地壳根、造山热与岩浆作用

简要讨论了近年来造山带及其岩浆作用研究的主要进展。造山带流变学结构与造山热和岩浆作用有着密切的耦合关系。年轻的山带往往存在地壳根,但古老的山带地壳根是否存在,取决于造山带的热状态和榴辉岩化的强度,只有缺乏流体和冷的造山带才保留有地壳根,例如古生代的南乌拉尔山和北美前寒武纪的南Trans-Hudson造山带。造山带的伸展塌陷往往伴随着幔源岩浆底侵、地壳软化、隆升和强烈岩浆作用。由于地幔浮力和造山热的作用,一些山带具有高的海拔和薄而热的地壳,属于具有长期活动性的构造带。研究表明,这些具有长期活动性的构造带,是建立在以前形成的热的、软化了的弧后区内。中国昆仑—秦岭—大别造山系北缘,古生代时期发育了与俯冲有关的弧岩浆带,而南缘发育了相近时代的与弧后伸展有关的双峰式岩浆带,构成古生代双岩浆带。该造山系早中生代的造山作用,就是在南缘的古生代弧后岩浆带基础上发展起来的。因此,该双岩浆带提供了造山热控制复合造山作用的实例。

Crustal roots, orogenic heat and magmatism

Major advances in the research of orogens and associated magmatism are briefly summarized in this paper. The rheological structure of orogens is closely coupled to orogenic heat and magmatism. Young orogens generally have crustal roots. For old orogens, however, whether crustal roots are preserved or not depends on thermal structure and the presence or absence within them of eclogitization processes. Among these orogens, crustal roots can only be preserved if they are fluid-absent and cold, such as the Paleozoic South Ural mountains and the Precambrian Trans-Hudson orogen. Extensional collapse is commonly accompanied by mantle-derived magmatic underplating, associated weakening of crust, uplift and voluminous magmatism. As a result of mantle buoyancy and orogenic heat, a number of mountain belts have high elevation and thin and hot crust, and thus become long-lived active mobile belts. It is suggested that these mobile belts are evolved from heated and weakened crust in former backarcs. On the northern margin of Kunlun-Qinling-Dabie ranges, a subduction-related magmatic arc of Paleozoic age (460～430 Ma) was developed. On the southern margin of the ranges, an intrusive belt of bimodal composition and of similar age (440～390 Ma) was formed as a result of back-arc extension. These can be considered as a paired magmatic belts. The earlier Mesozoic orogenesis of the Kunlun-Qinling-Dabie ranges was influenced by the southern back-arc belt of the paired magmatic belts, thus providing a typical case for composite orogenesis controlled by latent, i.e. inherited, orogenic heat.