High Closure Temperature Research Articles

Dating of Granulite Facies Metamorphism and Depletion: SIMS Chronometry of Micron-Scale Monazite Inclusion in Garnet

It is difficult to obtain precise ages that give reliable information on the timing of the highest temperature history in granulite terrains. This is because the isotopic closure temperatures of most minerals are significantly lower than those achieved during granulite grade metamorphism. Monazite is a common accessory mineral in highgrade metamorphic rocks and, unlike zircon, is rarely inherited. It appears to retain Pb even at high temperature (725__+25~ Copeland et al., 1988) and offers potential to provide reliable chronological information of the high-grade parts of metamorphic thermal history as already demonstrated (DeWolf et al., 1993; Zhu et al., 1993). Of particular importance is the relatively common occurrence of monazite as inclusions in garnet. Because garnet has a very high closure temperature for Pb-isotopes (> 800~ Mezger et al., 1989), it may serve to armour monazite inclusion and provide age information on the higher temperature part of the record. It may even be possible to obtain the age information on the prograde part of granulite metamorphism from the monazite inclusion in garnet. Here we report the results of in situ ion probe dating of monazite occurring as micron-scale inclusions in garnet as well as discrete grains of monazite in matrix from the samples of the Lewisian complex.

U–Pb isotopic dating of fluid infiltration and metasomatism during Dalradian regional metamorphism in Connemara, western Ireland

Abstract A metasomatic diopside rock occurs at the top of the dolomitic Connemara Marble Formation of western Ireland and contains titanite and K‐feldspar in addition to around 90% diopside (XMg= 0.90–0.97). U–Pb isotopic measurements on this mineral assemblage show that the titanite is both unusually uranium‐rich and isotopically concordant, with the result that a precise U–Pb age of 478 ± 2.5 Ma can be determined. The age is identical within error to a less precise Rb–Sr age of diopside–K‐feldspar of 483 ± 6 Ma. Petrological evidence indicates that the assemblage crystallized at c. 620° C close to or below the closure temperature of titanite. The age thus provides a precise estimate of the time of metamorphism; this age is 11 ± 3 Ma younger than the 490 Ma age for nearby gabbroic plutons which has previously been used to constrain the peak metamorphic age. This difference accords well with geological evidence that the gabbros were emplaced prior to the metamorphic peak. Analysis of minerals with high closure temperature from assemblages whose crystallization is unambiguously associated with a specific episode of fluid infiltration at the peak of metamorphism provides the basis for a new approach to dating metamorphism. The success of this approach is demonstrated by the results from Connemara.

Combined high-precision chronometry and geochemical tracing using accessory minerals: applied to the Central-Alpine Bergell intrusion (central Europe)

The behaviour of Nd, U, Th and Pb isotopes in granitic rocks is usually controlled by accessory minerals. In this study an approach is adopted which uses the initial 143 Nd 144 Nd as a geochemical tracer together with high-precision UThPb geochronology for accessory phases and their respective whole rock. This combination of techniques is applied to zircon, allanite, sphene, apatite and thorite from both a tonalite and a granodiorite from the Tertiary Alpine Bergell intrusion. Inherited isotopic signatures have been identified in bulk-zircon fractions by inverse discordance in the UPb system. In the case of the granodiorite, the initial ϵ Nd in the zircon is 1.77 units lower than the bulk rock. This result indicates that Nd isotopes in zircon may serve as a useful tool for tracing crustal precursors in silicic rocks. All other accessories yield an initial ϵ Nd equal to their respective whole rock and UThPb ages equal to or lower than the respective intrusion ages. This indicates either the absence of inherited cores, or that such cores have exchanged their Nd and Pb (in the case of apatite also Sr) by diffusion. Where available, experimental mineral melt equilibria and diffusion models are consistent with this behaviour. UPb geochronology of carefully selected, core-free zircons yields a precise intrusion age of 31.88 ± 0.09 Ma for the tonalite. In contrast, granodioritic zircons are partially reset due to radiation damage caused by high U concentrations. However, an alternative means of dating young intrusions is to use 232Th 208Pb ages of allanite. In the case of allanite the presence of a large excess of 206Pb derived from 230Th indicates that no total Pb loss has occurred since crystallization, and the Nd isotopes indicate the absence of inheritance. Using both the allanite age and a 238U 206Pb age for sphene an intrusion age of 30.13 ± 0.17 Ma is indicated for the granodiorite. Taken together with an allanite ThPb age for the tonalite, these results strongly support the high closure temperatures for the allanites studied here (⩾ 700–800°C at high post-magmatic cooling rates). All other accessory mineral ages are reset to values similar to hornblende 39Ar 40Ar ages, and reflect the effect of post-magmatic cooling superimposed on post-metamorphic cooling.

