| The Dabie orogenic belt in eastern China is formed as a result of a Triassic collision between the North China and Yangtze cratons. Since the discovery of coesite and micro-diamond inclusions in eclogites from the Dabie orogen in Central East China, this region is one of the most important targets for studying UHP metamorphism that documents continental subduction to mantle depths. After continental collision, it appears the strongest magmatism. Cretaceous post-collisional intrusive and volcanic rocks comprise47%of the surface exposure of the Dabie orogen. Dabie orogenic belt is bounded by the Xinyang-Shucheng fault to the north, Xiangfan-Guangji fault to the south, Shangcheng-Macheng fault to the west, and Tancheng-Lujiang fault to the east. Tancheng-Lujiang fault is a left-lateral syn-orogenic transform fault formed in236Ma and its major strike-slip offset in the Cretaceous period. It caused the Dabi-Sulu orogenic belt sinistral offset more than500km. The Dabie orogen belt experienced Triassic continental collision, deep subduction, Ultra-high pressure(UHP) metamorphism, exhumation of the UHP rocks and crustal extension and large-scale magmatism during the Early Cretaceous period (140-120Ma).North Dabie Complex(NDC) is the biggest tectonic units in the northern of the Dabie orogenic belt. It is bounded by the Xiaotian-Mozitan fault to the north, Wuhe-Shuihou fault to the south, Shangcheng-Macheng fault to the west, and Tancheng-Lujiang fault to the east. NDC was intruded by metamorphic intrusive rocks in Neoproterozoic, voluminous granites and mafic-ultramafic rocks in the early Cretaceous period. Geologists have made great efforts on dating emplacement age of granite. Most zircon U-Pb ages for the granites in the North Dabie fall between140Ma and120Ma, and a few ages is middle-Jurassic(160-140Ma). It is greater difference between the Sulu orogenic belt for it has Late Triassic, Jurassic and Cretaceous igneous intrusion.Many petrology, geochemistry, geophysics and geochronology studies were used to reveal the structure of the Dabie orogenic belt. The Dabie orogenic belt has crustal root during the Jurassic period and experienced extension-collapse tectonics, lithospheric thinning and lower crust delamination in the Early Cretaceous. Thest geological process are interwovenness connect with the tectonic regime transforms from compression to extension. The time and mechanism of transformation and the process of the lithospheric thinning are the key to discuss the orogenetic process and dynamics after collision.This thesis was based on the Tiantangzhai complex granites which is the biggest intrusion and contains maximum emplacement sequence in the north Dabie mountain. The purposes of this paper are:(1) to present emplacement sequences,(2) to present the composition and the depth of granitic magma,(3) to constraint the age of magma,(4) to present the ascent and emplacement mechanism,(5) to present the transformation time from compression to extension,(6) Crust-mantle interactions,(7) to present the process of lithospheric thinning. Therefore, this study through integrated approach (eg. macroscopic structure deformation, contact zone, shear zone deformation, microstructure, anisotropy of magnetic susceptibility(AMS), LA-ICP-MS and SHRIMP zircon U-Pb dating, whole rock major, trace and rare earth element, Sr-Nd isotope, Zircon Hf isotope and geo-thermobarometers) to achieve the research. We then discuss the emplacement characteristic of Cretaceous granite in North Dabie and the coupling of enormous granite invasion under the mechanism of lithospheric thinning in eastern China during the late Mesozoic, on which basis, we proposed a mode chart of the granite invasion in North Dabie orogenic belt. We got following major conclusions.1. macroscopic structure on tectonic deformation and the contact zones in the field, we found that the emplacement sequence of the complex massif, from early to late, ranges from Shigujian gneissoid quartz monzodiorite to E’gongbao augen granite to Jiuzihe banding monzonitic granite to Tiantangzhai phyric monzonitic granite to Erwan gneissic granite to Bodaofeng fine-grained monzonitic granite. In this sequence, the content of SiO2is gradually increasing, the deformation is gradually weakening, while the crystal habit of the phenocrysts in the massif is gradually automorphic; which indicates that the deep source rock which partial melted to form granite gradually became thinner and thinner during the process of the extension and collapsing of Dabie orogenic belt and the reduction of crust.And the massif emplacement can be divided into three phases, i.e.,(1)in the early stage of early Cretaceous, the tectonic deformation features of Shigujian pluton are consistent with that of the NE ductile shear zone, both of them are the deformation products of syntectonic. The ductile shear zone, as the passage for the ascent of magma emplacement, controlled pluton shape, emplacement characteristic and stress state, as well as reformed Shigujian pluton;(2)the deformation in E’gongbao pluton, Jiuzihe pluton, Tiantangzhai pluton and Erwan pluton, comparatively weak, only took place in NE and/or NW ductile shear zone with partly ductile deformation;(3) the deformation in the interrior of Baodaofeng is weak and no ductile shear zone can be found there. The first-phase and second-phase pluton that intruded Dabie complex like a network and the pluton pre-intruded did not typically appear to be pumiceous inflation of magma; the third-phase pluton, Bodaofeng fine-grained adamellite, with no Dabie complex inside, being abrupt contact with each other, intruded in the way of dyke or balloon distending.2.Evidences