Geochemistry and isotopic ratios of boninites and related rocks of the Izu-Bonin Forearc

Twenty-six samples representing the wide range of lithologies (low- and intermediate-Ca boninites and bronzite andesites, high-Ca boninites, basaltic andesites-rhyolites) drilled during Leg 125 at Sites 782 and 786 on the Izu-Bonin outer-arc high have been analyzed for Sr, Nd, and Pb isotopes. Nd-Sr isotope covariations show that most samples follow a trend parallel to a line from Pacific MORB mantle (PMM) to Pacific Volcanogenic sediment (PVS) but displaced slightly toward more radiogenic Sr. Pb isotope covariations show that all the Eocene-Oligocene samples plot along the Northern Hemisphere Reference Line, indicating little or no Pb derived from subducted pelagic sediment in their source. Two young basaltic andesite clasts within sediment do have a pelagic sediment signature but this may have been gained by alteration rather than subduction. In all isotopic projections, the samples form consistent groupings: the tholeiites from Site 782 and Hole 786A plot closest to PMM, the boninites and related rocks from Sites 786B plot closest to PVS, and the boninite lavas from Hole 786A and late boninitic dikes from Hole 786B occupy an intermediate position. Isotope-trace element covariations indicate that these isotopic variations can be explained by a three-component mixing model. One component (A) has the isotopic signature of PMM but is depleted in the more incompatible elements. It is interpreted as representing suboceanic mantle lithosphere. A second component (B) is relatively radiogenic (epsilon-Nd = ca 4-6; 206Pb/204Pb = ca 19.0-19.3; epsilon-Sr = ca -10 to -6)). Its trace element pattern has, among other characteristics, a high Zr/Sm ratio, which distinguishes it from the ìnormalî fluid components associated with subduction and hotspot activity. There are insufficient data at present to tie down its origin: probably it was either derived from subducted lithosphere or volcanogenic sediment fused in amphibolite facies; or it represents an asthenospheric melt component that has been fractionated by interaction with amphibole-bearing mantle. The third component (C) is characterized by high contents of Sr and high epsilon-Sr values and is interpreted as a subducted fluid component. The mixing line on a diagram of Zr/Sr against epsilon-Sr suggests that component C may have enriched the lithosphere (component A) before component B. These components may also be present on a regional basis but, if so, may not have had uniform compositions. Only the boninitic series from nearby Chichijima would require an additional, pelagic sediment component. In general, these results are consistent with models of subduction of ridges and young lithosphere during the change from a ridge-transform to subduction geometry at the initiation of subduction in the Western Pacific.

Chemical and isotopic compositions of zircons from amphibolites of the Kamchatksky Cape Peninsula

The paper reports the first data on geochemistry and U-Pb SHRIMP geochronology of zircons from garnet amphibolites whose fragments are hosted by the sole of the ophiolite complex of the Kamchatsky Cape, eastern Kamchatka. The zircons compose homogeneous sampling, have relatively small sizes, are anhedral, have no oscillatory zoning, and possess practically no inclusions. Chemical and photoluminescent characteristics of the zircons testify to their metamorphic genesis. U-Pb SHRIMP dates of the zircons (81.4+/-9.6 Ma) indicate that metamorphism of the amphibolite complex took place in Campanian, Late Cretaceous. These dates seem to correspond to the peak of high-pressure metamorphism, which is thought to be related to origin of an ophiolite complex of the suprasubduction type and its uplift within the Kronotsky Island arc.

SHRIMP U-Pb dating of high-grade migmatites and related magmatites from Northwestern Oates Land, East Antarctica

High- to very-high-grade migmatitic basement rocks of the Wilson Hills area in northwestern Oates Land (Antarctica) form part of a low-pressure high-temperature belt located at the western inboard side of the Ross-orogenic Wilson Terrane. Zircon, and in part monazite, from four very-high grade migmatites (migmatitic gneisses to diatexites) and zircon from two undeformed granitic dykes from a central granulite-facies zone of the basement complex were dated by the SHRIMP U-Pb method in order to constrain the timing of metamorphic and related igneous processes and to identify possible age inheritance. Monazite from two migmatites yielded within error identical ages of 499 +/- 10 Ma and 493 +/- 9 Ma. Coexisting zircon gave ages of 500 +/- 4 Ma and 484 +/- 5 Ma for a metatexite (two age populations) and 475 +/- 4 Ma for a diatexite. Zircon populations from a migmatitic gneiss and a posttectonic granitic dyke yielded well-defined ages of 488 +/- 6 Ma and 482 +/- 4 Ma, respectively. There is only minor evidence of age inheritance in zircons of these four samples. Zircon from two other samples (metatexite, posttectonic granitic dyke) gave scattered 206Pb-238U ages. While there is a component similar in age and in low Th/U ratio to those of the other samples, inherited components with ages up to c. 3 Ga predominate. In the metatexite, a major detrital contribution from 545 - 680 Ma old source rocks can be identified. The new age data support the model that granulite- to high-amphibolite-facies metamorphism and related igneous processes in basement rocks of northwestern Oates Land were confined to a relatively short period of time of Late Cambrian to early Ordovican age. An age of approximately 500 Ma is estimated for the Ross-orogenic granulite-facies metamorphism from consistent ages of monazite from two migmatites and of the older zircon age population in one metatexite. The variably younger zircon ages are interpreted to reflect mineral formation in the course of the post-granulite-facies metamorphic evolution, which led to a widespread high-amphibolite-facies retrogression and in part late-stage formation of ms+bi assemblages in the basement rocks and which lasted until about 465 Ma. The presence of inherited zircon components of latest Neoproterozoic to Cambrian age indicates that the high- to very-grade migmatitic basement in northwestern Oates Land originated from clastic series of Cambrian age and, therefore, may well represent the deeper-crustal equivalent of lower-grade metasedimentary series of the Wilson Terrane.

