A Geochemical Study Of Neoproterozoic Granites From The Jiuling Range In South China

Granite petrogenesis is one of the hotspots and frontiers in solid Earth science, especially in the origin of peraluminous granites. Although numerous studies have been devoted to granite petrogenesis, it is still unclear whether peraluminous granites are produced by reworking of crustal rocks or interaction between crust and mantle, what is the origin of mafic enclaves in peraluminous granites and how they have controlled on the compositions of host granites, how to distinguish different types of granites in peraluminous granites and what are causes for the diversity of geochemical compositions in granites. In this PhD thesis, we have focused on Neoproterozoic Jiuling peraluminous granitoid batholith and Xingzi granitic pluton at Jiuling range in the middle-eastern segment of the Jiangnan orogen, South China. An integrated study of petrology, mineralogy and geochemistry was carried out to decipher in the origin of these granitoid plutons. The results provide new insights not only into the formation mechanism for the origin of mafic enclaves in the peraluminous granitoid, but also into the mixing between different batches of felsic magmas in granite petrogenesis. The contribution from I-type and S-type granites is identified in the peraluminous pluton as well. These lend support to the common consensus that the peraluminous granitoid batholith primarily results from remelting of the crustal rocks under different physicochemical conditions.The origin of mafic enclaves in granites can provide significant information on granite petrogenesis.The Aozicun granites in the southeast of Jiuling granitoid batholith are mainly medium- to coarse-grained biotite granites. Mafic enclaves are widespread in the Aozicun granites. The sizes of these mafic enclaves range from less than one centimeter to about twenty centimeters. They are finer-grained and darker than the host granites. They have similar mineral assemblages with the host granites,but contain more biotites and less K-feldspars. Garnet debris occurs in some biotite aggregates (small mafic enclaves). They are relatively scarce in the small mafic enclaves and are typically associated with biotite. Some small garnets are rimmed by biotite. Other garnets are surrounded by biotite or contacted with biotite in their embayment.Furthermore, some small leptosomatic biotites (<10?m) coexist with xenomorphic quartz on the rims of the garnet. These petrographic characteristics show that these biotites are formed by consumption of garnet.These metasomatic biotites exhibit some chemical characteristics inherited from garnet. They are characterized by higher Mg# [=Mg/(Mg+Fe), molar] and MgO contents, lower K2O and TiO2 contents,lower (La/Yb)N and (Gd/Yb)N ratios and distinct negative Eu anomalies. In addition, a few primary biotite inclusions are also enclosed in some garnets, and they show similar compositions with these metasomatic biotites. Hence, the garnet fragments in the biotite aggregates should be peritectic garnets and these biotites that associated with garnet fragments are products of secondary reaction between the peritectic garnets and the granitic melts. This reaction can be defined as the back reaction of biotite dehydration-melting: Garnet+ K-feldspar + melt 1 + H2O =Biotite + Plagioclase + Quartz + melt 2.The host Aozicun granites are granodiorites with SiO2 contents of 62.94 to 67.91 wt.%. The mafic enclaves are dioritic with SiO2 contents of 59.12 to 61.81 wt.%. SIMS zircon U-Pb dating shows that the host granite amd mafic enclave have consistent zircon U-Pb ages of middle Neoproterozoic (823±3?824± 4 Ma) within the analytical errors. The host granites have whole-rock initial Sr isotope ratios of 0.708?0.712,?Nd(t) values of -3.34,?Hf(t)values of 4.10?7.49 and ?18O values of 10.70?11.81‰. The mafic enclaves have whole-rock initial Sr isotope ratios of 0.710?0.714, ?Nd(t) values of -0.75?-2.74, ?Hf(t)values of 4.26?4.69 and ?18O values of 10.50?10.90‰. In other words, the mafic enclaves and the host granites have similar Sr-Nd-Hf-O isotopes. Their enriched Sr-Nd isotopes and high O isotope compositions indicate that the host granites and mafic enclaves are mainly derived from the crustal source rather than the depleted mantle reservoir. Furthermore, the host granites and the mafic enclaves all display strong peraluminous feature (A/CNK > 1.1), indicating that their source rocks are mainly sedimentary rocks,which have undergone surface chemical weathering. In addition, mineral data show that some biotites in the mafic enclaves exhibit similar geochemical characteristics to these metasomatic biotites, and some have similar compositions with those in the host granites. The biotites in the enclaves contain no K-feldspar inclusions and more plagioclase inclusions than those in the host granites. In addition, zircons in the host granites and mafic enclaves have consistent Hf (?Hf(t): 2.7?5.9 vs. 3.8-7.8) and O (?18O: 8.23?9.99‰ vs.8.03?10.09‰) isotope compositions. The combined results document that the biotite-rich enclaves in the Jiuling Aozicun granites were formed by back-reaction of peritectic garnet aggregates with the host granitic melt at an increased water fugacity in the granitic magma system. Thermodynamic modeling indicates that the quantity of directly crystallized biotite is low (<5%). Therefore, it is inferred that most of the biotites in the S-type granites were formed by back-reaction of peritectic garnet entrained from the source.Mass-balance calculation shows that about 10% peritectic garnets are entrained into the Aozicun granites.And the fate of these peritectic garnets is converted into biotites through reaction with the granitic melt,which results in the high Mg and Fe contents in the Aozicun S-type granites. The result of the present