Geochemistry and age of metamorphosed felsic igneous rocks with A-type affinities in the Willyama Supergroup, Olary Block, South Australia, and implications for mineral exploration

Leucocratic quartzofeldspathic gneisses form a significant proportion of the lower part of the Palaeoproterozoic Willyama Supergroup sequence in the Olary Block, South Australia and have correlatives in the adjacent Broken Hill Block. Field and geochemical data demonstrate that these rocks were originally rhyolitic volcanics and granite, with A-type affinities consistent with magma production during intracratonic rifting, supporting tectonic models proposed for the Willyama Supergroup in the Broken Hill Block. Although the rocks have characteristic high-field-strength element enrichment, many have undergone extensive pervasive pre- or syn-metamorphic sodic alteration and are typically rich in albite.

Sensitive high resolution ion microprobe (SHRIMP) U-Pb zircon data tightly constrain the depositional and early intrusive history. Zircons from an interpreted metavolcanic rock containing relict quartz phenocrysts yield an age of 1699 ± 10 Ma, whereas a metagranitoid sample has an age of 1703 ± 6 Ma. These results are compatible with recent geochronological data on felsic metavolcanic rocks from the Broken Hill Block (Page and Laing, 1992) and are indicative of widespread magmatism during deposition of the Willyama Supergroup. Nd signatures for the two Olary Block samples imply the presence of a significant component from a depleted mantle source.

The A-type metavolcanic rocks are locally associated with small iron formations, some of which grade into stratiform barite-rich horizons. Although potentially favourable for sediment-hosted exhalative PbZn mineralisation, the Fe- and Ba-rich units, along with transgressive vein and breccia occurrences of Fe oxides ± quartz ± pyrite cutting both the metavolcanic and metagranitoid rocks, may be more prospective for epigenetic Cu-Au mineralisation related to later metamorphic and/or magmatic events. Partial melting of the A-type suite during high grade regional metamorphism at ˜ 1600 ± 20 Ma led to the formation of local volumes of sodic granitoids and pegmatites containing U-Th-Ti-REE-F mineralisation.     

The Broken Hill ore body,Australia

The paper reviews recent works on the Broken Hill ore body to see what data is available on the origin and history of the ore. A high grade metamorphism is recognizable (the Willyama Metamorphism) and this was followed by a number of retrograde metamorphic events. The characteristics of these events is described and this is followed by an analysis of the hypothesis that there is a stratigraphic control of Broken Hill type mineralization. It is concluded that any control that does exist is of a regional nature and that the evidence is equally as good for a restriction of mineralization to areas of high grade metamorphism. Many of the isotopic and geochronological results for Broken Hill are not diagnostic as far as ore genesis is concerned. They do present an internally consistent set of data that records three major events, the Willyama Metamorphism and introduction of the Broken Hill type mineralization at 1700 m. y., the intrusion of the Mundi Mundi Granite and cross cutting pegmatites at 1560 m. y., and retrograde metamorphism and introduction of the Thackaringa Mineralization at or prior to 500 m.y. The sulphides have been deformed in the retrograde schist zones and the ore therefore predates this event. But there is no evidence at the moment that unambiguously establishes that the sulphides were present prior to the Willyama Metamorphism. If it was present prior to or during the Willyama Metamorphism and there is no period of deformation prior to the first recognizable folds at Broken Hill, then the sulphide mass was initially grossly discordant with bedding.

