大别山高压?超高压片麻岩核?边结构锆石成因探讨*

大别山高压?超高压片麻岩中的锆石在阴极发光(CL)显示核?边结构：韵律环带的岩浆锆石核,较强发光的变质边。本文对核?边结构的锆石进行了包裹体、微区U?Pb年龄和微量元素研究。结果表明变质边由两种不同的机制形成：变质重结晶和变质增生。变质重结晶锆石的U?Pb年龄和稀土总量逐渐降低,稀土配分模式不变。变质增生锆石比变质重结晶锆石具有更低Th/U比值,重稀土严重亏损,Eu负异常不明显,低的Nb、Ta含量。     

大别山高压-超高压片麻岩锆石的拉曼光谱学研究

对大别山高压-超高压片麻岩的锆石进行了激光拉曼探针、阴极发光(CL)、U-Pb年龄和U、Th等微量元素的测定分析.研究结果表明:锆石拉曼光谱1008cm-1峰的半高宽普遍小于20cm-1,指示锆石结晶程度较好到中等程度的蜕晶化;早期继承锆石在三叠纪高压-超高压变质作用中经历了退火和重结晶作用,所受的放射性通量是从200Ma的早侏罗纪开始积累的,表明高压-超高压岩石在这一时间已出露地表;北大别白垩纪岩浆热事件对高压-超高压岩石的影响很小;大多数锆石数据都落在天然锆石的放射性破坏趋势(RDT)中,表明锆石在高压、超高压过程中基本发生了完全的重结晶.     

Timing of Variscan HP-HT metamorphism in the Moldanubian Zone of the Bohemian Massif: U-Pb SHRIMP dating on multiply zoned zircons from a granulite from the Dunkelsteiner Wald Massif, Lower Austria

In an attempt to better constrain the timing of Variscan HP-HT metamorphism in the SE Bohemian Massif we have dated zoned zircons from a garnet-kyanite granulite of granitic composition from the Dunkelsteiner Wald Massif, Lower Austria, by means of sensitive high-resolution ion microprobe (SHRIMP) technique. In order to combine isotopic information with crystal growth textures, CL and BSE images were systematically taken from the dated zircons. A characteristic threefold concentric zoning was found in many zircons. This involves pre-Variscan protolithic cores followed by two distinct metamorphic/anatectic overgrowth shells of Variscan age. The inner overgrowth shell is characterized by a weak CL but bright BSE signal, and yields high contents of uranium (0.1 to 0.2 wt.%). A pooled U-Pb Concordia age for this zone is 342.0?±?3.0?Ma (n?=?11, MSWD?=?0.12). The second, outer, overgrowth shell is always bright in the CL image, dark in the BSE image, and has generally low uranium contents (mostly <500?ppm). A pooled U-Pb Concordia age for this zone is 337.1?±?2.7?Ma (n?=?11, MSWD?=?0.22). These results imply that the Variscan HT crystallisation history of the Moldanubian granulites took place over a period of a few million years and was not an extremely rapid subduction-exhumation process. SHRIMP measurements in the protolithic cores yield a cluster of (sub)concordant ages between ??390 and 460?Ma and a few outliers at higher ages mostly represented by cores in cores. Core domains, which are large, homogeneous and with undisturbed igneous oscillatory zoning, yielded preferentially ages between 430 and 460?Ma. We therefore consider that granitic protolith formation took place at that time. The still older inner cores are interpreted as inherited into the granitic melt.     

