HR: 0800h
AN: V41C-0716    [Abstracts]
TI: Structural Geometry of an Exhumed UHP Terrane in the Sulu Orogen, China: Implications for Lower Crustal Deformation and Continental Collisional Processes
AU: * Kusky, T
EM: kusky@eas.slu.edu
AF: Earth and Atmospheric Sciences, Saint Louis University, St. Louis, MO 63108, United States
AU: Wang, L
AF: Dept. of Marine Geology, Ocean University of China, Qingdao, 266100, China
AU: Li, S
AF: Dept. of Marine Geology, Ocean University of China, Qingdao, 266100, China
AB: Yangkou Bay in the eastern Sulu Orogen, China, preserves significant exposures of UHP eclogites, representing pieces of continental and oceanic crust caught between the colliding North and South China continents in the Paleozoic-Mesozoic, subducted to depths > 200 km, then exhumed along thin shear zones. Narrow coesite- eclogite shear zones are folded and progressively overprinted by amphibolite and greenschist facies shear zones that become wider with lower grade. Detailed mapping shows eclogite facies fabrics (S₀ consisting of garnet+omphacite+phengite+rutile+quartz) preserved in relatively weakly deformed zones, where statically recrystallized garnet and omphacite are aligned with a weak shape preferred orientation. Phengite formed during an early eclogite-facies retrogression stage has a different orientation than S₀, meaning S₀ formed before the phengite grew. A thin layer of fine-grained hornblende and albite coronas grew on the omphacite. Elongated garnet, omphacite, phengite and rutile define S₁ and L₁. Garnet shows dynamic recrystallization and omphacite shows plastic deformation, representing eclogite facies deformation during early stages of exhumation. Mylonitic to ultramylonitic shear zones separate 5-10-meter-thick nappes of ultramafic-mafic UHP rocks and banded granitic gneiss. Ultramafic-mafic UHP rocks form a magmatic series ranging from dunite-gabbro- gabbroic eclogite- eclogite- retrogressed eclogite, bounded by ultramylonite, with interlayered, strongly foliated and lineated granitic and eclogitic mylonites. The protolith of these rocks was a mafic/ultramafic sequence, probably ophiolitic. In gabbroic to granitic mylonite most minerals except quartz and augite are sheared into thin bands. Quartz and augite are preserved as deformed porphyroblasts in a ductile matrix and are mantled by small grains of omphacite and garnet, which are present in the deformed tails of augite. Rutile is preferentially crystallized in pressure shadows. K-feldspar is dynamically recrystallized. UHP-HP minerals in the mantles, tails, and pressure shadows of the porphyroclasts confirm UHP deformation. Small lineated tonalite plugs intruded after F2 and before F3 between the banded granitoid-gabbro gneiss and ultramylonite. S₃ in amphibolite is defined by oriented mica and amphibole, while in lighter colored granitic gneiss, it is mainly defined by stretched quartz and feldspar. This foliation is widely developed in the country rocks parallel to compositional layering S₀. Dark mafic layers and amphibolites are stretched into lenses and ultimately, into thin layers parallel to S₀. F3 folds are defined by compositional layers and the regional F2 foliation that are both folded and deformed into isoclinal folds. Greenshist facies deformation and metamorphism overprints the granitoid gneiss. F3 is refolded by F4 and shows more angular fold styles whose fabrics developed together with greenschist facies retrogression. In hinges, biotite-chlorite crenulation lineations surround F3 fold axes, displaying radial patterns. In some cases, greenschist facies metamorphism and deformation are overprinted on both interlayered granitoid gneiss and retrogressed amphibolite facies eclogite layers. Isoclinal recumbent F3 folds are refolded by open folds. F4 folds with axial surfaces striking NW and dipping moderately SW exhibit a new greenshist facies foliation (S₄) that displaces the S₃ amphibolite facies foliation forming a crenulation foliation.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3625 Petrography, microstructures, and textures
DE: 8012 High strain deformation zones
DE: 8030 Microstructures
DE: 8032 Rheology: general (8160)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting