The Petrogensis And Tectonic Significance Of The Early Paleozoic And Late Mesozoic Volcanics From The Lhasa Terrane,South Tibet

The Lhasa terrane,located in southern Tibet,underwent not only the collisional orogeny between India and Asia during the Cenozoic,but also long-term and complex tectonic evolution during the pre-Cenozoic times.In this PhD thesis,we present a petrological,geochemical and geochronological study of the early Paleozoic acidic volcanics and late Mesozoic mafic-felsic volcanics from the northern segment of the Lhasa terrane.The obtained results reveal the origin and tectonic mechanism of these volcanics,and provide new insights into the tectonic evolution of the Gondwana supercontinent and the Tibetan Plateau.The early Paleozoic volcanic rocks are rhyolite and rhyolitic ignimbrite.They have varying but high SiO2(69.07-82.09 wt.%),A12O3(8.27-16.61 wt.%),TFe2O3(0.54-7.9 wt.%)and Zr(127-735ppm)contents,hgih K2O(4.19-9.15 wt.%)and very low Na2O 0.01-0.08 wt.%),belonging to ultrapotassic and peraluminous(A/CNK=1.08-2.22)rocks.They are enriched in Rb,Th,U,K,Pb,Zr and Hf,and show negative Ba,Nb,Ta,Sr,P and Ti anomalies,and slightly fractionated REE patterns with distinct negative Eu anomalies(0.41-0.58),typical of A-type granitoids.LA-ICP-MS U-Pb zircon dating implys that the early Paleozoic rocks have crystallization ages of 487-522 Ma.These zircons show variable sHf(t)values ranging from-10.2 to+1.6,and corresponding to two stage Hf model ages of 2099-1358 Ma.Combining with avaiable data,we conclude that the early Paleozoic ultrapotassic rhyolitic rocks were derived from partial melting of pelites under high-temperature to ultrahigh-temperature conditions in an extensional environment of active magmatic arc,and subsequently experienced mixing with mantle-derived magma and fractional crystallization.The Andean-type orogeny was probably related to the subduction of the proto-Tethyan oceanic slab beneath the northern margin of Gondwana supercontinent during the early Paleozoic.The late Mesozoic volcanic rocks include andesitic basalt,trachydacite and rhyolite.The andesitic basalts show calc-alkaline signature,and have low SiO2(51.42 wt.%),Cr(87.4 ppm),Ni(38.5 ppm)and Mg#(57).They are enriched in Th,U and Pb,and have negative Nb and Ta anomalies,comparable to arc-type basalts.The trachydacites and rhyolites have SiO2 of 62.56-67.92 wt.%and 75.19-77.87 wt.%,and Al2O3 of 14.66-15.13 wt.%and 11.89-13.13 wt.%,respectively.They have high K2O(3.15-4.79 wt.%and 3.93-5.26 wt.%,respectively)and Zr(365-567 ppm and 177-206 ppm,respectively)contents,and 10000Ga/Al(2.39-2.52 and 2.67-3.41,respectively)values.Almost all trachydacites and rhyolites are ferroan and calc-alkalic-alkalic,and enriched in large ion lithophile elements(LILEs,such as Th,U and Pb)and high field strength elements(HFSEs,such as Zr)and have negative Nb,Ta,P and Ti anomalies,typical of A-type granitoids.In addition,the rhyolites are characterized by high differentiation index(DI=96-98)and display strongly negative Eu anomalies(?Eu=0.02-0.08),indicating that they are highly fractionated A-type rhyolites.Zircons LA-ICP-MS dating indicates that trachydacite and rhyolite have crystallization ages of 110 Ma and 109 Ma,respectively.These zircons show ?Hf(t)values in the range of-10.2 to+3.7 and-8.7 to+6.7,corresponding to two stage Hf model ages of 1799-923 Ma and 1702-708 Ma,respectively.These features show that the andesitic basalts were most likely derived from moderate-degree(-20%)partial melting of a metasomatized lithospheric mantle source,and experienced orthopyroxene-dominated fractionated crystallization.The trachydacites and rhyolites were generated by mixing of ancient basement and mantle-derived melts.The magma of the rhyolites experienced significant fractional crystallization.The late Mesozoic volcanic rocks formed in an extensional regime during Andean-type orogeny,resulting from the slab roll-back and break-off and lithospheric delamination during the northward subduction of the Neo-Tethyan Ocean lithosphere along the southern margin of the Lhasa terrane or the slab break-off during the southward subduction of the Bangong-Nujiang Ocean lithosphere along the northern margin of the Lhasa terrane.