Rb-Sr Whole-rock Research Articles

Very fast exhumation of high-pressure metamorphic rocks with excess 40Ar and inherited 87Sr, Betic Cordilleras, southern Spain

In order to attempt to further constrain the age of the early Alpine tectonic evolution of the Mulhacén Complex and to explore the influence of inherited isotopes, micas from a small number of well-characterised rocks from the Sierra de los Filábres, with a penetrative tectonic fabric related to the exhumation of eclogite-facies metamorphic rocks, were selected for 40Ar/ 39Ar and Rb–Sr dating. A single phengite grain from an amphibolite yielded an 40Ar/ 39Ar laser step heating plateau age of 86.9±1.2 Ma (2 σ; 70% 39Ar released) and an inverse isochron age of 86.2±2.4 Ma with an 36Ar/ 40Ar intercept within error of the atmospheric value. Induction furnace step heating of a biotite separate from a gabbro relic in an eclogite yielded a weighted mean age of 173.2±6.3 Ma (2 σ; 95% 39Ar released). These ages are diagnostic of excess argon ( 40Ar XS) incorporation, as they are older than independent age estimates for the timing of eclogite-facies metamorphism and intrusion of the gabbros. 40Ar XS incorporation probably resulted from restricted fluid mobility in the magmatic rocks during their metamorphic recrystallisation. Rb–Sr whole-rock–phengite ages of graphite-bearing mica schists from Paleozoic rocks (Secano unit) show a dramatic variation (66.1±3.2, 40.6±2.6 and 14.1±2.2 Ma). An albite chlorite mica schist from the Mesozoic series of the Nevado–Lubrı́n unit has a whole-rock–mica–albite age of 17.2±1.9 Ma, which is within error of an 40Ar/ 39Ar plateau age published previously and of the youngest Rb–Sr age of the Paleozoic series obtained in this study. The significant spread in Rb–Sr ages implies that progressive partial resetting of an older isotopic system has occurred. The microstructure of the samples with pre-Miocene Rb–Sr ages reveals incomplete recrystallisation of white mica and inhibited grain growth due to the presence of graphite particles. This interpretation agrees with previously published, disturbed and slightly dome-shaped 40Ar/ 39Ar age spectra that may point similarly to the presence of an older isotope component. The progressively reset Rb–Sr system is a relic of Variscan metamorphism of the Paleozoic series of the Mulhacén Complex. In contrast, the origin of the ca. 17.2 Ma old sample from the Mesozoic series precludes any isotopic inheritance, in agreement with its pervasive tectono-metamorphic recrystallisation during the Miocene. Exhumation of the eclogite-facies Mulhacén Complex occurred in two stages with contrasting rates of about 22.5 mm/year during the early phase and 9–10 mm/year during the late phase; the latter with a cooling rate in the order of 330 °C/Ma.

Fluorite Deposits at Voznesenka in the Khanka Massif, Russia: Geology and Age of Mineralization

Abstract. A huge fluorite deposit at Voznesenka in the Khanka massif, Far East Russia is concluded to have formed at ca. 450 Ma in Late Ordovician time based on the K‐Ar ages for Li‐micas in the fluorite ore and greisenized leucogranite within the deposit. This conclusion is inconsistent with the current view of Devonian mineralization that stemmed from widely scattered whole‐rock Rb‐Sr isotope data for the heterogeneous leucogranite stocks influenced by strong alteration. The Voznesenka and neighboring fluorite deposits may have formed in Cambrian limestone in relation to the intrusion of the Li‐F‐rich felsic magma which has a similar chemistry to representative Li‐F‐rich felsic rocks including topaz granite and ongonite or topaz rhyolite; these rocks may be classified as a specific group of highly fractionated felsic magmas.Biotite granite plutons exposed in the Voznesenka district are divided in age into two groups based on the CHIME age data for zircon, monazite and xenotime: Ordovician and Permian. The Ordovician plutons seem to be coeval to the fluorite deposits and are characterized by F‐rich chemistry, reduced nature and association of tin mineralization with the deposition of fluorite and tourmaline. The biotite granite magmas of initially enhanced F contents could have been highly fractionated to form Li‐F‐rich leucogranite cupolas that provided fluorite deposits within the host limestone. Future prospecting for similar fluorite deposits is to be focused on areas of intersection between Ordovician Li‐F‐rich granite and Cambrian carbonate sequences.The Permian granite of southeastern margin of the Grodekovo batholith is characterized by lesser F content, oxidized nature and the lack of tin and fluorite mineralization in contrast to the Ordovician granite. The result of Permian age does not support the current view of Silurian age for the batholith and requires overall chronological reinvestigation in connection with the tectonic history of the Khanka massif because the Grodekovo is a representative of Paleozoic batholiths in Primorie.

