Petrogenesis And Geological Significance Of Early And Late Triassic Igneous Rocks In Guangxi And Guangdong

It is generally accepted that the South China Block experienced a tectonic regime transition from the Tethys to the paleo-Pacific tectonic domain during the Mesozoic,but the time frame for the transition and the initiation of paleo-Pacific subduction remains significantly controversial.In this paper,we selected the early Triassic granitic volcanic-subvolcanic-intrusive rock assemblage from the Qinzhou Bay area of Guangxi and the newly identified late Triassic(-early Jurassic)volcanic rocks from the Meizhou area of Guangdong(eastern Guangdong)as study objects,and conducted systematic petrographic,whole-rock geochemical,zircon U-Pb chronology and zircon Hf isotope studies,to explore the petrogenesis of magmatic rocks of different ages in different regions and their tectonic characteristicas.The main findings are as follows:The Banba formation rhyolites,the Jiuzhou granites and MMEs therein,and the Dashi-Taima granitic porphyries in the Qinzhou Bay area were formed from 247 ± 1to 245.7 ± 0.9 Ma during contemporaneous magmatic activity.The granitic rocks in this suite are characterized by high Si,K,Al-rich and Ca O and Mg O-poor,and are peraluminous;the trace element compositions show enrichment in the large ionophile elements such as Rb,K and Pb,and depletion in the high field strength elements such as Nb,Ta and Ti.All samples have similar and enriched zircon Lu-Hf isotopic compositions,with εHf(t)values ranging from-16.25 to-5.54 for the majority zircon grains,suggesting that they are mainly derived from remelting of Paleoproterozoic crustal basement materials.The similar geochemical composition of the different rock units reflects that they underwent similar magmatic evolutionary processes.The Songling formation intermediate-acid volcanic rocks were once recognized to be formed in early Jurassic.Zircon U-Pb dating results reveal that three volcanic samples,from the bottom layers of the formation,were formed at 204～200 Ma.Therefore,the continuous Late Mesozoic volcanism in South China can be bring temporally forward to late Triassic.All three volcanic samples show depleted zircon Hf isotopic compositions,with εHf(t)values varying between +9.2 to +5.7,+5.7 to+2.2 and +10.0 to +2.9,respectively.It suggests that the volcanic rocks were derived by partial melting of the Paleoproterozoic crustal basement,with obvious depleted mantle-derived involvement.Moreover,the depleted mantle-derived involvement became decreasing since late Triassic to middle Jurassic,and then became increasing over time.The early Triassic granitic rocks in the Qinzhou Bay area may have been formed by the decompressional melting of ancient peritectic crustal material due to the local tension of the South China Block in the tectonic environment of the paleo-Tethys subduction.The late Triassic volcanic rocks of South China identified in this paper are the result of early stage of paleo-Pacific subduction,suggesting that the onset of paleo-Pacific subduction was no later than the late Triassic.The early and late Triassic igneous rocks of the studied regions may correspond to the geological products of paleo-Tethys and paleo-Pacific subduction,respectively,during which a tectonic regime transition between the two tectonic domains occurred.