U---Pb, Sr and Nd evidence for grenvillian and latest proterozoic tectonothermal activity in the spitsbergen caledonides, arctic ocean

Within the Caledonian complexes of northwestern Spitsbergen, high PT formations provide U---Pb zircon ages of 965±1 Ma of a metagranite and 955±1 Ma of a corona gabbro, indicating the influence of Grenvillian activity in the area. Various isotopic systems suggest that these rocks were partially derived by reworking of ancient crust (as old as Archaean). Eclogites and felsic agmatite indicate latest Proterozoic magmatic or metamorphic events (625₋₅⁺² and 661±2 Ma, respectively) by U---Pb zircon dating. The eclogitic metamorphism age is not fully constrained and ranges between 540 and 620 Ma; this occurred prior to the superimposed Caledonian metamorphism, indicated by a part of the K---Ar and Rb---Sr mineral cooling ages. The new data and other evidence of Precambrian tectonothermal activity on Svalbard suggest that the Early Palaeozoic and Late Proterozoic successions exposed elsewhere on Svalbard may also be underlain by Grenvillian or older basement rocks. Relationships to other Grenvillian and older terrains in the Arctic are reviewed.     

A U-Pb zircon,and Rb-Sr and U-Th-Pb whole-rock study of a polymetamorphic terrane in the central Alps,Switzerland

U-Pb analyses of zircons from the southern paragneiss zone of the Gotthard massif in the central Alps indicate these rocks were derived from one or more source areas ≧ 1400 m.y. old and were strongly affected by both the Caledonian and Hercynian orogenies. Rb-Sr whole-rock analyses also appear to reflect the Hercynian event while Rb-Sr analyses of a metamorphic inclusion and a boudin indicate that these small-scale samples were affected by the Alpine orogeny. U-Pb whole rock data appear to reflect only the Hercynian event; these data, when corrected for primordial lead, furthermore yield an upper concordia intercept of 4500 m.y., possibly resulting from a U-Pb fractionation very early in the history of the earth. A more refined three-stage U-Pb evolution model yields an age of about 4480 m.y. rather than 4500 m.y. for this hypothesized early terrestrial differentiation. Geologically these data emphasize that: 1) the southern paragneiss zone of the Gotthard massif contains metasediments which have experienced the Caledonian orogeny and are at least 400 m.y. old; 2) the Hercynian episode in this region was pervasive indeed; and 3) the Alpine orogeny affected the Rb-Sr and U-Pb whole rock systems to a far lesser degree than the preceding orogenic episodes and apparently did not affect the U-Pb zircon systems investigated at all.     

Secondary geologically meaningless Rb---Sr isochrons, low Sr/Sr initial ratios and crustal residence times of high-grade gneisses

Dark green quartzo-feldspathic charnockitic ortho-gneisses from Arendal, southern Norway, have locally been decoloured in the vicinities of intrusive, undeformed granite sheets and pegmatite dikes. The result is a series of pink rocks which are macroscopically indistinguishable from primary augen granite gneisses. The main mineralogical changes associated with the secondary decolcuration occurred under relatively low-grade conditions, and mainly involved chlorite, serpentine ± actinolitic amphibole ± biotite growth. The alterations were initiated along narrow, irregularly spaced cracks. The primary ( 1540 m.y.) total rock Rb---Sr isotopic systems which relate to the charnockite mineralogy were disturbed, and sometimes reset, during the secondary event which occurred at 1060 m.y. Locality by locality, the decoloured samples yield linear arrays corresponding to 1457 ± 200 m.y., 1303 ± 50 m.y. and 1125 ± 185 m.y. The combined data (n = 19) give an apparent age of 1364 ± 43 m.y. with an ⁸⁷Sr/⁸⁶Sr initial ratio of 0.7011 ± 0.0018. This apparent age is unrelated to any geological event and there is no overlap at the 2δ level between the ⁸⁷Sr/⁸⁶Sr initial ratio and that defined by the primary event (0.70345 ± 0.00014). Any estimate of crustal residence time based on these decoloured samples would be in error. The weight of evidence is against significant involvement of Rb mobility in the isotopic resetting process which accompanied decolouration. A model involving modification of the ⁸⁷Sr/⁸⁶Sr ratios is preferred.     

