The Neoproterozoic Huangling granite batholith in the southern Huangling anticline is a composite plutonic complex with characteristics of multistage invasion. In the northern and western parts of the granitoids, it intrudes into the Xiaoyicun and Miaowan formations of the Kongling Group, respectively, whereas it is overlain with an angular unconformity by the Liantuo Formation and Nantuo Formation of Nanhua System and the Doushantuo Formation of Sinian System in the south and east, respectively (Fig. 1). The types of the intrusive rocks are gabbro, quartz-diorite, tonalite, trondhjemite, and monzogranite from west to east in southern Huangling granite batholith. The tonalite, in the southern Huangling granite batholith, is grouped into Sandouping suite (Ma et al., 2002) and is subdivided into Taipingxi, Xidianju, Yanwan, and Xiaoxikou units (rock-mass). However, the research results of 1:250 000 regional geological mapping of Yichang and Jianshi (2006, unpublished) and 1:50 000 regional geological mapping of Liantuo and Sandouping in 2011 (in this study) lead to the Sandouping suite being redivided into two units (Table 1): medium-grained biotite-hornblende tonalite of the Sandouping intrusion and medium-coarse-grained hornblende-biotite tonalite of the Jinpansi intrusion; the pyroxene diorite (metagabbro), which contains no or minor quartz, and the quartz diorite-tonalite were grouped into the Duanfangxi sequence (super-unit) and the Maoping sequence (super-unit), respectively. In addition, the rock name of the Taipingxi unit was revised to medium-coarse-grained biotite-hornblende ~83 km2, extending from north to south. It is the main intrusion in the Maoping superunit, and intrudes into the Xiaoyicun and Miaowan formations of Kongling Group in the north, and is overlain with an angular unconformity by sedimentary rocks from Liantuo Formation of Nanhua System in the south, and is invaded by medium-coarse-grained hornblende-biotite tonalite (Jinpansi intrusion) or medium-fine-grained trondhjemite (Luxiping intrusion) in the east. Moreover, in the internal part of Sandouping intrusion, it is invaded by fine-grained porphyritic biotite granite (Longtanping intrusion) (Fig. 2a), which contains tonalite xenoliths (Fig. 2b), and is invaded by late period veins and small stocks. The Jinpansi intrusion is distribution near Taipingxi with an area of ~5.2 km2 presenting a band shape with a NNW trend. It shows surging intrusive contact relationship with the Sandouping intrusion in the west, and is overlain with an angular unconformity by sedimentary rocks from Liantuo Formation of Nanhua System in the south, and is invaded by Luxiping intrusion in the east.
Table 1. Division and correlation of the Neoproterozoic granitic batholith in the Huangling anticline
In this study, the rock samples of isotope geochemistry and major and trace elements analysis were tested in the Ministry of Land and Resources of South Mineral Resources Supervision and Inspection Center, and the results are shown in Table 2. The major elements were analyzed by wet chemical method according to the GB/T 14506.28-1993 standard, whereas the analytical standards of the H2O+, CO2, and LOI were measured according to the GB/T14506.2-1993 standard, the GB 9835-1988 standard, and the LY/T 1253-1999 standard, respectively. Rare earth element (REE) and Y and trace elements of Th, Hf, Ta, Sc, and Co were tested by Plasma MS Excel methods with the standard of DZ/T0223-2001; however, the analytical standards of the trace elements of Sr, Ba, Rb, Nb, Zr, and Ga were followed by the X-ray fluorescence spectrometry method according to the JY/T016-1996 standard. The analysis process of Sr and Nd isotopic composition included three steps: dissolution, separation, and testing. The detailed information was referred to Ma et al. (2007).
Table 2. Representative major (wt.%), trace element (ppm), and Sr-Nd isotope compositions for the Huangling granitoids
Zircon separation of the samples was completed by the Ministry of Land and Resources of South Mineral Resources Supervision and Inspection Center. Analysis samples for dating were crushed to 80–120 mesh, washed dust by water, removed magnetic minerals such as magnetite with a magnet, selected zircons through heavy liquid methods, and picked out the zircons under the binocular microscope finally. Mount making, reflection light, cathodoluminescence (CL) images for zircon dating samples and measurements of U, Th, and Pb were completed at Beijing SHRIMP Center, Chinese Academy of Geological Sciences. The detailed analysis process was referred to the relevant literature (Song et al., 2002; Compston et al., 1992, 1984). In the laboratory, zircon standard samples were M257, which yielded an age of 416.8±1.1 Ma of 206Pb/238U; however, the content of U, Th, and Pb was heterogeneous. The U-Pb isotopic data were calculated by the isoplot software (Black et al., 2003; Ludwig, 2002), and the content of common Pb was corrected by the measured 204Pb content. The errors of all tested zircon points were 1σ, and the weighted mean age 206Pb/238U had 95% confidence interval.
