辽东半岛东北部宽甸地区南辽河群沉积时限的确定及其构造意义

摘要

辽东半岛东北部宽甸地区出露大面积南辽河群变质表壳岩系,本文通过对其中黑云石英片岩、含电气石浅粒岩和花岗质片麻岩进行精确的锆石LA-ICP-MS U-Pb定年及微区痕量元素分析,并结合锆石阴极发光(CL)图像研究来制约其原岩形成时代和变质时代,进而探讨胶-辽-吉活动带的大地构造属性.Cl图像显示锆石可以分为三类,第一类无核边结构,呈灰黑色均质特征；第二类发育核边结构,核部不发育或具弱生长环带,第三类锆石整体或者核部发育明显生长环带或具条痕状吸收特点,而后两类多数发育灰色均质边,与第一类特征一致.微区痕量元素分析结果显示,灰色均质锆石或边部具有高U(731.2×10-6～1383×10-6)、低Th(51.09～85.15 ×10-6)和Th/U(0.06～0.07)等特征,为变质成因；第二类锆石核部具有较高Th(97.68～219.7×10-6)和Th/U(0.21 ～0.27),为岩浆成因；第三类具有高Th(249.6×10-6～469.4×10-6)和Th/U(0.60 ～0.74),为岩浆成因.定年结果显示,所有测点均位于谐和线上或附近,三类锆石207Pb/206 Pb年龄分别介于1878～1903Ma,2011 ～ 2043 Ma和2082～3285Ma,前两者峰期年龄分别为1885Ma和2035Ma,表明该区南辽河群的原岩形成于～ 2035 Ma之后,而峰期变质作用应发生在～ 1885Ma,其沉积作用应发生于2035 ～ 1885Ma之间；第三类锆石年龄区域上与古元古代辽吉花岗岩、火山岩及古老结晶基底年龄相吻合,暗示它们为南辽河群提供重要物源.结合前人有关辽吉花岗岩及区域构造变形、变质作用等资料,本文研究认为辽东半岛东北部宽甸地区南辽河群应形成于伸展的构造环境.%South Liaohe Group widely occurred in Kuandian area of northeastern Liaodong Peninsula.LA-ICP-MS zircon U-Pb ages and micro trace-elements composition for the biotite-quartz schist,tourmaline-bearing leucoleptite and granitic gneiss from this group were carried out with the aim of restricting the ages of the protolith and metamorphism and the tectonic nature of the Jiao-Liao-ji Belt.The cathodoluminescence (CL) images show that the grain could be divided into three groups,i.e.,one group with features of showing low luminescence (cinereous) and structureless,one group with a structureless core surrounded by a cinereous rim,similar to grains of the former group,and the final ones with cores displaying striped absorption or fine-scale oscillatory growth zoning.The trace-element results suggest that the first group have low Th (51.09 × 10-6 ～ 85.15 × 10-6) and Th/U ratios (0.06 ～ 0.07),indicating a metamorphic origin,the second ones have high Th (97.68 × 10-6 ～ 219.7 × 10-6) and Th/U ratios (0.21 ～ 0.27),implying a magmatic origin,and the last ones show higher Th (249.6 ～469.4 × 10-6) and Th/U (0.60 ～0.74),suggesting a typical magmatic origin.The dating results show that the 207Pb/206pb ages of the two former ranging from 1878Ma to 1903Ma (peak at 1885Ma) and from 2011Ma to 2043Ma (peak at 2035Ma),respectively,suggesting that the protoliths of the South Liaohe Group was deposited at some time after 2035Ma and the peak metamorphism occurred at 1885Ma.Besides,the ages of the third ones vary from 2082Ma to 3285Ma,mainly consistent with the formation of regionally distributed granitoids and coeval volcanic rocks in the Jiao-Liao-Ji Belt and the granitoids of the basement,suggesting that these rocks provided the major component of the provenance for the South Liaohe Group.These new zircons ages for the South Liaohe Group and recent data for the Liaoji granite,combined with lithological,structural and metamorphic considerations,indicate that South Liaohe Group from Kuandian area could form under an extensional environment.