10:00 〜 10:15
[SIT20-05] Characterizing complex metamorphism and deformation processes of high-PT rocks occurred in exhumed subduction channel: a case study of the Yuli Belt in eastern Taiwan
キーワード：Subduction channel, Yuli Belt, Taiwan
High pressure/temperature rocks have been found at Earth’s surface from several field sites in the world. Commonly scientists argue that they are often associated with continental crust that once have been subducted into 50-100 km of depth. Many scientists also speculated that some high-PT rock suites, especially ultra-mafic rocks such as serpentinite bodies, might occur within the subduction channel, involving mantle or oceanic slab from one side or both sides of the deeply subducted continental materials.
Here we present a case study of the rocks from the Yuli Belt in eastern Taiwan. Based on detailed field mapping, with particular attention on the metamorphic deformation structures, along several river transects, we reconstruct a few geological cross sections of about 20 km long, to illustrate the rather complex geological structures of the Yuli Belt. The Yuli Belt is the rock unit which represents the late Cenozoic (mostly Miocene) sedimentary cover lying on the Paleo-Mesozoic Tailuko Belt, the metamorphic basement (core) likely had experienced subduction/metamorphism in Cretaceous time.
Many ultramafic blocks, in particular several large serpentinite bodies with diameter of hundreds of meters to kilometers, are distributed within the pelitic quartz-mica schist and reveal affinity of either intraplate basalt or mantle wedge. The rims of the serpentinite bodies show thermal or chemical interaction with the surrounding pelitic rocks, characterizing by metasomatism products such as talc, chlorite schist, and amphibolite (glaucophane) schist. Structurally, we are able to identify at least three sets of penetrative metamorphic fabrics, S1, S2 an S3. S1 usually parallel to the stratigraphic depositional layers, S0, which however is largely destroyed by pressure dissolution during D1 or D2 deformation event. S2 represents regionally the most dominant fabrics, often characterizing by axial plane cleavage of folded or crenulated S0/S1 compositional layers. Careful, detailed field observations allow us to trace the S2 across the transect across the entire Yuli Belt, showing a deeply dipping of 50-70° to W near the Shoufeng Shear zone (contact with the Tailuko Belt) to the west, and dramatically change to shallowly dipping toward the east in the rest of the belt. S3 seemingly is a late-stage product developed as a sub-horizontal fabric overprinted on locally kinked or crenulated S2 fabrics.
The deformation structures around the serpentinite bodies, including highly stretching rocks with mixed lithology, breccia schist, and incompetent pelitic schist with the main fabric S2 going around the serpentinite, suggest these rock suites have been experienced together probably within the subduction channel, where the Miocene turbidite of the Yuli Belt subducted into depth to mix with oceanic mafic rocks or mantle wedge.
Combining the P-T-t analyses and deformation fabrics evolution, we infer that some of the mafic blocks originated from the MOB, probably due to South China Sea opening around 17-15 Ma, approximately at the same time of the deposition of the turbidite of the Yuli Belt. Then these mixed rock suites(mélange) were dragged down through subduction channel to 40-60 km depth, where metamorphism and serpentinization occurred at the near-peak temperature of 500-550°C around 5-7 (?) Ma. At the same time, the D2 deformation event induce deformation structures, such as isoclinal fold of S1, axial plane cleavage S2, which likely also represents the dominant fabrics in the subduction channel at the depth. As the subduction rock suite of the Yuli Belt later exhumed rapidly but gradually up to shallow surface level, we suspect that a significant flower structure of S2 fabrics occurred due to convergence and collision between Philippine Sea plate and Eurasia. During the exhumation and flowering process, S3 sub-horizontal fabrics developed at the shallow level, together with widespread normal faulting of obliquely across-strike extension.
Here we present a case study of the rocks from the Yuli Belt in eastern Taiwan. Based on detailed field mapping, with particular attention on the metamorphic deformation structures, along several river transects, we reconstruct a few geological cross sections of about 20 km long, to illustrate the rather complex geological structures of the Yuli Belt. The Yuli Belt is the rock unit which represents the late Cenozoic (mostly Miocene) sedimentary cover lying on the Paleo-Mesozoic Tailuko Belt, the metamorphic basement (core) likely had experienced subduction/metamorphism in Cretaceous time.
Many ultramafic blocks, in particular several large serpentinite bodies with diameter of hundreds of meters to kilometers, are distributed within the pelitic quartz-mica schist and reveal affinity of either intraplate basalt or mantle wedge. The rims of the serpentinite bodies show thermal or chemical interaction with the surrounding pelitic rocks, characterizing by metasomatism products such as talc, chlorite schist, and amphibolite (glaucophane) schist. Structurally, we are able to identify at least three sets of penetrative metamorphic fabrics, S1, S2 an S3. S1 usually parallel to the stratigraphic depositional layers, S0, which however is largely destroyed by pressure dissolution during D1 or D2 deformation event. S2 represents regionally the most dominant fabrics, often characterizing by axial plane cleavage of folded or crenulated S0/S1 compositional layers. Careful, detailed field observations allow us to trace the S2 across the transect across the entire Yuli Belt, showing a deeply dipping of 50-70° to W near the Shoufeng Shear zone (contact with the Tailuko Belt) to the west, and dramatically change to shallowly dipping toward the east in the rest of the belt. S3 seemingly is a late-stage product developed as a sub-horizontal fabric overprinted on locally kinked or crenulated S2 fabrics.
The deformation structures around the serpentinite bodies, including highly stretching rocks with mixed lithology, breccia schist, and incompetent pelitic schist with the main fabric S2 going around the serpentinite, suggest these rock suites have been experienced together probably within the subduction channel, where the Miocene turbidite of the Yuli Belt subducted into depth to mix with oceanic mafic rocks or mantle wedge.
Combining the P-T-t analyses and deformation fabrics evolution, we infer that some of the mafic blocks originated from the MOB, probably due to South China Sea opening around 17-15 Ma, approximately at the same time of the deposition of the turbidite of the Yuli Belt. Then these mixed rock suites(mélange) were dragged down through subduction channel to 40-60 km depth, where metamorphism and serpentinization occurred at the near-peak temperature of 500-550°C around 5-7 (?) Ma. At the same time, the D2 deformation event induce deformation structures, such as isoclinal fold of S1, axial plane cleavage S2, which likely also represents the dominant fabrics in the subduction channel at the depth. As the subduction rock suite of the Yuli Belt later exhumed rapidly but gradually up to shallow surface level, we suspect that a significant flower structure of S2 fabrics occurred due to convergence and collision between Philippine Sea plate and Eurasia. During the exhumation and flowering process, S3 sub-horizontal fabrics developed at the shallow level, together with widespread normal faulting of obliquely across-strike extension.