Geochemical analysis results of samples from the Sandouping, Jinpansi, and Longtanping intrusions in the Neoproterozoic Huangling granitoids are shown in Table 2. The major elements of samples from the Sandouping and Jinpansi intrusives show that the content of SiO2 is from 62.27 wt.% to 64.27 wt.%, average 63.21 wt.%, and plot in the dioritegranodiorite fields in the Q'-F'-Anor diagram (Fig. 3) (Streckeisen and Le Maitre, 1979); Al2O3 from 16.94 wt.% to 17.46 wt.%, average 17.16 wt.%; K2O+Na2O from 5.31 wt.% to 5.75 wt.%, average 5.46 wt.%; and Na/K from 2.52 to 3.43. The values of consolidation index (SI), which vary from 13.22 to 16.95, average 15.73, and differentiation index (DI), which vary from 63.72 to 69.68, average 66.11, reflect that these samples have moderate-degree crystallization differentiation characteristics. The Rittmann index (δ=1.37–1.53) and SiO2-K2O diagram (Fig. 4a) reflect that the sam-ples belong to calc-alkaline series granites. Samples from the two intrusions have A/CNK values (Al2O3/CaO+Na2O+K2O) of 0.98–1.06, and the A/NK-A/CNK diagram (Fig. 4b) show that the samples appear as silica supersaturated type of metaluminous to weakly peraluminous calc-alkaline granites. Samples from the fine-grained porphyritic monzogranite of Longtanping intrusion, which intrude the tonalite-diorite of Sandouping rock unit, fall in the fields of monzogranite in the Q'-F'-Anor diagram (Fig. 3 (after Streckeisen and Le Maitre, 1979). The content of SiO2 is 73.16 wt.%, Al2O3 is 13.91 wt.%, K2O+Na2O is 7.23 wt.%, and Na/K mostly is 1.2. The values of SI and DI, which are 2.8 and 85.69, respectively, reflect that the Longtanping intrusion have higher-degree crystallization differentiation characteristics. A/CNK=1.06, Rittmann index δ=1.3, show that the samples belong to silica supersaturated type of peraluminous high-K granites.
Figure 3. Q'-F'-Anor diagram of the Huangling granitoids (after Streckeisen and Le Maitre, 1979).
The range of the total REE abundances of biotite-hornblende tonalite from Sandouping intrusion and hornblende-biotite tonalite from Jinpansi intrusion is from 42.62 ppm to 117.12 ppm (Table 2), and they have similar REE patterns. The Sm/Nd value is 017–0.24, and the two intrusions show significant enrichment in LREE, negative anomalies in Ce (Ce/Ce*=0.87–0.89), and weak positive anomalies in Eu (Eu/Eu*=1.0–1.2), except two groups (one group is 0.97 and another is 1.63). The Eu/Sm values range from 0.33 to 0.5, reflecting that the two units are homologous. The total REE abundance of biotite monzogranite from the Longtanping intrusion is low, and the REE distribution pattern is shown in Fig. 5a. There are significant enrichment in LREE, negative anomalies in Ce (Ce/Ce*=0.89), and weak positive anomalies in Eu (Eu/Eu*=1.03). The value of LREE/HREE ratio is 13.9, which is higher than the Sandouping and Jinpansi units, reflecting that the degree of magma differentiation for Longtanping intrusion is higher, and the curve slope of LREE is steeper also.
Figure 5. Chondrite-normalized REE distribution patterns (a) and primitive-normalized trace element spider diagram (b) of the Neoproterozoic Huangling granitoids (chondrite and primitive mantle data after Sun and McDonough, 1989).
Trace elements analysis results of biotitehornblende tonalite from Sandouping intrusion and hornblende-biotite tonalite from Jinpansi intrusion are shown in Table 2. The RbN/YbN values range from 1.1 to 3.62, are > 1, and suggest that the two intrusions are strongly enriched in compatible elements. From the spider diagram of trace element ratios (Fig. 5b), the granite samples are enriched in LILE of K, Rb, Ba, and Sr and immobile elements such as Zr but depleted in HFSE of Nb, Ta, Hf, Ti, and P and show that the trace element distribution patterns are characteristics of tectonic environment of active continental margin arc; these features suggest that the generation of the granites is related to plate subduction. They are enriched in K, Zr, and Sr and depleted in Nb, P, Th, and Ti, which indicate that the material source of granite is derived from crustal rocks and show the characteristics of crust-derived granites (Li C N, 1992; Pearce et al., 1984). The trace element contents of fine-grained porphyritic monzogranite of Longtanping intrusion are shown in Table 2. The RbN/YbN value is 10.37, which is > 1. It suggests that the intrusion is strongly enriched in compatible elements. There are significant differences of spider diagrams between the Sandouping, Jinpansi, and Longtanping intrusions. The Longtanping intrusion granites are depleted in, Nb, Sr, P, Ti, and Th and enriched in K and Zr, which indicate that the intrusion material source is derived from crustal rocks.
