新疆准噶尔北东缘造山后伸展及陆壳生长：来自哈旦逊杂岩体的岩石学及地球化学的证据

Postorogenic Extension and Continental Growth of the Northeastern Margin of the Juggar:Evidences from Petrography and Geochemistry of the Hadansun Intrusive Complex

哈旦逊岩体位于准噶尔北东缘玛因鄂博断裂以南,受断裂构造控制,呈近三角形。该岩体岩石类型较复杂,主要岩石组合为石英二长岩、二长岩、辉长岩、闪长岩、二长花岗岩、钾长花岗岩等。二长岩中锆石SHRIMPU-Pb测年结果为289.5±3.6Ma。除个别辉长岩样品为钙碱性系列外,其它所有样品属于碱性系列。辉长岩的稀土总量(∑REE)为43×10-6～88×10-6,Eu异常不明显,轻稀土富集(LaN/YbN=2～6),微量元素含量变化趋势一致且富集大离子元素,Cr、Ni含量低。中性岩的∑REE大于100×10-6,轻稀土元素表现出不同程度的富集(LaN/YbN=6～20),具有不明显或是弱的Eu正异常。微量元素含量变化大,但总体变化趋势一致,即表现出大离子亲石元素(LILE)富集而高场强元素(HFSE)亏损。花岗岩的稀土元素组成可以分两类,一类具有中等到弱的Eu负异常(δEu=0.7～1.0),轻稀土强烈富集(LaN/YbN=12～30),其微量元素的原始地幔标准化配分模式与中性岩近于一致;另一类具有显著的Eu负异常(δEu=0.1～0.3),轻重稀土的分异不明显(LaN/YbN=3～8)。在微量元素配分模式中,有显著的Ba、Sr、P、Ti的亏损。不同岩石类型显示出Sr初始值异常低(0.7036～0.7039)和正的εNd(t)初始值(～6.5)。综合野外观察、岩石学及地球化学特征,推测辉长岩部分来自被俯冲带流体交代的亏损岩石圈地幔,而花岗岩部分来自年轻的基性下地壳重熔,熔融发生的深度大约在30～40km,使其中的部分酸性岩浆出现典型的高Ba-Sr花岗岩的特征。基性岩浆和酸性岩浆在上升过程中发生混合,闪长岩及二长岩可能是它们混合的产物。哈旦逊岩体形成于造山后伸展构造背景,壳幔岩浆的混合可能是造成新生陆壳生长的一种重要方式。

The Hadansun intrusive complex is situated at the south of the Mayinebo fault in the northeastern margin of the Junggar.It takes a form of triangle which is possibly contributed to faults.The main rocks of the complex are complicated,including gabbro,monzonite,quartz monzonite,diorite,monzonitic granite and potassic granite.SHRIMP U-Pb dating of zircon from monzonite indicates a crystallized age of 289.5±3.6Ma.Except several gabbro samples,the other samples are alkaline.The gabbro has a total REE content ranging from 43×10-6 to 86×10-6,and enrichment of LREE（LaN/YbN=2～6）,with no distinct Eu anomalies.The contents of trace elements change consistently and LILE is also enriched with low Cr and Ni.The intermediate rocks have ∑REE higher than 100×10-6 and variable enrichment of LREE,and no Eu anomalies.The total REE content of the intermediate rock is more than 100×10-6,with various enrichment of LREE（LaN/YbN=6～20）and no distinct or weak Eu anomies.The intermediate rocks contain variable contents of trace elements,but generally have a similar variation trend such as systematic enrichment of LILE and depletion of HFSE.According to their REE characteristics,the granite samples can be subdivided in to two groups： one group,which is of weak Eu anomalies and highly enriched in LREE,shares similar trace elements distribution patterns with those of the intermediate rocks the other group have significantly negative Eu anomalies（δEu=0.1～0.3）,slightly enrichment of LREE and significant Ba,Sr,P,and Ti depletions on the trace elements distribution patterns.All the rocks from the Hadansun complex have very low initial Sr values（0.7036～0.7039） and coherent positive initial Nd values（～6.5）.Combined with field observations,geochemical and lithological characteristics suggest that the gabbro may be derived from depleted mantle which was possibly metamorphosed by the subduction-released fluid.The granite was derived from partial melting of the juvenile mafic lower crust at depth of 30～40 km,which resulted in that part of acid magma show natures of high Ba and Sr granite.Mixing of basic magma and acidic magma during moving up resulted in the formation of the intermediate rocks including diorite and monzonite.The Hardansun complex was formed at the post-collision tectonic setting.Magma mixing between the mantle-derived and crust-derived members was possibly of great importance to the Phanerozoic crustal growth.