西昆仑康西瓦西部早古生代侵入岩的岩浆混合作用

Magma mingling of Early Paleozoic intrusive rocks in western Kangxiwar, West Kunlun, China

三十里营房以北广泛发育早古生代侵入岩,据岩性可分为中基性、中酸性和酸性3类。中基性岩以辉石闪长岩为主,暗色矿物中常见辉石,斜长石多为中长石。中酸性岩以石英闪长岩为主,岩石组构不均一,广泛发育深源暗色包体。这些包体多数与寄主岩石界线清楚,少部分呈现过渡关系,镜下可见不平衡矿物共生和反相矿物包裹的现象,显示出岩浆混合作用的特征。酸性侵入岩以中粗粒二长花岗岩为主,岩石中含有少量包体,包体的特征与中酸性侵入岩中的相同。中基性岩和酸性岩分别代表了该区寒武纪第一次地幔分异、陆壳垂向增生和晚奥陶世—早志留世西昆仑地区造山后去根的过程中,第二次基性岩浆广泛贯入,其热源又引发了下地壳物质大规模部分熔融生成大量的花岗质岩浆,造成陆壳第二次垂向增生。

Early Paleozoic intrusive rocks are widespread north of Sanshilifang. They may be divided into the intermediate-basic, intermediate-acid and acid types. The intermediate-basic rocks consist predominantly of pyroxene diorite. The dark-colored minerals are commonly pyroxene and plagioclase is mostly andesine. Intermediate-acid rocks are dominated by quartz diorite, which has an inhomogeneous petrofabric and in which deep-seated, dark-colored xenoliths are commonly seen. Most of these xenohths have clear-cut boundaries with their host rocks, while a few show transitional phenomenon. Under the microscope, association of unbalanced minerals and enclosure by antiphase minerals may be observed and the features of mingling of magma are conspicuous. Acid intrusive rocks are composed mainly of medium- and coarse-grained monzogranite. They contain a small number of xenoliths, whose features are the same as intermediate-acid intrusive rocks. Intermediate-basic rocks and acid rocks represent two successive continental crustal vertical growths respectively. The first mantle differentiation and continental crustal vertical growth occurred in the study area in the Cambrian, while the second extensive intrusion of basic magma occurred during post-orogenic derooting in the West Kunlun in the Late Ordovician-Early Silurian and its heat source resuited in large-scale partial melting of lower crustal material, thus generating large amount of granitic magma and leading to the second vertical growth of continental crust.