胶北地体多期变质事件的P-T-t轨迹及其对胶-辽-吉带形成与演化的制约
P-T-t paths of the multiple metamorphic events of the Jiaobei terrane in the southeastern segment of the Jiao-Liao-Ji Belt (JLJB), in the North China Craton: Impication for formation and evolution of the JLJB
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摘要: 胶北地体位于华北克拉通东部陆块胶-辽-吉带南端,主要由闪长质-TTG-花岗质片麻岩、变质表壳岩系和变质镁铁-超镁铁质岩所组成。本文通过对胶北早前寒武纪变质岩系的岩石学、矿物化学、变质反应结构和序列、变质温度和压力估算与同位素年代学资料的综合研究和总结,得出以下重要结论:(1)与华北克拉通东部陆块其它地区太古宙变质基底类似,本区也存在~2500Ma区域性新太古代变质事件,且与本区2550~2500Ma岩浆作用在时间上非常接近,其变质作用发生的时间比岩浆作用要晚10~50Myr,指示本区~ 2500Ma区域性变质事件可能与大规模的幔源岩浆底侵作用存在密切的成因关系。(2)胶北还存在1950~1850Ma区域性古元古代变质事件,并导致了大量高压基性和泥质麻粒岩的形成,高压基性麻粒岩主要以不规则透镜体、变形岩墙群或岩脉群的形式赋存于闪长质-TTG-花岗质片麻岩之中,并集中分布在安丘-平度-莱西-莱阳-栖霞一带,大致沿北东-南西向断续带状分布,构成了一条长约300km的古元古代高压麻粒岩相变质带。(3)本区古元古代高压麻粒岩以记录近等温减压(ITD)及随后近等压降温(IBC)的顺时针P-T-t轨迹为特征,指示本区变质杂岩在古元古代晚期曾强烈地卷入了与俯冲-拼贴-碰撞造山有关的构造过程,并可能经历了如下复杂的构造演化:(I)在古元古代晚期2000~1950Ma,随着有限大洋地壳的持续俯冲作用,本区各类变质岩的原岩开始经历一次构造增厚事件,并导致了它们的原岩经历了早期绿片岩相-角闪岩相进变质作用;(II)1950~1870Ma,大洋地壳俯冲作用结束,本区开始发生弧-陆拼贴和陆-陆碰撞作用,大陆地壳持续缩短和加厚,在加厚下地壳或岛弧根部带约50km的深度,发生了区域性高压麻粒岩相变质作用,并导致了本区变基性岩和变泥质岩分别形成了石榴石+单斜辉石+斜长石±角闪石±石英±铁-钛氧化物和石榴石+蓝晶石+钾长石+斜长石+黑云母+石英+铁-钛氧化物+熔体的高压麻粒岩相矿物组合。(III)1870~1800Ma,在同碰撞峰期变质结束之后,本区造山作用进入了后碰撞构造折返-伸展演化阶段,先后经历了早期快速构造折返和晚期缓慢冷却降温两个构造热演化阶段。其中,在早期快速构造折返阶段,高压麻粒岩经历了峰后近等温或略微增温减压退变质作用的叠加,高压基性麻粒岩表现为沿石榴石边部形成了含斜方辉石的后成合晶。与此同时,早期快速构造折返阶段还伴随着热松弛和伸展作用,出现一系列的幔源基性岩浆活动,不仅导致了本区大量未经历高压麻粒岩相变质的变基性岩群的形成,同时也诱发了区内大规模的地壳深熔作用的发生。自温度高峰期之后,本区地壳岩石还经历了一个近等压冷却降温过程,并发生了区域性角闪岩相退变质作用,高压基性麻粒岩表现为石榴石和斜方辉石边部常出现含角闪石的退变边或后成合晶。最终,在1800Ma左右,本区含电气石花岗伟晶质岩脉的大量出现,则标志着胶北地体古元古代晚期(2000~1800Ma)俯冲-拼贴-碰撞造山作用的最终结束。Abstract: Jiaobei terrane (JBT), located in the southeastern segment of the Jiao-Liao-Ji Belt (JLJB), in the Eastern Block (EB) of the North China Craton (NCC), are composed predominantly of dioritic, tonalitic-trondhjemitic-granodioritic (TTG) and granitic orthogneisses, supracrustal rocks and meta-mafic-ultramafic rocks. The combination of petrography, mineral compositions, metamorphic reaction texstures and history, thermobarometry and geochronology of these metamorphic rocks reveals the following important conclusions: (1) Likely the other Archean terrane in the EB of the NCC, the Archean metamorphic rocks of the JBT have recorded ca.2500Ma regional metamorphic event, and both ca. 2500Ma metamorphic and magmatic events were almost synchronous and commonly metamorphism was later than about 10~50Myr, indicated that the ca. 2500Ma metamorphic event of the JBT is thought to be linked to underplating of large amounts of mantle-derived mafic magma. (2) Almost of all metamorphic rocks have recorded the 1950~1800Ma regional and high-grade metamorphic event, which led to the formation of the regional high pressure (HP) mafic and pelitic granulites. The HP mafic granulites were widely distributed within the dioritic-TTG-granitic orthogneisses as irregular lenses and as a set of deformed dyke/sill swarms along the Anqiu-Pingdu-Laixi-Laiyang-Qixia belt, and they with HP pelitic granulites