The Early Silurian diabase dikes in the northern margin of the Yili Block, southwestern CAOB: insight into rift-related magmtism

Abstract

The Central Asian Orogenic Belt (CAOB) is one of the largest orogenic collages in the world, and preserves important records of accretionary orogeny and Phanerozoic continental growth. The Yili Block is one microcontinent in southwest of CAOB, with Precambrain basement rocks exposed in the northern and southern margin. The Middle to Late Ordovician arc-type magmatic rocks were identified in the northern margin of the Yili Block with a subduction-related calc-alkaline affinity conclude that the southward subduction of the Junggar Ocran beneath the Yili Block, but the Silurian magmatism is rarely reported. Mafic dikes preserve a considerable amount of geological information about geodynamics, crustal evolution and transformation of the regional stress field. Multi-period basic dikes, including Neoproterozoic and Carboniferous, are exposed in the northern margin of the Yili Block, which record important information about the transformation process of regional tectonic system. Recently, we have identified early Silurian diabase dikes in the Precambrian metamorphic rocks in the Wustu area, Wenquan County, northern margin of Yili Block. This paper reports zircon U-Pb age and Lu-Hf isotopic compositions, whole-rock geochemistry and Sr-Nd isotopic compositions for the Wustu diabase dikes and its surrounding rocks. One diabase sample yielded a zircon U-Pb age of 442±7 Ma with positive εHf(t) values (+3.0~+9.1), and its surrounding rock sample (leucogranite) yielded a zircon U-Pb age of 901±3 Ma. The diabase samples have high TFe2O3 contents (8.34%~9.81%) and K2O+Na2O contents (5.72%~6.86%), low MgO contents (3.69%~4.38%) and TiO2 contents (1.69%~2.00%) and belong to the high-K calc-alkaline series. The samples are enriched in the large ion lithophile elements (LILEs, such as Rb, Th, U and K) and have negative anomalies in the high-field-strength elements (HFSEs, e,g. Nb, Ta and Ti), with low Nb/Th ratios (0.13~1.16), Nb/La ratios (0.42~0.45) and high Zr/Hf ratios (39.6~42.2). They also have high initial 87Sr/86Sr ratios (0.707369~0.708637) and positive εNd(t) values (+1.9~+3.6). Our results indicate that they were sourced from a metasomatic sub-continental lithospheric mantle, which mainly composed of spinel iherzolite and garnet iherzolite. The trace element contents and its ratios, such as Zr (212×10-6~242×10-6), Hf (5.16×10-6~6.02×10-6), Nb (6.69×10-6~9.24×10-6), Ta (0.60×10-6~0.81×10-6), Zr/Y (5.21~6.82) and Hf/Th (0.69~0.91), indicate that the diabase dikes formed in an extensional setting during the early Silurian. Finally, we propose that the extensional tectonic setting maybe relate to the change of the subducted slabs angle or tectonic regime transition induced by the collage of the Aktau-Wenquan continental domain to the Yili Block in the end of Ordovician.