Advances in our understanding of the gem corundum deposits of the West Pacific continental margins intraplate basaltic fields   总被引：2，自引：1，他引：1  

Ian Graham  Lin Sutherland  Khin Zaw  Victor Nechaev  Alexander Khanchuk 《Ore Geology Reviews》2008,34(1-2):200

Recent discoveries over the last decade of new gemfields, exploitation of new and existing deposits, and application of relatively new techniques have greatly increased our knowledge of the basalt-derived gem sapphire–ruby–zircon deposits. In this paper we focus on the Late Mesozoic to Cenozoic intraplate basaltic fields of the West Pacific continental margins. We review advances made in understanding the genesis of these deposits, based on the application of newer techniques. We also critically review existing data on the gem corundum deposits, in order to further refine a model for their origin.In some of the intraplate basaltic fields, corundum-bearing xenoliths have been found showing a range of PT formation conditions from 790 °C at 0.85 GPa to as much as 1100 to 1200 °C at 1.0 to 2.5 GPa. Although most magmatic sapphires contain syngenetic inclusions of columbite-group phases, zircon, spinel and rutile, some contain additional nepheline and K-feldspar, suggesting crystallization from more undersaturated alkaline magma while the Weldborough field of NE Tasmania also contains molybdenite and beryl, suggesting at least some interaction with more fractionated ‘granitic-type’ magmas. There is a large range in PT conditions calculated for the metamorphic rubies (from 780 to 940 °C, through 800 to 1150 °C up to 1000 to 1300 °C). Fluid/melt inclusion studies on magmatic corundums generally suggest that they formed in a CO₂-rich environment from a ‘syenitic’ melt under a range of T conditions from 720 to 880 °C up to 1000 to 1200 °C. Oxygen isotope studies reveal that typical magmatic corundums have values of + 4.4 to 6.9‰, whereas metamorphic corundums from the same basaltic host have lower values of + 1.3 to 4.2‰.Geochronological studies have shown that some fields produced a simple eruptive and zircon/corundum crystallization event while others had multiple eruptive events but only one or two zircon crystallization events. For a few fields, some corundums/zircons crystallized in storage regions and then remained relatively inert for periods of 200 to 400 Ma without significant change before transport to the surface in the Cenozoic. Tectonic studies of the Australian region suggest that many of the corundums crystallized from magmas related to episodic basaltic volcanism in a tectonic regime of extension, associated with the opening of the Tasman and Coral Seas. For the Asian region, the magmatic–polygenetic corundums within the basaltic fields largely crystallized in a tectonic regime of distributed E–W extension, whereas the metamorphic-metasomatic corundums crystallised in a transpressional regime associated with the collision of the Indian Plate with the Eurasian Plate (e.g., [Garnier, V., Giuliani, G., Maluski, H., Ohnenstetter, D., Deloule, E., 2003. Ar–Ar and U–Pb ages of marble-hosted ruby deposits from Central and South-east Asia. Geophysical Research Abstracts 5, 03751; Garnier, V., Giuliani, G., Ohnenstetter, D., and Schwarz, D., 2004. Les gisements de corindon: classification et genese. Les placers a corindon gemme. Le Regne Mineral 55, 7-47; Garnier, V., Ohnenstetter, D., Giuliani, G., Maluski, H., Deloule, E., Phan Trong, T., Pham Van, L., Hoang Quang, V., 2005a. Age and significance of ruby-bearing marble from the Red River Shear Zone, Northern Vietnam. Canadian Mineralogist 43, 1315–1329]).  相似文献   

Basement–cover relationships of the Kalak Nappe Complex, Arctic Norwegian Caledonides and constraints on Neoproterozoic terrane assembly in the North Atlantic region   总被引：1，自引：0，他引：1  

Christopher L. Kirkland  J. Stephen Daly  Martin J. Whitehouse 《Precambrian Research》2008,160(3-4):245-276

The Kalak Nappe Complex (KNC) has been regarded as Baltica passive margin metasediments telescoped eastwards onto the Baltic (Fennoscandian) Shield during the Caledonian Orogeny. Recent studies have questioned this interpretation, instead pointing to a Neoproterozoic exotic origin. In an effort to resolve this controversy we present a Sm–Nd and U–Th–Pb study of gnessic units, traditionally considered as the depositional basement, along with cover rock sediments and intrusives. Late Palaeoproterozoic gneisses now beneath the KNC were deposited after 1948 ± 33 Ma, before intrusion of the Tjukkfjellet Granite at 1796 ± 3 Ma, and were affected by later melting events at 1765 ± 9 and 1727 ± 9 Ma. These gneisses are interpreted as part of the Baltic Shield and underlie the KNC across a tectonic contact. An unconformity between psammites of the KNC and other paragneisses previously considered as its Precambrian basement is reinterpreted as a modified sedimentary contact between Neoproterozoic metasediments. These metasediments have statistically very similar detrital zircon populations with grains as young as 1034 ± 22, 1025 ± 32 and 1014 ± 14 Ma. The results indicate that the KNC sediments were deposited during the Neoproterozoic in basins along the Laurentian margin of eastern Rodinia and were not connected to Baltica via a depositional basement. Dating of the 851 ± 5 Ma Eidvågvatnet and 853 ± 4 Ma Nordneset granites shows that intrusive material associated with the Porsanger Orogeny (c. 850 Ma) affected a considerable region of the upper KNC terrane. Later Neoproterozoic events at 711 ± 6, 687 ± 12 and 617 ± 6 Ma are also recognised the latest of which may be an expression of rifting. Since early Neoproterozoic magmatism (c. 840–690 Ma) is unknown in Baltica, these results support an exotic origin for the KNC terranes.  相似文献   

