Constraining the prograde and retrograde P-T-t path of Eocene HP rocks by SHRIMP dating of different zircon domains: inferred rates of heating, burial, cooling and exhumation for central Rhodope, northern Greece

Ion microprobe (SHRIMP) dating was carried out on different zircon domains from metamorphic rocks of the HP-HT terrane of central Rhodope, northern Greece, to constrain the timing of prograde and retrograde stages within a single tectono-metamorphic cycle. A well determined P-T-t(relative) path for the metamorphic rocks of this terrane was used as a petrological basis for the geochronological investigations. Ion microprobe work was assisted by cathodoluminescence (CL) images of the zircon crystals. The geochronological results revealed that Hercynian continental crust was subducted during the Eocene. Several stages of the Eocene tectono-metamorphic cycle – including both the prograde and retrograde parts of the P-T path above ca 300 °C, 0.3 GPa – were dated using zircons from the following rock types: (1) A deformed quartz vein probably formed at ca 300 °C, 0.3 GPa. Zircons in this vein precipitated from a hydrothermal fluid; they yielded an age of 45.3 ± 0.9 Ma which corresponds to the time of a low-T prograde stage of metamorphism. (2) In kyanite eclogites, zircons were entirely reset during eclogite-facies metamorphism. Resetting was very probably enhanced by the presence of fluids derived by H₂O liberating reactions close to the P-peak. They yielded an age of 42.2 ± 0.9 Ma. (3) Orthogneisses surrounding the kyanite eclogites contained zircons with magmatic oscillatory zoned cores, which yielded Hercynian ages of 294 ± 8 Ma (age of granitic protolith formation), whereas CL-bright, metamorphic rims yielded, like the eclogite zircons, ages of 42.0 ± 1.1 Ma. Therefore, both the eclogites and orthogneisses are interpreted to have approached maximum depth at around 42 Ma. (4) In a leucosome of a migmatized orthogneiss, oscillatory zoned zircons yielded an age of 40.0 ± 1.0 Ma. At this time the rocks reached maximum temperatures during early decompression. (5) A late pegmatite crosscutting the schistosity of amphibolites contained oscillatory zoned zircons that yielded a crystallization age of 36.1 ± 1.2 Ma. Thus, the whole tectono-metamorphic cycle above ca 300 °C, 0.3 GPa lasted from 45.3 ± 0.9 Ma to 36.1 ± 1.2 Ma, that is 9.2 Ma with an extreme error value of 2.1 Ma. Based on combined SHRIMP and petrological data, the average rates of heating and burial during subduction (above ca 300 °C, 0.3 GPa) are >94 °C/Ma and >15 mm/a, respectively. Rates of cooling and exhumation (also above 300 °C, 0.3 GPa) are calculated to be >128° C/Ma and >7.7 mm/a. The Eocene age of metamorphism in central Rhodope implies that the terrane of, at least, central Rhodope and the Cyclades very probably was part of the same continental crust. Received: 5 October 1998 / Accepted: 18 January 1999     

P-T estimates and timing of the sapphirine-bearing metamorphic overprint in kyanite eclogites from Central Rhodope, northern Greece

Sapphirine-bearing symplectites that replace kyanite in eclogites from the Greek Rhodope Massif have previously been attributed to a high-pressure granulite-facies metamorphic event that overprinted the eclogitic peak metamorphic assemblage. The eclogitic mineralogy consisted of garnet, omphacitic pyroxene, rutile and kyanite and is largely replaced by low-pressure minerals. Omphacite was initially replaced by symplectites of diopside and plagioclase that were subsequently replaced by symplectites of amphibole and plagioclase. Garnet reacted during decompression to form a corona of plagioclase, amphibole and magnetite. Rutile was partly transformed to ilmenite and kyanite decomposed to produce a high-variance mineral assemblage of symplectitic spinel, sapphirine, plagioclase and corundum. The presence of quartz and corundum in the kyanite eclogites is evidence for the absence of bulk equilibrium and obviates a conventional analysis of phase equilibria based on the bulk-rock composition. To circumvent this difficulty we systematically explored the pressure-temperature-composition (P-T-X) space of a thermodynamic model for the symplectites in order to establish the pressure-temperature (P-T) conditions at which the symplectites were formed after kyanite. This analysis combined with conventional thermometry indicates that the symplectites were formed at amphibolite-facies conditions. The resulting upper-pressure limit (～0.7 GPa) of the sapphirine-producing metamorphic overprint is roughly half the former estimate for the lower pressure limit of the symplectite forming metamorphic event. Temperature was constrained (T ～ 720°C) using garnet-amphibole mineral thermometry. The P-T conditions inferred here are consistent with thermobarometry from other lithologies in the Rhodope Massif, which show no evidence of granulite-facies metamorphism. Regional geological arguments and ion-probe (SHRIMP) zircon dating place the post-eclogite-facies metamorphic evolution in Eocene times.     

