首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 734 毫秒
1.
Zircon from a lower crustal metapelitic granulite (Val Malenco, N‐Italy) display inherited cores, and three metamorphic overgrowths with ages of 281 ± 2, 269 ± 3 and 258 ± 4 Ma. Using mineral inclusions in zircon and garnet and their rare earth element characteristics it is possible to relate the ages to distinct stages of granulite facies metamorphism. The first zircon overgrowth formed during prograde fluid‐absent partial melting of muscovite and biotite apparently caused by the intrusion of a Permian gabbro complex. The second metamorphic zircon grew after formation of peak garnet, during cooling from 850 °C to c. 700 °C. It crystallized from partial melts that were depleted in heavy rare earth elements because of previous, extensive garnet crystallization. A second stage of partial melting is documented in new growth of garnet and produced the third metamorphic zircon. The ages obtained indicate that the granulite facies metamorphism lasted for about 20 Myr and was related to two phases of partial melting producing strongly restitic metapelites. Monazite records three metamorphic stages at 279 ± 5, 270 ± 5 and 257 ± 4 Ma, indicating that formation ages can be obtained in monazite that underwent even granulite facies conditions. However, monazite displays less clear relationships between growth zones and mineral inclusions than zircon, hampering the correlation of age to metamorphism. To overcome this problem garnet–monazite trace element partitioning was determined for the first time, which can be used in future studies to relate monazite formation to garnet growth.  相似文献   

2.
U-Pb isotopic analyses of eight single and multi-grain zircon fractions separated from a syenite of the Diana Complex of the Adirondack Mountains do not define a single linear array, but a scatter along a chord that intersects the Concordia curve at 1145 ± 29 and 285 ± 204 Ma. For the most concordant analyses, the207Pb/206Pb ages range between 1115 and 1150 Ma. Detailed petrographic studies revealed that most grains contained at least two phases of zircon growth, either primary magmatic cores enclosed by variable thickness of metamorphic overgrowths or magmatic portions enclosing presumably older xenocrystic zircon cores. The magmatic portions are characterized by typical dipyramidal prismatic zoning and numerous black inclusions that make them quite distinct from adjacent overgrowths or cores when observed in polarizing light microscopy and in backscattered electron micrographs. Careful handpicking and analysis of the “best” magmatic grains, devoid of visible overgrowth of core material, produced two nearly concordant points that along with two of the multi-grain analyses yielded an upper-intercept age of 1118 ± 2.8 Ma and a lowerintercept age of 251 ± 13 Ma. The older age is interpreted as the crystallization age of the syenite and the younger one is consistent with late stage uplift of the Appalachian region. The 1118 Ma age for the Diana Complex, some 35 Ma younger than previously believed, is now approximately synchronous with the main Adirondack anorthosite intrusion, implying a cogenetic relationship among the various meta-igneous rocks of the Adirondacks. The retention of a high-temperature contact metamorphic aureole around Diana convincingly places the timing of Adirondack regional metamorphism as early as 1118 Ma. This result also implies that the sources of anomalous hightemperature during granulite metamorphism are the syn-metamorphic intrusions, such as the Diana Complex.  相似文献   

3.
This contribution evaluates the relation between protracted zircon geochronological signal and protracted crustal melting in the course of polyphase high to ultrahigh temperature (UHT; T?>?900 °C) granulite facies metamorphism. New U–Pb, oxygen isotope, trace element, ion imaging and cathodoluminescence (CL) imaging data in zircon are reported from five samples from Rogaland, South Norway. The data reveal that the spread of apparent age captured by zircon, between 1040 and 930 Ma, results both from open-system growth and closed-system post-crystallization disturbance. Post-crystallization disturbance is evidenced by inverse age zoning induced by solid-state recrystallization of metamict cores that received an alpha dose above 35 × 1017 α  g?1. Zircon neocrystallization is documented by CL-dark domains displaying O isotope open-system behaviour. In UHT samples, O isotopic ratios are homogenous (δ18O = 8.91?±?0.08‰), pointing to high-temperature diffusion. Scanning ion imaging of these CL-dark domains did not reveal unsupported radiogenic Pb. The continuous geochronological signal retrieved from the CL-dark zircon in UHT samples is similar to that of monazite for the two recognized metamorphic phases (M1: 1040–990 Ma; M2: 940–930 Ma). A specific zircon-forming event is identified in the orthopyroxene and UHT zone with a probability peak at ca. 975 Ma, lasting until ca. 955 Ma. Coupling U–Pb geochronology and Ti-in-zircon thermometry provides firm evidence of protracted melting lasting up to 110 My (1040–930 Ma) in the UHT zone, 85 My (ca. 1040–955 Ma) in the orthopyroxene zone and some 40 My (ca. 1040–1000 Ma) in the regional basement. These results demonstrate the persistence of melt over long timescales in the crust, punctuated by two UHT incursions.  相似文献   