Neoproterozoic evolution of Western Ethiopia: igneous geochemistry, isotope systematics and U–Pb ages

New geochemical, isotopic and age data from igneous rocks complement earlier models of a long-lived and complex accretionary history for East African Orogen lithologies north of the Blue Nile in western Ethiopia, but throw doubt on the paradigm that ultramafic complexes of the region represent ophiolites and suture zones. Early magmatism is represented by a metavolcanic sequence dominated by pyroclastic deposits of predominantly basaltic andesite composition, which give a Rb–Sr whole-rock errorchron of 873±82 Ma. Steep REE patterns and strong enrichments of highly incompatible trace elements are similar to Andean-type, high-K to medium-K calc-alkaline rocks; εNd values between 4.0 and 6.8 reflect a young, thin continental edge. Interlayered basaltic flows are transitional to MORB and compare with mafic rocks formed in extensional, back-arc or inter-arc regimes. The data point to the significance of continental margin magmatism already at the earliest stages of plate convergence, in contrast with previous models for the East African Orogen. The metavolcanites overlap compositionally with the Kilaj intrusive complex dated at 866±20 Ma (U–Pb zircon) and a related suite of dykes that intrude thick carbonate-psammite sequences of supposedly pre-arc, continental shelf origin. Ultramafic complexes are akin to the Kilaj intrusion and the sediment-hosted dykes, and probably represent solitary intrusions formed in response to arc extension. Synkinematic composite plutons give crystallization ages of 699±2 Ma (Duksi, U–Pb zircon) and 651±5 Ma (Dogi, U–Pb titanite) and testify to a prolonged period of major (D1) contractional deformation during continental collision and closure of the ‘Mozambique Ocean’. The plutons are characterized by moderately peraluminous granodiorites and granites with εNd values of 1.0–2.0. They were coeval with shoshonitic, latitic, trachytic and rare trachybasaltic intrusions with very strong enrichments of highly incompatible trace elements and εNd of 0.4–8.0. The mafic end-member is ascribed to partial melting of enriched sub-continental mantle that carried a subduction component inherited from pre-collision subduction. Contemporaneous granodiorite and granite formation was related to crustal underplating of the mafic magmas and consequent melting of lower crustal material derived from the previously accreted, juvenile arc terranes of the East African Orogen.

Mantle–crust interactions during Variscan subduction in the Eastern Alps (Nonsberg–Ulten zone): geochronology and new petrological constraints

We have analyzed the Sm–Nd and Rb–Sr whole-rock and mineral isotope systematics of garnet peridotites and associated eclogites and migmatitic gneisses from the Nonsberg–Ulten zone of the Eastern Alps. The garnet peridotites include coarse-grained varieties, characterized by well-preserved to slightly modified mantle geochemical signatures, and finer-grained varieties enriched in amphibole and LILE. Hydration of some of the most strongly deformed, fine-grained peridotites by crustal fluids caused isotopic disequilibrium between the peridotite minerals, preventing accurate age determinations. The coarse-grained peridotites, the eclogites and the country migmatitic gneisses yield garnet–whole-rock and garnet–clinopyroxene Sm–Nd ages that indicate for all rock types an isotopic homogenization event at ca. 330 Ma. The similar ages suggest that all rock types shared a common history since the incorporation of the peridotites in the crust, and constrain the garnet-facies metamorphism of the peridotites, as well as partial melting of the crust, to an episode of crustal subduction at the end of the Variscan orogenic cycle.

ID-TIMS using the step-wise dissolution technique versus ion microprobe U–Pb dating of metamict Archean zircons from NE Madagascar

A geochronological study using the step-wise dissolution technique and in situ ion microprobe analysis is applied to late Archean metamict zircons of granitoids from northeastern Madagascar basement. Both techniques provide concordant U–Pb ages. Additionally, the ion microprobe study provided in situ measurements on the Pb isotopic composition of the metamict domains. Finally, Pb–Pb evaporation ages were found to be slightly but systematically younger than the dating obtained by the two other methods. Based on these data, granites from the Antongil belt were emplaced during at least three episodes separated by periods of magmatic inactivity of about 10 Ma at 2513, 2523 and 2532 Ma, respectively. These results are consistent with previous whole-rock Rb–Sr and U–Pb zircon dating from this part of Madagascar. The occurrence of this late Archean basement in northeastern Madagascar can be related to southern India where large volumes of contemporaneous granitoids were emplaced.

The Proterozoic Hustad igneous complex: a low strain enclave with a key to the history of the Western Gneiss Region of Norway

