| The continental lithosphere include the crust and sub-continental lithospheric mantle (SCLM). The study of the nature and evolution of the deep crust and SCLM is the core and frontier in solid Earth science research. Mantle-derived magmatic rocks and its carried crustal-mantle xenoliths are the important "petrology probe" and one of the fundamental term to know the nature and evolution of continental lithosphere. The formation, accretion and reconstruction of the South China Block have an important position in Chinese Earth science research. Compared with the coastal regions, the study of the inland regions of the South China have experienced more complex Phanerozoic evolution and the study on these regions research is weak. This thesis focuses on the Cretaceous ultrapotassic lamprophyres, Paleogene basaltic rocks and its carried mantle xenoliths in eastern and central Guangxi Province, and crustal xenoliths on Mesozoic basalts in southern Hunan province. Through the study of zircon U-Pb dating and Lu-Hf isotope, whole-rock and mineral geochemical, and Sr-Nd isotopic compositions to constraint the nature and evolution of the deep lithosphere under Hunnan-Guangxi region of South China Block.The mantle xenoliths carried in basaltic rocks is the directly sample of the SCLM, can be used to reflect the nature and evolution of the SCLM. In Hunnan-Guangxi region, the mantle xenoliths found in Cenozoic basaltic rocks on Pingnan area, eastern Guangxi Province, and Mashan area, central Guangxi Province, are including spinel harzburgites, clinopyroxene-poor lherzolites, lherzolites and olivine websterites. The mantle xenoliths are mainly spinel harzburgites and clinopyroxene-poor lherzolites, their mineral chemistry show moderately refractory [Mg# value of olivine (Mg#OI)= 90.2-91.3], whereas other rare lherzolite is more fertile (Mg#OI= 89.3). An olivine websterite is different to the peridotite xenoliths in mineral chemistry, and more fertile (Mg#OI= 87.5). The rare earth elements (REE) patterns of clinopyroxenes in these xenoliths vary from LREE-depleted, to flat, to LREE-enriched patterns, and commonly exhibit positive Sr anomalies and negative Nb, Zr and Ti anomalies. The peridotitic xenoliths mostly experienced moderate to high degree of melt extraction (F= 10-20%) and were modified by silicate metasomatism. During the evolution of SCLM, a small amount of pyroxenite was produced via peridotite-melt interaction, the melt could be Ca-rich silicate melts. During the basaltic lave erupted, because the disequilibrium between olivines and the basaltic host melts, the mantle xenoliths exist the reaction phenomenon, and formed the zoned olivines (Mg#OI= 83.7-88.8) in the harzburgites and zoned olivine xenocrysts (Mg#OI= 75.2-82). The temperature of the Guangxi peridotites is around 860-1080 ℃, mainly lower than 950 ℃, the temperature of the Guangxi peridotites is lower than the peridotites in Ningyuan-Daoxian area of southern Hunan (>950 ℃). Combined with the study of mantle xenoliths in southern Hunan Province, the mantle xenoliths revealed the nature of the lithospheric mantle beneath the Hunan-Guangxi region. Because of the multi-period geological event and long-term evolution of the lithospheric mantle, the lithospheric mantle has obvious heterogeneous beneath the Hunan-Guangxi region, featuring various types of peridotite and co-existing pyroxenite. The lithospheric mantle beneath the Hunan-Guangxi region is mainly ancient (Proterozoic) lithospheric mantle, represented by the harzburgites and clinopyroxene-poor lherzolites in mantle xenoliths in Guangxi Province, and little fertile lherzolites and pyroxenite. The lithospheric mantle beneath some region affected by the deep translithospheric fault zone, such as Ningyuan-Daoxian area in southern Hunan, are mainly fertile, accreted mantle during the Phanerozoic. While, in the region on the margin of translithospheric fault zone, the lithospheric mantle is mainly ancient mantle co-existing with little newly-accreted mantle, such as the fertile xenolith in eastern Guangxi Province.The mantle-derived magmatic rocks of different period and types in the same region could be used to constrain the petrogenesis, source characteristics and dynamic processes. This thesis studied the Cretaceous ultrapotassic lamprophyres and Paleogene basaltic rocks in eastern and central Guangxi Province.The Cretaceous ultrapotassic lamprophyres in Longchang area of central Guangxi Province have obvious lamprophyric texture, consist of biotite, olivine, clinopyroxene, K-feldspar and plagioclase. The lamprophyres have high Mg# value (68-71), low SiO2 (50.65-54.60 wt.%), especially high K2O (5.6-6.2 wt.