Petrographical classification and mineral assemblage of basement clasts from the Quaternary and Miocene sections of sediment core CRP-1 (Table 1)

Petrographical and mineral chemistry data are described for the mist representative basement lithologies occurring as clasts (pebble grain-size class) from the CRP-1 drillhole. Most pebbles consits of either undeformed or foliated biotite with or without hornblende monzogranites. Other rock types include biotite with or without garnet syenogranitr, biotite-hornblende granodiorite, tonalite, monzogranitic porphyries, haplogranite, quartz-monzonite (restricted to the Quaternary section), Ca-silicate rocks and biotite amphibolite (restricted to the Miocene strata). The common and ubiquitous occurence of biotite with or without hornblende monzogranite pebbles, in both the Quaternary and Miocene sections, apparently mirrors the dominance of these rock types in the granitoid assemblages which are presently exposed in the upper Precambrian-lower Paleozoic basement of the south Victoria Land. The other CRP-1 pebble lithologies show petrographical features which consitently support a dominant supply from areas of the Transantarctic Mountains located to the west and south-west of the CRP-1 site, and they thus furthercorroborate a model of local provenance for the supply of basement clasts to the CRP-1 sedimentary strata.

Mineralogy, mineral chemistry, and genesis of precious metal-bearing volcanogenic massive sulfide deposits in the Newfoundland Appalachians, Canada: the Ming deposit as example

The Ming deposit, Newfoundland Appalachians, is a metamorphosed (upper greenschist to lower amphibolite facies), Cambro-Ordovician, bimodalmafic volcanogenic massive sulfide (VMS) deposit that consists of several, spatially-associated, elongated orebodies composed of stratabound semimassive to massive sulfides and/or discordant sulfide stringers in a rhyodacitic footwall. Copper is the main commodity; however, the deposit contains precious metal-bearing zones with elevated Au grades. In this study, field observations, microscopy, and micro-analytical tools including electron microprobe, laser ablation inductively coupled plasma mass spectrometry, and secondary ion mass spectrometry were used to constrain the relative timing of precious metal emplacement, the physico-chemical conditions of hydrothermal fluid precipitation, and the sources of sulfur, precious metals, semi-metals and metals. The ore mineral assemblage is complex and indicates an intermediate sulfidation state. Pyrite and chalcopyrite are the dominant ore minerals with minor sphalerite and pyrrhotite, and trace galena, arsenopyrite and cubanite. Additional trace phases include tellurides, NiSb phases, sulfosalts, electrum, AgHg±Au alloys, and oxides. Silver phases and precious metals occur predominantly in semi-massive and massive sulfides as free grains, and as grains spatially associated with arsenopyrite and/or sulfosalts. Precious metal phases occurring between recrystallized pyrite and within cataclastic pyrite are rare. Hence, the complex ore assemblage and textures strongly suggest syngenetic precious metal emplacement, whereas metamorphism and deformation only internally and locally remobilized precious metal phases. The ore assemblage formed from reduced, acidic hydrothermal fluids over a range of temperatures (≈350 to below 260ºC). The abundance of telluride and Ag-bearing tetrahedrite, however, varies strongly between the different orebodies indicating variable ƒTe₂, ƒSe₂, mBi, and mSb within the hydrothermal fluids. The variations in the concentrations of semi-metals and metals (As, Bi, Hg, Sb, Se, Te), as well as Au and Ag, were due to variations in temperature but also to a likely contribution of magmatic fluids into the VMS hydrothermal system from presumably different geothermal reservoirs. Sulfur isotope studies indicate at least two sulfur sources: sulfur from thermochemically-reduced seawater sulfate and igneous sulfur. The source of igneous sulfur is the igneous footwall, direct magmatic fluid/volatiles, or both. Upper greenschist to lower amphibolite metamorphic conditions and deformation had no significant effect on the sulfur isotope composition of the sulfides at the Ming deposit.

