新疆巴里坤地区志留纪花岗岩的确定及其地质意义

Determination of Silurian granitic plutons in the Balikun area,Xinjiang and its implications

准噶尔盆地东部巴里坤地区怪石山二长花岗岩体和县煤矿北花岗闪长岩体分别获得440.6±3.7 Ma和412.8±3.3 Ma的LA-ICP-MS锆石U-Pb形成年龄,指示该区早志留世-早泥盆世一期重要的花岗岩浆活动。两岩体为准铝-弱过铝钙碱性及高钾钙碱性岩系,微量元素具有富集LILE、贫HFSE,亏损Nb、Ta、Ti、P以及Pb正异常的岛弧岩浆成因特征。其中,怪石山岩体高Si、富K,高Rb/Sr和Ga/A1比值、低K/Rb比值,强Eu负异常,代表其为岛弧演化较为成熟阶段岩浆演化的产物;县煤矿北二长花岗岩富Na、高Sr和Sr/Y比值,低Y和Yb,则指示其为岛弧演化晚期,洋盆俯冲消减闭合过程、岛弧区地壳增厚环境下部陆壳物质熔融的结果。此外,两个岩体正的锆石ε_(Hf)(t)(+7.64^+13.86)和老于其形成年龄的Hf二阶段模式年龄(T_(DM2)=1123~561 Ma)证明,它们主要源自新元古代晚期到早古生代早期的新生岛弧陆壳物质。

The Guaishishan to the north of coal mine in Balikun area, east Jungger, yield formation ages of 440.6±3.7 Ma and 412.8±3.3 Ma,respectively, indicating that there was an important granitic magmatism during the Early Silurian to Early Devonian. Both plutons are metaluminous-peraluminous and cale-alkalic series. They are characterized by the affinity of island arc magmatism with the enrichment of LILE, poor HFSE, and depletion of Nb, Ta, Ti, P as well as positive Pb anomaly. Of which,the Guaishishan pluton shows high contents of silica and potassium,and high Rb/Sr,low K/Rb ratios and obvious negative Eu anomaly, resulting from the high differentiation granodiorite formed in the stage of mature island arc. Whereas,the granodiorite pluton to the north of coal mine are enriched in Na and Sr with high Sr/Y ratio and low Y and Yb contents, evidently demonstrating that it was formed by the partial melting of lower continental crust during the thickened-crust process due to the oceanic subduetion during the closure of oceanic basin in the later stage of island arc. In addition, the two plutons have positive εHf（t）（ ＋7.64～ ±13.86）and TDM2 ages of 1123 -561 Ma slightly older than their formation ages,which prove that they were products derived by the partial melting of juvenile island arc materials formed from the late Neoproterozoic to the early stage of Early Paleozoie.