阿尔金江尕勒萨依榴辉岩和围岩锆石LA-ICP-MS微区原位定年及其地质意义

阿尔金江尕勒萨依榴辉岩及其直接围岩——石榴子石黑云母片麻岩锆石的阴极发光图像、微区原位LA-ICP-MS微量元素分析研究表明,榴辉岩锆石内部结构比较均匀,少数颗粒保留斑杂状残核;位于锆石斑杂状残核测点的重稀土相对富集,Th/U比值多大于0.4,为岩浆锆石的特征;位于锆石边部与内部结构均匀颗粒上的测点显示HREE近平坦型或弱亏损型的稀土配分模式,显示了与石榴石平衡共生的变质锆石特征;而石榴子石黑云母片麻岩的锆石具有核-幔-边结构,核部为碎屑锆石,幔部则为与石榴石平衡共生的变质锆石。LA-ICP-MS微区定年获得榴辉岩的变质年龄为(493±4.3)Ma,其原岩形成年龄为(754±9)Ma;石榴子石黑云母片麻岩的变质年龄为(499±27)Ma。榴辉岩的变质年龄滞后于其原岩的形成年龄约250Ma,并且榴辉岩与其直接围岩副片麻岩的变质年龄几乎完全一致,充分表明该超高压榴辉岩的形成是陆壳深俯冲作用的产物。     

Further evidence for an early Carboniferous (∼340 Ma) age of high-grade metamorphism in the Saxonian granulite complex

U–Pb and Pb–Pb zircon ages for metamorphic zircons from granulites in the Saxonian granulite complex are reported, using the SHRIMP ion microprobe, conventional multigrain and single-gain techniques and the evaporation method. This is complemented by a Pb–Pb evaporation age for a post-granulite granite emplaced into the schist mantle around the granulites during uplift of the complex. We also demonstrate that zircon ages are not reset during high-grade metamorphism, as commonly argued, but have a very high closure temperature and usually preserve the isotopic composition reflecting the time of their formation. Multifaceted zircons from four granulite samples that probably grew close to the peak of high-grade metamorphism yielded identical U–Pb and Pb–Pb ages of ～340?Ma which support previously published data and unambiguously show that the granulites formed during a lower Carboniferous event and not in the early Palaeozoic or Precambrian as previously suggested. Older cores in some of the metamorphic zircons reveal early Palaeozoic components at 470–485?Ma that we interpret as ages reflecting magmatic crystallization of the granulite precursors. One sample suggests an inherited component as old as ～1700?Ma. The post-granulite granite has a Pb–Pb evaporation age of 333.1±1.0?Ma, and the short time interval between granulite metamorphism and granite intrusion implies that uplift, crustal extension and cooling of the granulite complex occurred rapidly after peak metamorphic conditions.     

Constraining a SHRIMP U-Pb age: micro-scale characterization of zircons from Saxonian Rotliegend rhyolites

We present results of a detailed investigation of zircons from two rhyolites from St. Egidien and Chemnitz, Saxony, using a combination of microprobe techniques (SHRIMP ion probe, Raman microprobe, SEM: SE, BSE, and CL imaging). These rhyolites belong to the so-called “lower volcanics”, which is the older of two series of Late Variscan volcanic rocks occurring in the Saxonian Sub-Erzgebirge basin (Germany). The purpose of the present contribution is to demonstrate that detailed characterization of zircons, as provided by the different micro-techniques, facilitates soundest interpretation of geochronological data. The zircons (at most 40 to 80 m in size) show oscillatory growth zoning, with reversely correlated CL and BSE signal intensities. These zircons are interpreted to have grown during crystallization of the rhyolite because, apart from some cracking, they do not appear to have experienced any alteration since the time of their growth: The shapes of the zircons and their internal structures revealed by CL and BSE imaging appear to be magmatic, and neither annealing of the accumulated alpha-decay damage nor disturbance of the U-Pb system is observed. The SHRIMP ion probe measurements on the zircons gave a Permian ²⁰⁶Pb/²³⁸U age of 278 ± 5 Ma (95% confidence). The concordance of this age is supported by the correlation between the low degrees of metamictization (estimated from Raman parameters) and the accumulated alpha fluxes (calculated from SHRIMP data). The 278 Ma zircon age is interpreted to represent the age of the “lower rhyolites” series and, with that, the age of postkinematic Late Variscan volcanism in the Sub-Erzgebirge basin, which has been related to anorogenic extension and uplift as a result of intracontinental rifting. Because of genetic association of rhyolites in the Sub-Erzgebirge basin and Li-F granites and lamprophyres in the neighbouring Erzgebirge, the rhyolite age also indirectly contributes to the understanding of the geological history of the Erzgebirge. The 278 Ma age is the first absolute dating result for rhyolites from the Saxonian Sub-Erzgebirge basin. Received: 16 December 1997 / Accepted: 4 May 1998     

