辽南地块新太古代2.52～2.46Ga构造-热事件的识别及地质意义

胶-辽-吉活动/造山带是华北克拉通早前寒武纪基底三条古元古代构造带之一,对其邻区内太古宙地质体的岩石组合和时空分布的研究,可为制约胶-辽-吉带的构造属性以及华北克拉通东部在古元古代时期的构造格局提供重要依据。胶-辽-吉带的北侧为龙岗地块,南侧为辽南和狼林地块,本文对辽南地块中典型前寒武纪基底变质岩石进行了详细的野外地质、岩石学和锆石LA-ICP-MS U-Pb年代学研究,结果显示原岩为石英闪长岩或英云闪长岩的黑云角闪片麻岩(DL05-1)和侵入其中的花岗质脉体(DL05-2)分别形成于2523±11Ma(MSWD=0.62)和2505±13Ma(MSWD=0.67),原岩为TTG岩系的黑云斜长片麻岩(DL10-1)形成于2521±12Ma(MSWD=1.15)。2件斜长角闪岩样品(DL12-1和DL07-1)分别给出2521±16Ma(MSWD=1.9)和2464±20Ma(MSWD=0.72)的~(207)Pb/~(206)Pb加权平均年龄,前者的锆石形态、结构和Th-U体系组成具有高温岩浆成因的特征,因此2521±16Ma代表基性原岩的形成时代,后者的锆石没有明显的内部结构,且Th、U含量低(分别为6×10~(-6)~29×10~(-6)、9×10~(-6)~26×10~(-6))具变质重结晶锆石的特征,2464±20Ma代表区域变质事件的年龄。侵入斜长角闪岩的2件花岗片麻岩样品(DL12-2和DL07-2)207Pb/206Pb加权平均年龄分别为2498±18Ma(MSWD=1.4)和2.1Ga,1件钾质花岗岩样品(DL17-1)由于强烈的Pb丢失未能获得准确的形成年龄,根据野外产状和锆石U-Th-Pb组成推测该钾质花岗岩形成于新太古代晚期。虽然辽南地块内太古宙表壳岩的时空分布规律目前还不清楚,但本文的研究结果进一步确认,辽南地块内存在新太古代2.52Ga基性岩、闪长岩和TTG组合,它们的形成时代总体上不晚于共生的花岗岩(2.50Ga),而后辽南地块经历了新太古代晚期2.46Ga区域变质作用的改造。侵位时代接近2.1Ga花岗岩的存在表明辽南地块太古宙陆壳在形成后受到2.1Ga构造-热事件的影响。另一方面,本文研究的花岗质岩石样品中残余锆石核年龄分布于2.61~2.51Ga,表明辽南地块内古老大陆地壳物质的形成时代至少可以追溯至新太古代早期。辽南地块内太古宙岩浆、变质作用时限和陆壳物质的形成时间都与华北克拉通东部可以对比,具有华北的太古宙基底属性。与狼林地块相比,辽南地块内太古宙的岩石类型更加丰富,规模也更大;与胶-辽-吉活动/造山带北侧龙岗地块相比,辽南地块内尚没有发现大规模条带状铁建造(BIF)和形成于新太古代之前(2.8Ga)的岩石,且变质程度相对较低。目前的地质和年代学证据尚不支持辽南地块与胶-辽-吉带北侧的龙岗地块原属于同一陆壳基底。

The identification of the Neoarchean 2.52~2.46Ga tectono-thermal events from the Liaonan terrain and its geological significance

The Jiao-Liao-Ji mobile/orogenic belt is one of the three Paleoproterozoic tectonic belts in the Early Precambrian basement of the North China Craton (NCC). The chronological and spacial distributions of the Archean assemblages in the neighboring terrains are significant to constrain the nature of the Jiao-Liao-Ji belt and the Paleoproterozoic tectonic framework of the eastern NCC. Traditionally, the Precambrian terrain, situated in the north of the Jiao-Liao-Ji belt is named as Longgang Block and those in the south side are Liaonan Block and Rangnim massif. In this study, zircon LA-ICP-MS U-Pb dating combined with geology and lithology was carried out on the Early Precambrian metamorphic rocks in the Liaonan terrain. We report weighted mean ²⁰⁷Pb/²⁰⁶Pb zircon ages of 2523±11Ma (MSWD=0.62) and 2505±13Ma (MSWD=0.67) for meta-diorite/tonalite (DL05-1) and intruding granitic dike (DL05-2), respectively. The biotite plagioclase gneiss (DL10-1), derived from TTG series, formed at 2521±12Ma (MSWD=1.15). The amphibolite samples (DL12-1 and DL07-1) yield weighted mean ²⁰⁷Pb/²⁰⁶Pb zircon ages of 2521±16Ma (MSWD=1.9) and 2464±20Ma (MSWD=0.72), respectively. The morphology, texture and Th-U systematics indicate that the zircon grains of DL12-1 originate from high temperature magma. Thus, we interpret the 2521±16Ma as the formation age of the mafic protolith. Zircons of DL07-1 are characterized by no-inner textures and low Th (6×10^-6~29×10^-6) and U (9×10^-6~26×10^-6) contents, suggesting the origin of re-crystallization. 2464±20Ma is interpreted as the age of regional metamorphic event. The granitic rocks (DL12-2 and DL07-2), which intrude into the amphibolite, formed at 2498±18Ma (MSWD=1.4) and 2.1Ga, respectively. Because of strong Pb loss, the age of the K-rich granite (DL17-1) was not achieved during this study. According to the occurrence of the granite and the U-Th-Pb systematics of the zircons, we suggest the K-rich granite (DL17-1) should be formed at the Neoarchean. Although the distributions of the Archean supracrustal rocks are not clear, our geochronological studies further confirm that the 2.52Ga Neoarchean assemblages of mafic rocks, diorites and TTGs are preserved in the Liaonan terrain, which are generally older than the associated granites (2.50Ga). Subsequently, the Liaonan terrain is subjected to the Late Neoarchean (2.46Ga) regional metamorphism. The 2.1Ga granite indicates that the Archean crust of the Liaonan terrain was influenced by Paleoproterozoic tectono-thermal events. The inherited zircons from granite in this study are formed at 2.61~2.51Ga. It can be inferred that the crustal materials of the Liaonan terrain was at least formed during the Early Neoarchean. The magmatic and metamorphic events and the crustal formation time of the Archean assemblage of the Liaonan terrain can be generally comparable to the eastern basement of the NCC. Compared with the Rangnim massif, more types and volumes of the Archean rocks are identified from the Liaonan terrain. Different from the Longgang block situated in the north of the Jiao-Liao-Ji belt, no voluminous of banded iron formation (BIF) and >2.8Ga rocks have been identified in the Liaonan terrain, and the Liaonan terrain was subjected to relatively lower grade metamorphism. Current evidences do not support the point of view that the Liaonan terrain derived from a same crustal basement with the Longgang Block.