Zoning and recrystallization of phengitic micas: implications for metamorphic equilibration

White micas (phengites) in the metasediments of the Scottish Dalradian display a large range of compositions within single samples. The variations in the composition of these phengites are strongly controlled by their structural age, with early fabrics containing a paragonite-poor, celadonite-rich phengite whereas in later fabrics the micas are generally paragonite-rich and celadonite-poor. Retrograde phengite growth, identified using back scattered electron imaging, occurs as celadonite-rich rims on micas within all existing fabrics and appears to be preferentially developed along existing white mica-plagioclase grain boundaries. The presence of these chemically distinct phengite populations within single samples implies that chemical exchange between the individual micas was inefficient. It is proposed that diffusion-controlled exchange reactions in phengites have relatively high closure temperatures below which major element exchange is effectively impossible. This closed system behaviour of micas questions the ease with which phengites may equilibrate with other phases during prograde greenschist and lower amphibolite facies metamorphism. Many of the chemical variations preserved in phengites from such metamorphic rocks may reflect deformation/recrystallization controlled equilibria.

Tectonometamorphic evolution of the Sebadani eclogitic metagabbro and the Sambagawa schists, central Shikoku, Japan: 40Ar/39Ar mineral age constraints

Abstract The Sambagawa metamorphic belt exposed in central Shikoku records a high‐P–T metamorphic event. It is represented by the Oboke nappe and structurally overlying, internally imbricated, Besshi nappe complex. These major structural units are in ductile thrust contact. A melange is developed along a ductile internal tectonic contact within the Besshi nappe complex. Tectonic emplacement of a high‐T enclave (Sebadani eclogite) in the melange zone resulted in the development of a contact metamorphic aureole within the host Sambagawa rocks. 36Ar/40Ar versus 39Ar/40Ar isotope correlation ages recorded by hornblende from the Sambagawa basic schists which surround the Sebadani enclave are 83.4 ± 0.3 Ma (within contact aureole) and 83.6 ± 0.5 Ma (outside aureole). 40Ar/39Ar plateau ages recorded by muscovite from the same samples are 87.9 ± 0.3 and 89.3 ± 0.4 Ma. Amphibole from the amphibolite within the Sebadani enclave records isotope correlation ages of 93.7 ± 1.1 and 96.5 ± 0.7 Ma (massive interior) and 84.6 ± 1.2 Ma (marginal shear zone). Amphibole within the massive amphibolite is significantly higher in XMg than that within the host Sambagawa basic schists. The older ages recorded by amphibole within the Sebadani enclave are interpreted to date cooling through somewhat higher closure temperatures than which characterize the more Fe‐rich amphibole in surrounding schists. The younger amphibole age recorded within the marginal shear zone probably indicates that crystallization of amphibole continued until cooling through the relatively lower amphibole closure temperatures.These results, together with the previously published 40Ar/39Ar ages of the Sambagawa schists, suggest: (i) metamorphic culmination occurred in the Besshi nappe complex at c. 100–90 Ma; (ii) at c. 95 Ma the Besshi nappe complex was internally imbricated and tectonic enclaves were emplaced; (iii) at c. 85 Ma, the composite Besshi nappe was rapidly exhumed and tectonically emplaced over the Oboke nappe (which attained peak metamorphic conditions at c. 75 Ma); (iv) the Besshi and Oboke nappe complexes were further exhumed as a coherent tectonic unit and unconformably overlain by the Eocene Kuma Group at c. 50 Ma.