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Zusammenfassung Die Arbeit bringt Ergebnisse der neuesten Untersuchungen über die Broken-Hill-Lagerstätte anschließend an einen Überblick früherer Arbeiten. Der intensiven Willyama-Metamorphose folgen verschiedene retrograde Vorgänge, über die Einzelheiten mitgeteilt werden. Ferner wird die Hypothese einer stratigraphischen Kontrolle der Vererzung einer Prüfung unterzogen und festgestellt, daß die maßgeblichen Frakturen regionaler Natur sein müssen und daß die Beobachtungen somit eine Annahme der Vererzung der hochmetamorphen Gebiete ebenso stark stützt. Viele Resultate von Isotopenzusammensetzungen und geochronologischen Untersuchungen sind für Broken Hill nicht diagnostisch, wenigstens was die Erzgenese betrifft. Sie belegen 3 Hauptereignisse: die Willyama-Metamorphose und den Beginn der Vererzung um 1700.10⁶a, die Intrusion des Mundi Mundi Granits und querschlägiger Pegmatite um 1560.10⁶a und eine retrograde Metamorphose und den Eintritt der Thackaringa-Vererzung um oder vor 500.10⁶a. Die Sulfide sind in der Zone der retrograden Schiefer deformiert worden, was beweist, daß die Vererzung älter ist als dieses Ereignis. Es gibt aber z. Z. keinen Beweis, daß die Sulfide vor der Willyama Metamorphose vorhanden gewesen sind. Wenn sie vor oder während dieses Ereignisses vorhanden gewesen sind und wenn vor der Zeit der ersten deutlichen Falten in Broken Hill keine Deformation stattgefunden hat, dann war die Sulfidmasse ursprünglich in der Schichtung stark diskordant.

     

Geochronology of Precambrian granites and associated U-Ti-Th mineralization,northern Olary province,South Australia

Proterozoic granitoids and metamorphic rocks in the Olary province of the Willyama block of South Australia host ore-grade amounts of U-Th-Ti and U-Fe-Ti-Th minerals. U-Pb-Th isotope analyses on zircons from all granitoids associated with the Crocker Well brannerite deposit indicate that these granitoids were intruded within a short time span, close to the 1579.2±1.5 m.y. age of the brannerite-bearing host-rock. Though the early Paleozoic Delamerian orogeny was intense in this region, the zircon isotopic systems remained unaffected; rather, the best-defined zircon chords on concordia plots show a welldefined lower intercept of 43.8±6.5 Ma, which can only be associated with early Tertiary block faulting. Pb-U-Th isotope analyses on brannerite from the Crocker Well deposit and davidite from the Mt. Victoria deposit and the Radium Hill deposit yield badly scattered and discordant apparent ages that suggest a primary age at least as old as the age of the Crocker Well granitoids, followed by a severe disturbance in the early Paleozoic.     

Clay mineral,geochemical and Sr–Nd isotopic fingerprinting of sediments in the Murray–Darling fluvial system,southeast Australia

The Willyama Supergroup of the Broken Hill region in southern Australia consists of supracrustal sedimentary and magmatic rocks, formed between 1810 and 1600 Ma. A statistical analysis of nearly 2000 SHRIMP U–Pb zircon spot ages, compiled from published and unpublished sources, provides evidence for three distinct tectonostratigraphic successions and four magmatic events during this interval. Succession 1 includes Redan Geophysical Zone gneisses and the lower part of the Thackaringa Group (Cues Formation). These rocks were deposited after 1810 Ma and host granite sills of the first magmatic event (1710–1700 Ma). Succession 2 includes the upper Thackaringa Group (Himalaya Formation), the Broken Hill Group and the Sundown Group and was deposited between 1710 and 1660 Ma. These rocks all contain detrital zircons from the first magmatic event (1710–1700 Ma) and in some cases from the second magmatic event (1690–1680 Ma). The second magmatic event (1690–1680 Ma) was bimodal, resulted from crustal extension, and was coeval with deposition of the Broken Hill Group and deepening of the basin. With this event a mafic sill swarm focused in the Broken Hill Domain. Mafic sills lack any trace of inheritance, unlike the granitoids that commonly contain inherited zircons typical of the supracrustal sediments. Succession 3, the Paragon Group and equivalents were deposited after 1660 Ma, but before a regional metamorphic event at 1600 Ma. Metamorphism was closely followed by inversion of the succession into a fold‐and‐thrust belt, accompanied by a fourth late to post‐orogenic magmatic event (ca 1580 Ma) characterised by granite intrusion and regional acid volcanism (the local equivalents of the Gawler Range Volcanics in South Australia).     

