A Geochemical Study Of Late Triassic Granitoids From The Qinling Orogen

The Qinling orogen constitutes one of the major part in the Central Orogenic Belt of China. It has a complex tectonic evolution history involving oceanic subduction and arc-continental collision in the Paleozoic as well as continental collision in the Early Mesozoic that led to the final amalgamation between the North China Block (NCB) and the South China Block (SCB). Especially in the Early Mesozoic, the Qinling orogen experienced strong tectonic-magmatic activities and the Late Triassic granitoids were formed widely, which are related to the continental collision between the NCB and the SCB. These granitoid intrusions are the important products of tectonic evolution in the collisional orogen, and thus can have great significance for understanding the crustal architecture and geodynamic evolution of the Qinling orogen. Although many studies have been devoted to the petrogenesis of granitoids, it is still hotly debated on the source nature of granitoids and the cause of their geochemical variations. Furthermore, mafic microgranular enclaves (MMEs) are common in granitoids, and can provide significant information on the source nature and magmatic processes which are sometimes not readily available from the granitoids themselves. In this regard, the widespread Late Triassic granitoids and associated MMEs in the Qinling orogen provide us good natural samples to investigate these issues, and will further advance our understanding of granitoid petrogenesis. This PhD thesis focuses on petrology, mineralogy and geochemistry of typical Late Triassic granitoid intrusions from the Qinling orogen, which are located at the Guangtoushan and Huayang areas in the southern margin of the South Qinling zone (SQZ), at the Shahewan, Caoping and Zhashui areas in the northern margin of the SQZ, and at the Mishuling area in the West Qinling zone (WQZ). The results provide new constraints on the emplacement age, rock type, source nature and petrogenetic mechanism of the Late Triassic granitoids and associated MMEs.A combined study of zircon U-Pb ages and Lu-Hf isotopes, whole-rock major-trace elements and Sr-Nd-Hf isotopes as well as mineral O isotopes has been carried out for the Guangtoushan (GTS) and Huayang (HY) plutons along the Mianlue suture in the SQZ. The results indicate that these granites were derived from partial melting of metasedimentary rocks rather than metaigneous rocks as suggested by previous studies, and thus they belong to S-type granites rather than I-type granites. LA-ICPMS zircon U-Pb dating yields concordant ages of 207 ± 3 to 212 ± 4 Ma for their magma emplacement. The GTS granites are weakly to strongly peraluminous with A/CNK ratios of 1.05 to 1.15 and show high whole-rock?18O values of 9.52 to 11.20‰. The HY granites are also weakly to strongly peraluminous with A/CNK values of 1.04 to 1.14 and have high zircon ?18O values of 10.71 to 11.43‰ and whole-rock ?18O values of 12.42 to 13.77‰. On the other hand, the GTS granites exhibit low K2O/Na2O ratios of 0.5 to 1.32, low Rb/Sr ratios but high Sr/Ba ratios, variable Eu anomalies (Eu/Eu*=0.29-1.05), and low zircon saturation temperatures (Tzr = 653-766 ?). In contrast, the HY granites have high K2O/Na2O ratios of 1.01 to 2.82,high Rb/Sr ratios but low Rb/Ba ratios, conspicuous negative Eu anomalies (Eu/Eu* = 0.52-0.77) and relatively high zircon saturation temperatures (Tzr = 711-795 ?). Compared with compositions of granitic melts produced by experimental partial melting of various lithologies (amphibolites, greywackcs, pclites)under different melting conditions (fluid-present vs. fluid-absent), an integrated interpretation of all these geochemical features is that the GTS granites were produced by fluid-present partial melting of a mixture source of metapelite and metagreywacke at low temperatures, whereas the HY granites were generated by fluid-absent partial melting of a metagreywacke at relatively higher temperatures. In addition, the two granite plutons show variably high whole-rock (87Sr/86Sr)i ratios of 0.7047 to 0.7115, negative whole-rock?Nd(t) values of -9.8 to -6.8 and zircon eHf(t) values of -13.1 to -2.0, corresponding to two-stage Nd-Hf model ages of Mesoproterozoic to Paleoproterozoic, indicating that their source rocks have originated from weathering of ancient crust rocks. Furthermore, their ?Nd(t) values are comparable to those for sedimentary rocks in the southern margin of the SQZ and the northern margin of the SCB, suggesting a genetic link between them. Therefore, the GTS and HY granites originated from the sedimentary rocks rather than igneous rocks, and thus belong to S-type rather than ?