The Early Cretaceous Volcanism In The North China Craton And Their Impacts On The Jehol Biota

The coupling relationship between the Earth’s deep（lithosphere,asthenosphere,etc.）and surface（atmosphere,hydrosphere,biosphere,etc.）systems is a cutting-edge topic in the scientific research.The remarkable spatiotemporal correlation between the North China craton（NCC）destruction and the Jehol Biota evolution in the Early Cretaceous provides a natural laboratory for studying this key issue.Volcanism provides an important mechanism for transferring matter（including volatiles and nutrient elements）and heat from the Earth’s interior to its surface.The NCC destruction was accompanied by intense magmatic activities.However,although previous studies have emphasized the impacts of volcanism on the Jehol Biota,most of them focused on the role of volcanic ashes or pyroclastic flows in mass mortality events and exceptional preservation of fossils.It remains enigmatic how the volcanic activities in different stages affected the Jehol Biota evolution（especially the flourishing of the Jehol Biota）through releasing volatiles（e.g.,CO₂,H₂O,S and halogen）and nutrients（e.g.,P,Fe）.In this thesis,we studied the contents,origin and total emissions of volatiles（S,Cl,F and H₂O）from the rhyolites of the Zhangjiakou Formation（～140-130 Ma;origin and development of the Jehol Biota）,the basalts and high-Mg andesites of the Lower Yixian Formation（～125 Ma;flourishing of the Jehol Biota）,the trachydacite of the Upper Yixian Formation（～125-124 Ma;decay of the Jehol Biota）and the Pingzhuang basalts（～106 Ma）in the NCC.Combined with bulk-rock P contents,we further explored the possible coupling mechanism between the Early Cretaceous volcanism and the Jehol Biota evolution in the NCC.Bulk-rock major and trace elements and Sr isotope compositions indicate that the Early Cretaceous basalts have been little affected by later alteration and crustal contamination.The fractional crystallization of olivine or clinopyroxene（Cpx）is the main process that controls the compositional evolution of the basalts.Based on the H₂O contents of Cpx（measured by Fourier Transform Infrared Spectroscopy,FTIR）,the estimated H₂O contents in the primary melt of the Yixian and Sihetun basalts are 0.46±0.16 wt.%and 2.59±0.56 wt.%,which fall within the ranges of ocean island basalts（OIBs）and island arc basalts（IABs）,respectively.We quantified the S,Cl and F contents in the Yixian and Sihetun basalts by combining the volatile contents in Cpx phenocrysts（measured by secondary ion mass spectrometry,SIMS）and apatite inclusions（measured by electron probe microanalyzer,EPMA）with experimentally determined partition/exchange coefficients between Cpx/apatite and melts.Our results show that the S,Cl and F contents are 94±85 ppm,553±285 ppm and 1589±232ppm in the primary melts of Yixian basalts,and 259±261 ppm,604±269 ppm and506±90 ppm in the primary melts of Sihetun basalts.The calculated S and Cl contents are low and fall within the ranges of mid-ocean-ridge basalts（MORBs）and OIBs.Fluorine contents in the Yixian basalts are within the range of back-arc basin basalts（BABBs）,and those in the Sihetun basalts are overlapped with the range of MORBs.Using the partial melting model,the mantle source of the Yixian and Sihetun basalts was estimated to have low S（2-191 ppm）and Cl contents（9-129 ppm）,with the S contents almost falling within the range of the MORB source（100-165 ppm）.Combined with the high Cpx oxygen isotope compositions（7.18‰-9.17‰）,bulk-rock⁸⁷Sr/⁸⁶Sr（t）and trace element ratios（Ce/Pb,Nb/La）,we consider the contributions of recycled sediments and fluids derived from the subducted slab in the mantle source at～125 Ma,in accord with the flourishing stage of the Jehol Biota.The calculated H₂O,S and Cl contents in the primary melts of the Pingzhuang basalts were 5.40±1.43 wt.%,1365±1164 ppm and 2636±387 ppm respectively,all of which fall in the range of IABs.The mantle source of the Pingzhuang basalts has high S contents（0-617 ppm）and Cl contents（25-202 ppm）.Combined with Ba/Th,Ce/Pb and Sr-Nd isotope compositions,it suggests that components from dehydrated altered upper oceanic crust and dehydrated sediments had contributed to the mantle source of the Pingzhang basalts.We interpret the maximum S,Cl and F concentrations to be the most representative pre-eruptive magmatic volatile contents.The pre-eruptive magmatic S,Cl and F contents are 3288 ppm,7915 ppm,and 3132 ppm for rhyolites in the Zhangjiakou Formation（pre-flourishing stage of the Jehol Biota）;1083-2370 ppm,1277-5608 ppm,and 893-3746 ppm for basalts and high-Mg andesites in the Lower Yixian Formation（flourishing stage of the Jehol Biota）;and 1991 ppm,12315 ppm,and 2450 ppm for the trachydacite in the Upper Yixian Formation（post-flourishing stage of the Jehol Biota）.Our results show that the volcanic rocks in the non-flourishing stages have higher S and Cl contents than those in the flourishing stage,but the difference of F contents in these three stages is not obvious.A‘petrological method’was used to calculate the total masses of volatiles released in the volcanic activities.The total masses of released S,Cl and F were estimated to be 19-766 Gt,17-1168 Gt,and 9-175 Gt in the pre-flourishing stage（duration of ca.10 Ma）;1-14 Gt,0.6-10 Gt,and 0.6-9 Gt in the flourishing stage（duration of<0.3 Ma）;and 2-32 Gt,4-127 Gt,and 1-10 Gt in the post-flourishing stage（duration of<1 Ma）.These data clearly show that the volcanic activity in the flourishing stage produced the lowest total emissions and/or average fluxes of S,Cl and F.Compared with other volcanic events,including large igneous provinces（LIPs）and Toba volcano（75 ky）,we find that the average fluxes of S,Cl and F in the flourishing stage are much lower than those in all LIPs due to the limited total S,Cl and F emissions,which are similar to those of Toba with limited impacts on the environment and ecosystem.In contrast,the volcanic eruptions in the non-flourishing stages show both elevated concentrations and total emissions of S,Cl and F,which can reach the amounts of the devastating Deccan or Emeishan LIPs.However,given the average volatile fluxes of the non-flourishing stages are much lower than those in all LIPs,we consider the volatile emissions in the non-flourishing stages may have catastrophic effects on the local environment and ecosystem.Lavas can release essential nutrients,such as P for life through weathering after their solidification.We compiled the bulk-rock major compositions of the Early Cretaceous volcanic rocks in the eastern NCC of different stages.The results show that the flourishing stage is dominated by intermediate-mafic volcanic rocks with higher P₂O₅ contents（median value of 0.50 wt.%）compared to non-flourishing stages（median values of 0.11 wt.%and 0.24 wt.%）,Toba（median value of 0.04 wt.%）and all LIPs（median values of 0.20-0.38 wt.%）.Moreover,the intermediate-mafic rocks were easily weathering and the warm and humid climate undoubtedly contributed to transporting P into the ecosystem in the flourishing stage.The enriched P in the flourishing stage was probably attributed to the deep-Earth material cycle associated with the NCC destruction.Our data suggest the regional climatic and environmental impacts induced by volatile degassing were likely negligible in the flourishing stage.The enhanced volcanogenic P flux in the soil and lake increased primary productivity and promoted the greatest species diversification.Our study speculates the impacts of craton destruction on the biota evolution driven by volcanism,which provides a unique case to reveal the coupling relationship between the deep geodynamic processes and Earth surface ecosystem.