constitute a HP granulite facies metamorphic belt that trends northeast to southwest and extends for nearly 300km. (3) The Palaeoproterozoic regional HP granulite facies metamorphism was characterized by the clockwise P-T-t paths bearing near-isothermal decompressional (ITD) and near-isobaric cooling (IBC) processes, suggesting that the JBT have involved in the subduction-, aggregation- and collision-related tectonic processes during 1950~1800Ma, and the following metamorphic and tectonic processes were inferred: (I) During 2000~1950Ma, the JBT has experienced a crustal thickening event as a consequence of the subduction of the Paleoproterozoic small oceanic crust. As a result of tectonic burial and heat conduction, a greeschist facies to amphibolite facies prograde metamorphism occurred in the upper and middle crust region of the JBT. (II) During 1950~1870Ma, accompanying by the stop of the subduction the oceanic crust, the aggregation between the Paleoproterozoic arc and Archean continent and the collision between the Archean continents caused the regional HP granulite facies metamorphism, as represented by the mineral assemblages of garnet+clinopyxene+plagioclase±amphibole±Fe-Ti oxide±quartz and garnet+kyanite+K-feldspr+plagioclase+biotite+quartz+Fe-Ti oxide+liquid in the mafic granulites and pelitic granulites, respectively, occurred in around 50km region of the thickened lower crust or arc root at ca. 1900Ma. (III) During 1870~1800Ma, after syn-collisional and peak HP granulite facies metamorphic stage, the Paleoproterozoic orogeneiss of the JBT had entered into the post-collisional exhumation and extension stage. The post-collisional tectonic process can be divided into the two stages. During the early fast tectonical exhumation stage, the Jiaobei HP mafic granulites have experienced a nearly isothermal or slight heating decompression (ITD), which resulted in the overprinting of the regional post-peak middle- and low-pressure granulite-facies metamorphism, and the development of orthopyroxene-bearing symplectites in the HP mafic granulites. Meanwhile, the ca.1850Ma mafic dyke/sill swarms which haven't experienced HP granulite-facies metamorphism and partial melting of the crustal rocks were caused by the underplating of mantle-derived mafic magma during the early stage. After temperature peak, the late stage is dominated by slow isobaric cooling processes, which resulted in the overprinting of the regional amphibolite-facies metamorphism and the development of amphibole-bearing symplectites or rims around garnets and orthopyroxenes in the mafic granulites. Finally, the end of the 2000~1800Ma orogenesis was marked by the regional development of ca. 1800Ma tourmaline-bearing granitic pegmatites in the JBT.
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Key words:
- P-T-t paths /
- Multiple metamorphic events /
- Jiaobei terrane /
- Jiao-Liao-Ji belt /
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