Geochronology and Geochemistry of Granitoids from the Prydz Belt, East Antarctica, and Their Tectonic Implications
|School||Chinese Academy of Geological Sciences|
|Course||Mineralogy,petrology, Mineral Deposit Geology|
|Keywords||East Antarctica Prydz Belt Pan-African time post-collisional granitoids|
The Prydz Belt is one of the most important Pan-African mobile belts within the East Antarctic Shield. The belt provoked great interest to many geologists in recent years because it was related to the formation and evolution of the Gondwana suppercontinent during the Cambrian. However, at present the tectonic attribute of the Prydz Belt remain highly debated. Some argued for an intraplate orogen, whereas others argued for a suture. In this paper, Geochronology, geochemistry and petrogenesis of granitoids from the Prydz Belt have been investigated and their tectonic significance and behavior during the assembly of Gondwana suppercontinent have been discussed. The main conclusions from the thesis are as follows.1. SHRIMP U-Pb zircon analyses reveal that granitoids from the Prydz Belt in East Antarctica were intruded from 550 Ma to 490 Ma, immediately to about 60 Ma after the metamorphic peak. In the Grove Mountains, charnockite, charnockite dykes, granite and granite dykes were dated at 547 Ma, 533 Ma, 526~503 Ma and 501 Ma, respectively. In the Prydz Bay area, the Progress granite and the Dalkoy granite were dated at 530~500Ma, whereas the emplacement ages of the Landing granite and the Munro Kerr Mountains granite are as young as 500Ma. These diverse granitoids from charnockite, monzonite, quartz-monzonite to granite constitute a metaluminous to weakly peraluminous granitic serious. They also show the same chemical characteristics as the shoshonitic suits.2. The results of representative minerals by electron microprobe analyses suggest that orthopyroxene is ferrohypersthene in charnockites, and eulite in charnockites dykes, whereas amphibole is ferropargasite, and biotite is annite (expect for a few Mg-rich biotite) in all granitoids. The mineralogical characteristics indicate an affinity of crust-mantle mixed granites in a relatively H2O-poor and reduced environment. P-T calculations suggest that granitoids from the Prydz Belt were emplaced at 5~6.6kbar and 774~832℃. Using the equation given by Wones (1989), oxygen fugacities are estimated to be -12.16 to -13.65, which are slightly above the QFM (quartz-fayalite-magnetite) buffer.3. Granitoids from the Prydz Belt are geochemically characterized by having high K2O + Na2O contents, K2O/Na2O and Ga/Al ratios and Fe* values, and low MgO, Cr, Ni contents, in agreement with A-type granites. The Progress granite in the Larsemann Hills should be defined as aluminous A-type granite, rather than S-type granite as considered by formers. This granite may have undergone extremely fractionation and contamination by the country pelites. In the primitive-mantle-normalized trace elements diagrams, all the granitoids show LILE and HREE enriched patterns with variably trough at Sr, Nb-Ta, Ti and P, a distinctive feature of subduction-related magmas. In a few tectonic diagrams, most granitoids plot the field of within-plate granites, and some belong to the syn- and post-collision granites.4. Isotopically, granitoids from the Prydz Belt display very low initial Nd isotope compositions and high initial Sr isotope compositions. The initial Nd values vary over a narrow range of -13.37~ -9.17, while the initial Sr values vary considerably from 0.7075 to 0.7246. Nd-depleted mantle model ages of 2.0~2.3Ga imply their derivation from old palaeoproterozoic crustal sources. Petrographical, geochemical and isotopic evidence indicate that granitoids from the Prydz Belt were generated from a phlogopite-bearing subcontinental enriched lithospheric mantle. Granitoids in the Larsemann Hills have variable initial Sr values, probably implying an inhomogeneity of Sr isotope composition in the source. On the other hand, these granitoids have Nd-depleted mantle model ages same as those of paragneisses, suggesting a close genetic link between them.5. Both magmatism and high-grade metamorphism in the Prydz Belt are probably related to the continental collision, including lithospheric thinning, magmatic underplating and crustal relaxation. The granitoids were derived by partial melting of the underplating materials of mantle magmas from enriched subduction-modified lithosphere, accompanying by the contamination of crustal components. The Landing granite and Murro Kerr granite may the final represent welding of different terranes and therefore sign the end of the post-collisional orogenic episode in the whole Pan-African belt and the beginning of a next Wilson cycle.6. Syn- and post-collisional A-type granitoids from the Prydz Belt support the idea that Prydz Belt represents a collisional orogen and the East Antarctic Shield itself was finally amalgamated by different terranes during the Pan-African period. Accordingly, East Antarctic Shield as well as East Gondwana were not a united continental blocks before the Pan-African time.