4.
Recent work in Barrovian metamorphic terranes has found that rocks experience peak metamorphic temperatures across several grades at similar times. This result is inconsistent with most geodynamic models of crustal over‐thickening and conductive heating, wherein rocks which reach different metamorphic grades generally reach peak temperatures at different times. Instead, the presence of additional sources of heat and/or focusing mechanisms for heat transport, such as magmatic intrusions and/or advection by metamorphic fluids, may have contributed to the contemporaneous development of several different metamorphic zones. Here, we test the hypothesis of temporally focussed heating for the Wepawaug Schist, a Barrovian terrane in Connecticut, USA, using Sm–Nd ages of prograde garnet growth and U–Pb zircon crystallization ages of associated igneous rocks. Peak temperature in the biotite–garnet zone was dated (via Sm–Nd on garnet) at 378.9 ± 1.6 Ma (2σ), whereas peak temperature in the highest grade staurolite–kyanite zone was dated (via Sm–Nd on garnet rims) at 379.9 ± 6.8 Ma (2σ). These garnet ages suggest that peak metamorphism was pene‐contemporaneous (within error) across these metamorphic grades. Ion microprobe U–Pb ages for zircon from igneous rocks hosted by the metapelites also indicate a period of syn‐metamorphic peak igneous activity at 380.6 ± 4.7 Ma (2σ), indistinguishable from the peak ages recorded by garnet. A 388.6 ± 2.1 Ma (2σ) garnet core age from the staurolite–kyanite zone indicates an earlier episode of growth (coincident with ages from texturally early zircon and a previously published monazite age) along the prograde regional metamorphic T–t path. The timing of peak metamorphism and igneous activity, as well as the occurrence of extensive syn‐metamorphic quartz vein systems and pegmatites, best supports the hypothesis that advective heating driven by magmas and fluids focussed major mineral growth into two distinct episodes: the first at c. 389 Ma, and the second, corresponding to the regionally synchronous peak metamorphism, at c. 380 Ma.  相似文献   

5.
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.  相似文献   

6.
ABSTRACT The products of metamorphic fluid flow are preserved in zones within the marbles and metamorphosed semipelites of the Upper Calcsilicate Unit in the granulite portion of the Late Palaeoproterozoic Reynolds Range Group, northern Arunta Block, central Australia. The zones of retrogression, characterized by minerals such as wollastonite, grossular and clinohumite, local resetting of oxygen isotopic compositions and local major element metasomatism, were channelways for water-rich fluids derived from granulite facies metapelites. U–Th–Pb isotopic ages measured by the SHRIMP ion microprobe on zircon and monazite from a granulite facies semipelite, an early semiconcordant aluminous quartz-rich fluid-flow segregation and a late discordant quartz-rich segregation record some of the extended thermal history of the area. Zircon cores from the semipelite show its likely protolith to be an igneous rock 1812 ± 11 Ma old, itself derived from a source containing zircon as old as 2.2 Ga. Low-Th/U overgrowths on the zircon grew during granulite facies metamorphism at 1594 ± 6 Ma. Monazite cooled to its blocking temperature at 1576 ± 8 Ma. Zircon cores from the semiconcordant segregation are dominantly >2.3 Ga old, indicating that the source of the fluids was not the particular metamorphosed semipelite studied. Two generations of low-Th/U overgrowths on the zircon give indistinguishable ages for the older and younger of 1589 ± 8 and 1582 ± 8 Ma, respectively. The monazite age is the same, 1576 ± 12 Ma. Zircon from the late discordant segregation gave 1568 ± 4 Ma. Fluid flow occurred for at least 18 ± 3 (σ) Ma and ended 26 ± 3 (σ) Ma after the peak of metamorphism, suggesting a very slow cooling rate of ～3°C Ma–1. The last regional high-grade metamorphism in the Reynolds Range occurred at ～1.6 Ga, not ～1.78 Ga as previously thought. The high-grade event at ～1.78 Ga is a separate event that affected only the basement to the Reynolds Range Group.  相似文献   