Zircon U–Pb ages, Hf and O isotopes constrain the crustal architecture of the ultrahigh-pressure Dabie orogen in China     

Zi-Fu Zhao  Yong-Fei Zheng  Chun-Sheng Wei  Fu-Kun Chen  Xiaoming Liu  Fu-Yuan Wu 《Chemical Geology》2008,253(3-4):222-242

The crustal structure of the Dabie orogen was reconstructed by a combined study of U–Pb ages, Hf and O isotope compositions of zircons from granitic gneiss from North Dabie, the largest lithotectonic unit in the orogen. The results were deciphered from metamorphic history to protolith origin with respect to continental subduction and exhumation. Zircon U–Pb dating provides consistent ages of 751 ± 7 Ma for protolith crystallization, and two group ages of 213 ± 4 to 245 ± 17 Ma and 126 ± 4 to 131 ± 36 Ma for regional metamorphism. Majority of zircon Hf isotope analyses displays negative ε_Hf(t) values of − 5.1 to − 2.9 with crust Hf model ages of 1.84 to 1.99 Ga, indicating protolith origin from reworking of middle Paleoproterozoic crust. The remaining analyses exhibit positive ε_Hf(t) values of 5.3 to 14.5 with mantle Hf model ages of 0.74 to 1.11 Ga, suggesting prompt reworking of Late Mesoproterozoic to Early Neoproterozoic juvenile crust. Zircon O isotope analyses yield δ¹⁸O values of − 3.26 to 2.79‰, indicating differential involvement of meteoric water in protolith magma by remelting of hydrothermally altered low δ¹⁸O rocks. North Dabie shares the same age of Neoproterozoic low δ¹⁸O protolith with Central Dabie experiencing the Triassic UHP metamorphism, but it was significantly reworked at Early Cretaceous in association with contemporaneous magma emplacement. The Rodinia breakup at about 750 Ma would lead to not only the reworking of juvenile crust in an active rift zone for bimodal protolith of Central Dabie, but also reworking of ancient crust in an arc-continent collision zone for the North Dabie protolith. The spatial difference in the metamorphic age (Triassic vs. Cretaceous) between the northern and southern parts of North Dabie suggests intra-crustal detachment during the continental subduction. Furthermore, the Dabie orogen would have a three-layer structure prior to the Early Cretaceous magmatism: Central Dabie in the upper, North Dabie in the middle, and the source region of Cretaceous magmas in the lower.  相似文献   

Timing of UHP metamorphism in the Hong’an area,western Dabie Mountains,China: evidence from zircon U–Pb age,trace element and Hf isotope composition     

Yuan-Bao Wu  Shan Gao  Hong-Fei Zhang  Sai-Hong Yang  Wen-Fang Jiao  Yong-Sheng Liu  Hong-Lin Yuan 《Contributions to Mineralogy and Petrology》2008,155(1):123-133

The Hong’an area (western Dabie Mountains) is the westernmost terrane in the Qinling-Dabie-Sulu orogen that preserves UHP eclogites. The ages of the UHP metamorphism have not been well constrained, and thus hinder our understanding of the tectonic evolution of this area. LA-ICPMS U–Pb age, trace element and Hf isotope compositions of zircons of a granitic gneiss and an eclogite from the Xinxian UHP unit in the Hong’an area were analyzed to constrain the age of the UHP metamorphism. Most zircons are unzoned or show sector zoning. They have low trace element concentrations, without significant negative Eu anomalies. These metamorphic zircons can be further subdivided into two groups according to their U–Pb ages, and trace element and Lu–Hf isotope compositions. One group with an average age of 239 ± 2 Ma show relatively high and variable HREE contents (527 ≥ Lu_N ≥ 14) and ¹⁷⁶Lu/¹⁷⁷Hf ratios (0.00008–0.000931), indicating their growth prior to a great deal of garnet growth in the late stage of continental subduction. The other group yields an average age of 227 ± 2 Ma, and shows consistent low HREE contents and ¹⁷⁶Lu/¹⁷⁷Hf ratios, suggesting their growth with concurrent garnet crystallization and/or recrystallization. These two groups of age are taken as recording the time of prograde HP to UHP and retrograde UHP–HP stages, respectively. A few cores have high Th/U ratios, high trace element contents, and a clear negative Eu anomaly. These features support a magmatic origin of these zircon cores. The upper intercept ages of 771 ± 86 and 752 ± 70 Ma for the granitic gneiss and eclogite, respectively, indicate that their protoliths probably formed as a bimodal suite in rifting zones in the northern margin of the Yangtze Block. Young Hf model ages (T _DM1) of magmatic cores indicate juvenile (mantle-derived) materials were involved in their protolith formation. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

