5:15 PM - 7:15 PM
[SMP28-P07] Origin and Metamorphic History of Serpentinite in the Sangun-Renge Belt exposed at Sasaguri, Fukuoka Prefecture
Keywords:Serpentine, Cr-spinel, Ferritchromite, Exhumation
The Sangun-Renge Belt is a high-pressure metamorphic complex in southwestern Japan formed during the Late Paleozoic to Early Mesozoic. The serpentinites of Sangun-Renge Belt along with high-pressure metamorphic rocks are exposed around Narubuchi Dam in Sasaguri, Fukuoka Prefecture. This presentation will discuss the origin, metamorphic history and classification of Sasaguri serpentinite, as revealed by microstructural observations, petrography and geochemical analysis.
The serpentinites in the study area can be classified into the following four types according to the combination of serpentine minerals they contain: (1) Lizardite-Chrysotile serpentinite, (2) Lizardite-Chrysotile-Antigorite serpentinite, (3) Lizardite-Antigorite serpentinite, and (4) Antigorite serpentinite. Their distribution varies across the study area.
In the northern region of the study area, only Type-4 serpentinite is observed, which is characterized by coarse-grained antigorite with undulose extinction. A mylonitized zone formed by shear deformation can be identified, where the grain sizes of the antigorite are much finer (<100 µm) by dynamic recrystallization.
All four types of serpentinite are exposed to the south of the above area. Type-1 serpentinite shows bastite and mesh texture. Type-2 serpentinite retains mesh textures with minor antigorite. Type-3 serpentinite features ribbon texture of antigorite. Type-4 serpentinite preserves pseudomorphic textures after pyroxene, including schiller structures and pyroxene replaced by magnesite and magnesio-anthophyllite. Type-4 serpentinite exposed approximately 500 m south of Narubuchi Dam (southernmost region of the current study) is dominated by fine-grained antigorite with crosscutting veins. The minor minerals found in these serpentinites include Cr-spinel, magnetite, Cr-magnetite, chlorite, magnesio-anthophyllite, magnesite, talc-chlorite, tremolite, and hornblende, though their presence varies among the different types.
The accessory mineral Cr-spinel acts as a petrogenetic indicator and helps to reveal the protolith of serpentinite. Cr-spinel is present in all the serpentinites except those in the southernmost area. The Cr-spinel shows compositional zoning along grain boundaries, which is well-developed in Type-4 serpentinite in the northern part. Electron probe microanalysis (EPMA) of the Cr-spinel shows a progressive alteration sequence from pristine cores to ferritchromite rims, which are subsequently overgrown by magnetite. The Cr-spinel core has TiO2 contents of < 0.1 wt%, Cr#[Cr/(Cr + Al)] > 0.6 and Mg#[Mg/(Mg + Fe2+)] varying between 0.1-0.7. The depleted nature of these serpentinites is indicated by the high Cr# and low TiO2. The high Cr# and Mg# suggest a compositional affinity with fore-arc peridotites. The bulk rock chemical analyses show that the serpentinites (except in southernmost area) have very low Al2O3 (0.22-0.84 wt.%) and CaO (0.01-0.29 wt.%), and high MgO (34.75-39.63 wt%). The very low abundances of Al2O3 and CaO are similar to the fore-arc peridotite composition. The high contents of Ni (2011-2949 ppm), Cr (1877-3637 ppm) and Co (82-105 ppm) and depletion in the incompatible elements indicate the depleted nature of the parent rock.
In summary, the presence of both mesh and bastite textures in the serpentinite suggests that the protolith contained both olivine and pyroxene, indicating that it could have been harzburgite or lherzolite. The high Cr# and low TiO2 spinel and the low Al2O3 and CaO of the bulk serpentinites suggest the origin as forearc peridotite origin. The exhumation of the forearc peridotite has led to metamorphism at greenschist-amphibolite facies conditions (400-700°C), suggested by the ferritichromitization of Cr-spinel. Further exhumation has led to serpentinitization at significantly lower temperatures (250-400°C), as indicated by the magnetite overgrowth around the Cr-spinel grains.
The serpentinites in the study area can be classified into the following four types according to the combination of serpentine minerals they contain: (1) Lizardite-Chrysotile serpentinite, (2) Lizardite-Chrysotile-Antigorite serpentinite, (3) Lizardite-Antigorite serpentinite, and (4) Antigorite serpentinite. Their distribution varies across the study area.
In the northern region of the study area, only Type-4 serpentinite is observed, which is characterized by coarse-grained antigorite with undulose extinction. A mylonitized zone formed by shear deformation can be identified, where the grain sizes of the antigorite are much finer (<100 µm) by dynamic recrystallization.
All four types of serpentinite are exposed to the south of the above area. Type-1 serpentinite shows bastite and mesh texture. Type-2 serpentinite retains mesh textures with minor antigorite. Type-3 serpentinite features ribbon texture of antigorite. Type-4 serpentinite preserves pseudomorphic textures after pyroxene, including schiller structures and pyroxene replaced by magnesite and magnesio-anthophyllite. Type-4 serpentinite exposed approximately 500 m south of Narubuchi Dam (southernmost region of the current study) is dominated by fine-grained antigorite with crosscutting veins. The minor minerals found in these serpentinites include Cr-spinel, magnetite, Cr-magnetite, chlorite, magnesio-anthophyllite, magnesite, talc-chlorite, tremolite, and hornblende, though their presence varies among the different types.
The accessory mineral Cr-spinel acts as a petrogenetic indicator and helps to reveal the protolith of serpentinite. Cr-spinel is present in all the serpentinites except those in the southernmost area. The Cr-spinel shows compositional zoning along grain boundaries, which is well-developed in Type-4 serpentinite in the northern part. Electron probe microanalysis (EPMA) of the Cr-spinel shows a progressive alteration sequence from pristine cores to ferritchromite rims, which are subsequently overgrown by magnetite. The Cr-spinel core has TiO2 contents of < 0.1 wt%, Cr#[Cr/(Cr + Al)] > 0.6 and Mg#[Mg/(Mg + Fe2+)] varying between 0.1-0.7. The depleted nature of these serpentinites is indicated by the high Cr# and low TiO2. The high Cr# and Mg# suggest a compositional affinity with fore-arc peridotites. The bulk rock chemical analyses show that the serpentinites (except in southernmost area) have very low Al2O3 (0.22-0.84 wt.%) and CaO (0.01-0.29 wt.%), and high MgO (34.75-39.63 wt%). The very low abundances of Al2O3 and CaO are similar to the fore-arc peridotite composition. The high contents of Ni (2011-2949 ppm), Cr (1877-3637 ppm) and Co (82-105 ppm) and depletion in the incompatible elements indicate the depleted nature of the parent rock.
In summary, the presence of both mesh and bastite textures in the serpentinite suggests that the protolith contained both olivine and pyroxene, indicating that it could have been harzburgite or lherzolite. The high Cr# and low TiO2 spinel and the low Al2O3 and CaO of the bulk serpentinites suggest the origin as forearc peridotite origin. The exhumation of the forearc peridotite has led to metamorphism at greenschist-amphibolite facies conditions (400-700°C), suggested by the ferritichromitization of Cr-spinel. Further exhumation has led to serpentinitization at significantly lower temperatures (250-400°C), as indicated by the magnetite overgrowth around the Cr-spinel grains.