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164.
大别山榴辉岩中磷灰石的结构碳酸根 总被引:5,自引:0,他引:5
在大别山东南部的高压和超高压榴辉岩中发现磷灰石含有结构碳酸根。磷灰石主要以包裹体的形式存在于筛状变晶结构的石榴石中,其形成时间早于石榴石和榴辉岩的矿物组合。X射线衍射和傅立叶变换红外光谱研究表明,磷灰石中结构碳酸根替代「PO4」^3-,表现为在1455-1422cm^-1的吸收带,以及在150℃以下温度难以消除的在1630cm^-1的吸附水,但常规X射线衍射和傅立叶变换红外光谱难以给出其含量。用EA-MS连线技术测量给出结构碳酸根的含量在0.95-5.24 CO2wt%。榴辉岩磷灰石中结构碳酸根的鉴定和碳含量的定量测定证明,在超高压变质过程中存在含CO2的峰变质流体。 相似文献
165.
大别山太湖—马庙断裂带两侧变质地体的增温变质作用 总被引:16,自引:7,他引:9
大别山榴辉岩地体以太湖马庙断裂带为南界。该断裂带近东西走向, 向南倾斜。通过对石榴石中 Mg 含量由核部向边缘增高的生长环带构造的分析和详细的 P T 轨迹的计算表明, 断裂带北侧的榴辉岩和南侧的角闪岩及角闪片麻岩均经历了降压增温变质过程。热榴辉岩由地幔深处的折返和地温梯度的升高为可能的热源。 相似文献
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167.
J. S. Yang J. L. Wooden C. L. Wu F. L. Liu Z. Q. Xu R. D. Shi I. Katayama J. G. Liou S. Maruyama 《Journal of Metamorphic Geology》2003,21(6):551-560
This paper provides further evidence for the ongoing discussion as to whether the Dabie UHPM belt formed in Triassic or Palaeozoic time, and whether the Sulu UHPM belt formed in Triassic or Neoproterozoic time. Combined use of laser Raman spectrometer (LR), cathodoluminescence imaging (CL), and ion probe U–Pb in‐situ dating (SHRIMP) provided accurate ages of UHPM from rocks collected from Weihai, NE Sulu UHPM belt. LR was used to identify coesite and other UHP minerals as inclusions in zircon separates from an amphibolized peridotite and an eclogite. CL was used to examine the zoning structure of these zircon, and SHRIMP dating was performed on specific spots on zircon to obtain ages of different geological events. An age of 221 ± 12 Ma was obtained for coesite‐bearing zircon from the amphibolized peridotite; an age of 228 ± 29 Ma for eclogite was obtained from the lower intercept of a concordia plot. These ages are interpreted as the time of UHPM in the Weihai region. Ultramafic rocks to the east of Weihai yield a magmatic age at 581 ± 44 Ma. The zircon in the ultramafic rocks possibly also records a thermal event at c. 400 Ma, but no independent geological evidence for this event has been found. The eclogite protolith formed in the Middle Proterozoic (1821 ± 19 Ma), which is similar to the age of country rock gneisses of 1847–1744 Ma. The new geochronological data confirm that UHPM occurred in the Triassic in the Sulu area when subduction took the ultramafic body and the eclogite protolith, together with the adjacent supracrustal rocks, to mantle depths. 相似文献
168.
Microsampling Lu–Hf geochronology on mm‐sized garnet in eclogites constrains early garnet growth and timing of tectonometamorphism in the North Qilian orogenic belt 下载免费PDF全文
Hao Cheng Ying Zhou Kai‐Yang Du Ling‐Min Zhang Tian‐Yi Lu 《Journal of Metamorphic Geology》2018,36(8):987-1008
This study presents Lu–Hf geochronology of zoned garnet in high‐P eclogites from the North Qilian orogenic belt. Selected samples have ~mm‐sized garnet grains that have been sampled with a micro‐drill and analysed for dating. The Lu–Hf dates of bulk garnet separates, micro‐drilled garnet cores and the remnant, rim‐enriched garnet were determined by two‐point isochrons, with cores being consistently older than the bulk‐ and rim‐enriched garnet. The bulk garnet separates of each sample define identical garnet–whole rock isochron date of c. 457 Ma. Consistent U–Pb zircon dates of 455 ± 8 Ma were obtained from the eclogite. The Lu–Hf dates of the drilled cores and rim‐rich separates suggest a minimum garnet growth interval of 468.9 ± 2.4 and 452.1 ± 1.6 Ma. Major and Lu element profiles in the majority of garnet grains show well‐preserved Rayleigh‐style fractionated bell‐shaped Mn and Lu zoning profiles, and increasing Mg from core to rim. Pseudosection modelling indicates that garnet grew along a P–T path from ~470–525°C and ~2.4–2.6 GPa. The exceptional high‐Mn garnet core in one sample indicates an early growth during epidote–blueschist facies metamorphism at <460°C and <0.8 GPa. Therefore, the Lu–Hf dates of drilled cores record the early prograde garnet growth, whereas the Lu–Hf dates of rim‐rich fractions provide a maximum age for the end of garnet growth. The microsampling approach applied in this study can be broadly used in garnet‐bearing rocks, even those without extremely large garnet crystals, in an attempt to retrieve the early metamorphic timing recorded in older garnet cores. Given a proper selection of the drill bit size and a detailed crystal size distribution analysis, the cores of the mm‐sized garnet in most metamorphic rocks can be dated to yield critical constraints on the early timing of metamorphism. This study provides new crucial constraints on the timing of the initial subduction (before c. 469 Ma) and the ultimate closure (earlier than c. 452 Ma) of the fossil Qilian oceanic basin. 相似文献
169.
