The Early Paleozoic tectonic -thermal events in northern margin of Yili Block: Constraints from Toksai garnet amphibolite and its surrounding rocks

The Yili Block ( YB) is one microcontinent in southwest of Central Asian Orogenic Belt ( CAOB) with Precambrain basement rocks exposed in its northern and southern margins. The Middle to Late Ordovician arc -type magmatic rocks were identified from the Wenquan Group in the northern margin of the YB, which has a subduction-related calc-alkaline affinity of a continental arc environment. Although a southward subduction of the Junggar Ocean beneath the Yili Block was suggested, the related record of metamorphism was rarely reported. The Toksai garnet amphibolite identified from the Wenquan Group in the northern margin of YB records a clockwise P-T-t path, but the age and tectonic attributes of them are still unclear. Here, we presented systematic study of whole -rock geochemistry and zircon/monazite U-Pb age dating on the Toksai garnet amphibolite and its surrounding rocks in the northern margin of YB. The garnet amphibolites outcrop as tectonic lenticles in the Wenquan Group, which mainly contains garnet, hornblende and plagioclase. Its protolith belongs to tholeiite, with high TiO2 (3. 10% similar to 3. 89%) and low K2O. + Na2O ( 0. 76% similar to 2. 01%) contents, enrichment of large ionic lithophile and rare earth elements and depletion of high field strength elements, showing the geochemical characteristics of tholeiite in intra-continental rift setting ( Th/Ta = 1. 70 similar to 2. 76, Ta/Hf = 0. 23 similar to 0. 37). The geochemical characteristics of the magmatic rocks reveal that they are derived from an OIB-like mantle source. The garnet amphibolites also have low contents of Mg0 ( 4. 82% similar to 6. 40%), Cr ( 70. 8 x 10 (-6) 224 x 10 (-6)), Ni ( 9.68 x 10 (-6) 65.7 x 10 (-6)) and low values of Me( 34. 0 similar to 41. 3), Nb/U ( 14. 3 similar to 36. 3) and Nb/Ta ( 9. 70 similar to 16. 2), indicating that their protoliths are not primitive magma, but formed by separate crystallization of different mineral phases with a small amount of crustal contamination. The zircon U-Pb dating results suggest that the protolith of the garnet amphibolite was formed in the Middle to Late Neoproterozoic with a metamorphic age ending in the Late Ordovician (450 similar to 440Ma). The zircon and monazite from surrounding rocks also record the coeval tectonic thermal event. Consequently, it is inferred that the protolith of the garnet amphibolites may have formed in an intraplate rifting setting as a result of the breakup of Rodinia, which indicated that the Yili Block maybe a continental fragment separated from the Tarim Block during the Middle to Late Neoproterozoic. In the Middle to Late Ordovician, the Wenquan Group as a part of Aktau-Wenquan continental domain was involved in the continental -arc collision and continuing accretion in north of the Yili/Kazakhstan Block with the southward subduction of the Junggar-Balkhash oceanic lithosphere, and experienced high amphibolite facies metamorphism in the end of Ordovician.

Automated summary

This study presents a systematic analysis of the geochemical characteristics and age dating of garnet amphibolite and its surrounding rocks in the Yili Block. The results suggest that the garnet amphibolite originated in the Middle to Late Neoproterozoic and experienced high amphibolite facies metamorphism. This study also indicates that the Yili Block may be a continental fragment separated from the Tarim Block during the Middle to Late Neoproterozoic.