天山石炭-二叠纪后碰撞花岗质岩石地球化学研究

Geochemistry of the Carboniferous-Permian post-collisional granitic rocks from Tianshan

在对前人研究资料综合的基础上,重点对天山造山带不同构造单元中石炭—二叠纪后碰撞花岗岩地球化学特征进行了对比研究,认为天山石炭—二叠纪后碰撞花岗质岩石属高钾钙碱—钾玄岩系列,以钙碱系花岗质岩石类型为主。其中,发育在裂谷带中的石炭—二叠纪花岗质岩石较发育在古微地块中或其边缘的花岗质岩石富MgO、TiO2、Na2O,贫K2O、Nb、Th,两者具有不同的源区。裂谷带中的花岗质岩石多与火山岩同源,由含不同程度壳源组分的壳幔混合源岩浆形成,而古微地块花岗质岩石主要来自壳源。处于同一类构造位置但不同地区的花岗质岩石化学成分也不相同,如天山西端伊犁地块南缘的托木尔峰花岗质岩石较天山中东段南部的库鲁克塔格花岗质岩石富Fe、Mg、K、Nb、Sr,而贫Zr、Ba,它们均是壳源成因,反映了天山东、西地壳基底组分的不同。石炭—二叠纪裂谷带中的花岗质岩石在天山东、西段也有差别,东段的觉罗塔格和博格达花岗质岩石较天山西段伊犁裂谷中的花岗质岩石明显地富Nb、Th而贫Sr、Ba,反映出东、西天山石炭—二叠世花岗岩源区不同。花岗质岩石的正εNd值并不是来源于与基性、超基性岩浆活动有关的幔源组分,而是来自于幔源火山岩浆活动。

The geochemical study of Carboniferous-Permian post-collisional granitic rocks from different tectonic units in Tianshan reveals that they belong to high-K calc-alkalic or shohonitic series and are dominated by calc-alkalic granitic rocks. Compared to granitic rocks developed within or along the margin of paleo-microplate, the Carboniferous-Permian granitic rocks developing in rift setting are rich in MgO, TiO_2 and Na_2O and poor in K_2O, Nb, Th, indicating they are derived from different sources, maybe the former dominantly derived from the mantle and crust mixture sources and the latter mainly from the crust sources. The granitic rocks occurred on same tectonic units but in different sites also are different in geochemistry. For example, those from Temuer summit in the western end of Yili microplate have higher Fe, Mg, K, Nb, Th and lower Zr, Ba than those from Kuluketage microplate in the middle-eastern stage of Tianshan mountains, which shows the difference of the basement in eastern and western Tianshan mountains. On the other hand, the granitic rocks in Carboniferous-Permian rift belt have different geochemical characteristics in western and eastern Tianshan, those from Jueltag and Bodga in eastern Tianshan have higher Nb and Th and lower Sr and Ba, strongly suggests the different sources for granitic rocks of the western and eastern Tianshan. The granitic rocks and the ultrabasic-basic intrusive consist of the carboniferous-Permian bi-modal intrusive suite and the positive εNd values of granitic rocks can not be derived from the same mantle sources as the ultrabasic-basic rocks derived, but can be explained forming from the mantle sources which the Carboniferous-Permian volcanic rocks derived.