天山海西期不同类型花岗岩类岩石化学特征及其地球动力学意义

THE PETROCHEMICAL CHARACTERISTICS OF THE HERCYNIAN GRANITOIDS IN TIANSHAN AND ITS GEODYNAMIC IMPLICATINOS

通过研究把天山海西期花岗岩类分为 4类 :幔源 (M)型、同熔 (SY)型、改造型[混合改造 (MT)亚型、富铝改造 (PT)亚型和富钾改造 (KT)亚型 ]和 A型。对各类花岗岩的地质特征和岩石化学特征进行了分析 ,总结了其时空演化规律。随时间变新 ,从幔源型到 A型 ,基性程度降低、碱质含量增高、规模由小变大再变小。就同熔型花岗岩而言 ,基性程度中天山 >北天山 >南天山 ;富钾改造亚型与同熔型变化的特点相反 ,酸性程度中天山>北天山 >南天山 ;富铝改造亚型在天山各区呈现岩石化学特征基本相似的特点。天山海西期花岗岩类岩石化学特点及时空规律表明 :1)天山造山带花岗岩类的演化序列为 :幔源型 -同熔型 -改造型 (混合改造亚型 -富铝改造亚型 -富钾改造亚型 ) - A型。 2 )花岗岩类演化序列表征了造山带花岗岩成分定向演化的规律 ,是陆壳增生和其成熟度渐高的例证。 3)“造山岩”的含义应界定为同熔型和改造型花岗岩类 ,天山海西期花岗岩分布的不均一性由各构造单元的性质不同所造成。 4 )富铝改造亚型花岗岩类是陆块碰撞拼合的标志。 5 )富钾改造亚型花岗岩类花岗岩为碰撞造山减压达到高潮的标志。 6 ) A型花岗岩的发育是造山运动结束的标志。

Four genectic types of granitoids,i.e.mantle derived type (M type),syntexis type(SY type),trasformation type [T type,including migmatic(MT),peraluminous(PT) and K rich (KT) subtypes],and A type were classified in the Hercynian granitoids in Tianshan area.Their geological and petrochemical characteristics were studied.With the evolution of granitoids type to A type and with the ages of granitoids becoming young,the content of SiO 2 and alkali increased and the size of magmatic bodies dicreased.The basicity of syntexis type and the acidicity of K rich transformation granites are of the highest value in the central Tianshan,followed by northern Tianshan and southern Tianshan successively.The peraluminous transformation granitoids are of same petrochemical characteristics in the entire region of Tianshan.These petrochemical characteristics and their spatiotmeporal variations imply that:1)The developing sequence of the granitoids in the Tianshan collisional orogen is generally M typeSY typeMT typePT subtypeKT subtypeA type.2)The sequence shows the directionally compositional evolution of granitoids in a collisional orogen,which illustrates the vertical and lateral crust growth and the enhancement of crustal maturity.3)The concept of ″orogenic rock″ should include granitoids of the syntexis and transformation type,and the petrochemical and distributing heterogenity of granitoids were resulted from the nature and setting of various tectonic units.4)The PT subtype granitoids mark the collision of continental blocks.5)The KT subtype granitoids could be used as the marker of the climax of decompression geothermal increase.7)A type granitoids are the indicator of the end of orogenesis.