The Rb-Sr and Sm-Nd isotopic analysis results are shown in Table 3. The εNd(t) and εSr(t) values of tonalite from Sandouping intrusion range from -12.4 to -11.0 and from 20.2 to 32.2, respectively, and the fine-grained porphyritic biotite monzonitic granite are -16.3 and 82.9. In the εNd(t)-εSr(t) diagram (Fig. 6), all the representative samples fall in the field of Chinese lower crust, which indicates that the source region of granite formation is the lower crust.
Table 3. Rb-Sr and Sm-Nd isotopic composition and main parameters of the Neoproterozoic Huangling granitoids
Figure 6. εSr(t)-εNd(t) diagram of the Neoproterozoic Huangling granitoids (Zhu et al., 2001).
SHRIMP zircon U-Pb dating of granites are shown in Table 4. The characteristics of zircon grains from the granite samples are as follows: the columnar zircon is the predominant type, except a small amount of granular type, and their aspect ratios range mainly from 1:1 to 3:1. The analyzed zircon grains are uniformly transparent in color, and the CL images show that the zircon grains have clear oscillatory zoning (Fig. 7), which is the typical characteristic of magmatic origin (Chen et al., 2007; Wu and Zheng, 2004; Wysoczanski and Allibone, 2004; Corfu et al., 2003).
Figure 7. The CL images of the representative zircons (a, c, e) and SHRIMP zircon U-Pb concordia age diagrams (b, d, f) from the Neoproterozoic Huangling granitoids.
Medium-grained biotite-hornblende tonalite (Sample No.: PM053-11-1, sample location: N30°52′29″, E111°01′09″) from the Sandouping intrusion has 11 zircon grains analyzed. U and Th contents of zircon range from 70 ppm to 232 ppm and from 40 ppm to 114 ppm, respectively. The Th/U ratio is between 0.33 and 0.81, which is consistent with typical magmatic zircon. U-Pb ages of zircon from the tonalite sample are in the range of 833±14 and 905±15 Ma (Table 4). The CL images (Fig. 7a) show that No. 11 zircon grain has local characteristics of planar zoning, and the rim shows a wide alteration edge that cuts the magmatic zoning of the protolith zircon. These features indicate that the zircon grain may be affected by late hydrothermal alteration, and the U-Pb age (833±14 Ma) is younger than others. The core zoning of No. 7 zircon grain is curve-shaped and unsmooth and has erosion structure. The U-Pb age (905±15 Ma) is older than others as the zircon part of laser ablation contains the inherited core composition. The remaining nine grains all plot close to Concordia, yielding an age of 863±9 Ma (MSWD=0.88) (Fig. 7b), which is the formation time of Sandouping intrusion.
Medium-grained hornblende-biotite tonalite (Sample No.: PM054-2-1, sample location: N30°54′24″, E111°00′36″) from the Jinpansi intrusion has 13 zircon grains analyzed. U and Th contents of zircon range from 48 ppm to 191 ppm and from 25 ppm to 173 ppm, respectively. The Th/U ratio is between 0.46 and 0.96, which is consistent with typical magmatic zircon. U-Pb ages of zircon from the tonalite samples are in the range of 794±11 and 873± 8 Ma (Table 4). The CL images show that the No. 5 zircon grain has "core-mantle" structure, and the position of No. 5.2 point is located at the core of the grain, which shows the oscillatory zoning characteristics of the magmatic zircon, whereas the crystal edge of No. 5.1 analysis point has been rounded (Fig. 7c), and the content of Th and U is low. These features indicate that the zircon grain may be affected by the late hydrothermal alteration, and the U-Pb age is 803± 11 Ma, which could be used as the age of late hydro-thermal alteration. The No. 8 zircon grain with a black circle inside may be reconstructed by the late fluids, which cause lead loss, and the U-Pb age (794±11 Ma) is younger than others. The No. 2 zircon grain shows characteristics of partial curve shape and erosion structure. The U-Pb age is older (873±8 Ma) as the zircon part of laser ablation contains the inherited core composition. The remaining 10 grains all plot close to concordia, yielding an age of 842±10 Ma (MSWD=0.88) (Fig. 7d), which is the formation time of the Jinpansi intrusion.
Fine-grained porphyritic monzogranite (Sample No.: PM053-2-1, sample location: N30°52′32″, E111°00′28″) from Longtanping intrusion has 16 zircon grains to be analyzed. U and Th contents of zircon range from 70 ppm to 232 ppm and from 40 ppm to 114 ppm, respectively. The Th/U ratio is between 0.33 and 0.81, which is consistent with typical magmatic zircon. The CL images show that the No. 3 zircon grain has characteristics of partial planar zoning (Fig. 7e) and a wide alteration edge on the rim, which indicates that the zircon grain may be affected by late hydrothermal alteration; the U-Pb age (815±34 Ma) is younger than others. The plot of No. 5 zircon grain deviates from concordia. Therefore, these two points are excluded. The remaining 14 grains all plot close to concordia, yielding an age of 844±10 Ma (MSWD=0.71) (Fig. 7f), which is the formation time of the Longtanping intrusion.