Rb-Sr and Sm-Nd Mineral Isochron Ages of the Metamorphic Rocks in the Namaqualand Metamorphic Complex, South Africa

Rb-Sr and Sm-Nd isotopic studies were carried out for metamorphic rocks in the Namaqualand Metamorphic Complex, South Africa. The metamorphic rocks give the Rb-Sr mineral isochron ages (whole-rock - biotite - felsic fractions) of 844±85 Ma and 811.6±6.6 Ma for the lower granulite zone and of 776.5±5.4 Ma for the upper granulite zone. The rocks yield the Sm-Nd mineral isochron ages of 1071±18 Ma (whole-rock - garnet - felsic fractions) and 1067±158 Ma (whole-rock - hornblende - biotite rich fraction - felsic fractions) for the lower granulite zone and of 1052.0±3.6 Ma and 1002.5±1.4 Ma (whole-rock - garnet - felsic fractions) for the upper granulite zone. These age data suggest that the granulite facies metamorphism took place at 1060-1000 Ma, and that the rocks cooled down at 850-780 Ma. The Sr and Nd isotopic compositions of metamorphic rocks are different between the lower and upper granulite zones.     

Crustal residence history and garnet Sm–Nd ages of high-grade metamorphic rocks from the Windmill Islands area,East Antarctica

Nd whole-rock data from the Windmill Islands area yield early Proterozoic to middle Archaean Nd model ages. These crustal residence times are consistent with regional correlations with other parts of Antarctica (Bunger Hills, Denman Glacier area) and the Albany-Fraser Orogen of south-western Australia during the Mid-Proterozoic and thus support reconstructions with a continuous Mid-Proterozoic orogen in these areas. The new Nd isotope data provide strong evidence that no age boundary exists between the higher- and lower-grade parts of the Windmill Islands area, and that the metamorphic complex represents a single terrane with a common crustal history. The data support the notion of a time-link between the occurrence of intrusive charnockites (C-type magmas) and high-grade metamorphism. The magmatic rocks and orthogneisses in the area are interpreted to have a mixed source consisting of older crustal components, i.e. older sediments (ca. 3.2-2.6 Ga) and a younger mafic component (ca. 1.9 Ga). Two garnet Sm-Nd isochrons yield ages of 1156ᆥ Ma and 1137DŽ.5 Ma and are identical to SHRIMP U-Pb results on monazite from these samples. A garnet Sm-Nd age of 1123ᆡ Ma for the Ford granite is significantly younger than the SHRIMP U-Pb zircon age for this sample. The difference relates to the different closure temperature of each isotopic system and is thus interpreted as initial cooling after granulite facies metamorphism.     

大别山金河桥榴辉岩矿物O—Nd—Pb同位素体系及其对扩散速率的制约

对大别山太湖金河桥超高压榴辉岩作了矿物Sm-Nd内部等时线定年研究和激光氧同位素分析。石榴石+绿辉石Sm-Nd等时线给出了较低年龄210±3Ma,石榴石+金红石Sm-Nd等时线给出了较高年龄237±4Ma。岩相学观察发现,绿辉石具有角闪石退变质边。氧同位素分析表明,石榴石与绿辉石之间的氧同位素体系处于不平衡状态。据此,石榴石+绿辉石Sm-Nd同位素体系因退变质作用导致Nd同位素不平衡而给出不合理偏低年龄。较老的石榴石+金红石Sm-Nd年龄有可能指示了榴辉岩相前期阶段的时代,且在温度变质峰期没有使它们之间的Nd同位素再次均一化,它指示Nd在金红石中的扩散速率较慢,可能与石榴石相当。矿物对氧同位素测温得到,石英—石榴石对温度为695±35℃,石英—金红石对为460±15℃,与根据金红石U—Pb内部等时线估计的Pb扩散封闭温度470±50℃一致。对比表明,O在石榴石中的扩散速率与Nd相当或略低,而O和Pb在金红石中的扩散速率相近,且均比Nd快。     

