文档共享_Beijing SHRIMP Center

你好,请

Relationship between metamorphism and ore formation at the Sukhoi Log gold deposit hosted in black slates from the data of U-Th-Pb isotopic SHRIMP-dating of accessory minerals [查看] M.A.Yudovskaya、V.V.Distler、N.V.Rodionov、A.V.Mokhov、A.V.Antonov、S.A.Sergeev

Abstract—The formation conditions and age of the Sukhoi Log gold deposit are considered on the basis of new isotopic–geochemical data. The U–Pb isotopic study of zircon and monazite from highgrade ore and host black slates at the Sukhoi Log deposit was carried out with SIMS technique using a SHRIMP II instrument. Two generations of monazite are distinguished on the basis of optical and scanning electron microscopy, cathodoluminescence, and micro X-ray spectroscopy. Monazite I is characterized by black opaque porphyroblasts with microinclusions of minerals pertaining to metamorphic slates and structural attributes of pre and synkinematic formation. Monazite II occurs only within the ore zone as transparent crystals practically free of inclusions and as rims around monazite I. The REE contents are widely variable in both generations. Porphyroblastic monazite I differs in low U and Th (0.01–0.7 wt % ThO2) contents, whereas transparent monazite II contains up to 4 wt % ThO2. The average weighted U–Pb isotopic age of monazite I is 650 ± 8.1 Ma (MSWD = 1.6; n = 9) and marks the time of metamorphism or catagenesis. The U–Pb age estimates of synore monazite II cover the interval of 486 ± 18 to 439 ± 17 Ma. Zircons of several populations from 0.5 to 2.6 Ga in age are contained in the ore. Most detrital zircon grains have porous outer rims composed of zircon and less frequent xenotime with numerous inclusions of minerals derived from slates. The peaks of 206Pb/238U ages in the most abundant zircon populations fall on 570 and 630 Ma and correspond to the age of newly formed metamorphic mineral phases. The discordant isotopic ages indicate that the U–Th–Pb isotopic system of ancient detrital zircons was disturbed 470–440 Ma ago in agreement with isotopic age of monazite II and the Rb–Sr whole rock isochron age of black slates (447 ± 6 Ma). The new data confirm the superimposed character of the gold–quartz–sulfide mineralization at the deposit. Black shales of the Khomolkho Formation of the Bodaibo Synclinorium were affected by metamorphism over a long period; the peaks of metamorphism and catagenesis are dated at 570 and 650–630 Ma. The hightemperature ore formation was probably related to a hidden granitic pluton emplaced 450–440 Ma ago, that is, 200 Ma later than the events of greenschist metamorphism. Hercynian granitoid magmatism (320–270 Ma) did not exert a substantial effect on the U–Th–Pb isotopic system in accessory minerals from the ore and could not have been a major source of oreforming fluids.

Petrology and SHRIMP U-Pb zircon geochronology of Cordilleran granitoids of the Bariloche area, Argentina [查看] A.Castro、I.Moreno-Ventas、C.Fernández、G.Vujovich、G.Gallastegui、N.Heredia、R.D.Martino、R.Becchio、L.G.Corretgé、J.Díaz-Alvarado、P.Such、M.García-Arias、D.-Y.Liu

A petrological and geochronological study of Cordilleran granitoid intrusions in the Bariloche area (Argentina) point to a complex time-compositional evolution of magmatic processes in relation with oblique subduction of the Phoenix plate below the South America active margin during Jurassic times.The observed geochemical variations in both major and trace elements, together with the textural and mineralogical relations, point to a roughly defined, overall process of magmatic “filtering” linking all the intrusive batholithic rocks of the Bariloche area. These data suggest that the composition of the parental magma that underwent fractionation may be an intermediate magma with SiO2=58-60wt%,MgO=2.5 wt%, FeO=6.5 wt%, CaO=6.1. These are coincident with the typical compositions of evolved andesites. Magnetite, amphibole and plagioclase are the main phases involved in the fractionation process. According to Hbl thermobarometry, fractionation may have taken place, at least in part, at shallow pressures (P ¼ 0.5e1.5 kbar), possibly at the level of emplacement. The coupled observations of the two pressure dependent ratios, namely Sr/Y and La/Yb are pointing to a low-pressure, lowtemperature final fractionation dominated by only Pl. The geochronologic study by UePb SHRIMP zircon determinations of 14 samples from granites, tonalites and diorites yield a broad range of about 20 Ma,between 150 and 170 Ma at the Medium Jurassic. The batholith was accomplished by a protracted magmatic activity that lasted for about 20 Ma. This time is much longer than the time elapsed from intrusion to complete crystallization of shallow magma chambers. It is concluded that amalgamation of discrete magma pulses is the dominant process that built-up the batholith. The observed structures suggest that the fractures conditioning the emplacement of the magma batches were arranged en échelon and show a right-stepping. The resulting geometry is compatible with the activity of a largescale,sinistral, NeS trending, strike-slip fracture zone permitting the emplacement of each magma pulse.This major, strike-slip fault system should be deeply entrenched in the crust to allow intruding magmas generated and fractionated at depth. Because batholith generation is a direct consequence of subduction,structural relations and ages can be used to constraint the plate motion relations during Jurassic in this region of the South America active margin.