from microstructure and anisotropy of magnetic susceptibility fabric indicate that Shigujian pluton, paratectonic, emplaced under the compressional background. And the zircon U-Pb dating result is141±2.3Ma. The characteristics of major and trace elements of whole rocks present the pluton emplacement possessed a thinken crustal structure. Amphibole-Ti temperature and pressure gauge indicates that the source area, about36km deep, partially melted in the lower crust. Hence, the tectonic transformation of north Dabie, from squeezing to extenting, happened after141Ma.3. Zircon U-Pb dating results have shown that the three phases of composite pluton in Tiantangzhai emplaced in141Ma,139-132Ma,132-127Ma separately, representing the upper limit age of the tectonic transformation, starting point of enomorously granitic emplacement while lithosphere thinning and the completion of lithosphere thinning, respectively. In a zircon, inherited cores in late proterozoic, middle proterozoic, neoproterozoic, early paleozoic and triassic can have been discovered. Among which, zircon age in late and middle proterozoic represents metamorphic event of the source area; zircon age in neoproterozoic, from830Ma to695Ma, similar to the age of neoproterozoic tectonomagmatic event in the northern boundary of Yangtze plate, indicates that the source of granite is from the Yangtze plate; the geological significance of zircon age in early paleozoic remains unknown; and zircon age in triassic, from243Ma to203Ma, closing to the age of the continent-continent collision, deep thrusting and turning-back process of Dabie orogen, records the process of collisional orogeny. Dabie orogen experienced a rapidly collapsing tectonic process in late mesozoic. At141Ma, the tectonic regime transformed into extensional background; at135Ma, the extension reached to its fullest, and most north Dabie Cretaceous granite formed during this period, at125Ma, the extension and collapsion ended and Dabie orogen returned to normal crust in thickness, resulting in undeformed granitello and both acidic and basic veins. The thinning and collapsion of lithosphere in Dabie orogen completed just in a few million years.4. Results from lithogeochemistry state that all magma source of the Tiantangzhai complex pluton came from partial melting of the continental crust, and during the process, the crust increasingly contributed while the depth of magma chamber and emplacement became lower and lower. Shiguqian pluton, formed in the thickened crust, represented the beginning of tectonic regime transformation from compression to extension; pluton from E’gongbao, Jiuzihe, Tiantangzhai and Erwan represents the partialy melted product from the tectonic regime transformation; and Bodaofeng pluton formed from the melting of crust in normal depth after the transforamtion.5. Zircon Hf isotope reveal that all the values of εHf(t) are under zero, suggesting that all the source rock of granite came from partial melting of the continental crust rather than the juvenile mantle. The values of εHf(t) gradually decreases with the development of Tiantangzhai complex pluton in North Dabie and changes in the direction of discreasing negative values. This indicates that the decreasing of basic components and increasing of acidic ones in the source area. The two-stage model age of Hf in zircon indicates the source area of three phases granites respectively came from (1)2.9-3.1Ga basic rock in the thickened lower crust,(3)2.6-3.0Ga intermediate acidity upper part of the thickened lower crust, and (2)2.2Ga intermediate acidity rock in the thinned crust with normal thickness.6. All the evidences above together have showed that the source area of Shigujian magma is36km deep, and its intrusion distance is greater than25km; the source area of Tiantangzhai granites is45.5km deep, and its intrusion distance is38.5km; the source area of Bodaofeng pluton is15-17km, and its intrusion distance is about2.5km. These factes indicate that Dabie orogen remained thickened crust during the emplacement of Shigujian pluton, orogenic root existed, and the magma origined from basic material about36km deep in lower crust and obtained room for placement by active expanding and intrusion; orogenic root began to collapse when Tiantangzhai granites (like Jiuzihe pluton, Tiantangzhai pluton and Erwan pluton) formed, resulting in intermediate acidity materials in the lower crust falling into the mantle in the lithosphere and they ascended by magmatic intrusion and structural expanding; late Bodaofeng granitello originated from intermediate acidity materials in normal crust about12-17km deep after the collapsion and emplaced in a way of dike expanding.7.After the tectonic regime transformed from compression to extension, the thickened lithosphere thinning, collapsed and delaminated. Due to the heat provided by the upwelling of asthenosphere in the mantle, the lower crust in different depth and different source area partly melted. And the Tiantangzhai complex pluton resulted from this process. The lithosphere of Dabie orogen extended and thinned under the same dynamic background with the late-Mesozoic thinning of lithosphere in the eastern China, that is, after the Tethys tectonic region transformed into the Pacific tectonic region, whole eastern China was under extensional setting, the convection of mantle speeded up as the angle and the direction of the Pacific plate subducting into Eurasian plate changed. The asthenosphere underplated into the bottom of the lower crust, causing partial melting of the lower crust at different levels and of different components. |
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