5:15 PM - 7:15 PM
[SCG53-P08] Petrological study of gabbroic xenoliths in andesitic rocks, the Atsumi district, Yamagata Prefecture, NE Japan
Keywords:xenoliths, gabbro
In the Suzu area of the Atsumi district, Yamagata Prefecture, a dacitic to andesitic dike is distributed in association with the Atsumi dolerite sill, which intruded into the Miocene Nezugaseki Formation. The dike contains gabbroic xenoliths. Kondo et al. (1999) classified these gabbroic xenoliths into two types: olivine-bearing and non-olivine types, based on petrological characteristics. They inferred, based on Sr and Nd isotopic compositions and Sm-Nd mineral isochron dating, that these xenoliths originated from the Paleogene alkaline-basalt magma and the Late Cretaceous basalt magma, respectively, and concluded that neither type was genetically related to the Neogene Atsumi dolerite or the associated dacitic to andesitic dike. On the other hand, Tsuchiya et al. (2003) reported that the majority of the xenoliths consist of plagioclase, hornblende, clinopyroxene, and magnetite, with rare occurrences of those composed of olivine, clinopyroxene, plagioclase, and chromian spinel. They interpreted the former as xenoliths derived from the lower crust and the latter as cumulates of the doleritic magma. As seen above, different interpretations exist regarding the origin of the gabbroic xenoliths in the Atsumi district, and further investigation is required. Therefore, we examined the petrological characteristics of the gabbroic xenoliths.
At a coastal area near Kurotaki, approximately 350 m south-southwest of Suzu Fishing Port, the contact between the mudstone of the Nezugaseki Formation and the Atsumi dolerite sill is observed. The sill is intruded by an andesitic dike (SiO2 = 59–65 wt%). The dike contains gabbroic xenoliths ranging from a few centimeters to 40 cm in length. We collected approximately 20 xenoliths from this site and classified them into the following five types based on petrological characteristics:
Medium-grained olivine gabbro (high-Mg type: SiO2 = 50 wt%, MgO = 8.8 wt%): This type shows the equigranular texture and are composed of 41 vol% plagioclase (An87-89), 42 vol% clinopyroxene (Mg#79-81), and 14 vol% olivine. Olivine is almost entirely altered to serpentine and other secondary minerals.
Medium-grained olivine gabbro (low-Mg type: SiO2 = 48.6-48.7 wt%, MgO = 4.9-5.5 wt%): The interstices of euhedral plagioclase (64 vol%, An87) are filled by anhedral clinopyroxene (25 vol%, Mg#85) and olivine pseudomorph (9 vol%).
Coarse-grained gabbro: This type is composed of 24 vol% plagioclase (An84-85), 58 vol% clinopyroxene (Mg#74), 13 vol% titanomagnetite, and 4 vol% olivine. Olivine is almost entirely altered. Mineral grains are several centimeters in size.
Fine-grained gabbro (SiO2 = 48 wt%, MgO = 5.8 wt%): This type is consisting of fine-grained (grain size 0.2–1 mm) plagioclase (41 vol%, An71-81), clinopyroxene (10 vol%, Mg#73-76), apatite (0.6 vol%), and very fine-grained interstitial material (~44 vol%, grain size ~10 μm). The mineral assemblage of the interstitial material is clinopyroxene, plagioclase, magnetite, and saponite.
Amphibole gabbro: This type is composed of 80 vol% plagioclase (An84-89), 19 vol% hornblende, and 1 vol% titanomagnetite. Coarse-grained anhedral hornblende encloses euhedral plagioclase grains. The rims of some hornblende grains show opacitization.
In olivine-bearing gabbroic xenoliths, very fine-grained materials (~10 μm grain size) consisting of plagioclase (An60-67), clinopyroxene (Mg#72-79), titanomagnetite, and serpentine are observed in the interstices between coarse-grained minerals. The compositions of plagioclase and clinopyroxene in the very fine-grained interstitial materials are more evolved than their coarse-grained minerals. The bulk chemical composition of the very fine-grained interstitial materials is often more Mg-rich than the whole-rock composition of the xenoliths. In some areas adjacent to the fine-grained regions, coarse plagioclase grains show reverse zoning at their rims, and clinopyroxene exhibits irregular shapes and spongy textures.