Zircon U-Pb Age, Trace Element, and Hf Isotope Evidence for Paleoproterozoic Granulite-Facies Metamorphism and Archean Crustal Remnant in the Dabie Orogen

Zircon U-Pb age, trace elements, and Hf isotopes were determined for granulite and gneiss at Huaugtuling (黄土岭), which is hosted by ultrahigh-pressure metamorphic rocks in the Dabie(大别) orogen, east-central China. Cathodolumineseence (CL) images reveal core-rim structure for most zircons in the granulite. The cores show oscillatory zoning, relatively high Th/U and 176 Lu/177 Hf ratios, and high rare earth element (HREE)-enriched pattern, consistent with magmatic origin. They gave a weighted mean 207 Pb/206 Pb age of (2 766±9) Ma, dating magma emplacement of protolith. The rims are characterized by sector ur planar zoning, low Th/U and 176 Lu/177 Hf ratios, negative Euanomalies and flat HREE patterns, consistent with their formation under granulite-facies metamorphicconditions. Zircon U-Pb dating yields an age of (2 029±13) Ma, which is interpreted as a record ofmetamorphic event during the assembly of the supercontinent Columbia. The gneiss has a protolith ageof (1982±14) Ma, which is similar to the zircon U-Pb age for the granulite-facies metamorphism,suggesting complementary processes to granulite-facies metamorphism and partial melting. A fewinherited cores with igneous characteristics have 207 pb/206 Pb ages of approximately 3.53, 3.24, and 2.90Ga, respectively, suggesting the presence of Mesoarchean to Paleoarchean crustal remnants. A fewTriassic and Cretaceous metamorphic ages were obtained, suggesting the influences by the Triassiccontinental collision and postcollisional collapse in response to the Cretaceous extension. Comparingwith abundant occurrence of Triassic metamorphic zircons in ultrahigh-pressure eclogite and granitehydrous melt is evident for zircon growth in theHuangtuling granulite and gneiss during thecontinental collision. The magmatic protolithzircons from the granulite show a large variationin 176 Hf/177 Hf ratios from 0.280 809 to 0.281 289,corresponding to era(t) values of-7.3 to 6.3 andHf model ages of 2.74 to 3.34 Ga. The 2.90 Gainherited zircons show the similar Hf isotope features. These indicate that both growth of juvenile crust and reworking of ancient crust took place at the time of zircon formation. It is inferred that the Archean basement of the Yangtze block occurs in thenorth as the Dabie orogen, with ca. 2.90-2.95 Ga and 2.75-2.80 Ga as two major episodes of crustalformation.     

U–Pb Dating of Hydrothermal Zircons from the Neoproterozoic Liushanyan VMS Cu–Zn Deposit,Central China: Evidence for a Triassic Deformation Event

The Liushanyan deposit is an important volcanic‐host massive sulfide (VMS) Cu–Zn deposit in the Qinling‐Tongbai‐Dabie orogenic belt, central China, with reserve of 2.38 Mt Cu and 16.11 Mt Zn. Orebodies occur in the meta‐quartz keratophyre of the Liushanyan formation. In this paper, we present textural features and laser ablation ICP‐MS U–Pb dating results of zircons from the ore‐bearing mylonitized meta‐quartz keratophyre. The hydrothermal zircons are distinct from metamorphic zircons in this rock, showing low cathodoluminescence (CL) response and hydrothermal rims (black in CL images). They have relatively flat light rare earth element patterns and high La content and low (Sm/La)_N and Ce/Ce* values. These features are typical of hydrothermal zircons. The cores of metamorphic zircons yield a weighted mean ²⁰⁶Pb/²³⁸U age of 900 ± 26 Ma, interpreted as the volcanic and related VMS mineralizing age. Two much younger events are also recorded by zircons in this rock: (i) the Early Silurian amphibolites–greenschist facies metamorphism at 435 ± 26 Ma; and (ii) the growth of hydrothermal zircons at ca. 241 ± 1 Ma, associated with the ductile shear deformation. The Silurian metamorphic event is probably associated with the arc–continent collision, while the Triassic ductile deformation event formed in the final continent–continent collision setting.     

