A Geochemical Study Of Mesozoic Granites From The Nanling Range In South China

The South China Block has experienced strong tectonic-magmatic activities during the Mesozoic, producing widespread igneous rocks, most of which are peraluminous granites. Petrogenesis of peraluminous granites is not straightforward because they may be produced by partial melting of both metasedimentary rocks and metaigneous rocks in terms of experimental petrology. Studying the emplacement age, geochemical characteristics and source nature of Mesozoic granites in South China has great bearing on understanding of not only the regional tectonic evolution but also the petrogenesis of peraluminous granites. Although a number of studies have been devoted to the Mesozoic granites in South China, it remains controversial with respect to their petrogenesis due to their various origins and differences from typical S-or I-type granites in Australia. This doctoral dissertation deals with Mesozoic granites from the Nanling Range in South China, with main focus on the Triassic granites.A combined study of zircon U-Pb ages as well Hf-O isotopes and trace element, whole-rock major-trace elements and Sr-Nd isotopes, and biotite petrography and major elements has been carried out for the Luxi, Xiazhuang and Gaodong plutons from the Guidong complex and the Fucheng pluton. The results demonstrate that these peraluminous granites were produced from partial melting of metasedimentary rocks, and thus they belong to S-type granites rather than I-type granites. LA-ICPMS zircon U-Pb dating reveals that these granites were emplaced at 230±4 to 237±4 Ma. These granites contain different amounts of residual zircon cores, whose U-Pb ages are largely variable from 355 to 2379 Ma, indicating that the source rocks of Triassic granites contain crustal materials with different ages. Age peaks are distributed at ～440 Ma,～800 Ma and ～2355 Ma. The first two peaks are large and overlap with the main ages of early Paleozoic and middle Neoproterozoic igneous activities, respectively, in the South China Block. The SIMS In-situ O isotope analysis for the Triassic syn-magmatic zircons yield consistently high δ18O values of 8.8-11.4‰, comparable with δ18O values of 9.2-10.6‰ obtained by the laser fluorination method on bulk zircon grains. This suggests that the source rocks of Triassic granites are mainly supracrustal materials, which have experienced chemical weathering on the surface. The δ18O values (by SIMS) for residual zircons vary from 5.1 to 10.3‰. However, the residual zircons of ～440 Ma also show high 818O values of 8.6-10.3‰ similar to those of Triassic syn-magmatic zircons, whereas the other residual zircons older than 600 Ma exhibit relatively low δ18O values of 5.1-7.9‰. Therefore, the parental rocks of -440 Ma zircons probably share the similar petrogenesis with the Triassic granites but differ from those of other residual zircons. The trace element composition of Triassic zircons shiws a large range, and there are obvious correlations between Hf and Ti, Th, U, Th/U, Eu/Eu* and (Gd/Yb)N, which are probably caused by fractional crystallization. There is no correlation between Hf and δ18O values, implying that the isotope compositions remain nearly invariable during magma crystallization. The trace element variations in the residual zircons are relatively small and in the range of those for the Triassic zircons. All residual zircons share the similar chondrite-normalized REE patterns to the Triassic zircons, especially the -440 Ma residual zircons which can hardly be distinguished from the Triassic ones, demonstrating the similar origin of their parental rocks. The Triassic magmatic zircons exhibit enriched Hf isotope compositions with εHf(t) values of -12.6 to -5.4 and two-stage model ages (T2DM) of 1603 to 2031 Ma (calculated at t= 230 Ma). Zircon εHf(t) value and T2DM ages for the Fucheng granites are respectively slightly higher and younger than those of the other granites, suggesting slight differences in their source rocks.All the Triassic granites from the Luxi, Xiazhuang, Gaodong and Fucheng plutons have SiO2 contents higher than 65 wt.%. On the TAS plot, the Luxi granites are mainly distributed in the granodiorite field, whereas the rest granites are mainly distributed in the granite field. The all granites share the common feature of high-K calc-alkaline. Most of the Fucheng granites are ferroan, whereas the others are mostly magnesian. There are roughly linear correlations between SiO2 and the majority of major-trace element contents and ratios for the Triassic granites. Notably, they show a negative correlation between SiO2 and P2O5 and a positive correlation between SiO2 and A/CNK, both of which are akin to typical I-type granites. Granites in this study are all peraluminous with A/CNK>1.0, and they have K20/Na2O ratios higher than unity. They show right-tilted REE patterns and negative Eu anomalies to different degrees on the chondrite-normalized REE diagrams. On the