朝鲜半岛北部显生宙花岗岩成因研究及地质意义

Petrogenesis and geological implications of Phanerozoic granitoids at northern Korean Peninsula

朝鲜半岛北部图们江褶皱带和冠帽地块发育大面积的花岗岩,长期被认为形成于晚古生代二叠纪-早中生代三叠纪。采用锆石原位微区U-Pb测年技术(SIMS U-Pb),对6个代表性岩体的16个样品进行了侵位年龄测定,结合早期发表的年代学数据,确定朝鲜北部花岗岩主要形成于中生代的侏罗纪(199~173Ma),少部分为晚二叠世(265~263Ma)和早三叠世(248~240Ma)。岩石学及地球化学特征表明,朝鲜北部花岗岩以钙-碱性岩石系列为主,其成因类型为I型,且少数为高分异I型。锆石Hf-O同位素特征明显分为两类:图们江褶皱带及冠帽地块中的侏罗纪花岗岩、冠帽地块中晚二叠世片麻状花岗岩具有正的εH f(t)值(4.7~13.5)和年轻的二阶段Hf模式年龄(367~784Ma),O同位素组成δ^(18)O集中分布在5.7‰~7.4‰,表明其源区物质为新生地壳,且该地壳物质来自于亏损型的软流圈地幔;而冠帽地块中晚二叠世和早三叠世花岗岩通常具有负的εH f(t)值(-10.9~3.8)和古老的二阶段Hf模式年龄(889~1651Ma),O同位素组成变化较大,δ^(18)O分布在5.6‰~9.2‰,推测其主要为古老地壳物质重熔的产物。区域对比研究表明,图们江褶皱带和冠帽地块具有相同的地质演化历史,应归属于同一个整体,其上发育的花岗岩与我国吉黑造山带花岗岩在年代学格架、岩石学及地球化学特征、成因类型及岩浆源区都具有极大的相似性,可以进行对比。由此可见,朝鲜北部图们江褶皱带和冠帽地块为中亚造山带的组成部分,且朝鲜北部晚二叠世-早三叠世花岗岩形成于古亚洲洋闭合阶段,由于西伯利亚板块与华北板块的碰撞拼合,在华北地台北缘形成了同碰撞型花岗岩。侏罗纪中国东北及朝鲜开始进入环太平洋构造域演化阶段,在太平洋板块俯冲的挤压体制下形成大面积花岗岩。

The Tumangang belt and Kwanmo massif,located in northern part of the Korean Peninsula,are characterized by large volumes of granitoids and traditionally regarded as of Late Permian and Early Triassic ages.We obtained zircon SIMS U-Pb isotopic data of 16 samples from six representative granitic intrusions to determine their intrusive ages.New zircon U-Pb ages indicate that the granites in this area can be subdivided into three episodes： Late Permian（ 265 ~ 263 Ma）,Early Triassic（ 248 ~ 240 Ma） and Jurassic（199 ~ 173Ma）.Mineralogical and geochemical features suggest that all granites are I-and high fractionated I-types and most rocks belong to high calc-alkaline.Zircons from Jurassic granites in the Tumangang Belt and Kwanmo Massif,Late Permian gneissic granites in the Kwanmo Massif,are characterized by positive εH f（t） values（4.7 ~ 13.5）,fairly young tD M2（Hf）（367 ~ 784Ma） and uniform O isotopic composition（ δ^（18）O： 5.7 ‰ ~ 7.4 ‰）.Such Hf-O isotopic characteristics of these granitoids are likely indicative of a high proportion of juvenile material in their petrogenesis.The highest εH f（ t） value（13.5） need a source of juvenile underplated basalt derived from depleted asthenospheric mantle.Zircons from Late Permian and Early Triassic granites in Kwanmo Massif normally have lower εH f（t） values（-10.9 ~ 3.8）,older tD M2（Hf）（889 ~ 1651Ma） and large variation of O isotopic composition（δ^（18）O： 5.6 ‰ ~9.2 ‰）,indicating that they were derived from anataxis of old crustal basement materials.Regional comparison shows that the Tumangang belt and Kwanmo massif have the same geological evolution history.The granites of these areas have a great similarity with the granites in Jihei orogenic belt in the geochronological framework,petrological and geochemical characteristics,genetic type and magma source region,which indicate that the Tumangang belt and Kwanmo massif belong to the Cental Asian Orogenic Belt.Our studies suggested that Late Permian and Early Triassic granitoids belong to syn-collisional granites in northern margin of the North China Craton,which were most likely related to subduction of the Paleo-Asian Oceanic Plate beneath the North China Craton.In the Jurassic,Northeast China and Korea Peninsula transfer to Round Pacific Ocean tectonic field.With the effect of subduction of the Pacific Ocean Plate,a large area of Jurassic granites were developed.