碧溪岭岩体中石榴橄榄岩的锆石显微结构及离子探针定年

对碧溪岭岩体中石榴橄榄岩的锆石进行了BSE和CL显微结构分析，并在此基础上用离子探针进行了锆石微区U－Pb定年。锆石CL图像显示其有明显的核边结构，核部具岩浆锆石，边部是变质增生锆石的特征。结合锆石的外形特征认为，边部的变质锆石是在超高压变质作用中重结晶形成的。离子探针分析结果在一致曲线图中构成不一致线，得到上下交点 年龄分别为824±39Ma和254±38Ma。结合CL图像认为上交点824±39Ma的年龄代表其原岩形成年龄，下交点254±38Ma为基超高压变质作用的年龄。另有少量较年龄的捕虏晶锆石，可能来自岩浆上升过程中同化混染的围岩。     

CL-imaging and ion microprobe dating of single zircons from a high-grade rock from the Central Zone,Limpopo Belt,South Africa: Evidence for a single metamorphic event at ∼2.0 Ga

The combination of ion microprobe dating and cathodoluminescence (CL) imaging of zircons from a high-grade rock from the Central Zone of the Limpopo Belt were used to constrain the age of metamorphic events in the area. Zircon grains extracted from an orthopyroxene-gedrite-bearing granulite were prepared for single crystal CL-imaging and ion microprobe dating. The grains display complex zoning when using SEM-based CL-imaging. A common feature in most grains is the presence of a distinct core with a broken oscillatory zoned structure, which clearly appears to be the remnant of an original grain of igneous origin. This core is overgrown by an unzoned thin rim measuring about 10–30 μm in diameter, which is considered as new zircon growth during a single metamorphic event. Selected domains of the zircon grains were analysed for U, Pb and Th isotopic composition using a CAMECA IMS 1270 ion microprobe (Nordsim facility). Most of the grains define a near-concordant cluster with some evidence of Pb loss. The most concordant ages of the cores yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2689 ± 15 (2σ) Ma, interpreted as the age of the protolith of an igneous origin. The unzoned overgrowths of the zircon grains yielded a considerably younger weighted mean ²⁰⁷Pb/²⁰⁶Pb age of ∼2006.5 ± 8.0 Ma (2σ), and these data are interpreted to reflect closely the age of the ubiquitous high-grade metamorphic event in the Central Zone. This study shows clearly, based on both the internal structure of the zircons and the data obtained by ion microprobe dating, that only a single metamorphic event is recorded by the studied 2.69 Ga old rocks, and we found no evidence of an earlier metamorphic event at ∼2.5 Ga as postulated earlier by some workers.     

辽宁后仙峪硼矿区古元古代电气石岩: 锆石特征及SHRIMP定年

为了确定辽东硼矿含硼岩系中电气石岩的形成时代, 对后仙峪硼矿区电气石岩首次进行了锆石SHRIMP U-Pb年龄测定.阴极发光图像显示, 锆石具核-边结构, 锆石核部多具振荡环带, 为高Th/U值的岩浆碎屑锆石; 锆石边部无明显结构变化, 为变质成因.岩浆碎屑锆石和变质锆石的年龄分别为2 171~2 175 Ma和1 894~1 906 Ma.结合野外地质和岩相学研究, 认为后仙峪硼矿区电气石岩原岩为古元古代克拉通裂谷环境中源于地壳再循环组分混染的亏损地幔的中酸性岩浆活动引发海底喷流作用的产物, 其原岩形成时代为古元古代中期(2 171~2 175 Ma), 并在稍后(1 894~1 906 Ma)遭受变质作用叠加改造.      