Sm-Nd isotope systematics in garnet from different lithologies (Eastern Alps): age results, and an evaluation of potential problems for garnet Sm-Nd chronometry [Chem. Geol. 185 (2002) 255-281]

Interpretation of Sm-Nd garnet ages is frequently impaired by one of the following restrictions: (a) high-LREE inclusions, (b) isotopic disequilibrium, and (c) the uncertainty about the closure temperature. These issues are addressed by way of an evaluation of garnet Sm-Nd data from different rock types of the Austroalpine basement units, Eastern Alps, including metabasic eclogites, mica schist and paragneiss, metapegmatite and metagranite.Nd concentration in handpicked garnet varies between 0.021 and 23.1 ppm in metabasites, 0.49 and 17.4 ppm in metapelites and between 0.024 and 4.6 ppm in metapegmatites and metagranites. The overall range of ¹⁴⁷Sm/¹⁴⁴Nd is 0.15-2.5 in garnet from metabasites, 0.12-3.03 in metapelite garnet and 0.66-7.21 in Mn-rich garnet from metapegmatites and metagranites. A clear negative correlation between Nd concentration and Sm/Nd is observed in garnets from all these lithologies. Therefrom, it is concluded that even optically “clean” garnet separates may contain high-LREE microinclusions, such as epidote-allanite, zoisite, apatite, sphene, monazite or zircon. However, very low Nd concentrations correlated with low Sm/Nd as well as high Nd concentrations (>5 ppm) correlated with fairly high Sm/Nd ratios (0.8) have also been observed. Apart from replicate analyses within as well as between samples with a common PT-history, leaching experiments are a useful technique to elucidate any distorting influence of unequilibrated inclusions on the garnet age, especially if the observed Sm/Nd ratio is low (<0.5). Leaching of garnet separates with HCl (2.5, 5.8 M) produces no obvious element fractionation, but may improve Sm/Nd, and hence age precision, considerably. Isotopic disequilibrium between garnet and other matrix minerals is observed preferentially in basic eclogites, derived from gabbroic precursors.Sm-Nd garnet analysis allows the recognition of several distinct garnet-forming events in the Eastern Alps.(a) A Variscan high-P event is documented in metabasites from the northern-central Ötztal basement around 360-350 Ma, whereas garnet from sillimanite-bearing gneisses dates the Variscan thermal peak in the western part of the same subunit around 345-330 Ma.(b) A long-lived, Permian to Triassic event (285-225 Ma), correlated with crustal extension and low-P metamorphism, is documented by spessartine-rich garnet from metapegmatites as well as almandine-rich garnet cores from mica schist.(c) Age data of garnet from eo-Alpine (Cretaceous) deeply subducted rocks of the southern/eastern Austroalpine units are related to near-peak PT, eclogite- to amphibolite-facies metamorphic conditions (peak: 2 GPa/685 °C), and/or incipient isothermal decompression, due to fast, tectonically driven exhumation (110/100-85 Ma). At cooling rates of 20-30 °C/Ma (exhumation rates: 3-5 km/Ma), the Sm-Nd closure temperature (T_c) for mm-sized garnet in these rocks is estimated at 650-680 °C.     

Deep subduction of mantle-derived garnet peridotites from the Su-Lu UHP metamorphic terrane in China