Origin of ~2.5 Ga potassic granite from the Nellore Schist Belt,SE India textural, cathodoluminescence, and SHRIMP U-Pb data [查看] K.VijayaKumar、W.G.Ernst、C.Leelanandam、J.L.Wooden、M.J.Grove

In a geochemical and geochronological investigation of Archean and Proterozoic magmatism in the Nellore Schist Belt, we conducted SHRIMP U–Pb analyses of zircons from two cospatial granitic bodies at Guramkonda and Vendodu. The former is a Ba- and Sr-rich hornblende-bearing tonalite, whereas the latter is a Rb-, Zr-,Pb-, Th-, U-, and REE-rich biotite-bearing leucogranite. The Guramkonda tonalite displays a restitic texture with remnants of trapped granitic melt, whereas the Vendodu leucogranite contains residual/partially melted plagioclase grains. Both rock types contain two generations of zircon: tonalite contains a group of euhedral zoned zircons enclosed within plagioclase and a group of subhedral patchy zircons associated with trapped melt (quartz + feldspar matrix), and leucogranite also contains a group of doubly terminated euhedral zircons included within orthoclase as well as a group of zircons with visible cores mantled by later rim growth. Cathodoluminescence images also clearly document two distinctly textured varieties of zircon: the tonalite contains a population characterized by narrowly spaced uninterrupted oscillatory zoning and a second population lacking zoning but exhibiting a random distribution of dark (U-rich) and light (U-poor) regions; the leucogranite contains U-rich zoned zircons and U-poor zircon cores mantled by U-rich rims. The REE chemistry of zircon cores from the Vendodu leucogranite is very similar to the REE of zoned zircons from the Guramkonda tonalite. Zircon ages from both plutons exhibit bimodal distributions in U–Pb concordia diagrams. The tonalite defines an age of 2,521 Ma ± 5 Ma for zoned magmatic zircons and 2,485 Ma ± 5 Ma for unzoned newly precipitated zircons, whereas the leucogranite has an age of 2,518 Ma ± 5 Ma for U-poor zircon cores (relics of the tonalite pluton) and 2,483 Ma ± 3 Ma for U-rich zoned magmatic zircons. The trace element geochemistry of the ~2,520 Ma zircons is distinctly different from the ~2,485 Ma zircons, irrespective of the host rock. Our textural, CL image, and SHRIMP U–Pb analyses document the origin of the leucogranite by partial melting of the tonalite. High alkalis (Na2O ? K2O), Rb, Nb, HREE,FeOt/MgO and low Ca, Al, Ba, Sr, and large negative Eu anomalies characterize the leucogranite as a thermal minimum melt, whereas the very low K and Rb of the tonalite attests to its residual nature. We suggest that the leucogranite formed by high-T (900–950C), moderate-pressure (\10 kbar) dehydration partial melting of the tonalite under reducing conditions. The calculated source compositions of the leucogranite melt and the tonalite residue show strong similarities to melts that are considered to have been produced in a subduction-zone environment.The leucogranite probably formed in a post-collisional realm immediately after accretion of the tonalitic crust.

Neoarchean high-pressure metamorphism from the northern margin of the Palghat-Cauvery Suture Zone, southern India Petrology and zircon SHRIMP geochronology [查看] YohsukeSaitoh、ToshiakiTsunogae、M.Santosh、T.R.K.Chetty、KenjiHorie

