Geochronology, Geochemistry and Its Tectonic Significance of Chahe Granites in Hainan Island

INTRODUCTION

Granitic magmatic rocks are widely distributed in Hainan Island, with multi-phase event characteristics. Granites since the Mesozoic are mainly intrusive rocks such as Hercynian–Indosinian granites (which take about 40% of the island), Yanshanian granite porphyry and granodiorite. The second eruption phase is Late Yanshanian acid volcanic rocks. Some researchers have done some research on Mesozoic granite in Hainan Island, such as Ge (2003) believed that Qiongzhong and Dan County batholith granitoids belong to high-K and calc-alkaline granitiods with potassium feldspar phenocrysts; meanwhile they had the native orientation construct, and were the product of tectonic evolution with compressionrelaxation- compression. Qiongzhong batholith formed in 237 Ma, maybe in the relaxation stage after the Indosinian collision orogeny peak stage, while the Dan County batholith formed in 186 Ma, which was the end stage of orogeny. Yun et al. (2005) and Li et al. (2005) believed that the Qiongzhong granites formed in 233 Ma, belonged to high-K calc-alkaline series, and formed in regional compression collision environment. Previous studies on Indosinian granites have focused on Triassic, but very few on the granite formed in Late Permian. This article will report petrology and geochemical characteristics of granites in Chahe area, southwest of Hainan Island, and its geochronology combined to discuss the tectonic significance.

GEOLOGICAL SETTINGS

Chahe granites are exposed in Chahe Town in Changjiang City, Hainan Island (Fig. 1). The rocks are red, holocrystalline, medium to coarse-grained and massive structure. The main minerals are plagioclase, K-feldspar and quartz, and the minor minerals are biotites. On the rock surface, there can be seen red Kfeldspar, gray plagioclase and quartz. The plagioclase is platy, subhedral-anhedral, with cassette crystal twins and albite polysynthetic crystal, while the Kfeldspar is mainly microcline and microperthite, with the lattice crystal twins (Fig. 2a) and envelope crystal twins (Fig. 2b). The quartz is colorless and with irregular crystal, and the biotite is brown-light yellow. Accessory minerals are magnetite, allanite, phosphorite and zircon. The mafic minerals are late-stage crystals interstitial to the feldspar and quartz.

Figure  1.  The geological sketch map of Chahe granite in Hainan Island.

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Figure  2.  Microphotographs showing the crystal twins of K-feldspar. (a) Lattice crystal twins; (b) envelope crystal twins.

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SAMPLE AND METHODS

The zircons come from the granite above, and the specific sampling location is 19°14′10.5″N, 108°54′ 13.1″E. Zircons have been separated using standard density and magnetic separation techniques. Random zircon grains were handpicked under a Nikon binocular microscope and were mounted in epoxy in a 1.4 cm diameter circular grain mount. The mount was then polished to section the crystals in half for analysis. In order to characterize the internal structures of the zircons and to choose potential target sites for UPb dating, back scatter electron (BSE) images and cathodeluminescence (CL) images (Fig. 3) were obtained using a JEOLJXA-8100 microprobe at the State Key Laboratory of Mineral Deposit Research, Nanjing University, Nanjing. The operating conditions were: 15 k accelerating voltage, 20 n abeam current.

Figure  3.  CL microphotographs, dating spots and ²⁰⁶Pb/²³⁸U ages of part representative zircons from granite in Chahe.

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Zircon U-Pb analyses have been carried out at the State Key Laboratory of Mineral Deposit Research, Nanjing University, Nanjing. An agilent 7500a ICPMS equipped with new wave research 213 nm laser ablation system was used. The laser system delivers a beam of 213 nm UV light from a frequencyquintupled Nd: YAG laser. The ablated material is transported in a He carrier gas through 3 mm i.d. PVC tubing and then combined with Ar in a 30 cm³ mixing chamber prior to entering the ICP-MS for isotopic quantification. Analyses were carried out with a beam diameter of 30–40 μm, 5 Hz repetition rate, and energy of 10–20 J/cm². Data acquisition for each analysis took 100 s (40 s on background, 60 s on signal). Raw count rates for ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²³²Th and ²³⁸U were collected for age determination. Mass discrimination of the mass spectrometer and residual elemental fractionation were corrected by calibration against a homogeneous zircon standard, GEMOC/GJ-1 (609 Ma). Samples are analyzed in 'runs' of ca. 14 analyses, which include 10 unknowns, bracketed by 4 analyses of the standard. Detailed analytical procedures are similar to those described by Jackson et al. (2004). The raw ICP-MS data were exported in ASCII format and processed using GLITTER (ver. 4.0, Macquarie University). The age calculations and plotting of concordia diagrams were made using isoplot (ver. 2.49). The concentrations of U and Th in each analytical spot were derived by comparison of background-corrected count rates with mean count rates on the GJ-1 standard, which has well-known mean U and Th contents of 230 ppm and 15 ppm. A weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 610±11 (2σ, n=8) Ma of GJ-1 was obtained during this study, which is in good agreement with a highly precise ²⁰⁷Pb/²⁰⁶Pb age of 608.5±0.4 Ma by TIMS analysis of Jackson et al. (2004).

Major elements were analyzed using an ARL9800XP+X-ray fluorescence spectrometer (XRF) at the Centre of Modern Analysis, Nanjing University, following the procedures described by Franzini et al. (1972), and the analytical precision is generally less than 2%. Trace and REE elements were measured in the State Key Laboratory of Mineral Deposit Research, Nanjing University, using Finnigan Element II ICPMS, with precision better than 10% for most of the elements analyzed. The detailed analytical procedure followed Gao et al. (2003). The Sr and Nd isotopic compositions were measured using an England VG354 isotope mass spectrograph at the Centre of Modern Analysis, Nanjing University.