The Hustad Igneous Complex (HIC) is a concentrically zoned, high K calc-alkaline intrusive complex, with a core of quartz–monzonite and granite surrounded successively by monzodiorite and pyroxenite, preserved in a 10 km2 lens within the strongly ‘caledonised’ Western Gneiss Region (WGR) of Norway. Upper intercept UPb zircon ages of 1654±1 and 1653±2 Ma, obtained from the granite and the monzodiorite, respectively, overlap with a RbSr whole rock age of 1656±49 Ma (I.R.: 0.70193±0.0009, MSWD:6.6) obtained from the felsic members of the complex and a SmNd whole rock age of 1555±180 Ma (I.R.: 0.51067±13, MSWD:3.3). The UPb age of 1654±1 Ma is interpreted to date the time of intrusion. A 50 m thick dolerite dyke that can be followed for almost 4 km across the HIC yields a UPb zircon and baddeleyite age of 1251±3 Ma. The dyke also contains a secondary polycrystalline zircon generation, which together with baddeleyite defines a lower intercept age of 384±9 Ma. The Caledonian events were also marked by the emplacement of a swarm of pegmatites, which crosscut all the other units and provide a UPb zircon age of 401±1 Ma. A RbSr whole-rock age for the pegmatites is older at 452±26 Ma (I.R.: 0.7105, MSWD:4.3) indicating some initial isotopic heterogenity and formation of the pegmatites from material with a long crustal residence time. A concordant UPb monazite age of 390±2 Ma from the pegmatite and the above 384±9 Ma lower intercept age date metamorphism caused by percolating fluids. Post pegmatite deformation transformed the complex along shear zones into gneisses indiscernible from the surrounding gneisses. The HIC is characterised by high mg-numbers (0.39–0.80), high Ni and Cr values, and depletion of Nb and Ta relative to LIL elements. Phenocrysts of ortho- and clinopyroxene have high concentrations of Cr, supporting the Cr-rich nature of the complex. The suite is LREE-enriched, has a fractionated and U- shaped REE pattern (Cen 90–143, Ybn 12–40), eNd values ranging from +0.0 to +2.7, and the felsic members display weak positive Eu anomalies while the pyroxenite has a negative Eu anomaly. The complex bears a geochemical resemblance to the late-tectonic, high-BaSr granite complexes of West Scotland, and particularly to their volcanic counterparts, the Mg-rich Old Red Sandstone andesites. Overall the data suggest generation of the suite in a subduction environment from material derived by melting of a recently enriched, but previously long-term LREE-depleted mantle. The HIC is one of the few well-preserved remnants of the igneous protoliths of the 1686–1640 Ma gneisses that make up the WGR. In fact, the pyroxenite is compositionally identical to orthopyroxene eclogite found as lenses in the surrounding gneisses and forms a likely protolith to this enigmatic rock. The metamorphism had little influence on the RbSr and SmNd systems of the felsic rocks, but it appears to have affected more strongly those in the mafic rocks, probably as a result of the more intense mineralogical transformations and metasomatic loss of Rb and LREE during high-grade metamorphism.

Highly fractionated I-type granites in NE China (I): geochronology and petrogenesis

Northeastern (NE) China is the easternmost part of the Central Asian Orogenic Belt (CAOB), which is celebrated for its accretionary tectonics and the world's most important juvenile crust production in the Phanerozoic era. Abundant granitoids occur in the Great Xing'an, Lesser Xing'an and Zhangguangcai Ranges in NE China. This paper presents partial results of a series of studies on the granitoids from this region, aiming to understand their role in the building of new continental crust in eastern Asia. Three composite granite plutons (Xinhuatun, Lamashan and Yiershi) were chosen for geochemical and isotopic study in order to determine their emplacement ages and petrogenesis. Petrographically, they range from granodiorite (minor), monzogranite, syenogranite to alkali-feldspar granite. Quartz and perthitic feldspar are principal phases, accompanied by minor amounts of plagioclase, biotite (<5%) and other accessory minerals. In addition, many contain abundant miarolitic cavities which suggest that they were emplaced at shallow levels with extensive fractional crystallization. Geochemically, the granites are silica-rich, peraluminous and have high contents of alkalis. They invariably show enrichment in light rare earth elements (LREE) and significant negative Eu anomalies. All the granitic rocks demonstrate the characteristic negative anomalies in Ba, Nb, Sr, P, Eu, and Ti, and a positive anomaly in Pb in the spidergram. The emplacement of the Xinhuatun pluton took place at 184±4 Ma as revealed by zircon SHRIMP U–Pb data. This is also supported by the slightly younger Rb–Sr whole-rock (WR) isochron age of 173±3 Ma. A whole-rock (WR) Rb–Sr isochron age of 154±3 Ma was obtained for the Lamashan pluton, which is interpreted as close to the time of emplacement. The Yiershi pluton was intruded at about 140 Ma as evidenced by a zircon U–Pb age of 137±2 Ma and WR Rb–Sr isochron age of 143±5 Ma. Biotite-WR Rb–Sr isochrons and 40Ar/ 39Ar ages of feldspars allow us to estimate the cooling rate of each pluton. Geochemical data suggest that the rocks are highly fractionated I-type granites. Fractionation of biotite and feldspars was the principal process of magmatic differentiation and responsible for major element variation. Rb, Sr and Ba concentrations were controlled by feldspar separation, whereas REE elements were fractionated by accessory minerals, such as apatite, allanite and monazite.