We report the metamorphic pressure–temperature (P–T) history of mafic granulites from two localities in southern India, one from Kanja Malai in the northern margin and the other from Perundurai in the central domain of the Palghat–Cauvery Suture Zone (PCSZ). The PCSZ is described in recent models as the trace of the suture along which crustal blocks were amalgamated within the Gondwana supercontinent during Late Neoproterozoic–Cambrian. The mafic granulite from Kanja Malai yields P–T conditions of 750–800℃ and 8–12 kbar reflecting the partially retrograded conditions following a peak high-pressure(HP) metamorphic event. The common Grt + Cpx + Qtz assemblage in these rocks and lack of decompression texture suggest that peak metamorphism was probably buffered by Grt + Cpx + Opx + Pl + Qtz assemblage, following which the rocks were exhumed through a gradual P–T decrease. The mafic granulite from Perundurai (Grt + Cpx + Pl) contains Opx + Pl symplectite commonly occurring between garnet and clinopyroxene, suggesting the progress of reaction: Grt + Cpx z+ Qtz?Opx + Pl, with the Grt + Cpx + Qtz representing the peak metamorphic assemblage. The reaction microstructures and calculated P–T conditions suggest that the mafic granulites from Perundurai underwent peak HP metamorphism at P > 12 kbar and T = 800–900 C and subsequent isothermal decompression along a clockwise P–T path, in contrast to the P–T path inferred for Kanja Malai. The contrasting P–T paths obtained from the two localities suggest that whereas Perundurai is a part of the metamorphic orogen developed within the PCSZ during Gondwana assembly, the high-pressure granulites of Kanja Malai belong to a different orogenic regime.In order to evaluate this aspect further, we analyzed zircons in a charnockite and garnet-bearing quartzo-feldspathic gneiss associated with the HP granulites from Kanja Malai which yielded mean 207Pb/206Pb magmatic protolith emplacement ages of 2536.1 ± 1.4 Ma and 2532.4 ± 3.7 Ma, and peak metamorphic ages of 2477.6 ± 1.8 Ma and 2483.9 ± 2.5 Ma, respectively. These results closely compare with the available magmatic (2530–2540 Ma) and metamorphic (2470–2480 Ma) ages reported from charnockites in the Salem Block at the southern fringe of the Archean Dharwar craton, immediately north of the PCSZ. The Neoarchean/Paleoproterozoic ages obtained from Kanja Malai correlate with the tectonic history at the margin of the Archean craton. Although no age data are available for the Perundurai mafic granulite, the close correspondence of their P–T data and exhumation path with those reported for Late Neoproterozoic–Cambrian HP–UHT metamorphism within the PCSZ suggest that these rocks form part of the Gondwana-forming orogen.

Mississippian volcanism in the south-central Andes New U–Pb SHRIMP zircon geochronology and whole-rock geochemistry [查看] F.Martina、J.M.Viramonte、R.A.Astini、M.M.Pimentel、E.Dantas

In the northern extension of the Famatina and the southern Puna (NW Argentina) prominent rhyolitic volcanic rocks traditionally referred to as Ordovician are exposed, resting on metamorphic basement and covered by thick Late Paleozoic siliciclastic successions. We report new U–Pb SHRIMP ages from these rhyolites that show them to be of Mississippian (348–342 Ma) age, thus identifying a previously unknown volcanic event in this portion of western Gondwana. Whole-rock geochemistry and Sr–Nd isotopic analyses suggest a crustal source for these rocks but with a juvenile input (εNd(t) between −2.91 and −0.3, and TDM values between 1.09 and 1.1 Ga). This is different from the Early Paleozoic magmatism of western Argentina where crustal recycling took place without any involvement of mantle material. The Carboniferous magmatism is compatible with an extensional environment developed along the Terra Australis accretionary orogen as a result of tectonic switching processes. These rhyolites may be related to the coeval Mississippian A-type granites exposed to the east, in the Sierras Pampeanas, confirming the regional character of this magmatism.

Mid-to Late Cambrian docking of the Rı´o de la Plata craton to southwestern Gondwana age constraints from U–Pb SHRIMP detrital zircon ages from Sierrasde Ambato and Velasco (Sierras Pampeanas, Argentina) [查看] SEBASTIA´NO.VERDECCHIA、CESARCASQUET、EDGARDOG.BALDO、ROBERTJ.PANKHURST、CARLOSW.RAPELA、MARKFANNING、CARMENGALINDO