7.
Zircon cathodoluminescent imaging and SHRIMP U-Pb dating were carried out for metapelitic rocks (sapphirine-bearing granulites and garnet-cordierite gneisses) from the NW of Madurai, Southern India. The cathodoluminescence images reveal the complex, inhomogeneous internal structure having irregular-shaped core and overgrowths. Zircon grains have obliterated oscillatory zoning. SHRIMP U-Pb chronological results yield ages of 550±15 Ma and 530±50 Ma as a time of metamorphic overprint, and the age of 2509±12 Ma and 2509±30 Ma corresponding to a timing of protolith formation for sapphirine-bearing granulites and garnet-cordierite gneisses respectively. Zircon ages reflect that continental crust in the NW of Madurai region resulted from the recycling of Archaean protolith of an igneous origin similar to the preserved crust in the southern part of Dharwar craton. The present SHRIMP U-Pb zircon ages are in close agreement with earlier published Nd isotopic data which suggest an extended precrustal history of their protoliths. The abraded zircon grains indicate multiple recycling and repeated metamorphism that has ultimately resulted in present day continental crust exposed in Madurai region. These SHRIMP U-Pb zircon ages from metapelitic UHT granulites are also significant to understanding the architecture of the SGT during the amalgamation of Gondwana in Neoproterozoic time.  相似文献   

8.
The Madurai Block (MB) is the largest Precambrian crustal block in the Southern Granulite Terrane (SGT) of India and hosts rare cordierite- and orthopyroxene-bearing granulites. Investigations based on field study, petrology, metamorphic P–T estimation, and detrital zircon geochronology of these granulites are crucial for understanding the ultrahigh-temperature (UHT) metamorphism and crustal evolution in this block. Here we investigate the petrology and zircon U–Pb geochronology of two new localities of cordierite granulites at Kottayam (southern MB; SMB) and Munnar (central MB; CMB). Petrographic observations and phase equilibria modelling results indicate that these rocks experienced UHT metamorphism with the peak temperature exceeding 950℃ and involving clockwise P–T paths. The prograde mineral assemblages define the P–T conditions of 6.8–8.7 kbar and 750–875℃. The peak conditions are estimated using pseudosection modelling and geothermometry, which yield P–T estimates of 7.1–9.1 kbar and 955–985℃. The retrograde cooling and decompression are inferred at 860–790℃ and <6.5 kbar, respectively. Partial melting played an important role during metamorphism and contributed to the overgrowth around detrital zircons. The melt production process was probably related to biotite dehydration melting, and was mainly triggered by heating, with or without the effect of decompression. Detrital zircons in cordierite granulite samples from the two localities show similar age distributions and have dominantly Neoproterozoic ages (1024–760 Ma). The zircon cores show oscillatory zoning with a wide range of Th/U ratios (0.01–0.96), implying complex protoliths from multiple Neoproterozoic provenances from both southern and central domains of the MBs. Zircon rims and homogeneous bright zircons yield mean ages of 549 ± 5 Ma, 536 ± 6 Ma, and 544 ± 6 Ma, which are interpreted to represent zircon overgrowths during the post-peak cooling and decompression process. The timing of peak UHT metamorphism is constrained as 549–599 Ma, which coincides with the assembly of the Gondwana supercontinent.  相似文献   