U-Pb isotopic relationships in zircon suites from a para- and ortho-gneiss from the Ceneri Zone,Southern Switzerland

U-Pb isotopic analyses have been determined on four size splits from an orthogneiss zircon population and on seven size and magnetic splits from a paragneiss zircon population from the Ceneri Zone, Southern Switzerland. The orthogneiss zircon results suggest that a severe isotopic disturbance occurred in this region between 400–450 m. y. ago which, besides initiating new zircon growth, almost completely reconstituted older zircon. The U-Pb systems of the orthogneiss zircons are nearly concordant, except the coarsest size split where some older lead is present.The paragneiss zircon U-Pb systems have Precambrian apparent ages and appear to be an old detrital zircon suite which has been severely isotopically disturbed by the same Caledonian event which affected the orthogneiss zircons. Zircon U-Pb data-points of paragneiss size-and-magnetic fractions are linearly correlated on a concordia plot. The age significance of this is not fully understood, however these results indicate that Precambrian crustal rocks, at least 1,500 m. y. old, contributed detritus to the original sediments of the Alpine gneisses.The gneissic rocks in Southern Switzerland have been postulated as a possible source or root zone for petrologically and chemically similar gneisses in the Silvretta Nappe to the north. The close similarity of zircon U-Pb systems of orth- and para-gneisses between the two regions supports this hypothesis.     

Age differences between Rb---Sr whole rock and U---Pb zircon ages of syn- and postorogenic Svecokarelian granitoids in Sottunga, SW Finland

The U---Pb age on pooled data of zircnos from the postorogenic Svecokarelian Mosshaga granite is 1788 ± 11 Ma (2 σ). The corresponding Rb---Sr whole rock age is 1742 ± 34 Ma (2 σ). The Mosshaga granite intrudes a synorogenic granodiorite, which has a U---Pb zircon age of 1889 ± 10 Ma and a Rb---Sr whole rock age of 1850 ± 28 Ma. The age differences between the U---Pb zircon ages and Rb---Sr whole rock ages are concluded to be geologically significant. It is shown that the Mosshaga pluton cooled rapidly and as a consequence the Rb---Sr isotopic system closed at a time very close to the crystallization of the zircons. A commonly observed feldspar and biotite alteration in Svecokarelian granitoids indicates migration of Rb and Sr isotopes in the whole rock system which may lead to apparently younger Rb---Sr ages.     

Rb---Sr isotopic equilibrium during Sveconorwegian (= Grenville) deformation and metamorphism of the Orust dykes, S.W. Sweden

The Orust dykes and their regional correlatives are an important time marker in the polymetamorphic Östfold-Marstrand belt of S.E. Norway and S.W. Sweden. They were deformed and metamorphosed in the amphibolite facies during the Sveconorwegian (= Grenville) orogeny. Three narrow strongly deformed dykes yield Rb---Sr whole-rock ages (1106 ± 52 m.y., 1062 ± 90 m.y., 1000 ± 153 m.y.). The weighted mean age (1087 m.y.) is interpreted as the age of the first post-dyke deformation and the accompanying amphibolite facies metamorphism. High initial ⁸⁷Sr/⁸⁶Sr ratios (0.7074 ± 9, 0.710 ± 2, 0.713 ± 3, respectively) and other geochemical evidence point to considerable exchange of mobile elements between the dykes and the country rocks. The emplacement age is not known precisely. However, the dykes may be correlated with widespread mafic magnatism which accompanied crustal instability in the N. Atlantic region early in the Sveconorwegian (= Grenville) orogeny about 1200 m.y. ago.     

Correlations between metamorphic events and Rb-Sr ages in metasediments and eclogite from Western Tasmania

The Strathgordon area of low metamorphic grade (450±50°C, 4±1 Kb) and the eclogitebearing Lyell Highway-Collingwood River area, which is of higher grade (670±20°C, 11±1 Kb), have been studied by the Rb-Sr technique.