The presence of the very fine-grained interstitial materials in medium-grained olivine gabbro and fine-grained gabbro suggests that melt may have been present in the interstitial spaces of the xenoliths. Fine-grained gabbro, in which the volume of the very fine-grained materials exceeds 40 vol%, may have existed as a crystal mush when incorporated into the andesitic magma. In contrast, the nearly holocrystalline medium-grained olivine gabbro was likely captured by the magma in a nearly solidified state. The magnetite-bearing coarse-grained gabbro, having a lower Mg# than other xenoliths, suggests crystallization from a more evolved magma.
At a coastal area near Kurotaki, approximately 350 m south-southwest of Suzu Fishing Port, the contact between the mudstone of the Nezugaseki Formation and the Atsumi dolerite sill is observed. The sill is intruded by an andesitic dike (SiO2 = 59–65 wt%). The dike contains gabbroic xenoliths ranging from a few centimeters to 40 cm in length. We collected approximately 20 xenoliths from this site and classified them into the following five types based on petrological characteristics:
Medium-grained olivine gabbro (high-Mg type: SiO2 = 50 wt%, MgO = 8.8 wt%): This type shows the equigranular texture and are composed of 41 vol% plagioclase (An87-89), 42 vol% clinopyroxene (Mg#79-81), and 14 vol% olivine. Olivine is almost entirely altered to serpentine and other secondary minerals.
Medium-grained olivine gabbro (low-Mg type: SiO2 = 48.6-48.7 wt%, MgO = 4.9-5.5 wt%): The interstices of euhedral plagioclase (64 vol%, An87) are filled by anhedral clinopyroxene (25 vol%, Mg#85) and olivine pseudomorph (9 vol%).
Coarse-grained gabbro: This type is composed of 24 vol% plagioclase (An84-85), 58 vol% clinopyroxene (Mg#74), 13 vol% titanomagnetite, and 4 vol% olivine. Olivine is almost entirely altered. Mineral grains are several centimeters in size.
Fine-grained gabbro (SiO2 = 48 wt%, MgO = 5.8 wt%): This type is consisting of fine-grained (grain size 0.2–1 mm) plagioclase (41 vol%, An71-81), clinopyroxene (10 vol%, Mg#73-76), apatite (0.6 vol%), and very fine-grained interstitial material (~44 vol%, grain size ~10 μm). The mineral assemblage of the interstitial material is clinopyroxene, plagioclase, magnetite, and saponite.
Amphibole gabbro: This type is composed of 80 vol% plagioclase (An84-89), 19 vol% hornblende, and 1 vol% titanomagnetite. Coarse-grained anhedral hornblende encloses euhedral plagioclase grains. The rims of some hornblende grains show opacitization.
In olivine-bearing gabbroic xenoliths, very fine-grained materials (~10 μm grain size) consisting of plagioclase (An60-67), clinopyroxene (Mg#72-79), titanomagnetite, and serpentine are observed in the interstices between coarse-grained minerals. The compositions of plagioclase and clinopyroxene in the very fine-grained interstitial materials are more evolved than their coarse-grained minerals. The bulk chemical composition of the very fine-grained interstitial materials is often more Mg-rich than the whole-rock composition of the xenoliths. In some areas adjacent to the fine-grained regions, coarse plagioclase grains show reverse zoning at their rims, and clinopyroxene exhibits irregular shapes and spongy textures.
The presence of the very fine-grained interstitial materials in medium-grained olivine gabbro and fine-grained gabbro suggests that melt may have been present in the interstitial spaces of the xenoliths. Fine-grained gabbro, in which the volume of the very fine-grained materials exceeds 40 vol%, may have existed as a crystal mush when incorporated into the andesitic magma. In contrast, the nearly holocrystalline medium-grained olivine gabbro was likely captured by the magma in a nearly solidified state. The magnetite-bearing coarse-grained gabbro, having a lower Mg# than other xenoliths, suggests crystallization from a more evolved magma.