Mineral-chemical and isotopic studies of Namaqualand granulites,South Africa: A grenville analogue

The northwestern part of South Africa and southern South-West Africa/Namibia is amongst the most extensive granulite terranes in Africa. This work reports the results of electron microprobe studies of minerals from two-pyroxene, cordieriteorthopyroxene (-gedrite) (-sapphirine) and garnet and/or cordierite parageneses from Namaqualand, in the N.W. Cape Province of South Africa. Determined PT conditions of prograde metamorphism based on thermodynamic calculations are 800°–900° C and ca. 6–7 Kb; and it is argued that rocks of unusual composition, notably cordierite-orthopyroxene rocks, are restites after the extraction of granitic liquid from former argillites. This interpretation is consistent with previously published data on similar rocks, and with McCarthy's (1976) suggestion of extensive partial melting in the quartzofeldspathic rocks in the area. U-Pb isotopic studies of some 50 zircon fractions have been carried out and confirm an age of 1,200 m.y. for the high-grade regional metamorphism; but certain zircon populations record inherited ages greater than 1,700 m.y. Garnet-sillimanite rocks that contain retrograde kyanite reflect PT conditions of 550°–650° C and ca. 7–8 Kb; and constituent biotite has yielded a K-Ar age of ca. 950 m.y. These data, the regional stratigraphy and structure, and the mineralisation are compared with data from the Grenville Province of Canada. Notable similarities are the possible basement-cover relationships, and the calendar of tectonothermal events, while differences include the important stratiform base-metal mineralisation in the supracrustal sequence in Namaqualand, and the Cu-mineralisation in hypersthenebearing intrusives, emplaced some 1,100 m.y. ago, that are areally, and believed to be genetically, related to the granulite facies metamorphic regime.     

Constraints on the early metamorphic evolution of Broken Hill, Australia, from in situ U-Pb dating and REE geochemistry of monazite

The Broken Hill Pb-Zn deposit, New South Wales Australia, is hosted in granulite facies gneisses of the Southern Curnamona Province (SCP) that have long been known to record a polydeformational and polymetamorphic history. The details of this potentially prolonged tectonothermal history have remained poorly understood because of a historical emphasis on conventional (i.e. grain mount) U-Pb zircon geochronology to reveal details of the sedimentary, magmatic and metamorphic history of the rock that crops out in the vicinity of the city of Broken Hill. An alternative approach to unravelling the metamorphic history of the granulite facies gneisses in and around Broken Hill is to date accessory minerals, such as monazite, that participate in sub-solidus metamorphic reactions. We have taken advantage of the high spatial resolution and high sensitivity afforded by SHRIMP monazite geochronology to reconstruct the early history of the metamorphic rocks at Broken Hill. In contrast to previous studies, in situ analysis of monazite grains preserved in their original textural context in polished thin sections is used. Guided by electron microprobe X-ray maps, SHRIMP U-Pb dates for three distinct monazite compositional domains record pulses of monazite growth at c. 1657 Ma, c. 1630 Ma and c. 1602 Ma. It is demonstrated that these ages correspond to monazite growth during lower amphibolite facies, upper amphibolite facies and granulite facies metamorphism, respectively. It is speculated that this progressive heating of the SCP crust may have been driven by inversion of the upper crust during the Olarian Orogeny that was pre-heated by magmatic underplating at c. 1657 Ma.     

The Redan Geophysical Zone: Part of the Willyama Supergroup? Broken Hill,Australia

The Redan Geophysical Zone forms a regional magnetic high in contrast to the regional magnetic low defined by the main part of the Broken Hill Block. The magnetic rocks are interpreted to dip below the remainder of the Broken Hill Block and there has been speculation that they are significantly older than the Early Proterozoic Willyama Supergroup.

Evaluation of lithological mapping and aeromagnetic data permitted interpretation of a stratigraphic sequence within the Redan Geophysical Zone, consisting of three new formations: the Redan Gneiss, Ednas Gneiss and Mulculca Formation, plus the Lady Brassey Formation, part of the Thackaringa Group. The rocks are considered to belong to the lower part of the Willyama Supergroup and are not an older basement.

Although the Redan Geophysical Zone contains some rock types not found elsewhere in the Broken Hill Block, there are some lithological similarities with the lower part of the Willyama Supergroup: an abundance of albite‐rich rocks, the presence of quartz‐magnetite rocks with Cu and trace Co, and abundant amphibolite/ basic granulite in the Lady Brassey Formation.

