蒙古国东南部巴音苏赫图晚古生代-早中生代二长花岗岩地球化学特征及其对构造环境的约束

摘要

Located between the northern margin of Erlian - Hegenshan suture zone and the pericontinental accretion zone of the southeastern margin of the Siberia plate, the Bayinsukhtu granitic body is an intrusive complex composed of monzonitic granites emplaced at Carboniferous and Triassic-Jurassic respectively. Geochemical analyses suggest that both of the two periods of monzonitic granites are characterized by high contents of silica, A12O3 and alkali, low contents of calcium and magnesium. They are enriched in LILE and depleted in HFSE, with moderately enrichment of LREE, weak fractionation of LREE from HREE and negative Eu anomalies (δEu = 0. 35 - 0. 66). Based on the petrologic and geochemical features, the Carboniferous monzonitic granite is classified as mainly high-K, calc-alkaline, metaluminous to weakly peraluminous syn-orogenic I-type granite, while the latter is A-type granite. Isotope geochemistry of the Carboniferous granite shows that values of εNd(i) and ( 87Sr/86Sr)i range from 0.9 to 1. 5, and from 0. 700 62 to 0. 704 82, respectively, which indicates that the granitic magma was likely derived from a young mantle source or an accretionary island arc with some degree of crustal contamination. However, the Triassic-Jurassic granite has lower εNd(t) but higher (87Sr/86Sr); values relative to the Carboniferous grdnite, ranging from -0. 3 to 0. 3 and from 0. 709 96 to 0. 710 19, respectively. This indicates the granite derived from the same source with relatively high degree of crustal contamination. The dating of zircons from the monzonitic granites determined by LA-ICP-MS yielded a weighted age of (296 ±3. 5) Ma, indicating that in the late Carboniferous the studied area was a syn-collision pericontinental tectonic environment. According to geochemical analysis of Late Triassic - Early Jurassic granite, it is presumed that the syn -collisional magmatism lasted till Triassic-Jurassic, probably suggestive of a long duration of collision.%巴音苏赫图二长花岗岩位于西伯利亚板块东南缘陆缘增生带与二连一贺根山板块对接带北缘之间.该岩体由石炭纪及三叠—侏罗纪两期花岗岩组成,均以高硅、富AI203、K2O、Na20,贫MgO、CaO为特征.微量元素表现为富集LILE、亏损HFSE.弱富集LREE,Eu负异常明显.岩石学和地球化学综合研究表明:两期花岗岩早期为高钾钙碱性、准铝质-过铝质花岗岩碰撞期I型花岗岩,晚期花岗岩具有A型花岗岩特点.Sr-Nd同位素研究表明:石炭纪二长花岗岩(87Sr/86Sr);比值较低(0.700 62 ～0.704 82),εNd(t)为正值(0.9～1.5),岩浆来自于增生的岛弧或年轻的幔源物质,在岩浆上升过程中遭受了少量地壳物质的混染；晚三叠世一早侏罗世花岗岩(87Sr/86Sr)i比值较高(0.709 96 ～0.710 19),εNd(t)较低,为-0.3～0.3,该期次花岗岩与晚石炭世花岗岩同源,受地壳混染程度相对较高.应用LA-ICP-MS法测得石炭纪花岗岩U-Pb年龄为(296±3.5)Ma,结合区域构造演化特点及所测三叠纪花岗岩的地球化学特征认为,在晚石炭世,南蒙古额尔德尼查干地区已进入碰撞期构造环境,在三叠-侏罗纪仍有同碰撞花岗岩侵入,晚石炭世以后的碰撞期持续时间较长.