东北及邻区晚古生代大地构造属性新认识

哲斯腕足动物群是一个凉水型腕足动物群,土著种含量在75%以上,古生物地理区划属于北方生物地理大区的内蒙古区。自成一个地理区说明其北有重要地理隔绝:暗示存在一个较大地块(佳—蒙地块);凉水型性质则说明中二叠世这个稳定地块与华北板块之间存在一个足够宽的深海洋盆——古亚洲洋南支。在佳—蒙地块上,晚古生代地层发育较佳,保存较好。这些晚古生代地层围绕佳—蒙地块核心呈环带状分布,明显构成佳—蒙地块的大陆边缘沉积。在我国东北地区,出露的晚古生代海相地层主要为佳—蒙地块的南缘和北缘沉积。根据目前已有资料统计,中生代为东北及邻区花岗岩侵位的峰期,志留纪—泥盆纪为花岗岩浆活动相对沉寂的时期,花岗岩的这种侵位时代特征支持晚古生代存在一个稳定地块的设想。东北地区晚古生代地层主体未见遭受低绿片岩相区域变质的迹象,说明这一地区晚古生代地层主体未曾发生过强烈的褶皱造山、普遍的区域变质的地槽发展过程。东北及邻区为天然地震弱震区,其周缘被锡霍特—珲春、蒙古—鄂霍次克、阿尔泰—华北北缘强地震带所围,似乎可以得出这是一个构造相对稳定的区域、其间不曾存在晚古生代以来的板块构造缝合线的结论。东北地区中、新生代发生的大规模拆离—走滑使得原本较为完整的、稳定的块体发生了分割与位移。如此,一个完整的地块被切割成多个块体(即所称的众多"微板块")。东北及邻区晚古生代构成了一个巨大地块:佳—蒙地块。佳—蒙地块是北由蒙古—鄂霍次克缝合带、南由西拉木伦河—延吉缝合带、东由中锡霍特俯冲带所围限的一个晚古生代稳定的大地构造单元。

NEW UNDERSTANDING OF THE LATE PALEOZOIC TECTONICS IN NORTHEASTERN CHINA AND ADJACENT AREAS

The Zhesi brachiopod fauna represents a cold-water brachiopod paleobiogeographic region,belonging to Mongolia Province of the Boreal Realm.This fauna is recognized because it consists of 75% endemic brachiopod species.The high endemicity indicates biogeographic isolation,and suggests that there existed a geographic barrier to the north,possibly a large paleo-continent(the Jiamusi-Mongolia block).The cold-water feature of the Zhesi brachiopod fauna suggests that there was a wide deep ocean,i.e.the southern branch of Paleo-Asian Ocean,between the block and North China Plate during Middle Permian.In the Jiamusi-Mongolia Block,Late Paleozoic strata are well developed and preserved.These strata,which are distributed around the core of the Jiamusi-Mongolia Block,obviously represent sedimentation on the continental margins.Marine Paleozoic strata in the northeast China were mainly deposited on the southern and northern margins of the Jiamusi-Mongolia Block.Based on available data,granite intrusion in northeastern China and adjacent areas was peaked in the Mesozoic.The granitic magma activity was relatively weak in the Silurian-Devonian period.This supports the existence of a stable block during the Late Paleozoic.Most Late Paleozoic strata have not undergone strong regional metamorphism,suggesting that the Late Paleozoic strata in the region have not suffered strong folding orogeny and metamorphism.Northeastern China and adjacent areas,bounded by the Sikhote-Hunchun,Mongolia-Okhotsk and Altai-northern margins of North China,are weak natural earthquake areas.Therefore,the region is a relatively stable area in which there have been no plate tectonic suturings since the Late Paleozoic.In northeastern China,the large scale Mesozoic-Cenozoic detaching-slipping belts have cut and dislocated the whole stable block into many smaller blocks(i.e.so-called micro-plates).These large scale Mesozoic-Cenozoic detaching-slipping belts are regarded as subduction zones or plate sutures by previous researchers.Northeastern China and adjacent areas,bounded by the Mongolia-Okhotsk Suture Belt to the north,the Xilamulun-Yanji suture Belt to the south,and the Middle Sikhote Subduction Belt to the east,are a stable Late Paleozoic tectonic unit,which consists of a huge Late Paleozoic Block,named as Jiamusi-Mongolia Block.