%) and K2O/Na2O (3.0-3.8), showing the ultrapotassic characteristic. They have high compatible elements, such as Cr (345-414ppm) and Ni (206-349ppm). The rare earth elements (REE) show right-incline pattern, strongly enriched in LREE, large-ion lithophile elements (LILEs, such as Rb, Ba, Th, U), and depleted in high-field-strength elements (HFSEs, Nb, Ta, Zr, Hf), and have high (87Sr/86Sr)i (0.7136-0.7138) and negative εNd(t) values (-9.17--9.45). The lava of ultrapotassic lamprophyres probably were derived from low-degree partial melting of an EM2-type garnet-facies (>80 km) SCLM, not obvious affected by the crustal material when it ascend to the earth, and happened some fractional crystallization of olivine and pyroxene. The modeled SCLM, mainly a phlogopite-bearing harzburgite, represents a refractory mantle that was metasomatically enriched by subduction, and also suffered some carbonatite metasomatism.The Paleogene basaltic rocks in Pingnan area of eastern Guangxi and Mashan area of central Guangxi Province consisit of olivine, titanaugite, plagioclase and Ti-Fe oxide. They have lwo SiO2 (41.31-46.84 wt.%), high Fe2O3T (9.57-11.50 wt.%), TiO2 (2.16-2.75 wt.%), Mg# values in 56-64, and high in compatible elements, such as Cr and Ni. The Guangxi basaltic rocks are enriched in LILEs and LREE but positive Nb-Ta anomalies, and are similar to alkali oceanic-island basalts (OIB). These basaltic rocks have DM-type Sr-Nd isotopic signatures, with low (87Sr/86Sr)i (0.7042-0.7046) and high εNd(t) valuves (3.05-4.17). The basaltic lava not obvious suffered by the crustal contamination, some olivine and pyroxene, and little Ti-Fe oxide and apatite fractional crystallization happened. They probably were derived from the low-degree partial melting of fertile (asthenospheric) mantle in spinel- to spinel-garnet-facies lherzolite (<80 km), and happened little interaction between lithosphere and asthenosphere.The deep crustal xenoliths carried by volcanics are important to reveal the nature and evolution of covered deep crust. This thesis studied the felsic and mafic xenoliths on the Mesozoic Huziyan basaltic rock in Daoxian area of southern Hunan Province. The felsic xenoliths include felsic gneiss and felsic granulite.The Daoxian felsic gneiss xenolith show gneissic texture, and consist of biotite+quartz+ K-feldspar+plagioclase, show high SiO2 (70.17 wt.%), Al2O3 (13.97 wt.%), K2O (4.63 wt.%), and low MgO (1.87 wt.%) contents, in the range of Paleozoic granite in Hunan-Guangxi region. The calculated D.F. value was positive 1.12, indicated its protolith could be magmatic rock, this is conform to the petrographic observation. The felsic xenolith could be named biotite plagioclase K-feldspar gneiss or granitic gneiss. The REE partition curve of the felsic xenolith show LREE enriched and HREE flat pattern, no Eu anomaly, enriched in LILEs (Rb, Th), little Zr-Hf positive anomaly. In CL image, most zircons show core-rim, the core are show magmatic oscillatory zoning, the metamorphic rim are homogeneous. Little other zircons are metamorphic zircon show no zoning and homogeneous. All the data fall on a discordia line with an upper intercept age of-431 Ma and a lower intercept age of ～201 Ma. The zircon near the upper and lower intercept age show the weighted mean age are ～427 Ma and ～210 Ma, respecially. The zircons have (176Hf/177Hf)i of 0.282268-0.282512 and negative εHf(t) values of -10.8 to -4.2, respectively. Hf model ages TDM2 of ca.1.97-1.54 Ga. We interpret 431-427 Ma as the formation age of the xenolith and 210-201 Ma as a metamorphic age, which is consistent with the core-rim structures.The Daoxian felsic granulite is granoblastic texture, and consist of quartz+K-feldspar+ plagioclase+ orthopyroxene. Compared with felsic gneiss, the felsic granulite has low SiO2 (67.6 wt.%), Al2O3 (12.69 wt.%), high MgO (3.94 wt.%), K2O (5.87 wt.%). The trace elements show LREE enriched and HREE flat pattern, positive Eu anomaly, enriched in LILE (Rb, Ba, Th, U) and depleted in HFSEs (Nb, Ta, Zr), similar to lower crust. the zircon in felsic graulite usually magmatic and no zoning, show a a discordia line with an upper intercept age of ～923 Ma and a lower intercept age of ～202 Ma. The Lu-Hf isotope show uniformity, (176Hf/177Hf)i of 0.282190-0.282340, imply same origion, εHf of 0.5 to -11.9, TDM2 in 2.08-1.74 Ga. The granulite show the old crust material (-923 Ma) suffered early Mesozoic (-202 Ma) tectothermal event.The Daoxian mafic xenoliths are all belong to mafic granulite and granoblastic texture, and consist of orthopyroxene (hypersthene)+clinopyxene+plagioclase ± spinel ± ilmenite ± pyrite ± amphibole. The mafic granulite have similar SiO2 content (44.82-50.77 wt.%), but according to the contents of pyroxene and plagioclase, the mafic xenoliths can simple divided to two type:A type pyroxene-rich and B type plagioclase-rich two-pyroxene granulite. A type xenoliths are rich in pyroxene (pyroxene> 50 vol.%), and show low Al2O3 (16.40-19.49 wt.%), high MgO (9.78-13.16 wt.%), flat REE, depleted in Th, U, Nb, positive Sr anomaly, little Eu anomaly (Eu/Eu*=0.82-1.49), and negative Zr, Hf anomaly, but little have no Nb, Zr, Hf and Sr anomaly. B type xenoliths are rich in plagioclase, and show higher Al2O3 (20.66-26.20 wt.