The Late Paleozoic Geodynamical Environment Of Central Asia

Dykes, a special kind of intrusive rocks formed by brittle deformation in the crust and the magma activities from the deep, are very significant in the study of geodynamics.There are different opinions existing in the study of the Paleozoic tectonic evolution of Central Asia, especially the Late Paleozoic geodynamical environment of this area. Being different form earlier studies in Central Asia, this dissertation focus on the Late Paleozoic dark dykes distributed in this area. By analysis the regional stress field which result in the brittle fractures and filled by later dyke magma, we can obtain more reliable constrains in the study of the Late Paleozoic geodynamical environment of Central Asia.This study started with the remote sensing interpretation to investigate the spatial and temporal distribution of the dark dykes and their general characteristics. This is the first study of the distribution of dark dykes in whole Central Asia, and reveals that the dykes are not evenly distributed in time and space. Most of the darks were formed in the Late Paleozoic, except for the Neoproterozoic dykes in Kuruktage, and most dykes concentrated in west and north coast of Balkhash Lake, Chingis-Taerbahatai, eastern Junggar, eastern Junggar, Mongolian Altai, eastern Tianshan and Beishan. There is obvious relevance between the distribution of dykes and regional faults and sutures.In the study of western Junggar, the diorite dykes were formed in 302-303Ma, according to the zircon LA-ICP-MS U-Pb dating of 6 dyke samples(1 sample from the Karamay pluton and 5 samples from the Hongshan pluton). The dykes emplaced in the Karamay pluton can be classified into 3 groups by different strike. The group 1 with NW/SE strike was intruded in sinistral dextral shear fractures, and the group 2 with NE/SW strike was intruded in sinistral shear fractures. The group 3 ware formed in approximately N/S strike extensional fractures. This study firstly discover that the dykes emplaced in the Hongshan pluton can be classified into 2 groups, namely the eclipse shaped ring dykes and the non-ring dykes with different strikes. The stress interpretation reveals that from 315Ma to 303Ma, western Junggar was in a NNW/SEE direction regional press.In the study of eastern Tianshan, the dykes emplaced in Dananhu pluton, eastern Santoushan pluton and eyeball shaped pluton located in Kangguertage fault was analyzed. By zircon LA-ICP-MS U-Pb chronology, the age of dykes in eastern Santoushan pluton can be determined as 293Ma. The dykes in Dananhu pluton and eastern Santoushan pluton were formed in an approximately N/S regional press. The eyeball shaped pluton formed a huge porphyroclast system which indicate sinistral shear, and the dykes emplaced in this pluton may intruded in the extensional plane T.In the study of Beishan, we pay mare attention to the dykes emplaced in Shijinpo pluton. The zircon SHRIMP dating and LA-ICP-MS dating yield 374Ma and 287Ma of 2 samples from different dykes. Considering with the contact and cut relationship of different dykes, the dykes can be calssified into 3 groups by dated dyke ages. There were formed in 409-374Ma, ca.374Ma and ca.287Ma, respectively. Most dykes formed at ca.374Ma were curved in a large scale. They are fault related folds, according to the spatial and kinetic relationship between NE/SW direction faults. The dykes formed at ca.287Ma without obvious curved shape cut the fault related dyke folds at several places. The structural study worked out that from the Carboniferous to the Early Permian, this area was in a N/S regional press and sinistral shear occurred.By comprehensive combine the general characteristics of the distribution of dark dykes in Central Asia, and the regional stress field analysis in western Junggar, eastern Tianshan and Beishan, this study conclude that form the Late Carboniferous to the Early Permian the Central Asia was in a obvious large scale regional press condition, which suggest the existing of a uniformed regional stress field.On the basis of study in this dissertation and discussion with earlier opinions, we finally conclude that in the Late Paleozoic, specifically form the Late Carboniferous to the Early Permian, the whole Central Asia was in a N/S direction press. At this time, the paleo-oceans in Central Asia had almost been closed, the uniform continental crust was initially shaped, and the collision and press among different blocks were taking place.