New Structural, Geochronological and Geochemical Constraints on the Late Paleozoic Geodynamic Evolution of Northwestern Tianshan, NW China
Résumé
The Tianshan Belt is one of the main elements of the Central Asia Orogenic collage, which builds up the Eurasian continent during the Paleozoic. The Tianshan Belt is often divided into North, Central and South Tianshan domains, the Yili Block is a continental piece that wedges eastward between the North and Central Tianshan. Our study provides some new structural, geochronological and geochemical evidence to better understand the Late Paleozoic geodynamic amalgamation of the western part of the North Tianshan and the Yili Block. The North Tianshan is composed of relics of oceanic lithosphere and Mid-Carboniferous turbidite. Blocks of serpentinized peridotite, gabbro, pillow basalt, chert and detrital rocks derived from ophiolite make up an ophiolitic mélange, refered to as the `Bayingou ophiolite'. Sedimentological and structural features indicate that this mélange is a sheared olistostrome with exotic blocks, which experienced polyphase tectonic processes. In the ophiolitic mélange, Famennian-Tournaisian radiolarians in chert, N-MORB, OIB and IAT-type mafic rocks with SHRIMP U-Pb age of 325-A7 Ma on Zircon from gabbro, indicate that these ophiolites developed within a Late Devonian to Mid-Carboniferous oceanic basin. This mélange separates the turbidite into two parts. The northern one is deformed by north-verging recumbent folds, and the southern one is characterized by a syn-metamorphic ductile deformation. Sandstone and pelite exhibit a steeply dipping slaty cleavage and a sub-horizontal stretching and mineral lineation. Kinematic criteria indicate ductile dextral shearing. A new 40Ar/39Ar dating on biotite of deformed schist shows that the shearing took place around 250 Ma. The Paleozoic rocks of the Yili Block consist of Carboniferous platform sediments associated with volcanic and volcaniclastic rocks and intruded by granitoids. Geochemical analyses (ICP-MS) on 60 volcanic rocks and granites from 8 representative sections show that (1) these rocks belong to the calc-alkaline series; (2) they are enriched in LILE while depleted in HFSE, and display a moderate negative anomaly of Nb and Ta relative to Th and Ce. These characters are consistent with subduction-related magmas; (3) they emplaced in a continental active margin. Zircon ICPMS-LA U-Pb ages of the andesites, granodiorites and granites range from 360 to 310 Ma. On the basis of above evidences, a Late Paleozoic geodynamic evolution of Northwestern Tianshan is proposed as follows. During Late Devonian-Early Carboniferous, a southward directed subduction closed an oceanic basin located between the Northern Tianshan and Junggar continent, and generated the Yili continental margin arc magmatism. Within the basin, intraoceanic tectonics (possibly along transform faults) might account for the development of the ophiolitic mélange. In Late Carboniferous, the subduction was followed by collision of the Yili Block with the Junggar continent. During the Early Permian, this suture zone was re-activated by dextral wrench faulting. In the Yili Block, the Permian tectonics is also responsible for the opening of pull-apart basins and emplacement of post-orogenic alkaline magmas. Keywords: Structure, geochronology, geochemistry, geodynamic evolution, Late Paleozoic, Tianshan