Partial melting of deeply subducted continental crust during exhumation: insights from felsic veins and host UHP metamorphic rocks in North Qaidam,northern Tibet

A combined petrological, geochronological and geochemical study was carried out on felsic veins and their host rocks from the North Qaidam ultrahigh‐pressure (UHP) metamorphic terrane in northern Tibet. The results provide insights into partial melting of deeply subducted continental crust during exhumation. Partial melting is petrograpically recognized in metagranite, metapelite and metabasite. Migmatized gneisses, including metagranite and metapelite, contain microstructures such as granitic aggregates with varying outlines, small dihedral angles at mineral junctions and feldspar with magmatic habits, indicating the former presence of felsic melts. Partial melts were also present in metabasite that occurs as retrograde eclogite. Felsic veins in both the eclogites and gneisses exhibit typical melt crystalline textures such as large euhedral feldspar grains with straight crystal faces, indicating vein crystallization from anatectic melts. The Sr–Nd isotope compositions of felsic veins inside gneisses suggest melt derivation from anatexis of host gneisses themselves, but those inside metabasites suggest melt derivation from hybrid sources. Felsic veins inside gneisses exhibit lithochemical compositions similar to experimental melts on the An–Ab–Or diagram. In trace element distribution diagrams, they exhibit parallel patterns to their host rocks, but with lower element contents and slightly positive Eu and Sr anomalies. The geochemistry of these felsic veins is controlled by minerals that would decompose and survive, respectively, during anatexis. Felsic veins inside metabasites are rich either in quartz or in plagioclase with low normative orthoclase. In either case, they have low trace element contents, with significantly positive Eu and Sr anomalies in plagioclase‐rich veins. Combined with cumulate structures in some veins, these felsic veins are interpreted to crystallize from anatectic melts of different origins with the effect of crystal fractionation. Nevertheless, felsic veins in different lithologies exhibit roughly consistent patterns of trace element distribution, with variable enrichment of LILE and LREE but depletion of HFSE and HREE. There are also higher contents of trace elements in veins hosted by gneisses than veins hosted by metabasites. Anatectic zircon domains from felsic veins and migmatized gneisses exhibit consistent U–Pb ages of c. 420 Ma, significantly younger than the peak UHP eclogite facies metamorphic event at c. 450–435 Ma. Combining the petrological observations with local P–T paths and experimentally constrained melting curves, it is inferred that anatexis of UHP gneisses was caused by muscovite breakdown while anatexis of UHP metabasites was caused by fluid influx. These UHP metagranite, metapelite and metabasite underwent simultaneous anatexis during the exhumation, giving rise to anatectic melts with different compositions in various elements but similar patterns in trace element distribution.     

Polygenetic titanite records the composition of metamorphic fluids during the exhumation of ultrahigh‐pressure metagranite in the Sulu orogen

An integrated study of U–Pb ages and trace elements was carried out for titanite and zircon from ultrahigh‐pressure (UHP) metagranites in the Sulu orogen, east‐central China. The results provide constraints on the composition of metamorphic fluids during the exhumation of deeply subducted continental crust. Titanite has two domain types based on REE patterns and trace element variations, Ttn‐I and Ttn‐II respectively. These two domains show indistinguishable U–Pb ages of 232 ± 14 to 220 ± 8 Ma, in general agreement with anatectic zircon U–Pb ages of 223 ± 4 to 219 ± 2 Ma for the partial melting event during early exhumation. The Ttn‐I domains have significantly higher REE, Th, Ta and Sr, and higher Th/U ratios than the Ttn‐II domains, indicating that the two domains have grown from metamorphic fluids with different compositions. For the Ttn‐I domains, Zr‐in‐titanite thermometry yields high temperatures of 773–851 °C at 2.5 GPa, and petrographic observations reveal the presence of melt pseudomorphs. Thus, they are interpreted to have grown from hydrous melts in the early exhumation stage. In contrast, the Ttn‐II domains were texturally equilibrated with amphibolite facies minerals such as biotite and plagioclase and contain inclusions of plagioclase and quartz. The Zr‐in‐titanite thermometry yields lower temperatures of 627–685 °C at 1.0 GPa. In combination with their REE patterns, they are interpreted to have grown from aqueous solutions at amphibolite facies metamorphic conditions during further exhumation. The differences in Th and Sr contents are prominent between the Ttn‐I and Ttn‐II domains, signifying the compositional difference between the hydrous melts and aqueous solutions. Therefore, the polygenetic titanite in the UHP metamorphic rocks provides insights into the geochemical property of metamorphic fluids during the continental subduction‐zone processes.     