RESULTS

Zircon U-Pb Dating Results

Eighteen zircons U-Pb age dating results were got, and their analyses were carried out (Table 1). CL photos, dating spots and ²⁰⁶Pb/²³⁸U ages of part representative zircons are provided in Fig. 3. According to the CL photos (Fig. 3), the zircons of the granites are euhedral and have obvious magma ring. From 18 dating results, the Th/U ratios of all of the zircons are above 0.4, and have the feature of the classical magma zircons (Wu and Zheng, 2004), and lack any inherited cores. All of the analyses plot on or near the concordia curve (Fig. 4) and yield the ²⁰⁶Pb/²³⁸U age within 245±3 to 255±3 Ma. The weighted mean age for the seventeen analyses is 251±1.4 Ma (MSWD=0.89), which records the age of granite in Chahe, Hainan Island.

Table  1.  LA-ICP-MS U-Pb dating results of zircon from granite in Chahe

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Figure  4.  U-Pb concordia diagram for zircons for the granite in Chahe.

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Geochemistry

The major and trace element analyses for representative samples from Chahe granites are listed in Table 2. The rocks have higher SiO₂ contents (75.4 wt.%), and are alkali-rich, with K₂O+Na₂O ranging from 8.40 wt.% to 8.49 wt.%, and K₂O/Na₂O ratio ranging from 1.42 to 1.59, Fe₂O₃ ranging from 1.42 wt.% to 1.64 wt.%, whereas poor in MgO content (0.01 wt.%) and CaO content 0.87 wt.% to 0.96 wt.%. The A/CNK ratio is ranging from 1.09 to 1.11, with A.I. ranging from 0.83 to 0.86. It belongs to peraluminous granite. The rocks from Chahe can be contrasted to typical A-type granites (Eby, 1992), which also cont alkali mafic minerals and are rich in SiO₂, alkali, Fe₂O₃ while depleted in MgO and CaO.

Table  2.  Major (wt.%) and trace element (ppm) analytical results from Chahe granites

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The trace and rare elements results (Table 2 and Fig. 5) suggest that, Chahe granites have the characteristic of very low Sr and high Yb, with Sr content ranging from 23×10^-6 to 24×10^-6, Yb content ranging from 4.72×10^-6 to 8.35×10^-6. Depleting Sr is the main feature, meanwhile poor in Ba, P, Ti and Eu. The samples have flat REE chondrite-normalized patterns, with obvious negative Eu anomalies, whose shape like a swallow, and should belong to A-type granites (Zhang et al., 2008).

Figure  5.  Chondrite-normalized REE diagrams (a) and MORB-normalized spidergrams (b) for Chahe granites (chondrite values after Boynton, 1984; MORB values after Pearce et al., 1983).

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DISCUSSION

Previous studies show that, granites with very low Sr and high-Yb formed in very low pressure setting, which represent the product of crust thinning (Zhang et al., 2008; Wu F Y et al., 2007; Eby, 1992; Whalen et al., 1987). In Fig. 6, Chahe granites samples are basically in the region of A-type granites, and belong to A2-type granites, which are similar to the average continental crust and island arc basalt, the magma is from continental crust or derived by island arc magma, produced in the post-collision or postorogenic tectonic setting (Eby, 1992).

Figure  6.  10 000× Ga/A1 versus Na₂O+K₂O, K₂O/MgO, Zr and Nb diagrams of syenite in Fenjiezhou (a), (b), (c) and (d), after Whalen et al. (1987), and Nb-Y-3Ga, Nb-Y-Ce diagrams (e) and (f), after Eby (1992).

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At present, there are many different cognitions about the tectonic evolution of Hainan Island. Yang et al. (1989) thought that Hainan Island was the cracking block of Australia, Zhang et al. (1997) thought that Hainan Island was the part of SE China, but both of them also thought that the southern and northern two parts of Hainan Island were connected by the Jiusuo-Lingshui fault. Li et al. (2000a, b) thought that the southern part of Hainan Island was belonged to SE China, and the northern part belonged to Indo-China Peninsula, there was an ancient ocean between the two parts, which were connected by Changjiang-Qionghai fault. Liu et al. (2006) proposed the existence of a 'South Hainan Island suture', and indicated that there were two ancient oceans among the southern, middle and northern of Hainan Island, called southern ocean and northern ocean of Hainan Island. Now though the suture zone of each block of Hainan Island can't be identified consistently, most of the scholars think that the collision time was Triassic. The garnet acmite syenite from Sanya in Hainan Island formed in 244±7 Ma, and was in post-orogenic extensional setting (Xie et al., 2005), while the Wanning hornblende gabbro in Hainan Island formed in 241 Ma, and also was in post-orogenic setting (Tang et al., 2010), suggesting that Hainan Island was in post-orogenic extensional setting in Early Triassic, which means the collision time must be more earlier.

Chahe granites mentioned in this article formed in 251 Ma, and have all the mineralogy and geochemistry features which are consistent with A-type granite as talked above, such as richness in SiO₂, alkali, Fe₂O₃ while depleted in MgO and CaO, with low Sr and high Yb, and all the samples were in A-type region in the type diagrams. Since A-type can be recognized as the typical product of post-orogenic setting, then the forming of Chahe granites suggests that Hainan Island was already in post-orogenic setting in Late Permian.