Paper No. 185-6
Presentation Time: 8:00 AM-5:30 PM
GEOLOGICAL CONSTRAINTS ON THE CRUSTAL ARCHITECTURE OF THE CENTRAL OROGENIC BELT, NORTH CHINA CRATON, REVEAL NEOARCHEAN ALPINE-STYLE TECTONICS
KUSKY, Tim1, WANG, Lu2, NING, Wenbin3, HUANG, Bo4, WANG, Junpeng2, DENG, Hao2, ZHONG, Yating5 and PENG, Yaying5, (1)Center for Global Tectonics, State Key Lab for Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan, 430078, China, 388 Lumo Road, Wuhan, China, Hubei 430074, China; State Key Lab for Geological Processes and Mineral Resources, Center for Global Tectonics, China University of Geosciences Wuhan, Wuhan, China, (2)State Key Laboratory of Geological Processes and Mineral Resources and Center for Global Tectonics, China University of Geosciences, Wuhan, 68Jincheng Street, East Lake High-tech Devp. Zone, Wuhan, Hubei Province 430078, China, (3)State Key Lab for Geological Processes and Mineral Resources, China University of Geosciences (Wuhan), China University of Geosciences, Wuhan, 388 Lumo Road, Wuhan, 430074, China, (4)Badong National Observatory and Research Station for Geohazards, China University of Geosciences (Wuhan), China University of Geosciences, Wuhan, 388 Lumo Road, Wuhan, 430074, China, (5)State Key Laboratory of Geological Processes and Mineral Resources, and Center for Global Tectonics, School of Earth Sciences, China University of Geosciences Wuhan, 388 Lumo Road, Wuhan, 430074, China
Styles of tectonics on Earth through time are debated. North China Craton’s 1800 km-long Central Orogenic Belt (COB on figure) records the 2.5 Ga collision of a 2.7-2.5 Ga oceanic arc(s) with a collage of slightly older arcs in an accretionary orogen. The orogen has classical orogenic zones, including a 2.7-2.5 Ga arc(s), fore-arc MORB-boninitic-IAT sequences, far-travelled sub-horizontal sheath-folded nappes, poly-genetic mélanges, and fore-arc basinal assemblages. The belts form a classic medium-low dT/dP high dT/dP paired metamorphic belt. Ophiolitic fragments in the mélange range in size from individual outcrop-sized blocks and elongate several-km-long lenses in a metapelitic and locally serpentinitic or mafic matrix. Rock types include serpentinized harzburgite tectonites, rare lherzolites, pyroxenites, garnet pyroxenites, layered gabbro, massive gabbro, local dike complexes, basalts (locally pillowed), cherts, metalliferous deposits, and metapelites. Some blocks have fore-arc geochemical signatures, others have MORB signatures, locally separable into different mappable mélange belts. These belts were thrust over a 2.58-2.50 Ga passive margin-like sequence formed on the back-side of other recently accreted arcs forming the Eastern Block of the craton, and are associated with a 2.50/2.45 – 2.31 Ga foreland basin sequence. The thrusting is well-documented by a belt of Alpine-style sub-horizontal nappes resting allochthonously over the passive-margin sequence.
The ophiolitic fragments provide constraints on conditions in Archean subduction zones, oceanic lithosphere, and deeper mantle. An N-MORB garnet pyroxenite block in mélange experienced eclogite-facies conditions, yielding a dT/dP of 11℃/ km, one of the coldest recorded Archean geotherms. This is confirmed by an Alpine-type peridotitic block in the mélange independently yielding a dT/dP of 11℃/ km. Perhaps most remarkably, some podiform chromitites in harzburgites have yielded a suite of crustal and mantle inclusions, including TiO2(II) stable above 7.5 GPa (~270 km), and the UHP phase of chromite, chenmingite, stable above 14 GPa (~410). These inclusions are similar to those of Phanerozoic ophiolites, demonstrating the operation of deep crust-mantle recycling through plate tectonic processes for at least the past 2.7 Ga.
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