study is an important supplement to the peritectic mineral entrainment model, a vital imperfection of which is the absence of peritectic garnet in the S-type granites. We propose that most of the peritectic minerals are replaced by other minerals (e.g. biotite in our study) by back-reaction, thus directly contribute to the geochemical diversity of S-type granites.Mixing of felsic magmas originated from different sources may be common in granite petrogenesis,but difficult to unravel. Two types of rocks exist in the Jiuling peraluminous granitoid batholith. One is middle to coarse-grained granodiorite (termed as low-Si granitoids), and the other is middle to fine-grained high-Si biotitie granites (termed as high-Si granite). SIMS and LA-ICPMS zircon U-Pb dating indicates that the low-Si granitoids and high-Si granites have consistent emplacement ages at middle Neoproterozoic of ?820Ma. Although they show similar Sr-Nd-Hf isotope compositions, the low-Si granitoids display higher MgO, FeO, TiO2, Al2O3, MREE contents. And the whole-rock zirconium saturation temperatures show that the low-Si granitoids have relatively higher temperature (771?855?) than the high-Si granites(736?765?). The low-Si granitoids have whole-rock initial Sr isotope ratios of 0.706?0.720,?Nd(t) values of -2.74?-4.44, two-stage Nd model ages of 1.7-1.9 Ga, ?Hf(t) values of 3.93?6.57 and two-stage Hf model ages of 1.4?1.6 Ga. The high-Si granites have whole-rock initial Sr isotope ratios of 0.712?0.717, ?Nd(t)values of -3.20?-4.36, two-stage Nd model ages of 1.75?1.85 Ga, ?Hf(t) values of 1.99?3.44 and two-stage Hf model ages of 1.60?1.69 Ga.In addition, two groups of zircon and garnet are identified as either individual grains or core and rim in the same grain in the low-Si granitoids and high-Si granites. Group I zircons show high ?18O values(?18O>8‰, ave.10.0‰) and low ?Hf(t) values from -9.4 to 7.5(ave. 0.9), whereas Group II zircons exhibit lower ?18O values (?18O <8‰, ave. 6.7‰) and higher ?Hf(t) values from 0.0 to 10.6 (ave. 5.5). Group I and II zircons both occur in the magmatic zircon rim or magmatic zircon core. Group I garnets are higher in FeO and MgO but lower in CaO and MnO than Group ? garnets. Group I garnets show more remarkable negative Eu anomalies than Group ? garnets. Group I and Group ? garnets exhibit distinct ?18O values, in equilibrium with Group I and Group II zircons, respectively. Group I garnets all occur as cores in the zoned grains, and Group II garnets all occur as rims.The integrated analyses imply that the Jiuling peraluminous granitoid batholith is derived from partial melting of continental crustal rocks. Two groups of zircons and garnets crystallized from two distinct batches of felsic magmas. Batch I may be derived from ancient sedimentary rocks at higher temperature,whereas Batch H is probably formed by partial melting of relatively juvenile rocks at lower temperature.Therefore, the Jiuling batholith is produced by mixing between the two batches of felsic magmas.SIMS zircon U-Pb datings for five samples in different parts of the Xingzi granitoid pluton yield weighted 206Pb/238U ages from 819±5 Ma to 831 ±6 Ma,indicating that the Xingzi granitoid pluton is formed in the middle Neoproterozoic, similar to the Jiuling granitoid batholith. Most of the Xingzi granites are peraluminous with A/CNK> I .0.They have similar whole-rock Sr-Nd-Hf isotopes. In addition, there are no obvious correlations between A/CNK and P2O5 with SiO2 in their whole-rock covariance plots, similar to S-type granites. Besides, their synmagmatic zircons have ?18O values ranging from 5.7‰ to 11.3‰,which are higher than that of the mantle zircon. However, some other Xingzi granites associated with the gabbro dykes display metaluminous to peraluminous compositions. They present significant positive correlation between A/CNK and SiO2, and negative correlation between P2O5 and SiO2 in their whole-rock covariance plots. These are typical characteristics for I-type granites. Furthermore, the synmagmatic zircons of these samples have similar O isotopes with the mantle zircon. The results show that both S-type and I-type granites exists in the Xingzi granitoid pluton. The S-type granites have positive zircon ?Hf(t)ratios ranging from 0.60 to 8.23, whereas the zircon ?Hf(t) ratios in the I-type granites range from 1.43 to 11.71. Therefore, the Xingzi granitoid pluton is formed by mixing between S-type and I-type granitic magmas. What's more, the S-type granitic magma is derived from melting of the sediments of juvenile arc crust which experienced rapid weathering and deposition. The I-type granitic magma is mainly produced by partial melting of juvenile crust.The present study demonstrates that back reaction of peritectic garnets with the granitic melts is a reliable origin for the mafic enclaves in the peraluminous granitoid except the traditional residue model,mixing of mafic magma or the cumulate model. The metasomatic biotites that scattered into the host peraluminous granites can account for the high MgO+FeOT contents in S-type granites. The petrogenesis of the peraluminous granites is mainly attributed to partial melting of the continental crustal rocks, and incremental intrusion of granitic magmas with different compositions may be a primary mechanism.Mixing between different batches of felsic magmas with similar rheologies is more common in nature but difficult to be verified, because the whole-rock compositions are readily affected by other magmatic processes. The combined microscale study of different refractory minerals for their mineralogy and geochemistry is an effective way to reveal the mixing between felsic magmas. An integrated study of minralography, whole-rock geochemical compositions and zircon O isotopes can precisely and effectively distinguish I-type and S-type granites in a peraluminous granitoid pluton.