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
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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作者：
L. Ping
胶北地体位于胶辽冀带东南段，北中国克拉通东部地块，主要由闪长岩、英云闪长花岗闪长岩和花岗岩正长岩、表壳岩及变质镁铁质-超镁铁质岩石组成。通过对这些变质岩的岩石学、矿物组成、变质反应结构和历史、温压测量和年代学的综合研究，得出以下重要结论：(1)JBT太古宙变质岩与华北裂谷EB中的其他太古代地体相似，记录了约2500 Ma的区域变质事件，变质作用和岩浆作用几乎同步，变质作用一般晚于10~50 Ma；认为JBT约2500 Ma的变质事件与大量地幔基性岩浆的底侵作用有关。(2)几乎所有的变质岩都记录了1950~1800 Ma的区域和高级变质事件，导致了区域高压镁铁质和泥质麻粒岩的形成。高压镁铁质麻粒岩广泛分布在闪长-TTG-花岗岩正长岩中，呈不规则透镜状，沿安丘-平度-莱西-莱阳-栖霞带呈一组变形的岩墙/基岩群分布，与高压泥质麻粒岩共同构成一条由东北向西南延伸近300公里的高压麻粒岩相变质带。(3)古元古代区域高压麻粒岩相变质作用具有顺时针方向的P-T-t轨迹，具有近等温降压(ITD)和近等压冷却(IBC)作用，表明JBT参与了1950~1800 Ma与俯冲、聚集和碰撞相关的构造作用。并推断了以下变质作用和构造作用：(1)2000~1950 Ma，由于古元古代小洋壳的俯冲作用，JBT经历了地壳增厚事件；(2)1950~1870 Ma期间，伴随着洋壳俯冲作用的停止，古元古代弧体与太古代大陆的聚集和太古宙大陆之间的碰撞，导致区域高压麻粒岩相变质，其矿物组合分别为石榴石+单斜辉石+斜长石±角闪石±Fe-Ti氧化物±石英和石榴石+蓝晶石+钾长石+斜长石+石英+铁钛+泥质麻粒岩。(3)1870~1800 Ma，JBT古元古代造山岩经过同碰撞和高峰期高压麻粒岩相变质后，进入碰撞后折返和伸展阶段。碰撞后构造过程可分为两个阶段。在早期快速构造折返阶段，胶北高压镁铁质麻粒岩经历了近等温或微热减压作用(ITD)，导致区域峰后中低压麻粒岩相变质叠加，并在高压镁铁质麻粒岩中发育含斜方辉石的麻粒岩。同时，1850 Ma左右未经历高压麻粒岩相变质和地壳岩石部分熔融的基性岩墙/地台群是早期地幔基性岩浆底侵作用的产物。温度高峰过后，晚期以缓慢的等压冷却作用为主，导致区域角闪岩相变质作用叠加，镁铁质麻粒岩中石榴石和斜方辉石周围发育角闪石或角闪石。2000~1800 Ma造山作用的结束标志着JBT地区1800 Ma左右含电气石的花岗伟晶岩的区域发育。
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. 2500 Ma regional metamorphic event,and both ca. 2500 Ma metamorphic and magmatic events were almost synchronous and commonly metamorphism was later than about 10 ~ 50 Myr,indicated that the ca. 2500 Ma 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 the1950 ~ 1800 Ma 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 300 km.( 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 ~ 1800 Ma,and the following metamorphic and tectonic processes were inferred:( I)During 2000 ~ 1950 Ma,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 ~ 1870 Ma,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 50 km region of the thickened lower crust or arc root at ca. 1900 Ma.( III) During 1870 ~ 1800 Ma,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 granulitefacies metamorphism, and the development of orthopyroxene-bearing symplectites in the HP mafic granulites. Meanwhile, the ca. 1850 Ma 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 ~ 1800 Ma orogenesis was marked by the regional development of ca. 1800 Ma tourmaline-bearing granitic pegmatites in the JBT.