滇西梁河地区钾长花岗岩锆石LA-ICP-MS U-Pb定年及其地质意义

在详细区域地质调查的基础上,对滇西梁河地区具有代表性的两件钾长花岗岩样品中的锆石进行了阴极发光图像、微区原位LA-ICP-MS微量元素分析和U-Pb定年。结果表明,梁河地区钾长花岗岩锆石显示振荡环带的内部结构,指示其岩浆成因;两件样品的锆石具有较高的Th、U含量,稀土元素总量较高,轻稀土亏损、重稀土富集,明显的的Ce正异常和Eu负异常的稀土配分模式,说明其壳源岩浆锆石特征;两件样品的锆石206Pb/238U加权平均年龄分别是(60.7±0.6)Ma和(115.8±1.0)Ma,属古新世和早白垩世花岗岩。结合区域地质特征,腾冲地块在中生代晚期-新生代早期发生多期岩浆作用,揭示滇西三江地区特提斯演化的岩浆响应,为青藏高原东缘地区地质演化的动力过程提供约束。     

SHRIMP and electron microprobe chronology of UHT metamorphism in the Napier Complex,East Antarctica: implications for zircon growth at >1,000 °C

Zircons in ultra-high-temperature (UHT) metamorphosed paragneisses from Mt. Riiser-Larsen in the Napier Complex, East Antarctica, were dated by using ion microprobe (SHRIMP) and electron microprobe (EMP). Both SHRIMP and EMP analyses yield consistent 2520–2460 Ma age populations for garnet–orthopyroxene-bearing paragneiss and leucosomes enclosed within. The peak UHT event was dated at 2480 Ma by SHRIMP analyses on metamorphic zircons from the garnet–orthopyroxene paragneiss and those on magmatic zircons from the leucosomes which are interpreted to be formed at syn-UHT. As obtained by SHRIMP, the UHT metamorphic event was terminated no later than 2460 Ma. Minor 2520-Ma SHRIMP age suggests either the onset of prograde metamorphism or another high-grade metamorphic event unrelated to the UHT. EMP analyses on metamorphic zircons from sapphirine–quartz and osumilite-bearing magnesian paragneisses give c. 2500–2450 Ma ages. Inherited igneous zircon cores of the magnesian paragneisses yield relatively scattered EMP ages ranging over c. 3000–2650 Ma, suggesting that igneous materials of these ages sourced the protoliths of the paragneisses and that they were deposited during the interval c. 2650–2520 Ma.     

Trace element composition and Lu-Hf isotope systematics of zircon from plagiogneisses of the Kola superdeep well: Contribution of a Paleoarchean crust in Mesoarchean metavolcanic rocks

Zircons have been studied in three samples of Archean plagiogneisses from the Kola superdeep well (SG-3). The crystals consist of cores, magmatic shells, and metamorphic rims. The cores and shells are characterized by similar lowered concentrations of most trace elements, which is typical of zircons from plagiogranitoids, rocks of elevated basicity, and basites. At a wide range of Hf isotope characteristics, the cores and shells have similar average ¹⁷⁶Hf/¹⁷⁷Hf_i, which determines the close composition of their sources. The metamorphic rims have close ¹⁷⁶Hf/¹⁷⁷Hf_i ratio. The minimum age of the crustal contaminant of parental melts is estimated at 3.4 and 3.3 Ga for cores and 3.3–3.2 Ga for shells at almost equal proportions of mantle and crustal components in them. The contribution of Paleoarchean crust established in zircons from plagiogneisses of SG-3 using Lu-Hf isotope systematics is confirmed by the presence of 3.3and 3.4-Ga old zircons in surrounding TTG.     

