The Composition And Evolution Of The Deep Continental Crust Under The Central China

The deep continental crust, including the middle and lower crust, can be investigated by two methods:forward and inversion modelling. The first method contains the typical profiles and xenoliths directly from the deep crust, while the second method includes geophysical detection and geochemical trace from the source region of granitoid. The central China formed by three blocks including the South China Craton, the western and eastern blocks of the North China Craton, therefore it is the key area to understand the timing and mechanisms of tectonic interactions between these different blocks, then reveal the crustal composition and evolution of the central China. However, previous studies about the deep crust under this area mostly focus on the geophysical exploration, moreover, the central China lacks the typical deep crustal sections, so this paper selected the Xinyang felsic xenoliths from the deep crust and granitoids from the South Qinling orogen, to forward and inversion the deep crust, by detailed petrography, whole-rock geochemistry, the internal structure, inclusions, U-Pb geochronology, trace element and Lu-Hf isotope studies of zircon. Results from the xenoliths reveal that the deep crust beneath the central China contain the Hadean crustal component, then suffered the multistage modifications, while those of the granitoids uncover the differences between the western and eastern segments of the South Qinling orogenic belt and their tectonic evolution during the Proterozoic period. Combination with these results from the xenoliths and granitoids, respectively, can provide important constraints on the composition and evolution of the crust in the central China.The Xinyang felsic xenoliths include granulite and leptynite in lithology, and have the mineral assemblages of plagioclase (20-65%)+k-feldspar (13-45%)+quartz (20-45%)±garnet (1-6%)±pyroxene (2-3%), with granoblastic texture. They have high SiO2(71.0-76.07wt.%), Na2O (2.96-4.51wt.%) and K2O (2.25-8.41wt.%) contents, low MgO (0.09-0.41wt.%) and Fe2O3T (0.11-2.04wt.%) contents. Their A/CNK values range from1.09-1.36, indicating meta- luminous to slightly peraluminous rock series. They also show positive D.F. values (1.67-4.58) and similar CIA values (52-58) to the fresh granite. On the plot of Al2O3-CaO*+Na2O-K2O, they lie parallel and close to the join between plagioclase and k-feldspar, and within or near the field of the Phanerozoic igneous rocks from the North China Craton. Furthermore, petrographic observations also indicate that they have the residual felsic minerals. These suggest that the protoliths of these xenoliths may be the igneous rock. Based on the internal structure, trace elements and Lu-Hf isotope of zircon, zircons from the Xinyang felsic xenoliths can be divided into the inherited magmatic zircon and the metamorphic new growth zircon. Although the former with magmatic genesis, most of them also show no internal structure in CL images, a larger range of trace element contents and ratios (such as Ti and Th/U ratios), as well as light rare earth element patterns, suggesting that they may also suffer variable degrees of metamorphic recrystallization, whereas the latter display no zoning, similar trace element content and REE pattern, and give the Paleoproterozoic formation age. Inherited magmatic zircons from the Xinyang felsic xenoliths show distinct Hf isotopic characteristics, including unchanged176Hf/177Hf ratios through time, and the negative and positive correlations between the176Hf/177Hf ratios and time, which indicates that they may be subject to different metamorphic modifications, including solid-state recrystallization, low176Lu/177Hf ratio and high176Hf/177Hf ratio fluid/melt-assisted recrystallization and high176Lu/177Hf ratio and low176Hf/177Hf ratio fluid/melt-assisted recrystallization. Whole-rock REE pattern and spider diagram display that the protoliths of these xenoliths have different sources, most of which have a significant positive Eu, Ba, Sr and K anomalies, and depleted HREE patterns, suggesting feldspar cumulation and garnet residual in their source regions, while the others show negative Eu, Sr, and Ba anomalies, indicating fractional crystallization of feldspar, however these xenoliths have common negative Nb, Ta and Ti anomalies, suggesting their crustal source. The inherited magmatic zircons gives the Archean to Paleoproterozoic formation ages, including3.6Ga,～3.5Ga,～3.3Ga,～2.22Ga and～2.05Ga. Combined with their igneous protolith properties, these ages should represent the emplacement time of granitic magma, which derived from the partial melting of the deep crust beneath the central China during the Archean to Paleo-proterozoic period. However, most of the metamorphic new growth zircon have depleted HREE compared to their inherited magmatic zircons, and obviously negative Eu anomalies, and show the high formation temperature (zircon-Ti thermometer calculate that the average