| Granites formed at all stages of collisional orogenesis, the research of their petrogenesis can promote our understanding of the nature of the orogenic. lower crust, deep dynamic orogenic process and the interactions between crust and mantle during orogenic process. The Triassic Qinling-Dabie-Sulu orogenic belt is the most prominent tectonic features in central China, which was resulted from the continental-continental collision between the North China and Yangtze blocks, and led to the final formation of the China continents. However, this uniform collisional orogenesis caused obvious contrast between the western and eastern parts of the Qinling-Dabie-Sulu orogenic belt. In the Dabie-Sulu orogenic belt, there is large-scale of high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks, which were considered to be resulted from deep subduction (about 80～120km) and subsequent rapid exhumation of the Yangtze continental lithosphere beneath the North China Block. However, in the Qinling orogenic belt, the Late-Triassic collision caused large-scale high-K calc-alkaline granites and associated mafic enclaves. So some scientific questions were raised: (1) Why the uniform collision caused huge difference between the eastern and western parts of the of the Qinling-Dabie-Sulu orogenic belt? (2) Whether or not the occurrence of Triassic continental subduction in the Qinling orogenic belt. If their, how the nature and fate of the subducted continental lithosphere in the Qinling orogenic belt between differ from the Dabie-Sulu orogenic belt? Understanding these issues has great scientific significance for investigating the complete tectonic evolution of the Triassic Qinling-Dabie-Sulu orogenic belt. This thesis is focused on the significant difference between the eastern and western part of Qinling-Dabie-Sulu orogenic belt, trying to prove that the Late Triassic granitoids in the Qinling orogenic belt was resulted from partial melting of subducted Yangtze continental crust.This thesis focus on the petrogenesis and geodynamic implications of the Late-Triassic granites and associated mafic enclaves from the different tectonic unite of the Qinling orogenic belt(Dongjiangkou and Wulong pluton in the southern Qinling terrane, Yangba pluton from the northern margin of Yangtze block and Mishuling pluton from the western Qinling terrane), and the data of petrology, major-and trace element geochemistry, Sr-Nd-Pb isotopic composition, as well as zircon LA-ICP-MS U-Pb geo-chronology and Lu-Hf isotopic composition of typical granite pluton are presented in the thesis. We use these data to trace the source region and petrogenesis of the granites, discuss the role of mantle components in the genesis of the granitic magma. In combination with regional tectonic setting, we argue that the Late-Triassic granites in the Qinling orogenic belt were resulted from multi-stage partial melting of the subducted Yangtze continental crust during its exhumation process. These results provide a new research approach for the petrogenesis of high-K calc-alkaline adakitic granites in post-collisional setting. This thesis yielding the following results and understanding:1. Zircon U-Pb dating revealed a period of 235-200Ma for the formation of the Late-Triassic granites in the Qinling orogenic belt, and it can be divided into three stages according their petrology and geochemical features:the first stage (235～225Ma) formed minor quartz diorite; the second stage (225～210Ma) include large amount of high-K calc-alkaline granites and associated mafic enclaves; the third stage (～200Ma) formed minor biotite granite.Detailed zircon LA-ICP MS U-Pb dating on the granites from different tectonic unites of the Qinling orogenic belt revealed that:the Dongjiangkou granites from the eastern part of the south Qinling terrane have crystallization ages of 214±2 Ma to 222±2 Ma; The Wulong pluton is a composite pluton:quartz diorite from the pluton margin has crystallization ages of 233±2 Ma to 227±2 Ma, granodiorite from the intermediate zone has crystallization age of 218±2 Ma, K-feldspar megacrysts-bearing monzogranite has crystallization age of 207±2 Ma, this shows that the Wulong pluton was formed span a period of 25Ma, and the pluton was formed by increments of different batch of granitic magmas. The Yangba monzogtanite from northern margin of the Yangtze block have U-Pb age of 208±2 Ma. The Mishuling monzogranite from western Qinling terrane has zircon U-Pb age of 213±3 Ma.The above zircon U-Pb data show that Late-Triassic granitic magmatism in the Qinling orogenic belt span a period of 235Ma～200Ma, and three main pulse can be recognized according to their geochronology and petrological features:(1) the first pulse (235Ma to 225Ma) formed minor quartz diorite in the margin of the Wulong pluton; (2) the second pulse (220Ma to 210Ma) formed widespread high-K calc-alkaline