9.
Integrated textural and chemical characterisation of zircon is used to refine the U–Pb geochronology of the Archaean, ultra-high temperature Napier Complex, east Antarctica. Scanning electron microscope characterisation of zircon and the rare earth element compositions of zircon, garnet and orthopyroxene are integrated to place zircon growth in an assemblage context, thereby providing tighter constraints on the timing of magmatic and metamorphic events. Data indicate that magmatism occurred in the central and northern Napier Complex at ca. 2,990 Ma. A regional, relatively low-pressure metamorphic event occurred at ca. 2,850–2,840 Ma. Mineral REE data from garnet-bearing orthogneiss indicate that ca. 2,490–2,485 Ma U–Pb zircon ages provide an absolute minimum age for the ultrahigh temperature (UHT) foliation preserved in this rock. Internal zircon zoning relationships and estimated zircon-garnet DREE values from paragneiss suggest that an absolute minimum age of ultra-high temperature metamorphism is ca. 2,510 Ma, but that it is more likely to be older than ca. 2,545 Ma. We suggest that the high proportion of published zircon U–Pb data with ages between ca. 2,490–2,450 Ma reflects late, post-peak zircon growth and does not date the timing of peak UHT metamorphism.Electronic Supplementary Material Supplementary material is available for this article at  相似文献   

10.
刘福来  刘平华 《岩石学报》2009,25(9):2113-2131
北苏鲁仰口地区出露超高压的变辉长岩.锆石阴极发光图像和其内部矿物包体激光拉曼测试的联合研究结果表明,变辉长岩锆石具有弱发光效应的岩浆韵律环带的核和被改造的强发光效应的边.岩浆韵律环带的核部保存大量而复杂的矿物包体,包括普通辉石(Cpx)+斜方辉石(Opx)+斜长石(P1)+石英(Qtz)+黑云母(Bt)+钛铁矿(Ilm)+磷灰石(Ap);边部保存的矿物包体则相对较少,包括普通辉石(Cpx)+斜方辉石(Opx)+斜长石(Pl)+磷灰石(Ap).尽管岩浆韵律环带核部的稀土元素总合量比被改造的锆石边部明显偏高,但二者稀土元素配分模式具有明显的相似性,主要表现为轻稀土相对亏损,而重稀土明显富集,相应的(La/Yb)N=0.00015～0.00039,并具有明显的负Eu异常(Eu/Eu*=0.20～0.26)、相对明显的正Ce异常(Ce/Ce*=71.5～147.4)和较高的Th/U比值(1.97～2.69).上述特征表明,仰口地区变辉长岩中的锆石均为继承性的岩浆锆石,而没有新生的变质锆石.LA-(MC)-ICP-MS锆石原位U-Pb定年和Lu-Hf同位素分析结果表明,两件锆石样品Y1和Y2的年龄数据所构成的不一致线显示了十分接近的上交点和下交点年龄.其上交点年龄分别为785±15Ma(2σ)和784±12Ma(2σ),应代表原岩的形成时代,表明变辉长岩的原岩与Rodinia超大陆裂解的岩浆事件存在密切的成因关系;而下交点年龄分别为226±24Ma(2σ)和228±26Ma(2σ),与苏鲁其它类型超高压岩石中含柯石英锆石微区记录的变质年龄十分吻合,应代表变辉长岩的超高压变质时代.岩浆结晶锆石的核部具有明显偏高的176Lu/177Hf(0.00044～0.00291)和176Yh/177Hf(0.0165～0.1168)比值,而176Hf/177Hf比值变化于0.281956～0.282048之间,相应的εHf(t)=-8.5～-14.0,tDM2=2.03～2.32Ga,表明仰口地区变辉长岩的原岩起源于古元古代时期的富集地幔或发生部分熔融的下地壳残留体.被改造的岩浆结晶锆石的边部则具有明显偏低的176Lu/177Hf(0.00029～0.00060)和176Yh/177Hf(0.0112～0.0200)比值,而176Hf/177Hf(t)比值变化于0.281953～0.282002之间,相应的εHf(t)=-10.2～-11.9,tDM2=2.12～2.21Ga.与岩浆结晶锆石核部相比,被改造的岩浆锆石边部的176Lu/177Hf、176Yb/177Hf、176Hf/177Hf(t)比值和εHf(t)和tDM2值的变化范围更小,表明中-新三叠纪的超高压变质作用使岩浆结晶锆石边部的Lu-Hf同位素体系发生调整,更趋向于均一化.  相似文献   