Three deformational events, D₁ to D₃ correlate in time with two important P,T events, here termed D₂₋₂ and D₃, D₁₋₂ reflects the continuous prograde P,T environment up to the metamorphic maximum of the two areas (D₂), while D₂ is a superimposed deformational event occuring at lower P,T conditions. According to stratigraphical constraints D₃ must have occured before or about 550 m.y. ago.

It is possible from rather complex Rb-Sr data to indicate ages for the D_1–2 and D₃ events. Two-point total rock+phengite (mica) ages together with total rock isochrons using closely spaced samples show that the main metamorphic event D_1–22 occurrred at about 800 m.y. in both areas. Similar data suggest that the superimposed event D₃ occurred between 550 and 630 m.y. ago, probably near the lower age limit. This deformation (crenulation cleavage) was responsible for the local resetting of total rock and mineral systems.

Consistent with later Palaeozoic events, the Cambrian-early Ordovician Jukesian Movement and the Devonian Tabberabberan Orogeny, which affected some Rb-Sr mineral systems (phengite, amphibole and chlorite), have ages of 500±20 m.y. and 385-350 m.y. respectively.

Due to dispersion of the eclogite total rock data points along secondary isochrons it is difficult to estimate an original age. The age of the original minerals must however have been set by the 800 m.y. D_1–2 event, as the P,T history of the eclogite is the same as that of the surrounding schists.     

A SHRIMP U–Pb and LA-ICP-MS trace element study of the petrogenesis of garnet–cordierite–orthoamphibole gneisses from the Central Zone of the Limpopo Belt, South Africa

The Central Zone of the Limpopo Belt (South Africa) underwent high-grade metamorphism at 2.7–2.5 and 2.03 Ga. Quartz-rich, garnet-, cordierite-, biotite- and orthoamphibole-bearing, feldspar-free gneisses from the western Central Zone reached granulite-facies conditions (800 °C at 8–10 kbar) followed by decompression. Garnet from one such sample shows significant zonation in trace elements but little zonation in major elements. Zoning patterns suggest that the early prograde breakdown of REE-rich accessory phases contributed to the garnet trace element budget. Monazite from the sample yields a SHRIMP weighted mean ²⁰⁷Pb–²⁰⁶Pb age of 2028 ± 3 Ma, indistinguishable from a SHRIMP zircon age of 2022 ± 11 Ma previously measured on metamorphic overgrowths on 2.69 Ga igneous zircon cores. New zircon and monazite formed before, or at, the metamorphic peak, and occur as inclusions in garnet. Monazite appears to have formed through the breakdown of early allanite ± xenotime ± apatite. Trace element zoning patterns in garnet and the age of accessory phases are most consistent with a single tectonometamorphic event at 2.03 Ga.

The plagioclase and K-feldspar-free composition of the garnet–cordierite–orthoamphibole gneisses requires open system processes such as intense hydrothermal alteration of protoliths or advanced chemical weathering. In the studied sample, the 2.69 Ga igneous zircons show a prominent negative Eu anomaly, suggesting equilibrium with plagioclase, or plagioclase fractionation in the precursor magma. In contrast, the other minerals either show small negative (2.03 Ga monazite), no (2.02 Ga zircon and garnet) or positive Eu anomalies (orthoamphibole). This suggests that the unusual bulk compositions of these rocks were set in after 2.69 Ga but before the peak of the 2.03 Ga event, most probably while the protoliths resided at shallow or surficial crustal levels.     