Origin and Intensive Parameters in the Crystallization of Ultrapotassic Syenites: The Serra Do Man Pluton, Northeastern Brazil

The late Neoproterozoic ultrapotassic Serra do Man pluton intruded supracrustal schists and igneous rocks of the Alto Pajeu tectonostratigraphic terrane, in the Borborema province of northeastern Brazil. This quartz-alkali feldspar syenite is composed of microcline, albite, normally zoned aegirine-augite, winchite (formed after pyroxene), titanite and apatite, with the major-element geochemistry in consonance with early fractionation of pyroxene-titanite-apatite assemblage. Chondrite-normalized REE-patterns are LREE-enriched (La/Lu ∼ 20) with discrete negative Eu anomaly, and primitive mantle-normalized elemental concentration diagrams display enhanced negative Nb, P and Ti anomalies that are also observed in patterns of similar rocks in NE Brazil. Whole-rock d 18O values are rather high (avg. +8.7% SMOW) and in the +7.8 to +8.6% SMOW range when clinopyroxene-corrected. A five-point whole-rock Rb-Sr isochron yields an age of 580±28 Ma with high Sr o (0.7108), which together with eNd (0.58 Ga) value of -17 and t chur of 2.3 Ga allow hypothesizing a Paleoproterozoic enriched-mantle source for the syenites in the ultrapotassic province of the Alto Pajeu terrane. The aegirine-augite, titanite, and quartz assemblage attests to a high oxygen fugacity prevailing during magmatic crystallization. The almost complete absence of primary hydrated phases assures that the magma was rather anhydrous, and its viscosity, at the liquidus temperature, was relatively high. Density is slightly higher than expected for syenitic magmas (2.8 g/cm 3), being explained by a relatively high iron content of the magma. Geothermobarometry suggests shallow emplacement and temperatures of 700-750°C and 850-900°C, respectively, for the solidus and liquidus of the magma.

Rb–Sr and Sm–Nd isotopic study of the Puquı́ complex, Colombian Andes

The Puquı́ Complex is exposed in the northwestern part of the Colombian Central Cordillera and consists of a complex association of metamorphic rocks (amphibolite facies gneisses, migmatites, granulites, and amphibolites) intruded by the Puquı́ Tonalite, a large and mostly undeformed intrusion. The Puquı́ Tonalite is calc-alkaline and displays a mild peraluminous character, whereas the gneissic country rocks are strongly peraluminous (and interpreted as derived from a sedimentary protolith). A peculiar feature of this tonalitic body is the common presence of garnet (up to 7% modal proportion in some localities). The Rb–Sr data for the Puquı́ Gneiss yielded an isochron age of 306±11Ma with a high initial 87Sr/86Sr ratio (ca. 0.70980). This is interpreted as the result of Late Paleozoic metamorphic resetting of the Rb–Sr isotopic system of the sedimentary/metasedimentary protolith. This metamorphic event is also recognized in other parts of the Colombian Andes. Sm–Nd garnet–whole rock ages, however, indicate that these rocks have also been involved in a much younger metamorphic event, between ca. 140 and 120Ma. Sm–Nd model ages reveal a Precambrian age for the source of the original sediments (TDM model ages are in the range 1.33–1.53Ga). The Puquı́ Tonalite was emplaced at ca. 248±17Ma (whole-rock Rb–Sr isochron) with an initial 87Sr/86Sr ratio of ca. 0.70748, which suggests some contribution from older sialic material in the original magma. This is confirmed by the Sm–Nd isotopic data, which show εNd(248) between −5 and −6.15 and TDM model ages between 1.17 and 2.13Ga. These results suggest that the original tonalitic magma is strongly contaminated by the Puquı́ Gneiss. These preliminary Nd–Sr isotopic data suggest the presence of Precambrian crustal material in this part of the Colombian Central Cordillera. We also postulate that the Puquı́ Gneisses derived from Precambrian peraluminous crustal sources served as the contaminant of the tonalitic magma.

Rb- Sr isochron dating of granitoids from the Kazaure schist belt, Nigeria

The results of Rb-Sr isochron dating of granites from the crystalline basement complex of Kazaure Schist Belt are presented in this paper. Whole rock Rb-Sr radiometric age determination of five samples of a syntectonic coarse-grained porphyritic granite define a 4-point whole-rock Rb-Sr isochron (MSWD=1.2) corresponding to an age of 592 + 14 Ma, the initial 87Sr/86Sr ratio being 0.7097 + 0.0004. This is distinctly a Pan-African age and is taken as the time of emplacement and crystallization of a granitic magma formed from a reworking of a pre-existing continental crust. The initial 87Sr/86Sr ratio indicates a significant contribution of older crustal materials by partial melting to the granitic magma. Key Words: Kazaure Belt, Geochronology, Isochron, Pan-African, Crust. (Global Journal of Pure and Applied Sciences: 2002 8(3): 319-324)

Geochemical and geochronological constraints on the suture location between the North and South China blocks in the Dabie Orogen, Central China