Migmatites record multiple episodes of crustal anatexis and geochemical differentiation in the Sulu ultrahigh‐pressure metamorphic zone,eastern China

Partial melting of ultrahigh‐pressure (UHP) metamorphic rocks is common during collisional orogenesis and post‐collisional reworking, indicating that determining the timing and processes involved in this partial melting can provide insights into the tectonic evolution of collisional orogens. This study presents the results of a combined whole‐rock geochemical and zirconological study of migmatites from the Sulu orogen in eastern China. These data provide evidence of multiple episodes of crustal anatexis and geochemical differentiation within the UHP metamorphic rocks. The leucosomes contain higher concentrations of Ba and K and lower concentrations of the rare earth elements (REE), Th and Y, than associated melanosomes and granitic gneisses. The leucosomes also have homogenous Sr–Nd–O isotopic compositions that are similar to proximal (i.e. within the same outcrop) melanosomes, suggesting that the anatectic melts were generated by the partial melting of source rocks that are located within individual outcrops. The migmatites contain zircons with six different types of domains that can be categorized using differences in structures, trace element compositions, and U–Pb ages. Group I domains are relict magmatic zircons that yield middle Neoproterozoic U–Pb ages and contain high REE concentrations. Group II domains represent newly grown metamorphic zircons that formed at 230 ± 1 Ma during the collisional orogenesis. Groups III, IV, V, and VI zircons are newly grown anatectic zircons that formed at 222 ± 2 Ma, 215 ± 1 Ma, 177 ± 2 Ma, and 152 ± 2 Ma, respectively. The metamorphic zircons have higher Th/U and lower (Yb/Gd)_N values, flat heavy REE (HREE) patterns with no significantly negative Eu anomalies relative to the anatectic zircons, which are characterized by low Th/U ratios, steep HREE patterns, and negative Eu anomalies. The first two episodes of crustal anatexis occurred during the Late Triassic at c. 222 Ma and c. 215 Ma as a result of phengite breakdown. The other two episodes of anatexis occurred during the Jurassic period at c. 177 Ma and c. 152 Ma and were associated with extensional collapse of the collision‐thickened orogen. The majority of Triassic anatectic zircons and all of the Jurassic zircons are located within the leucosomes, whereas the melanosomes are dominated by Triassic metamorphic zircons, suggesting that the leucosomes within the migmatites record more episodes of crustal anatexis. Both metamorphic and anatectic zircons have elevated ε_Hf(t) values compared with relict magmatic zircon cores, suggesting that these zircons contain non‐zircon Hf derived from material with more radiogenic Hf isotope compositions. Therefore, the Sulu and Dabie orogens experienced different episodes of reworking during the exhumation and post‐collisional stages.     

Source of coesite inclusions within inherited magmatic zircon from Sulu UHP rocks, eastern China, and their bearing for fluid–rock interaction and SHRIMP dating

The 5-km deep Chinese Continental Scientific Drilling Main Hole penetrated a sequence of ultrahigh pressure (UHP)-metamorphic rocks consisting mainly of eclogite, gneiss and garnet-peridotite with minor schist and quartzite. Zircon separates taken from thin layers of schist and gneiss within eclogite were investigated. Cathodoluminescence images of zircon grains show that they have oscillatory zoned magmatic cores and unzoned to patchy zoned metamorphic rims. Zircon rims contain rare coesite and calcite inclusions whereas cores contain inclusions of both low- P minerals (e.g. feldspar, biotite and quartz) and coesite and other eclogite-facies minerals such as phengite and jadeite. The zircon cores give highly variable ²⁰⁶Pb/²³⁸U ages ranging from 760 to 431 Ma for schist and from 698 to 285 Ma for gneiss, and relatively high but variable Th/U ratios (0.16–1.91). We suggest that the coesite and other eclogite facies mineral inclusions in zircon cores were not magmatic but formed through metasomatic processes caused by fluids during UHP metamorphism, and that the fluids contain components of SiO₂, Al₂O₃, K₂O, FeO, MgO, Na₂O and H₂O. Metasomatism of the Sulu UHP rocks during continental subduction to mantle depths has partly altered magmatic zircon cores and reset isotopic systems. This study provides key evidence that mineral inclusions within magmatic zircon domains are not unequivocal indicators of the formation conditions of the respective domain. This finding leads us to conclude that the routine procedure for dating of metamorphic events solely based on the occurrence of mineral inclusions in zoned zircon could be misleading and the data should be treated with caution.     

高压-超高压变质岩石中不同成因的石榴石

石榴石是高压-超高压变质岩石中最重要的变质矿物之一,是研究俯冲带深部变质和熔融过程的理想研究对象.通过对俯冲带内不同条件下形成的石榴石进行详细研究,确定了岩浆成因、变质成因和转熔成因石榴石.岩浆石榴石是岩浆熔体在冷却过程中结晶形成,成分主要为锰铝榴石-铁铝榴石,通常含有石英、长石、磷灰石等晶体包裹体.变质石榴石是在亚固相条件下通过变质反应形成,包裹体为参与变质反应的矿物组合;进变质生长的石榴石通常显示核部到边部锰铝榴石降低的特征.转熔石榴石是在超固相条件下通过转熔反应形成,通常含有晶体包裹体,其中既有从转熔熔体结晶的矿物包裹体,也有转熔反应残留的矿物包裹体.对超高压变质岩石中转熔石榴石的识别,可以为深俯冲陆壳岩石的部分熔融提供重要的岩石学证据,是大陆俯冲带部分熔融研究的重要进展之一.      