temperature range from701℃to871℃, and the low176Lu/177Hf ratio compared to their magmatic zircons, implying their formation coeval with garnet and feldspar under granulite-facies conditions, while the others only display high formation temperature (723-827℃), suggesting that they formed at least upper amphibolite facies or granulite facies. zircon U-Pb dating show that the metamorphic new growth zircon formed at～2.33Ga,～2.13Ga,1.91-1.86Ga,1.78-1.71and～1.65Ga. These indicate that the deep crust under the central China had experienced intense modification during the Paleoproterozoic period, which may be related to the subduction and collision between the western and eastern blocks of the North China Craton. Therefore, the deep crust under the central China contain the Hadean crustal components (4.16-4.62Ga), then had experienced the Paleoarchean magmatism (～3.6Ga,～3.5Ga and-3.3Ga), and the Paleoproterozoic multistage thermal events (～2.33Ga,-2.22Ga,～2.13Ga,～2.05Ga,1.91～1.86Ga,1.78-1.71Ga and-1.65Ga).The granitoids from the western and eastern segments of the South Qinling orogenic belt are made up of plagioclase (10-65%)+k-feldspar (9-50%)+quartz (5-35%)+biotite (2-10%)±hornblende (2-10%) with porphyritic or granitic texture. Because they have similar major-and trace-element compositions, this paper will foucs on their radiogenic isotopes of zircon. The zircons from these granitoids have clear oscillatory zoning and high Th/U ratios (greater than0.2), and also show significant negative Eu anomalies, and HREE enrichment patterns, suggesting their magmatic genesis. Zircon U-Pb dating show that they formed at218-211Ma, indicating that the western and eastern granitoids formed in the Late Triassic period. These ages are slightly later than the UHP peak metamorphic ages of240-225Ma in the Dabie orogenic belt, indicating that those granitoids all formed in a post-collision setting. However the western and eastern granitoids have significantly different source regions in their isotopic compositions, the former have negative zircon εHf (t) values ranging from-20.9to-5.2, with an average of-10.3, and give the two-stage Hf model ages of1.58-2.57Ga, with an average of1.90Ga, indicating that they were probably generated by the partial melting of Paleoproterozoic or older deep crust, while the latter display negative to positive zircon εHf (t) values ranging from-5.4to6.8, with an average of-0.90, and show the two-stage Hf model ages of0.82-1.60Ga, with an average of1.31Ga, suggesting that their source regions were predominately Mesoproterozoic or older crustal materials. Combined with previous published whole-rock Sr-Nd-Pb isotopes, the western and eastern granitoids with significantly different zircon-Hf isotopes indicate that the boundary between them may be located between the Xiba and Wulong plutons, roughly as the Taibai-Chenggu line. Comprehensive studies of surface geology, geochemistry and geophysics, the western and eastern segments of the South Qinling orogenic belt are probably characterized by different terranes, therefore, this belt may be formed by tectonic interactions between its western and eastern segments. On the basis of previous studies, we generally established the evolution model of the South Qinling orogenic belt during the Proterozoic period:the western segment rifted from the North China Craton during the Paleoproterozoic to early Proterozoic period, then switched into the continent convergence environment with the eastern segment (the northern margin of the Yangtze block) during the late Mesoproterozoic to Neoproterozoic, finally the two segments joined during the late Neoproterozoic.The Xinyang felsic deep crustal xenoliths and the Triassic granitoids from the South Qinling all have revealed the composition and evolution of the deep crust under the central China. The Xinyang xenoliths provide the microscopic information and subtle evolution of the deep crust under the central China, such as fluid/melt activities on their protoliths and the fluid/melt with the characteristics of low176Lu/177Hf ratio and high176Hf/77Hf ratio or the high176Lu/177Hf ratio ratio and low176Hf/177Hf ratio. They also recorded the Hadean (4.16-4.62Ga) crustal components, then the Archean to Paleoproterozoic multi-stage reworking (-3.6Ga,-3.5Ga,-3.3Ga,-2.33Ga,-2.22Ga,-2.13Ga,-2.05Ga,1.86-1.91Ga,1.71～1.78Ga and～1.65Ga). While the granitoids reveal the integrated and macro information of the deep crust beneath the central China, such as their source rocks of different ages (the western granitoids are of1.58-2.57Ga and the eastern granitoids are of0.82-1.64Ga), and the difference between the deep crust under the western and eastern segments of the South Qinling orogenic belt. In short, the deep crustal xenoliths and granitoids provide the micro and subtle evolution, and the macro and comprehensive information, resepectively. Combining with these two aspects can provide more comprehensive information about the composition and evolution of the deep crust under the central China.