Zircon coronas around Fe–Ti oxides: a physical reference frame for metamorphic and metasomatic reactions     

Håkon Austrheim  Christine V. Putnis  Ane K. Engvik  Andrew Putnis 《Contributions to Mineralogy and Petrology》2008,156(4):517-527

Ilmenite in coronitic gabbros from the Bamble and Kongsberg sectors, southern Norway, is surrounded by zircons ranging in diameters from a fraction of a micrometer to 10 μm across. The zircons are inert during subsequent metamorphism (amphibolite- to pumpellyite–prehnite facies) and metasomatism (scapolitization and albitization) and can be found as trails in silicates (phlogopite, talc, chlorite, amphibole, albite, and tourmaline) in the altered rocks. The trails link up to form polygons outlining the former oxide grain boundary. This 3-dimensional framework of zircons is used to (a) recognize metasomatic origin of rocks, (b) quantify the mobility of elements during mineral replacement, (c) establish the growth direction of reaction fronts and to identify the reaction mechanism as dissolution–reprecipitation. Zircon coronas on Fe–Ti oxides have been described from a number of terrains and appear to be common in mafic rocks (gabbros and granulites) providing a tool for a better understanding of metasomatic and metamorphic reactions.  相似文献   

Variscan basic dykes in the Pelagonian (Northern Greece and south FYROM): Geodynamic significance based on petrological, geochemical and geochronological studies     

Athanassios Katerinopoulos   《Chemie der Erde / Geochemistry》2008,68(1):93-103

Dolerite dykes intruding Variscan plutonites were studied in terms of mineralogy, petrology, geochemistry and geochronology. The main mineral constituents were studied and the sequence of crystallization has been derived. The geochemical characteristic indicate mantle origin of the dolerites and magma sources different from the hosting granitoids. From SHRIMP analyses of five spots on four different zircon crystals, resulted a 292.0±4.1 Ma age that is interpreted as the time of crystallization of the dolerite. The hosting granitoids are probably the result of mixing between two possible end-members: enriched mantle and acid metaigneous or lower crustal metasediments.

The Variscan age of the dolerites, in combination with the geochemical characteristics, indicated that the enriched mantle basaltic material should be the source of the dolerite veins. These mantle-derived basaltic melts may represent the underplated material, which probably provided the necessary thermal input to the dehydration melting in the lower crust. The dolerites should have intruded the newly formed batholiths before or at the first stages of their uplift, recording the last events of the Variscan subduction.  相似文献   

Si diffusion in zircon     

D. J. Cherniak 《Physics and Chemistry of Minerals》2008,35(4):179-187

Self-diffusion of Si under anhydrous conditions at 1 atm has been measured in natural zircon. The source of diffusant for experiments was a mixture of ZrO₂ and ³⁰Si-enriched SiO₂ in 1:1 molar proportions; experiments were run in crimped Pt capsules in 1-atm furnaces. ³⁰Si profiles were measured with both Rutherford backscattering spectrometry (RBS) and nuclear reaction analysis with the resonant nuclear reaction ³⁰Si(p,γ)³¹P. For Si diffusion normal to c over the temperature range 1,350–1,550°C, we obtain an Arrhenius relation D = 5.8 exp(−702 ± 54 kJ mol⁻¹/RT) m² s⁻¹ for the NRA measurements, which agrees within uncertainty with an Arrhenius relation determined from the RBS measurements [62 exp(−738 ± 61 kJ mol⁻¹/RT) m² s⁻¹]. Diffusion of Si parallel to c appears slightly faster, but agrees within experimental uncertainty at most temperatures with diffusivities for Si normal to c. Diffusion of Si in zircon is similar to that of Ti, but about an order of magnitude faster than diffusion of Hf and two orders of magnitude faster than diffusion of U and Th. Si diffusion is, however, many orders of magnitude slower than oxygen diffusion under both dry and hydrothermal conditions, with the difference increasing with decreasing temperature because of the larger activation energy for Si diffusion. If we consider Hf as a proxy for Zr, given its similar charge and size, we can rank the diffusivities of the major constituents in zircon as follows: D _Zr < D _Si << D _{O, dry} < D _{O, ‘wet’}.  相似文献   