The boundary between the Redan Geophysical Zone and the remainder of the Broken Hill Block appears to be conformable, with no evidence of major faulting. Similarly no evidence of unconformities or major displacement of stratigraphic boundaries has been found within the Redan Geophysical Zone. Structural history, fold style and orientation, and metamorphic grade within the Redan Geophysical Zone are similar to adjacent areas of the Broken Hill Block.

It is concluded that the Broken Hill Block contains no outcropping equivalent of the first cycle of sedimentary/ igneous rocks recognized in the Early Proterozoic of northern Australia.

Albite‐quartz‐hornblende‐magnetite rocks unique to the Redan Geophysical Zone most likely comprised detritus derived directly from an intermediate volcanic suite. Some were altered considerably, while other rocks retained the dacite/andesite composition, except for the addition of Na, an increase in the oxidation state, and partial leaching of some of the more mobile elements. These modifications could have taken place in shallow alkaline evaporitic lakes.

The Redan Geophysical Zone contains some of the elements of a foreland basin adjacent to a continental volcanic arc: a thick stratigraphic sequence, oxidizing evaporitic conditions, and intermediate volcanic detritus. The change from intermediate‐acid volcanism in the earliest formations, to bimodal acid/basic volcanism in the Thackaringa and Broken Hill Groups could correspond with a change from initial continental arc volcanism into bimodal rift volcanism. The case for the arc volcanism is weakened, however, by the relative scarcity of rocks with andesitic compositions and the lack of basaltic andesite compositions. The alternative is that the intermediate to acid volcanism represents only a variation on the later bimodal rift volcanism.     

Broken Hill — A Precambrian hot spot?

Studies on the variations of metamorphism in the Precambrian granulite facies terrain of Broken Hill has revealed that these high-grade rocks are most frequently found in an elliptical zone known as the Broken Hill Basin. The density distribution of these granulite facies rocks, together with their lithological—structural—geophysical—mineralization relationships suggest local conditions of high-grade metamorphism. Several models to account for these observations are possible including lithological, structural-stratigraphic and mineralization controls. However in the light of current views of plate tectonics, a crustal-spreading hot spot model should also be seriously considered. The association of high-grade metamorphism and Broken Hill type mineralization is thought to be significant and this relationship could be applied in the search for ore deposits.     

Rb/Sr isotopic and geochemical evolution of a recrystallized shear (mylonite) zone at Broken Hill

Relatively narrow (1 to 1,000 metres) planar zones of intense shear deformation and retrograde metamorphism (retrograde shist zones, RSZ) are common in the Willyama Complex around Broken Hill. A Rb/Sr isotopic study of one of these zones has revealed an unexpected isotopic pattern. All of the analysed points lie to the left of and above the 1665 Ma isochron of the Potosi Gneiss host. This result indicates a different isotopic composition of the zone as a whole rather than a chemical redistribution of Potosi Gneiss within a closed system. The isochron plot scatter is similar to that of ubiquitous metamorphic pegmatites in the basement of the Broken Hill region. The shear zone observation, however, is shown to be unrelated to the pegmatite segregation process and requires the introduction, by elemental exchange, of a more radiogenic strontium, whilst maintaining relatively fixed Rb/Sr ratios. Significant proportions of SiO₂, K₂O and Ba were removed at the same time. Na₂O, Al₂O₃, TiO₂, MgO, P₂O₅, Zr, Ni, Sc, Y, Nb, Rb remain essentially constant after allowing for volume loss of the removed elements. CaO and Sr increase slightly overall.This pattern of elemental exchange is consistent with the breakdown of K-feldspar to muscovite and the essentially isochemical recrystallization of the other major phases (quartz, plagioclase and biotite) that are observed petrographically. Silica produced by the K-feldspar breakdown and a significant amount of the free quartz must also have been removed. Large scale fluid transport is required to achieve these results.The total rock isochron gives no indication of the age of the retrograde events. However two biotite-total rock joins indicate that the last internal isotopic redistribution occurred 458 Ma ago, shortly after the time of a regional low grade metamorphism. ⁸⁷Sr/⁸⁶Sr v 100/⁸⁶Sr plots confirm that a mixing process took place at about this time and not at 1,490 Ma or the present day.The introduced fluid had an ⁸⁷Sr/⁸⁶Sr ratio of about 0.794. If this figure still represents the original source value then it has come from what was long established mature crust at 458 Ma and not the mantle. The large fluid volumes required for the silica dissolution preclude a source in the local rock porosity, and suggest a meteoric fluid which has reacted extensively with the Willyama Complex metamorphics.     