Garnet peridotites from the southern Su‐Lu ultra‐high‐pressure metamorphic (UHPM) terrane, eastern China, contain porphyroblastic garnet with aligned inclusions comprising a low‐P–T mineral assemblage (chlorite, hornblende, Na‐gedrite, Na‐phlogopite, talc, spinel and pyrite). Orthopyroxene porphyroblasts show fine exsolution lamellae of clinopyroxene and minor chromite. A clinopyroxene inclusion in garnet shows some orthopyroxene exsolution lamellae. Both the rims of porphyroblastic pyroxene and garnet and the matrix pyroxene and garnet crystallized at the expense of olivine. This is interpreted as a result of metasomatism of the peridotites by an SiO₂‐rich melt at UHP conditions. A chromian garnet further overgrew on the rims of the garnet. The X_Mg values (Mg/(Mg+Fe)) of porphyroblastic garnet decrease from core to rim and vary in different peridotite samples, while the compositions of both the porphyroblastic and the matrix pyroxene are similar in terms of Ca–Mg–Fe. The Mg‐rich cores of porphyroblastic garnet and orthopyroxene record high temperatures and pressures (c. 1000 °C, ≥5.1 GPa), whereas the matrix minerals, including the rims of porphyroblasts, record much lower P–T (c. 4.2 GPa, c. 760 °C). Sm–Nd data give apparent isochron ages of c. 380 Ma and negative ε_Nd(0) values (c.?9). These dates are considered meaningless due to isotopic disequilibrium between garnet cores and the rest of the rocks. The isotopic disequilibrium was probably caused by metasomatism of the peridotites by melt/fluids derived from the coevally subducted crustal materials. On the other hand, the Rb–Sr isotopic systems of phlogopite and clinopyroxene appear to have reached equilibrium and record a cooling age of c. 205 Ma. It is suggested that the garnet peridotites were originally emplaced into a low‐P–T environment prior to the c. 220 Ma continental collision, during which they were subducted together with crustal rocks to mantle depth and subjected to UHP metamorphism. An important corollary is that at least some of the coevally subducted crustal rocks in the Su‐Lu terrane have been subjected to peak metamorphism at P–T conditions much higher than presently estimated (≥2.7 GPa, ≤800 °C).     

Sm-Nd isotopic evidence on the age of eclogitization in the Zermatt-Saas ophiolite

Rocks within the Zermatt-Saas ophiolite of the western Alps have undergone eclogite facies metamorphism during subduction prior to the Alpine collision. The metamorphic history of these rocks is well defined, with eclogitic assemblages being followed by the limited growth of blueschist assemblages of glaucophane and paragonite. Subsequent greenschist alteration occurs adjacent to faults, veins and metasediments. Away from such sources of water, retrogression is very limited. Sm-Nd isotopic analyses of an essentially unretrogressed eclogitic metabasalt suggest that eclogite facies metamorphism occurred at 52 ± 18 Ma. The large uncertainty is due to the presence of very small amounts of Nd-rich epidote present as inclusions within garnet. As the closure temperature of garnet to Sm & Nd is thought to be >600C, resetting due to post-high-pressure diffusion is thought to be insignificant. Given the fine-grained protolith to the sample analysed, and its extensive deformation under eclogite facies conditions, incomplete homogenization of pre-metamorphic isotopic variations is also considered unlikely to be responsible for the young age. A Tertiary age of eclogitization means that models of early Alpine evolution based on the cessation of high-pressure metamorphism in the Cretaceous need to be revised.     

Combined U-Pb dating and Sm-Nd studies on lower crustal and mantle xenoliths from the Delegate basaltic pipes, southeastern Australia

