The Neoproterozoic Tectono-Thermal Events In The Eastern Qinling Orogen, And Their Geo-Tectonic Significances

Qinlin Orogen, which lies between Yangtze Craton and North China Craton and occurs in thecentral part of China, has already been the highlight region since many years before and attracts somuch attention of geologists from all over the world. Particularly, along with the detailed study ofHP and UHP metamorphism in Qinlin and Dabie Orogens from 90's last century, more and moregeochronological information of Neoproterozoic tectono-thermal events has also been identifiedwithin these orogen. What are the exactly geological significances of these Neoproterozoictectono-thermal events? Do these geological events represent the existence of Neoproterozoicorogenic process? What is the relationship between this geo-tectonic process and Rodinia? For thepurpose to answer these kinds of questions, this paper was designed to take the eastern part ofQinling Orogen as the key region and focused the main research work on the different geologicalrecords of late-Mesoproterozoic to Neoproterozoic tectono-thermal events occurred in this area.And systematic petrological, geochemical and geochronological studies of these kinds ofgeological records such as ophiolite, granitoid intrusions, (meta-) volcanic-sedimentary rocks,have been carded out on the base of detailed geological survey work in the field, for the aim todetermine their geo-tectonic setting, forming ages and evolutionary sequences and then reveal thestructural signiricances of these late-Mesoproterozoic to Neoproterozoic tectono-thermal events inthe process of continental assembly and break-up.The data of this dissertation show that a lot of ultra-maric accumulations and mafic meta-volcanicrocks which exhibit the geochemical features of MORB are formed within Songshugou ophiolitewhich occurred in North Qinling, except for the remnant pyrolite. All of them are closelycorrelated in space, forming age and genetic relationship. The whole-rock and minerals fromultra-maric accumulations of orthopyroxene peridotite and dunite determine a Sm-Nd isotopicisochron defining an age of 1079±63 Ma (εNd (T)=+5). This is consistent with the forming ageof the protolith of marie meta-volcanic rocks within this ophiolite, demonstrating the existence oflate-Mesoproterozoic oceanic basin structure in Qinling Orogen. Moreover, the geochemical dataof these mafic volcanic rocks also show that the development of this oceanic basin was probablyresulted in the on-going break-up of continental-related basement and can be compared to animmature oceanic basin structure. In addition, the forming ages of Niujiaoshan, Dehe and Zhaigengranitoid intrusions have been determined using high precise SHRIMP and ID-TIMS zircon U-Pbisotopic dating method, giving ages of 954.6±5.1 Ma, 943±18 Ma and 914±10 Ma, respectively. The newly recognized gneissic granitoid intrusions occurred in Shicaogou of Xixia County andSongshugou of Shangnan County have the forming ages of 937±21Ma,843.6±4.4 Ma and844.4±3.1 Ma, respectively. All the petrological and geochemical data show that these granitoidintrusions are formed in continental collisional tectonic setting, exhibiting characters ofsyn-collisional granitoid in large scale as well as with some features of volcanic arc granitoids intrace element geochemistry. The recognition of Songshugou late-Mesoproterozoic ophiolite andthe early-Neoproterozoic syn-collisional granitoid intrusions demonstrates the existence of theNeoproterozoic orogen in Qinling Orogen. The early-Neoproterozoic orogenic events are thesignificant response of the assembly process of Rodinia. The presence of large scaledsyn-collisional intrusive activities with forming ages between～960 Ma and～900 Ma suggeststhat the final assembly of Rodinia occurred around～900 Ma, nearly 100 Ma younger than thetiming period of the typical Grenvillian orogen (1.3～1.0Ga). From this point, the convergentprocesses of Rodinia are diachronous, not solely constrained by the Grenvillian orogeny.Many different geological records related to the break-up events of Nanhua Period, such asYaolinghe Group and Yunxi Group bimodal volcanic rocks,Tuwushan A-type granite andLaojundian intrusive rocks, occur in South Qinling in the eastern part of Qinling Orogen. Theirforming ages range from～745 Ma to～710 Ma (these are 745.8±1.5 Ma, 713.7±6.6 Ma,717±10 Ma, 716.6±3.9 Ma and 711±11Ma, respectively). These geological events