A record of ultrahigh temperature metamorphism in the Dabie orogen during Triassic continental collision

Ultrahigh temperature (UHT) metamorphism is traditionally recognized by the development of characteristic mineral associations in Mg–Al-rich metapelitic rocks. However, recognition of UHT metamorphism in non-supracrustal rocks is more difficult. UHT metamorphic conditions are recorded by a migmatite from the North Dabie Terrane (NDT) of the Dabie orogen, east China. The migmatite is composed of intercalated layers of melanosome and K-feldspar-rich leucosome. Zircon grains in the migmatite have a core–rim structure comprising a metamorphic core and an anatectic rim. The metamorphic cores have low U contents (mainly <657 ppm) and low Th/U ratios (<0.2), and are depleted in heavy rare earth element (HREE). The metamorphic domains yield concordant ²⁰⁶Pb/²³⁸U ages ranging from 205.1 ± 4.8 Ma to 248.0 ± 4.1 Ma with a weighted mean of 217.7 ± 4.3 Ma (n = 20, MSWD = 4.2). They contain a granulite-facies inclusion assemblage of garnet + clinopyroxene + plagioclase + quartz + rutile. Conventional geobarometry and Ti-in-zircon thermometry constrain P–T conditions to approximately 11–12 kbar and 900–950 °C, suggesting UHT metamorphism. The discovery of Triassic UHT metamorphism in the Dabie orogen, which was previously best known for ultrahigh pressure metamorphism, provides new insights into the thermal structure and geodynamics of the orogeny during continental collision. The anatectic rims of zircon grains have relatively high U contents and low Th/U ratios (<0.14), and are enriched in HREE. They yield concordant ²⁰⁶Pb/²³⁸U ages of 133.6 ± 1.1 Ma to 156.4 ± 2.2 Ma, indicating that anatexis occurred during post-collisional collapse of the Dabie orogen.     

Multiple partial melting events in the Sulu UHP terrane: zircon U–Pb dating of granitic leucosomes within amphibolite and gneiss

Thin layers and lenses of granitic leucosome are widely distributed within amphibolites, paragneisses and orthogneisses of the Sulu UHP terrane. They are parallel to, or cross‐cut, foliations in the host rocks at different scales and show evidence of coalescence and migration to form centimetre‐ to decimetre‐scale segregations. Variously migmatized rocks extend at least 350 km from SW Sulu (Maobei) to NE Sulu (Weihai), in a band at least 50 km wide. A combined study of mineral inclusions, cathoduluminescence (CL) images, U–Pb LA‐ICP‐MS dates, and in‐situ trace element compositions of zircon provide clear evidence on the nature and timing of partial melting in these UHP rocks. Most zircon from the granitic leucosomes occurs as distinct overgrowths around inherited (igneous or metamorphic) cores or as new, euhedral crystals. The overgrowths and new crystals commonly show perfectly euhedral shapes, have pronounced oscillatory zoning and contain felsic mineral inclusions, such as Kfs + Pl + Qtz ± Ilm ± monazite (Mon). In contrast, the inherited igneous or metamorphic cores are rounded or irregular, contain low‐P or UHP mineral inclusions and show clear dissolution textures. These data suggest that the new zircon is anatectic in origin and that it grew during partial melting of the UHP rocks. The REE patterns of the anatectic zircon show steep slopes from the HREE to LREE with strongly to moderately negative Eu anomalies (Eu/Eu* = 0.31–0.72) and pronounced positive Ce anomalies (Ce/Ce* = 6.8–26.5). Abundant U–Pb spot analyses of the anatectic zircon reveal two discrete and meaningful ages of partial melting within the Sulu UHP terrane. Anatectic zircon from 12 granitic leucosomes within amphibolites, paragneisses, and orthogneisses from Sulu UHP slices II and III yields consistent mean U–Pb ages of 219.0 ± 1.2 to 218.3 ± 1.6 Ma, 218.8 ± 2.0 to 217.3 ± 1.7 Ma and 218.2 ± 1.4 to 215.0 ± 1.5 Ma, respectively. In contrast, anatectic zircon from six granitic leucosomes within paragneisses and orthogneisses from Sulu UHP slice III records younger mean U–Pb ages of 151.9 ± 1.3 to 151.1 ± 1.8 Ma and 155.9 ± 1.8 to 153.7 ± 1.7 Ma, respectively. These data imply that the Sulu UHP terrane experienced two Mesozoic partial melting events. The first partial melting event (219–215 Ma) was probably associated with a Late Triassic granulite facies stage of ‘hot’ exhumation, whereas the second (156–151 Ma) is interpreted as the result of Middle‐Late Jurassic extension and thinning of the previously thickened crust of the Sulu UHP terrane. Both partial melting events induced extensive retrograde metamorphism of the eclogites and their country rocks.     

U–Pb ages and trace elements in metamorphic zircon and titanite from UHP eclogite in the Dabie orogen: constraints on P–T–t path