Constraints of U-Pb zircon dating on the evolution of the Nan-Uttaradit suture zone in northern Thailand

Nan-Uttaradit suture zone in northern Thailand is a narrow N-S trending and discontinuous ophiolite belt along the Nan River (Barr and MacDonald, 1987). It was interpreted as the Paleo-Tethys oceanic remnants that separate Shan-Thai (Sibumasu) terrane and Indo-china terrane (Bunopas, 1981; Hada, 1999), and rein-terpreted as the boundary of Sukhothai (or Simao) terrane and the Indochina terrane that representing a segment of the back-arc basin (Barr and MacDonald, 1991; Ueno and Hisada, 2001; Metcalfe, 2006; Ferrari et al., 2008; Sone and Metcalfe, 2008). This zone is dominated by Carboniferous to Permian Pha Som Metamorphic Complex (Hess and Koch, 1975). The Pha Som Metamorphic Complex consists of several tectonostratigraphic slices of volcanic rocks, schists, meta-greywacke, serpentinite and bedded chert. And it is in fault contact with Pak Pat volcanic rocks. Both of Pha Som Metamorphic Complex and Pak Pat volcanic rocks are covered by the Upper Triassic and the Juras-sic red sandstones with angular unconformity. Previ-ous studies mainly focused on the amalgamation epi-sodes of the Sukhothai terrane and Indochina terrane. The Late Carboniferous to Early Permian age of the opening of the basin was proposed by some authors (Singharajwarapan and Berry, 2000; Metcalfe, 2006; Ferrari et al., 2008) on the basis of the regional strati-graphy, different dating of cherts, and schists from the Pha Som Metamorphic Complex.     

Metamorphic evolution and geochemistry of kyanite eclogites in central Rhodope,northern Greece

In central Rhodope of northern Greece, kyanite eclogites were discovered in the area of Thermes. They are strongly overprinted and exhibit a multi-stage development of minerals and mineral assemblages formed during successive stages of the exhumation. The initial high-pressure assemblage was garnet+omphacite+kyanite+zoisite+phengite+rutile+quartz. Corundum, Fe-Mg-spinel, sapphirine and högbomite occur as products of a first, high-temperature overprint, still at high pressures, whereas various symplectites [corundum-plagioclase (pl), spinel-pl, sapphirine-pl, clinopyroxene-pl, biotite-pl, amphibole-pl] grew during subsequent stages of the exhumation. Diablastic amphibole+plagioclase formed as end-products of the amphibolitization. According to geochemical data, the protoliths of the kyanite eclogites were basalts to basaltic andesites with “volcanic arc” affinities. For the high-pressure stage of metamorphism, minimum PT conditions were around 19 kbar, 700°C, while for the initial stages of the overprint, high-pressure granulite-facies conditions prevailed (T>800°C, at P>15 kbar). The PT conditions of the amphibolite facies were 8–11 kbar, 580–690°C. The kyanite eclogites of Thermes record the highest temperatures of metamorphism within the whole of Rhodope.     

北阿尔金地区古元古代岩体LA-ICP-MS锆石U-Pb年龄及其地质意义

北阿尔金是塔里木克拉通变质基底的主要出露区之一。对该区具有侵入接触关系的正长花岗岩和花岗片麻岩进行了LA-ICP-MS锆石U-Pb定年研究。正长花岗岩中的锆石多呈椭圆状,具有振荡环带结构,部分颗粒中可见老锆石残核,Th/U值较高,亏损轻稀土元素,富集重稀土元素,具有负Eu异常和正Ce异常的特点,表明该组锆石为岩浆成因。定年结果获得1903±13Ma和2506±55Ma两组年龄加权平均值,前者代表岩体的结晶年龄,后者为捕获锆石年龄,结合区域年代学资料,认为正长花岗岩岩浆侵入过程中可能捕获了太古宇米兰群的古老基底锆石。花岗片麻岩中16个测点的锆石~(207)Pb/~(206)Pb年龄集中于1802±28Ma,代表了岩体侵位时代,其余5个测点的锆石~(207)Pb/~(206)Pb年龄为1911~1951Ma,说明岩浆侵位过程中捕获了部分正长花岗岩的物质。区域地质与同位素年代学研究表明,北阿尔金地区广泛存在2.0~1.8Ga的构造-热事件。获得的花岗质岩石的1.9~1.8Ga的年龄结果,直接证实了北阿尔金存在约1.9Ga的岩浆作用,可能为古元古代Columbia超大陆汇聚事件在该地区的响应,为探讨塔里木板块前寒武纪构造-热事件演化历史提供了新资料。古元古代末期约1.8Ga的花岗片麻岩,代表了后造山伸展阶段的岩浆活动。     