are nearlyconsistent in time and tectonic setting with the formation of volcanic rocks and intrusions formedin the break-up process of Nanhua Period in north margin of Yangtze Craton, which demonstratesthat both of them are controlled by a uniform break-up tectonic system. These break-up events ofNanhua Period in Qinling Orogen are the significant response to the break-up process of Rodiniain Yangtze Craton. And～810 Ma to～710 Ma is the critical period of break-up process ofRodinia.The research results also show that all the early-Neoproterozoic syn-collisional granitoidintrusions only occur in north Qinling. That is to say the north Qinling was involved in thesubduction of the oceanic crust and subsequent collision of continental block to continental blockin the early-Neoproterozoic period. According to the fact that Songshugou oceanic basin wasoriginally formed within Yangtze Craton, the disappearance of Songshugou oceanic basin is thenthe result of subduction of ancient oceanic crust down to the north Qinling and south Qinling,respectively. However, there is not any coeval tectono-thermal event occurred in adjacent southmargin of North China Craton. This can be interpreted that the North China Craton does not getinvolved in the Neoproterozoic tectonic cycle at that time. So it is believed that thelate-Mesoproterozoic to early-Neoproterozoic geological events only represent the orogenicprocess between north Qinling and south Qinling. But there was no convergent process occurringbetween north Qinling and the adjacent south margin of North China Craton in Neoproterozoic era. Qinling Group is one of the most characteristic Precambrian metamorphic rock series occurred innorth Qinling. The studies of its forming age and metamorphism are quite helpful to therealization of Precambrian geological evolution of the Qinling Orogen. Firstly, the granulite faciesmetamorphic rocks have been recognized for the first time from Qinlin Group. One of them, theclinopyroxene- and hypersthene-bearing mafic granulite, occurs in association with calc-silicategranulite and graphite-bearing marble and gives a zircon U-Pb isotopic age of 512.4±9.2 Mawhich represents its metamorphic time. In combination with the fact that Qinling Group ischaracterized by the formation of aluminium- and carbon-rich meta-sedimentary rocks such asgraphite-bearing marble, graphite-bearing gneiss, sillimanite gneiss, graphite mica-schist andsillimanite garnet biotite quartz-schist, it seems to be concluded that the principal part of QinlingGroup is identical to the khondalite series. Moreover, the forming age of leucosome occurred inQinling Group has been determined giving an age of 499.3±4.3 Ma. This identification of theages of mafic granulite and leucosome further proves the strong impact of Caledonian orogen tothe Qinling Orogen.The forming ages of detrital magmatic zircons from sillimanite biotite plagioclase gneiss ofQinling Group have been obtained using SHRIMP method. All their ²⁰⁶Pb/²³⁸U apparent ages aredivided into two groups aged at 1200 Ma～1300 Ma and 1500 Ma～1800Ma, respectively. Andthe detrital zircons from mica quartz schist of Qinling Group give ²⁰⁶Pb/²³⁸U apparent agesconcentrated around 1100 Ma～1300 Ma, 1400 Ma～1800 Ma and＞2000 Ma, with a minimumage of～1122 Ma. These data suggest that the maximum forming age of Qinling Grouporiginated no more than 1100 Ma～1200 Ma and do not support the previous viewpoint thatQinling Group is originately formed in Palaeoproterozoic era. The sedimentary fragments ofQinling Group mainly come from the erosion of Mesoproterozoic and Paleoproterozoic rocks. Inaddition, one group of the detrital zircons from para-metamorphic gneiss of Douling Group gives²⁰⁷Pb/²⁰⁶Pb apparent ages ranging from 1635±22 Ma to 1672±25 Ma, demonstrating thatprotolith of Douling Group was formed later than 1635±22 Ma but not in Paleoproterozoicperiod.On the whole, all the geological events associated with the early-Neoproterozoic assembly andlate-Neoproterozoic break-up of Qinling Orogen represent the critical response within YangtzeCraton to the formation and break-up of Rodinia. The final assembly of Rodinia was finishedaround～900 Ma, 100 Ma younger than the timing period of the typical Grenvillian orogeny.And～810 Ma to～710 Ma is the critical period of break-up process of Rodinia. There was noconvergent process occurring between north Qinling and the adjacent south margin of North ChinaCraton in Neoproterozoic era...