西藏冈底斯带措勤地区则弄群火山岩锆石U-Pb年代学格架及构造意义   总被引：29，自引：12，他引：17  

朱弟成  周肃  莫宣学  赵志丹  许继峰  周长勇  孙晨光  王立全  陈海红  董国臣 《岩石学报》2008,24(3)

大规模的则弄群火山岩呈带状近东西向展布于西藏冈底斯带中北部地区.对出露状况较好的措勤地区则弄群火山岩进行了锆石U-Pb定年,以建立其年代学格架便于区域对比.措勤地区则弄群火山岩多数锆石具有生长振荡环带,部分锆石显示核边结构.在措勤达雄北西部上覆于中二叠统下拉组灰岩的1件则弄群英安岩样品的LA-ICP-MS锆石U-Ph年龄为130±1Ma,1件则弄群流纹岩样品的SHRIMP锆石U-Pb年龄为129±3Ma;在措勤达雄北东部与下白垩统多尼组碎屑岩呈断层接触的1件则弄群英安岩样品中的锆石具有两组LA-ICP-MS锆石U-Pb年龄,最年轻的一组(包括增生边)为121±1Ma,较老的一组(包括核部)为131±1Ma;在措勤南西部1件则异群流纹岩样品的LA-ICP-MS锆石U-Pb年龄为111±1Ma,与措勤地区花岗岩类的侵位时代相当.这些高质量的锆石U-Pb年代学数据表明,措勤地区的则弄群火山作用很可能开始于约130Ma,停息于约110Ma,持续时间约20Ma.区域对比表明,东西延伸约1000km的则弄群火山作用同时发生在约130Ma.区域上目前的年代学数据结合同时代岩浆作用的分布特征表明,冈底斯带中北部地区早白垩世岩浆作用不太可能由新特提斯洋壳向北的低角度或平板俯冲产生.  相似文献   

东天山黄山-镜儿泉过铝花岗岩矿物学、地球化学及年代学研究   总被引：3，自引：0，他引：3  

唐俊华  顾连兴  张遵忠  吴昌志  三金柱  汪传胜  刘四海  黎广荣 《岩石学报》2008,24(5):921-946

对东天山黄山-镜儿泉一带黄山南、镜儿泉、图拉尔根沟三个过铝花岗岩作了岩相学、矿物学、地球化学、sr-Nd同位素和锆石U-Pb年代学研究.锆石U-Pb LA-ICP-MS原位定年测得黄山南岩体结晶年龄为259.9±1.4Ma(MSWD=0.86),图拉尔根沟岩体结晶年龄为275.4±8.3Ma(MSWD=29),均侵位于二叠纪碰撞后伸展环境.三个过铝花岗岩均具有低锶同位素初始比值(Isr=0.6969～0.70396)、高εNd(t)值( 5.5～ 7.2)以及年轻的亏损地幔单阶段模式年龄(tDM=0.48～0.56Ga),表明其岩浆源区均为来源于亏损地幔的新生地壳岩石.这种新生地壳岩石可能为偏酸性的火山岩.三个岩体的矿物学和地球化学可分为两类:一类以黄山南白云母花岗岩为代表,为强过铝花岗岩(A/CNK>1.1),强烈亏损Ba、sr和Ti而富集Cs、Rb和K,具有高的Rb/sr(2.03～14.5)和Al2O3/TiO2(110～1592),低的Nb/Ta(3.24～6.76)比值,其稀土元素配分曲线呈"V"字形,显示强烈铕亏损(Eu/Eu*=0.04～0.55),表明这类岩体的直接源岩以泥质岩为主.另一类以图拉尔根沟二云母花岗岩为代表,为弱过铝(1相似文献   

新疆塔里木北缘色日牙克依拉克一带泥盆纪花岗质侵入体的确定及其地质意义   总被引：15，自引：7，他引：8  

朱志新  李锦轶  董连慧  王克卓  刘国忠  李亚萍  刘振涛 《岩石学报》2008,24(5)

位于新疆南天山色日牙克依拉克一带发育较多的中酸性侵入岩,前人均划为石炭纪侵入岩,作者对其中花岗闪长岩锆石进行SHRIMP定年,获得侵入岩形成年龄387±8Ma,另有一组捕获锆石的年龄为418.4±6.5Ma,通过对这些侵入岩的岩石学研究,结果显示该区泥盆纪侵入岩均具较典型活动陆缘侵入岩的特征.结合区域岩浆活动资料及前人成果,初步确定在塔里木北缘木札尔特、色日牙克依拉克、虎拉山南缘野云沟至库尔勒一带在古生代为一活动陆缘.  相似文献