西准噶尔北部晚古生代两期侵入岩的地球化学、Sr-Nd同位素特征及其地质意义

Geochemistry,Sr-Nd isotopes and tectonic implications of two generations of Late Paleozoic plutons in northern West Junggar,Northwest China

西准噶尔北部发育构造环境截然不同的晚古生代两期侵入岩体,它们的空间分布与地球化学特征存在明显差异。早石炭世侵入岩包括辉石闪长岩、二长闪长岩、花岗闪长岩、二长花岗岩和钾长花岗岩,仅分布于晚古生代扎尔玛-萨吾尔岩浆弧内,可能与额尔齐斯-斋桑洋的南向俯冲有关。它们的A/CNK和A/NK比值变化范围分别为0.86～1.03和1.24～1.86,属于偏铝质-弱过铝质岩石。Al2O3、CaO、Na2O含量较高,Na2O/K2O比值普遍大于1.2;LREE、HREE分馏明显,(La/Lu)N=4.50～26.03,无明显的Eu异常;富集大离子亲石元素(K、Rb、Ba)、Pb和Sr,亏损Nb、Ta、P和Ti,具有I型花岗岩的特征。早石炭世侵入岩的Sr同位素初始比值(ISr)为0.70408～0.70912,εNd(t)值为+6.09～+7.25,单阶段Nd模式年龄(tDM1)为527～593Ma。元素和同位素地球化学特征表明早石炭世岩体(不包括阿布都拉二长花岗岩)可能是受交代的亏损地幔部分熔融并发生岩浆混合之后再经过结晶分异形成的,而阿布都拉二长花岗岩可能来源于洋壳的部分熔融。晚石炭世-中二叠世侵入岩以钾长花岗岩为主,遍布西准噶尔北部及相邻的各个构造单元,是后碰撞岩浆活动的产物。它们的A/CNK和A/NK比值变化范围分别为0.92～0.99和1.03～1.27,属于偏铝质岩石。Al2O3、MgO、CaO的含量很低,全碱含量很高(8.6%～9.7%),Na2O/K2O比值为0.9～1.1;REE总量很高(平均值为196×10-6),LREE、HREE分镏程度较弱,(La/Lu)N=3.32～5.36,Eu负异常明显;强烈亏损Ba、Sr、P、Ti,中度亏损Nb和Ta,具有A型花岗岩的特征。εNd(t)值为+5.26～+7.26,单阶段Nd模式年龄(tDM1)为502～655Ma。元素和同位素地球化学特征表明晚石炭世-中二叠世A型花岗岩体可能是由被流体交代的亏损地幔熔融生成的玄武质岩浆在上升过程中(可能发生岩浆混合)发生高度结晶分异之后的产物,是后碰撞环境下的板片断离导致软流圈物质上涌的结果。

Two generations of Late Paleozoic plutons,Early Carboniferous subduction-related plutons and Late Carboniferous-Middle Permian post-collisional A-type granites,were recently recognized in northern West Junggar,Xinjiang,and they are distinguishable by their spatial distribution and geochemistry.Early Carboniferous plutons include diorites,granodiorites,and monzonitic and K-feldspar granites,and they are exclusively restricted within the Late Paleozoic Zharma-Saur arc.The formation of these plutons may be related to the southward subduction of the Irtysh-Zaysan Ocean between Kazakhstan in the south and Altai in the north.Geochemically,they are metaluminous to weak peralunimous,with A/CNK and A/NK ratios of 0.86 ～ 1.03 and 1.24 ～ 1.86,respectively,and are characterized by relative high Al2O3,CaO,Na2O and Na2O/K2O(> 1.2).They show strong fractionations between LREE and HREE((La/Lu) N =4.50 ～26.03),with negligible Eu anomalies,and enrichment in LILE(K,Rb,Ba),Pb and Sr and depletion in Nb,Ta,P and Ti.All they are typical of I-type granitoids.The Early Carboniferous plutons show a wide range of initial Sr isotopic ratios(ISr =0.70408 ～0.70912),relatively constant εNd(t) values(+6.09 ～ +7.25) and young Nd model ages(tDM1 =527 ～593 Ma).All these data suggest that Early Carboniferous plutons,except for the Abudula monzonitic granite,were probably formed by fractional crystallization of mantle-derived magma which was generated by partial melting of the metasomatized depleted mantle and underwent a magma mixing process.Additionally,the Abudula monzonitic granite was likely to be derived from partial melting of subducted slab.By contrast,Late Carboniferous-Middle Permian plutons,mainly K-feldspar granites,are widespread across all the tectonic units in northern West Junggar and adjacent areas,and they were the products of post-collisional magmatism.These plutons are metaluminous and have A/CNK and A/NK ratios of 0.92 ～ 0.99 and 1.03 ～ 1.27,respectively,low Al2O3,MgO,CaO,moderate Na2O/K2O ratios(0.9 ～1.1),high total alkali(8.6% ～9.7%) and REE concentrations(average 196 ×10-6),weak fractionations between LREE and HREE((La/Lu) N = 3.32 ～ 5.36) with striking negative Eu anomalies,significant depletion in Ba,Sr,P,Ti,Nb and Ta.All these are consistent with those of A-type granites.Their εNd(t) values(+ 5.26 ～ + 7.26) and young Nd model ages(tDM1 = 502 ～ 655Ma) indicate that Late Carboniferous-Middle Permian A-type granites might be produced by highly fractional crystallization of ascending basaltic magma,which was possibly originated from fluid-metasomatized depleted mantle and mixed with juvenile crustal melts,as a result of slab break-off in a post-collisional setting.