Laser ablation inductively coupled plasma mass spectrometry analyses of U–Pb isotopes and trace elements in zircon and titanite were carried out on epoxy mounts and thin sections for ultrahigh‐pressure (UHP) eclogite in association with paragneiss in the Dabie orogen. The results provide a direct link between metamorphic ages and temperatures during continental subduction‐zone metamorphism. Zircon U–Pb dating gives two groups of concordant ages at 242 ± 2 to 239 ± 5 Ma and 226 ± 2 to 224 ± 6 Ma, respectively. The Triassic zircon U–Pb ages are characterized by flat heavy rare earth element (HREE) patterns typical of metamorphic growth. Ti‐in‐zircon thermometry for the two generations of metamorphic zircon yields temperatures of 697 ± 27 to 721 ± 8 °C and 742 ± 19 to 778 ± 34 °C, respectively. We interpret that the first episode of zircon growth took place during subduction prior to the onset of UHP metamorphism, whereas the second episode in the stage of exhumation from UHP to HP eclogite facies regime. Thus, the continental subduction‐zone metamorphism of sedimentary protolith is temporally associated with two episodes of fluid activity, respectively, predating and postdating the UHP metamorphic phase. The significantly high Ti‐in‐zircon temperatures for the younger zircon at lower pressures indicate the initial ‘hot’ exhumation after the peak UHP metamorphism. There are two types of titanite. One exhibits light rare earth element (LREE) enrichment, steep MREE–HREE patterns and no Eu anomalies, and yields Zr‐in‐titanite temperatures of 551 to 605 °C at 0.5 GPa, and the other shows LREE depletion and flat MREE–HREE patterns, and gives Zr‐in‐titanite temperatures of 782–788 °C at 2.0 GPa. The former is amenable for U–Pb dating, yielding a discordia lower intercept age of 252 ± 3 Ma. Thus, the first type of titanite is interpreted to have grown in the absence of garnet and plagioclase and thus in the early stage of subduction. In contrast, the second one occurs as rims surrounding rutile cores and thus grew in the presence of garnet during the ‘hot’ exhumation. Therefore, there is multistage growth of zircon and titanite during the continental subduction‐zone metamorphism. The combined studies of chronometry and thermobarometry provide tight constraints on the P–T–t path of eclogites during the continental collision. It appears that the mid‐T/UHP eclogite facies zone would not only form by subduction of the continental crust in a P–T path slightly below the wet granite solidus, but also experience decompression heating during the initial exhumation.     

Aureoles of Pb(II)-enriched feldspar around monazite in paragneiss and anatectic pods of the Napier Complex,Enderby Land,East Antarctica: the roles of dissolution-reprecipitation and diffusion

Extraordinarily high Pb content in K-feldspar and plagioclase has been found contiguous to monazite in two occurrences in the ultrahigh-temperature Napier Complex of Antarctica. Monazite shows a variety of textures and compositions. In a garnet-sillimanite-orthopyroxene paragneiss at Mount Pardoe (Amundsen Bay), grains range 80–150 μm across and are anhedral; two grains are Th- and Si-dominant. In pods that crystallized from anatectic melts at 2500 Ma at Zircon Point, Casey Bay, monazite grains range 0.05 mm–1 cm in length and are highly variable in texture. The coarsest grains (>0.7 cm) are skeletal and euhedral, whereas the smallest grains are anhedral and associated with fine- to medium-grained quartz, K-feldspar, plagioclase, garnet, sillimanite and rutile in aggregates that form interstitial veinlets interpreted to be a second generation of anatexis during an event at 1100 Ma. The huttonite component (ThSiO₄) reaches 30 mole% in the cores of the coarsest skeletal grains, whereas other grains, particularly smaller ones, show complex and irregular zoning in Th and U. The latter zoning is attributed to dissolution-reprecipitation, which also resulted in complete Pb loss during the 1100 Ma event. In the paragneiss at Mount Pardoe, K-feldspar and myrmekitic plagioclase (An16) are found in a 70–80 μm band between monazite and orthopyroxene and contain up to 12.7 wt.% and 2.7 wt.% PbO, respectively, corresponding to 18.5% and 3.4% PbAl₂Si₂O₈ component, respectively. Cathodoluminescence of both feldspars increases with distance from a nearby monazite grain and is not correlated with Pb content. Incorporation of Pb in K-feldspar and plagioclase could be a result of diffusion, even though the monazite adjacent to feldspar apparently lost little Pb, i.e., Pb could have been transported by fluid from the Th-rich grains, which did lose Pb. In contrast to the paragneiss, cathodoluminescence correlates with Pb content of K-feldspar in aureoles surrounding skeletal monazite grains 0.7–1 cm across in anatectic pods at Zircon Point. Pb content of K-feldspar decreases monotonically to near detection limits within several millimetres of monazite grains; the greatest PbO concentration is attained in K-feldspar inliers and embayments in monazite, 8.8 wt.%, corresponding to 11.7% PbAl₂Si₂O₈ component. Fine-grained quartz in the K-feldspar suggests that the mechanism for Pb incorporation involved breakdown of feldspar: Pb²⁺ + 2(K,Na)AlSi₃O₈ → PbAl₂Si₂O₈ + 4SiO₂ + 2(K,Na)⁺ . The smooth decrease of Pb in the aureoles is not characteristic of dissolution-reprecipitation, which is characterized by abrupt changes of composition, and it seems more likely that Pb was incorporated in K-feldspar by diffusion at 1100 Ma. We suggest a model whereby fluid introduced during the 1100 Ma event flowed along grain boundaries and penetrated mineral grains. Temperatures were sufficiently high, i.e., 700°C, assuming burial in the mid-crust, for the fluid to induce localized melting of quartzofeldspathic matrix of the anatectic pods. Loss of radiogenic Pb was complete. Some penetration of K-feldspar by aqueous fluid is suggested by the presence of scattered galena specks and by rays of turbidity emanating from monazite. Aqueous fluid or water-rich granitic melt may have mediated the diffusion of Pb in feldspar, but it did not cause dissolution-reprecipitation. Although Pb was mobilized by aqueous fluid or water-rich granitic melt, it was not entirely flushed from the immediate vicinity of the monazite, but nearly half was incorporated in adjacent feldspar. Fluid activity that could cause Pb loss in monazite does not always leave an obvious trace, i.e., hydrous minerals, such as sericite, are very sparse, and biotite is absent in the anatectic pods at Zircon Point. Nonetheless, electron microprobe dating of monazite from the pods could not detect the 2500 Ma age of original crystallization determined by isotopic dating.     