阿尔金山北缘蛇绿混杂岩中辉长岩锆石SHRIMP U-Pb定年及其地质意义

阿尔金北缘阿克赛青崖子附近蛇绿混杂岩组成岩石包括蛇纹岩和辉长岩等,呈透镜状构造岩片产出。蛇纹岩以富Mg,贫Al₂O₃、CaO、TiO₂和∑REE为特征,与世界上典型蛇绿岩中变质橄榄岩的特征值一致。辉长岩SiO₂含量为49.06%~50.70%,具有高的MgO（7.16%~10.01%）和Fe ₂O₃（6.36%~12.85%）含量,TiO₂含量低到中等,为0.31%~1.02%,Na₂O和K₂O含量中等,平均分别为0.73%和0.75%,辉长岩在球粒陨石标准化稀土元素配分模式图上属平坦型或轻微富集型,无负Eu异常,与E-type洋中脊和OIB玄武岩类似。辉长岩的主量元素、微量元素特征表明其可能既保留了形成于MORB的背景信息,也记录了SSZ环境改造的结果,推测青崖子蛇绿混杂岩组合可能来自不同构造背景。蛇绿混杂岩中辉长岩的SHRIMP锆石U-Pb年龄测定,确定出青崖子附近蛇绿混杂岩所代表的洋壳形成于521±12Ma,为古生代早期。该年龄测定的结果还显示本区在472±10Ma有一次地质事件,可能代表了古生代的洋盆闭合的下限年龄。     

Tertiary granitoids of Rhodope, northern Greece: Magmatism related to extensional collapse of the Hellenic Orogen?

Many granites that occur in orogenic belts such as the Himalaya, the Hercynides or Caledonides are thought to result from anatexis of the lower parts of over-thickened thrust-stacked crustal units, just preceding termination of the orogenic cycle. Numerous Tertiary granitoids occur in the Rhodope Zone of northern Greece (and adjacent regions of Bulgaria), a belt regarded as an eastern extension of the Alpine System. These granites could also be regarded as syn- or late-tectonic crustal melts in that some have been emplaced as sheet-like bodies along major Alpine thrust planes, and show a strong marginal foliation parallel to the pervasive Alpine compressional fabrics. In detail, however, the geochemistry of the granitoids, their timing and relationships with country rocks, suggest they have been emplaced in an extensional regime that is related to the collapse of the Hellenic Orogen and, more particularly, to the crustal thinning that the Aegean region has suffered since the mid-Tertiary. Rather than being products of crustal thickening, it is suggested that their petrogenesis is related to the elevation of thermal gradients during the extensional phase, which initiated thermal breakdown of hydrous minerals in the lithosphere and lower crust that had existed metastably during compression and the subduction of cool ocean lithosphere beneath the region in the late Mesozoic-early Tertiary.     

南帕米尔北缘切实界别辉长岩LA-ICP-MS锆石U-Pb定年、地球化学特征及其地质意义

南帕米尔北缘切实界别地区广泛分布的基性岩体(岩脉)侵位于下二叠统中,LA-ICP-MS锆石U-Pb同位素定年获得其成岩年龄为269.5±1.3Ma,反映该地区存在中二叠世基性岩浆活动。岩体中SiO2含量为46.68%~49.20%,TiO2平均值为1.40%,(K2O+Na2O)在3.94%~5.83%之间,属于碱性系列;稀土元素配分模式呈LREE富集型,δEu值为0.92~1.31,无明显Eu异常;明显富集K、Rb、Sr、Ba等大离子亲石元素,相对亏损Nb、Ta、Ti、P等高场强元素及Th、U放射性生热元素,类似于板内玄武岩;微量元素构造环境判别图亦显示其形成于板内构造环境。与邻区二叠纪基性火山岩地球化学特征对比,结果表明,南帕米尔和南羌塘在二叠纪同处于板内伸展环境,而冈底斯带属于岛弧背景,这为帕米尔和青藏高原石炭纪—二叠纪构造单元的划分与对比提供了岩浆作用方面的依据。     

