The Evolution Of Late Mesozoic Volcanic-magmatic System In The Offshore Of Eastern Shandong Province

Eastern Shandong province which located in the juncture of the North China Craton and Yangtze Craton,has experienced fierce magmatic activities in late Mesozoic.The formation and evolution of magmatic rocks,and their temporal and spacial distribution have relationship with the fault activities,evolution of basins and their deep-earth dynamic processes.With the studies of late Mesozoic igneous rocks in Wulian and Zhucheng,and previous work,the evolution of late Mesozoic igneous rocks and their dynamic background can be studied.The volcanic rock types and distribution,textural characteristics,distribution of terrane and lithofacies,faults and dykes should be researched clearly by detailed records and descriptions of crossing paths and cross sections.We found that Taolin volcanic edifice is a caldera,which developed Laiyang group,Qingshan group and many intermediate-acidic intrusive mass,and we modified this area's geologic map based on our field investigation.The characteristics of major elements,trace elements and rare earth elements of igneous rocks in Taolin area show that its magmatic rocks are calc-alkaline series,and have a wide range of SiO₂?47.79⁷7.14%?.The spider diagram shows that the large ion lithophile elements?LILE?such as Rb,K,Ba,Th and U are relatively enriched,while high field strength elements?HFSE?like Nb,P and Ti are relatively loss;and part of acidic rocks are characterized by Ba and Sr depletion.REE patterns shows that igneous rocks are characterized by enrichment of LREE and depletion of HREE and weak Eu negative anomalies;part of acidic rocks have low REE content and obvious Eu negative anomalies.This area's samples experienced weak alteration and obvious crystallization differentiation and partial melting trend,and it might experience fractional crystallization of olivine,clinopyroxene,Ti-Fe oxide,apatite and plagioclase.During the ascending process,the intermediate-basic rocks and part of acidic rocks might experience different degree of crustal hybridization.In addition,the different colour bandings,compositional bandings,enclaves and mineral zonings suggest that part of the igneous rocks have experienced process of magma mixing during their formation.The intermediate-basic rocks generated from the partial melting of enriched mantle which was metasomated by fluid derived from subducted lower crust and contains low magnesium hornblende,then experienced crystallization differentiation and crustal contamination;besides,part of them have been mixed with other types of magma.However,acidic magma may derive from two ways:the high-Sr acidic magma evolved from the intermediate-basic magma and the low-Sr magma generated from the partial melting of middle or lower crust.The age distribution of late Mesozoic volcanic rocks in eastern Shandong range from131Ma to 72Ma,and the volcanic rocks's ages of Qingshan group lie between 126.2⁹1.4Ma.The intermediate-basic volcanic activities become younger from north-east direction to south-west,i.e.the intermediate-basic volcanic rocks which located in areas adjacent to the Tan-Lu fault zone have more younger magmatic ages.However,the acidic volcanic rock in Lingshan island is much younger than that in Jiaozhou and Laiyang.The low-Sr acidic magma in Taolin shares similar characteristics with acidic volcanic rocks in Lingshan island and Huangdao district,suggesting that these acidic volcanic rocks are likely to generated from the same origin and experienced similar magmatic processes.The dating date and field investigation illustrate that the low-Sr acidic magmatic activities have an earlier ages,and it was caused by the heating of upwelling mantle.Part of the intermediate-basic magma experienced magmatic mixing,while some of them evolved to high-Sr acidic magma with the fractional crystallization.The magmatic activities are consistent with the extension and thinning of lithosphere,and may be impacted by slab tear and rollback of paleo-Pacific plate.