Devonian to Carboniferous collision in the Greenland Caledonides: U-Pb zircon and Sm-Nd ages of high-pressure and ultrahigh-pressure metamorphism

A variety of eclogites from an east-west transect across the North-East Greenland eclogite province have been studied to establish the timing of high pressure (HP) and ultrahigh-pressure (UHP) metamorphism in this northern segment of the Laurentian margin. Garnet + omphacite ± amphibole + whole rock Sm-Nd isochrons from a quartz eclogite, a garnet + omphacite + rutile eclogite and a partially melted zoisite eclogite in the western HP belt are 401±2, 402±9 and 414±18 Ma, respectively. Corresponding sensitive high-resolution ion microprobe (SHRIMP) ²⁰⁶Pb/²³⁸U ages of metamorphic zircon in the same samples are 401±7, 414±13, and 393 ±10 Ma. Metamorphic zircon domains were identified using morphology, cathodoluminescence (CL) imaging, U, Th, Th/U and trace element contents. Zircon from the quartz eclogite and the garnet + omphacite + rutile eclogite are typical of eclogite facies zircon with rounded to subhedral shapes, patchy to homogenous CL domains, low U, and very low Th and Th/U. The partially melted eclogite contains euhedral zircons with dark, sector-zoned, higher U, Th and Th/U inherited cores. Three cores give a Paleoproterozoic ²⁰⁷Pb/²⁰⁶Pb age of 1,962±27 Ma, interpreted as the age of the leucogabbroic protolith. CL images of the bright overgrowths show faint oscillatory zoning next to homogenous areas that indicate zircon growth in the presence of a HP melt and later recrystallization. Additional evidence that zircon grew during eclogite facies conditions is the lack of a Eu anomaly in the trace element data for all the samples. These results, combined with additional less precise Sm-Nd ages and our earlier work, point to a Devonian age of HP metamorphism in the western and central portions of the eclogite province. An UHP kyanite eclogite from the eastern part of the transect contains equant metamorphic zircon with homogeneous to patchy zoning in CL and HP inclusions of garnet, omphacite and kyanite. These zircons have slightly higher U, Th and Th/U values than the HP ones, no Eu anomaly, and are thus comparable to UHP zircons in the literature. The ²⁰⁶Pb/²³⁸U age of these zircons is 360±5 Ma, much younger than the HP eclogites. The same sample gives a Sm-Nd age of 342±6 Ma. Unlike the HP eclogites, the Sm-Nd age of the UHP rock is ca. 20 Ma younger than the U-Pb zircon age and most likely records slow cooling through the closure temperature, since peak temperatures were in excess of 900°C. Widespread HP metamorphism of both the Laurentian and Baltica continental margins marks the culmination of this continent–continent collision in the Devonian. Carboniferous UHP conditions, though localized in the east, suggest a prolonged collisional history rather than a short-lived Scandian orogeny. The traditional Silurian Scandian orogeny should thus be extended through the Devonian.     

Multiple zircon growth during fast exhumation of diamondiferous, deeply subducted continental crust (Kokchetav Massif, Kazakhstan)