Abstract: The Early Palaeozoic stratigraphy and tectonic history of the Eastern Sierras Pampeanas of central Argentina are complicated by metamorphism and deformation resulting from the Pampean (545–510 Ma) and Famatinian (490–440 Ma) orogenies. We report U–Pb sensitive high-resolution ion microprobe dating of detrital zircons in two metasedimentary successions exposed at Quebrada de La Ce´bila (c. 28845’S, 66825’W): the Ambato and the La Ce´bila metamorphic complexes. The Ambato zircons record age peaks corresponding to Pampean (530±10 Ma), Brasiliano (c. 570 and c. 640 Ma), Grenville (c. 950 to c. 1025 Ma) and minor Neoarchaean ages. Similar peaks are also apparent in the La Ce´bila sample but it additionally contains Palaeoproterozoic zircons (c. 2.1 Ga) corresponding to the age of the Rio de la Plata craton, from which they are considered to have been sourced. Our interpretation is that the protolith of the Ambato complex was deposited prior to juxtaposition with the craton and is older than the Early Ordovician La Ce´bila metamorphic complex. We infer that the craton reached its current relative position in the Mid- to Late Cambrian, after the main Pampean tectonothermal event (530–520 Ma) and before deposition of the La Ce´bila protolith and the Achavil Formation (Sierra de Famatina), which contain comparable detrital zircon populations.

Mesoproterozoic juvenile maficeultramafic magmatism in the SW Amazonian Craton (Rio Negro-Juruena province) SHRIMP UePb geochronology and Nd-Sr constraints of the Figueira Branca Suite [查看] WilsonTeixeira、MauroC.Geraldes、ManoelS.D’Agrella-Filho、JoãoO.S.Santos、MárciaA.Sant’AnaBarros、AmarildoS.Ruiz、PauloC.CorrêadaCosta

The Figueira Branca Suite (FBS) comprises a layered maficeultramafic complex which together with maficefelsic plugs makes up a string of NW-trending intrusive bodies that are emplaced into the Jauru domain (Rio Negro-Juruena province; 1.80-1.60 Ga). This domain comprises Orosirian calc-alkaline rocks and coeval metamorphic volcanicesedimentary associations, intruded by voluminous granitoid plutons resulted from outboard Cachoeirinha (1587-1522 Ma) and Santa Helena (1485-1420 Ma) accretionary orogens that eventually created the Rondonian-San Ignacio province along the SW margin of the proto- Amazonian Craton. SHRIMP U-Pb age in zircon for one cumulatic gabbro from the FBS yielded a concordia intercept age of 1425.5±8.0 Ma (MSWD=1.11). Another gabbroic plug which crops out to the East gives a similar within error concordia intercept zircon age of 1415.9±6.9 Ma (MSWD=0.25),whereas a nearby monzogranite yields a concordia intercept zircon age of 1428.9±2.8 Ma(MSWD=1.30). All these results are crystallization ages and constrain an important intraplate magmatic event within the Orosirian continental crust at the time of outboard Santa Helena orogen. On the other hand, igneous titanite from another gabbro located to the West of the FBS yielded a weighted mean 207Pb/206Pb crystallization age of 1541±23 Ma (MSWD ¼ 0.74). Therefore this rock is not genetically associated with the FBS, as previously suggested by the field information. Additional Nd-Sr isotopic analyses of the FBS maficeultramafic rocks and coeval gabbro showed comparable εNd(1.42Ga) values (+3.0 to +4.7) and variable 3Sr(1.42Ga) ones (-39.1 to -8.1). These data plot in the depleted field quadrant of the Nd-Sr diagram, indicating a significant influence of the MORB end-member reservoir in the magma genesis. This interpretation is similarly supported by comparison of the Nd evolutionary path of the FBS with those that characterize the isotopic evolution of the Jauru crust and the Cachoeirinha and Santa Helena intrusive plutons. The bulk isotopic signature allows the genetic relationship among the Santa Helena Suite and the FBS and coeval rocks. The anorogenic character of the FBS is supported by the geologic framework of the Jauru domain, given that the emplacement took place under an extensional regime, associated with the recognized regional NW-trending structures. 40Are39Ar analyses were carried out in mafic rocks that are assigned to the FBS. Biotite from a gabbronorite yielded an ideogram age of 1222±5 Ma while a nearby troctolite yielded plateau biotite ages of 1275±4 Ma and 1268±4Ma.These ages are minimum estimates of the regional cooling of the FBS. One gabbro of the Alto Jauru Group yields a 40Ar-39Ar plateau age of 1781±15 Ma, interpreted as the time of regional cooling that succeeded continental accretion and metamorphism at Orosirian times. Later on heterogenous crustal thickening and uplift took place from West to East, as response from the Cachoeirinha and Santa Helena orogen dynamics, which is marked by the 1539-1510 Ma and 1452-1322 Ma 40Ar-39Ar age-patterns,respectively. The available apparent ages suggest that Cachoeirinha crust was subjected to a fast exhumation rate (ca. 50 m.y.), as estimated by the zirconetitaniteehornblendeebiotite time-path, while the Santa Helena crust seems to display a lower exhumation rate. Particularly, the youngest 40Ar-39Ar biotite age (1322 Ma) available for the Santa Helena crust signals the post-tectonic phase of the Rondonian-San Ignacio province.