primitive mantle-normalized spidergrams. the Triassic granites exhibit positive anomalies in Rb, Th, U, K and Pb, but negative anomalies in Ba, Sr, Nb, Ta, P and Ti, typical of arc-derived continental crust. The Fucheng granites have higher TiO2, P2Os and TiO2/MgO but lower Na2O and Mg# than the other granites at comparable SiO2 contents. Major element compositions of the Fucheng granites are consistent with those of A-type granitic melts produced by partial melting of crustal rocks at high temperatures in experimental petrology. Furthermore, they exhibits relatively high LREE contents,104*Ga/Al ratios and Zr+Nb+Ce+Y contents, in accordance with those of typical A-type granites. The all Triassic granites show consistent initial Nd isotopic compositions with εNd(t= 230 Ma) values of -11.0 to-9.5 but largely variable initial Sr isotopic compositions with (87Sr/86Sr); of 0.6007 to 0.7246, most of them are higher than 0.7100. The granite SiO2 contents are not correlated with either whole-rock εNd(t) values or zircon 818O values.The major element composition of biotites from the Luxi, Xiazhuang and Fucheng granites is consistent with that of typical peraluminous granites (e.g., enriched in Al2O3). The Mg# and A/CNK values of biotite keep in pace with those of whole-rock; the Luxi granites have the highest Mg# and the lowest A/CNK in both whole-rock and biotite, whereas the opposite is true for the Fucheng granites and both values of the Xiazhuang granites are moderate. In addition, the Fucheng biotite exhibits the highest contents of F.The combined results document that although the Triassic granites in the Nanling Range resemble the common 1-type granites on the plots of SiO2 vs. P2O5 and SiO2-A/CNK, their other crucial features, including peraluminous, high K2O/Na2O ratios, (87Sr/86Sr)i and zircon δ18O values, and abundant residual zircons, are all consistent with typical S-type granites derived from partial melting of metasedimentary rocks. These latter features depend on those of source rocks, whereas the relationships between major elements are largely controlled by magmatic processes. Therefore, all the Triassic granites used in this study are S-type granites. In addition, the Fucheng granites have common features as A-type granites in whole-rock major-trace elements and biotite major elements, indicating their derivation from high-temperature melting of metasedimentary rocks. Such petrogenetic processes as magma mixing, crustal assimilation, resitite unmixing and peritectic assemblage entrainment cannot explain the compositional variations in the Triassic granites from the Nanling Range. Instead, partial melting of heterogeneous metasedimentary rocks at various P-T conditions may have played a dominated role, with additional modification by fractional crystallization during magma emplacement. Based on this study, partial melting of metasedimentary rocks at relatively low temperatures can produce S-type granitic melts which are strongly peraluminous. After extraction of S-type granitic melts, restites may suffer partial melting at elevated temperatures to produce weakly peraluminous granitic melts and even A-type granitic melts. Fractional crystallization of granitic magmas produced under different conditions can result in regular compositional variations between many major and trace elements, some of which are even akin to common 1-type granites. But their essential sources do not change. and they are still the metasedimentary rocks despite the extraction of felsic melts to different extent.Magma mixing is a common process in granite petrogenesis. Mixing of magmas from different crustal rocks is particularly important in the formation of peraluminous granites. This is illustrated by a combined study of matrix and inclusion biotites from the Longyuanba granites. The major element composition of biotites in granites is primarily controlled by the composition of magmas from which they have crystallized. Biotite grains enclosed by quartz and feldspars in granites are naturally protected by their host minerals, so that their compositions are likely original and can potentially be used to track the magma mixing. Three granite samples from the Longyuanba complex have been used in this study:one two-mica granite and two biotite granites. The LA-ICPMS zircon U-Pb dating on two-mica granite 10SC74A, biotite granites 10SC71 and 10SC77 yield concordant ages, with the weighted averages of 240±3 Ma (2σ),239±2 Ma and 147±2 Ma, respectively. All the three samples have high SiO; contents of ≥68.0 wt.