Trace elements in zircon and coexisting minerals from low-T/UHP metagranite in the Dabie orogen: Implications for action of supercritical fluid during continental subduction-zone metamorphism

Simultaneously in-situ analyses of U–Pb isotopes and trace elements were carried out for zircons, in combination with the in-situ analyses of trace elements in coexisting minerals, from low-T/UHP metagranite in the Dabie orogen. The results provide geochemical evidence for the existence of supercritical fluid during continental subduction-zone metamorphism. The zircons are categorized into three types based on their patterns of REE distribution. Type I zircons show increasing enrichment from La to Lu, with prominent positive Ce anomalies and negative Eu anomalies, which are typical of magmatic zircon. Some of them display regular or blurred oscillatory-zoned texture and apparent ²⁰⁶Pb/²³⁸U ages of 341 to 780 Ma, suggesting metamorphic modification by solid-state recrystallization with no significant involvement of metamorphic fluid. Type II zircons share similar Th, U and HFSE contents and REE patterns to Type I zircons. However, they exhibit blurred oscillatory-zoned texture or are unzoned, have apparent ²⁰⁶Pb/²³⁸U ages of 348 to 709 Ma, and are LREE-enriched relative to Type I zircons. This suggests that they underwent metamorphic reworking by replacement recrystallization in the presence of metamorphic fluid. The LREE enrichment is due to the presence of microscale LREE-bearing mineral inclusions (such as apatite, monazite or epidote) in the zircons. Type III zircons, representing the majority of the present analyses, are characterized by spongy texture and consistent enrichment of LREE, HREE, Th, U and HFSE relative to Type I zircons. They yield nearly concordant U–Pb ages close to the discordia lower-intercept. The consistent enrichment of trace elements relative to the magmatic zircon indicates involvement of a special UHP metamorphic fluid that has a strong capacity to extract significant amounts of LREE, HREE, Th, U and HFSE from such accessory minerals as allanite, garnet, rutile and zircon. Because these minerals are stable in the field of hydrous melt in granite–water systems, they are not able to be decomposed during the exhumation of deeply subducted continental crust. Thus, a supercritical fluid is suggested to transport the LREE, HREE, Th, U and HFSE in the accessory minerals to recrystallized zircons. The mechanism of dissolution recrystallization is responsible for the spongy texture and the very high concentration of trace elements in this type of metamorphic zircons. Therefore, the action of supercritical fluid is evident under the low-T/UHP metamorphic conditions.     

苏鲁仰口超高压岩石SHRIMP锆石U/Pb定年与部分熔融时限

在大型碰撞造山带中,在陆壳物质深俯冲或快速折返早期,在超高压-高压条件下,易熔组分可能发生水致或脱水部分熔融,形成花岗质熔体。在超高压-高压条件下,苏鲁超高压岩石发生过部分熔融作用,形成长英质多晶体包裹体和不同尺度的花岗质岩石, 导致可观的地球化学效应。为确定苏鲁超高压岩石部分熔融的时限,对山东仰口超高压副片麻岩和其中平行片麻理的同构造钾质花岗岩脉进行了SHRIMP锆石U/Pb地质年代学、全岩地球化学和锆石内矿物包裹体的研究。副片麻岩的锆石具有典型的核-幔-边结构。核部锆石为碎屑锆石,²⁰⁶Pb/²³⁸U年龄大于282Ma,可能反映了副片麻岩的原岩包含不同成因的物质;幔部和边部的Th/U比都小于0.1,分别给出233±3Ma和214±4Ma的²⁰⁶Pb/²³⁸U 年龄,分别对应于超高压变质和角闪岩相退变质年龄。同构造花岗岩脉是富钾过铝质花岗岩(A/CNK=1.2),锆石也具有核-幔-边结构;核部锆石年龄与副片麻岩的核部锆石年龄相当,反映了该花岗岩脉的源区可能是变沉积岩;除幔部锆石的一个点具有²⁰⁶Pb/²³⁸U年龄为234.6±3.9Ma之外,其它幔部锆石位于谐和线附近,给出²⁰⁶Pb/²³⁸U年龄为220.8±2.9Ma, 该年龄代表着该花岗岩脉的形成年龄。上述数据表明,在仰口地区,超高压岩石的部分熔融作用早于角闪岩相退变质作用。     

Petrogenesis of metatexite and diatexite migmatites determined using zircon U–Pb age,trace element and Hf isotope data,Higo metamorphic terrane,central Kyushu,Japan