The Dabie orogen is a collision zone between the North China block (NCB) and South China block (SCB), and a famous ultrahigh pressure metamorphic belt in the world. Location of the suture between the NCB and SCB in the Dabie orogen is still a controversial issue. We identify that the metabasaltic volcanic rocks from the Dingyuan Formation of the Beihuaiyang zone located in the northernmost Dabie orogen have typical geochemical feature of island arc basalt, e.g. negative Nb, Ti and P anomalies in trace elemental spidergram and high LREE, Ba and Pb abundance. Their Sm-Nd and Rb-Sr whole-rock isochron ages are 446±23Ma and 444±31Ma, respectively. These ages are consistent with the age of the island arc volcanics in the North Qinling belt that was the south active continental margin of the NCB in the Paleozoic. These data, together with the similarity in metamorphic history between the Dingyuan Formation and North Qinling belt, suggest that the Dingyuan formation is a Paleozoic magmatic arc on the south margin of the NCB. The Nd isotopic model ages of the Devonian metasediments from the Foziling flysch formation of the Beihuaiyang zone (TDM = 1.7 to 1.9Ga) are younger than those of the northern Paleozoic sediment cover of the SCB (TDM = 1.9 to 2.1Ga). It suggests that more Phanerozoic volcanic material, which could be derived from the active continental margin of the NCB, was involved into the flysch formation. The Huwan tectonic melange zone located to the south of the Dingyuan Formation was formed in Triassic. The eclogites involved in this melange zone were formed in the Carboniferous (301 ± 13 Ma). Their protoliths could be middle Paleozoic oceanic crust or late Proterozoic to early Paleozoic island arc basalt. All these data together with the recent discovery of the Triassic eclogites in the Northern Dabie Zone place important constraints on the suture location between the NCB and SCB. It should follow the Huwan tectonic melange zone in the west and the boundary between the Northern Dabie zone and the Beihuaiyang zone in the east.

Two contrasting paleozoic magmatic belts in northern Inner Mongolia, China: petrogenesis and tectonic implications

The Solonker zone, extending from Solonker to Sonidzuoqi and Xilinhot in northern Inner Mongolia, China, has been considered as the suture between the Manchurides and Altaids. Two magmatic belts have been recognized along the suture zone: an older, variably deformed arc-related magmatic belt represented by the Baolidao magma suite, and a younger, collisional granite belt, represented by the Halatu plutons. The Baolidao suite is composed of gabbroic diorite, quartz diorite, tonalite and granodiorite. Hornblende is present in all rock types. The Halatu plutons consist mainly of two-mica adamellite and subordinate granodiorite and leucogranite. Hornblende is absent in all rock types. The bulk of Baolidao rocks were emplaced at ca. 310 Ma (zircon 206Pb/ 238U age). They are metaluminous to weakly peraluminous and possess high Na 2O/K 2O ratios from 2 to 10. They have no Eu anomalies in the REE patterns but show strong positive Sr anomalies in the primitive-mantle-normalized spidergrams. In contrast, the Halatu granites were formed at ≈230 Ma based on a whole-rock Rb–Sr isochron. They are weakly metaluminous to peraluminous and have rather uniform chemical compositions. In comparison with the Baolidao magma suite, they have much lower contents of FeO, MgO, CaO, TiO 2, Cr, V and Sr, but are richer in SiO 2, K 2O and Rb. They also exhibit distinct negative Sr, P, Ti and Nb anomalies in the spidergrams and significant negative Eu anomalies in the REE patterns, a feature typical of crustal melts. The most primitive rock type of the Baolidao suite (gabbroic diorite) has I Sr=0.7052 and ϵ Nd( T)=+2.4, whereas the other rock types show small variations in both I Sr (0.7056–0.7059) and ϵ Nd( T) (+0.7 to −0.1). Geochemical modelling suggests that the gabbroic diorite was derived from a metasomatized upper mantle and the other rock types were generated from the gabbroic diorite magma by fractional crystallization coupled with assimilation of older crustal material (AFC), and followed by hornblende-dominated fractionation in a closed system. The Halatu granites exhibit a range of I Sr (0.7048–0.7063) and ϵ Nd( T) (+1.0 to −2.2). They were derived from sources dominated by young mantle-derived rocks with variable amounts of ancient continental crust. The largely juvenile isotopic signature for both the Baolidao suite and Halatu granites indicates a significant addition of new continental crust in northern Inner Mongolia.

Géochronologie U–Pb et géochimie isotopique Sr–Nd des granitoïdes néoprotérozoïques des suites Galiléia et Urucum, vallée du Rio Doce, Sud-Est du Brésil

U–Pb zircon and monazite dating and whole-rock Sr–Nd isotopes have been applied on Neoproterozoic granitoids from the Atlantic shear belt, south-eastern Brazil. U–Pb zircon results on the Galiléia metaluminous and Urucum peraluminous suites indicate crystallisation ages of 594 ± 6 Ma and 582 ± 2 Ma respectively. Whole-rock Rb–Sr and Sm–Nd analyses on representative samples from both suites demonstrate that they mainly derive from melting of older crustal rocks. The occurrence of an old isotopic memory in the zircons from the Urucum granitoids around 2.2 Ga, in agreement with Nd model ages, suggests a Palaeoproterozoic age for the crustal protolith of these two suites, probably associated with the Transamazônico event. Owing to these new dating results and previous regional geochronological data, it can be proposed that the metaluminous magmatism ended around 590 Ma and that the collisional peraluminous magmatism associated with the Brasiliano orogeny occurred between 584 and 570 Ma.