Retrograde Schists of the Amphibolite Facies at broken hill,New South Wales

Aluminous, mafic, felsic, calcareous, and sulphide‐rich rocks have been involved in localized deformation and retrograde metamorphism at Broken Hill, western New South Wales, where retrograde schist‐zones intersect high‐grade, regional metamorphic rocks of the lower granulite facies (or the amphibolite‐granulite facies transition). Although technically retrograde, the schists contain mineral assemblages indicative of the lower amphibolite facies. The schist‐zones were formed by local folding, apparently as part of the third stage of deformation in the Broken Hill area.     

西藏康马片麻岩穹隆及其周围变质岩的主要特征

位于西藏康马县城北面的花岗片麻岩穹隆体,因其形态上类似一个侵入体,故早期的研究大都将其当作一个花岗岩体,其周围的变质岩则被认为是接触变质岩(周云生和张魁武,1981;张旗和李炤华,1981,涂光炽等,1981;李锦统等,1981)。目前,对穹隆体本身及其周围变质岩的性质和成因,仍然存在着争论。本文对康马片麻岩穹隆的性质及其成因机制进行了探讨,试图对认识拉轨岗日构造带的演化提供某些线索。     

U---Pb and Rb---Sr systematics in a polyorogenic segment of the Precambrian shield, central southern Norway

U---Pb analyses of zircon, monazite and sphene as well as Rb---Sr analyses of whole rocks and minerals have been carried out in an attempt to elucidate the evolution of the Precambrian shield underlying the Caledonian Jotun-nappe in central southern Norway. The earliest event recognized in the area is a high grade metamorphism at 1518 ± 17 m.y. which followed intense magma formation, igneous activity, erosion and sedimentation over a period of maximum 300 m.y. The Sveconorwegian cycle is characterized by the intrusion of two anatectic magma generations at 1014 ± 35 m.y. and 930±10 m.y. that disturbed and partly reset the isotopic systems of the country rocks. A post-magmatic fracturing stage at 875 m.y. concludes this cycle. Rb---Sr mineral systems were not completely equilibrated in the undeformed shield during the Caledonian event in contrast to new grown minerals in strongly deformed overlying Lower Paleozoic sediments which record the main deformation at 384±18 m.y. Zircon lower intercept ages ranging between 330–370 m.y. show that lead loss of zircon in rocks subjected to low grade metamorphic conditions may be the result of such processes as annealing, alteration, dilation or a combination of them.     

超高压变质岩的放射性同位素体系及年代学方法

深刻理解同位素在超高压变质及退变质过程中的地球化学行为对获得超高压变质岩准确并有明确意义的年龄值是非常重要的。对 Sm-Nd,Rb-Sr 同位素体系,只有变质矿物同位素体系达到平衡才能给出精确有意义的等时线年龄。研究表明,与副变质岩互层的细粒榴辉岩的高压变质矿物之间,或者强退变质岩石的退变质矿物之间,其 Nd,Sr 同位素可以达到平衡;然而高压变质矿物与退变质矿物之间 Nd,Sr 同位素不平衡。由于全岩样品总是含有数量不等的退变质矿物,因此石榴石 全岩 Sm-Nd 法或多硅白云母 全岩 Rh-Sr 法将有可能给出无地质意义的年龄。通常低温榴辉岩的高压变质矿物之间存在Nd 同位素不平衡。超高压变质岩多硅白云母所含过剩 Ar 主要源于榴辉岩原岩中角闪石在变质分解时释放出来的放射成因 Ar。因此,不含榴辉岩的花岗片麻岩多硅白云母基本不含过剩 Ar。对变质锆石成因的准确判断是正确理解锆石 U-Ph 年龄意义的关键。本文对不同成因锆石的判别标志及年龄意义做了总结,并指出将阴极发光图形,锆石痕量元素组成及矿物包裹体鉴定相结合是进行锆石成因鉴定的有效方法。高压变质或退变质增生锆石组成单一,是理想变质定年对象。然而变质重结晶锆石域常是重结晶锆石和继承晶质锆石的混合区,因而给出混合年龄。只有完全变质重结晶锆石才能给出准确变质时代。     