Petrogenetic modelling of in situ fractional crystallization in the zoned Loch Doon pluton,Scotland

The late Caledonian Loch Doon granitic intrusion ranges in composition from hypersthene diorite at the margin, through quartz diorite, granodiorite and granite to cordierite microgranite at its core. Petrogenetic modelling of trace element variations and least squares analysis of major elements indicate that two distinct magmas are involved, each magma controlled by crystallization of plag-opx-cpx-bio. Late stage rocks related to the second magma include the cordierite microgranites and aplites, which are interpreted as the final residue which crystallized rapidly after a build up and loss of volatiles.Analyses of whole rocks and minerals for REE's and other elements of moderate-high ionic potential indicate that these elements are strongly controlled by minor phase crystallization; apatite, zircon, sphene and allanite are dominant at intermediate compositions but other accessory minerals such as monazite and xenotime may also become important at acid compositions.It is probable that within each magma the mechanisms of crystal settling and filter pressing operated, the former being initially dominant, and the latter becoming more important with increasing degree of fractional crystallization.     

Retention of Precambrian Rb/Sr phlogopite ages through Caledonian eclogite facies metamorphism, Bergen Arc Complex, W-Norway

The Lindås Nappe, Caledonides W-Norway was affected by two major tectonometamorphic events. A Precambrian granulite facies event at T=800–900°C, P<10 kbar was followed by localized Caledonian eclogite facies (T=650–700°C and P>15 kbar) and localized amphibolite facies reworking. During the granulite–eclogite facies transition, anorthositic rocks were converted from garnet granulites to kyanite eclogites, while phlogopite-bearing spinel lherzolite reacted to garnet lherzolite. The eclogite and amphibolite facies reequilibration took place along shear zones and fluid pathways. In the unhydrated and undeformed parts, the minerals preserved their granulite facies composition with constant Fe/Mg ratios from core to rim, suggesting diffusional reequilibration. Rb/Sr age dating was carried out on relict granulite facies minerals from three lenses of ultramafites (Alvfjellet, Hundskjeften and Kvamsfjellet). Phlogopite from phlogopite lherzolite at Alvfjellet give 857±9 Ma, while clinopyroxene, amphibole, phlogopite and whole rock from a lherzolite at Hundskjeften yield an age of 842±12 Ma (MSWD=1.9). Clinopyroxene, feldspar, orthopyroxene phlogopite and whole rock from websterite, Kvamsfjellet, yield an age of 835±7 Ma (MSWD<1), while clinopyroxene, phlogopite and whole rock from a lherzolite from the same lens gives a result of 882±9 Ma. These results are interpreted as minimum ages for the granulite facies event and only slightly younger than, or overlap with previous U–Pb zircon ages (929±1 Ma) and Sm–Nd garnet–pyroxene ages (890–923 Ma) interpreted to date the end of the granulite facies event. By contrast, ages obtained for the eclogite and amphibolite facies range from 460 (U–Pb, sphene), 440 (Ar–Ar), 419 (U–Pb, zircon) to 410 Ma (Rb/Sr mineral ages).

These results demonstrate that the reopening temperature for the Rb/Sr system in phlogopite–biotite under dry and static high-pressure conditions is, in the given mineral assemblages, at least 650°C, considerably higher than the 300–400°C assumed as the closure temperature of this system. We ascribe this elevated reopening temperature to fluid absent conditions that prevented element transport and rehomogenization.     

Zircon U–Pb geochronology of gneissic rocks in the Yunkai massif and its implications on the Caledonian event in the South China Block

This paper presents new zircon U–Pb data and interpretations for the gneissic rocks in the Yunkai massif in order to constrain the timing and evolution of the Caledonian tectonothermal event in the South China Block (SCB). Magmatic and inherited zircons from the orthogneiss in the region, previously thought to be of Precambrian origin, yielded ²⁰⁶Pb/²³⁸U apparent ages of 421–441 Ma and 513–1343 Ma, respectively. Also a weighted mean ²⁰⁶Pb/²³⁸U age of 236.0 ± 3.1 Ma was obtained, interpreted as the metamorphic resetting age during the Indosinian tectonic event. Our analyses show that the paragneiss in the region, previously regarded as Proterozoic sedimentary rocks, contains detrital zircons of the Archaean to Paleozoic origin, of which the youngest zircons yielded the U–Pb age of  423 Ma. These data indicate that (1) the Proterozoic and Archaean components may exist beneath the Yunkai massif; (2) most of the metaigneous rocks are actually the Caledonian anatectic granites possibly overprinted by Indosinian ( 236 Ma) reactivation; (3) some paragneiss might have originally deposited during the Devonian time; and (4) a subsequently rapid uplifting took place after the emplacement of the Caledonian granites, revealed by the observation that the Devonian clastic strata uncomfortably overlie the Caledonian granites. In combination with other geochronological data and geological observations throughout the SCB, we propose that the Caledonian tectonothermal event around Silurian ( 450–400 Ma) might be a result from an intracontinental collision between the Yangtze and Cathaysian blocks in response to the subduction/collision between the North China block and SCB.     