Mafic rocks dominate the lower crustal and upper mantle xenolith suites within the Jurassic Delegate basaltic diatremes in the Paleozoic Lachlan Fold Belt, SE Australia. Two upper mantle mafic xenoliths from the Delegate pipes, a garnet pyroxenite and a garnet granulite (equilibrated at 1060 and 1140 °C, and 40–50 km), yield garnet-clinopyroxene Sm-Nd ages of 160 ± 4 Ma and 153 ± 10 Ma, respectively. Both ages are indistinguishable from the time of eruption of the diatremes, and are interpreted as showing continuous isotopic equilibrium within the mantle of Sm and Nd between garnet + clinopyroxene at temperatures ≥ 1050 °C. A lower crustal, 2-pyroxene granulite xenolith (equilibrated at 810–850 °C and ca. 25 km) yields a clinopyroxene + plagioclase + whole rock Sm-Nd isochron ages of 283 ± 26 Ma. This age probably reflects partial resetting of the isotopic systems of much older granulite during slow cooling, or after a heating event in the lower crust associated with the Jurassic magmatic activity represented by the basaltic host rock. Metamorphic zircons from the 2-pyroxene granulite xenolith were dated by the U-Pb method at 398±2 and 391 ± 2 Ma. These ages are considered to date granulite facies metamorphic events in the lower crust of the region. The age gap between the granulite facies metamorphism and granitoid plutonism in the region (420–410 Ma) indicates that the dated granulite is unlikely to represent residue after partial melting and magma extraction that generated the regional granitoids. It is suggested that these ages may record a relatively slow cooling following the cessation of mafic magmatic intrusion that formed the xenolith protoliths and that was probably the heat source responsible for granite production. At about 25 km, this thermal relaxation accounts for the change from an olivine + plagioclase + 2-pyroxene gabbroic assemblage into the granulite facies 2-pyroxene + plagioclase + spinel field. Received: 17 May 1995 / Accepted: 24 March 1997     

阿尔泰造山带变质岩系时代问题的讨论

针对目前阿尔泰地区变质岩系时代划分中年龄数据应用的一些问题,提出如下认识:富蕴县城西的石榴石片麻岩中锆石UPb上交点年龄2349±226Ma(2σ),初步确证了该区古元古代大陆地壳岩石的存在;克木齐群和富蕴群变质岩全岩SmNd等时线年龄代表其母岩形成时代。变质岩系和显生宙花岗岩类的Nd模式年龄,以及各种类型岩石中长石Pb二阶段模式t1年龄仅指出存在前寒武纪大陆地壳的可能性,没有真正的年龄意义,不能作为划分地层时代的依据。阿尔泰造山带是否存在1400Ma和700～900Ma的变质岩系岩石,至今仍缺乏可靠年龄数据的佐证     

康定杂岩Rb-Sr、Sm-Nd同位素系统及其意义

通过对康定—冕宁地区出露的英云闪长岩、黑云角闪斜长片麻岩、角闪变粒岩全岩及其中所分离出的角闪石、黑云母、斜长石、钾长石的Rb-Sr、Sm-Nd同位素的系统测定,结合岩石的锆石U-Pb年龄结果,确定这些变质杂岩由于经历了复杂的形成过程与变质历史,Rb-Sr、Sm-Nd同位素体系难以确定其结晶年龄。由单矿物与全岩Rb-Sr、Sm-Nd体系拟合的~700 M a的等时线年龄反映了角闪岩相-高角闪岩相的变质作用年龄。Sm-Nd同位素体系由于在变质作用过程中的部分开放性,很容易给出无意义的较老的混合年龄。康定杂岩结晶后并没有经历麻粒岩相变质作用,区域上所含的麻粒岩透镜体可能是新元古代(773~721 M a)期间由Rod in ia超大陆裂解产生的新生洋壳向扬子克拉通陆块俯冲消减过程的变质产物。俯冲到一定深度后,由于板片被拉断,软流圈上涌导致变质洋壳板片岩石、先前底侵变质的镁铁质岩石及扬子陆块长英质基底岩石发生部分熔融,以镁铁质岩石熔融产生的熔浆为主(>70%),与长英质基底岩石熔融产生的熔浆混合形成w(Na2O)/w(K2O)>1的TTG组合。     

Duration of the formation and sources of the granitoids of the Litsk-Araguba Complex, Kola Peninsula