Diamondiferous rocks from the Kokchetav Massif, Kazakhstan, represent deeply subducted continental crust. In order to constrain the age of ultra high pressure (UHP) metamorphism and subsequent retrogression during exhumation, zircons from diamondiferous gneisses and metacarbonates have been investigated by a combined petrological and isotopic study. Four different zircon domains were distinguished on the basis of transmitted light microscopy, cathodoluminescence, trace element contents and mineral inclusions. Mineral inclusions and trace element characteristics of the zircon domains permit us to relate zircon growth to metamorphic conditions. Domain 1 consists of rounded cores and lacks evidence of UHP metamorphism. Domain 2 contains diamond, coesite, omphacite and titanian phengite inclusions providing evidence that it formed at UHP metamorphic conditions (P>43 kbar; T~950 °C). Domain 3 is characterised by low-pressure mineral inclusions such as garnet, biotite and plagioclase, which are common minerals in the granulite-facies overprint of the gneisses (P~10 kbar; T~800 °C). This multi-stage zircon growth during cooling and exhumation of the diamondiferous rocks can be best explained by zircon growth from Zr-saturated partial melts present in the gneisses. Domain 4 forms idiomorphic overgrowths and the rare earth element pattern indicates that it formed without coexisting garnet, most probably at amphibolite-facies conditions (P~5 kbar; T~600 °C). The metamorphic zircon domains were dated by SHRIMP ion microprobe and yielded ages of 527LJ, 528NJ and 526LJ Ma for domains 2, 3 and 4 respectively. These indistinguishable ages provide evidence for a fast exhumation beyond the resolution of SHRIMP dating. The mean age of all zircons formed between UHP metamorphic conditions and granulite-facies metamorphism is 528Dž Ma, indicating that decompression took place in less than 6 Ma. Hence, the deeply subducted continental crust was exhumed from mantle depth to the base of the crust at rates higher than 1.8 cm/year. We propose a two-stage exhumation model to explain the obtained P-T-t path. Fast exhumation on top of the subducted slab from depth >140 to ~35 km was driven by buoyancy and facilitated by the presence of partial melts. A period of near isobaric cooling was followed by a second decompression event probably related to extension in a late stage of continental collision.     

华北克拉通胶北地体多期高级变质事件:来自石榴斜长角闪岩与花岗质浅色体锆石U-Pb定年与稀土元素的新证据

胶北是华北克拉通一个记录了多期高级变质事件的前寒武纪变质地体,确定其每期变质-深熔事件时代与性质对进一步认识胶北乃至华北克拉通早前寒武纪地质演化具有十分重要的科学意义.本文以栖霞石榴斜长角闪岩及其伴生的花岗质浅色体为研究对象,通过锆石内部矿物包体、CL图像、LA-ICP-MS U-Pb定年与稀土元素组成的综合研究,发现石榴斜长角闪岩（19LR53-1）中锆石可进一步划分为两类:第一类（新太古代晚期）锆石通常具有浑圆状晶形且相对弱的（灰黑色）的阴极发光效应,18个该类锆石微区记录了十分一致的207Pb/206Pb年龄,变化于2 540±58 Ma至2 439±54 Ma之间,相应的加权平均年龄为2 498±25 Ma,应代表石榴斜长角闪岩遭受新太古代晚期变质作用的时代;第二类锆石通常具有柱状晶形且相对强的（灰色-灰白色）的阴极发光效应,14个该类锆石微区记录了十分一致的207Pb/206Pb年龄,变化于1 906±54 Ma至1 821±60 Ma之间,相应的加权平均年龄为1 865±30 Ma,被解释为石榴斜长角闪岩遭受古元古代晚期退变质作用的时代.花岗质浅色脉体（19LR53-2）中绝大多数锆石具有不规则状晶形且相对弱的（灰黑色）阴极发光效应,有的含有少量长英质矿物包体,如钾长石+斜长石+石英+磷灰石,18个该类锆石微区记录的207Pb/206Pb年龄变化于2 521±48 Ma至2 453±42 Ma之间,相应的加权平均年龄为2 480±22 Ma,应代表胶北地体新太古代晚期地壳深熔作用的时代.综合本文与前人发表的同位素年代学资料可知,胶北地体太古宙变质基底岩石不仅经历了新太古代晚期~2 500 Ma高级变质作用与地壳深熔作用,而且还叠加了古元古代晚期~1 850 Ma的高级变质作用与地壳深熔作用,它们可能是华北克拉通两次克拉通化过程中的地质响应.      