New Zealand's Darran Complex and Mackay Intrusives; Rb-Sr whole-rock isochrons in the Median Tectonic Zone

The Cretaceous opening of the Tasman Sea, followed by uplift east of the Alpine Fault, has given access to the roots of a Mesozoic convergent Gondwana margin in western New Zealand. This long term margin contains the complex Median Tectonic Zone (MTZ) of recent authors and is pefectly exposed between Milford Sound and the Darran Range. In the western part of this profile, in the Milford Gneiss Complex, earlier MTZ history is obscured by Cretaceous metamorphism and structural transposition. However, the Pembroke Granulite is well preserved, and the metamorphic curtain gradually lifts toward the east. Rb-Sr whole-rock isochron dates and initial 87 Sr/ 86 Sr (I o )values are presented. The Darran Complex yields a whole rock isochron of 133 + or - 17 Ma, concordant with published zircon U-Pb and hornblende K-Ar results. The Mackay Intrusives (Mistake Diorite and Hut Suite) yield a whole rock isochron of 224 + or - 4 Ma (MSWD = 1.21) in agreement with published U-Pb dates of Mistake Diorite. Rafts of Christina Gneiss in Mackay Intrusives were subject to partial melting at the time of the Mackay Intrusive activity. However, the Darran Complex also appears spectacularly intruded by the Christina-Crosscut irruptives of Mackay granitoids, which plot precisely on the 224 Ma Mackay Rb-Sr whole rock isochron. Obvious ways of resolving this paradox are (1) that the Darran Complex has undergone thermal resetting after its intrusion by Mackay magma, and (2) the Christina-Crosscut irruptives are late, back-intrusive Darran differentiates containing assimilated Mackay material. These solutions fail on the grounds of heat transfer (Mackay isochron was not re-set) or of heat balance (surmised irruptives excessively contaminated for the available heat). A way past the constraints is the passive tectonic emplacement of the Mt. Crosscut irruptive as a dike-like roof pendant. The Mt. Christina irruptive is dated less reliably, is tectonically less modified, and may represent a late Darran liquid. A much older age and lower I o of the Darran correlative Rotoroa Complex may be inherited from a common Rotoroa/Darran protolith. I o increases systematically from Rotoroa to Western Darran (Milford/Pembroke), to Darran rocks, and finally to Western Fiordland Orthogneiss (WFO), which is chemically distinct from the Pembroke Granulite and Darran Complex.

Episodic Silicic Volcanism in Patagonia and the Antarctic Peninsula: Chronology of Magmatism Associated with the Break-up of Gondwana

New SHRIMP U–Pb zircon, Rb–Sr whole-rock, and 40Ar–39Ar data are presented for the Jurassic silicic volcanic rocks and related granitoids of Patagonia and the Antarctic Peninsula. U–Pb is the only reliable method for dating crystallization in these rocks; Rb–Sr is prone to hydrothermal resetting and Ar–Ar is additionally affected by initial excess 40Ar. Volcanism spanned more than 30 My, but three episodes are defined on the basis of peak activity: V1 (188–178 Ma), V2 (172–162 Ma) and V3 (157–153 Ma). The first essentially coincides with the Karoo–Ferrar mafic magmatism of South Africa, Antarctica and Tasmania. The silicic products of V1 are lower-crustal melts that have incorporated upper-crustal material. The geochemistry of V2 and V3 ignimbrites is more characteristic of destructive plate margins, but the presence of inherited zircon still points to a crustal source. The pattern of volcanism corresponds in space and in time to migration away from the Karoo mantle plume towards the proto-Pacific margin of Gondwana during rifting and break-up. The heat required to initiate bulk crustal fusion may have been supplied by the spreading plume-head, but thinning of the crust during continental dispersion would also have facilitated anatexis.

Direct Isotope Dating of W(−Y) Mineralization at Kyzyltau (Mongolian Altai): Preliminary Results

Absolute ages of granite magmatism, as well as of tungsten mineralization, are poorly constrained in the Mongolian Altai and adjacent areas. There are no reports focusing on special isotopic investigations of the tungsten deposits. For the deposits in the Achit nuur and the Zagaan-Shibetin tectonic zones, two concepts that are discussed in the literature assume Paleozoic or Mesozoic ages for mineralization and related granite magmatism. We report the first results of a combined Sm-Nd and Rb-Sr isotope investigation of rocks and vein minerals of the Ulaan uul tungsten deposit at Kyzyltau; the results suggest Paleozoic ages for vein mineralization and for albitization of the host granite. The Sm-Nd isotope system, and the structure of vein minerals used for isotopic dating, were only slightly affected by late alteration processes. Sm-Nd mineral isochrons for wolframite and fluorite from the veins define an age of 303 ± 17 Ma (MSWD = 1.8, ϵNd = +0.9 ± 0.2). The Rb-Sr isotope system of the vein-hosting granite was strongly influenced by alteration processes. The Rb-Sr whole-rock isochron (282.2 ± 2.4 Ma, Sri = 0.70667 ± 0.00032, MSWD = 0.53) is interpreted as a mixing line and the age so defined has no direct geological meaning. Nevertheless, using Rb-Sr model ages and data on the degree of alteration of the samples, an age somewhat below 316 Ma can be estimated for albitization of the vein-hosting granite. This age estimation is in good agreement with the Sm-Nd isochron age for the vein mineralization. High ϵNd values obtained for wolframite and fluorite from the ore veins indicate an important role for material derived from the upper mantle in the ore formation processes.