大别超高压变质带的进变质、超高压和退变质时代的准确限定:以双河大理岩中榴辉岩锆石SHRIMP U-Pb定年为例

在大别超高压变质带的双河地区存在一种特殊类型的榴辉岩,该类榴辉岩主要以似层状、条带状以及不规则透镜体赋存于大理岩中。矿物组成主要为石榴石、绿辉石以及少量的金红石、白云石、菱镁矿等。沿石榴石和绿辉石边缘常退变为角闪石+斜长石等,有的岩石完全退变为斜长角闪岩。激光拉曼和阴极发光综合分析表明,该类榴辉岩中的锆石可划分为两种类型:继承性碎屑锆石和变质锆石。继承性碎屑锆石十分少见,阴极发光图象具有明显的双层结构,即强发光的核和弱发光的边,核部和边部的包体矿物分别为Pl+Ap和Qtz+Pl。SHRIMP U-Pb定年结果表明。继承性碎屑锆石核部记录的207Pb／206Pb年龄为2701±15Ma,Th/U比值明显偏高为1．05,稀土元素配分模式显示重稀土明显富集,具有典型岩浆结晶锆石的特点;边部记录的207Pb／206Pb年龄为1801±12Ma-1753±22Ma,Th／U比值则明显偏低,为0．19-0．22之间,稀土元素配分模式显示重稀土相对平坦,具有典型变质锆石的特点。上述特征表明该类继承性碎屑锆石可能来源于太古代的基底,并经历了早元古代变质热事件的改造。新生的变质锆石无论是矿物包体还是阴极发光图象均与继承性碎屑锆石存在明显差异。有的变质增生锆石具有弱发光的核(阴极发光图象呈灰色)和强发光的边(阴极发光图象呈白色)。核部包体矿物组合为Qtz +Grt+Omp+Phe+Dol+Ap,具有典型石英榴辉岩相矿物组合特征,而边部则保存含柯石英的超高压包体矿物组合Coe+Grt +Omp+Mgs+Arg+Ap,表明该类锆石的核部和边部分别形成于俯冲进变质阶段和超高压变质阶段。另一部分变质增生锆石具有强发光的核(阴极发光图象呈白色)和弱发光的边(阴极发光图象呈黑色)。核部保存的标志性超高压包体矿物组合为Coe+Grt+Omp+Mgs+Arg+Ap,边部则保存Qtz+Cal等退变矿物组合,有的则缺乏矿物包体,表明该类锆石自超高压变质阶段开始生长,并经历了后期退变质作用的改造。从不同微区矿物包体组合的性质及其转变特征可以明显看出,自石英榴辉岩相进变质阶段到超高压峰期变质阶段存在下列转变反应:Qtz→Coe和Dol→Mgs+Arg;而自超高压峰期变质阶段到后期退变质阶段则存在下列退变反应:Coe→Qtz和Arg→Cal。SHRIMP U-Pb定年结果表明,含石英榴辉岩相矿物包体的锆石微区记录的206Pb／238U年龄为249-241Ma,加权平均值为244±4Ma,代表了深俯冲石英榴辉岩相进变质阶段的变质年龄;含柯石英等超高压矿物包体的锆石微区记录的206Pb／238U年龄为239-231Ma,加权平均值为234±3Ma,代表超高压阶段的峰期变质年龄;而含石英和方解石的退变边记录的206Pb／238U年龄为219-211Ma,加权平均值为216±6Ma,应代表后期折返阶段的角闪岩相退变质年龄。上述两类变质增生锆石微区的Th／U比值和稀土元素配分模式十分相似,Th／U比值变化于0．02- 0．18之间,稀土元素配分模式显示重稀土相对平坦,稀土元素总量明显低于继承性碎屑锆石,具有典型变质锆石的特点。根据锆石微区矿物包体的化学成分,采用Grt-Omp和Grt-Omp-Phe温压计,结合前人的变质反应实验资料的综合分析,确定榴辉岩的原岩在深俯冲过程的石英榴辉岩相进变质阶段的变质温压条件为T=588-668℃,P=1．7-1．8GPa;超高压峰期阶段的温压条件为T=784-849℃,P>5．5GPa;而构造折返过程中角闪岩相退变质阶段的温压条件为T=550-720℃,P=0．8～1．4GPa。由此可见,大别超高压变质岩的原岩——元古代(部分可能为太古代)的陆壳物质在早三叠纪发生俯冲至55- 60km深处,并经历了石英榴辉岩相变质作用。随后这些变质岩石继续深俯冲至165～175km的地幔深处,于中三叠纪发生了超高压变质作用,石英榴辉岩相矿物组合转变为超高压榴辉岩相矿物组合。最后这些超高压变质岩石发生构造折返,至晚三叠纪抬升到约30km的中下地壳深度,并经历了角闪岩相退变质作用的改造,超高压榴辉岩相矿物组合退变为角闪岩相矿物组合。由此推断,大别超高压变质带俯冲和折返速率分别为11-12km Myr-1和7．5-8．1km Myr-1。该项成果不仅确定了大别超高压变质地体的石英榴辉岩相进变质-超高压榴辉岩相峰期变质-角闪岩相退变质的年代谱系,而且对于重塑大别超高压变质地体的快速俯冲-折返的动力学模式有着重要的科学意义。     

