Magmatic And Metamorphic Records Of The Ailaoshan Tectonic Belt Since Cretaceous And Their Tectonic Implications

Three metamorphic belts of the Sanjiang area,Gaoligongshan,Chongshan and Ailaoshan metamorphic zones,are located in the southeastern margin of the eastern Himalayan Syntaxis,with an N-S or NW-SE trending extension from west to east,at which abundant geological signatures related to the Indian-Eurasian plate convergence have been preserved.Some important geological problems have yet been unsolved since Cretaceous,e.g.,(1)whether the Cretaceous mantle properties and their dynamics are controlled by the subduction of the Tethyan Ocean or the Pacific Ocean;(2)what is the relationship between the Cenozoic Indian-Eurasian collision and extruding mechanism of the Indochina block(e.g.,the rotation and occurrence of the ductile shear zone in the Yunnan-Burma area).Extensive magmatic activities have been developed since the Cretaceous in the Ailaoshan metamorphic zone on the eastern Sanjiang area.The temporal and spatial framework and petrogenesis of these magmatic rocks may provide key insights into the aforementioned scientific problems.In this dissertation,metabasites and dioritic-granitic rocks from the Ailaoshan tectonic zone since the Cretaceous were selected.This dissertation also reports the detailed field investigations and laboratory analysis of petrology,mineralogy,elemental geochemistry,LA-ICP-MS or SIMS Zircon U-Pb chronology,Sr-Nd-Hf isotope data.In combination with the regional geological data and previous achievements,it is proposed for the temporal and spatial distribution and petrogenesis of these rocks.The main achievements and progresses are listed as following.1.Proposing the tectonic model of episodic slab rollback and extension in response to the Neotethyan subductionIn this dissertation,we reported Cretaceous Nb-enriched diabases(~139 Ma),Nb-enriched(~68 Ma)and OIB-like(~73 Ma)amphibolites from the Ailaoshan tectonic zone in western Yunnan Province.The ~139 Ma diabases and ~68 Ma amphibolites display similar geochemical signatures with typical Nb-enriched basalts,e.g.,high Na2 O,TiO2,Nb(>7 ppm),(Nb/Th)PM,(Nb/La)PM and Nb/U ratios.Distinct from the adakite-associated Nb-enriched basalts,our Nb-enriched rocks were derived from the mixing source of OIB-like with arc-like components in the spinel stability field(<70 km),in spite of higher degree(~15%)partial melting for the Nb-enriched diabases.In comparison with the 139 Ma and 68 Ma Nb-enriched mafic rocks,the ~73 Ma Yaoshan amphibolites have lower Mg# values,higher TiO2 and Nb(20.0-26.0 ppm)contents.The Yaoshan samples show the OIB-like REE patterns and trace element spidergrams and were probably generated by a low degree partial melting of garnet-facies peridotite(>80 km).In combination with the Cretaceous to Early Eocene data of granitoids,three significant magmatic flare-ups can be identified involving 140-115 Ma,75-64 Ma and 55-50 Ma.Taking into account the spatial pattern of the Pacific and Neotethyan trenches(2400-5000 km and 320 km,respectively),it is inferred that these Cretaceous igneous rocks were geodynamically most likely related to subduction of the Neotethyan Ocean.It is herein proposed that a tectonic model of episodic slab rollback and back-arc extension in response to the Neotethyan subduction in western Yunnan Province.2.Probing the continuous shear process of the Ailaoshan-Red River shear zone using the late Eocene to early Miocene metamorphic signaturesLA-ICP-MS zircon U-Pb dating results of the migmatite,gneiss and amphibolite from the northwest section of Gasa and Mosa regions to the southeastern part of Jinping area show the Cenozoic metamorphism at 40~20 Ma.These metamorphic rocks can be divided into two types with one of which represented by newly migmatite veins and other by Cenozoic metamorphic gneiss and amphibolite.In combination with the U-Pb geochronology of these Cenozoic rocks and the Ar-Ar dating of the synkinematic minerals along the Ailaoshan tectonic belt,it is inferred that the metamorphism and synchronous by shearing was at ca.40~20 Ma and subsequent cooling at ~25~5 Ma.3.Determining the temporal pattern of Late Eocene to Early Miocene magmatism in the Ailaoshan-Red River shear zoneThe Cenozoic basic to acid magmatism in the Ailaoshan-Red River shear zone is mainly dated at 38~22 Ma.The typical magmatic zircons from the basic rocks yield the U-Pb age of 25.5 Ma.The monzonitic-dioritic-granitic rock-association can be divided into the low and high ?Nd(t)-?Hf(t)rocks with former being derived from Ailaoshan Proterozoic metamorphic metasedimentary rocks and later from partial melting of juvenile crust.In late Oligocene,the shearing tense is transformed from transcompression to transtension in response to the upwelling of the mantle,accompanied by the basic and I-and S-type granitic magmatism.4.Dividing the sinistral strike-slip shearing stages in the Ailaoshan-Red River shear zone and understanding the tectonic connection of the Sanjiang shear systemsOur data show that the magmatism and metamorphism along the Ailaoshan-Red River shear zone focus between ~40 Ma and ~20 Ma.The synthesis of the available data suggests three-staged sinistral strike-slip shearing.At ~40-~30 Ma,crustal anatexis played a crucial role for inducing the formation of migmatite and granite gneiss.At ~30-~22 Ma,strong shearing resulted in the generation numerous magmatic rocks including basic to intermediate acid rocks.After ~22 Ma,a process of weaken shearing is followed.Available data show that the Gaoligongshan zone between the Tengchong block and Baoshan block are characterized by dextral strike-slip shearing,synchronous with the sinsitral shearing of the Chongshan and Ailaoshan-Red River shear system.Thus it is most likely that these shearing systems were the products of convergence of the Indian-Eurasian plates,which is the major controlling factor inducing large-scale extrusion of the various fragments in SE Asia,even the opening of the South China Sea basin.