吉南白山地区早白垩世岩浆岩U-Pb年代学、岩石地球化学、Hf同位素证据:对华北克拉通破坏的制约

Evidences from zircon U-Pb geochronology,whole-rock geochemistry and Hf isotope of Early Cretaceous magmatic rocks in Baishan area,southern Jilin Province:Constraints on destruction of the North China Craton

对华北克拉通东北部早白垩世岩浆岩开展锆石U-Pb年代学、岩石地球化学及Hf同位素研究,能约束其岩石成因与构造背景,并为深入探讨晚中生代华北克拉通东北部岩石圈形成与演化提供依据。吉南白山地区的兴林花岗斑岩、红土崖安山质晶屑凝灰岩和江源含角砾流纹质岩屑晶屑凝灰岩的锆石为岩浆成因,结晶年龄分别为130±1Ma、128±1Ma和120±1Ma。兴林花岗斑岩和红土崖安山质晶屑凝灰岩具高SiO_(2)、Al_(2)O_(3)、Sr含量及高Na_(2)O/K_(2)O,Sr/Y和(La/Yb)_(N)比值,贫Y、Yb,富集LILE(如Ba、Th、U),亏损HFSE(Nb、Ti、P),具有微弱的负铕异常(δEu=0.82~0.94),具埃达克质岩石特征。兴林花岗斑岩富碱,低TiO_(2)含量,铝饱和指数A/CNK=1.06~1.14,表现为右倾型稀土配分模式,全岩锆饱和温度为798~815℃,属I型花岗岩。兴林花岗斑岩ε_(Hf)(t)介于-17.96~-0.19,两阶段模式年龄t_(DM2)=1197~2313Ma,指示兴林花岗斑岩源自古元古代-中元古代的加厚镁铁质下地壳部分熔融。红土崖安山质晶屑凝灰岩ε_(Hf)(t)介于3.37~5.47,两阶段模式年龄t_(DM2)=830~967Ma,暗示红土崖安山质晶屑凝灰岩是新元古代拆沉榴辉岩质下地壳部分熔融并交代地幔橄榄岩的产物。结合江源含角砾流纹质岩屑晶屑凝灰岩ε_(Hf)(t)(-2.69~1.00),两阶段模式年龄t_(DM2)=1115~1349Ma,可知吉南地区早白垩世岩浆岩ε_(Hf)(t)具有不均一性,岩浆源区存在多组分的特点,既有古老地壳,也存在新生地壳,指示多阶段地壳生长过程,暗示华北克拉通东北部陆壳在元古代演化过程中存在两次增生事件。综合研究表明,早白垩世古太平洋板块(伊泽奈崎)的俯冲回撤导致增厚的华北克拉通东北部地壳整体处于伸展的构造背景下,并自西向东发生拆沉减薄,拆沉下地壳的部分熔融形成区域广泛分布的埃达克质岩石。

This paper reports new geochemical data including zircon U-Pb ages with Hf isotopic data,whole-rock major and trace isotopic compositions for the Early Cretaceous magmatic rocks in the northeastern North China Craton(NCC),with the purpose of constraining its petrogenesis and tectonic setting.It also provides a basis for further discussion on the formation and evolution of the lithosphere in the northeastern NCC during the Late Mesozoic.Zircons in the Xinglin granite porphyry,Hongtuya andesitic crystal tuff and Jiangyuan rhyolitic breccia-bearing crystal-lithic tuff all show magmatic origin.Zircon U-Pb dating yield age are 130±1Ma,128±1Ma and 120±1Ma,respectively.The Xinglin granite porphyry and the Hongtuya andesitic crystal tuff have high content of SiO_(2),Al_(2)O_(3),Sr,high Na_(2)O/K_(2)O,Sr/Y and(La/Yb)_(N) ratios.In addition,they exhibit low Y and Yb.They are enriched in LILE(e.g.,Ba,Th and U),depleted in HFSE(e.g.,Nb,Ti and P)with weak negative Eu anomalies(δEu=0.82~0.94),showing they are typical of adakitic rocks.The Xinglin granite porphyry has high content of total-alkali,low content of TiO_(2),characterized by peraluminous-weakly peraluminous granite A/CNK=1.06~1.14,the zircon saturated temperature values of 798~815℃ and a distribution pattern of LREE enrichment with incline to the right,which indicates it is I-type granite.The zircon ε_(Hf)(t)values in the Xinglin granite porphyry ranged-17.96~-0.19,and the two-stage model ages yield t_(DM2)=1197~2313Ma,which imply that the Xinglin granite porphyry is derived from partial melting of the thickened mafic lower crust of Paleoproterozoic to Mesoproterozoic.The zircon ε_(Hf)(t)values in the Hongtuya andesitic crystal tuff ranged 3.37~5.47,and the two-stage model ages yield t_(DM2)=830~967Ma,indicating that the Hongtuya andesitic crystal tuff is a metasomatism product of the Neoproterozoic delaminated eclogite lower crust partial melting and mantle peridotite.Combined with the ε_(Hf)(t)values in the Jiangyuan rhyolitic breccia-bearing crystal-lithic tuff ranged-2.69~1.00,and the two-stage model ages yield t_(DM2)=1115~1349Ma,we conclude that the ε_(Hf)(t)values of magmatic rocks are heterogeneous in the southern Jilin Province,and the magmatic source area is characterized by multi-component origin,including both ancient crust and juvenile crust.We supposed that there were two accretion evolution events in the continental crust of the northeastern NCC during Proterozoic.Comprehensive studies have shown that subduction and slab roll-back of the Paleo-Pacific plate(Izanagi)led to the thickened the northeastern NCC being in an extensional tectonic setting during the Early Cretaceous,and the destruction of the northeastern NCC by delamination occurred spatially from west to east.The partial melting of delaminated lower continental crust resulted in a wide distribution of adakitic rocks.