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
Liu, P; Liu, F; Wang, F; Liu, C; Yang, H; Liu, J; Cai, J; Shi, J
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 ∼ 180 Ma orogenesis was marked by the regional development of ca. 1800Ma tourmaline-bearing granitic pegmatites in the JBT.
Jiao-Liao-Ji belt; Jiaobei terrane; Multiple metamorphic events; P-T-t paths; Feldspar; Garnets; Geochronology; Heat conduction; Igneous rocks; Isotherms; Minerals; Petrography; Regional planning; Silicate minerals; Structural geology; Tectonics; Titanium oxides; Amphibolite-facies metamorphism; Formation and evolutions; Mafic-ultramafic rocks; Metamorphic events; Metamorphic reactions; P-T-t path; Regional development; Metamorphic rocks; metamorphic facies; metamorphism; Proterozoic; tectonic setting; North China Block; Shandong