Isotopic ages and chemical and isotopic composition of the Archaean Turfloop Batholith, Pietersburg granite--greenstone terrane, Kaapvaal Craton, South Africa

Thegranodioritic to granitic Turfloop Batholith in the northern Kaapvaal Craton, South Africa, intrudes metamorphosed and deformed greenstone lithologies ofthe >2.9 Ga Pietersburg Group and the >2.8 Ga tonalitic to trondhjemtic BaviaanskloofGneiss. New results presented here include 19 Rb—Sr and 7 Sm—Nd whole-rock isotopic analyses and U—Pb analyses of zircon and titanite from the Batholith. Three U—Pb analyses of zircon define a discordia with an upper intercept of 2.78 ±0.01 Ga (2o) and a lower intercept of 0.61 ±0.13 Ga (2σ).The average of two U—Pb analyses of titanite is concordant at 2.76 ±0.015 Ga(2σ). A composite Rb—Sr whole-rock errorchron (MSWD = 50.6), for 18 new and 11 previously published analyses, yields an age of 2.78 ±0.015 Ga (2σ) and initial 87Sr/86Sr = 0.7013 ±0.0003 (2σ).Pervasive, sub-solidus alteration affected the rocks of the Batholith but because isotopic ages show it was early (deuteric), it contributed little or nothing to the excess scatter of data. Instead the scatter is attributed to variable 87Sr/86Sr ratios among the plutons comprising the Batholith. Whole-rock Sm—Nd isotopic data indicates an age of 2.72 ±0.04 Ga (2a), and initial 43Nd/144Nd ratio of 0.50902 ±0.00003 (2σ) (eNd = –172.72 Ga for depleted mantle). Again, excess scatter in these data (MSWD = 5.05) is attributed to variations in initial 43Nd/144Nd ratios of plutons comprising the Batholith. The closeness of ages derived using four different techniques strongly suggests that the Turfloop Batholith was emplaced at ~2.78 Ga. Isotopic, whole-rock major-, trace-, and rare-earth element data, the presence of mafic enclaves and emplacement into greenschist to amphibolite-facies rocks suggest that the various magmas giving rise to the Turfloop Batholith were generated when a deep-seated heat source, probably mafic magma, partially melted >2.8 Ga tonalitic and trondhjemitic lower crustal rocks similar in composition to the BaviaanskloofGneiss. The Turfloop Batholith was emplaced after deformation, northward thrusting, and low-grade metamorphism of the Pietersburggranite—greenstone terrane, thus these processes must have occurred prior to ~.2.78 Ga. The thrusting is possibly related to similarly directed thrusting of rocks of the Witwatersrand Supergroup to the south. Ifemplacement of theTurfloop Batholith also post-dates southwestward thrusting in the Southern Marginal Zone of the Limpopo Belt, that thrusting also occurred prior to ~2.78 Ga.

Sr and Nd isotopic compositions, age and petrogenesis of A-type granitoids of the Vernon Supersuite, New Jersey Highlands, USA

Voluminous late Mesoproterozoic monzonite through granite of the Vernon Supersuite underlies an area of approximately 1300 km 2 in the Highlands of northern New Jersey. The Vernon Supersuite consists of hastingsite±biotite-bearing granitoids of the Byram Intrusive Suite (BIS) and hedenbergite-bearing granitoids of the Lake Hopatcong Intrusive Suite (LHIS). These rocks have similar major and trace element abundances over a range of SiO 2 from 58 to 75 wt.%, are metaluminous to weakly peraluminous, and have a distinctive A-type chemistry characterized by high contents of Y, Nb, Zr, LREE, and Ga/Al ratios, and low MgO, CaO, Sr and HREE. Whole-rock Rb–Sr isochrons of BIS granite yield an age of 1116±41 Ma and initial 87 Sr/ 86 Sr ratio of 0.70389, and of LHIS granite an age of 1095±9 Ma and initial 87 Sr/ 86 Sr ratio of 0.70520. Both suites have similar initial 143 Nd/ 144 Nd ratios of 0.511267 to 0.511345 (BIS) and 0.511359 to 0.511395 (LHIS). Values of ε Nd are moderately high and range from +1.21 to +2.74 in the BIS and +2.24 to +2.95 in the LHIS. Petrographic evidence, field relationships, geochemistry, and isotopic data support an interpretation of comagmatism and the derivation of both suites from a mantle-derived or a juvenile lower crustal parent with little crustal assimilation. Both suites crystallized under overlapping conditions controlled by P– T– f H 2O . Lake Hopatcong magma crystallized at a liquidus temperature that approached 900°C and a pressure of about 6 kbar, and remained relatively anhydrous throughout its evolution. Initial P– T conditions of the Byram magma were ≥850°C and about 5.5 kbar. BIS magma was emplaced contemporaneous with, or slightly preceding LHIS magma, and both magmas were emplaced during a compressional tectonic event prior to granulite facies metamorphism that occurred in the Highlands between 1080 and 1030 Ma.