11.
Zircon crystals from a locally charnockitized Paleoproterozoic high-K metagranite from the Kerala Khondalite Belt (KKB) of southern India have been investigated by high-spatial resolution secondary ion mass spectrometry analysis of U–Th–Pb and rare earth elements (REE), together with scanning ion imaging and scanning ion tomography (depth-profiled ion imaging). The spot analyses constrain the magmatic crystallization age of the metagranite to ca. 1,850 Ma, with ultrahigh-temperature (UHT) metamorphism occurring at ca. 570 Ma and superimposed charnockite formation at ca. 520–510 Ma, while the ion imaging reveals a patchy distribution of radiogenic Pb throughout the zircon cores. Middle- to heavy-REE depletion in ca. 570 Ma zircon rims suggests that these grew in equilibrium with garnet and therefore date the UHT metamorphism in the KKB. The maximum apparent 207Pb/206Pb age obtained from the unsupported radiogenic Pb concentrations is also consistent with formation of the Pb patches during this event. The superimposed charnockitization event appears to have caused additional Pb-loss in the cores and recrystallization of the rims. The results of depth-profiling of the scanning ion tomography image stack show that the Pb-rich domains range in size from <5 nm to several 10 nm (diameter if assumed to be spherical). The occurrence of such patchy Pb has previously been documented only from UHT metamorphic zircon, where it likely results from annealing of radiation-damaged zircon. The formation of a discrete, heterogeneously distributed and subsequently immobile Pb phase effectively arrests the normal Pb-loss process seen at lower grades of metamorphism.  相似文献   

12.
Nepheline-bearing gneisses from the 75 km2 Tambani body in the Mozambique Belt of southern Malawi, are miaskitic biotite-nepheline monzodiorites, reflecting an absence of K-feldspar, alkali amphiboles or pyroxenes, and contain euhedral zircon megacrysts up to 5 cm across. The zircons contain U = 1–1,860 ppm, Th = 0–2,170 ppm and Y = 400–1,100 ppm, and very low concentrations of all other measured trace elements except Hf (HfO2 = 0.53–0.92 wt. %). Cathodoluminescence images reveal oscillatory sector growth zoning and no evidence for xenocrystic cores, indicating that the zircons represent primary magmatic crystallization products that have survived amphibolite grade metamorphism. U-Pb isotopic analyses (by TIMS) yield an upper intercept age of 730 ± 4 Ma (MSWD = 1.7), which we interpret as the time of magmatic crystallization of the zircons. This is coincident with 11 SHRIMP spot analyses, which yield a mean age of 729 ± 7 Ma (MSWD = 0.37). Metamorphism, at 522 ± 17 Ma as suggested by monazite, caused partial Pb-loss during local recrystallization of zircon. Lu-Hf isotopic data for three whole-rock samples of nepheline-bearing gneiss are collinear with those for zircon megacrysts, and correspond to an age of 584 ± 17 Ma (MSWD = 0.37. We interpret the Lu-Hf array to represent a mixing line defined by the Hf isotopic signature of primary zircon and that of the rock-forming minerals reset during metamorphic (re-)crystallization; hence the 584 Ma age is likely geologically meaningless. Given the well-defined association of nepheline syenites (and phonolitic volcanic equivalents) with continental rifting, we suggest that the Tambani body represents a magmatic product formed at 730 Ma during the break-up of the Rodinia supercontinent. The 522 Ma age is akin to other Pan-African metamorphic ages that record collisional suturing events during the final assembly of Gondwana. Zircon-bearing nepheline gneisses thus preserve a record of intra-continental rifting and of continental collision in southern Malawi.  相似文献   

13.
对胶东谭格庄地区一地质剖面进行了详细野外观察和锆石SHRIMP年代学研究.剖面主要是由奥长花岗质片麻岩和斜长角闪岩组成,两者互层产出并一起发生褶皱变形,在强变形域两者完全平行化.所以,在强变形地区,不同成分岩性的薄层状互层产出不能作为变质沉积岩的识别标志.奥长花岗质片麻岩(S1105)形成于2496±10Ma,遭受了古元古代早期和晚期变质.两个斜长角闪岩样品(S1238和S0812)都只发育变质锆石,年龄分别为1842±3Ma和1833±13Ma.该剖面岩石记录的两期变质作用在胶东地区具有普遍意义.结合前人研究,作者指出该地区太古宙末存在区域性变质深熔作用,早期地壳受到了强烈的再造;古元古代末的变质作用具有带状分布特点,与造山作用有关.  相似文献   