A geochemical and Rb---Sr study of the Proterozoic augen orthogneisses on the Molde peninsula, west Norway

Coarse augen gneisses in this part of the west Norway Basal Gneiss Complex are shown to be derived from a rapakivi-like, porphyritic, plutonic body intruded into a mid-Proterozoic mobile belt. The rocks are intermediate to granitic in composition with little disturbance of either major or trace elements by metamorphism or metasomatism. This suggests that the Rb---Sr age of 1477±21 m.y., with a low I.E. of 0.7042 reflects an intrusive event. The tectonic controls of the intrusion appear to be similar to those for the advanced differentiates of the Proterozc anorthositic suite, although amajor difference in respective H₂O contents resulted in differing Fe contents and resultant mineralogies. This example, with other supposed Proterozoic orthogeneisses Norway, suggests that plutonism at that time and in that area was more widespread than previously considered.     

Crystallization history of rhyolites at Long Valley, California, inferred from combined U-series and Rb-Sr isotope systematics

In this study, we present ⁸⁷Rb/⁸⁶Sr and ²³⁰Th/²³⁸U isotope analyses of glasses and phenocrysts from postcaldera rhyolites erupted between 150 to 100 ka from the Long Valley magmatic system. Both isotope systems indicate complex magma evolution with preeruptive mineral crystallization and magma fractionation, followed by extended storage in a silicic magma reservoir. Glass analyses yield a Rb-Sr isochron of 257 ± 39 ka, which can be explained by a feldspar-fractionation event ∼150 ky before eruption. Individual feldspar-glass pairs confirm this age result. A mineral ²³⁰Th-²³⁸U isochron in a low-silica rhyolite from the Deer Mountain Dome defines an age of 236 ± 1 ka, but the glass and whole rock do not lie on the isochron. U-Th fractionation of the rocks is controlled by the accessory minerals zircon and probably allanite, which crystallized at 250 ± 3 ka and 187 ± 9 ka, respectively. All major mineral phases contain accessory mineral phases; therefore, the mineral isochron represents a mixture of zircon and allanite populations. A precision of ±1 ka for the mixing array implies that the minor phases must have crystallized within this timescale. Longer periods of crystal growth would cause the mixing array to be less well defined. U-series data from other low- and high-silica rhyolites indicate younger accessory mineral crystallization events at ∼200 and 140 ka, probably related to the thermal evolution of the magma reservoir. These crystallization events are, however, only documented by the accessory minerals and had no further influence on bulk magma compositions. We interpret the indistinguishable age results from both isotope systems (∼250 ka) to record the fractionation of small magma batches by filter pressing from a much larger underlying magma volume, followed by physical isolation and extended storage at the top of the magma reservoir for up to 150 ky.     

Petrogenesis of the Lien peridotite and associated eclogites, Almklovdalen, Western Norway

Core and rim compositions of minerals in garnet-bearing assemblages in the Lien peridotite define a retrograde metamorphic trend from 820° C, 28.1 kbar, to 645° C, 17.6 kbar. Eclogites in Basal Gneiss near the peridotite contain a record of prograde metamorphism which converges with the retrograde trend of the ultramafic rocks. The Lien peridotite appears to have been derived from the upper mantle under eclogite facies conditions and emplaced into unusually thick continental crust during a Caledonian eclogite facies metamorphic event.     

