内蒙古查干敖包三叠纪碱性石英闪长岩的地球化学特征及成因

Geochemical characteristics and genesis of Triassic Chagan Obo alkaline quartz diorites in Inner Mongolia

近年来在西伯利亚板块南缘发现多处三叠纪碱性花岗岩,构成了一个碱性花岗岩带。产出在该碱性花岗岩带上的查干敖包石英闪长岩岩体,SHRIMP年龄为237Ma。本文通过元素地球化学和同位素地球化学分析,结果显示SiO2含量60.70%～62.67%,平均值61.76%;K2O+Na2O为9.18%～10.48%,平均值9.74%,且Na2O>K2O。这些岩石具有REE总量为(236.3～260.0)×10-6,平均246.5×10-6,右斜式稀土配分模式,LREE/HREE为18.91～20.11,平均19.48,显示微弱的Eu负异常的特征。它们也表现高的Sr和Ba含量、低的Y含量,其中Sr为1216×10-6～2028×10-6,平均为1707×10-6,Ba为1597×10-6～1947×10-6,平均为1717×10-6。Y为12.9×10-6～16.5×10-6,平均为15.3×10-6。206Pb/204Pb比值为18.172～18.529,平均值为18.314,207Pb/204Pb值为15.465～15.529,平均值为15.503,208Pb/204Pb值为37.831～38.120,平均值为38.016;初始锶比较集中,变化于0.70405～0.70411,平均0.70408;147Sm/144Nd的比值为0.0671～0.0679,平均0.06747;143Nd/144Nd的比值变化范围为0.512605～0.512631,平均值为0.512619。εNd(t)均为正值,变化范围为3.3～3.8,平均3.5。Pb、Sr、Nd同位素特征均显示地幔来源的特征。这些地球化学和同位素特征值指示查干敖包石英闪长岩属于碱性岩类,源于西伯利亚板块与中朝板块碰撞后,由残留的古老俯冲洋壳部分熔融并经过富钾的基性幔源物质污染而成,与中朝板块北缘的三叠纪碱性岩带属同一期次岩浆活动的产物。该成果为西伯利亚板块与中朝板块碰撞闭合的时限提供了新证据。

Many Triassic alkaline granites have been discovered in southern Siberian plate margin in recent years,and these alkaline granites construct an alkaline granite belt.The Chagan Obo quartz diorite is an important intrusion in this alkaline granite belt.Its formation age was determined as 237Ma using SHRIMP U-Pb method.These alkaline quartz diorites in this intrusion display their SiO2 contents of 60.70% to 62.67% with an average value of 61.76%,K2O＋Na2O contents of 9.18% to 10.48% with an average value of 9.74%,and Na2OK2O.These rocks are characterized by total rare earth concentrations from 236.3×10-6 to 260.0×10-6 with an average of 246.5×10-6 and right-decline REE distribution models with LREE/HREE ratios of 18.91 to 20.11 with an average value of 19.48 and slightly negative Eu anomaly.Similarly,they are also characterized by high Sr and Ba but low Y,exhibit Sr,Ba and Y concentrations of 1216×10-6 to 2028×10-6（average value of 1707×10-6） ,1597×10-6 to 1947×10-6（average value of 1717×10-6） ,and 12.9×10-6 to 16.5×10-6（average value of 15.3×10-6） ,respectively.These geochemical features suggest they belong to adackite-like lithological assemblage.The quartz diorite lithological assemblage shows higher 206Pb/204Pb（from 18.172 to 18.529 with an average value of 18.314） ,208Pb/204Pb（from 37.831 to 38.120 with an average value of 38.016） ,and εNd（t） （from 3.3 to 3.8 with an average value of 3.5） but low 207Pb/204Pb（from 15.465 to 15.529 with an average value of 15.503） and initial 87Sr/86Sr values（from 0.70405 to 0.70411 with an average value of 0.70408） ,which suggest that the original magma of the quartz diorites was derived from partial melting of residual subducted oceanic slabs after collision between Siberia and China-Korean plates.This original magma was contaminated by ultramafic and K-enriched alkaline fluids.Therefore,these quartz diorites are synchronous products of Triassic alkaline granitoid magmatism in northern margin of Sino-Korean plate.The paper provides new evidence for the end time of finally collision between Siberian and Sino-Korean plates.