| Because of occupying a particular geotectonic setting, i.e. siatuated at the intersection of Eurasia plate, Pacific plate and India-Australia plate, Hainan Island of South China has been a key "window" to accurately understand the Southeast Asia continental accretion, continental margin evolution and formation of the South China sea. Since the middle Cenozoic, large-scale magmatism and associated metallogeny occurred in South China including Hainan Island. Among these, the occurrence of the Cretaceous Qianjia composite pluton in the southwestern Hainan island also involved strong Mo, Pb, Zn and Ag-polymetallic mineralization, which led to a series of deposits such as the Shimenshan Mo-Pb-Zn, the Houwanling Pb-Zn, and the Kanshuling Ag-Pb-Zn polymetallic deposits. Because these deposits are hosted within the Qianjia pluton, both of they are ideal candidates to evaluate the relationship between the Late Mesozoic magmatism and the metallogenic events in South China.Based on geological investigation and petrological obervation, analyses on mineral chemistry, geochronology, whole rock geochemistry and Sm-Nd and isotopes were carried out on the Qianjia composite pluton, in order to reveal the potential source of the pluton and magama-forming mechanism, to discuss the geodynamic background of magmatism, to understand the relations between magmatism and mineralization, and to provide evidence for tectonic system and mineralizing process during the Late Mesozoic lithospheric evolution of Hainan island. The main conclusions are as follows:1. Qianjia composite pluton can be subdivided into four units of the Fubao, the Mumiantou, the Duowenling, and the Baohao. Lithologically, this four units comprise medium-grained porphyritic hornblende-biotite monzonitic granite, fine-grained biotite syenogranite, medium-grained hornblende-biotite granodiorite, and medium-grained hornblende-biotite monzonitic diorite or diorite, respectively. With respect to emplacement age, these units are also characterized by an order from Baohao/Duowenling, via Fubao, to mumiantou, from early to late. Quartz concentration is significantly different in each unit, which is defined by gradual increase from intermediate to acid units, i.e.5～10% in the Baohao,5～18% in the Duowenling,20～25% in the Fubao, and 25～28% in the Mumiantou. The Qianjia pluton also contains large amounts of enclaves and dykes. Mineral assemblages and morphological characteristics of the enclaves indicate a mixing process for generation of the Qianjia composite magma.2. The crystallization ages for the Qianjia composite pluton were determined by LA-ICPMS U-Pb dating on zircon. The dating results suggests a Early Late Cretaceous age of ca.100 Ma for the Qianjia pluton. Among these, the crystallization ages for the Fubao, Mumiantou, Duowenling and Baohao units, and the quartz diorite enclaves in the Baohao unit are 100±1Ma,100±1Ma, 99±1 Ma,101±1Ma, and 100±1 Ma respectively. In addition, the age for the grained granodiorite-porphyry dykes in the Fubao unit is 94-96 Ma. These Cretaceous ages are similar to those of other plutons in Hainan Island, such as 99.2±3.7 Ma for the Baocheng pluton,107.6±1.5 Ma for the Tunchang pluton,105.8±2.4 Ma for the Tianyahaijiao pluton,90-104 Ma for the Yaliang pluton. As a result, a series of important magmatism occurred in Hainan Island, from Late Early cretaceous to Early Late Cretaceous.3. As to the different units of the Qianjia composite pluton with the Cretaceous ages, they have the same mineral assemblages bisically composed of hornblende、biotite, plagioclase, alkali feldspar, and quartz, and show similar crystallization conditions. Mineral chemical characteristics indicate that hornblende was mainly from crust-mantle mixture, biotite formed in a high oxygen fugacity environment which accounted for a mixing origin and a calcium alkaline host rock, and plagioclase is characterized by both normal and reverse annulus. Collectively, the petrological and mineralogical data as well as the presence of annulus plagioclase indicate a crust-mantle mixing responsible for formation of the Qianjia composite pluton.4. The Qianjia composite pluton has variable contents of SiO2 (47.06～77.57%) and high concentrations of total alkali (K20+Na20=5.11～8.35%). The K20/Na2O ratios are from 0.54 to 1.65, and the A/CNK values from 0.70 to 1.10. These geochemical features show that the Qianjia composite pluton belongs to high-K calc-alkaline series and are metaluminous-peraluminous. The total REE contents (ΣREE) of the Qianjia composite pluton range from 91.93-267.47 ppm, with La/YbN ratios from 6.47 to 28.2 and δEu values from 0.60 to 0.84. This pluton is also rich in LREE and depleted in HREE. The (δ7Sr/δ6Sr)i ratios of ranging from 0.706 to 0.710 as well as the εNd(t) values of -4.63～-8.14 commonly imply that the Qianjia pluton was derived from a mixed crust-mantle source. The two-stage Nd model ages between 1.28 Ga and 1.46 Ga, slightly younger than the oldest Baoban Group (1430～1770 Ma) in Hainan Island, most likely suggested an involvement of mantle components into the source. The Qianjia pluton also has low FeOT/Mg ratios, low Zr, Nb, Y and LREE concentrations, and 1000Ga/Al ratios of <2.7, which are different from that of A-type granites, but similar to that of I-type granites. Further evidence shows that the Qianjia pluton likely was derived from partial melting of the high-K calc-alkaline metabasites of the Mesoproterozoic Baoban Group, but with addition of mantle components.5. Re-Os dating on molybdenite from the Shimenshan deposit gives a model age of ca.80 Ma, which represents the mineralization age. This age is approximately close to age of ca.94 Ma-84 Ma previously dated by Rb-Sr and K-Ar