崆岭杂岩中角闪岩类的年代学和地球化学

通过崆岭杂岩中角闪岩类的年代学和地球化学研究,以揭示黄陵结晶基底的形成及演化。崆岭杂岩主要由太古代的TTG片麻岩和角闪岩类岩石,以及早元古代孔兹岩系组成,角闪岩类以围岩或包体的形式存在于TTG片麻岩的周围和内部。角闪岩类围岩的全岩Sm-Nd等时线年龄为2 998.9 Ma,原生岩浆锆石的U-Pb年龄为3 013 Ma,均代表角闪岩类原岩的形成时间,且与包体状斜长角闪岩原岩的年龄(3.0 Ga)相同。说明以围岩或包体存在的角闪岩类,其原岩的形成年龄均为3.0 Ga。微量元素和Nd同位素地球化学特征指示,角闪岩类原岩形成的构造环境为大陆初始裂谷环境。角闪岩类岩石中变质新生锆石的U-Pb年龄为2 043 Ma,指示黄陵地区存在第Ⅲ期(2.1～1.9 Ga)角闪岩相热变质事件,且该期热变质作用将松散的陆源碎屑岩等变质为孔兹岩系,从而构成早元古代结晶基底,并与晚太古代稳定陆块焊结在一起,最终完成整个黄陵结晶基底的形成。     

Zircon trace element characteristics and ages in granulite xenoliths: a key to understanding the age and origin of the lower crust,Arkhangelsk kimberlite province,Russia

Garnet granulite and pyroxenite xenoliths from the Grib kimberlite pipe (Arkhangelsk, NW Russia) represent the lower crust beneath Russian platform in close vicinity to the cratonic region of the north-eastern Baltic (Fennoscandian) Shield. Many of the xenoliths have experienced strong interaction with the kimberlite host, but in others some primary granulite-facies minerals are preserved. Calculated bulk compositions for the granulites suggest that their protoliths were basic to intermediate igneous rocks; pyroxenites were ultrabasic to basic cumulates. A few samples are probably metasedimentary in origin. Zircons are abundant in the xenoliths; they exhibit complex zoning in cathodoluminescence with relic cores and various metamorphic rims. Cores include oscillatory zircon crystallized in magmatic protoliths, and metamorphic and magmatic sector-zoned zircons. Recrystallization of older zircons led to the formation of bright homogeneous rims. In some samples, homogeneous shells are surrounded by darker convoluted overgrowths that were formed by subsolidus growth when a change in mineral association occurred. The source of Zr was a phase consumed during a reaction, which produced garnet. Late-generation zircons in all xenoliths show concordant U–Pb ages of 1.81–1.84 Ga (1,826 ± 11 Ma), interpreted as the age of last granulite-facies metamorphism. This event completely resets most zircon cores. An earlier metamorphic event at 1.96–1.94 Ga is recorded by some rare cores, and a few magmatic oscillatory zircons have retained a Neoarchaean age of 2,719 ± 14 Ma. The assemblage of metaigneous and metasedimentary rocks was probably formed before the event at 1.96 Ga. Inherited magmatic zircons indicate the existence of continental crust by the time of intrusion of magmatic protoliths in the Late Archaean. The U–Pb zircon ages correspond to major events recorded in upper crustal rocks of the region: collisional metamorphism and magmatism 2.7 Ga ago and reworking of Archaean rocks at around 1.95–1.75 Ga. However, formation of the granulitic paragenesis in lower crustal rocks occurred significantly later than the last granulite-facies event seen in the upper crust and correlates instead with retrograde metamorphism and small-volume magmatism in the upper crust.     

吉林南部新太古代末地壳深熔作用——来自变质石英闪长岩及淡色花岗岩的证据

深熔作用是大陆地壳分异、元素迁移富集和混合岩化作用的主要机制和关键地质过程.吉南地区出露的太古宙基底普遍经历了角闪岩相-麻粒岩相变质及深熔作用,长英质淡色体及淡色花岗岩广泛分布.吉南和龙花岗-绿岩地体出露的太古宙变质石英闪长岩及相关的长英质浅色体和含斜方辉石(角闪石)淡色伟晶花岗岩的野外地质特征、相互关系及岩相学特征指...