Dating of granulites involved in the hercynian fold-belt of Europe: An example taken from the granulite-facies orthogneisses at La Picherais,Southern Armorican Massif,France

Situated within the crystalline metamorphic complex of Champtoceaux NE of Nantes, the orthogneiss of La Picherais (near St Mars-du-Désert, Loire Atlantique, France) show relicts of a granulite facies paragenesis. Comparison with other granulitic rocks in the Hercynian fold-belt suggest possible ages ranging from Lower Proterozoic to Phanerozoic. The Rb-Sr whole rock method yields an errorchron of 570±110 m.y. for the Picherais orthogneiss, whereas the U-Pb zircon method indicates an upper intersection on Concordia at 1,880±120 m.y. and a lower intersection at 423±10 m.y. Several interpretations are possible for these data: the granite emplacement age was (1) 1,900 m.y. ago. (2) more likely Upper Proterozoic — Lower Palaeozoic. The zircons concordant at 1,900 m.y. were either present in the granitic magma at its time of origin or were introduced into the magma during emplacement. These zircons could be derived from sedimentary horizons such as found in the Lower Ordovician sandstones of the Armorican massif whose zircon age data are presented here.     

Temporal and tectonic evolution of the granulite-eclogite association from the Bergen Arcs, western Norway

The U-Pb and Sm-Nd dating of deep crustal rocks from the Bergen Arcs system helps resolve enigmatic aspects of the tectonic evolution of the Caledonian Orogen in western Norway and yields insights into the arrested stages of eclogite development within the granulites of the area. The U-Pb dating of zircon from one of the eclogite facies shear zones yields an upper intercept age of 945 ± 5 Ma [all errors two standard deviations (2σ)], which is similar to other zircon ages from the granulite facies protolith. The age is interpreted to represent the time of late Proterozoic (Sveconorwegian) granulite metamorphism. The U-Pb ages of sphene and epidote show that the eclogites formed early in the evolution of the Caledonian Orogen (pre-Scandian phase) at about 460 Ma. An eclogite facies quartz vein yields a Sm-Nd whole rock-garnet isochron of 440 ± 12 Ma that may reflect the onset of cooling immediately after peak eclogite facies conditions, although the Sm-Nd systematics reveal some isotopic disequilibrium within the sample. In tandem with previous ⁴⁰Ar/³⁹Ar age determinations from, an adjacent eclogite of 450 Ma for hornblende and 430 Ma for muscovite, these data indicate that < 30 Ma elapsed between formation of the eclogites and the initial stages of cooling and exhumation to at least mid-crustal levels. This corresponds to minimum cooling rates of 14 °C/m.y. The timing relations suggest that the formation and exhumation of these eclogites from the overlying Caledonian Nappe wedge in western Norway are related to an early phase of crustal subduction during or somewhat before the major phase of continent-continent collision.

The short period of time between the formation of the eclogites and the initial stages of exhumation and rapid cooling is consistent with the only partial and localized transformation of the granulite to eclogite. Isolated occurrences of eclogite within the granulite, the formation of eclogite along metasomatic fronts and the formation of hydrous eclogite facies minerals within the “dry” granulite all point to the importance of fluids in the transformation and re-equilibration of the granulite to eclogite. Together, field and isotopic data demonstrate that both the localized and limited access of fluids and the rapid cycling of continental crust through the deepest portions of the orogen to upper crustal levels resulted in the preservation of the arrested stages of eclogite formation and survival of the granulites metastably through eclogite facies conditions.     

Isotopic age and metamorphic history of the banded gneiss at Danmarkshavn,East Greenland