Postorogenic granitoids of the Litsk-Araguba Complex compose a chain of intrusive bodies around 850 km² in area, which are confined to the NE-trending deep-seated fault zone. Results of U-Pb zircon dating indicate that the formation of granitoids of the Litsk-Araguba Complex lasted 28 ± 9 Ma. Note that the rocks of the first-fourth phases have similar age within (1774–1762 Ma), while quartz syenites of the fifth phase were formed much later (1746 ± 8 Ma). The study of Sm-Nd isotopic system revealed that the quartz syenites plot in the field of the Nd isotopic evolution of the lower crust represented mainly by the Paleoproterozoic garnet granulites with model ages T_Nd(DM) = 2.4–2.7 Ga and ?_Nd(T) from ?5.6 to ?6.3. It was found that the near-contact syenites of the Litsk Massif contain composite zircons with an age of 1758 ± 9Ma. They differ from zircons in coeval porphyraceous granites in lowered U and Th concentrations, which are close to those in zircons from the lower crustal garnet granulites of this region. These data in combination with internal structure of the crystals determine xenogenic lower-crustal origin of zircons from syenites and confirm geochemical data on the lower crustal input in the formation of granitoid melts.     

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, Sr_i = 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.     

Constraints on the petrogenesis of Martian meteorites from the Rb-Sr and Sm-Nd isotopic systematics of the lherzolitic shergottites ALH77005 and LEW88516

Detailed Rb-Sr and Sm-Nd isotopic analyses have been completed on the lherzolitic shergottites ALH77005 and LEW88516. ALH77005 yields a Rb-Sr age of 185 ± 11 Ma and a Sm-Nd age of 173 ± 6 Ma, whereas the Rb-Sr and Sm-Nd ages of LEW88516 are 183 ± 10 and 166 ± 16 Ma, respectively. The initial Sr isotopic composition of ALH77005 is 0.71026 ± 4, and the initial ε_Nd value is +11.1 ± 0.2. These values are distinct from those of LEW88516, which has an initial Sr isotopic composition of 0.71052 ± 4 and an initial ε_Nd value of +8.2 ± 0.6. Several of the mineral and whole rock leachates lie off the Rb-Sr and Sm-Nd isochrons, indicating that the isotopic systematics of the meteorites have been disturbed. The Sm-Nd isotopic compositions of the leachates appear to be mixtures of primary igneous phosphates and an alteration component with a low ¹⁴³Nd/¹⁴⁴Nd ratio that was probably added to the meteorites on Mars. Tie lines between leachate-residue pairs from LEW88516 mineral fractions and whole rocks have nearly identical slopes that correspond to Rb-Sr ages of 90 ± 1 Ma. This age may record a major shock event that fractionated Rb/Sr from lattice sites located on mineral grain boundaries. On the other hand, the leachates could contain secondary alteration products, and the parallel slopes of the tie lines could be coincidental.Nearly identical mineral modes, compositions, and ages suggest that these meteorites are very closely related. Nevertheless, their initial Sr and Nd isotopic compositions differ outside analytical uncertainty, requiring derivation from unique sources. Assimilation-fractional-crystallization models indicate that these two lherzolitic meteorites can only be related to a common parental magma, if the assimilant has a Sr/Nd ratio near 1 and a radiogenic Sr isotopic composition. Further constraints placed on the evolved component by the geochemical and isotopic systematics of the shergottite meteorite suite suggest that it (a) formed at ∼4.5 Ga, (b) has a high La/Yb ratio, (c) is an oxidant, and (d) is basaltic in composition or is strongly enriched in incompatible elements. The composition and isotopic systematics of the evolved component are unlike any evolved lunar or terrestrial igneous rocks. Its unusual geochemical and isotopic characteristics could reflect hydrous alteration of an evolved Martian crustal component or hydrous metasomatism within the Martian mantle.     

