Fabric kinematics of the ultrahigh-pressure metamorphic rocks from the main borehole of the Chinese Continental Scientific Drilling Project: Implications for continental subduction and exhumation

摘要: Abstract The 5158-m-deep main borehole of the Chinese Continental Scientific Drilling Project (CCSD-MH) penetrated granitic gneisses, paragneisses, eclogites, retrograde amphibolites and ultramafic rocks in Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. CCSD-MH consists four petro-structural units separated by three SE-dipping ductile shear zones DFa (835–1280 m), DFb (2010–2280 m) DFc (2920–3225 m), which are correspondent with regional northern UHP supracrustal zone. Using electron backscatter diffraction (EBSD) technique, we investigated lattice-preferred orientations (LPOs) omphacite, diopside quartz core samples from CCSD-MH. Omphacite eclogites garnet pyroxenites display very strong LPOs, characterized maximum concentration [001]-axes parallel to lineation (010)-poles normal foliation. Quartz para- mylonites/gneisses DFa, developed multiple slip systems. 40Ar/39Ar dating biotite gneisses yields 223–202 Ma, constrains formation ages high-temperature prism systems {m} {m}[c]. asymmetric LPOs diopside, olivine respect structural frame reveal deformation phases terrane. In Middle Triassic, northward subduction Yangtze plate depths > 100 km produced a top-to-the-south sense olivine, nearly N–S-striking foliation subhorizontal N–S-trending rocks. Late were exhumed lower crust top-to-the-NW sense, is consistent SE-plunging zones. Cretaceous middle when migmatization intrusion formed NE-striking antiform structure. As result, activation low-temperature basal (0001) top-to-the-SE south, but north.