In situ determination of U–Pb ages and Sr–Nd–Hf isotopic constraints on the petrogenesis of the Phalaborwa carbonatite Complex, South Africa [查看] Fu-YuanWu、Yue-HengYang、Qiu-LiLi、RogerH.Mitchell、J.BarryDawson、GüntherBrandl、MasakiYuhara

The Phalaborwa carbonatite Complex, situated in the northeastern part of South Africa, is characterized by copper and zirconium mineralization, and is composed principally of pyroxenites, phoscorite and carbonatite(banded and transgressive). The complex is transected by mafic dykes, and is geographically associated with a satellite syenite and minor granite intrusions. Zircon and baddeleyite U–Pb isotopic age determinations using CAMECA 1280 secondary ion mass spectrometry have shown that the outer pegmatitic pyroxenite at the Loolekop pipe was emplaced at 2060±4 Ma, and the main phoscorite at 2062±2 Ma. Both ages are identical to those of 2060±2 and 2060±1 Ma for the banded and transgressive carbonatites, respectively. The satellite syenite, which forms plug-like bodies outside of the border of the main complex, and the later mafic dyke have “similar” emplacement ages of 2068±17 and 2062±53 Ma, indicating that these intrusions were apparently near-synchronously emplaced. In contrast to other carbonatites, the Phalaborwa Complex is characterized by high initial Sr and low initial Nd and Hf isotopic compositions. In situ isotopic analyses of apatite,calcite, zircon and baddeleyite indicate that the primary magma was derived from an enriched mantle.As the complex was emplaced slightly earlier at ~2060 Ma than the nearby mafic phase of the Bushveld Complex (~2055 Ma), it is proposed that the Phalaborwa carbonatite magmatism was triggered by the same mantle plume activity, which partially melted the overlying lithospheric mantle. This contribution also highlights that isotopic studies used to constrain the genesis of ancient igneous complexes should concentrate on minerals with low parent/daughter elemental ratios, such as apatite and calcite for Sr isotopes, and zircon and baddeleyite for Hf isotopes.

Depositional History of the Chhattisgarh Basin, Central India Constraints from New SHRIMP Zircon Ages [查看] M.E.Bickford、AbhijitBasu、SarbaniPatranabis-Deb、PratapC.Dhang、JuergenSchieber

The Indian Shield includes the Singhbhum, Bastar, and East and West Dharwar cratons. Proterozoic sedimentary basins formed on these cratons have preserved rocks with a range of degrees of metamorphism and deformation. In the Chhattisgarh Basin, within the Bastar Craton, the ca. 2200–2500-m-thick Chhattisgarh Supergroup has been preserved in nearly pristine condition. Previous work has shown that the Sukhda Tuff, located about 2200 m from the base of the section, was formed ca. 1007 Ma. New U-Pb SHRIMP age determinations show that the Singhora Tuff, located about 100 m above the base of the basin, is not older than 1405±9 Ma. Thus, most of the Chhattisgarh Supergroup was deposited between 1400 and 1000 Ma. Age data for detrital zircons from sandstones show that,regardless of their stratigraphic position, there is a unimodal age peak near 2500 Ma, the typical age of adjacent granitic and rhyolitic basement rocks, indicating that these constituted the principal provenance of the sediments in the Chhattisgarh Basin. However, near the top of the succession, the Sarnadih Sandstone and a volcaniclastic sandstone near Sukhda Village, show a wide range of ages with peaks from ca. 1000 through 2680 Ma. The ca. 1000 Ma detrital zircons were probably derived from igneous sources similar to the Sukhda Tuff, but the zircons with other ages indicate a different source. Age data from rock units in the Central Indian Tectonic Zone to the north of the basin match this age spectrum better than any to the south of the basin and are consistent with a change in provenance direction to a northerly source late in the basin-filling cycle.

Depositional History of the Chhattisgarh Basin, Central India Constraints from New SHRIMP Zircon Ages A Reply [查看] M.E.Bickford、AbhijitBasu、SarbaniPatranabis-Deb、PratapC.Dhang、JuergenSchieber

The main issues in the discussion of our article(Bickford et al. 2011b) by Chakraborty et al. (2011)appear to be the stratigraphic position of the Singhora tuff and our interpretations of SHRIMP U-Pb zircon data. We address these issues but will not discuss stratigraphic classification, sedimentology,and plate tectonic considerations that are not relevant to the theme of our article.

© BJSHRIMP 2013 - bjshrimp.cn