14.
滇西哀牢山变质岩系锆石U-Pb定年及其地质意义   总被引：11，自引：7，他引：4  
哀牢山-红河构造带是滇西地区最著名的带状变质带之一,其主体由哀牢山深变质岩系(哀牢山岩群)组成,一直被认为是扬子陆块古元古代结晶基底.本文选取哀牢山深变质岩系内的花岗片麻岩(11 ALl7-1和11AL09-1)和石英岩(11AL08-1),以及邻区的花岗岩(11ALl2-1)进行LA-ICP-MS锆石U-Pb定年.结果显示,花岗片麻岩11 ALl7-1有岩浆和变质两类锆石,两者的206Pb/238U年龄加权平均值分别为700±6Ma(MSWD=1.4,n=14)和27.4±1.2Ma(MSWD=1.9,n=3),代表原岩形成时代和变质年龄.花岗片麻岩llAL09-1岩浆锆石206 pb/238U年龄为220±3Ma(MSWD=3.1,n=14),变质锆石年龄为31.2±2.3Ma(MSWD =6,n=5),分别代表原岩结晶时代和后期变质年龄.石英岩11AL08-1中所有锆石具有核-边结构,92颗锆石核部年龄集中分布在6组,分别为493～528Ma(n=42)、635 ～ 640Ma(n=2)、701～784Ma(n=44)、976 ～980Ma(n=2)、1839Ma(n=1)和2487Ma(n=1).92个核部分析点具有高的Th/U比值(＞0.23),指示岩浆来源.最年轻一组的42个核部年龄加权平均值为509Ma,代表石英岩原岩的最大沉积时代.7颗锆石变质边年龄为26～ 75 Ma内,代表变质年龄.花岗岩11 ALl2-1锆石206pb/238U年龄加权平均值为750±4Ma(MSWD =0.6),代表岩石形成时代.这些年龄表明哀牢山变质岩系是一个原岩复杂的变质杂岩带,它的原始物质至少包含新元古代～ 700Ma岩浆岩、～509 Ma沉积地层及220 ～ 240Ma的岩浆岩和地层,而不是以往认为的古元古代结晶基底.现今所见的哀牢山岩群“古老”岩石面貌主要是由地质历史上的浅变质或未变质的地层和岩浆岩在新生代26～31Ma发生变质变形作用改造的结果.哀牢山变质带的源区物质特征和主要岩浆事件与扬子陆块西缘十分相似,具有亲扬子的构造属性.  相似文献   

15.
北秦岭西段冥古宙锆石(4.1～3.9Ga)年代学新进展   总被引：15，自引：13，他引：2  
2007年王洪亮等报道在北秦岭西段火山岩中获得一粒年龄为4079±5Ma的冥古宙捕虏锆石。之后,对这一发现开展了深入的调查研究,我们除利用SHIMP技术方法对原4079Ma的锆石进行验证外,新获得了两粒~(207)Pb/~(206)Pb年龄为4007±29Ma和3908±45Ma捕获的变质成因锆石,表明早在4.0Ga已经有变质作用的发生,这或许说明在冥古宙时期地球已经具有相当规模和厚度的地壳。同时开展的岩石学研究表明,蕴含古老锆石的母岩属于火山碎屑熔岩类而不是火山熔岩。  相似文献   