methods. However, due to easy disturbance on Rb-Sr dating and K-Ar systerm by subsequent thermal events, those ages are 20 Ma younger than zircon U-Pb ages of ca.110 Ma in this study. Moreover, the existence of late-thermal events can help us to suggest that there is a hidden ore-related massif below Qianjia composite pluton. In addition, we found some I-type granites with age of 73 Ma in the Longlou town of northeastern of Hainan islands, which draws a conclusion that magmatic events may occur in Hainan islands at ca.94～73 Ma. Therefore, the Shimenshan deposit may be related to this magmatic event.6. The Re concentrations in the molybdenite Re-Os systerm of the Shimenshan deposit indicate that the ore-forming metals were mainly sourced from crust-mantle mixture. During the stage of mineralization, the δ34S values of the metal sulfides vary from 1.36%o to 5.73%o with an average value of 4.3%o. This approaches the range of sulfur from mantle-derived magma with 0±1‰, and probably implies that the sulfur was mainly derived from the mantle. The formation of the deposit may have experienced long time of magmatic-hydrothermal activities, with magma supplemented by some crustal sulfur during fractional crystallization.The result show that source of the Shimenshan deposit is crust-mantle mixture, with a predominant mantle.7. The Qianjia composite pluton occurred during the Early Late Cretaceous. The Late Cretaceous granites in South China were mainly present along the NE-trending southeast coastal area. During the end of Early Cretaceous (110-100 Ma), the granites mostly are Ca-alkaline I-type and have affinities to adakites. When it comes to the Early Late Cretaceous, the A-type granite was developed in this area. The magmatic activities became younger from inland to coastal area during Cretaceous. The NE-trending igneous rocks have been interpreted as the result of subduction of the Paleo-Pacific plate beneath South China continent. Therefore, the Qianjia pluton likely was related to this subduction event. The Mesozoic was the most important period of molybdenum polymetallic mineralization in eastern South China and adjacent areas. Mo mineralization at ca.100 Ma occured only in Fujian Province and southeast coastal areas, whereas the youngest Mo mineralization of ca.80-72 Ma ages was present in Hainan Island. Therefore, ages of Mo-polymetallic ore deposits are also getting younger from west to east and north to south, in eastern South China and adjacent areas.8. During the end of the late Permian to the beginning of early Triassic, the closure of Paleotethys resulted in the collision between Indosinian block and South China block. It also caused the Indosinian tectono-magmatic event in southern China. In Hainan Island, numerous granitic plutons were developed, such as Zhizhong, Wuzhishan, Jianfengling etc. Many metal ore deposits such as the Erjia and Baolun gold deposits, and the Lanyang tungsten deposit were also formed during this period. After the Indosinian collision orogeny, the South China continent underwent a tectonic transformation from the Tethys domain to the Pacific domain. The Ca-alkaline I-type granites in the coastal area were characterized by signature of the Pacific plate subduction systerm. The influence of the Indosinian orogeny on the South China block was not stopped until the early Jurassic. The subduction of the paleo-Pacific plate to the Eurasian plate resulted in an extensional environment in the inland of the South China block. It thus formed the intraplate magmatic activity in southern China and the arc magmatism in southeast coast during the early Mesozoic. With the constant subduction of the paleo-Pacific plate underbeneath the South China intra-continent, the extension stress state was switched to the compressive state. All these led to the Jurassic magmatism. The oceanic plate started rollback and back off during the early Cretaceous, which led the compressive state transfer to the extension stress state in southern China. Afterwards, the extension was enhanced and many A-type granites intruded during ca.135-120 Ma. During period of 120 Ma to110 Ma, with the influence of further subduction of paleo-Pacific plate, the extension stress state turned into the compressive state again, resulting the structural inversion of rifted basin of early Cretaceous in southern China. And the magmatisms also fell into the bottom. From 110 Ma, the magmatism was developed increasingly, but limited in the coastal area, except the inland area. The A-type granites (92～103Ma) and mafic dikes reappeared in the coastal area, marking the compressive state started transfering to the extension stress state. The Qianjia pluton emplaced around 100 Ma, and at the same time, the Ca-alkaline Ⅰ-type granites were widely developed in the southern China, such as the Taiwushan Granites in Fujian (97Ma), the Sifang Granites in Fujian, the Shipingchuan Granites (102Ma) in Zhejiang and the Penggushan biotite granites (104Ma). These granites indicated the beginning extension. During this period, a serious of Ⅰ-type granites formed in Hainan Island, as thee Yaliang, Shengyu, and Tianyahaijiao granites. They constituted a nearly E-trending, under the continental margin environment related to the oceanic plate subduction. Afterwards, the extension was further strengthened, and the bimodal volcanic rocks and numerous 100-80 Ma A-type granites as well as the mafic dikes occurred, marking that the lithosphere was turned into the strong extension stage. Thus the tectonic development of the South China block was controlled by the Pacific plate subduction, which resulted in the formation of magmatic arc and associated back-arc magmatism at ca.73 Ma. |
PDF Full Text Request