Metatexite and diatexite migmatites are widely distributed within the upper amphibolite and granulite facies zones of the Higo low‐P/high‐T metamorphic terrane. Here, we report data from an outcrop in the highest grade part of the granulite facies zone, in which diatexite occurs as a 3 m thick layer between 2 m thick layers of stromatic‐structured metatexite within pelitic gneiss. The migmatites and gneiss contain the same peak mineral assemblage of biotite + plagioclase + quartz + garnet + K‐feldspar with retrograde chlorite ± muscovite and some accessory minerals of ilmenite ± rutile ± titanite + apatite + zircon + monazite ± pyrite ± zinc sulphide ± calcite. Calculated metamorphic P–T conditions are 800–900 °C and 9–12 kbar. Zircon in the diatexite forms elongate euhedral crystals with oscillatory zoning, but no core–rim structure. Zircon from the gneiss and metatexite forms euhedral–subhedral grains comprising inherited cores overgrown by thin rims. The overgrowth rims in the metatexite have lower Th/U ratios than zircon in the diatexite and yield a ²⁰⁶Pb/²³⁸U age of 116.0 ± 1.6 Ma, which is older than the 110.1 ± 0.6 Ma ²⁰⁶Pb/²³⁸U age derived from zircon in the diatexite. Zircon from the diatexite has variable REE contents with convex upward patterns and flat normalized HREE, whereas the overgrowth rims in the metatexite and gneiss have steep HREE‐enriched patterns; however, both types have similar positive Ce and negative Eu anomalies. ¹⁷⁶Hf/¹⁷⁷Hf ratios in the overgrowth rims from the metatexite are more variable and generally lower than values from zircon in the diatexite. Based on U–Pb ages, trace element and Hf isotope data, the zircon rims in the metatexite are interpreted to have crystallized from a locally derived melt, following partial dissolution of inherited protolith zircon during anatexis, whereas the zircon in the diatexite is interpreted to have crystallized from a melt that included an externally derived component. By integrating zircon and petrographic data for the migmatites and pelitic gneiss, the metatexite migmatite is interpreted to have formed by in situ partial melting in which the melt did not migrate from the source, whereas the diatexite migmatite included an externally derived juvenile component. The Cretaceous high‐temperature metamorphism of the Higo metamorphic terrane is interpreted to reflect emplacement of mantle‐derived basalts under a volcanic arc along the eastern margin of the Eurasian continent and advection of heat via hybrid silicic melts from the lower crust. Post‐peak crystallization of anatectic melts in a high‐T region at mid‐crustal depths occurred in the interval c. 116–110 Ma, as indicated by the difference in zircon ages from the metatexite and diatexite migmatites.     

柴北缘鱼卡河榴辉岩围岩的变质时代及其地质意义

利用阴极发光和LA-ICP-MS原位分析方法,对柴北缘鱼卡河超高压榴辉岩的直接围岩——石榴石白云母角闪钠长片麻岩和含蓝晶石的石榴石云母片岩进行了详细的锆石微量元素和U-Th-Pb同位素分析。结果表明,石榴石白云母角闪钠长片麻岩中的锆石主体呈浑圆状形态,内部结构以扇状和冷杉树状为主,稀土总量和重稀土含量均较低,重稀土富集程度低,具明显正Eu异常,Th/U比值均小于0.1,指示其形成在与石榴石平衡共生而不含斜长石的变质条件下。而含蓝晶石的石榴石云母片岩中的锆石显示明显的核-边结构,核部显示碎屑锆石特征,边部为与石榴石共生的变质新生锆石。LA-ICP-MS原位定年获得两种围岩的峰期变质年龄分别为(431±3)Ma和(432±19)Ma。这两组年龄在误差范围内一致,而且与已获得的紧邻的两类超高压榴辉岩的变质年龄((436±3)Ma和(431±4)Ma)以及带内指示大陆深俯冲作用时代的都兰含柯石英副片麻岩锆石的柯石英微区年龄(424~432Ma)完全一致。参考这两类岩石与超高压榴辉岩的野外产状关系、含蓝晶石的石榴石云母片岩的原岩特征以及榴辉岩中超过变质年龄达300Ma的原岩残核的存在,共同表明,柴北缘鱼卡河超高压变质地体是典型的大陆深俯冲碰撞作用的产物,本区大陆深俯冲发生的时代为430~435Ma的早古生代。     

