Time scale of an early to mid-Paleozoic orogenic cycle of the long-lived Central Asian Orogenic Belt,Inner Mongolia of China:Implications for continental growth

摘要

We present a detailed,new time scale for an orogenic cycle(oceanic accretion-subduction-collision)that provides significant insights into Paleozoic continental growth processes in the southeastern segment of the long-lived Central Asian Orogenic Belt(CAOB).The most prominent tectonic feature in Inner Mongolia is the association of paired orogens.A southern orogen forms a typical arc-trench complex,in which a supra-subduction zone ophiolite records successive phases during its life cycle:birth(ca.497-477 Ma),when the ocean floor of the ophiolite was formed;(2)youth(ca.473-470 Ma),characterized by mantle wedge magmatism;(3)shortly after maturity(ca.461-450 Ma),high-Mg adakite and adakite were produced by slab melting and subsequent interaction of the melt with the mantle wedge;(4)death,caused by subduction of a ridge crest(ca.451-434 Ma)and by ridge collision with the ophiolite(ca.428-423 Ma).The evolution of the magmatic arc exhibits three major coherent phases:arc volcanism(ca.488-444 Ma);adakite plutonism(ca.448-438 Ma)and collision(ca.419-415 Ma)of the arc with a passive continental margin.The northern orogen,a product of ridge-trench interaction,evolved progressively from coeval generation of near-trench plutons(ca.498-461 Ma)and juvenile arc crust(ca.484-469 Ma),to ridge subduction(ca.440-434 Ma),microcontinent accretion(ca.430-420 Ma),and finally to forearc formation.The paired orogens followed a consistent progression from ocean floor subduction/arc formation(ca.500-438 Ma),ridge subduction(ca.451-434 Ma)to microcontinent accretion/collision(ca.430-415 Ma);ridge subduction records the turning point that transformed oceanic lithosphere into continental crust.The recognition of this orogenic cycle followed by Permian-early Triassic terminal collision of the CAOB provides compelling evidence for episodic continental growth.