南帕米尔北缘切实界别辉长岩LA-ICP-MS锆石U-Pb定年、地球化学特征及其地质意义

南帕米尔北缘切实界别地区广泛分布的基性岩体（岩脉）侵位于下二叠统中,LA-ICP-MS锆石U-Pb同位素定年获得其成岩年龄为269.5±1.3Ma,反映该地区存在中二叠世基性岩浆活动。岩体中SiO2含量为46.68%~49.20%,TiO2平均值为1.40%,（K2O+Na2O）在3.94%~5.83%之间,属于碱性系列;稀土元素配分模式呈LREE富集型,δEu值为0.92~1.31,无明显Eu异常;明显富集K、Rb、Sr、Ba等大离子亲石元素,相对亏损Nb、Ta、Ti、P等高场强元素及Th、U放射性生热元素,类似于板内玄武岩;微量元素构造环境判别图亦显示其形成于板内构造环境。与邻区二叠纪基性火山岩地球化学特征对比,结果表明,南帕米尔和南羌塘在二叠纪同处于板内伸展环境,而冈底斯带属于岛弧背景,这为帕米尔和青藏高原石炭纪—二叠纪构造单元的划分与对比提供了岩浆作用方面的依据。     

冀北承德盆地九龙山组凝灰岩夹层LA-ICP-MS锆石U-Pb定年及其地质意义

冀北承德盆地中生代侏罗纪地层序列较齐全,但作为关键层位之一的九龙山组一直缺乏高精度年代学的约束。笔者首次对九龙山组凝灰岩夹层进行了LA-ICP-MS锆石U-Pb高精度定年,获得凝灰岩中岩浆锆石²⁰⁶Pb/²³⁸U加权平均年龄164.1±1.3 Ma。综合研究及对比表明,九龙山组的时限为158~164Ma,属中侏罗世晚期至晚侏罗世早期,对应于国际地层表的牛津期至卡洛维期,其沉积时间相对短暂。在此基础上,更新了研究区侏罗系年代地层柱状图,该认识对于深入研究燕山地区中生代侏罗纪地层格架及构造岩浆演化过程具有重要意义。     

Evidence for UHP anatexis in the Shuanghe UHP paragneiss from inclusions in clinozoisite,garnet, and zircon

The Shuanghe garnet-bearing paragneiss from the Dabie ultra-high–pressure (UHP) orogen occurs as an interlayer within partially retrogressed eclogite. A first UHP metamorphic stage at 680°C, 3.8–4.1 GPa is documented by Zr-in-rutile temperatures coupled with phengite inclusions (Si = 3.55) in clinozoisite and grossular-rich garnet. Relic matrix phengite and phengite inclusions in zircon rims display lower Si of 3.42. Combined with garnet compositions and Ti-in-zircon temperatures, they provide evidence for a second UHP metamorphic stage at 800–850°C, ~3.8 GPa. Such isobaric heating at UHP conditions has not been documented so far from the adjacent eclogites and other rock types in the Dabie orogen and indicates proximity to the hot, convecting mantle wedge. The dominant mineral assemblage consisting of plagioclase, epidote, biotite, and amphibole provides evidence for widespread retrogression during the exhumation of the UHP paragneiss. Several types of polyphase mineral inclusions were identified. Phengite inclusions hosted by clinozoisite are partially replaced by kyanite and K-feldspar, whereas inclusions in host garnet consist of relic phengite, K-feldspar, and garnet, indicating limited sub-solidus dehydration of phengite by the reaction Ph→Kfs+Ky±Grt+fluid. Tightly intergrown K-feldspar and quartz are preserved as inclusions with sharp boundaries and radial cracks in garnet. Analyses of whole inclusions also show small enrichments in light rare earth elements. These inclusions are interpreted to be derived from melting of an inclusion assemblage consisting of Ph+Coe±Czo. A third type of polyphase inclusion consists of typical nanogranite (Ab+Kfs+Qz±Ep) inclusions in recrystallized metamorphic zircon. Ti-in-zircon thermometry and the Si content of phengite included in these zircon domains indicate that melting occurred at 800–850°C and 3.8–4.0 GPa during isobaric heating at UHP conditions. The partial melting event led to an equilibration of trace elements in garnet, phengite, and apatite. Using published partition coefficients between these minerals and hydrous granitic melt, the trace element composition of the UHP anatectic melt can be constrained. The melts are characterized by high LILE contents and pronounced relative enrichments of U over Th and Ta over Nb. The REE are below primitive mantle values, likely due to the presence of residual clinozoisite and garnet during partial melting. So far, no major granitic bodies have been found that share the same trace element pattern as the partial melts from the UHP anatexis of the Shuanghe paragneiss.     

