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Tectonic evolution of the Ailaoshan Fold Belt in Southwestern Yunnan, China

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Lai, CK (2012) Tectonic evolution of the Ailaoshan Fold Belt in Southwestern Yunnan, China. PhD thesis, University of Tasmania.

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Abstract

The Ailaoshan Fold Belt represents an important component of the geologically
complex South China–Indochina suture zone. The fold belt is made up of the Eastern
Ailaoshan (EAL) High–Grade Metamorphic Belt, the Paleozoic Central Ailaoshan
(CAL) Mélange, and the Carboniferous–Triassic Western Ailaoshan (WAL) volcanic
belts. This thesis presents a new, better constrained tectono–magmatic evolution
model for the Ailaoshan Fold Belt based on a detailed compilation of existing Chinese
literature, complemented by extensive new sampling, as well as geochronological
(U–Pb zircon and U–Th/Pb monazite), and whole rock and mineral geochemical data.
Contrasting with most published literature, magmatic rocks within the Paleozoic CAL
Mélange have been found to record two episodes of continental rifting and/or volcanic
passive margin development, one Late Devonian–Early Carboniferous, the other Late
Permian. There is no preserved record of the oceanic crust of the ocean basin which
opened as a result of the Late Devonian–Early Carboniferous rifting episode, and the
other magmatic suite recorded in the CAL is the earliest Mid–Triassic syn–collisional
granitic magmatism. The WAL volcanic belts, in contrast, include fault–bounded
slices of rocks formed during a Late Carboniferous–earliest Mid–Permian continental
rift magmatism, a Mid Permian arc/backarc basin magmatism, a Late Permian
syn–collisional granitic magmatism and an Early Triassic post–collisional magmatism.
Magmatism/metamorphism in the EAL High–Grade Metamorphic Belt has been
determined to have occurred during Neoproterozoic, Mid– to Late–Triassic and
Cenozoic events, with protoliths originating from various magmatic– and sedimentary
rocks in the Ailaoshan–western South China region.
A new regional tectonic model based on the newly compiled data suggests that
diachronous continental rift magmatism, leading to volcanic passive margin formation
and break up, occurred in the CAL and WAL during, respectively, Silurian–Late
Devonian/Early Carboniferous (ca. 430–400 Ma and ca. 370–325 Ma) and Late
Carboniferous/Early Permian–earliest Mid–Permian (ca. 300–270 Ma), before which
these terranes were attached to the western South China margin in Eastern Gondwana.
The volcanic passive margin development and the subsequent seafloor spreading
opened the Ailaoshan–Song Ma Paleotethys branch, also known as the
Ailaoshan–Song Ma Ocean.
Ridge push associated with WAL continental rifting may have facilitated the
west–vergent subduction of the Ailaoshan–Song Ma Ocean beneath eastern Indochina
since Early Permian (ca. 280 Ma), producing the Truong Son arc magmatism in
Vietnam. In the WAL, in contrast, Mid Permian (ca. 265 Ma) arc/backarc basin
magmatism may have only been active for a limited time before the WAL arc collided
with and was accreted onto eastern Indochina during late Mid–Permian to Late
Permian. This arc–continent collision has been accompanied by extensive Late
Permian WAL–Truong Son syn–collisional granitic magmatism (ca. 260–250 Ma)
and followed by Early Triassic WAL post–collisional magmatism (ca. 250–245 Ma).
Coevally (ca. 260 Ma), the Emeishan Large Igneous Province located in the western
South China Block, and may have extended to the CAL–Jinshajiang–Song Da
regions.
Continued subduction of the Ailaoshan–Song Ma Ocean remnants may have
eventually led to the South China–Indochina continent–continent collision,
commenced during Late Permian–Early Triassic in central Vietnam and propagated
northward to the CAL–Song Ma (northwestern Vietnam) region during earliest
Mid–Triassic to Late Triassic (ca. 230–200 Ma). This may have produced the major
regional unconformity recorded across SE Asia, together with the syn– and
post–collisional CAL and EAL magmatism/metamorphism.

Item Type: Thesis (PhD)
Keywords: Ailaoshan, tectonic, geochemistry, geochronology
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Date Deposited: 25 Feb 2013 04:06
Last Modified: 15 Sep 2017 00:59
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