Along the northern part of the East Greenland coast the Caledonian structures are superimposed on an older fold system called the Carolinidian. Traces of this Carolinidian belt are preserved in a few isolated fragments within the Caledonian fold belt. According to Haller (1970) one of these fragments exhibiting the typical Carolinidian NNW to NW-trending infrastructural folds is the peninsula of Germania Land which is accessible near the Danish weather station Danmarkshavn. The rock sampled there is a banded gneiss of granodioritic composition with steeply inclined, NNW-trending layers. Isotopic age determinations yielded essentially two groups of ages: 1) 3,000±150 m.y. (zircon suite and Rb/Sr whole rock analyses of layers) and 2) 320–380 m.y. (Rb/Sr mineral isochrons, U-Th-Pb on sphene, K/Ar on hornblende and biotite). The egg-shaped zircons support a sedimentary origin of the banded gneiss and in conjunction with the Rb/Sr whole rock ages determine the age of formation of the banded gneiss (or its last high grade metamorphism) some 3,000 m.y. ago. No other Precambrian metamorphism or orogeny is recorded in the rock. The ages between 320–380 m.y. date a thermal event of lower amphibolite facies grade related to a late Caledonian spasm.The new isotopic data reveal the existence of very old rocks in the hinterland — away from the direction of thrusting—of the East Greenland Caledonian belt. With respect to the age of the Carolinidian fold system three geological interpretations are compatible with the results of this study:

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.     

U-Pb zircon and Rb-Sr whole-rock dating of low-grade metasediments example: Montagne Noire (Southern France)

Three detrital, Proterozoic zircon suites extracted from siltstones progressively metamorphosed between chlorite- and staurolite-grade independently date the major Caledonian metamorphism within the gneiss dome of the Montagne Noire (Southern France). From this, the following conclusions concerning U-Pb systematics of zircons in low-, medium- and highgrade metamorphic rocks can be drawn:

1. Temperatures of at most 350–400 °C are sufficient to open U-Pb systems of metamict zircons or domains within zircons.
2. The observed open U-Pb system behaviour during metamorphism of the host rocks was found to be due to a low-temperature recrystallisation of highly radiation damaged zircon lattices, probably enhanced by high concentrations of fluid phases in the dehydrating rock volumes.
3. Recrystallisation of metamict zircons under low temperatures causes maximum U-Pb ages for the thermal climax of metamorphism of medium-and high-grade metamorphic rocks, as annealing and accompaning closing of U-Pb systems took place before the maximum temperatures of metamorphism were reached.
4. Low-temperature recrystallisation of old — generally Proterozoic—zircons can readily help to explain the fact that the numerous zircon suites from ancient shield areas yield “lower intercept ages” which are not correlated to any known geological event. Thus, either a weak thermal pulse, not necessarily registered by the mineral assemblage of the host rock, and/or elevated temperatures during burial in the crust might supply enough energy for a structural reordering and simultaneous lead loss of at least the most disordered lattice domains.

In contrast to the U-Pb zircon method, no unambiguous dating of the Caledonian main metamorphism was possible using the Rb-Sr whole-rock technique for phyllites and mica schists sampled in the same metamorphic profile from which the zircon samples were taken. The scatter of data points can best be explained by their rotation around a probable Caledonian isochron. This rotation very probably took place during the later Hercynian orogeny, not significantly affecting the slope of the least square regression line through the scattered data points.     

Isotope geochronological study of Late Palaeozoic granitic rocks in the Nanling Region,South China

Fourty-four isotopic ages have been determined by K-Ar and U-Th-Pb methods for Late Palaeozoic granitic rocks in the Nanling Region, South China. All dating values vary within the range of 231–348 m.y. From the obtained dates, further evidence has been found that there do exist Late Palaeozoic granitic rocks, which can be subdivided into Late Devonian and Permian granitic rocks. Within a Late Devonian terrain, there is a granitic pluton, namely granodiorite with a zircon U-Th-Pb age of 348 m.y., while ten granitic plutons have been recognized within a Permian terrain where granites are predominant, yielding biotite K-Ar ages of 236–289 m.y. (λ _β =4.72×10^?10yr.^?1,λ _K=5.57×10^?11yr.^?1) and zircon U-Th-Pb ages ranging from 231 to 280 m.y., respectively. It is obvious from the dates that intrusive activity of granitic magma extensively took place in the Nanling Region during Late Palaeozoic, although no records of orogenie movements have been found, indicating that the faults are the main factor controlling the activity of granitic magma, whereas the orogenic movements are not the only prerequisite for the formation of granitic magma and the intrusive activity.