Rejuvenation of ancient micro-continents during accretionary orogenesis: Insights from the Yili Block and adjacent regions of the SW Central Asian Orogenic Belt

Published :Jul 29,2020

In the Central Asian Orogenic Belt (CAOB), whether substantial juvenile additions associated with accretionary orogenesis are preserved is still a pending issue. The Yili Block (YB) is a micro-continent in the Western Tianshan, SW CAOB. Voluminous felsic rocks constitute two major belts stretching in the southern and northern margins of the YB. We synthetically compile up-to-date zircon U-Pb geochronological, elemental, and Nd-Hf isotopic data for felsic rocks from the YB and adjacent tectonic domains. Spatially, Hf-Nd isotopic mapping unveils an inboard-younging trend in Hf model age of the YB, which indicates relative ancient basement rocks in its northern and southern edges and the most juvenile crust beneath its centre. Temporally, the compiled zircon Hf isotopic dataset suggests alternating continental reworking and growth in the YB and adjacent regions during Paleozoic times. In combination with other evidence, we speculate that the Paleozoic continental evolution in the Yili Block and adjacent regions were associated with episodic advancing and retreating subduction of branches of the Paleo-Asian Ocean (i.e., North Tianshan Ocean in the north and South Tianshan Ocean in the south) and the following orogenic collage. Continental arc-type felsic rocks yield two major populations of ~460 to ~395 Ma and ~375 to ~310 Ma, respectively, implying two epochs of subduction events punctuated by a magmatic lull. In the first-stage subduction, tectonic switch from advancing to retreating subduction took place around ~450 Ma in the northern YB and ~420 Ma in the southern YB. The second-stage subduction was characterized by a period of trench advance (~ 375 to ~350 Ma in the northern YB and ~370 to ~350 Ma in the southern YB) at the initiation and the following Early Carboniferous trench retreat (~350 to ~310 Ma in the northern YB, and ~350 to ~322 Ma in the southern YB) associated with the development of back-arc basins. The final assembly of the Western Tianshan orogenic collage plausibly occurred during the Late Carboniferous. In the south, a “hard” collision followed the closure of the South Tianshan Ocean. On the contrary, the northern margin of the YB was likely collided “softly” with an immature/nascent island arc.

         The current study highlights a crucial link between supra-subduction extension triggered by trench retreat (slab rollback) and the continental growth of the Yili Block and adjacent regions. On a larger scale, such a long- term “rejuvenation” process, which was characterized by the gradual replacement of old basement by juvenile crust and associated with subduction zone retreat (rollback), has been documented in some other micro-con- tinents of the CAOB. The preservation of juvenile/mixed crust requires some continental margins that did not collide with any ancient micro-continent or craton (i.e., non-collisional or soft-collisional margins) even until the termination of accretionary orogenesis. The development of a series of oroclines is likely the principal me- chanism resulting in survival (preservation) of juvenile/mixed crust within the huge fossil orogen.