Ultramafic–mafic mélange, island arc and post-collisional intrusions in the Mayile Mountain, West Junggar, China: Implications for Paleozoic intra-oceanic subduction–accretion process
Ultramafic–mafic mélange, island arc and post-collisional intrusions in the Mayile Mountain, West Junggar, China: Implications for Paleozoic intra-oceanic subduction–accretion process
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DOI：
10.1016/j.lithos.2011.11.016
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发表时间：
2012-02
期刊：
影响因子：
3.5
通讯作者：
Zhao Xu;Bao-Fu Han;Rong Ren;Yin Zhou;Lei Zhang;Jia‐fu Chen;L. Su;Xian‐Hua Li;Dunyi Liu-
中科院分区：
文献类型：
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作者：
Zhao Xu;Bao-Fu Han;Rong Ren;Yin Zhou;Lei Zhang;Jia‐fu Chen;L. Su;Xian‐Hua Li;Dunyi Liu-
西准噶尔位于中亚造山带西南部，由古生代蛇绿混杂岩、弧和增生杂岩组成。西准噶尔南缘玛依勒山超镁铁质-镁铁质混杂岩带是玛依勒蛇绿混杂岩带的东北部。马伊勒山混种是由一个CA代表。长30 km，宽5 km，北-东北走向蛇纹岩-基质混杂岩，包封蛇纹化方辉橄榄岩、纯橄榄岩、铬铁岩、韦氏岩、单斜辉石岩、辉长岩、钠硅铝石和硅质岩块。上覆有中志留世绿片岩相和早-中泥盆世未变质火山-沉积地层。橄榄岩和单斜辉石岩中的铬尖晶石在化学性质上与岛弧熔岩和超俯冲带亏损橄榄岩中的铬尖晶石相似。超镁铁质-镁铁质岩石中的单斜辉石具有岛弧拉斑玄武岩和玻安岩的亲合性。超镁铁质-镁铁质岩石具有与俯冲作用相关的地球化学特征，如稀土元素配分模式平坦，轻稀土元素亏损（（La/Yb）N=0.66-0.89），大离子亲石元素富集，高场强元素亏损。超镁铁质-镁铁质混杂岩被侵入岩横切。锆石U-Pb定年揭示了四期岩浆活动：晚寒武世（501- 493 Ma）、早奥陶世（488- 485 Ma）、早志留世（444- 439 Ma）和晚石炭世-早二叠世（318- 296 Ma）。晚寒武世辉长岩、闪长岩和花岗岩、早奥陶世闪长岩和早志留世花岗闪长岩侵入混杂岩中，并早于上覆的中志留世和早-中泥盆世层序。富LILE和LREE（（La/Yb）N=3.77-21.45），亏损HFSE，具强烈的岛弧亲合性。而晚石炭世-早二叠世I型闪长岩、花岗闪长岩、花岗岩和A型碱长花岗岩则横切混杂岩和上覆地层。它们与西准噶尔广泛分布的后碰撞花岗岩类是同时代的。该混杂岩和侵入岩的资料表明，该混杂岩可能是早-中寒武世南倾俯冲-增生早期形成的SSZ型蛇绿混杂岩，晚寒武世在南部相继形成岛弧早奥陶世（488- 485 Ma）和早志留世（444- 439 Ma），是由向北的弧形迁移作用形成的。在中志留世，混杂岩和岛弧侵入体可能在弧后环境中被深海或半深海火山-沉积地层出露和覆盖，并在早-中泥盆世之前向南增生到另一岛弧和大陆边缘。早石炭世末大洋最终闭合后，晚石炭世-二叠纪碰撞后花岗岩类将西准噶尔的混杂岩、岛弧等构造单元缝合在一起。
The West Junggar, northwestern China, is located in the southwest of the Central Asian Orogenic Belt (CAOB) and consists of Paleozoic ophiolitic mélanges, arcs and accretionary complexes. The Mayile Mountain ultramafic–mafic mélange in the southern West Junggar is the northeastern part of the Mayile ophiolitic mélange. The Mayile Mountain mélange is represented by a ca. 30km long, 5km wide, north–northeast-striking serpentinite-matrix mélange, which encloses serpentinized harzburgite, dunite, chromitite, wehrlite, clinopyroxenite, gabbro, rodingite, and chert blocks. It is overlain by the Middle Silurian greenschist facies and Early–Middle Devonian unmetamorphic volcano-sedimentary strata. Chemically, chromian spinel from peridotite and clinopyroxenite is similar to those of island arc lava and depleted peridotite from supra-subduction zone (SSZ). Clinopyroxene from the ultramafic–mafic rocks shows the affinities of island arc tholeiite and boninite. The ultramafic–mafic rocks also show the subduction-related geochemical signatures, such as flat REE patterns with slight LREE depletion ((La/Yb)N=0.66–0.89), LILE enrichment and HFSE depletion. The ultramafic–mafic mélange is crosscut by intrusions. Zircon U–Pb dating reveals four phases of magmatism: Late Cambrian (501–493Ma), Early Ordovician (488–485Ma), Early Silurian (444–439Ma), and Late Carboniferous–Early Permian (318–296Ma). The Late Cambrian gabbro, diorite and granite, Early Ordovician diorite and Early Silurian granodiorite intrude the mélange in places and predate the overlying Middle Silurian and Early–Middle Devonian successions. They are I-type calc-alkaline and show strong island arc affinities, with LILE and LREE enrichment ((La/Yb)N=3.77–21.45) and HFSE depletion. On the contrary, the Late Carboniferous–Early Permian I-type diorite, granodiorite and granite and A-type alkali-feldspar granite crosscut both the mélange and overlying strata. They are coeval with the widespread post-collisional granitoids in the West Junggar. Our data for the mélange and intrusions suggest that this mélange might be a highly mixed SSZ-type ophiolitic mélange generated in the earliest stage of south-dipping subduction–accretion during the Early–Middle Cambrian, and then an island arc was successively built in the south in the Late Cambrian (501–493Ma), in the center in the Early Ordovician (488–485Ma) and in the north in the Early Silurian (444–439Ma), as a result of northward arc migration. In the Middle Silurian, the mélange and island arc intrusions might be exposed and covered by deep or hemi-deep sea volcano-sedimentary strata in a back-arc setting, and they might have been accreted southward to another arc and continental margin before the Early–Middle Devonian. After the ocean was ultimately closed at the end of the Early Carboniferous, the mélange and island arc as well as other tectonic units in the West Junggar were stitched together by the Late Carboniferous–Permian post-collisional granitoids.