[S-MP25] 変形岩・変成岩とテクトニクス

[SMP25-P01] ラダック東部のNidarオフィオライトで記録された複数の火成活動

*佐藤 成修¹、今山 武志¹、Dutta Dripta² (1.岡山理科大学、2.インド工科大学ボンベイ校)

The Nidar ophiolite complex (NOC) is exposed within the Indus suture zone (ISZ) in eastern Ladakh, India. The NOC comprises of a sequence of ultra-mafic rocks (base to top) gabbros, doleritic and basaltic rocks, and volcanic sedimentary assemblages. Previous studies considered that the NOC was formed by supra-subduction zone (SSZ) igneous activity in the early Cretaceous period (e.g. Ahmad et al. 2008), although multiple igneous activities from the late Jurassic to late Cretaceous periods are known in other ophiolites (e.g. Spongtang and Dras) from within the ISZ (e.g. Parsons et al., 2020). In this study, we identified the gabbro and dolerite of the NOC to be products of MORB, implying multiple igneous activities similar to that of the other ophiolites in the ISZ. In addition, metamorphic minerals are distinguished from the igneous ones based on detailed petrological work of the gabbro, dolerite, and basalt of the NOC. The gabbro mainly consists of clinopyroxene and plagioclase, and is subdivided into Type-1 and Type-2. The former is non-metamorphosed and clinopyroxene-rich, while the later is plagioclase-rich and contains epidote and chlorite. The Mg# [Mg/(Mg+Fe2+) ratio] of pyroxene in the rocks decrease down the stratigraphic level from gabbro (0.85-0.92) to dolerite (0.79), and basalt (0.66-0.77). The amphiboles in the dolerite show a compositional zoning – Mg-rich core (Mg#:0.73-0.90) surrounded by Fe-rich rim (Mg#:0.54-0.71). The Mg# of the actinolite and horblende grains in the basalt lie within 0.58-0.62. The plagioclase in the gabbro is bytownite (An: 70-90%), whereas the An content in the plagioclase grains of the dolerite varies from 5% to 89%. The euhedral plagioclase in the basalt are randomly oriented and rich in An (94-99%). In contrast, the anhedral grains are albitic (An: 0.2-2.2%). These indicate that the pyroxene and An-rich plagioclase are igneous minerals. The type-2 gabbro, dolerite and basalt experienced greenschist- to lower amphibolite-facies metamorphism to form An-poor plagioclase, Fe-rich hornblende, epidote, and chlorite. The SiO₂ content of Type-1 gabbro is 55 wt%, which is significantly higher than that of Type-2 gabbro (45 wt%). The Al₂O₃, MgO, and CaO contents decrease with increasing SiO₂, while that of the other major oxides increase. Six samples of the Type-1 gabbro yield flat REE patterns, which are consistent with those noted for SSZ-type igneous rocks (e.g. Ahmad et al. 2008). On the other hand, three samples of the Type-2 gabbro have low REE concentrations and are depleted in the LREE content, which is a characteristic of the MORB-type igneous rocks and not the SSZ rocks. Therefore, it is likely that the NOC is not a product of a single igneous event in the SSZ, but of multiple igneous activities including the formation of the MORB.