Origin of Mg-Metatholeiites of the Schirmacher Region,East Antarctica: Constraints from Trace Elements and Nd-Sr Isotopic Systematics

The mafic granulites of Schirmacher region, East Antarctica, the rocks under study, occur more or less as concordant sills or as lenses or as boudinage structures within the felsic rocks, charnockites or metapelites of the region. They show variation from garnet bearing two-pyroxene granulites and garnet free pyroxene granulites to transitional amphibolite-pyroxene granulites. Their major, trace, REE and isotopic chemistry are not distinct from each other and they represent Mg-basalts with MgO >7% and Al₂O₃ <16%.The majority of the analyzed samples plot in the tholeiitic field or show tholeiitic trends, suggesting their metatholeiitic nature as well as general preservation of original composition. The rocks are characterized by enriched large-ion lithophile elemental concentrations than that of mid-oceanic ridge basalts. Their high-field strength elements and heavy rare-earth elemental concentrations, however, are as that of mid-oceanic ridge basalts, a feature which is also reflected in the ratios of their large-ion lithophile elements against high-field strength elements and heavy rare-earth elements, wherein we find these ratios are higher than N-type MORB. Further, the rocks show negative Nb anomaly, high Th/Ta ratio and low La/Nb ratio, which are also characteristics of subduction-related magmatism.The isotopic studies carried out on these samples show that, the Sm-Nd and Rb-Sr dating did not yield much spread, but suggested a Sm-Nd metamorphic age of ∼960 Ma. Rb-Sr dating gave ages ∼886 Ma, suggesting the reworking of the Rb and Sr elements during subsequent tectonothermal overprinting. The Nd model ages (T_DM^Nd) of these rocks show a relatively restricted range of 1120 to 1357 Ma, suggesting mafic magmatism ∼1200 Ma. The positive eNd values (+4.22 to +6.07) shown by these rocks, represent a juvenile crustal fragment derived from melting of mantle precursors, without significant reworking of older crustal material. It is proposed that these rocks were produced by partial melting of a mantle source, characterized by LILE enrichment, related principally to dehydration of subducted oceanic crust.     

Retention of Sm–Nd isotopic ages in garnets subjected to high-grade thermal reworking: implications for diffusion rates of major and rare earth elements and the Sm–Nd closure temperature in garnet

Garnet is a vital mineral for determining constrained P–T–t paths as it can give both the P–T and t information directly. However, estimates of the closure temperature of the Sm–Nd system in garnet vary considerably leading to significant uncertainties in the timing of peak conditions. In this study, five igneous garnets from an early Proterozoic 2414 ± 6 Ma garnet—cordierite bearing s-type granite—which was subjected to high-T reworking have been dated to examine their diffusional behaviour in the Sm–Nd system. Garnets 8, 7, 6 and 2.5 mm in diameter were compositionally profiled and then dated, producing two-point Sm–Nd isochron ages of 2412 ± 10, 2377 ± 5, 2370 ± 5 and 2365 ± 8 and 2313 ± 11 Ma, respectively. A direct correlation exists between grain size and amount of resetting highlighting the effect of grain size on closure temperature. Major element EMPA and LA-ICPMS REE traverses reveal homogenous major element profiles and relict igneous REE profiles. The retention of REE zoning and homogenisation of major element zoning suggest that diffusion rates of REEs are considerably slower than that of the major cations. The retention of REE zoning and the lack of resetting in the largest grains suggest that Sm–Nd closure temperature in garnet is a function of grain size, thermal history and REE zoning in garnet.     

华中榴辉岩带地球化学和年代学研究

根据地质产状，华中高压超高压变质带中的榴辉岩可划分为与超基性岩伴生的P类和与片麻岩、斜长角闪岩等伴生的G类。它们具有不同的地球化学特征，但均为大洋基性火成岩经复杂成分演化作用的产物。全岩-矿物Sm－Nd和颗粒锆石207Pb-206Pb年代学研究表明，超高压变质作用很可能发生在加里东期（480Ma）；而印支期（265Ma）则可能是超高压变质岩的后期高压退变质改造阶段。     

