| In recent years, numerous investigations have been carried out on the Precambrian basement rocks within the NCC, particularly with regard to their tectonic feature, and the implications for Precambrian continental evolution. However, almost all of the researches focused on Archean rocks in the central and northeastern NCC, and relatively few studies were carried out on the Archean basement rocks in the southern margin of the NCC. The Taihua Complex outcrops in several terranes within the southern NCC, and consists of complicated Archean-Paleoproterozoic basement rocks, making it an ideal laboratory to study and constrain the crustal evolution of the southern NCC.In this study, the TTG gneisses, Precambrian granitic rocks and amphibolite of the Taihua complex have been selected. Based on the petrographical and geochemical studies and zircon U-Pb-Hf isotope dating, we discuss the geochronology, petrogenesis, and dynamic setting for rocks from Archean-Paleoproterozoic basement in Taihua complex. In addition, based on a summary of the published chronological, zircon Hf isotopic, and other geochemical dates in the southern NCC, we discuss the Archean to Paleoproterozoic (2.9?1.8 Ga) crustal growth and evolution of the crystalline basement in the southern NCC.1. Research for Taihua complex in Xiaoqinling area gets the main achievements as follows:(1) Through the combined analysis of zircon U-Pb and Lu-Hf isotopes, we first report the Archean magmatic events that occurred in the Xiaoqinling area.(2) Early Precambrian geological events in the Taihua complex of the Xiaoqinling area can be divided into five phases, including 2.8?2.7 Ga?2.6-2.5 Ga??2.3 Ga?1.9-1.8 Ga. The Archean magmatic events is dominated by the TTG gneisses, where is located in the northwestern part of the Xiaoqinling region. The 2.8-2.7 Ga TTGs is distributed mainly in the area of Wujiaping. The rock type include the granodioritic gneisses (2802±13 Ma and 2801±11Ma), the trondhjemitic gneisses (2786±12 Ma). An analysis of the petrogenesis showed that the rocks were generated by the partial melting of basalts underplated beneath continental crust. The 2.6-2.5 Ga TTGs is distributed mainly in the area of Dumutai. The rock type include the granodioritic gneisses (2523±19 Ma), the trondhjemitic gneisses(2511±9 Ma) and leucosomes(2521±26 Ma), they have been generated by partial melting of basalts underplated beneath continental crust. The zircon Lu-Hf isotope analysis of the rocks shows that most of zircons in the 2.85-2.70 Ga and 2.6-2.5Ga rocks consisting predominantly variable but positive ?Hf(t) values. We conclude that the magmatic events that occurred from2.9?2.7Ga and 2.6?2.6 Ga were very intense, and may represent the two marked episodes of continental crust growth events in the southern NCC.(3) Ca.2.3 Ga rocks include the granodioritic gneisses (2315±7.8 Ma), Amphibolites (2298±16 Ma) and K-rich granitic gneisses (2334±11 Ma ? 2359±29 Ma). An analysis of the petrogenesis showed that the rocks were generated by the partial melting of older crust materials and they were produced in the tectonic setting of crustal extension and thinning. According to the zircon Lu-Hf isotope analysis, we suggest that the ?2.3 Ga magmatism mainly reflect reworking of the ancient continental crust during Mesoarchean to Neoarchean and a small amount of crust growth in early paleoproterozoic.(4) Lu-Hf isotope analysis shows that the ?1.8 Ga magmatism represent mainly the reworking of old crust; the ca.1.8 Ga K-rich granitic gneisses were generated by the partial melting of prexistence older crust materials (TTQ metabasaltic or metasedimentary rocks) under low-level pressures (10-12 kbar; 753-800?) and the tectonic setting concerning the crustal extension and thinning.2. Research for Taihua complex in Lushan area gets the main achievements as follows:(1) Through the combined analysis of zircon U-Pb and Lu-Hf isotopes, we recognizated the ca.2.9 Ga TTG rocks occurred in the Xiaoqinling area. The rock type include the Tonalitic gneisses (2845±36Ma and 2888±34Ma), granodioritic gneisses (2902±11 Ma), the trondhjemitic gneisses (2872±19 Ma). The rock series experienced crystallization differentiation of a single parent magma and had a better affinity. They were generated by the partial melting of subducted oceanic crust in high-level pressures (20-25 kbar). In addition, Zircon Hf isotope analysis showed that the ?2.9 Ga magmatism reflect mainly crustal growth event in Lushan area.(2) The leucosomes in Lushan area show the form of narrow banded which parallel with TTG gneisses and Amphibolites. They were formed in the age of 2715±17Ma to 2758± 20Ma. An analysis of the petrogenesis showed that the rocks were generated by the partial melting of prexistence older crust materials, represented that the 2.9-2.8 Ga rocks have been underwent the anatexis occurred in 2.7 Ga. In addition, the K-rich granites formed in 2729±18 Ma, the rocks shows the feature of K-rich granites and they were generated by the partial melting of middle to upper crust materials under middle to low-level pressures (10-12 kba).3. Based on reviewed the published chronological, zircon Hf isotopic, and other geochemical dates in the southern NCC, we discussed crustal evolution features in the southern NCC. We get the mainly conclusion as follows:(1) The main crustal growth event in the southern NCC occurred in 2.9-2.7 Ga which can also be divided into two part. The former is represented for 2.90-2.8 Ga TTG gneisses in Lushan area. They were generated by the partial melting of subducted oceanic crust. The latter is represented for ?2.70 Ga TTG rocks where occurred in the area of Lushan, zhongtiao. The tectonic setting of those rocks concerning to be related to a worldwide superplume event occurred in the 2.9-2.7Ga.(2) 2.6?2.5 Ga magmatism in southern NCC mainly reflect reworking of the ancient continental crust during Mesoarchean to Neoarchean. However, there are also have a amount of juvenile crust addition in this time.(3) In early of Paleoproterozoic(2.3-1.8 Ga), the basement of southern NCC may have underwent a extension and thinning which caused crust reworking and led a large amount of granites intrude in this time. In addition, because of a mantle material addition, there also have a small amount of crust growth.(4) Based on the similarity of lithologic associations, metamorphic features, U-Pb-Hf isotopic compositions and dominant nearly NWW-SEE trending foliation of the orthogneisses in these areas, we suggest that the southern NCC at different locations underwent a similar crustal evolution history. It can be inferred that they share a common crystalline basement in Archean, where extending in a northwest-southeast direction for more than 1,000 km. However, due to the extensive reworking events during the Paleoproterozoic (2.45?1.8 Ga), only a small volume of Archean rocks were preserved. |
PDF Full Text Request