-type granites. It is inferred that these sedimentary rocks were subducted to the lower crustal depth during the Late Triassic collisional orogeny in the Qinling orogen, and gone through complicated burial and metamorphism, and then the partial melting of these metasedimentary rocks would produce the GTS and HY S-type granites under the proper P-T conditions.Continental granitoids usually exhibit petrological and geochemical diversities. The various models and hypotheses have been proposed to interpret the variations of geochemical compositions among different plutons even in single pluton, which are not only controlled by the composition and nature of source rocks but also influenced by magmatic processes and melting conditions. A combined study of zircon U-Pb ages and Lu-Hf isotopes, whole-rock major-trace elements and Sr-Nd isotopes as well as mineral chemistry and O isotopes was carried out for the Shahewan (SHW), Caoping (CP) and Zhashui (ZS)plutons along the Shangdan suture in the SQZ, model calculations were also performed to examine trace element fractionation during partial melting of crustal rocks. The results indicate that these granitoids are of I-type affinity and were generated by partial melting of metaigneous rocks, and record the reworking of continental crust. LA-ICPMS zircon U-Pb dating yields concordant ages of 208 ± 2 to 216 ± 3 Ma for the SHW, CP and ZS granitoids, in accordance with ca. 205-225 Ma emplacement ages for the vast majority of Triassic granitoids in the Qinling orogen. Furthermore, no relict zircons are identified by the CL imaging and U-Pb dating. The SHW and CP granitoids show high-K calc-alkaline and metaluminous features(A/CNK = 0.84-0.93), contain hornblende and MMEs occurring in the outer parts of intrusion, and have mantle-like zircon ?18O values of 4.71 to 5.72‰. The ZS granites show high-K calc-alkaline and metaluminous to weakly peraluminous features (A/CNK = 0.99-1.03), and almost have no hornblende and MMEs. They have slightly low zircon ?18O values of 4.60 to 4.83‰. On the other hand, the SHW and CP granitoids exhibit relatively low SiO2 contents (62.88-69.04 wt.%), high contents of FeOT, MgO and TiO2 but low ratios of Rb/Sr and Al2O3/TiO2 as well as slightly to negligibly negative Eu anomalies (?Eu =0.79-0.89) and relatively flat REE distribution patterns. In contrast, the ZS granites show relatively high SiO2 contents (69.32-75.94 wt.%), low FeOT, MgO and TiO2 contents but high Rb/Sr and Al2O3/TiO2 ratios as well as moderate negative Eu anomalies (?Eu = 0.63-0.81) and concave-up REE distribution patterns.All these granitoids show arc-like trace element distribution patterns with enrichment in LREE and LILE(e.g., Rb, K and Pb) but depletion in HFSE (e.g., Nb, Ta and Ti). Geochemical comparison and modeling indicate that these granitoids are obviously different from the adakitic rocks derived from partial melting of the Dabie thickened lower crust, the trace element compositions of these granitoids can be appropriately obtained by ?20% to 30% batch melting of the LCC. Compared with the composition of felsic melts experimentally produced by partial melting of various lithologies, an integrated interpretation of all these results indicates that these granitoids were derived from dehydration melting of metabasaltic sources, and subsequently experienced varying degrees of fractional crystallization. Furthermore, the melting temperature and thus the extent of partial melting would have played an important role in producing these granitoids. Mineral major elements indicate that the SHW and CP granitoids were crystallized under the conditions of variably low pressures of 0.7-2.3 kb, temperatures of 628-766 ? and higher fO2 than the ZS granites. On the other hand, all these granitoids show moderate whole-rock initial 87Sr/86Sr ratios of 0.7045 to 0.7055 and slightly negative ?Ne(t) values of -4.0 to -1.5 as well as slightly negative to positive zircon?Hf(t) values of -1.3 to 3.2, corresponding to two-stage Nd model ages of 1.05-1.38 Ga and Hf model ages of 1.04-1.39 Ga, suggesting that these granitoids were derived from reworking of Mesoproterozoic crustal materials. In addition, their whole-rock ?Nd(t) and zircon ?Hf(t) values are comparable to those for Neoproterozoic Yaolinghe metabasalts and mafic-ultramafic intrusions in the SQZ but inconsistent with those for Neoproterozoic Bikou metavolcanics and mafic-ultramafic intrusions in the northern margin of the Yangtze block. Together with their non-adakitic features, it is suggested that these granitoids were derived from partial melting of the mafic lower crust in the SQZ rather than the subducted Yangtze continental crust. Combined with regional geological background, it is indicated that the continental collision between the NCB and the SCB in the Late Triassic would have brought about reworking of the orogenic lower crust for the granitoid magmatism in the Qinling orogen.The partial melting of various crust rocks would produce different types of the granitoids, however,whether the partial melting of a common source rock at various P-T conditions can generate different types of granitoid or not? It remains unclear how such process influences the geochemical compositions of granitoid. Furthermore, MMEs are common in granitoids, they can provide significant information on the source nature and magmatic processes of granitoid. However, it remains hotly debated with respect to the petrogenesis of MMEs and their host granitoids. An integrated study involving petrology, mineralogy and geochemistry was carried out for the monzogranites and their hosting MMEs as well as biotite granites from the Mishuling pluton in the WQZ, central China. The results not only place constraints on the origin of MMEs but also provide insights into the source nature of granitoids. LA-ICPMS and SIMS zircon U-Pb dating yields concordant emplacement ages of 213 ± 2 to 219 ± 2 Ma for the monzogranites, biotite granites and MMEs from the Mishuling pluton, which show uniform U-Pb ages within analytical errors. All of them display arc-like trace element distribution patterns, similar whole-rock Sr-Nd-Hf-O isotopes and zircon Hf-O isotopes, suggesting their origination from a common crustal source. The clinopyroxene compositions and uniformly high in-situ zircon ?18O values of 7.32 to 9.21‰ as well as enriched Sr-Nd-Hf isotope compositions for the MMEs indicate that they cannot represent globules of the mixing between mantle-derived mafic and crust-derived felsic magmas. In addition, the MMEs have fine-grained igneous textures, and contain acicular apatite and abundant amphiboles. They also display metasomatic reaction textures where clinopyroxene was replaced by amphibole in different degrees, and contain plagioclase crystals with compositional disequilibrium. These geochemical and petrographic features suggest that the MMEs would be formed by the mixing or mingling of an intermediate magma with a felsic magma. On the other hand, the biotite granites are slightly peraluminous (A/CNK = 1.02), contain relict zircons of variable U-Pb ages and exhibit high in-situ zircon ?18O values of 7.59 to 8.77‰ indicating their derivation from the early-stage partial melting of metasedimentary rocks at relatively low temperatures. Compared with the biotite granites, the Mishuling host monzogranites show metaluminous features (A/CNK = 0.92-0.99) and contain amphibole, which are similar to those of typical I-type granites. However, they also show high in-situ zircon and whole-rock ?18O values of 7.87 to 9.21‰ and 9.42 to 10.89‰, respectively. These geochemical features indicate that the host monzogranites would be derived from late-stage partial melting of the metasedimentary restite after extraction of biotite granitic melts at elevated temperatures. Therefore,the sequential partial melting of metasedimentary rocks at different stages and temperatures would play an important role in dictating the geochemical diversity of the granitoids. In this regard, source nature, melting temperature and thus the degree of partial melting have played a substantial role in dictating granitoid compositions.