16.
Eclogite lenses in marbles from the Dabie-Sulu ultrahigh-pressure (UHP) terrane are deeply subducted meta-sedimentary rocks. Zircons in these rocks have been used to constrain the ages of prograde and UHP metamorphism during subduction, and later retrograde metamorphism during exhumation. Inherited (detrital) and metamorphic zircons were distinguished on the basis of transmitted light microscopy, cathodoluminescence (CL) imaging, trace element contents and mineral inclusions. The distribution of mineral inclusions combined with CL imaging of the metamorphic zircon make it possible to relate zircon zones (domains) to different metamorphic stages. Domain 1 consists of rounded, oblong and spindly cores with dark-luminescent images, and contains quartz eclogite facies mineral inclusion assemblages, indicating formation under high-pressure (HP) metamorphic conditions of T = 571-668℃and P = 1.7-2.02 GPa. Domain 2 always surrounds domain 1 or occurs as rounded and spindly cores with white-luminescent images. It contains coesite edogite facies mineral inclusion assemblages, indicating formation under UHP metamorphic conditions of T = 782-849℃and P > 5.5 GPa. Domain 3, with gray-luminescent images, always surrounds domain 2 and occurs as the outermost zircon rim. It is characterized by low-pressure mineral inclusion assemblages, which are related to regional amphibolite facies retrograde metamorphism of T = 600-710℃and P = 0.7-1.2 GPa. The three metamorphic zircon domains have distinct ages; sample H1 from the Dabie terrane yielded SHRIMP ages of 245±4 Ma for domain 1, 235±3 Ma for domain 2 and 215±6 Ma for domain 3, whereas sample H2 from the Sulu terrane yielded similar ages of 244±4 Ma, 233±4 Ma and 214±5 Ma for Domains 1, 2 and 3, respectively. The mean ages of these zones suggest that subduction to UHP depths took place over 10-11 Ma and exhumation of the rocks occurred over a period of 19-20 Ma. Thus, subduction from～55 km to > 160 km deep mantle depth took place at rates of approximately 9.5-10.5 km/Ma and exhumation from depths >160 km to the base of the crust at～30 km occurred at approximately 6.5 km/Ma. We propose a model for these rocks involving deep subduction of continental margin lithosphere followed by ultrafast exhumation driven by buoyancy forces after break-off of the UHP slab deep within the mantle.  相似文献   

17.
Growth of zircon with respect to that of garnet has been studied using a combination of petrography, U–Pb dating and oxygen isotope analysis. The aim is to document the mechanism and pressure–temperature conditions of zircon growth during metamorphism in order to better constrain the Tertiary metamorphic history of Naxos, Greece. Two metamorphisms are recognised: (1) an Eocene Franciscan metamorphism (M1) and (2) a widespread Miocene Barrovian metamorphism (M2) that increases from greenschist facies up to partial melting. An amphibolite sample contains zircon crystals characterised by a magmatic core and two metamorphic rims, denoted as A and B, dated at 200–270, 42–69, and 14–19 Ma, respectively. The first metamorphic rim A (δ18O = 7 ± 1‰) preserves the δ18O value of the magmatic core (6.2 ± 0.8‰), whereas rim B is characterised by higher δ18O values (7.8 ± 1.8‰). These observations indicate the formation of A rims by solid-state recrystallisation in a closed system with regard to oxygen and those of B in an open system. Compositional zoning in garnet is interpreted as the result of decompressional heating. Zircon B rims and garnet rims display similar δ18O values which indicates a contemporaneous growth of garnet and zircon rims during the Miocene Barrovian event (M2). Calcic gneiss and metapelite samples contain zircon crystals with single metamorphic overgrowths aged 41–57 Ma. δ18O values measured in zircon overgrowths (11.8 ± 1.4‰) from the calcic gneiss are similar to those measured in garnet rims (11.4 ± 1.1‰) from the same rock. This suggests that garnet rims and zircon overgrowths grew during the high pressure–low temperature event in equilibrium with prograde fluids. In the metapelite sample, δ18O values are similar in garnet cores (14.8 ± 0.2‰) and in zircon metamorphic overgrowths (14.2 ± 0.5‰). As zircon overgrowths have been dated at ca. 50 Ma by U–Pb, garnet cores and zircon overgrowths are interpreted to have grown during the high pressure event.

As demonstrated here for the island of Naxos, correlating the crystallisation of zircon with that of metamorphic index minerals such as garnet using stable isotope composition and U–Pb determination is a powerful tool for deciphering the mechanism of zircon growth and pin-pointing zircon crystallisation within the metamorphic history of a terrain. This approach is potentially hampered by an inability to verify the degree of textural equilibrium of zircon with other mineral phases, and the possible preservation (in metamorphic rims) of isotopic signatures from pre-existing zircon when they form by recrystallisation. Nevertheless, this study illustrates the application of this approach in providing key constraints on the timing and mechanism of growth of minerals important to understanding metamorphic petrogenesis.  相似文献   


