东昆仑夏日哈木矿区早泥盆世正长花岗岩锆石U-Pb年代学、地球化学及其动力学意义

Zircon U-Pb Geochronology and Geochemistry of the Early Devonian Syenogranite in the Xiarihamu Ore District from East Kunlun, with Implications for the Geodynamic Setting

夏日哈木矿区的正长花岗岩体呈岩株状出露于矿区北部。年代学研究表明,正长花岗岩中岩浆锆石LA-MC-ICP-MS U-Pb加权平均年龄为391.1±1.4Ma,MSWD=0.06,属早泥盆世。岩石学及化学成分显示其属于高钾钙碱性、弱过铝质系列花岗岩。岩体高硅(SiO2=75.55%~76.10%)、富碱(Na2O+K2O=8.23%~8.46%)、高铁镁比(FeOT/MgO=17.40~42.59)、贫钙(CaO=0.54%~0.69%)、贫镁(MgO=0.03%~0.09%);稀土配分曲线呈现"海鸥式"分布特征,显示强烈的Eu负异常(δEu=0.09~0.12);微量元素特征显示具有较高的Ga(24.3×10–6~26.9×10–6)、Zr(132×10–6~363×10–6)和Y(86.1×10–6~97.0×10–6)含量,较低的Sr(8.6×10–6~19.5×10–6)、Ba(14.0×10–6~37.9×10–6)含量,在微量元素原始地幔标准化蛛网图上显示明显的Sr、Ba、P、Eu和Ti的负异常。以上特征表明夏日哈木矿区正长花岗岩为铝质A型花岗岩。岩石具有高的Rb/Sr(介于14.97~38.26,平均值22.63)和Rb/Nb(介于13.84~16.13,平均值14.54),显示出壳源岩浆的成分特征。综合分析表明本区正长花岗岩为低压下长英质地壳部分熔融的产物。结合区域构造演化及构造判别,本文认为该区正长花岗岩形成于造山后伸展的构造环境。在晚志留世-早泥盆世,东昆仑地区构造体制经历了重要的转变,由挤压体制转变为造山后软流圈上涌、岩石圈减薄及地壳伸展。在伸展体制下,软流圈地幔上涌导致上覆长英质地壳直接部分熔融,形成本区A型正长花岗岩。

In this paper, we discuss the zircon U-Pb age, major and trace element geochemistry of the syenogranite intrusion in the north of the Xiarihamu ore district. Zircon U-Pb dating demonstrates that the weighted mean 2~6pb/238U age for the zircons from the syenogranite is 391.1＋1.4 Ma （MSWD = 0.06）, i.e., the Early Devonian. The syenogranite is slightly peraluminous and belongs to the high-K calc-alkaline series. The syenogranite is characterized by high SiOz （75.55%~76.10%）, high alkali （Na20＋K20=8.23%~8.46%）, high FeOT/MgO （FeOT/MgO=17.40N42.59）, low CaO （0.54%-0.69%） and MgO （0.03%-0.09%） contents. The syenogranite is characterized by a ＂sea-gull＂ REE pattern and significant negative Eu anomaly （~Eu=0.09-0.12）, as well as enrichment of Ga （24.3x 10^-26.9~ 10-6）, Zr （132.0~ 104 -363.0~10-6） and Y （86.1x10-6-97.0x10-6）, and depletion of Sr （8.6x10-6-19.5x10-6） and Ba （14.0~10-6-37.9~10-6）. And thus, the syenogranite belongs to aluminous A-type granite. The high Rb/Sr （ranging from 14.97 to 38.26, and the average value is 22.63） and Rb/Nb ratios （ranging from 13.84 to 16.13, and the average value is 14.54） of the rock indicate a crustal origin. Combined with the regional tectonic evolution and geochemical discrimination, we suggest that the syenogranite was formed in a post-orogenic setting. During the Late Silurian-Early Devonian, the tectonic regime in the East Kunlun area changed from compression to extension due to asthenosphere upwelling. Under the extensional setting, the upwelling asthenosphere provided enhanced heat and triggered the partial melting of the overlying felsic crust, and produced the A-type syenogranite.