Mercury in the Broken Hill (N.S.W., Australia) lead-zinc-silver lodes

In the Pb-Zn-Ag lodes at Broken Hill, significant Hg remains located in sphalerite and in tetrahedrite inclusions in galena, despite prograde granulite facies regional metamorphism that has resulted in recrystallization of the ore. The stratigraphically uppermost No. 3 lens has the highest Hg content, with sphalerite and galena concentrates containing up to 149,000 ppb and 198,000 ppb Hg, respectively, in Ag-Sb-rich parts of the lode.The No. 2 and 1 lenses and A lode have generally low Hg contents, but narrow tetrahedrite-rich sections contain up to 83,000 ppb Hg in sphalerite concentrates and 82,000 ppb in galena concentrates. The stratigraphically lowermost B lode has a moderately high Hg content with sphalerite concentrates containing from 4400 to 41,000 ppb Hg and galena concentrates from 30 to 21,000 ppb Hg.Programmed heating determinations showed Hg in sphalerite concentrates to be contained principally in the sphalerite structure, with subordinate amounts in tetrahedrite inclusions. Mercury in galena concentrates is almost wholly contained in tetrahedrite inclusions, with lesser amounts in dyscrasite and pyrargyrite.Piercement bodies show increased Pb, Ag, Sb, As and Hg contents due to preferential migration of galena and tetrahedrite. The Hg content of sphalerite in these bodies is approximately doubled relative to the source lode, and the Hg content of galena concentrates is increased sevenfold, due to an increase of tetrahedrite inclusions.Coarse-grained sulfide-gangue pods and pegmatites, which reflect partial melting and remobilization of parts of the lode, have a mean Hg content less than the same volume of host lode. Within these bodies Hg is strongly partitioned into sphalerite, which contains from 14,000 to 33,000 ppb Hg, whereas galena contains only from 10 to 570 ppb Hg, due to the absence of Hg-bearing tetrahedrite inclusions.Within the lode environment, trace amounts of Hg in veins formed during retrograde metamorphism are located in sphalerite which contains up to 29,000 ppb Hg.The presence of significant amounts of Hg in Ag-Sb-As minerals in a siderite-rich vein within the main lode and in the Consols lode (located 700 m from the main lode) indicates that Hg remains associated with these elements over a wide range of conditions.In the wall rocks, there is no Hg halo which is exclusive of disseminated sulfide minerals. Under high-grade regional metamorphism, trace amounts of Hg in the Broken Hill lode are retained within pre-metamorphism host minerals. However, mobilization of Hg does take place in veins associated with retrograde metamorphism after localized breakdown of tetrahedrite.     

Sm–Nd garnet dating of polyphase metamorphism: northern Coast Mountains, south-eastern Alaska, USA

Sm–Nd ages of garnet from the northern Coast Mountains of south-eastern Alaska, USA, constrain the timing of thermal events in polyphase metamorphic rocks of the western metamorphic belt and provide new data on the spatial extent of Cretaceous regional metamorphism. Bulk garnet–whole-rock Sm–Nd ages for a sillimanite-zone amphibolite (Taku Inlet) and a biotite-zone metapelite (Tracy Arm) are 77±17 Ma and 59±12 Ma, respectively. Garnet core–whole-rock (80±9 Ma), core–matrix (84±9 Ma), rim–whole-rock (59±4 Ma) and rim–matrix (62±4 Ma) ages were obtained from a sample collected 200  m west of a Palaeocene Coast plutonic–metamorphic complex sill-like pluton that separates medium-grade metamorphic rocks from high-grade metamorphic rocks and voluminous Tertiary plutons in the core of the orogen. The garnet core ages of c. 80 Ma indicate that the regional metamorphic grade reached garnet zone prior to the intrusion of the plutons and high-grade metamorphism of rocks to the east. Similar ages for the younger plutons, the youngest garnets and the rim of a multistage garnet ( c. 59 Ma) indicate a later episode of contact metamorphic garnet growth. Documentation of pre-71 Ma garnet-zone metamorphism along the western edge of the Coast plutonic–metamorphic complex confirms that Albian to Late Cretaceous metamorphism associated with crustal thickening affected this part of the orogen. The similarity of garnet Sm–Nd ages to independent age estimates for metamorphic events confirms that this technique provides useful estimates for the timing of Late Cretaceous to Tertiary thermal events. The c. 20  Myr difference between garnet core and rim ages suggests that the Sm–Nd isotope systematics of a single garnet grain can be used for distinguishing between multiple metamorphic events.     

