日本地球惑星科学連合2015年大会

講演情報

インターナショナルセッション(ポスター発表)

セッション記号 S (固体地球科学) » S-MP 岩石学・鉱物学

[S-MP08] Micro- to macro-scale deformation: petrologic, mineralogic, geophysical and geochemical checkpoints

2015年5月26日(火) 18:15 〜 19:30 コンベンションホール (2F)

コンビーナ:*Hafiz Ur Rehman(Department of Earth and Environmental Sciences, Graduate School of Science and Engineering, Kagoshima University)、Tatsuki Tsujimori(Institute for Study of the Earth's Interior, Okayama University)、Patrick J. O'Brien(Department of Earth and Environmental Sciences, Potsdam University, Germany)、David Mainprice(Geosciences Montpellier, Montpellier University 2, France)

18:15 〜 19:30

[SMP08-P01] ボヘミア山塊Moldanubian帯Nove Dvory産超高圧エクロジャイト中に含まれる4種類のCa角閃石の起源

*安本 篤史1平島 崇男1 (1.京都大学理学研究科)

キーワード:超高圧, エクロジャイト, 角閃石, 流体

Meta-mafic rocks underwent ultra-high pressure (UHP) conditions tend to be anhydrous with bi-mineralic mineral assemblage of garnet and omphacite (Schmidt & Poli, 1998). The studied sample, ND0107, collected at Nove Dvory, Czech Republic, also comprises mainly of garnet (Grt) and omphacite (Omp) with accessory minerals of rutile (Rt), ilmenite (Ilm), apatite (Apt). However, 4 types Ca-amphiboles (Amp-Ia, Ib, Ma, and Mb) are identified based on the different modes of occurrence. Amp grains identified in garnet are named Amp-I and those in the matrix are named Amp-M. Amp-I is subdivided into Amp-Ia and Amp-Ib. Amp-Ia (40um in long dimension) appears as a member of multi-phase solid inclusions (MSI) which show a sporadic distribution in garnet, and shows a close association with Omp maintaining UHP stage compositions with/without Rt. Amp-Ib (5-50um) appears as a member of aligned anhedral inclusions in Grt, associated with diopside (Di) and spinel (Spl), which are stable at relatively low pressure conditions. Amp-M occurs as a member secondary minerals developed between Grt and Omp, and it is subdivided into Amp-Ma and Amp-Mb based on whether they are associated with Pl or not. Amp-Ma (10um in long dimension) is developed interstitially between Grt and Omp and is not associated with plagioclase (Pl). The incipient stage of Amp-Ma formation can be represented by the alternation of idiomorphic Amp-Ma interspacing saw-blade shaped rim of garnet, and by subsequent transformation of saw-blade shaped part of garnet into fine-grained symplectite composed of Di, Spl, and hematite (Hem). Amp-Mb (>5-50um in long dimensions) is also developed interstitially between Grt and Omp but is associated with Pl. On the garnet-side of Amp-Mb, the fine-grained symplectite composed of Di, Spl, and Hem sometimes appears. The outline of the fine-grained symplectite resembles the saw-blade shaped rim of garnet which is interspaced by Amp-Ma, suggesting that Amp-Mb was formed at later stage of exhumation than Amp-Ma. Four types of Amp are mainly classified as pargasite (Prg). Amp-Ia and Ma are almost K-free but Amp-Ib and Mb contains up to 0.3pfu of K. Mg# ascends in the order of Amp-Ib, Ia, Mb, and Ma (70-82, 77-85, 80-93, 87-92, respectively). All types of Amp contain F as shown as follows; Ia: 0.07-0.2pfu; Ib: 0.02-0.28pfu; Ma: 0.10-0.16pfu; Mb: 0.05-0.16pfu, but a significant amount of Cl is identified only in Amp-Ib (0.10-0.30pfu). Modes of occurrence of Amp and compositions of neighboring Grt and Omp suggest that Amp-Ib, Ma, and Mb were generated by later stage fluid activity accompanying with decompositions of Grt and Omp. Since the studied eclogite is bi-mineralic at the UHP stage, K, F, and Cl in secondary Amp should be supplied by the infiltrated fluids. In contrast, Amp-Ia seems like to be equilibrated with Grt and Omp rather than to be generated by decomposition of Grt and Omp. The maximum pressure limit of Prg is known to be 2-3GPa (Gilibert, 1969), but it expands to higher pressures by incorporation of F in Prg (Holloway & Ford, 1975). High XF (= F/(F+Cl+OH) > 0.4) is required for Prg to be stable at the peak metamorphic conditions of Nove Dvory. However, Amp-Ia yields low XF (< 0.1), and thus is considered to be a prograde relict.