Provenance and tectonic setting of the Jiehekou Group in the Lüliang Complex: constraints from zircon U-Pb age and Hf isotopic studies

Abstract

The Trans-North China Orogen (TNCO) is considered to be a continent-continent collisional belt, along which the Eastern and Western blocks collided. The Lüliang Complex is located in the middle segment of the western margin of the TNCO, adjacent to the boundary between the Western Block and the TNCO. The Lüliang Complex can be divided into the granitoid plutons and supracrustal rocks. The Chijianling gneisses in the former give LA-ICP-MS U-Pb zircon age of 2180Ma and εHf(t) values between +0.52 and +4.30. The supracrustal rocks have been subdivided into the Jiehekou, Lüliang and Yejishan groups, of which the Jiehekou Group is mainly composed of meta-siltstones, meta-pelitic rocks, marbles and minor amphibolites. Geochemistry and Nd isotopic data from the meta-sedimentary rocks indicated that the source rocks are felsic, probably from the Archean upper crust and newly-formed arc felsic rocks. Combined with the previous data, our data suggested that the youngest detrital zircon age peaks from samples of the Jiehekou Group are consistent within the errors and gave an age of 2.0Ga, which is the maximum formation age of the group. On the other way, the similar isothermal decompression P-T path with other high-grade metamorphic rocks in the TNCO suggests that the Jiehekou Group also experienced the 1.85Ga metamorphic event, which is the minimum formation age of the group. Furthermore, due to similarities of the Jiehekou Group with the khondalitic rocks from the Western Block and remarkable difference with the Yejishan Group in the Lüliang Complex in the zircon U-Pb and Hf isotopic data, we suggest that the Jiehekou Group and the khondalitic rocks in the Western Block share a provenance. In a word, the Jiehekou Groups was formed between 2.0Ga and 1.85Ga on the eastern margin of the Western Blocks in a passive margin environment and was thrust eastward during the 1.85Ga collision with the Eastern Block.

華北克拉通中部造山帶被認為是由東西陸塊碰撞而產生的陸陸碰撞帶,而呂梁雜巖位則位于中部造山帶中段西緣,緊鄰中部造山帶與西部陸塊的邊界。總體上,呂梁雜巖可以分為花崗質巖體以及變質表殼巖兩大類,其中前者中的赤堅嶺片麻巖在本次研究中獲得的鋯石LA-ICP-MSU-Pb年齡為2180Ma,而εHf(t)值為+0.52～+4.30。變質表殼巖又可進一步劃分為界河口群、呂梁群以及野雞山群,其中界河口群主要由變泥砂質巖石、大理巖以及少量的斜長角閃巖組成。變質沉積巖的地球化學及Nd同位素特征說明它們的源巖為中酸性巖石,有可能是太古宙上地殼與新生酸性弧物質的混合。本次研究以及前人的研究數據都表明界河口群最下部奧家灘組中變質沉積巖的碎屑鋯石最小峰值年齡在2.0Ga左右,這為界河口群形成時代的下限做出了限制,而界河口群中斜長角閃巖的順時針近等溫降壓P-T軌跡則與中部造山帶中～1.85Ga的主期變質作用特征一致,這為界河口群形成時代的上限做出了限制。由于界河口群中碎屑鋯石的U-Pb年齡以及Hf同位素特征與同屬呂梁雜巖的野雞山群不同而與西部陸塊孔茲巖帶中的變質沉積巖具有相似性,因此我們推斷界河口群的源巖不是來自中部造山帶而是來自鄂爾多斯陸塊。綜上所述,在2.00～1.85Ga之間形成于鄂爾多斯陸塊東緣被動大陸邊緣盆地中的界河口群在～1.85Ga的東西陸塊碰撞過程中沿著韌性剪切帶向東逆沖從而進入中部造山帶中。