Sm/Nd,Rb/Sr,and 40Ar/39Ar Isotopic Systematics of the Ultrahigh-Pressure Metamorphic Rocks in the Dabie-Sulu Belt,Central China: A Retrospective View

Because of a complicated metamorphic history, the isotopic systematics of the ultrahigh-pressure (UHP) metamorphic rocks in the Dabie-Sulu belt, east China, appear to be rather different from what were expected. Depending on the degree of retrograde metamorphism and on the retentivity of isotopes, the radiogenic isotopic systematics in the UHP metamorphic rocks yielded a wide range of radiometric ages. Some of these ages are geologically meaningful, but others may not be. In some fine-grained UHP metamorphic rocks, Sm/Nd isotopic systematics appear to be in equilibrium among the UHP phases, showing the best estimate for the age of peak metamorphism at 226 ± 3 Ma. On the other hand, retrograde overprinting often makes the interpretation of isotopic data more difficult. It is common to find that the Sm/Nd and Rb/Sr isotopic systematics among the UHP phases and retrograde phases are not in equilibrium. Regression of isotopic data involving both UHP and retrograde minerals in isotopic correlation diagrams often yields geologically meaningless ages. Although ⁴⁰Ar/³⁹Ar dating of UHP metamorphic rocks has been reported not to be very helpful in establishing the thermal history because of the presence of excess argon, a good correlation between excess argon and rock type in the Dabie-Sulu belt would provide a criterion in identifying the possible sources of excess argon. By taking all the possible effects into consideration, a T-t path with two rapid cooling stages for UHP metamorphic rocks from the Dabie-Sulu belt can be postulated. An initial rapid cooling stage in the period from 226 to 219 Ma may have resulted from rapid exhumation of UHP metamorphic rocks immediately after the peak metamorphism. The second rapid cooling stage, from 450°C to 300°C, may have been caused by the exhumation of the entire terrane, including UHP metamorphic units and their host gneisses, during the period from 180 to 167 Ma.     

Correlation between the Wonominta and the Broken Hill Blocks in western New South Wales: Evidence from Nd isotope studies of metasediments

Nd isotope studies of the oldest metasedimentary rocks from the Wonominta Block, western New South Wales reveal that these samples have a model age (T_DM) of 1780–2010 Ma, slightly younger than that of low‐grade Willyama Supergroup metasediments (1920–2160 Ma), and significantly younger than those ages previously reported from high‐grade rocks of the Broken Hill Block (2200–2300 Ma). These differences have important implications for tectonic reconstruction in this region and support a model of transitional tectonics from the Broken Hill to Wonominta Blocks, as suggested by earlier geochemical studies of mafic rocks. Those studies revealed that the mafic rocks from the basal sequence of the Wonominta Block may have formed in a back‐arc basin, developed from a propagating rifting, an environment contiguous to that in which Willyama Supergroup was deposited. These results also carry significant implications for tectonic reconstruction of eastern Australia.     

Geochronology of Archaean gneisses and tonalites from north of the Frederikshåbs isblink,S.W. Greenland

