochronology of organic rich sediments: an evaluation of organic matter analysis methods     

David Selby  Robert A. Creaser 《Chemical Geology》2003,200(3-4):225-240

Rhenium and osmium in organic-rich sedimentary rocks are dominantly hydrogenous, but any nonhydrogenous component will influence the accuracy and precision of the Re–Os date obtained. To minimize the influence of any nonhydrogenous Re and Os, we evaluate analysis of isolated organic matter from the whole rock, together with whole rock analysis using a CrO₃–H₂SO₄ digestion medium instead of inverse aqua regia, for a black shale unit of the Exshaw Formation, Canada. This unit previously returned a whole rock Re–Os date of 358±10 Ma (Model 3) [Geochim. Cosmochim. Acta (2002)] using inverse aqua regia dissolution. Organic matter isolated from the whole rock matrix using the HF–BF₃ technique [Org. Geochem. 20 (1993) 249] yields scattered data and a Re–Os date of 449±220 Ma (Model 3, MSWD=616). The organic matter analyses show similar ¹⁸⁷Os/¹⁸⁸Os values, but significantly lower ¹⁸⁷Re/¹⁸⁸Os values in comparison to the whole rock analyses. We show that the Re–Os systematics of organic matter are altered during chemical isolation, and as such we suggest that the HF–BF₃ method should not be used for Re–Os analysis of organic matter. Whole rock Re–Os analysis using a CrO₃–H₂SO₄ digestion medium yields significantly better regression analysis compared with the inverse aqua regia method, and the Re–Os data identify two distinct initial ¹⁸⁷Os/¹⁸⁸Os values for the sample set. Separate regressions of these data yield precise dates [366.1±9.6, MSWD=2.2 and 363.4±5.6 Ma, MSWD=1.6 (Model 3)], which are indistinguishable from the age constraints for this formation (363.4±0.4 Ma, U–Pb monazite). Comparison of the Re–Os dates obtained from aqua regia and CrO₃–H₂SO₄ methods suggests that the former may contain nonhydrogenous Re and Os, whereas the CrO₃–H₂SO₄ method dominantly liberates hydrogenous Re–Os from organic matter, allowing for better stratigraphic age determinations and evaluation of the Os isotope composition of seawater.  相似文献   

Blind testing of rock varnish microstratigraphy as a chronometric indicator: results on late Quaternary lava flows in the Mojave Desert, California   总被引：2，自引：0，他引：2  

Tanzhuo Liu   《Geomorphology》2003,53(3-4):209-234

Rock varnish is a manganiferous dark coating ubiquitous in desert landscapes. To test the validity of varnish microstratigraphy as a chronometric indicator, varnish samples were collected from radiometrically dated and undated late Quaternary lava flows in Amboy, Cima, and Pisgah volcanic fields (AVF, CVF, PVF) in the Mojave Desert of California, western United States. Varnish microstratigraphies show a replicable layering sequence that appears to record regional climate changes that likely correspond in time to the Younger Dryas and Heinrich events in the North Atlantic region. Microstratigraphic patterns on these volcanic fields match patterns found in varnishes from other western US sites with available radiometric age constraints. Based on this regional chronology, varnishes from the A flow, H flow, and a stone pavement surface in the Cima volcanic field were estimated to be 16.5–24, 74–85, and 74–85 ka, respectively; these ages are consistent with previously published cosmogenic ³He ages of 18–20, 72–74, and 80–85 ka for these geomorphic surfaces. Varnishes from the I flow at Cima yielded a puzzling age estimate of 39 ka, which is consistent with an older ³He age of 37±6 ka reported for the I flow, but inconsistent with a younger ³He age of 31±7 ka and a cosmogenic ³⁶Cl age of 27±1.3 ka for the same flow. Reinterpretation of the original varnish age data, with knowledge of then available field mapping results of the I flow, suggests that the I cone is polycyclic and different flow units were probably unintentionally sampled in the field. The revised varnish ages of 30 and 39 ka for the I flow thus may be in good agreement with their corresponding ³He and ³⁶Cl ages. In a blind test of the method, varnishes from the Phase 1 flow at Pisgah, an unnamed flow (called here the I′ flow) at Cima, and the Amboy flow were estimated to be 24–30, 46–60, and 74–85 ka, respectively; these ages agree well with ³⁶Cl ages of 22.5±1.3, 46±2, and 79±5 ka reported for the same flows by Phillips [Geomorphology (2002).]. These test results provide convincing evidence that varnish microstratigraphy, once radiometrically calibrated, can be used as a valid dating tool to estimate surface exposure ages of desert landforms in the western US drylands.  相似文献   

元古宙蛇绿岩地质年代学研究方法探讨   总被引：4，自引：2，他引：2  

倪志耀  王仁民 《大地构造与成矿学》1998,22(4):339-345

确定元古宙蛇绿岩形成时代的最佳方法是同位素地质年代学研究。元古宙蛇绿岩往往遭受过多期热-构造事件的改造,其地质年代学研究应依据单颗粒锆石U－Pb稀释法或Pb－Pb蒸发法的年龄数据,在具体操作上须正确区分岩浆结晶锆石和变质重结晶锆石。Sm－Nd、Rb－Sr及Pb－Pb等时线年龄具有多解性的特点．不宜用于元古宙蛇绿岩的定年,其地质意义的解释应结合区域地质构造背景及地质地球化学特征的综合研究。  相似文献   

Geochemistry and zircon geochronology of paleoproterozoic granitoids: Further evidence on the magmatic and crustal evolution of the São Luís cratonic fragment, Brazil     