The geochronology of metamorphic processes

Summary Metamorphic processes are complex, and the geochronology of these processes will be complex as well. A primary requirement of metamorphic geochronology is adequate time resolution, if metamorphism is to be viewed as more than just an event. Currently, geochronologists can achieve time resolution of 10 6 years or better for Phanerozoic and some Proterozoic rocks. A second requirement of metamorphic geochronology is that the systematics of dating schemes be understood in the context of the geochemical or geodynamic environment being examined. The approach that has been used most successfully, at least in young rocks, exploits the temperature-dependent nature of daughter-isotope diffusion to make measurements of the thermal history of metamorphic belts. Specifically, the fission-track and 40 Ar/ 39 Ar methods can be used to trace the post-peak temperature P-T path of a metamorphic rock from 500 to 100°C. Some progress has also been made in using other systems such as U-Pb, Rb-Sr, and Sm-Nd to date minerals having higher closure temperatures. New technologies and techniques of interpretation, such as the use of 40 Ar/ 39 Ar age spectra to determine diffusion profiles, or the ion microprobe to determine age zoning in zircon, are making available a great deal of additional geochronological information. In general, however, before tight constraints on the evolution of metamorphic belts can be obtained, many more diffusion data for systems of geochronological interest are needed, as are better constraints on the stability relationships under metamorphic conditions of accessory minerals such as zircon.

Fluids in Granulite Facies Metamorphism: A Comparative Oxygen Isotope Study on the South India and Adirondack High-Grade Terrains

Oxygen isotope compositions have been determined for samples from the South India Archean to Pro-terozoic and northwest Adirondack late-Proterozoic high-grade terrains. $\delta^{18}O$ values of charnockites and related orthogneiss protolith from the prograde amphibolite facies to granulite facies transition zone in South India range from 5.9 to 8.4‰. Charnockites, khondalites (sillimanite gneisses), and retrogressed gneisses from the interior of the granulite facies terrain average 2‰ higher than orthogneisses of the transition zone. Individual minerals in incipient charnockitic net veins, as at Kabbaldurga, Karnataka, and Ponmudi, Kerala, have $\delta^{18}O$ identical to minerals of their non-charnockitic host rocks. Quartz $\delta^{18}O$ values of the northwestern Adirondack granites and gneisses average 3-4‰ higher than those from the Indian rocks. The two terrains are thus isotopically distinct, which may reflect differences in premetamorphic history or different degrees of communication with external oxygen reservoirs during metamorphism. The low $\delta^{18}O$ of the South India terrain, including metasedimentary rocks, suggests the pervasive influence of a deep crustal or upper mantle igneous reservoir, whereas the elevated $\delta^{18}O$ of the Adirondack rocks suggests that the isotopic effects of near-surface, low-temperature processes in the protoliths survived subsequent fluid-deficient granulite-facies metamorphism. The near-identity of $\delta^{18}O$ of individual minerals from charnockitic veins and non-charnockitic host gneiss in the South India terrain indicates either that fluids causing charnockitic alteration were nearly in isotopic equilibrium with the country rocks or that oxygen isotopes were set in premetamorphic processes and little altered by incipient charnockite formation. Therefore we do not have direct isotopic evidence of fluid action in the origin of the much-discussed South Indian charnockitic net-veining. Isotopic temperatures are 100°C or more lower than peak metamorphic temperatures in both terrains, as inferred from mineralogic thermometry, indicating extensive postmetamorphic re-equilibration to lower temperatures, even in the fluid-deficient Adirondack terrain. An exception seems to be the quartz-garnet pair, which yields temperatures of 750° to 850°C for South India. Diffusion theory calculations show high closure temperatures in garnet, providing an explanation for the refractory behavior of this pair. It is possible that quartz-garnet will provide a useful oxygen isotope thermometer for granulites when favorable circumstances for postmetamorphic preservation are realized.