The main rock types in the area north of the Frederikshåbs isblink are streaky gneisses, massive tonalites and ‘supracrustals’. The gneisses are thought to be the parent rocks of the tonalite and can be seen to merge into tonalite across a narrow zone of nebulite. Rb-Sr whole rock points from samples of gneiss and tonalite fall on a common isochron with an age of 2662 ± 116 m.y. (2σ) and initial ratio of 0.7032 ± 0.0008 (2σ) (half-life of ⁸⁷Rb = 50 b.y.). The uncertainties in the isochron could mask small age and initial ratio differences between the gneiss and tonalite. However, our present interpretation is that the isochron reflects a homogenization of Sr isotopes within and between the two rock types. The presence of two out of four K-feldspar points on the whole rock isochron is interpreted as evidence that the K-feldspar became closed to Sr isotope migration at the same time as the whole rocks. Subsequent local isotopic disturbance has resulted in a minor loss of radiogenic strontium from two of the samples. The interpretation of the K-feldspar as a product of the epidoteamphibolite facies metamorphism allows the conclusion that the whole rock-K-feldspar isochron is recording a Sr isotopic homogenization during this event and is not related to the formation of the gneiss or the tonalite. Rb-Sr closure ages of ca. 2515 m.y. for muscovite and ca. 1950 m.y. for biotite could be recording separate isotopic disturbances or the cessation of strontium isotope migration as the minerals cooled through their characteristic blocking temperatures. Zircons from both the gneiss and the tonalite have igneous morphological features. Their U-Pb systems are complex, however, and suggest a multistage history of isotopic disturbance. Whereas the zircon U-Pb and whole rock Rb-Sr results suggest a maximum age of approximately 3000 m.y. for the parent rocks of the gneiss and tonalite they do not entirely exclude the possibility that the rocks represent older crust in which the isotopic systems have been almost completely reset ca. 2700 m.y. ago.     

中国云南哀牢山含海蓝宝石伟晶岩的起源和成岩模拟实验研究

The Ailaoshan aquamarine-bearing pegmatites are associated with Proterozoic metamorphic rocks in the southern portion of the Ailaoshan fault-folded complex.The gem-bearing pegmatite mineralization zones of the region occur in areas generally consistent with the regional tectonic trend.The pegmatites are found in metamorphic rocks,migmatites and in the inner/outer contact zones of gneissoid granites. The Rb-Sr isochron drawn for the pegmatites is 26～31 Ma,(i.e.in Himalayan).The homogenization temperatures of melt and liquid inclusions in minerals vary from 185 to 920℃,which are comparable to the inclusions observed in banded migmatites and ptygmatic quartz veins in the surrounding metamorphic rocks. The mineralization fluids of the pegmatite were rich in HCO_3 and CO_2,and their compositional assemblages are comparable to metamorphic fluids.Results of H,O,C,Si etc.isotopic analyses and REE,and Be analyses indicates that the sources of mineralization components that formed the pegmatites are closely associated with metamorphic fluids and the enclosing metamorphic rocks. A pegmatite structure simulation experiment was conducted at high temperature and pressure(840℃and 1,500×105Pa.),with various metamorphic rock samples in a water-rich and volatile-rich environment.When the liquidus was reached,the temperature was gradually decreased at the rate of 5～10℃/day over a time period of three months.SEM energy-dispersive spectrum analyses were performed on the experimental products.A series of pegmatoid textures were observed including zonal texture,megacryst texture,drusy cavities,crystal druses,and vesicular texture along with more than ten types of minerals including plagioclase,microcline,quartz and biotite.Different metamorphic rock melts generated different mineral assemblages.Experiment results revealed that the partial melting of metamorphic rocks could form melts similar to pegmatite magmas. Based upon the geological characteristics,geochemistry,and pegmatite texture simulation experimental results,it is concluded that the mineralization components of Ailaoshan aquamarine-bearing pegmatites came from metamorphic rocks.The petrogenetic model for the origin of pegmatites is related to ultrametamorphism and metamorphic anatexis.     

A sulphur isotope study of the Broken Hill deposit,New South Wales,Australia

Sulphur isotopic compositions of sulphides within garnet-rich rocks and high-grade ore from the Broken Hill deposit, New South Wales, Australia, have been determined and show a range of values of –3.3 to +6.7 per mil. Thermochemical considerations, including the spread of values of ³⁴S, suggest that the deposit was derived from a mixed source of sulphur in which seawater, reduced by inorganic processes, mixed with magmatic sulphur or that sulphate from contemporaneous seawater was reduced biogenically at low temperatures. Thermochemical considerations also suggest that pyrrhotite formed by desulphidation of pyrite so that the original Fe-S-O assemblage was pyrite ± magnetite.³⁴S measurements show a broad range which is considered to be due to isotopic reequilibration during retrograde metamorphism and analytical and sampling technique. These data should not be used to indicate original temperatures of deposition or metamorphic temperatures associated with the various metamorphic events.