Evandro L. Klein  Renê Luzardo  Candido A.V. Moura  Richard Armstrong 《Precambrian Research》2008,165(3-4):221-242

The Tromaí Intrusive Suite is the predominant exposed unit of the São Luís cratonic fragment in northern Brazil. The suite forms batholiths and stocks of granitoids that were emplaced between 2168 ± 4 Ma and 2149 ± 4 Ma and intruded a 2240 ± 5 Ma old metavolcano-sedimentary sequence. The batholiths are composed of a variety of petrographic types that have been grouped in three sub-units, based on the predominant petrographic type, and named Cavala Tonalite, Bom Jesus Granodiorite, and Areal Granite, from the more primitive to the more evolved phases, in addition to subordinate shallow felsic intrusions. The Tromaí Suite is an expanded magmatic association comprising minor mafic rocks to predominantly intermediate and felsic, low- to high-K, and metaluminous to weakly peraluminous granitoids that follow a Na-enriched calc-alkaline trend. Combined rock association, geochronology, Nd isotopes, and geochemical signature indicate that the Tromaí Suite formed from magmas derived from juvenile protoliths modified by fractional crystallization. The juvenile protoliths included ocean plate, mantle wedge, and minor sediments. The data also indicate an intra-oceanic arc setting that possibly transitioned to a continental margin and that the Tromaí Intrusive Suite records the main accretionary stage of the Rhyacian orogen (ca. 2.24–2.15 Ma) that culminated with a collision stage at about 2.1 Ga and gave rise to the present day São Luís cratonic fragment. This time interval is coincident with the main period of crustal growth in the South American Platform and in the Paleoproterozoic terranes of the West African Craton. The beginning of this period is also coincident with the end of a period in which only minor amounts of juvenile crust is found worldwide.The Negra Velha Granite is a distinct unit that forms a few stocks that intruded the granitoids of the Tromaí Suite between 2076 and 2056 Ma ago. Negra Velha is an association of monzogranite and subordinate quartz–monzonite and syenogranite with an alkaline signature that shows high Rb–Sr–Ba enrichments, resembling shoshonitic associations. This granite represents the post-orogenic phase of the Rhyacian orogenesis.  相似文献   

Age and sources of gold mineralization in the Marmato mining district, NW Colombia: A Miocene–Pliocene epizonal gold deposit     

Colombo Celso Gaeta Tassinari  Fabio Diaz Pinzon  Juaquin Buena Ventura 《Ore Geology Reviews》2008,33(3-4):505-518

Epigenetic gold mineralization occurs in the Marmato mining district, within the Calima Terrain of the Setentrional Andes, Colombia. Regional rocks associated with this mineralization include: graphite- and chlorite-schists of the Arquia Complex; metamorphosed during the Cretaceous, Miocene sandstones, shales and conglomerates of the Amagá Formation; as well as pyroclastic rocks (clasts of basalt, andesites and mafic lavas) and subvolcanic andesitic/dacitic bodies of the Combia Formation (9 to 6 Ma). The subvolcanic Marmato stock hosts mesothermal and epithermal low-sulfidation Au–Ag ores in the form of distensional veins, stockwork, and quartz veinlets within brecciated zones. Ore minerals are pyrite, sphalerite and galena with subordinate chalcopyrite, arsenopyrite, pyrrhotite, argentite and native gold/electrum.Sericitized plagioclase from a porphyry dacite yielded a K–Ar age of 5.6 ± 0.6 Ma, interpreted as the age of ore deposition. This is in close agreement with the age of reactivation of the Cauca–Romeral Fault System (5.6 ± 0.4 Ma), which bounds the Calima Terrain. A porphyry andesite–dacite (6.7 ± 0.1 Ma), hosting the Au–Ag veins, shows a measured ⁸⁷Sr/⁸⁶Sr between 0.70440 and 0.70460, ε_Nd between + 2.2 and + 3.2 and ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of 18.964 to 19.028; 15.561 to 15.570; and 38.640 to 38.745, respectively. The ⁸⁷Sr/⁸⁶Sr and ε_Nd values of rocks from the Arquia Group range from 0.70431 to 0.73511 and − 12.91 to + 10.0, respectively, whereas the corresponding Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb) range from 18.948 to 19.652; 15.564 to 15.702; and 38.640 to 38.885, respectively. ⁸⁷Sr/⁸⁶Sr and ε_Nd values obtained on sulfides from the gold quartz veins, which occur at shallow and intermediate levels, range from 0.70500 to 0.71210 and from − 1.11 to + 2.40. In the deepest veins, ε_Nd values lie between + 1.25 and + 3.28 and the ⁸⁷Sr/⁸⁶Sr of calcite and pyrite fall between 0.70444 and 0.70930. The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of all mineralization are in the ranges 18.970 to 19.258; 15.605 to 15.726 and 38.813 to 39.208, respectively. Carbonates have an average ⁸⁷Sr/⁸⁶Sr ratio of 0.70445, which is within the range of values measured in the host dacite. The Sr isotopic data indicate that carbonic fluids have a restricted hydrothermal circulation within the host igneous body, while the Sr, Pb and Nd isotopic compositions of the sulfides suggest that the fluids not only circulated within the Marmato stock, but also throughout the Arquia Complex, inferring that these rocks offer a potential target for mineral exploration. Based on geological and geochronological evidence, the epizonal Marmato gold ores formed during the Miocene to Pliocene, as a result of cooling of the Marmato stock and reactivation along a crustal-scale fault zone related to thermal processes in an accretionary oceanic–continental plate orogen.  相似文献