Structure and tectonic evolution of the Pirin-Pangaion Structural Zone (Rhodope Massif,southern Bulgaria and northern Greece)

The Pirin-Pangaion Structural Zone occupies the south-western part of the Rhodope Massif. It consists of Proterozoic amphibolite facies metamorphic rocks of the Rhodopian Supergroup, and granitoids of Hercynian, Late Cretaceous and Palaeogene age. The pre-Hercynian structure of the zone is dominated by an interference pattern of three superimposed fold generations of NE-SW and NW-SE trends. These structures are cut by Hercynian granitoids, and the entire complex is affected by late Hercynian or early Alpine conical folds. The zone was overthrusted by the Ogražden and Kroussia Units (Serbo-Macedonian ‘Massif’) along the north-east vergent Mid-Cretaceous Strimon overthrust, and by the Central Rhodope Zone of the Rhodope Massif, along the south-west vergent Meso-Rhodopean Overthrust. With this thrusting event, the Pirin-Pangaion Structural Zone was brought together with the Serbo-Macedonian ‘Massif’ and the Central Rhodope Zone to form the Late Cretaceous Morava-Rhodope Zone, which acted as a ‘plateau’ along the southern edge of the Eurasian plate. Late Cretaceous granitoid magma of crustal origin intruded this zone, whereas north of it the Srednogorie volcanic island arc was the site of igneous activity with magmas originating in the upper mantle. The West Thrace Zone developed as a Palaeocene to Oligocene depression superimposed over the older basement obliquely to the southern periphery of the Rhodope Massif. In the Late Eocene and Early Oligocene, this depression represented a volcanic island arc with mantle-derived basic to intermediate magmas; contemporaneous granitoid magmas formed through crustal melting in the thickened crust of the Rhodope Massif (Pirin and Pangaion Units included). Early Miocene thrusting was most intense in the Pangaion Unit, and was followed by Late Miocene to Quaternary extension.     

四川九龙新火山花岗岩体单颗粒锆石LA-ICP-MS U-Pb定年及其地质意义

本文对四川九龙新火山花岗岩岩体中的锆石进行了详细的LA-ICP-MS U-Pb同位素年龄测定。该岩体中的锆石发育较好的环带结构,Th/U值较高(大于0.13),具有岩浆成因锆石的特征。研究结果表明,新火山岩体中锆石的206Pb/238U加权平均年龄为181.2±1.4Ma(MSWD=3.3),代表了新火山花岗岩的结晶年龄,为早侏罗世,反映了松潘-甘孜造山带燕山期岩浆活动的一个重要阶段,为进一步理解松潘-甘孜燕山期花岗质岩浆活动及其动力学背景等提供了有力的证据。     

西秦岭天水地区流水沟岩浆杂岩LA-ICP-MS锆石U-Pb测年和岩石地球化学特征

流水沟岩浆杂岩分布于西秦岭北缘天水关子镇地区,主要由变辉长岩、变辉长闪长岩和变闪长岩组成。为确定其构造属性,探讨西秦岭北缘构造演化过程,对其进行了LA-ICP-MS锆石U-Pb测年和主量、微量、稀土元素分析。结果表明,岩浆杂岩体的锆石为岩浆成因,其形成时代为548.7Ma±1.8Ma(MSWD=0.56)。流水沟岩浆杂岩具有SiO2变化范围大(52.47%～66.95%),低TiO2,低碱,Mg#值变化范围较大的特征,主量元素在SiO2-氧化物变异图解上具有较好的线性相关性。稀土元素配分曲线总体呈近平坦型,LREE/HREE比值多数在1.14～2.60之间,轻重稀土元素分馏较弱,δEu介于0.65～1.90之间,大多数具有弱的负Eu异常;在微量元素蛛网图上,大离子亲石元素Cs、Sr、Th、U富集,Rb、K和高场强元素Nb、P、Ti、Y亏损,Ti/Y=113～253,Th/Yb=0.23～2.85,Th/Nb=0.09～1.00,Hf/Th=0.41～2.97,显示了流水沟岩浆杂岩形成于岛弧环境。流水沟岩浆杂岩可能反映在中央造山系中存在一期与"泛非造山运动"有关的构造热事件,为中央造山系在新元古代晚期—早古生代的构造演化提供了证据。     