东昆仑东端扎那合惹地区元古宙蛇绿岩

扎那合惹地区元古宙蛇绿岩由超镁铁质堆晶岩组成，堆晶岩岩石化学成分、矿物组成变化较大。痕量元素、稀土元素表现出富集不相容元素的特征，而Nd同位素地球化学特征显示其源于亏损的地幔源区。超镁铁质堆晶岩Sm-Nd等时线年龄为1480±3Ma，与之紧密伴生的榴闪岩40Ar／39Ar坪年龄为（881.8±4.7）Ma（石榴石）和（852.2±1.0）Ma（角闪石），分别代表了古昆中洋扩张及俯冲消减的时限。     

Sr and Nd isotope geochronology,geologic history,and origin of the Adirondack Anorthosite

We have analyzed samples from the Adirondack Marcy massif for Rb-Sr and Sm-Nd isotopes in an attempt to determine directly the primary crystallization age of a Proterozoic massif-type anorthosite rock suite. The oldest age obtained (1288 ± 36Ma) is from a 4 point Sm-Nd isochron defined by igneous-textured whole-rock and mineral separate data from a local layered sequence gradational from oxiderich pyroxenite to leuconorite. This age is older than Silver's (1969) 1113 Ma zircon age of associated charnockites, but is within the window of permissible anorthosite ages based on previous geochronology and field relationships. As such, 1288 Ma may represent the time of crystallization of the massif. For the most part, however, both Sm-Nd and Rb-Sr isotopic systems did not survive granulite facies metamorphism. Internal isochrons based on whole rocks and minerals yield ages between 995 and 919 Ma. These isotopic data suggest that the granulite fades metamorphism experienced by the massif was a prograde event that occurred a minimum of 100 Ma and as much as 350 Ma after crystallization of the massif. The relatively large range in Rb abundance, and in calculated initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ (0.7039–0.7050) and ${}^{143}\text{Nd}{}^{144}\text{Nd}$ ratios among anorthosite suite rocks, particularly those at or near the contacts of the Marcy massif is explicable by variable contamination with “crustal” materials and/or fluids, derived from surrounding acidic metaplutonic rocks, paragneisses, and marbles. Despite uncertainies caused by crustal contamination and metamorphic resetting of primary ages, Marcy samples have epsilon Nd values between +0.44 and +5.08, implying a source for the massif with long-term depletion in light rare earth elements. A probable source material would be depleted mantle.     

Isotopic disequilibrium in ultrahigh-pressure and retrograde metamorphism of eclogite and gneiss from the Chinese Continental Scientific Drilling in the Sulu orogen, China: evidence from mineral Nd–Sr–O isotopic composition

Diffusion rates of Sr and O in minerals are often comparable while Nd has a lower diffusion rate during thermal overprint(s); thus, the O isotope systems between metamorphic minerals can serve as an indicator to evaluate whether equilibrium of Rb–Sr and Sm–Nd systems has been preserved in the metamorphic minerals that experienced retrograde metamorphism. This study presents a combination of investigation on Sm–Nd, Rb–Sr, and O isotopic compositions of minerals separated from ultrahigh-pressure eclogite and gneiss that were collected from the main hole of the Chinese Continental Scientific Drilling project located in the Sulu orogen, eastern China. Oxygen isotopic compositions of minerals from gneiss and eclogite yield two temperature groups of 620–740 and 460–590°C, representing diffusion cessation of isotopic exchange during the eclogite-facies recrystallization and later amphibolite-facies retrograde overprint. Rb–Sr mineral regressions of two eclogite samples give consistent Triassic ages of 244 Ma, corresponding to eclogite-facies metamorphism, while the same minerals do not yield meaningful Sm–Nd isochron ages. This phenomenon likely suggests that Rb–Sr isotopic equilibrium was achieved during eclogite-facies metamorphism and preserved during late amphibolite-facies retrogression. In contrast, Sm–Nd isotopic equilibrium between the minerals of eclogite was not achieved under UHP metamorphic conditions. Regressions of epidote and biotite of one gneiss sample give a Triassic Sm–Nd age of 243 ± 34 Ma, corresponding to the time of the eclogite-facies metamorphism, and a Jurassic mineral Rb–Sr age of 187.5 ± 1.8 Ma. These results imply that fluids have played an important role to achievement of the Sm–Nd isotopic equilibrium during eclogite-facies metamorphism and re-equilibration of the Rb–Sr isotopic system during later retrograde overprint.     