Paleozoic paleo-cover of the southern part of the fennoscandian shield-fission track constraints

Granitic intrusions older than 1200 Ma, from an area west of lake Vänern, south Sweden, show fission track ages of 680 ± 50 Ma (2 σ) (Late Precambrian) for sphene and 220 ± 10 Ma (Late Triassic) for apatite. The samples were taken at the present post-glacial erosion surface which is close to the sub-Cambrian peneplain. The c. 680 Ma sphene fission track ages are thought to be related to the uplift of the area which led to the establishment of this erosion surface at c. 600 Ma ago. The much younger apatite fission track ages and the confined spontaneous track length distributions along with regional stratigraphic constraints suggest that the basement was subsequently buried under 3–4 km of Paleozoic supracrustals. This induced in the basement a rise in temperature which obliterated the older apatite fission tracks, but not those in sphene which has a higher closure temperature. At the beginning of the Mesozoic, the area was uplifted again, the cover rocks were removed, and the basement rocks cooled slowly below the temperature range of fission track annealing in apatite.

Uranium-lead isotope systematics in a regionally metamorphosed tonalite from the eastern Alps

U-Pb isotopic analyses were made on sphene, three epidote fractions, apatite, K-feldspar and plagioclase from a 314-m.y.-old tonalite member of the Zentralgneis plutonic suite in the southeast Tauern Window. The tonalite reached temperatures in excess of 550°C during the Tertiary Alpine metamorphism. Apatite, fine-grained clinozoisite and feldspars equilibrated during the metamorphism, and the apatite yields an age of 22 m.y. which is 6 m.y. older than the Rb-Sr age of coexisting biotite. Sphene and coarse-grained iron-rich epidote did not reach equilibrium during Alpine metamorphism and the sphene data indicate crystallisation before 215 m.y. These minerals contain a large proportion of the uranium and thorium in the rock and the data thus imply that the present distribution of heat-producing elements in the tonalite was established long before the Alpine metamorphism. The very high closure temperatures for sphene and epidote implied by the data suggest they may be of value in dating metamorphism.

Stoichiometry measurements of GaAs by means of 15 MeV He ++ backscattering

We have undertaken petrologic and SHRIMP U-Th-Pb isotopic studies on zircons from basaltic eucrites (Yamato [Y]-75011, Y-792510, Asuka [A]-881388, A-881467 and Padvarninkai) with different thermal and shock histories. Eucritic zircons are associated with ilmenite in most cases and have subhedral shapes in unmetamorphosed and metamorphosed eucrites. Some zircons in highly metamorphosed eucrites with granulitic texture occur alone in pyroxene, and typically have rounded to subrounded shapes due to recrystallization. Superchondritic Zr/Hf ratios of eucritic zircons indicate that they crystallized from incompatible element-rich melts after crystallization of ilmenite. Concentrations of uranium and thorium in zircons in the unmetamorphosed eucrite Y-75011 are higher than those in metamorphosed eucrites.The U-Pb systems of eucritic zircons are almost concordant but some zircon grains show reverse discordance. Radiogenic lead-loss up to 48% from zircons is observed in the shock-melted eucrite Padvarninkai. The 207Pb-206Pb ages of zircon in Y-75011 (4550 ± 9 Ma, n = 5) are nearly identical, within analytical uncertainty, to the ages of zircons from the metamorphosed eucrite Y-792510 (4545 ± 15 Ma, n = 13), the highly metamorphosed eucrites A-881388 (4555 ± 54 Ma, n = 5) and A-881467 (4558 ± 13 Ma, n = 8), and the shock-melted eucrite Padvarninkai (4555 ± 13 Ma, n = 18). The averaged 207Pb-206Pb age of zircon from five eucrites analyzed in this study is 4554 ± 7 Ma (95% confidence limits, n = 49), indistinguishable from the averaged U-Pb age (4552 ± 9 Ma) of the same samples. Because of the high closure temperature of lead in zircon (Tclosure = ∼1050°C with a cooling rate of 0.2°C/yr), the 207Pb-206Pb ages of eucritic zircon do not represent metamorphic ages but crystallization ages of extrusive lavas.This fact strongly suggests that volcanism of the eucrite parent body occurred at a very early stage of the Solar System history, 7–20 Ma after CAI formation (4567.2 ± 0.6 Ma), thus basaltic eucrites crystallized from parental magmas within a short interval following the differentiation of their parent body. The U-Pb ages of eucritic zircons are older than the U-Pb, Sm-Nd and Rb-Sr ages of some basaltic eucrites, which is consistent with differences in closure temperatures of each isotopic system, and suggests that thermal and shock metamorphism affected the isotopic systems of pyroxene, plagioclase and phosphates.