Single-zircon evaporation ages and Rb–Sr dating of four major Variscan batholiths of the Urals: A perspective on the timing of deformation and granite generation

Using single-zircon stepwise-evaporation 207Pb/ 206Pb and whole-rock Rb–Sr methods, we have dated the main facies of four Variscan batholiths of the Urals — Syrostan, Verkhisetsk, Murzinka and Dzhabyk — from different geodynamic environments. Syrostan and Verkhisetsk are subduction-related batholiths of the accreted terrains in the west, closely related to the Main Uralian Fault. Syrostan is composed of metaluminous epidote-bearing synkinematic gabbros, diorites and granodiorites ( 87Sr/ 86Sr init≈0.7029–0.7035), dated at 333±3 Ma, and postkinematic granites ( 87Sr/ 86Sr init≈0.7035–0.7038) dated at 327±2 Ma, whose zircons occasionally have cores with an age of ∼1800 Ma. The presence of these relict zircon cores probably indicates the contribution of Russian Platform materials to subduction-related magmas. Verkhisetsk, 250 km north of Syrostan, comprises deformed amphibole- and epidote-bearing granodiorites ( 87Sr/ 86Sr init≈0.7042–0.7044), dated at 318±3 Ma, and undeformed granites ( 87Sr/ 86Sr init≈0.7044) that record a long history of recurrent partial melting and crystallization, from ∼310 Ma to ∼275 Ma. Murzinka and Dzhabyk are continental-type batholiths in the east Uralian zone. Dzhabyk, in the southern Urals, postdates the Kartali seismic reflector and is composed of several coeval intrusions of metaluminous monzonites ( 87Sr/ 86Sr init≈0.7049) and peraluminous granitoids ( 87Sr/ 86Sr init≈0.7043) dated at 291±4 Ma. Murzinka, 600 km north of Dzhabyk, also comprises two coeval intrusions of peraluminous granites with different chemical and isotopic compositions ( 87Sr/ 86Sr init≈0.7045 and 0.7093) dated at 254±5 Ma. These data indicate that the age of cessation of movements along the Main Uralian Fault and the transition from subduction to continent–continent collision migrated northwards. The age of granite magmatism also migrated gradually northwards and eastwards, suggesting an oblique convergence with a SE movement of the converging continent, so that the closure of the Uralian palaeo-ocean started in the south and then migrated progressively northwards.

Nature and longevity of hydrothermal fluid flow and mineralisation in granites of the Bushveld Complex, South Africa

The Lebowa Granite Suite of the Bushveld Complex is a large, 2054 Ma old, A-type batholith, characterised by numerous relatively small magmato-hydrothermal, polymetallic ore deposits. The mineralisation is represented by a three-stage paragenetic sequence: early magmatic Sn-W-Mo-F ores (600°C &gt; T &gt; 400°C), followed by a Cu-Pb-Zn-As-Ag-Au paragenesis (400°C &gt; T &gt; 200°C) and then late-stage Fe-F-U mineralisation (&lt; 200°C). The first stage of mineralisation (typified by the endogranitic Zaaiplaats tin deposit) is related to incompatible trace element concentration during crystal fractionation and subsequent fluid saturation of the magma. Evolution of the late magmatic fluids as they were channelled along fractures, as well as mingling with externally derived connate or meteoric fluids, resulted in the deposition of the second stage of mineralisation (typified by the fracture-related, endogranitic Spoedwel and Albert deposits and the exogranitic, sediment-hosted Rooiberg mine) which is dominated by polymetallic sulphide ores. As the externally derived fluid component became progressively more dominant, oxidation of the polymetallic sulphide assemblage and precipitation of hematite, pitchblende and fluorite occurred generally along the same fracture systems that hosted the earlier sulphide paragenesis.Small hydrothermal zircons trapped along quartz growth zones from the Spoedwel deposit yield a U-Pb concordia age of 1957 ± 15 Ma. Whole-rock Rb-Sr age determinations from the Lebowa Granite Suite fall in the range 1790 ± 114 Ma to 1604 ± 70 Ma and are interpreted to reflect alkali element mobility and isotopic resetting during exhumation of the Bushveld granite. In contrast to thermal modelling which indicates that hydrothermal activity should have ceased within 4 my of emplacement, isotopic evidence suggests that mineralisation was long-lived, but episodic, and that fluid flow events were linked to major periods of Palaeo- and Mesoproterozoic orogenic activity along the margins of the Kaapvaal Craton. During these orogenic episodes, fluid flow was enhanced by tectonically induced fluid over-pressuring and/or exhumation of the Bushveld Complex.