%), lower MgO (3.58-9.20 wt.%), right-inclined REE, and depleted in HREE, Th, U, Nb, obvious positive Sr, Eu anomaly (Eu/Eu*=1.28-4.63), and negative Zr, Hf anomaly. Zircon in mafic xenoliths usually have magmatic oscillatory zoning or no zoned characteristic. Zircon U-Pb dating result show two types:One type xenolith was younger lower crust material, having its protolith formation age of-205 Ma, and granulite facies metamorphic age of ～198 Ma. The Zircon Hf isotope of this type xenolith have (176Hf/177Hf); in 0.282760-0.282907, εHf values of 4.1-9.3 and the Hf model age of ca.0.56-0.26 Ga. The two xenoliths show the information of the reworking of the old material, that the old crustal material (～1206 Ma) was reworked and show metamorphic age of ～196 Ma. The zircon Hf-isotope compositions in these xenoliths are complex, the (176Hf/177Hf)i in 0.281306-0.282816, εHf values of -2.4-8.0, the Hf model age of ca.0.61-2.65 Ga.Through the study of the late Mesozoic to Cenozoic mantle-derived magmatic rocks and crustal-mantle xenoliths, it reflected several tectonic-thermal events (early Paleozoic, early Mesozoic, late Mesozoic and early Cenozoic) affected the deep lithosphere beneath the Hunan-Guangxi region in inland of South China Block:(1) In the post-orogenic stage of early Paleozoic Wuyi-Yunkai orogeny (Kwangsian) on South China, the delamination/collapse in the core of the orogeny induced the asthenosphere upwelling and the mantle-derived lava produced. The melting produce of the deep crust which heating by the mantle-derived lava, including the protolith of the felsic xenolith during 431-427 Ma. This imply the delamination/collapse in the post-orogenic stage of early Paleozoic orogeny has large influence on the southeastern South China, and the Hunan-Guangxi region in the non core region of the orogeny happened different level crust-mantle interaction and deep crustal material melting.(2) In late Paleozoic to early Mesozoic, the Permo-Triassic (Indosinian) orogeny has affected the lithosphere of the South China. The study of the Daoxian crustal xenoliths show that the asthenosphere upwelling and lithospheric extension was happened in southern Hunan province during the late period of Permo-Triassic (Indosinian) orogeny. The mantle-derived magmatism was not wide outcrop in the earth, but changed the deep lithosphere beneath the Hunan province:(a) mafic magma underplating generate newly accreted lower crust (-205 Ma), and some of the mafic magma have granulite facies metamorphism (-198 Ma); (b) some mantle-derived lava intruded into the different level of the crust, even erupted and form the basalts; (c) the mafic underplating could direct or indirect influence the deep crust, and happened the crust-mantle interaction, and induced the metamorphism (201-196 Ma) in the middle-lower crust, including the Meso-Neoproterozoic (1206-923 Ma) old crustal material and early Pleozoic (431-427 Ma) magmatic rocks; (d) the material and heat from mantle could also induced the melting of the crustal material, like the Triassic granite in the Hunan-Guangxi region.(3) During Mesozoic-Cenozoic time, the mantle replacement process not only affected the coastal region of the South China, but also influenced the inland of the South China, such as Hunan-Guangxi region:(a) During the Jurassic, the areas beneath the translithospheric fault zone, such as southern Hunan, was mainly affected by the mantle replacement, the ancient lithospheric mantle was wholly or partially replaced by the juvenile accreted mantle; (b) with the lithospheric extension and asthenosphere upwelling, during the late Mesozoic-early Cenozoic, the mantle-replacement process that affected the South China block could have extended into Guangxi Province, accreting minor fertile mantle. The majority ancient (Proterozoic) lithospheric mantle also preserved beneath the Guangxi Province, with minor, fertile, newly accreted lithospheric mantle. This imply the mantle replacement in Guangxi province is lighter than the Hunan region. (c) The inversion of late Mesozoic to early Cenozoic mantle-derived magmatic rocks show their mantle source transformed from the EM2-type enriched mantle to the asthenospheric mantle. The change of the mantle source is the response to the mantle replacement/lithosphere thinning beneath the Hunan-Guangxi region.(4) During the late Mesozoic-Cenozoic time, the deep lithosphere process happened beneath the Hunan-Guangxi region, inland of the South China Block, could related to dynamic effect from both the east and the west sides. It was driven by the two-sided dynamics around the Cathaysia block, with Pacific subduction in the east and the Eurasian-Indian plate collision in the west. The complex subduction dynamics led to complex asthenospheric convection, a regional lithospheric extension beneath the Hunan-Guangxi region, and the removal or modification of the ancient lithospheric mantle. |
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