granites and associated mafic enclaves, including the granodiorite and tonalite from the Dongjiangkou pluton, granodiorite and monzogranite from the Wulong pluton, monzogranite from the Yangba and Mishuling pluton; (3) the third pulse (～200Ma) formed biotite granites in the Guangyoushan and Yanzhiba pluton, and there is no mafic enclaves in these granites. Then, it can be concluded that Late-Triassic granitic magamtism in the Qinling orogenic belt is formed by multi-stage increment, but not short-lived (220 to 205Ma).2. Late-Triassic granites from the Qinling orogenic belt are mainly high-K calc-alkaline, and display adakitic affinity, e.g., enriched in Sr, Ba, extremely depleted in Y and HREE, without significant negative Eu anomalies. This thesis trace the petrogenesis and source rocks of the these granites through petrology, major-and trace-element geochemistry, Sr-Nd-Pb and zircon Lu-Hf isotopic composition, these result provide a new approach for the genesis of Late-Triassic granites in the Qinling orogenic belt:(1) these granites are formed in post-collisional setting, and their high K content may suggest that their source rock were metasomatized by K-rich fluid or melt; (2) their high-Mg# (Mg#＞45) values can not prove Late-Triassic thickened lower crust delamination in the Qinling area, but may be formed by interaction between subducted continental crust-derived adakitic magma and overlying mantle; (3) zircons from all the granites display large variation in Hf isotopic composition, suggesting they were derived from a homogeneous source region, including slices of Neo-Proterozoic mantle lithosphere, Mid-Proterozoic mafic lower crust and some ancient crust; (4) the high Sr/Y ratios may be inherited from their source rocks, it is not a precise evidence for the high-pressure condition. Above conclusion have important potential significance for the genesis of post-collisional high-K calc-alkaline granites in collision oeogenic belt.Major-and trace-element geochemistry revealed that Late-Triassic granites are mainly high-K calc-alkaline, and display adakitic affinity, e.g., enriched in Sr, Ba, extremely depleted in Y and HREE, without significant negative Eu anomalies. According their geochemical features, three main types can be recognized:(1) quartz diorite formed at 235Ma to 225Ma were characterized by low Si, high K, Mg contents, which was considered to be resulted from high degree (>40%) partial melting of mafic lower crust and undergone subsequent interaction with enriched mantle wedge during their ascent; (2) high-K calc-alkaline granidiorites, tonalite and monzogranite formed at 220Ma～205Ma also display adakitic geochemical features, the high Mg# adakitic granodiorite and tonalite from the Dongjiangkou provide exact evidences for partial melting of subducted continental crust:Exhumation of subducted continental crust that resulted from slab break off would melt in condition of aqueous fluids derived from the decomposition of hydrous minerals and heat from the upwelling asthenosphere, producing adakitic melts at relatively high pressure (amphibole±garnet stable field). During their ascent, the adakitic magma bodies extensively react with ambient peridotite to form pyroxenite and orthopyr-oxene-rich zone, this is evidenced by the outward increasing trend of MgO, Mg#, Cr, Ni P2O5 and Y contents, as well as Nb/Ta, Rb/Sr and Th/U ratios, indicating more mantle components in the pluton margin. Extensive melt-mantle interactions would make the Cr, Ni, Mg, Y, Rb and K transfer into the granitic melt, and produce the Mg-rich hybrid magmas. (3) biotite granite formed at～200Ma should be resulted from partial melting of middle orogenic crust, which was induced by fluids from lower crust or mantle lithosphere.3. Whole-rock Sr-Nd and zircon Lu-Hf isotopic composition for the Late-Triassic granites in the Qinling orogenic revealed a complex source rocks:mainly consisted by Mid-Proterozoic mafic lower crust with minor incorporation of Neo-Proterozoic mantle lithosphere and Early-Proterozoic upper crust. Meanwhile, whole-rock Pb isotopic composition of the granites suggested that there is different basement in the southern Qinling terrane and western Qinling terrane.Late-Triassic granites from the Qinling orogenic belt mainly have whole-rock Nd isotopic model ages of 1.1 to 1.3Ga, this is consistent with the Mid-Proterozoic crustal formation event in the Yangtze block, but clearly different from the North China block, suggested that these granites were mainly derived from the Yangtze continental crust. Zircons from all the Late-Triassic granites display large variation in Lu-Hf isotopic composition, their Hf isotopic model ages mainly focus on 1.1 to 1.3Ga, but still contain some zircons that have Hf isotopic model ages of-800Ma and 1400 to 1600Ma. The overall isotopic geochemical data suggest that the source rocks of the