Oxygen isotope equilibrium between eclogite minerals and its constraints on mineral Sm-Nd chronometer

Sm-Nd and oxygen isotope analyses were carried out for mineral separates of ultrahigh pressure eclogites from the Sulu terrane in eastern China. The results show a direct correspondence in equilibrium or disequilibrium state between the oxygen and Sm-Nd isotope systems of eclogite minerals. The omphacite-garnet pairs of oxygen isotope equilibrium at eclogite-facies conditions yield meaningful Triassic Sm-Nd isochron ages, whereas those of oxygen isotope disequilibrium give non-Triassic ages of geological meaninglessness. This can be reasonably interpreted by the fact that the rates of oxygen diffusion in garnet and pyroxene are lower than, or close to, those of Nd diffusion, and thus attainment of isotopic equilibrium in the omphacite-garnet O system suggests achievement of Nd isotope equilibrium in the same mineral pairs. The presence or absence of fluid in the eclogite protoliths is a major rate-controlling factor for isotopic equilibration during high-grade metamorphism. It appears that the state of oxygen isotope equilibrium between cogenetic minerals can provide a critical test for the validity of the Sm-Nd mineral chronometer. In addition, the exact timing of the ultrahigh pressure metamorphism in the Dabie-Sulu terranes is constrained at Early Triassic rather than Late Triassic.     

Geochronology of high-pressure mafic granulite dykes in SW Sweden: tracking the P–T–t path of metamorphism using Hf isotopes in zircon and baddeleyite

Although the U–Pb zircon chronometer has been widely used for dating metamorphism in moderate‐ to high‐grade rocks, it is generally difficult to link the U–Pb age of zircon to specific metamorphic reactions. In this study, the initial Hf isotopic composition of secondary zircon is compared with the evolution of Hf isotopic composition of the bulk sample, back‐projected from the measured value through time. This approach may enhance the interpretation of radiometric ages performed on metamorphic mineral assemblages. Here, U–Pb, Sm–Nd and Lu–Hf geochronology and thermobarometry have been integrated and applied to two metamorphosed diabase dykes in the Sveconorwegian orogen, SW Sweden. The dykes are located ～5 km east of the NNE‐trending Göta Älv deformation zone in the Idefjorden terrane, and trend parallel to this zone. The Lunden dyke is recrystallized into a coronitic, granulite facies assemblage. U–Pb isotopic analyses of baddeleyite in this dyke indicate an emplacement age of c. 1300 Ma. Thermobarometric techniques applied to garnet and omphacitic clinopyroxene coronas indicate high‐pressure metamorphism at ～15 kbar and ～740 °C. The growth of polycrystalline zircon at the expense of baddeleyite occurred at 1046 ± 6 Ma. The identical Hf isotopic composition of polycrystalline zircon and baddeleyite shows that the baddeleyite‐to‐zircon transition took place before Hf equilibration among the other metamorphic minerals and, hence the c. 1046 Ma age of polycrystalline zircon sets an upper age limit of metamorphism of this sample. The Haregården dyke is recrystallized into a granoblastic transitional upper amphibolite to granulite facies assemblage. The estimated P–T conditions are ～10 kbar and ～700 °C. Analyses of small (～30 μm), clear and round zircon in this sample yield a Concordia U–Pb age of 1026 ± 4 Ma, which is indistinguishable from the Lu‐Hf and Sm‐Nd mineral isochron ages of 1027 ± 9 and 1022 ± 34 Ma, respectively. This type of secondary zircon plots at the lower end of the Lu‐Hf isochron and indicates simultaneous growth with garnet at c. 1026 Ma, a time when Hf isotopic equilibrium among minerals must have been reached.