Search

The Lower Devonian Lianhuashan Formation is unconformly unlained by Cambrian strata due to Early Paleozoic Yunan orogeny and Kwangsian orogeny in Dayaoshan area of Guangxi. The new petrology and whole-rock geochemical data of Lower Devonian Lianhuashan Formation in Dayaoshan area were presented in order to constrain their provenance and sedimentary tectonic setting. The sandstones are composed of the predominated moderately sorted and rounded quartz and sedimentary lithic fragments, indicating a close detrital provenance. These samples have obviously high SiO2 contents ( 7 4. 19 % - 95. 35 % with an average of 85. 72%), low Al2 O3 contents 0. 23%-14. 73% with an average of 6. 48%), TiO2 contents (0. 09% -1. 05% with an average of 0. 38%) and Al2 O3 /SiO2 (0. 01- 0. 20). The sandstones have low REE contents ( (12. 48- 281. 10) X 10- 6 ) and are characterized by depletion in LILE, enrichments in HFSE and LREE ((La/ Yb)N is 3. 70- 29. 40) coupled with slightly negnative Eu anomaly (0. 66-1. 14 with an average of 0. 88). The high SiO2 /AlzO3 (5. 04-77. 28 with an average of 22. 30) implies the high grade of maturity. The wide ranges of ICV (0. 61- 2. 13 with an average of 1. 21) and high CIA ( 82. 13 - 91. 68) indicate that the source rocks contain recycled ancient sediments and first-cycle materials, which have experienced high degree of chemical weathering and alteration in the heat and humid climate environment of tropical or subtropical zone. Integrated with geochemistry, elastic zircon U-Pb geochronology, paleogeography and tectonic evolution, it is suggested that Lower Devonian Lianhuashan Formation is deposited in passive margin, and its provenance may be Dayaoshan-Damingshan area in the north, and Yunkai area in the south. The source rocks of Lower Devonian Lianhuashan Formation mainly comprise Cambrian Huangdongkou Formation, and minor Neoproterozoic sedimentary rocks of Cathaysia block and the granitoids formed in Kwangsian orogeny (470-435 Ma). [ABSTRACT FROM AUTHOR]

The discriminant indexes, petrogenesis and tectonic significance of unfractionated A-type granites have been widely recognized, while there has not been a universal consensus on the discriminant criterion of highly fractionated A-type granites. LA-ICP-MS zircon U-Pb dating shows that Dajin granites form at Early Paleozoic(～440 Ma), and have high Rb contents(>400×10-6), Rb/Sr and Rb/Ba values, and relatively low Zr/Hf values, illustrating that they are highly fractionated. According to the high zircon saturation temperature(819 ℃-863 ℃), and high Ga/Al(2.6-3.1), Zr+Nb+Ce+Y contents and the evolution trend of specific geochemical indexes, Dajin granites are comprehensively determined as highly fractionated A-type granites. Combining with geochemical and Nd-Hf isotopic compositions, it is suggested that Dajin granites are mostly likely generated by partial melting of the mafic lower crustal rocks, and experienced high degree fractional crystallization and contamination of a samall amount of the wall rock materials. As consequence to post-orogenic lithospheric delamination, the forming time of Dajin A-type granites represent the timing of geodynamic transition of Kwangsian orogeny from orogenic compression to post-orogenic extension in the northeastern Guangxi. [ABSTRACT FROM AUTHOR]