Late-Triassic granites in the Qinling orogenic belt are mainly consisted by Mid-Proterozoic continental crust, with minor incorporation of Neo-Proterozoic mantle lithosphere and Early-Proterozoic upper crust. Whole-rock Pb isotopic composition reveal that basement from the western Qinling terrane have clearly Yangtze affinity, and the basement from the southern Qinling terrane and Bikou terrane have transitional features between Yangtze and North China blocks.4. Mafic enclaves from the Late-Triassic granites have zircon U-Pb ages of 220 to 210Ma, some of the mafic enclaves also display high Sr, Sr/Y and depleted in Y and HREE, in combination with whole-rock Sr-Nd-Pb and zircon Lu-Hf isotopic data, it can be considered that these mafic enclaves may be produced by Late-Triassic reworking of Neo-Proterozoic enriched sub-continental lithospheric mantle, which is consisted by garnet iherzolite. The widespread mafic enclaves in the granites suggest a regional partial melting of sub-continental lithospheric mantle during Late-Triassic time, which may have great contribution to the genesis of granitic magma. The high-K calc-alkaline granites that formed at 220 to 210Ma contain widespread mafic enclaves, detailed zircon LA-ICP MS U-Pb dating for the mafic enclaves indicates that these mafic enclaves have identical ages with their host granites, suggesting Late-Triassic mafic magmatism in the Qinling orogenic belt. These mafic enclaves have mafic-intermediate, alkaline composition, enriched in LILEs and LREE, display negative anomalies in HFSEs, display geochemical features of island arc volcanic rocks, in addition, some of the mafic enclaves display high Sr, Sr/Y ratios and depleted in Y and HREE, suggesting garnet in their source residue. In combination with the results of whole-rock Sr-Nd-Pb and zircon Lu-Hf isotopic analysis, we argued that these mafic enclaves may be formed by reworking of Neo-Proterozoic sub-continental lithospheric mantle, and undergone magma minxing with their host granites. The widespread Late-Triassic mafic enclaves in the granites suggest a regional partial melting of sub-continental lithospheric mantle during Late-Triassic time, which may have great contribution to the genesis of granitic magma.5. This thesis put forward a new petrogenetic model for the genesis of Late-Triassic granites in the Qinling orogenic belt:they were formed by multi-stage partial melting of subducted Yangtze continental crust. the partial melting of continental crust may be caused by:(1)the clockwise rotation of the Yangtze Block cause extension setting in the Dabie area, which led to the rapid exhumation of HP-UHP metamorphic rocks, while compression setting in the Qinling area and cause extensive partial melting of subducted continental lithosphere, led to the widespread Late-Triassic granites magmatism and associated mafic magmatiam; (2) there is higher convergence rate in the Qinling area than those of the Dabie-Sulu area during the Triassic collision between the Yangtze and North China blocks; (3) continental lithosphere in the western and eastern part of the Yangtze block has different composition and thermal structure.In combination with regional tectonic setting, this thesis proposed a new petrogentic model to explain the genesis of Late-Triassic granites in the Qinling orogenic belt:(1) the sub-continental lithospheric mantle was metasomatized by Late-Paleozoic to Early-Mesozoic northward subduction of the Mianlue oceanic crust beneath the south Qinling terrane; (2) in Late-Triassic time, the Yangtze and North China blocks collided in the Qinling area, because of the regional convergence and gravity of high-density eclogitized oceanic lithosphere, led the Yangtze continental lithosphere subducted beneath the south Qinling terrane; (3) dense and refractory mafic lower crust that was trapped in mantle depth by continental subduction will melt at high temperature to produce the early (235 to 225 Ma) quartz diorite, subsequent interaction with mantle peridotite would elevate and their Mg# and metasomatized the overriding mantle wedge, in combination with the gravity of high-density oceanic lithosphere, this would led to the slab break-off in the Qinling area; (4) the slab break-off cause asthenosphere upwelling and exhumation of the subducted continental crust, this would led to the extensive partial melting of subducted continental crust and overlying enriched wedge, produce large-scale high-K calc-alkaline granites and associated mafic enclaves. (5) In the final stage, exhumation of subducted continental crust would cause lithospheric extension in the interior of the south Qinling terrane, led to the partial melting of middle-crust on the condition of fluids from mantle lithosphere or lower crust, produce biotite granite and the exposure of Foping metamorphic complex.
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