北苏鲁变辉长岩锆石U-Pb年龄和Lu-Hf同位素组成及其对源区的指示

在北苏鲁经历超高压变质的花岗片麻岩中零星分布着一些变质表壳岩包体或者残片,通常称之为荆山群。北苏鲁荆山群的年代学特征以及构造亲缘性仍不清楚。本文报道了威海市郊和海阳所附近的荆山群露头,有脉状变质辉长岩侵入到荆山群岩系中。对上述两处变质辉长岩中锆石的阴极发光照片、稀土元素配分模式和Th/U比值的综合分析表明,它们均为岩浆锆石,部分岩浆锆石具有弱发光效应和岩浆韵律环带的核部、以及被改造的强发光效应的边部,但未见新生的变质锆石。未被改造的岩浆锆石微区的LA-ICP-MS U-Pb定年结果表明,威海市郊变辉长岩(S4-2)的~(207)Pb/~(206)Pb年龄值变化范围是1831~1966 Ma,加权平均年龄为1870±34 Ma(MSWD=0.6);海阳所变辉长岩(H3)中绝大部分锆石的~(207)Pb/~(206)Pb年龄值变化范围是1769~1887 Ma,加权平均年龄为1839±37 Ma(MSWD=0.5),两件样品共同记录了一致的ca.1.85 Ga岩浆事件。另外,海阳所变辉长岩中3颗捕获的岩浆锆石分别测得ca.2.9 Ga、ca.2.3 Ga和ca.2.1 Ga等三组~(207)Pb/~(206)Pb年龄。两件样品ca.1.85 Ga岩浆锆石具有相似的Hf同位素组成,其ε_(Hf)(t)=-6.1~-10.7,t_(DM2)=2.74~2.98 Ga,捕获ca.2.1 Ga和ca.2.9 Ga锆石的ε_(Hf)(t)值和t_(DM2)年龄分别是-1.9和2.7 Ga、-4.8和3.5 Ga。锆石U-Pb年龄、Lu-Hf同位素特征表明,北苏鲁两处变辉长岩记录的岩浆事件与胶北前寒武纪陆壳的多期演化历史一致,变辉长岩及其围岩荆山群可能是源自胶北的外来地质体。     

柴北缘大陆深俯冲板片折返过程中的深熔作用研究

柴北缘锡铁山地区长英质（花岗）片麻岩普遍经历了不同程度的部分熔融作用,常见新生的花岗质浅色体呈层状、脉状或网络状分布于长英质片麻岩中,并显示出混合岩化的特征。岩相学观察结果显示长英质片麻岩保留了关键的深熔作用显微结构证据:（1）石榴石内部发育有钾长石、石英和斜长石组成的矿物包裹体;（2）长石颗粒边界出现由石英+钾长石±斜长石±白云母组成的楔形矿物集合体;（3）云母颗粒边界发育尖锐的、不规则的微斜长石,而且云母边界溶蚀明显,形成锯齿状不规则的边界;（4）石英、斜长石或钾长石颗粒边界发育圆珠状（stringofbeads）结构,而且颗粒边界或三联点中尖锐状微斜长石与周围矿物的形成较小的二面角。阴极发光图像和锆石U-Pb定年结果表明花岗质浅色体中的锆石具有明显的核、幔、边三层结构,而且具有明显不同的年龄结果。发光较强的继承性锆石岩浆核部的²⁰⁶Pb/²³⁸U年龄约为~910Ma,而且具有高的Th/U比值;弱发光的变质锆石幔的²⁰⁶Pb/²³⁸U年龄结果约为~450Ma。新生的锆石增生边中等程度发光,并发育震荡环带和较低的Th/U比值,与世界典型地区混合岩中深熔锆石的特征十分相似,其²⁰⁶Pb/²³⁸U年龄结果为432±3Ma。野外关系、显微结构特征和年代学的研究结果显示柴北缘锡铁山地区花岗质浅色体可能是其寄主岩石长英质片麻岩在折返到高压麻粒岩相条件下深熔作用的产物,而且白云母的脱水熔融是引发岩石发生深熔作用的主要机制。柴北缘地区已有的资料综合研究表明,大陆深俯冲板片在俯冲/碰撞和折返过程中可能经历了多重深熔作用。     

Time constraints on the tectonic evolution of the Eastern Sierras Pampeanas (Central Argentina)

The application of the SHRIMP U/Pb dating technique to zircon and monazite of different rock types of the Sierras de Córdoba provides an important insight into the metamorphic history of the basement domains. Additional constraints on the Pampean metamorphic episode were gained by Pb/Pb stepwise leaching (PbSL) experiments on two titanite and garnet separates. Results indicate that the metamorphic history recorded by Crd-free gneisses (M2) started in the latest Neoproterozoic/earliest Cambrian (553 and 543 Ma) followed by the M4 metamorphism at ~530 Ma that is documented in the diatexites. Zircon ages of 492 Ma in the San Carlos Massif correlate partly with rather low Th/U ratios (<0.1) suggesting their growth by metamorphic fluids. This age is even younger than the PbSL titanite ages of 506 Ma. It is suggested that the fluid alteration relates to the beginning of the Famatinien metamorphic cycle in the neighbouring Sierra de San Luis and has not affected the titanite ages. The PTt evolution can be correlated with the plate tectonic processes responsible for the formation of the Pampean orogene, i.e., the accretion of the Pampean basement to the Río de La Plata craton (M2) and the later collision of the Western Pampean basement with the Pampean basement.     

