一种可能的多陆块、小洋盆、弱俯冲的动力学特征及其花岗岩演化特点——以秦岭造山带核部花岗岩为例

A POSSIBLE DYNAMIC CHARACTERISTICS OF MORE CONTINENTAL BLOCKS, OCEAN BASIN AND WEAK SUBDUCTION AND THE GRANITE EVOLUTION ——AN EXAMPLE FROM THE GRANITES IN THE CORE OF THE QINLING OROGENIC BELT, CENTRAL CHINA

板块俯冲碰撞理论曾是花岗岩构造动力学、地球动力学研究的主线.如根据碰撞前的俯冲、同碰撞、后碰撞、碰撞后四个阶段提出了对应的四组花岗岩系列.很多造山带花岗岩具有这种演化(如由俯冲型Ⅰ型→碰撞型S型)的特点,从而常常成为人们的一种思维定势.但近年来的研究显示,花岗岩生成演化及其地球动力学背景具有多样性.我们的研究表明,秦岭造山带核部花岗岩在长达700Ma期间发育两期S→H(混合花岗岩,相当于Ⅰ型,Castro,1991;1999)型花岗岩演化.晋宁期较早为S型花岗岩体,多呈线形片麻岩体;略晚为Hss型花岗岩体;之后为Hm型(幔源组分较多).加里东—海西期中略早的为S型花岗岩;略晚的为H型花岗岩体.初步估算,H型花岗岩体中,幔源组分可达10%～70%,显示了由S→H(或Ⅰ)型花岗岩的连续过渡特点.这种S→H型花岗岩的演化可能揭示了在每次构造旋回中,早期以大陆内部物质的调整和再分配的作用为主;晚期以洋陆或壳幔相互作用为主,这有别于一般的板块俯冲→碰撞演化特点.按照目前的认识,这种S→H型花岗岩的演化至少有以下三种动力学解释:(1)大陆构造体制的俯冲碰撞(S型为主)→拆沉(H型为主);(2)或小洋盆构造体制的弱俯冲(岩浆少)→碰撞(S型为主)→拆沉(H型为主);(3)小洋盆构造体制的弱俯冲(初期S型为主,略后H型为主)→洋?

Subduction and collision theory of Plate tectonics is a basement of the study of granite tectonic dyna mics and geodynamics. Generally, it corresponds to plate tectonic evolution (e.g, Harris et al ., 1988). This classical evolution of granites seems to be consistent finely with classical plate tectonic evolution, so it is often accepted as a model. Indeed, in many plate tectonic evolution, so it is often accepted as a model. Indeed, in many orogenic belts, orogeny related granites show the evolution from subductionrelated calc alkaline granites (Ⅰ type ) to syn collision granites (S type). Recent studies, however, show that the cases are not always simply like that. This paper presents that the granites in the core of the Qinling orogenic belt are characterized by two phase evolutions from S to H type granites in Proterozoic and Paleozoic, respectively, corresponding to two phase subduction/collision events. In Proterozoic, early group consists of S type granites and later of metaluminous (H type) granites. In Paleozoic, also early group is composed of S type granites and later of H type granites. The lithology, geochemistry and enclaves suggest that these S type granites are (strong) peraluminous and derived from the Qinling group, and the H type granites, i.e. I type, are metaluminous and could have been generated from the mixing between mantle derived magma and crustal derived composition (Qinling group). For all of these H type granites, it is estimated that about 10%～70% mantle derived materials were involved in the formation of the granites. This granite evolution implies that continental crust reworking was dominant in the early stage of the tectonism and the reaction between the ocean and continent crust or between the continent crust and mantle was stronger in the later stage. This evolution and the tectonic dynamics seem to be different from classical tectonic plate. There may have several possible explanations for this, e.g., (a) continental collision to post collision delamination (or break off) in a continental regime; (b)subduction to break off (or delamination) in a small ocean plate regime. The comprehensive studies suggest that the latter may be more probable. For this, a dynamic feature of more continental blocks, small ocean basin and weak subdction and the corresponding granite evolution are proposed in this paper. It appears that not all the orogenic belts in China were developed from a classical plate tectonic regimes, i.e. from a great ocean plate. Therefore, this kind of dynamic proposed in this paper may have an important signficance.