%, and are peraluminous. Two-mica granite 10SC74A has the lowest εNa(t) value of -11.3, the oldest T2Dm age of 1.93 Ga, and the highest zircon δ18O value of 9.5%o and the highest A/CNK value of 1.18. Biotite granite 10SC77 has the highest εNd(t) value of -8.0, the youngest T2DM age of 1.58 Ga, and the lowest zircon δ18O value of 8.4%o. A/CNK value of this sample is 1.07. Biotite granites 10SC71 have a moderate εNd(t) value of -9.4, a T2DM age of 1.78, and a zircon δ18O value of 8.9%o, as well as an A/CNK value of 1.06.There are different occurrences of biotite in the target granites. Biotite in two-mica granite 10SC74A occurs not only as one of rock-forming minerals in matrix but also as inclusions in quartz and K-feldspar. Both the matrix and inclusion biotites have the same paragenetic minerals of muscovite and ilmenite. All biotites from this sample are termed as Bt-I. Similarly, there are three occurrences of biotite in biotite granite 10SC71:in feldspar, in quartz and in matrix. The biotite inclusions in feldspars are generally euhedral plates, have widths of 30-200μm with aspect ratios of about 3:1, and are termed as Bt-IIA, whereas biotites in quartz (Bt-ⅡB) and matrix (Bt-III) are usually subhedral flakes. In biotite granite 10SC77, fresh biotite inclusions in quartz can be categorized into two groups except chloritized biotites. Group A is euhedral, platy and about 50μm in size, and coexists with primary muscovite and ilmenite (also Bt-IIA). Group B is subhedral to anhedral, about 30-40μm in width and 60-80μm in length, and is enclosed independently in quartz (also Bt-ⅡB). Chemical compositions of biotites with different occurrences in two-mica granite 10SC74A have no distinction and are comparable to those of biotites from typical S-type granites. Bt-1 has higher Al2O3 but lower MgO contents than biotites from the biotite granites. A/CNK and Mg# values of Bt-ⅡA and Bt-IIB in 10SC77 are lower and higher than those of Bt-Ⅰ, respectively, and fall transitionally between compositional fields of biotites from typical S-and I-type granites. In 10SC77, Bt-IIA has lower MgO and TiO2 but higher A12O3, K2O and A/CNK than the Bt-IIB. Bt-IIA has compositions close to Bt-I. In 10SC71, inclusion biotites show much larger variations in composition than the matrix biotites. Furthermore, biotite inclusions in quartz (Bt-IIB) show higher Mg# but lower A/CNK than biotite inclusions in feldspar (Bt-IIA). Bt-Ⅲ has compositions between Bt-IIA and Bt-ⅡB.Two contrasting zircon populations are distinguished in biotite granite 10SC77. One is bright and shows oscillatory zonings in CL images, whereas the other is very dark and has no discernible zonings. The two zircon populations are either as individual grains or present together with the later overgrowing on the former, indicating that the dark zircons crystallized later than the bright ones. The two zircon populations show the same U-Pb ages but different SIMS δ18O values and trace element compositions. Bright zircons have higher δ18O values of 7.95± 0.34‰ (2σ) to 8.26±0.23‰ than those of 7.43±0.30‰ to 7.62±0.29‰ for the dark zircons. The bright zircons (high δ18O) show lower Hf, U, Th. and Y contents than the dark zircons (low δ18O), whereas (Gd/Yb)N values for the former is higher than those of the latter. The δ18O values are not correlated with U-Pb ages and trace element contents/ratios.Biotites in the Longyuanba granites are suggested to have directly crystallized from the granitic magmas and their original compositions have been retained. Two-mica granite 10SC74A has originated from metasedimentary rocks and belongs to S-type granites, whereas biotite granites 10SC71 and 10SC77 are partial melting products of major metasedimentary rocks and minor metaigneous rocks. During the formation of biotite granites, an early and reduced magma derived from partial melting of metasedimentary rocks, which is more peraluminous and has a higher δ18O value, was emplaced to crystallize the bright zircons, Bt-ⅡA, muscovite and ilmenite. Then a new magma pulse derived from partial melting of metaigneous rocks, which is relatively less peraluminous and has a lower δ18O value, was recharged to crystallize different minerals such as the dark zircons and Bt-ⅡB. Bt-Ⅲ probably has grown from the mixture of these two magma pulses. Since the two pulses of magma were derived from partial melting of crustal rocks, they may show close similarities in physicochemical properties, and thus they can mix so sufficiently that no macroscopical petrological features can be observed. Nevertheless, such contrasting magmas can be recorded by biotite inclusions and refractory minerals such as zircon.The present study demonstrates that the petrogenesis of peraluminous granites is primarily attributed to partial melting of continental crustal rocks, without involvement of mantle-derived magmas. Therefore, they record the reworking rather than growth of continental crust. Constraints from whole-rock geochemistry may sometimes give ambiguous conclusions about the nature of source rocks because whole-rock geochemistry is largely affected by magmatic processes. However, minerals such as biotite inclusion and zircon are able to unravel the nature of source rocks for granites. The compositions of source rocks play a dominant role in controlling the compositional variations of peraluminous granites, with subordinate influences by melting conditions, melting degrees and fractional crystallization.