西秦岭天水地区流水沟岩浆杂岩LA-ICP-MS锆石U-Pb测年和岩石地球化学特征

流水沟岩浆杂岩分布于西秦岭北缘天水关子镇地区,主要由变辉长岩、变辉长闪长岩和变闪长岩组成。为确定其构造属性,探讨西秦岭北缘构造演化过程,对其进行了LA-ICP-MS锆石U-Pb测年和主量、微量、稀土元素分析。结果表明,岩浆杂岩体的锆石为岩浆成因,其形成时代为548.7Ma±1.8Ma（MSWD=0.56）。流水沟岩浆杂岩具有SiO2变化范围大（52.47%～66.95%）,低TiO2,低碱,Mg#值变化范围较大的特征,主量元素在SiO2-氧化物变异图解上具有较好的线性相关性。稀土元素配分曲线总体呈近平坦型,LREE/HREE比值多数在1.14～2.60之间,轻重稀土元素分馏较弱,δEu介于0.65～1.90之间,大多数具有弱的负Eu异常;在微量元素蛛网图上,大离子亲石元素Cs、Sr、Th、U富集,Rb、K和高场强元素Nb、P、Ti、Y亏损,Ti/Y=113～253,Th/Yb=0.23～2.85,Th/Nb=0.09～1.00,Hf/Th=0.41～2.97,显示了流水沟岩浆杂岩形成于岛弧环境。流水沟岩浆杂岩可能反映在中央造山系中存在一期与＂泛非造山运动＂有关的构造热事件,为中央造山系在新元古代晚期—早古生代的构造演化提供了证据。     

西秦岭天水地区流水沟岩浆杂岩LA-ICP-MS锆石U-Pb测年和岩石地球化学特征

流水沟岩浆杂岩分布于西秦岭北缘天水关子镇地区,主要由变辉长岩、变辉长闪长岩和变闪长岩组成。为确定其构造属性,探讨西秦岭北缘构造演化过程,对其进行了LA-ICP-MS锆石U-Pb测年和主量、微量、稀土元素分析。结果表明,岩浆杂岩体的锆石为岩浆成因,其形成时代为548.7Ma±1.8Ma(MSWD=0.56)。流水沟岩浆杂岩具有SiO2变化范围大(52.47%~66.95%),低TiO2,低碱,Mg#值变化范围较大的特征,主量元素在SiO2-氧化物变异图解上具有较好的线性相关性。稀土元素配分曲线总体呈近平坦型,LREE/HREE比值多数在1.14~2.60之间,轻重稀土元素分馏较弱,δEu介于0.65~1.90之间,大多数具有弱的负Eu异常;在微量元素蛛网图上,大离子亲石元素Cs、Sr、Th、U富集,Rb、K和高场强元素Nb、P、Ti、Y亏损,Ti/Y=113~253,Th/Yb=0.23~2.85,Th/Nb=0.09~1.00,Hf/Th=0.41~2.97,显示了流水沟岩浆杂岩形成于岛弧环境。流水沟岩浆杂岩可能反映在中央造山系中存在一期与“泛非造山运动”有关的构造热事件,为中央造山系在新元古代晚期—早古生代的构造演化提供了证据。     

Gabbroic rocks in ophiolitic occurrences from East Othris, Greece: petrogenetic processes and geotectonic environment implications

East Othris area consists of scattered ophiolitic units, as well as ophiolitic mélange occurrences, which encompass gabbroic rocks. These rocks have been affected by low-grade ocean floor metamorphism (T?<?350°C and P?<?8?kbar). Based on their petrography, mineral chemistry and geochemistry gabbroic rocks have been distinguished into gabbros and diorites, with the latter being divided into two groups. Gabbros seem to have been formed from moderate to high partial melting degrees (~8–25%) of a highly depleted mantle source, while group (1) diorites have been differentiated after variable fractionation processes (up to 30%). Group (2) diorites seem to have been derived from low partial melting degrees (~3%) of a fertile or moderately depleted mantle source and with extensive fractionation processes (~50%). Geochemical results suggest that partial melting processes occurred at relatively shallow depths, in the plagioclase-spinel stability field, while amphibole chemistry data indicate shallow level crystallization. Chondrite and PM-normalized patterns, Th/Yb, and Nb/Th ratios as well as mineral chemistry analyses show that gabbros and group (1) diorites (with relatively low PM-normalized Nb and Ta values and negative Ti anomalies) suggest subduction processes, while group (2) diorites are MORB or BAB related. Some gabbros have been characterised as high-Mg, being compositionally similar to picrites or boninites. Variability in extent of partial melting of the mantle source and the different geotectonic environment affinities are consistent with a supra-subduction zone (SSZ) origin of the east Othris ophiolites. The fact that IAT related rocks are more abundant in east rather than in west Othris may possibly be explained by a slab rollback model retreating to the east within the Pindos oceanic basin.     