A combined study of major and trace elements, fluid inclusions and oxygen isotopes has been carried out on garnet pyroxenite from the Raobazhai complex in the North Dabie Terrane (NDT). Well‐preserved compositional zoning with Na decreasing and Ca and Mg increasing from the core to rim of pyroxene in the garnet pyroxenite indicates eclogite facies metamorphism at the peak metamorphic stage and subsequent granulite facies metamorphism during uplift. A P–T path with substantial heating (from c. 750 to 900 °C) after the maximum pressure reveals a different uplift history compared with most other eclogites in the South Dabie Terrane (SDT). Fluid inclusion data can be correlated with the metamorphic grade: the fluid regime during the peak metamorphism (eclogite facies) was dominated by N2‐bearing NaCl‐rich solutions, whereas it changed into CO2‐dominated fluids during the granulite facies retrograde metamorphism. At a late retrograde metamorphic stage, probably after amphibolite facies metamorphism, some external low‐salinity fluids were involved. In situ UV‐laser oxygen isotope analysis was undertaken on a 7 mm garnet, and impure pyroxene, amphibole and plagioclase. The nearly homogeneous oxygen isotopic composition (δ18OVSMOW = c. 6.7‰) in the garnet porphyroblast indicates closed fluid system conditions during garnet growth. However, isotopic fractionations between retrograde phases (amphibole and plagioclase) and garnet show an oxygen isotopic disequilibrium, indicating retrograde fluid–rock interactions. Unusual MORB‐like rare earth element (REE) patterns for whole rock of the garnet pyroxenite contrast with most ultra‐high‐pressure (UHP) eclogites in the Dabie‐Sulu area. However, the age‐corrected initial εNd(t) is ? 2.9, which indicates that the protolith of the garnet pyroxenite was derived from an enriched mantle rather than from a MORB source. Combined with the present data of oxygen isotopic compositions and the characteristic N2 content in the fluid inclusions, we suggest that the protolith of the garnet pyroxenite from Raobazhai formed in an enriched mantle fragment, which has been exposed to the surface prior to the Triassic metamorphism. 相似文献
170.
西藏拉萨地块MORB型榴辉岩的岩石地球化学特征 总被引:11,自引:3,他引:11
松多榴辉岩的SiO2、Al2O3含量分别为45.3%~50.6%、12.93%~14.27%,K2O Na2O为1.3%~2.8%,TFeO/MgO为1.11~2.02,CaO为10.4%~14.47%,K2O<0.3%,Na2O为0.97%~2.47%。总体上具有玄武质或辉长质岩石的成分特征,属于钙碱性系列,具有MORB的特征,表明其原岩来自大洋环境。松多榴辉岩大多数样品的稀土元素配分模式显示轻稀土元素亏损,无明显的Eu异常,重稀土元素平坦。微量元素蛛网图均显示从Zr到Yb属于平坦型,基本上未发生分馏。松多榴辉岩的LaN/YbN比值为0.19~0.56,LaN/CeN为0.75~0.81,LaN/SmN为0.37~0.81,三者均小于1;Hf/Th除了石英榴辉岩中的06Y-344样品为6.81外,其他样品为11.10~33.97,均大于8;Ce/Nb为2.72~19.02,均大于2;Th/Yb为0.01~0.10,均小于或等于0.1,也说明其原岩具有典型MORB的特征。锆石SHRIMPU-Pb定年获得松多榴辉岩的变质年龄为261Ma±5.3Ma,推测其原岩可能为古特提斯洋盆的残留。 相似文献