18.
Several types of growth morphologies and alteration mechanisms of zircon crystals in the high-grade metamorphic Ivrea Zone (IZ) are distinguished and attributed to magmatic, metamorphic and fluid-related events. Anatexis of pelitic metasediments in the IZ produced prograde zircon overgrowths on detrital cores in the restites and new crystallization of magmatic zircons in the associated leucosomes. The primary morphology and Th-U chemistry of the zircon overgrowth in the restites show a systematic variation apparently corresponding to the metamorphic grade: prismatic (prism-blocked) low-Th/U types in the upper amphibolite facies, stubby (fir-tree zoned) medium-Th/U types in the transitional facies and isometric (roundly zoned) high-Th/U types in the granulite facies. The primary crystallization ages of prograde zircons in the restites and magmatic zircons in the leucosomes cannot be resolved from each other, indicating that anatexis in large parts of the IZ was a single and short lived event at 299 ± 5 Ma (95% c. l.). Identical U/Pb ages of magmatic zircons from a metagabbro (293 ± 6 Ma) and a metaperidotite (300 ± 6 Ma) from the Mafic Formation confirm the genetic context of magmatic underplating and granulite facies anatexis in the IZ. The U-Pb age of 299 ± 5 Ma from prograde zircon overgrowths in the metasediments also shows that high-grade metamorphic (anatectic) conditions in the IZ did not start earlier than 20 Ma after the Variscan amphibolite facies metamorphism in the adjacent Strona–Ceneri Zone (SCZ). This makes it clear that the SCZ cannot represent the middle to upper crustal continuation of the IZ. Most parts of zircon crystals that have grown during the granulite facies metamorphism became affected by alteration and Pb-loss. Two types of alteration and Pb-loss mechanisms can be distinguished by cathodoluminescence imaging: zoning-controlled alteration (ZCA) and surface-controlled alteration (SCA). The ZCA is attributed to thermal and/or decompression pulses during extensional unroofing in the Permian, at or earlier than 249 ± 7 Ma. The SCA is attributed to the ingression of fluids at 210 ± 12 Ma, related to hydrothermal activity during the breakup of the Pangaea supercontinent in the Upper Triassic/Lower Jurassic. Received: 7 July 1998 / Accepted: 4 November 1998  相似文献   

19.
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.  相似文献   

20.
Granulite facies rocks from the northernmost Harts Range Complex (Arunta Inlier, central Australia) have previously been interpreted as recording a single clockwise cycle of presumed Palaeoproterozoic metamorphism (800–875 °C and >9–10 kbar) and subsequent decompression in a kilometre‐scale, E‐W striking zone of noncoaxial, high‐grade (c. 700–735 °C and 5.8–6.4 kbar) deformation. However, new SHRIMP U‐Pb age determinations of zircon, monazite and titanite from partially melted metabasites and metapelites indicate that granulite facies metamorphism occurred not in the Proterozoic, but in the Ordovician (c. 470 Ma). The youngest metamorphic zircon overgrowths from two metabasites (probably meta‐volcaniclastics) yield 206Pb/238U ages of 478±4 Ma and 471±7 Ma, whereas those from two metapelites yield ages of 463±5 Ma and 461±4 Ma. Monazite from the two metapelites gave ages equal within error to those from metamorphic zircon rims in the same rock (457±5 Ma and 462±5 Ma, respectively). Zircon, and possibly monazite ages are interpreted as dating precipitation of these minerals from crystallizing melt within leucosomes. In contrast, titanite from the two metabasites yield 206Pb/238U ages that are much younger (411±5 Ma & 417±7 Ma, respectively) than those of coexisting zircon, which might indicate that the terrane cooled slowly following final melt crystallization. One metabasite has a second titanite population with an age of 384±7 Ma, which reflects titanite growth and/or recrystallization during the 400–300 Ma Alice Springs Orogeny. The c. 380 Ma titanite age is indistinguishable from the age of magmatic zircon from a small, late and weakly deformed plug of biotite granite that intruded the granulites at 387±4 Ma. These data suggest that the northern Harts Range has been subject to at least two periods of reworking (475–460 Ma & 400–300 Ma) during the Palaeozoic. Detrital zircon from the metapelites and metabasites, and inherited zircon from the granite, yield similar ranges of Proterozoic ages, with distinct age clusters at c. 1300–1000 and c. 650 Ma. These data imply that the deposition ages of the protoliths to the Harts Range Complex are late Neoproterozoic or early Palaeozoic, not Palaeoproterozoic as previously assumed.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号