UHP metamorphism and exhumation of the Dabie Orogen, China: Evidence from SHRIMP dating of zircon and monazite from a UHP granitic gneiss cobble from the Hefei Basin

A SHRIMP U-Pb study of zircons separated from an ultrahigh-pressure (UHP) granitic gneiss cobble (sample DS12) from the Upper Jurassic Fenghuangtai Formation of the Hefei Basin, north of the Dabie Orogen, has identified three different domains: (1) cores, some of which show straight boundaries and strong oscillatory zoning, with Th/U ratios of 0.12-0.70 and an imprecise upper intercept age of 777 ± 220 Ma (MSWD = 2.4); (2) mantles, variable in shape and cathodoluminescence (CL) intensity and containing many UHP mineral inclusions, including coesite and omphacite, and with Th/U ratios of 0.02-0.26 and a weighted mean ²⁰⁶Pb/²³⁸U age of 244 ± 5 Ma (MSWD = 4.7); and (3) rims, which are more homogeneous and luminescent in CL than the mantles and contain fewer UHP mineral inclusions, with Th/U ratios of 0.01-0.05 and a weighted mean ²⁰⁶Pb/²³⁸U age of 226 ± 2 Ma (MSWD = 0.65). Monazites from the same sample do not contain coesite and omphacite inclusions, but they do show sector zones and are composed of two contrasting domains. The light domains in backscattered electron (BSE) images have a Th/U ratio of 7.3-18.9 and a weighted mean ²⁰⁶Pb/²³⁸U age of 221 ± 2 Ma (MSWD = 0.94). The dark domains in BSE have a Th/U ratio of 2.8-7.2 and have a weighted mean ²⁰⁶Pb/²³⁸U age of 218 ± 2 Ma (MSWD = 0.96). Based on these data and a review of the literature, it can be concluded that (1) the protolith of gneiss sample DS12 is a granitoid of Neoproterozoic age; (2) the ages of 244 and 226 Ma for the mantle and rim domains of the zircons are time records of two discrete stages in the UHP evolution of Dabieshan; (3) the ages of 221 and 218 Ma for the light and dark gray domains, respectively, of the monazites record the times of two stages of retrogressive metamorphism during exhumation of the UHP granitic gneiss to a higher crustal level; (4) the exhumation rate of the gneiss is estimated to be >6 km/Ma; and (5) the UHP metamorphic blocks of the Dabie Orogen provided significant detritus to the Hefei Basin in the Jurassic.     

新疆托云火山群SHRIMP锆石U-Pb年代学及其动力学意义

新疆西南天山托云盆地是一个新生代火山构造盆地。盆地内火山岩主要为玄武质岩石,呈锥状岩席、熔岩被、塌陷破火山口、火山颈和岩脉产出,常见流动自碎角砾岩。锥状岩席含有大量深源岩石包体,具有原生岩浆的性质,其 SHRIMP 锆石 U-Pb 法定年结果为48.1±1.6Ma。塌陷破火山口附近熔岩层底部的锆石(样品 A015D)给出比较集中的新太古宙和古元古代年龄,最老可达2.5Ga,可能代表了塔里木地块基底的卷入;而上部岩层的锆石(样品 A015E)年龄则比较分散,几乎关贯穿整个古生代(寒武纪和泥盆纪除外),是天山古造山带演化历史的记录。火山颈锆石(样品 A-D016)的年龄时间跨度最大,包括古元古代,古生代泥盆纪和二叠纪以及中生代三叠纪。SHRIMP 测年结果证实了托云火山群开始活动的时代为古近纪,与藏北新生代岩浆活动的起始时间大致相同,揭示了天山新生代板内造山过程与印度-亚洲大陆碰撞的内在联系。不同成因与形成时间的锆石共存,显示了西南天山岩石圈演化的复杂性,托云火山群的形成与岩石圈拆沉作用有关。