甘肃北山野马大泉志留纪花岗岩的成因和构造意义

马鬃山地体是北山造山带的主要构造单元之一,其广泛出露的花岗岩对于认识和完善北山古生代构造格架具有重要意义.对野马大泉一带的花岗岩进行LA-ICP-MS锆石U-Pb定年,获得了早-中志留世(441±4 Ma和425±2 Ma)的年龄.岩石地球化学显示它们均富钠、钙碱性和镁质花岗岩的特征,小于A型花岗岩的Ga/Al×10⁴值和Zr+Nb+Ce+Y值,且含角闪石二长花岗岩出现标志性矿物角闪石,属于Ⅰ型花岗岩.它们具有正的ε_Hf(t)值(-0.1～+7.6),两阶段Hf模式年龄集中于1.42～0.94 Ga,反映花岗质岩浆可能起源于中元古代(～1.4 Ga)地壳的部分熔融,并伴有幔源岩浆的参与.马鬃山地体志留纪花岗岩的地球化学显示形成于大陆弧的构造环境,且结合区域古生代花岗岩的时空分布指示相关的俯冲带在早石炭世发生北向后撤.

Petrogenesis and Tectonic Significance of Silurian Granites in Yemadaquan Area,Beishan, Gansu Province

The Mazongshan terrane is one of the main units of the Beishan orogenic collage. Among which there are abundant plutons, and they are of great significance for understanding and perfecting the Paleozoic tectonic framework of the Beishan orogenic collage. In this study, LA-ICP-MS zircon U-Pb dating yields the 206Pb/238U ages of 441±4 Ma and 425±2 Ma for the biotite monzogranite and hornblende monzogranite in Yemadaquan area respectively, which are the Early and Middle Silurian. The geochemical characteristics of the rocks show that they are sodium-rich, calc-alkalic, and magnesian. And their Ga/Al×104 and Zr+Nb+Ce+Y values are less than those of the A-type granite. Especially diagnostic mineral amphibole appears in hornblende monzogranite, meaning they belong to I-type granite. The ε_Hf(t) values of their zircons are positive, and the crustal Hf model ages are concentrated at 0.94-1.42 Ga, suggesting that the Early-Middle Silurian granitic magmas may be derived from the partial melting of the Mesoproterozoic (~1.4 Ga) crust, with the participation of mantle-derived magmas. The analyses from geochemistry and regional data show that the granites were formed in the tectonic setting of continental arc, and the spatial and temporal distribution of Paleozoic granites suggests that slab roll-back might occur in the Early Carboniferous.