滇西腾冲地块片麻状花岗岩的锆石U-Pb年龄

报道了滇西腾冲地块片麻状花岗岩的锆石U-Pb年龄。区域地质和岩相特征表明,片麻状花岗岩为岩浆成因,并经历了后期深熔作用。24颗锆石测得24个年龄数据中,1颗继承锆石的206Pb/238U年龄为537.5±5.6Ma,2颗岩浆锆石的206Pb/238U平均年龄为456Ma,3颗深熔锆石的206Pb/238U平均年龄为117Ma,18颗深熔锆石的Robust (Median) 206Pb/238U年龄为66.2+7/-4Ma。上述结果表明,花岗岩形成于早古生代,而后在白垩纪发生深熔作用,可能是特提斯演化过程中的岩浆响应。     

北秦岭超高压地体折返过程中的多期次部分熔融

本文对丹凤地区秦岭岩群含柯石英超高压变质地体长英质片麻岩中的混合岩化长英质浅色体和含石榴子石暗色包体的花岗质脉体进行了详细的矿物学、地球化学和锆石U-Pb年代学以及Lu-Hf同位素研究。其中,长英质浅色体显示了近原位熔融的高硅、富钾的过铝质花岗岩地球化学特征;锆石的CL图像呈灰黑色,均匀无结构或云雾状内部结构,Th/U比值0. 008,并含有钾长石、斜长石、石英和磷灰石等包裹体,显示深熔锆石的特征;花岗质脉体暗色包体中的石榴子石显示核-边成分环带,其中核部成分与秦岭岩群长英质片麻岩中石榴子石成分一致,边部Sps含量升高,显示熔体改造或退变质扩散特征,寄主花岗质脉体显示重稀土强烈亏损的与石榴子石平衡的熔体特征,指示它们是秦岭岩群含石榴子石长英质片麻岩部分熔融的产物。锆石LA-ICP-MS定年得到长英质浅色体和花岗质脉体的结晶年龄分别为445±4Ma和420±1Ma,明显晚于本区的超高压变质时代,而与折返过程中麻粒岩相和角闪岩相退变质叠加的时代基本一致。结合区域地质和前人的研究成果,提出秦岭岩群在深俯冲板块的折返过程中,分别在445Ma和420Ma发生了两期部分熔融作用。     

变基性岩部分熔融过程中榍石的微量元素效应:以南迦巴瓦混合岩为例

南迦巴瓦地区广泛出露的中下地壳变基性岩部分熔融形成的层状混合岩和淡色花岗岩,为研究部分熔融过程中榍石的地球化学行为对熔体的微量元素组成的影响提供了良好的机会。相对于源岩或熔融残留体,淡色体亏损Ti、V、REE、Y、Nb、Ta、U等元素,与混合岩中榍石的微量元素特征互补。混合岩、淡色体和榍石微量元素特征表明南迦巴瓦角闪岩部分熔融形成的淡色体的微量元素特征主要受控于榍石的地球化学行为。角闪岩脱水部分熔融过程中,由于长英质熔体的低Ti溶解度,榍石以未熔残留体形式存在于暗色体中,导致熔体亏损Ti、REE、Nb、Ta、V、U等元素和Sr/Y比值相对升高。关键元素在榍石和熔体之间的配分系数受熔体成分影响明显。角闪岩中变质榍石D_Nb/Ta<1,因此变质榍石残留导致熔体Nb/Ta相对于源岩升高;而高Si-Al花岗质熔体中榍石D_Nb/Ta>1,因此与高Si-Al熔体平衡的榍石的分离（转熔或结晶分异）将导致熔体Nb/Ta比值相对源岩降低。榍石在部分熔融过程中的微量元素效应为理解变基性岩部分熔融产生熔体的地球化学特征提供新的认识。     

北苏鲁威海地区超基性岩的原岩形成时代和超高压变质时代

超基性岩是苏鲁超高压变质地体中一类特殊且十分重要的岩石类型,它们通常呈规模不一的块状、条带状或不规则透镜状 (体) 赋存于区域大面积出露的花岗质片麻岩中。锆石中矿物包体激光拉曼测试、阴极发光图像分析和不同性质锆石LA-ICP-MS U-Pb定年等综合研究结果表明,北苏鲁威海地区含橄榄石辉石岩 (样品W1和W2) 中锆石的成因十分复杂,可进一步划分3种不同类型锆石。其中第一类锆石呈自形-半自形晶,阴极发光图像显示清晰的岩浆结晶环带,矿物包体主要为Ol+Cpx+Ap, 记录的²⁰⁷Pb/²⁰⁶Pb年龄为1835~1845Ma,应代表含橄榄石辉石岩的原岩形成时代;第二类为变质重结晶锆石,呈半自形-他形晶,阴极发光图像显示模糊的岩浆结晶环带,矿物包体与第一类完全一致,记录的²⁰⁶Pb/²³⁸U年龄变化范围大,为250~784Ma之间,表明部分继承性岩浆结晶锆石明显受到后期岩浆-变质热事件的影响而发生不完全重结晶和Pb丢失,进而使其记录的年龄相对偏新;第三类锆石呈他形晶,为典型的变质锆石,阴极发光图像十分均匀,矿物包体相对少见,主要为Grt+Cpx,记录的²⁰⁶Pb/²³⁸U年龄为230~234Ma, 且与苏鲁地体榴辉岩及其围岩中含柯石英锆石微区记录的超高压变质年龄 (225~235Ma) 十分一致,应代表含橄榄石辉石岩的峰期超高压变质时代。超基性岩中超高压变质锆石的准确识别表明苏鲁地体在峰期超高压变质阶段的确存在流体,流体的存在对超高压变质锆石的形成起着至关重要的作用。该项研究不仅准确厘定北苏鲁威海地区超基性岩的原岩形成时代和超高压变质时代,而且对于深入探讨苏鲁-大别超高压地体流体行为、演化规律及其水-岩相互作用机理具有重要的科学意义。