17:15 〜 19:15
[SCG51-P05] Heterogeneity of Plutonic Rocks in the Sodong Parat Peninsula, Ciletuh Mélange, Indonesia
キーワード：Plutonic rocks, Heterogeneity, Dynamic magmatic system, Ophiolite, Supra-subduction zone, Ciletuh
Plutonic rocks in the Sodong Parat Peninsula, part of the Ciletuh Melange (Indonesia), have been petrographically identified to exhibit variations in texture, structure, and mineralogy. In this study, detailed lithological mapping was conducted, supported by whole-rock geochemistry and mineral chemistry analyses. Laterally, in the east-west direction, the outcrops display lithological variations, forming a "layered" sequence with irregular contacts separating each lithology. These rocks also exhibit strong deformation features, such as shearing foliation and cataclastic textures that obscure their original characteristics. Additionally, they are characterized by the presence of enclaves, phacoids, and block-in-matrix structures, which are often composed of lithologies distinct from the surrounding rocks, further emphasizing their heterogeneity.
Lithological variations include gabbro, monzodiorite, gabbroic diorite, Fe-Ti gabbro, and trondhjemite. Among these, gabbroic diorite exhibits the most primitive characteristics, featuring adcumulate textures, sparse or absent apatite, labradorite-andesine (An50-59) plagioclase, higher bulk Mg# (62-70), and low TiO2 and P2O5 contents. In contrast, the more evolved lithologies such as monzodiorites are distinguished by the presence of apatite, Na-rich plagioclase (<An20), ilmenite, lower bulk Mg#, and moderate to high FeOt, TiO2, and P2O5 concentrations. Trondhjemite occurs as enclaves within sheared monzodiorite, characterized by a hypidiomorphic texture and composed of albite, interstitial quartz and biotite, with apatite and minor zircon. Fe-Ti gabbro appears as either discrete layers, enclaves or block-in-matrix structures, identified by poikilitic texture of plagioclase oikocrysts, ferro-augite and Fe-rich olivine (Mg#=12-13) chadacrysts, and abundant ilmenite.
Most of the samples exhibit metamorphism up to the amphibolite facies, characterized by the dominance of secondary amphibole minerals constituting over 30% of the rock's composition and accompanied by pervasive chlorite replacing primary pyroxene. Plagioclase exhibits deformation features, including kink-band and undulose extinction, and is extensively altered through albitization (<An20) and saussuritization, forming secondary minerals such as sericite, calcite, and epidote. Green-mineral veins, composed of a mixture of chlorite, epidote, and actinolite, commonly crosscut all outcrops.
The whole-rock major and trace element data indicate a tholeiitic affinity, exhibiting both MORB and island arc signatures, consistent with formation during fore-arc rifting in a supra-subduction zone environment. The co-occurrence of these rock types indicates a dynamic magmatic system involving mantle-derived mafic melts that undergo fractional crystallization, partial melting of hydrated mafic rocks, late-stage oxide enrichment and possible magma mixing.
Lithological variations include gabbro, monzodiorite, gabbroic diorite, Fe-Ti gabbro, and trondhjemite. Among these, gabbroic diorite exhibits the most primitive characteristics, featuring adcumulate textures, sparse or absent apatite, labradorite-andesine (An50-59) plagioclase, higher bulk Mg# (62-70), and low TiO2 and P2O5 contents. In contrast, the more evolved lithologies such as monzodiorites are distinguished by the presence of apatite, Na-rich plagioclase (<An20), ilmenite, lower bulk Mg#, and moderate to high FeOt, TiO2, and P2O5 concentrations. Trondhjemite occurs as enclaves within sheared monzodiorite, characterized by a hypidiomorphic texture and composed of albite, interstitial quartz and biotite, with apatite and minor zircon. Fe-Ti gabbro appears as either discrete layers, enclaves or block-in-matrix structures, identified by poikilitic texture of plagioclase oikocrysts, ferro-augite and Fe-rich olivine (Mg#=12-13) chadacrysts, and abundant ilmenite.
Most of the samples exhibit metamorphism up to the amphibolite facies, characterized by the dominance of secondary amphibole minerals constituting over 30% of the rock's composition and accompanied by pervasive chlorite replacing primary pyroxene. Plagioclase exhibits deformation features, including kink-band and undulose extinction, and is extensively altered through albitization (<An20) and saussuritization, forming secondary minerals such as sericite, calcite, and epidote. Green-mineral veins, composed of a mixture of chlorite, epidote, and actinolite, commonly crosscut all outcrops.
The whole-rock major and trace element data indicate a tholeiitic affinity, exhibiting both MORB and island arc signatures, consistent with formation during fore-arc rifting in a supra-subduction zone environment. The co-occurrence of these rock types indicates a dynamic magmatic system involving mantle-derived mafic melts that undergo fractional crystallization, partial melting of hydrated mafic rocks, late-stage oxide enrichment and possible magma mixing.