Representative analyses of garnet, plagioclase, orthopyroxene, ilmenite, hornblende and biotite for pre-tectonic mafic and metapelitic rocks in Gjelsvikfjella

A metamorphic petrological study, in conjunction with recent precise geochronometric data, revealed a complex P-T-t path for high-grade gneisses in a hitherto poorly understood sector of the Mesoproterozoic Maud Belt in East Antarctica. The Maud Belt is an extensive high-grade, polydeformed, metamorphic belt, which records two significant tectono-thermal episodes, once towards the end of the Mesoproterozoic and again towards the late Neoproterozoic/Cambrian. In contrast to previous models, most of the metamorphic mineral assemblages are related to a Pan-African tectono-thermal overprint, with only very few relics of late Mesoproterozoic granulite-facies mineral assemblages (M1) left in strain-protected domains. Petrological and mineral chemical evidence indicates a clockwise P-T-t path for the Pan-African orogeny. Peak metamorphic (M2b) conditions recorded by most rocks in the area (T = 709-785 °C and P = 7.0-9.5 kbar) during the Pan-African orogeny were attained subsequent to decompression from probably eclogite-facies metamorphic conditions (M2a). The new data acquired in this study, together with recent geochronological and geochemical data, permit the development of a geodynamic model for the Maud Belt that involves volcanic arc formation during the late Mesoproterozoic followed by extension at 1100 Ma and subsequent high-grade tectono-thermal reworking once during continent-continent collision at the end of the Mesoproterozoic (M1; 1090-1030 Ma) and again during the Pan-African orogeny (M2a, M2b) between 565 and 530 Ma. Post-peak metamorphic K-metasomatism under amphibolite-facies conditions (M2c) followed and is ascribed to post-orogenic bimodal magmatism between 500 and 480 Ma.

Geochemical composition of minerals obtained from ODP Sites 180-1109, 180-1117 and 180-1118

During the last 8 m.y. the Papuan Peninsula region of Papua New Guinea has been affected by extension which opened the Woodlark Basin. The present-day spreading tip is located at the foot of the Moresby Seamount, a crustal block whose northern flank is an active low-angle normal fault related to this extension. During Ocean Drilling Program Leg 180 (7 June-11 August 1998), 11 sites (1108-1118) were drilled along a north-south-trending transect across the Woodlark Basin just ahead of the spreading tip. Four of these sites (1118, 1109, 1114, and 1117) reached the crystalline basement, which is composed of diabase and gabbro. Sites 1118 and 1109, located on the Woodlark Rise, belong to the hanging wall block, and Sites 1114 and 1117, located on the crest of the Moresby Seamount, belong to the footwall block and the fault zone itself. Most of the basalt, diabase, and gabbro that were recovered show a well-preserved magmatic texture. The diabase, which is the most abundant rock type, has a coarse-grained ophitic texture composed of poikilitic clinopyroxene including radiating, locally skeletal plagioclase laths with interstitial iron oxide grains. Secondary mineralogy consists of chlorite, zeolite, calcite, albite, and quartz. The gabbro shows a medium-grained granular texture. The magmatic mineralogy consists of euhedral laths of plagioclase and anhedral interstitial clinopyroxene. Secondary mineralogy consists of a magnesio to actinolitic hornblende, chlorite, clinozoisite, zeolite, quartz, and calcite. The retrograde metamorphic evolution of both gabbro and diabase occurred under low amphibolite to subgreenschist facies conditions associated mainly with brittle deformation and the development of a local low-temperature shear zone. This shows no evidence for high thermal gradient in the crust during the continental rifting.

(Table) Argon isotope ratios and argon ages of rocks obtained from the Chernorud zone, western Baikal area

Structural-petrologic and isotopic-geochronologic data on magmatic, metamorphic, and metasomatic rocks from the Chernorud zone were used to reproduce the multistage history of their exhumation to upper crustal levels. The process is subdivided into four discrete stages, which corresponded to metamorphism to the granulite facies (500-490 Ma), metamorphism to the amphibolite facies (470-460 Ma), metamorphism to at least the epidote-amphibolite facies (440-430 Ma), and postmetamorphic events (410-400 Ma). The earliest two stages likely corresponded to the tectonic stacking of the backarc basin in response to the collision of the Siberian continent with the Eravninskaya island arc or the Barguzin microcontinent, a process that ended with the extensive generation of synmetamorphic granites. During the third and fourth stages, the granulites of the Chernorud nappe were successively exposed during intense tectonic motions along large deformation zones (Primorskii fault, collision lineament, and Orso Complex). The comparison of the histories of active thermal events for Early Caledonian folded structures in the Central Asian Foldbelt indicates that active thermal events of equal duration are reconstructed for the following five widely spiced accretion-collision structures: the Chernorud granulite zone in the Ol'khon territory, the Slyudyanka crystalline complex in the southwestern Baikal area, the western Sangilen territory in southeastern Tuva, Derbinskii terrane in the Eastern Sayan, and the Bayankhongor ophiolite zone in central Mongolia. The dates obtained by various isotopic techniques are generally consistent with the four discrete stages identified in the Chernorud nappe, whereas the dates corresponding to the island-arc evolutionary stage were obtained only for the western Sangilen and Bayankhongor ophiolite zone.