Genesis of Cambrian MORB-like basalts at the southeastern margin of the Altai orogenic belt and constraints on tectonic transformation processes on the region

In order to further deepen the understanding of the Early Paleozoic tectonic evolution of the Altai orogenic belt, zircon U-Pb chronology and geochemistry studies were carried out on the gabbro-plagioclase bodies in the Qinghe area at the southeastern margin of the Altai orogenic belt.The zircon U-Pb dating results of the pyroxene show that the formation age of this rock body is at the end of the Middle Cambrian (506±3 Ma, MSWD=0.91), and the Hf isotope analyses of zircon show that the value of ε_Hf (t) ranges from -2.01 to +4.38, and the values of its SiO₂ are 49.10% to 50.25%, K₂O is 0.10% to 0.30%, FeO^T is 7.02% to 8.27%, MgO is 8.15% to 8.70%, Mg^# value is 69 to 71, which belongs to the series of low-potassium zebra basalts. The total amount of rare earths in Qinghe gabbro is low (∑REE = 62.03×10^-6~71.38×10^-6), and it has a rare earth element partitioning pattern similar to that of mid-ocean ridge basalts (N-MORB), in which the internal fractionation of the light and heavy rare earths is not obvious (La/Sm) _N =0.55~0.86; (Gd/Yb) _N =1.38~1.50, and it has a standardised distribution curve in the primitive mantle. distribution curve, enriched in Ba, Sr and Pb elements, and deficient in Th, U, La, Ce, etc., and generally show consistent geochemical characteristics of typical MORB-like basalt phases. Based on the regional geological data, the gabbro-gabbro bodies in the Qinghe area are the products of the pre-arc subduction process, which reveals that the onset of the subduction of the paleoceanic crust of the Paleo-Asian Ocean in the Altai area should be earlier than or equal to that of the Middle Cambrian, and provides new clues for the timeframe of the onset of the subduction of the Paleo-Asian Ocean in the Altai area.