Petrogenesis Of The Late Mesozoic Volcanic Rocks In The South Segment Of Tan-Lu Fault Zone

The Tan-Lu Fault Zone(TLFZ)is the largest fault zone in East China,and an ideal subject of studying the destruction of the North China Craton(NCC)and the subduction history of Paleo-Pacific plate.In addition to the analyses of structural deformation and basin sedimentation,voluminous Cretaceous intrusion/eruption in the middle-south segment of TLFZ and rifted basins controlled by TLFZ are another important point to discuss the aforementioned hot issues.This dissertation constrain the chronological,magma sources,and relationship between petrogenesis of volcanic rocks and activities of TLFZ through a combined zircon U-Pb dating,whole-rock major and trace elements,whole-rock Sr-Nd-Pb and zircon Lu-Hf-O isotopes from Chaohu-Lujiang area,and Chuzhou basin in south segment of TLFZ,eastern China.Besides,this study also constrain the property of Cretaceous lithospheric mantle beneath the south segment of TLFZ and provide the petrological evidences for the Cretaceous Paleo-Pacific subduction history and TLFZ activities.Volcanic rocks from Chuzhou basin are mainly composed of andesites.Zircon U-Pb dating indicated the forming age was 128 Ma.These rocks have high alkali contents(?= 2.34-4.97)and Na2O/K2 O ratios with variable Si O2 ranging from 60.21 to 66.20 wt%,and are characterized by enriched LREE and LILEs(e.g.,Ba,U and K)and depleted HREE and HFSEs(e.g.,Nb,Ta and Ti).All samples exhibit typical features of adakitic rocks,such as low Yb(0.70-0.92 ppm)and Y(8.84-11.5 ppm)contents,high Sr(944-1212 ppm),Sr/Y(80.0-113)and(La/Yb)N(23.8-29.7)ratios.Additionally,the moderately high Mg O(2.39-3.48 wt%)and Mg#(43.0-60.8)indicate the andesites from Chuzhou Basin belong to high-Mg adakitic rocks.They have enriched Sr-Nd isotopic compositions((87Sr/86Sr)i = 0.70564 to 0.70579,?Nd(t)=-15.7 to-14.0),similar to the low-Mg adakitic rocks derived from thickened lower crust of Yangtze block.The?Hf(t)values of samples 11CZ08 and 11CZ09 are respectively from-25.9 to-19.4 and from-32.6 to-27.4,the corresponding TDM2(Hf)of them are respectively from 2818 to2413 Ma and 3236 to 2916 Ma,these situ-zircon Hf isotopes share the similarities with the Outang adakites and the ore-barren adakites from the south segment of TLFZ.Zircons from these rocks show elevated Ce4+/Ce3+ratios of 38-1260,Eu/Eu* values ranging from 0.68 to 1.20 and ?18O values from 5.36 to 7.00‰.Calculted Ti-in-zircon temperatures range from 613 to 728 ?.These features are similar to adakites fromLower Yangtze River Belt,but different from adakites from the south segment of TLFZ,indicating relatively high oxygen fugacity but low temperatures in their magma sources.These data suggest that the high-Mg adakitic rocks from Chuzhou basin were highly probable formed by melts produced by partial melting of the thickened lower crust of Yangtze block with garnet(± rutile),subsequently reacted with high oxygen fugacity and hydrous magmas from melting of enriched mantle,which were formed under the subduction of the Paleo-Pacific plate.We also identify two-stage bimodal volcanic rocks based on their diverse volcanic assemblages and precise isotopic ages.The Early Cretaceous(125-121 Ma)volcanic rocks are dominated by aluminous fractionated A-type rhyolites and potassic basalts,whereas the Late Cretaceous volcanic rocks(97-83 Ma)range in composition from porphyritic andesites and trachydacites to rhyolites.There are about 24 Myr magmatic quiescence between these two-stage volcanic rocks.The Early Cretaceous rhyolites display strongly negative Nd-Hf isotopic compositions(?Nd(t)=-16.3 to-16.1,?Hf(t)=-24.1 to-17.9).The most likely source rocks of the fractionated aluminous A-type rhyolites are metaigneous rocks of the Feidong Complex.The Early Cretaceous potassic basalts(K2O/Na2 O = 1.5-3.4,?Nd(t)=-9.0)exhibit more enriched isotopic compositions than the Late Cretaceous sodic porphyritic andesites(K2O/Na2 O = 0.6-0.8,?Nd(t)=-5.6,?Hf(t)=-15.1 to-6.8),indicating derivation from heterogeneously enriched lithospheric mantle.This heterogeneity is ascribed to derivation from more fertile and isotopically enriched mantle sources with lower solidus temperatures.Modeling of selected elements reveals that the Late Cretaceous trachydacites(?Nd(t)=-12.2 to-8.2,?Hf(t)=-16.5 to-6.4)originated by mixing between coeval mafic and felsic magmas.The most negative(?Nd(t)=-18.8 to-17.1,?Hf(t)=-20.5 to-4.7)isotopic compositions,along with Neoproterozoic inherited zircons within the Late Cretaceous rhyolites,indicate derivation from lower crust of the Yangtze block.The Cretaceous two-stage mafic rocks have arc-like trace element patterns and enriched whole-rock Nd and zircon Hf-O isotopes,indicating that the lithospheric mantle beneath the STLZ was isotopically enriched during the Late Cretaceous(96 Ma).Although the Late Cretaceous sodic mafic rocks are less isotopically enriched than the Early Cretaceous potassic mafic rocks,we preclude the contribution of asthenospheric material to the later ones.This indicates that the lithospheric mantle in the south segment of TLFZ underwent a transformation between the Late Cretaceous and the Cenozoic,which is significantly latter than the time of coastal area of the eastern NCC(from Early Cretaceous to Late Cretaceous).Consequently,the lithosphere mantle beneath the TLFZ accelerated by the juvenile asthenospheric lithosphere,which was metasomatized by slab-derived melts during rollback of the Paleo-Pacific plate,may be limited.We suggest that the Early Cretaceous bimodal volcanic rocks formed in an extensional backarc setting induced by retreat of the subducting Izanagi slab.However,the TLFZ became a focus of Late Cretaceous bimodal volcanism due to weak far-field extension caused by the slow northward motion of the Paleo-Pacific Plate.Moreover,the high velocity and low angle of subduction of the Izanagi slab were the main driving mechanisms behind an Early Cretaceous transpressional event that separated the two stages of bimodal volcanism.