| Daning granitic pluton mainly consist of quartz diorite, granodiorite and monzogranite, which is typical of Caledonian granite, Nanling. Predecessors carried out on lithology, mineralogy, geochemistry and mineralization and other aspects, but the formation age of rock and its mafic microgranular enclaves, petrogenesis, tectonic setting and other aspects of cognition remains controversial, which restricts the genesis of western Nanling Caledonian granites and to some extent explore the tectonic setting. In this thesis, igneous petrology, field rock cross-section measurements, combined petrology, major and trace element geochemistry, geochronology and Sr-Nd-Hf isotope methods, in identifying the formation age of Daning granitic pluton and mafic microgranular enclaves, discusses the diagenesis and evolution of them and contrast granite with the same period in the study area, to discuss the tectonic setting of middle Nanling.Daning granitic pluton mainly consists of quartz diorite, granodiorite and monzogranite, the first two has sharp contact and the latter two has transitional contact. Both of which have the similar mineral association, which mainly consists of quartz, feldspar, plagioclase, biotite and amphibole, and the only difference is the content of major mineral. Zircon LA-ICP-MS U-Pb dating of the Daning granodiorite and mafic enclaves is～440Ma, indicative of its generation during Early Silurian. The Daning granodiorite is characterized by relatively low silica content (SiO2of63.82-70.94wt.%), enriched in mafic component (FeOt of3.30-5.99wt.%, MgO=1.05～3.22wt.%), wide variations in A/NCK(0.93～1.46) and n(K20)/n(Na2O)(0.52～1.26), belongs to high-K calc-alkaline granites. In the primitive mantle normalized spider, granodiorite is relatively enriched in Rb, Th, U, Ta, Zr, Hf, LREE, whereas depleted in Nb, Sr, P and Ti, high in Rb/Sr (0.52～1.99) and Rb/Nb (7.12-10.09), low in Nb/Ta (0.16-0.21). Granodiorite is enriched in LREE, negative Eu anomaly (0.49-0.74),all of which indicate the degree of magmatic evolution is not high and fractional crystallization is not obvious. Granodiorite gives (87Sr/86Sr)i value of0.7139-0.7169, εNd(t) value of-9.09～7.86and Nd model ages of1.62～2.09Ga. The zircons crystallized in granodiorite g εHf(t) values of-4.5～-2.5and Hf model ages of1.16～1.24Ga, which suggest magma mixing. The provenance of granodiorite maybe origine from ancient crustal materialand felsic crust and a certain amount of basaltic magma. The formation age, geochemistry and Sr isotopes of monzogranite and granodiorite is similar, suggests they are homologous magmatic evolution of the product. Moyite is high silica content and low in mafic component, belongs to peraluminous granite, has similar Sr-Nd isotopic compositions with granodiorite, which suggests420Ma and440Ma two magma events of Daning have the same or similar source.Mafic microgranular enclaves are well developed in Daning granitic pluton. Enclaves belongs to dioritic rock and its mineral granularity is smaller than the host rock, development of acicular apatite, with typical igneous structures. Mafic microgranular enclaves is mainly ellipsoidal, the contact relationship between which and host rocks is sharp or transitional and in some region, mafic enlaves and the host rocks share consistent lineation structure.Mafic enclaves is more basic than the host rock, and is characterized by relatively low silica (SiO2of51.89～64.83wt.%), enriched in mafic component (FeOt of4.46～7.16wt.%, MgO of2.12～4.43wt.%), low in A/NCK (0.78～0.92), more alkaline than host rocks, belongs to shoshonitic series. Mafic enclaves and granodiorite have similar REE and trace element distribution patterns, being relatively enriched in HFSE and LREE, negative Eu anomaly (0.2～0.0.59). Mafic enclaves are high in Rb/Sr (0.52～1.99) and Rb/Nb (7.12～10.09), low in Nb/Ta (0.16～0.21). Rb/Sr (1.12～1.73) and Rb/Nb (0.18～0.21) of mafic enclaves is consistent with granodiorite, whereas its Nb/Ta is higher than the host rocks, indicating they have different source. Mafic enclaves give εNd(t) value of-6.39and Nd model ages of1.46Ga. The zircons crystallized in enclaves give lower εHf(t) values of-7.7～-4.1than the host rocks and similar Hf model ages of1.23-1.26Ga as host rock, suggesting mafic is the product of magma mixing. The source of mafic enclves has acid end-member with host rocks, and the other may have originated from basaltic magma formed enriched mantle. What’s wore, the fractal dimension of mafic microgranular enclaves in the interior and margin of granodiorite is1.293and0.991.The above data shows mafic microgranular enclaves is result of magma mixing which happened at the same time or after feldspar phenocrysts crystallized nearly complete the granitic magma. Magma mixing mainly happened in the interior of granodiorite, the degree of which is weak and is between in laminar and turbulent flow conditions.Series of granitoid rock, Mafic microgranular enclaves and geochemisty is similar in Caledonian Guilin, Dayongding, Yonghe, Taibao and Daning granitic pluton, northeast Guangxi, which suggests all the Caledonian granitic plutons of which belong to calc-alkaline I-type granites formed in the post-orogenic tectonic environment. What’s more, combined with the corresponding period in Daning lamprophyre and Guangdong high Mg basalt, it suggests that Caledonian granitic plutons of northeast Guangxi formed of the transition in post-collision tectonic setting to stretch transitional tectonic environment. |
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